Comparing the Photophysical Properties of Bridged and Unbridged Platinum(II) Cyclometalated Complexes

Platinum(II) complexes have been thoroughly researched for applications in bioimaging and optoelectronics, often searching for potential uses after observing a complex's unique properties. Here, we investigate the role of the bridging atom on the resulting photophysical properties, comparing two tetradentate complexes bearing O and N elemental bridges between 2-phenylpyridine (phpy) units to the well-known unbridged Pt(phpy)₂. Unlike the O-bridged complex, single-crystal X-ray diffraction demonstrated no Pt···Pt interactions for the N-bridged complex in the solid state. As anticipated, the introduction of a bridging atom turns on room-temperature phosphorescence. Absorption and emission spectroscopy combined with density functional theory (DFT) calculations explained that the blue-shifted spectra of the O-bridged complex are a result of HOMO stabilization by the more electronegative bridging atom. The minor differences were found to result in macroscopic differences in the properties of the bridged complexes in the solid state and when aggregated. Notably, the N-bridged complex experienced significant aggregation-caused quenching, whereas the O-bridged complex displayed red-to-green aggregation-induced emission switching resulting from excimer emission. In this work, we investigate the photophysical properties of the core structures for a popular family of Pt complexes in hopes of streamlining the rational design of new functional molecules.