Tohru Sato

The dihedral angle of π‐conjugated backbones critically influences the properties of organic semiconductors, yet whether and how side‐chain stereochemistry controls backbone torsion at the molecular level remains poorly understood. Here, we establish a direct relationship between the absolute configuration of 2‐ethylhexyl (EH) side chains and the preferred dihedral angles of CPDT–A–CPDT molecules (CPDT = cyclopenta[2,1‐ b :3,4‐ b ’]dithiophene; A = quinoxaline‐based acceptors). The absolute configurations of mono‐formylated CPDT (CPDT‐CHO) stereoisomers were determined by combining density functional theory calculations with experimental ¹ H NMR analysis, enabling stereochemical assignment of the corresponding CPDT–A–CPDT derivatives. A clear correlation between the chemical shift of the thiophene β proton adjacent to the acceptor unit (H _d ) and the CPDT–A dihedral angle established H _d as a spectroscopic probe of local backbone conformation. Analysis of stereochemically defined stereoisomers led to a steric‐interaction model in which EH side‐chain stereochemistry modulates the CPDT–A dihedral angle through differences in local steric interactions. The characteristic ninefold H _d splitting was assigned to distinct stereochemical environments and was reproduced in derivatives containing different quinoxaline acceptors, demonstrating the generality of this relationship. These findings provide a molecular design principle for controlling conjugated backbone conformations through side‐chain stereochemistry.

Polycyclic aromatic hydrocarbons are molecular frameworks that enable the development of efficient emitters owing to their extensive conjugation and rigid structure. Here, we introduce monohomotruxene (MHTX), designed by reducing the symmetry of homotruxene (HTX) and truxene (TX). This partial planarization greatly enhances the complex photophysics over symmetric, triply twisted HTX, substantially increasing the photoluminescence quantum yield (PLQY), a result we attribute to its enhanced molecular rigidity and symmetry breaking. MHTX displays an unconventional violation of Kasha’s rule, yielding excitation-dependent dual emission. Excitation of the S ₂ state under anaerobic conditions produces intense, 455–480 nm, ultralong room-temperature phosphorescence (RTP), lasting over 12 s at room temperature. In contrast, excitation into the S ₁ state yields prompt fluorescence and weak delayed fluorescence (DF) by triplet-triplet annihilation (TTA) in the neat film. Aggregate species also present a complex emissive behavior with weak orange phosphorescence and TTA-DF. These findings establish MHTX as a promising emissive scaffold, highlighting the potential for accessing complex excited-state dynamics and advanced photophysical functionalities.

Luminescent radicals are an emerging class of materials for organic electronics, bioimaging, and quantum applications. Circularly polarized luminescence (CPL) from luminescent radicals with propeller‐type chirality remains challenging because it is difficult to simultaneously achieve high photoluminescence quantum yield (PLQY), high stability, and high racemization barriers. A series of brominated chiral luminescent radicals, CzTTBrM, 2CzTTBrM, and 3CzTTBrM, was obtained by attaching carbazole donors to TTBrM. The donor‐acceptor‐type design induces red to near‐infrared (NIR) emission via charge transfer (CT) excited states with remarkably high PLQY (up to 76%) and high photostability. The new radicals possess high racemization barriers (Δ G ^‡ (353K) = 27.9–29.1 kcal/mol), allowing the isolation of enantiopure compounds at room temperature. The enantiopure radicals exhibit CPL with broad emission across the red to NIR range (650–800 nm) and display B _CPL values (0.76–1.1), nearly one order of magnitude higher than that of TTBrM (B _CPL = 0.16). Moreover, doping these radicals into polystyrene microspheres produces whispering gallery mode (WGM) resonances, marking the first observation of WGM emission from luminescent radicals. This strategy establishes a versatile platform for integrating spin, chirality, and luminescence, offering new opportunities for applications in electroluminescence, bioimaging, and quantum photonics.

Luminescent radicals are an emerging class of materials for organic electronics, bioimaging, and quantum applications. Circularly polarized luminescence (CPL) from luminescent radicals with propeller‐type chirality remains challenging because it is difficult to simultaneously achieve high photoluminescence quantum yield (PLQY), high stability, and high racemization barriers. A series of brominated chiral luminescent radicals, CzTTBrM, 2CzTTBrM, and 3CzTTBrM, was obtained by attaching carbazole donors to TTBrM. The donor‐acceptor‐type design induces red to near‐infrared (NIR) emission via charge transfer (CT) excited states with remarkably high PLQY (up to 76%) and high photostability. The new radicals possess high racemization barriers (Δ G ^‡ (353K) = 27.9–29.1 kcal/mol), allowing the isolation of enantiopure compounds at room temperature. The enantiopure radicals exhibit CPL with broad emission across the red to NIR range (650–800 nm) and display B _CPL values (0.76–1.1), nearly one order of magnitude higher than that of TTBrM (B _CPL = 0.16). Moreover, doping these radicals into polystyrene microspheres produces whispering gallery mode (WGM) resonances, marking the first observation of WGM emission from luminescent radicals. This strategy establishes a versatile platform for integrating spin, chirality, and luminescence, offering new opportunities for applications in electroluminescence, bioimaging, and quantum photonics.

We theoretically investigated vibronic coupling responsible for nonradiative transitions, i.e., internal conversion (IC) and intersystem crossing (ISC), in xanthone. The nonradiative decay pathway of aromatic ketones is often debated because of their fast ISC. Xanthone in the gas phase follows a pathway that obeys El-Sayed’s rule, namely, IC from the 1ππ* to 1nπ* states and ISC from the 1nπ* to 3ππ* states, of which a simple pathway is adequate for analyzing vibronic structures. We employed an expression for the nonradiative rate constant based on Fermi’s golden rule within the mixed-spin crude adiabatic approximation, which has the advantage that both IC and ISC can be considered as equally vibronically induced transitions. Our calculations showed that the IC from the 1ππ* to 1nπ* state was faster than ISC channels because of stronger vibronic coupling and less favorable spin–orbit (SO) coupling to nearby triplets. In addition, the ISC from 1nπ* to 3ππ* was faster than that from 1nπ* to 3nπ* because of the large structural displacement and small energy gap. This study can provide guidelines for determining whether IC or ISC dominates, depending on the balance between vibronic coupling, SO coupling, and the energy gap, thereby informing molecular design for controlled nonradiative decay. ISC can dominate its IC counterpart in a xanthone derivative by tuning the singlet–triplet energy gap and/or the SO coupling.

Organic luminescent radicals have emerged as promising open-shell emitters for advanced optoelectronic and sensing applications. Among them, the carbazole-substituted tris(2,4,6-trichlorophenyl)methyl radical (Cz-TTM) exhibits remarkable photostability and emission efficiency compared with the parent TTM radical. Nevertheless, Cz-TTM can be visibly degraded by indoor light irradiation and its photodegradation mechanism is poorly understood. Here, we systematically investigate the photochemical behavior of Cz-TTM in solutions, together with the influence of impurities on the photophysical properties of the compound. We found that emissive and non-emissive impurities are introduced depending on the purification methods and the light irradiation conditions. Impurities cause underestimation of photoluminescence quantum yield through competitive absorption. Photodegradation under continuous irradiation undergoes a multi-step reaction via metastable intermediates, which react with dissolved oxygen upon local excitation of the TTM radical moiety. Quantum-chemical calculations indicate a ring-closure followed by dechlorination within the TTM moiety as the key degradation pathway. The superior photostability of Cz-TTM to TTM originates from its stabilized first excited doublet energy level, which increases the activation energy for ring-closure reaction in Cz-TTM. These combined experimental and theoretical insights reveal the fundamental factors governing the photostability of TTM-type luminescent radicals and offer design principles for next-generation organic luminescent materials.

We theoretically investigated vibronic coupling responsible for nonradiative transitions, i.e., internal conversion (IC) and intersystem crossing (ISC), in xanthone. The nonradiative decay pathway of aromatic ketones is often debated because of their fast ISC. Xanthone in the gas phase follows a pathway that obeys El-Sayed’s rule, namely, IC from the 1ππ* to 1nπ* states and ISC from the 1nπ* to 3ππ* states, of which a simple pathway is adequate for analyzing vibronic structures. We employed an expression for the nonradiative rate constant based on Fermi’s golden rule within the mixed-spin crude adiabatic approximation, which has the advantage that both IC and ISC can be considered as equally vibronically-induced transitions. Our calculations showed that the IC from the 1ππ* to 1nπ* was faster than ISC channels because of stronger vibronic coupling and less favorable spin–orbit (SO) coupling to nearby triplets. Additionally, the ISC from 1nπ* to 3ππ* was faster than that from 1nπ* to 3nπ* because of the large structural displacement and small energy gap. This study can provide guidelines for determining whether IC or ISC dominates, depending on the balance between vibronic coupling, SO coupling, and the energy gap, thereby informing molecular design for controlled nonradiative decay. Specifically, for xanthone, the ISC from 1ππ* can dominate its IC counterpart by decreasing the energy gap or increasing the SO coupling between 1ππ* and an energetically close triplet.

Linear carbon cluster anions, such as C6−, have been considered to be promising candidate interstellar molecules. Recent experiments have demonstrated that in a collision-free vacuum environment, C6− exhibits fast radiative cooling from its highly vibrationally excited states through inverse internal conversion (IIC). Since IIC is driven by vibronic coupling, the understanding of vibronic structures of C6− is of theoretical significance. Here, we utilize time-dependent density functional theory to calculate the absorption spectrum of C6−, allowing us to reassign the peaks in the experimental spectrum to the vibronic progressions associated with the C2Πg ← X2Πu and D2Πg ← X2Πu electronic transitions. A vibronic coupling density analysis reasonably explains the strong vibronic peaks and validates the reassignment. We also discuss the potential softening caused by quadratic vibronic coupling in the excited states and the partially allowed vibronic transitions. In addition, we show that a pseudo-Jahn–Teller distortion occurs along the ν4(σu+) antisymmetric stretching mode due to the pseudo-degeneracy of the A and B states.

Pd oxide, which is epitaxially bonded to the ab surface of Sr₃Ti₂O₇, exhibits cyclic structural fluctuations and excellent catalytic activity for automotive exhaust gas purification.

Linear carbon cluster anions, such as C6–, have been considered to be promising candidate interstellar molecules. Recent experiments have demonstrated that in a collision-free vacuum environment, C6 exhibits fast radiative cooling from its highly vibrationally excited states through inverse internal conversion (IIC). Since IIC is driven by vibronic coupling, understanding of vibronic structures of C6– is of theoretical significance. Here, we utilize time-dependent density functional theory to calculate the absorption spectrum of C6–, allowing us to reassign the peaks in the experimental spectrum to the vibronic progressions associated with the C2Πg ← X2Πu and D2Πg ← X2Πu electronic transitions. A vibronic coupling density analysis reasonably explains the strong vibronic peaks and validates the reassignment. We also discuss the potential softening caused by quadratic vibronic coupling in the excited states and the partially allowed vibronic transitions. Additionally, we show that a pseudo-Jahn–Teller distortion occurs along the ν4(σu+) antisymmetric stretching mode due to the pseudo-degeneracy of the A and B states.