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 on the light absorbed by each form. If I is the photon flux, ε_A(λ) and ε_B(λ) are molar absorptivities, and Φ_AB and Φ_BA are the quantum yields for A→B and B→A, the steady state satisfies I ε_A Φ_AB [A] = I ε_B Φ_BA [B]. Thus the ratio [B]/[A] at PSS is (ε_A Φ_AB)/(ε_B Φ_BA), and the fraction of B is f_B = (ε_A Φ_AB)/(ε_A Φ_AB + ε_B Φ_BA). The composition at PSS depends on wavelength through ε and Φ, as well as light intensity and sample geometry (inner-filter effects).

This concept is central to designing and understanding photoswitchable molecules such as azobenzenes, spiropyrans, and diarylethenes, where illumination selects a desired isomer at PSS. While the two-state model captures the core behavior, real systems may involve more states, competing pathways, and deviations due to concentration, aggregation, or photobleaching. Photostationarity highlights how light, not just thermodynamics, controls the operational state of photoactive materials.