6690
J. Phys. Chem. B 2000, 104, 6690-6693
Photoinduced Evolution on the Conformational Landscape of Nonplanar Dodecaphenylporphyrin: Picosecond Relaxation Dynamics in the 1(π,π*) Excited State Jennifer L. Retsek,1a Steve Gentemann,1a Craig J. Medforth,1b Kevin M. Smith,1b Vladimir S. Chirvony,1c Jack Fajer,1d and Dewey Holten*,1a Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130, Department of Chemistry, UniVersity of California, DaVis, California 95616; Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F. Skaryna AVe. 70, 220072, Minsk, Belarus, and Department of Applied Science, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: April 25, 2000; In Final Form: May 25, 2000
The early events following photoexcitation of the nonplanar porphyrin 2,3,5,7,8,10,12,13,15,17,18,20dodecaphenylporphyrin (H2DPP) 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 H2DPP 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 H2DPP 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 H2DPP 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.
Introduction Steric crowding of peripheral substituents in porphyrins affords new classes of chromophores with significantly altered (photo)physical and chemical properties that result from conformational distortions of the macrocycles.2-6 Studies of these nonplanar model systems have provided insights into the potential effects of the skeletal deformations increasingly seen in crystal structures2f,7 of protein complexes containing tetrapyrrole chromophores. It is apparent that some nonplanar porphyrins (e.g., those based on 2,3,5,7,8,10,12,13,15,17,18,20dodecaphenylporphyrin (DPP) framework) can access more than one conformation in the ground electronic state,3 and that nonplanar porphyrins have a propensity for photoinduced conformational changes.4,5 For example, light-induced structural changes and vibrational-relaxation events have previously been studied for nonplanar nickel(II) porphyrins such as NiDPP and planar analogues such as nickel 5,10,15,20-tetraphenylporphyrin (NiTPP).5,8,9 Ultrafast (
