Organic Persistent Luminescence Materials:Multicolor-Tunable Afterglow And Ultralong Room Temperature Phosphorescence

Organic room-temperature phosphorescence(ORTP)materials are widely used in anti-counterfeiting,encryption,data storage,bioimaging and sensors because of their advantages of simple synthesis,low price and good biocompatibility.However,the weak spin-orbit coupling of pure organic materials leads to low intersystem crossing efficiency and fast nonradiative transition of triplet excitons,it is still challenging to obtain long-lived ORTP materials.In addition,afterglow materials with multi-color adjustable have important scientific and practical application value in anti-counterfeiting and data security applications.In this paper,a series of donor-acceptor organic compound with distorted structure are designed and prepared.Themulti-color adjustable afterglow characteristics of their crystals are studied.Then,the pure organic RTP materials with ultralong lifetime are obtained by dispersion in methyl methacrylate(MMA)solvent and in-situ polymerization,and the mechanism is deeply studied.1.Four donor-acceptor organic compounds 1?4 with distorted structure were designed and synthesized,the molecular structure was determined by nuclear magnetic hydrogen spectrum ¹H NMR?¹³C NMR carbon spectrum and time of flight mass spectrometry MALDI-TOF and the purity was determined by high performance liquid chromatography HPLC.The basic photophysical properties of compounds 1?4 were studied.The shoulder peaks of compounds 1?4 in the absorption absorption spectrum and the solvation effect indicated the intramolecular charge transfer characteristics,which was beneficial to inter-system crossing.The single crystals of compounds 1?4were cultured and the single crystal structures were determined based on X-ray crystallography with monoclinic and triclinic crystal system for 1 and 3,respectively;By studying the photophysical properties of the crystal,it is found that crystal 2 show different luminous colors at different delay times,from blue to purple,then purplish red for single crystal 2 and from blue to purple,then orange and finally brown for single crystal 3 Further,the time-resolved persistent luminescent spectra of single crystal 2 and3 identified that the color-tunable afterglow come from the dual emission of delayed fluorescence at around 415 nm,435 nm and phosphorescence at 550 nm and 600 nm.Moreover,the crystal 1?3 was applied in anticounterfeiting for their color tunable afterglow.2.The synthesized organic compounds 1?4 were dissolved into the MMA solvent by different mass percentages and then in-situ free radical polymerization was carried out to obtain a series of pure organic polymer room temperature phosphorescent materials P1?P4,in which the afterglow time of the P2 was up to 9 s,and the lifetime was up to 1.51s by transient decay tests.The phosphorescence spectra of polymers P1?P4 were similar to those of 1?4 in MMA solution at 77 K with barely redshift,which indicates that the compounds are monodisperse without aggregation in MMA.Moreover,the PMMA rigid environment can suppress the nonradiation and greatly improve its phosphorescence lifetime.To prove the universality of the method,compounds 5?8 were synthesized and characterized and also dispersed in MMA and in situ polymerization to obtain ultra-long room temperature phosphorescent materials P5?P8,in which the phosphorescence lifetime of P5 reached 1.64 s.Furthermore,the singlet S₁ and triplet T_n energy levels and their transition orbitals of the serial compounds are calculated by TD-DFT theory.It is found that the small energy level band gaps of S₁ and T₁,multiple triplet energy levels and moderate distortion angles are all beneficial to enhance room temperature phosphorescence and thus increase their lifetime.