The DFT Study On Structure And Nonlinear Optical Properties Of Compounds Involving Carborane With Thiophen And Triarylboranes

Novel molecular materials with optimal nonlinear optical （NLO） properties are stillattracting considerable attention because of their potential applications in fields includingoptical communications and computation, optical switching and limiting, data storage andretrieval, and sensors. Although efforts over several decades in the NLO field have resulted infar-ranging studies on the design, synthesis, structure, reactivity and application of thesematerials, a number of strategies have also been extensively studied as a part of this progress.The primary aim is to be able to reversibly switch and effectively modulate the NLO responseof a molecule. The methods to solve this problem are great diverse. It is well known that theformation of the structure of D-π-A is π bond bridged with electron donor group （D） andelectron-withdrawing groups （A）. Although this structure is simple, it can increase themolecular NLO response by enhancing the strengths of electron donor group andelectron-withdrawing group or appropriately modifying the structure of π-conjugated bridge.Recently, some novel concepts including oxidation/reduction, photoisomerization,photoinduced proton transfer, and protonation/deprotonation have been extensivelyinvestigated experimentally and theoretically, particularly for the redox switching of NLOproperties. It has also been reported that the boron （B） atom shows a strong affinity towardfluoride ion （F）, and the binding of F to the B center can switch and modulate the material’soptical properties.With the development of NLO study and the prospect of NLO material application, thetheoretical computations on a series of carborane derivatives have received more attention,owing to their special electron structures and favorable chemical stability. Allis et al. andTayler et al. studied the NLO properties of the system with12-vertex closed carboranebridged with electron donor or acceptor substituent group, the results indicated that the rigidcage structure of icosahedral carborane polyhedra is in favor of the first hyperpolarizability.This work employed density functional theory （DFT）, combined finite-field （FF） methodto calculate the structure and NLO properties of a series of compounds involving carboranewith thiophen and triarylboranes. The results as follw:1. NLO properties of seven compounds using carborane and benzene rings as the bridginggroups has been investigated at the B3LYP method. The results indicate that a larger sizebridging groups would be helpful in enhancing the polarizability; Electron delocalization ofbridging group has an important influence on the second-order NLO coefficients （the firsthyperpolarizabilities） as well as the molecular geometry, and the charge transfer has a greatercontribution to the NLO properties. Moreover, from the analysis of electronic spectrum andthe corresponding molecular orbit composition, it illustrates that the carborane can be not onlythe bridge for electron transmission, but also the electron withdrawing group.2. In this work, DFT-FF method has been adopted to analyze the second-order NLOproperties of the triarylborane （TAB） derivatives obtained by introducing different inductiveelectron 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 （βtot） of theRTAB molecules can be switched by binding one Fˉto the boron center （RTAB’） orone-electron reduction （RTAB"）. The DFT-FF calculations show that the βtotvalues of2′,3′and5′decrease while those of1′and4′increase compared with the values of their neutralmolecules, which was attributed to the fact that the charge transfers of2,3and5becomesmaller and those of1and4become larger by binding one Fˉion to the boron center,according to time-domain DFT （TD-DFT） analysis. However, the incorporation of oneelectron enhances the second-order NLO properties of the RTAB molecules remarkably,especially for system1. It is notable that the βtotvalue of reduced form1″is508.69×10-30esu,i.e., about578times larger than that of system1. Frontier molecular orbital （FMO） andnatural bond orbital （NBO） analyses suggest that the reversal of the charge distributionbetween the neutral molecules and their reduced forms leads to low HOMO-LUMO energygaps （E0） and thus large βtotvalues for the reduced forms.