(532dm) Single Molecule Studies of the Aldol Condensation of Nile Red Aldehyde with Silica-Bound Acetophenone

The dream of heterogeneous catalysis research is to directly link catalytic activity to surface properties. Towards this goal, the ability to see the catalytic reaction as it occurs is a key contribution. Single molecule (SM) spectroscopy has emerged as a method to measure nanoscale local activity under realistic reaction conditions. Spatial resolutions of less than 20 nm can be achieved using SM methods. However, a limitation of SM methods is the lack of suitable fluorescent probes that can be used with important catalytic reactions. In this work, we present, for the first time, the use of a fluorescent Nile Red derivate incorporating a reactive aldehyde - 9-diethylamino-5H-benzo[α]phenoxazin-5-one-2-carboxaldehyde (NR-CHO) to study aldol condensation. The dye reacts with silica-bound acetophenone over amine-functionalized silica catalysts, producing changes in fluorescence emission color and intensity when it forms its aldol products. These changes provide information on the distributions of local activity on films.

To study the effect of the relative distributions of amine and weakly acidic silanol sites on the chemical reactivity, we carried out the aldol reactions on films with a gradient in amine concentration. The results shown in Figure 1 indicate that the fluorescence activity (or aldol reactivity) is highest in the middle section of the films followed by the high-amine end. The amine gradient results are similar to previous works which showed that aldol reactions are best catalyzed by bifunctional catalysts with weak acid and medium strength basic sites.

A quasi-single molecule analysis, where the number of ‘fluorescence events’ (defined as successive frames with significant color change) was calculated for randomly selected areas on the films, showed that the fluorescence reactivity (9×10^-14 mol/m² s) is almost three times higher for the aldol reaction at the middle section of the amine gradients than at the high and low-amine ends.