Electron transport via saturated hydrocarbon bridges: 'exciplex' emission from flexible, rigid and semiflexible bichromophores
The photophysical properties are compared of systems containing electron donor- acceptor (D/A) pairs linked by saturated hydrocarbon bridges with various degrees of flexi- bility. It is concluded that even in fully extended conformations rapid (subnanosecond) pho- toinduced electron transfer can occur, thus providing a mechanism for quenching of 'local' fluorescence that is not restricted by the conformational dynamics of the bridge. Especially in solvents of low dielectric constant electrostatic forces strongly modify the conformational dynamics occurring after the initial charge separation (harpooning mechanism'). Further- more it is shown that the extended charge separated state may undergo radiative recombina- tion resulting in the observation of 'exciplex-like' emission. For flexibly bridged systems this allows the occurrence of multiple 'exciplex' emission from widely different conforma- tions ranging from fully extended to fully folded. The distance across which charge separa- tion and radiative recombination occur with significant rate can be extended by through-bond interaction (TJ3I) via the bridge, but even if the bridge structure and conformation do not al- low for important TBI these rates can be quite significant for bridges with a length up to that corresponding to an extended pentamethylene chain.