Pulsed zero-field electron paramagnetic resonance free induction decay (ZF EPR FID) techniques are employed to study the phase transition of p-terphenyl crystals (Tc = 193 K) by measuring selectively populated photoexcited triplet ZF transitions of dilute pentacene molecules in p-terphenyl single crystals. The sensitivity of ZF spectroscopy to small shifts in local magnetic fields enables the studies of guest-host configuration changes over a wide temperature range. Here we report the observation of guest pentacene (-h(14) and -d(14)) triplet ZF EPR FID spectra that disappear abruptly at Tc and of spectral broadening and shift below Tc. We interpret these spectral changes as evidence for guest couplings to host phenyl rings. Further, these data allow assignments of spectroscopic sites to crystallographic sites that occur in the phase transition.

No abstract

The pulsed electron paramagnetic resonance (EPR) technique using computer controlled three-dimensional (3D) near-zero fields (NZF, 0-1.5 mT) have previously been reported. This paper reports on a comprehensive NZF pulsed EPR study of pentacene-d 14 doped in p-terphenyl (PDPT) single crystals. Molecular symmetry axes are determined from the analyses of these NZF EPR spectra.