“The cover specifically shows the vital role of icosahedral boron clusters-based ligands as a highly efficient “antenna” for lanthanides. Our carborane-based ligand serves as a chromophore to absorbs photons and then sensitize mixed Eu³⁺/Tb³⁺ metals. Energy transfer from Eu³⁺ to Tb³⁺ occurred after irradiation, which results in time-dependent color change (from green to orange to red with time going on; represented as colored waves in the cover) and spectroscopic bar-code variation that could be potentially used for anticounterfeiting (e.g., in bank notes).” explains José Giner, ICMAB Researcher at the Inorganic Materials & Catalysis (LMI) Group.

Cover for "“Water-Stable Carborane-Based Eu3+/Tb3+ Metal–Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding”" in Chemistry of Materials

The cover highlights the article “Water-Stable Carborane-Based Eu³⁺/Tb³⁺ Metal–Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding”, which explores the rational design and synthesis of iso-reticular water stable carborane-based lanthanide MOFs (Ln-MOFs), that exhibit stoichiometrically dependent emission color variation from green to red and tunable energy transfer efficiency between different lanthanides.

“Owing to the exceptional hydrolytic stability of the carborane-based Ln-MOFs, luminescent properties could be well preserved after dispersing the materials into water, which allowed us to prepare a series of aqueous security inks, thus making our materials easily printable onto paper. Such a convenient printing method combined with the time-dependent color and spectroscopic bar-cording variations indicate that our materials could be potentially used for anticounterfeiting”, says José Giner.

The properties of these materials make them interesting for many potential optical and optoelectronic applications, such as in optical information multiplexing, LEDs, chemical sensing, and fluorescent bio-imaging for theragnostic purposes. The team has found its properties, however, are most useful on anticounterfiting efforts:

“Due to the colorless nature of our materials under daylight, various invisible patterns can be printed onto paper. Furthermore, the energy transfer efficiency between Eu³⁺ and Tb³⁺ could be finely regulated by changing the Eu/Tb ratio, thus variation speed in colors and spectroscopic bar-codes could be well controlled, demonstrating that the diverse time-dependent information can be encoded into the printed patterns”, according to José Giner.

The results achieved in this study are treading the unexplored territory of carboranes for use in the “antenna effect” for lanthanides. José Giner: “To the best of our knowledge, ours is the first study on the luminescent properties of carborane-based Ln-MOFs. Our study suggest that the observed extraordinary antennae effect is due to the presence of the icosahedral carborane fragment and clearly differs from all reported only-carbon based ligands. The present study provides an example to use carborane-based Ln-MOFs as precursors to prepare invisible security inks, which could be used for future anticounterfeiting technologies.”

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 mCB-Eu_yTb_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 mCB-Eu_yTb_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.

Reference Article

Water-Stable Carborane-Based Eu³⁺/Tb³⁺ Metal–Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding
Zhen Li, Rosario Núñez, Mark E. Light, Eliseo Ruiz, Francesc Teixidor, Clara Viñas, Daniel Ruiz-Molina, Claudio Roscini*, and José Giner Planas*
Chem. Mater. 2022, 34, 10, 4795–4808
doi: 10.1021/acs.chemmater.2c00323

More information:

• New MOF materials for advanced anticounterfeiting techniques (ICN2, 27 July 2022)