The environment around a molecule can have a pronounced effect on its excited state and is frequently used in nature to control and manipulate photophysical outcomes. One example is the first step of vision, where the photoisomerization of retinal is governed by the microenvironment of opsin proteins. Another example is the fluorophore of green fluorescent protein that is only emissive when packaged inside of the folded protein. Because photochemistry is a zero-sum competition of the rates of different excited state processes, the manipulation of those rates by the environment can select which pathway is dominant. In particular, a photo-reaction known as excited-state intramolecular proton transfer (ESIPT) has attracted considerable attention because the ESIPT process affords the potential for two different wavelengths of light from the same molecule depending on local environmental conditions (Fig. ). We sought to design a protein binding pocket that could predictably and selectively distinguish between different excited-state pathways of the ESIPT process. The result of this work is a fluorescent probe and that exhibits a change fluorescence signal by detecting subtle changes in a protein-protein interface.