INTRAMOLECULAR ELECTRON-TRANSFER IN DONOR-SPACER-ACCEPTOR SYSTEMS WITH ALKYLAMIDE SPACERS

Intramolecular electron transfer and exciplex formation have been inve stigated for a family of styrene-spacer-amine molecules in which the s pacer consists of a rigid amide group in the middle of a flexible alka ne chain. The amide is connected to the styrene by either one or two m ethylene units and to a trialkylamine by two to six methylene units. T he dependence of electron-transfer kinetics and exciplex formation hav e been investigated as a function of the length of the spacer, the ori entation of the amide, and the amine oxidation potential. Intramolecul ar quenching of the styrene fluorescence intensity and lifetime is obs erved for all of the styrene-spacer-amine molecules. Quenching of styr ene fluorescence is accompanied by the appearance of intramolecular st yrene-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 styr ene lifetime is observed for spacers with a total length of seven atom s, including the amide group. This unusual chain length dependence is attributed to the conformational requirements of the amide group.