Acid-Induced Degradation of Phosphorescent Dopants for OLEDs and Its Application to the Synthesis of Tris-heteroleptic Iridium(III) Bis-cyclometalated Complexes

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∧N)2(X∧O)] complex (C∧N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)­pyridine
(diFppy) and X∧O = 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∧N)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∧N1)­(C∧N2)­(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∧N ligands (C∧N1 = ppy; C∧N2 = 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∧N ligands. Finally, we use DFT/LR-TDDFT to rationalize
the impact of the two different C∧N ligands on the
observed photophysical and electrochemical properties.