Mechanistic Basis for Red Light Switching of Azonium Ions
- Miroslav Medved’Miroslav Medved’Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute (CATRIN), Palacký University, Šlechtitelů 241/27, Olomouc, 783 71 Czech RepublicDepartment of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovského 40, 974 01 Banská Bystrica, Slovak RepublicMore by Miroslav Medved’
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- Mariangela Di DonatoMariangela Di DonatoLENS, European Laboratory for Non-Linear Spectroscopy, via N. Carrara 1, 50019 Sesto Fiorentino, FI, ItalyCNR-ICCOM, via Madonna del Piano 10, 50019 Sesto Fiorentino, FI, ItalyMore by Mariangela Di Donato
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- Wybren Jan BumaWybren Jan BumaVan ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The NetherlandsInstitute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernooiveld 7c, 6525 ED Nijmegen, The NetherlandsMore by Wybren Jan Buma
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- Adèle D. LaurentAdèle D. LaurentNantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, FranceMore by Adèle D. Laurent
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- Lucien LameijerLucien LameijerStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AF Groningen, The NetherlandsMedical Imaging Center, University Medical Center Groningen, University of Groningen Hanzeplein 1, 9713GZ Groningen, The NetherlandsMore by Lucien Lameijer
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- Tomáš HrivnákTomáš HrivnákDepartment of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovského 40, 974 01 Banská Bystrica, Slovak RepublicPolymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41 Bratislava, Slovak RepublicMore by Tomáš Hrivnák
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- Ivan RomanovIvan RomanovVan ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The NetherlandsMore by Ivan Romanov
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- Susannah TranSusannah TranDepartment of Chemistry, University of Toronto, 80 St. George St., Toronto M5S 3H6, CanadaMore by Susannah Tran
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- Ben L. FeringaBen L. FeringaStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AF Groningen, The NetherlandsMore by Ben L. Feringa
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- Wiktor Szymanski*Wiktor Szymanski*Email: [email protected]Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AF Groningen, The NetherlandsMedical Imaging Center, University Medical Center Groningen, University of Groningen Hanzeplein 1, 9713GZ Groningen, The NetherlandsMore by Wiktor Szymanski
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- G. Andrew Woolley*G. Andrew Woolley*Email: [email protected]Department of Chemistry, University of Toronto, 80 St. George St., Toronto M5S 3H6, CanadaMore by G. Andrew Woolley
Abstract
Azonium ions formed by the protonation of tetra-ortho-methoxy-substituted aminoazobenzenes photoisomerize with red light under physiological conditions. This property makes them attractive as molecular tools for the photocontrol of physiological processes, for example, in photopharmacology. However, a mechanistic understanding of the photoisomerization process and subsequent thermal relaxation is necessary for the rational application of these compounds as well as for guiding the design of derivatives with improved properties. Using a combination of sub-ps/ns transient absorption measurements and quantum chemical calculations, we show that the absorption of a photon by the protonated E–H+ form of the photoswitch causes rapid (ps) isomerization to the protonated Z–H+ form, which can also absorb red light. Proton transfer to solvent then occurs on a microsecond time scale, leading to an equilibrium between Z and Z–H+ species, the position of which depends on the solution pH. Whereas thermal isomerization of the neutral Z form to the neutral E form is slow (∼0.001 s–1), thermal isomerization of Z–H+ to E–H+ is rapid (∼100 s–1), so the solution pH also governs the rate at which E/E–H+ concentrations are restored after a light pulse. This analysis provides the first complete mechanistic picture that explains the observed intricate photoswitching behavior of azonium ions at a range of pH values. It further suggests features of azonium ions that could be targeted for improvement to enhance the applicability of these compounds for the photocontrol of biomolecules.
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