Electrochemistry: A technique to form, to modify and to characterize organic conducting polymers

The optical properties of doped columnar phases formed either by a 1 : 1 mixture of an electron donor [2,3,6,7,10,11-hexa-(n-pentyloxy)triphenylene] and an electron acceptor (2,4,7-trinitrofluoren-9-one) or by a dimer in which the donor (triphenylene moiety) and the acceptor (trifluorenone moiety) are covalently linked are reported and compared with those of the solutions. In concentrated solutions a charge transfer (CT) absorption is observed for both donor–aceptor pairs corresponding to 2.51 and 2.43 eV, respectively. For the 1 : 1 mixture, CT complex formation in solution is greatly favoured by the presence of molecular aggregates. The polarized absorption spectra obtained with oriented thin films reveal that the charge transfer occurs along the alternate donor–acceptor stacking.

Charge-transfer absorption in doped columnar liquid crystals

Side-chain liquid crystalline polymers with laterally attached mesogenic group were synthesized. Polymers having polyoxetane main chain were obtained by cationic polymerization, and with polymethacrylate main chain by radical polymerization. Some of the polymers showed nematic phase, and polyoxetanes having mesogen with electron-withdrawing and electron-donating groups in the molecule showed a little more discrete mesophase.

Synthesis and thermal transition of side‐chain liquid crystalline polyoxetanes having laterally attached mesogenic group

Charge-transfer complexes formed by a discogenic electron donor (2,3,6,7,10,11-hexa-n-pentyloxytriphenylene) and a non-discogenic electron acceptor (2,4,7-trinitrofluoren-9-one) in non-polar solvents are studied by picosecond time-resolved absorption spectroscopy based on Kerr ellipsometry. The transient spectra obtained following laser excitation either in the charge-transfer band or in the lower lying local acceptor band correspond to the oxidized donor and reduced acceptor. Recombination of the geminate ion pair occurs with a first order rate constant of 1.7 × 1010 s–1. It is shown that the polarization of the transient species absorption is orthogonal to the charge-transfer transition. It is also demonstrated that the relative orientation between the donor and the acceptor in the geminate ion pair is not random.

Charge-transfer complexes of discogenic molecules : a time-resolved study based on kerr ellipsometry

Pulsed deuteron NMR spectroscopy has been used to examine the axial motion and the side-chain conformation in both oriented and unoriented mesophases of discotic liquid crystal main-chain polymers based on pentyloxy or heptyloxy substituted triphenylenes. Lineshape simulations show that the rotational motion of the triphenylene rings about the column axes can be described by an inhomogeneous distribution of reorientation angles around 45°. However, only about 60% of the discs are involved in such large amplitude motions; the remaining 40% have reorientation angles below 10°. This illustrates the severe restrictions imposed on the rotation of the discs by the interlinkage of the columns via the alkylene spacers. Furthermore, the simulations demonstrate that, at the α-carbon, the side chains show very little fast internal motion, but have a relatively complex conformation involving a disorder which does not change on the microsecond timescale. Since such a disorder is not present in the corresponding monomeric samples it is ascribed to the presence of the spacers. These results are also consistent with the presence of large sterical hindrances between the first side chains segments of adjacent discs, and they indicate a correlated reorientation of the discotic units within a column.

Volker Brzezinski

Papers

Microstructure of a Discotic Polymer as Revealed by Electron Diffraction and High-Resolution Imaging