MOVEMENT OF SMALL MOLECULES ON A SILICA SURFACE - A PHOTOPHYSICAL AND PHOTOCHEMICAL STUDY

Steady-state and time-resolved excitation measurements are utilized to investigate movement of small molecules on the heterogeneous surface of porous silica. Pyrene and its derivative 1-pyrenebutyric acid (PBA) are the principal adsorbates used for the creation of the excited and ionized states studied. Quenching of these states by ferrocene indicates that pyrene, like its analogue PBA, is immobilized on the surface for the duration of the quenching process (milliseconds) and movement is due to ferrocene. Additional investigations with perylene indicate that the molecules tend to be adsorbed in selected Positions on the SiO2 surface. These selected regions consist of silanol groups at high surface concentration. Heating preferentially removes these regions and decreases the extent of solute adsorption. The non-homogeneous adsorption behavior of non-heated SiO2 tends to lead to two basic modes of kinetics, initial static or rapid reaction, together with subsequent slower homogeneous kinetics. Both regimes are studied in the present work. The extent of the high density silanol is about 10(-6) mole g-1.