Nonlinear optical signature of nanostructural transition in ionic liquids

The homologous series of the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [CnC(1)im][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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The homologous series of ionic liquids [CnC(1)im][NTf2] with different alkyl chain lengths was studied using polarization-resolved second harmonic scattering (SHS). The analysis revealed an octupolar contribution dominating the SHS response, with orientational correlations observed on the nanometer scale even for the shortest alkyl chain length. There was a sharp transition in correlation range between ionic liquids with n = 4 and n = 6, indicating the dominant role of the alkyl chains.