On the importance of equatorial metal-centered excited states in the photophysics of cyclometallated Ir(iii) complexes

In the present contribution, the following three cyclometallated Ir(iii) complexes were theoretically investigated using density functional theory calculations to explain their different photophysical properties: [Ir(ppy)(2)(bpy)](+), where Hppy is 2-phenylpyridine and bpy is 2,2 & PRIME;-bipyridine, [Ir(ppy)(2)(pbpy)](+), where pbpy is 6-phenyl-2,2 & PRIME;-bipyridine, and [Ir(ppy)(2)(dpbpy)](+), where dpbpy is 6,6 & PRIME;-diphenyl-2,2 & PRIME;-bipyridine. Despite sharing the same molecular skeleton, with the only difference being the addition of one or two phenyl groups attached to the ancillary bpy ligand, the complexes show different emission quantum yields in CH2Cl2 solution (0.196, 0.049 and 0.036, respectively). Such a behavior was previously justified as a consequence of a different ability to non-radiatively decay through an axial metal-centered (MC) triplet state. In the present contribution, a new non-radiative decay path has been characterized to be mediated by the so-called equatorial MC states, in which an Ir-N-bpy bond is elongated instead of an Ir-N-ppy bond as observed in the axial MC states. The decay path involving the equatorial MC states is more favorable than that associated with the axial MC states, and the different ability to decay through the former better explains the photoemission properties exhibited by the three complexes.