Photochirogenic Cyclodimerization of 2-Anthracenecarboxylic Acid Mediated by Plasmonic Nanohelices

Chirality is a natural asymmetric phenomenon, and many of life’s building blocks (e.g., amino acids, DNA, RNA, proteins and sugars) exhibit homochirality, with one stereoisomeric configuration preferentially existing over its mirror image, leading to chirality-dependent interactions and functions in biological systems. The existence of biochemical homochirality raises a fundamental but unsolved question about the origin of molecular enantiopreference on Earth. Several intriguing chiral phenomena, such as spontaneous symmetry breaking, chirality amplification and asymmetric autocatalysis, are proposed to be relevant to biochemical homochirality.

To disclose the mysterious origin of biochemical homochirality, it is of vital interest to study how molecular chirality can be manipulated via absolute asymmetric syntheses. Chiral force is essentially required to trigger absolute asymmetric syntheses, and the unidirectional rotation and/or revolution of the Earth has been proposed to function as a chiral force. However, it is unclear how movement at such a macroscopic scale could substantially manipulate molecular chirality, considering the dramatic dimensional difference. It is evidently challenging to macroscopically manipulate molecular chirality, which intrinsically demands a chirality transfer from macroscopic rotation to molecules. To the PI’s best knowledge, experimental demonstration of such chirality transfer is lacking.

The proposed project will aim to verify the speculation that macroscopic rotation mediated by plasmonic nanohelices (NHs), which are generated by a substrate rotation- induced glancing angle deposition (GLAD), can serve as a chiral force to stimulate absolute asymmetric synthesis and to disclose the mechanism of chirality transfer. Clockwise and counterclockwise rotation of a substrate during GLAD of noble metals leads to a sculpture of the deposited plasmonic NHs with right- and left-handedness, respectively. Using the photocyclodimerization of 2-anthracenecarboxylic acid (AC) as a photochirogenic model operated on the plasmonic NHs, it is planned to verify that photochirogenic enantiopreference can be controlled by substrate rotation direction via the sculptured helical handedness, and that the proposed chiral force of macroscopic rotation is contributed from two chiral factors: helical surface-induced chiral arrangement of AC precursors and circularly polarized light chiroptically enhanced by chiral plasmonics excited from the plasmonic NHs. Based on the verified NH-mediated photochirogenic mechanism, the enantioselectivity of photochirogenic AC cyclodimerization will be maximized.

Success in this project may shed light on the origin of fundamental molecular enantiopreferences on Earth, to help in understanding the diverse homochirality-determined biological functions and phenomena. It may fundamentally pave the way to tackling various important chirality-related problems associated with health, food and the environment. Furthermore, this project will devise a new chiral force to trigger asymmetric synthesis/catalysis, which plays a vital role in producing single-enantiomer drugs and pesticides, natural products and vaccines without fatal side effects.