Novel extended viologen derivatives for photochromic and electrochromic dual-response smart windows

Smart windows, which could save building energy consumption by modulating indoor thermal and light intensity, have attracted considerable attention in recent years. Photochromic (PC) and electrochromic (EC) are two representative working modes of smart windows. Integrating PC and EC on a single smart window is beneficial for adapting to various environments and meeting individual needs. However, it remains challenging to develop PC and EC dual-responsive materials. In this work, we reported two extended viologen derivatives, EV-1 and EV-2, with both PC and EC properties. Photochromic devices (PCDs) and electrochromic devices (ECDs) based on these materials were fabricated successfully. Under UV light irradiation, the PCDs exhibited a significant color change from transparent to green (PCD-1) and blue (PCD-2) color, and the colored state can maintain a long time (2.73 h for PCD-1 and 41.72 h for PCD-2) after removing the UV light. Owing to the extension of the conjugate length of the chromophore as compared with traditional viologens using bipyridine as chromophores, the spectra response range of the devices was extended to the near-infrared (NIR) region. To obtain different light modulation capabilities, we fabricated electrochromic devices using propylene carbonate as solvent. These devices display different colored states (red for ECD-1 and purple for ECD-2) and spectra absorption compared to PCD-1 and PCD-2. Furthermore, a photo/electrochromic device (PECD) smart window system was developed to enhance light and thermal regulation. This system offers four working modes, including off mode, photochromic mode, electrochromic mode, and photo-electrochromic mode, which opens up new possibilities for expanding its applications.