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Issue 8, 2017
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Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

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Abstract

A new class of Ar2B–π–A dyads and A–π–B(Ar)–π–A triads that feature strong organic acceptor moieties (A = pyridalthiadiazole, PT) attached to a central triarylborane were synthesized via Stille cross-coupling of ArB(Th–SnMe3)2 (Th = thiophenediyl, Ar = 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethylphenyl) (FMes)) with one or two equivalents of dibromopyridalthiadiazole. Single-crystal X-ray crystallography data for the triad Mes*B(Th–PT–Br)2 indicate a highly coplanar conformation, which is ideal for extended π-conjugation and favors intermolecular π-stacking. Despite the presence of Br substituents, these compounds exhibit strong photoluminescence in THF solution with quantum yields reaching up to 52%. Further extension of conjugation by coupling with 2-hexylthiophene leads to additional bathochromic shifts to give a highly soluble and strongly red-emissive material. All these compounds undergo facile reduction, first of the PT substituents and then at more negative potentials for the borane moiety. Upon chemical reduction with Image ID:c6sc03097a-t1.gif in THF, an intramolecular charge transfer (ICT) pathway from the reduced PT moieties to boron is enabled and this results in a change of the color to blue. Theoretical calculations reveal that, due to the electron-withdrawing effect of the PT moieties, not only the PT-centered LUMOs themselves but also the LUMO+1 or LUMO+2, which show contributions from the p orbital of boron, experience a significant decrease in energy; they are much lower in energy than those of typical conjugated triarylboranes. The relatively low energy of both the PT-centered LUMOs and boron-centered LUMO+1 or LUMO+2 opens up multiple pathways for reaction with highly nucleophilic fluoride anions. Evidence for very strong F binding to boron is obtained in the case of the more sterically accessible FMes derivatives. Fluoride anion binding leads to an electron-rich borate moiety and as such generates an ICT pathway to the electron-deficient PT moieties; the direction of this ICT is opposite to that observed upon chemical reduction. For the Mes* derivatives, F binding is hindered, resulting in competing reduction of the PT acceptors. Finally, the electron acceptor character of the hexylthiophene derivative is exploited in electron-only diodes that show an average electron mobility of 6.4 ± 1.6 × 10−5 cm2 V−1 s−1.

Graphical abstract: Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

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Publication details

The article was received on 13 Jul 2016, accepted on 26 May 2017 and first published on 07 Jun 2017


Article type: Edge Article
DOI: 10.1039/C6SC03097A
Citation: Chem. Sci., 2017,8, 5497-5505
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    Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

    X. Yin, K. Liu, Y. Ren, R. A. Lalancette, Y. Loo and F. Jäkle, Chem. Sci., 2017, 8, 5497
    DOI: 10.1039/C6SC03097A

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