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-(5chloro-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−CC−CN−C (or OC−CC−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−CC−CN−C (or OC−CC−N−C) πconjugated 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.
The nonlinear optical (NLO) properties of a double photochrome molecular switch are reported for the first time by considering the four trans forms of a dithienylethene-indolinooxazolidine hybrid. The four forms are characterized by means of hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations. Experimental measurements provide evidence that the pH- and light-triggered transformations between the different forms of the hybrid are accompanied by large variations of the first hyperpolarizability, which makes this compound an effective multistate NLO switch. Quantum chemical calculations conducted at the time-dependent Hartree-Fock and time-dependent DFT levels agree with the experimental data and allow a complete rationalization of the NLO responses of the different forms. The HRS signal of the forms with an open indolinooxazolidine moiety are more than one order of magnitude larger than that measured for the other forms, whereas the open/closed status of the dithienylethene subunit barely influences the dynamic NLO properties. However, extrapolation of the NLO responses to the static limit leads to univocally distinguishable intrinsic responses for three of the various forms. This hybrid system thus acts as a highly efficient multistate NLO switch for eventual exploitation in optical memory systems with multiple storage and nondestructive readout capacity.
The first hyperpolarizability of the four trans forms of a dithienylethene indolinooxazolidine biphotochrome in acetonitrile solution has been evaluated by using two solvation models, an explicit and an implicit one. The implicit solvation model is the integral equation formalism of the polarizable continuum model (IEF-PCM), whereas in the explicit one, the solvent molecules are represented by point charges, of which the positions have been generated by Monte Carlo simulations whereas the solute is treated quantum mechanically. At optical frequencies, first hyperpolarizabilities calculated with the implicit solvation model are usually larger than those obtained with the multiscale approach. However, both approaches predict similar contrasts, indicating that implicit solvation models such as IEF-PCM are well-suited to describe the variations in the NLO responses of molecular switches. In addition, the analysis of the contrasts of first hyperpolarizabilities shows that the biphotochrome can act as a three-state NLO switch.
This work demonstrates that covalent organic frameworks (COFs) can exhibit large second-order nonlinear (NLO) responses and that these NLO responses can be modulated as a function of successive enol-imine/keto-enamine tautomerisms, leading to efficient solid-state second-order NLO switches. The proof of concept is given by evidencing, by means of periodic boundary condition (time-dependent) density functional theory calculations, the large amplitudes of the second-order NLO susceptibility, χ (2) , of two-dimensional COFs built from the assembly of tris(N-salicylideneaniline) units as well as their variations when switching between keto and enol forms. Calculations further demonstrate the key role of symmetry, that is, the distribution of enol and keto functions in the unit cell, on the χ (2) values as well as on their dipolar/octupolar character.
The geometries of the enol (E) and keto (K) forms of a crystalline salicylideneaniline molecular switch, (E)-2-methoxy-6-(pyridine-3-yliminomethyl)phenol (PYV3), have been determined using periodic density functional theory (DFT) calculations with a variety of exchange-correlation functionals (XCFs). They are compared to X-ray diffraction (XRD) data as well as to geometries obtained using empirical dispersion energy in the form of the second iteration of Grimme's scheme, either with its original parameters (DFT-D2) or with parameters revised for the solid state (DFT-D*). Using DFT, a good agreement with experiment on the unit cell parameters is obtained with the PBEsol, PBEsol0, and ωB97X XCFs. DFT-D2 contracts the unit cell too much with all considered XCFs, whereas DFT-D* lessens this effect thus allowing B3LYP, PBE, and PBE0 to achieve reasonable agreement with respect to XRD data. When considering molecular geometries, both DFT and DFT-D* have a similar effect on the bond lengths, both systematically underestimating (overestimating) the length of the single (double) bonds (within 0.003 Å), as well as on valence angles attaining differences of 2 • with respect to XRD data. The errors on the torsion angles are less spread out with DFT-D* (averaging 1 •) than DFT for which only PBEsol, PBEsol0, and ωB97X perform well. Finally, the relative keto-enol energies, ∆E KE , have been calculated, showing that the inclusion of dispersion energy stabilizes the keto form more than it does the enol form. This results in the PBE and PBEsol XCFs wrongly predicting the keto form as the most stable form.
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