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

Abstract

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.

Solvent-Controlled Morphology of Catalytic Monolayers at Solid–Liquid Interfaces

“…Similarly slow dynamics for the overall change in the IR spectrum of Re-carbonyl containing monolayers of Langmuir-Blodgett films have been presented very recently, but at the same time, time-dependent cross peaks in the 2D IR spectra and have been interpreted in the framework of chemical exchange between sample sub-structures. 48 However, due to the non-immobilized nature of the Langmuir-Blodgett film, that sample system is highly mobile and structural flexibility on a wide range of timescales can be rationalized. Taking both studies together, there exist now growing evidence for very slow structural transitions in monolayer samples at different types of interfaces, whether the molecules are immobilization covalently or not.…”

“…where indeed distinct cross peaks show up above and below the diagonal after a few picoseconds. We note in passing that such time-dependent cross peaks may in principle also originate from chemical exchange 48,51,52 , i.e. interconversion of the two species.…”

“…44 Very recently, the same group studied the dynamics of the same molecule in monolayers of Langmuir-Blodgett films, again observing complex IR spectra which evolve on a very slow second to hour timescales. 48 In the present paper, we also aim to clarify some of the confusion that exists about the IR spectroscopy of this important molecule on surfaces. Figure 1.…”

Enhancing the signal strength of surface sensitive 2D IR spectroscopy

“…30,31 More recently, in a method developed by Fayer and coworkers, the reflection at an interface is used to decrease the intensity of the local oscillator in a pump-probe geometry. 32,33 In their experiments, a probe beam impinges on the surface of a sample at an angle near Brewster's angle for that air/liquid or air/solid interface so that only a small percentage of the probe is reflected. Instead of measuring the transmitted probe, as usually done in 2D IR spectroscopy, the reflected probe is collected instead.…”

“…32 While this enhancement is not as large as plasmon enhanced 2D IR, [24][25][26][27][28][29] it is applicable to solution surfaces where interesting phenomena using metal carbonyl molecules has been observed. 33 In this article, we report a variation of 2D IR spectroscopy that combines the approaches of Fayer and Hamm (Fig. 1).…”