Excitation mechanism in the photoisomerization of a surface-bound azobenzene derivative: Role of the metallic substrate
- Fachbereich Physik, Freie Universitaet Berlin, Arnimallee 14, D-14195 Berlin (Germany)
Two-photon photoemission spectroscopy is employed to elucidate the electronic structure and the excitation mechanism in the photoinduced isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). Our results demonstrate that the optical excitation and the mechanism of molecular switching at a metal surface is completely different compared to the corresponding process for the free molecule. In contrast to direct (intramolecular) excitation operative in the isomerization in the liquid phase, the conformational change in the surface-bound TBA is driven by a substrate-mediated charge transfer process. We find that photoexcitation above a threshold h{nu}{approx_equal}2.2 eV leads to hole formation in the Au d-band followed by a hole transfer to the highest occupied molecular orbital of TBA. This transiently formed positive ion resonance subsequently results in a conformational change. The photon energy dependent photoisomerization cross section exhibit an unusual shape for a photochemical reaction of an adsorbate on a metal surface. It shows a thresholdlike behavior below h{nu}{approx_equal}2.2 eV and above h{nu}{approx_equal}4.4 eV. These thresholds correspond to the minimum energy required to create single or multiple hot holes in the Au d-bands, respectively. This study provides important new insights into the use of light to control the structure and function of molecular switches in direct contact with metal electrodes.
- OSTI ID:
- 21254779
- Journal Information:
- Journal of Chemical Physics, Vol. 129, Issue 16; Other Information: DOI: 10.1063/1.2997343; (c) 2008 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
BENZENE
CHARGE EXCHANGE
CROSS SECTIONS
ELECTRONIC STRUCTURE
ENERGY DEPENDENCE
EV RANGE 01-10
EXCITATION
GOLD
ISOMERIZATION
MOLECULAR ORBITAL METHOD
MOLECULAR STRUCTURE
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
PHOTOEMISSION
PHOTONS
SPECTROSCOPY
SURFACES