Intense millisecond-long red luminescence from heteroleptic Cu(I) 2,1,3-benzothiadiazole complexes

Intense red-emitting heteroleptic Cu(I) complexes were isolated using 2,1,3-benzothiadiazole (BTD) as N-donor ligand and triphenylphosphine, bis[(2-diphenylphosphino)phenyl] ether (DPEphos) or bis(diphenylphosphino)methane (dppm) as P-donors. The structures of two mononuclear and one dinuclear derivatives with one nitrogen and two phosphorus atoms in the Cu(I) coordination sphere were ascertained by means of X-ray diffraction, revealing an overall tetrahedral geometry at the solid state due to the direct interaction of the counterion with Cu(I). Upon excitation with UV and violet-blue light, the Cu(I) complexes exhibited emissions centred between 623 and 683 nm with photoluminescence quantum yields up to 46%. Lifetimes in the tens-thousands millisecond range were observed. The values resulted affected by the choice of the counterion, and an increase of up to 15% was observed passing from tetrafluoroborate to perchlorate in the mononuclear BTD complex with two PPh3 ligands. DFT calculations indicated that the lowest energy absorptions have charge transfer nature, from the {CuP2} fragments with the contributions of the P-bonded aryl substituents to an unoccupied π* orbital of BTD. The triplet→singlet emissions involve the metal centre, the phosphine ligands, and the occupied and empty orbitals of BTD.