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Synthesis, optical and electrochemical propertiesof 4,40 -bibenzo[c]thiophene derivatives†
Kotaro Obayashi,a Keiichi Imato, a Satoshi Aoyama,a Toshiaki Enoki,a Seiji Akiyama,b
Mio Ishida,b Seiji Suga,c Koichi Mitsudo c and Yousuke Ooyama *a
We designed and synthesized unsubstituted 4,40 -bibenzo[c]thiophene 4,40 -BBT and its silyl-substituted
derivatives 1,10 -Si-4,40 -BBT and 1,10 ,3,30 -Si-4,40 -BBT with one or two tert-butyldimethylsilyl groups oneach thiophene ring, as new p-building blocks in emitters, photosensitizers and semiconductors fororganic optoelectronic devices. The characterization of 4,40 -BBT, 1,10 -Si-4,40 -BBT and 1,10 ,3,30 -Si-4,40 BBT was successfully determined by FTIR, 1H and 13C NMR measurements, high-resolution mass
spectrometry (HRMS) analysis, photoabsorption and ﬂuorescence spectroscopy, cyclic voltammetry (CV)and density functional theory (DFT) calculations. Moreover, a single-crystal X-ray structural analysis was
successfully made for 1,10 -Si-4,40 -BBT and 1,10 ,3,30 -Si-4,40 -BBT. The photoabsorption and ﬂuorescenceﬂ0
maxima (labsmax and lmax) of the three 4,4 -bibenzo[c]thiophene derivatives in toluene exhibit
bathochromic shifts in the order of 4,40 -BBT (359 nm and 410 nm) < 1,10 -Si-4,40 -BBT (366 nm and 420nm) < 1,10 ,3,30 -Si-4,40 -BBT (371 nm and 451 nm). The HOMO and LUMO energy levels rise in the order of
4,40 -BBT (5.55 eV and 2.39 eV) < 1,10 -Si-4,40 -BBT (5.45 eV and 2.34 eV) < 1,10 ,3,30 -Si-4,40 -BBT
(5.34 eV and 2.30 eV), but the rise of the HOMO energy level is larger than that of the LUMO energy
level, resulting in the bathochromic shift of the photoabsorption band from 4,40 -BBT to 1,10 ,3,30 -Si-4,40 BBT. The ﬂuorescence quantum yields (Fﬂ) of 4,40 -BBT, 1,10 -Si-4,40 -BBT and 1,10 ,3,30 -Si-4,40 -BBT in

Received 13th February 2021
Accepted 19th May 2021
DOI: 10.1039/d1ra01189h
rsc.li/rsc-advances

toluene are 0.41, 0.41 and 0.36, respectively. It is worth mentioning that in the solid state 1,10 -Si-4,40 BBT and 1,10 ,3,30 -Si-4,40 -BBT show relatively high Fﬂ-solid values of 0.22 and 0.25, respectively, whereas
4,40 -BBT exhibits poor solid-state ﬂuorescence properties (Fﬂ-solid < 0.02). This work provides an
eﬃcient synthetic method for the 4,40 -bibenzo[c]thiophene derivatives and their photophysicalproperties in the solution and solid state, electrochemical properties and X-ray crystal structures.

Introduction
Benzo[b]thiophene as a p-building block has created considerable scientic interest in synthetic organic chemistry, photochemistry, electrochemistry and theoretical chemistry as well as
materials chemistry, because it is an especially crucial player for
functional materials due to the air-stability and commercial
availability (Fig. 1).1 Actually, benzo[b]thiophene derivatives are
key constituents of emitters, semiconductors and photosensitizers for organic optoelectronic devices, such as organic lightemitting diodes (OLEDs),2 organic eld-eﬀect transistorsaDepartment of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan. E-mail:yooyama@hiroshima-u.ac.jp; Fax: +81 82-424-5494

b

Science & Innovation Center, Mitsubishi Chemical Corporation,
Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa 227-8502, Japan

1000,

c
Division of Applied Chemistry, Graduate School of Natural Science and Technology,
Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan† Electronic supplementary information (ESI) available. CCDC [2055734,
2055735]. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1ra01189h

18870 | RSC Adv., 2021, 11, 18870–18880(OFETs),3 organic photovoltaics (OPVs)4 and dye-sensitized solar
cells (DSSCs).5 Furthermore, much eﬀort has been made towards
the construction and characterization of fused benzo[b]thiophene systems such as thienoacenes (e.g., [1]benzothieno[3,2-b][1]benzothiophene (BTBT), dinaphtho[2,3-b:20 ,30 -f]thieno[3,2-b]
thiophene (DNTT) and dianthra[2,3-b:20 ,30 -f]thieno[3,2-b]thiophene (DATT))6a and thiophene-fused naphtho[2,3-b:6,7-b]
dithiophene diimide (NDTI)6b in the past two decades. Indeed,
these benzo[b]thiophene derivatives have been used as organic
semiconductors with high carrier mobility and stability underambient conditions. Similarly, benzo[c]thiophene is also aninteresting p-building block (Fig. 1). For example, Wudl andHeeger have prepared poly(1,3-benzo[c]thiophene) (i.e., poly(isothianaphthene) (PITN)) and demonstrated that PITN can form
the aromatic and quinoidal states by the bond alternation.7 In thearomatic state, the benzo[c]thiophene unit contains a thiophenering in the structure. On the other hand, in the quinoidal state,
the unit contains the more stable benzene ring in the structure.
Thus, PITN has a low band gap (Eg ¼ 1.0–1.2 eV), which is about
1.0 eV lower than that of polythiophene (PT), because the polymer backbone of PITN intrinsically stabilizes its quinoidal state,© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

Paper

Chemical structures of benzo[b]thiophene, benzo[c]thiophene,
1,10 -bibenzo[c]thiophene 1,10 -BBT and 3,30 -silyl-disubstituted-1,10 bibenzo[c]thiophene 3,30 -Si-1,10 -BBT.

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Fig. 1that is, the contribution of the quinoidal state decreases the Eg.
However, unsubstituted benzo[c]thiophene has never been isolated so far due to the instability in air, in contrast to unsubstituted benzo[b]thiophene. Nevertheless, some benzo[c]thiophene derivatives with substituents on the thiophene ring
and/or the benzene ring such as 1,3-diarylbenzo[c]thiophenes,8–12
5,6-disubstituted benzo[c]thiophenes13 and 1,3,4,7- or 1,3,5,6tetrasubstituted benzo[c]thiophenes14–16 have been synthesized
and their optical and electrochemical properties were investigated, although much less substituted benzo[c]thiophene derivatives have been reported than substituted benzo[b]thiophene
derivatives. In particular, few 3,30 -disubstituted-1,10 -bibenzo[c]thiophenes as the 1,10 -dimer of bibenzo[c]thiophene have beendeveloped by Cava,17 Mohanakrishnan18 and Ono et al.,19 and they
revealed their synthetic methods and, optical and electrochemical properties, although there are no reports on thesynthesis and physical properties of unsubstituted 1,10 -bibenzo[c]thiophene 1,10 -BBT (Fig. 1). On the other hand, in our previous
work,20 we have designed and developed 1,10 -bis(tert-butyldimethylsilyl)-4,40 -bibenzo[c]thiophene (1,10 -Si-4,40 -BBT) as the 4,40 dimer of bibenzo[c]thiophene and the fused-bibenzo[c]thiophene(PHDT-Si), which is the rst report on the synthesis, characterization and optical and electrochemical properties of 4,40 bibenzo[c]thiophene and fused-bibenzo[c]thiophene derivatives(Fig. 2). PHDT-Si exhibits intense vibronic-structured photoabsorption (labsmax ¼ 598 nm, molar extinction coeﬃcient (3max) ¼
80 900 M1 cm1 in toluene) and uorescence (lmax ¼ 613 nm,
uorescence quantum yield (F) ¼ 0.74 in toluene) bands ina signicantly longer wavelength region and a smaller Stokes

Fig. 2 Chemical structures of 4,40 -bibenzo[c]thiophene 4,40 -BBT
(BBT-1), 1,10 -silyl-disubstituted-4,40 -bibenzo[c]thiophene 1,10 -Si-4,40 BBT (BBT-2), 1,10 ,3,30 -silyl-tetrasubstituted-4,40 -bibenzo[c]thiophene
1,10 ,3,30 -Si-4,40 -BBT (BBT-3) and fused-bibenzo[c]thiophene PHDTSi.

© 2021 The Author(s). Published by the Royal Society of Chemistry

RSC Advancesshi (409 cm1), compared to those of 1,10 -Si-4,40 -BBT (labsmax ¼
366 nm, lmax ¼ 420 nm, 3max ¼ 14 400 M1 cm1, F ¼ 0.41,
Stokes shi ¼ 3513 cm1 in toluene). It is worth noting here that
1,10 -Si-4,40 -BBT exhibits the labs
max in a shorter wavelength region
by 40 nm in comparison with that of the isomer 3,30 -bis(tertbutyldimethylsilyl)-1,10 -bibenzo[c]thiophene
3,30 -Si-1,10 -BBT
1
1
(labs
¼
406
nm,
3
¼
ca.
11
500
M
cm
in
CH
max
max
2Cl2) reported
by Cava et al.17a However, in order to use extensively andcommonly benzo[c]thiophene derivatives as p-building blocks inthe emitters, photosensitizers and semiconductors for organic
optoelectronic devices, it is necessary to develop eﬃcient andfacile synthetic methods for benzo[c]thiophene derivatives.
Therefore, the aim of this work is to provide the synthetic
strategy for 4,40 -bibenzo[c]thiophene derivatives and to reveal
their optical and electrochemical properties. With this aim, we
designed and synthesized unsubstituted 4,40 -bibenzo[c]thiophene (4,40 -BBT: abbr. as BBT-1) and its silyl-substituted derivatives 1,10 ,3,30 -Si-4,40 -BBT (abbr. as BBT-3) with two stericallyhindered tert-butyldimethylsilyl groups on each thiophene ringas well as 1,10 -Si-4,40 -BBT (abbr. as BBT-2) (Fig. 2). The characterization of BBT-1, BBT-2 and BBT-3 was successfully determined by FTIR, 1H and 13C NMR measurements, highresolution mass spectrometry (HRMS) analysis, photoabsorption and uorescence spectroscopy, cyclic voltammetry(CV) and density functional theory (DFT) calculations. This
work is the rst to achieve the synthesis, photophysical andelectrochemical characteristics of unsubstituted 4,40 -bibenzo[c]thiophene BBT-1. Moreover, we achieved the single-crystal X-ray
structural analysis of BBT-2 and BBT-3. Herein we report aneﬃcient synthetic method for the 4,40 -bibenzo[c]thiophene
derivatives and their photophysical properties in the solution
and the solid state, electrochemical properties and X-ray crystal
structures.

Results and discussion
Synthesis
4,40 -Bibenzo[c]thiophene (BBT-1) and its silyl-substituted
derivatives (BBT-2 and BBT-3) were synthesized according toa stepwise synthetic protocol (Scheme 1). Indeed, unsubstituted
4,40 -bibenzo[c]thiophene (BBT-1) was successfully prepared bytreatment of 1,10 ,3,30 -tetrahydro-[4,40 -bibenzo[c]thiophene] 2,20 -Scheme 1 Synthetic route to 4,40 -bibenzo[c]thiophene derivatives
4,40 -BBT (BBT-1), 1,10 -Si-4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT
(BBT-3).

RSC Adv., 2021, 11, 18870–18880 | 18871

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RSC AdvancesPapercharacterization of BBT-1, BBT-2 and BBT-3 was successfully
determined by FTIR, 1H and 13C NMR measurements andHRMS analysis. Therefore, this result proposes the stepwise
synthetic method for the introduction of substituents into thethiophene rings of the 4,40 -bibenzo[c]thiophene skeleton.

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X-ray crystal structures
A single-crystal X-ray structural analysis was successfully madefor BBT-2 and BBT-3 (Fig. 3), while unfortunately, we could not
obtain single crystals of BBT-1 with suﬃcient size to make the Xray structural analysis possible. The dihedral angles between
the two benzo[c]thiophene units in BBT-2 and BBT-3 are 50.7
and 67.7 (Fig. 3a–d), respectively, which show that the twounits in BBT-3 twist considerably due to the steric hindrance ofthe tert-butyldimethylsilyl groups at the 3,30 -positions,compared to those in BBT-2. The crystal structure of BBT-2 ismade up of dimer units composed of pairs of molecules (Fig. 3e
and g). There are two short interatomic contacts of less than
3.60 Å between a pair of molecules, that is, the interatomic
distance between C(3) in a benzo[c]thiophene unit and C(5) inthe other unit is ca. 3.58 Å. In the crystal structure of BBT-3, on
the other hand, there are no short p–p contacts of less than 3.60
Å between the neighboring molecules (Fig. 3f and h), whichindicates the absence of the p–p interactions between themolecules.
Photophysical properties in the solution and the solid stateFig. 3 Crystal structures of 1,10 -Si-4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si4,40 -BBT (BBT-3): (a) top view, (c) side view, (e) a top view of the pair ofmolecules and (g) molecular packing structure of BBT-2, and (b) top
view, (d) side view, (f) a top view of the pair of molecules and (h)molecular packing structure of BBT-3.

dioxide 1 (see ref. 20 for the synthesis) with lithium hexamethyldisilazide (LHMDS). In our previous work, we demonstrated
that BBT-2 with a tert-butyldimethylsilyl group on each thiophene ring was obtained by the reaction of 1 with tetramethylethylenediamine (TMEDA) and then nBuLi, followed bytreatment with tert-butyldimethylsilyl chloride (TBDMSCl).20 In
this work, we found that BBT-2 was also prepared by the reaction of BBT-1 with lithium diisopropylamide (LDA), followed bytreatment with TBDMSCl. It is worth noting here that the above
reactions from 1 or BBT-1 did not yield 3,30 -Si-4,40 -BBT with two
tert-butyldimethylsilyl groups at the 3,30 -positions. This result
indicates that the lithiation of BBT-1 preferentially occurs at the
1,10 -positions rather than the 3,30 -positions. On the other hand,
BBT-3 was not obtained directly from 1 even under the condition using TMEDA, nBuLi (10 eq.) and then TBDMSCl (10 eq.).
Finally, we prepared BBT-3 with two tert-butyldimethylsilylgroups on each thiophene ring by the reaction of BBT-1 or BBT-2
with LDA, followed by treatment with TBDMSCl. The

18872 | RSC Adv., 2021, 11, 18870–18880

The photoabsorption and uorescence spectra of BBT-1, BBT-2
and BBT-3 in toluene are shown in Fig. 4a, and their photophysical data are summarized in Table 1. The photoabsorption
spectra demonstrated that BBT-2 and BBT-3 exhibit an intense
photoabsorption band (labsmax ¼ 366 nm and 371 nm, respectively) with a relatively high 3max value (14 400 M1 cm1 and
21 300 M1 cm1, respectively) in a longer wavelength region by
7 nm and 12 nm, respectively, in comparison with that of BBT-1
1(labscm1), due to the electronmax ¼ 359 nm, 3max ¼ 7500 M
donating tert-butyldimethylsilyl group. The correspondinguorescent bands of the three uorophores appear in longer
wavelength regions in the order of BBT-1 (lmax ¼ 410 nm) < BBT2 (lmax ¼ 420 nm) < BBT-3 (lmax ¼ 451 nm), and thus, the Stokes

Fig. 4 (a) Photoabsorption (solid line) and ﬂuorescence (dotted line)

spectra of 4,40 -BBT (BBT-1), 1,10 -Si-4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si4,40 -BBT (BBT-3) in toluene. (b) Solid-state UV-vis diﬀuse reﬂection–
absorption (solid line) and ﬂuorescence (dotted line) spectra (lex ¼ 360nm) of the as-recrystallized BBT-1, BBT-2 and BBT-3.

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Table 1

Photophysical and electrochemical data and HOMO and LUMO energy levels of 4,40 -bibenzo[c]thiophene derivatives in the solution

Dye

a
labs
max /nm(3max/M1 cm1)lmaxb/nm (F)sc/ns

krd/s1knre/s1

knr/krf
Eox
onset /V

g
Eopt
/eV
g

HOMOh/eV

LUMOh/eV

4,40 -BBT
1,10 -Si-4,40 -BBT
1,10 ,3,30 -Si-4,40 -BBT

359 (7500)
366 (14 400)i
371 (21 300)410 (0.41)
420 (0.41)i
451 (0.36)

3.46
3.19i
3.59

1.18  108
1.29  108i
1.00  108

1.70  108
1.84  108i
1.78  108

1.44
1.43i
1.78

0.75
0.65i
0.54

3.16
3.11i
3.04

5.55
5.45i
5.34

2.39
2.34i2.30

aIn toluene. b In toluene. Fluorescence quantum yields (F) were determined by using a calibrated integrating sphere system (lex ¼ 359 nm, 366 nm
and 371 nm for 4,40 -BBT (BBT-1), 1,10 -Si-4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT (BBT-3), respectively). c Fluorescence lifetime. d Radiative rate+g
Optical energy
constant (kr ¼ F/s). e Nonradiative rate constant (knr ¼ (1  F)/s). f Onset (Eox
onset) versus Fc/Fc of the oxidation potential.
)
weredeterminedfromtheintersection
(393
nm,
399
nm
and
408
nm
for
BBT-1,BBT-2
and
BBT-3,
respectively)ofphotoabsorption
gaps (Eoptgand uorescence spectra in toluene. h Versus vacuum level. i Previous work (ref. 20).

shi (SS) values increase in the order of BBT-1 (3465 cm1) z
BBT-2 (3513 cm1) < BBT-3 (4781 cm1). The F values of BBT-1,
BBT-2 and BBT-3 are 0.41, 0.41 and 0.36, respectively, indicating
moderate uorescence properties. The relatively low F andlarge SS values of BBT-3 indicate signicant changes in themolecular and electronic structures between the ground andexcited states by the rotation or twisting of the two benzo[c]thiophene units, due to the steric hindrance of the tert-butyldimethylsilyl groups at the 3,30 -positions. The time-resolveduorescence spectroscopy of the three uorophores demonstrated that the uorescence lifetimes (s) are 3.46 ns for BBT-1,
3.19 ns for BBT-2 and 3.59 ns for BBT-3, and thus, there are little
diﬀerence in the s values between the three uorophores. The
radiative rate constant (kr ¼ 1.00  108 s1) for BBT-3 is slightly
smaller than those (1.18  108 s1 and 1.29  108 s1, respectively) for BBT-1 and BBT-2. In contrast, the nonradiative rate
constants (knr ¼ 1.70–1.84  108 s1) of the three uorophores
resemble each other. Consequently, the ratio of nonradiative
constant to radiative constant (knr/kr ¼ 1.78) for BBT-3 is larger
than those (1.44 and 1.43, respectively) for BBT-1 and BBT-2,
suggesting that the lower F value of BBT-3 is mainly due to thesmaller kr value compared to those of BBT-1 and BBT-2.
In order to investigate the solid-state photophysical properties of BBT-1, BBT-2 and BBT-3, we have measured the solidstate UV-Vis diﬀuse reection–photoabsorption and uorescence spectra for the solids (Fig. 4b), and their photophysicaldata are summarized in Table 2. Both BBT-2 and BBT-3 in thesolid state show a photoabsorption band at around 360 nm withan onset at 420–425 nm, which is similar to the corresponding
photoabsorption band of the two uorophores in toluene(Fig. 4a). On the other hand, the photoabsorption band of BBT-1

Table 2in the solid state is broadened in a longer wavelength region
with an onset of ca. 500 nm, in comparison with that in toluene.
The corresponding solid-state uorescence spectra revealed
that BBT-1 and BBT-2 show a uorescence band (l-solid¼
max
455 nm and 435 nm, respectively) in a longer wavelength regionby 45 nm and 15 nm, respectively, compared to those in toluene.
It is worth mentioning here that the uorescence band (lsolidmax ¼ 435 nm) of BBT-3 in the solid state appeared ina shorter wavelength region than that (lmax ¼ 451 nm) intoluene (Fig. 4a). The F-solid values of BBT-1, BBT-2 and BBT-3
in the solid state are <0.02, 0.22 and 0.25, respectively, which are
lower than those in toluene (Table 1). In particular, the uorescence properties of BBT-1 were strongly quenched in thesolid state. The bathochromic shis of labs
max and lmax and thelowering of F value by changing from the solution to the solidstate are quite common and explained in terms of the formationof intermolecular p–p interactions between the uorophores inthe solid state and consequent delocalization of excitons or
excimers.21 Thus, for BBT-1, the bathochromic shi of lmax andthe signicant lowering of F value by changing from thesolution to the solid state would be attributed to the formationof intermolecular p–p interactions between the uorophores inthe solid state. On the other hand, the relatively high F-solid
value of BBT-3 in the solid state is based on the fact that the
short p–p contacts of less than 3.60 Å between the neighboring
molecules were not observed in the crystal structure of BBT-3(Fig. 3f), which indicates the absence of the intermolecular p–p
interactions between the uorophores. Indeed, the four tertbutyldimethylsilyl groups at the 1,10 ,3,30 -positions can eﬀectively prevent the uorophores from forming intermolecular p–
p interactions in the solid state. In addition, for BBT-3, thePhotophysical data of 4,40 -bibenzo[c]thiophene derivatives in the solid stateDyelabs-solid
/nm
max

a
l-solid/nm (F-solid)maxs-solidb/ns

kr-solidc/s1

knr-solidd/s1

knr-solid/kr-solid

4,40 -BBT (BBT-1)
1,10 -Si-4,40 -BBT (BBT-2)
1,10 ,3,30 -Si-4,40 -BBT (BBT-3)

360
360
370

455 (<0.02)
435 (0.22)435 (0.25)

<1
1.61
2.33

—e
1.38  108
9.00  107—e
4.84  108
3.38  108

—e
3.55
3.76

a
Fluorescence quantum yields (F-solid) were determined by using a calibrated integrating sphere system (lex ¼ 360 nm for 4,40 -BBT (BBT-1), 1,10 -Si4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT (BBT-3)). b Fluorescence lifetime. c Radiative rate constant (kr-solid ¼ F-solid/s-solid). d Nonradiative rate
constant (knr-solid ¼ (1  F-solid)/s-solid). e Due to feeble solid-state uorescence properties.

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in the solid state are larger than those (1.84  108 s1 and 1.78
 108 s1, respectively) in toluene. The knr-solid/kr-solid values for
BBT-2 and BBT-3 in the solid state are 3.55 and 3.76, respectively, which are larger than those (1.43 and 1.78, respectively)
in toluene, suggesting that the non-radiative decay in the solidstate is accelerated. Consequently, the relatively low F-solid
values of BBT-2 and BBT-3 in the solid state are mainly due tothe larger knr-solid values compared to those in toluene.
Electrochemical properties
0

0

0

Fig. 5 Cyclic voltammograms of 4,4 -BBT (BBT-1), 1,1 -Si-4,4 -BBT
(BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT (BBT-3) in acetonitrile containing
0.1 M Bu4NClO4 at a scan rate of 100 mV s1. The arrow denotes the
direction of the potential scan.

hypsochromic shi of lmax by changing from the solution to thesolid state may be attributed to inhibition of structural relaxation in the excited state by restriction of the rotation or twistingof the two benzo[c]thiophenes in the solid state, due to the sterichindrance of the tert-butyldimethylsilyl groups at the 3,30 -positions between the neighboring uorophores. The time-resolveduorescence spectroscopy of the three 4,40 -bibenzo[c]thiophene
derivatives in the solid state demonstrated that the s-solid
values are 1.61 ns for BBT-2 and 2.33 ns for BBT-3, which areshorter than those (3.19 ns and 3.59 ns, respectively) in toluene.
However, the precise evaluation of the s-solid value (<1 ns) of
BBT-1 was diﬃcult due to its feeble solid-state uorescence
properties. The kr-solid values for BBT-2 and BBT-3 in the solidstate are 1.38  108 s1 and 9.00  107 s1, respectively, which
are almost equivalent to those (1.29  108 s1 and 1.00  108s1, respectively) in toluene. In contrast, the knr-solid values (4.84
 108 s1 and 3.38  108 s1, respectively) for BBT-2 and BBT-3

The electrochemical properties of BBT-1, BBT-2 and BBT-3 were
determined using CV in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (Bu4NClO4). The potentials wereinternally referenced to ferrocene/ferrocenium (Fc/Fc+). The
cyclic voltammograms of the three compounds are shown in
Fig. 5, and their electrochemical data and the HOMO andLUMO energy levels are summarized in Table 1. For all the threecompounds, an irreversible oxidation wave was observed at
0.88 V for BBT-1, 0.78 V for BBT-2 and 0.67 V for BBT-3, while
any obvious reduction wave did not appear within the potential
window. The oxidation waves for BBT-2 and BBT-3 are cathodically shied by 0.10 V and 0.21 V, respectively, compared tothat for BBT-1, indicating that the introduction of the tertbutyldimethylsilyl group into the benzo[c]thiophene skeleton
can lower the oxidation potential. The HOMO energy levels([Eoxonset + 4.8] eV) versus vacuum level were estimated from theonset potentials (Eox
onset ¼ 0.75 V for BBT-1, 0.65 V for BBT-2 and0.54 V for BBT-3) of the oxidation waves, and the LUMO energy
levels were estimated from the Eox
onset and intersections (opticalenergy gap: Eopt
¼ 3.16 eV for BBT-1, 3.11 eV for BBT-2 andg
3.04 eV for BBT-3) of the photoabsorption and uorescence
spectra in toluene. The HOMO energy level rises in the order of
BBT-1 (5.55 eV) < BBT-2 (5.45 eV) < BBT-3 (5.34 eV).

Fig. 6 (a) Optimized geometries (top and side views) of 4,40 -BBT (BBT-1), 1,10 -Si-4,40 -BBT (BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT (BBT-3) and (b)energy level diagram, HOMO and LUMO of BT and the three 4,40 -bibenzo[c]thiophene derivatives derived from DFT calculations at the B3LYP/631G(d,p) level. Numbers in parentheses are the experimental values.

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Similarly, the LUMO energy level rises in the order of BBT-1(2.39 eV) < BBT-2 (2.34 eV) < BBT-3 (2.30 eV). However,from BBT-1 to BBT-3, the rise of the HOMO energy level is larger
than that of the LUMO energy level. Consequently, the factreveals that the bathochromic shi of the photoabsorption
band from BBT-1 to BBT-2 and BBT-3 is mainly attributed to thedestabilization of the HOMO energy level through the introduction of the electron-donating tert-butyldimethylsilyl group
into the benzo[c]thiophene skeleton, resulting in a decrease in
the HOMO–LUMO band gap.

RSC AdvancesFig. 7 Photoabsorption spectra of BT, 4,40 -BBT (BBT-1), 1,10 -Si-4,40 BBT (BBT-2) and 1,10 ,3,30 -Si-4,40 -BBT (BBT-3) derived from TD-DFT
calculations.

Theoretical calculations
In order to examine the electronic structures of the 4,40 -bibenzo[c]thiophene derivatives, the molecular structures and molecular orbitals of BBT-1, BBT-2 and BBT-3 and benzo[c]thiophene(BT) as a reference were calculated using DFT at the B3LYP/631G(d,p) level22 (Fig. 6). The DFT calculations demonstratethat the calculated dihedral angles between the two benzo[c]thiophene units are 57.8 for BBT-1, 59.2 for BBT-2 and 96.8
for BBT-3, that is, the two units in BBT-3 twist considerably due
to the tert-butyldimethylsilyl groups at the 3,30 -positions,compared to those in BBT-1 and BBT-2 (Fig. 6a). Therefore, goodcorrelation was observed between the molecular structures
estimated by the DFT calculations and experimentally obtained
from the X-ray crystal structure analysis, although for BBT-3, thecalculated dihedral angle between the two benzo[c]thiophene
units is larger than that determined from the X-ray crystal
structure analysis (Fig. 3a and b). As shown in Fig. 6b, for thethree 4,40 -bibenzo[c]thiophene derivatives the HOMO are delocalized on each benzo[c]thiophene unit, as with the pattern of
BT. However, the LUMO for the three 4,40 -bibenzo[c]thiophene
derivatives are delocalized over the whole molecule through the
4,40 -positions. It was found that the HOMO energy levels of thethree 4,40 -bibenzo[c]thiophene derivatives are higher than that(5.38 eV) of BT, but their LUMO energy levels are lower thanthat (1.38 eV) of BT. The HOMO and LUMO energy levels risein the order of BBT-1 (5.29 eV and 1.55 eV) < BBT-2 (5.22 eVand 1.53 eV) < BBT-3 (5.07 eV and 1.52 eV), while from
BBT-1 to BBT-3, the rise of the HOMO energy level is larger thanthat of the LUMO energy level, resulting in a decrease in the
HOMO–LUMO band gap. Moreover, the time-dependent density
functional theory (TD-DFT) calculations indicate that thecalculated labs-calcdof BBT-1, BBT-2 and BBT-3 is 345 nm,
max
354 nm and 356 nm, respectively, which appears in a signicantly longer wavelength region than that (332 nm) of BT(Fig. 7). For BT and the three 4,40 -bibenzo[c]thiophene derivative, the S0 / S1 transitions are mainly attributed to the transitions from the HOMO to the LUMO (70% for BT, 67% for BBT1, 67% for BBT-2 and 68% for BBT-3). The corresponding
oscillator strength (f) value increases in the order of BT (0.07) <BBT-1 (0.11) < BBT-3 (0.14) < BBT-2 (0.16), and indeed, thecalculated 3calcd value also increases in the order of BT(3000 M1 cm1) < BBT-1 (7800 M1 cm1) < BBT-3(13 200 M1 cm1) < BBT-2 (15 600 M1 cm1). Thus, the DFT
calculations reveal that the bathochromic shi of the photoabsorption band from BBT-1 to BBT-2 and BBT-3 is ascribable to© 2021 The Author(s). Published by the Royal Society of Chemistry

the destabilization of the HOMO energy level through theintroduction of the electron-donating tert-butyldimethylsilylgroup into the benzo[c]thiophene skeleton. Consequently, theDFT calculations are in good agreement with the experimental
results from the CV and the photoabsorption and uorescence
spectral analyses, although the f and 3calcd values of BBT-3 areslightly lower than those of BBT-2, which is opposite in thevalues to the experimental results.

Conclusions
We have achieved a facile synthesis of 4,40 -bibenzo[c]thiophene
derivatives, unsubstituted 4,40 -bibenzo[c]thiophene 4,40 -BBT
and its silyl-substituted derivatives 1,10 -Si-4,40 -BBT and 1,10 ,3,30 Si-4,40 -BBT with one or two tert-butyldimethylsilyl groups oneach thiophene ring, and revealed their photophysical properties in the solution and in the solid state and electrochemical
properties. It was found that the bathochromic shi of thephotoabsorption band from 4,40 -BBT to 1,10 -Si-4,40 -BBT and
1,10 ,3,30 -Si-4,40 -BBT is mainly attributed to the destabilization of
the HOMO energy level through the introduction of theelectron-donating tert-butyldimethylsilyl group into the benzo[c]thiophene skeleton, resulting in a decrease in the HOMO–
LUMO band gap. The three 4,40 -bibenzo[c]thiophene derivatives
exhibit moderate uorescence properties in the solution (F ¼
ca. 0.4). Moreover, the silyl-substituted derivatives 1,10 -Si-4,40 BBT and 1,10 ,3,30 -Si-4,40 -BBT show relatively high F-solid value(0.22 and 0.25, respectively) in the solid state, compared to 4,40 BBT (F-solid < 0.02). The DFT calculations demonstrate that the
HOMO energy levels of the 4,40 -bibenzo[c]thiophene derivatives
are higher than that of benzo[c]thiophene (BT), while their
LUMO energy levels are lower than that of BT. Consequently,
this work opened up a new way for not only synthetic method
and photophysical and electrochemical properties of 4,40 bibenzo[c]thiophene derivatives with the diﬀerent substituentson the thiophene rings, but also development of fused-bibenzo[c]thiophene derivatives as functional dyes.

Experimental
GeneralMelting points were measured with an AS ONE ATM-02. IR
spectra were recorded on a SHIMADZU IRTracer-100 by ATR

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RSC Advances
method. 1H NMR and 13C NMR spectra were recorded ona Varian-500 FT NMR spectrometer and NMR solvent was used
as an external standard for calibration. High-resolution mass
spectral data by APCI and GC-EI were acquired on a Thermo
Fisher Scientic LTQ Orbitrap XL and JEOL JMS-T100 GCV 4G,
respectively. Recycling gel permeation chromatography (GPC)
were preformed using RI-detector (GL Science RI 704) andpump (GILSON 307 PUMP) with column (Shodex GPC H-2001L).
Photoabsorption spectra of solution were observed with a Shimadzu UV-3600 plus spectrophotometer. Photoabsorption
spectra of the solid were recorded by a Shimadzu UV-3600 plus
spectrophotometer with a calibrated integrating sphere system.
Fluorescence spectra of solution and the solid were measured
with a HORIBA FluoroMax-4 spectrouorometer. The uorescence quantum yields in solution and in the solid state were
determined using a HORIBA FluoroMax-4 spectrouorometerwith a calibrated integrating sphere system. Fluorescence decay
measurements were performed on a HORIBA DeltaFlex
modular uorescence lifetime system, using a Nano LED pulsed
diode excitation source (370 nm). Cyclic voltammetry (CV)curves were recorded in acetonitrile/Bu4NClO4 (0.1 M) solution
at a scan rate of 100 mV s1 with a three-electrode system
consisting of Ag/Ag+ as the reference electrode, a Pt plate as theworking electrode and a Pt wire as the counter electrode using
an Electrochemical Measurement System HZ-7000 (HOKUTO
DENKO).

Synthesis
4,40 -Bibenzo[c]thiophene (4,40 -BBT). To a THF solution (20mL) of 1,10 ,3,30 -tetrahydro-[4,40 -bibenzo[c]thiophene] 2,20 dioxide (1)20 (0.20 g, 0.66 mmol) under a nitrogen atmosphere at
0  C was added dropwise a 1.3 M THF solution of lithium hexamethyldisilazide (3.1 mL, 4.0 mmol). Aer stirring for 3 h, thereaction mixture was quenched with water, and then, thesolution was extracted with ethyl acetate. The ethyl acetate
extract was dried over anhydrous MgSO4, ltrated and concentrated. Recycling GPC (toluene as eluent) was performed to give
4,40 -BBT (0.09 g, yield 51%) as a light-yellow solid; decomposedat around 145  C; FT-IR (ATR): ñ ¼ 3103, 1686, 1172, 864 cm1;
1
H NMR (500 MHz, acetone-d6): d ¼ 7.19–7.24 (m, 4H), 7.59 (dd,J ¼ 1.1 and 3.4 Hz, 2H), 7.75 (dt, 2H), 8.00 (d, J ¼ 3.4 Hz,2H) ppm; 13C NMR (125 MHz, acetone-d6): d ¼ 117.96, 118.31,
122.51, 124.29, 124.35, 134.56, 138.45, 139.91 ppm; HRMS (GCEI): m/z (%): [M  H+] calcd for C16H9S2, 265.01457; found265.01442.
1,10 -Bis(tert-butyldimethylsilyl)-4,40 -bibenzo[c]thiophene(1,10 -Si-4,40 -BBT). (Method A: previous work)20 To a THF solution(60 mL) of 1 (0.50 g, 1.65 mmol) under an argon atmosphere at
80  C was added tetramethylethylenediamine (1.48 g, 9.92mmol). Aer stirring for 30 min, a 1.6 M hexane solution ofnBuLi (6.20 mL, 9.92 mmol) was added dropwise for 30 min,and then, a THF solution (10 mL) of tert-butyldimethylsilyl
chloride (0.75 g, 4.96 mmol) was added dropwise for 20 min.
Aer stirring for 12 h at room temperature, the reaction mixture
was quenched with water, and then, the solution was extractedwith ethyl acetate. The ethyl acetate extract was dried over

18876 | RSC Adv., 2021, 11, 18870–18880

Paper
anhydrous MgSO4, ltrated and concentrated. The residue waschromatographed on silica gel (hexane as eluent), and then,recycling GPC (toluene as eluent) was performed to give 1,10 -Si4,40 -BBT (0.33 g, yield 40%) as a light-yellow solid. (Method B)
To a THF solution (0.8 mL) of 4,40 -BBT (0.14 g, 0.53 mmol)
under a nitrogen atmosphere at 0  C was added dropwise
a 1.0 M hexane/THF solution of lithium diisopropylamide (1.6mL, 1.6 mmol). Aer stirring for 3 h, a THF solution (0.3 mL) of
tert-butyldimethylsilyl chloride (0.24 g, 1.6 mmol) was added
dropwise. The reaction mixture was further stirred for 12 h. The
reaction mixture was quenched with water, and then, thesolution was extracted with ethyl acetate. The ethyl acetate
extract was dried over anhydrous MgSO4, ltrated and concentrated. The residue was chromatographed on silica gel(dichloromethane : hexane ¼ 1 : 1 as eluent) and then on
alumina (dichloromethane : hexane ¼ 1 : 3 as eluent) to give
1,10 -Si-4,40 -BBT (0.095 g, yield 36%) as a yellow solid; mp 153–
155  C; FT-IR (ATR): ñ ¼ 2949 (aliphatic C–H str.), 2926
(aliphatic C–H str.), 2855 (aliphatic C–H str.), 1458 (Si–C(Ar)str.), 1360, 1250 (aliphatic Si–CH3 str.), 804 (aliphatic Si–CH3str.) cm1; 1H NMR (500 MHz, CDCl3): d ¼ 0.53 (s, 12H), 0.99 (s,18H), 7.18–7.23 (m, 4H), 7.78 (d, J ¼ 8.1 Hz, 2H), 7.81 (s,2H) ppm; 13C NMR (125 MHz, CDCl3): d ¼ 3.73, 18.37, 26.92,
123.17, 123.25, 123.50, 124.03, 128.34, 134.88, 140.20,
145.12 ppm; HRMS (APCI): m/z (%): [M + H+] calcd for
C28H39S2Si2, 495.20262; found 495.20303.
1,10 ,3,30 -Tetrakis(tert-butyldimethylsilyl)-4,40 -bibenzo[c]thiophene (1,10 ,3,30 -Si-4,40 -BBT). (Method A) To a THF solution (0.8mL) of 4,40 -BBT (0.14 g, 0.53 mmol) under a nitrogen atmosphere at 0  C was added dropwise a 1.0 M hexane/THF solutionof lithium diisopropylamide (4.2 mL, 4.2 mmol). Aer stirringfor 3 h, a THF solution (1.0 mL) of tert-butyldimethylsilyl chloride (0.62 g, 4.2 mmol) was added dropwise. The reaction
mixture was further stirred for 12 h. The reaction mixture wasquenched with water, and then, the solution was extracted with
ethyl acetate. The ethyl acetate extract was dried over anhydrous
MgSO4, ltrated and concentrated. The residue was chromatographed on silica gel (dichloromethane : hexane ¼ 1 : 3 aseluent) to give 1,10 ,3,30 -Si-4,40 -BBT (0.1 g, yield 26%) as a whitesolid. (Method B) To a THF solution (0.3 mL) of 1,10 -Si-4,40 -BBT
(0.07 g, 0.14 mmol) under a nitrogen atmosphere at 0  C was
added dropwise a 1.0 M hexane/THF solution of lithium diisopropylamide (1.4 mL, 1.4 mmol). Aer stirring for 3 h, a THF
solution (0.3 mL) of tert-butyldimethylsilyl chloride (0.21 g, 1.4mmol) was added dropwise. The reaction mixture was further
stirred for 14 h. The reaction mixture was quenched with water,
and then, the solution was extracted with dichloromethane. The
dichloromethane extract was dried over anhydrous MgSO4, ltrated and concentrated. The residue was chromatographed on
silica gel (hexane as eluent) to give 1,10 ,3,30 -Si-4,40 -BBT (0.04 g,
yield 40%) as a white solid; mp 172–173  C; FT-IR (ATR): ñ ¼
2955 (aliphatic C–H str.), 2926 (aliphatic C–H str.), 2855
(aliphatic C–H str.), 1470 (Si–C(Ar) str.), 1362, 1250 (aliphatic Si–
CH3 str.), 804 (aliphatic Si–CH3 str.) cm1; 1H NMR (500 MHz,
acetone-d6): d ¼ 0.87 (s, 3H), 0.04 (s, 3H), 0.57 (s, 3H), 0.62 (s,3H), 0.77 (s, 9H), 0.99 (s, 9H), 7.02 (dd, J ¼ 1.2 and 6.6 Hz, 2H),
7.19–7.22 (m, 2H), 7.99 (dd, J ¼ 8.8 Hz, 2H) ppm; 13C NMR (125© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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MHz, acetone-d6): d ¼ 3.78, 3.61, 18.97, 19.32, 27.15, 28.25,
123.12, 125.27, 128.83, 136.01, 136.12, 139.28, 147.78,
149.76 ppm; HRMS (APCI): m/z (%): [M+] calcd for C40H66S2Si4,
722.36775; found 722.36843.

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X-ray crystallographic analysis
The reection data were collected at 100 K on a Bruker AXS
SMART APEX II ULTRA diﬀractometer using monochromated
Mo-Ka (l ¼ 0.71073 Å). The structure was solved by the SHELXT
2014/5 method and rened based on full-matrix least squares
on F2 using SHELXL-2017/1. The non-hydrogen atoms wererened anisotropically. Hydrogen atoms were xed geometrically and not rened. Crystallographic data have been depositedin the Cambridge Crystallographic Data Centre (CCDC
2055734† for 1,10 -Si-4,40 -BBT and CCDC 2055735† for 1,10 ,3,30 Si-4,40 -BBT, respectively).
Crystal of 1,10 -Si-4,40 -BBT. A suitable single crystal of 1,10 -Si0
4,4 -BBT was grown by slow evaporation of acetone/ethanol
solution at room temperature for several days, as colorless
block crystal, air stable. Crystallographic data: C28H38S2Si2, M ¼
494.88, monoclinic, a ¼ 17.6427(9), b ¼ 11.2395(6), c ¼
14.1794(7) Å, b ¼ 95.121(1) , V ¼ 2800.5(2) Å3, Dcalcd ¼1.174 g cm3, space group P21/c (no. 14), Z ¼ 4, 17 974 reections measured, 6739 unique (Rint ¼ 0.027), which were used in
all calculations. The nal R1(reections) ¼ 0.0292(5949) [I >2s(I)], wR2(reections) ¼ 0.0743(6739). GOF ¼ 0.955 (Table S1†).
Crystal of 1,10 ,3,30 -Si-4,40 -BBT. A suitable single crystal of
0
1,1 ,3,30 -Si-4,40 -BBT was grown by slow evaporation of acetone/
ethanol solution at room temperature for several days, ascolorless plate crystal, air stable. Crystallographic data:
C40H66S2Si4, M ¼ 723.40, triclinic, a ¼ 7.5572(9), b ¼
13.6372(16), c ¼ 22.295(3) Å, a ¼ 72.5230(10) , b ¼ 89.9300(10) ,g ¼ 81.0900(10) , V ¼ 2162.7(4) Å3, Dcalcd ¼ 1.111 g cm3, space (no. 2), Z ¼ 2, 10 428 reections measured, 10 234
group P1
unique (Rint ¼ 0.041), which were used in all calculations. The
nal R1(reections) ¼ 0.0449(7519) [I > 2s(I)], wR2(reections) ¼
0.1092(10 234). GOF ¼ 1.011 (Table S1†).

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
This work was supported by Grants-in-Aid for ScienticResearch (B) from the Japan Society for the Promotion of
Science (JSPS) KAKENHI Grant Number 19H02754.

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