Illumination of 7a at an activating wavelength () of 365 nm cleaves the two nitrobenzyl groups to generate 7b, which then emits upon irradiation at an exciting wavelength () of 490 nm. As a result, a sequence of images of a glycerol drop containing 7a that recorded exciting at reveal fluorescence only after illumination at . Specifically, pulses at applied after 100 (a), 500 (b), and 1000 (c) ms from the onset of the frame sequence to the central region of the imaging field activate fluorescence. Interestingly, the fluorescent area widens over time indicating that the activated probes diffuse over this temporal scale. The drawing in (a) represents the tip of the optical fiber used to deliver the activating radiations. The scale bar in (b) corresponds to 10 μm (reproduced with permission from [161]).