Spin-selective Energy Transfer from Quantum Dots to Molecules: An Application in Stereoselective Organic Synthesis

Seminar | January 28 | 4-5 p.m. | 120 Latimer Hall

 Emily Weiss, Department of Chemistry, Northwestern University

 College of Chemistry

Tetrasubstituted cyclobutyl structures are precursors to, or core components of, many important bioactive molecules, including prospective drugs. Light-driven [2+2] cycloaddition is the most direct strategy for construction of these structures. [2+2] photocycloadditions that proceed through the triplet excited state can be triggered with visible light through excitation of a triplet sensitizer followed by triplet-triplet energy transfer (TT EnT). Achieving selectivities for a particular regioisomer or diastereomer of the cyclobutane product and for homo- vs. hetero-coupling within a mixture of reactive olefins still remains a challenge. Here, we discuss the use of colloidal CdSe quantum dots (QDs) as visible light absorbers, triplet exciton donors, and scaffolds to drive homo- (photodimerization) and hetero- (cross coupling) intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, with perfect and switchable regioselectivity and 97-98% diastereoselectivity for the previously minor syn-head-to-head (HH) or syn-head-to-tail (HT) configurations of the adducts. The diasteromeric ratios (d.r.) we achieve are a factor of 5 - 10 higher than those reported with all other triplet sensitizers. Furthermore, the size-tunable triplet energy of the QD enables regioselective hetero-intermolecular couplings through selective sensitization of only one of the reagent olefins.