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

Abstract

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) as the electron donor and [60]­fullerene (C₆₀) 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 tweezer-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 of the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)₂-[2]­catenate-C₆₀ system were compared with those of the (ZnP)₂-rotaxane-C₆₀ precursor and the previously reported ZnP-[2]­catenate-C₆₀. 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^•+/C₆₀^•– charge-separated radical pair (CSRP) state. However, the decay kinetics of the CSRP 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)­phen₂]⁺–C₆₀^•– CSRP state is more than four times longer in <b>3</b> (1.03 μs) than in <b>1</b> (0.24 μs) and approaches that in catenate <b>2</b> (1.1 μs). The results clearly showed that creation of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, when the Cu­(I) ion used as the template to assemble the (ZnP)₂–[Cu­(I)­phen₂]⁺–C₆₀ rotaxane was removed, it was evident that a major structural change had occurred. since charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as in polar solvents. Only ZnP and C₆₀ triplet excited states were observed upon laser excitation of the Cu-free rotaxane. These results highlight the critical importance of the central Cu­(I) ion for long-range ET processes in these nanoscale interlocked electron donor–acceptor systems