| Organic optical functional material has broad application prospects in the fields of lighting display,laser technology,information storage,energy conversion,and biomedicine.However,it has some disadvantages,such as poor photo-thermal stability,short lifetime,low luminous efficiency,reliance on the synthesis of complex molecular structures,which is difficult to purify the product with low yield.These problems restrict the development and application of organic optical functional materials,and there is an urgent need to design new energy-saving and environmentally friendly organic optical functional materials.The performance of optical functional material can be optimized by adjusting the aggregation state of material molecules.Organic charge transfer co-crystals are formed by the interaction of electron-rich electron donors and electron-deficient electron acceptors through?? stacking,hydrogen bonds,halogen bonds,van der Waals forces and other non-covalent bonds.In recent years,it has become a research central issue for the development of new organic optical functional material.The advantages are that the synthesis method is simple and green,the yield is high,the stability is high,and the photophysical properties of the material can be effectively controlled.In this paper,1,2,4,5-tetracyanobenzene(TCNB)is used as the electron acceptor,and polycyclic aromatic hydrocarbon derivatives containing chromophores such as naphthalene,phenanthrene and coronene are selected as the electron donor.Seven cocrystals were prepared by ball milling and suspension methods,including 2-hydroxy-1-naphthoic acid/TCNB,1-hydroxy-2-naphthoic acid/TCNB,3-hydroxy-2-naphthoic acid/TCNB,boron nitrogen-9,10-phenylphenanthrene/TCNB,boron nitrogen-5,6-phenylphenanthrene/TCNB and coronene/TCNB(including Form I and Form II).The single crystal structures analysis and the subsequent Hirsheld surface analysis were carried out.The results show that the ?? interaction is the main driving force for the formation of cocrystals,and other non-covalent bonding forces include hydrogen bonds and van der Waals forces are also included.The powder X-ray diffraction,thermodynamic properties,infrared spectroscopy,solid-state ultraviolet-visible absorption spectra,fluorescence emission spectra and fluorescence decay curves of a series of cocrystal samples were measured.The results show that all cocrystals have a homogeneous single crystal phase.The thermal stability of 2-hydroxy-1-naphthoic acid/TCNB,1-hydroxy-2-naphthoic acid/TCNB,3-hydroxy-2-naphthoic acid/TCNB,boron-9,10-phenylphenanthrene/TCNB and boron-5,6-phenylphenanthrene/TCNB is higher than the respective single component,while the melting points of the two crystal forms of coronene/TCNB are lower than that of coronene.The peak shift of the infrared spectrum shows that the electron cloud density of the benzene ring of TCNB increases after the formation of the cocrystal,which suggests that electrons may be transferred from the polycyclic aromatic hydrocarbon derivatives to TCNB.The electron spin resonance spectrum of the cocrystals show that the g factor is close to 2.0023,which proves that there are free electrons in the cocrystals.Compared with single components,the solid-state ultraviolet-visible absorption spectra of the respective cocrystals demonstrats broader absorption peak with different degrees.This is due to the reduction of the HOMO-LUMO energy gap caused by the effect of charge transfer in the cocrystals.As the number of aromatic ring units increases,the red shift of the absorption peak gradually increasesfrom 100 nm,200 nm to 300 nm.The color of the corresponding co-crystals changes from green,orange to red.At the same time,the photophysical properties of the solid physical mixture and the solution of the cocrystals were test.The results show that they are all simple superpositions of the spectra of single components,which indicates that the charge transfer effect between the electron donor and the acceptor in the cocrystal is the reason for the broadening and the red shift of the band.In addition,the fluorescence lifetime and fluorescence quantum yield of most cocrystals have been greatly improved.This paper reveals that the charge transfer state of organic cocrystals has an important influence on its optical properties,which provides certain theoretical guidance for the synthesis of organic charge transfer cocrystal materials with specific luminescence properties. |
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