Photoinduced electron transfer between coumarin dyes and N,N-dimethylaniline in imidazolium based room temperature ionic liquids: Effect of the cationʹs alkyl chain length on the bimolecular photoinduced electron transfer process

Photoinduced electron transfer (PET) reactions in room temperature ionic liquid (RTIL) have been paid great interests due to their potential applications in the field of solar energy conversion. In this work, we selected four commonly used coumarin dyes and studied the bimolecular PET reaction between respective coumarin dye and N,N-dimethylaniline (DMA) in [Emim][BF4], [Bmim][BF4], [Hmim][BF4] and [Dmim][BF4] by means of both the steady-state and time-resolved fluorescence spectroscopies. We found that the fluorescence quenching efficiency of Coumarin 152 (C152) by DMA increases with the increase of the cationʹs alkyl chain length of RTIL and that the fluorescence quenching efficiencies of Coumarin 102 (C102), Coumarin 151 (C151) and Coumarin 153 (C153) by DMA decrease with the increase of the cationʹs alkyl chain length of RTIL. We also found that the fluorescence lifetime of C152 increases with the increase of the cationʹs alkyl chain length of RTIL and that the fluorescence lifetimes of C102, C151 and C153 maintain constant with the increase of the cationʹs alkyl chain length of RTIL. In addition, we also found that the static quenching components in the fluorescence quenching of C152 by DMA in RTIL are dominated. These results demonstrate that the fluorescence lifetimes of selected fluorophores play important roles in their bimolecular PET fluorescence quenching processes in viscous media.