Confirming the relationship between first hyperpolarizability and the bond length alternation coordinate for merocyanine dyes

Calculation of the geometry, absorption spectrum, and first hyperpolarizability of 4,5-dicyanoimidazole derivatives in solution. A multiscale ASEC-FEG study

We investigate the effects of solvents on the geometry, absorption spectrum, and first hyperpolarizability of six push-pull molecules, each containing a 4,5-dicyanoimidazole group as an electron acceptor and a N,N-dimethylamino group as an electron donor, with systematically extended π-conjugated systems. Geometry optimizations in dichloromethane, methanol, water, and formamide under normal thermodynamic conditions were performed using the average solvent electrostatic configuration-free energy gradient method, which employs a discrete solvent model. The conformational structure of molecules is moderately affected by the environment, with the π-conjugated system becoming more planar in protic solvents. Solvent effects on the first hyperpolarizability result in marked increases that are in line with the red shifts of the absorption spectrum. The hyperpolarizability of smaller molecules within the set may be significantly influenced by the effects of geometry relaxation in highly polar protic solvents. The results illustrate the role of hydrogen bonds in the structure and electronic properties of push-pull molecules in protic environments. For smaller molecules, hydrogen bonds significantly contribute to enhancing the hyperpolarizability, but the effect of these specific interactions becomes less significant with the length of the π-conjugated system.

Theoretical investigation on the linear and nonlinear optical properties of DAPSH crystal

The linear polarizability, first and second hyperpolarizabilities of the asymmetric unit of DAPSH crystal are studied and compared with available experimental results. The polarization effects are included using an iterative polarization procedure, which ensures the convergence of the dipole moment of DAPSH embedded within a polarization field generated by the surrounding asymmetric units whose atomic sites are considered as point charges. We estimate macroscopic susceptibilities from the results of the polarized asymmetric units in the unit cell, considering the significant contribution of the electrostatic interactions in crystal packing. The results show that the influence of the polarization effects leads to a marked decrease of the first hyperpolarizability, compared with the respective isolated counterpart, which improves the concordance with the experiment. There is a minor influence of polarization effects on the second hyperpolarizability but our estimated result for the third-order susceptibility, related to the NLO process of the intensity dependent refractive index, is significant as compared with the results for other organic crystals, such as chalcone-derivatives. In addition, supermolecule calculations are conducted for explicit dimers in presence of the electrostatic embedding to illustrate the role played by the electrostatic interactions in the hyperpolarizabilities of the DAPSH crystal.

Unraveling the acid-base characterization and solvent effects on the structural and electronic properties of a bis-bidentate bridging ligand

Understanding the interactions and the solvent effects on the distribution of several species in equilibrium and how it can influence the ¹H-NMR properties, spectroscopy (UV-vis absorption), and the acid-base equilibria can be especially challenging. This is the case of a bis-bidentate bridging ligand bis(2-pyridyl)-benzo-bis(imidazole), where the two pyridyl and four imidazolyl nitrogen atoms can be protonated in different ways, depending on the solvent, generating many isomeric/tautomeric species. Herein, we report a combined theoretical-experimental approach based on a sequential quantum mechanics/molecular mechanics procedure that was successfully applied to describe in detail the acid-base characterization and its effects on the electronic properties of such a molecule in solution. The calculated free-energies allowed the identification of the main species present in solution as a function of the solvent polarity, and its effects on the magnetic shielding of protons (¹H-NMR chemical shifts), the UV-vis absorption spectra, and the acid-base equilibrium constants (pK_as) in aqueous solution. Three acid-base equilibrium constants were experimentally/theoretically determined (pK_a1 = 1.3/1.2, pK_a2 = 2.1/2.2 and pK_a5 = 10.1/11.3) involving mono-deprotonated and mono-protonated cis and trans species. Interestingly, other processes with pK_a3 = 3.7 and pK_a4 = 6.0 were also experimentally determined and assigned to the protonation and deprotonation of dimeric species. The dimerization of the most stable neutral species was investigated by Monte Carlo simulations and its electronic effects were considered for the elucidation of the UV-vis absorption bands, revealing transitions mainly with the charge-transfer characteristic and involving both the monomeric species and the dimeric species. The good matching of the theoretical and experimental results provides an atomistic insight into the solvent effects on the electronic properties of this bis-bidentate bridging ligand.

Unveiling the relationship between structural and polarization effects on the first hyperpolarizability of a merocyanine dye

The nonlinear optical response, more specifically the Hyper-Rayleigh Scattering (HRS) response of the Brooker's merocyanine, has been calculated at the time-dependent density functional theory level and rationalized in terms of the structural changes and polarization effects induced by applied external electric fields. The structural change leads to large changes in the HRS response, while only slight variations were observed due to the polarization effects on the fixed quinoid form. Considering both structural and polarization contributions concurrently, the HRS response is dominated by cooperative behavior of those effects for weak and intermediate electric field strengths. At the same time, the competition between both effects was a crucial factor in the region of strong electric fields. The obtained results can lead to an easier understanding for upcoming studies considering more realistic models of solvents where it is not simple to disentangle these contributions.

Molecular Dynamics Approach to Calculate the Thermodiffusion (Soret and Seebeck) Coefficients of Salts in Aqueous Solutions

An approach to investigate the physical parameters related to ion thermodiffusion in aqueous solutions is proposed herein by calculating the equilibrium hydration free energy and the self-diffusion coefficient as a function of temperature, ranging from 293 to 353 K, using molecular dynamics simulations of infinitely diluted ions in aqueous solutions. Several ion force field parameters are used in the simulations, and new parameters are proposed for some ions to better describe their hydration free energy. Such a theoretical framework enables the calculation of some single-ion properties, such as heat of transport, Soret coefficient, and mass current density, as well as properties of salts, such as effective mass and thermal diffusion, Soret and Seebeck, coefficients. These calculated properties are compared with experimental data available from optical measurements and showed good agreement revealing an excellent theoretical predictability of salt thermodiffusion properties. Differences in single-ion Soret and self-diffusion coefficients of anions and cations give rise to a thermoelectric field, which affects the system response that is quantified by the Seebeck coefficient. The fast and slow Seebeck coefficients are calculated and discussed, resulting in values with mV/K order of magnitude, as observed in experiments involving several salts, such as K⁺Cl^-, Na⁺Cl^-, H⁺Cl^-, Na⁺OH^-, TMA⁺OH^-, and TBA⁺OH^-. The present approach can be adopted for any ion or charged particle dispersed in water with the aim of predicting the thermoelectric field induced through the fluid. It has potential applications in designing electrolytes for ionic thermoelectric devices in order to harvest energy and thermoelectricity in biological nanofluids.

Applicability of DFT functionals for evaluating the first hyperpolarizability of phenol blue in solution

The first electronic hyperpolarizability (β) of phenol blue (PB) in several solvents in a wide range of dielectric constants is investigated using the density functional theory (DFT). The reliability of various exchange-correlation functionals is assessed by a comparison to reference Møller-Plesset second-order perturbation theory (MP2) calculations. The equilibrium geometry of PB in each solvent is obtained by using the average solvent electrostatic configuration/free energy gradient method, which performs optimizations on the free energy hyper-surface by employing iteratively the sequential quantum mechanics/molecular mechanics methodology. The dependence of β on the bond length alternation (BLA) coordinate is rationalized by means of the two-level model. Within the employed exchange-correlation functionals, the LC-BLYP functional shows the best performance for describing the static and dynamic MP2 results of β, which increases as the BLA diminishes, reaching a maximum in an intermediate value of BLA. The results also illustrate the role played by the difference between the ground- and excited-state dipole moments (Δμ) in determining the hyperpolarizability behavior in solution. Particularly, in the aqueous solution case, Δμ goes to around zero when BLA is near zero, leading to an abrupt decline in the β value. The DFT results of this study, therefore, indicate a clear relationship between the first hyperpolarizability and the BLA coordinate for the PB in solution, in agreement with experiment.

Assessing the structure and first hyperpolarizability of Li@B10H14 in solution: a sequential QM/MM study using the ASEC-FEG method

The structure and electronic properties of the lithium decahydroborate (Li@B₁₀H₁₄) complex in chloroform and water in normal thermodynamic conditions have been investigated using sequential QM/MM calculations by means of the average solvent electrostatic configuration (ASEC) and the Free Energy Gradient (FEG) methods. To obtain the structure of the Li@B₁₀H₁₄ complex in each of the solvents considered, we have performed geometry optimizations in solution using the ASEC-FEG method. The results show, for the first time with a realistic model of the molecular environment, that this alkali-metal-borane cluster is stable in chloroform but unstable in water. We have also explored the role of the electronic polarization of the solute due to solvent in the static first hyperpolarizability. The results show that, despite the reduction due to the effect of electrostatic polarization in chloroform, the Li@B₁₀H₁₄ complex still exhibits a large electronic first hyperpolarizability, with potential for application as a second-order nonlinear optical (NLO) material. In water, in contrast, the contribution of the excess electron for NLO responses is significantly affected by the electrostatic polarization effects. Therefore our results reveal that the influence of the environment must be considered in the design of new stable NLO materials.