PICOSECOND TO MICROSECOND PHOTODYNAMICS OF A NONPLANAR NICKEL PORPHYRIN - SOLVENT DIELECTRIC AND TEMPERATURE EFFECTS

The lifetime of the (d(z)(2),d(x -y)(2)) metal excited state of nickel (II) 5,10,15,20-tetra-tert-butylporphyrin (NiT(t-Bu)P) exhibits an ext raordinary dependence on solvent dielectric properties and temperature . At room temperature, the excited-state deactivation time varies from 2 ps in highly polar solvents to about 50 ns in nonpolar media. The l ifetimes increase to several microseconds in both polar and nonpolar s olvents near 80 K. In contrast, the (d,d) lifetimes of nominally plana r nickel porphyrins such as nickel tetraphenylporphyrin (NiTPP) vary o nly weakly with solvent dielectric properties and temperature, and typ ically fall in the range of 100 to 300 ps. All available evidence indi cates that NiT(t-Bu)P in solution is highly ruffled (nonplanar) in the ground electronic state. It is proposed that the photoinduced conform ational changes that occur in NiT(t-Bu)P in order to accommodate the e xcited-state electronic distribution are limited by the severe steric constraints imposed by the bulky meso tert-butyl substituents, and res ult in molecular and electronic asymmetry and thus a polar excited sta te. Solvent dielectric properties and temperature modulate these confo rmational excursions and thus the electronic deactivation rates by aff ecting the excited-state energies, porphyrin/solvent reorganizations, and the populations of low-frequency out-of-plane vibrations of the ma crocycle. The novel findings for this nonplanar nickel porphyrin demon strate the intimate connectivity that exists between the static and dy namic molecular structures of porphyrins and their ground- and excited -state electronic properties. Furthermore, the results obtained provid e insights into the interactions between tetrapyrrole chromophores and their host proteins, and suggest the potential use of nonplanar porph yrins as building blocks for molecular photonics applications.