The following information is supplemental data for J. R. Weinberg-Wolf, L. E. McNeil,
Shubin Liu, Christian Kloc, "Evidence of low intermolecular coupling in rubrene single crystals
by Raman spectroscopy," Journal of Physics--Condensed Matter 19, 276204 (2007). This work is supported by the National Science Foundation under grant # DMR-0505773.
Introduction:
Since the first organic light-emitting device (OLED) was
successfully fabricated using 8-hydroxyquinoline aluminum (Al3)
in 1987[1], interest in the optical properties of
molecular crystals for improving existing electronic devices has
grown. The first single-crystal organic semiconductor field-effect
transistor (FET)[2] (which used
alpha-hexathiophene) followed soon after, opening the door for the
development of a new commercial product: all-organic displays based
on organic light-emitting diodes and organic field-effect
transistors. Compared to liquid crystal display (LCD) technology,
organic transistors and discrete LED displays hold the potential for
devices with improved characteristics including lower power
requirements, better resolution, more mechanical flexibility, and
lower production costs (to name just a few benefits).
Research to date has focused on two distinct directions:
semiconducting polymers and organic small molecules. The former may
have the advantage of higher stability for practical applications,
but the latter, due to the feasibility of forming large single
crystals, seems to be more suitable for basic science studies.
Materials specifically composed of polycyclic aromatic compounds
such as the pi-conjugated oligothiophenes, oligoacenes and their
derivatives are of particular interest. These small molecules with
high levels of conjugation are particularly appealing as the typical
highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO)
separation is in the visible range. In the crystal state, these
materials have optical transitions in the visual range (important
for optical device applications) and can be tailored for specific
applications by modifications of the molecular structure (through
chemical substitution and or the addition of side groups).
Consequently, they can be used for a host of photonic devices. Some
of these molecules are also very stable, another requirement for a
successful device.
[1] C.W. Tang and S.A. VanSlyke, Applied Physics Letters 51,
913 (1987).
[2] G. Horowitz, F. Garnier, A. Yassar, R. Hajlaoui, and F. Kouki,
Advanced Materials 8, 52 (1996).
Computer calculations:
These calculations were performed using GAUSSIAN 03 . The
Hartree-Fock method was used to do a structural optimization and the
density functional theory (DFT) B3LYP method was used to calculate
the Raman frequencies, both with the 6-31G* basis set. The
calculation with GAUSSIAN 03 was repeated with both the
structural optimization and the frequency simulation done with the
DFT B3LYP method and the 6-31G9(d) basis set. All calculations were
performed on an SGI Origin 3800 with 64 CPUs and 128 GB memory
running the IRIX 6.5 OS.
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