Fluorescent probes for silica and reversed-phase silica surfaces: 1,3-Di-1-pyrenylpropane and pyrene

The fluorescence of the bifunctional probe molecule 1,3-di-1-pyrenylpropane (Py(3)Py), adsorbed on silica (in the presence and absence of coadsorbed 1-octanol) and on octadecylsilica, is investigated by utilizing photostationary and single-photon-counting methods. It is concluded that dynamic (intramolecular) excimer formation, taking place after excitation, occurs at low probe concentrations in the reversed octadecyl phase. A study of temperature effects yields a value of ∼40 kJ/mol for the effective activation energy of this process, indicating a relatively high viscosity for the reversed phase. Dynamic intramolecular excimers are also observed on silica surfaces in the presence of 1-octanol as coadsorbate. No excited-state rearrangements are observed for Py(3)Py on untreated (or partially C₁₈ silylated) silica, where the only path to excimers originates in ground-state intermolecular aggregates. Comparative experiments are also carried out with pyrene (Py) in the same systems. Py(3)Py appears to be a most convenient indicator for the degree of mobility freedom of adsorbed molecules.