Fig. 1. (a) Absorption titration spectra of 4 (10 μM) in a HEPES-buffer (10 mM, pH = 7.4) in the presence of increasing concentrations of Cu²⁺ (0–1 equiv.). (b) Fluorescence titration spectra of 4 (5 μM) in a HEPES-buffer (10 mM, pH = 7.4) in the presence of increasing concentrations of Cu²⁺ (0–1.2 equiv.). (c) Emission intensity at 434 nm in the presence of increasing concentrations of Cu²⁺ (0–1 equiv.), (d) Job's plot of compound 4 with Cu²⁺ according to the method of continuous variation. [4] + [Cu²⁺] = 10 μM. Excitation and emission were recorded at 254 nm and 434 nm, respectively.