Synthesis of Aryl‐Substituted Hexa‐Alkynyl Hexaazatrinaphthylenes via Sonogashira Coupling and Evaluation of Their Photophysical Properties

ABSTRACT

Hexaazatrinaphthylene (HATNA) has recently been a subject of great interest as a polycyclic heteroaromatic scaffold in materials science, specifically in the field of discotic liquid crystals (DLCs), battery cathodes, n‐type semiconductors, and so on. However, little is known about the photophysical properties of HATNA derivatives. Here we report the synthesis of a new class of hexa(arylethynyl) HATNA derivatives via consecutive Sonogashira coupling. This protocol showed good functional‐group tolerance and moderate to good yields were achieved. The obtained HATNA products showed favorable photophysical properties, with molar absorption coefficients up to 1.69 × 10⁵ (M⁻¹ cm⁻¹) and high quantum yields up to 0.73.

A new series of hexaalkynyl‐substituted hexaazatrinaphthylene (HATNA) derivatives was successfully synthesized via Sonogashira coupling between hexabromo‐HATNA and aryl‐substituted terminal alkynes. This protocol exhibited good functional‐group tolerance, affording the desired products in moderate to good yields. The resulting HATNA derivatives were confirmed to possess favorable photophysical properties, including high molar absorption coefficients and quantum yields of up to 0.73.

1. Introduction

Polycyclic aromatic compounds have attracted widespread interest due to their unique optical, electrical, and magnetic properties, which arise from their highly conjugated π‐systems. Among them, those with C ₃‐symmetric geometries generally exhibit favorable physicochemical characteristics and have been widely used in electronic devices as well as in the synthesis of bioactive and pharmaceutical compounds [1, 2, 3, 4, 5].

As a representative example, hexaazatrinaphthylene (HATNA) features a fully conjugated planar π‐system composed of three fused naphthalene units arranged around a central benzene core with six imine‐type nitrogen atoms symmetrically embedded. This configuration forms a rigid C ₃‐symmetric framework with extensive electron delocalization and a strong propensity for intermolecular π–π stacking. The highly conjugated planar structure endows HATNA with strong electron‐accepting ability and pronounced self‐assembly, giving its derivatives excellent optoelectronic properties, enhanced polymerization, and the formation of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) [6, 7, 8, 9]. In recent decades, extensive research has focused on HATNA derivatives as versatile building blocks in diverse applications, including n‐type semiconductors [10, 11], discotic liquid crystals (DLCs) [12, 13, 14], organic electrode materials [15, 16, 17], organogels [18], and photochemical hydrogen storage [19]. However, most of the reported works have concentrated on the electrical properties of HATNA or the formation of porous polymer structures, and little is known about its photophysical properties in the visible light range.

Since HATNA offers numerous advantages in materials science applications, functionalization of the HATNA skeleton has attracted considerable attention [20, 21, 22]. Meanwhile, extensive research has demonstrated that twisted structures in polycyclic aromatic compounds can significantly influence their electronic and chiroptical properties [23, 24, 25, 26]. Consequently, twisted HATNA derivatives have aroused growing interest, and can be obtained through backbone functionalization—for instance, by introducing rigid alkynyl substituents at the bay positions. In 2014, Mateo‐Alonso and co‐workers reported the synthesis of HATNAs with varying degrees of twist by incorporating rigid acetylenes with bulky silyl substituents (R = SiR₃ in Scheme 1) [27]. By tuning the size of the terminal silyl groups, the twist angles of HATNA could be significantly altered, resulting in shifted emission wavelengths and changes in the redox potentials of the twisted HATNAs. In the same year, Liu's group disclosed the synthesis of alkynylated HATNA derivatives with linear alkyl chains at the alkyne termini, which exhibited self‐directed molecular column orientation (R = alkyl in Scheme 1) [28]. However, in these reported studies, only a few examples were obtained in low to moderate yields, and no remarkable photophysical properties were described.

SCHEME 1.

Hexa‐alkynyl‐substituted HATNAs at their bay positions.

Herein, we report a new synthetic protocol for a new series of bay‐substituted hexa‐arylethynyl HATNA derivatives 3, obtained by consecutive Sonogashira coupling of hexabromo‐HATNA 1 with arylalkyne 2. This protocol exhibited good functional‐group tolerance, affording hexa‐arylethynyl HATNAs in moderate to good yields. By introducing aryl groups at the alkyne terminals, the resulting HATNA derivatives displayed favorable photophysical properties and achieved high quantum yields of up to 0.73 (Scheme 2).

SCHEME 2.

Synthetic protocol of new HATNA derivatives.

2. Results and Discussion

First, we optimized the reaction conditions using hexabromo‐HATNA 1 and p‐tolylacetylene 2a (Table 1). Conventional conditions for Sonogashira coupling were initially examined, affording the target compound 3a in 41% yield (entry 1). When the reaction was conducted without PPh₃, the yield significantly decreased (entry 2). In addition to THF, several other solvents were tested, and DMSO proved to be the most effective (entries 3–5). The reaction was then performed at different temperatures: 100°C gave the best results (entries 5–8). Replacement of diisopropylamine with triethylamine or N,N,N′,N′‐tetramethylethylenediamine (TMEDA) afforded 3a in only moderate yields (entries 9–10). The use of inorganic bases instead of amines proved ineffective (entry 11). Therefore, we determined that Entry 6 represented the optimal reaction conditions.

TABLE 1.

Optimization of Reaction Conditions ^a .

Entry	Base	Solvent	Temp./ °C	NMR Yield / %
1	i‐Pr2NH	THF	60	41
2 b	i‐Pr2NH	THF	60	20
3	i‐Pr2NH	1,4‐Dioxane	60	28
4	i‐Pr2NH	DMF	60	55
5	i‐Pr2NH	DMSO	60	63
6	i‐Pr2NH	DMSO	80	81(70 c )
7	i‐Pr2NH	DMSO	100	58
8	i‐Pr2NH	DMSO	140	2
9	Et3N	DMSO	80	50
10	TMEDA	DMSO	80	47
11 d	Cs2CO3	DMSO	80	53

^a Unless otherwise noted, the reaction was conducted at 25 µmol scale in solvent (1.5 mL), amine (1.5 mL) under argon. The NMR yield was determined using 1,1,2,2‐tetrachloroethane as an internal standard.

^b Reaction was conducted without the addition of PPh₃.

^c Isolated yield.

^d Twelve equivalent amounts of Cs₂CO₃ were used instead of amine.

Next, under the optimal conditions, we examined a variety of aryl acetylenes (Table 2). The use of phenyl acetylene 2b as the coupling partner afforded 3b in moderate yield. Substituent variation at the para‐position from methyl (3a) to electron‐donating (3c), electron‐withdrawing (3e, 3f), and bulky groups (3d) was generally well tolerated. Only in the case of the cyano group was a pronounced decrease in yield (3f). In addition to monosubstituted aryl acetylenes, derivatives bearing multiple substituents at the para‐ and meta‐positions also furnished the desired products 3g and 3h in high yields. In contrast, substitution at the ortho‐position exerted a clear negative effect on the reaction outcome (3i). Moreover, bulkier aryl acetylenes such as naphthyl and 1‐pyrenyl derivatives were well accommodated, giving products 3j–3l in good yields. Heteroaryl acetylene was also compatible with this protocol, affording the corresponding product 3m in moderate yield. We also evaluated the solubility of the obtained compounds using 3a as a representative example. Compound 3a showed good solubility in several common organic solvents: 1 mg dissolved in approximately 0.1 mL of dichloromethane (DCM), ∼1.5 mL of THF, and ∼3 mL of DMF. In contrast, its solubility in DMSO and acetone was significantly low, requiring more than 20 mL of solvent to dissolve 1 mg of 3a. Moreover, 3a was practically insoluble in alcohols such as methanol and ethanol.

TABLE 2.

Substrate Scope ^{a .}

^a The reaction was conducted at 25 µmol scale in DMSO (1.5 mL), diisopropylamine (1.5 mL) under argon.

Among the newly obtained hexa‐alkynyl HATNA derivatives, 3c and 3m were suitable for single‐crystal x‐ray structure analysis (Figure 1) [29]. Overlap between the terminal aryl groups of adjacent blades was clearly observed. The inter‐blade distances were measured to be 3.246 Å for 3c and 3.397 Å for 3m, indicating the presence of π–π stacking interactions between the aromatic groups, with only a slight influence from substituent bulkiness. From the side‐views, the twist angles between the blades were measured to be 23° for 3c and 22° for 3m, with almost no difference. Examination of the intermolecular packing revealed the presence of both (P,P,P) and (M,M,M) enantiomeric pairs in the crystals of 3c and 3m. These enantiomers were arranged in an orderly manner, with the terminal aryl groups of one enantiomer engaging in π–π stacking interactions with the benzene ring at the periphery of another HATNA skeleton. In 3c, the distance between the interacting units was 3.246 Å, whereas in 3m it was 3.733 Å, highlighting the influence of substituent bulkiness on intermolecular π–π stacking. Since enantiomeric pairs were observed in the single‐crystal structures, high‐performance liquid chromatography (HPLC) analysis was attempted using various chiral columns provided by Daicel Corporation. However, the chirality could not be confirmed, possibly due to the flipping of molecules in solution at room temperature [30].

FIGURE 1.

ORTEP Diagrams of 3c and 3m (thermal ellipsoids are shown at 50% probability level, hydrogen atoms are omitted for clarity).

The photophysical properties of the obtained hexa‐alkynyl HATNA derivatives were measured in dichloromethane, and the results are summarized in Table 3 and Figure 2. The maximum molar absorption coefficients (ε) were found to depend strongly on the substituents at the aryl groups on the alkyne termini. Derivatives bearing electron‐donating substituents exhibited higher ε values than those with electron‐withdrawing groups, while bulky aryl substituents led to a decrease in ε [31]. All samples displayed absorption bands in the long‐wavelength region (450–550 nm) (Figure 2a). The maximum emission wavelengths also showed a strong dependence on the electron density of the aryl groups, with electron‐withdrawing substituents inducing significant hypsochromic shifts (Figure 2b). Representative emission photographs of derivatives with para‐methoxy (3c), para‐methyl (3b), para‐fluoro (3e), and para‐cyano groups (3f) under 365 nm UV irradiation revealed distinct color changes from red to yellow (Figure 2c). Remarkably, all the obtained HATNA derivatives exhibit very large Stokes shifts, exceeding 200 nm. Besides dichloromethane, we also examined the absorption and emission behaviors in THF solution, where only a slight decrease in ε and a small hypsochromic shift of the absorption and emission maximum were observed (Figure S1 and Table S1). In addition, fluorescence quantum yields were measured for all samples in dichloromethane (DCM) solution and solid state (Table 4). Most derivatives exhibited moderate to high values in solution, with the highest reaching 0.73, markedly surpassing previously reported HATNA derivatives with precisely determined quantum yields [32]. Some of the samples also exhibited solid‐state fluorescence, with quantum yields of up to 0.22, which we attribute to aggregation enabled by intermolecular π–π stacking interactions. In general, HATNA derivatives bearing electron‐deficient substituents exhibited higher quantum yields than those with electron‐rich groups, while the quantum yield of the trimethoxyphenyl‐substituted HATNA 3h decreased dramatically. Notably, the pyrene‐substituted derivative 3l displayed the largest redshift, with a maximum emission wavelength of 669 nm. However, the quantum yield of 3l was lower than that of other HATNA derivatives. This reduction may be attributed to the bulkiness of the aryl substituent, which could hinder intramolecular π–π stacking interactions—believed to play a significant role in the favorable fluorescence properties of these new HATNA derivatives.

TABLE 3.

Photophysical Properties of Hexa‐alkynyl HATNAs 3.

Comp.	λ max(abs) (nm) (ε [× 104 M−1 cm−1]) a	λmax(em) (nm) b , c	Stokes Shift (nm)
3a	317(15.4), 335(12.8), 396(6.4), 500(1.6)	587	270
3b	313(8.5), 328(7.2), 383(4.0), 485(0.79)	567	254
3c	322(16.3), 344(13.4), 415(5.7), 525(1.9)	619	297
3d	319(16.9), 336 (15.2), 399(6.9), 506(1.8)	585	266
3e	311(11.4), 327(12.0), 383(6.6), 487(1.3)	569	258
3f	331(8.1), 376(4.7), 478(0.97)	547	216
3g	315(7.8), 332(sh), 391(3.2), 498(0.81)	575	260
3h	323(12.6), 418(3.8), 530(1.9)	643	320
3i	324(12.4), 342(10.8), 416(4.7), 531(1.7)	606	282
3j	347(9.8), 433(3.2), 551(1.1)	589	242
3k	331(8.7), 349(7.9), 400(sh), 510(1.0)	595	264
3l	334(18.0), 358(20.6)	669	335
3m	330 (13.7), 378(9.0), 480(sh)	558	228

^a 3a: 5.60×10⁻⁶ M; 3b: 2.79×10⁻⁶ M; 3c: 5.23×10⁻⁶ M; 3d: 2.86×10⁻⁵ M; 3e: 2.85×10⁻⁵ M; 3f: 2.47×10⁻⁶ M; 3g: 2.23×10⁻⁶ M; 3h: 2.79×10⁻⁶ M; 3i: 2.63×10⁻⁶ M; 3j: 2.60×10⁻⁵ M; 3k: 2.20×10⁻⁶ M; 3l: 2.39×10⁻⁶ M; 3m: 2.84×10⁻⁶ M.

^b Excitation wavelengths: 3a: 317 nm; 3b: 313 nm; 3c: 322 nm; 3d: 319 nm; 3e: 311 nm; 3f: 331 nm; 3g: 315 nm; 3h: 323 nm; 3i: 342 nm; 3j: 347 nm; 3k: 331 nm; 3l: 334 nm; 3m: 330 nm.

^c 3a: 5.60×10⁻⁶ M; 3b: 2.79×10⁻⁶ M; 3c: 5.23×10⁻⁶ M; 3d: 2.86×10⁻⁶ M; 3e: 2.85×10⁻⁶ M; 3f: 2.47×10⁻⁶ M; 3g: 2.23×10⁻⁶ M; 3h: 2.79×10⁻⁶ M; 3i: 2.63×10⁻⁶ M; 3j: 2.60×10⁻⁶ M; 3k: 2.20×10⁻⁶ M, 3l: 2.39×10⁻⁶ M; 3m: 2.84×10⁻⁶ M.

FIGURE 2.

UV–vis and Fluorescence Spectra of Hexaalkynyl HATNAs.

TABLE 4.

Fluorescence Quantum Yields of Hexa‐alkynyl HATNAs 3.

Comp.	Φ in DCM a , b	Φ in solid state b
3a	0.54	0.22
3b	0.54	0.08
3c	0.20	0.09
3d	0.55	0.04
3e	0.73	0.11
3f	0.65	0.03
3g	0.38	0.05
3h	0.01	—
3i	0.69	—
3j	0.28	0.06
3k	0.44	0.02
3l	0.02	—
3m	0.59	0.03

^a 3a: 5.60×10⁻⁶ M; 3b: 2.79×10⁻⁶ M; 3c: 5.23×10⁻⁶ M; 3d: 2.86×10⁻⁵ M; 3e: 2.85×10⁻⁵ M; 3f: 2.47×10⁻⁶ M; 3g: 2.23×10⁻⁶ M; 3h: 2.79×10⁻⁶ M; 3i: 2.63×10⁻⁶ M; 3j: 2.60×10⁻⁵ M; 3k: 2.20×10⁻⁶ M; 3l: 2.39×10⁻⁶ M; 3m: 2.84×10⁻⁶ M.

^b Excitation wavelengths: 3a: 317 nm; 3b: 313 nm; 3c: 322 nm; 3d: 319 nm; 3e: 311 nm; 3f: 331 nm; 3g: 315 nm; 3h: 323 nm; 3i: 342 nm; 3j: 347 nm; 3k: 331 nm; 3l: 334 nm; 3m: 330 nm.

3. Conclusion

We have developed a synthetic method for a new class of hexaalkynyl HATNA derivatives via Sonogashira coupling, starting from easily prepared hexabromo‐HATNA 1 and commercially available or readily accessible arylalkynes 2. This protocol exhibited broad functional‐group tolerance and afforded hexa(arylethynyl) HATNAs 3 with diverse substituents in moderate to high yields. Unlike in previous studies on HATNA, the hexa‐arylalkynyl derivatives 3 displayed favorable photophysical properties, achieving moderate to high fluorescent quantum yields in dichloromethane solutions, thereby highlighting their potential in photophysical applications of HATNA‐based materials.

Conflicts of Interest

The authors have nothing to report.

Supporting information

Experimental details, analytical data, and NMR spectral copies of substrates and products (PDF).Supporting File 1: asia70632‐sup‐0001‐SuppMat.pdf.

Supporting File 2: asia70632‐sup‐0002‐DataFile.zip.

Wu Y., Kumagai C., Sugimura N., and Shibata T., “Synthesis of Aryl‐Substituted Hexa‐Alkynyl Hexaazatrinaphthylenes via Sonogashira Coupling and Evaluation of Their Photophysical Properties.” Chemistry – An Asian Journal 21, no. 4 (2026): e01003. 10.1002/asia.202501003