Structural Dynamics at Monolayer−Liquid Interfaces Probed by 2D IR Spectroscopy

Monolayers functionalized with tricarbonyl1,10-phenanthroline rhenium chloride (RePhen(CO)3Cl) are studied in the presence of a variety of polar organic solvents (chloroform, tetrahydrofuran, dimethylformamide, acetonitrile), hexadecane, and water. The headgroup, RePhen(CO)3Cl, is soluble in the bulk polar organic solvents but not in hexadecane or water. The surface structural dynamics are studied using IR absorption, ultrafast two-dimensional infrared (2D IR) vibrational echoes, and heterodyne-detected transient gratings (HDTG). The headgroup is also studied in the bulk solvents. Immersion in solvent changes the environment of the surface bound RePhen(CO)3Cl as shown by a solventdependent symmetric CO stretch absorption frequency and line shape. The 2D IR spectroscopy is sensitive to spectral diffusion, which reports on the structural time dependence. In the absence of solvent (bare surface), the spectral diffusion takes place on a 37 ± 4 ps time scale, whereas in the presence of THF, MeCN, or CHCl3 it occurs on the 25 ± 6 ps time scale. However, in DMF, the spectral diffusion time constant is 15 ± 3 ps. In hexadecane the spectral diffusion slows to 77 ± 15 ps, slower than the bare surface. In D2O, the monolayer CO stretch is split into two bands, indicating distinct local structures, and the spectral diffusion of main band is again slower than the bare surface. HDTG spectroscopy is used to investigate the vibrational lifetime dynamics.