2-(4-dimethylaminophenyl)benzothiazolate (MeN-pbt)-cyclometalated platinum complexes containing four different picolinate ligands, [Pt(MeN-pbt)(R-pic-κ)] (R = 3-H , 3-NH , 3-OH , 4-COOH ) were prepared and examined for their photophysical properties, singlet oxygen production, and bioactivity. X-ray studies of and ·CHCl revealed aggregation to give 1D infinite chains. They showed phosphorescent emissions essentially associated with metal-perturbed ILCT excited states in - and mixed LL'CT/ILCT (L = MeN-pbt, L' = pic) in , in agreement with theoretical calculations. Their tendency to self-assemble was demonstrated in films (, ) and DMSO/HO (). Complexes - showed singlet oxygen photosensitization quantum yields (ϕ O) in the range of 13-17%. The biological activity toward selected cell lines in dark conditions and under 5 min irradiation at 450 nm was tested. Complex showed the highest phototoxicity with up to 10 times improvement of the antiproliferative activity upon irradiation, with EC values in the nanomolar range, related to overproduction of ROS in dark conditions, further enhanced upon irradiation. Complexes - did not bind to DNA, while the most potent complex demonstrated interaction with BSA and photooxidation of NADH. Finally, intracellular dose-dependent ROS production in MDA-MB-231 cells treated with was observed in the dark and further stimulated upon blue light irradiation.