Aryl-substituted symmetrical and unsymmetrical benzothiadiazoles

Abstract

A set of benzothiadiazoles (BTD) of the type D–π–A–π–D and D₁–π–A–π–D₂ were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical and electrochemical properties were studied. The substitution of anthracene on BTD improves its thermal stability, and lowers the HOMO–LUMO gap, which results in a red shift of the electronic absorption. The experimental optical gap values show good agreement with the calculated HOMO–LUMO gap.

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Introduction

The design and synthesis of donor–acceptor (D–A) molecular systems are of significant interest, because of their application in organic light emitting diodes (OLEDs), organic photovoltaic devices (OPVs), organic thin film transistors (OTFTs), and non-linear optics (NLO).^1,2 The photonic properties of donor–acceptor molecular systems can be tuned by varying the strength of the donor as well as the acceptor.^3,4 Benzothiadiazole is a strong acceptor and has been widely studied in OPVs and NLO.^5,6

Our group is interested in the design and synthesis of donor–acceptor molecular systems with low HOMO–LUMO gap.⁷ Recently we have reported symmetrical and unsymmetrical BTD based molecular systems of the type D–π–A–π–D, D–π–A–D, D–π–A–π–A–π–D and D–A–A–A–D.⁸ We were interested to see the effect of different aryl groups on the HOMO–LUMO gap and thermal stability of a BTD based donor–acceptor molecular system. In this contribution we wish to report BTD based molecular systems of the type D–π–A–π–D and D₁–π–A–π–D₂ with a planar anthracene unit, and non-planar triphenylamine and triarylborane units (Chart 1).

Chart 1 Aryl-substituted benzothiadiazoles.

Result and discussion

Synthesis

The synthetic route for aryl substituted benzothiadiazoles 2, 3, 8 and 9 are shown in Schemes 1, 2 and S1.† The dibromobenzothiadiazole 1, was synthesized by the bromination reaction of benzothiadiazole.⁹ The aryl substituted symmetrical benzothiadiazoles 2 and 3 were synthesised by the Pd-catalyzed Sonogashira cross-coupling reaction of dibromobenzothiadiazole 1, with 9-ethynylanthracene (4) and (4-ethynylphenyl)dimesitylborane (5) in 84% and 62% yield respectively.¹⁰

Scheme 1 Synthesis of BTDs 2 and 3.

Scheme 2 Synthesis of BTDs 8 and 9.

The Sonogashira cross-coupling reaction of 1 equivalent of dibromobenzothiadiazole 1, with 1.2 equivalent of (4-ethynylphenyl)diphenylamine resulted BTD 6 (chart) and intermediate 7 in 30% and 52% respectively (Scheme S1†).^1d,11 The aryl substituted unsymmetrical benzothiadiazoles 8 and 9 were synthesised by the Pd-catalyzed Sonogashira cross-coupling reactions of intermediate 7, with 9-ethynylanthracene (4) and 4-ethynylphenyl-dimesitylborane (5) in 74% and 72% yield respectively (Scheme 2).¹² Benzothiadiazoles 2, 3, 6, 8 and 9 were well characterized by ¹H, ¹³C NMR, and HRMS techniques.

Thermal stability

The thermal stability of donor–acceptor organic chromophore are significant for their practical applications. The thermal stability of BTDs 2, 3, 6, 8 and 9 were determined by thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹, under nitrogen atmosphere (Fig. 1). The decomposition temperatures (T_d) values for 5% weight loss of the BTDs are listed in Table 1. The trend in thermal stability follows the order 2 > 8 > 6 > 9 > 3. This indicates incorporation of planar anthracene in BTDs 2 and 8 improves the thermal stability. On the other hand incorporation of phenyl-dimesitylborane results in lower thermal stability.

Fig. 1 TGA plots of benzothiadiazoles 2, 3, 6, 8 and 9 at a heating rate of 10 °C min⁻¹, under nitrogen atmosphere.

Table 1 Photophysical and electrochemical data of BTDs 2, 3, 6, 8 and 9

BTD	Photophysical data^a						Electrochemical data^e		Td^g (°C)
	λabs (nm)	ε (M^−1 cm^−1)	λem (nm)	Φf	Stoke's shift (cm^−1)	Optical gap^d (eV)	Eox (V)	Ered (V)
a Absorbance measured in dichloromethane at 1 × 10^−5 M concentration; λabs: absorption wavelength; λem: emission wavelength; ε: extinction coefficient.b Calculated using quinine sulfate in 1 M H2SO4, Φf = 0.55, as standard.c Fluorescence quantum yield relative to rhodamine B in ethanol (0.65).d Determined from onset wavelength of the UV-vis absorption.e Recorded by cyclic voltammetry, in 0.1 M solution of TBAPF6 in DCM at 100 mV s^−1 scan rate versus SCE electrode.f For the irreversible redox process, the peak potential is quoted.g Decomposition temperatures for 5% weight loss under N2 atmosphere at heating rate of 10 °C min^−1.
2	508	31 344	613	0.32^c	3372	2.14	—	−1.21	456
3	419	58 500	502	0.38^c	3946	2.64	—	−1.29	162
6	484	38 000	663	0.19^b	5578	2.24	0.99^f	−1.28	379
8	494	32 885	672	0.22^b	5362	2.20	0.89^f	−1.26	405
9	470	54 316	692	0.27^b	6826	2.28	0.88^f	−1.31	184

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Theoretical calculations

In order to explore the electronic structure of the aryl substituted symmetrical and unsymmetrical BTDs, DFT (density functional theory) calculations were performed at the B3LYP/6-31G** level.¹³ The contours of the HOMO and LUMO of BTDs 2, 3, 6, 8 and 9 are shown in Fig. 2.

Fig. 2 HOMO and LUMO frontier orbitals of BTDs 2, 3, 6, 8 and 9 at the B3LYP/6-31G** level for C, N, S and H.

The computational results indicate the following: (a) in BTDs 2, 6 and 8 the HOMO orbitals are localized over anthracene, triphenylamine and benzo of the BTD unit, whereas the LUMO is delocalized over the benzothiadiazole. (b) In case of BTDs 3 and 8 the HOMO is delocalized over the triphenylamine, phenyl ring of phenyl-dimesitylborane and benzo of the BTD unit whereas the LUMO over the BTD and phenyl of the phenyldimesitylborane unit. (c) The HOMO–LUMO gap in BTDs 2, 3, 6, 8 and 9 are 2.15 eV, 2.70 eV, 2.32 eV, 2.22 eV and 2.35 eV respectively. The trend observed in the HOMO–LUMO gap exhibits the order 3 > 9 > 6 > 8 > 2. These results indicate that the incorporation of anthracene and triphenylamine unit lowers the HOMO–LUMO gap whereas phenyl-dimesitylborane unit increases the HOMO–LUMO gap (Fig. 3).

Fig. 3 Energy diagram of the frontier orbitals of benzothiadiazole 2, 3, 6, 8 and 9 estimated by DFT calculations.

Photophysical properties

The electronic absorption and emission spectra of benzothiadiazoles 2, 3, 6, 8 and 9 were recorded in dichloromethane at room temperature (Fig. 4), and the data are compiled in Table 1. Benzothiadiazoles 2, 3, 6, 8 and 9 exhibit a high energy absorption band between 300–350 nm, corresponding to π → π* transition.¹⁴ The charge transfer (CT) band in BTD 2, 3, 6, 7 and 8 were observed at ∼505 nm, ∼419 nm, ∼484 nm, ∼492 nm and ∼470 nm respectively.^14,15 The CT band exhibits larger red shift in anthracene substituted BTDs 2 and 8. The DFT results indicate that substitution of the anthracene units on the BTD core results in completely planar conformation of BTD 2. This results in strong electronic communication between the donor anthracene and the acceptor BTD core. However, the substitution of electron deficient phenyl-dimesitylborane unit on the acceptor BTD results is relatively weak electronic communication, which results in blue shift of the absorption spectrum. The trend observed in the optical gap exhibits the order 3 > 9 > 6 > 8 > 2. The experimental optical gap values show good agreement with the calculated HOMO–LUMO gap.

Fig. 4 Normalized electronic absorption and emission spectra of BTDs 2, 3, 6, 8 and 9 in dichloromethane.

The fluorescence spectra of BTD 2, 3, 6, 8 and 9 displays a single emission peak at 613 nm, 502 nm, 663 nm, 672 nm and 692 nm respectively. The BTDs 6, 8 and 9 bearing triphenylamine as one of the donor unit exhibit large Stoke's shift compared to BTDs 2 and 3. The effect of variation of π-conjugation through different aryl groups is also reflected from the colored solution (Fig. 5).

Fig. 5 Benzothiadiazoles 2, 3, 6, 8 and 9 illuminated under white light (daylight lamp).

Electrochemical properties

The electrochemical properties of compounds 2, 3, 6, 8 and 9 were explored by the cyclic voltammetric analysis in dichloromethane (DCM) solution using tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as supporting electrolyte. The cyclic voltammograms are presented in Fig. 6, S17 and S18,† and the data are listed in Table 1. The aryl-substituted benzothiadiazoles 2, 3, 6, 8 and 9 exhibit one reversible reduction wave corresponding to the BTD acceptor moiety.¹⁶ The reduction potentials of anthracene substituted BTDs 2 and 8 are −1.21 V and −1.26 V respectively whereas, the reduction potentials of phenyl-dimesitylborane substituted BTDs 3 and 9 are −1.29 V and −1.31 V respectively. The reduction potential data shows that the ease of reduction follows the order 2 > 8 > 6 > 3 > 9.¹⁷ The irreversible oxidation peaks corresponding to the oxidation of the triphenylamine unit for BTDs 6, 8 and 9 were observed in the region 0.88–0.99 V.¹⁸

Fig. 6 Cyclic voltammograms of BTDs 2, 3, 8 and 9 at 0.01 M concentration in 0.1 M TBAPF₆ in dichloromethane recorded at a scan rate of 100 mV s⁻¹.

Conclusion

In summary symmetrical and unsymmetrical donor–acceptor benzothiadiazole (BTD) were synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical, computational and thermal properties reveal that the substitution of anthracene unit results in red shifted absorption band, lower optical gap and improved thermal stability. The results obtained in this study will be useful for the design and synthesis low HOMO–LUMO gap molecular motifs for organic photovoltaics. The study towards the optoelectronic applications of anthracene based BTDs are currently ongoing in our laboratory.

Experimental section

Experimental details

Chemicals were used as received unless otherwise indicated. All the oxygen or moisture sensitive reactions were carried out under argon/nitrogen atmosphere. ¹H NMR spectra were recorded using a 400 MHz spectrometer. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using residual protonated solvent as an internal standard {CDCl₃, 7.26 ppm}. ¹³C NMR spectra were recorded using a 100 MHz spectrometer. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using the solvent as internal standard {CDCl₃, 77.0 ppm}. The ¹H NMR splitting patterns have been described as “s, singlet; d, doublet; t, triplet and m, multiplet”. UV-visible absorption spectra of all compounds were recorded in DCM. Emission spectra were taken in a fluoromax-4p fluorimeter from HoribaYovin (model: FM-100). The excitation and emission slits were 2/2 nm for the emission measurements. The density functional theory (DFT) calculation were carried out at the B3LYP/6-31G** level for C, N, S, H in the Gaussian 09 program. HRMS was recorded on TOF-Q mass spectrometer. Cyclic voltammograms and differential pulse voltammograms were recorded on electrochemical analyzer using glassy carbon as working electrode, Pt wire as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The scan rate was 100 mV s⁻¹ for CV. A solution of tetrabutylammonium hexafluorophosphate (TBAPF₆) in DCM (0.1 M) was employed as the supporting electrolyte. DCM was freshly distilled from CaH₂ prior to use. All potentials were experimentally referenced against the saturated calomel electrode couple. Under our conditions, the Fc/Fc⁺ couple exhibited E⁰ = 0.38 V versus SCE. Thermogravimetric analyses were performed on the Mettler Toledo thermal analysis system.

Preparation of benzothiazole 2

To a stirred solution of the 9-ethynylanthracene (2.5 mmol), and dibromo-BTD 1 (1 mmol) in THF, and TEA (1 : 1, v/v) were added [PdCl₂(PPh₃)₂] (20 mg, 0.028 mmol) and CuI (4 mg, 0.02 mmol) under an argon flow at room temperature. The reaction mixture was stirred for 12 h at 70 °C, and then cooled to room temperature. The solvent was then evaporated under reduced pressure, and the mixture was purified by SiO₂ chromatography with DCM/hexane (2 : 3, v/v) to obtain 2 as deep red solid (450 mg, yield: 84%). ¹H NMR (400 MHz, CDCl₃, δ in ppm): 8.95–8.93 (m, 4H), 8.53 (s, 2H), 8.08–8.06 (m, 6H), 7.74–7.70 (m, 4H), 7.60–7.56 (m, 4H); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): precipitate formation at higher concentration resulted poor spectrum; HRMS (ESI-TOF) m/z calcd for C₃₈H₂₀N₂S + H: 537.1420 [M + H]⁺, found 537.1409 [M + H]⁺.

Preparation of benzothiazole 3

To a stirred solution of the 4-ethynylphenyl-dimesitylborane (2.5 mmol), and dibromo-BTD 1 (1 mmol) in THF, and TEA (1 : 1, v/v) were added [PdCl₂(PPh₃)₂] (10 mg, 0.014 mmol) and CuI (2 mg, 0.01 mmol) under an argon flow at room temperature. The reaction mixture was stirred for 24 h at 70 °C, and then cooled to room temperature. The solvent was then evaporated under reduced pressure, and the mixture was purified by SiO₂ chromatography with DCM/hexane (1 : 3, v/v), followed by recrystallization in DCM/hexane (1 : 3) to obtain 3 as yellow solid (507 mg, yield: 62%). ¹H NMR (400 MHz, CDCl₃, δ in ppm): 7.81 (s, 2H), 7.64–7.61 (m, 4H), 7.55–7.53 (m, 4H), 6.84 (s, 8H), 2.32 (s, 12H), 2.01 (s, 24H); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): 154.3, 151.0, 140.9, 139.0, 136.0, 132.6, 131.4, 128.3, 126.9, 125.5, 117.2, 97.8, 87.0, 23.4, 21.2; HRMS (ESI-TOF) m/z calcd for C₅₈H₅₄B₂N₂S + Na: 855.4104 [M + Na]⁺, found 855.4063 [M + Na]⁺.

Preparation of benzothiadiazole 8

To a stirred solution of 9-ethynylanthracene (1.2 mmol), 7 (1 mmol) in THF, and TEA (1 : 1, v/v) were added [PdCl₂(PPh₃)₂] (10 mg, 0.014 mmol) and CuI (2 mg, 0.01 mmol) under an argon flow at room temperature. The reaction mixture was stirred for 18 h at 70 °C, and then cooled to room temperature. The solvent was then evaporated under reduced pressure, and the mixture was purified by SiO₂ chromatography with DCM/hexane (2 : 2, v/v), to obtain 8 as red solid (447 mg, yield: 74%). ¹H NMR (400 MHz, CDCl₃, δ in ppm): 8.91–8.88 (m, 2H), 8.51 (s, 1H), 8.07–8.05 (m, 2H), 7.99 (d, J = 7.5, 1H), 7.84 (d, J = 7.2, 1H), 7.71–7.67 (m, 3H), 7.58–7.52 (m, 4H), 7.33–7.28 (m, 4H), 7.16–7.03 (m, 7H); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): 154.7, 154.5, 148.7, 147.0, 135.0, 133.0, 132.97, 132.0, 131.8, 131.2, 121.8, 117.5, 117.0, 116.6, 115.0, 98.5, 97.0; HRMS (ESI-TOF) m/z calcd for C₄₂H₂₅N₃S + Na: 626.1661 [M + Na]⁺, found 626.1673 [M + Na]⁺.

Preparation of benzothiadiazole 9

To a stirred solution of the 4-ethynylphenyl-dimesitylborane (1.5 mmol), 7 (1 mmol) in THF, and TEA (1 : 1, v/v) were added [PdCl₂(PPh₃)₂] (10 mg, 0.014 mmol) and CuI (2 mg, 0.01 mmol) under an argon flow at room temperature. The reaction mixture was stirred for 20 h at 70 °C, and then cooled to room temperature. The solvent was then evaporated under reduced pressure, and the mixture was purified by SiO₂ chromatography with DCM/hexane (2 : 2, v/v), to obtain 9 as orange-red solid (540 mg, yield: 72%); ¹H NMR (400 MHz, CDCl₃, δ in ppm): 7.79 (d, 1H, J = 7.3 Hz), 7.75 (d, 1H, J = 7.3 Hz), 7.63 (d, 2H, J = 8.3 Hz), 7.54–7.49 (m, 4H), 7.32–7.28 (m, 4H), 7.15–7.01 (m, 8H), 6.84 (s, 4H), 2.32 (s, 6H), 2.01 (s, 12H); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): 154.3, 148.7, 146.9, 140.8, 138.9, 136.0, 133.0, 132.7, 131.9, 129.4, 128.2, 125.7, 125.3, 123.9, 121.7, 117.9, 116.3, 116.2, 114.8, 114.76, 98.6, 97.4, 95.1, 87.2, 84.8, 23.4, 21.4; HRMS (ESI-TOF) m/z calcd for C₅₂H₄₂BN₃S + H: 752.3274 [M + H]⁺, found 752.3279 [M + H]⁺.

Acknowledgements

RM thanks CSIR, and DST, New Delhi for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization data for all the new compounds. Copies of ¹H, ¹³C NMR, and HRMS spectra, electrochemical and DFT calculation data of new compounds. See DOI: 10.1039/c4ra15424j

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