Photomagnetic Effects DOI: 10.1002/ange.200704027 Reversible Phototuning of Ferromagnetism at Au–S Interfaces at Room Temperature** Masayuki Suda, Naoto Kameyama, Motohiro Suzuki, Naomi Kawamura, and Yasuaki Einaga* The design of molecular compounds that exhibit photo- induced magnetization and magnetic transitions is one of the main challenges in the field of materials science because of their possible application to future optical memory and switching devices. Photoinduced changes in magnetic order have been studied extensively in a variety of systems, including cyanometalate-based compounds, [1–3] LIESST (light-induced excited spin-state trapping) compounds, [4–6] diluted magnetic semiconductors, [7,8] and manganite films. [9] Although interesting photomagnetic phenomena have been reported in the above systems, most of the observations of such phenomena have been limited to operation at very low temperature. Other candidate systems that show photomag- netic effects occur in hybrid materials of organic photo- chromes and inorganic magnetic compounds whose magnetic properties are relatively superior to those of conventional compounds. [10–14] Based on such a strategy, a previous report has demonstrated room-temperature reversible photocontrol of ferromagnetic order in photochrome-modified FePt nano- particles. [15] However, the occurrence of these photomagnetic effects was limited to just the surface layers of the FePt nanoparticles. Hence, the design and synthesis of a new class of optically switchable magnetic compounds that exhibit both large magnetization changes and ferromagnetic order even at room temperature is still a challenging issue. Herein we propose a novel strategy that focuses on the two-dimensional ferromagnetism which appears “out of nowhere” at the interfaces between organic–inorganic hybrids such as self-assembled monolayer (SAM) films on gold. In a number of recent papers, the occurrence of ferromagnetism at Au–S interfaces has been observed. [16–19] The ferromagnetism has been associated with Au 5d localized holes that are the result of charge transfer from the Au surface atoms to the S atoms of the organic ligands when forming the Au  S bonds. Since charge transfer from Au to S atoms acts as a “trigger” for the generation of ferromagnetism, the magnitudes of the magnetic moments will vary with the metal work functions. It is well known that the metal work function is correlated with the surface dipole of organic layers, which arises from the cooperative effect of intrinsic molecular dipole moments. [20] Since the trans state and cis state of azobenzene-derivatized thiols have opposite dipoles, they can be used to decrease and increase the work function of gold, respectively. As a consequence, the use of SAM-containing photochromes grafted onto gold surfaces has been shown to have great potential for the photocontrol of magnetic proper- ties. Furthermore, reducing the volume of such materials down to the nanometer scale will expose the local ferromag- netic areas at the Au–S interfaces as explicit entities, which will not only leads to enhancement of the magnetism, but also opens the possibility that the magnetization could be control- lable by photochromes with high efficiency. We now report reversible photoinduced magnetization changes that were observed in gold nanoparticles passivated with azobenzene- derivatized ligands. Figure 1 shows TEM images and size distribution histo- grams for the AZ-passivated gold nanoparticles (AZ-Au NPs) prepared by the modified Brust method. [21] The AZ-Au NPs exhibit a narrow size distribution with an average size of 1.74 nm  0.29 nm. The powder X-ray diffraction pattern of the AZ-Au NPs (shown in the Supporting Information) is consistent with a face-centered cubic crystal structure for Au Figure 1. a)TEMmicrographofAZ-AuNPsdepositedfromatoluene dispersionontoacollodion-coatedcoppergrid;scalebar:10nm. b)SizedistributionoftheAZ-AuNPs;theaveragewasdeterminedto be 1.74  0.29nm. [*] M. Suda, N. Kameyama, Prof. Y. Einaga Department of Chemistry FacultyofScienceandTechnology Keio University 3-14-1 Hiyoshi, Yokohama 223-8522 (Japan) Fax:(+ 81)45-566-1697 E-mail:einaga@chem.keio.ac.jp Dr.M.Suzuki,Dr.N.Kawamura Japan Synchrotron Radiation Research Institute (JASRI/SPring-8) 1-1-1 Kouto, Sayo, 679-5198 (Japan) [**] This work was supported by the New Energy and Industrial TechnologyDevelopmentOrganization(NEDO)andaGrant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese govern- ment. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Zuschriften 166 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2008, 120, 166 –169