Arylpyrrolylidene-indanones as readily synthesised photoswitches offering dual- and single-colour fluorescence toggling with nearly quantitative <i>E</i> / <i>Z</i> isomerisation under visible light

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 fluorescence quantum yields. Herein, we present a new class of visible-light-responsive novel arylpyrrolylidene-indanone switches that permit high-intensity dual-colour fluorescence switching and quantitative photoisomerisation between exceptionally thermally stable <i>E</i>- and <i>Z</i>-isomers. The presence of an intramolecular hydrogen bond (-C[double bond, length as m-dash]O⋯H-N) renders the <i>Z</i>-isomer significantly more emissive, achieving a quantum yield (<i>Q</i> _f) of 0.5. These photoswitches are highly fatigue resistant and additionally display solvent viscosity-dependent fluorescence emission by the <i>E</i>-isomers as well. By speeding up the rotation of a C-C bond between pyrrole and C[double bond, length as m-dash]C with the addition of two large substituents at sp³-C, guided by the DFT calculations, the emission of the <i>E</i>-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.