Assessing density functional theory approaches for predicting the structure and relative energy of salicylideneaniline molecular switches in the solid state

The geometrical structures of salicylideneaniline (anil) molecular switches in the solid state have been determined using periodic structure calculations and a variety of density functional theory (DFT) exchange-correlation (XC) functionals, of which several have been tuned for the solid state. The first target was on predicting the unit cell and intramolecular geometrical parameters for three anil derivatives, i.e., the (E)-2-methoxy-6-(pyridine-3-yliminomethyl)phenol (PYV3) and N-(5-chloro-2-hydroxybenzylidene)-aniline (HC) crystals, where the enol (E) form is dominant in the crystalline state at low temperature (∼100 K), and the N-(5-chloro-2-hydroxybenzylidene)-hydroxyaniline (POC) crystal, which is mostly composed of the keto (K) form. The best performance for the unit cell parameters, in comparison with single-crystal X-ray diffraction (XRD) data, is achieved with XC functionals developed for the solid state (PBEsol and PBEsol0) as well as with ωB97X. On the other hand, the differences between the functionals are much smaller when considering the bond lengths and the valence angles so that the deviations with respect to XRD data in the bond length alternations of the key O-C=C-C=N-C (or O=C-C=C-N-C) π-conjugated segment are smaller than 0.02 Å for PBEsol0 and ωB97X. Similar trends are observed for the two polymorphic cocrystals of PYV3 with fumaric or succinic acid. The second target was the characterization of the variations of energy and structural parameters when switching between the enol and keto forms. All XC functionals predict that PYV3 presents a larger ΔE_KE value than HC, and as expected, both are larger than for POC. Still, only hybrid functionals correctly predict which form is the most stable in the crystalline state. Then, the bond length changes in the O-C=C-C=N-C (or O=C-C=C-N-C) π-conjugated segment that occur upon enol to keto transformation are similarly predicted by all functionals and are consistent with the reversal of the single/double bonds pattern. (Graph Presented).