Synthesis, electronic structure and spectral fluorescent properties of vinylogous merocyanines derived from 1,3-dialkyl-benzimidazole and malononitrile

• New highly dipolar merocyanines possessing reversed solvatochromism.
• Solvent effects correlate well with the fluorescence quantum yields.
• The (TD)DFT ground and excited states optimization with PCM solvent field.
• Vibronic interactions and cis-trans rotations – causes of fluorescence quenching.
• Lesser π-charge alternation in the S1 state causes decrease of solvatofluorochromism.

A vinylogous series of merocyanines were synthesized with 1,3-dibutyl-benzimidazole and malononitrile residues as the donor and acceptor terminal groups. These dyes do not comprise carbonyl groups, which are prone to the strong specific solvation by polar solvents up to hydrogen bond formation, and nevertheless they possess distinct reversed solvatochromism, i.e. their molecules have very high dipolarity. At that, they are soluble in a wide range of solvents from n-hexane to ethanol and do not aggregate readily. They were studied thoroughly by UV/Vis, fluorescence, IR, and NMR spectroscopy methods. Their structure and spectral properties in the ground and excited fluorescent states were modelled at the DFT level both in vacuum and in solvents of various polarities by using the PCM solvent field simulation. The calculations were performed using several hybrid functionals (B3LYP, CAM-B3LYP, and wB97XD) and the split-valence 6-31G (d,p) basis set.

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