Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that switch with visible light and are highly promising for applications ranging from smart materials to biological systems. However, the strong solvent dependence of the photoswitching kinetics limits their application. The nature of the photoswitching mechanism in different solvents is key for addressing the solvatochromism of DASAs, but as yet has remained elusive. Here, we employ spectroscopic analyses and TD-DFT calculations to reveal changing solvatochromic shifts and energies of the species involved in DASA photoswitching. Time-resolved visible pump-probe spectroscopy suggests that the primary photochemical step remains the same, irrespective of the polarity and protic nature of the solvent. Disentangling the different factors determining the solvent-dependence of DASA photoswitching, presented here, is crucial for the rational development of applications in a wide range of different media.

CEISAM UMR CNRS 6230 BP 92208 2 Rue de la Houssinière 44322 Nantes Cedex 3 France

Centre for Systems Chemistry Stratingh Institute for Chemistry University of Groningen Nijenborgh 4 9747 AG Groningen The Netherlands

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

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

Dipartimento di Chimica Università di Perugia via Elce di Sotto 8 06100 Perugia Italy

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

Istituto Nazionale di Ottica Largo Fermi 6 50125 Firenze Italy

Regional Centre of Advanced Technologies and Materials Department of Physical Chemistry Faculty of Science Palacký University in Olomouc 17 listopadu 1192 12 CZ 771 46 Olomouc Czech Republic

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

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