An ab initio multiconfigurational(CASPT2//CASSCF)approach has been employed to map radiative and nonradiative relaxation pathways for a cyclam-methylbenzimidazole fluorescent sensor and its complexes with Zn2+,Cd2+ and Cu2+ ions to provide an universal understanding of ON-OFF fluorescent mechanisms for the selective identification of these metal ions.A new SCT(1nπ*)state has been determined for the first time to originate from electron communication between the lone pair of the bridging atom and π* of the signaling unit,which is a typical 1nπ* transition but exhibits a significant charge transfer character and zwitterionic radical configuration.The competitive coexistence between radiative decay of the 1ππ* state localized in the signaling unit and the nonradiative relaxation of the SCT(1nπ*)state is demonstrated to be the key factor for the ON-OFF fluorescence emission.For the detection of completely filled metal ions with d10 configuration(i.e.Zn2+,Cd2+ etc.),the strength of the M2+-N(O)coordination bond has been quantitatively found to dominate the occurrence of electron transfer,producing to the recovery of the enhanced fluorescence(Zn2+)or the weak fluorescence emission(Cd2+).Once Cu2+(d9)is bound to the fluorescent sensor,a low energy d-d transition is appended to the signaling unit centered 3ππ* triplet state excitation.The fluorescence quenching for Cu2+ detection is quantitatively characterized by the regulation of non-adiabatic relays of two conical intersections and one signaling unit centered singlet/triplet crossing along a downhill ladder relaxation pathway.The present study contributes two theoretical models of the competitive coexistence of radiative/nonradiative decay channel in 1ππ*/SCT(1nπ*)states and a downhill ladder relaxation pathway through multi nonadiabatic relays,which will establish a benchmark for ON-OFF mechanisms of a fluorescent sensor that coordinates various transition metal ions with different electron configuration and radius.