Charge Stabilization in Axially Linked Donor – Aluminum(III) Porphyrin – Fullerene Reaction Center Models

Abstract

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge-separated state. Recently, we reported a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, Donor-AlPorFn-C60, were constructed by linking the fullerene (C60) and Donor to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF3, or AlPorF5) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF3), or 2,3,4,5,6-pentafluorophenyl (PhF5) groups in its meso positions. The C60 and Donor (BTMPA) units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge-separated state, BTMPA•+-AlPorFn-Ph-C60 •−, which lies energetically 1.50 eV above the ground state. Although the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA•+-AlPorFn-Ph-C60 •− radical pair was found to be very different in each of them. I will discuss some of these systems in terms of their design principles and photoinduced properties. Figure 1