Molecular Design,synthesis And Piezochromic Luminescent Properties Of Cationic Iridium(?)complexes

In recent years,stimuli-responsive materials whose color changes under external stimuli?light,electricity,heat,mechanical force,etc.?have attracted much attention.In particular,piezochromic luminescent?PCL?materials exhibiting changes of color can be realized by mechanical force.These materials have displayed the potential application prospects in the fields of mechanical sensing,safety ink and information storage due to their unique properties.After more than 20 years development,great progress has been made on the research of PCL materials.However,many problems are still unsolved in this field.Firstly,it is difficult to directly observe the molecular packing changes in the grinding process,therefore,the understanding of piezochromic phenomena still remains in the initial stage of transformation between crystalline state and amorphous state.Secondly,most of the PCL materials are obtained by accident,and the universal principle of molecular design is largely unexplored,which limits the further development of this field immensely.In addition,the novel PCL materials with multicolor switching and high contrast are rarely,which needs to be explored.In this paper,to solve these problems,we design and synthesize a series of PCL materials based on iridium?III?complexes,and their photophysical characteristics and piezochromic behavior have been investigated in detail,which may provide experience and reference for the future development of such materials.All the relative studies are outlined as follows.?1?Based on isomeric pyridine-triazole ligands,two cationic iridium?III?complexes were prepared,and the PCL behavior of these iridium?III?complexes were studied by contrast.The remarkably similar crystal structure of two iridium?III?complexes determines their similar changes of molecular packing upon grinding.Consequently,the difference in the PCL properties of the two complexes is mainly caused by their inherent properties.The results revealed that the complexes dominated by ³MLCT/³LLCT in the excited state have better PCL characteristics.This work skillfully selects isomers to explore the mechanism of piezochromism,which provides the experimental and theoretical basis for constructing the effective system of PCL materials.?2?We designed three iridium?III?complexes containing-CF₃ groups to investigate whether the introduction of-CF₃ group into molecules can achieve the piezochromism of iridium?III?complex.Based on the grinding experiment results,all of these three complexes display reversible PCL behaviour.The single crystal structures of complexes indicate that intermolecular C-H···F interactions exist.The powder X-ray diffraction?PXRD?patterns demonstrate that these weak interactions are destroyed,and the crystalline structures collapse under the external pressure,resulting in the PCL phenomenon.This work proves that the introduction of-CF₃ group into the system can effectively construct the intermolecular C-H···F interactions,and is beneficial for achieving the PCL characteristics of iridium?III?complexes.Moreover,this work not only enriches the system of PCL materials,but also paves the way for the design of new PCL materials.?3?An iridium?III?complex with polymorphism had been designed and synthesized by introducing heteroatoms and bulky mesityl groups into ligands.We obtain the tricolour luminescence switching of an iridium?III?complex for the first time,which utilizing the reversible transformation between two crystalline states and an amorphous state.The crystal structure and quantum chemical calculation indicate that the different degree of ligand distortion affects the orbital energy gap of the two crystals thereby resulting in the different luminescence color of complex.The significance of this work is not only to achieve the tricolour luminescence switching of an iridium?III?complex for the first time,but also to provide a valuable reference for the future development of excellent iridium?III?complexes with PCL properties.?4?A double component and high contrast PCL system?mix?was constructed by mixing complex 2 and organic small molecule m-BHCDCS.In particular,complex 2 is nota PCL material.An efficient F?rster energy transfer exist between the two components in the solid state before and after grinding.The minute crystals in the sample were destroyed after grinding,the molecular distance between complex 2 and m-BHCDCS was shortened,and the energy transfer process was obviously increased.Therefore,under mechanical force,the blue emission derived from complex 2 was almost completely quenched,while the red emission derived from m-BHCDCS was greatly enhanced,thus achieving the 110 nm shift of the maximum emission peak before and after grinding.This work reflects the flexibility of the double component system in material selection and the advantages in building excellent PCL system,which expands a new method for obtaining high-efficient PCL materials.