The electron paramagnetic resonance spectrum of the photoexcited triplet state of pentacene-d14 in pterphenyl crystals exhibits a doubling at temperatures below the phase transition temperature of p-terphenyl. The relative intensities and the separation of the two members of the doublets vary with temperature. We ascribe the spectral changes to the accessibility of each pentacene molecule to two conformations with unequal energies. The data are fitted to a two-conformation jumping model. The energy difference between these two conformations (AE) is calculated to be 53 f 4 cm-I. The activation energy (Nf) is calculated to be 71 * 4 cm-I with the Arrhenius constant A = (7.8 f 0.2) X IO6 SKI.
Abstract. The spin dynamics of the photoexcited triplet state of pentacene in three host crystals: benzoic acid, naphthalene and p-terphenyl, were measured from 5 to 300 K by the electron spin echo technique following N, laser excitation. The triplet kinetics (population, spinlattice relaxation and decay rates) of pentacene in the benzoic acid (BA) hosts differ from those observed in the other two host systems. A model which includes a strong guest-host interaction and proton tunneling in the tautomerization of BA hosts is used to explain the observations.
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