Room-temperature luminescence from Pd(ii) and Pt(ii) complexes: from mechanochromic crystals to flexible polymer matrices

Abstract

A series of Pd(II) (PdL_OMe, PdL_OHex) and Pt(II) (PtL_OMe, PtL_OHex) complexes bearing tetradentate ligands as dianionic luminophores were synthesized. Hence, the cyclometallating chelators were alternatively decorated with two n-hexyloxy (L_OHex) or two methoxy (L_OMe) moieties to promote crystallization and processability. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry as well as by single crystal X-ray diffractometric analysis (PtL_OMe and PdL_OMe). Steady state and time-resolved photoluminescence spectroscopic studies were carried out in crystalline phases, in fluid solutions at room temperature, in frozen glassy matrices at 77 K and in a flexible polymeric matrix (PMMA). PtL_OMe presents an intriguing mechanochromism resulting from the responsive metal–metal interactions involving adjacent monomeric units. Incorporation of the Pd(II) complexes into the polymeric matrix boosts their photophysical properties by stiffening of the coordination environment while reducing non-radiative deactivation pathways mediated by dissociative metal-centred states, which also become thermally inaccessible at 77 K.