Local Surface Environments and Their Effects on Molecular Encounter Rates at Silica/Solution Interfaces Studied by Quenching of Phosphorescence from a Silica-Immobilized Triplet-State Probe

• Published in: Langmuir Vol. 18; no. 11; pp. 4307 - 4313
• Main Authors: Shield, Stephanie R, Harris, Joel M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.05.2002
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la011144+

The triplet-state decay kinetics of erythrosin-ITC, chemically bound to aminated porous silica, provides information about the local environment of the liquid/solid interface and its influence on interfacial reaction kinetics. A dispersion in phosphorescence decay rates of the immobilized erythrosin was observed. From the pH response of the decay kinetics, two distinct populations appear to arise from domains of protonated amine and silanol sites on the silica surface. The decay rates of the excited-triplet populations are sufficiently slow to probe quenching encounters with solution-phase azulene, which can diffuse over relatively long distances during the lifetime of the excited-state. By variation of the solvent conditions to influence adsorption of azulene, it was found that adsorbed azulene does not participate in interfacial quenching. The quenching process, therefore, involves contact between solution-phase azulene and the immobilized probe. The rates of this process for the short-lived (solution-associated) erythrosin population were slower than free-solution rates by a factor of 4.3, which is expected from surface-immobilization of the probe. The results indicate that transport of azulene to this population is efficient and that the pore network through which the azulene diffuses is well connected over a distance scale of ∼0.5 μm. The quenching kinetics for the longer-lived (surface-associated) erythrosin were somewhat slower, possibly due to steric hindrance or local exclusion of azulene, or less efficient molecular transport over longer distances within the porous silica.