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Materials in which photoluminescence is modulated by redox processes are known as electrofluorochromic. Intrinsically switchable fluorophores, incorporating both redox and fluorescent moieties, could be ideal electrofluorochromic materials if they possess high fluorescence quantum yields in at least one of their redox states. Fluorescent liquid crystals with redox active centres could combine the above requirements with the advantage to work in bulk anisotropic phases. However, electrofluorochromic liquid crystals have not been reported yet because their synthesis is challenging due to aggregation-caused fluorescent quenching. Here we show the first examples of electrofluorochromic p-conjugated ionic liquid crystals based on thienoviologens. These ordered materials, combining ionic and electronic functions, are highly fluorescence in the bulk state (quantum yield460%). Their direct electrochemical reduction leads to fast and reversible bulk electrofluorochromic response in both columnar and smectic phases allowing for fluorescence intensity modulation and colour tuning.

A highly fluorescent electrofluorochromic gel with quantum yields as high as 67% is prepared by incorporating the thienoviologen fluorophore 4,4′‐(2,2′‐bithiophene‐5,5′‐diyl)bis(1‐nonylpridinium)bistriflimide into a polymeric matrix. The fluorescent emission spectrum of the gel at low percentages of thienoviologen shows a strong band at 530 nm. A new intense fluorescence band at 630 nm can be induced by fluorophore aggregation. Single layer electrofluorochromic devices were readily prepared by sandwiching the polymer gels between two indium tin oxide (ITO) electrodes. The fluorescence intensity can be easily modulated between a fluorescent and a quenched state, in a wide visible spectral range, by direct electrochemical reduction of the thienoviologen fluorophore. It exhibits three reduction states, each with different emission properties. The reversible interconversion among these states leads to a high electrofluorochromic stability of the device, exhibiting switching times of a few seconds and, to the best of our knowledge, the highest contrast ratio (337).

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Functional electrochromic materials that allow energy modulation both in the visible and in the near-infrared (NIR) spectral ranges are attracting increasing interest both for the fundamental scientific aspects related to their spectroelectrochemistry and for their technological applications. Vis-NIR dimmable windows based on these materials are very promising for tunable shading, thus allowing lighting and heat energy use saving. Organic mixed valence compounds (MVs) are an interesting class of small molecules with NIR electrochromism arising from optically induced intervalence charge transfer transitions (IVCT). Here, we report the synthesis and vis-NIR electrochromic properties of new organic mixed valence systems, with two and three amino redox centers bridged by a dibenzofulvene (DBF) unit. We studied the neutral and charged state characteristics of these MVs in solution by spectroelectrochemical experiments, theoretical TD-DFT investigations, and, in the solid state, through electrochromic devices (ECDs). We show that a fine-tuning of the electro-optical properties of these MVs can be obtained by different functionalization on the exocyclic fulvene bond of the DBF moiety, including the introduction of a third redox center, leading to compounds where all three redox centers participate in the electron transfer processes as a function of the applied voltage. As a proof-of-concept, the above MVs were used to form solid thermoplastic laminable films in order to fabricate transmissive-to-black switching electrochromic devices, with intermediate color switching characteristics, enabling us to cover all the color palette. Beyond this important exploitability in the vis region, useful in many applications, the most important characteristic of these devices is their absorption in almost the whole NIR range (800–2200 nm) through the excitation of highly charged radical species, which show intense IVCTs. Importantly, all the devices show high optical contrast, response times of a few seconds, and excellent switching stability over 10 000 cycles.