Charge delocalization and hyperpolarizability in ionic liquids

In this work the role that charge delocalization plays in the non-linear optical response of ionic liquids is evaluated. The first hyperpolarizability for the non-linear process of second harmonic generation (SHG) and second hyperpolarizability for the non-linear process of electro-optical Kerr-Effect (EOKE) of a large number of ionic liquid forming ions were estimated by means of density functional theory calculations. The results point to that both charge delocalization and molecular geometry are the key features that govern their hyperpolarizabilities. Our findings show that some of the most commonly used anions in ionic liquids are expected to present strong non-linear responses while common cations present a much more limited performance. However, this limitation can be overcome by a proper tailoring of cations to present charge delocalization over large molecular regions. The hypothesis of additivity of hyperpolarizabilities in ionic liquids is tested and exploited to obtain a map of second and third order non-linear susceptibilities of 1496 ion combinations. This map is intended to be a guide for future works on the hyperpolarizability of ILs. (C) 2021 The Authors. Published by Elsevier B.V.