NANOSECOND AND MICROSECOND STUDY OF PROBE REORIENTATION IN ORTHOTERPHENYL

Time-resolved optical measurements of probe reorientation in orthoterphenyl have been performed. Rotational correlation functions have been measured from 100 ps to 1 ms for anthracene and anthanthrene, while correlation functions were observed over a narrower time window for 9-phenylanthracene and 9,10-diphenylanthracene. The rotational correlation time of anthracene follows the temperature dependence of η/T, indicating that anthracene rotation is well coupled to the dynamics responsible for the main glass transition. At temperatures above 30°C, rotational correlation times depend upon probe size. In this region, orthoterphenyl behaves as a simple liquid. At temperatures near -5°C, observed rotational correlation times are roughly independent of probe size. These results are consistent with the existence of domains of associated orthoterphenyl molecules. At lower temperatures, near and below the glass transition temperature, a partial reorientation of anthracene associated with the β process is observed. These results imply that the β process in orthoterphenyl involves the partial reorientation of most molecules rather than the total reorientation of a small fraction of the molecules. © 1990 American Institute of Physics.