Natural photochromic minerals from the aluminosilicate family have been reported by geologists since several decades, calling this phenomenon “tenebrescence”. Up to now, these minerals, the hackmanite (Na8Si6Al6O24Cl2), the tugtupite (Na8Al2Be2Si8O24Cl2) and the scapolite ([Na,Ca]4Al3Si9O24[Cl,CO3]2), have been considered almost as curiosity object without a deep investigation of the photochromism mechanism. Based on quantum chemical calculations, we recently proved the hypothesized mechanism: Under UV-light exposure, a sulfur-based impurity transfers an electron toward a Cl- vacancy giving rise to a trapped electron (called F-center). This F-center gives the color to the mineral. Its return to the sulfur-based impurity corresponds to the bleaching of the mineral. While probably sharing the same mechanism of coloration and bleaching, these three minerals experience different properties in terms of colored form (see figure below), activation energy (i.e. the photon’s energy of the UV light), bleaching energy and stability of the colored form. Beyond these quantitative aspects, remain also the question of the unique reversibility of the photochromism of these natural minerals keeping in mind the difficulty to develop artificial, stable and reversible photochromic materials. In this presentation, we provide the complete description of the mechanism of the photochromism of the three minerals combining high level quantum chemical calculations (periodic calculations followed by embedded cluster excited state simulations) associated to experimental set-up especially designed to investigate them (the thermotenebrescence experiment). We bring the first understanding of the unique reversibility of the photochromism of these minerals: the electronic state inversion responsible of the photochromic phenomenon is due to an unusually strong atomic relaxation for the inorganic solid state. More precisely, the free volume offered by the aluminosilicate structure allows a sodium atom to move (by more than 1.3 Å!) to stabilize a trapped electron, giving the colour to the materials.

At the origin of the unique reversibility of the photochromism in natural minerals

ACS Spring 2022

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