Kinetic Study of the Photoexcited Triplet State of Free Base Porphyrins by EPR

An electron paramagnetic resonance (EPR) study of the photoexcited triplet state of four free base porphyrins is presented. The zero field splitting parameters (ZFS) |D| and |E| were calculated from the EPR spectra of the porphyrins dissolved in n‐octane matrices at 80°K. |D| = 0.0359 cm −1 , |E| = 0.0079 cm −1 for tetra phenyl porphyrin (H 2 TPP), |D| = 0.0432 cm −1 , |E| = 0.0037 cm −1 for tetra (per‐fluoro) phenyl porphyrin H 2 T (per‐F) PP, |D| = 0.0366 cm −1 , |E| = 0.0078 cm −1 for tetra (para‐chloro) phenyl porphyrin H 2 T(P‐Cl)PP, |D| = 0.0369 cm −1 , |E| = 0.0076 cm −1 for tetra (para‐methyl) phenyl porphyrin H 2 T(P‐Me)PP. The transient behavior of the EPR signal intensities in the last two porphyrins is discussed. The depopulation rate constants of the triplet sublevels k p , the ratio between the population rate constants A p (at zero field, p = x,y,z), and the spin lattice relaxation rate W within the triplet manifold, were calculated. k x = (12 ± 2) × 10 2 sec −1 , k y = (0.5 ± 0.1) × 10 2 sec −1 , k z = (1.2 ± 0.4) × 10 2 sec −1 , A x :A y :A z ≃ 0.63:0.01:0.33, W = (0.4 ± 0.1) × 10 4 sec −1 for H 2 T(P‐Cl)PP, k x = (7 ± 2) × 10 2 sec −1 , k y = (4 ± 1) × 10 2 sec −1 , k z = (1.5 ± 0.5) × 10 2 sec −1 , A x :A y :A z ≃ 0.56:0.31:0.13, W = (1.7 ± 0.4) × 10 3 sec −1 for H 2 T(P‐Me)PP.