García-Fernández Pablo, Moreno Miguel, Aramburu José Antonio
Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria , Avenida de los Castros s/n, 39005 Santander, Spain.
Inorg Chem. 2015 Jan 5;54(1):192-9. doi: 10.1021/ic502420j. Epub 2014 Dec 17.
The study of chemical factors that influence pigment coloring is a field of fundamental interest that is still dominated by many uncertainties. In this Article, we investigate, by means of ab initio calculations, the origin of the unusual bright blue color displayed by historical Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) pigments that is surprisingly not found in other compounds like BaCuSi2O6 or CaCuO2 containing the same CuO4(6-) chromophore. We show that the differences in hue between these systems are controlled by a large red-shift (up to 7100 cm(-1)) produced by an electrostatic field created by a lattice over the CuO4(6-) chromophore from the energy of the 3z(2)-r(2) → x(2)-y(2) transition, a nonlocal phenomenon widely ignored in the realm of transition metal chemistry and strongly dependent upon the crystal structure. Along this line, we demonstrate that, although SiO4(4-) units are not involved in the chromophore itself, the introduction of sand to create CaCuSi4O10 plays a key role in obtaining the characteristic hue of the Egyptian Blue pigment. The results presented here demonstrate the opportunity for tuning the properties of a given chromophore by modifying the structure of the insulating lattice where it is located.
对影响色素着色的化学因素的研究是一个具有根本重要性的领域，目前仍存在许多不确定性。在本文中，我们通过从头算计算，研究了历史悠久的埃及蓝（CaCuSi4O10）和汉蓝（BaCuSi4O10）颜料所呈现出的异常亮蓝色的起源，令人惊讶的是，在其他含有相同CuO4(6-)发色团的化合物如BaCuSi2O6或CaCuO2中并未发现这种颜色。我们表明，这些体系之间色调的差异是由一个静电场产生的大幅红移（高达7100 cm(-1)）所控制的，该静电场由晶格在CuO4(6-)发色团上产生，源于3z(2)-r(2) → x(2)-y(2)跃迁的能量，这是一种在过渡金属化学领域中广泛被忽视的非局部现象，并且强烈依赖于晶体结构。沿着这条线，我们证明，尽管SiO4(4-)单元本身并不参与发色团，但引入沙子以形成CaCuSi4O10在获得埃及蓝颜料的特征色调方面起着关键作用。本文给出的结果表明，通过改变发色团所在绝缘晶格的结构来调整给定发色团性质的可能性。
