The origins of fluorescence quenching by Hg(II) ion chelation and fluorescence enhancement by Zn(II) ion chelation to a PET sensor are investigated. Specifically, the fluorescence quenching and enhancing mechanisms associated with the ligand ADPA (N-(9-anthracenylmethyl)-N- (2-pyridinylmethyl)-2-pyridinemethanamine), protonated ADPA and metal bound (Zn(II) and Hg(II)) ADPA are studied via density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The study found that a structural change in the excited state of ADPA induces reordering of the frontier molecular orbitals, and the S-1 -> S-0 transition becomes a charge transfer transition from the fluorophore to the- tertiary nitrogen of the dipicolylamine (DPA) unit, which is forbidden. Protonation on the tertiary amine or chelation of Zn(II) prevents such changes, and the HOMO-LUMO transition is contained within the fluorophore. Therefore, fluorescence is restored. The chelation of Hg(II), on the other hand, promotes extensive interaction between the Hg(II) ion and the fluorophore, which is reflected in the short lig(H) fluorophore distance (3.11 angstrom). A noticeable structural change upon the So -> SI transition is observed in the Hg(II) ADPA system as well, where the resulting SI -> So transition becomes a charge transfer transition from mercury to the fluorophore and the fluorescence is thus quenched. Therefore, the present DFT/TDDFT calculations reproduce the fluorescence on off behavior associated with the entire ADPA family of complexes, which illustrates that the combination of DFT and TDDFT calculations, including excited state geometry optimization, can be a valuable tool to uncover the detailed fluorescence sensing mechanisms.