Bolaamphiphiles (“bolas”) containing two secondary amide groups at the ends of an oligomethylene
chain and two amino acid type headgroups were synthesized. The structures of crystals, of noncovalent
fibers, and of surface monolayers on gold strongly depended on odd−even effects. In the crystal structures
of alanine−alanine dipeptides with C11- and C12-α,ω-amino acid linkers, helical (even number of methylene
groups in the chain) or sheetlike (odd) arrangements of the headgroups were found. Bolas containing two
different amino acid end groups, namely, d- or l-alanine and l-lysine, connected by the same C11- and C12
linkers did not crystallize. Only the even-numbered bolas gave fibers. l- and d- configurations of alanine
headgroups affected the curvature of the fibers. Diamido bolas with terminal SH-groups self-assembled
on gold. Only those with even-numbered chains gave rigid monolayers. Simple stereochemical arguments
concerning the fitting of amide hydrogen bond chains on both ends of the bolas are given to explain the
observed differences in crystals, fibers, and monolayers.
Mixed monolayers made of steroid thiol molecules lying flat on the
surface and thin octadecanethiol
walls have been prepared on gold by subsequent chemisorption and
self-assembly procedures. Cyclovoltammetry of ferricyanide in bulk water showed 30% of the peak
current observed for naked gold electrodes.
1,2-trans-Cyclohexanediol interrupted ferricyanide
penetration into the steroid membrane gaps almost
quantitatively; its 1,2-cis diastereomer, on the other hand,
had practically no blocking effect. Glucose,
galactose, and mannose were also efficient blockers for ion penetration
into the hydrophobic gaps. Infrared
spectroscopy, quartz balance, radioactivity, impedance, and contact
angle measurements on the monolayers
and its physisorbed entrapments were used to characterize the membrane
system and the unique
physisorption process
of the highly water-soluble compounds
in hydrophobic
gaps. A model based on
the
fitting of polyols with a cyclohexane skeleton and equatorial OH-groups
into icelike water clusters and
the slowdown of diffusion processes in such rigid and shielded clusters
is proposed.
~. 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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