Enhanced singlet oxygen production under nanoconfinement using silica nanocomposites towards improving the photooxygenation’s conversion

In this contribution, the effect of physical immobilization of methylene blue (MB) into silica nanocomposites was investigated on the conversion and selectivity of the photooxygenation of anthracene and dihydroartemisinic acid (DHAA). Physically immobilized photocatalysts were synthesized through a developed Stöber method and were thoroughly characterized by UV–Vis, FTIR, XRD, XPS, SEM, TEM, HR-TEM, BET-BJH, and EDX analyses. Based on the TEM and UV–Vis results, it was determined that enhancement of the MB concentration as an organocatalyst for the Stöber reaction led to an increase in the size of the nanoparticles from 54 to 183 nm and a 21 nm blue shift in their UV–Vis spectra. Moreover, utilizing an immobilized MB as a photocatalyst for photooxygenation reactions under visible light led to a remarkable enhancement of 9% (i.e., from 89 to 98%) in the reaction conversion of anthracene photooxygenation compared to those using the same amount of homogenous MB. Nonetheless, a 5% reduction (i.e., 83 to 78%) in the selectivity of photooxygenation of DHAA was observed. These behaviors were rationalized through the nanoconfinement effects of pores with a narrow size distribution of 3.1 nm obtained through the HR-TEM and BET-BJH analyses, which led to a controlled aggregation of the MB molecules. Deep neural networks (DNNs) were applied to accurately predict the UV–Vis spectra and aggregation of the MB molecules. The results from time-dependent density functional theory (TD-DFT) calculations suggested that aggregation of the MB led to decreasing in intersystem crossing energy gap; hence, an increase in the ¹O₂ generation became possible. Finally, the finite element method (FEM) simulation revealed a 300 nm penetration depth of ¹O₂ around synthesized photocatalysts.