Support Curvature and Conformational Freedom Control Chemical Reactivity of Immobilized Species Tino Zdobinsky, Pradipta Sankar Maiti, and Rafal Klajn* Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel * S Supporting Information ABSTRACT: We show that bimolecular reactions between species confined to the surfaces of nanoparticles can be manipulated by the nature of the linker, as well as by the curvature of the underlying particles. F or many decades, enzymes have impressed scientists with the elegance with which they control chemical reactions. With their abilities to activate, preorganize, and increase local concentrations of substrate molecules, they have inspired chemists to design novel systems in which improved reactivities emerge. 1 In one example, Liu and co-workers induced dramatic acceleration of bimolecular reactions between substrates by preorganizing them on nucleic acid templates. 2 Other templates that were used to bring molecules together include self- assembled cages 3 as well as surfaces of inorganic materials. 4 However, increasing effective molarity alone is not enough to induce efficient bimolecular reactions; as we demonstrate in this study, lack of sufficient conformational freedom can drastically suppress chemical reactivity despite greatly increased local concentrations of the reactive moieties. Further, we show that reactivity of surface-confined species can be controlled by tailoring the flexibility of the linker chains, as well as by manipulating the curvature of the underlying nanoparticle (NP) surface. Ethynylanthracenes 5-7 are attractive substrates for studying the effect of molecular confinement on chemical reactivity. In the absence of the CC triple bond, anthracenes undergo the well-known 8-11 [4+4] dimerization when exposed to long-wave (λ ∼ 365 nm) UV irradiation. The presence of the triple bond at the 9-position, however, alters the reaction pathway so as to induce a selective [4+2] Diels-Alder dimerization 12-14 under the same reaction conditions. Recently, Weiss and co-workers have demonstrated that upon binding to Au(111) surfaces, thiol 1 dimerizes in the [4+4] fashion despite the presence of the triple bond; 14,15 that is, the “normal” behavior of unsubstituted anthracenes is restored. These researchers argued that the planar surface acts as a template favoring molecular arrangements ideal for the [4+4], but not for the [4+2], cycloaddition. Inspired by these findings and motivated by the many advantages of NPs over planar surfaces, 16 we synthesized thiolated 9-ethynylanthracenes 1-3 and investigated their photoreactivities within self-assembled monolayers on metallic nanospheres. Our initial experiments were based on 2.5 nm gold NPs functionalized with the previously reported 14,17-19 9-(4- mercaptophenylethynyl)anthracene (Figure 1b). These and other NPs investigated in this study were prepared via ligand exchange reaction using preformed, dodecylamine (DDA)- capped NPs 20 and excess of free thiol, followed by Received: November 13, 2013 Published: December 9, 2013 Figure 1. (a) Structural formula of 9-(4-mercaptophenyl-ethynyl)- anthracene 1. (b-d) TEM images of 1-functiona-lized 2.5 nm, 5.5 and 7.5 nm Au NPs, respectively. (e) UV-vis spectra of small-molecule 1 (in the form of a thioacetate; pure 1 is highly unstable), and the NPs shown in (b). The broad band at ∼550 nm is due to surface plasmon resonance. (f) UV-vis spectra of 1 released from 5.5 nm (dark gray) and 7.5 nm (black) Au NPs preirradiated under the same conditions (t = 21 h). Also shown (light gray) is a spectrum of 1 released from nonirradiated 2.5 nm NPs. The spectra were normalized with respect to the same initial concentration of 1. Communication pubs.acs.org/JACS © 2013 American Chemical Society 2711 dx.doi.org/10.1021/ja411573a | J. Am. Chem. Soc. 2014, 136, 2711-2714