Synthesis and characterization of luminescent bis-cyclometalated platinum(IV) complexes

Presented is the synthesis and characterization of a series of luminescent heteroleptic bis-cyclometalated platinum(IV) complexes. An oxidation-facilitated cyclometalation is employed to convert platinum(II) pendant species into bis-cyclometalated platinum(IV) dichlorides, which are transformed into the tris-chelated diimine complexes through ligand substitution. The structure-property relationship is probed by judiciously varying substituents on both the C(∧)N and the N(∧)N ligands resulting in a family of complexes exhibiting blue emission, long excited-state lifetimes, and highly efficient oxygen quenching. Excited-state properties are corroborated by static and time-dependent density-functional theory calculations of both the singlet and the triplet state.     

Oxygen reduction activity dependence on the mesoporous structure of imprinted carbon supports

Two mesoporous carbons (with 15 (CIC-15) and 26 nm (CIC-26) diameter pores) were synthesized using a silica colloid imprinting method, loaded with 10 wt.% Pt, and then evaluated (against Vulcan^? carbon (VC)) as oxygen reduction (ORR) catalysts for use in proton exchange membrane fuel cells. Both Pt/CICs reproducibly out-performed Pt/VC, with Pt/CIC-15 demonstrating higher ORR activity than Pt/CIC-26, despite its smaller pore size and lower surface area. Transmission electron tomography showed that the Pt nanoparticles (4–5 nm diameter) are fully deposited throughout the pores of the CICs and that the pore distribution in CIC-26 is partially ordered, while CIC-15 shows no ordering of its pores. Importantly, using the powerful imaging capabilities of transmission electron tomography, a first-time correlation is demonstrated between the ORR activity and the wall thickness of the carbon support materials. Pt/CIC-15 has significantly thicker walls, giving a lower measured electronic resistance, a lower ORR Tafel slope, and thus better performance overall compared to Pt/CIC-26.     

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.     

Flexible and stretchable electrodes for next generation polymer electronics: a review

Transparent conductive electrodes play a significant role in the fabrication and development of optoelectronic devices. As next generation optoelectronic devices tend towards mobile and wearable devices, the added attribute of flexibility or stretchability for these electrodes becomes increasingly important. However, mechanical requirements aside, transparent conductive electrodes must still retain high transparency and conductivity, with the metrics for these parameters being compared to the standard, indium tin oxide. In the search to replace indium tin oxide, two materials that have risen to the forefront are carbon nanotubes and silver nanowires due to their high transparency, conductivity, mechanical compliance, and ease of fabrication. This review highlights recent innovations made by our group in electrodes utilizing carbon nanotubes and silver nanowires, in addition to the use of these electrodes in discrete devices and integrated systems.     

From Ion‐Like Ethylzinc Aluminates to [EtZn(arene)2]+[Al(ORF)4]− Salts

Attempts to prepare previously unknown simple and very Lewis acidic [RZn]⁺[Al(OR^F)₄]^? salts from ZnR₂, AlR₃, and HO?R^F delivered the ion‐like RZn(Al(OR^F)₄) (R=Me, Et; R^F=C(CF₃)₃) with a coordinated counterion, but never the ionic compound. Increasing the steric bulk in RZn⁺ to R=CH₂CMe₃, CH₂SiMe₃, or Cp*, thus attempting to induce ionization, failed and led only to reaction mixtures including anion decomposition. However, ionization of the ion‐like EtZn(Al(OR^F)₄) compound with arenes yielded the [EtZn(arene)₂]⁺[Al(OR^F)₄]^? salts with arene=toluene, mesitylene, or o‐difluorobenzene (o‐DFB)/toluene. In contrast to the ion‐like EtZn(η³‐C₆H₆)(CHB₁₁Cl₁₁), which co‐crystallizes with one benzene molecule, the less coordinating nature of the [Al(OR^F)₄]^? anion allowed the ionization and preparation of the purely organometallic [EtZn(arene)₂]⁺ cation. These stable materials have further applications as, for example, initiators of isobutene polymerization. DFT calculations to compare the Lewis acidities of the zinc cations to those of a large number of organometallic cations were performed on the basis of fluoride ion affinity. The complexation energetics of EtZn⁺ with arenes and THF was assessed and related to the experiments.     

On the swelling behavior of poly(N-Isopropylacrylamide) hydrogels exposed to perfluoroalkyl acids

Per- and polyfluoroalkyl substances (PFAS) have rapidly accumulated in the environment due to their widespread use prior to commercial discussion in the early 21st century, and their slow degradation has magnified concerns of their potential toxicity. Monitoring their distribution is, therefore, necessary to evaluate and control their impact on the health of exposed populations. This investigation evaluates the capability of a simple polymeric detection scheme for PFAS based on crosslinked, thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels. Surveying swelling perturbations induced by several hydrotropes and comparable hydrocarbon analogs, tetraethylammonium perfluorooctane sulfonate (TPFOS) showed a significantly higher swelling ratio on a mass basis (65.5 ± 8.8 at 15°C) than any of the other analytes tested. Combining swelling with the fluorimetric response of a solvachromatic dye, nile red, revealed the fluorosurfactant to initiate observable aggregation (i.e., its critical aggregation concentration) at 0.05 mM and reach saturation (i.e., its charge neutralization concentration) at 0.5 mM. The fluorosurfactant was found to homogeneously distribute throughout the polymer matrix with energy dispersive X-ray spectroscopy, marking the swelling response as a peculiar nexus of fluorinated interfacial positioning and delocalized electrostatic repulsion. Results from the current study hold promise for exploiting the physiochemical response of PNIPAM to assess TPFOS's concentration.     

Limit points badly approximable by horoballs

For a proper, geodesic, Gromov hyperbolic metric space X, a discrete subgroup of isometries Γ whose limit set is uniformly perfect, and a disjoint collection of horoballs {H _j }, we show that the set of limit points badly approximable by {H _j } is absolutely winning in the limit set Λ(Γ). As an application, we deduce that for a geometrically finite Kleinian group acting on ${\mathbb{H}^{n+1}}$ , the limit points badly approximable by parabolics, denoted BA(Γ), is absolutely winning, generalizing previous results of Dani and McMullen. As a consequence of winning, we show that BA(Γ) has dimension equal to the critical exponent of the group. Since BA(Γ) can alternatively be described as the limit points representing bounded geodesics in the quotient ${\mathbb{H}^{n+1}/\Gamma}$ , we recapture a result originally due to Bishop and Jones.     

The Diffie–Hellman problem and generalization of Verheul’s theorem

Bilinear pairings on elliptic curves have been of much interest in cryptography recently. Most of the protocols involving pairings rely on the hardness of the bilinear Diffie–Hellman problem. In contrast to the discrete log (or Diffie–Hellman) problem in a finite field, the difficulty of this problem has not yet been much studied. In 2001, Verheul (Advances in Cryptology—EUROCRYPT 2001, LNCS 2045, pp. 195–210, 2001) proved that on a certain class of curves, the discrete log and Diffie–Hellman problems are unlikely to be provably equivalent to the same problems in a corresponding finite field unless both Diffie–Hellman problems are easy. In this paper we generalize Verheul’s theorem and discuss the implications on the security of pairing based systems.      

Cyclometalated platinum(II) diimine complexes: synthetically tuning the photophysical and electrochemical properties

Presented is the synthesis of an array of 16 heteroleptic phosphorescent diimine complexes of platinum(II) with electronically diverse ligand spheres and their full spectroscopic, photophysical, and electrochemical characterization. The complexes were found to exhibit luminescence (480-500 nm) in deaerated solutions at room temperature from a long-lived (3)LC state (τ = 2-3 μs) that exhibits significant metal character perturbed by a low-lying (1)MLCT state. Interestingly, emission from a (3)MLCT state was not observed as is the case with many other polypyridine-based d-block complexes. Electrochemical intermediates proved stable as multiple reversible reductions between -1 and -2 V vs. SCE were noted during cyclic voltammetry experiments unveiling the potential of these luminophores for use in a variety of optoelectronic and solar energy conversion applications.