Donor or Acceptor? How Selection of the Rylene Imide End Cap Impacts the Polarity of π-Conjugated Molecules for Organic Electronics

Three molecular semiconductors are compared and evaluated in organic field-effect transistors and organic solar cells. The molecules are constructed from the dyes diketopyrrolopyrrole (DPP), perylene diimide (PDI), and N-(alkyl)benzothioxanthene-3,4-dicarboximide (BTXI). The compound PDI–DPP–PDI (1) has previously been reported and used as a nonfullerene acceptor. The compounds PDI–DPP–BTXI (2) and BTXI–DPP–BTXI (3) were synthesized using direct (hetero)arylation methods and fully identified using NMR spectroscopy and mass spectrometry. All three compounds were characterized using UV–visible spectroscopy, cyclic voltammetry, and density functional theory calculations. Increasing the BTXI content results in a progressive destabilization of the electronic energy levels. For all compounds, no significant changes in the optical absorption spectra are observed when compared to a combination of the constituent optical absorption spectra. Compound 1 exhibits electron transport characteristics and functions as an electron acceptor in solar cells that produce a power conversion efficiency of 5%. Compound 2 exhibits unbalanced (electron transporting dominate) ambipolar charge transport characteristics and performs better as a nonfullerene acceptor in solar cells. Compound 3 exhibits balanced ambipolar charge transport characteristics and performs best as a donor in solar cell devices. The ability to tune the optical and charge-carrier transport characteristics of these panchromatic dyes through direct (hetero)arylation synthesis offers a distinctive way to create organic semiconductors that span a range of device performance metrics.