Volume 205, number 1 CHEMICAL PHYSICSLBTlBRS 2 April 1993 Ring-expanded porphyrins as an approach towards highly conductive molecular semiconductors Nagao Kobayashi ’ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Pharmaceutical Institute, Tohoku University, A obayama, Sendai 980, Japan W. Andrew Nevin ‘, Satoshi Mizunuma, Hiroshi Awaji and Minori Yamaguchi Central Research L&oratories, Kaneka Corporation. 2-80, I-chome, Yoshida-cho, Hyogdu, Kobe 652, Japan Received 24 December 1992;in final form 18 January 1993 As an approach to obtainiq molecular semiconductors with high conductivity, it was attempted to lowerthe activation energy of conduction by enlargiqthe n-conjugated systemin porphyrin materials. An anthraporphyrin (ZnTAnP) has been synthesized for the fast time, and the spectroscopic, electrochemicaland semiconductive properties have been characterizedas a function of increasing molecular size, in the order: tetraphenylporphyrin (ZnTPP), tetrabenzoporphyrin (ZnTBP), tetranaphthalopor- phyrin (ZnTNP), and ZnTAnP. With enlargement of the macrocycle,the Q band shifts to the red, and the absolute values of oxidation and reduction potentials become smaller. Room temperature conductivities of air-doped ZnTPP, ZnTBP, ZSNP, and ZnTAnP thin films are < lo-“, 4~ lo-lo, 3~ lo-’ and 1 x lo-’ W’ cm-‘, respectively. The smaller conductivity of ZnTAnP appears due to the formation of a stable charge transfer complex with oxygen. In addition to their important roles in nature, por- phyrins have recently attracted attention as prom- ising electric and electro-optic materials in a number of areas, such as molecular metals [ 11, photovoltaics [ 2 1, and non-linear optics [ 3 1. Generally, the wide separation (AE) of around 2.2 eV between the high- est-occupied (HOMO) and lowest-unoccupied (LUMO) molecular orbitals results in porphyrins having low conductivities (o), of the order of lo-lo- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH lo-18 n-1 cm-l , in the intrinsic or air-exposed states, with high activation energies of conduction (E,) of 0.5-l eV. To obtain larger o, the porphyrin material must be doped with a strong electron ac- ceptor, such as iodine [ 11, which introduces charge transfer states into the band gap. Recently, in the re- lated phthalocyanines (PC), Simon and co-workers reported the first examples of intrinsic molecular semiconductors [ 41. They found that the complexes LuPc, and LiPc, which contain radical PCrings, have A.5 values of less than 1 eV, and show high intrinsic ’ To whom correspondenceshould be addressed. conductivities of 10-4-10-5 W ’ cm-’ in thin-film form. As an alternative approach to obtaining high con- ductivity, we have attempted to lower E. by reducing AE through enlargement of the x-conjugated system of the porphyrin ring. Previous studies on electronic absorption spectra have indicated a lowering of the energy of the first x-f transition when one or two benzene rings are fused radiahy onto each pyrrole unit of the porphyrin skeleton to give a tetraben- zoporphyrin (TBP) or tetranaphthaloporphyrin (TNP ), respectively [ 5 1, while compressed powders of TNPs have been found to have higher Qthan TBPs [ 6 1. In this study, we have synthesized an anthra- porphyrin for the fust time, and carried out a sys- tematic comparison of the electrical, optical and electrochemical properties as a function of the por- phyrin ring expansion. The results point the way to- wards the realization of a new series of highly con- ducting molecular semiconductors. ZnTBP, ZnTNP, and zinc tetraanthraporphyrin (ZnTAnP) (scheme 1) were synthesized in a man- ner similar to the preparation of meso-phenylated 0009-2614/93/s 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved. 51