Synthesis of dithienosilole-based highly photoluminescent donor-acceptor type compounds

Article abstract of DOI:10.1039/c2dt32738d

Highly photoluminescent acceptor-donor-acceptor (A-D-A) and donor-acceptor (D-A) type compounds with a dithienosilole unit as the donor and perfluorotolyl or dimesitylboryl group(s) as the acceptor were prepared by the reaction of lithiated dithienosilole derivatives with perfluorotoluene or fluorodimesitylborane, respectively. The resulting A-D-A and D-A type compounds showed red-shifted UV absorption and PL bands compared to those of simple dithienosiloles having no acceptor units, reported previously, and were highly photoluminescent in the solid state as well as in solution. Solvatochromic behaviour that would arise from the intramolecular donor-acceptor interaction were observed for the D-A type compounds with respect to the UV absorption and PL spectra. In addition, it was found that bis(dimesitylboryl)dithienosilole and (dimesitylboryl)(methylthio)dithienosilole responded to coexisting fluoride anions, leading to clear UV absorption and PL spectral changes in solutions.

Full text of DOI:10.1039/c2dt32738d

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Synthesis of dithienosilole-based highly
photoluminescent donor–acceptor type compounds
Cite this: Dalton Trans., 2013, 42, 3646
Joji Ohshita,*a Yuta Tominaga,a Daiki Tanaka,a Yousuke Ooyama,a
Tomonobu Mizumo,a Norifumi Kobayashib and Hideyuki Higashimurab
Highly photoluminescent acceptor–donor–acceptor (A–D–A) and donor–acceptor (D–A) type compounds
with a dithienosilole unit as the donor and perfluorotolyl or dimesitylboryl group(s) as the acceptor were
prepared by the reaction of lithiated dithienosilole derivatives with perfluorotoluene or fluorodimesityl-
borane, respectively. The resulting A–D–A and D–A type compounds showed red-shifted UV absorption
and PL bands compared to those of simple dithienosiloles having no acceptor units, reported previously,
and were highly photoluminescent in the solid state as well as in solution. Solvatochromic behaviour that
would arise from the intramolecular donor–acceptor interaction were observed for the D–A type com-
pounds with respect to the UV absorption and PL spectra. In addition, it was found that bis(dimesityl-
boryl)dithienosilole and (dimesitylboryl)(methylthio)dithienosilole responded to coexisting fluoride
anions, leading to clear UV absorption and PL spectral changes in solutions.
Received 16th November 2012,
Accepted 7th December 2012
DOI: 10.1039/c2dt32738d
www.rsc.org/dalton
enhances the fluorescence properties.6 Indeed, we demon-
strated that DTSs featuring phosphorus-substituents exhibited
1. Introduction
Introduction of a heteroatom bridge, such as an organophos- high PL quantum efficiencies in the range of Φ = 0.22–0.99
phorus,1 -boron,2 or -germanium unit,3 at the 3,3′-position of and 0.33–0.79 as solutions and solids, respectively (Chart 1).7
bithiophene forming fused tricyclic systems has been exten- Interestingly, DTS1-P showed a higher quantum yield as a
sively studied. In these systems, better conjugation than that solid than that in chloroform, while the PL quantum yields in
in the parent bithiophene is expected primarily by fixing the solution were markedly improved by quenching the phos-
bithiophene π-electron system to be completely planar. In phorus lone pair electrons by oxidation and complex for-
addition, the electronic effects of the bridging heteroatom mation (DTS1-PO7a and DTS2-PAu7b). It was also found that
often provide opportunities to manipulate the bithiophene vapour-deposited films of DTS3-PO exhibited electrolumines-
properties and functionalities. In regard to this, we have intro- cence properties in organic light emitting diode (OLED)
duced silicon-bridged dithienosilole (DTS) system, in which systems, giving
a
blue-green emission.7a However, those
the bonding interaction between the silicon σ*- and bithio- phosphorus-substituted DTSs showed high quantum yields
phene π*-orbital lowers the LUMO energy level, leading to even
more enhanced conjugation.4 Currently, the DTS system
attracts much attention as the donor-components of donor–
acceptor type π-conjugated photovoltaic polymers.5 We have
recently studied the photoluminescent (PL) properties of DTS
compounds, expecting that the rigid tricyclic system mini-
mises the non-radiative decay based on molecular vibration
and that the introduction of sterically bulky substituents on
the ring silicon prevents the DTS chromophores from being
stacked intermolecularly to suppress the concentration
quenching. Furthermore, it has been demonstrated that the
introduction of silicon substituents on a chromophore often
aDepartment of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, Higashi-Hiroshima 739-8527, Japan. E-mail: jo@hiroshima-u.ac.jp;
Fax: +81 82 424 5494; Tel: +81 82 424 7743
bTsukuba Labs, Sumitomo Chemical Co. Ltd, 6 Kitahara, Tsukuba 300-3294, Japan
Chart 1 Phosphorus-substituted dithienosiloles and their PL quantum yields.
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either in solution or as solids, and none of them exhibited
efficient PL properties in the both phases.
On the other hand, recent attention has been focused on
the development of fluorescence compounds with donor–
acceptor (D–A) type structures, in which intramolecular charge
transfer (ICT) photoexcitation leads to strong UV absorption
and PL properties. Applications of the D–A compounds as
functional materials such as fluorescent probes, non-linear
optics, chromic materials, sensitizing dyes for dye-sensitized
solar cells, and two-photon absorption materials have been
also explored.8–10 In the hope of obtaining materials with high
PL properties based on the DTS system, we prepared A–D–A
and D–A type DTSs that possessed electron-deficient perfluoro-
tolyl and boryl substituents as the acceptor and relatively elec-
tron rich DTS and methylthio-DTS as the donor. As expected,
the present DTS compounds were highly photoluminescent in
the solid state as well as in solution, and the D–A type com-
pounds showed clear solvatochromic behaviour. In addition,
bis(dimesitylboryl)-DTS and (dimesitylboryl)(methylthio)-DTS
were found to be ionochromic towards fluoride anion (F−),
being potentially applicable to F−-sensing.
Scheme 1 Synthesis of perfluorotolyl- and dimesitylboryl-substituted DTS
derivatives.
mixture was stirred at room temperature for 10 h. After the
usual workup, the residue was subjected to silica gel column
chromatography eluting with hexane and the resulting crude
solids were recrystallized from ethanol to give 0.10 g (14%
yield) of DTS2-F as yellow fine needles: mp 204.2–206.1 °C;
APCI-MS m/z 890 (M−); 1H NMR (δ in CDCl3) 0.91 (t, 6H, J =
7.2 Hz, CH2CH2CH2CH3), 1.34 (m, 4H, CH2CH2CH2CH3), 1.58
2. Experimental
2.1 General procedure
(m, 4H, CH2CH2CH2CH3), 2.62 (t, 4H,
J = 8.0 Hz,
All reactions were carried out in dry argon. Diethyl ether used
as the reaction solvent was distilled from sodium–benzophe-
none ketyl and stored over activated molecular sieves until use.
Starting DTS compounds DTS1-Br,4a DTS2-Br,11 and DTS1-
BrS12 were prepared as reported in the literature. NMR spectra
were recorded using Varian 400MR and System 500 spec-
trometers. BF3·OEt2 was used as the external reference for 11B
NMR spectroscopy. APCI, APPI, and ESI mass spectra were
measured using a Thermo Fisher Scientific LTQ Orbitrap XL
spectrometer at the Natural Science Centre for Basic Research
and Development (N-BARD), Hiroshima University, while DI-EI
mass analysis was performed using a Shimadzu QP5050A spec-
trometer. UV absorption and PL spectra were measured using
Shimadzu UV-3150 and Hitachi F4500 spectrophotometers,
respectively. PL quantum yields were determined in an inte-
CH2CH2CH2CH3), 7.23 (d, 4H, J = 8.0 Hz, m-Ph), 7.57 (d, 4H,
J = 8.0 Hz, o-Ph), 7.92 (s, 2H, DTS ring H); 13C NMR (δ in
CDCl3) 13.94, 22.37, 33.44, 35.79, 118.33 (m), 119.48, 122.17,
126.54, 128.70, 128.94, 134.57 (t, J = 7.5 Hz), 135.44, 142.99,
143.38 (dm, J = 252 Hz), 144.59 (dm, J = 261 Hz), 146.16,
153.03 (m). Anal. Calcd for C42H28F14S2Si: C, 56.62; H, 3.17.
Found: C, 56.85; H, 3.35. The chemical shifts and coupling
patterns of the 13C NMR signals of the fluorocarbons are con-
sistent with the literature data for perfluorotolyl-arenes.13
2.3 Preparation of DTS1-B
To a solution of 0.32 g (0.64 mmol) of DTS1-Br in 10 mL of
diethyl ether was added 0.80 mL (1.3 mmol) of a 1.65 M
hexane solution of nBuLi at −80 °C. After the mixture was
stirred for 1 h at room temperature, 0.34 g (1.28 mmol) of
fluorodimesitylborane was added to the mixture at −80 °C and
the mixture was stirred at room temperature for 10 h. After
usual workup, the residue was subjected to silica gel column
chromatography eluting with hexane–chloroform = 3 : 1 to give
gration sphere attached to
a
Hamamatsu Photonics
C7473 multi-channel analyzer. The usual workup mentioned
below includes hydrolysis of the reaction mixture with water,
extraction of the organic products with toluene, washing the
extract with water, drying the washed extract over anhydrous
magnesium sulfate, and evaporation of the solvent, in that
order. The synthetic routes to A–D–A and D–A type DTSs are
summarized in Scheme 1.
0.36
g (66% yield) of DTS1-B as yellow powders: mp
316.0–317.8 °C; ESI-MS m/z 865.4 ([M + Na]+); 1H NMR (δ in
CDCl3) 2.14 (s, 24H, o-Me), 2.30 (s, 12H, p-Me), 6.82 (s, 8H,
Mes ring H), 7.32–7.56 (m, 6H, m- and p-Ph), 7.57 (br m, 4H,
2.2 Preparation of DTS2-F
To a solution of 0.50 g (0.81 mmol) of DTS2-Br in 15 mL of o-Ph), 7.62 (s, 2H, DTS ring H); 13C NMR (δ in CDCl3) 21.24,
diethyl ether was added 1.00 mL (1.57 mmol) of a 1.57 M 23.59, 128.16, 128.21, 130.43, 131.34, 135.40, 138.56, 140.79,
hexane solution of nBuLi at −80 °C. After the mixture was 141.22, 143.97, 144.96, 154.77, 162.34; 11B NMR (δ in CDCl3)
stirred for 1 h at room temperature, 0.35 mL (2.5 mmol) of per- 67.47 (br). Anal. Calcd for C56H56B2S2Si: C, 79.80; H, 6.70.
fluorotoluene was added to the mixture at −80 °C and the Found: C, 79.81; H, 6.72.
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2.4 Preparation of DTS1-FS
Table 1 Optical properties of perfluorotolyl- and dimesitylboryl-substituted
DTS derivatives
To a solution of 0.20 g (0.42 mmol) of DTS1-BrS in 20 mL of
diethyl ether was added 0.25 mL (0.41 mmol) of a 1.65 M
hexane solution of nBuLi at −80 °C and the mixture was
stirred at room temperature for 1 h. To this was added 0.20 mL
(1.2 mmol) of perfluorotoluene at −80 °C and the resulting
mixture was stirred at room temperature for 12 h. After the
usual workup, the residue was subjected to silica gel column
chromatography eluting with hexane–chloroform to give
0.070 g (27% yield) of DTS1-FS as yellow powders: mp
202.7–203.9 °C; 1H NMR (δ in CDCl3) 2.55 (s, 3H, SMe), 7.24 (s,
1H, HCvC(S)), 7.37–7.47 (m, 6H, m- and p-Ph), 7.62–7.65 (m,
4H, o-Ph), 7.89 (s, 1H, HCvC(C7F7)); 13C NMR (δ in CDCl3)
22.44, 118.55 (m), 119.50, 122.24, 127.31, 128.40, 130.50,
130.76, 133.90, 134.43 (t, J = 7.5 Hz), 135.38, 139.89, 141.11,
142.41, 143.78 (dm, J = 256 Hz), 144.57 (dm, J = 257 Hz),
150.54, 154.63; Exact APPI-MS Calcd for C28H15F7S3Si (M+):
609.00661. Found: 609.00604.
abs λmax/nm (ε/104)
in chloroform
Emission λem/nma
(Φ) in chloroformb
Compound
Solid
DTS2-F
DTS1-B
410 (2.6), 430 (2.1)
342 (1.1), 418 (4.7),
441 (5.3)
403 (2.8)
415 (2.6)
462, 481 (0.67)
460, 481 (0.90)
531 (0.60)
496 (0.83)
DTS1-FS
DTS1-BS
DTS1
497 (0.84)
486 (0.83)
409 (0.71)
511 (0.66)
501 (0.58)
c
356 (1.0)
—
a Excited at absorption maxima. b Measured at [substrate] = 1.8–5.2 ×
10−6 M. c Not measured.
Chart 2
2.5 Preparation of DTS1-BS
To a solution of 0.35 g (0.74 mmol) of DTS1-BrS in 20 mL of
diethyl ether was added 0.45 mL (0.74 mmol) of a 1.65 M
hexane solution of n-BuLi at −80 °C and the mixture was
stirred at room temperature for 1 h. To this was added a solu-
tion of 0.20 g (0.75 mmol) of fluorodimesitylborane in 5 mL
of ether at 0 °C and the resulting mixture was stirred at room
temperature for 12 h. After the usual workup, the residue was
subjected to silica gel column chromatography eluting with
hexane–chloroform to give 0.082 g (18% yield) of DTS1-BS as
These A–D–A and D–A type compounds were obtained as
stable yellow solids that could be handled under ambient con-
ditions. They were soluble in common organic solvents and
melted without decomposition.
3.2 Optical properties
Table 1 summarizes the optical properties of the DTS deriva-
tives thus prepared. The UV absorption and emission bands
were red-shifted from those of the phosphorus-substituted
DTSs (Chart 1)7 that showed absorption and emission bands
at λmax (CHCl3) = 361–380 nm, λem (CHCl3) = 422–465 nm, and
λem (powder) = 434–464 nm, and also from those of trimethyl-
silyl-substituted DTS1 (Chart 2 and Table 1), reported previous-
ly.4a It was also noted that the present DTS compounds
exhibited relatively high absorption coefficiencies, as com-
pared to DTS1. Interestingly, they showed high PL quantum
efficiencies in both solution and the solid state, in contrast to
the phosphorus-substituted DTSs reported previously, which
exhibited highly PL properties only in either the solution or
the solid state. This is presumably ascribed to the D–A and A–
D–A type structures of the present DTS compounds and also to
1
yellow powders: mp 222.7–224.3 °C; H NMR (δ in CDCl3) 2.16
(s, 12H, o-Me), 2.30 (s, 6H, p-Me), 2.51 (s, 3H, SMe), 6.83 (s,
4H, Mes ring H), 7.21 (s, 1H, HCvC(S)), 7.33–7.43 (m, 6H,
m- and p-Ph), 7.56–7.60 (m, 5H, o-Ph and HCvC(B)); 13C NMR
(δ in CDCl3) 21.23, 22.45, 23.57, 128.19, 128.21, 130.48, 131.23,
133.97, 135.38, 138.42, 141.17, 141.77, 143.41, 144.06, 151.71,
162.94; 11B NMR (δ in CDCl3 from BF3·Et2O) 66.81 (br); Exact
APPI-MS Calcd for C39H37BS3Si (M+): 640.19142. Found:
640.19141.
3. Results and discussion
3.1 Synthesis
As shown in Scheme 1, A–D–A type bis(perfluorotolyl)- and bis- the sterically large and rigid perfluorotolyl and dimesitylboryl
(dimesitylboryl)-substituted dithienosiloles (DTS1-F, DTS2-F, groups that prevent the DTS chromophores from being stacked
and DTS1-B) were prepared by the reactions of dilithio-DTSs and minimize the molecular vibration in the solid state.
with perfluorotoluene14 and fluorodimesitylborane, respect-
D–A type compounds DTS1-FS and DTS1-BS were solvato-
ively. The low yield of DTS2-F was ascribed to the formation of chromic, although A–D–A type DTS2-F and DTS1-B showed no
many unidentified by-products including non-volatile sub- evident UV absorption and PL spectral changes depending on
stances. We also attempted to prepare DTS1-F using a similar the solvents. As shown in Fig. 1, the UV absorption and PL
reaction. Although the GC-MS analysis of the reaction mixture maxima of DTS1-FS moved to shorter and longer wavelength
indicated the formation of the expected product in approxi- respectively, as the solvent polarity increased on going from
mately 20% yield, it could not be isolated in a pure form. chloroform (λem = 403 nm, λem = 497 nm) to DMSO (λem
Similar reactions of lithio(methylthio)dithienosilole prepared 369 nm, λem = 532 nm). Compound DTS1-BS showed smaller
from DTS1-BrS gave unsymmetrical D–A type compounds red shifts with solvent polarity in the PL spectrum from λem
=
=
DTS1-FS and DTS1-BS.
486 nm in chloroform to 509 nm in DMSO, and no obvious
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Fig. 1 UV absorption (a) and PL (b) spectra of DTS1-FS in different solvents.
changes in the UV absorption spectra. These chromic beha-
viours were probably due to the photo-induced intramolecular
charge transfer (ICT) of DTS1-FS and DTS1-BS. In fact, the PL
lifetimes were slightly but certainly extended from 2.5 and 2.6
ns in chloroform to 2.6 and 2.8 ns in DMSO for DTS1-FS and
DTS1-BS, respectively, suggesting stabilization of the photo-
excited states in relatively polar DMSO.
Fig. 2 UV absorption (a) and PL (b) spectral changes of DTS1-B in THF on
addition of Bu4NF–THF.
3.3 F−-sensing
Recently, triarylboranes have been studied as optical sensor
materials for F− ions,9,10 in which coordination of F− to the
Lewis acidic borane centre causes UV and/or PL spectral
changes. Compound DTS1-B also responded to the presence of
fluoride anions in solution. Thus, on addition of Bu4NF as the
F− source to a THF solution of DTS1-B, the UV absorption
spectra changed in two steps, as shown in Fig. 2a. When 0 to
10 equiv. of Bu4NF was added to DTS1-B, the absorption
bands were broadened a little and shifted to longer wave-
lengths, while further addition of Bu4NF resulted in a decrease
in the absorption intensity and blue shifts of the maxima. The
PL spectra showed similar behaviors including red-shifts at
low Bu4NF concentration (0–10 equiv.) and a subsequent blue-
shift of the maximum with an excess of Bu4NF (Fig. 2b).
Complex formation between DTS1-B and F− proceeds in
two steps (DTS1-B → DTS1-B-F → DTS1-B-F2), and supporting
evidence for this was provided by monitoring the process via
1H NMR spectroscopy. As shown in Fig. 3, new signals (Hc–Hf),
assignable to the unsymmetrical DTS1-B-F protons with a
borane–DTS–borate structure appeared on addition of Bu4NF,
of which Hc and Hd were high-field shifted from others,
reflecting the existence of the electron-rich borate unit.
Further addition of Bu4NF afforded two additional new signals
for symmetrical borate–DTS–borate DTS1-B-F2 (Hg and Hh) at
even higher field. In these spectra, the integration ratios
Fig. 3 1H NMR spectral changes of DTS1-B in CDCl3 (aromatic region) on
addition of 0 equiv. (a), 2.0 equiv. (b), 4.0 equiv. (c), and 30 equiv. (d) of Bu4NF–
THF (1.0 M). High field shifts of the signals in spectrum d compared to those of
c are likely due to the increased THF content of the solvent from ca. 5% to 30%.
Asterisks indicate 13C satellite bands.
always agreed with the assignments. The process was revers-
ible and addition of CaCl2 to the solution of DTS1-B and
Bu4NF led to recovery of the original 1H NMR spectra, together
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Fig. 4 HOMO (a) and LUMO (b) profiles of boryl and borate-substituted DTSs,
derived from DFT calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p)
level. Hydrogen atoms are omitted for clarity.
with precipitation of CaF2 from the solution, although some
unidentified low intensity signals were also observed. In
1
accordance with the H NMR spectral changes, a borate signal
appeared at about 4 ppm in the 11B NMR spectra on the
addition of Bu4NF.
Usually, complex formation between triarylboranes and F−
suppresses p–π interactions, leading to the blue-shift of the
absorption bands.9 An exception to this is the ionochromism
of bis(dimesitylboryl)arenes (R2B-Ar-BR2) towards F−, which
showed two-step spectral changes involving red-shifts of the
absorption bands followed by blue-shifts, similar to the
present case.10 It was suggested by Müllen et al. that the
R2B-Ar-R2BF− structure permitted ICT type interactions
between the boron and borate units, being responsible for the
red-shifts of the absorption bands at the early stage of the F−
addition.10b However, no quantum chemical approaches to
this phenomenon have been conducted. To learn more about
the ionochromic behaviour of DTS-B, we carried out DFT
calculations at the level of B3LYP/6-31+G(d,p)//B3LYP/6-31G-
(d,p)15,16 on model molecules DTS0-B, DTS0-B-F, and DTS0-B-F2,
shown in Fig. 4. The HOMOs and LUMOs of these compounds
originate from the DTS π-conjugated units featuring the boron
and borate orbital and the gaps decrease in the order of DTS0-
B-F2 (3.67 eV) > DTS0-B (3.28 eV) > DTS0-B-F (2.79 eV), in
accordance with the experimental observation for DTS1-B
described above. Clear p–π interaction between the borane and
DTS units is seen in the HOMO and LUMO profiles of DTS0-B.
On the contrary, those of DTS0-B-F2 are rather localized on the
DTS segment with only limited contribution of the borate
orbital. For DTS0-B-F, the LUMO is localized on the borane–
DTS fragment, while the HOMO is delocalized over the
borane–DTS–borate unit, suggesting the ICT excited state as
expected. These results qualitatively support the observed spec-
tral behaviour presented in Fig. 2, including the clear spectral
changes around the addition of 1 equiv. of Bu4NF. However,
the changes were not saturated upon addition of 2 equiv. of
Bu4NF, indicating that the coordination of F− to the borane
centre is not very strong and that they were at equilibrium.
The monoboryl compound DTS1-BS exhibited normal iono-
chromic changes in its UV and PL spectra as shown in Fig. 5,
Fig. 5 UV absorption (a) and PL (b) spectral changes of DTS1-BS in THF on
addition of Bu4NF–THF.
including monotonous blue-shifts of the bands. We examined
Job’s plots to investigate the stoichiometry of complex for-
mation. However, this was complicated, presumably due to the
formation of aggregated complexes and the stoichiometry
could not been determined.
4. Conclusions
In conclusion, we prepared new A–D–A and D–A type com-
pounds with a DTS unit as the donor and perfluorotolyl or
dimesitylboryl group(s) as the acceptor and demonstrated their
high absorption coefficiencies in solution and also highly PL
properties in the solid state as well as in solution. Of those,
the D–A type compounds exhibited efficient photoexcited ICT,
leading to the solvatochromic behaviour. We also found that
the dimesitylboryl-DTS system is potentially useful for F−-
sensing, which is of importance in the both chemical and bio-
logical fields of research.9
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (no. 2401)” (24102005) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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