Super-resolution fluorescence imaging and ultrahigh-density optical data storage, in particular, rely on the switches that demonstrate single- or (multi) dual-colour fluorescence toggling, with the latter option being more advantageous. However, a large number of switches are non-emissive owing to their fast photoisomerisation. Also, many fluorescent switches suffer from the necessity of high-energy UV light irradiation, low isomerisation yields, photodegradation, or the comparatively low fluorescent quantum yields. Herein, we present visible-light-addressable novel arylpyrrolylidene-indanone switches that permit high-intensity dual-colour fluorescence switching and quantitative photoisomerisation between thermally exceptionally stable E- and Z-isomers. The presence of an intramolecular hydrogen bond (-C=O—H-N) renders the Z-isomer significantly more emissive, achieving a quantum yield (Qf) of 0.5. These photoswitches are highly fatigue resistant and additionally display solvent viscosity-dependent fluorescence emission by the E-isomers as well. By speeding up the rotation of a C-C bond between pyrrole and C=C with the addition of two large substituents at sp3-C, guided by the DFT calculations, the emission of the E-isomer is almost completely lost, making these compounds single-colour (on/off) fluorescent photoswitches. Fluorescence photoswitching in 50% DMSO-PBS buffer works well, revealing the potential of these switches for biological applications, as well as for material uses.
