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Two novel D-A-π-A1 metal-free organic dyes of the KEA series containing benzo[d][1,2,3]thiadiazole (isoBT) internal acceptor, indoline donors fused with cyclopentane or cyclohexane rings (D), a thiophene as a π-spacer, and a cyanoacrylate as an anchor part were synthesized. Monoarylation of 4,7-dibromobenzo[d][1,2,3]thiadiazole by Suzuki-Miyamura cross-coupling reaction showed that in the case of indoline and carbazole donors, the reaction was non-selective, i.e., two monosubstituted derivatives were isolated in each case, whereas only one mono-isomer was formed with phenyl- and 2-thienylboronic acids. This was explained by the fact that heterocyclic indoline and carbazole fragments are much stronger donor groups compared to thiophene and benzene, as confirmed by cyclic voltammetry measurements and calculation of HOMO energies of indoline, carbazole, thiophene and benzene molecules. The structure of monoaryl(hetaryl) derivatives was strictly proven by NMR spectroscopy and X-ray diffraction. The optical and photovoltaic properties observed for the KEA dyes showed that these compounds are promising for the creation of solar cells. A comparison with symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 showed that the asymmetric nature of benzo[d][1,2,3]thiadiazole KEA dyes leads to a hypsochromic shift of the ICT band in comparison with the corresponding benzo[c][1,2,5]thiadiazole isomers. KEA dyes have a narrow HOMO-LUMO gap of 1.5–1.6 eV. Amongst these dyes, KEA321 recorded the best power efficiency (PCE), i.e., 5.17%, which is superior to the corresponding symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 (5.07 and 4.90%). [ABSTRACT FROM AUTHOR]

An efficient synthesis of 4,7‐dibromo[1,2,5]thiadiazolo[3,4‐d]pyridazine is reported. For the first time, palladium‐catalysed cross‐coupling reactions of a dihalo derivative was found to be a powerful tool for the selective formation of various mono‐ and diarylated derivatives of strongly electron‐accepting heterocycles. Suzuki–Miyaura coupling can be successfully employed for the preparation of mono‐arylated derivatives, whereas Stille coupling is useful for both mono‐ and diaryl(hetaryl)ated heterocycles. The cyclic voltammogram showed that 4,7‐dibromo[1,2,5]thiadiazolo[3,4‐d]pyridazine can be easily oxidized to form a stable anion radical. The calculated values of ELUMO confirmed that [1,2,5]thiadiazolo[3,4‐d]pyridazine is one of the strongest electron–acceptor systems. The 4,7‐dibromo derivative of highly electron‐deficient [1,2,5]thiadiazolo[3,4‐d]pyridazine was synthesized. Conditions for the selective formation of both mono‐ and di‐aryl(hetaryl)ated [1,2,5]thiadiazolo[3,4‐d]pyridazines by palladium‐catalysed cross‐coupling reactions were found. [ABSTRACT FROM AUTHOR]

Four new D-A-π-A metal-free organic sensitizers for dye-sensitized solar cells (DSSCs), with [1,2,5]thiadiazolo[3,4-d]pyridazine as internal acceptor, thiophene unit as π-spacer and cyanoacrylate as anchoring electron acceptor, have been synthesized. The donor moiety was introduced into [1,2,5]thiadiazolo[3,4-d]pyridazine by nucleophilic aromatic substitution and Suzuki cross-coupling reactions, allowing design of D-A-π-A sensitizers with the donor attached to the internal heterocyclic acceptor not only by the carbon atom, as it is in a majority of DSSCs, but by the nitrogen atom also. Although low values of power conversion efficiency (PCE) were found, a few important consequences were identified: (i) poor PCE data can be attributed to high electron deficiency of the internal [1,2,5]thiadiazolo[3,4-d]pyridazine acceptor due to lower light harvesting by the dye; (ii) the manner in which the donor was attached to the internal acceptor (by carbon or nitrogen) did not play an essential role in the photovoltaic properties of the dyes; (iii) dyes based on the novel donor 2,3,4,4a,9,9a-hexahydro-1H-1,4-methanocarbazolyl and 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H- carbazole moieties showed similar photovoltaic properties to dyes based on the well-known 4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indolyl building block, which opens the door for further optimization potential of new dye families. [ABSTRACT FROM AUTHOR]

Four new D-A-π-A metal-free organic sensitizers for dye-sensitized solar cells (DSSCs), with [1,2,5]thiadiazolo[3 ,4- d ]pyridazine as internal acceptor, thiophene unit as π-spacer and cyanoacrylate as anchoring electron acceptor, have been synthesized. The donor moiety was introduced into [1,2,5]thiadiazolo[3,4- d ]pyridazine by nucleophilic aromatic substitution and Suzuki cross-coupling reactions, allowing design of D-A-π-A sensitizers with the donor attached to the internal heterocyclic acceptor not only by the carbon atom, as it is in a majority of DSSCs, but by the nitrogen atom also. Although low values of power conversion efficiency (PCE) were found, a few important consequences were identified: (i) poor PCE data can be attributed to high electron deficiency of the internal [1,2,5]thiadiazolo[3,4- d ]pyridazine acceptor due to lower light harvesting by the dye; (ii) the manner in which the donor was attached to the internal acceptor (by carbon or nitrogen) did not play an essential role in the photovoltaic properties of the dyes; (iii) dyes based on the novel donor 2,3,4,4a,9,9a-hexahydro-1 H -1,4-methanocarbazolyl and 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H- carbazole moieties showed similar photovoltaic properties to dyes based on the well-known 4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indolyl building block, which opens the door for further optimization potential of new dye families., Competing Interests: The authors declare no conflict of interest.