José Veleta

The photoluminescence behavior of trivalent americium, Am(III), in mineral acid solutions is reported under direct-metal excitation at seven high-energy wavelengths (378–512 nm) in the ultraviolet and visible regions. This study revealed a dependence of the Am(III)-based visible emission profile on the chosen excitation wavelength. For example, excitation at the prominent absorption band at 504 nm (5L6′ ← 7F0′) produces the two expected emission bands in the visible region: one at approximately 600 nm (5D1′ → 7F0′) and another at 690 nm (5D1′ → 7F1′). When the excitation wavelength is shifted to 398 nm, targeting the 5D3′, 5H7′ ← 7F0′ transitions, the emission occurs almost exclusively at 600 nm. This behavior is unexpected and potentially highlights fundamental knowledge gaps in the current understanding of the intraconfigurational 5f–5f electronic transitions that give rise to Am(III) photoluminescence. Although a full interpretation of the results presented here are limited by the incomplete understanding of Am(III) speciation in mineral acid systems, this study explores several potential explanations for the observed behavior and challenges the widely held assumption of a single Am(III) emissive species in these systems.

We report the preparation and optical characterization of three solutions of Am(III) in common aqueous (aq.) mineral acids (HCl, HNO₃, and HBr), including absorption, emission, and Raman vibrational analyses. To our knowledge, this study provides the first detailed absorption spectra of Am(III) spanning the entire UV/vis/NIR region in these mineral acids reported since the 1960s. Using this high-resolution absorption data, provided in an open access format for the broader field, we build on prior work by Carnall and others to provide detailed optical analysis including all transition assignments. This work also includes the first reported absorption spectrum of Am(III) in aq. HBr. Characteristic Am(III) luminescence could be detected from all three samples. These are the first reports of emission from Am(III) in these acid systems, which build on prior emission reports collected from aq. HClO₄ solutions. The acquired emission spectra display anion-dependent shifting of the visible-region transitions by up to 21 nm from their predicted positions, akin to the nephelauxetic effect. The shift magnitude trends linearly according to acid pK_a and more generally according to the transition metal spectrochemical series. These ligand field effects, coupled with luminescence lifetime experiments, indicate that weak Am-X interactions persist in solution.

Frustration at carbon! Herein, we present a frustrated Lewis pair system derived from a water stable carbon-based Lewis acid, trioxatriangulene (TOTA⁺), and a variety of Lewis bases, which successfully promotes bond cleavage and molecule fixation.

We report the reactivity between the Lewis acidic trioxatriangulenium ion (TOTA+) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzed via variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe3, led to the formation of a stable Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P(tBu)3. The TOTA+-P(tBu)3 FLP was characterized as an encounter complex, and was found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of C–Br bond, and interception of Staudinger reaction intermediate. Moreover, TOTA+ and NHC was found to first undergo single-electron transfer (SET) to form [TOTA]•[NHC] •+, which was confirmed via electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC]+ adduct or a mixture of products depending the reaction condition used.

We report the reactivity between the Lewis acidic trioxotriangulenium ion (TOTA ⁺ ) and a series of Lewis bases such as phosphines and N -heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzed via variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe ₃ , led to the formation of a stable Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P( ^t Bu) ₃ . The TOTA ⁺ -P( ^t Bu) ₃ FLP was characterized as an encounter complex, and was found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of C–Br bond, and interception of Staudinger reaction intermediate. Moreover, TOTA ⁺ and NHC was found to first undergo single-electron transfer (SET) to form [TOTA] ^• [NHC] ^•+ , which was confirmed via electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC] ⁺ adduct or a mixture of products depending the reaction condition used.

Redox flow batteries (RFBs) represent a promising technology for grid-scale integration of renewable energy. Redox-active molecular pairs with large potential windows have been identified as key components of these systems. However, cross-contamination problems encountered by the use of different catholyte and anolyte species still limits the development of reliable organic RFBs. Herein, we report the first use of a helical carbenium ion, with three stable oxidation states, as electrolyte for the development of symmetric cells. Cyclic voltammo-amperometric studies were conducted in acetonitrile to assess the essential kinetic properties for flow battery performance and cycling stability of this molecule. The selected [4]helicenium ion was then evaluated by using mono- and bi-electronic cycling experiments, resulting in 745 and 80 cycles respectively, with near-perfect capacity retention. This helical carbenium ion based electrolyte achieved a proof-of-principle 2.12 V open circuit potential as an all-organic symmetric RFB.

Persistent organic radicals have gained considerable attention in the fields of catalysis and materials science. In particular, helical molecules are of great interest for the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of easily tunable and stable neutral quinolinoacridine radicals under anaerobic conditions by chemical reduction of their quinolinoacridinium cation analogs. The structures of these [4]helicene radicals were determined by X-ray crystallography. Density functional theory (DFT) calculations, supported by electron paramagnetic resonance (EPR) measurements, indicate that over 40% of spin density is located at the central carbon of our [4]helicene radicals regardless of their structural modifications. The localization of the charge promotes a reversible oxidation to the cation upon exposure to air. This unusual reactivity toward molecular oxygen was monitored via UV-Vis spectroscopy.

Redox flow batteries (RFBs) represent a promising technology for grid-scale integration of renewable energy. Redox-active molecular pairs with large potential windows have been identified as key components of these systems. However, cross-contamination problems encountered by the use of different catholyte and anolyte species still limits the development of reliable organic RFBs. Herein, we report the first use of a helical carbenium ion, with three stable oxidation states, as electrolyte for the development of symmetric cells. Cyclic voltammo-amperometric studies were conducted in acetonitrile to assess the essential kinetic properties for flow battery performance and cycling stability of this molecule. The selected [4]helicenium ion was then evaluated by using mono- and bi-electronic cycling experiments, resulting in 745 and 80 cycles respectively, with near-perfect capacity retention. This helical carbenium ion based electrolyte achieved a proof-of-principle 2.12 V open circuit potential as an all-organic symmetric RFB.

Persistent organic radicals have gained considerable attention in the fields of catalysis and material sciences. In particular, helical molecules are of great interest for the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of easily tunable and stable neutral quinolinoacridine radicals under anaerobic conditions by chemical reduction of their quinolinoacridinium cation analogs. The structures of these [4]helicene radicals were determined by X-ray crystallography. Density functional theory (DFT) calculations, supported by Electron paramagnetic resonance (EPR) measurements, indicate that over 40% of spin density is located at the central carbon of the [4]helicene radicals regardless of their structural modifications. The localization of the charge promotes a reversible oxidation to the cation upon exposure to air. This unusual reactivity toward molecular oxygen was monitored via UV-Vis spectroscopy.

Stable organic radicals have gained considerable attention in the fields of catalysis and material sciences. In particular, helical molecules are of great interest in the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of highly stable neutral quinolinoacridine radicals by chemical reduction of their quinolinoacridinium cation analogs. The crystal structures of these [4]helicene radicals were determined by X-ray diffraction. Electron paramagnetic resonance (EPR) measurements, supported by density functional theory (DFT) calculations, indicate that the unpaired electron is mostly localized, showing more than 40% of spin density located at the central carbon of the [4]helicene radicals. Quantitative conversion from neutral radical to cation is observed upon exposure to air, monitored via UV-vis spectroscopy. The successful photoreductive dehalogenation of aryl halides occurs in the presence of 10 mol% of [4]helicene radical under blue light.