Published: October 10, 2011 r2011 American Chemical Society 22965 dx.doi.org/10.1021/jp206889y | J. Phys. Chem. C 2011, 115, 22965–22975 ARTICLE pubs.acs.org/JPCC Remarkable Stability of High Energy Conformers in Self-Assembled Monolayers of a Bistable Electro- and Photoswitchable Overcrowded Alkene Oleksii Ivashenko, † Hella Logtenberg, ‡ Jetsuda Areephong, ‡ Anthony C. Coleman, ‡ Philana V. Wesenhagen, ‡ Edzard M. Geertsema, ‡ Nicolas Heureux, ‡ Ben L. Feringa,* ,†,‡ Petra Rudolf,* ,† and Wesley R. Browne* ,‡ † Zernike Institute for Advanced Materials and ‡ Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b S Supporting Information ’ INTRODUCTION Bringing molecular bistability from solution to surfaces is without doubt one of the most exciting challenges in molecular switching currently, not least because of the potential to control surface properties with external stimuli such as light and redox reactions. 1 Such “smart” surfaces, that is, surfaces that are stimuli responsive, hold considerable potential for application in areas as diverse as organic electronics, 2 magnetic memory 3 and logic, 4 biocompatibility, and cell culture on surfaces. 5 Achieving robust bistability, under repeated operation, either photo- or electro- chemically, is essential for real applications and ideally a large hysteresis in the switching and thermal bistability is preferable, as is key to magnetic memory, for example, in the light-induced spin crossover systems. 6 In this regard, the structurally simple over- crowded alkene bisthiaxanthylidene is an excellent candidate for redox control of surface properties due to its fascinating photo- chemically, thermally, and redox-driven switching behavior. 7 The photochemical, thermal, and electrochemical properties and the dynamic stereochemistry 8 of overcrowded alkenes 9 have captured the imagination of chemists for over 100 years. 10 Over the last decade, this class of functional compounds has seen application in the development of functional molecular materials, molecular switches, and devices. Most notable are the bistricyclic aromatic enylidenes (BAE) 11 and phenanthrylidenes 12 that were the basis for unidirectional light-driven molecular motors. 12b,d The thermal and photochemical properties of BAEs are highly sensitive to variations in molecular structure. 13,14 Indeed, even structurally simple BAEs show complex thermo-, photo-, and electrochromic behavior, such as the bisthiaxanthylidenes 1, for which we reported recently the unprecedented three-state- switching of luminescence between blue, red, and nonfluorescent states using a combination of electrochemical, photochemical, and thermal stimuli (Scheme 1). 7 The overcrowded alkene dimethyl-bisthiaxanthylidene (1) can adopt any of three distinct conformational states, anti-folded (1A), syn-folded (1B), and twisted (1C); with the anti-folded Received: July 19, 2011 Revised: October 6, 2011 ABSTRACT: Although bistability of molecular switches in solution is well established, achieving highly robust bistable molecular switching in self- assembled monolayers remains a challenge. Such systems are highly attrac- tive as components in organic electronics and molecular-based photo and electrochromic devices. Here we report a remarkably robust surface confined bisthiaxanthylidene redox switch that shows excellent bistability, manifested in reversible changes in spectroscopic and electrochemical properties and in physical properties such as water contact angle changes (ca. 30° difference in water contact angle between the two redox states of a bisthiaxanthylidene self-assembled monolayer). The effect of surface immobilization of bis- thiaxanthylidene on its photochromic, thermal and electrochemical properties is described. Surface immobilization is achieved by incorporating thiol- and alkylsiloxy-terminated “legs” on one of the tricyclic aromatic units. The molecular switch in its neutral and dicationic state, generated by bulk electrolysis, was characterized in solution, in the solid state and on surfaces, by UV/vis absorption, Fourier transform infrared, X-ray photoelectron, and Raman spectroscopies and by cyclic voltammetry. In solution, the redox switching to the dicationic state is achieved by oxidation at 1.2 V versus SCE. Reduction of the dication at <0.4 V results in initial formation of a highly unstable twisted conformation that reverts via a syn-folded conformational state to the most stable (anti- folded) conformer. Although the syn-folded state can be obtained by UV irradiation at <0 °C, the twisted conformation is not observable in solution, even at 200 K. Remarkably, in monolayers on electrodes this highly unstable form (which is generated by reduction of the dicationic state) is relatively stable even for several minutes. This stability is ascribed to the formation of densely packed monolayers in which the intermolecular interactions provide for a substantial barrier to thermal interconversion of the various conformational states.