Dynamics of p-Terphenyl Crystals at the Phase Transition Temperature: A Zero-Field EPR Study of the Photoexcited Triplet State of Pentacene in p-Terphenyl Crystals
Pulsed zero-field electron paramagnetic resonance free induction decay (ZF EPR FID) techniques are employedto study the phase transition of p-terphenyl crystals (Tc = 193 K) by measuring selectively populatedphotoexcited triplet ZF transitions of dilute pentacene molecules in p-terphenyl single crystals. The sensitivityof ZF spectroscopy to small shifts in local magnetic fields enables the studies of guest-host configurationchanges over a wide temperature range. Here we report the observation of guest pentacene (-h14 and -d14)triplet ZF EPR FID spectra that disappear abruptly at Tc and of spectral broadening and shift below Tc. Weinterpret these spectral changes as evidence for guest couplings to host phenyl rings. Further, these dataallow assignments of spectroscopic sites to crystallographic sites that occur in the phase transition.