Spin Multiplicity Control Of Nitrogen-containing Aromatic Compounds

Ylides play an important role in the field of synthetic chemistry and are the subject of vivid research activities in most diverse fields of chemistry.Their unique electronic structure has always been a very controversial topic.In this paper,the electronic structure of pyridinium ylides has been studied.it was found that pyridinium ylides exhibit radical characters with a singlet ground state and a thermally activated triplet state.We proposed the theory of single-electron transfer radicals to explain this phenomenon,and tried to use its radical properties to prepare organic ferromagnets.Phosphors also have triplet excited states.The difference is that phosphorescence is photoluminescence that the electron absorbes a photon,and then reach the triplet excited state through ISC process.Phosphorescence can be applied in the fields of photovoltaic devices,photocatalytic reactions,optical storage and molecular sensing owing to its long lifetime.During the course of my Ph.D.study,I have been focused on the study of the spin multiplicity control of nitrogen-containing aromatic compounds,including the free radical properties and phosphorescence properties of pyridinium compounds,and the room temperature phosphorescence research of naphthalimide compounds as follows.1.Pyridinium ylides are usually considered nucleophiles that can undergo various reactions involving electron pairs.However,it was found that ylides resulted from deprotonation of N-alkyl substituted pyridinium salts exhibit radical characters,with no discernable NMR signals but decent EPR spectra in both solution and the solid state.An observed correlation between lowered*energy level of the pyridinium ring and increased EPR activity indicates that thermally induced electron-transfer processes could be involved,where the variable-temperature EPR spectrum indicates a singlet ground state and a thermally activated triplet state for the ylide,?EST=-0.262 kcal/mol.The fact that high-resolution mass spectrum confirms the presence of oligomers of a less sterically hindered pyridinium ylide further points to a radical mechanism.2.The magnetism produced by the interaction of unpaired electrons is one of the most typical properties of organic free radicals.The spin arrangement caused by the interaction between unpaired electrons through ? electrons or heteroatom paired electrons lays the foundation for obtaining molecular-based magnetic materials.We have designed and synthesized a series of pyridinium ylide polymers connected by aliphatic chains.It is expected that free radicals can achieve ferromagnetic order through intermolecular and intramolecular exchange interactions,thereby obtaining organic ferromagnets.Although the results were not satisfactory,they inspired me.Next,we will try more effective linkers to obtain macroscopic ferromagnetic bodies based on pyridinium ylides.3.Stimulus-responsive organic phosphorescent materials can not only visually monitor the changes in the color and intensity of the luminescence under external stimuli,but also use the changes in emission lifetime under external stimuli as monitoring parameters.Acridinium salt exhibits ultralong green phosphorescence under alkaline conditions.As a result,we designed a "turn-on" alkali-responsive RTP sensor based on the property.The system only exhibits fluorescence properties in neutral environment,and there is no afterglow that can be observed with the naked eye even at 77 K.But in alkaline environment,a strong green afterglow with the lifetime of more than 1 s can be observed at low temperatures.4.Long-lived red room temperature phosphorescent materials have unique advantages in the field of bioimaging,but there are few related studies.Our research group previously reported that the naphthalimide system can achieve RTP bridged by intramolecular charge transfer state.In this article,we have designed two strategy of red room temperature phosphorescence based on naphthalimide:the first one is to dope the naphthalene diimide-based D-A system into the polymer matrix.The use has greater The conjugation of naphthalene diimide is greater which results in the red shift of luminescence,thereby inducing the red RTP.The second strategy is based on an organic guest-host system which converts the strong fluorescence of the guest into ultralong RTP via doping the naphthalimide guest into a tailored host.