Photochemistry

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Light as part of the electromagnetic spectrum

The term photochemistry is understood to mean chemical reactions that are initiated by the action of light . The basic requirement for this is absorption of the light by the molecule that is supposed to react. This means that the wavelength of the light used must match the absorption behavior of the molecule. In addition to direct excitation, there are also photoreactions in which a photosensitizer is first excited and this then transfers energy to the molecules to be reacted.

The absorption of a light quantum leads to energetically (electronically) excited states which, thanks to the excitation energy, can enter into chemical reactions. Chemical conversions compete with photophysical deactivation processes such as photo emission from the excited singlet state ( fluorescence ) or from the triplet state ( phosphorescence ) as well as radiationless deactivation . The relative extent to which the individual processes are carried out is expressed by the quantum yields . The sum of the quantum yields is 1 - except for chain reactions .

Since the absorption of a light quantum leads to an electronic excitation, reactions can be observed in the excited state that are not allowed in the electronic ground state of the molecule (cf. Woodward-Hoffmann rules , pericyclic reactions ). Photochemical reactions are often a good way to build complex and highly strained molecules.

In addition to the action of light, photochemistry can also be operated with higher-energy quanta . X-ray photochemistry and high-energy photochemistry, among others, deal with such photo-induced processes. Here, the place synchrotron an application in chemistry.

Homolytic cleavage of molecules A – B under the influence of light. Two radicals are created (right).

Examples of photochemical reaction types

  • Cleavages (bond homolysis), as they occur e.g. B. be observed in photoinitiators - u. U. followed by further fragmentation of the resulting radicals, cf. the splitting off of carbon monoxide from carbonyl compounds.
  • Photoisomerizations, such as. B. the formation of fulvene and benzvalene from benzene via the first excited singlet state or the formation of dewarbenzene from the second excited singlet state of benzene.
  • Electrocyclic reactions, such as the conversion of ergosterol to previtamin D, the cyclization of butadienes to cyclobutenes, the cyclization of cis -stilbene to dihydrophenanthrene or of diphenylamine to dihydrocarbazole.
  • Rearrangements, such as the isomerization of cycloheptatriene to toluene .
  • Light-induced chain reactions :
  • Photo-Fries shift of phenyl esters with the formation of ketones with a hydroxyl function in the ortho and / or para position on the phenyl ring.
  • [2 + 2] cycloadditions of alkenes in the course of a photodimerization or photocyclization , which lead to cyclobutanes (four-membered ring), cf. the formation of quadricyclane by exposure of norbornadiene or the photodimerization of cyclopentene .
  • [2 + 2] cycloaddition of alkenes and ketones in the Paternò – Büchi reaction .
  • α-cleavage of thiol esters to form aldehydes and disulfides.
  • Formation of thioxanthones through rearrangement reactions.
  • Isomerizations, such as, for example, cis - trans isomerizations (standard examples : cis / trans - stilbene , cis - / trans - azobenzene , maleic / fumaric acid ). For example, irradiating maleic acid and fumaric acid in both cases produces the same mixture of 75% maleic acid and 25% fumaric acid. The position of the photo equilibrium can be controlled by the excitation wavelength.
  • Photosensitized reactions, d. H. Photoisomerizations, photocycloadditions, with the addition of a photosensitizer , some of which are enantioselective.
  • Photo reactions in biology:
    • Photosynthesis is a well-known example of a chemical reaction with photochemical reaction steps .
    • Another example is seeing with the human eye. In the process, a photoisomerization of the rhodopsin responsible for light-dark vision takes place in the rods of the retina of the eye, more precisely, a light-controlled cis - trans isomerization of the 11- cis - retinal , which is a component of the chromophore rhodopsin.

The first photochemical experiments go back to Giacomo Luigi Ciamician . Alexander Schönberg wrote the first comprehensive books on preparative organic photochemistry .

Carrying out photochemical experiments

Carrying out photochemical experiments requires a number of prerequisites that result from the starting condition - absorption of light by the reactants. It must be known at which wavelength the photochemical excitation should take place. Corresponding information on the reactants can be found in tables or can be obtained by measuring the UV-VIS spectra . The next step is to ensure that a suitable light source is available. It must be ensured here that the light source delivers sufficient power in the relevant wavelength range, and under certain circumstances it must also be ruled out that wavelengths that lead to photochemical side reactions are excluded. The solvents used must be transparent in the relevant wavelength range (unless they act as sensitizers for the photoreaction). Furthermore, the solvents must not act as “quenchers” (take over singlet or triplet excitation energy of the reactants and thus deactivate the reactive species) and must be inert towards the species occurring. Oxygen usually leads to side reactions, which is why the reactions are usually carried out under protective gas (nitrogen, argon) (exception: reactions of singlet oxygen , in which oxygen is specifically passed through the reaction mixture). As the penetration depth of the light is usually only a few millimeters (see Lambert-Beer's law ), good mixing must be ensured. This can often be achieved in laboratory experiments by passing inert gas through, which is necessary anyway. In terms of safety, protection against UV radiation (eye damage, "sunburn") or the dissipation of e.g. B. high amounts of heat generated by high pressure lamps.

Apparatus for photochemical experiments

Reactor for photochemical experiments

Preparative photochemical experiments in the laboratory are preferably carried out in immersion equipment. For this purpose, a glass tube closed at the bottom is immersed in the solution to be exposed. In the glass tube can then, for. B. a mercury vapor lamp can be set. Ideally, the immersion tube is double-walled so that water can be passed through for cooling purposes. In good equipment, the outer tube is made of quartz, the inner tube can be replaced using a screw connection. In such structures, different inner tubes can be used in the sense of a filter to cut off UV radiation below a certain wavelength (300 nm for Pyrex or Solidex , approx. 350 nm for standard glasses ). The cooling liquid can also serve as a filter in double-walled inserts if appropriate (stable) dyes or metal salts are added.

The simplest experiments with small amounts of substance can be carried out in which substances z. B. irradiated in NMR tubes or cuvettes. The irradiation in cuvettes is common for photophysical examinations and can also be carried out at very low temperatures (liquid nitrogen) in suitable structures.

Light sources

Light sources for photochemical work can in principle be divided into continuous and discontinuous emitters. When making the selection, it is also important to consider which performance is required for the work. Continuous emitters deliver light in a wide range of wavelengths. Typical examples are black bodies (sun, light bulbs). Their spectra are characterized by a very wide spectral distribution, ranging from the infrared (thermal radiation) through the visible to the (near) UV range. However, the UV components of these lamps are low, so that other light sources are required for photochemical work. Gas discharge lamps based on hydrogen / deuterium or noble gases are suitable as continuous emitters in the UV range .

gas Emission range / nm
H 2 170-350
Hey 58-100
No 74-79
Ar 107-160
Kr 124-150
Xe 147-170

These lamps are mainly used in UV spectrometers (hydrogen / deuterium lamps) or z. B. in photoelectron spectrometers.

LED photolamp for microreactor (Dechema)

Discontinuous steel workers deliver light in the form of discrete lines. Typical representatives are lasers and metal halide lamps. With metal halide lamps, the individual lines are broadened or even overlaid by a continuum - with increasing operating pressure. For preparative photochemistry, the mercury vapor lamps are of paramount importance, lasers have found a broad use in mechanistic investigations. Low-pressure mercury lamps (0.01–1 mbar) are characterized by their predominant radiation at 254 nm (approx. 95%). Coating the lamps with suitable phosphors (analogous to fluorescent tubes) allows light sources with emissions around 300 nm or 350 nm to be provided. This is implemented in which the irradiation wavelength can be achieved by exchanging the lamp types. At higher operating pressures of the mercury lamps (0.1–100 bar), emissions at 297 nm, 334 nm, 365 nm, 404 nm, 436 nm as well as 546 nm and 577 nm increasingly predominate. Are metal salts added (iron, cadmium, thallium, indium ), further main emission lines can be generated. The advantage of high and ultra-high pressure lamps is their high power output. In industrial applications, lamps with an output of several tens of kW are used.

Solvents, Sensitizers, and Quenchers

A summary of solvents for photochemical work is shown in the table below. At the specified wavelengths, the light intensity is reduced by around 90% over a distance of 1 cm due to the self-absorption of the solvent.

solvent Wavelength / nm solvent Wavelength / nm
water 185 Acetonitrile 190
n -hexane 195 Ethanol 204
Methanol 205 Cyclohexane 215
Diethyl ether 215 1,4-dioxane 230
Dichloromethane 230 chloroform 245
Tetrahydrofuran 245 acetic acid 250
Ethyl acetate 255 Carbon tetrachloride 265
Dimethyl sulfoxide 277 benzene 280
toluene 285 Pyridine 305
acetone 330

The light absorption and emission take place predominantly while maintaining the multiplicity, i. That is, a molecule in the singlet state is converted to an excited singlet state or shows rapid fluorescence emission . The direct excitation from a singlet ground state to a triplet state is quantum-chemically "forbidden" and is therefore only observed to a subordinate extent. (For comparison: the radiant transition of a triplet into the ground state ( phosphorescence ) is forbidden and therefore a slower process compared to fluorescence.) So-called sensitizers are used if photochemical reactions are to proceed via triplet states . In these, a singlet-triplet transition ( intersystem crossing , ISC) takes place to a significant extent in the excited state . The sensitizer can then transfer its triplet excitation to a reactant, which then enters into chemical reactions in the triplet state. The phenomenon of energy transfer can also be used to gain insight into the mechanism of photoreactions. Molecules with known singlet or triplet energies are used here to “quench” (“quench”) the excited states that occur during reactions. If the energy of the quencher is below the energy of the state to be quenched, energy is transferred and the original photoreaction is interrupted. The principle can be applied to reactions in the singlet as well as in the triplet state. Some typical sensitizers and quenchers are listed below (energies of the first excited singlet and triplet states, quantum yields for the singlet-triplet intersystem crossing (Φ ISC )):

connection E T / (kJ mol −1 ) E S / (kJ mol −1 ) Δ (E S - E T ) / (kJ mol −1 ) Φ ISC
benzene 353 459 106 0.25
toluene 346 445 99 0.53
Methyl benzoate 326 428 102 -
acetone 332 372 40 0.9-1.00
Acetophenone 310 330 20th 1.00
Xanthone 310 324 14th -
Benzaldehyde 301 323 22nd 1.00
Triphenylamine 291 362 71 0.88
Benzophenone 287 316 29 1.00
Fluorene 282 397 115 0.22
Triphenylene 280 349 69 0.86
Biphenyl 274 418 144 0.84
Phenanthrene 260 346 86 0.73
Styrene 258 415 157 0.40
naphthalene 253 385 132 0.75
2-acetylnaphthalene 249 325 76 0.84
Biacetyl 236 267 31 1.00
Benzil 223 247 24 0.92
Anthracene 178 318 140 0.71
Eosin 177 209 32 0.33
Bengal pink 164 213 49 0.61
Methylene blue 138 180 42 0.52

See also

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  1. ↑ H. Baumgärtel: Synchrotron Radiation in Chemistry. In: Chemistry in Our Time. 28th year, No. 1, 1994, ISSN  0009-2851 , pp. 6-17.
  2. ↑ Jens Dreyer: Theoretical investigations on the photochemical reactivity of benzene and its isomers . Dissertation . WWU, Münster 1995.
  3. ^ Siegfried Hauptmann: Organic chemistry. 2nd Edition. VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1985, p. 297.
  4. ^ Siegfried Hauptmann: Organic chemistry. 2nd Edition. VEB German publisher for basic industry, Leipzig 1985, p. 207.
  5. ↑ Jürgen Martens, Klaus Praefcke , Ursula Schulze: Intramolecular Photo Friedel Crafts Reactions; a new synthetic principle for heterocycles1 . In: Synthesis . tape 1976 , no. 08 , 1976, p. 532-533 , doi : 10.1055 / s-1976-24110 .
  6. ↑ Jürgen Martens, Klaus Praefcke: Organic sulfur compounds, VII. Photochemical α-cleavage of thiobenzoic acid-Sp-tolylesters in solution . In: Chemical Reports . tape 107 , no. 7 , July 1, 1974, p. 2319-2325 , doi : 10.1002 / cber.19741070716 .
  7. ↑ Gerd Buchholz, Jürgen Martens , Klaus Praefcke: Photochemical thiaxanthone synthesis from 2-halo-thiobenzoic acid S-aryl esters1 . In: Synthesis . tape 1974 , no. 09 , 1974, p. 666-667 , doi : 10.1055 / s-1974-23399 . ; Gerd Buchholz, Jürgen Martens, Klaus Praefcke: 2- and 4-azathioxanthones by photo rearrangement of thionicotinic acid S-aryl esters . In: Angewandte Chemie . tape 86 , no. 15 , August 1, 1974, p. 562-563 , doi : 10.1002 / anie.19740861513 . Gerd Buchholz, Jürgen Martens, Klaus Praefcke: 2- and 4-Azathioxanthones by Photo Arrangement of S-Aryl Thionicotinates . In: Angewandte Chemie International Edition in English . tape 13 , no. 8 , August 1, 1974, p. 550–551 , doi : 10.1002 / anie.197405501 .
  8. ^ Siegfried Hauptmann : Organic chemistry. 2nd Edition. VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1985, p. 774.
  9. ^ A. Gilbert, J. Baggott: Essentials of Molecular Photochemistry . Blackwell, 1991, p. 168.
  10. ↑ Martin Vondenhof, Jochen Mattay : Radical ions and photochemical charge transfer phenomena, 28. 1,1'-Binaphthalene-2,2'-dicarbonitrile in photochemically sensitized enantiodifferentiating isomerizations. In: Chemical Reports. 123, 1990, pp. 2457-2459, doi: 10.1002 / cber.19901231232 .
  11. ↑ Alexander Schönberg : Preparative Organic Photochemistry . with a contribution by GO Schenk, Springer-Verlag, Berlin / Göttingen / Heidelberg 1958; Alexander Schönberg: Preparative Organic Photochemistry . in cooperation with GO Schenk, O.-A. Neumüller. 2., complete revised Edition of Preparative Organic Photochemistry . Springer-Verlag, Berlin / Heidelberg / New York 1968.
  12. ^ Wilhelm Nultsch: General botany: short textbook for physicians and scientists. 8th edition. Thieme Verlag, Stuttgart / New York 1986, ISBN 3-13-383308-1 , p. 257.
  13. ↑ G. von Bünau, T. Wolff: Photochemistry: Basics, Methods, Applications . VCH Verlagsgesellschaft, Weinheim / New York 1987, ISBN 3-527-26506-6 .
  14. ↑ rayonet.org
  15. ↑ a b J. Mattay, A. Giesbeck (Ed.): Photochemical Key Steps in Organic Synthesis. VCH, Weinheim / New York / Basel / Cambridge / Tokyo 1994, ISBN 3-527-29214-4 .