To explore the influence of surface packing densities on the interfacial photochemical kinetics, the surface-selective second harmonic generation (SHG) technique was used to investigate the kinetics of two-photon induced [2 + 2] photocycloadditions of a styrylquinoline alkoxy derivative within the Langmuir–Blodgett (LB) monolayers. The laser power dependence experiment revealed that this interfacial photodimerization is a first-order reaction, which implies that the photoexcitation is the rate-limiting step. Interestingly, a comparison of photodimerization kinetics at different surface packing densities shows a nonmonotonic distribution of reaction rate constants, which can be attributed to a result of combined effects of the topochemical mechanism and steric hindrance. The atomic force microscopy measurements and theoretical calculations were also employed to help understand the [2 + 2] photocycloaddition mechanisms. The results presented in this work demonstrate that the surface packing density plays an important role in regulating the interfacial photoreactions within the LB monolayers composed of the conjugated aromatic molecular systems.

The real-time second harmonic generation was employed to investigate the [2+2] photodimerization of styrylquinoline (SQ) derivatives in Langmuir-Blodgett (LB) monolayers deposited from aqueous subphases with different acidity. It was discovered that the photodimerization rate constant significantly decreased upon the addition of acid. The additional atomic force microscopy measurements revealed that surface morphologies were correlated to the photodimerization kinetics. These experimental results provide direct evidence for the topo-chemical mechanisms of [2+2] photodimerization in LB films. The current study demonstrates that the intermolecular interactions and aggregation structures play important roles in the photochemical properties at surfaces.Download high-res image (87KB)Download full-size imageThe real-time second harmonic generation was employed to reveal the acidic effect on the photodimerization kinetics of styrylquinoline derivatives in Langmuir-Blodgett monolayers.

The salt effects on molecular orientation at air/liquid methanol interface were investigated by the polarization-dependent sum frequency generation vibrational spectroscopy (SFG-VS). We clarified that the average tilting angle of the methyl group to be θ = 30° ± 5° at the air/pure methanol surface assuming a δ-function orientational distribution. Upon the addition of 3 mol/L NaI, the methyl group tilts further away from the surface normal with a new θ = 41° ± 3°. This orientational change does not explain the enhancement of the SFG-VS intensities when adding NaI, implying the number density of the methanol molecules with a net polar ordering in the surface region also changed with the NaI concentrations. These spectroscopic findings shed new light on the salt effects on the surfaces structures of the polar organic solutions. It was also shown that the accurate determination of the bulk refractive indices and Raman depolarization ratios for different salt concentrations is crucial to quantitatively interpret the SFG-VS data.The average tilting angle of the methyl group at the air/liquid methanol interface was found to change by more than 10° when 3 mol/L NaI was added into the liquid bulk.

Sum frequency generation spectroscopy (SFG) has been widely used to study the interfacial chemistry of aqueous salt solutions of biological or environmental importance. Most of the SFG data analysis used the same bulk refractive index for different salt concentrations despite of the variations of the refractive indices. Here we systematically investigate the influence of the refractive index on the SFG intensities at various experimental conditions. It is discovered that the SFG intensities are the most sensitive to the refractive index at solid/liquid interfaces nearby the total internal reflection geometries. At air/liquid interfaces, the effect of the refractive indices is also nonegligible. Consequently some important SFG results, such as the response of water structures to the ionic strength at the SiO2/aqueous interfaces, are necessary to be reevaluated. These conclusions on the effect of the small variations of the refractive index are generally useful for the common practice of SFG data analysis.A small change of the bulk refractive indices with the elevating salt concentrations was found to significantly affect the sum frequency generation intensities.

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids (RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium ([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy (SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the (N)-CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up (N)-CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.