Chiral

Quantum effect could explain how chiral molecules interact: Electron spin polarization promotes recognition between molecules of similar chirality.

Biomolecules from small amino acids to large DNA helices are chiral, and how they interact depends on their chirality. A newly identified quantum effect could help explain how biomolecules’ chirality persists. When two molecules interact, their electron clouds reorganize. In chiral molecules, that reorganization is accompanied by electron spin polarization that enables molecules of the same chirality to interact more strongly than molecules of opposite chirality, reports a research team led by Ron Naaman and Jan M. L. Martin of the Weizmann Institute of Science and David H. Waldeck of the University of Pittsburgh (Proc. Natl. Acad. Sci. USA 2017).

“The mechanism that they have demonstrated is different from any that was previously reported,” comments David N. Beratan of Duke University. “If the idea holds up, it could entirely change the way we think about molecular recognition in biological and organic chemistry.”

In an article in the Proceedings of the National Academy of Science (doi: 10.1073/pnas.1611467114), David Waldeck and colleagues in Israel propose a mechanism for the enantioselectivity (chiral specificity) of non-covalent interactions between chiral molecules. Their study examines how the non-covalent interactions between molecules give rise to enantioselective interaction energies. Non-covalent interactions do not involve the formation of a bond; rather they include electrostatic interactions between permanent or induced dipoles as the electron clouds of the molecules rearrange and the purely quantum exchange interaction as the wavefunctions of the molecules overlap. Their two part study shows experimentally that charge redistribution in chiral molecules is accompanied by spin polarization and it shows theoretically that the exchange interactions for homochiral (both molecules have the same handedness) interactions differ from heterochiral ones.

The latest study in David Waldeck's group, published in ACS Nano Letters, demonstrates that chiral imprinted CdSe quantum dots (QDs) can act as spin selective filters for charge transport.

Semiconductor quantum dots remain an attractive material for photovoltaics because of their solution processability and potential for multiple exciton generation; enabling a promising route for the realization of low cost, high efficiency solar cells. In addition, previous experiments have shown that spin selective charge transport can enhance the photoconversion efficiencies of organic bulk heterojunctions. The present work therefore explores whether chiral induced spin selectivity (CISS) can be used as an alternative approach to affect charge transport through quantum dot films and demonstrates that quantum dot thin films composed of chiral semiconductors preferentially transmit electrons with a particular spin orientation.