Preparation Of Donor-acceptor Type Aggregation-Induced Emission Luminogens Based On Amination Reaction Of Aryl Nitriles Through C-C Bond Activation

The research on luminescent molecules originated from the early last century,while their photo-physical behavior is almost studied in solution state.Due to the planar and rigid structure,they often emit strongly in solution,while the emission is quenched by the ?–? stacking interaction.This is called as aggregation-caused quenching(ACQ).Aggregation-induced emission(AIE)is a novel photo-physical behavior which was termed by Tang,et al in 2001.It refers to a unique behavior that a series of propellershaped molecules which are weakly or non-luminescent in their diluted solution,but greatly enhance their emissions by aggregation.AIE overcome the shortcomings from the ACQ,therefore boosting rapid development in the fields of organic optoelectronics and bio-imaging.Although many studies based on phenyl-substituted silole and tetraphenylethylene as the AIE core have been reported,the intrinsic disadvantages of these AIE molecules should not be ignored.For example,siloles are troublesome in preparationand purification,and unstable under basic conditions.On the other hand,the double bonds in tetraphenylethylene often worse their stabilities under photo and heat.Herein,we selected 2,3-Dicyano-5,6-diphenylpyrazine(DCDPP)as the AIE core by avoiding the above shortcomings.Based on the active cyano group in its structure,we carried out the following work:Firstly,we developed a catalyst-free amination reaction through C-C activation of arylonitriles.The reactions can work efficiently only in the presence of amines in solvent under heat condition.Compared with the previously reported transition metalcatalyzed amination reactions based on C-halogen bond,C-O bond,and C-H activation,the catalyst-free amination reaction exhibits obvious advantages in economic and environmental conservation,and it can facilely obtains aryl amine compounds used in medicine and materials areas in large-scale production.Secondly,Thanks to the amination reaction,a series of D-A type AIE molecules bearing different substitutions are obtained.These AIE molecules show wide emissions and high luminescent efficiencies in aggregate state.Interestingly,the suppression of intramolecular flapping plus penyl ring rotation play important roles in the AIE effect.It enriches the AIE mechanism.The multiple intramolecular and intermolecular interactions can aid to lock the molecular motion in aggregate state to brighten the emissions.Notably,increasing the intermolecular interactions by three-dimensional physical connection and local hydrogen bond density are beneficial for molecular motion suppression.These AIE molecules exhibit low biological toxicity and superior targeting ability towards lipid droplets in He La cells.Taking Py-5 as an example,it successfully demonstrated its utility in fluorescently discriminating the fatty liver tissue from normal liver tissue.Thus,the unique D-A type AIE molecules can act as fluorescent probes to visualize lipid droplet associated biological behavior and fatty liver diseases.Our catalyst-free amination method exhibit great potential in tailoring various fluorescent materials for diversified applications.Finally,we also developed this reaction with aryl amines,and got three D-A type AIE molecules with highly twisted molecular configuration and longer fluorescence lifetime.Among them,Py-Px and Py-BLE emit weakly in the solution,while the emission is intensified significantly in the aggregates.By contrast due to the TICT effect,Py-Cz also emits strongly in the solution with its emission is red-shifted and weakened as the solvent polarity is increased.This is attributable to the TICT effect.However,its emission is still enhanced remarkably by aggregation.The crystal structures exhibit that the Py-Cz has the highest luminescent efficiency because of its rich intermolecular hydrogen bonds and self-locked conformation,which effectively suppressed its non-radiative transition.Although Py-Px and Py-BLE possess a selflocked conformation,they have less intermolecular interaction and large intermolecular gap so that the molecules can still dissipate the excited-state energy through molecular motion.As a result,the luminescent efficiencies in the aggregated state are relatively weak.