Orthogonal luminescence lifetime encoding by intermetallic energy transfer in heterometallic rare-earth MOFs

Abstract

Lifetime-encoded materials are particularly attractive as optical tags, however examples are rare and hindered in practical application by complex interrogation methods. Here, we demonstrate a design strategy towards multiplexed, lifetime-encoded tags via engineering intermetallic energy transfer in a family of heterometallic rare-earth metal-organic frameworks (MOFs). The MOFs are derived from a combination of a high-energy donor (Eu), a low-energy acceptor (Yb) and an optically inactive ion (Gd) with the 1,2,4,5 tetrakis(4-carboxyphenyl) benzene (TCPB) organic linker. Precise manipulation of the luminescence decay dynamics over a wide microsecond regime is achieved via control over metal distribution in these systems. Demonstration of this platform's relevance as a tag is attained via a dynamic double encoding method that uses the braille alphabet, and by incorporation into photocurable inks patterned on glass and interrogated via digital high-speed imaging. This study reveals true orthogonality in encoding using independently variable lifetime and composition, and highlights the utility of this design strategy, combining facile synthesis and interrogation with complex optical properties.

Fig. 1. Concept of controllable energy transfer for lifetime modulation in EuGdYb-based trimetallic compositions.

a Direct ligand excitation results in modulated energy transfer, illustrating the relationships between each component. b Examples of complex intermetallic energy transfer in three distinct EuGdYb-based compositions with different elemental ratios and the direct impact on lifetime modulation.

Fig. 2. Information regarding the composition and crystallinity of the reported compounds.

a Ternary diagram of metal content showing the composition of each compound in the study. b Powder X-ray diffraction patterns for each compound reported in this work, highlighting their consistent crystallinity and the slight variations resulting from different metallic compositions. c Scanning electron microscope images and energy dispersive spectroscopy maps for the four trimetallic compounds 10–13, highlighting the morphology of the materials and the uniform distribution of metals within each crystal.

Fig. 3. Detailed MOF structural information and photophysical properties evaluation of the rare-earth MOF family.

a Ball-and-stick representation of MOF structure showing the intra-cluster and inter-cluster intermetallic distances in compound 1, EuTCPB. Atom color scheme: C gray, O red, Yb green. H atoms are omitted for clarity. b Energy transfer diagram illustrating the relationships between each component of the compounds reported here when the ligand is excited directly (λ_ex = 337 nm). Colored arrows were chosen to approximate emission color of each transition. c Emission spectra for select compounds in the visible range, with the excitation wavelength 394 nm; all peaks associated with Eu. d Emission spectra for select compounds in the NIR range, with the excitation wavelength 394 nm; all peaks associated with Yb.

Fig. 4. Fluorescence decay data for reported compounds.

a–c Visible decay curves for all compounds containing Eu; d A ternary diagram of the different compounds with colors corresponding to the lifetime of each. These show the relative effects of Gd and Yb content on emission lifetimes of Eu in these materials.

Fig. 5. Information regarding metal distribution and its effect on the reported MOFs.

a Illustration of the effect of composition on visible lifetime for different compounds. The lifetime of a compound is determined by the probability of a Yb metal center (blue sphere) neighboring an Eu metal center (red sphere) and receiving energy from it. The presence of a Gd metal center (green sphere) reduces the probability that an Eu center will have a neighboring Yb center. b Calculated RE PDOS near the electronic band edges for three representative heterometallic clusters 3Eu-6Yb (left), 3Eu-3Gd-3Yb (center), and 1Eu-7Gd-1Yb (right). The PDOS identify the relative electron density localized on each of the three RE elements; Eu (red), Yb (blue), and Gd (green). Each panel highlights the two primary peaks of RE electron density in the conduction band (black dashed box with labeled vertical arrows 1 and 2) which participate in excited state relaxation mechanisms. The three panels are organized by relative luminescence decay time as measured in this experimental work.

Fig. 6. Representation of an encoded message utilizing the braille alphabet in a 96-well plate.

a A dynamic tag that shows the transition from undifferentiated dots to the message over time based on different compound lifetimes. b A double-encoded dynamic tag that shows an initial encoding based on emission intensity via compound concentration in each well, with a final encoding based on different compound lifetimes. The intensity of each red dot is based on experimental data. Threshold operations are the process of choosing an intensity value as a cutoff for reading a dot as lit versus unlit for the purpose of decoding.

Fig. 7. A demonstration of patterning and interrogating a tag made with compounds 11 and 12 in an ink.

a Diagram detailing stencil design and etching. b Stencil composed of painters’ tape on glass. c Inked Sandia thunderbird logo under UV light, held in hand for scale. d, e Inked logo under ambient and UV light respectively. f Diagram of the laser excitation and digital high-speed imaging setup used to capture the decay of the logo. g Time-lapse of tag emission after pulsed laser excitation, showing two distinct lifetimes for the thunderbird and border respectively. h Decay data curves and fit curves for each section of the logo, derived from digital high-speed images, compared to the decay of pure compound 11 and 12 reported above. In every image, the pattern was printed on a 25 mm × 25 mm substrate.