title: Direct Observation of Photoinduced Tautomerization in Single Molecules at a Metal Surface.
abstract: Molecular switches are of fundamental importance in nature, and light is an important stimulus to selectively drive the switching process. However, the local dynamics of a conformational change in these molecules remain far from being completely understood at the single-molecule level. Here, we report the direct observation of photoinduced tautomerization in single porphycene molecules on a Cu(111) surface by using a combination of low-temperature scanning tunneling microscopy and laser excitation in the near-infrared to ultraviolet regime. It is found that the thermodynamically stable trans configuration of porphycene can be converted to the metastable cis configuration in a unidirectional fashion by photoirradiation. The wavelength dependence of the tautomerization cross section exhibits a steep increase around 2 eV and demonstrates that excitation of the Cu d-band electrons and the resulting hot carriers play a dominant role in the photochemical process. Additionally, a pronounced isotope effect in the cross section (∼100) is observed when the transferred hydrogen atoms are substituted with deuterium, indicating a significant contribution of zero-point energy in the reaction. Combined with the study of inelastic tunneling electron-induced tautomerization with the STM, we propose that tautomerization occurs via excitation of molecular vibrations after photoexcitation. Interestingly, the observed cross section of ∼10(-19) cm(2) in the visible-ultraviolet region is much higher than that of previously studied molecular switches on a metal surface, for example, azobenzene derivatives (10(-23)-10(-22) cm(2)). Furthermore, we examined a local environmental impact on the photoinduced tautomerization by varying molecular density on the surface and find substantial changes in the cross section and quenching of the process due to the intermolecular interaction at high density.
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Böckmann
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Liu
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Mielke
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, Warsaw 01-224, Poland.
authors.forename: S
authors.initials: S
authors.lastname: Gawinkowski
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, Warsaw 01-224, Poland.|Faculty of Mathematics and Natural Sciences, College of Science, Cardinal Stefan Wyszyński University , Dewajtis 5, 01-815 Warsaw, Poland.
authors.forename: J
authors.initials: J
authors.lastname: Waluk
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.|Department of Physical Chemistry, University of Graz , Heinrichstrasse 28, 8010 Graz, Austria.
authors.forename: L
authors.initials: L
authors.lastname: Grill
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Wolf
authors.affiliation: Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society , Faradayweg 4-6, 14195 Berlin, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Kumagai
keywords: Molecular switch; metal surface; photochemistry; scanning tunneling microscopy; single molecule; tautomerization