Time-Resolved Electron Spin Resonance of Gallium and Germanium Porphyrins in the Excited Triplet State

摘要

Gallium and germanium porphyrin complexes in the lowest excited triplet (T_1) state have been studied by time-resolved electron spin resonance (TRESR). It is found that for Ge(TPP)(OH)_2 (TPP=dianion of tetraphenylporphyrin) intersystem crossing (ISC) from the lowest excited signlet (S_1) state to the T_(1x) and T_(1y) sublevels is faster than that to the T_(1z) sublevel (T_(1x), T_(1y), and T_(1z) are sublevels of the T_1 state), while the ISC of ZnTPP and Ga(TPP)(OH) is selective to the T_(1z) sublevel. This is interpreted by a weak interaction beween that d_(#pi#) orbital of germanium and LUMO(e_g) of the porphyrin ligand, resulting in small spin-orbit coupling (SOC). The interpretation is supported by molecular orbital calculations. The ISC of Ge(OEP)(OH)_2 (OEP=dianion of octaethylporphyrin and Ge(Pc)(OH)_2 (Pc=dianion of tetra-tern butylphthalocyanine) is found to be selective to the T_(1z) sublevel in contrast to Ge(TPP)(OH)_2. This dependence on the porphyrin ligand is reasonably explained by a difference between the ~3(a_(1u)e_g) (the OEP and Pc complexes) and~3(a_(2u)e_g)(the TPP complex) configurations. This is the first observation of a difference in selective ISC between the ~3(a_(1u)e_g) and ~3(a_(2u)e_g) configurations. The TRESR spectrum of Ge(TPP)Br_2 is different from those of Ge(TPP)Cl_2 and Ge(TPP)(OH)_2, and is interpreted by SOC between the T_1 and T_2 states. From ESR parameters the square of the coefficient of the e_g orbital on bromine is evaluated as T_1 and T_2 states. From ESR parameters the square of the coefficient of the e_g orbital on bromine is evaluated as 0.018 in the T_1 state.r