Precise control of the spatial organization of nanoscopic building blocks, such as nanoparticles, over multiple length scales is a bottleneck in the 'bottom-up' generation of technologically important materials. Only a few approaches have been shown to achieve nanoparticle assemblies without surface modification. We demonstrate a simple yet versatile approach to produce stimuli-responsive hierarchical assemblies of readily available nanoparticles by combining small molecules and block copolymers. Organization of nanoparticles into one-, two- and three-dimensional arrays with controlled inter-particle separation and ordering is achieved without chemical modification of either the nanoparticles or block copolymers. Nanocomposites responsive to heat and light are demonstrated, where the spatial distribution of the nanoparticles can be varied by exposure to heat or light or changing the local environment. The approach described is applicable to a wide range of nanoparticles and compatible with existing fabrication processes, thereby enabling a non-disruptive approach for the generation of functional devices.

Nanocomposites, composed of organic and inorganic building blocks, can combine the properties from the parent constituents and generate new properties to meet current and future demands in functional materials. Recent developments in nanoparticle synthesis provide a plethora of inorganic building blocks, building the foundation for constructing hybrid nanocomposites with unlimited possibilities. The properties of nanocomposite materials depend not only on those of individual building blocks but also on their spatial organization at different length scales. Block copolymers, which microphase separate into various nanostructures, have shown their potential for organizing inorganic nanoparticles in bulk/thin films. Block copolymer-based supramolecules further provide more versatile routes to control spatial arrangement of the nanoparticles over multiple length scales. This review provides an overview of recent efforts to control the hierarchical assemblies in block copolymer-based hybrid nanocomposites.

Small organic molecules with strong intermolecular interactions have a wide range of desirable optical and electronic properties and rich phase behaviors. Incorporating them into block copolymer (BCP)-based supramolecules opens new routes to generate functional responsive materials. Using oligothiophene-containing supramolecules, we present systematic studies of critical thermodynamic parameters and kinetic pathway that govern the coassemblies of BCP and strongly interacting small molecules. A number of potentially useful morphologies for optoelectronic materials, including a nanoscopic network of oligothiophene and nanoscopic crystalline lamellae, were obtained by varying the assembly pathway. Hierarchical coassemblies of oligothiophene and BCP, rather than macrophase separation, can be obtained. Crystallization of the oligothiophene not only induces chain stretching of the BCP block the oligothiophene is hydrogen bonded to but also changes the conformation of the other BCP coil block. This leads to an over 70% change in the BCP periodicity (e.g., from 31 to 53 nm) as the oligothiophene changes from a melt to a crystalline state, which provides access to a large BCP periodicity using fairly low molecular weight BCP. The present studies have demonstrated the experimental feasibility of generating thermoresponsive materials that convert heat into mechanical energy. Incorporating strongly interacting small molecules into BCP supramolecules effectively increases the BCP periodicity and may also open new opportunities to tailor their optical properties without the need for high molecular weight BCP.

Organic small molecule semiconductors have many advantages over their polymer analogues. However, to fabricate organic semiconductor-based devices using solution processing, it is requisite to eliminate dewetting to ensure film uniformity and desirable to assemble nanoscopic features with tailored macroscopic alignment without compromising their electronic properties. To this end, we present a modular supramolecular approach. A quaterthiophene organic semiconductor is attached to the side chains of poly(4-vinylpyridine) via noncovalent hydrogen bonds to form supramolecular assemblies that act as p-type semiconductors in field-effect transistors. In thin films, the quaterthiophenes can be readily assembled into microdomains, tens of nanometers in size, oriented normal to the surface. The supramolecules exhibited the same field-effect mobilities as that of the quaterthiophene alone (10−4 cm2/(V·s)). Since the organic semiconductors can be readily substituted, this modular supramolecular approach is a viable method for the fabrication of functional, nanostructured organic semiconductor films using solution processing.

We fabricate a field-effect transistor by covalently functionalizing PbS nanoparticles with tetrathiafulvalenetetracarboxylate. Following experimental results from cyclic voltammetry and ambient-pressure X-ray photoelectron spectroscopy, we postulate a near-resonant alignment of the PbS 1Sh state and the organic HOMO, which is confirmed by atomistic calculations. Considering the large width of interparticle spacing, we observe an abnormally high field-effect hole mobility, which we attribute to the postulated resonance. In contrast to nanoparticle devices coupled through common short-chained ligands, our system maintains a large degree of macroscopic order as revealed by X-ray scattering. This provides a different approach to the design of hybrid organic-inorganic nanomaterials, circumvents the problem of phase segregation, and holds for versatile ways to design ordered, coupled nanoparticle thin films.

Block copolymer (BCP)-based supramolecular systems provide a versatile approach to manipulate functional structures spanning several nanometers to macroscopic length scales. Most studies to date focused on supramolecules containing asymmetrically end-functionalized small molecules, and it remains challenging to obtain molecular control over small molecule ordering within the BCP microdomain. Here we designed symmetrically end-functionalized bis-phenol quarter­thiophene (BP4T) small molecules and systematically investigated how the end-group chemistry of the small molecules affects the supramolecular assembly process and the resulting morphology. Bifunctionalized small molecules can bridge two adjacent polymer blocks and lead to macroscopically aligned hierarchical assemblies at much higher degree of ordering than previously observed for asymmetrically functionalized small molecule analogues. The supramolecular morphology is very sensitive to the stoichiometry between the BP4T and polymer repeat unit because of the specific molecular organization within BCP microdomain. Furthermore, similar thermoresponsiveness of supramolecule, i.e., ∼40% change in the supramolecular periodicity during the heating and cooling cycles, can be obtained at BP4T loading stoichiometry of 0.5, much smaller than that of asymmetrically functionalized small molecule. These results clearly demonstrate that supramolecular assemblies can be readily manipulated by engineering the small molecule chemistry. Present studies provide basic design principles and an effective route to fabricate well-defined hierarchical assemblies for functional and stimuli-responsive nanomaterials.

Using organic semiconductor-containing supramolecule, this study investigates how the small molecule (SM) phase behavior as well as the loading rate and size of the NPs can affect their coassembly in supramolecular frameworks containing strongly interacting molecules. The NPs can be incorporated into the supramolecular microdomains even from a macrophase-separated state. However, the assembly of nanocomposites based on strongly interacting SMs is found to be distinct from both supramolecular nanocomposites based on simple alkyls and supramolecules based on strongly interacting SMs without NPs. The strong crystallization of the SMs tends to force the NPs into less crystalline domains of the comb block, where the SMs are less abundant. The larger NPs favor assembly in the center of the comb domains, parallel to the BCP lamellar axis, to minimize the entropic penalty of deforming the polymer chain, while the smaller particles assemble into rows in between the comb lamellae, perpendicular to the axis of the BCP lamellae. These studies present a versatile method for the coassembly of strongly interacting, functional SMs with NPs for the fabrication of nanocomposite devices.

Small molecules (SMs) with unique optical or electronic properties provide an opportunity to incorporate functionality into block copolymer (BCP)-based supramolecules. However, the assembly of supramolecules based on these highly crystalline molecules differs from their less crystalline counterparts. Here, two families of organic semiconductor SMs are investigated, where the composition of the crystalline core, the location (side- vs end-functionalization) of the alkyl solubilizing groups, and the constitution (branched vs linear) of the alkyl groups are varied. With these SMs, we present a systematic study of how the phase behavior of the SMs affects the overall assembly of these organic semiconductor-based supramolecules. The incorporation of SMs has a large effect on the interfacial curvature, the supramolecular periodicity, and the overall supramolecular morphology. The crystal packing of the SM within the supramolecule does not necessarily lead to the assembly of the comb block within the BCP microdomains, as is normally observed for alkyl-containing supramolecules. An unusual lamellar morphology with a wavy interface between the microdomains is observed due to changes in the packing structure of the small molecule within BCP microdomains. Since the supramolecular approach is modular and small molecules can be readily switched out, present studies provide useful guidance toward access supramolecular assemblies over several length scales using optically active and semiconducting small molecules.