Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity

Abstract

Previously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double‐cell magnetic circular dichroism and magnetic ROA experiments with copper‐porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

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. 20 We also see the RET mechanisms to be responsible for the massive ROA induced chirality, but first, look at the complexity of RETs that can occur here.…”

“…Please do not adjust margins Please do not adjust margins The RROA and ECD sign concordance has not been observed until last minute where the systems have been interpreted without conformational and excited state analyses. 9 Wu et al have argued that the induced chirality in ROA comes from the interference of ECD and polarized Raman scattering, and have shown that the different ECD profiles within the ROA range, differently modified the induced chirality ROA bands. 9 As a result, the induced chirality ROA bands of solvent were monosigned and concordant with the sign of the involved ECD band.…”

“…9 Wu et al have argued that the induced chirality in ROA comes from the interference of ECD and polarized Raman scattering, and have shown that the different ECD profiles within the ROA range, differently modified the induced chirality ROA bands. 9 As a result, the induced chirality ROA bands of solvent were monosigned and concordant with the sign of the involved ECD band. This was the case for their chiral Cu complexes for which constant ECD intensity in the scattering range was observed.…”

“…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.…”

“…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. Oddly enough, the effect has been observed even without a physical contact between the solute and solvent.…”