The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air−water interface have been measured. These spectra were affected significantly by compression, and the observed changes were ascribed to the formation and structural rearrangement of aggregated species on the water surface during compression. The spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded film region are oriented with their C 2-axis tilted away from the surface normal on angle θ distributed in the range of 31−39°. The rotational distribution around the C 2-axis was assumed to be 45−60° according to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and the structural rearrangement within the monolayer during compression was described.

Ultrafast change in refractive index after excitation by a femtosecond laser pulse was observed for dilute Auramine O solutions in low-viscosity solvents by detecting the change as an ultrafast lensing effect (ULE). The decay of the ULE signals was found to consist of two components, and both their relaxation times depended on solvents. This is the first reported observation of the faster component by this technique. The slower component had a similar relaxation time to that of the fluorescence lifetime. Molecular orbital calculation results attributed the faster to relaxation in the lowest excited singlet state S 1 by way of internal rotation and the slower to the subsequent relaxation from S 1 to the ground state. Solvent dependence of the experimentally determined relaxation time was strongly correlated with the molecular weight of the solvents rather than their bulk viscosity. A simplified model explaining these results was proposed in which solute and solvent molecules interact only via hydrogen bonding, the strength of which is solvent independent.

A new method has been proposed to determine the equilibrium between the bulk and the surface by directly measuring surface concentrations using laser two-photon ionization. This method has been applied to pyrenebutyric acid. The surface concentrations depended on the pH of the solution and were analyzed on the basis of two equilibrium constants and two distribution coefficients. Most pyrenebutyric acid stays on the surface at pH = 2.2. The equilibrium constant, pK a, of pyrenebutyric acid on the water surface was determined to be 7.85 ± 0.13, and this value is shifted to higher value than that in the bulk (4.76). The distribution coefficient of the neutral pyrenebutyric acid was determined as (5.9 ± 2.8) × 10-2 m, and that of pyrenebutyric anion as (4.82 ± 0.1) × 10-5 m. The ratio of the distribution coefficient of the neutral pyrenebutyric acid to that of pyrenebutyric anion was determined to be (1.2 ± 0.6) × 103. These findings indicate that the equilibrium shifts toward the neutral form on the water surface. Laser two-photon ionization was found to be a sensitive and powerful technique to analyze equilibrium on the surface and that between the surface and the bulk.

Orientational arrangement of long-chain Fluorescein O322 molecules within the monolayer at the air/water interface induced by the compression process was studied by surface second harmonic generation. Compression does not change the average tilt angle of the chromophore in any significant way, but it induces the alignment of the chromophore along a preferred direction perpendicular to the compression direction, originating the in-plane optical anisotropy. A model has been proposed on the orientational ordering of Fluorescein O322 molecules within the monolayer induced by the compression process.

Pressure-induced structure ordering of long-chain Fluorescein O322 molecules within the monolayer at the air/water interface was studied by rotating a sample trough using the second-harmonic generation technique. Azimuthal angle dependences of the polarized second-harmonic intensity (I pp , I ps , I ss , and I sp ) were measured for different degrees of monolayer compression. I pp and I ss showed a 2-fold pattern, whereas I ps and I sp showed a 4-fold pattern. A set of theoretical equations was derived to interpret the observed pattern. The observed patterns agreed with the equations based on C 2 v symmetry, and the monolayer showed a definite crystalline structure in the 0.21 nm2/molecule region. These results showed that the transition of an initially disordered and heterogeneous dye monolayer into a fairly regular crystalline-like structure was characterized by continuous changes in the structure ordering of constituent molecules maintaining C 2 v symmetry.

Using a confocal fluorescence microscope, we found a significant temporal fluctuation in the fluorescence spectrum of porphine molecules on the water surface. The spectra fluctuated when the accumulation time was 300 ms and the pH was within a certain range. No fluctuation was observed with a longer accumulation time (36 s) or on a solid surface. The present observation indicates that the distribution of porphine on the water surface is heterogeneous and that the Brownian motion of the surface species induces spectral fluctuation. A neutral form of porphine produces domains at a higher surface coverage.

Mixed insoluble monolayers of bis(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic acid (ArAc) at the air−water interface were characterized by surface pressure−area isotherm, second harmonic generation (SHG), and reflection spectroscopy studies. The analysis of surface pressure−area isotherms of mixed RhC18/ArAc layers provided evidence that there was a high degree of miscibility of film components and a strong interaction among them. A new band was found to appear on the blue side of the reflection spectrum after an increase in both the concentration of ArAc in the layer and the applied surface pressure. This new band was assigned to dye aggregates, where spontaneous formation in the gaslike region was promoted by the presence of ArAc and induced by compression in the region of the continuous monolayer. Both in gaslike and in compressed film, the formation of blue-shifted aggregates coincided with the increase of the SHG response of the monolayer. Resonance SHG enhancement was proven to be the main reason for this observation due to electronic resonance of aggregates with the incident light frequency.

Orientation of pyrenehexadecanoic acid (PyHA) on the water surface has been investigated using surface compression, fluorescence spectra, and two-photon ionization measurements. PyHA stays as a monomer at surface densities lower than 0.025 nmol/cm2, while at high surface densities excimer emission was observed. The two-photon ionization signal had a peak at 90° (p-polarized laser beam) upon the rotation of the polarization of the incident laser beam. The difference in the signal intensity generated by p-polarized and s-polarized lights became smaller as surface density increased. It is suggested that the tilt angle of the transition moment of the pyrene ring is ca. 70° at a low surface density, and it increases with the surface density increase. Problems in determining orientation of the molecules on water are discussed.