Control over Charge Transfer through Molecular Wires by Temperature and Chemical Structure Modifications

A series of electron donor,acceptor arrays contaning pi conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene(C-60) 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 dearly confirm that, although the C-60 units are connected to the ZnP donor through pi 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 pi conjugated pathway from the donor to the acceptor Photoexcitation of ZnP-oFL(n)-C-60 results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP center dot+ oFL(n) C-60(center dot-)], 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 (beta) of 0 097 angstrom(-1) The correlation beta to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker pi-electron system break the homogeneous pi conjugation pattern, leading to higher values of beta