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for a cooperative cyclization reaction, yielding cyclic products over
other possible linear polymer structures. As the difference in twist
angle (
) increases, the intermolecular assembly codes fade and
heterochromophoric cyclic compounds become less probable than
the other random collision products.
In all cases, the distance between the two cooperative reaction
centers is the same, roughly 39 Å away. The nanoenvironment of
the reaction centers is also the same because both are bound to
tetraethylene glycol linkers. Electronic effects such as electron
induction or hyperconjugation from the perylene unit (through
bond effects) cannot contribute to the reactivity difference or alter
the reaction course because the reaction center is
q
14 Å away
from the perylene unit. At such an isolated position, perylene can
hardly impart any steric repulsion (through space effects). The
experimental results demonstrate that molecular self-assembly
invokes selective coding phenomena and the reactions abide by the
molecular codes.
The restrictive cyclic architectures force the chromophores
into close proximity independent of concentration. Just as optical
absorption and NMR report the extent of homogeneous self-
assembly, they also report the extent of folding in covalently bound
heterodimers. The parameters
r
and
d
for compounds
2D 2H
obs
are summarized in Scheme 5.9. Despite the restrictive architectures,
cyclic dimers with incompatibly coded chromophores (
2H
) are
essentially unfolded, whereas compatibly coded structures (
) are
strongly folded. Both absorption and NMR divulge that molecular
shapes behave like codes in reactions and otherwise.
Obviously, monomers with similar twist angles tend to self-
organize independently of other coded structures. The next natural
step tests whether or not such molecular codes can direct chemical
reactions to form specific products. Homochromophoric exotic
catenane and ring structures were previously synthesized from
starting compounds
2D
using disulfide linkages [58,59]. From
homogenous mixtures of reactive monomers, planar
1I
and
1J
1I
formed
monomer ring
1
, dimer ring
2A
, and concatenated rings
2 x 2
, while
highly twisted compound
1J
only yielded the homochiral dimer ring
2B
.
To validate the molecular code concept, we employed a 1:1
heterogeneous reaction mixture of compounds
1I
and
1J
, where
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