Tsuyoshi Kawai

Photoresponsive systems capable of generating ion pairs provide promising opportunities for light-controlled catalysts, yet the structural factors governing ion-pair formation and stability remain poorly understood. Here we report a series of terarylene derivatives (1o–3o) that undergo photoinduced cyclization to generate ion pairs with distinct reactivity profiles. Spectroscopic studies combined with density functional theory calculations reveal that ion-pair formation strongly depends on the leaving group and solvent polarity. While 1o readily generates a highly stable cationic species upon irradiation even in nonpolar media, 2o and 3o exhibit alternative pathways involving neutral closed intermediates or other photoinduced processes. These differences give rise to markedly different photochemical behaviors in polar and nonpolar media. Notably, the photoinduced ion pairs exhibit catalytic activity in ring-opening polymerization reactions. Whereas 1o promotes self-initiated cationic polymerization, 2o and 3o require external protic initiators under the conditions examined, while the specific catalytic role of the counteranion was assumed as a working hypothesis. These findings provide structure–reactivity insights for controlling photoinduced ion-pair formation and demonstrate their potential as light-activated catalysts.

The influence of the positional placement of chiral handles on a molecular core upon self-organization and chiroptical properties is investigated using phenylalanine-substituted oligo(p-phenyleneethynylene)s (OPEs) through joint chiroptical and molecular dynamics simulation studies. The linear OPE systems with d- and l-phenylalanine units appended at the meta- and para-positions of the terminal benzene ring exhibit exciton-coupled and induced circular dichroism (CD), respectively, upon self-assembly under identical experimental conditions. Meta-substituted OPEs form ordered helical stacks in 50% (v/v) water in methanol, facilitating on-resonance intermolecular transition dipole moment coupling that produces bisignate CD signals and strong circularly polarized luminescence (CPL). In contrast, para-substituted OPEs assemble into ladder-type structures under the same conditions, leading to off-resonance coupling between the terminal chiral handles and the OPE core, resulting in weak monosignate CD signals that are devoid of CPL. The potential of meta-substituted OPEs as temperature-dependent chiroptical switches is also demonstrated.

Despite significant advances, simultaneously achieving a high emission dissymmetry factor (glum) and high luminescence efficiency remains a major challenge in the field of circularly polarized luminescent (CPL) materials. Here, we report a dual strategy that combines liquid crystal doping with naphthalimide-based thermally activated delayed fluorescence (TADF) emitters and controlled surface alignment to amplify CPL in chiral nematic liquid crystal (N*LC) systems. Two emitters were investigated: 1, bearing a conventional octyl chain, and 2, functionalized with cyanobiphenyl mesogenic units to enhance compatibility with the nematic host. Both compounds show TADF behavior within the LC matrix. Controlled homogeneous and homeotropic alignment of the N* phase enabled the systematic investigation of boundary-induced effects on CPL performance. Homogeneous alignment proved particularly effective, delivering high glum values of up to 0.14 even at low dopant loading (0.5 wt%), maintaining stable chiroptical responses over 30 days. These findings highlight macroscopic LC alignment as a critical, yet underexplored, parameter governing CPL performance and establish practical design principles for next-generation CP-TADF/LC materials targeting photonic and display device applications.

Photochemical and thermal peri-cyclization and cycloreversion reactions of organic polyenes are typically governed by orbital symmetry considerations. Here, we report two aryl-imidazoisoindole derivatives featuring benzene and thiophene rings that undergo reversible photochromic reactions via 10π cyclization and cycloreversion. The synthetic study was fully characterized using NMR, mass spectroscopy, and X-ray analyses, which are further supported by quantum chemical calculations based on density functional theory (DFT). Both molecules exhibit a reversible color change under UV and visible light irradiation. Photochemical quantum yields are presented, showing suppression with solvent polarity. The colored ring-closed form displayed a spontaneous fading reaction, whose thermodynamic parameters are well characterized. The thiophene-based substance exhibited a lifetime of 67 h, which was markedly reduced upon protonation of the imidazole ring. Notably, small amounts of acid markedly enhanced the spontaneous fading reaction through a catalytic mechanism. DFT calculations clearly supported the ground-state structures and the major contribution of the neutral mesomeric form in both substances. The reaction pathway and transition state for the thermal reaction were also characterized, which rationally support the major contribution of reversible 10π-photocyclization/cycloreversion reactions.

We report new photochromic diarylethenes incorporating a curved aromatic corannulene unit as a side aryl group: 1-(2-methoxycorannulenyl)-2-(2-methyl-5-phenyl-3-thienyl)­perfluorocyclopentene (1) and its sulfone derivative 1-(2-methoxycorannulenyl)-2-(1,1-dioxide-2-methyl-5-phenyl-3-thienyl)­perfluorocyclopentene (2). These compounds are compared with our previously reported compound 1,2-bis­(2-methoxycorannulenyl)­perfluorocyclopentene (3). Both 1 and 2 exhibited photochromic color changes with photochemical quantum yields of 0.44 (cyclization) and 0.025 (cycloreversion) for 1 and 0.54 (cyclization) and 0.034 (cycloreversion) for 2. The open-ring isomers, 1o and 2o, displayed fluorescence at room temperature, which is attributed to locally excited (LE) and intramolecular charge transfer (ICT) states, respectively. Whereas the closed-ring isomer 1c exhibited no fluorescence, 2c showed orange fluorescence at 615 nm with a fluorescence quantum yield of 0.010 indicating fluorescence wavelength switching. Furthermore, 1o and 3o exhibited phosphorescence at 80 K in 2-methyltetrahydrofuran (2-MTHF) promoting a phosphorescence switching nature, while 2 o /2c did not. This difference can be rationalized by the LE character of the excited states and efficient intersystem crossing facilitated by the curved corannulene moiety.

Circularly polarized luminescence (CPL) is a powerful tool to probe the chiroptical properties of molecular systems in the excited state. Here, we demonstrate the evolution of excited‐state chiroptical property in silver nanoclusters (NCs) by CPL measurements. The use of bidentate thiolate ligand, 1,3‐benzenedithiol (BDT), with the aid of triphenylphosphine (PPh ₃ ), affords racemic [Ag ₂₉ (BDT) ₁₂ (PPh ₃ ) ₄ ] ³⁻ NCs (Ag ₂₉ NCs) with intrinsic chirality in the exterior cage composed of a silver(I)‐dithiolate coordination framework, that are separable by chiral column chromatography. We have recently reported that the post‐synthetic modification of negatively charged Ag ₂₉ NCs with a cationic silver(I) complex on the exterior cage stabilizes a specific excited state with a triplet character, enhancing photoluminescence (PL) in the near‐infrared (NIR) region. While the same modification on the separated enantiomeric NCs does not change the circular dichroism (CD) profile so much, the CPL activity is dramatically enhanced with an increase in the anisotropy factor (| g _lum | value) from −3 to 6 × 10 ⁻³ , demonstrating the excited‐state dependent CPL property of Ag ₂₉ NCs. In combination with the picosecond PL decay study, the low‐temperature CPL measurement indicates the time development of excited‐state property, suggesting the possible evolution of CPL activity.

Circularly polarized luminescence (CPL) is a powerful tool to probe the chiroptical properties of molecular systems in the excited state. Here, we demonstrate the evolution of excited‐state chiroptical property in silver nanoclusters (NCs) by CPL measurements. The use of bidentate thiolate ligand, 1,3‐benzenedithiol (BDT), with the aid of triphenylphosphine (PPh ₃ ), affords racemic [Ag ₂₉ (BDT) ₁₂ (PPh ₃ ) ₄ ] ³⁻ NCs (Ag ₂₉ NCs) with intrinsic chirality in the exterior cage composed of a silver(I)‐dithiolate coordination framework, that are separable by chiral column chromatography. We have recently reported that the post‐synthetic modification of negatively charged Ag ₂₉ NCs with a cationic silver(I) complex on the exterior cage stabilizes a specific excited state with a triplet character, enhancing photoluminescence (PL) in the near‐infrared (NIR) region. While the same modification on the separated enantiomeric NCs does not change the circular dichroism (CD) profile so much, the CPL activity is dramatically enhanced with an increase in the anisotropy factor (| g _lum | value) from −3 to 6 × 10 ⁻³ , demonstrating the excited‐state dependent CPL property of Ag ₂₉ NCs. In combination with the picosecond PL decay study, the low‐temperature CPL measurement indicates the time development of excited‐state property, suggesting the possible evolution of CPL activity.

Theta-solvent-driven self-assembly of poly(methyl methacrylate) enables selective and stable dispersion of single-walled carbon nanotubes.