Nonlinear optical signature of nanostructural transition in ionic liquids

The homologous series of the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [CnC1im][NTf2] (n = 2,4,6,8 and 10) was studied by polarization-resolved second harmonic scattering (SHS). The analysis of the polarization plots clearly demonstrates that the SHS response is dominated by an octupolar contribution. Orientational correlations are observed on the nanometer scale in all the investigated ionic liquids, even for the shortest alkyl chain length. A radial distribution of the nonlinear optical sources is evidenced. A sharp transition from short to longer range correlations occurs between ionic liquids with n = 4 and 6, signature of the dominating role taken by the alkyl chains. The SHS technique therefore provides structural observation in liquids at scales well-below the diffraction limit and these findings open new interesting perspectives in the domain.