Mono-and Dicarbonyl-Bridged Tricyclic Heterocyclic Acceptors: Synthesis and Electronic Properties

A series of trialkylsilyl-substituted 2,2'-dithiophene, 4,4'-di-n-hexy1-2,2'-dithiophene, 5,5'-dithiazole, and 2,2'-diselenophene with carbonyl (2a-d) and alpha-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 (Bu4NPF6)-Bu-n 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 a-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 pi-stacking interactions, and S center dot center dot center dot 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.