Unsymmetrical Dendrimers as Highly Efficient Light-Emitting Materials: Synthesis, Photophysics, and Electroluminescence

Highly efficient light-emitting materials can be archived by a rational design that involves rigid dendron and π-conjugated chromophores. This strategy is exemplified by three unsymmetrical dendrimers 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(9- anthracenyl)-2,2′-bithiophene (1), 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenyl)-2,2′ -bithiophene (2), and 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenylethynyl)-2,2′- bithiophene (3). Thermogravimetric analysis and differential scanning calorimetry suggest that unsymmetrical dendrimers 1 and 2 have good thermal stability (>400 °C) and high glass transition temperature (>130 °C) which make them promising candidates for highly stable organic light-emitting devices. The comparative study on their UV−vis absorption and fluorescence spectra in solution and as thin films indicates that the unsymmetrical dendrimers can form intermolecular π−π stacking in the solid state. Electroluminescence (EL) devices using them as emitters were made, and it was found that they emit intense green light, and the maximum luminance was 8659 cd/m2 at 13.5 V with a maximum efficiency of 4.9 cd/A for 1. The further comparative study on the EL performance of 1−3, combined with their single crystal structures, demonstrated that with felicitously chosen π-conjugated chromophores and better control of the space between the chromophore and dendron highly efficient light-emitting materials will be realized.