The Properties Of Ultralong Organic Phosphorescence Materials Based On Phenothiazine Derivatives

Recently,ultralong organic phosphorescence(UOP)due to their unique photophysical properties has been widely used in data encryption,information storage,biological imaging,sensing,organic light-emitting diode(OLED),and other fields.Currently,researchers have developed a variety of methods to obtain highly efficient organic afterglow materials,including small molecule crystals,metal-organic frameworks(MOFs),hydrogen-bonded organic frameworks(HOFs),organic ionic crystals,carbon dots(CDs),co-crystals,host-guest doping,polymers,and so on.Among the reported strategies,small molecule crystals are indispensable for obtaining ultralong organic phosphorescence,because small molecules have the advantages of simple synthesis,low cost,and easy modification.To date,many studies have been conducted on the relationship between the UOP behavior and molecule packing,but how molecule packing affects UOP and the underlying mechanism are still unclear.In general,organic phosphorescent materials with high efficiency have a relatively short lifetime.Despite great success have been achieved for improving the quantum yield of phosphorescent materials,there are few studies on organic compounds with high efficiency as well as long emission lifetime.In addition,most aftergolw materials are excited by ultraviolet light,which greatly limits their applications in other fields,especially in biology.Compared with ultraviolet light,visible light has less phototoxicity,deeper penetrability for biological analysis and biological imaging,and more accessibility for practical applications.Although great efforts have been devoted,visible-light-excited UOP remains a formidable challenge.At present,there are few kinds of small molecule with UOP behavior and lack of more universal design strategies.In particular,both high efficiency and ultralong lifetime of organic phosphorescent materials need to be developed.Therefore,we have studied the following work.This paper includes three parts,taking phenothiazine derivatives as the research object.Phenothiazine is an unique non-planar butterfly structure with certain rigidity and flexibility.The unique structure can inhibit a possible intensive intermolecular ?-? stacking and reduce triplet-triplet quenching,which is beneficial for improving emission indensity.The work contents are outlined as follows:(1)Molecule packing plays an important role in organic afterglow emission,but how molecule packing affects UOP and the underlying mechanism are still unclear.In order to deeper investigating the relationship between molecule packing and UOP behavior,we designed and synthesized a series of phenothiazine derivatives using simple methyl groups as functional moieties.On the premise of not affecting the electronic structure of molecule,we adjusted the solid packing and further explored the effective molecule packing styles.By combining the theoretical calculations and experiments,the main factors influencing the photophysical processes of UOP in molecule stacking modes were explored.It provides a new guidance for the preparation of pure organic phosphors in the future.Among these compounds,the dimethyl substituted compound m-DMOPP achieved the best performance with a phosphorescence lifetime of 375 ms and a quantum efficiency of 1.9%.Furthermore,in virtue of the excellent UOP characters of the compounds,we have successfully applied them to information security and 3D patterning.(2)Pure organic phosphors with both high efficiency and ultralong lifetime are still a great challenge.Based on the first work,we also adopted the basic unit of phenothiazine-benzene,incorporated different number of methyl groups and added bromine atoms at the same time,and designed and synthesized a series of highly efficient and long-lived organic afterglow materials.The bromine atoms can greatly enhance the intersystem crossing(ISC).The methyl units were beneficial for building multiple intramolecular interactions.A close network packing was formed on the basis of intramolecular interactions along a,b,and c axis directions,which provided the dense and rigid molecular alignment under the connections of the entire space,thereby inhibiting the nonradiative transition of the triplet excitons.Together,the quantum efficiency and phosphorescence lifetime can be greatly enhanced.In comparison to the compound without bromine atom,the phosphorescence quantum efficiency increased from0.8% to 11.1% and the lifetime prolonged from 335 ms to 391 ms under ambient conditions.Besides,considering the highly efficient UOP features,the potential applications of these compounds in encryption and anticounterfeiting were investigated.(3)To achieve room temperature phosphorescent emission of pure organic small molecules,an external force(introducing functional moieties or doing in the medium)is usually required to increase ISC and reduce the non-radiative relaxation.Furthermore,the UOP materials need to be excited by ultraviolet light.To overcome the shortage of small molecule afterglow materials,considering the structural characters of phenothiazine and the possible role of self-assembly by hydrogen bonds,a supramolecular grid architecture via self-assembly was obtained by direct oxidation of phenothiazine through one step reaction.This extremely simple molecule can self-assemble through hydrogen bonds without the introduction of any substituents,thus realizing highly efficient fluorescence-room temperature phosphorescence dual-emissions.The lifetime was up to 305 ms and the phosphorescence quantum efficiency reached 4%.Interestingly,they can also achieve long-lived phosphorescence under ambient conditions excited by readily available visible light.The experimental results showed that the rigid molecule conformation and highly ordered spatial grid structure can not only restrict the molecular motion and inhibit the non-radiative transition but also promote the ISC process,thus achieving efficient and long-lived room temperature phosphorescence.Moreover,the applications of multiple encryption and decryption were successfully realized by using the remarkable characteristics of ultralong organic phosphorescence under visible light excitation.