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. In this work, we investigated a series of donor and acceptor 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 (TA) spectroscopy, we were able to understand the variable photoreactivity of different stilbene donor-acceptor complexes. We found that the back electron transfer (BET) 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.