Molecular near-field antenna effect in resonance hyper-Raman scattering: Intermolecular vibronic intensity borrowing of solvent from solute through dipole-dipole and dipole-quadrupole interactions

Abstract

We quantitatively interpret the recently discovered intriguing phenomenon related to resonance Hyper-Raman (HR) scattering. In resonance HR spectra of all-trans-β-carotene (β-carotene) in solution, vibrations of proximate solvent molecules are observed concomitantly with the solute β-carotene HR bands. It has been shown that these solvent bands are subject to marked intensity enhancements by more than 5 orders of magnitude under the presence of β-carotene. We have called this phenomenon the molecular-near field effect. Resonance HR spectra of β-carotene in benzene, deuterated benzene, cyclohexane, and deuterated cyclohexane have been measured precisely for a quantitative analysis of this effect. The assignments of the observed peaks are made by referring to the infrared, Raman, and HR spectra of neat solvents. It has been revealed that infrared active and some Raman active vibrations are active in the HR molecular near-field effect. The observed spectra in the form of difference spectra (between benzene/deuterated benzene and cyclohexane/deuterated cyclohexane) are quantitatively analyzed on the basis of the extended vibronic theory of resonance HR scattering. The theory incorporates the coupling of excited electronic states of β-carotene with the vibrations of a proximate solvent molecule through solute-solvent dipole-dipole and dipole-quadrupole interactions. It is shown that the infrared active modes arise from the dipole-dipole interaction, whereas Raman active modes from the dipole-quadrupole interaction. It is also shown that vibrations that give strongly polarized Raman bands are weak in the HR molecular near-field effect. The observed solvent HR spectra are simulated with the help of quantum chemical calculations for various orientations and distances of a solvent molecule with respect to the solute. The observed spectra are best simulated with random orientations of the solvent molecule at an intermolecular distance of 10 Å.

β‐Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy

“…If the overcompensation were to be avoided, a scaling factor s ≈0.90 must be applied to the solvent contribution before subtraction when t >0, whereas s =1.00 when t <0. The imbalance is reproduced consistently and is therefore significant; it implies that the β‐carotene solute and nearby n‐ hexane solvent molecules are electronically coupled, similar to what has been observed using hyper‐Raman scattering by Hamaguchi and coworkers …”

“…The imbalance is reproduced consistently and is therefore significant;i ti mplies that the b-carotene solute and nearby nhexane solventm olecules are electronically coupled, similart o what has been observed using hyper-Raman scattering by Hamaguchiand coworkers. [61,62] The question is which opticalp rocess in b-carotenei sr esponsible, upon actinic excitation,f or the observed change of solventR amana ctivityn earby?I st he solvente nvironment "normal" (i.e. uncoupled) for t < 0a nd then becomes coupled by excitation of the solute (case a)?O ra lternatively, is the solvent's Ramans pectrum enhanced when b-caroteneo ffers its full S 2 !…”

Local electric field factors by a combined charge-transfer and point–dipole interaction model

“…For example, solvent vibrational modes have been observed in the resonance Raman and resonance hyper-Raman spectra of small molecules. 87,88 In addition, there is much current interest in the plasmonic states of aromatic molecules, 89,90 where one expects to see enhanced elds and hot spots.…”

Local Field Factors and Dielectric Properties of Liquid Benzene

“…In surface-enhanced Raman scattering (SERS), − local field enhancement is observed in plasmonic nanoparticles showing very strong scattering of light due to the localized surface plasmon resonances. − In addition to metal nanoparticles, the plasmonic character of some aromatic molecules such as linear acenes also enhances the local field factor . In resonance Raman and resonance hyper-Raman spectra, , the intermolecular vibronic coupling between solute molecules and neighboring solvent molecules causes enhanced local fields, a phenomenon called the molecular near-field effect …”