212 Abstracts

coo1 ULTRAFAST ELECTRONIC AND VIBRATIONAL RELAXATIONS OF PHOTOEXCITED HEME ANALOGS IN THE CONDENSED AND GAS PHASES

J. Rodriguez: C. Kirmaier,b and D. Holtenb “Physics Department, Centenary College, Shreveport, LA 71134 bChernisv Department, Washington University, St. Louis, MO 63130

The photo-induced dissociation of a diatomic ligand such as 0, or CO from the heme active sites of hemoglobin and myoglobin, and the subsequent stages of ligand-heme recombination have been studied extensively by time-resolved optical techniques. The early stages of the process is known to be quite complex and may involve electronic, vibrational and conformational relaxations, and geminate recombination. Deducing the involvement of these processes and their contributions to the time-resolved optical and vibrational data is quite difficult. Insights into the underlying factors can be provided by studies on simple metallo- porphyrin systems. We have been particularly interested in ultrafast electronic relaxations in metalloporphyrin complexes, the time scale of vibrational relaxation and the effects of excess vibrational energy on electronic spectra, and the role that the medium may play in these phenomenon. Results from some of our recent studies in these areas will be presented. For example, a number of porphyrinic systems -- monomers, dimers, donor/acceptor complexes -- all show a complex, wavelength-dependent evolution of the absorption changes following an ultrafast and highly energetic electronic relaxation [1,2]. We believe that this behavior in large part reflects the redistribution and dissipation of excess vibrational energy. Interesting effects of excess vibrational energy on electronic emission are also observed, and do not follow the expectations based on a simple hot-band picture [2]. Collectively, the observations suggest that the presence of excess vibrational energy may result in anharmonic expansion of the porphyrin, which perturbs the electronic spectra directly and possibly indirectly via a perturbed effect of the medium on the complex. We also will report on our initial ultrafast studies of porphyrins in the gas phase. We find that transferring a porphyrin from the condensed to gas phase can have a dramatic effect on the excited state relaxation behavior, and that the changes involve more than the expected increase in the time for vibrational cooling. 1. J. Rodriguez and D. Holten, J. Chem. Phys., 91, 3525 (1989). 2. J. Rodriguez, C. Kirmaier and D. Holten, J. Chem. Phys., 94, 6020 (1991).