Thermoresponsive Platinum(II) 2,6-Di(pyrid-2-yl)pyrazine Complexes with Unusual Aggregation Behavior upon Heating

A series of alkynylplatinum(II) complexes with 2,6-di(pyrid-2-yl)pyrazine ligand has been synthesized and characterized. Interestingly, the complexes have been found to exhibit unprecedented thermochromic behavior with absorption maxima shifting to the longer-wavelength region upon increasing temperature. This phenomenon is significantly different from that in conventional platinum(II) systems, where an elevated temperature typically promotes the dissociation of aggregates and diminishes MMLCT absorptions. Transmission electron microscopy (TEM) and variable-temperature ¹H NMR spectroscopy suggest a thermoresponsive morphological transformation behavior from rods to circular morphologies, driven by the preferential formation of π-π and Pt···Pt interactions over hydrogen bonding interactions upon the disruption of hydrogen bonding at high temperature, as supported by solution-state Fourier transform infrared (FT-IR) spectroscopy together with computational studies. This unusual optical and morphological transformation is rarely observed, as it represents a stark contrast to the deaggregation behavior upon increasing temperature typical of the control and commonly found platinum(II) complexes and organic compounds.