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

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-1 H - indene-1,3( 2H )-dione, and 2-(2-methylene-3-oxo-2,3-dihydro-1 H -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.


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. 9,11Chromophores 2-4 were synthesized via Knoevenagel condensation of the thienaldehyde (1) with malononitrile, indan-2,3-dione and 2-(3-oxo-2,3-dihydroindene-1ylidene)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-2,3dione with its respective derivatives proceeds smoothly by heating under reflux in ethanol to yield (4) and nbutanol to yield (3), and does not require any basic catalyst. 13,14More detailed descriptions of the processes used to prepare aldehyde (1) and 3-(dicyanomethylidene)indan-1-one are presented in the Supplementary Material section.UV-vis absorption and fluorescence spectra of compounds 2-4 were recorded for their CHCl 3 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.

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Absorption maxima (λ max ) 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 (E g opt ) of dyes 2-4 have been calculated on the basis of the absorption-edge values (λ onset ); 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.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 2ethylhexyl 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 CH 2 Cl 2 containing Et 4 NClO 4 (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 (E ox 1 , E ox 2 ), the values of which depend on the nature of the electron-withdrawing group.The values of oxidation potentials, as well as the values of the onset oxidation potentials (E ox 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 (E ox 2 ) 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.The empirical HOMO and LUMO energy levels (E HOMO, cv ; E LUMO, cv ) of the dyes 2-4 were obtained using the equations 15 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. 12Dyes 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-C61butyric 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. 17In 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 FireFly package. 18Geometries of all compounds have been fully optimized without symmetry constraints at the density-functional theory (DFT) (B3LYP) [19][20][21][22] level with the Def2-TZVP basis set. 23It 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).The calculated energies of HOMO and LUMO (E HOMO, th ; E LUMO, th ) listed in Table 2 are rather close to the values obtained through the cyclic-voltammetry measurements.The calculated HOMO levels were 0.15-0.32eV lower than the experimentally obtained levels, whereas the calculated LUMO levels were 0.27-0.4eV 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 (E gap, th ) are also in agreement with the E g opt values obtained from the UV-Vis data.

Conclusions
In summary, we have synthesized and characterized three new push-pull organic dyes bearing the thieno[2,3b]indole electron-excessive system and three different electron-withdrawing fragments.The main spectral and 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
General. 1 H and 13 C NMR spectra were recorded on a Bruker Avance III HD 400 (400 MHz) spectrometer in CDCl 3 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, 0.060-0.2mm).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 CHCl 3 ).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 Et 4 NClO 4 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.