*2.3. Molecular Structure*

The structure of one of the obtained azo dyes was confirmed by X-ray crystal structure analysis (CCDC1946289). All atoms of the studied 2-[4-(*N,N*-dimethylamino)phenylazo]-5-(4-methoxyphenyl) -1,3,4-thiadiazole (**5b**) occupied general positions (Figure 2).

**Figure 2.** Molecular structure of compound **5b** plotted with 50% probability of displacement ellipsoids. Hydrogen atoms are drawn as spheres with arbitrary radii.

– – – – – – – – – The five- and six- membered rings were practically planar (the largest deviating atom C2, C7, C14 stuck out 0.0060(8), 0.0079(9), 0.0073(9) Å from the weighted least squares plane containing the respective atom), but the whole molecule is considerably bent. The benzene rings containing C3 and C11 atoms were inclined at 10.11(7) and 24.90(6) ◦ to the central 1,3,4-thiadiazole ring, respectively and mutually at 26.85(6) ◦ . The *N,N*-dimethylamino group was inclined at 11.53(14) ◦ to the neighboring benzene ring, which is a larger value than is typically observed for similar systems (4.6%–5.8 ◦ ) [30]. However, it still falls within the range of 2.3%–18.8 ◦ (the angle values of median population). The C–S bonds (Table 2) were shorter than typical single C–S bonds (i.e., a mean value of 1.819 Å for a bond between an sp 3 carbon atom and a divalent sulfur) and similar to the values observed for slightly delocalized systems containing sp 2 carbon atoms (i.e., a mean value of 1.741 Å) [31]. A similar effect exists for an N–N bond (a formal bond length of 1.454 Å) and opposite effects were observed for slightly longer C=N bonds (Table 2) and double C=N bonds (i.e., a mean value of 1.279 Å [32]). Combined, these effects demonstrate small, but observable delocalization of electron density within the 1,3,4-thiadiazole ring. The azo N=N bond (Table 2) shows a transitional character between a well-localized and a completely delocalized system (mean values of 1.245 and 1.304 Å) [33]. This, along with the shortening of the neighboring C–N bonds, also demonstrates the considerable degree of delocalization of electron density within the C–N=N–C moiety. Due to the absence of classical hydrogen bond donors, the studied compounds are connected only by weak C–H•••A non-classic hydrogen bonds (where A denotes the following acceptors: N, O, S and π electrons) [34] and π•••π stacking interactions (Table 3, Table 4) [35].

**••• Table 2.** Selected structural data of studied compound **5b**.


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**••• •••**

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**Table 3.** Non-classic hydrogen bonds and the first level graph motifs in the studied compound. Cg(C3) indicates centroid of ring containing C3 atom.

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Symmetry transformations used to generate equivalent atoms: (i) −x + 1, −y + 1, −z − 2; (ii) x + 1, y, z; (iii) x, −y + 1.5, z − 0.5. − − x, − −

The unitary graph set is composed of R<sup>2</sup> 2 (22)C(17)S(5)C(2) motifs of the lowest degree. The ring motif assembles the molecules into supramolecular dimers (Figure 3), and the C(17) chain motif extends these dimers to a supramolecular ribbon extending along the crystallographic axis (Figure 4). This ribbon is constructed from alternating R<sup>2</sup> 2 (22)R<sup>4</sup> 4 (26) ring motifs of a binary graph set of the lowest degree. The neighboring ribbons are interconnected by C–H•••π and π•••π interactions (Table 3, Table 4) and form a three-dimensional network assembled by non-covalent interactions. – **•••**π π**•••**π

**Figure 3.** Supramolecular dimers forming a R<sup>2</sup> 2 (22) motif of a unitary graph set. Symmetry codes as in Table 3. Non-classic hydrogen bonds are indicated by dotted lines.

α β **Table 4.** Interactions in the studied compound. Each ring is indicated by one atom, which belongs solely to this ring. α is a dihedral angle between planes I and J, β is an angle between Cg(I)-Cg(J) vector and normal to plane I and dp is the perpendicular distance of Cg(I) on ring J plane.


enerate equivalent atoms: (iv) − , −y+1, −z+1;(v) −x, −y+1, − Symmetry transformations used to generate equivalent atoms: (iv) −x+1, −y+1, −z+1;(v) −x, −y+1, −z+1.

**Figure 4.** Part of molecular packing showing a supramolecular ribbon extending along the crystallographic axis. Hydrogen bonds indicated by dotted lines.

## **3. Experimental**
