Fluorescence, phosphorescence, and delayed fluorescence of benzil in imidazolium ionic liquids

Temperature dependence of the emission behaviour of benzil has been studied in three imidazolium ionic liquids differing in their polarity and viscosity. Room temperature absorption and steady-state emission spectra suggest that the ground and excited state conformers of benzil in ionic liquids are similar to those in conventional organic solvents. The non-degassed solutions of benzil in ionic liquids show phosphorescence at room temperature in contrast to conventional solvents where phosphorescence is commonly observed in degassed conditions. This study reveals that a thermally activated reverse intersystem crossing (T1→S1) process is responsible for the drastic change in phosphorescence intensity with temperature in ionic liquids. The rate constant () of this process is found to be dependent on the polarity of the media and is 5 times higher in most polar ionic liquids. The evidence of the presence of multiple conformers of benzil in frozen conditions is obtained from the excitation wavelength dependence of the phosphorescence spectra.