About Supramolecular Assemblies of π-Conjugated Systems

Poly(aryleneethynylene)s: Syntheses, Properties, Structures, and Applications

The synthesis, spectroscopy, and fluorescence quenching behavior of pentiptycene-derived phenyleneethynylene polymers, 1−3, are reported. The incorporation of rigid three-dimensional pentiptycene moieties into conjugated polymer backbones offers several design advantages for solid-state (thin film) fluorescent sensory materials. First, they prevent π-stacking of the polymer backbones and thereby maintain high fluorescence quantum yields and spectroscopic stability in thin films. Second, reduced interpolymer interactions dramatically enhance the solubility of polymers 1−3 relative to other poly(phenyleneethynylenes). Third, the cavities generated between adjacent polymers are sufficiently large to allow diffusion of small organic molecules into the films. These advantages are apparent from comparisons of the spectroscopic and fluorescence quenching behavior of 1−3 to a related planar electron-rich polymer 4. The fluorescence attenuation (quenching) of polymer films upon exposure to analytes depends on seve...

/pdf/fluorescent-porous-polymer-films-as-tnt-chemosensors-57bxm0urtq.pdf

Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects

A chemosensor is a molecular device designed to detect a specific molecule or class of molecules.1a,b Research in this field is poised for considerable advances in the coming years with the advent of diverse methods for analyte detection and new developments in the field of molecular recognition. To date, the most common signal transduction schemes utilize optical or electrical methods.1c Fluorescence is a highly sensitive optical transduction method, and analyte binding events that produce an attenuation, enhancement, or wavelength shift in the emission can be used to produce a functional sensor.1d Changes in absorption spectra, while less sensitive, have also been extensively used.1e Redox processes are widely used in electrical-based transduction methods, and typical systems function in either potentiometric or amperometric modes.1f Conductometric detection schemes based on SnO2, conducting polymers,1h and phthalocyanines1i have also been investigated. As shown in Scheme 1, a chemosensor is composed of two functional elements, a receptor and a reporter group, which need not be separate in identity. When the equilibrium between the analyte and receptor is rapid, sensors can be produced that provide a real-time response, which continuously varies with the concentration of the analyte. The detection sensitivity is determined by both the ability to measure the transduction event and the association constant of the receptor-analyte complex. As a result, when pursuing higher sensitivity one may seek instrumentation improvements and/or endeavor to increase the magnitude of the association constant of the receptor-analyte complex. The standard approach to higher association constants is to design highly preorganized receptors that do not pay a high entropic penalty for complexation. The downside of this approach is that preorganization and high association constants generally result in slow dissociation kinetics. A molecular chemosensor with slow kinetics or an irreversible response cannot yield a reversible real-time response. Molecular systems displaying irreversible or slow behavior are nonetheless useful, but are properly called dosimeters or indicators. Methods may be developed to allow irreversible systems to function in a sensory device. For example, the device can be “reset” by chemical, electrochemical, photochemical, or physical events. These processes can cause the analyte to dissociate from the receptor or can result in the replacement of the indicator (chemosensor) molecules. Such approaches have the disadvantage of introducing additional complexity into the sensor devices. This Account describes an approach to enhancing the sensitivity of chemosensors in effect by “wiring chemosensory molecules in series” (Scheme 2). Recent work from my research laboratory has shown that this molecular wire approach provides a universal method by which to obtain signal amplification relative to single molecule systems. I will use the term molecular wire interchangeably with conducting polymer. For the sake of clarity, some representative conducting polymers are shown in Scheme 3. These materials are insulators in their neutral (undoped) Timothy M. Swager is a native of Montana and received a B.S. from Montana State University in 1983 and a Ph.D. from the California Institute of Technology in 1988. After a postdoctoral fellowship at the Massachusetts Institute of Technology (1988-1990), he began his independent academic career at The University of Pennsylvania and was promoted to Professor in 1996. He is currently a Professor of Chemistry at the Massachusetts Institute of Technology. His interests in supramolecular chemistry include the design of electronic polymers, sensors, and liquid crystals, molecular recognition, and catalysis. Scheme 1

/pdf/the-molecular-wire-approach-to-sensory-signal-amplification-3a05lvtynr.pdf

The Molecular Wire Approach to Sensory Signal Amplification

Fluorescence is a leading signal transduction method for the formation of chemosensory devices.1,2 Conjugated polymer-based fluorescent chemosensors have been proven to further amplify this inherently sensitive method.2 Although solid-state (thin film) fluorescent chemosensory devices are generally required, most systems have limited solid state utility with the lack of stable (reproducible) spectroscopic features and low fluorescence quantum yields, presumably due to the excimer formation and selfquenching.3,4 These processes have been an impediment to other applications of conjugated polymers and general solutions also have implications for electroluminescent polymer devices.4 We report herein an approach to stable fluorescent conjugated polymer films by the incorporation of the rigid three-dimensional pentiptycene moieties,5 1, which prevent π-stacking or excimer formation. In comparison to the planar model polymer 3, thin films of the pentiptycene polymer, 2, display enhanced fluorescence quantum yield and stability. Moreover, their potential for the trace detection of high explosives such as 2,4,6-trinitrotoluene (TNT) is also reported.6 Polymer 2 has unprecedented high sensitivity as an artificial fluorescent chemosensor for the vapors of TNT and 2,4-dinitrotoluene (DNT).

/pdf/porous-shape-persistent-fluorescent-polymer-films-an-512re0bfni.pdf

Porous Shape Persistent Fluorescent Polymer Films: An Approach to TNT Sensory Materials

Intrazeolite organometallics and coordination complexes: internal versus external confinement of metal guests

Intrazeolite Organometallics and Coordination Complexes: Internal versus External Confinement of Metal Guests

Intrazeolite site-selective far-IR cation probe

The incorporation of the half-sandwich complexes CpM(CO){sub 2}, Cp*M(CO){sub 2}, and CpM(C{sub 2}M{sub 4}){sub 2} (M = Rh, Ir; Cp = C{sub 5}H{sub 5}; Cp* = (CH{sub 3}){sub 5}C{sub 5}), as well as CpRh(C{sub 2}H{sub 4})CO and (acac)Rh(CO){sub 2}, into zeolites has been investigated. Initial studies have focused on the immobilization and characterization of the species, primarily in alkali-metal Y zeolites, through in situ optical reflectance and vibrational spectroscopy. Size exclusion studies in Na{sub 12}-A (4.2 {angstrom}) and K{sub 8}Na{sub 4}-A (3.4 {angstrom}) zeolites demonstrate internal rather than external confinement of the organometallic. Two distinct anchoring sites within the supercages are observed for the carbonyl systems. Site 1 interaction is attributed to a cation-ring coordination, while site 2 is assigned to a single cation interaction with the oxygen atoms of both carbonyl ligands. The population distribution between sites 1 and 2 varies with the nature of the organometallic guest, the exchanged cation, and the Si:Al ratio of the zeolite. The {sup 13}CO exchange reactions of encapsulated CpM(CO){sub 2} and Cp*M(CO){sub 2} show faster rates compared to solution-phase work in those alkali-metal Y zeolites that exhibit population of both sites 1 and 2. Preliminary studies indicate {sup 13}CO pressure dependent exchangemore » kinetics for CpIr(CO){sub 2} in K-Y zeolite. This has been attributed to an intrazeolite associative exchange mechanism which is enhanced by site 1 occupation. Similarly, fast room temperature CO substitution of ethylene from intrazeolite CpM(C{sub 2}H{sub 4}){sub 2} yields the dicarbonyl in the same sites and distributions as encountered for a freshly CpM(CO){sub 2} impregnated sample. The thermal properties of the entrapped organometallics have also been probed and the anchored half-sandwich products, CpM and Cp*M, tentatively identified.« less

Intrazeolite half-sandwich complexes of rhodium and iridium: spectroscopy and reactivity patterns

Intrazeolite Site-Selective Far-IR Cation Probe.

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