Triarylborane-Based Helical Donor-Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (3-B(Mes)₂ -[4]helix-9-N(p-Tol)₂ 1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (3-B(Mes)₂ -[5]helix-12-N(p-Tol)₂ 2) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φ_f =0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (Φ_f =0.18 for [4]helicene; Φ_f <0.05 for [n]helicenes (n≥5)). In the solid state, the Φ_f values are higher (Φ_f =1.00 and 0.55 for 1 and 2, respectively) than those in solution, most likely due to the restrictions of molecular motions. The S₁ ←S₀ transitions of 1 and 2 are predominately HOMO→LUMO transitions. Upon excitation with UV light, the interplanar angle between the two terminal aryl rings of the [5]helix core of 2 decreases (S₁ state compared with S₀ state), which is similar to placing a spring under an external force.

Twisted Polycyclic Aromatic Systems Prepared by Annulation of Bis(arylethynyl)arenes with Biphenylboronic Acids

Twisted polycyclic aromatic hydrocarbons (PAHs) were prepared by the successive rhodium-catalyzed annulation and dehydrogenative cyclization of bis(arylethynyl)arenes with di-tert-butylbiphenyl-2-ylboronic acid. The molecular structures of the PAHs were determined by single-crystal XRD analysis. The PAHs showed up to four fjord regions, and the twisting angle was 46.7°. The nonplanarity (NP) and harmonic oscillator model of aromaticity (HOMA) were calculated by using the structural data obtained from XRD analysis. The PAHs derived from dialkynyl naphthalenes showed low planarity and HOMA of the central ring. The optical properties of the PAHs were investigated by UV/Vis absorption and fluorescence spectroscopy analyses. The absorption and emission maxima of the PAHs with a larger planar region appeared at a longer wavelength. DFT calculations support that the absorption band at λ≈450 nm can be mainly attributed to the HOMO-LUMO transition.

D-π-A triarylboron compounds with tunable push-pull character achieved by modification of both the donor and acceptor moieties

The push-pull character of a series of donor-bithienyl-acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7-tetramethyljulolidine as a donor and B(2,6-Me2 -4-RC6 H2)2 (R=Me, C6 F5 or 3,5-(CF3)2 C6 H3) or B[2,4,6-(CF3 )3 C6 H2]2 as an acceptor. Ir-catalyzed C-H borylation was utilized in the derivatization of the boryl acceptors and the tetramethyljulolidine donor. The donor and acceptor strengths were evaluated by electrochemical and photophysical measurements. In solution, the compound with the strongest acceptor, B[2,4,6-(CF3)3 C6 H2]2 ((FMes)2 B), has strongly quenched emission, while all other compounds show efficient green to red (ΦF =0.80-1.00) or near-IR (NIR; ΦF =0.27-0.48) emission, depending on solvent. Notably, this study presents the first examples of efficient NIR emission from three-coordinate boron compounds. Efficient solid-state red emission was observed for some derivatives, and interesting aggregation-induced emission of the (FMes)2 B-containing compound was studied. Moreover, each compound showed a strong and clearly visible response to fluoride addition, with either a large emission-color change or turn-on fluorescence.