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Article

Heteroaryl-Fused Triazapentalenes: Synthesis and Aggregation-Induced Emission

by
Yingchun Wang
1,2,
Thanh Chung Pham
2,
Jianjun Huang
2,
Junfeng Wu
1,* and
Wim Dehaen
2,*
1
Henan Key Laboratory of Water Pollution Control and Rehabilitation Technology, Henan University of Urban Construction, Pingdingshan 467036, China
2
Sustainable Chemistry for Metals and Molecules, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
*
Authors to whom correspondence should be addressed.
Molecules 2025, 30(1), 156; https://doi.org/10.3390/molecules30010156
Submission received: 30 September 2024 / Revised: 30 November 2024 / Accepted: 30 December 2024 / Published: 3 January 2025
(This article belongs to the Special Issue Heterocyclic Chemistry with Applications (Second Edition))
Browse Figures
Versions Notes

Abstract

:
A pyridine-fused triazapentalene shows weak fluorescence in solution and is readily accessible via nitrene-mediated cyclization. In this study, a modified Cadogan reaction was used to synthesize HetATAP 1. Palladium-catalyzed reactions have been used as post-functionalization methods. Interestingly, modified Suzuki cross-couplings with various boronic acids resulted in poor to moderate yields of HetATAPs 2–5 which were arylated at the azole moiety. Direct CH arylation of HetATAP 1 gave the products with the same regiochemistry in satisfactory yields. The structures of HetATAPs 2–5 were confirmed using NMR analysis. In addition, the photophysical properties of HetATAPs 1–5 were studied under various conditions. Particularly, the emission of HetATAPs 2–5 is enhanced in the solid and aggregate state.

1. Introduction

Heteroaryl-fused triazapentalenes (HetATAPs), more specifically N-heteroaryl triazapentalenes (N-HetATAPs), show fluorescent properties that are of interest for potential applications [1,2,3,4,5,6,7]. Currently, only two general synthetic pathways toward HetATAPs have been reported [2,3,8,9,10,11], as shown in Scheme 1. The first published procedure mostly focuses on the deoxygenation of 1-(o-nitro(hetero)aryl)pyrazoles and the thermolysis or photolysis of 1-(o-azido(hetero)aryl)pyrazoles [10,11]. In particular, HetATAP has been studied by the Bettinetti group, forming in unreactive media starting from 2-azido-1-(3,5-dimethylpyrazol-1-yl)phenazine 1. The triazapentalene 2 was produced by intramolecular trapping of the formed nitrene. On the other hand, the synthetic methodology of Guillaumet et al. involves a deoxygenation reaction using nitro(pyrazolyl)pyridines 3, which leads to pyridine-fused triazapentalenes 4 [3]. Although the deoxygenation reaction was only slightly influenced by the substituents at the pyrazole core, the modification at the nitropyridine part had a considerable impact on the reaction result. Later in 2009, the same group reported a library of pyrazine-fused triazapentalenes 6 by the thermolysis of 2-azido-3-(pyrazol-1-yl)pyrazines 5, starting from nitropyrazoles and 2-azido-3-chloropyrazine [2]. The second reported pathway toward compound 8 begins with coupling reactions or nucleophilic aromatic substitutions with halogenated heterocyclic amines and pyrazole derivatives, followed by oxidative cyclization (Scheme 1, Suzenet et al.) [6,7]. Involving a radical mechanism under basic conditions in the presence of hypervalent iodine, the direct formation of an intramolecular N–N bond takes place to form the desired HetATAP derivatives. Unfortunately, in function of the substitutions on the fused ring and the position of the amine, the reaction yields were variable.
Applying the oxidative intramolecular N–N bond formation, Suzenet et al. studied the photophysical properties of HetATAP derivatives [6]. However, most exhibit modest to poor fluorescence quantum yields in solution. The variable quantum yields can be attributed to either the charge of the nitrogen atoms in the heteroaryl moiety or substituents in the azole moiety. Introducing two nitrogen atoms in the fused ring and/or an electron-withdrawing group on the azole ring in the case of R3 position can improve the quantum yields of the resulting dyes in solution [6,7].
In 2001, Tang et al. reported that the aggregation of silole molecules induced light emission, which was opposite to the well-known aggregation-caused quenching (ACQ) [12]. Aggregation-induced emission enhancement (AIEE) has sparked considerable interest because of its broad potential applications in physics [13], chemistry [14], biology [15,16,17,18], and material science [19,20,21,22]. However, as the formation of delocalized excitons or excimers may lead to enhanced non-radiative inactivation of the excited state, most organic molecules show fluorescence ACQ in their aggregated or solid state. The reported organic fluorescence dyes exhibit AIEE characteristics, such as siloles [12,23,24,25], tetraphenylethylene (TPE) [26,27], 1-cyano-trans1,2-bis(4′-methylbiphenyl)ethylene (CN-MBE) [28], 1,4-di[(E)-2-phenyl-1-propenyl]benzene (PPB) [29,30], and salicylaldehyde azine derivatives [31,32].
To date, the photophysical properties of HetATAP derivatives have only been reported in solution. The successful applications of HetATAPs are limited by their use as fluorescent probes in living cells and fluorescent dyes inserted into LDH host structures and polymers [1,5,7,33]. Because of the growing interest in HetATAP fluorophores, the initial goal of the present work was to expand the variety of substitution patterns on the heteroaryl moiety of triazapentalenes via post-functionalization strategies and study the photophysical properties of HetATAP in solids. Furthermore, a few reports mention that the benzotriazole moiety in the molecular structure could be used as an acceptor in donor–acceptor–donor (D–A–D) conjugated systems [34] and a triazole-based probe has also been used for optical imaging of cellular targets with high Stokes shifts [35]. For this purpose, the readily available Cadogan reaction was chosen as the synthetic pathway to obtain HetATAP 1, and the feasibility of the palladium-catalyzed cross-coupling reactions was demonstrated. Herein, we describe the synthesis of HetATAP derivatives and their fluorescence emission in solid-state and aggregate conditions. To the best of our knowledge, the photophysical properties of solid/aggregate-state HetATAPs have not been reported until now [1].

2. Results and Discussion

2.1. Synthesis

To start our investigation, the desired HetATAP 1 was obtained using a modified Cadogan reaction (see SI Scheme S1) [3]. In an attempt to arylate heteroaryl-fused triazapentalene, procedures previously reported for the arylation of 1,3a,6a-triazapentalene with palladium catalysis were applied to HetATAP 1 [36]. The reported procedure with a modified Suzuki cross-coupling reaction (palladium-catalyzed reaction 1) was first carried out taking advantage of the presence of a chlorine atom on the pyridine moiety and was performed with one equivalent of phenylboronic acid in the presence of CuI in DMF at 60 °C for 12 h. Surprisingly, a CH arylation occurred instead of the excepted coupling reaction (Scheme 2a). The unexpected compound HetATAP 2 was formed and its structure was confirmed via NMR analysis. In addition, we have sequentially attempted a modified Stille coupling reaction (palladium-catalyzed reaction 2) with PhSnBu3 and direct palladium-catalyzed CH arylation with bromobenzene (palladium-catalyzed reaction 3) using toluene as solvent, which provided the same product (Scheme 2b,c). Similar reaction yields were obtained for HetATAP 2 from the palladium-catalyzed reaction 1 and 3, but only 13% from the palladium-catalyzed reaction 2. Hence, to extend the substituents, the palladium-catalyzed reactions 1 and 3 were carried out with different boronic acids and bromoarenes, respectively (Table 1). The reaction yields of the palladium-catalyzed reaction 3 were quite satisfying, and consequently higher than those of palladium-catalyzed reaction 1. In 2008, Gorelsky et al. reported the mechanism of the direct CH bond cleavage in the presence of Pd(OAc)2 and potassium carbonate for the (electron-rich) heteroarenes [37]. As mentioned above, the direct palladium-catalyzed CH arylation conditions were adopted for the arylation of 1,3a,6a-triazapentalene, where arylation may occur via a concerted metalation-deprotonation in the presence of carbonate. For this process, the same reaction pathway is considered to have happened. Furthermore, the substitution position was the same as the one of dibenzo-fused triazapentalenes (Scheme 2d). We assume that adding a nitrogen atom in the fused ring would not be enough to change the charge characteristics at each location of the triazapentalene core.

2.2. Photophysical Measurements

2.2.1. Fluorescence in the Solid-State

The absorption and emission spectra were taken for HetATAP 1 and its postmodified products HeTATAPs 2–5 in solid-state at room temperature. The colors of these compounds under the naked eye and a UV lamp are collected in Figure 1a, and the emission spectra for HetATAPs 1–5 are shown in Figure 1b. The photophysical parameters are listed in Table 2. The color of the compounds shows slight differences. Observing the brightness of all compounds under UV light, HetATAPs 2 and 5 are less intense compared to HetATAP 1, while HetATAPs 3 and 4 are slightly more intense, suggesting different fluorescence quantum yields (ΦF, 5–20%) which were obtained by the directed measurement with integrating spheres (Table 2). Compared to the precursor HetATAP 1, the absorption spectra of HetATAPs 2–4 are redshifted and the emission spectra are slightly blue-shifted. Only HetATAP 5 (R=N(CH3)2) exhibited an absorption band at 458 nm and an emission band at 588 nm, which was assigned to the electron-rich character of the TAP core. The presence of additional functional groups that could disturb the molecular planarity of HetATAP 1 might be responsible. In HetATAPs 2–5, the Stokes shift is smaller than that of HetATAP 1, suggesting more coplanarity.

2.2.2. Spectral Properties in Solution

The UV–Vis absorption and fluorescence emission spectra of HetATAPs 1–5 in toluene (Tol), tetrahydrofuran (THF), acetonitrile (ACN), and distilled water (1%THF) (DW99), with a concentration of 20 µM, are shown in Figure 2. The respective photophysical properties data of HetATAPs 1–5 for the solvents used in the study are presented in Table 3. All of the HetATAP derivatives show absorption spectra in a similar wavelength region that changes little with the variation in solvent (Table 3). Their absorption peaks (λabs) appear at 388–456 nm, which were assigned to the π-π* electronic transitions of these molecules. With increasing solvent polarity (from Tol up to DW99), a general blue shift is observed, but the extent of the spectral shift is relatively small (12–33 nm).
All the HetATAP derivatives studied in this work showed negligible luminescence emission in organic solvents (e.g., Tol, THF, and ACN). The HetATAP 1 exhibited similar emission bands (463 nm, 450 nm, 461 nm) in THF, can, and DW99, respectively (Table 3). Meanwhile, this compound displayed a very low fluorescent quantum yield (ФF ≤ 0.5%) and small Stokes shifts (5–71 nm) in various solvents. The luminescence behaviors of the HetATAPs 2–5 solutions vary with the solvent interestingly. Only HetATAP 3 exhibited a weak emission band at 469 nm (ФF = 0.1%) in THF, which was assigned as locally-excited (LE) emission of the TAP core. The fluorescent signals of all HetATAP derivatives in this study were detected in DW99. HetATAP 2 and HetATAP 3 gave the same emission peak ((λems = 541 nm) and similar emission spectra (Figure 2b,c). Compared with HetATAP 3 ((λems = 541 nm, ФF = 2.3%), HetATAP 2 displayed a relatively high fluorescent quantum yield (ФF = 8%) and smaller Stokes shift (112 nm) (Table 3). HetATAP 4 and HetATAP 5 show absorption peaks at 428 and 423 nm, respectively, suggesting that the former is more conjugated than the latter. Figure 2d,e show the emission spectra of HetATAP 4 and HetATAP 5, respectively, and these both present green emissions. When these compounds were excited by 400 nm light, the maximum emission peaks of the compounds appeared at 568 nm and 565 nm, respectively. Further, the fluorescent quantum yield (ФF) of HetATAP 4 and HetATAP 5 in DW99 solution was shown to be 13.1% and 2.6%. HetATAP 4 (R = COOEt) exhibits the highest fluorescent quantum yield in DW99 (Table 3), suggesting that the electron-rich character of the TAP core and the electron-withdrawing group (EWG) with help the electron delocalization, favoring the resonance structure [38]. Furthermore, by comparing the fluorescence quantum yields of these compounds in solution ФF(solution) and solid-state ФF(solid), especially for those compounds with ФF(solid)/ФF(solution) > 1, including HetATAP 3–5, we estimated that they might show effective AIE behavior.

2.3. Fluorescence in the Aggregated State

To verify the AIEE properties of HetATAPs 2–5, fluorescence emission spectra and images (under 365 nm light) were recorded in THF/water mixtures (Figure 2f and Figure 3). As shown in Figure 2f and Figure 3, weak fluorescent signals are seen at fw ≤ 60% because the HetATAPs 2–5 molecules are generally dissolved in these mixtures. The fluorescent intensity starts to rise for fw > 60%, where the solvating power of the mixture is lowered to such an extent that the luminogen molecules begin to aggregate. As fw reached 80% for HetATAPs 2 and 4, the fluorescent spectra showed a sharp increase compared to HetATAPs 2 and 4 (Figure 2f). However, when fw increased up to 80% (HetATAP 5), 85% (HetATAP 4), or 90% (HetATAPs 2 and 3), the emission exhibited a decreasing trend owing to more aggregates precipitated might form during this process. To summarize, HetATAPs 1–4 demonstrates either no or minimal LE fluorescence emission (e.g., HetATAP 3 in THF) under non-aggregated conditions. However, fluorescence emission is significantly enhanced in the presence of aggregated states, which can be assigned to the TICT effect.

3. Materials and Methods

3.1. Chemical and Materials

All chemicals were purchased from Acros Organics (Geel, Belgium), Merck (Boston, MA, USA), J&K Scientific (Beijing, China), Fluorochem (Derbyshire, UK), and TCI Europe (Antwerp, Belgium) and used as received. Solvents were dried prior to use. For column chromatography, 70–230 mesh silica gel 60 (Acros) was used as the stationary phase.

3.2. Instrumentation

1H and 13C NMR spectra were recorded on a Bruker Avance 300, Bruker Avance III HD 400, or a Bruker Avance II+ 600 spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) referenced to tetramethylsilane (0.00 ppm) as an internal standard for samples in CDCl3, or to the respective solvent for samples in DMSO-d6 (2.50 ppm). 13C NMR spectra were referenced to the respective solvent signals (CDCl3, 77.16 ppm). High-resolution mass spectra were acquired on a quadrupole orthogonal acceleration time-of-flight mass spectrometer (Synapt G2 HDMS, Waters, Milford, MA, USA). Melting points (not corrected) were determined using a Reichert Thermovar apparatus.
UV–Vis absorption spectra were recorded on a PerkinElmer (Shelton, CT, USA) Lambda 950 spectrophotometer using blank correction. Fluorescence and excitation spectra were recorded on a HORIBA Jobin Yvon Fluorolog FL3-22 fluorimeter (HORIBA, Minami ku, Japan), and corrections for the excitation beam intensity, the wavelength-dependent sensitivity of the detector, and the optical path were applied.

3.3. Synthesis

3.3.1. General Procedure for Palladium-Catalyzed Reaction 1

To an oven-dried reaction tube equipped with a magnetic stirring bar HetATAP 1 (0.2 mmol, 1 equivalent), boronic acids (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent) and DMF (2 mL) were added. The mixture was left and stirred for 12 h at 60 °C in an aluminum heating block. The crude reaction mixture was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL) and brine (1 × 20 mL). The organic layer was subsequently dried over magnesium sulfate and concentrated under reduced pressure. Further purification by column chromatography, using a PE-DCM gradient as the eluent, afforded the solid compounds.

3.3.2. Palladium-Catalyzed Reaction 2 of Compound HetATAP 1

HetATAP 1 (24.3 mg, 0.1 mmol, 1 equivalent), PhSnBu3 (44.2 mg, 0.12 mmol, 1.2 equivalent), and Pd2dba3 (5 mg, 0.005, 0.05 equivalent) were added to an oven-dried reaction tube equipped with a magnetic stirring bar and dissolved in toluene (1 mL) under a nitrogen atmosphere. The reaction mixture was left and stirred at 70 °C for 24 h. The reaction mixture was diluted with EtOAc (20 mL) and washed with water (2 × 20 mL) and brine (1 × 20 mL). The organic layer was subsequently dried over magnesium sulfate and concentrated under reduced pressure. Further purification by column chromatography, using a PE-DCM (1:1–0:1) gradient as the eluent, afforded a yellow solid HetATAP 2 (4 mg, 13%).

3.3.3. General Procedure for Palladium-Catalyzed Reaction 3

To an oven-dried reaction tube equipped with a magnetic stirring bar HetATAP 1 (0.2 mmol, 1 equivalent), bromoarene reagents (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent) and dry toluene (4 mL) were added. The mixture was left and stirred for 24 h at 110 °C in an aluminum heating block. The reaction mixture was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL) and brine (1 × 20 mL). The organic layer was subsequently dried over magnesium sulfate and concentrated under reduced pressure. Further purification by column chromatography, using a PE-DCM gradient as the eluent, afforded the solid compounds.

3.3.4. Experimental Details and Characterization Data

HetATAP 1: Yellow solid. Yield 89.5 mg, 74%. Mp 192–195 °C. 1H NMR (400 MHz, CDCl3) δ 8.35 (d, J = 5.5 Hz, 1H), 7.98 (s, 1H), 7.80–7.73 (m, 1H), 7.67 (d, J = 8.5 Hz, 1H), 7.48 (d, J = 7.0 Hz, 2H), 7.33 (d, J = 8.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 138.77, 138.40, 135.91, 125.03, 124.57, 123.66, 123.55, 121.16, 120.60, 118.70, 111.37, 98.33. HRMS (ESI-Q-TOF): m/z [M + H]+ calculated for C12H7Cl1N4: 243.0432, found: 243.0431.
HetATAP 2: (1) Prepared following the general procedure for palladium-catalyzed reaction 1: HetATAP 1 (0.2 mmol, 1 equivalent), phenylboronic acid (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent) and DMF (2 mL), 12 h. Purification by column chromatography, using a PE-DCM gradient (1:1–0:1) as the eluent, afforded HetATAP 2 (38 mg, 53%) as a yellow solid. (2). Prepared following the procedure for palladium-catalyzed reaction 2: HetATAP 1 (24.3 mg, 0.1 mmol, 1 equivalent), PhSnBu3 (44.2 mg, 0.12 mmol, 1.2 equivalent), Pd2dba3 (5 mg, 0.005, 0.05 equivalent), 24 h. Purification by column chromatography, using a PE-DCM gradient (1:1–0:1) as the eluent afforded HetATAP 2 (4 mg, 13%). (3). Prepared following the general procedure for palladium-catalyzed reaction 3: HetATAP 1 (0.2 mmol, 1 equivalent), bromobenzene (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent) and dry toluene (4 mL), 24 h. Purification by column chromatography, using a PE-DCM (1:1–0:1) gradient as the eluent, afforded the solid compounds HetATAP 2 (42 mg, 66%) as a yellow solid. Mp 192–194 °C. 1H NMR (400 MHz, CDCl3) δ 8.52–8.40 (m, 1H), 8.35 (dd, J = 8.5, 1.2 Hz, 2H), 8.22–8.09 (m, 1H), 7.75 (d, J = 8.5 Hz, 1H), 7.67–7.59 (m, 2H), 7.61–7.52 (m, 2H), 7.48–7.38 (m, 1H), 7.35 (d, J = 8.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 138.90, 138.71, 129.12, 128.14, 127.88, 126.57, 125.28, 124.00, 123.90, 123.02, 121.22, 120.83, 119.15, 111.53. HRMS (ESI-Q-TOF): m/z [M + H]+ calculated for C18H11ClN4: 319.0744, found: 319.0741.
HetATAP 3: (1). Prepared following the general procedure for palladium-catalyzed reaction 1: HetATAP 1 (0.2 mmol, 1 equivalent), 4-methoxyphenylboronic acid (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent) and DMF (2 mL), 12 h. Purification by column chromatography, using a PE-DCM gradient (1:1–0:1) as the eluent, afforded HetATAP3 (32.9 mg, 47%) as a yellow solid. (2). Prepared following the general procedure for palladium-catalyzed reaction 3: HetATAP 1 (0.2 mmol, 1 equivalent), 4-bromoanisole (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent) and dry toluene (4 mL), 24 h. Purification by column chromatography, using a PE-DCM (1:1–0:1) gradient as the eluent, afforded the solid compounds HetATAP 3 (43.4 mg, 62%) as a yellow solid. Mp 248–250 °C. 1H NMR (400 MHz, CDCl3) δ 8.45–8.35 (m, 1H), 8.26 (d, J = 8.9 Hz, 1H), 8.11–8.06 (m, 1H), 7.69 (d, J = 8.5 Hz, 1H), 7.57–7.48 (m, 1H), 7.31 (d, J = 8.5 Hz, 1H), 7.15 (d, J = 8.9 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 159.28, 138.93, 138.37, 136.12, 129.97, 128.19, 125.05, 123.93, 122.88, 121.04, 120.61, 120.33, 119.11, 114.65, 111.49, 110.98, 55.45. HRMS (ESI-Q-TOF): m/z [M] calculated for C19H13ClN4O: 348.0778, found: 348.0778.
HetATAP 4: (1). Prepared following the general procedure for palladium-catalyzed reaction 1: HetATAP 1 (0.2 mmol, 1 equivalent), 4-ethoxycarbonylphenylboronic acid (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent) and DMF (2 mL), 12 h. Purification by column chromatography, using a PE-DCM gradient (1:1–0:1) as the eluent, afforded HetATAP 4 (27.3 mg, 35%) as a yellow solid. (2). Prepared following the general procedure for palladium-catalyzed reaction 3: HetATAP 1 (0.2 mmol, 1 equivalent), ethyl 4-bromobenzoate (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent) and dry toluene (4 mL), 24 h. Purification by column chromatography, using a PE-DCM (1:1–0:1) gradient as the eluent, afforded the solid compounds HetATAP 4 (39.8 mg, 51%) as a yellow solid. Mp 249–251 °C. 1H NMR (400 MHz, CDCl3) δ 8.53–8.48 (m, 2H), 8.48–8.43 (m, 1H), 8.29–8.24 (m, 3H), 8.24–8.15 (m, 1H), 7.88 (dd, J = 8.2, 1.3 Hz, 1H), 7.64–7.48 (m, 2H), 7.39 (dd, J = 8.2, 4.8 Hz, 1H), 4.44 (q, J = 7.1 Hz, 2H), 1.45 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 166.23, 139.56, 139.37, 137.33, 132.86, 130.24, 128.54, 125.55, 125.39, 124.20, 123.92, 123.09, 121.26, 119.58, 119.00, 111.41, 109.35, 61.04, 14.41. HRMS (ESI-Q-TOF): m/z [M + H]+ calculated for C21H15ClN4O2: 391.0956, found: 391.0954.
HetATAP 5: (1). Prepared following the general procedure for palladium-catalyzed reaction 1: HetATAP 1 (0.2 mmol, 1 equivalent), 4-(dimethylamino)phenylboronic acid (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent) and DMF (2 mL), 12 h. Purification by column chromatography, using a PE-DCM gradient (1:1–0:1) as the eluent, afforded HetATAP 5 (22.1 mg, 30%) as a yellow solid. (2). Prepared following the general procedure for palladium-catalyzed reaction 3: HetATAP 1 (0.2 mmol, 1 equivalent), 1-bromo-4-(dimethylamino)benzene (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent) and dry toluene (4 mL), 24 h. Purification by column chromatography, using a PE-DCM (1:1–0:1) gradient as the eluent, afforded the solid compounds HetATAP 5 (53.8 mg, 73%) as an orange–yellow solid. Mp 115–117 °C. 1H NMR (300 MHz, CDCl3) δ 8.49–8.26 (m, 1H), 8.19 (d, J = 8.5 Hz, 2H), 8.17–7.98 (m, 1H), 7.64 (d, J = 8.5 Hz, 1H), 7.58–7.45 (m, 2H), 7.28 (d, J = 8.0 Hz, 1H), 6.93 (d, J = 8.5 Hz, 2H), 3.06 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 149.99, 136.30, 127.88, 124.75, 124.01, 123.88, 122.85, 120.84, 119.79, 119.51, 115.65, 112.52, 111.44, 40.41. HRMS (ESI-Q-TOF): m/z [M] calculated for C20H16ClN5: 361.1094, found: 361.1090.

4. Conclusions

In summary, novel heteroaryl-fused triazapentalene derivatives were successfully synthesized using palladium-catalyzed reactions. The yields of palladium-catalyzed reaction 1 were suboptimal, whereas the direct CH arylation reaction proved to be an effective strategy. The photophysical properties of HetATAPs 2–5 revealed fluorescence in both the solid-state and DW99 solution. However, in organic solvents, only HetATAP 3 exhibited weak fluorescence in THF. Notably, incorporating a C-4-phenylethoxycarbonyl group on the indazole moiety of HetATAP significantly enhanced electron delocalization in the resonance structure, resulting in relatively high fluorescent quantum yields in both the solid-state and DW99 solution. Furthermore, an investigation of the aggregation-induced emission behavior of HetATAPs 2–5 demonstrated that all these materials exhibited AIE properties, highlighting the potential of this approach for developing new organic solid fluorescent materials.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30010156/s1.

Author Contributions

Conceptualization, Y.W. and W.D.; methodology, Y.W., J.H. and T.C.P.; validation, Y.W. and J.W.; investigation, Y.W., J.H. and T.C.P.; writing—original draft preparation, Y.W. and J.W.; writing—review and editing, Y.W. and W.D.; supervision, W.D.; funding acquisition, W.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Research Council of KU Leuven through projects C14/19/78 and C14/19/079 (FUEPONA). Y.W. received a doctoral fellowship from the China Scholarship Council (201706920044) and a research fellowship from Henan University of Urban Construction (990/K-Q2024015). Mass spectrometry was made possible by the support of the Hercules Foundation of the Flemish Government (20100225-7) and NMR spectroscopy by Hercules grants I002720N and G0D6221N. The purchase of the diffractometer was supported by the Hercules Foundation through project AKUL/09/0035.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials; further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors acknowledge Bart Van Huffel for technical assistance with the NMR spectrometers and Jef Rozenski for the HRMS measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Molecules 30 00156 sch001
Scheme 1. Synthetic pathways toward heteroaryl-fused triazapentalenes (HetATAPs) [2,3,6,7,10,11] and the numbering of the triazapentalene (TAP) core.
Scheme 1. Synthetic pathways toward heteroaryl-fused triazapentalenes (HetATAPs) [2,3,6,7,10,11] and the numbering of the triazapentalene (TAP) core.
Molecules 30 00156 sch001
Molecules 30 00156 sch002
Scheme 2. The synthetic pathways towards HetATAPs 2–5. (a). Palladium-catalyzed reaction 1, (b). Palladium-catalyzed reaction 2, (c) Palladium-catalyzed reaction 3, (d). Electrophilic substitution of dibenzo-fused triazapentalene.
Scheme 2. The synthetic pathways towards HetATAPs 2–5. (a). Palladium-catalyzed reaction 1, (b). Palladium-catalyzed reaction 2, (c) Palladium-catalyzed reaction 3, (d). Electrophilic substitution of dibenzo-fused triazapentalene.
Molecules 30 00156 sch002
Molecules 30 00156 g001
Figure 1. The color of compounds observed with the naked eye and by irradiation with a UV lamp (a) (365 nm) and normalized fluorescence emission spectra of HetATAPs 1–5 in the solid-state (b).
Figure 1. The color of compounds observed with the naked eye and by irradiation with a UV lamp (a) (365 nm) and normalized fluorescence emission spectra of HetATAPs 1–5 in the solid-state (b).
Molecules 30 00156 g001
Molecules 30 00156 g002
Figure 2. (a–e) Normalized UV–Vis absorption and fluorescence emission spectra of HetATAPs 1–5 in toluene (Tol), tetrahydrofuran (THF), acetonitrile (ACN), and distilled water (1%THF) (DW99) (20 µM); (f) fluorescence emission intensity ration (I/I0) of HetATAPs 2–5 (10 µM) in THF/DW (0–99%) solvent mixture.
Figure 2. (a–e) Normalized UV–Vis absorption and fluorescence emission spectra of HetATAPs 1–5 in toluene (Tol), tetrahydrofuran (THF), acetonitrile (ACN), and distilled water (1%THF) (DW99) (20 µM); (f) fluorescence emission intensity ration (I/I0) of HetATAPs 2–5 (10 µM) in THF/DW (0–99%) solvent mixture.
Molecules 30 00156 g002
Molecules 30 00156 g003
Figure 3. (a–d) Fluorescence emission spectra and image (under 365 nm light) of HetATAPs 2–5 (10 µM) in THF/DW (0–99%) solvent mixture.
Figure 3. (a–d) Fluorescence emission spectra and image (under 365 nm light) of HetATAPs 2–5 (10 µM) in THF/DW (0–99%) solvent mixture.
Molecules 30 00156 g003
Table 1. The reaction yields of various pathways towards HetATAPs 2–5.
Table 1. The reaction yields of various pathways towards HetATAPs 2–5.
HetATAPRYield (%)
abc
2H531366
3OMe47- a62
4COOEt35- a51
5N(Me)230- a73
a Reaction was not carried out. Experimental conditions: (a) palladium-catalyzed reaction 1: HetATAP 1 (0.2 mmol, 1 equivalent), boronic acids (0.24 mmol, 1.2 equivalent), Pd(OAc)2 (27 mg, 0.12 mmol, 0.6 equivalent), CuI (15.2 mg, 0.08 mmol, 0.4 equivalent), SPhos (10 mg, 0.023, 0.12 equivalent), and DMF (2 mL), 12 h; (b) palladium-catalyzed reaction 2: HetATAP 1 (24.3 mg, 0.1 mmol, 1 equivalent), PhSnBu3 (44.2 mg, 0.12 mmol, 1.2 equivalent), Pd2dba3 (5 mg, 0.005, 0.05 equivalent), 24 h; (c) palladium-catalyzed reaction 3: HetATAP 1 (0.2 mmol, 1 equivalent), bromoarene reagents (0.2 mmol, 1.0 equivalent), Pd(OAc)2 (2.4 mg, 0.01 mmol, 0.05 equivalent), Cy3HBF4 (7.8 mg, 0.02 mmol, 0.1 equivalent), pivalic acid (6.6 mg, 0.03, 0.3 equivalent), K2CO3 (88 mg, 0.6 mmol, 3.0 equivalent), and dry toluene (4 mL), 24 h.
Table 2. Fluorescence properties of HetATAPs 1–5 in the solid-state.
Table 2. Fluorescence properties of HetATAPs 1–5 in the solid-state.
Molecules 30 00156 i001λabs,max (nm)λem,max (nm)Stokes Shift (cm−1)ΦF (%)
HetATAP14605844600 ± 5010
HetATAP25255761680 ± 505
HetATAP35205701680 ± 5012
HetATAP45205711710 ± 5020
HetATAP54855883610 ± 505
Table 3. Photophysical properties of HetATAPs 1–5 measured in different solutions.
Table 3. Photophysical properties of HetATAPs 1–5 measured in different solutions.
λabs(nm)/ε (cm−1·M−1·10−3)λems (nm)/ФF (%)
TolTHFACNDW99THFACNDW99
HetATAP1407/49.4402/41.1398/35.4388/31.7463/0.2450/0.2461/0.5
HetATAP2441/35.1438/38.4433/36.0429/10.6--541/8.0
HetATAP3445/30.4441/34.0436/30.0415/14.6469/0.1-541/2.3
HetATAP4450/59.3446/59.3443/52.9428/18.0--568/13.1
HetATAP5456/34.9454/34.1449/30.7423/13.1--565/2.6
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Wang, Y.; Pham, T.C.; Huang, J.; Wu, J.; Dehaen, W. Heteroaryl-Fused Triazapentalenes: Synthesis and Aggregation-Induced Emission. Molecules 2025, 30, 156. https://doi.org/10.3390/molecules30010156

AMA Style

Wang Y, Pham TC, Huang J, Wu J, Dehaen W. Heteroaryl-Fused Triazapentalenes: Synthesis and Aggregation-Induced Emission. Molecules. 2025; 30(1):156. https://doi.org/10.3390/molecules30010156

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Wang, Yingchun, Thanh Chung Pham, Jianjun Huang, Junfeng Wu, and Wim Dehaen. 2025. "Heteroaryl-Fused Triazapentalenes: Synthesis and Aggregation-Induced Emission" Molecules 30, no. 1: 156. https://doi.org/10.3390/molecules30010156

APA Style

Wang, Y., Pham, T. C., Huang, J., Wu, J., & Dehaen, W. (2025). Heteroaryl-Fused Triazapentalenes: Synthesis and Aggregation-Induced Emission. Molecules, 30(1), 156. https://doi.org/10.3390/molecules30010156

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