Structural and Photophysical Differences in Crystalline Trigonal Planar Copper Iodide Complexes with 1,2-Bis(methylpyridin-2-yl)disilane Ligands

We synthesized trigonal planar Cu(I) iodide complexes with 1,2-bis(methylpyridin-2-yl)disilane ligands L1–L4 and investigated how the substitution position of the methyl group on the pyridine ring in σ–π conjugation affects their structure and physical properties. The structures were characterized by NMR, elemental analysis, and single-crystal X-ray diffraction. In the crystalline state, the methylpyridyl groups of CuIL1–CuIL3 were coordinated with Cu(I) in an anticlinal conformation relative to the Si–Si σ bond, whereas those of CuIL4 were coordinated with Cu(I) in a synperiplanar conformation relative to the Si–Si σ bond. The conformational difference in the crystalline state was influenced by the N−Cu−N bite angle and the emission wavelength. CuIL1–CuIL3 exhibited blue-green emission (λem: 476–494 nm), and CuIL4 exhibited green-yellow emission (λem: 512 nm) with high emission quantum yields (Φ: 0.59–0.86) in the crystalline state at 293 K. These Cu(I) complexes exhibited thermally activated delayed fluorescence from the S1 state at 293 K and phosphorescence from the T1 state at 77 K in the crystalline state. The optical properties in the crystalline state were discussed by DFT and TD-DFT calculations. These complexes also displayed aggregation-induced emission in THF–water solution (fw > 80%), although they did not show emission in dehydrated THF.