Iron-Catalyzed Tandem Cyclization of Diarylacetylene to a Strained 1,4-Dihydropentalene Framework for Narrow-Band-Gap Materials

Carbon bridging in a form of a strained 1,4-dihydropentalene framework is an effective strategy for flattening and stabilizing oligophenylenevinylene systems for the development of optoelectronic materials. However, efficient and flexible methods for making such a strained ring system are lacking. We report herein a mild and versatile synthetic access to the 1,4-dihydropentalene framework enabled by iron-catalyzed single-pot tandem cyclization of a diarylacetylene using FeCl2 and PPh3 as catalyst, magnesium/LiCl as a reductant, and 1,2-dichloropropane as a mild oxidant. The new annulation method features two iron-catalyzed transformations used in tandem, a reductive acetylenic carboferration and an oxidation-induced ring contraction of a ferracycle under mild oxidative conditions. The new method provides access not only to a variety of substituted indeno[2,1-a]indenes but also to their thiophene congeners, 4,9-dihydrobenzo[4,S]pentaleno[1,2-b]thiophene (CPTV) and 4,8-dihydropentaleno [1,2-b:4,5-b']-dithiophenes (CTV). With its high highest occupied molecular orbital level and narrow optical gap, CTV serves as a donor unit in a narrow-band-gap non-fullerene acceptor, which shows absorption extending over 1000 nm in the film state, and has found use in a near-infrared photodetector device that exhibited an external quantum efficiency of 72.4% at 940 nm.