Picosecond Studies of Quinone-Substituted Monometalated Porphyrin Dimers: Evidence for Superexchange-Mediated Electron Transfer in a Photosynthetic Model System

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Journal of the American Chemical Society


Time-resolved studies are reported for a series of quinone-substituted porphyrin monomsrs and monometalated phenyl-linked dimers. Irradiation of the simple porphyrin monomer systems Ph-Zn-Q, Ph-H2-Q, and 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 ~10 ps to relax. Somewhat more complex behavior is observed for the two regioisomeric monometalated porphyrin dimers {formula-omited} (gable) and Zn-H2-Q (flat), although complete recovery is again observed within ~ 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 {formula-omited} 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 {formula-omited} 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(π, π*) 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 {formula-omited} 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. © 1991, American Chemical Society. All rights reserved.

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