THEORETICAL STUDY ON THE SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF C,B-SUBSTITUTED CARBORANE CONJUGATED DERIVATIVES

To systemically investigate structure-property relationship and design excellent nonlinear optical (NLO) material, the second-order NLO properties of a series of C,B-substituted carborane conjugated derivatives have been studied by density functional theory (DFT). The static first hyperpolarizabilities (beta(tot)) were calculated at the M05-2X/6-31+G* level of theory. The results show that the beta(tot) values gradually increase with the increasing of the conjugation length, especially the introduction of ferrocene. It is found that 1,3-benzo-o-carborane-ferrocene (2h) has the largest first hyperpolarizability (55.968 x 10(-30) esu), which is 150 times larger than that of benzocarborane (1a). This means that the static first hyperpolarizabilities of the studied compounds can be substantially increased by structural modification. A basis for understanding the origin of these large NLO responses is proposed based on consideration of the frontier molecular orbitals (FMOs), orbital energy, transition energy of the studied compounds, and the two-state mode. The lower transition energy and larger oscillator strength play an important role in increasing the first hyperpolarizability value. This study may evoke possible ways to design preferable NLO materials.