Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ un...
VanSnickSven-PhD thesis
1.
Faculty of Science
Departement of Chemistry
Molecular Design and Synthesis
Synthesis and characterization of novel indolo[3,2-b]carbazoles
towards organic photovoltaic applications
Promotor:
Prof. dr. Wim Dehaen
Doctoral thesis
Sven Van Snick
April 2012
3. Members of the jury
Prof. dr. Mark Van der Auweraer
Prof. dr. Mario Smet
Prof. dr. Guy Koeckelberghs
Prof. dr. Paul Heremans
Prof. dr. Dirk Vanderzande
Prof. dr. Wim Dehaen (Promotor)
III
5. Acknowledgements
At the end of an important chapter in his life, one tends to reflect on the past years
and the key moments that brought him where he is at that moment. Without a
doubt the key moment that has led me to write all of the following in this thesis is
the first meeting I had with professors Wim Dehaen and Paul Heremans, as their
confidence in my abilities resulted in my enrollment at KULeuven. I’m grateful to
them for offering me the chance to have a unique experience while gradually
taking the first steps to what is hopefully a successful career. As many of you,
readers, can testify, research can hardly ever be associated with smooth sailing, so
again I feel the need to thank Wim for keeping me focused and motivated.
Professor Mark Van der Auweraer, Karel, Adis and Helmut I would like to
acknowledge for their help with the characterization of some of the materials I
synthesized and Liliana for the thorough analysis of the crystal structures. Without
your help this thesis would never have seen the light of day.
Directeur de Recherche Eric Rose and his team I thank for presenting me the
opportunity to join his research group. It was reassuring to see that even for a short
time synthesis by my hands resulted in the desired compounds.
Professor Mario Smet and Dirk, thanks a lot for your unconditional and moral
boosting greetings every day.
I also acknowledge my students Théo and Melissa for their motivation and
dedication to this project and the help they gave me with synthesizing the ICZs.
Professor Shouchun Yin I would like to thank for the very nice company and
support during the first two years at the department. I could not have had a nicer
lab partner than you. In the same spirit I thank Joice Thomas, also for his advice
concerning synthesis and his understanding.
Sachin, Vaibhav, Claudia, BK, Jitender, Jalpa, Bo, Yi, Hua, Nithya, Seva, Olga,
Veronica, THX lot for your hospitality and I have to say I couldn’t have had a
V
6. better chance to taste and enjoy genuine Indian, Chinese, Italian, Russian and
Spanish cuisine than by trying the various dishes you prepared.
A special thanks goes out to Karel-Simon and siblings and Walhalla for keeping
me in touch with the fun side of life.
Finally, to the last person I mention here I express the greatest gratitude for
providing a refuge and supporting me with almost all imaginable, little, but very
important things. Mom, merci, you took a lot of weight off my shoulders.
Undoubtedly I forgot to acknowledge a few people here. To them I say: don’t take
it personally! I know it’s easier said than done, but when I meet you, I will surely
remember to make it up to you.
And now…off to chapter 1.
VI
7. Samenvatting
Een van de belangrijkste aspecten in de ondersteuning en werking van een
ontwikkelde maatschappij is de toegang tot een veilige en hernieuwbare bron van
energie. Een veelbelovend alternatief voor het gebruik van fossiele brandstoffen is
de onmiddellijke omzetting van zonne-energie in elektrische stroom door het
gebruik van fotovoltaïsche cellen. Ondanks de commerciële beschikbaarheid van
siliciumgebaseerde fotovoltaïsche materialen, wordt dezer dagen veel aandacht
geschonken aan de ontwikkeling van andere performante producten, zoals
organische materialen.
In dit werk worden indolo[3,2-b]carbazolen onderzocht als potentiële kandidaat
voor gebruik in op organische materialen gebaseerde fotovoltaïsche cellen.
In hoofdstuk 1 wordt de synthese van 6,12-difenyl-5,11-dihydroindolo[3,2-
b]carbazool besproken. We tonen aan dat deze verbinding eenvoudig en in goede
opbrengst gesynthetiseerd kan worden in drie opeenvolgende stappen. Ten eerste,
condensatie van indool met een geschikt aromatisch aldehyde onder zuur-
gekatalyseerde omstandigheden, vervolgens alkylering om de oplosbaarheid te
verhogen en ten laatste oxidatie en halogenering.
In hoofdstuk 2 wordt de reactiviteit onderzocht van de in hoofdstuk 1 besproken,
gehalogeneerde verbinding. Twee courante, transitiemetaal gekatalyseerde reacties,
nl. de Suzuki-Miyaura koppeling en de Sonogashira koppeling, worden daartoe
aangewend. De formylering wordt eveneens behandeld en geeft aanleiding tot een
verbinding die zowel gebruikt wordt als substraat in de Horner-Wadsworth-
Emmons condensatiereactie met gefunctionaliseerde fosfonaten en in de Corey-
Fuchs reactie ter vorming van 2,8-diethynyl-indolo[3,2-b]carbazool.
Laatstgenoemde wordt aangewend als substraat voor de Sonogashira koppeling en
de Cu-gemedieerde azide-alkynkoppeling.
Teneinde de spectroscopische eigenschappen van de gesynthetiseerde indolo[3,2-
b]carbazolen aan te passen, wordt 2,8-diethynyl-indolo[3,2-b]carbazool gekoppeld
VII
8. met twee andere chromoforen (BODIPY en DPP) wat resulteert in indolo[3,2-
b]carbazool gebaseerde donor-acceptor materialen. De condensatie van het
geformyleerde indolo[3,2-b]carbazool met 2,4-dimethylpyrrool wordt besproken,
maar blijkt niet succesvol.
In Hoofdstuk 3 wordt een inleidende studie gedaan naar de vorming van
indolo[3,2-b]carbazoolgebaseerde polymeren. We tonen aan dat
gefunctionaliseerde indolo[3,2-b]carbazolen gepolymeriseerd kunnen worden langs
twee moleculaire assen. Ten eerste, door gebruik te maken van een goed oplosbaar
comonomeer, kan een “6-12” polymeer gevormd worden. Eveneens wordt het
geformyleerde indolo[3,2-b]carbazool gepolymeriseerd gebruik makende van de
Horner-Wadsworth-Emmons condensatiereactie. Als laatste wordt de
polymerisatie van 2,8-diethynyl-indolo[3,2-b]carbazool met BODIPY en DPP
besproken.
De verdere studie van de optoelektronische eigenschappen wordt momenteel
uitgevoerd in samenwerking met de afdeling spectroscopie van het Departement
Chemie (KU Leuven).
VIII
9. Abstract
One very important factor for sustaining a developed society is indisputably the
access to a safe and renewable source of energy. On this account, a promising
alternative to fossil fuels is provided by the direct conversion of solar energy to
electricity through the use of photovoltaic cells. While various silicon based
photovoltaic devices have already been commercialized, much attention is being
devoted to the development of other performant products, such as organic material
based devices. Unlike the former, the use of organic materials allows for a cheaper
production of large area, flexible devices. Many such organic materials are
currently under investigation towards this goal.
In this work, indolo[3,2-b]carbazoles are investigated as potential candidates
towards use in organic photovoltaic devices.
In Chapter 1 we discuss the synthesis of the parent 6,12-diphenyl-5,11-
dihydroindolo[3,2-b]carbazole. We show that this compound can easily be
synthesized in good yield, using three consecutive steps. Firstly, condensation of
indole with a suitable aromatic aldehyde under acid catalysis yields the insoluble
6,12-diaryl-5,6,11,12-tetrahydroindolo[3,2-b]carbazole. The solubility can be
induced by N-alkylation with suitable alkyl halides. In the final step, the soluble
compound is both oxidized and halogenated affording the properly functionalized
2,8-dihalo-5,11-dialkyl-6,12-diphenyl-5,11-dihydroindolo[3,2-b]carbazole.
In Chapter 2 we explore various ways to further expand on the reactivity of this
2,8-dihalo-indolo[3,2-b]carbazole. Two common transition metal catalyzed
reactions, the Suzuki-Miyaura and the Sonogashira coupling are successfully
investigated, giving rise to previously unreported indolo[3,2-b]carbazoles. The
formylation was also performed and gave rise to the 2,8-diformyl-indolo[3,2-
b]carbazole. This compound was used as a substrate in the Horner-Wadsworth-
Emmons condensation with functionalized phosphonates, resulting in novel and
more complex indolo[3,2-b]carbazoles. Furthermore, the 2,8-diformyl-indolo[3,2-
IX
10. b]carbazole was converted to the 2,8-diethynyl-indolo[3,2-b]carbazole using the
Corey-Fuchs reaction. The resulting compound could easily be reacted under
standard Sonogashira reaction conditions and demonstrated to be a good substrate
in the Cu- mediated azide-alkyne coupling.
In an attempt to influence the spectroscopic properties of the synthesized
indolo[3,2-b]carbazoles, the 2,8-diethynyl-indolo[3,2-b]carbazole was coupled
with other chromophores (BODIPY and DPP) giving rise to indolo[3,2-b]carbazole
based donor-acceptor materials which demonstrated a large bathochromic shift in
the absorbance. The condensation of 2,8-diformyl-indolo[3,2-b]carbazole with 2,4-
dimethylpyrrole was also investigated in an effort to directly couple BODIPY onto
the indolo[3,2-b]carbazole, this was however unsuccessful.
In Chapter 3 a preliminary study towards the formation of indolo[3,2-b]carbazole
based polymeric materials was done. We show that a properly functionalized
indolo[3,2-b]carbazole can be polymerized along two different molecular axes.
Firstly, by using a highly soluble 1,4-dialkyne spacer a ”6-12”- polymer could be
synthesized with moderate molecular weight and a small bathochromic shift in the
absorbance. Secondly, the 2,8-diformyl-indolo[3,2-b]carbazole was polymerized
using the Horner-Wadsworth-Emmons condensation, resulting in a low molecular
weight material with only a small bathochromic shift. The 2,8-diethynyl-
indolo[3,2-b]carbazole was polymerized with two different chromophores, namely
DPP and BODIPY, resulting in two low molecular mass materials, which showed a
large bathochromic shift in the absorbance.
A further study of the optoelectronic properties is currently being undertaken in
collaboration with the spectroscopy research group at the Department of Chemistry
(KU Leuven).
X
11. List of Abbreviations
AcCl Acetyl chloride
AcOH Acetic acid
ASTM American Society for Testing and Materials
atm atmospheric pressure
BHJ bulk heterojunction
BODIPY 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
CI chemical ionization
CIE Commission Internationale de L’éclairage
CuAAC Cu- mediated alkyne azide coupling
D – A donor-acceptor
DCM dichloromethane
DDQ 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
DMF N,N-dimethylformamide
DMSO dimethyl sulfoxide
DMSO-d6 deuterated dimethyl sulfoxide
DNA deoxyribonucleic acid
DPP Diketopyrrolopyrrole, 3,6-bis(4-bromophenyl)-2,5-
dihexylpyrrolo[3,4-c]pyrrole-1,4-dione
DSSC dye sensitized solar cell
EBU European Broadcasting Union
EI electron impact
EM electromagnetic
EL electroluminescence
ESI electrospray ionization
EtOAc ethyl acetate
EtOH ethanol
FET Field effect transistor
HOMO highest occupied molecular orbital
HRMS high resolution mass spectrometry
XI
12. HWE Horner-Wadsworth-Emmons
ICZ 5,11-dihydroindolo[3,2-b]carbazole
iPr2NH N-isopropylpropan-2-amine
IR infrared
KHMDS Potassium hexamethyldisilazane
KOH Potassium hydroxide
KOtBu Potassium tert-butoxide
LAH Lithium aluminum hydride
LED light emitting diode
LUMO lowest unoccupied molecular orbital
Mn number average molecular mass
MS Mass spectrometry
MsCl Methanesulfonyl chloride
n-BuLi n-butyllithium
NBS N-bromosuccinimide
NEt3 triethylamine
NIS N-iodosuccinimide
NMR nuclear magnetic resonance
NPB N,N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’-
diamine
OLED organic light emitting diode
OP organic photovoltaics
OPC organic photovoltaic cell
OTFT organic thin film transistor
P3HT Poly(3-hexylthiophene)
PDI polydispersity index
Ph phenyl
PL photoluminescence
P(OEt)3 triethyl phosphite
PSC polymer solar cell
rt room temperature
SQ 3,4-dihydroxycyclobut-3-ene-1,2-dione (Squaric acid)
XII
13. TFT thin film transistor
THF tetrahydrofuran
TLC thin layer chromatography
TM transition metal
TMS trimethylsilyl
UV-Vis ultra violet- visible
XIII
19. I. Introduction
I.1. Preface
Throughout the evolution of human species, chemistry has always played a vital
role in aiding to secure its position in the world. An unmistakable role is reserved
here for organic chemistry in general and the heterocyclic chemistry branch in
particular. With its virtually innumerous, well-established and still improving
synthetic procedures, together with the existence of an immense library of organic
building blocks, a very powerful tool arises to plan for and synthesize whatever
chemical structure the chemist can envision within the framework of his goals.
The vastness of possibilities created by this inexhaustible source of knowledge has
sparked many areas in which its potential has already clearly been demonstrated.
Amongst many others, the relatively new field of organic electronics is a testimony
to this claim.
I.2. Organic electronics
I.2.1. Origin and definition
While the basic idea from which molecular electronics has grown was postulated in
the late 1940s by Mulliken, the actual birth of organic electronics is often
associated with the discovery of the highly conductive polyacetylene in 1977.1,2
Although polyacetylene had already been prepared in 1958 by Nata et al., further
research into this polyene was restricted, mainly due to the low solubility of the
product.3
Nonetheless, Shirakawa and co-workers persisted and were able to obtain
polyacetylene as a film comprised of entangled microfibers in 1974.4
The full
potential of polyacetylene became clear when Heeger et al. discovered a dramatic
increase in the conductivity of polyacetylene films upon oxidation with halogens
(Cl2, Br2 and I2).2
The conductivity of this “doped” polymer had increased by a
factor 105
, resulting in a conductivity of the order 103
S/m, which makes it
comparable to the conductivity of common metals for electric and electronic
applications, such as Cu (107
S/m). In 2000, Heeger, Shirakawa and MacDiarmid
1
20. received the Nobel Prize in Chemistry for the development of electrically
conductive polymers.5
Ever since, continuous efforts have been made to explore
new classes of organic compounds that exhibit high conductivity and other
properties which could make them serve as potential conducting materials,
including polypyrroles, polyanilines, polythiophenes, etc.6
In its most rudimentary sense, organic electronic materials can be described as
electronic devices which owe their working to the action of an active layer built up
from organic molecules. In essence this means that the device consists of one or
more active components, based on carbon (C). Although carbon is crucial in this
definition, the active layers rarely contain only carbon (besides H). Other elements
are also present. These so called heteroatoms ( N, O, S, Se, Te,…) are often very
important for the specific characteristics of various electronic devices.7
I.2.2. Applications
In the quest for more economical and environmentally friendly materials for use in
both low-and high-tech applications, organic molecule based devices have received
a great deal of attention. This can largely be ascribed to the special requirements,
such as flexibility, low power consumption, device fabrication, etc. which can be
determinative for the viability of the application.8
The accessibility of the required
building blocks can also advocate the choice for carbon based electronics, since
this will be reflected in the cost of the production process. Furthermore, the low
energy requirements for producing organic electronics, in comparison with the
metal based counterparts, are of great environmental importance.9
These considerations have lead to the development, optimization and production of
organic molecule based optoelectronic devices.
2
21. I.2.2.1. Organic Light Emitting Diodes (OLEDs)
Light emitting diodes (LEDs) are a class of semiconductors that emit light, mainly
in the visible part of the electromagnetic spectrum. The workings of a LED are
based on the principle of electroluminescence (EL), i.e. the radiative recombination
of holes and electrons under the influence of an electric field. The first report of EL
in organic compounds was made by Bernanose in 1955, using gonacrin and
acridine as luminescent materials.10
Subsequent research eventually led to the
fabrication of the first real OLED by Tang et al. in 1987.11
As the name suggests,
OLEDs contain organic molecule based active layers, either monomeric or
polymeric, through which the charge carriers (holes and electrons) are transported.
In the emissive layer the charges recombine and emit light with a wavelength
depending on the position of the energy levels of the two layers (Figure I.1).
Figure I.1. OLED device.
Continuous efforts made in the next two decades resulted in the better
understanding of the workings of OLEDs and allowed for the development of more
performant organic layers.12
For display applications, red, green and blue OLEDs
are highly desired. According to the RGB (Red, Green and Blue) additive color
+
-
e- e- e- e-
e- e- e-
e- e- e- e-
h+e- 2 h+e- 2 h+e-
h+ h+ h+ h+
h+ h+ h+ h+
1
3
1
3
Conductivelayer (p-type)
Emissivelayer (n-type)
Cathode
Transparant anode
1 Charges migrateunderthe
influence of an electric field.
2 Charges recombinein the
emissiveLayer.
3 Emission oflight through the
conductivelayer.
3
22. model, the combination of these three colors can produce a broad array of other
colors.
In 2010, Ma et al. reported on the synthesis of novel red (I.1) and green (I.2)
emitting small molecules characterized by luminous efficiencies of 31.6 cd/A and
7.2 cd/A, respectively and with CIE coordinates close to the EBU standard, which
are amongst the highest reported efficiencies for colored OLEDs.13
An efficient
blue emitter I.3 was reported recently by Kieffer and co-workers, showing a
luminous efficiency of 13.2 cd/A (Figure I.2).14
Figure I.2. Efficient red, green and blue OLED materials.
N
N
NC CN
R R
R
R
R R
R
R
R = C6H13
BLUE
GREEN
RED
N
R
N
S
N
R
R
R R
I.3
I.2
I.1
NN
R RR R
4
23. I.2.2.2. Organic Thin Film Transistors (OTFTs)
In 1930, Lilienfeld was awarded a patent concerning a method and apparatus for
controlling electric currents and hence introduced the concept of the field effect
transistor (FET).15
The first thin film transistor (TFT), making use of the field
effect in a CdS semiconductor was reported by Weimer in 1962.16
Much later, in
1987, Koezuke et al. prepared the first organic thin film transistor (OTFT) with
polythiophene as an active semiconducting material.17
In figure I.3 a typical
geometry of an OTFT is depicted. In such a device, the flow of charge carriers
between the source (S) and the drain (D) is mediated by an organic semiconducting
channel. In case the channel consists of a p-type semiconductor, e.g. pentacene,
and a negative voltage is applied to the gate (G), the semiconducting channel
between the source and the drain becomes conductive, resulting in the flow of
charge between these two contacts (Figure I.3).18
Figure I.3. OTFT device (bottom gate).
Research towards improvement of the device characteristics is situated mainly in
the development of new organic conductive materials and is ongoing until today.19
S D
G
+
h+ h+ h+
1
3
2 2 2 2
Semiconductor(p-type)
Isolatinglayer
Substrate
Electrode
1 Negativepotentialover thegate.
2 P-typesemiconductor
becomes conductingchannel.
3 Chargeflows between S and D.
-
5
24. I.2.2.3. Organic Photovoltaics (OP)
I.2.2.3.1. Dye Sensitized Solar Cells (DSSCs)
The DSSC, often referred to as the Grätzel cell, was introduced by Grätzel and co-
workers in 1991.20
In comparison with existing solar cell technologies at that time,
the Grätzel cell immediately became competitive.21
Indeed, the first device
displayed a power conversion efficiency of 12% and was exceptionally stable
under illumination, as compared to the other organic photovoltaic architectures.
The typical DSSC is comprised of nanometer sized TiO2 particles onto which an
organic dye is adsorbed. The organic dye acts a photosensitizer and transfers the
photogenerated electron to the conductive TiO2 upon excitation by Vis-light. The
circuit is completed by the electron transfer from the anode to an electrolyte.
Mostly the electrolyte is the I2/I3
-
redox couple, however, other electrolytes such as
ionic liquids have been successfully employed in DSSC devices.22
Finally, the
redox couple effects the electron transfer back to the photooxidized dye (Figure
I.4).
Figure I.4. DSSC device.
In the last two decades the device architecture and the operating principles have
not radically changed and the main focus has been on finding more efficient dyes.
Mostly these photosensitive dyes are based on ruthenium as the coordination metal
6
~3
Electrolytesolution
Organicdye
Anode
Transparant cathode
1 Electron transferto TiO2.
2 Electron transferto cathode.
3 Poweringofload.
4 Electron transferto electrolyte.
5 Electron transferto dye.
I2 / I3
-
<
>
5
2
4
1
TiO2
6
25. for the organic ligands, but also other metals have been employed.23
The highest
power conversion efficiencies were reported for the porphyrin derivative I.4, which
serves as the photo-sensitizer in DSSCs (η = 12.3%) (Figure I.5).24
Figure I.5. Organic dyes in DSSCs.
I.2.2.3.2. Organic Photovoltaic Cells (OPCs)
In comparison with the rather high conversion efficiency of DSSCs, the polymer
based organic photovoltaic cells (PSCs) are less efficient at converting solar power,
but are considered to be more promising candidates in future applications.25
Since
they do not require a liquid electrolyte this greatly benefits the production process,
making it possible to print the constituent layers on flexible substrates in a roll-to
roll process.8a
The breakthrough OPC design dates back to 1985 when Tang et al. reported the
first heterojunction photovoltaic cell, with a 1% power conversion efficiency.26
With this bi-layer device a novel device architecture was suggested that was crucial
in determining the photovoltaic properties namely, the interface between two
different layers (p-type and n-type) provides a dissociation site for the photo-
generated charges (holes and electrons), which are then transported through the
respective active layers to be collected at the electrodes.
N
N N
N
N
C8H17O OC8H17
OC8H17C8H17O
C6H13O
C6H13O
COOHZn
I.4
Ru
HOOC
COO-
HOOC
COO-
SCN
SCN
7
26. The next step in the evolution of OPCs was made in 1995 with the report of the
first bulk heterojunction (BHJ) OPC by Holmes and co-workers.27
The cornerstone
in this BHJ concept is the interpenetrating network formed between two different
active layers. This effectively increases the interface contact area and provides the
necessary energy offset for charge separation, resulting in improved charge
dissociation (Figure I.6).28
Figure I.6. PSC device.
On the molecular level, the formation of a bound hole-electron pair (exciton)
involves several subsequent steps leading to the extraction of electrical charges at
the corresponding electrodes. Firstly, a photon with sufficient energy to bridge the
band gap (ΔEg) causes the excitation of a valence electron from the HOMO level to
the LUMO level of the photosensitive dye (figure I.7).
h+e-
e-
h+
2
1
4
4
3
6
6
~
5
5
6
Holeconductor (+)
Electron conductor(-)
High workfunctionelectrode
Low workfunctionelectrode
1 Photogenerated exciton.
2 Diffusion ofexciton to dissociation site.
3 Separationofexciton into holes and
electrons.
4 Diffusion ofcharges.
5 Collection ofcharges.
6 Poweringofa load.
8
27. Figure I.7. Photo excitation creating an exciton.
In contrast with inorganic photovoltaic cells, organic materials have a low
dielectric constant, preventing the immediate separation of the exciton in its
constituent charges. An energy barrier has to be overcome to create these separated
charges. This preferably takes place at the interface of the hole conductor (organic
dye) and the electron conductor and requires an energy offset between the two
LUMO levels involved (Figure I.8).
Figure I.8. Exciton dissociation at the donor/acceptor interface.
1
E (eV)
0
ITO
Al
D D* A
1 Photoexcitation
ITO
Al
D* A
1
2
1 Energy off-set
2 Band gap ΔEg
E (eV)
0
9
28. Before and after charge separation, several deactivation pathways can cause the
charges to recombine, but ideally they successfully migrate towards their
corresponding electrodes to be extracted. Although a more detailed discussion of
the physical principles and the relevant quantities involved in the formation of
charges through absorption of photons is beyond the scope of this thesis, one
quantity is imperative in relation to the goals of this work. From the Planck relation
it is clear that the band gap ΔEg is in direct relation with the wavelength of the
incident photons, and that a smaller band gap implies the need for photons of
longer wavelength (or lower energy). Currently, large efforts are being made
towards the synthesis of small band gap materials. The reason for this originates
from the specific composition of solar light. Because solar light is polychromatic, it
contains photons with different wavelength (different energy) and also, their
numbers differ. This can easily be illustrated in a plot of the solar photon flux in
function of the wavelength (Figure I.9)
Figure I.9. Solar irradiance under AM 1.5 conditions. Data obtained from ASTM.
From this plot it can be seen that substantially more photons with lower energy
penetrate earth’s atmosphere and are thus available to be absorbed by the organic
dye in the OPC. As a consequence, low band gap materials are highly desired to
benefit from this large amount of photons. A popular approach to create small band
10
29. gap materials is through the synthesis of donor-acceptor materials, which act as the
photosensitive dye. A simple molecular orbital scheme explains this concept.
When a donor (electron rich) segment and an acceptor (electron withdrawing)
segment are covalently linked, four new molecular orbitals arise. Distribution of
the lowest energy electrons in the HOMO orbital of the donor and acceptor, over
the newly formed molecular orbitals according to the Aufbau principle, create a
new HOMO and LUMO level with decreased energy spacing or, lower band gap
(Figure I.10).
Figure I.10. Creation of small band gap materials.
Research in the last 15 years has been dedicated to optimizing the BHJ
morphology and on the tuning of the energy levels of the active layers involved.
Extensive reviews dealing with all aspects involved have recurrently appeared in
the literature.29
To date, the highest power conversion efficiency obtained with a
polymer based BHJ OPC is 8.3%.30
The most performant polymer for use in
polymer solar cells is to date still poly(3-hexylthiophene) (P3HT) and has been the
focus of research in the last ten years (Figure I.11).31
ΔEg,D
ΔEg,D-A
D D-A-dyad A
LUMOA
LUMOD
HOMOA
HOMOD
1 Band gap ofthefree
donorΔEg,D .
2 Decreased band gap of
the donor-acceptordyad
ΔEg,D-A .
2
1
11
30. Figure I.11. Poly(3-hexylthiophene) or P3HT.
All the aforementioned organic compounds represent only a small fraction of
active materials which have been suggested by the scientific community. Several
reviews on this matter present in the literature give a more detailed image of
possible materials.32
I.2.3. Active layers in OPCs
Concerning the application of organic molecules for organic photovoltaics, most
appear to be comprised of thiophene or thiophene derivatives as leading moiety.33
Although the use of thiophene based organic electronics is wide-spread, the search
for new materials plays a key role in further improving overall device
efficiencies.34
Amongst the nitrogen containing heterocycles, (poly)carbazoles have been
explored to some extent as efficient components for organic electronics.35
Carbazole is known to possess electron donating properties and is thus a candidate
for organic electronic applications which require p-type behavior, this is
particularly the case for organic photovoltaics.36
Over the years, the photovoltaic
efficiencies of carbazole and polymers containing carbazole donors have shown an
steady increase, with reports of efficiencies as high as 6.5%.37
Another promising candidate related to carbazole are the indolo-
[3,2-b]carbazoles (ICZs, I.5) because of their resemblance to pentacene (I.6)
(Figure I.12).
S
**
n
P3HT
12
31. Figure I.12. Structural similarity between ICZs and pentacene.
The latter is a well-studied organic hole conductor reaching hole mobilities up to
>5 cm²/Vs and has hence been implemented in OTFTs and OPCs.38
Pentacene
suffers however from a low solubility in organic solvents and poor stability
towards environmental conditions.39
Because of the low solubility, deposition on a substrate is preferably done by
sublimation.40
This process requires state of the art equipment to deposit pentacene
in high purity and is very energy consuming. Alternatives have been suggested and
researched, nonetheless straightforward implementation of pentacene in organic
electronics remains a cumbersome practice.41
Even though the solubility of plain ICZs is also quite low, this can easily be
overcome by functionalizing the molecule in various positions, without interfering
with its stability towards atmospheric conditions. Several functionalized ICZs have
already been designed and synthesized to serve as active layer in various
optoelectronic devices. Early reports of the use of ICZs in OLEDs indicated good
hole transporting mobilities and excellent electrochemical stability, resulting in a
device performance equivalent to NPB (a well-recognized hole transporter and
often cited as the reference OLED material) based ones.42
As a soluble alternative to other p-type molecules in OFETs, ICZs have
demonstrated high hole mobilities in an OFET configuration (up to 0.22 cm²/Vs).43
High power conversion efficiencies were reported for polymer based OPCs
containing functionalized ICZs as electron donor.44
The best result was obtained for
N
H
H
N
indolo[3,2-b]carbazole pentaceneI.5 I.6
13
32. a pyrazino[2,3-g]quinoxaline – ICZ polymer I.7 which demonstrated power
conversion efficiencies as high as 3.2% (Figure I.13).45
Figure I.13. Efficient ICZ pyrazino[2,3-g]quinoxaline – ICZ polymer in PSCs.
Even in DSSCs, properly functionalized ICZs show efficiencies of more than 7%.46
I.3. Indolocarbazoles
I.3.1. Origin and definition
Indolocarbazoles are a class of N-containing heterocycles for which five structural
isomers can be defined, depending on the linkage between the constituent indole
and carbazole moieties (Figure I.14).
N
N
C6H13
C6H13
C6H13
S
S
S
C6H13
NN
N N
S
C6H13
ICZ in PSC.
h = 3.2%
m = 0.7, n = 0.3
m
n
I.12
14
33. Figure I.14. Indolocarbazole structural isomers.
As reported in the literature, so far only three ICZ-isomers have been isolated from
natural sources, namely indolo[3,2-a]carbazole, indolo[2,3-a]carbazole and
indolo[3,2-b]carbazole.47
These isomers are also accessible through organic
syntheses and have been synthesized with success in no small part by the
contributions made to the field by Jan Bergman et al.48
Indolo[2,3-c] and [2,3-b]carbazoles are thus far only obtainable through synthetic
means.49
Concerning the applications of ICZs a lot of attention has been dedicated to
indolo[2,3-a]carbazoles, and indolo[2,3-c]carbazoles to a lesser extent, because
these isomers have shown to possess DNA topoisomerase I inhibitory activity.50
Indolo[2,3-b]carbazoles and indolo[3,2-a]carbazoles are only scarcely mentioned
in literature, probably because there is no apparent connection to any application
neither biological nor technological.
N
H
H
N
N
H
N
H
N
H
N
H
N
H
HN
HN NH
indolo[3,2-b]carbazoleindolo[2,3-b]carbazole
indolo[3,2-a]carbazoleindolo[2,3-a]carbazole
indolo[2,3-c]carbazole
I.5
15
34. The indolo[3,2-b]carbazole isomer on the contrary seems to be very diverse in
properties and has been studied for its biological activity. Moreover, this isomer
has also been investigated for use in electronic applications as mentioned in the
previous paragraph and illustrated by several examples in Figure I.15.
16
35. Figure I.15. Indolo[3,2-b]carbazoles in organic electronics.
N
N
C6H13
C6H13
C6H13
S
S
S
C6H13
NN
N N
S
C6H13
ICZ in PSC.
h = 3.2%
m = 0.7, n = 0.3
m
n
N
N
C8H17
C8H17
ICZ in OFET.43
μ = 0.22 cm2
/Vs
N
N
C6H13
O
O
ICZ in OLED.42b
η = 3,6 cd/A
N
N
S
S
H17C8
C8H17
C6H13
C6H13
COOH
CN
ICZ in DSSC.46
η = 7.3%
I.9
I.10 I.11
I.12
17
36. The reported indolo[3,2-b]carbazoles notwithstanding, research into this class of
promising semiconductors seems to have lost its momentum as evidenced by the
drop in the number of research papers concerning further development and
characterization of these ICZs in the last five years. One very probable reason for
this decline can be attributed to the tedious and cumbersome synthetic procedures
towards efficient construction of the indolo[3,2-b]carbazole core structure and
further functionalizing of this molecule, limiting the option to tune the
optoelectronic characteristics of this substrate.
In the following paragraph a short description of the most interesting synthetic
routes towards indolo[3,2-b]carbazoles will be highlighted. Although a full
coverage of every pathway would surely demonstrate the complexity and
sometimes problematic nature (overall synthetic yield) of ICZ syntheses, only the
most interesting milestones in the progress towards elegant and high yielding
syntheses will be discussed.
I.3.2. ICZ syntheses
The first literature report on the synthesis of indolo[3,2-b]carbazoles originates
from 1960 when the parent ICZ was prepared by Grotta et al. by
cyclodehydrogenation of N,N’-diphenyl-p-phenylenediamine I.12 over a Pt/MgO
catalyst at 560°C, yielding the 5,11-dihydroindolo[3,2-b]carbazole I.5 in 10% yield
(Scheme I.1).51
Scheme I.1. ICZ synthesis by Grotta et al.
NHHN N
H
H
N
Pt/MgO, 560°C
10%
I.12 I.5
18
37. Three years later, Robinson obtained ICZ I.5 in 35% yield through double Fischer
indolisation of cyclohexane-1,4-dione bisphenyl hydrazone I.13 (Scheme I.2).52
Although this synthesis is quite dated, it is thus far the most popular one towards
substituted ICZs.53
Scheme I.2. ICZ synthesis by Robinson.
The condensation of indole with formaldehyde under acidic conditions was
investigated by Bergman in 1970.54
In this pathway indole supposedly reacts with
formaldehyde under acid catalysis to produce 3,3’-bisindolylmethane I.14.
Subsequent isomerization to 2,3-bisindolylmethane I.15 and reaction with another
equivalent of aldehyde produces the intermediate I.16 which then yields ICZ I.5 in
the presence of air, light and a sensitizer. If the latter conditions are not met,
trimeric and tetrameric species are formed (Scheme I.3).
NN
HN
NH
AcOH, H2SO4
35%
N
H
H
N
I.13 I.5
19
38. Scheme I.3. ICZ synthesis by Bergman.
In 1986, a noteworthy improvement to this synthetic pathway was proposed by
Pindur et al.55
When trying to prepare ICZ I.17 from 3,3’-bisindolylmethane I.14,
ICZ I.5 was formed in 80% yield instead. This result can be explained by the acid
catalyzed Plancher rearrangement, which is mechanistically related to the Wagner-
Meerwein rearrangement (Scheme I.4).56
N
H N
HN
H
N
HN
N
N
H
H
N
HCHO
H+
I.5
I.15
indole
HN NH
I.14
HCHO
H+
I.16
H
N
NH
Air, light
Sensitizer
22%
NH
20
39. Scheme I.4. ICZ synthesis by Pindur et al.
Although this reaction yield is one of the highest reported for the synthesis of
indolo[3,2-b]carbazoles it remains a questionable result. Literature concerning the
reproduction or further exploration of this pathway appears to be nonexistent.
The self-condensation of 2-(benzotriazol-1-yl-methyl)indole I.18 in the presence of
ZnCl2 was reported in 1995 by Katritzky et al. resulting in ICZ I.19 in 64%
isolated yield (Scheme I.5). The ICZ precursor I.18 however, needs to be
synthesized in three steps from 1-propargylbenzotriazole, eventually providing the
ICZ I.19 in 35% overall yield.57
R = Alkyl.
Scheme I.5. ICZ synthesis by Katritzky et al.
The most efficient ICZ synthesis was reported by Tholander et al. in 1999 and
involves the Pd- assisted cyclisation of diethyl 2,5-dianilinoterephthalate. This
HN NH
N
H
N
H
N
H
H
N
HC(OEt)3, H+
+ 80%
I.14
I.17 I.5
N
H
N
R
N
N
ZnCl2
N
H
H
N
3 steps
R
R
64%
I.19I.18
Bt
Bt = benzotriazole
21
40. short reaction path provides the ICZ I.20 in very good yield (83%), but requires
stoichiometric amounts of Pd(OAc)2 (Scheme I.6).58
Scheme I.6. ICZ synthesis by Tholander et al.
In 2006, Gu et al. described the synthesis of novel functionalized 5,11-
dihydroindolo[3,2-b]carbazoles by a three stage “one-pot” procedure.59a-c
Indole
was condensed with an aliphatic aldehyde, providing 3,2’-bis(indolyl)alkanes I.15
prior to the acid-catalyzed ring closure with triethyl orthoformate (Scheme I.7).
Scheme I.7. ICZ synthesis by Gu et al. R = CH3, C5H11, C11H23.
The resulting 6-monosubstituted 5,11-dihydroindolo[3,2-b]carbazoles were
subsequently functionalized at various positions. Formylation and bromination
were readily performed, the latter, however, appeared to be non selective (Scheme
I.8).
NHHN
N
H
H
N2.2 eq.Pd(OAc)2
83%
O
O
OEt
EtO
O OEt
OEtO
HOAc, 100°C
I.20
N
H
RCHO
HN
H
N
R
N
H
H
N
R
I2
(EtO)3CH
H+
36-50%
I.15 I.20
22
41. Scheme I.8. Reactivity of 6-pentyl-5,11-dihydroindolo[3,2-b]carbazole.
It was further reported that the similar condensation of indole with aromatic
aldehydes (R = aryl in Scheme I.7) under these conditions was less successful,
caused by the formation of an insoluble precipitate. This precipitate was eventually
identified as the 6,12-diphenyl-5,6,11,12-tetrahydroindolo[3,2-b]carbazole.59d
N
H
H
N
C5H11
N
H
H
N
C5H11
N
H
H
N
C5H11
N
H
H
N
C5H11
POCl3/DMF
NBS
Br2
CHO
Br
Br
Br
Br
Br
Br
Br
Br
50%
20%
40%
I.20 I.21
I.22
I.23
23
43. Objectives
The ultimate aim of this research project is to evaluate the potential of indolo[3,2-
b]carbazoles to serve as charge generating layer in organic photovoltaic cells.
Given the limited amount of research dedicated to this substrate for use in OPCs,
many obstacles will have to be overcome to arrive at functional ICZs.
Firstly, construction and functionalisation of the backbone to yield the soluble and
adequately functionalized ICZ building blocks will be addressed. Despite the
sizeable number of proposed synthetic routes towards functionalized indolo[3,2-
b]carbazoles reported in literature, all of them are characterized by long syntheses
and sometimes vigorous reaction conditions and thus often provide the desired
compound only in low yield. As a continuation of the work done by our research
group on the synthesis of 6,12-diarylindolo[3,2-b]carbazoles and their analogues
we will plan for a robust synthetic protocol that allows to prepare the indolo[3,2-
b]carbazoles in a straightforward way and with increased yield by the use of cheap
starting materials and building blocks.
To increase the solubility in organic solvents two different strategies will be
explored. Several reactive positions on the ICZ core structure are available to attain
this goal; positions 2,4,8 and 10 are amenable to Friedel-Crafts alkylation and the 5
and 11 positions can easily be alkylated with linear or branched chains. The
presence of alkyl substituents will not only induce the necessary solubility, but also
prevent the oxidation of the backbone (Figure O.1).
25
44. R = Phenyl and 4-bromophenyl.
Figure O.1. Possible positions for functionalisation towards increasing the
solubility of ICZs.
When the backbone structure is formed we will investigate the reactivity of this
ICZ towards functionalization at the remaining reactive positions. Based on the
inherent reactivity features of this class of compounds we firstly explore
functionalisation of the 2-and 8-positions, since these are expected to be easily
accessible. Once a properly functionalized substrate has been formed, several
transition metal catalyzed coupling procedures, such as the Suzuki-Miyaura, the
Sonogashira and other reaction pathways will be used to create novel ICZ-based
materials which will subsequently be spectroscopically characterized to evaluate
their performance concerning the absorption of Vis-light. We plan to further tune
the optoelectronic properties of the ICZs by implementing well-known and
effective literature procedures. This includes the synthesis of donor/acceptor –
dyads to increase the spectral coverage of the ICZs.
Decoration of the ICZ backbone in the 3-and 9-positions will also be attempted,
starting from a properly 2,8-difunctionalized ICZ. Conjugation along the 3,9-
difunctionalized ICZ is expected to be better and furthermore results in a larger
quinoid contribution to the overall system. It is expected that this larger quinoid
contribution will result in a bathochromic shift of the absorbance (Figure O.2).
N
H
H
N
R
R
2
5
8
4
10
11
26
45. R = C6H13
Figure O.2. Possible positions for functionalisation towards tuning the
optoelectronic properties of ICZs.
Because throughout the synthesis of the ICZ backbone the opportunity arises to
create bifunctionalized derivatives we will test this substrate as a basis for
polymerization along two different molecular axes, with the aim of expanding the
conjugation and tune the optoelectronic properties of this ICZ, giving rise to
polymers amenable for use as active layer in OPC devices (Figure O.3).
R = H, C6H13.
Figure O.3. Molecular axis for polymerization of the ICZ.
N
N
2
3
8
9
R
R
N
N
2
8
6
12
R
R
27
47. Chapter 1 Synthesis of soluble 2,8-
difunctionalized 6,12-diaryl-5,11-
dihydroindolo[3,2-b]carbazole
1.1. “One-pot” synthesis of the indolo[3,2-b]carbazole
backbone
In 2009 Gu et al. reported on the synthesis of 6,12-diaryl-5,11-dihydroindolo[3,2-
b]carbazoles by condensation of indole and an aromatic aldehyde under acid
catalysis.59d
Hydrogen iodide (57% w/w) was shown to be an efficient catalyst for
the reaction, yielding the expected ICZs in moderate yield, using either electron
rich, electron poor and sterically hindered aromatic aldehydes.
In the first reaction step an insoluble precipitate consisting of a mixture of
5,6,11,12-tetrahydroindolo[3,2-b]carbazole 1.2 and the desired oxidized analogue
1.1 was formed. This mixture, containing up to 50% of the non-oxidized ICZ 1.2
could easily be transformed into pure oxidized ICZ 1.1 by suspending the
precipitate in fresh acetonitrile and adding I2 as oxidant. (Scheme 1.1)
i) CH3CN, HI (57% w/w), 80°C, 14 hrs.
ii) CH3CN, I2, 80°C, 14 hrs.
Scheme 1.1. Synthesis of 6,12-diaryl-5,11-dihydroindolo[3,2-b]carbazoles.
N
H
+
R
O
H
N
H
H
N
R
R
N
H
H
N
R
R
+ N
H
H
N
R
R
i ii
1.1a R = phenyl
1.1
1.2
1.1b R = 4-bromophenyl
29
48. As a consequence of the low solubility of the ICZs, simple filtration and washing
with cold acetonitrile yielded the oxidized ICZs in moderate yield.
1.2. Solubilizing the ICZs
1.2.1. t-Butylation
A noteworthy obstacle in the manipulation of the ICZs is the low solubility of these
compounds in common organic solvents. The lack of sufficient solubility can be
attributed to the structure of the backbone, which is almost completely flat. This
results in an intense π-π-stacking interaction between neighboring units.60
Low solubility greatly inhibits further treatment of any organic compound, hence
solubilizing the ICZ is of primary importance before conducting subsequent
reaction steps. An effective way to overcome this was by the introduction of t-butyl
groups on the ICZ backbone. These bulky substituents prevent the aforementioned
π-π-stacking and increase interaction with the solvent through Van der Waals
interactions. This strategy was applied to ICZs 1.1 by Friedel-Crafts type
alkylation, using t-butyl chloride in CHCl3 and ZnCl2 as Lewis acid, resulting in
1.3a in 77% yield and 1.3b in 78% yield. The t-butylated compounds showed
greatly improved solubility in CHCl3 and thus gave rise to a soluble precursor 1.3b
for further reactions (Scheme 1.2).
30
49. i) CHCl3, t-BuCl, ZnCl2, reflux, overnight.
Scheme 1.2. t-Butylation of ICZs 1.1.
1.2.2. N-alkylation
A second manner to increase the solubility was also explored. It was previously
demonstrated that the introduction of long linear or branched alkyl chains on the
backbone of indolo[3,2-b]carbazoles can affect the solubility of these compounds
without affecting the optoelectronic properties.61
The two nitrogen atoms of the
indole moieties of the ICZ are excellent anchoring points for these solubilizing
alkyl chains. Not only is this strategy effective in inducing increased solubility, but
it also leaves the remaining reactive positions ortho, and para to the indole
nitrogen in the backbone untouched, allowing for further decoration of the
molecule. The concept was firstly applied to ICZ 1.1a. Starting material 1.1a was
suspended in DMSO and treated with a 50% NaOH solution, followed by the
addition of hexyl bromide, affording the N-alkylated ICZ 1.4 in 78% yield. The
mono-alkylated ICZs were not observed in the reaction mixture (Scheme 1.3). The
same reaction conditions were expanded to other ICZs and were also used to attach
longer alkyl chains to these ICZs. All alkylations proceeded uneventfully, but the
reaction yields appeared to decrease upon increasing alkyl chain length,
presumably caused by the lower reactivity of these longer chains. (Table 1.1).
N
H
H
N
R
R
N
H
H
N
R
R
i
1.1a R = phenyl
1.1b R = 4-bromophenyl
1.3a R = phenyl, 77%
1.3b R = 4-bromophenyl, 78%
31
50. i) DMSO, NaOH (50%), alkyl bromide, rt, 3 hrs.
Scheme 1.3. N-Alkylation of oxidized ICZs.
Table 1.1. Reaction yield of the double N-alkylation.
ICZ R1
R2
Yield (%)
1.4 H C6H13 78
1.5 H C12H25 55
1.6 Br C6H13 78
1.7 Br C12H25 50
1.3. Functionalizing the soluble ICZs
With the availability of two types of soluble ICZs, we set out to functionalize these
at the remaining reactive positions. In contrast to the N-alkylated ICZs, the t-
butylated analogues have no free sites left and thus further functionalisation is not
possible. ICZ 1.3b was however polymerized to yield a material with a 6,12-axis
backbone, which will be discussed in Chapter 3.
N
H
H
N
N
N
i
R2
R2
Table 1.1
R1
R1
R1
R1
1.1a R1
= H
1.1b R1 = Br 1.4-1.7
32
51. 1.3.1. Bromination of ICZs
Haloaryls are very versatile reagents in various transition metal catalyzed C-C-
coupling reactions, such as the Suzuki-Miyaura and Sonogashira reactions.62
The
importance of these coupling reactions is reflected by the impressive variety of
structural motives that can be introduced on an equally great number of substrates,
using straightforward reaction conditions and catalytic amounts of Pd.63
In relation to the goals we envisioned that bromination of the ICZs would allow us
to introduce substituents at these positions by using transition metal catalyzed
reactions, and thus create the means to influence the optoelectronic properties of
the ICZs through polymerization with other suitable monomers. To this end ICZ
1.5 was treated with 2 eq. of NBS in AcOH. Unexpectedly, no brominated ICZs
were observed in the reaction mixture. When Br2 was used as a halogen source, a
complex mixture containing di- and tetrabromo- ICZs was obtained with only
small amounts of dibrominated ICZ present, as evidenced by proton NMR
(Scheme 1.4). The formation of multiple brominated 6-monosubstituted
indolo[3,2-b]carbazoles upon treatment with Br2 was reported earlier by Gu et
al.59a
i) AcOH, NBS, rt.
Scheme 1.4. Bromination of ICZ 1.5.
N
N
N
N
C12H25
C12H25
C12H25
C12H25
1.5 1.8
i
+
Br
Br
33
52. In an ultimate effort to obtain ICZ 1.8 we looked back at the synthesis towards this
compound and decided to remove the difficulties encountered during the
bromination reaction by condensing benzaldehyde with the commercially available
5-bromoindole. Since this would leave us with the bromines already in place, only
oxidation of the primary reaction mixture with I2 and subsequent alkylation would
be necessary to afford ICZ 1.8. The reaction was performed using the same
reaction conditions as described in paragraph 1.1 and resulted in a white precipitate
which was isolated by filtration. Proton NMR and ESI-MS analysis showed no
trace of the expected compound. It was observed that the precipitate rapidly
decomposed when in solution (Scheme 1.5).
i) CH3CN, HI (57%w/w), 60°C, 14 hrs.
Scheme 1.5. Alternate synthesis of 2,8-dibromo ICZs.
The result obtained here was quite surprising, especially because the synthesis of
the analogous 2,8-dibromo-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole 1.9a was
reported earlier.59a
This asymmetric indolo[3,2-b]carbazole was prepared by
condensation of 5-bromoindole and the aliphatic aldehyde hexanal and subsequent
reaction with triethyl orthoformate in methanol. The reaction proceeded as
expected but with reduced yield (26%) as compared to the synthesis of the
reference molecule 6-pentyl-5,11-dihydroindolo[3,2-b]carbazole 1.9b (45%),
illustrating that the proposed reaction condition for the condensation is not very
successful when applied to substituted indoles (Scheme 1.6).
N
H
Br
+
O
N
H
H
N
Br
Br
i
+
34
53. i) CH3CN, HI/I2, rt, 14 hrs.
ii) CH3OH, (EtO)3CH, CH3SO3H, rt, 14 hrs.
Scheme 1.6. Synthesis of 6-pentyl-5,11-dihydroindolo[3,2-b]carbazoles 1.11. (Gu
et al.59a
)
1.3.2. Formylation of ICZs
As an alternative to haloaryls, formylated compounds, i.e. compounds containing
one or more aldehyde functional groups, are very versatile substrates in various
condensation reactions, such as the Wittig reaction.64
Introduction of this reactive
moiety on the ICZ-backbone structure would surely allow for further
diversification of this molecule and could prove to be a good alternative to the
failed bromination as mentioned in the previous paragraph.
A popular approach giving rise to formylated (hetero)aryls is the Vilsmeier-Haack
reaction.65
This protocol relies on the reactive species 1.10, created in situ by
reaction between DMF (also present as the solvent) and POCl3, and an activated
arene. Thus, the Vilsmeier reagent was added to a solution of ICZ 1.11 in DMF. To
prevent N-formylation, ICZ 1.1a was methylated prior to use. The reaction mixture
was heated at reflux for 14 hours but no desired products could be identified in the
N
H
HN NH
C5H11
N
H
H
N
C5H11
N
H
H
N
C5H11
i
ii
X
X X
X
X
X
X
1.9b X = H, 45%
1.9a X = Br, 26%
35
54. reaction mixture, in fact, only starting material 1.11 was present after work-up
(Scheme 1.7).
i) DMF, POCl3, reflux, 14 hrs.
Scheme 1.7. Formylation of ICZ 1.11.
Although other methods towards formylated heteroaryls exist in the literature we
chose not to pursue these based on the combined information from the
halogenation and the formylation reactions, indicating the low reactivity of the
oxidized ICZ towards these common electrophiles. Instead we investigated a
different approach to the desired compounds.
1.4. Alternate synthesis of the 2,8-difunctionalized ICZ
building block
Based on the fact that almost no information on the functionalisation of the ICZ-
backbone after its formation exists in the literature, combined with the well-
described halogenations of indoles66
and carbazoles, we envisioned to halogenate
the ICZ prior to its oxidation. In this case the reactivity of the substrate should
approximate the reactivity of indole and hence we would be able to avoid the
previously encountered problems.
From the results obtained by Gu et al. on the synthesis of symmetric 6,12-
diphenyl-5,11-dihydroindolo[3,2-b]carbazoles it was apparent that longer reaction
times and elevated temperatures favoured the oxidized ICZ 1.1 over the non-
N
N
N
Ni
+
N
Cl
Cl
+
1.11 1.10 1.12
O
O
36
55. oxidized ICZ 1.2 when performing the condensation reaction (paragraph 1).59a
As a
consequence, the absence of heat should have an opposite effect and thus provide
for larger amounts of non-oxidized ICZs. We found that the condensation between
indole and benzaldehyde, catalyzed by HI (57% w/w) but this time at room
temperature, not 80°C, yielded a white precipitate that could easily be isolated
from the mother liquor by filtration in near quantitative yield (97%). Proton-NMR
analysis of this substance revealed the presence of two singlets at 5.59 and 5.69
ppm which could be assigned to the benzylic protons at the basis of the two phenyl
groups in the ICZ-backbone. From this we concluded that the reaction proceeded
as expected and provided the tetrahydro-ICZ as a mixture of the cis and trans
isomers of 5,6,11,12-tetrahydroindolo[3,2-b]carbazole in a 1:2 ratio, according to
the 1
H-NMR spectrum (Scheme 1.8).
i) CH3CN, HI, 57% (w/w), rt, 14 hrs.
Scheme 1.8. Synthesis of 5,6,11,12-tetrahydroindolo[3,2-b]carbazole.
This tetrahydro-ICZ also displayed low solubility in common organic solvents and
was therefore alkylated at the free nitrogens present in the compound. The same
strategy was adopted as previously discussed (paragraph 1.2.2) using alkyl halides
of varying length. An interesting result was observed when tetrahydro-ICZ 1.13
was treated with short chain alkyl halides, such as methyl iodide and propyl
bromide. As expected, the starting compound 1.13 was alkylated, but unexpectedly
N
H
+
CHO
N
H
H
N
Ph
H
Ph
H
N
H
H
N
Ph
H
H
Ph
+
i
97%
37
56. also oxidized as shown by the disappearance of the benzylic protons in the 1
H-
NMR spectrum. While no oxidizer was present during the alkylation reaction, we
suspect that adventitious oxygen, the reaction solvent DMSO or a combination of
both are responsible for this peculiar result. ICZs 1.14a and 1.14b were thus
synthesized and isolated by filtration in 90 % and 86% yield, respectively (Scheme
1.9).
i) DMSO, NaOH (50%), alkyl halide, rt, 3 hrs.
Scheme 1.9. Alkylation of tetrahydro-ICZ 1.13 with short alkyl chains.
A different observation was made when longer alkyl chains where attached. The
alkylation reaction with both hexyl bromide and dodecyl bromide proceeded as
desired, yielding tetrahydro-ICZs 1.15a and 1.15b in 67% and 50%, respectively.
In this case, no oxidation took place, providing the alkylated, but non-oxidized
5,6,11,12-tetrahydro-ICZs 1.15 in good yields. During the purification of 5,11-
dihexyl-ICZ 1.15a by crystallization, a mixture of both the cis- and trans-ICZ was
isolated, however, the same purification method allowed only for the more stable
trans-5,11-didodecyl-ICZ 1.15b to be recovered, explaining the lower yield of the
latter in comparison with 5,11-dihexyl-ICZ 1.15a (Scheme 1.10). Presumably the
cis-derivative of ICZ 1.15b is not easily crystallized due to its lower symmetry, as
compared to the trans-derivative of 1.15b, effectively hampering the crystallization
of the former.
N
H
H
N
N
N
i
R
R
1.13 1.14a R = CH3, 90%
1.14b R = C3H7, 86%
38
57. i) DMSO, NaOH (50%), alkyl bromide, rt, 3 hrs.
Scheme 1.10. Alkylation of 1.13 with long alkyl chains.
In an effort to explore the possibility of also oxidizing these tetrahydro-ICZs 1.15,
the reaction was allowed to proceed longer than the time necessary for alkylation,
but even after 24 hours no trace of any oxidized ICZs 1.15 could be identified,
neither by NMR, nor ESI-MS. A plausible explanation for this could be the
solubility difference between the short alkyl tetrahydro-ICZs 1.14 on one hand and
the long alkyl tetrahydro-ICZs 1.15 on the other. While the short alkyl ICZs 1.14
demonstrate some solubility in DMSO, the longer ones surely do not, preventing
their oxidation. An alternate explanation could be the steric hindrance arising
during oxidation. When oxidation occurs, the backbone becomes planar and the
phenyl substituents at the 6- and 12-position rotate out off plane to minimize steric
interaction. Based on the results obtained here, this process becomes more difficult
when the alkyl chain length becomes longer.
With the availability of the non-oxidized, tetrahydro- ICZ 1.15 we set out to test
our hypothesis concerning the halogenation of these indole-like substrates.
Compound 1.15a was treated with four equivalents of NBS in acetic acid and at
room temperature. Upon addition of the NBS suspension a dark green solution was
immediately formed. The reaction was allowed to continue for another one hour,
followed by aqueous work-up. A yellow precipitate was eventually isolated in near
quantitative yield after crystallization from ethyl acetate. Proton NMR analysis of
N
H
H
N
N
N
i
R
R
1.13 1.15a R = C6H13, 67%
1.15b R = C12H25, 50%
39
58. this product confirmed our hypothesis. Two doublets were identified at 7.12 and
7.39 ppm, implying the dibromination of ICZ 1.15a. Furthermore, the two singlets
corresponding to the benzylic protons were no longer present in the 1
H-spectrum,
indicative of the oxidation of the ICZ. Based on the results obtained during
previous research (paragraph 1.3.1) we conclude that bromination occurs prior to
oxidation. Conclusive confirmation regarding the compound was given by ESI-
MS, showing the mass of the dibrominated ICZ 1.16a. Thus, in one reaction step,
both bromination and oxidation were accomplished, effectively cancelling the need
for an additional oxidative step. The same reaction protocol was followed for the
synthesis of the ICZ 1.16b, providing the expected, alkylated ICZ 1.16b in
comparable yield (Scheme 1.11).
i) AcOH, NBS, rt, 1 hr.
Scheme 1.11. Bromination of ICZ 1.15.
Single crystal X-ray diffraction of compound 1.16a (crystallized from ethyl
acetate) shows that 1.16a crystallises in the monoclinic space group P21/c with half
a molecule in the asymmetric unit. The torsion angle of the pendant phenyl ring is
ca. 84º with respect to the indolocarbazole core (Figure 1.1).
N
N
R
R
i
N
N
R
R
Br
Br
1.15 1.16a R = C6H13, 82%
1.16b R = C12H25, 72%
40
59. Figure 1.1. The molecular structure of 1.16a with the atom-numbering scheme
(unlabelled atoms are related to the labeled ones by the symmetry operation: -x, 1-
y, 1-z); displacement ellipsoids are drawn at 50% probability level. Side chains
were found disordered over two positions – omitted on the picture for clarity.
There are no π-π interactions present in the molecular packing, but there is an
abundance of weak C-H…
π hydrogen bonds with the donor atoms originating
exclusively from the side chains, which are located in close proximity of the ICZ
backbones of the adjacent molecules. These weak bonds involve all rings of the
neighbouring ICZ cores (with C…
centroid distances ranging from 3.6 to 3.8 Å) and
result in the formation of 2D layers parallel to the bc plane (Figure 1.2).
41
60. Figure 1.2. Capped-stick representation of the molecular packing for 1.16a shown
down the b axis.
Because the selective bromination requires a 5,6,11,12-tetrahydro- ICZ and these
compounds could only be obtained through N-alkylation of 1.13 with long alkyl
chains, the flexibility of this reaction pathway towards other N-functionalized,
brominated ICZs is restricted. To overcome this limitation we also investigated the
bromination of the non-alkylated ICZ 1.13 in an attempt to obtain a 2,8-dibromo-
ICZ with two free nitrogens (Scheme 1.12).
i) AcOH, NBS, rt, 1 hr.
Scheme 1.12. Bromination of ICZ 1.13.
N
H
H
N
i
N
H
H
N
Br
Br
1.13 1.17
Br
Br
42
61. We adopted the same reaction conditions, using NBS in AcOH. Initially, the
reaction proceeded as expected, resulting in the formation of a yellow precipitate
consisting of a major product, contaminated with two other products.
Chromatographic purification was quite cumbersome caused by the absence of
solubilizing groups, but eventually produced a pure compound. Single crystal X-
ray diffraction revealed the presence of four bromines on the ICZ core structure
with a peculiar substitution pattern, namely meta and para to the indole nitrogen.
ICZ 1.17 crystallises (from ethyl acetate) as a solvate in the monoclinic space
group P21/c with two independent halves of the molecule and one molecule of
ethyl acetate in the asymmetric unit. The ethyl acetate is hydrogen bonded with one
of the nitrogen atoms of the IC core with a N…
O distance of 2.869(9) Å
(Figure1.3).
Figure 1.3. The molecular structure of 1.17 with the atom-numbering scheme
(unlabelled atoms are related to the labelled ones by the symmetry operation: -x,
1-y, 1-z); displacement ellipsoids are drawn at 50% probability level.
The molecules are stacked by π-π interactions between the terminal indole units of
the ICZ cores of alternating, crystallographically independent molecules along the
b axis (with centroid-centroid distances of ca. 3.7 Å). These columns are
interconnected further - by a net of C-Br…
π interactions such as: C4-Br17…
centroid
43
62. of C2-C7 (terminal benzene ring of the IC core) and C5-Br18…
centroid of C29-
C34 (one of the pendant phenyl rings) - into 2D layers parallel to the bc plane with
C…
centroid distances of 3.558(3) Å and 3.662(3) Å respectively. These layers in
turn are interconnected by means of weak C-H…Br hydrogen bonding (C14-
H14…
Br36 with a C…
Br distance of 3.543(10) Å) into a 3D supramolecular
assembly (Figure 1.4).
Figure 1.4. Packing diagram of 1.17 shown down the b axis.
Because multiple brominated ICZs are of little importance in further reactions, we
decided to not further explore this reaction.
1.5. Conclusions
In this first chapter the synthesis of soluble 2,8-dibrominated ICZs with variable
alkyl chains was discussed. Building on the experience and as a continuation of the
work done by our research group on the construction of the ICZ backbone we have
shown two different strategies to increase the solubility of 5,11-dihydroindolo[3,2-
b]carbazoles.
44
63. t-Butylation yields a highly soluble ICZ that, however, has limited use for further
reactions, because the four reactive positions on the ICZ-backbone are taken by the
four t-butyl groups.
N-Alkylation on the other hand provides the ICZs with ample and tunable
solubility depending on the length of the chosen alkyl halide. This reaction is a
very straightforward and high yielding pathway towards soluble 5,11-
dihydroindolo[3,2-b]carbazoles and leaves the four reactive backbone positions
untouched, and thus allows further decoration of the substrate with various
functional groups.
Functionalisation of the soluble ICZs was also explored. In this case we found that
halogenation was greatly affected by the oxidation state of the ICZ. The oxidized
ICZ-derivative 1.5 displayed no affinity towards NBS or the Vilsmeier reagent.
When Br2 was used, a complex reaction mixture of multiple brominated ICZs was
formed. The non-oxidized, alkylated derivative (5,6,11,12-tetrahydro-ICZ)
however, proved to possess sufficient reactivity towards NBS, not only allowing
for the introduction of two reactive moieties on the ICZ-backbone, but also
resulting in the oxidation of the backbone, affording the versatile building block
2,8-dibromo-5,11-dihexyl-6,12-diphenyl-5,11-dihydroindolo[3,2-b]carbazole in
high overall yield.
The reactivity of this building block in various transition metal catalyzed coupling
reactions and other well-known reactive pathways will be discussed in chapter 2.
45
65. Chapter 2 Synthesis of functionalized ICZs
2.1. Transition metal catalyzed coupling reactions
In general organic synthesis, the transition metal (TM) catalyzed C-C-coupling
reactions represent a very diverse array of highly efficient strategies that allow for
C-C bond formations between an equally diverse variety of substrates, by the use
of a transition metal. Many TMs, such as Ni, Zn, Cu, Fe, etc. can be employed,
however most commonly Pd is used in TM-catalyzed reactions.70
The action of the
metal is such that it can move through the catalytic cycle multiple times
(denounced by its turnover number) and hence only small amounts need to be
present in the reaction mixture.
With the 2,8-dibromo-ICZ 16a now available we investigated the possibility of
applying two common TM-catalyzed reactions, the Suzuki-Miyaura and the
Sonogashira coupling, on this substrate.
2.1.1. Suzuki-Miyaura cross-coupling
The Suzuki-Miyaura reaction is described as the Pd-catalyzed coupling reaction
between a haloaryl and an organoboron derivative. For the reaction to work, the
organoboron reaction partner needs to be activated by a base (Scheme 2.1).
Scheme 2.1. Suzuki-Miyaura catalytic cycle.
For the optimization of this coupling reaction with ICZ 1.16a, phenylboronic acid
(R = H) was used as the coupling partner. Firstly DMF/H2O (4:1) was used as the
solvent for this reaction and Cs2CO3 as the base, but no expected products were
formed. Compound 1.16a was recovered in quantitative yield. When switching to
R X L2Pd
R
X
PdL4
L2Pd
R
OH
L2Pd
R
R'
R R'
B(OH)2R'
MOH
B(OH)3
-
M+
R'
MOH
-MX -PdL4
47
66. the alternate solvent system THF/H2O (4:1) and K2CO3 as the base, compound
2.1a was isolated in 36% yield, but only 50% of the starting ICZ had reacted. We
soon found the base to be determinative for the reaction outcome. When a
combination of Cs2CO3 in THF/H2O (4:1) was used, the yield of 2.1a was
increased to 44%. An increase in the amount of catalyst only had a limited effect
on the reaction outcome. Initially, the yield increased by roughly 20% using a five-
fold increase in Pd(PPh3)4, however, further increase in the amount of catalyst did
not provide more product (entry 4 and 5). Finally, KOH in THF/H2O (4:1) resulted
in the complete conversion of 1.16a in 30 minutes and allowed for ICZ 2.1a to be
isolated in 92% yield (Scheme 2.2).
Scheme 2.2. Synthesis of ICZs 1.19.
Table 2.1. Optimization of the Suzuki-Miyaura coupling.
Entry Solvent
(4:1)
Catalysta
(mol%)
Base Time Yield
(%)
1 DMF/H2O 10 Cs2CO3 16 hrs. -
2 THF/H2O 1 K2CO3 16 hrs. 36
3 THF/H2O 1 Cs2CO3 16 hrs. 44
4 THF/H2O 5 Cs2CO3 16 hrs. 63
5 THF/H2O 10 Cs2CO3 16 hrs. 65
6 THF/H2O 10 KOH 2 hrs. 90
7 THF/H2O 5 KOH 30 min. 92
a
In all cases Pd(PPh3)4 was used as the Pd-source.
N
N
C6H13
C6H13
Br
Br
N
N
C6H13
C6H13
R
R
Table 2.1
1.16a 2.1a R = Phenyl, 92%
2.1b R = 4-bromophenyl, 0%
2.1d R = 2-thienyl, 0%
48
67. Encouraged by this result we planned to couple several functionalized boronic
acids with ICZ 1.16a using these optimized reaction conditions. In contrast with
phenylboronic acid, functionalized boronic acids would not only expand the
conjugation of the ICZ, but they would also provide additional anchoring groups to
the ICZ in para conjugation with the backbone. Remarkably, the coupling with 4-
bromophenyl boronic acid did not proceed. Presumably the boronic acid was
polymerized instead of coupled with the ICZ, since an isoluble precipitate was
formed in the reaction vessel after overnight reflux. This result is indicative for the
low reactivity of ICZ 1.16a in the TM- catalyzed reaction. This was further
substantiated when 2-thienylboronic acid was used. Also in this case no trace of the
coupled product was identified in the reaction mixture and ICZ 1.16a was
recovered in near quantitative yield.
2.1.2. Sonogashira cross-coupling
Although the Suzuki-Miyaura reaction is a very suitable means to directly couple
two (hetero)aryls via a C-C-linkage, it is not a common strategy to introduce
unsaturated aliphatic building blocks. Because alkynes (C C) can act as short
electronically conjugated linkers between various heteroaryls, they can prove to be
very useful in the synthesis of conjugated ICZ polymers.
The Sonogashira reaction offers a convenient way to effect this. Much like the
Suzuki-Miyaura reaction, the Sonogashira reaction relies on Pd as the catalytic
species. However, instead of a boronic acid an in situ generated Cu-acetylide is
used as alternate coupling reagent ( Scheme 2.3).
Scheme 2.3. Sonogashira catalytic cycle.
R X L2Pd
R
X
PdL4
R'
:B, Cu(I)
R' Cu
L2Pd
R
R'
-PdL4
R R'
49
68. To explore the possibility of introducing the acetylene moiety on the ICZ
backbone, 1.16a was treated with TMS-acetylene in a THF/NEt3 mixture, a
catalytic amount of CuI (4 mol%) and PdCl2(PPh3)2 (10 mol%). We found that
even when the reaction proceeded for long times (>24 hrs) at reflux temperature no
desired compound was present in the reaction mixture, also in this case only
starting ICZ 1.16a was recovered. The same reaction conditions were also tried
with phenylacetylene, but also in this case only starting material was recycled. By
analogy with the results obtained during the optimization of the Suzuki-Miyaura
coupling we conclude that the reactivity of the 2,8-dibromo- ICZ 1.16a is not
sufficient. It appears that the Pd cannot easily be inserted in the halo-aryl (slow
oxidative addition) (Scheme 2.4).
i) THF/NEt3, Pd(PPh3)2Cl2, R-acetylene.
Scheme 2.4. Sonogashira coupling of acetylenes with ICZ 1.16a.
A commonly used strategy to facilitate the oxidative addition is the use of alternate
Pd sources (Pd coordinated with other ligands). However, we explored an
alternative route to avoid the low reactivity of ICZ 1.16a.
2.1.3. Synthesis of 2,8-diiodo-ICZ and subsequent TM-catalyzed
coupling
The previous paragraphs illustrated the reluctance of ICZ 1.16a to participate in the
Suzuki-Miyaura and the Sonogashira reaction. We reasoned that the lack of
N
N
C6H13
C6H13
Br
Br
i
N
N
C6H13
C6H13
R
R
+
1.16a 2.2a R = TMS
2.2b R = Ph
50
69. reactivity of the latter towards Pd in the oxidative addition step was causing this
result. Because iodoaryls are considered more reactive in TM-catalyzed reactions
we planned to re-adjust the synthesis of the ICZ so that this time two iodine atoms
would be present on the backbone, instead of two bromines. Hence, the same
reaction as described in Chapter 1 (paragraph 1.4) was performed on ICZ 1.13, but
this time iodosuccinimide was used as the halogen source. The reaction proceeded
uneventfully and yielded the 2,8-diiodo-ICZ 2.3 in 84% yield. We subsequently
performed the Sonogashira coupling. Starting ICZ 2.3 was solubilized in a
THF/iPr2NH (1:1) mixture, CuI (5mol%) and Pd(PPh3)4 (10mol%) were added,
followed by the addition of phenylacetylene. As indicated by TLC, the starting
compound had completely reacted in six hours. Aqueous work-up and purification
by column chromatography resulted in ICZ 2.2b in 74% yield (Scheme 2.5).
i) THF/iPr2NH, CuI, Pd(PPh3)4, R-acetylene, 80°C, 6 hrs.
Scheme 2.5. Sonogashira coupling of acetylenes with ICZ 2.3.
TMS-acetylene also reacted with 2.3 in the expected manner, providing the
expected compound 2.2a in 70% yield.
Because the Suzuki-Miyaura coupling with functionalized boronic acids did not
proceed on the 2,8-dibromo-ICZ 1.16a, we tried the same reaction on the more
reactive 2,8-diiodo-ICZ 2.3. Three boronic acids were successfully coupled to the
N
N
C6H13
C6H13
I
I
i
N
N
C6H13
C6H13
R
R
2.3 2.2a R = TMS,70%
2.2b R = Ph, 74%
51
70. ICZ backbone, illustrating the potential of the 2,8-diiodo-ICZ towards popular
TM-catalyzed reactions (Scheme 2.6).
i) THF/H2O, KOH, Pd(PPh3)4, arylboronic acid, reflux, 2 hrs.
Scheme 2.6. Suzuki-Miyaura coupling of ICZ 2.3 with (hetero)aryls.
Under the reported conditions ICZ 2.1b was obtained in 38% yield, much lower
than the model ICZ 2.1a. Presumably, a substantial amount of 4-bromobenzene
boronic acid reacted with itself under these conditions. The lower yield of 2.1c
could be explained as a consequence of the Cannizzaro side reaction which can
take place between KOH and the aldehyde moiety.
N
N
C6H13
C6H13
I
I
i
N
N
C6H13
C6H13
2.3 2.1b R = Br,38%
2.1c R = CHO, 40%
R
R
i
N
N
C6H13
C6H13
2.1d, 57%
S
S
52
71. 2.2. Synthesis and reactivity of 2,8-diformyl ICZ
From the previous chapter it is clear that formylation of the oxidized ICZ 1.11 is
not possible using the standard Vilsmeier-Haack reaction conditions. The aldehyde
functional group is however a far too important functional group to just dismiss it,
since it allows for multiple different follow-up reactions that can further expand the
conjugation of the ICZ backbone.
2.2.1. Synthesis of 2,8-diformyl ICZ
Based on the findings we made during the halogenations of the non-oxidized ICZs
1.15 we planned to formylate the latter using the Vilsmeier-Haack reaction. Thus
ICZ 1.15a was treated with an excess of Vilsmeier reagent and reacted at elevated
temperature (160°C) for five hours, but no formylated ICZs could be identified in
the reaction mixture (Scheme 2.7).
i) DMF, POCl3, 160°C, 5 hrs.
Scheme 2.7. Formylation of the non-oxidized ICZ 1.15a.
In an ultimate attempt to obtain the thus far elusive 2,8-diformyl-ICZ we chose to
increase the nucleophilicity of the ICZ by lithiation of the 2,8-dibromo-ICZs 1.16
at lowered temperature, followed by the addition of DMF. The aryllithium in the 2-
and 8-position of 1.16 easily reacted with the carbonyl moiety of DMF, expelling
dimethylamine (lithium salt) and thus provided the functionalised products, 2,8-
i
N
N
C6H13
H13C6
OHC
CHO
+
N
N
C6H13
H13C6
1.15a
53
72. diformyl-5,11-dialkyl-6,12-diphenyl-5,11-dihydroindolo[3,2-b]carbazoles 2.4a and
2.4b. The products could be easily isolated in 82% and 44%, respectively by
filtration after crystallisation from ethyl acetate (Scheme 2.8).
i) THF, n-BuLi, DMF, -78°C, 30 min.
Scheme 2.8. Synthesis of 2,8-diformyl-ICZs 1.16.
It is likely that the lower yield of 2.4b as compared to 2.4a stems from the
increased solubility of 2.4b in the crystallization solvent. Attempts to purify the
latter compound through chromatographic separation resulted in the decomposition
of the ICZ.
The diformylated ICZ 2.4a crystallizes (from ethyl acetate) in the triclinic space
group P1 with half a molecule in the asymmetric unit, whereas the other half is
generated by an inversion centre. The torsion angle of the pendant phenyl ring with
respect to the indolocarbazole backbone is ca. 87º (Figure 2.1).
N
N
R
R
Br
Br
N
N
R
R
i
O
O
2.4a R = C6H13, 82%
2.4b R = C12H25, 44%
1.16a R = C6H13
1.16b R = C12H25
54
73. Figure 2.1. The molecular structure of 2.4a with the atom-numbering scheme
(unlabelled atoms are related to the labeled ones by the symmetry operation: -x, 2-
y, 1-z); displacement ellipsoids are drawn at 50% probability level.
The introduction of an aldehyde group (positions 2,8 of the ICZ unit), through the
presence of O atoms, has a big influence on the molecular packing, which is very
different from the one observed for 1.16a. This results in the formation of 1D
double-stranded chains expanding along the b axis, which are directed by weak
C16-H16…
O24i
hydrogen bonds (symmetry operation i: -x, 1-y, 1-z) between the
pendant phenyl rings (positions 6,12 of the ICZ unit) and the aldehyde groups, with
a C…
O distance of 3.280(3) Å. These are additionally stabilised by face-to-face π-
π
interactions between terminal benzene rings of the ICZ core from neighbouring
molecules in the chain (with a centroid…
centroid distance of 3.671(1) Å). The 1D
motive is further interconnected in the a direction to form 2D layers held together
by weak C-H…
π interactions between the alkyl side chains and the terminal
benzene rings of the ICZ backbone (C17…
centroid of C2-C7 = 3.503(3) Å,
symmetry operation: 1-x, 2-y, 1-z) (Figure 2.2).
55
74. Figure 2.2. Representation of the interactions participating in the formation of 2D
layers shown down the b axis; the centroids of the terminal rings from the ICZ
core are shown as orange balls, C-H…
O (red), π-π (purple), C-H…
π (blue).
In the following paragraph the reactivity of the 2,8-diformyl-ICZ will be further
explored in various condensation reactions and the viability of this compound for
polymerization will be evaluated in Chapter 3.
2.2.2. Horner-Wadsworth-Emmons (HWE) reaction
Concerning the reactivity of aldehydes, the base-induced condensation reaction
with phosphonium salts (Wittig reaction) or the variant where phosphonates are
used are well-known procedures towards the formation of olefins. Within the
framework of our goals, it would prove valuable to explore this reaction. To this
end, ICZ 2.4a was reacted with benzyltriphenylphosphonium chloride in THF and
n-BuLi at 0°C. While the ylide was readily formed, no condensation with 2.4a took
place. In literature it was reported that Wittig reactions with stabilized ylides
proceed slowly.67
Alternatively, the condensation of phosphonate carbanions
proceeds more readily. This Horner-Wadsworth-Emmons modification to the
56
75. Wittig reaction benefits from the higher nucleophilicity of the phosphonate
carbanion to yield olefins with predominantly “E” configuration.
The procedure was performed with diethyl benzylphosphonate in THF together
with the milder base KOtBu. This time the condensation took place easily and
afforded the double condensation product 2.5 in 74% yield (Scheme 2.9).
i) THF, KOtBu, BnP(=O)(OEt)2, 0°C, 90 min.
Scheme 2.9. Synthesis of 2,8-bis(2-phenylethenyl)-ICZ 2.5.
The Horner-Wadsworth-Emmons reaction showed to be a reliable method to yield
previously unreported ICZs in high yield and hence we expanded on this by
condensing the 2,8- diformyl-ICZ 2.4a with other phosphonates.
While functionalized phosphonates are commercially available they can be
synthesized easily from the corresponding benzylic bromides in the Michaelis-
Arbuzov reaction (conditions “i” in Scheme 2.10) or through simple substitution
with diethyl phosphate in the Michaelis-Becker reaction (conditions “ii” in Scheme
2.10).68
N
N
C6H13
H13C6
N
N
C6H13
C6H13
i
O
O
74%
2.4a 2.5
57
76. i) P(OEt)3, reflux, 14 hrs.
ii) THF, KHMDS, HP(=O)(OEt)2, rt, 14 hrs.
Scheme 2.10. Synthesis of functionalized phosphonates.
Using the Michaelis-Becker approach, diethyl (3,5-dibenzyloxybenzyl)-
phosphonate 2.6 was synthesized from 3,5-dihydroxybenzoic acid in 4 steps (73%
overall yield), diethyl (3,5-dimethoxybenzyl)phosphonate 2.7 was synthesized
from 3,5-dimethoxybenzoic acid in 3 steps (73% overall yield) and diethyl (4-
cyanobenzyl)phosphonate 2.8 was synthesized from 4-cyanobenzyl bromide in
81% yield (Scheme 2.11). Except for the commercially available 4-cyanobenzyl
bromide, the other benzylic bromides were synthesized from their corresponding
benzoic acids, according to a literature procedure.102
i Or ii
R
Br
R
P
O
EtO
OEt
58
77. Scheme 2.11. Synthesis of functionalized phosphonates.
The subsequent coupling of these phosphonates with the 2,8-diformyl ICZ yielded
the expected compounds in good yields (Scheme 2.12).
OBnBnO
P(OEt)2
O
OCH3CH3O
P(OEt)2
O
OHHO
OHO
4 steps
73%
3 steps
73%
OCH3H3CO
OHO
P(OEt)2
O
CN
81%
CN
Br
2.6
2.7
2.8
59
78. i) THF, KOtBu, 0°C, 90 min.
Scheme 2.12. HWE condensation with functionalized phosphonates.
Table 2.2. Reaction yields of the HWE condensation.
Compound R1
R2
Yield (%)
2.9a H OBn 60
2.9b H OCH3 60
2.9c CN H 76
Although the synthesis of phosphonates is very straightforward, it is rather time
consuming to prepare these starting materials. It would prove to be more efficient
if the reactivity of both reaction partners in the HWE reaction was reversed. If the
phosphonate moiety would be present on the ICZ, the expansion of the number of
possible ICZs would be only restricted by the amount of commercially available
aldehydes, which is quite extensive. Furthermore this would greatly improve the
versatility of the ICZ substrate towards synthesis of interesting compounds for
optoelectronic applications.
Because the Michaelis-Arbuzov reaction is often high yielding and very easy to
implement, we used the retrosynthetic approach on the phosphonylated ICZ 2.12.
In order to obtain the latter it becomes clear that an ICZ with two benzylic alcohols
N
N
C6H13
H13C6
N
N
C6H13
C6H13
i
O
O
2.4a 2.9a-c
Table 2.2
R1
R2
R2
R1
R2
R2
60
79. is required. Such a compound could be prepared from 2,8-diformyl- ICZ 2.4a
(Scheme 2.13).
Scheme 2.13. Tetrosynthesis towards the phosphonylated ICZ 28.
The benzylic alcohol ICZ 2.10 was obtained in 70% yield by reduction of ICZ 2.4a
with LAH in THF. Subsequent bromination of the benzylic position was initially
performed with the procedure reported by Appel (CBr4/PPh3)69
, but no desired
products could be isolated from the reaction mixture, only starting compound was
present. An alternate approach was tried using HBr (33%), but this lead to a
complex reaction mixture in which no brominated products could be identified.
Upon consulting the literature we came across a research paper by Marder and co-
workers describing the synthesis of phosphonates.70
It was reported that electron
rich substrates were difficult to phosphonylate using the standard Michael-Arbuzov
reaction conditions, owing to the instability of the corresponding benzylic halide.71
N
N
C6H13
C6H13
N
N
C6H13
C6H13
Br
Br
P
P
O
OEtEtO
O
EtO OEt
N
N
C6H13
C6H13
OH
HO
N
N
C6H13
C6H13
O
O
2.12
2.11
2.4a 2.10
61
80. Instead, a direct phosphonylation of a benzylic alcohol was successfully attempted
by refluxing the substrate in P(OEt)3 and an excess of I2. Presumably the reaction
involves the intermediates (EtO)3PI+
I-
and/or (EtO)2POI, which have been known
to form when P(OEt)3 is treated with I2.72
Similarly, we treated ICZ 2.10 with 4 eq.
I2 in refluxing P(OEt)3 and were pleased to find that all starting material had been
converted into the phosphonylated ICZ 2.12. The product could be isolated and
purified by flash column chromatography (70% yield).
i) P(OEt)3, I2, reflux, 90 min.
Scheme 2.14. Phosphonylation of ICZ 2.10.
The subsequent condensation with benzaldehyde was also investigated using the
same conditions as described above, but ICZ 2.12 seemed to be unreactive towards
benzaldehyde. Since KOtBu could possibly be inadequate to create the dianion we
switched to n-BuLi in THF. This time, a deep purple color was observed upon
addition of the base, presumably indicative for the formation of the dianion.
However, when benzaldehyde was reacted with the dianion, no expected
condensation products were formed upon quenching with water, only starting ICZ
2.12 could be recovered from the reaction mixture (Scheme 2.15).
N
N
C6H13
C6H13
i
HO
OH
N
N
C6H13
C6H13
P
P
O
OEtEtO
O
EtO OEt
70%
2.10 2.12
62
81. i) THF, KOtBu or n-BuLi, benzaldehyde.
Scheme 2.15. Alternative synthesis of 2,8-bis(2-phenylethenyl)- ICZ 2.5.
Possibly the reaction failed to yield any desired products because the ICZ-
phosphonate 2.12 is quite electron rich and hence the formation of an
oxaphosphetane is difficult. A similar finding was made by Reichwein et al.when
trying to synthesize olefins from non stabilized β-hydroxyphosphonates.73
2.2.3. BODIPY – ICZ-triad
In an effort to bathochromically shift the absorbance of a chromophore to longer
wavelengths (decrease the band gap) the most popular approach is to couple
electron withdrawing units with electron donating fragments in a donor-acceptor
pattern. The resulting D-A-interaction can give rise to charge transfer bands which
are often broad and intense.74
Many such systems have been explored for use in
OPCs and demonstrate improved absorption in the Vis-wavelength region of the
EM-spectrum.75
In contrast with the plethora of donor materials that have been
used in such D-A-complexes, the diversity of acceptors is quite limited and mostly
encompasses electron withdrawing units such as (benzo)thiadiazoles,
diketopyrrolopyrrole (DPP).76
During the past five years our lab has had the privilege of acquiring a significant
amount of knowledge concerning the synthesis and properties of 4,4-difluoro-4-
N
N
C6H13
C6H13
i
+
N
N
C6H13
C6H13
P
P
O
OEtEtO
O
EtO OEt
2.12 2.5
63
82. bora-3a,4a-diaza-s-indacenes (BODIPYs).77
BODIPYs are a class of dyes with
strong absorption in the Vis region and have relatively long excited state lifetimes.
They are generally well soluble in organic solvents and display a good
photostability.78
Given these properties, BODIPYs have been the focus of research
for a diverse range of applications such as chemosensors, biomolecule labeling,
energy transfer cassettes and even as photosensitizer in DSSCs and BHJ OPCs.79
Towards the purpose of increasing the spectral coverage of the ICZ absorption, we
planned for the synthesis of a BODIPY-ICZ-triad, using ICZ 2.4a as a substrate.
In general, the BODIPY core structure can be formed through the acid catalyzed
condensation of an aldehyde and pyrrole, followed by oxidation to the
dipyrromethene. Subsequent complexation with boron trifluoride etherate yields
the BODIPY (Scheme 2.16).80
Scheme 2.16. General synthetic pathway towards BODIPY.
Similarly to this procedure, we treated ICZ 2.4a with an excess of 2,4-
dimethylpyrrole and TFA as catalyst. As the reaction proceeded, DDQ (oxidizer)
was added and the reaction progress was monitored until oxidation was complete.
Subsequently complexation with BF3.Et2O was performed, providing a polar
substance. Eventually, purification resulted in a minute amount of a highly
fluorescent material, supposedly the BODIPY – ICZ-triad, but 1
H-NMR was
inconclusive because no characteristic signals of either the ICZ or the BODIPY
could be identified (Scheme 2.17).
N
H
+
CHO
H+
NH HN
[O]
NH N
N N
B
F2
BF3.OEt
B
64
83. i) CH2Cl2, 2,4-dimethylpyrrole, TFA, DDQ, NEt3, BF3.Et2O, rt, 5 days.
Scheme 2.17. Synthesis of the BODIPY – ICZ triad 2.13.
2.2.4. Corey-Fuchs reaction
In paragraph 1.3 the possibility to introduce two alkyne functional groups in the
2,8-position on the ICZ backbone was demonstrated. All the reported examples
however are internal alkynes, restricting further use for these compounds in follow-
up reactions. Although the TMS protected dialkyne ICZ 2.2a can give rise to the
free alkynes (R = H), we chose a different route towards the external alkynes.
In 1972 Corey and Fuchs reported on a convenient synthesis towards alkynes using
aldehydes as a substrate.81
We applied a similar method on the 2,8-diformyl ICZ
2.4a by adding a solution of 2.4a in CH2Cl2 to a solution of CBr4 and PPh3 in
CH2Cl2. The condensation proceeded uneventfully and yielded the
tetrabromovinyl-ICZ 2.14 in 83% yield. Treatment of the latter with an excess of
nBuLi in THF resulted in the formation of the 2,8-diethynyl- ICZ 2.15 in 65%
yield (Scheme 2.18).
N
N
C6H13
C6H13
N
N
C6H3
C6H13
i
O
O
2.4a 2.13
N BF2
N
NF2B
N
+
65
84. i) CH2Cl2, CBr4, PPh3, 0°C, 1 hr.
ii) THF, n-BuLi, -78°C, 30 min.
Scheme 2.18 Synthesis of 2,8-diethynyl-ICZ 32.
As a follow-up reaction we investigated the Cu-mediated alkyne-azide coupling
(CuAAC). If successful, the ICZ could then be coupled with a suitable bifunctional
azide to obtain conjugated polymers. The 1,4-disubstituted 1,2,3-triazoles have
previously been reported as stable units within polymers and were already explored
as functional moieties within conjugated materials.82
To synthesize our model
compound we treated ICZ 2.15 with benzyl azide under standard CuAAC coupling
conditions. The latter was synthesized according to a literature procedure.83
Overnight reflux in THF resulted in a yellow precipitate. Further work-up and
purification by crystallization yielded ICZ 2.16 in 46% yield (Scheme 2.19).
Although these un-optimized conditions provided the bistriazole in moderate yield
and further improvements could still be made on this account, we planned for a
better use of the acetylene functionalized ICZs.
N
N
C6H13
C6H13
O
O
N
N
C6H13
H13C6
Br
Br
Br
Br
N
N
C6H13
H13C6
i ii
65%83%
2.4a 2.152.14
66
85. i) THF, Cu, CuI, NEt3, benzyl azide, reflux, 14 hrs.
Scheme 2.19. Synthesis of 2,8-bis(1-benzyl-1H-1,2,3-triazol-4-yl)-ICZ 2.16.
2.2.5. BODIPY – π – ICZ
In 2009, Marrocchi et al. studied the potential of acetylenic spacers as charge
mediator in conjugated materials for BHJ OPCs. Substitution of olefinic for
acetylenic spacers resulted in higher power conversion efficiencies.84
An extensive
review covering developments in the use of acetylenic spacers in semiconducting
materials has recently appeared in the literature.85
In paragraph 2.2.4 we described the direct coupling of BODIPY on ICZ 2.4a.
Given the importance of charge-transfer complexes as an effective means to
decrease the optical band gap, resulting in a bathochromic shift of the absorbance,
we made another attempt to link the BODIPY to the ICZ through the acetylenic π-
spacers provided by ICZ 2.15, using a Sonogashira coupling. Both the BODIPY
chromophore86a
and the 2,8-dialkyne-ICZ86b
have already independently been
successfully subjected to the Sonogashira coupling. BODIPY 2.17 was prepared
according to a literature procedure.86a
Both starting compounds readily reacted
under standard Sonogashira conditions and provided the BODIPY-ICZ-triad which
could be isolated by column chromatography in 24% yield (Scheme 2.20).
N
N
C6H13
C6H13
i
N
N
C6H13
H13C6
N
N
N
N
N
N
Bn
Bn
46%
2.15 2.16
67
86. i) THF/iPr2NH (1/1), CuI, Pd(PPh3)4, reflux, 90 min.
Scheme 2.20. Synthesis of the BODIPY – π – ICZ triad 2.18.
The normalized UV-Vis absorption spectra of the ICZ 2.15, the BODIPY 2.17 and
the triad 2.18 were recorded in dilute THF solutions and show a peak absorption at
525 nm for the triad 2.18, corresponding to a bathochromic shift of 29 nm,
compared to the absorption maximum of the parent BODIPY 2.17 with a
maximum at 496 nm. The molar extinction coefficient at the maximum amounts to
6.5±0.5x104
M-1
cm-1
. Furthermore the features of the absorption band are changed
significantly: the band is broadened and the maximum no longer corresponds to the
0-0 vibronic band as in the parent BODIPY. Closer inspection reveals a shoulder
around 555 nm which can be associated with the 0-0 vibronic transition. Both the
shift of the 0-0 transition over 60 nm as well as the significant changes of the
N
N
C6H13
C6H13
N
N
C6H13
C6H13
N
BF2N
N
N
F2B N
F2B N
I
+
i
2.15
2.18
2.17
68
87. features of the absorption band suggest a charge transfer nature of the transition
(Figure 2.3).
Figure 2.3. Normalized absorption spectra of ICZ 2.15, BODIPY 2.17 and the
triad 2.18 in THF (concentration: 10-6
M).
In toluene and chloroform a small red shift of the absorption band is observed
which can be attributed to increased polarizability of the solvent.87
The less intense absorption signal at 430 nm can be attributed to the ICZ
chromophore. The much smaller red shift of this transition is probably due to the
fact it is oriented along the short axis of the ICZ and hence less influenced by the
conjugation.
Excitation of 2.18 at 515 nm, in the BODIPY centered absorption band yields in
toluene fluorescence with a maximum around 650 nm and a fluorescence quantum
yield of 9±1 % (Figure 2.4).
69
88. Figure 2.4. Fluorescence spectrum of 2.18 in toluene at a concentration of 10-6
M
(λexc = 515 nm).
In chloroform and THF the emission maximum is shifted to 725 and 730 nm
(Figure 2.5).
Figure 2.5. Fluorescence spectra of 2.18 in chloroform and THF at a
concentration of 10-6
M (λexc = 515 nm).
The large Stokes shift as well as the major red shift of the emission upon
increasing the solvent polarity suggests an excited state with a significant dipole
70
89. and hence charge transfer character. Upon excitation at 515 nm the fluorescence
quantum yield drops to 0.5 and 0.2 % in chloroform and THF respectively.
Previous studies indicate a similar behavior for BODIPY conjugated to an electron
donor, other than ICZ 2.15.88
Upon excitation at 410 nm in the S0→S1 absorption band of the ICZ, a
fluorescence spectrum with a maximum around 650 nm and a fluorescence
quantum yield of 8±1 %, comparable to that observed for direct excitation of the
BODIPY moiety, is observed. Furthermore around 430-440 nm a very weak
secondary maximum with a fluorescent quantum yield of 0.08 % is observed
(Figure 2.6).
Figure 2.6. Fluorescence spectra of 2.18 in toluene at a concentration of 10-6
M
(λexc = 410 nm).
The latter is attributed to residual ICZ. This can either be due to incomplete energy
transfer or an ICZ impurity. As the fluorescence quantum yield of free ICZ 2.15
amounts to 30±3 %. This means that (assuming incomplete energy transfer) the
energy transfer is at least for 99.5 % efficient. As the singlet decay rate of ICZ is of
the order of 5x107
s-1
, 99.5 % efficiency for the energy transfer suggests a rate
constant of 1010
s-1
for the energy transfer. Apparently this is quite slow for two
directly conjugated chromophores. This can be explained in terms of the separating
71
![Faculty of Science
Departement of Chemistry
Molecular Design and Synthesis
Synthesis and characterization of novel indolo[3,2-b]carbazoles
towards organic photovoltaic applications
Promotor:
Prof. dr. Wim Dehaen
Doctoral thesis
Sven Van Snick
April 2012](https://image.slidesharecdn.com/b6d4be78-db1f-47e0-a3dc-f489ead05136-150516164907-lva1-app6892/85/vansnicksvenphd-thesis-1-320.jpg)
