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 Phi(PL) = 90% and tau(300 K) = 1.16 mu s for the mononuclear complex 5, and Phi(PL) = 95% and tau(300 K) = 0.44 mu s 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 T-1 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 T-1 substates. With the help of TD-DFT calculations, we rationalize that the drastic changes in the T-1 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.