Enhanced Optoelectronic Performances of the Cocrystals between 2,2′-Bi(1,8-naphthyridine) and Iodine through Strong Halogen Bonds

An organic semiconductor cocrystal between 2,2′-bi(1,8-naphthyridine) and I2 has been investigated using a joint experimental and theoretical method. Different from the doping of I2 in many organic semiconductors, strong I···N halogen bonds with interaction energies of −18.51 kcal/mol are formed in this organic semiconductor cocrystal. The formation of the strong halogen bonds increases the sublimation temperature of I2 from 40 to 100 °C. On the other hand, the formation of the strong halogen bonds changes the crystalline order of 2,2′-bi(1,8-naphthyridine) from the herringbone pattern to the 1D slipped-π-stack arrangement and leads to a smaller energy gap between the frontier molecular orbitals, which is beneficial for charge transport. The optoelectronic measurements further demonstrate that the cocrystal thin film has much higher electrical conductivity as compared to the crystal thin film of pure 2,2′-bi(1,8-naphthyridine).