Laser induced picosecond electron transfer with participation of excited states for porphyrins covalently linked with electron acceptor

Abstract

The dynamics and pathways of relaxation processes in meso-ortho-nitrophenyl substituted octaethylporphyrins OEP-Ph(o-NO 2 ) and PdOEP-Ph(o-NO 2 ) occurring with the participation of the S 1 and T 1 states was studied in polar (dimethylformamide) and nonpolar (toluene) solvents at 295 and 77 K using pico- and nanosecond laser kinetic spectroscopy. At 295 K, steric interactions between bulky β-alkyl substituents and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the porphyrin macrocycle (donor) and NO 2 -group (acceptor), thus leading to an efficient photoinduced electron transfer (PET). For free-base OEP-Ph(o-NO 2 ), PET occurs only via the porphyrin S 1 state within 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S 1 ∼∼>T 1 is low probable. For metallocomplex PdOEP-Ph(o-NO 2 ), PET involves both S 1 and T 1 states. In the last case, the direct PET from the T 1 state to CT state also occurs within picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K). Rate constants for PET with participation of T 1 states are by 3-5 times smaller with respect to those found for PET occurring via the S 1 state. For both compounds, the observed long-lived component (250-700 ns) in decays of the transient T 1 -T n absorption is due to the recombination processes of radical-ion pairs whose lifetime decreases with an increase of the surrounding polarity and is hardly dependent on the presence of molecular oxygen in the solution. For both compounds, PET is completely absent in rigid solutions at 77 K.