De novo design for functional amorphous materials: synthesis and thermal and light-emitting properties of twisted anthracene-functionalized bimesitylenes

Abstract

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 °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 °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., λ_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 Å)/1% 2:MADN (400 Å)/TPBI (400 Å)/LiF (10 Å)/Al (1500 Å) for 2 yields the highest luminance of ~13900 cd/m² at ~17.5 V, a maximum luminance efficiency of 7.4 cd/A at 4.5 V, and a power efficiency of ~5.3 lm/W at 4.0 V. Further, at a brightness of 800 cd/m² and a current density of 13.8 mA/cm², 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.