Hydrogen-Bond-Assisted Excited-State Deactivation at Liquid/Water Interfaces

Abstract

' 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...

Ultrafast Excited-State Dynamics of Eosin B: A Potential Probe of the Hydrogen-Bonding Properties of the Environment

“…As a consequence, EB appears to be a valuable dynamic probe of the hydrogen-bonding properties in bulk environments and at liquid interfaces, as demonstrated by a recent TRSSHG study. 15 ' EXPERIMENTAL SECTION Samples. Eosin B (EB, disodium salt, Sigma-Aldrich) and eosin Y (EY, disodium salt, Acros Organics) were used without further purification.…”

Interfacial Generation of a Carbanion: The Key Step of PTC Reaction Directly Observed by Second Harmonic Generation

“…In this method, the surface between two immiscible liquids is illuminated with intense laser pulses (more details are given in the supporting information), which results in nonlinear generation of light with twice the frequency (half the wavelength) of the incident wave, the second harmonic (SH) light. Only molecules located in the interfacial region affect this process because SH generation is forbidden in bulk liquid due to symmetry reasons . The maxima of SHG spectra (defined here as a dependence of the maximum of the SH generation yield on half of the wavelength of the incident light) for each species adsorbed at the surface are located at similar wavelengths as the maxima of electronic absorption spectra of these species in solution.…”

“…SHG is a well‐established surface‐sensitive spectroscopic technique . In this method, the surface between two immiscible liquids is illuminated with intense laser pulses (more details are given in the supporting information), which results in nonlinear generation of light with twice the frequency (half the wavelength) of the incident wave, the second harmonic (SH) light.…”

Photoinduced Electron Transfer Reactions: From the Elucidation of Old Problems in Bulk Solutions Towards the Exploration of Interfaces

“…[45] We have used this property to probe hydrogen bonding at liquid-liquid interfaces. [46] Measurements at liquid-liquid interfaces can also be used to obtain mechanistic information on the deactivation pathway of an excited molecule. This has been done with an hemicyanine, whose S 1 state decays non-radiatively through large amplitude motion (Fig.…”