Near Infrared Phosphorescent Dinuclear Ir(III) Complex Exhibiting Unusually Slow Intersystem Crossing and Dual Emissive Behavior

A dinuclear iridium(III) complex Irk shows dual emission consisting of near infrared (NIR) phosphorescence (lambda(max )= 714 nm, CH2Cl2, T = 300 K) and green fluorescence (lambda(max) = 537 nm). The NIR emission stems from a triplet state (T-1) localized on the ditopic bridging ligand ((LC)-L-3). Because of the dinuclear molecular structure, the phosphorescence efficiency (Phi(PL) = 3.5%) is high compared to those of other known red/NIR-emitting iridium complexes. The weak fluorescence stems from the lowest excited singlet state (S-1) of (LC)-L-1 character. The occurrence of fluorescence is ascribed to relatively slow intersystem crossing (ISC) from state S-1 ((LC)-L-1) to the triplet manifold. The measured ISC rate corresponds to a time constant tau(ISC) of 2.1 ps, which is an order of magnitude longer than those usually found for iridium complexes. This slow ISC rate can be explained in terms of the LC character and large energy separation (0.57 eV) of the respective singlet and triplet excited states. Irk is internalized by live HeLa cells as evidenced by confocal luminescence microscopy.