Synthesis and characterization of solution-processible anthracene- based deep-blue fluorescent dendrimers

Abstract

Anthracene has been intensively studied as an attractive building block for deep-blue fluorescent emitters due to its wide band gap and high photoluminescence quantum yield (PLQY) in dilute solution. In solid state, however, these anthracene derivatives seem to aggregate or crystallize, leading to the quenched fluorescence, red-shifted emission and poor device lifetime. Therefore, there are few reports about anthracene-based deep-blue emitters suitable for solution-processed undoped devices. To this end, herein we have designed and synthesized a series of solution-processible deep-blue fluorescent dendrimers (Ent1, Ent2 and Ent3) by using 9,10-diphenylanthracene (DPA) as the central core and 1st to 3rd generation oligocarbazole as the peripheral dendron. Unlike DPA that tends to crystallize in the solid state, Ent1~Ent3 are thermally stable, showing very high glass transition temperatures up to 194−371oC without any related signals from the crystallization or melting processes. As determined by atomic force microscopy (AFM), in addition, both the pristine and annealed films of Ent1~Ent3 have a fairly smooth surface with a root-mean-square roughness of 0.33-0.47 nm, indicative of their morphology stability. Meanwhile, the introduction of dendrons can effectively suppress the intermolecular interactions and thus enhance the film PLQY from 46% of Ent1 to 52% of Ent2 and 67% of Ent3. Correspondingly, as a result of the much richer electron-cloud density from the higher generation dendron, the highest occupied molecular orbital (HOMO) energy level is increased from −5.51 eV of Ent1 to −5.37 eV of Ent2 and −5.34 eV of Ent3, which is beneficial for the hole injection. Solution-processed undoped organic light-emitting diodes (OLEDs) are subsequently fabricated with a configuration of ITO\PEDOT:PSS(40 nm)\Ent1~Ent3(40 nm)\TPBI(55 nm)\LiF(1 nm)\Al(100 nm). All the dendrimers display a bright deep-blue emission with Commission International De L’Eclairge (CIE) coordinates of (0.16, 0.11), (0.16, 0.10) and (0.16, 0.09) for Ent1, Ent2 and Ent3, respectively. Noticeably, the turn-on voltage is firstly down from 4.2 V of Ent1 to 3.8 V of Ent2, and then up to 4.8 V of Ent3. This observation means that, albeit the facilitated hole injection, the electron injection would become more difficult in Ent3 owing to its shallow lowest unoccupied molecular orbital (LUMO) level. Therefore, among Ent1~Ent3, more balanced charge injection/transporting and efficient recombination could be expected for the 2nd generation dendrimer Ent2. Consequently, Ent2 obtains the best device performance, revealing a maximum luminous efficiency of 2.63 cd/A, a maximum power efficiency of 2.07 lm/W and a maximum external quantum efficiency of 2.48%. Our results demonstrate a promising approach to the development of solution processible deep-blue fluorescent emitters for undoped OLEDs applications.