## 1 Introduction  
Metallophthalocyanines (MPcs) are widely used not only as organic dyes but also as organic semiconductors, owing to their high air and chemical stabilities. They are being actively studied as components of organic electronics, in applications such as organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaic cells [1; 2; 3]. MPcs are used as hole transport/injection/extraction materials because the energy level of their highest occupied molecular orbital (HOMO), consisting of the \(\pi\) orbital of the phthalocyanine (Pc), matches well with the work function of ordinary hole injection/extraction electrodes such as Au or indium-tin-oxide (ITO) [4; 5; 6]. They show p-type semiconducting characteristics, with a field effect hole mobility of around 10\({}^{-2}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) in thin film transistors and around 1 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) in single crystal transistors [1; 7].  
Introduction of electron-withdrawing groups onto the peripheral benzene rings of Pc is a valid approach for tuning the electronic properties of MFCs. For instance, F\({}_{16}\)CuPc, a fully fluorinated CuPc shows n-type characteristics with an electron mobility comparable to its unsubstituted counterpart, CuPc, a p-type material [8; 9]. Therefore, F\({}_{16}\)CuPc has been widely used as an n-type material in organic electronics [10; 11; 12; 13].  
For n-type organic semiconductors, in order to obtain a low energy barrier at the interface, the position of the lowest unoccupied molecular orbital (LUMO) should be close to the work function of the electrode. Moreover, a large \(\pi\)-\(\pi\) overlap of the LUMOs between the adjacent molecules in the crystal is preferred [14], because electron transport in the crystalline state occurs through the overlapped LUMOs. As for the LUMO level of F\({}_{8}\)CuPc (Figure 1), the reported value, \(-3.91\) eV [15], is similar to that of C\({}_{60}\) and its derivatives [16; 17]. This implies that the LUMO level of F\({}_{8}\)CuPc is positioned optimally to enable it to function as an n-type organic semiconductor, and that F\({}_{8}\)CuPc can be potentially used as an electron transport/injection/extraction material in organic electronics, similar to C\({}_{60}\). Indeed, the junction between F\({}_{8}\)CuPc and p-type MPc has been reported to exhibit a rectification effect [18; 19]. In contrast, the highest occupied molecular orbital (HOMO) level of F\({}_{8}\)CuPc has been reported to be \(-6.06\) eV [15]. This value is far from the work function of Au or ITO [5; 6]. Therefore, we expected F\({}_{8}\)CuPc to be an n-type semiconductor. However, recently, thin film or single crystal transistors composed of F\({}_{8}\)CuPc have been reported to show ambipolar characteristics in air [20; 21].  
In this study, we grew a single crystal of F\({}_{8}\)CuPc, and determined its crystal structure to estimate the \(\pi\)-\(\pi\) overlap between the frontier orbitals of adjacent molecules by the extended H\(\ddot{\rm{u}}\)ckel calculation. We have also examined the charge transport properties of OFETs based on F\({}_{8}\)CuPc thin films, and discovered that the transistor is unstable in air, whereas it shows stable n-type transport properties in vacuum.