Specific solute-solvent interactions and dual fluorescence of electron donor substituted bis-pyrazoquinoline in binary mixed solvents

Some extended electron donor and acceptor substituted π-electron systems, composed of a dimethyl- or diethylanilino (DMA or DEA, respectively) group as electron donor and bis-pyrazoloquinoline (DPPQ) or bis-pyrazolopyridine (DMPP) derivatives as acceptors, were studied with the aim to elucidate the contributions of various solute interactions with solvent molecules upon intramolecular charge separation. Steady state and time resolved fluorescence studies were performed for DMA-DPPQ, DMA-DMPP and DEA-DMPP in alcohols and their binary mixtures with heptane. The decay kinetics of the fluorescence of DMA-DPPQ was also studied in neat 1-propanol in dependence of the temperature between 170 and 293 K. Several different fluorescent species, which were ascribed to different complexes with alcohol molecules, contribute to the fluorescence of the various compounds in these binary mixtures and account for the complex fluorescence decay. A simplified consecutive model is proposed to explain the decay behaviour of the excited species.