Intramolecular Photoinduced Electron Transfer in Zwitterionic Quinolinium Dyes – Experimental and Theoretical Studies

Quinolinium cations and quinolinium betaines were investigated in the representative solvents water and acetonitrile at room temperature using stationary and time-resolved fluorescence spectroscopy (Single-Photon-Counting-method). Experimental results reveal that sulfoalkyland carboxyalkyl-quinolinium compounds display a strikingly different behavior in the two solvents. Furthermore, the fluorescence lifetime depends on the length of the spacer for the sulfoalkyl compounds in acetonitrile and the carboxyalkyl compounds in water, respectively. This suggests an intramolecular interaction of the anionic headgroups with the quinolinium system in the excited state. To support this idea, different positions at the chromophore are substituted by a methylgroup in order to perturb the proposed interaction. With the intention to understand the dynamics of the postulated photoinduced electron transfer from the anionic group onto the excited quinolinium chromophore, semiempirical quantum chemical calculations were performed on the species using the PM3 hamiltonian including solvent effects by a self consistent reaction field (SCRF). We show that the Marcus theory of electron transfer may serve as a theoretical basis for a natural interpretation of the dynamic fluorescence quenching behavior.