Phenylenevinylene terpyridine (mepvpt) shows chelation enhanced fluorescence (CHEF) with Cd(II) and chelation quenched fluorescence (CHQF) with Zn(II), respectively. To understand the behaviors, we studied their intrinsic optical properties using DFT/TDDFT methods. The results show that fluorescence quantum yields (FQY) of mepvpt, mepvpt-Cd and mepvpt-Zn are low due to high ISC rates from higher excited states rather than the S1 excited state. When mepvpt chelates Cd(II), the molecular structure becomes more planar and S3,4 → S0 radiation rates become higher than that of mepvpt, which results in CHEF. When mepvpt chelates Zn(II), a new S4 → S0 emission with low oscillator strength occurs and high S4 → Tn ISC rates appear, which leads to CHQF. This proposed mechanism of metal fluorescence enhancing/quenching suggests a design strategy for single-molecular multi-analyte sensors.