Local structure of ionic liquids probed by self-quenching of thiobenzophenone

Self-quenching of the triplet excited state of thiobenzophenone ((3)TBP*) in molecular solvents, a sodium dodecyl sulfate (SDS) micellar solution, and ionic liquids (ILs) was measured using nanosecond laser flash photolysis. In the molecular solvents and ILs, (3)TBP* single-exponentially decayed, and the decay was accelerated with increasing TBP concentration. Self-quenching rate constants (k(SQ)) obtained in the ILs were smaller than those in the molecular solvents with similar solvent viscosities, indicating that molecular diffusion is inhibited in the ILs. In the SDS micellar solution, the decay of (3)TBP* showed two components. The fast decay component was attributed to self-quenching of (3)TBP* which presents inside micelles containing two or more TBP molecules, while the slow decay component was from deactivation of single (3)TBP*. The decay of (3)TBP* in an IL, N,N,N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)amide (TMPA TFSA), had only one decay component, showing that unlike the micelles, the local structure of TMPA TFSA does not have a long lifetime. The cage lifetime of TMPA TFSA was estimated to be between 17 and 20 ns.