Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 186

Importantly, both solution-phase and single-molecule emission
spectra provide the extent to which the red-emitting exciton state
is delocalized. The -stack emission involves delocalized excitons

that relax radiatively while emitting red fluorescence to the
Franck

π

−

Condon ground state, which is in a repulsive high vibrational
level of the relaxed ground state and eventually loses its energy
nonradiatively, reorganizing to the lowest vibronic level (

∆ E

) [67].
When a locally excited (LE) state evolves into an exciton, there is
another energy loss (

g

∆ E

) to delocalization. The two-energy losses

e

∆ E + ∆ E →

0 vibronic
band of the monomer. Thus, the number of chromophores involved
in the delocalized state governs the exciton emission wavelength.

( ) account for the exciton red shift from the 0

e e

-stack emission peaks, which
are broad, closely overlapping, and separated by only 15 nm [68].
The linear and cyclic dimers have identical spectral shape peaked
at 624 nm in the solution (DCM:MeOH, 4:1), indicating that the
excitons have identical

These oligomers exhibit two

π

π

-stack structures regardless of linear or
cyclic motifs. Similarly, linear tetramer and concatenated tetramer
have identical spectral shape with a 638 nm maximum; the four
chromophores also form identical exitonic structures regardless of
linear or concatenated architectures. Comparing ensemble emission
for all foldamers (dimer through undecamer), the

π

-stack emission
shows no further red shift beyond the tetramer. This is in contrast to
the

0 − 0 0 − 1

absorbance ratio, which continues to decrease through
the undecamer, demonstrating that the stack length continues
to increase in higher oligomers. Thus, solution-phase ensemble
measurements clearly reveal that the maximum delocalization
length is essentially achieved with four perylene units, although the
wave-like excited state may tail slightly into adjacent chromophores.
Additionally, the minimum delocalization length is achieved with
just two chromophores; the excitons delocalize from two to four
chromophores and longer

A / A

π

-stacks generate no further red-shifted

fluorescence.

5.7.2 Single-Molecule Studies of Linear Foldamers

Nature uses exquisitely folded biopolymers such as proteins to
generate an astonishing array of novel biological functions. Despite
its importance, our current understanding of folding of synthetic
polymers is rather limited. Several reasons contribute to this