Photosensitizer in a molecular bowl: Steric protection enhancing the photonuclease activity of copper(II) scorpionates

Steric encumbrance caused by the tripodal ligand in the ternary tris(3-phenylpyrazolyl)borate copper(II) heterocyclic base complexes [Cu(B)(Tp(Ph))](CIO4) (B = dipyridoquinoxaline, dipyridophenazine) leads to efficient cleavage of supercoiled DNA to its relaxed form upon exposure to red light at 632.8 and 694 nm as a result of protection of the photosensitizer in the molecular bowl of the {Cu(Tp(Ph))} moiety, which generates singlet oxygen as the reactive species in a type-II process.