Combined theoretical and time-resolved photoluminescence investigations of [Mo₆Brⁱ₈Br^a₆]²⁻metal cluster units: evidence of dual emission

Abstract

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.

Mo6 cluster-based compounds for energy conversion applications: comparative study of photoluminescence and cathodoluminescence

“…This leads to significant differences in the excited-state populations between optical and e-beam excitation, and results in different spectral shapes due to different non-radiative energy losses occurring before emission. Taking into account lifetime of the excited states, re-absorption can also occur [29]. As it can be seen in Supporting Figure 2, there is a good match between the CL and PL spectra in the high-energy region (>1.68 eV).…”

“…As there is no observable PL emission for 3 , it will not be discussed further. It may also be noted that, for all the clusters except 4 , there is an inflexion point around 750 nm that may be related to self-absorption, as quantum chemical calculations performed on [] 2− relaxed excited states show absorption transitions in the NIR region (∼600, 750, 900 nm) [29]. It may be noted that the position of the inflexion point found for PL (750 nm) is different compared to that found for CL (725 nm).…”

“…On the other hand, we reported the design of transparent inorganic thin films based on octahedral Mo 6 MCs deposited on indium tin oxide (ITO) glass by electrophoretic deposition (EPD) within a short time interval and at low cost [21]. Recently, multicomponent emission of [] 2− cluster units has been reported [29]. Several excited triplet states resulting from complex de-excitation processes (breaking Kasha’s rule) have been evidenced.…”

Time-gated luminescence bioimaging with new luminescent nanocolloids based on [Mo6I8(C2F5COO)6]2−metal atom clusters

“…Indeed, the trifluoroacetate family cluster compounds is reported to have a high rate of singlet-oxygen formation. 51,52 These points should be improved and further investigated. This toxicity could be suppressed or at least strongly reduced by adding a silica layer by seed-growth process at the surface of the nanoparticles.…”

“…Phosphorescence decay profiles were fitted to a twoexponential decay with long and short lifetime components. cluster units (L = carboxylate group), the recent joint work of 51 nanocolloids were compared to those of the Cs2Mo6I14@SiO2 Kitamura and Sokolov evidences that photo-luminescent dispersed in aqueous media. The same concentration of clusters and similar condition experiments were used in order to compare the intensity of both colloidal solutions.…”