Design,Synthesis And Applications Of Halide-Containing Organic Persistent Luminescent Materials

In the past few years,great achievement has been made in many aspects of organic persistent luminescent（OPL）materials,such as material design,spectrum regulation,mechanism research and application exploration,for their advantages of long luminescence lifetimes,low manufacturing cost,flexibility,and bendability.However,most of the OPL materials only show one emission peak,limiting the wide application of materials,and there is a lack of universal molecular design strategy to regulate the luminescent color and emission lifetime at the same time.In order to solve these problems,herein,pure organic long afterglow materials with dual emission property were constructed based on the halogen-modified strategy,and the application scenarios of these materials were explored.Firstly,a dihydroindo[3,2-b]carbazole compound was found to exhibit dual persistent phosphorescence emission.And then,trifluoromethylation and oxidation strategies were adopted to adjust the molecular structure and crystal stacking to regulate the OPL emission wavelength and lifetime.Secondly,bromine atoms were introduced into the aromatic rings and alkyl chains to realize the OPL emission.The intensity and lifetime of persistent thermal activation delayed fluorescence（TADF）and room temperature phosphorescence were tuned based on the internal/external heavy atom effect of bromine atoms.Finally,bromine-containing OPL materials were applied for X-ray imaging owing to their high triplet exciton utilization efficiency,and the photophysical properties under X-ray excitation of these materials were studied.The contents are as follows:1.Achieving dual persistent phosphorescence emission via inter-/intramolecular charge transferA series of indolo[3,2-b]carbazoles analogues were designed and synthesized with distorted molecular configuration to make the structures of the donor and acceptor perpendicular to each other.The dual persistent phosphorescence emission was achieved in the single-component materials via the inter-/intramolecular charge transfer characteristics.Intramolecular charge transfer contributes the small energy gap between the singlet and triplet excited states,which will promote the intersystem crossing process and lead to blue phosphorescence emission from monomers.C-H···πinteractions in crystals boost the intermolecular charge transfer process and promote the yellow phosphorescence emission from aggregates.In addition,trifluoromethylation and oxidation strategies were adopted to regulate the molecular stacking distance in the crystal,and the effects of halogen-containing groups and crystal stacking on room temperature phosphorescence properties were explored.2.Tuning OPL emission by introducing alkyl and aryl bromide atomsThe phosphorescent emission from the monomer of indolo[3,2-b]carbazoles derivatives was tuned to 500 nm by N-alkylation reaction.And then,bromine atoms were introduced into the alkyl chains and aromatic rings to regulate the reverse intersystem crossing（RISC）and triplet-triplet energy transfer process,respectively,realizing the persistent TADF and room temperature phosphorescence emission.The monomers act as the reservoir of triplet excitons,providing the long-lived excitons for dual persistent emission.The aromatic bromine atoms promote the RISC process and TADF emission due to the internal heavy atom effect and enhance charge transfer among chromophores.The alkyl bromide atoms suppress the non-radiative transition by inhibiting the molecular vibration of the alkyl chains,and promote the electronic transition rates between singlet and triplet states by the external heavy atom effect on adjacent chromophores.Finally,bromine atoms were introduced into the aromatic rings and alkyl chains simultaneously to realize dual OPL emission with lifetimes over 0.2seconds and the quantum yield over 10%.3.Applying bromine-containing OPL materials to X-ray imagingBromide atoms were introduced to dihydroindo[3,2-b]carbazole rings directly to improve the radiative efficiency of triplet excitons and enhance the X-ray photon absorption cross section of the OPL materials,which will contribute to achieve efficient radioluminescence properties.The compounds exhibit persistent TADF and phosphorescence emission with lifetimes over 10 ms under excitation of 350 nm ultraviolet light at room temperature.Owing to the heavy atoms on the chromophores,persistent emission is dominating in the photoluminescence signal of the materials,with an OPL efficiency of 27.1%.The luminescence of the compounds under excitation of X-ray is mainly the phosphorescence emission with a maximum light yield of 4867 photon/Me V,and the detection limit of X-ray dose rate is as low as 20.73 n Gy s^-1.Finally,the films of the bromine-containing compound were applied in the X-ray imaging of different objects with a spatial resolution larger than 3 lp/mm.