Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Abstract

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Figure 1

Crystal structure of mCB-Tb. (a) View of the coordination of mCBL1 to the three independent Tb atoms that are repeated along the structure to provide one-dimensional (1D) inorganic rod-shaped chains and (b, c) two perpendicular views of the extended 3D framework along the b and a axes, respectively. Green polyhedra represent the Tb coordination spheres and H atoms are omitted for clarity. Color code: B, pink; C, gray; O, red; N, dark blue; and Tb, green.

Figure 2

Solid-state emission spectra of mCB-Eu (a) and mCB-Tb (b) under continuous-wave irradiation (λ_ex = 280 nm) at room temperature. Insets: optical microscopy images of the corresponding crystals (λ_exc = 280 nm). (c) Photograph of the hand-painted logo of the Institut de Ciència de Materials de Barcelona (ICMAB) with mCB-Eu and mCB-Tb crystals (λ_ex = 254 nm).

Figure 3

Energy diagram of the singlet and triplet states calculated using TDDFT with the B3LYP functional. The orbitals involved in such processes of the mCBL1 ligand are shown.

Figure 4

(a) Photographs of the powders of the mixed mCB-Eu_yTb_1–y (λ_ex = 254 nm); (b) selection of steady-state emission spectra of the powders of mixed mCB-Eu_yTb_1–y with various Eu/Tb molar ratios (λ_ex = 280 nm) (see Figure S14 for the spectra of all mCB-Eu_yTb_1–y series); (c) photograph of the hand-painted logo of the Institut de Ciència de Materials de Barcelona (ICMAB) with mCB-Tb (green), mCB-Eu_0.1Tb_0.9 (yellow), and mCB-Eu (red) crystals; and (d) color coordinates drawn onto the 1931 CIE chromaticity diagram for the mixed mCB-Eu_yTb_1–y. Inset: luminescence microscopy images of the mCB-Tb (green), mCB-Eu_0.1Tb_0.9 (yellow), and mCB-Eu (red) crystals.

Figure 5

(a) Luminescence decays of Tb (λ_em = 541 nm) in the different MOFs (λ_exc = 280 nm); (b) luminescence decays of Eu (λ_em = 614 nm) in the different MOFs (λ_exc = 280 nm); (c) comparison of the luminescence decay of mCB-Eu_0.6Tb_0.4 with mCB-Eu; and (d) average lifetimes, ET quantum yield, and luminescence quantum yield trends against the Eu³⁺ fraction.

Figure 6

Schematic diagram of the energy absorption to the singlet state (S₀) of the mCBL1 ligand, transfer to the triplet state (T₁), energy transfer, and emission processes of mCB-Eu_yTb_1–y.

Figure 7

Time-dependent emission spectra of (a) mCB-Eu_0.01Tb_0.99, (b) mCB-Eu_0.1Tb_0.9, and (c) mCB-Eu_0.6Tb_0.4 powders at various time delays and (d–f) corresponding CIE coordinates (λ_ex = 266 nm). Time-dependent bar codes of (g) mCB-Eu_0.01Tb_0.99, (h) mCB-Eu_0.1Tb_0.9, and (i) mCB-Eu_0.6Tb_0.4 (λ_ex = 355 nm).

Figure 8

(a) Spray-coated mCB-Eu_0.01Tb_0.99 using a prepatterned mask to illustrate the logo on Institut de Nanociencia i Nanotecnologia, (b) time-dependent emission spectra of the printed mCB-Eu_0.01Tb_0.99, (c) corresponding color coordinates in the 1931 CIE diagram, and (d) time-dependent bar codes of the printed mCB-Eu_0.01Tb_0.99 (λ_ex = 355 nm).