Second-Harmonic Probe of Pressure-Induced Structure Ordering within Long-Chain Fluorescein Monolayer at the Air/Water Interface

Abstract

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.

Microscopic Organization of Long-Chain Rhodamine Molecules in Monolayers at the Air/Water Interface

“…Given that the four-fold symmetry of I ps and I sp is evident, the symmetry of the orientational distribution of RhC 18 molecules in the plateau region of the isotherm can be ascribed to C 2V . 12 These data on the SHG rotation anisotropy measurements revealed an interesting feature. The solid lines in Figures 5 and 6 are the simulated curves based on the assumption that the monolayer is composed of well-ordered single microcrystallites of RhC 18 with a C 2V symmetry.…”

“…12,16 A Teflon-coated trough of 100 cm 2 was mounted on a rotational stage. The frequency-doubled output of a Nd 3+ :YAG-laser (532 nm, s-polarized, 40 ps) was used as the excitation light.…”

“…12 Using the procedure explicated elsewhere, 12 the determination of the packing symmetry in the RhC 18 monolayer was also attempted to gain further insight into the organization of the rhodamine dye molecules at the air/water interface. The measurements were done with RhC 18 monolayers at areas of 0.83-0.4 nm 2 /molecule.…”

“…Meanwhile, it was shown that the direct determination of the microstructure and the symmetry of packing of xanthene dye molecules in the monolayer at the air/water interface could be accomplished with the azimuthal angle second-harmonic generation (SHG) method. 12 Thus, using the advantage of this method together with other ones, we attempted a systematic study of the RhC 18 monolayer behavior.…”

“…Monolayers of amphiphilic molecules bearing a chromophoric moiety, in particular, those belonging to the xanthene dye family, have attracted considerable attention for many years for both scientific and practical reasons. − As their photophysical properties are sensitive to the organization of chromophores at the interface, dye monolayers provide ideal model systems to study orientational arrangement, molecular aggregation, and energy transfer processes between molecules residing in restricted geometry. − Besides, dye monolayers are used as precursors of multilayered films built up with the Langmuir−Blodgett (LB) deposition technique . Therefore, the study of the chromophore arrangement at the air/water interface, which provides the ability to elucidate, predict, and control 2D molecular organization, is crucial in the intelligent design of well-ordered and defect-free films for various applications in such areas as molecular electronics, optical devices, and sensors.…”