The molecular architecture of Langmuir-Blodgett (LB) films of saturated fatty acids of different lengths transferred onto flat solid supports (mica, SiO, highly oriented pyrolytic graphite) was investigated with the scanning force microscope (SFM) under ambient conditions. Intrinsic defects in the LB monolayer have been used to measure the film thickness, which changes by an apparent increment of 0.2 nm per methylene group. The influence of different solid supports on monolayer morphology was determined. The investigations were extended to three-dimensional superstructures by deposition of successive monolayers. Reorganization of the bilayer in contact with the aqueous subphase forms regions with different thickness. Molecular resolution was achieved with the SFM on films with more than two layers. We have also determined unit cell parameters of the LB film surface of different fatty acid multilayers. The lattice constants did not change significantly, but the amplitude of the surface corrugations increases with the length of the aliphatic chain. Energetic contributions to monolayer stiffness are discussed. on solid substrates are not always free of defects (e.g. holes) even on a microscopic scale.4 The scanning force microscope,6 SFM, provides a new tool for studying a variety of surface phenomena on a molecular level. It has been
~. 0.0 -2.0 -1 .o 0.0 1 .o 2.0 transition energy / 1368 cm-1 Fig. 2. Simulated spectrum of a model PIC aggregate ( N = 20). The model Hamiltonian was diagonalized for 2000 different configurations obtained from a Gaussian random number generator. The parallel polarized absorption (solid curve) shows the J-band and a vibronic side band at higher energies. The perpendicularly polarized absorption (broken curve) represents transitions to states at the upper band edge k = K which carry intensity for aggregate structures with two inversion related molecules in the elementary cell (insert). They are absent for simpler aggregate structures.taken into account. Its sign alternates and it gives intensity to exciton states near the upper band edge k = n. As in earlier simulations this leads to a broad absorption band at the energy position of the monomer absorption (broken curve of Fig. 2). This additional absorption band is absent for simpler aggregate structures with only one molecule per unit cell. The simulated spectra are not very sensitive to the distance dependence of the excitonic coupling, even if only next neighbor interaction is taken into account. We performed numerical simulations to study the effect of diagonal disorder on the vibronic structure of small PIC aggregates. The results are similar to earlier calculations based on a one-site approximation of the spectral functionr5] and show a vibronic satellite of the J-band and a broad perpendicularly polarized band at higher energies for structures with two inversion symmetric molecules in the elementary cell. Quantum chemical calculations support the picture of a Frenkel exciton moving along the aggregate and give reasonable numbers for the excitonic couplings. In addition, they reveal two deviations from the simple exciton model. Firstly two excitations can be found simultaneously on one molecule forming a higher excited singlet state and secondly the rather strong electron transfer coupling matrix element points to the interaction between excitonic and charge separated states which is of large interest for possible applications in solar energy
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