Solvation dynamics and probe rotation in glass-forming liquids

We have measured optical depolarization and solvation dynamics using triplet state molecular probes in various glass-forming organic liquids of low molecular weight. Optical depolarization yields the single particle rotational correlation function r(t) of the guest molecules, while mechanical solvation dynamics reflect the time resolved shear modulus G(t) specifically of those host molecules which surround the probe. Using the ratio m_guest/m_host of molecular weights as an indicator for the relative sizes, it is observed that large m_guest/m_host values lead to much slower and more exponential probe rotation compared with the structural relaxation of the environment. As m_guest approaches m_host, the time dependencies of r(t) and G(t) become very similar. It appears that a larger and relatively immobile guest molecule has little effect on the otherwise cooperative dynamics of the surrounding solvent.