Enhancing electrochromism and photoluminescence in carbazole-viologens via conjugation extension and dimerization suppression

Abstract

Viologens exhibit reversible redox properties and remarkable color response, making them core materials in electrochromism. However, their low conjugation compromises charge transfer efficiency, undermines redox reversibility, and causes fluorescence quenching, thereby limiting their applications. This study addresses this challenge by proposing a synergistic design strategy of 'aromatic bridging–flexible side chains.' Incorporating electron-rich carbazole groups into the viologen backbone as a rigid bridging structure expands the molecular conjugation network; while tuning the length of the alkyl side chains on both sides suppresses molecular aggregation. Following this strategy, we designed and synthesized carbazole-viologen derivatives substituted with hydroxypropyl (HICDP) and hydroxyhexyl (HICDH) groups. A systematic investigation revealed how the carbazole unit and side-chain length regulate their electrochromic and photoluminescent properties. Experiments showed that the carbazole bridging unit expanded the conjugation system, enabling HICDH to achieve an optical contrast of ΔT = 77.4 % at 700 nm. Even after 100 cycles, HICDH retained approximately 70 % of its optical contrast. The increased side-chain length in carbazole-viologen derivatives increased the distance between radicals, suppressed dimer formation, and enhanced stability. Meanwhile, the material exhibited excellent photoluminescent properties. Theoretical calculations further confirmed that the side-chain extension strategy suppressed radical dimerization through steric hindrance. The hexyl side chain reduced the HOMO-LUMO bandgap to facilitate electron transitions while increasing the molecular dihedral angle to minimize non-radiative recombination. The extension of the side chains effectively suppressed non-radiative transition pathways, ultimately enhanced both the photochromic and electrochromic performance.