Highly Luminescent Hetero-Ligand MOF Nanocrystals with Engineered Stokes Shift for Photonic Applications

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. The Stokes shift value is crucial in photonic devices and applications because, at a first approximation, it enables to estimate if a specific emitter would be affected by significant reabsorption of the generated light. If its value is lower or similar to the bandwidth of the absorption and emission spectra, the consequent intrinsic extensive ‘<i>inner-filter’</i> effect can heavily limit the lighting performance of bulk photonic 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 spectral bandwidths the system the inner filter effects are avoided. These reabsorption-free materials are highly desirable for several applications. For example, in fluorescence imaging large Stokes shift optical probes allow 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, large Stokes shift emitters are undoubtedly the most promising materials 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 good light output intensity without effects on the scintillation pulse timing, as required by the most advanced medical imaging techniques such as time-of-flight positron emission tomography and high-rate high-energy physics experiments.<br/>High efficiency, large Stokes shift emission is obtained by realizing <i>hetero</i>-ligand Metal-Organic Framework (MOF) nanocrystals. Two fluorescent conjugated polyacene ligands of equal molecular length and connectivity, yet complementary photophysical properties, are co-assembled by zirconium oxy-hydroxy clusters, generating highly crystalline MOF nanoparticles. The fast diffusion of singlet molecular excitons in the framework, 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 100 ps time scale. In the optimized composition, MOF nanocrystals show a fluorescence quantum efficiency of about 70% with an actual Stokes shift of 750 meV. 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. These features allowed to realize a prototypal fast nanocomposite scintillator [1] that shows an enhanced performance with respect to the <i>homo</i>-ligand nanocrystals,[2] achieving benchmark values competing with those of some commercial inorganic and organic systems.<br/>[1] Hajagos, T. J., at al. <i>Adv. Mater. </i><b>30</b>, 1706956 (2018).<br/>[2] J. Perego, I. Villa, at al. <i>Nature Photonics</i> <b>15</b>, 393–400 (2021).