Iridium(III) complexes with orthometalated phenylimidazole ligands subtle turning of emission to the saturated green colour

Article abstract of DOI:10.1007/s10895-010-0737-7

A series of novel six iridium complexes (1-6) bearing two substituted phenylimidazole and an additional acetylacetone as the third co-auxilary ligand are reported. The lowest absorption band for all iridium complexes consist of a mixture of heavy atom Ir(

Full text of DOI:10.1007/s10895-010-0737-7

J Fluoresc (2011) 21:507–519
DOI 10.1007/s10895-010-0737-7
ORIGINAL PAPER
Iridium(III) Complexes with Orthometalated
Phenylimidazole Ligands Subtle Turning of Emission
to the Saturated Green Colour
Jayaraman Jayabharathi & Venugopal Thanikachalam &
Kanagarathinam Saravanan & Natesan Srinivasan
Received: 27 July 2010 /Accepted: 28 September 2010 /Published online: 16 October 2010
#
Springer Science+Business Media, LLC 2010
Abstract A series of novel six iridium complexes (1–6)
bearing two substituted phenylimidazole and an additional
acetylacetone as the third co-auxilary ligand are reported. The
lowest absorption band for all iridium complexes consist of a
mixture of heavy atom Ir(III) enhanced MLCT and π-π*
transitions and the phosphorescent peak wavelength can be
fine-tuned to cover the spectral range 455–518 nm with high
quantum efficiencies. The peak wavelength of the dopants can
be finely tuned depending upon the electronic properties of
the substituents. On the basis of onset potentials of the
oxidation and reduction, the HOMO-LUMO energies were
calculated and the reported iridium complexes emit green light
with exceeding higher efficiency.
efficiency (hint) of as high as ~100% could theoretically be
achieved, these heavy-metal containing emitters would be
superior to their fluorescent counterparts in future OLED
applications [4–11]. As a result, there is a continuous trend
of shifting research endeavors of these heavy transtion metal
complexes.
Since the manufacture of a full colour display requires
the usage of emitters with all three primary colours, blue,
green and red rationally tuning the emission wavelength of
heavy-metal phosphorescent emitters over the entire visible
range has emerged as an important ongoing research task
[12]. Reports on the red-emitting complexes with the Os(II)
[13–18] and Pt(II) [19, 20] elements have been documented
in the literature. In this paper, we report a systematic
design, synthesis and characterization of green-emitting Ir
(III) complexes containing substituted phenylimidazole
ligands for which, the high rigidity of the ligand framework
would significantly reduce the non-radiative transitions.
3
3
.
.
Keywords MLCT transition Colour tuning DFT
.
.
calculation HOMO-LUMO orbital Transphobia
Introduction
Organometallic complexes possessing a third-row transition-
metal element are crucial for the fabrication of highly efficient
organic light-emitting diodes (OLEDs) [1–3]. The strong
spin-orbit coupling induced by a heavy-metal ion such as
iridium(III) promotes an efficient intersystem crossing from
the singlet to the triplet excited-state manifold which then
facilitates strong electroluminescence by the harnessing of
both singlet and triplet excitons after the initial charge
recombination. Since internal phosphorescence quantum
Experimental
Materials and Methods
Iridium(III) trichloride hydrate (IrCl3·3H2O, Sigma-Aldrich
Ltd.), 2-ethoxyethanol (H5C2OC2H4OH, S.D. fine) and all
the other reagents used without further purification.
Optical Measurements and Compositions Analysis
The ultraviolet-visible (UV-vis) spectra of the phosphorescent
Ir(III) complexes were measured in an UV-vis spectrophotom-
eter (Perkin Elmer Lambda 35) and corrected for background
absorption due to solvent. Photoluminescence (PL) spectra
were recorded on a (Perkin Elmer LS55) fluorescence
:
:
:
J. Jayabharathi (*) V. Thanikachalam K. Saravanan
N. Srinivasan
Department of Chemistry, Annamalai University,
Annamalainagar 608002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

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spectrometer. The solid-state emission spectra were recorded
on fluoromax 2 (ISA SPEX) xenon-Arc lamp as a source.
NMR spectra were recorded on Bruker 400 MHz NMR
spectrometer. MS spectra (EI and FAB) were recorded on a
Varian Saturn 2200 GCMS spectrometer. Cyclic voltammetry
(CV) analysis were performed by using CHI 630A potentios-
tap electrochemical analyzer. Measurements of oxidation and
reduction were undertaken using 0.1 M tetra(n-butyl)ammo-
nium- hexafluorophosphate as the supporting electrolyte, at
scan rate of 0.1VS−1. The potentials were measured against an
Ag/Ag+ (0.01 M AgNO3) reference electrode using ferro-
cene/ferrocenium (CP2Fe/CP2Fe+) as the internal standard.
The onset potentials were determined from the intersection of
two tangents drawn at the rising current and background
current of the cyclic voltammogram.
4,5-Dimethyl-1-(3′,5′-dimethoxyphenyl)-2-
phenyl-1H-imidazole (dmdmppi)
1
Yield: 45%. H NMR (400 MHz, CDCl3): δ 2.29 (s, 3H),
2.05 (s, 3H), 3.73 (s, 6H), 6.32–7.43 (aromatic protons).
13C NMR (100 MHz, CDCl3): δ 9.36, 12.58, 55.32, 100.21,
106.20, 125.13, 127.45, 127.65, 127.83, 130.68, 133.34,
139.38, 144.79, 161.01. Anal. calcd. for C19H20N2O2: C,
73.93; H, 6.54; N, 9.08. Found: C, 73.23; H, 6.18; N, 8.79.
MS: m/z 308.3, calcd. 308.38.
4,5-Dimethyl-1-(3′,5′-dimethoxyphenyl)-2-
(p-fluorophenyl)-1H-imidazole (dmdmpfpi)
1
Yield: 45%. H NMR (400 MHz, CDCl3): δ 2.28 (s, 3H),
2.04 (s, 3H), 3.74 (s, 6H), 6.30–7.40 (aromatic protons).
13C NMR (100 MHz, CDCl3): δ 9.40, 12.58, 55.44, 100.32,
106.25, 114.85, 115.06, 125.23, 127.00, 129.55, 133.38,
139.27, 143.98, 161.18. Anal. calcd. for C19H19N2O2F: C,
69.92; H, 5.87; N, 8.58. Found: C, 69.12; H, 5.37; N, 8.24.
MS: m/z 326.3, calcd. 326.37.
General Procedure for the Synthesis of Ligands
The various substituted phenylimidazole ligands were
prepared from an unusual four components assembling of
1,2-dione, ammonium acetate, arylamine and an arylalde-
hyde as shown in Scheme 1 [21].
4,5-Dimethyl-1-(4′-t-butylphenyl)-2-(p-fluorophenyl)-1H-
4,5-Dimethyl-1,2-diphenyl-1H-imidazole (dmdpi)
imidazole (dmtbpfpi)
1
1
Yield: 48%. H NMR (400 MHz, CDCl3): δ 2.07 (s, 3H),
Yield: 52%. H NMR (400 MHz, CDCl3): δ 1.32 (s, 9H),
2.35 (s, 3H), 7.22 (m, 5H), 7.37 (m, 2H), 7.48 (m, 3H). 13
C
1.92 (s, 3H), 2.26 (s, 3H), 6.82 (t, 2H, J=8.4 Hz), 7.04 (d,
2H, J=8 Hz), 7.27 (m, 2H), 7.41 (d, 2H, J=8 Hz). 13C
NMR (100 MHz, CDCl3): δ 9.40, 12.58, 31.0, 72.0,
110.30, 125.31, 127.08, 133.89, 142.68. Anal. calcd. for
C21H23N2F: C, 78.17; H, 7.13; N, 8.69. Found: C, 78.34;
H, 7.03; N, 6.71. MS: m/z 322.2, calcd. 322.39.
NMR (100 MHz, CDCl3): δ 9.50, 12.58, 71.50, 114.2,
117.3, 128.20, 129.58, 144.35. Anal. calcd. for C17H16N2:
C, 82.21; H, 6.45; N, 11.28. Found: C, 82.07; H, 6.32; N,
11.04. MS: m/z 248.03, calcd. 248.13.
4,5-Dimethyl-1-(4′-methoxyphenyl)-2-
phenyl-1H-imidazole (dmmppi)
General Procedure for the Synthesis of Iridium
Complexes (1–6)
1
Yield: 50%. H NMR (400 MHz, CDCl3): δ 2.29 (s, 3H),
2.02 (s, 3H), 3.85 (s, 3H), 6.91–7.10 (aromatic protons).
13C NMR (100 MHz, CDCl3): δ 9.39, 12.59, 55.32, 114.50,
125.51, 127.41, 127.87, 128.78, 130.48, 130.74, 133.12,
145.08, 159.23. Anal. calcd. for C18H18N2O: C, 77.67; H,
6.52; N, 10.06. Found: C, 77.14; H, 6.32; N, 9.87. MS: m/z
278.2, calcd. 278.36.
The phenylimidazole based cyclometalated iridium com-
plexes 1–6 were readily synthesized from IrCl3·nH2O and
the phenylimidazole ligands to give the corresponding
dimeric species via the Nonoyama route [22] followed by
the treatment with acetylacetone in the presence of K2CO3
as shown in Scheme 1.
Iridium(III)bis(4,5-dimethyl-1,2-diphenyl -1H-
imidazolato-N,C2′)(acetylacetonate) (Ir(dmdpi)2(acac)), 1
4,5-Dimethyl-1-(4′-methoxyphenyl)-2-
(p-fluorophenyl)-1H-imidazole (dmmpfpi)
1
Yield: 68%. H NMR (400 MHz, CDCl3): δ 7.89 (dd,
2H, J=5.0, 9.0 Hz), 7.71 (dd, 2H, J=5.5, 8.5 Hz), 7.35 (dd,
2H, J=8.5, 11.0 Hz), 7.20 (d, 4 H), 7.06 (t, 2H, J=8.5 Hz),
7.14 (t, 3H, J=8.5 Hz), 6.52 (d, 3H, J=8.5 Hz), 5.31 (s,
1H), 2.24 (s, 3H), 2.06 (s, 3H), 1.36 (s, 6H), 1.20 (s, 6H).
13C NMR (100 MHz, CDCl3): δ 9.41, 10.52, 23.91, 24.48,
25.74, 31.19, 31.31, 34.65, 115.92, 116.13, 116.31, 119.18,
126.12, 126.62, 127.44, 130.06, 130.13, 130.99, 131.06,
136.57, 151.58, 171.66. Anal. calcd. for C39H37IrN4O2: C,
1
Yield: 40%. H NMR (400 MHz, CDCl3): δ 2.28 (s, 3H),
1.99 (s, 3H), 3.85 (s, 3H), 6.85–7.35 (aromatic protons).
13C NMR (100 MHz, CDCl3): δ 9.50, 12.69, 55.48, 114.71,
114.93, 115.14, 128.90, 129.80, 130.42, 133.21, 144.36,
159.45. Anal. calcd. for C18H17N2OF: C, 72.95; H, 5.78; N,
9.45. Found: C, 72.24; H, 5.36; N, 8.98. MS: m/z 296.5,
calcd. 296.35.

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509
Scheme 1 Synthesis of ligands
and iridium complexes (C^N)2Ir
(acac) 1–6
1
59.60; H, 4.75; N, 7.13. Found: C, 59.30; H, 4.35; N, 7.08.
MS: m/z 786.05, calcd. 786.25.
Iridium(III)bis(4,5-dimethyl-1-(4′-methoxyphenyl)-2-
phenyl-1H-imidazolato-N,C2′) (acetylacetonate) (Ir
(dmmppi)2(acac)), 2
Yield: 62%. H NMR (400 MHz, CDCl3): δ 7.35 (m,
2H), 7.26 (m, 2H), 7.05 (m, 4H), 6.53 (m, 2H), 6.47 (dd,
2H, J=0.92, 7.53 Hz), 6.38 (m, 2H), 6.15 (dd, 2H, J=0.88,
7.73 Hz), 5.28 (s, 1H), 3.91 (s, 6H), 2.20 (s, 6H), 2.00 (s,
6H), 1.76 (s, 3H), 1.60 (s, 3H), 1.26 (s, 3H). 13C NMR

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8H), 5.29 (s, 1H), 3.90 (s, 6H), 2.12 (s, 3H), 2.15 (s, 3H),
1.98 (s, 6H), 1.76 (s, 6H). 13C NMR (100 MHz, CDCl3): δ
9.31, 10.41, 28.26, 55.56, 115.03, 101.04, 106.38, 115.03,
128.91, 129.36, 129.49, 156.30, 160.18, 184.29. Anal. calcd.
for C41H39F2IrN4O4: C, 55.83; H, 4.46; N, 6.35. Found: C,
55.62; H, 4.35; N, 6.28. MS: m/z 881.40, calcd.
882.26.
Iridium(III)bis(4,5-dimethyl-1-(3′,5′-dimethoxyphenyl)-
2-phenyl-1H-imidazolato-N,C2′)(acetylacetonate) (Ir
(dmdmppi)2(acac)), 4
1
Yield: 78%. H NMR (400 MHz, CDCl3): δ 7.24 (s,
2H), 6.61 (t, 2H, J=2.5 Hz), 6.54 (t, 2H, J=2.5 Hz), 6.45 (t,
2H, J=2.5 Hz), 6.24 (dd, 2H, J=7.5, 11.0 Hz), 6.13 (d, 2H,
J=2.5 Hz,), 6.07 (dd, 2H, J=3.5, 13.0 Hz), 5.29 (s, 1H),
3.80 (s, 6H), 3.77 (s, 6H), 2.16 (s, 6H), 2.03 (s, 6H), 1.77
(s, 3H), 1.56 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 9.63,
14.09, 22.68, 29.69, 55.74, 101.84, 106.30, 119.56, 126.0,
132.08, 134.11, 138.76, 161.89, 163.86, 187.02. Anal.
calcd. for C43H45IrN4O6: C, 57.00; H, 5.01; N, 6.18.
Found: C, 56.82; H, 4.98; N, 6.02. MS: m/z 905.82, calcd.
906.30.
Fig. 1 The UV-vis absorption spectra of the complexes 1–6 in CH2Cl2
(100 MHz, CDCl3): δ 9.32, 10.47, 28.32, 29.67, 55.54,
100.67, 114.89, 119.33, 121.57, 123.37, 126.55, 128.23,
128.84, 129.45, 132.38, 134.03, 137.22, 144.86, 157.15,
160.05, 183.92. Anal. calcd. for C41H41IrN4O4: C, 58.21;
H, 4.88; N, 6.62. Found: C, 57.96; H, 4.72; N, 6.48. MS:
m/z 846.60, calcd. 846.28.
Iridium(III)bis(4,5-dimethyl-1-(3′,5′-dimethoxyphenyl)-
2-(p-fluorophenyl)-1H-imidazolato-N,C2′)(acetylacetonate)
(Ir(dmdmpfpi)2(acac)), 5
Yield: 72%. 1H NMR (400 MHz, CDCl3): δ 6.63 (t, 2H,
J=2.26 Hz), 6.57 (t, 2H, J=1.97 Hz), 6.47 (t, 2H, J=
1.93 Hz), 6.62 (dd, 2H, J=5.88, 8.52 Hz), 6.15 (m, 2H),
6.10 (dd, 2H, J=2.56, 10.41 Hz), 5.31 (s, 1H), 3.82 (s, 6H),
3.79 (s, 6H), 2.18 (s, 6H), 2.05 (s, 6H), 1.79 (s, 3H), 1.26
Iridium(III)bis(4,5-dimethyl-1-(4′-methoxyphenyl)-2-(p-
fluorophenyl)-1H-imidazolato-N,C2′)(acetylacetonate) (Ir
(dmmpfpi)2(acac)), 3
Yield: 75%. 1H NMR (400 MHz, CDCl3): δ 7.32 (d, 1H,
J=2.76 Hz), 7.24 (t, 1H, J=2.16 Hz), 7.04 (m, 4H), 6.1 (m,
Table 1 Photophysical properties of iridium complexes 1–6
Complex
Absorptiona (λ, nm) (log ε)
Emissionb (λ, nm)
Quantum yield (ϕ)
Lifetime (μs)
kr
knr
Ir(dmdpi)2(acac), 1
230.0 (4.77)
331.0 (4.07)
455
0.08
2.3
0.03
0.40
393.0 (3.23)
Ir(dmmppi)2(acac), 2
Ir(dmmpfpi)2(acac), 3
Ir(dmdmppi)2(acac), 4
Ir(dmdmpfpi)2(acac), 5
Ir(dmtbpfpi)2(acac), 6
231.0 (4.63)
328.0 (3.93)
502
508
510
518
498
0.28
0.34
0.43
0.49
0.25
2.4
2.3
1.8
1.5
2.2
0.12
0.15
0.24
0.33
0.11
0.30
0.28
0.32
0.34
0.34
389.0 (3.26)
260.0 (4.60)
356.0 (3.62)
450.0 (2.83)
269.0 (4.65)
369.0 (4.22)
489.0 (3.34)
228.0 (4.79)
335.0 (4.07)
356.0 (3.23)
222.0 (4.72)
301.0 (4.12)
392.0 (3.20)
a
UV-vis absorption measured in CH2Cl2 solution, concentration=1×10−5 M.
Photoluminescence measured in CH2Cl2 solution, concentration=1×10−4 M.
b

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511
Theoretical Calculations
All calculations were performed using density functional
theory (DFT) as implemented in the Gaussian 03 program
[23]. The geometry optimization was carried out by B3LYP
level using LANL2Z basis set [24].
Results and Discussion
Absorption Spectra
Figure 1 shows the absorption spectra of iridium complexes
1–6, the intense band observed in the ultraviolet region
(220–270 nm) can be assigned to the allowed ligand-
centered (LC) (π-π*) transition [25] of the phenylimidazole
ligand since free ligands show absorption around similar
wavelength region. Somewhat weaker bands are observed
in the lower part of energy around 300–490 nm are not
observed in the spectra of free ligands. The band position,
size and extinction coefficients suggest that these are metal-
to-ligand charge transfer (MLCT) transitions [10, 25–30]
and the same assignment is likely here [25–28]. Therefore
metal-to-ligand charge transfer transitions 1MLCT and
3MLCT have been resolved in the range of 300–490 nm
as indicated in Table 1. Absorption in the range around
300–369 nm for all the complexes correspond to the
1
transition of the MLCT state as evident from its extinction
coefficient of the order 103. The long tail toward lower
energy around 380–489 nm are assigned to 3MLCT
transitions and gains intensity by mixing with the higher
1
lying MLCT transitions through the spin-orbit coupling of
Iridium(III) [27, 28].
Fig. 2 a The photoluminescence emission spectra of the complexes 1,
2, 3 and 6 in CH2Cl2 and b the photoluminescence emission spectra of
the complexes 4 and 5 in CH2Cl2
Photoluminescence Properties
(s, 3H). 13C NMR (100 MHz, CDCl3): δ 9.25, 10.26, 29.69,
55.72, 100.97, 102.06, 106.22, 106.47, 119.95, 120.11,
123.01, 123.31, 132.26, 133.16, 137.94, 161.63, 184.27.
Anal. calcd. for C43H43F2IrN4O6: C, 54.82; H, 4.60; N,
5.95. Found: C, 54.63; H, 4.38; N, 5.78. MS: m/z 941.98,
calcd. 942.04.
The emission spectra in CH2Cl2 at ambient temperature for
all six iridium complexes 1–6 are shown in Figs. 2a, b and
Table 1 summarizes the luminescent data of complexes 1–6.
In solution, complexes 1–6 have emission maxima around
455, 502, 508, 510, 518 and 498 nm, respectively.
Introduction of substituent [31] in the para and meta
positions of the phenyl ring attached to the nitrogen of the
imidazole ring produces the red shift in the emission spectra
by 43–63 nm. In the case of p-methoxy substituted iridium
complexes [Ir(dmmppi)2(acac) (2) and Ir(dmmpfpi)2(acac)
(3)] red shift was observed when compared with unsub-
stituted iridium complex Ir(dmdpi)2(acac) (1) by means of
47 and 53 nm, respectively. This may be due to the
mesomeric effect exerted by the p-methoxy substituent so
that resonance interaction between the imidazole ring and
the methoxy substituted aryl ring increased and this causes
the red shift in the emission spectra (Figs. 2a and b). When
Iridium(III)bis(4,5-dimethyl-1-(4′-t-butylphenyl)-2-(p-
fluorophenyl)-1H-imidazolato-N,C2′)(acetylacetonate) (Ir
(dmtbpfpi)2(acac)), 6
1
Yield: 72%. H NMR (400 MHz, CDCl3): δ 7.56 (m,
2H), 7.55 (d, 2H, J=4.5 Hz), 7.26 (m, 2H), 6.07 (m, 8H),
5.31 (s, 1H), 2.19 (s, 6H), 2.00 (s, 6H), 1.99 (s, 3H), 1.60
(s, 3H), 1.42 (s, 18H). 13C NMR (100 MHz, CDCl3): δ
9.51, 12.92, 29.30, 32.52, 72.89, 100.68, 112.82, 125.8,
131.12, 138.81, 160.92, 187.32. Anal. calcd. for
C47H51F2IrN4O2: C, 60.43; H, 5.50; N, 6.00. Found: C,
60.16; H, 5.28; N, 5.83. MS: m/z 934.12, calcd. 934.36.

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513
Fig. 4 HOMO-LUMO energy levels of the complexes 1–6
solvents than in non-polar solvents which leads to red shift
of emission with increasing solvent polarity [32]. The
photoluminescent peak of solid state (representative spec-
trum of complex 1 is shown in Fig. 3) of all complexes are
almost similar to that of emission in non-polar solvent (n-
hexane) which shows that there is very little or no influence
of molecular interaction on the excited state of iridium
complexes in the solid state [32].
Fig. 3 The solid state and the solvatochromic emission spectra of the
complex 1
HOMO-LUMO Energies
on dimethoxy group introduced [Ir(dmdmppi)2(acac) (4)
and Ir(dmdmpfpi)2(acac) (5)] the emission is still maximum
due to the additional electronic effect imparted by the
methoxy substitutents at 3′ and 5′.
The electrochemical properties of the cyclometalated
iridium complexes were examined by cyclic voltammetry.
The redox potentials were measured relative to an internal
ferrocene reference (Cp2Fe/Cp2Fe+=0.45 V versus SCE in
CH2Cl2 solvent) [33, 34] are given in Table 3. As revealed
previously [35, 36] the reductions occur primarily on the
more electron accepting heterocyclic portion of the cyclo-
metalated C^N ligands whereas the oxidation process is
generally considered to largely involve the Ir-phenyl centre.
The energies of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) were calculated using the following Eqs. 1 and 2
[33, 34] and the calculated values are given in Table 3.
Solvatochromism of the Complexes 1–6
The absorption peak of the complexes 1–6 are almost same
in different solvents. This suggests that the polarity of the
solvent has very little influence on the ground state energy
level of the complexes (Table 2). However variation in the
emission peak of the complexes 1–6 was observed in
different solvents. The representative spectra of complex 1
are shown in Fig. 3. The emission peak of the complexes 1–
6 are 436–487 nm in hexane, 459–524 nm in ethanol, 455–
518 nm in CH2Cl2 and 461–527 nm in CH3CN. The peak
shift may be due to the stronger interaction between the
solvents and the excited state molecules. The excited state
of all iridium complexes are more stabilized in polar
EHOMO ¼ 4:4 þ E
ð1Þ
ð2Þ
ðonsetÞ
ELUMO ¼ EHOMO ꢀ 1239=labs
The iridium complexes show reversible oxidation behav-
iour and these complexes exhibit HOMO levels ranging of
Table 3 Cyclic voltammetry
data of the complexes 1−6
Complex
E
(V)
HOMO (eV)
LUMOa (eV)
Eg (eV)
(onset)
Ir(dmdpi)2(acac), 1
Ir(dmmppi)2(acac), 2
Ir(dmmpfpi)2(acac), 3
0.20
0.38
0.48
0.50
0.54
0.22
−5.00
−5.18
−5.28
−5.30
−5.34
−5.02
−2.50
−2.82
−2.98
−3.08
−3.24
−2.52
2.50
2.36
2.30
2.22
2.10
2.50
Ir(dmdmppi)2(acac), 4
a
Ir(dmdmpfpi)2(acac), 5
Ir(dmtbpfpi)2(acac), 6
Measurement was carried out in
CH2Cl2 solution, concentration=
1×10−3 M.

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Quantum Yield and Photochemical Properties
All these complexes 2–6 having larger quantum yield
ranging from 0.25–0.49 than unsubstituted iridium complex
Ir(dmdpi)2(acac) (0.08) (1) (Table 1). The quantum yield
has a tendency to shift to higher with increasing the
maximum emission peak wavelength (Fig. 5). The PL
quantum yield for all iridium complexes 1–6 were
measured in dichloromethane using coumarin 47 in ethanol
as a standard [39] according to the Eq. 3,
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
hunk
hstd
funk ¼ fstd
ð3Þ
Aunk
Where funk is the fluorescence quantum yield of the
sample, fstd is the fluorescence quantum yield of the
standard, Iunk and Istd are the integrated emission intensities
of the sample and the standard, respectively. Aunk, and Astd
are the absorbances of the sample and the standard at the
excitation wavelength, respectively. ηunk and ηstd are the
indexes of refraction of the sample and standard solutions.
The quantum yield for complexes 3, 4 and 5 are close to the
value of 0.40 for Ir(ppy)3 and the photoluminescent lifetime
for all iridium complexes 1–6 are reported in Table 1
(Fig. 6). Since these values are comparable to that of Ir
(ppy)3 (0.40) [40, 41] which strongly support that these
iridium complexes 1–6 are highly phosphorescent emitters.
The radiative and non-radiative rate constants kr and knr
are calculated from the phosphorescence yield (Φp) and the
phosphorescence lifetime (τ) using the following Eq. 4,
Fig. 5 The plot of quantum yield versus emission peak wavelength of
the complexes 1–6
5.00–5.34 eV. The HOMO level of complexes 3–5 are
higher than those reported for other iridium complexes
[5.2 eV for Ir(ppy)2(acac) and 5.16 eV for Ir(btp)2(acac)]
[12, 25, 36, 37]. The LUMO energies were calculated based
on the HOMO energies and the lowest-energy absorption
edges of the UV-vis absorption spectra [34]. The calculated
energy gap (Eg=HOMO-LUMO) (Fig. 4) for complexes 2–
5 are minimum whereas complexes [Ir(dmdpi)2(acac) (1)
and Ir(dmtbpfpi)2(acac) (6)] exhibit maximum energy gap.
These results reveal that the methoxy substituted complexes
2–5 show emission with the maximum wavelength [502
(2), 508 (3), 510 (4) and 518 nm (5)] (minimum Eg)
whereas complexes [Ir(dmdpi)2(acac) (1) and Ir
(dmtbpfpi)2(acac) (6)] (maximum Eg) exhibit emission with
shorter wavelength [455 (1) and 498 nm (6)].
Φp ¼ Φiscfkr=ðkr þ knrÞg
ð4Þ
Where, Φisc is the intersystem crossing yield. For iridium
complexes Φisc is safely assumed to be 1.0 because of the
strong spin-orbit interaction caused by heavy atom effect of
iridium [27, 28]. Thus,
Effect of Substituent in the Phenyl Ring on HOMO
In the present study iridium complexes 2–6 have electron
releasing substituents on the phenyl ring attached to the
nitrogen atom of the phenylimidazole ligand and electron
withdrawing substituent (i.e., fluorine) on the phenyl ring
attached to the carbon atom of the imidazole ligand. It is
obvious that the HOMO level of complexes 1–6 is strongly
affected by a kind and number of substituent on the phenyl
ring attached to the carbon atom of the imidazole moiety
(Table 3). Comparison of HOMO values of the iridium
complexes reveals that the HOMO values are higher for 2,
3, 4 and 5 than complexes 1 and 6. This may be due to the
inductive effect exerted by substituent causes stabilization
of iridium complexes through the carbon atom-iridium
bonding. Therefore the HOMO stability and the emission
energy gap are controlled by the nature of substituents and
its inductive influence on the aromatic ring [38].
Fig. 6 The lifetime spectra of the complexes 1–6 in CH2Cl2

J Fluoresc (2011) 21:507–519
515
Fig. 7 Quantum yield and decay rate constant of the complexes. a Radiative decay rate constant versus emission peak wavelength, b non-
radiative decay rate constant versus emission peak wavelength and c the plot of ln(knr) versus emission energy gap
with increase in the red shift however the non-radiative
kr ¼ Φp=t
ð5Þ
ð6Þ
decay rate constant (knr) dose not show monotonous change
i.e., nearly same for all complexes [38] (Figs. 7a and b).
The plot of ln(knr) versus the energy gap for complexes
1–6 (Fig. 7c) shows no linear relationship and there is no
agreement with the “Energy gap Law”. The energy gap law
predicts that the rate of non radiative decay increases when
the energy gap decreases. This relation is based on the
vibrational overlap between the ground state and the
excited state and knr is a function of a Franck-Condon
overlap integral [38, 39, 42, 43].
Lastly the radiative decay rate constant of complexes Ir
(dmdmppi)2(acac) (4) and Ir(dmdmpfpi)2(acac) (5) are
higher than those of complexes 1–3 and 6. This may be
due to the changes of the mixing states. Consequently
complexes Ir(dmdmppi)2(acac) (4) and Ir(dmdmpfpi)2(a-
cac) (5) have large quantum yield rather than complexes 1–
3 and 6. Thompson et al. [19] have concluded that shorter
lifetime and stronger trans effect would cause lower
knr ¼ 1=t ꢀ Φp=t
t ¼ ðkr þ knrÞꢀ1
From the viewpoint of the relationship between maximum
emission peak wavelength of photoluminescent spectra and
decay rate constants, two trends are evident for the iridium
complexes 1–6. The radiative rate constant (kr) increases
ð7Þ
Fig. 8 The HOMO-LUMO orbital picture for the complex Ir
(dmmpfpi)2(acac) (3)
Fig. 9 Solution colour of the photoluminescence for complexes 4 and 5

516
J Fluoresc (2011) 21:507–519
Table 4 Selected bond distance (Å) and bond angles (°) of iridium complexes 3 and 5
Complex
Atom(1)−Atom(2)
Distance (Å)
Atom(1)−Atom(2)−Atom(3)
Bond angles (°)
Ir(dmmpfpi)2(acac), 3
Ir(1)–C(10)
Ir(1)–C(4)
Ir(1)–N(28)
Ir(1)–N(2)
Ir(1)–O(9)
Ir(1)–O(8)
Ir(1)–C(8)
Ir(1)–C(25)
Ir(1)–N(2)
Ir(1)–N(4)
Ir(1)–O(6)
Ir(1)–O(7)
2.0197
2.0513
2.0161
2.0472
2.1695
2.1265
2.0197
2.0160
2.0512
2.0471
2.1695
2.1264
Ir(1)–N(4)–C(16)
Ir(1)–C(8)–C(33)
Ir(1)–N(6)–C(28)
Ir(1)–N(4)–C(10)
113.99
116.54
49.81
49.57
Ir(dmdmpfpi)2(acac), 5
N(4)–Ir(1)–C(8)
N(4)–Ir(1)–C(25)
Ir(1)–N(4)–C(31)
Ir(1)–N(2)–C(14)
94.04
79.19
115.00
113.99
quantum efficiency in the iridium complexes and similar
trend is observed in the case of para substituted iridium
complexes Ir(dmmppi)2(acac) (2) and Ir(dmtbpfpi)2(acac)
(6). The Ir(dmmppi)2(acac) (2) complex have longer
emission decay lifetime and consequently much larger
luminescence efficiency than the Ir(dmtbpfpi)2(acac) (6)
complex having shorter lifetime and longer quantum
efficiency.
π*) and ΦT(MLCT) [44, 45]. Therefore it can be concluded
that the complexes 1, 2, 3 and 6 have excited state with
3
large contribution of π-π* whereas the complexes 4 and 5
3
have excited state with large contribution of MLCT [38,
44, 46, 47].
Theoretical Approaches
The Mixing of Excited States
The Mixing of Excited States
The photophysical properties of complexes 1–6 reveals that
the vibrational sideband pattern of the photoluminescence
spectra were observed for the complexes 1, 2, 3 and 6
whereas broad shape of the luminescence spectra were
observed for complexes 4 and 5 as shown in Figs. 2a and b.
Phosphorescent lifetime of the complexes 4 and 5 were
obviously shorter than those of the complexes 1, 2, 3 and 6
and the radiative decay constant of complexes 1, 2, 3 and 6
are smaller than complexes 4 and 5 as shown in Table 1 and
the same trend was observed by Okada et al. [38].
From the above results (the differences of vibrational
structure, kr, molar absorption co-efficient of singlet-triplet
absorption peak and phosphorescent lifetime), the degree of
mixing is taken into consideration to understand the
photochemical differences of these complexes according
to following Eq. (8)
Calculations were performed using density functional
theory (DFT) as implemented in the Gaussian-03 program
[23]. The geometry optimization was carried out by B3LYP
level using LANL2Z [24] basis set. The geometry optimi-
zation were carried out for Ir(dmmppi)2(acac) (2) and Ir
(dmdmppi)2(acac) (4). The well known iridium complex Ir
3
(ppy)3 was used as a reference that had MLCT dominant
lowest excited state [38] for which the calculated Mulliken
charge difference on iridium atom between the ground state
and the lowest triplet excited state was 0.45. The
calculation were done for 2 and 4 and the calculated
Mulliken charges are 0.31 for Ir(dmmppi)2(acac) (2) and
0.43 for Ir(dmdmppi)2(acac) (4) and this result strongly
supports that 3π-π* is dominant for complex Ir
(dmmppi)2(acac) (2) owing to small reduction of Mulliken
charge on iridium when compared with Ir(ppy)3 and
3MLCT is dominant for complex Ir(dmdmppi)2(acac) (4)
[23] owing to close to the value of Ir(ppy)3.
»
ΦT ¼ aΦTðp ꢀ p Þ þ bΦTðMLCTÞ
ð8Þ
Where, a and b are the normalized co-efficients and ΦT (π-
3
HOMO-LUMO Orbitals of Ir(dmmpfpi)2(acac) (3)
π*) and ΦT(MLCT) are the wave function of π-π* and
3MLCT excited states, respectively. For the iridium com-
plex, the wave function of the triplet state (ΦT) responsible
for phosphorescence and Eq. 8 implies that the excited
triplet state of the iridium complexes are mixture of ΦT(π-
The DFT calculations suggest that the highest occupied
molecular orbital (HOMO) of these complexes are mainly
localized on the phenyl rings of the phenylimidazole ligand

J Fluoresc (2011) 21:507–519
517
emission spectra and it turns out that this expectation
correlates well with the emission data [25].
Description of the Structure of Complexes Ir
(dmmpfpi)2(acac) 3 and Ir(dmdmpfpi)2(acac) 5
From the optimized structures of iridium complexes, Ir
(dmmpfpi)2(acac) (3) and Ir(dmdmpfpi)2(acac) (5) some
selected bond lengths are presented in Table 4. From the
Table 4 it was concluded that these complexes exhibit an
octahedral geometry around iridium and prefers cis-C,C
and trans-N,N chelate disposition instead of trans-C,C and
trans-N,N chelate. Electron rich phenyl rings normally
exhibit very strong influence and trans effect. Therefore,
the trans-C,C arrangement is expected to be thermodynam-
ically higher in energy and kinetically more labile [49].
This well known phenomenon referred to as transphobia
[49, 50]. The Ir−Cav bond of these complexes [Ir−Cav=
2.018Å for Ir(dmmpfpi)2(acac) (3) and 2.013Å for Ir
(dmdmpfpi)2(acac) (5)] are found to be shorter than Ir−
Nav bond [Ir−Nav=2.049Å for Ir(dmmpfpi)2(acac) (3) and
Ir(dmdmpfpi)2(acac) (5)]. The Ir−Cav bond length is similar
to those in the analogues complexes reported [51, 52].
Furthermore the Ir−Nav bond lengths also fall within the
range of values for those of similar type of reported
complexes [47, 50]. The Ir−O bond lengths [2.049Å for
Ir(dmmpfpi)2(acac) (3) and 2.200Å for Ir(dmdmpfpi)2(a-
cac) (5)] are longer than the mean Ir−O bond length (2.088
Å) reported and these observations reflect the trans
influence of the phenyl groups. All other bond lengths
and bond angles are analogous to the similar type of
complexes [51, 52]. The optimized structure of complexes
Ir(dmmpfpi)2(acac) (3) and Ir(dmdmpfpi)2(acac) (5) are
shown in Fig. 10.
Fig. 10 The optimized structure of the complexes Ir(dmmpfpi)2(acac)
(3) Ir(dmdmpfpi)2(acac) (5)
and iridium center. On the contrary, the lowest unoccupied
molecular orbital (LUMO) is mainly localized on the
phenyl ring attached to the carbon of the imidazole ring.
The HOMO-LUMO orbital picture for the complex Ir
(dmmpfpi)2(acac) (3) is given in Fig. 8. The orbital picture
predicts that variation is the electronic properties of the
ligands should have an effect on the energy of the excited
state and thereby confirmed the existence of remarkable
photoinduced charge transfer properties [48].
Conclusions
We have synthesized a series of Ir(III) complex dopants
using various substituted imidazole ligands. These com-
plexes exhibit different quantum efficiencies in solution
depending upon the nature of substituents. The wavelength
can be tuned by 63 nm depending upon the electronic
properties of the substituents in the ligand. Some of the
complexes discussed here showed 3MLCT predominant
mixing states for their lowest excited triplet states. But the
Colour Tuning Based on DFT Calculations
On the basis of DFT calculation [23], the HOMO is
localized the imidazole ring and iridium center and LUMO
is localized on the phenyl ring attached to the carbon of the
imidazole ring (Fig. 8). Therefore it is decided to substitute
the electron withdrawing substituent (i.e., fluoro) at para
position of the phenyl ring attached to the carbon of the
imidazole ring and electron releasing substituent in the
phenyl ring attached to the nitrogen of the imidazole ring
for colour tuning (Fig. 9). From the emission peaks
(Table 1) it was concluded that methoxy substituent on
the benzaldeyde would cause a larger red shift in the
3
3
degree of mixing between MLCT and π-π* states of the
excited states varied. From DFT calculation, the HOMO-
LUMO orbital picture was determined and effort towards
the development of RGB colour complexes using different
substituents are currently underway and investigation of
device studies for these iridium complexes are also
currently in progress.

518
J Fluoresc (2011) 21:507–519
Acknowledgements One of the authors Dr. J. Jayabharathi, Reader in
Chemistry, Annamalai University is thankful to Department of Science and
Technology [No. SR/S1/IC-07/2007], University Grants Commission (F.
No. 36-21/2008 (SR)) for providing fund to this research work and Prof. C.
H. Cheng, Chairman, National Science Council, National Tsing Hua
University for my post-doctoral research work.
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