Dynamics of Molecular Monolayers with Different Chain Lengths in Air and Solvents Probed by Ultrafast 2D IR Spectroscopy

The ultrafast dynamics of functionalized alkylsilane monolayers with two different alkyl chain lengths, C-11 and C-3, are studied by 2D IR vibrational echo spectroscopy. Terminal sites of the monolayers are functionalized with an IR probe, tricarbonyl (1,10-phenanthroline) rhenium chloride (RePhen(CO)(3)Cl), to report on the structural dynamics of monolayer-air and monolayer-solvent interfaces. Frequency-frequency correlation functions (FFCF) of symmetric CO stretching mode were extracted from 2D IR spectra. The FFCF provides information on the time evolution of surface structures that contribute to the inhomogeneously broadened IR absorption spectrum of the CO stretch by quantifying spectral diffusion. To elucidate the detailed structural dynamics with accurate time constants, FFCF decays were monitored to 60 ps, a major improvement over prior experiments. Without the presence of solvents, C-3 monolayers have significantly slower spectral diffusion (66 ps) than C-11 monolayer (38 ps). This difference supports the previous postulate that spectral diffusion associated with the molecular monolayer in air involves the structural dynamics of the tethering alkyl chains. With the C-11 and C-3 monolayers immersed in dimethylformamide (DMF), in which RePhen(CO)(3)Cl is soluble, the FFCFs both samples display biexponential decays with the time constants of 5.6 and 43 ps for C-11 and 5.9 and 63 ps for C-3. The slower time constants are in good agreement with the spectral diffusion time constants observed in the absence of solvent, indicating that these slower components still reflect the monolayers' dynamics, which are not affected by the presence of the solvent. Because the faster time constants were independent of chain length and similar to that of the RePhen(CO)(3)Cl headgroup in bulk DMF solution, they are attributed to the dynamics of the interfacial DMF molecules. When monolayers were immersed in hexadecane, in which RePhen(CO)(3)Cl is not soluble, slower dynamics were again observed for the C-3 monolayer than the C-11 monolayer. The orientational dynamics of the IR probes were studied using polarization selective heterodyne detected transient grating experiments as well.