Probe 1 has been synthesized as the first ratiometric fluorescent cobalt probe, and it exhibits several highly favorable features, in particular a remarkable ratiometric fluorescence response at two wavelengths. Importantly, the electronic energy transfer (EET) efficiency of the probe is modulated by the energy acceptor molar absorptivity variations on cobalt binding, which then transforms to a large ratiometric fluorescence response at two wavelengths. This signaling mechanism should open a new avenue for the development of powerful ratiometric probes with large emission signal ratios at two wavelengths for exciting applications in many fields.