Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens

As a promising electrochromic material, viologens have attracted increasing attention due to their high redox activity and adjustable electrochromic capability. In order to investigate the effect of alkyl substituents on electrochromic behavior, four alkyl-substituted viologens and a benzyl-substituted viologen were synthesized, namely 1,1'-dioctyl-4,4'-bipyridinium dibromide (OV), 1,1'-didekyl-4,4'-bipyridinium dibromide (DeV), 1,1'-didodecyl-4,4'-bipyridinium dibromide (DoV), 1,1'-dihexadecyl-4,4'-bipyridinium dibromide (HV), and 1,1'-dibenzyl-4,4'-bipyridinium dibromide (BV). The different photophysical and electrochemical properties of these viologens were attributed to their deviation in spatial structure caused by different substituents. Compared with benzyl-substituted BV, a slight blueshift occurred for the absorption peaks of alkyl-substituted viologens from 262 to 257 nm with the increase in alkyl chain length. Moreover, the first redox couple increased positively, and the dimerization of the compound decreased gradually, accompanied by the decrease in optical contrast and distinct chromatic difference. A comparison of chromatic and optical contrasts indicated that OV had the longest coloring response time (R_Tc), while it was shortest for HV. The bleaching response time (R_Tb) of viologen films gradually decreased with the alkyl chain length, and the OV film had the shortest R_Tb. Furthermore, when increasing the length of the alkyl chain, the cycling stabilities of alkyl viologens increased gradually. In addition, the OV film exhibited the best contrast after 200 continuous cycles.