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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Photophysical Properties and Electronic Structure of Porphyrins Bearing Zero to Four meso -Phenyl Substituents: New Insights into Seemingly Well Understood Tetrapyrroles

Six free base porphyrins bearing 0–4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S 1 ) decreases with an increase in the number of meso-phenyl groups; the values in toluene are H 2 P-0 (15.5 ns) > H 2 P-1 (14.9 ns) > H 2 P-2c (14.4 ns) > H 2 P-2t (13.8 ns) ~ H 2 P-3 (13.8 ns) > H 2 P-4 (12.8 ns), where “H 2 P” refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and “c” and “t” refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are H 2 P-0 (0.049) < H 2 P-1 (0.063) ~ H 2 P-2c (0.063) < H 2 P-2t (0.071) < H 2 P-3 (0.073) < H 2 P-4 (0.090). Similar trends occur in DMF. All radiative and non-radiative (internal conversion and intersystem crossing) rate constants for S 1 decay increase with increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S o → S 1 absorption manifold. Here, the trend is reproduced by time-dependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S o → S 1 radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S o → S 1 absorption wavelength. Polarizations of the S 1 and S 2 excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy and life-sciences research.

14 SOLAR ENERGY↗
DOE OSTI2023

Well-Defined Iron Sites in Crystalline Carbon Nitride

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula C 6 N 9 H 2 Fe 0.4 Li 1.2 Cl, abbreviated PTI/FeCl 2 ) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl 2 /KCl flux, followed by anaerobic rinsing with methanol. X-ray diffraction, X-ray absorption and Mössbauer spectroscopies, and SQUID magnetometry indicate that there are tetrahedral high-spin iron(II) sites throughout the material, all having the same geometry. As a result, the material is active for electrocatalytic nitrate reduction to ammonia, with a production rate of ca. 0.1 mmol cm –2 h –1 and Faradaic efficiency of ca. 80% at −0.80 V vs RHE.

Anions↗
DOE OSTI2022

Rapid atom-efficient polyolefin plastics hydrogenolysis mediated by a well-defined single-site electrophilic/cationic organo-zirconium catalyst

Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H 2 with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH 2 units)·mol(Zr) –1 ·h –1 at 200 °C/2 atm H 2 pressure. Under similar solventless conditions, polyethylene-co–1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß-alkyl transfer rather than the more common σ-bond metathesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗