The binding of transition metals, such as Zn 2+ , Ni 2+ , and Cd 2+ , with protein residues often leads to proton displacement. [1] This process occurs, for example, in the reaction centers (RCs) of photosynthetic bacteria, [2] and leads to conformational changes and modification of the pK a values of the surrounding amino acid residues. These changes eventually slow down the photosynthetic process. A similar deprotonation event also takes place in cation-diffusion facilitators (CDFs), in which the coordination-coupled deprotonation (CCD) provides the energetic basis for metal efflux.[3] The CCD process has considerable potential for molecular switches, [4] as it opens the way to a new and far-reaching switching mechanism in which acid modulations [5] can be brought about without the addition of protons.[6] This possibility could be of interest, for example, in cases in which molecular switches need to be activated under mild or neutral conditions. The CCD process in RCs and CDFs is associated with metal binding to histidine residues. Therefore, NÀH-containing molecular switches, such as the pH-activated hydrazone-based rotary switches [7] that we have been developing, might be suitable for mimicking this bioinorganic process. As it happens, the coordination of transition metals to hydrazones can lead, in certain cases, to N À H deprotonation.[8] With this possibility in mind, we set out to develop a new switching mechanism that takes advantage of the CCD process.We recently reported the four-step pH-activated switching cycle of a tristable hydrazone-based molecular switch (QPH) with a quinolinyl group as the stator and an ethyl 2-pyridylacetate derivative as the rotor.[7b] This molecular switch can be viewed as a tridentate ligand that can accommodate a transition metal by coordination with either the pyridine or carbonyl moiety in the rotor, the quinolinyl group in the stator, and the imine or N À H nitrogen atoms in the axle. We explored the binding of this molecular switch to Zn II , which has been shown to bring about CCD in RCs, CDFs, and the deprotonation of hydrazones, [8] to discern whether the deprotonation process could be useful in activating the molecular rotor.The UV/Vis spectrum of QPH in CH 3 CN shows an absorption maximum at l max = 393 nm (Figure 1). Upon titration with zinc(II) perchlorate (Zn(ClO 4 ) 2 ), the absorption maximum shifted bathochromically with an accompanying hyperchromic change that reached saturation at 8 equivalents of Zn 2+ (l max = 452 nm). This shift in UV/Vis absorption is an indication that Zn 2+ coordinates strongly with QPH. A Job plot (see Figure S4 in the Supporting Information) derived from the UV/Vis spectra shows a vertex around 0.65, which suggests a 2:1 binding stoichiometry between QPH and Zn 2+ .[9] The fact that no isosbestic points were observed during the UV/Vis titrations indicates that more than two chromophoric species are involved in the process, and thus the binding stoichiometry cannot be 1:1. Binding constants were determined as K 1 = 5.7 10 5 m ...

A triazolopyridinium salt chemodosimeter has been developed that displays a 60-fold enhancement in fluorescence upon reaction with cyanide. The novel, fast, selective and sensitive reaction-based indicator relies on the pseudopericyclic ring opening of the bridgehead nitrogen-containing detector.

The binding of transition metals, such as Zn 2+ , Ni 2+ , and Cd 2+ , with protein residues often leads to proton displacement. [1] This process occurs, for example, in the reaction centers (RCs) of photosynthetic bacteria, [2] and leads to conformational changes and modification of the pK a values of the surrounding amino acid residues. These changes eventually slow down the photosynthetic process. A similar deprotonation event also takes place in cation-diffusion facilitators (CDFs), in which the coordination-coupled deprotonation (CCD) provides the energetic basis for metal efflux. [3] The CCD process has considerable potential for molecular switches, [4] as it opens the way to a new and far-reaching switching mechanism in which acid modulations [5] can be brought about without the addition of protons. [6] This possibility could be of interest, for example, in cases in which molecular switches need to be activated under mild or neutral conditions. The CCD process in RCs and CDFs is associated with metal binding to histidine residues. Therefore, NÀH-containing molecular switches, such as the pH-activated hydrazone-based rotary switches [7] that we have been developing, might be suitable for mimicking this bioinorganic process. As it happens, the coordination of transition metals to hydrazones can lead, in certain cases, to N À H deprotonation. [8] With this possibility in mind, we set out to develop a new switching mechanism that takes advantage of the CCD process.We recently reported the four-step pH-activated switching cycle of a tristable hydrazone-based molecular switch (QPH) with a quinolinyl group as the stator and an ethyl 2pyridylacetate derivative as the rotor. [7b] This molecular switch can be viewed as a tridentate ligand that can accommodate a transition metal by coordination with either the pyridine or carbonyl moiety in the rotor, the quinolinyl group in the stator, and the imine or N À H nitrogen atoms in the axle. We explored the binding of this molecular switch to Zn II , which has been shown to bring about CCD in RCs, CDFs, and the deprotonation of hydrazones, [8] to discern whether the deprotonation process could be useful in activating the molecular rotor.The UV/Vis spectrum of QPH in CH 3 CN shows an absorption maximum at l max = 393 nm (Figure 1). Upon titration with zinc(II) perchlorate (Zn(ClO 4 ) 2 ), the absorption maximum shifted bathochromically with an accompanying hyperchromic change that reached saturation at 8 equivalents of Zn 2+ (l max = 452 nm). This shift in UV/Vis absorption is an indication that Zn 2+ coordinates strongly with QPH. A Job plot (see Figure S4 in the Supporting Information) derived from the UV/Vis spectra shows a vertex around 0.65, which suggests a 2:1 binding stoichiometry between QPH and Zn 2+ . [9] The fact that no isosbestic points were observed during the UV/Vis titrations indicates that more than two chromophoric species are involved in the process, and thus the binding stoichiometry cannot be 1:1. Binding constants were determined as K 1 = 5.7...