DNA Cleavage on Photoexposure at the d−d Band in Ternary Copper(II) Complexes Using Red-Light Laser

Ternary copper(II) complexes [Cu(L1)B](ClO4) (1, 2) and [Cu(L2)B](ClO4) (3, 4), where HL1 and HL2 are tridentate
NSO- and ONO-donor Schiff bases and B is a heterocyclic base, viz. dipyrido[3,2-d:2‘,3‘-f]quinoxaline (dpq, 1 and
3) or dipyrido[3,2-a:2‘,3‘-c]phenazine (dppz, 2 and 4), were prepared and their DNA binding and photoinduced
DNA cleavage activity studied. Complex 1, structurally characterized by single-crystal X-ray crystallography, shows
an axially elongated square-pyramidal (4 + 1) coordination geometry in which the monoanionic L1 binds at the
equatorial plane. The NN-donor dpq ligand exhibits an axial−equatorial binding mode. The complexes display
good binding propensity to calf thymus DNA, giving a relative order 2 (NSO-dppz) > 4 (ONO-dppz) > 1 (NSO-dpq)
> 3 (ONO-dpq). They cleave supercoiled pUC19 DNA to its nicked circular form when treated with 3-mercaptopropionic
acid (MPA) by formation of hydroxyl radicals as the cleavage active species under dark reaction conditions. The
photoinduced DNA cleavage activity of the complexes was investigated using UV radiation of 365 nm and red light
of 633, 647.1, and 676.4 nm (CW He−Ne and Ar−Kr mixed gas ion laser sources) in the absence of MPA. Complexes
1 and 2, having photoactive NSO-donor Schiff base and dpq/dppz ligands, show dual photosensitizing effects
involving both the photoactive ligands in the ternary structure with significantly better cleavage properties when
compared to those of 3 and 4, having only photoactive dpq/dppz ligands. Involvement of singlet oxygen in the
light-induced DNA cleavage reactions is proposed. A significant enhancement of the red-light-induced DNA cleavage
activity is observed for the dpq and dppz complexes containing the sulfur ligand when compared to their earlier
reported phen (1,10-phenanthroline) analogue. Enhancement of the cleavage activity on photoexposure at the d−d
band indicates the occurrence of metal-assisted photosensitization processes involving the LMCT and d−d band
in the ternary structure.