Effect of the side-chain length and NLO properties of the N-ethyl-N,N,N-trialkylammonium ionic liquids – A DFT study

•The NLO properties of the cations show a linear increament as the alkyl side-chain lengths enlarge.•The hyperpolarizability of the ionic liquids are stabilized when the anion interact with the cations.•The obtained NLO property values are large enough to be considered in new optoelectronic devices. In this work, the structural, electronic, and nonlinear optical properties of the new ionic liquids N-ethyl-N,N,N-trihexylammonium adipate (CPA6) and N-ethyl-N,N,N-trioctylammonium ethyl sulfate (ESA8) and their derivatives are reported. The effect of the length of the alkyl side-chains on the ionic liquids within the framework of the Density Functional Theory was analyzed. It was determined that the interaction energy, the dipole moment, the polarizability and the hyperpolarizability of the ionic liquids ESAn and CPAn (n = 2, …, 14) and the corresponding pure former cations, show a strong dependency on the number of ethyl radicals added or removed from the side-chains. The alkyl side-chain length is a more important factor than the interaction with the anion, where the adipate anion presents better values of the nonlinear optical properties concerning the ethyl sulfate anion. We found that the cations exhibit a direct proportional dependence on n. Meanwhile, as n increases, the anionic contributions become negligible.