Modification of Molecular Assemblies of Azobenzene Derivatives by Electrical Stimuli

In recent years, molecular switches such as azobenzene adsorbed on metal surfaces have attracted considerable attention due to their potential use in nanotechnology, information storage or molecular electronics [1,2,3]. In particular, the 3,3’,5,5’-tetra-tert-butylazobenzene (TBA) molecule is a promising candidate for such studies, due to the four lateral tert-butyl-groups which act as “spacer leg” to reduce the electronic coupling between the active part of the molecule (azobenzene) and the metal surface. This azobenzene derivative switches reversibly between the two isomeric states (<i>trans</i>-TBA and <i>cis</i>-TBA). This isomerization has already been demonstrated by scanning-tunneling microscopy (STM) using either the electric field induced by the STM tip [4] or by exposure to UV or blue light [5].<br/>Here we report another effect of electric field for one-monolayer TBA coverage on Au(111), both characterized by STM and non-contact AFM (nc-AFM) with sub-molecular resolution using stiff (length extensionalv resonator) probes. These studies enable to observe and characterize an in-situ modification of the formed TBA self-assembly under electrical stimuli.<br/><br/>After the TBA evaporation, molecules are organized on the Au(111) surface in the <i>trans</i> isomer with four lobes corresponding to the 4 “legs”, with an apparent height of 0.25 ± 0.02 nm. STM imaging at 77K reveals domains only visible in negative polarity; by scanning these TBA assemblies at -2V, bright spots appear with an apparent height of 0.36 ± 0.05 nm confirmed by nc-AFM. These spots are usually associated to the <i>cis</i>-TBA induced by the <i>trans</i>-TBA to <i>cis</i>-TBA isomerization under the presence of an electric field [4]. For domains imaged with both polarities in STM, an in-plane rotation of TBA network is observed in STM and nc-AFM after scanning at negative polarity (-1V). The origin of this in-situ in-plane rotation will be discussed. These results show how crucial are the intramolecular and molecular-substrate forces control and understanding in the perspective of molecular electronics.<br/><br/>[1] Huang, X. H.; Li, T., <i>Journal of Materials Chemistry C </i><b>2020,</b> <i>8</i> (3), 821-848.<br/>[2] Molecular Switches 2nd. B.L. Feringa, W. R. B., Ed. Wiley-VCH, Weinheim, Germany: <b>2011</b>.<br/>[3] Thomas, L., Arbouch, I., Guérin, D., Wallart, X., Van Dyck, C., Mélin, T., Cornil, J., Vuillaume, D., Lenfant, S., Nanoscale <b>2021</b> 13, 14, 6977-6990.<br/>[4] Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L., E <i>JACS </i><b>2006,</b> <i>128</i> (45), 14446-14447.<br/>[5] Comstock, M. J.; Levy, N.; Cho, J.; Berbil-Bautista, L.; Crommie, M. F.; Poulsen, D. A.; Frechet, J. M. J., <i>APL </i><b>2008,</b> <i>92</i> (12), 123107-3