Polymorphism and Mechanochromism in 2-Phenylbenzothiazole Cyclometalated Pt^II Complexes with Chelating N^∧O Ligands

Abstract

New cyclometalated Pt^II complexes with 2-phenylbenzothiazole (pbt) and two different picolinate ligands [Pt(pbt)(R-pic-κ<i>N,O</i>)] (R = H (<b>1</b>), OH (<b>2</b>)) were prepared. In contrast to <b>1</b>, the OH substituent group on <b>2</b> allows modulation of the packing in the solid state through donor-acceptor H-bonding interactions with the CH₂Cl₂ solvent. Thus, three pseudopolymorphs of <b>2</b> with different aggregation degrees were isolated, including yellow <b>2-Y</b>, orange-red <b>2-R</b> (<b>2·0.5CH</b>_<b>2</b><b>Cl</b>_<b>2</b>) and black <b>2-B</b> (<b>2·0.75CH</b>_<b>2</b><b>Cl</b>_<b>2</b>) with emissions at 540, 656, and 740 nm, respectively, in the solid state at 298 K. <b>2-R</b> and <b>2-B</b> can be transformed to the pristine solid <b>2</b>. Studies of their crystal structures show that <b>1</b> and <b>2-Y</b> stack in columns with only π···π stacking interactions, whereas <b>2-R</b> displays strong aggregated 1D infinite chains based on Pt···Pt and π···π stacking interactions, consistent with the colors and the photophysical properties, measured in several media. Interestingly, <b>1</b> and <b>2</b> exhibit reversible mechanochromic behavior with high contrast in the color and color emission upon mechanical grinding due to a phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Theoretical calculations indicate that Pt···Pt contacts are more relevant in the trimers and tetramers than in the dimers, particularly in their T₁ states, associated with a change from a ³IL/³MLCT transition in the monomer to ³MM(L+L')CT in the oligomers. Noncovalent interaction (NCI) theoretical studies indicate that the π···π stacking among chelates also exerts a strong influence in the metal-metal-to-ligand charge transfer transition character.