Tuning the semiconducting nature of bis(phthalocyaninato) holmium complexes via peripheral substituents

The semiconducting properties of the heteroleptic and homoleptic bis(phthalocyaninato) holmium complexes bearing electron-withdrawing phenoxy substituents at the phthalocyanine periphery, namely Ho(Pc)[Pc(OPh)(8)] (1) and Ho[Pc(OPh)(8)](2) (2) [Pc = unsubstituted phthalocyaninate; Pc(OPh)(8) = 2,3,9,10,16,17,23,24-octaphenoxyphthalocyaninate] have been investigated comparatively. Using a solution-based Quasi-Langmuir-Shafer (QLS) method, the thin solid films of the two compounds were fabricated. The structure and properties of the thin films were investigated by UV-vis absorption spectra, X-ray diffraction (XRD) and atomic force microscopy (AFM). Experimental results indicated that H-type molecular stacking mode with the common preferential molecular "edge-on" orientation relative to the substrate has been formed, and the intermolecular face-to-face pi-pi interaction and film microstructures are effectively improve by increasing the number of phenoxy substituents of the Pc periphery within the double-decker complexes. The electrical conductivity of Ho(Pc)[Pc(OPh)(8)] films was measured to be approximately 4 orders of magnitude larger than that of Ho[Pc(OPh)(8)](2) films, indicating significant effect of peripheral electron-withdrawing phenoxy groups on conducting behaviour of bis(phthalocyaninato) holmium complexes. In addition, the gas sensing behaviour of the QLS films of 1 and 2 toward electron donating gas, NH3, was investigated in the concentration range of 15-800 ppm. Surprisingly, contrary responses towards NH3 were found for the QLS films of 1 and 2. In the presence of NH3, the conductivity of the films of Ho(Pc)[Pc(OPh)(8)] (1) decreased while the conductivity of the films of Ho[Pc(OPh)(8)](2) (2) increased. This observation clearly demonstrated the p- and n-type semiconducting nature for 1 and 2, respectively. Furthermore, compared to the heteroleptic 1 having a hole mobility of 1.7 x 10(-4) cm(2) V-1 s(-1), homoleptic 2 exhibits an electron mobility as high as 0.54 cm(2) V-1 s(-1). Therefore, the inversion of the semiconducting nature of the double-deckers from p- to n-type can be successfully and easily realized just by increasing the number of peripheral phenoxy groups attached to the conjugated Pc cores.