Figure 6: Illumination of 4a and 5a at an activating wavelength ( ) encourages the formation of 4b and 5b, respectively. The photogenerated isomers can be excited selectively with irradiation at an appropriate exciting wavelength ( ) to emit light in the form of fluorescence. Both transformations are reversible. The original nonemissive state 4a is regenerated thermally, while 5a is reformed photochemically.