Enhanced vibrational solvatochromism and spectral diffusion by electron rich substituents on small molecule silanes

Abstract

Fourier transform infrared and two-dimensional IR (2D-IR) spectroscopies were applied to two different silanes in three different solvents. The selected solutes exhibit different degrees of vibrational solvatochromism for the Si-H vibration. Density functional theory calculations confirm that this difference in sensitivity is the result of higher mode polarization with more electron withdrawing ligands. This mode sensitivity also affects the extent of spectral diffusion experienced by the silane vibration, offering a potential route to simultaneously optimize the sensitivity of vibrational probes in both steady-state and time-resolved measurements. Frequency-frequency correlation functions obtained by 2D-IR show that both solutes experience dynamics on similar time scales and are consistent with a picture in which weakly interacting solvents produce faster, more homogeneous fluctuations. Molecular dynamics simulations confirm that the frequency-frequency correlation function obtained by 2D-IR is sensitive to the presence of hydrogen bonding dynamics in the surrounding solvation shell.

Influence of Solvent Swelling on Ultrafast Structural Dynamics in Polydimethylsiloxane Thin Films by Two-Dimensional IR Spectroscopy

“…Recent work from our group has shown that the solvatochromic effect for the silicon hydride can be explained by a vibrational Stark effect with the addition of specific solvent−solute interactions in the case of hydrogen and halogen bond donors. 64 The FTIR peak widths in Figure 1a are slightly more sensitive to the different solvent environments, increasing in the order of none (green) < pentane (blue) < 2-propanol (black) < chloroform (red) and consistent with our previous report (excluding the solvent-free sample). 64 For comparison, Figure 1b shows the FTIR spectra of the oligomeric cross-linking species in the same solvents prior to being polymerized into the PDMS elastomer.…”

“…64 The FTIR peak widths in Figure 1a are slightly more sensitive to the different solvent environments, increasing in the order of none (green) < pentane (blue) < 2-propanol (black) < chloroform (red) and consistent with our previous report (excluding the solvent-free sample). 64 For comparison, Figure 1b shows the FTIR spectra of the oligomeric cross-linking species in the same solvents prior to being polymerized into the PDMS elastomer. The hydride on the cross-linking oligomer is only slightly more solvatochromic with its frequency shifts spanning a range of 4 cm −1 and has the same trend except for a higher frequency in pentane than without solvent.…”

“…In previous work, these three solvents were found to span the range of solvatochromic shifts for silicon hydride vibrations. 64 The ν Si−H on PDMS is nearly insensitive to the different solvent environments. A slight solvatochromic trend (less than a wavenumber) is seen in order of increasing frequency: chloroform (red) < 2-propanol (black) < pentane (blue) < no solvent (green).…”

“…The insensitivity of ν Si−H can be interpreted in the context of solvatochromism on small silane molecules that are in these same solvents 64 and that are bound to a porous silica surface. 71 When we compare the PDMS elastomer to the silica sol−gel study performed by our group previously, 70 the Si−H frequency is much lower in frequency, suggesting that the silicon atom in the elastomer is more electron rich than it is in the silica sol−gels.…”

“…The frequency of the silicon hydride mode in the PDMS systems is closer to that for triphenylsilane (TriPS) than to those for the silica sol−gels and trimethoxysilane (TriMOS). 70,64 The electron withdrawing methoxy ligands of TriMOS lead to a greater charge separation and electrostatic coupling to the solvent reaction field, whereas TriPS does not have either of these benefits. In the PDMS elastomer and the cross-linking oligomer, the silicon atom bearing the hydride is bound to two oxygen atoms and one methyl group, likely rendering it less polarized and therefore less sensitive to the solvent environment.…”