Bright and Photostable TADF-Emitting Zirconium(IV) Pyridinedipyrrolide Complexes: Efficient Dyes for Decay Time-Based Temperature Sensing and Imaging

Luminescence thermometry represents a technique of choice for measurements in small objects and imaging of temperature distribution. However, most state-of-the-art luminescent probes are limited in spectral characteristics, brightness, photostability, and sensitivity. Molecular thermometers of the new generation utilizing air and moisture-stable zirconium(IV) pyridinedipyrrolide complexes can address all these limitations. The dyes emit pure thermally activated delayed fluorescence without any prompt fluorescence and show a unique combination of attractive features: a) visible light excitation and emission in the orange/red region, b) high luminescence brightness (quantum yields approximate to 0.5 in toluene and 0.8-1.0 in polystyrene matrix), c) excellent photostability, d) suitability for two-photon excitation and e) mono-exponential decay on the order of tens to hundreds of microseconds with strongly temperature-dependent lifetimes (between -2.5 and -2.9% K-1 in polystyrene at 25 degrees C). Immobilization in gas-blocking polymers yields sensing materials for self-referenced decay time read-out that are manufactured in two common formats: planar optodes and water-dispersible nanoparticles. Positively charged nanoparticles are demonstrated to be suitable for nanothermometry in live cells and multicellular spheroids. Negatively charged nanoparticles represent advanced analytical tools for imaging temperature gradients in samples of small volumes such as microfluidic devices.