Enhanced translation of probe molecules in supercooled o-terphenyl: Signature of spatially heterogeneous dynamics?

A holographic fluorescence recovery after photobleaching technique has been used to measure translational diffusion coefficients D-T for four probes in supercooled o-terphenyl (OTP). D-T values from 10(-6) to 10(-14) cm(2)/s were observed in the temperature range from T-g+8 K to T-g+90 K (T-g=243 K). In agreement with previous reports, the translational diffusion of probe molecules which are the same size as OTP molecules has a significantly weaker temperature dependence than T/eta. However, as the size of the probe molecule is increased the temperature dependence of D-T tracks T/eta increasingly well. The transition between a weak temperature dependence to that of T/eta occurs over only a factor of 3 in probe size. Previous work established that the rotational correlation times tau(c) of these four probes in OTP tracks eta/T. The product D-T tau(c) is independent of temperature for the largest probe but increases almost 2 orders of magnitude for the smaller probes as T-g is approached. A strong correlation is observed between this enhancement of translational diffusion and the KWW beta values obtained from rotation measurements; probes with small beta values show enhanced translation. These observations are qualitatively explained by spatially heterogeneous dynamics. The probes studied are rubrene, 9,10-bis(phenylethynyl)anthracene, tetracene, and anthracene. (C) 1996 American Institute of Physics.