Picosecond dynamics of photoinduced electron transfer involving singlet and triplet states in nitroporphyrin molecules

The dynamics of relaxation processes in meso-ortho-nitrophenyl-substituted octaethylporphyrins OEP-Ph(o-NO₂) and PdOEP-Ph(o-NO₂) occurring with the participation of the S ₁ and T ₁ states is studied in polar (dimethylformamide) and nonpolar (toluene) media at 295 and 77 K using pico-and nanosecond laser kinetic spectroscopy. It is shown that, at 295 K, the steric interactions between β-alkyl substituents in porphyrins and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the donor and the acceptor, which ensures an efficient photoinduced electron transfer (PET). It is ascertained that, in free-base OEP-Ph(o-NO₂), the PET occurs only via the S ₁ state of porphyrin in time intervals of 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S ₁ ? T ₁ is unlikely. In the case of the metal complex PdOEP-Ph(o-NO₂), the PET involves both S ₁ and T ₁ states. In the PdOEP-Ph(o-NO₂) molecule, direct PET from the T ₁ state to the charge-transfer state also occurs in the picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K), with the rate constants being 3–5 times smaller in comparison with the corresponding values found for the PET occurring via the S ₁ state of this compound. For both compounds, the experimentally observed long-lived component in the decay kinetics of the induced T ₁-T _n absorption (250–700 ns) is due to the recombination processes in ion-radical pairs whose lifetime decreases with an increase in the polarity of the medium and is almost independent of the presence of molecular oxygen in the solution. The PET is completely absent at 77 K in the compounds under investigation when they are dissolved in rigid solutions.