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CC BY-NC-ND 4.0 · Organic Materials 2021; 03(02): 337-345
DOI: 10.1055/s-0041-1730899
Emerging Stars in Organic and Polymer Materials
Short Communication

Carbonyl-to-Alkyne Electron Donation Effects in up to 10-nm-Long, Unimolecular Oligo(p-phenylene ethynylenes)

Sinu C. Rajappan  ‡   #
a   University of Vermont, Departments of Chemistry and Materials Science, 82 University Place, Burlington, VT 05405, United States
,
Olav Vestrheim‡   #
a   University of Vermont, Departments of Chemistry and Materials Science, 82 University Place, Burlington, VT 05405, United States
,
Mona Sharafi
a   University of Vermont, Departments of Chemistry and Materials Science, 82 University Place, Burlington, VT 05405, United States
,
Jianing Li 
a   University of Vermont, Departments of Chemistry and Materials Science, 82 University Place, Burlington, VT 05405, United States
,
Severin T. Schneebeli 
a   University of Vermont, Departments of Chemistry and Materials Science, 82 University Place, Burlington, VT 05405, United States
› Author Affiliations Funding Information This work was supported by the Army Research Office (Grant 71015-CH-YIP awarded to S.T.S.). J.L. was partially supported by an NSF CAREER award (Grant CHE-1945394). The UVM Mass Spectrometry facilities were supported by National Institutes of Health (Grants S10-OD018126 and P30-GM118228). Part of the computational facilities was also supported by an NSF CAREER award (Grant CHE-1848444 awarded to STS).
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Abstract

We synthesized some of the longest unimolecular oligo(p-phenylene ethynylenes) (OPEs), which are fully substituted with electron-withdrawing ester groups. An iterative convergent/divergent (a.k.a. iterative exponential growth – IEG) strategy based on Sonogashira couplings was utilized to access these sequence-defined macromolecules with up to 16 repeating units and 32 ester substituents. The carbonyl groups of the ester substituents interact with the triple bonds of the OPEs, leading to (i) unusual, angled triple bonds with increased rotational barrier, (ii) enhanced conformational disorder, and (iii) associated broadening of the UV/Vis absorption spectrum. Our results demonstrate that fully air-stable, unimolecular OPEs with ester groups can readily be accessed with IEG chemistry, providing new macromolecular backbones with unique geometrical, conformational, and photophysical properties.

Key words

sequence-defined macromolecules - shape-defined macromolecules - Sonogashira coupling - iterative exponential growth - density functional theory - π-conjugation

Supporting Information

Supporting Information for this article is available online at https://doi.org/10.1055/s-0041-1730899.


# These authors have contributed equally to this work.


Supporting Information



Publication History

Received: 24 January 2021

Accepted: 30 April 2021

Article published online:
18 June 2021

© 2021. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/)

Georg Thieme Verlag KG
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  • References and Notes

  • 1 New address: Sinu C. Rajappan, School of Polymer Science and Engineering, University of Southern Mississippi, 118 College Drive, Hattiesburg, MS 39406, USA
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  • 22 General synthetic procedure for IEG growth deprotection: For TIPS deprotection, 4 M solutions of the TIPS-protected derivatives 3–6 (1.0 equiv) in anhydrous CH2Cl2 were prepared under an inert-gas atmosphere. Next, a 1 M solution of TBAF (1.5 equiv) in THF was added at room temperature and the reaction mixtures were stirred at room temperature for 1–2 h. Upon completion (monitored by TLC), the reaction mixtures were diluted with CH2Cl2 (25 mL) and then washed with water (3 × 20 mL) and brine (20 mL). Next, the organic layers were dried over anhydrous sodium sulfate, filtered, and the solvent was evaporated under reduced pressure. Any remaining catalyst was removed via a short flash column over silica gel (eluents: ethyl acetate/hexane mixtures). The TIPS-deprotected derivatives of the tetramer 5 and the octamer 6 were further purified with size-exclusion chromatography (stationary phase: Bio-Beads™ SX-1 Resin, eluent: CH2Cl2) before being carried forward to the Sonogashira coupling steps. Activation (via diazotization–iodination): Following a procedure adopted from Ref. 23, the aniline derivatives 3–6 (1.0 equiv) were dissolved in acetonitrile to form 0.1 M solutions. For compounds with low solubility in acetonitrile, toluene (10 vol%) was added as a co-solvent. Next, a 6 M aqueous HCl solution (10 vol% of the total reaction solvent) was added to the reaction mixtures and the reaction mixtures were cooled in an ice-bath. Diazotization was then initiated by adding an aqueous solution of NaNO2 (1.1 equiv) dropwise to the reaction mixtures at a reaction temperature of <5 °C. The reaction mixtures were then stirred at ∼0 °C for 15 minutes and subsequently added to ice-cold solutions of KI (3 equiv) in water. The resulting mixtures were again stirred at 0 °C for 1 h and then extracted with ethyl acetate. The combined organic layers were washed with water and with brine, dried over anhydrous sodium sulfate, filtered, and the solvent was evaporated under reduced pressure. Finally, the crude activated (iodinated) derivatives of 3–6 obtained in this manner were run through short silica gel columns (eluent: ethyl acetate/hexane solvent mixtures) and then directly carried forward to the Sonogashira coupling reactions. Sonogashira couplings: To oven-dried reaction flasks were added (i) the iodo-derivative (1.0 equiv), (ii) Pd(PPh3 4 (3 mol%), and (iii) CuI (6 mol%). The reaction flasks were then evacuated and backfilled with argon three times with standard Schlenk techniques. Next, the reaction flasks were charged with anhydrous DMF (12 mL) and triethylamine (2.0 equiv) followed by the TIPS-deprotected acetylene compounds (1.3 equiv). Finally, the reaction mixtures were stirred overnight at 70 °C. The progress of the reactions was monitored by TLC. Upon completion, the reaction mixtures were cooled to room temperature, filtered through Celite® 545, and washed with ethyl acetate. The filtrates were diluted with water and the products were extracted with ethyl acetate. The combined organic layers were washed with water and brine, dried over anhydrous magnesium sulfate, filtered, and the solvent was evaporated under reduced pressure. The crude Sonogashira-coupled materials 4–7 were purified by flash column chromatography (eluents: ethyl acetate in hexane mixtures) and, for the longer derivatives 5–7, subsequent size-exclusion chromatography (stationary phase: Bio-Beads™ SX-1 Resin, eluent: CH2Cl2)
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  • 25 Synthesis and characterization data of the dimer 4: Following the general reaction procedure for IEG growth (see: Ref. 22), the monomer 3 (2.0 mmol in 5 mL CH2Cl2) was deprotected with TBAF to afford 1.11 g (95% yield) of the TIPS-deprotected derivative of 3. At the same time, 3 (2.0 mmol) was activated following the general diazotization/iodination procedure to afford 1.06 g (57% yield) of the iodinated derivative of 3. The TIPS-deprotected (2.6 mmol) and iodinated (2.0 mmol) derivatives of 3 were then coupled together under Sonogashira coupling conditions to complete the IEG cycle, as detailed in the general IEG procedure. The crude product was purified by flash column chromatography (eluent: 0–20 vol% ethyl acetate in hexanes) to afford 1.72 g (88% yield) of the dimer 4. 1H NMR (500 MHz, CDCl3) δ 8.19 (s, 1 H), 8.11 (s, 1 H), 8.04 (s, 1 H), 7.22 (s, 1 H), 6.06 (s, 2 H), 4.29–4.20 (m, 8 H), 1.74 (dt, J = 12.0, 5.8 Hz, 2 H), 1.67 (td, J = 12.6, 6.3 Hz, 2 H), 1.55–1.28 (m, 28 H), 1.27–1.20 (m, 4 H), 1.16–1.14 (m, 21 H), 0.98–0.82 (m, 24 H). 13C (1H) NMR (125 MHz, CDCl3) δ 167.23, 165.90, 165.32, 165.22, 149.93, 137.99, 136.99, 136.85, 135.58, 135.15, 134.02, 123.84, 122.29, 118.67, 104.26, 99.30, 96.10, 89.23, 68.28, 68.19, 68.15, 67.49, 60.52, 38.95, 38.92, 38.87, 30.60, 30.48, 30.44, 29.12, 29.06, 29.05, 29.03, 24.06, 23.94, 23.90, 23.82, 23.08, 23.04, 18.79, 17.83, 14.18, 14.14, 12.42, 11.49, 11.14, 11.03. HRMS characterization for 4 was obtained after TIPS deprotection: HRMS (neg. ESI) calcd. for C52H74NO8 −: m/z = 840.5420 [M – H]−; found: 840.5421
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  • 26 Synthesis and characterization data of the tetramer 5: Following the general reaction procedure for IEG growth (see: Ref. 22), the dimer 4 (1.0 mmol in 8 mL CH2Cl2) was deprotected with TBAF to afford 0.828 g (75% yield) of the TIPS-deprotected derivative of 4. At the same time, 4 (1.1 mmol) was activated following the general diazotization/iodination procedure to afford 1.038 g (62% yield) of the iodinated derivative of 4. The TIPS-deprotected (1.3 mmol) and iodinated (1.0 mmol) derivatives of 4 were then coupled together under Sonogashira coupling conditions to complete the IEG cycle, as detailed in the general IEG procedure. The crude product was purified by flash column chromatography (eluent: 0–20 vol% ethyl acetate in hexanes) to afford 0.910 g (82% yield) of the tetramer 5. 1H NMR (500 MHz, CDCl3) δ 8.28 (s, 1 H), 8.27 (s, 1 H), 8.24 (s, 1 H), 8.23 (s, 1 H), 8.21 (s, 1 H), 8.16 (s, 1 H), 8.12 (s, 1 H), 7.23 (s, 1 H), 6.07 (s, 2 H), 4.34–4.23 (m, 16 H), 1.79–1.67 (m, 8 H), 1.52–1.23 (m, 64 H), 1.16 (s, 21 H), 0.98–0.93 (m, 6 H), 0.91–0.84 (m, 42 H). 13C (1H) NMR (125 MHz, CDCl3) δ 167.09, 165.67, 165.01, 164.95, 164.91, 164.76, 149.82, 137.88, 136.87, 136.14, 136.01, 135.86, 135.61, 135.34, 134.54, 134.38, 134.07, 124.32, 123.24, 123.02, 122.57, 121.95, 118.55, 113.21, 109.86, 103.93, 100.16, 96.47, 95.07, 94.72, 94.47, 94.15, 89.19, 68.27, 68.25, 68.18, 68.06, 67.38, 38.82, 38.80, 38.78, 38.74, 30.47, 30.44, 30.32, 29.00, 28.95, 28.92, 23.94, 23.84, 23.78, 23.73, 22.95, 22.92, 18.66, 14.06, 14.03, 11.35, 11.00, 10.91. ∼46 13C (1H) NMR resonances coincide with other signals. HRMS (pos. ESI) calcd. for C113H168NO16Si+: m/z = 1823.2127 [M + H]+; found: 1823.2124
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  • 27 Synthesis and characterization data of the octamer 6: Following the general reaction procedure for IEG growth (see: Ref. 22), the tetramer 5 (0.50 mmol in 10 mL CH2Cl2) was deprotected with TBAF and the product was purified further via size exclusion chromatography (stationary phase: Bio-Beads™ SX-1 Resin, eluent: CH2Cl2) to afford 0.900 g (93% yield) of the TIPS-deprotected derivative of 5. At the same time, 5 (0.32 mmol) was activated following the general diazotization/iodination procedure to afford 0.413 g (67% yield) of the iodinated derivative of 5. The TIPS-deprotected (0.27 mmol) and iodinated (0.21 mmol) derivatives of 5 were then coupled together under Sonogashira coupling conditions to complete the IEG cycle, as detailed in the general IEG procedure. The crude product was purified by flash column chromatography (eluent: 0–20 vol% ethyl acetate in hexanes) to afford 0.200 g (27% yield) of the octamer 6. 1H NMR (500 MHz, CDCl3) δ 8.29 (dd, J = 5.0, 2.8 Hz, 10 H), 8.25 (s, 1 H), 8.23 (s, 1 H), 8.21 (s, 1 H), 8.16 (s, 1 H), 8.12 (s, 1 H), 7.24 (s, 1 H), 6.06 (s, 2 H), 4.34–4.22 (m, 32 H), 1.78–1.68 (m, 16 H), 1.52–1.22 (m, 128 H), 1.16 (s, 21 H), 0.98–0.93 (m, 9 H), 0.92–0.84 (m, 87 H). MS (MALDI, DCTB matrix) calcd. for C217H311NNaO32Si+: m/z = 3494.2401 [M + Na]+; found: 3494.3000
    PubMed
  • 28 Synthesis and characterization data of the hexadecamer 7: Following the general reaction procedure for IEG growth (see: Ref. 22), the tetramer 6 (0.017 mmol in 5 mL CH2Cl2) was deprotected with TBAF and the product was purified further via size exclusion chromatography (stationary phase: Bio-Beads™ SX-1 Resin, eluent: CH2Cl2) to afford 0.056 g (95% yield) of the TIPS-deprotected derivative of 6. At the same time, 6 (0.029 mmol) was activated following the general diazotization/iodination procedure to afford 0.073 g (53% yield) of the iodinated derivative of 6. The TIPS-deprotected (0.017 mmol) and iodinated (0.015 mmol) derivatives of 6 were then coupled together under Sonogashira coupling conditions to complete the IEG cycle, as detailed in the general IEG procedure. The crude product was purified by flash column chromatography (eluent: 0–20 vol% ethyl acetate in hexanes) and further via size exclusion chromatography (stationary phase: Bio-Beads™ SX-1 Resin, eluent: CH2Cl2) to afford 0.020 g (19% yield) of the hexadecamer 7. 1H DOSY NMR (500 MHz, CDCl3, polystyrene standard, see Figure S1 for the calibration curve): M̅ w = 6.9 kDa (expected: 6.8 kDa)
    PubMed
  • 29 See Supplementary Figures S12 and S13 for the 13C (1H) NMR spectra (125 MHz, CDCl3, 298 K) as well as for the 1H–13C HMBC NMR spectra (500 MHz, CDCl3, 298 K) of 6 and 7. With over 200 carbon atoms in 6 and over 400 carbon atoms in 7, a large percentage of carbon signals is coinciding and/or is showing relatively weak signal-to-noise ratios. Yet, there are no carbon signals observed in the 80–85 ppm regions, where one would expect to find 13C resonances for potential homocoupled diacetylene byproducts (see, e.g., Ref. 33 for the 13C (1H) NMR spectra of ester-containing diacetylene derivatives with similar structures). Taken together with the observed (see: Refs. 27 and 28) molecular weights – and the fact that the 1H NMR resonances corresponding to the TIPS protecting groups are clearly observed at ∼1.16 ppm with the proper integrations – this finding excludes the formation of homocoupled diacetylene derivatives as potential side-products
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  • 32 Since a racemic mixture of 2-ethylhexyl bromide was used for the synthesis, the OPEs are present as a mixture of diastereoisomers, which could further contribute to the observed line-broadening of the UV/Vis spectra
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  • 33 Vestergaard M, Jennum K, Sørensen JK, Kilså K, Nielsen MB. J. Org. Chem. 2008; 73: 3175
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