Gallium and germanium porphyrin complexes in the lowest excited triplet (T1) 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 singlet (S1) 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 T1 state), while the ISC of ZnTPP and Ga-(TPP)(OH) is selective to the T(1z) sublevel. This is interpreted by a weak interaction between the d(π) 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-tert-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)Br2 is different from those of Ge(TPP)Cl2 and Ge(TPP)(OH)2, and is interpreted by SOC between the T1 and T2 states. From ESR parameters the square of the coefficient of the e(g) orbital on bromine is evaluated as 0.018 in the T1 state.
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