Synthesis of 1,2-Bis(2-aryl‑1<i>H</i>‑indol-3-yl)ethynes via 5-<i>exo</i>-Digonal Double Cyclization Reactions of 1,4-Bis(2-isocyanophenyl)buta-1,3-diyne with Aryl Grignard Reagents

New π-conjugated 1,2-bis­(2-aryl-1<i>H</i>-indol-3-yl)­ethynes <b>1a</b>–<b>j</b> having various substituents on the two aryl groups were efficiently synthesized via unusual 5-<i>exo</i>-digonal double isocyanide-acetylene cyclization reactions of 1,4-bis­(2-isocyanophenyl)­buta-1,3-diyne <b>3</b> and aryl Grignard reagents (R-MgBr, R = C₆H₅ (<b>1a</b>), 4-H₃CC₆H₄ (<b>1b</b>), 2-H₃CC₆H₄ (<b>1c</b>), 3-MeOC₆H₄ (<b>1d</b>), 3-(CH₃)₂NC₆H₄ (<b>1e</b>), 4-F₃CC₆H₄ (<b>1f</b>), 4-FC₆H₄ (<b>1g</b>), 3-FC₆H₄ (<b>1h</b>), 4-PhOC₆H₄ (<b>1i</b>), and 2-Naph (<b>1j</b>)) in 19–85% yields. The UV–vis spectra were rationalized in detail using time-dependent DFT and single point calculations. The fluorescence emission peaks for <b>1a</b>–<b>j</b> were observed at around 450 nm. Especially for <b>1f</b> and <b>1j</b>, those spectra displayed broad emission bands and relatively large Stokes shifts (3977–4503 cm^–1), indicating the contribution of an intramolecular charge transfer. The absolute quantum yields (0.50–0.62) of <b>1a</b>–<b>j</b> were higher than those of parent <b>8</b> (0.19) and 2-phenyl-1<i>H</i>-indole (0.11). The electrochemical features for <b>1a</b>–<b>j</b> were investigated by cyclic voltammetry. The frontier molecular orbital levels for <b>1a</b>–<b>j</b> were estimated based on the combination of oxidation potentials, UV–vis, and DFT calculated data. The structural property of 1,2-bis­(2-phenyl-1<i>H</i>-indol-3-yl)­ethyne <b>1a</b> was characterized by several spectroscopic methods and finally determined by X-ray analysis of a single crystal of <b>1a</b> recrystallized from ethyl acetate. The structural features of <b>1a</b>–<b>j</b> were also supported by DFT calculations