Nonlinear Optical Switching Behavior in the Solid State: A Theoretical Investigation on Anils

The linear (χ (1) ) and second-order nonlinear (χ (2) ) optical properties of two anil crystals, [ N -(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) and N -(3,5-di- tert -butylsalicylidene)-4-aminopyridine, denoted 4A and 4P, respectively], as well as the optical contrasts upon switching between their enol (E) and keto (K) forms, have been investigated by combining the molecular responses calculated using quantum chemistry methods and an electrostatic interaction scheme to account for the local field effects. It is found that intermolecular interactions impact differently the K/E optical contrasts in the two systems, which illustrates the importance of the supramolecular organization on the macroscopic responses. In 4A, the surrounding effects on the (hyper)polarizabilities are similar in the enol and keto forms, leading to optical contrasts very close to those of the isolated molecule. In contrast, an enhancement of the second-order susceptibility is observed in the keto form of 4P, leading to a large χ (2) ( K )/χ (2) ( E ) contrast. Moreover, the χ (2) ( 4A )/χ (2) ( 4P ) ratio for the most stable enol forms is obtained to be in good agreement with previous experimental investigations, which supports the reliability of the computational procedure.