O₂‑Triggered Directional Switching of Photocurrent in Self-Assembled Monolayer Composed of Porphyrin- and Fullerene-Terminated Helical Peptides on Gold

Directional switching of photocurrent generation in response to oxygen is attained with the self-assembled monolayer (SAM) composed of porphyrin- and fullerene-terminated helical peptides. The anodic photocurrent of the porphyrin SAM under argon gas is successfully switched over to the cathodic photocurrent in the presence of oxygen gas only in the copresence of the fullerene-terminated helical peptide. The first-principle calculations explain that the cathodic photocurrent is promoted as a result of suppression of the anodic photocurrent due to the small electron coupling between the lowest unoccupied molecular orbitals of fullerene and the amide moieties of electron mediating helix peptides

Chemical Reactivity in Nucleophilic Cycloaddition to C₇₀: Vibronic Coupling Density and Vibronic Coupling Constants as Reactivity Indices

The chemical reactivity in nucleophilic cycloaddition to C₇₀ is investigated on the basis of vibronic (electron-vibration) coupling density and vibronic coupling constants. Because the <i>e</i>₁^″ LUMOs of C₇₀ are doubly degenerate and delocalized throughout the molecule, it is difficult to predict the regioselectivity by frontier orbital theory. It is found that vibronic coupling density analysis for the effective mode as a reaction mode illustrates the idea of a functional group embedded in the reactive sites. Furthermore, the vibronic coupling constants for localized stretching vibrational modes enable us to estimate the quantitative reactivity. These calculated results agree well with the experimental findings. The principle of chemical reactivity proposed by Parr and Yang is modified as follows: the preferred direction is the one for which the initial vibronic coupling density for a reaction mode of the isolated reactant is a minimum

Isolation and Characterization of Persistent Radical Cation and Dication of 2,7-Bis(dianisylamino)pyrene

Orbital interaction between 2,7-pyrenylene and two nitrogen redox-active centers effectively reduces the energy difference between HOMO and HOMO–1, both of which were distributed over the two nitrogen centers. In fact, one- and two-electron oxidation of 2,7-bis­(dianisylamino)­pyrene <b>3</b> generated a persistent radical cation and a persistent dication, respectively, and we succeeded in the isolation and single crystal X-ray structural analyses of all three oxidation states. The radical cation was considered as a spin and charge delocalized mixed-valence compound with a semiquinoidal structure. The dication was in an open-shell singlet state with a small singlet–triplet energy gap. The molecular and electronic structures for all three oxidation states of <b>3</b> were studied in comparison with the data reported for each oxidation state of closely related bis­(triarylamine)­s, of which structures were determined by X-ray crystallography

<i>Meta</i>–<i>Para</i>-Linked Octaaza[1₈]cyclophanes and Their Polycationic States

Octaazacyclophanes, octaaza­[1₈]<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>-cyclophane (<b>2</b>) and octaaza­[1₈]<i>m</i>,<i>p</i>,<i>p</i>,<i>p</i>,<i>m</i>,<i>p</i>,<i>p</i>,<i>p</i>-cyclophane (<b>3</b>), as ring-size extended congeners of tetraaza­[1₄]<i>m</i>,<i>p</i>,<i>m</i>,<i>p</i>-cyclophane were synthesized, and the electronic states of their polycationic species were investigated by quantum chemical calculations, electrochemical measurements (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), UV–vis–NIR spectroelectrochemical measurements, and pulsed electron spin resonance (ESR) spectroscopy. These octaazacyclophanes exhibited multiredox activities depending on different linkage patterns along the macrocyclic molecular skeletons, and both molecules were oxidizable up to their respective octacations. Spectroelectrochemical measurements demonstrated that <i>p</i>-phenylenediamine (PD) moieties in <b>2</b> could be converted from the semiquinoidal structure to the quinoidal sturcture with increasing oxidation number, whereas higher oxidation states of <b>3</b> did not show definite quinoidal deformation of PD moieties. A pulsed ESR spectrum gave evidence about formation of the almost pure spin-triplet state for <b>3</b>²⁺, whereas the high-spin states of <b>2</b>²⁺ and <b>2</b>⁴⁺ are virtually degenerate with the competing low-spin states even at low temperatures, probably due to the fragility of spin-coupling pathway caused by facile conformational changes

Radical Cation of an Oligoarylamine Having a Nitroxide Radical Substituent: A Coexistent Molecular System of Localized and Delocalized Spins

A trimer derivative of oligotriarylamine bearing a nitroxide radical substituent as a localized spin center {<i>N</i>,<i>N</i>-bis­[4-(di-4-anisylamino)­phenyl]-<i>N</i>-[3-<i>tert</i>-butyl-5-(<i>N</i>-<i>tert</i>-butyl-<i>N</i>-oxylamino)­phenyl]­amine (<b>1</b>)} was characterized by electrochemical, spectroelectrochemical, and electron paramagnetic resonance spectroscopic measurements. The first and second oxidations of <b>1</b> occurred from the triamine moiety, leaving the nitroxide radical moiety intact. The delocalized polaronic state in the triamine moiety was generated by one-electron oxidation of <b>1</b>, indicating the coexistence of localized and delocalized spins on <b>1</b>⁺, where an intramolecular antiferromagnetic interaction was detected

Induced-Dipole-Directed, Cooperative Self-Assembly of a Benzotrithiophene

A benzotrithiophene derivative possessing phenylisoxazoles self-assembled to form stacks. The molecule isodesmically self-assembled in chloroform, whereas it self-assembled in a cooperative fashion in decalin and in methylcyclohexane. Thermodynamic studies based on isodesmic, van der Schoot, and Goldstein–Stryer mathematical models revealed that the self-assembly processes are enthalpically driven and entropically opposed. An enthalpy–entropy compensation plot indicates that the assembly processes in chloroform, decalin, and methylcyclohexane are closely related. The enthalpic gains in less-polar solvents are greater than those in more-polar solvents, resulting in the formation of large assemblies in decalin and in methylcyclohexane. The formation of large assemblies leads to cooperative assemblies. The elongation process is enthalpically more favored than the nucleation process, which drives the cooperativity of the self-assembly. DFT calculations suggested that a hexameric assembly is more stable than tetrameric or dimeric assemblies. Cooperative self-assemblies based on intermolecular interactions other than hydrogen bonding have rarely been reported. It is demonstrated herein that van der Waals interactions, including induced dipole–dipole interactions, can drive the cooperative assembly of planar π-conjugated molecules

Strategy for Designing Electron Donors for Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters are promising dopants for organic light-emitting diodes, including those containing highly twisted donor–acceptor-type structures. However, highly twisted structures limit the variety of chemical structures applicable as TADF emitters. We present a strategy for designing electron donors that can eliminate this requirement and increase the structural diversity of TADF emitters. Using this strategy, we developed an electron donor containing carbazolyl and diphenylamino groups by carefully controlling its electron-donating ability. By combining this donor with a quinoxaline-based acceptor, we obtained the efficient green TADF emitter, <i>N</i>³,<i>N</i>³,<i>N</i>⁶,<i>N</i>⁶-tetraphenyl-9-(4-(quinoxalin-6-yl)­phenyl)-9<i>H</i>-carbazole-3,6-diamine (DACQ), without a highly twisted structure. DACQ exhibits high photoluminescence and electroluminescence efficiencies, comparable to those of a highly twisted TADF emitter containing the same electron-accepting unit. Quantum chemical calculations showed that the diphenylamino groups within the carbazolyl moiety effectively withdraw the HOMO distribution. This reduces the singlet–triplet energy gap, thus inducing TADF. The photophysical properties of TADF compounds depend on the twisting angle between the electron-donating and accepting units. Eliminating the highly twisted structure increases the diversity of potential TADF emitters and allows their photophysical properties to be controlled by changing the twisting angle

Coplanar Oligo(<i>p</i>‑phenylenedisilenylene)s as SiSi Analogues of Oligo(<i>p</i>‑phenylenevinylene)s: Evidence for Extended π‑Conjugation through the Carbon and Silicon π‑Frameworks

A series of oligo­(<i>p</i>-phenylenedisilenylene)­s (Si-OPVs <b>1</b>–<b>4</b>), silicon analogues of oligo­(<i>p</i>-phenylenevinylene)­s, up to the tetramer have been synthesized and isolated by the introduction of a newly developed protecting group [(HexO)­MEind] for improving their solubility. The experimental and theoretical studies of the Si-OPVs <b>1</b>–<b>4</b> demonstrate the fully extended π-conjugation of the Si-OPV main chains. Single crystal X-ray analyses of the monomer <b>1</b> and the dimer <b>2</b> revealed the highly coplanar Si-OPV backbones facilitating the effective extension of the π-conjugation, which has further been validated by the significant increases in the absorption maxima from 465 nm for the monomer <b>1</b> to 610 nm for the tetramer <b>4</b>. The absorption maxima exhibit an excellent fit to Meier’s equation, leading to the estimation of an effective conjugation length (ECL) of 9 repeat units (<i>n</i>_ECL = 9) and the absorption maximum of 635 nm for the infinite chain (λ_∞ = 635 nm). In sharp contrast to other nonemissive disilenes, the Si-OPVs <b>2</b>–<b>4</b> show an intense fluorescence from 613 to 668 nm at room temperature with the quantum yields up to 0.48. All the data presented here provide the first evidence for the efficient extended π-conjugation between the SiSi double bonds and the carbon π-electron systems over the entire Si-OPV skeleton. This study reveals the possibility for developing the conjugated disilene π-systems, in which the SiSi double bonds would be promising building blocks, significantly optimizing the intrinsic photophysical and electrochemical properties of the carbon-based π-conjugated materials

Pseudo Jahn–Teller Origin of Buckling Distortions in Two-Dimensional Triazine-Based Graphitic Carbon Nitride (g‑C₃N₄) Sheets

Due to its direct band gap and light mass, the recently synthesized triazine-based, graphitic carbon nitride (TGCN) is considered a promising material for future microelectronics. However, despite the structural similarity with completely planar carbon-only graphene, TGCN sheets are different because of the presence of buckling distortions making the TGCN sheets nonplanar. In this article, we show that the sufficiently strong coupling between the unoccupied molecular orbitals (UMOs) with occupied molecular orbitals (OMOs) leads to pseudo Jahn–Teller distortions (PJT) and consequent buckling of TGCN layers. Doping the TGCN with doubly charged cations such as Be²⁺ can suppress the PJT distortions resulting in a completely planar structure. A proper understanding of the mechanism of the PJT effect in TGCN is crucial for tailoring properties that are relevant for practical applications

Highly Efficient Blue Electroluminescence Using Delayed-Fluorescence Emitters with Large Overlap Density between Luminescent and Ground States

The use of thermally activated delayed-fluorescence (TADF) allows the realization of highly efficient organic light-emitting diodes (OLEDs) and is a promising alternative to the use of conventional fluorescence and phosphorescence. Recent research interest has focused on blue TADF emitters. In this study, we use quantum mechanics to reveal the relationship between the molecular structures and the photophysical properties of TADF emitters and derive a direction for the molecular design of highly efficient blue TADF emitters. Theoretical analyses show that the luminescence efficiency of TADF emitters largely depends on the overlap density (ρ₁₀) between the electronic wave functions of the ground state and the lowest excited singlet state. By increasing ρ₁₀, we develop an efficient sky-blue TADF emitter material, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)­phenyl)-9′-phenyl-9<i>H</i>,9′<i>H</i>-3,3′-bicarbazole (BCzT). When doped into a host layer, BCzT produces a high photoluminescence quantum yield of 95.6%. From the transient photoluminescence decays of the doped film, the efficiency of excited triplet state conversion into light is estimated to be 76.2%. An OLED using BCzT as a sky-blue emitter produces a maximum external quantum efficiency (EQE) of 21.7%, which is much higher than the EQE range of conventional fluorescent OLEDs (5–7.5%). The high EQE is a result of the high triplet-to-light conversion efficiency of BCzT. Our material design based on ρ₁₀ distribution provides a rational approach for developing TADF emitters for high-efficiency blue OLEDs