“…Because the susceptibility of each constituent is unknown, eq 2 is insoluble, and an accurate determination of the molecular orientation of every species is impossible. Similar findings on the SHG signal with the increase of the surface coverage were reported for xanthene dyes at the air−water interface and for an LB film of a coumarin dye, in which signal changes were related to extended conjugation of the chromophores.…”
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.
“…Because the susceptibility of each constituent is unknown, eq 2 is insoluble, and an accurate determination of the molecular orientation of every species is impossible. Similar findings on the SHG signal with the increase of the surface coverage were reported for xanthene dyes at the air−water interface and for an LB film of a coumarin dye, in which signal changes were related to extended conjugation of the chromophores.…”
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.
“…For long-chain amphiphiles such as stearic acid, the final stage of compression gives a “solid” phase, in which the hydrocarbon tails are all aligned, pointing out of the aqueous phase, and the hydrophilic headgroups are close packed. The relationship between the observed two-dimensional phase behavior and interadsorbate interactions has been elucidated spectroscopically. − 2 A typical π− A isotherm for long-chain fatty acids, in this case, stearic acid. The arrows indicate “kinks” in the isotherm due to phase transitions; the two-dimensional phases are labeled.…”
“…In most cases, the SSHG or SSFG experiments do not deliver straightforward information about interfaces, and the interpretation of the results is usually difficult and sometimes ambiguous. This especially applies to SSHG, which generally provides limited spectral information, although it is a convenient and relatively simple tool for studying the orientation, − the concentration, − and the dynamics of molecules at interfaces. − …”
' INTRODUCTIONOver the past few years, second-order nonlinear optical techniques, such as surface second harmonic generation (SSHG) 1À3 and surface sum frequency generation (SSFG), 4,5 have become powerful and extensively applied tools for studying the chemical and physical properties of interfaces. These methods are intrinsically insensitive to isotropic media, and they thus overcome the main difficulty of optical studies of interfaces, namely, the fact that the linear response from the relatively small number of molecules located in the interfacial region is completely hidden by the signal from the overwhelming number of bulk molecules.The information obtained in these experiments may have a significant value for various fields of chemistry and biochemistry, 6,7 because interfaces constitute the local environment of numerous chemical reactions and physical phenomena, including common and important processes for everyday life, such as the corrosion of metal surfaces or the formation of foams and emulsions. On the other hand, water interfaces play a key role in biological processes taking place at cell membranes, including the transport of medicines and the infection of cells by viruses. In this case, knowledge of the physical and chemical properties of the interface is essential for the design of more effective drugs.In most cases, the SSHG or SSFG experiments do not deliver straightforward information about interfaces, and the interpretation of the results is usually difficult and sometimes ambiguous. This especially applies to SSHG, which generally provides limited spectral information, although it is a convenient and relatively simple tool for studying the orientation, 8À13 the concentration, 14À16 and the dynamics of molecules at interfaces. 17À25 A powerful approach to investigate physical and chemical properties of interfaces is the use of a "dynamic probe", namely, a molecule with an excited-state dynamics (for instance, its S 1 lifetime) that depends on some properties of the environment. Once the dynamics of this molecule and its dependence upon a given environment property is well-characterized by bulk spectroscopy, information on this property at the interface can then be deduced from time-resolved SSHG (TRSSHG) experiments. A good example of this approach is the measurement of the S 1 state lifetime of malachite green to characterize the microscopic friction at air/liquid and liquid/liquid interfaces. 8,26 Because the deactivation pathways of the excited state of this dye were well-understood and the dependence of its S 1 state lifetime on solvent viscosity was established, it was possible to determine the friction exerted on this molecule at various interfaces by TRSSHG. In this approach, two aspects are simultaneously investigated: (1) the properties of the interface and (2) how these properties affect the behavior of solute molecules located at the interface. A proper understanding of the interfacial process requires both issues to be addressed.We report here on the application of the xanthene...
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