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

• Published in: Journal of materials chemistry 2012-11, Vol.22 (41), p.22142-22149
• Main Authors: Chen, Yanli, Li, Dapan, Yuan, Na, Gao, Jian, Gu, Rongmin, Lu, Guifen, Bouvet, Marcel
• Format: Article
• Language: English
• Subjects: Atomic force microscopy; Holmium; Molecular structure; N-type semiconductors; Stacking; Thin films; Tuning
• ISSN: 0959-9428; 1364-5501
• DOI: 10.1039/c2jm35219b

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) sub(8)] (1) and Ho[Pc(OPh) sub(8)] sub(2) (2) [Pc = unsubstituted phthalocyaninate; Pc(OPh) sub(8) = 2,3,9,10,16,17,23,24-octaphenoxyphthalocyaninate] have been investigated comparatively. Using a solution-based Quasi-Langmuir-Shaefer (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) sub(8)] films was measured to be approximately 4 orders of magnitude larger than that of Ho[Pc(OPh) sub(8)] sub(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, NH sub(3), was investigated in the concentration range of 15-800 ppm. Surprisingly, contrary responses towards NH sub(3) were found for the QLS films of 1 and 2. In the presence of NH sub(3), the conductivity of the films of Ho(Pc)[Pc(OPh) sub(8)] (1) decreased while the conductivity of the films of Ho[Pc(OPh) sub(8)] sub(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 super(-4) cm super(2) V super(-1) s super(-1), homoleptic 2 exhibits an electron mobility as high as 0.54 cm super(2) V super(-1) s super(-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.