The reported compounds are obtained in one-pot reactions where the respective substrates: the Er III complex and the ligand are combined in pentane, stirred at room temperature for a few minutes and left for spontaneous crystallization overnight (Figure ). {[Er III (BHT)3]3(dtepy)2} . 3C5H12 (1) crystallizes in the form of small yellow prism crystals that change color to greenish-blue when exposed to LED light of the stereomicroscope. {[Er III (BHT)3(azopy)}nnC5H12 (2), on the other hand, forms larger purple/violet crystals that do not appear to be photochromic. The preparation of both compounds requires strict control of the reaction stoichiometry. Any deviation from the provided procedure (see the experimental section for details) leads to by-products, which are visible as additional reflections in the powder X-ray diffraction (PXRD) experiments. These by-products could not be identified at this point. If the reaction is followed to the letter, the experimental PXRD patterns recorded at room temperature are identical with those simulated from the single-crystal X-ray diffraction (scXRD) obtained at 270 K (Figure ). .798 Å are within the proper range for the photocyclization reaction to occur even in the solid state (shorter than the 4.2 Å limit). Due to the linear geometry, there are two non-equivalent Er III centers in the molecule. The two terminal ones are four-coordinate and adopt a distorted trigonal pyramidal geometry with three O-coordinated BHT and one N-coordinated dtepy ligand. The central cation is five-coordinate with three O-coordinated BHT ligands in the equatorial plane and two N-coordianted dtepy ligands occupying the axial positions of the distorted trigonal bipyramid. The dtepy molecules act as bridging ligands connecting the terminal Er III ions with the central one via the nitrogen atoms of the pyridine groups. The intramolecular distances between the central and terminal Er III ions are 19.357(1) and 19.597(1) Å. The distance between the two photoreactive carbon atoms of the thiophene rings are within the 3.7-3.8 Å range as indicated in Figure , which according to the literature reports, enables the photo-cyclization reaction to occur even in the solid state. Table gathers selected bond lengths and angles of the coordination spheres of the Er III ions. The Er-O bond lengths in the case of the terminal erbium atoms are within the 2.056(5)-2.094(4) Å range, which is similar to the previous reports for lanthanide-aryloxide complexes. Noteworthy, the analogous distances for the central erbium are slightly longer (2.095(5)-2.145(5) Å). Similar elongation can be observed for Er-N bonds: the terminal units show Er-N of 2.459(6)-2.461(5) while the central ones are longer 2.499(6)-2.519 (5). The distortion from the ideal trigonal pyramidal geometry is caused mainly by unusually large O-Er-O angles 125.22 (17)° for O32-Er3-O33 and 122.37 (19)° for O21-Er2-O22. As aforementioned, the bipyramidal geometry of the central Er III is also distorted. The N-Er-N angle deviates from 180° by ca. 30° and the O-Er-O angles are larger than the expected 120° (i.e. 136.04(18) ○ for O13-Er1-O12). .