Figure S1. Job plot of HBC and Zn²⁺. The total concentration of HBC and Zn²⁺ was 100 μM.

Figure S2. Positive‐ion electrospray ionization mass spectrum of HBC (100 μM) upon addition of 1.0 equiv of Zn(NO₃)₂.

Figure S3. Benesi‐Hildebrand plot of HBC (at 538 nm), assuming 1:1 stoichiometry for association between HBC and Zn²⁺.

Figure S4. Competitive selectivity of HBC (30 μM) toward Zn²⁺ (14 equiv) in the presence of other metal ions (14 equiv).

Figure S5. ¹H NMR titration of HBC with Zn²⁺.

Figure S6. Fluorescence intensity (at 538 nm) of HBC as a function of Zn²⁺ concentration ([HBC] = 30 μmol/L and [Zn²⁺] = 0.0–42.0 μmol/L).

Figure S7. (a) The theoretical excitation energies and the experimental UV–Vis spectrum of HBC. (b) The major electronic transition energies and molecular orbital contributions for HBC (H = HOMO and L = LUMO).

Figure S8. (a) The theoretical excitation energies and the experimental UV–Vis spectrum of HBC‐Zn²⁺. (b) The major electronic transition energies and molecular orbital contributions for HBC‐Zn²⁺ (H = HOMO and L = LUMO).

Figure S9. Molecular orbital diagrams and excitation energies of HBC and HBC‐Zn²⁺ complex.