Synthesis and photophysical properties of 3,5-diaryl-2-heteroarylthiophenes

Article abstract of DOI:10.1016/j.dyepig.2019.107646

A series of 3,5-diaryl-2-heteroarylthiophenes have been synthesized via Fiesselmann type reaction between alkynones and easily available heteroarylmethanethiols or heteroarylmethyl carbamimidothioates. A correlation between compounds structure and efficie

Full text of DOI:10.1016/j.dyepig.2019.107646

DyesandPigments170(2019)107646
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of 3,5-diaryl-2-heteroarylthiophenes
Ieva Karpavičienėa, Mantas Jonušisa, Kaspars Leduskrastsb, Indrė Misiūnaitėa, Edgars Sunab,
Inga Čikotienėa,∗
a Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania
b Latvian Institute of Organic Synthesis, Aizkraukles 21, LV-1006, Riga, Latvia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Trisubstituted monomeric thiophene
Fiesselmann reaction
Photoluminescence quantum yields
Twisted intramolecular charge transfer
Solvatochromism
A series of 3,5-diaryl-2-heteroarylthiophenes have been synthesized via Fiesselmann type reaction between al-
kynones and easily available heteroarylmethanethiols or heteroarylmethyl carbamimidothioates. A correlation
between compounds structure and efficiency of luminescence has been established. These compounds exhibited
photoluminescence with quantum yields up to 83%. An evidence of twisted intramolecular charge transfer and
singlet emissive states have been demonstrated.
1. Introduction
solutions (Fig. 1).
Among a variety of methods for the synthesis of 2,3,5-trisubstituted
Substituted thiophenes represent an important class of building
blocks for the development of new pharmaceuticals [1,2] and optical
materials [3–7]. There are numerous reports on the optoelectronic
applications of thiophene-containing oligomeric or polymeric materials
[8–13]. In contrast, the literature on the optoelectronic properties of
monomeric thiophenes is relatively scarce. Although there are a few
reports on the emissive properties of monomeric thiophenes [14], and
these studies have led to the application of thiophenes in aggregation
induced emission [15] and three-dimensional optical storage [16], the
relationship between the structure and emissive behavior of monomeric
thiophenes has not been systematically studied. It has been only shown
that the introduction of benzimidazole (or structurally related phe-
nanthroimidazole) [14] in the position 2 of the thiophene ring has
helped to increase photoluminescence quantum yields (PLQY) to near
unity in solution [14b]. In addition, Mueller has also demonstrated that
aryl substituents that are not conjugated with the thiophene core have
almost no effect on the emissive properties of oligomeric thiophenes
[13]. Hence, we decided to utilize the beneficial effect of benzimidazole
in position 2 and aryl substituents in positions 3 and 5 of thiophene
scaffold in our study of emissive properties. The relationship between
electronic and emissive properties of 2,3,5-trisubstituted thiophenes
was systematically explored by the introduction of electron with-
drawing groups (EWG) or electron donating groups (EDG) in aryl
moieties (Fig. 1.). We were pleased to find that the proper choice of
EWGs in the R1 and R2 positions and EDG in the R3 position of 2,3,5-
trisubstituted thiophenes allowed for PLQY of 83% to be achieved in
thiophenes, Fiesselmann reaction is especially suitable. Early examples
of Fiesselmann reaction involved a base-catalyzed condensation of
acetylene dicarboxylates with mercaptoacetate to furnish 2,3,5-trisub-
stituted thiophenes (Scheme 1) [17]. The reaction experienced a re-
naissance in the late 90ties, when a tandem Michael addition/in-
tramolecular Knoevenagel condensation between the appropriate 1,3-
disubstituted propynones and mercaptoacetates was reported (Scheme
1) [18]. Later developments were mostly focused on various approaches
for the in situ generation of ynones/ynoates under the Fiesselmann re-
action conditions. Accordingly, Mueller elaborated the Fiesselmann
reaction as a three-component or pseudo-five-component synthesis of
2,3,5-trisubstituted thiophenes and oligothiophenes [11–13] and Wu
recently reported a four-component Fiesselmann reaction [19]. Im-
portantly, easy-to-oxidize and relatively toxic mercaptans have been
used as sulphur-containing building blocks in most of the above men-
tioned examples. Herein we report the use of heteroaryl substituted S-
functionalized thioureas as a stable, non-toxic and easy-to-handle al-
ternative to thiols [20] in the Fiesselmann reaction (Scheme 1).
2. Experimental section
2.1. General information
1H and 13C NMR spectra were recorded at 400 MHz and 100.6 MHz
with Bruker Ascend™ spectrometer. Chemical shifts are reported in
parts per million (ppm) using the residual protic solvent resonance as
∗ Corresponding author.
E-mail address: inga.cikotiene@chgf.vu.lt (I. Čikotienė).
https://doi.org/10.1016/j.dyepig.2019.107646
Received 1 April 2019; Received in revised form 16 June 2019; Accepted 17 June 2019
Availableonline21June2019
0143-7208/©2019ElsevierLtd.Allrightsreserved.

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
2.2.2.2. 5-Chloro-2-(3,5-diphenylthiophen-2-yl)-1H-benzo[d]imidazole
(3ab). Yellowish powder, m.p. 205–209 оC (iPrOH). Yield: 55%. 1H
NMR (400 MHz, CDCl3) δ: 12.29 (1H, s), 7.82 (2H, d, J = 7.3 Hz), 7.78
(1H, s), 7.73–7.35 (10H, m), 7.22 (1H, d, J = 8.1 Hz) ppm. 13C NMR
(100 MHz, CDCl3) δ: 147.7, 145.3, 142.8, 135.2, 133.3, 129.8, 129.2,
129.1, 129.0, 128.7, 128.5, 127.5, 126.9, 126.0, 123.1, 120.5, 118.4,
113.5 ppm. HRMS (ESI) calcd. for C23H16ClN2S (MH+): 387.0717;
found 387.0719.
Fig. 1. General structure of luminescent 2,3,5-trisubstituted thiophenes.
2.2.2.3. 5,6-Dichloro-2-(3,5-diphenylthiophen-2-yl)-1H-benzo[d]
imidazole (3ac). Yellowish powder, m.p. 232–233 оC (iPrOH). Yield:
60%. 1H NMR (400 MHz, DMSO‑d6) δ: 12.32 (1H, br. s), 7.89–7.68 (5H,
m), 7.56–7.31 (8H, m) ppm. 13C NMR (100 MHz, DMSO‑d6) δ: 148.9,
145.7, 143.2, 139.5, 135.1, 133.2, 129.8, 129.2, 129.1, 129.1, 128.6,
127.6, 126.1, 125.6, 125.1, 116.8 ppm. HRMS (ESI) calcd. for
the internal standard (CHCl3: δ 7.28 (77.0), DMSO: δ 2.50 (39.5)) un-
less otherwise stated. Data is reported as follows: chemical shift, mul-
tiplicity (s = singlet, d = doublet, t = triplet, q = quartet, br = broad,
m = multiplet), coupling constants (Hz) and integration). Mass spectra
were recorded with a TSQ Endura™ spectrometer (electrospray ioniza-
tion) in positive mode. Melting points of synthesized compounds were
monitored in open capillary with “Stuart SMP 10” apparatus. Reaction
process was monitored by TLC using Silicagel 60 F254 Merc plates. For
purification Silicagel 60 (40–63 μm), hexane or petroleum ether (PE),
ethyl acetate (EA), toluene (Tol), chloroform (CHCl3), acetonitrile
(MeCN) and their mixtures were used as eluents. Hexane and petroleum
ether were distilled before use.
The solution absorption and emission, as well as solid state emission
spectra were measured on the FS5-Edinburgh instruments spectro-
fluorometer in ambient atmosphere and room temperature. The PLQYs
were determined by the absolute method at ambient temperature using
an integrating sphere. Compounds were excited in the wavelength they
absorbed in. The temperature measurements were conducted in a
cryostat using EtOH/liquid nitrogen as coolant.
C
23H15Cl2N2S (MH+): 421.0328; found 421.0332.
2.2.2.4. 4-(3,5-diphenylthiophen-2-yl)pyridine (3ad). Yellowish plates,
m.p. 107–110 оC (EtOH). Yield: 67%. 1H NMR (400 MHz, DMSO‑d6)
δ: 8.50 (2H, d, J = 5.0 Hz), 7.79–7.75 (2H, m), 7.70 (1H, s), 7.47 (2H, t,
J = 7.6 Hz), 7.43–7.34 (6H, m), 7.22 (2H, dd, J = 4.6, 1.5 Hz) ppm. 13
C
NMR (100 MHz, DMSO‑d6) δ: 150.5, 144.2, 141.7, 141.4, 135.8, 134.0,
133.3, 129.7, 129.3, 129.2, 128.8, 128.3, 128.0, 125.9, 123.2 ppm.
HRMS (ESI) calcd. for C21H16NS (MH+): 314.0998; found 314.0998.
2.2.2.5. 2-(3,5-bis(4-methoxyphenyl)thiophen-2-yl)-1H-benzo[d]
imidazole (3ba). Yellow crystals, m.p. 255–258 °C (EtOH). Yield: 92%.
1H NMR (400 MHz, CDCl3) δ: 9.18 (1H, br. s), 7.56 (2H, d, J = 8.0 Hz),
7.50–7.42 (4H, m), 7.20–7.13 (3H, m), 6.99 (2H, d, J = 8.0 Hz), 6.91
(2H, d, J = 4.8 Hz), 3.85 (3H, s), 3.83 (3H, s) ppm. 13C NMR (100 MHz,
CDCl3) δ: 159.8, 146.8, 145.9, 141.8, 130.1, 128.1, 127.1, 126.2, 125.6,
122.8, 114.7, 114.4, 55.3 ppm. HRMS (ESI) calcd. for C25H21N2O2S
(MH+): 413.1318; found 413.1322.
2.2. Synthesis of compounds
2.2.1. Synthesis of starting materials
2.2.2.6. 2-(3,5-bis(4-methoxyphenyl)thiophen-2-yl)-5-chloro-1H-benzo[d]
imidazole (3bb). Yellow powder, m.p. 235–237 оC (EtOH). Yield: 58%.
1H NMR (400 MHz, DMSO‑d6) δ: 12.03 (1H, br. s), 7.72 (2H, d,
J = 8.6 Hz), 7.60–7.53 (2H, m), 7.50 (1H, d, J = 8.3 Hz), 7.41 (2H, d,
J = 8.6 Hz), 7.19 (1H, dd, J = 8.5, 1.6 Hz), 7.03 (2H, d, J = 8.7 Hz),
6.98 (2H, d, J = 8.6 Hz), 3.81 (3H, s), 3.79 (3H, s) ppm. 13C NMR
(100 MHz, DMSO‑d6) δ: 160.0, 159.6, 148.1, 145.2, 142.5, 130.3,
127.7, 127.7, 127.4, 127.4, 126.8, 126.3, 126.0, 124.1, 124.1, 122.7,
122.7, 115.1, 114.6, 55.8, 55.6 ppm. HRMS (ESI) calcd. for
1,3-Diarylpropynones 1a-f were synthesized according to known
procedure [21] from corresponding alkynes and aroyl chlorides. (1H-
benzo[d]imidazol-2-yl)methanethiols were prepared according to
known procedure [22] in cyclization of aryl-1,2-diamine and mercap-
toacetic acid and various substituted methyl carbamimidothioates were
synthesized from corresponding alcohols in two step synthesis ac-
cording the literature procedures [23] in good to excellent yield.
C
25H20ClN2O2S (MH+): 447.0928; found 447.0934.
2.2.2. Synthesis of 3,5-diaryl-2-heteroarylthiophenes (3) and 2-
heteroaryl-3,5-diaryl-2,3-dihydrothiophen-3-ols
(4).
General
procedure
2.2.2.7. 2-(3,5-bis(4-methoxyphenyl)thiophen-2-yl)-5,6-dichloro-1H-
benzo[d]imidazole (3bc). Yellow crystals, m.p. 122–127 °C (Rf = 0.3,
PE:EA = 4:1). Yield 74%. 1H NMR (400 MHz, CDCl3) δ: 8.76 (1H, br. s),
7.79 (1H, s), 7.59 (2H, d, J = 8.4 Hz), 7.43 (2H, d, J = 8.8 Hz), 7.31
(1H, s), 7.14 (1H, s), 7.05 (2H, d, J = 8.4 Hz), 6.94 (2H, d, J = 8.8 Hz),
3.90 (3H, s), 3.85 (3H, s) ppm. 13C NMR (100 MHz, CDCl3) δ: 160.1,
160.0, 148.9, 146.8, 142.7, 132.7, 130.1, 127.8, 127.2, 126.0, 125.7,
124.6, 120.2, 114.9, 114.5, 111.8, 55.4 ppm. HRMS (ESI) calcd. for
To the 25 ml round bottomed flask the corresponding hetero-
arylmethanethiol or heteroarylmethyl carbamimidothioate
(0.45 mmol) and 36 mg NaOH (0.9 mmol) were added followed by 5 ml
of ethanol. Reaction mixture was refluxed 1 h under argon atmosphere,
then the corresponding 1,3-diarylpropynone 1 (0.3 mmol) was added
and refluxing was continued until full completion (monitored by TLC).
The solvent was evaporated under reduced pressure; the residue was
portioned between DCM and water. The organic layer was separated,
then washed with water, dried with sodium sulfate and concentrated
under reduced pressure. The resulting solid material was purified by
crystallization or column chromatography.
C
25H19Cl2N2O2S (MH+): 481.0539; found 481.0545.
2.2.2.8. 4-(3,5-bis(4-methoxyphenyl)thiophen-2-yl)pyridine
(3bd). Yellow crystals, m.p. 120–125 оC (Rf = 0.5, Tol:EA = 1:1).
Yield: 89% 1H NMR (400 MHz, CDCl3) δ: 8.47 (2H, br. s), 7.57 (2H,
d, J = 8.0 Hz), 7.24–7.22 (2H, m), 7.20–7.17 (3H, m), 6.94 (2H, d,
J = 8.0 Hz), 6.89 (2H, d, J = 8.0 Hz), 3.84 (6H, s) ppm. 13C NMR
(100 MHz, CDCl3) δ: 159.8, 159.2, 149.9, 144.5, 142.3, 141.3, 137.9,
132.5, 132.5, 130.1, 128.5, 127.1, 126.4, 114.5, 114.2, 55.4, 55.3 ppm.
HRMS (ESI) calcd. for C23H19NO2S (MH+): 374.1209; found 374.1212.
2.2.2.1. 2-(3,5-diphenylthiophen-2-yl)-1H-benzo[d]imidazole
(3aa). Yellowish powder, m.p. 210–215 оC (iPrOH). Yield: 62%. 1H
NMR (400 MHz, CDCl3) δ: 8.89 (1H, s), 7.79 (1H, d, J = 7.8 Hz), 7.69
(2H, d, J = 7.3 Hz), 7.62–7.49 (5H, m), 7.43 (2H, t, J = 7.5 Hz),
7.39–7.31 (2H, m), 7.29–7.23 (1H, m), 7.20 (2H, m) ppm. 13C NMR
(100 MHz, CDCl3) δ: 146.0, 143.1, 141.8, 136.0, 133.4, 129.4, 129.1,
129.0, 128.8, 128.4, 126.6, 125.8, 123.3, 122.6, 119.5, 110.5 ppm.
HRMS (ESI) calcd. for C23H17N2S (MH+): 353.1106; found 353.1109.
2.2.2.9. 2-(3,5-bis(4-methoxyphenyl)thiophen-2-yl)-1-methyl-1H-
imidazole (3be). Brown oil (Rf = 0.6, Tol:EA = 1:1). Yield: 30%. 1H
2

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
Scheme 1. Fiesselmann reaction.
NMR (400 MHz, CDCl3) δ: 7.55 (2H, d, J = 8.0 Hz), 7.38 (1H, s),
7.16–7.15 (2H, m), 7.14 (1H, s), 6.91 (2H, d, J = 8.0 Hz), 6.84–6.83
(2H, m), 6.81 (1H, s), 3.81 (3H, s), 3.77 (3H, s), 3.00 (3H, s) ppm. 13C
NMR (100 MHz, CDCl3) δ: 159.6, 159.0, 145.4, 142.2, 141.5, 129.0,
128.9, 128.6, 127.1, 126.6, 123.4, 123.1, 121.7, 114.4, 114.2, 55.4,
55.2, 33.3 ppm. HRMS (ESI) calcd. for C22H21N2O2S (MH+): 377.1318;
found 377.1312.
(400 MHz, CDCl3) δ: 9.13 (1H, br. s), 7.76 (1H, s), 7.72 (2H, d,
J = 8.0 Hz), 7.60 (2H, d, J = 8.0 Hz), 7.54 (2H, d, J = 8.8 Hz), 7.34
(1H, s), 7.16 (1H, s), 6.92 (2H, d, J = 8.8 Hz), 3.84 (3H, s) ppm. 13C
NMR (100 MHz, CDCl3) δ: 160.3, 148.0, 147.6, 142.6, 141.0, 139.3,
130.9 (2JC-F = 32.6 Hz), 130.7, 129.3, 127.2, 126.3 (3JC-F = 3.5 Hz),
125.5, 125.4, 125.2, 123.8 (1JC-F = 270.9 Hz), 114.6, 112.0, 55.4 ppm.
HRMS (ESI) calcd. for C25H16Cl2F3N2OS (MH+): 519.0307; found
519.0315.
2.2.2.10. 2-(5-(4-methoxyphenyl)-3-(4-(trifluoromethyl)phenyl)thiophen-
2-yl)-1H-benzo[d]imidazole (3ca). Yellow crystals, m.p. 260–270 оC
(Rf = 0.4, PE:EA = 3:1). Yield: 42%. 1H NMR (400 MHz, DMSO‑d6) δ:
12.62 (1H, s), 7.82–7.67 (7H, m), 7.65–7.59 (1H, m), 7.44–7.36 (1H,
m), 7.26–7.14 (2H, m), 7.05 (2H, d, J = 8.7 Hz), 3.81 (3H, s) ppm. 13C
NMR (100 MHz, DMSO‑d6) δ: 160.1, 145.9, 145.4, 143.9, 140.6, 139.6,
135.3, 129.9, 128.5 (2JC-F = 31.3 Hz), 127.5, 126.9, 126.2, 126.0 (3JC-
F = 3.7 Hz), 125.8, 124.8 (1JC-F = 270.0 Hz), 123.3, 122.3, 119.2,
115.2, 112.1, 55.8 ppm. HRMS (ESI) calcd. for C25H18F3N2OS (MH+):
451.1086; found 451.1091.
2.2.2.13. 4-(5-(4-methoxyphenyl)-3-(4-(trifluoromethyl)phenyl)thiophen-
2-yl)pyridine (3cd). Amber oil (Rf = 0.3, PE:EA = 2:1). Yield: 51%. 1H
NMR (400 MHz, CDCl3) δ: 8.47 (2H, d, J = 6.0 Hz), 7.52 (2H, d,
J = 8.4 Hz), 7.55 (2H, d, J = 8.8 Hz), 7.41 (2H, d, J = 8.0 Hz), 7.22
(1H, s), 7.14 (2H, d, J = 6.0 Hz), 6.93 (2H, d, J = 8.8 Hz), 3.82 (3H, s)
ppm. 13C NMR (100 MHz, CDCl3) δ: 160.0, 149.9, 145.4, 141.7, 139.7,
139.7, 134.1, 129.7 (2JC-F = 32.3 Hz), 129.3, 127.1, 125.9, 125.7 (3JC-
F = 3.7 Hz), 125.6, 124.1 (1JC-F = 270.5 Hz), 123.0, 114.5, 55.4 ppm.
HRMS (ESI) calcd. for
412.0981.
C
23H17F3NOS (MH+): 412.0977; found
2.2.2.11. 5-Chloro-2-(5-(4-methoxyphenyl)-3-(4-(trifluoromethyl)phenyl)
thiophen-2-yl)-1H-benzo[d]imidazole (3 cb). Yellow crystals, m.p.
200–205 °C (Rf = 0.2, CHCl3:MeCN = 98:2). Yield: 64%. 1H NMR
(400 MHz, CDCl3) δ: 9.03 (1H, br. s), 7.72 (2H, d, J = 8.0 Hz), 7.69
(1H, s), 7.61 (2H, d, J = 8.0 Hz), 7.55 (2H, d, J = 8.4 Hz), 7.26–7.15
(3H, m), 7.16 (1H, s), 6.92 (2H, d, J = 8.0 Hz), 3.84 (3H, s) ppm. 13C
NMR (100 MHz, CDCl3) δ: 160.1, 147.1, 140.6, 139.4, 139.4, 130.6
(2JC-F = 32.6 Hz), 130.6, 129.3, 127.2, 126.2 (3JC-F = 3.5 Hz), 125.9,
125.7, 125.2, 125.1, 123.8 (1JC-F = 270.6 Hz), 114.6, 113.7, 55.4 ppm.
HRMS (ESI) calcd. for C25H17ClF3N2OS (MH+): 485.0697; found
485.0701.
2.2.2.14. 2-(3-(4-methoxyphenyl)-5-(4-(trifluoromethyl)phenyl)thiophen-
2-yl)-1H-benzo[d]imidazole (3da). Yellow crystals, m.p. 120–125 оC
(Rf = 0.4, PE:EA = 3:1). Yield: 64%. 1H NMR (400 MHz, CDCl3) δ: 9.49
(1H, br. s), 7.64 (2H, d, J = 8.0 Hz); 7.56 (2H, d, J = 8.0 Hz), 7.48 (2H,
br. s), 7.36 (2H, d, J = 8.4 Hz), 7.27 (1H, s), 7.23–7.20 (2H, m), 6.91
(2H, d, J = 8.8 Hz), 3.79 (3H, s) ppm. 13C NMR (100 MHz, CDCl3) δ:
146.2, 143.8, 142.0, 136.7, 130.0 (2JC-F = 32.6 Hz), 130.0, 127.8,
127.7, 127.5, 126.0 (3JC-F = 3.6 Hz), 125.8, 124.0 (1JC-F = 270.4 Hz),
123.1, 114.7, 55.3 ppm. HRMS (ESI) calcd. for C25H18F3N2OS (MH+):
451.1086; found 451.1090.
2.2.2.12. 5,6-Dichloro-2-(5-(4-methoxyphenyl)-3-(4-(trifluoromethyl)
phenyl)thiophen-2-yl)-1H-benzo[d]imidazole (3 cc). Yellow crystals,
m.p. 227–231 °C (Rf = 0.3, CHCl3:MeCN = 98:2). Yield 62%. 1H NMR
2.2.2.15. 4-(3-(4-methoxyphenyl)-5-(4-(trifluoromethyl)phenyl)thiophen-
2-yl)pyridine (3dd). Yellow crystals, m.p. 120–125оC (Rf = 0.3,
Tol:EA = 2:1). Yield: 67%. 1H NMR (400 MHz, CDCl3) δ: 8.50 (2H, d,
3

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
J = 6.0 Hz), 7.74 (2H, d, J = 8.4 Hz), 7.66 (2H, d, J = 8.4 Hz), 7.41
(1H, s), 7.24 (2H, d, J = 8.8 Hz), 7.22 (2H, d, J = 6.0 Hz), 6.90 (2H, d,
J = 8.8 Hz), 3.84 (3H, s) ppm. 13C NMR (100 MHz, CDCl3) δ: 159.4,
149.9, 142.4, 141.9, 141.5, 137.0, 134.9, 130.1, 129.9 (2JC-
F = 32.5 Hz), 128.4, 127.9, 126.1 (3JC-F = 3.7 Hz), 125.8, 124.0 (1JC-
F = 270.1 Hz), 114.3, 55.3 ppm. HRMS (ESI) calcd. for C23H17F3NOS
(MH+): 412.0977; found 412.0981.
imidazol-2-yl)-5-(4-(trifluoromethyl)phenyl)-2,3-dihydrothiophen-3-ol
(4de). Brown oil (Rf = 0.45, Tol:EA = 2:1). Yield: 44%.1H NMR
(400 Hz, DMSO‑d6) δ: 7.82 (2H, d, J = 8.4 Hz), 7.79 (2H, d,
J = 8.8 Hz), 7.44 (2H, d, J = 8.8 Hz), 7.16 (1H, s), 6.93 (1H, s), 6.90
(2H, d, J = 8.4 Hz), 6.53 (1H, s), 5.42 (1H, s), 3.75 (3H, s), 3.46 (3H, s)
ppm. 13C NMR (100 MHz, DMSO‑d6) δ: 159.0, 142.7, 138.9 (1JC-
F = 271.9 Hz), 137.3, 129.4 (2JC-F = 31.8 Hz), 128.3, 127.5, 127.3,
126.7, 126.1 (3JC-F = 3.6 Hz), 123.4, 123.1, 113.8, 88.4, 55.5, 55.5,
53.5, 33.0 ppm.
2.2.2.16. 2-(3,5-bis(4-(trifluoromethyl)phenyl)thiophen-2-yl)-1H-benzo
[d]imidazole (3ea). Yellow crystals, m.p. 130–135 оC (Rf = 0.5,
PE:EA = 4:1). Yield: 44%. 1H NMR (400 MHz, CDCl3) δ: 9.17 (1H, br.
s), 7.53–7.49 (5H, m), 7.47–7.42 (5H, m), 7.26–7.21 (3H, m) ppm. 13C
NMR (100 MHz, CDCl3) δ: 145.3, 144.4, 140.6, 138.6, 136.0, 130.2
(2JC-F = 33.0 Hz), 130.4 (2JC-F = 33.0 Hz), 128.9, 128.5, 128.3, 127.9,
127.1, 126.0 (3JC-F = 3.7 Hz), 125.9 (3JC-F = 3.7 Hz), 125.6, 123.8 (1JC-
F = 270.0 Hz), 123.7 (1JC-F = 271.0 Hz), 123.5 ppm. HRMS (ESI) calcd.
for C25H15F6N2S (MH+): 489.0855; found 489.0859.
2.2.2.23. (2R,3S) or (2S,3R)-2-(1-methyl-1H-imidazol-2-yl)-3,5-bis(4-
(trifluoromethyl)phenyl)-2,3-dihydrothiophen-3-ol
(4ee). Brown
oil
(Rf = 0.3, Tol:EA = 4:1). Yield 41%. 1H NMR (400 MHz, CDCl3) δ:
7.76 (2H, d, J = 8.0 Hz), 7.72–7.65 (6H, m), 7.62 (2H, d, J = 8.0 Hz),
7.07 (1H, s), 6.86 (1H, s), 6.33 (1H, s), 5.15 (1H, s), 3.47 (3H, s) ppm.
13C NMR (100 MHz, CDCl3) δ: 142.3, 139.9 (1JC-F = 266.0 Hz), 139.8
(1JC-F = 272.0 Hz), 137.9, 136.5, 131.0 (2JC-F = 32.0 Hz), 130.0 (2JC-
F = 32.0 Hz), 129.6, 129.0, 127.4, 127.0, 126.2, 125.6 (3JC-F = 3.7 Hz),
125.2 (3JC-F = 3.7 Hz), 121.6, 88.6, 52.4, 32.5 ppm.
2.2.2.17. 4-(3,5-bis(4-(trifluoromethyl)phenyl)thiophen-2-yl)pyridine
(3ed). Yellow crystals, m.p. 115–130 оC (Rf = 0.4, Tol:EA = 2:1).
Yield: 50%. 1H NMR (400 MHz, CDCl3) δ: 8.54 (2H, d, J = 8.0 Hz),
7.75 (2H, d, J = 8.0 Hz), 7.68 (2H, d, J = 8.0 Hz), 7.63 (2H, d,
J = 8.0 Hz), 7.44 (2H, d, J = 8.0 Hz), 7.44 (1H, s), 7.19 (2H, d,
J = 8.0 Hz) ppm. 13C NMR (100 MHz, CDCl3) δ: 150.1, 143.2, 141.1,
139.9, 139.1, 136.7, 136.6, 130.2 (2JC-F = 33.0 Hz), 130.0 (2JC-
F = 32.0 Hz), 129.3, 127.8, 126.2 (3JC-F = 3.7 Hz), 125.9, 125.8 (3JC-
F = 3.7 Hz), 124.0 (1JC-F = 271.0 Hz), 124.0 (1JC-F = 270.0 Hz),
123.1 ppm. HRMS (ESI) calcd. for C23H14F6NS (MH+): 450.0746;
found 450.0751.
2.2.3. Conversion of compound 4de to compound 3de
Compound 4de (40 mg, 0.093 mmol) was refluxed in 5 ml of EtOH
in the presence of 2 drops of concentrated H2SO4. Reaction was mon-
itored by TLC. After the completion of reaction, solvent was evaporated
under reduced pressure, the residue was treated with DCM and water.
Organic layer was collected, dried with Na2SO4 giving 25 mg of crude
oil 2-(3-(4-methoxyphenyl)-5-(4-(trifluoromethyl)phenyl)thiophen-2-
yl)-1-methyl-1H-imidazole 3de. 1H NMR (400 MHz, CDCl3) δ: 7.76 (2H,
d, J = 8.0 Hz), 7.68 (2H, d, J = 8.0 Hz), 7.50 (1H, s), 7.37 (1H, d,
J = 1.2 Hz), 7.17 (2H, d, J = 8.0 Hz), 6.99 (1H, d, J = 1.2 Hz), 6.89
(2H, d, J = 8.0 Hz), 3.83 (3H, s), 3.11 (3H, s) ppm.
2.2.2.18. 2-(3,5-bis(4-(pentyloxy)phenyl)thiophen-2-yl)-1H-benzo[d]
imidazole (3fa). White powder, m.p. 95–100 оC (iPrOH). Yield: 72%.
1H NMR (400 MHz, CDCl3) δ: 9.00 (1H, br. s), 7.58 (2H, d, J = 8.6 Hz),
7.45 (2H, d, J = 8.5 Hz), 7.29 (1H, s), 7.26–7.12 (3H, m), 7.15 (1H, s),
7.02 (2H, d, J = 8.6 Hz), 6.93 (2H, d, J = 8.7 Hz), 4.08–3.95 (4H, m),
1.90–1.78 (4H, m), 1.56–1.36 (8H, m), 1.04–0.91 (6H, m) ppm. 13C
NMR (100 MHz, CDCl3) δ: 159.5, 159.4, 147.0, 145.9, 143.2, 141.8,
133.5, 130.1, 128.0, 127.1, 126.1, 125.6, 125.6, 123.0, 122.4, 119.3,
115.3, 115.0, 110.4, 68.2, 68.1, 29.0, 28.9, 28.2, 28.2, 22.5, 22.4, 14.0,
14.0 ppm. HRMS (ESI) calcd. for C33H37N2O2S (MH+): 379.2268; found
379.2270.
3. Results and discussion
3.1. Synthesis
The diarylalkynones 1a–f were prepared by Sonogashira coupling
reaction of commercially available aroyl chlorides with aryl alkynes
[21]. The cross-coupling reaction proceeded in good to high yields with
substrates containing both electron-donating and electron withdrawing
substituents (Table 1). Next, the construction of thiophene ring via
Fiesselmann-type reaction [11] between alkynones and easily available
heteroarylmethanethiols was attempted.
First, we performed a short screening of solvents and bases for the
Fiesselmann cyclization. Although DBU is routinely used as a base in
the cyclization, in our hands the reaction between alkynone 1b and
thiol 2a in refluxing ethanol in the presence of 1 equiv. of DBU gave
only 19% of the desired thiophene 3ba (Table 2, entry 1). The yield of
3ba was increased up to 37% and 51% when excess of thiol 2a (3 and 6
2.2.2.19. (2R,3S)
or
(2S,3R)-2-(1H-benzo[d]imidazol-2-yl)-5-(4-
methoxyphenyl)-3-(4-(trifluoromethyl)phenyl)-2,3-dihydrothiophen-3-ol
(4ca). Brown oil (Rf = 0.2, PE:EA = 4:1). Yield: 10%. 1H NMR
(400 MHz, CDCl3) δ: 7.62–7.60 (4H, m), 7.55–7.52 (2H, m),
7.49–7.44 (4H, m), 6.84 (2H, d, J = 8.0 Hz), 6.02 (1H, s), 5.46 (1H,
s), 3.79 (3H, s) ppm.
2.2.2.20. (2R,3S) or (2S,3R)-2-(1H-benzo[d]imidazol-2-yl)-3,5-bis(4-
(trifluoromethyl)phenyl)-2,3-dihydrothiophen-3-ol
(4ea). Brown
oil
(Rf = 0.3, PE:EA = 4:1). Yield: 50%. 1H NMR (400 MHz, CDCl3) δ:
7.64–7.58 (8H, m), 7.54–7.25 (4H, m), 6.23 (1H, s), 5.47 (1H, s) ppm.
Table 1
The synthesis of alkynones 1.
2.2.2.21. (2R,3S)
or
(2S,3R)-5-(4-methoxyphenyl)-2-(1-methyl-1H-
imidazol-2-yl)-3-(4-(trifluoromethyl)phenyl)-2,3-dihydrothiophen-3-ol
(4ce). Brown oil (Rf = 0.4, Tol:EA = 4:1). Yield: 38%. 1H NMR
(400 MHz, CDCl3) δ: 7.73 (2H, d, J = 8.4 Hz), 7.57 (2H, d,
J = 8.0 Hz), 7.50 (2H, d, J = 8.8 Hz), 7.03 (1H, s), 6.89 (2H, d,
J = 8.8 Hz), 6.82 (1H, s), 6.10 (1H, s), 5.08 (1H, s), 3.83 (3H, s),
3.42 (3H, s) ppm. 13C NMR (100 MHz, CDCl3) δ: 160.4, 142.7, 141.7
(1JC-F = 265.0 Hz), 137.8, 129.7 (2JC-F = 32.0 Hz), 128.8, 128.1, 127.2,
126.3, 125.1 (3JC-F = 4.0 Hz), 123.1, 122.0, 113.9, 88.7, 55.4, 55.3,
52.4, 32.5 ppm.
Entry
R1
R2
Product (yield, %)
1
2
3
4
5
6
H
OMe
CF3
OMe
CF3
OC5H11
H
1a (98)
1b (78)
1c (80)
1d (62)
1e (60)
1f (71)
OMe
OMe
CF3
CF3
OC5H11
2.2.2.22. (2R,3S)
or
(2S,3R)-3-(4-methoxyphenyl)-2-(1-methyl-1H-
4

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
Table 3
The Fiesselmann type reaction between alkynones 1 and thiols or thioates 2.4
Table 2
Optimization of the Fiesselmann reaction conditions.
#
Deviation from initial conditions
Yield, %
Entry
Starting material
RCH2SX
Product (Yield,
%)
1
2
3
4
5
6
Nonea
3 eq of 2a
6 eq of 2a
THF instead of EtOH
DMF instead of EtOH
NaOH instead of DBU
19
37
51
63
54
92
1
2
3
1a (R1 = R2 = H)
1b (R1 = R2 = OCH3)
1c (R1 = CF3,
3aa (62)
3ba (92)
3ca (42), 4ca
(10)
2a
R2 = -OCH3)
4
5
1d (R1 = OCH3,
3da (64)
a
Initial reaction conditions: 1.0 equiv. DBU, 1.0 equiv. 1b, 1.5 equiv. 2a,
R
2 = CF3)
EtOH, reflux.
1e (R1 = R2 = CF3)
3ea (44), 4ea
(50)
equiv, respectively) was used (Table 2, entries 2–3). However the iso-
lation and purification of the product became challenging. Change of
the solvent to THF or DMF did not improve the conversion (Table 2,
entries 4–5). After extensive screening of various bases and solvents we
were pleased to find that the yield of thiophene 3ba can be increased to
92% by the use of sodium or potassium hydroxide in ethanol (Table 2,
entry 6).
6
7
8
9
10
11
12
1f (R1 = R2 = OC5H11
)
3fa (72)
3 ab (55)
3bb (58)
3 cb (64)
3ac (60)
3bc (74)
3cc (62)
1a
1b
1c
1a
1b
1c
2b
2c
13
14
15
16
17
18
1a
1b
1c
1d
1e
1a
3ad (67)
3bd (89)
3cd (51)
3dd (67)
3ed (50)
3ae (13), 4ae
(17)a
3be (30)
4ce (38)
4de (44)
4ee (41)
With optimized reaction conditions in hand, we elaborated on the
scope of alkynones and thiols (Table 3, entries 1–9). Yields of thio-
phenes 3 depended on the electronic nature of both alkynones and
thiols. The highest yield (92%) was observed for the most electron rich
methoxy-substituted alkynone 1b (Table 3, entry 2). In contrast, the
lowest yields (42–44%) were observed for the electron deficient alky-
nones 1c,e bearing trifluoromethyl substituents (Table 3, entries 3, 5).
Similar relationship between electronic effects and yields of thiophenes
was observed for thiols, with the more electron deficient thiol 2b pro-
viding lower yields than 2a (Table 3, entries 1–2 vs entries 7–8).
In the meantime, the use of thiols in the Fiesselmann thiophene
synthesis is compromised by their inherent toxicity and propensity to
oxidize to disulfide side-products that decreases yields of the desired
thiophenes and complicates their purification. We were pleased to find
that the more stable, easy-to-handle and readily available (see
Experimental Section 2.2.1) carbamimidothioates 2c-e can be used as
an alternative to thiols in the Fiesselmann-type cyclization (Table 3,
entries 10–19) affording the desired thiophenes in low to high yields.
Interestingly, the reactions between alkynones 1c,e and thiol 2a
(Table 3, entries 3 and 5) furnished intermediates 4ca and 4ea as major
products. In addition, the methylimidazole (2e) ring–bearing starting
material exclusively provided dihydrothiophen-3-ols 4ce–ee instead of
the desired thiophenes 3ce–ee even after prolonged heating (Table 3,
entries 20–22). Gratifyingly, all of the intermediates 4 can be dehy-
drated into the corresponding thiophenes 3 by the heating of acidified
solutions (see example in Experimental Section 2.2.3). Slow dehydra-
tion process was also observed in DMSO solution after several weeks.
2d
2e
19
20
21
22
1b
1c
1d
1e
a
Inseparable mixture, yields calculated via 1H NMR integration.
PLQY, but a substantial increase in the solid state PLQY (Table 4, entry
2). However, the introduction of EWG in the position 3 and EDG in the
position 5 resulted in a considerable increase of PLQY in the solution
(Table 4, entry 3) and in the solid state PLQY. Mutual exchange of the
EWG and EDG substituents resulted in a decrease in both the solution
and the solid state PLQY (Table 4, entry 4). If both aryl moieties in
positions 3 and 5 possess EWGs substituents, the solution PLQY is in-
creased, but the solid state PLQY is lowered (Table 4, entry 5). In-
troduction of OC5H11 groups in the positions 3 and 5 of aryl moieties
(Table 4, entry 6) gave similar results as compared to the unsubstituted
Ph groups. Thus, we can conclude that the introduction of EWG-sub-
stituted arenes in position 3 and EDG-substituted aromatic moieties in
position 5 of the thiophene ring results in higher PLQY in thiophenes 3.
Next, the electronic properties of the benzimidazole core were al-
tered by introducing chloro substituents (Table 4, entries 7–12). The
PLQY increased when relatively electron deficient chloro-benzimida-
zoles 3 ab–cb and 3ac–cc were present in the molecule. The highest
PLQY in solution (82.6%) and solid state (30.9%) was observed for 3 cc
with the most electron deficient benzimidazole core in a series. We also
wanted to see whether the attachment of heterocycles other than ben-
zimidazole to the position 2 of thiophene would affect the PLQY. Thus,
the pyridine and N-methylimidazole was introduced in the position 2
(Table 4, entries 13–18). A sharp decrease of PLQY was observed for
luminophores bearing pyridine and N-methylimidazole substituent,
highlighting the importance of the benzimidazole subunit. However,
3cd (Table 4, entry 15) still possessed relatively high (33.9%) PLQY in
solution, possibly owing to the proper choice of EWG in the position 3
3.2. Photophysical properties
Photophysical properties of thiophenes 3 (Table 4) were studied in
MeCN solution at ca. 10−6 M concentration. UV/Vis Absorption and
emission spectra were measured under ambient atmosphere and at
room temperature (for attenuation coefficients see SI page SI56). PLQY
were measured using an integrating sphere. To evaluate the electronic
effects on the emission in both solution and solid state, thiophene 3aa
(Table 4, entry 1) was chosen as a benchmark compound. The presence
of EDG in both positions 3 and 5 resulted in a decrease in the solution
5

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
Table 4
Photophysical properties of thiophenes 3.
Entry
1
Compound
λ max (abs), nma
237, 266, 347
λ max (em), nmb
Stokes shifts (nm)
86
QY, % (solution)
QY, % (solid)
8.5
433 sn;
456 sd
38.9
2
235, 278, 356
445 sn;
467 sd
89
28.0
17.0
3
4
5
234, 274, 358
238, 280, 354
238, 272, 352
454 sn;
459 sd
96
90
90
77.5
28.6
53.5
22.0
4.5
444 sn;
457 sd
442 sn;
512 sd
2.1
6
234, 280, 356
441 sn;
464 sd
85
33.8
1.8
7
8
235, 266, 350
234, 280, 360
435 sn;
459 sd
85
85
45.0
42.5
17.5
11.6
445 sn;
463 sd
9
235, 274, 360
460sn;
467 sd
100
77.4
30.6
10
11
232, 268, 355
234, 280, 368
434 sn;
475 sd
79
99
63.1
34.6
52.2
15.1
467 sn;
465 sd
12
236, 275, 365
462sn;
478 sd
97
82.6
30.9
13
14
265, 329
416 sn;
414 sd
87
2.1
4.0
2.0
0.9
252, 280, 340
440 sn;
538 sd
100
(continued on next page)
6

I. Karpavičienė, et al.
Table 4 (continued)
DyesandPigments170(2019)107646
Entry
15
Compound
λ max (abs), nma
236, 270, 340
λ max (em), nmb
Stokes shifts (nm)
120
QY, % (solution)
QY, % (solid)
11.9
460 sn;
550 sd
33.9
16
242, 284, 332
436 sn;
104
1.9
8.4
436, 525 sd
17
18
240, 268, 326
236, 272, 318
406 sn;
446 sd
80
2.0
1.4
6.1
NA
420
102
a
Absorption bands in bold correspond to the excitation wavelength.
Emission spectra written in solution, otherwise indicated as Sn-in solution or Sd-in solid state.
b
Fig. 2. Solvatochromism (A), viscosity (B), oxygen quenching (C) and variable temperature experiments (D) for 3cd.
and EDG in the position 5 of the thiophene core. Additionally, ag-
gregation induced emission enhancement (AIEE) was also observed for
3dd and 3ed.
emission maxima were also shifted from 434 to 467 nm, when going
from 3ac to 3bc. Similar bathochromic shifts were observed for 2-
pyridinyl thiophenes bearing electron-rich 4-methoxyphenyl sub-
stituent in various positions of the thiophene subunit (Table 4, entries
14–16). Stokes shifts for all materials were in the range of 79–120 nm
(see Table 4).
All thiophenes in MeCN solution feature broad emission peaks
without any distinctive bands (Table 4). The broad nature of emission
peaks is indicative of twisted intramolecular charge transfer (TICT)
phenomenon [24]. To verify whether TICT is responsible for the ob-
served emission, a solvatochromism study was conducted because
sensitivity of TICT towards solvent effects is well-known [24,25].
Thiophene 3cd was used in the solvatochromism study due to higher
solubility and relatively high intensity of the emission (33.9% PLQY,
The benzimidazole ring-containing compounds showed three char-
acteristic absorption bands at 232–238 (band 1); 266–280 (band 2) and
347–368 (band 3) nm respectively (Table 4, entries 1–12). However,
only one broad emission band with maxima at 433–467 nm was ob-
served in MeCN solutions for these compounds. The presence of elec-
tron-donating substituent in the 5-aryl ring resulted in bathochromic
shift of absorption bands 2 and 3 as well as of the emission peak
maxima. This becomes clear by comparing 3ac (Table 4, entry 10) and
3bc (Table 4, entry 11), where introduction of electron-donating sub-
stituents results in the shift of absorption band 2 from 268 to 280 nm,
and the shift of absorption band 3 from 355 to 368 nm. In addition,
7

I. Karpavičienė, et al.
DyesandPigments170(2019)107646
entry 15, Table 4). The emission maxima for 3cd featured a shift from
440 nm in dioxane to 468 nm in DMSO (Fig. 2A), thus providing an
evidence for the TICT. TICT states are also highly susceptible to visc-
osity of a solvent: enhanced emission is generally observed in solvents
with higher viscosity [26]. Accordingly, the emission intensity of 3cd
was measured in various EtOH/glycerine mixtures (Fig. 2B). The ob-
served correlation between the emission intensity and the viscosity of
the solvent provided further support of TICT.
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4. Conclusions
In summary, we have demonstrated that odorless, stable, easy-to-
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containing building blocks for the assembly of 2,3,5-trisubstituted
thiophene luminophores via the Fiesselmann type reaction. We have
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and electron rich arenes in position 5 of the thiophene core not only
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luminophores, but also enhances PLQY of the less emissive, pyridine-
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Acknowledgements
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
Appendix A. Supplementary data
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