Langmuir and Langmuir-Blodgett films of amphiphilic hexa-peri-hexabenzocoronene: New phase transitions and electronic properties controlled by pressure

N. Reitzel, T. Hassenkam, K. Balashev, T.R. Jensen, P.B. Howes, K. Kjær, A. Fechtenkötter, N. Tchebotareva, S. Ito, K. Müllen, T. Bjørnholm

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    We present the synthesis as well as the structural and electronic properties of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC), which contains one alkyl substituent that is terminated with a carboxylic acid group. The molecules form well-defined Langmuir films when spread from a solution at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal two crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (xi = 200-400 Angstrom) pi -stacked lamellae of HBC molecules tilted approximate to 45 degrees relative to the surface normal is observed in the low-pressure phase. In this phase. the HBC molecules pack in a rectangular two-dimensional unit cell with a = 22.95 Angstrom and b = 4.94 Angstrom. In the high-pressure phase, coherence from the it stack is lost. This is a consequence of stress induced by the crystallization of the substituent alkyl chains into a hexagonal lattice, which has a trimerized superstructure in one direction: a = 3 x b = 15.78 Angstrom. b = 5.26 Angstrom, gamma = 120 degrees, A = 71.9 Angstrom (2) = 3 x 23.9 Angstrom (2). Thin monolayer films can be transferred to solid supports by the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) with atomic resolution reveals the crystalline packing of alkyl chains in the high-pressure phase. Kelvin force microscopy (KFM) shows a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in the highly coherent low-pressure phase, which is in contrast to the localized molecular orbitals present in the high-pressure phase. The highly coherent pi stack is expected to sustain a very high charge-carrier mobility.
    Original languageEnglish
    JournalChemistry: A European Journal
    Volume7
    Issue number22
    Pages (from-to)4894-4901
    ISSN0947-6539
    DOIs
    Publication statusPublished - 2001

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