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

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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 considerably in mineral oil and at low temperature. The solvent dependence of the ground-state absorption spectrum combined with the dependence of the excited-state kinetics on detection wavelength, viscosity, and temperature suggests that NiDPP has a number of accessible conformers and that the energies of these conformers and the barriers between them differ with the electronic state. Such conformers probably differ in the type and degree of nonplanar distortion (e.g., saddle or ruffled) and in the orientations and solvent interactions of the phenyl rings. The results presented here for NiDPP, together with previous time-resolved data on metal-free nonplanar porphyrins, suggest that key properties of nonplanar tetrapyrroles include ready access to multiple conformers and a propensity for photoinduced conformational changes. Collectively, the results of spectroscopic studies on nonplanar porphyrins suggest that static and dynamic functional properties of tetrapyrrole cofactors in vivo may be strongly modulated by the steric constraints imposed by a protein matrix.

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