Wavelength-dependent triplet state populating mechanisms of naphthalene in heavy-atom hosts

We have made zero-field optically-detected magnetic resonance measurements on the triplet state of naphthalene-d₈ incorporated in the heavy-atom host crystals p-dichlorobenzene, p-dibromobenzene, and s-tetrachlorobenzene. The dependence of the relative triplet sublevel populating rates on the exciting wavelength is interpreted in terms of the electronic excitation transfer mechanisms. We find that upon excitation of the host S₁ ← S₀ transition in p-dichlorobenzene, energy transfer to the guest T₁ takes place via the guest S₁, whereas in the other host crystals the intermediate is mainly the host T₁ band. Upon host T₁ ← S₀ excitaton, trapping takes place from a spin-aligned excitor band in s-tetrachlorobenzene, whereas the initial alignment is lost prior to trapping in the dihalobenzene hosts.