Spectroscopic and Theoretical Investigation of Color Tuning in Deep-Red Luminescent Iridium(III) Complexes

A series of heteroleptic, neutral iridium(III) complexes of the form [Ir(L)2(N^O)] (where L = cyclometalated 2,3-disubstituted quinoxaline and N^O = ancillary picolinate or pyrazinoate) are described in terms of their synthesis and spectroscopic properties, with supporting computational analyses providing additional insight into the electronic properties. The 10 [Ir(L)2(N^O)] complexes were characterized using a range of analytical techniques (including 1H, 13C, and 19F NMR and IR spectroscopies and mass spectrometry). One of the examples was structurally characterized using X-ray diffraction. The redox properties were determined using cyclic voltammetry, and the electronic properties were investigated using UV–vis, time-resolved luminescence, and transient absorption spectroscopies. The complexes are phosphorescent in the red region of the visible spectrum (λem = 633–680 nm), with lifetimes typically of hundreds of nanoseconds and quantum yields ca. 5% in aerated chloroform. A combination of spectroscopic and computational analyses suggests that the long-wavelength absorption and emission properties of these complexes are strongly characterized by a combination of spin-forbidden metal-to-ligand charge-transfer and quinoxaline-centered transitions. The emission wavelength in these complexes can thus be controlled in two ways: first, substitution of the cyclometalating quinoxaline ligand can perturb both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO, Cl atoms on the ligand induce the largest bathochromic shift), and second, the choice of the ancillary ligand can influence the HOMO energy (pyrazinoate stabilizes the HOMO, inducing hypsochromic shifts).

page 46 Table S2  page 47   Table S3  page 48   Table S4  page 49   Table S5  page 50   Table S6  page 51   Table S7  page 52   Table S8  page 53   Table S9 page 54 Experimental 1 H, 19 F{ 1 H} and 13 C{ 1 H} NMR spectra were recorded on an NMR-FT Bruker 500 and 400 MHz spectrometer and recorded in CDCl3, acetone-d6, acetonitrile-d3 and DMSO-d6. 1 H and 13 C{ 1 H} NMR chemical shifts (δ) were determined relative to residual solvent peaks with digital locking and are given in ppm. Coupling constants are quoted in Hz.
Low-resolution mass spectra were obtained by the staff at Cardiff University. Highresolution mass spectra were carried out at the EPSRC National Mass Spectrometry

Transient absorption measurements
Transient absorption measurements were carried out using an Edinburgh Instruments LP920 spectrometer. All spectra were collected using a pump wavelength of 355 nm (third harmonic of a Continuum Surelite II Nd:YAG laser system). The probe light for these measurements was a Xenon lamp, affording spectral generation between 300 < l < 800 nm. Wavelength dependent spectra were recorded with a 2.05 nm spectral resolution, collected using an Andor ICCD camera, and integrated over the first 500 ns after the pump laser pulse. The spectra are presented as DODXe lamp, which is simply referred to as DOD. Lifetime data was generated using a photomultiplier to collect time resolved signals, with the bandwidth of these data being identical to the camera resolution (2.05 nm). The lifetime data is fit using the Origin 2017 software package, and each data set is fit using a monoexponential function, with no evidence of multiexponential components. Uncertainties in lifetimes are taken from the Least-Squares fitting algorithm, and are not indicative of the uncertainties in multiple fits or data sets.

Cyclic voltammetry
Electrochemical studies were carried out using a Parstat 2273 potentiostat in conjunction with a three-electrode cell.

Computational methods
Electronic structure calculations were all performed using density functional theory within the Gaussian 09 computational chemistry suite. 3 All calculations were performed using the Stuttgart-Dresden (SDD) effective core potential and basis set in the treatment of the iridium, 4 in combination with a 6-31G* basis set for all other light atoms. 5 Full geometry optimizations were performed for the cationic complexes utilizing the self-consistent reaction field model (SCRF) which treats the solvent implicitly as a dielectric continuum. In all cases the solvent chosen was chloroform, consistent with that utilized in the both final synthesis and in the majority of the spectroscopic measurements. Chloroform is characterized by an electrical permittivity (ε) of 4.7113 within the calculations. This computational method models the solvent as surrounding a cavity in which the solute resides, and this cavity is characterized using an integral equation formalism for the polarizable continuum model (IEFPCM).
This model represents the system in equilibrium during, for example, an optimization routine: in all excited state calculations a non-equilibrium solvent model is used.
All geometry optimizations were performed using an ultrafine grid and very tight convergence criteria, and the minima were confirmed as stationary points through the computation of harmonic vibrational frequencies, each of which showed no imaginary components. These stationary points were used in single point TD-DFT calculations to compute vertical excitation energies. All TD-DFT calculations were undertaken using a linear response approach. All TD-DFT calculations were also performed with a long range corrected hybrid functional (CAM-B3LYP).
Phosphorescence and spin-forbidden absorption bands were investigated using unrestricted density functional theory to compute parameters associated with the first triplet state (T1), using an identical methodology as for the singlet states.
Decomposition of the molecular orbital character was performed using the GaussSum software package. 6 Crystal structure overlays with optimised computational structures has been performed using the Chimera software package, which has also been used to calculate root mean squared deviation (RMSD) values for these comparative structures. 7

Data collection and processing
Suitable crystals of [Ir(L7)2(pic)] were selected and data collected following a standard method. 8