Crystal Engineering Using Polyiodide Halogen and Chalcogen Bonding to Isolate the Phenothiazinium Radical Cation and Its Rare Dimer, 10-(3-Phenothiazinylidene)phenothiazinium

Utilizing facile one-electron oxidation of 10H-phenothiazine by molecular diiodine, the solid-state structure of the 10H-phenothiazinium radical cation was obtained in three cation:iodide ratios, as well as its THF and acetone solvates. Oxidation of 10H-phenothiazine with molecular diiodine in DMSO or DMF provided the structure of the radical coupling product 10-(3-phenothiazinyldene)phenothiazinium, which has not been crystallographically characterized to date. The radical cations were balanced by a mixture (I₇ )^- , (I₅ )^- , (I₃ )^- , and I^- anions, where a variety of chalcogen, halogen, and hydrogen bonding interactions stabilize the structures to reveal these interesting cationic species.

Promoting Photocatalytic Hydrogen Evolution Activity of Graphitic Carbon Nitride with Hole-Transfer Agents

Visible light-driven photocatalytic reduction of protons to H2 is considered a promising way of solar-to-chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photocatalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydrogen (H2 ) evolution process. Here, for the first time, the hole-transfer dynamics from Pt-loaded sol-gel-prepared graphitic carbon nitride (Pt-sg-CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H2 production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H2 production for 3 days. A detailed charge-transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time-resolved photoluminescence and in situ electron paramagnetic resonance studies.

Photochromic Radical Complexes That Show Heterolytic Bond Dissociation

Photochromic materials have been widely used in various research fields because of their variety of photoswitching properties based on various molecular frameworks and bond breaking processes, such as homolysis and heterolysis. However, while a number of photochromic molecular frameworks have been reported so far, there are few reports on photochromic molecular frameworks that show both homolysis and heterolysis depending on the substituents with high durability. The biradicals and zwitterions generated by homolysis and heterolysis have different physical and chemical properties and different potential applications. Therefore, the rational photochromic molecular design to control the bond dissociation in the excited state on demand expands the versatility for photoswitch materials beyond the conventional photochromic molecular frameworks. In this study, we synthesized novel photochromic molecules based on the framework of a radical-dissociation-type photochromic molecule: phenoxyl-imidazolyl radical complex (PIC). While the conventional PIC shows the photoinduced homolysis, the substitution of a strong electron-donating moiety to the phenoxyl moiety enables the bond dissociation process to be switched from homolysis to heterolysis. This study gives a strategy for controlling the bond dissociation process of the excited state of photochromic systems, and the strategy enables us to develop further novel radical and zwitterionic photoswitches.

N10 -carbonyl-substituted phenothiazines inhibiting lipid peroxidation and associated nitric oxide consumption powerfully protect brain tissue against oxidative stress

During some investigations into the mechanism of nitric oxide consumption by brain preparations, several potent inhibitors of this process were identified. Subsequent tests revealed the compounds act by inhibiting lipid peroxidation, a trigger for a form of regulated cell death known as ferroptosis. A quantitative structure–activity study together with XED (eXtended Electron Distributions) field analysis allowed a qualitative understanding of the structure–activity relationships. A representative compound N‐(3,5‐dimethyl‐4H‐1,2,4‐triazol‐4‐yl)‐10H‐phenothiazine‐10‐carboxamide (DT‐PTZ‐C) was able to inhibit completely oxidative damage brought about by two different procedures in organotypic hippocampal slice cultures, displaying a 30‐ to 100‐fold higher potency than the standard vitamin E analogue, Trolox or edaravone. The compounds are novel, small, drug‐like molecules of potential therapeutic use in neurodegenerative disorders and other conditions associated with oxidative stress.

The fate of phenothiazine-based redox shuttles in lithium-ion batteries

The stability and reactivity of the multiple oxidation states of aromatic compounds are critical to the performance of these species as additives and electrolytes in energy-storage applications. Both for the overcharge mitigation in ion-intercalation batteries and as electroactive species in redox flow batteries, neutral, radical-cation, and radical-anion species may be present during charging and discharging processes. Despite the wide range of compounds evaluated for both applications, the progress identifying stable materials has been slow, limited perhaps by the overall lack of analysis of the failure mechanism when a material is utilized in an energy-storage device. In this study, we examined the reactivity of phenothiazine derivatives, which have found interest as redox shuttles in lithium-ion battery applications. We explored the products of the reactions of neutral compounds in battery electrolytes and the products of radical cation formation using bulk electrolysis and coin cell cycling. Following the failure of each cell, the electrolytes were characterized to identify redox shuttle decomposition products. Based on these results, a set of decomposition mechanisms is proposed and further explored using experimental and theoretical approaches. The results highlight the necessity to fully characterize and understand the chemical degradation mechanisms of the redox species in order to develop new generations of electroactive materials.

On the microwave-assisted synthesis of acylphenothiazine derivatives — Experiment versus theory synergism

The microwave-assisted synthesis of a series of acylphenothiazine derivatives is described. 10H-Phenothiazine-3-carbaldehyde derivatives were obtained in moderate yields by the Duff formylation reaction, and 10-acetyl-phenothiazine derivatives were obtained in excellent yields by acetylating phenothiazine derivatives with acetic anhydride. A theoretical explanation for the chemoselectivity and regioselectivity of these acylation reactions applied to phenothiazine substrates was attempted by molecular-modeling analyses based on molecular mechanics, and semi-empirical and DFT calculations.

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.

Oxidation kinetics of phenothiazine and 10-methylphenothiazine in acidic medium

The rate of phenothiazine degradation in an acidic oxygen-saturated medium was studied. 3H-Phenothiazine-3-one and phenothiazine 5-oxide are produced by parallel reactions, and 7-(10'-phenothiazinyl)-3H-phenothiazine-3-one is produced in a more complex manner. The overall phenothiazine degradation rate appears to be pH independent up to pH 7.0. The degradation kinetics of 10-methylphenothiazine were studied after isolation and identification of its degradation products, 10-methylphenothiazine 5-oxide and 3H-phenothiazine-3-one. The main degradation product is 10-methylphenothiazine 5-oxide; but at low pH values and high temperatures, more 3H-phenothiazine-3-one is formed. The degradation rate of 10-methylphenothiazine is pH independent up to pH 7.

Mechanism for phenothiazine oxidation

The mechanism of phenothiazine degradation was studied by following the degradation of 3,10'-diphenothiazine in ethanol-water mixtures as well as the electrochemical oxidation of phenothiazine. A mechanism, including the formation of an oxidized dimer and some polymers, is suggested.