Unveiling the significance of adduct formation between thiocarbonyl Lewis donors and diiodine for the structural organization of rhodanine-based small molecule semiconductors

Rhodanine vinyl bithiophene (BTR) was synthesized and characterized both spectroscopically and structurally. The reaction of BTR with molecular iodine led to the 1 : 1 “spoke” adduct BTR·I2, formed by interaction of the rhodanine thiocarbonyl group with a diiodine (I2) molecule. The elongation of the I–I bond in the adduct with respect to solid-state I2 and the Raman response in the low-energy region (ν = 150 cm−1) clearly indicate BTR·I2 to be a weak CT adduct. Hybrid-DFT calculations showed that the adduct formation narrowed the HOMO–LUMO gap in BTR·I2 as compared to BTR, while the extended network of secondary interactions, including type-I halogen bonds (XB), results in the formation of an extended 3D network. As a consequence, the room temperature conductivity of BTR·I2 increased with respect to BTR, testifying for a more efficient molecular packing for charge percolation, with the formation of charge carriers in the crystal being facilitated by the presence of I2. It is worth noting that the single-crystal junction device operates at room temperature, in air, and no variation of conductivity over time was observed, indicating that no loss of diiodine occurred during measurements. These results clearly indicate the formation of thiocarbonyl–diiodine CT adducts and their potential as a solid additive for modulating the organization of small molecule semiconductors.