5-Alkyl-8-hydroxyquinolines: Synthesis and Application in Dye-Sensitized Solar Cells

Abstract

The use of co-adsorbents in dye-sensitized solar cells (DSCs) increases both the power-conversion efficiency and long-term stability of these devices. Co-adsorbents usually consist of a hydrophobic moiety attached on a carboxylic or phosphoric acid terminal anchoring group, which chemisorbs on the semiconductor surface. In this work, the synthesis of a new family of 8-quinolinol derivatives bearing alkyl chains of variable length at the 5-position is described and their comparative efficiency as effective bidentate co-adsorbents in DSCs is evaluated. The key step towards their straightforward modular synthesis is a Suzuki coupling between 5-chloro-8-methoxyquinoline and alkyltrifluoroborates. The new compounds showed better performance as co-adsorbents in terms of cell efficiency as compared to their alkylcarboxylic acid analogues, with the best results obtained from the derivative bearing the longer dodecyl alkyl chain.

Key words

dye-sensitized solar cells - co-adsorbents - hydroxyquinoline - Suzuki coupling - organotrifluoroborates

• References and Notes

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• 19 Representative Synthetic Procedure for the Dodecyl Derivative 1c
  Potassium Dodecyltrifluoroborate (7c)
  Dry and degassed Et₂O (20 mL) was added in flame-dried grinded magnesium turnings (300.0 mmol, 3.60 g, flame activated) under argon. Then, 1-bromododecane (50.0 mmol, 12.46 g) was added dropwise, resulting in a gentle refluxing mixture, and stirred for 1 h. The as-prepared Grignard reagent solution was added dropwise under Ar to a cooled (–78 °C), dry, and degassed THF (50 mL) solution of trimethyl borate (75 mmol, 7.79 g), and the mixture was stirred for 1 h at that temperature and at r.t. for another 1 h. After cooling to 0 °C, a 4.5 M aq KHF₂ solution (205 mmol, 45.5 mL) was added dropwise, and the resulting mixture was stirred for 30 min. The solvents were evaporated, the residue was extracted with hot acetone (3 × 50 mL), and the combined extracts were filtered. The filtrate was condensed to minimum volume solution, the product precipitated out after addition of Et₂O (30 mL), filtered, and dried at 45 °C in vacuo (2 × 10^–3 mbar), furnishing pure 7c as a white powder (7.00 g, 50%). ¹H NMR (500 MHz, DMSO-d ₆): δ = 1.30–1.05 (m, 22 H), 0.88–0.82 (m, 3 H). ¹³C NMR (126 MHz, DMSO-d ₆): δ = 33.13, 31.24, 29.44, 29.21, 29.15, 29.06, 28.98, 28.66, 25.62, 22.02, 13.88. 5-Dodecyl-8-methoxyquinoline (3c) A degassed mixture of 2 (3.07 mmol, 0.59 g), 7c (3.38 mmol, 934 mg), Na₂CO₃ (9.21 mmol, 0.98 g), Pd(OAc)₂ (0.25 mmol, 0.06 g), and RuPhos (0.50 mmol, 0.23 g) in toluene–water (5:1, 15.3 mL) was stirred at 100 °C under argon for 24 h. After cooling to r.t., water (15 mL) was added, and the mixture was extracted with EtOAc (3 × 15 mL). The combined organic layers were dried (MgSO₄) and the solvents evaporated. The residue was purified by column chromatography (EtOAc–PE = 1:1) yielding 3c as yellow oil (0.90 g, 90 %, R_f = 0.5). ¹H NMR (200 MHz, CDCl₃): δ = 8.93 (dd, J = 4.2, 1.6 Hz, 1 H), 8.32 (dd, J = 8.6, 1.6 Hz, 1 H), 7.45 (dd, J = 8.6, 4.2 Hz, 1 H), 7.28 (d, J = 7.9 Hz, 1 H), 6.97 (d, J = 7.9 Hz, 1 H), 4.07 (s, 3 H), 2.99–2.92 (m, 2 H), 1.75–1.60 (m, 2 H), 1.45–1.25 (m, 20 H), 0.90–0.84 (m, 3 H). ¹³C NMR (50 MHz, CDCl₃): δ = 153.82, 148.69, 140.59, 132.48, 130.73, 127.86, 126.07, 121.27, 107.12, 55.95, 32.03, 31.15, 29.76, 29.74, 29.64, 29.46, 22.80, 14.25. 5-Dodecyl-8-quinolinol (1c) A solution of methoxyquinoline 3c (2.75 mmol, 900 mg) in aq HBr (48%, 11.8 mL) was stirred at reflux for 22 h. After cooling to r.t., the mixture was washed with Et₂O (30 mL), the organic washing was decanted, and the aqueous phase was basified with NaHCO₃ (pH 8) and extracted with Et₂O (3 × 30 mL). The combined organic phases were dried (MgSO₄), the solvent was evaporated, and the residue was purified with column chromatography (EtOAc–PE = 1:9) affording 1c as pale yellow powder (373 mg, 43 %, R_f = 0.6). ¹H NMR (200 MHz, CDCl₃): δ = 8.78 (dd, J = 4.2, 1.4 Hz, 1 H), 8.35 (dd, J = 8.5, 1.4 Hz, 1 H), 8.30 (br s, 1 H), 7.46 (dd, J = 8.5, 4.2 Hz, 1 H), 7.28 (d, J = 7.8 Hz, 1 H), 7.10 (d, J = 7.8 Hz, 1 H), 2.99–2.91 (m, 2 H), 1.74–1.60 (m, 2 H), 1.42–1.26 (m, 18 H), 0.91–0.85 (m, 3 H). ¹³C NMR (50 MHz, CDCl₃): δ = 150.56, 147.33, 138.78, 132.95, 129.50, 127.11, 127.11, 121.38, 109.47, 32.07, 31.94, 31.40, 29.81, 29.78, 29.68, 29.50, 22.84, 14.27. ESI-HRMS: m/z calcd for C₁₇H₂₄NO: 314.2484; found: 314.2478 [M + H]⁺.