Supramolecular glasses with color-tunable circularly polarized afterglow through evaporation-induced self-assembly of chiral metal-organic complexes

Abstract

The fabrication of chiral molecules into macroscopic systems has many valuable applications, especially in the fields of optical displays, data encryption, information storage, and so on. Here, we design and prepare a serious of supramolecular glasses (SGs) based on Zn-L-Histidine complexes, via an evaporation-induced self-assembly (EISA) strategy. Metal-ligand interactions between the zinc(II) ion and chiral L-Histidine endow the SGs with interesting circularly polarized afterglow (CPA). Multicolored CPA emissions from blue to red with dissymmetry factor as high as 9.5 × 10^-3 and excited-state lifetime up to 356.7 ms are achieved under ambient conditions. Therefore, this work not only communicates the bulk SGs with wide-tunable afterglow and large circular polarization, but also provides an EISA method for the macroscopic self-assembly of chiral metal-organic hybrids toward photonic applications.

Fig. 1. Schematic illustration of Zn–L SGs fabrication.

a Preparation of Zn–L and RB-doped SGs (RB: Rhodamine B), and photos of the obtained Zn–L and RB-doped SGs in the shape of a sphere and/or a round sheet taken under the sunlight, and before and after UV-light turned off. Scale bar: 0.8 cm. b Multicolored CPA emissions from chiral SGs. Spiral lines represent CPA-active signals. c C2H2-type zinc finger motif. d Schematic representation of the SGs assembly.

Fig. 2. The molecular structure of the Zn–L complex, and the thermal properties and morphologies of the SGs.

a The HR-ESI-MS spectrum of the Zn–L complex. The inset was the illustration of the molecular structure of the Zn–L complex. b The hydrogen-bonding network in the Zn–L crystal structure. The orange and purple dashed lines respectively represent intra/intermolecular hydrogen bonds. There were intra/intermolecular hydrogen bonds between the complexes, as well as hydrogen bonds between the complexes and the solvent water molecules. The Zn–L complex showed a twist configuration in the crystal, owing to the presence of alkyl chains. c The DSC traces of Zn–L-1/2/3 SGs. d UV-Vis-NIR transmittance spectra results for Zn–L/D-2 SGs. The Zn–L/D-2 SGs were highly transparent (above ~90%) in the visible and NIR ranges (gray area). Their bands in the cyan and blue areas originated from the Zn–L complexes and water absorption, respectively. e The PXRD patterns of Zn–L-1/2/3/4, Zn-D-2 and Zn-L-RB-1/2 SGs. Note: Zn–L-4 glass corresponds to Zn–L mass content of 90.5 wt%. f The representative TEM image of Zn–L SG.

Fig. 3. Representative images of ultralong RTP observed in the glasses.

a Images of spherical Zn–L-2 and Zn-L-RB-1/2 SGs taken before and after the excitation turned off across a time scale of 0 to 10 s. b Images of thin Zn–L-3, Zn-L-Cl/ClO₄/C₂O₄ and Cd/Eu/Tb-L SGs taken before and after 365 nm irradiation turned off under ambient conditions. Scale bar: 0.8 cm.

Fig. 4. Photophysical properties of Zn–L SGs and RB doped SGs.

a, b Delayed PL spectra and lifetime profiles of Zn–L-1/2/3/4 SGs excited by 365 nm at room temperature. c, d Delayed PL mapping spectra and lifetime profiles of Zn–L-2 SG excited by 365 nm at different temperatures. e Delayed PL spectra of Zn–L-2 SG excited by different wavelengths at room temperature. f Phosphorescence lifetime decay profile of Zn-D-2 SG excited by 365 nm at room temperature. g, h Delayed PL spectra and lifetime profiles of Zn-L-RB-1/2 SGs excited by 365 nm at room temperature.

Fig. 5. The CPL properties of the SGs.

a–c The CPL spectra of Zn–L/D-2, Zn–L/D-RB-1 and Zn–L/D-RB-2 SGs excited by 365, 380, and 400 nm, respectively. “DC” is analogous to “PL intensity”. d–f The CD and UV-Vis absorption spectra of these SGs.

Fig. 6. Theoretical analyses of color-tunable CPA from SGs.

a Calculated energy bandgaps for the selected L-His monomer extracted from the L-His single crystal (CCDC: 1206542), and Zn–L monomer and aggregates extracted from Zn–L single crystal. The blue and orange lines illustrate the energy values of HOMO and LUMO, respectively. b TD-DFT calculated energy level diagrams of selected L-His monomer, as well as Zn–L monomer and dimer in L-His and Zn–L single crystal, respectively. c, d Proposed mechanisms of the CPA emissions from Zn–L SGs and RB doped SGs. Cyan ball represents the Zn–L complex. The black dotted line symbolizes non-covalent interaction between the complexes—the shorter the line, the stronger the interaction. S₀, S₁, and T₁ represent the ground state, the lowest singlet, and triplet excited states, respectively. T₁*' and T₁*'' are the stabilized triplet excited states originating from varied emission species with different aggregation degrees, respectively. Fluo. and Phos. are the abbreviations of fluorescence and phosphorescence, respectively. ISC and PRET are the abbreviations of intersystem crossing and phosphorescence resonance energy transfer, respectively. The color of the spiral lines and the lines representing the energy states is similar to the corresponding luminous color.

Fig. 7. Demonstration of multicolored CPA emissions of SGs for the potential application in displays under ambient conditions.

a The photos of Zn–L-2 SG in the shape of a deer taken before and after 365/395 nm irradiation turned off in the dark environment. b The photos of RB-doped SGs in the shape of deer taken before and after 365 nm irradiation turned off in the dark environment. Scale bar: 0.5 cm.