Synthesis and characterisation of 3-H-indolo-2-dimethinehemicyanine dyes obtained by the condensation of (1,3,3-trimethylindolin-2-ylidene)acetaldehyde with anilines in acetic acid

Twenty, 1,3,3- trimethyl-2-[2-(2-Y-, 3- or 4-X-phenylamino)ethenyl]-3-H-indolium salts 1c–1z were synthesised and characterised by their spectroscopic properties (UV–VIS, IR and NMR), ionisation constants (pKa), and solvatochromic behaviour. The assigned 3-H-indolo-2-dimethinehemicyanine structure was proven on the basis of NMR data; the strong deshielding of the NH proton from the phenylamino moiety and of the N(1)CH3 protons from the 3-H-indole moiety (11.5–13.43 and 3.59–3.74 ppm, respectively) indicates partial positive charge at the nitrogen atoms of these groups. Such partial positive charge at both nitrogen atoms from the end of the trimethine chain is compatible with an electron delocalization through the cationic polymethine chain, which is characteristic of hemicyanine dyes. The positive charge at the mentioned NH group is also sustained by the obtained pKa values (8.05–9.37). The study concerning the solvatochromic behaviour revealed a small hypsochromic shift for all investigated dyes.