Quenching of Excited States of Pyrene by Tetranitromethane on SiO2 Surfaces: Effect of Reaction Temperature, Silica Pore Size, and Surface Silanol Content

Abstract

Quenchers which adsorb strongly to the silica gel surface show limited quenching of the excited states of coadsorbed donors, and when quenching does occur it is predominantly static quenching. This is observed for several quenchers including nitromethane, tetranitromethane, and methyl viologen. The strong adsorption of tetranitromethane (TNM) to the surface silanol groups is indicated by the significant adsorption from cyclohexane solutions and the lack of any desorption with continuous evacuation of the sample after addition from solution or by vapor transfer. The strong adsorption of TNM is observed in the quenching characteristics of several adsorbed donors including the singlet and triplet excited states of pyrene. The quenching mechanism is partially determined by the binding characteristics of the coadsorbed electron acceptors and partially by the surface structure of the porous silica gel support material. Removing the high density of surface silanol groups changes the adsorption characteristics reducing the adsorption efficiency of both pyrene and TNM. The photophysics and photochemistry between coadsorbed donor and acceptors are also affected by reducing the silanol group concentration. The singlet quenching mechanism changes from predominantly static to increasingly dynamic as the silanol surface concentration is reduced. TNM movement on the surface is indicated by the loss of quenching reaction with decreased sample temperatures. The pyrene triplet excited state is quenched by coadsorbed TNM at lower TNM concentrations than are required for singlet quenching. The dynamic quenching mechanism of the triplet increases significantly using dehydroxylated silica gel. Excited-state quenching occurs by electron transfer on the surface with the formation of the pyrene cation. The strong binding of tetranitromethane to the silica gel surface also changes the photoinduced reaction products. Nitropyrene is formed along with nitroform indicating the reaction mechanism involves charge transfer with pyrene cation formation by a similar mechanism as occurs in acetonitrile.