Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry Letters  

Two-dimensional electronic spectroscopy (2DES) has been widely employed to probe the dynamics of electronic excitations. However, conventional 2DES often encounters challenges in spectral assignment due to the intrinsic limitations of transient absorption detection. To overcome these issues, fluorescence-detected 2DES has been anticipated, and broadband excitation sources can enhance its performance. In this study, we have developed a two-dimensional fluorescence excitation (2DFLEX) spectroscopy by combining white-light excitation with Kerr-gate detection. This approach exploits the white light generated from a high-repetition-rate Yb-amplifier laser, providing both broad spectral coverage and excellent stability. Importantly, 2DFLEX exhibits intrinsic selectivity for stimulated emission, enabling the isolation of excited-state dynamics. This technique represents a significant advancement in fluorescence-detected 2DES and opens new opportunities for elucidating complex electronic processes. 2DFLEX is a spectroscopic technique that integrates excitation-spectrum measurements with time-resolved photoluminescence and has the potential to become a novel standard analytical method for photofunctional materials.

DOI： 10.1021/acs.jpclett.5c03373

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Inorganic Chemistry  

Elongation of the triplet metal-to-ligand charge transfer excited state (MLCT) lifetimes in Fe(II) complexes is particularly important because of their 3d⁶ electronic configuration that is analogous to that of the well-studied photoactive Ru polypyridyl complexes. Toward extending ³MLCT state lifetimes, the use of strongly σ-donating NHC ligands is known to suppress unwanted deactivations through d-d states. Hexa-NHC coordination environments seem highly promising to provide strong ligand fields; however, Fe(II) hexa-NHC complexes are unstable in air and are oxidized to Fe(III) oxidation states under ambient conditions. In this study, we sought an air-stable Fe(II) hexa-NHC complex with a slightly weaker σ-donor from a triazole-based NHC ligand, and its photoexcited state dynamics were determined by femtosecond transient absorption spectroscopy. The obtained Fe(II) state is stable under ambient conditions (E(Fe^3+/2+) = 0.33 V vs Fc^+/0), and its photoexcited state characteristics are similar to those of reported electrochemically reduced Fe(II) hexa-NHC complexes. Our findings revealed that Fe(II) oxidation states can be stabilized while keeping a strong ligand field nature of NHCs. Although the ³MLCT lifetime remains short-lived (1.4 ps), air-stable Fe(II) hexa-NHC complexes are amenable to further exploration of the long-lived ³MLCT states in Fe(II) complexes.

DOI： 10.1021/acs.inorgchem.5c05069

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of the American Chemical Society  

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.

DOI： 10.1021/jacs.5c06020

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Aggregate  

Organic lasers hold great promise for enabling a new class of future optoelectronics. Consequently, the development of new organic semiconductors as gain media has recently been the subject of significant interest. The molecular design principle based on Einstein coefficients has been validated for achieving high gain, with para-phenylene-vinylene scaffolds recognized as one of the most crucial frameworks. In this study, we develop a stilbene tetramer derivative, QSBCz, which has significantly increased conjugation compared to the highly efficient laser material, BSBCz, resulting in a remarkably high radiative decay rate and a large gain cross-section. However, we find that the optical losses play a significant role in the light amplification of QSBCz. Indeed, a comprehensive understanding and suppression of detrimental optical loss pathways throughout the lasing process are essential, whereas the losses intrinsically associated with molecules have not been well considered. Although host–guest systems are helpful in preventing concentration quenching in aggregated states, this study reveals notable losses when using common host molecules such as 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) and mCBP. In contrast, a BSBCz derivative is successfully employed as the host, leading to improved stimulated emission amplification. These findings indicate the importance of host–emitter interactions in lasing properties and highlight the necessity to optimize host materials for developing new laser dyes.

DOI： 10.1002/agt2.70030

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/physrevb.109.195412