PICOSECOND STUDIES OF QUINONE-SUBSTITUTED MONOMETALLATED PORPHYRIN DIMERS - EVIDENCE FOR SUPEREXCHANGE-MEDIATED ELECTRON-TRANSFER IN A PHOTOSYNTHETIC MODEL SYSTEM

Time-resolved studies are reported for a series of quinone-substituted porphyrin monomers and monometalated phenyl-linked dimers. Irradiation of the simple porphyrin monomer systems Ph-Zn-Q, Ph-H2-Q in toluene at 295 K elicits charge separation to produce the oxidized free-base (H2) or zinc (Zn) porphyrin and the reduced quinone (Q) within the 350-fs excitation flash. (Ph is the phenyl spacer utilized in the porphyrin dimers). Charge recombination occurs with a time constant of 3-6 ps, returning the system to the electronic ground state but in an excited nuclear configuration that takes approximately 10 ps to relax. Somewhat more complex behavior is observed for the two regioisomeric monometalated porphyrin dimers Zn-H2-Q (gable) and Zn-H2-Q (flat), although complete recovery is again observed within approximately 15 ps of excitation and ascribed to charge separation/recombination between the quinone and the adjacent H2 subunit. In contrast, very different photodynamic behavior is found for the regioisomeric monometalated dimers H2-Zn-Q and H2-Zn-Q, in which the central Zn porphyrin forms a built-in energy barrier between the H2 subunit and the quinone acceptor. In particular, a slow step having a time constant of 55-75 ps is observed at 295 K, in addition to the fast (< 15 ps) charge separation/recombination process involving the quinone and adjacent Zn subunit. From the absorption changes accompanying the 55-75-ps process in the H2-Zn-Q and H2-Zn-Q systems, and their absence in the other complexes, it is concluded that the slower process involves a quinone-induced deactivation of the lowest 1(pi, pi*) state of the free-base subunit to the ground state. The time constant for this slower process is only weakly dependent on temperature (and solvent), increasing for H2-Zn-Q in 2-MTHF from 55 ps at 295 K to 106 ps at 77 K. This observation, coupled with an energetic analysis, indicates that net H2* to Q electron transfer does not involve a thermally activated step. Rather, the results suggest that it takes place by a direct Zn porphyrin mediated superexchange mechanism. Additionally, the results suggest that in all complexes charge separation/recombination between the porphyrin and adjacent quinone involve vibrationally excited states.