Photochemical reaction

Norrish type II reaction

In the area of photochemistry, a photochemical reaction is a chemical reaction that is induced by light. Photochemical reactions are valuable in organic and inorganic chemistry because they proceed differently than thermal reactions. Photochemical reactions are not only very useful but also can be a serious nuiscance, as in the photodegradation of many materials, e.g. polyvinyl chloride. A large-scale application of photochemistry is photoresist technology, used in the production of microelectronic components. Vision is initiated by a photochemical reaction of rhodopsin.

Experimental set-up

Photochemical reactions require a light source that emits wavelengths corresponding to an electronic transition in the reactant. In the early experiments (and in everyday life), sunlight was the light source, although it is polychromatic. Mercury-vapor lamps are more common in the laboratory. Low pressure mercury vapor lamps mainly emit at 254 nm. For polychromatic sources, wavelength ranges can be selected using filters. Alternatively, LEDs and Rayonet lamps emit monochromatically.

Schlenk tube containing slurry of orange crystals of Fe₂(CO)₉ in acetic acid after its photochemical synthesis. The mercury lamp (connected to white power cords) can be seen on the left, set inside a water-jacketted quartz tube.

The emitted light must of course reach the targeted functional group without being blocked by the reactor, medium, or other functional groups present. For many applications, quartz is used for the reactors as well as to contain the lamp. Pyrex absorbs at wavelengths shorter than 275 nm. The solvent is an important experimental parameter. Solvents are potential reactants and for this reason, chlorinated solvents are avoided because the C-Cl bond can lead to chlorination of the substrate. Strongly absorbing solvents prevent photons from reaching the substrate. Hydrocarbon solvents absorb only at short wavelengths and are thus preferred for photochemical experiments requiring high energy photons. Solvents containing unsaturation absorb at longer wavelengths and can usefully filter out short wavelengths. For example, cyclohexane and acetone "cut off" (absorb strongly) at wavelengths shorter than 215 and 330 nm, respectively.

Excitation

Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state. The photon can be absorbed directly by the reactant or by a photosensitizers, which absorbs the photon and transfers the energy to the reactant. The opposite process is called quenching when a photoexited state is deactivated by a chemical reagent.

Most photochemical transformations occur through a series of simple steps known as primary photochemical processes. One common example of these processes is the Excited State Proton Transfer (ESPT).

Organic photochemistry

Examples of photochemical organic reactions are electrocyclic reactions, photoisomerization and Norrish reactions.

Alkenes undergo many important reactions that proceed via a photon-induced π to π* transition. The first electronic excited state of an alkene lack the π-bond, so that rotation about the C-C bond is rapid and the molecule engages in reactions not observed thermally. These reactions include cis-trans isomerization, cycloaddition to other (ground state) alkene to give cyclobutane derivatives. The cis-trans isomerization of a (poly)alkene is involved in retinal, a component of the machinery of vision. The dimerization of alkenes is relevant to the photodamage of DNA, where thymine dimers are observed upon illuminating DNA to UV radiation. Such dimers interfere with transcription. The beneficial effects of sunlight are associated with the photochemically induced retro-cyclization (decyclization) reaction of ergosterol to give vitamin D. In the DeMayo reaction, an alkene reacts with a 1,3-diketone reacts via its enol to yield a 1,5-diketone.

In an industrial application, about 100,000 tonnes of benzyl chloride are prepared annually by the gas-phase photochemical reaction of toluene with chlorine. The light is absorbed by chlorine molecule, the low energy of this transition being indicted by the yellowish color of the gas. The photon induces homolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical:

Cl₂ + hνn → 2 Cl^.

C₆H₅CH₃ + Cl^. → C₆H₅CH₂^. + HCl

C₆H₅CH₂^. + Cl^. → C₆H₅CH₂Cl

Inorganic and organometallic photochemistry

Inorganic compounds are also photoreactive. Perhaps the most pervasive reaction is the generation of singlet oxygen by photosensitized reactions of triplet oxygen. Typical photosensitizers include tetraphenylporphyrin and methylene blue. The resulting singlet oxygen is an aggressive oxidant, capable of converting C-H bonds into C-OH groups.

Inorganic complexes are also photoreactive. These reactions can entail cis-trans isomerization. More commonly photoreactions result in dissociation of ligands, since the photon excites an electron on the metal to an orbital that is antibonding with respect to the ligands. Thus, metal carbonyls that resist thermal substitution undergo decarbonylation upon irradiation with UV light. UV-irradiation of a THF solution of molybdenum hexacarbonyl gives the THF complex, which is synthetically useful:

Mo(CO)₆ + THF → Mo(CO)₅(THF) + CO

In a related reaction, photolysis of iron pentacarbonyl affords diiron nonacarbonyl (see figure):

2 Fe(CO)₅ → Fe₂(CO)₉ + CO

History

Although bleaching has long been practiced, the first photochemical reaction was described by Trommsdorf in 1834. He observed that crystals of the compound α-santonin when exposed to sunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal.

The first step is a rearrangement reaction to a cyclopentadienone intermediate 2, the second one a dimerization in a Diels-Alder reaction (3) and the third one a intramolecular [2+2]cycloaddition (4). The bursting effect is attributed to a large change in crystal volume on dimerization.