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

Abstract

We report the synthesis, characterisation, photophysical and electrochemical properties of a series of cationic cyclometallated Ir(III) complexes of general formula [Ir(ppy)(2)(phen)]PF(6) (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 pi 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(ox)(1/2)=+1.18 and +1.20 V, respectively, versus Ag/Ag(+) in CH(2)Cl(2)) assigned to the metal-centred Ir(III)/Ir(IV) couple. The complexes exhibit strong absorption in the UV region in solution spectra, due to spin-allowed ligand-centred (LC) (1)pi-pi* 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 lambda(max) approximately 595 nm assigned to a combination of (3)MLCT and (3)pi-->pi* 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 (tau=2.56 and 2.57 micros in MeCN, respectively) compared to monofluorenyl analogues 6 and 15 (tau=1.43 and 1.39 micros, respectively). DFT calculations of the geometries and electronic structures of complexes 6', 9' (for both singlet ground state (S(0)) and triplet first excited (T(1)) states) and 18 have been performed. In the singlet ground state (S(0)) 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 (T(1)) 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(styrene) sulfonate). A maximum brightness efficiency of 9 cd A(-1) has been attained at a bias of 9 V for 17 with a Ba/Al cathode. The devices operated in air with no reduction in efficiency after storage for one week in air.

Luminescent Ionic Transition‐Metal Complexes for Light‐Emitting Electrochemical Cells

“…The photophysical, electrochemical, and theoretical studies showed that addition of the bulky groups does not affect the emitting excited state, in agreement with other reports. [182,183] LECs using these complexes exhibit a roughly sixfold increase in the EQE when compared with the archetype complex without phenyl and phenolic ether groups. Additionally, other important device parameters, such as turn-on time, luminance, and stability improved upon increasing the size of the bulky groups.…”

“…[182,183] The addition of bulky groups and high luminances (1000 cd m À2 at 3 V). Notably, there are limits in the use of large substituents because driving voltages tend to increase as a consequence of larger intermolecular separation.…”

Photophysical Properties of Oligo­(phenylene ethynylene) Iridium(III) Complexes Functionalized with Metal‐Anchoring Groups

“…The topologies of the molecular orbitals of Ir2 and Ir4 ( Figure S4) are identical to those of Ir1. Similarly to those of related ppy-based cyclometallated Ir complexes, [31][32][33] the HOMO is composed of a mixture of Ir III dπ orbitals (t 2g ) and phenyl π orbitals with little contribution from the pyridine rings of the cyclometallated ligands. As the family of complexes Ir1-Ir4 only differs in the diimine ligand, the energy of the HOMO remains almost constant along the series and is slightly more stable for Ir3 and Ir4 (Figure 6, b).…”

Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

“…[32,34] The groups of both Bryce and Schanze reported that incorporation of π-conjugated substituents, such as fluorenyl and 4-ethynyl-N,N-dihexylaniline substituents on the N ∧ N ligand led to a mixture of 3 MLCT (metal-to-ligand charge transfer) and 3 π,π* characters in the lowest triplet excited state (T 1 state) and thus increased the T 1 lifetimes. [17,35] Our group also demonstrated that introducing the benzothiazolylfluorenyl (BTF) substituents to the N ∧ N ligand dramatically increased the T 1 state lifetimes for both Pt II diimine acetylide complexes [23] and heteroleptic Ir III complexes. [18,19,27] However, incorporation of the BTF motif into the C ∧ N ligands in the Ir III complex resulted in a reduced T 1 lifetime.…”