Published: July 27, 2011 r2011 American Chemical Society 14010 dx.doi.org/10.1021/ja204034g | J. Am. Chem. Soc. 2011, 133, 14010–14016 ARTICLE pubs.acs.org/JACS InorganicÀOrganic Hybrid Vesicles with Counterion- and pH-Controlled Fluorescent Properties Dong Li, †,§ Jie Song, ‡,§ Panchao Yin, † Silas Simotwo, † Andrew J. Bassler, † YuYu Aung, † James E. Roberts, † Kenneth I. Hardcastle, ‡ Craig L. Hill,* ,‡ and Tianbo Liu* ,† † Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States ‡ Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States b S Supporting Information ’ INTRODUCTION Smart supramolecular assemblies that respond to single or multiple external stimuli, such as temperature, 1 ionic strength, 2 pH, 3 redox, 4 light, 5 ultrasound, 6 and magnetic field, 7 are of great interest for their potential applications in drug delivery, oil recovery, and design of new sensors and catalysts. 8À13 While most smart supramolecular assemblies are constructed from organic molecules or block copolymers, incorporating functional inorganic components (such as quantum dots, silica, transition metal ions, etc.) into organic building blocks to make functional hybrid assemblies represents an interesting new direction. 5,6,14 Polyox- ometalates (POMs), a large group of early transition metal oxide clusters, represents structurally well-defined inorganic molecules with diversified structures and properties. 15À17 The synergistic combination of POMs and organic components gives these hybrids noteworthy properties of potential utility. 18,19 For in- stance, polymers with grafted POMs can be used in photovoltaic cells or in trapping magnetic nanoparticles, 20,21 and surfaces that are covered with hybrid POMs (SAMÀPOMÀpyrene) show specific cell adhesion ability. 22 Organo-POM hybrids 23À27 have been shown to exhibit unusual spectroscopic properties 28 or catalyze oxidation reactions, 29À31 organometallic POM clusters have been tested for norbornene and cyclohexene oxygenation as precatalysts, 32 and a recent study on a photoresponsive surfactant- encapsulated POM complex has shown an interesting dynamic structure transition in solution. 33 The new hybrids with surfactant- like amphiphilic solution properties are particularly interesting be- cause they can be compatible with both hydrophilic and hydrophobic environments and therefore can enhance the applications of the functional clusters. Interesting supramolecular structures such as micelles, vesicles, liquid crystals, and 2D- films have been reported in various amphiphilic hybrid solutions. 34À36 The self-assembly is dependent upon solvent polarity, 37 the hydrocarbon chain length, 38 the type and length of organic linkers, 39 and the architecture of the hybrids, as well as temperature. Meanwhile, the development of fluorescent tags or probes opens new opportunities for smart molecules. Applying fluor- escent probes to investigate biochemical microenvironments is a powerful technique that can often provide unique and critical information. Pyrene, a highly symmetrical polyaromatic hydro- carbon fluorophore possessing restricted modes of motion, can exhibit fine structure in its absorbance and fluorescence spectra at room temperature. 40 One important application of the pyrene fluorescence stems from its ability to probe the polarity of the local microenvironment, either in a hydrophobic or hydrophilic media, from the change of specific emission peaks in the spectrum. 41,42 Therefore, amphiphilic hybrid POM clusters with pendent pyrene fluorescent probes are of potential interest for the construction of smart supramolecular assemblies via macro- ion-counterion interaction. It is an interesting question how the hybrid surfactants arrange themselves to form closely packed regions in the supramolecular structures. These hybrids differ from conventional surfactants in Received: May 12, 2011 ABSTRACT: Two inorganicÀorganic hybrid clusters with one or two covalently linked pyrene fluorescent probes, [(n-C 4 H 9 ) 4 N] 2 - [V 6 O 13 {(OCH 2 ) 3 C(NH(CO)CH 2 CH 2 CH 2 C 16 H 9 )}{(OCH 2 ) 3 - CÀ(NH 2 )}] ((TBA + ) 2 1) and [(n-C 4 H 9 ) 4 N] 2 [V 6 O 13 {(OCH 2 ) 3 - C(NH(CO)CH 2 CH 2 CH 2 C 16 H 9 )} 2 ] ((TBA + ) 2 2), respectively, are synthesized from Lindqvist type polyoxometalates (POMs). The incorporation of pyrene into POMs results in amphiphilic hybrid molecules and simultaneously offers a great opportunity to study the interaction between hybrid clusters and their counterions. 2D-NOESY NMR and fluorescence techniques have been used to study the role of counterions such as tetrabutyl ammonium (TBA) in the vesicle formation of the hybrid clusters. The TBA + ions not only screen the electrostatic repulsions between the POM head groups but also are involved in the hydrophobic region of the vesicular structure where they interrupt the formation of pyrene excimers that greatly perturbs the luminescence signal from the vesicle solution. By replacing the TBA + counterions with protons, the new vesicles demonstrate interesting pH-dependent fluorescence properties.