| As a typical third-order nonlinear optical effect,two-photon absorption(TPA)has found applications in fields as broad as optical,optoelectronics and medical treat,for the advantages of higher three-dimensional spatial selectivity,stronger penetration and less light damage.For decades,extensive explorations on single component chromophores with large two-photon absorption cross section(σTPA)were carried out based on intramolecular approach,such as organic conjugated chromophores(from simple dipolar to quadrupolar,even dendritic octupolar dyes),metal-organic derivatives and fluorescent protein chromophores,etc.,aiming to increase the intramolecular electron delocalization.However,a significant increase in molecular complexity may not achieve a synergistic increase in σTPA and synthesis complexity is inevitable.Instead of intramolecular charge transfer,in recent years,co-crystallization strategy has been proposed to design and prepare nonlinear optical(NLO)materials,which is expected to achieve broad two-photon excitation window,large σTPA and strong two-photon excited fluorescence emission.In the typical intermolecular charge transfer organic cocrystals,electron-rich(donor)and electron-deficient(acceptor)molecules are assembled into a densely packing superstructures in segregated or mixed stacking.Compared with single-component organic molecular synthesis based on intramolecular charge transfer,with the advantages of simple preparation,abundant raw materials,and even realizing two-photon excited fluorescence(TPEF)property by using TPA-free components,organic cocrystals greatly expanded the sources of TPEF materials theoretically.However,the TPEF cocrystals prepared are rather rarer than expectation up to new.It is difficult to achieve effective co-crystallization between different molecules,which depends largely on growth methods,structural matching,and the assembly driving force between donor and acceptor molecules.In addition,the expected crystal structure may not be obtained due to the rearrangement of component molecules,which results in poor cocrystal performance far short of practical application requirements.In our study,we successfully prepared three new types of cocrystals with TPEF property by designing the donor and acceptor components scientifically.In order to achieve good optical properties,the solubility,molecular structure,electronic structure,self-assembly ability and intermolecular charge transfer ability of components were primarily evaluated.These new types of organic cocrystals in high quality were obtained through solvent evaporation method,solidgrinding method and so on.After that,the crystal structure properties,intermolecular interactions,intermolecular charge transfer properties and linear/nonlinear optical properties of cocrystal samples were investigated by combining experimental tests with theoretical calculations.From macro-and micro-perspective,the co-crystallization mechanisms was explored.The origin of nonlinear optical property was revealed by analyzing the relationship among molecules stacking,the intermolecular charge transfer and the optical property.The researches of this thesis play important roles in understanding the relationship among the physical properties of organic cocrystals,the molecular structures as well as the intermolecular interactions of components.The explorations of new types organic cocrystal systems in this thesis also provide some reference value for the development of organic cocrystals in the fields of optical,electrical,magnetism and other fields such as energetic materials and pharmaceutical.The main research contents of this thesis are as follows:(1)Using the biphenyl aromatic heterocyclic compound 2,2’:6’,2’-terpyridine(Tpy)as donor and 1,2,4,5-tetracyanobenzene(TCNB)as acceptor with the mole ratio of 1:1,the blue fluorescent TPEF cocrystal TTC in good stability and phase purity was obtained via solvent evaporation method.The intermolecular π-π stacking and hydrogen bond C-H…N strongly drived the self-assembly of TTC,and thus Tpy-TCNB molecule pairs exhibited alternating close mixed stacking.The excited state charge transfer characteristics were demonstrated graphically,and π-π*excitation characteristics with complete intermolecular charge transfer were exhibited in Tpy-TCNB molecule pairs.TPEF spectra showed that the TTC cocrystal exhibited wide two-photon excitation window and stable up-conversion fluorescence.We explained the TPA property of TTC cocrystal as follows:the delocalization of π conjugated electrons in the whole Tpy-TCNB cocrystal system was induced by the long-range intermolecular charge transfer along the stacking direction of donor-acceptor molecules,which would lead to electron polarization in the supramolecular structure.The results from our experiments could provide a theoretical basis for the preparation of two-photon absorption materials by intermolecular strategy.The co-crystallization strategy could achieve nonlinear optical property by using highly ordered mixed stacking with heterogeneous molecules,and could be a good tool to explore the structure-optical relationship.(2)The polycyclic aromatic heterocyclic compound Acridine(Acr)as donor and 1,2,4,5tetracyanobenzene(TCNB)as acceptor,with solvent evaporation method,the green fluorescent cocrystal ATC with high TPEF property and high fluorescence quantum yield(PLQY=:22.48%)was successfully prepared.In ATC cocrystal,donor-acceptor molecules exhibited π…πstacking with close distance along a specific stacking direction,close to the completely parallel and overlapping molecular dipole stacking,resulting in strong coupling between adjacent πorbitals,thus,the charge transfer between donor-acceptor molecules would be strongly promoted.The existences of intermolecular charge transfer properties in the ground and excited states of ATC cocrystal were demonstrate by experimental and theoretical calculations.It can be inferred that the optical absorption was derived from the transition from the ground state CT0 to the new charge transfer excited state CTn,rather than from the ground state to the excited state of donor or acceptor molecules.TPEF measurements showed that ATC cocrystal exhibited a wide two-photon excitation window and strong two-photon excitation fluorescence.The quantum chemistry calculation with Dalton and analysis of two-state model revealed that the charge transfer between Acr and TCNB molecules greatly increased the change of the permanent dipole moment between the ground state and excited state,and significantly enhanced the electron polarization of ATC cocrystal system,which is fundamentally beneficial to the TPA performance of ATC cocrystal.(3)ADC cocrystal was successfully prepared with Acridine(Acr)and 3,5-Dinitrobenzoic acid(DNB)(molar ratio of 1:2)by solid-grinding.We inferred that the close π-π stacking interaction between Acr-DNB layers and the strong hydrogen bond interaction caused by the introduction of functional groups(-COOH and-NO2)both provided the structural basis for the super-strong co-crystallization behavior and intermolecular charge transfer properties.The cocrystallization mechanisms of Acr and DNB in solid-grinding and non-solid-grinding were illuminated by gas diffusion and surface diffusion theory.ADC exhibited strong TPEF property with wide two-photon excitation window,which was explained by intermolecular CT in the Xand Z-axis directions.Our experimental results demonstrated that the non-luminescent components can be grown into TPEF cocrystal with high PLQY by the simple solid-grinding process through proper selection and design of components materials. |
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