The effect of solvent on the excited vibronic states and first hyperpolarizability of “push–pull” merocyanines

The absorption and fluorescence spectral properties of several merocyanines with very high first hyperpolarizabilities in fluid solutions are reported. These molecules contain a cyanodicyanomethylidenedihydrofuran electron acceptor group and with either a pyridinylidene or quinolinylidene electron donor. The molecules also contain a bulky substituent group which has been incorporated in order to hinder molecular aggregation. No effect of merocyanine concentration on the absorption spectral distributions of these compounds was observed in any of the solvents studied indicating there was no molecular aggregation in solution. The absorption spectra of these compounds display changes in their vibronic structure in solvents with different polarities/polarizabilities while the fluorescence spectra exhibit the standard solvent polarity stabilizing behaviour.The first hyperpolarizabilities decreased with decreasing solvent polarity for merocyanines with a quinolinylidene electron donor and this trend was reversed for compounds with a pyridinylidene electron donor. This is consistent with changes in electron delocalization within the merocyanines induced by changes in the dielectric of the solvent.