The DFT Study On Redox-switchable Second-order NLO Properties Of The 12-vertex Carborane Derivatives

Molecular nonlinear optical(NLO) materials have attracted considerable interest owing to their potential applications for optical signal communication, laser technology, optical sensing and storage. Carboranes are known for their extraordinary electronic structures, complex overall molecular architectures, steric groups and remarkable thermal stability, which can be regarded as efficient building blocks to gain excellently luminescent electronic pharmacologic and functional molecular materials. Furthermore, carboranes compounds possess high polarizable electron and extensive transparency in UV region, which can be potential candidate for NLO materials.Based on the unique bonding and electronic properties of carboranes, we evaluate redox-switchable second-order NLO properties of 12-vertex carborane derivatives employing density functional theory(DFT). It includes the following results:(1) The second-order NLO properties of 12-vertex [1-R-CB11-Me11]- carboranes were investigated by using DFT method. The results indicate that both the electron-donating(-withdrawing substituents) and reversible redox reactions of molecules can affect the molecular geometry. By the analysis of natural bond orbital(NBO) charge and electron spin density, it can conclude that the molecular oxidation center is in carborane cage. The oxidation reactions have a significant effect on changing the electronic character of carborane. Meanwhile, the oxidization of the carborane anion is more helpful to enhance the totb value, for instance, the amino derivative of carborane. Therefore, the redox reaction of this carborane can be used to adjust the second-order NLO response effectively.(2) Much effort has been devoted to investigate the molecular geometries, electronic structures, redox properties and NLO properties of Ir complexes involving o-, m- or p-carborane groups by DFT methods. The switchable second-order NLO properties were induced by redox process of these complexes. It is found that the mainly coordination bonds of Ir complexes have changed during the oxidation process. Our calculations reveal that the oxidation reactions have a significant influence on the second-order NLO response owing to the change of the charge transfer pattern. The totb values of oxidized species are at least ~9 times for set I and ~5 times for set II as large as that of their corresponding parent complexes. Introduction of carborane groups into ppy(phenylpyridine) ligands can enhance the second-order NLO responses with 1.2~1.6 times by metal-to-ligand charge transfer(MLCT) transition between Ir atom and carborane. The totb of complex 2 [(ppy)2Ir(phen)]+(phen=phenanthroline) is 3.3 times as large as that of complex 1(ppy)2Ir(acce)(acce=acetylacetonate), which is caused by ligand-to-ligand charge transfer(LLCT) between ppy ligands and ancillary ligand. Therefore, it can be concluded that the second-order NLO response can be effectively enhanced by oxidation reaction.