Electron spin echo observations of the photoexcited triplet state of 0.1 mol % pentacene-h14 and -d14 in p-terphenyl crystals at room temperature are presented. Theory is presented for calculation of the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit. Echo envelope modulations due to proton and deuteron hyperfine interactions in the pentacene molecule have been observed. The echo decay data are used to calculate triplet state decay parameters.

The pulsed EPR free induction decay (FID) signals of the photo-excited pentacene triplet state are reported for three mixed crystals at room temperature: pentacene-h14 in p-terphenyl, pentacene-h14 in benzoic acid, and pentacene-d14 in p-terphenyl. The recorded FID signals have relatively long decay times of about four microseconds, presumably due to the reduced hyperfine interactions in the zero magnetic field. The time domain FID signals transform to spectral components typically narrower than 500 kHz, allowing us to determine the pentacene triplet zero field splitting parameters to better accuracy than previously reported. Further, a new experimental technique using the high speed magnetic field jumping capability enables us to examine the anisotropic hyperfine and quadrupole interactions.

The triplet kinetics of pentacene imbedded in p-terphenyl and benzoic acid crystals have been studied by electron spin echo spectroscopy in the temperature range of 77–300 K. Population inversions have been observed for both H0‖‖x and H0‖‖z in the benzoic acid crystal but only for H0‖‖z in the p-terphenyl crystal at room temperature. Detailed analyses showed that the triplet population and decay of the pentacene are very much host and temperature dependent. Dimer formation and tautomerization (proton transfer of the benzoic acid dimer) may be responsible for the peculiar triplet kinetics of the pentacene in the benzoic acid crystal.

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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.

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