Photophysics of Bis(chlorophyll)cyclophanes: Models of Photosynthetic Reaction Centers

The title cyclophanes are dimers of methyl mesopyropheophorbide a that are connected by two covalent linkages. Insertion of zero, one, or two magnesium ions in the cyclophane provides a series of compounds that are useful in exploring artificial photosynthesis. In contrast to recently studied cofacial porphyrins, the macrocycles of these dimers have orthogonal transition moments. This precludes strong interaction in spite of their close distance. The ground-state absorbance and emission spectra of the dimetalated and nonmetalated cyclophanes are similar to those of the corresponding monomers but show small red shifts, broadening, and hypochromism of the lowest energy (Φ –* Φ*) transition. Difference spectra for absorbance of the excited state are presented for the monomers and the monometalated cyclophane. A weak vibronic coupling mechanism is proposed to account for the absorbance band broadening and is indicated by the circular dichroism observed. The fluorescence yields of the cyclophanes are quenched by a factor of 2 with respect to the monomers. This is due primarily to a decrease in the radiative rate. As the dielectric strength of the solvent is increased, further quenching of the fluorescence and triplet yields of only the monometalated cyclophane is found. The data presented elucidate the photophysics of these chlorophyll dimers and provide the basis for further study of the photochemistry of a donor-acceptor system with restricted conformational freedom.