Temporal behavior of the singlet molecular oxygen emission in imidazolium and morpholinium ionic liquids and its implications.

Abstract

The solvent effect of ionic liquids on the lifetime of singlet molecular oxygen, O2((1)Δg), was investigated by means of time-resolved near-IR emission spectroscopy. O2((1)Δg) was generated by photosensitization of methylene blue in morpholinium and imidazolium ionic liquids, both comprising various alkyl chains of different lengths. The measured time profiles of O2((1)Δg) luminescence for the (1)Δg → (3)Σg(-) transition were represented by a single-exponential decay function. The phosphorescence lifetime was found to be correlated with the alkyl chain length of the morpholinium ionic liquids. This observation was interpreted by considering efficient quenching of O2((1)Δg) through energy transfer to the high-frequency C-H stretching modes of the N-alkyl chain in the morpholinium cation. Interestingly, we found that O2((1)Δg) quenching by the C-H stretching modes of the ring of the cations was remarkably depressed. The kinetics of the O2((1)Δg) emission decay process was discussed on the basis of the heterogeneous structure of ionic liquids consisting of polar and nonpolar domains.