A hallmark feature of cancer cells is the elevated need of transition metals such as iron and copper, which are attractive targets for therapeutic and diagnostic applications.1 Formazan-based compounds are widely used as ligands in coordination chemistry and can be delivered to cancer cells through reduction of tetrazolium cations.2 As such, tetrazolium compounds act as prochelators that are activated in the reducing environments typical of malignant cells. Previous work on this scaffold demonstrated iron(II) chelation, strong lipophilicity, and antiproliferative activity of these complexes in cancer through intracellular iron deprivation that triggered apoptosis.3 This project aims to develop formazanate transition metal complexes tailored for cancer imaging. Achieving optical penetration depth necessitates chemical sensors that respond at bathochromically shifted wavelengths. A library of formazan ligands featuring a water-solubilizing carboxylate group was characterized and their iron(II) and copper(II) affinity constants were investigated. The photophysical properties were rationalized through time-dependent density functional theory. The main insight from this analysis is that flatter coordination modes result in planar, extended π systems that have lower optical transition energy and thus red-shifted optical parameters. This work extends the original iron chelation strategy to copper sequestration to achieve square planar coordination modes. The structure-activity relationships were used to develop a new series of formazan ligands to explore new coordination modes, electronic structures, and improve aqueous solubility. Cytotoxicity and prochelator activation via reduction were studied in cultured cancer cells. Preliminary results show near-infrared absorbtion for copper(II) formazanate complexes. This work suggests that formazan ligands can be viable biomedical imaging agents for the study of cancer biology and future diagnostic applications.