Magnetic susceptibility, heat capacity, and optical conductivity of LiPc and LiPcI

The magnetic susceptibility, using de and electron spin resonance (ESR) met hods, the specific heat, and the infrared properties of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated c ompound LiPcI have been investigated for temperatures 1.5 K less than or eq ual to T less than or equal to 300 K. LiPc has a half-filled conduction ban d and is expected to be an organic metal. However, due to the strong Coulom b repulsion the system is a one-dimensional Mott-Hubbard insulator with a H ubbard gap of 0.75 eV as inferred from optical measurements. The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagneti c spin chain. The spill susceptibility, as determined by ESR experiments, a nd the magnetic contribution to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant J approximate to 40 K. LiPcI is an i ntrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR ex periments it is silent, indicating that all the unpaired electrons have bee n removed from the macrocycle via doping with iodine.