The photophysics of two dyes from the xanthene family, eosin B (EB), and eosin Y (EY) has been investigated in various solvents by femtosecond transient absorption spectroscopy, first, to clarify the huge disparity of the EB fluorescence lifetimes reported in literature, and, second, to understand the mechanism responsible for the ultrafast excited-state deactivation of EB in water. The excited-state lifetime of EB was found to be much shorter in water and in other protic solvents, due to the occurrence of hydrogen-bond assisted nonradiative deactivation. This mechanism is associated with the hydrogen bonds between the solvent molecules and the nitro groups of EB, which become stronger upon optical excitation due to the charge-transfer character of the excited-state. This process is not operative with EY, where the nitro groups are replaced by bromine atoms. Therefore, the excited-state lifetime of EB in solution is directly related to the strength of the solvent as a hydrogen-bond donor, offering the possibility to build a corresponding scale based on the fluorescence quantum yield or lifetime of EB. This scale of hydrogen-bonding strength could be especially useful for studies of liquid interfaces by time-resolved surface second harmonic generation

The present study reports on the effect caused by sodium salts added to a solution of malachite green in a liquid/liquid interfacial system probed by the time-resolved surface second harmonic generation (TRSSHG) technique. This effect is known as "salting-out effect" and is shown to reveal two main issues: salts added to the bulk, first, cause a reduction of the dye solubility and, second, stimulate the adsorption of malachite green cations at the interface, changing the equilibrium constant between the dye molecules adsorbed at the interface and those being dissolved in the bulk. The increased adsorption at the interface is observed in the TRSSHG experiment as a relative increase of the aggregates' contribution to the measured time profile. However, depending on the nature and properties of salt anions, the mechanisms responsible for enhancing the population of interfacial aggregates can differ. This study explains such mechanisms for NaCl and NaSCN: addition of NaCl leads to an increase of the malachite green adsorption at the interface followed by the formation of aggregates, whereas the addition of NaSCN leads rather to the formation of aggregates already in the bulk with their further migration toward the interface. A simple quantitative description of the salting-out effect based on a modified Frumkin-Fowler-Guggenheim model also has been proposed. It has been shown to give a good agreement with the experiment with NaCl, i.e., when the formation of dye aggregates in the bulk solution can be neglected

' 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...

The excited-state dynamics of rhodamine 6G (R6G) has been investigated in aqueous solution using ultrafast transient absorption spectroscopy and at the dodecane/water interface using the femtosecond time-resolved surface second harmonic generation (SSHG) technique. As the R6G concentration exceeds ca. 1 mM in bulk water, both R6G monomers and aggregates are excited to a different extent when using pump pulses at 500 and 530 nm. The excited-state lifetime of the monomers is shortened compared to dilute solutions because of the occurrence of excitation energy transfer to the aggregates, which themselves decay nonradiatively to the ground state with a ca. 70 ps time constant. At the dodecane/water interface, both monomers and aggregates contribute to the SSHG signal to an extent that depends on the bulk concentration, the pump and probe wavelengths, and the polarization of probe and signal beams. The excited-state lifetime of the monomers at the interface is of the order of a few picoseconds even at bulk concentrations where it is as large as several nanoseconds. This is explained by the relatively high interfacial affinity of R6G that leads to a large interfacial concentration, favoring aggregation and thus rapid excitation energy transfer from monomers to aggregates.

The excited-state dynamics of the cationic dye malachite green (MG) and of the dianionic dye eosin B at the dodecane/water interface has been investigated using femtosecond time-resolved surface second harmonic generation (TR-SSHG). By using different probe wavelengths, the contributions of monomeric and aggregated MG to the signal could be spectroscopically distinguished. The effect of the addition of a small amount of surfactants was found to strongly depend on the relative charges of surfactant and dye. For surfactant/dye pairs with opposite charges, the TR-SSHG signal is dominated by the contribution from aggregates, whereas for pairs with the same charges, the signal intensity becomes vanishingly small. These effects are explained in terms of electrostatic interactions between surfactants and dyes that favor either attraction of the dye toward the interface or its repulsion toward the bulk. As a very similar behavior is observed with MG upon addition of NaSCN, we conclude that, in this case, this effect reflects the affinity of SCN¯ for the interface. On the other hand, the guanidinium cation was found to have a different effect than that of a positively charged surfactant on the SSHG signal of MG, indicating this cation does not accumulate in the interfacial region.