Optoelectronic and thermal properties of highly fluorescence emissive 2,2′-distyryl-[3,3′]-bithiophenes

Article abstract of DOI:10.1016/j.dyepig.2017.01.042

Five newly designed organic small molecules (DSBTs, 1–5) with 3,3′-bithienyl unit had been successfully synthesized with 3-bromothiophene as the starting material. In both UV–vis absorption and fluorescence emission investigation, compound 3 with nitro-substitution exhibited the largest bathochromic shift and narrowest optical bandgap among 1–5. Further photophysical studies showed that 3 had obvious solvatochromism and aggregation-induced emission (AIE) property. The electrochemical properties and thermal stabilities studies demonstrated that 3 had high electron affinity and good thermal stability, which in combination with favorable optical properties indicated the prospect of 3 as a promising material in optoelectronic devices. Finally, theoretical calculations on structures, frontier molecular orbitals, UV–vis spectra and electronic transitions gave a good acknowledge of the relationship between optoelectronic properties and molecular structures.

Full text of DOI:10.1016/j.dyepig.2017.01.042

Accepted Manuscript
Optoelectronic and thermal properties of highly fluorescence emissive 2,2′-distyryl-
[3,3′]-bithiophenes
Fengkai Cheng, Yan Yin, Guozhen Zhang, Yuan Wang, Wei Deng, Fanhong Wu
PII:
S0143-7208(16)30935-4
10.1016/j.dyepig.2017.01.042
DYPI 5741
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 October 2016
Accepted Date: 17 January 2017
Please cite this article as: Cheng F, Yin Y, Zhang G, Wang Y, Deng W, Wu F, Optoelectronic and
thermal properties of highly fluorescence emissive 2,2′-distyryl-[3,3′]-bithiophenes, Dyes and Pigments
(2017), doi: 10.1016/j.dyepig.2017.01.042.
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ACCEPTED MANUSCRIPT
S
NO2
O2N
S
Obvious stokes shifts, solvatochromism and AIE behavior were observed.

ACCEPTED MANUSCRIPT
Optoelectronic and thermal properties of highly
fluorescence emissive 2,2’-distyryl-[3,3']-bithiophenes
Fengkai Cheng a, Yan Yin a,b,*, Guozhen Zhang c,*, Yuan Wanga, Wei Deng a,
Fanhong Wua
a School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Hai
Quan Rd., Shanghai 201418, PR China
b Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic
Chemistry, Chinese Academy of Science, 345 Ling Ling Rd. Shanghai 200032, PR China
c Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical
Physics, University of Science and Technology of China, Hefei 230026, Anhui, PR China
*Corresponding authors: Tel:+86 21 60877220, Fax:+86 21 60877230 (Y. Yin)
E-mails: yinyan@sit.edu.cn(Y. Yin); guozhen@ustc.edu.cn (G. Z. Zhang)
Abstract
Five newly designed organic small molecules (DSBTs, 1-5) with 3,3'-bithienyl unit had been
successfully synthesized with 3-bromothiophene as the starting material. In both UV-vis
absorption and fluorescence emission investigation, compound 3 with nitro-substitution exhibited
the largest bathochromic shift and narrowest optical bandgap among 1-5. Further photophysical
studies showed that 3 had obvious solvatochromism and aggregation-induced emission (AIE)
property. The electrochemical properties and thermal stabilities studies demonstrated that 3 had
high electron affinity and good thermal stability, which in combination with favorable optical
properties indicated the prospect of 3 as a promising material in optoelectronic devices. Finally,
theoretical calculations on structures, frontier molecular orbitals, UV-vis spectra and electronic
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transitions gave a good acknowledge of the relationship between optoelectronic properties and
molecular structures.
Keywords: 3,3’-Bithiophene; Solvatochromism; Aggregation-induced emission (AIE);
Photoluminescence; Fluorescence emission
Introduction
Research on highly fluorescence emissive organic molecules has received increasing interest in
recent year for the various applications in sensors, displays, storage and photoelectronic
devices[1-9]. But significant aggregation-caused quenching (ACQ) largely limits practical
applications of fluorophores while aggregation in the condensed phase[10]. Since the discovery of
aggregation-induced emission (AIE) phenomenon by Tang group in 2001[11], the studies of AIE
systems have opened a new route to develop highly-emissive organic materials which usually
suffer from the severe ACQ effect[12-15]. AIE active molecules basically have two or more
conjugated-aromatic moieties joined through rotatable C-C single bond. This free intramolecular
rotation along the C-C single bond occurs only in the solution, whereas in the aggregated state it
gets inhibited. The C-C single bond rotation is responsible for AIE phenomenon[16-18].
Conjugated organic compounds with thiophene ring have attracted a lot of attention in recent
years for the frequency applications in organic field effect transistors, organic light-emitting
diodes, and dye-sensitized solar cells[19-29], but there were few reports of 3,3'-bithiophenes
although molecules with 3,3'-bithiophene backbone were of π-conjugated properties and the
rotation of C-C single bond between the two thiophene rings was restricted while alfa or
beta-positions of 3,3'-bithiophene were substituted. As a continuous research interest on
3,3'-bithiophenes[30], 2,2’-distyryl-[3,3']-bithiophenes (DSTBs, 1-5) were synthesized from
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commercially available 3-bromothiophene, and the photophysical, electrochemical, and thermal
properties of DSTBs were then investigated. Finally, quantum chemistry calculations were studies
and the theoretical results were in good accordance with the experimental results. According to
our results, compound 3 was a competitive candidate for optoelectronic devices owing to its
strong UV-vis absorption and fluorescence emission, high electron affinity and good thermal
stability.
2. Experimental
2.1. General methods
All reagents and chemicals were purchased from commercial suppliers and used without further
purification unless otherwise specified. Ether and N,N-dimethyl formamide (DMF) were pre-dried
and distilled before used. Thin layer chromatography (TLC) analyses were performed with 1 mm
thick precoated silica plates (shanghai version). Column chromatography was carried out on silica
gel (200-300 mesh). NMR spectra were recorded on Bruker 500MHz with chloroform-d (CDCl3)
or dimethysulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as the internal
standard. The HRMS spectra were acquired on a Solaril X70 FT-MS apparatus. The IR spectra
were performed with a Nicolet 6700 FT-IR spectrometer in the rage of 375-7800 cm-1. The
melting points were measured on a SGW X-4 melting point instrument. The UV-vis absorption
spectra were performed with a Phenix UV1902PC spectrophotometer. Fluorescence spectra were
recorded on a Hitachi F-4600 fluorescence spectrophotometer equipped with a 150 W xenon lamp
as the excitation source. Differential scanning calorimetry (DSC) and thermo-gravimetric (TG)
analysis were performed on Netzsch STA 449F3 analyzers at a heating rate of 10 oC min-1. Cyclic
voltammetry (CV) study was performed in dichloromethane solution with 0.1
M
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tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as the supporting electrolyte. Pt disk,
Platinum wire and saturated calomel electrode (SCE) were used as working electrode, counter
electrode and reference electrode, respectively. Ferrocene-ferrocenium (Fc/Fc+) redox couple
Fc
1/ 2
(E
= 0.52 V vs SCE) was selected as the internal standard. All the measurements were
recorded at room temperature with a scan rate of 50 mV s-1. The fluorescence quantum yields (Φx)
were recorded at room temperature and calculated using the following equations: Фx = Фst * (Ast /
Ax) * (Ix / Ist) * (nx / nst)2. Ast and Ax were the absorbencies of reference sample and synthesized
compounds, respectively. Ist and Ix were the integration of PL emission spectra for reference
sample and synthesized compounds, respectively. nst and nx were the refractive index of the
solvents used for the reference sample and synthesized compounds, respectively. Φst was the
fluorescence quantum yields of reference sample, and 9,10-bis-phenylethynyl-anthracene in
cyclohexane was used as the reference.
2.2. Theoretical calculations
All quantum chemistry calculations were performed on Gaussian 09 program [31]. Geometry
optimizations of all molecules were carried out in gas phase at the B3LYP/6-31G(d) level [32].
The following vibrational frequency calculations at the same level confirmed that these structures
were genuine minima on the potential energy surface (PES). Then time-dependent density
functional theory (TD-DFT)[33] calculations were conducted on the basis of the optimized
structures to simulate their excited state properties that can predict their UV-vis absorptions.
TD-DFT calculations were carried out at the B3LYP/6-31+G(2df,p) level in conjunction with a
polarizable continuum model (PCM)[34] of chloroform that accounts for the solvation effect
implicitly in the experiment setup.
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2.3. Synthesis of compounds 1-5
A mixture of [3,3'-bithiophene]-2,2'-dicarbaldehyde (1 mmol), triphenylphosphonium bromide
salt (2.2 mmol), and DBU (2.2 mmol) in CHCl3 (16 mL) was refluxed for 6 h. Then the reaction
mixture was quenched with 1 mol/L HCl (30 mL). The organic phase was extracted with CH2Cl2
(3 × 30 mL) and the combined organic phases were washed with saturated sodium carbonate
solution (15 mL) and NaCl solution (30 mL), dried over anhydrous sodium sulfate, and
concentrated under reduced pressure to give a residue. Finally, the residue was purified through
column chromatography on silica gel.
o
1
2,2'-Distyryl-[3,3']bithiophenyl (1). Yellow-green solid. Yield: 62%. Mp: 52-54 C. H NMR
(500 MHz, DMSO-d6) δ 7.61 (d, J = 5.0 Hz, 2H), 7.39 (d, J = 5.0 Hz, 4H), 7.27 (d, J = 5.0 Hz,
4H), 7.07 (d, J = 5.0 Hz, 2H), 7.04 (d, J = 5.0 Hz, 2H), 6.64 – 6.55 (m, 2H), 6.44 (dd, J1 = 15.0 Hz,
J2 = 10.0 Hz, 2H). 13C NMR (125 MHz, DMSO-d6) δ 138.3, 136.94, 134.81, 130.92, 129.21,
129.11, 128.27, 126.60, 125.35, 121.31. IR (KBr, cm-1): 3102, 3057, 3025, 2923, 2852, 1662
(C=C on olefinic), 1597, 1491, 1449, 1263, 862, 693. HRMS calcd for C24H19S2 [M+H]+:
371.0806, found: 371.0809.
2,2'-Bis-[2-(4-cyano-phenyl)-vinyl]-[3,3']bithiophenyl (2). Dark green solid. Yield: 44%. Mp:
o
140-142 C. 1H NMR (500 MHz, CDCl3) δ 7.60 (d, J = 10.0 Hz, 4H), 7.40 (d, J = 10.0 Hz, 4H),
7.18 (d, J = 5.0 Hz, 2H), 6.93 (d, J = 5.0 Hz, 2H), 6.54 (dd, J1 = 10.0 Hz, J2 = 5.0 Hz, 4H). 13C
NMR (125 MHz, CDCl3) δ 141.85, 136.68, 134.76, 132.17, 129.53, 129.37, 128.12, 125.27,
124.80, 118.93, 111.04. IR (KBr, cm-1): 3102, 2958, 2920, 2310, 2274 (C≡N), 2225, 1601 (C=C
on olefinic), 1559, 1541, 1457, 852, 669. HRMS calcd for C26H17N2S2 [M+H]+: 421.0831, found:
421.0834.
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2,2'-Bis-[2-(4-nitro-phenyl)-vinyl]-[3,3']bithiophenyl (3). Dark red solid. Yield: 84%. Mp:
o
185-187 C. 1H NMR (500 MHz, CDCl3) δ 8.17 (d, J = 10.0 Hz, 4H), 7.43 (d, J = 10.0 Hz, 4H),
7.19 (d, J = 5.0 Hz, 2H), 6.94 (d, J = 9.0 Hz, 2H), 6.61 (d, J = 10.0 Hz, 2H), 6.54 (d, J = 10.0 Hz,
2H). 13C NMR (125 MHz, CDCl3) δ 146.97, 143.88, 136.77, 134.68, 129.64, 129.45, 127.78,
125.43, 125.23, 123.71. IR (KBr, cm-1): 3103, 3076, 2955, 2924, 2851, 1618 (C=C on olefinic),
1590, 1510 (N-O), 1339, 855, 715. HRMS calcd for C24H18N2O4S2 [M+H]+: 462.0604, found:
462.0607.
2,2'-Bis-[2-(4-tert-butyl-phenyl)-vinyl]-[3,3']bithiophenyl (4). Red solid. Yield: 55%. Mp:
76-78 oC. 1H NMR (500 MHz, DMSO-d6) δ 7.44 – 7.28 (m, 8H), 7.12 – 6.89 (m, 4H), 6.54 (d, J =
10.0 Hz, 2H), 6.41 (d, J = 10.0 Hz, 1H), 6.37 (d, J = 10.0 Hz, 1H), 1.27 (s, 18H). 13C NMR (125
MHz, DMSO-d6) δ 150.83, 138.75, 133.97, 130.58, 129.34, 128.84, 126.42, 125.90, 125.72,
125.57, 34.82, 31.57. IR (KBr, cm-1): 3103, 3024, 2961, 2927, 2867, 1670 (C=C on olefinic),
1507, 1458, 955, 852, 707, 664. HRMS calcd for C32H35S2 [M+H]+: 483.2101, found: 483.2095.
2,2'-Bis-[2-(4-methoxy-phenyl)-vinyl]-[3,3']bithiophenyl (5). Orange solid. Yield: 39%. Mp:
56-58 oC. 1H NMR (500 MHz, CDCl3) δ 7.30 (d, J = 5.0 Hz, 4H), 7.11 (d, J = 5.0 Hz, 2H), 6.99 (d,
J = 5.0 Hz, 2H), 6.89 (d, J = 10.0 Hz, 4H), 6.55 – 6.44 (m, 4H), 3.85 (s, 6H). 13C NMR (125 MHz,
CDCl3) δ 159.14, 136.20, 135.99, 130.25, 129.58, 129.45, 129.04 124.03, 121.97, 113.83, 55.24.
IR (KBr, cm-1): 3103, 3000, 2955, 2932, 2834, 1663 (C=C on olefinic), 1605, 1510, 1462, 1248
(C-O), 1174, 1033, 849, 709. HRMS calcd for C26H22O2S2 [M+H]+: 431.1132, found: 431.1134.
3. Results and discussion
The synthetic route of targeted compounds 1-5 are depicted in Scheme 1. Intermediates
3,3’-bithiophene (6), 2,2'-dibromo-3,3'-bithiophene (7), and [3,3'-bithiophene]-2,2'-dicarbaldehyde
6

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(8) were prepared according to the previous literature [35], and triphenylphosphonium bromide
salts were obtained from the corresponding benzyl bromides and triphenylphosphine according to
Krohnke procedure[36]. Finally, targeted compounds 1-5 were obtained from 8 and corresponding
phosphonium salts by Wittig reaction [37-39] and characterized by 1H NMR, 13C NMR, FT-IR and
HRMS. Although compounds 1-5 shared the same 2,2'-distyryl-[3,3'] bithiophenyl π-conjugated
central core, they have different small substitutions on the phenyl rings. Then the highly pure
products 1-5 were applied to the following optoelectronic and thermal investigations.
Scheme 1. Synthesis of compounds 1-5
3.1. Optical properties
Fig.1. depicted the absorption and emission spectra of 1-5 in chloroform solution at room
temperature and the photophysical data were collected in Table 1. All compounds exhibited a
strong absorption maximum (λmax) in the visible region (327 - 383 nm) corresponding to the
intramolecular charge transfer (ICT) between the donating unit and acceptor moiety and a
relatively weak peak in the near-UV region (~290 nm) due to the π-π* transitions of these π
conjugated compounds. The λmax in the visible region red- shifted from 327 nm to 332 nm, 338 nm,
344 nm, and 383 nm for 1, 4, 5, 2, and 3, respectively, which can be ascribed to the different
electron density dispersed on the molecules. Obviously, 3 substituted with NO2 had the strongest
electron-acceptor ability, which enhanced the ICT between the donating unit and NO2 acceptor
moiety, and as a result to shift the λmax to the long wavelength (ICT behavior of 3 was further
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studied in theoretical calculations section as Fig. 8).The molar extinction coefficient (ε) of the
strong absorption maximum appeared at 25.3 - 41.2 mM-1 cm-1, indicating that these metal-free
organic compounds had excellent light harvesting ability, and 3 with NO2 group had the largest
light harvesting ability (ε= 41.2 mM-1 cm-1) among compounds 1-5. The optical band gap (E ogpt
)
in CHCl3 solution was determined from the onset of the absorption spectra. Both small electronic
withdrawing substitution (CN or NO2) and electronic donating substitution (t-Bu or OCH3) at the
π–conjugated system caused decreased E ogpt values. 3 with very electronic withdrawing group
(NO2) had the narrowest optical band gap (E opt = 2.62eV for 3 vs 3.09eV for 1, 2.93eV for 2,
g
3.03eV for 4, and 3.00eV for 5).
All DSBTs showed obvious fluorescence emission and 3 exhibited distinct bathochromic shift
(emission maxima λem = 571 nm for 3 vs. 445 nm for 1, 484 nm for 2, 446 nm for 4, and 458 nm
for 5) as well as different fluorescence intensity (Table 1 and Fig. 1b). Irradiated with UV lamp
(λex = 365 nm), the fluorescence color of compounds 1, 2, 4, and 5 in CHCl3 solution (1.15×10-5 M)
at room temperature was blue, however 3 emitted yellow fluorescence (Fig. 3). Introduction of
small electronic donating groups was favored for ΦF values (ΦF = 0.48 for 5 and 0.67 for 4 vs.
0.28 for 1) and electronic withdrawing substitution caused low ΦF values (ΦF = 0.06 for 2 with
–CN group). However compound 3 with very electronic –NO2 group exhibited the similar ΦF
values as 1 probably due to the strong influence of nitro group on ICT as well as the rotations of
the conjugated C-C single band. The large Stokes shifts (7699 - 8597 cm-1) were observed, which
indicated large differences (geometric, electronic and vibrational) between the Franck-Condon
state and the emissive excited state.
Table 1
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Photophysical data of 1-5a
E ogpt b,eV
UV-vis λmax, nm
λ
em,nm Stokes shiftsc, cm-1
d
Compd.
ΦF
(ε, mM-1 cm-1)
288 (28.4), 327 (30.1)
287 (24.7), 344 (35.5)
282 (31.1), 383 (41.2)
290 (19.1), 332 (25.3)
289 (33.3), 338 (40.1)
1
2
3
4
5
3.09
2.93
2.62
3.03
3.00
445
8109
8409
8597
7699
7752
0.28
0.06
0.29
0.67
0.48
484
571
446
458
a All spectra were recorded in CHCl3 solution (1.15×10-5 M) at room temperature.
b Calculated from the onset of the absorption spectra in solution.
c Calculated with the less energetic absorption band.
d Determined relative to 9,10-bis-phenylethynyl-anthracene in cyclohexane (ΦF = 1.00).
Fig. 1. (a) Absorbance spectra (b) Emission spectra of 1-5 in CHCl3 at room temperature.
3.2. Solvatochromism
For 3 exhibited special optical properties among 1-5, such as obviously red shift in both UV-vis
absorption and fluorescence emission, the largest stokes shifts and the narrowest optical band gap,
the absorption and emission behavior of 3 in a variety of aprotic solvents were then explored to
obtain further insights into the photophysics of 3 (Table 2). No significant solvent polarity
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dependent spectral shifts were observed in the UV-vis absorption spectra, which indicated that the
ground states were not significantly stabilized by the coordination of solvent molecules due to a
relatively small dipole moment [40-41].
Fig. 2 showed the emission spectra of 3 in a variety of aprotic solvents with 396 nm as the
excitation wavelength and the emission data were collected in Table 2. With the increment of
solvent polarity, a distinctly shift of λem was observed (λem = 484 nm - 632 nm)). The emission
intensity enhanced with the increment of the solvent polarity from hexane to chloroform, but a
further increment of solvent polarity also lead to a sharp decrease until no obvious emission was
observed in CH3CN solution. The ΦF of 3 had the same rule as emission intensity and the highest
ΦF was 0.29 with CHCl3 as the solution. The similar solvatochromic behavior was observed by
Fayed and he owed this phenomenon to biradicaloid charge transfer, proximity effect and
conformational change[42].
Table 2
Absorption maxima and emission maxima of 3 in different aprotic solvents (1.15×10-5 M) at room
temperature (λex = 396 nm).
Solvent
λmax, nm
λem, nm
ΦF
Hexane
358
Toluene
378
THF
376
CHCl3
383
Acetone
371
CH3CN
370
484
496
530
571
581
614
0.02
0.04
0.22
0.29
0.02
<0.01
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Fig. 2. The emission spectra of 3 in different aprotic solvents (1.15×10-5 M) at room temperature.
3.3. AIE behavior
Increasing the water fractions (fw) in THF/H2O mixed solvent could change the existing forms
of the molecules from a homogenous solution in pure THF to aggregated luminogen particles in
the water mixtures which can lead to the changes in PL spectra. Since the excitation and
absorption spectra of 3 in CHCl3 were quite different, formation of aggregation in solution can
therefore be assumed to occur [43]. The fluorescence behaviors of 3 in THF/H2O mixtures were
then studied with different fw (Fig. 3). With the increment of the fw, the color of the emissive
fluorescence changed from green to orange at fw = 60%, and then turned into green when fw
reached 90% (Fig. S3). The PL intensity decreased dramatically and reached the lowest intensity
when fw reached 60%. However, when the fw reached 90%, the PL intensity was 9.2 times higher
than that in 60%. Interestingly, compared with the sample in pure THF, remarkable red shifts of
λem was observed when fw below 60%, while a slight blue shifts of λem was found when fw was
between 70% and 80%.These shifts can be attributed to typical twisted intramolecular charge
transfer (TICT) effects arising from increased solvent polarity through increases in the water
fraction[44, 45]. Increasing the water fraction from 0% to 60% and to 90% gives the ΦF values
from 0.22 to 0.02 and to 0.23, respectively. Obviously, addition of water can effectively influence
the present conditions of compound 3, so 3 possess AIE behavior.
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Fig. 3. PL spectra (a) and fluorescence quantum yield change (b) of 3 in THF / water mixture (10-5
M) with different water fractions. Excitation wavelength = 396 nm.
Since 3 was AIE active, we then studied its solid state fluorescence properties. Thin film was
prepared[46] and Fig. 4 showed the emission spectra of compound 3 in both CHCl3 solution and
thin film irradiated with Xe lamp (λex = 396 nm) at room temperature. The λem in film exhibited ~
60 nm red shifts compared with in CHCl3 solution, which may be due to nitro-group played the
role of auxochrome and effectively dispersed electron density throughout the whole molecule and
enlarged the conjugation length, which led to a red shift.
Fig. 4. Emission spectra of 3 at room temperature.
3.4. Electrochemical properties
Although being interfered by random errors caused by intermolecular interaction and solvent
effect, the CV experimental data are still indispensable to characterize optoelectronic properties
and provide an important guide for better understanding the structure-property relationship
[47-49].CV experiments were performed on a CHI660E electrochemical workstation, and the
electrochemical data of compound 1, 3 and 4 were summarized in Table 3. The CV graphs obeyed
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the quasi reversible nature and the first onset oxidation potentials (Eox vs SCE) were found to be
0.78 eV, 1.00 eV and 0.69 eV for 1, 3 and 4, respectively. Obviously, 3 with nitro substitution had
more positive oxidation potential than 1 and 4. Besides, the HOMO levels for 1, 3 and 4 were
-5.52 eV, -5.74 eV and -5.43 eV, respectively. It is reasonable to conclude that the HOMO level
could be lifted up by electron donors, which is consistent with several pioneering studies[50-51].
The excited state redox potentials (LUMO) of 1, 3 and 4 were calculated as -2.43 eV, -3.12 eV and
-2.40 eV, respectively.
Table 3
Electrochemical data of 1, 3 and 4a
E ogpt , eV
Compd.
Eox (vs SCE),eV
HOMOcv, eV
-5.52
LUMOcv, eV
-2.43
0.78
1.00
0.69
3.09
2.62
3.03
1
3
4
-5.74
-3.12
-5.43
-2.40
aHOMOCV = -(Eox + 4.74) eV, LUMOcv=HOMOcv + E opt
g
3.5. Thermal properties
The thermal properties of 3 were evaluated by DSC and TG with a heating rate of 10 oC min-1
under nitrogen. The DSC and TG curves of compound 3 were plotted in Fig. 5. An obvious heat
o
absorption phenomenon was founded at 169 C in DSC curve. In TG curve, the decomposition
o
temperature (Td) was 253 C (based on 5% weight loss). It is known that some electro-optic
materials with chromophores must be thermally stable enough to withstand encountered high
temperatures (>200 oC) in electric field poling and subsequent processing of
chromophore/polymer materials. The good thermal stability of 3 made it suitable for practical
application in the optoelectronic devices.
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Fig. 5. The TG and DSC curves (inset) of 3.
3.6. Theoretical calculations
To further understand the relationship between optoelectronic properties and molecular
structures, we investigated structures, frontier molecular orbitals (FMO), UV-vis spectra and
electronic transitions by the means of quantum chemistry calculations. Geometry optimization for
compounds 1, 3, and 4 with B3LYP/6-31g(d) level exhibited that the dihedral angles between
O
O
O
thiophenyl rings of 1, 3, 4, and 3,3’-bithiophene were 50.23O , 50.27 , 51.22 and 34 ,
respectively (Table 4). It was apparent that a substitution could appreciably increase the dihedral
angle between two thiophenyl rings and this change was quite consistent among various
substitution groups. In addition, these dihedral values suggested that a remarkable non-planar
feature of substituted 3,3’-bithiophenes that may minimize the stacking of molecules and avoid
intermolecular interactions that were commonly presented in luminescent materials.
Table 4
Predicted data by Gaussian 09 program
E(HOMO)
,
E(LUMO)
,
△
E(HOMO-LUMO)
,
△E(S0-S1)
,
λ(S0
,
S1)
ꢀ
Compd. Dihedral,O
f
eV
eV
eV
eV
nm
431
517
1
3
50.23
51.22
-5.50
-5.94
-2.19
-3.18
3.31
2.76
2.88
2.40
0.88
0.94
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4
50.27
-5.40
-2.12
3.28
2.84
436
1.01
Table 4 also showed the information about frontier molecular orbitals and electronic transitions.
Pred.
We found that the NO2 substitution could lower the energy of HOMO (E(HOMO ) = -5.50 eV for
1, -5.94 eV for 3 and -5.40 eV for 4. Table 4), which was in well agreement with the CV results
cv
(E(HOMO ) = -5.52 eV for 1, -5.74 eV for 3 and -5.43 eV for 4). HOMO-LUMO gap is a commonly
used descriptor to characterize the ICT occurring in chromophore molecules [52-53]. Our
calculations also showed that the NO2 substitution may reduce the energy gap of HOMO-LUMO
Pred..
(ꢀE(HOMO-LUMO
= 3.31 eV for 1, 2.76 eV for 3 and 3.28 eV for 4, respectively. Table 4),
)
which was consistent with the energy gaps obtained from UV-vis absorption (Eoptg = 3.09 eV for 1,
2.62 ev for 3 and 3.03 ev for 4, respectively).
The UV-vis adsorption feature was associated to electronic transitions between central π system
and the substitutions. Our TD-DFT calculation showed that the calculated λ(S0
and ꢀE(S0-S1)
S1)
ꢀ
values were in accordance with the optical experiments (λ(S0 S1) = 431 nm for 1, 517 nm for 3 and
ꢀ
436 nm for 4, and ꢀE(S0-S1) = 2.88 eV for 1, 2.40 eV for 3 and 2.84 eV for 4, Table 4 and Fig. S4).
To further explain the special red shift of 3, the ICT behavior of 3 was explored by
characterization of semi quantitative charge transfer distance of ꢀr value[54] as well as
electron-hole figures (Fig.6) generated by Multiwfn [55].The calculated ꢀr was 1.63 A, and a clear
separation of electron and hole area were found, which implied a significant ICT characteristics
for 3 at S1 state. However no obvious ICT behaviors were found in compounds 1and 4.
Fig.6. The ICT behavior of 3 at S1 state
15

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Conclusions
In summary, we successfully designed and synthesized five organic small molecules with
π-conjugated 2,2'-distyryl-[3,3'] bithiophenyl moieties. The photophysical experiments
demonstrated that these symmetric compounds possessed broaden light absorption and emission
spectra as well as considerable ΦF values. Compound 3 emitted distinctly yellow light in CHCl3
opt
g
and orange light in thin film. Besides, the lowest E
, a strong positive emission
solvatochromism, and AIE behavior were observed for 3. The electrochemical and thermal
experiments further demonstrated the high electron affinity and good thermal stability of 3.
Theoretical studies showed that the photoelectrochemical properties measured by experiments and
predicted by Gaussian 09 were in good agreement. These results indicated that it was a meaningful
strategy to explore our work aiming at symmetric 3,3’-bithiophene system, and compound 3 may
find versatile applications in relevant optoelectronic devices.
Acknowledgement
The work was supported by National Natural Science Foundation of China (Grant
No.21502117), Shanghai key laboratory of molecular catalysis and functional materials research
funds (2015MCIMKF01), and Shanghai Municipal Education Commission (Plateau Discipline
Construction Program). ZGZ was grateful for the Special Program for Applied Research on Super
Computation of the NSFC-Guangdong Joint Fundꢁthe second phase). Support from Prof. Guanjun
Wang and Prof. Gang Zhao was also greatly appreciated.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016
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Highlights
2,2’-Distyryl-[3,3']-bithiophenes (DSTBs) were designed and synthesized.
Compound with NO2 substitution exhibited strong UV-vis absorption and fluorescence emission,
high electron affinity and good thermal stability.
Theoretical calculations on structures, frontier molecular orbitals, UV-vis spectra and electronic
transitions were performed.