The Calculated Study On Nonlinear Optical Properties Of Series Derivatives Of Indeno [1,2-b] Fluorene

During the past several decades, a lot of valuable work has been carried out todesign and synthesize the nonlinear optical (NLO) materials. Furthermore, organicmolecules become good NLO materials. The organic NLO materials have beenfocused in the current researches owing to their potential use in a variety of materialsapplications such as organic light-emitting diodes, thin-film transistors, and solar cells.A majority of studies of organic NLO properties have focused on the acenes such asthe pentacene and its derivatives, but these molecules are susceptible to oxidative andphotolytic degradation. However, compared with the pentacene and its derivatives,indeno[1,2-b]fluorene skeleton is a closely resembled molecule containing five-membered rings, which is more stable and could produce relatively larger chargetransfer (CT). This analysis indicates that indeno[1,2-b]fluorene derivatives could be anew member of the family of high-performance NLO materials.Recently, lithium salts have been widely used to tune the NLO responsesbecause of their CT and ionic bond. For improve NLO response of Li salts, we canadd the Li atom number or change Li atom position. For this reason, theindeno[1,2-b]fluorenes are checked as the object of study of NLO material. The NLOproperties of the studied molecules and Li salts are calculated by the quantumchemistry method, and the results are as follows:1. The polarizabilities (αs) and second hyperpolarizabilities (γs) of a series of6,12-diethynylindeno[1,2-b]fluorene derivatives were investigated by densityfunctional theory (DFT) CAM-B3LYP method, and the conclusions are as follow:(1) A series of6,12-diethynylindeno[1,2-b]fluorene possess considerably largesecond hyperpolarizabilities.(2) Introducing ethynyl silyl to the molecules can increase the αsand γsvalues.Also, the γsvalues are related to the2,8-disubstituted R groups (R=H, F, CH3) ofindeno[1,2-b]fluorene molecules. When R is methyl, the molecule has much larger αsand γsvalues.(3) Moreover, according to time-dependent density functional theory (TD-DFT)calculations, with respect to indeno[1,2-b]fluorene series, the maximum absorptionwavelengths of the molecules introducing ethynyl silyl show the bathochromic shiftdue to the increasing conjugation, while blue shift of maximum absorption wavelengths are observed in the molecules introducing oxygen atoms because of the decreasing conjugation caused by the distorted structure.(4) When the6-and12-hydrogen atoms of indeno[1,2-b]fluorene molecules are replaced by ethynyl silyl or oxygen atoms, the geometrical structures of these molecules changed, and thus the NLO properties are affected. The configurations of c series have much larger influence.2. The structures with all real frequencies and static first hyperpolarizabilities (βtot) of a series of Lin@indeno[1,2-b]fluorene (n=1-6) salts have been calculated by the second-order M(?)ller-Plesset (MP2) theory, and the conclusions are:(1) The results show that substituting H atoms by Li atoms onto the π-conjugated indeno[1,2-b]fluorene molecule dramatically enhances the βtot values. The order of the βtot values is βtot(2)<βtot(1)<βtot(4)<βtot(5)<βtot(6)<βtot(3).(2) System3(three Li atoms at2,3and4positions) possesses the largest βtot values (18640a.u.), which is20times larger than that of system2(Li atoms at2and3positions).(3) CIS results show the larger f△μ/△E3value the larger βtot value. For example, the f△μ/△E3value of system3is the largest among all systems, which leads to the largest βtot value.(4) The analysis of MP2frequency-dependent β(ω)) values shows that the dispersion effect has less influence on the series of Lin@indeno[1,2-b]fluorene at frequence of0.005～0.034a.u.. The low frequency of incident light should be chosen to measure the molecular second-order NLO coefficients in experiment.