Multiple-Stimuli Fluorescent Responsive Metallo-Organic Helicated Cage Arising from Monomer and Excimer Emission

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This study presents a metallo-cage ( MTH ) featuring a triple helical motif that displays a unique dual emission. This emission arises from both intramolecular monomer and intermolecular excimer, respectively. The distorted molecular conformation of MTH and the staggered stacking mode for MTH excimer were verified through single crystal X-ray diffraction analysis. These structural features facilitate the switch between monomer and excimer emission, which is induced by changes in concentration and temperature. Significantly, adjusting the equilibrium between these two states in MTH enables the production of vibrant white light emission in both solution and solid state. Moreover, when combined with a PMMA (polymethyl methacrylate) solution, the resulting thin films can serve as straightforward fluorescence thermometer and materials for thermally activated information encryption. Physical sciences/Chemistry/Supramolecular chemistry/Self-assembly Physical sciences/Chemistry/Coordination chemistry metallo-organic cage self-assembly excimer monomer multiple-stimuli Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Materials exhibiting luminescence with responsiveness to stimuli have garnered consistent attention in fundamental research and practical applications. These applications span a range of areas, including sensing, bioimaging, data security, and display devices. [ 1 – 7 ] Following Kasha’s excitonic coupling theory, [ 8 – 10 ] the photophysical properties of fluorescent materials are directly influenced by both the extent of chromophores aggregation and the relative orientation of chromophores within these aggregates. [ 11 – 13 ] Hence, external stimuli can alter the photophysical properties (emission/color) of fluorescent materials by inducing twists or rearrangement in chromophore aggregates. Nevertheless, achieving fluorescence tuning or controlling is highly challenging due to the robust stability of conventional organic or inorganic luminophores, which are typically unresponsive to various stimuli. Metallo-organic cages offer a promising avenue to address the challenges indicated above. Utilizing coordination-driven self-assembly allows for carefully constructing supramolecular architectures with precisely defined sizes and shapes. This is achieved through the controllability and directionality of dative bonds. [ 14 – 26 ] Encapsulation within metallo-organic cages can significantly alter the physicochemical properties of enclosed guest chromophores. Klajn et al. investigated the noncovalent encapsulation of three different BODIPY dyes within the cavity of a flexible metal − organic cage. Taking advantage of the dynamic nature of encapsulation, the manipulation of optical properties through external stimuli has been realized by reversible switching between H- and J-aggregates of BODIPY dyes. [ 27 ] However, this approach is constrained by the size and shape compatibility requirement between hosts (cages) and guest dyes. A notable characteristic of metallo-assemblies is their sensitivity to changes in microenvironments, attributed to the reversible nature of coordination force. [ 28 – 34 ] This sensitivity is advantageous for the stimuli-induced rearrangement of chromophores’ aggregation. [ 35 – 36 ] Stang et al. reported on two tetragonal prismatic coordination cages exhibiting solvent-dependent aggregation behaviour for tuning emission wavelengths. [ 37 ] These uncommon yet significant examples illustrate that metallo-cages can be promising platforms for developing stimuli-responsive luminescent materials. However, supramolecular fluorescent materials still face challenges due to the absence of precise control over the aggregation mode of metallo-cages assisted by various external stimuli. In this context, we present a systematically designed axially-twisted metallo-organic cage featuring a triple helicate motif, denoted as [Zn 3 L 2 ] or MTH . This structure was comprehensively characterised through NMR, ESI-MS, and single-crystal X-ray diffraction analysis. MTH exhibits noteworthy concentration- and temperature-dependent luminescent behaviour, exhibiting dual emissions of monomer and excimer [ 38 – 41 ] fluorescence in solution. The confirmation from single crystal X-ray diffraction data establishes the distorted conformation of MTH , preventing direct face-to-face π-π staking induced fluoresence quenching. A single axially-twisted pillar in two MTH s adopted a “head-to-tail” configuration, forming an excimer with moderate stability. This configuration facilitates a switchable supramolecular fluorescence system between monomer and excimer states triggered by varying concentrations or temperatures. Moreover, by incorporating the metallo-organic cage MTH into the phase change materials, specifically PMMA, a dual-mode emission with temperature variation has been successfully realized in the solid state (Fig. 1 ). This achievement has been further applied as a fluorescence temperature indicator and as a thermally-activated information encryption material. Results Synthesis and characterization of metallo-cage MTH。 Initially, ligand L was synthesized through a 3-fold Suzuki–Miyaura coupling reaction featuring a C 3h symmetric 5,5’,10,10’,15,15’-hexaethyltruxene core (Supplementary Figs. 1, 3–13). Subsequently, ligand L (1.0 eq.) and Zn(NO 3 ) 2 ·6H 2 O (1.5 eq.) were stirred in a mixed solvent (CH 3 OH:CHCl 3 = 1:1) at 60°C for 8 h, resulting in a clear solution. After cooling to room temperature, an excess of methanolic NH 4 PF 6 solution was introduced to induce precipitation. The precipitate was thoroughly washed with deionized water, and methanol before being dried under a vacuum. In the last step, the product was obtained in high yield (95%) as a pale-white yield (95%) as a pale-white solid (Fig. 2 a). The proton nuclear magnetic resonance ( 1 H NMR) spectrum of MTH displayed two sets of terpyridine (tpy) signals originating from two singlets at 8.79 ppm and 8.73 ppm, with an integration ratio of 1:2. These signals were attributed to the proton 3’,5’. The existence of two distinct connectivities suggested a low symmetric structure (Fig. 2 b), possibly arising from the presence of some steric congestion. This hindrance affects the rotation of the three arms of the metallo-organic cage MTH , resulting in temporal low symmetry. The characteristic doublets derived from the 6,6’’ protons of tpy experienced a significant upfield shift, attributed to the electron shielding effect caused by the pseudo-octahedral bis(terpyridine) complex. [ 42 ] Proton assignments were facilitated by both 2D COSY and NOESY NMR (Supplementary Figs. 14–19). Despite the complex 1 H NMR signals, the proton diffusion-ordered NMR spectroscopy (DOSY) experiment exhibited a single band at log D = -9.47 for MTH , indicating the formation of a single discrete species in solution (Supplementary Fig. 20). [ 43 ] Subsequently, the composition of MTH was validated through electrospray ionization mass spectrometry (ESI-MS), revealing a series of peaks corresponding to charged ions [Zn 3 L 2 (PF 6 ˉ) 6-n ] n+ (n = 6, 5, 4, 3, 2). From these peaks, a molecular weight of 4387.26 Da for MTH can be deduced (Fig. 2 c, Supplementary Fig. 2). Furthermore, the travelling wave ion mobility mass spectrometry (TWIM-MS) plot depicted a series of bands with narrow drift time distributions for each charge state of MTH , ranging from 3 + to 6+ (Fig. 2 d). This observation suggests the presence of a single species and rules out the possibility of other isomers and conformers. [ 44 ] Moreover, colorless and transparent bulk crystals, suitable for X-ray crystallography, were acquired by slowly diffusing ethyl acetate into an acetonitrile solution of MTH at 15°C for two weeks (Supplementary Figs. 21–22, Table 1). MTH crystallized into the triclinic space group P -1. MTH exhibits a twisted helical geometry in its solid-state single crystal structure where the truxene planes of ligand L serve as the upper and lower faces with a distance of 18.9 Å. Its three terpyridine arms twist clockwise ( P ) or anti-clockwise ( M ) to coordinate with the metallic zinc, functioning as the three strands in the helical structure, resulting from significant steric hindrance from ortho substitution. The crystal structure reveals axial twist angles of 125°, 127°, and 127° for the three strands, respectively (Supplementary Fig. 23). Tunable fluorescence emission at different concentrations and temperatures. After comprehensive structural characterizations of MTH , we conducted detailed photo-property studies (Supplementary Figs. 27, 41–43). Initially, steady-state fluorescence emission measurements of MTH were carried out in a diluted DMF solution at a concentration of 1 × 10 − 5 M to investigate the monomer emission in the solution state. A sole blue emission at a short wavelength was observed (F1, λ max ∼ 428 nm), which could be attributed to the local excited (LE) state of two luminescent moieties (tpy and truxene). As the concentration of MTH increased, it exhibited distinctive dual emission characteristics: the F1 emission gradually decreased, and a new orange emission (F2, λ max ∼ 580 nm) emerged (Fig. 3 a). The ratiometers of the dual bands (F1, F2) changed with concentrations, indicating that the dual emissions were linked to the intermolecular interaction of MTH . The corresponding emission spectra unequivocally confirm this at higher concentrations (5 × 10 − 4 M), where the F2 band is maximized while the F1 band is minimized. Therefore, the F1 and F2 bands were reasonably attributed to MTH -monomer and MTH -excimer emission, respectively. More interestingly, the optical features of MTH in solution exhibited a rare room temperature white-light emission [ 45 – 46 ] at a specific concentration, where the dual emission bands virtually covered the entire visible spectral region (~ 400–700 nm). At a concentration of 2.1 × 10 − 5 M, MTH emitted pure white light with coordination (0.32, 0.33) (Fig. 3 b,) in the 1931 Commission Internationale de L’Eclairage (CIE) chromaticity diagram, remarkably close to the value of theoretical white light (0.33, 0.33) (Supplementary Fig. 28). In contrast to the prior methods involving mixing or doping to achieve white light, MTH achieved single-molecule white light emission by straightforwardly adjusting the solution concentration (Fig. 3 f). It is crucial to note that the entire structure of the metal-cage MTH remained intact upon dilution in DMF solution, as confirmed by ESI-MS (Supplementary Fig. 46). Then, the luminescence efficiency at various concentrations was also investigated. As depicted in Fig. 3 c, blue luminescence's fluorescence quantum yield (QY) reaches up to 17.36% at low concentrations, decreasing to 7.81% at high concentrations with orange luminescence. Moreover, time-resolved fluorescence spectra of MTH in DMF solution were monitored at 428 nm (F1) and 580 nm (F2) at a concentration of 10 − 5 M, primarily dominated by the typical monomer and excimer emissions of MTH , respectively (Supplementary Figs. 29–35). Upon 320 nm excitation and 428 nm monitoring for monomer emission, a short fluorescence lifetime of (5.7 ns) was detected. In contrast, the fluorescence lifetime of excimer emission measured at 580 nm increased to 18.3 ns (Supplementary Table 2). This finding aligns with the steady-state observation, confirming the co-existence of MTH -monomer and MTH -excimer (Supplementary Figs. 36–40). Moreover, we aimed to manipulate MTH 's monomer and excimer emission by varying temperature, a crucial and fundamental physical parameter, in both the solution state and a suitable matrix. Subsequently, a temperature-dependent fluorescence measurement of MTH at a high concentration (3 × 10 − 5 M) of DMF solution was conducted. The intensity of F1 and F2 emissions of MTH exhibited opposite temperature responsiveness (F1: positive, F2: negative) as the temperature increased from 298 K to 400 K. This led to a color change in luminescence from orange to blue, consistent with the previously observed concentration-induced emission change process (Fig. 3 d). This phenomenon reflects that the formed excimer gradually dissociates into a monomeric structure at higher temperatures. Elevated temperature 1 H NMR spectra confirm the thermodynamic stability of metallo-cage MTH (Supplementary Fig. 47). From a thermodynamics standpoint, the exothermic reaction from monomers to excimer at elevated temperatures increases the number of monomers, thereby rationalizing the positive temperature reactivity of monomer emission. [ 52 ] Moreover, a small energy difference between monomer and excimer, experimentally determined to be 6.4 kcal/mol (for detailed calculations, please refer to Supplementary Information), further demonstrates that MTH excimer can readily transform into monomeric molecules at high temperatures. [ 53 ] The CIE diagram (Fig. 3 e) shows that the orange emission attributed to the excimer shifts towards the blue-emitting monomer in a nearly linear trend with increasing temperature. Notably, MTH once again achieved white light emission at 320 K (Fig. 3 f). Therefore, white light emission based on a single molecule can be accomplished by utilising multiple external stimuli (temperature and concentration) in the solution state. The optical properties of luminescent materials are closely linked to the molecular conformation [ 47 – 48 ] as well as the stacking mode [ 49 – 50 ] of inner chromophores. To gain in-depth insights about MTH , we investigated the single crystal structures of MTH . The benzene ring and tpy unit were not coplanar in their crystal structure, as the benzene ring rotated by 32.9° along the C-C single bond (Fig. 4 a, Supplementary Fig. 24). The twisted conformation of MTH can hinder face-to-face tight π-π stacking, which tends to form an irreversible aggregate with high stability. Additionally, a pair of MTH molecules were stacked in a “head-to-tail” mode to form a dimer within two adjacent unit cells, where the apical benzene ring and lateral pyridine ring engaged π∙∙∙π interactions with a distance of 3.45 Å. The considerable high slip angles observed θ (72.3°) indicate the creation of an excimer, leading to a red-shifted in emission (from 428 nm to 580 nm) (Figs. 4 b, 4 c). [ 37 , 51 ] The staggered stacking mode of aromatic rings produced a moderately stable excimer, allowing for the switching of monomer and excimer emissions through external stimuli, as indicated earlier, such as concentration variations (Supplementary Figs. 25–26). Tunable fluorescence emission at solid state. Encouraged by these results, an attempt was made to investigate temperature-dependent fluorescence emission in the solid state. An appropriate substrate, PMMA [ 54 – 56 ], was chosen as it can alter the molecular microenvironment, facilitating solid-liquid transition as the temperature changes. The PMMA film of MTH was created by blending a DMF solution of MTH with a PMMA solution, establishing a stable emission environment for MTH after cooling at room temperature (Fig. 5 b). As expected, the resulting solutions were applied to a prepared substrate, and their doped states exhibited a single orange-colored fluorescence emission at ~ 550 nm after cooling (Fig. 5 a). As expected, when the obtained “Sun” shaped films were heated from room temperature to 400 K, it was observed that the luminescence color gradually changed from orange to yellow to green and finally cyan blue. Simultaneously, the corresponding maximum emission peak shifted from 550 to around 480 nm. This observation is generally consistent with the temperature-dependent fluorescence emission of MTH in the solution state. This demonstrates that MTH ’s PMMA film can transition from excimer to monomer molecules at high temperatures, establishing a temperature-responsive fluorescence system in the rigid solid state. Consequently, this visually and directly distinguishable fluorescent color change can be utilized as a simple fluorescent thermometer (Fig. 5 b, Supplementary Figs. 44–45). Furthermore, its application as a message encryption method has been explored. Morse code, a traditional encryption technique used to convey messages, employs various dots and dashes to represent different letters. As illustrated in Fig. 5 c, an encrypted Morse cipher was created using MTH ’s PMMA film, which is closely linked to temperature. When exposed to sunlight or UV light at room temperature, the cipher displays an incorrect message, and only after being heated up to 50 ℃ it reveals the correct message under UV light (the hidden word “Chemistry”). This phenomenon is attributed to temperature-controlled excimer-monomer conversion of MTH . The innovative creation of the cryptograph opens up new possibilities for applying supramolecular structures in information transmission and encryption. Discussion In summary, a twisted metallo-organic cage, MTH , featuring a triple helical structure, was designed and synthesized. Its fluorescence emission, tunable from orange to blue, was successfully achieved by adjusting the concentration and temperature. Moreover, single-molecule white light emission was realized in both systems. The single-crystal X-ray diffraction analysis unveiled the distorted conformation and staggered stacking aggregation mode of MTH , resulting in a moderately stable MTH excimer. Thus, regulated by external stimuli (concentration and temperature), tunable fluorescence-emission of MTH can be achieved through the switch of monomer and excimer emission in the solution state. Furthermore, a thin film was obtained by incorporating a MTH solution with a PMMA solution, exhibiting multiple functions such as temperature monitoring and thermally activated information encryption. This work expands the potential applications of metallo-organic supramolecular materials and provides new perspectives for designing and preparing multipurpose intelligent luminescent materials for targeted practical applications. Declarations Data availability Crystallographic data for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under the deposition numbers 2300228. Copies of these data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. The authors declare that the data supporting the findings of this study are provided within the article and its Supplementary Information file. All data are available from the authors upon reasonable request. Acknowledgements This research was supported by the National Natural Science Foundation of China (22101061 to Z.Z., 21971257 to P.W.), Guangzhou Basic and Applied Basic Research of City and University (Institute) Joint Funding Project (SL2022A03J01050 to P.W., SL2022A03J00929 to Z.Z., 202201022174 to T.-Z.X.), the Guangdong Provincial Pearl River Talents Program (2019QN01C243 to T.-Z.X.), the Science and Technology Projects in Guangzhou (202201010664 to T.W.), the Youth Project of Guangdong Natural Science Foundation (2021A1515110696 to Q.L.), the Characteristic Innovation Project of Guangdong Universities (2022KTSCX094 to Q.L.). The authors thank the Single crystal Xray diffraction tests from the Modern Analysis and Testing Center of Guangzhou University. The authors would like to express their gratitude to EditSprings (https://www.editsprings.cn) for the expert linguistic services provided. Author contributions ‡ Z.Z. and Q.B. contributed equally to this work. Z.Z., P.W. and T.W. conceived the project. Z.Z. performed the synthesis and characterization experiments. Q.B. performed the MS measurements and PL measurements. H. Z., C. Z. and G. N. performed the NMR studies. Z.Z. analyzed and organized the data. W.Z. and H.L. did the variable-temperature fluorescence tests. E.H. analyzed the crystal structure. Q.B. and Z.Z. drafted the manuscript. T.X. revised the manuscript. Z.Z., P.W. and T.W. drafted the manuscript. T.W. and P.W. oversaw the project. All authors discussed the results any commented on the paper. Competing interests The authors declare no competing interests. Additional information Supplementary information is available at … or from the author. 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Supplementary Files NCSIfinal.docx metalloorganiccageMTH.cif Cite Share Download PDF Status: Published Journal Publication published 23 Aug, 2024 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYDACCQglB6HYSNBiTLqWxAaitfDPbj66mXdHXXp//xkDhg9lh4EiDQQsuXMs7TbvmcO5M27kGDDOOHcYKHIAvxYDiRyz27ltB3I3SPAYMPO2HQaKJBDSkv8NqKUu3YD/jAHzX+K05LABtTAnGDDkGDAzEqNF4kaa2e2/Zw4bzriRVnCw51w6j8QNAlr4ZyQ/uzlzR508f//hjQ9+lFnL8c8goAUMGBsg9AEg5iFCPZKWUTAKRsEoGAVYAQDJmkLmL5ZKDAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-1988-7604","institution":"Guangzhou University","correspondingAuthor":true,"prefix":"","firstName":"Pingshan","middleName":"","lastName":"Wang","suffix":""},{"id":287146358,"identity":"cd4602c5-597e-4f43-88ed-9cff6d83c480","order_by":1,"name":"zhe zhang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"zhe","middleName":"","lastName":"zhang","suffix":""},{"id":287146359,"identity":"12d5b9a7-e0a7-4aac-89b6-384b66b81363","order_by":2,"name":"Qixia Bai","email":"","orcid":"","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Qixia","middleName":"","lastName":"Bai","suffix":""},{"id":287146360,"identity":"54f1c460-1972-4555-9007-391d6b311e01","order_by":3,"name":"Zirui Zhai","email":"","orcid":"","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Zirui","middleName":"","lastName":"Zhai","suffix":""},{"id":287146361,"identity":"9f4f89ae-6b7e-4e0e-ae97-6a1e9c44db2e","order_by":4,"name":"Qingwu Long","email":"","orcid":"","institution":"Shunde Polytechnic","correspondingAuthor":false,"prefix":"","firstName":"Qingwu","middleName":"","lastName":"Long","suffix":""},{"id":287146362,"identity":"6697eb77-112c-49bd-afb9-47505312add5","order_by":5,"name":"Ermeng Han","email":"","orcid":"","institution":"Central South University","correspondingAuthor":false,"prefix":"","firstName":"Ermeng","middleName":"","lastName":"Han","suffix":""},{"id":287146363,"identity":"4c59e2ee-4f40-4ca5-b8ce-214571bd84a3","order_by":6,"name":"He Zhao","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"He","middleName":"","lastName":"Zhao","suffix":""},{"id":287146364,"identity":"9314f320-b26d-4fa0-b2a5-acf4c4b9149a","order_by":7,"name":"Chuang-Wei Zhou","email":"","orcid":"","institution":"Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Chuang-Wei","middleName":"","lastName":"Zhou","suffix":""},{"id":287146365,"identity":"c42e2150-131d-4f72-b6b1-ffcbd9e69159","order_by":8,"name":"Haobo Lin","email":"","orcid":"","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Haobo","middleName":"","lastName":"Lin","suffix":""},{"id":287146366,"identity":"6f878ad8-d55a-4348-a6a1-c176c93b2c0d","order_by":9,"name":"Wei Zhang","email":"","orcid":"","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Zhang","suffix":""},{"id":287146367,"identity":"adbe3a96-8997-4e2e-8aa6-413e075be443","order_by":10,"name":"Guo-Hong Ning","email":"","orcid":"https://orcid.org/0000-0002-5640-9062","institution":"Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Guo-Hong","middleName":"","lastName":"Ning","suffix":""},{"id":287146368,"identity":"cfeeecfc-3107-4d59-a216-12fb5cc1ad3c","order_by":11,"name":"Ting-Zheng Xie","email":"","orcid":"","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Ting-Zheng","middleName":"","lastName":"Xie","suffix":""},{"id":287146369,"identity":"3bff8730-705d-4780-90b1-e286e8955ccb","order_by":12,"name":"Tun Wu","email":"","orcid":"https://orcid.org/0000-0002-8721-6167","institution":"Guangzhou University","correspondingAuthor":false,"prefix":"","firstName":"Tun","middleName":"","lastName":"Wu","suffix":""}],"badges":[],"createdAt":"2024-03-26 08:30:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4168269/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4168269/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-024-51792-x","type":"published","date":"2024-08-23T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":54114688,"identity":"72c9a09f-cf2b-4ce2-9e70-8d51455836f6","added_by":"auto","created_at":"2024-04-04 19:23:49","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":258594,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of monomer and excimer conversion of metal-organic cage \u003cstrong\u003eMTH\u003c/strong\u003ecausing by external stimulus.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/0eccda9e72766856e372b277.png"},{"id":54114689,"identity":"52250934-ce7b-4726-8686-e9e12cf6afd6","added_by":"auto","created_at":"2024-04-04 19:23:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":275654,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis and characterization of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e. (a) Self-assembly of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e. (b) \u003csup\u003e1\u003c/sup\u003eH NMR spectra (400 MHz, 300 K) of ligand \u003cstrong\u003eL\u003c/strong\u003e in CDCl\u003csub\u003e3\u003c/sub\u003e and metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e in CD\u003csub\u003e3\u003c/sub\u003eCN. (c) ESI-MS spectra of metallo-organic cage \u003cstrong\u003eMTH \u003c/strong\u003ewith inset showing the observed and simulated isotopic pattern of the 4+ charge state. (d) TWIM-MS plot of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e. (e) Front, top and side views of the single crystal structure of metallo-organic cage \u003cstrong\u003eMTH.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/5073d29d297e25162fccfa93.png"},{"id":54114692,"identity":"a634a224-60df-46dc-a7ab-2ebe784ef35c","added_by":"auto","created_at":"2024-04-04 19:23:50","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":292185,"visible":true,"origin":"","legend":"\u003cp\u003eTunable fluorescence emission of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e at different concentrations and temperatures. (a) PL spectra, (b) CIE 1931 chromaticity diagram (the crosses signify the luminescence color coordinates) (c) Absolute fluorescence quantum yields of metallo-cage \u003cstrong\u003eMTH \u003c/strong\u003ein DMF at different concentrations (λ\u003csub\u003eex\u003c/sub\u003e= 320 nm). (d) PL spectra, (e) CIE 1931 chromaticity diagram (the crosses signify the luminescence color coordinates) of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e in DMF at different temperature (λ\u003csub\u003eex\u003c/sub\u003e= 320 nm, c= 3 × 10\u003csup\u003e-5\u003c/sup\u003e M, 300 K). (f) Fluorescence photographs of metallo-organic cage \u003cstrong\u003eMTH \u003c/strong\u003eat different concentrations and temperatures.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/acc2f76ab34a3e6f63b572b6.png"},{"id":54114691,"identity":"7e952ee3-a8fd-4be5-a4b7-ed964f776281","added_by":"auto","created_at":"2024-04-04 19:23:50","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":391885,"visible":true,"origin":"","legend":"\u003cp\u003eSingle crystal structure of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e. (a) The twisted conformation of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e. (b) Stacking mode of metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003ewithin two neighbouring unit cells. (c) Arrangement of\u003cstrong\u003e MTH\u003c/strong\u003e molecules in the single crystal structure.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/4b93782ac3638f89be03ddf3.png"},{"id":54114690,"identity":"d8764091-3cfd-4f79-928b-83da39d6f583","added_by":"auto","created_at":"2024-04-04 19:23:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":677831,"visible":true,"origin":"","legend":"\u003cp\u003eTunable fluorescence emission in solid state. (a) PL spectra of PMMA films with metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003e at different temperatures. (b) Fluorescence photographs of PMMA films with the “Sun” shape for metallo-organic cage \u003cstrong\u003eMTH\u003c/strong\u003eat different temperatures. (c) Thermally activated information encryption with the Morse code by PMMA films of metallo-organic cage \u003cstrong\u003eMTH.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/8677ab978b9287304c356c34.png"},{"id":54114696,"identity":"d03fb5e5-445e-44d9-9e62-79b31f504718","added_by":"auto","created_at":"2024-04-04 19:23:55","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":12385230,"visible":true,"origin":"","legend":"","description":"","filename":"NCSIfinal.docx","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/05aff95f848a79765d464221.docx"},{"id":54114693,"identity":"bd4e8e81-32ce-453c-93b8-ae111470bdb4","added_by":"auto","created_at":"2024-04-04 19:23:50","extension":"cif","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":9378487,"visible":true,"origin":"","legend":"","description":"","filename":"metalloorganiccageMTH.cif","url":"https://assets-eu.researchsquare.com/files/rs-4168269/v1/30817f11c1003072149fd324.cif"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Multiple-Stimuli Fluorescent Responsive Metallo-Organic Helicated Cage Arising from Monomer and Excimer Emission","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMaterials exhibiting luminescence with responsiveness to stimuli have garnered consistent attention in fundamental research and practical applications. These applications span a range of areas, including sensing, bioimaging, data security, and display devices.\u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e Following Kasha\u0026rsquo;s excitonic coupling theory, \u003csup\u003e[\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e the photophysical properties of fluorescent materials are directly influenced by both the extent of chromophores aggregation and the relative orientation of chromophores within these aggregates. \u003csup\u003e[\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e Hence, external stimuli can alter the photophysical properties (emission/color) of fluorescent materials by inducing twists or rearrangement in chromophore aggregates. Nevertheless, achieving fluorescence tuning or controlling is highly challenging due to the robust stability of conventional organic or inorganic luminophores, which are typically unresponsive to various stimuli.\u003c/p\u003e \u003cp\u003eMetallo-organic cages offer a promising avenue to address the challenges indicated above. Utilizing coordination-driven self-assembly allows for carefully constructing supramolecular architectures with precisely defined sizes and shapes. This is achieved through the controllability and directionality of dative bonds.\u003csup\u003e[\u003cspan additionalcitationids=\"CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22 CR23 CR24 CR25\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e Encapsulation within metallo-organic cages can significantly alter the physicochemical properties of enclosed guest chromophores. Klajn et al. investigated the noncovalent encapsulation of three different BODIPY dyes within the cavity of a flexible metal\u0026thinsp;\u0026minus;\u0026thinsp;organic cage. Taking advantage of the dynamic nature of encapsulation, the manipulation of optical properties through external stimuli has been realized by reversible switching between H- and J-aggregates of BODIPY dyes.\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e However, this approach is constrained by the size and shape compatibility requirement between hosts (cages) and guest dyes. A notable characteristic of metallo-assemblies is their sensitivity to changes in microenvironments, attributed to the reversible nature of coordination force.\u003csup\u003e[\u003cspan additionalcitationids=\"CR29 CR30 CR31 CR32 CR33\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e This sensitivity is advantageous for the stimuli-induced rearrangement of chromophores\u0026rsquo; aggregation.\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e Stang et al. reported on two tetragonal prismatic coordination cages exhibiting solvent-dependent aggregation behaviour for tuning emission wavelengths.\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e These uncommon yet significant examples illustrate that metallo-cages can be promising platforms for developing stimuli-responsive luminescent materials. However, supramolecular fluorescent materials still face challenges due to the absence of precise control over the aggregation mode of metallo-cages assisted by various external stimuli.\u003c/p\u003e \u003cp\u003eIn this context, we present a systematically designed axially-twisted metallo-organic cage featuring a triple helicate motif, denoted as [Zn\u003csub\u003e3\u003c/sub\u003e\u003cb\u003eL\u003c/b\u003e\u003csub\u003e2\u003c/sub\u003e] or \u003cb\u003eMTH\u003c/b\u003e. This structure was comprehensively characterised through NMR, ESI-MS, and single-crystal X-ray diffraction analysis. \u003cb\u003eMTH\u003c/b\u003e exhibits noteworthy concentration- and temperature-dependent luminescent\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ebehaviour, exhibiting dual emissions of monomer and excimer\u003csup\u003e[\u003cspan additionalcitationids=\"CR39 CR40\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e fluorescence in solution. The confirmation from single crystal X-ray diffraction data establishes the distorted conformation of \u003cb\u003eMTH\u003c/b\u003e, preventing direct face-to-face π-π staking induced fluoresence quenching. A single axially-twisted pillar in two \u003cb\u003eMTH\u003c/b\u003es adopted a \u0026ldquo;head-to-tail\u0026rdquo; configuration, forming an excimer with moderate stability. This configuration facilitates a switchable supramolecular fluorescence system between monomer and excimer states triggered by varying concentrations or temperatures. Moreover, by incorporating the metallo-organic cage \u003cb\u003eMTH\u003c/b\u003e into the phase change materials, specifically PMMA, a dual-mode emission with temperature variation has been successfully realized in the solid state (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). This achievement has been further applied as a fluorescence temperature indicator and as a thermally-activated information encryption material.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and characterization of metallo-cage MTH。\u003c/h2\u003e \u003cp\u003eInitially, ligand \u003cb\u003eL\u003c/b\u003e was synthesized through a 3-fold Suzuki\u0026ndash;Miyaura coupling reaction featuring a \u003cem\u003eC\u003c/em\u003e\u003csub\u003e3h\u003c/sub\u003e symmetric 5,5\u0026rsquo;,10,10\u0026rsquo;,15,15\u0026rsquo;-hexaethyltruxene core (Supplementary Figs.\u0026nbsp;1, 3\u0026ndash;13). Subsequently, ligand \u003cb\u003eL\u003c/b\u003e (1.0 eq.) and Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO (1.5 eq.) were stirred in a mixed solvent (CH\u003csub\u003e3\u003c/sub\u003eOH:CHCl\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1:1) at 60\u0026deg;C for 8 h, resulting in a clear solution. After cooling to room temperature, an excess of methanolic NH\u003csub\u003e4\u003c/sub\u003ePF\u003csub\u003e6\u003c/sub\u003e solution was introduced to induce precipitation. The precipitate was thoroughly washed with deionized water, and methanol\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ebefore being dried under a vacuum. In the last step, the product was obtained in high yield (95%) as a pale-white yield (95%) as a pale-white solid (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The proton nuclear magnetic resonance (\u003csup\u003e1\u003c/sup\u003eH NMR) spectrum of \u003cb\u003eMTH\u003c/b\u003e displayed two sets of terpyridine (tpy) signals originating from two singlets at 8.79 ppm and 8.73 ppm, with an integration ratio of 1:2. These signals were attributed to the proton 3\u0026rsquo;,5\u0026rsquo;. The existence of two distinct\u0026thinsp;\u0026lt;\u0026thinsp;tpy-Zn(II)-tpy\u0026thinsp;\u0026gt;\u0026thinsp;connectivities suggested a low symmetric structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), possibly arising from the presence of some steric congestion. This hindrance affects the rotation of the three arms of the metallo-organic cage \u003cb\u003eMTH\u003c/b\u003e, resulting in temporal low symmetry. The characteristic doublets derived from the 6,6\u0026rsquo;\u0026rsquo; protons of tpy experienced a significant upfield shift, attributed to the electron shielding effect caused by the pseudo-octahedral bis(terpyridine) complex.\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e Proton assignments were facilitated by both 2D COSY and NOESY NMR (Supplementary Figs.\u0026nbsp;14\u0026ndash;19). Despite the complex \u003csup\u003e1\u003c/sup\u003eH NMR signals, the proton diffusion-ordered NMR spectroscopy (DOSY) experiment exhibited a single band at log \u003cem\u003eD\u003c/em\u003e = -9.47 for \u003cb\u003eMTH\u003c/b\u003e, indicating the formation of a single discrete species in solution (Supplementary Fig.\u0026nbsp;20).\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e Subsequently, the composition of \u003cb\u003eMTH\u003c/b\u003e was validated through electrospray ionization mass spectrometry (ESI-MS), revealing a series of peaks corresponding to charged ions [Zn\u003csub\u003e3\u003c/sub\u003e\u003cb\u003eL\u003c/b\u003e\u003csub\u003e2\u003c/sub\u003e(PF\u003csub\u003e6\u003c/sub\u003eˉ)\u003csub\u003e6-n\u003c/sub\u003e]\u003csup\u003en+\u003c/sup\u003e (n\u0026thinsp;=\u0026thinsp;6, 5, 4, 3, 2). From these peaks, a molecular weight of 4387.26 Da for \u003cb\u003eMTH\u003c/b\u003e can be deduced (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, Supplementary Fig.\u0026nbsp;2). Furthermore, the travelling wave ion mobility mass spectrometry (TWIM-MS) plot depicted a series of bands with narrow drift time distributions for each charge state of \u003cb\u003eMTH\u003c/b\u003e, ranging from 3\u0026thinsp;+\u0026thinsp;to 6+ (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). This observation suggests the presence of a single species and rules out the possibility of other isomers and conformers.\u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eMoreover, colorless and transparent bulk crystals, suitable for X-ray crystallography, were acquired by slowly diffusing ethyl acetate into an acetonitrile solution of \u003cb\u003eMTH\u003c/b\u003e at 15\u0026deg;C for two weeks (Supplementary Figs.\u0026nbsp;21\u0026ndash;22, Table\u0026nbsp;1). \u003cb\u003eMTH\u003c/b\u003e crystallized into the triclinic space group \u003cem\u003eP\u003c/em\u003e-1. \u003cb\u003eMTH\u003c/b\u003e exhibits a twisted helical geometry in its solid-state single crystal structure where the truxene planes of ligand \u003cb\u003eL\u003c/b\u003e serve as the upper and lower faces with a distance of 18.9 \u0026Aring;. Its three terpyridine arms twist clockwise (\u003cem\u003eP\u003c/em\u003e) or anti-clockwise (\u003cem\u003eM\u003c/em\u003e) to coordinate with the metallic zinc, functioning as the three strands in the helical structure, resulting from significant steric hindrance from ortho substitution. The crystal structure reveals axial twist angles of 125\u0026deg;, 127\u0026deg;, and 127\u0026deg; for the three strands, respectively (Supplementary Fig.\u0026nbsp;23).\u003c/p\u003e \u003cp\u003e \u003cb\u003eTunable fluorescence emission at different concentrations and temperatures.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAfter comprehensive structural characterizations of \u003cb\u003eMTH\u003c/b\u003e, we conducted detailed photo-property studies (Supplementary Figs.\u0026nbsp;27, 41\u0026ndash;43). Initially, steady-state fluorescence emission measurements of \u003cb\u003eMTH\u003c/b\u003e were carried out in a diluted DMF solution at a concentration of 1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M to investigate the monomer emission in the solution state. A sole blue emission at a short wavelength was observed (F1, λ\u003csub\u003emax\u003c/sub\u003e \u0026sim; 428 nm), which could be attributed to the local excited (LE) state of two luminescent moieties (tpy and truxene). As the concentration of \u003cb\u003eMTH\u003c/b\u003e increased, it exhibited distinctive dual emission characteristics: the F1 emission gradually decreased, and a new orange emission (F2, λ\u003csub\u003emax\u003c/sub\u003e \u0026sim; 580 nm) emerged (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The ratiometers of the dual bands (F1, F2) changed with concentrations, indicating that the dual emissions were linked to the intermolecular interaction of \u003cb\u003eMTH\u003c/b\u003e. The corresponding emission spectra unequivocally confirm this at higher concentrations (5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e M), where the F2 band is\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003emaximized while the F1 band is minimized. Therefore, the F1 and F2 bands were reasonably attributed to \u003cb\u003eMTH\u003c/b\u003e-monomer and \u003cb\u003eMTH\u003c/b\u003e-excimer emission, respectively. More interestingly, the optical features of \u003cb\u003eMTH\u003c/b\u003e in solution exhibited a rare room temperature white-light emission\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e at a specific concentration, where the dual emission bands virtually covered the entire visible spectral region (~\u0026thinsp;400\u0026ndash;700 nm). At a concentration of 2.1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M, \u003cb\u003eMTH\u003c/b\u003e emitted pure white light with coordination (0.32, 0.33) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb,) in the 1931 Commission Internationale de L\u0026rsquo;Eclairage (CIE) chromaticity diagram, remarkably close to the value of theoretical white light (0.33, 0.33) (Supplementary Fig.\u0026nbsp;28). In contrast to the prior methods involving mixing or doping to achieve white light, \u003cb\u003eMTH\u003c/b\u003e achieved single-molecule white light emission by straightforwardly adjusting the solution concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). It is crucial to note that the entire structure of the metal-cage \u003cb\u003eMTH\u003c/b\u003e remained intact upon dilution in DMF solution, as confirmed by ESI-MS (Supplementary Fig.\u0026nbsp;46). Then, the luminescence efficiency at various concentrations was also investigated. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, blue luminescence's fluorescence quantum yield (QY) reaches up to 17.36% at low concentrations, decreasing to 7.81% at high concentrations with orange luminescence. Moreover, time-resolved fluorescence spectra of \u003cb\u003eMTH\u003c/b\u003e in DMF solution were monitored at 428 nm (F1) and 580 nm (F2) at a concentration of 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M, primarily dominated by the typical monomer and excimer emissions of \u003cb\u003eMTH\u003c/b\u003e, respectively (Supplementary Figs.\u0026nbsp;29\u0026ndash;35). Upon 320 nm excitation and 428 nm monitoring for monomer emission, a short fluorescence lifetime of (5.7 ns) was detected. In contrast, the fluorescence lifetime of excimer emission measured at 580 nm increased to 18.3 ns (Supplementary Table\u0026nbsp;2). This finding aligns with the steady-state observation, confirming the co-existence of \u003cb\u003eMTH\u003c/b\u003e-monomer and \u003cb\u003eMTH\u003c/b\u003e-excimer (Supplementary Figs.\u0026nbsp;36\u0026ndash;40).\u003c/p\u003e \u003cp\u003eMoreover, we aimed to manipulate \u003cb\u003eMTH\u003c/b\u003e's monomer and excimer emission by varying temperature, a crucial and fundamental physical parameter, in both the solution state and a suitable matrix. Subsequently, a temperature-dependent fluorescence measurement of \u003cb\u003eMTH\u003c/b\u003e at a high concentration (3 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M) of DMF solution was conducted. The intensity of F1 and F2 emissions of \u003cb\u003eMTH\u003c/b\u003e exhibited opposite temperature responsiveness (F1: positive, F2: negative) as the temperature increased from 298 K to 400 K. This led to a color change in luminescence from orange to blue, consistent with the previously observed concentration-induced emission change process (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). This phenomenon reflects that the formed excimer gradually dissociates into a monomeric structure at higher temperatures. Elevated temperature \u003csup\u003e1\u003c/sup\u003eH NMR spectra confirm the thermodynamic stability of metallo-cage \u003cb\u003eMTH\u003c/b\u003e (Supplementary Fig.\u0026nbsp;47). From a thermodynamics standpoint, the exothermic reaction from monomers to excimer at elevated temperatures increases the number of monomers, thereby rationalizing the positive temperature reactivity of monomer emission.\u003csup\u003e[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e Moreover, a small energy difference between monomer and excimer, experimentally determined to be 6.4 kcal/mol (for detailed calculations, please refer to Supplementary Information), further demonstrates that \u003cb\u003eMTH\u003c/b\u003e excimer can readily transform into monomeric molecules at high temperatures.\u003csup\u003e[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e The CIE diagram (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) shows that the orange emission attributed to the excimer shifts towards the blue-emitting monomer in a nearly linear trend with increasing temperature. Notably, \u003cb\u003eMTH\u003c/b\u003e once again achieved white light emission at 320 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). Therefore, white light emission based on a single molecule can be accomplished by utilising multiple external stimuli (temperature and concentration) in the solution state.\u003c/p\u003e \u003cp\u003eThe optical properties of luminescent materials are closely linked to the molecular conformation\u003csup\u003e[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e as well as the stacking mode\u003csup\u003e[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e of inner chromophores. To gain in-depth insights about \u003cb\u003eMTH\u003c/b\u003e, we investigated the single crystal structures of \u003cb\u003eMTH\u003c/b\u003e. The benzene ring and tpy unit were not coplanar in their crystal structure, as the benzene ring rotated by 32.9\u0026deg; along the C-C single bond (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, Supplementary Fig.\u0026nbsp;24). The twisted conformation of \u003cb\u003eMTH\u003c/b\u003e can hinder face-to-face tight π-π stacking, which tends to form an irreversible aggregate with high stability. Additionally, a pair of \u003cb\u003eMTH\u003c/b\u003e molecules were stacked in a \u0026ldquo;head-to-tail\u0026rdquo; mode to form a dimer within two adjacent unit cells, where the apical benzene ring and lateral pyridine ring engaged π∙∙∙π interactions with a distance of 3.45 \u0026Aring;. The considerable high slip angles observed \u003cem\u003eθ\u003c/em\u003e (72.3\u0026deg;) indicate the creation of an excimer, leading to a red-shifted in emission (from 428 nm to 580 nm) (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec).\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e The staggered stacking mode of aromatic rings produced a moderately stable excimer, allowing for the switching of monomer and excimer emissions through external stimuli, as indicated earlier, such as concentration variations (Supplementary Figs.\u0026nbsp;25\u0026ndash;26).\u003c/p\u003e \u003cp\u003e \u003cb\u003eTunable fluorescence emission at solid state.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eEncouraged by these results, an attempt was made to investigate temperature-dependent fluorescence emission in the solid state. An appropriate substrate, PMMA\u003csup\u003e[\u003cspan additionalcitationids=\"CR55\" citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e],\u003c/sup\u003e was chosen as it can alter the molecular microenvironment, facilitating solid-liquid transition as the temperature changes. The PMMA film of \u003cb\u003eMTH\u003c/b\u003e was created by blending a DMF solution of \u003cb\u003eMTH\u003c/b\u003e with a PMMA solution, establishing a stable emission environment for \u003cb\u003eMTH\u003c/b\u003e after\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ecooling at room temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). As expected, the resulting solutions were applied to a prepared substrate, and their doped states exhibited a single orange-colored fluorescence emission at ~\u0026thinsp;550 nm after cooling (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eAs expected, when the obtained \u0026ldquo;Sun\u0026rdquo; shaped films were heated from room temperature to 400 K, it was observed that the luminescence color gradually changed from orange to yellow to green and finally cyan blue. Simultaneously, the corresponding maximum emission peak shifted from 550 to around 480 nm. This observation is generally consistent with the temperature-dependent fluorescence emission of \u003cb\u003eMTH\u003c/b\u003e in the solution state. This demonstrates that \u003cb\u003eMTH\u003c/b\u003e\u0026rsquo;s PMMA film can transition from excimer to monomer molecules at high temperatures, establishing a temperature-responsive fluorescence system in the rigid solid state.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConsequently, this visually and directly distinguishable fluorescent color change can be utilized as a simple fluorescent thermometer (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, Supplementary Figs.\u0026nbsp;44\u0026ndash;45). Furthermore, its application as a message encryption method has been explored. Morse code, a traditional encryption technique used to convey messages, employs various dots and dashes to represent different letters. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, an encrypted Morse cipher was created using \u003cb\u003eMTH\u003c/b\u003e\u0026rsquo;s PMMA film, which is closely linked to temperature. When exposed to sunlight or UV light at room temperature, the cipher displays an incorrect message, and only after being heated up to 50 ℃ it reveals the correct message under UV light (the hidden word \u0026ldquo;Chemistry\u0026rdquo;). This phenomenon is attributed to temperature-controlled excimer-monomer conversion of \u003cb\u003eMTH\u003c/b\u003e. The innovative creation of the cryptograph opens up new possibilities for applying supramolecular structures in information transmission and encryption.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn summary, a twisted metallo-organic cage, \u003cb\u003eMTH\u003c/b\u003e, featuring a triple helical structure, was designed and synthesized. Its fluorescence emission, tunable from orange to blue, was successfully achieved by adjusting the concentration and temperature. Moreover, single-molecule white light emission was realized in both systems. The single-crystal X-ray diffraction analysis unveiled the distorted conformation and staggered stacking aggregation mode of \u003cb\u003eMTH\u003c/b\u003e, resulting in a moderately stable \u003cb\u003eMTH\u003c/b\u003e excimer. Thus, regulated by external stimuli (concentration and temperature), tunable fluorescence-emission of \u003cb\u003eMTH\u003c/b\u003e can be achieved through the switch of monomer and excimer emission in the solution state. Furthermore, a thin film was obtained by incorporating a \u003cb\u003eMTH\u003c/b\u003e solution with a PMMA solution, exhibiting multiple functions such as temperature monitoring and thermally activated information encryption. This work expands the potential applications of metallo-organic supramolecular materials and provides new perspectives for designing and preparing multipurpose intelligent luminescent materials for targeted practical applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCrystallographic data for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under the deposition numbers 2300228. Copies of these data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. The authors declare that the data supporting the findings of this study are provided within the article and its Supplementary Information file. All data are available from the authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the National Natural Science Foundation of China (22101061 to Z.Z., 21971257 to P.W.), Guangzhou Basic and Applied Basic Research of City and University (Institute) Joint Funding Project (SL2022A03J01050 to P.W., SL2022A03J00929 to Z.Z., 202201022174 to T.-Z.X.), the Guangdong Provincial Pearl River Talents Program (2019QN01C243 to T.-Z.X.), the Science and Technology Projects in Guangzhou (202201010664 to T.W.), the Youth Project of Guangdong Natural Science Foundation (2021A1515110696 to Q.L.), the Characteristic Innovation Project of Guangdong Universities (2022KTSCX094 to Q.L.). The authors thank the Single crystal Xray diffraction tests from the Modern Analysis and Testing Center of Guangzhou University. The authors would like to express their gratitude to EditSprings (https://www.editsprings.cn) for the expert linguistic services provided.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e\u0026Dagger;\u003c/sup\u003eZ.Z. and Q.B. contributed equally to this work. Z.Z., P.W. and T.W. conceived the project. Z.Z. performed the synthesis and characterization experiments. Q.B. performed the MS measurements and PL measurements. H. Z., C. Z. and G. N. performed the NMR studies. Z.Z. analyzed and organized the data. W.Z. and H.L. did the variable-temperature fluorescence tests. E.H. analyzed the crystal structure. Q.B. and Z.Z. drafted the manuscript. T.X. revised the manuscript. Z.Z., P.W. and T.W. drafted the manuscript. T.W. and P.W. oversaw the project. All authors discussed the results any commented on the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information is available at \u0026hellip; or from the author.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003e\u003cspan\u003eWenger OS (2013) Vapochromism in Organometallic and Coordination Complexes: Chemical Sensors for Volatile Organic Compounds. Chem Rev 113:3686\u0026ndash;3733\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003eSun H et al (2014) Smart responsive phosphorescent materials for data recording and security protection. 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Angew Chem Int Ed 62:e202215652\u003c/span\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":true,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"metallo-organic cage, self-assembly, excimer, monomer, multiple-stimuli","lastPublishedDoi":"10.21203/rs.3.rs-4168269/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4168269/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEffectively controlling monomer and excimer emission in a singular luminous platform is challenging due to their highly stable structures in solution, solid, or doped states when subjected to external stimuli. This study presents a metallo-cage (\u003cb\u003eMTH\u003c/b\u003e) featuring a triple helical motif that displays a unique dual emission. This emission arises from both intramolecular monomer and intermolecular excimer, respectively. The distorted molecular conformation of \u003cb\u003eMTH\u003c/b\u003e and the staggered stacking mode for \u003cb\u003eMTH\u003c/b\u003e excimer were verified through single crystal X-ray diffraction analysis. These structural features facilitate the switch between monomer and excimer emission, which is induced by changes in concentration and temperature. Significantly, adjusting the equilibrium between these two states in \u003cb\u003eMTH\u003c/b\u003e enables the production of vibrant white light emission in both solution and solid state. Moreover, when combined with a PMMA (polymethyl methacrylate) solution, the resulting thin films can serve as straightforward fluorescence thermometer and materials for thermally activated information encryption.\u003c/p\u003e","manuscriptTitle":"Multiple-Stimuli Fluorescent Responsive Metallo-Organic Helicated Cage Arising from Monomer and Excimer Emission","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-04 19:23:44","doi":"10.21203/rs.3.rs-4168269/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d0d532f2-e102-4a7b-81e8-1216e8735759","owner":[],"postedDate":"April 4th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":30226270,"name":"Physical sciences/Chemistry/Supramolecular chemistry/Self-assembly"},{"id":30226271,"name":"Physical sciences/Chemistry/Coordination chemistry"}],"tags":[],"updatedAt":"2024-08-24T07:07:51+00:00","versionOfRecord":{"articleIdentity":"rs-4168269","link":"https://doi.org/10.1038/s41467-024-51792-x","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2024-08-23 04:00:00","publishedOnDateReadable":"August 23rd, 2024"},"versionCreatedAt":"2024-04-04 19:23:44","video":"","vorDoi":"10.1038/s41467-024-51792-x","vorDoiUrl":"https://doi.org/10.1038/s41467-024-51792-x","workflowStages":[]},"version":"v1","identity":"rs-4168269","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4168269","identity":"rs-4168269","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}