The low, medium and high concentration luminescence and luminescence-excitation spectra for alkyl substituted hexa-peri-hexabenzocoronene (HBC-C8,2) and hexa(4-n-dodecylphenyl) substituted hexa-peri-hexabenzocoronene (HBC-PhC12) are presented. A study of the concentration dependence of the optical properties of these self-assembling molecular nanowires, in the low to medium concentration regime, associates the spectrum at ~ 10-13 M with the single molecule, and indicates that previously published spectra of HBC's by others were the product of aggregation phenomena. The insertion of an exo-phenyl group between the HBC core and the alkyl side chains, as in HBC-PhC12, was found not to extend the conjugation, but did increase the inhomogeneous broadening of the single molecule luminescence. The continued presence of HBC-PhC12 single molecules, at high concentration, implies that HBC-C8,2 aggregates are thermodynamically more stable than HBC-PhC12 aggregates. In conclusion, the spectroscopic properties of both derivatives were found to be very sensitive to aggregation at low concentration and strongly correlated to the observed macroscopic physical properties.
Thomas Hassheider, Stephan Benning, Marcus Lauhof, Reinhold Oesterhaus, Sonia Alibert-Fouet, Harald Bock, John Goodby, Mark Watson, Klaus Muellen, Heinz-Siegfried Kitzerow
Columnar liquid crystals are useful for the construction of organic light emitting diodes (OLED) due to their high charge carrier mobility and their ability of developing defect-free superstructures. We investigated the electroluminescent properties of various mesogenic derivatives of triphenylene, pyrene, perylene, benzoperylene and hexa-peri-hexabenzocoronene (HBC) which combine good charge carrying properties and luminescence. The wavelengths of maximum electroluminescence range from about 400 nm to 720 nm, depending on the core and the substitution of the molecules. Nearly white emission can be obtained in samples containing two or more emitting layers. The perylene core is so efficient that even a single organic layer between an ITO anode and an aluminum cathode can give luminance values up to 100 cd/m2. In addition to the electroluminescence, we present results on the photovoltaics in hetero junction structures of the compounds. Samples with two organic layers made of a HBC- and a perylene derivative show an open circuit voltage Voc of about 0.8 V, a short circuit current Isc of a few μA/cm2, and a filling factor FF of about 31 %.
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