Electronic structure and solvatochromism of merocyanines based on N,N-diethylthiobarbituric acid

A series of merocyanines based on N,N-diethylthiobarbituric acid containing heterocycles of various electron-donating properties have been synthesized. With the aim of studying the dependence of their electronic structure on the main structural factors (i.e. change of the terminal groups and the length of polymethine chain), and also on the solvation, their solvatochromism has been investigated in a wide range of solvents with different polarity. Moreover, their electronic structure has been studied by NMR spectroscopy and quantum-chemical calculations. It has been found a rather significant influence of the polymethine chain length on the electronic structure of merocyanines in solvents in which the condition close to the ideal polymethine state is not achieved. By analysis of the UV/vis absorption spectra in non-polar n-hexane and low-polar toluene the important role of solvation for achievement of the cyanine limit is also shown. Only for dyes with the strong electron-donating group – the benzimidazole end group – a uniformity of bond orders in the chromophore is achieved even in n-hexane with minimum participation of the solvent. These conclusions are confirmed by the data of 1H and 13C NMR spectroscopy. Attempts of correlations of the data observed with the results of quantum-chemical calculations appeared unsatisfactory, which seems to be connected to both, the lack of suitable calculation methods, and the neglect of the solvent influence.