Synthesis and characterization of Cu(I) isocyanide complexes exhibiting reversible luminescence

Herein, we describe the synthesis of Cu(I) isocyanide complexes, namely, the [CuI(PDI)] (PDI = 1,4-phenylene diisocyanide) dimer and [Cu2I2(PPh3)2(PDI)2] (PPh3 = triphenylphosphine), which exhibit weak orange (quantum yield Φ = 1%) and intense pale blue (Φ = 13%) emissions in the solid state under UV irradiation, respectively. Upon grinding, the luminescence of the [CuI(PDI)] dimer does not change, whereas that of [Cu2I2(PPh3)2(PDI)2] changes to a weak olive (Φ = 4%) emission. Treatment of the ground [Cu2I2(PPh3)2(PDI)2] with an organic solvent and subsequent drying restore its original pale blue emission, which is indicative of reversible luminescent mechanochromism. Moreover, both the [CuI(PDI)] dimer and [Cu2I2(PPh3)2(PDI)2] exhibit thermochromism, i.e., their emissions change to a very intense green emission at 77 K. In particular, time-dependent density functional theory calculations reveal that [Cu2I2(PPh3)2(PDI)2] could be assigned to luminescence induced by halide-to-ligand charge transfer.