Ultrafast Scintillating Hetero-Ligand Metal Organic Frameworks with Engineered Stokes Shift for Detection of Ionizing Radiation

The Stokes shift is an important property of luminescent materials, defined as the energy difference between the absorption band maximum and the emission spectrum maximum frequencies. Its extent crucial in photonic devices because it enables to estimate in a first approximation if a specific emitter would be affected by reabsorption of its luminescence. If the Stokes shift is lower or similar to the bandwidth of the absorption and emission spectra, the consequent ‘<i>inner-filter’</i> effect can heavily limit the lighting performance of bulk devices, and, in the worst cases, it can also affect the kinetics of the luminescence generation. Conversely, if the Stokes shift is larger than the system spectral bandwidths this effect is avoided. Thus, reabsorption-free materials are highly desirable for several applications such as fluorescence imaging, enabling to obtain high contrast images with limited excitation stray light, avoiding the use of expensive filtering component or time-consuming image post-processing. For solar applications, to realize luminescent solar concentrators without reabsorption of the condensed radiation. Similarly, the sensitivity of scintillating detectors for ionizing radiation would greatly benefit from the use of fast emitters with no reabsorption showing maximum light output intensity without effects on the scintillation pulse timing, as required by the most advanced medical imaging, detection and metrology techniques.<br/>High efficiency, large Stokes shift emission and scintillation is obtained by realizing <i>hetero</i>-ligand Metal-Organic Framework (MOF) nanocrystals. [1] [2] Two fluorescent conjugated polyacene ligands of equal molecular length and connectivity, yet complementary photophysical properties, are co-assembled by hafnium oxy-hydroxy clusters, generating highly crystalline MOF. The fast diffusion of singlet molecular excitons in the framework given by the fine tuning of the ligand molecular orbitals properties, coupled to the achieved fine matching of co-ligands absorption and emission properties, enables to achieve an ultrafast activation of the low energy emitting ligands by diffusion-mediated non-radiative energy transfer in the ps time scale. [3] [4] In the optimized composition, MOF nanocrystals show an excellent scintillation yield of 13000 ph MeV^-1, with an actual Stokes shift of 1.3 meV and emission lifetime of 700 ps. This large Stokes shift suppresses the reabsorption of fast emission issue in bulk devices, pivotal for a plethora of applications in photonics and photon managing spacing from solar technologies, imaging, and detection of high energy radiation. [2] Coupled to the ultrafast emission properties of the system, this allowed to realize a prototypal metascintillator for high-resolution/hi-rate of g-rays detection.<br/>[1] Perego, Villa, et al. <i>Nature Photonics</i> <b>15</b>, 393 (2021)<br/>[2] Orfano, M., et al. <i>Nature Photonics </i><b>17</b>, 672–678 (2023)<br/>[3] Perego, J., et al. <i>Nature Communications</i><b> 13</b>, 3504 (2022)<br/>[4] Orfano at al<i> Adv. Funct. Mater</i>. 2404480 (2024)