Viologen cations are excellent electro- and photochromic materials. They generally have no response or very low sensitivity to bases. In this paper, three compounds, 1,1′-bis(2-oxo-2-phenylethyl)-4,4′-bipyridinium (viologen) with different substituents, including H (1), Cl (2), and OH (3), were synthesized. All three, especially 1 and 2, have very high sensitivity to base in both solution and solid state in air atmosphere. These viologens are responsive not only to bases but also to solvent polarity. NMR shows 1 became enolic and then a radical, whereas 3 is colored only in the radical form. These results are in agreement with EPR spectra. Crystal structures show that the C–C that links two pyridinium and N–C distances in coplanar pyridinium in the colored (radical) form is clearly longer than that of the pale-yellow form, indicating that the color is due to the viologen radical. Viologens containing an electron-withdrawing phenacetyl group are the most sensitive compounds for fast, naked eye detection of base and solvent polarity.Keywords: air-stable; electronic effect; solvatochromic; viologen; visual base sensor;

A new neuromelanin-like ketocatechol-containing iminodiacetic acid ligand, (N-(3,4-dihydroxyl)phenacylimino)diacetic acid (H4L), which is also quite similar to compounds found in insect cuticle, has been synthesized and characterized. The X-ray crystal structure of H4L has been successfully determined. Proton binding and coordination with Fe(III), Cu(II), and Zn(II) have been studied by potentiometric titrations and UV-vis spectrophotometry in aqueous solution. UV spectra of H4L in the absence and presence of different metal ions indicate complexes formed with the catechol moiety of H4L in aqueous solution. Visible spectra and NMR reveal that H4L with Fe(III), Cu(II), and Zn(II) can all give stable mono-(ML) and dinuclear complexes [M(ML)]. Fe(III) can also form {Fe(FeL)2} and {Fe(FeL)3} species with sufficient base. The process is accompanied by a drastic color change from light blue to deep-blue to wine-red. The Fe(III)–Cu(II) heteronuclear complex also exists in aqueous solution whose spectra are similar to the homonuclear Fe(III) complex. However, the spectra of {Fe(CuL)} shifted to a longer wavelength and {Fe(CuL)2} and {Fe(CuL)3} shifted to a shorter wavelength. Keto–enol tautomerism was observed in weak basic aqueous solution as indicated by 1H NMR spectra. The reaction products of Cu(II) complex with H2O2 depend on the H2O2 concentration and pH value. Low concentrations of H2O2 oxidize H4L to a series of semiquinone and quinone compounds with absorption maxima at 314–400 nm, while a high concentration of H2O2 oxidizes H4L to colorless muconic acid derivatives. NaIO4 gives different oxidase products, but no 2,4,5-trihydroxyphenylalanine quinone (TPQ)-like hydroxyquinone can be found.

•Cationic pyridinium (or pyridione) is not well explored as a ligand to construct coordination polymer.•The ligand in 1–5 exist in 4(1H)-pyridinone form rather than commonly presented 4-oxypyridinium form.•π-π interaction between pyridinone rings exists in 1–4 and is stronger than that between two benzene rings.•The pyridinone oxygen is the strongest coordination atom, or it forms strongest H-bond.•Different from viologen (pyridinum) compound, all the complex do not change color upon heating or light irradiation.Five new coordination polymers, [Cd(η6-L)]n (1), [Cd(η4-L)(H2O)2]n (2), [Co(η4-L)(H2O)2]n (3), {[Cd(η4-L)(H2O)1.5]·H2O}n (4), and {[Cu2(η4-L)2(H2O)2]·2H2O}n (5) were synthesized by reactions of corresponding metal salts with 4-(4-oxypyridinium-1-yl)phthalic acid (H2L) hydrothermally. Ligands in complexes are deprotonated even in the absence of a base. Metal ions in these complexes are all in an octahedral environment. The coordination number of ligand varies from 4 to 6. Each ligand binds three or four metal ions. High temperature synthesis tends to give ligands with larger coordination numbers. 1–5 are all 2D double-layer coordination polymers. Complex 2 and 3 are isostructural. The ligands in 1–5 exist in 4(1H)-pyridinone form rather than commonly presented 4-oxypyridinium forms. Without water molecules, complex 1 is stacked through π–π interaction between 4(1H)-pyridinone moieties in adjacent layers. Compound 2 is connected by intrermolecular hydrogen bonds via coordinated water molecules to form a 3D structure. All capable protons in 2 and 3 form H-bonds. π–π interaction between pyridinone rings exists in 1–4 and is stronger than that between two benzene rings. 4 packed into 3D via H-bond and π–π interaction while 5 packs into 3D via H-bond only. The pyridinone oxygen either coordinates to metal ion or form very strong H-bond with water molecule at O⋯O distance of 2.58 Å. Of all these complexes, anhydrous 1 with non-redox Cd(II) can stable up to 410 °C and is the most thermally stable while redox active Cu(II) complex 5 is stable under 210 °C. All the Cd(II) complexes have ligand centered emission at 430 and 480 nm. According to bond lengths, ligand in complexes exists as pyridinone form rather than the commonly presented oxopyridinium form. Different from viologen (pyridinum) compound, the entire complex does not change color upon heating or light irradiation.

An aspartic acid-functionalized water-soluble perylene bisimide, N,N′-di(2-succinic acid)-perylene-3,4,9,10-tetracarboxylic bisimide (PASP) was synthesized and characterized. It has absorbance maximum A0–0 and A0–1 at 527 and 498 nm (ε ≈ 1.7 × 104 L cm–1 mol–1) respectively in pH 7.20 HEPES buffer. Two quasi-reversible redox processes with E1/2 at −0.17 and −0.71 V (vs Ag/AgCl) respectively in pH 7–12.5 aqueous solutions. PASP can react with Na2S in pure aqueous solution to form monoanion radical and dianion species consecutively. PASP–• has EPR signal with g = 1.998 in aqueous solution, whereas PASP2- is EPR silent. The monoanion radical formation is a first-order reaction with k = 8.9 × 10–2 s–1. Dianion species formation is a zero-order reaction and the rate constant is 4.3 × 10–8 mol L–1 s–1. The presence of H2O2 greatly increases the radical formation rate constant. PASP as a two-electron transfer reagent is expected to be used in the water photolysis.Keywords: anion radical; electron transfer; perylene bisimide;

A new neuromelanin-like ketocatechol-containing iminodiacetic acid ligand, (N-(3,4-dihydroxyl)phenacylimino)diacetic acid (H4L), which is also quite similar to compounds found in insect cuticle, has been synthesized and characterized. The X-ray crystal structure of H4L has been successfully determined. Proton binding and coordination with Fe(III), Cu(II), and Zn(II) have been studied by potentiometric titrations and UV-vis spectrophotometry in aqueous solution. UV spectra of H4L in the absence and presence of different metal ions indicate complexes formed with the catechol moiety of H4L in aqueous solution. Visible spectra and NMR reveal that H4L with Fe(III), Cu(II), and Zn(II) can all give stable mono-(ML) and dinuclear complexes [M(ML)]. Fe(III) can also form {Fe(FeL)2} and {Fe(FeL)3} species with sufficient base. The process is accompanied by a drastic color change from light blue to deep-blue to wine-red. The Fe(III)–Cu(II) heteronuclear complex also exists in aqueous solution whose spectra are similar to the homonuclear Fe(III) complex. However, the spectra of {Fe(CuL)} shifted to a longer wavelength and {Fe(CuL)2} and {Fe(CuL)3} shifted to a shorter wavelength. Keto–enol tautomerism was observed in weak basic aqueous solution as indicated by 1H NMR spectra. The reaction products of Cu(II) complex with H2O2 depend on the H2O2 concentration and pH value. Low concentrations of H2O2 oxidize H4L to a series of semiquinone and quinone compounds with absorption maxima at 314–400 nm, while a high concentration of H2O2 oxidizes H4L to colorless muconic acid derivatives. NaIO4 gives different oxidase products, but no 2,4,5-trihydroxyphenylalanine quinone (TPQ)-like hydroxyquinone can be found.