The Structures And Nonlinear Optical Properties Of Helical Chiral Conjugation: A Theoretical Study

Since the discovery of second-harmonic generation,more and more nonlinear optical(NLO)phenomena have been studied.Now it has beome an important role in the field of optical communication,optical gate,data storage and so on.Helical molecules always exhibited some special features.For some helical species,their non-planar conjugation is expected to solve the confliction between the transparency and excellent NLO property of a material,which is important for its application.Therefore in this work,three types of helical conjugated structures were theoretically studied,namely,sexipyridine,helicene and chiral single-wall carbon nanotube.On the one hand,the effective NLO molecules were expected to be obtained.On the other hand,some strategies such as redox,solvent effect,external electric field,chirality match and so on were tried to control the stability and photoelectrical properties of a molecule.For single helix of sexipyridine(sexipy),adding alkali metal atoms into the cavity of the molecule could generate significant charge transfer from metal to ligand especially on radial direction,while large difference of electric dipole moments between ground and excited state led to great increase of molecular first-order static hyperpolarizability.Different conditions could greatly influence the molecular NLO property.In Li-doped single helix Li₂-sexipy,redox was a high-effeciency NLO switch,and solvent effect would tune the magnitude of molecular first-order hyperpolarizability as well.Further more,different alkali metal atoms would change the molecular first-order static hyperpolarizability.Especially in hetero-dinuclear helix Li Na-sexipy,significant increase of first-order hyperpolarizability compared with Li₂-sexipy was observed.Then single helix of sexipyridine was extended into double helix.The relative slip betwee two helical chains brought different conformations,and zigzag-shape tendency of hyperpolarizability was observed.Meanwhile,the intensity of absorption at about 500 nm UV-Vis absorption spectra also generally increased.We also found that different size of alkali metal atoms such as Li and Na,or cooperation of external electric field and electron-donating(-NH₂)and-withdrawing groups(-NO₂)could change the variation tendency of molecular stability.It is different from sexipyridine that helicene is ortho-fused benzenoid polycyclic hydrocarbons.It is always more regid.To explore the effect of environmental factor on structure and properties of helicene,[8]cethrene as well as hexabenzo[a,c,k,m,u,w]trinaphthylene(HBTN)modified by-NH₂and-NO₂were selected to respectively studied the variation of their structures,stabilities and optical activities in solvent and external electric field.According to the consequences of theoretical calculation,it could be found that different stabilities of right-and left-handed[8]cethrene molecules were observed in different ratios of solvent molecules.Changed diradical characteristics were also obtained with ratio changed.For modified HBTN,the cooperation of external electric field and substituents led to the generation of molecular handnesses.Then the helical conjugation was further extended,and chiral Carbon Nanotube(CNT)was selected.To explore the influence of different combinations of chiral molecules on structure and NLO property of a system,alanine(Ala)were tried to add into chiral CNT.As a result,different combination led to change of difference of dipole monment between ground and excited state.In detail,right-handed CNT with(D)-Ala(or left-handed CNT with(L)-Ala)brought larger first-order hyperpolarizability.In addition for(D)-Ala@(10,5)and(L)-Ala@(10,5),their hyperpolarizability difference could be tuned by changing length and(or)chirality of CNT.These results suggested the effect of chirality match on molecular NLO property.