Host-guest-templated [2+2] photodimerizations are often used to improve the selectivity of [2+2] reactions and to provide photoresponsive supramolecular crosslinks in polymeric systems. Host-guest chemistry, however, has not been used to achieve synthetically valuable cross-[2+2] photocycloadditions. 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 absorption bands in the visible and near IR regions. Irradiation of the charge transfer band of a methylated azastilbene, 4,4’-stilbenedicarboxylate, and cucrbit[8]uril ternary complex led to selective cross-[2+2] photocycloaddition between the stilbenes. Using density functional theory (DFT) and transient absorption (TA) spectroscopy, we were able to understand the variable photoreactivity of the various 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 thanks to electrostatic stabilization of the ground state, allowing [2+2] to effectively compete with this relaxation pathway. Such electrostatic interactions will be critical to consider for any future developments or applications of host-guest-templated cross-[2+2] photoclyoadditions.

Selective cross-[2+2] photocyloaddition of stilbenes in a cucurbit[8]uril host

ACS Spring 2022

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