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 Td=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. Indole‐based helicenes HTPI, HBTNI, HIPD and HPI were facilely prepared through three‐ or four‐step reactions. The compounds emitted violet‐blue light with maxima ranging from 391 nm to 415 nm in hexane. Furthermore, they all undergo two‐photon absorption (TPA) and excited fluorescence, and HTPI showed the maximum TPA with a cross‐section of 171.5 GM at 770 nm.