Density Functional Theory Study of Substitution Effects on the Second-Order Nonlinear Optical Properties of Lindquist-Type Organo-Imido Polyoxometalates

Emna Rtibi,Benoit Champagne

doi:10.3390/sym13091636

Emna Rtibi, Benoit Champagne

Open Access

https://doi.org/10.3390/sym13091636

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Journal: Symmetry	Publication Date: Sep 6, 2021
Citations: 6	License type: CC BY 4.0

Affiliation: University of Namur, University of Carthage

Abstract

Density functional theory and time-dependent density functional theory have been enacted to investigate the effects of donor and acceptor on the first hyperpolarizability of Lindquist-type organo-imido polyoxometalates (POMs). These calculations employ a range-separated hybrid exchange-correlation functional (ωB97X-D), account for solvent effects using the implicit polarizable continuum model, and analyze the first hyperpolarizabilities by using the two-state approximation. They highlight the beneficial role of strong donors as well as of π-conjugated spacers (CH=CH rather than C≡C) on the first hyperpolarizabilities. Analysis based on the unit sphere representation confirms the one-dimensional push-pull π-conjugated character of the POMs substituted by donor groups and the corresponding value of the depolarization ratios close to 5. Furthermore, the use of the two-state approximation is demonstrated to be suitable for explaining the origin of the variations of the first hyperpolarizabilities as a function of the characteristics of a unique low-energy charge-transfer excited state and to attribute most of the first hyperpolarizability changes to the difference of dipole moment between the ground and that charge-transfer excited state.

Highlights

Polyoxometalates (POMs) are nanomolecular metal-oxides made of metal atoms (M)from groups VB and VIB in a high oxidation state
We extend our previous investigation by addressing one aspect of the structure-nonlinear optical (NLO) property relationships in Lindqvist-type POMs: the modulation of the donor/acceptor character of the organic linker either by changing the substituent on the terminal phenyl ring or by changing the π-conjugated character by replacing the CC triple bond by a CC double bond (Figure 1)
Following reference [29], density functional theory and time-dependent calculations have been performed on an extended series of Lindquist-type organo-imido polyoxometalates derivatized with ligands bearing donor or acceptor groups to unravel the relationships between their structures and first hyperpolarizabilities

Summary

Introduction

From groups VB (often, V) and VIB (often Mo or W) in a high oxidation state. These anionic nanoclusters are built from MOx y- oxyanion polyhedra linked together by shared. In the case of Lindqvist-type organo-imido-substituted hexamolybdates bearing π-conjugated ligands, these investigations demonstrated that the polyanion clusters act as electron acceptors because the first hyperpolarizability (β), which characterizes the SHG response at the molecular scale, is enhanced when these organic linkers are substituted by electron donors, in other words, when they present a strong non-centrosymmetry. These experimental characterizations have been carried out using the hyper-Rayleigh scattering (HRS). These are bulkier than in 10, a diphenylamino (14) or a carbazole (15) substituent

Computational Methods

Ground State Equilibrium Geometries

Method

First Hyperpolarizabilities

UV-Absorption Spectra and Interpretation of the β Values

Conclusions and Outlook