Time-resolved EPR and ENDOR study of the photoexcited triplet state of free-base tetraphenylchlorin in a crystalline toluene matrix

A time-resolved EPR and ENDOR study of the photoexcited triplet state of the free-base tetraphenylchlorin has been made in a polycrystalline toluene matrix at 120 K. Crystallization of the toluene results in a partially aligned sample. The nature of the orientation of the solute molecules is investigated by time-resolved EPR spectroscopy using the anisotropy of the zero-field splitting tensor of the triplet state as the observable parameter. It is determined that 55% of the triplet molecules are oriented in a single crystal-like domain with the triplet z-axes oriented within 15° of the magnetic field. In the ENDOR study selective excitation, of only those molecules which have their triplet z-axes parallel to the magnetic field, has permitted the measurement of the Azzcomponent of the hyperfine coupling tensor of protons, in the reference frame of the zero-field splitting tensor. The sign and magnitude of the matrix couplings are also determined. The use of the partially oriented sample drastically enhanced the signal intensity over that achieved in a randomly oriented sample by increasing the number of molecules with their triplet z-axes parallel to the magnetic field. Additionally, time-resolved ENDOR spectroscopy allowed the hyperfine interactions to be determined at far higher temperatures than usual for the study of triplet states.