Abstract
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.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 6201-6210 |
| Number of pages | 10 |
| Journal | Organic and Biomolecular Chemistry |
| Volume | 20 |
| Issue number | 31 |
| DOIs | |
| State | Published - Apr 6 2022 |
Funding
This research was primarily supported by an Army Research Office Young Investigator Program grant (#72846-CH-YIP to J. A. K.). C. T. E. acknowledges support from a National Science Foundation Graduate Research Fellowship (grant no. DGE-1842165). C. O. acknowledges support from an Academic Year Undergraduate Research Grant from Northwestern University. S. P., M. C. H., and E. A. W. acknowledge support from the Northwestern University Materials Research Science and Engineering Center (MRSEC) (NSF DMR-1720139). This work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN). This research was supported in part through the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology.
ASJC Scopus subject areas
- Biochemistry
- Physical and Theoretical Chemistry
- Organic Chemistry