Transformation From "Inert" To "Active" Photo-Controlled Third-Order Nonlinear Switch Of Azobenzene Compounds

The photo-controllable third-order nonlinear optical?NLO?switches are easy to transform between two distinct chemical forms,further leading to reversible third-order NLO properties variations under external light stimuli.The materials have drawn ever-increasing attention due to considerable research potential in the emerging field of nonlinear optics.We call this class of materials which produce reversible NLO conversion directly under external light stimulus as"active"photo-controllable NLO switches.The other type of materials,which contain photosensitive groups but cannot express directly switching properties under light conditions,are named as“inert”photo-controllable NLO switches.However,the“inert”materials are not generally considered as the subject of NLO switches,so this kind of materials are generally neglected.In fact,the“inert”photo-controllable NLO switches can exhibit excellent NLO switching properties if the“inert”materials are activated.In this paper,the carboxylic acid-substituted azo derivative H₂L,which have no direct photo-controlled NLO switch.Its potential light-controlled switch behavior is successfully activated by removing the factors affecting the light response behavior,and the photo-controllable third-order NLO behavior of the activated material was investigated.The thesis mainly studies from the following two parts:1.In the article,an"inert"azo photo-controlled NLO switching material H₂L with carboxylic acid groups is reported.Since H⁺is ionized from its carboxylic acid group in solution,which affects its cis-trans isomerism expression under light stimulation,we successfully activated the light-controlled switching behavior of"inert"materials by introducing Lewis bases to shield H⁺effects.When activated materials are exposed to UV light,the third-order nonlinear absorption properties produce change from reverse saturated absorption to saturation absorption,and the third-order nonlinear refraction behaviors produce transition from self-defocusing to strong self-focusing.Moreover,this light-controlled switching materials have good reversibility and fatigue resistance.The results of density functional theory calculations of compound?CH₃?₂L show thatproduces rearrangement of the electronic structure in its frontier molecular orbital after illumination.We think this significant third-order nonlinear transformation is due to the rearrangement of the electronic structure in the frontier molecular orbitals,which makes them produce different response mechanisms under laser stimulation.2.Metal complexes are a class of metal organic framework formed by bridging metal ions with organic ligands.When the azobenzene derivative H₂L of carboxylic acid substitution coordinates with metal ions to form metal complexes to achieve the function of removing H⁺,which may realize the activation of the“inert”third-order NLO switching material.We synthesized five complexes{[Zn₂L₂?H₂O?₃]·2DMA)}_n?1?,[Cd L?H₂O??DMF?]_n?2?,{[Cd L?4,4'-bipy?H₂O]·H₂O}_n?3?,[Zn L?4,4'-bipy?]_n?4?and[Mn L?4,4'-bipy?]_n?5?.We first investigated the photo-responsive behavior of complexes 1-5,and the results showed that both complexes 1-5 can produce reversible cis-trans isomerization reactions.Secondly,we conducted ¹H NMR test with complexes 1 and 4 as examples,the results showed that complexes 1 and 4 have coordination bonds in solution.Finally,the third-order NLO properties of complex 1-5before and after UV illumination were studied by Z-scan experiments.The results show that complexes 1-4 show both reverse saturated absorption behavior and no refraction behavior of complexes 2-4 before illumination besides the complex 1 shows week self-defocusing behavior;but after illumination,complexes 1-4 exhibit opposite third-order NLO behavior?saturated absorption and strong self-focusing behavior?.While complex5 has no third-order nonlinear performance before and after illumination,this may be due to poor dissolution of complex 5.On the whole,metal complexes can be removed H⁺,thus achieving the“inert”photo-controlled third-order NLO switching activation of the“inert”azobenzene derivatives.The interference of H⁺in the ligand molecules can be removed by forming metal complex with the metal,so as to achieve the activation of"inert"azobenzene derivatives photo-controlled third-order NLO switches.To sum up,"inert"materials exhibit excellent photo-controlled third-order NLO switching performance after being modified,so the researches on the such materials will open up a new field of NLO switches.