Electronic and vibronic properties of a discotic liquid-crystal and its charge transfer complex.

Discotic liquid crystalline (DLC) charge transfer (CT) complexes combine visible light absorption and rapid charge transfer characteristics, being favorable properties for photovoltaic (PV) applications. We present a detailed study of the electronic and vibrational properties of the prototypic 1:1 mixture of discotic 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) and 2,4,7-trinitro-9-fluorenone (TNF). It is shown that intermolecular charge transfer occurs in the ground state of the complex: a charge delocalization of about 10(-2) electron from the HAT6 core to TNF is deduced from both Raman and our previous NMR measurements [L. A. Haverkate, M. Zbiri, M. R. Johnson, B. Deme, H. J. M. de Groot, F. Lefeber, A. Kotlewski, S. J. Picken, F. M. Mulder, and G. J. Kearley, J. Phys. Chem. B 116, 13098 (2012)], implying the presence of permanent dipoles at the donor-acceptor interface. A combined analysis of density functional theory calculations, resonant Raman and UV-VIS absorption measurements indicate that fast relaxation occurs in the UV region due to intramolecular vibronic coupling of HAT6 quinoidal modes with lower lying electronic states. Relatively slower relaxation in the visible region the excited CT-band of the complex is also indicated, which likely involves motions of the TNF nitro groups. The fast quinoidal relaxation process in the hot UV band of HAT6 relates to pseudo-Jahn-Teller interactions in a single benzene unit, suggesting that the underlying vibronic coupling mechanism can be generic for polyaromatic hydrocarbons. Both the presence of ground state CT dipoles and relatively slow relaxation processes in the excited CT band can be relevant concerning the design of DLC based organic PV systems.

On the morphology of a discotic liquid crystalline charge transfer complex.

Discotic liquid crystalline (DLC) charge transfer (CT) complexes, which combine visible light absorption with rapid charge transfer characteristics within the CT complex, can have a great potential for photovoltaic applications when they can be made to self-assemble in a bulk heterojunction arrangement with separate channels for electron and hole conduction. However, the morphology of some liquid crystalline CT complexes has been under debate for many years. In particular, the liquid crystalline CT complex built from the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) and discotic molecules has been reported to have the TNF "sandwiched" either between the discotic molecules within the same column or between the columns within the aliphatic tails of the discotic molecules. We present a detailed structural study of the prototypic 1:1 mixture of the discotic 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6) and TNF. Nuclear magnetic resonance (NMR) line widths and cross-polarization rates are consistent with the picosecond time scale anisotropic thermal motions of the HAT6 and TNF molecules previously observed. By computational integration of Rietveld refinement analyses of neutron diffraction patterns with density experiments and short-range structural constraints from heteronuclear 2D NMR, we determine that the TNF molecules are vertically oriented between HAT6 columns. The data provide the insight that a morphology of separate hole conducting channels of HAT6 molecules can be realized in the liquid crystalline CT complex.

Conformation, defects, and dynamics of a discotic liquid crystal and their influence on charge transport.

Future applications of discotic liquid crystals (DLCs) in electronic devices depend on a marked improvement of their conductivity properties. We present a study of 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) and show how local conformation, structural defects, and thermal motions on the picosecond time scale strongly affect the efficient charge transport in DLCs. A direct and successful comparison of classical molecular dynamics (MD) simulations with both neutron powder diffraction and quasielastic neutron scattering (QENS) give a full insight into the structural and dynamical properties of HAT6. The local conformation of HAT6 molecules is characterized by a mutual rotation (twist) angle of about 37° and typically a mutual aromatic-core distance of 3.4 Å instead of the average distance of 3.65 Å usually quoted. We show that a considerable number of structural traps is present in HAT6, which persist at the picosecond time scale. We find that the high disorder in the mutual positions of the aromatic cores is an important factor contributing to the limited conductivity of HAT6 compared to larger DLCs.