Synthesis Of Halogen Coordinated Polymers And Study Of Their Multicolor Phosphorescence Propertie

Tunable multicolor room temperature phosphorescence（RTP）has attracted much attention due to its potential applications in the fields of anti-counterfeiting encryption,multicolor display and multi-channel biomarkers.However,the majority of reported tunable multicolor phosphorescence is mainly realized in multi-component systems,the observed emission is generally determined by the lowest excited state according to Kasha’s rule,thus it is a huge challenge to achieve excitation-dependent tunable multicolor RTP from single-component compounds.Metal-organic coordination polymers have become the preferred construction platform for multicolor RTP materials due to their structural designability and their rich selection of metals,ligands and guests.In this kind of coordination polymers,the triplet excitons can be well stabilized owing to the restricted motion of aromatic ligands（as the luminescent center）on the basis of coordination.When halogen ions are introduced into the complexes,their heavy atom effect will promote the generation of triplet excitons.Halogen ions also can act as electron donors to generate multiple luminescent centers through charge transfer with imidazole electron acceptor ligands.Excitation-dependent tunable multicolor RTP in single-component compounds can be attributed to the emission of ligands and their coemission with halogen atoms involving the charge transfer process.Therefore,in this paper,1,3,5-tri（2-methyl-1H-imidazole-1-yl）benzene（timb）and 1,3,5-tri（imidazol-1-ylmethyl）-2,4,6-trimethylbenzene（titmb）with potential luminescence properties were used as ligands to prepare a series of metal-organic coordination polymers containing halogen ions,and the luminescence properties of these materials were studied in detail.The specific work is as follows:（1）Four metal-organic coordination polymers with RTP and photochromism properties were designed and synthesized based on ligand timb:[Zn₈（timb）₁₂Cl₈][Zn Cl₄]₃·2Cl（1）,[Zn（timb）Br]·（HCOO）·H₂O（2）,[Zn（timb）I][Zn I₃]（3）and[Zn（timb）I]·I·H₂O（4）.Complexes 1-4 all display two-dimensional layered structures with halogen ions presenting in the form of terminal coordination or counter ions.The four complexes represent cationic host framework and the charges are balanced by free[Zn Cl₄]^2-、HCOO^-、[Zn I₃]^-and I^-guests.The close intermolecular interactions established between the coordinated halogens and N on ligand timb imidazoly provide convenience for charge transfer.Complexes 1-4 have multiple luminescent centers such as ligand-centeredπ-π*and charge transfer from halogen to ligand,and exhibit excitation-dependent multicolor RTP,and their color can be dynamically tuned from cyan to green-yellow/yellow-orange.In addition,it is obvious to observe that 1-4 change from white to light/dark yellow upon xenon lamp irradiation.Photochromic behavior is caused by free radicals which are generated by electron transfer between hosts and guests.（2）Flexible ligand titmb has a variety of geometric configurations.During the assembly process,it can adopt flexible coordination modes according to the coordination environment of metal ions,which is expected to construct inorganic-organic coordination polymers with novel structures.Four metal-organic coordination polymers with multicolor phosphorescence were synthesized based on ligand titmb:[Cd₅（titmb）₄Cl₁₀]·H₂O（5）、Cd₂（titmb）Br₄（6）、Cd（titmb）I₂（7）、[Zn（titmb）Br]·Br（8）.The halide ions act as terminal and bridging coordination mode in 5,the terminal coordination mode in 6 and 7,and the terminal coordination and counter ions in 8.Complexes 5-7exhibit different fluorescence and phosphorescence upon varying excitation wavelengths at room temperature,but complex 8 only represent different phosphorescent emission.Multicolor luminescence can be attributed to the emission of ligand titmb and its coemission with halogen atoms involving the charge transfer process.In addition,compared with complexes 1-4,no photochromic phenomenon was found in complexes5-8,which may be attributed to the following reasons:（1）There is no host-guest interaction or effective hydrogen bonding between the hosts and guests to provide effective pathways for electron transfer to generate free radicals.（2）Compared with rigid ligand molecules,flexible ligands exhibit more active molecule motion,which is adverse to stabilizing free radicals.