Dynamics of solvent relaxation in room temperature ionic liquids

In this Letter, the steady state and time resolved emission behaviour of two solvatochromic Coumarin dyes (Coumarin 153 and Coumarin 152) in two room temperature ionic liquids ([bmim][PF6] and [hmim][PF6]) are reported. The solvent relaxation in these ionic liquids is biphasic and retards drastically compared to conventional solvents of similar polarity. The motion of the ions is responsible for solvation in these ionic liquids. The rotational relaxation of both the dyes is also retarded in these ionic liquids in comparison to that of conventional solvents. The high viscosity of these ionic liquids is possibly responsible for the slow rotational relaxation.