Preparation And Properties Of Organic Cocrystals Of Fused Ring Compound

The design and synthesis of organic functional cocrystal materials based on crystal engineering have attracted the attention of researchers.Organic cocrystals which formed by the assembly of organic molecules in a fixed stoichiometric ratio through non-covalent interactions.The synthetic methods of organic cocrystals are simple,efficient,and low-cost.It not only possesses the properties of a single molecule,but also exhibits novel properties unique to the assembly,showing great potential in the fields of fluorescence emission and photothermal conversion.Indepth exploration of the packing mode between organic molecules and the interrelationship between crystal structure and cocrystal properties is not only beneficial to enhance the understanding of the structure-activity relationship of organic cocrystals,but also enables the efficient synthesis of organic cocrystal materials with specific functions.In this paper,a series of fused-ring compound organic cocrystal materials are designed and synthesized from the perspective of crystal engineering,trying to explore the relationship between the structure of the organic cocrystal and the properties of the materials.It mainly includes the following two aspects:1.The chemical structure of 1,2,4,5-tetracyanobenzene（TCNB）contains four strongly electron-withdrawing cyano groups conjugated to the benzene ring.The whole molecule exhibits strong electron-withdrawing properties and is a typical accptor,the highest occupied molecular orbital（HOMO）of TCNB is-8.57 eV,the lowest unoccupied molecular orbital is 3.92 e V（LUMO）.After TCNB is cocrystallized with electron-rich organic molecules,TCNB is often used as an electron acceptor,and electron-rich organic molecules are used as electron donors to form organic cocrystals through charge transfer.The LUMO is mainly derived from the LUMO of TCNB,and its energy gap is greatly reduced compared with that of a single component,resulting in a significant increase in the probability of electronic transitions and a significant red shift in the characteristic peaks of absorption and emission spectrum.In this paper,three organic cocrystals,CZ/TCNB,Cl CZ/TCNB and Br CZ/TCNB were synthesized using TCNB as electron acceptor and fused ring compounds such as carbazole（CZ）and its halides as electron donors.The solid-state characterization was carried out by X-ray diffraction and thermal analysis,and the UV-Vis absorption and fluorescence spectrum were studied.The results showed that,compared with the donor,the absorption and emission peaks of the cocrystals have a red shift of about 200 nm,and the luminescence lifetime is significantly improved,and the quantum yield is also improved to varying degrees.The structure of the cocrystals shows that π-π interaction is one of the most important driving forces for the formation of cocrystals,and the tightly packed chargetransfer columnar structure is the structural basis for the altered luminescence properties of carbazole and its halides.2.7,7,8,8-tetracyanodimethane（TCNQ）,another classical electron acceptor containing four strongly electron-withdrawing cyano groups,has a HOMO energy of-7.39 e V relative to TCNB,the LUMO energy is-4.68 e V.Due to its lower LUMO energy level,as an electron acceptor and a donor to form a cocrystal,an organic cocrystal with a smaller energy gap will be formed,although it will lead to a further increase in the probability of electronic transition As well as a further red-shift of the characteristic peaks of the absorption spectrum,the probability of electron nonradiative transitions of the excited state at the same time may be greatly increased,thereby exhibiting a photothermal conversion effect.In this paper,four organic cocrystals,AN/TCNQ,CZ/TCNQ,Cl CZ/TCNQ and Br CZ/TCNQ were synthesized using TCNQ as electron acceptor and fused ring compounds such as anthracene（AN）,carbazole and their halides as donors.Single crystal and powder X-ray diffraction,differential calorimetric scanning analysis,and infrared spectroscopy were used for solid-state characterization,crystal structure,Hirshfeld surface analysis,DFT theoretical calculation,UV-Vis absorption spectroscopy,fluorescence spectroscopy,and photothermal conversion properties were completed.The results show that the four cocrystals all form a classic ···DADADA···charge-transfer columnar structure throughπ-π interactions,and the UV-Vis absorption spectrum of the cocrystals has a very large red shift of about 500 nm relative to the single donor.However,due to the ultra-low energy gap of 1.46 e V-1.87 e V,the electrons return to the ground state through nonradiative transitions,and no photon emission was observed for the four cocrystals.The results of the photothermal conversion test showed that the photothermal conversion efficiency of AN/TCNQ,CZ/TCNQ,Cl CZ/TCNQ and Br CZ/TCNQ are 58.3%,53.7%,48.5% and 32.0%,respectively,which are basically consistent with their energy gaps,that is,the smaller the energy gap of the cocrystal,the greater the photothermal conversion efficiency;the four cocrystals can be successfully applied to photothermal imaging technology.In addition,this paper is the first time to apply cocrystal materials to photosensitive electronic switching technology.In this paper,it is found that by changing the composition of organic cocrystals,the luminescence properties and photothermal conversion effects of organic cocrystal materials can be regulated,which provides theoretical guidance for the rational design and controlled synthesis of specific functional materials based on organic cocrystals.