Luminescent dinuclear Cu(I) complexes, [Cu2X2(dpypp)(2)] [Cu-X; X = Cl, Br, I; dpypp = 2,2'(phenylphosphinediyl)dipyridine], were successfully synthesized by a solvent-assisted mechanochemical method. A trace amount of the assisting solvent plays a key role in the mechanochemical synthesis; only two solvents possessing the nitrile group, CH3CN and PhCN, were effective for promoting the formation of dinuclear Cu-X. X-ray analysis revealed that the dinuclear structure with no Cu center dot center dot center dot Cu interactions, bridged by two dpypp ligands, was commonly formed in all Cu-X species. These complexes exhibited bright green emission in the solid state at room temperature (phi = 0.23, 0.50, and 0.74; lambda(em) = 528, 518, and 530 nm for Cu-Cl, Cu-Br, and Cu-I, respectively). Emission decay measurement and TD-DFT calculation suggested that the luminescence of Cu-X could be assigned to phosphorescence from the triplet metal-to-ligand charge-transfer ((MLCT)-M-3) excited state, effectively mixed with the halide-to-ligand charge-transfer ((XLCT)-X-3) excited state, at 77 K. The source of emission changed to thermally activated delayed fluorescence (TADF) with the same electronic transition nature at room temperature. In addition, the CH3CN-bound analogue, [Cu-2(CH3CN)(2)(dpypp)(2)](BF4)(2), was successfully mechanochemically converted to Cu-X by grinding with solid KX in the presence of a trace amount of assisting water.