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.

Raman and ROA analyses of twisted anthracenes: connecting vibrational and electronic/photonic structures

“…It has been demonstrated that these results are compatible with strong resonances with more than one electronic state. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Indeed, the ECD spectrum in this region exhibits at least two negative bands, weaker than the first intense positive feature.…”

“…The description of this phenomenon in ROA is embraced by the General Unrestricted (GU) theory, 11 with which any ROA experiment can be explained. [12][13][14][15] The simplest case is the limit of strong resonance with a singleelectronic-state (SES), where one electronic state modulates the molecular polarizability. [16][17] According to the SES-RROA theory, in the strong resonance limit only one ROA invariant (linear combinations of tensors products which are independent of the molecular axis) remains for all the polarization form.…”

Chiral detection by induced surface-enhanced Raman optical activity

“…Apart from the SERS/SEROA effect, interference of circular dichroism and ROA has been lately discovered as another mechanism of chirality induction in ROA experiments. 55,56 This mechanism can be nevertheless probably ruled out as the circular dichroism is immeasurably weak (Fig. S5, ESI †).…”

Recognition of the True and False Resonance Raman Optical Activity

“…A chirality induction/transfer can be observed either when achiral molecules are in a close proximity to the chiral ones [19] or due to a plasmonic resonance. [8] A strong induced optical activity under resonance conditions was reported to occur from chiral metal complexes [26] or dyes [22,27,28] to numerous (a)chiral organic solvents. The phenomenon was initially explained by a phenomenological model named "ring of fire", [26] partially predicting the spectra, in particular signs of the ROA bands.…”

“…More recently, an interference between the ROA and ECD better explained the effect, including prediction of band intensities. [27] The importance of ECD became evident in double-cell experiments, where there was no chemical contact between the solute and the solvent.…”