Dual visible/NIR emission from organometallic iridium(III)complexes

Article abstract of DOI:10.1016/j.jorganchem.2019.04.019

A series of four substituted benzo[g]quinoxaline species have been synthesised and utilised as cyclometalating ligands for iridium(III). The ligands (L1-L4)were synthesised and isolated in good yield following the condensation of 2,3-diaminonaphthalene wi

Full text of DOI:10.1016/j.jorganchem.2019.04.019

Accepted Manuscript
Dual visible/NIR emission from organometallic iridium(III) complexes
Kaitlin A. Phillips, Thomas M. Stonelake, Peter N. Horton, Simon J. Coles, Andrew J.
Hallett, Sean P. O'Kell, Joseph M. Beames, Simon J.A. Pope
PII:
S0022-328X(19)30159-7
DOI:
https://doi.org/10.1016/j.jorganchem.2019.04.019
JOM 20769
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 12 February 2019
Revised Date: 15 April 2019
Accepted Date: 20 April 2019
Please cite this article as: K.A. Phillips, T.M. Stonelake, P.N. Horton, S.J. Coles, A.J. Hallett, S.P. O'Kell,
J.M. Beames, S.J.A. Pope, Dual visible/NIR emission from organometallic iridium(III) complexes, Journal
of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2019.04.019.
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ACCEPTED MANUSCRIPT
Dual visible/NIR emission from organometallic iridium(III) complexes
a
a
b
b
c
Kaitlin A. Phillips, Thomas M. Stonelake, Peter N. Horton, Simon J. Coles, Andrew J. Hallett, Sean P.
c
a
a
O’Kell, Joseph M. Beames* and Simon J. A. Pope*
a
School of Chemistry, Main Building, Cardiff University, Museum Avenue, Cardiff, United Kingdom, CF10
AT. Tel: +44(0) 29 20879316, Fax: +44(0) 29 20874030, email: fallis@cardiff.ac.uk;
3
popesj@cardiff.ac.uk
b
UK National Crystallographic Service, Chemistry, Faculty of Natural and Environmental Sciences,
c
University of Southampton, Highfield, Southampton, England, United Kingdom, SO17 1BJ;
Aerospace, Brecon House, Cwmbran, United Kingdom, NP44 3AB
STG
Abstract
A series of four substituted benzo[g]quinoxaline species have been synthesised and utilised as
cyclometalating ligands for iridium(III). The ligands (L1-L4) were synthesised and isolated in good
yield following the condensation of 2,3-diaminonaphthalene with benzil and three of its derivatives.
The substituent modulated electronic properties of L1-L4 were dominated by intraligand π−π*
transitions, with the fluorescence profile demonstrating vibronic features attributed to the highly
conjugated nature of the chromophore. Iridium(III) complexes of the form [Ir(L) (bipy)]PF were
2
6
synthesised from L1-L4 in two steps. The electronic properties of the complexes reveal absorption
in the UV-vis. region with spin forbidden metal-to-ligand charge transfer (MLCT) transitions
possibly contributing at longer wavelengths to ca. 600 nm. Steady state luminescence (aerated,
room temperature) on solutions of the complexes showed dual emissive properties in the visible and
near-infra red (NIR) regions. Firstly, a vibronically structured emission in the visible region
(centred ca. 525 nm) was attributed to ligand centred fluorescence. Secondly, a broad emission peak
in the NIR (ca. 950 nm) which extended to around 1200 nm was observed with corresponding
lifetimes of 116-162 ns, indicative of triplet excited state emission.
Introduction
The development and study of molecular species that are luminescent in the near-infra red (NIR)
region continues to attract significant attention. Such studies have been motivated by the many
1
applications (some postulated, others realised) that can benefit from NIR luminescent materials. A
1

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range of optoelectronic devices can utilise NIR wavelengths. In the realm of bioimaging, it has long
been argued that the use of fluorescence microscopy as a research and diagnostic tool can greatly
2
benefit from the use of NIR excitation and emission wavelengths. The optical properties of
biological tissue are such that its relative transparency in parts of the NIR region can greatly
improve optical imaging potential. Achieving NIR luminescence from molecular species has
therefore become an ongoing challenge. From a photophysical perspective, consideration of the
nature of the excited emitting state and suppression of non-radiative deactivation pathways
(particularly where the energy gap between the ground and excited state is small) is a key challenge.
Lanthanide coordination compounds have been successfully developed in this regard with
Nd(III), Er(III) and Yb(III) species demonstrating long-lived emission in the 880-1550 nm
3
window. Uniquely, such species possess metal centred (4f) excited states that can give rise to NIR
emission. Whilst demonstrating attractive emission properties, the limiting aspect of such systems is
4
overcoming the inherently poor molar absorption coefficients associated with 4f-4f transitions.
Over the last two decades a wide range of functionalised ligand systems have been developed to
5
address these challenges.
In contrast, only a relatively small number of reports have described NIR emission from d-
metal complexes. Most commonly such observations are often defined at low temperature (i.e. in a
frozen matrix) or under deoxygenated conditions. Of course, both approaches seek to minimise non-
radiative quenching of the excited emitting state. For several decades, Cr(III) complexes have been
2
known to demonstrate emission from a d-centred excited state ( E) which typically emits around
6
7
8
7
80 nm, and work continues to maximise these properties. A small number of Ru(II), Os(II), and
9
Pt(II) complexes have also been reported to demonstrate luminescence in the NIR region.
Strategies can include lowering the energy of charge transfer (CT) excited states or facilitating the
population of low-lying ligand-centred triplet states through efficient spin orbit coupling (SOC)
facilitated by the heavy metal atom of the complex.
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Cyclometalated Ir(III) complexes have also, very recently, been investigated in this context.
Neutral Ir(III) complexes based on 1-(benzo[b]thiophen-2-yl)-isoquinolinate show emission
between 680-850 nm and have been successfully processed into phosphorescent organic light-
1
0
emitting devices. Ir(III) complexes with boron-dipyrromethane (BODIPY) conjugated ligands
have shown interesting NIR absorption and emission properties with particularly long triplet state
1
1
lifetimes. Wong and co-workers have demonstrated highly tuneable emission properties for Ir(III)
1
2
complexes using both conjugation and substitution strategies on the cyclometalating ligands. This
has allowed the emission to be tuned in the range 420-729 nm with successful application shown
within light emitting diodes. Two further reports of cationic Ir(III) complexes that incorporate
1
3
benzo[g]quinoxaline cyclometalated ligands have been previously described by Chen et al and
1
4
Sun. The extended conjugation of these ligands, versus the red wavelength emission observed
1
5
from previous studies on 2-phenylquinoxaline analogues, appears to bathochromically shift the
emission wavelength into the NIR region. Building on these reports, we herein describe a further
series of Ir(III) organometallic complexes based on substituted benzo[g]quinoxaline ligands,
providing further evidence that these species are viable NIR emitters, even under ambient
conditions (room temperature, aerated solution).
N
N
N
N
L1
L2
Br
Br
OMe
OMe
N
N
N
N
L3
L4
Scheme 1. The benzo[g]quinoxaline ligands synthesised in this study for Ir(III) complexation.
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Results and Discussion
Synthesis and characterisation
The ligands (L1-L4) were synthesised in a single step by the condensation reaction of 2,3-
diaminonaphthalene with a benzil derivative in the presence of acetic acid. The ligands were
isolated in good yield and characterised using a range of analytical and spectroscopic techniques.
All relevant data is presented in the experimental section. Good quality single crystals were
obtained for the dibromo-substituted derivative L3 following slow evaporation of a concentrated
chloroform solution. The orange blade shaped crystals were submitted to diffraction studies and the
structure was refined in the P2 /n space group and there is a single molecule in the asymmetric unit.
1
The data confirmed the proposed structure (Figures 1 and 2), with a packing arrangement supported
by some long range intermolecular π-π interactions.
Figure 1: X-ray crystal structure of L3. Ellipsoids are drawn at a 50 % probability. Crystal Data:
C H Br N , M = 490.19, monoclinic, P2 /n (No. 14), a = 5.7963(2) Å, b = 21.8021(9) Å, c =
24
14
2
2
r
1
°
°
3
15.1661(6) Å,
β
= 91.211(3) ,
α
=
γ
= 90 , V = 1916.15(13) Å , T = 100(2) K, Z = 4, Z' = 1,
-1
µ
(MoK ) = 4.243 mm , 19907 reflections measured, 4396 unique (R = 0.0501) which were used
α
int
in all calculations. The final wR was 0.1219 (all data) and R was 0.0491 (I > 2(I)).
2
1
4

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Figure 2: Packing diagram for L3 obtained from the X-ray crystal structure. Ellipsoids are drawn at
a 50 % probability.
PF6
PF6
N
N
N
N
N
N
N
N
Ir
Ir
N
N
N
N
[
Ir(L1) (bipy)]PF6
[Ir(L2) (bipy)]PF6
2
2
PF6
PF6
Br
MeO
N
N
N
N
N
Br
MeO
MeO
N
N
Br
Ir
Ir
N
N
N
N
N
Br
MeO
[
Ir(L3) (bipy)]PF6
[Ir(L4) (bipy)]PF6
2
2
Scheme 2. The structures of the iridium(III) complexes synthesised in this study.
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1
6
The complexes were synthesised according to the well known Nonoyama route that first isolates
the chloro-bridged dimeric Ir(III) species, [(L) Ir-(µ-Cl )-Ir(L) ]. Subsequent splitting of the dimer
2
2
2
with 2,2′-bipyridine gave the target cationic complexes. HRMS data was obtained for each of the
1
complexes and showed the correct isotopic distribution in all cases (Fig. 3). In addition to this, H
NMR spectroscopy was particularly insightful for the determination of the coordination of the
ligands to the iridium centre. This was typically noted by the upfield shift of the proton adjacent to
1
the cyclometalating carbon atom (for example, Fig. 4). In the H NMR spectra of [Ir(L2) (bipy)]PF
2
6
and [Ir(L4) (bipy)]PF , there were two unique methyl environments (note that they are equivalent in
2
6
the free ligand) indicative of cyclometalation in the expected manner (i.e. the benzo[g]quinoxaline
ligands do not act in a bridging manner). The frequency separations in these inequivalent methyl
resonances was approximately 0.5 ppm.
Figure 3. Example of a high resolution mass spectrum obtained for [Ir(L3) (bipy)]PF showing the
2
6
cationic fragment (main) and observed versus theoretical isotopic distribution pattern (inset).
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1
Figure 4. H NMR spectrum of [Ir(L1) (bipy)]PF that shows the upfield shift of the ligand proton adjacent
2
6
to the cyclometalating carbon atom.
UV-vis. and luminescence spectroscopy
The solution state absorption spectra of the ligands L1-L4 (Fig. 5) were recorded as chloroform
-5
solutions at 1 × 10 M and were comparable to related literature examples that are based on the
1
1-13
4
-1
-1
benzo[g]quinoxaline chromophore.
All ligands showed strong (ε >10 M cm ) π–π* transitions
in the UV region at 200-400 nm. A shoulder feature extended beyond 450 nm but again is attributed
to low energy π–π* transitions although weaker n–π* transitions may also contribute in this region.
The phenyl substituent (-H, -Me, -Br, -OMe) of the ligand clearly influences the positioning of
these bands, with the methoxy substituted L4 possessing the most bathochromically shifted
features, which was attributed to the increased conjugation induced by the -OMe substituents. The
corresponding emission spectra obtained for these ligands shows that they are all luminescent in the
visible region from 400-550 nm showing some vibronic structure to the peak shape. Again the
nature of the substituent influences the peak positioning, and in this case the benzil derivative (L1)
7

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possesses the longest emission wavelength. The recorded lifetimes for each ligand were found to be
1
<
5 ns, consistent with fluorescence from a π–π* emitting state.
L1
L2
L3
L4
L1
L2
L3
L4
1
1
1
40000
20000
00000
80000
60000
40000
20000
0
300
400
400
500
600
Wavelength/ nm
Wavelength/ nm
-5
Figure 5. UV–vis absorption (left) and emission (right) spectra of the ligands (10 M CHCl ).
3
UV-vis. spectroscopy on solutions of the complexes revealed spectra with multiple contributions
across the UV and visible regions (Fig. 6; Table 1). The majority of peaks at wavelengths than 250-
450 nm are attributed to different ligand-centred transitions that are perturbed upon coordination to
the Ir(III) centre, as well as contributions from spin allowed metal-to-ligand charge transfer bands
1
(
MLCT).
140000
120000
100000
C1
C2
C3
C4
80000
60000
40000
20000
0
250
350
450
550
650
750
Wavelength / nm
-5
Figure 6. UV–vis absorption spectra of the complexes (10 M CHCl ; C1 = [Ir(L1] (bipy)]PF , C2
3
2
6
=
[Ir(L2] (bipy)]PF , C3 = [Ir(L3] (bipy)]PF , C4 = [Ir(L4] (bipy)]PF ).
2
6
2
6
2
6
8

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The ancillary bipyridine ligand π–π* absorptions are also likely to contribute ca. 270-290 nm. When
compared to the ligand spectra, each of the complexes showed additional absorption bands across
the visible region that extended > 600 nm. These weaker absorption bands may comprise spin
3
17
forbidden MLCT bands. These Ir(III) complexes therefore demonstrate strong bathochromic
shifts in the visible region relative to related complexes based upon substituted 2-phenylquinoxaline
1
5
ligands. This is attributed to the lowering of the MLCT transitions due to the added conjugation of
the ligand benzo[g]quinoxaline ligands. The subtle variation between the complexes are again
indicative of the electronic character of the substituents; as with the free ligands the methoxy-
substituted variant [Ir(L4] (bipy)]PF possessed the lowest energy absorption features.
2
6
C1
C2
C3
C4
5
00
600
700
Wavelength/ nm
Figure 7. Visible emission spectra for the complexes (aerated CHCl , λ = 405 nm; C1 =
3
exc
[
Ir(L1] (bipy)]PF , C2 = [Ir(L2] (bipy)]PF , C3 = [Ir(L3] (bipy)]PF , C4 = [Ir(L4] (bipy)]PF ).
2 6 2 6 2 6 2 6
The luminescence spectra for the complexes were recorded on aerated samples at room temperature
Table 1). In all cases an excitation wavelength of 470 nm was used as this corresponds to strong
(
absorption bands in all of the complexes reported herein. The first collection of emission spectra
9

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were recorded between 490-750 nm and revealed a defined vibronic progression to the peak shape
that was reproduced for each of the four complexes, but differentiated from the corresponding free
ligands (Fig. 7). In fact these emission spectra for the complexes are virtually superimposable, with
little variance in the wavelength positioning of these bands across the series. Time-resolved
measurements provided the emission lifetimes for the visible emission which were all short-lived (<
10 ns). Taken together the data implies that these features may be due to intraligand transitions
associated with the coordinated phenyl benzo[g]quinoxaline units (see further discussion in
computational section).
Interestingly, these observations do not correspond with the luminescence properties of
1
5
closely related 2-phenylquinoxaline complexes of Ir(III), which we have previously described,
3
which were shown to be phosphorescent around 620-650 nm and attributed to a MLCT emitting
state. Therefore, further luminescence studies on the complexes investigated the possibility of
emission in the NIR region. Again, aerated solutions were studied at room temperature and, using
an excitation wavelength of 505 nm, spectra were collected between 850-1300 nm.
[a]
[a, b]
c]
[a, d]
Compound
λabs / nm
λem / nm
λem / nm
[
[e]
(
τ
/ ns )
(
τ
/ ns )
L1
L2
L3
L4
386, 309, 276
485 (1.4)
480 (1.1)
490 (1.8)
480 (1.1)
520 (4.9)
520 (7.0)
520 (2.9)
520 (< 1)
-
-
-
-
391, 323, 312, 276
393, 318, 280
402, 332, 319, 276
[
[
[
[
Ir(L1) (bipy)]PF6
554sh, 490, 415, 329, 286
915 (119)
912 (148)
928 (116)
949 (162)
2
Ir(L2) (bipy)]PF6
554sh, 491, 445, 420, 388, 367, 334, 283
554sh, 500, 439, 415, 340, 287
554sh, 455, 348, 285
2
Ir(L3) (bipy)]PF6
2
Ir(L4) (bipy)]PF6
2
a
Table 1. Absorption and emission data for the ligands and complexes. Recorded in aerated CHCl ;
3
b
c
d
e
λexc = 405 nm; λ = 295 nm; λ = 505 nm; λ = 355 nm.
exc
exc
exc
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0

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As shown in Figure 8, under these conditions, each of the complexes demonstrated luminescence in
the NIR region peaking around 915-950 nm. The appearance of the emission peak is very broad and
tails to ca. 1250 nm. This is consistent with the findings of previous work on related complex
1
2,13
structures using different benzo[g]quinoxaline based ligands.
Each of the spectra includes an
additional sharp emission peak centred at 1274 nm which is characteristic of the radiative decay that
1
3
accompanies the spin relaxation of singlet oxygen ( O → O ). The photogeneration of singlet
2
2
oxygen in solution, firstly corroborates the use of aerated solvent for the measurements, and
secondly, implies the presence of an excited triplet state on a sensitizer molecule. Further, time-
resolved measurements monitoring the decay kinetics of the NIR emission band revealed emission
lifetimes in the range of 116-162 ns, each indicative of a triplet excited state. Critically, these
lifetimes are significantly longer than those attributed to the visible emission of the complexes
(measured under identical sample conditions).
C1
C2
C3
C4
900
1000
1100
1200
1300
Wavelength
Figure 8. Steady state near-IR emission spectra of the complexes. The peak at 1274 nm is due to
1
the photogeneration of, and subsequent emission from, O . (Aerated CHCl , λ = 505 nm; C1 =
2
3
exc
[
Ir(L1] (bipy)]PF , C2 = [Ir(L2] (bipy)]PF , C3 = [Ir(L3] (bipy)]PF , C4 = [Ir(L4] (bipy)]PF ).
2 6 2 6 2 6 2 6
1
1

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Density Functional Theory (DFT) Calculations
The geometries of all the complexes were optimised in Gaussian09 using the B3LYP functional
with a combination of 6-31G* basis sets for the ligand atoms and the Stuttgart-Dresden (SDD) core
1
8
potential/basis set for the iridium metal core. Solvent effects were included implicitly using the
self-consistent reaction field (SCRF) and polarized continuum (IEFPCM) models. Time dependent
DFT (TD-DFT) calculations were performed on the stationary points to determine the first five
singlet excited states for each complex, using the same basis sets and the long range corrected
CAM-B3LYP functional. Phosphorescence and spin forbidden absorption bands were computed
using unrestricted DFT calculations, computing parameters for the first triplet state of the
complexes (T ). Figure 8 shows the superimposition of the calculated molecular geometries of the
1
complexes in both singlet and triplet excited states. Whilst the geometries look broadly similar it is
notable that there are differing degrees of distortion within the coordinated ligand fragments of the
two excited states. The composition of the complexes’ HOMOs and LUMOs are detailed in Table
2.
HOMO
[
a]
Compound
Ir (5d)
26%
C^N (1)
36%
C^N (2)
36%
bipy
1%
[
[
[
[
Ir(L1)
Ir(L2)
Ir(L3)
Ir(L4)
2
2
2
2
(bipy)]PF
(bipy)]PF
(bipy)]PF
(bipy)]PF
6
6
6
6
26%
13%
16%
37%
44%
40%
36%
42%
43%
1%
0%
1%
LUMO
[
[
[
[
Ir(L1)
Ir(L2)
Ir(L3)
Ir(L4)
2
2
2
2
(bipy)]PF
(bipy)]PF
(bipy)]PF
(bipy)]PF
6
6
6
6
3%
3%
3%
3%
48%
47%
49%
47%
47%
48%
46%
49%
1%
2%
3%
2%
Table 2. Predicted MO compositions of the HOMO and LUMO of the complexes.
1
2

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In all cases the cyclometalated ligands are predicted to contribute strongly to both molecular
orbitals, with Ir contributions of 13-26% for the HOMO. Orbital contributions to the excited states
(see ESI) show that a range of transitions are predicted for each. The lowest calculated energy
excited state is dominated by the HOMO → LUMO transition, which may comprise both MLCT
and intra(cyclometalated)ligand contributions (Table 2). It is noteworthy that the DFT calculations
suggest that bipyridine localised orbitals primarily contribute to the upper lying LUMO+2 and
LUMO+3, neither of which feature strongly as participants in contributing transitions to the
predicted excited states. Ligand centred transitions, more specifically transitions localized on the
benzo[g]quinoxaline ligands, are assigned as the dominant contributors to the visible complex
emission. This is consistent with the similarity in vibronic band shapes between the free ligand
emission (Fig. 2) and the complex emission band (Fig. 3). Both of these emission bands show clear
-1
vibrational progressions, with a vibrational spacing of ~1300 cm . This vibrational frequency is
consistent with quinoxalene (predominantly pyrazine) ring breathing modes, which are the most
infrared active modes for these ligands. The electronic emission spectrum of the free ligand has
been simulated using the Franck-Condon-Herzberg-Teller approach implemented in the Gaussian09
1
9 , 20
software package.
Both ground and excited states have been optimized using the
DFT//B3LYP/6-31G(d) method, with the excited state being optimized using TD-DFT. The Franck-
Condon progression has then been calculated using the vibrational frequencies from the two states,
-1
at a temperature of 300 K, and convoluted with a HWHM of 500 cm . The computed spectrum is
shown below (Fig. 9) in comparison with the experimental spectra of the free ligand L1, and the
complex [Ir(L1] (bipy)]PF .
2
6
1
3

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Figure 9. Franck-Condon simulation of the L1 A-X transition (blue), overlaid against the
experimental emission spectra of the free ligand L1 (red), and the complex [Ir(L1] (bipy)]PF6
2
(black). The spectra have been offset by their respective vibronic origin, and are therefore displayed
-1
as emission energy (cm ) relative to zero. The grey bars are a guide to the eye, showing different
vibronic features.
The spectra shown in Fig. 9 show remarkable agreement between simulation and experiment. The
simulated spectrum has been deliberately slightly under convoluted to highlight the overlap of the
vibronic peak positions. This agreement not only confirms the assignment of the vibronic features,
but further illustrates the similarity between L1 within the complex [Ir(L1] (bipy)]PF and as its
2
6
free ligand. The vibrational spacing is approximately consistent between the free ligand and the
complex, but there is a clear change in the Franck-Condon factors associated with the progression.
This is equally consistent with the assignment: the metal binding to the ligand alters the bond
lengths of the pyrazine ring, slightly lengthening the C-N bonds (C=N, 1.315 Å; C-N, 1.37 Å in the
free ligand, C=N, 1.347 Å; C-N, 1.39 Å in the complex) which will alter the Franck-Condon
overlap between S and S .
0
1
1
4

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Figure 10. A comparison of the calculated geometries of the singlet and triplet excited states for
[
Ir(L3) (bipy)]PF (left, RMSD = 0.185 Å) and [Ir(L4) (bipy)]PF (right, RMSD = 0.149 Å).
2 6 2 6
Further photochemical properties of the complexes have also been computed: the wavelengths of
selected calculated transitions are shown in Table 3, including those that are spin forbidden and thus
relate to an excitation (T ← S , at S minimum energy geometry) and phosphorescence (T → S , at
1
0
0
1
0
T minimum energy geometry). The overlap between the S and T minimum energy geometries are
1
1
1
highlighted in Fig. 10. The calculated data corresponds well with the experimental results and
importantly predicts that the emission should occur in the NIR region around 962-1019 nm. Both
the spin allowed and spin forbidden excitations correlate well with the primary features of the
complexes’ absorption spectra. The DFT results suggest that the variations in ligand structure
induce differences in emission energy, but these subtleties were not apparent in the broadened
features of the experimental data. It is also intriguing to see that such subtle changes in geometry
within the triplet state, i.e. geometric relaxation after ISC, lead to dramatic changes in the T – S0
1
energy gap. Tempering or expanding upon this shift will form the basis of future work.
1
5

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Compound
S ← S / nm
T ← S / nm
T → S / nm
1 0
1
0
1
0
[
[
[
[
Ir(L1)
Ir(L2)
Ir(L3)
Ir(L4)
2
2
2
2
(bipy)]PF
(bipy)]PF
(bipy)]PF
(bipy)]PF
6
6
6
6
433
434
428
428
557
602
590
583
993
980
Ta
ble
1019
962
3
.
Calculated vertical transitions for the complexes.
Conclusion
Extending the conjugation of cyclometalating ligands on iridium(III) can bathochromically shift
luminescence from organometallic complexes into the NIR spectral region. For Ir(III), substituted
benzo[g]quinoxaline type cyclometalting ligands are appropriate choices to achieve this and the
study shows that supporting DFT calculations can reliably predict the spin forbidden transitions that
dominate the lowest energy absorptions and phosphorescent emission properties of these
complexes.
Experimental
All reactions were performed with the use of vacuum line and Schlenk techniques. Reagents were
1
13
1
commercial grade and were used without further purification. H and C{ H} NMR spectra were
1
13
1
run on NMR-FT Bruker 400 or 250 spectrometers and recorded in CDCl . H and C-{ H} NMR
3
chemical shifts (δ) were determined relative to internal TMS and are given in ppm. Low-resolution
mass spectra were obtained by the staff at Cardiff University. High-resolution mass spectra were
carried out by at the EPSRC National Mass Spectrometry Service at Swansea University. UV-Vis
-5
studies were performed on a Jasco V-570 spectrophotometer as MeCN solutions (10 M).
Photophysical data were obtained on a JobinYvon-Horiba Fluorolog spectrometer fitted with a JY
TBX picosecond photodetection module as MeCN solutions. A Hamamatsu R5509-73 detector
(cooled to −80 °C using a C9940 housing) was used for NIR luminescence measurements. For the
NIR lifetimes the pulsed laser source was a Continuum Minilite Nd:YAG configured for 355 nm
1
6

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output. For all NIR emission data, a 850 nm band pass filter was used. The pulsed source was a
Nano-LED configured for 295 nm output operating at 1 MHz. All luminescence lifetime profiles
were obtained using the JobinYvon-Horiba FluoroHub single photon counting module and the data
fits yielded the lifetime values using the provided DAS6 deconvolution software.
Data collection and processing
X-ray diffraction datasets were measured on a Rigaku AFC12 diffractometer equipped with
enhanced sensitivity (HG) Saturn724+ CCD detector mounted at the window of an FR-E+
SuperBright rotating anode generator (Mo Kα, λ = 0.71075 Å) with VHF Varimax optics (70 µm
2
1
22
focus) using CrysAlisPro software for data collection and reduction.
Structure analysis and refinement
2
3
2
o
The structures were solved by direct methods using Superflip and refined on F by full-matrix
2
4
25
least-squares refinements using SHELXL within the OLEX2 suite. All non-hydrogen atoms
were refined with anisotropic displacement parameters, and all hydrogen atoms were added at
calculated positions and refined using a riding model with isotropic displacement parameters based
on the equivalent isotropic displacement parameter (U ) of the parent atom.
eq
Density Functional Theory
The geometries of all the complexes were optimised in Gaussian09 using the B3LYP functional
with a combination of 6-31G* basis sets for the ligand atoms and the Stuttgart-Dresden (SDD) core
16
potential/basis set for the iridium metal core. Solvent effects were included implicitly using the
self-consistent reaction field (SCRF) and polarized continuum (IEFPCM) models, and in all cases
the solvent chosen was chloroform, characterized by an electrical permittivity of ε = 4.7113.
Molecular orbital calculations were performed using the same basis sets and functionals, and
1
7

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decomposition analysis was performed on the frontier orbitals using the GaussSum software
package. TD-DFT calculations were performed on the stationary points to determine the first five
singlet excited states for each complex, using the same basis sets and the long range corrected
CAM-B3LYP functional. Phosphorescence and spin forbidden absorption bands were computed
using unrestricted DFT calculations, computing parameters for the first triplet state of the
complexes (T ). The Franck-Condon simulations were performed without implicit solvation,
1
however it is clear from the agreement between experiment and theory that this does not
significantly alter the results. Additionally, the Franck-Condon progressions are harmonic in nature.
Small shifts in peak positions would be expected using anharmonic modes (the harmonic scaling
2
6
factor for this functional and basis set is ca.0.962), however these shifts are expected to be small
relative to the peak widths. The Chimera software package was used to visualise the complexes and
2
7
compare singlet and triplet geometries.
Synthesis
Synthesis of 2,3-diphenylbenzo[g]quinoxaline (L1)
To a stirred solution of benzil (322 mg, 1.6 mmol) in ethanol (15 mL) was added 2,3-
diaminonaphthalene (250 mg, 1.6 mmol) and acetic acid (0.5 mL). The reaction mixture was heated
at reflux under a nitrogen atmosphere for 24 hours. The reaction mixture was cooled to room
1
temperature and a precipitate collected by filtration to give L1 as a brown solid (0.34 g, 64%). H
NMR (300 MHz, CDCl ): δ 8.68 (s, 2H, CH), 8.05 (dd, J = 3.21 Hz, 6.39 Hz, 2H), 7.50 (m, 6H),
3
H
1
3
1
7
1
.30 (m, 6H) ppm. C{ H} NMR (101 MHz, CDCl ): δ 154.2, 139.2, 138.0, 134.1, 129.8, 129.0,
3
C
28.6, 128.3, 127.6, 126.8 ppm. HRMS found m/z 333.1387; calcd m/z 333.1386 for C H N . UV-
24
16
2
3
-1
-1
vis. (CHCl ) λ
(ε / dm mol cm ): 386 (25200), 309 (71500), 276 (114100) nm. Selected IR
3
max
(solid) υmax = 1607, 1508, 1441, 1346, 1248, 1175, 1013, 976, 876, 836, 760, 741, 691, 552, 500.0,
-1
4
90, 467 cm .
1
8

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Synthesis of 2,3-di-p-tolylbenzo[g]quinoxaline (L2)
As L1 but with 4,4’-dimethylbenzil (376 mg, 1.6 mmol) and 2,3-diaminonaphthalene (250 mg, 1.6
1
mmol). Product collected as a brown solid (0.41 g, 71%). H NMR (300 MHz, CDCl ) δ 8.73 (s,
3
H
2H), 8.12 (dd, J = 3.2, 6.5 Hz, 2H), 7.57 (dd, J = 3.1, 6.7 Hz, 2H), 7.49 (d, J = 8.0 Hz, 4H), 7.19 (d,
1
3
1
J = 8.0 Hz, 4H), 2.41 (s, 6H) ppm. C{ H} NMR (101 MHz, CDCl ) δ 154.3, 139.0, 138.0, 136.5,
3
C
134.0, 129.8, 129.0, 128.6, 127.4, 126.6, 21.4 ppm. HRMS m/z found 361.1699; calcd m/z 316.1699
3
-1
-1
for C H N . UV-vis. (CHCl ) λ (ε / dm mol cm ): 391 (24800), 323 (48600), 312 (54200),
2
6
20
2
3
max
2
8
76 (134500) nm. Selected IR (solid) υmax = 1607, 1508, 1445, 1346, 1247, 1174, 1109, 1013, 972,
-1
78, 835, 819, 760, 559, 500, 490, 471, 421 cm .
Synthesis of 2,3-bis(4-bromophenyl)benzo[g]quinoxaline (L3)
As L1 but with 4,4’-dibromobenzil (581 mg, 1.6 mmol) and 2,3-diaminonaphthalene (250 mg, 1.6
1
mmol). Product collected as a light brown solid (0.56 g, 72%). H NMR (400 MHz, CDCl ) δ 8.66
3
H
(s, 2H), 8.05 (dd, J = 3.2, 6.5 Hz, 2H), 7.52 (m, 2H), 7.46 (d, J = 8.4 Hz, 4H), 7.38 (d, J = 8.4 Hz,
1
3
1
4
1
H) ppm. C{ H} NMR (101 MHz, CDCl ) δ 152.6, 137.9, 137.8, 134.3, 131.7, 131.4, 128.6,
3 C
27.7, 127.0, 123.9 ppm. HRMS found m/z 490.9569; calcd m/z 490.9576 for C H Br N . UV-vis.
2
4
15
2
2
3
-1
-1
(
CHCl ) λ (ε / dm mol cm ): 393 (11400), 318 (34300), 280 (46400) nm. Selected IR (solid)
3 max
υmax = 1605, 1510, 1445, 1344, 1246, 1173, 1109, 1053, 1011, 972, 880, 833, 795, 758, 745, 723,
-1
6
56, 646, 623, 573, 557, 548, 532, 498, 471, 409 cm .
Synthesis of 2,3-bis(4-methoxyphenyl)benzo[g]quinoxaline (L4)
As L1 but with anisil (427 mg, 1.6 mmol) and 2,3-diaminonaphthalene (250 mg, 1.6 mmol).
1
Product collected as a light brown solid (0.50 g, 80%). H NMR (300 MHz, CDCl ) δ 8.61 (s, 2H,
3
H
CH), 8.02 (dd, J = 3.01, 6.42 Hz, 2H), 7.48 (d, J = 8.31 Hz, 6H), 6.83 (d, J = 8.43, 4H), 3.78 (s, 6H)
1
3
1
ppm. C{ H} (101 MHz, CDCl ) δ 160.4, 153.8, 138.0, 133.9, 131.8, 131.3, 128.5, 127.2, 126.5,
3
C
1
9

ACCEPTED MANUSCRIPT
1
13.8, 55.4 ppm. HRMS found m/z 393.1593; calcd m/z 392.1598 for C H N O . UV-vis. (CHCl )
26 21 2 2 3
3
-1
-1
λmax (ε / dm mol cm ): 402 (16900), 332 (24200), 319 (25600), 276 (110300) nm. Selected IR
(solid) υmax = 1605, 1578, 1541, 1508, 1445, 1414, 1344, 1275, 1246, 1173, 1109, 1051, 1015, 976,
-1
8
91, 880, 853, 833, 795, 756, 747, 656, 646, 623, 592, 573, 557, 532, 525, 496, 474, 421 cm .
Synthesis of complexes
Synthesis of [Ir(C^N) Cl ] IrCl .xH O (1 eq.) and ligand, L (2 eq.) were dissolved in 2-
2
2 2
3
2
ethoxyethanol (10 mL) and the reaction mixture heated at reflux for 48 hours. The reaction was then
cooled to room temperature and water (30 mL) was added to form a dark brown precipitate. The
solid was collected by filtration to yield [(L) Ir(µ-Cl )Ir(L) ].
2
2
2
Synthesis of [Ir(L1) (bipy)][PF ]
2
6
[
(L1) Ir(µ-Cl )Ir(L1) ] (195 mg, 0.11 mmol) and 2,2’-bipyridyl (36 mg, 0.23 mmol) were dissolved
2 2 2
in 2-ethoxyethanol (10 mL) and heated at reflux for 24 hours under a nitrogen atmosphere. The
reaction mixture was then cooled to room temperature and a saturated solution of aqueous
ammonium hexafluorophosphate was added. Upon formation of a red precipitate, the mixture was
filtered and the precipitate washed with water and diethyl ether. The crude solid was purified by
column chromatography using DCM followed by DCM/MeOH (9:1). Product collected as first red
band with DCM/MeOH and dried in vacuo to give [Ir(L1) (bipy)][PF ] as a red solid (38 mg, 15%).
2
6
1
H NMR (400 MHz, CDCl ) δ 8.65 (d, J = 5.6 Hz, 2H), 8.55 (s, 2H), 8.31 (d, J = 8.8 Hz, 2H), 8.12
3
H
(app. t, J = 8.8 Hz, 2H), 7.91 (d, J = 8.4 Hz, 2H), 7.89-7.77 (m, 6H), 7.75 (s, 2H), 7.64-7.60 (m,
6
H), 7.44 (app. t, J = 8.8 Hz, 2H), 7.36 (app. t, J = 6.4 Hz, 2H), 7.17 (s, 2H), 6.98 (d, J = 8.8 Hz,
H), 6.70 (app. t, J = 5.6 Hz, 2H), 6.61 (app. t, J = 7.2 Hz, 2H), 6.50 (d, J = 7.2 Hz, 2H) ppm.
2
1
3
1
C{ H} NMR (101 MHz, CDCl ) δ 164.5, 155.8, 154.3, 153.8, 147.9, 144.3, 139.6, 134.6, 133.7,
3
C
1
7
32.5, 131.5, 130.4, 129.4, 129.4, 128.9, 128.8, 128.2, 128.0, 127.6, 125.4, 122.0, 119.5, 114.4,
7.4, 76.7 ppm. HRMS found m/z 1011.2777; calcd m/z 1011.2786 for C H IrN . UV-vis.
5
8
38
8
2
0

ACCEPTED MANUSCRIPT
3
-1
-1
(
CHCl ) λ (ε / dm mol cm ): 554 (2100), 490 (3900), 415 (10700), 329 (24200), 286 (33200)
3 max
-1
nm. Selected IR (solid) υmax = 1607, 1445, 1350, 1253, 835, 760, 738, 696, 574, 557, 500, 468 cm .
Synthesis of [Ir(L2) (bipy)][PF ]
2
6
As [Ir(L1) (bipy)][PF ] but with [(L2) Ir(µ-Cl )Ir(L2) ] (56 mg, 0.03 mmol) and 2,2’-bipyridyl (10
2
6
2
2
2
1
mg, 0.07 mmol). Product collected as a red solid (44 mg, 62%) H NMR (400 MHz, CDCl ) δ 8.67
3
H
(
d, J = 6.60 Hz, 2H), 8.59 (s, 2H), 8.53 (d, J = 8.8 Hz, 2H), 8.21 (dd, J = 7.72, 8.80 Hz, 2H), 7.98
d, J = 8.80 Hz, 2H), 7.78 (m, 8H), 7.50 (m, 6H), 7.42 (app. t, J = 7.70, 2H), 7.24 (d, J = 7.72 Hz,
(
2H), 7.03 (d, J = 7.72 Hz, 2H), 6.65 (d, J = 8.84 Hz, 2H), 6.43 (s, 2H), 2.57 (s, 6H), 1.94 (s, 6H)
13
1
ppm. C{ H} NMR (101 MHz, CDCl ) δ 164.8, 156.0, 148.0, 142.5, 141.7, 141.1, 140.5, 136.9,
3
C
136.3, 135.1, 134.0, 133.4, 132.3, 130.0, 129.1, 128.8, 128.5, 128.1, 128.0, 127.7, 127.5, 125.8,
1
23.8, 122.0, 21.8, 21.7 ppm. HRMS found m/z 1067.3406; calcd m/z 1067.3413 for C H IrN .
6
2
46
6
3
-1
-1
UV-vis. (CHCl ) λ (ε / dm mol cm ): 554 (2500), 491 (4900), 445 (9700), 420 (11000), 388
3
max
(14900), 367 (16900), 334 (28300), 283 (69500) nm. Selected IR (solid) υmax = 1586, 1503, 1445,
1
4
398, 1359, 1314, 1257, 1211, 1177, 1138, 1069, 1042, 978, 833, 772, 635, 579, 556, 513, 471,
-1
24, 407 cm .
Synthesis of [Ir(L3) (bipy)][PF ]
2
6
As [Ir(L1) (bipy)][PF ] but with [(L3) Ir(µ-Cl )Ir(L3) ] (150 mg, 0.06 mmol) and 2,2’-bipyridyl
2
6
2
2
2
1
(
20 mg, 0.13 mmol). Product collected as a red solid (75 mg, 61%). H NMR (400 MHz, CDCl ) δ
3 H
8
.82 (d, J = 8.80 Hz, 2H), 8.56 (s, 2H), 8.52 (d, J = 6.40 Hz, 2H), 8.21 (app. t, J = 8.40 Hz, 2H),
.94 (d, J = 7.20 Hz, 3H), 7.77 – 7.68 (m, 5H), 7.62 (s, 2H), 7.49 (app. t, J = 8.40 Hz, 3H), 7.39
7
1
3
1
(
app. t, J = 6.00 Hz, 3H), 7.11 (d, J = 7.20 Hz, 3H), 6.97 (m, 5H), 6.62 (s, 2H) ppm. C{ H} NMR
(
101 MHz, d6-acetone) δ 164.5, 155.8, 153.4, 149.6, 144.1, 141.0, 138.8, 137.5, 137.4, 136.8,
C
134.1, 133.7, 133.6, 132.3, 131.5, 129.3, 129.1, 128.5, 128.3, 127.9, 127.5, 126.5, 126.0, 124.8,
1
24.2, 122.4, 78.3 ppm. LRMS found m/z 1326.90; calcd m/z 1326.92 for C H Br IrN . UV-vis.
58
34
4
6
2
1

ACCEPTED MANUSCRIPT
3
-1
-1
(
CHCl ) λ (ε / dm mol cm ): 554 (2400), 500 (5100), 439 (10700), 415 (11500), 340 (26000),
3 max
287 (34800) nm. Selected IR (solid) υmax = 1607, 1512, 1445, 1348, 1248, 1174, 1109, 1013, 878,
-1
8
35, 760, 559, 500 cm .
Synthesis of [Ir(L4) (bipy)][PF ]
2
6
As [Ir(L1) (bipy)][PF ] but with [(L4) Ir(µ-Cl )Ir(L4) ] (100 mg, 0.09 mmol) and 2,2’-bipyridyl
2
6
2
2
2
1
(
16 mg, 0.11 mmol). Product collected as a red solid (27 mg, 21%). H NMR (400 MHz, CDCl ) δ
3 H
8
7
5
5
1
1
1
3
1
.63 (d, J = 4.64 Hz, 2H), 8.45 (s, 2H), 8.43 (d, J = 8.00 Hz, 2H), 8.14 (app. t, J = 9.28 Hz, 2H),
.87 (d, J = 8.12 Hz, 2H), 7.76 (m, 6H), 7.62 (s, 2H), 7.40 (dd, J = 6.96, 8.12 Hz, 2H), 7.32 (dd, J =
.80, 9.28 Hz, 2H), 7.27 (s, 2H), 7.24 (s, 4H), 7.11 (d, J = 8.52 Hz, 2H), 6.94 (d, J = 8.16 Hz, 2H),
1
3
1
.99 (s, 2H), 3.91 (s, 6H), 3.26 (s, 6H) ppm. C{ H} NMR (101 MHz, CDCl ) δ 164.2, 161.4,
3
C
56.0, 148.2, 141.1, 137.2, 136.7, 136.2, 134.4, 134.0, 133.1 132.1, 130.5, 128.9, 128.4, 128.1,
27.5, 125.6, 121.6, 119.8, 114.7, 108.3, 55.6, 54.8 ppm. HRMS found m/z 1131.3193; calcd m/z
3
-1
-1
131.3209 for C H IrN O . UV-vis. (CHCl ) λ (ε / dm mol cm ): 554 (10000), 455 (31700),
6
2
46
6
4
3
max
48 (55200), 285 (120700) nm. Selected IR (solid) υmax = 1580, 1503, 1447, 1400, 1358, 1246,
-1
224, 1173, 1134, 1020, 978, 870, 837, 810, 772, 687, 519, 471 cm .
Acknowledgements
Cardiff University (Knowledge Economy Skills Scholarship to K.A.P.) and STG Aerospace are
thanked for financial support. The staff of the EPSRC Mass Spectrometry National Service
(University of Swansea) for providing MS data. We thank the staff of the EPSRC Mass
Spectrometry National Service (Swansea University) and the UK National Crystallographic Service
at the University of Southampton.
Supplementary Material: CCDC1896846 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif
2
2

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synthesis of cyclometalated iridium(III) complexes that incorporate conjugated and
substituted quinoxaline ligands
emission properties of the complexes reveals near infrared luminescence in aerated
solution