Quantum chemical studies on tuning the second-order nonlinear optical molecular switching of triarylborane derivatives

In this work, density functional theory （DFT） combined with the finite field （FF） method has been adopted to analyze the second- order nonlinear optical （NLO） properties of the triarylborane （TAB） derivatives obtained by introducing different inductive elec- tron groups into the phenylene ring of the TAB （RTAB, where R=2-C6H5-C2B10H10（1）, R=F（2）, R=Me（3）, R=NO2（4）, R=NH2（5））. The static first hyperpolarizabilities （Rot） of the RTAB molecules can be switched by binding one F- to the boron center （RTAB＇） or one-electron reduction （RTAB”）. The DFT-FF calculations show that the βtot values of 2＇, 3＇ and 5＇ decrease while those of 1＇ and 4＇ increase compared with the values of their neutral molecules, which was attributed to the fact that the charge transfers of 2, 3 and 5 become smaller and those of 1 and 4 become larger by binding one F^- ion to the boron center, according to time-domain DFT （TD-DFT） analysis. However, the incorporation of one electron enhances the second-order NLO properties of the RTAB molecules remarkably, especially for system 1. It is notable that the βtot value of reduced form 1” is 508.69×10 ^-30 esu, i.e. about 578 times larger than that of system 1. Frontier molecular orbital （FMO） and natural bond orbital （NBO） analyses suggest that the reversal of the charge distribution between the neutral molecules and their reduced forms leads to low HOMO-LUMO energy gaps （E0） and thus large βtot values for the reduced forms.