Paramagnetic Resonance Absorption in Naphthalene in Its Phosphorescent State

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Abstract
Paramagnetic resonance absorption has been observed in naphthalene in solid solution in a single crystal of durene when irradiated with light from an A-H6 mercury lamp. The fine structure of the spectrum at a frequency of ∼22.7×109 cps may be described by the spin-Hamiltonian H=|β|H·g·S+DSz2+E(Sx2−Sy2),,with S=1,g=2.0030±0.0004 (isotropic),D/hc=±0.1003±0.0006 cm−1,E/hc=∓0.0137±0.0002 cm−1.Observations at a different frequency, ∼9.7×109 cps, and also measurements on perdeuteronaphthalene are described by essentially the same parameters. The fine structure data identify the spectrum as arising from a triplet state and leave no doubt as to the triplet character of the phosphorescent state of naphthalene. The zero-field splitting of the triplet has its origin in the magnetic dipole-dipole interaction between the two unpaired electrons in the triplet state. These experimental results are in agreement with the theoretical estimates. Hyperfine structure has been observed and is interpretable on the basis of the anisotropic hyperfine interaction constants for a C–H fragment given by McConnell. The π-electron spin density on the α-carbons is found to be 0.209. The hyperfine structure results identify the spectrum as arising from naphthalene. In addition they show that the triplet state is 3B2u+ and that the naphthalene molecules are oriented in the crystals such that they are not only coplanar with the durene molecule which they replace but also have their long and short twofold axes in this plane parallel, respectively, to the long and short axes of durenes.

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