FEMTOSECOND TRANSIENT GRATING STUDIES OF CHLOROPHYLLS AND A CHLOROPHYLL-BASED ELECTRON DONOR-ACCEPTOR MOLECULE

Ultrafast, polarization selective, transient grating studies are perfo rmed on several chlorophylls and an electron donor-acceptor molecule, Zn DMPPH-PI, that consists of a zinc methyl 13(1)-desoxopyropheophorbi de a donor covalently linked to a pyromellitimide acceptor. The covale nt linkage between the donor and acceptor constrains the distance and geometric relationship between them. The ability of the experiment to determine the nature of a particular physical phenomenon through separ able polarization characteristics of the diffracted signal is used to analyze photophysical and photochemical processes in these molecules. This is achieved for cases in which the observed physical processes ha ve overlapping spectral contributions, a situation where linear dichro ism experiments are very difficult to interpret. Careful analysis of t he absorption grating data allows us to determine rotational anisotrop y values for the molecules in solution, two-photon absorption-state sy mmetries, and the relative orientation of the donor and acceptor in th e Zn DMPPH-PI molecule. These results demonstrate complete separation of the contributions to the time dependent diffracted signal from the two radical ion pair products formed in the electron transfer reaction .