Photoreduction of 9,10-Anthraquinone Derivatives: Transient Spectroscopy and Effects of Alcohols and Amines on Reactivity in Solution¶

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide

This study addresses the fundamental processes that drive the photomineralization of dissolved organic matter (DOM) to carbon dioxide (CO2), deconvoluting the role of direct and sensitized photolysis. Here, a suite of DOM isolates and model compounds were exposed to simulated sunlight in the presence of various physical and chemical quenchers to assess the magnitude, rate, and extent of direct and sensitized photomineralization to CO2. Results suggest that CO2 formation occurs in a biphasic kinetic system, with fast production occurring within the first 3 h, followed by slower production thereafter. Notably, phenol model chromophores were the highest CO2 formers and, when conjugated with carboxylic functional groups, exhibited a high efficiency for CO2 formation relative to absorbed light. Simple polycarboxylated aromatic compounds included in this study were shown to be resistant to photomineralization. Quencher results suggest that direct photolysis and excited triplet state sensitization may be largely responsible for CO2 photoproduction in DOM, while singlet oxygen and hydroxyl radical sensitization may play a limited role. After 3 h of irradiation, the CO2 formation rate significantly decreased, and the role of sensitized reactions in CO2 formation increased. Together, the results from this study advance the understanding of the fundamental reactions driving DOM photomineralization to CO2, which is an important part of the global carbon cycle.

Long-Lived Triplet Charge Separated State and Thermally Activated Delayed Fluorescence in a Compact Orthogonal Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyad

A long-lived triplet charge separated state (³CS state lifetime: 0.56 μs) was observed in a compact electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond (AQ-PTZ). The ¹CS state energy (2.0 eV in cyclohexane) is lower than those of the ³AQ (2.7 eV) or the ³PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the ¹CS state energy (2.7 eV in cyclohexane), thermally activated delayed fluorescence (TADF) was observed [τ = 17.7 ns (99.9%)/1.5 μs (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectra confirm the electron spin multiplicity of the ³CS state, and the zero-field-splitting (ZFS) parameters |D| and |E| are 48.2 mT and 11.2 mT, respectively. These results are useful for design of compact electron donor-acceptor dyads to access the long-lived ³CS state and study the TADF mechanism.

Visible-Light Photocatalytic Oxidation of DMSO for RAFT Polymerization†

The solvent is an important, yet often forgotten part of a reaction mechanism. Many photochemical polymerizations are carried out using dimethyl sulfoxide (DMSO) as a way to promote the solubility of both the reactants and products, but its reactivity is rarely considered when initiation mechanisms are proposed. Herein, the oxidation of DMSO by an excited-state quinone is used to form initiating radicals resulting in the polymerization of methacrylate monomers, and the polymerization can be controlled with the addition of a chain transfer agent. This process leads to the formation of polymers with narrow molecular weight distribution, and the polymerization is able to be carried out in the presence of oxygen. A visible light absorbing substituted anthraquinone is synthesized, and nanosecond transient absorption spectroscopy is used to monitor the intermediates involved in the initiation mechanism. Photoproduct analysis indicates formation of methyl radicals as a result of DMSO oxidation. Furthermore, we show that the solvent outcompetes the chain transfer agent for interacting with the excited-state anthraquinone. These observations have a broad impact on photoinduced polymerizations performed in DMSO as many photocatalysts are strong oxidants in the excited state and are capable of reacting with the solvent. Therefore, the role of the solvent needs to be more carefully considered when proposing mechanisms for photoinduced polymerizations in DMSO.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Reversible Emitting Anti-Counterfeiting Ink Prepared by Anthraquinone-Modified β-Cyclodextrin Supramolecular Polymer

Intelligent fluorescent materials have been paid more and more attention due to their wide application in information encryption and anti-counterfeiting materials. Herein, a supramolecular polymer is constructed through the host-guest interaction of anthraquinone-modified β-cyclodextrin (AQ-β-CD) in aqueous solution. Thanks to the hydrophobic microenvironment of the cyclodextrin cavity and the shielding effect on oxygen molecules, the anthraquinone group, as the guest molecule, can rapidly produce 9,10-anthracenediol (QH2) with strong fluorescence by photoreduction. Interestingly, the generated anthracenediol group is still sensitive to oxygen and can be converted to anthraquinone by oxygen. Significantly, aqueous solution of AQ-β-CD supermolecular polymer is used as emitting ink, which decrypts the information by ultraviolet light and encrypts the information in the air.