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Liquid crystals

Also known as the “fourth state of matter”, the liquid crystal phase (mesophase) is present between the solid and liquid phases and shares some of the properties of both states of matter. In liquids, the molecules exist in a highly disordered state in which all molecular directions are equivalent and therefore no order is detected (isotropic). As we move towards the highly ordered crystal lattice, the molecules become completely anisotropic and arrange themselves in a fixed order and/or position. In a liquid crystal (LC), the molecules will arrange themselves due to anisotropic interactions to give a phase that presents some degree of order either positional or orientational whilst preserving its fluidity.

Liquid crystals can be divided into two main groups; thermotropic and lyotropic. Thermotropic LC’s display a stable mesophase at a specific temperature range, whilst lyotropic mesophases only occur upon the addition of a solvent to the LC material. The thermotropic LC’s can be sub-classified into calamitic and discotic liquid crystals depending of the shape of the molecule. As shown in the figure (left), particular examples of phthalocyanines are known to be discotic mesogens. The most commonly known mesogenic Pc materials are composed of an electron-rich aromatic core with aliphatic chains attached to the periphery. These mesogens were first investigated in the 1980’s, when it was discovered that the flexible chains incorporated led to an  enhanced solubility in most organic solvents and most importantly promoted columnar-type LC formation.

In contrast,calamitic-type LCs show more diversity in their self-assembled phases; they can display either nematic, smectic and cholesteric phases with each one allowing different properties to emerge (optical anisotropy, ferroelectricity, chirality, etc.). In organic electronic applications, the arrangement of the LC materials is determining for the charge mobility. Columnar mesophases (bottom left) show only one-dimensional charge transport, whils smectic-type mesophases are able to conduct the charges in two-dimensions (bottom right).

During my postdoctoral post I worked mostly on discotic mesogens for OPV applications (read more here). A second part of my project concerned the preparation of donor-acceptor liquid crystalline blends using perfluorinated  Pcs and a fullerene derivative (PCBM).  We found that by incorporating the perfluoroalkyl groups in both donor (Pc) and acceptor (fullerene derivative) we are able to control better the self-assembly of the mixtures by exploiting the fluorophobic and fluorophilic interactions which occur within the liquid crystalline donor-acceptor array. In this case, intermolecular (fluorophilic/phobic) interactions were observed to prompt a stabilization of the Colh mesophase and allowed the incorporation of up to 25 molar% of the fullerene acceptor; a two-fold increase in comparison to present alkylated Pc analogues.

Other mesogenic blends studied were triphenylene-gold nanorod composites (collab. T. Hegmann). These were analysed exhaustively by POM (see below) XRD and DSC to investigate the miscibility and possible alignment induced by the discotic-triphenylene mesophase to the GNRs. I also measured their charge transport properties, which evidenced that the insertion of the GNRs ( 1% wt) induces a tighter packing of the host mesogen with a better alignment, and as a result  displays improved charge mobility.

Textures observed under POM (left-open and right-cross polariser)of the thin films of un-doped H4TP (a), DLC-GNR/H4TP 1wt% (b) and DLC-GNR/H4TP 2wt% (c) spontaneous homeotropic alignment can be observed clearly for the 1wt% mixture.

I also studied covalently linked, donor-acceptor discotic mesogens; based on perylene-triphenylene units linked by aliphatic chains.  These dyad (perylene-triphenylene) and triad (perylene-triphenylene-perylene) derivatives were expected to show nano-segregated columnar structures, and had the final aim to produce ambipolar charge transport materials. The desired nano-segregated structure was achieved by the triad molecule only and unfortunately, the mobility measured was not ambipolar.

Each one of these examples demonstrate the potential that molecular design has, to steer and control (to a certain point) the nano-structuration of organic semiconductive materials. Nevertheless, we are still unable to achieve practical and highly-efficient materials based on LCs for optoelectronic applications.

 

PUBLICATIONS on this TOPIC:

 Self-organization behavior of Donor-Linker-Acceptor Dyads and Triads Based on Triphenylene and Perylene Diimide Cores as a Template for Nanostructured Semiconducting Materials, Y. Xiao, X. Su, L. Sosa-Vargas, E. Lacaze, B. Heinrich, B. Donnio, D. Kreher, F. Mathevet, A.-J. Attias, Cryst. Eng.Comm, 201618, 4787-4798.

Solvent effect on columnar formation in solar-cell geometryJ. H. Park ; L. Sosa-Vargas ; Y. Takanishi ; K. H. Kim ; Y. S. Kim ; Y. W. Park ; J. Yamamoto ; M. Labardi ; J. P. F. Lagerwall ; Y. Shimizu ; G. Scalia; Proc. SPIE 9769, 2016, Emerging Liquid Crystal Technologies XI, 97690T .

Liquid crystalline and charge transport properties of novel non-peripherally octasubstituted perfluoroalkylated phthalocyanines L. Sosa-Vargas, F. Nekelson, D. Okuda, M. Takahashi, Y. Matsuda, Q. Dao, Y. Hiroyuki, A. Fujii, M. Ozaki and Y. Shimizu, J. Mater. Chem. C. 20153, 1757-1765.

Discotic liquid crystal functionalized gold nanorods: 2- and 3D self-assembly and macroscopic alignment as well as increased charge carrier mobility in hexagonal columnar liquid crystal hosts affected by molecular packing and π- π interactions, X. Feng, L. Sosa-Vargas, S. Umadevi, T. Mori, Y. Shimizu and T. Hegmann,  Adv. Funct. Mater. 20158, 1161.

Miscibility and Phase Separation in LC Semiconductor Blends, Y. Shimizu, Y. Matsuda, T. Nakao, L. Sosa-Vargas, M. Takahashi, H. Yoshida, A. Fujii, M. Ozaki, Proc. SPIE 9004, Emerging Liquid Crystal Technologies IX2014, 900408.

 

Shimizu group 2012          UBIQEN-AIST Kansai

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