Macrocyclic chromophores, electronic properties

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

Incorporation of electronegative flnorine atoms in benzene rings of phthalocyanine macrocycle has a considerable impact on its electronic structure and spectral properties. The electronic absorption spectra of fluorinated phthalocyanines are typical for symmetrical metal phthalocyanines (Table 3). They contain an intense Q-band in the visible region (650-715 nm) corresponding to the lowest degenerated it-it transition of the macrocyclic jt-chromophore, and a less intense and broader Soret band in the UV-region (300-380 nm). 

A series of compounds, aimed to be used as fluorescent probes for biological ions, have been synthesized and the fluorescent properties of their free and ion-bound forms were studied. The fluorescent properties of these probes are due to the extended conjugation of chromophores such as substituted coumarins and benzothiazols, whereas their ion chelating properties are due to the presence of ionophores such as [1,10]phenanthrolin-5-amine, N,N-bis(2-pyridinylmethyl)amine, mono- and polyaza macrocyclics, and polycarboxylate moieties. Based on their structural features and their spectral profile, these probes are classified as Photoinduced Electron Transfer (PET) or Photoinduced Charge Transfer (PCT) indicators. Their ion selectivity is discussed in terms of the ionophore structure and the extent of conjugation in their framework. 

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