Donor-acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activatable negative photochromes. They are closely related to (mero)cyanine dyes with the sole difference being a hydroxy group in the polyene chain. The presence or absence of the hydroxy group has far-reaching consequences for the photochemistry of the compound: cyanine dyes are widely used as fluorescent probes, whereas DASAs hold great promise for visible light-triggered photoswitching. Here we analyze the photophysical properties of a DASA lacking the hydroxy group. Ultrafast time-resolved pump-probe spectroscopy in both the visible and IR region show the occurrence of E-Z photoisomerization on a 20 ps time scale, similar to the photochemical behavior of DASAs, but on a slower time scale. In contrast to the parent DASA compounds, where the initial photoisomerization is constrained to a single position (next to the hydroxy group), 1H NMR in situ-irradiation studies at 213 K reveal that for nonhydroxy DASAs E-Z photoisomerization can take place at two different bonds, yielding two distinct isomers. These observations are supported by TD-DFT calculations, showing that in the excited state the hydroxy group (pre)selects the neighboring C2-C3 bond for isomerization. The TD-DFT analysis also explains the larger solvatochromic shift observed for the parent DASAs as compared to the nonhydroxy analogue, in terms of the dipole moment changes evoked upon excitation. Furthermore, computations provide helpful insights into the photoswitching energetics, indicating that without the hydroxy group the 4π-electrocyclization step is energetically forbidden. Our results establish the central role of the hydroxy group for DASA photoswitching and suggest that its introduction allows for tailoring photoisomerization pathways, presumably both through (steric) fixation via a hydrogen bond with the adjacent carbonyl group of the acceptor moiety, as well as through electronic effects on the polyene backbone. These insights are essential for the rational design of novel, improved DASA photoswitches and for a better understanding of the properties of both DASAs and cyanine dyes.

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 Ugo Schiff Università di Firenze via della Lastruccia 13 50019 Sesto Fiorentino Italy

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

INO Largo Fermi 6 50125 Firenze Italy

LENS via N Carrara 1 50019 Sesto Fiorentino 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 1098 XH Amsterdam The Netherlands

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Solvent Effects on the Actinic Step of Donor-Acceptor Stenhouse Adduct Photoswitching