Nanothermometer Based on Polychlorinated Trityl Radicals Showing Two-Photon Excitation and Emission in the Biological Transparency Window: Temperature Monitoring of Biological Tissues

Nanothermometers are emerging probes as biomedical diagnostic tools. Especially appealing are nanoprobes using NIR light in the range of biological transparency window (BTW) since they have the advantages of a deeper penetration into biological tissues, better contrast, reduced phototoxicity and photobleaching. This article reports the preparation and characterization of organic nanoparticles (ONPs) doped with two polychlorinated trityl radicals (TTM and PTM), as well as studies of their electronic and optical properties. Such ONPs having inside isolated radical molecules and dimeric excimers, can be two-photon excited showing optimal properties for temperature sensing. Remarkably, in TTM-based ONPs the emission intensity of the isolated radical species is unaltered increasing temperature, while the excimer emission intensity decreases strongly being thereby able to monitor temperature changes with an excellent thermal absolute sensitivity of 0.6-3.7% K-1 in the temperature range of 278-328 K. The temperature dependence of the excimeric bands of ONPs are theoretically simulated by using electronic structure calculations and a vibronic Hamiltonian model. Finally, TTM-doped ONPs as ratiometric NIR-nanothermometers are tested with two-photon excitationwith enucleated pig eye sclera, as a real tissue model, obtaining a similar temperature sensitivity as in aqueous suspensions, demonstrating their potential as NIR nanothermometers for bio applications.

Exploring the Luminescence, Redox, and Magnetic Properties in a Multivariate Metal-Organic Radical Framework

Persistent neutral organic radicals are excellent building blocks for the design of functional molecular materials due to their unique electronic, magnetic, and optical properties. Among them, triphenylmethyl radical derivatives have attracted a lot of interest as luminescent doublet emitters. Although neutral organic radicals have been underexplored as linkers for building metal-organic frameworks (MOFs), they hold great potential as organic elements that could introduce additional electronic properties within these frameworks. Herein, we report the synthesis and characterization of a novel multicomponent metal-organic radical framework (PTMTCR@NR-Zn MORF), which is constructed from the combination of luminescent perchlorotriphenylmethyl tricarboxylic acid radical (PTMTCR) and nonemissive nonradical (PTMTCNR) organic linkers and Zn(II) ions. The PTMTCR@NR-Zn MORF structure is layered with microporous one-dimensional channels embedded within these layers. Kelvin probe force microscopy further confirmed the presence of both organic nonradical and radical linkers in the framework. The luminescence properties of the PTMTCR ligand (first studied in solution and in the solid state) were maintained in the radical-containing PTMTCR@NR-Zn MORF at room temperature as fluorescence solid-state quenching is suppressed thanks to the isolation of the luminescent radical linkers. In addition, magnetic and electrochemical properties were introduced to the framework due to the incorporation of the paramagnetic organic radical ligands. This work paves the way for the design of stimuli-responsive hybrid materials with tunable luminescence, electrochemical, and magnetic properties by the proper combination of closed- and open-shell organic linkers within the same framework.

Dark State of the Thiele Hydrocarbon: Efficient Solvatochromic Emission from a Nonpolar Centrosymmetric Singlet Diradicaloid

In this work, a comprehensive investigation of the photoinduced processes and mechanisms linked to the luminescence of a novel nonperchlorinated Thiele hydrocarbon (TTH) is presented. Despite the comparable diradical character of TTH (y0 = 0.32-0.44) and the unsubstituted Thiele hydrocarbon (TH) (y0 = 0.30), the polyhalogenated species is inert and photostable, showing an intense deep-red/near-infrared (NIR) fluorescence (photoluminescence quantum yield (PLQY) = 0.84 in toluene) even at room temperature and in the solid state (PLQY = 0.19). TTH displays a large Stokes shift (307 nm in benzonitrile) and solvatochromic behavior, which is unusual for a centrosymmetric, nonpolar, and low-conjugated species. These outstanding emission features are interpreted through quantum-chemical calculations, indicating that its fluorescence arises from the low-lying dark doubly excited zwitterionic state, typically found at low excitation energies in diradicaloids, acquiring dipole moment and intensity by state mixing via twisting around the strongly elongated exocyclic CC bonds of the excited p-quinodimethane (pQDM) core, with a mechanism similar to sudden polarization occurring in olefins. Such a mechanism is derived from ns and fs transient absorption measurements.

Unraveling the Reaction Interfaces and Intermediates of Ru-Catalyzed LiOH Decomposition in DMSO-Based Li-O2 Batteries

Investigation of LiOH decomposition in nonaqueous electrolytes not only expands the fundamental understanding of four-electron oxygen evolution reactions in aprotic media but also is crucial to the development of high-performance lithium-air batteries involving the formation/decomposition of LiOH. In this work, we have shown that the decomposition of LiOH by ruthenium metal catalysts in a wet DMSO electrolyte occurs at the catalyst-electrolyte interface, initiated via a potential-triggered dissolution/reprecipitation process. The in situ UV-vis methodology devised herein provides direct experimental evidence that the hydroxyl radical is a common reaction intermediate formed in several nonaqueous electrolytes; this method is applicable to study other battery systems. Our results highlight that the reactivity of the hydroxyl radical toward nonaqueous electrolyte represents a major factor limiting O₂ evolution during LiOH decomposition. Coupling catalysts restraining hydroxyl reactivity with electrolytes more resistant to hydroxyl radical attack could help improve the reversibility of this reaction.

OUP accepted manuscript

Aprotic lithium–oxygen (Li–O₂) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li–O₂ batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li–O₂ batteries and are regarded as an advanced solution. To fully unlock the capability of Li–O₂ batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li–O₂ batteries.

Revealing the catalytic pathway of a quinone-mediated oxygen reduction reaction in aprotic Li-O2 batteries

Soluble redox species that facilitate the oxygen reduction reaction by mediating the LiO₂ intermediate and consequently the formation of the Li₂O₂ have attracted considerable interest for Li-O₂ batteries. Based on extensive radical studies, this work discloses a distinct solution reaction route when a quinone derivative was employed as a redox mediator.

Organic Polyradicals as Redox Mediators: Effect of Intramolecular Radical Interactions on Their Efficiency

The spin-spin interactions between unpaired electrons in organic (poly)radicals, especially nitroxides, are largely investigated and are of crucial importance for their applications in areas such as organic magnetism, molecular charge transfer, or multiple spin labeling in structural biology. Recently, 2,2,6,6-tetramethylpiperidinyloxyl and polymers functionalized with nitroxides have been described as successful redox mediators in several electrochemical applications; however, the study of spin-spin interaction effect in such an area is absent. This communication reports the preparation of a novel family of discrete polynitroxide molecules, with the same number of radical units but different arrangements to study the effect of intramolecular spin-spin interactions on their electrochemical potential and their use as oxidation redox mediators in a Li-oxygen battery. We find that the intensity of interactions, as measured by the d₁/d electron paramagnetic resonance parameter, progressively lowers the reduction potential. This allows us to tune the charging potential of the battery, optimizing its energy efficiency.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Current Challenges and Routes Forward for Nonaqueous Lithium-Air Batteries

Nonaqueous lithium-air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.

Promoting Li-O2 Batteries With Redox Mediators

Li-O₂ batteries have a high theoretical specific energy, 3500 Wh kg^-1 ; however, its practical capacity is far below this value and limited by the passivation with the insulating discharge product Li₂ O₂ . The nonconductive nature of Li₂ O₂ also impedes the charging process, leading to a low coulombic efficiency and high overpotential on charge even at a moderate rate. To address these challenges, redox mediators could be used both during discharge and charge to transfer electrons between O₂ /electrode surface or Li₂ O₂ /electrode surface to overcome the passivation of Li₂ O₂ , which would facilitate the discharge and charge process. The capacity and current density were significantly improved using the redox mediators, thus representing a promising strategy to achieve a high energy density for Li-O₂ batteries.

Molecular Sieve Induced Solution Growth of Li2O2 in the Li-O2 Battery with Largely Enhanced Discharge Capacity

The formation of the insulated film-like discharge products (Li₂O₂) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li-O₂ battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li-O₂ battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li₂O₂ as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li₂O₂ with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li-O₂ battery by adding the inorganic additives.

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O₂ batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H₂O on the oxygen chemistry in a nonaqueous Li-O₂ battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li⁺). When water and the quinone are used together in a (largely) nonaqueous Li-O₂ battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li₂O₂, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li₂O₂ crystals can grow up to 30 μm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O₂ by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li⁺ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O₂ battery is obtained.

Functional and stability orientation synthesis of materials and structures in aprotic Li–O2batteries

This review presents the recent advances made in the functional and stability orientation synthesis of materials/structures for Li–O₂batteries.

Using polyoxometalates to enhance the capacity of lithium–oxygen batteries

Improving Li–O₂ batteries with the highly stable Keggin type-polyoxometalate α-SiW₁₂O₄₀⁴⁻.

Excimers from stable and persistent supramolecular radical-pairs in red/NIR-emitting organic nanoparticles and polymeric films

For the first time, using a carbon free-radical, excimeric emission from stable and persistent supramolecular radical-pairs has been observed.