Topological and Conformational Effects on Electron Transfer Dynamics in Porphyrin-[60]Fullerene Interlocked Systems

The effect of molecular topology, and conformation on the dynamics of photoinduced electron transfer (ET) processes has been studied in interlocked electron donor-acceptor systems, specifically rotaxanes with zinc(II)-tetraphenylporphyrin (ZnP) electron donor and [60]fullerene (C(60)) as the electron acceptor. Formation or cleavage of coordinative bonds was used to induce major topological and conformational changes in the interlocked architecture. In the first approach, the tweezers-like structure created by the two ZnP stopper groups on the thread was used as a recognition site for complexation of 1,4-diazabicyclo[2.2.2]octane (DABCO), which creates a bridge between the two ZnP moieties on the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)(2)-[2]catenate-C(60) system were compared with those of the (ZnP)(2)-rotaxane-C(60) precursor and the previously reported ZnP-[2]catenate-C(60). Steady-state emission and transient absorption studies showed that a similar multistep ET pathway emerged for rotaxanes and catenanes upon photoexcitation at various wavelengths, ultimately resulting in a long-lived ZnP(•+)/C(60) (•-) charge separated radical pair state. However, the decay kinetics of the latter states clearly reflect the topological differences between the rotaxane, the catenate, and DABCO-complexed-catenate architectures. The lifetime of the long-distance ZnP(•+)-[Cu(I)phen(2)](+)-C(60) (•-) charge separated state is more than four times longer in 3 (1.03 µs) than in 1 (0.24 µs) and approaches that in catenate 2 (1.1 µs). The results clearly showed that adoption of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, the Cu(I) ion used as template to assemble the (ZnP)(2)-[Cu(I)phen(2)](+)-C(60) rotaxane was removed, and structural analysis suggested a major topographical change occurred, such that charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as polar solvents. Only ZnP and C(60) triplet excited states were observed upon laser excitation. These results highlighted the critical importance of the central Cu(I) ion for long range ET processes in these large interlocked electron donor-acceptor systems.