The Nonlinear Optical Properties Of Dimethyldihydropyrene (DHP) Derivatives:A Theoretical Study

Due to the wide applications of nonlinear optics in laser technology,material structure analysis,all-optical information processing,optical storage and other fields,the design and research of novel nonlinear optical(NLO)materials has become an important frontier and research hotspot.Recently,organic photochromic systems can be reversibly transformed between two stable configurations under light or heat stimulation,so they have been applied in optical recording,sensor,intelligent display window and so on.Notably,the transformation of these molecular configurations can also cause significant changes in the absorption spectra,which may lead to large differences in the NLO response,showing NLO switching characteristics.Thus,organic photochromic molecules will be applied in molecular-scale memory devices with multiple storage and nondestructive reading capacity.In this paper,based on density functional theory(DFT),the relationships between the structure and NLO property of negative photochromic molecule switches,dimethyldihydropyrene(DHP)and donoracceptor Stenhouse adducts(DASAs),were systematically discussed by means of analytical derivative method and sum-over-states method.Moreover,the effects of substituent modification,structural isomerization and environmental factors on the NLO properties of these compounds were investigated systematically.The microscopic mechanism of NLO response was further elucidated by analyzing the geometrical structure,electronic structure,electronic transition form,UV-Vis absorption spectrum and hyperpolarizability density.The main research contents of this paper are as follows:(1)Based on DFT and TD-DFT,we investigated the second-order NLO properties of DHP derivatives with D-π-A structure.The origin of NLO properties was revealed from the aspects of geometric structure,electronic structure,charge distribution,electronic transition property and the first hyperpolarization density.The effects of substituent position,photoisomerization process and environmental factors(polarization environment and frequency dispersion)on the first hyperpolarizability of the system have been investigated systematically.An important factor that regulates the NLO properties of the system is clarified: the π-conjugation difference in the process of isomerization.(2)Based on the DHP molecular framework,we have studied and compared the secondorder NLO properties of DHP derivatives with protonated donor and acceptor groups.The results showed that the protonation of the acceptor can improve the NLO performance of the system,while the protonation of the donor reduces it.In particular,the introduction of quinoxaline acceptor can effectively enhance the second-order NLO response of the compound.Importantly,the NLO response of the compound can be significantly changed(more than one order of magnitude)before and after protonation,showing good pH-regulated NLO switching characteristics.In addition,the compound showed the possibility of being three-state NLO switch,under the combined action of photoisomerization and protonation.The work is not only expected to expand the application fields of DHP derivatives,but also provides important ideas for the design of NLO switches.(3)Further,we designed and studied the structure and second-order NLO properties of a series of DHP-AB(azobenzene)double photochromic group mixed systems.The synergistic effect of two photochromic groups on the NLO properties was revealed from the aspects of geometric structure,electronic structure and electronic transition form.In addition,the effects of polarization environment and frequency dispersion were also explored to simulate a real experimental environment.The results showed that the second-order NLO properties of the system can be regulated effectively under the synergistic action of DHP and AB groups,and it is hopeful to obtain a new type of NLO switch.(4)Based on the above work,we also investigated the second-order NLO properties of donor-acceptor Stenhouse adducts(DASAs)molecular switches,with the aim of understanding the structural characteristics of molecular switches suitable for second-order NLO switches.The results showed that the molecular switches with conjugativity and planarity completely destroyed shows the potential as efficient NLO switches.In addition,NLO switching efficiency can be regulated by the properties of acceptor groups and the polarization environment.This paper revealed the relationship between the structure and NLO properties of some photochromic molecules at the microscopic level.We hoped that these works can provide theoretical guidance for the design and experimental synthesis of high-performance NLO switching materials.