Acenes can be thought of as one-dimensional strips of graphene and they have the potential to be used in the next generation of electronic devices. However, because acenes larger than pentacene have been found to be unstable, it was generally accepted that they would not be particularly useful materials under normal conditions. Here, we show that, by using a physical vapour-transport method, platelet-shaped crystals of hexacene can be prepared from a monoketone precursor. These crystals are stable in the dark for a long period of time under ambient conditions. In the crystal, the molecules are arranged in herringbone arrays, quite similar to that observed for pentacene. A field-effect transistor made using a single crystal of hexacene displayed a hole mobility significantly higher than that of pentacene. This result suggests that it might be instructive to further explore the potential of other higher acenes.

The sterically congested tetraarylpyrenes 1-3, which can be readily accessed by Suzuki coupling, exhibit no-aggregation (pi-stacking) behavior in both solution and solid states. The indisposed tendency of 1-3 toward crystallization and their moderate molecular dimensions permit exploitation as blue light emitting materials in OLEDs with respectable device performances.

Acenes are a class of aromatic hydrocarbons composed of linearly fused benzene rings. Noteworthy features of these molecules include their extended flat structure and the narrow gap between the HOMO and LUMO energy levels. However, the preparation of larger acenes, those that are larger than pentacene, has been challenging. These molecules are relatively unstable and have low solubility in typical solvents. Recently researchers have developed a new synthesis route for higher acenes using stable and soluble "precursors," which generate these structures on demand by either heating or irradiation of light. Using this method, nonsubstituted hexacene, heptacene, octacene, and nonacene were successfully prepared. In this Account, we summarize the preparation of nonsubstituted acenes from corresponding precursors, describe their physical properties, and discuss potential applications including potential usage in organic semiconductor devices. We first introduced the concept of using a precursor in the work with pentacene. Overall, we divide this methodology into two categories: masking pentacene itself with a dienophile to form a cycloadduct and the construction of higher acenes through conventional synthetic procedures. For the first category, a diverse array of dienophiles could be chosen, depending on the processing needs, especially for use in field-effect transistors (FETs). For the second category, researchers synthesized the pentacene precursor molecules using a multistep procedure. Upon proper activation, these molecules expel small fragments to generate pentacene readily. This strategy enabled the production of pentacene andunprepared higher acenes ranging from hexacene to nonacene. This new method provides a way to unravel the fascinating chemistry of higher acenes.

The unique structural attributes inherent to D(2d)-symmetric rigid tetraarylbimesityls render their close packing in the solid state difficult. We have exploited the indisposed tendency of such modules based on the bimesityl scaffold toward crystallization to design a novel class of amorphous functional materials with high glass transition temperatures and thermal stability (T(d) > 400 degrees C). It is shown that a variety of 2- and 4-fold anthracene-functionalized bimesityls, 1-7, that exhibit excellent amorphous properties (T(g) = ca. 190-330 degrees C) can be readily prepared via facile Pd(0)-mediated cross-coupling strategies. As the communication between the bimesityl core and the anchored anthracenes is negligible or only marginal, the trends observed for luminescence of model constituent anthracenes are reproduced in the condensed-phase photoluminescence and electroluminescence of 1-7. In other words, the emission characteristics, i.e., lambda(max) and quantum yields, are readily modulated via appropriate modification of the fluorophores. The functional behavior of this unique class of amorphous materials based on the bimesityl scaffold is demonstrated by fabrication of OLED devices. The 2-fold functionalized derivatives 1 and 2 lend themselves to sublimation techniques, so that the electroluminescence is captured with high efficiencies at low turn-on voltages (3.5-6.5 V). The device ITO/NPB (400 A)/1% 2:MADN (400 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A) for 2 yields the highest luminance of approximately 13,900 cd/m(2) at 17.5 V, a maximum luminance efficiency of approximately 7.4 cd/A at 4.5 V, and a power efficiency of approximately 5.3 lm/W at 4.0 V. Further, at a brightness of 800 cd/m(2) and a current density of 13.8 mA/cm(2), the device is found to exhibit excellent luminance efficiency of 5.8 cd/A, external quantum efficiency of 4.3% with a power efficiency of 2.2 lm/W, and pure blue light with a CIE(x,y) (x = 0.13, y = 0.18). The performance characteristics of the devices fabricated for 1 and 2 are remarkable. Although the 4-fold functionalized systems did not permit sublimation leading to spin-coating as a means for device fabrication, the observed electroluminescence for 4 and 5 attests to a broader scope and applicability of this new category of amorphous molecules for application in OLEDs.