Journal of the American Chemical Society, Vol.130, No.51, 17320-17333, 2008
De Novo Design for Functional Amorphous Materials: Synthesis and Thermal and Light-Emitting Properties of Twisted Anthracene-Functionalized Bimesitylenes
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 angstrom)/1 % 2:MADN (400 angstrom)/TPBI (400 angstrom)/LiF (10 angstrom)/Al (1500 angstrom) for 2 yields the, highest luminance of similar to 13 900 cd/m(2) at 17.5 V, a maximum luminance efficiency of similar to 7.4 cd/A at 4.5 V, and a power efficiency of similar to 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 Im/W, and pure blue light with a CIEx,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.