Synthesis and characterization of fluorinated azadipyrromethene complexes as acceptors for organic photovoltaics

Homoleptic zinc(II) complexes of di(phenylacetylene)azadipyrromethene (e.g., Zn(WS3)₂) are potential non-fullerene electron acceptors for organic photovoltaics. To tune their properties, fluorination of Zn(WS3)₂ at various positions was investigated. Three fluorinated azadipyrromethene-based ligands were synthesized with fluorine at the para-position of the proximal and distal phenyl groups, and at the pyrrolic phenylacetylene moieties. Additionally, a CF₃ moiety was added to the pyrrolic phenyl positions to study the effects of a stronger electron withdrawing unit at that position. The four ligands were chelated with zinc(II) and BF₂⁺ and the optical and electrochemical properties were studied. Fluorination had little effect on the optical properties of both the zinc(II) and BF₂⁺ complexes, with λ_max in solution around 755 nm and 785 nm, and high molar absorptivities of 100 × 10³ M^-1cm^-1 and 50 × 10³ M^-1cm^-1, respectively. Fluorination of Zn(WS3)₂ raised the oxidation potentials by 0.04 V to 0.10 V, and the reduction potentials by 0.01 V to 0.10 V, depending on the position and type of substitution. The largest change was observed for fluorine substitution at the proximal phenyl groups and CF₃ substitution at the pyrrolic phenylacetylene moieties. The later complexes are expected to be stronger electron acceptors than Zn(WS3)₂, and may enable charge transfer from other conjugated polymer donors that have lower energy levels than poly(3-hexylthiophene) (P3HT).