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[chemical reaction: see text]. Treatment of N-substituted 2,5-dimethylpyrroles 2 with an equilibrated mixture of disulfur dichloride and DABCO in chloroform at 0 degrees C gives pentathiepinopyrroles 3 in moderate yields; further reaction of 3 with the same mixture at room temperature leads, in an extensive reaction cascade, to bis(dithiolo)pyrroles 4 in high yield; 2 can be converted into 4 in a one-pot operation under unusually mild conditions.

The parent bis [1,2]dithiolo [1,4]thiazine-3,5-dione 8, -3,5-dithione 11, the unsymmetrical 3-oxo-5-thione 10 and the bis[1,2]dithiolopyrrole-3,5-dione 9 are synthesised by acid catalysed cleavage of their various N-benzyl, ethyl, ethoxycarbonylethyl and propanoic acid derivatives. These N-alkyl compounds are prepared in the usual way from the appropriate N-alkyldiisopropylamine and S 2 Cl 2 . N-Benzyl derivatives 2 and 5 of the thiazine and pyrrole diones give 8 (100%) and 9 (58%) respectively with conc. H 2 SO 4 in dilute DCM solution, and the N-ethyl thiazine derivatives of the dione 12 and the keto-thione 13 give 8 (89%) and 10 (75%) respectively in conc. H 2 SO 4 at 120 ЊC. Ethyl 3-(diisopropylamino)propanoate 16 with S 2 Cl 2 gives the three thiazines 17 (30%), 18 (15%) and 19 (13%), and 17 and 19 are converted into the pyrroles 20 (95%) and 21 (90%) respectively by thermal extrusion of sulfur in refluxing xylene. All five ethyl esters, 17-21, are hydrolysed with aqueous acid to 22-26 respectively in 92-100% yield, and the N-propanoic acids 22-25 with hot conc. hydrochloric or hot 80-90% sulfuric acid are dealkylated to the corresponding parent products 8-11. It is also shown that the hydrolysis and dealkylation steps 17→22→8 can be combined in one operation (75%).

This paper presents an improved synthesis of 4,7-dibromobenzo[d][1,2,3]thiadiazole from commercially available reagents. According to quantum-mechanical calculations, benzo[d][1,2,3]thiadiazole (isoВТD) has higher values of ELUMO and energy band gap (Eg), which indicates high electron conductivity, occurring due to the high stability of the molecule in the excited state. We studied the cross-coupling reactions of this dibromide and found that the highest yields of π-spacer–acceptor–π-spacer type compounds were obtained by means of the Stille reaction. Therefore, 6 new structures of this type have been synthesized. A detailed study of the optical and electrochemical properties of the obtained π-spacer–acceptor–π-spacer type compounds in comparison with isomeric structures based on benzo[c][1,2,5]thiadiazole (BTD) showed a red shift of absorption maxima with lower absorptive and luminescent capacity. However, the addition of the 2,2′-bithiophene fragment as a π-spacer resulted in an unexpected increase of the extinction coefficient in the UV/vis spectra along with a blue shift of both absorption maxima for the isoBTD-based compound as compared to the BTD-based compound. Thus, a thorough selection of components in the designing of appropriate compounds with benzo[d][1,2,3]thiadiazole as an internal acceptor can lead to promising photovoltaic materials.