Photochemical probes of conformational mobility and reactivity on monolayer-protected nanoparticles

The unimolecular and bimolecular photochemical reactions of a series of aryl ketones have been used to probe the monolayer-protected nanoparticle (MPN) environment. The incorporation of aryl ketones into MPNs significantly influences the fragmentation/cyclization product distribution for the Norrish-Yang type II photoreaction as illustrated by the exclusive generation of the fragmentation product, whereas the analogous reaction in solution results in a 5:1 ratio of the fragmentation: cyclization products. In addition, the conversions for both the unimolecular Norrish-Yang type II reaction and the bimolecular Diels-Alder reaction of a photodienol with a solution phase dienophile are dependent on the binding site of the aryl ketone on the MPN surface, where the core is comprised of edge, vertex and terrace sites.; In general, aryl ketones on MPNs with core sizes greater than 2 nm react incompletely. The inability for the aryl ketone probes to react quantitatively was shown to be the result of differential reactivity at the various sites of the MPN core. Selective population of the specific sites with probe aryl ketones that undergo unimolecular or bimolecular photochemical reactions show that aryl ketones anchored predominantly at the edge and vertex sites are significantly more reactive than those predominantly occupying terrace sites. That is, there is site selective reactivity.; To provide further evidence for the differential reactivity at the various sites on the MPN surface, aryl ketones capable of the Norrish-Yang type II photoreaction were anchored to MPNs with smaller cores (1.7 +/- 0.4 nm), medium (2.2 +/- 0.4 nm) and larger cores (4.5 +/- 0.7 nm). The extent of photoreaction decreases as the MPN core size increases. This size effect on reactivity was attributed to the decreasing ratio of the more reactive edge and vertex sites present on the MPNs as they increase in size.; In a separate investigation acyl and alkyl radicals generated in the presence of the MPN were shown to efficiently liberate substrates from both MPNs and two-dimensional self-assembled monolayers (SAMs). This radical abstraction was shown to facilitate an increase in core size when carried out on MPNs. As such, it proposed that this reaction could serve in applications where substrates can be controllably-released from metal surfaces and also provide a mild new synthetic route to larger MPN cores.; Overall, this work provides a better fundamental understanding of the factors that control the conformational mobility and reactivity of MPNs of different sizes.