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 tripler state of the free-base tetraphenyl-chlorin has been made in a polycrystallin e 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 tripler mo lecules are oriented in a single crystal-like domain with the triplet z-axes oriented within 15 degrees of the magnetic field. In the ENDOR study selective excitation, of only those molecules which have their t riplet z-axes parallel to the magnetic field, has permitted the measur ement of the A(zz) component of the hyperfine coupling tensor of proto ns, in the reference frame of the zero-field splitting tenser. The sig n and magnitude of the matrix couplings are also determined. The use o f the partially oriented sample drastically enhanced the signal intens ity over that achieved in a randomly oriented sample by increasing the number of molecules with their triplet z-axes parallel to the magneti c field. Additionally, time-resolved ENDOR spectroscopy allowed the hy perfine interactions to be determined at far higher temperatures than usual for the study of tripler states.