**5. Conclusions and outlook**

Even though the super atom model could provide a qualitative description of the luminescence properties, the exact picture to contain the dynamics and kinetics of electron transition of the excited state remains blurred. By judiciously manipulating the interactions at the interface between cluster and zeolite framework together with a careful calculation of time-dependent density functional theory, a few proposed kinetic scheme claim to elucidate the long-lived emitting state. Simply considering an excited state intersystem crossing process followed by a radiative decay to the ground state would not fully explain the presence of a complex array of decay components observed on a fs and ps time scales, decay observed in the photophysics of many clusters. Ultrafast decay components would indeed indicate the occurrence of an intersystem crossing phenomena but can also suggest internal conversion or fast structural relaxation which are so common for molecules. The ns components would suggest decays between states of the same spin multiplicity, like fluorescence. Are these components considered at all when concluding the excited state dynamics? Why do these emissions indicate a Stokes shift as large as 12–15000 cm−1? Are there other intermediate states that are not yet experimentally revealed? When all is said and done, are all the fruits from the tree harvested?

As shown above, the lack of full understanding of the photophysics of metal clusters resides in its optical properties which depend on so many factors including cluster size, temperature, surface ligands geometry, cluster assembly structure, humidity, etc. One think is clear. Dedicated time-resolved spectroscopy techniques like transient absorption and fluorescence up-conversion at the femtosecond scale should come more consistently into play to provide precise and reliable information. Only then a complete picture of the relaxation and decay pathways of excited electrons immediately after the population of the Franck-Condon state can be achieved. More modern time-resolved spectroscopy techniques, like for instance X-ray diffraction based on X-Ray Free-Electron Laser (XFEL) methods, could elucidate changes in structural dynamics immediately after excitation. Most importantly, with respect to the paramount importance, universality and complexity of the model, the experimental part, be that spectroscopy, diffraction or ESR techniques, should take the first place in providing the arguments while modeling and computation could shortly support the experimental findings. Only when a complete picture of the excited state processes is achieved one can tune the electronic structure and thus interfere with the optical properties of the metals clusters. Hence, the advance achieved will be of immediate interest to a broader pool of researchers and will open real pathways for practical applications.

*Advances in Geopolymer-Zeolite Composites - Synthesis and Characterization*
