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1. Electron Transfer in Rhodamine–TiO2 Complexes Studied as a Function of Chalcogen and Bridge Substitution

   https://doi.org/10.1021/acs.jpcc.9b11049  

   Zachary Piontkowski, Yu-Chen Wang ( January 2020 , The journal of physical chemistry)

   Many emerging light-harvesting systems for solar-energy capture depend on absorption of light by molecular dyes and subsequent electron transfer to metal-oxide semiconductors. However, the inhomoge- neous electron-transfer process is often misunderstood when analogies from bimolecular electron transfer are used to explain experimental trends. Here, we develop and apply a theoretical methodology that correctly incorporates the semiconductor density of states and the system reorganization energies to explain observed trends in a series of molecular sensitizers. The effects of chalcogen and bridge substitution on the electron transfer in rhodamine− TiO2 complexes are theoretically investigated by combining density functional theory (DFT)/time-dependent DFT calculations and Fermi’s golden rule for the rate constant. It is shown that all dyes exhibit τeT < 4 ps. Dyes with thiophene bridges exhibit shorter τeT (∼1 ps) than dyes with phenylene bridges (∼4 ps). When the planes of the dye core and bridge are fixed at coplanarity, the dye−TiO2 coupling strength is found to increase by a factor of ∼2 when compared with the Franck− Condon geometry. However, the donor energy level of coplanar dyes falls significantly below the TiO2 conduction band edge so that, despite enhanced coupling, electron transfer is slowed to ∼20 ps. Similar results appear formore »the excited triplet states of these dyes, showing that the intersystem crossing to low energy triplet states can increase electron-transfer time constants to 60−240 ps. These results are compared to the results of previous photocatalytic hydrogen generation and dye-sensitized solar cell experiments.« less
2. Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from Cu ^I Cu ^I Cu ^I to Cu ^II Cu ^II Cu ^II states

   https://doi.org/10.1039/D0SC05441K  

   Zhang, Weiyao ; Moore, Curtis E. ; Zhang, Shiyu ( March 2021 , Chemical Science)

   One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu I (MeCN) 4 ]PF 6 , and paraformaldehyde affords a mixed-valent [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu 3 (μ 3 -OH)] 3+ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 and its solvent-exposed analog [ TREN3 Cu II Cu II Cu II (μ 3 -O)](PF 6 ) 4 suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [ TREN4 Cu I Cu I Cu I (μ 3 -OH)](PF 6 ) 2 can reduce O 2 under acidic conditions. The geometric constraints provided bymore »the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu I Cu I Cu I ( 4a ), Cu II Cu I Cu I ( 4b ), and Cu II Cu II Cu I ( 4c ) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10 5 to 10 6 M −1 s −1 ) were observed for both Cu I Cu I Cu I /Cu II Cu I Cu I and Cu II Cu I Cu I /Cu II Cu II Cu I redox couples, approaching the rapid electron transfer rates of copper sites in MCO.« less
3. Photochemistry of carbon nitrides and heptazine derivatives

   https://doi.org/10.1039/D1CC02745J  

   Hwang, Doyk ; Schlenker, Cody W. ( September 2021 , Chemical Communications)

   We explore the photochemistry of polymeric carbon nitride (C 3 N 4 ), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (Δ E ST ). In violation of Hund's multiplicity rules, the lowest energy singlet (S 1 ) is stabilized relative to the lowest triplet (T 1 ) electronic excited state. Exploiting this unique inverted Δ E ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted Δ E ST , paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibilitymore »of optically controlling the photochemistry of Hz derivatives using ultrafast pump–push–probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary “push” excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted Δ E ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.« less
4. Open-air green-light-driven ATRP enabled by dual photoredox/copper catalysis

   https://doi.org/10.1039/D2SC04210J  

   Szczepaniak, Grzegorz ; Jeong, Jaepil ; Kapil, Kriti ; Dadashi-Silab, Sajjad ; Yerneni, Saigopalakrishna S. ; Ratajczyk, Paulina ; Lathwal, Sushil ; Schild, Dirk J. ; Das, Subha R. ; Matyjaszewski, Krzysztof ( October 2022 , Chemical Science)

   Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis ofmore »polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions.« less
5. Copper-based redox shuttles supported by preorganized tetradentate ligands for dye-sensitized solar cells

   https://doi.org/10.1039/c9dt04030g  

   Rodrigues, Roberta R. ; Lee, Joseph M. ; Taylor, Natalie S. ; Cheema, Hammad ; Chen, Lizhu ; Fortenberry, Ryan C. ; Delcamp, Jared H. ; Jurss, Jonah W. ( January 2020 , Dalton Transactions)

   Three copper redox shuttles ([Cu( 1 )] 2+/1+ , [Cu( 2 )] 2+/1+ , and [Cu( 3 )] 2+/1+ ) featuring tetradentate ligands were synthesized and evaluated computationally, electrochemically, and in dye-sensitized solar cell (DSC) devices using a benchmark organic dye, Y123 . Neutral polyaromatic ligands with limited flexibility were targeted as a strategy to improve solar-to-electrical energy conversion by reducing voltage losses associated with redox shuttle electron transfer events. Inner-sphere electron transfer reorganization energies ( λ ) were computed quantum chemically and compared to the commonly used [Co(bpy) 3 ] 3+/2+ redox shuttle which has a reported λ value of 0.61 eV. The geometrically constrained biphenyl-based Cu redox shuttles investigated here have lower reorganization energies (0.34–0.53 eV) and thus can potentially operate with lower driving forces for dye regeneration (Δ G reg ) in DSC devices when compared to [Co(bpy) 3 ] 3+/2+ -based devices. The rigid tetradentate ligand design promotes more efficient electron transfer reactions leading to an improved J SC (14.1 mA cm −2 ), higher stability due to the chelate effect, and a decrease in V lossOC for one of the copper redox shuttle-based devices.