The Rainbow Arching over the Fluorescent Thienoviologen Mesophases

Abstract

Thermofluorochromic materials exhibit tunable fluorescence emission on heating or cooling. They are highly desirable for applications ranging from temperature sensing to high-security anti-counterfeiting. Luminescent matrices based on liquid crystals are very promising, particularly those based on liquid crystals with intrinsic fluorescence. However, only a few examples have been reported, suggesting ample margins for development in the field, due to the wide range of fluorophores and supramolecular organizations to be explored. Moreover, thermofluorochromic liquid crystals can be tailored with further functionalities to afford multi-stimuli responsive materials. For the first time, herein we report the thermofluorochromism of thienoviologen liquid crystals, already known to show bulk electrochromism and electrofluorochromism. In particular, we studied their photophysics in the 25 °C–220 °C range and as a function of the length of the N-linear alkyl chains, m (9 ≤ m ≤ 12 C atoms), and the type of anion, X (X = OTs-, OTf-, BF4-, NTf2-). Interestingly, by changing the parameters m, X and T, their fluorescence can be finely tuned in the whole visible spectral range up to the NIR, by switching among different mesophases. Importantly, by fixing the structural parameters m and X, an interesting thermofluorochromism can be achieved for each thienoviologen in a homologous series, leading to a switch of the emitted light from red to green and from white to blue as a consequence of the temperature-induced variation in the supramolecular interactions in the self-assembled phases.

Novel Star-Shaped Viologens Containing Phenyl and Triphenylamine Moieties for Electrochromic Applications

“…Woodward et al synthesized and characterized viologens containing thiazolo [5,4-d]-thiazole as a bridging group and found that the obtained material showed an emission of light with a wavelength corresponding to the blue color and a reversible color change from yellow to dark blue as a result of the electroreduction [20]. The thienoviologens showed thermofluorochromism, which involves the switching of the emitted light from red to green and from white to blue as a result of the temperature-induced variation in the supramolecular interactions in the self-assembled phases [21]. Described by Veltri et al, N-aryl-substituted viologens based on bis(4-alkylphenyl)bipyridinium cations were found to exhibit liquid-crystalline properties and colorless-to-green electrochromism [22].…”