Noble metal clusters of sizes comparable to the Fermi wavelength are known to exhibit molecule-like transitions owing to the discretion of the density of states. In the present article, the important factors influencing the reactivity of excited gold (Au) clusters are examined from the viewpoint of molecular photochemistry. The investigation of the differently sized Au Clusters embedded in it polymer film using single-molecule fluorescence spectroscopy facilitates the further understanding oft heir size-dependent photoreactivity. In addition, it was discovered that the spin multiplicity of the excited State (i.e., singlet or triple excited state) governs the photoreactivity.