Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid

Abstract

We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888 ][NTf2 ]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888 ][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

Probing the heterogeneity of molecular level organization of ionic liquids: a comparative study using neutral Nile red and cationic Nile blue sulfate as fluorescent probes for butyrolactam-based protic ionic liquids

“…35 Recently, photophysics and rotational dynamics of Nile blue A in ([S 222 ][NTf 2 ]) and ([P(C 6 ) 3 C 14 ][NTf 2 ]) ILs have been reported by Aninda et al, and they noticed that the rotational diffusion of Nile blue A is influenced by H-bonding and electrostatic interactions. 36 Very recently, Rajesh et al observed similar interactions for Nile red in a neat 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide RTIL and binary IL mixture. 37 Although fluorescent molecular probes have been extensively employed to study the structural organization of aprotic ionic liquids (APILs), there is a distinct lack of explicit investigations into such studies for PILs.…”