Enhanced luminescent iridium(III) complexes bearing aryltriazole cyclometallated ligands

Article abstract of DOI:10.1021/ic2014013

Herein we report the synthesis of 4-aryl-1-benzyl-1H-1,2,3-triazoles (atl), made via "Click chemistry" and their incorporation as cyclometallating ligands into new heteroleptic iridium(III) complexes containing diimine (N^{^}N) ancillary ligands 2,2′-bipyridine (bpy) and 4,4′-di-tert-butyl-2,2′-bipyridine (dtBubpy). Depending on decoration, these complexes emit from the yellow to sky blue in acetonitrile (ACN) solution at room temperature (RT). Their emission energies are slightly blue-shifted and their photoluminescent quantum efficiencies are markedly higher (between 25 and 80%) than analogous (C^{^}N)₂Ir(N ^{^}N)⁺ type complexes, where C^{^}N is a decorated 2-phenylpyridinato ligand. This increased brilliance is in part due to the presence of the benzyl groups, which act to sterically shield the iridium metal center. X-ray crystallographic analyses of two of the atl complexes corroborate this assertion. Their electrochemistry is reversible, thus making these complexes amenable for inclusion in light-emitting electrochemical cells (LEECs). A parallel computational investigation supports the experimental findings and demonstrates that for all complexes included in this study, the highest occupied molecular orbital (HOMO) is located on both the aryl fragment of the atl ligands and the iridium metal while the lowest unoccupied molecular orbital (LUMO) is located essentially exclusively on the ancillary ligand.

Full text of DOI:10.1021/ic2014013

ARTICLE
pubs.acs.org/IC
Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole
Cyclometallated Ligands
Sꢀebastien Ladouceur, Daniel Fortin, and Eli Zysman-Colman*
Dꢀepartement de chimie, Facultꢀe des Sciences, Universitꢀe de Sherbrooke, 2500 Blvd de l’Universitꢀe, Sherbrooke J1K 2R1, Canada
S Supporting Information
b
ABSTRACT: Herein we report the synthesis of 4-aryl-1-benzyl-1H-1,2,
3-triazoles (atl), made via “Click chemistry” and their incorporation as
cyclometallating lig∧ands into new heteroleptic iridium(III) complexes con-
taining diimine (N N) ancillary ligands 2,20-bipyridine (bpy) and 4,40-di-
tert-butyl-2,20-bipyridine (dtBubpy). Depending on decoration, these com-
plexes emit from the yellow to sky blue in acetonitrile (ACN) solution at
room temperature (RT). Their emission energies are slightly blue-shifted
and their photoluminescent quantum efficiencies are markedly higher
(between 25 and 80%) than analogous (C∧N)2Ir(N∧N)+ type complexes,
where C∧N is a decorated 2-phenylpyridinato ligand. This increased
brilliance is in part due to the presence of the benzyl groups, which act to
sterically shield the iridium metal center. X-ray crystallographic analyses of
two of the atl complexes corroborate this assertion. Their electrochemistry is
reversible, thus making these complexes amenable for inclusion in light-emitting electrochemical cells (LEECs). A parallel
computational investigation supports the experimental findings and demonstrates that for all complexes included in this study, the
highest occupied molecular orbital (HOMO) is located on both the aryl fragment of the atl ligands and the iridium metal while the
lowest unoccupied molecular orbital (LUMO) is located essentially exclusively on the ancillary ligand.
’ INTRODUCTION
triazole-based ligands into luminophore design would seem parti-
cularly astute given their facile and generalized synthesis via “Click”
chemistry, their similar binding motifs to well-studied pyridine sub-
strates, and the predicted blue-shift in emission energy compared to
pyridine conferred by the more electron-rich triazole (pKa(H2O)
1,2,3 triazole = 9.3; pKa(H2O) pyridinium = 5.2).
Since the seminal work of Watts and co-workers1 in the late
1980s to early 1990s, interest in the photophysical and electro-
chemical properties of cationic heteroleptic iridium complexes of
the form [(ppy)2Ir(bpy)]+, where ppyH is 2-phenylpyridine and
bpy is 2,20-bipyridine, has increased tremendously.2 Owing to the
large ligand field splitting and the strong spin orbit coupling
present in these iridium(III) complexes, color tuning is rendered
facile for these phosphors. This class of compounds has thus been
readily incorporated into the construction of a myriad of devices.3
The principal strategy for emission energy tuning of iridium-
(III)-based luminophores has reliedonthedecorationof either the
cyclometallating ligands and/or the ancillary ligand with either
electron-donating or electron-withdrawing groups. Far less ex-
plored is modification of the heterocycle itself. Those heterocycles
investigated to date as ligands about iridium include: 3-Pyridyl-1,
2-pyrazoles,4 1-aryl-1,2-pyrazoles,4c,d,5 pyridyl-imidazoles,5a,6 phenyl-
imidazoles,7 pyridyl-1,2,4-triazoles,4a,c,8 5-aryl-1,2,4-triazoles,9 and
pyridyl-1,2,3,4-tetrazoles.4a,10 There is a particular paucity of
examples for the use of aryl-1,2,3-triazoles (I) and/or pyridyl-1,2,
3-triazoles (II) inluminophoric complexes, iridium or otherwise.11
In fact, concurrent with our own efforts, only very recently have
I12 or II13 (R = alkyl, benzyl, phenyl or steroidyl) been inves-
tigated as ligands about iridium(III), notably by the de Cola and
Schubert groups. In light of the increasing interest in the use
of triazole ligands on iridium, a detailed structureꢀproperty
investigation would seem to be warranted. The incorporation of
In this paper, we report the synthesis, photophysical and
electrochemical properties of a series of [(atl)2Ir(N∧N)]+(PF6)ꢀ
complexes (Chart 1), where atl is an aryl-1H-1,2,3-triazole cyclo-
metallating ligand and N∧N is a diimine ligand. The properties of
these complexes are compared to archetypal [(C∧N)2Ir(N∧N)+]-
(PF6)ꢀ complexes, wherein C∧N is either a 2-phenylpyridinato
(ppy, complexes 1a and 1b) or a 2-(2,4-difluorophenyl)-5-methyl-
pyridinato (dFMeppy, complexes 3a and 3b) ligand. We demon-
strate that the electrochemical gap can be easily tuned resulting in
complexes that emit from the yellow to sky blue. To our knowledge,
the complexes presented herein are among the brightest known
at each of their respective wavelengths in acetonitrile solution.
We attribute the increased quantum yields to steric shielding of
Received: July 6, 2011
Published: September 30, 2011
r
2011 American Chemical Society
11514
dx.doi.org/10.1021/ic2014013 Inorg. Chem. 2011, 50, 11514–11526
|

Inorganic Chemistry
ARTICLE
ꢀ
Chart 1. Structure of Heteroleptic Iridium Complexes
Bearing Chelating Aryltriazole (atl) Ligands
Synthesis of [(C∧N)2Ir(N∧N)]+PF6 Complexes. Method A.
This protocol proceeds via isolation of the μ-dichloro bridged iridium-
(III) dimer described by Nonoyama16 followed by its cleavage with 2
equiv of ancillary N∧N ligand. The desired complex was obtained as its
ꢀ
PF6 salt after anion metathesis using NH4PF6. Each complex was
purified by flash chromatography on silica gel using a gradient of acetone
in DCM (0ꢀ10% acetone/DCM). The purified complex was redis-
solved in a minimum amount of MeOH and precipitated again by adding
it to an NH4PF6 solution (10 mL; 1 g/10 mL). The solid complex was
filtered, washed with water, and dried under vacuum. This protocol was
used for the synthesis of 1a, 1b, 2a, 3a, and 3b.
Method B. This protocol does not require dimer isolation. The
corresponding C∧N ligands (0.58 mmol, 2.2 equiv.) and IrCl3 (0.27
mmol, 1.0 equiv.) were placed in a mixture of 2-EtOEtOH and H2O
(6/1) for a concentration of about 0.04 M of IrCl3. The suspension was
degassed repeatedly and placed under N2 and heated at 110 °C for about
19 h. The corresponding N∧N ligands (0.30 mmol, 1.1 equiv.) were then
added, and the solution was stirred at 110 °C for about 19 h. The colored
solution was left to cool to room temperature (RT), diluted with water,
and the resulting aqueous phase was washed with portions of Et2O. The
aqueous phase was then heated to 70 °C for about 10 min to remove
trace Et2O and then cooled to RT. To the reaction mixture was added
a solution of NH4PF6 (10 mL, 6.13 mmol; 1 g/10 mL) with the sub-
sequent formation of a precipitate. Each complex was purified by flash
chromatography on silica gel using a gradient of DCM in acetone
(0ꢀ10%). The purified complex was redissolved in a minimum amount
of MeOH and precipitated again by adding it to an NH4PF6 solution
(10 mL; 1 g/10 mL). The title complex was filtered, washed with water,
and dried under vacuum. This protocol was used for the synthesis of 1b,
2a, 2b, 4a, and 4b.
the benzyl groups (I, R = Bn) about the iridium center. Crystal
structure analysis of two of the complexes corroborates this
hypothesis. Cyclic voltammetry studies reveal reversible oxida-
tion and reduction waves, making these complexes appropriate
for incorporation into light-emitting electrochemical cells (LEECs).
A combined density functional theory (DFT) and time-dependent
DFT (TD-DFT) study elucidates the origin of the transitions
involved in both absorption and emission for these com-
plexes. These transitions were found to be an admixture of metal-
to-ligand charge transfer/ligand-to-ligand charge transfer (MLCT/-
LLCT) transitions, similar to transitions found in analogous
[(ppy)2Ir(bpy)]+ complexes.
20
(1a) Iridium(III) Bis[2-phenyl-pyridine-N,C ]-2,20-bipyri-
dine Hexafluorophosphate. A yellow solid was obtained (304.7
mg, 83% - Method A). Rf: 0.33 (Acetone/DCM (1/9)). 1H NMR (400
MHz, CDCl3) δ (ppm): 8.66 (d, J = 8.2 Hz, 2H), 8.13 (td, J = 8.0, 1.6
Hz, 2H), 7.92 (t, J = 6.4 Hz, 4H), 7.76 (td, J = 8.0, 1.3 Hz, 2H), 7.68 (d,
J = 7.8 Hz, 2H), 7.49 (d, J = 5.4 Hz, 2H), 7.40 (t, J = 6.1 Hz, 2H), 7.03 (t,
J = 6.7 Hz, 4H), 6.91 (td, J = 7.4, 1.2 Hz, 2H), 6.29 (d, J = 7.5 Hz, 2H).
13C NMR (75 MHz, CDCl3) δ (ppm): 155.8, 150.1, 148.6, 139.8,
138.1, 131.7, 130.8, 130.0, 128.0, 127.0, 126.3, 125.4, 124.8, 123.4, 122.7,
119.6. HR-MS (C32H24N4Ir+) Calculated: 657.16247; Experimental:
657.16195. The characterization generally matches that found in the
literature.17
’ EXPERIMENTAL SECTION
General Procedures. Commercial chemicals were used as supplied.
All experiments were carried out with freshly distilled anhydrous solvents
obtained from a Pure Solv solvent purification system from Innovative
Technologies except where specifically mentioned. Triethylamine (Et3N)
and di-iso-propylamine (iPr2NH) were distilled over CaH2 under a nitrogen
atmosphere. Solvents employed for the Click reaction were used without
further purification: these consisted of commercial lab grade methanol and
deionized water, which was accessible in house. Aryltriazoles were prepared
following a one-pot protocol14 while 2-(2,4-difluorophenyl)-5-methylpyridine-
(dFMeppyH) was obtained via a Kr€ohnke condensation.15
All reactions were performed using standard Schlenck techniques
under an inert (N2) atmosphere, save for the Click reactions. Flash
column chromatography was performed using silica gel (Silia-P from
Silicycle, 60 Å, 40ꢀ63 μm). Analytical thin layer chromatography
(TLC) was performed with silica plates with aluminum backings (250
μm with indicator F-254). Compounds were visualized under UV light.
1H and 13C NMR spectra were recorded on either a Bruker Avance
spectrometer at 400 and 100 MHz, respectively, or a Bruker Avance
spectrometer at 300 MHz and 75 MHz, respectively. The following
abbreviations have been used for multiplicity assignments: “s” for singlet,
“d” for doublet, “t” for triplet, “m” for multiplet, and “br” for broad.
Deuterated cholorform (CDCl3) was used as the solvent of record
except where noted below. Spectra were referenced to the solvent peak.
Melting points (Mp) were recorded using open-end-capillaries on a
Meltemp melting point apparatus and are uncorrected. GC-MS samples
were separated on a Shimadzu QP 2010 Plus equipped with a HP5-MS
30 m ꢁ 0.25 mm ID ꢁ 0.25 μm film thickness column. Accurate mass
measurements were performed on a LC-MSD-TOF instrument from
Agilent Technologies in positive electrospray mode. Protonated molec-
ular ions (M+H)+ were used for empirical formula confirmation. Spectra
were recorded at the Universitꢀe de Montrꢀeal Mass Spectrometry facility.
20
(1b) Iridium(III) Bis[2-phenyl-pyridine-N,C ]-4,40-di-tert-
butyl-2,20-bipyridine Hexafluorophosphate. A yellow solid
was obtained (228.8 mg, 76% - Method B). Rf: 0.50 (DCM). Mp:
1
219 °C. H NMR (400 MHz, CDCl3) δ (ppm): 8.37 (d, J = 1.8 Hz,
2H), 7.88 (d, J = 7.9 Hz, 2H), 7.82 (d, J = 5.8 Hz, 2H), 7.75 (dt, J = 7.8,
1.5 Hz, 2H), 7.66 (dd, J = 7.8, 1.1 Hz, 2H), 7.60 (dd, J = 5.1, 0.7 Hz, 2H),
7.38 (dd, J = 5.9, 1.9 Hz, 2H), 7.08 (ddd, J = 7.3, 5.9, 1.4 Hz, 2H), 7.00
(td, J = 7.6, 1.2 Hz, 2H), 6.89 (td, J = 7.4, 1.3 Hz, 2H), 6.29 (dd, J = 7.6,
1.0 Hz, 2H), 1.42 (s, 18H). 13C NMR (101 MHz, CDCl3) δ (ppm):
167.8, 164.0, 155.9, 151.0, 149.9, 149.3, 143.9, 138.2, 131.9, 130.8, 125.5,
124.8, 123.7, 122.6, 121.8, 119.6, 110.0, 35.9, 30.5. HR-MS
(C40H40N4Ir+) Calculated: 769.28767; Experimental: 769.28811.
The characterization generally matches that found in the literature.18
20
(2a). Iridium(III) Bis[10-phenyl-1,2,3-triazolato-N,C ]-2,20-
bipyridine Hexafluorophosphate. A yellow solid was obtained
(128.3 mg, 50% - Method B). Rf: 0.52 (Acetone/DCM (1/9)). Mp:
182 °C. 1H NMR (400 MHz, CD3CN) δ (ppm): 8.46 (dt, J = 8.1, 1.1
Hz, 2H), 8.16ꢀ8.07 (m, 6H), 7.51 (ddd, J = 7.5, 1.4, 0.5 Hz, 2H), 7.47
(ddd, J = 7.6, 5.5, 1.2 Hz, 2H), 7.42ꢀ7.34 (m, 6H), 7.23ꢀ7.18 (m, 4H),
6.99 (td, J = 7.5, 1.2 Hz, 2H), 6.85 (td, J = 7.5, 1.4 Hz, 2H), 6.25 (ddd, J =
7.6, 1.1, 0.5 Hz, 2H), 5.46 (d, J = 1.9 Hz, 4H).13C NMR (75 MHz)
δ (ppm): 158.1, 157.5, 152.2, 147.0, 140.0, 136.7, 135.4, 133.4, 130.0,
11515
dx.doi.org/10.1021/ic2014013 |Inorg. Chem. 2011, 50, 11514–11526

Inorganic Chemistry
ARTICLE
129.7, 129.2, 128.8, 128.3, 124.6, 123.6, 123.4, 120.8, 56.1. HR-MS
(C40H32N8Ir+) Calculated: 817.23737; Experimental: 817.2382.
160.0 (d, J= 13.1 Hz), 157.2 (s), 156.6 (d, J= 12.7 Hz), 153.2 (s), 152.0 (s),
151.4 (d, J = 3.9 Hz), 135.2 (s), 129.8 (d, J = 10.7 Hz), 125.5 (s), 122.7 (s),
122.0 (s), 115.7 (d, J = 18.3 Hz), 98.8 (t, J = 25.6 Hz), 56.3 (s), 36.4 (s),
30.4 (s). HR-MS (C48H44N8F4Ir+) Calculated: 1001.32488; Experimen-
tal: 1001.32511.
20
(2b). Iridium(III) Bis[10-phenyl-1,2,3-triazolato-N,C ]-4,40-
di-tert-butyl-2,20-bipyridine Hexafluorophosphate. A yellow
solid was obtained (228.5 mg, 63% - Method B). Rf: 0.9 (DCM). Mp:
245 °C. 1H NMR (400 MHz, CD3CN) δ (ppm): 8.45 (d, J = 1.8 Hz,
2H), 8.12 (s, 2H), 7.98 (d, J = 5.8 Hz, 2H), 7.51 (dd, J = 7.5, 1.3 Hz, 2H),
7.48 (dd, J = 5.9, 2.0 Hz, 2H), 7.43ꢀ7.33 (m, 6H), 7.29ꢀ7.14 (m, 4H),
6.98 (td, J = 7.5, 1.2 Hz, 2H), 6.84 (td, J = 7.5, 1.4 Hz, 2H), 6.24 (dd, J =
7.5, 0.6 Hz, 2H), 5.47 (s, 4H), 1.43 (s, 18H). 13C NMR (75 MHz,
CD3CN) δ (ppm): 164.7, 158.1, 157.4, 151.6, 147.5, 136.6, 135.4,
133.4, 129.9, 129.7, 129.2, 128.8, 125.3, 123.5, 123.4, 121.8, 120.8, 105.7,
56.1, 36.4, 30.4. HR-MS (C48H48N8Ir+) Calculated: 929.36257; Ex-
perimental: 929.36251.
X-ray Crystallography. The crystals for 2a, 4a were grown by slow
evaporation of a dichloromethane/acetone solvent mixture while crys-
tals for 3b were grown from a chloroform solution. One single crystal of
0.30 mm ꢁ 0.20 mm ꢁ 0.05 mm for 2a, one single crystal of 0.20 mm ꢁ
0.30 mm ꢁ 0.50 mm for 4a, and one single crystal of 0.20 mm ꢁ
0.30 mm ꢁ 0.70 mm for 3b were each individually mounted using a glass
fiber on the goniometer. Data were collected on an Enraf-Nonius CAD-
4 automatic diffractometer at the Universitꢀe de Sherbrooke using ω
scans at 198(2) K. The DIFRAC20 program was used for centering,
indexing, and data collection. One standard reflection was measured
every 100 reflections, no intensity decay was observed during data
collection. The data were corrected for absorption by empirical methods
based on ψ scans and reduced with the NRCVAX21 programs. They
were solved using SHELXS-9722 and refined by full-matrix least-squares
on F2 with SHELXL-97.22 The non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were placed at idealized calculated
geometric position and refined isotropically using a riding model. A
summary of the refinement parameters and the resulting factors for 2a,
4a and 3b is given in Supporting Information, Table S1 and ORTEP
drawings for each of 2a, 3b and 4a at 30% probability are given in Figures
S1, S2 and S3, respectively.
Density Functional Theory (DFT) Calculations. Calculations
were performed with Gaussian 0923 at the Universitꢀe de Sherbrooke
with Mammouth super computer supported by le Rꢀeseau Quꢀebꢀecois de
Calculs de Haute Performances. The DFT24 and TD-DFT25 were calcu-
lated with the B3LYP26 method; excited-state triplet geometries were
calculated using the unrestricted B3LYP method (UB3LYP). The 3-21G*27
basis set was used for C, H and N, and the VDZ (valence double ζ) with
SBKJC effective core potential27a,28 basis set was used for Iridium. The
predicted phosphorescence wavelengths were obtained by energy dif-
ferences between the Triplet and Singlet optimized states.29 The energy,
oscillator strength and related MO contributions for the 100 lowest
singletꢀsinglet excitations were obtained from the TD-DFT/Singlets
output file. The calculated absorption spectra was visualized with
GaussSum 2.1 (fwhm: 1000 cmꢀ1).30
Photophysical Characterization. All samples were prepared in
either HPLC grade acetonitrile (ACN) or a mixture of spectroscopic
grade methanol (MeOH) and ethanol (EtOH), with concentrations on
the order of 25 μM. Absorption spectra were recorded at RT and at 77 K
in a 1.0 cm capped quartz cuvette and an NMR tube, respectively, using a
Shimadzu UV-1800 double beam spectrophotometer. Molar absorptiv-
ity determination was verified by linear least-squares fit of values
obtained from at least three independent solutions at varying concen-
trations with absorbances ranging from 0.01ꢀ2.6 M. Steady-state emis-
sion spectra were obtained by exciting at the longest wavelength
absorption maxima using a Horiba Jobin Yvon Fluorolog-3 spectro-
fluorometer equipped with double monochromators and a photomul-
tiplier tube detector (Hamamatsu model R955). Emission quantum
yields were determined using the optically dilute method.31 A stock
solution for each complex with an absorbance of about 0.5 was prepared
and then four dilutions were obtained with dilution factors of 40, 20,
13.3, and 10 resulting in optical dilution absorbances of about 0.013,
0.025, 0.038, and 0.05, respectively. The BeerꢀLambert law was
assumed to remain linear at the concentrations of the solutions. The
emission spectra were then measured after the solutions were rigorously
degassed with solvent-saturated nitrogen gas (N2) for 20 min prior to
spectrum acquisition using septa-sealed quartz cells from Starna. For
each sample, linearity between absorption and emission intensity was
verified through linear regression analysis and additional measurements
(3a) Iridium(III) Bis[2-(2,4-difluorophenyl)-5-methylpyri-
dine-N,C ]-2,20-bipyridine Hexafluorophosphate. A yellow
20
solid was obtained (162.6 mg, 68% - Method A). Rf: 0.43 (Acetone/
1
DCM (1/9)). Mp: 215 °C. H NMR (300 MHz, CDCl3) δ (ppm):
8.80 (d, J = 8.2 Hz, 2H), 8.29ꢀ8.15 (m, 4H), 7.93 (d, J = 4.1 Hz, 2H),
7.63 (d, J = 6.9 Hz, 2H), 7.50 (t, J = 6.4 Hz, 2H), 7.19 (d, J = 1.0 Hz, 2H),
6.62ꢀ6.51 (m, 2H), 5.65 (dd, J = 8.3, 2.4 Hz, 2H), 2.18 (s, 6H). 13C
NMR (75 MHz, CD3CN) δ (ppm): 165.7 (d, J = 12.2 Hz), 163.6 (d, J =
12.5 Hz), 162.1 (dd, J = 33.5, 9.6 Hz), 160.1 (d, J = 12.8 Hz), 156.6 (s),
154.7 (d, J = 5.3 Hz), 151.8 (s), 150.0 (s), 141.1 (s), 140.6 (s), 135.5 (s),
129.5 (s), 125.9 (s), 124.1 (d, J = 19.5 Hz), 114.7 (d, J = 17.6 Hz), 99.5 (t,
J = 27.1 Hz), 18.1 (s). HR-MS (C34H24N4F4Ir+) Calculated:
757.15608; Experimental: 757.15793.
(3b) Iridium(III) Bis[2-(2,4-difluorophenyl)-5-methylpyri-
dine-N,C ]-4,40-di-tert-butyl-2,20-bipyridine Hexafluoro-
20
phosphate. A yellow solid was obtained (275.7 mg, 57% - Method A).
Rf: 0.33 (DCM). Mp: 359 (dec.) °C. 1H NMR (400 MHz, CD3CN)
δ (ppm): 8.52 (d, J = 1.9 Hz, 2H), 8.22 (dd, J = 8.5, 1.7 Hz, 2H), 7.89 (d,
J = 5.9 Hz, 2H), 7.76 (dd, J = 8.5, 1.8 Hz, 2H), 7.54 (dd, J = 5.9, 2.0 Hz,
2H), 7.38ꢀ7.33 (m, 2H), 6.68 (ddd, J = 12.6, 9.4, 2.4 Hz, 2H), 5.75 (dd,
J = 8.7, 2.4 Hz, 2H), 2.14 (s, 6H), 1.44 (s, 18H).13C NMR (101 MHz,
CD3CN) δ (ppm): 165.3 (s), 164.0ꢀ162.4 (m), 162.1 (d, J = 7.3 Hz),
160.6 (d, J = 13.0 Hz), 156.5 (s), 155.4 (d, J = 6.4 Hz), 151.2 (s), 149.9
(s), 141.0 (s), 135.4 (s), 129.0 (dd, J = 4.3, 2.9 Hz), 126.3 (s), 124.0 (d,
J = 19.5 Hz), 123.3 (s), 114.6 (dd, J = 17.6, 2.9 Hz), 99.3 (t, J = 26.3 Hz),
36.5 (s), 30.4 (s), 18.1 (s). HR-MS (C42H40N4F4Ir+) Calculated:
869.28128; Experimental: 869.28462. The characterization generally
matches that found in the literature.19
(4a). Iridium(III) Bis[10-(40,60-difluorophenyl)-1,2,3-triazo-
20
lato-N,C ]-2,20-bipyridine Hexafluorophosphate. A yellow so-
lid was obtained (185.6 mg, 64% - Method B). Rf: 0.45 (Acetone/DCM
(1/9)). Mp: 183 °C. 1H NMR (300 MHz, CD3CN) δ (ppm): 8.46 (d,
J = 8.1 Hz, 2H), 8.21ꢀ8.03 (m, 6H), 7.50 (t, J = 6.4 Hz, 2H), 7.40ꢀ7.31
(m, 6H), 7.24ꢀ7.05 (m, 4H), 6.67 (td, J = 10.1, 1.6 Hz, 2H), 5.77 (dd,
J = 8.9, 1.7 Hz, 2H), 5.48 (s, 4H).13C NMR (75 MHz, CD3CN)
δ (ppm): 165.6 (d, J = 11.7 Hz), 162.2 (d, J = 10.6 Hz), 160.0 (d, J = 12.4
Hz), 157.3 (s), 156.6 (d, J = 12.8 Hz), 153.2 (s), 152.6 (s), 150.9 (d, J =
4.9 Hz), 140.6 (s), 135.2 (s), 129.9 (d, J = 13.5 Hz), 128.7 (d, J = 7.3 Hz),
124.8 (s), 122.7 (s), 115.8 (d, J = 18.5 Hz), 105.7(s), 98.9 (t, J = 25.5
Hz), 56.3 (s). HR-MS (C40H28N8F4Ir+) Calculated: 889.19968;
Experimental: 889.19969.
(4b). Iridium(III) Bis[10-(40,60-difluorophenyl)-1,2,3-triazo-
20
lato-N,C ]-4,40-di-tert-butyl-2,20-bipyridine Hexafluoropho-
sphate. A yellow solid was obtained (513.7 mg, 44% - Method B). Rf:
0.20 (Et3N/DCM/Hexanes (4/48/48)). Mp: 236 °C. 1H NMR (300
MHz, CD3CN) δ (ppm): 8.44 (d, J = 1.9 Hz, 2H), 8.17 (d, J = 1.4 Hz,
2H), 7.98 (d, J = 5.8 Hz, 2H), 7.48 (dd, J = 5.9, 2.0 Hz, 2H), 7.38ꢀ7.30
(m, 6H), 7.24ꢀ7.15 (m, 4H), 6.66 (td, J = 9.9, 2.2 Hz, 2H), 5.77 (dd, J =
8.9, 2.2 Hz, 2H), 5.49 (s, 4H), 1.43 (s, 18H).13CNMR(75MHz, CD3CN)
δ (ppm): 165.56 (d, J = 11.1 Hz), 165.3 (s), 162.2 (d, J = 10.8 Hz),
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Table 1. Structure and Isolated Yields of
[(atl)2Ir(N∧N)]+(PF6)ꢀ Complexes
isolated yield
Figure 1. ORTEP perspective representations of complexes 2a, 4a, and
3b (ellipsoids at 50% probability). The counterion and solvent mol-
ecules have been omitted for clarity.
compound
R
R1
method Aa
40%
method Ba
2a
H
H
H
50%
63%
64%
44%
2b
4a
t-butyl
H
40,60-difluoro
40,60-difluoro
4b
t-butyl
a See text.
Table 2. Selected Bond Distances (Å) and Angles (deg) for la,
3b, 2a, and 4a
complex
IrꢀCC∧N
laa
3b
2a
4a
2.004
2.024
2.042
2.047
2.129
2.136
172.1
80.1
2.037
2.057
1.988
2.040
2.158
2.194
171.6
78.7
1.980
2.039
1.988
2.000
2.106
2.150
174.4
96.2
1.999
2.055
1.988
1.988
2.123
2.134
173.2
94.5
IrꢀNC∧N
IrꢀNN∧N
NC∧NꢀIrꢀNC∧N
CC∧NꢀIrꢀNC∧N
80.7
79.4
98.2
97.7
NN∧NꢀIrꢀNN∧N
76.2
74.7
76.3
77.0
a Data taken from ref 17.
were acquired until the Pearson regression factor (R2) for the linear fit of
the data set surpassed 0.9. Individual relative quantum yield values were
calculated for each solution and the values reported represent the slope
obtained from the linear fit of these results. The equation Φs = Φr(Ar/As)-
(Is/Ir)(ns/nr)2 was used to calculate the relative quantum yield of the
sample, where Φr is the absolute quantum yield of the reference, n is the
refractive index of the solvent, A is the absorbance at the excitation
wavelength, and I is the integrated area under the corrected emission curve.
The subscripts s and r refer to the sample and reference, respectively.
A solution of [Ru(bpy)3](PF6)2 in ACN (Φr = 0.095)32 was used as the
external reference. The experimental uncertainty in the emission quantum
yields is conservatively estimated to be 10%, though we have found that
statistically we can reproduce PLQYs to 3% relative error.
Time-resolved excited-state lifetime measurements were determined
using the time-correlated single photon counting (TCSPC) option of
the Jobin Yvon Fluorolog-3 spectrofluorometer. A pulsed NanoLED at
341 nm, (pulse duration <1 ns; fwhm = 14 nm), mounted directly on the
sample chamber at 90° to the emission monochromator, was used to
excite the samples, and photons were collected using a FluoroHub
single-photon-counting detector from Horiba Jobin Yvon. The lumines-
cence lifetimes were obtained using the commercially available Horiba
Jobin Yvon Decay Analysis Software version 6.4.1, included within the
Figure 2. (a) Normalized absorption spectra for 1a, 1b, 2a, and 2b in
ACN at 298 K; (b) Normalized absorption spectra for 3a, 3b, 4a, and 4b
in ACN at 298 K.
fluorometer. Lifetimes were determined though an assessment of the
goodness of its mono exponential fit by minimizing the chi-squared
function (χ2) and by visual inspection of the weighted residuals (see
Supporting Information, Figure S4).
Electrochemical Characterization. Cyclic voltammetry mea-
surements were performed on an Electrochemical Analyzer potentiostat
model 600D from CH Instruments. Solutions for cyclic voltammetry
were prepared in ACN and degassed with ACN-saturated nitrogen
bubbling for about 15 min prior to scanning. Tetra(n-butyl)ammonium-
hexafluorophosphate (TBAPF6; ca. 0.1 M in ACN) was used as the
supporting electrolyte. It was recrystallized twice from EtOH and dried
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Figure 3. (a) Normalized emission spectra for 1a, 1b, 2a, and 2b in deaerated ACN at 298 K; (b) Normalized emission spectra for 1a, 1b, 2a, and 2b in
deaerated MeOH:EtOH at 77 K; (c) Normalized emission spectra for 3a, 3b, 4a, and 4b in deaerated ACN at 298 K; (d) Normalized emission spectra
for 3a, 3b, 4a, and 4b in deaerated MeOH:EtOH at 77 K.
under vacuum prior to use. A nonaqueous Ag/Ag+ electrode (silver wire in
a solution of 0.1 M AgNO3 in ACN) was used as the pseudoreference
electrode; a glassy-carbon electrode was used for the working electrode
and a Pt electrode was used as the counter electrode. The redox
potentials are reported relative to a saturated calomel (SCE) electrode
with a ferrocenium/ferrocene (Fc+/Fc) redox couple as an internal
reference (0.40 V vs SCE).33
characterized by X-ray crystallography. The X-ray structure of
each is monoclinic and each crystallized within the P21/c space
group. Single crystals for 3b were also obtained by slow evapora-
tion of a chloroform solution. The X-ray structure for 3b is mono-
clinic, crystallizing within the P21/n space group. A summary of
the crystal data and CIF files are available in the Supporting
Information. Selected bond lengths and angles for 2a and 4a and
3b are reported in Table 2 and are compared with those for
[(ppy)2Ir(bpy)]PF6, 1a. Perspective views for each are shown in
Figure 1 and clearly confirm a pseudo-octahedral geometry
containing a cis-configuration of the aryl groups and a trans
disposition of the triazole cycles in the atl ligands.
The IrꢀCatl, IrꢀNatl, and IrꢀNbpy bond lengths of complexes
2a and 4a are similar to those of 1a or 3b. The structural param-
eters for 4a in particular mirror those recently reported by De Cola
and co-workers for the related [(dFphtl)2Ir(dMebpy)]PF6, where
dMebpy = 4,40-dimethyl-2,20-bipyridine.12c The presence of the
fluorineatomsin3b and 4a weakens the IrꢀCC∧N bond somewhat
’ RESULTS AND DISCUSSION
Synthesis and Structure. Complex 2a was synthesized by
cleaving the μ-dichloro-bridged iridium(III) dimer [(phtl)2Ir
(μ-Cl)]2 (phtl = 4-phenyl-1-benzyl-1H-1,2,3-triazole) with 2
equiv of the diimine ligand (N∧N) in refluxing 2-ethoxyethanol
followed by anion metathesis of the chloride salt with NH4PF6,
analogous to the procedure developed by Nonoyama (Method A).16
Isolation of the corresponding fluorinated dimer precursors for
4aꢀ4b proved unsuccessful using this procedure. The hetero-
leptic complexes were instead obtained via a one-pot two step
procedure (Method B) in which IrCl3 was combined with 2 equiv
of dFphtl [dFphtl = 4-(2,4-difluorophenyl)-1-benzyl-1H-1,2,3-
triazole] in 2-ethoxyethanol and heated for 19 h at 110 °C,
followed by addition of 1 equiv of N∧N ligand and further
heating at 110 °C for 19 h. Upon anion metathesis with NH4PF6
the heteroleptic complexes could be isolated in good yield
(Table 1). Given its simplicity, this protocol was extended to
the synthesis of 2b. All complexes were characterized by 1H and
13C NMR, which confirmed their C2 symmetry, and HRMS
analyses. Additionally, the crystal structures for 2a and 4a and 3b
were obtained to verify the structure.
resulting in a slight lengthing when compared (average IrꢀCC∧N
=
2.047 and 2.027 Å for 3b and 4a, respectively) to 2a (average
IrꢀCatl = 2.010Å). The average CatlꢀIrꢀNatl bite angle for 2a and
4a is, respectively, 97.4° and 96.1°, which is substantially larger
than those found in 1a,17 3b or in [(ppz)2Ir(dtBubpy)]+(PF6)ꢀ
(average CppzꢀIrꢀNppz = 80.7°), where ppz = 1-phenylpyrazoyl
and dtBubpy = 4,40-di-t-butyl-2,20-bipyridine.5b The NatlꢀIrꢀNatl
bond angle measured between the two triazolyl moieties (174.4°
for 2a and 173.2° for 4a) resembles that found for the
NC∧NꢀIrꢀNC∧N in 1a and 3b (172.1° and 171.6°, respectively).
The NbpyꢀIrꢀNbpy bond angle is consistent and was found in all
four complexes to be about 76°.
Single crystals for each of 2a and 4a were grown by slow
diffusion in a mixed solvent DCM/acetone system and were
Solution-State Photophysical Properties. Absorption and
emission spectra are compiled in Figures 2 and 3, respectively,
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and the data are summarized in Table 3. All complexes were
found to be readily soluble in ACN, and this is the solvent of
record for all RT spectroscopic and electrochemical measure-
ments. Low temperature measurements were conducted in a 1:1
MeOH/EtOH mixture to promote the formation of a glass state.
It is important to note that the spectroscopic properties reported
in this study for 1a,3,29 1b,18,29,34 and 3b34,35 generally reproduce
those found in the literature. Absorption and emission spectra for
each of the individual complexes can be found in the Supporting
Information, Figures S5ꢀS12.
The absorption profiles obtained at RT for 2a, 2b and 4a, 4b
resemble, respectively, those of 1a, 1b and 3a, 3b (Figure 2).
Each of the absorption spectra of the 8 complexes in this study
exhibits intense bands (ε > 104 Mꢀ1 cmꢀ1) in the ultraviolet
region, between 200 and 275 nm. Each of these bands has been
1
assigned to spin-allowed πꢀπ* ligand-centered (1LC) transi-
tions centered on both the cyclometallating and the ancillary
ligands. The 1LC band for 2a and 2b are blueshifted, respectively,
by about 30 and 20 nm compared to 1a and 1b. Similarly, the
1LC band for both 4a and 4b is hypsochromically shifted by
15 nm compared to 3a and 3b. Hypochromic bands found
at lower energy have been assigned to both spin-allowed
and spin-forbidden admixtures of metal-to-ligand charge-transfer
(1MLCT and 3MLCT) and ligand-to-ligand charge transfer
(1LLCT and 3LLCT) transitions.1a,c,3,36 The lower absorptivities
observed for the spin-allowed CT transitions are the result of
poor spatial overlap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) (see below for electron density map). These charge
transfer-type bands were found to be more intense for the atl
C∧N ligands than for the arylpyridine ligands (see absorptivities
in Table 3).
The RT (298 K) photoluminescence (PL) spectra were
obtained in dearated ACN and are shown in Figures 3a and 3c.
Emission energy maxima range from 498 to 602 nm, resulting in
perceived emission colors of sky-blue to yellow. The presence of
the atl fragment results in a blue-shift of 630 cmꢀ1 for 2a
compared to 1a and 471 cmꢀ1 for 2b compared to 1b.18,29 For
the fluorinated analogues, the blue shift is 480 and 549 cmꢀ1
,
respectively, for 4a compared to 3a and 4b compared to 3b. The
incorporation of the dtBubpy groups results in a relative blueshift
150 and 625 cmꢀ1, respectively, for the nonfluorinated (2b
compared to 2a) and fluorinated (4b compared to 4a) com-
plexes. In all cases the PL spectra is broad and featureless,
indicative of CT processes. The lack of fine structure in the
emission spectra at RT for 4a and 4b distinguishes it from other
charged heteroleptic iridium complexes emitting at similar
wavelengths that exhibit vibronic structure.4e,13b Stokes shifts
inferior to 7300 cmꢀ1 clearly demonstrate the phosphorescent
nature of the emission. The overall effect of the replacement of a
ppy-type ligand for an atl-type ligand is a blue shift of between
471 and 630 cmꢀ1
.
The PL spectra at 77 K recorded in the MeOH/EtOH glass
state are shown in Figures 3b and 3d. The emission maximum
ranged from 463 to 542 nm, and each complex exhibits the
characteristic rigidochromic blueshift compared to PL spectra
obtained at RT. The PL spectra for 2a and 2b are broad and
featureless. The spectrum for 2b distinguishes itself from 1b in
that in the latter, fine vibronic structure is observed. All fluori-
nated analogues (3a, 3b, 4a, and 4b) were blueshifted and each
spectrum exhibited fine structure, indicative of emission from
different vibronic levels in the excited state and a corresponding
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Table 4. Electrochemical Properties of 1ꢀ4a
ligands
E1/2,ox
ΔEp
ΔE
E1/2,red
ΔEp
complex
C∧N
ppy
N∧N
bpy
(V vs SCE)
(mV)
(V vs SCE)
(V VS SCE)
(mV)
1a
lb
1.27
1.31
1.28
1.26
1.55
1.51
1.61
1.60
56
2.65
2.71
2.75
2.82
2.89
2.94
3.01
3.09
ꢀ1.38
ꢀ1.40
ꢀ1.47
ꢀ1.56
ꢀ1.34
ꢀ1.43
ꢀ1.40
ꢀ1.49
55
87
78
84
84
74
69
75
ppy
dtBubpy
bpy
106
69
2a
2b
3a
3b
4a
4b
phtl
phtl
dtBubpy
bpy
84
dFMeppy
dFMeppy
dFphtl
dFphtl
109
98
dtBubpy
bpy
105
96
dtBubpy
a Measured in ACN (ca. 1.5 mM) with NBu4PF6 (ca. 0.1 M) as the supporting electrolyte. Potentials (V) are reported vs an SCE standard electrode and
were calibrated using an internal standard Fc/Fc+ redox couple (0.40 V in ACN); see ref 33.
increase in LC character for these transitions.5b,37 Monoexpo-
nentially decaying emission lifetimes for 2a, 2b, 4a, and 4b are
similar to those found for 1a, 1b, 3a, and 3b and are on the order
of 4ꢀ6 μs. Low temperature PL spectra show a moderate-to-
large hypsochromic shift resulting from impeded movement of
solvent molecules in the glass state (Table 3). This lifetime
behavior and the expected blue shift in emission energy suggest
that the character of the emission at 77 K remains predominantly
factor of about 2.3 in the nonradiative rate constant. Thus, there
seems to be an interplay between the tert-butyl groups and the atl
ligand. The effect of fluorine atom incorporation into the com-
plexes promotes a further decrease in knr, in keeping with the
behavior predicted by the energy gap law. The similar values
obtained for kr for 3a, 3b, 4a, and 4b would suggest that emission
in these four complexes is occurring from a similar excited state.
Electrochemistry. Cyclic voltammetry measurements (CV),
obtained in ACN, were performed to probe the nature of the
frontier molecular orbitals of the complexes. A summary of the
redox potentials is shown in Table 4, measured relative to an
internal ferrocene standard (Fc+/Fc = +0.40 V vs SCE in ACN).
Generally, electrochemical processes were found to be either
reversible or quasi-reversible one-electron processes, and CV
spectra were found to be reproducible regardless of scan rate
(50ꢀ200 mV/s). The redox potentials measured for 1a and 1b18
mirror those found in the literature, while that for 3b34 is offset
anodically by about 0.16 V, perhaps owing to a difference in
working electrode (glassy carbon vs Pt). Compounds 1a, 1b, 2a,
and 2b exhibited oxidations in the range of 1.26ꢀ1.31 V while
compounds 3a, 3b, 4a, and 4b showed oxidations between 1.51
and 1.61 V. The oxidation is assigned to occur on the metal
center and on the C∧N ligands. The more difficult oxidation of
the metal and C∧N ligands for 3a, 3b, 4a, and 4b is expected
given the electron-withdrawing nature of the fluorine atoms.
Generally, the oxidation wave for the atl complex (E1/2,ox) was
slightly anodically shifted compared to its apy-type congener
(apyH = 2-arylpyridine). This anodic shift was more pronounced
at about 0.08 V for the comparison between fluorinated com-
plexes (3a and 3b vs 4a and 4b) compared to the comparison
between nonfluorinated complexes (1a and 1b vs 2a and 2b; shift
of about 0.02 V). Oddly, whereas the E1/2,ox for the oxidation of
2a was found to be at 1.28 V, the E1/2,ox for the analogous
[(a)2Ir(bpy)](PF6) (5) was measured to be 1.23 V in DCM vs
SHE, which translates to 1.41 V vs SCE (where H-a = 1-decyl-4-
phenyl-1H-[1,2,3]triazole).12b This result would suggest that the
benzyl group is in fact playing an active role in destabilizing the
HOMO in 2a compared to the decyl group in 5. For comparison,
cationic heterolepetic iridium complexes bearing phenylpyra-
zole, ppz, cyclometallating ligands, exhibited oxidation potentials
at 1.41 V vs SCE.5b
3
3
an admixture of MLCT and LLCT. The reorganization of
solvent molecules at elevated temperatures about the complex
aide in the stabilization of CT states prior to emission.
PL Quantum yields (PLQY, Φpl) for atl complexes ranged
from 25 to 80%. The observed increase in quantum yields is due
to a diminution of nonradiative rates with increasing emission
energy; an example of the so-called energy-gap law. Striking is the
comparison in quantum yield between atl-based complexes and
ppy-based complexes. For instance, 2a is nearly three times as
bright as 1a while 4b is among the brightest charged heteroleptic
iridium complexes emitting in the green-blue region, particularly
given that measurements were carried out in ACN.4e,5,6,10,27e,34,38
The observed increase in quantum yields is ascribed to the
presence of the benzyl groups on the atl ligand, which sterically
shield the iridium about the bipyridine and inhibit nonradiative
intermolecular charge recombination.3,39 Whereas this strategy
has been used effectively by placing large groups on the ancillary
ligand,3,5a,39,40 incorporation of bulky groups on the cyclometal-
lating ligand that shield the environment of the ancillary ligand
has, to our knowledge, rarely been previously reported.12a,c The
PL quantum yields decrease markedly in aerated ACN solution.
For example the quantum yield for 2a in aerated solution was
found to be 11%.
Emission intensity decays monoexponentially at 298 K with
lifetimes that are in the submicrosecond-microsecond regime, an
indication of the triplet character of the emission process. Whereas
1a and 2a exhibit similar lifetimes, the lifetime of 2b is twice aslong
as1b, ostensiblyowingtoa synergisticrigidification of thecomplex
because of a favorable interaction between the benzyl and t-butyl
groups. Fluorinated complexes, which displayed large PLQYs,
exhibited longer lifetimes than nonfluorinated analogues. There
was little discrimination in lifetimes, however, between 4a and 4b.
From the PLQY and lifetimes, it is possible to determine the
radiative (kr) and nonratiative (knr) rate constants. The increased
quantum yield in going from 1a to 2a seems to be due to a 3.5 fold
increase in the radiative rate constant. By contrast the greater
brilliance of 2b compared to 1b is ascribed to a decreased by a
The replacement of bpy for dtBubpy results in a slight cathodic
shift of about 0.02 V in the oxidation wave but an expected much
larger cathodic shift of about 0.07 V in the first reduction wave,
an indication that the LUMO is in large part localized on the
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ancillary ligand. The first reduction for each of the complexes is
thus assigned to occur on one of the pyridine rings of the N∧N
ligand. Surprisingly, the replacement of ppy-type ligands for atl-
type ligands results in a large cathodic shift of 0.06ꢀ0.16 V of the
reduction wave. This would indicate that unlike complexes 1a,
1b, 3a, and 3b, which have been shown to have a LUMO
localized essentially only on the ancillary ligand, the LUMO for
2a, 2b, 4a, and 4b possesses contributions from the metal and/or
the cyclometallating ligand. The incorporation of fluorine atoms
onto the C∧N ligand results in a anodic shift of the reduction
band of about 0.06 V, much smaller than the anodic shift of about
0.28 V for the oxidation wave.
A comparison of the emission energies (λmax in eV) at 298 K
with the ΔEred‑ox reveals the expected linear monotonic relation-
ship (Figure 4). The presence of atl-type ligands, the inclusion of
the dtBubpy ligand, and the incorporation of fluorine atoms each
have the net effect of increasing the ΔEred‑ox
.
Computational Modeling. To gain greater insight into the
photophysical and electrochemical properties of the atl-based
complexes, a combined DFT/TD-DFT24,25 investigation was
undertaken at the B3LYP26 level of theory using the SBKJC-DVZ
basis set27a,28 for Ir and the 3-21G* basis set27 for all other heavy
atoms using Gaussian 09 suite.23 We have previously shown this
computational approach to be effective in predicting ground- and
excited state properties of (C∧N)2Ir(N∧N)+-type complexes.3
Figures 5 and 6 show the relative orbital energies and their
electronic distribution character for each of the 8 complexes.
From an evaluation of these MOs, we can assign the origin of the
dominant absorption and emission transitions that are observed
in the photophysical study. For all the complexes, the HOMO
corresponds to a mixture of phenyl πC∧N and Ir d orbitals. The
metal contributions for the HOMO, HOMO-1, and HOMO-2
are of dxy, dyz, and dxz symmetry, in accordance with ligand field
theory for the “t2g” orbitals of a metal bearing octahedral
symmetry (see Supporting Informatio). The LUMO is princi-
pally composed of a π*N∧N orbital with very minor contributions
from the Ir d orbital. The small changes in E1/2,red between 1a, 1b
and 3a, 3b (or 2a, 2b and 4a, 4b) are a testament to this latter
assignment. Thus, there is little electronic overlap between these
two MOs. The LUMO+1, which is π*N∧N in nature, is
Figure 4. PL emission energy (eV) at RT in ACN compared to ΔEred‑ox
(V) determined by cyclic voltammetry in ACN.
Figure 5. Energy level schemes for the KohnꢀSham orbitals of 1a, 1b, 2a, and 2b, including the most relevant KohnꢀSham orbitals and the
HOMOꢀLUMOenergygap (see SupportingInformationforcomputational details). SelectedMOs havebeencoloredtoaideintheanalysis. Red indicates
that the electron density is primarily localized on the πbpy with minor Ir dπ contributions, green on the πppy or πatl with minor Ir dπ contributions.
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Figure 6. Energy level schemes for the KohnꢀSham orbitals of 3a, 3b, 4a, and 4b, including the most relevant KohnꢀSham orbitals and the
HOMOꢀLUMO energy gap (see Supporting Informatio for computational details). Selected MOs have been colored to aide in the analysis. Red
indicates that the electron density is primarily localized on the πbpy with minor Ir dπ contributions, green on the πppy or πatl with minor Ir dπ
contributions.
Figure 7. Calculated energies of the 1HOMO and 1LUMO and
Figure 8. Comparison between the calculated HOMOꢀLUMO energy
gap obtained from the vertical transitions by TD-DFT and the ΔEred‑ox
measured by cyclic voltammetry for each of the 8 complexes, divided
into two families based on the identity of the ancillary (N∧N) ligand.
The linear fit has also been included.
3HSOMO for each of the 8 complexes under investigation with the
corresponding HOMOꢀLUMO gap. The lines connecting the data
points are placed to increase clarity only.
significantly destabilized by between 0.81 and 0.96 eV, which
explains the difficulty in observing the second reduction wave in
the CV of these compounds. Probing further the origin of higher
energy unoccupied orbitals reveals that they are dominated by
π*C∧N contributions. In fact the greatly destabilized eg* orbitals
do not even present themselves in the first 7 lowest unoccupied
MOs. In the related complex, 5, Schubert found that the 5dz2 and
5dx2ꢀy2 metal-based orbitals to be at LUMO+11 and LUMO+13,
respectively. This expected behavior is due to the strong ligand
field splitting character of the cyclometallating ligands and the
third row nature of iridium.
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Figure 9. Experimental spectra (red) and simulated (fwhm: 1000 cmꢀ1, blue) absorption spectra for (a) 2a; (b) 2b; (c) 4a; (d) 4b. The oscillator
strengths, calculated by TD-DFT (TD-B3LYP/SBKJC-DVZ on Ir and TD-B3LYP/3-21G* on other heavy atoms), are represented by unbroadened
vertical lines of the calculated singletꢀsinglet transitions (green).
Table 6. Comparison between Calculated Emission Energies
(See Text) and Measured Emission Energies at 77 K in l:l
MeOH/EtOH
atl in the LUMO. The replacement of the bpy for the dtBubpy
ligand results in an expected destabilization of both the HOMO
and the LUMO with a resulting small increase in the overall
energy gap. As had been previously demonstrated,34,41 the
incorporation of fluorine atoms results in a net increase in the
HOMOꢀLUMO gap resulting from stabilization of both fron-
tier MOs, with the energy of the HOMO being more significantly
influenced. The calculated increase in the energy gap, corrobo-
rated both by CV and spectroscopic measurements (see Tables 3
and 4), is due to the synergistic effect of the addition of fluorine
atoms, the incorporation of electron releasing t-butyl groups, and
the presence of the triazole moiety. Thus, 4b has the largest
calculated energy gap of 3.46 eV.
emission energy
calculated
77 K
nm
errora
(%)
298 K
nm
complexes
eV
nm
eV
eV
la
2.21
2.42
2.25
2.52
2.40
2.61
2.67
2.86
561
513
550
491
516
475
464
434
2.29
2.62
2.46
2.49
2.72
2.72
2.77
2.86
542
473
503
498
455
456
448
433
3.4
8.0
8.9
1.3
12.7
4.1
3.5
0.1
2.06
2.10
2.14
2.16
2.35
2.42
2.41
2.49
602
591
580
575
527
512
514
498
lb
2a
2b
3a
3b
4a
4b
Table 5 lists the dominant low energy singletꢀsinglet vertical
excitations, their oscillator strengths, and a synopsis of their
corresponding assignments for each of the 8 complexes under
study. A comparison of the calculated and experimental absorp-
tion spectra for 2a, 2b and 4a, 4b is presented in Figure 9. The
calculated absorption spectra agree well with experiment, not-
withstanding that predicted spectra obtained by TD-DFT meth-
ods are frequently red-shifted.42 In all 8 complexes, the HOMOꢀ
LUMO transition is not permissible (f e 0.0002) nor was it observed
in the experimental absorption spectra. The low intensity absorp-
tion transitions observed at >300 nm can now be assigned unequiv-
ocally as mixed singlet MLCT/LLCT transitions. The tailings at
about >420 nm, where singletꢀsinglet transitions are dark, can
then be inferred to be spin-forbidden triplet excitations. From an
a Relative error between calculated emission energy and experimental
results at 77 K.
The characterization for these complexes mirrors that found
for similar ppy-type and atl-type charged heteroleptic iridium
complexes.3,12b The effect of substitution on the relative energies
of the HOMO and LUMO and its impact on the HOMOꢀLU-
MO gap are summarized in Figure 7. The replacement of the
ppy-type ligand for the atl-type ligand results in destabilization of
both the HOMO and the LUMO energies with a resulting net
increase in the overall energy gap, supporting that observed by
CV (Figure 8). The large influence on the LUMO energy is
surprising given the paucity of electron density localized on the
3
analysis of the electron density distribution of the HSOMOs,
these transitions can in part be assigned as 3MLCT/3LLCT (see
Jablonski plots in Supporting Information, Figures S17ꢀS24).
The strongly absorbing higher energy transitions are LC in
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dx.doi.org/10.1021/ic2014013 |Inorg. Chem. 2011, 50, 11514–11526

Inorganic Chemistry
ARTICLE
nature, wherein the localization of electron density can either be
C∧N-based or N∧N based.
acknowledges CFI (Canadian Foundation for Innovation), NSERC
(the National Sciences and Engineering Research Council of
Canada), FQRNT (Le Fonds quꢀebꢀecois de la recherche sur la
nature et les technologies), and the Universitꢀe de Sherbrooke for
financial support. S.L. acknowledges the Universitꢀe de Sher-
brooke for an institutional scholarship.
The PL spectra at RT shown in Figure 3 are clearly broad and
featureless, thus indicating that the character of emission remains
essentially unchanged and is charge-transfer in nature. This assign-
ment is supported by evaluation of the nature of the 3HSOMO for
each of the complexes (see Supporting Information). Though we
also estimated emission energies by calculating the vertical energy
differences between the lowest triplet state and the S0 singlet
ground state, a more precise method is to take the difference
between the total energy of the complex in the T1 state, whose
geometry is optimized at that state, and the total energy of the
complex in the ground S0 state,optimizedatthatstateusingUB3LYP
methods without symmetry restrictions.3 These results are sum-
marized in Table 6 and are compared to the measured E0,0 band
obtained at 77 K in 1:1 MeOH/EtOH. Predictions are generally
accurate though are slightly bathocromically shifted from experi-
mental values.
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’ ASSOCIATED CONTENT
S
Supporting Information. Summary of crystallographic
b
parameters and ORTEP diagrams for 2a, 3b, and 4a. Absorption
and emission spectra at 298 and 77 K for each of the 8 complexes.
Cyclic voltammograms for each of the 8 complexes. Full computa-
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tables of 100 lowest energy transtions calculated by TD-DFT and
tables of calculated emission energies and isodensity surface plots
for selected orbitals of each of the 8 complexes. 1H and 13C NMR
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: Eli.Zysman-Colman@USherbrooke.ca.
’ ACKNOWLEDGMENT
(12) (a) Felici, M.; Contreras-Carballada, P.; Smits, J. M. M.; Nolte,
R. J. M.; Williams, R. M.; De Cola, L.; Feiters, M. C. Molecules 2010,
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Computational time on the Mammouth supercomputer at the
Universitꢀe de Sherbrooke was underwritten by the RQCHP
(rꢀeseau quꢀebꢀecois de calculs de haute performance). E.Z.-C.
11525
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Inorganic Chemistry
ARTICLE
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