Carbon nanothreads are the most exciting carbon based nanomaterials recently discovered. Obtained by compressing aromatics around 20 GPa, they are characterized by potentially exceptional mechanical properties. The reaction mechanisms have been partly elucidated through computational studies and x-ray diffraction experiments. However, in all these studies, the electronic modifications to which the molecule is subjected with increasing pressure are neglected as also if, and to which extent, the electronic excited states are involved in the high-pressure reactivity. In fact, the pressure increase induces remarkable changes in the electronic properties of molecular crystals, which are often directly related to the reaction's onset and path. We report the pressure evolution of the two-photon induced emission spectrum of crystalline stilbene, the archetype of a class of molecules from which double-core nanothreads are obtained, with the twofold purpose of gaining insight into the reaction mechanism and monitoring if the structural changes observed in x-ray diffraction studies have a detectable counterpart in the electronic properties of the system. The freezing of the spectral diffusion observed on rising pressure is ascribed to a hampered conformational rearrangement because of the larger stiffness of the local environment. The transition to the high pressure phase where the nanothreads form is revealed by the slope change of the pressure shift of all spectral components, while the progressive intensification with pressure of the 0-0 transition suggests a strengthening of the ethylenic bond favoring the charge delocalization on the benzene moieties, which is likely the trigger of the chemical instability.