Acid-induced degradation of phosphorescent dopants for OLEDs and its application to the synthesis of tris-heteroleptic iridium(III) bis-cyclometalated complexes

Etienne Baranoff*, Basile F E Curchod, Julien Frey, Rosario Scopelliti, Florian Kessler, Ivano Tavernelli, Ursula Rothlisberger, Michael Grätzel, Md Khaja Nazeeruddin

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

135 Citations (Scopus)

Abstract

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir(2-(2,4-difluorophenyl)pyridine) 2(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Brønsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir(C λN) 2(X λO)] complex (C λN = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (diFppy) and X λO = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium(III) dimer [{Ir(C λN) 2(μ-Cl)} 2]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium(III) complexes [Ir(C λN 1)(C λN 2)(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium(I) complex, [{Ir(COD)(μ-Cl)} 2], and a stoichiometric amount of two different C λN ligands (C λN 1 = ppy; C λN 2 = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir(ppy)(diFppy)(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir(ppy) 2(acac)] and [Ir(diFppy) 2(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir(ppy)(diFppy)(μ-Cl)} 2], which can be used as starting material for the preparation of a new tris-heteroleptic iridium(III) complex based on these two C λN ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C λN ligands on the observed photophysical and electrochemical properties.

Original languageEnglish
Pages (from-to)215-224
Number of pages10
JournalInorganic Chemistry
Volume51
Issue number1
DOIs
Publication statusPublished - 2 Jan 2012

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