The lowest photoexcited triplet state of subphthalocyanine in solid and fluid environments. Time-resolved electron paramagnetic resonance studies

Time-resolved EPR spectra of the lowest excited triplet state of boron subphthalocyanine chloride have been measured in toluene at various temperatures (5-360 K). On the basis of the observed and simulated spectra, electronic structure and molecular motion of the triplet state (T-1) were analyzed, both in solid and fluid solution. The simulations were carried out using a model, which considers a temperature-dependent exchange between sites having different zero-field splitting (ZFS) parameters and molecular orientations. The ZFS parameter, D, was nearly the same at all temperatures examined. At very low temperatures, below 20 K, the spectrum was analyzed by a static model. At 40-120 K, two conformers with different ZFS parameter, E, have been found. The population ratio between the two conformers showed strong temperature dependence. These conformers were attributed to Jahn-Teller states and were identified by their different ZFS parameters. The exchange rate and activation energy of the conformers were compared with similar experiments performed in solid solution. Further increase in temperature (130-160 K) resulted in noticeable change in the spectra. However, at this temperature range the spectra could not be analyzed quantitatively because of the unstable crystal structure of toluene (soft glass). Above 163 K, the solvent turns slowly into fluid and the spectra were strongly dependent upon temperature. In this range of temperatures, molecular rotations occur, initially around the out-of-plane z-axis, and, as temperature rises, also around the in-plane x- and y-axes. Anisotropic exchange rates were obtained from the spectral simulation and were analyzed by a population exchange between the Jahn-Teller states combined with anisotropic rotations. Anisotropic spin-lattice (T-1) and spin-spin (T-2) relaxation times were also obtained and discussed. The rotations become isotropic above 263 K, where the spectrum exhibits a single sharp Lorentzian line and is analyzed in terms of the dipolar spin interaction.