This document summarizes research on the synthesis and characterization of nanobuilding blocks based on functionalized silsesquioxane cages. The researchers synthesized a series of [o-4-RStyrPhSiO1.5] cages of different sizes and degrees of functionalization via Heck coupling reactions. They characterized the compounds using various analytical techniques and studied their photophysical properties, including absorption, emission, and two-photon absorption. Unexpected photophysical behaviors were observed that depended on the functional group, cage geometry and degree of substitution. Density functional theory modeling supported the hypothesis that asymmetric bromination during synthesis led to asymmetrically functionalized cages with unusual polarization effects, explaining the observed photophysical properties.

1. Synthesis and Characterization of Nanobuilding Blocks [o-
RStyrPhSiO1.5]10,12 (R = Me, MeO, NBoc, and CN). Unexpected
Photophysical Properties Arising from Apparent Asymmetric Cage
Functionalization as Supported by Modeling Studies
M. Bahrami,†,‡
J. C. Furgal,§
H. Hashemi,∥
M. Ehsani,‡
Y. Jahani,‡
T. Goodson, III,§
J. Kieffer,∥
and R. M. Laine*,†,∥
†
Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States
‡
Department of Plastic Processing, Iran Polymer and Petrochemical Institute, 14965/115, Tehran, Iran
§
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
∥
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States
*S Supporting Information
ABSTRACT: The photophysics of [o-4-RStyrPhSiO1.5]8 [R =
Me, OMe, NBoc, and CN] was reported previously. Here we
report studies on [o-4-RStyrPhSiO1.5]10,12, [o-4-RStyrPh-
SiO1.5]3−[PhSiO1.5]7, and [o-4-RStyrPhSiO1.5]6[PhSiO1.5]6 to
explore cage size, geometry, and partial substitution effects on
photophysical properties. All compounds were characterized
by traditional methods including solution spectroscpy and two-photon absorption (TPA) cross sections and except R = NBoc
offer Td5% ≥ 400 °C/air. All exhibit absorption and emission spectra similar to the T8 cages but with some important differences
in TPA cross sections. The R-stilbenes appear to interact in the excited state through the cage, exhibiting emission spectra red-
shifted from the parent stilbenes. TPA studies show novel behavior that is functional group, geometry, and substitution number
dependent. Thus, NBoc TPA cross sections/moiety increase, with decreasing numbers of functional groups from 8 to 3 for PhT10 and
10 to 6 for PhT12 where [NBocStyrPhSiO1.5]8 TPA/moiety ≈0. In contrast, CN cages offer TPA/moiety values slightly greater on
going from 3 to 8 (PhT10) and 6 to 10 (PhT12). NBoc TPA data are best explained if bromination occurs asymmetrically, leading
to asymmetric functionalization and exceptional polarization in partially substituted cages as symmetrically substituted cages
exhibit opposing polarizations. In sum, all the individual induced transition dipoles on excitation mutually cancel. In contrast, both the
cage and CN are strongly electron withdrawing such that no significant polarization is observed/expected when asymmetrically
functionalized. Both NBoC and CN substituents offer red shifts greater than Me and MeO T10,12, suggesting extended
conjugation without polarization. Asymmetric bromination is supported by DFT modeling studies where initial o-Br/o-H
bonding stabilizes incoming Br2 by 300 mEv.
■ INTRODUCTION
The first step in designing novel hybrid nanocomposites now
used in multiple applications is to develop reproducible, high-
yield syntheses to well-defined nano “building blocks” that
allow properties tailoring.1−16
Silsesquioxanes (SQs), e.g.,
[PhSiO1.5]n (n = 8, 10, 12), offer excellent potential as well-
defined, 3-D nanobuilding blocks where careful modification
can provide control of nanostructure assembly and thereby
target properties. In previous studies, we functionalized
[BrnPhSiO1.5]8 (n = 1−3, Scheme 1) via Heck cross-coupling
with 4-R-styrenes (R = Me, Acetoxy, N-Boc), developing
libraries of compounds with well-defined photophysical proper-
ties.17
The original objective was to establish the general effects of
types and densities of functional groups/unit volume on the
photophysical properties of Oh symmetry [4-RStyrnPhSiO1.5]8
cages as they seem to exhibit 3-D electronic communication
between the cage and conjugated moieties in the excited state
evidenced by significant red shifts in emission λmax vs the simple
chromophore.17−20
The current studies were further motivated
by our finding of unusually large two-photon absorption (TPA)
cross sections for [p-4-NH2C6H4CHCHC6H4CH
CHSiO1.5]8 cages (100 GM/moiety),17
whereas follow-on
studies on [NBoCStyrPhSiO1.5]8 where the stilbene vinyl is
on the ortho carbon offered TPA values of ≈0/moiety ascribed
to the Oh symmetry of the molecule, resulting in self-canceling
polarization (see below for further discussion). Note that this
contrasts with {[(NBocStyryl)3Ph]8SiO1.5}8, which offers 12
GM/moiety (total number of moieties =24) presumably
because this system has much less symmetry.17
The design of organic compounds with very large or small
TPA cross sections is a field that still needs extensive
Received: March 19, 2015
Revised: June 18, 2015
Article
pubs.acs.org/JPCC
© XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b02678
J. Phys. Chem. C XXXX, XXX, XXX−XXX

2. experimental and theoretical studies, especially for 3-D
structured molecules. The current studies offer a novel
opportunity to compare sets of 3-D structured molecules with
essentially the same composition [4-RStyr]n[PhSiO1.5]10,12 with
D5h and D2d rather than Oh symmetries, respectively, and with
different degrees of substitution. Efforts to characterize their
photophysical properties also provide an additional mechanism
to probe for 3-D conjugation as well as assessing effects on TPA
behavior.
In particular, we are interested in their photophysical
properties as a means to develop components for photovoltaic
(PV) and/or organic light-emitting diode (OLED) applica-
tions.13,21−24
To this end, we are especially interested in the
dominant factors governing TPA responses as a means to
design new structures with higher TPA cross sections. In
addition to PV and OLED applications, we recognize that
molecular materials with large TPA cross sections have proven
of crucial importance in applications including optical limiting,
3D microfabrication, up-conversion lasing, photodynamic
therapies, optical data storage, and biomedical imaging.25−38
Thus, below we briefly present the synthesis of functionalized
Ph10SQs and Ph12SQs via Heck coupling of brominated SQs
with styrene derivatives (Scheme 1). In our previous paper, sets
of stilbene-SQs were synthesized using the corresponding
brominated Ph8SQs generating compounds with some of the
highest densities of functional groups per unit volume.17
A further objective was to compare electron-withdrawing
(acceptor) moieties (e.g., 4-cyanostilbene) in the ortho position
with donor moieties (e.g., NBoc), on cage phenyls for
comparison with donor analogs.40
Our measurements and
complementary computational analyses suggest that the
introduction of both NBoc and cyano groups leads to better
electronic communication between the core and the organic
moieties as evidenced by greater red shifts in emission as
compared to Me and MeO derivatives. Our results also point to
novel strategies for improving TPA cross sections.
■ EXPERIMENTAL SECTION
Materials. Dichloromethane (CH2Cl2) was purchased from
Fisher Scientific and distilled from CaH2 under N2 prior to use.
Dioxane and THF were purchased from Fisher Scientific and
distilled from Na/benzophenone under N2 prior to use.
[BrPhSiO1.5]12, [Br0.5PhSiO1.5]12, [BrPhSiO1.5]10, and
[Br0.5PhSiO1.5]10 were synthesized using previously reported
methods.17
All other chemicals were purchased from Sigma-
Aldrich, Fisher Scientific, or Strem Chemicals, Inc., and used as
received.
Analytical Methods. Gel Permeation Chromatography
(GPC). Analyses were done on a Waters 440 system equipped
with Waters Styragel columns (7.8 × 300, HT 0.5, 2, 3, 4) with
RI detection using a Waters refractometer and THF as solvent
and polystyrene standards and toluene as references.
Scheme 1. o-Br8OPS, Br16OPS, and Br24OPS Heck Cross-Coupling Studies17
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3. Thermogravimetric Analyses (TGA/DTA). All TGA/DTA
analyses were run on a 2960 simultaneous DTA−TGA
instrument (TA Instruments, Inc., New Castle, DE). Samples
(15−25 mg) were loaded in alumina pans and ramped at air/60
mL min−1
/10 °C min−1
to 1000 °C.
(MALDI-TOF) Mass Spectrometry. MALDI-TOF was done
on a Micromass MALDI micro MX equipped with a 337.1 nm
nitrogen laser in positive-ion reflectron mode using poly-
(ethylene glycol) as a calibration standard, dithranol as the
matrix, and AgNO3 as the ion source. Samples were prepared
by mixing solutions of 5 parts matrix (10 mg mL−1
in THF), 5
parts sample (1 mg mL−1
in THF), and 1 part AgNO3 (2.5 mg
mL−1
in water) and blotting the mixture on the target plate.
The resulting spectra were averaged and smoothed once using
the Savitzky−Golay algorithm. The baseline was subtracted
using a 99th-order polynomial, and the spectra were centered
using a channel width of half the full width at half-maximum.
UV−vis spectra were recorded on a Shimadzu UV-1601
UV−vis spectrometer in CH2Cl2. Samples were dissolved in
CH2Cl2 and diluted to a concentration (10−3
−10−4
M) where
the absorption maximum was less than 10% for a 1 cm path
length.
Photoluminescence Spectrometry. Photoluminescent spec-
tra were taken on a Fluoromax-2 fluorimeter in THF. Samples
from UV−vis spectroscopy were diluted (10−5
to 10−7
M) to
avoid excimer formation and fluorimeter detector saturation.
Photoluminescence Quantum Yields (ΦPL). ΦPL was
determined by a comparison method between a standard and
the sample.41
Each sample was compared for ΦPL with 1,4-
bis(2-methylstyryl)benzene (Bis-MSB) at different wave-
lengths, in order to account for the most similar concentration
between the standard and sample. The solutions were diluted
to three sets of concentrations with absorption ranging from
0.02 to 0.08, to reduce fluorimeter saturation and excimer
formation. The total area of emission for each sample and
standard was calculated by first subtracting out the background
signal and then calculating the area. The experiments were
repeated at least two times and were averaged. To obtain the
best accuracy, the slope of a plot of emission versus absorption
was determined and calculated according to the equation
Φ = Φx A A F F n n s( ) ( / )( / )( / ) ( )PL s x x s x s
2
PL
where ΦPL is the quantum yield; A is the absorption at the
excitation wavelength; F is the total integrated emission; and n
is the refractive index of the solution, which due to low
concentration can be approximated as the refractive index of
the solvent. Subscripts x and s refer to the sample and
reference, respectively. These measurements may have some
error due to the sensitivity of the fluorescence spectropho-
tometer and other environmental conditions.
Two-Photon Excited Fluorescence Measurements. In order
to measure the two-photon absorption cross sections, we
followed the two-photon excited fluorescence (TPEF)
method.42
A 10−4
M Coumarin 307 (7-ethylamino-6-methyl-
4-trifluoro-methyl-coumarin) solution in methanol was used as
the reference for measuring TPA cross sections. The laser used
for this study was a Spectraphysics Millenia Diode-pumped
system coupled to a Kapteyn-Murnane Mode-Locked Ti:sap-
phire laser tunable from 790 to 820 nm, with 800 nm excitation
used in the current study. The beam was directed into the
sample cell (quartz cuvette, 0.5 cm path length), and the
resultant fluorescence was collected in a direction perpendicular
to the incident beam. A 10 cm focal length plano-convex lens
was used to direct the collected fluorescence into a
monochromator. The output from the monochromator was
coupled to a PMT. The photons were converted into counts by
a photon counting unit. A log plot between collected
fluorescence photons and input intensity gave a slope of two,
ensuring a quadratic dependence. The intercept allowed for
calculation of the two-photon absorption cross sections.
Synthetic Methods. General Heck Reaction of
[BrPhSiO1.5]8[PhSiO1.5]2. To a dry 10 mL Schlenk flask under
N2 was added 0.50 g (0.3 mmol, 2.4 mmol-Br) of
[BrPhSiO1.5]8[PhSiO1.5]2 (75% ortho), 22 mg (0.046 mmol)
of Pd[P(t-Bu3)]2, and 21 mg (0.023 mmol) of Pd2(dba)3. 1,4-
Dioxane (3 mL) was then added by syringe, followed by
NCy2Me (3.7 mmol, 0.8 mL) and R-styrene (8.70 mmol). The
mixture was stirred at 70 °C for 48 h. The reaction solution was
then quenched by filtering through 1 cm Celite followed by
rinsing with 5 mL of THF. The solution was then precipitated
into 200 mL of methanol and filtered. The solid was then
redissolved in 10 mL of THF and filtered again through Celite
to remove any remaining Pd particles and then precipitated into
200 mL of methanol, filtered through paper, and dried under
vacuum.
General Heck Reaction of [BrPhSiO1.5]3[PhSiO1.5]7. To a dry
10 mL Schlenk flask under N2 was added 0.50 g (0.3 mmol, 2.4
mmol-Br) of [BrPhSiO1.5]3[PhSiO1.5]7, 22 mg (0.046 mmol) of
Pd[P(t-Bu3)]2, and 21 mg (0.023 mmol) of Pd2(dba)3. 1,4-
Dioxane (3 mL) was then added by syringe, followed by
NCy2Me (3.7 mmol, 0.8 mL) and R-styrene (8.70 mmol). The
mixture was stirred at 70 °C for 48 h. Then the reaction
solution was quenched by filtering through 1 cm Celite. Then
Celite was washed with 5 mL of THF. The solution was then
precipitated into 200 mL of methanol and filtered. The solid
was redissolved in 10 mL of THF and filtered again through
Celite to remove any remaining Pd particles and then
precipitated into 200 mL of methanol, filtered through paper,
and dried under vacuum.
General Heck Reactions of [BrPhSiO1.5]10[PhSiO1.5]2. To a
dry 10 mL Schlenk flask under N2 was added 0.50 g (0.3 mmol,
2.4 mmol-Br) of [BrPhSiO1.5]10[PhSiO1.5]2, 22 mg (0.046
mmol) of Pd[P(t-Bu3)]2, and 21 mg (0.023 mmol) of
Pd2(dba)3. 1,4-Dioxane (3 mL) was then added by syringe,
followed by NCy2Me (3.7 mmol, 0.8 mL) and R-styrene (8.70
mmol).17
The mixture was stirred at 70 °C for 48 h. Then the
reaction solution was quenched by filtering through 1 cm
Celite. Then Celite was washed with 5 mL of THF. The
solution was then precipitated into 200 mL of methanol and
filtered. The solid was redissolved in 10 mL of THF and filtered
again through Celite to remove any remaining Pd particles and
then precipitated into 200 mL of methanol, filtered through
paper, and dried under vacuum.
General Heck Reaction of [BrPhSiO1.5]6[PhSiO1.5]6. To a dry
10 mL Schlenk flask under N2 was added 0.50 g (0.3 mmol, 2.4
mmol-Br) of [BrPhSiO1.5]6[PhSiO1.5]6, 22 mg (0.046 mmol) of
Pd[P(t-Bu3)]2, and 21 mg (0.023 mmol) of Pd2(dba)3. 1,4-
Dioxane (3 mL) was then added by syringe, followed by
NCy2Me (3.7 mmol, 0.8 mL) and R-styrene (8.70 mmol). The
mixture was stirred at 70 °C for 48 h. Then the reaction
solution was quenched by filtering through 1 cm Celite. Then
Celite was washed with 5 mL of THF. The solution was then
precipitated into 200 mL of methanol and filtered. The solid
was redissolved in 10 mL of THF and filtered again through
Celite to remove any remaining Pd particles and then
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J. Phys. Chem. C XXXX, XXX, XXX−XXX
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4. precipitated into 200 mL of methanol, filtered through paper,
and dried under vacuum.
Removal of Residual Pd Catalyst. To a dry 50 mL Schlenk
flask under N2 was added 1.0 g of RStyrxPhSQ dissolved in 10
mL of toluene and 0.1 g of N-acetyl-L-cysteine dissolved in 1
mL of THF. The solution was stirred overnight at room
temperature and then filtered through a short silica gel column
to remove the insoluble Pd−cysteine complex. The filtrate was
then concentrated by rotary evaporation and precipitated into
200 mL of methanol or hexane. The product was filtered and
dried at 50 °C overnight.
Solubilities. All stilbene derivatives were soluble in solvents
with moderate polarities including THF, dioxane, and CH2Cl2.
None were soluble in nonpolar solvents, e.g., hexane. The same
behavior was observed with polar solvents like methanol. The
only exceptions are the NBoc-amino derivatives, which are
soluble in methanol and precipitate in hexane.
Computational Methodology. To develop an under-
standing of the asymmetric bromination process, the
physisorption of Br2 to the already brominated T8 silsesquiox-
ane (SQ) cage was first investigated using local orbitals in a full
potential representation, within the framework of density
functional theory and the generalized gradient approximation
methods as implemented in Gaussian 03.43
B3LYP as the
exchange-correlation functional was chosen.44
Maximum
degrees of freedom are given to the structures; therefore,
they are optimized without any symmetry using 6-31G*
contracted Gaussian basis set with polarization functions.45,46
Because of the important role of the long-range interactions in
Br2 adsorption to the T8 cage, the initial adsorption calculations
were verified with the Vienna ab Initio Simulation Package
(VASP),47,48
with added van der Waals long-range inter-
actions.49
Projector-augmented-wave (PAW) potentials50
were
used to mimic the ionic cores, while the generalized gradient
approximation (GGA) in the Perdew−Burke−Ernzerhof51
(PBE) rendition was employed for the exchange and
correlation functional. A conjugate gradient algorithm was
used to relax the ions and the lattice vectors. Ionic and
electronic relaxation were performed by applying a convergence
criteria of 10−2
eV/Å and 10−4
eV, respectively. Convergence
with respect to the plane wave cutoff is checked carefully.
■ RESULTS AND DISCUSSION
As noted in the Introduction, in previous papers we synthesized
4-Rstilbene derivatives based on [BrxPhSiO1.5]8 (x = 1−3) (R =
Me, MeO, NHBoc, Cl, and Acetoxy per Scheme 1). These
molecules show unique photophysical behavior suggesting that
the same functional groups offer different absorption and
emission behaviors depending on chromophore densities,
degree and type of conjugation, and steric intractions between
the cages and the functional groups.17
The 4-Rstilbenes where x = 1 start from [BrPhSiO1.5]8 where
Br substitution is 85% ortho. In related studies we also
examined the photophysical behavior of 4-Rstilbenes synthe-
sized from essentially pure [p-IPhSiO1.5]8 permitting a contrast
between ortho and para substitution.18
We recently analyzed ortho bromination computationally,
revealing quite strong orbital associations between incoming
Br2 and the cage LUMOs for the T8,10,12 systems that drive the
process.39
Modeling indicates that the cage LUMO extends
beyond the cage face engaging the incoming Br2 to promote
ortho bromination. We believe that similar interactions arise
between the extended cage LUMOs and ortho-substituted
moieties conjugated to the cage as supported by the work
discussed below and published previously.39
Feher and Budzichowski report 13
C chemical shifts for
[RphenylSiO1.5]8 in parallel with their Hammett substituent
Scheme 2. Heck Coupling Synthesis of (RStyr)x(PhSiO1.5)10 from (Br)x(PhSiO1.5)10 (x ≈ 3 or 8)
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D

5. constants, indicating that the SiO1.5 unit offers electron-
withdrawing behavior equivalent to CF3.52
Thus, the mostly
ortho-functional groups generated here likely are affected by
the electron-withdrawing cages. For example, the 15 nm blue-
shifted absorption in [MeStyrPhSiO1.5]8 probably arises due to
the influence of the prominent cage LUMO on the o-stilbene
electronic excitations.17,39
Similar blue shifts are not seen for
the analogous [p-MeStyrPhSiO1.5]8.18
The objectives of the current study are to
(1) Expand our previous study to the larger cages,19,53
[PhSiO1.5]10,12 possessing equivalent chemical structures, but
with different symmetries and degrees of functionality. We
report here on structure−property relationships for selected
multifunctional derivatives.
(2) Functionalize partially brominated cages: [o-
BrSiO1.5]x[PhSiO1.5]10/12−x (x = 3, 6) via Heck coupling to
introduce selected o-stilbene moieties as done for the fully
brominated [BrxPhSiO1.5]8 compounds.17
In these studies, 4-
CNstyrene was chosen as a model electron-accepting moiety as
previous studies found it equivalent to C6F5.52
The 4-R electron
donor moieties were Me, MeO, and NBoc for comparison with
our previous work.17,18
In future studies, these baseline systems
can be compared with cages with push−pull or pull−pull
electronic/photonic interactions across cage faces and/or
through cage structures.
(3) Characterize two-photon absorption (TPA) cross
sections as a function of “chromophore density/unit volume”
and cage symmetry for the compounds produced.
Scheme 2 provides an overview of the chemistries explored
to realize partial or nearly full mono ortho bromination of
[PhSiO1.5]10,12. Materials are named by the following
convention as derived from MALDI-TOF peak assignments
(see below); for example, stilbene SQs are labeled as
(RStyr)x(PhSiO1.5)10 from (Br)x(PhSiO1.5)10 (x ≈ 3 or 8).
Table 1 lists substitution patterns determined by F−
/H2O2
cleavage,42
used to produce corresponding phenols as reported
previously.39
Ortho bromination of the [PhSiO1.5]8 cage is typically 85%,
whereas ortho selectivity for [PhSiO1.5]10 is 72 ± 2% and that
for [PhSiO1.5]12 is closer to 60%. The reduction in selectivity is
ascribed to steric effects.39
In contrast ortho bromination of
PhSiCl3 is only 5% pointing to the special effects of the cage on
bromination.39
In general, all syntheses run using conditions developed for
[o-BrPhSiO1.5]8 were straightforward.54
The resulting com-
pounds were characterized by MALDI-TOF mass spectrome-
try, GPC, NMR, and TGA. The following sections are
organized according to these analytical techniques followed
by comparative studies of photophysical properties. Exemplary
spectra are provided for each section with additional spectra
given in the Supporting Information (SI).
MALDI-TOF spectra of all compounds except the NBoc
derivatives were obtained. Figures 1−5 show representative
spectra (see also Figures S1−S16, SI). The spectra obtained are
consistent with the substitution patterns found for the starting
materials and do not reveal any extraneous impurities, e.g., cage
fragments. The small discrepancies in average functionality (∼
± 10%) compared with starting brominated cages come from
variations in MALDI-TOF peak intensities due to slight
variations in ionizing laser intensity, detector sensitivity to
chemical structure, or differences in ease of ionization of
individual isomers, with the latter being most likely.
MALDI-TOF in Figures S5−S16 (SI) show sets of molecular
species (e.g., RStyr9Ph12SQ, RStyr10Ph12SQ, RStyr11Ph12SQ,
etc.) corresponding to both fully monobrominated cages
coupled with significant amounts of cages missing one or two
bromines as expected given our objective of avoiding
dibrominated phenyls. Efforts made to produce “half”
brominated cages gave an envelope of products averaging 2−
4 groups/cage, e.g., MeStyr2.7Ph10SQ (Figure 2). Note that
MALDI-TOF data show different percentages of the 2- and 4-
mer, likely due to both substitutional variations and different
ionization potentials for the individual cages as seen
previously.54
The RStyrxPh10SQ and RStyrxPh12SQ substitution
patterns correspond well with those found for BrxPh10SQ and
BrxPh12SQ (Figures 1−5) with no residual Br detected.
MALDI-TOF of NBoc compounds was not feasible because
of facile fragmentation, as observed previously.17
Figure 5
provides a representative spectrum for 4-MeStyr9.8Ph12SQ.
Parent peaks are seen at m/z = 2817.9. Similar data were
obtained for all compounds (Figures S5−S16, SI).
GPC analysis (see Table 2) reveals the low-PDI peaks
(∼1.03 ± 0.02) give molecular weights smaller than proposed
structures, as expected for spherical, rigid structures.18,20,55,56
Figure 6 is a representative GPC. Figures S17−S20 (SI)
provide traces for other functionalized compounds. The narrow
MW distributions indicate cage structures are retained after
synthesis. In some cases, a small second peak at ca. twice the
molecular weight of the first peak was also detected. This
“dimer” peak arises from small amounts of coupled SQ cores,
i.e., double Heck reactions on the same double bond.18
TGAs of RStyrxPhySQ (except for the NBoc moiety) offer
excellent thermal stabilities with Td5% ≥ 400 °C/air. Figure 7
provides representative TGAs with analogous data presented in
Figures S21−S24 (SI). Theoretical ceramic yields were
calculated from average formulas from MALDI-TOF data and
agree well with those found experimentally. The mass loss from
NBoc groups at ≈200 °C was used to estimate the average
functionality per SQ molecule, as done previously.17
Photophysical Properties. Theoretical studies suggest
that conjugated π-centers play a prominent role in enhancing
TPA cross sections.57
The electronic communication between
cages and R groups seen previously is also seen here,17
suggesting the potential to enhance TPA cross sections in 3-D.
To systematically probe 3-D conjugation and TPA cross
sections in the cages we have used simple 4-R-substituted
Table 1. Bromination of [PhSiO1.5]x (x = 8, 10, or 12) and
F−
/H2O2 Si−C Cleavage Productsa
characterization
SQ
max. in
MALDI temp. °C
area %b
ortho
b
% 2-bromo-
phenol16
Ph8SQ BrPh8SQ 30−40 85 ≈90
Ph10SQ Br4Ph10SQ 30−40 78 75
Ph10SQ Br9Ph10SQ 30−40 75 75
Ph12SQ Br6Ph12SQ 40 60 60
Ph12SQ Br6Ph10SQ 40 60 60
Ph12SQ Br9Ph10SQ 50−55 62 60
PhSiCl3 BrPhSiCl3 5b
I8Ph8SQ I8Ph8SQ
0 ± 5 90% 4-iodophenolI10Ph10SQ I10Ph10SQ
I12Ph12SQ I12Ph12SQ
a
From GC analysis of F−
/H2O2 cleavage of the phenyl Si−C bond.16
b
70% meta, 25% para.17
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6. stilbene probes whose synthesis and characterization are
discussed just above.
Figure 8 provides UV−vis absorption and emission spectra
for Table 3 [o-4-RStyrPhSiO1.5]x. All R = Me, Methoxy, NBoc,
and CN show λmax absorptions at 320 ± 20 nm and emissions
at λmax 420−450 nm, respectively. trans-4-Methylstilbene has
absorption λmax at 298/311 nm, similar to the MeStilbeneSQs,
but with emission λmax = 352 vs λmax >400 nm for all
MeStilbeneSQs, a 50 nm red shift.
N-tert-Butoxycarbonylation or NBoc protection converts
primary amines to urethane derivatives, reducing basicity and
electron-donating potential. Thus, the NBoc compounds have
15−20 nm red shifts in absorption and emission arising from
the electron-withdrawing nature of Boc. In contrast unpro-
Figure 1. MALDI-TOF of Br2.5Ph10SQ.
Figure 2. MALDI-TOF of MeStyr2.7Ph10SQ.
Figure 3. MALDI of methoxyStyr2.7Ph10SQ.
Figure 4. MALDI-TOF of Br9.4Ph12SQ.
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7. tected NH2 groups offer exceptional red-shifts up to 120 nm;
however, unprotected amines are basic and react with traces of
water to promote cage degradation.20
Therefore, NBoc
protection is necessary. For electron-withdrawing o-cyano-
stilbene groups, we expect to see blue shifts in both absorption
and emission spectra; however, the o-4-cyanoStyr9Ph12SQ
absorption and emission spectra are similar to o-4-methox-
yStyr9.8Ph12SQ but offer quite different TPA cross sections,
behavior that is discussed below.
Figure 5. MALDI-TOF of MeStyr9.8Ph12SQ.
Table 2. GPC, TGA Analyses, and Isolated Yields for MethoxyStyrxPhySQ, MeStyrxPhySQ, NBoc-StyrxPhySQ, and
CyanoStyrxPhySQ (x = 3, 6, 8, 10; y = 10, 12)
ceramic yield (%) GPC
actual calcd Td5% (°C) Mn Mw PDI yield (%)
MeStyr2.7Ph10SQ 34.7 36.7 318 1752 1813 1.035 78
MethoxyStyr2.7Ph10SQ 35.1 36.5 423 1735 1767 1.018 73
NBocStyr4Ph10SQ 26.7 27.6 204 1578 1604 1.016 51
CyanoStyr5.3Ph10SQ 32.3 30.6 280 1428 1435 1.005 70
MeStyr9.3Ph10SQ 26.6 25.2 408 2322 2428 1.045 77
MethoxyStyr6.5Ph10SQ 28.3 27.9 412 1904 1981 1.041 74
NBocStyr7Ph10SQ 21.5 21.1 196 3734 3849 1.031 53
CyanoStyr9.5Ph10SQ 23.7 24.0 323 1653 1688 1.022 68
MeStyr6.9Ph12SQ 30.0 30.7 423 2210 2267 1.025 78
MethoxyStyr6.6Ph12SQ 28.2 29.7 275 2738 2808 1.026 74
NBocStyr5Ph12SQ 27.5 27.1 204 3829 3945 1.030 52
CyanoStyr6.3Ph12SQ 28.1 30.7 280 2521 2607 1.034 69
MeStyr9.8Ph12SQ 25.4 26.8 428 2582 2650 1.026 76
MethoxyStyr10.2Ph12SQ 26.5 24.8 425 2750 2816 1.024 75
NBocStyr8Ph12SQ 21.7 21.7 200 4435 4354 1.018 51
CyanoStyr9Ph12SQ 25.6 26.7 390 2671 2749 1.029 69
Figure 6. GPC trace of NBocStyr5Ph12SQ.
Figure 7. TGAs of (a) MeStyrxPhySQ and (b) NBocStyrxPhySQ.
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8. The Table 3 data provide the UV−vis absorption and PL
emission spectra recorded in Figures S33−36 (SI) for the [o-4-
methylStyrPhSiO1.5]x[PhSiO1.5]y, [o-4-methoxyStyrPh-
SiO1.5]x[PhSiO1.5]y, [o-4-NBocStyrPhSiO1.5]x[PhSiO1.5]y, and
[o-4-cyanoStyrPhSiO1.5]x[PhSiO1.5]y in THF. Within each
series, similar trends are observed for the same R group.
What is surprising is that the level of functionality has little
effect on the quantum yields in all except the NBoc SQs. The
reason for this trend is not completely understood; however, as
we add functionality the interactions between the chromo-
phores become stronger, perhaps self-quenching the expected
increases in ΦPL.58,59
The best ΦPL values obtained are for the
−NBoc and −CN moieties at up to 30%.
One unique observation is that the −CN compounds show
red-shifts in absorption and emission equal to methoxy groups
suggesting that conjugation is more important than electron-
donating or -accepting capacity. In support of this idea, we have
previously reported that introducing a C6F5 ring to a
vinylstilbene system [C6F5CHCHC6H4CHCH2SiO1.5]8
19
blue shifts absorptions (λmax = 313 vs 330 nm) but red shifts
emissions (λmax = 433 vs 390 nm) vs [p-C6H4CHCH−
C6H4CHCH2SiO1.5]8.19
If charge transfer (CT) contribu-
tions give rise to this behavior, we would then be obliged to
explain why NBoc, CN, and C6F5 all promote CT behavior.
CT behavior can be tested by switching to a more polar
solvent. A 5% THF/90% CH3CN solvent solubilizes o-
RStyr9Ph12SQ effectively. Figure 9 (Tables S7 and S8, SI)
reveals a range in λmax from 426 ± 1 to 451 ± 1 nm. NBoc
systems exhibit less structured, more intense emissions at λmax
≈ 451 nm, whereas for Me, the emission is less intense at λmax
≈ 426 nm. We believe that the 2x higher emission intensities of
the NBoc and CN derivatives at constant 0.41 μM
concentrations mirror solvent polarity effects.19
However, λmax
for these two sets of compounds remains unchanged in
Figure 8. (a) Absorption and (b) emission of all [o-RPhSiO1.5]10[PhSiO1.5]2 (THF, normalized to 1).
Table 3. Spectral Data for RStyrxPh10SQ and RStyryPh12SQ in THF
SQ abs. λmax (nm) emiss. λmax (nm) E (M−1
cm−1
) ΦPL δ (GM) 800 nm δ/group (GM)
trans-4-methyl stilbene17
298, 311 355
MeStyr2.7Ph10SQ 298, 312 400, 420 1.30 × 1005
0.02 1
MeStyr9.3Ph10SQ 298, 312 400, 420 3.14 × 1005
0.05 3
MeStyr6.9Ph12SQ 298, 312 400, 420 2.22 × 1005
0.03 2
MeStyr9.8Ph12SQ 298, 312 400, 420 1.86 × 1005
0.07 2
MethoxyStyr2.7Ph10SQ 317 412, 431 8.94 × 1004
0.01 7
MethoxyStyr6.5Ph10SQ 317 412, 431 1.42 × 1005
0.02 3
MethoxyStyr6.6Ph12SQ 317 412, 431 3.57 × 1005
0.01 7
MethoxyStyr10.2Ph12SQ 317 412, 431 2.25 × 1005
0.03 3
o-NBocStyr8Ph8SQ17
317 386, 422, 445 0.05 1 ∼0
NBocStyr4Ph10SQ 330 422, 451 7.63 × 1004
0.3 88 22
NBocStyr7Ph10SQ 330 422, 451 8.21 × 1004
0.3 38 5.4
NBocStyr5Ph12SQ 330 422, 451 2.07 × 1005
0.3 146 29
NBocStyr8Ph12SQ 330 422, 451 1.90 × 1004
0.3 84 10
CyanoStyr5.3Ph10SQ 327 413, 435 2.69 × 1005
0.1 40 7.5
CyanoStyr9.5Ph10SQ 327 413, 435 2.76 × 1005
0.3 62 6.5
CyanoStyr6.3Ph12SQ 327 413, 435 8.30 × 1004
0.1 28 4.4
CyanoStyr9Ph12SQ 327 413, 435 1.01 × 1005
0.2 46 5.1
Figure 9. Emission of o-RStyr9Ph12SQ in CH3CN at constant 0.41 μM
concentration.
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9. CH3CN. Only very small effects are observed in the cyano
moiety emissions on going from less polar THF to pure
CH3CN (Figure S37, SI). Application of Occam’s Razor
suggests that some CT may occur for CN, but extended
conjugation better explains the effects of these moieties. TPA
data discussed below seem to support these choices.
Finally, longer excitation wavelength effects on emission
behavior were assessed (Figures 10 and 11). Figure 10 for o-
CyanoStyr9.5Ph10SQ at 400 vs 327 nm generates a new peak at
491 nm, slightly more intense than the original unshifted
emission but at 9x the intensity. Figure 11 shows the same
trend for o-MeStyr9.3Ph10SQ.
The rigid, 3-D nature of these mostly ortho-substituted
compounds should limit exciplex formation via interdigitation
as seen previously.23
However, the strong second peak at 491
nm suggests excimer formation especially in the Ph10SQs. A
better explanation is that the exciplex arises due to chromophore
overlap on the same cage between two ortho substituents. The
crystal structure of [o-BrPhSiO1.5]8 shows a strong predilection
for two Br’s to reside above a single face pointing at each
other.54
The introduction of two stilbenes in the same positions
likely predisposes them to form internal excimers on excitation.
Indeed, even [PhSiO1.5]10 and [PhSiO1.5]12 cages form internal
excimers with some evidence suggesting 3-D excimer
formation.23
As noted above,39
ortho substitution in Ph10SQs is higher
than Ph12SQs. Thus, one might anticipate that an “internal”
excimer peak in Ph10SQs would lead to a stronger emission at
491 nm than in Ph12SQ. No excimer peaks appear for the MeO
or NBoc analogs likely because these functional groups are
better solvated preventing excimer formation.
One objective in the present study is to probe photophysical
properties vs “functional group density/unit volume”. In
addition to the comprehensive steady-state absorption and
emission spectra summarized in Figures 8−11 and Figures
S33−37 (SI) for these series of [o-4RStyrPhSiO1.5]x, Table 3
provides TPA cross sections to compare polarization and
investigate linear and nonlinear absorption properties for both
the Ph10SQ and Ph12SQ derivatives at 800 nm excitation.
As seen in Table 3, no significant changes in cross section are
observed for Me and MeO within each series of compounds as
is expected for these poorly polarizable groups. However, for
the NBoc compounds, the TPA cross sections decrease with
increasing numbers of chromophores, and with T12 showing a
slightly higher cross section likely due to the increased
polarizability of this partially substituted system. Two factors
may come into play to explain this observation. The first is that
the TPA calculation depends on the polarization of the
molecule. Even though more chromophores may be attached to
a fully functionalized system, increasing the overall absorption
potential, cage symmetry can work against the measured TPA cross
sections. Thus, in these truly 3-D substituted systems, in sum all
the individual induced transition dipoles on excitation mutually
cancel, greatly lowering the overall net effect. However, a
partially substituted cage−chromophore system will be
polarized in the direction of higher functionalization and
increase the overall TPA cross-section. However, in a roughly
spherical system, on a statistical basis, one would anticipate that
even with partial substitution, self-canceling behavior would
also result, which is not the case here.
This unique result is best explained by asymmetric
bromination of the phenylSQ, where bromination occurs
selectively on the same side of the molecule first. One can
envision, based on our recent paper,39
that electronic or
electrostatic interactions favor strong interactions between
already ortho-brominated phenyls and incoming bromine. This
would then place stilbene functionalization on the same side of
the cage escaping the proposed self-canceling behavior seen for
example with [NBocStyrPhSiO1.5]8 with TPA values of 0
(Table 3).17
Additionally, the partially substituted NBoc cages
offer ΦPL values of 30% vs 5% for [NBocStyrPhSiO1.5]8.17
This
is further support for the asymmetric bromination and the
transition dipole moment enhancement observed. There may also
be a contribution from cage symmetry differences although this
remains conjecture at this point.
A second important observation arises because quantum
efficiency (ΦPL) is part of TPA cross section calculations and is
nearly constant as chromophore numbers increase. If the cage
trend of decreasing ΦPL with increasing cage size were
observed, the fully substituted cage TPA values should be
similar to the partially substituted systems.24
Figure 12 gives a
representative example of the structural and electronic motifs
present in the NBoc system.
This trend is not straightforward for all the chromophores.
To the contrary, we observe a slight increase in the overall
Figure 10. Emission of o-cyanoStyr9.5Ph10SQ on excitation at 327 and
400 nm (THF, 0.41 μM).
Figure 11. o-MeStyr9.3Ph10SQ emission on excitation at 312 and 380
nm (THF, 0.41 μM).
Figure 12. Representative Push−π−Pull−π−Push system for
NBocStyrxPh10SQ.
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10. cross-section in the CNStyrxPhySQ series with increasing
functional group population in the same volume. However, if
we compare the TPA/chromophore values for each cage size
the extent of polarization enhancement seen in the NBoc
system does not occur. This could be attributed to two
possibilities: the polarization of the cyano group has less
influence on the overall transition dipole orientation in the
molecule, or the cyano groups increase the number of electron-
deficient styrenyl centers dispersed around the molecule.
Above, we note that the cyano groups do not seem to exhibit
CT behavior, in support of the first possibility.
A comparison of [CNStyrPhSiO1.5]10 with [CNStyrPh-
SiO1.5]12 reveals a slightly higher cross-section possibly due to
the better polarization from spatial orientation in [SiO1.5]10.24
However, a better explanation is that ortho bromination in
[SiO1.5]10 occurs 72% of the time and only 60% of the time for
[SiO1.5]12, which supports the observed TPA differences but also
gives more credence to the modeling studies below, where ortho
substitution is needed for asymmetric bromination. Still another
explanation is that the cyanostilbene may “reverse” the
transition dipole direction from being CN-to-cage to cage-to-
CN due to its strong electron-withdrawing nature (Figure 13).
The literature notes that increasing the electron-poor nature of
the aromatic core slightly increases TPA efficiencies.59
Many π-conjugated molecules show strong donor−acceptor
interactions leading to large changes in transition dipole
moments on excitation, thereby offering significant TPA cross
sections60,61
as found for the NBoc and CN derivatives. One
limitation to the current studies arises because our TPA
experimental setup was not ideal for studying the current
molecules in their strongest absorption regions. For example,
we could only use 800 nm excitation to probe samples with
more extended absorption spectra (greater functionalization)
than those with less functionality. Hence the cross sections
appear low especially for the Me moieties. All cross sections
would be higher at ≤720 nm excitation. Unfortunately our
setup did not allow for such studies.
The possibility of finding a rare example of asymmetric
bromination is intriguing. To assess the energetics of
bromination where one phenyl is already ortho brominated,
we extended our original modeling efforts.17
To this end, we
developed a model (see Experimental section) of the
adsorption of Br2 with an ortho-brominated SQ cage surface
and its possible interaction with Br at other ortho positions.
Total-energy calculations are used to elucidate the initial
adsorption energetics of Br2 on the ortho-brominated T8
molecule.
Energetics are mapped using adsorbed Br2 (Br2 and Br3) to
probe all symmetrically distinct sites and relative orientations
arriving at a similar, well-defined adsorption behavior (Figure
14). The calculated adsorption energy is well above ambient
(∼300 meV), indicating this configuration is very stable. Key to
our results is the observation that Br2 bonds with ortho Br
(Br1) and H (H1). The Br1−Br2, Br2−Br3, and Br3−H1
distances are 3.5, 2.35, and 2.97 Å, respectively.
Br−Br halogen “bonding” distances found in the crystal
structures of BrxOPS54
are ≈3.5 Å, in excellent agreement with
our DFT-calculated value of 3.51 Å. In our previous study, first-
principles calculations suggested that the Br−H distance is
about 2.80 Å for cages that self-brominate.39
In Figure 14, Br2
interacts with H1 and H2 both at ortho positions and Br1
simultaneously. The Br3−H1 distance is somewhat greater at
2.97 Å. Furthermore, halogen−halogen “bonding” is quite
strong compared to the hydrogen bonding in a similar
structure.62
Thus, it appears that symmetric bromination is unlikely, first
because there are strong Br1−Br2 and Br3−H1 interactions.
Second, although the Br2−H2 (3.03 Å) distance is slightly
longer than Br3−H1 (2.97 Å), both are ortho hydrogens, and
there is no evidence for H−Br interactions between any other
aromatic hydrogen. Third, once the Br2 is adsorbed, its bond
Figure 13. Representative pull−π−pull′−π−pull system with electron-
poor nature of aromatic core for CNStyrxPh10SQ.
Figure 14. Br2 physisorption on the ortho-brominated T8 cage is shown: (a) top view and (b) side view. The Br2 interacts with the Br (Br1···Br2) via
halogen bonding and H (Br3···H1) via hydrogen bonding. Carbon atoms are shown by gray, oxygen by red, silicon by gray, bromine by purple, and
hydrogen by white. All distances are in Angstrom. The bottom of the cage is hydrogen terminated to have a better top view.
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11. length is increased (from 2.28 to 2.35 Å) by 3%. This is due to
the Br3−H1 interaction from one side and Br1−Br2 and Br2−
H2 interactions from another side. Fourth, the calculated ∼300
meV stabilization of the structure in Figure 14, provides a
reasonable energetic argument that the initial ortho bromine
directs or assists the second ortho bromination to occur on a
neighboring phenyl. These results suggest that asymmetric
bromination would be reasonable and might even be more
pronounced if a second ortho bromine were present on the
same cage face.
■ CONCLUSIONS
Our objectives here were to continue to build a detailed and
fully encompassing picture of the photophysical properties of
the very novel 3-D molecules, the [RPhSiO1.5]8,10,12 cages. Our
motivation was multifold. The most important driver was the
fact that in a number of our studies we found strong evidence
for interaction of cage LUMOs with conjugated moieties
attached directly to cage silicons.17,18
We also have now
determined in a number of these studies and on modeling the
bromination reactions of these cages19
that the LUMOs stick
out beyond the cage face and as such can interact with
segments of the functional groups that likely lie above cage
faces.
The current studies explore the electronic interactions of
ortho 4-substituted stilbenes used as model probes with the
larger cages [o-RPhSiO1.5]10,12 first to confirm that they also
have LUMOs that interact with conjugated moieties and to
provide comparative studies with the T8 cage studies described
previously.17,18
To this end, as expected from the recent bromination studies,
the larger cages exhibit photophysical behavior consistent with
the presence of LUMOs at an energetic level akin to those in
the T8 system. Again, evidence is found that the ortho stilbenes
sit above the cage face, and their absorptions can be blue-
shifted from the simple organic compound; whereas their
emission spectra are red-shifted as anticipated from the earlier
studies.
Several important findings with respect to the observed red
shifts are that the CN compound is red-shifted as much as the
MeO compound which is contrary to what might be anticipated
based on “donor/acceptor” behavior in simple conjugated
organics. This is despite a blue shift in its absorption. This red
shift is not the result of charge-transfer behavior given that a
shift to more polar CH3CN (from THF) results in no change
in emission λmax. This behavior is similar to that seen with C6F5
(previously studied)19
derivatives. Thus, it must be concluded
that the red-shift is a result of extended conjugation only.
The most important observation here is the TPA data
obtained for the NBoc SQs. In these systems, and contrary to
what might be expected, TPA cross sections increase, with
decreasing numbers of functional groups. Given that [NBoc-
StyrPhSiO1.5]8 exhibits a TPA cross-section of 0, we believe that
the only explanation for this set of results arises from
asymmetric functionalization of the cages.
The argument is that for a 3-D molecule that is fully
functionalized with highly polarizable groups the excited state
polarizations generated oppose each other and self-cancel as
likely happens for [NBocStyrPhSiO1.5]8. Furthermore, for the
more fully functionalized [NBocStyrPhSiO1.5]10,12 cages, the
NBoc groups would also be arranged in a more or less 3-D
array and would also self-cancel. Symmetry arguments can be
used to explain why the Ph10SQ derivatives show higher TPA
cross sections than the Ph12SQ analogue, suggesting larger
changes in transition dipole moment (μ) and enhanced
nonlinear susceptibility on optical excitation.
Finally, even for partially substituted cages, if the polarizable
groups were randomly attached to the cage then self-canceling
would also be expected. However, the TPA cross sections for
the partially substituted cages are much higher on a per moiety
basis. This can only be explained as a consequence of
asymmetric substitution during bromination.
On the basis of our bromination modeling studies it may be
reasonable to suggest that a first ortho bromine helps promote
addition of a second Br to an adjacent Ph group, thereby
promoting asymmetric bromination. This has important
implications for making “Janus”-type cages using other function-
alities.
The fact that the partially substituted CN systems do not
show this same exceptional TPA behavior likely resides in the
fact that the CN group competes with the cage for electron
density, limiting the degree of polarization that would be seen
in TPA measurements.
■ ASSOCIATED CONTENT
*S Supporting Information
Further characterization information and spectroscopic data of
synthesized compounds. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.jpcc.5b02678.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: talsdad@umich.edu.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The synthesis, separations, and spectroscopic work was
supported by the U.S. Department of Energy (DOE), Office
of Basic Energy Sciences, as part of the University of Michigan
Center for Solar and Thermal Energy Conversion Energy
Frontier Research Center, No. DE-SC0000957. The NMR
characterization and student support was supported by Intel
Corporation through contract number SRC MSR-Intel Task
2170.001. RML would like to thank the Technion Dept of
Mechanical Engineering, Haifa, Israel for a Lady Davis
Fellowship where portions of this manuscript were written.
We would especially like to thank Ashley Green who did some
of the TPA cross-section studies.
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