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External Stimuli Triggered Self-assembly of Dynamic Nanomaterials

Periodic Reporting for period 1 - ESTIMABLeNANO (External Stimuli Triggered Self-assembly of Dynamic Nanomaterials)

Reporting period: 2016-04-01 to 2018-03-31

"Hybrid nanostructures comprising semiconductor and plasmonic metal components have emerged as on of the most promising materials for next generation (photo)catalysis, optoelectronics and nanophotonics. One of the most promising way to achieve such complex systems are self-assembly processes utilizing supramolecular systems by which the individual chemical entities can be organized stepwisely into more complex matter via weak and specific yet dynamic interactions.

The original assembly approach developed within the project provided constructs with well-defined geometries that allowed to provide light in a highly controllable manner leading to model tunable systems for tracking of light induced charge-transfer processes both in colloids as well as in the solid state. Thereby, the research within the project paved the way to novel nanoparticulate constructs with potential application in emerging technologies.

Research carried out within the project allowed for the first time for incorporation of semiconducting NCs into host–guest chemistry of cucurbit[n]urils (CB[n]s), rigid macrocycles which appeared as one of the most promising self-assembly motifs acting here as: (i) molecular ""handcuffs"" enabling ultra-fine nanoengineering of NC interfaces tethering reversibly a variety of chemical entities and (ii) molecular nanojuctions bridging nanoparticulate constituents to form hybrid inorganic-organic nanomaterials providing high control over morphology of interstitial spaces (Figure 1).

The presented project demonstrated that photoactive semiconducting NCs can serve as photo‑switchable nanoparticulate building blocks for the rich host‑guest chemistry of CB[n]. The dynamic interfaces of semiconducting NCs were successfully modified in a controllable manner through the use of CB[n] as a binding motif resulting in interfacial supramolecular nanosystems, whose assembly can be controlled either by classical chemical factors or through favorable interaction with light."
"The main goal of this project comprised the development of multi-stimuli responsive supramolecular systems based on nanocrystals (NCs) and their subsequent use in the controlled assembly of complex organic-inorganic architectures and reversible modulation of their properties. This was accomplished by employing one of the most promising self-assembly motifs based on the barrel-shaped cyclic oligomers of cucurbit[n]urils (CB[n]s) that can bind within their cavity external stimuli (ES) responsive molecules, i.e. derivatives of viologen (MV) and/or photochromic guest molecules displaying light-control binding affinity for CB[n] molecular cavity. Particular attention was paid to the novel concept of self-assembly of inorganic building blocks and modulation of their interfaces triggered by photo-induced reduction of electron-accepting derivatives localized on the NC' surfaces.

The research strategy has been carefully designed to proceed in a stepwise manner and divided into four Work Packages (WP1-WP4). The first WP involved the synthesis, purification and characterization of a variety of metal and metal chalcogenide NCs. Within this part of the project a variety of selected NCs were synthesized, functionalized and structurally characterized. These NCs served further as nanostructured building blocks for investigation of assembly processes in colloids (WP2 and WP3) as well as nanofabrication of devices (WP4).

Within WP2, these well-characterized NCs where combined with monotopic ligands, i.e. short organic ligands or homopolymers both terminated by a variety of MV derivatives and/or second guest molecules, to achieve ES-controlled supramolecular discrete nanoparticulate assemblies in water. Research carried out in this part of the project allowed for the incorporation of semiconducting NCs into host–guest chemistry of cucurbit[n]urils (CB[n]s), as one of the most promising self-assembly motifs that can act as molecular ""handcuffs"" able to bind two various chemical entities leading in this way to more complex systems. The reversibility of these complex CB[n]–based organic–inorganic nanoarchitectures formation processes was achieved through classical chemical factors as well as by light and dioxygen (Figure 1).

The self-assembly of inorganic NCs into ES-responsive organic-inorganic hybrid networks was further evaluated in WP3. We demonstrated that semiconducting and metallic NCs can be brought together into hybrid homo- and heterogeneous nanosystems by simple aprotic and rigid binding motifs, cucurbit[n]urils (CB[n]). The CB[n]-mediated assembly provided an efficient way to well-defined nano-, micro- and macro-sized constructs.

Furthermore, the assembly processes developed within WP1, 2, and 3 provided the unique opportunity to study photo-induced electron transfer processes within the hybrid organic-inorganic nanosystems both in colloidal systems as well as in the solid state. The presented synthetic approach provided prototipical hybrid constructs for in situ tracking of redox processes at nanoscale in a water environment."
Hybrid nanostructures comprising semiconductor and plasmonic metal components have emerged as one of the most promising materials for next generation (photo)catalysis, optoelectronics and nanophotonics.
The original assembly approach developed within the project provided constructs with well-defined geometries that allowed to provide light in a highly controllable manner leading to model tunable systems for tracking of light induced charge-transfer processes both in colloids as well as in the solid state.

In addition to the fundamental insight of assembly of NCs mediated by CB[n] macrocycles and practical aspects brought up in within the project, there are several directions for which assembly approach presented is serving now as a beneficial tool. The on demand CB[n]-mediated assembly of semiconducting and plasmonic NCs within their colloidal suspensions can lead to highly porous hybrid scaffolds of extremely low density in which the dispersion of nanoparticulate building blocks can be controlled and permanent. These constructs are of great interest for complex heterogeneous catalysis and photovoltaics on account of their extended surface area and combined micro- meso- and macroporosity. On the other hand, the high affinity of CB[n]-based molecular junctions to semiconducting NCs provides the possibility of depositing of quantum dots on a variety of substrates of interesting geometries. This may open new possibilities for both (photo)catalysis as well as tracking of electron transfer in well defined spaces. Moreover, layer-by-layer supramolecular deposition of semiconducting NCs may enable straightforward preparation of nanofabricated devices. The rich host-guest chemistry of CB[n] macrocycles gives rise to confined hot-spots between NCs, where not only metal but now semiconducting nanoparticles as well can be involved. Furthermore, the combination of pre-aggregated clusters based on NCs may open an unique pathway to complex hybrid anisotropic structures, that are of great interest of hybrid catalysis and artificial photosynthesis, where two or more photocatalysts can be held together in a dynamic manner.
Figure 1