Physicochemical studies of green phosphorescent light-emitting materials from cyclometalated heteroleptic iridium(III) complexes

Article abstract of DOI:10.1016/j.saa.2011.02.052

A series of novel imidazole ligands were synthesized and characterized. Phosphorescence studies of series of heteroleptic cyclometalated iridium(III) complexes reveal that these complexes possess dominantly ³MLCT and ³π-π* excited st

Full text of DOI:10.1016/j.saa.2011.02.052

Spectrochimica Acta Part A 79 (2011) 338–347
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Physicochemical studies of green phosphorescent light-emitting materials from
cyclometalated heteroleptic iridium(III) complexes
Jayaraman Jayabharathi∗, Venugopal Thanikachalam,
Natesan Srinivasan, Marimuthu Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel imidazole ligands were synthesized and characterized. Phosphorescence studies of
series of heteroleptic cyclometalated iridium(III) complexes reveal that these complexes possess dom-
Received 28 January 2011
Received in revised form 19 February 2011
Accepted 25 February 2011
inantly 3MLCT and ␲–␲* excited states and the solvent shifts of these complexes are interpreted by
3
Richardt–Dimroth and Marcus solvent functions. The results consistent with prior assignments on the
absorption band to a metal-to-ligand charge transfer excited state associated with chelating ligand. Emis-
sion kinetic studies exploited that the radiative transition (kr), increases with increasing ꢁem and linear
correlation exists between ln(knr) and energy gap. Electronic transition theory is applied to study the
effect of Eg and ꢀQe on non-radiative transition (knr). With a larger ꢀQe, favouring vibrational overlap
Keywords:
Green emitters
HOMO–LUMO
ꢀQe
Energy gap law
Quantum efficiency
and leading to a larger value for knr
.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Tuning phosphorescence wavelength and enhancing phos-
phorescent quantum yield in these complexes is attractive for
both fundamental research and practical applications [11–20].
Our ongoing investigation [18–21] of phosphorescent imidazole
Electrophosphorescent materials incorporating complexes of
heavy metals have been aroused particularly attention in last
decade due to their extremely high efficiency as electrolumines-
cent emitters. They mainly include square planar d8 or octahedral
d6 complexes of heavy-metals such as Pt(II), Ru(II), Os(II), and Ir(III)
[1–6]. Of these materials, iridium complexes have been regarded
as the most appropriate phosphorescent materials because of
their relatively short lifetime and high quantum efficiency. These
ˆ
based cyclometalated iridium(III) complexes (CN)2Ir(LX), we have
observed that electron releasing substituent on the imidazole lig-
ands have a remarkable influence on the electrochemical and
photophysical properties. These complexes show unusual high
HOMO (highest occupied molecular orbital) levels and exhibit
exceedingly high efficiency even at relatively low dopant con-
centration. Our interest in the development of highly efficient
phosphors for applications in OLEDs prompted us to synthesize
heteroleptic iridium complexes and investigate their photophys-
ical properties. In the present study the methyl group as electron
releasing moiety was chosen due to the emissions of iridium com-
plex come from phosphorescence with triplet character of both
ligand centered (␲–␲*) and MLCT (metal-to-ligand charge transfer)
transitions. The phosphorescence is strongly affected by the triplet
ˆ
Ir complexes generally contain two cyclometalated ligands (CN)
and a single bidentate, monoanionic ancillary ligand, or with
three cyclometalated ligands. The cyclometalated ligands used
in the complexes are well known as heterocycle derivatives
that coordinated to the metal center via formation of Ir–N and
Ir–C bonds. Therefore, design of ligands for Ir complexes is of
great importance in order to achieve high efficiency and color
purity for organic light emitting diodes (OLEDs). To date, most
researches were focused on the design and synthesis of the
cyclometalated ligand, such as changing ␲ conjugation, introduc-
ing electron donating or withdrawing group on the appropriate
position of the aromatic ring [7–10], and adopting rigid struc-
ture.
ˆ
energy of the ortho-chelating CN ligands. Therefore substituents,
even methyl groups could play a key role in the alteration of the
triplet energy and although a methyl group is a weaker electron
donating group than other functional groups (e.g. −OCH3, NR2, etc.).
Incorporating a methyl group as a substituent on the imidazole
ring has an influence on the HOMO/LUMO level (lowest unoccupied
molecular orbital), as well as 3MLCT state of the complex. Herein we
described the results of our systematic investigation on the prepa-
ration, structural characterization, photophysical properties and
electrochemical behaviour of cyclometalated iridium complexes.
∗
Corresponding author. Tel.: +91 4144 239523; mobile: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.02.052

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
339
ˆ
Scheme 1. Synthesis of iridium complexes (CN)2Ir(acac) 1–4.
at scan rate of 0.1 Vs−1. The potentials were measured against
an Ag/Ag+ (AgCl) reference electrode using ferrocene/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 voltammo-
gram.
2. Experimental
2.1. Materials and methods
Iridium(III)trichloride trihydrate (IrCl3·3H2O, Sigma–Aldrich
Ltd.), 2-ethoxyethanol (H5C2OC2H4OH, S.D. fine) and all other
reagents used without further purification.
2.3. General procedure for the synthesis of ligands
2.2. Optical measurements and compositions analysis
The various substituted 2-arylimidazole ligands were prepared
from an unusual four components assembling of butan-2,3-dione,
ammonium acetate, methyl substituted aryl amine and substituted
aryl aldehyde [18–21].
The ultraviolet–visible (UV–vis) spectra of the phosphorescent
Ir(III)complexes were measured in an UV–vis spectrophotometer
(Perkin Elmer Lambda 35) and corrected for background absorp-
tion due to solvent. Photoluminescence (PL) spectra were recorded
on a (Perkin Elmer LS55) fluorescence spectrometer. The solid-state
emission spectra were recorded on fluoromax2 (ISA SPEX) Xenon-
Arc lamp as a source. NMR spectra were recorded on a Bruker
400 MHz NMR spectrometer. MS spectra (Fast Atom Bombard-
ment) were recorded on a Varian Saturn 2200 GC–MS spectrometer.
Cyclic Voltametry (CV) analyses were performed by using CHI
630A potentiostat electrochemical analyzer. Measurements of
oxidation and reduction were undertaken using 0.1 M tetra(n-
butyl)ammoniumhexafluorophosphate as a supporting electrolyte,
2.3.1. 4,5-dimethyl-2-phenyl-1-p-tolyl-1H-imidazole (dmpti)
Yield: 48%. 1H NMR (400 MHz, CDCl3): ı 2.42 (s, 3H), 2.31 (s,
3H), 2.02 (s, 3H), 7.02–7.36 (aromatic protons). 13C NMR (100 MHz,
CDCl3): ı 9.57, 12.76, 21.19, 125.45, 127.54, 127.63, 127.97, 128.01,
130.11, 130.92, 133.41, 135.34, 138.37, 145.15. Anal. calcd. for
C18H18N2: C, 82.41; H, 6.92; N, 10.68. Found: C, 82.12; H, 6.02; N,
10.08. MS: m/z 262.15, calcd. 262.00.

340
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
2.3.2. 2-(4-Fluorophenyl)-4,5-dimethyl-1-p-tolyl-1H-imidazole
(fpdmti)
Yield: 40%. 1H NMR (400 MHz, CDCl3): ı 2.01 (s, 3H), 2.29 (s,
3H), 2.43 (s, 3H), 6.87–7.34 (aromatic protons). 13C NMR (100 MHz,
CDCl3): ı 9.53, 12.69, 21.18, 114.92, 115.10, 125.44, 127.16, 129.89,
130.20, 133.35, 135.13, 138.57, 144.28, 161.26, 163.23. Anal. calcd.
for C18H17N2F: C, 77.12; H,6.11; N, 9.99. Found: C, 76.24; H, 5.98;
N, 8.99. MS: m/z 280.00, calcd. 280.14.
2.3.3.
4,5-Dimethyl-1-(3,5-dimethylphenyl)-2-phenyl-1H-imidazole
(dmdmppi)
Yield: 45%. 1H NMR (400 MHz, CDCl3): ı 1.98 (s, 3H), 2.27 (s,
3H), 2.29 (s, 6H), 6.77–7.36 (aromatic protons). 13C NMR (100 MHz,
CDCl3): ı 9.65, 12.77, 21.27, 125.54, 125.59, 127.59, 127.97, 128.03,
130.02, 130.07, 133.53, 137.88, 139.33, 144.98. Anal. calcd. for
C19H20N2: C, 82.57; H, 7.29; N, 10.14. Found: C, 82.01; H, 6.89; N,
9.12. MS: m/z 276.00, calcd. 276.16.
Fig. 1. The UV–vis absorption spectra of the complexes 1–4 in CH2Cl2.
2.3.4. 2-(4-Fluorophenyl)-4,5-dimethyl-1-(3,5-dimethylphenyl)-
1H-imidazole
(fpdmdmpi)
9.22, 10.23, 21.25, 29.70, 119.49, 120.37, 122.14, 124.30, 125.97,
127.10–127.42, 131.29, 133.24, 134.09, 136.36, 140.08, 144.73,
149.38, 153.41, 156.05, 157.28. Anal. calcd. for C44H42IrN5O2: C,
61.09; H, 4.89; N, 8.10. Found: C, 60.89; H, 4.53; N, 8.01. MS: m/z
865.30, calcd. 864.29.
Yield: 45%. 1H NMR (400 MHz, CDCl3): ı 1.99 (s, 3H), 2.27 (s,
3H), 2.31 (s, 6H), 6.77–7.34 (aromatic protons).13C NMR (100 MHz,
CDCl3): ı 9.5, 12.69, 21.15, 114.81–163.41(aromatic carbons). Anal.
calcd. for C19H19N2F: C, 77.52; H, 6.51; N, 9.52. Found: C, 77.01; H,
6.02; N, 9.21. MS: 294.00, calcd. 294.15.
2.4.4. Iridium(III)bis(4,5-dimethyl-1-(3,5-di-methylphenyl)-2-
fluorophenyl-1H-imidazolato-N,C2ꢀ)(picolanate), Ir(fpdmdmpi)
(pic), 4
2.4. General procedure for the synthesis of iridium complexes
(1–4)
Yield: 65%. 1H NMR (400 MHz, CDCl3): ı 1.32 (s, 3H), 1.93 (d, 6H),
2.18 (s, 3H), 2.40 (s, 6H), 2.44 (d, 6H), 6.07–8.04 (aromatic protons).
13C NMR (100 MHz, CDCl3): ı 9.20, 10.15, 21.18, 29.70, 119.29,
120.16, 123.45, 124.25, 125.26–127.39, 131.33, 132.38–140.35,
149.29, 153.23, 155.21, 156.43, 160.92, 162.90. Anal. calcd. for
The 2-aryl imidazole based cyclometalated iridium complexes
1–4 were synthesized from IrCl3·3H2O and the 2-aryl limidazole lig-
ands to give the corresponding dimeric species via the Nonoyama
route [22] followed by the treatment with picolinic acid in the
presence of K2CO3 as shown in Scheme 1.
C44H40IrN5F2O2: C, 58.65; H, 4.47; N, 7.77. Found: C, 58.37; H, 4.36;
N, 7.12. MS: m/z 901.28, calcd. 900.79.
2.4.1. Iridium(III)bis(4,5-dimethyl-1-(p-methylphenyl)-2-
phenyl-1H-imidazolato-N,C2ꢀ)(picolanate), Ir(dmpti)(pic),
1
3. Results and discussion
3.1. Photophysical properties
Yield: 65%. 1H NMR (400 MHz, CDCl3): ı 1.28 (s, 6H), 1.96 (s, 6H),
2.20 (s, 3H), 2.50 (s, 3H), 6.01–8.04 (aromatic protons). 13C NMR
(100 MHz, CDCl3): ı 9.12, 9.16, 9.21, 10.25, 22.69, 119.50, 120.36,
122.10, 124.17, 127.20–136.42, 139.83, 146.89, 149.36, 156.20,
157.36. Anal. calcd. for C42H38IrN5O2: C, 60.27; H, 4.58; N, 8.37.
Found: C, 60.13; H, 4.46; N, 8 16. MS: m/z 837.27, calcd. 836.69.
The absorption bands of the series of complexes (1–4) show
two kinds of bands (Fig. 1). The intense bands observed around
270 nm in the ultraviolet part of the spectrum can be assigned
to the allowed ligand centered (␲–␲*) transitions [23] and some-
what weaker bands also observed in the lower part of energy
(ꢁmax < 370 nm). The band position, size and extinction coefficient
of the bands observed in the range 370–447 nm suggest that these
are MLCT transitions (1MLCT and 3MLCT) [24,25]. According to our
previous papers [18–21], weak bands located at longer wavelength
have been assigned to the 1MLCT ← S0 and 3MLCT ← S0 transi-
tions of iridium complexes. Thus the broad absorption shoulders
at 386 and 438 nm observed for 1 are likely to be ascribed to the
1MLCT ← S0 and 3MLCT ← S0 transitions, respectively. The intensity
of the 3MLCT ← S0 transition is close to that of 1MLCT ← S0 transi-
tion, suggesting that 3MLCT ← S0 transition is strongly allowed by
S–T mixing of spin–orbit coupling [12] and similar observations are
made for other complexes 2–4. Absorption in the range of around
370 nm for complexes (1–4) corresponds to the transition of the
1MLCT state as evident from its extinction coefficient of the order
103. The absorption like long tail toward lower energy and higher
wavelength around 428 nm is assigned to 3MLCT transitions and
gains intensity by mixing with the higher lying 1MLCT transitions
through the spin–orbit coupling (Fig. 2) of iridium(III) [17]. Both
singlet MLCT (1MLCT) and triplet MLCT (3MLCT) bands are typi-
cally observed for these complexes in all solvents. In order for these
2.4.2. Iridium(III)bis(4,5-dimethyl-1-(p-methylphenyl)-2-
fluorophenyl-1H-imidazolato-N,C2ꢀ)(picolanate), Ir(fpdmti)(pic),
2
Yield: 65%. 1H NMR (400 MHz, CDCl3): ı 1.28 (s, 3H), 1.32 (s, 3H),
1.89 (s, 3H), 1.93 (s, 3H), 2.17 (s, 3H), 2.51 (d, 3H), 2.53 (s, 3H), 7.23
(m, 3H), 7.29 (m, 1H), 7.40 (m, 5H), 7.89 (m, 2H), 8.30 (m, 1H), 6.07
(dd, 1H), 6.22 (m, 4H), 6.38 (dd, 1H). 13C NMR (100 MHz, CDCl3):
ı 9.13, 9.19, 10.16, 21.35, 119.46, 120.16, 123.31, 124.35, 127.33,
127.83, 128.16, 128.27, 130.56, 130.98, 132.53, 133.31, 133.41,
136.85, 139.89, 140.10, 149.27, 153.17, 155.37, 156.53, 160.93,
162.87. Anal. calcd. for C42H36IrF2N5O2: C, 57.78; H, 4.16; N, 8.02.
Found: C, 56.93; H, 4.03; N, 7.93. MS: m/z 873.25, calcd. 872.69.
2.4.3. Iridium(III)bis(4,5-dimethyl-1-(3,5-di-methylphenyl)-2-
phenyl-1H-imidazolato-N,C2ꢀ)(picolanate), Ir(dmdmppi) (pic),
3
Yield: 65%. 1H NMR (400 MHz, CDCl3): ı 1.28 (s, 3H), 1.35 (s,
3H), 1.95 (s, 3H), 1.92 (s, 3H), 2.20 (s, 3H), 2.43 (d, 3H), 2.38 (d, 3H),
6.20–8.32 (aromatic protons). 13C NMR (100 MHz, CDCl3): ı 9.12,

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
341
Fig. 2. spin–orbit coupling of heavy-metal facilitated triplet emission.
iridium (III) complexes to be useful as phosphors EL devices, strong
spin–orbit coupling must be present to efficiently mix the singlet
and triplet excited states. Clear evidences for mixing of the singlet
and triplet excited states are seen in the absorption (Fig. 1) of these
complexes.
3.2. Phosphorescence spectra
Fig. 4. The solid state and the solvatochromic emission spectra of complex 1.
3.2.1. Mixing of excited states (LC and MLCT)
Phosphorescence of mononuclear metal complexes originates
from the ligand-centered excited state, metal-centered excited
state and MLCT excited state. For the cyclometalated iridium(III)
complexes, the wave function of the excited triplet state ˚T,
responsible for the phosphorescence is expressed as,
in shape. The luminescence spectra of complexes 1 and 3 reveal
broad shape of the luminescence spectra (Fig. 3) whereas the vibra-
tional sideband pattern of the photoluminescence spectra were
observed for the complexes 2 and 4 (Fig. 3). Complexes 1 and 3
have excited state with large contribution of 3MLCT whereas com-
plexes 2 and 4 have excited state with large contribution of 3␲–␲*.
All these complexes show emission at 540, 550, 553 and 560 nm
in dichloromethane, respectively (Table 1). The emission spectra of
complexes 1 and 3 are slightly blue-shifted in comparison with that
of complexes 2 and 4. [Ir(dmpti)2(pic)] (1) is blue shifted in com-
parison with that of Ir(dmdmppi)2(pic) (3), which is the result of
the introduction of electron withdrawing fluorine substituent into
the phenyl ring attached to C(2) carbon of the imidazole ring in
complexes 2 and 4. The Stokes shift (Table 1) of 1–4 was calculated
in wave numbers from the difference between the lowest energy
absorption maximum and the emission maximum of each complex.
The observed large Stokes shift in hydroxyl solvents may be due to
the significant structural differences between the ground state and
excited state of these iridium complexes [28].
˚
T
= a ˚T(ꢂ − ꢂ∗) + b
˚
T
(MLCT)
(1)
where ‘a’ and ‘b’ are the normalized co-efficients, ˚T (␲–␲*) and
˚
T
(MLCT) are the wave function of 3(␲–␲*) and 3(MLCT) excited
states, respectively. For these iridium complexes, the wave function
of the triplet state (˚T) responsible for the phosphorescence and
Eq. (1) implies that the excited triplet state of these iridium com-
plexes are mixture of ˚T (ꢂ–␲*) and ˚T (MLCT) [26]. The triplet
3
state is attributed to dominantly ␲–␲* excited state when a > b
and dominantly 3MLCT excited state when b > a.
The phosphorescence spectra of these complexes (1–4) obtained
at 298 K show significant broad shape and also vibronic fine
structure (Fig. 3). According to our previous studies [18–21], phos-
phorescence spectra from the ligand centered 3␲–␲* state display
vibronic progressions, while those from the 3MLCT state are broad
3.2.2. Marcus and Richardt–Dimroth solvent functions –
solvatochromism
Solvatochromism is a potentially important probe in accessing
charge distributions in both ground and excited states and also
yield information regarding charge distributions in Franck–Condon
states produced by absorption. Solvent shifts in the absorption and
emission of ortho-metalated iridium complexes 1–4 were mea-
sured (Table 1) in different solvents. Variation in the absorption and
emission of the complexes 1–4 was observed in different solvents.
The representative emission spectra of complex [Ir(dmpti)2(pic)] 1
is shown in Fig. 4. The emission peak of complexes 1–4 are around
528 nm in n-hexane (non-polar), 570 nm in ethanol (polar protic
solvent), 540 nm in CH2Cl2 (medium polarity) and 580 nm in ace-
tonitrile (a strongly polar aprotoic solvent). 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 solvents than in non-polar solvents
which lead to red shift of emission with increasing solvent polarity
[24] (Fig. 4). The photoluminescent peak of solid state (representa-
tive spectrum of complex 1 is shown in Fig. 4) of all complexes
Fig. 3. The photoluminescence emission spectra of the complexes 1–4 in CH2Cl2.

342
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
Table 1
Photoluminescence spectral data of various solvents and solid emission spectra of complexes 1–4.
Solvent
Absorptiona (ꢁ, nm) (log ε)
Emissionb (ꢁ, nm)
Stokes shift (cm−1
)
1
2
3
4
1
2
3
4
1
2
3
4
n-Hexane
269.0 (3.785)
386.0 (3.224)
438.0 (2.268)
270.0 (3.956)
375.0 (3.412)
428.0 (2.284)
287.0 (3.887)
388.0 (3.026)
447.0 (2.373)
274.0 (3.667)
370.0 (3.190)
438.0 (2.416)
278.0 (3.720)
376.0 (3.278)
430.0 (2.323)
276.0 (3.661)
378.0 (3.189)
440.0 (2.415)
272.0 (3.750)
387.0 (3.249)
431.0 (2.516)
280.0 (2.651)
380.0 (3.276)
426.0 (3.417)
278.0 (3.873)
388.0 (3.192)
437.0 (2.410)
283.0 (3.694)
371.0 (3.308)
438.0 (2.240)
282.0 (3.982)
379.0 (3.084)
436.0 (2.363)
272.0 (3.794)
370.0 (3.056)
427.0 (2.402)
286.0 (3.886)
386.0 (3.019)
440.0 (2.307)
275.0 (3.666)
376.0 (3.118)
436.0 (2.398)
278.0 (3.974)
380.0 (3.283)
426.0 (2.422)
277.0 (3.664)
366.0 (3.030)
444.0 (2.350)
273.0 (3.730)
382.0 (3.283)
426.0 (2.562)
280.0 (3.741)
378.0 (3.164)
428.0 (2.479)
272.0 (3.857)
383.0 (3.171)
432.0 (2.636)
282.0 (3.668)
373.0 (3.179)
432.0 (2.407)
286.0 (3.883)
380.0 (3.589)
430.0 (2.408)
273.0 (3.775)
378.0 (3.113)
438.0 (2.416)
280.0 (3.685)
380.0 (3.120)
448.0 (2.425)
276.0 (3.780)
381.0 (3.123)
433.0 (2.543)
280.0 (3.706)
377.0 (3.196)
430.0 (2.308)
278.0 (3.698)
370.0 (3.088)
440.0 (2.425)
275.0 (3.875)
386.0 (3.242)
432.0 (2.309)
281.0 (3.869)
383.0 (3.244)
430.0 (2.218)
276.0 (3.872)
384.0 (3.126)
430.0 (2.140)
284.0 (3.657)
376.0 (3.082)
434.0 (2.536)
284.0 (3.973)
382.0 (3.075)
431.0 (2.369)
274.0 (3.857)
380.0 (3.121)
436.0 (2.406)
278.0 (3.667)
382.0 (3.085)
453.0 (2.438)
277.0 (3.656)
383.0 (3.090)
430.0 (2.232)
280.0 (3.691)
370.0 (3.088)
432.0 (2.334)
279.0 (3.664)
372.0 (3.096)
439.0 (2.451)
276.0 (3.868)
385.0 (3.121)
436.0 (2.432)
282.0 (3.674)
380.0 (3.185)
432.0 (2.447)
274.0 (3.650)
380.0 (3.185)
432.0 (2.247)
283.0 (3.622)
380.0 (3.087)
434.0 (2.244)
528
530
537
541
3892
4068
4634
4244
3973
4880
5173
4870
5511
5834
6074
5947
4718
Benzene
530
534
540
542
552
560
570
578
582
526
532
540
546
547
555
563
572
580
583
532
538
545
549
551
560
567
574
582
585
536
544
547
550
560
562
568
576
581
588
540
4496
3645
4312
4806
4611
5345
5930
5582
5649
4622
4209
4621
5292
4505
5712
5882
5907
5995
4553
3794
5074
5291
4985
5330
5787
5936
6035
1,4-dioxane
Chloroform
Dichloromethane
Ethyl acetate
1-Butanol
Ethanol
Methanol
Acetonitrile
Solid emission spectra
a
UV–vis absorption measured in solution concentration = 1 × 10−5 M.
Photoluminescence measured in solution concentration = 1 × 10−4 M.
b
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 [27]. Measurement of absorption solvatochromism
has been interpreted with Marcus and Richardt–Dimroth solvent
functions to estimate the transition dipoles associated with low
lying MLCT excited state. The linear correlation (Fig. 6) of solvent
shift of absorption band positions of iridium complexes 1–4 with
Richardt–Dimroth solvent ET parameters is indicative of the fact
that the dielectric solute solvent interactions are responsible for
the observed solvatochromic shift for the iridium complexes. The
observed linear correlation (Fig. 5) of solvent shift of absorption
band positions of iridium complexes 1–4 with Marcus [28] opti-
cal dielectric solvent function [(1 − Dop)/(2Dop + 1)] (Fig. 6) reveal
that transition dipoles associated with absorption and the direc-
tion of excited dipole is opposite to that of the ground state-dipole
(Fig. 7). Application of Marcus theory interpretation of solvent shift
absorption data indicate that the absorption bands in the range
370–447 nm in these ortho-metalated Ir (III) complexes are due to
MLCT transitions to the chelating ligand. Further linear correlation
suggests as a result of the absorption that charge is transferred
from the region around the ortho-metalated carbon atom to the
region around the bridgehead of the chelating ligand and ground
state dipole pointed toward the ortho-metalated carbon atom and
an excited state dipole pointed toward the chelating ligand [28].
Fig. 5. Solvatochromic absorption of 1–4 with Richardt–Dimroth solvent ET param-
eters.
route of the excited state of these complexes (1–4) and their radia-
tive and non-radiative decay are studied in detail (Table 2). All
these complexes (1–4) exhibit appreciable phosphorescence quan-
tum yields at room temperature under nitrogen atmosphere. These
values appear to be high for (fpdmdmpi)2Ir(pic) (4) complex [26].
Moreover, radiative lifetime of these complexes fall in the range
of 1.1–2.4 ␮s. Among these complexes, a significant feature of the
highly emitting iridium complex (fpdmdmpi)2Ir(pic) (4) has short
triplet excited state lifetime of 1.1 ␮s.
3.3. Electronic transition theory
The absolute PL quantum yields were measured by compar-
ing fluorescence intensities (integrated areas) of a standard sample
(Coumarin 46) and the unknown sample [18,21]. The main decay

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
343
Table 2
Photophysical properties of iridium complexes 1–4.
Complex
ꢃa
␶ (ꢄs) 298 K
kr (␮s−1
)
knr (␮s−1
)
HOMOb (eV)
LUMOc (eV)
Eg (eV)
Eonset (V)
1
2
3
4
0.30
0.35
0.41
0.46
2.4
2.0
1.7
1.1
0.13
0.17
0.22
0.42
0.29
0.33
0.36
0.49
−5.09
−2.20
2.89
2.65
2.39
2.01
0.21
0.25
0.42
0.55
−5.05
−5.22
−5.35
−2.40
−2.83
−3.34
Фunk is quantum yield of the sample, Фstd is 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. Áunk and Ástd are the indexes of refraction of the sample and
standard solutions.
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
I
I
A
Á
Á
std
a
unk
std
unk
˚
unk
= ˚std
A
std
unk
b
c
EHOMO = 4.8 + E(onset)
ELUMO = EHOMO − 1239/ꢁabs
.
Fig. 6. Solvatochromic absorption of 1–4 with Marcus optical dielectric solvent
function.
Fig. 8. Schematic representation of emission kinetics.
explained by non-radiative path to the low-lying n–␲* state of the
nitrogen atoms.
From the plot (Fig. 9a) of the phosphorescence yield (˚) versus
the maximum emission energy of phosphorescence (ꢁem), it was
found that the quantum yields range from 0.25 to 0.40 and roughly
tend to increase with an increase in ꢁem. Fig. 9b indicates that the
kr values of 1–4 increases with an increase of ꢁem. According to the
theory of the electronic transition, the kr value is proportional to the
square of the electric dipole transition moment (MT–S). First-order
perturbation theory gives an approximate expression for MT–S [30]
Fig. 7. Direction of ground state and excited state dipole moment in Iridium com-
plex.
The radiative (kr) and non-radiative (knr) rate constants (Fig. 8)
are calculated from the phosphorescence yield ˚p and lifetime
,
ꢂ
ꢃ
kT
kr + knr
−1
˚
= ˚
(2)
(3)
p
ISC
= (kr + knr
)
ꢄ
1
MT−S
=
ˇn
<
1˚n|M| ˚0
(4)
Here, ˚ISC is the intersystem-crossing yield. For the iridium
complexes ˚ISC is safely assumed to be 1.0 because of the strong
spin–orbit interaction caused by heavy atom effects of iridium [29].
kr and knr are the radiative and non-radiative deactivation, f is the
life time of the S1 excited state. The low quantum yields could be
1
1
where
˚
n
and ˚ are the wave functions of Sn and S0 states,
0
respectively and M is the electric dipole vector with the use of the
spin–orbit coupling operator (Hso) and the wave function of the

344
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
Fig. 9. (a) Plot of emission peak wavelength versus quantum yield. (b) Plot of emission peak wavelength versus kr. (c) Plot of emission peak wavelength versus knr
.
lowest excited triplet state (3ꢅ1), ␤n is formulated as
The physical construct of multiphonon nonadiabatic description
of electron transfer developed by Freed and Jortner [34], Englman
and Jortner [35] and Jortner [36] describes, the relation between a
non-radiative transition knr and the energy gap (Eg) is given in Eq.
(7) [31].
3
˚
1
ˇn =< 1˚n|HSO
|
(5)
(1En
−
3E1)
where 1En and 3E1 are the energies of Sn state and the lowest excited
ꢅ
ꢆ
ꢅ
ꢆ
triplet state, respectively. Now, Eq. (4) is simply expressed as
−ꢆEg
h¯ ωM
Eg
SMh¯ ωM
knr ˛ exp
ꢆ = ln
− 1
(7)
1
3
1
1
{ꢁ ˚n|HSO| ˚1ꢂꢁ ˚1|M| ˚0ꢂ}
MT–S
=
=
(1E1
3E1)
−
3E1)
(6)
where the energy gap (Eg) is related to the zero-point energy (ZPE)
separations of two coupled surfaces. The h¯ ωM term describes, in
the limit of a single configuration co-ordinates model, the average
energy of the vibrational model that couples the final state of the
initial state. The degree to which the two surfaces are vibronically
coupled is gauged by the Huang–Rhys factor (SM),
˛
(1E1
−
1
3
1
Eq. (6) predicts that, when ˛ = (ꢁ ˚n|HSO| ˚1ꢂꢁ|M| ˚0ꢂ) is approx-
imately constant, Since MT–S kr increases with a decrease in the
energy difference (1E1 − E1), kr also increases with a decrease
3
in the energy difference (Table 2). In the present study, kr and
knr increases with an increase in ꢁem (Fig. 9b and c). This fact
is explained by assuming that the S1 energy does not differ sig-
nificantly among the complexes 1–4 and the energy difference
ꢅ
ꢆ
1
2
MωM
SM
=
(ꢀQe)2
(8)
h¯
where M is the reduced mass of the oscillator, ␻ is the funda-
M
(1E1 − E1), increases with decrease in ꢁem. Actually in complexes
3
mental frequency and ωM represents the difference between the
ground- and excited-state equilibrium geometries with respect to
the specified nuclear (i.e., vibrational) coordinate. The changes in
both E0 and ꢀQe have effect on vibrational overlap knr is shown pic-
torially in Fig. 10. On comparison with the central potential energy
surface diagram (B), a decrease in the energy gap at constant ꢀQe
(C) results in an increase in the vibrational overlap between the
excited state and ground states surfaces. Conversely, increasing the
energy gap has the opposite effect, i.e., decrease in vibrational over-
lap leading to a smaller value for knr [energy gap law (Fig. 11a)].
With a larger ꢀQe (A), favouring vibrational overlap and leading to
1–4, the S1 ← S0 absorption bands are located at 426–432 nm. But
phosphorescence S0 ← T1 exhibits large bathochromic shift rang-
ing from 528 to 588 nm. These data (Fig. 9b) provide the evidence
that the energy difference (1E1 − E1), increases with the decrease
3
in ꢁem and therefore, kr increases with an increase in ꢁem [30].
3.4. Energy gap law vs drop in knr –potential energy surface
diagram
It has been found that the behaviour of most transition metal
systems can be accommodated by the energy gap law [30–33].
a larger value for knr
.

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
345
Fig. 10. Graphical illustration of the factors influencing vibrational overlap for non-radiative excited state decay.
Fig. 11. (a) Plot of knr versus emission energy gap. (b) Plot of ln(knr) versus emission energy gap.
The plot of ln(knr) versus energy gap for complexes 1–4 (Fig. 11b)
shows that there is good qualitative agreement with the “Energy
gap law”. The Energy gap law predicts that the rate of non-radiative
decay increases when the energy gap separating the ground and
excited state decreases. The relation is based on the vibrational
overlap between the ground state and the excited state, knr is a
function of Franck–Condon overlap integral. These complexes hav-
ing similar excited states and vibrational coupling, a simplified form
of the energy gap law is obtained that predicts a linear correlation
between ln(knr) and the energy gap [14]. The slope of −0.59 eV−1 is
similar in magnitude to those found for iridium(III) complexes [26]
which have the 3MLCT excited states for the lowest excited triplet
states. This correlation suggests that the change of non-radiative
decay rate constant is owing to the energy gap of the complexes
[37].
on the aromatic ring. The photophysical study of these complexes
demonstrates that the electron releasing substituent increases the
absorption and emission energies of complexes by stabilizing the
HOMO level. Besides increasing the emission energy, the lower
HOMO energies decrease the energy separation between the 1MLCT
and 3LC states, which inturn modified the excited state properties
of the iridium complexes.
The redox potentials of the cyclometalated iridium com-
plexes were measured relative to an internal ferrocene reference
(Cp2Fe/Cp2Fe+ = 0.45 V versus SCE in CH2Cl2 solvent) [38,39]. As
revealed previously [40,41], the reductions occur primarily on the
more electron accepting heterocyclic portion of the cyclometa-
lated 2-arylimidazole ligands (LUMO contribution) whereas the
oxidation process is to largely involved in the Ir-phenyl center
(HOMO contribution). The calculated energies of the highest occu-
pied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) are given in Table 2.
3.5. Effect of substituents on tuning wavelength
The iridium complexes show reversible oxidation behaviour
and these complexes exhibit HOMO levels ranging of 5.09–5.35 eV.
The LUMO energies were calculated based on the HOMO energies
and the lowest-energy absorption edges of the UV–vis absorption
spectra [39] (Fig. 12). These results reveal that the mono-methyl
substituted complexes 1 and 2 show emission with the shorter
Substituent effect of the d orbital (t2g) stabilization of iridium
metal through the carbon atom-iridium bonding and this identi-
fies with the inductive effect of the substituents. Therefore, the
HOMO stability and the emission energy gap are controlled by
the nature and number of substituents and its inductive influence

346
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 338–347
states of the excited states varied. The solvent shifts are interpreted
in terms of Richardt–Dimroth and Marcus solvent functions. The
results consistent with prior assignments on the absorption band
to a metal-to-ligand charge transfer excited state associated with
chelating ligand. Electronic transition theory is applied to study the
effect of Eg and ꢀQe on non-radiative transition (knr). With a larger
ꢀQe, favouring vibrational overlap and leading to a larger value for
knr. Effort toward the development of RGB color complexes using
different substituents are currently underway and investigation of
device studies for these iridium complexes are also currently in
progress.
Acknowledgements
One of the authors Dr. J. Jayabharathi, Associate Professor,
Department of Chemistry, Annamalai University is thankful to
Department of Science and Technology [No. SR/S1/IC-07/2007] and
University Grants Commission (F. No. 36-21/2008 (SR)) for provid-
ing funds to this research work and Prof. C.H. Cheng, Chairman,
National Science Council, National Tsing Hua University, Taiwan,
ROC for my post-doctoral research work.
Fig. 12. Normalized sigma plot of absorption and emission of 1–4 in CH2Cl2.
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