Zwitterionic acceptor moieties: Small reorganization energy and unique stabilization of charge transfer products

A series of fulleropyrrolidine- and fulleropyrrolidinium-based donor-acceptor ensembles were tested in light of intrinsic reorganization energies for photoinduced electron transfer events. Among these complexes, the fulleropyrrolidinium ions are particularly well-suited for the role of an acceptor moiety. In fact, once reduced, fulleropyrrolidinium ions exhibit a zwitterionic character, due to the contemporary presence of a positive pyrrolidinium ion together with the fullerene radical ion. We demonstrate that this zwitterionic character of the acceptor plays a central role in improving acceleration of charge separation and deceleration of charge recombination. While the charge separation occurs with dynamics close to the top region of the Marcus parabola, the charge recombination rates are pushed deeply into the inverted region of the parabolic dependence. Overall, we determined reorganization energies (lambda) for the fulleropyrrolidinium-based ensemble that are appreciably smaller than those seen for the fulleropyrrolidine-based analogue, while keeping the electronic coupling element constant (!). Remarkable is the oxygen effect on the zwitterionic fulleropyrrolidinium acceptor-not seen on the fulleropyrrolidine acceptor-in the charge-separated radical pair.