Impact of Hydrophobic/Hydrophilic Balance on Aggregation Pathways, Morphologies, and Excited-State Dynamics of Amphiphilic Diketopyrrolopyrrole Dyes in Aqueous Media

We
report the thermodynamic and kinetic aqueous self-assembly of
a series of amide-functionalized dithienyldiketopyrrolopyrroles (TDPPs)
that bear various hydrophilic oligoethylene glycol (OEG) and hydrophobic
alkyl chains. Spectroscopic and microscopic studies showed that the
TDPP-based amphiphiles with an octyl group form sheet-like aggregates
with J-type exciton coupling. The effect of the alkyl chains on the
aggregated structure and the internal molecular orientation was examined
via computational studies combining MD simulations and TD-DFT calculations.
Furthermore, solvent and thermal denaturation experiments provided
a state diagram that indicates the formation of unexpected nanoparticles
during the self-assembly into nanosheets when longer OEG side chains
are introduced. A kinetic analysis revealed that the nanoparticles
were obtained selectively as an on-pathway intermediate state toward
the formation of thermodynamically controlled nanosheets. The metastable
aggregates were used for seed-initiated supramolecular assembly, which
allowed establishing control over the assembly kinetics and the aggregate
size. The sheet-like aggregates prepared using the seeding method
exhibited coherent vibration in the excited state, indicating a well-ordered
orientation of the TDPP units. These results underline the significance
of fine tuning of the hydrophobic/hydrophilic balance in the molecular
design to kinetically control the assembly of amphiphilic π-conjugated
molecules into two-dimensional nanostructures in aqueous media.