Theoretical Model of Polarization Effects on Third-Order NLO Properties of the Stilbazolium Derivative Crystal.

The linear and nonlinear properties of the stilbazolium derivative, 2-[2-(3-hydroxy-4-methoxy-phenyl)-vinyl]-1-methyl-pyridinium naphthalene-2-sulfonate dihydrate crystal (VSNS), were investigated using an iterative electrostatic embedding scheme and density functional theory (DFT). The dipole moment and second hyperpolarizability of the VSNS molecule are sharply influenced by its crystalline phase. Standard DFT global hybrids such as B3LYP and M06 are strongly benefited by the effects of crystalline polarization. The performance can be further improved by making small changes in the amount of Hartree-Fock exchange included in the hybrid, delivering good hyperpolarizability and spectroscopy.

Explicit Aqueous Solvation Treatment of Epinephrine from Car-Parrinello Molecular Dynamics: Effect of Hydrogen Bonding on the Electronic Absorption Spectrum.

The electronic absorption spectrum of the neurotransmitter epinephrine (EPN) in water solution is studied, combining ab initio Car-Parrinello molecular dynamics (CPMD) with a quantum mechanical approach within the framework of the time-dependent density functional theory (TDDFT) scheme. By selecting 52 uncorrelated snapshots, the excitation modes were calculated at the LC-ωPBE/6-31+G(d) level of theory, using an optimal range-separation parameter ω, determined by means of the gap-tuning scheme in the presence of the solvent molecules. By comparing with static approaches (vacuum and implicit solvation), we show here that explicit solvation treatment dramatically enhances the photophysical properties of the EPN, especially because of the more realistic dynamic description of the molecular geometry. The agreement between the simulated and experimental spectra is demonstrated when TDDFT calculations are performed with the optimally tuned version of the DFT hybrid, not only improving the relative intensities of the absorption bands but also the λmax position. These results highlight that accounting for the nuclear motions, that is, thermal effects (of both chromophore and solvent molecules), is imperative to predict experimental absorption spectra. In this paper, we have addressed the critical importance of explicit solvation effects on the photophysics of the EPN, raking in performance when the simulation is performed based on first-principles molecular dynamics such as CPMD.

Bond Ellipticity Alternation: An Accurate Descriptor of the Nonlinear Optical Properties of π-Conjugated Chromophores.

Well-defined structure-property relationships offer a conceptual basis to afford a priori design principles to develop novel π-conjugated molecular and polymer materials for nonlinear optical (NLO) applications. Here, we introduce the bond ellipticity alternation (BEA) as a robust parameter to assess the NLO characteristics of organic chromophores and illustrate its effectiveness in the case of streptocyanines. BEA is based on the symmetry of the electron density, a physical observable that can be determined from experimental X-ray electron densities or from quantum-chemical calculations. Through comparisons to the well-established bond-length alternation and π-bond order alternation parameters, we demonstrate the generality of BEA to foreshadow NLO characteristics and underline that, in the case of large electric fields, BEA is a more reliable descriptor. Hence, this study introduces BEA as a prominent descriptor of organic chromophores of interest for NLO applications.