Phonon-induced site relaxation of tetracene guests in p-terphenyl single crystals

Site relaxation processes of tetracene-h12 and tetracene-d12 guests in p-terphenyl crystals are examined using high-resolution fluorescence and fluorescence excitation spectroscopy. Two of the four sites, the so-called O3 and O4 sites, undergo this process and their phonon sidebands observed in the excitation spectra are characterized by an exceptionally long and well resolved progression of a low-frequency (pseudo-) local phonon mode superimposed on a broad background. Perdeuteration of tetracene results in a frequency reduction of the progression forming mode which identifies this local phonon as a guest librational mode. The latter is suggested to induce the site relaxation via guest reorientation processes. The intensity distribution in the phonon sideband of the O3 site indicates a reorientational angle of ∼21° about the L-axis for the tetracene guest in the excited singlet state.