Indole-, benzofuran- and benzothiophene-containing
analogues of ceramidonines and fused
3,9-diazaperylenes: Synthesis and physicochemical
properties of these new polycyclic azaarenes

Nicolas Mast, Ludmilla Verrieux, William Erb, Nicolas Richy, Olivier Mongin, Jean-Pierre Hurvois, Thierry Roisnel, Arnaud Fihey, Boris Le Guennic, Florence Mongin

► To cite this version:

Nicolas Mast, Ludmilla Verrieux, William Erb, Nicolas Richy, Olivier Mongin, et al.. Indole-, benzofuran- and benzothiophene-containing analogues of ceramidonines and fused 3,9-diazaperylenes: Synthesis and physicochemical properties of these new polycyclic azaarenes. Dyes and Pigments, 2025, 235, pp.112612. 10.1016/j.dyepig.2024.112612. hal-04928904

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# Indole-, benzofuran- and benzothiophene-containing analogues of ceramidonines and fused 3,9-diazaperylenes: Synthesis and physicochemical properties of these new polycyclic azaarenes

Nicolas Mast, Ludmilla Verrieux, William Erb, Nicolas Richy, Olivier Mongin*,
Jean Pierre Hurvois, Thierry Roisnel, Arnaud Fihey*, Boris Le Guennic, Florence Mongin*

<figure><img src="image_2.png" /></figure>

Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000, Rennes, France

## ARTICLE INFO

**Keywords:**

Ceramidonine

Diazaperylene

Anthraquinone

Photosensitisation

Fluorescence

## ABSTRACT

Ceramidonine analogues and fused 3,9-diazaperylenes were synthesised from aminoanthraquinones and 2-iodinated heteroaromatics by a copper-promoted *N*-arylation-cyclisation sequence. Their photophysical properties, determined experimentally and analysed through computation, revealed either efficient photosensitisers or fluorescent compounds, respectively.

## 1. Introduction

The incorporation of heteroatoms such as nitrogen, oxygen and sulfur into polyaromatic hydrocarbons (PAHs) allows the tuning of their physicochemical (e.g. electronic and photophysical) properties beyond the scope of all-carbon analogues [1–3].

Among these polycyclic heteroaromatic compounds, naphtho[3,2,1-kl]acridin-9-ones (known as ceramidonines) [4–7] and isomeric pyridine-fused benzanthrones [8] can exhibit promising dyeing or fluorescence properties, and also show strong potential for application in liquid crystal systems (Fig. 1, top left and middle) [1,2]. Our first objective in this work will be to replace the upper benzo ring of ceramidonine with an electron-rich aromatic heterocycle (indole, benzofuran or benzothiophene) and to study the physicochemical properties of these new hexacyclic compounds (Fig. 1, bottom left).

In addition, 3,9-diazaperylene has been shown to possess superior electron acceptor and fluorescence properties [9], which can be modulated by introducing substituents [10] or fused rings [7] (Fig. 1, top right). Therefore, our second objective will be to fuse this pentacyclic unit with the same heteroaromatics as above in order to study the photophysical properties of the resulting scaffolds (Fig. 1, bottom right).

## 2. Results and discussion

### 2.1. Synthesis

The first synthesis of ceramidonine was reported over a century ago by cyclodehydration of 1-arylaminoanthraquinones under acidic conditions [11]. The precursors were initially obtained by aromatic nucleophilic substitution between an aniline and 1-chloroanthraquinone [12–14]. More recently, related substrates have also been prepared by palladium-catalysed cross-coupling [6,15,16]. However, access to our targets requires a different approach, as 2-aminoindole, 2-amino-benzofuran and 2-aminobenzothiophene are not common substrates in such cross-coupling reactions. We therefore considered an alternative synthesis using commercially available 1-aminoanthraquinone (1). Indeed, short syntheses of ceramidonines have been documented from 1, notably by reaction with benzyne intermediates [17,18] and by Chan-Lam coupling-cyclisation sequences [19]. However, we preferred to apply a copper-promoted *N*-arylation-cyclisation sequence that we had developed from 1 and 2-iodo-*N*-methylindole [20].

Consequently, 1 was treated with each of the 2-iodinated heterocycles in the presence of activated copper and potassium carbonate in dimethylsulfoxide (DMSO) at 160 °C for 24 h. However, only the furan

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>*** Corresponding author.</footnote>

<footnote>E-mail addresses: olivier.mongin@univ-rennes.fr (O. Mongin), arnaud.fihey@univ-rennes.fr (A. Fihey), florence.mongin@univ-rennes.fr (F. Mongin).</footnote>
(a) Previously reported derivatives of benzanthrone, ceramidonine and 3,9-diazaperylene:

<figure><img src="image_3.png" /></figure>

(b) This work:

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) Derivatives of benzanthrone (left), ceramidonine (middle) and 3,9-diazaperylene (right) with reported physicochemical properties; (b) Compounds reported in this work.</figcaption></figure>

analogue of ceramidonine **2a** could be isolated in satisfactory yield under these conditions. By lowering the reaction temperature to 120 °C, the benzothiophene and *N*-methylindole analogues **2b** and **2c** were finally obtained in yields of 33 % and 78 %, respectively (Scheme 1).

To obtain 3,9-diazaperylenes fused with benzofuran, benzothiophene and *N*-methylindole, a double *N*-arylation-cyclisation was carried out from 1,5-diaminoanthraquinone (3). The expected products **4a-c** were isolated in yields ranging from 15 % to 95 %, consistent with the expected reactivities of *N*-methylindole, benzofuran and benzothiophene in copper-promoted *N*-arylation and subsequent cyclisation [21]. The products **4'a-c**, resulting from a single *N*-arylation-cyclisation, were also obtained in low yields (Scheme 2).

Compound **4b** crystallised in the orthorhombic space group (F DMSO-d₆ 2), each of its benzothieno[2,3-b]quinoline units being almost planar and twisted relative to each other by 24°, deforming the central ring (Fig. 2).

## 2.2. Electrochemical properties

As very few structures related to compound series **2** and **4** have been reported, we initially examined the electrochemical properties of **2a-c**, **4a** and **4c** by cyclic voltammetry (CV) in degassed dimethylformamide (DMF) using Ag/AgCl as the reference electrode. The main results are

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Synthesis of benzofuran, benzothiophene and *N*-methylindole analogues of ceramidonines from 1-aminoanthraquinone. MS: molecular sieves. [a] Added every 2 h.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Synthesis of benzofuran-, benzothiophene- and *N*-methylindole-fused 3,9-diazaperylenes from 1,5-diaminoanthraquinone. [a] Added every 2 h.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 2.** ORTEP diagrams (30 % probability) of **4b** (CCDC 2393840). Left: top view. Right: side view.</figcaption></figure>

summarised in Table 1. All the reduction processes, formation of the radical anion followed by the dianion, were found to be reversible, with ratios between cathodic CV peak current ($i_{pc}$) and anodic CV peak current ($i_{pa}$) close to unity. Compounds **2a** and **2b** exhibited close first/ second reduction potentials, while the replacement of O/S by NMe (compound **2c**) resulted in cathodic shifts of 110–120 mV. Compared with **2a**, the first reduction of **4a** was slightly more difficult due to the absence of the carbonyl group, but the second reduction was easier. A similar trend was observed between **2c** and **4c**.

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

Electrochemical data (in V) of compounds **2a-c**, **4a** and **4c**.[a]</caption><thead><tr><th rowspan="2">Compound</th><th colspan="3">1st reduction</th><th colspan="3">2nd reduction</th></tr><tr><th>E<sub>pc</sub><sup>[b]</sup></th><th>E<sub>pa</sub><sup>[c]</sup></th><th>i<sub>pc</sub>/i<sub>pa</sub></th><th>E<sub>1</sub>/<sub>2</sub><sup>[d]</sup></th><th>E<sub>pc</sub><sup>[b]</sup></th><th>E<sub>pa</sub><sup>[c]</sup></th><th>i<sub>pc</sub>/i<sub>pa</sub></th><th>E<sub>1</sub>/<sub>2</sub><sup>[d]</sup></th></tr></thead><tbody><tr><td>2a</td><td>-</td><td>-</td><td>0.95</td><td>-</td><td>-</td><td>-</td><td>0.90</td><td>-</td></tr><tr><td>2b</td><td>0.85</td><td>0.75</td><td>-</td><td><strong>0.80</strong></td><td>1.40</td><td>1.31</td><td>-</td><td><strong>1.36</strong></td></tr><tr><td>2c</td><td>-</td><td>-</td><td>0.95</td><td><strong>0.81</strong></td><td>1.41</td><td>1.32</td><td>-</td><td><strong>1.37</strong></td></tr><tr><td>4a</td><td>-</td><td>-</td><td>0.93</td><td>-</td><td>1.53</td><td>1.43</td><td>-</td><td><strong>1.48</strong></td></tr><tr><td>4c</td><td>-</td><td>-</td><td>1.01</td><td>-</td><td>-</td><td>-</td><td>0.85</td><td>-</td></tr><tr><td></td><td>0.89</td><td>0.79</td><td>-</td><td><strong>0.84</strong></td><td>1.35</td><td>1.27</td><td>-</td><td><strong>1.31</strong></td></tr><tr><td></td><td>-</td><td>-</td><td>1.10</td><td>-</td><td>-</td><td>-</td><td>1.05</td><td>-</td></tr><tr><td></td><td>1.02</td><td>0.94</td><td>-</td><td><strong>0.98</strong></td><td>1.48</td><td>1.40</td><td>-</td><td><strong>1.44</strong></td></tr></tbody></table>

<sup>a</sup> Potentials values in DMF (3 mM for **2a-c**; 1 mM for **4a** and **4c**) given relative to Ag/AgCl, scan rate = 100 mV s⁻¹.

<sup>b</sup> *E*<sub>pc</sub>: cathodic peak potential.

<sup>c</sup> *E*<sub>pa</sub>: anodic peak potential.

<sup>d</sup> *E*<sub>1/2</sub>=½(*E*<sub>pa</sub> + *E*<sub>pc</sub>).
## 2.3. Photophysical properties

The experimental and computed photophysical properties of ceramidonine analogues **2a-c** and **4'a-b** and 3,9-diazaperylene derivatives **4a-c** are gathered in Table 2. All of these polycycles absorb in the visible range, their lower energy band being situated in the violet for **2a-b** and in the blue-green for the other compounds (Fig. 3). TD-DFT (Time-Dependent Density-Functional Theory) computations (see Supplementary data for additional data and computational details) describe these first absorption bands ($S_0 \to S_1$ excitations) as a well delocalised $\pi \to \pi^*$ electronic transition, showing a more or less pronounced charge transfer (CT) character depending on the chromophore (see the plot of the electronic density difference between the two states in Fig. S3). While symmetric **4a-c** present a transition density homogeneously delocalised over the whole extended $\pi$ backbone and very small CT indicators (see charge ($q_{CT}$) and length ($d_{CT}$) associated to the CT phenomena in Table S5) leading to a first transition being the most intense, compounds **2a-c** and **4'a-b** exhibit a clear CT ($q_{CT}$ of ca. 0.5–0.6 e⁻ and $d_{CT}$ around 3–4 Å) due to the presence of the acceptor carbonyl unit, resulting in a less intense first band. For **2a-b**, a second intense computed transition is found close to the $S_0 \to S_1$ (see Table S3) and is responsible for the experimental maximum of the first absorption band.

The maxima of the first absorption bands range between 419 nm (2a) and 541 nm (4c). For each of the three series, this value increases depending on the heteroatom X of the 5-membered ring, in the order O < S < NMe, and the introduction of an amino auxochromic substituent in compounds **4'a-b** leads to a strong bathochromic shift (ca. 50 nm) in comparison with the related unsubstituted derivatives **2a-b**. Diazaperylenes **4a-c** show a structured lower energy band with increased molar extinction coefficients, in relation with their planar and rigid benzothienoquinoline moieties, associated to vibronic contributions (vide infra).

The maximum wavelengths of the emission bands range between 462 nm (for 2a) and 619 nm (for 4'b). The corresponding computed electronic transitions $S_1 \to S_0$ for the emission process in the series show

similar $\pi$ delocalised/CT character as the ones described for the absorption, with a very small reorganisation of the $S_1$ relaxed geometry (see Fig. S2 for the computed geometries and Fig. S3 for the electronic density differences of the transitions).

Polycycles of the two first series present much larger Stokes shifts (ca. 2000-4000 cm⁻¹), in contrast with diazaperylenes (only ca. 400 cm⁻¹), indicative, as discussed above with TD-DFT results, of the stronger intramolecular CT in the former (that bear electron-withdrawing carbonyl and electron-donating amino groups) than in the latter.

Ceramidonine analogues **2a-c** and **4'a-b** exhibit unstructured emission bands, whereas 3,9-diazaperylene derivatives **4a-c** display a well resolved vibronic structure, indicative of a more rigid relaxed excited state. Vibronic spectra have been computed for **4a-c** (see Fig. S5) in the harmonic approximation framework (see computational details in the Supplementary data). Several vibronic transitions contribute to the shoulders observed experimentally. The vibrational modes mostly responsible for this band shape correspond to torsions between the two moieties of the 3,9-diazaperylene derivatives and deformation of the central ring (see Fig. S5 and animated pictures provided in the Supplementary data).

These compounds also differ in their fluorescence efficiency: ceramidonine analogues **2a-c** exhibit weak fluorescence with quantum yields ranging between 0.1 % and 18 % (with an increase in the O < S < NMe order), while 3,9-diazaperylene derivatives **4a-c** are highly fluorescent (77–80 %). Lifetimes are in nanosecond range in agreement with a fluorescence emission, and follow the same O < S < NMe trend. Radiative decay rates ($k_r^{exp}$) increase by one order of magnitude when going from the less emissive compound (2a) to the most emissive one (4a), in line with the parallel increase of the molar extinction coefficients (Strickler-Berg relationship). For compounds **2c** and **4a-c**, both $k_r^{exp}$ and $k_{nr}^{exp}$ (approximated in our theoretical model as solely an Internal Conversion rate constant, $k_{IC}^{calc}$) values are well reproduced by computations, leading to satisfying qualitative description of the fluorescence quantum yields (see Table 2), and hinting that IC is the main

<table><caption>**Table 2**

Photophysical properties of ceramidonine analogues **2a-c**, **4'a-b** and 3,9-diazaperylene derivatives **4a-c** in dichloromethane.</caption><thead><tr><th>Cpd</th><th>λ<sup>exp</sup><sub>abs</sub><br>(nm)</th><th>λ<sup>calc</sup><sub>abs</sub><br>(nm)</th><th>ε<sub>max</sub><br>(M<sup>-1</sup> cm<sup>-1</sup>)</th><th>λ<sup>exp</sup><sub>em</sub><br>(nm)</th><th>λ<sup>calc</sup><sub>em</sub><br>(nm)</th><th>Stokes shift<br>[e] (cm<sup>-1</sup>)</th><th>Φ<sup>exp</sup><sub>F</sub><br>[r]</th><th>Φ<sup>calc</sup><sub>F</sub><br>[b]</th><th>τ<br>(ns)</th><th>k<sup>exp</sup><sub>r</sub><br>(s<sup>-1</sup>)</th><th>k<sup>calc</sup><sub>r</sub><br>(s<sup>-1</sup>)</th><th>k<sup>exp</sup><sub>nr</sub><br>(s<sup>-1</sup>)</th><th>k<sup>calc</sup><sub>IC</sub><br>(s<sup>-1</sup>)</th><th>Φ<sup>exp</sup><sub>Δ</sub><br>[r]</th></tr></thead><tbody><tr><td>2a</td><td>399/419</td><td>373/419</td><td>4200</td><td>462</td><td>498</td><td>2220</td><td>0.001</td><td>0.81</td><td>0.1</td><td>1.0 × 10<sup>7</sup></td><td>1.2 × 10<sup>8</sup></td><td>1.0 × 10<sup>9</sup></td><td>2.9 × 10<sup>7</sup></td><td>0.90</td></tr><tr><td>2b</td><td>412/447<br>(sh)</td><td>400/456</td><td>11400</td><td>508</td><td>535</td><td>2690</td><td>0.02</td><td>0.63</td><td>0.3</td><td>6.5 × 10<sup>7</sup></td><td>5.3 × 10<sup>7</sup></td><td>3.2 × 10<sup>9</sup></td><td>3.2 × 10<sup>7</sup></td><td>0.90</td></tr><tr><td>2c</td><td>484</td><td>489</td><td>6700</td><td>583</td><td>576</td><td>3510</td><td>0.18</td><td>0.29</td><td>10.8</td><td>1.7 × 10<sup>7</sup></td><td>2.9 × 10<sup>7</sup></td><td>7.6 × 10<sup>7</sup></td><td>7.0 × 10<sup>7</sup></td><td>0.34</td></tr><tr><td>4'a</td><td>484</td><td>485</td><td>10200</td><td>599</td><td>589</td><td>3970</td><td>0.05</td><td>0.36</td><td>1.2</td><td>4.1 × 10<sup>7</sup></td><td>5.1 × 10<sup>7</sup></td><td>7.8 × 10<sup>8</sup></td><td>9.0 × 10<sup>7</sup></td><td>0.08</td></tr><tr><td>4'b</td><td>493</td><td>498</td><td>8500</td><td>619</td><td>622</td><td>4130</td><td>0.03</td><td>0.27</td><td>0.7</td><td>4.3 × 10<sup>7</sup></td><td>4.1 × 10<sup>7</sup></td><td>1.4 × 10<sup>9</sup></td><td>1.1 × 10<sup>8</sup></td><td>0.04</td></tr><tr><td>4a</td><td>497</td><td>505</td><td>38500</td><td>506</td><td>615</td><td>360</td><td>0.80</td><td>0.85</td><td>5.3</td><td>1.5 × 10<sup>8</sup></td><td>1.1 × 10<sup>8</sup></td><td>3.8 × 10<sup>7</sup></td><td>1.8 × 10<sup>7</sup></td><td>0.14</td></tr><tr><td>4b</td><td>529</td><td>537</td><td>17500</td><td>543</td><td>661</td><td>490</td><td>0.77</td><td>0.78</td><td>8.6</td><td>9.0 × 10<sup>7</sup></td><td>6.8 × 10<sup>7</sup></td><td>2.7 × 10<sup>7</sup></td><td>2.0 × 10<sup>7</sup></td><td>0.15</td></tr><tr><td>4c</td><td>541</td><td>537</td><td>21800</td><td>553</td><td>650</td><td>400</td><td>0.78</td><td>0.69</td><td>10.8</td><td>7.2 × 10<sup>7</sup></td><td>7.6 × 10<sup>7</sup></td><td>2.0 × 10<sup>7</sup></td><td>3.4 × 10<sup>7</sup></td><td>0.03</td></tr></tbody></table>

<footnote>a Maximum of the first absorption band.</footnote>

<footnote>b See computational details in Supplementary data.</footnote>

<footnote>c Molar extinction coefficient at $\lambda_{abs}^{exp}$.</footnote>

<footnote>d Emission maximum.</footnote>

<footnote>e Stokes shift = (1/λ<sub>abs</sub><sup>exp</sup> - 1/λ<sub>em</sub><sup>exp</sup>).</footnote>

<footnote>f Fluorescence quantum yield using quinine bisulfate in 0.5 M H₂SO₄ as a standard ($\Phi_F^{exp} = 0.546$).</footnote>

<footnote>g Fluorescence lifetime.</footnote>

<footnote>h Radiative ($k_r$) and nonradiative ($k_{nr}$) decay rates derived from fluorescence quantum yield and lifetime values: $k_r^{exp} = \Phi_F^{calc}/\tau$ and $k_{nr}^{exp} = (1-\Phi_F^{exp})/\tau$.</footnote>

<footnote>i Internal Conversion rate constant.</footnote>

<footnote>j Singlet oxygen quantum yield obtained by direct measurement of the characteristic ¹O₂ luminescence at 1272 nm, using tetraphenylporphyrin in dichloromethane as a standard ($\Phi_Δ = 0.60$).</footnote>
<figure><img src="image_8.png" /><figcaption>Fig. 3. Absorption (left) and emission (right) spectra of ceramidonine analogues 2a-c and 4'a-b (top) and 3,9-diazaperylene derivatives 4a-c (bottom) in dichloromethane.</figcaption></figure>

process of non-radiative de-excitation. Quantum yields for 4'a-b are as expected less correctly reproduced by the harmonic vibronic model (underestimated $k_{IC}^{calc}$) due to the NH₂ groups inducing anharmonic rotations [22]. Strikingly, for 2a-b, the computed $k_{IC}^{calc}$ values are at least two orders of magnitude smaller than $k_{nr}^{exp}$, as a mark of the presence of other non-radiative deactivation channels.

On this basis, triplet states have been also investigated theoretically (see Table S6 and Fig. S6). For 2a-c, a triplet state ($T_2$ or $T_3$) is found lower and close (ca. 0.2 eV) in energy to the $S_1$, along with non-negligible $S_1-T_2$ (or $S_1-T_3$) spin-orbit coupling constant (SOC) in the following order: 2a > 2b = 2c. On the opposite for 4a-c, only the $T_1$ state is found lower than $S_1$, with a large energy difference (~1.0 eV), preventing any Inter-System Crossing (ISC) and explaining their efficient emission. This prompted us to measure the singlet oxygen quantum yield of ceramidonine analogues 2a-c, which was found to be 90% for 2a and 2b and 34% for 2c, confirming that ISC is indeed the main non-radiative de-excitation pathway of the less fluorescent compounds.

### 3. Conclusion

In this work we have prepared new polycyclic arenes containing heteroatoms by using a N-arylation-cyclisation sequence from 1-amino-anthraquinone or 1,5-diaminoanthraquinone and studied their properties. While the photophysical investigation of the fused 3,9-diazaperylenes revealed efficient fluorescent behaviour, the ceramidonine analogues appear to be promising photosensitisers and their potential for photodynamic therapy will be studied in further work.

All data are included in the Supplementary data (see below). Animated pictures of the vibrational modes involved in the vibronic couplings are available online for download.

### CRediT authorship contribution statement

**Nicolas Mast:** Writing – review & editing, Investigation. **Ludmilla Verrieux:** Writing – original draft, Investigation. **William Erb:** Writing – review & editing, Investigation. **Nicolas Richy:** Investigation. **Olivier Mongin:** Writing – original draft, Investigation. **Jean Pierre Hurvois:** Writing – review & editing, Investigation. **Thierry Roisnel:** Writing –

review & editing, Investigation. **Arnaud Fihey:** Writing – original draft, Funding acquisition. **Boris Le Guennic:** Writing – review & editing, Funding acquisition. **Florence Mongin:** Writing – original draft, Project administration, Funding acquisition.

### Declaration of competing interest

There is no conflict of interest to declare.

### Acknowledgements

The authors gratefully acknowledge the financial support by the Université de Rennes (PhD grant to N. M.). We also thank Rennes Métropole (Spectrum 100 PerkinElmer spectrometer) and the Fonds Européen de Développement Régional (FEDER; D8 Venture Bruker-AXS diffractometer). We thank Guillaume Calvez for his help in the singlet oxygen measurements.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dyepig.2024.112612.

### Data availability

Data will be made available on request.

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