The Closer, the Better: Engineering RGB Thermometric Performance in Tb3+, Eu3+-Doped MOFs

Technical Summary: The Closer, the Better: Engineering RGB Thermometric Performance in Tb3+, Eu3+-Doped MOFs

Problem Statement
While luminescence thermometry based on ratiometric intensity ratios is a well-established technique, conventional implementations typically require spectrometers to resolve emission bands, increasing the complexity and cost of the measurement setup. A more user-friendly alternative involves monitoring thermally induced changes in the emission color, which can be observed visually or captured by standard digital cameras. However, the rational design of such "visual" or "filter-free" thermometers is hindered by a lack of clear understanding regarding the relationship between the structural characteristics of the phosphor material, the underlying energy-transfer processes, and the resulting thermometric performance. Specifically, there is a need to establish how the host matrix influences the interionic distance between lanthanide activators (Tb3+ and Eu3+) and how this distance dictates the efficiency of energy transfer and subsequent thermal sensitivity.

Methodology
The study systematically investigated a series of Tb3+, Eu3+-co-doped metal-organic frameworks (MOFs) to correlate structural features with thermometric performance. Four distinct MOF systems were synthesized and characterized:

1. Gd-BTC: A gadolinium-based framework with trimesic acid (BTC) ligands, serving as a reference where Ln3+ ions substitute optically inactive Gd3+ nodes.
2. Zr-BTC, Zr-BDC, and Zr-TA: Zirconium-based frameworks utilizing trimesic acid (BTC), terephthalic acid (BDC), and L-(+)-tartaric acid (TA) as organic ligands, respectively.

The materials were synthesized via hydrothermal or solvothermal methods. Comprehensive characterization was performed using:

• Structural Analysis: Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray (EDX) spectroscopy, Powder X-ray Diffraction (PXRD), and Fourier Transform Infrared (FTIR) spectroscopy.
• Thermal Analysis: Thermogravimetric (TG) and Differential Scanning Calorimetry (DSC) analysis.
• Spectroscopic Evaluation: Temperature-dependent emission and excitation spectra (83–413 K) were recorded using a fluorescence spectrometer.
• Thermometric Assessment: Performance was evaluated using three distinct readout methods:
  1. Luminescence Intensity Ratio (LIR): Based on the ratio of integrated emission intensities of Tb3+ and Eu3+ bands.
  2. CIE 1931 Chromaticity Coordinates: Tracking the shift in color coordinates as a function of temperature.
  3. RGB Camera-Based Analysis: Analyzing the intensity ratios of the Blue (B), Green (G), and Red (R) channels captured by a standard digital camera to simulate filter-free thermal sensing.

Key Contributions and Results

• Structural Influence on Energy Transfer: The study established a direct correlation between the average interionic distance (Ln3+-Ln3+) and the efficiency of phonon-assisted Tb3+→Eu3+ energy transfer. The Gd-BTC host, where ions substitute Gd3+ nodes, exhibited the shortest interionic distance (~4.75 Å), facilitating the most efficient energy transfer. In contrast, Zr-based MOFs exhibited longer distances (ranging from ~7.34 Å to ~11.7 Å) due to the specific localization of ions within Zr-oxo clusters or interstitial sites.
• Emission Color Tuning: The efficiency of energy transfer directly dictated the emission color. Shorter interionic distances (Gd-BTC) resulted in dominant Eu3+ red emission due to efficient energy transfer from Tb3+, whereas longer distances (Zr-BDC, Zr-TA) favored Tb3+ green emission due to inefficient transfer. The CIE coordinates and color difference (CD) values showed a monotonic relationship with the metal-metal distance, confirming that emission color can be rationally tuned by controlling the MOF structure.
• Thermometric Performance (Spectroscopic):
  • Gd-BTC:Tb3+, Eu3+: Demonstrated effective ratiometric sensing below 250 K with a maximum relative sensitivity ($S_R$) of 0.62% K⁻¹ at 100 K.
  • Zr-BTC:Tb3+, Eu3+: Showed a maximum $S_R$ of 0.31% K⁻¹ below 300 K.
  • Zr-TA:Tb3+, Eu3+: Exhibited the highest sensitivity at higher temperatures, reaching ~0.58% K⁻¹ above 323 K.
  • The study noted that while these materials function as thermometers, their relative sensitivities are modest compared to state-of-the-art ratiometric systems reported in literature.
• RGB Camera-Based Readout: A novel approach utilizing the spectral sensitivity of digital camera channels was proposed. The analysis revealed that the Red (R) channel captures primarily Eu3+ emission, while the Blue (B) channel captures Tb3+ emission. The Green (G) channel, however, overlaps significantly with both, making the R/G ratio less sensitive to temperature changes. Conversely, the B/G ratio provided a more monotonic and effective temperature response.
  • Zr-TA:Tb3+, Eu3+ emerged as the superior candidate for this method, exhibiting a B/G ratio decrease of over 50% across the temperature range and a maximum relative sensitivity of 0.28% K⁻¹ at room temperature.
  • This sensitivity was found to be higher than that of the widely studied Gd-BTC reference material when using the same camera-based readout methodology.

Significance and Claims
The paper claims to provide a foundational framework for the rational design of visual luminescence thermometers. By establishing a direct link between the MOF host structure (specifically the interionic distance) and thermometric performance, the authors demonstrate that structural engineering can be used to tailor sensing characteristics.

The primary significance of the work lies in the validation of a filter-free, camera-based thermal sensing platform. The study highlights that by selecting the appropriate MOF structure (specifically Zr-TA) and utilizing the B/G intensity ratio from standard digital cameras, it is possible to achieve effective temperature sensing without complex spectroscopic instrumentation. This approach offers a simple, rapid, and low-cost alternative for thermal mapping and point sensing. The authors modestly conclude that while the absolute sensitivity values are not record-breaking, the methodology opens a pathway for optimizing filter-free thermal sensing platforms through structure-guided design.