Hyperbranched and viologen-functionalized polyglycerols: preparation, photo- and electrochromic performance

Covalently bonding viologen groups at the chain-ends of hyperbranched polyglycerol would enhance the performance of viologen chromophores to external photo- and potential stimuli, because the amorphous, soft and hyperbranched polyether in globular conformation inhibits the aggregation or crystallization of the highly concentrated and ionic viologen chromophores, and also stabilizes the viologen cation radicals. Hyperbranched and functional polyglycerol containing viologen chromophores (HPG-V2+) are prepared by firstly capping the terminal hydroxyl groups of hyperbranched polyglycidol (HPG) with chloroacetyl, and then with the chloroacetyl groups and 4,4'-bipyridine to form the viologen chromophores on the chain-ends of HPG. The evolution of chemical structures from glycidols to HPG-V2+ is carefully characterized. The HPG-V2+ exhibits both photo- and electrochromism with high color contrast, short response time, and well stability: upon UV irradiation, the HPG-V2+ film changes its color from pale yellow to purple, while optical transmittance at 540 nm decreases from >= 90% to <10%; to the stimuli of pulsed 2.5 V electrical potential, the solution type electrochromic cell based on HPG-V2+ responds with reversible color changes from transparent to violet within about 0.52 s, much faster than the previously reported viologen-containing electrochromic polymers. The photochromism of HPG-V2+ is effective even in atmosphere by sunshine: when exposing to sunshine for several minutes the color of the HPG-V2+ film turned to purple, while low molecular weight viologen analogue exhibits only electrochromism.