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.

CNR ICCOM via Madonna del Piano 10 50019 Sesto Fiorentino FI Italy

Department of Chemistry Faculty of Natural Sciences Matej Bel University Tajovského 40 974 01 Banská Bystrica Slovak Republic

Department of Chemistry University of Toronto 80 St George St Toronto M5S 3H6 Canada

Institute for Molecules and Materials FELIX Laboratory Radboud University Toernooiveld 7c 6525 ED Nijmegen The Netherlands

LENS European Laboratory for Non Linear Spectroscopy via N Carrara 1 50019 Sesto Fiorentino FI Italy

Medical Imaging Center University Medical Center Groningen University of Groningen Hanzeplein 1 9713GZ Groningen The Netherlands

Nantes Université CNRS CEISAM UMR 6230 F 44000 Nantes France

Polymer Institute Slovak Academy of Sciences Dúbravská cesta 9 845 41 Bratislava Slovak Republic

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Šlechtitelů 241 27 Olomouc 783 71 Czech Republic

Stratingh Institute for Chemistry University of Groningen Nijenborgh 4 9747AF Groningen The Netherlands

Van 't Hoff Institute for Molecular Sciences University of Amsterdam Science Park 904 1098 XH Amsterdam The Netherlands

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