Analysis of the transient EPR signals in the photoexcited triplet state. Application to porphyrin molecules

Abstract

Anomalous EPR line intensities observed in the photoexcited triplet state of porphyrins at 100°K are discussed. A kinetic model consisting of the singlet ground state and the first excited triplet state is developed. The EPR signal intensities are expressed in terms of the kinetic rate constants and are given as linear combinations of two exponents. With these expressions the transient effects on the line intensities can be explained. Experimental results for two porphyrin molecules H₂TPP (tetraphenyl porphyrin free base) and H₂(PF)TPP (pentafluorotetraphenyl porphyrin free base) are presented and compared with the theory. From the analysis we evaluated the spin lattice relaxation rate W, the depopulation rate constants k_p and the ratio between the population rate constants A_p: For H₂TPP, k_x =(6±2)×10² sec⁻¹, k_y =(3±1)×10² sec⁻¹, k_z =(1.5±0.5)×10² sec⁻¹, A_x:A_y:A_z≃1:0.6:0.2, and W =3.3×10³ sec⁻¹. For H₂(PF)TPP, k_x =(5±2)×10² sec⁻¹, k_y =(1.4±0.4)×10² sec⁻¹, k_z =(1.0±0.3)×10² sec⁻¹, A_x:A_y:A_z≃1:0.2:0.3, and W =1.7×10⁴ sec⁻¹.