Fluorescence quenching by electron transfer in quinolinium betaines. Semiempirical and experimental studies

Intramolecular fluorescence deactivation of photoexcited N-alkylated quinolinium ions and their betaine derivatives with a sulfonato group at the chain end are investigated by theie fluorescence lifetimes and by semiempirical calculations. Due to an attractive interaction between the anionic headgroup and the positively charged aromatic body cyclic conformers are formed which show a lengthened Cα(β)H and a shortened O/3.H distance. Intramolecular fluorescence quenching is regarded as an electron transfer (ET) process between the sulfonato group and the aromatic π system. The role of the CH/3.O stretching vibration as an accepting mode during ET quenching is discussed.