Advanced materials for molecular electronic, optoelectronic and photonic applications associate several molecular segments with complementary functionalities in the same (macro)molecule. A typical molecular architecture involves one or several p-conjugated units conferring the semiconducting/photophysical properties, linkers that join and space these p-conjugated units, and peripheral units modifying the physical state and the properties of the material. Most often, linkers and peripheral units are constituted by aliphatic chains, but other nature of chains are also used: siloxane, oligo(ethylene oxide),… Such a molecular architecture combining "rigid" p-conjugated units and "soft" chains offers multiple advantages for the design of the functional materials:
Solubility and processability. Molecules consisting in the bare conjugated systems are often insoluble solids melting at high temperature, with limited possibilities of handling and immutable features. In contrast, the implementation of such conjugated systems in molecular architectures containing peripheral chains generates solubility in organic solvent and softening into soft matter states. Getting solubility is a major goal of functional material research, not least as it gives access to versatile, relatively low-cost and up-scalable film deposition techniques from solutions. The soft matter states can be used to modify the deposited films and improve the device performances through thermal treatment and mechanical alignment, in particular.
Structure. Rigid conjugated units and soft chains are intrinsically incompatible, so that identical segments from neighboring molecules self-assemble into domains by repelling the antagonistic segments into juxtaposed domains. The result of this nanosegregation process are structures in which the domains of self-assembled p-conjugated units form semiconducting pathways that are isolated from each other by the insulating chains. The charge transport along these pathways could be one-dimensional for thread-like p-conjugated domains surrounded by chains, two-dimensional for p-conjugated layers alternating with chains or three-dimensional for networks of p-conjugated domains intermingled with chains. The structure might be multisegregated if the molecular architecture involves different p-conjugated units that nanosegregate in individual domains. Within domains, the interactions of p-conjugated units, and in particular their spacing, overlapping and respective orientation are driven by the choice of the linkers and the peripheral chains. These structural features determine the efficiency of charge transport between neighboring p-conjugated units, and thus the semiconducting properties of the material. Photophysical properties are also affected: the close-stacking of p-conjugated units can for instance cause the quenching of luminescence properties.
Morphology. Thin films deposited from solutions most often exhibit a spontaneous orientation of the nanosegregated structure relatively to the film surface. This orientation is reproducible but it can be influenced by the conditions of film preparation and processing. Its impact on electrooptic device performances can be tremendous, especially for one- and two-dimensional semiconductors. Indeed, the charge transport is optimal when the semiconducting pathways join the electrodes of the device, i.e. when the pathways are vertical and the electrodes are disposed of both sides of film, or when the pathways are horizontal and the electrodes are contained in the thickness of the film. Conversely, improper orientation of pathways results in strongly degraded device performances, since the path towards electrodes is interrupted by insulating chains.
In summary, molecular architectures are characterized by their versatility and their multifaceted control over material performances. Due to the strong impact of structure and morphology on properties, their characterization is inescapable for the evaluation of new materials and device fabrication procedures. The research institutions of the STELORG consortium offer facilities and skills to study the structure and the self-assembly in the bulk state. For the investigation of thin film structure and morphology in device-close conditions, experimental projects grouping the needs of the consortium in a transversal topic are regularly submitted to a synchrotron facility and lead to one experimental session per year, in the average.
The prediction of the likely structure and morphology guides the molecular design and considerably fosters the development of new materials. Although the detailed self-assembly and the accurate structural parameters cannot be anticipated, valuable predictions can be made on the main characteristics of the self-assembly, as well as on the effect of individual molecular variations. The prediction methodology uses libraries of features of molecular segments and of self-assembly rules that were created and are continuously enriched from experimental studies on previous systems. Beyond the tailored development of molecular architectures, the collected data and methods are rationalized to deepen the knowledge on self-assembly processes and soft matter structures.
Contact: Benoit HEINRICH