Synthesis of 1,2-Bis(2-aryl‑1H‑indol-3-yl)ethynes via 5-exo-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-1H-indol-3-yl)­ethynes 1a–j having various substituents on the two aryl groups were efficiently synthesized via unusual 5-exo-digonal double isocyanide-acetylene cyclization reactions of 1,4-bis­(2-isocyanophenyl)­buta-1,3-diyne 3 and aryl Grignard reagents (R-MgBr, R = C6H5 (1a), 4-H3CC6H4 (1b), 2-H3CC6H4 (1c), 3-MeOC6H4 (1d), 3-(CH3)2NC6H4 (1e), 4-F3CC6H4 (1f), 4-FC6H4 (1g), 3-FC6H4 (1h), 4-PhOC6H4 (1i), and 2-Naph (1j)) 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 1a–j were observed at around 450 nm. Especially for 1f and 1j, 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 1a–j were higher than those of parent 8 (0.19) and 2-phenyl-1H-indole (0.11). The electrochemical features for 1a–j were investigated by cyclic voltammetry. The frontier molecular orbital levels for 1a–j were estimated based on the combination of oxidation potentials, UV–vis, and DFT calculated data. The structural property of 1,2-bis­(2-phenyl-1H-indol-3-yl)­ethyne 1a was characterized by several spectroscopic methods and finally determined by X-ray analysis of a single crystal of 1a recrystallized from ethyl acetate. The structural features of 1a–j were also supported by DFT calculations.本研究通过1,4-双(2-异氰基苯基)丁-1,3-二炔（3）与芳基格氏试剂（R-MgBr，其中R分别为苯基对应1a、4-甲基苯基对应1b、2-甲基苯基对应1c、3-甲氧基苯基对应1d、3-二甲基氨基苯基对应1e、4-三氟甲基苯基对应1f、4-氟苯基对应1g、3-氟苯基对应1h、4-苯氧基苯基对应1i、2-萘基对应1j）的非常规5-外向双中心双异氰基-乙炔环化反应，高效制备了两个芳基上带有不同取代基的新型π共轭1,2-双(2-芳基-1H-吲哚-3-基)乙炔类化合物1a–j，产率范围为19%~85%。借助含时密度泛函理论（time-dependent DFT, TD-DFT）与单点能计算，对其紫外-可见（UV–vis）光谱进行了详细的解析与归属。1a–j的荧光发射峰均位于约450 nm处；其中1f与1j的发射光谱呈现宽化发射带及较大的斯托克斯位移（Stokes shift，3977~4503 cm⁻¹），表明其存在分子内电荷转移过程。1a–j的绝对量子产率（0.50~0.62）均高于母体化合物8（0.19）与2-苯基-1H-吲哚（0.11）的绝对量子产率。本研究采用循环伏安法对1a–j的电化学特性开展了研究，结合氧化电位、紫外-可见光谱数据与密度泛函理论计算结果，估算得到1a–j的前线分子轨道能级。通过多种光谱表征手段对1,2-双(2-苯基-1H-吲哚-3-基)乙炔（1a）的结构进行了表征，并通过其在乙酸乙酯中重结晶得到的单晶X射线衍射分析，最终确定了其分子结构。密度泛函理论计算结果同样佐证了1a–j的结构特征。