含不同共轭桥的TCNQ开闭壳层体系NLO性质的DFT研究

DFT Study on Nonlinear Optical Properties of Open and Closed Shell Systems of TCNQ Involving Different Conjugation Bridges

采用密度泛函理论(DFT)结合有限场(FF)方法,对不同共轭桥连接的四氰代二甲基苯醌(TCNQ)开壳层和闭壳层电子态的非线性光学(NLO)系数进行计算,并以乙烯桥为例讨论共轭链长度与NLO性质的关系.结果表明:开壳层体系的极化率和二阶超极化率值都大于闭壳层体系,且共轭桥的共轭性越强,体系的极化率和二阶超极化率越大;在自由基体系中,单重态的二阶超极化率随双自由基成分y和自旋多重度的增加而增大.体系的共轭链增长,BLA(BondLength Alternation,共轭分子中相邻单、双键键长差的平均值)逐渐减小,双自由基成分y逐渐增大,体系的二阶超极化率也逐渐增大.

Nonlinear optical coefficients of the closed and open shell systems of TCNQ connecting different conjugation bridges were computed by using density functional theory（DFT） combining with the finite field（FF） approach.Take the ethylene for an example to discuss the relationship between conjugation length and the NLO properties.The results show that the polarizability and second hyperpolarizability values of open shell systems are larger than those of closed shell systems.Moreover,the polarizabilities and second hyperpolarizabilities become large with the increased conjugation of the conjugation bridge.For the diradical systems,the second hyperpolarizabilities of the singlet increase with increasing the diradical character and spin multiplicity.With the increase of the conjugation bridge length,BLA（Bond Length Alternation,defined as the average of the change of length between adjacent single and double bonds in conjugate molecule） gradually decrease while the diradical character and second hyperpolarizabilities gradually increase.