Mesomorphism and Luminescence Properties of Platinum(II) Complexes with Tris(alkoxy)phenyl-Functionalized Pyridyl Pyrazolate Chelates

A series of new mesomorphic platinum(II) complexes 1-4 bearing pyridyl pyrazolate chelates are reported herein. In this approach, pyridyl azolate ligands have been strategically functionalized with tris(alkoxy)phenyl groups with various alkyl chain lengths. As a result, they are ascribed to a class of luminescent metallomesogens that possess distinctive morphological properties, such as their intermolecular packing arrangement and their associated photophysical behavior. In CH₂Cl₂, independent of the applied concentration in the range 10^-6-10^-3M, all Pt^II complexes exhibit bright  phosphorescence centered at around 520nm, which is characteristic for monomeric Pt^II complexes. In stark contrast, the single-crystal X-ray structure determination of [Pt(C4pz)₂] (1) shows the formation of a dimeric aggregate with a notable Pt⋯Pt contact of 3.258 Å. Upon heating, all Pt^II complexes 1-4 melted to form columnar suprastructures, for which similar intracolumnar Pt⋯Pt distances of approx. 3.4-3.5 Åare observed within an exceptionally wide temperature range (>250°C), according to the powder XRD data. Upon casting into a neat thin film at RT, the luminescence of 1-4 is dominated by a red emission that spans 630-660nm, which originates from the one-dimensional, chainlike structure with Pt-Pt interaction in the ground state. Taking complex 4 as a representative, the emission intensity and wavelength were significantly decreased and blueshifted, respectively, on heating from RT to 250°C. Further heating to liquefy the sample alters the red emission back to the green phosphorescence of the monomer. The results highlight the pivotal role of tris(alkoxy)phenyl groups in the structural versus luminescence behavior of these Pt^II complexes.