Synthesis, characterization and photophysical properties of polyfunctional phenylsilsesquioxanes: [o-RPhSiO1.5]8, [2,5-R 2PhSiO1.5]8, and [R3PhSiO 1.5]8. compounds with the highest number of functional units/unit volume

Article abstract of DOI:10.1039/c1jm11701g

The availability of pure samples of o-Br₈OPS, 2,5-Br ₁₆OPS, and Br₂₄OPS provides a rare opportunity to synthesize sets of corresponding stilbene derivatives: o-RStyr₈OPS, RStyr₁₆OPS, and RStyr₂₄OPS where R = 4-methyl (Me), Boc-protected 4-amino (NBoc), or 4-acetoxy (Ace). These derivatives show unique UV-Vis absorption and photoluminescent behavior that points to interesting interactions between the organic tethers and the silsesquioxane cage. o-RStyr₈OPS shows blue-shifts in the absorption spectra compared to p-MeStyr₈OPS, suggesting that the stilbene groups sit over and interact with the face of the electrophilic silsesquioxane cage as is the case with the parent molecule, o-Br₈OPS. The emission spectra of o-RStyr₈OPS are similar to p-MeStyr₈OPS indicating similar excited states involving the core LUMO. RStyr₁₆OPS exhibits absorption and emission spectra as well as Φ_PL similar to 1,4-distyrylbenzene, pointing to disruption in conjugation with the silsesquioxane cage because of steric interactions. RStyr₂₄OPS offers absorption maxima that are blue-shifted and emission maxima that are red-shifted relative to RStyr₁₆OPS. We speculate that RStyr ₂₄OPS is so sterically hindered that interactions with the cage face must occur. NBocStyr₂₄OPS and AceStyr₂₄OPS show moderate Φ_PL and high two photon cross-section values, leading us to conclude that there are two excited states of nearly equivalent energy in these molecules with similar decay rates: a normal radiative π-π* transition and charge transfer involving the silsesquioxane cage. These same functional groups can be anticipated to offer much greater two photon absorption if different methods can be found for protecting the free amine from oxidation or replacing the acetoxy group (e.g. perhaps using alkyl or aryl groups).

Full text of DOI:10.1039/c1jm11701g

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PAPER
Synthesis, characterization and photophysical properties of polyfunctional
phenylsilsesquioxanes: [o-RPhSiO1.5]8, [2,5-R2PhSiO1.5]8, and
[R3PhSiO1.5]8. compounds with the highest number of
functional units/unit volume†
ab
Santy Sulaiman,b Jin Zhang,b Theodore Goodson, IIIbc and Richard M. Laine
*
Received 19th April 2011, Accepted 26th May 2011
DOI: 10.1039/c1jm11701g
The availability of pure samples of o-Br8OPS, 2,5-Br16OPS, and Br24OPS provides a rare opportunity
to synthesize sets of corresponding stilbene derivatives: o-RStyr8OPS, RStyr16OPS, and RStyr24OPS
where R ¼ 4-methyl (Me), Boc-protected 4-amino (NBoc), or 4-acetoxy (Ace). These derivatives show
unique UV-Vis absorption and photoluminescent behavior that points to interesting interactions
between the organic tethers and the silsesquioxane cage. o-RStyr8OPS shows blue-shifts in the
absorption spectra compared to p-MeStyr8OPS, suggesting that the stilbene groups sit over and interact
with the face of the electrophilic silsesquioxane cage as is the case with the parent molecule, o-Br8OPS.
The emission spectra of o-RStyr8OPS are similar to p-MeStyr8OPS indicating similar excited states
involving the core LUMO. RStyr16OPS exhibits absorption and emission spectra as well as FPL similar
to 1,4-distyrylbenzene, pointing to disruption in conjugation with the silsesquioxane cage because of
steric interactions. RStyr24OPS offers absorption maxima that are blue-shifted and emission maxima
that are red-shifted relative to RStyr16OPS. We speculate that RStyr24OPS is so sterically hindered that
interactions with the cage face must occur. NBocStyr24OPS and AceStyr24OPS show moderate FPL and
high two photon cross-section values, leading us to conclude that there are two excited states of nearly
equivalent energy in these molecules with similar decay rates: a normal radiative p–p* transition and
charge transfer involving the silsesquioxane cage. These same functional groups can be anticipated to
offer much greater two photon absorption if different methods can be found for protecting the free
amine from oxidation or replacing the acetoxy group (e.g. perhaps using alkyl or aryl groups).
the heat capacity of silica often improving thermal stabilities
up to 100 ꢀC higher compared to all-organic compounds
Introduction
In the past few years, we and others have sought to use the
with structures similar to SQs or its organic tethers13–15 or
3-D nature of functionalized silsesquioxanes (SQs)1–3 to
model compounds such as organic triethoxysilanes. In
develop novel hybrid organic/inorganic components for
addition, these compounds are easily made in 0.1–1 kg
electronic and photonic devices.2,4–12 The rationale for this
quantities in short periods of time.16,17
effort comes from the fact that the T8 and Q8 derivatives,
Perhaps most importantly, recent work indicates that the cage
see Fig. 1, place functional groups in each octant in Carte-
is not an innocuous insulating component but rather appears to
sian space providing good-to-excellent solubility allowing
offer 3-D conjugation in the excited state promising novel
facile purification. Furthermore, the central cage provides
semiconductor properties.10,11 In the latter case we have shown
that the pure methyl-stilbene SQs, [o-MeStyrPhSiO1.5]8
(o-MeStyr8OPS) and [p-MeStyrPhSiO1.5]8 (p-MeStyr8OPS),
aMaterials Science and Engineering, University of Michigan, 2300
Hayward St, Ann Arbor, USA. E-mail: talsdad@umich.edu; Fax: +01
734-7634788; Tel: +01 734-7646203
show red-shifts in their emission behavior of up to 80 nm relative
to molecular trans-stilbene as seen in Fig. 2.11
bMacromolecular Science and Engineering, University of Michigan, 2300
Hayward St, Ann Arbor, USA. E-mail: talsdad@umich.edu; Fax: +01
734-7634788; Tel: +01 734-7646203
cDept of Chemistry, University of Michigan, 930 N. University, Ann Arbor,
MI, 48109-1055. E-mail: tgoodson@umich.edu
Furthermore, we also find that the two photon absorption
cross-sections (TPA) per moiety of methoxy- and amino-vinyl-
stilbene SQs, [p-MeOStilCH ¼ CHSiO1.5]8 and [p-NH2StilCH ¼
CHSiO1.5]8, are quite high relative to model compounds (see
Table 1). In these instances, the observed behaviors can be
directly correlated with the formation of charge transfer (CT)
† Electronic Supplementary Information (ESI) available: Spectroscopic
data for compounds synthesized. See DOI: 10.1039/c1jm11701g/
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Fig. 1 3-D symmetrical T8 and Q8 compounds.
excited states wherein the peripheral MeO- and NH2 groups
donate electron density to the cage in the excited state as sup-
ported by solvatochromism studies.10,11
diameters of approximately 1.5 nm.19 In contrast, for example,
PAMAM dendrimers20 with comparable numbers of functional
groups (see Table 2) obtained only at much greater diameters and
only as surface functional groups.
These results coupled with our recent discovery of methods of
making very pure [o-BrPhSiO1.5]8, [2,5-Br2PhSiO1.5]8,
[Br3PhSiO1.5]8, provided the impetus to explore the functionali-
zation of these compounds.18 In principle, these brominated
[PhSiO1.5]8 or OPS (octaphenylsilsesquioxane) derivatives offer
the potential to introduce 8, 16 or z 24 functional groups in the
same volume. Furthermore, these functional groups are expected
to offer extended conjugation via the central phenyl ring and
through the center of the cage. Also of potential interest is the
fact that the molecules synthesized here have some of the highest
densities of functional groups per unit volume possible.
Assuming spherical shapes, these polyfunctional SQs have
We have chosen functional groups per unit volume of indi-
vidual molecules and compared them to dendrimers as opposed
to total densities of molecules per cc. In our crystal structures,19
we do not see interdigitation; thus, these values are realistic at all
length scales.
The extended conjugation in each functional group coupled
with the potential for 3-D conjugation, considerable charge
transfer behavior and the fact that in earlier studies we saw
exceptional two-photon absorption cross-sections suggest
possible applications in optical limiting and enhanced solar
energy harvesting. As we report below, the results of these studies
provide unexpected properties that may be useful for these
applications.
Experimental section
Synthetic methods
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
Table 1 TPA properties of SQ derivatives.10,11
Sample
d (GM)
d/moiety (GM)
lmax nm
[p-MeStilSiO1.5 8
[p-Me2NStilSiO1.5 8
[StilCH ¼ CHSiO1.5 8
[p-MeOStilCH ¼ CHSiO1.5 8
[p-NH2StilCH ¼ CHSiO1.5 8
]
11
211
25
110
810
1.2
26
3
14
101
735
755
705
705
720
]
]
]
]
Table 2 Physical characteristics of PAMAM dendrimers.20
No. of
surface groups
Molecular
weighta
Generation
Diameter (nm)b
0
1
2
3
4
8
16
32
517
1.5
2.2
2.9
3.6
1430
3256
6909
a
Molecular weight is based on defect-free, ideal-structure, amine-
terminated dendrimers. Molecular dimensions determined by size-
exclusion chromatography.
b
Fig. 2 UV-Vis absorption and PL emission of trans-stilbene and [p-Me
(H)StyrPhSiO1.5]8 in THF.11
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distilled from Na/benzophenone under N2 prior to use.
[o-BrPhSiO1.5 8 (o-Br8OPS), [2,5-Br2PhSiO1.5 8 (Br16OPS), and
[Br3PhSiO1.5 8 (Br24OPS) were synthesized using previously
reported methods.18 All other chemicals were purchased from
Sigma-Aldrich, Fisher Scientific, or Strem Chemicals, Inc., and
used as received.
remove any remaining Pd particles, and re-precipitated into
200 ml methanol. The product is further purified by column
chromatography. Analytical data are presented in the discussion
section below.
]
]
]
Removal of residual Pd catalyst. To a dry 50-mL Schlenk flask
under N2 was added 1.0 g of RStyrxOPS 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 concen-
trated by rotary evaporation and precipitated into 200 mL of
methanol or hexane. The product was filtered and dried in vacuo
overnight.
Synthesis of 4-NBocStyrene. To a dry 50-mL Schlenk flask
under N2 was added a solution of 2.0 g (12 mmol) di-tert-butyl-
dicarbonate in 10 mL THF, followed by a solution of 1.2 g
(10 mmol) 4-vinylaniline in 10 mL THF. The mixture was then
refluxed for 24 h. After cooling to room temperature, the reac-
tion mixture was partitioned between 25 mL of DI H2O and
25 mL of CH2Cl2. The layers were separated and the aqueous
layer was washed twice with 25 mL of CH2Cl2. The organic layer
was then washed with 25 mL of DI H2O, dried over Na2SO4, and
concentrated to give a yellow residue, which was further purified
to by column chromatography on silica eluting with hexane/ethyl
acetate (70 : 30) to give the product as white waxy solid.
Deprotection of NBocStyrxOPS. To a dry vial under N2 was
added 10 mg of NBocStyrxOPS dissolved in 1 mL of THF and
1 mL of 37% HCl solution. The reaction was stirred overnight at
room temperature. The reaction was then neutralized with
saturated NaHCO3 solution and extracted with CH2Cl2 (3 ꢁ
10 mL). The combined organic layer was washed with DI H2O
(3 ꢁ 10 mL), dried over Na2SO4, and the solvent evaporated in
vacuo to give light brown solid as product. Analytical data are
provided in the results section.
General Heck reaction of o-Br8OPS. To a dry 10-mL Schlenk
flask under N2 was added 0.50 g (0.3 mmol, 2.4 mmol-Br) of
o-Br8OPS, 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 (3.7mmol, 0.8 ml) and R-styrene
(8.70 mmol). The mixture was stirred at 70 ꢀC for 24 h and then
quenched by filtering through 1 cm Celite, which was washed
with 5 ml THF. The solution was then precipitated into 200 ml
methanol, filtered, and the solid re-dissolved in 10 ml THF. This
solution was then filtered again through a 1 cm Celite column to
remove any remaining Pd particles, and re-precipitated into
200 ml methanol. Analytical data are presented in the discussion
section below.
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 Waters
refractometer and THF as solvent. The system was calibrated
using polystyrene standards and toluene as reference.
General Heck reaction of Br16OPS. To a dry 10-mL Schlenk
flask under N2 was added 0.50 g (0.22 mmol, 3.5 mmol-Br) of
2,5-Br16OPS, 22 mg (0.046 mmol) of Pd[P(t-Bu3)]2, and 21 mg
(0.023 mmol) of Pd2(dba)3. 1,4-dioxane (5 ml) was then added by
syringe, followed by NCy2Me (5.6 mmol, 1.2 ml) and R-styrene
(10.5 mmol). The mixture was stirred at 70 ꢀC for 24 h and then
quenched by filtering through 1 cm Celite, which was washed
with 5 ml THF. The solution was then precipitated into 200 ml
methanol, filtered, and the solid re-dissolved in 10 ml THF. This
solution was then filtered again through a 1 cm Celite column to
remove any remaining Pd particles, and re-precipitated into
200 ml methanol. The product is further purified by column
chromatography. Analytical data are presented in the discussion
section below.
Thermogravimetric analyses. TGAs were run on a SDT Q600
Simultaneous Differential DTA-TGA Instrument (TA Instru-
ments, Inc., New Castle, DE). Samples (15–25 mg) were loaded
in alumina pans and ramped at 10 ꢀC minꢂ1 to 1000 ꢀC under dry
air with a flow rate of 60 mL/min.
Matrix-assisted laser desorption/time-of-flight spectrometry.
MALDI-TOF was done on a Micromass TofSpec-2E equipped
with a 337 nm nitrogen laser in positive-ion reflectron mode
using poly(ethylene glycol) as calibration standard, dithranol as
matrix, and AgNO3 as ion source. Samples were prepared by
mixing solutions of 5 parts matrix (10 mg mLꢂ1 in THF), 5 parts
sample (1 mg mLꢂ1 in THF), and 1 part AgNO3 (2.5 mg mLꢂ1 in
water) and blotting the mixture on target plate.
General Heck reaction of Br24OPS. To a dry 10-mL Schlenk
flask under N2 was added 0.50 g (0.17 mmol, 3.9 mmol-Br) of
Br24OPS, 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 (6.2 mmol, 1.32 ml) and R-styrene
UV-Vis spectrometry. UV-Vis spectra were taken on a Shi-
madzu UV-1601 UV-Vis transmission spectrometer in CH2Cl2.
Samples were dissolved in CH2Cl2 and diluted to a concentration
(10ꢂ3-10ꢂ4 M) where the absorption maximum was less than 10%
for a 1 cm path length.
ꢀ
(12 mmol). The mixture was stirred at 70 C for 24 h and then
quenched by filtering through 1 cm Celite, which was washed
with 5 ml THF. The solution was then precipitated into 200 ml
methanol, filtered, and the solid re-dissolved in 10 ml THF. This
solution was then filtered again through a 1 cm Celite column to
Photoluminescence spectrometry. Photoluminescent spectra
were taken on a Fluoromax-2 fluorimeter in THF. Samples from
UV-VIS spectroscopy were diluted (10ꢂ5 to 10ꢂ7 M) to avoid
excimer formation and fluorimeter detector saturation.
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Two photon studies
Steady state measurements. All compounds were dissolved in
CH2Cl2 (Sigma-Aldrich, spectrophotometric grade) for carrying
out the optical measurements. The absorption spectra of the
molecules were measured using an Agilent (Model No. 8341)
spectrophotometer. In order to measure the molar extinction
coefficients, the original stock solutions were diluted to 10ꢂ6 M.
The fluorescence spectra were acquired using a Spex-fluorolog
spectrofluorimeter. The quantum yields of the molecules were
measured using a known procedure.21 Bis-MSB [p-bis(o-meth-
ylstyryl)benzene] has been used as the standard. The absorbance
was limited to less than 0.03. The solutions were purged with
argon for 3 min prior to measuring their emission spectra. Then,
the following relation was used to measure the quantum yield:21
Ð
JðnÞdn ðJaÞ n2
S
Ð
fF ¼ ðfF Þ
S
JSðnÞdn Ja n2S
where,
(fF)S ¼ Quantum yield of the standard
Ð
J(ꢀn)dꢀn ¼ Area under the fluorescence emission curve for the
sample
Scheme 1 Heck coupling studies on o-Br8OPS, 2,5-Br16OPS and
Br24OPS. Preparation of selected functionalized StyreneOPS for
comparison of photophysical properties.18
Ð
JS(ꢀn)dꢀn ¼ Area under the fluorescence emission curve for the
standard.
(Ja)S ¼ Absorbance of the standard
Ja ¼ Absorbance of the sample
n2 ¼ Refractive index of the solvent used for the sample.
iodination studies where double iodination was not observed.
With our recent discovery of routes to pure o-Br8OPS, Br16OPS,
and Br24OPS,18 we can now extend our studies to making sets of
octa-, hexadeca- and tetraicosa-styrenyl substituted OPS
compounds. The motivation, as noted above, is to extend our
knowledge concerning the cage-moiety interactions as it affects
photophysical properties. Scheme 1 shows the general reaction
schemes explored.
n2 ¼ Refractive index of the solvent used for the standard.
s
Two-photon excited fluorescence measurements. In order to
measure the two photon absorption cross sections, we followed
the two photon excited fluorescence (TPEF) method.22 A 10ꢂ4
M
Coumarin
307
(7-ethylamino-6-methyl-4-trifluoro-methyl-
coumarin) solution in methanol was used as the reference for
measuring TPA cross-sections at different wavelengths. The laser
used for the study was a Mai Tai Diode-pumped Mode-Locked
Ti:sapphire laser, which is tunable from 700 to 1000 nm. The
beam was directed on to 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 100 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 logarithmic plot between collected
fluorescence photons and input intensity gave a slope of two,
ensuring a quadratic dependence. The intercept enabled us to
calculate the two photon absorption cross sections at different
wavelengths.
(1)
23
(2)
Results and discussion
In previous studies, we used Heck coupling as a means to
introduce a variety of functional groups to brominated- (reaction
1) and iodinated- (reaction 2) OPS.17,23 In these studies, we had
not as yet learned to control bromination selectivity and opted
for compounds with less than eight bromines to minimize the
potential for double substitution at a single phenyl group. Even
so, double bromination was still observed at 3–5% of the total
bromine incorporated. This was the motivation for moving to the
We begin with a discussion of our synthetic methods as
a prelude to discussing characterization and thereafter photo-
physical properties for a set of three different StyreneOPS
systems: the para-methyl, -acetoxy and Boc-protected amine.
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Synthesis methods
a starting point for further functionalization to make supramo-
lecular or 3-D nanoarchitectures.
Heck coupling at the ortho position is relatively slow because of
steric interactions, see reaction (3) and Fig. 3. As a consequence,
most reactions were run at 70 ꢀC/24 h, in contrast to our earlier
studies where the reactions were run at room temperature
(48 h).11,12 Tables 6–8 list general characterization data for the
compounds prepared. In general, purities were quite high as
witnessed by the ceramic yields, which are close to those calcu-
lated, Tables 7, 8. Table 8 provides data for NBocStyrxOPS
separately as the analyses need some explanation.
It is well-known that SQ cages have high affinity for the
transition metals commonly used as catalyst in coupling reac-
tions, such as palladium and ruthenium.24 The Heck coupling
products of BrxOPS still contain residual palladium catalyst,
even after several precipitation steps, as evidenced by the grayish
tint of the products. Further removal of residual palladium was
achieved by treating the crude products with the well-known
palladium complexing additive N-acetyl-L-cysteine.25 The palla-
dium-cysteine complex is soluble in polar solvents such as THF
and methanol, but highly insoluble in non-polar solvents such as
toluene. Therefore, the palladium-cysteine complex can be
removed either during filtration of the original reaction mixture,
as it precipitated out of toluene, or during the precipitation as it
goes into solution in methanol.
Heck coupling reactions between crystalline BrxOPS (x ¼ 8,
16, 24) and the target p-RStyrene were run under much more
rigorous conditions compared to our previous work using
BrStyrenylOS.10,23 The close proximity of the ortho-Br groups to
the SQ core presents a challenge in obtaining complete conver-
sion to the Heck coupling product. This was achieved using
higher concentrations of p-RStyrene and elevated temperatures.
The Heck coupling reactions of BrxOPS produced some
unwanted byproducts, which according to the GPC analyses are
most likely dimeric in nature (i.e. two SQ molecules bridged by
an organic tether). These byproducts were not observed in our
earlier work with BrStyrenylOS.10 We believe that dimer
formation arises because of the harsher reaction conditions used
in this set of experiments, especially the high concentration of
reagents in the reaction mixture. The byproducts were removed
either by column chromatography or selective precipitation to
give pure products per GPC analyses.
(3)
Solubilities
As noted above, we chose three different R groups in this
work. The simplest methyl derivatives are used for comparison
with the small molecule analog and as a baseline for the other
derivatives. NH2VinylStilbeneOS previously synthesized from
p-BrStyrenylOS shows CT behavior in solution, and we would
like to investigate how this phenomenon is affected by increases
in the number of chromophores in the NH2StryxOPS systems.
However, due to the basic nature of –NH2 groups and the
instability of SQ cages in base, –Boc protection was used to
prevent unwanted degradation during synthesis. Deprotection
was possible for the octa-ortho-compound but the higher func-
tionality congeners were found to oxidize rapidly during depro-
tection and thus photophysical properties were measured for the
NBoc derivatives. The acetoxy- group was chosen as it can be
hydrolyzed to give hydroxyl- groups, which can be used as
All of the synthesized compounds are soluble in moderately
polar organic solvents such as THF, 1,4-dioxane, CH2Cl2, and
CHCl3. MeStyrxOPS and AceStyrxOPS are insoluble in highly
polar solvents such as methanol, and in nonpolar solvents such
as hexane. NBocStyrxOPS are soluble in methanol, and were
therefore purified by precipitation into hexane.
Molecular characterization
MALDI-ToF spectra of RStyrxOPS are shown in Figures S1-S6.
It should be noted that although the compounds synthesized in
this work are labeled as having 8, 16, or 24 groups per molecule,
those numbers reflect an average of the actual number of func-
tional groups present per molecule. The starting materials used in
this work (o-Br8OPS, Br16OPS, and Br24OPS) were purified by
multiple recrystallization; however, MALDI-ToF spectra of
these materials still show small amounts of other brominated
species (e.g. Br7OPS, Br15OPS, Br22OPS, etc.) present in the
system.18
Tables 3–5 detail the different molecular species present in
RStyrxOPS and their percentages based on MALDI-ToF data.
Even though each set of compounds (o-RStyr8OPS, RStyr16OPS,
and RStyr24OPS) was synthesized using starting material from
the same batch, their MALDI-ToF data show different compo-
sitions of molecular species. For example, MALDI-ToF data for
o-MeStyr8OPS and o-AceStyr8OS show different percentages for
the 7-mer and the 8-mer, and o-NH2Styr8OS show the presence
of the 9-mer, which is not present in the other two compounds.
We attribute these results to differences in ionization potentials
of the compounds rather than substitutional variations.
Fig. 3 o-RStyrxOPS bond lengths and angles exaggerated for clarity.
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Table 3 Molecular species present in MeStyrxOPS
o-MeStyr8OPS
MeStyr16OPS
MeStyr24OPS
Species
o-MeStyr7OPS
o-MeStyr8OPS
%
5
95
Species
%
Species
%
18.5
5.1
47.0
5.8
23.6
MeStyr14OPS
MeStyr15OPS
MeStyr16OPS
MeStyr17OPS
Average: 15.7
2.4
30.5
62.2
4.9
MeStyr22OPS
MeStyr22BrPh1OPS
MeStyr23OPS
MeStyr23BrOPS
MeStyrenyl24OPS
Average: 7.95
Table 4 Molecular species present in NH2StyrxOPS
Table 7 TGA data for MeStyrxOS and AceStyrxOS
o-NH2Styr8OPS
NH2Styr16OPS
Species
NH2Styr16OPS
Ceramic yield (%)
Calc. with
ave FWa
Species
%
89.6
10.4
%
100
R group
Me
x
Actual
Calc.
Td5% (ꢀC)
o-NH2Styr8OPS
o-NH2Styr9OPS
Average: 8.1
8
16
24
24.6
17.2
13.5
24.5
16.6
13
24.6
16.8
12.9
367
417
386
Average: 16.0
Ace
8
16
24
20.3
13
5.7
20.8
13.4
10.2
21.1
13.3
N.A.
397
359
344
Table 5 Molecular species present in AceStyrxOPS
o-AceStyr8OPS
AceStyr16OPS
a
Average FW calculated from MALDI-ToF data (Table 3).
Species
%
Species
%
o-AceStyr7OPS
o-AceStyr8OPS
23.6
76.4
AceStyr15OPS
AceStyr16OPS
AceStyr17OPS
Average: 16.05
10.6
74.4
15.1
Table 8 TGA data for NBocStyrxOPS
Average: 8.1
Boc group
mass (%)
Ceramic yield (%)
Actual Calc. Calc. with ave FWa Td5% (ꢀC) Actual Calc.
x
8
Complete conversion of the –Br groups is observed in the
MALDI-ToF spectra for almost all the compounds, with the
exception of MeStyr24OPS, which shows small amounts of
unreacted –Br groups. This is not completely unexpected,
considering the steric hindrance associated with having three –Br
groups on a phenyl ring attached to the SQ core, especially for
o-Br groups.
15.4
17.3
10.2
8
17.2
10.2
N.A.
177/ꢃ400 29
190/ꢃ440 35
184/ꢃ450 38
29.2
35.9
38.6
16 11.5
24
7
a
Average FW calculated from MALDI-ToF data (Table 2).
The MALDI-ToF data of NBocStyrxOPS show a series of low
intensity peaks close to the mass of the deprotected species. We
were able to collect MALDI-ToF data for two of the
deprotected compounds: o-NH2Styr8OS and NH2Styr16OPS. No
MALDI-ToF data were obtained for NH2Styr24OPS and
AceStyr24OPS, most probably due to their complex structures,
which makes them unstable under the UV laser of MALDI-ToF
such that they fragment. GPC and ceramic yield data for these
two compounds (Tables 6 to 8) suggest that nearly all of the –Br
groups were converted to the organic derivatives.
Table 6 MALDI-ToF and GPC data for RStyrxOPS
m/z (Ag+ adduct)
GPC
Mn
GPC analyses of RStyrxOPS (Table 6) show that these mate-
rials exhibit narrow molecular weight distributions, indicating
that they retain their silica core structures following reaction. As
expected, the methyl derivatives show the lowest molecular
weights due to their simple structures. Within each set of func-
tional groups, the molecular weight increases as the number of
functional groups per molecule increases. The values of Mn and
Mw, as measured by GPC, are smaller than those measured by
MALDI-TOF, but expected from GPC characterization of rigid,
spherical molecules using flexible, linear standards, based on
previous results.10–12
R group
Me
x
8
MALDIa Calc.
Mw
FW
PDI
2068.6
2070.7
2983.8
3821.3
1212 1225 1962.8 1.01
2032 2057 2875.9 1.01
2522 2565 3705.2 1.01
16 2890.4b
24 3812
1969.2b c
1971.7b c 2382 2431 2771.6 1.02
,
,
NBoc
8
16 3013.9c
24 N A
3015.7c
3845.1c
3749 3809 4509.7 1.02
4290 4390 6030.6 1.02
Ace
8
2422.2
16 3702.7
24 N A
2422.7
3704.1
4877.6
1535 1570 2314.9 1.02
2542 2614 3596.2 1.03
3593 3893 4717.4 1.08
TGA data for Me- and AceStyrxOPS, Fig. 4, show mass-loss
onset temperatures greater than 350 ꢀC. Table 7 details the actual
and theoretical ceramic yields for Me- and AceStyrxOPS. We
have also included theoretical ceramic yields calculated using
a
b
c
Highest intensity peak detected. As H+ adduct. As deprotected
species (NH2StyrxOPS).
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Fig. 6 Absorption and emission spectra for MeStyrxOPS.
Fig. 4 TGA data in air (10 ꢀC minꢂ1) for MeStyrxOPS (top) and
AceStyrxOPS (bottom).
Fig. 7 Absorption and emission spectra for NBocStyrxOPS.
Fig. 5 TGA data in air (10 ꢀC minꢂ1) for NBocStyrxOPS.
Fig. 8 Absorption and emission spectra for AceStyrxOS.
First, the absorption and emission spectra for RStyr16OPS are
red-shifted from those for o-RStyr8OPS as expected because of
the longer conjugation length. However, the absorption spectra
for RStyr24OPS are blue-shifted from those for RStyr16OPS,
whereas the emission spectra are red-shifted. Furthermore, for
RStyr8OPS FPL are typical of those seen previously at ca. 0.04
whereas for the RStyr16OPS FPL are an order of magnitude
higher at 0.40–0.60. Unexpectedly the FPL for RStyr24OPS are
0.10–0.30 lower than RStyr16OPS. This is surprising because it
might be anticipated that RStyr24OPS would exhibit more
extensive conjugation resulting in red-shifts in both the absorp-
tion and emission spectra relative to those of RStyr16OPS. This
should lead to more stabilization in the excited state leading to
higher FPL values.
average FW from MALDI-ToF data. With the exception of
AceStyr23OPS, the actual and theoretical values of Me- and
AceStyrxOPS are in close agreement with each other, confirming
that the compositions of the compounds are in accord with the
MALDI-ToF data.
TGA data for NBocStyrxOPS (Fig. 5) has two distinct mass
ꢀ
loss steps: The first step at ꢃ200 C arises from the loss of the
–Boc protecting group, while the second step at ꢃ400 ꢀC points
to the oxidative decomposition of the phenyl groups generating
silica as the final ceramic product. Table 8 summarizes the
thermal decomposition data of NBocStyrxOPS. The mass loss
from the –NBoc groups can be used to estimate the average
number of these functional groups per SQ molecule.
Since the actual and theoretical mass loss from the –NBoc
groups are comparable, we can assume that all of the –Br groups
are converted to the –NBocStyrene groups, in agreement with the
MALDI-ToF data.
In order to explain these results we must first compare the
observed behavior with their organic analogs.27–29 The compar-
ative data are also provided in Table 9. Thus, the absorption
spectrum of o-MeStyr8OPS has the same lmax as p-methyl-
stilbene (Table 9). This is z 20 nm blue-shifted from the
absorption lmax of p-MeStyr8OPS synthesized from p-I8OPS.11
In contrast, the emission lmax for p-methylstilbene is 355 nm
compared to 400 nm and 420 nm for p-MeStyr8OPS and 405 nm
for o-MeStyr8OPS.
Photophysical properties
The UV-Vis absorption and photoluminescence spectra for
RStyrxOPS are shown in Fig. 6–8. The photophysical data is
summarized in Table 9. Within each series of compounds with
the same R group, we observed the same trends as discussed
below. While consistent in their trends, there are still some
important surprises.
A comparison of 1,4-distryenylbenzene with MeStyr16OPS
shows similar absorption lmax of 353 nm and 349 nm, respec-
tively, suggesting similar ground states. The emitting lmax are
408 nm vs. 431 nm, which may be accounted for in part by the
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Table 9 Photophysical data for RStyrxOPS (THF, CH2Cl2 peak positions are identical)
Dn Stoke’s
shift (cmꢂ1
R group
x
Abs. lmax (nm)
Emis. lmax (nm)
FPL
%
)
d GM
d/group GM
p-MeStilbene25
298, 311
353
355
385, 408
9
62
1,4-Distyrylbenzene26
1,2,4-Tristyrenylbenzene27
1,3,5-Tristyrenylbenzene28
p-MeStyr8 OPSa
320, 355b
297
320
412 (toluene) 384 (hexane)
400, 422
4
Me
o-8
16
24
303
349
329
405
407, 431, 457
444
4
57
39
8312c
5451c
7873c
NBoc
o-8
16
24
317
364
346
386, 422, 445b
429, 453
5
43
8
7849c
5398c
7210c
1
50
270
ꢃ0
ꢃ3
440,b461
ꢃ12
Acetoxy
o-8
16
24
300
347
317
402, 422
407, 429, 451
435, 453b
4
45
19
9637c
5508c
8557c
ꢃ0
ꢃ1
221
ꢃ0
ꢃ0
ꢃ10
a
b
p-MeStyr8OPS synthesized from p-I8OPS.12 Shoulder. c There are several emission bands; the emission maximum (underlined) is used to calculate the
Stoke’s shift.
methyl substituent. In addition, the FPL values are quite similar
at 62% and 57% (ethanol vs. THF).
highly electrophilic. In fact, Bowers et al. calculate that the highly
electrophilic behavior extends beyond the cage,30 supporting the
suggestion that the cage face polarizes Br2, promoting ortho-
substitution, and the possibility that the styrenyl group prefers to
sit above the electrophilic cage face.18 This latter interaction
could explain the blue shift relative to the para analog
(p-MeStyr8OPS).
Finally, comparing 1,2,4-tristyrenylbenzene, 1,3,5-tristyr-
enylbenzene and MeStyr24OPS, we see that the all meta 1,3,5-
tristyrenylbenzene has an absorption lmax at z 300 nm whereas
the 1,2,4-isomer has an absorption lmax at z 320 nm. The
emission lmax for the 1,2,4-isomer is not reported but that for the
1,3,5-isomer is 412 nm in toluene. The data suggest that the
absorption and emission max for MeStyr24OPS are quite red-
shifted from the 1,3,5-tristyrenylbenzene analog but may be
similar to or red-shifted from the 1,2,4-isomer as well. Thus, the
blue shift for MeStyr24OPS seems explainable in terms of the
organic analogs. However, the FPL values remain difficult to
explain.
However, the emission spectrum of o-MeStyr8OPS is clearly
red-shifted from that of p-methylstilbene and similar to that of
p-MeStyr8OPS. The quantum yield of o-MeStyr8OPS is also
similar to p-MeStyr8OPS. In our previous work, we
attributed the red-shifts in the absorption and emission spectra of
p-MeStyr8OPS compared to molecular stilbene to 3-D conjuga-
tion as involving the LUMO of the SQ cage.11 It appears that the
excited states of both o- and p-MeStyr8OPS are quite similar
allowing for the argument that both excited states access the
same LUMO inside the SQ cage.
Multiple theoretical calculations suggest that the cage is highly
electrophilic,30 which is supported by the fact that it auto-cata-
lyzes bromination as discussed in the accompanying paper.18
Further support for this comes from our recent studies offering
considerable evidence for charge transfer transitions in the
excited state leading, for example, to quite high two photon
absorption cross sections as discussed below.10,11 As also noted in
the accompanying paper, the cage interacts with aromatic ring
substituents as if it were a electron withdrawing substituent
equivalent to a CF3 or NO2 group.31
The absorption and emission spectra of NBocStyrxOPS are
only slightly red-shifted from those of MeStyrxOPS and
AceStyrxOPS. This is expected given that the amino groups are
masked by the Boc protecting groups, which are electron with-
drawing. We would expect much larger red-shifts from free
amino groups due to their strong donor characteristics, as seen
previously.10,11 Normally, para-alkoxy substituents exhibit
good-to-excellent electron donating properties; however, the
AceStyrxOS absorption and emission spectra are similar to
MeStyrxOS suggesting roughly equivalent electron-donating
abilities. Thus the electron-withdrawing characteristics of the
acyl group seems to balance the normal electron donating
properties of a para oxygen substituent, but the TPA data
suggest this is not quite a correct assessment (see below).
The absorption and emission spectra of RStyr16OPS are both
red-shifted from those of o-RStyr8OPS as expected considering
the longer conjugation lengths. The absorption maxima and
quantum yields for RStyr16OPS are comparable to those for the
small molecule analog, 1,4-distyrylbenzene, while the emission
spectra are red-shifted from 1,4-distyrylbenzene (20–45 nm
Consequently, one might expect to see such an electron with-
drawing effect on the stilbene substituents prepared here
(RStyr8OPS) whether they are ortho- or para-substituents. The
implication is that the cages should cause considerable blue shifts
in both the absorption and emission maxima. This is contrary to
what is seen. Therefore this argument cannot be used to explain
the blue shift in the absorption data for the ortho-stilbene
(o-MeStyr8OPS). The only other possible explanation that seems
valid is that the styrenyl group in the ortho position is forced to
spend some of its time above the cage face. The crystal structure
of the starting [o-BrPhSiO1.5] indicates that the bromo group sits
over the face of the very electrophilic cage.18 Indeed, this inter-
action might actually be stabilized if the cage is considered to be
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depending on the R-group). The emission spectra of RStyr16OPS
exhibit structured bands typical for conjugated aromatic
compounds. These compounds also have low two-photon cross-
sections (GM values, see Table 9). All of these observations, as
briefly noted above, suggest that the HOMO and LUMO of
RStyr16OS are similar to those of 1,4-distyrylbenzene without
any contributions from the SQ cage.28 The simplest explanation
for this behavior is that the presence of the second styrenyl group
on each corner of the cage causes overcrowding around the cage,
forcing the distyrenylphenyl groups away from the cage face. It
also may interrupt the 3-D interaction with the cage in the excited
state. Thus we observe photophysical behavior expected for an
electronically isolated organic fragment. This contrasts with the
RStyr24OPS compounds.
Fig. 10 Interactions of ‘‘fragments’’ of tristyrenylphenyl groups on each
corner of RStyr24OS.
that for o-RStyr8OPS that tails into the absorption spectrum for
RStyr16OPS. Each has a lmax in between the maxima of
o-RStyr8OPS and RStyr16OPS. This implies that the absorption
spectra for each RStyr24OPS actually consist of contributions
from all ‘‘fragments’’ of the tristyrenylphenyl groups. Basically
we observe a sum of the interaction between the ortho-stilbenes
with the face of the SQ cage (Fig. 10a) with the longer conju-
gation of the distyrylbenzene units (Fig. 10b) coupled with the
disruption in overall conjugation caused by the meta-positioned
styrenyl (a.k.a. phenylenevinylene) units (Fig. 10c). The presence
of the third styrenyl group on each corner of the cage causes even
more crowding around the cage and some of the organic groups
are forced to approach the face of the cage, thus giving rise to an
interaction (blue shift) between the cage face and the organic
groups observed in the absorption spectra.
The blue-shifts in the RStyr24OPS absorption spectra
compared to those of RStyr16OPS likely arise from the
meta-styrenyl groups present on each central phenyl ring
(see Fig. 9). As discussed in the accompanying paper on the
polybromination of OPS,18 Br substitution starts at the
ortho-position (position 1 in Fig. 9). The second Br- adds (almost
exclusively) para- to the first Br- (position 5 in Fig. 9). We believe
that at this point, the steric hindrance will prevent addition of
a third Br to the other ortho-position (position 6, Fig. 9).
Therefore, addition of a third Br would be expected to occur at
either of the two other positions on the phenyl rings (positions 3
and 4 in Fig. 9), as supported by the X-ray single crystal data
from the accompanying paper, which shows partial occupancy at
both positions. This means that there will always be two styrenyl
groups meta to each other in RStyr24OPS.
Previous work on 1,2,4-tristyrylbenzene28 suggests that its
absorption spectrum can be considered as a summation of the
1,3-distyrylbenzene and 1,4-distyrylbenzene spectra, with
a maximum (320 nm) in between the maxima of the two dis-
tyrylbenzenes (300 nm and 355 nm) and a shoulder corre-
sponding to the absorption maximum of 1,4-distyrylbenzene
(355 nm). The authors attribute this phenomenon to interactions
between the two conjugated distyrylbenzene ‘‘fragments’’ of the
molecule. However, no rationalization as to what these interac-
tions might be is available in the literature.
The emission spectra for RStyr24OPS are red-shifted from
their respective RStyr16OPS and show no band structure. Their
quantum yields are slightly lower, but the two-photon cross-
section values are much higher for the NBoc and Acetoxy
groups, see Table 9 and Fig. 11. The data suggest that there are
actually two pathways for the transition from the excited to the
ground state: charge transfer (CT) and normal radiative emission
from a p* state.
The CT effects explain the lack of structure in their emission
spectra and the two high TPA values (see below). Normally CT
funnels energy away from luminescence and gives rise to low
quantum yields. However, in the case of RStilbene24OS, the FPL
values are not as low as expected if CT is the only mechanism for
excited-to-ground state transition. The normal p* radiative
emission provides another option, and explains the FPL values of
RSyr24OS. For both mechanisms to be active, both processes
must have roughly equivalent decay rates and energies, otherwise
one would dominate over the other. Indeed, this suggests that
one might promote the CT transition by using a more polar
solvent to stabilize the CT transition state and perhaps improve
the TPA values at the expense of the FPL values. Such studies are
planned in the future.
Related work suggests that meta-substituted phenyl-
enevinylene units in meta-distyrylbenzene and oligo(phenyl-
enevinylene) can be considered ‘‘conjugative insulators’’ that
disrupt the interaction between phenylenevinylene units.28,29 The
1,3,5-tristyrylbenzene absorption lmax
enevinylene units are meta-, is 297 nm (toluene or n-hexane) and
the emission lmax at 412 nm in toluene.29
, where all phenyl-
In comparison, we begin by considering the widths of the
absorption bands for RStyr24OPS. For each series of R-groups,
the absorption spectrum of RStyr24OPS has an onset similar to
One final comment about these results should be made.
Because of the very high density of functional groups, it is
possible that some form of 3-D through space interactions are
also involved in their absorption and emission behavior. At this
point, we have no proof but believe the possibility should be
mentioned as higher densities of functional groups affect
absorption and emission behavior in cyclic polystyrenes albeit
mostly causing blue shifts.32
As mentioned above, efforts to deprotect NBocStyrxOPS to
give NH2StyrxOPS for MALDI-ToF samples led to rapid
oxidation of the amino groups because of the aqueous acidic
reaction conditions. Attempts to deprotect the NBoc groups
using trifluoroacetic acid were also not successful. Our
Fig. 9 Two possible configurations of RStilbene24OS corner.
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previous work with hexadecaaminophenylsilsesquioxane,
[(NH2)2PhSiO1.5]8, shows it to be very basic and very susceptible
to air oxidation at room temperature.33 Thus, we were concerned
that similar efforts to generate samples for photophysical studies
would risk including contributions from oxidized materials.
Therefore, we decided that it was best to assess the photophysical
properties of the NBocStyrxOPS and AceStyrxOPS as discussed
below.
with an increase in the number of chromophores attached to the
phenyl group. As seen in Table 9, the cross-sections ‘‘per
chromophore’’ in RStyr24OPS are also enhanced by the
additional chromophores in comparison with RStyr16OPS and
o-RStyr8OPS systems.
For NBocStyr24OPS and NBocStyr16OPS, the cross-section
increases by a factor of 4.4, while the cross-section increase per
chromophore is by a factor of 3. This seems to correlate well with
the increase of the cross-section per chromophore, which seems
to correlate well with the steady-state measurements of Table 9.
In the case of RStyr16OPS, although the absorption measure-
ments showed a red shift, the fluorescence emissions and
quantum yields suggest that the system possesses less charge
transfer character. The literature suggests that molecular systems
with symmetric charge transfer from the ends of a conjugation to
the middle gives enhanced TPAs,36 such as found here for
RStyr16OPS and RStyr24OPS. Similar trends are observed in
AceStyrxOPS systems as well. Note that o-NBocStyr8OPS and
o-AceStyr8OPS with cross-sections below 1 GM, do not appear
to be promising two photon materials, but it may be that on
deprotection they will offer TPAs equivalent to the para systems
we have studied before.8 Finally, time-resolved measurements are
underway in order to understand the actual mechanism of the
charge transfer character of these systems.37
Two-photon excited fluorescence measurements
The interaction of the functional groups at the corners of the
cage can be very strong and it has been shown previously that this
interaction can lead to new optical properties.11 The possibility of
enhancement of the cage macromolecule’s two-photon absorp-
tion cross-section may result in possible applications in not only
optical limiting and sensor protection but the enhanced transi-
tion moment of these systems may also lead to enhanced solar
energy harvesting devices. In this section we discuss the linear
and nonlinear optical properties of this new class of SQs and
probe their two-photon absorption properties both in magnitude
and spectral width.
Table 9 provides much of the data taken in the current studies.
The steady state absorption and emission spectra shown in
Fig. 6–8 are displayed comprehensively in Figure S7. As noted
above, our efforts were directed towards understanding the
photophysical effects of increasing functional group density in
the same volume. Our previous studies provide a baseline for the
studies conducted here.10,11 In addition to the steady state studies
summarized in the Table 9 data, we also carried out TPA
measurements to investigate the charge transfer character.
Two-photon excited-state fluorescence (TPEF) measurements
with a femtosecond laser were used to measure TPA cross-
sections as a function of wavelength. It is clear from Fig. 11 that
the NBocStyrxOPS compounds offer superior TPA cross-
sections when compared with the AceStyrxOPS compounds for
RStyr16OPS and RStyr24OPS. This can be explained by the
greater strength of the donor group and by changes in the dipole-
moment.34,35 It was also noted that TPA cross sections increase
Conclusions
A series of stilbene-SQs (RStyrxOPS, x ¼ 8, 16, 24; R ¼
4-methylstyrenyl, 4-acetoxystyrenyl, Boc-protected 4-amino-
styrenyl) have been synthesized from the corresponding
bromophenyl-SQs, [o-BrPhSiO1.5]8, [2,5-Br2PhSiO1.5]8, and
[Br3PhSiO1.5]8. These molecules have some of the highest densi-
ties of functional groups/unit volume of any molecules even when
compared to silsesquioxane dendrimers.3–12,38–40 The blue shift in
the absorption spectra and red shift in the emission spectra of
o-RStyr8OPS compared to the small molecule analog suggests
interactions between the electrophilic face of the SQ cage with the
ortho-positioned organic group. In contrast, RStyr16OPS exhibit
absorption and emission spectra as well as quantum efficiencies
typical of simple organic compounds, most likely because of
steric interaction forcing the organic groups away from the cage
and disrupts the 3-D conjugation involving LUMO inside the SQ
cage. RStyr24OPS exhibits photophysical behavior that shows
characteristics of both regular p–p* transition and charge
transfer, suggesting that there are two excited states of nearly
equivalent energy with similar decay rates, involving the LUMO
inside the SQ cage as charge acceptor site. We expect these
compounds to offer much greater TPA properties if suitable
alternative methods of protecting the free amine from oxidation
or replacing the acetoxy group are found (e.g. with perhaps using
alkyl or aryl groups).
Acknowledgements
We would like to thank NSF for funds through grant CGE
0740108 with early work being funded by NSF IGERT Grant
DGE-9972776. Early work was also supported in part by Canon
Ltd and Mayaterials Inc. Most recently our work on the
Fig. 11 TPA cross section measurements of the investigated
chromophores.
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19 M. F. Roll, ‘‘Symmetric Functionalization of Polyhedral
Phenylsilsesquioxanes as a Route to Nano-Building Blocks.’’ PhD
dissertation, University of Michigan, 2010. Molecular diameters
were calculated based on single crystal X-ray data for
silsesquioxane molecules with similar structures [Styr8OPS and
(PhChC)8OPS] assuming cubic unit cells.
photophysical and spectroscopic analysis of the title compounds
was supported by a DOE Center Grant Award No. DE-
SC0000957.
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