Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens

Abstract

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 (RTc), while it was shortest for HV. The bleaching response time (RTb) of viologen films gradually decreased with the alkyl chain length, and the OV film had the shortest RTb. 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.

Self-Assembled Tetraphenylethene-Based Nanoaggregates with Tunable Electrochemiluminescence for the Ultrasensitive Detection of E. coli

“…28 Both the absorption of π−π* transition and TICT transition have the obvious red shift to 284 and 368 nm, respectively, due to the stronger electron-donating−accepting interaction of the emitting center of C X , resulting from the existence of the cationic unit with the electron-accepting properties. 29 The FL emission of all C X has significant red shifts to 618, 611, and 610 nm for C 3,5,6 compared to that of TPP at 428 nm (Figure 1C), and C 3 with a short alkyl chain has a longer FL emission wavelength because of its stronger electron absorption capacity and narrow energy gap, which is well consistent with the reference. 28,30 The presence of positively charged quaternary ammonium and pyridinium groups enables the hydrophilicity of C X , and their TPE group provides the hydrophobicity, leading to the amphiphilic property of C X .…”

“…The UV absorption spectrum of TPP demonstrates an intense peak at 246 nm due to the absorption of the π–π* transition, and a weak absorption peak at 314 nm is ascribed to the absorption of the twisted intramolecular charge transfer (TICT) transition from the electron-donating TPE group to the electron-accepting pyridinium unit (Figure B) . Both the absorption of π–π* transition and TICT transition have the obvious red shift to 284 and 368 nm, respectively, due to the stronger electron-donating–accepting interaction of the emitting center of C X , resulting from the existence of the cationic unit with the electron-accepting properties . The FL emission of all C X has significant red shifts to 618, 611, and 610 nm for C 3,5,6 compared to that of TPP at 428 nm (Figure C), and C 3 with a short alkyl chain has a longer FL emission wavelength because of its stronger electron absorption capacity and narrow energy gap, which is well consistent with the reference. , …”