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## Creator

Takuma Nakai, Kaori Shima, Mengfei Wang, [Sunao Shoji](https://orcid.org/0000-0002-0329-1136), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), Koji Fushimi, Yasuchika Hasegawa, [Yuichi Kitagawa](https://orcid.org/0000-0003-1487-2531)

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This is the peer reviewed version of the following article: T. Nakai, K. Shima, M. Wang, S. Shoji, T. Nakanishi, K. Fushimi, Y. Hasegawa, Y. Kitagawa, Angew. Chem. Int. Ed.. 2025, 64, e202513236, which has been published in final form at https://doi.org/10.1002/anie.202513236. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Dynamic Molecular Triplet Excitons Tune Lanthanide Emission Lifetime](https://mdr.nims.go.jp/datasets/8fa8d5f4-d3ee-4be6-b7df-81b35610f903)

## Fulltext

((Title))COMMUNICATION          1  Dynamic Molecular Triplet Excitons Tune Lanthanide Emission Lifetime Takuma Nakai,[a] Kaori Shima,[a] Mengfei Wang,[b,c] Sunao Shoji,[d] Takayuki Nakanishi,[e] Koji Fushimi,[b] Yasuchika Hasegawa,[b,c] and Yuichi Kitagawa*[b,c] [a] T. Nakai, K. Shima Graduate School of Chemical Sciences and Engineering, Hokkaido University Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628 (Japan). [b] Dr. M. Wang, Dr. K. Fushimi, Dr. Y. Hasegawa, Dr. Y. Kitagawa Faculty of Engineering, Hokkaido University Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628 (Japan). E-mail: y-kitagawa@eng.hokudai.ac.jp [c] Dr. M. Wang, Dr. Y. Hasegawa, Dr. Y. Kitagawa Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021 (Japan). [d] Dr. S. Shoji Faculty of Engineering, Nara Women’s University Kitauoya-Nishimachi, Nara 630-8506 (Japan). [e] Dr. T. Nakanishi National Institute for Material Science Namiki, Tsukuba, Ibaraki 305-0044 (Japan).  Supporting information for this article is given via a link at the end of the document.  Abstract: Optical codes generated from the emission lifetimes of lanthanide complexes have received considerable attention for their potential application in multiplexed bioimaging and anti-counterfeiting. However, such tunable lifetimes are limited to short ranges (0.01−1 ms). Herein, we describe a method that extends the emission lifetime using dynamic molecular triplet excitons in aggregation systems composed of lanthanide complexes. To demonstrate the conceptual method, molecular crystals comprising lutetium (Lu3+) and luminescent lanthanide (europium: Eu3+ or terbium: Tb3+) complexes with two organic ligands (2,2,6,6-tetramethyl-3,5-heptanedionate and 2,7-bis(diphenylphosphoryl)-phenanthrene) were prepared. These molecular crystals exhibited persistent emission via slow triplet exciton migration between the phenanthrene units. The extended degree of the lifetime could be controlled between 1 and 152 ms by ligand modifications. This proposed lifetime-tuning method should significantly expand the encoding capacity of lanthanide complexes.  Optical encoding is a technique to generate multiple codes by combining optical parameters such as ratiometric emission intensity and emission lifetime of luminophores.[1-4] This technique plays a crucial role in multiplexed bioimaging[5-8] and anti-counterfeiting applications.[9-12] Among the luminophores for optical encoding, trivalent lanthanide complexes are promising candidates for optical encoding owing to their distinguishable sharp emission bands (Full Width at Half Maximum (FWHM) ≈ 10 nm) originating from 4f-4f transitions.[13-21] These complexes also exhibit bright emission originating from the high absorption abilities of their organic ligands via energy transfer from the molecular triplet exciton.[22-24]  Figure 1. a: Conceptual design of this study. Black and red balls represent Lu3+ and Eu3+ (or Tb3+), respectively. b: Emission lifetime tuning range of the lanthanide complexes in a previous study (0.01−1 ms) and this study (1−152 ms). c: Chemical structures of Eu3+-(or Tb3+-)-incorporated Lu3+ molecular crystals for proof-of-concept. COMMUNICATION          2  Recently, emission lifetime has become an emerging parameter for the effective optical encoding of lanthanide complexes because it offers detection capabilities independent of ambient background.[13-15,19] Gallis et al. achieved data storage and document security using lanthanide metal-organic frameworks (MOFs) with tunable Eu3+ emission lifetime by controlling the energy transfer from Eu3+ to Yb3+.[13] Lu et al. demonstrated multiplexed bioimaging by tuning the Eu3+ emission lifetime based on the energy transfer from Eu3+ to an organic ligand.[15] However, these lifetime-tuning methods are derived from a quenching system for lanthanide emissions, and the tuning range is narrow (0.01−1 ms, Figure 1b). The development of a new method for extending the lifetime of lanthanide complexes will significantly expand their encoding capacities. Herein, we describe a new method for emission lifetime tuning utilizing the slow triplet exciton migration between the organic ligands in aggregated lanthanide complexes (Figure 1a). This slow exciton migration results in delayed formation of lanthanide-emitting states, leading to an extended emission lifetime. To demonstrate the proof-of-concept, luminescent Eu3+- (or Tb3+-)-incorporated Lu3+ molecular crystals comprising one neutral phenanthrene-based ligand (dpph: 2,7-bis(diphenylphosphoryl)phenanthrene), Figure 1c) and six anionic β-diketonate ligands (tmh: 2,2,6,6-tetramethyl-3,5-heptanedionate) were designed. The dpph ligand, encapsulated by the bulky anionic tmh ligands, exhibited a long-lived triplet state (τ = 47 ms[25]), which was selected to facilitate an efficient high triplet-triplet energy transfer (TTET) between the dpph ligands. Non-luminescent Lu3+ (lowest excited state: ~80,000 cm-1 [26]) also promotes TTET in the crystals [27], resulting in delayed emission of Eu3+ (or Tb3+) (see Supplementary Note 1 for a discussion of systems with other non-luminescent lanthanide ions). The delayed emission lifetime can be tuned by controlling the Eu3+ (or Tb3+) molar ratio. Another type of phenanthrene-based ligand (dcph: 2,7-bis(dicyclohexylphosphoryl)phenanthrene) was prepared to demonstrate tunable optical codes via ligand modification. The present dynamic triplet exciton system significantly expands optical encoding capacities to develop multiplexed bioimaging and anti-counterfeiting applications.  Figure 2. X-ray crystal structure of Lu2-dpph viewed along b-axis (50% probability ellipsoids). The distances were calculated based on the centroids of phenanthrene units. The [Lu2(tmh)6dpph] (where Lu2-dpph) was prepared using a previously reported method.[25] Single-crystal X-ray diffraction (XRD) analysis showed seven-coordinated dinuclear structures, consisting of two Lu(tmh)3 units connected by a dpph ligand (Crystallographic data are listed in Table S1, Supporting Information). In the X-ray crystal structure, each Lu2-dpph unit is surrounded by two adjacent Lu2-dpph units (Figure 2 and S14). The distances between the phenanthrene units were determined to be 1.02 and 1.05 nm. The preparation and identification of [Eu2(tmh)6dpph] (Eu2-dpph) and [Tb2(tmh)6dpph] [25] (Tb2-dpph) were similar to those of Lu2-dpph (Figure S15, crystallographic data are shown in Table S1). The photophysical measurements were performed in the crystalline state. The emission spectrum (Figure 3a) of Eu2-dpph exhibits emission bands at approximately 578, 587, 611, 648, and 701 nm, which are assigned to the 5D0→7FJ (J = 0, 1, 2, 3, and 4, respectively) transitions of Eu3+. For Tb2-dpph, emission bands at approximately 490, 548, 582, 616, and 659 nm were observed. These emission bands are attributed to the 5D4→7FJ (J = 6, 5, 4, 3, and 2, respectively) transitions of Tb3+.[25] The excitation spectra showed characteristic vibronic signals at approximately 346 and 362 nm in the UV region (Figure S16), indicating the effective emission via energy transfer from the dpph ligands. The emission decay curves of Eu2-dpph and Tb2-dpph (Figure 3b) were fitted to single-exponential function. Emission lifetimes for Eu2-dpph and Tb2-dpph were independent of the excitation wavelength (dpph excitation: τEu = 0.55 ms, τTb = 0.83 ms (λex = 356 nm) and lanthanide excitation: τEu = 0.55 ms (λex = 510 nm, Figure S18), τTb = 0.87 ms (λex = 495 nm)). These results indicate the absence of delayed emission via intermolecular triplet exciton migration (Figure 3c and Figure S19, details of the excited state dynamics are given in Supplementary Note 2), which is caused by a very fast energy transfer from the organic ligands to the lanthanide ions.[28-30]  Figure 3. a: Emission spectra (λex = 356 nm, crystalline state, 293 K) of Eu2-dpph (upper) and Tb2-dpph (lower) under degassed condition. b: Emission decay curves (λex = 356 nm, crystalline state, 293 K) of Eu2-dpph (upper, λem = 611 nm) and Tb2-dpph (lower, λem = 548 nm) under degassed condition. c: Schematic image of lanthanide emission in the Eu2-dpph and Tb2-dpph crystals. Red balls represent Eu3+ or Tb3+. COMMUNICATION          3  Mixed lanthanide crystals, [EuxLu2-x(tmh)6dpph] (EuxLu2-x-dpph) and [TbxLu2-x(tmh)6dpph] (TbxLu2-x-dpph), were prepared by complexation of the dpph ligand with [Lu(tmh)3] and either [Eu2(tmh)6] or [Tb2(tmh)6] in methanol. The Eu3+ or Tb3+ ions were mixed at concentrations of 1, 5, or 10 mol%. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) revealed the molar ratios for EuxLu2-x-dpph (x = 0.010, 0.028, or 0.084) and TbxLu2-x-dpph (x = 0.013, 0.060, or 0.140). The powder X-ray diffraction (PXRD) patterns of the lanthanide mixed crystals were similar to that of Lu2-dpph (Figure S20), suggesting similar crystal structures. The emission spectral shapes of EuxLu2-x-dpph and TbxLu2-x-dpph (Figure 4a and S21) are almost identical to those of Eu2-dpph and Tb2-dpph, respectively. Their photophysical properties are summarized in Table 1. The emission photographs of Eu0.010Lu1.990-dpph and Tb0.013Lu1.987-dpph are depicted in Figure 4b. EuxLu2-x-dpph exhibits a significantly extended emission decay time range compared with that of Eu2-dpph (Figure 4c and S22), which increases with decreasing Eu3+ molar ratios. The decay curves are fitted to triple-exponential functions (Table S3). In contrast to the single-exponential decay observed in Eu2-dpph and Tb2-dpph, the multi-exponential decay behavior in EuxLu2-x-dpph and TbxLu2-x-dpph suggests the presence of intermolecular triplet exciton migration (Figure 4d). The average emission lifetime (τavg) for Eu0.010Lu1.990-dpph, Eu0.028Lu1.972-dpph, Eu0.084Lu1.916-dpph is estimated to be 7.42, 3.54, and 1.22 ms, respectively. TbxLu2-x-dpph also shows similar photo-physical properties, with the τavg values for Tb0.013Lu1.987-dpph, Tb0.060Lu1.940-dpph, and Tb0.140Lu1.860-dpph being 7.80, 2.14, and 1.31 ms, respectively. Thus, we successfully demonstrated a method for extending the emission lifetime via triplet exciton migration in an aggregation system composed of lanthanide complexes. Table 1. Photophysical properties of lanthanide molecular crystals. τavg: average emission lifetime; Φtot: total emission quantum yield based on ligand excitation; IB: brightness. Compounds τavg[a] / ms Φtot[b] / % IB [d] / M-1 cm-1 Eu2-dpph 0.55 9.8±0.7 56.4 Eu0.084Lu1.916-dpph 1.22 23.5±0.1 135 Eu0.028Lu1.972-dpph 3.54 19.9±0.9 114 Eu0.010Lu1.990-dpph 7.42 14.0±1.3 80.5 Eu0.005Lu1.995-dcph 148 3.0±2.0 26.2 Tb2-dpph 0.83[c] 67.9±0.8 390 Tb0.140Lu1.860-dpph 1.31 38.2±0.7 220 Tb0.060Lu1.940-dpph 2.14 34.7±0.8 200 Tb0.013Lu1.987-dpph 7.80 18.5±0.8 106 Tb0.011Lu1.989-dcph 152 28.7±1.3 250 [a] λex = 356 nm, λem = 612 nm (for EuxLu2-x-dpph), 608 nm (for Eu0.005Lu1.995-dpph) or 548 nm (for TbxLu2-x-dpph, Tb0.011Lu1.989-dcph), under degassed condition, crystalline state. [b] λex = 356 nm, N2 atmosphere, crystalline state. [c] Ref. [25]. [d] λex = 356 nm. The optical codes were also tunable via ligand modification. The preparation and identification of Lu2-dcph, Eu0.005Lu1.995-dcph and Tb0.011Lu1.989-dcph were performed using the same procedures as those for Lu2-dpph, EuxLu2-x-dpph and TbxLu2-x-dpph, respectively. The PXRD patterns of Eu0.005Lu1.995-dcph and Tb0.011Lu1.989-dcph were similar to those of Lu2-dcph (Figure S23), suggesting similar crystal structures. The emission spectrum of Eu0.005Lu1.995-dcph (Figure S24) depicts emission bands at approximately 578, 594, 608, 655, and 701 nm were observed, which are assigned to the 5D0→7FJ (J = 0, 1, 2, 3, and 4, respectively) transitions of Eu3+. For Tb0.011Lu1.989-dcph, emission bands at approximately 489, 548, 581, 616, and 679 nm were observed. These emission bands are attributed to the 5D4→7FJ (J = 6, 5, 4, 3, and 2, respectively) transitions of Tb3+. The emission lifetimes of Lu2-dcph, Eu0.005Lu1.995-dcph, Tb0.011Lu1.989-dcph are significantly extended (τavg = 246, 148, and 152 ms, respectively; Table1, Figure 4b, and Supplementary Note 3). To reveal the origin of the significantly extended emission lifetime, single-crystal XRD analysis for Lu2-dcph was conducted (Figure S26, crystallographic parameters are shown in Table S4). From the X-ray crystal structure, one phenanthrene unit in the Lu3+ complex is surrounded by two adjacent phenanthrene units at a distance of 1.14 nm (Figure S27), which is larger than those in Lu2-dpph (1.02 and 1.05 nm). According to the Dexter-type TTET model, the TTET rate exhibits exponential decrease with increasing donor-acceptor distance.[31] Previous studies indicate that even a 0.1 nm change in the long-range region (> 1.0 nm) can lead to an effective change in the TTET rate.[32-34] The increased distance of approximately 0.1 nm can contribute to the extended lanthanide emission lifetime. Moreover, deactivation rate from the triplet state is also a factor contributing to extended emission lifetime. The total emission quantum yield (Φtot) for Tb3+ emission of Tb2-dcph (Φtot = 63.0%, Supplementary Note 4) is marginally lower than that of the Tb2-dpph (Φtot  = 67.9%, Table 1). Conversely, the Tb3+ emission quantum yield for Tb0.011Lu1.989-dcph (Φtot = 28.7%) is higher than that of Tb0.013Lu1.987-dpph (Φtot = 18.5%). These results indicate that more efficient intermolecular TTET occurred in the Tb0.011Lu1.989-dcph crystal, which is considered to be a smaller deactivation rate from the dcph triplet state in the complex unit. Thus, these combined effects of increased intermolecular distance and long-lived triplet state synergistically contribute to the significantly extended lanthanide emission lifetime in the present triplet exciton migration system. The space-fill drawing models for Lu2-dpph and Lu2-dcph are shown in Figure S30 to discuss the origin of the long-lived T1 state in Lu2-dcph. The space-fill drawing model for Lu2-dcph indicates that the central phenanthrene ring is more sterically protected by the surrounding bulky tmh ligands than that in Lu2-dpph. This enhanced steric protection is considered to suppress the torsional distortion of the phenanthrene unit, thereby suppressing access to conical intersections.[35]              COMMUNICATION          4   Figure 4. a: Emission spectra (λex = 356 nm, degassed condition, crystalline state, 293 K) of Eu0.010Lu1.990-dpph (orange line) and Tb0.013Lu1.987-dpph (blue line). b: Emission photographs (λex = 375 nm, degassed condition, crystalline state, 293 K) of Eu0.010Lu1.990-dpph, Tb0.013Lu1.987-dpph, Eu0.005Lu1.995-dcph, and Tb0.011Lu1.989-dcph. c: Emission decay curves (λex = 356 nm, degassed condition, crystalline state, 293 K) of Eu0.010Lu1.990-dpph (upper, orange line, λem = 612 nm), Eu2-dpph (upper, red line), Tb0.013Lu1.987-dpph (lower, blue line, λem = 548 nm), and Tb2-dpph (lower, green line). d: Energy diagrams of Eu0.010Lu1.990-dpph and Tb0.013Lu1.987-dpph. COMMUNICATION          5  The emission intensity is also a critical factor for optical encoders. For TbxLu2-x-dpph, the total emission quantum yield depends on the molar ratio of Tb3+ (Table 1, Φtot = 67.9% (x = 2), 38.2% (x = 0.140), 34.7% (x = 0.060), 18.5% (x = 0.013)). The brightness values (IB) are estimated using the following equation:[36] 𝐼B =  𝜀 × 𝜙tot where 𝜀 is the absorption coefficient (Figure S31). The IB values are estimated to be 390 (x = 2), 220 (x = 0.140), 200 (x = 0.060), and 106 (x = 0.013) M-1 cm-1, respectively (Table 1). Despite the dilution of the Tb3+ ion, the brightness of TbxLu2-x-dpph is much higher than that expected from the dilution degree, indicating effective photosensitization via intermolecular energy transfer between the lanthanide complexes.  The total emission quantum yields for EuxLu2-x-dpph change characteristically with decreasing Eu3+ molar ratios (Φtot = 9.8% (x = 2), 23.5% (x = 0.084), 19.9% (x = 0.028), and 14.0% (x = 0.010)). The photophysical analyses reveal that the low Φtot for Eu2-dpph (9.8%) is due to its low photosensitization efficiency (ηsens = 17.8%, Supplementary Note 2). According to previous reports, the low photosensitization efficiency for Eu2-dpph is attributed to the formation of ligand-to-metal charge transfer (LMCT) states in the lower-energy region than that for the ligand excited singlet state.[37-40] A relatively high HOMO level of the tmh ligands (−5.12 eV[25]) promotes the formation of LMCT states in the visible region, which can induce energy transfer quenching from the dpph singlet state. The formation of dpph singlet states in the Eu3+ complex leads to energy transfer quenching to a LMCT state, resulting in inefficient photosensitized Eu3+ emission. The mixed crystals (EuxLu2-x-dpph) suppress the formation of the dpph singlet state in the Eu3+ complex units by intermolecular photosensitization via TTET, resulting in higher brightness values (IB = 135 (x = 0.084), 114 (x = 0.028), and 80.5 (x = 0.010) M-1 cm-1) than that of Eu2-dpph (IB = 56.4 M-1 cm-1). The total emission quantum yields for Eu0.005Lu1.995-dcph and Tb0.011Lu1.989-dcph were estimated to be 3.0% and 28.7%, respectively (Table 1). Their IB values were calculated as 26.2 and 250 M-1 cm-1 for Eu0.005Lu1.995-dcph and Tb0.011Lu1.989-dcph, respectively. Although the Eu3+ and Tb3+ concentrations in the mixed lanthanide crystals are significantly lower than those in pure lanthanide crystals ([Eu2(tmh)6dcph]: IB = 45.3 M-1 cm-1 and [Tb2(tmh)6dcph]: IB = 549 M-1 cm-1, Supplementary Note 4), the IB values of Eu0.005Lu1.995-dcph and Tb0.011Lu1.989-dcph are much higher than those expected based on the dilution degree from [Eu2(tmh)6dcph] and [Tb2(tmh)6dcph], which indicates effective photosensitization via TTET. In this study, Eu3+-Lu3+ and Tb3+-Lu3+ mixed lanthanide crystals were prepared to demonstrate a method for controlling the emission lifetime using an aggregation system. The emission lifetime was effectively extended by slow intermolecular triplet exciton migration in the crystals. The emission lifetime (Eu3+: 0.55−148 ms, Tb3+: 0.83−152 ms) can be markedly tuned by varying the molar ratio of Eu3+ (or Tb3+) or modifying the ligands. The present mechanism is expected to allow greater extension of emission lifetime (> 200 ms) using organic ligands with longer-lived triplet excitons. Long-lived triplet exciton migration also provides high brightness for an effective optical encoder. This method significantly expands the optical encoding capacity of lanthanide complexes. Water-soluble nanoparticles and amorphous films in aggregated states will be key designs for the development of applications in multiplexed bioimaging and anti-counterfeiting, respectively. The proposed dynamic triplet exciton system can significantly impact on the fields of bio-chemistry and analytical chemistry. Supporting Information The authors have cited additional references within the Supporting Information.[41-44] Acknowledgements This work was partially supported by a Grants-in-Aid for Scientific Research (grant numbers JP24K21774, JP23H04863, and JP20H02748) and by a JSPS Research Fellow (JP24KJ0312). This work was also supported by the Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from the Japan Science and Technology Agency (JST), Japan (Grant Number JPMJTR23T5), and the Institute for Chemical Reaction Design and Discovery (ICReDD), established by the World Premier International Research Center Initiative (WPI) of MEXT, Japan. Keywords: lanthanide • optical encoding • molecular crystals Author Contributions Y.K. designed the research. T.Nakai and K.S. performed syntheses and measurements. T.Nakanishi and Y.H. supported the optical measurements. T.Nakai and Y.K. wrote the paper. All authors discuss and review the paper. References [1] H. H. Gorris, O. S. Wolfbeis, “Photon-Upconverting Nanoparticles for Optical Encoding and Multiplexing of Cells, Biomolecules, and Microspheres“ Angew. Chem. Int. Ed. 2013, 52, 3584–3600. [2] Y. Fan, S. Wang, F. Zhang, “Optical Multiplexed Bioassays for Improved Biomedical Diagnostics“ Angew. Chem. Int. 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