Long-Lived Charge-Separated States in ZnS/Na-MOR Zeolite upon trans-Stilbene Adsorption

Electron transfer (ET) processes are of fundamental importance in many photochemical processes in biological and chemical systems. Here, semiconductor nanoparticles of ZnS (as electron relay) and trans-stilbene molecules (t-St) (as electron donor), both confined within the porous volume of mordenite (MOR), are combined to mimic photosynthetic processes. ZnS nanoparticles were synthesized by cationic exchange between the counterion of the zeolite and ZnS precursors in solution. The characterization of the ZnS/Na-MOR composite was performed by powder X-ray diffraction (XRD), diffuse reflectance UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS). These complementary techniques highlight the successful formation of ZnS nanoparticles at the surface of Na-MOR. The electron transfer mechanisms taking place after the incorporation and photoionization of t-St molecules were evaluated using diffuse reflectance UV-vis spectroscopy. The results show the formation of long-lived t-St(center dot+)@ZnS/Na-MOR center dot- charge-separated states (lifetime = 18 min; k = 0.0553 min(-1)) whose stability is linked to the nature of the new charge compensating cation and to the close proximity of the ZnS nanoparticles, which are probably located within the zeolite framework. Indeed, the augmentation of the transient species lifetime was attributed to an electron transfer from the t-St molecule toward the ZnS conduction band.