Control over charge transfer through molecular wires by temperature and chemical structure modifications

A series of electron donor−acceptor arrays containing π-conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene (C60) as electron acceptor have been prepared by following a convergent synthesis. The electronic interactions between the electroactive species were determined by cyclic voltammetry, UV−visible, fluorescence, and femto/nanosecond transient absorption spectroscopy. Our studies clearly confirm that, although the C60 units are connected to the ZnP donor through π-conjugated oFL frameworks, no significant electronic interactions prevail in the ground state. Theoretical calculations predict that a long-range electron transfer occurs primarily due to a maximized π-conjugated pathway from the donor to the acceptor. Photoexcitation of ZnP-oFLn-C60 results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP•+-oFLn-C60•−], with lifetimes in the microsecond time regime. Temperature-dependent photophysical experiments have shown that the charge-transfer mechanism is controllable by temperature. Both charge separation and charge recombination processes give rise to a molecular wire behavior of the oFL moiety with an attenuation factor (β) of 0.097 Å−1. The correlation β to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker π-electron system break the homogeneous π-conjugation pattern, leading to higher values of β.