We present heterodyne-detected total internal reflection vibrational sum frequency generation (HD-TIR VSFG) spectroscopy for CaF2/liquid interfaces. With this technique, absolute orientations at solid/liquid interfaces can be determined by measuring complex χ(2) spectra of the buried interfaces. We applied the technique to CaF2/sodium dodecyl sulfate (SDS) solution interfaces, and directly determined the polar orientations of the water molecules and surfactants at the interface based on the phase information on VSFG spectra, which is unavailable in conventional homodyne-detected TIR VSFG spectroscopy. The reorientation of the interfacial water molecules was observed as a change of the sign of Im[χ(2)] depending on the surfactant concentration.

Heterodyne-detected vibrationally electronically doubly resonant chiral sum-frequency generation (HD-DR chiral SFG) spectra were observed for the first time on monolayers. Langmuir monolayers of R- and S-binaphthyl amphiphiles on water exhibited complex chiral vibrational sum frequency generation (VSFG) spectra whose amplitudes were approximately the same, but whose phases were different by π. By comparing the electronic resonance profiles of the chiral VSFG signal amplitude and phase with simulations based on a nonadiabatic resonance Raman theory, we concluded that the chiral VSFG signals from the monolayers originated not from the antisymmetric Raman tensor but from the symmetric one. This is the first experimental evidence that the symmetric Raman tensor indeed contributes to the chiral VSFG signals from oriented monolayer systems.

Heterodyne-detected (phase-sensitive) vibrational sum frequency generation spectroscopy was used to investigate molecular structures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers on water (Langmuir monolayer) and monolayers on a fused silica substrate (Langmuir–Blodgett [LB] monolayer). The spectral features in the CH stretching region depended on the phase of the Langmuir monolayer, which was controlled by the molecular area on water. From the spectral changes, the molecular structure of the monolayer in each phase was deduced at the molecular level. We discovered that when we compared Langmuir and LB monolayers, both of which correspond to a similar surface pressure, the LB monolayer tended to have fewer gauche defects and have less tilted terminal methyls in the n-pentadecyl groups than the corresponding Langmuir monolayer. In addition, weak vibrational bands, which have been hardly seen by the conventional (homodyne-detected) VSFG spectroscopy, were clearly observed with their phases, or arguments, for the first time.

Heterodyne-detected vibrationally electronically doubly resonant chiral sum frequency generation (HD-DR chiral SFG) spectroscopy has been developed for the study of chiral molecules with chromophores. The method enables us to detect and distinguish chiral molecules with high sensitivity and to obtain information on molecular vibrations. Strong enhancement due to the electronic resonance improves the sensitivity, and heterodyne detection ensures that the signal intensity is linear to the sample concentration. Detection of HD-DR chiral SFG signal from a dilute solution of binaphthol with 20 mM concentration and tens of nanometers thickness was demonstrated. Taking advantage of the enantiomer-dependent sign and linearity of the signal to the concentration, molecular concentrations and enantiomeric excesses were accurately evaluated. HD-DR chiral SFG is expected to have widespread application in the study of molecular chirality of thin films or samples of a very small quantity.

CN stretch bands of singlet and triplet cyclopentane-1,3-diyl diradicals (1,3-di(4-cyanophenyl)-2,2-dimethoxyoctahydropentalene-1,3-diyl and 1,3-di(4-cyanophenyl)-2,2-dimethyloctahydropentalene-1,3-diyl) were observed by time-resolved IR spectroscopy. CN stretching wavenumbers of the singlet and triplet diradicals were downshifted by 10 and 19 cm–1 from those of their corresponding ring-closed compounds, which are in closed-shell electronic states, respectively. The observed downshifts are attributed to the bond-order decrease in the CN bonds due to the contribution from a resonance structure that has cumulative double bonds (C═C═N•) at the para-positions of the radical carbons. This resonance structure is only possible when the molecules have unpaired electrons; thus, the wavenumber downshift can be regarded as an experimental manifestation of the radical character of the diradicals. The observed CN stretching wavenumbers indicate that the radical character of the singlet diradical is less significant than that of the triplet. The smaller radical character is ascribed to the contribution from the zwitterionic and π-single bonding resonance structures in the singlet diradical. Unrestricted DFT calculations at the B3LYP level of theory with the 6-31G(d) basis set reproduced the small/large relationship between the wavenumber downshifts of the singlet and triplet diradicals; however, the shift of the singlet diradical was overestimated.

We first demonstrated chiral vibrational sum frequency generation (VSFG) in the heterodyne detection, which enables us to uniquely determine chiral second-order nonlinear susceptibility consisting of phase and amplitude and distinguish molecular chirality with high sensitivity. Liquid limonene was measured to evaluate the heterodyne-detected chiral VSFG developed in this study. R-(+)- and S-(−)-limonene showed clearly opposite signs in the complex spectra of the second-order nonlinear susceptibility in the CH stretching region. This is the first report of the chiral distinction by VSFG without any a priori knowledge about chiral and achiral spectral response. Furthermore, from the phase of the chiral VSFG field measured in the heterodyne detection, the origin of the chiral signal was ascribed to the bulk limonene. The heterodyne detection also improves detection limits significantly, allowing us to observe weak chiral signals in reflection. The heterodyne-detected chiral VSFG can provide information on absolute molecular configuration.Keywords: chiral distinction; nonlinear spectroscopy; SFG spectroscopy; vibrational spectroscopy;

Heterodyne-detected (phase-sensitive) vibrational sum frequency generation spectroscopy was used to investigate molecular structures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers on water (Langmuir monolayer) and monolayers on a fused silica substrate (Langmuir–Blodgett [LB] monolayer). The spectral features in the CH stretching region depended on the phase of the Langmuir monolayer, which was controlled by the molecular area on water. From the spectral changes, the molecular structure of the monolayer in each phase was deduced at the molecular level. We discovered that when we compared Langmuir and LB monolayers, both of which correspond to a similar surface pressure, the LB monolayer tended to have fewer gauche defects and have less tilted terminal methyls in the n-pentadecyl groups than the corresponding Langmuir monolayer. In addition, weak vibrational bands, which have been hardly seen by the conventional (homodyne-detected) VSFG spectroscopy, were clearly observed with their phases, or arguments, for the first time.