Mono- and dicarbonyl-bridged tricyclic heterocyclic acceptors: Synthesis and electronic properties

A series of trialkylsilyl-substituted 2,2′-dithiophene, 4,4′-di-n-hexyl-2,2′-dithiophene, 5,5′-dithiazole, and 2,2′-diselenophene with carbonyl (2a-d) and α-dicarbonyl bridges (3a-d) were prepared from readily available dihalides, using double lithiation followed by trapping with N,N-dimethylcarbamoyl chloride or diethyl oxalate (or N,N-dimethylpiperazine-2,3-dione), respectively. Cyclic voltammetry reveals that the first half-wave reduction potentials for this series of compounds span a wide range, from -1.87 to -0.97 V vs the ferrocene/ferrocenium couple at 0 V (0.1 M ⁿBu₄NPF₆ in THF). A significant increase of the first half-wave reduction potential (by 0.50-0.67 V) was observed on substitution of the monocarbonyl bridge with α-dicarbonyl. Adiabatic electron affinity (AEA, gas phase) trends determined via density functional theory (DFT) calculations are in good agreement with the electrochemical reduction potentials. UV-vis absorption spectra across the series show a weak absorption band in the visible range, corresponding to the HOMO'LUMO transition within a one-electron picture, followed by a more intense, high-energy transition(s). Single-crystal X-ray structural analyses reveal molecular packing features that balance the interplay of the presence of the bulky substituents, intermolecular π-stacking interactions, and S.O intermolecular contacts, all of which affect the DFT-evaluated intermolecular electronic couplings and effective charge-carrier masses for the crystals of the tricyclic cores.