Partial Switching in Anil Crystals: Impact on the Second-Order Nonlinear Optical Properties

The influence of partial switching on the linear (χ⁽¹⁾) and second-order nonlinear (χ⁽²⁾) optical (NLO) properties of photochromic crystals has been investigated using time-dependent density functional theory (TD-DFT) calculations. A periodic boundary conditions approach was applied to two experimentally well-characterized anil crystals 4A and 4P, which undergo tautomeric conversion between keto-amine and enol-imine forms. This enables the direct assessment of their macroscopic optical responses as a function of keto/enol (K/E) distribution. Calculations highlight that: (i) the ground-state energies and linear electric susceptibilities evolve linearly with the K/E ratio, indicating additive behavior; (ii) the band gaps, however, show system-specific trends: additive in 4A but modulated by single-molecule switching in 4P; and (iii) the second-harmonic generation (SHG) χ⁽²⁾ tensors exhibit both additive and nonadditive characteristics, with dominant components of 4P, such as χ_ZZZ⁽²⁾, showing a near-linear evolution. The good agreement with experimental SHG data for the pure K/E forms supports the reliability of the crystalline orbital TD-DFT approach for predicting NLO contrasts in solid-state photochromic systems as a function of the yield of conversion.