In an attempt to shed light on the environmental effects on the deactivation channels of the PYP chromophore, radiative and non-radiative deactivation mechanisms of the anionic p-coumaric acid methyl ester (pCE⁻) in the gas phase and water solution are compared at the CASPT2//CASSCF/cc-pVDZ level and, when necessary, at the CASPT2//CASPT2/cc-pVDZ level. We find that the solvent produces dramatic modifications on the free energy profile of the S1 state. Two twisted structures that are minima in the gas phase could not be localized in aqueous solution. Furthermore, the relative stability of minima and conical intersections (CIs) is reverted with respect to the gas phase values, affecting the prevalent de-excitation paths. As a consequence of these changes, three competitive de-excitation channels are open in aqueous solution: the fluorescence emission from a planar minimum on S1, the trans–cis photoisomerization through a CI that involves the rotation of the vinyl double bond and the non-radiative, non-reactive, de-excitation through the CI associated with the rotation of the single bond adjacent to the phenyl group. In the gas phase, the minima are the structures with lower energy, while in solution the CI^β structure, characterized by a large charge separation, is strongly stabilized by interactions with water molecules and becomes the structure with the lowest energy on S1. These facts explain the low fluorescence signal of pCE⁻ in aqueous solution and the presence of partial trans–cis photoisomerization in this system.