Study of the\nPhotophysical Properties and the DNA\nBinding of Enantiopure [Cr(TMP)<sub>2</sub>(dppn)]<sup>3+</sup> Complex

The preparation,\nelectrochemistry and photophysical properties\nof a heteroleptic chromium(III) polypyridyl complex <i>rac-</i>[Cr(TMP)<sub>2</sub>(dppn)]<sup>3+</sup> (<b>1</b>) containing\ntwo 3,4,7,8-tetramethyl-1,10-phenanthroline (TMP) ligands and the\nπ-extended benzodipyrido[3,2-a:2′,3′-<i>c</i>]phenazine (dppn) ligand are reported. The visible absorption spectrum\nof <b>1</b> reveals distinct bands between 320 and 420 nm characteristic\nof dppn-based ligand-centered transitions, with <b>1</b> found\nto be nonemissive in aqueous solution but weakly luminescent in aerated\nacetonitrile solution. Transient visible absorption (TrA) spectroscopy\nreveals that 400 nm excitation of <b>1</b> leads to initial\npopulation of a ligand-to-metal charge transfer (LMCT) state which\nevolves within tens of ps to form a dppn-localized intraligand (<sup>3</sup>IL) state which persists for longer than 7 ns and efficiently\nsensitizes singlet oxygen. Chiral resolution and DNA binding of the\nlambda and delta enantiomers of <b>1</b> to four different DNA\nsystems is reported. In all cases the lambda enantiomer shows greater\naffinity for DNA and in particular AT-rich DNA. Thermal denaturation\nreveals that the lambda enantiomer stabilizes the DNA more. There\nis also a greater stabilization of the AT-containing DNA sequences\ncompared to GC DNA.