| Nonlinear optical?NLO?materials have become one of the key materials in the information age because they can be widely used in many fields such as optical information processing,optical computing and optical communication.Therefore,the research of NLO materials has attracted much attention.Many efforts and attempts have been made to design and synthesize various functional materials with desirable properties,and made important research progress.Nevertheless,the design and development of NLO materials with superior performance remain a core issue,especially for second-order NLO materials.The intrinsic non-centrosymmetric structures of chiral compounds can meet the basic requirements for second-order NLO materials.Preliminary studies have shown that chiral organic compounds exhibit unique advantages and broad development prospects in this field.However,deep understanding their NLO origin and revealing the key factors enhancing the NLO response are crucial issues that need to be solved urgently.Quantum chemical calculation has been a powerful tool to rationalize the experimental phenomena and design new materials with outstanding properties.Therefore,it is necessary to calculate the microscopic electronic structure of the chiral organic compound,and establish the relationship between the material structure and NLO response.These studies will provide meaningful theoretical guidance for further designing novel NLO materials with large NLO coefficients.In this thesis,the structures,linear and nonlinear optical properties of axial/helical chiral organic compounds are studied by employing density functional theory?DFT?.The effective ways of structural modification for improving the NLO response of such materials are discovered.The primary contents read as follows:1.The ultraviolet visible absorption?UV-vis?and electronic circular dichroism?CD?spectra,charge transport,and second-order NLO properties of four axial chiral conjugated macrocyclic compounds are investigated through DFT calcualtions.The simulated UV-vis/CD spectra are in good agreement with the experimental ones,which can be used to assign the electronic transition properties and absolute configuration?AC?with high confidence.The electronic structures,charge transport and photophysical properties of cyclic and acyclic molecules are compared and analyzed.Subtle structural modifications can obviously enhance second-order NLO response,indicating that the studied compounds have the potential to be excellent second-order NLO materials.This work provides a new perspective for the design of chiral macrocyclic compounds with NLO activity.The designed compounds are expected to exhibit excellent performances in organic photovoltaics in view of their low band gaps,large oscillator strengths and small electron reorganization energies.2.The photophysical properties origin of axial chiral boron heptaaryldipyrromethene?heptaaryl-BODIPY?have been explored with the aid of DFT method.At the same time,the other five compounds on the basis of heptaaryl-BODIPY are designed.The calculated electronic absorption and emission wavelengths of heptaaryl-BODIPY are in agreement with experimental ones,allowing us to reliably assign its electronic transition properties.The studied compounds show remarkably large second-order NLO coefficients,up to 118.71×10-30esu.Their NLO response values closely associate with positions and electronic natures of substituent groups.Moreover,the electron reorganization energies are comparable to tris?8-hydroxyquinolinato?aluminium?Alq?which is a typical electron transport material.Intriguingly,the studied compounds are the potential excellent fluorescent probe materials from the standpoint of large Stokes shift and high emission efficiency.3.The ground-and excited-state structures,UV-vis/CD and emission spectra,optical rotation?OR?,and second-order NLO properties of helical chiral tetraphenylethylene?TPE?derivatives have been studied by the theoretical calculations.The experimental UV-vis/CD spectra and OR values are well reproduced,allowing us to reliably assign the electronic transition and determine the AC.Intriguingly,TPE derivatives are excellent candidates for the second-order NLO materials in view of the large first hyperpolarizability values and intrinsic asymmetric structures.The effect of substitution positions and substituent types on NLO activity is also explored.The intramolecular charge transfer cooperativity is achieved through involvement of the donor and acceptor substituent groups.Moreover,the designed compounds are potential materials for the fluorescent probe.Our work can provide a theoretical basis for finding more excellent NLO materials and fluorescent probe materials.4.The structures,electronic spectra,second-order NLO response and charge transport properties of helical chiral[7]helicene and five quinoxaline-fused-[7]helicene derivatives are systematically studied based on the theoretical calculations.Their electronic transition properties and ACs can be assigned with high confidence in view of good agreement between theoretical spectra and experimental ones.It is found that the chiroptical origin of the studied compounds mainly originate from exciton coupling between quinoxaline,phenyl or4-methoxyphenyl groups and[7]helicene.More interestingly,these derivatives possess high second-order NLO coefficient,reaching to 32.96×10-30 esu,which is about 190 times as large as the organic urea molecule.The bandwidth of the valence band of quinoxaline-fused-[7]helicene is comparable to that of the conduction band,and slightly larger than that of Alq,becoming the potential candidate as an ambipolar charge transport material. |
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