Carbon-bridged oligophenylenevinylene (COPVn; n = 1, CPV; Figure 1a) has found its broad applications in optoelectronic materials as the incorporation of strained 1,4-dihydropentalene framework in an oligoarylenevinylene system is an effective strategy to flatten and to kinetically stabilize the conjugated system for the design of organic electronic materials. However, there lacks efficient and flexible methods for making such a strained ring system. Here we report 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 (Figure 1b). 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. We successfully applied this method to synthesize a variety of substituted indeno[2,1-a]indenes and their thiophene congeners, 4,9-dihydrobenzo[4,5]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 external quantum efficiency of 72.4% at 940 nm (Figure 1c).

Iron-catalyzed tandem cyclization of diarylacetylene to a strained 1,4-dihydropentalene framework for narrow-band-gap materials

ACS Spring 2022

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