ON-OFF Mechanism of a Fluorescent Sensor for the Detection of Zn(II), Cd(II), and Cu(II)Transition Metal Ions

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 metal ion (Zn2+, Cd2+, and Cu2+) complexes to provide an universal understanding of ON OFF fluorescent mechanisms for the selective identification of these metal ions. The photoinduced electron transfer (PET) between the receptor and the signaling unit is quantitatively attributed for the first time to a newly generated transition of S-0 -> S-CT((1)n pi*), which is a typical (1)n pi* excitation but exhibits a significant charge transfer character and zwitterionic radical configuration. The present study contributes the two theoretical models of the competitive coexistence of radiative/nonradiative decay channel in (1)pi pi*/S-CT((1)n pi*) states for the detection of metal ions with d(10) configuration (i.e., Zn2+, Cd2+, etc.) and a downhill ladder relaxation pathway through multi nona-diabatic relays for the probing of d(9) cations (Cu2+, etc.). These computational results will establish a benchmark for ON-OFF mechanisms of a fluorescent sensor that coordinates various transition metal ions with different electron configuration and radius.