PROPERTIES OF SINGLET EXCITED N-(PYRIMIDIN-2-ONE-4-YL)PYRIDINIUM CHLORIDE AND STRUCTURALLY RELATED ORGANIC CATIONS

The UV absorption and fluorescence spectra, fluorescence quantum yield s and lifetimes of four new fluorescent N-(pyrimidin-2-one-4-yl)pyridi nium and N-(pyrimidin-2-one-4-yl)methylimidazolium cations were measur ed in several solvents at room temperature and in ethanol glasses at 7 7 K. The spectroscopic studies were supplemented by theoretical calcul ations of the electronic transition energies, oscillator strengths and dipole moment changes by the INDO/S CI method. It is concluded that t he lowest excited singlet state of the compounds studied has charge tr ansfer character. The significantly different charge distribution in t he S-1 state and the rotational relaxation of the excited molecules ar e responsible for the large Stokes shift observed in the spectra of py ridinium cations. N-(Pyrimidin-2-one-4-yl)methylimidazolium cations ar e more efficient fluorophores than pyridinium cations and this is due to the increased rate of non-radiative decay in the tatter compounds.