Third-order nonlinear optical properties of choline chloride based deep eutectic solvents: Theoretical and experimental studies

In this work, the preparation of three deep eutectic solvents (DES) based on choline chloride as hydrogen bond acceptor and urea, malonic acid, and phenylpropionic acid as hydrogen bond donor is reported. These series of DES were used as models to measure, for the first time, the third-order optical nonlinearities in eutectic solvents. The nonlinear refractive index (n(2)) originated from the fast electronic response was determined by the close aperture Z-scan technique under excitation of laser ultrafast pulses at 800 nm, resulting in values of the order of similar to 3 x 10(-16) cm(2) W-1, which are similar to those reported recently for ionic liquids. These values are also similar to those exhibited by some traditional polar protic organic solvents such as water and ethanol. At relatively low energy pulses, the DES presented only pure refractive response, but at high energies two of them exhibited nonlinear absorption. On the other hand, ab initio theoretical calculations using density functional theory (DFT) allowed to determine the static and dynamic second hyperpolarizabilities for the DES with M11/6-311G(d,p) level of theory. The tendency of the theoretical results agrees satisfactorily with the values of n(2) obtained experimentally. Our results suggest that the presence of aromatic cloud makes that choline chloride:phenylpropionic acid contributes to produce relatively high n(2) values. This study provides relevant information to be considered when DES are intended as an alternative to standard organic solvents in the development of photonic materials.