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 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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Room temperature magnetic resonance studies of the photoexcited triplet state of tetracene in p-terphenyl crystals have been carried out by the electron spin echo technique. The studies yield the following structural and dynamical information: (1) zero-field splittings; D/hc=±0.0551(1) and E/hc=∓0.0042(1) cm−1; (2) spin densities: ρ1(α)=0.055, ρ2(β)=0.016, ρ5(γ)=0.217, ρ13=−0.025, and ρ15=−0.027; and (3) relative triplet population rates: px≊py≳pz.

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