title: Effect of the steric molecular structure of azobenzene on the formation of self-assembled monolayers with a photoswitchable surface morphology.
abstract: The growth processes of self-assembled monolayers (SAMs) of two azobenzene disulfides formed on flat gold surfaces were studied to confirm the effect of the intermolecular interactions between azobenzene molecules on the light-triggered surface morphologies of the SAMs. Scanning tunneling microscopy (STM), atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) absorption spectroscopy were employed to study the SAMs and their growth processes. The SAMs studied were of bulky-substituted azobenzene disulfide (Et-2S), and nonsubstituted azobenzene disulfide (Me-2S), formed on a gold-covered substrate, and had a twisted and a planar structure, respectively. STM-based imaging of the initial stage of the self-assembly of the Et-2S molecules revealed that cleavage of the disulfide bond occurred on the gold surface, and phase-separated domains composed of azobenzenethiolate and dodecanethiolate were formed. Time-dependent AFM-based imaging illustrated the mechanism through which the Et-2S SAM grew-it was through the formation of dendritic aggregates and islands-eventually resulting in phase-separated domains with a wormlike structure. This wormlike structure showed noticeable changes in its surface morphology upon irradiation with UV and visible light. On the other hand, while the growth process for the Me-2S SAM was similar to that of the Et-2S SAM, the final Me-2S SAM had smooth domains whose morphology did not exhibit photoswitchability. The TD and XP spectra of the SAMs showed that the number of adsorbed Et-2S molecules reached a point of saturation after a 24 h long immersion while the number of Me-2S molecules increased even after a 336 h long immersion. Furthermore, the area occupied by the azobenzene moiety in the Et-2S SAM was constant regardless of the immersion time, whereas that in the Me-2S SAM decreased with the immersion time. These results indicated that the strength of the interactions between the azobenzene molecules significantly influenced the aggregate-forming ability in SAMs.
authors.affiliation: Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8502, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Ishikawa
authors.forename: Eisuke
authors.initials: E
authors.lastname: Ito
authors.forename: Mina
authors.initials: M
authors.lastname: Han
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hara
mesh_terms: D000327:Adsorption; D001391:Azo Compounds / Q000737:chemistry*; D004220:Disulfides / Q000737:chemistry; D006046:Gold / Q000737:chemistry; D008027:Light; D008958:Models, Molecular; D008968:Molecular Conformation; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013499:Surface Properties
chemical_list: D001391:Azo Compounds; D004220:Disulfides; D013438:Sulfhydryl Compounds; D006046:Gold; C009850:azobenzene