Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 197

shown red-shifted -stack emission. However, their fluorescence

is not any redder than the tightly folded

π

. In these systems, trap
states were speculated to determine the average exciton domain
length and similarly individual photobleached chromophores would
fragment the extended arrays and limit exciton domains. The same
π

4A

-stack emission has been described when perylene monomers
self-assemble into -stacks in the solution [76]. These also do not

show emission wavelength maximums longer than

π

. The model is
similar to ours in that photo-excitation generates a relaxed state from
which emission takes place. Yet in these systems, it is impossible to
determine the minimum number of monomers required to achieve
such an emission or the maximum number of monomers possible,
forming an exciton that subsequently emits remarkably red-shift
fluorescence.
The exciton diffusion length and coherent domain length along
conjugated polymers

4A

attracts intensive experimental and theoretical
studies and debate and are currently believed to range between 20
nm and 1.2

m and 4.5 and 11 nm, respectively [67,77]. The domain
length across

µ

has not been directly measured,
although Barton and coworkers recently estimated the molecular
wire domain in DNA to be 4

p -stacked systems

5 stacked base pairs [78]. The foldamer
structural series presented here offers unique opportunities
to determine the maximum exciton coherent domain length in
π

−

-stacked perylene chromophores. Unlike the self-assembled or
polymer systems mentioned earlier, each foldamer has the precise
known number of chromophoric monomers.
Here, single-molecule experiments have determined that the
peak maximum red shift

∆ l

= 112 nm is achieved with just four
chromophores, and that no longer wavelength emissions have
been found when the foldamer

max

-stacks have been increased from
tetramer to undecamer. Ensemble solution phase measurements
confirm this apparent peak maximum red shift,

π

∆ l

.

max

5.8 Concluding Remarks

Molecular self-assemblies constitute a relatively new field of
studies, yet data about molecular self-assemblies have exploded
in the last three decades. Hitherto, most efforts focused on using
molecular self-assemblies as methods to fabricate specific targeted
nanostructures. However, molecular self-assemblies are expected to