  Four-nitrogen-containing 5,6,13,14-Tetraazapentacene (BTANC) has attracted
attention as a new n-type organic semiconductor with a rigid crystalline phase
due to intermolecular CH-N hydrogen bonding. However, in the thin film
transistor of BTANC, poor carrier transport properties and low stability in the
ambient condition have been reported so far; thus further refining and
understanding of the thin film of BTANC will be required. Here, by means of
carefully-controlled vacuum deposition of BTANC in the narrow window of
temperature avoiding impurity sublimation and thermal degradation of molecules,
we produced a well-defined monolayer on Cu(111) for molecular-level
investigations. Synchrotron photoemission of the monolayer revealed a
noticeable alteration of the chemical state of N atoms, which is unexpected for
the pure BTANC molecule. In addition, molecular imaging of the monolayer by
scanning tunneling microscope (STM) revealed that the molecular packing
structure in the monolayer significantly differed from that in the single
crystal of BTANC. These observations can be interpreted as a result of the
partial hydrogenation of N atoms in BTANC and the emergence of the NH-N type
intermolecular hydrogen bonding in the monolayer. These findings will provide a
general remark and strategy to control the molecular packing structure and
electronic property in the molecular films of the nitrogen-containing acenes,
by means of controlled hydrogenation.
