Organic Charge Transfer Cocrystal:Design,Assembly And Optoelectronic Functionality

Compared with traditional covalent synthesis,organic cocrystals are composed of two or more components through non-covalent bonds with the advantages of low cost,easily tunable structure,and solution processing,etc.,offering the opportunity to construct functional materials.Meanwhile,organic cocrystals are crystalline solid material,and its building units present a clear and ordered three-dimensional network structure in the cocrystal,providing a platform for unveiling the structure-property relationships at a molecular level,and thus giving a general guideline for the design and assembly of high-performance functional materials.Among them,the charge transfer cocrystals refer to the type of which free electron can delocalize between the donors and the acceptors,wherein the charge transfer interactions not only can be as the assembly driving force for the coctystals,but also provide more chance to explore some novel functionalities in cocrystals.This paper is mainly based on organic charge transfer cocrystal: design,assembly and optoelectronic functionality.The main contents are as follows:1.We have successfully prepared a charge transfer TTC cocrystal of trans-1,2-diphenylethylene(TSB)as the donor and 1,2,4,5-tetracyanobenzene(TCNB)as the acceptor.The intermolecular charge transfer interactions in cocrystals narrow the singlet-triplet energy gap and therefore facilitate reverse intersystem crossing for thermally activated delayed fluorescenece(TADF)for the first time.Those findings open up a new way for the future design and development of novel TADF materials.2.We have realized fabrication of a charge transfer STC cocrystal with onedimensional morphology,wherein the 4-styrylpyridine(Spe)and 1,2,4,5-tetracyanobenzene(TCNB)are chose as the donor and acceptor,respectively.The STC cocrystals have 1:1 stoichiometry with a mixed-stack packing style,which contribute to the intermolecular charge transfer interactions in the supramolecular architecture,thereby inducing electronic polarization of STC system,and the two-photon absorption properties of STC cocrystals are realized through the assembly of simple building blocks.It should be noted that the STC cocrystals possess two-photon absorption properties that are absent in the individual components.Moreover,the two-photon absorption characteristics in other charge transfer cocrystals are also found,indicating that cocrystal engineering is an effective method for developing nonlinear optical materials for future optoelectronic applications.3.In this paper,the reversible nonlinear optical switching behaviors of the cocrystal have been studtied under acid-base stimulation.The STC cocrystals can be transformed into a three-component pyridine salt(STC-H)under the stimulation of an acidic atmosphere.The stacking structure of STC-H has changed relative to that of STC cocrystal,and the degree of charge transfer of the STC-H has been reduced accordingly.Therefore,the fluorescence spectrum and two-photon absorption spectrum of STC-H are blue-shifted relative to those of the STC cocrystal.Moreover,STC-H can be combined with basic molecules to transform into the original STC cocrystal,and the spectral behaviors also return to the original state.In addition,the nonlinear optical behaviors of Spe crystals can also change under external acid stimulation,which achieve the controllable regulation of nonlinear optical behavior.