In recent years products utilizing organic light-emitting diodes (OLEDs) have become commercially viable in both display and lighting applications. Much of this success stems from development of highly efficient OLEDs employing phosphorescent iridium(III) complexes as the emissive species. A common property of these emitters is their low solubility which limits their processing to relatively expensive and energy inefficient vacuum thermal evaporation.
In existing technologies the high expense of processing is overcome by the large unit cost of the current applications; smartphones and televisions. This is particularly encumbering for large-area device applications, such as lighting, as the cost of vacuum thermal evaporation increases disproportionally with device area. If OLEDs are to become feasible to be employed on a wider range of electronics then another processing technique must be adopted. Of particular interest are solution processing techniques which could allow affordable roll-to-roll processing.
Previous work has shown that functionalization of the core phosphor with branched units, commonly referred to as dendrons, can be a successful strategy to enhance solubility with the additional benefit of decreasing inter-chromaphore interactions between excited emissive species. Poly(dendrimer)s also inherit the advantages of dendrimers along with improved film forming properties. Most of the work published within the literature focuses on green-emitting species based on the archetypical iridium(III) complex, Ir(PPy)3. Solution processable materials with deep blue- or red-emission are not as well investigated.
The work covered here describes a series deep red-emitting iridium(III) complexes functionalized with first-generation carbazolyl-dendrons; all exhibiting solution photoluminescent quantum yields in the range of 61-87% with comparable film quantum yields within a blend.
In addition to being functionalized with the same carbazolyl-dendron all iridium(III) complexes within the series utilize the 2-thienylquinoline emissive ligand. The series includes two fac-homoleptic dendrimers, two trans-N,N`, cis-C,C`-heteroleptic dendrimers and a poly(dendrimer).
The two homoleptic iridium(III) complexes are functionalized with the carbazolyl-dendrons at different positions resulting in a large variation in shape and size. The two heteroleptic iridium(III) complexes utilize the same dendronized ligands as the homoleptic complexes but with one replaced by acetoacetone. The resulting variations in shape and electronic characteristics greatly alters the photophysical and electroluminescent properties of the materials.
The poly(dendrimer) is a homopolymer synthesized by ring-opening metathesis polymerization from a norbornenyl-monomer with a pendant iridium(III) complex tethered through the emissive 2-thienylquinoline ligand and two dendronized phenylpyridine ancillary ligands (Mn=164 kDa, Dispersity= 2.4).