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 extraordinary 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 lifetimes increase to several microseconds in both polar and nonpolar solvents near 80 K. In contrast, the (d,d) lifetimes of nominally planar nickel porphyrins such as nickel tetraphenylporphyrin (NiTPP) vary only weakly with solvent dielectric properties and temperature, and typically fall in the range of 100 to 300 ps. All available evidence indicates that NiT(t-Bu)P in solution is highly ruffled (nonplanar) in the ground electronic state. It is proposed that the photoinduced conformational changes that occur in NiT(t-Bu)P in order to accommodate the excited-state electronic distribution are limited by the severe steric constraints imposed by the bulky meso tert-butyl substituents, and result in molecular and electronic asymmetry and thus a polar excited state. Solvent dielectric properties and temperature modulate these conformational excursions and thus the electronic deactivation rates by affecting the excited-state energies, porphyrin/solvent reorganizations, and the populations of low-frequency out-of-plane vibrations of the macrocycle. The novel findings for this nonplanar nickel porphyrin demonstrate the intimate connectivity that exists between the static and dynamic molecular structures of porphyrins and their ground- and excited-state electronic properties. Furthermore, the results obtained provide insights into the interactions between tetrapyrrole chromophores and their host proteins, and suggest the potential use of nonplanar porphyrins as building blocks for molecular photonics applications.