Covalently Linked 5,6,11,12-Tetraazanaphthacene Dimer and Its Triptycene-Capped Derivatives as Electron Acceptors

The development of electron transport and n-type materials is still largely dominated by a limited number of organic semiconductors, with fullerenes at the forefront. In contrast, substantial progress has been made in developing hole transport and p-type materials. Therefore, expanding the range of electron acceptors, making them solution-processable, and elucidating their structural arrangement by X-ray crystallography is essential. We synthesised 2,2'-bi-(5,6,11,12-tetraazanaphthacene) (bi-TANC) and its triptycene end-capped derivative, 2,2'-bi(8,13-dihydro-8,13-[1,2]benzenonaphtho-5,6,15,16-tetraazanaphthacene) (bi-TpTANC), as electron acceptors. Bi-TANC exhibits a herringbone-like crystal packing with intermolecular π-π overlap, which is observed in typical organic n-type semiconductors. However, it showed poor solubility, similar to larger acenes. In contrast, bi-TpTANC exhibited favourable solubility, and its electrochemistry in solution was investigated. In the cyclic voltammogram of bi-TpTANC, reversible redox waves corresponding to 3-step/4-electron transfer were observed at -0.795 V (1e-), -0.927 V (1e-), and -1.44 V (2e-) as half-wave potentials. The redox wave associated with the two-electron transfer on the negative low-potential side indicates the presence of through-bond charge delocalisation in the monoanionic state. Furthermore, the LUMO level of bi-TpTANC is -4.1 eV, which indicates its potential as a promising air-stable n-type material.

Interface Structures and Electronic States of Epitaxial Tetraazanaphthacene on Single-Crystal Pentacene

The structural and electronic properties of interfaces composed of donor and acceptor molecules play important roles in the development of organic opto-electronic devices. Epitaxial growth of organic semiconductor molecules offers a possibility to control the interfacial structures and to explore precise properties at the intermolecular contacts. 5,6,11,12-tetraazanaphthacene (TANC) is an acceptor molecule with a molecular structure similar to that of pentacene, a representative donor material, and thus, good compatibility with pentacene is expected. In this study, the physicochemical properties of the molecular interface between TANC and pentacene single crystal (PnSC) substrates were analyzed by atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), and photoelectron spectroscopy. GIXD revealed that TANC molecules assemble into epitaxial overlayers of the (010) oriented crystallites by aligning an axis where the side edges of the molecules face each other along the [1¯10] direction of the PnSC. No apparent interface dipole was found, and the energy level offset between the highest occupied molecular orbitals of TANC and the PnSC was determined to be 1.75 eV, which led to a charge transfer gap width of 0.7 eV at the interface.