Aromatic bridge-modulated viologen derivatives for electrochromic devices with high coloration efficiency and long-term stability

Dibenzo[b,d]thiophene and dimethoxybenzene bridges tune viologen charge transport. • DMBBH delivers 72.0% optical contrast and 183.90 cm 2 /C coloration efficiency. • DMBBH retains 61.8% optical contrast after continuous cycling for 10000 s. Organic electrochromic materials have shown considerable potential for applications in smart windows, biosensors, and energy-storage devices owing to their tunable molecular structures, facile preparation, and rich color variation. Conventional viologen compounds are susceptible to over-reduction in their reduced states, which induces irreversible structural changes and limits their application in bifunctional electrochromic-photoluminescent devices. Viologen molecules possess excellent structural designability, and their optical and electrochemical properties can be precisely modulated by modifying substituent structures, counterions, and conjugation lengths, giving them irreplaceable advantages in electrochromic applications. In this study, an aromatic-bridging strategy was adopted, in which dibenzo[b,d]thiophene and dimethoxybenzene units were introduced between pyridine rings to modulate molecular conjugation and charge transport, while hydroxyhexyl side chains were incorporated to improve molecular solubility and intermolecular interactions. The experimental results demonstrate that the successfully prepared 4,4′-(2,5-dimethoxy-1,4-phenylene)bis(1-(6-hydroxyhexyl)pyridin-1-ium) dibromide (DMBBH) exhibits outstanding performance. DMBBH enables a reversible transition from a transparent state to a deep-red state, with an optical contrast of ΔT = 72.0%, a high coloration efficiency of η = 183.90 cm 2 /C, coloration (τ c )/bleaching (τ b ) response times of 9.5 s and 51 s, respectively, and an optical contrast that remains at 61.8% after 10000 s of cycling. This work demonstrates the key role of aromatic bridging structures in stabilizing the excited states of viologen molecules and optimizing charge-transport pathways, providing a new design strategy for future high-performance bifunctional organic optoelectronic devices. DMBBH-based electrochromic devices show reversible transparent-to-deep-red switching, with ΔT = 72.0%, η = 183.90 cm 2 /C, τ c /τ b  = 9.5/51 s, and stable operation over 10000 s. Download: Download high-res image (290KB) Download: Download full-size image