Precision Photochemistry: Every Photon Counts

Abstract Photochemistry is undergoing a precision transformation. Through technological advancements, such as the advent of light emitting diodes and monochromatic lasers, chemists are now able to use photons not only as an energy source but also as a tool for directing photochemical processes with both wavelength and spatiotemporal precision. Enabled by these technologies, the discovery that photochemical action often does not align with molar extinction has catalysed the growth of the research field that we coin Precision Photochemistry . We explain how precision photochemistry stands on four fundamental pillars: molar extinction, wavelength‐dependent quantum yield, concentration of the chromophores, and the length of the irradiation. Each of these four pillars are intrinsically linked and dictate the experimental conditions that should be used (e.g., wavelength, light intensity, and solvent system), as we demonstrate through simulations of a photochemical uncaging system. Building on these pillars, we propose a concrete definition for Precision Photochemistry and highlight important fields within chemistry that will benefit from careful consideration of them. Finally, we address key experimental considerations that must be taken into account when conducting precision photochemistry including the light source, the reaction setup, and the method for determining (wavelength‐dependent) quantum yields. These factors are critical in furthering the development of the field of Precision Photochemistry .