The DFT Studies On Second-order Nonlinear Optical Properties Of BODIPY Derivatives

Nonlinear optical（NLO）materials have attracted significant research interest,which become one of the vital materials nowadays,for their potential utility,for example,optical information processing,photochromic switches,optical data storage,development of laser components and advanced optoelectronic device.Great efforts and contribution have been made to the field of designing and synthesizing superior performance NLO materials and made important results.Among the NLO materials,organic NLO materials are highly favoured over the inorganic ones owing to their hold the structural flexibility and the ability to maximize NLO response over the molecular framework and exhibit distinctive advantage and good development prospect.However,with progress of times and development of society,understanding NLO origin and finding effective methods to improve the second-order NLO response have become the most urgent problems to be solved.Thus,we investigated the geometric structure,electronic structure and NLO properties of molecules by quantum chemical calculation method,it is significant to provide theoretical guidance for searching excellent NLO materials.In this paper,the second-order NLO properties of a series of BODIPY complexes were systematical investigated by density functional theory（DFT）and time-dependent DFT（TD-DFT）methods.The main works are as follws:（1）The structure and second-order NLO properties of a series of methylene dipyrrole and fluoroboron dipyrrole（BODIPY）molecules were calculated and analyzed by DFT method.The results showed that the tops and bottoms of methylene dipyrrole and BODIPY molecules introduce different aromatic ring substituents can change the dipole moment and adjust their second-order NLO responses.Thereinto,the second-order NLO response of the molecule with D-π-A structure is larger.The charge transfer（CT）characteristics of BODIPY molecules introduced-BF₂ group are enhanced from 2-D CT to 1-D CT,and the second-order NLO responds is decreased.By the analyses of frontier molecular orbital composition and electron spectra of a series of molecules,it can be seen that the charge transfer of two types molecule mainly occur at the top and the bottom substituent as well as the methylene dipyrrole.In addition,the absorption spectra cause the different degrees of bathochromic-shift when the different substituents with electron-withdrawing ability are introduced.Among them,the absorption spectrum of D-π-A structure occur the obvious red-shift than other molecules.（2）Based on molecular structural design of BODIPY,nine BODIPY derivatives decorated with vinyl,phenyl,dimethylamino,nitro and aldehyde group substituents,respectively,have been systematically investigated by the DFT method.The purposes of this paper are to study the effect of various substituents and their positions on geometric structures,electronic structures and second-order NLO properties.In addition,the effects of polarizable environment and electric fields on the first hyperpolarizabilities(β_tot)values have been investigated.It is found that the incorporation of strong electron-donating dimethylamino could induce a significant enhancement of the β_totvalues,and the β_totvalues of the bilateral α-position substitution are much larger with respect to the unilateral α-position substitution,beacause those substituents are able to expend the BODIPYs’ delocalized π-electron system to different extents.Time-dependent DFT results suggest that the increasement of the β_totvalues is related to the intramolecular charge transfer from substituents to BODIPY core.Thus,forming a D-π-A model is favorable for increasing the β_totvalues,where BODIPY was used as electron accepting.Hence,we hope this work will be beneficial to further investigating versatile and novel NLO materials.（3）The geometric structures,electronic structures and second-order NLO properties of a series of unilateral and bilateral BODIPY derivatives have been systematically investigated by the DFT method.The results showed that the incorporation of strong electron-donating dimethylamino could induce a significant enhancement of the β_totvalues,and the β_totvalues of the bilateral α-position substitution are much larger with respect to the unilateral α-position substitution,beacause those substituents are able to expend the BODIPYs’ delocalized π-electron system to different extents.Moreover,the polarizable environment and electric fields also have influence on the β_totvalues of the BODIPY derivatives.Time-dependent DFT results suggest that the increasement of the β_totvalues is related to the charge transfer from local BODIPY core to the substituents to BODIPY core.In addition,forming a D-π-A model is favorable for increasing the β_totvalues,where BODIPY was used as electron accepting.Hence,we hope this work will be beneficial to further investigating versatile and novel NLO materials.