Distannylated isothianaphthene: A versatile building block for low bandgap conjugated polymers

A novel 1,3-distannylated isothianaphthene (1) was synthesized in high yield by dilithiation of isothianaphthene (ITN) followed by treatment with trimethyltin chloride. Compound 1 is stable in the absence of oxygen and was fully characterized by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. We used 1 as a building block for the synthesis of a series of conjugated polymers with alternating structures through Stille-type coupling reactions with dibromo and diiodo compounds. The resulting polymers were characterized by size exclusion chromatography, multinuclear NMR spectroscopy, mass spectrometry, and differential scanning calorimetry. The HOMO-LUMO energy bandgaps of these materials, estimated from UV-vis spectroscopy and cyclic voltammetry (CV), ranged from 1.66 to 2.56 eV depending on the chemical nature of the dihalogenated units. Bulk heterojunction photovoltaic (PV) devices constructed using blends of the ITN-containing copolymers and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) gave power conversion efficiencies ranging from 0.12% to 0.43%.