Synthesis of 5,9‐Diaza[5]helicenes

Cyclopenta-fused polyaromatic hydrocarbons: synthesis and characterisation of a stable, carbon-centred helical radical

An air- and moisture-stable helical radical with seven six- and five-membered rings arranged alternately was synthesized by cyclizations in a suitably ortho,ortho'-substituted terphenyl and re-establishment of its conjugation. Mesityl groups at the five-membered rings prevent radical reactions. This cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) was characterized by X-ray crystallographic analysis, EPR and UV/Vis spectroscopy, and by cyclic voltammetry. Further properties and spectra were determined by quantum chemical calculation (spin densities, orbital energies, UV/Vis/NIR and ECD spectra). It turned out that this radical is best described with its radical centre being in the outer five-membered rings, which allows for the largest number of fully intact benzene rings. Its triradical character is rather small and can be neglected. The five-membered rings show significant antiaromatic character, which is highest in the central ring.

Synthesis of Diaza[5]helicenes by ortho,ortho′-Fusion of ortho-Terphenyls

Double ortho-fusion in suitably substituted ortho-terphenyls was used for the synthesis of diaza[5]helicenes. Bis(carboxamido)-substituted ortho-terphenyls can be condensed to 5,9- and 6,9-diaza[5]helicenes, where substituents at the 6,10- and 5,10-positions, respectively­, are introduced with the carboxamido groups. While a twofold­ coupling sequence with intermediate protection of one amino group has to be used for 5,9-diaza[5]helicenes, a more concise sequence avoiding the protection leads to 6,9-diaza[5]helicenes. The simple heating of ortho,ortho′-diazidoterphenyls furnishes 5,8-dihydroindolo[2,3-c]carbazoles, i.e., [5]helicenes with alternating benzene and pyrrole rings.

Bischler-Napieralski Synthesis of 6-Alkynyl Phenanthridines Based on Tf2O-Promoted Electrophilic Activation of N-Aryl-2-propynamides

An efficient method for the synthesis of 6-alkynyl phenanthridines was developed. The method offered the first example to use 2-propynamides as substrates in the Bischler-Napieralski reaction and to create alkynylnitrilium triflates as new active intermediates in organic synthesis.

Rhodium-Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene-Like Molecules

The enantioselective synthesis of aza[6] and [7]helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2+2+2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo- and enantioselective synthesis of an S-shaped double aza[6]helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo[6]helicene-like molecule at 80 °C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80 °C. This double aza[6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

Indole-Based Aza[n]helicenes (n=5, 6) with Violet-Blue Fluorescence and Two-Photon Absorption (TPA)

A series of indole-based helicenes, namely, 15-hexyl-15H-tetraphenyl[1,2-e]indole (HTPI), 14-hexyl-14H-benzo[4',5']thieno [2',3':7,8]naphtha[1,2-e]indole (HBTNI), 7-hexyl-7H-indolo[5,4-k] phenanthridine (HIPD) and 3-hexyl-3H-phenanthro[4,3-e]indole (HPI) were successfully synthesized by three- or four-step reactions. They exhibited good solubility and high thermal stability of T_d =247, 388, 294 and 251 °C, respectively. These compounds emit violet-blue light with maximum emission peaks at 415, 397, 397 and 391 nm in hexane. Among them, HBTNI had excellent thermal stability, narrow and sharp emission peaks, and highest photoluminescence quantum yield (0.32). Thus, HBTNI was an ideal candidate for the violet-blue emitters of OLEDs. Furthermore, the four compounds had two-photon absorption and two-photon excited fluorescence. HTPI showed the maximum TPA with the TPA cross-section (δ) of 171.5 GM at 770 nm. They were rare examples of helicenes with both violet-blue emission and TPA.