Electronic coupling and electron transfer in hydrogen-bonded mixed-valence compounds

Electron transfer provided by hydrogen bonds represents a unique and highly significant area of research, as it has a crucial role to play in a wide variety of chemical and biological systems. The hydrogen-bonded mixed-valence system, in the form of donor-hydrogen bond-acceptor, provides an ideal platform for exploring thermally-induced electron transfer across this non-covalent unit. Over the past decades, ongoing progress has been made in this field. Here we critically assess some studies on the qualitative and quantitative evaluation of electronic coupling and thermal electron transfer across hydrogen bond interface. Additionally, selected experimental examples are discussed in terms of intervalence charge transfer, with particular attention paid to the proton-coupled and often overlooked proton-uncoupled electron transfer pathway in hydrogen-bonded mixed-valence systems. We further highlight the major limitations of this research area and suggest potential directions for future exploration.

NIR-responsive reversible phase transition of supramolecular hydrogels for tumor treatment

Locally administrable drugs with controllable release on external cues hold great promise for antitumor therapy. Here, we report an injectable, supramolecular hydrogel (SHG), where the drug release can be controllably driven by near infrared (NIR) irradiation. The SHGs are formed by electrostatic interactions with LAPONITE® (XLG), in which upconverting nanoparticles (UCNPs) modified with α-cyclodextrin (α-CD) are used as the core, and azobenzene quaternary ammonium salts (E-azo) are further assembled through host-guest interactions. The hydrogel demonstrates reversible phase transition between gel and sol states and photothermal conversion capability. In detailed in vitro and vivo trials, drug-loaded SHGs successfully suppressed invasion by cancer cells. Phase transitions that are regulated by NIR light and promote drug release using photothermal effects, highlighting the considerable potential of supramolecular hydrogels in anticancer therapies, especially for treatments requiring long-term, on-demand drug supply in clinics.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half-Cells in Quantum Cellular Automata

Realization of molecular quantum cellular automata (QCA), a promising architecture for molecular computing through current-free processes, requires improved understanding and application of mixed-valence (MV) molecules. In this report, we present an electrostatic approach to creating MV subspecies through internalizing opposite charges in close proximity to MV ionic moieties. This approach is demonstrated by unsymmetrically attaching a charge-responsive boron substituent to a well-known organometallic MV complex, biferrocenium. Guest anions (CN^- and F^- ) bind to the Lewis acidic boron center, leading to unusual blue-shifts of the intervalence charge-transfer (IVCT) bands. To the best of our knowledge, this is the first reported example of a zwitterionic MV series in which the degree of positive charge delocalization can be varied by changing the bound anions, and serves to clarify the interplay between IVCT parameters. The key underlying factor is the variable zero-level energy difference in the MV states. This work provides new insight into imbuing MV molecules with external charge-responsiveness, a prerequisite of molecular QCA techniques.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr₃) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr₃)^•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr₃ to NAr₂ sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

Evidence of proton-coupled mixed-valency by electrochemical behavior on transition metal complex dimers bridged by two Ag+ ions

H-Bonded metal complex dimers with reversible redox behaviour, which are connected by a low-barrier hydrogen bond (LBHB) with a very low energy barrier for proton transfer, can provide a unique mixed-valency state stabilized by the proton-coupled electron transfer (PCET) phenomenon. Using cyclic voltammetry measurements, newly prepared [ReIIICl2(PnPr3)2(Hbim)]2 (2) and [OsIIICl2(PnPr3)2(Hbim)]2 (3) existing as H-bonded dimers in a CH2Cl2 solution showed a four-step and four-electron transfer containing two mixed-valency states of ReIIReIII and ReIIIReIV, and OsIIOsIII and OsIIIOsVI, respectively. Furthermore, [ReIIICl2(PnPr3)2(Agbim)]2 (4) and [OsIIICl2(PnPr3)2(Agbim)]2 (5), bridged by two Ag+ ions instead of two H-bonding protons, were prepared, and their electrochemical behaviours changed to a two-step and four-electron transfer. It is clear that the H-bonded complex dimers 2 and 3, connected by an LBHB, can be electrochemically stabilized into unique pairs of mixed-valency states by PCET, and the H-bonding proton transfer also controls the electrochemical redox behaviour.

A study of asymmetrical mixed-valent Mo2-Mo2 complexes in the class III regime

Three novel asymmetrical dimolybdenum dimers, [Mo₂(DAniF)₃]₂(μ-OOCCOS) (DAniF = N,N'-di(p-anisyl)formamidinate) ([OO-OS]), [Mo₂(DAniF)₃]₂(μ-S₂CCO₂) ([SS-OO]), and [Mo₂(DAniF)₃]₂(μ-SSCCOS) ([SS-OS]), have been synthesized and characterized by either single-crystal X-ray crystallography or ¹H NMR spectroscopy. The structural asymmetry for these compounds gives rise to a redox asymmetry, which enlarges the potential separation (ΔE_1/2) between the two [Mo₂] units. The mixed-valance (MV) species [OO-OS]⁺, [SS-OO]⁺ and [SS-OS]⁺, prepared by one-electron chemical oxidation of the neutral precursors, exhibit an intense and symmetrical intervalence charge transfer (IVCT) absorption band in the near-IR region, along with the high energy metal (δ) to ligand (π*) (ML) and ligand (π) to metal (δ) charge transfer (LMCT) absorptions. The LMCT band, which is absent in the neutral precursors, is reflective of the cationic [Mo₂]⁺ unit in the MV species; therefore, it is evidenced that in the MV complexes optical electron transfer from the electron donor to acceptor occurs, while the thermal process is energetically unfavorable. The C(1)-C(2) bonds (1.44-1.48 Å) that connect the two [Mo₂] units are significantly shorter than a C-C single bond, showing that the two Mo₂ centers are strongly coupled. For the series, TD-DFT calculations show that the molecular orbitals have an unsymmetrical charge density distribution over the two dimolybdenum sites. For each of the complex systems, the calculated orbital energy gaps, SOMO(δ - δ)-LUMO(bridging ligand π*), HOMO-8(bridging ligand π)-SOMO(δ - δ) and SOMO(δ - δ)-HOMO-1(δ + δ), are in good agreement with the observed MLCT, LMCT and IVCT absorption band energies, respectively. The consistency in energy between the IVCT band and the SOMO(δ - δ)-HOMO-1(δ + δ) gap permits assignment of the MV complexes to Class III in the Robin-Day scheme.

Anion-regulated electronic communication in a cyclometalated diruthenium complex with a urea bridge

A combined study of electrochemical measurements, intervalence charge transfer analysis, and DFT calculations suggests that the degree of urea-mediated electronic coupling between two cyclometalated ruthenium sites is enhanced by the coordination of urea with Br^- or Cl^-via hydrogen bonding. In contrast, the redox waves of the diruthenium complex become highly irreversible in the presence of relatively strong basic anions such as H₂PO₄^-, F^-, or OAc^-. This work demonstrates that the anion-urea interaction can be employed to regulate the electronic coupling and electron transfer between redox-active sites, suggesting the potential applications of the urea-functionalized diruthenium complex in anion sensing and stimuli-responsive molecular electronics.