Second Harmonic Generation from Mixed Films of Rhodamine Dye and Arachidic Acid at the Air−Water Interface:  Correlation with Spectroscopic Properties

Abstract

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.

PalmitateLuciferin: A Molecular Design for the Second Harmonic Generation Study of Ion Complexation at the Air–Water Interface

“…These fluctuations are attributed to the nonhomogeneity of the molecular monolayer. The formation of molecular aggregates during monolayer compression is common and has been reported previously ,, for various types of molecules. In the case of the PL monolayer, BAM measurements, see Figure , were performed to illustrate this point.…”

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

“…2,[5][6][7][8][9][10][11]13 In particular, it was found that the RhC 18 chromophore adopted a tilted orientation at the air/water interface. 11 It was also reported that a variety of aggregated species, whether fluorescent or nonfluorescent dimers and aggregates, were formed in RhC 18 monolayers during compression and in RhC 18 -lipid LB films.…”

“…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.…”

“…Recently, pure RhC 18 monolayer at the air/water interface as well as RhC 18 -lipid mixed films deposited on glass substrata have been investigated by a variety of techniques. ,− , In particular, it was found that the RhC 18 chromophore adopted a tilted orientation at the air/water interface . It was also reported that a variety of aggregated species, whether fluorescent or nonfluorescent dimers and aggregates, were formed in RhC 18 monolayers during compression and in RhC 18 -lipid LB films. ,− ,, However, though these data shed some light on the interchromophoric interactions and excitation energy-transfer processes, they did not provide the key to understanding the fundamental properties of the microscopic organization of the RhC 18 molecules in the monolayer.…”

“…Recently, pure RhC 18 monolayer at the air/water interface as well as RhC 18 -lipid mixed films deposited on glass substrata have been investigated by a variety of techniques. ,− , In particular, it was found that the RhC 18 chromophore adopted a tilted orientation at the air/water interface . It was also reported that a variety of aggregated species, whether fluorescent or nonfluorescent dimers and aggregates, were formed in RhC 18 monolayers during compression and in RhC 18 -lipid LB films. ,− ,, However, though these data shed some light on the interchromophoric interactions and excitation energy-transfer processes, they did not provide the key to understanding the fundamental properties of the microscopic organization of the RhC 18 molecules in the monolayer. It was proposed that compression caused an arrangement of the RhC 18 molecules at the air/water interface with a long-range order in tilt and azimuthal orientation of the chromophore polarization axes, , but this hypothesis has not yet been proved by any experimental data.…”