The DFT Study On Second-order NLO Property Of The12-vertex Closo-carborane And Closo-metallacarborane

The field of nonlinear optics (NLO) has experienced an ever-increasing interest owing tomultiple applications in information processing, electro-optical switching andtelecommunications. While commercial nonlinear optical materials are still largely inorganic,organic compounds and metal-organic complexes have attracted great attention. Due to theirthree-dimensional aromatic structures, unique physical and chemical properties, and highchemical and thermal stability, carboranes and metallacarboranes derivatives have been atopic of great interest for chemical researchers. In addition, syntheses and theoreticalresearches are emerging in an endless stream. Recent studies suggest that carboranes andmetallacarboranes derivatives possess prominent second-order NLO properties, indicatingthat they are potential NLO materials.The second-order NLO properties of12-vertex o-carboranes and metallacarboranesderivatives were investigated by quantum chemical calculations. The results show that:(1) The NLO properties and the electronic spectra of a series of12-vertex carboraneNi(II) complexes were investigated by using the density functional theory (DFT) method. Theresults indicate that structural changes of two P(CH3)3ligands affect slightly the distancesbetween some atoms of complexes, and values of Ni(II) complexes increase with theincreasing of the conjugation of substituents and spatial volume of complexes. Enhancing theconjugation and changing the structures of P(CH3)3ligands have obvious influence on thesecond-order NLO coefficients, and the totvalue of complex5b which is substituted byaniline group is the largest. Moreover, from the analysis of electronic spectrum and thecorresponding molecular orbit composition, it illustrates that the contribution to thesecond-order NLO coefficient mainly comes from intraligand charge transfer andligand-to-metal charge transfer.(2) DFT calculations have been carried out to investigate the substituent effects andredox switching of the second-order nonlinear optical (NLO) properties of a series of thetwo-dimensional (2D) diaryl-substituted benzenes/carboranes. The calculated static firsthyperpolarizabilities (tot) of the neutral compounds increase with replacing the centralbenzene with carborane, extending the π-conjugated bridge, and introducing an electron donorinto carbazole moiety. Particularly, a dramatic enhancement of the totvalues for theoxidized/reduced species is observed. The totvalues of one-electron oxidized/reduced speciesfor compound3are14times and32times as large as that of its neutral counterpart,respectively. Furthermore, the electronic structure analysis shows that the carbazole performsas an oxidized center and the benzene/carborane performs as a reduced center. Interestingly,the second-order NLO responses are two-dimensional in characteristic with the largeoff-diagonal tensor values. This work not only provides more penetrating insight into thesecond-order NLO properties of2D compounds, but also obtains an efficient approach fordevising redox-switchable NLO materials.