Synthesis And Properties Of Pure Organic Room-Temperature Phosphorescence Materials Based On Nitrogen Heteroatom

Recently,organic room-temperature phosphorescence（RTP）materials have rapidly become a hot topic due to their tremendous potential applications in anti-counterfeiting,sensing,optical recording,fingerprint recognition,bio-imaging,information security,and organic light emitting devices（OLEDs）.However,it is normally hard to realize pure organic RTP emission under ambient conditions（atmospheric,room-temperature conditions）as the radiative transition of phosphorescence is a spin-forbidden process.Without heavy atoms（usually noble metals）,the spin-orbit coupling（SOC）coefficient of pure organic materials is pretty weak,which inhibits the formation of triplet excitons through the intersystem crossing（ISC）process.And it also leads to a lower radiative transition rate of triplet excitons,which can hardly compete with non-radiative transition processes such as molecular vibration and rotation,and oxygen quenching.Enhancing SOC and suppressing non-radiative transitions are both essential means to achieve efficient pure organic RTP.The former is mainly achieved by introducing heavy atoms,heteroatoms（e.g.O,N,and S）,heteroatom-containing groups（e.g.aldehyde,cyano,ester,and carboxyl）and increasing the distortion of the molecule,while the latter is accomplished by crystallization,H-aggregation,J-aggregation,host-guest doping,etc.Over the past few decades,the development of RTP was fast and numerous RTP materials emerged,but the development of pure organic RTP materials remains preliminary and many issues require urgent solutions.This promising field has attracted considerable attention and many efforts have been carried out.In this thesis,a series of pure organic RTP materials/systems with nitrogen heteroatom were designed and synthesized by employing suitable strategies for molecular modification,while the relationships between molecular structure,crystal packing mode,host-guest interactions,external stimuli and RTP performances were explored,providing critical guidelines for the subsequent design of efficient RTP molecules.The main contents are as follows:In Chapter 2,three molecules were constructed by introducing cyano and halogens into biphenyl.Among them,the bromine and cyano-substituted molecule named Br-BPh N exhibits distinct phase-dependent RTP properties,with the phosphorescence efficiency（Φ_P）of 9.1%for crystal 1（high-quality crystal）obtained by slow vacuum gradient sublimation,which drops to 6.0%and 1.4%for crystal 2（low-quality crystal）obtained by rapid solvent evaporation and amorphous solid.Since the atomic number of Cl and F is much smaller than Br,no RTP phenomenon was observed in Cl-BPh N and F-BPh N crystals.Nevertheless,their photoluminescence efficiencies(Φ_PL)reached36.0%and 56.0%,respectively,which were significantly improved compared with 10.0%for Br-BPh N,demonstrating that the introduction of heavy atoms could not only promote SOC but also induce luminescence quenching.Theoretical calculations and experimental results suggest that the rigid environment triggered by multiple intermolecular interactions in the crystalline state can effectively enhance the RTP property of the Br-BPh N molecule.Regarding the promotion of SOC/ISC by introducing internal heavy atoms,Br is superior to Cl/F.In Chapter 3,a stimuli-responsive RTP molecule DMAB was obtained by the incorporation of the nitrogen atom-containing dimethylamino and the boronic ester group in one benzene ring.The maximum emission peak of crystalline DMAB locates at 387 nm with theΦ_PL of 41.0%,while theΦ_P is up to 35.2%.The emission in the region of 470-600 nm possesses a unique ultralong RTP（URTP）characteristic.The afterglow persists for 14 s under ambient conditions after 1 min of continuous UV irradiation(365 nm,25 m W cm^-1)and spontaneously recovers to the initial state after50 min,displaying a reversible recycle feature.In addition,DMAB crystal features a special mechanoluminescence（ML）phenomenon,and detailed studies suggest that the ML is supposed to originate from the triplet state of the DMAB monomer.The photo-induced URTP is interpreted by the oxygen consumption during illumination,while both its efficient RTP and ML depend intimately on the packing mode and plentiful interactions inside the crystal.The derivatives DPh B and Cz B lack analogous ML and RTP properties with reducedΦ_PL,which is consistent with the presence ofπ···π/C-H···πinteractions in crystals and the expansion of molecular skeletons.DMAB crystal has been successfully employed in oxygen sensing and anti-counterfeiting applications thanks to its photo/oxygen sensitivity.In Chapter 4,the dimethylamine-containing molecules CDA and FDA are used as guests,while PPF and PPT with the diphenylphosphine oxide act as hosts.A series of highly efficient RTP systems were prepared by integrating the RTP host and guest into a doped system through the dissolution and removal of the solvent.As expected,all blends display intense phosphorescence under ambient conditions,and the phosphorescence behavior can be optimized by altering the doping concentration.The concentration of the guest was adjusted from 2 wt%to 98 wt%and the emission was100%contributed by phosphorescence for 10 wt%blends.The PPF-based systems offer higherΦ_P,capable of reaching 40%,a superior value in the reported RTP systems.Furthermore,systems with a guest concentration of 1 wt%generate long afterglow at77 K.Regarding the CDA-based system,the afterglow derives from the blend,whereas that for the FDA-based system originates from the triplet state of the guest.The experimental results show that the strong suppression of the non-radiative transitions by the rigid environment of the host as well as the charge transfer（CT）character state and large SOC coefficients of the doped system is advantageous for achieving efficient RTP.Host-guest doping method provides the advantages of simplified preparation,low cost,and adjustable properties,and its amorphous potential enables significant feasibility in practical applications.This work is significant for the expansion of RTP systems.In Chapter 5,a pair of isomers with different RTP natures,Cis-CS₂ and Trans-CS₂,were attained based on the bis-triphenylamine skeleton.Both the Cis-CS₂ crystal and the ground solid demonstrate instantaneous RTP in response to UV under ambient conditions,with theΦ_Ps of 21.4%and 14.2%,respectively.This efficient RTP is facilitated by the compact packing and substantial intermolecular interactions in the crystal as well as the introduction of heteroatoms.Intriguingly,Trans-CS₂,the isomer of Cis-CS₂,shows the absence of RTP in the crystalline state under ambient conditions,while the systems are conferred with photo-induced RTP effect upon doping it into the polymethyl methacrylate（PMMA）polymer.It only requires approximately 20 s of UV irradiation(365 nm,25 m W cm^-1)to activate RTP and spontaneously recovers to its original state after 5 min.TheΦ_PL of photo-stimulated PMMA films is up to 40.0%and theΦ_P can reach 9.96%.The photo-induced RTP phenomenon should be attributed to the intense suppression of molecular motions by the PMMA matrix and the consumption of oxygen during UV irradiation.Benefiting from their exclusive RTP features,Cis-CS₂ and Trans-CS₂ have been successfully used in anti-counterfeiting applications.