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

A dinuclear
iridium­(III) complex IrIr shows dual emission
consisting of near infrared (NIR) phosphorescence (λmax = 714 nm, CH2Cl2, T = 300
K) and green fluorescence (λmax = 537 nm). The NIR
emission stems from a triplet state (T1) localized on the
ditopic bridging ligand (3LC). Because of the dinuclear
molecular structure, the phosphorescence efficiency (ΦPL = 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 (S1) of 1LC character. The occurrence
of fluorescence is ascribed to relatively slow intersystem crossing
(ISC) from state S1 (1LC) to the triplet manifold.
The measured ISC rate corresponds to a time constant τISC 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. IrIr is internalized by live HeLa cells as evidenced by confocal luminescence
microscopy.