Monolayers, interfaces, and very thin films are fundamentally interesting, and they play important roles in many naturally occurring and technological processes. Molecular dynamics at an interface will be very different from those in a bulk material. The advent of acousto-optic pulse shaping methods has greatly expanded the applicability of 2D IR spectroscopy to an increasing range of important materials. However, it is still very difficult to study a monolayer or a very thin film because of the small number of molecules in the sample. We developed a new 2D IR method for monolayers and thin films. The near-Brewster’s angle reflection geometry enhances the 2D IR signal by as much as a factor of 50. The new method was used to study the dynamics and structure of a monolayer at the air/water interface. The surfactant, tricarbonylchloro-9-octadecylamino-4,5-diazafluorenerhenium(I) (TReF18) monolayer at the air/water interface contains a very useful vibrational probe, the head group ReCl(CO)3fluorene, which is located at the interface. 2D IR data at two surface densities, low (area 90 Å2) and high (area 60 Å2), are discussed. In addition, a water soluble version of the vibrational probe head group, ReCl(CO)3bipydicarboxylate (TReBDC), was studied in bulk water for comparison. Water H-bonds to the carbonyls. TReBDC and TReF18 both display time dependent spectral diffusion that is biexponential. For TReBDC the fast component is 1.5 ps, the previously measured time for water hydrogen bond rearrangement within error. For the monolayer, the value is 3.1 ps, a substantial slowing of hydrogen bond rearrangement in the interfacial water layer. The slower component for both molecules is caused by fluctuations in the number of water molecules hydrogen bonded to the three carbonyls. The hydrogen bond number fluctuations are much slower at the interface than in bulk water. At low density, there is a single band in the 2D spectrum. At high density, three bands are observed, two bands for minor structures in addition to the main band. Each minor structure band has chemical exchange peaks with the main band showing that the minor and major structures are in equilibrium with a chemical exchange time of roughly 40 ps. However, the minor structure bands are not always present. By taking 8 second data sets, the appearance and disappearance of the minor structures were tracked. Structural fluctuations in the monolayer cause the minor structures to come and go.

Monolayer dynamics at the air/water interface:  From ultrafast to ultraslow dynamics

255th National Meeting (2018)

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