Dynamic Photophysical Properties of Conformationally Distorted Nickel Porphyrins. 1. Nickel(II) Dodecaphenylporphyrin

Abstract

Transient absorption measurements with subpicosecond resolution have been performed on nickel(II) dodecaphenylporphyrin (NiDPP), a highly nonplanar tetrapyrrole. Following photoexcitation, NiDPP deactivates by the pathway (π,π*) → (d,d) → ground state, the route proposed previously for planar analogues. However, the (π,π*) state has now been spectrally and kinetically resolved, and evidence is presented that the deactivation proceeds mainly in the singlet manifold. The lifetime of the 1(π,π*) state of NiDPP is about 0.7 ps in all solvents investigated and increases only slightly as the temperature is reduced. The (d,d) state exhibits complex spectral evolution over the following 20 ps or more, which is interpreted in terms of vibrational relaxation and cooling, together with changes in the conformation of the porphyrin macrocycle. The ligand-field excited state decays with a lifetime of about 120 ps in toluene, but this decay time, like the vibrational and conformational dynamics in the (d,d) state, slows...