The intramolecular π-π stacking interaction does not always work for improving the stabilities of light-emitting electrochemical cells

Recently, the stabilities of light-emitting electrochemical cells (LECs) based on cationic iridium(III) complexes with controlled intramolecular pi-pi stacking interactions by adopting pendant phenyl rings have been dramatically enhanced compared with those of the complexes without pi-pi stacking interactions within the molecule. Herein, a novel cationic iridium complex [Ir(ppy)(2)(pyphmi)]PF6]ppy = 2-phenylpyridine, pyphmi = 1-pyridyl-3-phenylimidazolin-2-ylidene-C,C-2,C-] which exhibits intramolecular pi-stacking interaction has been prepared and its X-ray crystal structure has been investigated. Unexpectedly, however, the corresponding LECs based on [Ir(ppy)(2)(pyphmi)]PF6 do not show significantly enhanced stabilities compared to the LECs based on [Ir(ppy)2(pymi)]PF6 [pymi = 1-pyridyl-3-methylimidazolin-2-ylidene-C,C-2,C-] without pendant phenyl rings within the molecule. This phenomenon is attributed to the very long centroid-centroid distance between the it-stacked phenyl rings, which results from the larger tension of substituted five-membered ring moiety (imidazolin-2-ylidene). In addition, irreversible oxidation and reduction processes would also decrease the electrochemical stability of [Ir(PPY)(2)(pyphmi)]PF6. Thus, intramolecular pi-pi stacking interaction using pendant phenyl rings is not always effective to improve the stability of LECs. (C) 2012 Elsevier B.V. All rights reserved.