ABSTRACT
BACKGROUND: Polycyclic aromatic hydrocarbons (PAHs) with molecular mass 302 Da are the most investigated PAHs within the high molecular weight PAHs class. This PAH class contributes to a significant portion of the mutagenic and/or carcinogenic response associated to the PAH fraction present in environmental and combustion-related samples. Several reasons prevent the routine analysis of 302 Da PAHs in environmental samples, including large number of possible isomers, limited number of commercially available reference standards, and low concentration levels. RESULTS: These studies search for a newly synthetized dibenzo-fluoranthene of molecular mass 302 Da, namely dibenzo[b,l]fluoranthene, in a standard reference material (SRM 1597a) from the National Institute of Standards and Technology. The eluting behavior of dibenzo[b,l]fluoranthene is investigated under reversed-phase liquid chromatographic conditions for its determination via absorption and fluorescence detection. Vibrationally resolved spectra and fluorescence lifetimes recorded from octane matrices at 77 K and 4.2 K allow for its qualitative and quantitative analysis at the parts-per-trillion concentration levels. Its unambiguous determination is then reported for the first time in the SRM 1597a. SIGNIFICANCE AND NOVELTY: Of the 89 possible 302 Da PAH isomers, only 23 isomers have been identified in SRMs and/or environmental samples. The determination of dibenzo[b,l]fluoranthene in the SRM 1597a takes a step forward to fulfilling this gap.
ABSTRACT
J-dimer emission is an emergent property that occurs when pairs of ground state fluorophores associate, typically in a dilute solution medium. The resulting fluorescence is shifted with respect to the monomer. J-dimer emission, however, has never been observed in concentrated dispersions or in the solid state. We posited that multivariate (MTV) MOFs with double interwoven structures would help to isolate these dimers within their crystalline matrix. Using this strategy, J-aggregate density was controlled during crystallization by following a substitutional solid solution approach. Here, we identified the presence of J-dimers over the entire composition range for interwoven PIZOF-2/NNU-28 structures with variable amounts of a diethynyl-anthracene aggregate-forming link. We produced bulk crystals that systematically shifted their fluorescence from green to red with lifetimes (up to 13 ns) and quantum yields (up to 76%) characteristic of π-π stacked aggregates. Photophysical studies also revealed an equilibrium constant of dimerization, K D = 1.5 ± 0.3 M-1, enabling the first thermodynamic quantification of link-link interactions that occur during MOF assembly. Our findings elucidate the role that supramolecular effects play during crystallization of MTV MOFs, opening pathways for the preparation of solid-state materials with solution-like properties by design.
ABSTRACT
Systematically tuning the conductivity of metal-organic frameworks (MOFs) is key to synergizing their attractive synthetic control and porosity with electrochemical attributes useful in energy and sensing technologies. A priori control of charge transfer is possible by exploiting the solid-solution properties of MOFs together with electronic self-exchange enabled by redox pendants. Here we introduce a new strategy for preparing redox-active MOF thin-film electrodes with finely tuned redox pendant content. Varying the ratios of alkyl-ferrocene containing redox-active and inactive links during MOF synthesis enabled the fabrication of electrodes with tunable redox conductivity. The prepared MOF electrodes display maximum electron conductivity of 1.10 mS m-1, with crystallographic and electrochemical stability upon thousands of redox cycles. Electroanalytical studies demonstrated that the conductivity follows solution-like diffusion-controlled behavior with nonlinear electron diffusion coefficients consistent with charge hopping and percolation models of spatially fixed redox centers. Our studies create new prospects in the design and synthesis of redox-active MOFs with targeted properties for the design of advanced electrochemical devices.
ABSTRACT
A library of 12 dibenzo- and naphtho-fluoranthene polycyclic aromatic hydrocarbons (PAHs) with MW = 302 (C24H14) was synthesized via a Pd-catalyzed fluoranthene ring-closing reaction. By understanding the various modes by which the palladium migrates during the transformation, structural rearrangements were bypassed, obtaining pure PAHs in high yields. Spectroscopic and electrochemical characterization demonstrated the profound diversity in the electronic structures between isomers. Highlighting the significant differences in emission of visible light, this library of PAHs will enable their standardization for toxicological assessment and potential use as optoelectronic materials.
ABSTRACT
Covalent organic frameworks (COFs) usually crystallize as insoluble powders, and their processing for suitable devices is thought to be limited. We demonstrate that COFs can be mechanically pressed into shaped objects having anisotropic ordering with preferred orientation between hk0 and 00l crystallographic planes. Five COFs with different functionality and symmetry exhibited similar crystallographic behavior and remarkable stability, indicating the generality of this processing. Pellets prepared from bulk COF powders impregnated with LiClO4 displayed room temperature conductivity up to 0.26 mS cm(-1) and high electrochemical stability. This outcome portends use of COFs as solid-state electrolytes in batteries.
