Dehydrohelicenes and Other Polycyclic Heteroaromatic Molecules: Electrochemical Syntheses, Derivatization, and Optical Properties

概要

Dehydrohelicenes are some of the most attractive chiroptical materials with unique helical chirality that can be implemented in various material-based applications, such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs).1 However, there are no prior reports on their direct construction involving asymmetric methods. In chapter 2, the first electrochemical approach for synthesizing heterodehydro[7]helicenes 3 was introduced via a sequential protocol of chemoselective hetero-coupling of 3-hydoxycarbazoles 1 and 2-naphthols 2, followed by dehydrative cyclization, and intramolecular carbon-carbon bond formation. This study presents one of the most efficient and facile protocols to prepare these unique scaffolds under mild conditions affording up to 86% yield. Moreover, the re-optimization and fine-tuning of the electrochemical conditions afforded different unique polycyclic heteroaromatics (PHAs) in moderate to high yields. In chapter 3, the first efficient enantioselective synthesis of heterodehydro[7]helicene 3 by a chiral vanadium-catalyzed hetero-coupling to give 4 followed by an electrochemical oxidative transformation was presented. This sequential reaction could be carried out in gram scale.

In chapter 4, The optical properties of these synthesized PHAs, especially heterodehydro[7]helicenes were studied and revealed interesting features. An intense blue-colored circularly polarized luminescence (CPL) with luminescence dissymmetry factor |glum| = 2.5 × 10–3 at 433 nm which is the highest CPL among all reported dehydrohelicenes. 2 Thermodynamic and kinetic studies of the racemization barrier of heterodehydro[7]helicenes indicated significant chiral stability with ΔG‡ > 140 kJ mol-1 and t1/2 > ca. 1.6 x 104 years at 25 °C. Based on these results, new designs of multiple helicene and dehydrohelicenens, which apply the concepts of symmetry and assembly of chiral chromophores, were placed and then synthesized to boost the chiroptical features. Some of the suggested designs are predicted to improve the |glum| values according to the theoretical TD-DFT calculations.

1) Borissov, A.; Maurya, Y. K.; Moshniaha, L.; Wong, W.-S.; Żyła-Karwowska, M.; Stepien, M. Recent advances in heterocyclic nanographenes and other polycyclic heteroaromatic compounds. Chem. Rev. 2021, 122, 565-788.

2) Maeda, C.; Nomoto, S.; Akiyama, K.; Tanaka, T.; Ema, T. Facile Synthesis of azahelicenes and diaza[8]circulenes through the intramolecular Scholl reaction. Eur. J. Chem. 2021, 27, 15699-15705.