Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

Abstract

Supramolecular light-absorbing nanostructures are useful building blocks for the design of next-generation artificial photosynthetic systems. Development of such systems requires a detailed understanding of how molecular packing influences the material’s optoelectronic properties. We describe a series of crystalline supramolecular nanostructures in which the substituents on their monomeric units strongly affects morphology, ordering kinetics, and exciton behavior. By designing constitutionally-isomeric perylene monoimide (PMI) amphiphiles, the effect of side chain sterics on nanostructure crystallization was studied. Molecules with short amine linked alkyl-tails rapidly crystallize upon dissolution in water, while bulkier tails require the addition of salt to screen electrostatic repulsion and annealing to drive crystallization. A PMI monomer bearing a 3-pentylamine tail was found to possess a unique structure that results in strongly red-shifted absorbance, indicative of charge-transfer exciton formation. This particular supramolecular structure was found to have an enhanced ability to photosensitize a thiomolybdate, [(NH4)2Mo3S13], catalyst to generate hydrogen gas.

Supramolecular Energy Materials

“…When dissolving PMI‐L5 in aqueous media, supramolecular ribbon‐shaped polymers with nanoscale dimensions were found to have widths that can be enhanced with thermal annealing (≈80 °C) and the addition of NaCl (50 × 10 −3 m ) ( Figure ). [ 9c,59 ] The crystallization process is caused by charge screening of the deprotonated carboxylate anions with additive polyelectrolytes or by the addition of NaCl—an effect known from natural stabilization of charged protein surfaces. [ 60 ] Repulsive interactions on the outer ribbon surface bearing a high surface‐charge density are neutralized by a gradual layer of solvated counter ions and thereby allow dense packing of the hydrophobic cores.…”

“…By employing a combination of chemical derivatization, spectroscopy, X‐ray scattering, rheology, and microscopy techniques, it was possible to elucidate several design rules essential for highly efficient photocatalysis based on supramolecular hydrogels. The structural features of the screening included, 1) a variation of the linker length of the alkyl spacer between the hydrophilic head group and the π‐system, [ 9a ] 2) installation of sterically demanding groups at the 9‐position of PMI‐L5, [ 59 ] 3) tuning the molecular dipole moment and frontier orbital energy levels by various electron‐withdrawing and electron‐donating groups at the 9‐position, [ 9d,e ] 4) variations in the crystal phase of the assembly of the same derivative, [ 9c ] and 5) in‐depth analysis of the crystal packing and unit cell within the supramolecular ribbons. [ 9b ] The photocatalytic system has further been optimized by introducing other metal catalysts into the self‐assembled hydrogels, such as Na 2 Mo 3 S 13 [ 9b–e,59 ] for efficient hydrogen production or an Fe‐porphyrin [ 9e ] derivative for CO 2 reduction.…”

“…The impact of molecular packing within soluble nanostructures and the resulting opto‐electronic properties were further revealed in a remarkable series of 9‐( N ‐alkyl)‐PMI‐L5 derivatives. [ 59 ] The alkyl residues, introduced by Buchwald–Hartwig coupling, [ 66 ] were selected to have varying sterical demand and thereby influence crystal packing while having minimum electronic influence on the chromophore unit. In fact, unsubsituted PMI‐L5 shows an antiparallel slip‐stacked dimer in the single‐crystal phase because of the absence of any steric disturbance by substituents ( Figure a).…”

Impact of charge switching stimuli on supramolecular perylene monoimide assemblies

“…6,39,40 In contrast, the absorbance spectrum when the hydroxyl group is deprotonated (1 0 ) displays a broad peak (l max 643 nm) which resembles previously reported non-crystalline PMI nanostructures. 6,43 The pK a of the hydroxyl group was determined to be $9 by tracking the relative absorbance at 460 and 643 nm aer incubating 1 in various buffers with pH values ranging from 3 to 11 (ESI Fig. 5 †).…”

“…This represents a rather modest efficiency for a PMI based system due to the inability of compound 1 to support charge transfer exciton formation, a critical component for efficient photocatalysis. 39,40,43,50 Photosensitization was also tested using a recently developed water-soluble iron porphyrin (Fe-p-TMA) catalyst. 51 Fe-p-TMA has been shown to electrochemically reduce CO 2 to CO at potentials as low as À1.1 volts (SCE) in solutions with pH 7 and reduce protons to H 2 in acidic media.…”