Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

The synthesis and
photophysical characterization of two osmium(II)
polypyridyl complexes, [Os(TAP)2dppz]2+ (1) and [Os(TAP)2dppp2]2+ (2) containing dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene)
ancillary ligands, are reported. The complexes exhibit complex electrochemistry
with five distinct reductive redox couples, the first of which is
assigned to a TAP-based process. The complexes emit in the near-IR
(1 at 761 nm and 2 at 740 nm) with lifetimes
of >35 ns with a low quantum yield of luminescence in aqueous solution
(∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT
and GC oligodeoxynucleotides with high affinities. In the presence
of natural DNA, the visible absorption spectra are found to display
significant hypochromic shifts, which is strongly evident for the
ligand-centered π–π* dppp2 transition at 355 nm,
which undergoes 46% hypochromism. The emission of both complexes increases
upon DNA binding, which is observed to be sensitive to the Δ
or Λ enantiomer and the DNA composition. A striking result is
the sensitivity of Λ-2 to the presence of AT DNA,
where a 6-fold enhancement of luminescence is observed and reflects
the nature of the binding for the enantiomer and the protection from
solution. Thermal denaturation studies show that both complexes are
found to stabilize natural DNA. Finally, cellular studies show that
the complexes are internalized by cultured mammalian cells and localize
in the nucleus.