Single-Colour, Visible Light Activation and Excitation of the Luminescence of a ‘Switch-On’ Dye and Enhancement by Silver Nanoparticles

•Novel dihydrofluorescein probes can be easily prepared by a chemical reduction of substituted fluorescein derivatives.•The reduced probes can be photochemically reoxidized by illumination with visible light.•While the dihydrofluorescein derivatives are non-fluorescent, the oxidized photochemical products are highly emissive and can be excited using the same wavelength used for their activation.•Silver nanoparticles (AgNP) can enhance the photochemical transformation and the observed emission. We synthesized a fluorogenic methylated dihydrofluorescein derivative in two convenient steps from commercial fluorescein. This species can switch to the corresponding oxidized form upon visible-light illumination at an activating wavelength of 455 nm, where the photoinduced transformation culminates in the activation (‘switch on’) of the fluorescence, which can be observed by exciting the photoproduct at the same wavelength of activation. Thus, a single wavelength can be used to simultaneously activate and excite fluorescence. Mechanistic investigations suggest that singlet oxygen does not contribute to the formation of the fluorescent oxidation product. The single-colour activation/excitation mode of this model structure can effectively mimic a pseudo two-colour system and can find broad application in the development of photonic materials operated with simple optical setups. The activated fluorescence can be concomitantly enhanced by triangular silver nanoparticles according to the principles of metal-enhanced fluorescence (MEF).