In-situ second-harmonic generation and luminescence measurements for structural characterization of ruthenium-polypyridine complex monolayers with two and four aliphatic tails at the air/water interface

Langmuir monolayer assemblies of ruthenium-polypyridine complexes having two and four aliphatic tails were studied by in-situ optical second-harmonic generation (SHG) and luminescence measurements, together with surface pressure-molecular area (lambda-A) isotherms. As the monolayer was compressed, the four-tailed complex showed a distinct phase transition at a 30-40 mN/m region from phase I (1.3 nm(2) mean molecular area) to phase II (1.1 nm(2)). Excitation with 1064-nm light gave an SHG signal at 532 nm that appeared almost synchronously with the increase of surface pressure, but the p-polarized component of the SHG signal, induced by either p- (parallel to the plane of incidence) or s- (perpendicular to the plane of incidence) polarized fundamental light, showed a different profile upon compression of the monolayer. In the phase II region, the p-polarized component increased steeply, whereas the s-polarized component decreased to almost zero. A significant blue-shift and a longer lifetime of the luminescence signal were also observed in phase II. Thus, a tightly packed structure in which the main hyperpolarizability tensor is directed roughly perpendicular to the water surface is suggested for phase II. For the two-tailed complex, on the other hand, no appreciable phase transitions were observed, only a liquid-expanded phase (0.9 nm(2)). The results are ascribed to differences in the number of long aliphatic tails.