Two-laser two-color time-resolved EPR study on higher-excited-state triplet-singlet intersystem crossing of porphyrins and phthalocyanines

Photoinduced effects on the electron spin polarization (ESP) in the lowest excited triplet (T1) states of porphyrins (PORs) and phthalocyanines (PCs) have been observed with a two-color time-resolved (TR) electron paramagnetic resonance (EPR) technique in a glassy matrix at low temperatures. On single-color excitation with the wavelengths of the ground state absorptions of PORs and PCs, polarized EPR spectra due to the corresponding T1 state were observed. The polarization patterns match well with interpretation as anisotropic intersystem crossing (ISC) induced by the spinorbit coupling between the singlet excited (S1) and the triplet states. In contrast, two-color excitation led to a change of the phase of the T1 state polarization pattern to the opposite. The observed ESP in the T1 state resulting from the excitation to the upper triplet state (Tn) was interpreted in terms of anisotropic ISC between the Tn and S1 states. From the analysis of the ESP, changes in the quantum yields of the reverse ISC processes were determined at different temperatures. The results could be best interpreted by the existence of thermal pathways with small activation energy in the relaxation processes.