Scientific Results

Tailoring Photoisomerization Pathways in Donor-Acceptor Stenhouse Adducts: The Role of the Hydroxy Group

Year: 2018

Authors: Lerch MM., Medved M., Lapini A., Laurent AD., Iagatti A., Bussotti L., Szymanski W., Buma WJ., Foggi P., Di Donato M., Feringa B.

Autors Affiliation: Centre for Systems Chemistry, Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen, 9747 AG, Netherlands; Department of Radiology, University of Groningen, University Medical Center Groningen, Hanzeplein 1, Groningen, 9713 GZ, Netherlands; LENS (European Laboratory for Non Linear Spectroscopy), via N. Carrara 1, Sesto Fiorentino, 50019, Italy; INO (Istituto Nazionale di Ottica), Largo Fermi 6, Firenze, 50125, Italy; Dipartimento di Chimica Ugo Schiff, Università di Firenze, via della Lastruccia 13, Sesto Fiorentino, 50019, Italy; Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, Perugia, 06100, Italy; Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University in Olomouc, 17. listopadu 1192/12, Olomouc, CZ-771 46, Czech Republic; Department of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovského 40, Banská Bystrica, SK-97400, Slovakia; CEISAM, UMR CNRS 6230, BP 92208, 2 Rue de la Houssinière, Nantes, Cedex 3, 44322, France; Van

Abstract: Donor-acceptor Stenhouse adducts (DASAs) arc 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), H-1 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 arc 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 4 pi-electrogclization 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.

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY A

Volume: 122 (4)      Pages from: 955  to: 964

More Information: The authors gratefully acknowledge financial support from Laserlab-Europe (LENS002289), the Ministry of Education, Culture and Science (Gravitation program 024.001.035), The Netherlands Organization for Scientific Research (NWO-CW, Top grant to B.L.F., VIDI grant no. 723.014.001 for W.S.), the European Research Council (Advanced Investigator Grant, no. 227897 to B.L.F.) and the Royal Netherlands Academy of Arts and Sciences Science (KNAW). M.M. acknowledges the Czech Science Foundation (project no. 16-01618S), the Ministry of Education, Youth and Sports of the Czech Republic (grant LO1305) and the Grant Agency of the Slovak Republic (VEGA project No. 1/0737/17). This research used computational resources of (1) the GENCI-CINES/IDRIS, (2) CCIPL (Centre de Calcul Intensif des Pays de Loire), (3) a local Troy cluster, and (4) the HPCC of the Matej Bel University in Banska Bystrica by using the infrastructure acquired in projects ITMS 26230120002 and 26210120002 supported by the Research and Development Operational Programme funded by the ERDF. The Swiss Study Foundation is acknowledged for a fellowship to M.M.L. We thank P. van der Meulen for support with the temperature dependent NMR in situ irradiation studies and T. Tiemersma-Wegman for ESI-MS analyses.
KeyWords: Design for testability; Hydrogen bonds; Isomers; Light; Organic compounds; Probes, Electrocyclization; Electronic effects; Fluorescent probes; Photochemical behaviors; Photophysical properties; Solvatochromic shifts; Td-dft calculations; Time resolved pump-probe spectroscopies, Excited states
DOI: 10.1021/acs.jpca.7b10255

Citations: 14
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