Dicationic and monocationic benzobisthiazolium salts as potential NLO chromophores

Abstract Dicationic salts derived from linear and angular benzobisthiazole skeleton were prepared by methylation under microwave conditions. The following condensation with aromatic aldehydes afforded two-armed conjugated molecules, which represent quadrupolar systems. Monocationic angular derivatives were also prepared to study the differences in properties between the quadrupolar and dipolar structures. Absorption and fluorescence characteristics were determined, in the case of dipolar derivatives solvatochromism was studied as well. Quantum chemical calculation of absorption spectra and nonlinear optical properties (first and second hyperpolarizability) were performed. The predicted values correlate well with the experimental ones and helped to explain the split character of the main band in absorption spectra of quadrupolar angular derivatives. Although high values of second hyperpolarizability are predicted for the linear compounds, their very low solubility in common solvents is a strongly limiting factor when practical use is considered. On the contrary, the applicability of one dicationic and one monocationic angular dye, with better solubility, was successfully verified in confocal laser scanning microscopy with nonlinear excitation.