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Abstract

This work investigates the isomerization behavior of photochromic molecules upon adsorption onto a noble metal surface. 2-photon photoemission (2PPE) has been employed to study the corresponding electronic structure and the charge charier dynamics. In the case of tetra-tert-butyl-Azobenzol (TBA) adsorbed on Au(111) the isomerization processes has been demonstrated based on the light and temperature induced changes in the electronic structure. Here, 2PPE proved to be an ideal tool for the analysis of the underlying excitation mechanism, the reaction kinetics as well as the efficiency of the photoinduced switching.
The initial excitation leading to the photoisomerization of TBA/Au(111), in contrast to the free molecule, does not occur via a direct optical transition between the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO). Instead a thermally assisted, substrate mediated process has been identified. Therefore a hole is created in the gold d-bands upon photoexcitation of an electron. Subsequently the hole is transferred to the HOMO of the TBA initiating a conformational change. As for the free TBA a second thermally driven isomerization channel is available which converts the meta stable cis-isomer into the trans configuration. Due to the influence of the substrate-adsorbate interaction the range of molecular motions is strongly limited. Moreover the energy barrier separating the two isomers in the ground state is lowered by a factor of four. To elucidate the influence of the substrate on the photoisomerization TBA is adsorbed onto an Ag(111) surface. Although the absorption behavior is almost identical to the results obtained from the Au(111) substrate, no switching has been observed. Here, the absence of a substrate mediated photoisomerization can be correlated to the differences in the band structures and in particular the energetic position of the respective d-bands. For silver these are located in an energy region where they cannot overlap with the with the HOMO of the TBA which is involved in the substrate mediated isomerization process. In further experiments the infuence of different functional groups on the isomerization behavior has been investigated. Therefore TBA is replaced by the isoelectronic, likewise tetra-tert-butyl substituted molecules imine (TBI) and stilbene (TDS). These enable the analysis of the switching process with respect to the properties of the free molecule which differ signifcantly for the three used species, viz. the available isomerization pathways, like
e.g. rotation or inversion, and the height of the ground state barrier. In the corresponding 2PPE spectra of TBI and TBS adsorbed on Au(111) no light induced changes have been observed and thus no photoisomerization could be identified. The results presented in this work show that the adsorption of the studied molecular switches leads to a significant change of their isomerization behavior. Two important aspect arise from the substrate-adsorbate interaction: The first is the influence of the electronic coupling between molecular orbitals and the metal band structure. On one hand it can reduce the lifetime of the excited molecular state which is a possible reason for the suppressed isomerization upon intramolecular excitation. On the other hand it creates additional excitation pathways via a substrate mediated charge transfer. The second aspect is the geometric confinement of the molecules. It can suppress molecular motions required for an isomerization and thus presents another possible explanation for the observed quenching of switching process initiated by intramolecular excitations.