Photostationary State
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The photostationary state of a reversible photochemical
reaction Reaction may refer to a process or to a response to an action, event, or exposure. Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation) Biology and ...
is the
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chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
composition under a specific kind of
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
irradiation Irradiation is the process by which an object is exposed to radiation. An irradiator is a device used to expose an object to radiation, most often gamma radiation, for a variety of purposes. Irradiators may be used for sterilizing medical and p ...
(usually a single
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
of visible or UV
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
). It is a property of particular importance in photochromic compounds, often used as a measure of their practical efficiency and usually quoted as a ratio or percentage. The position of the photostationary state is primarily a function of the irradiation parameters, the absorbance spectra of the chemical species, and the quantum yields of the reactions. The photostationary state can be very different from the composition of a mixture at thermodynamic equilibrium. As a consequence, photochemistry can be used to produce compositions that are "contra-thermodynamic". For instance, although ''cis''-stilbene is "uphill" from ''trans-''stilbene in a thermodynamic sense, irradiation of ''trans''-stilbene results in a mixture that is predominantly the ''cis'' isomer. As an extreme example, irradiation of benzene at 237 to 254 nm results in formation of benzvalene, an isomer of benzene that is 71 kcal/mol higher in energy than
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
itself.


Overview

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 In physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department o ...
of the reactant with respect to the excitation source (σ), the
quantum yield In particle physics, the quantum yield (denoted ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. \Phi(\lambda)=\frac Applications Fluorescence spectroscopy The fluorescence ...
of reaction (Φ), and the
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
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_ and a "backwards" reaction of B \rightarrow A at a rate proportional to \sigma_b \times \phi_. 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_ / \sigma_b \times \phi_ 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 band In spectroscopy, an absorption band is a range of wavelengths, frequency, frequencies or energies in the electromagnetic spectrum that are characteristic of a particular transition from initial to final state in a substance. According to quantum ...
s. Compounds that may be readily switched in this way find utility in devices such as molecular switches and
optical data storage Optical storage refers to a class of data storage systems that use light to read or write data to an underlying optical media. Although a number of optical formats have been used over time, the most common examples are optical disks like the ...
.


Practical considerations

*Quantum yields of reaction (and to a lesser extent, absorption cross sections) are usually temperature and environment-dependent to some extent, and the photostationary state may therefore depend slightly on
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
and
solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
as well as on the excitation. *If thermodynamic interconversion of A and B can take place on a similar timescale to the photochemical reaction, it can complicate experimental measurements. This phenomenon can be important, for example in photochromatic
eyeglasses Glasses, also known as eyeglasses (American English), spectacles (Commonwealth English), or colloquially as specs, are Visual perception, vision eyewear with clear or tinted lens (optics), lenses mounted in a frame that holds them in front ...
.


References

{{reflist Photochemistry