Transfer and Amplification of Chirality Within the “Ring of Fire” Observed in Resonance Raman Optical Activity Experiments

Abstract

We report extremely strong chirality transfer from achiral nickel complex to solvent molecules detected as Raman optical activity (ROA). Electronic energies of the complex were in resonance with the excitation-laser light. The phenomenon was observed for aw ide range of achiral and chiral solvents. Forchiral 2-butanol, the induced ROAwas even stronger than the natural one.T he observations were related to so-called quantum (molecular) plasmons that enable as trong chiral Rayleigh scattering of the resonating complex. According to am odel presented here,t he maximal induced ROAi ntensity occurs at ac ertain distance from the solute,i nathreedimensional "ring of fire", even after rotational averaging. Most experimental ROAsigns and relative intensities could be reproduced. The effect might significantly increase the potential of ROAspectroscopyinbioimaging and sensitive detection of chiral molecules.Most important molecules in living organisms are chiral and thus sensitive to circularly polarized light. This sensitivity was first explored by Pasteur, [1] and since then an amazing variety of chiroptical methods has been developed for fundamental studies and practical applications. [2] One of the youngest and most dynamically evolving tools is Raman optical activity (ROA). [3] It can provide rich stereochemical information because of the large number of vibrational bands usually exhibited by chiral molecules,a nd is directly applicable to solutions.However,the ROAeffect is weak, and alarge amount of the sample is usually needed for the analysis.T his has prompted vigorous searches for enhancement techniques, utilizing,f or example,m olecular resonance, [4] nanoplasmons (resonating metallic nanoparticles), [5] and molecular aggregation. [6] Interesting phenomena such as ROAi nduction in achiral reporter molecules in the presence of nanoplasmons or asolvent-signal enhancement under hyper-Raman scattering have been observed. [5c,7,8] Even though many properties of the nanoplasmons can be explained by established electro-magnetic and chemical theories, [9] the ROAenhancement and many chirality-transfer effects have so far resisted detailed interpretation.In this context, we believe that the strong chirality transfer under resonance conditions described in this work significantly contributes to the understanding of molecular interactions with chiral light. We report strong ROAofarange of solvents induced by achiral nickel complex under resonance conditions.C ontrary to common belief,t he chirality transfer appears quite general, and the induced ROAs ignals were even stronger than those of the Ni complex itself.F or 2butanol, ac hiral solvent, the induced ROAi ntensity was stronger than the natural one.Ther esonance-ROA phenomenon itself,a na rea of intense experimental and theoretical interest, can be rationalized only in simple cases. [4a, 10] None of them, however,i s applicable to the observations presented here.B ased on extensive experimental data and model calculations,weshow that the effect is not caused by spe...

On Raman optical activity sign-switching between the ground and excited states leading to an unusual resonance ROA induced chirality

“…Previously, the ROA induced chirality effect with huge solvent and small solute bands as in Figures 1 and 2, was explained by resonance energy transfer (RET) from solute to solvent in the first solvation sphere, 7 by the plasmon RET in the Ni complex solutions, 8 and by an interference between ECD and ROA effects. 9 In RET, the energy excess in the excited moiety (donor) is transferred to the acceptor one via virtual photon emission governed primarily by dipole-dipole couplings.…”

“…3,4 In VCD, the effect has been known since the early 2000s, 5,6 while in ROA, it was only rarely reported. [7][8][9] In 2016, Šebestík et al interpreted strong ROA chirality induced from chiral HQ helicene dye to achiral solvents 7 in terms of transfer of energy from HQ to proximate solvent molecules sharing the HQ electronic space. Quite recently Li et al interpreted strong ROA induced chirality, from a chiral Ni-complex to solvent molecules, 8 by SERS-like transfer of energy from plasmons inducing a 100-times ROA enhancement.…”

“…[7][8][9] In 2016, Šebestík et al interpreted strong ROA chirality induced from chiral HQ helicene dye to achiral solvents 7 in terms of transfer of energy from HQ to proximate solvent molecules sharing the HQ electronic space. Quite recently Li et al interpreted strong ROA induced chirality, from a chiral Ni-complex to solvent molecules, 8 by SERS-like transfer of energy from plasmons inducing a 100-times ROA enhancement. In the newly published Wu et al study, 9 the induced chirality has been interpreted as arising from an interference between ECD and ROA effects.…”

Recent Trends in Chiroptical Spectroscopy: Theory and Applications of Vibrational Circular Dichroism and Raman Optical Activity

“…Sometimes during a resonance ROA experiment ROA signal of the solvent can be observed, even when the solvent itself is not optically active. This “chirality transfer” from a chiral resonating molecule to the non‐chiral environment was described for the “helquad” organic dye as well as for some transition metal complexes . For a nickel complex, the transfer was attributed to a “ring of fire”, a space around the resonating complex and relatively far from it, where the solvent provides the biggest Raman and ROA signal.…”

“…For a nickel complex, the transfer was attributed to a “ring of fire”, a space around the resonating complex and relatively far from it, where the solvent provides the biggest Raman and ROA signal. For simple solvents, the signs of ROA bands could be even reproduced using a phenomenological transition polarizability model (Figure ) . However, the model cannot explain the large CID ratios observed in the experiments.…”