Light is an exceptional external stimulus for establishing precise control over the properties and functions of chemical and biological systems, which is enabled through the use of molecular photoswitches. Ideal photoswitches are operated with visible light only, show large separation of absorption bands and are functional in various solvents including water, posing an unmet challenge. Here we show a class of fully-visible-light-operated molecular photoswitches, Iminothioindoxyls (ITIs) that meet these requirements. ITIs show a band separation of over 100 nm, isomerize on picosecond time scale and thermally relax on millisecond time scale. Using a combination of advanced spectroscopic and computational techniques, we provide the rationale for the switching behavior of ITIs and the influence of structural modifications and environment, including aqueous solution, on their photochemical properties. This research paves the way for the development of improved photo-controlled systems for a wide variety of applications that require fast responsive functions.

Department of Chemistry 'Ugo Shiff' University of Florence via della Lastruccia 3 13 50019 Sesto Fiorentino Italy

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

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

Faculty of Natural Sciences Department of Chemistry Matej Bel University Tajovského 40 SK 97400 Banská Bystrica Slovak Republic

Faculty of Science and Engineering Centre for Systems Chemistry Stratingh Institute for Chemistry University of Groningen Nijenborgh 7 9747 AG Groningen The Netherlands

Faculty of Science Regional Centre of Advanced Technologies and Materials Palacký University in Olomouc Šlechtitelů 27 CZ 771 46 Olomouc Czech Republic

INO Istituto Nazionale di Ottica Largo Fermi 6 50125 Firenze Italy

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

University of Nantes CEISAM UMR CNRS 6230 BP 92208 2 Rue de la Houssiniere 44322 Cedex 3 Nantes France

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

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Getting a molecular grip on the half-lives of iminothioindoxyl photoswitches

Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism