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Exploring Structure-Property Relations of B,S-Doped Polycyclic Aromatic Hydrocarbons through the Trinity of Synthesis, Spectroscopy, and Theory.

Tao JinLukas KunzeStefanie BreimaierMichael BolteHans-Wolfram LernerFrieder JäkleRainer F WinterMarkus BraunJan-Michael MewesMatthias Wagner
Published in: Journal of the American Chemical Society (2022)
Polycyclic aromatic hydrocarbons (PAHs) are prominent lead structures for organic optoelectronic materials. This work describes the synthesis of three B,S-doped PAHs with heptacene-type scaffolds via nucleophilic aromatic substitution reactions between fluorinated arylborane precursors and 1,2-(Me 3 SiS) 2 C 6 H 4 /1,8-diazabicyclo[5.4.0]undec-7-ene (72-92% yield). All compounds contain tricoordinate B atoms at their 7,16-positions, kinetically protected by mesityl (Mes) substituents. PAHs 1 / 2 feature two/four S atoms at their 5,18-/5,9,14,18-positions; PAH 3 is a 6,8,15,17-tetrafluoro derivative of 2 . For comparison, we also prepared the skewed naphtho[2,3- c ]pentaphene-type isomer 4 . The simultaneous presence of electron-accepting B atoms and electron-donating S atoms results in a redox-ambiphilic behavior; the radical cations [ 1 • ] + and [ 2 • ] + were characterized by electron paramagnetic resonance spectroscopy. Several low-lying charge-transfer states exist, some of which (especially S-to-B and Mes-to-B transitions) compete on the excited-state potential-energy surface. Consistent with the calculated state characters and oscillator strengths, this competition results in a spread of fluorescence quantum yields (2-27%). The optoelectronic properties of 1 change drastically upon addition of Ag + ions: while the color of 1 in CH 2 Cl 2 changes bathochromically from yellow to red (λ max from 463 to 486 nm; -0.13 eV), the emission band shifts hypsochromically from 606 to 545 nm (+0.23 eV), and the fluorescence quantum yield increases from 12 to 43%. According to titration experiments, higher order adducts [Ag n 1 m ] n + are formed. As a suitable system for modeling Ag + complexation, our calculations predict a dimer structure ( n = m = 2) with Ag 2 S 4 core, approximately linear S-Ag-S fragments, and Ag-Ag interaction. The computed optoelectronic properties of [Ag 2 1 2 ] 2+ agree well with the experimentally observed ones.
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