Experimental and theoretical studies of the influence of solvent polarity on the spectral properties of two push-pull oxazol-5-(4H)-one compounds.

Spectral and photophysical properties of two derivatives of the 2-phenyl-1,3-oxazol-5(4H)-ones were studied in 17 solvents of different polarity. These compounds have either push-pull non-centrosymmetric or C3-symmetric structures with electron-withdrawing groups (2-phenyl-oxazolone) introduced onto the triphenylamine. It has been found that their spectral and photophysical properties depend on the structure of the compounds and on the solvent polarity. The non-radiative relaxation process is facilitated by an increase of the solvent polarity. The changes in the electronic absorption and fluorescence maximum positions with solvent polarity were analyzed applying different solvent polarity parameters based on Lippert-Mataga, McRae, Bakhshiev and Kawski theories or ETN scale. The long-wavelength absorption band positions exhibit a slight dependence on the solvent, whereas the fluorescence spectra demonstrate substantial positive solvatochromism. It was found that the position of the electronic absorption band depends mainly on the solute polarizability (related to the solvent refraction index function f(n2)=(n2-1)/(2n2+1)), whereas the solvent polarity influences the position of the fluorescence band. Quantum chemical calculations of the transition energies and dipole moments at the DFT level have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be 10.8 D and 13.0 D according to Bakhshiev's model. The experimental values of Δμ were compared to that one obtained from theoretical calculations for various solvents.