Designing NHC–Copper(I) Dipyridylamine Complexes for Blue Light-Emitting Electrochemical Cells

Abstract

This study presents the influence of various substituents on the photophysical features of heteroleptic copper(I) complexes bearing both N-heterocyclic carbene (NHC) and dipyridylamine (dpa = dipyridylamine skeleton corresponding to ligand L1) ligands. The luminescent properties have been compared to our recently reported archetypal blue emitting [Cu(IPr)(dpa)][PF6] complex. The choice of the substituents on both ligands has been guided to explore the effect of the electron donor/acceptor and "push-pull" on the emission wavelengths and photoluminescence quantum yields. A selection of the best candidates in terms of their photophysical features were applied for developing the first blue light-emitting electrochemical cells (LECs) based on copper(I) complexes. The device analysis suggests that the main concern is the moderate redox stability of the complexes under high applied driving currents, leading to devices with moderate stabilities pointing to a proof-of-concept for further development. Nevertheless, under low applied driving currents the blue emission is stable, showing performance levels competitive to those reported for blue LECs based on iridium(III) complexes. Overall, this work provides valuable guidelines to tackle the design of enhanced NHC copper complexes for lighting applications in the near future.

Role of the Bridging Group in Bis‐Pyridyl Ligands: Enhancing Both the Photo‐ and Electroluminescent Features of Cationic (IPr)CuI Complexes

“…The 2,2′‐bis‐pyridyl derivatives are six‐membered chelates, in contrast to the most commonly encountered five‐membered ring chelate ligands, such as 1,10‐phenanthroline (phen) or 2,2′‐bipyridine (bipy) . Unfortunately, LECs based on the archetypal complex 1 performed poorly, that is, with a luminance of 6 cd m −2 and efficacy of 0.004 cd A −1 , owing to the need of high applied currents (50 mA) . This scenario changes for devices with an emitter bearing a modified version of the dpa ligand, that is, luminance of 20 cd m −2 and efficacy of 0.22 cd A −1 at applied currents of 5 mA …”

“…Unfortunately, LECs based on the archetypal complex 1 performed poorly, that is, with a luminance of 6 cd m −2 and efficacy of 0.004 cd A −1 , owing to the need of high applied currents (50 mA) . This scenario changes for devices with an emitter bearing a modified version of the dpa ligand, that is, luminance of 20 cd m −2 and efficacy of 0.22 cd A −1 at applied currents of 5 mA …”

“…Among these cationic NHC copper(I) complexes, we and others have reported cationic three‐coordinated copper(I) complexes, in which the “N^N ligand” was a six‐membered ring chelate with two pyridine rings and a borate or an amine as bridging group. In our previous studies, we reported a blue‐emitting [Cu(IPr)(dpa)][PF 6 ] complex 1 (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene, dpa=di(2‐pyridyl)amine) and its application in LECs . Here, we have identified key structural and electronic parameters that clearly affect the photoluminescence features of this novel family of compounds.…”

“…In a few words, the dpa ligand leads to a non‐coplanar geometry between the diamine ligand and the imidazole ring of the NHC . The photoluminescence properties can also be modified by attaching electron‐donating and/or ‐accepting groups at the periphery of the dpa ligand …”

Low‐Cost, Organic Light‐Emitting Electrochemical Cells with Mass‐Producible Nanoimprinted Substrates Made Using Roll‐to‐Roll Methods

“…[7][8][9][10][11][12][13] PLECs have simple singlelayer structures, which can be fabricated using roll-to-roll methods, using solutions containing both FCPs and ion conductors. [11,[22][23][24][25][26][27] Realizing high efficiency organic lighting will require more effective light outcoupling technologies. [7][8][9][10]20] PLECs have many other advantages, e.g., they can be fabricated using air-stable electrodes and thicker active layers.…”