Crystal−Amorphous−Crystal Transformation of a Photoluminescent Platinum(II) Complex: Solid-State Color Tuning via Dihedral-Angle Variations

Abstract

A new photoluminescent platinum(II) complex, cis-[PtCl₂(ppt)], where ppt = 2-(2-pyridyl)-4-phenylthiazole, has been synthesized and characterized. Two polymorphs of the complex were isolated with high phase purity: an orange crystalline phase O-Cryst and a yellow crystalline phase Y-Cryst, each exhibiting distinct solid-state photoluminescence. Partially amorphous phases derived from the respective polymorphs, O-Ground and Y-Ground, were prepared by mechanical grinding. Upon exposure to acetone vapor, both amorphous phases recrystallized exclusively to the O-Cryst. The structural difference among the crystalline and amorphous phases was investigated by powder X-ray diffraction and pair distribution function (PDF) analysis, with the latter clearly revealing the local molecular packing that is inaccessible by conventional diffraction techniques. Density functional theory calculations correlated the relative energies and molecular geometry in the respective solids with the observed photoluminescence color variations. The difference in the dihedral angles between the pyridyl and phenyl rings of the ppt ligand was found to strongly govern the availability of π-delocalized orbitals, thereby accounting for the polymorphism-dependent color variation. The stimuli-induced crystal−amorphous−crystal transformation described here provides a straightforward design principle for fine-tuning of the solid-state photoluminescence while emphasizing the enormous utility of PDF analysis for elucidating local structures and the mechanisms of phase transformation in molecular crystals.