Raman optical activity (ROA) spectroscopy is hampered by low sensitivity, with limited possibilities for enhancing the signal. In the present study, we report a new mechanism whereby chirality is enhanced using the resonance resulting from supramolecular aggregation. We have named this mechanism aggregation-induced resonance Raman optical activity (AIRROA). As an example, we study J-aggregates of astaxanthin (AXT), which show strong absorption of circularly polarized light in the range of ROA excitation. The implications of aggregation-induced signal enhancement for chiroptical spectroscopy are discussed.

New methods for enhancing the Raman optical activity (ROA) signal are desirable due to the low efficiency of ROA, demanding otherwise high sample concentrations, high laser powers, and/or long acquisition times. Previously, we have demonstrated a new phenomenon, aggregation-induced resonance ROA (AIRROA), that produces significant enhancement of the ROA signal provided that the excitation wavelength coincides with the absorption of the measured species and that the electronic circular dichroism (ECD) signal in the range of this absorption is nonzero. In this work, analyzing three very different supramolecular astaxanthin aggregates (H1, H2, and J), we confirm the phenomenon and demonstrate that aggregation itself is not enough to enhance the ROA signal and that the above-mentioned conditions are necessary for induction of the resonance ROA effect. Additionally, by analyzing the changes in the ECD spectra of the H1 assembly, we demonstrate that the supramolecular helicity sign switches with time, which is dependent on the prevalence of kinetic or thermodynamic stabilization of the obtained aggregates.

Carotenoid microcrystals,e xtracted from cells of carrot roots and consisting of 95 %o fa chiral b-carotene, exhibit avery intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is an ewly observed phenomenon in nature,a nd may be related to asymmetric information transfer from the chiral seeds (small amount of a-carotene or lutein) present in carrot cells.T oc onfirm this hypothesis,w es ynthesized several model aggregates from various achiral and chiral carotenoids.Because of the sergeantand-soldier behavior,asmall number of chiral sergeants (a-carotene or astaxanthin) force the achiral soldier molecules (b-o r1 1,11'-[D 2 ]-b-carotene) to jointly form supramolecular assemblies of induced chirality.T he chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co-crystallization of chiral and achiral analogues.

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