Co-facial magnesium porphyrin dimer complexed with fullerene: Photosynthetic reaction center model of 'special pair' self-assembled to electron acceptor

Using a self-assembled supramolecular approach, a closer model for the photosynthetic reaction center 'special pair' assembled with an electron acceptor conjugate, is reported. As the 'special pair' donor, magnesium meso-(benzo-15-crown-5)porphyrin was self-assembled with K⁺ to form a highly stable Mg porphyrin dimer with a co-facial geometry arrangement, which is a better electron donor since it is nearly 130 mV easier to oxidize compared to the earlier reported Zn porphyrin dimer analog. Further, pyridine and alkyl ammonium cation-functionalized fullerene was attached via axial coordination and crown ether-alkyl ammonium cation complexation to form the four-fold, donor-acceptor complex. The self-assembled supramolecular complex, constructed using this approach, possesses defined geometry and orientation. Efficient charge separation from the singlet excited state of the Mg porphyrin dimer to the fullerene entity and a relatively slow charge recombination, were revealed by time-resolved emission and nanosecond transient absorption spectral studies, supporting the impending exploitation of these novel biomimetic model systems for light energy harvesting applications.