Published papers

Pentacene derivatives carrying 3,4,5-trialkoxyphenylethynyl groups with different alkyl chain lengths (OCnH2n+1; n = 6, 12, 16) at the 6- and 13-positions of pentacene (denoted as PcCn) were newly synthesized to examine chain-length effects on self-assembly, optical, and electronic properties. Upon fast injection of a cyclohexane solution of PcCn into isopropyl alcohol, PcC6 and PcC12 underwent controlled self-assembly to give rise to spherical and fibrous nano-objects, respectively, while this self-assembly procedure could not be applied to PcC16 because of its poor solubility. Powder X-ray diffraction (PXRD) analysis showed that the nanofiber of PcC12 consists of a lamellar structure, where an effective overlap between the pentacene chromophores occur, resulting in significantly broadened and red-shifted absorption spectral features, compared with those observed for molecularly dispersed PcC12 in cyclohexane. In contrast, based on PXRD analysis, the fundamental assembly pattern of the nanoparticle of PcC6 was characterized by a two-dimensional hexagonal structure. Although a shorter core-to-core separation of 3.35 Å was observed for the nanoparticle, judging from the absorption properties, the pentacene chromophores involved in the PcC6 assembly do not effectively interact with each other. Interestingly, femtosecond transient absorption spectroscopy indicated that PcC6 in the spherical assembly tends to undergo deactivation of the singlet excited state, while PcC12 in the fibrous assembly shows ultrafast intermolecular singlet fission (apparent rate constant kSF ≥ 1012 s-1). A field-effect transistor (FET) fabricated using the nanofiber of PcC12 and an ion gel as the active layer and gate materials, respectively, exhibited a p-type behavior with a carrier mobility as high as 2.4 × 10-4 cm2 V-1 s-1, despite the fact that fibrous materials, where nanoscale conducting domains discontinuously aggregate as an entity, are not favorable for FET applications. Through this study, we demonstrate that a proper design of the side-chain length enables a pentacene chromophore to assemble into an arrangement beneficial for exhibiting optical and electronic properties.

Replacement of the carbon–carbon bonds of antiaromatic compounds with polar boron–nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B4N4-heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B4N4-heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

A series of ion pairs based on a bidipyrrin-AuIII complex that acts as a stable helical π-electronic cation have been prepared via ion-pair metathesis. The helical cation, which exhibits NIR absorption and phosphorescence emission, formed solid-state ion-pairing assemblies, whose assembling modes depended on the properties of coexisting counteranions.

A series of tetracene (Tc)-alkanethiol-functionalized gold nanoclusters (Aum: m = 25, 38) dyads with varying alkyl chain lengths (n = 5, 11) (denoted as Tc-Cn-Aum) were synthesized to investigate the excited-state dynamics through the triplet excited state of these gold nanoclusters. Transient absorption measurements revealed bidirectional intramolecular energy transfer processes, including the initial singlet-singlet energy transfer (S-SEnT) from Tc to Au25, followed by the subsequent triplet-triplet energy transfer (T-TEnT) from Au25 to Tc in Tc-Cn-Au25 (n = 5, 11), with nearly similar rate constants regardless of the alkyl chain length (n). In the case of Tc-C5-Au38, intramolecular S-SEnT from Tc to Au38 was similarly observed; however, T-TEnT from Au38 to Tc was not detected. The differences in excited-state dynamics between Tc-C5-Au25 and Tc-C5-Au38 can be attributed to variations in the triplet energies of Aum. These results clearly demonstrate the triplet character of Au25 and Au38

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole–dipole interactions.

Singlet fission (SF), which produces two triplet excitons from a singlet exciton, has been identified as a novel nanointerface for efficient (photo)energy conversion. This study aims to control exciton formation in a pentacene dimer through intramolecular SF using hydrostatic pressure as an external stimulus. We reveal the hydrostatic-pressure-induced formation and dissociation processes of correlated triplet pairs (TT) in SF by means of pressure-dependent UV/vis and fluorescence spectrometry and fluorescence lifetime and nanosecond transient absorption measurements. The photophysical properties obtained under hydrostatic pressure suggested distinct acceleration of the SF dynamics by microenvironmental desolvation, the volumetric compaction of the TT intermediate based on solvent reorientation toward an individual triplet (T1), and pressure-induced shortening of T1 lifetimes. This study provides a new perspective on the control of SF by hydrostatic pressure as an attractive alternative to the conventional control strategy for SF-based materials.

Chiral 1,1′-bi-2-naphthol (BINOL)-substituted boron dipyrromethene (BODIPY) derivatives with different numbers of phenyl groups at the 2 and 6 positions (denoted as nPh-B-BODIPY; n = 0, 1, 2) were newly synthesized to examine the aggregation-enhanced emission and circularly polarized luminescence (CPL) in the red and near-infrared (NIR) regions. Upon injection of THF solution of nPh-B-BODIPY into H2O, 0Ph-B-BODIPY underwent controlled self-assembly to produce fibrous nanoarchitectures, whereas spherical nanoparticles were formed in 1Ph-B-BODIPY and 2Ph-B-BODIPY. Absorption and CD spectra of 0Ph-B-BODIPY nanofibers demonstrated a red-shifted and split spectrum corresponding to the 0-0 band, suggesting strong interaction between neighbouring 0Ph-B-BODIPY units. Such fibrous assemblies exhibited a broad emission spectrum with multiple bands ranging from the visible to the near-infrared (NIR) region due to the multiple aggregate states. Consequently, 0Ph-B-BODIPY nanofibers demonstrated a broad circularly polarized luminescence (CPL) spectrum in the red to NIR regions together with the enhanced dissymmetry factors (glum) relative to 0Ph-B-BODIPY (monomer) and 1Ph-B-BODIPY nanoparticles.

The active control of singlet fission (SF) by external stimuli presents a major challenge in current chemistry, although it is yet in the initial stages. In this study, we synthesized a series of SF-active chromophore pentacene dimers connected by the flexible linkers and investigated the ground- and excited-state properties, under hydrostatic pressure as an external stimulus. The flexibilities of the linkers were elucidated by the density functional theory. Hydrostatic pressure-UV/vis and hydrostatic pressure-circular dichroism spectroscopies showed the absence of a ground-state conformational change, indicating that the active switching of SF observed in this study comes from the excited-state dynamics. The hydrostatic pressure-fluorescence lifetime and hydrostatic pressure-nanosecond transient absorption measurements revealed the active inversion of SF process (acceleration vs. deceleration) and independent triplet (T1) formation, in addition to the shortened T1 lifetimes and non-decreasing T1 quantum yields with elevating hydrostatic pressure. This study offers a valuable guideline through the critical molecular design revealed herein for further developing smart SF-controllable soft materials induced by hydrostatic pressure stimulations.

Quinoidal molecules based on dipyrrolyldiketone boron complexes (QPBs), in which pyrrole units were connected by a partially conjugated system as a singlet spin coupler, were synthesized. QPB, which was stabilized by the introduction of a benzo unit at the pyrrole β-positions, formed a closed-shell tautomer conformation that showed near-infrared absorption. The deprotonated species, monoanion QPB- and dianion QPB2-, showing over 1000 nm absorption, were formed by the addition of bases, providing ion pairs in combination with countercations. Diradical properties were observed in QPB2-, whose hyperfine coupling constants were modulated by ion-pairing with π-electronic and aliphatic cations, demonstrating cation-dependent diradical properties. VT NMR and ESR along with a theoretical study revealed that the singlet diradical was more stable than the triplet diradical.

Development of near-infrared (NIR)-light-driven photocatalytic hydrogen (H2) production is indispensable for an efficient use of solar energy in a sustainable society. Herein, a strategy is reported for achieving higher quantum yields of NIR-light-driven photocatalytic H2 production using salts of diprotonated dodecaphenylporphyrin (H4DPP2+(X−)2), showing large saddle distortion as organic photosensitizers and platinum nanoparticles as catalysts. When a perchlorate ion (ClO4−) is employed as a counter ion (X−) of H4DPP2+(X−)2, the quantum yield of the photocatalytic H2 production reaches to 36% by excitation at 750 nm, which is the highest value among those of NIR-light-driven H2 production reported so far. Such a drastic enhancement of the quantum yield is achieved by selection of X− as an appropriate counter anion of H4DPP2+, judging from second-order rate constants of photoinduced electron transfer (ET) from electron donors to 3(H4DPP2+(X−)2)* and the reorganization energy of ET for H4DPP2+(X−)2.

Hexacene (Hc) is highly promising for singlet fission (SF). However, the number of SFs in Hc is extremely limited. As far as Hc dimers in solution are concerned, there is no report on the observation of the dissociation process from a correlated triplet pair (TT) to an individual one. The emphasis in this study is on the first observation of the quantitative TT generation together with the orientation-dependent photophysical discussions for TT dissociation usingpara- andmeta-phenyl-bridged Hc dimers. Moreover, the activation enthalpies of Hc dimers in TT dissociation are smaller than those of pentacene (Pc) dimers, whereas the relative entropic contributions for Gibbs free energy of activation are much larger than the enthalpic ones in both Hc and Pc dimers. This implies that the vibrational motions are responsible for the intramolecular conformation changes associated with the TT dissociation. Consequently, “enthalpy-entropy compensation” has a large impact on the rate constants and quantum yields.

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Unlike previously well-studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short-step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady-state spectroscopic analyses revealed that the close proximity of the non-conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time-resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.

© 2018 the Owner Societies. Photophysical control and switching on organic-inorganic hybrid interfaces are of great interest in diverse fundamental and applicative research areas. 6,13-Bis(triisopropylsilylethynyl)pentacene (TP) is well-known to exhibit efficient singlet fission (SF) for generation of high-yield triplet excited states in aggregated forms, whereas perylenediimide (PDI) ensembles show the characteristic excimer formation. Additionally, a combination of pentacene (electron donor: D) and PDI (electron acceptor: A) is expected to undergo an efficient photoinduced electron transfer (PET), and absorption of two chromophores combined covers the entire visible region. Therefore, the concentration-dependent mixed self-assembled monolayers (SAMs) composed of two chromophores enable us to control and switch the photophysical processes on a surface. In this work, a series of mixed SAMs composed of TP and PDI units on gold nanoclusters (GNCs) were newly synthesized by changing the relative molecular concentration ratios. Structural control of mixed SAMs on a gold surface based on the concentration ratios was successfully achieved. Time-resolved femtosecond and nanosecond transient absorption measurements clearly demonstrate photophysical control and switching of the above competitive reactions such as SF, electron transfer (ET) and excimer formation. The maximum quantum yields of triplet states (ΦT = ∼170%) and electron transfer (ΦET = ∼95%) were quantitatively evaluated by changing the concentration ratios. The rate constants of SF and excimer processes are largely dependent on the concentration ratios, whereas the rate constants of ET processes approximately remain constant. These findings are also discussed based on the statistical framework of the assembly of chromophores on the gold surface.

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Singlet fission (SF) is expected to improve photoenergy conversion systems by generating two electrons from one photon. Pentacenes meet the energy-level matching condition between a singlet and two triplet states: [E(S1)≥2E(T1)]. However, the molar absorption coefficients of pentacenes in the approximately 400–500 nm region are limited, whereas quantum dots, such as CdSe/ZnS (QD), possess high fluorescence quantum yields and particle-size-dependent fluorescence wavelengths. Thus, a combination of QD (D) and pentacene (A) provides a system of both an enhanced light-harvesting efficiency throughout the solar spectrum and an efficient conversion of the harvested light into the triplet states by SF. Based on these points, m-phenylene-bridged triisopropylsilane (TIPS)-pentacene dimer-functionalized QD (denoted as m-(Pc)2-QD) was synthesized to examine the sequential photoinduced process from energy transfer to SF. In femtosecond transient absorption measurements, initial energy transfer from QD to pentacene (quantum yield: 87 %) and subsequent SF were efficiently observed. The quantum yield of triplet states of pentacene units (ΦΤ) based on the excitation of QD attained is 160±6.7 %.

Copyright © 2017 American Chemical Society. Coronenediimide derivatives with five- or six-membered maleimide groups [denoted as Cor(5Im)2 and Cor(6Im)2] were employed as electron acceptors to examine the electron-transfer reduction properties through photochemical and electrochemical measurements. In steady-state absorption and fluorescence measurements, the spectra of Cor(5Im)2 and Cor(6Im)2 became remarkably broadened and red-shifted as compared to pristine coronene (Cor). These results are supported by electrochemical measurements and DFT calculations. The rate constants of photoinduced intermolecular electron transfer from various donor molecules to 3Cor(5Im)2∗ or 3Cor(6Im)2∗ are determined by nanosecond transient absorption measurements. Although the back-electron-transfer reactions examined in this study proceed with the diffusion-limited rate constant in benzonitrile (PhCN), the rate constants of forward electron-transfer reactions (ket) increase with an increase in the driving force of electron transfer (-ΔGet) to approach the diffusion-limited rate constant. When the driving force dependence of ket was fit on the basis of the Marcus theory of electron transfer, the reorganization energy (λ) of the electron-transfer reduction of Cor(5Im)2 and Cor(6Im)2 are determined to be 0.77 and 1.15 eV, respectively. A new covalently perylene-linked donor-acceptor dyad was also synthesized to investigate the dynamics of ultrafast photoinduced intramolecular electron transfer.

© 2018 American Chemical Society. Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m-(Pc)2] were synthesized to examine intramolecular orientation-dependent multiexciton dynamics, especially focusing on singlet fission (SF) and recombination from correlated triplet pairs [(TT)]. Absorption and electrochemical measurements indicated strong intramolecular couplings of o-(Pc)2 relative to m-(Pc)2. Femtosecond and nanosecond TA measurements successfully demonstrated efficient SF in both dimers. In contrast, the dissociation process from the (TT) to the individual triplets [(2 × T)] was clearly observed in m-(Pc)2, which is in sharp contrast to a major recombination process in o-(Pc)2. Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and dissociation proceed from the quintet state of 5(TT) in m-(Pc)2. The rate constant of the SF was 2 orders of magnitude greater in o-(Pc)2 than that in m-(Pc)2 and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units.

© 2018 American Chemical Society. Singlet fission (SF) theoretically enables the performance of the sequential photoenergy conversion process starting from the singlet state and leading to electron transfer (ET) with the radical ion pair quantum yield approaching 200%. Additionally, the long lifetime of the triplet state opens the possibility for an intermolecular ET process in a diffusion-limited reaction. However, the quantitative two-electron transfer process through SF has yet to be reported. Herein we demonstrate the quantitative sequential process involving SF and leading to intermolecular two-electron transfers using 2,2′-biphenyl-bridged tetracene dimer (Tet-BP-Tet: SF and electron donor) and chloranil (Ch: electron acceptor). The high-yield and long-lived individual triplet excited states of Tet-BP-Tet by SF (φ T = 175 ± 5% and τ T = 0.29 ms) resulted in the quantitative two-electron transfer process (φ ET = 173 ± 5%) with Ch in benzonitrile.

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim A homoleptic zinc(II) helicate organized by a pair of achiral dipyrromethene ligands through zinc(II) coordination was synthesized to evaluate the chiroptical properties. This zinc complex showed strong exciton-coupled chiroptical responses from the helical configuration with a large absorption dissymmetry factor |gabs| (up to 0.20). More importantly, intense polarized luminescence in the far-red region (700–850 nm) with a fluorescence quantum yield ΦFL of 0.23 was observed for this helicate with a dissymmetry factor |glum| of 0.022, the largest value among rare-earth- and precious-metal-free small molecules. These unprecedentedly large g values were supported by theoretical calculations.

© 2017 American Chemical Society. Tetragermatetrathia[8]circulene has successfully been synthesized from tetraiodotetrathienylene through palladium-catalyzed germylation and rhodium-catalyzed intramolecular dehydrogenative cyclization. A single-crystal X-ray diffraction analysis elucidated that tetragermatetrathia[8]circulene has a highly symmetric, planar, and rigid structure. DFT calculations suggested that LUMO of tetragermatetrathia[8]circulene is stabilized due to σâÏ€∗ interactions between σ∗ orbitals of the Ge-C bonds and Ï€∗ orbitals of the thiophene rings. The excited dynamics of tetragermatetrathia[8]circulene have also been found to be significantly affected by the heavy-atom effect of the germanium atom.