Optimizing photon conversion routes in cinnamate derivatives.

E-Z photoisomerisation is a key relaxation process in biological chromophores, supporting processes such as vision and photoprotection. Here we use such pathways to optimise photon-to-heat conversion processes. To this purpose, we synthesise methyl cinnamate derivatives in which steric and electronic substitutions modulate excited-state decay. Femtosecond spectroscopy in solution and gas phase shows how para-methoxy substitution facilitates access to a conical intersection, while steric hindrance at the central double bond manoeuvres excited population towards a barrierless pathway back to the ground state, leading to derivatives isomerising on a timescale approaching that of cis-11-retinal, involved in vision. By associating substitution patterns with potential energy surface topography determined by multireference quantum chemical calculations, we identify how nonradiative decay can be accelerated in cinnamate scaffolds. These results advance mechanistic insights into cinnamate photophysics and pave the way for the rational design of efficient photon converters for photo-switching, heat generation, and light-filtering applications.