Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 180

The most remarkable result of the twisted cyclization reaction
is that only homochiral cyclic dimers form directly and exclusively
from the monomer racemic mixture. The perylene units first undergo
molecular recognition, dynamically self-assembling conducive
to subsequent disulfide bond formation. Homochiral assembly
directs nearly simultaneous disulfide bond formation leading

directly to cyclic 2B , whereas heterochiral assembly directs stepwise

disulfide coupling via intermediate linear dimer 2D , precluding

heterocyclization.
If instead the first disulfide bond formed primarily by random
intermolecular collision without self-templating, the second

disulfide bond must similarly form by random intramolecular

collision. Generally, intramolecular reactions are greatly accelerated
over intermolecular reactions owing to reduced activation entropy.
However, the linear heterochiral dimer 2D never directly cyclizes

into the cyclic heterochiral dimer 2K , while the linear homochiral

dimer is never isolated because it rapidly proceeds to cyclic

homochiral dimer

2E

from cyclizing? In the
absence of molecular codes, steric hindrance between heterochiral
perylene units could conceivably discourage favorable S

2B . What prevents 2D

−

S contact.

However, modeling shows minimal differences between 2D and 2E

in intramolecular steric resistance unless the perylene are cofacially
stacked (

∼

3.5 A apart), leaving many other available configurations
with likely intramolecular thiol collision probability. This is supported
by a nearly temperature independent

∼

K 1 for the interconversion eq

between , showing that the heterotemplate is similarly

favored thermodynamically to the homotemplate. Thus, a random
intramolecular collision between thiols would be nearly equally
favored in each and the reaction product would be a 1:1 ratio of

2B and 2K

2B

to is not a reaction product, intramolecular assembly

must precede cyclization, where the folded linear heterochiral
dimer

2K . Since 2K

structure unfavorably orients the remaining thiols,
effectively blocking the reaction. The same code responsible for
intramolecular folding must also direct intermolecular dimerization,
as the molecular code is not selectively turned on and off. What
heteroconfiguration could cause such an unfavorable orientation?
Modeling shows longitudinally offset stacking or perpendicular
cofacial rotation could greatly reduce the likelihood of the 90

2D

°− °

90

offset elbow C C bond formation. Regardless, the appearance

of strictly heterochiral linear dimers (

−

S

−

S

−

2D ) and homochiral cyclic

dimers ( 2B ) elucidates unique molecular codes.