The fractal nature of the surfaces of porous silicas as revealed in electronic energy transfer between adsorbates: Comparison of three donor/acceptor pairs

Electronic energy transfer between adsorbed rhodamine 6G (R6G, donor) and malachite green (MG, acceptor) was studied on nonporous silica, on silica with average pore diameter of 60 Å (Si-60), on Si-100 and on Si-200. The decay profiles obey the Klafter-Blumen equation for the survival probability in fractal environments, with fractal dimensions D of 2.1, 2.3, 2.5, and 2.7, respectively, in good agreement with the results obtained with two other pairs, rhodamine-B/MG and R6G/R6G (in a depolarization experiment). The invariance of D to changes in the critical Förster radius, suggests that the derived D values reflect structural scale invariance properties of the supports and in particular the surface fractal dimension as "seen" by surface/molecule interaction. This interpretation compares favorably with other surface studies of SiO₂.