Panchromatic Fluorescence Emission from Thienosquaraines Dyes: White Light Electrofluorochromic Devices

Electrofluorochromic devices (EFCDs) that allow the modulation of the light emitted by electroactive fluorophores are very attractive in the research field of optoelectronics. Here, the electrofluorochromic behaviour of a series of squaraine dyes was studied for the first time. In solutions, all compounds are photoluminescent with maxima located in the range 665–690 nm, characterized by quantum yields ranging from 30% to 4.1%. Squaraines were incorporated in a polymer gel used as an active layer in all-in-one gel switchable EFCDs. An aggregation induced quenching occurs in the gel phase, causing a significant decrease in the emission quantum yield in the device. However, the squaraines containing the thieno groups (thienosquaraines, TSQs) show a panchromatic emission and their electrofluorochromism allows the tuning of the fluorescence intensity from 500 nm to the near infrared. Indeed, the application of a potential difference to the device induces a reversible quenching of their emission that is significantly higher and occurs at shorter switching times for TSQs-based devices compared to the reference squaraine dye (DIBSQ). Interestingly, the TSQs fluorescence spectral profile becomes more structured under voltage, and this could be explained by the shift of the aggregates/monomer equilibrium toward the monomeric species, due to electrochemical oxidation, which causes the disassembling of aggregates. This effect may be used to modulate the colour of the fluorescence light emitted by a device and paves the way for conceiving new electrofluorochromic materials based on this mechanism.