Heterogeneous Dynamics and Domains in Supercooled<i>o</i>-Terphenyl: A Single Molecule Study

Abstract

Single molecule spectroscopy was used to characterize the rotation of fluorescent probe molecules in supercooled o-terphenyl (OTP) just above the glass transition. Rotational motions of spatially isolated individual probe molecules were followed in real time, revealing dynamics that reflect a mosaic of spatially heterogeneous environments. The short-time molecular motions in each environment are found to be diffusional, taking place through a Brownian rotational process characterized by a single rotational correlation time τC. The distribution of rotational diffusion constants for the heterogeneous environments becomes larger as the temperature approaches Tg, manifesting increased heterogeneity as OTP is cooled toward the glass transition. After many molecular rotations, the molecule's rotational time changes abruptly. This switch appears instantaneous on the time scale of molecular rotation and is indicative of a rapid rearrangement of the molecule and its local environment. The time required for the environment to change, τEx, is on average 15 times larger than the τC, and nearly 300 times slower than the α-relaxation time in OTP. The ensemble average correlation, 〈τC〉, and exchange times, 〈τEx〉, show a similar temperature dependence, both of which are consistent with the temperature dependence predicted by the Debye Stokes Einstein equation.