The present paper deals with the photophysical properties of columnar liquid crystals formed by hexakis-(alky1oxy)triphenylenes. Absorption and fluorescence spectra of solutions are analyzed on the basis of quantum chemical calculations performed by the CS-INDO-CI (conformations spectra-intermediate neglect of differential overlap-configuration interaction) m e t h d . the absorption maximum is due to the SO -Sq transition while fluorescence originates from the weak SO -S1 transition. In columnar aggregates, the former transition corresponds to delocalized excited states while the latter corresponds to localized ones; calculation of intermolecular interactions shows that, at the temperature domain of the mesophases, all the molecules have the same excitation energy and, therefore, no spectral diffusion of the fluorescence is expected, in agreement with the time-resolved emission spectra. Excitation transfer is investigated by studying the fluorescence decays of mesophases doped with energy traps. Their analysis is made by means of Monte Carlo simulations considering both intracolumnar and intercolumnar jumps and using four different models for the distance dependence of the hopping probability. The best description is obtained with a model based on the extended dipole approximation and taking into account molecular orientation.
X-ray and atomic force microscopy observations reveal features of
the surface morphology of Langmuir−Blodgett (LB) films from discotic donor−acceptor molecules with strong
core-to-core interactions caused
by a charge-transfer (CT) complexation. The compounds are designed
to be twin and triple molecules
composed of chemically connected donor (triphenylene) and
acceptor (trinitrofluorenone) fragments which
display liquid crystalline structure in the bulk state. The LB
films from these compounds possess edge-on
orientation of molecules within monolayers with a smooth surface on a
submicron scale but with substantial
macroscopic imperfections. These imperfections are represented by
the micron size fractures and ruptures
apparently caused by local deformations and tensions during transfer of
the discotic monolayers from the
air−water interface to a solid substrate. It is suggested that
this is due to the excessive rigidity of the
monolayers caused by the stiffness of the columns with positional
intracolumnar ordering enhanced by
the strong intracolumnar CT interactions.
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