Molecular p‑Doping in Thiophene Diketopyrrolopyrrole Thin Films through Formation of a Charge Transfer Complex

Molecular doping can increase conductivity in organic semiconductor solids through the addition of an electron donor or acceptor into the organic host matrix, when ground state charge transfer interactions are induced between the matrix and the dopant that facilitate carrier transport. Aryl-substituted diketopyrrolopyrroles are common motifs in both small molecules and polymers used for organic electronics, but their molecular doping has only begun to be explored. In this work, we describe a ground state charge transfer complex between a bis­(thiophene) diketopyrrolopyrrole (TDPP) as the donor and 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) as the acceptor. DDQ is a low-cost, relatively stable electron acceptor that we propose is a promising molecular dopant for TDPP-based organic materials. We report the structure of the TDPP/DDQ 1:1 co-crystal, within which we observe strong π–π interactions not only between TDPP and DDQ but also between TDPPs. In thin films prepared from a 1:1 precursor solution, we characterize the energy of the low-energy charge transfer band with UV–Vis absorption spectroscopy and observe a shift in the DDQ nitrile stretch to a lower frequency through Fourier transform infrared (FTIR) spectroscopy, confirming the presence of the ground state charge transfer complex. Single-crystal bond lengths and density functional theory are used to estimate the degree of charge transfer.