Dipyridoquinoxaline (dpq) complexes of bivalent 3d-metal ions, viz., [Fe-II(dpq)(3)](PF6)(2) (1), [Co-II(dpq)(3)](ClO4)(2) (2), [Ni-II(dpq)(3)](ClO4)(2) (3). [Cu-II(dpq)(2)(H2O)](ClO4)(2) (4), [Zn-II(dpq)(3)](ClO4)(2) (5), and [Zn-II(dpq)(2)(DMF)(2)](ClO4)(2) (5a) (DMF = N,N-dimethylformamide), are prepared and their photoinduced DNA cleavage activity studied. Structural characterization for the complexes 1 and 5a is done by single-crystal X-ray crystallography. All the complexes show efficient binding propensity to calf thymus DNA with a binding constant (K) value of similar to 10(5) M-1. Complexes 1, 2, and 4 show metal-based cyclic voltammetric responses at 1.2, 0.4, and 0.09 V (vs SCE) in DMF 0.1 M [(Bu4N)-N-n](ClO4) assignable to the respective Fe-III/Fe-II, Co-III/Co-II, and Cu-II/Cu-I couples. The Ni-II and Zn-II complexes do not show any metal-based redox process. The dpq-based reductions are observed in the potential range of -1.0 to -1.7 V vs SCE. DNA melting and viscosity data indicate the groove-binding nature of the complexes. Control experiments using distamycin-A suggest a minor groove-binding propensity of the complexes. The complexes exhibit photoinduced cleavage of supercoiled pUC19 DNA in UV light of 365 nm. The diamagnetic d(6)-Fe-II and d(10)-Zn-II complexes are cleavage-inactive on irradiation with visible light. The paramagnetic d(7)-Co-II and d(9)-Cu-II complexes exhibit efficient DNA cleavage activity on photoirradiation at their respective d-d band. The paramagnetic d(8)-Ni-II. complex displays only minor DNA cleavage activity on irradiation at its d-d band. The DNA cleavage reactions at visible light under aerobic conditions involve the formation of hydroxyl radical. The Co-II complex shows photocleavage of DNA under an argon atmosphere. Theoretical calculations on the complexes suggest a photoredox pathway in preference, to a type-2 process forming singlet oxygen for the visible-light-induced DNA cleavage activity of the 3d-metal complexes. The theoretical data also predict that the photoredox pathway is favorable for the 3d(7)-Co-II and 3d(9)-Cu-II complexes to exhibit DNA cleavage activity, while the analogous 3d(6)-Fe-II and 3d(8)-Ni-II complexes are energetically unfavorable for the exhibition of such activity under visible light. The Co-II and Cu-II complexes are better suited for designing and developing new metal-based PDT agents than their cleavage-inactive Fe-II, Ni-II, and Zn-II analogues.