Density Functional Theory Investigation On The Nonlinear Optical Property Of O-carborane Derivatives

Nonlinear optical （NLO） materials have stimulated a great deal of current interest as theirwide applications in the field of optical communications, optical computing, opticalinformation processing, photoelectric conversion, optical switching, and data conversion etc.Currently, a great development of NLO materials has appeared in the design, synthesis, andapplication. However, the design of materials with large nonlinear optical response is stillfacing great challenges. One of which is the effective improvements between the nonlinearoptical response and spectral red shift. With regard to this, it has been committed to differentapproaches to design or adjust the potential compound with a large nonlinear optical response.These approaches mainly include: using molecules with electron donor-acceptor,strengthening the donor or acceptor group to improve the NLO response, extending π-conjugated systems, enhancing the degree of intramolecular charge transfer,adopting thepush-pull molecules containing a donor and an acceptor connected via a π-conjugatedlinker（D-π-A）, etc. In addition, some novel concepts also include such as theoxidation/reduction, the proton transfer, and the protonated/deprotonated etc. It is reportedthat the boron atom has a certain degree of electron deficient nature and can form the threeand/or four coordination compound. Through the modification of the group, its opticalcharacteristics can be effectively adjusted.In recent years, the carborane and its derivatives have aroused widespread concern for itsspecial physical and chemical properties. Early in the19th century, Mingos and Michl et al.have investigated the nonlinear optical properties of a series of compounds for the mother ofthe closed-type carborane. The results show that the molecular nonlinear optical response canbe effectively improved by the appropriate modifications of the structures.This work employed finite-field （FF） method combined with density functional theory（DFT） to calculate the molecular structures, electronic spectra and second-order NLOproperties of a series of carbon/boron substituted o-carborane derivatives. The results asfollow:1. By M05-2X method, we investigated the nonlinear optical properties of1,2and1,3-bis-substituted o-carborane and the introduction of ferrocene groups leading to the changes inNLO properties. The results show that the1,3-dehydro-o-carborane is a more suitable NLOmaterial than1,2substituted o-carborane. The nonlinear optical property of1,3-bis-substituted o-carborane significantly increased by introducting the ferrocene group.Intramolecular charge transfer degree has a greater impact on the second-order nonlinear optical response. Electronic spectrum and the frontier molecular orbital analysis shows thatthe carborane character with electron withdrawing nature and ferrocene character withelectron-donating nature.2. Based on the special physical and chemical properties of the carborane, the influence of theconjugate length, the electron donating groups and electron withdrawing groups on thenonlinear optical properties of the o-carborane have been investigated. The results show thatincreasing the length of π-conjugated is an effective way to increase the firsthyperpolarizability. However, comparing with the electron-accepting group, the introductionof electron donating groups can effectively improve the first hyperpolarizability of o-carborane derivatives.3. A series of two-dimensional （2D） nonlinear optical molecules have been obtained byintroducing different electron-donating group in carborane molecules. The2D NLO responsehas been investigated by DFT method for the single and multi-branched carborane. Theresults show that: comparing with the single-branch carborane, the nonlinear optical responseof multi-branched carborane is significantly increased. For the multi-branch o-carborane, theenhancements of the strength of the donor group leads to a large first hyperpolarizabilityvalue. Electronic transitions to crucial excited states show that y-polarized transition,contributed to the off-diagonal second-order polarizability tensor (β_zyy), possesses lowertransition energy compared with z-polarized transition accounted for the diagonal second-order polarizabiliy tensor (β_zzz), which leads to the large in-plane nonlinear anisotropy(μ=β_zyy/β_zzz) value, as well as good2D second-order NLO properties.