Influence of connectivity and porosity on ligand-based luminescence in zinc metal-organic frameworks

Applications of metal-organic frameworks (MOFs) require close correlation between their structure and function. We describe the preparation and characterization of two zinc MOFs based on a flexible and emissive linker molecule, stilbene, which retains its luminescence within these solid materials. Reaction of trans-4,4'-stilbene dicarboxylic acid and zinc nitrate in N,N-dimethylformamide (DMF) yielded a dense 2-D network, 1, featuring zinc in both octahedral and tetrahedral coordination environments connected by trans-stilbene links. Similar reaction in N,N-diethylformamide (DEF) at higher temperatures resulted in a porous, 3-D framework structure, 2. This framework consists of two interpenetrating cubic lattices, each featuring basic zinc carboxylate vertices joined by trans-stilbene, analogous to the isoreticular MOF (IRMOF) series. We demonstrate that the optical properties of both 1 and 2 correlate with the local ligand environments observed in the crystal structures. Steady-state and time-resolved spectroscopic measurements reveal that the stilbene linkers in the dense structure 1 exhibit a small degree of interchromophore coupling. In contrast, the stilbenoid units in 2 display very little interaction in this low-density 3-D framework, with excitation and emission spectra characteristic of monomeric stilbenes, similar to the dicarboxylic acid in dilute solution. In both cases, the rigidity of the stilbene linker increases upon coordination to the inorganic units through inhibition of torsion about the central ethylene bond, resulting in luminescent crystals with increased emission lifetimes compared to solutions of trans-stilbene. The emission spectrum of 2 is found to depend on the nature of the incorporated solvent molecules, suggesting use of this or related materials in sensor applications.     

Photolytic interference affecting two-photon laser-induced fluorescence detection of atomic oxygen in hydrocarbon flames

This study reports on photochemical interferences affecting atomic oxygen detection using two-photon laser-induced fluorescence at 226 nm. In contrast to previous studies in which molecular oxygen was proven to be the relevant photochemical precursor molecule in a hydrogen-fueled flame, the present investigations were carried out in a laminar diffusion flame of methane and air. The most significant interferences were found at the fuel side of the flame in the absence of molecular oxygen, and vibrationally excited carbon dioxide was identified as the most probable precursor molecule for the photochemical production of oxygen atoms. Received: 11 December 2002 / Revised version: 10 March 2003 / Published online: 16 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-925/294-2595, E-mail: tbsette@sandia.gov     

Time-, wavelength-, and polarization-resolved measurements of OH (A 2Σ+ ) picosecond laser-induced fluorescence in atmospheric- pressure flames

Laser-induced fluorescence of OH (A ²Σ⁺, v’=1) was measured in hydrogen/oxygen and hydrogen/air/nitrogen flames using laser pulses of 80 psec duration. A 2D signal acquisition scheme simultaneously employed wavelength, temporal, and polarization resolution. The signals emitted in different rotational branches exhibit polarization-dependent intensities, depending on the rotational branch of the absorption line used. It is possible to select experimental conditions such that rotational and vibrational relaxation as well as electronic quenching can be monitored simultaneously. Advantages and limitations of the experimental approach are discussed. Numerical simulations are presented of the LIF spectra affected by energy transfer. Received: 29 March 1999 / Revised version: 14 June 1999 / Published online: 27 October 1999     

Temperature- and species-dependent quenching of NO A 2Sigma+(v'=0) probed by two-photon laser-induced fluorescence using a picosecond laser

We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A 2Sigma+(v'=0) state of NO. Cross sections were measured for gas temperatures ranging from 294 to 1300 K for quenching by NO(X (2)Pi), H(2)O, CO(2), O(2), CO, N(2), and C(2)H(2). The A 2Sigma+(v'=0) state was populated via two-photon excitation with a picosecond laser at 454 nm, and the decay rate of the fluorescence originating from A 2Sigma+(v'=0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H(2)O, CO(2), and O(2) at temperatures less than 1300 K.     

Fixed-frequency cavity ringdown diagnostic for atmospheric particulate matter

A nonresonant cavity ringdown diagnostic to measure light attenuation from atmospheric particulate matter at 532- and 355-nm wavelengths is described. The presence of atmospheric particulate is clearly detectable with this technique, as demonstrated by experimental results. The extinction cross section is higher at 355 than at 532 nm, although we were able to purchase significantly higher-reflectivity optics at 532 nm. The expected advantage at 355 nm is thus lost. This new technique is compared with a commercially available instrument, and sensitivity limitations are discussed.     

Analysis of 205-nm photolytic production of?atomic hydrogen in?methane flames

We investigate the 205-nm photolytic production of atomic hydrogen in methane flames. This process represents a significant interference in two-photon, laser induced-fluorescence (TP-LIF) detection of atomic hydrogen in flames. Relative TP-LIF profiles of the photolytically produced H atoms were measured using a pump-probe technique in atmospheric-pressure, premixed CH₄/O₂/N₂ flames. A high-fluence, non-resonant, nanosecond pump laser created H atoms by photodissociating flame constituents, and a copropagating, non-perturbing picosecond laser probed the photolytically produced Hatoms via TP-LIF. Spatial profiles of photolytically produced H atoms indicate that both intermediate and product species contribute to the interference in all flames. Excellent agreement between simulated and measured interference signals is observed in the product region of the flames. Vibrationally excited H₂O is the dominant source of interference in the product region, but an additional contribution is attributed to vibrationally excited OH radicals. In the flame-front region, CH₃ is the dominant precursor, and photodissociation of C₂H₂ becomes increasingly important in rich flames. Mechanisms for sequential photodissociation of CH₃ and C₂H₂ are presented, indicating that complete dissociation at 205 nm of both precursors is feasible.     

Laser imaging system for determination of three-dimensional scalar gradients in turbulent flames

An imaging system for the measurement of three-dimensional (3D) scalar gradients in turbulent hydrocarbon flames is described. Combined line imaging of Raman scattering, Rayleigh scattering, and CO laser-induced fluorescence (LIF) allows for simultaneous single-shot line measurements of major species, temperature, mixture fraction, and a one-dimensional surrogate of scalar dissipation rate in hydrocarbon flames, while simultaneous use of two crossed, planar LIF measurements of OH allows for determination of instantaneous flame orientation. In this manner the full 3D scalar dissipation can be estimated in some regions of a turbulent flame on a single-shot basis.     

Stability and activity of carbon nanofiber-supported catalysts in the aqueous phase reforming of ethylene glycol

Nickel, cobalt, copper and platinum nanoparticles supported on carbon nano-fibers were evaluated with respect to their stability, catalytic activity and selectivity in the aqueous phase reforming of ethylene glycol (230 C, autogenous pressure, batch reactor). The initial surface-specific activities for ethylene glycol reforming were in a similar range but decreased in the order of Pt (15.5 h1 ) >Co(13.0 h1 ) >Ni(5.2 h1 ) while the Cu catalyst only showed low dehydrogenation activity. The hydrogen molar selectivity decreased in the order of Pt (53%)>Co(21%)>Ni (15%) as a result of the production of methane over the latter two catalysts. Over the Co catalyst acids were formed in the liquid phase while alcohols were formed over Ni and Pt. Due to the low pH of the reaction mixture, especially in the case of Co (as a result of the formed acids), significant cobalt leaching occurs which resulted in a rapid deactivation of this catalyst. Investigations of the spent catalysts with various techniques showed that metal particle growth is responsible for the deactivation of the Pt and Ni catalysts. In addition, coking might also contribute to the deactivation of the Ni catalyst.