Unraveling Hidden Correlations between Molecular Diffusivity and Reactivity in Ruthenium Complex-Modified Mesoporous Silica

Mesoporous silicas (MPSs) are widely used as host materials for applications involving molecular sorption, separation, storage, and (photo)catalysis. Herein, we report single-molecule, single-particle approaches for exploring molecular diffusion and photochemical reactions in ruthenium (Ru) complex-modified MPS particles. Single-particle fluorescence recovery after photobleaching (FRAP) experiments revealed that Ru complexes are mobile in MPS with diffusion coefficients in the range of 10(0)-10(2) nm(2) s(-1), which vary among the particles and locations, and their diffusivity decreases with increasing amount of sulfonated phenyl (SPh) groups, which act as adsorption sites for positively charged Ru complexes, on the silica surface. From emission lifetime measurements and photocatalytic activity tests using a fluorogenic probe at the single-particle, single-molecule levels, it was established that a series of reactions including quenching of excited Ru complexes by O-2 and subsequent generation of (OH)-O-center dot are completed within the same particle. By combining the above methods, a positive correlation between molecular diffusivity and reactivity was validated, suggesting the importance of well-ordered nanochannels with optimized pore diameters and adequate environments for efficient heterogeneous (photo)catalysis.