The combined time-resolved photoluminescence (PL) and theoretical study performed on luminescent [Mo6Br(i)8Br(a)6](2-)-based systems unambiguously shows that their NIR-luminescence is due to at least two emissive states. By quantum chemical studies, we show for the first time that important geometrical relaxations occur at the triplet states either by the outstretching of an apex away from the square plane of the Mo6 octahedron or by the elongation of one Mo-Mo bond. Experimental PL measurements demonstrate that the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes. Temperature and excitation wavelength dependence of the two components of the luminescence spectra is representative of multiple competitive de-excitation processes in contradiction with Kasha's rule. Our results also demonstrate that the relaxation processes before and after emission can be tracked via fast time-resolved spectroscopy. They also show that the surroundings of the luminescent cluster unit and the excitation wavelength could be modulated for target applications.
Accepted 04 Dec 2013International audienceThe hybrid strategy is a powerful approach to design functional materials by combining inorganic dyes with an organic matrix. However, introducing high contents of inorganic species within the hybrid material is a real challenge that requires a perfect balance between the interactions of both components to avoid mainly phase segregation problems. Based on our demonstration on an anionic molybdenum cluster, we present a general method to introduce high contents of such class of nanometre sized inorganic molecular deep red dyes in a polymer matrix. Our strategy exploits the physical interactions between the organic and inorganic parts of the hybrid material and allows a high cluster rate to be introduced (up to 50 wt%) in the polymer matrix. The resulting hybrids are remarkably stable even after several months of ageing. Moreover, the Mo clusters maintain their intrinsic deep red luminescence properties while the polymer organic matrix fully maintains its processability, thanks to the di-anionic character of the Mo6 clusters. Such materials show promising prospects in applications needing deep red emitters
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