Photoisomerization of azobenzene.[1]

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


Photoisomerization of the retinal eye allows for vision.

Photoisomerizable substrates have been put to practical use, for instance, in pigments for rewritable CDs, 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.


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, norbornadiene converts to quadricyclane via ~300nm 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.[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-pi-methane rearrangement.

Coordination chemistryEdit

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 alsoEdit


  1. ^ a b Natansohn, Almeria; Rochon, Paul (2002). "Photoinduced Motions in Azo-Containing Polymers". Chemical Reviews. 102 (11): 4139–4176. doi:10.1021/cr970155y. PMID 12428986.
  2. ^ "Photoisomerization". IUPAC Compendium of Chemical Terminology. 2009. doi:10.1351/goldbook.P04622. ISBN 978-0-9678550-9-7.
  3. ^ Mammana, A.; et al. (2011). "A Chiroptical Photoswitchable DNA Complex" (PDF). Journal of Physical Chemistry B. 115 (40): 11581–11587. doi:10.1021/jp205893y. PMID 21879715.
  4. ^ Mokdad, A; Belof, J; Yi, S; Shuler, S; McLaughlin, M; Space, B; Larsen, R (2008). "Photophysical Studies of the Trans to Cis Isomerization of the Push−Pull Molecule: 1-(Pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene (mepepy)". Journal of Physical Chemistry B. 112 (36): 8310–8315. Bibcode:2008JPCA..112.8310M. doi:10.1021/jp803268r. PMID 18700732.
  5. ^ Vachon, J.; et al. (2014). "An ultrafast surface-bound photo-active molecular motor". Photochemical and Photobiological Sciences. 13 (2): 241–246. doi:10.1039/C3PP50208B. PMID 24096390.
  6. ^ Janossy, I.; Szabados, L. (1 October 1998). "Optical reorientation of nematic liquid crystals in the presence of photoisomerization". Physical Review E. 58 (4): 4598. Bibcode:1998PhRvE..58.4598J. doi:10.1103/PhysRevE.58.4598.
  7. ^ Chen, Alan G; Brady, David J (1992). "Real-time holography in azo-dye-doped liquid crystals". Optics Letters. 17 (6): 441–3. Bibcode:1992OptL...17..441C. doi:10.1364/OL.17.000441. PMID 19784354.
  8. ^ Kato, Jun-ichi; Yamaguchi, Ichirou (1996). "Nonlinear spatial filtering with a dye-doped liquid-crystal cell". Optics Letters. 21 (11): 767–769. Bibcode:1996OptL...21..767K. doi:10.1364/OL.21.000767. PMID 19876152.
  9. ^ Maly, Kenneth E; Wand, Michael D (2002). "Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant". Journal of the American Chemical Society. 124 (27): 7898–7899. doi:10.1364/OPEX.13.002358. PMID 19495125.
  10. ^ Waldeck, David H. (1991). "Photoisomerization dynamics of stilbenes". Chemical Reviews. 91 (3): 415–436. doi:10.1021/cr00003a007.
  11. ^ Klajn, Rafal (2014). "Spiropyran-based dynamic materials". Chem. Soc. Rev. 43 (1): 148–184. doi:10.1039/C3CS60181A. PMID 23979515.
  12. ^ Dubonosov, Alexander D.; Bren, Vladimir A.; Chernoivanov, V. A. (2002). "Norbornadiene–quadricyclane as an abiotic system for the storage of solar energy". Russian Chemical Reviews. 71 (11): 917–927. Bibcode:2002RuCRv..71..917D. doi:10.1070/RC2002v071n11ABEH000745.
  13. ^ Kazem-Rostami, Masoud; Akhmedov, Novruz G.; Faramarzi, Sadegh (2019). "Spectroscopic and computational studies of the photoisomerization". Journal of Molecular Structure. 1178: 538–543. Bibcode:2019JMoSt1178..538K. doi:10.1016/j.molstruc.2018.10.071.
  14. ^ D. M. Roundhill (1994). Photochemistry and Photophysics of Metal Complexes. Springer. ISBN 978-1-4899-1495-8.
  15. ^ Gã¼Tlich, P. (2001). "Photoswitchable coordination compounds". Coordination Chemistry Reviews. 219–221: 839–879. doi:10.1016/S0010-8545(01)00381-2.