Photoinduced Tautomerisation of ESIPT-Capable Iridium(III) Complexes with Rationally Designed Acyclic Diaminocarbene Ligands

A series of ESIPT-capable Ir^III-(acyclic diaminocarbene species) (ESIPT = Excited-state intramolecular proton transfer) exhibiting strong photoluminescence properties is described. The emission profile is strongly influenced by the nature of the azaheterocyclic fragment in the diaminocarbene ligand: pyrazine-derived species display phosphorescence bands red-shifted by approximately 100 nm compared to their pyridine analogues. This redshift is attributed to the luminescence of tautomerized species formed via an ESIPT process, wherein the iridium center enhances the basicity of the pyrazine ring, facilitating proton transfer from the C_carbene-NH groups. This interpretation is supported by the solvatochromic emission behavior of complexes prepared and corroborated by density functional theory calculations. Prepared Ir^III-(acyclic diaminocarbene species) complexes represent the first example of metal-organic luminophores in which the ESIPT mechanism involves direct participation of the metal center, resulting in orange emission.