Ultrafast dynamics and structure at aqueous interfaces by second harmonic generation

Femtosecond time-resolved second harmonic generation studies of the barrierless isomerization of an organic dye, malachite green (MG), have been carded out at several aqueous interfaces. A comparison of the dynamics at the air/aqueous, alkane/aqueous and silica/aqueous interfaces, indicates increased friction and increased water structure at the aqueous interfaces relative to bulk water, in support of molecular simulations, with the silica/aqueous interface being the most structured. The dynamics are slower at all of these interfaces than in bulk water, by a factor of three to five in the case of the air/aqueous and alkane/aqueous interfaces, and almost an order of magnitude in the case of the silica/aqueous interface. These investigations also indicate that the generally accepted isomerization model of twisting of the three aromatic rings about the central carbon atom requires modification in that the synchronous twisting of all three aromatic rings is not necessary for rapid internal conversion from the excited to ground electronic state. In contrast to MG, the dynamics of the activated photoisomerization of the cyanine dye, 3,3'-diethyloxadicarbocyanine iodide (DODCI), is faster at the air/aqueous interface than in bulk aqueous solution. The different dynamics of MG and DODCI suggest that the interface friction must be described in terms of the orientation and solvent structure in the vicinity of the chromophores involved in the isomerization process.