Picosecond dynamics and mechanisms of photoexcited Cu(II)-5,10,15,20-meso-tetrakis(4-N-methylpyridyl)porphyrin quenching by oxygen-containing Lewis-base solvents

Deactivation of the lowest excited triplet (pi,pi*) state, (3)(pi,pi*), of the cationic water-soluble Cu(II)-5,10,15,20-meso-tetrakis(4-N-methylpyridyl)porphyrin (CuTMpyP4) was studied by femtosecond transient absorption spectroscopy in three oxygen-containing solvents (Lewis bases) of various polarity, water, methanol, and dimethyl sulfoxide (DMSO). In all of these solvents, the (3)(pi,pi*) state depopulation follows biexponential kinetics. A majority of the (3)(pi,pi*) state population (similar to80%) deactivates very quickly with a time constant of about 1-2 ps to give rise to formation of an exciplex (CuTMpyP4)*-L between the porphyrin in its excited (d,d) state and a solvent molecule, L, the latter playing the role of porphyrin axial ligand. The exciplex lifetime is found to depend on the solvent dielectric constant E and increases from 7 ps in water (epsilon = 78.3) to 27 ps in methanol (epsilon = 32.7), through 23 ps in DMSO (epsilon = 46.5). A minor part of the initial (3)(pi,pi*) state population (similar to20%) deactivates to the ground state, without any detectable intermediate, with time constants of 25, 8, and 11 ps in water, DMSO, and methanol, respectively. These rather fast pathways (picosecond time scale) of excitation deactivation to the ground state are interpreted in terms of quenching influence of some low-lying intramolecular charge-transfer states that belong to four- and five-coordinate CuTMpyP4. A partitioning mechanism of (3)(pi,pi*) state CuTMpyP4 molecules into two populations decaying by different paths, that is, through exciplex formation and "directly" to the ground state, is proposed.