Revisiting Cu(I)‐Dppm Chemistry: Bright Thermally Activated Delayed Fluorescence Luminescence, Anion Dependance, Mechanochromism, and Singlet Oxygen Generation of a Dinuclear Cu(I) Homoleptic Complex

The one‐step syntheses of highly luminescent Cu(I) dimetallic complexes are reported using Cu(I) salts and the commercially available dppm ligand. Three crystalline phases A – C , obtained either with PF 6 − ( A and B : [Cu 2 (μ 2 ‐dppm) 3 ](PF 6 ) 2 ) or TEF − ( C : [Cu 2 (μ 2 ‐dppm) 3 ](TEF) 2 ) anions, display distinct conformations and crystal packings that strongly influence photophysical properties. All phases show intense solid‐state thermally activated delayed fluorescence with room temperature quantum yields of nearly 100%. Emission energies and decay times are modulated by packing effects. Phases A and B exhibit pronounced mechanochromic luminescence upon gentle grinding, while phase C is mechanically insensitive, presumably due to its bulky and flexible counter‐anions. Density functional calculation/time‐dependent density functional calculations suggest that crystal packing impacts the structural molecular relaxation, in agreement with mechanochromic behaviors. In aerated CH 2 Cl 2 , these complexes generate singlet oxygen with quantum yield of up to 30%, highlighting their potential as photosensitizers.