Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 175

other planar monomers forming a self-assembled structure favorable
for cooperative disulfide bond formation and cyclic products.
This hypothesis was validated previously because no reactive
intermediates were observed, no matter how fast the reaction was
quenched. The twisted chiral molecules, however, have two options:
homotemplate (R

−

R or S

−

S

π

-stack) versus heterotemplate (R

−

S

π

-stack). Obviously, the homotemplate and the heterotemplate have
different self-assembled structures and thus control the reaction
pathway differently, although both reactions are accelerated by the
self-assembly effect.
Experimental results reveal that the homotemplate directs
cooperative disulfide bond formation, yielding a cyclic dimer

,
whereas the heterotemplate endures a stepwise reaction, first
producing a linear heterodimer

2B

(Scheme 5.12). This linear
heterodimer equilibrates with its counterpart, the linear homodimer
2E

2D

, which then folds according to the homotemplate molecular
code rather than the heterotemplate molecular code. Since
homotemplate molecular code favors the reaction pathway toward
homocyclic formation, 2E is converted to the cyclic homodimer,

2B . The cyclization of the homodimer effectively removes it from

the equilibrium, causing more (Le Chatelier's

principle). Thus, the conversion rate is greatly increased when
compared to cyclic dimer interconversion.
Why did the twisted-monomer reaction yield no measurable
concatenated rings? According to the concatenation mechanism,
the cyclic dimer reacted with two linear monomers simultaneously
and produced the concatenated rings. Planar perylene derivatives
have identical complementary intermolecular codes. The twisted
perylene derivatives, however, are different; the racemic mixture
creates two molecular codes. A concatenated tetramer prefers
R,R,R,R or S,S,S,S configurations sterically. The probability of
forming such a homochiral tetramer is only one-eighth of the planar
concatenated rings because homodimers and linear monomers have
only ½ probability of participating in the reaction for a racemic
mixture. In addition, steric hindrance probably further discourages
the formation of such a template in twisted structures that leads to
concatenated rings. These analyses reduce the theoretical yield of
the concatenated rings to less than a few percent, a level difficult to
isolate.

2D to convert to 2E