Photoinduced Evolution on the Conformational Landscape of Nonplanar Dodecaphenylporphyrin:  Picosecond Relaxation Dynamics in the ¹(π,π*) Excited State

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