Synthesis And AIE Mechanism Of Benzylidenedione

Since the concept of aggregation-induced emission(AIE)was first proposed in 2001,a large number of AIE materials have been developed.These unique solid-state organic luminescence materials show excellent practical application in various fields such as optoelectronic devices,chemical sensing and bio-imaging owing to their high photoluminescence quantum yields,excellent photostability and rich chemical tailorability.Conventional luminogens are designed preferentially with polycyclic conjugated framework and planar structures,which usually emit strongly in the well-dissolved solution states,while weak or totally quenched emission usually occurs when aggregated or in the solid states with the strong intermolecular π-π interactions inducing the aggregation-caused quenching(ACQ)effect.To address this issue,thousands of typical AIE luminogens(AIEgens)have been designed and synthesized,and the major design strategy is incorporating molecular rotors,such as tetraphenylethylene(TPE)and triphenylamine(TPA),into conjugated luminophore.AIEgens are weakly or almost non-emissive in solution due to dissipating exciton energy by the rotation of peripheral molecular motors,while activate radiative transitions in the solid states owing to restriction of intramolecular motions(RIM)and weakened intermolecular π-π stacking by backbone distortion and twisted molecular rotors.Unlike the typical AIEgens with a propeller structure and rotors such as TPE,several unconventional rotorfree AIEgens was found.However,they also adopted non-planar conformations in their crystal and their intermolecular distances exceeded the typical π-πstacking distance(3.5 A)that usually quenched the fluorescence,which hindered their ACQ characteristics.For most AIEgens,their starting materials are expensive and their subsequent synthesis is complex and time-consuming,which is not conducive to further promotion and application.However,in this study,it was found that some of the benzylidenedione molecules,which remained completely flat in the aggregation state,could also show the AIE effect.Benzylidenedione is simple to synthesize and may have further applications in organic fluorescent materials.More importantly,there is a significant difference in structure between benzylidenedione and traditional AIE molecules,which have no free rotor structure and maintain a small crystal plane spacing and good planarity in the crystal aggregation state.Therefore,through the specific study of benzylidenedione,we can further broaden the mechanism explanation of AIE phenomenon and propose more new strategies for the design of AIE molecules.The main research contents and conclusions are as follows:(1)A series of benzal dimethylbarbituric acid(BAs)were designed and synthesized by Claisen ester condensation reaction using barbituric acid and different substituted methoxybenzaldehyde.They all have excellent light absorption properties and good light stability.The four BAs emit almost no emission in solution,due to the non-radiative transition that consumes the energy of the excited molecules.However,only B-4 emits strongly in the solid state and has significant AIE effect.This is due to the restriction of intramolecular motion and the blocking of non-radiative transition pathways.By density functional theory calculation and single crystal analysis,it is found that B-1 and B-2 are not emitted or weakly emitted mainly because of their small molecular oscillator strength,while B-3 is almost not emitted because of the overly tight π-π packing caused by H-aggregation.The crystal spacing of B-4 with obvious AIE effect is less than 3.5 A,and the crystal arrangement shows long distance planar arrangement and tight π-π packing,which is inconsistent with the traditional view.(2)By Claisen ester condensation reaction,a series of benzal meldrum’s acid(MAs)and a series of benzal indanedione(IDs)were designed and synthesized using meldrum’s acid,indanedione and methoxybenzaldehyde with different substitutions.They all have excellent light absorption properties and good light stability.The three MAs and the three IDs emit almost no emission in solution,due to the non-radiative transition that consumes the energy of the excited molecules.While M-2,ID-1,ID-2 and ID-3 emit strongly in the solid state,and have significant AIE effect.This is due to the restriction of intramolecular motion and the blocking of non-radiative transition pathways.By density functional theory calculation and single crystal analysis,it is found that M-1 has almost no emission due to the relatively tight π-π packing caused by H-aggregation,and M-3 has a weak fluorescence emission due to excessive intermediate methoxy-substitution.The crystal spacing of M-2 with obvious AIE effect is less than 3.5 A,and the crystal arrangement shows long planar arrangement and tight π-π packing,which is also inconsistent with the traditional view.After B-4,the completely planar M-2 also has the property of AIE,which further indicates that the completely planar AIE molecule may exist widely in the benzylidenedione molecule.