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David Kreher, assistant professor

Conjugated liquid crystal polymers for organic electronics, controlled free radical polymerizations for Non Linear Optics (NLO) applications...

The self-organization of p-conjugated organic materials forming highly ordered supramolecular architectures has been extensively investigated in the last two decades in view of optoelectronic applications(i). Indeed, the control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement of charge transport properties and achievement of high charge carrier mobilities. These wellordered materials are currently either self-organized semiconducting polymers(ii) or liquid crystals(iii). Indeed, on the one hand previous studies have shown that well-defined polymer architectures such as a regioregular poly(3-alkylthiophenes)(iv) promote self-organizations in a two-dimensional sheet-like lamellar structures due to the high planarity of the polymer chains.
More recently it has been reported that a semiconducting main-chain liquidcrystalline thieno[3,2-b]thiophene polymer exhibited enhanced charge-carrier mobility, when crystallized from the mesophase, due to the formation of large, well organized lamellar domains(v). On the other hand, it has been demonstrated that discotic mesogens self-assembling into columns can exhibit high charge carrier mobilities(vi). However the supramolecular architecture resulting from the phase separation at the nanoscale of two covalently linked p-conjugated systems able to self-assemble individually in a lamellar and a columnar nanostructure respectively, has never been investigated to the best of our knowledge.

In this context, we endeavored to investigate the self-organization of a side-chain liquid crystal (SCLC) semiconducting polymer where (i) the backbone is a p-conjugated polymer and (ii) the side groups are p-conjugated discotic mesogens. This way, we expected the. self-organization represented below.

 

Such materials and such organization could then be interested to improve the electronic characteristics of different devices, due to the fact that by surface treatment for example, we can expect to obtain in fine columns in two essential directions, either oriented parallele or perpidencular to a substrate (electrode), respectively to target field effect transistor or photovoltaics applications.

 

 

 

Here, we present our preliminary results on the design, synthesis, and structural characterization of a new side chain liquid crystal polymer based on a regioregular poly(3-decylthiophene) as polymer backbone postfunctionalized with discotic triphenylene moieties as side groups. This polymer was prepared according to the McCullough's procedure, an efficient method based on Grignard methathesis (GRIM) leading to well defined polymeric architectures, with a very narrow polydispersity and controlled molecular weights. Here is its chemical structure.

 

 

The thermal behavior of discotic side-chain polythiophene PTTn was examined by Polarized-light Optical Microscopy (POM), Differential Scanning Calorimetry study (DSC) and temperature-dependent small-angle Xray diffraction experiments. As a result we found show that this strategy leads to the supramolecular selfassembly of this SCLC semiconducting polymer in a peculiar lamello-columnar mesophase where a 2D oblique columnar lattice perforates a lamellar structure.
To our knowledge, it is the first First example of perforated lamello-columnar liquid crystal polymer with both a p-conjugated main chain and p-conjugated discotic side groups.

But back to the initial objective, we see that we don't obtain the expected self-organization in bulk. In a near future we will consequently play with different parameters (length of the polymer, dilution of the discs, length and type of the linker between the backbone and the disc, to try to observe lamello-lamellar organization, which would be better for organic electronics applications.

Moreover and in particular in view of incorporating them in field effect transistors to evaluate their mobility, we will try to modify either the backbone or the type of disc grafted, in order to target ambipolar materials, which means LC polymers able to carry both types of charges separately, electrons and holes (illustration below) intrinsically or after lightening.

 

  • (i) Special issue, Adv. Mater. 2006, 18, 1227.
  • (ii) Sirringhaus, H. et al., Nature 1999, 40, 685.
  • (iii) Sergeyev, S. et al., Chem. Soc. Rev. 2007, 36, 1902.
  • (iv) Osaka, I. et al., Acc. Chem. Res. 2008, 41, 1202.
  • (v) McCulloch, I et al., Nature Materials 2006, 5, 328.
  • (vi) Van de Craats, A. M. et al., Adv. Mater. 1999, 11, 1469.

 

Polymers : two principal projects

1. Novel Liquid Crystal Polymers for Organic Electronics
    The control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement
    of charge transport properties and achievement of high charge carrier mobilities...
2. Innovating Polymers for Non Linear Optics Applications
    Two years ago, we developed an approach based on Self-Doubled (SD) process to build organic lasers emitting in the blue-UV range
    of the spectrum, knowing that SD lasing procedure is the intracavity generation of a second harmonic signal by the active medium
    emitting the fundamental laser radiation...

 


Significant personal publications