Ultrafast to Ultraslow Dynamics of a Langmuir Monolayer at the Air/Water Interface Observed with Reflection Enhanced 2D IR Spectroscopy

Monolayers play important roles in naturally occurring phenomena and technological processes. Monolayers at the air/water interface have received considerable attention, yet it has proven difficult to measure monolayer and interfacial molecular dynamics. Here we employ a new technique, reflection enhanced two-dimensional infrared (2D IR) spectroscopy, on a carbonyl stretching mode of tricarbonylchloro-9-octadecylamino-4,5-diazafluorenerhenium(I) (TReF18) monolayers at two surface densities. Comparison to experiments on a water-soluble version of the metal carbonyl headgroup shows that water hydrogen bond rearrangement dynamics slow from 1.5 ps in bulk water to 3.1 ps for interfacial water. Longer time scale fluctuations were also observed and attributed to fluctuations of the number of hydrogen bonds formed between water and the three carbonyls of TReF18. At the higher surface density, two types of TReF18 minor structures are observed in addition to the main structure. The reflection method can take usable 2D IR spectra on the monolayer within 8 s, enabling us to track the fluctuating minor structures' appearance and disappearance on a tens of seconds time scale. 2D IR chemical exchange spectroscopy further shows these structures interconvert in 30 ps. Finally, 2D spectral line shape evolution reveals that it takes the monolayers hours to reach macroscopic structural equilibrium.