pH-Directed Aggregation to Control Photoconductivity in Self-Assembled Perylene Bisimides

Abstract

Perylene bisimides (PBIs) are one example of useful p-conjugated molecules that can be used in optoelectronic devices as n-type materials with strong visible light absorption. PBIs can self-assemble into a range of structures, but it is rare to be able to control the packing such that the same PBI can form either H-type or J-type aggregates. This is important because the conductivity pathways and optoelectronic properties are directly affected by this packing. Here, we show that we can control the packing of a single PBI functionalized with an amino acid by a subtle change in pH. Under one set of conditions, H-type aggregates form a gel when the pH is decreased. At a slightly different set of starting conditions, J-type aggregates are formed, but they cannot form a gel when the pH is lowered. We show that films formed from the self-assembled structures have very different photoconductive properties.

Insight into the self-assembly of water-soluble perylene bisimide derivatives through a combined computational and experimental approach

“…When functionalised with suitable substituents, they can form gels, liquid crystals or be dried down to form amorphous thin films. The resulting materials can act as photoconductors, [1][2][3][4][5][6] with very long charge carrier lifetimes even in the presence of air, 3 as hydrogen evolution photocatalysts, [7][8][9] and battery electrolytes, 10 as well as materials that structurally respond to electrochemical reduction 11 or illumination. 12 All these properties arise from the interplay between their propensity to self-assemble and their rich redox and photochemistry.…”

“…The alkyl-chain functionalised PBIs can form organogels [13][14][15][16] upon lowering the solution temperature or increasing the PBI concentration. PBIs functionalised with oligo(ethylene glycol) groups can form hydrogels upon mixing with water 17,18 and PBIs functionalised with ionisable groups can form hydrogels [2][3][4][5][6]12,[19][20][21][22] upon neutralisation of the charge. Both organic and water-soluble functionalised PBIs have also reported to be able to form columnar liquid crystalline systems.…”

“…Both organic and water-soluble functionalised PBIs have also reported to be able to form columnar liquid crystalline systems. [23][24][25][26][27][28][29] PBIs derivatised with amino acids at the imide nitrogen positions, [2][3][4][5][6]12,21 can be easily prepared by reacting perylene-3,4,9,10-tetracarboxylic dianhydride with an amino acid at high temperature in the presence of imidazole, and are examples of PBIs functionalised with ionisable groups and the focus of this paper. Such amino acid functionalised PBIs are soluble in water at high pH, when the terminal carboxylic groups are deprotonated, and can form hydrogels upon lowering the pH and hence neutralisation of the carboxylic groups.…”

“…Previous work has shown that the UV-Vis spectra and physical properties of the formed gel or dried down solution change with the exact amino acid group used. 5,6 The amino acid functionalised PBIs display relatively subtle changes in the UV-Vis spectrum upon a reduction in pH. 3,5,6 The alkyl-chain functionalised PBIs in contrast show a quite dramatic change in the UV-Vis absorption and fluorescence spectrum upon decreasing the temperature of a hot solution.…”

Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

“…revealed photoconductivity of PDI thin films could be dramatically altered by controlling the extent of H and J aggregation of chromphores. 30 Our group recently developed amphiphilic PMI molecules that readily self-assemble in water to form ribbon-shaped supramolecular assemblies. 31 The molecules stack in an antiparallel fashion within the cross-section of the ribbons, allowing for strong interactions among molecular dipoles.…”