Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design

The
design and detailed photophysical study of two novel Ir­(III)
complexes featuring mono- and dinuclear design are presented. Emission
quantum yield and decay times in solution are ΦPL = 90% and τ­(300 K) = 1.16 μs for the mononuclear complex 5, and ΦPL = 95% and τ­(300 K) = 0.44
μs for the dinuclear complex 6. These data indicate
an almost 3-fold increase in the phosphorescence rate for dinuclear
complex 6 compared to 5. Zero-field splitting
(ZFS) of the T1 state also increases from ZFS = 65 cm–1 for the mononuclear complex to ZFS = 205 cm–1 for the dinuclear complex and is accompanied by a drastic shortening
of the individual decay times of T1 substates. With the
help of TD-DFT calculations, we rationalize that the drastic changes
in the T1 state properties in the dinuclear complex originate
from an increased number of excited states available for direct spin–orbit
coupling (SOC) routes as a result of electronic coupling of Ir–Cl
antibonding molecular orbitals of the two coordination sites.