Theoretical Investigation On Li-doped Structure And Nonlinear Optical Properties Of Conjugated And Non-conjugated Systems

The last three decades have witnessed progress in the design and synthesis ofhigh-performance nonlinear optical (NLO) materials for their potential use intelecommunication, information storage, optical computing, optical switching, and signalprocessing. Among these investigations, the most typical molecular engineering schemesare based on intramolecular charge transfer processes from a donor toward an acceptormoiety through a π-electron conjugated path, which can be expressed as D-π-A.Significantly, in2000, a new molecular engineering strategy (D-S-A) for NLO introducedby Zyss is different from that of D-π-A. In D-S-A form, S is referred to “through-space”electronic interaction (a novel kind of noncovalent charge transfer), such as arene-areneinteraction.On the other hand, doping Li is an important method to enhance the NLO responses.Many valuable papers show that the NLO responses can be dramatically enhanced bydoping Li atoms. For example, Yu reported the first attempt on the interaction betweenthe Li atom and the π-conjugated aromatic ring. As a result, the first hyperpolarizabilitiesof Li-doped aromatic rings are obviously enhanced. What’s more, Papadopoulosconfirmed that the lithiation effect can lead to an enormous increase in the secondhyperpolarizability of the smaller π-conjugated benzene.Recently, both lithium (Li) salts and the electrides are formed by doping Li atominto ligand (L) complexes. Among them, the Li salt is an ionic compound in whichlithium as the cation (the charge of Li is close to+1), but the electride is also an ioniccompound with excess electron (electron is the anion and the charge of Li much smallerthan the+1). Thus, the Li salt can be expressed as Li+L-and electride can be consideredas (LiL)+e-. Both of the Li salts and Li electrides are formed by Li interacting with ligand(L) complexes. Inspired by the above discussion, three interesting question attracted ourattention:1Alkali metal doped effect on structure-property of adamantane.2The interaction between the Li atom and the intermediate cavity of D-S-A.3How does the location of Li atom effect structure-property?In the present work, we investigate Li-doped structure and nonlinear opticalproperties of conjugated and non-conjugated systems by using quantum chemicalcalculations. The research results suggest:1. Compared with highly symmetric graphene and single-walled carbon nanotubes(SWCNT), adamantane (Ad) only possesses σ-bonds. In this work, three complexesAd-Li,-Na and-K were obtained by alkali metal (Li, Na and K) substitution the H atom of methine position in Ad according to quantum chemical calculations. Interestingly, thesubstitution by alkali metals leads to absorption within visible region. The absorptioncorresponding to the maximum wavelength of the complexes shows a red shift trend fromthe Li to the K complex. This trend indicates that the crucial transition energy becomessmaller which leads to a larger nonlinear optical response. Among these three structures,the largest first static hyperpolarizability of the K complex was found to be76626au,which is about45times than a prototypical second-order NLO molecule of p-nitroaniline(βtot=1679au). Thus, our results show that alkali metal substituted Ads may be novelpotential candidates for high-performance NLO materials.2. Three complexes(1-Li-NO2,1-Li-Mid and1-Li-NH2) were designed by doping Liatom into the different location(above, middle and below) of the novel noncovalent“through-space” electronic interaction (S) of Donor-S-Acceptor (1) to explore theirstructure-property relationships. The results show that doping Li atom can obviouslyenhance the first hyperpolarizability (βtot) of1. The results also show that doping one Liatom has great influence on the aromaticity of intermediate cavity in1. For example, themost negative NICS value (-52.0ppm) of1-Li-Mid is about4times larger thanNICS(1.00) and NICS(2.00) in1. The results of electron-density differences indicate thatelectron clouds absorbed by intermediate cavity in the1-Li-Mid lead to the morenegative NICS value. What’s more, the NBO charges of Li atoms in1-Li-NO2and1-Li-NH2are0.84which are similar to1indicating that1-Li-NO2and1-Li-NH2may benovel Li salts. While in1-Li-Mid, the charge of Li (0.45) is obvious smaller than0.84because intermediate cavity hinders the polarization of the Li atom. Interestingly, thecrucial transition direction of1-Li-NH2is from donor group to acceptor group, which isopposite to those of1-Li-NO2and1-Li-Mid. The highest occupied molecular orbital(HOMO) of1-Li-NH2shows that doping one Li atom enhances the π-π interactionresulting in the largest βtotvalue(1.50×105) which is about16times larger than that of1(9.13×103) and is also obviously larger than those of1-Li-Mid(5.81×104) and1-Li-NO2(6.45×104au). The results indicate that1-Li-NH2can be considered as a novelhigh-performance NLO material and the location of Li atom can modulate NLO response.3.1-Li-PNA,2-Li-PNA,3-Li-PNA and4-Li-PNA were obtained by lithium atomarc-shaped doped into the p-nitroaniline (PNA) to explore which is a Li salt and Lielectride. The results show that1-Li-PNA is a typical Li salt with larger natural bondorder (NBO) charge of Li (0.859is close to1) and diffuse electron cloud in the lowestunoccupied molecular orbit (LUMO) and the2-Li-PNA is also a Li salt which is similarto1-Li-PNA. Interestingly,4-Li-PNA is a typical Li electride with excess electron cloudin the singly-occupied molecular orbits (SOMO) and smaller NBO charge of Li (0.160).Significantly,3-Li-PNA possesses both Li salt and electride characteristics. The electron-density differences and NBO charges analyses of3-Li-PNA indicate that, thevalence electron of the Li atom is ejected out under the action of lone pair of N atom of-NH2in PNA. Simultaneously, the valence electron of the Li in3-Li-PNA is partly pulleddown by C4atom. This unusual molecule shows the largest first hyperpolarizability(2.9×106au), which is about2600times than that of PNA. Further, the vertical ionizationpotentials (VIP) and the interaction energy (Eint) indicate that3-Li-PNA is less stable thanLi salts (1-Li-PNA and2-Li-PNA), but is more stable than the Li electride (4-Li-PNA).The present investigation reveals how to distinguish a compound with Li salt or Lielectride characteristic. Our work may also be beneficial to experimentalists for designingand synthesizing high-performance NLO materials with both characteristics of Li salt andelectride characteristics.