Multi-stimuli Responsive Dendritic Macromolecules

Organic photochromic compounds,which undergo reversible structural changes and subsequently reversible changes in spectral absorption,volume,polarity and solubility,have attracted much attention in recent decades due to their promising applications in optical data storage,chemical sensing,photocontrollable biological process.Unlike most of photochromic molecules,such as azobenzene,spiropyrans,diarylethenes,which need the high-energy UV light to trigger the isomerization,donoracceptor Stenhouse adducts(DASAs)switch from a conjugated,colored,and hydrophobic form to a ring-closed,colorless,and zwitterionic structure on irradiation with visible light and have high fatigue resistance under ambient conditions.Unfortunately,reversible switching of DASAs only occurs in organic solvents.Photoswitching of DASAs in a reversible manner in aqueous phases remains a fascinating challenge,which limits the application in biomedical filed.The OEG-based dendronized polymers have numerous advantages: excellent water solubility,thermoresponsibility,cylindrical molecular shape,and high rigidity.Its unique topological structure and amphiphilicity of OEG is supposed to offer a microenviroment which can mediate the isomerization balance of DASAs.Herein,a series of OEG-based dendronized copolymers carrying DASAs pendants are designed and prepaired,photochromism in aqueous solutions and the structural effects on the reversible switching process was investigated in details.The details are as following:(1)DASAs-containing amphiphilic dendritic macromolecules Me-G1-DASAand EtG1-DASA were designed and synthesized.Their structures were characterized by using 1H NMR.The isomerization and photo-responsive behaviors in various organic solvents and aqueous solutions were characterized by UV/Vis spectroscopy.(2)A series of OEG-based dendronized copolymers immobilizing by DASAs moieties were designed and synthesized,respectively.The structures of intermediate products,target monomers and copolymers were characterized by using 1H NMR,and the molecular weights of copolymers were measured by GPC.The morphology and size of copolymers were measured by AFM,TEM,DLS.The photoswitching mechanism was studied as well.It was found that reversible color switching of DASAs in aqueous solutions can be achieved by immobilization of DASAs to dendronized Et-OEG-based polymers,which simultaneously endowed the copolymers with excellent thermoresponsibilty.More interesting is that the performance of these dendronized copolymers in the bulk state is different from their aqueous solutions.The unique topology structure and the ratio of hydrophilicity and hydrophobicity showed important influences on reversible switching of DASAs in aqueous solutions.According to the AFM,TEM and DLS results,we presume that the cylindric topology structure and appropriate ratio of hydrophilicity and hydrophobicity in the copolymer form a suitable microenvironment which acts as a buffer driving reversible switching of DASAs in aqueous solutions.This work paves the way for photoswitching of DASAs inside living organisms using visible light.