Steric Engineering of Cyclometalated Pt(II) Complexes toward High-Contrast Monomer–Excimer-Based Mechanochromic and Vapochromic Luminescence

Ligands play a crucial role in the supramolecular photoluminescence properties of Pt(II) square-planar complexes. To improve the luminescence color responses of N∧C∧N cyclometalated Pt(II) complexes to external stimuli such as mechanical stress and chemical vapors, we have conducted a steric engineering of the previous systems 1a – 1d [ Inorg. Chem. 2017, 56, 4978–4989] by introducing two tert -butyl groups to the tridentate ligand to form complexes 2a – 2c . Unlike the “too low” or “too high” steric hindrance of the NCNPt core in 1a – 1d, the combined steric effects of the tert -butyl groups at one side and the pentiptycene group at the other side of the NCNPt core in 2b are “just right” for generating as-prepared powders with pure monomer (green) emission or pure excimer (red) emission, depending on the rate of precipitation from solutions. The synergistic steric effects are also beneficial to the solid-state luminescence quantum efficiency (30–36%). As a result of the differences in steric interactions and thus in the relative monomer vs excimer emission intensity, each complex of 2a – 2c performs a two-step luminescence mechanochromism and vapochromism with different color patterns. This work provides an intriguing example of steric engineering of Pt(II) complexes toward highly emissive molecular solids with high-contrast mechanochromic and vapochromic luminescence.