Microwave-assisted facile and expeditive syntheses of phosphorescent cyclometallated iridium(III) complexes

Article abstract of DOI:10.1016/j.poly.2013.01.045

The syntheses of greenish-blue light emitting [Ir(ppy)₂(dppel)] (2), [Ir(ppy)₂(dppp)] (3) and [Ir(ppy)₂(dppe)] (4) [ppy, 2-phenylpyridine; dppel, 1,2-bis(diphenylphosphino)ethylene; dppp, 1,3-bis(diphenylphosphino)propane;

Full text of DOI:10.1016/j.poly.2013.01.045

Polyhedron 53 (2013) 286–294
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Microwave-assisted facile and expeditive syntheses of phosphorescent
cyclometallated iridium(III) complexes
b
b
b
Parvej Alam a, Inamur Rahaman Laskar a, , Clàudia Climent , David Casanova , Pere Alemany ,
⇑
Maheswararao Karanam c, Angshuman Roy Choudhury c, J. Raymond Butcher d
a Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani 333 031, Rajasthan, India
b Departament de Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, Barcelona 08028, Spain
c Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Mohali, Sector 81, S.A.S. Nagar, Manauli PO, Mohali, Punjab 140306, India
d Department of Inorganic & Structural Chemistry, Howard University, Washington, DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses of greenish-blue light emitting [Ir(ppy)2(dppel)] (2), [Ir(ppy)2(dppp)] (3) and [Ir(ppy)2
(dppe)] (4) [ppy, 2-phenylpyridine; dppel, 1,2-bis(diphenylphosphino)ethylene; dppp, 1,3-bis(diphenyl-
phosphino)propane; dppe, 1,2-bis(diphenylphosphino)ethane] complexes were carried out using [(ppy)2-
Received 18 September 2012
Accepted 3 January 2013
Available online 8 February 2013
Ir(l-Cl)2Ir(ppy)2] (1) as a starting material. These complexes were characterized by elemental analyses
and NMR (1H, 13C and 31P) spectral studies. A single-crystal X-ray diffraction study confirmed a distorted
octahedral geometry for 3. Complexes 2–4 were found to exhibit blue-shifted emission as compared to
[Ir(ppy)2(acac)] (acacH = acetylacetone) and [Ir(ppy)2pic] (pic = 2-picolinic acid) because of the presence
Keywords:
Cyclometallated Ir(III) complex
Microwave
Heteroleptic complexes
Ancillary ligands
2-Phenylpyridine
Photoluminescence
DFT
of strongly p-accepting, Ph2P^PPh2 units. The solution quantum efficiency for 2–4 was measured and 2
showed the highest quantum efficiency. Ground state geometry optimizations for 2–4 were performed
using density functional theory (DFT) with the B3LYP hybrid functional and excitation energies for low
lying singlet and triplet excited states were obtained via time-dependent DFT (TDDFT) calculations. Fur-
ther, complexes 1–4 were synthesized by a Microwave Irradiation technique (MW) in a reasonably
shorter time. This facile and expeditive synthetic route has been extended and successfully verified for
other heteroleptic complexes of Ir(III) with varying different bidentate [(N^N) (5), (O^O) (6), (N^O) (7)]
and monodentate [PPh3 (8)] ancillary ligands.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Heteroleptic complexes of iridium(III) [2b], typically consisting
of two units of cyclometallated ligands along with one unit of
Heavy-metal based cyclometallated complexes are receiving
great attention as potential phosphorescent materials for applica-
tion in organic light emitting diodes (OLEDs) [1]. The presence of
a heavy metal in these complexes induces strong spin-orbit cou-
pling which results in the population of the lowest triplet excited
states, giving rise to high quantum efficiency. There are many re-
ports of this phenomenon being observed for new cyclometallated
complexes of 5d-transition metals such as Re(I), Os(II), Ir(III) and
Pt(II), but the prime interest for the design and syntheses of new
complexes for application in OLED technology is mainly limited
to Ir(III) containing complexes due to their superior quantum
efficiency, ease of tuning the emission wavelength throughout
ancillary ligand, [Ir(C^N)2(A–A)], have been observed to have a dis-
tinct set of advantages in comparison to their homoleptic analogs,
[Ir(C^N)3] [2a,3] (where HC^N and A–A symbolize cyclometallated
and ancillary ligands, respectively). In contrast to homoleptic com-
plexes that usually give mixtures of fac- and mer-isomers which
are tedious to separate out, heteroleptic complexes with a particu-
lar ancillary ligand in most cases produce only one isomer, i.e. the
trans-N,N-iridium(III) complex. However, there are several reports
which clearly show the formation of the other isomer, i.e. cis-N,N-
iridium(III) complexes [4]. Furthermore, heteroleptic complexes
introduce the possibility of facile and versatile derivatizations, so
that easy color tuning is possible, whilst the ancillary ligand hardly
interferes in the phosphorescent color. For the realization of effi-
cient full color RGB display systems, it is essential to obtain high
efficiency and color purity for the three primary blue, green and
red emitting materials. While there have been many reports of effi-
cient pure green and red light-emitting complexes, the search for
true blue emitters is more difficult [5]. The incorporation of a
the visible range and the availability of
synthetic route [2].
a straight-forward
⇑
Corresponding author.
E-mail addresses: ir_laskar@bits-pilani.ac.in (I.R. Laskar), p.alemany@ub.edu (P.
Alemany), angshurc@iisermohali.ac.in (A.R. Choudhury), rbutcher@howard.edu (J.
Raymond Butcher).
strongly
p-accepting PPh2 unit in the coordination sphere of an
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.045

P. Alam et al. / Polyhedron 53 (2013) 286–294
287
Ir(III) complex increases the HOMO–LUMO separation [6], opening
the way to the synthesis of new blue-emitting complexes. Further-
more, the strong bonding of a P-donor ligand to Ir(III) increases the
energy of d–d⁄ excitations on the metal ion, minimizing their inter-
ference with the radiative transition from lower lying excited
states. These two properties of P-donating ligands motivated the
search for new blue-emitting Ir(III) complexes using Ph2P^PPh2
type ancillary ligands.
scribed by Clark and Reid [9]. The crystal structure was solved
using SHELXS97 [10], and refined using SHELXL97 [10], through SHELXTL
.
2.3. Syntheses
The dichloro-bridged iridium(III) complex [(ppy)2Ir(l-Cl)2
Ir(ppy)2] [2b], 1, and the mononuclear iridium(III) complexes 2–5
[2b], 6–7 [11] and 8 [12] were synthesized following the conven-
tional routes as described in the literature. Alternatively, these
complexes have also been synthesized as described below, by an
alternative microwave irradiation technique (MW) developed in
our laboratory.
In most cases, heteroleptic complexes are synthesized with a
traditional, lengthy two step processes [2b], whereas the tris-cyclo-
metallated iridium(III) complexes are obtained by refluxing in
glycerol (at ꢀ200 °C) for 12–24 h [2a]. In an alternative approach,
these complexes can be synthesized by using a microwave (MW)
technique which reduces the time drastically as compared to the
conventional routes. There are several reports [7] found in the lit-
erature which describe the rapid syntheses of tris cyclometallated
iridium(III) complexes based on the MW technique in the time
range of 5–15 min. Constable et al. [8] synthesized ionic heterolep-
tic cyclometallated iridium(III) complexes from a dinuclear chloro-
bridged iridium(III) precursor in 1.5 h using the same technique.
Here, we have reported the MW-triggered syntheses of a series
of heteroleptic cyclometallated complexes with different ancillary
ligands in an expeditive way. In addition, the syntheses and char-
acterization of three greenish-blue emitting complexes with three
different P-coordinated chelating ligands (dppel, 1,2-bis(diphenyl-
phosphino)ethylene; dppp, 1,3-bis(diphenylphosphino)propane;
dppe, 1,2-bis(diphenylphosphino)ethane), retaining the same
chromophoric ligand of 2-phenylpyridine in each case, have been
reported. Further, their photophysical properties have been stud-
ied and these complexes have been modeled using quantum chem-
istry methods to get a deeper understanding of their excited state
properties.
[Ir(ppy)2(dppel)](PF6), 2: 1H NMR (400 MHz, CDCl3) d: 8.50 (m,
J = 56.4 Hz, 2H), 7.70 (t, J = 8.3 Hz, 4H), 7.51–7.23 (m, 14H), 6.98
(m, J = 16.2, 9.0 Hz, 2H), 6.96–6.84 (m, 4H), 6.77 (t, J = 6.8 Hz,
4H), 6.48 (t, J = 8.7 Hz, 4H), 6.22 (t, J = 5.8 Hz, 4H). Anal. Calc for
C48H38N2P3F6Ir (MW = 1041.3): C, 55.3; H, 3.6; N, 2.6. Found: C,
55.0; H, 3.5; N, 2.6%.
[Ir(ppy)2(dppp)](PF6), 3: 1H NMR (400 MHz, CDCl3) d: 8.67 (d,
1H), 8.18 (d, J = 5.5 Hz, 1H), 8.01 (dd, J = 12.1, 7.6 Hz, 1H), 7.62 (t,
J = 8.0 Hz, 2H), 7.44 (dt, J = 15.2, 7.3 Hz, 5H), 7.38–7.21 (m, 8H),
7.02 (m, 7H), 6.93–6.69 (m, 5H), 6.60 (dt, J = 13.4, 6.4 Hz, 2H),
6.53–6.43 (m, 1H), 6.29 (t, J = 8.3 Hz, 1H), 6.24 (s, 1H), 5.96 (d,
J = 6.6 Hz, 1H), 2.91 (dd, J = 109.4, 37.0 Hz, 4H), 2.13–1.72 (m,
2H). Anal. Calc for C49H42N2P3F6Ir (MW = 1057.3): C, 55.6; H, 4.0;
N, 2.6. Found: C, 55.3; H, 4.1; N, 2.4%.
[Ir(ppy)2(dppe)](PF6), 4: 1H NMR (400 MHz, CDCl3) d: 7.62 (dd,
J = 16.1, 7.3 Hz, 5H), 7.55–7.48 (m, 4H), 7.38 (t, J = 7.8 Hz, 3H),
7.30 (t, J = 10.5, 4.2 Hz, 2H), 7.23 (m, 6H), 6.98 (t, J = 7.2 Hz, 2H),
6.90 (m, 4H), 6.82 (dd, J = 7.6, 5.8 Hz, 4H), 6.58 (t, J = 8.5 Hz, 3H),
6.26 (m, 3H), 3.82 (m, J = 29.2, 9.9 Hz, 2H), 2.74 (d, J = 9.9 Hz,
2H). Anal. Calc for C48H40N2P3F6Ir (MW = 1043.3): C, 55.2; H, 3.8;
N, 2.7. Found: C, 55.0; H, 3.9; N, 2.4%. Crystallized in the ortho-
rhombic P212121 space group with a = 9.7798(2), b = 20.5092(6),
2. Experimental
c = 23.1578(4) Å, V = 4644.9, Z = 4, Z0 = 1, Dcalc = 1.493 Mg mꢁ3
k = Mo K (Mo K
) = 3.036 mmꢁ1, T = 123 K. Total 26750 reflec-
tions, 18200 unique, Rint = 0.0331. Refinement of 14398 reflections
(463 parameters) with I > 2 (I) converged at final R1 = 0.0634 (R1
,
a,
l
a
2.1. Materials
r
Iridium(III) chloride was purchased from the Alfa Aesar
Company Ltd.; 2-phenyl pyridine, 2-ethoxyethanol, triphenylphos-
phine, picolinic acid, acetylacetone, 1,10 phenanthroline, 1,2-bis
(diphenylphosphino)ethane, cis-1,2-bis(diphenylphosphino)ethyl-
ene, bis(diphenylphosphino)propane and potassiumhexafluoro-
phosphates were purchased from the Sigma Aldrich Company;
while methanol and acetone were purchased from the Merck
company.
all data = 0.0854), wR2 = 0.1531 (wR2 all data = 0.1664), Good-
ness-of-fit = 1.068, two phenyl rings on two different P atoms are
disordered. Earlier an occurrence of the structure of this complex
[8] was reported to be monoclinic, P21/n (non-conventional space
group), having different cell dimensions, though all the bond
lengths and bond angles around the Ir atom are found to be similar
in both structures.
[(ppy)2Ir(l-Cl)2Ir(ppy)2], 1: Iridium(III)chloride hydrate (0.17
mmol) and 2-phenyl pyridine (0.33 mmol) were dissolved in
2-ethoxyethanol and water (3:1) in a vial, which was placed in a
microwave reactor for 30 min at 135 °C. The yellow precipitate
that was obtained after half an hour was separated and washed
with ethylacetate, hexane and ethanol, successively for several
times, giving a yield of 1 of 36%.
[Ir(ppy)2(dppel/dppp/dppe/phnan)](PF6), 2–5: 0.09 mmol of 1 and
0.18 mmol of the ancillary ligands were mixed in 2-ethoxyethanol
in a vial and placed into a microwave reactor (MW) for 15 min,
maintaining a temperature of 135 °C. Then, the solvent was evap-
orated under reduced pressure and the crude product collected.
In a vial, 1 eq. of crude product and 2 eq. of KPF6 were dissolved
in a mixture of methanol and acetone (1:1), and the reaction mix-
ture was placed in a microwave reactor at 65 °C for 15 min. The
chloride counter ions from the ionic complexes were exchanged
with PF6ꢁ, then the solvent was evaporated under reduced pres-
sure and the product was purified by column chromatography.
Yield 70–82%.
2.2. Characterization
Microwave reactions were carried out in a CEM Discover. 1H, 13
C
and 31P NMR spectra were recorded using a 400 MHz Brucker NMR
spectroscope. UV–Vis absorption spectra were recorded in a Shi-
madzu Spectrophotometer, model UV-1800. The emission spectra
were recorded on a Horiba Jobin Yvon (Fluoro Max-4) spectrofluo-
rometer. Elemental analyses were obtained on an Elementar VARIO
III analyzer. The quantum yields (U) of complexes 2–4 were mea-
sured by comparing the integrated photoluminescence intensities
and the absorbance values with reference to quinine sulfate (QS).
The quinine sulfate (
the complexes 2–4 were dissolved in degassed dichloromethane.
U = 0.55) was dissolved in 0.1 M H2SO4 and
2.2.1. X-ray single crystal diffraction study
The crystal data were collected at 123 K using Mo Ka radiation
on an Xcalibur Ruby Geminy diffractometer equipped with an en-
hanced Mo X-ray source. Absorption corrections were made using
CRYSTALISPRO Version 1.171.35.11 (2011) following the method de-
[Ir(ppy)2(acac/pic)], 6–7: 0.09 mmol of 1, 0.18 mmol ancillary
ligands and ꢀ85–90 mg of sodium carbonate were mixed in

288
P. Alam et al. / Polyhedron 53 (2013) 286–294
Scheme 1. Schematic presentation for the syntheses of dinuclear iridium(III) complex 1 and the mononuclear iridium(III) complexes 2–8 via conventional and microwave
routes.
Table 1
Comparative study of the times and yields of the complexes synthesized by two
different routes.
Complex
Time (min.) MW vs. Conv.
Yield (%) MW vs. Conv.
1
30.0/1440.0
15.0/720.0
15.0/720.0
15.0/720.0
15.0/720.0
15.0/720.0
15.0/720.0
15.0/720.0
36.0/35.0
82.0/85.0
74.0/75.0
72.0/78.0
85.0/78.0
84.0/86.0
93.0/79.0
53.0/83.0
1?2
1?3
1?4
1?5
1?6
1?7
1?8
evaporated under reduced pressure and the crude product was col-
lected and purified by column chromatography. Yield 53%.
Fig. 1. ORTEP diagram for complex 4 showing the octahedral geometry around Ir(III)
(using 50% probability ellipsoids).
2.4. Computational details
Geometry optimizations for complexes 2–4 in their ground
state were carried out using quantum chemical calculations based
on density functional theory (DFT) with the B3LYP [13] hybrid
functional. A basis set of double-f quality (LANL2DZ) and the effec-
tive core potential of Hay and Wadt [14a–c] were used for iridium.
For all other atoms, all-electron 6-31 + G(d) [14d] basis sets have
been used. The optimized ground state geometries were used to
calculate vertical excitation energies for low lying singlet and trip-
let excited states via time-dependent DFT calculations (TDDFT)
2-ethoxyethanol in a vial and placed into a microwave reactor
(MW) for 15 min, maintaining the temperature at 135 °C. The sol-
vent was then evaporated under reduced pressure and the crude
product collected. The product was purified by column chromatog-
raphy. Yield 70–80%.
[Ir(ppy)2(Cl)PPh3], 8: In a vial, 0.09 mmol 1 and 0.18 mmol tri-
phenylphosphine were mixed in 2-ethoxyethanol and the mixture
introduced in the MW at 135 °C for 15 min. The solvent was then

P. Alam et al. / Polyhedron 53 (2013) 286–294
289
using the same functional and atomic basis sets. All ground state
geometry optimizations and excited state calculations were per-
formed for a solution in dichloromethane (e = 8.93), including sol-
vent effects via the Integral Equation Formalism-Polarizable
Continuum Model (IEF-PCM) [15]. All calculations described in this
report have been done using the GAUSSIAN 09 suite of programs [16].
1.0
0.8
0.6
0.4
0.2
0.0
2
3
4
3. Results and discussion
The cyclometallated dinuclear iridium(III) precursor 1 and the
phosphorous-coordinating mononuclear iridium(III) complexes
2–4 were synthesized through an established conventional route
(Scheme 1). Iridium(III)chloride hydrate and phenylpyridine were
mixed in 2-ethoxyethanol and refluxed for ꢀ24 h, which resulted
in the dichloro bridged iridium(III) complex 1. Subsequently, the
bridging dichlorides from complex 1 were replaced by ancillary
phosphorous coordinated chelating ligands (dppel, dppp, dppe)
200
300
400
500
Wavelength (nm)
Fig. 2. Solution absorbance spectra for complexes 2, 3 and 4 (DCM, 10ꢁ5 M).
Fig. 3. 1H NMR spectra for complex 4 synthesized following the MW synthesis (above) and the conventional process (below), showing similar spectral patterns.

290
P. Alam et al. / Polyhedron 53 (2013) 286–294
Fig. 4. Absorbance (left) and emission (right) spectra for complex 2 synthesized following the two alternative synthetic procedures; the same emission color (right, inset) has
been observed on exciting the two solutions at 364 nm. (Color online.)
(Scheme 1) by refluxing the mixture for ꢀ12 h, forming cationic
Table 2
Geometrical parameters for the coordination environment of iridium in the three
optimized ground-state structures in dichloromethane solution.
cyclometallated iridium(III) complexes. The counter monochloride
ion in the cationic complex was exchanged with one equivalent of
Molecule
Bond distancesa (Å)
Angles (°)
hexafluoro phosphate in an acetone/methanol mixture (1:1)
through microwave irradiation, which resulted in the desired com-
plexes 2–4. Complexes 2–4 were characterized by elemental anal-
yses and NMR spectra (1H, 13C, 31P). The structure of
[Ir(ppy)2(dppe)](PF6) 3 was determined by X-ray single crystal
structure analysis. The ORTEP diagram of 3 is presented in Fig. 1.
Crystallographic data and selected geometrical parameters are
shown in Tables S1 and S2, respectively. The molecule possesses
a distorted octahedral geometry with a cis-C,C-trans-N,N configura-
tion. The Ir–P bond distances found for 3 are somewhat larger than
those reported for similar complexes of iridium(III) [17], an elonga-
tion that can be rationalized considering the strong trans-effect ex-
erted by the ‘C’ positioning trans to the ‘P’ atom.
Ir–N
Ir–P
Ir–C
N–Ir–N
P–Ir–P
C–Ir–C
2
3
4
2.109
2.110
2.106
2.479
2.539
2.499
2.059
2.061
2.059
168.6
169.6
168.7
82.9
88.9
83.0
86.2
82.8
84.8
aAverage value of the two Ir-X distances in each compound.
was also found to be drastically reduced in comparison with the
syntheses via conventional routes (Table 1). The same complex, ob-
tained following the two alternative routes, was characterized by
NMR spectra (Figs. 3 and S6). The NMR peaks [1H, 13C and 31P (in
some cases)] for each pair of complexes (obtained through the con-
ventional and MW routes) match well with each other, supporting
the hypothesis that the same complex is being formed in the two
alternative routes (Figs. S3–S10). The six-coordinated cyclometal-
lated iridium(III) complex with monodenate triphenylphosphine
as coordinating ligand was also synthesized via the conventional
and MW routes (Scheme 1). The absorption and emission proper-
ties were recorded for the products synthesized following the
two different routes (Fig. 4). The same emission color and the spec-
tral pattern obtained for both cases support the identity of the two
The conventional synthesis of mononuclear iridium(III) com-
plexes 2–4 involves a lengthy route (32–36 h) (Table 1). In an alter-
native approach, these complexes were prepared using
a
microwave irradiation based technique (MW) that reduces the to-
tal reaction time drastically, with a slight variation in the product
yield (Table 1). This MW mediated synthetic technique has been
extended to other iridium(III) complexes (Scheme 1). In these
cases, the ancillary chelating ligands were varied to N^N, N^O
and O^O types (Scheme 1) and the observed total reaction time
5
6
7
8
2
4
3
400
500
600
700
800
Wavelength (nm)
Ph Ph
P
Ir
Ph Ph
P
Ir
Ph Ph
P
Ir
N
N
Cl
N
O
N
N
N
N
N
N
PF6
PF6
N
O
Ir
PF6
Ir
P
Ir
PF6
P
Ir
P
PPh3
Ph Ph
Ph Ph
Ph Ph
O
O
2
2
2
2
2
2
2
Fig. 5. A wide range of emission colors can be observed from iridium(III) complexes synthesized through the new microwave-based procedure. (Color online.)

P. Alam et al. / Polyhedron 53 (2013) 286–294
291
Fig. 6. Molecular orbital diagram showing the highest occupied and lowest unoccupied molecular orbitals for 4.
region, up to 380 nm. Based on previous reports on cyclometallat-
ed complexes of iridium(III) [18], these can be assigned to intrali-
gand 3p p⁄ and spin-allowed 1MLCT transitions. In addition, the
–
spectra of all these complexes exhibit weaker absorptions that ex-
tend up to 450 nm which can be assigned to 3MLCT transitions. The
band-edge of the 3MLCT transitions for these complexes has been
observed to be blue-shifted in comparison to what is found for
the well-known acetylacetonate/picolinate analogs (Fig. S1). This
observation can be explained by the coordination of strongly
p-
accepting phosphorous atoms to iridium(III) in 2–4 which results
in an increase of the HOMO–LUMO separation in these complexes.
This fact is also the origin of the blue-shifted emission spectra of
complexes 2–4 relative to their acetylacetonate/picolinate analogs
(Fig. S1). Complexes 2–4 do not exhibit a solvatochromic effect
(Fig. S2). This behavior, in combination with the observed struc-
tured emission spectra, supports the supposition that the nature
Fig. 7. HOMO and LUMO of 2, 3 and 4.
of the lowest excitations consists basically of 3p p⁄ transitions
–
with a significant admixture of 3MLCT transitions. This hypothesis
is also supported through quantum chemical modeling of these
complexes, described below. The relative quantum efficiency (in
DCM) for the complexes 2, 3 and 4 have been measured and the ob-
served values are 0.99, 0.48 and 0.40, respectively. The phospho-
rous-coordinated iridium(III) complexes, 2–4 and 8 exhibit a
structured emission and the color is greenish-blue [457 and
489 nm (2); 465 and 496 nm (3); 457 and 488 nm (4)] and green
[484 and 512 nm (8)], respectively, whereas the non-phosphorous
coordinated complexes 6–7 and 5 show a broad emission with yel-
lowish-green [503 nm (6); 518 nm (7)] and yellow [560 nm (5)]
emission colors, respectively (Fig. 5).
Table 3
Vertical excitation energies and oscillator strengths calculated for the lowest lying
singlet states for the three studied complexes (2–4).
Molecule
D
(nm)
E, eV
Osc.
Str.
%HOMO–
LUMO
EHOMO
(eV)
ELUMO
(eV)
2
3
4
3.39 (366)
3.39 (365)
3.39 (366)
0.076
0.058
0.072
97.4
96.7
97.3
ꢁ6.28
ꢁ6.33
ꢁ6.30
ꢁ2.19
ꢁ2.22
ꢁ2.21
complexes. The wide ranges of emission color that can be obtained
for complexes synthesized via the MW route, as shown in Fig. 5, ni-
cely illustrate that this alternative synthetic procedure can be em-
3.1. Computational modeling of the photophysical properties of
complexes 2–4
ployed for
a large variety of emitting materials for OLED
applications in an efficient way.
Their solution UV–Vis absorption spectra (in DCM, 10ꢁ5 M)
show intense bands appearing in the range 200–300 nm (Fig. 2).
3.1.1. Ground state geometries
Optimization of the ground-state molecular structures of the
three Ir(III) complexes 2–4 in dichloromethane solution yield very
similar coordination environments for the iridium atom (Fig. S11
and Table 1) in all three cases. The optimized values for the Ir–X
These bands can be assigned to spin-allowed 1p p⁄ transitions,
–
within the ligands. These ligand-centered bands are accompanied
by weaker transitions at lower energies, extending to the visible

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P. Alam et al. / Polyhedron 53 (2013) 286–294
Table 4
distances (Table 2) are in good agreement with those obtained for
similar complexes [5,19], both from X-ray diffraction and theoret-
ical calculations. As expected, the only significant difference in the
geometry of the coordination environment of iridium in the three
complexes is the simultaneous opening of the P–Ir–P angle and
closing of the C–Ir–C angle in 3 in order to fit the larger propane
bridge between the two phosphorus atoms. This also leads to a
slightly larger Ir–P distance in this case. There is, however, practi-
cally no difference between the other two cases with two-carbon
ethane or ethene bridges. Of these two observed changes, the most
important one is the opening of the P–Ir–P angle by almost 6°.
Vertical excitation energies calculated for the lowest lying singlet and triplet states
for the three studied complexes (2–4).
Molecule
States
T1
D
E, eV (nm)
Assignments
2
2.85 (434)
HOMOꢁ3 ? LUMO+1 (6%)
HOMOꢁ2 ? LUMO (12%)
HOMOꢁ1 ? LUMO+1 (22%)
HOMOꢁ1 ? LUMO+3 (2%)
HOMO ? LUMO (44%)
HOMOꢁ3 ? LUMO (9%)
HOMOꢁ2 ? LUMO + 1 (10%)
HOMOꢁ1 ? LUMO (29%)
HOMO ? LUMO + 1 (36%)
HOMO ? LUMO (97%)
T2
2.86 (433)
S1
T1
3.39 (366)
2.84 (437)
3.1.2. Frontier molecular orbitals
3
HOMOꢁ3 ? LUMO + 1 (6%)
HOMOꢁ2 ? LUMO (10%)
HOMOꢁ2 ? LUMO + 1 (3%)
HOMOꢁ1 ? LUMO (4%)
HOMOꢁ1 ? LUMO + 1 (15%)
HOMO ? LUMO (35%)
HOMO ? LUMO + 1 (10%)
HOMOꢁ3 ? LUMO (9%)
HOMOꢁ3 ? LUMO + 1 (3%)
HOMOꢁ2 ? LUMO + 1 (6%)
HOMOꢁ1 ? LUMO (22%)
HOMOꢁ1 ? LUMO + 1 (7%)
HOMO ? LUMO (7%)
In (Fig. 6) we show an isovalue representation of the molecular
orbitals that have a significant participation in the low-lying exci-
tations for complex 4. The calculated HOMO–LUMO gap is approx-
imately 4.1 eV for each of the complexes 2–4. A more detailed
picture comparing the HOMO and the LUMO for the three com-
plexes is given in Fig. 7. As is evident from this picture, while there
T2
2.85 (435)
is a significant participation of iridium d orbitals together with p-
type orbitals of the phenyl rings of the 2-phenylpyridyl ligands in
the HOMO, the LUMO is on the contrary dominated by p⁄-type
orbitals of the pyridine rings of the 2-phenylpyridyl ligands. There
is no significative participation of the phosphine ligands to any of
the orbitals involved in the low-lying excitations for any of the
three complexes.
HOMO ? LUMO + 1 (28%)
HOMO ? LUMO (97%)
S1
T1
3.39 (365)
2.85 (435)
4
HOMOꢁ3 ? LUMO + 1 (7%)
HOMOꢁ2 ? LUMO (12%)
HOMOꢁ1 ? LUMO + 1 (21%)
HOMOꢁ1 ? LUMO + 3 (2%)
HOMO ? LUMO (42%)
HOMOꢁ3 ? LUMO (10%)
HOMOꢁ2 ? LUMO + 1 (9%)
HOMOꢁ1 ? LUMO (27%)
HOMO ? LUMO + 1 (35%)
HOMO ? LUMO (97%)
3.1.3. Excited states
If we consider the excitation from the ground state to the first
excited singlet state S1, our results (Table 3) indicate that for all
three complexes 2–4 the electronic transition can be considered
basically as a simple HOMO to LUMO electron promotion. Accord-
ing to this finding and considering the representation of the molec-
ular orbitals involved in the transition (Fig. 7), it can be deduced
that this transition has significant MLCT character from the d orbi-
T2
S1
2.86 (434)
3.39 (366)
tals of the iridium atom to the
nylpyridyl ligands, mainly to p⁄ orbitals of the C5N pyridine rings.
It is also worth noting the existence of some degree of
to p⁄ tran-
sition in these ligands, with electronic charge being transferred
from the orbitals of the C6 phenyl rings of the 2-phenylpyridyl
p system of the coordinated 2-phe-
p
Table 5
p
MLCT character for the transitions from the ground state (S0) to the lowest excited
singlet (S1) and triplet states (T1).
ligands to p⁄ orbitals of the C5N pyridine rings. The calculated exci-
tation energies are practically identical for all three compounds, in
good agreement with the experimental absorption spectra (Fig. 2).
Our calculations show that the oscillator strength is very similar
for 2 and 4, the two complexes involving two-atomic bridges be-
tween the phosphine atoms, while it is predicted to be significantly
lower for 3, the complex with the propane bridge. In order to inter-
pret the emission spectra, we have also calculated vertical excita-
tion energies to the lowest lying triplet state (T1 in Table 4). The
computed vertical transition energies to T1 at the Frank–Condon
region are close to 435 nm for the three complexes, slightly blue
shifted with respect to the measured wavelengths (ꢀ460 nm).
Our calculations do not take into consideration the geometry relax-
ation of the T1 state, which is expected to reduce the S0/T1 com-
puted energy gaps. The experimental photoluminescence spectra
(Fig. 4) present several emission peaks approximately separated
by 1450 cmꢁ1 (Fig. 4). This structure has been described in previ-
ous studies on similar iridium compounds, and has been attributed
to the vibrational progressions related to the stretching of C–C
double bounds at the aromatic ligands [20]. In order to interpret
the emission spectra, we have also calculated vertical excitation
energies to the lowest lying triplet states (Table 4). Our calcula-
tions show for the three complexes the existence of two practically
degenerated triplet states in the 430 nm region. If we consider that
geometry relaxation in the excited states should lead to somewhat
lower energies of these states, as well as to somewhat different
Molecule
S
0 ? S1 (%)
S
0 ? T1 (%)
2
3
4
34
42
38
8
10
11
Fig. 8. Electron density differences between the S1 and S0 states (left) and between
the T1 and S0 states (right) for 4. Blue corresponds to negative values (higher
electron density in the ground state) while green corresponds to positive ones
(higher electron density in the excited state). (Color online.)

P. Alam et al. / Polyhedron 53 (2013) 286–294
293
energies for the two triplet states, these results seem to be in fair
agreement with the two emission peaks observed in the experi-
mental emission spectra. Unfortunately our calculations, that do
not include spin–orbit corrections, do not allow us to predict the
intensity of these spin-forbidden emission lines in order to explain
the differences observed experimentally. The character of the tran-
sitions from the ground state (S0) to the lowest excited singlet (S1)
and triplet states (T1) has been analyzed by comparing the atomic
Mulliken populations for each fragment in the molecule. The re-
sults, shown in Table 5, indicate a significant MLCT character for
the S0?S1 excitation, which is similar for all three molecules,
although it is interesting to note that the MLCT character is signif-
icantly lower in the case of the complex with an unsaturated car-
bon bridge, namely 2.
The nature of the MLCT character of these transitions can be
easily visualized by plotting the difference between the charge
densities of the two states involved in the transition. In Fig. 8 we
show these difference plots for complex 4. From this representa-
tion, it can be easily seen that the most important charge transfer
in the S0?S1 excitation, responsible for the absorption process, is
between the iridium atom and the two pyridine rings that are di-
rectly coordinated to the central metal atom. From this type of rep-
resentation it is evident that the ancillary phosphine ligands do not
play any significant role in the photophysics involving the low ly-
ing excited states of these complexes.
geometries, crystal data and structural refinement, including se-
lected bond lengths and bond angles for complex 4, are deposited.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.01.045.
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