Modulation of the excited state intramolecular electron transfer reaction and dual fluorescence of crystal violet lactone in room temperature ionic liquids

Abstract

The influence of polarity, viscosity, and hydrogen bond donating ability of the medium on the fluorescence behavior of crystal violet lactone (CVL), which undergoes excited state electron transfer reaction and exhibits dual fluorescence from two different electronic states, termed as CT_A and CT_B, has been studied in six different room temperature ionic liquids (ILs) using steady state and time-resolved emission techniques. It is shown that the excited state CT_A → CT_B transformation and dual fluorescence of CVL can be controlled by appropriate choice of the ILs. While dual fluorescence of CVL is clearly observed in pyrrolidinium IL, the molecule exhibits a single fluorescence band in ammonium IL. While the second emission from the CT_B state can barely be seen in 1,3-dialkylimidazolium ILs, dual fluorescence is quite prominent in 1-butyl-2,3-dimethylimidazolium IL, [bmMim][Tf₂N]. These contrasting results have been explained taking into account the hydrogen bonding interactions of the 1,3-dialkylimidazolium ions (mediated through the C(2)-hydrogen) with CVL and the viscosity of the ILs. The excited state CT_A → CT_B reaction kinetics has been studied in IL by monitoring the time-evolution of the CT_B emission in [bmMim][Tf₂N]. The solvation dynamics in this IL has been studied by following the dynamic fluorescence Stokes shift of C153, which is used as a probe molecule. A comparison of the excited state reaction time and solvation time suggests that the rate of the CT_A → CT_B reaction in moderately viscous ILs is primarily dictated by the rate of solvation. Very little or negligible excitation wavelength dependence of the emission behavior of CVL can be observed in these ILs.