Density Functional Theory Investigation on the Second-Order Nonlinear Optical Properties of Chlorobenzyl-o-Carborane Derivatives

The structures and second-order nonlinear optical (NLO) properties of a series of chlorobenzyl-o-carboranes derivatives (112) containing different push-pull groups have been studied by density functional theory (DFT) calculation. Our theoretical calculations show that the static first hyperpolarizability (beta tot) values gradually increase with increasing the pi-conjugation length and the strength of electron donor group. Especially, compound 12 exhibits the largest beta tot (62.404x10-30 esu) by introducing tetrathiafulvalene (TTF), which is about 76 times larger than that of compound 1 containing aryl. This means that the appropriate structural modification can substantially increase the first hyperpolarizabilities of the studied compounds. For the sake of understanding the origin of these large NLO responses, the frontier molecular orbitals (FMOs), electron density difference maps (EDDMs), orbital energy and electronic transition energy of the studied compounds are analyzed. According to the two-state model, the lower transition energy plays an important role in increasing the first hyperpolarizability values. This study may evoke possible ways to design preferable NLO materials.