2D-IR Spectroscopy of Porous Silica Nanoparticles: Measuring the Distance Sensitivity of Spectral Diffusion

Two-dimensional infrared spectroscopy (2D-IR) was used to investigate the sensitivity of a vibrational mode to spectral diffusion on the nanometer length scale. A confined volume within a porous silica nanoparticle was used as the model system. Utilizing a novel grafting technique, silane probes were covalently secured to the pore surface for a series of templated mesoporous silica nanoparticles with variable pore sizes. 2D-IR measurements determined that isopropanol exhibited two dominant time scales of ultrafast motions within the pores with time scales of a few picoseconds and tens of picoseconds. The unique perspective of the probe on a surface revealed that the vibrational dynamics of the reporting mode were perturbed by molecules up to μ5 nm away. The data were modeled well by a 1/d² distance dependence for the coupling of solvent dynamics to frequency fluctuations and a logistic radial distribution of fast and slow dynamic populations.