photostationarity
Photostationarity, or the photostationary state (PSS), refers to the steady-state distribution of species in a photoactive system under continuous illumination. At the PSS, the rates of forward and backward photoinduced reactions balance, so the concentrations of the involved isomers remain constant over time even though the system is being driven by light. This is a dynamic, non-thermal balance rather than a true thermodynamic equilibrium.
For a simple two-state photoisomerization A ⇌ B under monochromatic light, the forward and backward rates depend
This concept is central to designing and understanding photoswitchable molecules such as azobenzenes, spiropyrans, and diarylethenes,