“Circularly Polarized Light can Override and Amplify Asymmetry in Supramolecular Helices” by Jun Su Kang, Sungwoo Kang, Jong-Min Suh, Park Soon Mo, Dong Ki Yoon, Mi Hee Lim, Woo Youn Kim, and Myungeun Seo was published in the Journal of the American Chemical Society. This work was done in collaboration with the Woo Youn Kim group for density functional theory calculations, the Mi Hee Lim group for the EPR study, and the Dong Ki Yoon group for AFM and UV-Vis-NIR analyses.

Homochirality on earth has evolved to favor one enantiomer out of the mirror-image pair exclusively. Such predominance of one enantiomer implies that there were (at least) two main events in the prebiotic stage: symmetry breaking in the racemic mixture and amplification of the asymmetry. Circularly polarized light (CPL) has been considered a possible deterministic source that could slightly enrich one isomer by an asymmetric photochemical reaction. Because the different isomers show different extinction coefficients in the absorption of l– and r-CPL expressed by the anisotropic factor g, a slight enantiomeric excess (e.e.) can be created. The chiral bias can be transferred to other molecules and amplified through helical self-assembly to result in supramolecular chirality. While it would be reasonable to assume that the handedness of the incident CPL would be random in this light-driven chirality induction and amplification process, the everlasting homochirality suggests that there would be a point of “no return” once the e.e. is accumulated over a critical point. While CPL and molecular chirality could simultaneously influence the emergence of supramolecular chirality before this point, it has been an open question which handedness the supramolecular polymer will follow.

Here, we addressed the question using a photoinduced supramolecular polymerization system, achiral and chiral triarylamine derivatives. We first revealed the mechanism by which CPL controls supramolecular helWe addressed the question using a photoinduced supramolecular polymerization system with achiral and chiral triarylamine derivatives. We first investigated the mechanism of supramolecular chirality induction by CPL in the achiral molecules. We showed that symmetry breaking occurs in the radical cation state produced by photoinduced oxidation, which triggers nucleation for supramolecular polymerization. The enantiophobic interaction during nucleation and elongation promotes the formation of supramolecular helices with a specific screw sense following the CPL handedness. We then presented four possible scenarios by co-assembling the achiral molecules (as a “soldier”) with a small number of the chiral derivative (as a “molecular sergeant”) in the exposure to CPL. Matching the handedness of CPL with the chirality of the molecular sergeant amplified supramolecular chirality cooperatively, but a mismatch attenuated asymmetry. CPL could even override the molecular chirality at the supramolecular level when low molecular sergeant loading. This suggests that in a CPL-dominant regime below the critical bias, it would be possible to decouple supramolecular chirality from molecular chirality even in the presence of the opposite sergeant (at least up to some extent). When the molecular sergeant exceeded a critical amount, the helicity determined by the molecular chirality persisted regardless of the CPL handedness.

The paper is available at the following link: https://doi.org/10.1021/jacs.1c11306.

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