Back electron transfer rates determine the photoreactivity of donor-acceptor stilbene complexes in a macrocyclic host.

Host-guest 2 : 1 complexation of photoreactive alkene guests improves the selectivity of [2 + 2] photodimerizations by templating alkene orientation prior to irradiation. Host-guest chemistry can also provide 1 : 1 : 1 complexes through the inclusion of electronically complementary donor and acceptor guests, but the photoreactivity of such complexes has not been investigated. We imagined that such complexes could enable selective cross-[2 + 2] photocycloadditions between donor and acceptor stilbenes. In pursuit of this strategy, we investigated a series of stilbenes and found 1 : 1 : 1 complexes with cucurbit[8]uril that exhibited charge-transfer (CT) absorption bands in the visible and near-IR regions. Irradiation of the CT band of an azastilbene, 4,4'-stilbenedicarboxylate, and cucurbit[8]uril ternary complex led to a selective cross-[2 + 2] photocycloaddition, while other substrate pairs exhibited no productive chemistry upon CT excitation. Using transient absorption spectroscopy, we were able to understand the variable photoreactivity of different stilbene donor-acceptor complexes. We found that back electron transfer following CT excitation of the photoreactive complex is positioned deep in the Marcus inverted region due to electrostatic stabilization of the ground state, allowing [2 + 2] to effectively compete with this relaxation pathway. Control reactions revealed that the cucurbit[8]uril host not only serves to template the reaction from the ground state, but also protects the long-lived radical ions formed by CT from side reactions. This protective role of the host suggests that donor-acceptor host-guest ternary complexes could be used to improve existing CT-initiated photochemistry or access new reactivity.

Internal Catalysis in Dynamic Hydrogels with Associative Thioester Cross-Links.

Thioesters are an essential functional group in biosynthetic pathways, which has motivated their development as reactive handles in probes and peptide assembly. Thioester exchange is typically accelerated by catalysts or elevated pH. Here, we report the use of bifunctional aromatic thioesters as dynamic covalent cross-links in hydrogels, demonstrating that at physiologic pH in aqueous conditions, transthioesterification facilitates stress relaxation on the time scale of hundreds of seconds. We show that intramolecular hydrogen bonding is responsible for accelerated exchange, evident in both molecular kinetics and macromolecular stress relaxation. Drawing from concepts in the vitrimer literature, this system exemplifies how dynamic cross-links that exchange through an associative mechanism enable tunable stress relaxation without altering stiffness.