Absorption of radiation by reactants of a reaction at equilibrium increases the rate of forward reaction without directly affecting the rate of the reverse reaction. The
rate of a photochemical reaction is
proportional to the
absorption cross section of the reactant with respect to the
excitation source (σ), the
quantum yield of reaction (Φ), and the
intensity of the irradiation. In a reversible photochemical reaction between compounds A and B, there will therefore be a "forwards" reaction of A \rightarrow B at a rate proportional to \sigma_a \times \phi_{A\rightarrow B} and a "backwards" reaction of B \rightarrow A at a rate proportional to \sigma_b \times \phi_{B \rightarrow A}. The ratio of the rates of the forward and backwards reactions determines where the equilibrium lies, and thus the photostationary state is found at: \sigma_a \times \phi_{A\rightarrow B} / \sigma_b \times \phi_{B \rightarrow A} If (as is always the case to some extent) the compounds A and B have different
absorption spectra, then there may exist wavelengths of light where σa is high and σb is low. Irradiation at these wavelengths will provide photostationary states that contain mostly B. Likewise, wavelengths that give photostationary states of predominantly A may exist. This is particularly likely in compounds such as some photochromics, where A and B have entirely different
absorption bands. Compounds that may be readily switched in this way find utility in devices such as
molecular switches and
optical data storage. == Practical considerations ==