Construction And Photochromic Properties Of Dipyridyltriazole-based Cd(Ⅱ) Complexes

Electron transfer（ET）organic-inorganic hybrid photochromic materials based on electron donor-acceptor units have gained considerable interest for their excellent photostimuli-responsive ability and potential applications in light switching,sensing,display,photocatalysis,information storage,energy conversion,and so on.Although great achievement has been made in facilitating the photoinduced electron transfer（PET）prosess by modulating the matching relationship of donating-accepting ability and interfacial relation of electron donors and acceptors,most of the ET photochromic materials display narrow photo responsive range（mainly response to UV light or X-ray）and short-lived colored states owing to the formation of highly active organic radicals,which limit the practical applications of ET photochromic materials.Therefore,how to broaden the photo responsive range and achive long-lived colored states is of great significance for the commercial applications of ET photochromic materials in various fields.Dipyridyltriazole derivatives contain three aromatic rings and multiple N-donor coordination sites.The coordination/chelation of dipyridyltriazole with metal ions can not only enhance the electron deficiency of acceptors but also result in high coplanarity orπ-conjugation of aromatic rings that conducive to the stability of photo generated radicals.In this paper,by employing multidentate ligands 4-amino-3,5-bis（2-pyridyl）-1,2,4-triazole（2-abpt）and 4-amino-3,5-bis（3-pyridyl）-1,2,4-triazole（3-abpt）as electron acceptors and electron-riching halogen atoms or benzene dicarboxylic acids as electron donors,seven crystallinephotochromiccomplexesareprepared,[Cd₄（2-abpt）₂（H₂O）₂Cl₈]·2H₂O（1）,[Cd₄（2-abpt）₂（H₂O）₂Br₈]·2H₂O（2）,Cd₄（2-abpt）₂I₈（3）,[Cd₃（2-bpt-H）₂（pa）₂]·H₂O（4）,[Cd（2-bpt-H）（ip）_0.5]（5）,[Cd₃（2-abpt）₂（tp）₃]（6）and[Cd₂（3-abpt）（ip）₂]·2DMF（7）.The influence of electron donors on the molecular configuration of acceptors,the interfacial relation of electron donor-acceptor,PET and photochromic behaviors is investigated and the relationship of composition-structure-function is explored.The main conclusions are as follows:1.By introducing multidentate chelating ligand 2-abpt into the Cd X₂ system,three discrete tetranuclear complexes 1-3 have been synthesized,in which 1 and 2 are isostructual.Three complexes exhibit halogen-modulated photochromism and photoluminescence.Importantly,the colored photoproducts of 1 and 2 possess ultra-stability（can be stable at least 6 months in dark at ambient environment）and solvent erasable behavior.The ultralong-lived colored states could be assigned to the close packing structures formed by strong hydrogen-bonding interactions,while solvent erasable behavior may result from the disturbance of solvation effect to the hydrogen-bonding networks that lead to the quenching of radicals by O₂ in air2.Three complexes 4-6 with different stacking modes are constructed by coordinating electron-riching phthalic acid（H₂pa）,isophthalic acid（H₂ip）and terephthalic acid（H₂tp）and electron-deficient 2-abpt with metal Cd（Ⅱ）.4 shows 1D chain,while 5 and 6 present 2D layer structures.The distance between two carboxyl groups in benzene ring affects the molecular configuration of 2-abpt and the removal of the amino group in triazole ring,which further modulate the charge transfer（CT）interactions between electron donors and acceptors as well as the PET and photochromic behaviors.Complexes 4-6 display a wide photoresponsive range（response to X-ray,UV and visible light）and high sensitivity to visible light,which could be attributed to the promoting effect of CT absorption in the visible region on PET.Interestingly,under irradiated by green light（490-570 nm）,4-6 can form a rare intermediate state of Cd（I）with white color,which provides a good platform for better understanding the PET process.3.A 3D MOF（7）based on 3-abpt ligand is constructed from the assembly of 3-abpt,H₂ip and Cd（Ⅱ）.7 shows dual photo-/thermochromism,while the desolvated MOF display both photochromism and solvatochromism.Moreover,the colored samples all have a long-lived life time（at least 3 months in dark in ambient condition）,but can be decolored quickly by visible light,exhibiting excellent photoreversible behavior.