Study On Synthesis And Photochromic Properties Of Bipyridinium-Based Crystalline Compounds

Photochromic materials have promising applications in the fields of molecular switching,information security,sensing and molecular machines.However,most of these materials only respond to ultraviolet light and have difficulty responding to other wavelengths such as natural light,visible light and infrared light,which greatly limits their potential applications.Bipyridinium-based materials have good electron acceptance and photoresponsiveness and are a potential candidate for broad-band photoresponsiveness,which is becoming a hot topic for research on photochromic materials.Seven compounds of bipyridinium-based materials with sensitive photoresponsiveness,good reversibility and different photoresponsiveness were synthesized by self-assembly of a flexible coordination electron-deficient N,N’-dipropionic acid-4,4’-bipyridinium（CV）ligand with carboxylic acid-based auxiliary ligands and metal cations.This paper presents a detailed study on their microstructure,photochromic properties,photoluminescence switching and inkless printing,and discusses their conformational relationships in depth by combining theoretical calculations with microstructure.It is divided into four main sections as follows.1.A photo-responsive charge-assisted hydrogen-bonded organic network（H₂CV）（H₂BTE C）（1）,was synthesized by choosing electron-deficient N,N’-dipropionic acid-4,4’-bipyridiniu m（CV）and electron-rich benzotetracarboxylic acid（BTA）.Under the irradiation of soft X-ray,ultraviolet or natural light,1 rapidly changes from colourless to blue and kinetic calculations show that its photochromism meets first-order dynamics.Interestingly,the colored crystals 1P with long-lived colored radicals will not fade after being placed for more than two months at room temperature in air,owing to the fact that the photogenerated viologen radicals can be sta bilized by the abundant hydrogen bonds,the coplanar pyridinium rings and the slow electron-hole recombination.Hence,its application to inkless printing.2.Two different photochromic materials were synthesized by controlling halogen:[Cd₂Cl₄（CV）（H₂O）₂]_n（2）and[Cd Br₂（CV）]_n（3）.They are all three-dimensional supramolecular framework materials with different light-responsive properties,which are self-assembled by one-dimensional Zig Zag chains relying on hydrogen bonds.2 has faster photosensitivity,is darker,and responds quickly to X-rays,while 3 responds slowly to ultraviolet light.Microstructural and theoretical calculations suggest that the reason for this is most likely due to the fact that 2 has a slightly weaker ground-state charge transfer and a shorter electron transfer distance,which is more favorable for photoinduced electron transfer.In addition,the complex 2 was irradiated with Cu Kαray at a radiation dose rate of 120Gy/min.The color of the sample changed from colorless to dark blue with irradiation dose increasing,and the absorption intensity in the visible region changed accordingly,especially the intensity change at 660 nm was very significant,and the calculation showed that the X-ray detection limit was as low as 2.89 Gy,which is known to be the lowest among X-ray induced color change.The lowest detection limit among the bipyridine-based materials is known,which indicates the high sensitivity of our material to X-ray detection.3.Three complexes with different photoresponsiveness:{[Zn₂（BDC）₂（CV）]·4H₂O}_n（4）,[Zn₂（5-MIPA）₂（CV）（H₂O）₂]_n（5）,{[Zn₃（TAC）₂（CV）]·4H₂O}_n（6）.4 exhibits a 2D metal-organic layer with a[4,4]topology by the[Zn₂O₈C₂]SBU,in which a[Zn₃O₁₂]SBU is surrounded by 2-connected CV linkers and 2-connected BDC linkers.While 5 is a one-dimensional chain constructed with the[ZnO₄]SBU is surrounded by 2-connected CV linkers and 2-connected 5-MIPA linkers,and 6 exhibits a 3D 2,3,8-connected structure with a{6¹⁸·8⁹·10}{6³}₂{6}topological network,in which a trinuclear[Zn₃O₁₂]SBU is surrounded by 2-connected CV linkers and 3-connected TAC linkers.4,5,and 6 exhibit different photoresponsiveness,which is related to non-optically responsive auxiliary ligands.When the H atom at position 5 on the auxiliary ligand BDC is replaced with the electron donor methyl group,the electron donating ability of the auxiliary ligand is enhanced and more conducive to photo-induced electron transfer,which is one of the reasons why complex 5 is more photosensitive and more deeply coloured than 4.When the H atom at position 5 on the BDC is replaced with a carboxyl group,the carboxyl oxygen atom acts as an electron donor,making the electron transfer channels shorter and more numerous,which facilitates the photo-induced electron transfer from the auxiliary ligand to the N atom of the CV,which is one of the reasons why 6 has more outstanding photochromic properties than 4.4.A multifunctional complex{[Eu₂（BTA）（H₂BTA）（CV）（H₂O）₄]·2H₂O}_n（7）was synthesized by solvothermal method.7 is a 3D framework with{4¹⁶·6²⁰·8⁸·10}{4⁴·6²}₂{6}constructed from 10-connected[Eu₂O₁₆]nodes,2-connected CV linkers and 4-connected BTA linkers.Only under ultraviolet light irradiation,the colour of 7 changes from colorless to blue,and the coloring process conforms to first-order kinetics.In addition,7 also has excellent fluorescent switching properties and is a potential switching material.