ICYS Annual Report 2023

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ICYS Annual Report 2023complexes scaffolds but also Yuichi HIRAI1. Outline of Research2. Research ActivitiesTable 2. Summary of the crystallographic information obtained from SCXRD analysis and DFT calculations.References 1) Y. Hirai, ACS Appl. Opt. Mater. 2 (2024) 1025-1045. 2) Y. Zhuang, R.J. Xie, Adv. Mater. 33 (2021) 1-33. 3) Y. Xie, Z. Li, Chem. 4 (2018) 943-971. 4) J.C.G. Bünzli, K.L. Wong, J. Rare Earths. 36 (2018) 1-41. 5) Y. Sagara, T. Kato, Nat. Chem. 1 (2009) 605-610. 6) H. Ito et al. J. Am. Chem. Soc. 138 (2016) 6252-6260. 7) Y. Hirai et al. Adv. Opt. Mater. 2203139 (2023) 1-7. 8) Y. Hasegawa et al. Chem.-A Eur. J. 24 (2018) 1956-1961. 9) Y. Hirai et al, Mater. Lett. 384 (2025) 138112.Fig. 1. A schematic of Ln(III) mechanophores.Materials exhibiting photon emission under mechanical stimuli, including mechanoluminescence (ML) and mechanoresponsive luminescence (MRL), have gained attention for converting mechanical signals into photophysical outputs over a broad spectral range.1) Although these mechanically driven light-emitting materials are promising for stress sensing, healthcare devices, and security marking,2,3) deeper insights into their underlying mechanisms remain limited. Lanthanide(III) (Ln(III)) molecular materials constitute a major family of ML-active systems,4) whereas MRL is predominantly observed in fluorescent organics and d-block metal complexes.5,6) Ln(III) complexes exhibit unique emission characteristics, including environment-independent wavelengths and line-like profiles, due to their shielded f orbitals. As a result, developing Ln(III)-based mechanically responsive luminophores is a growing field in materials science. Recently, we described the first Ln(III) dinuclear complex bearing tmh (2,2,6,6-tetramethylheptane-3,5-dionate) and dpdf (2,7-bis(diphenylphosphoryl)-9,9-dimethylfluorene) ligands (Ln2(tmh)6(dpdf)), which displayed MRL via a ratiometric emission shift between Eu(III) and Tb(III) ions.7) Notably, its constrained dpdf linkage also yielded bright ML despite minimal photoluminescence (PL).To investigate structural–mechanical–photophysical correlations further, we focused on varying the flexibility of Ln(tmh)3-based bulky dinuclear complexes. Here, we highlight how free, locked, and locked + bent bridging ligands (dpbp: 4,4’-bis(diphenylphosphoryl)biphenyl,8) dphp: 2,7-bis(diphenylphosphoryl)-9,10-dihydrophenanthrene) impact their mechanical responsiveness (Figure 1).9) This study broadens our understanding of how structural modifications govern mechanically induced luminescence in Ln(III) systems. By systematically controlling these bridging motifs, we reveal new insights into the interplay between structural rigidity, mechanical force transfer, and luminescence modulation.Three types of dinuclear Ln(III) complexes with tmh ligands bridged by dpbp, dphp, and dpdf ligands, were prepared. Here, tmh serves as an ideal antenna ligand for Tb(III) excited states, while it quenches the Eu(III) excited states via ligand-to-metal charge transfer (LMCT) states, resulting in low PL efficiencies.The ML activity was confirmed in the “locked” and “locked + bent” systems, regardless of their centrosymmetry in space groups (P-1 and Fdd2, respectively) and the existence of aforementioned quenching processes. On the other hand, the heterodinuclear Tb(III)/Eu(III) demonstrated ratiometric MCL in all cases, resulting in red-shifted emissions in their ground forms. Therefore, the “locked” system is the second example of a dual ML/MCL-active Ln(III) complex. Single crystal X-ray diffraction (SCXRD) analysis also revealed differences in structural relaxation trends upon heating (Table 2), which may cause non-gradual change in photophysical processes, such as Tb-to-Eu energy transfer, f-f transitions, sensitization, and quenching pathways. The temperature-dependent PL is typically analyzed using the crystal structure determined at a specific temperature (i.e. 77 K, RT); however, structural changes upon heating may play a crucial role in explaining these unique photophysical properties. This study showcases the importance of controlling the degrees of freedom within a molecule to not only construct rationally designed mechanical induce counterintuitive photophysical properties.Research Digest 13Conformational Freedom and LuminescenceResponsivity of Lanthanide (III) Mechanophores