Rose Bengal (RB), a xanthene dye, incorporated into mesostructured silica nanoparticles (MSNs) exhibits efficient singlet oxygen ((1)O2) generation when illuminated with 540 nm green light which is particularly promising for PDT applications. Several systems with different RB loadings were synthesized and fully characterized by means of spectroscopic techniques in combination with a computational study, to optimize the amount of RB in order to avoid the formation of aggregates that is detrimental for a high (1)O2 delivery.