Theoretical and experimental comparative study of nonlinear properties of imidazolium cation based ionic liquids

Charge delocalization and hyperpolarizability in ionic liquids

“…In that work, the slope differences were proposed to be originated by changes in the conjugation of the heterocycles produced by the steric effects associated to the increase of the alkyl chain. The increase of first and second hyperpolarizabilities with the alkyl chain length is in agreement with the results obtained in recent computational works for [N 2;k;k;k ] + based cations [52] as well as imidazolium and pyridinium based cations [49,50,35]. With regard to the numerical agreement of first hyperpolarizability, our figures are slightly higher but compatible with those given in the above mentioned works, specially taken into account that we are comparing with static calculations, sometimes involving smaller basis sets and using different definitions of first hyperpolarizability.…”

“…Higher quantitative agreement is found for the second hyperpolarizability of the [C 4 mim][Oaf] given in Ref. [35], which is just a 20% larger than the one in this work. In relation to experimental measurements, available data indicate that our simulations of second order susceptibility overestimate that of the experiments in one order of magnitude.…”

“…All the calculations were performed on properly optimized geometries and, except indicated otherwise, on isolated ions in gas phase. The level of theory was chosen in accordance with that used in recent works dealing with the NL optical properties of ILs, which rely on the same level of theory [35] or combinations of the B3LYP potential with related basis sets [50,49].…”

“…As other organic molecules, ILs can show different non-linear (NL) optical phenomena, however, the number of works devoted to experimentally determine NL optical magnitudes in ILs is still scarce [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. In contrast, several characterization techniques based on NL optics were successfully applied to ILs for studying their ultra-fast dynamics [36][37][38][39], bulk [23,40], or interface [41][42][43][44][45][46][47][48] structure.…”

“…In contrast, several characterization techniques based on NL optics were successfully applied to ILs for studying their ultra-fast dynamics [36][37][38][39], bulk [23,40], or interface [41][42][43][44][45][46][47][48] structure. Recently, some computational works have shed more light in the magnitude of the NL optical response of ILs by calculating the first and second hyperpolarizabilities of some ILs as well as of their forming ions [23,[49][50][51]35,52,53]. However, despite these pioneering works, further investigation on the origins of this NL response is required to determine reliable rules to design ILs with tailored NL optical properties.…”