Synthesis And Studies On Fluorescence Properties Of Organic Charge Transfer Luminescent Cocrystals Based On Carbazole Molecule

The charge-transfer cocrystal is the one formed between electron-rich donor(D)and electron-deficient acceptor(A)charge transfer and other interactions.It can not only retain the inherent properties of single components,but also may exhibit novel properties that the building blocks do not possess in cocrystal formation process.In the fields of dyes,the charge transfer cocrystal is beneficial to better understand the structure-luminescence relationshi then is of great significance to the adjustment of luminescent color,the research of photophysical theory and the development of optical functional materials.In this paper,with carbazole derivatives as donors and 1,2,4,5-tetracyanobenzene(TCNB)molecule as acceptor,a library of cocrystals were designed and fabricated,and optical properties of these cocrystal products were explored.1.With the same acceptor molecule,the fluorescence wavelength and color of the charge transfer cocrystals can be adjusted by controlling the planarity and electron-donating abilities of the donor molecules.Carbazole derivative molecules Br Cz,Me Cz,DCB and In Cz were selected as donors and traditional molecule TCNB as acceptor,four new luminescent cocrystal materials C1-C4 were synthesized by utilizing solution method,then the structure and non-covalent bond interactions,thermal stability and photophysical properties of cocrystal products were studied.Single crystal X-ray diffraction tests showed that under non-covalent bonding(charge transfer,?-?interaction,hydrogen bonding and C-H���?interactions,etc.),the crystal products present a D-A mixed stacking mode,wherein a sandwich face-to-face stacking structure is conductive to promoting charge transfer interaction.Longer?-conjugated DCB and In Cz have stronger electron-donating ability(lower ionization potential values)than Br Cz and Me Cz,and compared to the twisted structure of Br Cz and DCB,the?-?interaction is stronger in the cocrystals(shorter D-A distance/higher C���C percentage)formed by the planar rigid monomer Me Cz and In Cz.The charge transfer effect in C1-C4 experienced gradually increase as a result of the difference in structure of donors,which induced the fluorescence wavelength to redshift from 582 nm to 642 nm with the luminescent colors cover from orange-yellow to red.However,the strong-donating ability and the planar rigity of the donor can enhance the charge transfer and?-?interaction of cocrystal then quench the fluorescence of the cocrystal,thus resulting in low fluorescent quantum yield(PLQY).Beneficial to the distorted and low-conjugated molecular structure of Br Cz,as well as moderate electron-donating ability,the PLQY of C1 is up to 18.7%.2.We compared the structure and photophysical properties of Br Cz-TCNB cocrystal and Br Cz single crystal to explore the effect of cocrystal formation on the luminescence mechanism of the system.The structure of Br Cz-TCNB changed significantly in comparison to Br Cz,wherein inter-molecular charge transfer and?-?interactions were appeared due to the D-A alternating stacking mode.Different from the state in solution and film,C1 not only displayed charge transfer in the excited state but also in the ground state between Br Cz and TCNB in crystal;The fluorescence spectra of the monomer doped in PMMA confirmed that the excited-state charge transfer process increased with the shortening of the intermolecular distance.The heavy atom effect increased the degree of spin-orbit coupling between S₁ and T₁ The Br Cz crystal displayed dual emission of both fluorescence and phosphorescence at room temperature.The corporation of strong electron-acceptor TCNB increased the charge transfer degree and RISC rate in the system.The cocrystal presented TADF characteristics with prompt and delayed fluorescence.Attributed to the enhanced exciton utilization,the PLQY of cocrystal Br Cz-TCNB is higher than Br Cz single crystal.All these investigations indicates that the cocrystal strategy enable to adjust the energy and exciton distribution of the system,change the luminescence mechanism of the binary-component system,improve the exciton utilization and generate new properties that the monomer does not possess,which opens the door for us to design and synthesize TADF materials.