Photoisomerization

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Photoisomerization of azobenzene[1]

In chemistry, photoisomerization is a form of isomerization induced by photoexcitation.[2] Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a reversible process.

Applications

Photoisomerization of the compound retinal in the eye allows for vision.

Photoisomerizable substrates have been put to practical use, for instance, in

DVDs, and 3D optical data storage solutions. In addition, interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches,[3][4] molecular motors,[5] and molecular electronics
.

Another class of device that uses the photoisomerization process is as an additive in

liquid crystals to change their linear and nonlinear properties.[6] Due to the photoisomerization is possible to induce a molecular reorientation in the liquid crystal bulk, which is used in holography,[7] as spatial filter[8] or optical switching.[9]

Methyl red molecule, a common azo dye used in liquid crystal doping

Examples

Azobenzenes,[1] stilbenes,[10] spiropyrans,[11] are prominent classes of compounds subject to photoisomerism.

Photoisomerization of norbornadiene to quadricyclane.

In the presence of a catalyst,

UV radiation . When converted back to norbornadiene, quadryicyclane’s ring strain energy is liberated in the form of heat (ΔH = −89 kJ/mol). This reaction has been proposed to store solar energy (photoswitchs).[12]

Photoisomerization behavior can be roughly categorized into several classes. Two major classes are trans–cis (or E–Z) conversion, and open-closed ring transition. Examples of the former include stilbene and azobenzene. This type of compounds has a double bond, and rotation or inversion around the double bond affords isomerization between the two states.[13] Examples of the latter include fulgide and diarylethene. This type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the di-π-methane rearrangement.

Coordination chemistry

Many complexes are often photosensitive and many of these complexes undergo photoisomerization.[14] One case is the conversion of the colorless cis-bis(triphenylphosphine)platinum chloride to the yellow trans isomer.

Photoisomerization of PtCl2(PPh3)2

Some coordination complexes undergo change in their spin state upon illumination, i.e. these are photosensitive spin crossover complexes.[15]

Light-induced spin-crossover of [Fe(pyCH2NH2)3]2+, which switches from high and low-spin

See also

References

  1. ^
    PMID 12428986
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  2. ISBN 978-0-9678550-9-7
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  3. S2CID 33375716
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  4. PMID 18700732
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  5. S2CID 23165784
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  6. S2CID 26508261
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  7. S2CID 20923350
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  8. PMID 19876152
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  9. PMID 19495125
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  10. doi:10.1021/cr00003a007
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  11. PMID 23979515
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  12. S2CID 250890545
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  13. S2CID 105312344
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  14. ISBN 978-1-4899-1495-8
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  15. doi:10.1016/S0010-8545(01)00381-2
    .
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