Analysis of associated structures of rhodamine B adsorbed at interfaces by second harmonic generation spectroscopy

We have developed an apparatus for the measurement of second harmonic generation (SHG) spectra that provide direct evidence on the electronic structures of interfacial species. By applying the measurement apparatus, we have studied molecular association of rhodamine B (RB) at a heptane/water interface and trimethylchlorosilane (TMS) modified-quartz/water interfaces with a different coverage. The observed SHG spectra at the heptane/water interface showed red-shift of the peak maxima compared to the absorption maximum of monomeric RB in the bulk aqueous phase, and these spectra indicated predominant formation of in-plane associates in which the electronic transition dipoles of each monomer were arranged parallel to one another. At the TMS-modified-quartz/ water interface, spectral shape of SHG spectra was almost same with that found in the heptane/water interface when the surface density of TMS was high. In contrast, with decreasing the surface density of TMS, the red-shifted peak due to the in-plane associates become weaker, and other peaks due to a sandwich dimmer appeared. We confirmed that the association form of RB depended on the surface density of TMS.