Room-temperature NIR phosphorescence of new iridium (III) complexes with ligands derived from benzoquinoxaline

Article abstract of DOI:10.1139/V05-253

A new series of new iridium (III) complexes (1-5) bearing ligands derived from benzoquinoxaline were designed and synthesized. X-ray structural analyses of 1 reveal a distorted octahedral geometry around the Ir atom in which the pyrazolate chelate is located opposite to the cis-oriented carbon donor atoms of benzoquinoxaline, while the benzoquinoxaline ligands adopt an eclipse configuration and their coordinated nitrogen atoms and carbon adopt trans-and cis-orientation, respectively. Complexes 1-5 exhibit moderate NIR phosphorescence with peak maxima located at around 910-930 nm. As supported by the TDDFT approach, the transition mainly involves benzoquinoxaline ³π-π* intraligand charge transfer (ILCT) and metal (Ir) to benzoquinoxaline charge transfer (MLCT) of which the spectroscopy and dynamics of relaxation have been thoroughly investigated. The relatively weak NIR emission can be tentatively rationalized by the low energy gap of which the radiationless deactivation may be governed by nearly temperature-independent, weak-bonding motions in combination with a minor channel incorporating small torsional motions associated with phenyl ring in the benzoquinoxaline sites.

Full text of DOI:10.1139/V05-253

309
Room-temperature NIR phosphorescence of new
iridium (III) complexes with ligands derived from
benzoquinoxaline1
Hsing-Yi Chen, Cheng-Han Yang, Yun Chi, Yi-Ming Cheng, Yu-Shan Yeh,
Pi-Tai Chou, Hsi-Ying Hsieh, Chao-Shiuan Liu, Shie-Ming Peng, and
Gene-Hsiang Lee
Abstract: A new series of new iridium (III) complexes (1–5) bearing ligands derived from benzoquinoxaline were de-
signed and synthesized. X-ray structural analyses of 1 reveal a distorted octahedral geometry around the Ir atom in
which the pyrazolate chelate is located opposite to the cis-oriented carbon donor atoms of benzoquinoxaline, while the
benzoquinoxaline ligands adopt an eclipse configuration and their coordinated nitrogen atoms and carbon adopt trans-
and cis-orientation, respectively. Complexes 1–5 exhibit moderate NIR phosphorescence with peak maxima located at
3
around 910–930 nm. As supported by the TDDFT approach, the transition mainly involves benzoquinoxaline π–π*
intraligand charge transfer (ILCT) and metal (Ir) to benzoquinoxaline charge transfer (MLCT) of which the spectros-
copy and dynamics of relaxation have been thoroughly investigated. The relatively weak NIR emission can be tenta-
tively rationalized by the low energy gap of which the radiationless deactivation may be governed by nearly
temperature-independent, weak-bonding motions in combination with a minor channel incorporating small torsional mo-
tions associated with phenyl ring in the benzoquinoxaline sites.
Key words: phosphorescence, NIR, iridium, benzoquinoxaline, isoquinoline, bipyridine, pyrazolate, acetylacetonate.
Résumé : On a préparé une nouvelle série de complexes de l’iridium(III) (1–5) portant des ligands dérivés de la ben-
zoquinoxaline. L’analyse des données structurales du composé 1, obtenues par diffraction des rayons X, révèle la pré-
sence d’une géométrie octaédrique déformée autour de l’atome d’iridium dans laquelle le chélate pyrazolate se trouve à
l’opposé des atomes de carbone donneurs de la benzoquinoxaline orientés en cis, alors que les ligands benzoquinoxa-
line adoptent une conformation éclipsée et que leurs atomes d’azote coordonnés adoptent respectivement des orienta-
tions cis et trans. Les complexes 1–5 présentent de la phosphorescence modérée dans le proche infrarouge (PIR) avec
un pic maximal situé autour de 910 à 930 nm. Tel que le prédit l’approche TDDFT, la transition implique principale-
3
ment un transfert de charge interligand (TCIL) π–π* de la benzoquinoxaline et un transfert de charge du métal (iri-
dium) à la benzoquinoxaline (TCML) dont on a étudié à fond la spectroscopie et la dynamique de relaxation.
L’émission relativement faible dans le proche infrarouge pourrait être rationalisée en fonction d’une discontinuité de
faible énergie de laquelle la désactivation sans rayonnement pourrait être orientée par des mouvements de faible liaison,
pratiquement indépendants de la température, en combinaison avec une voie mineure qui incorporerait de faibles mou-
vements de torsion du noyau phényle dans les sites benzoquinoxalines.
Mots clés : phosphorescence, PIR, iridium, benzoquinoxaline, isoquinoléine, bipyridine, pyrazolate, acétylacétonate.
[Traduit par la Rédaction] Chen et al. 318
Received 30 June 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 21 March 2006.
This paper is dedicated to Professor Arthur J. Carty; he has been our mentor, collaborator, and friend for the past 15 years.
H.-Y. Chen, C.-H. Yang, and Y. Chi.2 Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan.
Y.-M. Cheng, Y.-S. Yeh, P.-T. Chou,3 S.-M. Peng, and G.-H. Lee. Department of Chemistry and Instrumentation Center,
National Taiwan University, Taipei 106, Taiwan.
H.-Y. Hsieh.4 Department of Food Nutrition, Chung Hwa College of Medical Technology, Tainan 717, Taiwan.
C.-S. Liu. Department of Chemistry, SooChow University, Taipei 111, Taiwan.
1This article is part of a Special Issue dedicated to Professor Arthur Carty.
2Corresponding author (e-mail: ychi@mx.nthu.edu.tw).
3Corresponding author (e-mail: chop@ntu.edu.tw).
4Corresponding author (e-mail: simn1628@ms26.hinet.net).
Can. J. Chem. 84: 309–318 (2006)
doi:10.1139/V05-253
© 2006 NRC Canada

310
Can. J. Chem. Vol. 84, 2006
line (bpiq)H, were prepared according to the literature meth-
ods (8). Commercially available chemicals were used with-
out further purification. All reactions were monitored by
TLC with Merck silica gel plates (0.20 mm with fluorescent
indicator UV254). Compounds were visualized with 254 or
365 nm UV irradiation. Column chromatography was car-
ried out using silica gel purchased from Merck (230–
400 mesh). Mass spectra were obtained on a JEOL SX-102A
instrument operating in electron impact (EI) or fast atom
Introduction
Transition metal complexes, which exhibit strong emis-
sion in fluid solution at room temperature, have been exten-
sively investigated owing to their fundamental interest and
key roles in the design of novel systems for solar energy
conversion (1), telecommunication and other optical technol-
ogies (2), as well as fabrication of high efficiency phospho-
rescent organic light emitting devices (OLEDs) (3). So far,
most of the reported luminescent metal complexes emit in
the visible region, while studies of emission properties in the
NIR region are relatively rare. One reason lies in that de-
creasing the energy gap toward NIR may trigger a much
faster radiationless decay channel because of the interaction
between ground and emitting states, i.e., the so-called en-
ergy gap law, giving rise to poor quantum efficiencies (4).
The first class of complexes showing notable NIR lumi-
nescence should be credited to the lanthanide complexes in-
corporating the Nd(III), Yb(III), and Er(III) ions (5). Sharp
emission signals for lanthanide complexes are expected sim-
ply because the 4f orbitals in the lanthanide elements are
buried deep within the atom and would not be seriously cou-
pled with the ligand vibration motions, as opposed to the
broad d–d transitions of the transition metals. As a second
class, the Pd porphyrinate types of complexes are docu-
mented as exhibiting moderate NIR phosphorescence in the
fluid state at room temperature, a property which is desirable
for application as oxygen sensors for biomedical applica-
tions in vivo (6). The third class consists of the polypyridyl
complexes involving the Re(I), Ru(II), and Os(II) metal ions
with the d6 electronic configuration (7). Their systematic
synthesis has remained as the outstanding issue in view of
applications. These complexes typically absorb appreciably
in the high energy region of the spectra, accompanied by the
MLCT absorptions at lower energy, as well as a certain de-
1
bombardment (FAB) mode. H and 13C NMR spectra were
recorded on a Varian INOVA-500 instrument; chemical
shifts are quoted with respect to the internal standard tetra-
1
methylsilane for H and 13C NMR data. Elemental analysis
was carried out with a Heraeus CHN-O rapid elementary an-
alyzer.
Cyclic voltammetry (CV)
CV measurements were performed using a BAS 100 B/W
electrochemical analyzer. The oxidation and reduction mea-
surements were recorded, respectively, in anhydr. CH2Cl2
and anhydr. THF containing 0.1 mol/L TBAPF6 as the sup-
porting electrolyte, at a scan rate of 100 mV s–1. The poten-
tials were measured against a Ag/Ag+ (0.01 mol/L AgNO3)
reference electrode with ferrocene as the internal standard.
Preparation of 2,3-diphenyl benzo[g]quinoxaline
An anhydrous ethanol solution (30 mL) of 2,3-naphthal-
enediamine (1.0 g 6.33 mmol) and benzil (1.34 g,
6.33 mmol) was heated to reflux for 5 h. Then the solution
was gradually cooled to room temperature. The solid depos-
ited from solution was then filtered, washed with a few
drops of ethanol, and dried in vacuum overnight to afford
1.8 g of yellow-green 2,3-diphenyl benzo[g]quinoxaline
(5.42 mmol, 86%), denoted as (dpbq)H. The yellow-colored
derivative 2,3-di-p-tolyl benzo[g]quinoxaline, (dtbq)H, was
obtained from 4,4′-dimethylbenzil in a similar fashion.
3
gree of contribution from direct MLCT absorption, a result
of the heavy atom enhanced spin-orbit coupling. Accord-
ingly, NIR phosphorescent emissions are achievable for sys-
tems with extended electronic delocalization between the
metal and ligands in both ground and excited states, making
them important as light harvesting sensitizers for solar cell
applications.
As an extension of our contribution to the third-row metal
phosphorescent materials, we have embarked on the devel-
opment of Ir(III)-based metal complexes showing detectable
luminescence in the NIR spectral region. The key designing
feature is to utilize a diphenyl substituted benzoquinoxaline
ligand that allows formation of a highly extended π conjuga-
tion. The choice of Ir as the central atom renders the advan-
tages of linking the benzoquinoxaline ligand through facile
orthometalation, affording a desirable metal–chelate bonding
interaction and a presumably rigid molecular skeleton. Along
this line, the emission gaps are expected to shift significantly
toward the lower energy, i.e., the NIR region, without drastic
reduction to the corresponding emission efficiency.
Spectral data for (dpbq)H
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.72 (s, 2H), 8.07
(dd, 2H, JHH = 6.4, 2.8 Hz), 7.56 (d, 4H, JHH = 7.2 Hz), 7.52
(dd, 2H, JHH = 6.4, 2.8 Hz), 7.38–7.32 (m, 6H). MS (EI)
m/z: 332 [M+]. Anal. calcd. for C24H16N2: N 8.43, C 86.72,
H 4.85; found: N 8.48, C 86.63, H 5.05.
Spectral data for (dtbq)H
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.69 (s, 2H), 8.09–
8.06 (m, 2H), 7.54–7.51 (m, 2H), 7.46 (d, 4H, JHH = 10 Hz),
7.15 (d, 4H, JHH = 10 Hz), 2.37 (s, 6H). MS (EI) m/z: 360
[M+]. Anal. calcd. for C26H20N2: N 7.77, C 86.64, H 5.59;
found: N 7.41, C 86.65, H 5.26.
Preparation of [(dpbq)2IrCl]2
The chloride-bridged complex [(dpbq)2IrCl]2 was synthe-
sized according to the method reported by Nonoyama (9),
involving treatment of IrCl3·3H2O (1.0 g, 2.84 mmol) with
(dpbq)H (2.83 g, 8.5 mmol) in 2-methoxyethanol (30 mL)
for 24 h. Then the solution was treated with water (30 mL)
to induce precipitation of a dark brown solid. The solid was
then filtered, washed with methanol, and dried under vac-
uum (1.8 g, 1.22 mmol, 71%).
Experimental section
General experiments
The pyrazole ligands, 3-trifluoromethyl-5-(2-pyridyl)pyra-
zole (fppz)H and 1-(5-tert-butyl-2H-pyrazol-3-yl)isoquino-
© 2006 NRC Canada

Chen et al.
311
(m, 8H), 7.68–7.64 (m, 8H), 7.49 (t, 2H, JHH = 3.6 Hz), 7.40
(t, 2H, JHH = 7.6 Hz), 7.03 (d, 2H, JHH = 8.0 Hz), 6.75 (t,
2H, JHH = 7.6 Hz), 6.63 (t, 2H, JHH = 7.2 Hz), 6.54 (d, 2H,
JHH = 7.6 Hz). MS (FAB, 193Ir) m/z: 1011 [M+]. Anal. calcd.
for C58H38F4IrN6: N 7.65, C 63.45, H 3.49; found: N 7.71, C
63.25, H 3.57.
Preparation of [(dtbq)2IrCl]2
On the basis of a procedure similar to that described for
[(dpbq)2IrCl]2, the p-tolyl-substituted complex [(dtbq)2IrCl]2
was synthesized in ~75% yield.
Preparation of [(dpbq)2Ir(fppz)] (1)
A mixture of [(dpbq)2IrCl]2 (300 mg, 0.168 mmol), 3-
trifluoromethyl-5-(2-pyridyl)pyrazole (fppzH, 80 mg,
0.38 mmol), and Na2CO3 (100 mg, 0.94 mmol) in 2-
methoxyethanol (20 mL) was heated to reflux for 4 h. An
excess of deionized water was added after cooling the solu-
tion to room temperature (RT). The precipitate was collected
by filtration and washed with methanol (10 mL), followed
by diethyl ether (10 mL). Black crystals of [(dpbq)2Ir(fppz)]
(1) were obtained from a mixture of CH2Cl2 and methanol at
RT (210 mg, 0.19 mmol, 58%).
Preparation of [(dtbq)2Ir(fppz)] (4)
Procedures identical with that of 1 were followed, using
200 mg of [(dtbq)2IrCl]2 (0.106 mmol), 113 mg of (fppz)H
(0.53 mmol), and 100 mg of Na2CO3 (0.94 mmol) in 20 mL
of 2-methoxyethanol. Black crystals of [(dtpz)2Ir(fppz)] (4)
were obtained from a mixture of CH2Cl2 and methanol at RT
(120 mg, 0.107 mmol, 50%).
Spectral data for 4
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.69 (s, 1H), 8.57
(d, 1H, JHH = 8.0 Hz), 8.49 (s, 1H), 8.44 (s, 1H), 7.98 (s,
1H), 7.91 (d, 1H, JHH = 8.0 Hz), 7.86 (d, 1H, JHH = 8.0 Hz),
7.81 (d, 2H, JHH = 5.2 Hz), 7.72 (d, 1H, JHH = 8.0 Hz), 7.54
Spectral data for 1
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.77 (s, 1H), 8.64
(d, 1H, JHH = 5.6 Hz), 8.54 (s, 1H), 8.49 (s, 1H), 8.05 (s,
1H), 7.94–7.88 (m, 4H), 7.75 (d, 1H, JHH = 8.0 Hz), 7.72–
7.61 (m, 5H), 7.61–7.55 (m, 3H), 7.45–7.38 (m, 4H), 7.35 (t,
1H, JHH = 6.0 Hz), 7.27–7.32 (m, 2H), 7.25–7.20 (m, 3H),
6.78 (d, 1H, JHH = 7.6 Hz), 6.72 (t, 1H, JHH = 8.0 Hz), 6.65
(t, 1H, JHH = 7.6 Hz), 6.60 (t, 1H, JHH = 7.2 Hz), 6.50–6.47
(m, 2H), 6.36 (d, 1H, JHH = 7.2 Hz). MS (FAB, 193Ir) m/z:
1067 [M+]. Anal. calcd. for C57H35F3IrN7: N 9.19, C 64.15,
H 3.31; found: N 9.09, C 64.21, H 3.62.
(t, 1H, JHH = 1.2 Hz), 7.50–7.41 (m, 5H), 7.40 (t, 3H, JHH
=
1.2 Hz), 7.31–7.24 (m, 4H), 7.20–7.12 (m, 3H), 6.63 (s, 1H),
6.57 (d, 1H, JHH = 8.0 Hz), 6.48 (d, 1H, JHH = 8.0 Hz), 6.44
(s, 1H), 6.20 (s, 1H), 2.54 (s, 3H), 2.51 (s, 3H), 1.86 (s, 3H),
1.79 (s, 3H). MS (FAB, 193Ir) m/z: 1123 [M+]. Anal. calcd.
for C61H43F3IrN7: N 8.73, C 65.23, H 3.86; found: N 8.64, C
64.89, H 3.81.
Preparation of [(dtbq)2Ir(acac)] (5)
A mixture of [(dtbq)2IrCl]2 (200 mg, 0.106 mmol),
acetylacetone (acacH, 300 mg, 3.0 mmol), and Na2CO3
(100 mg, 0.94 mmol) in CH2Cl2 (20 mL) was stirred at RT
for 24 h. After stopping the reaction, the solution was
washed with water (3 × 50 mL), dried over MgSO4, and con-
centrated to dryness. Black crystals of [(dtbq)2Ir(acac)] (5)
were recrystallized from CH2Cl2 and hexane at RT (90 mg,
0.89 mmol, 42%).
Preparation of [(dpbq)2Ir(bpiq)] (2)
Procedures identical to that of 1 were followed, using
200 mg of [(dpbq)2IrCl]2 (0.112 mmol), 71 mg of (bpiq)H
(0.28 mmol), and 60 mg of Na2CO3 (0.57 mmol) in 20 mL
of 2-methoxyethanol. Black crystals of [(dpbq)2Ir(bpiq)] (2)
were obtained from a mixture of CH2Cl2 and hexane at RT
(74 mg, 0.085 mmol, 33%).
Spectral data for 2
1H NMR (400 MHz, CDCl3, 298 K) δ: 9.00 (s, 1H), 8.50–
Spectral data for 5
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.84 (s, 2H), 8.65
(s, 2H), 8.03 (d, 2H, JHH = 8.0 Hz), 7.94–7.89 (m, 6H),
7.49–7.42 (m, 8H), 7.06 (d, 2H, JHH = 8.4 Hz), 6.41 (d, 2H,
JHH = 8.4 Hz), 6.32 (s, 2H), 4.63 (s, 1H), 2.52 (s, 6H), 1.79
(s, 6H), 1.61 (s, 6H). MS (FAB, 193Ir) m/z: 1010 [M+]. Anal.
calcd. for C57H45IrN4O2: N 5.55, C 67.77, H 4.49; found: N
5.51, C 67.63, H 4.83.
8.43 (m, 5H), 8.30 (s, 1H), 7.97 (b, 2H), 7.87 (d, 2H, JHH
8.0 Hz), 7.82 (d, 1H, JHH = 8.0 Hz), 7.72 (d, 1H, JHH
=
=
8.0 Hz), 7.69–7.58 (m, 7H), 7.52 (t, 1H, JHH = 7.2 Hz),
7.44–7.28 (m, 4H), 7.26–7.18 (m, 5H), 6.74–6.66 (m, 2H),
6.64–6.53 (m, 3H), 6.45 (t, 1H, JHH = 7.6 Hz), 6.31 (d, 1H,
JHH = 8.0 Hz), 1.37 (s, 9H). MS (FAB, 193Ir) m/z: 1105
[M+]. Anal. calcd. for C64H46F3IrN7: N 8.87, C 69.54, H
4.19; found: N 8.79, C 69.48, H 4.56.
X-ray structural analysis
Single crystal X-ray diffraction data were measured on a
Bruker Smart CCD diffractometer using (Mo Kα) radiation
(λ = 0.710 73 Å). The data collection was executed using the
SMART program. Cell refinement and data reduction were
made with the SAINT program. The structure was deter-
mined using the SHELXTL/PC program and refined using
full-matrix least-squares. All non-hydrogen atoms were re-
fined anisotropically, whereas hydrogen atoms were placed
at the calculated positions and included in the final stage of
refinements with fixed parameters. The crystallographic re-
finement parameters for complexes 1 and 3 are summarized
in Table 1, and selected bond distances and angles of these
complexes are listed in Tables 2 and 3, respectively.
Preparation of [(dpbq)2Ir(bpy)] (3)
A mixture of [(dpbq)2IrCl]2 (200 mg, 0.11 mmol), 2,2′-
bipyridine (bpy, 45 mg, 0.29 mmol), and AgBF4 (100 mg,
0.51 mmol) in CH2Cl2 (25 mL) was stirred at RT for 3 h.
The solution was filtered and the filtrate was evaporated to
give the crude product. Black crystals of [(dpbq)2Ir(bpy)][BF4]
(3) were obtained from a mixture of CH2Cl2 and hexane at
RT (110 mg, 0.09 mmol, 41%).
Spectral data for 3
1H NMR (400 MHz, CDCl3, 298 K) δ: 8.71 (d, 2H, JHH
=
4.8 Hz), 8.60 (s, 2H), 8.55 (d, 2H, JHH = 8.8 Hz), 8.20 (t,
2H, JHH = 7.6 Hz), 7.96 (d, 2H, JHH = 8.0 Hz), 7.86–7.80
© 2006 NRC Canada

312
Can. J. Chem. Vol. 84, 2006
Table 1. Crystal data and refinement parameters for complexes 1 and 3.
Compound
Formula
1
3
C57H35F3N7Ir·CH2Cl2
C58H38N6Ir·BF4·CH2Cl2
Molecular weight
Crystal system
Space group
Crystal size (mm3)
a (Å)
1 152.05
Triclinic
P-1
1 182.88
Triclinic
P-1
0.4×0.15×0.12
11.9568(4)
12.0739(5)
17.1667(7)
74.820(1)
82.535(1)
84.788(1)
2 367.4(2)
2
0.18×0.15×0.05
16.3626(6)
17.2949(6)
19.5430(7)
73.631(1)
72.197(1)
70.003(1)
4 850.1(3)
4
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (g cm–3)
1.616
1.620
Temp (K)
295(2)
2.994
31 690
10 878 (R(int) = 0.0419)
0.0445, 0.0928
150(1)
2.927
64 361
22 242 (R(int) = 0.0680)
0.0873, 0.1475
µ (mm–1)
Reflections collected
Independent reflections
RF, Rw(F2) (all data)
RF, Rw(F2) [I > 2σ(I)]
GOF
0.0360, 0.0860
0.0606, 0.1319
1.144
1.043
2
4
Note: RF = Σ||Fo – Fc||/Σ|Fo| and Rw(F2) = [Σw|Fo – Fc2|2/Σw(Fo )]1/2
.
Table 2. Selected bond lengths (Å) and angles
(°) for complex 1.
Table 3. Selected bond lengths (Å) and angles
(°) for complex 3.
Bond lengths (Å)
Bond lengths (Å)
Ir—N(1)
Ir—N(4)
Ir—C(10)
C(25)···N(2)
C(49)···N(1)
Ir—N(2)
2.201(3)
2.065(3)
1.990(4)
3.013(7)
3.165(6)
2.116(3)
2.047(3)
1.991(4)
3.381(6)
3.439(6)
Ir(1)—N(1)
Ir(1)—N(5)
Ir(1)—C(1)
C(23)···N(6)
C(47)···N(5)
Ir(1)—N(3)
Ir(1)—N(6)
Ir(1)—C(25)
C(23)···C(58)
C(47)···C(49)
2.062(6)
2.171(6)
1.992(7)
2.995(8)
3.146(9)
2.076(5)
2.160(6)
1.999(6)
3.336(8)
3.438(8)
Ir—N(6)
Ir—C(34)
C(25)···N(3)
C(49)···C(1)
Bond angles (°)
N(1)-Ir-N(2)
N(6)-Ir-C(34)
N(2)-Ir-C(10)
N(4)-Ir-C(10)
N(1)-Ir-C(34)
N(4)-Ir-N(6)
Bond angles (°)
N(1)-Ir(1)-C(1)
N(5)-Ir(1)-N(6)
N(6)-Ir(1)-C(1)
N(3)-Ir(1)-C(25)
N(1)-Ir(1)-N(3)
N(5)-Ir(1)-C(25)
75.02(13)
79.47(15)
174.19(14)
79.21(15)
170.85(14)
178.16(12)
79.4(3)
75.2(2)
171.7(2)
79.3(2)
173.7(2)
174.9(2)
The crystallographic data for complexes 1 and 3 (exclud-
corded with a Hitachi (U-3310) spectrophotometer. The NIR
emission spectra were acquired by exciting the sample solu-
tion under a rectangular excitation configuration using an Ar
ion laser (488 or 514 nm, Innova 90, Coherent Inc., Santa
Clara, California) and the continuous wave was chopped by
ing structure factors) have been deposited.5
Spectral and dynamic measurement
Steady-state absorption and emission spectra were re-
5 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 5002. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 276875 and 276876 contain the crystal-
lographic data for this manuscript. These data can be obtained, free of charge, via http://www.ccdc.cam.ac.uk/conts/retrieving.html (Or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or de-
posit@ccdc.cam.ac.uk).
© 2006 NRC Canada

Chen et al.
313
Scheme 1.
a mechanic chopper operated at 200 Hz. To minimize the re-
absorption effect, the excitation beam was focused as closely
as possible to the front wall of the quartz cell. The NIR
emission was measured by the photomultiplier tube
(Hamamatzu R5509-72, operated at –80 °C) coupled with a
Lock-in amplifier (SR830, Stanford Research Systems,
Sunnyvale, California) and a monochromator (SpectraPro-
275, Acton Research Corporation, Acton, Massachusetts) in
that the grating was blazed at 1200 nm. IR125 (Exciton Inc.,
Dayton, Ohio) with Φ ~ 0.11 (λmax ~ 925 nm) in DMSO so-
lution was used as a standard to calculate the emission quan-
tum yield (10). For the NIR lifetime measurement, the
sample was excited by a 532 nm pulse (2nd harmonic of
Nd:YAG, 6 ns). The emission trace was detected by a photo-
multiplier tube (Hamamatzu R5509-72) operated at –80 °C
and recorded by a sampling oscilloscope (TDS 3012,
Tektronix, Beaverton, Oregon). An average of 100 shots was
taken in the lifetime measurement, unless specified else-
where.
R
R
R
O
O
NH2
NH2
N
N
+
R
R = H; (dpbq)H
R = Me; (dtbq)H
Scheme 2.
N
N
N
N
N
N
Ir
Ir
N
N
CF3
N
But
N
Computational methodology
2
R
R
Calculations on the electronic ground state of complexes 1
and 3 were carried out using B3LYP density functional the-
ory (11, 12). A double-ζ quality basis set consisting of Hay
and Wadt’s effective core potentials (LANL2DZ) (13) was
employed for the Ir atom and a 6-31G* (14) basis for the H,
C, N, and F atoms. A relativistic effective core potential
(ECP) replaced the inner core electrons of Ir(III), leaving the
outer core (5s25p6) electrons and the 5d6 valence electrons.
Time-dependent DFT (TDDFT) calculations (15) using the
B3LYP functional were then performed based on the struc-
tural optimized geometries. Typically, the lowest 10 triplet
and 10 singlet roots of the nonhermitian eigenvalue equa-
tions were obtained to determine the vertical excitation ener-
gies. Oscillator strengths were deduced from the dipole
transition matrix elements (for singlet states only). The
ground-state B3LYP and excited-state TDDFT calculations
were carried out using Gaussian03 (16).
2
(2)
(1), R = H
(4), R = Me
-
BF4
N
O
N
N
N
N
Ir
Ir
O
N
2
2
(3)
(5)
bipyridine (bpy), and acetylacetone (acacH) yielded the cor-
responding Ir(III) complexes, [(dpbq)2Ir(bpiq)] (2),
[(dpbq)2Ir(bpy)][BF4] (3), and [(dtbq)2Ir(acac)] (5), respec-
tively (see Scheme 2).
Results and discussion
All iridium complexes were easily soluble in organic sol-
vents such as CH2Cl2, THF, and acetone, and the corre-
sponding solutions showed negligible decomposition for a
period of over 24 h, upon exposure to air. Routine character-
izations were conducted using microanalysis, mass spec-
trometry, and 1H NMR spectroscopy. Moreover, the fppz
complex 1 was further characterized by single crystal X-ray
diffraction analysis. Its ORTEP diagram is depicted in
Fig. 1, while crucial bond distances and angles are listed in
Table 1.
As depicted in Fig. 1, complex 1 reveals a distorted octa-
hedral geometry around the Ir atom, consisting of two cyclo-
metalated phenylbenzoquinoxaline fragments and one (2-
pyridyl) pyrazolate ligand. The unique pyrazolate chelate is
located opposite to the cis-oriented carbon donor atoms of
benzoquinoxaline, while the benzoquinoxaline ligands adopt
an eclipse configuration and their coordinated nitrogen at-
oms (N(4) and N(6)) and carbon atoms (C(10) and C(34))
adopt trans and cis orientation, respectively. This coordin-
ative arrangement is akin to those of the parent phenyl
pyridine ligands observed for the chloride-bridged dimer
Syntheses and structural characterization
The preparation of highly conjugated ligand chromophores
is necessary to achieve the NIR-emitting iridium complexes.
As depicted in Scheme 1, the required ligands can be syn-
thesized by treatment of 2,3-naphthalenediamine and
dibenzil to produce 2,3-diphenyl-or di-p-tolyl-substituted
benzo[g]quinoxaline in good yields. This synthetic approach
follows the literature procedure utilized for the analogous
diphenylquinoxaline, and is best prepared from related o-
phenylenediamine and the corresponding α-diketone in
refluxing ethanol (17).
Subsequent reactions of IrCl3·nH2O with (dpbq)H or
(dtbq)H in refluxing ethoxyethanol afforded chloride-bridged
dimers [(dpbq)2IrCl]2 or [(dtbq)2IrCl]2, respectively. These
dimer complexes can easily react with the pyrazole chelate
ligand (fppz)H in the presence of Na2CO3 to afford black-
colored complexes [(dpbq)2Ir(fppz)] (1) and [(dtbq)2Ir(fppz)]
(4), where (fppz)H = 3-trifluoromethyl-5-(2-pyridyl)pyrazole.
Similar reactions towards other bidentate chelates such as
1-(5-tert-butyl-2H-pyrazol-3-yl)isoquinoline ((bpiq)H), 2,2Ј-
© 2006 NRC Canada

314
Can. J. Chem. Vol. 84, 2006
Fig. 1. (a) ORTEP diagram of 1 with thermal ellipsoids shown at the 30% probability level; fluorine atoms of the CF3 substituent
were omitted for clarity. (b) Same ORTEP diagram with an emphasis on the spatial distortion of dpbq ligands.
(b)
(a)
complex [(ppy)2Ir(µ-Cl)]2 (18), the diketonate complex
(ppy)2Ir(acac) (19), and even the pyrazolate derivatives, sug-
gesting that the incoming fppz ligand occupies the positions
of the bridging chloride ligands in its starting material,
[(dpbp)2IrCl]2. Moreover, the uncoordinated and cyclo-
metalated phenyl groups show twisting angles of 40°–54°,
as well as unusually large deformation angles of 23°–28°
with respect to the middle benzoquinoxaline fragments. The
deformation angles reveal an unfavorable steric interaction
between the chelating ligands and can be gauged by the
least-squares calculation using planes defined by the
cyclometalated phenyl group and the middle C6 hexagon of
the benzoquinoxaline. It appears to us that the nonbonding
repulsion between benzoquinoxaline and pyridyl pyrazolate
ligands is the main driving force causing such a large distor-
tion at the cyclometalated phenyl groups, as most of the
interligand C···C and C···N nonbonding contacts listed in Ta-
ble 2 are shorter than a limited value of ca. 3.4 Å, i.e., the
sum of the van der Waals radii of the carbon atoms.
is believed that the oxidation process mainly occurred at the
metal center and, in part, with a slight contribution from the
surrounding chelate ligands (20). Thus, the formal oxidation
potential was strongly dependent on the electronic environ-
ment of the Ir(III) cation, i.e., a better electron donor would
shift the oxidation potential to a less positive value. This hy-
pothesis was verified by the increased π conjugation of the
anionic bpiq ligand in 2, which decreased the oxidative po-
tential from 0.77 (in 1) to 0.61 V (in 2), while the attach-
ment of the 2,2′-bipyridine ligand in 3 led to the complete
suppression of the metal oxidation signal owing to its
cationic nature. Moreover, replacement of the phenyl sub-
stituents with tolyl substituents of benzoquinoxaline also
slightly altered the oxidation potential, as supported by the
fact that the oxidation potential of 4 is cathodically shifted
to 0.70 V in comparison with that of the parent complex 1.
Owing to the presence of an extra methyl substituent, this
result could possibly be attributed to a slightly better σ-do-
nor ability of the cyclometalated tolyl group.
For a further comparison, the X-ray structural study on
the cationic dipyridine complex 3 was also examined. Com-
parison of the ORTEP diagram (Fig. 2) and the metric pa-
rameters (Table 3) with those of 1 shows no significant
variation on the overall molecular architecture, except for
the formation of an approximate C2 rotational axis that bi-
sects the bipyridine ligand and passes through the central Ir
metal atom. This increasing molecular symmetry is consis-
Upon switching to the cathodic sweep in THF, two revers-
ible reduction processes, with potentials ranging from –1.33
to –1.96 V, were detected for all complexes (1–5). We as-
signed these signals to a sequential reduction of the cyclo-
metalated benzoquinoxaline fragments. For dtbq complex 4,
the first reduction peak is shifted to –1.65 V in comparison
with that of the dpbq counterpart 1 (–1.54 V) because of the
electron-releasing effect of the para-methyl substituents.
Conversely, complex 3 showed the lowest reduction poten-
tials, owing to its cationic nature, while complexes 2 and 5
gave the highest reduction potential among the whole series.
The results indicate that the third ancillary ligand also sig-
nificantly affects the redox properties of these Ir complexes,
possibly via their enhanced donor capability as well as the
extended π conjugation. Qualitatively, this observation is
consistent with both the photophysical properties and theo-
retical predictions elaborated as follows.
1
tent with the simplification of the H NMR spectral pattern
observed in solution at RT (see the Experimental section).
Electrochemistry
The electrochemical properties of these Ir complexes were
investigated by CV using ferrocene as the internal standard,
and the respective redox data are listed in Table 4. During
the anodic scan, all iridium complexes exhibited a reversible
oxidation wave with potentials in the region 0.50–0.77 V. It
© 2006 NRC Canada

Chen et al.
315
Fig. 2. (a) ORTEP diagram of 3 with thermal ellipsoids shown at the 50% probability level. (b) Same ORTEP diagram with an empha-
sis on the spatial distortion of dpbq ligands.
(a)
(b)
Fig. 3. The absorption and emission spectra of complexes 1–5 in
CH2Cl2. The peak intensities were normalized. λex = 514 nm.
Table 4. One-electron redox potentials of complexes 1–5.
E1/2 (V)a
E1/2 (V)a
ox
red
1
–
The arrow indicates the peak for O2 1∆g(v′ = 0) → Σg (v′′ = 0)
Complex
1
2
3
4
5
0.77 [100]b
0.61 [80]
—
0.70 [100]
0.50 [90]
–1.54 [110], –1.81 [100]
–1.64 [100], –1.91 [100]
–1.31 [80], –1.61 [80]
–1.65 [110], –1.91 [120]
–1.73 [90], –1.96 [100]
1276 nm emission.
40 000
1
2
3
aPotential values referenced vs. Fc/Fc+.
4
5
30 000
20 000
10 000
0
bData in square bracket denotes the |Epa – Epc| values in mV.
Photophysical properties
Figure 3 shows the absorption and emission spectra of
complexes 1–5 in CH2Cl2. As supported by CV measure-
ments as well as the frontier orbital analyses (see the follow-
ing section), it is reasonable to assign the first S0–S1
transition to absorption peaks around 500–600 nm (ε ~ 5 ×
103 (mol/L)–1 cm–1) to a metal to ligand (benzoquinoxaline
part) charge transfer (MLCT) overlapping with the inter-
ligand phenyl to benzoquinoxaline transition. Due to a large
molecular framework, the composition of higher electronic
states is complicated, so that any corresponding assignments
for complexes 1–5 may not be meaningful. Furthermore, the
Ir-metal-enhanced spin-orbit coupling should result in
nonnegligible absorptivity for the triplet transition, as indi-
cated by a corresponding small absorption hump around
650–700 nm. However, absorption bands ascribed to the
triplet manifolds could not be distinctly resolved mainly ow-
ing to their hiding inside the UV region of the strong singlet
transitions. Detailed discussion regarding the transition in
the triplet manifold will be given in the Theoretical approach
section.
400
600
800
1000
1200
1400
Wavelength (nm)
negligible aggregation effect in fluid solution. To reveal any
differences in the spectral profile, the emission spectra were
normalized at the peak wavelength. In comparison, for all
complexes studied, only a slight variation was observed in
view of the emission spectral feature and peak position. As
listed in Table 5, the relaxation dynamics of 1–5 are O2 con-
centration dependent. For example, the decay time of com-
plex 1 was measured to be 286 ns in the degassed CH2Cl2,
while it was decreased to 200 ns upon aeration. In the O2 de-
pendent study, in which the O2 concentration was monitored
by its vapor pressure filled in the vacuum line and deduced
by Henry’s law, a straight Stern–Volmer plot could be for-
mulated (not shown here), and an oxygen-quenching rate
constant of 1.7 × 109 (mol/L)–1 s–1 was deduced, which is
nearly one-nineth of the diffusion-controlled rate of 2.0 ×
1010 (mol/L)–1 s–1 calculated from the Stokes–Einstein equa-
Figure 3 also depicts the emission spectra of complexes
1–5 in CH2Cl2. The spectral feature, quantum efficiency and
relaxation dynamics (vide infra) of complexes 1–5 show
concentration independence up to 10–3 mol/L, indicating a
© 2006 NRC Canada

316
Can. J. Chem. Vol. 84, 2006
Table 5. Photophysical properties of complexes 1–5 in CH2Cl2 at room temperature.
Complex
λ
abs (nm) (ε, 103 (mol/L)–1 cm–1)
λem (nm)
Φaer (×103)a
τaer (ns)
τ
deg (ns)
6.48
9.53
3.59
10.2
21.8
1
2
3
4
5
535 (7), 491 (10), 414 (29)
575 (5), 464 (10), 414 (27)
540 (5), 484 (9), 412 (26)
541 (9), 491 (13), 422 (40)
570 (7), 508 (10), 421 (31)
916
922
922
916
904
200
195
245
209
216
286
276
403
302
290
aQuantum efficiencies were measured in aerated CH2Cl2.
Fig. 4. HOMO and LUMO of complex 1 involved in the low-lying transition.
HOMO
LUMO
Table 6. The calculated lowest excited singlet and triplet states of complex 1.
State
Assignment
λ (nm)
E (eV)
f
S1
T1
HOMO → LUMO (+95%)
HOMO–1 → LUMO (+42%)
HOMO → LUMO (+37%)
HOMO–1 → LUMO+1 (23%)
HOMO–2 → LUMO (11%)
618.3
743.6
2.01
1.67
0.0067
-0
tion in CH2Cl2. The result is consistent with the O2-quench-
ing triplet state according to the theory of electron-exchange
type (Dexter type) of energy transfer expressed as (21):
vide a clue that the observed phosphorescence possesses, in
3
part, a π–π* character plausibly originating from a ligand-
center chromophore of long conjugation such as
benzoquinoxaline. Nevertheless, the involvement of other
kO
3
1
2
T + O2 ⎯⎯⎯→ S0 + O2
3
transitions such as MLCT is also possible. Furthermore, the
The overall spin must be conserved upon forming a
collisional complex. Accordingly, the possibility of each col-
lision producing O2 is statistically one-nineth. Perhaps the
observation of rather small changes in spectral features, as
well as peak positions, implies that frontier orbitals respon-
sible for the T1–S0 transition may be mainly derived from
the same origin for complexes 1–5. It is thus of fundamental
importance to gain more detailed insights into the transition
properties of the phosphorescence based on the theoretical
approaches elaborated as follows.
1
strongest support for the O2 quenching is given by the obser-
1
1
vation of O2 (1∆g (v′′ = 0) → Σ– (v′ = 0) emission with a
peak wavelength at ~1276 nm (22) gfor complexes 1–5 in aer-
ated CH2Cl2 (see the arrow in Fig. 3). The results, in combi-
nation with the rather small radiative decay rate of <106 s–1,
lead us to unambiguously assign the dominant emission in
complexes 1–5 to be of phosphorescence character. The
spectra show a distinctly 0–0 vibronic onset of the transition,
Theoretical approach
Theoretical confirmation of the photophysical properties
of complex 1 was provided by the DFT calculations. In this
study, the crystallographic data for 1 was used as the frame-
work for the extended Hückel formalism, and the calculation
3
a profile that normally appears in a π–π* transition of aro-
matic chromophores with high symmetry. The results pro-
© 2006 NRC Canada

Chen et al.
317
Fig. 5. The luminescence spectrum of 1 at room temperature (᭿)
and in the 77 K CH2Cl2 matrix (∆). Note that both spectra were
analyzed using the same absorbance and excitation wavelength
(514 nm). Thus, the intensity ratio can be directly estimated
from the spectral integration.
was performed via a CACAO program (see the Experimen-
tal section). Figure 4 depicts the features of the HOMO to
HOMO–3 and LUMO to LUMO+3 frontier orbitals mainly
involved in the lower-lying transition, while the descriptions
and the energy gaps of each transition are listed in Table 6.
The validity of the TDDFT approach can be readily per-
ceived by the calculated T1 → S0 emission peak at 745 nm,
which is very close to the 0–0 onset peak (~785 nm) ob-
served experimentally. Based on the results (see Fig. 4 and
Table 6), the electron densities of the HOMO are mainly lo-
cated on the Ir(III) metal center and, to a small extent, the
cyclometalated phenyl ligand, while a large portion of the
electron densities of HOMO–1 and HOMO–2 is attributed to
the π orbitals of benzoquinoxaline overlapping with certain
dπ orbitals of the Ir(III) atom. Conversely, the electron den-
sities of LUMO and LUMO+1 are distributed mostly on the
benzoquinoxaline moiety. The S0 → S1 and S0 → S2 transi-
tions mainly incorporate HOMO → LUMO (+95%) and
HOMO → LUMO+1 (+93%), respectively, and thus are
dominated by MLCT (dπ (Ir) → π* (benzoquinoxaline))
overlapping with π–π* ILCT (mainly cyclometalated phenyl
site (π) → benzoquinoxaline pyridine site (π*)) in character.
As supported by the data shown in Table 6, the higher ex-
cited states in the singlet manifold, S0 → Sn (n > 3), are rela-
tively much more complicated, the results of which make a
definitive frontier orbital assignment meaningless. In con-
trast, the lowest electronic transition in the triplet manifold,
i.e., S0 → T1, mainly incorporates the ligand center
8
7
6
5
4
3
2
1
0
700 800 900 1000 1100 1200 1300 1400
Wavelength (nm)
ics of which should be temperature dependent, partly induces
the radiationless transition. However, one ought to realize
that because of the much smaller NIR energy gap, excitation
that causes any weakening of the bond strength may result in
crossing of the potential energy surface between T1 and S0
states and eventually trigger the rapid radiationless deactiva-
tion. In the case of complexes 1–5, these nearly temperature-
independent, weak vibrational motions play a key role in ac-
counting for the dominant radiationless deactivation.
3
3
benzoquinoxaline π–π* transition and MLCT (dπ (Ir) → π*
(benzoquinoxaline)).
The contribution of the π–π* transition is manifested by
3
the low-temperature (77 K) approach to complex 1. Figure 5
shows the luminescence spectrum of 1 in a 77 K CH2Cl2
matrix. Two salient remarks can be pointed out promptly ac-
cording the spectral layout. First, in comparison to that mea-
sured at 298 K, the enhancement of the vibronic progression
at 77 K is obvious, showing the notable population switch-
Conclusion
3
3
ing from perhaps MLCT/3π–π mixing character to the π–π
dominating state under this condition. Nevertheless, one
cannot eliminate the possibility that the increase in vibronic
resolution upon cooling may just indicate an increase in ri-
gidity of the molecule, making the vibrational levels more
identical to one another, as well as the disappearance of hot
bands. Secondly, the phosphorescence intensity at 77 K is
increased ~2.5-fold with respect to that at 298 K. Similar
structural phosphorescence with 2- to ~3-fold increases in
intensity was observed for complexes 2–5 in 77 K CH2Cl2.
Thus, temperature-dependent radiationless deactivation path-
ways may only play a minor role to the relaxation dynamics.
Moreover, as listed in Table 5, substitution at non-benzo-
quinoxaline sites causes neither spectral change nor large al-
ternation of the quantum efficiency. The latter implies that
the large amplitude vibration or any torsional motion at non-
benzoquinoxaline sites, which generally quenches emission
in the NIR region, are not actively involved in the deactiva-
tion process. This viewpoint can be rationalized by the negli-
gible contribution of non-benzoquinoxaline ligands to both
HOMO (or HOMO–1 and HOMO–2) and LUMO (or
LUMO+1). Since complexes 1–5 all bear aryl substituents at
benzoquinoxaline sites, it is plausible that the torsional mo-
tion of phenyl (or tolyl) substituents, the relaxation dynam-
In summary, we report the syntheses and characterization
of a new series of NIR Ir(III) complexes (1–5). These com-
plexes exhibit moderate NIR phosphorescence with peak
wavelengths around 910–920 nm at RT. As supported by the
TDDFT approach, the transition mainly involves benzoiso-
3
quinoline π–π* intraligand charge transfer and metal (Ir) to
benzoisoquinoline charge transfer. The relatively weak NIR
emission can be tentatively rationalized by the low energy
gap of which the radiationless deactivation may be governed
by the large amplitude vibration and, in part, the phenyl tor-
sional motions at the benzoquinoxaline sites. Knowing that
current interest in Ir(III) types of phosphorescence emitter is
focused on the Ir(III)-type OLED, to switch gears in re-
search orientation, future applications of the titled NIR com-
plexes and (or) relevant derivatives may be motivated by the
strategy of using these complexes in the fabrication of pho-
tovoltaic devices (23). Focus on this approach is in progress.
Acknowledgment
We thank the National Science Council of Taiwan, Repub-
lic of China, for financial support (grant Nos. NSC 91-2119-
M-002-016) and (NSC 91-2113-M-007-006).
© 2006 NRC Canada

318
Can. J. Chem. Vol. 84, 2006
13. (a) P.J. Hay and W.R. Wadt. J. Chem. Phys. 82, 270 (1985);
(b) W.R. Wadt and P.J. Hay. J. Chem. Phys. 82, 284 (1985);
(c) P.J. Hay and W.R. Wadt. J. Chem. Phys. 82, 299 (1985).
14. P.C. Hariharan and J.A. Pople. Mol. Phys. 27, 209 (1974).
15. (a) C. Jamorski, M.E. Casida, and D.R. Salahub. J. Chem.
Phys. 104, 5134 (1996); (b) M. Petersilka, U.J. Grossmann,
and E.K.U. Gross. Phys. Rev. Lett. 76, 1212 (1996); (c) R.
Bauernschmitt, R. Ahlrichs, F.H. Hennrich, and M.M. Kappes.
J. Am. Chem. Soc. 120, 5052 (1998); (d) M.E. Casida. J.
Chem. Phys. 108, 4439 (1998); (e) R.E. Stratmann, G.E.
Scuseria, and M.J. Frisch. J. Chem. Phys. 109, 8218 (1998).
16. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N.
Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
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, P.Y. Ayala, K. Morokuma, G.A. Voth,
P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui,
A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov,
M.A.G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Mar-
tin, D.J. Fox, T. Keith, Al-Laham, C.Y. Peng, A. Nanayakkara,
M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W.
Wong, C. Gonzalez, and J.A. Pople. Gaussian 03. Revision
C.02 [computer program]. Gaussian, Inc., Wallingford, Conn.
2004.
References
1 (a) B. O’Regan and M. Graetzel. Nature (London), 353, 737
(1991); (b) A. Hagfeldt and M. Gratzel. Acc. Chem. Res. 33,
269 (2000); (c) D. Kuciauskas, M.S. Freund, H.B. Gray, J.R.
Winkler, and N.S. Lewis. J. Phys. Chem. B, 105, 392 (2001);
(d) M. Gratzel. Nature (London), 414, 338 (2001).
2. (a) R.J. Mears and S.R. Baker. Opt. Quantum Electron. 24,
517 (1992); (b) E. Desurvire. Phys. Today, 97, 20 (1994);
(c) N. Ono, S. Ito, C.H. Wu, C.H. Chen, and T.C. Wen. Chem.
Phys. 262, 467 (2000).
3. (a) M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S.
Sibley, M.E. Thompson, and S.R. Forrest. Nature (London),
395, 151 (1998); (b) Y.-L. Tung, S.-W. Lee, Y. Chi, Y.-T. Tao,
C.-H. Chien, Y.-M. Cheng, P.-T. Chou, S.-M. Peng, and C.-S.
Liu. J. Mater. Chem. 15, 460 (2005); (c) 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. 125, 12971 (2003); (d) Y.-L.
Tung, S.-W. Lee, Y. Chi, L.-S. Chen, C.-F. Shu, F.-I. Wu, A.J.
Carty, P.-T. Chou, S.-M. Peng, and G.-H. Lee. Adv. Mater. 17,
1059 (2005).
4. (a) E.M. Kober, J.V. Caspar, R.S. Lumpkin, and T.J. Meyer. J.
Phys. Chem. 90, 3722 (1986); (b) L. Della Ciana, W.J.
Dressick, D. Sandrini, M. Maestri, and M. Ciano. Inorg.
Chem. 29, 2792 (1990).
5. (a) T.J. Foley, B.S. Harrison, A.S. Knefely, K.A. Abboud, J.R.
Reynolds, K.S. Schanze, and J.M. Boncella. Inorg. Chem. 42,
5023 (2003); (b) W.-K. Wong, A. Hou, J. Guo, H. He, L.
Zhang, W.-Y. Wong, K.-F. Li, K.-W. Cheah, F. Xue, and
T.C.W. Mak. J. Chem. Soc. Dalton Trans. 3092 (2001); (c) A.
Vogler and H. Kunkely. Top. Curr. Chem. 213, 143 (2001);
(d) N.M. Shavaleev, L.P. Moorcraft, S.J.A. Pope, Z.R. Bell, S.
Faulkner, and M.D. Ward. Chem. Eur. J. 9, 5283 (2003);
(e) A.M. Klonkowski, S. Lis, M. Pietraszkiewicz, Z. Hnatejko,
K. Czarnobaj, and M. Elbanowski. Chem. Mater. 15, 656 (2003).
6. (a) I.B. Rietveld, E. Kim, and S.A. Vinogradov. Tetrahedron,
59, 3821 (2003); (b) V.V. Rozhkov, M. Khajehpour, and S.A.
Vinogradov. Inorg. Chem. 42, 4253 (2003).
7. (a) A. Juris, V. Balzani, S. Campagna, G. Denti, S. Serroni, G.
Frei, and H.U. Guedel. Inorg. Chem. 33, 1491 (1994); (b) A.
Mamo, A. Juris, G. Calogero, and S. Campagna. Chem.
Commun. (Cambridge), 1225 (1996); (c) M.K. Nazeeruddin,
R. Humphry-Baker, M. Gratzel, and B.A. Murrer. Chem.
Commun. (Cambridge), 719 (1998).
17. G.B. Caygill, R.M. Hartshorn, and P.J. Steel. J. Organomet.
Chem. 382, 455 (1990).
18. F.O. Graces, K.A. King, and R.J. Watts. Inorg. Chem. 27, 3464
(1988).
19. 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. 123, 4304 (2001).
20. (a) Y. Ohsawa, S. Sprouse, K.A. King, M.K. DeArmond, K.W.
Hanck, and R.J. Watts. J. Phys. Chem. 91, 1047 (1987);
(b) P.J. Hay. J. Phys. Chem. A, 106, 1634 (2002); (c) 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.
44, 1344 (2005).
21. D.L. Dexter. J. Chem. Phys. 21, 836 (1953).
8. (a) W.R. Thiel and J. Eppinger. Chem. Eur. J. 3, 696 (1997);
(b) S.P. Singh, D. Kumar, B.G. Jones, and M.D. Threadgill. J.
Fluorine Chem. 94, 199 (1999).
22. For example, see: P.T. Chou, Y.C. Chen, C.Y. Wei, S.J. Chen,
H.L. Lu, and T.H. Wei. J. Phys. Chem. A, 101, 8581 (1997),
and refs. therein.
9. M. Nonoyama. J. Organomet. Chem. 86, 263 (1975).
10. R.C. Benson and H.A. Jues. J. Chem. Eng. Data, 22, 379
(1977).
11. C. Lee, W. Yang, and R.G. Parr. Phys. Rev. B, 37, 785 (1988).
12. A.D. Becke. J. Chem. Phys. 98, 5648 (1993).
23. (a) P.A. Anderson, F.R. Keene, T.J. Meyer, J.A. Moss, G.F.
Strouse, and J.A. Treadway. J. Chem. Soc. Dalton Trans. 3820
(2002); (b) M.K. Nazeeruddin, S.M. Zakeeruddin, J.-J. Lagref,
P. Liska, P. Comte, C. Barolo, G. Viscardi, K. Schenk, and M.
Graetzel. Coord. Chem. Rev. 248, 1317 (2004).
© 2006 NRC Canada