Oxovanadium(iv) complexes [VOCl(B)(2)]Cl (1-3) of phenanthroline bases (B), viz. 1,10-phenanthroline (phen in ), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq in ) and dipyrido[3,2-a:2',3'-c]phenazine (dppz in ), have been prepared, characterized and their DNA and protein binding, photo-induced DNA and protein cleavage activity and photocytotoxicity have been studied. Complex , structurally characterized by X-ray crystallography, shows the presence of a vanadyl group in VOClN(4) coordination geometry. The dpq ligand displays a chelating mode of binding with a N-donor site trans to the oxo-group. The chloride ligand is cis to the oxo-group. The one-electron paramagnetic complexes show a d-d band near 715 nm in 15% DMF-Tris-HCl buffer. The complexes are redox active exhibiting a V(iv)/V(iii) redox couple within -0.5 to -0.7 V vs. SCE in 20% DMF-Tris-HCl/0.1 M KCl. The complexes bind to calf thymus (CT) DNA in the order: (dppz) > (dpq) > (phen). The binding data reveal the groove and/or partial intercalative DNA binding nature of the complexes. The complexes show "chemical nuclease" activity in the dark in the presence of 3-mercaptopropionic acid or hydrogen peroxide via a hydroxyl radical pathway. The dpq and dppz complexes are efficient photocleavers of DNA in UV-A light of 365 nm forming reactive singlet oxygen ((1)O(2)) and hydroxyl radical ( OH) species. Complexes and also show DNA cleavage activity in red light (>750 nm) by an exclusive OH pathway. The complexes display a binding propensity to bovine serum albumin (BSA) protein giving K(BSA) values in the range of 7.1 x 10(4)-1.8 x 10(5) M(-1). The dppz complex shows BSA and lysozyme protein cleavage activity in UV-A light of 365 nm via OH pathway. The dppz complex exhibits significant PDT effect in human cervical cancer HeLa cells giving IC(50) values of 1.0 microM and 12.0 microM in UV-A and visible light, respectively (IC(50) = >100 microM in the dark).