Temperature-Dependent Semiconducting Behavior of an Organic Cocrystal Driven by the Stacking Mode of Interaction of a 4,4′-Bipyridine Molecule

Organic cocrystals based on H-bonding as well as π-stacking interactions between 4,4′-bipyridine–pyromellitic acid (1) and 4,4′-bipyridine–phthalic acid (2) are reported. Cocrystals 1 and 2 were fully characterized by single-crystal X-ray diffraction and NMR and IR spectroscopy. The single-crystal X-ray diffraction shows the H stacking pattern of the adjacent 4,4′-bipyridine molecules in the construction of a 3D chain structure for cocrystal 1. Cocrystal 2, however, formed a zigzag 3D chain where the adjacent 4,4′-bipyridyl molecules are involved in a J-stacking mode. Experimental conductivity measurements of the cocrystals 1 and 2 with a Keithley 4200 SCS parameter analyzer showed the temperature-dependent semiconducting behavior in the case of cocrystal 1, whereas cocrystal 2 remained as an insulator. The favorable H-stacking interaction of 4,4′-bipyridine molecules which is the prime origin of semiconductivity in cocrystal 1 may become out of phase due to the free rotation along the C–C bond of 4,4′-bipyridine with an increase in temperature. Although the semiconducting behavior of a material increases with increasing temperature and decreases in resistivity, in the case of cocrystal 1 due to the probable phase transition of the 4,4′-bipyridyl molecules the material became an insulator with an increase in temperature from 20 °C to higher temperature, whereas the semiconducting behavior was restored after cooling the crystals to 20 °C again. The theoretical study conducted with the optimized structures of 1 and 2 showed the higher electron hopping rate in the case of cocrystal 1 as compared to 2 which can account for the charge conduction in the case of 1