Study of the Photophysical Properties and the DNA Binding of Enantiopure [Cr(TMP)₂(dppn)]³⁺ Complex

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