R. Dabestani et al., 1,3-DI-2,2'-NAPHTHYLPROPANE AS A FLUORESCENT-PROBE FOR SILICA SURFACES, Journal of photochemistry and photobiology. A, Chemistry, 92(3), 1995, pp. 201-206
The bifunctional molecule 1,3-di-2,2'-naphthylpropane, N(3)N, has been
investigated as a thermodynamic surface interaction probe of silica s
urfaces. Adsorption of N(3)N onto a silica surface from cyclohexane fo
llows 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 t
o 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 f
luorescence studies reveal that excimer emission increases concurrentl
y with a decrease in monomer emission as the temperature is raised. Fr
om an Arrhenius plot of the data an activation energy of about 2.2 kca
l mol(-1) is calculated for the intramolecular excimer formation on si
lica surface. Gas phase molecular mechanics calculations predict an ac
tivation 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 conf
ormation may not be the predominant non-excimer emitting species prese
nt on silica surface.