The Design And Performance Study Of Photoresponsive Aggregation-Induced Emission Molecules Based On Salicylaldehyde Schiff Base

Aggregation-induced emission(AIE)molecules exhibit excellent luminescence properties and have received considerable attention from researchers in the fields of chemistry,materials science,and life sciences.AIE molecules with photochromic properties have important applications in optoelectronic devices,smart anticounterfeiting,and biological diagnosis and treatment.In this work,we designed and synthesized two new types of AIE molecular systems with photochromic properties based on salicylaldehyde Schiff base structural units,and investigated the photochromic mechanism in detail,and realized various color change properties such as reversible photochromism,photo-induced emission change,and photo-induced circularly polarized luminescent change.The main work is organized into two parts as follows:Fluoran salicylaldehyde hydrazone metal complex(FSHMC)is a kind of recently reported photoresponsive system with simple synthesis,diverse colors,and superior photochromic properties.Here,we first synthesized a class of FSHMC molecules based on the typical AIE group,tetraphenylethylene(TPE),and the obtained complex 1-Zn exhibited reversible photochromic and photo-induced emission change properties under UV light irradiation.Mechanistic studies showed that the photochromism of complex 1-Zn was caused by the ring-opening reaction of the rhodamine B part under UV irradiation.Compound 1 exhibited typical AIE properties with different luminescence properties in the aggregated and dispersed states,which made 1-Zn also exhibit different photoresponsive properties in different states.In the dispersed state,the TPE part of 1-Zn did not emit light and only the rhodamine B part exhibited red fluorescence after UV irradiation.In the aggregated state,1-Zn generates fluorescence resonance energy transfer(FRET)from the TPE part to the rhodamine B part after UV irradiation,thus exhibiting different photoresponsive properties from those in the dispersed state.This work provides a new strategy for the design and synthesis of photochromic materials with AIE properties.Additionally,we introduced chiral structures into the design and synthesis of salicylaldehyde Schiff base photochromic systems and constructed a series of chiral AIE molecules(2-4)based on 1,1’-binaphthyl-2,2’-diamine salicylaldehyde Schiff bases.Racemates((rac)-2-(rac)-4)and monochiral enantiomers((R)-2-(R)-4)exhibited very different photoresponsive properties.Under UV irradiation,(rac)2-(rac)-4 molecules did not exhibit photochromism,while(R)-2～(R)-4 showed significant photochromism.Mechanistic studies showed that the different crystal accumulation patterns of both are the primary reason for this difference.The single crystal structures of(rac)-2-(rac)-4 molecules are more tightly packed than those of chiral(R)-2-(R)-4 molecules.The solubility of the former are much lower than those of the latter while the melting point are much higher,suggesting that the former have stronger intermolecular interactions and thus inhibits the enol isomerization reaction of the molecules under UV irradiation.Interestingly,due to the AIE properties of the compounds,(R)-2-(R)-4 exhibited significant circularly polarized luminescence signal in the excited state,and become a kind of potential photo-induced circularly polarized luminescent color-change material.This work expands the functions of salicylaldehyde Schiff base type photochromic compounds and provides a new strategy and reference for the design of photochromic systems with photo-induced circularly polarized luminescent change properties.