Consider the emission of 2-(2'-hydroxyphenyl)benzoxazole (1, Fig. ). Upon absorbance of light, 1 forms the excited-state enol 1-E*. Emission from 1-E* (enol emission) back to the ground state (1) can produce a photon of light, which is the "normal" fluorescence process of numerous dyes. Alternatively, the structure of 1 is such that an intramolecular isomerization can occur in the excited state to produce excited keto form 1-K* from 1-E* (Fig. , ESIPT step). Emission can then take place from 1-K* (keto emission) to produce 1-K in the ground state, which rapidly isomerizes back to 1. (i) The excited-state isomerization from 1-E* to 1-K* is ultrafast (~ps)-faster than fluorescence from the enol tautomer (~ns)-leading to a predominance of keto fluorescence. Because the energy gap between the keto forms (S1'→S0') is smaller than for the enol forms (S1→S0), keto fluorescence typically leads to a >100 nm red-shift relative to enol fluorescence and the excitation wavelength. The ESIPT process therefore produces remarkably large Stokes shifts relative to "normal" fluorophores (e.g fluorescein, rhodamine, BODIPY, etc.).