Influence of the structure of electron-donating aromatic units in organosilicon luminophores based on 2,1,3-benzothiadiazole electron-withdrawing core on their absorption-luminescent properties

Article abstract of DOI:10.1016/j.dyepig.2018.03.043

A series of new linear π-conjugated oligomers consisting of five conjugated rings based on 2,1,3-benzothiadiazole (BTD) electron-withdrawing group and various combinations of 2,5-thiophene and 1,4-phenylene electron-donating units with terminal trimethyls

Full text of DOI:10.1016/j.dyepig.2018.03.043

Accepted Manuscript
Influence of the structure of electron-donating aromatic units in organosilicon
luminophores based on 2,1,3-benzothiadiazole electron-withdrawing core on their
absorption-luminescent properties
Maxim S. Skorotetcky, Evgeniya D. Krivtsova, Oleg V. Borshchev, Nikolay M.
Surin, Evgeniya A. Svidchenko, Yuriy V. Fedorov, Sergey A. Pisarev, Sergey A.
Ponomarenko
PII:
S0143-7208(17)32545-7
10.1016/j.dyepig.2018.03.043
DYPI 6628
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 13 December 2017
Revised Date: 21 March 2018
Accepted Date: 21 March 2018
Please cite this article as: Skorotetcky MS, Krivtsova ED, Borshchev OV, Surin NM, Svidchenko EA,
Fedorov YV, Pisarev SA, Ponomarenko SA, Influence of the structure of electron-donating aromatic
units in organosilicon luminophores based on 2,1,3-benzothiadiazole electron-withdrawing core on their
absorption-luminescent properties, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.03.043.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Influence of the structure of electron-donating aromatic units in organosilicon luminophores
based on 2,1,3-benzothiadiazole electron-withdrawing core on their absorption-luminescent
properties
Maxim S. Skorotetcky a, Evgeniya D. Krivtsovaa, Oleg V. Borshchev a, Nikolay M. Surin a, Evgeniya
A. Svidchenko a, Yuriy V. Fedorov b, Sergey A. Pisarev c, Sergey A. Ponomarenko a, d
*
a Enikolopov Institute of Synthetic Polymeric Materials of Russian Academy of Sciences,
Profsoyuznaya St. 70, Moscow 117393, Russia
b Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, Vavilova
st. 28, Moscow 119991, Russia
CInstitute of Physiologically Active Compounds of Russian Academy of Sciences, Severny Pr. 1,
Chernogolovka 142432, Russia
d Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia
*e-mail: ponomarenko@ispm.ru
Highlights
•
•
•
•
•
Four new organosilicon luminophores based on benzothiadiazole are synthesized
They possess high efficiency and large Stokes shift in solution and polymer matrix
An approach to tune their optical properties in wide spectral range is reported
The nearest donor fragments have the greatest influence on the spectral properties.
TMS groups accelerate fluorescence, increase solubility and extinction coefficient.
Keywords: organosilicon luminophores, fluorescence, conjugated oligomers, benzothiadiazole,
large Stokes shift

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A series of new linear π-conjugated oligomers consisting of five conjugated rings based on
2,1,3-benzothiadiazole (BTD) electron-withdrawing group and various combinations of 2,5-
thiophene and 1,4-phenylene electron-donating units with terminal trimethylsilyl groups has
been synthesized and characterized. Investigation of their absorption-luminescent properties
revealed that all of them possess high photoluminescence (PL) quantum yield and large Stokes
shift both in diluted solutions and polymer matrix, while in the solid state their PL efficiency
decreases. It was found that the main factor influencing the optical properties of the molecules
obtained is the type of aromatic fragment directly attached to the central BTD moiety.
Changing the chemical structure of the donor aromatic fragments from 1,1’-biphenyl to 2,2’-
bithiophene allows tuning the PL spectral maximum of the luminophores in wide range from
510 to 660 nm. It was shown that the presence of trimethylsilyl groups in these luminophores is
responsible for their increased solubility, enhanced molar extinction coefficients and shortening
the excited state lifetime without decreasing the PL efficiency.
1. Introduction
Luminophores as functional materials are successfully applied in many fields of science and
technology. Great progress has been made in the development of various electronic devices
based on organic materials such as displays [1,2], organic light-emitting transistors [3–5],
luminescent down-shifting layers for photovoltaic [6]. 2,1,3-Benzothiadiazole (BTD) derivatives
play an important role among such materials due to their unique optical and electronic
properties. They are normally form efficient luminophores [7–11], characterized by high
chemical and thermal stability, also widely used in organic photovoltaic materials as strong
electron-withdrawing moiety [12,13]. In addition, compounds based on BTD due to their
relatively high reduction potential and electron affinity are widely used as emissive layers in
organic light-emitting diodes (OLEDs) [14,15].

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Typically, BTD-based luminescent materials are symmetrical molecules composed by a
donor−acceptor− donor (D-A-D) framework with an electron-poor benzothiadiazole central
core connected to two electron-rich substituents. 4 and 7 positions of BTD unit are usually
preferred for direct π-extension and has a very positive effect both on PL spectra and
photostability [16,17]. More importantly, the absorption-luminescent properties of such
molecules can be tuned by the electronic properties of the extended donor substituents [18–
20].
On the one hand, high potential of luminescent BTDs derivatives was already demonstrated and
recently reviewed [21]. However, there is very little information available about the
interactions between the BTD acceptor and various donor substituents. On the other hand, σ-π
interactions of the silylene group with the conjugated chromophores is well known [22,23].
Introduction of silicon atoms into conjugated organic compounds can lead to the following
advantages: high thermal stability; high solubility in many organic solvents and therefore good
processability from solutions, which is necessary for obtaining various devices of organic
electronics by inexpensive printing methods; and the most important – the presence of specific
electronic effects [24]. These include an increase in the conjugation due to direct influence of
silicon on the conjugated system, which is the reason for an increase in the molar extinction
coefficient and a shift of the optical spectra maxima to the long-wavelength region [25,26]. In
addition, organosilicon luminophores, in contrast to their non-silicon analogues, can exhibit an
increased photoluminescence quantum yield (PLQY) [27]. Therefore, organosilicon derivatives
of BTD-based luminophores are promising materials both from the fundamental and applied
points of view. In this work the synthesis and systematic study of optical and thermal properties
of four novel BTD-based luminophores, being linear conjugated oligomers with a conjugation
length of five aromatic rings with the central BTD acceptor core containing various
combinations of 1,4-phenylene and 2,5-thiophene units with terminal trimethylsilyl (TMS)

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groups are reported (Fig.1). Influence of the TMS groups on physical properties of the
luminophores was revealed by comparing compound F1 with its full analogue F1a without TMS
groups described earlier [18,19,28].
S
N
N
N
Si
Si
F1
S
N
Si
Si
S
S
Si
Si
F2
S
N
N
N
S
S
S
S
F3
S
N
S
Si
Si
S
F4
Fig. 1. Chemical structures of novel organosilicon luminophores based on 2,1,3-
benzothiadiazole core synthesized and investigated in this work.
2. Experimental section
2.1. Materials
Hexane solutions of 1.6M and 2.5M n-butyllithium (CAUTION: solution can ignite in contact
with water and carbon dioxide [29], this compound should be handled under nitrogen with
special equipment[30]), trimethylchlorosilane (CAUTION: Reacts violently or explosively with
water [31,32]), 2-bromothiophene, sodium carbonate, magnesium and 1,4-dibromobenzene
were obtained from Acros organics; tetrakis(triphenylphosphine)palladium(0) Pd(PPh3)4, [1,1′-
bis(diphenylphosphino)ferrocene] dichloropalladium(II), 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (IPTMDOB) were obtained from Sigma-Aldrich and used as received. 2-
(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-5-(trimethylsilyl)-thiophene
(2)
[33],
(4'-
bromobiphenyl-4-yl)(trimethyl)silane (3) [34], trimethyl[4-(2-thienyl)phenyl]silane (5), 2,2'-

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bithien-5-yl(trimethyl)silane (6) [35], trimethyl{4-[5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-2-thienyl]phenyl}silane (9) [26] and 4,7-dibromo-2,1,3-benzothiadiazole (11) [8] were
synthesized as described earlier. THF, diethyl ether, toluene and ethanol were dried and
purified according to the standard techniques and then used as the solvents.
2.2. Characterization
1
The H NMR spectra were recorded on a Bruker WP250 SY spectrometer (250.13 MHz) using
the residual signal of CDCl3 (δ 7.27 ppm) as the internal standard. The 13C and 29Si NMR spectra
were recorded on a Bruker Advance II 300 spectrometer at working frequencies 75 MHz and 60
1
MHz, respectively. In the case of H NMR spectroscopy, the compounds to be analyzed were
taken in the form of 0.5% solutions in CDCl3. In the case of 13C or 29Si NMR spectroscopy, the
compounds to be analyzed were taken in the form of 1-3% solutions in CDCl3. The spectra were
then processed on the computer using the ACD Labs software.
Elemental analysis of C, H, N elements was carried out using CHN automatic analyzer CE1106
(Italy). Experimental error is 0.30−0.50%. The burning was done in the Sheninger flask using
alkaline solution of hydrogen peroxide as an absorbent. Spectrophotometry technique was
used for the Si analysis.
Thermogravimetric analysis was carried out in dynamic mode in 30-800 °C interval using
Mettler Toledo TG50 system equipped with M3 microbalance allowing measuring the weight of
the samples in 1-150 mg range with 1 µg precision. Heating/cooling rate was chosen to be 20
°C/min. Every compound was studied twice: in air and under nitrogen flow of 200 mL/min.
GPC analysis was performed on a Shimadzu instrument with a RID10AVP refractometer and a
SPD-M10AVP diode matrix as detectors using 7.8 × 300 mm2 Phenomenex columns (USA) filled
with the Phenogel sorbent with pore size of 500 Å and THF as the eluent. In the case of column
chromatography, silica gel 60 (“Merck”) was taken. For thin layer chromatography (TLC),

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“Sorbfil” (Russia) plates were used. The solvents were removed under vacuum (1 mBar) at 80
°C. All reactions, unless stated otherwise, were carried out in an inert atmosphere using
anhydrous solvents.
The absorption spectra were recorded on a Shimadzu UV-2501PC (Japan) spectrophotometer in
the standard 10 mm photometric quartz cuvette using THF solutions with the concentrations of
10-5 M. A scanning spectrofluorimeter ALS01M (Russia) with registration in single photon
counting mode at successive time intervals and automatic adjustment of the intensity of the
measured emission was used for the registration of photoluminescence spectra (PL).
Measurements were carried out for several optical densities in the range from 0.06 to 0.12
absorbance units in 10 mm cuvette, measurement geometry – 90°. Measurements of the PL
spectra in polystyrene (PS) and polycrystalline thin films were carried out in the integrating
sphere. The photoluminescence quantum yield (PLQY) was measured by comparing the integral
PL intensity of 10-6 M diluted solutions of luminophores in THF with the integral PL intensity of
the standard as described elsewhere [36]. As the standards in measuring the PLQY a solution of
1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) in cyclohexane (PLQY=1) and a solution of
rhodamine 6G in ethanol (PLQY=1) were used. Measurements of the PL lifetime were made by
the pulsed method in single photon counting mode on the Fluorolog-3 spectrofluorimeter
(HORIBA Scientific).
Density functional theory (DFT) calculations were performed using the ORCA v. 4.0.1 software
package using the B3LYP5/6-31G[d] approximation [37–39]. Optimization of the singlet ground
state geometries was carried out without any restriction on their symmetry.
2.3. Synthesis
General procedure for the Suzuki coupling.

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A flask was charged with organoboron precursor, 4,7-dibromo-2,1,3-benzothiadiazole (11) and
Pd(PPh3)4 (1-5 mol %). The mixture was degassed, toluene, an aqueous solution of Na2CO3 (2.0
M) and ethanol was added and heated to reflux until the reaction was complete. Completeness
of the reaction was controlled by GPC analysis. After completion of the reaction, the mixture
was cooled down to room temperature, the bottom aqueous phase was taken out by a syringe
and organic phase was concentrated and filtrated over a thin pad of silica gel in toluene, which
was evaporated by a Rotorvapour.
[5-(4-Bromophenyl)-2-thienyl](trimethyl)silane (4). This compound was obtained by the
general procedure for the Suzuki coupling using trimethyl[5-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-2-thienyl]silane (2) (5.74 g, 20.3 mmol), 1,4-dibrombenzene (14.39 g, 61
mmol), Pd(PPh3)4 (70 mg, 0.1 mmol) and aqueous solution (2M) of Na2CO3 (10 ml) in toluene-
ethanol mixture (150/10 mL). The reaction mixture was stirred at reflux for 17 hours. The crude
product was purified by recrystallization from ethanol to give pure compound 4 as a white solid
(4.23 g, 66.8%). 1H NMR (J Hz, CDCl3): δ [ppm] 0.33 (9H, s), 7.20 (1H, d, J = 3.4), 7.34 (1H, d, J =
3.4), 7.48 (4H, s).
Trimethyl[4'-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl-4-yl]silane (7). A 1.6 M
solution of n-butyl lithium (3.2 mL, 5.2 mmol) in hexane was added dropwise to a solution of
(4'-bromobiphenyl-4-yl)(trimethyl)silane (3) (1.5 g, 4.9 mmol) in anhydrous THF (35 mL),
keeping the temperature below -70 °C. The reaction mixture was stirred for 1 hour at -75 °C
and IPTMDOB (1.2 g, 6.45 mmol) was added in one portion. The reaction mixture was stirred
for 30 min at -78°C and the temperature was slowly allowed to rise to 0 °C. After completion of
the reaction, 100 mL of diethyl ether and 100 mL of ice water containing 5 mL of 1 M HCl were
added. The organic phase was separated, washed with water, and dried over sodium sulfate.
The solvent was evaporated to give 1.8 g of white crystals, containing 82% of compound 7 and

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18% of the initial compound 3 (according to GPC). The product was used in the subsequent
1
synthesis without further purification. H NMR (J Hz, CDCl3): δ [ppm] 0.30 (9H, s), 1.36 (12H, s),
7.59-7.64 (6H, overlapping peaks), 7.88 (2H, d, J = 7.7).
Trimethyl{5-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-2-thienyl}silane (8). This
compound was obtained by the method described above for compound 7 using 1.6 M solution
of n-butyl lithium in hexane (4 mL, 6.45 mmol), [5-(4-bromophenyl)-2-thienyl](trimethyl)silane
(4) (3.34 g, 6.0 mmol) and IPTMDOB (1.3 g, 7.0 mmol) in anhydrous THF (100 mL). The product
containing 89% of compound 8 according to GPC was used in the subsequent synthesis without
1
further purification. H NMR (J Hz, CDCl3): δ [ppm] 0.35 (9H, s), 1.37 (12H, s), 7.23 (1H, d, J =
3.4), 7.44 (1H, d, J = 3.4), 7.63 (2H, d, J = 8.2), 7.82 (2H, d, J = 8.2)
Trimethyl[5'-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2'-bithien-5-yl]silane (10). This
compound was obtained by the method described above for compound 7 using 2.5 M solution
of n-butyl lithium in hexane (5 mL, 12.6 mmol), 2,2'-bithien-5-yl(trimethyl)silane (6) (3 g, 12.6
mmol) and IPTMDOB (2.34 g, 12.6 mmol) in anhydrous THF (60 mL). The product containing
98% of compound 10 according to GPC was used in the subsequent synthesis without further
purification. 1H NMR (J Hz, CDCl3): δ [ppm] 0.34 (9H, s), 1.36 (12H, s), 7.14 (1H, d, J = 3.7), 7.23-
7.33 (2H, overlapping peaks), 7.53 (1H, d, J = 3.7).
4,7-Bis[4'-(trimethylsilyl)biphenyl-4-yl]-2,1,3-benzothiadiazole (F1). This compound was
obtained by the general procedure for the Suzuki coupling using compound 7 (8.7 g, 25.9
mmol), 4,7-dibromo-2,1,3-benzothiadiazole (11) (3.45 g, 11.7 mmol), Pd(PPh3)4 (300 mg, 0.26
mmol) and aqueous solution (2M) of Na2CO3 (20 mL) in toluene-ethanol mixture (200/20 mL).
The reaction mixture was stirred at reflux for 27 hours. The crude product was purified by
recrystallization from toluene to give pure compound F1 as a lime green solid (3.8 g, 55.7%), mp
= 261°C. 1H NMR (J Hz, CDCl3): δ [ppm] 0.34 (18H, s), 7.69 (8H, m, J1 = 11.6, J2 = 7.9), 7.81 (4H, d,

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J = 8.2), 7.88 (2H, s), 8.09 (4H, d, J = 8.2). 13C NMR (CDCl3): δ [ppm] -1.08 126.45, 127.39,
128.02, 129.62, 132.89, 133.92, 136.37, 139.60, 140.97, 141.14, 154.12. 29Si NMR (CDCl3): δ
[ppm] -3.90. Anal. Calcd. (%) for C36H36N2SSi2: C, 73.92; H, 6.20; N, 4.79; S, 5.48%; Si, 9.60.
Found: C, 73.87; H, 6.07; N, 4.71; Si, 9.50.
4,7-Bis{4-[5-(trimethylsilyl)-2-thienyl]phenyl}-2,1,3-benzothiadiazole (F2). This compound was
obtained by the general procedure for the Suzuki coupling using compound 8 (2.5 g, 7.0 mmol),
4,7-dibromo-2,1,3-benzothiadiazole (11) (0.92 g, 3.14 mmol), Pd(PPh3)4 (81 mg, 0.07 mmol) and
aqueous solution (2M) of Na2CO3 (5 mL) in toluene-ethanol mixture (50/5 mL). The reaction
mixture was stirred at reflux for 15 hours. The crude product was purified by recrystallization
from toluene-hexane (1:6) mixture to give pure compound F2 as a yellowish solid (1.4 g, 75%),
1
mp = 226 °C. H NMR (J Hz, CDCl3): δ [ppm] 0.38 (18H, s), 7.26 (2H, d, J = 3.4), 7.47 (2H, d, J =
3.5), 7.78-7.87 (6H, overlapping peaks), 8.03 (4H, d, J = 8.5), 13C NMR (CDCl3): δ [ppm] -0.09,
124.75, 126.15, 127.78, 128.19, 128.99, 129.62, 132.67, 134.45, 135.08, 136.28, 140.60, 149.07,
154.05. 29Si NMR (CDCl3): δ [ppm] -6.46. Anal. Calcd. (%) for C32H32N2S3Si2: C, 63.38; H, 5.40; N,
4.69; S, 16.11%; Si, 9.41. Found: C, 63.51; H, 5.27; N, 4.74; Si, 9.29.
4,7-Bis{5-[4-(trimethylsilyl)phenyl]-2-thienyl}-2,1,3-benzothiadiazole (F3). This compound was
obtained by the general procedure for the Suzuki coupling using compound 9 (2.05 g, 5.7
mmol), 4,7-dibromo-2,1,3-benzothiadiazole (11) (600 mg, 2.0 mmol), Pd(PPh3)4 (198 mg, 0.17
mmol) and aqueous solution (2M) of Na2CO3 (5 mL) in toluene-ethanol mixture (50/5 mL). The
reaction mixture was stirred at reflux for 50 hours. The crude product was purified by
recrystallization from toluene-hexane (1:3) mixture to give pure compound F3 as a red solid
(0.89 g, 73%), mp = 303 °C. 1H NMR (J Hz, CDCl3): δ [ppm] 0.31 (18H, s), 7.43 (2H, d, J = 3.8), 7.56
(4H, d, J = 8.0), 7.68 (4H, d, J=8.0), 7.87 (2H, s), 8.11 (2H, d, J = 3.9). 13C NMR (CDCl3): δ [ppm] -
1.14, 124.23, 125.16, 125.42, 125.98, 128.76, 133.99, 134.56, 138.86, 140.40, 145.80, 152.73.

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29Si NMR (CDCl3): δ [ppm] -3.98. Anal. Calcd. (%) for C32H32N2S3Si2: C, 63.38; H, 5.40; N, 4.69; S,
16.11%; Si, 9.41. Found: C, 64.74; H, 5.53; N, 4.46; Si, 9.21.
4,7-Bis[5'-(trimethylsilyl)-2,2'-bithien-5-yl]-2,1,3-benzothiadiazole (F4). This compound was
obtained by the general procedure for the Suzuki coupling using compound 10 (6.5 g, 17.86
mmol), 4,7-dibromo-2,1,3-benzothiadiazole (11) (2.5 g, 8.5 mmol), Pd(PPh3)4 (206 mg, 0.18
mmol) and aqueous solution (2M) of Na2CO3 (13.4 mL) in toluene-ethanol mixture (100/10 mL).
The reaction mixture was stirred at reflux for 45 hours. The crude product was purified by
recrystallization from toluene-hexane (1:1) mixture to give pure compound F4 as a red solid
1
(3.14 g, 83.1%), mp = 205 °C. H NMR (J Hz, CDCl3): δ [ppm] 0.36 (18H, s), 7.18 (2H, d, J = 3.5),
7.28 (2H, d, J = 3.9), 7.35 (2H, d, J = 3.5), 7.82 (2H, s), 8.03 (2H, d, J = 3.9). 13C NMR (CDCl3): δ
[ppm] -0.10, 124.70, 125.23, 125.37, 125.73, 128.42, 134.88, 138.26, 138.97, 140.63, 142.33,
152.63. 29Si NMR (CDCl3): δ [ppm] -6.40. Anal. Calcd. (%) for C28H28N2S5Si2: C, 55.22; H, 4.63; N,
4.60; S, 26.32; Si, 9.22. Found: C, 55.24; H, 4.69; N, 4.73; Si, 9.22.

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3. Results and discussion
3.1. Synthesis
Br
Br
O
Br
S
S
Si
Si
1
Si
B
O
Toluene, 2M Na2CO3
EtOH, reflux
2
4
1) n-BuLi
O
O
2) IPTMDOB
Si
Ar
R
Ar
B
THF, -78°C
3, Ar = 2Ph
4, Ar = T-Ph 6, Ar = 2T
5, Ar = Ph-T
7, Ar = 2Ph
8, Ar = T-Ph
9, Ar = Ph-T
10, Ar = 2T
S
N
N
S
Br
Br
N
N
O
O
11
Si Ar
Ar Si
Si Ar
B
R
Toluene, 2M Na2CO3
EtOH, reflux
F1, Ar = 2Ph
F2, Ar = T-Ph F4, Ar = 2T
F3, Ar = Ph-T
7-10
Si
S
Si
Br
Br
Si Ar
2Ph
T-Ph
2T
=
H
R = Br or H
S
S
Si
Si
H
S
Ph-T
Fig. 2. Scheme of the synthesis of compounds F1-F4.
As an object for the study, linear organosilicon BTD-based oligomers in which symmetric donor
fragments have different pairs of 1,4-phenylene and 2,5-thiophene units were selected. Their
synthesis was carried out in several main steps (Fig.2). First, using various combinations of
organometallic and cross-coupling reactions, trimethylsilyl derivatives 3-6 were obtained. The
introduction of the trimethylsilyl group was carried out through the reaction of
organomagnesium and organolithium precursors with trimethylchlorosilane. Then organoboron
precursors 7-10 were obtained and used in the Suzuki cross-coupling reaction with 4,7-
dibromo-2,1,3-benzothiadiazole (11). After recrystallization, pure compounds F1-F4 as solids
having various colors from lime green to red with the isolated yields of 56-83% were obtained.

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1
GPC, H, 13C and 29Si NMR spectroscopy together with the elemental analysis were used to
characterize the structure of the intermediates and final compounds (see ESI, Fig. S1-S16). All of
them gave satisfactory analysis and spectroscopic data corresponding to their expected
molecular structures. In particular, signals of TMS groups in the region of -0.32… -0.38 ppm at
1H NMR spectra, in the region of -0.09 … -1.14 ppm at 13C NMR spectra and in the region of -
3.90 … -6.46 ppm at 29Si NMR spectra are present at the corresponding NMR data of
compounds F1-F4. A detailed description of these data can be found in the Experimental
Section. According to the GPC data the purity of all the compounds obtained was higher than
99%.
In the case of compounds 7 and 10, an attempt was made to obtain the corresponding
organosilicon precursors 3 and 6 and corresponding organoboron compounds in a one-pot
synthesis from commercially available precursors. This would greatly simplify the preparation of
luminophores F1 and F4 due to elimination of the purification stage for compounds 3 and 6. For
this purpose, the starting bifunctional 4,4′-dibromobiphenyl and 2,2′-bithiophene were lithiated
with one equivalent of n-butyl lithium at -78 °C, stirred for one hour and chlorotrimethylsilane
was added in one portion. Slow addition of n-butyllithium and keeping temperature of the
reaction mixture below -70 °C made it possible to obtain almost completely their monolithium
derivatives. After adding of chlorotrimethylsilane and 30 min of stirring at this temperature,
GPC analysis showed the formation of 91% and 98% of compounds 3 and 6, respectively.
Thereafter, the second equivalent of n-butyllithium solution was added, the reaction mixture
was stirred at -78 °C for one hour and IPTMDOB was added in one portion. After isolation, the
yield of organoboron precursors 7 and 10 according to GPC analysis was 87% and 96%,
respectively. They were used for Suzuki cross-coupling reaction to produce compounds F1 and
F4. However, during their purification, it was not possible to eliminate the low-molecular
weight impurities. We assume that during the synthesis of compounds 7 and 10, organoboron

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precursors without a trimethylsilyl group were also produced as by-products. It was supported
by similar absorption spectrum of the low-molecular weight impurities and the final
luminophores from 3D GPC analysis using diode array detector. In the case of purification after
the separate stage of preparation of organosilicon compounds 3 and 6 during further synthesis
of F1 and F4 the formation of these impurities was not observed and pure compounds were
obtained by simple recrystallization.
3.2. Thermal behavior
Table 1. Thermal properties and solubility of compounds F1-F4.
ΔS*10-3,
J g-1 K-1
137
S,
Compound Тm, °C ΔH, J g-1
ТN2, °C Тair, °C
g L-1
5.2
1.5
8.5
1.5
7.3
F1
F1a
F2
261
249
226
303
205
73
100
72
364
352
370
392
340
358
332
372
362
357
192
144
148
132
F3
85
F4
63
Тm - Melting temperature; S - solubility measured in toluene; TN2 and Tair - decomposition temperature (at 5%
weight loss) according to TGA in the inert atmosphere (N2) and in the air; ΔH - melting enthalpy; ΔS - melting
entropy
Thermal properties of the luminophores synthesized were investigated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). According to the TGA data, the 5%
weight loss for all the compounds under study both in the air and in the inert atmosphere
occurs above 340 °C (Table 1 and Fig. S22-S26 in ESI). This suggest a very high stability of the
luminophores investigated both under thermal and thermo-oxidative conditions. Despite the
different chemical structure, their decomposition temperatures are close to each other.
Bearing in mind high thermal stability of the TMS group, these results suggest that the most
thermally unstable fragment in such molecules is the BTD unit. Analyzing dependence of the
change in the weight of the sample with increasing temperature (Fig. S22-S26 in ESI), we can
conclude that for all these compounds a partial sublimation is characteristic both in an inert

ACCEPTED MANUSCRIPT
atmosphere and in the presence of atmospheric oxygen. This is confirmed by close
decomposition temperatures and very low residual weights, even in the inert atmosphere.
Introduction of TMS groups does not have any noticeable effect on the stability of the
compounds as can be seen from the comparison of the thermal properties of luminophores F1
and its non-silicon analogue F1a (Table 1).
The DSC curves indicate that all these compounds are crystalline (Fig. S17-S21 in ESI). The
melting points were determined from the maxima at the first DSC heating scans (Table 1). The
subsequent cooling and second heating show similar melting temperatures to those obtained
at the first DSC heating scans. Compound F3 among all the organosilicon compounds
investigated has the highest melting point (Тm = 303 °C) and melting enthalpy (ΔH = 85 J g-1). In
contrast to the other compounds, it also has the lowest solubility in organic solvents and the
largest change in the melting entropy (ΔS = 148 mJ g-1 K-1), which also indicates its high degree
of crystallinity. The solubility of luminophores F1-F4 correlates well with their melting points. As
the solubility increases, the melting temperature decreases. Comparison of compound F1a with
its organosilicon analogue F1 showed that the former has the highest melting entropy and
entropy. Moreover, its’ solubility is 3.5 times lower than those of compound F1, which indicates
very good solubilizing ability of TMS groups.
3.3. Optical properties
The absorption and PL spectra of F1-F4 were investigated in diluted THF solutions (Fig.3-4, S27),
polystyrene (PS) matrix (Fig.S27-S29 in ESI) and polycrystalline thin films having a thickness of
50-100 nm (Fig.S27, S30-S32 in ESI). The data are summarized in Table 2. On all the absorption
spectra of both diluted THF solutions and polycrystalline films there are two absorption bands
related to the singlet electron-vibrational transitions – long wavelength and short wavelength.
For all molecules, the first peak corresponds to transition S0 → S1. The short wavelength

ACCEPTED MANUSCRIPT
absorption peak corresponds to transition S0 → S1+n. In this case, the value of n for all molecules
is different and varies from 2 to 5. More detailed information on electron-vibrational transition
properties calculated by DFT method is presented in Table S1 in ESI. The distance between
these maxima varies from 5000 to 9000 cm-1. The large distance between the maxima observed
max
is typical for the luminophores with a BTD core. The positions of the absorption maxima (
λ
)
abs
ε
and the values of the corresponding molar extinction coefficient ( max ) are given in Table 2.
Wavenumber, cm-1
32k 30k 28k 26k 24k
22k
20k
18k
16k
F1
F2
F3
F4
2,5
2,0
1,5
1,0
0,5
0,0
300
400
500
Wavelength, nm
600
Fig. 3. Absorption spectra of diluted THF solutions of compounds F1 - F4.
Wavenumber, cm-1
24k 22k
20k
18k
16k
14k
F1
F2
F3
F4
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
Wavelength, nm
700
800

ACCEPTED MANUSCRIPT
Fig. 4. PL spectra of diluted THF solutions of compounds F1 - F4.

ACCEPTED MANUSCRIPT
Table 2. Absorption and PL properties of F1-F4.
max *103,
L mol-1 cm-1
ε
τPL kPL
,
ns
max
abs
max
Stokes shift, cm-1
PLQY a, %
λ
, nm
λ
, nm
lum
*108,
s-1
THF
PS solid THF
PS
solid THF
21.7
PS THF PS solid THF PS Solid b
22
60
401 407 380
302 307 317
399
F1
F1a
F2
5600 4600 6200
516 502 496
84
83
73
77
65
85
100 5.4 1.56
59.2
20.2
54.6
27
-
-
5500
-
-
-
512
-
-
-
-
17
3
6.0 1.38
4.4 1.65
6.9 1.11
5.8 1.12
299
415 422 417
321 325 320
495 502 498
350 349 355
513 521 516
364 368 358
26
5600 4500 4800
4100 3600 3700
4200 3600 4400
540 521 521
620 611 612
656 641 668
75
66
57.3 55.3
33.3 33
F3
43.4 42.4
32.1 30.7
F4
82
3
35
32.6
max
abs
max
a The error in measuring the absolute value of PLQY is ±6%. b The relative PLQY calculated with respect to the F1 sample;
λ
and
λ
- absorption and luminescence maxima,
lum
respectively; ε
τ
k
PL
max - molar extinction coefficient;
- PL lifetime in THF;
PL rate constant in THF.
PL

ACCEPTED MANUSCRIPT
As can be seen from the data presented, the energy of the electronic transitions in the series of
BTD-based luminophores depends on the position of the phenylene and thiophene rings
relative to the central BTD core. Thus, F1 with 1,1’-biphenyl fragments has the shortest
absorption wavelength, while replacement of the outer phenyl rings by thiophene heterocycles
in F2 leads to a bathochromic shift of the S0 → S1 transition by 800 cm-1, and the S0 → S1+n
transition by 2000 cm-1. For its structural isomer F3, differing only in the position of the
thiophene and phenylene rings, with the thiophene rings adjacent to the BDT core, much
greater bathochromic shift is observed: 4700 cm-1 for S0 → S1 and 4500 cm-1 for S0 → S1+n . In
the case of F4 containing 2,2’-bithiophene units, the bathochromic shift is maximal: 5400 cm-1
for S0 → S1 and 5600 cm-1 for S0 → S1+n
.
Comparison of the absorption spectrum of diluted THF solutions of the luminophores with a
polymer matrix, a slight redshift is observed. At the same time, the molar extinction coefficients
hardly change (Table 2, Fig.S27-S29 in ESI). The absorption bands related to the S0 → S1 and S0
→ S1+n transitions in the polycrystalline films of compounds F1-F4 in addition to the main
maximum also have a long-wave shoulder (Fig. S30, S32 in ESI). Splitting of the absorption
bands can be connected to the resonance interaction of the molecules closely packed in the
crystal upon their excitation (Davydov splitting) [40]. Table 3 shows the results of
approximating the absorption bands corresponding to the electronic transitions S0 → S1 in F1-
F4 by two Gaussian functions (Fig. S32). The distance between the positions of the functions
maxima is 2400 +/- 300 cm-1. The contribution of the long-wavelength peak to the integrated
intensity of the absorption band of F1 molecule is about 2%. In the spectrum of the
polycrystalline film of F2 it reaches 18%, F3 - 23% and F4 - 49%. Thus, replacement of the
phenylene by thiophene rings in these luminophores leads to a gradual increase in the splitting
of the long-wavelength absorption band of the spectra of the polycrystalline films.

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Table 3. Splitting of the absorption band for the electronic transition S0 → S1 in polycrystalline
films of F1-F4 by Gaussian functions.
The fraction of the
integrated intensity between peaks,
Distance
Peak maximum
position, nm
Compound
Peak
of the band
0.98
cm-1
1
2
1
2
1
2
1
2
379
423
413
460
507
568
491
552
F1
F2
F3
F4
2750
0.02
0.82
2470
2120
2270
0.18
0.77
0.23
0.61
0.49
Diluted THF solutions of the luminophores investigated have a bright photoluminescence
(Fig.5). Their PL spectra obtained upon excitation by the light with a wavelength corresponding
to the position of the maximum of the long-wave band of the absorption in THF are shown in
Fig. 4 (for PS matrix and polycrystalline films see Fig.S27, S29 and S31 in ESI). The PL spectra of
solutions in THF, polystyrene matrix, and polycrystalline thin films have similar spectral
distributions with a single maximum (Fig. S27). A large Stokes shift is observed for all the
compounds investigated, which is typical for luminophores based on BTD core. The Stokes shift
values for all these compounds ranges from 3600 to 6200 cm-1. For the polymer matrix, the
Stokes shift is less than those for the diluted solutions in THF. This effect occurs because upon
transition from a diluted THF solution to a polymer matrix the absorption spectrum undergoes
a bathochromic and the PL spectrum has a hypsochromic shift.

ACCEPTED MANUSCRIPT
Fig. 5. Photo of 10-3M solutions in THF and powders of F1-F4 illuminated by 365 nm black light.
Table 2 gives the values of the photoluminescence quantum yield (PLQY), which varies from 65
to 84% in THF, and from 66 to 85% in PS matrix. In the case of F4, the PLQY increases from 65%
τ
in THF solution to 82% in PS matrix. The PL lifetime ( PL ) measured for diluted solutions of the
compounds in THF lies in the range of 4.4-6.9 ns. For F3 and F4 the PL lifetime is slightly higher
k
than those of F1 and F2 molecules. Comparison of the PL rate constant ( PL ) values obtained
PLQY = kPL ×τPL
for F1-F4 molecules according to formula
shows that when the phenylene
fragment is replaced by a thiophene the PL rate constant is decreased (Table 2).
In polycrystalline thin films, the highest PL intensity has luminophore F1. The values of PLQY
shown in Table 2 were calculated as a ratio of the integral PL intensity of compounds F2-F4 to
the integral intensity of compound F1. The integral PL intensity was calculated according to
D
(
λext
)
λ
Ν
λ
( )
is
Formula 1, where
is the optical density at the excitation wavelength
and
ext
PL
spectral distribution of PL intensity. The integration was carried out over the entire PL
spectrum.
−1
(
λext )
(1)
Ι =
(
1−10−D
)
ΝPL
(
λ dλ
)
∫
∆
λ
In the case of F2 the relative PLQY in thin polycrystalline film decreases 6 times. In the
polycrystalline films of compounds F3 and F4 photoluminescence is almost absent. Thus, for the

ACCEPTED MANUSCRIPT
compounds with thiophene fragments (F2-F4), PL quenching is observed in the aggregated
state. This may be due to intrinsic reasons, i.e. by H-aggregation [40], but also by trapping,
which is very common in polycrystalline phases [41]. Influence of the terminal TMS groups on
the spectral properties was investigated by comparing luminophore F1 and its non-silicon
analogue F1a in diluted THF solutions (Fig.6, Table 2). Although the spectra of these compounds
are almost similar, a bathochromic shift of the absorption maxima by 125-330 cm-1 and of the
PL maxima by 150 cm-1 is observed. The main influence of the silicon atoms on the absorption
spectrum of compound F1 appears as an increase in the molar extinction coefficient by 10%.
The PLQY of both luminophores are very high, 83-84%, and hardly depends on the presence of
TMS groups. However, for compound F1 a higher PL rate constant was found, which can be
explained by decrease in the lifetime of the excited state by 10% from 6.0 to 5.4 ns. Thus, the
introduction of silicon has a positive effect on both absorption and photoluminescence
properties.
Wavenumber, cm-1
40k 35k 30k
60k
25k
20k
15k
Abs / Lum of F1a
Abs / Lum of F1
2,5
2,0
1,5
1,0
0,5
0,0
S
N
N
N
50k
40k
30k
20k
10k
0
Si
Si
F1
S
N
F1a
250 300 350 400 450 500 550 600 650 700
Wavelength, nm
Fig. 6. Absorption and PL spectra of diluted solutions in THF of compounds F1 and F1a.
3.4. Theoretical calculations

ACCEPTED MANUSCRIPT
Calculated parameters for the organosilicon luminophores using density functional modeling
was performed in order to rationalize the observed properties in terms of geometry and
electronic structure (Table 4, S1). These data are in a good agreement with the results obtained
by optical spectroscopy method: gradual decrease in the energy gap in the transition from
phenylene to thiophene derivatives, pronounced donor-acceptor character of the compounds.
It was found that the energy gap (Eg) is reduced in the thiophene derivatives F3 and F4 when
compared to the phenylene compounds F1 and F2. This is mostly due to increase in the highest
occupied molecular orbital (HOMO) level together with the electron donating properties of the
substituents in benzothiadiazole core. The analogous effect of trimethylsilyl substitution is
appeared to be secondary, less in an order of its magnitude.
Table 4. Calculated optimized total energy (Etot), FMO levels and the Eg values for the
organosilicon luminophores using density functional modeling (B3LYP/6-31G[d]; ORCA v. 4.0.1).
Etot, Hatree
HOMO, eV
LUMO, eV
Eg, eV
F1
F2
-2479.1131420
-5.44
-2.32
3.12
-3120.6170309
-3120.6102507
-3762.1135698
-5.26
-4.98
-4.85
-2.35
-2.55
-2.58
2.91
2.43
2.27
F3
F4
The contour orbital diagrams for the frontier molecular orbitals (FMO) are shown in Figure 7. In
all cases the lowest unoccupied molecular orbitals (LUMO) is composed mostly on the acceptor
BTD fragment. On the other hand, HOMOs show sufficient conjugation between different

ACCEPTED MANUSCRIPT
donor fragments. There are no essential difference observed in the orbital shape caused by the
trimethylsilyl substitution.

ACCEPTED MANUSCRIPT
HOMO
LUMO
F1
F2
F3
F4
Fig. 7. Diagrams of the frontier orbitals of the F1-F4 (contour values ±0.025).

ACCEPTED MANUSCRIPT
The influence of the trimethylsilyl substitution upon the energy levels of FMO, as well as upon
the Eg gap value, was found to be relevant to control the photophysical properties of the
luminophores. For this study DFT calculations of a series of non-TMS analogues of the dyes F1a
- F4a were also performed (see Table S2, Fig. S33). In particular, the Eg gap of the unsubstituted
dyes is narrowed in their trimethylsilyl derivatives. This appeared mostly due to the slight
increase in the energy of HOMO levels, in its turn uncovering the σ-donor effect of the silyl
substituent. The LUMO levels do not show any clear trend under these circumstances.
4. Conclusions
Synthesis of four new conjugated linear oligomers based on BTD electron-accepting core with
4'-(trimethylsilyl)biphenyl-4-yl,
4-[5-(trimethylsilyl)-2-thienyl]phenyl,
5-[4-
(trimethylsilyl)phenyl]-2-thienyl and 5'-(trimethylsilyl)-2,2'-bithien-5-yl electron-donating
groups is described. Various combinations of thiophene and phenylene fragments in these
molecules were used to understand their influence on the absorption-luminescent and thermal
properties. It was found that although luminophores F2 and F3 are full isomers, their spectral
properties are very different. The investigation of absorption and PL spectra has shown that
determining factor influencing the spectral characteristics of the luminophores obtained is the
nature of the aromatic ring directly attached to the central BTD unit. When the more electron-
donating thiophene ring is separated from the BTD core by phenylene ring, its effect on the
spectral properties is less prominent than if it is directly connected to the BTD core. This is
clearly seen on the properties of isomers F2 and F3 when both the absorption and PL maxima
are shifted by 2400 cm-1 for luminophore F2 with the thiophene rings adjacent to the BDT core.
It is shown that by simply changing the structure of the conjugated donor fragment, it is
possible to tune the PL maximum of the luminophores in the 510-660 nm spectral range. All the
luminophores investigated show the intensive PL in diluted THF solution and PS polymer matrix

ACCEPTED MANUSCRIPT
with PLQY up to 84-85%, while only F1 have strong PL in polycrystalline thin film. Apparently,
this should be related to the strong interactions in the crystals and particular parking of the
molecules containing thiophene rings (H-aggregation), but also by trapping and require further
investigations. The introduction of trimethylsilyl groups allows increasing the molar extinction
coefficients and accelerating the PL by approximately 10%. At the same time, there is very little
effect on the position of absorption and PL maxima and the PL quantum yield. High thermal
stabilities of all these compounds having the temperature of 5% decomposition of 340 – 392 °C
under nitrogen and 357 – 372 °C in the air, which is partly caused by sublimation, indicate that
they can be applied in high temperature optical devices and deposited on the substrate by
vacuum sublimation technique. The introduction of trimethylsilyl groups does not affect the
thermal stability and extremely increases solubility of the luminophores.
Acknowledgments
This work was performed in the framework of leading science school NSh-5698.2018.3.
Synthetic work was supported by Foundation of President of the Russian Federation (project
МК- 364.2017.3). Optical properties were studied under support of Russian Science Foundation
(grant 15-12-30031). The authors thank G.V. Cherkaev for NMR and P.V. Dmitryakov for TGA
and DCS measurements.
Supplementary data
1
Supplementary data associated with this article such as H, 13C and 29Si NMR spectra, TGA and
DSC curves and additional spectral figures can be found in the online version.
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