Temperature and pressure variations of d–d luminescence band maxima of bis(pyridylalkenolato)palladium(ii) complexes with different ligand substituents: opposite-signed trends

Abstract

Luminescence spectra of two d⁸-configured bis(pyridylalkenolato)palladium(II) complexes, [Pd{PyCHC(C₃F₇)O}₂] and [Pd{PyCHC(CH₃)O}₂], are presented at variable temperature and pressure. Bands are assigned as d–d transitions. The heptafluoropropyl and methyl substituents on the ligands have different steric demands, influencing luminescence spectra. Broad bands with maxima at approximately 12 700 cm⁻¹ (790 nm) for ligands with heptafluoropropyl substituents and 12 100 cm⁻¹ (830 nm) for ligands with methyl substituents and widths of approximately 2100 cm⁻¹ for both complexes are observed at 80 K. Quenching of the luminescence is observed as temperature increases. The maxima of [Pd{PyCHC(C₃F₇)O}₂] show a shift of −0.9 ± 0.1 cm⁻¹ K⁻¹ due to broadening of the spectra to lower energy. The luminescence maxima of [Pd{PyCHC(CH₃)O}₂] shift in the opposite direction by +7.2 ± 0.7 cm⁻¹ K⁻¹. Shifts with different signs are also obtained from variable-pressure luminescence spectra, with values of +13 ± 2 cm⁻¹ kbar⁻¹ and −15 ± 7 cm⁻¹ kbar⁻¹ for [Pd{PyCHC(C₃F₇)O}₂] and [Pd{PyCHC(CH₃)O}₂], respectively. The pressure-induced decrease is unusual and likely caused by intermolecular interactions involving the palladium(II) center and a vinylic proton of a neighboring complex.