Synthesis and Emitting Properties of the Blue‐Light Fluorophores Indolizino[3,4,5‐ab]isoindole Derivatives

Abstract

AbstractIndolizino[3,4,5‐ab]isoindoles 3a–3k have been synthesized by [8+2] cycloaddition reactions of the corresponding indolizines 2a–2k with benzyne in moderate yields. Cycloadditions of pyrrolo[2,1‐a]isoquinolines 4a–4c with benzyne gave the benzo[6,7]pyrrolizino[3,4,5‐ab]isoquinolines 5a–5c. Compounds 7 and 9 were similarly prepared by the reactions of 6H‐[1]benzopyrano[3,4‐a]indolizine‐6‐one (6) and 6H‐[1]benzopyrano[3′,4′:3,4]pyrrolo[2,1‐a]isoquinolin‐6‐one (8) with benzyne in 93 and 62 % yields, respectively. The electronic structures of these compounds have been calculated by the DFT method at the B3LYP/6‐31G level. Compounds 3a–3j fluoresce in the blue region (λFmax = 439–475 nm) with high quantum yields (ΦF = 0.49–0.77 in ethanol). Compounds 5a–5c and 9 are fluorescent in the green region with quantum yields of 0.44–0.77 in ethanol and compound 7 emits blue light with a quantum yield of 0.59. The electrochemical properties of these compounds have been investigated by cyclic voltammetry. Indolizino[3,4,5‐ab]isoindoles without a substituent at C‐2 (compounds 3a, 3c, 3d) or with an alkyl substituent in the framework (3i, 3k) undergo irreversible anodic oxidations and reversible cathodic reductions. Compounds 3e and 3g with an electron‐withdrawing group at C‐2, on the other hand, undergo quasi‐reversible oxidations at 1.2–1.3 V (SCE) and reversible reductions at –1.5 to –1.9 V (SCE). Therefore, compounds 3e and 3g may be promising candidates to serve as blue‐light emitters with both hole‐ and electron‐transporting ability in monolayer OLED devices, while the other indolizino[3,4,5‐ab]isoindoles (3a–3d, 3f, 3h–3i) are potential blue‐light emitters with electron‐transporting ability. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)