Design, synthesis, and characterization of functionalized silepins: High quantum yield blue emitters

Article abstract of DOI:10.1021/om2000597

The synthesis of a family of 1,1-dimethyldibenzo[b,f]silepin derivatives, substituted with donor groups of various donor strength and conjugation length in the positions meta or para to the silepin silicon, is presented. The compounds were characterized b

Full text of DOI:10.1021/om2000597

ARTICLE
pubs.acs.org/Organometallics
Design, Synthesis, and Characterization of Functionalized Silepins:
High Quantum Yield Blue Emitters
Lauren G. Mercier,† Shunsuke Furukawa,‡ Warren E. Piers,*,† Atsushi Wakamiya,§ Shigehiro Yamaguchi,^
Masood Parvez,† Ross W. Harrington,|| and William Clegg||
†Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
‡Department of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan 113-0033
§Institute for Chemical Research, Kyoto University Uji, Kyoto, Japan 611-0011
^Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya, Japan, 464-8602
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, U.K., NE1 7RU
S Supporting Information
b
ABSTRACT: The synthesis of a family of 1,1-dimethyldibenzo-
[b,f]silepin derivatives, substituted with donor groups of various
donor strength and conjugation length in the positions meta or
para to the silepin silicon, is presented. The compounds were
characterized by NMR, UV-vis, and fluorescence spectroscopy, as
well as with cyclic voltammetry and DFT computations. One of
the meta series was characterized by X-ray crystallography. All
compounds show red-shifts in onset absorption and emission
maxima compared to the parent 1,1-dimethyldibenzo[b,f]silepin,
but all exhibit blue fluorescence with quantum yields ranging from 0.46 to 0.93 for the meta series and from 0.09 to 0.46 for the para
derivatives. The data suggest that, for the meta series, the primary absorption is of π-π* character, while in the para series, for strongly
donating groups, transitions from low-lying π orbitals to the LUMOþ1 orbital, which has contributions from the C-Si-C σ* orbitals,
are charge-transfer in character. The phenyl-substituted derivatives can be converted to borepins via transmetalation with BBr3, but other
strategies are necessary to prepare borepins with other donor groups.
Chart 1
’ INTRODUCTION
Organic electronics comprises a large area of research, and
many novel materials have been prepared that have applications
in organic light-emitting diodes (OLEDs), organic field effect
transistors (OFETs), and nonlinear optic materials (NLOs).1-6
Organic electronics are generally poorer performing than inor-
ganic materials in terms of electron transport, and lack of longterm
stability can also detract from their use in certain applications.
However, optoelectronic devices based on organic molecules and
polymers have significant advantages over inorganic materials, includ-
ing a wider array of processing options, cost effectiveness, and the
ability to tune the photophysical properties of a class of molecules
through synthesis and derivatization. Such modifications can lead to
fine control over, for example, the HOMO-LUMO gap of the
material, as well as the absolute energies of these frontier orbitals.
Perturbation of the HOMO-LUMO orbitals in π-conjugated
molecules can be accomplished by modification of the material’s
basic structural framework and extension of conjugation by
appropriate annulation or substitution patterns. Another strategy
is to replace carbon atoms with heteratoms, such as silicon.7
For example, 1,1-dialkyl siloles and substituted derivatives
(I, Chart 1), silicon analogues of the fluorene framework, have
been extensively studied and display strong fluorescence and
good carrier mobilities.8-26
Absorption maxima are red-shifted compared to the all-carbon
analogue fluorene, since the σ* orbitals of the exocyclic Si-CH3
groups interact with the π* orbital of the butadiene fragment and
lower LUMO energy.17,27 Silabenzenes (II) also have interesting
photoluminescence properties, but are less explored due to
their more challenging synthesis and high reactivity.28,29
Tokitoh utilized kinetic stabilization of the silabenzene
Received: January 21, 2011
Published: February 28, 2011
r
2011 American Chemical Society
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Scheme 1
Table 1. Conditions and Yields of Suzuki, Stille, and Sono-
gashira Coupling Reactions
Figure 1. Displacement ellipsoid diagram (50% probability level) for the
molecular structure of m-1-bTh. Selected bond distances (Å): Si(1)-
C(9), 1.872(3); Si(1)-C(18), 1.874(3); Si(1)-C(31), 1.865(3); Si-
(1)-C(32), 1.860(3); C(9)-C(14), 1.404(4); C(14)-C(15),
1.470(4); C(15)-C(16), 1.343(4); C(16)-C(19), 1.475(4); C(18)-
C(19), 1.413(4). Selected bond angles (deg): C(9)-Si(1)-C(18),
105.00(12); C(9)-Si(1)-C(31), 108.98(14); C(9)-Si(1)-C(32),
113.26(14); C(18)-Si(1)-C(31), 111.92(14); C(18)-Si(1)-C(32),
109.07(14); C(31)-Si(1)-C(32), 108.63(16); Si(1)-C(9)-C(14),
122.0(2); C(9)-C(14)-C(15), 124.2(3); C(14)-C(15)-C(16),
130.5(3); C(15)-C(16)-C(19), 132.3(3); C(16)-C(19)-C(18),
125.7(3); Si(1)-C(18)-C(19), 119.5(2). Selected torsion angles
(deg): S(1)-C(4)-C(5)-S(2), -150.59(19); S(2)-C(8)-C(11)-
C(10), 16.7(4); C(17)-C(22)-C(23)-S(3), -168.9(2); S(3)-
C(26)-C(27)-S(4), 171.58(16).
chromophore by incorporation of
a
2,4,6-tris[bis-
Recently, we reported the use of a seven-membered
borepin core in the blue-emitting boron-containing acene
analogues, exemplified by dibenzo[b,f]borepin IV.57 In order
to tune the HOMO-LUMO gap of these intriguing materi-
als, we sought to functionalize the periphery of the dibenzo-
borepin core in the positions meta and para to boron, as
indicated in Chart 1. We thus became interested in the
corresponding dibenzo[b,f]silepins as potential precursors,
reasoning that the carbon-silicon bonds of the silepin
framework would better withstand the conditions necessary
to forge C-C bonds via cross-coupling protocols than the
stannepin precursors we utilized in our initial study.57 In the
meantime, Tovar and co-workers reported that suitably
(trimethylsilyl)methyl]phenyl (Tbt) group to prepare silaben-
zene and higher silaacenes.30-38 Another rarely studied
organosilicon framework is the seven-membered-ring systems,
silepins (III).39-56 From a photophysical standpoint they are
potentially interesting compounds due to the possibility of
aromaticity through Si(3dπ)-C(2pπ) conjugation, although
UV-vis55 and mass spectral analysis39 do not indicate that this
type of electronic communication is significant in published
examples. However, red-shifts in absorption39,40,55 and fluor-
escence compared to the all-carbon congener, suberene, were
seen and are attributed to contribution from the Si-Me σ
bonds to the cycloheptatriene π system.55
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Figure 2. Representative absorption/emission spectra measured in CH2Cl2 (5 ꢀ 10-5 M): (a) absorbance and (b) fluorescence spectra of m-1-Ph and
m-1-bTh; (c) absorbance and (d) fluorescence of p-1-Ph and p-1-bTh.
sterically shielded borepins themselves could serve as sub-
strates for cross-coupling functionalization reactions without
breaching the integrity of the seven-membered core of the
molecule and reported their conversion to a number of
functionalized borepin targets.58,59 We show here that con-
version of a silepin framework to a borepin function is
possible in some instances, but is not generally successful;
however, with several meta- and para-substituted derivatives
of III in hand, a comparative study between these silepin-
based materials and their borepin analogues became possible.
Herein we describe the synthesis of a number of novel
donor-acceptor-donor (D-A-D) dibenzo[b,f]silepin-
based organic materials that exhibit bathochromic shifts
compared to both suberene and the parent, unsubstituted
1,1-dimethyldibenzo[b,f]silepin (III). Comprehensive photo-
physical and electrochemical measurements, and supporting
computational studies, show that for dibenzo[b,f]silepins
with various donor groups in the position meta to silicon,
silicon acts to rigidify the stilbene framework, while substitu-
tion in the para position puts the donor groups in direct
conjugation with the silicon atom, giving rise to charge-
transfer character in their absorption spectra.
aryl salt and a differentiated dihalobenzaldehyde, in keeping with
the synthetic strategy we developed for the preparation of
dibenzoborepin57 and also used by Tovar.58 The chlorobenzal-
dehydes were synthesized from the corresponding aryl iodide
using methodology developed by Knochel,60 while the phospho-
nium salts were derived from benzyl bromide derivatives formed
via NBS bromination of a dihalotoluene precursor (Scheme S1).
Bromobenzaldehydes and bromo-substituted phosphonium salts
were prepared from the corresponding commercially available
carboxylic acids (Scheme S2). The Wittig reactions afforded the
tetrahalo precursors to the meta and para silepins in excellent
yields of 95% and 88% as shown in Scheme 1. The differential
halogen substitution was necessary to ensure selective dilithium
halogen exchange using nBuLi in ether, allowing the aryl
dilithium species generated in situ to be trapped with dimeth-
yl silicon dichloride to give the compounds m-Cl (56%) and
p-Br (88%) in workable yields. The addition of TMEDA58
reduces the presence of polymeric byproduct and facilitates
purification.
With these precursors in hand, various C-C coupling proto-
cols were exploited to install a variety of aryl substituents onto the
dibenzo[b,f]silepin core (Table 1). Suzuki coupling reactions
were carried out with standard conditions61 to give fair yields for
strongly donating (m/p-1-PhOMe, m/p-1-PhNPh2) and mod-
erately donating (m/p-1-Ph) aryl groups. Stille coupling
conditions62 were used to prepare thiophene and bithiophene
derivatives (m/p-1-Th, m/p-1-bTh), while Sonogashira proce-
dures63 gave the two phenylacetylene-substituted compounds
(m/p-1-CCPh). The parent, unsubstituted dibenzo[b,f]silepin
’ RESULTS AND DISCUSSION
Synthesis. The synthetic chemistry relevant to the pre-
paration of the dibenzo[b,f]silepins is shown in Scheme 1.
Synthetic routes to necessary halogenated dibenzo[b,f]silepins
involve both a Wittig reaction between a dihalogen phosphonium
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Table 2. Photophysical and Electrochemical Data for meta- and para-Substituted Dibenzo[b,f]silepinsa,b
compound
suberene
abs λonset [nm]
abs λmax [nm] (log(ε))
fluor λmax [nm]
φF
τf [ns]
knr (1 - φF)/τf) [108 ns]
E1/2 [V]
332
287(4.22)
229(4.41)
293(4.08)
243(3.89)
384
1
336
379
359
0.14
0.90
0.15
1.6
57.3
0.63
(373)
(339)
413
m-1-Ph
326(3.59)
267(3.72)
229(3.75)
333(3.71),
282(3.67)
357(4.40)
305(4.31)
229(4.40)
349(3.37)
300(3.31)
389(3.89)
361(3.89)
253(3.55)
347(3.65)
299(3.69)
265(3.51)
305(3.74)
265(4.47)
278(3.78)
227(3.26)
339(3.82)
308(3.78)
253(3.74)
228(3.68)
292(3.95)
228(3.57)
350(3.42)
259(3.09)
227(3.03)
310(3.74)
291(3.80)
226(3.47)
-2.10
394
m-1-PhOMe
m-1-PhNPh2
385
435
424
0.93
0.91
1.4
2.1
0.50
0.43
-2.09
(407)
473
-2.22
-2.00
þ1.08 (2e-)
-2.19
m-1-Th
408
458
415
0.77
0.46
1.4
1.6
5.7
(438)
463
-1.93
m-1-bTh
0.94
-1.92
(488)
-1.80
m-1-CCPh
403
407
0.77
1.2
1.9
-2.01
-1.81
(429)
p-1-Ph
338
334
390
371
0.38
0.22
0.43
1.4
5.4
5.9
4.4
-2.16
-2.23
(355)
391
p-1-PhOMe
p-1-PhNPh2
1.4
447
394
0.97
-2.18
þ0.96c
p-1-Th
354
398
0.09
0.10
2.0
4.6
-2.23
-2.24
p-1-bTh
424
0.47
19.1
(404)
p-1-CCPh
346
375
0.36
3.8
1.7
-2.03
a Absorption and fluorescence were measured in 5 ꢀ 10-5 M DCM solution. b Cyclic voltammetry was carried out at a scan rate of 100 mV/s in THF
with 1 mM substrate and 0.1 M [NBu4][PF6] as the supporting electrolyte. Ferrocene (Fc/Fcþ) was used as an internal standard and set to 0.54 V.
c Cyclic voltammetry was carried out at a scan rate of 100 mV/s in DCM with 1 mM substrate and 0.1 M [NBu4][PF6] as the supporting electrolyte.
Ferrocene (Fc/Fcþ) was used as an internal standard and set to 0.47 V.
was prepared for comparison by a similar procedure, using a
dibromostilbene precursor as shown in eq 1.
of the silepin silicon was not strongly affected by the substitution
differences. Unsubstituted 1 was observed at -10.9 ppm relative
to SiMe4; the meta derivatives were slightly downfield of this
value (-10.2 to -10.4 ppm), while the para series appeared
marginally upfield (-10.6 to -11.4 ppm). In common organic
solvents (hexanes, toluene, methylene chloride) the compounds
give pale yellow or colorless solutions. Photostability studies in
solution indicate that they slowly, but not completely, decom-
pose when exposed in a light box to 254 nm light for >12 h. Drop
cast solid films of p-1-PhOMe demonstrated even less photo-
degradation (<3-4%) when irradiated for 24 h at 254 nm. The
primary products from these photoreactions appear to be formed
via photoejection of “SiMe2”, based on the mass spectrometric
All of the π-extended silepins reported here are air and
moisture stable and were purified by column chromatography.
They were fully characterized using NMR spectroscopy and high-
resolution mass spectrometry. In the 29Si NMR, the chemical shift
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compounds, it is apparent that as donating group strength and
conjugation length increase, there is a bathochromic shift in both
the longest absorption and fluorescence maxima compared to
unsubstituted 1 (Table 2). Compound m-1-Ph displays the
weakest red-shift in both onset absorption (43 nm) and emission
maxima (54 nm), while introduction of more extensively con-
jugated groups as in m-1-PhOMe and m-1-PhNPh2 shows larger
shifts in absorption (up to 56 nm) and emission (up to 80 nm).
The phenylacetylene (m-1-CCPh) and thiophene (m-1-Th)
derivatives display moderate changes, while an increase in donor
strength and conjugation length in m-1-bTh gives a very large
red-shift of 122 nm in onset absorption and of 56 nm in emission
maxima. Solvatochromism studies (Table S1) show small posi-
tive effects when a strongly donating group is present, for example
in m-1-NPh2. In this instance, the HOMO surface is mainly
associated with the donor group, while the LUMO is largely
located on the dibenzo[b,f]silepin core, resulting in a degree of
charge separation in the excited state. Other members of the meta
family exhibit little solvatochromism, since the HOMOs and
LUMOs are more diffusely conjugated over the entire frame-
work, resulting in little change in the dipole moments of ground
versus excited states. All of the meta series compounds show high
fluorescence quantum yields and short fluorescence lifetimes,
which indicates short nonradiative decay rate constants.
In the para series of substituted silepins, the donating substit-
uents are introduced in a position that puts them in conjugation
with the silicon atom and not only extends the π framework but
also increases charge-transfer character from the donating groups to
the silepin ring. Solvatochromism patterns in this series (Table S1)
show large red-shifts in polar solvents for silepins substituted
with strongly donating groups, which indicates significant charge
transfer in the excited state. This is supported by a Lippert-
Mataga plot65-67 (Figure S3), which revealed large changes in
dipole moment between ground and excited states for p-1-
PhNPh2 and p-1-PhOMe (35.0 and 18.4 D, respectively). In
the meta-substituted family, smaller differences were seen for
m-1-PhNPh2 and m-1-bTh (23.8 and 14.2 D). Compared to the
meta-substituted compounds, smaller bathochromic shifts in
onset absorption and emission maxima are seen for the para
series. For example, compound p-1-Ph exhibits similar onset
absorptions and emission maxima to 1; this is probably due to the
orthogonality of the phenyl rings, as indicated by the computed
structure for this derivative (see below). Introduction of an
electron-donating group to the phenyl ring, as seen for p-1-
PhOMe and p-1-PhNPh2, shows large shifts in emission maxima
relative to 1 (32 and 54 nm, respectively), suggesting a higher
level of electronic communication from these aryl groups to the
silepin core. Compounds p-1-Th and p-1-bTh show strong red-
shifts in both onset absorption (18 nm, 62 nm) and fluorescence
(35, 65 nm) profiles. The para series shows moderate quantum
yields, lower than those seen in the meta series, and longer
fluorescence lifetime; therefore the rates of nonradiative decay
processes are larger.
Figure 3. Representative HOMO and LUMO orbital surfaces for m-1-
Ph (left) and p-1-Ph (right) calculated at the B3LYP/6-31G(d,p) level
using Gaussian 03.
detection of substituted phenanthrenes. However, these reactions
are not clean, and rigorous determination of the photodecom-
position pathways has not been done.
Despite the fact that all of these silepins are solids under
ambient conditions, X-ray quality crystals were difficult to come
by; the crystal habit of the compounds tends toward long needles
that do not strongly diffract. The connectivity in compound p-1-
Ph was established positively, but the R-factor of ∼30% results
in metrical data with a low level of precision (see Figure S1).
However, one exception was the compound m-1-bTh, whose
structure was successfully determined; its molecular structure is
shown in Figure 1, along with selected metrical parameters. The
molecule assumes an overall bent shape due to the boat-like
conformation of the central silepin ring, where C(15)-C(16)
forms the stern and the Si(1)Me2 unit the prow. As a result, the
angle between the planes defined by the C6 rings flanking the
silepin core is 123.18(9)ꢀ. The bond distance of 1.343(4) Å for
C(15)-C(16) is typical for a CdC bond, but due to the strain
found in the seven-membered ring, the C(14)-C(15)-C(16)
and C(15)-C(16)-C(19) angles are 10-12ꢀ greater than the
ideal value of 120ꢀ. Ring strain also results in longer bonds from
C(9) and C(18) to Si(1) than those to the sp3-hybridized
carbons C(31) and C(32). The bithiophene units exhibit good
conjugation with the aromatic rings of the dibenzosilepin, as
judged by torsion angles about C(8)-C(11) and C(22)-C(23)
that are less than 17ꢀ away from coplanarity. The bithiophene
torsion angles about C(4)-C(5) and C(26)-C(27) are some-
what different, with the former being about 30ꢀ from coplanarity
in contrast to the latter, which has an S(3)-C(26)-C(27)-
S(4) dihedral angle of 171.58(16)ꢀ.
Computations. The absorption behavior of these compounds
as described above would seem to suggest that the meta series
essentially functions as a rigidified stilbene chromophore in
which the dimethylsilyl group serves as an anchoring group with
little ability to perturb the HOMO-LUMO character of the
molecule. In contrast, in the para series absorptions with more
charge-transfer character begin to compete with the π-π*
transitions that dominate the meta compounds. This notion is
supported by TD-DFT calculations using Gaussian 03, which for
Photophysical and Electrochemical Data. The absorption
and emission spectra for m/p-1-Ph and m/p-1-bTh are shown in
Figure 2 as typical examples of these two families of compounds;
a complete summary of the photophysical and electrochemical
data obtained is given in Table 2.64 Within the meta series of
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Table 3. Molecular Orbital Energies, Energy Gaps, and Oscillator Strengths Taken from Calculations Run at the B3LYP/6-31G(d,p)
Levela
Compound
HOMO-1 [eV]
HOMO [eV]
LUMO [eV]
LUMOþ1 [eV]
transition energyb [eV]
oscillator strengthb (f)
m-1-Ph
-6.27
-5.89
-4.97
-6.12
-5.69
-6.10
-6.28
-5.79
-5.19
-6.08
-5.61
-6.05
-5.32
-5.40
-4.81
-5.58
-4.99
-5.66
-5.95
-5.76
-5.18
-5.81
-5.56
-6.03
-1.65
-1.56
-1.60
-1.92
-2.17
-2.21
-1.56
-1.44
-1.53
-1.88
-1.82
-1.88
-0.56
-0.69
-0.65
-1.15
-1.65
-1.45
-1.18
-0.973
-1.21
-1.43
-1.81
-1.72
3.31
0.9338
m-1-PhOMe
m-1-PhNPh2
m-1-Th
3.42
0.9042
2.83
1.1773
3.28
0.8417
m-1-bTh
2.70
1.2913
m-1-CCPh
p-1-Ph
3.08
1.2555
3.98 (4.64)c
3.83 (4.38)c
3.29 (3.52)c
3.55 (4.21)c
3.56
0.3505 (1.4586)c
0.0221 (0.9645)c
0.0633 (1.0863)c
0.1067 (1.3879)c
1.2030
p-1-PhOMe
p-1-PhNPh2
p-1-Th
p-1-bTh
p-1-CCPh
3.68 (3.85)c
0.0030 (1.6983)c
a Frontier molecular orbital energies from optimized DFT calculations. b Oscillator strengths and energy of transitions from TD-DFT calculations.
c Oscillator strengths and energy of transitions from filled orbitals to the LUMOþ1 orbital.
Table 4. Frontier Orbital Energies Derived from Electro-
chemical Data and UV-Vis Onset Absorptions for meta- and
para-Substituted Silepins
compound
LUMOa [eV]
HOMOb [eV]
Egc [eV]
m-1-Ph
-2.1
-2.1
-2.2
-2.3
-2.4
-2.4
-2.0
-1.9
-2.1
-1.8
-2.1
-2.1
-5.4
-5.3
-5.0
-5.3
-5.1
-5.5
-5.7
-5.6
-5.3
-5.3
-5.2
-5.7
3.3
3.2
2.9
3.0
2.7
3.1
3.6
4.1
3.2
3.5
3.1
3.6
m-1-PhOMe
m-1-PhNPh2
m-1-Th
m-1-bTh
m-1-CCPh
p-1-Ph
p-1-PhOMe
p-1-PhNPh2
p-1-Th
Figure 4. Representative cyclic voltammetry (CV) curves for (a) m-1-
Ph and (b) p-1-Ph measured at 100 mV/s in THF with 1 mM substrate
and 0.1 M [NBu4][PF6] as the supporting electrolyte. Ferrocene (Fc/
Fcþ) was used as an internal standard and set to 0.54 V.
p-1-bTh
p-1-CCPh
a Calculated from Epc of first reduction wave referenced to Fc/Fcþ (E1/2
=
-4.8 eV). b Calculated from E1/2 and onset absorption energy. c Onset
absorption energy.
the meta series show strong oscillator strengths for HOMO to
LUMO absorptions that correspond to π-π* transitions based
on the fully conjugated stilbene framework (Figure S4). This is
exemplified by the HOMO/LUMO orbitals for m-1-Ph shown
in Figure 3 and the computed HOMO/LUMO energies and
oscillator strength data given in Table 3. For the corresponding
p-1-Ph isomer, the HOMO/LUMO combination is again sug-
gestive of a π-π* transition, but in these compounds, the
LUMOþ1 orbital is very close in energy to the LUMO and has
strong C-Si-C σ* character, showing silicon does more than
simply rigidify the stilbene and acts as an acceptor group
(Figure 3, Figure S5). Indeed, the TD-DFT analysis shows
that these HOMO to LUMOþ1 transitions have high oscilla-
tor strengths (Table 3).
Electrochemistry. Cyclic voltammetry was carried out on all
the silepins reported here to obtain a measurement of the
HOMO/LUMO gap. All compounds exhibited reversible one-
electron reduction waves; for several members of the meta series,
a second reduction was also observed (Figure 4, Figure S6).
Overall, the meta compounds were more easily reduced than the
para compounds, meaning their LUMO orbitals were more
stabilized. Ionization potentials (HOMO) and electron affinities
(LUMO) were derived from Epc (Fc/Fcþ E1/2 = -4.80 eV)68
and onset absorption (Table 4). For the meta-substituted
silepins, the energy of the HOMO increases with conjugation
length and LUMO energy is lowered as the groups interact favo-
rably with the silepin core, for example in m-1-bTh and m-1-
CCPh. In the para series, compounds p-1-BTh and p-1-CCPh
have the most stabilized LUMO levels due to the extended
conjugation framework in these derivatives. The HOMO en-
ergies see similar modifications to those in the meta series, and
red-shifts observed are mainly derived from conjugation length.
The HOMO/LUMO gap resulting from these measurements are
in excellent agreement with those obtained from the TD-DFT
computations (see Table 3).
Comparison with Substituted Borepins. In a closely related
study,59 Tovar and Caruso report a series of meta- and para-
substituted dibenzoborepins with a similar array of substituents
to those reported here for the silepins 1, and a comparison of the
behavior of the two families of compounds is instructive. In
general, the directly comparable systems (those with meta and
para PhOMe, Th, bTh, and CCPh substituents) show that the
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substantial curvature in the tricyclic ring system,57 the borepin
core in m-2-Ph is completely planar, in keeping with computa-
tional prediction. Here, incorporation of the meta phenyl groups
disrupts the packing interactions that result in the previously
observed curvature. The bond distances within m-2-Ph are as
expected, and the internal ring angles for the seven-membered
ring exhibit similar patterns to those described above for m-1-
bTh. The torsion angles given show that the two phenyl groups
are twisted out of coplanarity with the dibenzoborepin core to an
extent typical of biphenyl compounds.
Although this particular silepin to borepin conversion was
successful, when NPh2 groups were present, side reactions
between BBr3 and the Lewis basic groups precluded conversion
to the borepins via this route. The other derivatives would likely
suffer from this issue also; thus, the Tovar strategy of carrying out
coupling reactions directly on the dihaloborepin precursors
appears to be more general than a pathway that sequences the
coupling reactions before the transmetalation step.
Figure 5. Displacement ellipsoid diagram (50% probability level) for
the molecular structure of m-2-Ph. Selected bond distances (Å): B(1)-
C(1), 1.564(3); B(1)-C(14), 1.564(3); B(1)-C(27), 1.588(3);
C(1)-C(6), 1.427(3); C(6)-C(7), 1.438(3); C(7)-C(8), 1.345(3);
C(8)-C(9), 1.442(3); C(9)-C(14), 1.427(3). Selected bond angles
(deg): C(1)-B(1)-C(14), 127.42(17); C(6)-C(7)-C(8), 133.1(2);
C(7)-C(8)-C(9), 133.8(2). Selected torsion angles (deg): C(4)-
C(3)-C(15)-C(20), -35.4(3); C(11)-C(12)-C(21)-C(22), -
36.1(3).
’ CONCLUSIONS
onset absorptions for the silepins are at higher energy, indicating
a larger HOMO/LUMO gap in the silicon compounds. This can
be attributed to the boron-centered LUMO in the Lewis acidic
borepins. In both families of compounds, substitutions result in
red-shifts when compared to the parent, unsubstituted silepin
(1), and both families exhibit larger red-shifts for the meta series
than the para derivatives. In general, the silepins exhibit greater
quantum yields (the values in Table 1 are absolute quantum
yields) and larger Stokes shifts than the borepins. The larger
Stokes shifts in the silepins may be rooted in bent-to-planar
geometry changes69 from ground-state structures to excited
states; in the planar borepins, these changes are likely to be less
significant. However, the photophysical properties between
these families of chromophores are not phenomonologically
very different, but because in the planar borepins the empty p
orbital on boron is conjugated with the rest of the π system, the
character of the HOMO/LUMO absorptions in the boron
heterocycles have less charge-transfer character. In the silepins,
the chromophores are more similar to rigidified stilbenes, and
charge-transfer character emerges only when strongly donating
substituents are incorporated and HOMO f LUMOþ1 tran-
sitions dominate.
A family of meta- and para-substituted silepins were prepared
via standard aryl coupling reactions and exhibited red-shifted
absorption and emission compared to the parent 1,1-dimethyl-
dibenzo[b,f]silepin (1). In the meta position, these changes have
been attributed to an increase in overall conjugation of the stilbene
framework where silicon rigidifies the system, raising the energy
of the HOMO level. The degree of bathochromic shifting is
dependent on the nature of the donor group, which lowers the
LUMO level as it becomes more energetically compatible with
silepin core. When electron donor groups are coupled in the para
position, smaller bathochromic shifts in onset absorption and
fluorescence are observed since the HOMO-LUMO gap is
mostly affected by the raising of HOMO energy. For strongly
donating groups (e.g., in m-1-NPh2), strong positive solvato-
chromism is observed in the para case, suggesting charge-transfer
character in some of these systems; computations indicate this
involves excitation into the LUMOþ1 levels, which have a strong
contribution from the σ* orbitals of the core C-Si-C bonds.
The para series also show larger nonradiative decay processes
since they have lower quantum yields and longer fluorescence
lifetimes than the meta series and are less easily reduced. Overall,
these compounds exhibit strong blue fluorescence and are air and
moisture stable—excellent candidates for applications as organic
materials.
Conversion of Silepins to Borepins. Synthetic routes to both
families proceed through differentially halogenated tetrahalostil-
benes, with the borepins requiring an extra transmetalation step
from the precusor stannepins. In fact, the silepins can also be used as
precurors to borepins; as shown in eq 2, m-1-Ph can be converted to
the B-mesityl borepin m-2-Ph after quenching the in situ-generated
borepin bromide with mesityllithium. X-ray quality crystals of this
material were obtained after purification by column
’ EXPERIMENTAL SECTION
General Procedures and Equipment. All reactions and product
manipulations were performed under a purified argon atmosphere using
vacuum line techniques or in an MBraun glovebox unless otherwise
specified. All solvents were dried over the appropriate drying agents
(activated alumina, CaH2, Na/benzophenone) and vacuum distilled
prior to use. Silica gel column chromatography was carried out on Silia-P
Flash silica gel (particle size 40-63 mm) from Silicycle. Potassium tert-
butoxide, nBuLi (1.6 M in hexanes), and 4-bromo-2-iodobenzoic acid
(Trans World Chemicals Inc.) were used as received. Triphenylpho-
sphine, dichlorodimethylsilane, 1-bromo-4-chlorobenzene, dimethylfor-
mamide, 4-chloroaniline, and 4-chlorotoluene were used as received
from Aldrich. Dimethylformamide was dried with magnesium sulfate,
distilled under reduced pressure, and stored over molecular sieves.
These materials were employed to prepare the tetrahalogenated pre-
cursors to the silepins m/p-1-Ar as depicted in Schemes S1 and S2; a
chromatography, and a displacement ellipsoid diagram is given in
Figure 5, along with selected metrical parameters. In contrast
to the parent, unsubstituted dibenzoborepin, which exhibited
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ARTICLE
description of the synthesis of unknown materials is also given in the SI.
All NMR spectra were recorded from solutions in dry, oxygen-free
CDCl3 on a Bruker UGI-400 MHz, Bruker RDQ-400 MHz, or Bruker
DRY-400 MHz spectrometer operating at 400 MHz (1H), 79 MHz
(29Si), or 100 MHz (13C) at 25 ꢀC unless otherwise specified. Chemical
shifts are reported relative to SiMe4 (29Si) or to CDCl3 calibrated to
7.26 ppm (1H) and 77.0 ppm (13C). Where required, 1H and 13C peak
Synthesis of p-1-Br. nBuLi (1.6 M in hexanes, 1.1 mL) was added to a
solution of (Z)-1,2-bis(5-bromo-2-iodophenyl)ethene (0.50 g, 0.85 mmol)
in diethyl ether (43 mL) at -78 ꢀC, and the solution turned orange-red
in color. After 30 min, tetramethylethanediamine (0.28 mL, 1.87 mmol)
was added, and the solution was stirred for 2 h at -78 ꢀC. Neat
dimethylsilicon dichloride (0.11 mL, 0.94 mmol) was added dropwise,
and the solution eventually turned yellow. It was allowed to gradually
warm to room temperature over 16 h, at which time the reaction was
quenched with a mixture of water and ether (1:1) and the aqueous layer
extracted with ether. Combined organic extracts were washed with
hydrochloric acid (0.2 N), water, and brine, dried with anhydrous
MgSO4, filtered, and the solvent removed in vacuo. The crude product
was purified by column chromatography (SiO2, hexanes) to yield p-1-Br
(0.23 g, 68%) as a yellow gum-like substance. 1H NMR (CDCl3): δ 7.52
(d, JHH = 1.8 Hz, 2H), 7.47 (dd, JHH = 7.9 Hz, 1.8 Hz, 2H), 7.40 (d, JHH
= 7.9 Hz, 2H), 6.90 (s, 2H), 0.48 (s, 6H) ppm. 13C{1H} NMR (CDCl3):
δ 142.8, 135.7, 134.0, 132.8, 132.1, 130.5, 123.7, -4.8 ppm. 29Si{1H}
NMR (CDCl3): δ -10.2 ppm. HRMS (EI): calcd for C16H14SiBr2
[Mþ] 393.9211, found 393.9204.
General Procedure for Suzuki Coupling Reactions. The
Suzuki couplings were accomplished using a procedure based on that
reported by Buchwald and co-workers.61 A clean dried flask was charged
with m-1-Cl or p-1-Br (0.05 mmol), K3PO4 (0.20 mmol), boronic acid
(0.15 mmol), Pd(OAc)2 (4 mol %), and SPhos (8 mol %). Degassed
water (0.1 mL) was added under an atmosphere of argon, and the
mixture was stirred for 5 min. Toluene (1 mL) was then added, and the
suspension heated to reflux for 16 h. The reaction was cooled to room
temperature and filtered over silica, and solvent was removed in vacuo.
Crude product was purified by column chromatography.
1
assignments were made using H-1H COSY or 1H-13C HMQC 2D
spectra. High-resolution mass spectra were obtained on a Kratos MS-80
spectrometer operating in electron impact (EI) mode, HR-MALDI
spectra were recorded on a Bruker Autoflex III MALDI-Tof/Tof with
Smartbeam laser system, and HR-APCI were obtained on a Bruker
Escquire 3000 with ESI, APCI, and nanospray sources, LC (Agilent 1100
with autosampler), and direct syringe infusion. Fluorescence spectra
were obtained on a Horiba Jobin Yvon FluoroMax-4, and UV-visible
spectra were obtained on a Varian Cary 5000 UV-vis-NIR spectro-
photometer operating in single-beam mode. Absolute fluorescence
quantum yield values were measured using an Edinburgh Instruments
FLS92 calibrated integrating sphere system. Fluorescence lifetime
experiments were performed on the Edinburgh Instruments FLS920
using a reconvolution fit. High-resolution mass spectra were obtained on
a Kratos MS-80 spectrometer operating in electron impact (EI) mode.
X-ray crystallographic analyses were performed on suitable crystals
coated in Paratone oil and mounted on Oxford Diffraction Gemini A
Ultra and Nonius KappaCCD diffractometers. Computations were
performed using the Gaussian 03 suite of programs.70
Synthesis of 1. The procedure below is modified from a recent
literature report.58 nBuLi (1.6 M in hexanes, 0.59 mL) was added to a
solution of (Z)-1,2-bis(2-bromophenyl)ethene (159 mg, 0.47 mmol) in
diethyl ether (23 mL) at -78 ꢀC, and the solution turned yellow. After
30 min, tetramethylethanediamine (0.15 mL, 1.03 mmol) was added,
and the solution was stirred for 2 h at -78 ꢀC. Neat dimethylsilicon
dichloride (0.06 mL, 0.52 mmol) was added dropwise, and the solution
eventually turned yellow. It was allowed to gradually warm to room
temperature over 16 h, at which time the reaction was quenched with a
mixture of water and ether (1:1) and the aqueous layer extracted with
ether. Combined organic extracts were washed with hydrochloric acid
(0.2 N), water, and brine, dried with anhydrous MgSO4, filtered, and
the solvent was removed in vacuo. The crude product was purified by
column chromatography (SiO2, hexanes) to yield 1 (84.3 mg, 76%) as a
yellow oil. 1H NMR (CDCl3): δ 7.60 (ddd, JHH = 7.2 Hz, 1.2 Hz, 1.2 Hz
2H), 7.40-7.35 (m, 6H), 7.00 (s, 2H), 0.52 (s, JSi-H = 120.1 Hz, 6H)
ppm. 13C{1H} NMR (CDCl3): δ 141.4, 137.2, 133.1, 132.5, 129.4,
128.9, 127.3, -4.7 ppm. 29Si{1H} NMR (CDCl3): δ -10.9 ppm.
HRMS: calcd for C16H16Si [Mþ] 236.1021, found 236.1025.
Synthesis of m-1-Cl. nBuLi (1.6 M in hexanes, 8.4 mL) was added
to a solution of (Z)-1,2-bis(2-bromo-4-chlorophenyl)ethene (2.6 g,
6.4 mmol) in diethyl ether (320 mL) at -78 ꢀC, and the solution
turned orange then brown in color. After 30 min, tetramethylethane-
diamine (2.1 mL, 14.1 mmol) was added, and the solution was stirred for
2 h at -78 ꢀC. Neat dimethylsilicon dichloride (0.85 mL, 7.04 mmol)
was added dropwise, and the solution eventually turned yellow. It was
allowed to gradually warm to room temperature over 16 h, at which time
the reaction was quenched with a mixture of water and ether (1:1) and
the aqueous layer extracted with ether. Combined organic extracts were
washed with hydrochloric acid (0.2 N), water, and brine, dried with
anhydrous MgSO4, filtered, and the solvent was removed in vacuo.
The crude product was purified by column chromatography (SiO2,
hexanes) to yield m-1-Cl (1.10 g, 56%) as an off-white solid. 1H NMR
(CDCl3): δ 7.48 (d, JHH = 2.0 Hz, 2H), 7.33 (dd, JHH = 8.4 Hz, 2.0 Hz,
2H), 7.29 (d, JHH = 8.4 Hz, 2H), 6.92 (s, 2H), 0.50 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 139.4, 138.8, 134.2, 132.3, 132.2, 131.0,
129.1, -5.0 ppm. 29Si{1H} NMR (CDCl3): δ -10.2 ppm. HRMS (EI):
calcd for C16H14SiCl2 [Mþ] 304.0242, found 304.0249.
Data for m-1-Ph. The crude product was purified by column
chromatography (SiO2, hexanes -3% EtOAc/hexanes) to yield m-1-Ph
1
(12.6 mg, 65%) as a white solid. H NMR (CDCl3): δ 7.80 (d, JHH
=
2.0 Hz, 2H), 7.60 (ddd, JHH = 7.2, 1.3, 1.3 Hz, 6H), 7.48-7.43 (m, 6H),
7.36 (tdd, JHH = 7.3, 1.5, 1.5 Hz, 2H), 7.05 (s, 2H), 0.60 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 141.0, 140.5, 140.2, 137.4, 132.8, 131.3, 130.1,
128.8, 127.8, 127.3, 127.2, -4.5 ppm. 29Si NMR (CDCl3): δ -10.3 ppm.
HRMS (EI): calcd for C28H24Si [Mþ] 388.1647, found 388.1647.
Data for p-1-Ph. The crude product was purified by column chroma-
tography (SiO2, hexanes -3% EtOAc/hexanes) to yield p-1-Ph (19.2 mg,
88%) as a white solid. 1H NMR (CDCl3): δ 7.67 (dd, JHH = 7.7, 0.4 Hz,
2H), 7.62 (dd, JHH = 1.7, 0.9 Hz, 2H), 7.61-7.60 (m, 2H), 7.59-7.56 (m,
2H), 7.56 (d, JHH = 1.7 Hz, 2H), 7.44 (dddd, JHH = 7.4, 1.2, 1.2, 0.9 Hz,
4H), 7.34 (tdd, JHH = 6.7, 2.0,1.7 Hz, 2H), 7.10 (s, 2H), 0.58 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 141.9, 141.8, 140.9, 136.0, 133.5, 133.1, 128.8,
128.2, 127.4, 127.2, 126.2, -4.5 ppm. 29Si NMR (CDCl3): δ -10.9 ppm.
HRMS (EI): calcd for C28H24Si [Mþ] 388.1647, found 388.1646.
Data for m-1-PhOMe. The crude product was purified by column
chromatography (SiO2, 10-50% dichloromethane/hexanes) to yield
1
m-1-PhOMe (15.8 mg, 70%) as a yellow solid. H NMR (CDCl3): δ
7.78 (d, JHH = 2.0 Hz, 2H), 7.58-7.54 (m, 6H), 7.45 (d, JHH = 8.0 Hz,
2H), 7.03 (s, 2H), 7.00 (ddd, JHH = 6.7, 2.6, 2.4 Hz, 4H), 3.86 (s, 6H),
0.59 (s, 6H) ppm. 13C{1H} NMR (CDCl3): δ 159.2, 140.0, 139.7, 137.3,
133.6, 132.6, 130.8, 130.1, 128.1, 127.4, 114.2, 55.3, -4.6 ppm. 29Si
NMR (CDCl3): δ -10.4 ppm. HRMS (APCI): calcd for C30H28O2Si
[Mþ þ H] 449.193133, found 449.193574.
Data for p-1-PhOMe. The crude product was purified by column
chromatography (SiO2, 10% ethyl acetate/hexanes) to yield p-1-
1
PhOMe (11.1 mg, 95%) as a yellow solid. H NMR (CDCl3): δ 7.63
(d, JHH = 7.6 Hz, 2H), 7.57 (s, 2H), 7.53 (d, JHH = 8.0 Hz, 6H), 7.08 (s,
2H), 6.98 (d, JHH = 8.4 Hz, 4H), 3.85 (s, 6H), 0.56 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 159.3, 141.8, 141.5, 135.4, 133.5, 133.4,
133.1, 128.1, 127.8, 125.8, 114.2, 55.4, -4.5 ppm. 29Si NMR (CDCl3): δ
-11.4 ppm. HRMS (APCI): calcd for C30H28O2Si [MþþH]; 449.
193113, found 449.193289.
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Data for m-1-PhNPh2. The crude product was purified by gel
permeation chromatography (DCM, 4 mL/min, tR = 9.23 min) to yield
m-1-PhNPh2 (from 0.100 mmol, 46.4 mg, 64%) as a bright yellow
133.4, 133.3, 127.9, 126.4, 124.6, 124.52, 124.5, 124.1, 123.7, -4.6 ppm.
29Si NMR (CDCl3): δ -10.9 ppm. HRMS (EI): calcd for C32H25S4Si
[M þ Hþ] 565.060287, found 565.060839.
solid. 1H NMR (CDCl3): δ 7.78 (d, JHH = 2.0 Hz, 2H), 7.57 (dd, JHH
=
General Procedure for Sonogoshira Coupling Reactions.
The Sonogoshira couplings were accomplished using a procedure based
on that reported by Buchwald and co-workers.63 A clean dried two-neck
flask was charged with m-1-Cl or p-1-Br (0.05 mmol), PdCl2(MeCN)2
(4 mol %), XPhos (12 mol %), and Cs2CO3 (0.93 mmol). Acetonitrile
(1 mL) was added and the solution stirred at room temperature for
30 min, after which phenylacetylene (0.91 mmol) was added via syringe
and the solution heated to reflux for 36 h. The reaction was cooled
to room temperature and filtered over silica, and solvent removed in
vacuo. The crude product was purified by column chromatography or
preparatory GPC.
8.0 Hz, 2.0 Hz, 2H), 7.49 (d, JHH = 8.4 Hz, 4H), 7.44 (d, JHH = 8.0 Hz,
2H), 7.30-7.25 (m, 8H), 7.15 (d, JHH = 8.0 Hz, 12H), 7.04 (dd, JHH
=
7.4 Hz, 7.4 Hz, 4H), 7.01 (s, 2H) 0.59 (s, 6H) ppm. 13C{1H} NMR
(CDCl3): δ 147.7, 147.3, 140.2, 139.6, 137.3, 134.9, 132.7, 130.8,
130.1, 129.3, 127.8, 127.3, 124.5, 123.9, 123.0, -4.5. 29Si NMR
(CDCl3): δ -10.3 ppm. HRMS (APCI): calcd for C52H43N2Si [Mþ
þ
H]; 723.319002, found 723.316968.
Data for p-1-PhNPh2. The crude product was purified by column
chromatography (SiO2, 10% EtOAc/hex) to yield p-1-PhNPh2 (6.3 mg,
41%) as fine white needles. 1H NMR (CDCl3): δ 7.63 (d, JHH = 7.7 Hz,
2H), 7.58 (d, JHH = 1.4 Hz, 2H), 7.53 (dd, JHH = 7.7 Hz, 1.8 Hz, 2H),
7.46 (ddd, JHH = 8.8 Hz, 2.4 Hz, 2.4 Hz, 4H), 7.29-7.25 (m, 8H), 7.14-
7.11 (m, 12H) 7.06 (s, 2H), 7.03 (ddd, JHH = 7.32 Hz, 1.2 Hz, 1.2 Hz,
4H), 0.55 (s, 6H) ppm. 13C{1H} NMR (CDCl3): δ 147.6, 147.4, 141.8,
141.4, 135.5, 134.7, 133.5, 133.1, 129.3, 127.82, 127.8, 125.7, 124.5,
123.7, 123.0, -4.5. 29Si NMR (CDCl3): δ -11.4 ppm. HRMS (APCI):
calcd for C52H43N2Si [Mþ þ H] 723.319002, found 722.319744.
General Procedure for Stille Coupling Reactions. The Stille
couplings were accomplished using a procedure based on that reported
by Fu and co-workers.62 A clean dried flask was charged with m-1-Cl or
p-1-Br (0.05 mmol), CsF (0.22 mmol), stannane (0.13 mmol), Pd-
(PPh3)4 (5 mol %), and PtBu3 (10 mol %). Dioxane (1 mL) was added
and the suspension heated to reflux for 36 h. The reaction was cooled to
room temperature and filtered over silica, and solvent was removed in
vacuo. The crude product was purified by column chromatography.
Data for m-1-Th. The crude product was purified by column chroma-
tography (SiO2, 5-20% DCM/hexanes) to yield m-1-Th (13.2 mg, 66%)
as a yellow solid. 1H NMR (CDCl3): δ 7.80 (d, JHH = 2.0 Hz, 2H), 7.61
Data for m-1-CCPh. The crude product was purified by gel
permeation chromatography (DCM, 4 mL/min, tR = 10.71 min) to
1
yield m-1-CCPh (12.6 mg, 43%) as a bright yellow solid. H NMR
(CDCl3): δ 7.75 (d, JHH = 1.6 Hz, 2H), 7.58-7.55 (m, 4H), 7.53 (d,
JHH = 1.6 Hz, 2H), 7.37-7.35 (m, 8 H), 6.98 (s, 2H), 0.56 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 140.9, 137.0, 135.8, 133.2, 132.0, 131.6,
129.6, 128.4, 128.3, 123.3, 122.6, 90.8, 89.6, -4.7 ppm. 29Si NMR
(CDCl3): δ -10.6 ppm. HRMS (APCI) calcd for C32H25Si [M þ
Hþ]; 437.172004, found 437.172249.
Data for p-1-CCPh. The crude product was purified by column
chromatography (SiO2, 0-10% DCM/hexanes) to yield p-1-CCPh
(10.9 mg, 50%) as a bright yellow solid. 1H NMR (CDCl3): δ 7.56-7.53
(m, 8H), 7.49 (dd, JHH = 7.6 Hz, 1.2 Hz, 2H), 7.36-7.34 (m, 6H), 6.98
(s, 2H), 0.52 (s, 6H) ppm. 13C{1H} NMR (CDCl3): δ 141.3, 137.3,
133.1, 132.6, 132.3, 131.6, 130.1, 128.3, 124.0, 123.1, 90.0, 89.1, -4.7
ppm. 29Si NMR (CDCl3): δ -10.4 ppm. HRMS (MALDI): calcd for
C32H25Si [M þ Hþ] 437.1720, found 437.1719.
Synthesis of Mesityl Borepin m-2-Ph. In a glovebox a clean dry
round-bottom flask was charged with m-1-Ph (115 mg, 0.30 mmol) and
BBr3 (2 mL). The dark brown solution was heated in a sealed bomb to
50 ꢀC for 2 days, at which point liquids were removed in vacuo. The
solids were taken into a glovebox and dissolved in toluene. A suspension
of MesLi (76 mg, 0.60 mmol) in toluene (1 mL) was added dropwise,
and the suspension stirred for 16 h. Solvent was removed in vacuo, and
the product purified by column chromatography (SiO2, 0-15% EtOAc/
hexanes). X-ray quality crystals were obtained by recrystallization in
toluene/hexanes. 1H NMR (CDCl3): δ 8.32 (d, JHH = 1.2 Hz, 2H), 7.97
(dd, JHH = 8.0 Hz, 2.4 Hz, 2H), 7.89 (dd, JHH = 8.0 Hz, 2H), 7.52-7.49
(m, 4H), 7.43-7.39 (tdd, JHH = 7.4 Hz, 1.2 Hz, 1.2 Hz 4H), 7.41 (s,
2H), 7.32 (tdd, JHH = 7.4 Hz, 2.0 Hz, 1.6 2H), 6.91 (s, 2H), 2.37 (s, 3H),
1.99 (s, 6H) ppm. 11B NMR (CDCl3): δ 43.2 ppm.
(dd, JHH = 8.0 Hz, 2.0 Hz, 2H), 7.39 (d, JHH = 8.0 Hz, 2H), 7.33 (dd, JHH
3.6 Hz, 1.1 Hz, 2H), 7.29 (dd, JHH = 5.1 Hz, 1.1 Hz, 2H), 7.09 (dd, JHH
=
=
5.1 Hz, 3.6 Hz, 2H), 6.97 (s, 2H), 0.59 (s, 6H) ppm; 13C{1H} NMR
(CDCl3): δ 144.3, 140.7, 137.4, 133.4, 132.7, 130.2, 130.0, 128.1, 126.6,
125.0, 123.2, -4.7 ppm. 29Si NMR (CDCl3): δ -10.3 ppm. HRMS (EI):
calcd for C24H20S2Si [Mþ] 400.0776, found 400.0793.
Data for p-1-Th. The crude product was purified by column chroma-
tography (SiO2, 5-20% DCM/hexanes) to yield p-1-Th (15.8 mg, 96%)
as a yellow solid. 1HNMR(CDCl3):δ7.62 (dd, JHH =1.1Hz, 1.1Hz, 2H),
7.59 (s, 2H), 7.58 (d, JHH = 0.4 Hz, 2H), 7.34 (dd, JHH = 5.0 Hz, 1.2 Hz,
2H), 7.29 (dd, JHH = 5.1 Hz, 1.2 Hz, 2H), 7.08 (dd, JHH = 5.1 Hz, 3.6 Hz,
2H), 7.05 (s, 2H), 0.54 (s, 6H) ppm. 13C{1H} NMR (CDCl3): δ 144.1,
141.8, 136.4, 135.0, 133.4, 133.2, 128.0, 126.8, 125.1, 124.9, 123.4, -4.6.
29Si NMR (CDCl3): δ -11.0 ppm. HRMS (APCI): calcd for C24H21S2Si
[Mþ þ H] 401.084845, found 401.084072.
’ ASSOCIATED CONTENT
Data for m-1-bTh. The crude product was purified by column
chromatography (SiO2, 5-20% DCM/hexanes) to yield m-1-bTh (12.6
S
Supporting Information. Synthetic procedures for the
b
mg, 43%) as a bright yellow solid. 1H NMR (CDCl3): δ 7.80 (d, JHH
=
tetrahalostilbene starting materials, connectivity structure for
p-1-Ph, full UV-vis and emission spectra for all compounds,
including solvatochromism data, HOMO and LUMO surfaces
for all compounds, cyclic voltammograms for all compounds,
and Cartesian coordinates for computed structures of all com-
pounds. Supplementary crystallographic data for m-1-bTh (CCDC
801215) and m-2-Ph (801216) in the form of .cif files. This
material is available free of charge via the Internet at http://
pubs.acs.org.
2.0 Hz, 2H), 7.60 (dd, JHH = 8.1 Hz, 2.0 Hz, 2H), 7.39 (d, JHH = 8.1 Hz,
2H), 7.25 (d, JHH = 3.8 Hz, 2H), 7.22 (dd, JHH = 4.9 Hz, 1.1 Hz, 4H),
7.16 (d, JHH = 3.8 Hz, 2H), 7.04 (dd, JHH = 5.0 Hz, 3.7 Hz, 2H), 6.97 (s,
2H), 0.61 (s, 6H) ppm. 13C{1H} NMR (CDCl3): δ 143.0, 140.8, 137.5,
137.4, 137.0, 133.1, 132.8, 130.3, 129.7, 127.9, 126.2, 124.7, 124.4, 123.8,
123.7, -4.7 ppm. 29Si NMR (CDCl3): δ -10.2 ppm. HRMS (APCI):
calcd for C32H25S4Si [M þ Hþ] 565.060287, found 565.058261.
Data for p-1-bTh. The crude product was purified by column
chromatography (SiO2, 0-20% DCM/hexanes) to yield p-1-bTh
(14.6 mg, 52%) as a yellow solid. 1H NMR (CDCl3): δ 7.60 (d, 2H),
7.58-7.55 (m, 4H), 7.25 (d, JHH = 3.2 Hz, 2H), 7.22 (dd, JHH = 5.2 Hz,
1.2 Hz, 2H), 7.21 (dd, JHH = 3.6 Hz, 2H), 7.15 (d, JHH = 3.6 Hz, 2H),
7.06 (s, 2H), 7.03 (dd, JHH = 5.2 Hz, 3.6 Hz, 2H), 0.54 (s, 6H) ppm.
13C{1H} NMR (CDCl3): δ 142.7, 141.9 137.4, 137.0, 136.5, 134.7,
’ AUTHOR INFORMATION
Corresponding Author
*Tel: 403-220-5746. Fax: 403-289-9488. E-mail: wpiers@
ucalgary.ca.
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’ ACKNOWLEDGMENT
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This work was supported by the National Science Engineer-
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Promotion of Science, Alberta Innovates, the Xerox Research
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