|Physics Department | Center for Photonics and Nanoelectronics | Lehigh University|
Figure 1: A single rubrene molecule with defined molcular axes (top.) Facets of single-crystal rubrene (bottom.)
Figure 2: A rubrene single crystal's ab plane
The optical absorption and photoluminescence spectra of organic molecular crystals depend on the optical properties of the molecules, on the way the molecules are arranged in the crystal matrix, and how the molecules interact with each other. The rubrene single crystal has a large optical anisotropy that has a strong influence on the absorption and luminescence spectra that are observed under different experimental conditions. Although transport properties of rubrene single crystals have been extensively studied, considerably fewer studies have explored their optical properties.
The rubrene molecule is a polycyclic aromatic hydrocarbon which has a backbone structure made of benzene rings. Rubrene is different from other organic molecules such as anthracene, tetracene, and pentacene because it has phenyl side groups (see figure 1.).
Rubrene crystals can be grown using vapor transport method. Rubrene molecules are sublimated inside a 1 inch diameter fused silica tube, where pure argon gas is made to flow, at a low rate, through a temperature gradient that slowly allows the walls of the silica tube to cool, causing the vapor-phase molecules to assemble into a crystalline form. Rubrene crystals have an orthorhombic unit cell with lattice parameters a = 14.4A, b = 7.18A, and c = 26.9A (see Fig. 3.) The most common crystal shapes are platelets with extended surfaces perpendicular to the c-direction and crystals elongated in the b direction but with short dimensions in the a and c directions. As-grown rubrene crystals have facets that form a typical angle of 63.5 degrees to the b axis when observing the ab plane, and 75 degrees with respect to the b axis from the bc plane (see figure 3.)
Figure 3: Two facets of the rubrene single crystal, abplane view seen top left, and bc plane on top right. Molecular packing structure of rubrene is seen to be a herring bone pattern (bottom image.)
A major difference between rubrene and tetracene crystals is that is the molecular arrangement: In rubrene rubrene every single molecule in the crystal is oriented in such away that the tetracene backbone has its short axis (The 'M' direction in Fig. 1) parallel to the crystallographic c-axis, which is the axis perpendicular to the plane of the drawing in Fig. 3. Combine this with the fact that selection rules based on molecular symmetries tell us that photon absorption and emission with no phonon involvement is only allowed for light polarized along this 'M' direction, and you have a recipe for enormous anisotropy of optical properties. With both absorption and emission of c-polarized light very strong and dominant.
This molecular orientation is just one of may compelling properties of rubrene crystals. Rubrene has a high room-temperature charge carrier mobility for an organic crystal (10 - 40 cm^2/Vs for holes in field-effect transistors) and a high photoconductivity (which is actually caused by triplet exciton dissociation!). The high hole mobility values in rubrene crystals are found along the b axis, and we have found long-range diffusion of triplet excitons in the same direction. This can be explained by the fact that the rubrene molecules form molecular columns along the b direction in which the herringbone pattern has efficient pi-orbital overlap.
List of our group's rubrene-related publications:
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