Mechanistic Insight into the Thermochromic Emission of One-Dimensional Platinum(II) and Palladium(II) Complex Crystals

Self-assembled Pt(II) complexes have attracted increasing interest because of their bright and colorful luminescence, as well as their stimuli-responsive properties resulting from metallophilic interactions. This review focuses on the temperature-responsive luminescent behavior (i. e., thermochromic emission) of self-assembled one-dimensional Pt(II) complexes from the viewpoint of the structure-photophysics relationship. The thermochromism of Pd(II) complexes, which have the same d⁸ electronic configuration as Pt(II) complexes, is also summarized to gain a better understanding of the detailed thermochromic emissions. The mechanism of the thermochromic emissions of Pt(II) and Pd(II) complexes can be understood on the basis of two main temperature-dependent factors: (i) the energy change of the assembly, which induces excited state delocalization over two or more molecules (i. e., excited oligomers), and (ii) the thermal equilibrium between these excited oligomers. The threshold for the metal⋅⋅⋅metal distance, at which the latter factor becomes more dominant, is also discussed.