The first hyperpolarizability of nitrobenzene in benzene solutions: investigation of the effects of electron correlation within the sequential QM/MM approach.

The first hyperpolarizability of nitrobenzene in benzene solutions is evaluated by adopting the sequential-Quantum Mechanics/Molecular Mechanics approach at different correlated wavefunction and density functional theory levels of approximation in order to compare these methods for predicting the solvent effects and in particular the effects of nitrobenzene concentration, which modifies the polarization field due to the surrounding. The liquid configurations are generated using Monte Carlo simulations and the surrounding molecules are represented by point charges, defining an electrostatic embedding. At all levels of approximation, the higher the concentration in nitrobenzene, the larger the first hyperpolarizability of the targeted molecule. At optical frequencies (λ = 1064 nm), for the whole range of concentrations, increasing the amount of Hartree-Fock exchange in the exchange-correlation functional leads to the following observations (i) ß∥ and ßHRS decrease attaining a minimum at the HF level, (ii) the octupolar component to ßHRS increases, (iii) the HRS ßsolv/ßisol ratio increases, and (iv) the EFISHG ßsolv/ßisol ratio displays a less systematic behavior. Considering the static properties, for which reference CCSD(T) values have been evaluated, M05-2X, LC-BLYP (µ = 0.33), and M11 are the most reliable exchange-correlation functionals for predicting both ßHRS and its evolution as a function of the nitrobenzene concentration whereas in the case of ß∥, these are M05-2X, LC-BLYP (µ = 0.28 and 0.33), and CAM-B3LYP.

Explicit versus implicit solvation effects on the first hyperpolarizability of an organic biphotochrome.

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.

QM/MM investigation of the concentration effects on the second-order nonlinear optical responses of solutions.

A multiscale approach combining quantum mechanics (QM) and molecular mechanics methods has been employed to investigate the effects of solute-solute interactions and therefore of concentration on the first hyperpolarizability of solutions of nitrobenzene in benzene. First, spatial distributions of solute and solvent molecules are generated using Monte Carlo simulations where the intermolecular interactions are described using the Lennard-Jones potentials and Coulomb terms. Then, a reduced number of statistically-uncorrelated configurations are sampled and submitted to time-dependent Hartree-Fock calculations of the first hyperpolarizability. When only one molecule is described quantum-mechanically and is embedded in the electrostatic polarization field of the solution described by point charges, ßHRS and ß// as well as the depolarization ratio increase in parallel with the concentration in nitrobenzene. This effect is attributed to the increase of the polarization field associated with the presence of polar nitrobenzene molecules in the surrounding. Then, the first solvation shell is considered explicitly in the QM calculation to address solute-solute interactions effects. When the number of nitrobenzenes in the first solvation shell increases, ßHRS and ß// normalized to the number of nitrobenzene molecules decrease and this decrease attains roughly 50% when there are 3 nitrobenzene molecules in the first solvation shell. These drastic reductions of the first hyperpolarizability result from (partial) centro-symmetric arrangements between the nitrobenzene molecules, as supported by the relationship between ß and the angle between the nitrobenzene charge transfer axes. Moreover, these ß decreases originate mostly from the reduction of the dipolar ß component, whereas the octupolar one is rather constant as a function of the nitrobenzene concentration.