Fluorescent properties of merocyanines based on 1,3-indandione

Abstract

We have studied the fluorescent properties of vinylogous series of merocyanine dyes based on 1,3-indandione and heterocycles of different electron-donating powers. Using a set of solvents with polarities lying in a wide range, we have analyzed the dependence of their solvatofluorochromism on the key structural parameters—the donor heterocyclic group and the length of the polymethine chain. It has been found that the signs of solvatochromism and solvatofluorochromism of merocyanines under study coincide. However, the solvent more weakly affects the position and the shape of their fluorescence bands than in the case of absorption spectra, especially, for negatively solvatochromic derivatives of 1,3-diphenylbenzimidazole. We have found that, upon passage from polar aprotic solvents to alcohols, the fluorescence quantum yields of dyes under investigation (irrespective of the sign of their solvatochromism) decrease. We have performed quantum-chemical calculations of merocyanine molecules by the DFT/B3LYP/6-31G(d,p) and TDDFT methods, including calculations taking into account the polarity of the medium by the PCM method. Based on the analysis of electronic transitions, we have explained the fluorescence quenching of indandione merocyanines in alcohol, which is unusual for carbonyl-containing intraionic dyes.