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

We present the synthesis as well as the structural and electronic propertie s of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC ), which contains one alkyl substituent that is terminated with a carboxyli c acid group. The molecules form well-defined Langmuir films when spread fr om a solution at the air-water interface. Grazing-incidence X-ray diffracti on (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal t wo crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (xi = 200-4 00 Angstrom) pi -stacked lamellae of HBC molecules tilted approximate to 45 degrees relative to the surface normal is observed in the low-pressure pha se. 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-pressu re phase, coherence from the it stack is lost. This is a consequence of str ess induced by the crystallization of the substituent alkyl chains into a h exagonal 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 transfer red 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) sho ws a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in th e 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.