New strategies and synthetic routes to synthesize fluorescent atomic quantum clusters

Table 2.1 shows a summary of the photophysical parameters of metal clusters obtained by different approaches. One can try to rationalize the results observed in Table 2.1. For this purpose we will try to apply the spherical Jellium model to obtain an approximate description of the observed dependence of the emission energy on the number of atoms, N, in metal clusters. According to this simple model, the band gap of a cluster with N atoms will be given by the simple relation, Eg = EFermi/N1/3, (where EFermi = Fermi energy of the bulk metal, and Eg = band gap energy of the cluster). If we assume that the band gap energy is proportional to the fluorescence emission or excitation energy, then we can predict a linear relationship between such energies and N-1/3. Figures 2.18 and 2.19 show that, indeed, there is a relatively good linear relationship for all clusters displayed in Table 2.1. The fitted values of the slopes were: 10.0 ± 0.05 eV (excitation energy) and 8.0 ± 0.7 eV (emission energy) for copper clusters, and 6.6 ± 0.3 eV (excitation energy) and 5.0 ± 0.2 eV (emission energy) for the rest of the clusters (mainly gold and silver). It can be seen that the value obtained from the emission energy matches reasonably well with the EFermi value for bulk gold and silver (5.3 eV), and for bulk copper (7.0 eV). Moreover, from the fittings it can be predicted that ΔE = Eexc - Eemis = 1.6N-1/3 for Au and Ag, and 2.0N-1/3 for Cu. This indicates that this simple Jellium model can be used to obtain an approximate value for the band gap energy of clusters from their emission energies. At the same time, these results show that an approximate estimation of the number of atoms of the cluster can also be made from this simple model, a result already pointed out by some authors.49-51,77 In the figures one can see that there are some deviations from the fitted curves, indicating that there are, as expected, other factors - different from the number of atoms - contributing to the band gap energy of clusters. From inspection one can see that the major deviations come from cases where large Stokes shifts were reported, which can be related to some charge-transfer processes (between the clusters and the ligands), which cannot be accounted for by the very simple Jellium model because it does not take into account the influence of the ligands. In summary, it can be said that remarkable progress has been made during the last decade in the aqueous synthesis of fluorescent metal clusters. Multiple methods have been developed to synthesize metal clusters, allowing clusters to be obtained with different sizes and protected with different capping agents (such as biomolecules, proteins, etc.), providing different luminescent properties. Clusters with high QYs have been obtained; generally observing the highest values for clusters obtained in dendrimers and DNA templates. These capping agents seem to preserve better the inherent fluorescent properties of metal clusters protecting them against cluster-cluster interactions or possible interactions with the solvent. It is also important to point out the generally great photostability obtained for many metal clusters. This important aspect, together with their tiny sizes, their biocompatibility and their relatively high QYs (some of them reaching values close to those of QDs), make them a very attractive alternative to traditional fluorophores, with significant potential applications as biological labels or optoelectronic devices. (Table Presented).