Wataru Ota

Pentaphenylphenyl-substituted diphenylpyridylmethyl radicals combine high photoluminescence quantum yields and remarkable photostability with independently controllable electronic structures at the D ₀ - and D ₁ -optimised geometries.

This study derives expressions for the photophysics of a molecule exhibiting the Herzberg–Teller (HT) effect. The derivation is based on Fermi’s golden rule, and the Longuet-Higgins representation, where the electron–photon and vibronic couplings are treated as perturbations, is used. All the vibrational modes are included, thus enabling the assignment of vibronic progressions in the spectrum originating from Franck–Condon factors. A forbidden transition is regarded as the photon absorption or emission accompanied by phonon absorption or emission, clarifying the contribution of vibronic structures to the transition. The derived expressions are applied to the absorption from S0 to S1 and fluorescence from S2 to S0 for azulene, a molecule known to exhibit the HT effect. Using the S4 state as an intermediate state, the calculated fluorescence spectrum and rate constant agree well with the experimental results. The in-plane stretching vibration of the five-membered ring contributes to phonon emission, whereas that at the junction of the five-and seven-membered rings contributes to the acceptance of the excess excitation energy. Furthermore, the vibronic coupling density analyses reveal that the vibronic couplings originate from the five-membered ring. This study presents a theory that allows for a detailed assignment of vibronic structures involved in the HT effect, providing design guidelines for HT molecules.

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.

Systematic investigation of multi-substituted carbazole-dendronized radicals reveals the relationship between dendron architecture and luminescence.

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.

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.

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.

The nonlinear optical phenomenon of two-photon absorption (TPA) has numerous applications in scientific fields. However, designing materials with efficient TPA that meet the technical requirements is challenging. The fundamental optical properties of a material depend on its density of states (DOS). In this study, we achieved efficient TPA by controlling the DOS via tuning the orbital energy levels of gold clusters and molecular ligands. We synthesized Au₃₆(NP)₂₄ with an atomically precise Au₃₆ core and 24 naphthalenethiol (NP) ligands and evaluated the orbital hybridization effect on the TPA cross-section, σ⁽²⁾, of 6000 GM under 800 nm excitation. The synthesized Au₃₆(NP)₂₄ exhibited a considerably high σ⁽²⁾, originating from the DOS, suitable for near-double resonance enhancement because of the hybridization between the Au₃₆ and (NP)₂₄ orbitals. This orbital hybridization approach between functional molecules and a metal kernel provides a versatile design strategy for tuning the DOS of materials for various purposes including nonlinear optics.

Luminescent radicals are gathering much attention as a new class of luminescent material. We have synthesized a new carbazole oligomer (mono, di, tri, tetra) substituted luminescent tris (2,4,6-trichlorophenyl) methyl (TTM) radicals. The photoluminescence (PL) spectra showed emission stemmed from the excited state charge transfer character. Despite the increase in the highest occupied molecular orbital level of the carbazole oligomer associated with increasing the oligomer length, the PL maxima showed a blue shift from di- to tetra-oligomer substitution. This unexpected behavior was explained by quantum chemical calculations, including electron–electron repulsion. The electron–electron repulsion on the donor orbital decreases when the oligomer length increases and leads to the blue shift of the emission. The blue shift of the emission accompanied by an increase in the photoluminescence quantum yield by substituting a large π-conjugated donor to a TTM radical would be a unique method to control the photophysical property of the luminescent radical.