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

Abstract

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–Shäfer (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 π–π 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 × 10⁻⁴ cm² V⁻¹ s⁻¹, homoleptic 2 exhibits an electron mobility as high as 0.54 cm² 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.