Vibronic transitions in the excitation and emission spectra of hexakis(antipyrine)europium(III) tri-iodide and tri-perchlorate crystals

Abstract

Optical emission and emission excitation spectra are reported for crystals of Eu(AP)₆I₃ and Eu(AP)₆(ClO₄)₃ (where AP denotes an antipyrine ligand molecule). Emission results are reported for the ⁷ F _0,1,2 ← ⁵ D ₀ (luminescent) transition regions, and emission excitation results are reported for the ⁷ F ₀ → ⁵ D _0,1,2 (absorptive) transition regions. Particular attention is focused on vibronic transitions occurring between crystal-field (electronic) levels with the simultaneous excitation (or de-excitation) of a phonon or vibrational mode. The ⁷ F ₀ → ⁵ D ₂ excitation and ⁷ F ₂ ← ⁵ D ₀ emission regions exhibit large numbers of resolved vibronic lines with displacement-from-origin frequencies ranging from 22 to 1613 cm^-1. Vibronic transitions characterized by at least 48 different vibrational (or discrete phonon) frequencies are observed. Several intense vibronic lines are assigned to relatively low-frequency (< 200 cm^-1) vibrational modes associated with the octahedral EuO₆ coordination clusters, but the great majority of vibronic lines are assigned to vibrational modes associated with internal ligand (antipyrine) atomic motions. Several of the vibronic transitions promoted by internal ligand vibrations are nearly as intense as those promoted by EuO₆ moiety modes. The Eu(AP)₆I₃ and Eu(AP)₆(ClO₄)₃ systems exhibit identical vibronic structure in their spectra, and this structure is essentially independent of temperature between 295 and 10K (except for line narrowing and redistributions of relative line intensities attributable to changes in relative energy-level populations). However, loss of inversion symmetry at the europium sites appears to occur at T < 100 K in each system. No-phonon (origin) lines in the ⁷ F ₀ ↔ ⁵ D ₀, ⁷ F ₀ → ⁵ D ₂, and ⁷ F ₂ ← ⁵ D ₀ transition regions gradually exhibit (weak) electric-dipole intensity as sample temperature is lowered from ∼ 100 K to 10 K.