A water soluble diarylethene (DAE) derivative that displays exceptionally intense fluorescence from the
colorless open form has been synthesized and characterized using UV/vis spectroscopy and
fluorescence microscopy. We show that the bright emission from the open form can be rapidly switched
using amplitude modulated red light, that is, by light at wavelengths longer than those absorbed by the
fluorescent species. This is highly appealing in any context where undesired background fluorescence
disturbs the measurement, e.g., the autofluorescence commonly observed in fluorescence microscopy.
We show that this scheme is conveniently applicable using lock-in detection, and that robust amplitude
modulation of the probe fluorescence is indeed possible also in cell studies using fluorescence microscopy
