Prediction Of Nonlinear Optical Properties Research Articles

Gauge-Invariant Excited-State Linear and Quadratic Response Properties within the Meta-Generalized Gradient Approximation.

Gauge invariance is a fundamental symmetry connected to charge conservation and is widely accepted as indispensable for any electronic structure method. Hence, the gauge variance of the time-dependent kinetic energy density τ used in many meta-generalized gradient approximations (MGGAs) to the exchange-correlation (XC) functional presents a major obstacle for applying MGGAs within time-dependent density functional theory (TDDFT). Replacing τ by the gauge-invariant generalized kinetic energy density τ̂ significantly improves the accuracy of various functionals for vertical excitation energies [R. Grotjahn, F. Furche, and M. Kaupp. J. Chem. Phys. 2022, 157, 111102]. However, the dependence of the resulting current-MGGAs (cMGGAs) on the paramagnetic current density gives rise to new exchange-correlation kernels and hyper-kernels ignored in previous implementations of quadratic and higher-order response properties. Here we report the first implementation of cMGGAs and hybrid cMGGAs for excited-state gradients and dipole moments, as well as an extension to quadratic response properties including dynamic hyperpolarizabilities and two-photon absorption cross sections. In the first comprehensive benchmark study of MGGAs and cMGGAs for two-photon absorption cross sections, the M06-2X functional is found to be superior to the GGA hybrid PBE0. Additionally, two case studies from the literature for the practical prediction of nonlinear optical properties are revisited and potential advantages of hybrid (c)MGGAs compared to hybrid GGAs are discussed. The effect of restoring gauge invariance varies depending on the employed MGGA functional, the type of excitation, and the property under investigation: While some individual excited-state equilibrium structures are significantly affected, on average, these changes result in marginal improvements when compared against high-level reference data. Although the gauge-variant MGGA quadratic response properties are generally close to their gauge-invariant counterparts, the resulting errors are not bounded and significantly exceed typical method errors in some of the cases studied. Despite the limited effects seen in benchmark studies, gauge-invariant implementations of cMGGAs for excited-state properties are desirable from a fundamental perspective, entail little additional computational cost, and are necessary for response properties consistent with cMGGA linear response calculations such as excitation energies.

On the Performance of Hybrid Functionals for Non-linear Optical Properties and Electronic Excitations in Chiral Molecular Crystals: The Case of Butterfly-Shaped Dicinnamalacetone.

Purely organic chiral molecular assemblies in the solid state hold great potential for non-linear optical applications. Herein, a newly synthesised molecular system is reported, namely, dicinnamalacetone, an otherwise planar molecule that crystallises in a disordered non-centrosymmetric form with four different conformations having an overall predominance of a particular helicity. A combined experimental and theoretical approach, including single-crystal X-ray diffraction, Kurtz-Perry and ab initio methods, is employed to characterise the system and benchmark the performance of hybrid functionals for the prediction of non-linear optical properties and electronic excitations. Comparison of experiment and theory points to a particular set of hybrid functionals that provides an optimal description of this molecular system.

Finite Field Method for Nonlinear Optical Property Prediction Using Rational Function Approximants.

The finite field (FF) method is a quick, easy-to-implement tool for the prediction of nonlinear optical properties. Here, we present and explore a novel variant of the FF method, which uses a rational function to fit a molecule's energy with respect to an electric field. Similarly to previous FF methods, factors crucial for the method's accuracy were tuned. These factors include the number of terms in the function, the distribution of fields used to construct the approximation, and the initial field in the approximation. It was found that the approximant form that best fits the energy has four numerator terms and three denominator terms. To determine a reasonable field distribution, the common ratio of a geometric progression was optimized to √2. Finally, an algorithm for determining a good initial field guess was devised. The optimized FF method was used to compute the polarizability and second hyperpolarizability for a set of 121 molecules and the first hyperpolarizability for a set of 91 molecules. The results from this were compared to a previous polynomial-based FF method. It was found that using a rational function gives higher errors compared to the polynomial model. However, unlike the polynomial model, no subsequent refinement steps were needed to obtain usable results. An overall comparison of the behavior of the two methods also shows that the rational function is less sensitive to the chosen initial field, making it a good choice for new quantum chemistry codes.

Functionalized azo-based iminopyridine rhenium complexes for nonlinear optical performance

The nonlinear optical (NLO) performance of rhenium containing azo-based iminopyridine complexes is studied. For this scope, the proven methods of Maker fringe and Z-scan are used. The second and third harmonic generation (SHG and THG) measurements in produced guest-host polymeric films are performed and the corresponding high order susceptibilities are obtained. The Z-scan measurements in solutions are employed for the investigation of nonlinear refraction and nonlinear absorption properties of studied complexes. The enhanced NLO parameters of the Re-containing azo-based iminopyridine complexes comparing to their corresponding ligands confirm the positive role of metal complexation. Moreover, the polarity of connected lateral group has a considerable influence on NLO response of entire complex, in particular, on its NLO refraction behavior. This study promotes the better understanding of the structure-property relationship with possible prediction of NLO properties in such complexes as the design of such type of metal-organic systems has an essential impact on their NLO performance.

Assessment of long-range corrected functionals for the prediction of non-linear optical properties of organic materials

We calculate the hyperpolarizability of the prototypical chromophore p-nitroaniline in gas phase using different long-range corrected Density Functional Theory (LC-DFT) methods and compare it with experimental data. While traditional DFT methods are inadequate for predicting hyperpolarizabilities, LC-DFT functionals provide very good agreement with experiment (less than 10% error) for this archetypal organic molecule with large nonlinear optical (NLO) susceptibility. Additionally, we use the LC-ωPBE functional to calculate the structures and predict the hyperpolarizabilities of two recently synthesized potential organic NLO materials. The structures are compared with experimental measurements and found to be in excellent agreement.

Electronic Properties of a New Two-Photon Absorbing Fluorene Derivative: The Role of Hartree–Fock Exchange in the Density Functional Theory Design of Improved Nonlinear Chromophores

One- and two-photon absorption (2PA) properties of a new fluorene derivative with diphenylamino and 2-(2′-hydroxyphenyl)benzothiazole substituents were investigated theoretically using the density functional theory approach with different types of functionals and experimentally by two-photon induced fluorescence methodology. The effect of the exchange-correlation functional choice on the description of 2PA properties of fluorenes was analyzed. The best agreement of the experimental and theoretical 2PA spectra was obtained with a custom modification of the hybrid M05 functional that included 35% of Hartree−Fock exchange. This functional is recommended for reliable prediction of nonlinear optical properties of conjugated molecules. A chemical modification of the studied compound is suggested to increase the 2PA cross section.

Can liquids be macroscopically polar?

We will discuss the meaning of piezo-, pyro-, ferro- and antiferroelectricity (defined for solids) and their possible appearance in the liquid state. We will do this using general symmetry principles as a starting point. The arguments will also be used for the prediction of non-linear optical properties in the liquid state.