Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 184

Absorbance spectra confirm that the molecules are mostly
folded in the ground state, but fluorescence data report significant
unfolding during the excitation. This discrepancy is consistent with
a photoinduced unfolding mechanism: Following the absorption
of a photon, the excitation may delocalize across two or more
chromophores in a stack to form a traveling exciton state, which
sometimes produces broad red emission. Alternatively, the excitation
may cause the folded oligomer to partially unfold, generating one or
more uncoupled chromophores that may emit green monomer-like
emission. It is unclear whether or not the photon which causes the
oligomer to unfold directly results in emission, or whether or not
the molecule absorbs another photon during the ultra-fast transient
unfolding and emits green monomer-like fluorescence. In either case,
linear foldamers have high degrees of freedom, can actively undergo
dynamic conformation exchange, and emit relatively strong green
fluorescence. Conversely, the cyclic complements cannot completely
unfold in the same manner as the linear foldamers because
structurally imposed constraints limit conformation dynamics. Thus,
cyclic dimer and concatenated tetramer have less freedom, remain
mostly folded as

π

-stack nanosolenoids, and emit predominantly red

fluorescence.
The relative population of transiently unfolded green-emitting
chromophores in the excited state was estimated from the ensemble
spectrabycomparingtherelativegreenandredemissioncomponents.
Accounting for differences in quantum yields and lifetimes [66], we
estimate that one transiently unfolded chromophore is potentially
10 times brighter than a

π

-stack emission. Owing to such differences
and uncertainty in the brightness, an alternative method is to
compare the monomer-like emission intensity for the linear and
cyclic dimers to the free monomer emission. This method does
not depend on red emission, and provides the relative population
of chromophores emitting monomer-like fluorescence in the
excited state. We calculate that

∼

20% of 2A fluorophores exhibit

monomer-like emission while only 3% of chromophores display

such emission. Thus, both dimers reveal an increase in unfolded
spectroscopic signatures between the

2A

states,
with the linear dimer exhibiting much greater dynamics owing to
its less-constrained architecture. Similarly,

ground and excited

exhibit only
5% and 1% monomer emission, respectively. These architecture-
dependent differences in the folding equilibrium between the

4A and 2 x 2