Absorption and static emission properties of monometalated quinone-substituted porphyrin dimers: evidence for “superexchange” mediated electron transfer in multicomponent photosynthetic model systems

Abstract

The absorption and static emission properties of several selectively metalated quinone-substituted porphyrin dimers and control compounds are described. Whereas the unsubstituted metal-free and bis-zinc dimers 16, 19, 17, and 20 give rise to fluorescence emission spectra characteristic of typical monomeric free-base and zinc-containing porphyrins (e.g. 13 and 14), the corresponding monometalated dimers ( 15 and 18) display emission spectra characteristic only of free-base porphyrins, indicating that rapid exothermic energy transfer takes place between the metalated and free-base subunits in these latter systems. In the case of the control monomers, direct covalent attachment of a quinone subunit (to give 9–12) serves to quench all detectable fluorescence emission. Similar results are obtained in the case of the metal-free ( 3), bis-zinc ( 4), and “distal” monometalated materials ( 2) in either 2-methyl THF or toluene at room temperature. This suggests that in these systems net electron transfer from the porphyrin dimer ensemble to the quinone is fast compared to the rate of fluorescence emission. In the case of the “proximal” monometalated complex, 1, however, a weak but detectable fluorescence signal is observed when the sample is irradiated in toluene at room temperature, indicating that the built-in energy barrier provided by the “proximal” zinc porphyrin subunit is slowing the rate of net electron transfer. Nonetheless, even in the case of this system, the rate of net electron transfer remains exceedingly high, suggesting that the central or “proximal” metalated porphyrin serves to mediate the electron transfer process. The quantum yield for fluorescence for this material in 2-methyl THF is essentially temperature independent, increasingly by only a factor of 3 upon cooling from room temperature to 77 K. This suggests that that net electron transfer from the “distal” free-base subunit to the quinone is not thermally activated but takes place by a direct superexchange mediated process. A similar conclusion is derived from analogous studies of the “flat” compounds 5–8. Here, however, the quantum yields for fluorescence are higher through-out the series suggesting that net electron transfer is slower for these more open photosynthetic models.