[Ir(N^N^N)(C^N)L] +: A new family of luminophores combining tunability and enhanced photostability

Article abstract of DOI:10.1021/ic402411g

The relatively unexplored luminophore architecture [Ir(N^N^N)(C^N)L] ⁺ (N^N^N = tridentate polypyridyl ligand, C^N = 2-phenylpyridine derivative, and L = monodentate anionic ligand) offers the stability of tridentate polypyridyl coordination along with the tunability of three independently variable ligands. Here, a new family of these luminophores has been prepared based on the previously reported compound [Ir(tpy)(ppy)Cl] ⁺ (tpy = 2,2′:6′,2″-terpyridine and ppy = 2-phenylpyridine). Complexes are obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate ligand's field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir(ppy)₂(bpy)]⁺ (bpy = 2,2′-bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to elucidate experimental properties.

Full text of DOI:10.1021/ic402411g

Article
pubs.acs.org/IC
[Ir(N^N^N)(C^N)L]+: A New Family of Luminophores Combining
Tunability and Enhanced Photostability
Danielle N. Chirdon,† Wesley J. Transue,† Husain N. Kagalwala,† Aman Kaur,‡ Andrew B. Maurer,†
Tomislav Pintauer,‡ and Stefan Bernhard*,†
†Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
S
* Supporting Information
ABSTRACT: The relatively unexplored luminophore archi-
tecture [Ir(N^N^N)(C^N)L]+ (N^N^N = tridentate poly-
pyridyl ligand, C^N = 2-phenylpyridine derivative, and L =
monodentate anionic ligand) offers the stability of tridentate
polypyridyl coordination along with the tunability of three
independently variable ligands. Here, a new family of these
luminophores has been prepared based on the previously
reported compound [Ir(tpy)(ppy)Cl]+ (tpy = 2,2′:6′,2″-
terpyridine and ppy = 2-phenylpyridine). Complexes are
obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical
analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate
ligand’s field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride
with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and
quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation
reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir(ppy)2(bpy)]+ (bpy = 2,2′-
bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox
processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to
elucidate experimental properties.
emissive triplet state is not limited by the energy ceiling
imposed on ruthenium(II) materials. Instead, emission is
completely tunable via ligand modifications. This versatility
has expanded the possibilities for electron and energy transfer
studies while installing iridium as a key material for
phosphorescent light emitting diodes, solar fuel generation,
light-driven organic chemistry, and a myriad of other
applications.10−14
As with their ruthenium analogues, iridium(III) compounds
often incorporate cyclometalating (C^N) and polypyridyl
(N^N or N^N^N) ligands. Common structural motifs include
[Ir(N^N)3]3+,15 [Ir(N^N^N)2]3+,16 [ Ir(C^N)3],17 and [Ir-
(C^N)2(N^N)]+. Among these, the latter heteroleptic structure
is particularly popular with derivatives of [Ir(ppy)2(bpy)]+ (ppy
= 2-phenylpyridine) emerging as leading small molecule
photosensitizers for photocatalytic hydrogen evolution.8a,b
Interest in these compounds as photosensitizers and as
luminophores in general stems from their mixed metal to
ligand and interligand charge transfer (MLCT/ILCT) excited
state, which can be quenched oxidatively or reductively and can
be readily tuned. The LUMO involved in excitation resides on
the polypyridyl ligand while the HOMO is distributed over the
INTRODUCTION
■
Luminescent metal complexes continue to be important
synthetic targets with applications ranging from catalysis and
optoelectronics to biological imaging and gas sensing.
Phosphorescent complexes are of particular interest due to
their long-lived excited states and favorable spin statistics in
organic light emitting devices (OLEDs). The archetype of
phosphorescent compounds is [Ru(bpy)3]2+ (bpy = 2,2′-
bipyridine), which has been studied with a plethora of
derivatives in thousands of research publications;1 however,
ruthenium(II) complexes are almost exclusively orange
emitting. Their emissive charge transfer triplet state
(3MLCT) is deactivated via thermal population of a low-lying
metal-centered triplet state (3MC).2,3 Because the deactivating
3MC becomes more accessible with increases in the energy of
the emissive state, efforts to blue-shift emission are largely
thwarted by severe quenching. Significant interest in replacing
ruthenium(II) luminophores has resulted, and popular
candidates include square-planar d8 platinum(II) complexes4
as well as tetrahedral d10 copper(I) compounds.5 Period 6 d6
metals have also been explored with work focusing on metals
which offer larger ligand field splitting than ruthenium(II) such
as rhenium(I),6 osmium(II),7 iridium(III),8 and platinum(IV).9
In iridium(III) complexes, large ligand field splitting renders
thermal population of deactivating metal states unlikely, so the
Received: September 23, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Common tridentate ligand arrangements which share the same basic coordination environment as the popular photosensitizer
[Ir(ppy)2(bpy)]+.18−20
metal and the cyclometalating ligand. The spatial separation of
the frontier orbitals enables independent control of their
energies.
Unfortunately, the diimine ligand in [Ir(C^N)2(N^N)]+
luminophores is a source of photodegradation; it is susceptible
to dissociation when charge transfer populates its π* orbitals.
To circumvent this weakness, the Bernhard lab has investigated
[Ir(phbpy)2]+ (phbpy = 6′-phenyl-2,2′-bipyridine, Figure 1),
which mimics the two carbon and four nitrogen coordination
environment of [Ir(ppy)2(bpy)]+ while placing the LUMO on
more stable tridentate ligands.18 The new luminophore shows
Figure 2. [Ir(N^N^N)(C^N)L]+ type complexes previously reported
enhanced photostability, but its photophysics are ultimately
by Obara et al. (left)24 and Bexon et al. (right).21
altered in a way which diminishes its potential for device or
catalytic applications. A variety of other tridentate ligand
EXPERIMENTAL SECTION
arrangements maintaining the same basic coordination environ-
ment have been reported such as [Ir(C^N^C)(N^N^N)]+ and
[Ir(C^N^N)(N^C^N)]+ (Figure 1).19,20 These structures can
exhibit significant excited state lifetimes and spatial HOMO−
LUMO separation similar to that in [Ir(ppy)2(bpy)]+. Yet, their
synthesis is complicated by the multiple tridentate and
bidentate binding modes available to their ligands.21,22
Moreover, they tend to be less tunable than complexes with
bidentate coordination since the extensive π systems of their
tridentate ligands are more difficult to modify and more
resistant to the impacts of simple, synthetically accessible
changes.
■
General. 2-Phenylpyridine, 2,2′:6′,2″-terpyridine, IrCl3·4H2O, and
all solvents were used as received from commercial sources. The F-
mppy and MeO-mppy ligands (Figure 3) were synthesized according
to the procedure of Lowry et al.8c 4′-Phenyl-2,2′:6′,2″-terpyridine was
prepared as described by Wang and Hanan except the reaction mixture
was stirred in the dark under argon, the reaction time was extended to
25 h, and product was recrystallized from an ethanol/water mix
(98.5% EtOH v/v).26 4′-(2,4-Dimethoxyphenyl)-2,2′:6′,2″-terpyridine
was prepared based on the procedure of Song et al.27 with a slight
modification: the reaction was stirred under argon for the given run
time and then was pushed forward by adding excess aqueous
ammonium acetate before refluxing in open air for 3.5 h.28 1H and
13C NMR spectra were recorded on Bruker Avance 300 and 500 MHz
spectrometers. ESI-MS was performed with 50−60 μM acetonitrile
and methanol solutions using a Thermo-Fisher LCQ instrument.
Elemental analyses were conducted by Robertson Microlit Labo-
ratories (Ledgewood, NJ).
The relatively unexplored structure [Ir(N^N^N)(C^N)L]+
(L = anionic monodentate ligand) provides an interesting
alternative for development of luminophores and photo-
sensitizers. It has the potential to combine the stability of a
N^N^N-centered LUMO with the tunability of a C^N moiety
and the added versatility of a third independently variable
ligand. To the best of our knowledge, although a wide range of
[Ir(N^N^N)(N^N)Cl]2+ derivatives have been produced,23
only three compounds of the form [Ir(N^N^N)(C^N)L]+ have
been previously reported.21,24,25 Two of these are featured in
Figure 2 while the third, [Ir(tpy)(ppy)Cl]+ (tpy = 2,2′:6′,2″-
terpyridine), is the starting point for this work. Sato et al. have
recently introduced [Ir(tpy)(ppy)Cl]+ as a photocatalyst for
CO2 reduction, but it has yet to be fully characterized.25 Here,
an entire series of compounds is based on that complex to
methodically show the impact of individual ligand modifications
on electrochemical and photophysical properties. Predictions of
these properties and details of the electronic structure are
obtained through DFT computations. Limitations of the
computational models are also revealed. Lastly, testing of the
new materials as photosensitizers in photocatalytic hydrogen
evolution is included as proof of enhanced photostability.
Synthesis of [Ir(N^N^N)Cl3]. Following the work of Collin et
al.,16b the appropriate terpyridine ligand was combined with IrCl3·
4H2O (1.1−1.2 equiv) and ethylene glycol (∼14 mL/mmol ligand) in
a two neck 25 mL round-bottom flask. This flask was placed under an
argon atmosphere before stirring at 160 °C (1a) or 150 °C (1b, 1c) in
the dark for 10 (1b, 1c) to 12 (1a) minutes. After cooling to room
temperature, a dark red precipitate was collected via vacuum filtration
and was washed with water, ethanol, and diethyl ether.
1
[Ir(tpy)Cl3] (1a). Yield: 55%. H NMR (300 MHz, DMSO-d6): δ
9.22 (d, J = 5.5 Hz, 2H), 8.76 (d, J = 8.2 Hz, 2H), 8.72 (d, J = 7.8 Hz,
2H), 8.27 (t, J = 7.9 Hz, 2H), 8.23 (t, J = 8.1 Hz, 1H), 7.97 (dd, J =
7.5, 5.6 Hz, 2H). 13C NMR (126 MHz, DMSO-d6): δ 159.0, 157.2,
153.0, 140.4, 139.7, 128.3, 125.0, 123.5.
[Ir(phtpy)Cl3] (1b). Yield: 59%. 1H NMR (300 MHz, DMSO-d6): δ
9.23 (dd, J = 5.6, 1.1 Hz, 2H), 9.11 (s, 2H), 8.92 (d, J = 7.9 Hz, 2H),
8.31 (td, J = 7.9, 1.5 Hz, 2H), 8.25−8.18 (m, 2H), 7.98 (ddd, J = 7.7,
5.5, 1.3 Hz, 2H), 7.75−7.67 (m, 2H), 7.65−7.59 (m, 1H). 13C NMR
(126 MHz, DMSO-d6): δ 159.2, 157.3, 153.0, 151.1, 140.2, 135.2,
130.9, 129.2, 128.4, 128.3, 125.3, 121.1.
[Ir(dMeO-phtpy)Cl3] (1c). Yield: 65%. 1H NMR (300 MHz,
DMSO-d6): δ 9.22 (dd, J = 5.6, 1.1 Hz, 2H), 8.86 (s, 2H), 8.77 (d,
B
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Cyclometalating and polypyridyl ligands utilized in the new family of [Ir(N^N^N)(C^N)L]+ luminophores.
J = 7.9 Hz, 2H), 8.26 (td, J = 7.9, 1.5 Hz, 2H), 7.96 (ddd, J = 7.8, 5.5,
1.3 Hz, 2H), 7.72 (d, J = 8.5 Hz, 1H), 6.87−6.79 (m, 2H), 3.94 (s,
3H), 3.91 (s, 3H). 13C NMR (126 MHz, DMSO-d6): δ 162.7, 159.3,
158.1, 156.5, 153.0, 149.5, 140.2, 132.3, 128.2, 125.0, 123.3, 117.5,
106.1, 98.8, 56.0, 55.7.
PF6). Elem. Anal. Calcd for [IrC28H23N4OCl]PF6: C, 41.82; H, 2.88;
N, 6.97. Found: C, 41.63; H, 2.73; N, 6.68.
Synthesis of [Ir(tpy)(C^N)(CN)](PF6) Compounds. [Ir(tpy)Cl3]
(80 mg, 0.15 mmol) and the appropriate cyclometalating ligand (c.
0.70 mmol) were placed under an argon atmosphere in a test tube, and
ethylene glycol (5 mL) was injected. The reaction mixture was stirred
overnight at 180 °C in darkness. After cooling to 90 °C in the dark, the
reaction mix was injected with KCN (20 mg, 0.30 mmol) in minimal
water (0.2 mL) and was left to stir at 90 °C for an additional 1−2 h.
The mixture was then cooled to room temperature and poured into
water (20 mL). The resulting suspension was washed with chloroform
(5 × 25 mL) and vacuum filtered to break up the subsequent
emulsion. NH4PF6 (∼100 mg) was added to the aqueous layer before
it was stirred at room temperature for 1 h. Yellow precipitate was then
isolated by vacuum filtration and washed with water and diethyl ether.
[Ir(tpy)(ppy)(CN)](PF6) (3a). Yield: 45%. 1H NMR (300 MHz,
acetone-d6): δ 10.02 (ddd, J = 5.8, 1.6, 0.8 Hz, 1H), 8.99 (d, J = 8.2
Hz, 2H), 8.81 (dq, J = 7.9, 0.7 Hz, 2H), 8.68 (t, J = 8.2 Hz, 1H), 8.50
(dt, J = 8.2, 1.0 Hz, 1H), 8.37 (ddd, J = 8.4, 7.6, 1.6 Hz, 1H), 8.30 (td,
J = 7.9, 1.5 Hz, 2H), 8.07 (ddd, J = 5.7, 1.5, 0.7 Hz, 2H), 7.96 (dd, J =
7.9, 1.2 Hz, 1H), 7.83 (ddd, J = 7.4, 5.8, 1.4 Hz, 1H), 7.65 (ddd, J =
7.7, 5.6, 1.4 Hz, 2H), 7.00 (td, J = 7.6, 1.2 Hz, 1H), 6.83 (td, J = 7.4,
1.2 Hz, 1H), 5.98 (dd, J = 7.6, 1.2 Hz, 1H). 13C NMR (126 MHz,
acetone-d6): δ 167.5, 157.9, 155.1, 154.9, 153.9, 152.1, 140.2, 140.0,
139.9, 130.5, 129.7, 129.0, 125.9, 125.2, 124.6, 121.1. MS (m/z; ESI,
MeOH): 579.5 (100%, M − PF6 − CN), 606.3 (60%, M − PF6).
Elem. Anal. Calcd for [IrC27H19N5]PF6·1/2(C2H3N): C, 43.61; H,
2.68; N, 9.99. Found: C, 43.84; H, 2.42; N, 10.00.
Synthesis of [Ir(tpy)(C^N)Cl](PF6) Compounds. [Ir(tpy)Cl3]
(80 mg, 0.15 mmol) and the appropriate cyclometalating ligand (c.
0.70 mmol) were placed under an argon atmosphere in a test tube
before ethylene glycol (5 mL) was injected. The resulting mixture
stirred overnight at 180 °C in darkness. After cooling in the dark, the
reaction mixture was added to water (20 mL) and a yellow precipitate
was seen. This solid could not be readily isolated by vacuum filtration,
so the aqueous suspension was washed with chloroform (5 × 25 mL),
and the subsequent emulsion was dissipated via vacuum filtration.
Excess NH4PF6 (100 mg) was then added to the cloudy aqueous layer,
and the mixture was stirred at room temperature for 1 h. Yellow
precipitate was isolated by vacuum filtration and washed with water
and diethyl ether.
[Ir(tpy)(ppy)Cl](PF6) (2a). Yield: 61%. 1H NMR (300 MHz,
acetone-d6): δ 10.11 (ddd, J = 5.9, 1.6, 0.8 Hz, 1H), 8.89 (d, J = 8.2
Hz, 2H), 8.73 (ddd, J = 8.1, 1.4, 0.8 Hz, 2H), 8.57 (dd, J = 8.5, 7.8 Hz,
1H), 8.47 (dt, J = 8.3, 1.1 Hz, 1H), 8.31 (ddd, J = 8.2, 7.5, 1.6 Hz, 1H),
8.23 (td, J = 7.9, 1.5 Hz, 2H), 7.93 (dd, J = 7.8, 1.4 Hz, 1H), 7.89 (ddd,
J = 5.7, 1.6, 0.8 Hz, 2H), 7.80 (ddd, J = 7.4, 5.8, 1.4 Hz, 1H), 7.60
(ddd, J = 7.8, 5.6, 1.4 Hz, 2H), 6.96 (td, J = 7.6, 1.2 Hz, 1H), 6.78 (td,
J = 7.5, 1.4 Hz, 1H), 6.14 (dd, J = 7.5, 1.1 Hz, 1H). 13C NMR (126
MHz, acetone-d6): δ 166.5, 158.3, 155.8, 151.9, 150.9, 144.0, 142.0,
140.2, 140.1, 130.6, 130.5, 128.9, 125.6, 125.1, 124.2, 124.1, 124.0,
120.5. MS (m/z; ESI, MeOH): 579.5 (100%, M − PF6 − Cl), 615.1
(28%, M − PF6). Elem. Anal. Calcd for [IrC26H19N4Cl]PF6·1/
2(C4H10O): C, 42.19; H, 3.03; N, 7.03. Found: C, 41.97; H, 3.02; N,
6.89.
1
[Ir(tpy)(F-mppy)(CN)](PF6) (3b). Yield: 44%. H NMR (300 MHz,
acetone-d6): δ 9.83 (dt, J = 2.2, 0.8 Hz, 1H), 8.99 (d, J = 8.2 Hz, 2H),
8.81 (ddd, J = 8.2, 1.4, 0.7 Hz, 2H), 8.68 (dd, J = 8.5, 7.8 Hz, 1H), 8.38
− 8.24 (m, 3H), 8.19 (ddd, J = 8.4, 2.0, 0.8 Hz, 1H), 8.07 (ddd, J =
5.6, 1.5, 0.7 Hz, 2H), 8.03 − 7.93 (m, 1H), 7.65 (ddd, J = 7.8, 5.6, 1.4
Hz, 2H), 6.73 (td, J = 8.9, 2.6 Hz, 1H), 5.63 (dd, J = 8.6, 2.6 Hz, 1H),
2.65 (s, 3H). 13C NMR (126 MHz, acetone-d6): δ 157.9, 154.9, 153.5,
152.2, 140.9, 140.2, 140.1, 135.3, 129.1, 126.9 (d, J = 8.7 Hz), 126.0,
124.9, 120.6, 116.0 (d, J = 17.6 Hz), 111.4 (d, J = 22.8 Hz), 17.6. MS
(m/z; ESI, MeOH): 611.5 (100%, M − PF6 − CN), 638.2 (86%, M −
PF6). Elem. Anal. Calcd for [IrC28H20N5F]PF6: C, 42.97; H, 2.58; N,
8.95. Found: C, 42.72; H, 2.29; N, 8.69.
[Ir(tpy)(F-mppy)Cl](PF6) (2b). Yield: 57%. 1H NMR (300 MHz,
acetone-d6): δ 9.92 (s, 1H), 8.92 (d, J = 8.2 Hz, 2H), 8.75 (ddd, J =
8.1, 1.4, 0.7 Hz, 2H), 8.61 (dd, J = 8.5, 7.8 Hz, 1H), 8.34 (d, J = 8.4
Hz, 1H), 8.27 (td, J = 7.9, 1.5 Hz, 2H), 8.16 (ddd, J = 8.4, 2.1, 0.8 Hz,
1H), 7.97 (dd, J = 8.6, 5.5 Hz, 1H), 7.91 (ddd, J = 5.7, 1.5, 0.7 Hz,
2H), 7.62 (ddd, J = 7.8, 5.6, 1.4 Hz, 2H), 6.73 (td, J = 8.8, 2.5 Hz, 1H),
5.83 (dd, J = 9.0, 2.5 Hz, 1H), 2.63 (s, 3H). 13C NMR (126 MHz,
acetone-d6): δ 158.3, 155.8, 152.0, 150.5, 140.9, 140.3, 140.2, 134.3,
128.9, 126.6 (d, J = 9.2 Hz), 125.7, 124.3, 120.0, 116.8 (d, J = 18.9
Hz), 110.9 (d, J = 22.9 Hz), 17.6. MS (m/z; ESI, MeOH): 611.5
(100%, M − PF6 − Cl), 647.1 (36%, M − PF6). Elem. Anal. Calcd for
[IrC27H20N4FCl]PF6: C, 40.94; H, 2.54; N, 7.07. Found: C, 40.87; H,
2.38; N, 6.93.
Synthesis of [Ir(phtpy)(ppy)Cl](PF6) (2d). [Ir(phtpy)Cl3] (72
mg, 0.119 mmol), ppy (71.25 μL, 0.499 mmol), and ethylene glycol
(3.7 mL) were combined in a test tube. The tube was placed under
argon atmosphere and stirred at 174 °C for 17 h in darkness. Water
(8.0 mL) was added to the cooled reaction mixture, and the resulting
aqueous solution was extracted with DCM (4 × 6.5 mL) as well as
ether (2 × 8.0 mL). The aqueous phase was gently heated to remove
residual ether and was filtered. NH4PF6 (80 mg) was added, and the
resulting orange precipitate was collected via vacuum filtration.
Collected solid was washed with water and recrystallized through
1
[Ir(tpy)(MeO-mppy)Cl](PF6) (2c). Yield: 47%. H NMR (300 MHz,
acetone-d6): δ 9.87 (s, 1H), 8.90 (d, J = 8.2 Hz, 2H), 8.73 (ddd, J =
8.1, 1.3, 0.7 Hz, 2H), 8.57 (dd, J = 8.5, 7.8 Hz, 1H), 8.24 (td, J = 8.0,
1.6 Hz, 3H), 8.08 (ddd, J = 8.5, 2.1, 0.8 Hz, 1H), 7.91 (ddd, J = 5.7,
1.5, 0.7 Hz, 2H), 7.84 (d, J = 8.6 Hz, 1H), 7.62 (ddd, J = 7.8, 5.6, 1.4
Hz, 2H), 6.56 (dd, J = 8.6, 2.5 Hz, 1H), 5.51 (d, J = 2.5 Hz, 1H), 3.50
(s, 3H), 2.59 (s, 3H). 13C NMR (126 MHz, acetone-d6): δ 163.8,
160.5, 158.3, 155.9, 151.9, 150.0, 143.4, 140.6, 140.1, 140.0, 136.6,
133.0, 128.8, 126.3, 125.6, 124.1, 119.2, 116.1, 108.5, 54.2, 17.6. MS
(m/z; ESI, MeOH): 623.4 (100%, M − PF6 − Cl), 659.2 (31%, M −
1
vapor diffusion of pentane into acetone. Yield 50%. H NMR (300
MHz, acetone-d6): δ 10.13 (dq, J = 5.9, 0.7 Hz, 1H), 9.23 (s, 2H), 8.94
(dq, J = 8.1, 0.7 Hz, 2H), 8.48 (d, J = 8.0 Hz, 1H), 8.33 (m, 1H),
8.30−8.22 (m, 4H), 7.97- 7.89 (m, 3H), 7.82 (dd, J = 7.5, 1.4 Hz, 1H),
7.78−7.66 (m, 3H), 7.61 (dd, J = 8.0, 1.3 Hz, 2H), 6.96 (td, J = 7.5,
1.0 Hz, 1H), 6.78 (td, J = 7.5, 1.4 Hz, 1H), 6.27 (dd, J = 7.7, 0.9 Hz,
C
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1H). 13C NMR (126 MHz, acetone-d6): δ 159.4, 156.8, 153.3, 152.9,
151.9, 141.1, 140.9, 132.0, 131.6, 131.4, 130.4, 129.8, 129.1, 126.7,
126.0, 125.1, 124.9, 122.7, 121.5. MS (m/z; ESI, MeOH): 655.5
(100%, M − PF6 − Cl), 691.2 (97%, M − PF6). Elem. Anal. Calcd for
[IrC32H23N4Cl]PF6: C, 45.96; H, 2.77; N, 6.70. Found: C, 45.78; H,
2.58; N, 6.50.
and treated as riding atoms during subsequent refinement, with
Uiso(H) = 1.2Ueq(C) or 1.5Ueq (methyl C). The methyl groups were
allowed to rotate about their local 3-fold axes. Crystal Maker 8.2 was
used to generate molecular graphics.
Electrochemistry. Cyclic voltammograms were collected using a
CH-Instruments Electrochemical Analyzer 600C potentiostat with a
three electrode system consisting of a 1 mm2 platinum disk working
electrode, a coiled platinum counter electrode, and a silver wire
pseudoreference.32 Potential scans were performed under an argon
purge in acetonitrile solutions containing 0.10 M tetrabutylammonium
hexafluorophosphate as supporting electrolyte and 0.5−0.8 mM
analyte. An internal ferrocene standard was added to each solution,
and potentials were referenced to SCE by setting the ferrocene
oxidation to 0.40 V.33 Unless otherwise noted, positive scan polarity
and a scan rate of 0.10 V/s were employed.
Computational Methodology. All DFT calculations were carried
out with the Gaussian 03 suite.34 Ground-state and excited-state
geometries (both singlet and triplet) for all complexes were evaluated
computationally using Becke’s three-parameter exchange functional
(B3)35 in conjunction with the Lee, Yang, and Parr (LYP)36,37
nonlocal functional using the default thresholds for gradient
convergence. The LANL2DZ basis set was employed for all
calculations.38 The geometry optimization for complexes 2a and 3a
was performed with a Cs symmetry restriction, but the same minimum
energy and optimal coordinates were found without any constraints.
To model the complexes immediately after radiative relaxation, the
energy of a singlet electron configuration in the triplet geometry was
calculated. For each complex, time-dependent DFT (TD-DFT)
calculations were performed at the optimized ground-state geometry,
calculating the energy and oscillator strength for each of the 70 lowest
singlet excitations. Prediction of UV−vis absorption spectra was
accomplished using GaussSum 2.0.39 Electronic transitions were
expanded as Gaussian curves with a full-width at half-maximum
(fwhm) for each peak set at 4000 cm−1.
Photophysical Characterization. Photophysical measurements
were conducted at room temperature on 10 μM acetonitrile solutions
which were purged with argon for 5 min in capped quartz cuvettes.
UV−vis absorption spectra were obtained with a Shimadzu UV-1800
spectrophotometer. Photoluminescence was measured using a
Fluorolog-3 spectrophotometer equipped with dual monochromators
and a photomultiplier tube (PMT) at right angle geometry. All
compounds were excited at 380 nm. In order to calculate excited state
lifetimes, samples were pulsed at 337 nm with a Stanford Research
Systems NL 100 N2 laser as emission decay was monitored with an
oscilloscope and converted into a linear regression via a Labview
interface. Quantum yields were determined using a 10 μM
[Ru(bpy)3]2+ reference and the expression ϕs = ϕref (Is/Iref)(Aref/
As)(ηs/ηref)40 where ϕs is the sample’s quantum yield, ϕref is the
reference’s established quantum yield (6.2%),41 Is and Iref represent
maximum emission intensities for the sample and reference, As and Aref
are sample and reference absorbances at the excitation wavelength, and
ηref and ηs represent the refractive indices of the solvents used in
reference and sample absorbance measurements. Emission intensities
were corrected for the detector’s response over the spectral range.
Stern−Volmer analysis was performed by measuring emission from
solutions under atmospheres containing 10%, 30.7%, or 100% O2 at
ambient pressure. Slopes of generated Stern−Volmer plots gave
pressure-based Stern−Volmer constants (KsvP), which were converted
to quenching constants kq based on the expressions KsvP = KsvKh and
Ksv = kqτ0 where Ksv is the concentration-based Stern−Volmer
constant, Kh is the Henry’s Law constant for O2 solubility in
acetonitrile (8.92 × 10−6 M mbar−1),42 and τ0 is the excited state
lifetime under argon.43
Synthesis of [Ir(phtpy)(ppy)Cl](PF6) Derivatives (2e, 2f, 2g).
The appropriate [Ir(N^N^N)Cl3] compound (0.122 mmol) was
combined with cyclometalating ligand (3−4 equiv) and ethylene glycol
(∼3.8 mL) in a test tube. The tube was purged with argon and was
stirred while heating at 179 °C for 18.2 h in the dark. After cooling for
several hours, the reaction mixture was diluted with ethanol (∼3 mL)
and then water (∼6 mL). The resulting solution was extracted with
ether (4 × 10.5 mL), giving an aqueous layer, which was filtered and
heated to remove residual ether. NH4PF6 (95 mg) was added, and
precipitate was collected via vacuum filtration. Solid was washed with
ether and water before it was recrystallized through vapor diffusion
recrystallization (ether/acetonitrile for 2f, pentane/acetone for 2e and
2g).
1
[Ir(phtpy)(F-mppy)Cl](PF6) (2e). Yield: 63%. H NMR (300 MHz,
acetone-d6): δ 9.94−9.91 (m, 1H), 9.22 (s, 2H), 8.95 (dq, J = 8.2, 0.7
Hz, 2H), 8.37−8.23 (m, 5H), 8.16 (dd, J = 8.4, 0.6 Hz, 1H), 7.97 (dd,
J = 8.7, 5.6 Hz, 1H), 7.92 (dq, J = 5.7, 0.7 Hz, 2H), 7.77−7.67 (m,
3H), 7.62 (dd, J = 7.9, 1.4 Hz, 2H), 6.72 (td, J = 8.9, 2.6 Hz, 1H), 5.96
(dd, J = 8.9, 2.6 Hz, 1H), 2.62 (s, 3H). 13C NMR (126 MHz, acetone-
d6): δ 159.5, 156.8, 152.9, 151.5, 141.9, 141.1, 132.0, 130.4, 129.8,
129.1, 127.6, 127.5, 126.8, 123.0, 120.9, 117.9, 117.8, 111.9, 111.7, 49.8
(solvent impurity), 18.6. MS (m/z; ESI, MeOH): 687.4 (69%, M −
PF6 − Cl), 723.1 (100%, M − PF6). Elem. Anal. Calcd for
[IrC33H24N4FCl]PF6: C, 45.65; H, 2.79; N, 6.45. Found: C, 45.58;
H, 2.49; N, 6.29.
[Ir(phtpy)(MeO-mppy)Cl](PF6) (2f). Yield: 56%. 1H NMR (300
MHz, acetone-d6): 9.90−9.86 (m, 1H), 9.21 (s, 2H), 8.94 (dq, J = 8.2,
0.5 Hz, 2H), 8.31−8.20 (m, 5H), 8.08 (ddd, J = 8.5, 2.1, 0.5 Hz, 1H),
7.92 (dq, J = 5.6, 0.7 Hz, 2H), 7.84 (d, J = 8.7 Hz, 1H), 7.77−7.67 (m,
3H), 7.62 (dd, J = 7.8, 1.5 Hz, 2H), 6.56 (dd, J = 8.5, 2.5 Hz, 1H), 5.63
(d, J = 2.5 Hz, 1H), 3.50 (s, 3H), 2.59 (s, 3H). 13C NMR (126 MHz,
acetone-d6): δ 159.4, 156.9, 152.9, 151.0, 141.5, 140.9, 131.9, 130.4,
129.7, 129.1, 127.2, 126.7, 122.8, 120.1, 117.3, 109.2, 55.1, 18.5. MS
(m/z; ESI, MeOH): 699.5 (31%, M − PF6 − Cl), 735.1 (100%, M −
PF6). Elem. Anal. Calcd for [IrC34H27N4OCl]PF6: C, 46.39; H, 3.09;
N, 6.36. Found: C, 46.17; H, 3.10; N, 6.38.
1
[Ir(dMeO-phtpy)(F-mppy)Cl](PF6) (2g). Yield: 75%. H NMR (300
MHz, acetone-d6): 9.95−9.91 (m, 1H), 9.04 (s, 2H), 8.80 (dq, J = 8.2,
0.5 Hz, 2H), 8.32 (d, J = 8.2 Hz, 1H), 8.24 (td, J = 7.9, 1.5 Hz, 2H),
8.15 (ddd, J = 8.4, 2.1, 0.5 Hz, 1H), 7.96 (dd, J = 8.5, 5.5 Hz, 1H),
7.90−7.85 (m, 3H), 7.59 (ddd, J = 7.9, 5.6, 1.4 Hz, 2H), 6.91−6.82
(m, 2H), 6.72 (td, J = 8.9, 2.5 Hz, 1H), 5.92 (dd, J = 9.0, 2.5 Hz, 1H),
4.06 (s, 3H), 3.97 (s, 3H), 2.62 (s, 3H). 13C NMR (126 MHz,
acetone-d6): δ 155.8, 152.9, 151.5, 141.8, 141.1, 133.2, 129.6, 126.5,
124.9, 120.9, 107.2, 99.7, 56.5, 56.2, 18.6. MS (m/z; ESI, MeOH):
747.4 (43%, M − PF6 − Cl), 783.2 (100%, M − PF6). Elem. Anal.
Calcd for [IrC35H28N4O2FCl]PF6: C, 45.29; H, 3.04; N, 6.04. Found:
C, 45.00; H, 2.82; N, 5.88.
X-ray Crystallography. Single crystals of 2c were grown for X-ray
structure determination via slow diffusion of pentane into acetone.
Resulting crystals contained one molecule of acetonitrile and ethylene
glycol along with disordered unspecified electron density located in the
lattice. This was removed by the SQUEEZE/PLATON technique,29
and the estimated volume of the solvent accessible void per unit cell is
240 Å3, indicating unrefined, disordered solvent molecules. The X-ray
intensity data was collected at 150 K using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex II CCD
diffractometer. Data reduction included absorption corrections by the
multiscan method using SADABS.30 Crystal data and detailed
experimental conditions are given in the Supporting Information
(Tables S1−S5). Structures were solved by direct methods and refined
by full-matrix least-squares using the SHELXTL 6.1 bundled software
package.31 The H atoms were positioned geometrically (aromatic C−
H = 0.93 Å, methylene C−H = 0.97 Å, and methyl C−H = 0.96 Å)
Photocatalytic Hydrogen Generation. Photocatalytic hydrogen
generation was carried out following a fixed protocol described by
Cline et al.13d Solutions containing 0.075 mM photosensitizer (PS)
and 300 nmol of K2PtCl4 were made in 40 mL screw top vials (VWR)
with 10 mL of solvent composed of 8 mL of acetonitrile (ACN), 1 mL
of water, and 1 mL of triethylamine (TEA). Control vials were also
prepared without either the PS, the catalyst, or TEA. All vials were
D
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthetic Pathway and Labels for Ir(III) Complexes
a
Reaction conditions: (a) IrCl3·4H2O, ethylene glycol, argon atmosphere, 150−160 °C, 10−12 min. (b) C^N ligand, ethylene glycol, argon
atmosphere, 174−180 °C, overnight. (c) KCN (aq), ethylene glycol, argon atmosphere, 90 °C, 1−2 h.
placed in a 16 well, water-cooled photoreactor mounted on an orbital
shaker and equipped with pressure transducers as well as bottom LED
illumination (Luxeon V Dental Blue LEDs, LXHL-LRD5 with
collimating optics Fraen FHS-HNB1-LL01-H). With the transducers
screwed onto the vials, the vials were vacuum degassed and filled with
Ar gas 7 times before the vial headspace was allowed to come to
atmospheric pressure. The orbital shaker was then started (100 rpm)
and samples were illuminated at room temperature. H2 generation was
monitored by conversion of pressure transducer readings into pressure
traces using a LabView PC interface. Illumination was continued till no
further increase in the traces was observed. Analysis of the reaction
headspace of each vial was performed using a residual gas analyzer
(RGA), precalibrated using 10% and 30% H2/Ar mixtures.
Ligand synthesis was based on literature procedures carried out
in open air,26,27 but use of argon atmosphere during
condensation was found to limit byproducts. Air, meanwhile,
was required for ring closure in 4′-(2,4-dimethoxyphenyl)-
2,2′:6′,2″-terpyridine (dMeO-phtpy).28
X-ray Crystallography. Crystallography was critical for
unambiguously identifying the obtained stereoisomer of the
complexes. Possible isomers position the ppy with its
cyclometalating carbon either cis or trans to the monodentate
ligand. The structure determined from a single crystal of 2c
(Figure 4) shows that MeO-mppy is coordinated with its
RESULTS AND DISCUSSION
■
Synthesis and Design. The family of compounds prepared
here (Scheme 1, Figure 3) can be divided into three subsets
based on their tridentate and monodentate ligands: 2a−2c
(N^N^N = 2,2′:6′,2″-terpyridine and L = Cl−), 2d−2f
(N^N^N = 4′-phenyl-2,2′:6′,2″-terpyridine derivative and L =
Cl−), and 3a and 3b (N^N^N = 2,2′:6′,2″-terpyridine and L =
CN−). Within each subset, the phenylpyridine was function-
alized in order to alter its electron density and determine
whether it is a key site for HOMO modulation as in other
cyclometalated iridium(III) materials.44−47 Compound 2g was
more specifically designed with an electron poor cyclo-
metalating ligand across from an electron rich tridentate ligand
to explore the possibility of creating a “push−pull” force on
electrons.
All chloro complexes (2a−2g) were readily synthesized in
two steps: Tridentate ligand was added to IrCl3·4H2O to give
[Ir(N^N^N)Cl3], which was then cyclometalated with excess
ppy or ppy derivative. Exclusion of light as well as control of
run time and temperature were critical to the purity of products
in both steps. Cyano complexes were formed from correspond-
ing chloro materials through cyanide−chloride exchange
directly after the cyclometalation reaction in the same pot.
Although isomer formation previously complicated preparation
of the similar complex [Ir(Mebip)(ppy)Cl]+ (Figure 2),24 one
stereoisomer was readily obtained here from all cyclometalation
and ligand exchange reactions.
Figure 4. Crystal structure of 2c shown with 30% thermal ellipsoids.
Hydrogen atoms, the counterion (PF6 ), and solvent molecules are
omitted for clarity.
−
cyclometalating carbon trans to the chloride ligand, an
orientation seen previously in the isolated isomer of [Ir-
(Mebip)(ppy)Cl]+ (Figure 2).24 The strong trans influence of
the cyclometalated carbon in 2c causes the Ir−Cl bond to be
significantly elongated (2.4457(6) Å) when compared to
similar iridium complexes with noncyclometalating bidentate
ligands: [Ir(tpy)(bpy)Cl]2+ (2.335 Å) or [Ir(tpy)(dmbpy)Cl]2+
(2.357 Å).48,49 The bite angle (N1−Ir1−C1 = 80.69(9)°) and
bond lengths (Ir1−C1 = 2.021(2) Å and Ir1−N1 = 2.068(2)
Phenylterpyridine (phtpy) ligands were prepared using a
modified Krohnke pathway in which condensation of 2-
̈
acetylpyridine and a benzaldehyde is followed by ring closure.
E
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. Electrospray ionization mass spectra collected for 2d in (A) acetonitrile and (B) methanol. Peak m/z values are given with black labels
while assignments are made in blue. Spectra were collected with 50−60 μM solutions.
Figure 6. Frontier orbital diagram constructed for 2a (left) and 3a (right) from DFT calculations performed with Gaussian software34 and displayed
via Molekel.52 Cs symmetry labels are provided for the orbitals.
Å) for MeO-mppy are very similar to the values reported for
ppy in [Ir(ppy)3].50 The iridium(III) center in 2c adopts a
distorted octahedral geometry due to constraints imposed by
the planar tpy ligand. The tpy is coordinated to the iridium via
all three of its nitrogen atoms in a meridional arrangement with
relatively small bite angles (N2−Ir1−N3 = 80.38(9)° and N3−
Ir1−N4 = 80.29(9)°). Lengths of Ir−N bonds involving tpy’s
two peripheral pyridine rings (Ir1−N2 = 2.040(2) Å and Ir1−
N4 = 2.035(2) Å) are longer than the iridium bond with the
central ring (Ir1−N3 = 1.944(2) Å) as observed previously in
compounds of the type [Ir(tpy)2]3+ and [Ir(tpy)(bpy)-
Cl]2+.16b,49,51 All bond lengths and angles are available in the
Supporting Information (Tables S4 and S5).
mass of the monodentate ligand suggesting that I represents the
product of monodentate ligand dissociation, [Ir(phtpy)-
(ppy)]2+. Grouping II, meanwhile, is consistent with a complex
in which an acetonitrile solvent molecule replaces the
monodentate ligand. Its separation from the [Ir(phtpy)-
(ppy)]2+ grouping (I) matches acetonitrile’s mass within
resolution limits, and it is completely eliminated by
replacement of acetonitrile with less coordinating methanol
solvent. Furthermore, elimination of II markedly increases the
abundance of I confirming I and II as competing forms of an
electrospray ionization product. Mass spectra for all com-
pounds mirror the three features of 2d with variation limited to
relative abundances.
Mass Spectrometry. The electrospray ionization mass
spectrum collected for 2d in acetonitrile is presented in Figure
5 as a representative example for 2a−3b. It shows the expected
M − PF6 signal grouping (m/z = 691.2), but it also contains
two other groupings labeled I (m/z = 655.5) and II (m/z =
695.5). Since NMR results show a single set of peaks, I and II
must stem from compounds which are not intrinsic to the
sample and instead form during electrospray ionization.
Separation between I and the M − PF6 grouping equals the
Static DFT Calculations. The nuclear geometries and
electronic structures of the complexes were investigated with
DFT calculations using the B3LYP functional and the
LANL2DZ basis set. DFT calculations of 2a’s possible isomers
estimate that the trans isomer observed for 2c via
crystallography is 8.6 kcal/mol more stable than its counterpart
having ppy’s cyclometalating carbon cis to the chloride.
Frontier orbitals generated for 2a and 3a are compared in
Figure 6. The LUMO in both complexes is exclusively located
F
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
on the terpyridine ligand and exhibits A′ symmetry. The A″
HOMO resembles that of the well-studied [Ir(ppy)2(bpy)]+
complexes showing large involvement of the ppy’s phenyl ring
and an iridium d orbital.44,45,47 However, prominent con-
tributions are also made by an antibonding interaction between
the metal and the monodentate ligand.
Replacement of the chloride ligand with a stronger field
cyanide capable of backbonding reduces the HOMO energy by
0.24 eV and subsequently broadens the HOMO−LUMO gap.
Substitution of ppy adjusts calculated HOMO energies within a
0.433 eV range. Other tuning strategies, meanwhile, are
predicted to have mixed results. Addition of an aryl moiety
to terpyridine has little impact failing to extend the LUMO
beyond terpyridine’s three coordinating pyridyl rings except in
2g. There, the LUMO is predicted to include dMeO-phtpy’s
methoxy substituents.
Electrochemistry. Rich electrochemistry was observed for
the iridium compounds via cyclic voltammetry (Tables 1 and
Table 1. Electrochemical Properties of
[Ir(N^N^N)(C^N)Cl]PF6 Compounds
a
a
oxidation
reduction
d
I: E1/2/V
II:
III: Ered/V
(ΔE/mV)
Figure 7. (A) Cyclic voltammogram of 2a which exemplifies the main
redox features of all compounds with a chloro monodentate ligand
(2a−2g). Roman numeral labels denote three distinct types of
reduction. The inset displays the first two reductions of 2a scanned
without observing process III. (B) Cyclic voltammogram of 3a
representing the redox behavior of the cyano complexes. All cyclic
voltammograms were collected at 0.10 V/s using a three electrode
system in 0.10 M N(n-Bu)4PF6 (MeCN) solutions. Potentials are
referenced to SCE via an internal ferrocene standard (fc/fc+ = 0.40
V).33
compound
Epa/V
(ΔE/mV)
Epc/V
c
e
2a
2b
2c
2d
2e
2f
1.72, 2.09
1.78, 2.01
1.45
−1.09 (61)
−1.07 (69)
−1.09 (73)
−1.06 (78)
−1.05 (71)
−1.07 (75)
−1.10 (64)
−1.35
−1.39
−1.41
−1.39
−1.37
−1.42
−1.40
−1.98
−1.92
e
f
−1.95(85)
e
1.71
−1.95
−1.92
−1.95
e
1.77, 2.08
1.45
e
b
e
2g
1.63, 1.71
−1.98
a
Potentials were measured in 0.10 M N(n-Bu)4PF6 (MeCN) solution
with a three electrode system scanning at 0.10 V/s. Potentials are given
b
in V vs SCE, and peak separation is given in mV. Shoulder.
c
d
Resolution from solvent oxidation is poor. Irreversible. No anodic
(CO)3Cl].53 Interestingly, both derivatives of the rhenium
compound and the parent chloro compound (2a) have been
used as catalysts for CO2 reduction.25,54 Their redox behavior
may thus be related to catalytic performance and is given
particular consideration for the compounds studied here.
For all compounds except 2g, the first irreversible oxidation
can be assigned as a metal-centered process with some
participation of the ppy and monodentate ligands. First
oxidation potentials reflect the effect of ppy modification on
the HOMO and decrease expectedly as the ppy’s electron
density increases within the chloro or cyano series. Comparison
between the two series reveals that the strong field cyano ligand
causes the first oxidation to become significantly more difficult
stabilizing the HOMO as predicted via DFT. Cyanide’s strong
field effect precludes observation of a second oxidation in 3a
and 3b. However, such a process is seen in select chloro species
where the ppy orbitals are sufficiently low in energy to avoid
significant overlap with and stabilization of frontier metal
orbitals. In specially designed 2g, oxidation is different than in
the other compounds consisting of two poorly resolved
irreversible waves (Figure S3 in the Supporting Information).
These likely represent typical metal-centered oxidation as well
as oxidation of the electron rich dimethoxyphenyl moiety in
dMeO-phtpy. The order of the processes cannot be
unambiguously assigned, but calculations suggest that the
ligand moiety is most easily oxidized (Supporting Information),
which reflects the goal of 2g’s design to push electrons away
from that ligand.
e
reversal of II is resolved from the reverse of I. Irreversible. A
competing oxidative process supersedes reversal of reduction at 0.10
V/s and gives an anodic peak shifted relative to the reduction by +0.20
V. Quasi-reversible. Reversal of reduction is observed despite a
competing process.
f
Table 2. Electrochemical Properties of
[Ir(N^N^N)(C^N)CN]PF6 Compounds
a
a
compound
oxidation: Epa/V
reduction: E1/2/V (ΔE/ mV)
b
3a
3b
1.90
−1.10 (65), −1.68 (68)
−1.07 (71), −1.66 (75)
b
2.04
a
Potentials are given in V vs SCE and were collected under the same
b
experimental conditions used for 2a−2g. Resolution from the solvent
oxidation is poor.
2). All materials show at least one irreversible oxidation while
many of the chloro compounds (Figure 7a) also show a second
oxidation near the solvent window’s positive edge. In addition
to oxidation, cyano compounds exhibit two reversible
reductions (Figure 7b) and members of the chloro series
show three reductions of varying reversibility: a first reversible
reduction (I), a second irreversible process (II), and a third
reduction which is typically irreversible (III) (Figure 7a). The
electrochemical features of the chloro compounds are strikingly
similar to those described previously for [Re(dmbpy)-
G
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
First oxidations observed for the chloro and cyano complexes
are significantly higher in potential and less reversible than
those in more established bis-cyclometalated architectures
[Ir(C^N)2(bpy)]+ (EOX ≈ +1.25 V vs SCE)46 and [Ir-
(C^N^C)(phtpy)]+ (EOX ≈ +1.09 V vs SCE).19b The
irreversibility may stem directly from dissociation of the
monodentate ligand, which is shown to be possible for both
chloride and cyanide via mass spectrometry. The possibility of
cyanide dissociation is further supported by the previous report
on [Re(dmbpy)(CO)3Cl] ascribing the irreversibility of its
metal oxidation to dissociation of an isoelectronic CO ligand as
removal of metal electron density weakens backbonding.53
In all of the compounds, the first reduction occurs on the
polypyridyl ligand as in both [Ir(ppy)2(bpy)]+ and [Ir-
(C^N^C)(phtpy)]+ structures.19b,44,45 Accordingly, alterations
to the ppy and monodentate ligand have little impact on the
potential of the reduction. The potential also remains fairly
constant despite additions of various phenyl groups to tpy. As
predicted by computations, these groups are poor tuning tools
and do not participate in the LUMO with tpy’s three
coordinating pyridines. The disconnect between the added
phenyl groups and the coordinating pyridines arises from the
lack of coplanarity between those moieties as evidenced by the
crystal structure of [Ir(ttpy)(bpy)Cl]2+ (ttpy = 4′-(4-tolyl)-
2,2′:6′,2″-terpyridine).51
process (Figure S5 in the Supporting Information). Spectro-
scopic evidence is needed to confirm coordination changes
suspected to occur during redox processes.
Measured redox potentials have been used to calculate
excited state oxidation and reduction potentials, which are
critical for photosensitization (Table 3). The excited states of
Table 3. Calculated Excited State Redox Potentials55
a,
b
a,c
compound
E([M*]+/[M]2+)/V
E([M*]+/[M]0)/V
2a
2b
2c
2d
2e
2f
−0.57
−0.56
−0.77
−0.56
−0.53
−0.77
−0.67
−0.60
−0.52
1.20
1.27
1.13
1.21
1.25
1.15
1.20
1.40
1.49
0.68
2g
3a
3b
d
[Ir(ppy)2(bpy)](PF6)
−0.85
a
b
All potentials are given in V vs SCE. E([M*]+/[M]2+) = EOX − Eλem
.
c
d
E([M*]+/[M]0) = Ered+ Eλem
. From ref 46.
2a−3b are not as highly reducing as those of iridium
photosensitizers for hydrogen evolution (see [Ir(ppy)2(bpy)]-
(PF6) in Table 3).44,46 Nevertheless, the excited state reduction
potentials show that the excited chloro and cyano compounds
are stronger oxidants than some successful iridium materials
indicating a possibility for effective photosensitization via
reductive quenching.44,46
UV−Vis Absorption Spectroscopy. The absorption
spectra of compounds 2a, 2g, and 3a in acetonitrile are
depicted in Figure 8 as representative examples for the new
The second reduction in [Ir(ppy)2(bpy)]+ and [Ir(C^N^C)-
(phtpy)]+ is generally assigned to the cyclometalating ligand
rather than to a second reduction of the polypyridyl
group.19b,44,45 A similar assignment seems logical for the new
family of complexes, but their second reduction is surprisingly
dependent on the monodentate ligand. Although the process is
completely reversible in cyano complexes, it is consistently
irreversible for chloro compounds regardless of whether the
scan is reversed before or after the third reduction. Moreover,
the process occurs more readily in the chloro series despite the
greater electron deficiency of the iridium−cyanide bond which
should facilitate reduction. Ligand dissociation is again
suspected to play a key role. Cyanide is unlikely to dissociate
and cause irreversibility during reduction due to strengthening
of backbonding. Chloride, meanwhile, remains vulnerable as
witnessed in [Re(dmbpy)(CO)3Cl],53 and its dissociation is
forecast by DFT calculations showing metal−chloride bond
elongation with increases in negative charge. Chloride’s weak
binding is further implicated in the irreversibility of the second
reduction by the full reversibility of that process in the model
compound [Ir(tpy)(ppy)(py)](PF6)2 where pyridine (py)
binds more strongly than chloride (Figure S4 in the Supporting
Information). Lastly, loss of chloride is consistent with the
relative ease of the second reduction in the chloro series:
Dissociation of chloride from a singly reduced complex lowers
the overall negative charge alleviating repulsion of an incoming
second electron.
Figure 8. UV−vis absorption spectra for 2a (green), 2g (purple
dashes), and 3a (pink dashes). The portion of the spectrum from 400
to 580 nm is enlarged in the inset to show details of the weakest, low
energy transition. All spectra were collected at room temperature for
10 μM acetonitrile solutions.
Observation of a third reduction in chloro but not cyano
complexes can also be explained by lowering of overall negative
charge upon chloride loss. Yet, discussion of a similar process
by Breikss et al. does not apply here, and the process cannot be
unambiguously assigned at this time.53 Reversal of the
reduction is largely superseded by another process signified
by an anodic peak shifted from the reduction by ∼+0.20 V. An
increase in reversibility is observed at high scan rates (1.5 V/s)
and is the only significant difference when reductions of 2a are
measured in noncoordinating THF instead of acetonitrile.
Thus, acetonitrile binding may give rise to the competing
family of yellow to orange chromophores. The spectra lack any
strong, defining features as observed for many iridium(III)
complexes with cyclometalating and polypyridyl ligands
including the similar [Ir(tpy)(tphbpy)Cl]+ structure (Figure
2, right).17c,21 All chloro complexes (2a−2g) exhibit a weak
transition around 500 nm which borders a well-defined spectral
feature (λ ≈ 390 nm) most intense for 2g. Past this feature,
higher absorptivity is observed in a region crowded with
indistinct transitions. This region is not altered by ppy
decoration but does change upon extension of tpy’s π system
H
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 9. (A) Calculated (blue) and experimental (green) UV−vis absorption spectra for 2a. The region from 400 to 580 nm is enlarged in the inset
to show that the weakest low energy transition is predicted by the calculations. (B) Calculated (blue) and experimental (pink) UV−vis absorption
spectra for 3a. Calculated spectra were generated from Gaussian TD-DFT calculations using Gaussum software with fwhm = 4000 cm−1. Calculated
oscillator strengths are included as vertical lines.
(Figures S6 and S7 in the Supporting Information). Higher
energies are marked by a strong peak (λ = 234 nm, ε = 56,000−
38,000 M−1 cm−1) which remains fairly constant for all chloro
compounds (2a−2g). Upon substituting cyanide for chloride,
this high energy absorption blue-shifts slightly and loses
definition while the region of dense transitions becomes
more defined showing two distinct features (λ ≈ 280 and 320
nm). Notably, the spectra for cyano materials lack the low
energy features near 390 and 500 nm which are prevalent in all
of the chloro complexes.
MLCT/ILCT character promoting from the HOMO and
nearly identical HOMO − 1 to the tpy-based LUMO.
Moreover, although the spectrum of 3a appears to be missing
the prominent low-energy MLCT/ILCT band occurring at 389
nm in 2a, such a transition is calculated for 3a. It simply is blue-
shifted to ∼370 nm and blends more with the region of dense
higher energy transitions. In 3a, that region still involves a
mixture of MLCT/ILCT and ligand-centered transitions, but
tpy-centered transitions are less prevalent. Instead, they are
blue-shifted contributing more to the band at 225 nm which
causes it to differ slightly from the band at 235 nm for 2a. The
significant blue-shifting of transitions, especially prominent low
energy MLCT/ILCT transitions, once again reflects the
stabilization of occupied frontier orbitals by strong field
cyanide.
In order to elucidate the character of observed electronic
transitions, TD-DFT calculations were performed. For the
parent chloro compound (2a), the calculations model the low
energy region of the spectrum accurately and assign the weakest
feature appearing near 500 nm as a mixed MLCT/ILCT
transition from the HOMO to the LUMO. Based on its low
intensity, this transition has been assigned as a spin forbidden
Emission Spectroscopy. All of the investigated iridium-
(III) complexes strongly phosphoresce in acetonitrile in colors
ranging from light orange to a greenish blue (Figure 10). The
21,24
3
excitation to MLCT for similar compounds.
This assign-
ment may apply here as well but cannot be predicted by TD-
DFT. The stronger band near 390 nm is characterized as mixed
MLCT/ILCT (HOMO → LUMO + 1 and HOMO − 1 →
LUMO). At higher energies between 240 and 330 nm, the
calculated spectrum does not accurately depict the crowd of
transitions (Figure 9). However, it shows three signals with
high oscillator strengths (λ = 285, 295, 324 nm) which suggest
that mixed MLCT/ILCT excitations such as HOMO − 4 →
LUMO + 3 appear throughout the region and dominate its
lower energies near 324 nm. Meanwhile, mixed MLCT/tpy-
centered transitions (HOMO − 6 → LUMO + 1) also
contribute near 295 nm, and the highest energy end of the
region includes transitions from chloride and ppy π orbitals to
ppy π* orbitals (HOMO − 4 → LUMO + 2). Participation of
tpy-centered transitions is consistent with changes observed in
the crowded region upon extension of the tpy π system. At
higher energies near 234 nm, the experimental absorbance is
well represented in the calculations which show that it consists
of some ILCT transitions from ppy to tpy but predominately
involves transitions from chloride p orbitals and ppy π orbitals
to ppy π* orbitals. Existence of similar ppy and chloride orbitals
throughout the chloro series thus explains the consistency of
the 234 nm band. The TD-DFT assignments largely agree with
those made in other [Ir(N^N^N)(C^N)L]+ materials,21,24 and
they reveal a strong resemblance between the lowest excited
states in the new compounds and excitations in [Ir-
(ppy)2(bpy)]+ type complexes.45
Figure 10. Room temperature emission spectra measured in 10 μM
acetonitrile solutions following excitation at 380 nm.
broad and featureless emission bands are consistent with charge
transfer and are blue-shifted in comparison to emission from
analogous [Ir(ppy)2(bpy)]+ complexes.8c In those analogues,
the HOMO largely consists of metal orbitals which are
significantly destabilized relative to ligand orbitals due to
strong σ donation from two cyclometalating carbons. However,
as evidenced by the higher metal oxidation potentials in 2a−3b,
their iridium orbitals are less destabilized, which allows for a
lower-lying HOMO with more equal anionic ligand and metal
contributions. This overrides the slight LUMO stabilization
suggested by reduction potentials to produce the blue-shift.
The increased ligand-based character of the HOMO also causes
Despite differences in the experimental spectra for 2a and 3a,
TD-DFT calculations for 3a show that its transitions mirror the
nature of their 2a analogues. The first excitation of 3a is not a
pure HOMO−LUMO transition as in 2a, but it maintains
I
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the kr rate constants of 2a−3b to be low relative to the rate in
[Ir(ppy)2(bpy)]+ (kr = 1.89 × 105 s−1).8c
impressive in general when compared to similar iridium
materials.8c,17c,46
Within the tpy and phtpy subsets of the chloro series, ppy
substituents predictably alter the emission maxima: Blue-shifts
accompany HOMO stabilization by fluorine, and red-shifts
highlight the opposite effect caused by methoxy. Meanwhile,
replacement of the tridentate ligand tpy with phtpy produces
only a slight red-shift. Quantum yields decrease with the
HOMO−LUMO gap, and emission lifetimes of the chloro
compounds correlate reasonably with the HOMO−LUMO gap
in accordance with the energy gap law (Figure 11). “Push−pull”
Earlier studies highlight the aptitude of DFT calculation
protocols for modeling emission maxima in cyclometalated
iridium(III) complexes.8c,17a,58 Similar methodologies were
tested here for the new family of luminophores, and results
are fully detailed in the Supporting Information (Figures S9 and
S10). The most involved approach utilizes the difference in the
SCF energy of the triplet excited and singlet ground states at
the optimized triplet geometry.8c,58a These calculated energies
only correlate with luminescence maxima within subsets of very
similar complexes while the overall correlation coefficient (R2 =
0.66) is unsatisfactory. Surprisingly, a less common approach
taking the total energy difference between singlet ground and
triplet excited states at their respective optimized geometries58
offers the best model for emission energies across the entire
family (R2 = 0.89, Figure 12).
Figure 11. Application of the energy gap law. A linear correlation
(blue) is identified to show that all chloro compounds obey the law for
a single excited state while the cyano compounds deviate from the
trend.
compound 2g has a relatively long lifetime and strays slightly
from the energy gap correlation, which may be an effect of
differences in its HOMO detected electrochemically and
computationally. Cyano compounds clearly have higher knr
values than predicted from their emission energies by the
energy gap law correlation of the chloro compounds in Figure
11. This may occur because the metal−cyanide bond oscillates
Figure 12. Correlation between experimental emission energy and a
computational predictor of emission: the difference in the total
energies for T1 and S0 calculated at their respective optimized
geometries.
at a higher frequency (v ≈ 507 cm−1 in [Ir(CN)6]3−)56 than the
Photocatalytic Hydrogen Evolution. In order to
demonstrate the photostability of the new luminophores, they
were used as photosensitizers for photocatalytic water
reduction in acetonitrile solvent with TEA as a sacrificial
reductant and K2PtCl4 as a catalyst precursor. Figure 13 depicts
the H2 evolution traces for the complexes and the
corresponding photosensitizer turnover numbers (TON).
Control reactions are shown to produce negligible H2
indicating that the photosensitizer, catalyst, and light are all
essential for water reduction. Differences in the photoreaction
conditions employed here and heavily optimized conditions
̃
weaker metal−chloride bond (v ≈ 293−302 cm−1 in
̃
[IrCl6]3−)57 resulting in more broadly spaced ground state
vibrational levels and better overlap of those levels with the
excited state. Such overlap can bolster vibrational deactivation
pathways insignificant in the chloro series. Deactivation
3
through MC states may also become possible since metal−-
cyanide backbonding stabilizes metal orbitals as evidenced by
HOMO stabilization and subsequent blue-shifting of emission
in 3a and 3b. Table 4 displays all emission lifetimes and
radiative decay rates. Lifetimes are longest in 2g and 3b and are
Table 4. Photophysical Properties
emission
λmax
/
knr/105
kr/105
complex
absorption λmax /nm (intensity/104 M−1 cm−1
)
nm
τ/μs
Φ/%
s−1
s−1
2a
2b
2c
2d
2e
2f
234 (5.61), 270 (3.38), 279sh (3.16), 301 (2.45), 328 (1.70), 389sh (0.52), 499sh (0.11)
237 (5.11), 270 (3.46), 280sh (3.06), 300 (2.39), 327 (1.89), 388sh (0.40), 495sh (0.09)
234 (4.24), 254 (3.32), 280 (3.22), 307 (2.62), 327 (2.07), 359sh (0.94), 494sh (0.11)
238 (3.80), 275 (3.73), 283 (3.75), 300 (3.04), 330sh (1.50), 369 (0.71), 398 (0.66), 505sh (0.16)
238 (4.37), 275 (4.30), 284 (4.12), 301 (3.45), 327sh (1.89), 360 (0.76), 393 (0.61), 501 (0.07)
233 (4.46), 284 (4.76), 306sh (3.64), 329sh (2.26), 362 (1.25), 398sh (0.80), 507sh (0.30)
235 (4.43), 274 (4.12), 282 (4.04), 324sh (1.97), 370 (1.15), 396 (1.15), 493sh (0.11)
225sh (3.86), 272 (3.42), 280 (3.33), 320 (1.82), 368sh (0.49)
541
530
558
547
539
558
538
496
485
1.674
2.849
0.580
1.433
2.149
0.789
3.486
2.874
3.118
15.0
28.6
3.2
5.07
2.51
16.70
6.06
3.51
12.09
2.14
2.94
2.42
0.90
1.00
0.54
0.92
1.15
0.58
0.73
0.54
0.78
13.2
24.7
4.6
2g
3a
3b
25.2
15.5
24.5
226 (3.86), 271 (3.85), 280 (3.65), 319 (2.13), 367sh (0.37)
J
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 13. Performance of complexes 2a−3b as photosensitizers for
photocatalytic hydrogen evolution. Reactions were performed in 9 mL
of 8:1 ACN/H2O with 0.075 mM photosensitizer, 300 nmol of
K2PtCl4, and 1 mL of TEA sacrificial reductant. The amount of
hydrogen evolved is noted along with the number of turnovers of the
iridium photosensitizer. Control runs are indicated.
Figure 14. Stern−Volmer plots showing emission quenching as a
function of oxygen partial pressure for 2b (purple) and 3b (pink) in
acetonitrile.
used in previous studies complicate direct comparison. Still, the
photosensitizer turnover numbers achieved by the new
luminophores are generally less than those obtained with
members of the prevalent [Ir(ppy)2(bpy)]+ family in less
coordinating THF with palladium or rhodium catalyst.13d,59 In
acetonitrile, though, those well-known photosensitizers decom-
pose via displacement of their bpy ligands (Ir TON < 325 with
Pd catalyst, Ir TON < 100 with Pt catalyst) failing to match the
strong performance observed for the new materials in the
highly coordinating solvent.59,60 Thus, the use of a tridentate
ligand instead of bipyridine seems to impart robustness to 2a−
3b.
In the new family, the cyanide-containing complexes (3a and
3b) generate more H2 than chloro complexes with 3b achieving
up to 1440 turnovers. These compounds likely outlast their
chloro analogues because they decompose less readily through
monodentate ligand dissociation as suggested by the full
reversibility of their reduction processes. Meanwhile, 2g is a
standout among chloro complexes giving fast initial reaction
rates and 1300 turnovers. The three best photosensitizers (2g,
3a, 3b) have the longest excited state lifetimes, which improve
performance by expanding the time frame for transfer of excited
electrons to the catalyst. Performance of 2g may also be
strengthened by its “push−pull” design or unique HOMO
character.
Oxygen Quenching. Photosensitizers which are sensitive
to paramagnetic quenching by O2 cannot be envisioned for
oxygen-generating systems such as complete water splitting. In
order to evaluate susceptibility of the new complexes to this
common photosensitizer weakness, Stern−Volmer analysis was
completed for two complexes with long-lived excited states, 2b
and 3b. Emission was measured under different percentages of
oxygen in acetonitrile. The measured slopes of the Stern−
Volmer plots shown in Figure 14 indicate that the excited states
of both 2b (KsvP = 0.00634 mbar−1, kq = 2.49 × 108 M−1 s−1)
and 3b (KsvP = 0.00807 mbar−1, kq = 2.90 × 108 M−1 s−1) are
more resistant to oxygen quenching than the excited states of
many other luminophores.9a,61 For comparison, in acetonitrile,
the KsvP values for the archetypical dye [Ru(bpy)3]2+ and the
iridium(III) photosensitizer [Ir(ppy)2(bpy)]+ are 0.024 mbar−1
(kq = 2.9 × 109 M−1 s−1) and 0.026 mbar−1 (kq = 8.8 × 109 M−1
s−1) respectively.9a
CONCLUSION
■
A family of luminophores has been prepared to further explore
the structure [Ir(tpy)(ppy)Cl]+ presented previously as a
catalyst for CO2 reduction. The new luminophores were readily
synthesized as a single isomer which orients ppy’s cyclo-
metalating carbon trans to the monodentate ligand. Adding aryl
groups to the tpy ligand fails to tune the LUMO located there.
However, the HOMO composed of monodentate ligand, metal,
and ppy orbitals is significantly stabilized by increases in the
electron deficiency of the ppy and the field strength of the
monodentate ligand. Choice of monodentate ligand also affects
stability during redox processes: Both chloride and cyanide
appear to dissociate upon oxidation and electrospray ionization,
but only chloride leads to instability during reduction. The
HOMO of the investigated luminophores is more ligand-based
and stabilized than the HOMO of popular [Ir(ppy)2(bpy)]+
photosensitizers producing a blue-shifted emission. Lifetimes
and quantum yields of emission are impressive, especially in
cyano complexes and the specially designed push−pull
derivative. Those compounds are also leaders in photo-
sensitization of photocatalytic hydrogen evolution. Meanwhile,
all of the compounds show enhanced photostability by turning
over more than other iridium photosensitizers in harsh
coordinating solvent. In the future, compounds with the
explored [Ir(tpy)(ppy)Cl]+ motif may be considered as
photosensitizers for complete water splitting based on their
photostability as well as the relatively high tolerance of their
excited state for oxygen.
ASSOCIATED CONTENT
* Supporting Information
■
S
All NMR and UV−vis absorption spectra along with selected
mass spectra and DFT results. Additional electrochemical
measurements including a scan rate dependence illustration.
Detailed crystallographic parameters and X-ray crystallographic
data for 2c in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bern@cmu.edu.
Notes
■
The authors declare no competing financial interest.
K
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(19) (a) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.;
Scandola, F. Inorg. Chem. 2004, 43 (6), 1950−1956. (b) Polson, M.;
Ravaglia, M.; Fracasso, S.; Garavelli, M.; Scandola, F. Inorg. Chem.
2005, 44, 1282−1289.
ACKNOWLEDGMENTS
■
The CMU authors acknowledge support from the National
Science Foundation through CHE-1055547, and D.N.C. is
grateful for support from the U.S. Department of Energy Office
of Science Graduate Research Fellowship. The Duquesne
University authors are supported by the National Science
Foundation CAREER Award CHE-0844131.
(20) Whittle, V. L.; Williams, J. A. G. Inorg. Chem. 2008, 47, 6596−
6607.
(21) Bexon, A. J. S.; Williams, J. A. G. C. R. Chim. 2005, 8, 1326−
1335.
(22) Williams, J. A. G.; Wilkinson, A. J.; Whittle, V. L. Dalton Trans.
2008, 6513−6515.
REFERENCES
■
(23) (a) Yoshikawa, N.; Matsumura-Inoue, T. Anal. Sci. 2003, 19,
761−765. (b) Yoshikawa, N.; Yamabe, S.; Kanehisa, N.; Kai, Y.;
Takashima, H.; Tsukahara, K. Eur. J. Inorg. Chem. 2007, 1911−1919.
(24) Obara, S.; Itabashi, M.; Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii,
Y.; Nozaki, K.; Haga, M. Inorg. Chem. 2006, 45, 8907−8921.
(25) Sato, S.; Morikawam, T.; Kajino, T.; Ishitani, O. Angew. Chem.,
Int. Ed. 2013, 52, 988−992.
(26) Wang, J.; Hanan, G. S. Synlett 2005, No. 8, 1251−1254.
(27) Song, C.; Ye, Z.; Wang, G.; Yuan, J.; Guan, Y. Chem.Eur. J.
2010, 16 (22), 6464−6472.
(28) Constable, E. C.; Lewis, J.; Liptrot, M. C.; Raithby, P. R. Inorg.
Chim. Acta 1990, 178, 47−54.
(1) Juris, A.; Balzani, V.; Berigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85−277.
(2) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem.
Soc. 1982, 104, 4803−4810.
(3) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98 (16),
4853−4858.
(4) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205−268.
(5) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr.
Chem. 2007, 280, 69−115.
(6) Kirgan, R. A.; Sullivan, B. P.; Rillema, D. P. Top. Curr. Chem.
2007, 281, 45−100.
(7) Kumaresan, D.; Shankar, K.; Vaidya, S.; Schmehl, R. H. Top. Curr.
Chem. 2007, 281, 101−142.
(29) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13.
(30) Sheldrick, G. M. SADABS Version 2.03; University of Gottingen:
̈
(8) (a) Lowry, M. S.; Bernhard, S. Chem.Eur. J. 2006, 12, 7970−
7977. (b) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U.
S. Adv. Mater. 2009, 21, 4418−4441. (c) Lowry, M. S.; Hudson, W. R.;
Pascal, R. A., Jr.; Bernhard, S. J. Am. Chem. Soc. 2004, 126, 14129−
14135.
Gottingen, Germany, 2002.
̈
(31) Sheldrick, G. M. SHELXTL 6.1, Crystallographic Computing
System; Bruker Analytical X-Ray System: Madison, WI, 2000.
(32) Zysman-Colman, E.; Slinker, J.; Parker, J.; Malliaras, G.;
Bernhard, S. Chem. Mater. 2008, 20, 388−396.
(9) (a) Jenkins, D. M.; Bernhard, S. Inorg. Chem. 2010, 49, 11297−
11308. (b) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.;
Stoeckli-Evans, H. Inorg. Chem. 1996, 35, 4889−4895.
(10) Wenger, O. S. Coord. Chem. Rev. 2009, 253, 1439−1457.
(11) Lo, K. K. W.; Zhang, K. Y.; Leung, S. K.; Tang, M. C. Angew.
Chem., Int. Ed. 2008, 47, 2213−2216.
(12) (a) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A
2009, 113, 5927−5932. (b) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.;
Parker, J. B.; Bernhard, S.; Abruna, H. D.; Malliaras, G. G. J. Mater.
(33) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910.
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; 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.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02;
Gaussian Inc.: Wallingford, CT, 2004.
(35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(36) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter
Mater. Phys. 1988, 37, 785−789.
(37) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett.
1989, 157, 200−206.
(38) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(39) (a) O’Boyle, N. M. GaussSum 2.0; Dublin City University:
Dublin, Ireland, 2006. Available at http://gaussum.sourceforge.net/.
(b) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput.
Chem. 2008, 29, 839−845.
̃
Chem. 2007, 17, 2976−2988.
(13) (a) Gartner, F.; Sundararaju, B.; Surkus, A. E.; Boddien, A.;
̈
Loges, B.; Junge, H.; Dixneuf, P. H.; Beller, M. Angew. Chem., Int. Ed.
2009, 48, 9962−9965. (b) Wang, C.; deKrafft, K. E.; Lin, W. B. J. Am.
Chem. Soc. 2012, 134, 7211−7214. (c) Metz, S.; Bernhard, S. Chem.
Commun. 2010, 46, 7551−7553. (d) Cline, E. D.; Adamson, S. E.;
Bernhard, S. Inorg. Chem. 2008, 47, 10378−10388. (e) DiSalle, B. F.;
Bernhard, S. J. Am. Chem. Soc. 2011, 133, 11819−11821.
(14) (a) Condie, A. G.; Gonzalez-Gomez, J. C.; Stephenson, C. R. J.
J. Am. Chem. Soc. 2010, 132, 1464−1465. (b) Lalevee, J.; Peter, M.;
́
Dumur, F.; Gigmes, D.; Blanchard, N.; Tehfe, M. A.; Morlet-Savary,
F.; Fouassier, J. P. Chem.Eur. J. 2011, 17, 15027−15031.
(c) Rueping, M.; Zhu, S. Q.; Koenigs, R. M. Chem. Commun. 2011,
47, 8679−8681. (d) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J.
Am. Chem. Soc. 2009, 131, 10875−10877.
(15) (a) Flynn, C. M., Jr.; Demas, J. N. J. Am. Chem. Soc. 1974, 96
(6), 1959−1960. (b) Demas, J. N.; Harris, E. W.; Flynn, C. M., Jr.;
Diemente, D. J. Am. Chem. Soc. 1975, 97 (13), 3838−3839.
(16) (a) Ayala, N. P.; Flynn, C. M.; Sacksteder, L.; Demas, J. N.;
DeGraff, B. A. J. Am. Chem. Soc. 1990, 112, 3837−3844. (b) Collin, J.-
P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.; Barigelletti, F.;
Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009−5016.
(17) (a) 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, 7377−7387. (b) Tsuboyama, A.; Iwawaki, H.; Furugori, M.;
Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.;
Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc.
2003, 125, 12971−12979. (c) Baranoff, E.; Yum, J.-H.; Graetzel, M.;
Nazeeruddin, Md. K. J. Organomet. Chem. 2009, 694, 2661−2670.
(18) Tinker, L. L.; Bernhard, S. Inorg. Chem. 2009, 48 (22), 10507−
10511.
(40) Abergel, R. J.; D’Aleo, A.; Leung, C. N. P.; Shuh, D. K.;
Raymond, K. N. Inorg. Chem. 2009, 48, 10868−10870.
(41) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583−
5590.
(42) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, 2006.
(43) Morris, K. J.; Roach, M. S.; Xu, W.; Demas, J. N.; DeGraff, B. A.
Anal. Chem. 2007, 79, 9310−9314.
L
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(44) 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−
5719.
(45) Chirdon, D. N.; McCusker, C. E.; Castellano, F. N.; Bernhard,
S. Inorg. Chem. 2013, 52, 8795−8804.
(46) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.;
Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502−7510.
(47) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti,
F. Top. Curr. Chem. 2007, 281, 143−203.
(48) Yoshikawa, N.; Yamabe, S.; Kanehisa, N.; Kai, Y.; Takashima,
H.; Tsukahara, K. Inorg. Chim. Acta 2009, 362 (2), 361−371.
(49) Yoshikawa, N.; Sakamoto, J.; Kanehisa, N.; Kai, Y.; Matsumura-
Inoue, T.; Takashima, H.; Tsukahara, K. Acta Crystallogr., Sect. E:
Struct. Rep. Online 2003, 59 (10), 830−382.
(50) Breu, J.; Stossel, P.; Schrader, S.; Starukhin, A.; Finkenzeller, W.
J.; Yersin, H. Chem. Mater. 2005, 17 (7), 1745−1752.
(51) Yutaka, T.; Obara, S.; Haga, M.; Yokoyama, Y.; Sakai, K. Acta
Crystallogr., Sect. E: Struct. Rep. Online 2005, 61 (7), m1357−m1359.
(52) Varetto, U. Molekel: 5.4.0.8; Swiss National Supercomputing
Centre: Lugano, Switzerland.
(53) Breikss, A. I.; Abruna, H. D. J. Electroanal. Chem. 1986, 201,
̃
347−358.
(54) Hawecker, J.; Lehn, J. M.; Ziessel, R. J. Chem. Soc., Chem.
Commun. 1983, 536−538.
(55) Kalyanasundaram, K. Photochemistry of Polypyridine and
Porphyrin Complexes; Academic Press: San Diego, CA, 1992.
(56) Swanson, B. I.; Jones, L. H. Inorg. Chem. 1974, 13 (2), 313−316.
(57) Herbelin, F.; Herbelin, J. D.; Mathieu, J. P.; Poulet, H.
Spectrochim. Acta 1966, 22, 1515−1522.
(58) (a) Donato, L.; Abel, P.; Zysman-Colman, E. Dalton Trans.
2013, 42, 8402−8412. (b) Ladouceur, S.; Fortin, D.; Zysman-Colman,
E. Inorg. Chem. 2011, 50, 11514−11526.
(59) Curtin, P. N.; Tinker, L. L.; Burgess, C. M.; Cline, E. D.;
Bernhard, S. Inorg. Chem. 2009, 48, 10498−10506.
(60) Tinker, L. L.; McDaniel, N. D.; Curtin, P. N.; Smith, C. K.;
Ireland, M. J.; Bernhard, S. Chem.Eur. J. 2007, 13 (31), 8726−8732.
(61) Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am. Chem. Soc.
1977, 99 (11), 3547−3551.
M
dx.doi.org/10.1021/ic402411g | Inorg. Chem. XXXX, XXX, XXX−XXX