Here we describe the synthesis of seven new copolymer series
prepared by copolymerization
of hexyl isocyanate with seven new chiral azo chromophores with an
isocyano functionality. The resulting
copolyisocyanates (Nylon 1) possess a helical polymer conformation.
In the cases in which the chiral
group and polymer chain are linked to the same phenyl ring (copolymer
series VI−IX) a transfer of
chirality
to the polymer chain is possible. I.e. one helical twist sense is
preferred in this case. By variation of the
chiral center, polymers with preferable right- or left-handed helical
conformation are accessible. The
photochemical isomerization of the azo chromophore (trans → cis) is
possible in high yields (>90%). This
leads, depending on the nature of the chiral center, either to an
increase of the chiral interaction (further
increase of the amount of already preferred helix conformation) or to a
decrease of the chiral interaction
(more racemic situation). Based on these results, a picture for
the interplay of chiral side chains and
helical polymer chain is developed. Measurements of circular
dichroism (CD) spectra in the range of the
azo-chromophore absorption prove the dominant role of the helical
polymer chain. They show, in addition,
that the chromophores can form a chiral superstructure mediated by the
polymer chain. Some of these
polymers show strong changes of their optical rotation during
photoisomerization. This can be caused
either by a shift of the equilibrium between right- and left-handed
helical segments or by Cotton effects
of the azo chromophores.
The adsorption of heptahelicene, a helically shaped polyaromatic hydrocarbon (C(30)H(18)), on a Cu(111) surface was studied by means of thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) at temperatures between 130-1,000 K under ultrahigh vacuum (UHV) conditions. The molecule in the monolayer remains intact up to 400 K. Above that temperature it decomposes in several steps into carbon and hydrogen, desorbing subsequently as H(2). In the saturated monolayer of the racemate the enantiomers are separated into two different domains on the surface which are mirror images of each other. After adsorption of one enantiomer only, no mirror domains were observed.
The thermal cis−trans isomerization of disperse red 1 in hybrid
polymers has been
investigated. We employed transient absorption spectroscopy in
order to measure the reaction kinetics
of doped and functionalized systems of three matrices. The
matrices show different rigidity. In most of
the cases the thermal decay of the cis-isomers can be fitted by a sum
of two first-order reactions, a slow
and a fast one, which are interpreted on the basis of rotational and
translational relaxation processes of
chain segments. We found that the rate constants of the two
reactions increase and the fraction of the
fast reacting cis-isomers (x
b) decreases with
increasing rigidity of the matrix. The apparent influence
of
the matrix rigidity on x
bwhich is associated
with the cooperative translational movement of chain
segmentsis greater in doped systems than in the functionalized ones
due to a more rigid organic network.
Chromophores which are freely distributed in a polymer can be more
easily enclosed in the synthesis,
thus being less disturbing in the buildup process of the organic
polymer. The polymer around the
chromophore is more regular, and a cooperative translational motion of
adjacent chain segments is more
likely.
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