Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 139

we observed upfield shifts of both Ha and Hb. Second, we observed
the chemical shift separation (

∆ d δ − δ

(Hb)) between Ha and
Hb increases with the concentration. Third, the peaks of Ha and
Hb begin to broaden with additional fine structures, indicating the
presence of self-assembled dimer and higher oligomers. The NMR
results demonstrate that free monomer, self-assembled dimer,
trimer, and so forth undergo rapid exchange on NMR time scale. The
fact that NMR resonance peaks are well resolved confirms that the
system is in a dynamic equilibrium consisting of free monomer and
various self-assembled oligomers. The critical concentration for the
formation of self-assembled nanostructures is

= (Ha)

1 mM, which is in
good agreement with the optical absorption results.

∼

1

Figure 5.4 H NMR spectra reveal the dynamic self-assemblies of

a monomer ( ) (b) in the

solution. The concentrations of the monomer are 2.3 (top),
18 (middle), and 142 mM (bottom) while the concentrations
of the dimer are 2.3 (top), 23 (middle), and 94 mM (bottom).
Note that the chemical shift separation between Ha and
Hb of the folded dimer (b) is much larger than that of free
monomer (a) around the critical concentration (

1A ) (a) and a folded dimer ( 2A

C ) of the C

self-assembly.

Folded aromatic chromophores have essentially the same
NMR characteristics of those self-assembled

π

-stacks except much
pronounced effects. Therefore, the chemical shifts of Ha and Hb
groups move upfield with increased separation (

∆ d

) between Ha and
Hb. Moreover, we expect fine splitting in Ha and Hb since the two
chromophoric units are not equivalent. Indeed, the details of these