Mechanistic Study On Aromatic Nitrogen-heterocycle And Tetraphenylethylene-based AIE Luminogens

The elucidation of AIE mechanism and the development of novel AIE-active luminescent material with high performance have cast our constant focus and effort.With enthusiastic contribution from researchers in both experiments and theoretical simulation,the underlying mechanism of AIE has been unified as Restriction of Intramolecular Motion(RIM),whose quantum origin is restriction of non-radiative decay.We have dedicated my all works towards the application of such theoretical model to exploring the structure-performance relationship of new types of AIEgens based on aromatic nitrogen-heterocycle.And we have creatively explored the non-conventional excited-state features of the tetraphenylethylene,a classical kind of AIEgen.We aim to consummate the understanding of AIE mechanism and put forth theoretical directions for the design of novel AIEgens through our theoretical researches.AIEgens based on the aromatic tetraphenylpyrazine developed by our group in 2015 have improved the stability during utilization of AIE materials to a great extent.TPP derivatives with electron-donating groups have been developed as luminescent materials with high performance.But specific design guidance for TPP derivatives is still lacked.We have performed computational simulation on a series of TPP derivatives with single-,diagonal-and full-substituted methoxyl,thiophene,and triphenylamine groups.The analysis of radiative and non-radiative processes has shown that TPP derivatives with full-substituted weak electron-donors,diagonal-substituted moderate electron-donors and single-substituted strong electron-donors gain relatively high emission efficiency in solid states due to their balance between rigidity and flexibility,which not only leads to enhanced radiative decay rates,but also prevents molecules from emission-quenching ?-? stacking.Hence,balance between rigidity and flexibility is the key stone in designing TPP derivatives.Inspired by the excellent performance of TPP derivatives,we have designed and synthesized a novel type of AIEgens based on the rarely explored tetraphenylpyrimidine(TPPM).The analysis of electron-vibration coupling has shown that severe high-frequency bending motions of bond angles in the pyrimidine ring in TPPM dominate the non-radiative decay process.Attaching electron-donating methoxyl and phenyl groups to TPPM extends the conjugation and rigidifies the molecule skeleton and thus mitigates bending motion of the central core.And the electronic transition dipole moments of methoxyl and phenyl derivatives are greatly enhanced,inducing faster radiative decay.Thus,the emission efficiency is promoted in both the solution and the solid state.Meanwhile,due to the photo-induced electron transfer process enhanced by the hydrogen bond between the pyrimidine rings and explosive PA molecules,TPPM and its derivatives obtain high sensitivity in explosive detection.Traditional AIEgens possess through-bond ‘two-dimensional' conjugated structures.We have creatively developed a non-conventional type of AIEgens with through-space ‘three-dimensional' conjugated structures.Such through-space conjugation can produce enhanced visible luminescence in groups with no through-bond conjugation.With combining experimental and theoretical investigation,we have found that excited-state aromatic interaction in in the classical tetraphenylethylene can induce a through-space excited state(TSE state).And due to the stabilized TSE state and weakened non-radiative decay in solid states by RIM,TPE gain the enhanced emission of visible light.