Theoretical Studies On The Regulation Of Nonlinear Optical Properties By Metal Ions And Organic Molecular Configuration/Conformation

Organic nonlinear optical（NLO）materials have received great attention due to their versatility in molecular structure,high nonlinear optical coefficient,and short electronic excitation response time,and so on.The factors that affect the NLO properties of organic materials include:electron delocalization,conjugated chain length,dispersion effect,charge transfer and substituents,and so on.Generally,organic compounds with large nonlinear polarizability have large non-localizedπ-electron systems.The larger theπ-electron system is,the larger the second-order nonlinear optical response will be.The introduction of electron-donating groups（D）and electron-withdrawing groups（A）at different positions of the system can lead to the large intramolecular charge transfer.In addition,it is possible that the non-diagonal component of the first hyperpolarizability can be increased by changing the shapeanddimensionofthemolecules.Moreover,thecontradictionof nonlinearity-transparency can be effectively solved.In this paper,the density functional theory（DFT）combined with analytical derivative methods are used to explore the effects of the NLO response by introducing different kinds of metal cations and modifying the configuration and conformation of organic molecules.The ratio of the second-order NLO response can be used in the detection of cations based on the NLO property.In addition,the regulating effects of configuration and conformation on second-order NLO responses have provided new insights into the design of NLO materials,and have great significances for the design of thermally activated delayed fluorescence（TADF）materials.The main conclusions observed are as follows:（1）The NLO properties of 7-substituted coumarin NC7 and its metal cation compounds were studied by DFT method.The introduction of metal cations increases the conjugation of the whole system,especially for the transition metal complexes NC7*Fe²⁺and NC7*Zn²⁺.The first hyperpolarizability value increases significantly due to its lower excited energy and larger oscillator strength of the main excited state.（2）The electron properties and static second-order NLO responses of spirooxazine SP and open-form metallic merocyanine complexes MC*M were studied by DFT method.The results show that the introduction of Ca²⁺into spiroxazine can increase the first hyperpolarizability by about 21 times,due to the obvious charge transfer from indolin to oxazine ring,the larger charge distribution and lower transition energy of the crucial excited state.This large change in second-order NLO ratio further verified the ability of spirooxazine/merocyanine systems to selectively detect alkaline earth metal cations.（3）The structures and NLO properties of the benzothiazolyl-attached macrocycle L and its metal cation complexes were studied.It was found that the addition of Hg²⁺induced a significant decrease inβ_tot value,and selective recognition of Hg²⁺by the macrocycle L can be achieved based on the NLO response.In addition,the internal coordination type Hg（L）（ClO₄）₂ and the external coordination type Hg（L）（Cl）₂ formed by different anions introduced into the complex L*Hg²⁺were realized the potential application of NLO response to anionic metal cation sensors.（4）The through-space conjugation system（U shape）comprising D and A bridged by the heteroatom and the conventional conjugated D-A molecules（L shape）have been designed to investigate the effect of modifying the connected bridge（conjugation and the heteroatom）between D and A units on the second-order NLO and TADF properties.It was found that the L-type compound has a largeβ_tot value,which can be used as a candidate for an ideal NLO material.However,the singlet-triplet gaps(ΔE_ST)of the U-shaped molecules are smaller than that the corresponding L-type compounds,indicating that U-shaped molecules exhibite the excellent TADF properties.In addition,theβ_tot andΔE_ST values of two different conformations（T type and V type）of D-A-D compounds were calculated.The results show that the V-typed compounds exhibit excellent second-order NLO properties.Therefore,it is indicated that the configuration/conformation of organic molecules can also regulate the second-order NLO response,and it is also important for the design of TADF materials.