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Organogelators and π-conjugated self-assembled systems

Organogelators and π-conjugated self-assembled systems

While a wealth of organogelators have been synthesized, their thermodynamics properties and the mechanisms of gelation in relation to the morphology/structure of the self-assemblies formed in solvents, are only marginally understood. In particular, establishing correlations between molecular structure of the gelating molecules and the shape/structure of the assemblies is an important issue. The group has shown that the interactions between esters in a series of analogue diamide-ester gelators are the most determining (see chemical structure in Fig.1.). The ester length controls the shape of the aggregates : flat ribbons, twisted ribbons or nanotubes and, in the last case, the diameter of the tubes. A second important contribution is the precise determination of a complete phase diagram for a model organogelator. The combination of DSC, rheology and OM, revealed the presence of a miscibility gap, underlining the inherent complexity of phase diagrams of organogelators. In the case of amino-acid gelators, a so-called “jamming” transition has been evidenced by rheological measurements to explain the gelation mechanism.[1]

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Fig.1.:From left to right : chemical structure of BHPB-10 ; Self-assembled
nanotubes formed by BHPB-10 (freeze fracture TEM) ; Temperature−concentration phase diagram for BHPB10/transdecalin for a cooling rate of −0.25 °C/min.

Finally, the group investigated the structure formation in pi-conjugated semiconductors based on perylene-bisimide (PBI). The group has shown that a PBI organogelator behaves as a reversible stimuli responsive material (Thesis A. Sarbu). TEM and GIXD studies demonstrated that the property modification results from a structural reorganization from columnar to supramolecular helical assemblies. [2]

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Fig.2. : Illustration of the structural reorganisation from a columnar to a helical stacking of a N,N’-substituted perylenebisimide organogelator, responsible for the change in optical absorption (form I : green and form II : red).

References :
[1] N. M. Sangeetha, et al. ACS Nano, 2012.Simon, F.-X. ; et al. Soft Matter 2013. Collin, D. et al. Soft Matter, 2013. Christ, E. ; et al. Langmuir 2016.
[2] A. Sarbu, et al. J. Mater. Chem.C, 2015.

Hydrogen bonded semiconductors

Securing the world’s population energy supply in a renewable way is one of the biggest challenges of our generation. Solar energy is a great alternative to solve fossil fuel exhaustion and climate change. Organic solar cells are attractive due to their light weight, flexibility and scalability, but several issues need to be solved to make this technology competitive. One of the main challenges is achieving the optimal active layer morphology. So far, none of the common materials used form the desired structures needed. Therefore, the search of new materials is necessary. In this project a library of hydrogen-bonded (HB) materials is proposed since recently, the use of HB semiconductors has shown 50% improvement in device efficiency.[1, 2] However, this is just the tip of the iceberg because only one HB unit was used. A study of the HB strength, number and position within the molecular structure is proposed to explore the role of self-assembly in device morphology and final efficiency.

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1. Aytun, T., Barreda, L., Ruiz-Carretero, A., Lehrman, J. A. & Stupp, S. I. Improving Solar Cell Efficiency through Hydrogen Bonding : A Method for Tuning Active Layer Morphology. Chem. Mater. 27, 1201–1209 (2015).
2. Ruiz-Carretero, A. et al. Stepwise self-assembly to improve solar cell morphology. J. Mater. Chem. A 1, 11674 (2013).