Finely Tuned Blue Iridium Complexes with Varying Horizontal Emission Dipole Ratios and Quantum Yields for Phosphorescent Organic Light‐Emitting Diodes

Abstract

Two new heteroleptic blue emitting Ir(III) complexes, EtOCN-FIracac and tBuCN-FIracac, and a homoleptic Ir(III) complex, tBu-FCNIr, consisting of 2′,4″-difl uorophenylpyridine (dfppy) with a cyano (CN) group at the 5′ position of the phenyl and an ethoxy (EtO-) or tert -butyl (tBu) group at the 4 position of the pyridine as the main ligands, are synthesized and characterized for phosphorescent organic light-emitting diodes (PhOLEDs). The dual functionalization of the dfppy core with the electron-withdrawing CN and electron-donating EtO-or tBugroups leads to improved blue color purity by compensating the bathochromic shift originating from the ancillary ligand of the heteroleptic Ir(III) complex and contributes to the enhancement of the horizontal ratio of emitting dipoles (Θ), which is one of the promising approaches to improve the effi ciency of PhOLEDs. PhOLEDs based on tBuCN-FIracac and EtOCN-FIracac show one of the highest effi ciencies ever achieved for blue color, with maximum external quantum effi ciencies (EQEs) of 22.3% (39.2 cd A −1 ) and 21.5% (35.1 cd A −1 ), respectively, with a simple device structure; a similar device based on tBu-FCNIr displays a maximum EQE of 16.2% (23.5 cd A −1 ). The performance of PhOLEDs based on the two heteroleptic Ir(III) complexes are highly improved compared to tBu-FCNIr, due to the increased θ and photoluminescence quantum yield (PLQY). Adv. Optical Mater. 2015, 3, 211-220 www.MaterialsViews.com www.advopticalmat.de 212 wileyonlinelibrary.com

Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials

“…Angle-dependent p-polarized emission measurements of doped films by photoluminescent excitation are used to determine the net orientation of the TDVs of emissive dopants 4,12,23,25,[36][37][38] . We define the value Θ as the ratio of power radiated by vertical components of the contributing TDVs to the total power radiation.…”

“…Heteroleptic complexes of the formula (C ∧ N) 2 Ir(O ∧ O), where C ∧ N is a cyclometalated ligand and O ∧ O is a diketonate ligand such as acetylacetonate (acac), have previously been observed to demonstrate higher EQEs than their homoleptic Ir(C ∧ N) 3 analogues, because their average TDVs are disproportionately horizontal relative to the substrate in solid films 11,[16][17][18][19][20][21][22][23][24][25] . There are numerous reports of other dopants with ancillary ligands, especially acac and its close analogues, which feature net parallel TDV orientations to various degrees in doped films (see Supplementary Table 1 for tabulated literature data) [22][23][24][25] .…”

Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes

“…Thereafter, the search began for other iridium complexes exhibiting nonisotropic alignment of their TDMVs. As a consequence, several other molecules were found to display behavior similar to IrðmdqÞ 2 ðacacÞ, and the importance to light outcoupling was demonstrated [81,86,87,[209][210][211][212][213][214][215][216][217][218]. Using the feature of predominantly horizontal alignment of the TDMVs of such Ir complexes subsequently allowed OLED efficiencies for direct…”

Lessons learned in tuning the optoelectronic properties of phosphorescent iridium(iii) complexes

“…21 Several examples of these groups include: trifluoromethyl, 22 sulfonyl, 23 phosphonium, 24 diarylphosphine oxide 23b and nitrile. 21,25 In addition, there have also been efforts to change the aryl ring of the C^N ligands altogether from phenyl to a heterocycle such as pyridine, 26 pyridinium 27 or even pyrimidine. 28 The nitrogen atoms in these heterocycles stabilise the HOMO by inductively withdrawing electron density away from the metal centre ( Figure 5).…”

“…explored. An early example of the use of methoxy groups to mesomerically red-shift emission was reported by Watts et al, 34 who showed that the emission of the triscyclometalated complex fac-[Ir(ppy) 3 ] (24) could be red-shifted if -OMe groups were appended para to the Ir-C bond (25) or blue-shifted if they were in a meta relationship 26 Our study explored decoration of the C^N ligands with an increasing number of -OMe groups in a systematic fashion to red-shift the emission and explore the competing σwithdrawing effects with the π-donating effects of these substituents (29a -32b). 36 The electrochemistry follows the trend found for the emission properties.…”