Sigma-spacer regulated thiophenyl triazine conjugates: synthesis and crystal, electronic and luminescent properties

Abstract

A water-accelerated Pd catalyzed Suzuki–Miyaura cross-coupling reaction was developed for the synthesis of thiophenyl triazine conjugates (TTCs). The compounds (2a–2e and 3a–3e) with five σ-spacers were obtained and fully characterized. The X-ray analysis of 2a, 2d, and 3d found that the length of the intermolecular H bond was 3.465(3) Å and the π–π interactions ranged from 3.6538(2)–3.9519(1) Å. Five σ-spacers of TTCs had an effect on their emissions from blue (363 nm) to yellow (530 nm) whilst 2- and 3-thiophenyl chromophores finely tune the emission from 530 to 504 nm. The DFT computation showed the TTCs had low HOMO (−6.8 eV) and LUMO (−2.5 eV) energy and high triplet energy (E_T, 2.4 eV). The energy level and gap of TTCs were regularly modulated by the σ-spacer.

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Introduction

1,3,5-Triazine is a fascinating π-electron-deficient aromatic building block, which has been widely employed as a C_2V electron acceptor (A) for designing D–A luminescent organic molecular devices. Introducing a 1,3,5-triazine unit into the backbone of the conjugated molecules not only improved the abilities of electron-injection and electron-transportation of the molecules,¹ but also increased the heat resistance of the molecules.² Accordingly, to explore new triazine conjugates with a modulated electron donating spacer is essential for developing photophysical organic devices. Thiophenes as electron donor derivatives have drawn great interest due to their low electron affinity (EA = −1.17 eV) and high ionization potential (IP = 8.87 eV).³ The thiophenyl derivatives were widely used in liquid-crystalline materials,^4–6 OLEDs,⁷ photosensitizers⁸ and more^9–11 in the past few decades. Strohriegl et al. reported a series of asymmetric triazine derivatives with a flexible link-age (C–O bond) between the triazine core and the donor moiety for blue PHOLEDs with high glass transition temperature (up to 170 °C) and triplet energy (up to 2.96 eV).¹² In addition, asymmetric triazines with donors connected directly to the triazine core (acceptor) can benefit both from the D–A architectures and from the fine turning of the energy levels of the materials.¹⁰ What’s more, the introduction of σ-spacer not only have a benefit from the asymmetric aspect, but also can have a fine modulation of singlet and triplet excited states.¹³

Palladium catalyzed C–C and C–N cross-coupling reactions played a vital role for the rapid construction of TTCs (Scheme 1). From the pre-prepared 5-bromothio-phene-2-carbonitrile, Takuma Yasuda et al.³ constructed the propeller-shaped TTCs bearing different π-conjugated groups via Pd catalyzed Suzuki–Miyaura cross-coupling reaction,¹⁴ Stille cross-coupling reaction^15,16 and Buchwald–Hartwig amination^17,18 respectively. Chen and Huang et al.⁷ prepared a kind of conjugated asymmetric donor-substituted TTC via the Pd catalyzed nucleophilic substitution of the cyanuric chloride with magnesium thiophenyl bromide and lithium carbazole. At present, few synthetic efforts were devoted to the new-type moderate synthesis of TTCs with sophisticated structures. Herein, we reported water-accelerated Pd catalyzed Suzuki–Miyaura cross-coupling reactions for the preparation of TTCs bearing σ-spacers such as methoxyl (CH₃O), phenoxyl (PhO), 1-naphthoxyl (1-NaphthO), diphenylaminyl (N-Ph₂) and morpholinyl (N-Morph), by which 10 examples of 2-TTCs (2a–2e) and 3-TTCs (3a–3e) were obtained up to 99% yield. The crystal structures and luminescent properties of thiophen-2-yl triazine conjugates (2-TTCs) and thiophen-3-yl triazine conjugates (3-TTCs) were investigated, respectively. The experiment results indicated that five σ-spacers of CH₃O, PhO, N-Morph, 1-NaphthO, N-Ph₂ finely tuned the HOMO, LUMO, E_g and E_T of TTCs, demonstrating a regular fluctuation. It was also found that the 2- and 3-position of thiophenyl group had a significant impact on luminescence property of TTCs.

Scheme 1 Diversity oriented synthesis of TTCs.

Results and discussion

Synthetic strategy and characterization

Ten σ-spacer regulated luminescent TTCs were synthesized via a sequent two-step strategy (Scheme 2). In the first step, the five σ-spacers of CH₃O, PhO, 1-NaphthO, N-Ph₂ and N-Morph were introduced into triazine core by a mild nucleophilic substitution reactions with high yield (80–92%). In the second step, the water-accelerated Pd catalyzed Suzuki reaction were employed for the synthesis of 2- and 3-TTCs up to 99% yield. The reaction condition of the water-accelerated Pd catalyzed Suzuki–Miyaura cross-coupling reaction were optimized in Table 1.

Scheme 2 Synthetic strategy of σ-spaced TTCs.

Table 1 Water-accelerated Pd catalyzed Suzuki–Miyaura cross-coupling reaction^a

Entry	Catalyst	Solvent	Co-	Yield^b (%)
a Reaction conditions: 1a 0.5 mmol, thiophen-2-yl boronic acid 1 mmol, Na2CO3 3 equiv., catalyst (2% mmol Pd), 60 °C, 1 h, under air.b Yield was determined by ^1H NMR.c K2CO3 as base.
1	Pd(PPh3)4	CH3CN (5 mL)	—	20
2	Pd(PPh3)4	CH3CN (3 mL)	H2O (2 mL)	96
3	Pd(PPh3)4	THF (3 mL)	H2O (2 mL)	95
4	Pd(PPh3)4	Toluene (3 mL)	H2O (2 mL)	10
5^c	Pd(PPh3)4	CH3CN (3 mL)	H2O (2 mL)	85
6	Pd(PPh3)4	CH3CN (2 mL)	H2O (3 mL)	5
7	Pd(PPh3)4	CH3CN (1 mL)	H2O (4 mL)	—
8	Pd(PPh3)4	—	H2O (5 mL)	—
9	Pd2(dba)3	CH3CN (3 mL)	H2O (2 mL)	—
10	PdCl2(PPh3)2	CH3CN (3 mL)	H2O (2 mL)	99
11	PdCl2(PPh3)2	CH3CN (1.5 mL)	H2O (1 mL)	97

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In the preliminary experiment, the Suzuki–Miyaura coupling reaction of thiophen-2-yl boronic acid and 1a was in very low reactivity and efficiency under standard reaction condition (Table 1, entry 1, 20%). Fortunately, we found adding small amount of water significantly improved efficiency of the cross-coupling reaction (Table 1, entry 2, 96%). In the participation of co-solvent water, the CH₃CN performed best as the solvent (Table 1, entries 2–4). And the Na₂CO₃ was found to be optimum base in the coupling reaction (Table 1, entries 2 and 5). It was noted that the most efficient and accelerating effect of the water proportion was 2 : 3 (V : V_Org) (Table 1, entries 6–8). The P-ligand-free Pd₂(dba)₃ failed in the reaction (Table 1, entry 9) while the PdCl₂(PPh₃)₂ performed better in the reaction even in the half volume of solvent (Table 1, entries 10 and 11).

To our knowledge, the water-accelerated effect was mainly attributed to the water-promoted activation of palladium catalysis.¹⁹ It was well known that the formation of an active Pd(0) complex is most chiefly and commonly accomplished in the Pd catalyzed Suzuki catalytic cycle. The activation could be monitored visually by color change as shown in Fig. 1. In the participation of water, the Pd^IICl₂(PPh₃)₂ was easily and rapidly converted to the Pd(0) catalysis in five minutes. However, in the absence of water the reduction did not proceed. This showed that water played a direct role in the formation of the active Pd(0) catalyst to accelerate the Suzuki cross-coupling reaction.

Fig. 1 Visualization of water-promoted activation of palladium catalysis (PdCl₂(PPh₃)₂ 0.02 mmol, Na₂CO₃ 1.5 mmol, H₂O 2 mL, CH₃CN 3 mL, 60 °C).

With the optimized condition in hand, we finally install the thiophene group to the 1,3,5-triazine backbone bearing the different σ spacers (Table 2). With the decrease of the electron donating ability of σ spacers, the yields of 2-TTCs decreased (2a > 2b > 2c). When the steric hindrances of them decreased, the yields of 2-TTCs increased (3d < 3e). Meanwhile, water-accelerated catalytic system was also employed in the synthesis of 3-TTCs. The yields of 3-TTCs were slightly lower than those of 2-TTCs and were seriously influenced by the electron donating ability and steric hindrances of σ spacers in the same way as well. These compounds were readily soluble in common organic solvents by virtue of the attached flexible σ-spacers. All the compounds were fully characterized by ¹H and ¹³C NMR spectroscopy, HRMS (ESI) and DSC (ESI†). We also employed the method to synthesize the propeller-shaped TTCs (Scheme 3).

Table 2 Water-accelerated Pd-catalyzed Suzuki reaction to TTCs^a,b

a Reaction conditions: substituted triazine 0.5 mmol, thiophen-yl boronic acid 1 mmol, Na₂CO₃ 3 equiv., PdCl₂(PPh₃)₂ 2% mmol, CH₃CN 3 mL, H₂O 2 mL, 60 °C 2 h, under air.b Isolated yield.c 1 h.

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Scheme 3 Synthesis of the propeller-shaped TTCs.

Crystal structure

The crystal structure of 2a (coplanar CH₃O σ-spacer/2-TTC), 2d (non-coplanar N-Ph₂ σ-spacer/2-TTC) and 3d (non-coplanar N-Ph₂ σ-spacer/3-TTC) were obtained by X-ray single crystal analysis (Fig. 2). Suitable single crystals for X-ray diffraction were obtained by slow evaporation of CDCl₃ solution. All the three compounds crystallized as colourless cubes in the monoclinic system, in space group C2/c (2a) and P2₁/c (2d and 3d). In the molecular structures of them, one donor thiophene (Th1) and acceptor triazinal (T) units were almost coplanar (1.7–6.7) whilst the other one of those (Th2⋯T) were non-coplanar (5.4–24.1). These crystallographic observation suggest that the degree of conjugation between Th1⋯T and Th2⋯T were different. With the decrease of the dihedral angle for Th⋯T (Th2⋯T > Th1⋯T), the degree of conjugation between them was increased and the conjugated group showed shorter bond length (L1 > L2) (Fig. 2e).

Fig. 2 (a) Molecular structure and weak H bond of 2a. (b) Diagram of the π–π interactions between two neighboring molecules in every three layers. (c) Molecular structure of the 2d. (d) Molecular structure of 3d. (e) The comparison of the dihedral angle for Th⋯T and C–S bond length between 2a, 2d and 3d. (C gray; N blue; O red; S yellow).

It is noteworthy that each two molecules of 2a were connected by weak H-bond (Fig. 2a). There are intermolecular π–π interactions between two neighboring molecules in every three layers (Fig. 2b). When the non-plane diphenylamino group was introduced, the H-bond and the π–π interactions were destroyed to form the structure of 2d (Fig. 2c). These crystallographic observation suggest that the conjugated degree of it was decreased (the dihedral angle for Th⋯T: 2a < 2d). The π–π interactions in every three layers were destroyed and became into an intermolecular force in every two layers (Fig. S2†). In the following, the thiophen-3-yl group took the place of the thiophen-2-yl group. Its conjugated degree was decreased further more (the dihedral angle for Th⋯T: 2a < 2d < 3d) which made the unit cell more loose (Table S1†) to form the structure of 3d (Fig. 2d).

Electronic structure

To modulating singlet and triplet excited states through σ spacers, theoretical calculation was executed in fact. From the HOMOs and LUMOs of TTCs shown in Fig. 3, the LUMOs have high distribution density on thiophen-yl triazine core, whereas the HOMOs are tuned by the different σ-spacers. This D–A architecture of TTCs can lead to separated electron density distribution between the HOMOs and LUMOs. This separation provides the material with a charge-transfer (C-T) state, efficient hole- and electron-transporting properties and the prevention of reverse energy-transfer, which enables the related TTCs to have excellent potential as host materials.^20,21 Since the LUMOs of TTCs are dominated by thiophen-yl triazine core, their energy levels of LUMOs are similar with little fluctuation, whereas for HOMOs, a larger fluctuation was observed. This independent modification of HOMOs and LUMOs of TTCs with various σ-spacers can greatly facilitate the molecular design of the desired materials. Due to the different σ-spacers, the energy of HOMO increased in the following order: CH₃O-TTCs ≈ PhO-TTCs < N-Morpth-TTCs < 1-NaphthO-TTCs ≈ N-Ph₂-TTCs while the E_g (LUMO–HOMO) and E_T performed in the opposite way. Comparing the different position isomerism, the HOMOs of the 2-TTCs are higher while the LUMOs and E_g of the 3-TTCs are higher. The low LUMOs of all TTCs suggest properties such as good electron ejection and transportation of the material, which is very crucial for organic optoelectronic devices.

Fig. 3 The computed HOMOs and LUMOs of σ-spacered TTCs.

Optical properties

The normalized UV/vis absorption in CH₂Cl₂ solution (3 × 10⁻⁵ M) of TTCs were shown in Fig. 4 and summarized in Table 3 where the thermal properties of TTCs were also summarized.

Fig. 4 Normalized UV/vis absorption in CH₂Cl₂ solution (top) and photoluminescent spectra in CH₂Cl₂ solution (right lower) and solid powder (left lower).

Table 3 Optical and thermal properties of TTCs

Product	UVλmax [nm]		PLλmax [nm]		Tm	Td
	CH2Cl2		CH2Cl2	Powder	[°C]	[°C]
2a	274	315	363	451	102.2	222.8
2b	275	314	367	398	103.1	227.9
2c	277	313	503	406	161.5	275.8
2d	268	305	530	476	215.2	255.2
2e	266	300	437	410	166.8	240.1
3a	271		363, 443	426	95.4	244.8
3b	273		381	427	120.7	222.1
3c	278		481	421	150.2	275.1
3d	272		504	451	212.2	297.7
3e	265		407	400	183.8	224.5

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In the absorbance spectra, the absorption bands around 265 nm were observed for all the TTCs which can be associated with n–π* transition, while the bands around 300 nm just observed for the 2-TTCs which should be assigned to π–π* transition of the conjugated backbone.^22,23 The photoluminescent (PL) emission spectra in CH₂Cl₂ solution and solid powder of TTCs were also shown in Fig. 4 and summarized in Table 3. The TTCs exhibit emissions from blue to yellow in the CH₂Cl₂ solution according to the different σ-spacers of the 1,3,5-triazine. With the increase of the electron-donating ability of the σ-spacers (CH₃O < PhO < N-Morph < 1-NaphthO < N-Ph₂), a blue shift was observed (363–530 nm and 363–504 nm). In the solid powder, the emissions of TTCs are blue shifted up to 97 nm to form stable blue emissions in comparison with that in the solution state, probably due to the better stabilizing effects of the solvent on the base state (S₀) than the excited state (S₁) of the D–A structure of TTCs.²⁴ The abnormal red shifted emissions were probably caused by the hydrogen bonds or π–π stacking interactions which can enhance the conjugate such as the molecular structure of 2a (Fig. 2).

Experimental

General methods and materials

All manipulations were performed in an atmosphere of air. Tetrahydrofuran and acetonitrile were dried and purified by routine procedures while other solvents for reactions were used without distilled. All reagents were purchased from the commercial approaches and used without further purification unless specified otherwise. Flash chromatography was performed using 200–300 mesh silica gel with the indicated solvent system according to standard techniques and analytical thin layer chromatography was carried out using 250 μm commercial silica gel plates. Visualization of the developed chromatogram was performed by UV/vis absorbance. ¹H and ¹³C NMR spectra were recorded on a Bruker EQUINX55 (400 MHz for ¹H; 101 MHz for ¹³C) spectrometer in CDCl₃. For ¹H NMR, tetramethylsilane (TMS) served as internal standard (δ = 0) and ¹H NMR chemical shifts are reported in ppm downfield of tetramethylsilane and referenced to residual solvent peak (CDCl₃ at 7.26 ppm) unless otherwise noted. The data are reported as follows: chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet and m = multiplet), and coupling constant in Hz. For ¹³C NMR, CDCl₃ was used as internal standard (δ = 77.16) and spectra were obtained with complete proton decoupling. HRMS (ESI) analysis was performed and (HRMS) data were reported with sodium mass/charge (m/z) ratios as values in atomic mass units. Differential scanning calorimetry (DSC) was run on a Pyris 1 DSC (Perkin Elmer Co.) thermal analyst system under a heating rate of 20 °C min⁻¹ and an argon flow rate of 50 cm³ min⁻¹. Ultraviolet/visible (UV/vis) spectra were recorded on an UV-3600 SHIMADZU UV/vis-NIR spectrophotometer and fluorescence spectra were obtained using a RF-5301PC spectrofluorophotometer with a xenon lamp as a light source. The concentrations of the compound solutions (in CH₂Cl₂) were adjusted to 3 × 10⁻⁵ M.

X-ray structure determination. Single crystals suitable for X-ray analysis were obtained by diffusion of petroleum ether into a dichloromethane solution. X-ray diffraction data was collected at 100 K with a Bruker AXS area detector with Mo Kα radiation (λ 0.71073 Å). The SADABS program package was used for the data collection and unit cell determination. The structure was solved by direct methods and refined on F2 by full-matrix least squares analysis with SHELXTL-97 software.

Computational methods

Theoretical calculations were performed on Gaussian 09 program with the Becke's three-parameter exchange functional along with the Lee Yang Parr's correlation functional (B3LYP) using 6-31G (d) basis sets. The ground and lowest triplet-state geometries were fully optimized and these optimized stationary points were further characterized by harmonic vibration frequency analysis to ensure that real local minima had been found. The properties of the designed model compounds, such as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy, energy gap (E_g), and triplet-state energy (E_T) were derived from the computed results.

Synthesis of 2,4-dichloro-6-methoxy-[1,3,5]-triazine (1a)²⁵. Cyanuric chloride (13.0 g, 70 mmol) was added to a solution of sodium hydrogen carbonate (6.5 g, 77 mmol) in methanol (480 mL) at 0 °C and the resulting mixture was stirred for 2 h at room temperature. After a solid was removed by filtration, the filtrate was concentrated in vacuo to afford 1a as a white solid (10.0 g, 63 mmol, 82%). The NMR data can be found in the literature.

2,4-Dichloro-6-phenoxy-[1,3,5]-triazine (1b)²⁶. Cyanuric chloride (10.0 g, 54.22 mmol) were dissolved in 100 mL of THF. One equivalent (7.01 g, 54.22 mmol) of N-ethyldiisopropylamine (DIPEA) was added under stirring. The resulting solution was cooled to 0 °C and one equivalent (5.103 g; 54.22 mmol) of phenol was subsequently added. After 30 minutes, the colour of the solution changed from yellow to colourless and a yellow precipitate (identified N-ethyldiisopropylamine hydrochloride salt) appeared which was separated from the reaction mixture by filtration. The filtrate was concentrated in vacuo to afford 1b as a white solid (12.04 g, 49.8 mmol, 92%). The NMR data can be found in the literature.

2,4-Dichloro-6-(naphthalen-1-yloxy)-[1,3,5]-triazine (1c). Cyanuric chloride (6.25 g, 33.75 mmol) were dissolved in 15 mL of acetone, and a solution of 1-naphthol (4.84 g 33.57 mmol) in 15 mL of acetone was slowly added drop wise at 0 °C. Subsequently, 8.4 mL of 4 M sodium hydroxide solution were slowly added drop wise. The mixture was stirred at 0 °C for 1 h, then warmed to room temperature and stirred for a further 1 h. The aqueous phase was removed and 10 mL of 5% NaOH solution and 10 mL of chloroform were added to the organic phase, the aqueous phase was removed again and the organic phase was washed twice with 4 mL each time of 5% NaOH solution and three times with 9 mL each time of water, and dried over sodium sulfate. The mixture was filtered and the filtrate was concentrated in vacuo to afford 1c as a white solid (7.84 g, 26.85 mmol, 80%).

4,6-Dichloro-N,N-diphenyl-[1,3,5]-triazin-2-amine (1d)²⁷. Cyanuric chloride (1.845 g, 10.00 mmol), Na₂CO₃ (2.12 g, 20.00 mmol) and 20 mL acetone were fed into a 100 mL glass flask equipped with a mechanical agitator. Diphenylamine (1.42 g, 8.40 mmol) dissolved in 10 mL acetone, and the obtained solution was added drop wise into the flask. The reaction temperature was kept at 0 °C, and the mixture was stirred for about 2 h until yellowish precipitate formed. The product was washed several times by cold water until no chloride could be detected to afford 1d as a white solid (2.43 g, 7.66 mmol, 91.2%). The NMR data can be found in the literature.

4-(4,6-Dichloro-[1,3,5]-triazin-2-yl)morpholine (1e)²⁸. Cyanuric chloride (2.77 g, 15 mmol) and Na₂CO₃ (1.59 g, 15 mmol) were dissolved in THF (50 mL) and the mixture was cooled to 0 °C. Then, morpholine (1.31 g, 15 mmol) dissolved in THF (30 mL) was added drop wise in 30 min. After the completion of the addition, the cloudy mixture was warmed to room temperature and kept for 1 h. The mixture was filtered and the filtrate was collected. The solid obtained from rotary evaporation of the solvent under reduced pressure, was dissolved in 100 mL CHCl₃, washed with H₂O two times and dried over anhydrous Na₂SO₄ for 2 h. Then the inorganic salt was removed by filtration. The filtrate was concentrated in vacuo and performed by column chromatography with ether/hexane (mixture ratio is 1 : 3) as eluent to afford 1e as a white solid (2.89 g, 12.3 mmol, 82%). The NMR data can be found in the literature.

Synthesis of 2-methoxy-4,6-di(thiophen-2-yl)-[1,3,5]-triazine (2a). A stock solution of thiophen-2-yl boronic acid (0.1319 g, 1 mmol 97%), 1a (0.0900 g, 0.5 mmol) and PdCl₂(PPh₃)₂ (0.0141 g 0.02 mmol) in CH₃CN (3 mL) was prepared in a 50 mL round-bottom tube equipped with a magnetic stir bar, and was added a solution of Na₂CO₃ (0.1590 g, 1.5 mmol) in H₂O (2 mL) for 1 hour at 60 °C. On completion, water (20 mL) was added and the resulting mixture was extracted three times with EA (10 mL per time), dried over magnesium sulfate and concentrated to be performed by column chromatography with ether/EA (mixture ratio is 20 : 1) as eluent to afford 2a as a white solid (0.1363 g, 0.495 mmol, 99%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.24 (d, J = 3.7 Hz, 2H), 7.62 (d, J = 6.0 Hz, 2H), 7.20 (t, 2H), 4.16 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 170.11, 168.40, 140.01, 131.45, 130.82, 127.36, 53.99. HRMS (ESI) m/z: [M + Na]⁺ calcd for C₁₂H₉N₃NaOS₂: 298.0079. Found: 298.0090.

2-Phenoxy-4,6-di(thiophen-2-yl)-[1,3,5]-triazine (2b). The reaction of 1b with thiophen-2-yl boronic acid provided 2b as a white solid (0.1350 g, 0.4 mmol, 80%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.15 (s, 1H), 7.61 (d, J = 4.5 Hz, 1H), 7.48 (t, J = 7.5 Hz, 1H), 7.33 (d, J = 7.4 Hz, 2H), 7.20–7.15 (m, 1H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 172.41, 171.32, 153.42, 142.14, 134.35, 133.69, 130.77, 129.91, 127.17, 123.18. HRMS (ESI) m/z: [M + H]⁺ calcd for C₁₇H₁₂N₃OS₂: 338.0416. Found: 338.0428.

2-(Naphthalen-1-yloxy)-4,6-di(thiophen-2-yl)-[1,3,5]-triazine (2c). The reaction of 1c with thiophen-2-yl boronic acid provided 2c as a white solid (0.1298 g, 0.335 mmol, 67%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.06 (s, 2H), 8.00 (d, J = 8.1 Hz, 1H), 7.93 (d, J = 7.9 Hz, 1H), 7.83 (d, J = 8.2 Hz, 1H), 7.59–7.45 (m, 5H), 7.41 (d, J = 7.3 Hz, 1H), 7.16–7.09 (m, 2H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 170.02, 148.04, 140.68, 134.71, 132.88, 132.27, 128.42, 127.90, 127.07, 126.41, 125.99, 125.39, 121.78, 118.08. HRMS (ESI) m/z: [M + H]⁺ calcd for C₂₁H₁₄N₃OS₂: 388.0573. Found: 388.0587.

N,N-Diphenyl-4,6-di(thiophen-2-yl)-[1,3,5]-triazin-2-amine (2d). The reaction of 1d with thiophen-2-yl boronic acid provided 2d as a white solid (0.1444 g, 0.35 mmol, 70%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.97 (d, J = 3.4 Hz, 2H), 7.49 (d, J = 4.8 Hz, 2H), 7.47–7.37 (m, 8H), 7.29 (t, J = 6.5 Hz, 2H), 7.10 (t, 2H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 168.98, 144.55, 143.63, 132.95, 132.31, 130.20, 129.53, 129.22, 127.58. HRMS (ESI) m/z: [M + Na]⁺ calcd for C₂₃H₁₆N₄NaS₂: 435.0709. Found: 435.0720.

4-(4,6-Di(thiophen-2-yl)-[1,3,5]-triazin-2-yl)morpholine (2e). The reaction of 1e with thiophen-2-yl boronic acid provided 2e as a white solid (0.1371 g, 0.415 mmol, 83%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.12 (s, 2H), 7.52 (s, 2H), 7.15 (s, 2H), 3.99 (s, 4H), 3.80 (s, 4H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 168.72, 165.79, 143.89, 132.48, 131.96, 129.50, 68.22, 45.06. HRMS (ESI) m/z: [M + H]⁺ calcd for C₁₅H₁₅N₄OS₂: 331.0682. Found: 331.0695.

2-Methoxy-4,6-di(thiophen-3-yl)-[1,3,5]-triazine (3a). The reaction of 1a with thiophen-3-yl boronic acid provided 3a as a white solid (0.1308 g, 0.475 mmol, 95%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.56 (d, J = 2.7 Hz, 2H), 8.00 (d, J = 5.0 Hz, 2H), 7.41 (t, 2H), 4.18 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 173.06, 171.41, 141.33, 132.80, 129.05, 127.65, 56.34. HRMS (ESI) m/z: [M + Na]⁺ calcd for C₁₂H₉N₃NaOS₂: 298.0079. Found: 298.0090.

2-Phenoxy-4,6-di(thiophen-3-yl)-[1,3,5]-triazine (3b). The reaction of 1b with thiophen-3-yl boronic acid provided 3b as a white solid (0.1265 g, 0.375 mmol, 75%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.45 (d, J = 2.2 Hz, 1H), 7.88 (d, J = 4.9 Hz, 1H), 7.46 (t, J = 7.7 Hz, 1H), 7.33 (dt, J = 11.7, 4.9 Hz, 3H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 172.96, 171.83, 153.56, 141.06, 133.29, 130.82, 129.07, 127.75, 127.15, 123.24. HRMS (ESI) m/z: [M + H]⁺ C₁₇H₁₂N₃OS₂: 338.0416. Found: 338.0429.

2-(Naphthalen-1-yloxy)-4,6-di(thiophen-3-yl)-[1,3,5]-triazine (3c). The reaction of 1c with thiophen-3-yl boronic acid provided 3c as a white solid (0.1259 g, 0.325 mmol, 65%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.39 (d, J = 2.8 Hz, 2H), 8.00 (d, J = 8.3 Hz, 1H), 7.94 (d, J = 8.1 Hz, 1H), 7.88–7.81 (m, 3H), 7.56 (d, J = 7.9 Hz, 1H), 7.52 (d, J = 8.3 Hz, 1H), 7.50–7.45 (m, 1H), 7.41 (d, J = 7.5 Hz, 1H), 7.33 (dd, J = 5.0, 3.2 Hz, 2H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 172.14, 170.56, 148.19, 139.61, 134.75, 131.87, 127.96, 127.64, 127.14, 126.48, 126.45, 126.28, 126.00, 125.46, 121.80, 118.12. HRMS (ESI) m/z: [M + Na]⁺ calcd for C₂₁H₁₄N₃OS₂: 388.0573. Found: 388.0587.

N,N-Diphenyl-4,6-di(thiophen-3-yl)-[1,3,5]-triazin-2-amine (3d). The reaction of 1d with thiophen-3-yl boronic acid provided 3d as a white solid (0.1403 g, 0.34 mmol, 68%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.29 (d, J = 2.8 Hz, 2H), 7.75 (d, J = 5.0 Hz, 2H), 7.42 (d, J = 6.0 Hz, 8H), 7.31 (dd, J = 5.2, 2.9 Hz, 4H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 169.48, 167.73, 144.80, 142.28, 131.80, 130.24, 129.15, 127.54, 127.24. HRMS (ESI) m/z: [M + Na]⁺ calcd for C₂₃H₁₆N₄NaS₂: 435.0709. Found: 435.0724.

4-(4,6-Di(thiophen-2-yl)-[1,3,5]-triazin-2-yl)morpholine (3e). The reaction of 1e with thiophen-3-yl boronic acid provided 3e as a white solid (0.1403 g, 0.34 mmol, 78%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.43 (d, J = 2.7 Hz, 2H), 7.92 (d, J = 5.0 Hz, 2H), 7.36 (t, 2H), 4.00 (t, 4H), 3.79 (t, 4H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 169.25, 166.40, 142.52, 131.35, 129.01, 127.21, 68.23, 45.05. HRMS (ESI) m/z: [M + H]⁺ calcd for C₁₅H₁₅N₄OS₂: 331.0682. Found: 331.0690.

Synthesis of 2,4,6-tri(thiophen-2-yl)-1,3,5-triazine (4a). A stock solution of thiophen-2-yl boronic acid (0.2237 g, 1.70 mmol 97%), cyanuric chloride (0.0951 g, 0.5 mmol) and PdCl₂(PPh₃)₂ (0.0212 g 0.03 mmol) in CH₃CN (3 mL) was prepared in a 50 mL round-bottom tube equipped with a magnetic stir bar, and was added a solution of Na₂CO₃ (0.2385 g, 2.25 mmol) in H₂O (2 mL) for 1.5 hours in reflux. On completion, water (20 mL) was added and the resulting mixture was extracted three times with EA (10 mL per time), dried over magnesium sulfate and concentrated to be performed by column chromatography with ether/EA (mixture ratio is 40 : 1) as eluent to afford 4a as a white solid (0.1005 g, 0.307 mmol, 61%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.28 (d, J = 3.7 Hz, 2H), 7.63 (d, J = 4.9 Hz, 2H), 7.22 (t, J = 8.7 Hz, 1H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 167.87, 141.58, 132.47, 131.84, 128.57.

2,4,6-Tri(thiophen-3-yl)-1,3,5-triazine (5a). The reaction of cyanuric chloride with thiophen-3-yl boronic acid provided 5a as a white solid (0.0892 g, 0.281 mmol, 56%). ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.61 (d, J = 2.7 Hz, 1H), 8.06 (d, J = 5.0 Hz, 1H), 7.43 (t, 1H); ¹³C NMR (101 MHz, CDCl₃) δ (ppm): 168.52, 140.58, 131.12, 127.77, 126.38.

Conclusions

In summary, ten examples of σ-spacers regulated luminescent TTCs have been synthesized via a water-accelerated Suzuki cross-coupling method which provided it appeared to be a facile synthesis of more complex D–A molecules and bearing multiple triazinal groups. This will greatly promote the research of the triazine-based materials. The prepared TTCs exhibit good solubility and high thermostability for device fabrications. The D–A compounds with separated electron density distribution show low LUMO as revealed by the theoretical investigations. The DFT computations and optical property demonstrated that the modulation of σ-spacers is a pretty rational strategy for building promising optoelectronic materials and the 3-TTCs can be applied to new optoelectronic materials. Given these promising properties of these compounds, development of new photophysical materials with structurally complexity as well as the excellent photoproperty is underway.

Acknowledgements

This work was supported by the 111 Project (B14041), the grant from National Natural Science Foundation of China (21271124, 21272186, 21371112, 21446014), the Fundamental Funds Research for the Central Universities (No. GK201501005, GK201503029, 2016CSY002), the grant from Fundamental Doctoral Fund of Ministry of Education of China (20120202120005), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R33).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1430747, 1430748 and 1435751. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13795d

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