Circularly polarized luminescence-active low molecular weight lanthanide gels: tunable emission including white light

Abstract

Circularly polarized luminescence (CPL)-active lanthanide gels are seldom reported, despite their vast potential for various applications. Here, we report CPL-active lanthanide gels with a high dissymmetry factor (g_lum) and a high quantum yield of 59%. This is the first report of both left- and right-handed CPL-active lanthanide gels by employing simple low molecular weight chiral gelators. Moreover, tunable CPL signals, including white light-emitting CPL, were obtained. We can further upgrade the material's applicability as a potential anti-counterfeit up to a 4-tier security tag.

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Polarization of light is a fascinating phenomenon that is ubiquitous in nature and plays an essential role in many aspects of life, including the vision of many animals.^1,2a,b Circularly polarized light, a subclass of polarized light, is created by first converting unpolarized light into linearly polarized light using a linear polarizer, and then a quarter-wave plate is used to transform it into left- or right-circularly polarized light.^2a,b Many luminescent chiral compounds emit preferably right- or left-handed circularly polarized light when excited with unpolarized light, giving circularly polarized luminescence (CPL). CPL-active materials have garnered a lot of interest recently due to their potential applications in sensors, bioimaging, displays, optical data storage, 3D displays, information encryption, optical telecommunication, and quantum computing.^2–7 Considering their broad application, various types of CPL materials have been developed. Among them, supramolecular self-assembled soft materials have shown great potential.⁸ Supramolecular gels have tunable/stimuli-responsive properties, flexibility, biocompatibility, and easy processability, which widen and make their applications versatile.^9,10 Hence, special efforts have been made to develop CPL-active supramolecular gels.

CPL-active supramolecular gels require a chiral component and a luminescent component. In supramolecular gels, the chirality of the materials usually originates from the chiral organic gelator molecules with complicated structures. These chiral molecules typically need multi-step and asymmetric synthesis, which is always a challenge. A simpler chiral molecule with a simple synthetic route or a naturally occurring molecule will be helpful in this respect. It will make the CPL-active material scalable to a large scale and economical.

Various kinds of luminescent molecules/compounds have been used to develop CPL-active supramolecular gels, most of them are organic chromophores^8a,11,12 and a few are metal complexes.^13,14 One group of emissive materials that are widely used as highly efficient CPL materials are lanthanides (Ln).^7,15–17 Lanthanide complexes with a chiral ligand that can sensitize lanthanide emission have been extensively used to develop CPL materials due to their high dissymmetry factor (g_lum). The high g_lum value is due to their forbidden electric dipole f–f transitions and allowed magnetic dipole.¹⁸ Moreover, lanthanide luminescence has the additional advantages of narrow emission bands and long luminescence lifetimes.¹⁹ Moreover, unlike other chromophores, lanthanide-based materials, such as frameworks, cages, helicates, nanoparticles, and helices, have been major candidates for CPL research.^14,20–23 However, the design and synthesis of such complexes usually require multi-step synthesis and harsh conditions, which create a major hurdle to develop these materials.

Some of these issues were countered by our earlier report of a lanthanide luminescent gel with sodium salt of cholic acid or sodium cholate (NaCh) that shows CPL.¹⁶ While the preparation of this material was easy, a major problem with our material was that we did not have the opposite isomer of NaCh. NaCh has 11 chiral centres, so any attempt to synthesise its enantiomer will be almost impossible. The absence of an enantiomer will create a huge disadvantage in any applications using this material. Another CPL-active luminescent lanthanide gel, reported by Prof. Gunnlaugson and his group, faced a similar problem as the chiral gelator is based on a g-quadruplex and does not have the opposite enantiomer.¹⁷ Thus, CPL-active lanthanide gels with both enantiomers have not been reported, and we believe it is of utmost importance.

In this work, we report a CPL-active small molecule trivalent lanthanide gel whose colour can be tuned to give various colours, including white light emission. The chirality of the material was induced by malic acid (MA), an abundant naturally available material. Malic acid was found to form a gel with lanthanide ions, and using 1,10-phenanthroline (Phen) as a sensitizer, we were able to generate green, red, yellow, and white colour CPL emissive materials (Fig. 1). Since both L and D-malic acids are easily available, we were able to obtain both left- and right-handed CPL signals of the lanthanide gel. Using the colour and the handedness combination, we were able to obtain at least a 4-tier security tag. Finally, being naturally available or easily synthesised from naturally available materials, the material developed will be quite cheap.

Fig. 1 Schematic representation of the synthetic pathway of malic acid–lanthanide gels with tunable CPL emission, including white light emission.

Inspired by the earlier report of a lanthanide forming gel with citric acid,²⁴ we planned to develop a chiral version of such a simple gel. We decided to check the gelation with malic acid, which has close structural similarity to citric acid and has a chiral centre as well. Malic acid was found to form a gel with Tb(III) in DMF (Table S1). The critical gelation concentration was found to be 180 mM Tb and 270 mM MA (Table S2). The gel was prepared by mixing 0.3 M malic acid, 0.2 M terbium(III) acetate and 10 mM Phen solutions (Fig. S1 and Table S3), followed by gentle shaking (details are given in the Experimental section of the SI). A translucent gel was formed, and the gelation was checked using an inversion test. The gel formation was confirmed by measuring its rheological properties. The frequency sweep at 1 Pa strain and 25 °C was performed. The G′ and G′′ values at 1 rad s⁻¹ were found to be 63 470 Pa and 6980 Pa, respectively. The higher value of the storage modulus (G′) than the loss modulus (G′′), almost 10 times, shows that the material is highly viscous and has soft solid-like properties (Fig. 2a). The yield stress of the gel was measured by carrying out a strain sweep at a fixed frequency of 1 rad s⁻¹ at 25 °C. The yield stress was found to be 10% (Fig. 2b), which indicates that the gel is highly resistant to deformation and has strong structural integrity. The morphology of the gel was studied using a field emission scanning electron microscope (FE-SEM). The gel was made up of aggregate particles (Fig. 2c, d and Fig. S2).

Fig. 2 (a) Frequency sweep (the inset shows the image of the gel in ambient light). (b) Strain sweep. (c and d) FE-SEM images of the Tb : L-MA : Phen gel.

Tb³⁺ ions by themselves are very weakly luminescent due to their forbidden f-f transition. In order to make the Tb³⁺ ion luminescent, we added 1,10-phenanthroline, a well-known Tb-sensitizer.^16,25,26 Phenanthroline binds to the Tb³⁺ ion (Fig. S3) and sensitizes it, making the gel green emissive (inset image in Fig. 3a) with the characteristic peaks of the Tb³⁺ ion at 490 nm, 545 nm, 584 nm, and 622 nm corresponding to the ⁵D₄–⁷F₆, ⁵D₄–⁷F₅, ⁵D₄–⁷F₄, and ⁵D₄–⁷F₃ transitions, respectively (Fig. 3a). The luminescence lifetime of Tb³⁺ ion emission was observed to be long, and it was found to be 1.5 ms (Fig. S4 and Table S4). The sensitization of Tb by phenanthroline was proven by the similar spectral features of Tb excitation spectra and phenanthroline absorption (Fig. S5). The fluorescence lifetimes of phenanthroline in the presence and absence (in the Y : L-MA : Phen gel, Fig. S6)²⁷ of Ln³⁺ ions were also measured (Fig. 3b). The fluorescence lifetime of the donor decreases from 8.76 ns to 1.70 ns for Tb : L-MA : Phen (Fig. 3b and Table S5), thus showing energy transfer. Finally, the luminescence quantum yield (QY) of the material was measured and found to be a whopping 59%, which is very high for such a material. Thus, we have a lanthanide gel with a chiral gelator with a very high luminescence QY, which indicates that the material might show CPL if the chirality of the gelator was transferred to the luminescent Tb³⁺ ion.

Fig. 3 (a) Luminescence spectra of Tb : L-MA : Phen at λ_ex = 290 nm, the inset shows the photograph of the Tb : L-MA : Phen gel under a long-wave (365 nm) UV lamp. (b) Lifetimes of phenanthroline in the absence and presence of Tb³⁺ and Eu³⁺ ions at λ_ex = 290 nm and λ_em = 370 nm, respectively. (c) CD spectra of Phen in Tb : D-MA : Phen and Tb : L-MA : Phen. (d) CPL of the Tb³⁺ ion in Tb : D-MA : Phen and Tb : L-MA : Phen gels.

For our material, being a supramolecular gel, the gelation is driven by various non-covalent interactions. The chiral gelator, malic acid, interacts with the lanthanide ion through the carboxylic group as revealed by the IR data (Fig. S7). Through this interaction, we believe that the chirality of malic acid will be transferred to the Tb³⁺ ion. We examined the circular dichroism (CD) of the gelator and observed a CD peak at ∼215 nm (Fig. S8). A similar CD signal was also observed in our gel system, indicating the chiral nature of our material (Fig. 3c).²⁸ As the absorption peaks of the Tb³⁺ ion are difficult to observe due to its very low molar extinction coefficient, we cannot observe any CD signal.

We have a chiral lanthanide gel with a good luminescence QY. Hence, we instigated the CPL of the supramolecular gel. We measured the CPL of both Tb : L-MA : Phen and Tb : D-MA : Phen gels, and not to our surprise, the materials show very good CPL signals (Fig. 3d). The presence of mirror image CPL signals with enantiomers and no CPL with a racemic mixture of the gelators (Fig. S9) rules out the chances of any linear dichroism effect in our material, which is usually prominent for such a material. The g_lum value of the material is quite good and is ±0.5 × 10⁻² at 527 nm (Fig. S10 and Table S6). Thus, using a simple low molecular weight gelator, we were able to prepare a CPL-active lanthanide gel with both left- and right-handed circularly polarized emission. The easy availability of the gelators, as well as having both the enantiomers, will widen the scope of the applications of this material.

We expanded the scope of our material by using other lanthanides too. The Eu³⁺ ion was also found to form a gel with MA under similar conditions to those of the Tb³⁺ ion. The gel shows red luminescence (Fig. 4a, inset) with a QY of 38.05% having a long luminescence lifetime (Fig. S11) as expected from the Eu³⁺ based material. The characteristic peaks of Eu³⁺ at 592 nm, 616 nm, and 698 nm corresponding to the ⁵D₀–⁷F₁, ⁵D₀–⁷F₂, and ⁵D₀–⁷F₄ emission transitions, respectively, were also observed from its luminescence spectra (Fig. 4a). Identical to the Tb : L-MA : Phen gel, we observed a good CPL signal from the Eu gel as well (Fig. 4b). The fluorescence lifetime of phenanthroline decreases to 1.23 ns from 8.76 ns (Table. S5).

Fig. 4 (a) Luminescence spectra of Eu : L-MA : Phen (inset: image of the gel under a long-wave UV lamp). (b) CPL of the Eu³⁺ ion in Eu : D-MA : Phen and Eu : L-MA : Phen gels. (c) Luminescence spectra of Tb/Eu : L-MA : Phen (inset: image of the yellow colour emitting gel under a long-wave UV lamp). (d) CPL of Tb³⁺ and Eu³⁺ ions in Tb/Eu : D-MA : Phen and Tb/Eu : L-MA : Phen gels. (e) Luminescence spectra of Tb/Y/Eu : L-MA : Phen (inset: image of the white light emitting gel under a long-wave UV lamp). (f) CPL of Tb/Y/Eu : D-MA : Phen and Tb/Y/Eu : L-MA : Phen gels. All the luminescence spectra were obtained at an excitation wavelength of λ_ex = 290 nm.

The g_lum value of the gel was even better than that of the Tb : L-MA : Phen gel, and it was found to be ∼±1.2 × 10⁻² at 585 nm (Fig. S12 and Table S6), more than twice that of the Tb : L-MA : Phen gel. The higher g_lum value of the Eu : L-MA : Phen gel is attributed to the allowed magnetic dipole nature of the ⁵D₀-⁷F₁ transition of Eu³⁺ emission.²⁹ Thus, we were able to obtain green and red colour-emitting CPL-active lanthanide gels with both- left and right-handed CPL. We further tuned the luminescence colour of the material by mixing phenanthroline containing Tb and the Eu : L-MA gel. A yellow colour emitting CPL-active gel was obtained when the Tb and Eu ratio was kept at 19 : 1 (Fig. 4c and d). Interestingly, by introducing the Y³⁺ ion in the system, we were also able to generate a CPL-active white light-emitting gel. Phen-Y³⁺ generated the blue emission, and the green and red colours came from the Tb³⁺ and Eu³⁺ luminescence. At a ratio of 4 : 5 : 1 Tb³⁺/Y³⁺/Eu³⁺ ions, we obtained a white light emissive gel (Fig. 4e and Fig. S13), which is CPL active (Fig. 4f). Thus, we were able to obtain green, red, yellow, and white light emitting CPL-active small molecule-based lanthanide gels.

The designed materials have the potential to be used in the development of a 4-tier security tag. To comprehensively elucidate the claim, a pattern was developed using eight different gel materials, giving four different colours under a UV (∼365 nm) lamp – red (Eu : L-MA : Phen and Eu : D-MA : Phen gels), blue (from the comparatively short-lived organic chromophore Phen in the Y : L-MA : Phen gel), yellow (mixed Tb/Eu : L-MA : Phen and Tb/Eu : D-MA : Phen gels), and green (Tb : L-MA : Phen, a racemic mixture of Tb : (D + L)-MA : Phen and Tb : D-MA : Phen gels), as shown in Fig. 5a. The pattern was encrypted on optically dead paper. The revelation of the security features is explained at four different levels. (i) First, when observed without any polarizer, all the components in the pattern will be emissive. For instance, they can be observed by using a standard UV document reader. (ii) Second, when observed using a left circular polarizer (L-CP), materials that emit left-handed CPL will be emissive, i.e., L-MA gels, as shown in the second image of Fig. 5b. (iii) Similarly, the CPL-active components prepared with D-MA gels can be selectively observed using a right circular polarizer (R-CP), as depicted in the third image of Fig. 5b, while the racemate will be shown by both the polarizers with only a change in intensity. (iv) Finally, taking advantage of the long-lived luminescence of the lanthanide emission, we can employ the time-resolved imaging technique to add another security tier. Using a time-resolved imaging/photography technique, the short-lived phenanthroline (M-blue) emission will be removed, leaving only the long-lived lanthanide emission (fourth image in Fig. 5).³⁰ Thus, our materials can be used as advanced anticounterfeiting agents with an upgradation up to 4-tier security features, unlike the widely used conventional single-tier security system. The 4-tier security was possible due to the availability of both the enantiomers of the gelator, which was not the case with the previously reported CPL-active lanthanide gels.^16,17

Fig. 5 Design and composition of a 4-tier embedded security pattern. (a) Long-wave UV lamp-illuminated image of the actual pattern encrypted on optically dead paper, and schematic representations of the compositions. (b) Proposed imaging sequence of detecting the security tag using UV, circular polarizers, and time-resolved detection.

In summary, we developed a highly luminescent lanthanide-based gel with malic acid using 1,10-phenanthroline as a sensitizer. The QY of the Tb gel was 59%, which is one of the highest for such a material. The chirality of the malic acid was found to be transferred to the Tb³⁺ ion, thus making the Tb³⁺ ion CPL active. Since both D and L malic acids are available, we were able to develop a CPL-active Tb gel with both left- and right-handed CPL signals, which, to our knowledge, has not been reported before. We were able to tune the emission of the CPL-active material by using different lanthanides, i.e., Eu³⁺ ions, which give red colour CPL. By mixing Ln³⁺ ions, we were able to generate yellow and white light CPL-active materials, too. The g_lum value of the material goes up to 10⁻², which is quite good for such a material. Thus, we were able to develop both left- and right-handed CPL-active Ln gels with tunable emission. Moreover, the simplicity of the preparation and easy availability of the gelator will make the material cheap, expanding its applications. Finally, we believe that the material can be developed into a 4-tier security tag system, which is much better than the single-tier security that is commonly used.

Author contributions

L. T. S. performed the experiments, analysed the data, and wrote the manuscript. N. B. D. helped in performing the experiments and absorbance measurements. D. D. helped in circular dichroism (CD) and nanosecond lifetime measurements. S. G. helped in performing CPL and FE-SEM measurements. R. L. supervised the whole project and wrote and revised the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI: materials, methods, procedures, supporting figures, and tables. See DOI: https://doi.org/10.1039/d5dt01650a.

Acknowledgements

We thank Prof. Subi J. George and JNCASR, Bengaluru, for allowing us to use their instrumentation facilities. R. L. thanks DST-ANRF/SERB (Grant No. SRG/2023/000288) and DST-FIST (Grant No. SR/FST/CS-I/2023/328) for their funding. L. T. S. thanks Manipur University for a fellowship. We thank Dr Saraswathi C. for the lifetime and quantum yield measurements.

References

1. (a) S. Bian and O. Arteaga, Sci. Rep., 2025, 15, 8885; (b) P. T. Weir and M. H. Dickinson, Curr. Biol., 2012, 22, 21; (c) Y. L. Gagnon, R. M. Templin, M. J. How and N. J. Marshall, Curr. Biol., 2015, 25, 3074–3078; (d) R. M. Templin, M. J. How, N. W. Roberts, T.-H. Chiou and J. Marshall, J. Exp. Biol., 2017, 220, 3222–3230.
2. (a) Y. Deng, M. Wang, Y. Zhuang, S. Liu, W. Huang and Q. Zhao, Light: Sci. Appl., 2021, 10, 76; (b) D. Zhai, J. Jiang, C. Yuan, D. Wang, Y. Jiang and M. Liu, Adv. Opt. Mater., 2023, 11, 2300161; (c) Y. Imai, Y. Nakano, T. Kawai and J. Yuasa, Angew. Chem., Int. Ed., 2018, 57, 8973–8978; (d) C. Chen, J. Chen, T. Wang and M. Liu, ACS Appl. Mater. Interfaces, 2016, 8, 30608–30615; (e) S. Bera, Umesh and S. Bhattacharya, Chem. Sci., 2024, 15, 13987–13997.
3. P. Stachelek, S. S. Buitrago, B. L. Maroto, R. Pal and S. de la Moya, ACS Appl. Mater. Interfaces, 2024, 16, 67246–67254.
4. C. Wang, H. Fei, Y. Qiu, Y. Yang, Z. Wei, Y. Tian, Y. Chen and Y. Zhao, Appl. Phys. Lett., 1999, 74, 19–21.
5. X. Zhan, F.-F. Xu, Z. Zhou, Y. Yan, J. Yao and Y. S. Zhao, Adv. Mater., 2021, 33, 2104418.
6. K. Fu, D.-H. Qu and G. Liu, J. Am. Chem. Soc., 2024, 146, 33832–33844.
7. (a) Y. Zhang, S. Yu, B. Han, Y. Zhou, X. Zhang, X. Gao and Z. Tang, Matter, 2022, 5, 837–875; (b) N. F. M. Mukthar, N. D. Schley and G. Ung, J. Am. Chem. Soc., 2022, 144, 6148–6153; (c) O. G. Willis, F. Petri, G. Pescitelli, A. Pucci, E. Cavalli, A. Mandoli, F. Zinna and L. D. Bari, Angew. Chem., Int. Ed., 2022, 61, e202208326.
8. (a) Z. Shen, T. Wang, L. Shi, Z. Tang and M. Liu, Chem. Sci., 2015, 6, 4267–4272; (b) J. Liu, Z.-P. Song, L.-Y. Sun, B.-X. Li, Y._-Q. Lu and Q. Li, Responsive Mater., 2023, 1, e20230005; (c) J. Feng, Z. Gu, W. Ma, S. Jiang, Y. Jiao, L. Liu, B. Xu and W. Tian, J. Phys. Chem. C, 2021, 125, 21270–21276.
9. M. Maity and U. Maitra, Eur. J. Org. Chem., 2017, 1713–1720.
10. (a) J. Chen and X. Zou, Bioact. Mater., 2019, 4, 120–131; (b) S. Banerjee, R. K. Das and U. Maitra, J. Mater. Chem., 2009, 19, 6649–6687; (c) M. M. Calvo, O. Kotova, M. E. Möbius, A. P. Bell, T. McCabe, J. J. Boland and T. Gunnlaugsson, J. Am. Chem. Soc., 2015, 137, 1983–1992.
11. (a) C. Chen, J. Chen, T. Wang and M. Liu, ACS Appl. Mater. Interfaces, 2016, 8, 30608–30615; (b) S. Du, X. Zhu, L. Zhang and M. Liu, ACS Appl. Mater. Interfaces, 2021, 13, 15501–15508.
12. (a) Z. Sun, J. Zhong, H. Tang, L. Wang and D. Cao, Adv. Opt. Mater., 2025, 13, 2402460; (b) K. L. Reddy, J. P. Mathew, S. Maniappan, C. Tom, E. Shiby, R. K. Pujala and J. Kumar, Nanoscale, 2022, 14, 4946–4956.
13. (a) H.-H. Zhang, J. Jing, S.-X. Wu, Y.-P. Luo, S.-S. Sun, D.-S. Zhang, Z.-F. Shi and X.-P. Zhang, Dyes Pigm., 2022, 201, 110228; (b) X. Zhou, X. Su, C. Jiang, Q. Huang, X. Zhou and W. Miao, Eur. J. Inorg. Chem., 2025, 28, e202400726.
14. R. Carr, N. H. Evans and D. Parker, Chem. Soc. Rev., 2012, 41, 7673–7686.
15. (a) A. Homberg, F. Navazio, A. L. Tellier, F. Zinna, A. Fürstenberg, C. Besnard, L. D. Bari and J. Lacour, Dalton Trans., 2022, 51, 16479–16485; (b) S. Ruggieri, S. Mizzoni, C. Nardon, E. Cavalli, C. Sissa, M. Anselmi, P. G. Cozzi, A. Gualandi, M. Sanadar, A. Melchior, F. Zinna, O. G. Willis, L. D. Bari and F. Piccinelli, Inorg. Chem., 2023, 62, 8812–8822; (c) D. F. Caffrey, T. Gorai, B. Rawson, M. M. Calvo, J. A. Kitchen, N. S. Murray, O. Kotova, S. Comby, R. D. Peacock, P. Stachelek, R. Pal and T. Gunnlaugsson, Adv. Sci., 2024, 11, 2307448.
16. R. Laishram, S. Sarkar, U. Maitra and S. J. George, J. Mater. Chem. C, 2024, 12, 15418–15422.
17. O. Kotova, C. O'Reilly, S. T. Barwich, L. E. Mackenzie, A. D. Lynes, A. J. Savyasachi, M. Ruether, R. Pal, M. E. Möbius and T. Gunnlaugsson, Chem, 2022, 8, 1395–1414.
18. M. Tsurui, R. Takizawa, Y. Kitagawa, M. Wang, M. Kobayashi, T. Taketsugu and Y. Hasegawa, Angew. Chem., Int. Ed., 2024, 63, e202405584.
19. J.-C. G. Bünzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048–1077.
20. M. Zeng, A. Ren, W. Wu, Y. Zhao, C. Zhan and J. Yao, Chem. Sci., 2020, 11, 9154–9161.
21. Y. Zhong, Z. Wu, Y. Zhang, B. Dong and X. Bai, InfoMat, 2023, 5, e12392.
22. H.-Y. Wong, W.-S. Lo, K.-H. Yim and G.-L. Law, Chem, 2019, 5, 3058–3095.
23. B. Marydasan, K. Suryaletha, A. M. Lena, A. Sachin, T. Kawai, S. Thomas and J. Kumar, J. Mater. Chem. C, 2022, 10, 13954–13963.
24. A. Sebastian, M. K. Mahato and E. Prasad, Soft Matter, 2019, 15, 3407–3417.
25. G. Accorsi, A. Listorti, K. Yoosaf and N. Armaroli, Chem. Soc. Rev., 2009, 38, 1690–1700.
26. P. G. Sammes and G. Yahioglu, Chem. Soc. Rev., 1994, 23, 327–334.
27. The sample without the Tb³⁺ ion did not form a gel, so in order to measure the lifetime of the donor or sensitizer, phenanthroline, without the acceptor, we replaced the Tb³⁺ ion with the Y³⁺ ion. The Y³⁺ ion has various resemblances in properties with lanthanides (including the Tb³⁺ ion), but it is not emissive. The gel was blue emissive (Fig. S6), which is from the sensitizer, i.e., 1,10-phenanthroline.
28. A 6 : 1 MeOH/H₂O mixed solvent was used for circular dichroism (CD) measurements for phenanthroline in Tb : L-MA : Phen and Tb :  D-MA : Phen gels since the absorption range for the DMF solvent overlapped with that of phenanthroline.
29. (a) Y. Okayasu, K. Wakabayashi and J. Yuasa, Inorg. Chem., 2022, 61, 15108–15115; (b) Y. Kitagawa, M. Tsurui and Y. Hasegawa, ACS Omega, 2020, 5, 3786–3791.
30. (a) L. E. MacKenzie and R. Pal, Nat. Rev. Chem., 2021, 15, 109–124; (b) D. F. D. Rosa, P. Stachelek, D. J. Black and R. Pal, Nat. Commun., 2023, 14, 1537.

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