Why do the luminescence maxima of isostructural palladium(II) and platinum(II) complexes shift in opposite directions?

Abstract

Temperature-dependent luminescence spectra for a series of palladium(II) and platinum(II) complexes with thiocyanate, halide, and dithiocarbamate ligands are presented. All complexes show broad d−d luminescence. Crystal structures are reported for (n-Bu4N)2[Pt(SCN)4] and (n-Bu4N)2[Pd(SCN)4] at 150 and 250 K, for the palladium(II) dimethyldithiocarbamate (MeDTC) complex [Pd(MeDTC)2] at 150 and 300 K, and for its platinum(II) analog at 100 and 300 K. The structures of (n-Bu4N)2[Pt(SCN)4], (n-Bu4N)2[Pd(SCN)4], and [Pt(MeDTC)2] show similar volume increases with temperature. In contrast, the luminescence band maxima of palladium(II) and platinum(II) complexes have opposite shifts with increasing temperature. (n-Bu4N)2[Pd(SCN)4] shows a shift of −2.0 cm−1/K and [Pd(MeDTC)2] a shift of −1.1 cm−1/K, while both platinum(II) complexes have a positive shift of +1.6 cm−1/K. Calculated luminescence spectra with adjustable parameters reproduce the experimental spectra. The variation of their parameters with temperature shows the origin of different trends. Temperature-dependent luminescence spectra of [Pd(SCN)4]2− and [Pt(SCN)4]2− in polymer films of polyvinyl alcohol were measured. No clearcut shifts of maxima were observed for either compound, and their spectra are broader due to the disordered environment.