Nature of the lowest electron transitions in the linear conjugated systems derivatives of 2,6-diphenylthiapyrylium: Cationic trimethinecyanine, neutral ethylene, its dication and cation-radical

A series of linear conjugated systems of different types containing the 2,6-diphenylthiapyrylium residues as terminal groups have been synthesized and investigated by the combined spectral and quantum-chemical methods. The analysis of calculated by ab initio and semi-empirical methods of their molecular geometry and electron structure, and nature of the lowest electron transitions has shown that four types, cationic cyanine, neutral polyene, polyene-dication and cationic polyene-radical differ fundamentally each other by the charge distribution and alternation of bond lengths along π-electron system, as well as by nature of their lowest electron transitions and hence of shapes of the absorption bands. It was established the cyanine-like similarity of the “middle” high intensive and narrow spectral band for the cationic polyene-radical and cyanine dye, whereas the polyene with the close electron shell exhibit the comparatively wide longwavelength bands.

► Polyenes and polymethine containing 2,6-diphenylthiapyrylium have been synthesized. ► Their electron structure and the lowest transitions have been investigated. ► Cyanine nature of “middle” absorption band for the polyene-radical was established.