Self-assembled π-conjugated chromophores: preparation of one- and two-dimensional nanostructures and their use in photocatalysis

Abstract

Photocatalytic systems have attracted research interest as a clean approach to generate energy from abundant sunlight. In this context, developing efficient and robust photocatalytic structures is crucial. Recently, self-assembled organic chromophores have entered the stage as alternatives to both molecular systems and (in)organic semiconductors. Nanostructures made of self-assembled π-conjugated dyes offer, on the one hand, molecular customizability to tune their optoelectronic properties and activities and on the other hand, provide benefits from heterogeneous catalysis that include ease of separation, recyclability and improved photophysical properties. In this contribution, we present recent achievements in constructing supramolecular photocatalytic systems made of chromophores for applications in water splitting, H₂O₂ evolution, CO₂ reduction, or environmental remediation. We discuss strategies that can be used to prepare ordered photocatalytic systems with an emphasis on the effect of packing between the dyes and the resulting photocatalytic activity. We further showcase supramolecular strategies that allow interfacing the organic nanostructures with co-catalysts, molecules, polymers, and (in)organic materials. The principles discussed here are the foundation for the utilization of these self-assembled materials in photocatalysis.

Fig. 1. Organic dye aggregates used in photocatalysis have several advantages compared to their monomeric or semiconductor counterparts.

Fig. 2. Overview of self-assembled chromophores and their use in photocatalysis. (a) Schematic representation of the self-assembly of a chromophore into an aggregate. (b) Schematic energy diagram for the exciton coupling of dyes with transition dipole moments and allowed transitions in J-type and H-type dimers. (c) Schematic representation of molecular versus semiconductor photocatalytic processes. (d) Self-assembled dye nanostructures combine advantages of homogeneous molecular photosensitizers (molecular engineering of monomers) and heterogeneous catalysts (stability, recyclability, and processability) for sacrificial photocatalysis.

Fig. 3. Chromophore aggregation triggered in aqueous media through various protocols. (a) Precipitation, charge screening, acid–base neutralization, and surfactant-assisted self-assembly. (b) Aggregation of dyes through hydrophobic collapse and π–π stacking aided through additional interactions, which include complementary charges, coordination interactions, and hydrogen bonding.

Fig. 4. Representative examples of self-assembled nanostructures used in photocatalysis prepared by precipitation. Abbreviations: THF: tetrahydrofuran, TEOA: triethanolamine, and AA: ascorbic acid.

Fig. 5. Illustrative examples in which charge screening was employed to self-assemble amphiphilic chromophores and used for the photoproduction of solar fuels.

Fig. 6. Representative examples of supramolecular nanostructures obtained through acid–base neutralization. Abbreviations: TEA: triethylamine and AcOH: acetic acid.

Fig. 7. Illustrative examples of photocatalytic nanostructures obtained through surfactant-assisted self-assembly.

Fig. 8. Hydrogen bonding can be used to aid the supramolecular organization of chromophores.

Fig. 9. Schematic representation of self-assembled chromophores that can be co-assembled with co-catalysts, small molecules, polymers, or (in)organic materials.

Fig. 10. Aggregation leading to the formation of kinetic and thermodynamic products, which can have different photocatalytic activities.

Luka Đorđević