Identification of the EPR signal of S2− in green ultramarine pigments

Abstract

Green and blue ultramarine pigments are characterized by the sodalite structure Na(6)(Al(6)Si(6)O(24)) and colored inserted species. These chromophores are sulfur species: S(3)(-) (blue) and S(2)(-) (yellow). Both radicals are encapsulated inside the β-cages. They contribute to the EPR spectrum of ultramarine pigments. The well-known strong EPR signal observed in all ultramarine pigments Continuous-Wave (CW) spectra has long been assigned to S(3)(-) (g = 2.029). In contrast, the S(2)(-) contribution is still subject to controversy because its signal in ultramarine pigments was not resolved even at low temperature in CW-EPR experiments. In this study, we identify unambiguously for the first time by CW-EPR and field sweep-echo detected (FS-ED) EPR the signal of S(2)(-) in ultramarine pigments and we determine its tensor components: g(1) = 2.69(6), g(2) = 2.03(4) and g(3) = 1.86(4).

Synthesis and mechanical properties of new fibre-reinforced composites of inorganic polymers with natural wool fibres

“…Several pigments can be formed by the insertion polylsulphide species such as S 2 -, S 6 -, or S 3 -into the cages of sodalite Na 6 (Al 6 Si 6 O 24 ) or a related aluminosilicate structure [34,35]. The UV-visible spectrum of the coloured wool (Fig.…”

“…The UV-visible spectrum of the coloured wool (Fig. 5a) contains two peaks, at *630 and *430 nm, assigned to S 3 -and S 2 -respectively in green and blue ultramarine pigments, in which the blue colour contains the S 3 -chromophore while S 2 -produces a yellow colour [34][35][36][37]. It has been also reported that sodium sulphide Na 2 S produced by the reaction of alkali with wool fibres is a precursor of S 3 - [38].…”

Specific Induced Circular Dichroism and Enhanced B to Z Transitions of Duplexes Stabilized by Chromophore‐Linked Alkynylnucleoside Residues

“…The presence of S 2 · – radicals in zeolite pigments can be established through their diffuse reflectance UV/Vis band at about 395 nm and the strong Resonance Raman band for their ν 3 stretching mode at about 592 cm –1 74. It causes a broad and poorly resolved ESR signal, which is superimposed on the S 3 · – signal at low temperatures in green ultramarines.…”

“…Due to the frequent presence of an additional broad ESR signal caused by Fe III impurities, assignment of the S 2 · – contribution was long the subject of controversy. However, comparison of the cw‐ESR spectra of a thermally treated industrial green ultramarine containing S 2 · – and S 3 · – with that of an S 2 · – ‐free green zeolite pigment (GZP) with encapsulated S 6 2– dianions and S 3 · – radicals4 has recently enabled the unambiguous identification of the S 2 · – signal ( g iso = 2.19) of the former pigment at 4 K and the determination of its strongly anisotropic tensor components g 1 = 2.69, g 2 = 2.03 and g 3 = 1.86 74…”

A wide‐line low frequency electron paramagnetic resonance spectrometer

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Many pigments used in paintings are paramagnetic. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Many pigments used in paintings are paramagnetic.…”