Self-Assembly of Light-Harvesting Crystalline Nanosheets in Aqueous Media

A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV–vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).



They have designed a family of amphiphilic PDI derivatives that self-assemble into crystalline 2D structures in aqueous solutions. This assembly motif is generated by hierarchical mode of two distinct hydrophobic interaction types induced by an aromatic core and alkyl groups, suggesting a simple design strategy to obtain crystalline organic assemblies in water. The self-assembled materials largely preserve their ordered structure in the dry state. Assemblies based on 2 and 3 show promising light-harvesting characteristics, with 2 exhibiting exciton diffusion comparable to solid crystalline films. The light absorption properties, ordered 2D morphology and nanoscale thickness of the nanosheets appear to be useful for fabrication of light-harvesting systems. Remarkably, relatively simple molecular systems can be designed to undergo self-assembly into extended ordered assemblies with advantageous photonic properties using water as an assembling medium. The ability to rationally design and efficiently assemble aromatic crystalline systems in aqueous media may lead to water-based photonic and electronic materials employing facile, cost-efficient, and environmentally friendly fabrication and processing.

Reference:
Self-Assembly of Light-Harvesting Crystalline Nanosheets in Aqueous Media
Chen Shahar, Jonathan Baram, Yaron Tidhar, Haim Weissman, Sidney R. Cohen, Iddo Pinkas, and Boris Rybtchinski
ACS Nano Article ASAP