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New donor–π–acceptor pyrimidine-based dyes comprising an amide moiety as an anchoring group have been designed. The dyes were synthesized by sequential procedures based on the microwave-assisted Suzuki cross-coupling and bromination reactions. The influence of the dye structure and length of π-linker on the photophysical and electrochemical properties and on the photovoltaic effectiveness of dye-sensitized solar cells was investigated. An increase in efficiency with a decrease in the length of π-linker was revealed. The D1 dye with only one 2,5-thienylene-linker provided the highest power conversion efficiency among the fabricated dye sensitized solar cells.

New push-pull chromophores based on thieno[3,2-b]indole were synthesized. The energy levels of the outer molecular orbitals and the mobility of electrons and holes in thin films prepared from solutions of these compounds were determined experimentally.

New dyes of the 3-[5-(4-alkyl-4H-thieno[3,2-b]indol-2-yl)thiophen-2-yl]-2-cyanoacrylic acid series were prepared based on 2-(thien-2-yl)-substituted thieno[3,2-b]indole by its sequential N-alkylation, formylation, and condensation of the resulting aldehydes with cyanoacetic acid. The possible application of these dyes as sensitizers of mesoporous titanium dioxide thin films in dye-sensitized solar cells (DSSC) was shown. It was established that the efficiency of DSSC based on the dye IS 4 is superior to that of a similar element based on the commercial dye N3 [cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(ii)].

Received: 27.03.2020. Accepted: 20.10.2020. Published:30.12.2020. We propose a simple method of TiO2 paste preparation from titania powder (Degussa) and organic binders (terpineol, ethyl cellulose) for making a continuous photoactive layer of a dye-sensitized solar cell (DSSC). The prepared paste was characterized by using thermogravimetric and X-ray diffraction methods for comparison with commercial paste (Solaronix). The TiO2 layer parameters for applying and annealing were optimized by varying the layer thickness and using different masks. The surface morphology of annealed layers was controlled by optical microscopy. Before TiO2 paste applying and after annealing, the conductive glass (fluorine-tin oxide — FTO) was treated by TiCl4 hydrochloric acid solution. The structure of DSSCs were composed FTOglass / TiO2 layer sensitized Ruthenium complex (N719 dye)/ iodide-based electrolyte / Pt counter electrode/ FTO glass. The DSSC photovoltaic characteristics were measured under AM 1.5G irradiation and demonstrated to be close to those of photoanodes based on the prepared and commercial pastes. The team of authors is grateful to the Russian Science Foundation for financial support, grant No. 17‑79‑20165. A. S Steparuk would like to acknowledge the financial support for the analytical studies of synthesized compounds from the Ministry of Education and Science of the Russian Federation within the framework of the State Assignment for Research (project no. AAAA-A19‑119012490006‑1).

A number of 2-(hetero)aryl-substituted thieno[3,2-b]indoles have been successfully prepared using an efficient transition-metal-free strategy, involving the Fiesselmann synthesis of methyl 5-(hetero)aryl-3-hydroxythiophene-2-carboxylates from 2-bromo-3-(hetero)arylacrylates and methyl thioglycolate, and the transformation of the synthesized 3-hydroxyesters into the corresponding thiophen-3(2H)-ones, followed by their treatment with arylhydrazines to directly form the targeted structures via Fischer indolization. At the same time, structural variety of the obtained thieno[3,2-b]indoles has been achieved due to a wide range of available starting materials, including both 2-bromo-3-(hetero)arylacrylates and arylhydrazines. In addition, two π-extended molecules, namely 1,4-bis(4H-thieno[3,2-b]indol-2-yl)benzene and 2,5-bis(4H-thieno[3,2-b]indol-2-yl)thiophene, have been synthesized in line with the current approach towards 2-(hetero)arylated thieno[3,2-b]indoles.

A series of 6H,7H-chromeno[3′,4′:4,5]thieno[3,2-b]indol-6-ones, bearing thieno[3,2-b]indole and coumarin parts in their fused molecules, were readily prepared using one-pot procedure based on reaction of 3-aminothieno[3,2-c]coumarins, in situ formed from 3-aminothieno[3,2-c]coumarin-2-carboxylates, with arylhyrazines according to the Fischer indole synthesis. In turn, the synthesis of methyl 3-aminothieno[3,2-c]coumarin-2-carboxylates were carried out in two steps from available 4-hydroxycoumarins. To this end, 4-hydroxycoumarins were successively treated with POCl3-DMF complex and hydroxylamine hydrochloride to obtain 4-chloro-3-cyanocoumarins. The latter substrates further reacted with methyl thioglycolate in the presence of NaOMe to give the required 3-aminothiophene-2-carboxylate precursors.

During this study, series of benzo[4,5]thieno[3,2-b]indoles, pyrido[3′,2’:4,5]thieno[3,2-b]indoles and pyrazino[2′,3’:4,5]thieno[3,2-b]indoles were efficiently synthesized from benzo- and pyrido- or pyrazino-fused 3-aminothiophene-2-carboxylates, respectively, using one-pot two-step strategy based on the Fischer indolization reaction. The essence of this synthetic approach is acid-promoted reaction of the 3-aminothiophene intermediates, in situ generated from the corresponding ring-fused 3-aminothiophene-2-carboxylates, with arylhydrazines to give arylhydrazones of thiophen-3(2H)-ones, followed by their indolization to afford thieno[3,2-b]indole-cored molecules.

A convenient protocol for one-pot synthesis of thieno[3,2-b]indoles, bearing aromatic, thien-2-yl or styryl fragments at C-2 position, from easily accessible 5-substituted 3-aminothiophene-2-carboxylates using the Fischer indolization reaction, was developed during this study. Two main steps of this approach are the saponification of the starting 3-aminoesters with sodium hydroxide and next treatment of the crude 3-aminoacids sodium salts with arylhydrazines in glacial acetic acid solution. The latter step includes in situ decarboxylation of the freed 3-aminothiophene-2-caboxylic acids to the 3-aminothiophenes and their acid promoted reaction with arylhydrazines to initially form arylhydrazones of 5-substituted thiophene-3(2H)-ones, which smoothly cause indolization to afford the desired thieno[3,2-b]indoles.