Photoinduced evolution on the conformational landscape of nonplanar dodecaphenylporphyrin: Picosecond relaxation dynamics in the (1)(pi,pi*) excited state

The early events following photoexcitation of the nonplanar porphyrin 2,3,5 ,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H2DPP) have been examined using transient absorption spectroscopy. These studies were motivated by pr evious results indicating that photoinduced conformational changes underlie many of the perturbed photophysical properties of nonplanar porphyrins. It is shown here that H2DPP undergoes conformational/vibrational relaxation a nd equilibration in the (1)(pi,pi*) excited state that takes similar to 10 ps, and that this process is not observed for the nominally planar 5,10,15, 20-tetraphenylporphyrin system. The relaxation dynamics in H2DPP likely inv olve out-of-plane deformations of the porphyrin macrocycle and motions of t he peripheral phenyl rings and solvent molecules. The light-induced behavio r of H2DPP can thus be described by the following sequence of events: photo excitation produces the unrelaxed (1)(pi,pi*) excited state that evolves in to the relaxed (1)(pi,pi*) excited state in similar to 10 ps, followed by d ecay to the (3)(pi,pi*) excited state plus the ground state in similar to 1 ns.