Photophysical studies of the fluorophores in the presence of Cu 2+ : Spectrophotometric titration was used to examine the UV-Vis absorption spectrum of ligand 4 in 10 mM HEPES buffer solution at pH 7.4 (Fig. ). A prominent band with an absorption peak at 368 nm was visible. With increasing addition of Cu 2+ (from 0.1-0.5 equiv.) the absorption maximum at 368 nm increased, and adding more equivalents did not elicit additional changes indicating a 2:1 complexation with Cu 2+ . further changes, suggesting that 4 is complexed with Cu 2+ in a 2:1 stoichiometric ratio. Further, the 2 : 1 stoichiometry was confirmed by a Job's plot (Fig. ) and the HRMS data for 6 and 7 (Fig. and S8, respectively).