1,3-Di-2,2'-naphthylpropane as a fluorescent probe for silica surfaces

The bifunctional molecule 1,3-di-2,2'-naphthylpropane, N(3)N, has been investigated as a thermodynamic surface interaction probe of silica surfaces. Adsorption of N(3)N onto a silica surface from cyclohexane follows a Freundlich adsorption isotherm, demonstrating a range of weak interactions between the molecule and surface active sites. A moderate excimer emission (lambda(max) = 400 nm) observed at room temperature for low probe concentrations (below 5% of a monolayer) is attributed to an intramolecular process. Our results indicate that at low surface coverages more than 70% of the isolated bichromophoric molecules exist in an intramolecular excimer form. Thermodynamic data obtained from fluorescence studies reveal that excimer emission increases concurrently with a decrease in monomer emission as the temperature is raised. From an Arrhenius plot of the data an activation energy of about 2.2 kcal mol(-1) is calculated for the intramolecular excimer formation on silica surface. Gas phase molecular mechanics calculations predict an activation energy of 2.3 kcal mol(-1) for the interconversion of an anti-gauche conformer (non-excimer emitting) to a gauche-gauche conformer responsible for intramolecular excimer emission. Since the calculation does not take into account surface interactions, the anti-gauche conformation may not be the predominant non-excimer emitting species present on silica surface.