Towards Water Soluble Mitochondria-Targeting Theranostic Osmium(II) Triazole-Based Complexes

The complex [Os(btzpy)2][PF6]2 (1, btzpy = 2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine) has
been prepared and characterised. Complex 1 exhibits phosphorescence (λem = 595 nm, τ = 937 ns,
φem = 9.3% in degassed acetonitrile) in contrast to its known ruthenium(II) analogue, which is
non-emissive at room temperature. The complex undergoes significant oxygen-dependent quenching
of emission with a 43-fold reduction in luminescence intensity between degassed and aerated
acetonitrile solutions, indicating its potential to act as a singlet oxygen sensitiser. Complex 1
underwent counterion metathesis to yield [Os(btzpy)2]Cl2 (1
Cl), which shows near identical optical
absorption and emission spectra to those of 1. Direct measurement of the yield of singlet oxygen
sensitised by 1
Cl was carried out (φ (
1O2) = 57%) for air equilibrated acetonitrile solutions. On the
basis of these photophysical properties, preliminary cellular uptake and luminescence microscopy
imaging studies were conducted. Complex 1
Cl readily entered the cancer cell lines HeLa and U2OS
with mitochondrial staining seen and intense emission allowing for imaging at concentrations as
low as 1 µM. Long-term toxicity results indicate low toxicity in HeLa cells with LD50 >100 µM.
Osmium(II) complexes based on 1 therefore present an excellent platform for the development of
novel theranostic agents for anticancer activity.