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

Abstract

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.

Fabrication of Two-Dimensional Platelets with Heat-Resistant Luminescence and Large Two-Photon Absorption Cross Sections via Cooperative Solution/Solid Self-Assembly

“…This observation indicated that the solid self-assembly predominantly occurred along the a -axis, where the intermolecular distances were reduced. The infrared (IR) absorption spectrum of the green-emissive 2D platelets showed prominent absorption peaks at 3006, 3030, and 3063 cm –1 (Figure S7), which are characteristic of intermolecular hydrogen bonds between the benzimidazole groups . Based on these results, we conclude that new hydrogen bonds were formed between neighboring fluorophores 1 after the release of 1-propanol, causing the aforementioned size changes.…”

“…The infrared (IR) absorption spectrum of the green-emissive 2D platelets showed prominent absorption peaks at 3006, 3030, and 3063 cm −1 (Figure S7), which are characteristic of intermolecular hydrogen bonds between the benzimidazole groups. 28 Based on these results, we conclude that new hydrogen bonds were formed between neighboring fluorophores 1 after the release of 1-propanol, causing the aforementioned size changes. Although this process did not result in crystal collapse in the small 2D platelets (Figure 2c), it caused some degree of destruction in the bulky crystals, rendering them unsuitable for single-crystal analysis.…”

Continuous Making, Stretch Breaking, and Exciton Dynamics of a Red-Emissive Organic Submicrometer-Sized Triangular Pyramid

“…21 The most common strategy adopted for designing a self-assembly is the introduction of hydrophilic chains into a hydrophobic π-moiety for tuning hydrophobic/hydrophilic balance. 5 nine). 20,22,23 Other chromophores are also reported to form emissive aggregates such as perylene diimides, and porphyrins.…”

“…The design of photoluminescent organic nanostructure by self-assembly of π-conjugated molecules is highly challenging . Effective manipulation of intermolecular noncovalent interactions is crucial to attain unique nanostructures. − The hydrophobic–hydrophilic balance thus dictates the nature of ordering of structures. − Striking a such balance is being done by ticking the functional groups of monomers. Among the hydrophilic interactions, hydrogen bonding that is formed by the localized hydroxyl groups of monomers induced by temperature and pH of the medium offers a greater degree of control to tune the morphology vis-a-vis photonic properties of self-assembly. − Hydrogen-bonded self-assembled nanostructures are omnipresent in living systems such as native protein, helical DNA, cellulosic fibers, and other derived structures. − Inspired by their monomer structures, we have selected a natural product, pyrogallol as a precursor in synthesis of fluorescent monomer because it consists of localized hydroxyl groups that are capable of rapidly and robustly forming hydrogen bonds. − …”

“…Long-range electron/hole transportation is advantageous for optoelectronic applications such as solar cells, organic transistors, and others . The most common strategy adopted for designing a self-assembly is the introduction of hydrophilic chains into a hydrophobic π-moiety for tuning hydrophobic/hydrophilic balance . However, photoluminescent molecular aggregate which was introduced by Kasha are formed by primarily cyanine-based dyes (pseudoisocyanine, merocyanine, and thiacarbocyanine, indotricarbocyanine). ,, Other chromophores are also reported to form emissive aggregates such as perylene diimides, and porphyrins. − These molecular aggregates irrespective of their ordering (J or H) have inherent challenges: (i) very weak photoluminescence and (ii) highly expensive monomers.…”