Synthesis and Structural Characterization of a Novel Symmetrical 2,10-Bis-Styryl-1-Benzopyrylium Dye

Abstract

A novel symmetrical 2,10-bis-styryl-1-benzopyrylium dye was synthesized through two steps: In the first reaction an aldol condensation between 2,4,6-trihydroxybenzaldehyde and 3,4-dihydroxy­methylstyrylketone gave 5,7,3′,4′-tetrahydroxy-2-styryl-1-benzopyrylium; and in the second step this latter reacted with the same ketone by a cycloaddition reaction yielding the symmetrical 2,10-bis(3,4-di­hydroxystyryl)-pyrano-7-hydroxy-1-benzopyrylium. The dye was fully characterized by LC-DAD/ESI-MS and NMR 1D and 2D techniques, and a formation pathway was proposed. The novel dye obtained revealed extended π-conjugation (λ_max 556 nm) and catechol groups in its composition, which is very important for applications as a photosensitizer for dye-sensitive solar cells (DSSCs).

Key words

2,10-bis-styryl-1-benzopyrylium - dye - cycloaddition - NMR - LC-DAD/ESI-MS

• References and Notes

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• 18 HPLC-DAD HPLC analyses were performed on a Merck-Hitachi L-7100 (Merck, Darmstadt, Germany) apparatus with a 150 × 4.6 mm i.d. reversed-phase ODS C18 column (Merck, Darmstadt) at 25 °C; detection was carried out using a L-7450A diode array detector (DAD). The eluents were A: H₂O/HCOOH (9:1) and B: MeCN. The gradient consisted of 0–35 % B for 50 min at a flow rate of 0.5 mL/min. The column was washed with 100% B during 10 min and then stabilized with the initial conditions during another 10 min.
• 19 LC-DAD/ESI-MS LC-DAD/ESI/MS analyses were performed on a Finnigan Surveyor series liquid chromatograph equipped with Finnigan LCQ (Finnigan Corp., San Jose, Calif., USA) mass detector and an API source using an ESI interface. The samples were analyzed on a reversed-phase column (150 × 4.6 mm, 5 μm, C18) at 25 °C using the same eluents, gradients, and flow rates referred for HPLC analysis. The capillary voltage was 4 V and the capillary temperature 275 °C. Spectra were recorded in positive and negative ion modes between m/z 120 and 1500. The mass spectrometer was programmed to do a series of three scans: a full mass (MS), a zoom scan of the most intense ion in the first scan (MS²), and a MS-MS of the most intense ion using relative collision energy of 30 and 60 (MS³).
• 20 H NMR (600.13 MHz) and ¹³C NMR (125.77 MHz) spectra were recorded in DMSO-d6/TFA (9:1) on a Bruker Avance 600 MHz NMR spectrometer at 303 K and with TMS as an internal standard (chemical shifts (δ) in ppm, coupling constants (J) in Hz). Multiplicities are recorded as singlets (s), doublets (d), and multiplets (m). ¹H chemical shifts were assigned using 2D NMR (COSY, NOESY) experiment while ¹³C resonances were assigned using 2D NMR techniques (gHMBC and gHSQC). The delay for the long-range C/H coupling constant was optimized to 7 Hz.
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• 24 2-(3,4-Dihydroxystyryl)-5,7-dihydroxy-1-benzopyrylium Chloride (3) 2,4,6-Trihydroxybenzaldehyde (1, 0.28 mmol) and 3,4-dihydroxystyrylmethylketone (2, 2.80 mmol, 10 equiv) were dissolved in 3mL of AcOEt/MeOH (2:1, v/v). After cooling to 0 °C, TMSCl (20 equiv) was added and the solution stirred for 30 min at 0 °C and then at room temperature overnight. The crude product was pre-purified in a Buchner funnel loaded with C18 silica gel and eluted with acidified aqueous solution of 20–50 % of MeOH. Product in pure form was achieved by column chromatography using C18 silica gel with 40 % of acidified MeOH. A dark pink powder was obtained with 90 % of yield (>99 % purity determined by LC/MS). ¹H NMR (600.13 MHz, DMSO-d6/TFA 9:1): δ = 8.93 (d, J = 8.5 Hz, 1 H, H4), 8.24 (d, J = 15.8 Hz, 1 H, Hβ), 7.62 (d, J = 8.5 Hz, 1 H, H3), 7.35–7.27 (m, 3 H, Hα, H6′ and H2′), 6.91 (d, J = 8.2 Hz, 1 H, H5′), 6.86 (d, J = 1.6 Hz, 1 H, H6), 6.68 (d, J = 1.7 Hz, 1 H, H8). ¹³C NMR (125.77 MHz, DMSO/TFA 9:1): δ = 170.8 (C2), 170.5 (C7), 159.3 (C5 or C8a), 158.4 (C5 or C8a), 152.1 (C3′ or C4′), 149.0 (Cβ), 147.5 (C4), 146.6 (C3′ or C4′) 126.7 (C1′), 125.1 (C2′), 116.3 (C6′), 114.9 (Cα), 113.4 (C3), 112.5 (C4a), 112.1 (C5′), 102.1 (C8), 95.0 (C6). LC-DAD/ESI-MS: [M]⁺ m/z 297 λ_max 539 nm.
• 25 2,10-Bis(3,4-dihydroxystyryl)-pyrano-7-hydroxy-1-benzopyrylium Chloride (4) 2-(3,4-Dihydroxystyryl)-5,7-dihydroxy-1-benzopyrylium chloride (3, 0.20 mmol) and 3,4-dihydroxystyrylmethylketone (2, 2.01 mmol, 10 equiv) were dissolved in 50 mL of EtOH 30 % solution. The reaction mixture was stirred at 37 °C and pH 3.5 for 10 d. The crude product was pre-purified in a Buchner funnel loaded with C18 silica gel and eluted with acidified aqueous solution of 20–70 % of MeOH. Product was isolated by column chromatography using C18 silica gel with 60 % of acidified MeOH. A dark purple powder was obtained with 23 % of yield (95 % purity determined by LC/MS). ¹H NMR (600.13 MHz, DMSO-d6/TFA 9:1): δ = 7.86 (d, J = 15.8 Hz, 2 H, Hβ′and Hβ′), 7.23 (s, 2 H, H2′ and H2′′), 7.17 (d, J = 8.1 Hz, 2 H, H6′ and H6′′), 7.11–7.05 (m, 6 H, Hα, Hα′, H6, H8, H3, and H9), 6.87 (d, J = 8.1 Hz, 2 H, H5′ and H5′′). ¹³C NMR (125.77 MHz, DMSO-d6/TFA 9:1): δ = 166.6 (C2, C7 and C10), 153.2 (5 and 8a), 150.5 (C4 and C4′′), 146.3 (C3′ and C3′′), 143.6 (Cβ and Cβ′), 126.7 (C1′ and C1′′), 123.4 (C6′ and C6′′), 116.4 (C5′ and C5′′), 115.7 (Cα and Cα′), 115.5 (C2′ and C2′′), 107.0 (C4a), 105.0 (C3 and C9), 100.2 (C6 and C8). LC-DAD/ESI-MS: [M]⁺ m/z 455 λ_max 556 nm.