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With the aim of developing photostable near-infrared cell imaging probes, a convenient route to the synthesis of heteroleptic Os II complexes containing the Os(TAP) 2 fragment is reported. This method was used to synthesize the dinuclear Os II complex, [{Os(TAP) 2 } 2 tpphz] 4+ (where tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2''-h:2‴,3'''-j]phenazine and TAP = 1,4,5,8- tetraazaphenanthrene). Using a combination of resonance Raman and time-resolved absorption spectroscopy, as well as computational studies, the excited state dynamics of the new complex were dissected. These studies revealed that, although the complex has several close lying excited states, its near-infrared, NIR, emission (λ max = 780 nm) is due to a low-lying Os → TAP based 3 MCLT state. Cell-based studies revealed that unlike its Ru II analogue, the new complex is neither cytotoxic nor photocytotoxic. However, as it is highly photostable as well as live-cell permeant and displays NIR luminescence within the biological optical window, its properties make it an ideal probe for optical microscopy, demonstrated by its use as a super-resolution NIR STED probe for nuclear DNA.