title: Electronic and vibronic properties of a discotic liquid-crystal and its charge transfer complex.
abstract: 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.
authors.affiliation: Reactor Institute Delft, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands.
authors.forename: Lucas A
authors.initials: LA
authors.lastname: Haverkate
authors.affiliation: Institut Laue Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Zbiri
authors.affiliation: Institut Laue Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Johnson
authors.affiliation: Vibrational Spectroscopy Facility, School of Chemistry, The University of Sydney, NSW 2008, Australia.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Carter
authors.affiliation: ChemE-NSM, Faculty of Chemistry, Delft University of Technology, 2628BL∕136 Delft, The Netherlands.
authors.forename: Arek
authors.initials: A
authors.lastname: Kotlewski
authors.affiliation: ChemE-NSM, Faculty of Chemistry, Delft University of Technology, 2628BL∕136 Delft, The Netherlands.
authors.forename: S
authors.initials: S
authors.lastname: Picken
authors.affiliation: Reactor Institute Delft, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands.
authors.forename: Fokko M
authors.initials: FM
authors.lastname: Mulder
authors.affiliation: Bragg Institute, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia.
authors.forename: Gordon J
authors.initials: GJ
authors.lastname: Kearley