Charge transfer fluorescence of benzoxazol derivatives: Investigation of solvent effect on fluorescence of these dyes

Fluorescence of derivatives containing the N,N-dimethylaniline group attached to 2-phenyl-benzoxazole via double-bounded spacer shows extremely strong dependence of the position of the fluorescence maximum on solvent polarity. This indicates a strong charge transfer character of the emitting singlet state of these dyes. Electron transfer parameters such as solvent dependent reorganization energies and free energy changes for the back electron transfer process have been estimated from the analysis of the charge transfer fluorescence spectra. Surprisingly, the internal reorganization energy decreases with increasing solvent polarity. The same trend shows the values of the internal reorganization energy estimated from the temperature fluorescence measurements. This may indicate that the electronic structure of the dye has a biradical character already in the ground state in polar solvents. It has been also found that major channel of the nonradiative deactivation of the excited singlet state is an isomerisation of the compound leading to the isomer cis.