Intramolecular electron transfer in donor-spacer-acceptor systems with alkylamide spacers

Intramolecular electron transfer and exciplex formation have been investigated for a family of styrene-spacer-amine molecules in which the spacer consists of a rigid amide group in the middle of a flexible alkane chain. The amide is connected to the styrene by either one or two methylene units and to a trialkylamine by two to six methylene units. The dependence of electron-transfer kinetics and exciplex formation have been investigated as a function of the length of the spacer, the orientation of the amide, and the amine oxidation potential. Intramolecular quenching of the styrene fluorescence intensity and lifetime is observed for all of the styrene-spacer-amine molecules. Quenching of styrene fluorescence is accompanied by the appearance of intramolecular styrene-amine exciplex fluorescence in nonpolar solvents. The low energy of the exciplex fluorescence emission maximum in nonpolar solvents is attributed to internal solvation by the polar amide group. A maximum in the quantum yield for exciplex fluorescence and minimum in the styrene lifetime is observed for spacers with a total length of seven atoms, including the amide group. This unusual chain length dependence is attributed to the conformational requirements of the amide group.