Published papers

Hexaboraheptacene (B6-hept), featuring a periodic arrangement of boron atoms, together with its shorter homologues tetraborapentacene (B4-pent) and diboraanthracene (B2-ant), were synthesized via cyclocondensation of o-bis(dihydroboryl)arenes. The reactive boron centers are effectively protected by 2,4,6-triisopropylphenyl (Tip) groups, enabling handling under ambient conditions. As the number of boron bridges increases, the electron-accepting ability as well as the multistep reversible redox properties are progressively enhanced. B6-hept exhibits pronounced dual fluorescence in solution, in which relative intensities depend on the excitation wavelength and solvent viscosity. DFT calculations suggest that this phenomenon originates from two interconvertible conformers, namely, the bent-zigzag and twisted forms, which result from a delicate balance between the extended π-system and the steric demands of the Tip groups. This interpretation is supported by the fact that B4-pent displays only weak dual emission, while B2-ant shows none. This study not only introduces a new class of heteroacenes with potential applications in optoelectronics and electrocatalysis, but also provides insight into steric-design strategies for tuning the conformational landscapes of boron-containing π-systems.

Although long-range intramolecular electron transfer has been extensively demonstrated in covalently linked donor–acceptor conjugates, intramolecular singlet fission (ISF) over linker lengths significantly exceeding 15 Å has not yet been observed. Moreover, no studies have shown linker-length-dependent control of the singlet correlated triplet pair (TT) dynamics, where singlet TT undergoes spin conversion to quintet TT or dissociation into two individual triplet states (T1 + T1) via quintet TT. Here, we report pentacene dimers bridged by two different polyyne-based molecular wires, in which long-range ISF is successfully observed over linker lengths of up to ∼35 Å. As the linker length increases, we further demonstrate tunable singlet TT dynamics, achieving systematic control over pathways: spin conversion to quintet TT, and dissociation of quintet TT into T1 + T1. Transient absorption measurements reveal that the individual triplet yields increase with increasing linker length, while the nearly quantitative formation of the 1TT is maintained. The small attenuation factors for ISF and for recombination of singlet TT to the ground state indicate that the polyyne linker functions as an efficient molecular wire. Thermodynamic analysis and time-resolved electron paramagnetic resonance measurements clearly reveal that vibration-driven structural modulation of the polyyne-based molecular wire in the singlet TT directly controls the branching between (i) spin conversion from singlet TT to quintet TT and (ii) dissociation into T1 + T1 via quintet TT following spin conversion. These findings establish molecular wires as effective platforms for long-range ISF and provide a mechanistic foundation for engineering singlet TT dynamics across extended molecular distances.

Singlet exciton fission (SF), in which one singlet exciton (S1) generates two triplet excitons (T1), is one of the few available options for exceeding the theoretical limit of solar energy use, the so-called Shockley–Queisser limit. Although most SF systems applicable to Si solar cells exhibit a thermodynamically unfavourable endothermic process, a few approaches exist for overcoming the endothermic SF process. Here we discovered that combining SF molecules with quantum dots (QDs) is an alternative for improving the endothermic SF efficiency. Inter-material orbital hybridization between SF molecules and QDs harnesses the endothermic SF. Theoretical calculations and transient-absorption measurements of tetracene–CdX (X = Te, Se or S) QD composites showed that several orbital hybrid levels that formed due to orbital hybridization act as intermediate levels to facilitate SF. This result highlights alternative factors affecting SF efficiency beyond the molecular packing of SF molecules on the QD surface.

A newly synthesized multi-bladed benzene (2o) bearing two oxaboraanthracenyl and two N-carbazolylphenyl units undergoes highly cooperative binding with multiple methanol molecules (Hill coefficient > 3.5).

The development of highly efficient near-infrared (NIR) emissive materials presents tremendous opportunities across diverse applications, from biological imaging to advanced optoelectronics. While planar aromatic dyes have traditionally dominated this field, we report herein the successful design and synthesis of a novel bowl-shaped NIR dye, TPA-TOS, incorporating the rarely emissive trioxosumanene (TOS) as the acceptor and triphenylamine (TPA) as the donor. In polar solvents, TPA-TOS exhibited weak luminescence confined to the UV region; however, in nonpolar environments, it demonstrated intense NIR emission spanning the 650 to 850 nm range. Remarkably, a quantum yield exceeding 60% in the NIR range was achieved, attributed to an intriguing photo-relaxation pathway. This mechanism involves exciton relaxation from localized excitation to the charge transfer (CT) state, mediated by thermally activated delayed fluorescence (TADF) between CT singlet and triplet states, along with room-temperature phosphorescence (RTP), ultimately transitioning to a long-lived charge-separated (CS) state. The outstanding amphiphilic nature, high quantum yield of NIR dual emission enabled by TADF and RTP, and the generation of a long-lived CS state are likely driven by the pronounced spin–orbit coupling facilitated by the curved sumanene skeleton. This work establishes TPA-TOS as a platform for NIR materials and insights into non-planar photophysics.

Singlet fission (SF) is a spin-allowed multiexciton generation (MEG) process, where one singlet exciton (S1) splits into two triplet excitons (2T1) in two nearby molecules (theoretical maximum triplet quantum yield: 2). In contrast, bi- and multiexciton states of quantum dots (QDs) have been generated by exciting them at high excitation density (multiple-photon excitation). Here, we propose combining these materials for the integrated MEG (iMEG) process using 6,13-bis(triisopropylsilylethynyl)pentacene (TP) dimer [(TP)2]-modified CdSe QD (CdSeQD) hybrids. Upon photoexcitation of CdSeQD with multiple-photon excitation, a sequential photoinduced process from the multiexciton state (CdSeQD) to SF [(TP)2] occurred through singlet-singlet energy transfer (EnT) from CdSeQD to TP. The number of triplet excitons generated per CdSeQD (NT) increased up to ∼4.9 ± 0.7 at higher excitation intensities. Our proposed inorganic-organic hybrid system demonstrates a novel exciton amplification process for various future uses, such as solar energy conversion, optoelectronics, and biological applications.

Multi-exciton generation of organic chromophores, i.e., singlet fission (SF), is highly promising for photoenergy conversion such as photocatalysis and photovoltaics. Among the reported high-yield SF systems, many systems possess only short-lived triplet excited states derived from correlated triplet pair intermediates, but not individual doubled triplet excitons. However, high-yield and long-lived individual triplet exciton generation is favorable for achieving efficient photoenergy conversion. In this Focus Review, we discuss the molecular design strategy for high-yield and long-lived individual triplet exciton generation through intramolecular SF utilizing a series of acene derivatives such as hexacene, pentacene, and tetracene. In addition to clarifying the structural and vibronic characters required for high-yield individual triplet exciton generation, specific examples of the photoenergy conversion far exceeding 100% quantum yields of subsequent electron-transfer products and singlet oxygen generation are presented.

The conversion of a high-energy photon into two excitons using singlet fission (SF) has stimulated a variety of studies in fields from fundamental physics to device applications. However, efficient SF has only been achieved in limited systems, such as solid crystals and covalent dimers. Here, we established a novel system by assembling 4-(6,13-bis(2-(triisopropylsilyl)ethynyl)pentacen-2-yl)benzoic acid (Pc) chromophores on nanosized CdTe quantum dots (QDs). A near-unity SF (198 ± 5.7%) initiated by interfacial resonant energy transfer from CdTe to surface Pc was obtained. The unique arrangement of Pc determined by the surface atomic configuration of QDs is the key factor realizing unity SF. The triplet-triplet annihilation was remarkably suppressed due to the rapid dissociation of triplet pairs, leading to long-lived free triplets. In addition, the low light-harvesting ability of Pc in the visible region was promoted by the efficient energy transfer (99 ± 5.8%) from the QDs to Pc. The synergistically enhanced light-harvesting ability, high triplet yield, and long-lived triplet lifetime of the SF system on nanointerfaces could pave the way for an unmatched advantage of SF.

A soluble heptacene derivative, 5,9,14,18-tetrakis(triisopropylsilylethynyl)-heptacene (TIPS-Hep), was synthesized in situ from an α-diketone precursor via the Strating–Zwanenburg reaction, and its photophysical properties were comprehensively characterized in both solution and thin-film states. In deoxygenated dilute tetrahydrofuran (THF) solution, TIPS-Hep exhibited monomeric behavior. Absorption peaks were observed at 671, 737, 820, and 970 nm, accompanied by a shoulder at 870 nm, while fluorescence peaks were observed at 894 and 961 nm with a fluorescence lifetime (τS) of <100 ps. Nanosecond transient absorption spectroscopy revealed a triplet lifetime (τT) of 19 μs and a triplet quantum yield (ΦISC) of ∼2%. The short excited-state lifetime in dilute solution was estimated to be 87 ps by femtosecond transient absorption spectroscopy and precludes efficient molecular encounters, indicating that singlet fission is unlikely and that intersystem crossing serves as the dominant pathway for triplet formation. This interpretation is supported by transient decay analysis, which revealed no kinetic signatures associated with SF. In contrast, solution-processed thin films of TIPS-Hep, generated in situ by photoirradiation of a spin-coated α-diketone precursor, exhibited faster and enhanced formation of the triplet excited states. Since multiple excited species are involved, several kinetic models were examined, and a sequential model based on a SF pathway (S1 + S0 → TT → 2T1) was identified as the most consistent with the experimental results. Global analysis of femtosecond transient absorption data with the sequential SF model assuming three species revealed high yields of correlated triplet pairs (TT, 75% at the maximum value of 100%) and independent triplets (T1, 20% at the maximum value of 200%). These results provide valuable mechanistic insights into the excited-state dynamics of higher acenes.

Organic-inorganic hybrid molecular architectures utilizing self-assembled monolayers (SAMs) of organic chromophores (e.g., acene derivatives) are systematically discussed to examine the photophysical properties together with covalently-linked dimeric and oligomeric forms. Multi-exciton generation (i.e., singlet fission) and light energy conversion processes that occur in a reaction site composed of chemically modified organic chromophores on the surface of inorganic nanomaterials such as gold nanocluster, gold nanorod and quantum dot are described.

Here reports an photoinduced glycosylation of trichloroacetimidate donors and several alcohols using 3,11-dimethoxydinaphthothiophene as an organo Lewis photoacid catalyst upon long-wavelength UV-LED light (385 nm) irradiation. The reaction proceeded under mild reaction conditions smoothly to give the corresponding glycosides in good to high yields. In addition, the present glycosylation method was applicable to a wide range of trichloroacetimidate donors and acceptors. Furthermore, the dinaphthothiophene catalyst could be recovered and reused without any loss of efficiency. Moreover, this glycosylation method was applied successfully to the total synthesis of a biologically active natural product, cholesteryl 6-O-myristoyl-α-glucoside.

Rotaxane-based mechanophores that exploit spatial separation between a luminophore and a quencher are attractive due to their high structural design flexibility, enabling high-contrast changes in fluorescence intensity. However, it remains unclear whether their quenching mechanism is predominantly governed by photoinduced electron transfer (PET) or ground-state charge-transfer (CT) complex formation. This study unveils the quenching mechanism using rotaxane mechanophores incorporating π-extended anthracene, tetracene, or pentacene. In toluene, the quenching efficiency decreases with increasing π-conjugation of the fluorophore. Steady-state and transient absorption spectroscopy clarify that the fluorescence quenching of the anthracene-containing rotaxane is primarily due to PET, with a minor contribution from CT complex formation. In contrast, no clear CT complex formation is observed for the tetracene- and pentacene-containing mechanophores. PET moderately quenches the fluorescence for the tetracene-based system, while the low PET efficiency in the pentacene-containing mechanophore results in minimal quenching. Polyurethane elastomer films containing the anthracene-based mechanophore exhibit a significant increase in fluorescence intensity upon mechanical deformation. In contrast, almost no activation is observed for the pentacene-based mechanophore embedded in polyurethane. These findings clarify that PET is the primary quenching mechanism in rotaxane-based mechanochromic mechanophores, offering valuable insights for the future design of supramolecular mechanophores.

We demonstrated for the first time the determination of triplet quantum yields and molar absorption coefficients of ultrasmall gold nanoclusters specifically [Au25(PET)18]− with phenylethanethiolate (PET) ligands using two sensitization experiments.

Molecular assemblies featuring two-dimensionality have attracted increasing attention, whereas such structures are difficult to construct simply relying on spontaneous molecular assembly. Here, we present two-dimensional assemblies of acene chromophores achieved using a tripodal triptycene supramolecular scaffold, which have been shown to exhibit a strong ability to assemble molecular and polymer motifs two-dimensionally. We designed pentacene and anthracene derivatives sandwiched by two triptycene units. These compounds assemble into expected two-dimensional structures, with the pentacene chromophores having both sufficient overlap to cause singlet fission and space for conformational change to facilitate the dissociation of a triplet pair into free triplets, which is not the case for the anthracene analog. Detailed spectroscopic analysis revealed that the pentacene chromophore in the assembly undergoes singlet fission with a quantum yield of 88 ± 5%, giving rise to triplet pairs, from which free triplets are efficiently generated (ΦT = 130 ± 8.8%). This demonstrates the utility of the triptycene-based scaffold to design functional π-electronic molecular assemblies.

We herein report a new set of supramolecular nanotools for the generation and modulation of singlet fission (SF) of noncovalent/covalent pentacene dimers. Two molecules of a pentacene monomer with bulky substituents are facilely encapsulated by a polyaromatic capsule, composed of naphthalene-based bent amphiphiles, in water. The encapsulated noncovalent dimer converts to otherwise undetectable triplet pairs and an individual triplet in high quantum yields (179% and 53%, respectively) even under high dilution conditions. Within the capsule, a covalently linked pentacene dimer with bulky groups generates two triplet pair intermediates in parallel, which are hardly distinguished in bulk solution, in excellent total quantum yield (196%). The yield of the individual triplet is enhanced by 1.6 times upon encapsulation. For both types of pentacene dimers, the SF features can be readily tuned by changing the polyaromatic panels of the capsule (i.e., anthracene and phenanthrene).

We present a novel design concept of molecular dimers for quantitative individual triplet yield (φT) through intramolecular singlet fission. We synthesized a series of tetracene (Tc) dimers bridged by different phenylene-based linkers, focusing on the conformational flexibility in addition to the electronic coupling. In transient absorption measurements, a 4,4′-biphenyl-bridged Tc dimer with weaker electronic coupling and larger conformational flexibility exhibited the quantitative φT: 196 ± 12% at high excitation energy. Moreover, decoupled spins associated with conformational change were directly characterized by time-resolved electron paramagnetic resonance. Strong interplay of the geometry change and electronic localization was revealed on the triplet pair dissociation through magnitudes of spin-spin exchange couplings. Thus, a synergetic role of conformational flexibility and electronic coupling in quantitative φT was clarified.

We report the successful synthesis of tetramesityldiborane(4) (Mes4B2) through the reductive coupling of a dimesitylborinium ion. Owing to the steric protection conferred by the mesityl groups, Mes4B2 shows exceptional chemical stability and remains intact in water. Single-crystal X-ray analysis revealed that Mes4B2 has an orthogonal geometry, where the B–B center is completely hidden by the mesityl groups. Remarkably, Mes4B2 emits dual fluorescence at 460 and 620 nm, both in solution and in the solid state. Theoretical calculations showed that Mes4B2 in the excited S1 state adopts a twisted or planar geometry, which is responsible for the shorter- or longer-wavelength fluorescence, respectively. The intensity ratio of the dual fluorescence is sensitive to the viscosity of the medium, which suggests that Mes4B2 has potential as a ratiometric viscosity sensor.

Tetracene cyclophanes: a series of cyclic tetracene dimers bridged by two flexible ethylene glycol units demonstrated enhanced intramolecular singlet fission through through-space orientations by suppressing the H-type excited complex.

A series of ferrocene(Fc)-bridged pentacene(Pc)-dimers [Fc−Ph(2,n)−(Pc)2: n=number of phenylene spacers] were synthesized to examine the tortional motion effect of Fc-terminated phenylene linkers on strongly coupled quintet multiexciton (5TT) formation through intramolecular singlet fission (ISF). Fc−Ph(2,4)−(Pc)2 has a relatively small electronic coupling and large conformational flexibility according to spectroscopic and theoretical analyses. Fc−Ph(2,4)−(Pc)2 exhibits a high-yield 5TT together with quantitative singlet TT (1TT) generation through ISF. This demonstrates a much more efficient ISF than those of other less flexible Pc dimers. The activation entropy in 1TT spin conversion of Fc−Ph(2,4)−(Pc)2 is larger than those of the other systems due to the larger conformational flexibility associated with the torsional motion of the linkers. The torsional motion of linkers in 1TT is attributable to weakened metal-ligand bonding in the Fc due to hybridization of the hole level of Pc to Fc in 1TT unpaired orbitals.

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)n: n=2, 4, 6]. Then, a comparison of (Tc)n and Tc-hetero-oligomer [TcF3-(Tc)4-TcF3] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF3 units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: ΦTrT=176 %).