Introduction

Multi-N-heterocycles are versatile nanobuilding blocks for the construction of nanostructures and also for molecular recognition. By the formation of complexes via hydrogen or dative bonds with the nitrogen atoms, unique structures can be fabricated from these materials. In addition, the precise molecular recognition based on complexation has been accomplished with various targets involving biomolecules.1, 2, 3, 4 Furthermore, recent works have revealed the useful photo-catalytic activity of carbon nitrides, which possess network structures composed of three-fused multi-N-heterocycles.5 Notably, the decomposition of water with visible light irradiation has been achieved with carbon nitrides, without requiring the use of rare metals. Despite the utility of multi-N-heterocycles, the poor solubility of these compounds often occludes the analysis of their molecular structure and reaction mechanisms. Consequently, the applicability of the materials is critically limited. Improving the solubility and processability of these materials is essential to advance the fundamental chemistry and explore the applications of multi-N-heterocycles.

Azaphenalenes are a class of fused three-ring compounds in which the carbon at the center is replaced by nitrogen (Scheme 1).6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 Compared with phenalene, it was previously shown that azaphenalene has higher stability, even in the neutral state, because the unpaired electrons of the center nitrogen participate can be conjugated to form 14π electronic systems. Indeed, from the first synthesis of 9b-azaphenalene in 1976,7 the series of azaphenalenes have been isolated in a neutral form.6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 Moreover, their optical properties and electronic structures have been examined.7 However, to the best of our knowledge, few studies on the conjugation of azaphenalenes and other functional molecules have been executed. In particular, the influence of conjugation on the electronic structures of azaphenalenes has hardly been examined. Thus, the introduction of azaphenalenes into polymers and the investigation of their material properties are of great significance for the development of new functional materials, including those with interesting optical properties derived from the electronic structure of azaphenalenes.

Herein, we report the synthesis and optical properties of an organic solvent-soluble polymer incorporating the dimethyl-1,3,4,6,9b-pentaazaphenalene (5AP) unit. Initially, dibromo-substituted 5AP was prepared as a monomer. An alternating polymer with fluorene was obtained by Suzuki–Miyaura coupling reactions. The optical properties of the synthesized polymer were measured to understand the electronic structures of the polymer. Accordingly, from comparative studies with the model compounds, electronic interactions between 5AP and fluorene units were observed. Delocalization of the highest occupied molecular orbital (HOMO) through the 5AP unit was suggested from computer modeling of the frontier orbitals of the 5AP-containing polymer main chain.

Experimental Procedure

General

1H nuclear magnetic resonance (NMR) and 13C NMR spectra were measured with a JEOL EX-400 (400 MHz for 1H and 100 MHz for 13C) spectrometer (JEOL Ltd., Tokyo, Japan). Coupling constants (J value) are reported in Hertz. 1H and 13C NMR spectra used tetramethylsilane as an internal standard. The number-average molecular weight (Mn) and the molecular weight distribution (weight-average molecular weight/number-average molecular weight (Mw/Mn)) values of all polymers were estimated by size-exclusion chromatography, performed on a TOSOH G3000HXI (TOSOH Co., Ltd., Tokyo, Japan) system equipped with three consecutive polystyrene gel columns (TOSOH gels: α−4000, α−3000 and α−2500) and an ultraviolet detector at 40 °C. The system was operated at a flow rate of 1.0 ml min−1 with CHCl3 as an eluent. The system was calibrated with polystyrene standards. Ultraviolet–visible spectra were recorded on a Shimadzu UV-3600 spectrophotometer (Shimadzu Co., Ltd., Kyoto, Japan). Fluorescence emission spectra were recorded on a HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer (HORIBA Ltd., Kyoto, Japan), and the absolute quantum yield was calculated with the integrating sphere on the HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer in chloroform. Cyclic voltammetry spectra were recorded on a BAS ALS electrochemical analyzer model 600D (BAS Inc., Tokyo, Japan). Molecular orbital calculations were performed using Gaussian 09 software package.

5AP

A glyme solution (10 ml) containing 2,6-diaminopyridine (8.73 g, 80.0 mmol) and methyl-N-cyanoacetimidate (15.67 g, 160.0 mmol) was refluxed overnight, after which volatiles were removed under vacuum.15 The residual solid was recrystallized twice from methanol and chloroform to obtain the product (6.02 g, 30.2 mmol, 38%) as a red crystal. 1H NMR (CDCl3, p.p.m.) 7.27 (t, J=8.3 Hz, 1H), 6.09 (d, J=8.3 Hz, 2H), 2.03 (s, 6H). 13C NMR (CDCl3, p.p.m.) 177.66, 159.53, 154.23, 145.74, 110.73, 25.60. High resolution mass spectrometry (positive mode, electron spray ionization) HRMS (p-ESI) calcd. for C10H9N5+H+: m/z=200.0931; found: m/z=200.0928.

7,9-Dibromo-2,5-dimethyl-1,3,4,6,9b-pentaazaphenalene

A chloroform solution (250 ml) of 5AP (4.00 g, 20.1 mmol) and N-bromosuccinimide (10.7 g, 60.3 mmol) was refluxed for 3 days. The residual solution was evaporated under vacuum. After flash chromatography on silica gel (9:1 CHCl3/EtOAc, Rf=0.3), followed by recrystallization from ethanol, the product (3.82 g, 10.7 mmol, 53%) was obtained as a purple powder. 1H NMR (CDCl3, p.p.m.) 7.85 (s, 1H), 2.14 (s, 6H). 13C NMR (CDCl3, p.p.m.) 179.15, 150.93, 150.67, 102.49, 26.06. HRMS (p-ESI) calcd. for C10H7Br2N5+H+: m/z=355.9141; found: m/z=355.9137.

7-Bromo-2,5-dimethyl-1,3,4,6,9b-pentaazaphenalene

A chloroform solution (10 ml) of 5AP (200 mg, 1.0 mmol) and N-bromosuccinimide (178 mg, 1.0 mmol) was refluxed for 2 h. The residual solution was evaporated under vacuum. The product (202 mg, 0.73 mmol, 73%) was obtained as a purple powder after recrystallization from ethanol. 1H NMR (CDCl3, p.p.m.) 7.54 (d, J=8.8 Hz, 1H), 6.03 (d, J=8.9 Hz, 1H), 2.12 (s, 3H), 2.05 (s, 3H). 13C NMR (CDCl3, p.p.m.) 179.19, 177.84, 159.61, 153.65, 151.51, 148.56, 110.70, 103.03, 26.07, 25.69. HRMS (p-ESI) calcd. for C10H9BrN5+H+: m/z=278.0036; found: m/z=278.0033.

Fluorene (FL)-5AP-FL

A solution containing tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3; 1 mg, 0.001 mmol), 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (S-Phos; 3 mg, 0.007 mmol), cesium carbonate (Cs2CO3; 0.40 g, 1.23 mmol), 7,9-dibromo-2,5-dimethyl-1,3,4,6,9b-pentaazaphenalene (35.7 mg, 0.10 mmol) and 9,9-didodecylfluorene-2-boronic acid (87 mg, 0.20 mmol) in 1 ml of toluene and 1 ml of H2O was stirred at 85 °C for 2 days under an argon atmosphere. The product was precipitated by adding 50 ml of methanol and then washing with methanol twice to obtain a dark blue solid (60 mg, 61%). 1H NMR (CDCl3, p.p.m.) 7.73–7.67 (m, 5H), 7.60 (s, 2H), 7.39 (dd, J=8.0, 1.5 Hz, 2H), 7.37–7.29 (m, 6H), 2.00 (s, 6H), 1.96 (t, J=8.3 Hz, 8H), 1.18 (m, 8H), 1.05 (m, 32H), 0.81 (t, J=7.0 Hz, 12H), 0.70 (m, 8H). 13C NMR (CDCl3, p.p.m.) 177.00, 151.19, 151.08, 150.20, 146.30, 141.25, 140.45, 133.16, 127.30, 126.89, 126.80, 123.46, 123.13, 122.94, 119.83, 119.46, 55.02, 40.27, 31.75, 30.16, 29.26, 25.76, 23.89, 22.56, 14.04. HRMS (p-ESI) calcd. for C68H89N5+H+: m/z=976.7191; found: m/z=976.7177.

5AP-FL-5AP

A solution containing Pd2(dba)3 (1 mg, 0.001 mmol), S-Phos (3 mg, 0.007 mmol), Cs2CO3 (0.40 g, 1.23 mmol), 7-bromo-2,5-dimethyl-1,3,4,6,9b-pentaazaphenalene (56 mg, 0.20 mmol) and 9,9-didodecylfluorene-2,7-diboronic acid (59 mg, 0.10 mmol) in 1 ml of toluene and 1 ml of H2O was stirred at 85 °C for 2 days under an argon atmosphere. After the reaction was refluxed for 2 days, the resulting mixture was poured into brine, extracted with cyclopentyl methyl ether and dried with anhydrous Na2SO4. After the solvent was removed, the crude product was purified by silica gel column chromatography with an ethyl acetate/chloroform (1:1) eluent to afford the product (60 mg, 67%). 1H NMR (CDCl3, p.p.m.) 7.68 (d, J=7.9 Hz, 2H), 7.50 (s, 2H), 7.47 (d, J=8.5 Hz, 2H), 7.34 (d, J=8.0 Hz, 2H), 6.21 (d, J=8.5 Hz, 2H), 2.04 (s, 6H), 1.99 (s, 6H), 1.92 (m, 4H), 1.31–0.98 (m, 36H), 0.86 (t, J=6.8 Hz, 6H), 0.72 (br, 4H). 13C NMR (CDCl3, p.p.m.) 177.45, 177.38, 160.36, 153.33, 151.95, 150.66, 146.09, 141.65, 140.49, 133.30, 126.90, 123.24, 119.60, 110.59, 103.35, 99.55, 54.98, 40.14, 31.85, 30.22, 29.64, 29.56, 29.50, 29.27, 26.77, 26.64, 25.89, 25.56, 23.97, 22.62, 14.07. HRMS (p-ESI) calcd. for C57H72N10+H+: m/z=897.5993; found: m/z=897.6014.

5AP-co-FL

A solution containing Pd2(dba)3 (2 mg, 0.002 mmol), S-Phos (6 mg, 0.015 mmol), Cs2CO3 (0.80 g, 2.46 mmol), 7,9-dibromo-2,5-dimethyl-1,3,4,6,9b-pentaazaphenalene (68 mg, 0.19 mmol) and 9,9-didodecylfluorene-2,7-diboronic acid (112 mg, 0.19 mmol) in 1 ml of toluene and 1 ml of H2O was stirred at 85 °C for 3 days under an argon atmosphere. The product was obtained as a deep blue solid (110 mg, 83%) after adding 50 ml of methanol to precipitate the compound and then washing twice with methanol. 1H NMR (CDCl3, p.p.m.) 7.72 (1H), 7.69 (2H), 7.62 (2H), 7.36 (2H), 2.01 (10H), 1.20 (40H), 0.86 (6H). 13C NMR (CDCl3, p.p.m.) 117.11, 161.07, 151.16, 150. 75, 150.25, 146.31, 140.60, 133.52, 133.12, 127.35, 127.05, 123.63, 123.50, 123.24, 123.01, 119.85, 119.72, 119.48, 55.09, 40.30, 40.23, 31.92, 30.38, 30.20, 29.76, 29.63, 29.58, 29.44, 29.34, 25.79, 24.15, 23.94, 22.68, 14.12. Anal. Calcd. for C49H68N5: H, 9.80; C, 80.61; N, 9.5. Found: H, 7.95; C, 66.29; N, 5.91.

Results and Discussion

We designed a monomer based on the tri-s-triazine skeleton because of its relatively high stability in the series of azaphenalenes.15, 16 The two carbons at the α-position (C8 and C9) of tri-s-triazine were replaced with nitrogen. The C4 and C5 positions of the monomer were substituted with methyl groups to improve stability and enhance reactivity. To prepare the monomer, 5AP was halogenated at the α-carbons (C8 and C9) for the coupling reaction.17 From X-ray crystal structure analysis, it was found that the sum of the bond angle around N1 was approximately 360°.24, 25, 26, 27 These data indicated that 5AP was a highly planar molecule. Such planarity is favorable to realize the delocalization of electrons in aromatic compounds. In addition, the unpaired electrons of N1 were delocalized in the cyclic system of 5AP.27 These data indicate that a 14π electronic conjugation system, including the unpaired electrons at the N1 position, was obtained.

Polymerization of the dibromo-substituted 5AP17 with fluorenyldiboronic acid via Suzuki–Miyaura coupling was performed (Scheme 2). After re-precipitation from methanol to remove the metal species and small molecules derived from the monomers, the polymeric products were obtained. A series of measurements were performed to characterize the products. The number-average molecular weight (Mn) of the polymer determined from gel permeation chromatography analysis was 3600 (degree of polymerization=5), and the polydispersity index was 1.45. The polymer showed good solubility in conventional organic solvents such as chloroform, dichloromethane and tetrahydrofuran. The introduction of nitrogen was confirmed by elemental analysis, suggesting that the product had the expected structure. By increasing the reaction time (4 days), a larger molecular weight polymeric product was obtained (Mn=10 600, polydispersity index =1.69, degree of polymerization=14), whereas the solubility was only slightly reduced. Because the degree of polymerization in the shorter polymer was greater than five, the two polymer products could be used to evaluate the effect of extending the polymer main chain on the optical properties. Two model compounds, FL-5AP-FL and 5AP-FL-5AP, were also synthesized by Suzuki–Miyaura coupling. From the 1H and 13C NMR spectra, similar signal patterns involving 5AP and the fluorene units on the polymer and the model compounds were obtained. Thus, we concluded that the polymer and the model compounds possessed the designed chemical structures, including 5AP, and were of sufficient purity for the evaluation of optical properties.

The electronic structures of the synthesized 5AP derivatives at the ground states were investigated by ultraviolet–visible absorption measurements. Figure 1 shows the spectra of chloroform solutions (1.0 × 10−5 M, repeat unit reference). The optical properties are summarized in Table 1. 5AP presented absorption bands with peaks from 260 to 370 nm (π–π*) as well as very weak absorption bands from 500 to 600 nm (lone pair orbital at the nitrogen center-π*). The model compounds 5AP-FL-5AP and FL-5AP-FL showed absorption bands with peaks at approximately 370 nm (π–π* of the fluorene unit) and approximately 580 nm (lone pair orbital-π* of the 5AP unit). Interestingly, 5AP-co-FL exhibited significantly redshifted absorption bands compared with those of 5AP and the model compounds. In particular, a shift of the absorption band at approximately 370 nm of 5AP-co-FL, assigned as the π–π* transition in the fluorene unit, was observed compared with that of 5AP-FL-5AP. The optical properties of meta-substituted diphenylacetylene were previously examined with a dendritic system.28, 29, 30 A peak shift was hardly observed by extending the chain length. Thus, comparing to that work, these results obtained here for the polymer suggest that electronic interactions can occur through the polymer main chain involving the 5AP unit. The HOMO levels of 5AP derivatives can be perturbed by the inductive effect.10 Substituent effects could also be responsible for the peak shift of the absorption band at approximately 580 nm. The absorption spectrum of the longer polymer was measured, and less significant changes were observed (Supplementary Figure 1). Thus, the results suggest that electronic interactions can occur within five repeating units.

Figure 1
figure 1

Ultraviolet–visible spectra of the dimethyl-1,3,4,6,9b-pentaazaphenalene (5AP) derivatives in CHCl3 (1.0 × 10−5 M) and time-dependent density functional theory calculated (B3LYP/6–31G++(d)) oscillator strengths (color bar). ɛ=molar extinction coefficient and f=oscillator strength.

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Table 1 Optical properties of the 5AP derivatives
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The emission properties of the 5AP derivatives were examined in CHCl3 (Figure 2). According to a previous report,10 it is known that the S0-S1 transition of 5AP is attributed to a lone pair orbital-π* transition. When excited with light at the peak wavelength of the strong absorption band in the ultraviolet region (π–π*), which was attributed to the S0-S2 transition, 5AP derivatives showed weak emission. Although the quantum yields of these emissions were low (ΦPL⩽0.01), the emission properties represent the electronic interactions at the excited states. 5AP also presented weak emission, with the peaks at approximately 390 nm. Importantly, the emission bands obtained from 5AP-co-FL and 5AP-FL-5AP were at longer wavelengths compared with 5AP. These results suggest the existence of electronic interactions involving the 5AP units in the excited states. The energy level of the non-bonding electrons of the nitrogen in 5AP is higher than the HOMO of fluorene. Therefore, photo-induced electron transfer can occur in the excited states, and 5AP works as a quencher in the polymers.

Figure 2
figure 2

Normalized photoluminescence spectra of dimethyl-1,3,4,6,9b-pentaazaphenalene (5AP) derivatives in CHCl3 (1.0 × 10−5 M). (Excited at absorption maxima, respectively).

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To further understand the nature of 5AP and the electronic structures, we performed theoretical calculations for 5AP, FL-5AP-FL and 5AP-FL-5AP using the density functional theory method at the B3LYP/6-31G++(d) level.31 The long alkyl chains were replaced with methyl groups simplify the calculation. Figure 3 illustrates the diagrams of each HOMO and lowest unoccupied molecular orbital (LUMO) of FL-5AP-FL and 5AP-FL-5AP. In agreement with the results of the optical measurements, HOMOs were expanded through the 5AP and fluorene moieties. It is assumed that these delocalized HOMOs could be responsible for the redshifts of the absorption and the emission bands.

Figure 3
figure 3

Molecular orbital diagrams for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of FL-5AP-FL and 5AP-FL-5AP (B3LYP/6-31G++(d)). 5AP, dimethyl-1,3,4,6,9b-pentaazaphenalene.

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The electronic oscillator strength (f) was calculated using the time-dependent density functional theory method at the B3LYP/6-31G++(d) level (Figure 1).31 The calculated energy levels of each orbital showed good agreement with the experimental values. The absorption bands were assigned according to the calculation results. The bands between 500 and 600 nm should correspond to the HOMO→LUMO transition from the main chain to the 5AP moiety. The π–π* transition of the main chain observed at 384 nm in 5AP-FL-5AP was assigned to the HOMO→LUMO+2 transition, and the band at 364 nm in FL-5AP-FL was assigned to the HOMO→LUMO+1 transition (Figure 4).

Figure 4
figure 4

Molecular orbital diagrams for the lowest unoccupied molecular orbital (LUMO)+1 and LUMO+2 of FL-5AP-FL and 5AP-FL-5AP (B3LYP/6-31G++(d)). 5AP, dimethyl-1,3,4,6,9b-pentaazaphenalene.

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The energy gaps between the HOMO and LUMO levels were estimated using the data from the cyclic voltammetry and optical measurements (Figure 5 and Table 2). In the cyclic voltammetry measurements, the energy gaps were calculated from the peak positions. From the absorption measurements, the desired values were evaluated from the onset of the absorption bands, with the peak at approximately 400 nm assigned to the π−π* transition from the HOMO to LUMO+1 or +2, rather than assigning the small absorption bands at approximately 550 nm to the HOMO−LUMO gaps. The results showed that the energy gaps of the models calculated by both methods were smaller than that of 5AP. Furthermore, the polymer presented the narrowest energy gap of the 5AP derivatives, as reported in a previous study.10 These data also support the existence of electronic interactions through the polymer main chains involving 5AP.

Figure 5
figure 5

Cyclic voltammograms (electrolyte solution: 0.1 M Bu4NPF6 in tetrahydrofuran; scan rate: 0.1 V s−1; reference electrode: Ag/Ag+).

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Table 2 Electronic energy levels of the 5AP derivatives
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Conclusions

We synthesized an organic solvent-soluble polymer composed of the 5AP unit. From the series of optical measurements and computational calculations, electronic interactions between 5AP and the fluorene co-monomer were suggested. These results imply that the optical properties of 5AP units can be tuned via electronic coupling with the co-monomers through the polymer main chains. Furthermore, because of the structural similarity of azaphenalene to the monomer unit of carbon nitride, we expect that the 5AP-containing polymers might be useful as a soluble analog to carbon nitride. Studies on the photo-catalytic ability of 5AP-containing polymers are in progress.

scheme 1

Chemical structures of phenalene derivatives. 5AP, dimethyl-1,3,4,6,9b-pentaazaphenalene.

scheme 2

Synthesis of the polymer and model compounds. aReagents and conditions: (a) Pd2(dba)3, S-Phos, Cs2CO3, toluene, H2O, 85 °C, 3 days.