Electrofluorochromism in π-conjugated ionic liquid crystals

Abstract

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.

Ionic Switch Induced by a Rectangular–Hexagonal Phase Transition in Benzenammonium Columnar Liquid Crystals

“…The fluid LC assembled structures can be changed in response to external stimuli such as light irradiation, 5 the addition of chemicals, 6 application of mechanical pressure 7 and application of an electric field. 8 The switch of functions using induced structural changes is an emerging research area. [5][6][7][8][9] Switching of electric current and photoluminescence has been studied for π-conjugated LC materials.…”

“…8 The switch of functions using induced structural changes is an emerging research area. [5][6][7][8][9] Switching of electric current and photoluminescence has been studied for π-conjugated LC materials. 3,4,9 However, achieving switching of ionic conductivity is a challenging task because of the difficulty in the design of nanostructured materials forming ion-channels with stimuli responsiveness.…”

Luminescent ion pairs with tunable emission colors for light-emitting devices and electrochromic switches

“…For example, Chidichimo and co-workers have presented π-conjugated ionic liquid crystals in which the direct electrochemical reduction leads to a reversible electrochromic luminescence (ECL) response. 34 Beneduci and co-workers have shown an ECL-active polymer gel based on the conversion of the redox states, in which the emission properties of the polymer depend on the fluorophore contents and the voltages applied. 35 We have also reported a series of phosphorescent iridium( iii ) complexes containing a protonating functional group, which exhibited evident emission color change induced by an electric field.…”

n-Type Columnar Liquid Crystal Combining Ionic and Electronic Functions

“…Recently, we presented a new class of extended viologen liquid crystals, called thienoviologens, in which a bithiophene group links the two N,N -dialkylated pyridinium units ( Figure 1) [21,22,23]. The introduction of the bithiophene bridge between the pyridinium rings and of the pro-mesogenic alkyl chains in the N,N positions, leads to compounds that combine the electron acceptor properties typical of viologens salts with new functional properties arising from both the extended π -conjugated core and their liquid crystalline properties [23].…”

“…The introduction of the bithiophene bridge between the pyridinium rings and of the pro-mesogenic alkyl chains in the N,N positions, leads to compounds that combine the electron acceptor properties typical of viologens salts with new functional properties arising from both the extended π -conjugated core and their liquid crystalline properties [23].…”