Synthesis,Assembly And Spectral Property Of Triblock Copolymer With Terminal Rhenium Complex

Metal complexes have received tremendous attention and play an important role in the field of chemistry.Rhenium complexes as the typical metal complexes,can form octahedral space structure with ligand.It shows rich photophysical properties because there are many kinds of change transfer in the process of electron transition.Pluronic triblock copolymer is made up of polyethylene oxide(PEO)and polypropylene oxide(PPO):PEO-PPO-PEO.This kind of copolymer has a high sensitivity towards aqueous solution.Although there are many studies of rhenium complexes reported in recent years,the rhenium complexes triblock copolymer system is relatively less explored.A series of rhenium complexes connected at the ends of the PEO-PPO-PEO block copolymer was designed and synthesized.The spectral properties and the assembly behavior of this system in water solution by changing the temperature,concentration,solvents and additives were studied.For synthesis,traditional pyridine-2-benzimidazole ligand has a variety of electronic orbitals,and the complexes formed with metal centers should show interesting photophysical properties.However,there are a lot of problems in the chemical synthesis because the-NH group in the imidazole ring has a very low reactivity,resulting in the difficulty for the ligand to connect to the polymer.In this study,the design of new ligand,4-hydroxy-pyridine-2-benzimidazole can solve this problem.It occupies the more reactive phenolic hydroxyl group compared with the traditional pyridine-2-benzimidazole ligand.First of all,the chemical reactivity of phenolic hydroxyl is much higher than that of amines.Secondly,phenolic hydroxyl in the position outside of the ring and nitrogen in the imidazole ring would result in advantages in the aspect of reducing steric hindrance.The synthetic mechanism of these two kinds of ligands involves nucleophilic reagent O~–and N~–in the alkaline environment respectively,then the attack of the center carbon atoms.The space is very crowded because the carbon atom is connected with five groups,so the nitrogen in the imidazole ring is very unfavorable to react.Therefore,we choose this new kind of ligands 4-hydroxy pyridine-2-benzenimidazole and synthesized it successfully,and then connected it to the both sides of PEO-PPO-PEO block copolymer.The original nitrogen atom in imidazole ring is retained,allowing control of the properties of the complexes.Finally,the coordination reaction was done with five carbonyl bromine rhenium and characterized by NMR spectroscopy.For the self-assembly property,dynamic light scattering experiment was performed to analyze the state of target compounds in aqueous solution.It was found that the target compounds have three states in aqueous solution:distributed in solution as unit(1-3 nm),micelle(about 20 nm),large aggregate(90 nm).Micellar structure for typical of PEO-PPO-PEO triblock copolymer is like a sphere,centered on the structure of hydrophobic PPO nuclear,with hydrophilic PEO structure at the peripheral.Large aggregate is composed of multiple micelles together.We tested dynamic light scattering data of compounds in aqueous solution at different temperature,concentrations and additives.We found that when the temperature is very low,the state of the target compounds is very chaotic,coexistence of a variety of state in solution,this phenomenon will be weakened by the rise of temperature and concentration.At room temperature,target compounds will be at a relatively stable state of micelles in the water,when heated to a higher temperature,the size will change accordingly.And it also has a large influence on assembly properties of compounds by adding silicotungstic acid,because the additives can affect not only on ligand of the rhenium complexes,also the interaction between EO chains and water,change the EO chain in water solubility,and transform the assemble the morphology of the target compounds.For spectral properties,ultraviolet-visible absorption spectroscopy and emission spectroscopy were performed for the target compounds.In the absorption,there are two absorption peaks at 210 nm and 320 nm.One peak in emission spectrum at550-600 nm is assigned as the phosphorescence of rhenium complex.Firstly,we vary the temperature to regulate the morphology of target compounds.It can dissolve in water as unit at low temperature.With the increase in temperature,the compound aggregates to form micelles.The change of microenvironment of the rhenium complex would affect its self-assembly properties.For this reason,we study the effect of temperature with spectral properties of target compounds.In addition,we also test the absorption spectrum and emission spectrum of target compounds in different organic solvents to study the solvent effect to the spectral property.