The present invention is a new class of fluorescent sensor with unprecedented sensitivity, selectivity and synthetic efficiency for the identification and qualitative and quantitative detection of transition metal ions. The chemosensing properties are based on an internal charge transfer mechanism or photo-induced electron transfer and are effective in vitro. The chemosensor comprises one or more electron donating dipyrrin functionalities linked to an electron withdrawing functionality. The chemosensor forms a stable transition metal complex through internal charge transfer between the electron donating functionality and the target transition metal, which results in measurable fluorescence. Specific examples for sensing zinc and copper ions are described.