The Preparation of Dithieno[3,2-b:4,5-c’]germole, and Its Application as a Donor Unit in Conjugated D–A Compounds

Abstract

Group 14 metalloles have attracted much attention as core structures of conjugated functional materials. In this work, we prepared dithieno[3,2-b:4,5-c’]germole as a new unsymmetrically condensed dithienogermole and benzo[4,5]thieno[2,3-c]germole as the benzene-condensed analog. The electronic states and properties of these unsymmetrically condensed germoles are discussed on the basis of the results of optical and electrochemical measurements with the help of quantum chemistry calculations on the simplified model compounds. The Stille cross-coupling reactions of bromodithieno[3,2-b:4,5-c’]germole with di(stannylthienyl)- and di(stannylthiazolyl)benzothiadiazole provided conjugated donor–acceptor compounds that exhibited clear solvatochromic behavior in the photoluminescence spectra, indicating the potential application of the dithieno[3,2-b:4,5-c’]germole unit as an electron donor in donor–acceptor systems.

1. Introduction

Functional organic π-conjugated molecules have garnered significant attention because of their wide-ranging applications, including their use in optoelectronic devices. Group 14 metalloles have been extensively studied as core structures of conjugated functional materials [1]. For example, conjugated compounds and polymers containing silole units, such as poly(silole)s, are anticipated to exhibit effective conjugation, making them suitable for both n- and p-type semiconductor materials depending on the substituent [2]. This is explained by the low-lying LUMO energy levels due to σ*-π* orbital interactions and the high planarity of the silole system. Dithienosilole is a typical silole-containing building block that is used for the development of conjugated organic materials [3,4,5,6]. However, most research activities have focused on symmetrically condensed dithienosiloles, and less is known for unsymmetrical dithienosiloles [7,8,9]. H. Wang and coworkers have prepared dithieno[2,3-b:4,5-c’]silole derivatives and produced conjugated polymers using them as the monomers (Figure 1a) [8,9]. In this unsymmetrical unit, one thiophene ring can be incorporated into conjugated compounds and polymeric systems, and the introduction of a functional group (R) on the other thiophene ring enables the fine-tuning of the electronic state of the dithienosilole unit. Meanwhile, within the same group as silicon, germanium-based congeners known as dithienogermoles are relatively underexplored despite exhibiting properties that are comparable to or even superior to those of their Si analogs, arising from their relatively strong intermolecular interactions [10,11,12,13]. Furthermore, dithienogermoles generally demonstrate a higher stability than dithienosiloles [14]. Therefore, the exploration of unsymmetrical dithienogermoles as a promising building block for the development of functional organic optoelectronic device materials would facilitate the efficient construction of germole-containing conjugated compounds and polymers and have significant implications.

In the course of our studies on unsymmetrically condensed metalloles [15,16], we prepared dithieno[3,2-b:4,5-c’]germole (uDTG) as the first example of group 14 dithieno[3,2-b:4,5-c’]metalloles. We anticipate that uDTG would show enhanced conjugation compared with the dithieno[2,3-b:4,5-c’]siloles (Figure 1), thereby paving the way for the development of building units of conjugated functional materials with finely tuned electronic states. We also prepared benzo[4,5]thieno[2,3-c]germole (BTG). The optical and electrochemical properties of these thiophene-condensed unsymmetrical germole derivatives were investigated and discussed with the help of quantum chemical calculations. To explore the usefulness of uDTG as the building unit of conjugated compounds and polymers, we synthesized two donor–acceptor (D–A) compounds possessing uDTG as the donor and benzo(2,1,3)thiadiazole (BT) as the acceptor, which were linked by thiophene or thiazole units. These compounds showed clear solvatochromic behavior arising from their intramolecular D–A interaction, and their photoluminescence (PL) bands shifted to lower energies as the solvent polarity increased. The compounds described herein are expected to shed light on the design and application of new building blocks of conjugated compounds and polymers.

2. Results and Discussion

2.1. Synthesis

The synthetic strategy for unsymmetrically condensed benzothieno- and dithienogermoles is presented in Scheme 1. Compound 1 was prepared according to the previously reported method [17] and used for the synthesis of BTG. For the formation of uDTG derivatives, compound 4 was first obtained by a Negishi cross-coupling reaction of compounds 2 and 3 in 65% yield [18,19]. The dilithiation of 4 by n-BuLi, followed by a ring-forming reaction with dichlorodi(n-octyl)germane furnished a mixture from which uDTG-Si was isolated in 65% yield by preparative gel permeation chromatography (GPC). uDTG-Si was then treated with tetrabutylammonium fluoride (TBAF) to give desilylation product uDTG in 90% yield. The bromination of uDTG-Si with NBS gave uDTG-Br in 53% yield. This compound was further subjected to palladium-catalyzed Stille coupling with 5 and 6 to provide uDTG-BTT and uDTG-BTTz as dark red solids in 30% and 20% yields, respectively. The low yields of these compounds were due to the low reactivity of the starting materials and difficulty in purification. We found that the starting materials remained in the reaction mixtures by NMR spectral analysis.

2.2. Optical and Electrochemical Measurements

We examined the optical and electrochemical properties of the prepared BTG and uDTS derivatives to clarify their electronic states. The UV–vis absorption spectra of BTG, uDTG, uDTG-Si, uDTG-BTT, and uDTG-BTTz in dichloromethane are shown in Figure 2a, and the corresponding photophysical data are summarized in

Table 1. Optical and electrochemical properties of triazologermoles.

Compound	UV abs. λmax a/nm	PL λmax a,b/nm (Φf)	HOMO c/eV	LUMO d/eV	LUMO–HOMO e/eV
BTG	280	323 (<2%)	−6.27	−2.31	3.96
uDTG	303	347 (3.8%)	−5.50	−1.73	3.77
uDTG-Si	315	360 (<2%)	−5.52	−1.90	3.62
uDTG-BTT	329, 520	653 (33%)	−5.08	−3.04	2.04
uDTG-BTTz	331, 501	634 (41%)	−5.16	−3.05	2.11

^a In dichloromethane at room temperature. ^b Excited at the absorption maximum. ^c Derived from CV anodic onset potential. ^d E (LUMO) = E (HOMO) + Eg. ^e Optical band gap (Eg).

. As shown in Figure 2a, BTG displayed an absorption maximum at 280 nm. Upon replacing the condensed benzene ring of BTG with a thiophene ring, an absorption maximum appeared at 303 nm with a bathochromic shift of 23 nm for uDTG. A further bathochromic shift was observed for uDTG-Si, giving an absorption maximum at 315 nm. Their PL spectra showed similar bathochromic shifts, as presented in Figure 2b and Table 1. This bathochromic shift from BDG to uDTG is in agreement with that observed for symmetrically condensed dibenzo and dithienosiloles (DBG and DTG in Figure 3) [20,21]. The absorption band of uDTG-Si appeared at lower energy than that of uDTG similar to a previous report on DTG derivatives that showed a bathochromic shift of the absorption band by similar silyl substitution (DTG-Si in Figure 3) [22]. Hypsochromic shifts of the absorption bands of uDTG derivatives relative to those of the symmetrically condensed DTG analogs [20,21,22] are likely due to the less extended π-conjugation of the α,β-linked bithiophene units in uDTG and uDTG-Si compared with the α,α-linked bithiophene units in DTG and DTG-Si.

D–A interaction was noted in uDTG-BTT and uDTG-BTTz, which are composed of uDTG and BT units as the donor and acceptor units, respectively, linked by thiophene or thiazole units. The UV–vis absorption spectra of these compounds showed two π–π* transition bands around 330 nm and 500 nm. The former is ascribable to the local excitation, whereas the latter is likely based on the D–A interaction that leads to photoexcited intramolecular charge transfer (ICT). The absorption maxima in the low-energy region were observed at 520 nm and 501 nm for uDTG-BTT and uDTG-BTTz, respectively. The blue-shifted absorption maximum of uDTG-BTTz relative to that of uDTG-BTT is explained by the less extended conjugation of imine-containing thiazole than thiophene [23,24]. The more electron-deficient property of thiazole may also affect the donor–acceptor interaction of these compounds. The optical band gaps of the unsymmetrically condensed thienogermoles were estimated from the absorption edges, decreasing in the order of BTG > uDTG > uDTG-Si > uDTG-BTTz > uDTG-BTT, as shown in Table 1.

Monomeric compounds BTG, uDTG, and uDTG-Si showed PL bands around 323–360 nm with low PL efficiencies. In contrast, compounds uDTG-BTT and uDTG-BTTz exhibited strong PL bands at 653 nm and 634 nm, respectively. Many donor–acceptor compounds with a benzothiadiazole donor unit with red-shifted absorption and PL bands with sufficient PL efficiencies for optical applications have been reported [23,24,25]. To understand the ICT behaviors of uDTG-BTTz and uDTG-BTT, the PL spectra were measured in different solvents, and the results are shown in Figure 4, Figures S1 and S2 and Table S1. The PL band shifted to the longer wavelength region with increasing solvent polarity, clearly indicating that the photoexcited states of uDTG-BTT and uDTG-BTTz were more polar than their respective ground states, reflecting the ICT behavior. From the slope of the Lippert–Mataga plot, the dipole moment changes (Δμ) were estimated to be 13.03 D and 15.57 D for uDTG-BTT and uDTG-BTTz, respectively.

The HOMO and LUMO energy levels of conjugated materials are important parameters for electronic applications. Cyclic voltammogram (CV) measurements were performed in a typical three-electrode electrochemical cell with Pt as the working electrode in water-free acetonitrile at a scan rate of 50 mV s⁻¹ (Figure S3) to determine the energy levels. The HOMO energy levels determined from the onsets of the first oxidation peaks are −6.27 eV for BTG, −5.50 eV for uDTG, and −5.52 eV for uDTG-Si. uDTG-BTTz possessed a lower HOMO energy level than uDTG-BTT owing to the more electron-deficient nature of thiazole than thiophene [23,24]. The LUMO energy levels of the compounds were calculated from the anodic onsets and the optical band gaps and are listed in Table 1. The HOMO and LUMO of uDTG-BTT are at –5.08 eV and −3.04 eV, whereas those of uDTG-BTTz are at −5.16 eV and −3.05 eV, providing similar bandgaps of 2.04 and 2.11 eV, respectively.

2.3. Quantum Chemical Calculations

To obtain further insight into the effects of molecular structure and electron distribution on the spectroscopic and electrochemical properties of these compounds, their electronic structures were investigated with the Gaussian 16 suite of quantum chemical simulation programs. The structures were optimized in vacuo using DFT calculations at the B3LYP/6-31G(d,p) level of theory. The visualized HOMO and LUMO distributions and the calculated energy levels are shown in Figure 5. During the calculations, the compounds were optimized, where octyl chains on the germanium atom were replaced with methyl groups to simplify the calculations. The orbital levels of the small molecules were in good agreement with the experimental data, except that calculations predicted a higher HOMO energy level for uDTG-Si than for uDTG. This differs from the experimental data that uDTG-Si showed the anodic onset at a higher potential than uDTG, although the difference was not large. It is also seen that the vinylene unit at the C5 position of germole contributes less to the HOMO and LUMO of uDTG system than other C=C units, reflecting the unsymmetrical structure, thereby leading to less extended conjugation compared with symmetrical DTG. The LUMO/HOMO energy levels of uDTG-containing D–A compounds uDTG-BTT and uDTG-BTTz were estimated to be −2.59/−4.80 and −2.83/−5.07, respectively. Both the HOMO and LUMO of uDTG-BTT lie at higher levels than those of uDTG-BTTz, agreeing with the experimental data, reflecting the more electron-deficient property of thiazole. The LUMOs of these D–A compounds are mainly found in the acceptor BT units, whereas the HOMOs are distributed in the uDTG fragments, as shown in Figure 5. This result indicates that ICT occurs from the uDTG donor to the BT acceptor upon photoexcitation. It is interesting to see that the thiophene and thiazole linkers also affect the HOMO and LUMO energy levels to make their fine-tuning possible.

3. Experimental Section

3.1. General Preparation

All synthetic reactions were performed in an atmosphere of dry argon. Tetrahydrofuran (THF) and diethyl ether that were used as reaction solvents were dried by stirring with calcium hydride overnight at room temperature. These solvents were then distilled and stored with activated molecular sieves in the dark. Compounds 1, 2, 3, 5, and 6 were prepared by the literature procedures [17,18,19,26,27]. All other chemicals employed were commercially available and used as received. All spectral and electrochemical measurements were performed at room temperature. The spectrometer models used for the measurements are as follows: NMR spectra: Varian 400-MR; UV–vis absorption spectra: Hitachi U-2910; PL spectra: HORIBA FluoroMax-4; APCI-mass spectra: Thermo Fisher Scientific LTQ Orbitrap XL. Cyclic voltammograms (CVs) were obtained on an Autolab analyzer using a typical three-electrode electrochemical cell in an acetonitrile solution containing 0.1 M of tetrabutylammonium hexafluorophosphate as a supporting electrolyte at a scan rate of 50 mV s⁻¹ at room temperature in a nitrogen atmosphere with platinum disk and wire as the working counter electrode, respectively. An Ag/Ag⁺ electrode was used as the reference electrode, calibrated by ferrocene.

3.2. Procedures for Synthesis

3.2.1. Synthesis of BTG

To a solution of 1 (0.416 g, 1.308 mmol) in 10 mL of diethyl ether was added 3.27 mL (5.232 mmol) of a pentane solution of t-BuLi (1.6 M) at −78 °C. After the solution was stirred at this temperature for 2 h, 0.508 g (1.373 mmol) of dichlorodi(n-octyl)germane in 5 mL of ether was added by dropping. The reaction mixture was stirred at room temperature overnight. After being quenched with H₂O (20 mL), the reaction mixture was extracted with CHCl₃ (3 × 20 mL), and the extract was dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was subjected to preparative GPC eluting with toluene to give 0.388 g (0.800 mmol, 61% yield) of BTG as a colorless oil. ¹H NMR (400 MHz, CDCl₃) δ 7.68 (d, J = 8.0 Hz, 1H), 7.53 (d, J = 7.2 Hz, 1H), 7.48 (d, J = 2.2 Hz, 1H), 7.37–7.32 (m, 1H), 7.33 (d, J = 2.2 Hz, 1H), 7.22–7.17 (m, 1H), 1.47–1.43 (m, 4H), 1.28–1.19 (m, 24H), 0.86 (t, J = 7.2 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 152.8, 144.8, 143.2, 141.4, 134.0, 129.3, 128.6, 126.7, 122.3, 115.0, 33.1, 32.0, 29.4, 29.3, 25.5, 22.8, 14.9, 14.3. HR-MS (APCI) Calcd for C₂₆H₄₁Ges: [M + H]⁺: 459.21353, Found: 459.21353.

3.2.2. Synthesis of 4

To a solution of 2 (0.775 g, 2.006 mmol) in 20 mL of THF was added 1.25 mL (2.006 mmol) of a 1.6 M hexane solution of n-BuLi at −78 °C, and the reaction mixture was stirred at this temperature for 1 h. Zinc chloride (2.4 mL, 2.407 mmol, 1 M in THF) was added, and the resulting mixture was further stirred at −78 °C for 1 h. Then, the reaction mixture was warmed to room temperature. After stirring for 1 h, 3 (0.607 g, 2.006 mmol) and Pd(PPh₃)₄ (0.116 g, 0.1003 mmol) were added and the reaction mixture was heated at 80 °C overnight. The solvent was evaporated under reduced pressure, and the residue was directly subjected to column chromatography on silica gel eluting with hexane for purification to give 4 in 65% yield (0.629 g, 1.304 mmol) as a white solid. mp 89.6–90.8 °C. ¹H NMR (400 MHz, CDCl₃) δ 6.73 (s, 1H), 2.50 (s, 3H), 0.42 (s, 9H), 0.17 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ 146.77, 142.49, 141.28, 140.88, 139.60, 128.13, 122.70, 111.91, 15.67, −0.10, −0.68. HR-MS (APCI) Calcd for C₁₅H₂₂Br₂S₂Si₂: [M]⁺: 479.90682, Found: 479.90671.

3.2.3. Synthesis of uDTG-Si

To a solution of 4 (0.500 g, 1.036 mmol) in 20 mL of THF was added a 1.6 M hexane solution of n-BuLi (1.328 mL, 2.124 mmol) at −78 °C, and the reaction mixture was stirred at this temperature for 2 h. Dichlorodi(n-octyl)germane (0.421 g, 1.139 mmol) was added to the reaction mixture at −78 °C. After stirring overnight at room temperature, the solvent was evaporated under reduced pressure, and the residue was directly purified by preparative GPC eluting with toluene to give uDTG-Si in 65% yield (0.249 g, 0.673 mmol) as a colorless oil. ¹H NMR (400 MHz, CDCl₃) δ 6.71 (s, 1H), 2.54 (s, 3H), 1.41–1.33 (m, 4H), 1.25–1.18 (m, 24H), 0.86 (t, J = 7.2 Hz, 6H), 0.48 (s, 9H), 0.33 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ 154.77, 154.56, 147.72, 147.06, 146.09, 140.98, 131.40, 128.02, 33.08, 31.97, 29.37, 29.23, 25.76, 22.81, 16.16, 15.59, 14.25, 0.81, 0.35. HR-MS (APCI) Calcd for C₃₁H₅₇GeS₂Si₂: [M + H]⁺: 623.26465, Found: 623.26624.

3.2.4. Synthesis of uDTG

To a solution of uDTG-Si (0.399 g, 0.642 mmol) in THF (15 mL) was added a solution of TBAF (0.642 mmol, 1 M) in THF at 0 °C, and the reaction mixture was stirred for 10 min at this temperature. After evaporation of the solvent, the residue was diluted with 10 mL of ether acetate and hydrolyzed with 3 × 10 mL water. The organic layer and the extracts were combined and dried over anhydrous magnesium sulfate. The residue was subjected to preparative GPC to give uDTG as a colorless oil in 90% yield (0.275 g, 0.578 mmol). ¹H NMR (400 MHz, CDCl₃) δ 7.21 (d, J = 2.2 Hz, 1H), 7.04 (d, J = 2.2 Hz, 1H), 6.70 (s, 1H), 2.52 (s, 3H), 1.47–1.41 (m, 4H), 1.33–1.20 (m, 20H), 1.17–1.10 (m, 4H), 0.87 (t, J = 7.0 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 146.72, 146.26, 145.83, 143.85, 141.02, 128.10, 127.97, 112.08, 32.95, 32.02, 29.37, 29.30, 25.56, 22.81, 15.65, 15.20, 14.22. HR-MS (APCI) Calcd for C₂₅H₄₁GeS₂: [M + H]⁺: 479.18560, Found: 479.18591.

3.2.5. Synthesis of uDTG-Br

A mixture of uDTG-Si (0.425 g, 0.683 mmol) and NBS (0.122 g, 0.683 mmol) in 15 mL of THF was stirred at 0 °C in the dark. After 4 h, the reaction was quenched with water and extracted with CH₂Cl₂. The organic layer and the extract were combined and washed with saturated NaCl aqueous solution (2 × 10 mL). After the organic phase was dried over anhydrous sodium sulfate and the solvent was removed under reduced pressure, the residue was purified by silica gel column chromatography eluting with hexane to give uDTG-Br as a yellow oil in 53% yield (0.228 g, 0.363 mmol). ¹H NMR (400 MHz, CDCl₃) δ 6.75 (s 1H), 2.54 (s, 3H), 1.38–1.33 (m, 4H), 1.27–1.16 (m, 24H), 0.87 (t, J = 7.0 Hz, 6H), 0.31 (s, 9H). ¹³C NMR (100 MHz, CDCl₃) δ 152.76, 147.66, 146.55, 146.33, 144.37, 142.47, 127.81, 104.57, 33.00, 31.96, 29.86, 29.34, 25.58, 22.65, 16.37, 15.65, 14.25, 0.43. HR-MS (APCI) Calcd for C₂₈H₄₈BrGeS₂Si: [M + H]⁺: 627.13564, Found: 629.13599.

3.2.6. Synthesis of uDTG-BTT

uDTG-Br (14.5 mg, 0.023 mmol) and 5 (10.14 mg, 0.016 mmol) were dissolved in toluene (4 mL) that had been degassed by argon bubbling for approximately 4 h. Pd₂(dba)₃ (0.42 mg, 0.46 μmol) and P(o-tolyl)₃ (0.56 mg, 1.84 μmol) were added to the solution, and the solution was heated to reflux with stirring for 3 days. After removal of the solvent under reduced pressure, the crude product was purified by GPC eluting with toluene to afford uDTG-BTT (6.7 mg, 0.048 mmol, 30%) as a dark red solid. ¹H NMR (400 MHz, CDCl₃) δ 8.20 (d, J = 3.9 Hz, 2H), 7.92 (s, 2H), 7.49 (d, J = 3.9 Hz, 2H), 6.71 (s, 2H), 2.46 (s, 6H), 1.44–1.37 (m, 8H), 1.32–1.11 (m, 48H), 0.87 (t, J = 7.0 Hz, 12H), 0.36 (s, 18H). ¹³C NMR (100 MHz, CDCl₃) δ 152.82, 148.29, 147.54, 146.43, 146.21, 145.06, 144.26, 142.36, 139.40, 136.51, 131.70, 128.76, 128.57, 127.75, 125.91, 32.99, 31.92, 29.85, 29.34, 25.56, 22.63, 16.38, 15.66, 14.26, 0.43. HR-MS (FD) Calcd for C₇₀H₁₀₀Ge₂N₂S₇Si₂: [M]⁺: 1396.38881, Found: 1396.38577.

3.2.7. Synthesis of uDTG-BTTz

uDTG-BTTz was synthesized in a similar fashion to that described for uDTG-BTT above, using compound 6 (29.93 mg, 0.034 mmol) instead of compound 5. uDTG-BTTz was obtained as a dark red solid in 20% yield (0.95 mg, 0.68 μmol). ¹H NMR (400 MHz, CDCl₃) δ 8.85 (s, 2H), 8.29 (s, 2H), 6.72 (s, 2H), 2.54 (s, 6H), 1.41–1.35 (m, 8H), 1.28–1.19 (m, 48H), 0.86 (t, J = 7.0 Hz, 12H), 0.34 (s, 18H). ¹³C NMR (100 MHz, CDCl₃) δ 167.09, 154.06, 152.58, 150.01, 147.48, 146.36, 146.15, 144.19, 143.35, 142.29, 137.10, 135.60, 132.79, 127.52, 122.11, 33.00, 31.96, 29.34, 29.20, 25.58, 22.86, 16.37, 15.65, 14.25, 0.44. HR-MS (FD) Calcd for C₆₈H₉₈Ge₂N₄S₇Si₂: [M]⁺: 1398.37930, Found: 1398.37938.

3.3. DFT Calculations

DFT calculations were carried out using the Gaussian 16 program [28] with the GaussView 6.0 graphical interface (Gaussian Inc., Wallingford, CT, USA), based on the B3LYP functional [29] and the 6–31G (d,p) basis set for all atoms. No solvents were involved in the calculations.

4. Conclusions

We have synthesized unsymmetrically condensed benzothienogermole BTG and dithienogermole uDTG and thoroughly investigated their optical and electrochemical properties. DFT calculations have also been carried out. Replacing the benzene ring in BTG with a thiophene ring changes the electronic states to enhance conjugation in uDTG derivatives. We have also explored the potential application of uDTG as a donor unit in conjugated D–A materials by synthesizing D–A compounds uDTG-BTT and uDTG-BTTz featuring the combination with a benzothiadiazole acceptor unit. uDTG-BTT and uDTG-BTTz exhibited extended conjugation and clear solvatochromic behavior arising from their intramolecular D–A interaction. These results indicate the potential application of the dithieno[3,2-b:4,5-c’]germole unit as an electron donor in D–A systems. Studies on the preparation of D–A materials based on the uDTG system including conjugated polymers are in progress, the results of which will be reported elsewhere.