*2.2. Spectral Characterization*

– – – – – – – – – – – – <sup>−</sup> – – <sup>−</sup> – − – – – − The structures of the final diazotization products of 2-amino-5-aryl-1,3,4-thiadiazole and their subsequent coupling with aniline, *N,N*-dimethylaniline and phenol were confirmed by typical spectroscopic methods ( <sup>1</sup>H-NMR, <sup>13</sup>C-NMR, UV-Vis, FT-IR and HRMS). In the <sup>1</sup>H-NMR spectra of 2-arylazo-5-aryl-1,3,4-thiadiazoles (**4a**–**e**, **5a**–**e**, **6a**–**e**), the characteristic signals become from protons located in the benzene rings associated with the 1,3,4-thiadiazole core and appeared in the range between 6.74–8.40 ppm. Signals of the characteristic amino group (7.06–7.18 ppm), *N,N*-dimethylamino group (~3.18 ppm), *t*-butyl (~1.33 ppm) and methoxy (~3.86 ppm) were also observed. In the <sup>13</sup>C-NMR spectra the characteristic peaks from 1,3,4-thiadiazole carbon atoms C-2 and C-5 were observed at 164–166 ppm and 178–181 ppm, respectively. Signals due to methoxy group (~55.5 ppm), *t*-butyl group (30.8–34.8 ppm) and *N,N*-dimethylamino group (40.0–40.2 ppm) were observed upfield in the spectra. The formation of azo dyes **4a**–**e**, **5a**–**e**, **6a**–**e** was further shown by HRMS and elemental analyses.

– − – – – → → the σ→ The IR spectra of all prepared dyes were recorded from 4000 and 650 cm−<sup>1</sup> . For the dyes **4a**–**e**, two bands, respectively at 3400–3310 cm−<sup>1</sup> and 3201–3190 cm−<sup>1</sup> , were visible, which were attributed to the presence of a free amino group. Such bands were not observed in the case of derivatives **5a**–**e**, representing a group of tertiary amines. In series **6a**–**e**, a broad band appeared from 3500–3100 cm−<sup>1</sup> , which confirms the presence of a hydroxyl group. All dyes showed a weak band at 1505–1525 cm−<sup>1</sup> for an azo group (N=N).

The UV-Vis spectra of three series of 2-arylazo-5-aryl-1,3,4-thiadiazole dyes (**4a**–**e**, **5a**–**e**, **6a**–**e**) measured in MeOH exhibit several (four in most cases) absorption maxima (Figure 1, Table 1). The same is observed for calculated spectra (Figure 1, Table 1) which are in good comparison with those measured. All maxima result from multiple different transitions and include both n→π\* and π→π\* transitions. For series **5** and **6** with compounds possessing a t-butyl group, additionally the σ→π\* transitions are observed. Due to complexity of electronic transition creating each absorption maximum, there is no one evident dependence between the substituents and absorption maximum position/absorption coefficient (Figure S1). Nevertheless, some dependences exist within groups with one kind of R or G substituent. The change of the G substituent causes the red shift of the most intense absorption maximum (appearing in the visible regions of 490–507 nm for **4a**–**e**, 515–530 nm for **5a**–**e** and 405–415 nm for **6a**–**e**) in order of G substituents: OH, NH2, NMe2. This sequence is in

agreement with electron donating properties of the studied substituents. These red shifts are well reproduced in calculated spectra (Table 1). For the compounds with amino substituents (G=NH2, NMe2) this maximum is the most red-shifting for a compound possessing a nitro group (Table 1). For the compounds with hydroxy group (G=OH) the red shift forced by the presence of a nitro group (**6c**) is between some other red shifted compounds, and it is smaller than that shift observed for the compound **6b**. This effect is also reflected in the calculated spectra (Table 1). The simultaneous presence of a hydroxy group (G=OH) and methoxy one (R=MeO) in **6b** is an explanation of this phenomenon. For each series, the methoxy moiety induces red-shift effect (toward most of the compounds), but for amino substituents G=NH2, NMe<sup>2</sup> it was smaller than for their hydroxy counterpart G=OH (influence of this group could only be noticed for calculated spectra). Increase of the red shift for stronger electron donating EDG substituents (G=NMe<sup>2</sup> and NH2) is larger than caused by methoxy moiety and overrule strong electron withdrawing properties of nitro substituent. In case of the weaker EDG substituent (G=OH) with electron donating properties close to the methoxy substituent, the electron withdrawing properties of nitro substituent dominate at some point, and cause above described difference in red shifts. For each compound, the most intense (global) maximum is attributed to HOMO–LUMO (H→L) transitions (Table 1, Supplementary Materials, Figures S2–S4). For compounds possessing the nitro substituent these experimentally observed absorption maxima result also from other than H→L transitions, possessing relatively large oscillator strengths (Table 1, Figures S2–S4). In all cases, these transitions do not engage the antibonding orbitals of the nitro group (Table 1). For the second most intense absorption maximum (experimentally observed at lower wavelengths), the changes of its shift between the corresponding compounds (e.g., **4a** vs. **5a** vs. **6a**) are not as high as those observed for the most intense maxima. Those maxima appear in the ultraviolet regions of 237–267 nm for **4a**–**e**, 243–256 nm for **5a**–**e** and 243–260 nm for **6a**–**e** and they results from multiple π→ π\* and n→ π\* transitions involving antibonding orbitals of all substituents (i.e., NH2, NMe2, OH, MeO, NO<sup>2</sup> and Br, Table 1). The experimental and calculated spectra show some minor differences. For compounds with amino groups (G=NH2, NMe2), the calculated spectra do not contain absorption maxima observed experimentally in the range of 284–299 nm. This may result from the interaction of compound molecules with the solvent (i.e., formation of specific hydrogen bonds) defectively reproduced in continuous model of solvation.

− – – – **Figure 1.** UV-Vis absorption spectra of the studied azo dyes in MeOH at a concentration of 4.0 × 10 −5 -mol/L at room temperature: (**Series 4**) Absorption spectra of azo dyes **4a**–**e** containing 4-aminophenylazo group; (**Series 5**) absorption spectra of azo dyes **5a**–**e** containing 4-(*N,N*-dimethylamino)phenylazo group; (**Series 6**) absorption spectra of azo dyes **6a**–**e** containing 4-hydroxyphenylazo group. Respective calculated spectra are shown as well.

**Table 1.** Most important electronic transitions. H letter indicates Highest Occupied Molecular Orbital HOMO, L indicates Lowest Unoccupied Molecular Orbital LUMO and +/− (number) represent subsequent orbitals above HOMO and LUMO, respectively. The logarithm of molar absorption coefficients log ε and oscillator strengths are given in parenthesis under the wavelengths, respectively for experimental and calculated maxima. The letters **a**–**e** stand for the particular compound in a given series (**4**, **5**or**6**).



**Table 1.** *Cont.*


**Table 1.** *Cont.*


**Table 1.** *Cont.*

Used abbreviations: n—non-bonding orbital; S-ring—thiadiazole ring; C6-ring—benzene ring; \*—antibonding orbital.
