PhD Defense Presentation of Marina Freitag

1. Marina Freitag
Advisor: Prof. Elena Galoppini
Rutgers University – Newark
Ph.D. defense presented on: Sep 28th 2011

2. Outline
1. Introduction
1. Nanostructured Metal Oxide Interfaces
2. Background
3. The Cucurbituril Family
4. Inclusion of Methylviologen in Cucurbit[7]uril
2. Electrochromic properties of Viologen Cucurbituril complexes
1. Redox Active Compounds on TiO2
2. Viologens and their Synthesis
3. Host-guest Complexes
4. Electrochromic Devices
5. Characterization
6. Conclusions
3. Fluorescence properties of Tolyl-Viologen derivatives
1. Introduction
2. Viologen derivative synthesis
3. NMR titration
4. Emission titration
5. Quantum yield
6. Lifetime
7. Characterization
8. Conclusions
2

3. Metal Oxide Nanoparticles
Synthesis, characterization, and modification of nanoparticles and nanostructures
Differences in the bandgap
between metals, semiconductors
(metal oxides) and insulators
3

4. Surface Functionalization Methods
Cathode (Au)
Electrolyte
For LED, solar cell and sensors
applications, nanoparticle TiO2 layer
functionalization is necessary
Anode (FTO)
Glass
DSSC
trapping in
cavities
covalent binding
physisorption
• Alternative method to attach molecules to semiconductors
• Shield the guest from the heterogeneous interface
• Lead to new chemical, photophysical and electrochemical properties
4

5. The Hosts
e-
e-
U. H. Brinker, J.-L. Mieusset, Molecular Encapsulation: Organic Reactions in Constrained Systems, John Wiley and
Sons, 2010
M. Freitag , E. Galoppini, Energy & Environmental Science, 2011 5

6. Pioneering Work
• Possible to find matching host
• High complexation constants
• Structural modification of guest with
anchor group is not needed
• Prevents aggregates
• Improves Chemical stability
• Prevents quenching
• Shields guest from semiconductor
surface
H. Choi et al., Angew. Chem. Int. Ed., 2009, 48, 1.
P. Piotrowiak et al., Pure Appl. Chem., 2003, 75, 1061
C. Pagba et al., J. Am. Chem. Soc. 2004, 126, 9888. 6
S. A. Haque et al., Adv. Mater., 2004, 16, 1177.1

7. Cucurbituril Family
• Macrocyclic host consisting of glycoluril repeating units
• Particularly high affinity for positively charged compounds
• Commercially available 7

8. Cucurbit[7]uril
• Solubilization and prevention of
aggregation with CB[7]
• Negative electrostatic potential, leads
to a preference of cationic guests
• Condensation of glycoluril with
formaldehyde
• Changing the temperature of the
reaction to between 750C and 900C
to access other sizes of cucurbiturils
(CBs)
• Fractional dissolution and fractional
crystallization to separate different
cucurbituril homologues
8

9. Electrochromism of Viologens
• Highly efficient excited-state electron acceptor E°(MV2+*/MV•+) = 3.65 V
• Fast photoreduction (< 250 fs)
• Reversible color change
H. J. Kim, W. S. Jeon, Y. H. Ko and K. Kim, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5007.
K. Kim, N. Selvapalam, Y. H. Ko, K. M. Park, D. Kim, K. J. Kim, Chem. Soc. Rev., 2007, 36, 267.
9

10. Viologen@CB[7] Complexes
• Thermodynamically and kinetically stable
complexes in aqueous solutions
• Reversible electrochemical behavior in
solution
• Ion-dipole interaction between positively
charged guests and carbonyl oxygen's at the
portals of CB[7]
Kim, K. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5007 10

11. Outline
1. Introduction
1. Nanostructured Metal Oxide Interfaces
2. Background
3. The Cucurbituril Family
4. Inclusion of Methylviologen in Cucurbit[7]uril
2. Electrochromic properties of Viologen Cucurbituril complexes
1. Redox Active Compounds on TiO2
2. Viologens and their Synthesis
3. Host-guest Complexes
4. Electrochromic Devices
5. Characterization
6. Conclusions
3. Fluorescence properties of Tolyl-Viologen derivatives
1. Introduction
2. Viologen derivative synthesis
3. NMR titration
4. Emission titration
5. Quantum yield
6. Lifetime
7. Characterization
8. Conclusions
11

12. Alkyl Viologen Derivatives Bound to MOn
1V
• Use of anchoring groups for binding on TiO2
• Electrolyte: LiClO4, Ferrocene, γ-Butyrolactone
• Problems and side effects: dimerization,
insolubility of the neutral species
Cummins, D., Boschloo, G., Ryan, M., Corr, D., Rao, S. N.Fitzmaurice, D. J. Phys. Chem. B, 2000, 104, 1144
12

13. Guests: Viologens
13

14. Alkyl Viologen Synthesis
Synthesis of phosphonated viologen (4) and carboxylated viologen (5) by
Menshutkin reaction
Cummins, D., Boschloo, G., Ryan, M., Corr, D., Rao, S. N.Fitzmaurice, D. J. Phys. Chem. B, 2000, 104, 1144 14

15. Aryl Viologen Synthesis MTV2+
MTV2+ (2) was synthesized as shown, using an adaptation of the Zincke reaction
by Yamaguchi and coworkers to obtain asymmetric viologen derivatives.
Yamaguchi, I., Higashi, H., Shigesue, S., and Shingai, S. Tetrahedron Lett. 2007, 48, 7778
Zincke, T. H. and Weisspfenning, G. J. Liebigs Ann. Chem. 1913, 396, 103
15

16. Cucurbituril complexes bound to MOn
• Role of Cucurbituril: prevention of side
reactions and as binding unit to TiO2
• Binding from aqueous solution
• Inclusion confirmed by 1H NMR in
solution and UV-VIS of electrochromic
windows
FTO-TiO2
16

17. Complexation Constant for MTV2+
Benesi-Hildebrand method
17

18. NMR Complexation
18

19. NMR Complexation
19

20. Complexation and Binding
Chemisorption (or physisorption) of MV2+@CB[7] or MTV2+@CB[7] was done by
immersing the films in an aqueous solution with the complex (0.5 mM) for 24 h.
MOn preparation:
sol-gel technique
TiO2/ ZrO2 Binding
24h
MOn blank Viologen@CB[7]/MOn
• Equimolar amounts of MV2+ (or MTV2+) and CB[7] were dissolved in distilled
water (100 mL) to form the guest@host complex and stirred overnight.
• Formation of the complexes in solution was monitored by 1HNMR in D2O.
• It was observed that the complexation is fast (minutes).
20

21. Construction of ECW
Vinyl mask TiO2 film Binding of the complex
FTO substrate
Counter electrode
Final device
Assembly with
thermoplastic
Surlyn
21

22. Cyclic Voltammetry in Solution
E11/2,V E21/2, V CV in 0.1 M phosphate buffer
Compound (ΔEp, mV (ΔEp, mV (pH 7.0) of 0.05 mM solutions
vs Ag/AgCl) vs Ag/AgCl)
MV2+ -0.661 -0.975
MV2+@CB[7 -0.681 -0.992
MTV2+ -0.704
MTV2+@CB[7 -0.767 22

23. Cyclic Voltammetry of ECW
MV2+ @CB[7] ECW
MTV2+ @CB[7] ECW
• The CVs of electrochromic windows prepared from MV2+@CB[7] and
MTV2+@CB[7]
• Both complexes show a semi-reversible two-electron reduction process
• The complex formation and the physisorption to TiO2 are necessary to
bring the unsubstituted viologens close to the semiconductor surface
• Control experiment: free Methylviologen shows no binding to MOn
23

24. Absorption Spectra in Solution
Dication Radical Cation
1.0 1.00
2+
MV .+
MV
2+
0.8
MV @CB[7] 0.75
.+
MV @CB[7]
Absorbance a.u.
Absorbance a.u.
0.6
0.50
0.4
0.25
0.2
0.0 0.00
200 300 400 500 500 600 700 800
Wavelength [nm] Wavelength [nm]
• One-electron-reduced species were measured in solution at -0.6 V in the
presence and absence of one equivalent of CB[7]
• Broad absorption band centered at 600 nm 24

25. Absorption Spectra in Solution
Dication Radical Cation
1.0 1.00
2+ .+
MTV MTV
2+ .+
MTV @CB[7] MTV @CB[7]
0.8
0.75
Absorbance a.u.
Absorbance a.u.
0.6
0.50
0.4
0.25
0.2
0.00
0.0
500 600 700 800
200 300 400 500
Wavelength [nm] Wavelength [nm]
• The spectra of the complexes were essentially identical to those of the
free compounds. 25

26. Absorption Spectra of ECW Colored State
• Absorption spectra of MV2+@CB[7]/TiO2 andMTV2+@CB[7]/TiO2 measured in an
electrochromic window after application of -0.6 V
• Broad band around 600 nm
• Consistent with the absorption spectrum of the corresponding radical cations
• Coloration was reversible for over 20 switching cycles between bleached and
colored state 26

27. Conclusions
• Complexes of methylviologen (MV2+) and 1-methyl-1’-p-tolyl-4,4’-
bipyridinium dichloride (MTV2+) were encapsulated in a molecular host, CB[7],
and physisorbed onto the surface of TiO2 nanoparticle films
• Proof-of-concept demonstrated the electrochromic properties of two viologen
guests encapsulated inside a cucur[7]bituril host where the host was bound
to the surface of the semiconductor
• No need for synthetic modifications of the molecules with binding groups
• Electrochromic windows were prepared using viologen@CB[7]-modified TiO2
films cast on FTO electrodes.
• These windows exhibited reversible color switching upon application of -0.8
27
V, corresponding to the formation of intensely blue radical cations

28. Outline
1. Introduction
1. Nanostructured Metal Oxide Interfaces
2. Background
3. The Cucurbituril Family
4. Inclusion of Methylviologen in Cucurbit[7]uril
2. Electrochromic properties of Viologen Cucurbituril complexes
1. Redox Active Compounds on TiO2
2. Viologens and their Synthesis
3. Host-guest Complexes
4. Electrochromic Devices
5. Characterization
6. Conclusions
3. Fluorescence properties of Tolyl-Viologen derivatives
1. Introduction
2. Viologen derivative synthesis
3. NMR titration
4. Emission titration
5. Quantum yield
6. Lifetime
7. Characterization
8. Conclusions
28

29. Fluorescence Properties Tolyl-Viologen in CB[7]
29

30. Cucurbituril Encapsulation of Fluorescent Dyes
• Increase of quantum yield
• Longer lifetimes
• Enhanced photostability
Marquez, C.; Huang, F.; Nau, W. M. IEEE Trans. • Protected towards quenchers
NanoBiosci. 2004, 3, 39.
• Prevents disaggregation
• Solubilization
• Change in absorption and emission
properties
A. Hennig, M. Florea, D. Roth, T. Enderle, W. M. Nau,
Supramolecular Chemistry, 2007 (19) 55–66
30

31. The Viologens
“For example, MV2+ will quench the fluorescence of species
such as excited state [Ru(bipy)3] 2+.
A few workers have claimed that methyl viologen dication will
fluoresce, although it is now considered that much of the
‘fluorescence’ reported is in fact due to minute traces of p. 211
highly fluorescenig ’oxo’-viologen compounds as impurities.”
Fluorescent
OXO-Bipyridilium
Compounds
BUT…
The fluorescence and excited state properties of MV2+ were thoroughly
characterized for the first time by Kohler and coworkers a decade ago.
A.W.-H. Mau, J.M. Overbeek, J.W. Loder, W.H.F. Sasse, J. Chem. Soc., Faraday Trans. 2, 1986,82, 869. 31

32. Host and Guests
32

33. Synthesis
12
14
33

34. 1H NMR Titration
Viologen region
1H NMR Titration of DTV2+ (1µM) with CB[7]
In 0.05 M NaCl / D2O
In D2O
34

35. Binding Modes within the Host Cavity
35

36. Stoichiometry - Host - Guest
2+ Job's Plot
DTV with CB[7] 70000 in Water
45000 in 0.05 M NaCl
Integrated Fluoresence Intensity a.u.
Integrated Fluorescence Intensity a.u.
40000 60000
35000
50000
30000
40000
25000
20000 30000
15000
20000
10000
1:1 1:2 in 0.05M NaCl 10000
0.33
5000
in water
0 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.2 0.4 0.6 0.8 1.0
2+
CB[7]/[DTV ] nDTV /(nDTV +nCB[7])
2+ 2+
1:2
36

37. Emission Properties of the Complex
2+
DTV @CB[7] Absorption
FL= 470 nm
2+
DTV @CB[7] Emission 450
1.2
400
1.0 135 nm
350
MCB[7]
Normalized Intensity a.u.
300
0.8
Emission a.u.
250
0.6
200
0.4 150
100
ex= 350 nm
0.2
max=335 nm em=470 nm 50 em= 505 nm
2+
0.0 0 1M DTV
250 300 350 400 450 500 550 600 650 400 450 500 550 600 650
Wavelength [nm] Wavelength [nm]
37

38. Quantum Yield
ex [nm] FL [nm]  
DTV2+ 350 505 0.02 < 20 ps
DTV2+@CB[7]
1:1 350 482 0.12 0.4 ns
DTV2+@CB[7]
1:2 350 470 0.29 0.65 ns
DTV2+@CB[7]
1:3 350 470 0.28 0.70 ns
Tryptophan* 300 355 0.14
* Lakowicz, J. R. Principles of Fluorescence Spectroscopy,1999, Kluwer Academic / Plenum Publishers
44

39. Conformation Effect
140
2+
DTV in PMMA
em= 465 nm
120
PMMA
PMMA
100
Emission a.u.
80
60
40
20
ex= 350 nm
0
400 450 500 550 600
Wavelength [nm]
Emission spectra of DTV2+ in polymer matrix of PMMA
• Structural factors (delocalized π-electrons, electron donating groups,
chelation effect, pH etc )
• Quenching
39

40. DFT and CIS Calculations
• S0, S1 and T1 states of DTV2+
• Indicating the inter-ring dihedral angles
• The (S0-S1) and (S1-T1) and
intramolecular reorganization energies
• The three semi planar rings exhibit
alternating bond lengths.
• Bonds with length changing by more
than 0.025 Å, contraction in red and
lengthening in green
40

41. Electrochromic Properties
UV-VIS spectra of DTV2+ in water before and UV-VIS spectra of DTV2+ @CB[7] in water
after reduction before and after reduction
1.5
1.5
2+
1.0 DTV DTV.+ 2+ .+
1.0
DTV DTV
Absorbance a.u.
Absorbance a.u.
330 nm 400 nm
335 nm 400 nm
0.5
0.5
0.0
0.0
300 400 500 600
300 400 500 600
Wavelength [nm] Wavelength [nm]
• Viologen was reduced electrochemically by applying -1.0 V
and chemically with 1M NaOH in Methanol
41

42. Electrochromic Properties
Emission spectra of DTV2+ in water before and Emission spectra of DTV2+ @CB[7] in water
after reduction before and after reduction
25
600 em=475 nm
em=527 nm
20
500
2+
DTV
2+ DTV
Intensity a.u.
15 400
Intensity a.u.
ex=350 nm
300
10
200
ex=350 nm
5
.+ 100
DTV .+
DTV
0 0
400 450 500 550 600 650 400 450 500 550 600 650
Wavelength [nm] Wavelength [nm]
• Viologen was reduced electrochemically by applying a
voltage of -1.0 V and chemically with 1M NaOH in Methanol
42

43. Complex on Metal Oxide Surface
2+
8 DTV @CB[7] on ZrO2
em= 480 nm
7
6 •Fluorescence spectra of the
5
DTV2+@CB[7] bound to ZrO2
Emission a.u.
4
•ZrO2 is an insulator, similar
3
morphology to the semiconductor
2
TiO2
1
ex= 350 nm
0
400 450 500 550 600
Wavelength [nm]
43

44. Conclusions
Use of Cucurbituril as a Host
• Encapsulation of fluorescent dyes in Cucurbituril can lead to numerous
applications
• Benefits of encapsulation: solubilization, deaggregation, photostabillization
• Pronounced enhancement of fluorescence
• Prolonged lifetime
• Restriction of conformational changes
• Binding unit to the metal oxide nanoparticles
New Viologen Derivative DTV2+
• First viologen derivative showing fluorescent properties
• DFT calculations confirm conformational effect
• Position of Methyl group in benzylic positions is key component
• Restriction of molecular mobility is necessary to enhance fluorescence (CB[7]
or Polymer matrix) 44

45. Acknowledgements
Advisor: Prof. Elena Galoppini
Thesis committee: Prof. Phillip Huskey of Rutgers University, Newark
Prof. Jenny Lockard of Rutgers University, Newark
Prof. Angel E. Kaifer of University of Miami, Florida
Collaborators: Prof. Piotr Piotrowiak, Prof. Lars Gundlach – Laser measurements
Prof. Carlo Bignozzi, Dr. Stefano Caramori – Research visit 2009
Prof. Richard Mendelsohn, Dr. Carol Flach – FT-IR-ATR Instrument
Research group: Prof. Jonathan Rochford, Prof. Olena Taratula, Dr. Sujatha Thyagarajan,
Dr. Yongyi Zhang, Andrew Kopecky, Keyur Chitre, Yan Cao
and Agnieszka Klimczak
Chemistry dept.: Prof. Philip W. Huskey, Prof. Frank Jordan and Prof. John Sheridan and all
professors and staff, former and current members of Rutgers Chemistry
Department Newark and especially to Judy Slocum, Monika Dabrowski
Lorraine McClendon, Paulo Vares and Maria Araujo
Financial support: Donors of the American Chemical Society Petroleum Research Fund
(ACS PRF #46663-AC10).
45

46. Thank you!

47. 47