Investigating the Effect of Steric Crowding in Phosphorescent Dendrimers

Abstract

A simple convergent procedure has been developed for the formation of sterically encumbered phosphorescent dendrimers. The procedure is demonstrated with the preparation of a first-generation dendrimer composed of a fac-tris(2-phenylpyridyl)iridium(III) core and three dendrons. Each dendron is comprised of a branching phenyl unit with a further four phenyl groups attached. The lack of surface groups on the dendrons was found to reduce solubility and also reduced the level of control over the intermolecular interactions of the emissive and electroactive core in films. The 6-fold decrease in photoluminescence quantum yield in going from solution (69%) to the solid state (11%) showed that there were strong intermolecular interactions of the emissive cores in the solid state. Single-layer devices with the dendrimer blended with 4,4′-bis(N-carbazolyl)biphenyl showed an external quantum efficiency of 1.7% (5.4 cd/A) at 100 cd/m 2 and 11.4 V, giving a power efficiency of 1.5 lm/W.

Synthesis and characterization of one star-shaped polymer with charged iridium complex as luminescent core

“…On the other hand, there was a significant irreversible reduction peak at about À 3.18 V. It has been reported that oxidation processes of Ir cyclometalated compounds were assigned to metal-centered orbital and/or to s bond Ir-C orbitals while the reduction process were assigned to ppy-centered orbital [9,52]. Therefore, the potential peak at about À 3.18 V was assigned to ppy reductions and the similar phenomenon (ppy reduction potential between 2.5-3.2 V) was also noted by other reporters [19,53]. However, we cannot find any obvious reduction peaks of the monomer and the complex.…”

“…In order to reduce the concentration quenching, bulky groups were introduced into the complex to provide steric hindrance. Ir complex with dendritic structures were reported with satisfactory photoluminescence and electroluminescence quantum yield [16][17][18][19][20][21].…”

Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems

“…Like the polymeric systems, dendritic structures can be classified as conjugated [50,51,55,[174][175][176][177][178][179][180] and nonconjugated [31,56,[181][182][183] systems. In most cases, a heteroleptic or homoleptic facial tris-cyclometallated iridium(III) complex constitutes the core with two [173,174] or three [50,51,55,56,[172][173][174][175][176][177][178][179][180] ligand-tethered dendritic arms forming the shell. Lin et al reported on a number of bis(cyclometallated) Ir III complexes coordinating dendrimer-functionalized acetoacetonate derivatives as ancillary ligand.…”

“…Based on such tris-cyclometallated Ir III complexes (Scheme 1), Zhou et al fabricated devices exhibiting efficient red emission (h ext ¼ 11.6%, h P ¼ 3.7 lm W À1 ) with high color purity (CIE x,y-coordinates 0.70, 0.30). [50] Samuel and co-workers focused in particular on phenyl-based configurations, [55,56,[172][173][174][175][176][177] while You et al presented a complex equipped with tetraphenyl silyl moieties. [51] The possibility to construct highly defined and very dense assemblies is the most beneficial aspect of dendritic systems.…”

The Development of Light‐Emitting Dendrimers for Displays

“…5). [66] The addition of surface groups to dendrimers like 19 addressed this problem and good device efficiencies (> 10 %) could be obtained in bilayer devices in which the dendrimer was blended with (CBP). [67] Another direction for dendron development has been to incorporate charge-transporting moieties into the dendron structure, while leaving the emissive chromophore at the core of the dendrimer the same.…”