Cationic Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue, Green, and Red Electroluminescent Devices

A series of cationic Ir(III) complexes with the general formula (C∧N)2Ir(N∧N)+PF6- featuring bis-cyclometalated
1-phenylpyrazolyl-N,C2‘ (C∧N) and neutral diimine (N∧N, e.g., 2,2‘-bipyridyl) ligands were synthesized and their
electrochemical, photophysical, and electroluminescent properties studied. Density functional theory calculations
indicate that the highest occupied molecular orbital of the compounds is comprised of a mixture of Ir d and
phenylpyrazolyl-based orbitals, while the lowest unoccupied molecular orbital has predominantly diimine character.
The oxidation and reduction potentials of the complexes can be independently varied by systematic modification
of either the C∧N or N∧N ligands with donor or acceptor substituents. The electrochemical redox gaps (Eox − Ered)
were adjusted to span a range between 2.39 and 3.08 V. All of the compounds have intense absorption bands in
the UV region assigned to 1(π−π*) transitions and weaker charge-transfer (CT) transitions that extend to the
visible region. The complexes display intense luminescence both in fluid solution and as neat solids at 298 K that
is assigned to emission from a triplet metal−ligand-to-ligand CT (3MLLCT) excited state. The energy of the 3MLLCT
state varies in nearly direct proportion to the size of the electrochemical redox gap, which leads to emission colors
that vary from red to blue. Three of the (C∧N)2Ir(N∧N)+PF6- complexes were used as active materials in single-layer light-emitting electrochemical cells (LECs). Single-layer electroluminescent devices were fabricated by spin-coating the Ir complexes onto an ITO−PEDOT/PSS substrate followed by deposition of aluminum contacts onto
the organic film. Devices were prepared that give blue, green, and red electroluminescence spectra (λmax = 492,
542, and 635 nm, respectively), which are nearly identical with the photoluminescence spectra of thin films of the
same materials. The single-layer LECs give peak external quantum efficiencies of 4.7, 6.9, and 7.4% for the blue,
green, and red emissive devices, respectively.