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15N NMR study of protonation position determination
in 2-(6-methoxynaphthalen-2-yl)vinyl diazine
chromophores

Antonin Lyčka, Guillaume Noirbent, Sylvain Achelle

► To cite this version:

Antonin Lyčka, Guillaume Noirbent, Sylvain Achelle. 15N NMR study of protonation position determination in 2-(6-methoxynaphthalen-2-yl)vinyl diazine chromophores. Journal of Molecular Structure, 2026, 1350, pp.143980. 10.1016/j.molstruc.2025.143980. hal-05300894

HAL Id: hal-05300894

https://hal.science/hal-05300894v1

Submitted on 7 Oct 2025

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¹⁵N NMR study of protonation position determination in 2-(6-methoxynaphthalen-2-yl)vinyl diazine chromophores

Antonin Lyčka<sup>a,*</sup> <sup>●</sup>, Guillaume Noirbent<sup>b</sup>, Sylvain Achelle<sup>b,*</sup> <sup>●</sup>

ARTICLE INFO

Keywords:

Diazines

¹⁵N NMR

Protonation

Intramolecular charge transfer

ABSTRACT

The ¹H-¹⁵N HMBC spectra were successfully used for protonation position determination in 4-[(E)-2-(6-methoxynaphthalen-2-yl)vinyl]pyrimidine (1), 2-[(E)-2-(6-methoxynaphthalen-2-yl)vinyl]quinoxaline (2), 2-[(E)-2-(6-methoxynaphthalen-2-yl)vinyl]pyrazine (3), and 3-[(E)-2-(6-methoxynaphthalen-2-yl)vinyl]pyridazine (4). Mono protonation was proved. In three cases (compounds 1, 2 and 4) only one nitrogen was protonated while in pyrazine compound 3 the equilibrium between regioisomers was observed.

1. Introduction

Diazines and their benzo derivatives have been extensively studied for their photophysical properties [1–8]. Depending on the relative positions of the nitrogen atoms, the three diazine isomers can be distinguished: pyridazine (1,2-diazine), pyrimidine (1,3-diazine), and pyrazine (1,4-diazine). Among the main benzodiazine derivatives, the most notable are quinazoline (benzopyrimidine) and quinoxaline (benzopyrazine). (Fig. 1).

Push–pull styryldiazine derivatives are characterized by their strong photoluminescent properties, resulting from the intramolecular charge transfer between electron-donating and electron-withdrawing groups [3,9–12]. Styryldiazine derivatives have found applications in (bio) chemical sensing [13–16] and are characterized by their two-photon absorption properties, which are particularly useful in bioimaging and microfabrication [17–19]. In addition, certain styrylpyrimidines, styrylpyridazines, and styrylquinoxalines can be incorporated into second-order nonlinear optical (NLO) chromophores, functioning either as electron-withdrawing units or as π-conjugated linkers [18,20–23].

Azaheterocycles are known for their halochromism properties [24–29]. In particular push–pull diazine chromophores exhibit photophysical properties that are highly sensitive to protonation [11,30–33]. Diazines are commonly considered as mono base (pyridazine pKa ≈ 2.3, pyrimidine pKa ≈ 1.1, pyrazine pKa ≈ 0.65, quinoxaline pKa ≈ 0.56) [34]. Protonation of the first nitrogen atom leads to a marked decrease in the basicity of the second nitrogen, due to the electron-withdrawing inductive effect of the resulting quaternary ammonium center (in case

of pyrimidine pKa ≈ -6.3) [24]. Protonation of the diazine ring significantly enhances the electron-withdrawing character of the resulting diazinyln fragment by engaging the lone pair of one of the nitrogen atoms. This modification leads to red-shifted absorption spectra [11,24,30–32]. In contrast to their pyridazine, pyrazine, and quinoxaline analogues, certain styrylpyrimidine and arylvinylpyrimidine derivatives remain luminescent upon protonation, exhibiting red-shifted emission [24,35]. By careful control of partial protonation, white light emission can be observed in both solution and solid state [24]. The protonation has been demonstrated as a way to significantly and reversibly increase the two-photon absorption properties of styrylpyrimidines [36]. Moreover it has been demonstrated that the protonated form of N,N-diphenylaminostyrylpyrimidines are observed upon oxidation [37]. Up to now, the position of protonation was not experimentally determined. In case of 4-styrylpyrimidine and 4-(arylvinyl)pyrimidine chromophores, theoretical calculation tends to indicate protonation in N1 position (the less sterically hindered) [35,38]. On the other hand, 2-(arylvinyl)pyrazine, 2-(arylvinyl)quinazoline and 3-(arylvinyl)pyridazine protonation is expected to occur the nitrogen closest to the vinyl moiety, with a non-negligible proportion of protonation in N4 position of the pyrazine derivative [35,38]

¹H and ¹³C NMR spectra are frequently used for characterisation of chemical compounds, contrary to application of ¹⁵N NMR spectra. The reason is that the sensitivity of ¹⁵N NMR is much lower compared to ¹H and ¹³C NMR spectroscopy due to low natural abundance of ¹⁵N isotope and its low γ value. On the other hand, ¹⁵N NMR spectroscopy is capable of providing original and unique pieces of information in cases when

<footnote><sup>a</sup> University of Hradec Králové, Faculty of Science, Rokitanského 62, CZ-500 03 Hradec Králové, Czech Republic</footnote>

<footnote><sup>b</sup> Univ. Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes), UMR 6226, F-35000 Rennes, France</footnote>

<footnote>* Correspondence authors.</footnote>

<footnote>E-mail addresses: antonin.lycka@uhk.cz (A. Lyčka), Sylvain.achelle@univ-rennes1.fr (S. Achelle).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Structure of (benzo)diazine moieties.</figcaption></figure>

nitrogen atoms are directly involved in chemical processes or transformations of compounds [39,40]. The greatest changes in ¹⁵N chemical shifts and J(¹⁵N, ¹H) coupling constants can be observed in tautomeric systems and protonation of aromatic and heterocyclic compounds [39, 40].

In this study, thanks to a series of 1D and 2D ¹⁵N NMR analyses, the position of protonation of various methoxynaphthyl substituted vinyl-diazine derivatives 1–4 (Scheme 2) was unequivocally demonstrated. This work that complements the previous theoretical work significantly enhances the understanding of the effect of protonation on the photo-physical properties of diazine derivatives.

## 2. Results and discussion

The structures of the studied compounds are shown in Fig. 2. First, ¹H and ¹³C NMR spectra including appropriate 2D experiments were measured in deuterated chloroform and analysed so that one could understand and interpret ¹H-¹⁵N HMBC spectra. The aim of this paper is protonation position determination in asymmetric 2-(6-methoxynaphthalen-2-yl)vinyl diazine compounds and ¹H-¹⁵N HMBC spectra were used for this purpose because the greatest changes were expected for ¹⁵N chemical shifts. Having done this, CF₃COOH was added to chloroform solution and all measurements were repeated. ¹H and ¹³C chemical shifts obtained in chloroform are collected in Table 1, those in chloroform and 0.050 mL of CF₃COOH in Table 2 and all ¹⁵N chemical shifts are presented in Table 3. The structure of the protonated compounds is shown in Fig. 3.

Fig. 4 shows ¹H-¹⁵N HMBC spectrum of compound 1 in CDCl₃ where two very similar ¹⁵N chemical shifts can be observed. The correlation of

<table><caption>**Table 1**

¹H and ¹³C chemical shifts in compounds 1–4 in 0.7 mL of CDCl₃.</caption><thead><tr><th rowspan="2">H/C or N<br>No.</th><th colspan="2">1</th><th colspan="2">2<sup>a</sup></th><th colspan="2">3</th><th colspan="2">4</th></tr><tr><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th></tr></thead><tbody><tr><td>2</td><td>9.16</td><td>158.8</td><td>–</td><td>150.8</td><td>–</td><td>151.4</td><td>–</td><td>–</td></tr><tr><td>3</td><td>–</td><td>–</td><td>9.03</td><td>144.4</td><td>8.64</td><td>143.7</td><td>–</td><td>158.4</td></tr><tr><td>4</td><td>–</td><td>162.4</td><td>–</td><td>–</td><td>–</td><td>–</td><td>7.63</td><td>123.8</td></tr><tr><td>5</td><td>7.30</td><td>118.5</td><td>–</td><td>142.4</td><td>8.53</td><td>144.3</td><td>7.43</td><td>126.4</td></tr><tr><td>6</td><td>8.65</td><td>157.2</td><td>–</td><td>141.4</td><td>8.38</td><td>142.4</td><td>9.04</td><td>149.5</td></tr><tr><td>7</td><td>7.09</td><td>124.6</td><td>7.41</td><td>124.4</td><td>7.20</td><td>123.1</td><td>7.40</td><td>124.2</td></tr><tr><td>8</td><td>7.99</td><td>137.7</td><td>7.96</td><td>136.5</td><td>7.86</td><td>135.3</td><td>7.81</td><td>135.3</td></tr><tr><td>9</td><td>–</td><td>130.9</td><td>–</td><td>131.2</td><td>–</td><td>131.4</td><td>–</td><td>131.2</td></tr><tr><td>10</td><td>7.89</td><td>128.9</td><td>7.92</td><td>128.6</td><td>7.87</td><td>128.2</td><td>7.88</td><td>128.2</td></tr><tr><td>11</td><td>–</td><td>128.8</td><td>–</td><td>128.8</td><td>–</td><td>128.8</td><td>–</td><td>128.8</td></tr><tr><td>12</td><td>7.74</td><td>129.9</td><td>7.75</td><td>129.9</td><td>7.74</td><td>129.9</td><td>7.74</td><td>129.8</td></tr><tr><td>13</td><td>7.16</td><td>119.3</td><td>7.16</td><td>119.3</td><td>7.15</td><td>119.2</td><td>7.16</td><td>119.3</td></tr><tr><td>14</td><td>–</td><td>158.5</td><td>–</td><td>158.4</td><td>–</td><td>158.3</td><td>–</td><td>158.4</td></tr><tr><td>15</td><td>3.92</td><td>55.3</td><td>3.92</td><td>55.3</td><td>3.92</td><td>55.3</td><td>3.91</td><td>55.3</td></tr><tr><td>16</td><td>7.14</td><td>105.9</td><td>7.12</td><td>105.9</td><td>7.12</td><td>105.9</td><td>7.13</td><td>105.9</td></tr><tr><td>17</td><td>–</td><td>135.2</td><td>–</td><td>135.0</td><td>–</td><td>134.9</td><td>–</td><td>134.9</td></tr><tr><td>18</td><td>7.73</td><td>127.4</td><td>7.74</td><td>127.4</td><td>7.73</td><td>127.3</td><td>7.76</td><td>127.4</td></tr><tr><td>19</td><td>7.73</td><td>124.1</td><td>7.79</td><td>124.0</td><td>7.73</td><td>124.0</td><td>7.76</td><td>124.0</td></tr></tbody></table>

a8.06/129.0 (HC-20), 7.68/129.1 (HC-21), 7.74/130.2(HC-22), 8.06/129.0 (HC-23).

nitrogen N-3 and proton H-7 via ³J(¹⁵N, ¹H) is of key importance for assignment allowing the differentiation between these two ¹⁵N resonances. Nitrogen N-1 is too far from proton H-7 and, thus, no correlation with this nitrogen is observable. ¹H-¹⁵N HMBC spectrum of compound 1 after addition of 0.05 mL of CF₃COOH shows two quite different resonances. It is well known from literature [39,40] that protonation of “pyridine-like” nitrogens result in shifts up to ca 145 ppm to lower frequencies. Correlation of nitrogen N-3 and proton H-7 via ³J(¹⁵N, ¹H) is visible for nitrogen whose change of ¹⁵N chemical shift is rather small (-13.1 ppm) and, thus, nitrogen N-1 must undoubtedly be protonated (change -94.2 ppm, Table 3). A small change of ¹⁵N chemical shift of N-3 can be ascribed to a change of substituent chemical shift effect (SCS) since (N-1)⁺-H influences ¹⁵N chemical shift of N-3 in a different manner as N-1 did (the same is observed for ¹H and ¹³C chemical shifts, see data in Tables 1 and 2).

Fig. 5 shows quite analogous situation to Fig. 4. The only difference is that in compound 2, nitrogen N-1 is protonated based on the correlation of N-1 and H-7 in ¹H-¹⁵N HMBC spectrum of compound 2 being shifted to lower frequency. The protonation differences are -117.9 ppm (N-1)

<figure><img src="image_5.png" /><figcaption>Fig. 2. Structure of studied compounds 1–4.</figcaption></figure>
<caption>**Table 2**</caption>

¹H and ¹³C chemical shifts in compounds **1–4** in 0.7 mL of CDCl₃ and 0.05 mL of CF₃COOH.

<table><thead><tr><th rowspan="2">H/C or N<br>No.</th><th colspan="2">1</th><th colspan="2">2<sup>a</sup></th><th colspan="2">3</th><th colspan="2">4</th></tr><tr><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th><th><sup>1</sup>H</th><th><sup>13</sup>C</th></tr></thead><tbody><tr><td>2</td><td>9.24</td><td>150.4</td><td>-</td><td>146.4</td><td>-</td><td>153.1</td><td>-</td><td>-</td></tr><tr><td>3</td><td>-</td><td>-</td><td>9.76</td><td>143.8</td><td>9.12</td><td>138.2</td><td>-</td><td>158.0</td></tr><tr><td>4</td><td>-</td><td>170.1</td><td>-</td><td>-</td><td>-</td><td>-</td><td>8.46</td><td>131.2</td></tr><tr><td>5</td><td>7.73</td><td>118.9</td><td>-</td><td>139.3</td><td>8.94</td><td>142.9</td><td>8.18</td><td>134.6</td></tr><tr><td>6</td><td>8.75</td><td>147.2</td><td>-</td><td>131.0</td><td>8.62</td><td>135.0</td><td>9.09</td><td>148.1</td></tr><tr><td>7</td><td>7.30</td><td>120.4</td><td>7.61</td><td>114.3</td><td>7.31</td><td>117.4</td><td>7.38</td><td>114.2</td></tr><tr><td>8</td><td>8.51</td><td>150.4</td><td>8.46</td><td>150.9</td><td>8.05</td><td>143.7</td><td>8.01</td><td>146.9</td></tr><tr><td>9</td><td>-</td><td>128.8</td><td>-</td><td>129.3</td><td>-</td><td>129.6</td><td>-</td><td>128.9</td></tr><tr><td>10</td><td>8.07</td><td>133.5</td><td>8.16</td><td>134.5</td><td>7.93</td><td>131.4</td><td>7.98</td><td>132.3</td></tr><tr><td>11</td><td>-</td><td>129.4</td><td>-</td><td>128.8</td><td>-</td><td>128.6</td><td>-</td><td>128.7</td></tr><tr><td>12</td><td>7.83</td><td>131.1</td><td>7.86</td><td>131.5</td><td>7.74</td><td>130.6</td><td>7.80</td><td>130.8</td></tr><tr><td>13</td><td>7.25</td><td>119.9</td><td>7.24</td><td>120.2</td><td>7.18</td><td>119.8</td><td>7.21</td><td>119.9</td></tr><tr><td>14</td><td>-</td><td>159.9</td><td>-</td><td>160.4</td><td>-</td><td>159.3</td><td>-</td><td>159.6</td></tr><tr><td>15</td><td>3.99</td><td>55.6</td><td>4.02</td><td>55.7</td><td>3.95</td><td>55.5</td><td>3.98</td><td>55.6</td></tr><tr><td>16</td><td>7.22</td><td>106.8</td><td>7.22</td><td>106.9</td><td>7.14</td><td>106.4</td><td>7.18</td><td>106.6</td></tr><tr><td>17</td><td>-</td><td>137.3</td><td>-</td><td>137.9</td><td>-</td><td>136.3</td><td>-</td><td>136.9</td></tr><tr><td>18</td><td>7.77</td><td>128.3</td><td>7.81</td><td>128.8</td><td>7.76</td><td>128.0</td><td>7.80</td><td>128.4</td></tr><tr><td>19</td><td>7.79</td><td>124.2</td><td>7.86</td><td>123.6</td><td>7.72</td><td>123.8</td><td>7.72</td><td>123.7</td></tr></tbody></table>

a 8.20/121.0 (HC-20), 8.13/136.2 (HC-21), 8.06/133.0 (HC-22), 8.34/128.2 (HC-23).

and -5.7 ppm (N-4).

The situation for compound **3** (Fig. 6) is considerably different from previous two cases. Rather great shifts were observed for both nitrogens and we interpret this fact as equilibrium between two regioisomers (see Fig. 3). This might be explained by a balance between electronic effects, which favor protonation at the conjugated N-1 position, and steric hindrance, which disfavors it. The equilibrium is shifted to the form where nitrogen N-1 is preferentially protonated.

Fig. 7 shows the changes of ¹⁵N chemical shifts due to protonation of

compound **4**. The changes in ¹⁵N chemical shifts of both nitrogen atoms are huge: -124.8 ppm for N-2 and -50.0 ppm for N-1 (Table 3).

For comparison, we prepared 1-methylpyridazin-1-ium iodide **5** using CH₃-I (by mixing of pyridazine and methyl iodide 1:1 (both liquids) and heat gently in water bath without any solvent for one hour). Raw solid consists of the expected 1-methylpyridazin-1-ium iodide and ca 10 molar % of starting pyridazine. We measured ¹H-¹⁵N HMBC and found the following results in DMSO-D6 (1-methylpyridazin-1-ium iodide is negligibly soluble in CDCl₃) and it was advantageous to have ¹⁵N resonances of both compounds in one spectrum (Fig. 8).

¹⁵N chemical shift of both nitrogen (due to the symmetry) in pyridazine was +16.0, while ¹⁵N chemical shift of N-1 was -132.1 and -27.1 for N-2 (proving mono methylation).

$$ \Delta = \delta(^{15}\text{N})_{\text{CDCl}_3 + \text{CF}_3\text{COOH}} - \delta(^{15}\text{N})_{\text{CDCl}_3} = -148.1 \text{ for N-1 in compound 5 is comparable with that for N-2 (-124.8 ppm) in compound 4. It is clear that "ortho" effect of alkylation on N-2 (-43.1 ppm) in compound 5 is also huge and very similar to -50.0 ppm for N-1 in compound 4, i.e. much bigger than those in our pyrimidine and pyrazine compounds. Protonation effect could be slightly different compared with methylation.} $$

When we add both differences together, we got the “total” difference -174.8 ppm for **4** and -191.2 ppm for 1-methylpyridazin-1-ium iodide, i. e., these values are rather similar. Compound **4** is a pull-push type of compound and this fact might be responsible for a possible redistribution of the protonation effect. Basic conclusion is that protonation takes place (at least strongly predominantly) on N-2 in **4**. Of course, some effect of solvent cannot be excluded, however, we could not measure 1-methylpyridazin-1-ium iodide in CDCl₃.

### 3. Conclusion

In this study, we successfully identified the site of protonation in methoxynaphthyl-substituted vinyldiazine derivatives using 1D and 2D

<caption>**Table 3**</caption>

¹⁵N chemical shifts in compounds **1–4** in CDCl₃ and CDCl₃ + 0.05 mL of CF₃COOH.

<table><thead><tr><th rowspan="2">N No.</th><th colspan="3">1</th><th colspan="3">2</th><th colspan="3">3</th><th colspan="3">4</th></tr><tr><th>CDCl<sub>3</sub></th><th>CDCl<sub>3</sub> + CF<sub>3</sub>COOH</th><th>Δ<sup>a</sup></th><th>CDCl<sub>3</sub></th><th>CDCl<sub>3</sub> + CF<sub>3</sub>COOH</th><th>Δ<sup>a</sup></th><th>CDCl<sub>3</sub></th><th>CDCl<sub>3</sub> + CF<sub>3</sub>COOH</th><th>Δ<sup>a</sup></th><th>CDCl<sub>3</sub></th><th>CDCl<sub>3</sub> + CF<sub>3</sub>COOH</th><th>Δ<sup>a</sup></th></tr></thead><tbody><tr><td>1</td><td>-95.1</td><td>-194.0</td><td>-98.9</td><td>-69.5</td><td>-187.4</td><td>-117.9</td><td>-62.7</td><td>-91.8</td><td>-29.1</td><td>16.5</td><td>-66.5</td><td>-50.0</td></tr><tr><td>2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>10.1</td><td>-114.7</td><td>-124.8</td></tr><tr><td>3</td><td>-99.8</td><td>-108.2</td><td>-8.4</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>4</td><td>-</td><td>-</td><td>-</td><td>-54.3</td><td>-60.0</td><td>-5.7</td><td>-50.2</td><td>-109.2</td><td>-59.0</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

$$ a \Delta = \delta(^{15}\text{N})_{\text{CDCl}_3 + \text{CF}_3\text{COOH}} - \delta(^{15}\text{N})_{\text{CDCl}_3} $$

<figure><img src="image_6.png" /><figcaption>1</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>2</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>5</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>3</figcaption></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>4</figcaption></figure>

<caption>**Fig. 3.** Structure of protonated forms of compounds **1–4** obtained upon addition of CF₃COOH in CDCl₃ and structure of compound **5**.</caption>
<figure><img src="image_12.png" /><figcaption>Fig. 4. ¹H-¹⁵N HMBC spectrum of compound 1 in 0.7 mL of CDCl₃ (left) and 0.7 mL of CDCl₃ and 0.05 mL of CF₃COOH (right).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 5. ¹H-¹⁵N HMBC spectrum of compound 2 in 0.7 mL of CDCl₃ (left) and 0.7 mL of CDCl₃ and 0.05 mL of CF₃COOH (right).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 6. ¹H-¹⁵N HMBC spectrum of compound 3 in 0.7 mL of CDCl₃ (left) and 0.7 mL of CDCl₃ and 0.05 mL of CF₃COOH (right).</figcaption></figure>

¹⁵N NMR spectroscopy. The large downfield shifts in ¹⁵N resonances upon CF₃COOH addition confirmed protonation predominantly at the N-1 position for compounds 1 and 2, at the N-2 position for compound 4,

while compound 3 exhibited behavior consistent with a protonation equilibrium. The observed protonation-induced shifts highlight the utility of ¹⁵N NMR in probing subtle electronic changes in diazine
<figure><img src="image_15.png" /><figcaption>Fig. 7. ¹H-¹⁵N HMBC spectrum of compound 4 in 0.7 mL of CDCl₃ (left) and 0.7 mL of CDCl₃ and 0.05 mL of CF₃COOH (right).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 8.** ¹H-¹⁵N HMBC spectrum of 1-methylpyridazin-1-ium iodide 5 containing ca 10 molar % of pyridazine in 0.7 mL of DMSO-D₆.</figcaption></figure>

systems. These results also reinforce the importance of considering both
structure and substitution patterns when predicting protonation
behavior in diazine systems and provide valuable insight into the
structure-property relationships of diazine-based push-pull chromo-
phores. In addition to their fundamental significance, these new findings
will aid in the design of new diazine-based molecular materials for white
light emission and anticounterfeiting applications through controlled
protonation. Future research will focus on the study of amino-
substituted styryldiazine derivatives that possess an additional proton-
ation site.

**4. Experimental**

**4.1. Synthesis**

Compounds 1–4 were prepared according to previously published
data [29].

1-Methylpyridazin-1-ium iodide (5) was prepared by gentle heating of equimolar amounts of pyridazine and methyl iodide for one hour.

**4.2. NMR spectroscopy**

The ¹H, ¹³C and ¹⁵N NMR spectra were recorded on a Bruker Avance III HD 400 spectrometer operating at 400.13 MHz for ¹H, 100.62 MHz for ¹³C and at 40.56 MHz for ¹⁵N and using liquid nitrogen-cooled 5-mm Prodigy cryo probe (Bruker) at 300 K. The samples were dissolved in deuterated chloroform. The ¹H and ¹³C chemical shifts were referenced to internal TMS (δ = 0.0). The ¹⁵N chemical shifts were referred to the signal of external nitromethane placed in a co-axial capillary (δ = 0.0). All 2D experiments (gradient-selected (gs)-COSY, gs-NOESY, gs-HMQC, gs-HSQC, gs-HMBC) were performed using manufacturer's software (TOPSPIN 3.5) [41–43]. All measurements were repeated after addition of 0.050 mL of CF₃COOH into deuterated chloroform solution.

CRediT authorship contribution statement

**Antonin Lyčka:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Data curation, Conceptualization.
**Guil-laume Noirbent:** Investigation. **Sylvain Achelle:** Writing – review & editing, Writing – original draft, Supervision, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgements

A. L. thanks University of Hradec Králové for financial support. ANR is acknowledged though the TNT-sensor-IA project (grant ANR-23-ASTR-0021-02). S. A. Is grateful to EUR LUMOMAT project and the investments for the future program (ANR-19-EURE-0012)

**Supplementary materials**

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2025.143980.

**Data availability**

Data will be made available on request.
References

[1] S. Achelle, N. Plé, A. Turck, RSC Adv. 1 (2011) 364–388.

[2] G.N. Lipunova, E.V. Nosova, V.N. Charushin, O.N. Chupakhin, Curr. Org. Synth. 15 (2018) 793–814.

[3] S. Achelle, J. Rodríguez-López, F. Robin-le Guen, Org. Biomol. Chem. 21 (2023) 39–52.

[4] E.V. Verbitskiy, G.N. Lipunova, E.V. Nosova, V.N. Charushin, Dyes Pigm. 220 (2023) 111763.

[5] P. Meti, H.-H. Park, Y.-D. Gong, J. Mater. Chem. C 8 (2020) 352–379.

[6] S. Achelle, C. Baudequin, N. Plé, Dyes Pigm. 98 (2013) 575–600.

[7] E.V. Nosova, S. Achelle, G.N. Lipunova, V.N. Charushin, O.N. Chupakhin, Russ. Chem. Rev. 88 (2019) 1128–1178.

[8] S. Achelle, M. Hodée, J. Massue, A. Fihey, C. Katan, Dyes Pigm. 200 (2022) 110157.

[9] S. Achelle, A. Barsella, C. Baudequin, B. Caro, F. Robin-le Guen, J. Org. Chem. 77 (2012) 4087–4096.

[10] F. Lincker, D. Kreher, A.-J. Attias, J. Do, E. Kim, P. Hapiot, N. Lemaître, B. Geffroy, G. Ulrich, R. Ziessel, Inorg. Chem. 49 (2010) 3991–4001.

[11] V. Schmitt, S. Moschel, H. Detert, Eur. J. Org. Chem. (2013) 5655–5669.

[12] S. Achelle, J. Rodríguez-López, F. Robin-le Guen, J. Org. Chem. 79 (2014) 7564–7571.

[13] A. Boländer, D. Kieser, C. Voss, S. Bauer, C. Scön, S. Burgold, T. Bittner, J. Hölzer, R. Heyny-von Haussen, G. Mall, V. Goetschy, C. Czech, H. Knust, R. Berger, J. Herms, I. Hilger, B. Schmidtt, J. Med. Chem. 55 (2012) 9170–9180.

[14] J.-P. Malval, M. Cranney, S. Achelle, H. Akdas-Kiliç, J.-L. Fillaut, N. Cabon, F. Robin-le Guen, O. Soppera, Y. Molard, Chem. Commun. 55 (2019) 14331–14334.

[15] X.-S. Ma, Y.-Z. Cui, Y.-Q. Ding, F.-R. Tao, B. Zheng, R.-H. Yu, W. Huang, Sens. Act. B 238 (2017) 48–57.

[16] N. Wu, L. Zhang, X. Zhang, Q. Zhang, J. Liu, Y. Li, X.-X. Yan, Y. Liang, J. Zhang, M. Cui, Mol. Pharmaceutics 20 (2023) 5865–5876.

[17] M. Rumi, S.J.K. Pond, T. Meyer-Friedrichsen, Q. Zhang, M. Bishop, Y. Zhang, S. Barlow, S.R. Marder, J.W. Perry, J. Phys. Chem. C 112 (2008) 8061–8071.

[18] B. Liu, H.-L. Zhang, J. Liu, Y.-D. Zhao, Q.-M. Luo, Z.-L. Huang, J. Mater. Chem. 17 (2007) 2921–2929.

[19] M. Fecková, P. le Poul, F. Bureš, F. Robin-le Guen, S. Achelle, Dyes Pigm. 182 (2020) 108659.

[20] A.A. Kalinin, L.N. Islamova, S.M. Sharipova, G.M. Fazleeva, A.I. Gaysin, A. G. Shmelev, A.E. Simanchuk, S.A. Turgeneva, A.V. Sharipova, A.S. Mukhtarov, T. A. Vakhonina, O.D. Fominykh, S.L. Mikerin, M.Y. Balakina, New J. Chem. 47 (2023) 2296–2306.

[21] L.N. Islamova, A.A. Kalinin, A.A. Shustikov, G.M. Fazleeva, A.I. Gaysin, A. G. Shmelev, A.E. Simanchuk, N.I. Shalin, A.V. Sharipova, O.B. Babaeva, T. A. Vakhonina, O.D. Fominykh, A.R. Khamatgalimov, S.L. Mikerin, M.Y. Balakina, Dyes Pigm. 216 (2023) 111316.

[22] M. Klikar, P. le Poul, A. Rúzička, O. Pytela, A. Barsella, K.D. Dorkenoo, F. Robin-le Guen, F. Bureš, S. Achelle, J. Org. Chem. 82 (2017) 9435–9451.

[23] T.-R. Cheng, C.-H. Huang, L.-B. Gan, C.-P. Luo, A.-C. Yu, X.-S. Zhao, J. Mater. Chem. 8 (1998) 931–935.

[24] S. Achelle, J. Rodríguez-López, F. Bureš, F. Robin-le Guen, Chem. Rec. 20 (2020) 440–451.

[25] S. Glang, T. Rieth, D. Borchmann, I. Fortunati, R. Signorini, H. Detert, Eur. J. Org. Chem. (2014) 3116–3126.

[26] T. Gao, B. Shen, Y. Liang, X. Liu, H. Gao, Y. Xiao, J. Mol. Struct. 1327 (2025) 14241.

[27] T.-L. Zhang, Z.-B. Fan, C.-C. Zhang, H.-Y. Lin, D.-H. Qu, F. Tong, Dyes Pigm. 229 (2024) 112313.

[28] L.N. Kloeters, L. May, B. Tengen, L. May, T.J.J. Müller, Dyes Pigm. 206 (2022) 110564.

[29] U. Giovanella, E. Cariati, E. Lucenti, M. Pasini, F. Caleotti, C. Botta, ChemPhysChem 18 (2017) 2157–2161.

[30] H.-H. Park, P. Meti, Y.-D. Gong, Dyes Pigm. 190 (2021) 109320.

[31] M. Lamoria, M.D. Milton, Dyes Pigm. 213 (2023) 111182.

[32] S. Lee, S.-H. Kim, S. Lee, Y. Lee, Y.S. Lee, K.-H. Yang, K.-K. Wang, W.-S. Han, Dyes Pigm. 194 (2021) 109613.

[33] M. Li, Y. Yuan, Y. Chen, ACS Appl. Mater. Interfaces 10 (2018) 1237–1243.

[34] R. Milcent, Chimie Organique Hétérocyclique, EDP Science, Les Ulis, 2003, p. 605.

[35] S. Achelle, J. Rodríguez-López, C. Katan, F. Robin-le Guen, J. Phys. Chem. C 120 (2016) 26989–26995.

[36] S. Lampros, K. Alexandros, I. Polyzos, T. Bonnaud, G. Noirbent, S. Achelle, M. Fakis, ChemPhotoChem 9 (2025) e202400278.

[37] O. Alévèque, S. Achelle, L. Sanguinet, ChemPhotoChem 7 (2023) e202200201.

[38] M. Hodée, A. Lenne, J. Rodríguez-López, F. Robin-le Guen, C. Katan, S. Achelle, A. Fihey, New J. Chem. 45 (2021) 19132–19144.

[39] R. Marek, A. Lyčka, Cur. Org. Chem. 7 (2002) 35–66.

[40] A.Lyčka R.Marek, E. Kolehmainen, E. Sieveren, J. Toušek, Cur. Org. Chem. 11 (2007) 1154–1205.

[41] S. Berger, S. Braun, 200 and More Basic NMR Experiments: A Practical Course, 3rd ed., Wiley-VCH, Weinheim, 2004. ISBN: 978-3-527-31067-8.

[42] T.D.W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, 3rd ed., Elsevier, 2016. ISBN: 9780080999869.

[43] N.E. Jacobsen, NMR Data Interpretation Explained, Wiley, New Jersey, 2016. ISBN: 9781118370223.