To generate a metal quenched ESIPT-OFF probe 6, we reacted probe 4 with 0.5 equiv. of CuCl2 at room temperature (61% yield). The formation of this complex in a 2:1 stoichiometry was confirmed by HRMS data (Fig. ). Furthermore, we confirmed the correct stoichiometry by changes in photophysical properties and Job's plot analysis (see next section). While the Cl substituent in 6 was mainly installed to enable further potential modifications, we also generated a non-chlorinated version to study and compare the impact of substitution on the photophysical properties and selectivity regarding different anions. An identical complexation of 5 with Cu 2+ in a 2:1 stoichiometry was conducted, resulting in complex 7.