Syntheses And Chromic Properties Of Pyridine N-Oxides And Their Complexes

Electron transfer(ET)photochromic materials have attracted intense interest due to their appealing applications on smart windows,information storage,photocatalysis,solar energy conversion,etc.Stable free radicals are not only conductive to the practical application of photochromic materials,but also play an important role in biological processes,organic synthesis and spintronics.However,the organic radicals generated from photoinduced ET process are usually unstable in air.Therefore,improving the stability of radicals and exploring new photochromic materials with long-lived charge-separated states have been the research frontier in the field of photochromic materials.In the first part of this thesis,the photochromic behaviors of two pyridine nitrogen oxides and the stability of corresponding organic radicals are investigated.In the second part,cadmium halide complexs based on the pyridine N-oxides are synthesized and their photochromic mechanism and properties are studied.The main results of this thesis are as follows:1.Unprecedented photochromic phenomenon of two pyridine N-oxides in the crystalline state are observed:4-cyanopyridine N-oxide(1)and 4-carboxypyridine N-oxide(2).The crystal structures before and after irradiation,various spectroscopy data and DFT calculations show that two organic molecules undergo intramolecular ET and generate stable triplet double radicals upon irradiated by UV light.The existence of strong electron-withdrawing group(CN)facilitates the improvement of photochromic performance and stability of organic radicals.2.Using organic compound 1 as ligand,four complexes with photochromic properties are synthesized:[Cd I₂(H₂O)(4-CPO)]_n(3),[Cd Br₂(H₂O)₂(4-CPO)₂](4),[Cd Br₂(4-CPO)₂]_n(5)and[Cd Cl₂(4-CPO)₂]_n(6).The electron-donating ability of O atom on 1 decreases markedly after coordination with cadmium.Therefore,photochromism of complexes 3-6 is attributed to the occurrence of intermolecular ET from halide ions(Cl^?,Br^?,I^?)to pyridine ring and generation of colored radicals.The different halide ions and loss of coordinated water lead to the change in the distance of ET pathway(X…N_py)and?-?interactions among pyridine rings,which further influence the photochromic properties of complexes.3.Using organic compound 2 as ligand,two isostructural complexes are synthesized:[Cd Br(H₂O)(4-CAPO)]_n(7)and[Cd Cl(H₂O)(4-CAPO)]_n(8).Upon irradiated by UV light,complex 7 undergoes intermolecular ET with hydrogen bond C-H…O as ET pathway and shows photochromism.While complex 8 is nonphotochromic owing to unsuitable ET pathway.4.Compared with organic compounds 1 and 2,the degree of photocoloration of complexes 3-7 decreases,which may be related to the color of generated single and double radicals.Except for 3,complexes 4-7 can generate long-lived charge-separated states under UV light irradiation.