Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

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.