Single-colour, visible light activation and excitation of the luminescence of a ‘switch-on’ dye and enhancement by silver nanoparticles

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).