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Abstract
The optical and magnetic properties of the lowest triplet state (T 1) of phthalazine were investigated in benzoic acid (BAC), p-dichlorobenzene (DCB) and p-dibromobenzene (DBB) by means of phosphorescence spectroscopy, ODMR and E.P.R. at liquid helium temperatures. Well resolved phosphorescence spectra were observed in DCB and DBB and a vibrational analysis of the spectra was made. The transition energies of the T 2(nπ *) and the S 1(nπ *) states were estimated from the excitation spectra. The energy gaps between T 1 and T 2 were estimated to be 2150, 694 and 788 cm−1 in BAC, DCB and DBB, respectively.
The sublevel schemes and the zero field splittings (ZFS) were determined. The observed changes of the ZFS on going from the polar to the non-polar hosts are discussed in terms of the vibronic and spin-orbit interactions with the nearby T 2(nπ *) state. The most radiative sublevel was found to be T2 in DCB, but T y in BAC. This difference is attributed to the shifts of the energy levels and the reduction of the molecular symmetry in the polar host. In the non-radiative decay T y was found to be the most active. Tremendous increases in the non-radiative decay rate constants were observed on going from BAC to DCB. Possible explanations for these observations are presented.
The external heavy atom effect on the phosphorescence was observed in the case of DBB. The mechanisms enhancing the radiative decay rates, including those for the vibronic bands are discussed.