Effect of Mutual Position of Electron Donor and Acceptor on Photoinduced Electron Transfer in Supramolecular Chlorophyll-Fullerene Dyads

In this study we have explored the influence of mutual position of chlorin electron donor and fullerene C-60 electron acceptor on photoinduced electron transfer. Two zinc-chlorin-aza-[18]crown-6 compounds and three pyrrolidino[60]fullerenes with alkyl aminium and varying coordinative moieties were synthesized and used for self-assembling of a set of complexes via two-point binding. The aza[18]crown-6 moieties were connected to chlorins via amide linker either at 13(4) or 17(4) position, hence, being attached on different sides of the chlorin plane. Furthermore, in the former case, the linker holds the crown closely spaced, whereas, in the latter, the linker gives more space and conformational freedom for the crown with respect to the chlorin macrocycle. The coordinative moieties at fullerene site, 3-pyridine, 4-pyridine, and 3-furan, were built by utilizing the Prato reaction. The two-point binding drove the molecules into specific complex formation by self-assembling; aminium ion was chelated by crown ether, while zinc moiety of chlorin was coordinated by pyridine and furan. Such pairing resulted in distinct supramolecular chlorin-fullerene dyads with defined distance and orientation. The performed computational studies at DFT level in solution, with TPSS-D3/def2-TZVP//def2-SVP, indicated different geometries and binding energies for the self-assembling complexes. Notably, the computations pointed out that for all the studied complexes, the donor acceptor distances and binding energies were dictated by chirality of pyrrolidino ring at C-60. The selective excitation of chlorin chromophore revealed efficient emission quenching in all dyads. The ultrafast spectroscopy studies suggested a fast and efficient photoinduced charge transfer in the dyads. The lifetimes of the charge separated states range from 55 to 187 ps in o-dichlorobenzene and from 14 to 60 ps in benzonitrile. Expectedly, the electron transfer rate was found to be critically dependent on the donor-acceptor distance; additionally, the mutual orientation of these entities was found to have significant contribution on the rate.