Fluoride binding to an organoboron wire controls photoinduced electron transfer

Luminescence Tunable Europium and Samarium Complexes: Reversible On/Off Switching and White-Light Emission

Single-molecule functional materials with luminescence tunable by external stimuli are of increasing interest due to their application in sensors, display devices, biomarkers, and switches. Herein, new europium and samarium complexes with ligands having triphenylamine (TPA) groups as the redox center and 2,2'-bipyridine (bpy) as the coordinating groups and diketonate (tta) as the second ligand have been constructed. The complexes show white-light emission in selected solvents for proper mixtures of the emission from Ln³⁺ ions and the ligands. Meanwhile, they exhibit reversible luminescence switching on/off properties by controlling the external potential owing to intramolecular energy transfer from the Ln³⁺ ions to the electrochemically generated radical cation of TPA^•+. Time-dependent density functional theory (TD-DFT) calculations have been performed to study the electronic spectra. The proposed intramolecular energy transfer processes have been verified by density functional theory (DFT) studies.

Chemical control of photoinduced charge-transfer direction in a tetrathiafulvalene-fused dipyrrolylquinoxaline difluoroborate dyad

Chemical regulation of a distinct photoinduced charge flow over multiple pathways present in an electron donor–acceptor dyad is demonstrated.

Electronic Communication as a Transferable Property of Molecular Bridges?

Electronic communication through molecular bridges is important for different types of experiments, such as single-molecule conductance, electron transfer, superexchange spin coupling, and intramolecular singlet fission. In many instances, the chemical structure of the bridge determines how the two parts it is connecting communicate, and does so in ways that are transferable between these different manifestations (for example, high conductance often correlates with strong antiferromagnetic spin coupling, and low conductance due to destructive quantum interference correlates with ferromagnetic coupling). Defining electronic communication as a transferable property of the bridge can help transfer knowledge between these different areas of research. Examples and limits of such transferability are discussed here, along with some possible directions for future research, such as employing spin-coupled and mixed-valence systems as structurally well-controlled proxies for understanding molecular conductance and for validating first-principles theoretical methodologies, building conceptual understanding for the growing experimental work on intramolecular singlet fission, and developing measures for the transferability of electronic communication as a bridge property.

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.

Push-pull isomers of indolizino[6,5,4,3-def]phenanthridine decorated with a triarylboron moiety

1,3-Dipolar cycloaddition reactions between a new azomethine ylide and three BPhMes₂-functionalized internal alkynes produced three pairs of fluorescent push-pull regioisomers, which show distinct electronic and photophysical properties. All the six compounds are found to exhibit charge-transfer (CT) fluorescence, and some of which show rare and interesting temperature "turn-on" fluorescence.

The opposite and amplifying effect of B ← N coordination on photophysical properties of regioisomers with an unsymmetrical backbone

1,3-Dipolar cycloaddition of pyrido[2,1-a]isoindole with internal alkynes functionalized by a BMes2ph and an N-aromatic heterocycle leads to the formation of two types of regioisomers (major a and minor b) that have distinct physical and photophysical properties. Examination on 5 pairs of regioisomers unveils that the major isomers consistently have a smaller optical energy gap and emission energy than the corresponding minor isomers, which is greatly amplified by the formation of an internal B ← N bond. The regioisomers with a B ← N bond display contrasting temperature-dependent structural dynamics and response to fluoride ions, owing to an entropy-driven or fluoride initiated B ← N bond rupture/ring-opening process and the different B ← N bond strength. The opposite inductive effect and the Lewis pair properties of the dichotomic substituent units are responsible for the contrasting properties of the regioisomers in this system.

Probing Lewis acid-base interactions in single-molecule junctions

A novel strategy to regulate the tunneling mechanism for charge transport through an organoborane wire via Lewis acid-base interactions has been developed. A change from LUMO- to HOMO-dominated charge transport upon the addition of the fluoride is verified both experimentally and theoretically.

Fluoride-ion-binding promoted photoinduced charge separation in a self-assembled C60 alkyl cation bound bis-crown ether-oxoporphyrinogen supramolecule

A bis-crown ether-oxoporphyrinogen was newly synthesized and self-assembled concurrently with C₆₀ alkyl ammonium cations at the crown ether sites and F^- anions (through hydrogen bonding) at the oxoporphyrinogen core. Ultrafast photoinduced charge transfer processes within the donor-acceptor conjugate were promoted by fluoride ion binding and this was established using various spectroscopic methods and transient absorption studies.

Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

A new class of Ar₂B-π-A dyads and A-π-B(Ar)-π-A triads that feature strong organic acceptor moieties (A = pyridalthiadiazole, PT) attached to a central triarylborane were synthesized via Stille cross-coupling of ArB(Th-SnMe₃)₂ (Th = thiophenediyl, Ar = 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethylphenyl) (^FMes)) with one or two equivalents of dibromopyridalthiadiazole. Single-crystal X-ray crystallography data for the triad Mes*B(Th-PT-Br)₂ indicate a highly coplanar conformation, which is ideal for extended π-conjugation and favors intermolecular π-stacking. Despite the presence of Br substituents, these compounds exhibit strong photoluminescence in THF solution with quantum yields reaching up to 52%. Further extension of conjugation by coupling with 2-hexylthiophene leads to additional bathochromic shifts to give a highly soluble and strongly red-emissive material. All these compounds undergo facile reduction, first of the PT substituents and then at more negative potentials for the borane moiety. Upon chemical reduction with in THF, an intramolecular charge transfer (ICT) pathway from the reduced PT moieties to boron is enabled and this results in a change of the color to blue. Theoretical calculations reveal that, due to the electron-withdrawing effect of the PT moieties, not only the PT-centered LUMOs themselves but also the LUMO+1 or LUMO+2, which show contributions from the p orbital of boron, experience a significant decrease in energy; they are much lower in energy than those of typical conjugated triarylboranes. The relatively low energy of both the PT-centered LUMOs and boron-centered LUMO+1 or LUMO+2 opens up multiple pathways for reaction with highly nucleophilic fluoride anions. Evidence for very strong F^- binding to boron is obtained in the case of the more sterically accessible ^FMes derivatives. Fluoride anion binding leads to an electron-rich borate moiety and as such generates an ICT pathway to the electron-deficient PT moieties; the direction of this ICT is opposite to that observed upon chemical reduction. For the Mes* derivatives, F^- binding is hindered, resulting in competing reduction of the PT acceptors. Finally, the electron acceptor character of the hexylthiophene derivative is exploited in electron-only diodes that show an average electron mobility of 6.4 ± 1.6 × 10^-5 cm² V^-1 s^-1.

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.

Unusual distance dependences of electron transfer rates

There are regimes in which electron transfer rates increase with increasing donor–acceptor distance, leading to rate maxima at large donor–acceptor separations.