Theoretical study on tertiary amine-fluorophore photoinduced electron transfer (PET) systems

A series of tertiary amine-fluorophore systems, have been investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. These systems have been proposed as fluorescent sensors for organophosphorus (OP) nerve agent mimics [J. Am. Chem. Soc. 2006, 128, 4500]. The different fluorophores considered are pyrene, coumarin, peryline and coronene. Excitation energies have been determined at the ground state DFT-optimum geometry, with and without the presence of a solvent, yielding information on the absorption spectra of these systems as well as the type of excitation contributing to the relevant excited states. In all these systems, in the initial form, excitations are found characterized by a transfer of electron from the amine to the fluorophore, consistent with the observed quenching of fluorescence. This type of excitations are absent in the corresponding quaternary ammonium systems, resulting from reaction with an OP nerve agent mimic, which is also consistent with the observed fluorescence. A simple rule is proposed for the prediction of the strength of the PET process, based on the degeneracy of the HOMO orbitals calculated for the separate constituent molecules of a sensor system.