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.

Physical Aspects of Radiation-Induced Processes on SiO2, γ-Al2O3, Zeolites, and Clays

“…Quencher molecules that adsorb strongly to the silica gel surface exhibited limited quenching of excited states of co-adsorbed donors, and quenching tends to be predominantly static. Several quenchers including nitromethane, tetranitromethane, and methylviologen 79 behave in this way. The strong adsorption of TNM controls the quenching characteristics of several adsorbed donors, such as the singlet and triplet excited states of pyrene.…”

“…As carbon tetrachloride does not adsorb strongly on the porous silica gel surface, evacuation of the CCl 4 −SiO 2 sample readily removes CCl 4 from the gas phase with surface desorption occurring only after several minutes of evacuation. , Again, the desorption time depends on the pore size and surface silanol group concentration. Similar data have been also obtained for O 2 but only at low temperatures, as the desorption at room temperature 56 is too rapid.…”

Steady-State Fluorescence of Dye-Sensitized TiO2 Xerogels and Aerogels as a Probe for Local Chromophore Aggregation

“…The spectra exhibit vibronic bands in the 370−400 nm region that are characteristic of pyrene monomer emission 35 with somewhat higher emission intensity observed with the covalently bound chromophore. The relative intensities of the highest and lowest frequency bands, referred to as I 1 and I 5 , respectively, have been correlated to the local solvent polarities with a decreased I 1 /( I 1 + I 5 ) ratio indicating a reduction in local solvent polarity. −

1 Steady-state fluorescence for covalently bound 1 (solid line) and occluded (dashed line) pyrene for titanate xerogel samples aged 3 weeks as monoliths prepared from 10 -5 M sol solutions containing a 99:1 ratio of Ti(OCHMe 2 ) 4 /CH 3 Ti(OCHMe 2 ) 3 . Excitation wavelength = 343 nm under air.

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“…The spectroscopy of dye-modified gels can be used not only as probes to characterize the internal structure of these porous host matrices but also to monitor the influence of the local environment on excited-state properties of the included dye. This technique was first applied to sol−gels by Avnir et al who monitored the fluorescence emission of an included pyrene throughout sol formation, gelation, and condensation. , Two aspects of the excited-state dynamics of pyrene were found to be especially significant with respect to the physical characterization of a sol−gel-derived host matrix. ,− First, pyrene forms an excimer that emits at wavelengths significantly longer than those characteristic of an isolated pyrene monomer. Second, the emission signal attributed to pyrene monomer exhibits vibronic structure that is highly sensitive to environmental polarity.…”

“…Second, the emission signal attributed to pyrene monomer exhibits vibronic structure that is highly sensitive to environmental polarity. Because a metal oxide sol−gel contains a finite number of terminal bonds, hydroxy and alkoxy functional groups are present on the internal surfaces of the metal oxide matrix, ,, enhancing local polarity. Thus, the geometric constraints imposed by the nature of the sol, by the swelling of the gel matrix, and by the interactions of included pyrene probes with surface functional groups will impact the observed fluorescent response.…”