The lowest excited states of copper porphyrins

Abstract

Relaxation processes of excited copperporphyrins were studied with relevance to the structure of the substates of the lowest excited states. Lifetimes of luminescence at room temperature were determined as 17, 29, 69, and 105 ns for T(EtO)PPCu [T(EtO) PP: 5, 10, 15, 20‐tetra (p‐ethoxyphenyl)porphin], TPPCu(TPP: 5, 10, 15, 20‐tetraphenylporphin), TFPPCu[TFPP: 5, 10, 15, 20‐tetra(pentafluorophenyl)porphin], and OEPCu(OEP: 2, 3, 7, 8, 12, 13, 17, 18‐octaethylporphin) in toluene, respectively. Emission intensities and lifetimes of OEPCu and TFPPCu measured as a function of temperature show a variation ascribed to a Boltzmann distribution between the lowest trip–doublet and –quartet with an energy gap of 300–400 cm− 1. The anomalous temperature dependence for TPPCu and T(EtO)PPCu is explained by a larger energy gap and larger vibronic distortions in the excited state. The difference in behavior is attributed to the orbital nature of the triplet: ‖2 , 4[b 3 1(a 1 e)]〉 for OEPCu and TFPPCu but ‖2 , 4[b 3 1(a 2 e)]〉 for TPPCu and T(EtO)PPCu. The assumption of a low energycharge transfer state is not necessary for our analysis.