Synthesis of Pyridinothienogermoles as Unsymmetrically Condensed

After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to silica gel column chromatography eluting wi...
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Synthesis of Pyridinothienogermoles as Unsymmetrically Condensed Germoles Joji Ohshita,* Michitaka Sugino, Yousuke Ooyama, and Yohei Adachi Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

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ABSTRACT: Pyridinothienogermoles (PyTGs) were prepared by the reaction of dilithiopyridylthiophene with dichlorodi(noctyl)germane and dichlorodiphenylgermane, and their electronic states were investigated by optical measurements and density functional theory (DFT) calculations. Bromination of trimethylsilyl-substituted PyTG with N-bromosuccinimide gave PyTG bromide, and palladium-catalyzed Stille coupling reactions of bromide with diphenyl[(trimethylstannyl)phenyl]amine and N-[(trimethylstannyl)phenyl]carbazole provided donor−acceptor compounds with pyridine as the acceptor and triphenylamine or phenylcarbazole as the donor. These compounds showed clear solvatochromic properties in the photoluminescence (PL) spectra. Bi(pyridinothienogermole) was prepared by the Stille coupling of DPyG bromide and trimethylstannyl-substituted DPyG. The Lewis basicity of the pyridine ring made it possible to use these DPyG derivatives as ligands for complex formation. Protonation and complex formation of the PyTG pyridine unit with tris(pentafluorophenyl)borane enhanced the donor−acceptor interaction, shifting the UV absorption and PL bands to lower energies.

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INTRODUCTION Group 14 metalloles, such as siloles and germoles, have received attention as core structures of conjugated functional materials.1,2 The often-observed interaction between Si/Ge σ*-orbitals and endocyclic butadiene π*-orbitals (σ*−π* conjugation) stabilizes the lowest unoccupied molecular orbital (LUMO), resulting in enhanced electron affinity and reduced highest occupied molecular orbital (HOMO)−LUMO energy gaps.3 These siloles and germoles have been studied as core fragments of conjugated functional materials as Tamao and co-workers demonstrated the utility of siloles as electron-transporting materials for multilayered organic light-emitting diodes (OLEDs).4 The condensation of siloles and germoles with heterobiaryls has also been studied in an effort to manipulate their properties and thus tune their functionalities.1 Of those, dithienosilole (DTS) and dithienogermole (DTG) (Chart 1) are currently utilized as building units of conjugated functional materials for optoelectronic devices, such as OLEDs, organic transistors, and organic photovoltaic cells.2,5 It has also been demonstrated that condensed tricyclic systems could be used as highly luminescent materials and sensors. Bifuran-,6 biselenophene-,7 and biiodole-condensed8 siloles and germoles were prepared to realize the fine-tuning of chemical behavior and properties by changing the heterobiaryl structures. The condensation of electron-deficient bipyridyl with silole/germole yielded dipyridinosilole (DPyS) and dipyridinogermole (DPyG) systems (Chart 1).9 It is noteworthy that the σ*−π* conjugation is operative even in highly electron-deficient © XXXX American Chemical Society

Chart 1. Siloles and Germoles Condensed with Heterobiaryls

bipyridyl systems, resulting in enhanced electron deficiency of the DPyS and DPyG derivatives compared to noncondensed 4,4′-bipyridyl. It was also demonstrated that spiro(dipyridinogermole)(dithienogermole)s exhibited photosensitizing effects on singlet oxygen generation.10 Viologen derivatives were also prepared by treating diphenyldipyridinogermole with methyl iodide, and these compounds showed stable electrochromic properties.11 However, most of the condensed siloles and germoles reported so far possess symmetrical tricyclic systems, and studies of unsymmetrical Received: January 21, 2019

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DOI: 10.1021/acs.organomet.9b00036 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Preparation PyT1Br2. A mixture of 1.28 g (4.00 mmol) of 3bromo-4-(trimethylstannyl)pyridine, 0.442 mL (4.00 mmol) of 2,3dibromothiophene, 78.6 mg (1.70 mol %) of Pd(PPh3)4, 13.0 mg (1.70 mol %) of CuI, and 20 mL of toluene was heated at 220 °C for 1 h in a sealed vial under microwave irradiation, and the resulting mixture was poured into water. The organic layer was separated and the aqueous layer was extracted with toluene. The organic layer and the toluene extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to silica gel column chromatography eluting with hexane/ethyl acetate = 6:1 to give 230 mg (18% yield) of the title compound as a yellow oil: 1H NMR (400 MHz, δ in acetone-d6): 7.22 (d, 1H, thiophene, J = 5.3 Hz), 7.50 (d, 1H, pyridine, J = 4.8 Hz), 7.80 (d, 1H, thiophene, J = 5.3 Hz), 8.66 (d, 1H, pyridine, J = 4.8 Hz), 8.88 (br s, 1H, pyridine); 13C NMR (125 MHz, δ in acetone-d6): 111.78, 123.24, 127.98, 131.33, 134.78, 142.53, 143.80, 149.40, 153.13; HRMS m/z: 317.85864 [M + H]+ (calcd 317.85822). Preparation of PyT2Br2. This compound was obtained in a fashion similar to that described above using 2,3-dibromo-5-(trimethylsilyl)thiophene instead of 2,3-dibromothiophene in 18% yield as a yellow oil: 1 H NMR (400 MHz, δ in acetone-d6): 0.38 (s, 9H, Me3Si), 7.35 (s, 1H, thiophene), 7.48 (dd, 1H, pyridine, J = 4.9, 0.6 Hz), 8.66 (d, 1H, pyridine, J = 4.9 Hz), 8.88 (br s, 1H, pyridine); 13C NMR (125 MHz, δ in acetone-d6): −0.39, 113.00, 122.94, 127.75, 137.83, 139.53, 142.69, 143.80, 149.38, 153.15; HRMS m/z: 389.89816 [M + H]+ (calcd 389.89775). Preparation of 1. To a solution of 0.11 g (0.36 mmol) of 3-bromo4-(3-bromothiophen-2-yl)pyridine in 18 mL of THF, 0.462 mL (0.716 mmol) of a 1.55 M hexane solution of n-butyllithium was added dropwise at −85 °C. After stirring the mixture at this temperature for 1 h, 0.13 g (0.36 mmol) of dichlorodi(n-octyl)germane was added. The mixture was heated at 90 °C for 1 h and then hydrolyzed with water. The organic layer was separated and the aqueous layer was extracted with chloroform. The organic layer and the chloroform extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to silica gel column chromatography eluting with hexane/ ethyl acetate = 1:1 to give 11.2 mg (7% yield) of 1 as a yellow oil: 1H NMR (400 MHz, δ in acetone-d6): 0.85 (t, 6H, CH3 in n-octyl, J = 7.0 Hz), 1.19−1.35 (m, 24H, n-octyl), 1.46 (q, 4H, n-octyl, J = 8.0 Hz), 7.31 (d, 1H, thiophene, J = 4.8 Hz), 7.43 (dd, 1H, pyridine, J = 5.0, 0.9 Hz), 7.69 (d, 1H, thiophene, J = 4.8 Hz), 8.52 (d, 1H, pyridine, J = 5.0 Hz), 8.68 (br s, 1H, pyridine); 13C NMR (125 MHz, δ in CDCl3): 14.08, 14.32, 22.61, 25.43, 29.07, 29.14, 31.79, 32.80, 116.23, 129.24, 130.01, 135.92, 144.24, 150.37, 150.76, 151.26, 152.65; HRMS m/z: 460.20859 [M + H]+ (calcd 460.20878). Preparation of 2 and 3. Compound 2 was obtained as a yellow oil in 35% yield in a fashion similar to that described above using 3-bromo4-[3-bromo-5-(trimethylsilyl)thiophen-2-yl]pyridine instead of 3bromo-4-(3-bromothiophen-2-yl)pyridine. Column chromatography was performed using hexane/ethyl acetate = 3:1 and silica gel pretreated with hexamethyldisilazane to avoid the potential desilylation of 2. Data for 2: 1H NMR (400 MHz, δ in acetone-d6): 0.36 (s, 9H, Me3Si), 0.85 (t, 6H, CH3 in n-octyl, J = 7.1 Hz), 1.20−1.34 (m, 24H, n-octyl), 1.48 (q, 4H, n-octyl, J = 7.9 Hz), 7.44 (dd, 1H, pyridine, J = 5.0, 1.0 Hz), 7.46 (s, 1H, thiophene), 8.53 (d, 1H, pyridine, J = 5.0 Hz), 8.68 (d, 1H, pyridine, J = 1.0 Hz); 13C NMR (125 MHz, δ in CDCl3): −0.03, 14.07, 14.28, 22.62, 25.43, 29.06, 29.17, 31.80, 32.82, 116.41, 136.34, 136.66, 145.56, 145.92, 150.11, 150.71, 152.78, 156.59; HRMS m/z: 532.24854 [M + H]+ (calcd 532.24830). Compound 3 was obtained as a brown solid in 24% yield in a fashion similar to that described above using 3-bromo-4-[3-bromo-5(trimethylsilyl)thiophen-2-yl]pyridine and dichlorodiphenylgermane. Compound 3 was separated by a gel permeation chromatography (GPC) column using toluene as the eluent. Data for 3: 1H NMR (400 MHz, δ in acetone-d6): 0.38 (s, 9H, Me3Si), 7.42−7.47 (m, 6H, phenyl), 7.56 (dd, 1H, pyridine, J = 5.1, J = 1.0 Hz), 7.64−7.66 (m, 4H, phenyl), 7.71 (s, 1H, thiophene), 8.53 (d, 1H, pyridine, J = 5.1 Hz), 8.91 (d, 1H, pyridine, J = 1.0 Hz); 13C NMR (125 MHz, δ in CDCl3): −0.03, 116.72, 128.67, 129.99, 133.43, 134.31, 134.62, 136.33, 142.84,

tricyclic systems have been limited to rather simple cases, including siloles and germoles condensed with phenylpyridine (BPyS), 12 phenylthiophene (BTS), 13 phenylcarbazole (BCzS),14 phenylindole (BInS and BInG),13,15 phenylbenzofuran (BBFS and BBFG),15 phenylbenzothiophene (BBTS and BBTG),15,16 and benzofuranylbenzothiophene (BFBTS and BFBTG),17 as shown in Chart 1. In the present work, we prepared pyridinothienogermole (PyTG) derivatives that have a germole condensed with a pyridylthiophene unit, hoping that the condensation with these donor−acceptor (D−A) systems would pave the way for the development of new building units of conjugated functional materials having finely tuned electronic states. The PyTG derivatives were examined by optical measurements and density functional theory (DFT) calculations of model compounds, and their electronic states were compared with those of DTG and DPyG. We also prepared a PyTG bromide that underwent transformation via the palladium-catalyzed Stille coupling reaction to form a dimer and triphenylamine- and phenylcarbazole-substituted PyTGs for efficient extension of conjugation. These PyTG derivatives exhibited clear solvatochromic properties arising from their intramolecular D−A interaction, and their UV absorption and photoluminescence (PL) bands were shifted to lower energies as solvent polarity was increased. These derivatives were also investigated with respect to the complex formation with trifluoroethanol, trifluoroacetic acid, and tris(pentafluorophenyl)borane. The optical properties of these PyTG-based complexes are also described.

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EXPERIMENTAL SECTION

General. All reactions were carried out in dry argon atmosphere. N[4-(Trimethylstannyl)phenyl]carbazole, 10c N,N-diphenyl-4(trimethylstannyl)aniline,10c and pyridylthiophene18 were prepared as reported in the literature. Tetrahydrofuran (THF), diethyl ether, and toluene used as the reaction solvents were distilled from CaH2 and P2O5, respectively, and stored over activated molecular sieves until use. Microwave irradiation was carried out using a Biotage model Initiator+ reactor. Nuclear magnetic resonance (NMR) spectra were recorded on Varian 400−MR and System−500 spectrometers. High-resolution electrospray ionization mass spectra were measured on a Thermo Fisher Scientific LTQ Orbitrap XL spectrometer at N−BARD, Hiroshima University. UV absorption and PL spectra were measured on Hitachi U−2910 and HORIBA FluoroMax−4 spectrophotometers, respectively. PL quantum yields excited at 350 nm were determined using a JASCO F−6300−H spectrometer attached to a JASCO ILF− 533 integrating sphere unit (φ = 100 mm). Preparation of 3-Bromo-4-(trimethylstannyl)pyridine. A solution of lithium diisopropyl amide was prepared by mixing 9.0 mL (64 mmol) of diisopropylamine and 24.0 mL (63.6 mmol) of a hexane solution of 2.65 M n-butyllithium in 211 mL of THF at 0 °C for 1 h. To this was added 6.20 mL (63.6 mmol) of 3-bromopyridine at −80 °C, and the mixture was stirred at this temperature. After 1 h, 13.9 g (69.8 mmol) of trimethyltin chloride was added and the resulting mixture was stirred at room temperature overnight. The solvent was evaporated and the residue was dissolved in hexane and then poured into water. The organic layer was separated and the aqueous layer was extracted with hexane. The organic layer and the hexane extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated. Recrystallization of the residue from hexane gave 15.3 g of the title compound as a brown solid (75% yield): 1H NMR (400 MHz, δ in acetone-d6): 0.43 (s, 9H, Me3Sn), 7.39 (dd, 1H, pyridine, J = 4.5, 0.8 Hz), 8.44 (d, 1H, pyridine, J = 4.5 Hz), 8.58 (br s, 1H, pyridine); 13C NMR (125 MHz, δ in acetone-d6): −8.22, 132.25, 132.27, 147.43, 150.53, 157.03; HRMS m/z: 321.92499 [M + H]+ (calcd 321.92479); mp 69.0−70.6 °C. B

DOI: 10.1021/acs.organomet.9b00036 Organometallics XXXX, XXX, XXX−XXX

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(s, 1H, pyridine), 8.74 (br s, 1H, pyridine); 13C NMR (125 MHz, δ in CDCl3, at 50 °C): 13.98, 14.56, 22.61, 25.51, 29.10, 29.17, 31.83, 32.81, 109.80, 120.19, 120.38, 123.69, 126.05, 126.54, 127.42, 127.59, 133.54, 137.49, 140.94, 146.15, 148.06, 150.36, 150.73, 150.93, 152.68; HRMS m/z: 701.29828 [M + H]+ (calcd 701.29793). Preparation of D-1. A mixture of 0.10 g (0.19 mmol) of 4, 0.080 g (0.19 mmol) of N,N-diphenyl-4-(trimethylstannyl)aniline, 6.6 mg (3.0 mol %) of Pd(PPh3)4, 1.1 mg (3.0 mol %) of CuI, and 1.9 mL of toluene was heated in a sealed vial at 160 °C for 1 h under microwave irradiation, and the resulting mixture was poured into water. The organic layer was separated and the aqueous layer was extracted with toluene. The organic layer and the toluene extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to silica gel column chromatography eluting with hexane/ethyl acetate = 3:1 to give 34.7 mg (26% yield) of D-1 as a yellow solid: 1H NMR (400 MHz, δ in acetone-d6): 0.83 (t, 6H, CH3 in n-octyl, J = 7.0 Hz), 1.20−1.34 (m, 24H, n-octyl), 1.50 (q, 4H, n-octyl, J = 7.9 Hz), 7.05−7.13 (m, 8H, phenyl), 7.32−7.36 (m, 4H, phenyl and phenylene), 7.41 (dd, 1H, pyridine, J = 5.0, 1.0 Hz), 7.57 (s, 1H, thiophene), 7.65 (d, 2H, phenyl, J = 8.9 Hz), 8.53 (d, 1H, pyridine, J = 5.0 Hz), 8.68 (d, 1H, pyridine, J = 1.0 Hz); 13C NMR (125 MHz, δ in CDCl3, 25 °C): 14.07, 14.36, 22.62, 25.45, 29.07, 29.17, 31.80, 32.81, 123.26, 123.41, 123.46, 124.62, 124.99, 125.05, 126.77, 126.81, 128.10, 129.32, 145.94, 147.36, 147.70, 149.10, 150.55, 150.73, 152.40; 13C NMR (125 MHz, δ in CDCl3, 50 °C): 13.98, 14.53, 22.60, 25.49, 29.09, 29.16, 31.82, 32.81, 123.36, 123.54, 124.76, 125.08, 126.91, 128.32, 129.38, 146.02, 147.54, 147.90, 149.25, 149.32, 150.63, 150.84, 152.52 (two carbons are missing likely because of overlapping); HRMS m/z: 703.31409 [M + H]+ (calcd 703.31358); mp 105.0−106.7 °C.

147.21, 150.24, 151.56, 153.10, 157.62; HRMS m/z: 460.06042 [M + H]+ (calcd 460.06050); mp 151.2−152.6 °C. Preparation of 4. To a solution of 641 mg (1.20 mmol) of PyTGSi was added 215 mg (1.20 mmol) of N-bromosuccinimide (NBS) in several portions at 0 °C. After the mixture was stirred at room temperature overnight, an aqueous solution of Na2S2O4 was added. The organic layer was separated and the aqueous layer was extracted with chloroform. The organic layer and the chloroform extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to silica gel column chromatography eluting with hexane/ ethyl acetate = 3:1 to give 446 mg (69% yield) of 4 as a brown oil: 1H NMR (400 MHz, δ in acetone-d6): 0.85 (t, 6H, CH3 in n-octyl, J = 7.0 Hz), 1.19−1.37 (m, 24H, n-octyl), 1.46 (q, 4H, n-octyl, J = 7.8 Hz), 7.38 (s, 1H, thiophene), 7.40 (dd, 1H, pyridine, J = 5.0, 1.0 Hz), 8.54 (d, 1H, pyridine, J = 5.0 Hz), 8.72 (d, 1H, pyridine, J = 1.0 Hz); 13C NMR (125 MHz, δ in CDCl3): 14.07, 14.37, 22.60, 25.34, 29.02, 29.12, 31.77, 32.75, 115.83, 116.09, 132.70, 134.44, 144.87, 149.73, 150.89, 151.46, 152.63; HRMS m/z: 538.11890 [M + H]+ (calcd 538.11929). Preparation of 5. To a solution of 0.35 g (0.66 mmol) of 4 in 16 mL of THF, 0.2380 mL (0.6569 mmol) of hexane solution of 2.76 M nbutyllithium was added slowly at −80 °C. After the resulting mixture was stirred at this temperature for 1 h, 0.16 g (0.80 mmol) of trimethyltin chloride was added. The mixture was further stirred at room temperature overnight and then poured into water. The organic layer was separated and the aqueous layer was extracted with chloroform. The organic layer and the chloroform extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to GPC eluting with toluene to give 0.20 g (50% yield) of 5 as a brown oil: 1H NMR (400 MHz, δ in acetone-d6): 0.41 (s, 9H, Me3Sn), 0.85 (t, 4H, CH3 in n-octyl, J = 6.6 Hz), 1.20−1.33 (m, 24H, n-octyl), 1.48 (q, 4H, n-octyl, J = 7.2 Hz), 7.39 (s, 1H, thiophene), 7.40 (d, 1H, pyridine, J = 5.0 Hz), 8.51 (d, 1H, pyridine, J = 5.0 Hz), 8.66 (s, 1H, pyridine, J = 1.0 Hz); 13C NMR (125 MHz, δ in CDCl3): −8.13, 14.07, 14.32, 22.61, 25.45, 29.06, 29.16, 31.79, 32.83, 116.44, 136.22, 137.84, 143.47, 145.26, 150.08, 150.70, 152.75, 157.13; HRMS m/z: 624.17383 [M + H]+ (calcd 624.17357). Preparation of B-1. A mixture of 0.18 g (0.33 mmol) of 4, 0.20 g (0.33 mmol) of 5, 11.4 mg (3.0 mol %) of Pd(PPh3)4, 1.9 mg (3.0 mol %) of CuI, and 3.3 mL of toluene was heated at 200 °C for 1 h in a sealed vial under microwave irradiation, and the mixture was poured into water. The organic layer was separated and the aqueous layer was extracted with toluene. The organic layer and the toluene extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to GPC eluting with toluene to give 37.6 mg (12% yield) of B1 as a yellow solid: 1H NMR (400 MHz, δ in acetone-d6): 0.83 (t, 12H, CH3 in n-octyl, J = 6.9 Hz), 1.21−1.41 (m, 48H, n-octyl), 1.51 (q, 8H, n-octyl, J = 7.5 Hz), 7.43 (d, 2H, pyridine, J = 5.0 Hz), 7.56 (s, 2H, thiophene), 8.55 (d, 2H, pyridine, J = 5.0 Hz), 8.71 (d, 2H, pyridine, J = 1.0 Hz); 13C NMR (100 MHz, δ in CDCl3): 14.07, 14.37, 22.62, 25.41, 29.06, 29.17, 31.79, 32.80, 116.06, 126.99, 135.39, 141.56, 145.97, 149.96, 150.11, 150.83, 152.52; HRMS m/z: 917.39636 [M + H]+ (calcd 917.39462); mp 62.0−63.3 °C. Preparation of C-1. A mixture of 0.18 g (0.33 mmol) of 4, 136 mg (0.33 mmol) of N-[4-(trimethylstannyl)phenyl]carbazole, 11.6 mg (3.0 mol %) of Pd(PPh3)4, 1.9 mg (3.0 mol %) of CuI, and 3.4 mL of toluene was heated in a sealed vial at 200 °C for 1 h under microwave irradiation, and the resulting mixture was poured into water. The organic layer was separated and the aqueous layer was extracted with toluene. The organic layer and the toluene extract were combined and washed with water. After drying over anhydrous magnesium sulfate, the solvent was evaporated and the residue was subjected to GPC eluting with toluene to give 64.3 mg (27% yield) of C-1 as a yellow oil: 1H NMR (400 MHz, δ in acetone-d6): 0.84 (t, 6H, CH3 in n-octyl, J = 6.7 Hz), 1.22−1.43 (m, 24H, n-octyl), 1.54 (q, 4H, n-octyl, J = 7.2 Hz), 7.29−7.44 (m, 2H, phenyl), 7.44−7.51 (m, 5H, phenyl, phenylene, and pyridine), 7.73 (d, 2H, phenylene, J = 8.7 Hz), 7.83 (s, 1H, thiophene), 8.07 (d, 2H, phenyl, J = 8.8 Hz), 8.24 (d, 2H, phenyl, J = 7.0 Hz), 8.58

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RESULTS AND DISCUSSION Synthesis. PyTG derivatives were prepared as shown in Scheme 1. Dilithiation of dibromo(pyridylthiophene)s (PyT1Br2 and PyT2Br2) with n-BuLi, followed by ring-forming reactions with dichlorodi(n-octyl)germane, gave mixtures containing the expected PyTG derivatives, unsubstituted pyridylthiophenes, and many other unidentified products. Scheme 1. Synthesis of PyTG Derivatives and 1H NMR Chemical Shifts of 3

C

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voltammograms (CVs) of pyridylthiophenes and PyTG derivatives, whereas the introduction of amino substituents provided clear, reversible anodic couples. The UV absorption and PL bands tended to shift to lower energies in the order of DPyG > PyTG > DTG, although DPyG-1 was nearly nonemissive in THF. Lowering of the PL efficiency by introducing pyridine in the same order as above seemed to reflect the characteristics of bipyridyl that shows no PL in THF at room temperature.9 The introduction of triarylamine substitution on the thiophene ring efficiently enhanced the conjugation. This is likely due to the intramolecular D−A interaction between the triarylamine donor and the pyridine acceptor, as supported by DFT calculations (vide infra). Dimer B-1 also exhibited enhanced conjugation. Reflecting the D−A structures, the PyTG-based compounds exhibited solvatochromic behaviors that were markedly enhanced by the introduction of electrondonating amino groups, as shown in Figures 1 and S1. Of these,

From the mixtures, 1 and 2 were isolated in 7 and 35% yields, respectively, by silica gel column chromatography. The rather low yields were likely due to the thermal instability of the dilithiated species. Phenyl-substituted 3 was also prepared. Bromination of 2 with NBS gave 4, which further underwent palladium-catalyzed Stille coupling reactions with stannylarenes to form bi(pyridinothienogermole) (B-1), (carbazolylphenyl)(pyridinothienogermole) (C-1), and (diphenylaminophenyl)(pyridinothienogermole) (D-1). In the formation of C-1 and D1, homocoupled dimer B-1 was generated as the byproduct. All these compounds are stable under ambient conditions and can be handled without special care except 5, which must be stored in a refrigerator. These PyTG-based compounds were characterized by spectrometry. The 1H NMR signal of 3 thiophene proton was shifted downfield relative to that of DTG-1 even though both compounds have the same phenyl substitutions on the germanium atom,19 whereas 3 pyridine protons were shifted upfield relative to those of DPyG-1,9 as shown in Scheme 1 and Chart 2. This clearly indicates that the thiophene ring and the Chart 2. Structures of DTG-1, DTG-6, DPyG-1, DPyG-6, and PyT and 1H NMR Chemical Shifts of DTG-1 and DPyG-1.

pyridine ring function were used as the electron donor and acceptor, respectively, in the PyTG system to diminish the electron richness and deficiency of the respective rings. Optical and Electrochemical Properties. We examined the optical and electrochemical properties of the prepared PyTG derivatives to clarify their electronic states. Table 1 shows the Table 1. Optical Properties of Pyridylthiophenes and Condensed Germoles in THF comp

UV abs λmax/nm (edge/eV)

PL λmax/nm (Φ/%)

1 2 B-1 C-1 D-1 PyT1Br2 PyT2Br2 DTG-1 DPyG-1 PyT

300 (3.7) 324 (3.6) 403 (2.8) 360 (3.0) 391 (2.8) 264 (3.8)b 269 (3.7) 350 (3.5) 269 (3.9) 277 (3.9)

348 ( DPyG-6 when electron-rich thiophenes were replaced one at a time with electron-deficient pyridine. The lowering of the π-orbital energy levels was greater than that of the π*-orbital energy levels; therefore, the π−π* transition energies were increased in the order of DTG-6 < 6 < DPyG-6, in accordance with the optical properties of real compounds that show high-energy shifts of the UV absorption and PL maxima in the order of DTG-1 < 1 and 2 < DPyG-1. In their LUMOs, σ*−π* conjugation was noted between the germanium σ*orbital and the biaryl π*-orbital. Figure 3 depicts the energy levels and profiles of the HOMOs and the LUMOs of B-6, C-6, and D-6. Dimerization of the PyTG system in B-6 enhanced the conjugation, which led to the reduced HOMO−LUMO energy gap (π−π* transition energy) by raising the HOMO energy level and lowering the LUMO energy level by 0.58 and 0.66 eV, respectively, relative to those of 6. In contrast, the introduction of amino groups in C-6 and D-6 affected the HOMO−LUMO energy gaps mainly by raising the HOMO energy levels rather than lowering the LUMO energy levels. This is more evident for D-6, which has the LUMO lying at an energy level lower by only 0.15 eV and the HOMO lying at

Scheme 2. Hydrogen Bonding of PyTG with CF3CH2OH with Optimized Geometry and Orbital Profiles of 6CF3CH2OH Complex, Derived from DFT Calculations

enhancing the intramolecular D−A interaction by reducing pyridine electron density. The structure of the 6-trifluoroethanol complex was optimized by DFT calculations and is shown in Scheme 2. The 6-trifluoroethanol complex has a smaller HOMO−LUMO energy gap of 4.47 eV than 6. In the real system, other trifluoroethanol molecules assist the interaction by forming secondary hydrogen-bonding to further enhance the red shift. A similar process has been proposed for pyridine derivatives, such as carbolines.21 As shown in Figure 4, the 1H NMR spectra and the UV absorption spectra changed gradually depending on the ratio of the THF/trifluoroethanol mixed E

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Organometallics

Figure 4. 1H NMR spectra of 1 in (acetone-d6)/CF3CH2OH (a) and its UV absorption spectra in THF (b).

solvent. The signals of pyridine α-protons were shifted to higher fields as the acetone-d6/trifluoroethanol ratio was decreased, whereas the signals of β-proton were slightly shifted to lower fields. This contrasts the result that all the 1H NMR signals of pyridine DPyG shifted to lower fields upon the formation of pyridinium salt with methyl iodide.11 Presumably, the hydrogen bonding affects less significantly the pyridine electron density than the pyridinium formation, and the reduced deshielding effects of the nitrogen nonbonding lone pair are primarily responsible for the high field shifts of the pyridine α-proton signals. After the 1H NMR spectrum was measured in acetoned6/trifluoroethanol = 1, the solvent was evaporated and the spectrum was measured again in acetone-d6. The 1H NMR spectrum indicated no decomposition when PyTG was dissolved in trifluoroethanol-containing solvent. The formation of hydrogen bonds was clearly supported by the enhanced PL quantum yields of 1 (Φ = 37%), 2 (Φ = 32%), and B-1 (Φ = 41%) in trifluoroethanol, which reflect the reduced lone-pair character of the pyridine nitrogen atom. Indeed, the n-orbital was stabilized by 0.63 eV to be HOMO − 3 upon the formation of the trifluoroethanol-DPyG-6 complex. On the other hand, the PL quantum yields were suppressed (Φ < 2%) for C-1 and D-1 that have less contribution of pyridine on the PL properties because of the PL red shifts. The PL spectra show a red-shifted broad band, likely because of exciplex formation, as reported for carboline derivatives.21 The emission colors in trifluoroethanol were green for B-1 and red for C-1 and D-1, respectively, while still blue for 1 and 2. In THF containing 1% trifluoroacetic acid, we observed redshifted absorption bands (Figure 5) that are likely due to the formation of pyridinium salts, which pronounced D−A interaction. Quinoid structures seemed to also contribute to the red shift (Scheme 3). The complex formation with tris(pentafluorophenyl)borane was also examined and found to give rise to red-shifted absorption and PL bands, similarly to the case of trifluoroacetic acid (Figures 6 and S2). Titration experiments revealed the existence of isosbestic points in both UV absorption and PL spectra, suggesting the formation of 1:1 complexes (Figure S3), even though B-1, C-1, and D-1 have two Lewis basic nitrogen centers in their molecules. The generation

Figure 5. UV absorption (a) and PL (b) spectra of PyTG derivatives in THF containing 1% trifluoroacetic acid.

of a positive charge on the other nitrogen atom upon the formation of 1:1 complexes seemed to suppress the formation of 1:2 complexes (Scheme 3). It is also likely that C-1 and D-1 formed only 1:1 complexes preferentially using the higher basicity pyridine center. A similar 1:1 complex formation was reported for aminocarboline.22 The binding constants determined by titration were around 105 M−1 (Figure S3). The emission colors of the borane complexes in THF were green for B-1 and red for C-1 and D-1, respectively. Lewis Basicity of PyTG. The UV absorption and PL spectra of 3 and DPyG-1, both of which have the same phenyl substitutions on the germanium atom, were recorded in THF and trifluoroethanol, for comparison. Compound 3 underwent clear changes; a new absorption band at 370 nm and a new PL band at approximately 410 nm were observed when the spectra were measured in trifluoroethanol, as observed for 1, 2, and B-1 (Figure 7). In contrast, DPyG-1 showed only a slightly enhanced absorption at 280 nm in trifluoroethanol and no PL regardless of the solvent (THF or trifluoroethanol) (Figure S4). The 1H NMR spectra of 3 showed solvent-dependent peak shifts, similarly to those of 1 presented in Figure 4a, and different from DPyG-1 that exhibited no evident solvent-dependent 1H NMR spectral changes (Figure S5). These results are indicative of the higher Lewis basicity of PyTG than DPyG, likely because of the electron donation from thiophene to pyridine in PyTG.

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CONCLUSIONS In conclusion, we prepared PyTG derivatives and investigated their electronic states by optical and electrochemical measurements and DFT calculations. The electronic states of the PyTG derivatives were in between those of DTG and DPyG analogs, clearly indicating possible fine-tuning of the electronic states and the functionalities when electron-rich thiophenes were replaced one at a time with electron-deficient pyridine. Electron donation as a result of triarylamine substitution led to efficient D−A F

DOI: 10.1021/acs.organomet.9b00036 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 3. Interaction of PyTG Derivatives with Trifluoroacetic Acid and Tris(pentafluorophenyl)borane

molecular design of new germole-based conjugated functional materials. The PyTG derivatives showed Lewis basicity because of the pyridine lone pair and formed complexes with trifluoroethanol, trifluoroacetic acid, and tris(pentafluorophenyl)borane to shift the UV absorption bands and the PL maxima to the longer wavelength region, suggesting potential applications to sensor materials. Higher Lewis basicity of PyTG than that of DPyG was also demonstrated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00036. Solvent-dependent UV absorption and PL spectra of 2, B1, and C-1; UV absorption and PL spectral changes of B-1 and C-1 in THF on adding tris(pentafluorophenyl)borane; determination of biding constants of B-1, C-1, and D-1 with B(C6F5)3, based on titration experiments on UV absorption spectra; UV absorption and PL spectra of DPyG-1 in THF and CF3CH2OH; 1H NMR spectra of 3 and DPyG-1 in acetone-d8 and acetone-d8/CF3CH2OH; and NMR spectra of new compounds (PDF) Computed optimized geometries (Mol2) (ZIP)

Figure 6. UV absorption (a) and PL (b) spectral changes of D-1 in THF containing (C6F5)3B (0−10.0 equiv).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Joji Ohshita: 0000-0002-5401-514X Yohei Adachi: 0000-0001-8311-5046 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant number JP17H03105. Figure 7. UV absorption and PL spectra of 3 in THF and trifluoroethanol.

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DOI: 10.1021/acs.organomet.9b00036 Organometallics XXXX, XXX, XXX−XXX