Fine tuning of emission color of iridium(iii) complexes from yellow to red via substituent effect on 2-phenylbenzothiazole ligands: Synthesis, photophysical, electrochemical and DFT study

Article abstract of DOI:10.1039/c1dt10305a

Four novel iridium(iii) complexes bearing biphenyl (7a-7c) or fluorenyl (7d) modified benzothiazole cyclometallate ligands are synthesized. In comparison with the yellow parent complex, bis(2-phenylbenzothiozolato-N,C ^2′) iridium(iii) (acetylacetonate) [(pbt)₂Ir(acac)] (λ_PLmax = 557 nm, φ_PL = 0.26), 7a-7d show 20-43 nm bathochromic shifted orange or red phosphorescence in solution, with maximum photoluminescence (PL) quantum yield of 0.62, and PL lifetime of 1.8-2.0 μs. Meanwhile, the resulting complexes also exhibit intense orange or red phosphorescence of λ_PLmax = 588-611 nm in solid films. The complex 7c with two tert-butyl substituents possesses the highest phosphorescent efficiency both in dilute solution and thin solid films, therefore may be a prospective candidate for both doping and host emitting electrophosphorescent material. Furthermore, despite the observation of severe oxygen quenching for 7a-7d in solution, 7a and 7c even show efficient emission intensity quenching by oxygen in their solid state due to the existence of void channels in crystals; consequently, they are promising molecular oxygen sensor reagents. Electrochemical measurement and DFT calculation results suggest that all these chelates own declined LUMOs of 0.1 eV relative to that of (pbt) ₂Ir(acac) owing to the contribution of the phenyl substituents; whereas only 7d shows a more destabilized HOMO (～0.1 eV) compared with the parent chelate.

Full text of DOI:10.1039/c1dt10305a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 7153
www.rsc.org/dalton
PAPER
Fine tuning of emission color of iridium(III) complexes from yellow to red via
substituent effect on 2-phenylbenzothiazole ligands: synthesis, photophysical,
electrochemical and DFT study†
Ming Li,a Hui Zeng,a Yanyan Meng,a Huiqin Sun,b Song Liu,a Zhiyun Lu,*a,c Yan Huanga and Xuemei Pu*a
Received 23rd February 2011, Accepted 18th April 2011
DOI: 10.1039/c1dt10305a
Four novel iridium(III) complexes bearing biphenyl (7a–7c) or fluorenyl (7d) modified benzothiazole
cyclometallate ligands are synthesized. In comparison with the yellow parent complex,
bis(2-phenylbenzothiozolato-N,C2¢) iridium(III) (acetylacetonate) [(pbt)2Ir(acac)] (lPLmax = 557 nm,
jPL = 0.26), 7a–7d show 20–43 nm bathochromic shifted orange or red phosphorescence in solution,
with maximum photoluminescence (PL) quantum yield of 0.62, and PL lifetime of 1.8–2.0 ms.
Meanwhile, the resulting complexes also exhibit intense orange or red phosphorescence of
lPLmax = 588–611 nm in solid films. The complex 7c with two tert-butyl substituents possesses the highest
phosphorescent efficiency both in dilute solution and thin solid films, therefore may be a prospective
candidate for both doping and host emitting electrophosphorescent material. Furthermore, despite the
observation of severe oxygen quenching for 7a–7d in solution, 7a and 7c even show efficient emission
intensity quenching by oxygen in their solid state due to the existence of void channels in crystals;
consequently, they are promising molecular oxygen sensor reagents. Electrochemical measurement and
DFT calculation results suggest that all these chelates own declined LUMOs of 0.1 eV relative to that of
(pbt)2Ir(acac) owing to the contribution of the phenyl substituents; whereas only 7d shows a more
destabilized HOMO (~0.1 eV) compared with the parent chelate.
red iridium chelates has restrained their application in full color
Introduction
displays and biological sensors as well.
During the past twenty years, phosphorescent iridium complexes
Since the excited states in Ir complexes are ligand-related, the
have drawn great interest due to their high quantum efficiency and
general strategies for exploiting red phosphors are the employment
color diversity, which make them promising for various applica-
of cyclometallate ligands with deliberately extended aromatic p-
tions such as phosphorescent light-emitting diodes (PhOLEDs),1
biological labelling agents2 and chemosensors.3 In comparison
with the well-developed green iridium complexes,4 red ones
compromising both efficiency and color purity are still scarce,5
because the emission should originate from a smaller energy gap
transition, in which more nonradiative pathways may exist from
strong p–p interaction or charge transfer between ligands, hence
decrease the PL quantum yield.6 The relative poor performance of
conjugation systems, such as 1-phenylisoquinoline (piq),7 2,3-
diphenylquinoxaline (dbq),8 2-phenylbenzoquinoline (pbq),9 2-(2-
benzo[4,5-a]thienyl) pyridine (btp),10 etc., and the modification on
conjugated lengths of the ancillary ligands.11 On the other hand,
the emission color of Ir(III) chelates can be fine-tuned to red via
rational modification on CŸN ligands by substituents.12 For ex-
ample, Ir(III)-ppy complexes (ppy: 2-phenylpyridine) are found to
have their HOMOs mainly dominated by mixtures of Ir d-orbitals
and p-orbitals of phenyl moieties, consequently the addition of
electron-donating groups to the phenyl segment would generally
lead to red-shifted emission due to the destabilized HOMOs;
whereas the LUMOs of the complexes principally localize on the
p-orbitals of pyridyl groups, hence the incorporation of electron-
withdrawing groups with the pyridyl moiety would mostly lead
to declined LUMO, and accordingly redder emission.13 Based
on the above strategy, several red Ir-ppy type complexes have
been demonstrated,14 yet this principle is found to show some
deviation in the case of both Ir-ppy analogues15 and chelates
with other CŸN ligands.16 For instance, compared with the yellow
parent compound (pbt)2Ir(acac) (lPLmax = 557 nm, jPL = 0.26),17
aCollege of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
E-mail: luzhiyun@scu.edu.cn; Fax: +86-28-85410059. E-mail: xmpuscu@
scu.edu.cn; Fax: +86-28-85412907
bAnalytical and Testing Center, Sichuan University, Chengdu, 610064, P. R.
China
cKey Laboratory of Green Chemistry and Technology, Minister of Education,
Sichuan University, Chengdu, 610064, P. R. China
† Electronic supplementary information (ESI) available: Crystallographic
data of 7c and 7d are given in CIF format and their CCDC reference
numbers are 796972 and 805245. 1H NMR spectra of 7a–7d and the
composition of some frontier molecular orbitals in the ground state of
each complex are given in Figure S1–S5 and Table S1–S6. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10305a
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7153–7164 | 7153
©

View Article Online
the addition of an electron-donating –OCH3 onto the para-site
of the phenyl of pbt would, however, induce a 16 nm blue-shifted
PL with lPLmax of 541 nm;16 while the alteration of the –OCH3
substituent to the meta-position would result in a ~50 nm red-
shifted PL emission (609 nm).12 Additionally, the introduction of
electron-withdrawing –CF3 or –F to the para-site of the phenyl
in pbt would bring 7 nm bathochromic or 16 nm hypsochromic
shifts in their lPLmax, respectively.16 Thus for (pbt)2Ir(acac)-type
complexes, the tuning of emission color through substituents on
CŸN ligands still relies on certain serendipitous discoveries, and
the relationship between emission color and substituents has not
yet been elucidated.
In pursuing Ir(III) complexes for successful electrophosphores-
cence or chemo/biosensor application, the other key requirement
is to attain high emission quantum yields (QY). This would, in-
evitably, be influenced by the substituents introduced, and the col-
lective substituent effects on jPL are not easily predicted.18 For ex-
ample, despite a few reports stating tert-butyl modified transition-
metal chelates show slightly improved19 or decreased20 PLQYs
in solution, our previous work has confirmed that tert-butylated
tively, under nitrogen atmosphere at a heating rate of 10 ◦C
min-1. The purity of key intermediates and target molecules were
determined by HPLC (Agilent 1100). The UV-Vis absorption
spectra were acquired from 10-5 mol L-1 2-methyltetrahydrofuran
(2-MeTHF) solution on a SHIMADZU UV-2100 spectropho-
tometer. The photoluminescence spectra of the chelates in 10-5 mol
L-1 2-MeTHF solution and thin solid films were recorded on
a HITACHI F-7000 fluorescence spectrophotometer at 298 K,
while thin film samples were acquired by spin-coating from their
chlorobenzene solution (10 mg mL-1) at a speed of 2000 rpm on
quartz substrates. PL efficiency measurements were carried out
at room temperature in both argon degassed (by several freeze-
pump-thaw cycles) or aerated 10-5 mol L-1 dichloromethane and
2-MeTHF solution, using 10-5 mol L-1 Ru(bpy)3Cl2 in water
as the reference under excitation of 420 nm. PL lifetimes of
the compounds were obtained by exponential fit of emission
decay curves recorded on a FLS920 spectrofluorimeter (Edinburgh
Instruments) under excitation of 440 nm at 298 K. The absolute
PL quantum efficiencies of the thin films were determined with an
integrating sphere (IS80 form Labsphere) together with a digital
photometer (S370 from UDT) under ambient conditions, and
the excitation wavelength was 440 nm. To investigate the oxygen
quenching properties of the chelates in solid state, vacuum-dried
crystalline samples were loaded in a rubber-stopper-equipped
quartz cuvette, and the variation of PL emission intensity at lPLmax
was monitored under excitation of 420 nm at a time interval of 1 s
when allowing oxygen to flow over the argon-degassed samples,
and the solid-state PL spectra of the complexes under pure argon
and oxygen were collected with a HITACHI F-7000 fluorescence
spectrophotometer. Cyclic voltammetry (CV) measurement was
carried out on a PARSTAT 2273 electrochemical workstation at
room temperature in argon-purged 5 ¥ 10-4 mol L-1 anhydrous
CH2Cl2 solution with tetrabutylammonium perchlorate (0.1 mol
L-1) as the supporting electrolyte at a scanning rate of 100 mV
s-1. The CV system was constructed using a platinum plate, a
Ag/AgNO3 (0.1 mol L-1 in acetonitrile) electrode and a platinum
wire as the working electrode, quasi-reference electrode and
counter electrode, respectively. Each measurement was calibrated
with a ferrocene/ferrocenium (Fc/Fc+) redox couple as internal
standard.
(pbt)2Ir(acac) exhibits remarkably enhanced solution PLQY (jPL
:
0.51 vs 0.26).21 Likewise, the attachment of –F, –CF3, and –OCH3
to C4-site of phenyl in pbt is also found to have positive effects
on PLQY,16 whereas the 2-(3-methoxyphenyl)benzothiazole-based
analogue shows much lowered emission QY relative to that of
(pbt)2Ir(acac).12 Therefore, no generalized approaches have been
well-established to predict the emission QY of complexes based on
(pbt)2Ir(acac) skeleton.
Although complexes bearing modified pbt ligands show versatile
tunability for both emission color and emission QY, only a few
efforts have been made to exploit efficient red (pbt)2Ir(acac)-
type phosphors.12,22 Herewith, we report the synthesis and char-
acterization of four novel iridium chelates with (pbt)2Ir(acac)
framework. All of them show orange or red emission in both
solution and solid state, and the tert-butylation on phenyl is
likely to benefit the enhancement of PLQY. Moreover, they all
exhibit molecular oxygen quenching character in solution, and
two of them even show efficient emission intensity quenching
in solid state. Further investigation on electronic effects of the
substituents are carried out through electrochemical experiments
and theoretical computation as well.
X-Ray crystallographic Analysis
Experimental section
The crystallographic data for 7c and 7d reported here had
been deposited in the Cambridge Database (CCDC 796972 and
805245). The determination of the unit cell and data collection
for the crystals were preformed on a Rigaku Saturn CCD
diffractometer or a Xcalibur E X-ray single crystal diffractometer
General information and materials
All the chemicals commercially available were used without further
purification unless otherwise stated. All the solvents were of
analytical grades and freshly distilled prior to use. Anhydrous
tetrahydrofuran (THF) and carbon tetrachloride were obtained by
treatment with sodium and diphosphorous pentoxide, respectively,
˚
equipped with graphite monochromator Mo-Ka (l = 0.71073 A)
radiation. The data collection was executed using CrystalClear23a
program for 7c and CrysAlisPro23b program for 7d. Structures
were solved by direct method and successive Fourier difference
syntheses (SHELXS-97), and were refined by full-matrix least-
squares procedure on F2 with anisotropic thermal parameters for
all non-hydrogen atoms (SHELXL-97).24 Packing analysis of the
crystal cells was carried out using Platon,25a and the pictorial
representations of the void spaces in the crystals were outlined
with Mercury25b program.
1
followed by distillation. H NMR and 13C NMR spectra were
recorded on a Bruker AVANCE-400 spectrometer in CDCl3
using TMS as internal standard. High resolution MS spectra
were obtained from a Q-TOF Priemier ESI mass spectrometer
(Micromass, Manchester, UK). Elemental analysis was carried out
with a Carlo Erba 1106 elemental analyzer. Differential scanning
calorimetry (DSC) and thermogravimetic analysis (TGA) were
performed on DSC Q100 and TGA Q500 instruments, respec-
7154 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Computational method
recrystallized from ethanol before use. Intermediates 1b,31 2,32
3,33 4a34 and 4b34 were synthesized according to the literature.
The parent compound (pbt)2Ir(acac) was synthesized following a
reported procedure17 with purity of 99.8%.
For the singlet ground states (S0) of complexes (pbt)2Ir(acac) and
7a–7d, B3LYP26 geometry optimization were performed using
LANL2DZ27 basis set for Ir and 6-31G(d) basis sets for C,
H, S, N and O atoms. All the geometries were confirmed as
stationary structure by the presence of only real frequencies at
the same level of theory. Based on the optimized geometries, time-
dependent density functional theory (TDDFT)28 method within
the framework of the IEF-PCM Model29 in THF media was
employed to calculate the lowest 20 singlet-singlet and singlet–
triplet excitations at the same level. All calculations were carried
out with Gaussian 09 software.30
4¢-Bromo-3,5-di-tert-butylbiphenyl (1c). To a solution of 1a
(1.0 g, 4.3 mmol) and tert-butyl chloride (1.2 mL, 12.8 mmol)
in 10 mL of carbon tetrachloride was added 0.1 g (0.8 mmol)
anhydrous aluminum chloride. The mixture was stirred at 0 ◦C
for 4 h, then the reaction was quenched with water. The organic
phase was washed with saturated sodium bicarbonate solution (2
¥ 50 mL), water (50 mL), and brine (50 mL) in turn, then dried
over MgSO4 and concentrated in vacuo. The resulting white solid
was recrystallized from ethanol to afford w◦hite needles with yield
of 46%, and purity of 99.6%. M.p.: 120–121 C. 1H NMR (CDC13,
400 MHz, d (ppm)): 7.55 (d, J = 8.4 Hz, 2H, ArH), 7.44–7.46 (m,
3H, ArH), 7.34 (s, 2H, ArH), 1.37 (s, 18H, t-BuH).
Synthesis
The synthetic procedures for intermediates and objective
molecules are shown in Scheme 1. 4-Bromobiphenyl (1a) and
fluorene were available commercially from Aldrich Co., and
Methyl 9,9-dimethyl-9H-fluorene-2-carboxylate (3). To a solu-
tion of 9,9-dimethyl-9H-fluorene (2) (1.0 g, 5.15 mmol) and oxalyl
Scheme 1 The synthetic routes of the objective iridium(III) complexes, and the sketch map of the five chelates studied here as well. Reagents and reaction
conditions for the synthetic procedures: (i) (CH3)3CCl, AlCl3, CCl4, 0 ◦C; (ii) (a) Mg, THF, reflux, (b) CO2, H+; (iii) (a) (COCl)2, CH2Cl2, AlCl3, 0 ◦C, (b)
CH3OH, CH2Cl2; (iv) CH3OH, THF, KOH, reflux; (v) (a) SOCl2, reflux, (b) o-aminothiophenol, NMP, 100 ◦C; (vi) IrCl3·nH2O, 2-ethoxyethanol (aq),
110 ◦C; (vii) acetylacetone, 2-ethoxyethanol, Na2CO3, 110 ◦C.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7153–7164 | 7155
©

View Article Online
chloride (1.29 g, 10.3 mmol) in 10 mL dichloromethane was added
2.04 g (15.45 mmol) of aluminum chloride at 0 ◦C. After stirring
for 4 h, the mixture was poured into 100 mL water with crushed
ice followed by extraction with ethyl acetate (3 ¥ 20 mL). The
organic layer was washed with brine, dried over Na2SO4, then
the solvent was removed to yield white residue. It was dissolved
into 40 mL dichloromethane and 10 mL methanol, followed by
2 h stirring at room temperature, then concentrated in vacuo. The
crude product was purified by column chromatography to afford
white solid (eluent: dichloromethane/n-hexane = 1/1), yield: 78%.
1H NMR (CDC13, 400 MHz, d (ppm)): 8.11 (s, 1H, ArH), 8.05
(d, J = 8 Hz, 1H, ArH), 7.78–7.75 (m, J = 4 Hz, 2H, ArH), 7.47
(t, J = 3.6 Hz, 1H, ArH), 7.37 (t, J = 4 Hz, 2H, ArH), 3.96 (s, 3H,
MeH), 1.52 (s, 6H, MeH).
ArH), 7.92 (d, J = 8 Hz, 1H, ArH), 7.82 (d, J = 8 Hz, 1H, ArH),
7.77–7.79 (m, J = 8 Hz, 1H, ArH), 7.52–7.46 (m, J = 2.4 Hz, 1H,
ArH), 7.41–7.36 (m, J = 2 Hz, 3H, ArH), 1.58 (s, 6H, MeH).
General procedure for the synthesis of target iridium(III) com-
plexes 7a–7d. Dichloro-bridged iridium(III) complexes (6a–6d)
were prepared by refluxing IrCl3·nH2O (1 mmol) with cyclomet-
allate ligands 5 (2.4 mmol) in a mixture of 2-ethoxyethanol and
water (3 : 1) under argon for 24 h.35 The precipitate was filtered
and washed with 10 mL of 1 mol L-1 HCl and 3 ¥ 15 mL methanol
in sequence, then dried over vacuo to afford 6. Target compounds
of 7a–7d were prepared by refluxing the chloride-bridged dimers
6 (0.1 mmol), acetylacetone (0.3 mmol) and sodium carbonate
(1 mmol) in 10 mL of 2-ethoxyethanol under argon for 12 h.
After cooled down, the precipitates were collected and purified
from flash chromatography through silica column using CHCl3
as eluent, followed by more than three times recrystallization
procedure to afford satisfied purity, then dried for 24 h at 100 ◦C
under vacuum of 1.5 kPa.
3¢,5¢-Di-tert-butylbiphenyl-4-carboxylic acid (4c). 3¢,5¢-Di-
tert-butylbiphenyl magnesium bromide was prepared from 1c
(3.44 g, 10 mmol) with magnesium (0.27 g, 11 mmol), then
reacted with 10 g dry ice followed by treatment with 1 mol L-1
hydrochloric acid to yield white precipitate. The crude product
was recrystallized from hexane to afford white needles. Yield: 54%;
1H NMR (CDC13, 400 MHz, d (ppm)): 8.20 (d, J = 8.4 Hz, 2H,
ArH), 7.70 (d, J = 8.4 Hz, 2H, ArH), 7.50 (s, 1H, ArH), 7.47 (s,
2H, ArH), 1.37 (s, 18H, t-BuH).
Bis[2-(biphenyl-4-yl)benzothiazolato-N,C2¢]iridium(III)(acetyl-
acetonate) (7a). Red solid, recrystallized from a mixture of
benzene and methanol. Yield: 45%; purity: 98.9%. 1H NMR
(CDC13, 400 MHz, d (ppm)): 8.17 (d, J = 3.6 Hz, 1H, ArH), 8.16
(s, 1H, ArH), 7.92 (s, 1H, ArH), 7.90 (d, J = 3.6 Hz, 1H, ArH), 7.70
(d, J = 7.6 Hz, 2H, ArH), 7.45 (t, J = 4.6 Hz, 4H, ArH), 7.14–7.18
(m, 10H, ArH), 7.08 (d, J = 8 Hz, 2H, ArH), 6.62 (s, 2H, ArH),
5.16 (s, 1H, acacH), 1.79 (s, 6H, acacH). 13C NMR (100 MHz,
CDCl3, d (ppm)): 185.81, 179.83, 150.94, 148.54, 142.17, 141.30,
141.23, 133.45, 131.45, 128.31, 127.33, 127.26, 127.20, 127.05,
125.99, 125.11, 122.36, 120.81, 120.20, 101.70, 28.40. TOF-MS:
m/z 887.1354 (M + Na+); Calcd for Mw + Na+: 887.1342. Anal.
Calcd for C43H31IrN2O2S2: C, 60.05; H, 3.74; N, 3.19; S, 7.28.
Found: C, 59.77; H, 3.62; N, 3.24; S, 7.42.
9,9-Dimethyl-9H-fluorene-2-carboxylic acid (4d). A solution
of 3 (2.52 g, 10 mmol) in hot methanol (20 mL) and THF
(20 mL) was added into 30 mL 1 mol L-1 aqueous KOH. The
mixture was refluxed for 3 h, then neutralized by concentrated
hydrochloric acid to pH = 1.0. The white precipitate was collected
and recrystallized from methanol. Yield: 88.6%.
General procedure for the synthesis of cyclometallate ligands
5a–5d16. 10 mmol of carboxylic acid (4) was refluxed with
20 mL thionyl chloride for 3 h, then excessive thionyl chloride
was removed in vacuo. The residue was dissolved in 20 mL N-
methylpyrrolidinone (NMP), then added dropwise into a solution
of 10 mmol of o-aminothiophenol in 20 mL NMP under inert
atmosphere, followed by stirring at 100 ◦C for 1 h. After
cooled down, the reactant was poured into water followed by
neutralization with 7 mol L-1 aqueous ammonia to pH = 8~9. The
precipitate was filtered and recrystallized from alcohol to afford
intermediate 5.
Bis[2-(4¢-tert-butylbiphenyl-4-yl)benzothiazolato-N,C2¢]iridium-
(III)(acetylacetonate) (7b). Red solid. It was purified by
recrystallization from a mixture of dichloromethane and benzene,
and may contain 1 : 1 cocrystallized benzene even after drying in
vacuum. Yield: 30%; purity: 99.2%. 1H NMR (CDC13, 400 MHz,
d (ppm)): 8.14–8.17 (m, 2H, ArH), 7.90–7.92 (m, 2H, ArH), 7.67
(d, J = 8 Hz, 2H, ArH), 7.44–7.47 (m, 4H, ArH), 7.20–7.22 (d,
4H, ArH), 7.36 (s, ~5H, ArH of benzene), 7.06–7.10 (m, 6H,
ArH), 6.64 (s, 2H, ArH), 5.15 (s, 1H, acacH), 1.79 (s, 6H, acacH),
1.25 (s, 18H, t-BuH). 13C NMR (100 MHz, CDCl3, d (ppm)):
185.83, 179.98, 151.07, 150.09, 148.56, 142.04, 141.12, 138.33,
133.32, 131.51, 128.41, 127.31, 126.93, 126.09, 125.35, 125.05,
122.40, 120.72, 120.23, 101.73, 34.46, 31.37, 28.40. TOF-MS:
m/z 999.2588 (M + Na+); Calcd for Mw + Na+: 999.2606. Anal.
Calcd. for C51H47IrN2O2S2·C6H6: C, 64.81; H, 5.14; N, 2.63; S,
5.98. Found: C, 64.93; H, 5.07; N, 2.66; S, 6.08.
2-(4¢-tert-Butylbiphenyl-4-yl)benzo[d]thiazole
(5b). White
plate crystal. Yield: 74.6%; m.p.: 217–220 ◦C; 1H NMR (CDC13,
400 MHz, d (ppm)): 8.15 (d, J = 8 Hz, 2H, ArH), 8.08
(d, J = 8 Hz, 1H, ArH), 7.91 (d, J = 8 Hz, 1H, ArH), 7.72 (d, J =
8.4 Hz, 2H, ArH), 7.61 (d, J = 8.4 Hz, 2H, ArH), 7.41–7.51 (m,
3H, ArH), 7.39 (t, J = 8 Hz, 1H, ArH), 1.37 (s, 9H, t-BuH).
2-(3¢,5¢-Di-tert-butylbiphenyl-4-yl)benzo[d]thiazole
(5c).
White needles. Yield: 68.6%; m.p.: 165–167 ◦C; 1H NMR
(CDC13, 400 MHz, d (ppm)): 8.16 (d, J = 8 Hz, 2H, ArH), 8.10
(d, J = 8 Hz, 1H, ArH), 7.91 (d, J = 7.6 Hz, 1H, ArH), 7.72 (d,
J = 8.4 Hz, 2H, ArH), 7.47–7.52 (m, 4H, ArH), 7.39 (t, J = 8 Hz,
1H, ArH), 1.40 (s, 18H, t-BuH).
Bis[2-(3¢,5¢-di-tert-butylbiphenyl-4-yl)benzothiazolato-N,C2¢]iri-
dium(III)(acetylacetonate) (7c). Red solid, recrystallized from
a mixture of benzene and methanol. Yield: 36%; purity: 98.9%.
1H NMR (CDC13, 400 MHz, d (ppm)): 8.22 (d, J = 8.4 Hz, 2H,
ArH), 7.86 (d, J = 8 Hz, 2H, ArH), 7.65 (d, J = 7.6 Hz, 2H,
ArH), 7.41–7.52 (m, 4H, ArH), 7.25 (s, 2H, ArH), 7.10 (d, J =
8 Hz, 2H, ArH), 7.00 (s, 4H, ArH), 6.62 (s, 2H, ArH), 5.19 (s,
1H, acacH), 1.82 (s, 6H, acacH), 1.16 (s, 36H, t-BuH). 13C NMR
(400 MHz, CDCl3, d (ppm)): 185.78, 179.87, 151.05, 150.43,
2-(9,9-Dimethyl-9H-fluoren-2-yl)benzo[d]◦thiazole (5d). Yellow
columncrystal. Yield: 78.6%;m.p.:134–137 C; 1H NMR (CDC13,
400 MHz, d (ppm)): 8.23 (s, 1H, ArH), 8.11 (d, J = 8.4 Hz, 1H,
ArH), 8.06 (d, J = 0.4 Hz, 1H, ArH), 8.04 (d, J = 0.8 Hz, 1H,
7156 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 Crystal data and structure refinement for complex 7c and 7d
Compounds
7c
7d
Empirical formula
Formula weight
C71H75IrN2O2S2·2C6H6
C49H40IrN2O2S2
944.14
1244.65
Temperature
Wavelength
113(2) K
0.71073 A
150(2) K
0.71073 A
˚
˚
Crystal system
Space group
a
b
Triclinic
P1
11.046(2) A
15.200(3) A
18.650(4) A
Triclinic
P1
12.028(13) A
13.348(17) A
17.714(2) A
¯
¯
˚
˚
˚
˚
˚
˚
c
a
88.90(3) deg
81.91(3) deg
87.03(3) deg
95.58(11) deg
96.78(10) deg
b
g
102.79(10) deg
3
3
˚
˚
Volume
3095.8(11) A
2731.6(6) A
Z
2
2
Calculated density
Absorption coefficient
F(000)
1.335 mg m-3
1.148 mg m-3
2.270 mm-1
1280
2.552 mm-1
944
Crystal size
0.20 ¥ 0.18 ¥ 0.10 mm
22776
10867 [R(int) = 0.0325]
1.73 to 25.02◦
1.065
0.42 ¥ 0.40 ¥ 0.35 mm
23069
11108 [R(int) = 0.0273]
2.98 to 26.37◦
1.092
Reflections collected
Independent reflections
q-range for data collection
Goodness-of-fit on FŸ2
Final R indices [I > 2s(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0269, wR2 = 0.0647
R1 = 0.0295, wR2 = 0.0665
R1 = 0.0297, wR2 = 0.0767
R1 = 0.0341, wR2 = 0.0792
-3
-3
˚
˚
1.063 and -1.763 e·A
0.819 and -0.674 e·A
148.66, 143.18, 140.87, 140.14, 133.67, 131.46, 127.37, 125.90,
125.11, 122.24, 121.70, 121.15, 120.51, 120.24, 101.73, 34.76,
31.39, 28.45. TOF-MS: m/z 1089.3977 (M + H+); Calcd for Mw +
H+: 1089.3960. Anal. Calcd for C59H63IrN2O2S2: C, 65.35; H,
5.92; N, 2.53; S, 5.95. Found: C, 65.10; H, 5.83; N, 2.57; S, 5.89.
poor yield of 12% in the procedure for preparing 1b,31 while the
reaction conditions are optimized with prolonged reaction time
and excessive feeding ratio of anhydrous AlCl3 to achieve a much
improved yield of 46% finally.
Single crystals of complex 7c and 7d are obtained from slow
vapor diffusion of methanol into their benzene or CHCl3 solution,
and 2 equivalent crystallized benzene molecules can be found in
the unit cell of 7c. The crystallographic refinement parameters,
selected bond distances as well as bond angles of 7c and 7d are sum-
marized in Table 1 and 2, respectively, and their crystal structures
are shown in Fig. 1. It is clear that both the two complexes reveal
Bis[2-(9,9-dimethyl-9H-fluoren-2-yl)benzothiazolato-N,C2¢]iri-
dium(III)(acetylacetonate) (7d). Red solid, recrystallized from a
mixture of chloroform and methanol. Yield: 42%; purity: 99.2%.
1H NMR (CDC13, 400 MHz, d (ppm)): 8.13 (s, 1H, ArH), 8.11
(d, J = 4 Hz, 1H, ArH), 7.97 (d, J = 4 Hz, 1H, ArH), 7.96 (s,
1H, ArH), 7.67 (s, 2H, ArH), 7.45 (t, J = 4 Hz, 4H, ArH), 7.28
(d, J = 7.6 Hz, 2H, ArH), 7.12–7.16 (m, 2H, ArH), 7.08 (s, 4H,
ArH), 6.74 (s, 2H, ArH), 5.18 (s, 1H, acacH), 1.79 (s, 6H, acacH),
1.46 (s, 6H, CH3), 1.40 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3,
d (ppm)):185.81, 180.32, 154.85, 151.15, 146.99, 146.89, 141.51,
140.70, 138.62, 131.20, 127.53, 127.42, 126.43, 125.94, 124.89,
122.27, 120.40, 120.24, 120.06, 101.80, 46.08, 28.48, 27.45, 27.31.
TOF-MS: m/z 945.1849 (M + H+); Calcd for Mw + H+: 945.2160.
Anal. Calcd for C49H39IrN2O2S2: C, 62.33; H, 4.16; N, 2.97; S, 6.79.
Found: C, 62.80; H, 4.01; N, 2.97; S, 6.61.
˚
Table 2 Selected bond lengths (A) and angles (deg) for 7c and 7d
˚
Bond lengths (A)
Bond angles (deg)
7c
Ir(1)–C(13)
Ir(1)–C(40)
Ir(1)–N(1)
Ir(1)–N(2)
Ir(1)–O(1)
Ir(1)–O(2)
O(1)–C(56)
O(2)–C(58)
1.992(3)
1.992(3)
2.045(2)
2.044(2)
2.1573(19)
2.131(2)
1.270(3)
1.266(3)
C(13)–Ir(1)–N(2)
C(40)–Ir(1)–N(1)
N(2)–Ir(1)–O(1)
N(2)–Ir(1)–N(1)
N(1)–Ir(1)–O(1)
O(2)–Ir(1)–O(1)
N(2)–Ir(1)–O(2)
N(1)–Ir(1)–O(2)
95.01(10)
94.70(11)
88.10(8)
173.20(8)
96.34(8)
87.84(8)
99.14(9)
86.20(9)
Results and discussion
Synthesis and characterization
All these heteroleptic complexes are synthesized from reaction
of IrCl3·nH2O with cyclometallate ligands to form chloride-
bridged complexes [(CŸN)2IrCl]2, followed by treatment with
acetylacetone. The synthetic routes are outlined in Scheme 1. The
key intermediates of carboxylic acids for CŸN ligands are prepared
through Grignard reaction in the case of 4a–4c, and Friedel–
Crafts acylation for 4d; while the aryl bromides 1b and 1c are
prepared by Friedel–Crafts tert-butylation using 1a as reactant.
It is noteworthy that 1c just exists as byproduct initially with
7d
Ir(1)–C(14)
Ir(1)–C(47)
Ir(1)–N(1)
Ir(1)–N(2)
Ir(1)–O(1)
Ir(1)–O(2)
O(1)–C(1)
O(2)–C(3)
2.001(3)
1.993(3)
2.058(3)
2.055(3)
2.158(2)
2.155(2)
1.267(4)
1.272(4)
C(14)–Ir(1)–N(2)
C(47)–Ir(1)–N(1)
N(2)–Ir(1)–O(1)
N(2)–Ir(1)–N(1)
N(1)–Ir(1)–O(1)
O(2)–Ir(1)–O(1)
N(2)–Ir(1)–O(2)
N(1)–Ir(1)–O(2)
93.39(12)
93.60(12)
101.64(10)
171.16(10)
84.78(10)
87.40(9)
86.57(10)
99.90(10)
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7153–7164 | 7157
©

View Article Online
(tbt)2Ir(acac), implying 7c and 7d have less efficient p-conjugation
system relative to (tbt)2Ir(acac).37 The biphenyl moieties in 7c
exhibit twisted geometry with dihedral angles of 33.59 (0.14)◦ and
44.72 (0.14)◦, while in the case of 7d, the dihedral angles between
the two phenyl in fluorenyl are 9.11 (0.18)◦ and 2.35 (0.17)◦.
Thus 7d bearing one nearly coplanar CŸN ligands should have
more extended conjugation system. Additionally, spacious cavities
accommodating four benzene molecules can be observed between
two neighboring molecules of 7c in the crystal cell (vide Fig. 1a),
validating that 7c possesses a bulky volume. Consequently, the
molecular aggregation and concentration quenching of 7c could
be reduced effectively in its solid state.
Examination of the molecular packing of the structures of 7c
shows the existence of narrow channels containing two cocrystal-
lized benzene molecules per Ir unit. It was speculated that if the
crystals of 7c could lose all solvents without loss of crystallinity,
more void spaces would be released after removal of benzene to
form open vacant channels which may allow the diffusion of small
molecules such as oxygen into and out of the crystals, hence enable
efficient emission quenching in crystalline form.38 While in the case
of 7d, only small isolated voids could be found (vide Fig. 1b), so we
expect it has less oxygen sensing character in comparison with 7c.
Thermal properties
DSC results reveal that all the complexes show decomposition
prior to melting transition with no obvious glass transition
peaks. TGA of the complexes indicates that all of the compounds
are stable in air without crystallized solvent molecules except
for 7b, and their decomposition temperatures (at 5 wt% loss)
are determined as 318~362 ◦C, suggesting that they have good
thermostability (thermograms and data are shown in Fig. 2 and
Table 4). It is worthy of note that the initial weight loss of ~7% at
254 ◦C of 7b should be due to the loss of benzene from 7b·benzene
(1 : 1) complex that is formed in the recrystallization procedure,
which can be further confirmed by the elementary analysis data
1
and H NMR spectrum (the intense signal at d = 7.36 ppm, vide
ESI Figure S2,† the sample has been dried for 24 h at 100 ◦C
under vacuum of 1.5 kPa before the measurement). Interestingly,
although the X-ray crystallographic analysis of 7c shows two
equiv. of crystallized benzene in its unit cell, no analogous
Fig. 1 The ORTEP drawing, crystal cell structure and packing diagram
viewed down the a axis of crystal structures of 7c (a) and 7d (b).
distorted octahedral geometries around the Ir atom with the two
CŸN ligands adopting C,C-cis, N,N-trans configurations, which
are akin to those for analogous iridium complex (tbt)2Ir(acac)
{tbt: 2-(p-tolyl)benzo[d]thiazole}.36 In 7d, the bond lengths of
the two Ir–O bonds are almost identical, while in the case of
7c, they differed slightly from each other [2.131(2) and 2.1573(19)
36
˚
A], suggesting the phenyl groups in 7c exist trans-influence. The
dihedral angles between the benzothiazole and the aryl rings
are 9.99 (0.11)◦ and 14.49 (0.12)◦ for 7c, while 8.50 (0.14)◦ and
18.71 (0.14)◦ for 7d, both are larger than those observed in
Fig. 2 TGA thermograms of (pbt)2Ir(acac) and 7a–7d.
7158 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 3 Photophysical data of the complexes studied here
lPLmax (nm) lPLmax (nm)
lifetime
b
Compound
Absorbance lmax (nm) (log e)a solutionb
filmc
jPL
j
PL(%)e t (ms)d
kr (s-1)e
knr (s-1)e
kq[O2] (s-1)f
(pbt)2Ir(acac) 313(4.4), 327(4.5), 358(3.9),
557
578
578
577
600
584
0.26 (0.06) 0.47
0.24 (0.03) 0.75
0.33 (0.04) 1.13
0.62 (0.04) 1.77
0.33 (0.02) 0.47
1.8 (0.08) 1.44 ¥ 105 4.11 ¥ 105 1.18 ¥ 107
1.8 (0.13) 1.33 ¥ 105 4.22 ¥ 105 0.75 ¥ 107
1.9 (0.12) 1.74 ¥ 105 3.53 ¥ 105 0.80 ¥ 107
2.0 (0.15) 3.10 ¥ 105 1.90 ¥ 105 0.64 ¥ 107
2.0 (0.11) 1.65 ¥ 105 3.35 ¥ 105 0.89 ¥ 107
447(3.7), 488(3.6)
7a
7b
7c
7d
311(4.7), 331(4.8), 341(4.8),
417(4.0), 464(3.9), 501(3.8)
313(4.6), 344(4.7), 417(4.0),
463(3.8), 501(3.6)
313(4.5), 344(4.7), 418(4.0),
465(3.8), 503(3.7)
598, 634sh
593
588
322(4.5), 355(4.7), 425(4.1),
478(3.8), 516(3.7)
611
a UV-Vis absorbance is determined in 10-5 mol L-1 2-MeTHF solution at 298 K. b PL emission spectra are acquired in argon degassed 10-5 mol L-1
2-MeTHF solution. PL efficiencies in deaerated 10-5 mol L-1 dichloromethane and as delivered 2-MeTHF solution (values in parenthesis) are determined
using Ru(bpy)3Cl2 as the standard (j = 0.042 and 0.028 in deaerated and as delivered water solution)41 under excitation of 420 nm. c Thin films are
spin-coated from 10 mg mL-1 chlorobenzene solution, PL spectra are obtained under irradiation of 420 nm. The absolute PL quantum yields of the
compounds in thin solid films are measured in an intergrating sphere under ambient conditions. d Lifetime is determined in argon degassed and as
delivered (values in parenthesis) 10-5 mol L-1 2-MeTHF solution. e The radiative and overall nonradiative constants kr and knr are calculated through the
equition kr = jPL/t and knr = (1 - jPL)/t.42 f Oxygen quenching rates are determined in as delivered 10-5 mol L-1 2-MeTHF solution, which are calculated
from the equation kq[O2] = (1 - j)/t.44
Table 4 Electrochemical and thermal data of the iridium complexes
Td (◦C)g
ox
Compound
E1/2 (V)a,b
Eg (eV)c
HOMO (eV)d
LUMO (eV)e
HOMO (eV)f
LUMO (eV)f
(pbt)2Ir(acac)
0.51
0.49
0.50
0.49
0.41
2.23
2.13
2.12
2.12
2.07
-5.31
-5.29
-5.30
-5.29
-5.21
-3.08
-3.16
-3.18
-3.17
-3.16
-5.28
-5.29
-5.27
-5.26
-5.17
-1.80
-1.91
-1.89
-1.87
-1.91
326
355
353
362
318
7a
7b
7c
7d
ox
a Oxidation potential values are measured in CH2Cl2 solution containing 5 ¥ 10-4 mol L-1 of the iridium(III) complexes. b E1/2 = 1/2(Epa + Epc), potential
values are reported versus Fc/Fc+. c Eg are estimated from the onset wavelength of the optical absorption bands. d HOMO energies are deduced from the
.
e LUMO energies obtained from the equation LUMO = HOMO + Eg. f Obtained from B3LYP calculations within the
ox
equation HOMO = 4.8 + E1/2
framework of the IEF-PCM model in THF media. g Temperature with 5 wt% loss (12 wt% loss in the case of 7b).
benzene complex is obtained after vacuum drying at 100 ◦C.
These observations give a clue that complex 7b with tightly held
benzene molecules should have smaller void spaces relative to 7c,
therefore it is expected to exhibit less efficient oxygen quenching
property.
Photophysical data
All these Ir(III) complexes exhibit two distinguishable absorption
bands in UV-Vis region. The intense ones in 300~380 nm with log
e > 4 are generally assigned to the spin-allowed 1(p–p*) transition
due to the contributions from cyclometallate ligands,39 while the
weaker low energy features at 420~550 nm are generally ascribed
to the metal to ligand charge-transfer (1MLCT and 3MLCT)
3
and the spin–orbit coupling enhanced (p–p*) transition.40 The
absorption curves of 7a–7d resemble one another in shape, yet
differ slightly from that of (pbt)2Ir(acac), implying 7a–7d may
have similar excitation characters. It can be seen from Fig. 3 and
Table 3 that 7a–7d show bathochromic absorption relative to that
of (pbt)2Ir(acac) in both the two major bands, and 7d exhibits
the most red-shifted ones. Thus the replacement of phenyl into
biphenyl in CŸN ligands would result in comparatively narrow
band gap, while the fluorenyl group would bring about the most
extended conjugation system for the chelate.
Fig. 3 Normalized absorption and PL spectra of the iridium complexes
studied here in 10-5 mol L-1 2-MeTHF solution at 298 K. PL spectra are
obtained under irradiation of 420 nm.
All these iridium complexes exhibit intense orange/red phos-
phorescence in solution at room temperature. In accordance
with the UV-Vis absorption spectra, the lPLmax of 7a, 7b and 7c
(~578 nm) red-shift for 20 nm relative to (pbt)2Ir(acac); while
complex 7d possesses the reddest phosphorescence with PL peak at
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7153–7164 | 7159
©

View Article Online
600 nm. PLQY of (pbt)2Ir(acac) in argon-degassed dilute solution
is determined as 0.26, which is consistent with the literature report
(using fac-Ir(ppy)3 as standard reference).17 Its phenyl-modified
derivative 7a shows a jPL of 0.24; whilst 7b and 7c bearing bulky
tert-butyl modified cyclometallate ligands have enhanced jPL of
0.33 and 0.62, respectively; and that of 7d with rigid fluorenyl is
calculated to be 0.33. As the five chelates possess analogical triplet
lifetime (1.8~2.0 ms) in solution, the radiative and nonradiative rate
constants kr and knr of (pbt)2Ir(acac), 7a, 7b and 7d, calculated from
the equations kr = jPL/t and knr = (1 - jPL)/t,42 are quite similar,
whereas 7c owns much higher kr and lower knr, simultaneously.
Because 7a–7c have much resembled PL spectra in solution, which
indicates tert-butyl may contribute little electronic effect to the
radiative decay processes, the favorable PL efficiency of 7c may be
attributed to the reduction of the interaction of the luminescent
parts of molecules.43
In comparison with those determined in degassed solution,
the jPL of 7a–7d in their air-saturated solutions drop greatly
to 0.02~0.04, and the lifetime is shortened dramatically to
0.08~0.15 ms as well, therefore the deteriorated PLQYs are ascribed
to triplet oxygen quenching. The oxygen quenching rate of
these chelates, based on the equation44 kq[O2] = (1 - j)/t, are
calculated to be 0.64~1.18 ¥ 107 s-1. Since the lifetime, PLQY
and kq[O2] of our target molecules are comparable to those
of Ir(ppy)3,44 which has been demonstrated successfully to be
the first Ir-complex oxygen sensor,45 our complexes 7a–7d are
prospective phosphorescent chemosensor reagents for dissolved
oxygen measurement in solution or polymer dispersed matrices.
To investigate their potential use as non-doping host phosphors
in PhOLEDs, and solid molecular oxygen sensors as well, solid-
state emission spectra of 7a–7d are acquired (shown in Fig. 4).
Similar to those observed in solution, 7a–7d display red-shifted PL
spectra compared with their prototype in solid films with lPLmax of
588~611 nm, each accompanied with a shoulder peak tailing to the
lower energy region, and their emission colors range from orange
to red. The PL spectra collected from solid films are broader
with more bathochromic effect relative to those acquired from
solution, which may be due to the aggregation of phosphorescent
chromophores.46 It is found that (pbt)2Ir(acac) owns the largest
difference of 27 nm between the lPLmax in solution and film, while
those for 7a–7d are 20 nm, 15 nm, 11 nm and 11 nm, successively,
hence 7a–7d should possess less molecular stacking.
Though in most cases, no severe triplet oxygen quenching
could be observed in the solid state of phosphorescent complexes
owing to the immobilization of dissolved oxygen,38a,44 the spacious
void channels found in the crystal structure of 7c evokes our
interest to investigate emission intensity quenching character of
the target compounds (depicted in Fig. 5). Surprisingly, all five
complexes show some emission quenching in oxygen relative to
argon, and the intensity quenched fraction (I0 - I)/I0 for 7c (lPLmax
= 594 nm), as expected, is the highest among all these compounds
[(I0 - I)/I0 = 0.68]; that of 7b which has smaller void spaces due
to the existence of tightly cocrystallized benzene is much lower
[(I0 - I)/I0 = 0.28];. while that of 7d possessing isolated small voids
is as low as 0.19. Interestingly, 7a exhibits a rather high intensity
quenched fraction [(I0 - I)/I0] of 0.57 and (pbt)2Ir(acac) also
shows some quenching character with [(I0 - I)/I0] of 0.23. Thus
the distorted biphenyl moiety should be the key component for
constructing porous crystalline iridium complexes. This is, to our
knowledge, the first report on the discovery of iridium complexes
that show efficient emission intensity quenching by oxygen in
condensed solid states. As these compounds show intense PL in
solid state, they have promising application as crystalline oxygen
sensors.
Fig. 5 Plot of (I0 - I)/I0 vs time for the argon predegassed crystalline
samples under oxygen purging. I0 represents the intensity of lPLmax of
each compound in argon; while I is that of oxygen purged sample. The
l
PLmax monitored for (pbt)2Ir(acac), 7a–7d is 591, 599, 600, 594 and 613 nm,
consecutively. The inset figure depicts the emission spectra of solid complex
7c under argon and air, showing the quenching of the emission in the
presence of oxygen.
As far as absolute quantum efficiency in solid films is concerned,
(pbt)2Ir(acac) and 7d have relatively low QYs of 0.47%; 7a bearing
twisted geometry has an increased QY of 0.75%; while 7b and
7c with bulky molecular volumes show much improved PLQY of
1.13% and 1.77% (vide Table 3). It should be pointed out here that
the measurement of absolute PLQYs in nitrogen is precluded by
our experimental limitations, and therefore has to be carried out in
air. As each complex studied here is found to show some emission
quenching by oxygen, the QYs reported here should be lower than
those measured in nitrogen atmosphere.
Fig. 4 Phosphorescence spectra of the iridium complexes in thin solid
films at 298 K (under irradiation of 420 nm).
7160 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
All these preliminary investigations on photophysical property
suggest that 7a–7d are attractive candidates for both oxygen sensor
agents and emitting phosphors in PhOLEDs.
the Fc/Fc+ redox couple (whose energy level is 4.80 eV below
vacuum),36 and those of LUMO are deduced from the HOMO
energies and optical bandgaps.47
The oxidation wave values of 7a–7c are quite akin to that
of (pbt)2Ir(acac), therefore the additional twisted phenyls would
bring little influence tothe HOMO ofthe complexes; yet 7d bearing
coplanarly bonded phenyl has an elevated HOMO of ~0.1 eV. On
the other hand, all of the four target complexes show unexpected
declined LUMOs of ~0.1 eV relative to (pbt)2Ir(acac), indicating
that the para-substituented phenyls, either twisted or coplanar
with the adjacent phenyl moieties of pbt, contributes similarly to
the destabilization of LUMOs.
Electrochemical data
The electrochemical behaviors of these Ir(III) complexes are
investigated by cyclic voltammetry in argon purged 5 ¥ 10-4 mol
L-1 CH2Cl2 solution with Fc/Fc+ redox couple as an internal
reference. The cyclic voltammograms of (pbt)2Ir(acac), 7c and
7d are shown in Fig. 6. During the anodic scan, each of these
complexes exhibits a reversible one-electron oxidation wave in the
range of 0.41~0.51 V, which is mainly attributed to the Ir(III/IV)
metal center.47 No reduction wave has been detected because of
the limited range available in CH2Cl2. The HOMO energy levels
of the compounds can be roughly estimated by comparison with
Theoretical calculation
DFT calculations are performed using Gaussian09 software to
gain insights into the origin of the variable property induced
by substitution. The optimized structures for the complexes
along with the numbering of important atoms, main geometrical
parameters calculated, and some experimental data of 7c and
7d derived from X-ray crystallography can be found in ESI.†
The close similarity between calculated and experimental values
reveals the reliability of our computation. Consistent with the
experimental observations for 7c and 7d, all the biphenyl moieties
in the optimized structures for 7a–7c derived from calculation,
show twisted geometry with dihedral angles of ca. 37◦, while the
fluorenyl segments in 7d are calculated to be planar with torsion
angles of ca. 0◦.
The calculated HOMO energies, as shown in Table 4, accurately
reproduce the experimental data. (pbt)2Ir(acac) and 7a–7c bear-
ing additional distorted phenyl substituents possess analogous
HOMO energy levels, while the introduction of a coplanar one
(7d) would lead to elevated HOMO for 0.09 eV. Similar to
those observed in Ir-ppy type complexes, the HOMOs of the
objective Ir-pbt type complexes are dominated by Ir-d orbitals
and p-orbitals of phenyl moieties directly connected to Ir atom
Fig. 6 Cyclic voltammograms of (pbt)2Ir(acac), 7c and 7d. The oxidation
potentials are determined relative to Ag/Ag+ in 5 ¥ 10-4 mol L-1 CH2Cl2
solution, using ferrocene as internal reference.
Fig. 7 Isodensity plots of the HOMO (top) and LUMO (bottom) for (pbt)2Ir(acac) and 7a–7c.
The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 7153–7164 | 7161
This journal is
©

View Article Online
7162 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
(orbital compositions are shown in Tables S2–S6 in ESI†). The
contributions from Ir-d orbitals are calculated to be 49% for
(pbt)2Ir(acac), 46~47% for 7a–7c, and 42% for 7d, and those
from phenyl-p orbitals are nearly identical (33–34%). It can be
seen obviously from Fig. 7 that the additional phenyl or tert-
butyl substituents of 7a–7c have little contribution (2–3%) to
their HOMOs; yet in the case of 7d, the coplanar phenyl and
the methylene groups show visible donation to its HOMO (6%).
Therefore, (pbt)2Ir(acac) and 7a–7c exhibit comparable features
when considering the contribution from both center atom and
substituents in their ground states, while 7d displays higher
HOMO energy.
The substituent effects on LUMOs, however, differs obviously
from those on HOMOs, since different substituents on benzoth-
iazole (viz., biphenyl, tert-butyl modified biphenyl and fluorenyl)
would result in similarly lowered LUMO energies of 0.07~0.11 eV
relative to that of (pbt)2Ir(acac). According to the calculation
results, all the LUMOs of the five complexes predominantly
localize on the p* orbitals of benzothiazole segments [68% for
(pbt)2Ir(acac), 60% for 7a–7c, and 56% for 7d (vide Tables S2–S6
in ESI†)]. Accompanied with the decreased donation composition
of benzothiazole units, increased compensative contributions from
the substituents (for 7a–7c, ca. 8~9%, for 7d, ca. 13%) can be
found. It is noticeable that although the calculated LUMOs
are significantly higher than those estimated from experiment
(depicted in Table 4), the variation trends for both LUMO and
HOMO–LUMO gap of the complexes show perfect reproduction
between the calculation and experiment values. Thus compared
with (pbt)2Ir(acac), the decreased band gaps for 7a–7c can be
merely assigned to their declined LUMOs, while 7d having
simultaneously destabilized HOMO and stabilized LUMO owns
the lowest band gap.
To further investigate the nature of absorption and emission
process involved in these phosphors, 20 low-lying singlet and
triplet states of the complexes are calculated based on their
optimized geometries of S0 in THF solvent using TDDFT
method, and the transition bands and characters of the relevant
excited states are listed in Table 5. The calculated strongest
absorption bands of the five compounds are comparable to their
experimental values. Furthermore, our calculation reveals that the
strongest absorption bands of 7a–7c are dominated (84~90%)
by the HOMO-2→LUMO/LUMO+1 transition with p(CŸN +
acac)→p*(CŸN) configurations. That of 7d shows a HOMO-
3→LUMO predominated (77%) p(CŸN)→p*(CŸN) transition; yet
(pbt)2Ir(acac) is found to have its strongest absorption band dom-
inated (90%) by the HOMO-5→LUMO+1 transition showing a
mixed MLCT/p→p* feature (vide Table 5 and Tables S2–S6†).
This may account for the different shapes of absorption curves
observed experimentally between (pbt)2Ir(acac) and 7a–7d at 300–
380 nm. The lowest singlet excited-states are calculated to locate
at 453 nm for (pbt)2Ir(acac), 462 nm for 7a–7c, and 477 nm for
7d, which are analogous to their experimental values (447, 464,
463, 465 and 478 nm, successively), and all of them are assigned
to HOMO→LUMO and HOMO→LUMO+1 transitions with
MLCT/LC characters, where LUMO and LUMO+1 are calcu-
lated to be a degenerate couple with predominantly p* delocalized
feature on benzothiazole moieties. Moreover, both the strongest
transition bands and the lowest singlet excited-states of 7a–7d are
calculated to show bathochromic effect relative to (pbt)2Ir(acac),
and 7d has the lowest transition energy. All these computational
results are consistent with the experimental facts.
The vertical excitation bands and the orbitals of the complexes
involved in the dominant excitations for the lowest triplet states
are depicted in Table 5. The T1 states for all these complexes are
similar in main configurations with only slightly different weight.
The largest contribution of them (67%–77%) can be assigned to the
HOMO–LUMO transitions, displaying MLCT/p→p* characters
derived from d(Ir)-p(CŸN + acac)→p*(CŸN). The emission peaks
calculated from T1 states locate in 512, 532, 534, 530 and 562 nm
for (pbt)2Ir(acac) and 7a–7d, successively. Despite the relatively
large derivation from their actual lPLmax data, the calculated 18–
22 nm red-shift for 7a–7c and 50 nm for 7d relative to (pbt)2Ir(acac)
are in accordance with the experimental facts (ca. 20–21 nm for
7a–7c, and 43 nm for complex 7d).
Conclusion
The substituent effects of four (pbt)2Ir(acac)-type complexes on
emission color and jPL have been investigated. By attaching an
extra phenyl substituent to the para-site of phenyl in pbt, the
emission color of the complexes could be fine-tuned to orange
and red, and coplanar one is demonstrated to be more effective for
achieving bathochromic shift of PL emission. The incorporation
of tert-butyls with the chelates is proven to be an impactful way
for the enhancement of PL efficiency. Both CV measurement and
DFT calculation results reveal that the addition of a distorted
phenyl substituent to phenyl segment of pbt ligands would lead
to a declined LUMO rather than elevated HOMO; while the
alteration of the twisted phenyl into a coplanar one would lead
to both higher HOMO and lower LUMO. All these objective
molecules show emission quenching by molecular oxygen in
solution, and two of them even show efficient oxygen quenching in
solid state. As all these complexes have appropriate PL quantum
yields, emission colors and emission lifetime, they are prospective
electrophosphors and oxygen sensing agents.
Acknowledgements
The authors acknowledge the financial support for this work
by the National Natural Science Foundation of China (Projects
20872103, 50803040 and 20973115). We are grateful to the
analytical and testing center of Sichuan University for providing
NMR, X-ray single crystal analysis, UV-Vis and PL data for the
intermediates and objective compounds.
References
1 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson and S. R. Forrest, Nature, 1998, 395, 151; (b) E. L. Williams,
K. Haavisto, J. Li and G. E. Jabbour, Adv. Mater., 2007, 19, 197; (c) B.
Tong, Q. Mei, S. Wang, Y. Fang, Y. Meng and B. Wang, J. Mater.
Chem., 2008, 18, 1636.
2 (a) K. K.-W. Lo, C.-K. Chung, T. K.-M. Lee, L.-K. Lui, K. H.-K.
Tsang and N. Zhu, Inorg. Chem., 2003, 42, 6886; (b) M. Yu, Q. Zhao,
L. Shi, F. Li, Z. Zhou, H. Yang, T. Yi and C. Huang, Chem. Commun.,
2008, 2115.
3 (a) H. Chen, Q. Zhao, Y. Wu, F. Li, H. Yang, T. Yi and C. Huang, Inorg.
Chem., 2007, 46, 11075; (b) J. S. Y. Lau, P. K. Lee, K. H. K. Tsang,
C. H. C. Ng, Y. W. Lam, S. H. Cheng and K. K. W. Lo, Inorg. Chem.,
2009, 48, 708.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7153–7164 | 7163
©

View Article Online
4 (a) C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J.
Appl. Phys., 2001, 90, 5048; (b) H. Sasabe, Y. J. Pu, K. Nakayama and
J. Kido, Chem. Commun., 2009, 6655; (c) Z. Jiang, Y. Chen, C. Yang,
Y. C a o, Y. Ta o, J. Q i n a n d D. M a , Org. Lett., 2009, 11, 1503.
5 (a) R. Meerheim, R. Nitsche and K. Leo, Appl. Phys. Lett., 2008, 93,
043310; (b) J. H. Jou, S. M. Shen, S. H. Chen, M. H. Wu, W. B. Wang,
H. C. Wang, C. R. Lin, Y. C. Chou, P. H. Wu and J. J. Shyue, Appl.
Phys. Lett., 2010, 96, 143306; (c) Y. Tao, Q. Wang, L. Ao, C. Zhong, J.
Qin, C. Yang and D. Ma, J. Mater. Chem., 2010, 20, 1759.
6 F. M. Hwang, H. Y. Chen, P. S. Chen, C. S. Liu, Y. Chi, C. F. Shu, F.
I. Wu, P. T. Chou, S. M. Peng and G. H. Lee, Inorg. Chem., 2005, 44,
1344.
7 (a) Y. T. Huang, T. H. Chuang, Y. L. Shu, Y. C. Kuo, P. L. Wu, C. H.
Yang and I. W. Sun, Organometallics, 2005, 24, 6230; (b) Y. Y. Lyu, J.
Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee and K. Char,
Adv. Funct. Mater., 2009, 19, 420.
8 (a) Z. Liu, M. Guan, Z. Bian, D. Nie, Z. Gong, Z. Li and C. Huang, Adv.
Funct. Mater., 2006, 16, 1441; (b) R. Meerheim, S. Scholz, S. Olthof,
G. Schwartz, S. Rebastianeineke, K. Walzer and K. Leo, J. Appl. Phys.,
2008, 104, 014510/1.
24 G. M. Sheldrick SHELXL-,97, Program for Refinement of Crys-
tal StructuresUniversity of Go¨ttingen, Go¨ttingen, Germany,
1997.
25 (a) A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7; (b) I. J. Bruno, J. C.
Cole, P. R. Edgington, M. K. Kessler, C. F. Macrae, P. McCabe, J.
Pearson and R. Taylor, Acta Crystallogr., Sect. B: Struct. Sci., 2002,
B58, 389.
26 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) C. Lee, W. Yang
and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
27 P. J. Hay, J. Phys. Chem. A, 2002, 106, 1634.
28 C. Jarmorski, M. E. Casida and D. R. Salahub, J. Chem. Phys., 1996,
104, 5134.
29 E. Cance`s, B. Mennucci and J. Tomasi, J. Chem. Phys., 1997, 107, 3032.
30 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J.
Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R.
Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S.
S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvado, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J.
B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09; A.
02 ed.; Gaussian, Inc.: Wallingford CT, 2009.
9 J. Qiao, L. Duan, L. Tang, L. He, L. Wang and Y. Qiu, J. Mater. Chem.,
2009, 19, 6573.
10 (a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am.
Chem. Soc., 2001, 123, 4304; (b) M. L. Xu, G. Y. Wang, R. Zhou, Z. W.
An, Q. Zhou and W. L. Li, Inorg. Chim. Acta, 2007, 360, 3149.
11 (a) A. B. Tamayo, S. Garon, T. Sajoto, P. I. Djurovich, I. M. Tsyba, R.
Bau and M. E. Thompson, Inorg. Chem., 2005, 44, 8723; (b) L. He, L.
Duan, J. Qiao, D. Zhang, L. Wang and Y. Qiu, Org. Electron., 2010,
11, 1185; (c) C. F. Li, G. P. Yong and Y. Z. Li, Inorg. Chem. Commun.,
2010, 13, 179.
31 S. Murphy, X. Yang and G. B. Schuster, J. Org. Chem., 1995, 60,
2411.
32 X. Zhan, C. Risko, F. Amy, C. Chan, W. Zhao, S. Barlow, A. Kahn, J.
L. Bre´das and S. R. Marder, J. Am. Chem. Soc., 2005, 127, 9021.
33 F. Picard, T. Schulz and R. W. Hartmann, Bioorg. Med. Chem., 2002,
10, 437.
34 I. Mutule and E. Suna, Tetrahedron, 2005, 61, 11168.
35 (a) F. O. Garces, K. A. King and R. J. Watts, Inorg. Chem., 1988, 27,
3464; (b) S. Sprouse, K. A. King, P. J. Spellane and R. J. Watts, J. Am.
Chem. Soc., 1984, 106, 6647.
36 L. Chen, C. Yang, J. Qin, J. Gao and D. Ma, Inorg. Chim. Acta, 2006,
359, 4207.
37 S. Takizawa, Y. Sasaki, M. Akhtaruzzaman, H. Echizen, J. Nishida, T.
Iwata, S. Tokitoac and Y. Yamashita, J. Mater. Chem., 2007, 17, 841.
38 (a) K. A. McGee, D. J. Veltkamp, B. J. Marquardt and K. R. Mann, J.
Am. Chem. Soc., 2007, 129, 15092; (b) K. A. McGee, B. J. Marquardt
and K. R. Mann, Inorg. Chem., 2008, 47, 9143.
39 S. Jung, Y. Kang, H. S. Kim, Y. H. Kim, C. L. Lee, J. J. Kim, S. K. Lee
and S. K. Kwon, Eur. J. Inorg. Chem., 2004, 17, 3415.
40 (a) A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani,
S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino
and K. Ueno, J. Am. Chem. Soc., 2003, 125, 12971; (b) J. P. Duan, P. P.
Sun and C. H. Cheng, Adv. Mater., 2003, 15, 224.
41 (a) J. Van, Houten and R. J. Watts, J. Am. Chem. Soc., 1975, 97, 3843;
(b) J. Van, Houten and R. J. Watts, J. Am. Chem. Soc., 1976, 98, 4853.
42 E. M. Kober, J. V. Caspar, R. S. Lumpkin and T. J. Meyer, J. Phys.
Chem., 1986, 90, 3722.
43 (a) H. Z. Xie, M. W. Liu, O. Y. Wang, X. H. Zhang, C. S. Lee, L. S.
Hung, S. T. Lee, P. F. Teng, H. L. Kwong, H. Zheng and C. M. Che, Adv.
Mater., 2001, 13, 1245; (b) T. Qin, J. Ding, L. Wang, M. Baumgarten,
G. Zhou and K. Mu¨llen, J. Am. Chem. Soc., 2009, 131, 14329.
44 W. Holzer, A. Penzkofer and T. Tsuboi, Chem. Phys., 2005, 308,
93.
12 W. C. Chang, A. T. Hu, J. P. Duan, D. K. Rayabarapu and C. H. Cheng,
J. Organomet. Chem., 2004, 689, 4882.
13 (a) A. B. Tamayo, B. D. Alleyne, P. I. Djurovich, S. Lamansky, I. Tsyba,
N. N. Ho, R. Bau and M. E. Thompson, J. Am. Chem. Soc., 2003,
125, 7377; (b) C. H. Yang, S. W. Li, Y. Chi, Y. M. Cheng, Y. S. Yeh,
P. T. Chou, G. H. Lee, C. H. Wang and C. F. Su, Inorg. Chem., 2005,
44, 7770; (c) K. Dedeian, J. Shi, N. Shepherd, E. Forsythe and D. C.
Morton, Inorg. Chem., 2005, 44, 4445; (d) S. Bettington, M. Tavasli, M.
R. Bryce, A. Beeby, H. Al-Attar and A. P. Monkman, Chem.–Eur. J.,
2007, 13, 1423.
14 (a) M. Tavasli, S. Bettington, M. R. Bryce, H. A. Al Attar, F. B. Dias,
S. King and A. P. Monkman, J. Mater. Chem., 2005, 15, 4963; (b) C.
H. Yang, C. H. Chen and I. W. Sun, Polyhedron, 2006, 25, 2407.
15 (a) G. Zhou, C. L. Ho, W. Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin,
T. B. Marder and A. Beeby, Adv. Funct. Mater., 2008, 18, 499; (b) G.
J. Zhou, Q. Wang, W. Y. Wong, D. Ma, L. Wang and Z. Lin, J. Mater.
Chem., 2009, 19, 1872.
16 I. R. Laskar and T. M. Chen, Chem. Mater., 2004, 16, 111.
17 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I.
Tsyba, M. Bortz, B. Mui, R. Bau and M. E. Thompson, Inorg. Chem.,
2001, 40, 1704.
1 8 Y. Yo u a n d S. Y. P a r k , Dalton Trans., 2009, 1267.
19 (a) Md. K. Nazeeruddin, R. T. Wegh, Z. Zhou, C. Klein, Q. Wang, F.
De Angelis, S. Fantacci and M. Gra¨tzel, Inorg. Chem., 2006, 45, 9245;
(b) H. J. Bolink, E. Coronado, R. D. Costa, N. Lardie´s and E. Ort´ı,
Inorg. Chem., 2008, 47, 9149.
20 S. Bernhard, J. A. Barron, P. L. Houston, H. D. Abrunˇa, J. L. Ruglovksy,
X. Gao and G. G. Malliaras, J. Am. Chem. Soc., 2002, 124, 13624.
21 X. Wei, J. Peng, J. Cheng, M. Xie, Z. Lu, C. Li and Y. Cao, Adv. Funct.
Mater., 2007, 17, 3319.
22 T. Peng, H. Bi, Y. Liu, Y. Fan, H. Gao, Y. Wang and Z. Hou, J. Mater.
Chem., 2009, 19, 8072.
23 (a) Rigaku/MSC (2005). CrystalClear, Rigaku/MSC Inc., The Wood-
lands, Texas, USA; (b) Oxford Diffraction (2009). CrysAlisPro CCD
and CrysAlisPro RED, including ABSPACK. Oxford, Diffraction Ltd,
Yarnton, England.
45 E. Vanderdonckt, B. Camerman, F. Hendrick, R. Herne and R.
Vandeloise, Bull. Soc. Chim. Belg., 1994, 103, 207.
46 H. A. Bronstein, C. E. Finlayson, K. R. Kirov, R. H. Friend and C. K.
Williams, Organometallics, 2008, 27, 2980.
47 K. R. J. Thomas, M. Velusamy, J. T. Lin, C. H. Chien, Y. T. Tao, Y. S.
Wen, Y. H. Hu and P. T. Chou, Inorg. Chem., 2005, 44, 5677.
7164 | Dalton Trans., 2011, 40, 7153–7164
This journal is
The Royal Society of Chemistry 2011
©