Polymorphism and Mechanochromism in 2-Phenylbenzothiazole Cyclometalated PtII Complexes with Chelating NO Ligands

New cyclometalated PtII complexes with 2-phenylbenzothiazole (pbt) and two different picolinate ligands [Pt(pbt)(R-pic-KN,O)] (R = H (1), OH (2)) were prepared. In contrast to 1, the OH substituent group on 2 allows modulation of the packing in the solid state through donor-acceptor H-bonding interactions with the CH2Cl2 solvent. Thus, three pseudopolymorphs of 2 with different aggregation degrees were isolated, including yellow 2-Y, orange-red 2-R (2 center dot 0.5CH2Cl2) and black 2B (2 center dot 0.75CH2Cl2) with emissions at 540, 656, and 740 nm, respectively, in the solid state at 298 K. 2-R and 2-B can be transformed to the pristine solid 2. Studies of their crystal structures show that 1 and 2-Y stack in columns with only K center dot center dot center dot K stacking interactions, whereas 2-R displays strong aggregated 1D infinite chains based on Pt center dot center dot center dot Pt and K center dot center dot center dot K stacking interactions, consistent with the colors and the photophysical properties, measured in several media. Interestingly, 1 and 2 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 center dot center dot center dot 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 K center dot center dot center dot K stacking among chelates also exerts a strong influence in the metal-metal-to-ligand charge transfer transition character.