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

Abstract

In this work, density functional theory (DFT) combined with the finite field (FF) method has been adopted to analyze the secondorder nonlinear optical (NLO) properties of the triarylborane (TAB) derivatives obtained by introducing different inductive electron groups into the phenylene ring of the TAB (RTAB, where R=2-C 6 H 5 -C 2 B 10 H 10 (1), R=F(2), R=Me(3), R=NO 2 (4), R=NH 2 (5)). The static first hyperpolarizabilities ( tot ) 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  tot 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  tot 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 (E 0 ) and thus large  tot values for the reduced forms. Novel molecular materials with optimal nonlinear optical (NLO) properties are still in great demand because of their primary roles in applications in fields including optical communications and computation, optical switching and limiting, data storage and retrieval, and sensors [1-4]. Recently, triarylborane (TAB) and its derivatives have attracted increasing attention because of their strong photophysical properties, and have been making greater contributions in the fields of luminescence [5][6][7][8], optoelectronics (organic light-emitting diodes and electroluminescence (OLED/EL), nonlinear optical and two-photon absorption) [9][10][11][12], catalytic polymerization [13][14][15] and anion sensors [16][17][18], because their intriguing electronic structures come from the p  - conjugation through the vacant p-orbital on the boron atom (p(B)) [19,20]. It is well known that the molecular second-order NLO properties depend not only on the nature of the -conjugation but also on the strengths of the donor and acceptor groups [21]. Thus, in this report, we have introduced the conventional electron donor or acceptor substituent groups to the phenylene ring of the TAB to optimize its NLO properties. We have also studied closo-C 2 B 10 H 12 carborane with an aryl group from the perspective of its use as a novel substituent on the phenylene ring of TAB. It was found that not only it does the carborane enhance Lewis acidity by the formation of an electron deficient cage, but also it leads to high stabilization of the vacant p(B) for the TAB used in the experiment [22]. Al...