Cationic Bis-cyclometallated Iridium(III) Phenanthroline Complexes with Pendant Fluorenyl Substituents: Synthesis, Redox, Photophysical Properties and Light-Emitting Cells

We report the synthesis, characterisation, photophysical and electrochemical properties of a series of cationic cyclometallated IrIII complexes of general formula [Ir(ppy)2(phen)]PF6 (ppy=2‐phenylpyridine, phen=a substituted phenanthroline). A feature of these complexes is that the phen ligands are substituted with one or two 9,9‐dihexylfluorenyl substituents to provide extended π conjugation, for example, the 3‐[2‐(9,9‐dihexylfluorenyl)]phenanthroline and 3,8‐bis[2‐(9,9‐dihexylfluorenyl)]phenanthroline ligands afford complexes 6 and 9, respectively. A single‐crystal X‐ray diffraction study of a related complex 18 containing the 3,8‐bis(4‐iodophenyl)phenanthroline ligand, revealed an octahedral coordination of the Ir atom, in which the metallated C atoms of the ppy ligands occupy cis positions. The complexes 6 and 9 displayed reversible oxidation waves in cyclic voltammetric studies (${E{{{\rm ox}\hfill \atop 1/2\hfill}}}$=+1.18 and +1.20 V, respectively, versus Ag/Ag+ in CH2Cl2) assigned to the metal‐centred IrIII/IrIV couple. The complexes exhibit strong absorption in the UV region in solution spectra, due to spin‐allowed ligand‐centred (LC) 1π–π* transitions; moderately intense bands occur at approximately 360–390 nm which are red‐shifted with increased ligand length. The photoluminescence spectra of all the complexes were characterised by a broad band at λmax≈595 nm assigned to a combination of 3MLCT and 3π→π* states. The long emission lifetimes (in the microsecond time‐scale) are indicative of phosphorescence: the increased ligand conjugation length in complexes 9 and 17 leads to increased lifetimes for the complexes (τ=2.56 and 2.57 μs in MeCN, respectively) compared to monofluorenyl analogues 6 and 15 (τ=1.43 and 1.39 μs, respectively). DFT calculations of the geometries and electronic structures of complexes 6′, 9′ (for both singlet ground state (S0) and triplet first excited (T1) states) and 18 have been performed. In the singlet ground state (S0) HOMO orbitals in the complexes are spread between the Ir atom and benzene rings of the phenylpyridine ligand, whereas the LUMO is mainly located on the phenanthroline ligand. Analysis of orbital localisations for the first excited (T1) state have been performed and compared with spectroscopic data. Spin‐coated light‐emitting cells (LECs) have been fabricated with the device structures ITO/PEDOT:PSS/Ir complex/Al, or Ba capped with Al (ITO=indium tin oxide, PEDOT=poly(3,4‐ethylenedioxythiophene), PSS=poly