Synthesis, optical and electrochemical properties of new thieno[2,3-b]indole-based dyes

Article abstract of DOI:10.24820/ark.5550190.p010.454

Three new push-pull dyes consisting of a thieno[2,3-b]indole ring system as an electron donor, thiophene spacer as a π-bridge, and electron-withdrawing moieties such as 2-methylene malonitrile, 2-methylene-1Hindene- 1,3(2H)-dione, and 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, have been synthesized and studied for their application in organic electronics devices. Investigation of their optical and electrochemical properties reveal that these compounds possess narrow band gaps (1.7-2 eV) and an effective absorption in the visible spectral range (440-740 nm). Therefore, these chromophores can be regarded as promising light-harvesting materials.

Full text of DOI:10.24820/ark.5550190.p010.454

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Organic Chemistry
Arkivoc 2018, part v, 0-0
Synthesis, optical and electrochemical properties of
new thieno[2,3-b]indole-based dyes
Artur N. Bakiev,a* Roman A. Irgashev,d,e Elena V. Shklyaeva,b,c Aleksandr N. Vasyanin,b
George G. Abashev,a,b Gennady L. Rusinov,d,e and Valery N. Charushin d,e
a Institute of Technical Chemistry, Ural Division of the Russian Academy of Sciences,
Ac. Koroleva Str. 3, Perm 614 990, Russia
b Perm State University, Bukireva Str. 15, Perm 614 990, Russia
c Institute of Natural Sciences of Perm State University, Genkel Str. 4, Perm 614990, Russia
d Postovsky Institute of Organic Synthesis, Ural Division of the Russian Academy of Sciences,
Kovalevskaya Str. 22 / Akademicheskaya Str. 20, Ekaterinburg 620 990, Russia
e B. N. Yeltsin Ural Federal University, Mira Str. 19, Ekaterinburg 620 002, Russia
E-mail: gabashev@psu.ru
Received 12-25-2017
Abstract
Accepted 03-02-2018
Published on line 05-31-2018
Three new push-pull dyes consisting of a thieno[2,3-b]indole ring system as an electron donor, thiophene
spacer as a π-bridge, and electron-withdrawing moieties such as 2-methylene malonitrile, 2-methylene-1H-
indene-1,3(2H)-dione, and 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, have been
synthesized and studied for their application in organic electronics devices. Investigation of their optical and
electrochemical properties reveal that these compounds possess narrow band gaps (1.7-2 eV) and an effective
absorption in the visible spectral range (440-740 nm). Therefore, these chromophores can be regarded as
promising light-harvesting materials.
Keywords: Thieno[2,3-b]indoles, thiophene-2-carbaldehydes, isatins, Knoevenagel condensation, push-pull
structures, chromophores, light-harvesting dyes
DOI: https://doi.org/10.24820/ark.5550190.p010.454
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Introduction
Small π-conjugated organic molecules bearing pyrrole and thiophene rings are the focus of acute attention
due to their promising applications as electro- and photo-luminescent, charge-transport and light-harvesting
materials for high-performance electronic devices such as organic light-emitting diodes (OLEDs), organic field-
effect transistors (OFETs), and organic photovoltaic cells (OPVs).1-5 Nevertheless, literature data concerning the
applications of π-conjugated compounds based on thieno[3,2-b]indole and thieno[2,3-b]indole (TI) subunits as
active components for organic photovoltaic (OPV) devices are rather limited. For example, two series of push-
pull metal-free dyes containing the electron-rich carbazole (MK 1-8)6-7 and thieno[3,2-b]indole (MKZ 39-41)8
moieties (Fig. 1) have been synthesized, studied and further used as effective sensitizers for high-performance
dye-sensitized solar cells (DSSC).
Figure 1. Structures of carbazole- and thienoindole-based dyes for OPVs.
Recently, we have reported a convenient two-step synthetic approach to construct 2-(hetero)aryl-
substituted thieno[2,3-b]indoles using the corresponding acetylated (hetero)arenes and 1H-indole-2,3-dione
(isatin) derivatives as starting compounds.9 Furthermore, 2-thien-2-yl-substituted thieno[2,3-b]indoles were
successfully applied for preparation of the IK 1-6 series of dyes (Fig. 1). The DSSCs based on these sensitizers
have exhibited power-conversion efficiencies (PCE) up to 6.3%.10,11 It has also been reported that the planar
and bulk heterojunction solar cells (HSC) based on a donor-acceptor couple, consisting of the NTU-2 dye
(Fig.1) as an electron-donor, light-harvesting component, and C70-fullerene as an electron-acceptor
component, also exhibited high PCE values of 5.2%.12 Therefore, the incorporation of the thieno[2,3-b]indole
fragment as an electron-donating subunit into the π-conjugated systems for use as new photo- and electro-
active organic materials, and their further investigation, appear to be important issues for the field of organic
photovoltaics.
Results and Discussion
Herein, we report convenient syntheses of three new thieno[2,3-b]indole-based D-π-A chromophores, and the
investigation of their photophysical and electrochemical properties. The key stages of the reaction procedures
are outlined in Figure 2. 5-{8-(2-Ethylhexyl)-8H-thieno[2,3-b]indole-2-yl}thiophene-2-carbaldehyde (1) was
prepared in three steps from the synthetically available 1-(2-ethylhexyl)isatin and 2-acetylthiophene according
to the previously described procedures.9,11 Chromophores 2-4 were synthesized via Knoevenagel condensation
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of the thienaldehyde (1) with malononitrile, indan-2,3-dione and 2-(3-oxo-2,3-dihydroindene-1-
ylidene)malononitrile, respectively. It has been found that this aldehyde readily reacts with malononitrile in
refluxing glacial acetic acid in the presence of pyrrolidine as a base to yield (2), while the reaction of indan-2,3-
dione with its respective derivatives proceeds smoothly by heating under reflux in ethanol to yield (4) and n-
butanol to yield (3), and does not require any basic catalyst.13,14 More detailed descriptions of the processes
used to prepare aldehyde (1) and 3-(dicyanomethylidene)indan-1-one are presented in the Supplementary
Material section.
CN
CN
CH2(CN)2
S
S
AcOH,
pyrrolidine
reflux
N
ETH
2
O
O
O
O
S
O
S
n-BuOH
reflux
S
S
N
N
3
1
ETH
ETH
O
O
ETH = 2-Ethylhexyl
CN
S
NC
CN
S
NC
EtOH
60 °C
N
ETH
4
Figure 2. Key stages in syntheses of TI-based dyes 2-4.
UV-vis absorption and fluorescence spectra of compounds 2-4 were recorded for their CHCl3 solutions
(2×10-5 mol L-1). The corresponding absorption and fluorescence spectra are presented in Figure 3 and the
results are summarized in Table 1. The main absorption bands in the visible region of 440-740 nm, with high
molecular-absorption coefficients, correspond to the intramolecular charge-transfer (ICT) transitions
proceeding in a dye molecule, whereas the absorption bands in the 240-440 nm region can be attributed to
the π-π* transitions.
Figure 3. UV-vis absorption (left) and fluorescence (right) spectra of the TI-dyes in CHCl3 solutions 2-4.
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Absorption maxima (λmax) of the studied compounds are successively red-shifted going from dye (2) to dye
(4), reflecting the increase in ICT in the subsequent structures. Thus, the acceptor moiety of dye (4) causes the
most significant bathochromic shift of the absorption band due to its strong electron-withdrawing nature. At
the same time, an electron-deficient fragment of dye (4) has the more π-extended structure when compared
with the structures of acceptor moieties in dyes (2) and (3). This fact also favors a more efficient
intramolecular charge-transfer processes. The optical band gaps (Egopt) of dyes 2-4 have been calculated on the
basis of the absorption-edge values (λonset); these compounds possess a narrow band gap of 2.03 eV, 1.91 eV
and 1.69 eV, respectively. The data obtained are also presented in Table 1.
Table 1. Optical data for dyes 2-6
opt
λmax
(nm)
243,
λonset λem Stokes shift Eg
ε
Dye
(nm) (nm) nm (cm-1) (eV)*
(M-1×cm-1)
20448, 9810,
CN
S
NC
N
Me
2
288,
336, 537
249,
612 632
95 (2799)
2.03 6494, 37120
5 (MTI-DCV) 14
31128,
3
4
294,
349, 570
242,
647 671 101 (2640) 1.91 10565, 8159,
29100
27572,
321,
732 766 127 (2594) 1.69
28486,
O
369, 639
452
10409, 21835
N
S
5
6
2.38
1.86
62000 Me
71000
CN
NC
566
6 (MTI-DCI) 14
* Egopt = 1240/ λonset
Thiophene is one of the most commonly used π–spacers for push-pull molecules which are among the
most important building blocks for organic electronics materials, since its planar structure allows extended
π-conjugation in a system. Our investigation has confirmed that incorporation of a thiophene moiety between
electron-excessive and electron-deficient parts of a corresponding chromophore, e.g., compounds (5) and (6)
(Table 1), results in significant changes in the dye's absorption spectrum, including red shifts of the major
absorbance peaks, and absorption-edge values. These changes can be clearly seen if one compares the
spectral data of two pairs of dyes, i.e., (2) and (5)14 and (4) and (6).14 It is important to notice that, in our case,
the absorption intensity falls. At the same time, the influence of an alkyl substituent at the thieno[2,3-b]indole
nitrogen atom cannot be excluded. In the case of compounds (2) and (4), the +I inductive effect of a 2-
ethylhexyl chain enhances the basicity of the thieno[2,3-b]indole group to a greater extent than does a methyl
group [compounds (5) and (6)].
The electrochemical behavior of all obtained compounds was estimated by cyclic voltammetry (CV) in the
1:1 mixture of dry MeCN and CH2Cl2 containing Et4NClO4 (0.23 g, 10−3 mol) as the supporting electrolyte. (The
concentration of the compounds was fixed at 1x10-3 M to avoid aggregation.) A more detailed description of
the CV experiment is presented in the Experimental Section. Some examples of the obtained cyclic
voltammograms are displayed in Figure 4. Each CV curve shows two irreversible oxidation waves with
oxidation-peak potentials (Eox1, Eox2), the values of which depend on the nature of the electron-withdrawing
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group. The values of oxidation potentials, as well as the values of the onset oxidation potentials (Eox onset), are
summarized in Table 2. The presence of the strong electron-accepting group in a dye molecule leads to the
expected positive shift in the values of oxidation potentials. Therefore, the string of anodic peak potentials
(Eox2) associated with formation of a radical cation, are as follows: 1.59 V (2), 1.62 V (3), 1.73 V (4).
HOMO and LUMO energy-level values, determined from experimental cyclic voltammetry, theoretical
calculations, and band gap energy values found from UV-vis measurements, are summarized in Table 2.
Figure 4. Comparison of CV curves of the dyes 2-4 (CH3CN/CH2Cl2, Et4NClO4, 10-3M, 50 mV/s, 1st cycle).
The empirical HOMO and LUMO energy levels (EHOMO, cv; ELUMO, cv) of the dyes 2-4 were obtained using the
equations15 presented in Supplementary Material Section.
It should be noted that the optical and electrical characteristics of compounds 2-4 are comparable with
those of known organic dyes such as NTU-2 dyes.12 Dyes 2-4 possess sufficiently high molar-absorption
coefficients in the visible light region (Table 1), which is one of the most important and necessary
characteristics of the light-harvesting materials for solar-cell applications. In addition, their HOMO levels
(Table 2) are lower than the value of the indium-tin oxide function (WF) (-4.7 eV),16 which is a conductive layer
in HSC photoanodes. This makes the regeneration of oxidized dye molecules, after injection of excited
electrons into an acceptor layer, possible. The latter process is energetically permitted for [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM-60), which is one of the most commonly used electron-acceptor materials for
HSC, since LUMO energy levels of dyes 2-4 (Table 2) are higher than the LUMO level (-3.7eV) of PCBM-60.17
Table 2. Electrochemical data of dyes 2-4
1
2
opt
Eox
,
Eox Eox
EHOMO, cv ELUMO, EHOMO, th ELUMO,th Egap, th
Eg
onset
Dye
V
(V)
(V)
(eV)
-5.33
-5.21
-5.26
cv, (eV)
-3.28
-3.37
-3.59
(eV)
-5.65
-5.36
-5.55
(eV)
-3.01
-2.77
-3.24
(eV)
2.64
2.59
2.30
(eV)
2.03
1.91
1.69
2
3
4
1.07 1.59 0.94
1.18 1.62 0.82
1.26 1.73 0.87
In order to obtain additional information on the electronic structure of compounds 2-4, quantum chemical
calculations of the energy and geometry of their frontier molecular orbitals have been performed using the
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FireFly package.18 Geometries of all compounds have been fully optimized without symmetry constraints at
the density-functional theory (DFT) (B3LYP)19-22 level with the Def2-TZVP basis set.23 It becomes clear that
electronic distribution of the HOMO of compounds 2-4 is delocalized over the entire molecule with the
exception of a phenylene unit in compounds (3) and (4). The LUMO orbitals are delocalized substantially over
the acceptor moieties and thiophene spacer of compounds, and partially over the thiophene unit of a
thieno[2,3-b]indole moiety (Figures 5-7).
Figure 5. Calculated HOMO and LUMO electron density of dye (2).
Figure 6. Calculated HOMO and LUMO electron density of dye (3).
Figure 7. Calculated HOMO and LUMO electron density of dye (4).
The calculated energies of HOMO and LUMO (EHOMO, th; ELUMO, th) listed in Table 2 are rather close to the
values obtained through the cyclic-voltammetry measurements. The calculated HOMO levels were 0.15-0.32
eV lower than the experimentally obtained levels, whereas the calculated LUMO levels were 0.27-0.4 eV
higher. Compound (4) shows the smallest energy gap (2.3 eV) while compound (2) shows the largest (2.64 eV)
energy gap. The calculated values of energy gaps (Egap, th) are also in agreement with the Egopt values obtained
from the UV-Vis data.
Conclusions
In summary, we have synthesized and characterized three new push-pull organic dyes bearing the thieno[2,3-
b]indole electron-excessive system and three different electron-withdrawing fragments. The main spectral and
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electrochemical characteristics of these compounds have been elucidated, and these data have been used to
estimate their HOMO / LUMO energy levels and energy-band gaps. It has been found that the dyes possess
narrow band gaps of 2.03 eV, 1.91 eV and 1.69 eV, respectively, and an effective absorption in the visible
spectral region. The absorption maxima of their UV-spectra are located in the range of 440-740 nm. Therefore,
the obtained push-pull chromophores can be considered promising light-harvesting materials for planar as
well as bulk heterojunction solar cells. Further studies are in progress.
Experimental Section
1
General. H and 13C NMR spectra were recorded on a Bruker Avance III HD 400 (400 MHz) spectrometer in
CDCl3 with hexamethyldisiloxane (0.055 ppm) as an internal standard. Elemental analysis was carried out using
a CHNS-932 LECO Corp analyzer. IR spectra were recorded on a Spectrum Two FTIR spectrometer (Perkin
Elmer) for chloroform solutions of the samples. Reaction course and purity of both starting and resulting
compounds were monitored by thin-layer chromatography on Sorbfil plates. The mixtures were separated and
target products purified using column chromatography on silica gel (Lancaster, Silica Gel 60, 0.060–0.2 mm).
Mass spectra of the starting compounds were recorded on an Agilent Technologies 6890N/5975B instrument
(ionization electron energy 70 eV). UV-Vis absorption spectra were recorded for chloroform solutions of
compounds in 10 mm cuvettes using a Shimadzu UV-2600 spectrophotometer. Fluorescence spectra were
recorded using a Shimadzu RF-5301 PC spectrofluorophotometer (excitation wavelength 220 nm, cuvette
dimensions 10×10 mm, solvent - dried CHCl3). Electrochemical measurements were carried out on a
potentiostat/galvanostat (ZRA Interface 1000) using a three-electrode cell with carbon-pyroceramic working
electrode, a Pt wire counter electrode, and a Ag/Ag+ reference electrode in a 0.1 M solution of Et4NClO4 in
acetonitrile-dichloromethane mixture (1:1, v/v). Potential scan rate was 50 mV/s.
Quantum chemical calculations of molecular orbital energies using B3LYP/6-31G(d) were performed using the
Firefly software on a PGU Tesla supercomputer.
2-(3-Oxo-2,3-dihydroindene-1-ylidene)malononitrile was synthesized according to the synthetic procedure
described in [11].
{[(8-(2-Еthylhexyl)-8H-thieno[2,3-b]indol-2-yl)]thiophen-2-ylmethylidene}propanedinitrile (2). The mixture
of aldehyde (1) (198 mg, 0.5 mmol), CH2(CN)2 (66 mg, 1 mmol) and pyrrolidine (0.02 ml) was heated under
reflux in AcOH (5 ml) for 4 h, then cooled and diluted with MeOH (10 ml). The resulting solid was filtered off,
washed with MeOH and dried, affording the dye (2) as a black powder. Yield: 156 mg (70%), mp 104-106 оС. IR,
cm−1: 2222 (C≡N) (CHCl3). 1H NMR (CDCl3, 400 MHz) : 0.82 (3H, t, 3JHH 8 Hz), 0.89 (3H, t, 3JHH 7.5 Hz) 1.34 (8H,
m), 2.08 (1H, m), 4.04 (2H, d, 3JHH 7.5 Hz), 7.16 (1H, d, 3JHH 4.2 Hz), 7.19 (1H, t, 3JHH 8.0 Hz), 7.27 (1H, d, 3JHH 8.3
Hz), 7.31 (1H, t, 3JHH 8.0 Hz), 7.50 (1H, d, 3JHH 4.2 Hz), 7.74 (1H, s), 7.76 (1H, d, 3JHH 7.8 Hz). 13C NMR (CDCl3, 400
MHz,) δ = 10.6, 13.9, 22.6, 24.2, 28.7, 31.9, 39.1, 50.7, 74.1, 109.7, 113.8, 114.7, 119.2, 122.4, 123.2, 125.9,
129.8, 130.8, 131.8, 140.4, 141.0, 142.7, 146.3, 147.0, 149.5, 152.0. Anal. calcd. for C26H25N3S2 (443.63): C,
70.39; H, 5.68; N, 9.47; S, 14.46. Found: C, 70.32; H, 5.65; N, 9.53; S, 14.41 %.
2-({5-[8-(2-Ethylhexyl)-8H-thieno[2,3-b]indol-2-yl}thiophen-2-yl]methylidene)-2,3-dihydro-1H-indene-1,3-
dione (3). 1,3-Indanedione (40 mg, 0.30 mmol,) and aldehyde (1) (90 mg, 0.25 mmol) were dissolved in n-
butanol (3ml). After 3 h of reflux, the reaction mixture was cooled down to room temperature. The crude
product was filtered off and purified by column chromatography (silica gel, eluent – mixture CH2Cl2/Hex, 1:1)
о
to get the desired compound (3) as a purple solid, yield 90 mg (68%). M.p. 115-117 С. IR, cm−1: 1716 (C= О)
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1
3
3
(CHCl3). H NMR (CDCl3, 400 MHz) : 0.80 (3H, t, JHH 7.3 Hz), 0.92 (3H, t, JHH 7.6 Hz), 1.27 (8H, m), 2.11 (1H,
m), 4.05 (2H, d, 3JHH 7.5 Hz), 7.18 (1H, t, 3JHH 8.0 Hz), 7.26 (1H, d, 3JHH 8.4 Hz), 7.31 (1H, t, 3JHH 8.1 Hz), 7.38 (1H,
3
3
3
d, JHH 8.3 Hz), 7.69-7.72 (2H, m), 7.77 (1H, d, JHH 7.6 Hz), 7.80 (1H, s), 7.83 (1H, d, JHH 4.1 Hz), 7.73 (s, 1H),
7.91 (2H, m). 13C NMR (CDCl3, 400 MHz,) δ = 10.6, 13.9, 22.6, 24.2, 28.6, 31.8, 39.1, 50.7, 119.7, 116.7, 119.9,
120.2, 121.9, 122.5, 122.7, 125.1, 127.2, 128.8, 132.6, 134.5, 134.7, 135.3, 136.1, 137.8, 140.5, 141.9, 142.6,
144.0, 151.4, 154.6, 158.8, 188.5, 192.5. Anal. calcd. for C32H29NO2S2 (523.71): C, 73.39; H, 5.58; N, 2.67; S,
12.25. Found: C, 73.36; H, 5.53; N, 2.71; S, 12.15 %.
2-[2-({5-[8-(2-Ethylhexyl)-8H-thieno[2,3-b]indol-2-yl]thiophen-2-yl}methylidene)-3-oxo-2,3-dihydro-1H-
inden-1-ylidene]propanedinitrile (4). 3-(Dicyanomethylene)indan-1-one (30 mg, 0.18 mmol,) was added to a
solution of aldehyde (1) (100 mg, 0.25 mmol) in ethanol (6 ml) at 60 °C. The reaction mixture was then stirred
for 5 min, cooled down to room temperature, concentrated in vacuo and purified by chromatography on silica
gel (eluent: CH2Cl2/Hex, 1:1) to get the desired compound as a dark blue solid, yield 80 mg (56 %). M.p. 194-
196 оС. IR, cm−1: 2220 (C≡N), 1690 (C= O) (CHCl3). 1H NMR (CDCl3, 400 MHz)ppm: 0.86 (3H, t, 3JHH 8), 0.91 (3H,
t, 3JHH 7.6), 1.36 (8H, m), 2.11 (1H, m), 4.04 (2H, d, 3JHH 7.5 Hz), 7.20 (1H, t, 3JHH 8.4 Hz), 7.26 (1H, d, 3JHH 4.2 Hz),
7.32 (1H, t, 3JHH 8.0 Hz), 7.38 (1H, d, 3JHH 8.8 Hz), 7.68 (2H, t, 3JHH 9.6 Hz), 7.73 (1H, d, 3JHH 4.4 Hz), 7.84(2H, d,
3JHH 8.0 Hz), 7.88 (1H, s), 8.63 (2H, d, 3JHH 8.0 Hz), 8.78 (1H, s). 13C NMR (CDCl3, 400 MHz,) δ = 10.5, 14.0, 22.9,
24.2, 29.3, 31.8, 38.9, 50.6, 76.4 112.1, 113.7, 116.3, 120.0, 122.1, 123.4, 124.2, 125.5, 125.9, 128.0, 128.7,
130.8, 132.2, 134.1, 134.7, 135.6, 140.4, 143.7, 146.8, 149.5, 153.8, 156.8, 158.7, 159.7, 167.6, 190.9. Anal.
calcd. for C35H29N3OS2 (571.75): C, 73.52; H, 5.11; N, 7.35; S, 11.22. Found: C, 73.49; H, 5.07; N, 7.27; S, 11.16
%.
Acknowledgements
This research study was financially supported by the Russian Science Foundation (Project No. 16-13-10435).
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