TY - JOUR
T1 - On the Dynamic Origin of the Pyrene Excimer Emission on Silica Surfaces with Varying Pore Size Distribution
AU - Wellner, Edna
AU - Ottolenghi, Michael
AU - Avnir, David
AU - Huppert, Dan
PY - 1986
Y1 - 1986
N2 - Steady-state and time-resolved fluorescence measurements were performed with pyrene (Py) adsorbed on silicas with varying average pore sizes (60, 200, 500, 1000, and 2500 A), as well as on the nonporous Si02 surfaces of fume-silica (Aerosil) and quartz. The following phenomena were studied: (a) growing-in of the excimer fluorescence on a nanosecond time scale; (b) monomer and excimer fluorescence excitation spectra; (c) dependence of the relative excimer/monomer fluorescence intensity on time, after preparation of samples. It is found that, regardless of pore size distribution (psd), the excimer growing-in shows a fast (30-50%) subnanosecond component, common to all Si02 surfaces. A slower (50-70%) component is also detected, showing an increase in rise time from ≃1.2 ns for Si-60 to ≃4 ns for the macroporous and nonporous materials. Both fast and slow processes are due to a distinct nonequilibrium population of close-lying ground-state molecules. Different monomer and excimer excitation spectra (commonly interpreted in terms of ground-state aggregates leading to “static” excimerization) are apparent only in Si-60, -200, and -500. A substantial fraction of the so-called “static” excimers are formed on a nanosecond time scale. Due to equilibration of adsorption, the excimer emission decreases in intensity (to almost zero in the cases of Si-2500 and Aerosil) on an ~ 102-h time scale. The equilibration rates increase gradually with increase of average psd. All observations are interpreted in terms of an interplay between the basic Si-OH/Py and Py/Py interactions and the geometry, i.e., the fractal dimensions and psd of the investigated materials. Heterogeneity of adsorption sites is due mainly to the geometrical parameters.
AB - Steady-state and time-resolved fluorescence measurements were performed with pyrene (Py) adsorbed on silicas with varying average pore sizes (60, 200, 500, 1000, and 2500 A), as well as on the nonporous Si02 surfaces of fume-silica (Aerosil) and quartz. The following phenomena were studied: (a) growing-in of the excimer fluorescence on a nanosecond time scale; (b) monomer and excimer fluorescence excitation spectra; (c) dependence of the relative excimer/monomer fluorescence intensity on time, after preparation of samples. It is found that, regardless of pore size distribution (psd), the excimer growing-in shows a fast (30-50%) subnanosecond component, common to all Si02 surfaces. A slower (50-70%) component is also detected, showing an increase in rise time from ≃1.2 ns for Si-60 to ≃4 ns for the macroporous and nonporous materials. Both fast and slow processes are due to a distinct nonequilibrium population of close-lying ground-state molecules. Different monomer and excimer excitation spectra (commonly interpreted in terms of ground-state aggregates leading to “static” excimerization) are apparent only in Si-60, -200, and -500. A substantial fraction of the so-called “static” excimers are formed on a nanosecond time scale. Due to equilibration of adsorption, the excimer emission decreases in intensity (to almost zero in the cases of Si-2500 and Aerosil) on an ~ 102-h time scale. The equilibration rates increase gradually with increase of average psd. All observations are interpreted in terms of an interplay between the basic Si-OH/Py and Py/Py interactions and the geometry, i.e., the fractal dimensions and psd of the investigated materials. Heterogeneity of adsorption sites is due mainly to the geometrical parameters.
UR - http://www.scopus.com/inward/record.url?scp=0001557563&partnerID=8YFLogxK
U2 - 10.1021/la00071a016
DO - 10.1021/la00071a016
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AN - SCOPUS:0001557563
SN - 0743-7463
VL - 2
SP - 616
EP - 619
JO - Langmuir
JF - Langmuir
IS - 5
ER -