Hikaru Kuramochi

Dithieno[ b , f ]arsepins are compact dual-emissive fluorophores. Excited-state planarization through coupled phenyl rotation and arsenic translocation creates two emissive minima, enabling ratiometric viscosity sensing over a wide range.

Photosynthetic organisms thrive across remarkably diverse light environments, from dim, spectrally filtered habitats to high-irradiance niches. Their antenna complexes have evolved structural and excitonic features that tailor energy capture and delivery to local conditions, making comparative studies of noncanonical antennas essential for understanding the diversity of evolved designs. Here, we investigate the far-red light-harvesting complex from an Antarctic alga, Prasiola crispa (Pc-frLHC), whose ring-shaped undecameric (11-subunit) antenna absorbs strongly near ∼710 nm yet still drives photosystem II. Using femtosecond transient absorption with selective excitation of chlorophyll (Chl) b (645 nm), Chl a (675 nm), and far-red Chls (740 nm), we resolve a rapid, directional energy transfer cascade that funnels population into the lowest excitonic level of the far-red Chl trimer. Transient absorption anisotropy directly resolves a characteristic hopping time between the far-red Chl trimers of ∼11.5 ps, while exciton–exciton annihilation kinetics under increased fluence at far-red excitation provide independent corroboration. Spectral dynamics and anisotropy further reveal the existence of a higher-lying excitonic state near ∼670 nm within the far-red Chls. Thermal access to this ∼670 nm state enables repeated cycling that facilitates the uphill transfer toward Chl a despite a sizable energy gap of ∼580 cm−1. These findings clarify how Pc-frLHC exploits the excitonic coupling and the ring-mediated transport to harvest redshifted light for photosystem II excitation, offering general principles for engineering photofunctional systems adapted to spectrally limited environments.

Luminescent organic radicals often exhibit a red-shifted, broad, and structureless emission band when assembled in a host matrix. This emission has been attributed to pairs of neighboring photoexcited and ground-state radicals forming an excimer. However, the electronic character of this "excimer-like" (EX) state has not been definitively clarified through experiments. Here we present an experimental characterization of this EX state using a spatially confined luminescent diradical as a model for radical dimers. Comprehensive spectroscopic investigations revealed that the EX state of the diradical is a zwitterionic (ZI) state with a closed-shell singlet electronic configuration generated by intramolecular symmetry-breaking radical-to-radical charge transfer. In combination with previous magnetoluminescence studies, we experimentally show that the formation of the ZI state is specific to the singlet spin state. Our results enable a detailed understanding of the photophysics and photochemistry of radical multichromophoric systems for future spin photonics applications.

Vibrational chiroptical spectroscopy is a valuable tool for investigating the three-dimensional structure of various biomolecules, such as proteins and nucleic acids, in the solution phase. Since the functions of these biomolecules are closely related to their dynamic behavior, the extension of vibrational chiroptical spectroscopy to time-resolved measurements is a promising approach for elucidating the mechanisms responsible for biological functions. However, the implementation of vibrational chiroptical spectroscopy with an ultrashort pulsed laser source—a crucial step toward time-resolved chiroptical spectroscopy—presents significant technical challenges due to its inherently low signal intensity and the difficulty of precise polarization control. Here, we report a sensitive and effective method to measure vibrational Raman optical activity (ROA) spectra using sub-10-fs pulses, which we call time-domain Raman optical activity (TD-ROA) spectroscopy. Specifically, ROA spectra were obtained by measuring a time-dependent change in circular birefringence accompanied by coherent nuclear wavepacket motion. We performed proof-of-principle TD-ROA measurements with (+)- and (−)- β -pinene in the spectral range from 400 to 1200cm ⁻¹ , which shows the capability of the method to discriminate enantiomers. Our approach paves the way for tracking chiral nuclear motion that is correlated to the activation of various biomolecular functions.

Femtosecond time-domain Raman spectroscopy and DFT calculations examine the excited triplet [Au(CN) ₂ ⁻ ] oligomers from the dimer to pentamer, revealing stronger Au–Au bonds in the central part and a unique nature of the photo-induced p _z –p _z σ bonds.

Molecular symmetry is a central design element in functional materials, yet its dynamic modulation in the excited state and its consequences for optoelectronic properties remain largely unexplored, particularly in main-group p-block element complexes. We address this knowledge gap by investigating unique Al(III) dinuclear triple-helical complexes that combine high symmetry with twisted π-conjugated systems and achieve exceptional optical properties of unusually large Stokes shifts and high photoluminescence quantum yields. Using transient absorption spectroscopy with a 10 fs pump pulse, we detected coherent vibrational oscillations overlapped with transient absorption/stimulated emission signals. Analysis of the dephasing times of oscillatory signals revealed photoexcitation-triggered Jahn-Teller distortions in these high-symmetry p-block complexes, evidenced by a specifically short dephasing time constant of 410 fs associated with intraligand twisting vibrations. Our findings demonstrate that excited-state symmetry breaking, strongly coupled with intraligand twisting vibrations, is crucial in determining the remarkable photofunctional properties of large Stokes shifts and high photoluminescence quantum yields. This work elucidates the fundamental mechanisms underlying the performance of these Al(III) complexes and provides a conceptual framework for designing next-generation photofunctional materials by harnessing dynamic symmetry changes.

Rhodopsins, an important group of photoreceptor proteins, possess a retinal chromophore. It has long been believed that the photoisomerization of the chromophore is the initial event triggering its photocycle. However, we recently reported that protein structural changes around the chromophore precede photoisomerization in bacteriorhodopsin (BR). In this study, we performed deep-ultraviolet femtosecond stimulated Raman measurements for H⁺-pump rhodopsin (RxR) and photosensor rhodopsin (NpSRII) to investigate whether these ultrafast protein dynamics are common among rhodopsins. We observed that the protein structural changes occur within 0.2 ps after photoexcitation and precede the chromophore photoisomerization, similar to the case for BR. This strongly indicates that the protein structural change precedes the photoisomerization in rhodopsins, regardless of the difference in their origins and functions. Furthermore, we found that only limited protein changes occur on the time scale of the photoisomerization, suggesting that the protein environment is already optimized for the structural change of the chromophore. These observations raise the possibility that the ultrafast protein change arranges the environment around the chromophore to facilitate the photoisomerization.

Excited-state aromaticity is one of the most widely applied concepts in the field of chemistry, often used as a rational guideline for predicting conformational changes of cyclic π-conjugated systems induced by photoexcitation. Yet, the details of the relationship between the corresponding photoinduced electronic and structural dynamics have remained unclear. In this work, we applied femtosecond transient absorption and time-resolved time-domain Raman spectroscopies to track the nonequilibrium planarization dynamics of a cyclooctatetraene (COT) derivative associated with the excited-state aromaticity. In the femtosecond time-resolved Raman data, the bent-to-planar structural change was clearly captured as a continuous peak shift of the marker band, which was unambiguously identified with ¹³C labeling. Our findings show that the planarization occurs after a significant change in the electronic structure, suggesting that the system first becomes aromatic, followed by a conformational change. This work provides a unique framework for understanding the excited-state aromaticity from a dynamical aspect.

Molecular design of dual‐fluorescent probes requires precise adjustment of the energy levels of two excited states and the energy barrier between them. While the hybridized local and charge‐transfer (HLCT) state has been recently focused as an important excited state for high emission efficiency with a tunable energy level, a dual emission involving the HLCT state has been only achieved with the excited‐state intramolecular proton transfer (ESIPT) system. Here, a series of dual‐fluorescent molecules involving an HLCT excited state with the excited‐state conjugation enhancement (ESCE) motif is presented as the first case. The energy level of the HLCT state has been adjusted by changing substituents and solvents, separately from the ESCE energy level. The HLCT‐ESCE molecular design with tunable fluorescence properties proposes a new strategy for the development of advanced fluorescent probes.

Excited-state aromaticity is one of the most widely applied concepts in the field of chemistry, often used as a rational guideline for predicting conformational changes of cyclic π-conjugated systems induced by photoexcitation. Yet, the details of the relationship between the corresponding photoinduced electronic and structural dynamics have remained unclear. In this work, we applied femtosecond transient absorption and time-resolved time-domain Raman spectroscopies to track a nonequilibrium planarization dynamics of cyclooctatetraene (COT) derivative associated with the excited-state aromaticity. In the femtosecond time-resolved Raman data, the bent-to-planar structural change was clearly captured as a continuous peak shift of the marker band, which was unambiguously identified with 13C-labeling. Our findings show that the planarization occurs after a significant change in the electronic structure, suggesting that the system first becomes aromatic, followed by a conformational change. This work provides a unique framework for understanding the excited-state aromaticity from a dynamical aspect.