Synthesis and characterization of metallo-cage MTH。
Initially, 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. In the last step, the product was obtained in high yield (95%) as a pale-white yield (95%) as a pale-white solid (Fig. 2a). The proton nuclear magnetic resonance (1H 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 < tpy-Zn(II)-tpy > connectivities suggested a low symmetric structure (Fig. 2b), 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 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. 2c, 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. 2d). 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, 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. 3a). 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. 3b,) 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. 3f). 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. 3c, 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. 3d). This phenomenon reflects that the formed excimer gradually dissociates into a monomeric structure at higher temperatures. Elevated temperature 1H 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. 3e) 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. 3f). 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. 4a, 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. 4b, 4c).[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. 5b). 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. 5a).
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. 5b, 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. 5c, 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.