Effect of axially projected oligothiophene pendants and nitro-functionalized diimine ligands on the lowest excited state in cationic Ir(III) bis-cyclometalates

Article abstract of DOI:10.1021/ic202573y

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^{^}N)₂(N^{^}N)]PF ₆9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C^{^}N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5″ positions with the ppy and mesityl groups. The diimine ligands (N^{^}N) are 2,2′-bipyridine (bpy) or 4-NO₂-bipyridine (4-NO₂-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO₂ functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO₂ substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO₂ substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ_ex = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ_em = 0.11. In stark contrast the NO₂ and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO₂ and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO₂ substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.

Full text of DOI:10.1021/ic202573y

Article
pubs.acs.org/IC
Effect of Axially Projected Oligothiophene Pendants and Nitro-
Functionalized Diimine Ligands on the Lowest Excited State in
Cationic Ir(III) bis-Cyclometalates
Kyle R. Schwartz, Raghu Chitta, Jon N. Bohnsack, Darren J. Ceckanowicz, Pere Miro,
́
Christopher J. Cramer,* and Kent R. Mann*
Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota
55455, United States
S
* Supporting Information
ABSTRACT: The novel terthiophene (3T) oligomer 6 and a
series of cationic Ir(III) bis-cyclometalates [Ir(C∧N)2(N∧N)]-
PF6 9−12 were prepared. The synthesis, characterization,
electrochemical, and photophysical properties are reported.
The cyclometalating ligands (C∧N) are 2-phenylpyridinato
(ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in
the 5 and 5″ positions with the ppy and mesityl groups. The
diimine ligands (N∧N) are 2,2′-bipyridine (bpy) or 4-NO2-
bipyridine (4-NO2-bpy). Hybrid metal-organic complexes 11
and 12 bear 3T-pendants ligated through the ppy cap, 10 and
12 contain NO2 functionalized diimines, whereas 9 contains
neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO2 substituent and
pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing
effect of the NO2 substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10
and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in
complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λex = 400 nm)
of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with ϕem = 0.11. In stark contrast the
NO2 and 3T functionalized complexes 10−12 display near total quenching of luminescence. Computations of the ground and
excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both
the NO2 and 3T substituents play an important role in excited state deactivation of complexes 10−12. A substantial electronic
contribution of the NO2 substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient
nonradiative decay pathway for the excited state. Spin−orbit coupling effects of the Ir(III) ion lead to efficient population of the
low lying, nonluminescent, triplet states centered on the 3T-pendants.
through ligand modification have been exploited for the
fabrication of OLEDs as well LECs.4d,7e,9 LECs utilize ionic
luminescent materials in the active layer, bringing charged
transition metal species to the forefront. This is to the
advantage of the field as the synthetic steps required to yield
cationic Ir(III) bis-cyclometalates are relatively straightforward
and isomer free compared their neutral tris-cyclometalated
congeners.2a,4h,7e,9e,n,10
Taking into consideration the many favorable attributes of
cationic Ir(III) bis-cyclometalates, their use in devices for light-
to-energy or energy-to-light does not come without challenges,
such as minimizing concentration based quenching (triplet-
triplet annihilation) and enhancing spectral coverage of these
phosphorescent materials. These obstacles have been ap-
proached using several strategies. Aggregation can be
INTRODUCTION
■
Over the past several decades the properties of electro and
photoredox active transition metal species have been
extensively investigated. Most notable among this class of
molecules are the Ru(II) polypyridyls, the most successful
inorganic sensitizers for dye-sensitized solar cell (DSSC)
applications.1 The isostructural Ir(III) cyclometalates have
also found use in a variety of applications ranging from
biolabeling agents2 and oxygen sensors3 to electron transfer
arrays,4 photocatalytic hydrogen production,5 and light-harvest-
ing materials.6 Perhaps the true strength of Ir(III) cyclo-
metalates is found as phosphorescent materials in organic light-
emitting diodes (OLEDs)6a,7 and most recently for the
fabrication of electroluminescent devices known as light-
emitting electrochemical cells (LECs).8
The desirable photophysical properties of Ir(III) cyclo-
metalates, such as high emission quantum yields, favorable
excited state lifetimes, photostability, and spectral tuning
Received: November 29, 2011
Published: April 19, 2012
© 2012 American Chemical Society
5082
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
Figure 1. Molecular structures of cationic Ir(III) bis-cyclometalates. Control complexes 9, 10, and 3T-pendant complexes 11, 12.
discouraged and charge transport enhanced by adjusting the
amount of phosphorescent dopant found in the supportive hole
transporting matrix11 or by employing a single layer of a
sterically encumbered phosphorescent material.8a,g,h,j,12 Pre-
vious work from others, as well as our own research lab, has
been concerned with the synthesis, photophysical and electro-
chemical properties of various Ru(II) and Os(II) polypyridyls
functionalized with oligothiophenes. These studies highlight the
importance of organic chromophores in the construction of
photoredox active dyads.13 We expect enhanced spectral
coverage to be provided by strongly absorbing oligothiophene
chromophores and the possibility of hole transfer after selective
hole generation at the metal chromophore, enhancing charge
separation. With these considerations in mind we have
designed a metal-organic hybrid motif consisting of a cationic
Ir(III) bis-cyclometalate possessing axially projected steric bulk
in the form of π-conjugated, hole transporting, photoredox
active oligothiophene chromophores. The interplay of the
metal and organic chromophores in the excited state makes this
motif an interesting candidate for light-to-energy or energy-to-
light devices. To our knowledge this is the first example of a
cationic Ir(III) bis-cyclometalate to utilize axially projected
photoredox active pendants in this manner.
Herein we present the synthesis, characterization, and studies
of the photophysical and electrochemical properties of a series
of cationic Ir(III) bis-cyclometalates of the type [Ir-
(C∧N)2(N∧N)]PF6 (C∧N = cyclometalating ligand, N∧N =
diimine ligand) as shown in Figure 1. Two control complexes
([Ir(ppy)2(bpy)]PF6 (9) and [Ir(ppy)2(4-NO2-bpy)]PF6 (10))
were synthesized to aid in understanding the photophysical and
electrochemical properties of the series. These complexes
provide a reference for the systematic introduction of the
functionalized terthiophene (3T) pendants and electron
withdrawing NO2 substituents found in complexes [Ir(3T-
ppy)2(bpy)]PF6 (11) and [Ir(3T-ppy)2(4-NO2-bpy)]PF6 (12)
respectively. The 3T-pendants, selected for their well
documented spectroscopic and electrochemical properties,14
were functionalized with n-Bu chains and capped with mesityl
groups to increase solubility and decrease aggregation. The 3T-
pendants were placed para to the heteroatom of the pyridyl
moiety to maximize axial projection (Figure 1); functionaliza-
tion at this location has not been well studied for this class of
compounds.6a,9k,15 Finally, the NO2 substituent in complexes
10 and 12 is predicted to lower the energy gap of the MLCT
transition, acting as a trap for the lowest triplet excited state.
This stabilization should also make reduction of the diimine
facile and enhance charge separation.4h,7e,9d,f,j,l,p,16
EXPERIMENTAL SECTION
■
Safety Note. Caution! Appropriate safety measures should be
followed when using the organolithium and organostannane reagents
described here due to their pyrophoric and toxic natures, respectively.
General Considerations. NMR spectra were recorded on a
Varian Unity or Varian Inova 300 MHz instrument. High-resolution
mass spectrometry was performed on a Bruker BioTOF II mass
spectrometer, Hewlett-Packard Series II Model 5890 gas chromato-
graph/Finnigan MAT 95 mass spectrometer, or Bruker Biflex III
MALDI-TOF. All solvents and reagents used for synthetic procedures
were purchased from commercially available sources and used without
further purification. The following chemicals were used as provided:
IrCl3·3H2O (Johnson Matthey), 2-phenylpyridine (Hppy) (Aldrich),
2,2′-bipyridine (bpy) (TCI Amercia), NaPF6 (Aldrich).
Compounds 2,2′-bipyridine-N-oxide,17 4-nitro-2,2′-bipyridine-N-
oxide,18 and 3′,4′-dibutyl-2,2′:5′,2″-terthiophene (3′,4′-n-Bu-3T)19
were synthesized according to the reported literature procedures.
The cyclometalated Ir(III) dichloro-bridged dimers, [Ir(ppy)2(μ-Cl)]2
(7) and [Ir(3T-ppy)2(μ-Cl)]2 (8) were prepared using the method of
Watts from IrCl3·3H2O in a mixture of 2-methoxyethanol and water
with slight modification.20 A series Ir[(C∧N)(N∧N)]+ (9−12)
complexes was prepared by cleavage of the corresponding dimers
5083
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
−
with an appropriate diimine ligand and subsequent metathesis to PF6
H2O (v/v, 35:11.6 mL) was degassed and added to the flask via
cannulation. The reaction mixture was refluxed for 24 h then cooled to
r.t., after which H2O (20 mL) and hexanes (50 mL) were added. The
resulting aqueous phase was extracted with hexanes (3 × 75 mL). The
combined organic extracts were washed with water (3 × 100 mL), and
then dried over Na2SO4. The solvent was removed via rotary
evaporation to yield the crude product as a brown oily solid. The
crude product was purified via column chromatography (alumina,
salts as previously described with slight modifications.2a Details for all
procedural modifications are provided below.
4-Nitro-2,2′-bipyridine (1). Compound 1 was synthesized
according to the literature procedure with slight modifications.21
PCl3 (16.1 mL, 25.3 g, 0.185 mol) was added by syringe to a three
neck round-bottom flask containing 4-nitro-2,2′-bipyridine-N-oxide
(1.0 g, 4.6 mmol), and the reaction mixture was refluxed (85 °C)
under an inert gas atmosphere for 24 h. The reaction mixture was
allowed to cool, then poured onto ice and stirred for 30 min. The
reaction mixture was brought to a pH of ∼11 through the dropwise
addition of aq. NaOH, followed by extraction into CHCl3. The organic
extract was dried over Na2SO4 and evaporated to give 1 as a white
1
hexanes) to yield 4 as a yellow oil. Yield: 82%. H NMR (300 MHz,
CD2Cl2): δ 7.34 (dd, 1H, J = 5.4 Hz, 1.2 Hz), 7.16 (dd, 1H, J = 3.9 Hz,
1.2 Hz), 7.14 (d, 1H, J = 3.6 Hz), 7.08 (1H, dd, J = 5.4 Hz, 3.6 Hz),
6.95 (s, 2H), 6.76 (d, 1H, J = 3.6 Hz), 2.79−2.70 (m, 4H), 2.31 (s,
3H), 2.17 (s, 6H), 1.60−1.50 (m, 4H), 1.48−1.43 (m, 4H), 0.98−0.90
(m, 6H). HRMS (GC-MS EI) [M·]+: calcd for C23H34S3 478.1823,
found 478.1790.
1
solid. Yield: 97%. H NMR (300 MHz, CD2Cl2): δ 9.13 (dd, 1H, J =
2.4, 0.6 Hz), 8.95 (dd, 1H, J = 5.4, 0.6 Hz), 8.73 (ddd, 1H, J = 4.8, 1.8,
0.9 Hz), 8.48 (ddd, 1H, J = 7.8, 1.2, 0.9 Hz), 8.01 (dd, 1H, J = 5.1, 2.1
Hz), 7.89 (ddd, 1H, J = 7.8, 7.8, 1.8 Hz), 7.42 (ddd, 1H, J = 7.5, 4.5,
1.2 Hz).
5-((Tributyl)stannyl)-5″-mesityl-3′,4′-dibutyl-2,2′:5′,2″-ter-
thiophene (5). 4 (2.10 g, 4.39 mmol) was added to an oven-dried
Schlenk flask and pumped/purged with Ar. THF (30 mL) was added
and the solution was cooled to −78 °C with a dry ice/isopropanol
bath. n-butyllithium (2.40 mL, 4.82 mmol) was added dropwise over
45 min, and the solution was stirred for an additional 30 min at −78
°C, followed by the addition of tributyltin chloride (1.50 mL, 5.27
mmol). The solution was warmed to r.t. and stirred for 1 h, followed
by the addition of H2O (10 mL) and hexanes (10 mL). The resulting
organic phase was separated, and the aqueous phase extracted with
hexanes (3 × 20 mL). The combined organic extracts were washed
with H2O (2 × 50 mL) and a saturated solution of aq. NaHCO3 (2 ×
50 mL), and then dried over MgSO4. The solvent was removed via
rotary evaporation to yield 5 as a yellow-brown oil Yield: 82%. The
material as isolated was considered a reaction intermediate that was
pure enough for use without purification. 1H NMR (300 MHz,
CD2Cl2): δ 7.27 (d, 1H, J = 3.3 Hz), 7.13 (d, 2H, J = 3.6 Hz), 6.95 (s,
2H), 6.75 (d, 1H, J = 3.6 Hz) 2.78−2.71 (m, 4H), 2.31 (s, 3H), 2.17
(s, 6H), 1.63−1.30 (m, 20H), 1.17−1.11 (m, 6H), 1.0−0.90 (m, 15H).
4-(3′,4′-Dibutyl-5″-mesityl-2,2′:5′,2″-terthiophen-5-yl)-2-
phenylpyridine (H3T-ppy, 6). 5 (1.08 g, 1.40 mmol), 2 (0.30 g, 1.3
mmol), and tetrakis(triphenylphosphine)palladium(0) (0.08 g, 0.07
mmol) were added to an oven-dried Schlenk flask and pumped/
purged with Ar. DMF (25 mL) was degassed and added to the flask via
cannulation, and the reaction mixture was heated to 100 °C for 48 h.
The solution was cooled to r.t., transferred to a separatory funnel
followed by the addition of H2O (50 mL) and ethyl acetate (50 mL).
The resulting organic phase was separated and the aqueous phase
extracted with ethyl acetate (4 × 50 mL). The combined organic
extracts were washed with water (4 × 100 mL) and a saturated
solution of aq. NaHCO3 (1 × 75 mL), and then dried over Na2SO4.
The solvent was removed via rotary evaporation to yield the crude
product as a red-brown oil. The crude material was purified via column
chromatography (silica gel, CH2Cl2:hexanes 75:25 to 100 CH2Cl2) to
yield 6 as a thick red oil. Yield: 78%. 1H NMR (300 MHz, CD2Cl2) δ
8.66 (dd, 1H, J = 5.4 Hz, 0.6 Hz), 8.07 (ddd, 1H, J = 5.7 Hz, 3.0 Hz,
1.5 Hz), 7.94 (dd, 1H, J = 1.8 Hz, 0.9 Hz), 7.59 (d, 1H, J = 3.9 Hz),
7.48 (m, 5H), 7.22 (d, 1H, J = 3.9 Hz), 7.17 (d, 1H, J = 3.6 Hz), 6.95
(s, 2H), 6.77 (d, 1H, J = 3.6 Hz), 2.82−2.74 (m, 4H), 2.31 (s, 3H),
2.17 (s, 6H), 1.61−1.42 (m, 8H), 1.00−0.94 (m, 6H). HRMS (ESI-
4-Bromo-2-phenylpyridine (2). Compound 2 was synthesized
according to the literature procedures with slight modifications.22
A
solution of 4-bromopyridine (10 g, 0.052 mol) in dry tetrahydrofuran
(THF, 200 mL) was cooled to −78 °C to which a solution of
phenylmagnesium chloride (2 M) in THF (65 mL) was added
dropwise at −78 °C and allowed to stir for 15 min. Phenyl-
chloroformate (8.0 mL, 10 g, 0.064 mol) in THF (20 mL) was then
added dropwise over 10 min at −78 °C after which the reaction
mixture was allowed to warm to room temperature (r.t.). The reaction
mixture was cooled to 0 °C and quenched with 20% NH4Cl followed
by stirring at 0 °C for 2 h and extraction into diethylether. The
combined extracts were washed with H2O, 20% HCl, and H2O. The
organic layer was dried over Na2SO4 and evaporated under vacuum to
yield a pale yellow material. The residue was dissolved in dry toluene
(200 mL) and o-chloronil (15.86 g, 0.06451 mol) dissolved in glacial
acetic acid (120 mL) was added dropwise, and the resultant red
solution was stirred at r.t. for 36 h. The solution was made basic using
aq. 10% NaOH, filtered through Celite and washed with H2O. The
organic layer was extracted three times using aq. 10% HCl. The
aqueous layers were made basic with NaOH and extracted with
CH2Cl2. The combined organic layers were dried over Na2SO4, and
the solvent was removed under vacuum to give the crude compound as
a red oil. Purification of the crude compound by column
chromatography (silica gel, hexanes:ethylacetate 98:2−95:5 v/v)
yielded 2 as a pale yellow oil. Yield: 79%. 1H NMR (300 MHz,
CDCl3): δ 8.52 (d, 1H, J = 5.1 Hz), 7.99−7.96 (m, 2H), 7.91 (d, 1H, J
= 1.8 Hz), 7.53−7.45 (m, 3H), 7.41 (dd, 1H, J = 5.4, 1.8 Hz).
5-Bromo-3′,4′-dibutyl-2,2′:5′,2″-terthiophene (3). Compound
3 was synthesized according to the literature procedure with slight
modifications.23 3′,4′-n-Bu-3T (4.0 g, 0.011 mol), was added to a
round-bottom flask and pumped/purged with Ar (3 times).
Dimethylformamide (DMF, 25 mL) was added to the flask, and the
resulting solution purged for 15 min, then cooled to 0 °C. A solution
of N-bromosuccinimide (2.0 g, 0.011 mol) in DMF (6 mL) was then
added dropwise over 45 min. The reaction mixture was stirred for 3 h
at 0 °C, warmed to r.t. and allowed to stir an additional 21 h, followed
by the addition of water (20 mL) and CH2Cl2 (15 mL). The resulting
organic phase was separated, and the aqueous phase extracted with
CH2Cl2 (3 × 15 mL). The combined organic extracts were washed
with H2O (3 × 15 mL) and brine (1 × 20 mL), and then dried over
MgSO4. The solvent was removed via rotary evaporation to yield the
crude product as a brown-yellow oil. The crude product was purified
via column chromatography (silica gel, hexanes) to yield 3 as a yellow
oil. Yield: 79%. 1H NMR (300 MHz, CD2Cl2): δ 7.35 (dd, 1H, J = 5.1
Hz, 1.2 Hz), 7.14 (dd, 1H, J = 3.6 Hz, 1.2 Hz), 7.08 (dd, 1H, J = 5.1
Hz, 3.6 Hz), 7.04 (d, 1H, J = 3.9 Hz), 6.90 (d, 1H, J = 3.9 Hz), 2.73−
2.65 (m, 4H), 1.58−1.38 (m, 8H), 0.97−0.90 (m, 6H).
+
TOF) [M + H] : calcd for C40H42NS3 632.2474, found 632.2464.
[Ir(3T-ppy)2Cl]2 (8). A solution of 6 (0.30 g, 0.48 mmol) in 2-
methoxyethanol (15 mL) was added to a flask containing IrCl3·3H2O
(0.08 g, 0.2 mmol) and H2O (5 mL). The reaction mixture was
allowed to reflux under an inert gas atmosphere for 24 h. After cooling
to r.t. a red precipitate was collected by filtration. The reaction solid
was dried by suction and dissolved off the frit with CH2Cl2 then
concentrated by rotary evaporation. Precipitation was induced by
addition of diethyl ether under a stream of N2. The solid was filtered
and washed with diethyl ether. Compound 8 was obtained as a brick
red solid in a crude yield of 70% and used without further purification.
1H NMR (300 MHz, CD2Cl2): δ 9.28 (d, 4H, J = 6.3 Hz), 8.15 (s,
4H), 7.70 (d, 4H, J = 7.5 Hz), 7.59 (d, 4H, J = 3.9 Hz), 7.25 (d, 4H, J
= 3.9 Hz), 7.14 (d, 4H, J = 3.6 Hz), 7.02 (dd, 4H, J = 6.3, 1.8 Hz), 6.94
(s, 8H), 6.89 (t, 4H, J = 7.5 Hz), 6.73 (d, 4H, J = 3.6 Hz) 6.68 (t, 4H, J
5-Mesityl-3′,4′-dibutyl-2,2′:5′,2″-terthiophene (4). 3 (2.5 g,
5.7 mmol), 2,4,6-trimethylphenyl boronic acid (1.12 g, 6.83 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.53 g, 0.46 mmol), and
K2CO3 (2.4 g, 0.017 mol) were added to a round-bottom flask and
pumped/purged with Ar (3 times). A mixture of dimethoxyethane and
5084
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
= 7.5 Hz), 6.12 (d, 4H, J = 7.8 Hz), 2.71 (q, 16H, J = 7.2 Hz), 2.32 (s,
12H), 2.16 (s, 24H), 1.46 (m, 32H), 0.989 (t, 12H, J = 6.6 Hz), 0.910
(t, 12H, J = 7.2 Hz). MS (MALDI-TOF) [M − C80H80Cl2IrN2S6]+:
calcd for C80H80IrN2S6 1453.427, found 1453.427.
standard three electrode setup consisting of a glassy-carbon working
electrode, a platinum auxiliary electrode, and a Ag/AgCl reference
electrode containing 1.0 M KCl. Supporting electrolyte tetrabuty-
lammonium hexafluorophosphate (TBAPF6, Sigma-Aldrich) was
recrystallized from ethanol, dried and stored in a desiccator prior to
use. All compounds studied by electrochemical analysis were prepared
as 0.5 mM solutions in acetonitrile, dried over an activated alumina
column and deaerated, containing 0.1 M TBAPF6. Potentials are
reported vs aqueous Ag/AgCl and are not corrected for the junction
potential. The E°′ values for the ferrocenium/ferrocene couple for
concentrations similar to those used in this study were +0.40 V for
acetonitrile solutions at a glassy carbon electrode. A thorough
description of the experimental setup and conditions used here has
been described previously by our group.24
X-ray Structural Determination. The data for the structural
determination were collected in the X-ray Crystallographic Lab in the
LeClair-Dow Instrumentation Facility (Department of Chemistry,
University of Minnesota). A single crystal of compound 10 was
secured to a glass capillary and mounted on a Siemens SMART
platform CCD diffractometer for a data collection at 173(2) K using a
graphite monochromator and Mo Kα radiation (λ = 0.71073 Å). An
initial set of cell constants was calculated from 51 reflections harvested
from three sets of 20 frames such that orthogonal wedges of reciprocal
space were surveyed. Final cell constants were determined from a
minimum of 4023 strong reflections from the actual data collection.
Data were collected to the extent of 1.5 hemispheres to a resolution of
0.77 Å. Four major sections of frames were collected with 0.30° steps
in ω. The intensity data were corrected for absorption and decay using
SADABS.25 The space group Pnma was determined based on
systematic absences and intensity statistics. A direct-methods solution
provided the positions of most non-hydrogen atoms. Full-matrix least-
squares/difference Fourier cycles were performed to locate the
remaining non-hydrogen atoms. All non-hydrogen atoms were refined
with anisotropic displacement parameters, and all hydrogen atoms
were placed in ideal positions and refined as riding atoms with relative
isotropic displacement parameters. All calculations were performed
using the SHELXTL suite of programs.26 Further details on the
structural refinement and treatment of disordered solvent of
crystallization and the NO2 substituent are provided in the Supporting
Information.
Optical Spectroscopy. Absorption spectra of the 3T oligomer
and cationic Ir(III) bis-cyclometalates were collected for acetonitrile
solutions using a 1.0 cm path length cell with a Cary 14
spectrophotometer running the OLIS globalworks software suite.
Photoluminescence experiments were obtained using an excitation
wavelength of 400 nm in acetonitrile using a front face geometry, with
optical densities of solutions >2.0. Data was collected on a Spex
Fluorolog 1680 0.2 m double spectrofluorimeter, equipped with a
Hamamatsu R928 photomultiplier tube, running Datamax software.
Solutions of the cationic Ir(III) bis-cyclometalates were deaerated for
15 min prior to collection using an argon purge. All spectra were
corrected for the wavelength dependence of the detector.
Luminescence quantum yields (ϕem) were measured relative to
Coumarin 485 (C485) in acetonitrile solutions (ϕem = 0.28);
estimated uncertainties in ϕem measurements are 20% or better.
Computational Details. All geometries were fully optimized using
DFT with the M06-L functional.27 The 6-31G(d,p) basis set28 was
used for all nonmetal atoms and the Stuttgart/Dresden (SDD) basis
set29 relativistic effective core potential was used for the Ir(III) metal
centers. The n-Bu groups found in the 3′ and 4′ positions of the
terthiophene pendants were replaced with hydrogen atoms for all
computations. No symmetry restrictions were imposed while
determining the fully optimized ground state geometries. Integral
evaluation made use of the grid defined as ultrafine in the Gaussian 09
program.30 The nature of all stationary points was verified by analytic
computation of vibrational frequencies.31 Solvent effects for
acetonitrile were modeled with SMD.32 To obtain the vertical
excitation energies of the three lowest singlet and triplet excited
states of the studied complexes time-dependent DFT (TD-DFT)33
calculations were performed using the M06-2X functional34 at the
[Ir(ppy)2(bpy)]PF6 (9). [Ir(ppy)2Cl]2 (0.17 g, 0.16 mmol), and
2,2′-bipyridine (0.053 g, 0.34 mmol) were refluxed under an inert gas
atmosphere in a 1:1 mixture of CH2Cl2:CH3OH for 6 h. The reaction
mixture was allowed to cool to r.t. and then concentrated by rotary
evaporation. Diethyl ether was added to induce precipitation followed
by filtration and washing with diethyl ether. Solid was then dissolved in
CH3OH. Addition of NaPF6 (0.31 g) to the solution while stirring
resulted in a bright yellow precipitate that was filtered and washed with
H2O and diethyl ether and then dried in vacuo giving compound 9 as a
yellow solid. Yield: 92%. 1H NMR (300 MHz, CD2Cl2): δ 8.49 (d, 2H,
J = 8.4 Hz), 8.13 (ddd, 2H, J = 7.5, 7.5, 1.5 Hz), 8.03 (ddd, 2H, J = 5.4,
1.5, 0.6 Hz), 7.97 (d, 2H, J = 7.8 Hz), 7.80 (dd, 2H, J = 7.5, 1.5 Hz),
7.76 (ddd, 2H, J = 9.0, 7.5, 0.9 Hz), 7.49 (ddd, 2H, J = 5.4, 1.5, 0.9
Hz), 7.46 (dd, 2H, J = 5.4, 1.2 Hz), 7.08 (ddd, 2H, J = 7.5, 7.5, 1.5 Hz)
6.93 (ddd, 2H, J = 7.5, 6.0, 1.5 Hz), 6.94 (ddd, 2H, J = 7.5, 7.5, 1.2
Hz), 6.31 (ddd, 2H, J = 7.5, 1.2, 0.6 Hz). HRMS (ESI-TOF) [M]+:
calcd for C32H24IrN4 657.1625, found 657.1638.
[Ir(ppy)2(4-NO2-bpy)]PF6 (10). The synthesis of complex 10 was
conducted in a manner similar to that of complex 9 with the exception
of amounts used, [Ir(ppy)2Cl]2 (0.09 g, 0.09 mmol), and 1 (0.04 g, 0.2
mmol). Compound 10 was obtained as a brown-red solid. Yield: 79%.
1H NMR (300 MHz, CD2Cl2): δ 9.09 (d, 1H, J = 2.1 Hz), 8.61 (d, 1H,
J = 8.4 Hz), 8.33 (d, 1H, J = 6.3 Hz), 8.21 (ddd, 1H, J = 8.1, 8.1, 1.8
Hz), 8.11 (dd, 1H, J = 5.7, 2.1 Hz), 8.08 (d, 1H, J = 5.4 Hz), 7.97 (d,
2H, J = 8.1 Hz), 7.81 (ddd, 2H, J = 7.5, 7.5, 1.5 Hz), 7.76 (d, 2H, J =
7.8 Hz), 7.58 (ddd, 1H, J = 7.8, 5.4, 1.2 Hz), 7.54 (d, 1H, J = 6.0 Hz),
7.47 (d, 1H, J = 5.4 Hz), 7.03 (m, 6H), 6.32 (d, 1H, J = 7.2 Hz), 6.45
(d, 1H, J = 7.8 Hz). HRMS (ESI-TOF) [M]+: calcd for C32H23IrN5O2
702.1476, found 702.1474.
[Ir(3T-ppy)2(bpy)]PF6 (11). [Ir(3T-ppy)2Cl]2 (0.15 g, 0.050
mmol), and 2,2′-bipyridine (0.02 g, 0.1 mmol) were refluxed under
an inert gas atmosphere in a 2:1 mixture of CH2Cl2:CH3OH for 6 h.
The reaction mixture was allowed to cool to r.t. and the solvent was
removed by rotary evaporation. The sticky residue was sonicated in
diethyl ether to give a solid that was filtered and washed with diethyl
ether. The solid was dissolved in CH3OH and filtered before the
addition of NaPF6 (0.12 g) to the stirred solution. Concentration of
the solution resulted in a precipitate that was collected by filtration and
washed with H2O and then dried in vacuo to give compound 11 as an
orange-brown solid. Yield: 51%. 1H NMR (300 MHz, CD2Cl2): δ 8.52
(d, 2H, J = 8.1 Hz), 8.15 (ddd, 2H, J = 7.5, 7.5, 1.2 Hz), 8.08 (d, 4H, J
= 6.0 Hz), 7.84 (d, 2H, J = 7.2 Hz), 7.62 (d, 2H, J = 3.9 Hz), 7.50 (t,
2H, J = 6.6 Hz), 7.44 (d, 2H, J = 6.3 Hz), 7.24 (d, 2H, J = 3.9 Hz),
7.17 (d, 2H, J = 3.6 Hz), 7.17 (d, 2H, J = 8.4 Hz), 7.12 (d, 2H, J = 8.4
Hz), 7.00 (ddd, 2H, J = 8.4, 8.4, 1.8 Hz), 6.95 (s, 4H), 6.78 (d, 2H, J =
3.6 Hz), 6.49 (d, 2H, J = 7.5 Hz), 2.78 (q, 8H, J = 8.4 Hz), 2.31 (s,
6H), 2.17 (s, 12H), 1.51 (m, 16H), 0.979 (t, 6H, J = 7.2 Hz), 0.937 (t,
6H, J = 7.2 Hz). HRMS (ESI-TOF) [M]+: calcd for C90H88IrN4S6
1609.4957, found 1609.4914.
[Ir(3T-ppy)2(4-NO2-bpy)]PF6 (12). The synthesis of complex 12
was conducted in a manner similar to that of complex 11 with the
exception of amounts used, [Ir(3T-ppy)2Cl]2 (0.09 g, 0.03 mmol), and
1 (0.01 g, 0.06 mmol). Compound 12 was obtained as an orange-
brown solid. Yield: 51%. 1H NMR (300 MHz, CD2Cl2): δ 9.12 (d, 1H,
J = 2.1 Hz), 8.64 (d, 1H, J = 8.1), 8.38 (d, 1H, J = 5.7 Hz), 8.23 (ddd,
1H, J = 8.1, 8.1, 1.5 Hz), 8.14 (ddd, 1H, J = 6.0, 6.0, 2.4 Hz), 8.09 (s,
4H), 7.86 (d, 2H, J = 6.9 Hz), 7.63 (d, 2H, J = 3.9 Hz), 7.60 (d, 1H, J
= 6.3 Hz), 7.48 (d, 1H, J = 6.3 Hz), 7.41 (d, 1H, J = 6.3 Hz), 7.24 (dd,
2H, J = 4.8, 0.9 Hz), 7.17 (d, 2H, J = 3.6 Hz), 7.15(m, 3H), 7.03 (q,
2H, J = 6.6 Hz), 6.95 (s, 4H), 6.78 (d, 2H, J = 3.6 Hz), 6.50 (d, 1H, J =
7.2 Hz), 6.46 (d, 1H, J = 7.5 Hz), 2.79 (q, 8H, J = 8.4 Hz), 2.31 (s,
6H), 2.17 (s, 12H), 1.50 (m, 16H), 0.977 (t, 6H, J = 7.2 Hz), 0.936 (t,
6H, J = 7.2). HRMS (ESI-TOF) [M]+: calcd for C90H87IrN5O2S6
1654.4808, found 1654.4813.
Electrochemical Measurements. Electrochemical experiments
were performed with a BAS 100B electrochemical analyzer using a
5085
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
Scheme 1. Synthesis of 3T-Ligand and Cationic Ir(III) Bis-Cyclometalates
M06-L optimized ground-state (S0) geometry. Acetonitrile non-
equilibrium solvatochromic effects were evaluated via a linear response
formalism using SMD cavities.33a
the synthesis of 9 an 10 and 2:1 for the synthesis of 11 and 12)
to generate the monomeric [Ir(C∧N)2(N∧N)]+ species as Cl−
salts. These salts were subsequently metathesized by treatment
with NaPF6 to give PF6− as the counterion.2a Complexes 9−12
were characterized by 1H NMR spectroscopy as well as HR-ESI
RESULTS AND DISCUSSION
■
1
mass spectrometry. Ir(III) complexes 9−12 display H NMR
Synthesis of Ligands and Complexes. Syntheses of the
3T oligomer 6, its corresponding Ir(III) complexes 11−12, and
the Ir(III) control complexes 9−10 are outlined in Scheme 1.
The diimine ligand 4-nitro-2,2′-bipyridine (1),21 precursor 4-
bromo-2-phenylpyridine (2)22 and 5-bromo-3′,4′-dibutyl-
2,2′:5′,2″-terthiophene (3)23 were synthesized with slight
modifications to previously reported literature procedures.
Compound 3 was reacted with mesitylboronic acid ((Mes)B-
(OH)2) under Suzuki coupling conditions to obtain the
monomesityl-capped terthiophene 4 in good yield (82%).
The monostannylated terthiophene 5 was prepared in
quantitative yield by reaction of the monolithiated derivative
of 4, generated in situ, with tributyltin chloride. Compound 5
was then coupled to 2 using standard Stille coupling conditions
to obtain the desired cyclometalating 3T oligomer 6 in good
yield (78%).
spectra consistent with a C,C-cis/N,N-trans coordination mode
for the cyclometalating ligands. As expected for molecules of C2
symmetry, 9 and 11 show a single resonance for each set of
aromatic 1H signals, 12 total for 9, and 15 total for 11.
1
Complexes 10 and 12 display more complicated H NMR
spectra as introduction of the NO2 substituent results in a loss
1
of symmetry. The number of H signals observed for the
aromatic region is increased, 15 total for 10, and 19 total for 12.
In the case of 10 further confirmation of structural assignment
was provided by single crystal X-ray crystallography.
X-ray Crystallography. Single crystals of 10 were grown
by slow evaporation from a CH2Cl2 solution. Relevant
crystallographic data for 10 is shown in Table 1, and a list of
selected bond lengths and angles (experimental and calculated)
can be found in Table 2. The structure obtained for 10 displays
two ppy cyclometalating ligands in the C,C-cis/N,N-trans or
“mer like” arrangement about a pseudo-octahedrally coordi-
nated Ir(III) ion (Figure 2). The 4-NO2-bpy diimine ligand
occupies the remaining two coordination sites with both N
atoms trans to the carbanions of the ppy ligands. Ir(III)-ligand
bond lengths for the diimine and cyclometalating ligands are in
good agreement with distances found for similar [Ir-
(C∧N)2(N∧N)]+ species.2b,7e,9p,16b,35 As is typically observed
within cyclometalating ligands the Ir−C bonds (∼2.02 Å) are
The desired series of complexes 9−12 was synthesized via
conventional synthetic procedures for cationic Ir(III) bis-
cyclometalates, with control complexes 9−10 (79−92%), and
the 3T-pendant complexes 11−12 (51%), isolated in good to
moderate yields. The dichloro-bridged dimers [Ir(C∧N)2(μ-
Cl)]2 7 and 8, were prepared using the method described by
Watts et al., substituting 2-ethoxyethanol for 2-methoxyetha-
nol.20 Dimers 7 and 8 were cleaved with the appropriate
diimine ligand in a mixture of refluxing CH2Cl2:MeOH (1:1 for
5086
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
Table 1. Crystallographic Data and Refinement Parameters
for [Ir(ppy)2(4-NO2-bpy)]PF6 (10)
empirical formula
C32 H23 F6 Ir N5 O2 P
red-orange, plate
orthorhombic
Pnma
crystal color, morphology
crystal system
space group
a, Å
21.287(4)
26.420(5)
13.113(3)
90
b, Å
c, Å
α, deg
β, deg
90
γ, deg
volume (V), Å3
90
7375(2)
Z
8
formula weight, g mol−1
density (calculated), g cm−3
temperature, K
absorption coefficient (μ), mm−1
F(000)
731.76
1.525
173(2)
3.728
3296
Figure 2. Thermal ellipsoid plot of [Ir(ppy)2(4-NO2-bpy)]PF6 (10).
θ range, deg
1.54 to 27.51
−27 ≤ h ≤ 27
−34 ≤ k ≤ 34
−17 ≤ l ≤ 16
75379
Major component of disordered NO2 substituent is shown. Hydrogen
index ranges
−
atoms and PF6 counterion have been removed for clarity.
Surprisingly the presence of a NO2 substituent on the diimine
ligand appears to have no observable effect on the Ir1−N4 or
the Ir1−C11 distances. The average N−O bond length found
for atoms O1, O2, and N5 is 1.220(11) Å, which compares well
with the average values attained from the CSD36 (1.213(12) Å)
and the DFT optimized ground state geometry (1.229 Å). The
NO2 substituent was nearly coplanar with the bpy ligand, with a
small torsion angle of 10.6(4)°, as measured from the O2−
N5−C25−C24 unit. A slight twist in the NO2 substituent is
commonly observed for transition metal complexes with NO2
functionalized phenanthroline and bipyridine ligands in the
coordination sphere.37b,38
Frontier Orbital Analysis. Theoretical calculations were
performed in support of experiment to assess the optimized
ground state geometry and electronic structure of both the
control (9−10) and 3T-pendant (11−12) complexes. Calcu-
lations at the DFT level of theory allowed the investigation of
the stepwise addition of 3T-pendants and NO2 substituents.
Contour plots representing the frontier orbitals for the series
(9−12) can be found in Figure 3. Control complexes 9 and 10
exhibit electron density assigned to the highest occupied
molecular orbital (HOMO) localized over the Ir(III) ion and
the phenyl ring of the cyclometalating ligands. The HOMO is
virtually unchanged on addition of the NO2 substituent. The
lowest unoccupied molecular orbital (LUMO) is found to
reside almost exclusively on the π-system of the diimine ligand
(bpy/4-NO2-bpy) in 9 and 10, greatly stabilized by the NO2
substituent.
reflections collected
independent reflections
8645 [Rint = 0.1145]
0.0493, 11.7270
0.9126, 0.3316
8645/3/446
0.0548, 0.1038
0.1095, 0.1181
1.052
a
weighting factors, a, b
max, min transmission
data/restraints/parameters
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
GOF
largest diff. peak, hole e Å−3
1.037, −0.788
a
2
2
w = [ σ2(Fo ) + (aP)2 + (bP)]−1, where P = (Fo +2Fc2)/3.
Table 2. Selected Bond Lengths and Angles for [Ir(ppy)2(4-
NO2-bpy)]PF6 (10)
bond length (Å)
bond angle (deg)
atoms experimental calculated
atoms
experimental calculated
Ir1−N1
Ir1−N2
Ir1−N3
Ir1−N4
2.043(5)
2.038(5)
2.137(5)
2.139(5)
2.031(6)
2.013(7)
2.087
2.089
2.217
2.209
2.018
2.020
N1−Ir1−
80.7(2)
79.6(3)
76.5(2)
124.7(7)
79.8
79.7
74.2
125.1
C22
N2−Ir1−
C11
N3−Ir1−
N4
O1−N5−
O2
Ir1−
C11
Ir1−
C22
N5−O1
N5−O2
1.240(8)
1.199(8)
1.229
1.229
Addition of 3T-pendants to complexes 11 and 12 does little
to affect the energy or nature of the molecular orbital (MO)
localized on the Ir(III)-(Ph−
)
C N
moiety, assigned as the
∧
slightly shorter than those of the Ir−N bonds (∼2.04 Å), while
the Ir−N bonds (∼2.14 Å) to the diimine ligand are longer
than those observed for the cyclometalating ligands (Table 2).
2
HOMO−2 in the 3T-pendant complexes (Figure 3). In both
complexes 11 and 12 calculations show a higher lying pair of
degenerate MOs representing the HOMO and HOMO−1
delocalized over the 3T π-system. The small contribution of
electron density to the HOMO or HOMO−1 from the
cyclometalating ligand cap indicates poor interaction between
the 3T-pendant and the Ir(III) ion in the ground state.
∧
Lengthening of the Ir-N(N N) is regularly observed in these
complexes because of the strong trans-influence of the
cyclometalated carbanions.2b,7e,9p,16b,35
A search of the Cambridge Crystallographic Database
(CSD)36 indicates 10 is the first Ir(III) species among five
transition metal complexes to contain a NO2 functionalized
bipyridine ligand to be characterized crystallographically.37
Analysis of the lowest unoccupied orbitals for the series (9−
12) shows a MO of similar composition localized on the
5087
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
Figure 3. Contour plots of frontier orbitals (0.002 au) for control complexes (9−10) and 3T-pendant complexes (11−12) in the ground state.
Occupied orbitals are shown in blue/red, virtual orbitals are in cyan/green. Hydrogen atoms omitted for clarity.
a
b
Table 3. Photophysical and Electrochemical Properties of 3T Oligomer 6 and Cationic Ir(III) Complexes 9−12
E1/2(ox), V
E1/2(red), V
c
d
e
compound
λabs, nm
λem, nm
ϕem
Ir(IV)/Ir(III)
3T+/3T
1.04
bpy/bpy−
6
252, 378
498
603
599
0.11
0.11
9
255, 265, 312, 344 375, 415, 470
252, 266, 305, 381 400, 530
263, 300, 421
1.29
1.35
1.36
1.38
−1.37
−0.52
−1.25
−0.40
10
11
12
0.005
0.001
0.0003
579, 756
592
1.10
1.10
263, 308, 418
a
b
Spectra recorded in ACN solutions. Potentials measured vs Ag/AgCl 1.0 M KCl at room temperature in dry degassed 0.1 M TBAPF6 ACN
solution, scan rate 100 mV/s. Deaerated solutions at 298 K, λex = 400 nm. Determined relative to Coumarin 485 in ACN (ϕem = 0.28). Redox
c
d
e
couple also contains significant contribution from the cyclometalating ligands.
diimine ligand. These MOs are assigned as the LUMO in 9 and
the LUMO+2 in 11, while the diimine based MOs on the NO2
functionalized compounds are greatly stabilized, leading to their
assignment as LUMO in both 10 and 12. For the 3T-pendant
complexes 11 and 12, calculations show a pair of nearly
degenerate MOs delocalized over the 3T-pendant and pyridyl
ring of each cyclometalating ligand. In the case of 11, these
orbitals (LUMO/LUMO+1) lie close in energy to the diimine
based MO, while in complex 12 these orbitals (LUMO+1/
LUMO+2) are significantly higher in energy (−2.833 eV, not
shown) than the NO2 stabilized diimine MO.
Electrochemistry of the Control Complexes. The
electrochemical properties of the cationic Ir(III) bis-cyclo-
metalates (9−12) investigated here are in agreement with the
DFT simulations discussed above. Results of the electro-
chemical investigation in acetonitrile (ACN) for control
complexes 9 and 10 are given in Table 3 and Figure 4. Both
complexes 9 and 10 displayed electrochemical behavior typical
of [Ir(C∧N)2(N∧N)]+ type complexes.4g,7e,8j,9d,n,10 Quasi-
reversible single electron processes within the solvent window
were observed for 9 and 10, centered at 1.29 and 1.35 V,
respectively. Addition of the NO2 substituent causes a nominal
positive shift (60 mV) in the oxidation of 10 relative to 9,
5088
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
12 were also examined in ACN by cyclic voltammetry (Table 3,
Figure 4). No reductions were observed for the 3T oligomer
within the solvent window, as expected for similar aryl capped
terthiophene species,14b,e,g but the oligomer did exhibit rich
oxidation chemistry, undergoing two reversible, and one quasi-
reversible oxidation processes. The first 3T oligomer oxidation
is centered at 1.04 V, and is assigned to the formation of the 3T
radical cation.13e,14b,c,e,f Further oxidation produces a dicationic
species.
As observed for complexes 9 and 10, each of the 3T-pendant
complexes undergoes a quasi-reversible reduction located at
−1.25 and −0.40 V respectively for 11 and 12. Generation of
more highly reduced species at more negative potentials during
electrochemical experiments conducted with 11 and 12 gave
evidence for precipitation on the electrode surface. These
additional reductions were not assigned. A noteworthy
similarity in the electrochemical data is the 850 mV positive
shift observed for the first reduction of the diimine ligand in the
NO2 functionalized 10 and 12, relative to complexes 9 and 11.
The similarity in the reduction chemistry of the 3T-pendant
complexes is fully supported by computational results, which
present diimine MOs nearly identical to those of the analogous
control complexes.
The 3T-pendant complexes 11 and 12 revealed three
oxidation processes rather than the single process observed
for the control complexes 9 and 10 (Table 3). In the 3T-
pendant complexes the first quasi-reversible oxidation at 1.10 V
is assigned to the isopotential one electron oxidation of both
3T-pendants to form the respective 3T radical cations. This
process is nearly unchanged in potential from the analogous
process in the free 3T oligomer, suggesting the ground states of
the 3T-pendant complexes and 3T oligomer are comparable.
The second process at ∼1.37 V is assigned to the oxidation of
Figure 4. Cyclic voltammograms in ACN for control complexes 9, 10
and 3T-pendant complexes 11, 12. Anodic currents are positive and
plotted downward. Scan rate of 100 mV/s.
consistent with the minimal disturbance in their HOMO
energies, as predicted by DFT. A single reduction centered at
−1.37 V for complex 9, is replaced by two reductions for
complex 10, centered at −0.52 and −1.16 V. A third reduction
at −1.42 V may be due to the successive reduction of the
doubly reduced species. These reductive processes are assigned
to ligand based reductions of the diimine ligand found in 9 and
10.
the Ir(III)-(Ph−C N)2 moiety, only slightly shifted from the
∧
values observed for the same process in control complexes
(Table 3). The third oxidation process located at ∼1.54 V is
attributed to a second oxidation of the 3T-pendant to form the
dicationic terthiophene species.13e,14b,e,24 This process is not
reversible on the CV time scale and occurs at potentials close to
the second oxidation making it difficult to resolve electro-
chemically. Ordering of the first two oxidative processes are in
accord with the composition of the HOMO/HOMO−1, and
HOMO−2 predicted by DFT calculations.
Electronic Spectroscopy and Excited State Analysis.
UV−vis absorption spectra collected in ACN for cationic Ir(III)
bis-cyclometalates 9−12, and the free 3T oligomer 6, are shown
in Figure 5. Corresponding photophysical and spectral data
have been summarized in Table 3. To assist with the
interpretation of excited state properties, TD-DFT calculations
were performed for all molecules in the series (9−12) on their
fully optimized ground-state geometries. An energy level
diagram representing the three lowest singlet and triplet
excited states of complexes 11 and 12 is shown in Figure 6 (see
Supporting Information for detailed assignment of states). The
free 3T oligomer exhibits a relatively simple electronic
spectrum with only two strongly absorbing features. The high
energy band, located at 252 nm, is assigned to the π → π*
transition localized on the Hppy cap of 6. The broad, intense
absorption at lower energy (378 nm) is identified as the π →
π* transition characteristic of π-conjugated thiophene
oligomers.
The most notable difference between the electrochemistry of
the control complexes is the large positive shift (850 mV) in the
first reduction of 10 relative to 9. Significant perturbation of the
first reduction potential and LUMO energy of 10 clearly
demonstrates the strong electron withdrawing character of the
NO2 substituent. A positive shift in the location of the first
reductive process is well documented for similar complexes
coordinating NO2 substituted ligands, although the terminus of
the electron has been a topic of debate.9d,39 Recent work to
determine the reduction site of 4-NO2-bpy (1) and its Pt(II)
coordination complex Pt(4-NO2-bpy)Cl2 was conducted by
Murray et al. with a combination of theoretical calculations,
cyclic voltammetry, as well as UV/vis, NIR, and EPR
spectroelectrochemistry.40 Their results indicate that after
electrochemical reduction or photoexcitation (MLCT, dπPt(II)
∧
→ π*N N) the major component of electron density is localized
on the NO2 substituted pyridyl ring in both the bound and the
unbound ligand. We adopt their assignment for the first
reduction in complex 10 as the one electron reduction of the
NO2 substituted pyridyl ring of the coordinated 4-NO2-bpy
ligand. It is likely that the reduction at more negative potentials
in 10 is the addition of the second electron to the unsubstituted
pyridyl ring, slightly stabilized by induction.
Electrochemistry of the 3T Oligomer and 3T-Pendant
Complexes. The electrochemical properties of the 3T
oligomer 6 and its corresponding Ir(III) complexes 11 and
Control complexes 9 and 10 presented absorption spectra
representative of comparable cationic Ir(III) bis-cyclometalate-
5089
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
occur at 326 (9) and 388 (10) nm. Experiment shows intense
absorption bands and shoulders present at high energy (320−
350 nm), assigned to 1LC transitions (π → π*) on the
cyclometalating and diimine ligands. At lower energy a series of
weak absorption shoulders (340−440 nm) followed by a set of
very weak absorptions (450−470 nm) can be found. The bands
1
located between 340−440 nm are assigned to LLCT/1MLCT
∧
∧
transitions (πph− → π*N N/dπIr(III) → π*N N), while those
found at wavelengths longer than 450 nm likely result from
transitions to the corresponding triplet states. The exact
ordering of these bands is not known for complexes 9 and
10. The long wavelength absorption band centered at 530 nm
for complex 10 is likely due to transitions terminating on the
significantly stabilized MO of the NO2 substituted diimine
ligand (see inset Figure 5).
Interpretation of the electronic spectra for the 3T-pendant
complexes 11 and 12 becomes increasingly more complicated
than for either the 3T oligomer or the control complexes.
Vertical excitations to the lowest energy singlet state S1 in the
3T-pendant complexes are described as a 1π → π* transition by
TD-DFT. Calculated values for excitation of the S1 state are
446 (11) and 448 (12) nm. Experimentally the ultraviolet
region of complexes 11 and 12 display the same intense high
energy absorptions as 9 and 10, consistent with the 1LC
transitions (π → π*) on the cyclometalating and diimine
ligands. The remaining portions of the spectra are dominated
1
by the characteristic π → π* transition of the 3T-pendants, as
observed in the 3T oligomer. The absorbance of this band is
also increased (∼ ×1.5) because of the existence of two 3T-
pendants within the same molecule. Presence of the NO2
substituent on the diimine ligand of 12 yields a long wavelength
absorption near 540 nm analogous to the band observed in 10.
The NO2 substitution appears to have no effect on the overall
position of the absorption bands corresponding to the 3T-
pendants. The broad absorptions attributed to the 3T-pendants
in complexes 11 and 12 overlap substantially with the position
of the charge transfer bands observed in the control complexes.
Observation and assignment of those bands expected to be
present beneath the 3T-pendant absorptions would require
additional experiments and data treatment beyond the scope of
this study.
A noteworthy red-shift (∼40 nm) in the absorption of the
3T-pendants 1π → π* transition is observed upon complexation
of 6 to the Ir(III) ion in both 11 and 12. Behavior such as this
has been consistently observed for coordination of ligands
functionalized with oligothiophene pendants to transition metal
centers,13h rationalized as stabilization of the oligothiophene
excited state resulting from increased conjugation with the aryl
caps along with the metal cations inductive effect.24b We believe
this reasoning can be applied as the likely cause of the red-shift
described here for complexes 11 and 12.
Figure 5. Normalized UV−vis absorption spectra in ACN solutions at
room temperature of (a) control complexes 9, 10. (b) 3T oligomer 6
and 3T-pendant complexes 11, 12. Insets depict long wavelength
absorbance of 10 and 12.
s.2a,4h,7e,9b,e,n,10a,41 Vertical excitations to the lowest energy
singlet state S1 in the control complexes are described as a
1
mixed LLCT/1MLCT transition by TD-DFT, computed to
Aside from complex 9, whose luminescence has been
previously reported,2a,9b,e,n,10b,41a,e the remaining cationic
Ir(III) bis-cyclometalates display extremely weak luminescence
at room temperature. Quantum yields obtained for the NO2
and 3T functionalized compounds are 2−3 orders of magnitude
smaller than that of control complex 9. In the case of the 3T-
pendant complexes accurate measurement of quantum yield or
time dependent decay dynamics using our experimental
configuration were not possible because of extremely low
luminescence intensities.
The experimentally observed luminescence of control
complexes 9 and 10 (Table 3, Figure 7) is described by TD-
Figure 6. Energy level diagram for the lowest excited states in the 3T-
pendant complexes (11−12).
5090
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
it is reasonable to infer that the HOMO in complex 10 is
relatively undisturbed by the NO2 substituent. Conversely, the
very large shift in the reduction potential and computed energy
of the LUMO for 10 suggests significant stabilization of this
MO, as has been previously reported.9d,39,40 These results
emphasize the ability to independently modify the energy of
HOMO and LUMO. Specific functionalization of the cyclo-
metalating and diimine ligands then allows for the potential to
create a low lying trap state for triplet energy.
Introduction of 3T-pendants into the system provides
additional photoredox chromophores with MOs that may
produce dramatic changes in the excited state dynamics.
Relative to both the control complexes and the 3T oligomer an
overall positive shift for the oxidation of the 3T-pendants and
the Ir(III)-(Ph−
) moiety in complexes 11 and 12 is
∧
C N
2
apparent, but minimal. The minimal disturbance in the
oxidation potential of the 3T-pendants is indicative of minor
electronic involvement between the 3T and Ir(III) chromo-
phores in the ground state. Further evidence of the weak
interaction is found in the contour plots of the HOMO/
HOMO−1 for complexes 11 and 12. This type of weak
electronic coupling is commonly observed in metal-oligothio-
phene systems, attributed to decreased conjugation between
the ligated aryl caps and their oligothiophene pendants.24b,45
This suggests that coordination of the ligand containing the 3T
oligomer is unlikely to be a significant contributor to the
changes in the electrochemical processes observed at positive
potentials. Rather, this shift to positive potentials is likely a
consequence of the increased positive charge imparted on the
complexes through sequential oxidation.
Figure 7. Luminescence spectra of 3T oligomer 6 and control
complexes 9, 10 in deaerated ACN solutions at 298 K. λex = 400 nm.
DFT as an excitation to the lowest triplet excited state T1 with
3LLCT/3MLCT character, computed to arise at 381 (9) and
524 (10) nm. For the nonluminescent 3T-pendant complexes
TD-DFT predicts excitations to the 3T centered T1 and T2
states (Figure 6), a degenerate pair of low lying triplet states, 3π
→ π* in character. Calculations place the energy of these
degenerate triplet states 1.96 eV above the ground. This value is
in excellent agreement with the values measured and computed
for the lowest triplet excited state of the unfunctionalized 3T
oligomer, ranging from 1.76−1.92 eV.42
A partial decrease in emission intensity has been observed for
Ir(III) complexes similar to 10 coordinating electron with-
drawing diimine ligands bearing COOH or COOR substituent-
s.4h,7e,9p In cases where the strong electron withdrawing NO2
substituent is used, near total quenching of charge transfer
phosphorescence is reported for Ru(II), Re(I), Pt(II), and
Ir(III) species.9d,39a,43 Strong involvement of the NO2
substituent with the excited state and its vibrational modes
are believed to be the major deactivation pathway of radiative
decay in 10.39a There is also evidence in NO2 aromatics for
excited state deactivation due to structural reorganization,
involving intramolecular twisting of the NO2 substituent out of
the aromatic ring plane.44 The weak emission measured for 10
is in nearly the same location as that of control complex 9
(Table 3). Electrochemical measurements and analysis of
absorption band energies for 10 suggest excitation of the
1LLCT/1MLCT state and subsequent decay from the
3LLCT/3MLCT after rapid intersystem crossing to occur at
much lower energies than observed. Considering the position
and the extremely low quantum yield we cannot be certain of
the origin of luminescence from 10; for example, the
luminescence may originate from an alternate triplet state on
the diimine ligand. In any event, efficient deactivation of
excitation energy likely occurs through a triplet state partially
localized on the NO2 substituent.
The 3T-pendants have a more pronounced effect on the
excited states of complexes 11 and 12. The absence of strong
luminescence from the 3T-pendant complex 11 is striking as
both the free 3T oligomer 6 and the control compound 9
display strong fluorescence and phosphorescence respectively,
both with ϕem = 0.11. For uncoordinated oligothiophene
chromophores fluorescence from the initial photoexcited
1π−π* state is observed, with intersystem crossing to the
3
lowest energy nonemissive π−π* state representing the major
nonradiative pathway.42a It has also been shown that
asymmetric capping of 3T oligomers in the 5 or 5″ position
with electron withdrawing substituents enhances the rate of
nonradiative decay (knr) in polar solvents by imparting a charge
transfer character from the 3T-backbone to the polarized caps,
14d
1
deactivating fluorescence from the π−π* state. In complex
11 the presence of a positively charged Ir(III) ion coordinated
to the ppy cap of the 3T oligomer serves as an electron
withdrawing moiety in the excited state, as indicated by the
significant contribution from the pyridyl ring of the cyclo-
metalating ligands in the contour plot of the LUMO/LUMO
+1.
On the basis of TD-DFT results and the disparity between
the extinction coefficients of the 3T and Ir(III) chromophore,
photoexcitation of the 3T-pendant complexes efficiently
1
populates the 3T π−π* state, although contributions of a
charge transfer character cannot be mutually excluded. We
1
suggest that following photoexcitation, the π−π* state rapidly
undergoes intersystem crossing to the lowest energy excited
triplet state facilitated by the strong spin−orbit coupling of the
Ir(III) ion. TD-DFT finds the lowest energy excited state to be
the 3T based 3π−π* state, where the excitation energy is left to
DISCUSSION
■
Electrochemistry and DFT calculations for the first oxidation
and reduction processes in complexes 9 and 10 correlate with
the relative positioning of the HOMO and LUMO,
respectively. From the electrochemical and theoretical results,
decay nonradiatively.8a,46 Existence of the dark π−π* states
3
below the potentially emissive Ir(III) based triplet states found
5091
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
in the control complexes further supports deactivation of
potential radiative decay pathways. This relative positioning of
excited triplet states has been observed in other metal-
oligothiophene systems, resulting in quenching of oligomer
fluorescence and metal centered phosphorescence, supporting
the quenching observed here.13a,b,j,46a
is not particularly beneficial for light-harvesting in the visible
region, but use of longer wavelength absorbers such as the 3T
oligomers used in this study may still provide worth in light-
generating devices such as LECs.
ASSOCIATED CONTENT
* Supporting Information
■
The 3T/NO2 functionalized compound 12 is similar to 11
and its photophysical behavior can be rationalized using the
arguments outlined above for both complexes 10 and 11. In
summary, attempts to design a trap for the lowest triplet excited
state through functionalization of the diimine ligand with a
NO2 substituent did not lower the energy of the diimine MO
(Figure 7, T3 for 12) enough to shift the resulting
S
Crystallographic data in CIF format for 10, additional structural
discussion, detailed assignment of TD-DFT transitions, and
atom coordinates for DFT optimized geometries of complexes
9−12. This material is available free of charge via the Internet
at http://pubs.acs.org.
3
3LLCT/3MLCT state below the 3T π−π* state. As a result
AUTHOR INFORMATION
Corresponding Author
*E-mail: krmann@umn.edu (K.R.M.), cramer@umn.edu
(C.J.C.).
■
it is no surprise that neither fluorescence nor phosphorescence
is observed in the 3T-pendant complexes.
CONCLUSIONS
■
Notes
We have carried out electrochemical and photophysical
investigations that include theoretical calculations on a series
of cationic Ir(III) bis-cyclometalated complexes (9−12)
incorporating 3T-pendants and the potent electron with-
drawing NO2 substituent. Differences arising from the system-
atic introduction of 3T-pendants and the NO2 substituent are
illustrated dramatically in the cyclic voltammetry experiments.
The potentials for the one electron oxidative processes of the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank the University of Minnesota
and Victor G. Young of the X-ray crystallographic Facility.
Experimental work was funded by a grant from the Chemical
Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, Office of Science, U.S. Department of Energy,
under Award DE-FG02-07ER15913. Theoretical work was
funded by the National Science Foundation (CHE-0952054).
3T-pendants and the Ir(III)-(Ph−C N)2 moiety are unaffected
∧
by each other, but the one electron reductive process of the
diimine ligand is greatly affected by addition of the NO2
substituent. These trends are confirmed by DFT calculations
that show almost no disturbance in the Ir(III)-(Ph−C N)2 based
∧
REFERENCES
■
MO upon addition of 3T-pendants, while considerable
stabilization of the diimine based MO is evident upon addition
of a NO2 substituent. Both electrochemical and DFT results
suggest that the series should be capable of providing a low
lying trap state localized on the NO2 functionalized diimine
ligand.
The excited state properties of the series and the supportive
TD-DFT calculations tell a different story. Examination of the
absorption spectra of 3T-pendant complexes (11 and 12)
shows complete overlap of the bands originating from both the
metal and organic chromophores ruling out selective excitation.
Aside from control complex 9 and the uncomplexed 3T
oligomer 6, NO2 functionalized complex 10 is weakly
luminescent and no luminescence is observed upon photo-
excitation of the 3T-pendant complexes at 400 nm in room
temperature ACN solutions. TD-DFT calculations suggest that
the absence of luminescence from the 3T-pendant complexes
results because the lowest excited triplet state is found on the
3T-pendants rather than on the diimine ligand, regardless of
the NO2 substituent.
The systems studied here have merit as potential light-
harvesting materials that have been improved through the
addition of the strongly absorbing 3T-pendants as secondary
chromophores to increase spectral efficacy; however, further
tuning of the system energetics will be necessary to direct the
excitation energy to a triplet state capable of charge injection.
This latter adjustment has proven somewhat difficult to achieve
as cationic Ir(III) bis-cyclometalates have higher energy triplet
excited states than their Ru(II) polypyridyl counterparts. The
use of UV-absorbing pendant chromophores is then required to
raise the triplet state energy of the pendant to prevent energy
funneling into a noneffective lowest energy state. This strategy
(1) (a) Asbury, J. B.; Ellingson, R. J.; Ghosh, H. N.; Ferrere, S.;
Nozik, A. J.; Lian, T. J. Phys. Chem. B 1999, 103, 3110. (b) Bisquert, J.;
Cahen, D.; Hodes, G.; Ruehle, S.; Zaban, A. J. Phys. Chem. B 2004,
108, 8106. (c) Figgemeier, E.; Hagfeldt, A. Int. J. Photoenergy 2004, 6,
127. (d) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.;
Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. J. Am.
Chem. Soc. 2008, 130, 10720. (e) Gratzel, M. J. Photochem. Photobiol.
2004, 164, 3. (f) Hagfeldt, A.; Graetzel, M. Acc. Chem. Res. 2000, 33,
269. (g) Heimer, T. A.; Heilweil, E. J.; Bignozzi, C. A.; Meyer, G. J. J.
Phys. Chem. A 2000, 104, 4256. (h) Kamat, P. V.; Haria, M.;
Hotchandani, S. J. Phys. Chem. B 2004, 108, 5166. (i) Nazeeruddin, M.
K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito,
S.; Takeru, B.; Graetzel, M. J. Am. Chem. Soc. 2005, 127, 16835.
(j) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Lagref, J. J.; Liska, P.;
Comte, P.; Barolo, C.; Viscardi, G.; Schenk, K.; Graetzel, M. Coord.
Chem. Rev. 2004, 248, 1317. (k) O’Regan, B.; Graetzel, M. Nature
1991, 353, 737. (l) Yum, J.-H.; Chen, P.; Gratzel, M.; Nazeruddin, M.
K. ChemSusChem 2008, 1, 699.
(2) (a) Lo, K. K.-W.; Chan, J. S.-W.; Lui, L.-H.; Chung, C.-K.
Organometallics 2004, 23, 3108. (b) Lo, K. K.-W.; Hui, W.-K.; Chung,
C.-K.; Tsang, K. H.-K.; Lee, T. K.-M.; Li, C.-K.; Lau, J. S.-Y.; Ng, D.
C.-M. Coord. Chem. Rev. 2006, 250, 1724. (c) Lo, K. K.-W.; Tsang, K.
H.-K.; Sze, K.-S.; Chung, C.-K.; Lee, T. K.-M.; Zhang, K. Y.; Hui, W.-
K.; Li, C.-K.; Lau, J. S.-Y.; Ng, D. C.-M.; Zhu, N. Coord. Chem. Rev.
2007, 251, 2292. (d) Shao, F.; Elias, B.; Lu, W.; Barton, J. K. Inorg.
Chem. 2007, 46, 10187. (e) Zhang, K. Y.; Lo, K. K.-W. Inorg. Chem.
2009, 48, 6011.
(3) (a) Borisov, S. M.; Klimant, I. Anal. Chem. 2007, 79, 7501. (b) Di
Marco, G.; Lanza, M.; Mamo, A.; Stefio, I.; Di Pietro, C.; Romeo, G.;
Campagna, S. Anal. Chem. 1998, 70, 5019. (c) Fernandez-Sanchez, J.
F.; Roth, T.; Cannas, R.; Nazeeruddin, M. K.; Spichiger, S.; Graetzel,
M.; Spichiger-Keller, U. E. Talanta 2007, 71, 242. (d) Koese, M. E.;
Crutchley, R. J.; DeRosa, M. C.; Ananthakrishnan, N.; Reynolds, J. R.;
Schanze, K. S. Langmuir 2005, 21, 8255. (e) Mak, C. S. K.; Pentlehner,
D.; Stich, M.; Wolfbeis, O. S.; Chan, W. K.; Yersin, H. Chem. Mater.
5092
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
2009, 21, 2173. (f) Marin-Suarezdel Toro, M.; Fernandez-Sanchez, J.
F.; Baranoff, E.; Nazeeruddin, M. K.; Graetzel, M.; Fernandez-
Gutierrez, A. Talanta 2010, 82, 620. (g) Xie, Z.; Ma, L.; de Krafft, K.
E.; Jin, A.; Lin, W. J. Am. Chem. Soc. 2010, 132, 922.
Inorg. Chem. 2007, 46, 8533. (e) Garces, F. O.; King, K. A.; Watts, R. J.
Inorg. Chem. 1988, 27, 3464. (f) Hay, P. J. J. Phys. Chem. A 2002, 106,
1634. (g) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc.
1985, 107, 1431. (h) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-
Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.;
Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (i) Li, X.-N.; Wu,
Z.-J.; Zhang, H.-J.; Liu, X.-J.; Zhou, L.; Li, Z.-F.; Si, Z.-J. Phys. Chem.
Chem. Phys. 2009, 11, 6051. (j) Lowry, M. S.; Bernhard, S. Chem.
Eur. J. 2006, 12, 7970. (k) Lowry, M. S.; Hudson, W. R.; Pascal, R. A.,
Jr.; Bernhard, S. J. Am. Chem. Soc. 2004, 126, 14129. (l) Nazeeruddin,
M. K.; Gratzel, M. Struct. Bonding (Berlin) 2007, 123, 113.
(m) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.;
Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790.
(n) Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck, K.
W.; Watts, R. J. J. Phys. Chem. 1987, 91, 1047. (o) Tamayo, A. B.;
Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.;
Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377.
(p) Waern, J. B.; Desmarets, C.; Chamoreau, L.-M.; Amouri, H.;
Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Inorg. Chem. 2008,
47, 3340.
(10) (a) Bandini, M.; Bianchi, M.; Valenti, G.; Piccinelli, F.; Paolucci,
F.; Monari, M.; Umani-Ronchi, A.; Marcaccio, M. Inorg. Chem. 2010,
49, 1439. (b) King, K. A.; Watts, R. J. J. Am. Chem. Soc. 1987, 109,
1589. (c) Ladouceur, S. b.; Fortin, D.; Zysman-Colman, E. Inorg.
Chem. 2010, 49, 5625.
(11) (a) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M.
E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (b) Baldo, M. A.;
O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.;
Forrest, S. R. Nature 1998, 395, 151. (c) Chen, X.; Liao, J.-L.; Liang,
Y.; Ahmed, M. O.; Tseng, H.-E.; Chen, S.-A. J. Am. Chem. Soc. 2003,
125, 636. (d) D’Andrade, B. W.; Baldo, M. A.; Adachi, C.; Brooks, J.;
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 1045.
(e) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
(4) (a) Baranoff, E.; Barigelletti, F.; Bonnet, S.; Collin, J.-P.; Flamigni,
L.; Mobian, P.; Sauvage, J.-P. Struct. Bonding (Berlin) 2007, 123, 41.
(b) Baranoff, E.; Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Ventura, B.;
Flamigni, L. Inorg. Chem. 2004, 43, 3057. (c) Baranoff, E.; Griffiths, K.;
Collin, J.-P.; Sauvage, J.-P.; Ventura, B.; Flamigni, L. New J. Chem.
2004, 28, 1091. (d) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.;
Barigelletti, F. Top. Curr. Chem. 2007, 281, 143. (e) Flamigni, L.;
Collin, J.-P.; Sauvage, J.-P. Acc. Chem. Res. 2008, 41, 857. (f) Flamigni,
L.; Ventura, B.; Baranoff, E.; Collin, J.-P.; Sauvage, J.-P. Eur. J. Inorg.
Chem. 2007, 5189. (g) Geiss, B.; Lambert, C. Chem. Commun. 2009,
1670. (h) Hanss, D.; Freys, J. C.; Bernardinelli, G.; Wenger, O. S. Eur.
J. Inorg. Chem. 2009, 4850.
(5) (a) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008,
47, 10378. (b) Fukuzumi, S. Eur. J. Inorg. Chem. 2008, 1351.
(c) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. J. Am. Chem. Soc. 2010,
132, 1496. (d) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.;
Anderson, T. H.; Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502.
(e) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R. A.,
Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712.
(6) (a) Baranoff, E.; Yum, J.-H.; Graetzel, M.; Nazeeruddin, M. K. J.
Organomet. Chem. 2009, 694, 2661. (b) Baranoff, E.; Yum, J.-H.; Jung,
I.; Vulcano, R.; Gratzel, M.; Nazeeruddin, M. K. Chem.Asian. J.
2010, 5, 496. (c) Huang, J.; Yu, J.; Guan, Z.; Jiang, Y. Appl. Phys. Lett.
2010, 97, 143301. (d) Mayo, E. I.; Kilsa, K.; Tirrell, T.; Djurovich, P.
I.; Tamayo, A.; Thompson, M. E.; Lewis, N. S.; Gray, H. B. Photochem.
Photobiol. Sci. 2006, 5, 871. (e) Ning, Z.; Zhang, Q.; Wu, W.; Tian, H.
J. Organomet. Chem. 2009, 694, 2705.
(7) (a) Bolink, H. J.; Coronado, E.; Garcia Santamaria, S.; Sessolo,
M.; Evans, N.; Klein, C.; Baranoff, E.; Kalyanasundaram, K.; Graetzel,
M.; Nazeeruddin, M. K. Chem. Commun. 2007, 3276. (b) Bolink, H. J.;
De Angelis, F.; Baranoff, E.; Klein, C.; Fantacci, S.; Coronado, E.;
Sessolo, M.; Kalyanasundaram, K.; Graetzel, M.; Nazeeruddin, M. K.
Chem. Commun. 2009, 4672. (c) Bulovic, V.; Gu, G.; Burrows, P. E.;
Forrest, S. R.; Thompson, M. E. Nature 1996, 380, 29. (d) Chou, P.-
T.; Chi, Y. Chem.Eur. J. 2007, 13, 380. (e) Tamayo, A. B.; Garon, S.;
Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2005, 44, 8723. (f) Wong, W.-Y.; Ho, C.-L. Coord. Chem.
Rev. 2009, 253, 1709. (g) You, Y.; Park, S. Y. Dalton Trans. 2009,
1267.
C. E.; Kohler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004,
̈
126, 7041. (f) Schulz, G. L.; Chen, X.; Chen, S.-A.; Holdcroft, S.
Macromolecules 2006, 39, 9157. (g) Wang, L.; Lei, G.; Qiu, Y. J. Appl.
Phys. 2005, 97, 114503. (h) Yan, Q.; Yue, K.; Yu, C.; Zhao, D.
Macromolecules 2010, 43, 8479.
(12) (a) Ding, J.; Lu, J.; Cheng, Y.; Xie, Z.; Wang, L.; Jing, X.; Wang,
̈
F. Adv. Funct. Mater. 2008, 18, 2754. (b) Qin, T.; Ding, J.; Wang, L.;
Baumgarten, M.; Zhou, G.; Mullen, K. J. Am. Chem. Soc. 2009, 131,
̈
14329.
(13) (a) Bair, J. S.; Harrison, R. G. J. Org. Chem. 2007, 72, 6653.
́ ́
(8) (a) Costa, R. D.; Fernandez, G.; Sanchez, L.; Martín, N.; Ortí, E.;
(b) Barbieri, A.; Ventura, B.; Flamigni, L.; Barigelletti, F.; Fuhrmann,
Bolink, H. J. Chem.Eur. J. 2010, 16, 9855. (b) Costa, R. D.; Ortí, E.;
Bolink, H. J.; Graber, S.; Housecroft, C. E.; Constable, E. C. Adv.
Funct. Mater. 2010, 20, 1511. (c) Costa, R. D.; Ortí, E.; Bolink, H. J.;
Graber, S.; Schaffner, S.; Neuburger, M.; Housecroft, C. E.; Constable,
E. C. Adv. Funct. Mater. 2009, 19, 3456. (d) Graber, S.; Doyle, K.;
Neuburger, M.; Housecroft, C. E.; Constable, E. C.; Costa, R. D.; Orti,
E.; Repetto, D.; Bolink, H. J. J. Am. Chem. Soc. 2008, 130, 14944.
(e) Liu, S.-J.; Zhao, Q.; Fan, Q.-L.; Huang, W. Eur. J. Inorg. Chem.
2008, 2008, 2177. (f) Rodriguez-Redondo, J. L.; Costa, R. D.; Orti, E.;
Sastre-Santos, A.; Bolink, H. J.; Fernandez-Lazaro, F. Dalton Trans.
2009, 9787. (g) Rothe, C.; Chiang, C.-J.; Jankus, V.; Abdullah, K.;
Zeng, X.; Jitchati, R.; Batsanov, A. S.; Bryce, M. R.; Monkman, A. P.
Adv. Funct. Mater. 2009, 19, 2038. (h) Shan, G.-G.; Zhu, D.-X.; Li, H.-
B.; Li, P.; Su, Z.-M.; Liao, Y. Dalton Trans. 2011, 40, 2947. (i) Su, H.-
C.; Chen, H.-F.; Wu, C.-C.; Wong, K.-T. Chem.Asian. J. 2008, 3,
1922. (j) Zeng, X.; Tavasli, M.; Perepichka, I. F.; Batsanov, A. S.;
Bryce, M. R.; Chiang, C.-J.; Rothe, C.; Monkman, A. P. Chem.Eur. J.
2008, 14, 933.
G.; Bauerle, P.; Goeb, S.; Ziessel, R. Inorg. Chem. 2005, 44, 8033.
̈
(c) Hjelm, J.; Handel, R. W.; Hagfeldt, A.; Constable, E. C.;
Housecroft, C. E.; Forster, R. J. Inorg. Chem. 2005, 44, 1073.
(d) Moorlag, C.; Wolf, M. O. Polym. Prepr. 2007, 48, 543.
(e) Pappenfus, T. M.; Mann, K. R. Inorg. Chem. 2001, 40, 6301.
(f) Sauvage, F.; Fischer, M. K. R.; Mishra, A.; Zakeeruddin, S. M.;
Nazeeruddin, M. K.; Baeuerle, P.; Graetzel, M. ChemSusChem 2009, 2,
761. (g) Steen, R. O.; Nurkkala, L. J.; Angus-Dunne, S. J.; Schmitt, C.
X.; Constable, E. C.; Riley, M. J.; Bernhardt, P. V.; Dunne, S. J. Eur. J.
Inorg. Chem. 2008, 2008, 1784. (h) Stott, T. L.; Wolf, M. O. Coord.
Chem. Rev. 2003, 246, 89. (i) Yin, J.-F.; Chen, J.-G.; Lu, Z.-Z.; Ho, K.-
C.; Lin, H.-C.; Lu, K.-L. Chem. Mater. 2010, 22, 4392. (j) Ziessel, R.;
Bauerle, P.; Ammann, M.; Barbieri, A.; Barigelletti, F. Chem. Commun.
2005, 802.
(14) (a) Beek, W. J. E.; Janssen, R. A. J. Adv. Funct. Mater. 2002, 12,
519. (b) Casado, J.; Pappenfus, T. M.; Miller, L. L.; Mann, K. R.; Orti,
E.; Viruela, P. M.; Pou-Amerigo, R.; Hernandez, V.; Lopez Navarrete,
J. T. J. Am. Chem. Soc. 2003, 125, 2524. (c) Facchetti, A.; Yoon, M.-H.;
Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem.
Soc. 2004, 126, 13480. (d) Huss, A. S.; Pappenfus, T.; Bohnsack, J.;
Burand, M.; Mann, K. R.; Blank, D. A. J. Phys. Chem. A 2009, 113,
10202. (e) Pappenfus, T. M.; Burand, M. W.; Janzen, D. E.; Mann, K.
R. Org. Lett. 2003, 5, 1535. (f) Roncali, J. Acc. Chem. Res. 2009, 42,
1719. (g) Tian, H.; Shi, J.; He, B.; Hu, N.; Dong, S.; Yan, D.; Zhang, J.;
Geng, Y.; Wang, F. Adv. Funct. Mater. 2007, 17, 1940. (h) Won, Y. S.;
(9) (a) Colombo, M. G.; Brunold, T. C.; Riedener, T.; Guedel, H. U.;
Fortsch, M.; Buergi, H.-B. Inorg. Chem. 1994, 33, 545. (b) Colombo,
M. G.; Hauser, A.; Guedel, H. U. Inorg. Chem. 1993, 32, 3088.
(c) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts, R.
J. Inorg. Chem. 1991, 30, 1685. (d) Dragonetti, C.; Falciola, L.;
Mussini, P.; Righetto, S.; Roberto, D.; Ugo, R.; Valore, a. A.; De
Angelis, F.; Fantacci, S.; Sgamellotti, A.; Ramon, M.; Muccini, M.
5093
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094

Inorganic Chemistry
Article
Yang, Y. S.; Kim, J. H.; Ryu, J.-H.; Kim, K. K.; Park, S. S. Energy Fuels
2010, 24, 3676. (i) Xia, P. F.; Feng, X. J.; Lu, J.; Tsang, S.-W.;
Movileanu, R.; Tao, Y.; Wong, M. S. Adv. Mater. 2008, 20, 4810.
(15) Araya, J. C.; Gajardo, J.; Moya, S. A.; Aguirre, P.; Toupet, L.;
Williams, J. A. G.; Escadeillas, M.; Le Bozec, H.; Guerchais, V. New J.
Chem. 2010, 34, 21.
(16) (a) Bolink, H. J.; Coronado, E.; Costa, R. D.; Lardies, N.; Orti,
E. Inorg. Chem. 2008, 47, 9149. (b) He, L.; Duan, L.; Qiao, J.; Wang,
R.; Wei, P.; Wang, L.; Qiu, Y. Adv. Funct. Mater. 2008, 18, 2123.
(c) Zhao, Q.; Liu, S.; Shi, M.; Wang, C.; Yu, M.; Li, L.; Li, F.; Yi, T.;
Huang, C. Inorg. Chem. 2006, 45, 6152.
(b) Bush, P. M.; Whitehead, J. P.; Pink, C. C.; Gramm, E. C.; Eglin, J.
L.; Watton, S. P.; Pence, L. E. Inorg. Chem. 2001, 40, 1871. (c) Chen,
X.-F.; Cheng, P.; Liu, X.; Zhao, B.; Liao, D.-Z.; Yan, S.-P.; Jiang, Z.-H.
Inorg. Chem. 2001, 40, 2652. (d) Lam, S. C.-F.; Yam, V. W.-W.; Wong,
K. M.-C.; Cheng, E. C.-C.; Zhu, N. Organometallics 2005, 24, 4298.
(39) (a) Basu, A.; Weiner, M. A.; Strekas, T. C.; Gafney, H. D. Inorg.
̌
̌ ̌
Chem. 1982, 21, 1085. (b) Canivet, J.; Suss-Fink, G.; Stepnicka, P. Eur.
̈
J. Inorg. Chem. 2007, 2007, 4736. (c) Morotti, T.; Pizzotti, M.; Ugo, R.;
Quici, S.; Bruschi, M.; Mussini, P.; Righetto, S. Eur. J. Inorg. Chem.
2006, 2006, 1743. (d) Stepnicka, P.; Ludvík, J.; Canivet, J.; Suss-Fink,
̈
G. Inorg. Chim. Acta 2006, 359, 2369.
(40) Murray, P.; Jack, L.; McInnes, E. J. L.; Yellowlees, L. J. Dalton
Trans. 2010, 39, 4179.
(17) Demnitz, F. W. J.; D’heni, M. B. Org. Prep. Proced. Int. 1998, 30,
467.
(41) (a) Colombo, M.; Hauser, A.; Gudel, H. Top. Curr. Chem. 1994,
̈
(18) Zhu, S. S.; Kingsborough, R. P.; Swager, T. M. J. Mater. Chem.
1999, 9, 2123.
(19) Wang, C.; Benz, M. E.; LeGoff, E.; Schindler, J. L.; Allbritton-
Thomas, J.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1994, 6,
401.
171, 143. (b) Colombo, M. G.; Guedel, H. U. Inorg. Chem. 1993, 32,
3081. (c) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.;
Encinas, S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385. (d) Lafolet,
F.; Welter, S.; Popovic, Z.; Cola, L. D. J. Mater. Chem. 2005, 15, 2820.
(e) Wilde, A. P.; King, K. A.; Watts, R. J. J. Phys. Chem. 1991, 95, 629.
(20) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647.
́
(42) (a) Rubio, M.; Merchan, M.; Ortí, E. ChemPhysChem 2005, 6,
1357. (b) Wasserberg, D.; Marsal, P.; Meskers, S. C. J.; Janssen, R. A.
J.; Beljonne, D. J. Phys. Chem. B 2005, 109, 4410.
(43) (a) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.;
Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
(b) Villegas, J. M.; Stoyanov, S. R.; Huang, W.; Rillema, D. P. Inorg.
Chem. 2005, 44, 2297.
(21) Wenkert, D.; Woodward, R. B. J. Org. Chem. 1983, 48, 283.
(22) Comins, D. L.; Mantlo, N. B. J. Org. Chem. 1985, 50, 4410.
(23) Araki, K.; Endo, H.; Masuda, G.; Ogawa, T. Chem.Eur. J.
2004, 10, 3331.
(24) (a) Graf, D. D.; Mann, K. R. Inorg. Chem. 1997, 36, 150.
(b) Graf, D. D.; Mann, K. R. Inorg. Chem. 1997, 36, 141.
(25) (a) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
(b) Sheldrick, G. SADABS, v.2.03; Bruker AXS: Madison, WI, 2002.
(26) SHELXTL, v.6.1; Bruker AXS: Madison, WI, 2001.
(27) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101.
(28) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; John Wiley & Sons: New York, 1986.
́
(44) (a) Crespo-Hernandez, C. E.; Burdzinski, G.; Arce, R. J. Phys.
Chem. A 2008, 112, 6313. (b) Farztdinov, V. M.; Schanz, R.;
Kovalenko, S. A.; Ernsting, N. P. J. Phys. Chem. A 2000, 104, 11486.
(c) Kovalenko, S. A.; Schanz, R.; Farztdinov, V. M.; Hennig, H.;
Ernsting, N. P. Chem. Phys. Lett. 2000, 323, 312. (d) Mohammed, O.
F.; Vauthey, E. J. Phys. Chem. A 2008, 112, 3823. (e) Takezaki, M.;
Hirota, N.; Terazima, M.; Sato, H.; Nakajima, T.; Kato, S. J. Phys.
Chem. A 1997, 101, 5190.
(29) Andrae, D.; Haußermann, U.; Dolg, M.; Stoll, H.; Preuß, H.
̈
Theor. Chim. Acta 1990, 77, 123.
(45) Laye, R. H.; Couchman, S. M.; Ward, M. D. Inorg. Chem. 2001,
40, 4089.
(46) (a) Kozhevnikov, D. N.; Kozhevnikov, V. N.; Shafikov, M. Z.;
Prokhorov, A. M.; Bruce, D. W.; Williams, J. A. G. Inorg. Chem. 2011,
50, 3804. (b) Spaenig, F.; Olivier, J.-H.; Prusakova, V.; Retailleau, P.;
Ziessel, R.; Castellano, F. N. Inorg. Chem. 2011, 50, 10859.
(30) Frisch, M. J.; Trucks, G. W.; Shchlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09;
Gaussian, Inc.: Wallingford, CT, 2009.
(31) Cramer, C. J. Essentials of Computational Chemistry: Theories and
Models, 2nd ed.; John Wiley & Sons, Ltd: Chichester, U.K., 2004.
(32) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
2009, 113, 6378.
(33) (a) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997.
(b) Cammi, R.; Mennucci, B.; Tomasi, J. J. Phys. Chem. A 2000, 104,
5631.
(34) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157.
(35) (a) Jiang, W.; Gao, Y.; Sun, Y.; Ding, F.; Xu, Y.; Bian, Z.; Li, F.;
Bian, J.; Huang, C. Inorg. Chem. 2010, 49, 3252. (b) Zhao, Q.; Liu, S.;
Shi, M.; Li, F.; Jing, H.; Yi, T.; Huang, C. Organometallics 2007, 26,
5922.
(36) Allen, F. H. Acta Crystallogr., Sect. B 2002, B58, 380.
(37) (a) Chen, C.-T.; Lin, T.-Y. J.; Chen, C.-H.; Lin, K.-J. J. Chin.
Chem. Soc. 2000, 47, 197. (b) Kinnunen, T.-J. J.; Haukka, M.;
Nousiainen, M.; Patrikka, A.; Pakkanen, T. A. J. Chem. Soc., Dalton
Trans. 2001, 2649.
(38) (a) Busby, M.; Gabrielsson, A.; Matousek, P.; Towrie, M.; Di
Bilio, A. J.; Gray, H. B.; Vlcek, A. Inorg. Chem. 2004, 43, 4994.
̌
5094
dx.doi.org/10.1021/ic202573y | Inorg. Chem. 2012, 51, 5082−5094