Synthesis and characterization of metallo-organic cage MTHInitially, ligand L was synthesized through a 3-fold Suzuki-Miyaura coupling reaction featuring a C3h symmetric 5,5’,10,10’,15,15’-hexaethyltruxene core (Supplementary Figs. 1, 3–13). Subsequently, ligand L (1.0 eq.) and Zn(NO3)2·6H2O (1.5 eq.) were stirred in a mixed solvent (CH3OH: CHCl3 = 1:1) at 60 °C for 8 h, resulting in a clear solution. After cooling to room temperature, an excess of methanolic NH4PF6 solution was introduced to induce precipitation.The precipitate was thoroughly washed with deionized water and methanol before being dried under a vacuum. Then, the product was obtained in high yield (95%) as a pale-white solid (Fig. 1a). The proton nuclear magnetic resonance (1H NMR) spectrum of MTH displayed two sets of terpyridine (tpy) signals originating from two singlets at 8.83 ppm and 8.77 ppm, with an integration ratio of 1:2. These signals were attributed to the proton 3’,5’. The existence of two distinct <tpy-Zn(II)-tpy> connectivities suggested a low symmetric structure (Fig. 1b), 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) complex42. Proton assignments were facilitated by both 2D COSY and NOESY NMR (Supplementary Figs. 14–19). Despite the complex 1H 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 [Zn3L2(PF6ˉ)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. 1c, Supplementary Fig. 2). Furthermore, the traveling 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. 1d). This observation suggests the presence of a single species and rules out the possibility of other isomers and conformers44.Fig. 1: Synthesis and characterization of metallo-organic cage MTH.a Self-assembly of metallo-organic cage MTH. b 1H NMR spectrum of ligand L in CDCl3 (400 MHz, 300 K) and metallo-organic cage MTH in CD3CN-d3 (600 MHz, 300 K). c ESI-MS spectra of metallo-organic cage MTH with inset showing the observed and simulated isotopic pattern of the 4+ charge state. d TWIM-MS plot of metallo-organic cage MTH. e Front, top and side views of the single crystal structure of metallo-organic cage MTH.Moreover, light yellow 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, Supplementary 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 (Fig. 1e). 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 temperaturesAfter 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, λmax ∼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, λmax ∼580 nm) emerged (Fig. 2a). 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 room temperature white-light emission45,46 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. 2b) 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. 2f). Then, the luminescence efficiency at various concentrations was also investigated. As depicted in Fig. 2c, 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 (5.4 ns) was detected. In contrast, the fluorescence lifetime of excimer emission measured at 580 nm increased to 18.6 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).Fig. 2: Tunable fluorescence emission of metallo-organic cage MTH at different concentrations and temperatures.a PL spectrum (λex = 320 nm, 300 K, in DMF), b CIE 1931 chromaticity diagram (the crosses signify the luminescence color coordinates), c Absolute fluorescence quantum yields of metallo-organic cage MTH at different concentrations. Data were means ± Standard Deviation (SD) (n = 3). Source data are provided in the Supplementary Figs. 29–35. d PL spectrum (λex = 320 nm, c = 3 × 10−5 M, in DMF), e CIE 1931 chromaticity diagram (the crosses signify the luminescence color coordinates) of metallo-organic cage MTH in DMF at different temperature. f Fluorescence photographs of metallo-organic cage MTH at different concentrations (300 K) and temperatures (c = 3 × 10−5 M) in DMF.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 300 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. 2d). This phenomenon reflects that the formed excimer gradually dissociates into a monomeric structure at higher temperatures. 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 emission47. Moreover, a small energy difference between monomer and excimer, experimentally determined to be 6.4 kcal/mol (Supplementary Note, Supplementary Fig. 59), further demonstrates that MTH excimer can readily transform into monomeric molecules at high temperatures48. The CIE diagram (Fig. 2e) 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. 2f). Therefore, white light emission based on a single molecule can be accomplished by utilizing multiple external stimuli (temperature and concentration) in the solution state (Supplementary Fig. 45). It is crucial to note that concentration gradient ESI-MS, solution state elevated temperature 1H NMR (600 MHz, DMF-d7), TGA analysis and variable temperature UV/Vis absorption spectra of metallo-organic cage MTH confirmed the stability of structure and photophysical properties (Supplementary Figs. 49–55).The optical properties of luminescent materials are closely linked to the molecular conformation49,50 as well as the stacking mode11,13 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. 3a, 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) (Fig. 3b, 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 or temperature variations (Supplementary Figs. 25, 26). Based on the single-crystal structure of metallo-orgainc cage MTH, frontier molecular orbitals involved in the electronic transitions for the investigated molecules obtained by density functional theory (DFT) calculations have been provided (Supplementary Figs. 57, 58 and Supplementary Tables 3, 4).Fig. 3: Single crystal structure of metallo-organic cage MTH.a The twisted conformation of metallo-organic cage MTH. b Stacking mode of metallo-organic cage MTH within two neighboring unit cells. c Arrangement of MTH molecules in the single crystal structure.Tunable fluorescence emission at solid stateEncouraged by these results, an attempt was made to investigate temperature-dependent fluorescence emission in the solid state (Supplementary Fig. 44). An appropriate substrate, PMMA52,53,54, 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. 4b). 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. 4a).Fig. 4: Tunable fluorescence emission in solid state.a PL spectrum of PMMA films with metallo-organic cage MTH at different temperatures (λex = 320 nm). b Fluorescence photographs of PMMA films with the Sun shape for metallo-organic cage MTH at different temperatures. c Thermally activated information encryption with the Morse code by PMMA films of metallo-organic cage MTH.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. 4b, Supplementary Figs. 46–47). 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. 4c, 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 °C it reveals the correct message under UV light (the hidden word Chemistry). It is worth mentioning that the encryption experiments can be performed for 5 cycles without degradation (Supplementary Fig. 56). This phenomenon is attributed to temperature-controlled excimer-monomer conversion of MTH. Poly(methyl methacrylate) (PMMA) is a material that is strongly affected by temperature changes. Single-crystal structure of metallo-organic cage MTH confirmed the staggered stacking mode (slip angles θ = 72.3°) of aromatic rings rather than face-to-face stacking, indicating a moderate stable excimer. When heating the film, the fluidity of the PMMA film is greatly enhanced, helping to separate the otherwise stacked molecules of metallo-organic cage MTH and increase the intermolecular distance. After cooling to room temperature, PMMA material returned to solid-state, can shorten the intermolecular distance of metallo-organic cage MTH and promote their excimer state. So, heating destroyed the stacking of metallo-organic cage MTH, leading to the disappearance of excimer fluorescence and generation of monomer fluorescence. To further test the hypothesis, control experiment has been conducted by doping metallo-organic cage MTH at low concentration into the solution of poly(methyl methacrylate) (PMMA) in DMF, in which luminophores are not close and against the intermolecular interactions55,56. The obtained film by low concentration doping showed blue fluorescence emission. And, there is no change in the corresponding fluorescence emission with the increasing of temperature, further supported non-interacting of metallo-organic cage MTH molecules in the monomeric state (Supplementary Fig. 48). The innovative creation of the cryptograph opens up possibilities for applying supramolecular structures in information transmission and encryption.
