Single molecule probe reports of dynamic heterogeneity in supercooled ortho-terphenyl.

The rotational dynamics of three perylene diimide dyes are studied on the single molecule (SM) level in ortho-terphenyl (OTP) near the glass transition temperature (T(g)). At all temperatures probed, spanning 1.03-1.06 T(g), each of the three probes exhibits rotational correlation times, τ(c), that span more than a decade, consistent with the presence of spatially heterogeneous dynamics in OTP. No trend is found as a function of temperature, but a trend as a function of probe is observed: Average probe rotational correlation time scales inversely with breadth of SM τ(c) distribution, with faster probes exhibiting broader τ(c) distributions. This implies that dynamic exchange occurs on and below time scales associated with probe rotation. Extrapolating FWHM of rotational relaxation times to the structural relaxation time of the host shows that the τ(c) distribution would span nearly two decades in the limit of no probe temporal averaging. Comparison with SM measurements in glycerol suggests that OTP demonstrates a greater degree of spatially heterogeneous dynamics in this temperature range than does glycerol.