Benzobis(imidazolium)-cucurbit[8]uril complexes for binding and sensing aromatic compounds in aqueous solution

The utilities of benzobis(imidazolium) salts (BBIs) as stable and fluorescent components of supramolecular assemblies involving the macrocyclic host, cucurbit[8]uril (CB[8]), are described. CB[8] has the unusual ability to bind tightly and selectively to two different guests in aqueous media, typically methyl viologen (MV) as the first guest, followed by an indole, naphthalene, or catechol-containing second guest. Based on similar size, shape, and charge, tetramethyl benzobis(imidazolium) (MBBI) was identified as a potential alternative to MV that would increase the repertoire of guests for cucurbit[8]uril. Isothermal titration calorimetry (ITC) studies showed that MBBI binds to CB[8] in a 1:1 ratio with an equilibrium association constant (K(a)) value of 5.7×10(5) M(-1), and that the resulting MBBI·CB[8] complex binds to a series of aromatic second guests with K(a) values ranging from 10(3) to 10(5) M(-1). These complexation phenomena were supported by mass spectrometry, which confirmed complex formation, and a series of NMR studies that showed the expected upfield perturbation of aromatic peaks and of the MBBI methyl peaks. Surprisingly, the binding behavior of MBBI is strikingly similar to that of MV, and yet MBBI offers a number of substantial advantages for many applications, including intrinsic fluorescence, high chemical stability, and broad synthetic tunability. Indeed, the intense fluorescence emission of the MBBI·CB[8] complex was quenched upon binding to the second guests, thus demonstrating the utility of MBBI as a component for optical sensing. Building on these favorable properties, the MBBI·CB[8] system was successfully applied to the sequence-selective recognition of peptides as well as the controlled disassembly of polymer aggregates in water. These results broaden the available guests for the cucurbit[n]uril family and demonstrate potentially new applications.     

Fast oscillations in an optical parametric oscillator

We report on the observation of fast oscillations at frequencies of a few MHz in a triply resonant optical parametric oscillator. These oscillations can appear alone, or superimposed on slow oscillations due to thermo-optical instabilities, and display a great variety of waveforms. The analysis of the regimes observed experimentally leads us to conjecture that the mechanism responsible for this instability is not the Hopf bifurcation of the single-mode mean-field model, but that it is based on the interaction of two signal fields oscillating in cavity modes with neighboring frequencies. This interpretation is supported by numerical simulations of the mean-field model with two coupled modes, which reproduce well the behaviors observed experimentally. We also find chaotic solutions of this model, which unveils another possible scenario leading to deterministic chaos in this system.     

Mechanically Modulating the Photophysical Properties of Fluorescent Protein Biocomposites for Ratio‐ and Intensiometric Sensors

Mechanically sensitive biocomposites comprised of fluorescent proteins report stress through distinct pathways. Whereas a composite containing an enhanced yellow fluorescent protein (eYFP) exhibited hypsochromic shifts in its fluorescence emission maxima following compression, a composite containing a modified green fluorescent protein (GFPuv) exhibited fluorescence quenching under the action of mechanical force. These ratio‐ and intensiometric sensors demonstrate that insights garnered from disparate fields (that is, polymer mechanochemistry and biophysics) can be harnessed to guide the rational design of new classes of biomechanophore‐containing materials.     

N-heterocyclic carbene enabled synthesis of conjugated polymers

The Suzuki polycondensation of a dihalogenated 4,5-diphenylimidazole (2) with a fluorenyl diboronic acid diester followed by methylation afforded a conjugated poly(imidazolium) copolymer (P2) in 93% yield. Upon exposure to strong base, P2 was converted in situ to the corresponding poly(N-heterocyclic carbene) P3, as evidenced by ¹H NMR spectroscopy and a trapping experiment involving sulfur that afforded the corresponding poly(thiourea) P4. Similarly, treating a solution of P2 with KOtBu and [Ir(1,5-cyclooctadiene)Cl]₂ afforded a conjugated polymer bearing pendant Ir complexes (P5) in 63% yield. Thermal and photophysical analyses of the aforementioned polymers revealed that they were thermally stable with tunable fluorescence properties, features which poise them for use in various electronic and sensing applications. The presented methodology is expected to facilitate the synthesis of a broad range conjugated organometallic polymers from a common and readily accessible precursor.     

Atom Transfer Radical Polymerization in the Solid‐State

Poly(2‐vinylnaphthalene) was synthesized in the solid‐state by ball milling a mixture of the corresponding monomer, a Cu‐based catalyst, and an activated haloalkane as the polymerization initiator. Various reaction conditions, including milling time, milling frequency and added reductant to accelerate the polymerization were optimized. Monomer conversion and the evolution of polymer molecular weight were monitored over time using ¹H NMR spectroscopy and size exclusion chromatography, respectively, and linear correlations were observed. While the polymer molecular weight was effectively tuned by changing the initial monomer‐to‐initiator ratio, the experimentally measured values were found to be lower than their theoretical values. The difference was attributed to premature mechanical decomposition and modeled to accurately account for the decrement. Random copolymers of two monomers with orthogonal solubilities, sodium styrene sulfonate and 2‐vinylnaphthalene, were also synthesized in the solid‐state. Inspection of the data revealed that the solid‐state polymerization reaction was controlled, followed a mechanism similar to that described for solution‐state atom transfer radical polymerizations, and may be used to prepare polymers that are inaccessible via solution‐state methods.     

A systematic study of stereochemical effects in homologous poly(alkenamer)s: Dewar benzene versus norbornene

Two diastereomeric derivatives of norbornene, dimethyl (1R,2R,3S,4S)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate and dimethyl (1R,2S,3S,4S)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate, were synthesized and polymerized using ring-opening metathesis polymerization (ROMP). For comparative purposes, diastereomeric derivatives of Dewar benzene, dimethyl (1R,2S,3R,4S)-bicyclo[2.2.0]hex-5-ene-2,3-dicarboxylate and dimethyl (1R,2S,3S,4S)-bicyclo[2.2.0]hex-5-ene-2,3-dicarboxylate, were also synthesized and polymerized using ROMP. The polymerization reactions proceeded in a controlled manner as evidenced in part by linear relationships between the monomer-to-catalyst feed ratios and the molecular weights of the polymer products. Chain extension experiments were also conducted which facilitated the formation of block copolymers. Although the poly(norbornene) derivatives exhibited glass transition temperatures that were dependent on their monomer stereochemistry (cis: 115°C vs. trans: 125°C), more pronounced differences were observed upon analysis of the polymers derived from Dewar benzene (cis: 70°C vs. trans: 95°C). Likewise, microphase separation was observed in block copolymers that were prepared using the diastereomeric monomers derived from Dewar benzene but not in block copolymers of the norbornene-based diastereomers. The differential thermal properties were attributed to the relative monomer sizes as reducing the distances between the polymer backbones and the pendant stereocenters appeared to enhance the thermal effects.     

Mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors

The performance of mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors (EDLCs) was evaluated in the presence of a variety of electrolytes, including room temperature ionic liquids (ILs).