Light-driven molecular switches in azobenzene self-assembled monolayers: effect of molecular structure on reversible photoisomerization and stable cis statew Mina Han,* a Daisuke Ishikawa, a Takumu Honda, a Eisuke Ito b and Masahiko Hara* ab Received 19th October 2009, Accepted 16th March 2010 First published as an Advance Article on the web 8th April 2010 DOI: 10.1039/b921801g Both the reversible trans 2 cis photoisomerization and slow thermal back cis-to-trans isomerization of azobenzene- functionalized self-assembled monolayers on gold surfaces have been achieved by rationally designed single-component azobenzene thiol. Design and fabrication of a smart surface capable of control- lable and reversible switching between two states in response to light have attracted much attention in the fields of photo- chemistry, nanotechnology, biology, organic chemistry, and so on. 1 Among the various types of photochromic molecules, considerable interest has been focused on azobenzene due to its structural change between the trans and cis forms and its ease of chemical modification. 2 Controlling the trans 2 cis photoisomerization and the lifetime of the two different states is highly desirable for potential applications into optical information storage and an optical signal generated by photo- switching between the trans and cis states. In particular, the lifetime of the cis form is greatly influenced by the surrounding environment as well as the molecular structure. 1a,3 Even though many azobenzene molecules in dilute organic solutions undergo reversible photoisomerization and show a lifetime of the cis form on the time scale of minutes to hours, it has been reported that densely packed azobenzene thiol self- assembled monolayers (SAMs) are photochemically unreactive on flat gold surfaces. 4,5 In this context, efforts to afford free volume required for the conformational change between the trans and cis forms during photoisomerization have been made by fabricating (1) unsymmetrical azobenzene disulfide SAMs 6 and (2) mixed azobenzenethiol and alkanethiol SAMs. 7 How- ever, another instability problem accompanied by cleavage of the S–S disulfide and phase separation through favorable intermolecular interactions between the same chemical com- ponents emerges in these SAMs consisting of two components on a gold surface. 6b,8 Moreover, obviously different from investigations of the effects of free volume on the trans-to-cis photoconversion, the effects of molecular structure on the lifetime of cis-azobenzene in combination with reversible photoisomerization on solid surfaces have not yet been reported. We have designed single-component azobenzene substituted with sterically bulky ethyl groups at the ortho positions with respect to the azo group (Et-SH, Fig. 1 and see ESIw), which exhibits excellent reversible photoswitching properties as well as slow thermal cis-to-trans isomerization over a one-day period in SAMs. A comparison with Me-SH containing methyl group at the meta position has also been made to clarify the influence of free volume and molecular structure on both the trans-to-cis photoisomerizability and the lifetime of the cis form. Ortho-diethylated azobenzene (Et-SH) in dichloromethane solution showed a strong p–p* absorption band at 354 nm (e = 2.6  10 4 L mol 1 cm 1 ) in the as-prepared initial state (Fig. S1 and Table S1, see ESIw). NMR and UV-vis absorp- tion spectroscopic studies indicated that approximately 95% of the cis form was produced at a photostationary state of UV light irradiation at 365 nm. Subsequent irradiation with visible light at 436 nm induced cis-to-trans isomerization, giving a photostationary state containing both the major trans form (B70%) and the minor cis form. 1,9 Alternatively, thermal back cis-to-trans isomerization of Et-SH proceeded much Fig. 1 Upper: Molecular structure. Lower: Schematic representation of molecular switches of Et-SH SAMs on a gold surface. a Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. E-mail: mrhan@echem.titech.ac.jp, masahara@echem.titech.ac.jp; Fax: 81-45-924-5447; Tel: 81-45-924-5447 b Flucto-Order Functions Research Team, RIKEN-HYU Collaboration Research Center, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan w Electronic supplementary information (ESI) available: Detailed experimental procedures (instrumentation, monolayer preparation, synthesis and characterization), UV-vis absorption spectra of dichloro- methane solutions, XPS data, absorption spectra of Me-SH SAMs. See DOI: 10.1039/b921801g 3598 | Chem. Commun., 2010, 46, 3598–3600 This journal is  c The Royal Society of Chemistry 2010 COMMUNICATION www.rsc.org/chemcomm | ChemComm Published on 08 April 2010. Downloaded by NIMS Namiki Library on 04/09/2014 09:19:20. View Article Online / Journal Homepage / Table of Contents for this issue