Synthesis And Chromic Properties Of Complexes Based On Carboxybenzylate Viologen Ligands

Chromic materials will reversibly change their color when they are subjected to different forms of external stimuli,and have potential applications in various fields,including information storage,information protection,the erasable copy papers,smart windows,switches,and so on.However,most of the synthesized functional materials are monochromic,which severely limits their further application and development.Therefore,the development of multifunctional chromic materials with simple synthesis and excellent chromic properties has been a hot topic of research.Viologens have attracted much attention in the fields of photochromism and electrochemistry due to their good redox activity,unique electron acceptability and loss characteristics,and biological sensitivity.Considerable efforts have been devoted to designing and modifying viologen groups to construct chromic materials with fascinating structures and excellent properties.Through literature research and research of our group,we found that the factors affecting the chromic property of viologen-based materials are mainly reflected in three aspects:solvent molecules,auxiliary ligands and metal ions.When the electron-rich solvent enters the crystal lattice,it can form a donor-acceptor relationship with the electron-deficient viologen ligand.Auxiliary ligands of different types,shapes and sizes coordinate with the main ligand,thereby affecting the change of the crystal structure.Metal ions can enrich the crystal structure due to their different coordination modes,which can control the interfacial relationship between electron donors and acceptors.However,at this stage,there are relatively few studies on the regulation of crystal structure and chromic properties by metal ions.Based on the above considerations,two examples of 1,1’-bis（3-carboxybenzyl）-4,4’-bipyridine-dichloride（H₂bcbpyCl₂）and1,1’-bis（4-carboxybenzyl）-4,4’-bipyridine-dichloride（H₂bpybcCl₂）with similar structures and containing flexible backbones were selected as functional ligands in this thesis,and 11examples of complexes were successfully prepared by self-assembly with different kinds of auxiliary ligands and transition metal ions by solvent evaporation method at room temperature:{[Co（H₂O）₆][Co（BTEC）（H₂O）₄]·（bcbpy）·4H₂O}_n（1）;{[Ni（H₂O）₆][Ni（BTEC）（H₂O）₄]·（bcbpy）·4H₂O}_n（2）;{[Zn（H₂O）₆][Zn（BTEC）（H₂O）₄]·（bcbpy）·4H₂O}_n（3）;{[Co_0.5（bcbpy）_0.5（H₂O）₂]（p-BDC）_0.5·H₂O}_n（4）;{[Ni（bcbpy）（H₂O）₄]（p-BDC）·2H₂O}_n（5）;{[Co₄（bpybc）₆（H₂O）₁₂]·（OH）₂·（p-BDC）₃·22H₂O}_n（6）;{[Ni₄（bpybc）₆（H₂O）₁₂]·（OH）₂·（p-BDC）₃·22H₂O}_n（7）;{[Mn₄（bpybc）₆（H₂O）₁₂]·（OH）₂·（p-BDC）₃·22H₂O}_n（8）;[Cd（bpybc）_0.5（HBTC）（H₂O）]_n（9）;Co（bpybc）_0.5（HBTC）（H₂O）₄（10）;Ni（bpybc）_0.5（HBTC）（H₂O）₄（11）.The synthesis,crystal structure and chromic properties of these complexes are also described in detail,and the effect of metal ions on the chromic properties of these complexes is further investigated by comparison.The main contents are as follows.1.Taking H₂bcbpyCl₂ as the functional ligand,introducing 1,2,4,5-benzenetetracarboxylic acid and then coordinating with the transition metal ions Co²⁺,Ni²⁺and Zn²⁺to construct three complexes with the same structure.Effects of the transition metal ions on the chromic properties of the complexes under isostructural conditions were comparatively studied.It was found that the electron-withdrawing ability of metal ions,that is,the strength of electronegativity,has a positive effect on the photochromic properties of these complexes.In addition,complex 3 is sensitive to sunlight,UV light and X-rays.2.Inspired by complex 3,using H₂bcbpyCl₂as the functional ligand,1,4-terephthalic acid（p-H₂BDC）was introduced to coordinate with transition metal ions Co²⁺and Ni²⁺to synthesize complexes of 4 and 5 with the same structure.Similar to complex 3,both 4 and 5are sensitive to sunlight,UV light and X-rays.In addition,4 also exhibits hydrochromism,which is caused by the d-d transition in the Co²⁺coordination field.Under the same reaction conditions,three cases of complexes 6-8 with the same structure were successfully constructed by selecting H₂bpybcCl₂,which is similar in structure to H₂bcbpyCl₂,as the functional ligand.These three isostructural complexes only respond to UV light,but all have thermochromic behavior caused by water loss,which is the result of the synergistic effect of electron transfer and d-d transition in the ligand field.3.H₂bpybcCl₂was selected as the functional ligand,1,3,5-trimesic acid（H₃BTC）was selected as the auxiliary ligand,and Cd²⁺,Co²⁺,and Ni²⁺were introduced to participate in the coordination,three complexes 9-11 with different structures and chromic properties were successfully constructed.Crystal structure analysis showed that complex 9 has a two-dimensional layered structure,and complexes 10 and 11 have the same zero-dimensional structure.Complex 9 only has electron-transfer（ET）photochromic behavior,10 has photochromic behavior caused by ET and dehydration caused by d-d transition in the coordination field,while 11 has both ET photo and thermally-induced chromic behavior.The results show that the metal ion still play a significant role in regulating the structure and chromic behavior of these complexes.