Effect of donor and acceptor end-groups on electronic structure and spectral-fluorescent properties of merocyanines in frozen ethanol

Abstract

UV–vis absorption and fluorescence spectra of a series of merocyanines with varying heterocyclic donor and carbonyl-comprising acceptor groups have been studied in frozen ethanol at 77 K. Due to a greater polarity of a frozen solvent matrix in comparison with the liquid solvent, the electronic structure of donor–π–acceptor molecules in the former becomes much more dipolar. The resulting spectral effects depend on the donor–acceptor abilities of the terminal groups. In the case of positively solvatochromic indole-based dyes comprising the 1,3-indandione or barbituric acceptors, going from 293 to 77 K results in a narrowing and bathochromic shifts of absorption bands. Vice versa, for more dipolar reversely and negatively solvatochromic merocyanines comprising the thiobarbituric acceptor-group, the solvent freezing leads to a broadening and hypsochromic shifts of the absorption bands, which is very unique, taking into account that the solute vibrations/rotations should decrease in a frozen solvent. In all cases, the fluorescence bands are shifted hypsochromically at cryo-temperature and are closely mirror-like to the absorption ones, which, along with the smaller Stokes shifts, has been explained by the inhibited solvent rearrangement in the frozen matrix, resulting in a greater similarity of the ground state S0 and the fluorescent excited state S1. The deductions about the electronic structure of the studied dyes in the ground and excited states and its dependence on the donor–acceptor properties of the end-groups correlate well with the results of the PCM/(TD)-DFT calculations performed within the four-level diagram of electronic transitions.