  Discotic liquid crystalline (DLC) charge transfer (CT) complexes combine
visible light absorption and rapid charge transfer characteristics within the
CT complex, 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 groundstate 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 (Reference 32), 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
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 CT dipoles and relatively slow
relaxation processes in the CT band can be relevant concerning the design of
DLC based organic PV systems.
