Cyclometalated iridium(III) complexes as photosensitizers for long-range electron transfer: Occurrence of a coulomb barrier

Article abstract of DOI:10.1002/ejic.200900673

Six cyclometalated iridium(III) complexes were investigated to assess their potential as photosensitizers for long-range electron transfer, and two of them were incorporated directly into covalent donor-bridge-acceptor molecules. The influence of ligarid substitutions on the excited-state properties and the photoredox behavior of the iridium complexes was explored by optical absorption, steady-state and time-resolved luminescence spectroscopy, as well as by electrochemical methods. Bimolecular electron transfer between the photoexcited complexes and 10-methylphenothiazine and methylviologen was found to be only weakly dependent on the ligand substitutions. Intramolecular long-range electron transfer from phenothiazine to photoexcited iridium(III) in the dyads is slow due to the occurrence of a Coulomb barrier. Consequently, an electron-transfer photoproduct is only observable in the transient absorption spectrum, of a donorbridge-acceptor molecule with a fluorinated photosensitizer that exhibits a very long excited-state lifetime. A flashquench technique is necessary for detection of an electrontransfer product in the dyad with a non-fluorinated photosensitizer. The occurrence of a Coulomb barrier associated with intramolecular (excited-state) long-range electron transfer in the dyads with cyclometalated iridium(III) photosensitizers represents an important difference to previously investigated similar donor-bridge-acceptor molecules with photosensitizers based on d⁶ metal diimine complexes.

Full text of DOI:10.1002/ejic.200900673

FULL PAPER
DOI: 10.1002/ejic.200900673
Cyclometalated Iridium(III) Complexes as Photosensitizers for Long-Range
Electron Transfer: Occurrence of a Coulomb Barrier
[a]
[a]
[b]
[a][‡]
David Hanss, Jonathan C. Freys, Gérald Bernardinelli, and Oliver S. Wenger*
Keywords: Electron transfer / Donor-acceptor systems / Time-resolved spectroscopy / Iridium / Photochemistry
Six cyclometalated iridium(III) complexes were investigated
to assess their potential as photosensitizers for long-range
electron transfer, and two of them were incorporated directly
into covalent donor–bridge–acceptor molecules. The influ-
ence of ligand substitutions on the excited-state properties
and the photoredox behavior of the iridium complexes was
explored by optical absorption, steady-state and time-
Consequently, an electron-transfer photoproduct is only ob-
servable in the transient absorption spectrum of a donor–
bridge–acceptor molecule with a fluorinated photosensitizer
that exhibits a very long excited-state lifetime. A flash-
quench technique is necessary for detection of an electron-
transfer product in the dyad with a non-fluorinated photosen-
sitizer. The occurrence of a Coulomb barrier associated with
resolved luminescence spectroscopy, as well as by electro- intramolecular (excited-state) long-range electron transfer in
chemical methods. Bimolecular electron transfer between the
photoexcited complexes and 10-methylphenothiazine and
methylviologen was found to be only weakly dependent on
the ligand substitutions. Intramolecular long-range electron
transfer from phenothiazine to photoexcited iridium(III) in
the dyads is slow due to the occurrence of a Coulomb barrier.
the dyads with cyclometalated iridium(III) photosensitizers
represents an important difference to previously investigated
similar donor–bridge–acceptor molecules with photosensi-
tizers based on d metal diimine complexes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
6
tems,[8] cyclometalated iridium(III) complexes are almost
unexplored in this respect. A cyclometalated iridium(III)
complex has been used recently as a photosensitizer for
Introduction
[
9]
The Ru(bpy)32+ (bpy = 2,2Ј-bipyridine) complex and its
congeners play a unique role in inorganic photochemistry
due to their favorable luminescence and photoredox proper-
ties.[ In recent years, isoelectronic cyclometalated iridi-
um(III) complexes have received increasing attention, and
it has become clear that their photophysical and electro-
chemical properties can be controlled almost deliberately
[10]
long-range energy transfer,
and another one for photo-
[11]
triggering of a proton-coupled electron-transfer reaction.
Yet, the applicability of cyclometalated iridium(III) photo-
sensitizers for kinetic investigations of long-range electron-
transfer reactions remains to be explored. This is an attract-
ive research target in view of the fact that the energy of the
emissive iridium excited state can be tuned over a much
1]
[2]
through structure and ligand variations. Many applica-
tions for which ruthenium(II) α-diimine complexes have
been used in the past have in the meantime been im-
plemented also with cyclometalated iridium(III) species.
greater range than is the case for ruthenium(II) polypyridyl
[2,12]
complexes.
The associated greater tunability of excited-
state redox potentials is interesting for driving-force investi-
gations of long-range electron-transfer rates. In principle, it
is conceivable that in this respect cyclometalated iridi-
um(III) complexes will excel over the traditionally used ru-
thenium(II) photosensitizers. What is more, some cyclomet-
[3]
Examples are iridium-based oxygen sensors, luminescent
[4]
DNA intercalators, sensitizers for photocatalytic water
[
5]
[6]
splitting, dye-sensitized solar cells, and electrolumines-
[
7]
cent devices. However, although ruthenium(II) complexes
have been used extensively as photosensitizers for long-
range electron transfer both in artificial and biological sys-
alated iridium(III) complexes are known to have signifi-
2+ [5a]
cantly longer-lived excited states than Ru(bpy)3
.
This is
expected to be advantageous for long-range charge-transfer
a] University of Geneva, Department of Inorganic, Analytical and reactions originating from these states, simply because the
[
Applied Chemistry
0 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
b] University of Geneva, Laboratory of X-ray Crystallography
inherent excited-state decay processes are less competitive.
However, it is known that the lowest-lying excited state of
many cyclometalated iridium(III) complexes is not a pure
metal-to-ligand charge transfer (MLCT) state as the case
3
[
[
2
4 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
‡] New address: Georg-August-Universität Göttingen, Institut für
Anorganische Chemie
2+
[2,12,13]
Tammannstraße 4, 37077 Göttingen, Germany
Fax: +49-551-39-3373
for Ru(bpy)
3
-type complexes,
which could result in
different photoredox behavior. Exploring the above-men-
tioned and related issues has been the motivation for the
research presented in this paper.
E-mail: oliver.wenger@chemie.uni-goettingen.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900673.
4850
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4850–4859

Cyclometalated Iridium(III) Complexes as Photosensitizers
We report here on the photophysical and photoredox metals and the two chloride bridging ligands.[2,12a,16] Thus,
properties of the eight cyclometalated iridium(III) com- only one geometrical isomer is formed upon complexation
plexes shown in Scheme 1. Six of them are simple com- of the α-diimine ligand in the final synthetic step. Indeed,
1
plexes with fluorinated (IrF series on the right) and non- all our H NMR spectroscopic data are consistent with the
fluorinated (IrMe series on the left) cyclometalating ligands. exclusive formation of one isomer.
Their bipyridine ligands are either unsubstituted or substi-
Single crystals including the IrMe_ester cation could be
tuted with electron-withdrawing or electron-donating grown by slow diffusion of (liquid) diisopropyl ether into a
groups. The two remaining complexes are donor–bridge– dichloromethane solution. The X-ray crystal structure
acceptor molecules composed of a phenothiazine (PTZ) do- analysis revealed no unusual features; all bond lengths and
nor, a p-xylene bridge, and a cyclometalated iridium(III) angles in the IrMe_ester cation shown in Figure 1 are in
species acting as an acceptor upon photoexcitation. We line with those observed previously for related species.[
chose this particular donor–bridge moiety for three reasons: Characteristic for this class of complexes are the relatively
2b,17]
(i) The p-xylene bridge is rigid, thereby keeping the donor– short Ir–N distances involving nitrogen atoms from the cy-
acceptor couple at a fixed distance, contrary to what would clometalating ligands and comparatively long Ir–N dis-
be expected for flexible bridging units. (ii) The methyl sub- tances involving α-diimine nitrogen atoms. Here, the respec-
stituents on the bridge impede complete planarization of tive distances are 2.016(5)/2.032(5) Å and 2.129(8)/
the phenothiazine–xylene–bipyridine moiety,[ thereby li- 2.145(0) Å, respectively. A noteworthy observation is the
miting its degree of π conjugation. (iii) We have previously fact that the pyridyl and tolyl rings in the cyclometalating
14]
[15]
studied an analogous ruthenium(II) dyad that will allow ligands are significantly more coplanar than the two pyridyl
2
+
direct comparison of Ru(bpy)3 and cyclometalated iridi- rings of the bipyridine ligand. The respective dihedral
um(III) photosensitizers for long-range electron transfer. angles are 0.5 and 3.8° in the two 2-(p-tolyl)pyridine ligands
Generally, an electron transfer is considered “long-range” and 11.5° in the bipyridine ligand. Examination of the com-
when it occurs over a distance greater than 10 Å.[ In the plete crystal structure reveals the presence of a dichloro-
dyads from Scheme 1, the phenothiazine-to-iridium dis- methane molecule in immediate vicinity to the bipyridine
8]
tance is 10.6 Å.
ligand. Thus, the above comparatively large torsion angle is
likely a consequence of crystal packing.
Figure 1. Crystal structure of the IrMe_ester complex. The thermal
ellipsoids for the image represent the 50% probability limit. All
hydrogen atoms are omitted for clarity.
Excited-State Properties
The optical absorption properties of the complexes from
Scheme 1 are very similar to those of previously investi-
Scheme 1. Formulas of the molecules investigated in this work.
[
3b,13]
gated analogous complexes.
As summarized in Table 1,
intense bands due to spin-allowed π–π* transitions on the
ligands are observed in the UV region, whereas the weaker
absorptions in the blue spectral range are assigned to
MLCT transitions. However, unlike in Ru(bpy)3 where
the MLCT bands are spectrally well separated from intrali-
gand absorptions, in the cyclometalated iridium(III) com-
Results and Discussion
2
+
Structural Aspects
The synthesis of all complexes reported here passes plexes of this work, no isolated MLCT bands can be ob-
through an intermediate that is a dichlorido-bridged metal served. Rather, they are noticeable as long and relatively ill-
dimer. In this species, the two cyclometalating C atoms of defined tails to the more intense π–π* absorptions. This
each iridium(III) center are in the plane formed by the two makes it difficult to detect a clear trend in how substituent
Eur. J. Inorg. Chem. 2009, 4850–4859
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4851

D. Hanss, J. C. Freys, G. Bernardinelli, O. S. Wenger
FULL PAPER
changes affect the MLCT energy by using absorption data the cyclometalating ligand stabilize the predominantly
only. Much more promising for this is analysis of the lumi- metal-localized HOMO by removing electron density from
nescence data presented in Figure 2.
the metal center; hence, the shorter emission wavelengths
of the IrF/IrF_ester molecules with respect to the IrMe/
[
2d,5a]
IrMe_ester complexes.
drawing substituents on the bipyridine ligand stabilize the
Conversely, electron-with-
Table 1. Optical absorption (λabs) and emission (λem) wavelengths,
relative luminescence intensities (φrel), and luminescence lifetimes
[
2d,5a]
predominantly α-diimine-localized LUMO;
hence, the
(τ) in freeze–pump–thaw deoxygenated acetonitrile solution at
room temperature.
redshifts observed upon attachment of the ester groups.
Electron-donating substituents on the α-diimine have been
λ
abs [nm]
λ
[nm]
em
φ
rel
τ
[ns]
[
2d,5a,12,18]
3
–1
–1
shown to lead to the opposite effect,
which was
(ε, 1000dm mol cm )
the motivation for the synthesis of the IrF_PTZ and IrMe_
PTZ complexes. Prior work focused on 4,4Ј-dimethylamino-
substituted bipyridine, as substitution in the para position
IrMe
405 (12.3), 351 (12.0),
12 (16.6), 253 (40.7)
415 (3.7), 381 (6.0),
03 (29.8), 269 (32.8)
405 (12.1), 384 (14.6),
00 (48.9), 272 (58.7)
-PTZ 405 (5.1), 377 (7.4), 324 (36.7),
77 (56.1), 256 (90.0)
412 (2.7), 379 (4.6), 310 (15.6),
75 (30.0), 259 (28.2)
405 (6.8), 351 (8.9),
04 (16.2), 257 (31.8)
403 (7.4), 340 (16.2), 302 (19.9),
83 (24.2), 258 (55.1)
408 (3.0), 313 (40.3),
72 (52.0), 256 (95.3)
581
0.18
0.03
–
230
36
3
IrMe_ester
IrMe_PTZ
669
–
3
to the coordinating nitrogen atoms is expected to lead to
–
[18]
the strongest effect.
Our motivation for exploring 5,5Ј-
3
substitution of bipyridine with a tertiary amine roots in the
fact that such ligands are synthetically more readily access-
IrMe-xy
1
587
483[a]
565
–
0.16
1.00
0.05
–
180
2280
602
–
2
IrF
ible: thanks to an efficient palladium(0) catalyzed N–C cou-
2
[19]
pling reaction,
the 5,5Ј-diphenothiazine-2,2Ј-bipyridine
IrF_ester
IrF_PTZ
can be made in only two reaction steps. However, our irid-
ium complexes with these ligands turn out to be nonlumi-
nescent. Nanosecond transient absorption spectroscopy
fails to provide evidence for the formation of phenothiazine
3
2
IrF-xy
1
-PTZ
508
0.10
612
2
[20]
3
radical cations. It is possible that a low-lying π–π* state
3
[
a] Corresponds to the wavelength of the barycenter of the struc- of the new ligand causes complete quenching of the MLCT
tured (double-maximum) emission band.
luminescence. The absorption and luminescence properties
of these complexes are concentration independent between
–4
–6
10
and 10 .
We note that the unsubstituted IrMe complex displays
absorption and photoluminescence spectra that are vir-
+
tually identical to those of the [Ir(phenylpyridine) (bpy)]
complex,
2
[
3b]
that is, the electron-donating effect of the ad-
ditional methyl groups in the IrMe complexes is negligible.
No significant additional information can be expected from
the investigation of a third series of complexes with phenyl-
pyridine instead of tolylpyridine cyclometalating ligands.
Our preference for the tolylpyridine ligand is due to practi-
cal reasons: the additional methyl group improves the solu-
1
bility of the complexes and the H NMR spectra are easier
to interpret.
The emission bandshapes in Figure 2 reveal an important
difference between the IrF species and all other complexes
investigated here: the IrF luminescence band is structured,
Figure 2. Luminescence spectra obtained from room-temperature
acetonitrile solutions of the six emissive complexes from Scheme 1. signaling that the emissive excited state is a mixed MLCT/
Excitation occurred at 390 nm. Emission intensities are normalized,
and only the IrF/IrF-xy -PTZ and the IrMe/IrMe-xy -PTZ inten-
1 1
sities can be compared mutually.
intraligand state, whereas all other luminescence bands are
unstructured, indicating relatively pure MLCT emissions.
DFT calculations can offer quantitative insight into the
electronic structures of cyclometalated iridium(III) com-
[
21]
Except for the IrMe_PTZ and IrF_PTZ compounds, all plexes, but this is beyond the scope of this paper.
complexes investigated here are emissive in room-tempera- Inspection of the relative emission intensities and life-
ture acetonitrile solution. In Figure 2, the intensities of the times listed in Table 1 shows that there is a correlation be-
four simple complexes are normalized for clear visualiza- tween luminescence wavelength on the one hand and inten-
tion of spectral band shifts; only the emission intensities of sity and lifetime on the other hand: the shorter the emission
the two dyads are to scale with their respective reference wavelength, the higher the luminescence intensity and the
complexes.
longer the excited-state lifetime, but this remains a qualita-
The emission band shifts that occur with the different tive observation. Quantitative analysis using an energy gap
ligand substitutions are in accord with prior investi- law model is not reasonable, as there are too important de-
[
2,12]
gations:
electron-withdrawing fluorine substituents on viations from a strictly exponential correlation of lumines-
4852
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Eur. J. Inorg. Chem. 2009, 4850–4859

Cyclometalated Iridium(III) Complexes as Photosensitizers
Table 2. Ground- and excited-state electrochemical potentials (E, V vs. SCE) and energies of the electronic origins (E00, eV) of various
iridium complexes in acetonitrile solution.
E(IrIV/IrIII
)
E(L/L–)
E
00
[a]
E(IrL
2+/*IrL
+ [b]
)
E(*IrL +
/IrL )
0 [c]
3
3
3 3
IrMe
IrMe-xy
IrF
1.28[d]
1.32
–1.38[d]
–1.43
–1.37
2.37
2.36
2.69
2.67
–1.09
–1.04
–1.00
–1.17
0.99
0.93
1.32
1.43
1
-PTZ
1.69[
1.50
e]
[e]
IrF-xy
1
-PTZ
–1.24
3 3
a] Estimated based on room-temperature absorption and emission data. [b] Calculated using the approximation E(IrL /
+/*IrL +) ≈ E(IrIV
2
[
III
+
0
–
[22]
[5a]
3 3
Ir ) – E00. [c] Calculated using the approximation E(*IrL /IrL ) ≈ E(L/L ) + E00. [d] From ref. [e] From ref.
cence intensity and lifetime with emission energy. There are a better excited-state electron acceptor than the IrMe unit,
three key observations from the luminescence intensity and whereas both complexes should be similarly good excited-
lifetime data in Table 1 and Figure 2: (i) The emissive ex- state donors.
cited state of IrF lives an order of magnitude longer than
that of IrMe. (ii) In the IrMe-xy -PTZ dyad the lumines-
cence is hardly quenched with respect to the IrMe reference
1
Bimolecular Electron Transfer
complexes. (iii) In the IrF-xy -PTZ dyad the excited-state is
1
In order to test the above predictions regarding the ex-
cited-state redox potentials, we investigated the lumines-
cence behavior of IrF and IrMe in the presence of reduc-
tive and oxidative quenchers. 10-Methylphenothiazine
quenched substantially relative to the IrF reference. The lat-
ter two observations suggest that photoinduced phenothi-
azine-to-iridium electron transfer is much more efficient for
the fluorinated photosensitizer than for the non-fluorinated
complex. Additional support for this hypothesis will be pro-
vided by experimental data presented below.
+
[1]
[
E(MePTZ /MePTZ) = 0.73 V vs. SCE] and methylviol-
2+
+
[1]
ogen [E(MV /MV ) = –0.46 V vs. SCE] were chosen for
this purpose. Bimolecular quenching experiments show that
reductive quenching with 10-methylphenothiazine is signifi-
cantly more important for the IrF complex than for IrMe.
The Stern–Volmer constants obtained from the slopes of
Regarding the excited-state properties of the ester-substi-
tuted complexes, we note that their emissive excited states
are strongly quenched with respect to those of the unsubsti-
tuted parent complexes. Particularly, the IrMe_ester lifetime
is rather short and already close to the experimental limit
imposed by nanosecond laser equipment. Therefore, we re-
stricted all subsequent photoredox investigations to the IrF,
IrMe, IrF-xy -PTZ, and IrMe-xy -PTZ compounds.
linear regression fits to the experimental data are K
=
SV
–1
–1
3
935  for IrF and K = 536  for IrMe (see Support-
SV
[23]
ing Information).
This seems to suggest that the fluori-
nated complex is indeed a more potent photooxidant than
IrMe, as suspected above from estimation of the excited-
state redox potentials. However, calculation of the rate con-
1
1
Ground- and excited-state redox potentials of these four
complexes are summarized in Table 2. The non-fluorinated
compounds are about 300 mV more readily oxidized than
the fluorinated complexes (first column), but one-electron
reduction occurs at nearly identical potentials for all four
complexes (second column). The electron-withdrawing
fluoro and trifluoromethyl substituents pull electron density
away from the metal center, making it more difficult to oxi-
stants (k ) for bimolecular electron transfer from the Stern–
Q
Volmer constants and the luminescence lifetimes (Table 1)
yields two essentially diffusion-limited values that differ
only by 35% between IrF and IrMe (Table 3). Thus, the
more efficient luminescence quenching (or higher KSV
value) observed for IrF is a consequence of the longer ex-
cited-state lifetime of this complex, which makes bimolecu-
lar quenching reactions more competitive with inherent ex-
cited-state depopulation through other (nonradiative and
radiative) processes.
[
2d,5a]
dize.
Reduction, in contrast, involves mainly bipyr-
idine-localized orbitals that are unaffected by substitution
of the cyclometalating ligands. Using the energies of the
electronic origins of the emissive excited states (third col-
umn) it is possible to arrive at reasonable estimates for the Table 3. Rate constants for oxidative and reductive quenching of
excited-state redox potentials.[1] The resulting values are re- the excited states of the IrMe and IrF complexes and Ru(bpy)
2+
3
2
+
in acetonitrile solution at room temperature. The Ru(bpy)
3
data
ported in the last two columns of Table 2 and illustrated
graphically in the modified Latimer diagrams of Scheme 2.
The key conclusion from this is that the IrF unit should be
are from ref.[ MePTZ = 10-methylphenothiazine, MV = methyl-
1]
2+
viologen.
k
Q
(MePTZ) [–1 s–1]
k
Q
(MV2+) [–1 s–1]
9
9
IrMe
IrF
Ru(bpy)
2.3ϫ10
1.3ϫ10
1.7ϫ10
2.4ϫ10
9
8
1.7ϫ10
2
+
9
9
3
1.6ϫ10
Oxidative quenching with methylviologen yields similar
–
1
Stern–Volmer constants for the fluorinated (K = 388  )
SV
–1
and non-fluorinated (KSV = 300  ) complexes. Because
the reactive excited state is 10 times longer-lived in the for-
Scheme 2. Latimer diagrams for the IrMe and IrF complexes based
on the electrochemical and spectroscopic data from Table 2.
mer, this yields rate constants (k ) for bimolecular quench-
Q
Eur. J. Inorg. Chem. 2009, 4850–4859
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4853

D. Hanss, J. C. Freys, G. Bernardinelli, O. S. Wenger
FULL PAPER
ing that differ by one order of magnitude (Table 3). Thus, PTZ· formation in the IrF-xy -PTZ dyad by transient ab-
+
1
IrMe turns out to be a somewhat better photoreductant sorption spectroscopy, and we are forced to estimate the
than IrF, at least for the methylviologen acceptor.
rate constant (kET) for phenothiazine-to-iridium excited-
In short, the results from the bimolecular quenching ex- state electron transfer based on luminescence lifetimes. A
periments demonstrate that our cyclometalated iridium(III) common approach is to use the approximation given in
[
26]
complexes exhibit almost equally efficient photoredox Equation (1).
chemistry with 10-methylphenothiazine donors and methyl-
viologen acceptors Ϫ despite the fact that the fluorinated
–
1
–1
k
ET = τdyad – τref
(1)
complex has a reactive excited state that is not of pure
MLCT character. The bimolecular quenching behavior of
the two cyclometalated iridium(III) complexes is similar to
In the present case, τdyad = 612 ns and τ = 2280 ns
Table 1), which yields kET ≈ 1.2ϫ10 s (Scheme 3).
ref
2
+
that observed for Ru(bpy)3 (Table 3, bottom line).
6 –1
(
Intramolecular Long-Range Electron Transfer
In a preceding paragraph we attributed the luminescence
quenching observed for the IrF-xy -PTZ and IrMe-xy -
1
1
PTZ dyads relative to their reference complexes (Table 1) to
reductive excited-state quenching caused by the phenothi-
azine donors. The transient absorption data shown in Fig-
ure 3 provide direct evidence for the formation of electron-
transfer products following photoexcitation of the dyads.
The dashed trace in the left panel of Figure 3 is a transient
–
4
absorption spectrum of a 10  solution of IrF-xy -PTZ in
1
deoxygenated acetonitrile. The data was acquired in a 10-
µs time window starting 2 µs after excitation with 10-ns
laser pulses at 457.9 nm.[
24]
Scheme 3. Schematic diagrams illustrating the energetics and kinet-
ics of the photoinduced processes occurring in the IrMe-xy -PTZ
and IrF-xy -PTZ dyads.
1
1
Strictly analogous transient absorption experiments were
performed with the non-fluorinated IrMe-xy -PTZ dyad,
1
but in this case the phenothiazine radical cation consistently
escaped detection. Yet, thermodynamic considerations led
to the conclusion that in this dyad, too, excited-state PTZ-
to-iridium electron transfer should occur: on the basis of
the electrochemical potentials for one-electron reduction of
the IrMe complex (–1.38 V vs. SCE, Table 2) and the poten-
tial for one-electron oxidation of phenothiazine (≈+0.8 V
[
1]
II
·+
vs. SCE), we estimate that the Ir Me-xy -PTZ charge-
1
Figure 3. Left: transient absorption spectra obtained from 0.1 m separated state is roughly 2.2 eV above the electronic
1
solutions of IrF and IrF-xy -PTZ following excitation at 457.9 nm
ground state (Scheme 3). The emissive MLCT excited state
of IrMe is about 2.36 eV above the ground state (Table 2),
and consequently excited-state electron transfer should be
with 10-ns laser pulses.[ Detection occurred in a time window
ranging from 2 to 12 µs after the laser pulse. Right: transient ab-
sorption spectra measured on deoxygenated acetonitrile solutions
24]
[
27,28]
containing 0.1 m iridium complex (IrMe or IrMe-xy
5
1
-PTZ) and exergonic by about 160 meV (Scheme 3).
Interestingly,
for the luminescence lifetime data of IrMe-xy -PTZ and
0 m methylviologen (MV2 ). Excitation occurred at 457.9 nm
+
1
with laser pulses of ca. 10 ns duration, detection occurred in a 10-
µs time window starting 500 ns after the excitation pulse.
6 –1
IrMe (Table 1), Equation (1) yields kET ≈ 1.2ϫ10 s ,
which is the same value as for the fluorinated dyad.
On the basis of the luminescence lifetimes of the IrF and
An absorption peaking at ca. 520 nm and extending to IrMe reference complexes (Table 1) we estimate that the in-
shorter wavelengths is observed. This is the typical absorp- herent excited-state decay of the photosensitizer fragments
·
+
[20,25]
5
tion of the phenothiazine radical cation (PTZ ).
As in IrF-xy -PTZ and IrMe-xy -PTZ occurs with 4.39ϫ10
1 1
6
–1
expected, the spectral signature of this electron-transfer and 4.35ϫ10 s , respectively (Scheme 3). Thus, it be-
product remains unobserved for a reference solution con- comes obvious that in IrF-xy -PTZ the excited-state elec-
1
taining only the IrF parent complex (solid line in the left tron-transfer process is reasonably competitive with other
panel of Figure 3). Unfortunately, the presence of lumines- deactivation pathways, whereas in the non-fluorinated dyad
cent impurities impedes direct measurement of the rate of it must be poorly competitive. In fact, this finding reflects
4854
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Eur. J. Inorg. Chem. 2009, 4850–4859

Cyclometalated Iridium(III) Complexes as Photosensitizers
the results from the Stern–Volmer experiments and the
The rate constants extracted from the above analyses of
larger Stern–Volmer constant (KSV) observed for the IrF/ the various bi- and intramolecular phototriggered electron-
PTZ couple with respect to the IrMe/PTZ couple. It is transfer processes deserve further discussion, because their
therefore plausible that excited-state electron transfer does relative magnitudes are striking in several respects. First, it
occur also in the non-fluorinated dyad, but the kinetics are is somewhat surprising to see that 10-methylphenothiazine
such that a significant population of the charge-separated reductively quenches the excited states of IrF and IrMe with
state never builds up; hence, our difficulties in detecting the rate constants (k ) near the diffusion-controlled limit
Q
·
+
III
PTZ species in transient absorption spectroscopy.
(Table 3), but the intramolecular PTZ-to-Ir excited-state
In certain Ru(bpy)3 /phenothiazine donor–bridge–ac- electron transfers in the IrF-xy -PTZ and IrMe-xy -PTZ
2
+
1
6
1
–1
ceptor systems excited-state electron transfer is also difficult dyads proceed only with kET ≈ 10 s (Scheme 3; Scheme 5,
to detect because its driving force is very low.[
15,29]
This top). Second, in a structurally very similar dyad comprised
problem can be overcome by photochemical generation of of the same phenothiazine donor, a p-xylene bridge, and a
III
a highly oxidizing Ru species by using a quencher such as rhenium(I) tricarbonyl diimine acceptor (Scheme 5, second
[
15,30]
I
methylviologen.
We have applied such a flash-quench from top), intramolecular PTZ-to-Re excited-state electron
9
–1 [14a]
technique to IrMe-xy -PTZ in an attempt to find direct transfer occurs with kET ≈ 10 s ,
which is three orders
1
spectroscopic evidence for electron-transfer photoproducts
in this dyad as well. The principle of this experiment is illus-
trated in Scheme 4. The photosensitizer (present at 0.1 m
concentration) is excited with a 10-ns laser pulse (“flash”),
which induces an almost instantaneous bimolecular elec-
2+
tron transfer (“quench”) with methylviologen (MV ) be-
cause this quencher is present in large excess (50 m). The
IV
resulting Ir species is strongly oxidizing and based on the
[27,28]
redox potentials from Table 2,
intramolecular PTZ-to-
Ir electron transfer is expected to be exergonic by ca.
.5 eV. Indeed, the transient absorption data in Figure 3
right panel) provide spectroscopic evidence for this intra-
IV
0
(
molecular charge transfer. Both data sets (solid and dashed
traces) were measured in a 10-µs time window starting
500 ns after selective iridium excitation at 457.9 nm. The
experiment with the IrMe reference complex (solid line)
gives an absorption band with a maximum near 600 nm,
which is due to one-electron reduced methylviologen
·
+ [29a,31]
(
MV ).
Expectedly, for the IrMe-xy -PTZ dyad this
1
·
+
MV band is observed as well (dashed line), but it is over-
lapped by an additional absorption peaking at 520 nm that
can be attributed to the phenothiazine radical cation. This
signal builds up still within the 10-ns laser pulse, implying
8
–1
a rate constant greater than 10 s for the intramolecular
PTZ-to-Ir electron transfer across the p-xylene bridge.
IV
Scheme 5. Excited-state PTZ-to-IrIII electron transfer is slow in the
1 1
IrF-xy -PTZ and IrMe-xy -PTZ dyads (top). In an analogous rhe-
I
nium(I)-based dyad (second from top), the PTZ-to-Re excited-
state electron transfer is three orders of magnitude faster despite
identical donor–acceptor distance and similar driving force
(
–∆GET), but in this case the MLCT excitation does not impose a
Coulomb barrier to long-range electron transfer. Ground-state
IV
III
PTZ-to-Ir and PTZ-to-Ru electron transfers across the same
distance and similar –∆GET are also three orders of magnitude
faster (bottom half), because in this case the MLCT excited elec-
tron is abstracted by methylviologen and there is no Coulomb bar-
rier associated with long-range electron transfer either.
Scheme 4. Illustration of the flash-quench technique used to induce
IV
1
phenothiazine-to-Ir electron transfer in the IrMe-xy -PTZ dyad.
Eur. J. Inorg. Chem. 2009, 4850–4859
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4855

D. Hanss, J. C. Freys, G. Bernardinelli, O. S. Wenger
FULL PAPER
of magnitude faster than PTZ-to-IrIII excited-state electron photosensitization of long-range electron transfer, the
transfer in the new dyads. Third, the flash-quench triggered greater tunability of the excited-state properties of the cy-
ground-state electron transfers in IrMe-xy -PTZ (i.e., PTZ- clometalated iridium(III) complexes with respect to
1
IV
2+
to-Ir charge transfer) and an analogous phenothiazine– Ru(bpy)3 -type compounds appears useful: fluorination of
III
xylene–ruthenium dyad (i.e., PTZ-to-Ru charge transfer) the cyclometalating ligand lengthens the excited-state life-
are also three orders of magnitude more rapid (Scheme 5, time by about an order of magnitude relative to complexes
[
15a]
bottom half),
although the driving forces (–∆GET) asso- with non-fluorinated ligands, thereby making excited-state
ciated with these charge transfers are similar to that for ex- electron transfer more competitive with inherent excited-
cited-state electron transfer in IrF-xy -PTZ.
state decay.
1
These three observations can be explained by the elec-
The long excited-state lifetime (≈2.3 µs) of the fluori-
tronic structure of the MLCT excited state in the cyclomet- nated photosensitizer IrF is clearly advantageous for kinetic
alated iridium(III) complexes. As discussed above, in the investigations of intramolecular long-range electron trans-
respective MLCT state the excited electron is located on fer, but in our case this is outweighed by the fact that intra-
the bipyridine ligand, not on the cyclometalating ligands. molecular reductive quenching of this excited state in IrF-
Formally, the long-range electron transfer from phenothi- xy -PTZ is associated with a large Coulomb barrier: The
1
azine to the MLCT-excited iridium center therefore corre- MLCT-excited electron is located on the bipyridine ligand
sponds to electron transfer across a negatively charged that connects the phenothiazine donor to the metal ac-
bridge (Scheme 5, top right), which is known to decelerate ceptor, thereby imposing an electrostatic barrier to intramo-
[
32]
electron transfer.
However, this effect is only important lecular long-range electron transfer. The result is an elec-
6
–1
when the phenothiazine donor is attached covalently to the tron-transfer rate constant of only ca. 10 s , which com-
bipyridine ligand, that is, for bimolecular electron transfer pares to ca. 10 s in three closely related donor–bridge–
9
–1
between 10-methylphenothiazine and IrF it is unimportant; acceptor molecules where there is no such electrostatic bar-
[
14a,15a]
hence, the high rate for this intermolecular charge-transfer rier.
This includes flash-quench triggered intramolec-
process. In the rhenium(I) dyad from Scheme 5, MLCT ex- ular electron transfer in the non-fluorinated IrMe-xy -PTZ
1
citation also involves the α-diimine ligand, but in this case dyad. Thus, the flash-quench method leading to phenothi-
this is only an auxiliary ligand, and the donor-bridge moi- azine-to-iridium(IV) (ground state) intramolecular electron
ety is attached to the metal through a pyridine ligand. Thus, transfer has several advantages over simple photoexcitation
the MLCT-excited electron does not perturb PTZ-to-Re that induces phenothiazine-to-iridium(III) excited-state
long-range electron transfer, and the rate constant of this electron transfer: (i) electron-transfer products can be ob-
process is three orders of magnitude higher than in IrF-xy1- served directly even in systems with photosensitizers that
PTZ. When using the flash-quench technique, the MLCT- have relatively short-lived excited-states; (ii) the flash-
excited electron is abstracted by methylviologen.[
29a,30]
Con- quench triggered intramolecular electron transfer is very ra-
sequently, in neither of the two cases of the bottom half of pid because it is not associated with a large Coulomb bar-
Scheme 5 (IrMe dyad and ruthenium dyad) there results a rier. An important technical advantage is that transient ab-
IV
negative charge on the bridge, and the PTZ-to-Ir and sorption spectroscopy is easier to perform with the flash-
III
PTZ-to-Ru
electron transfers are very rapid (kET
≈
quench method because there is usually no interference
with the luminescence from the sensitizer or strongly emis-
9
–1
10 s ).
A similar Coulomb barrier phenomenon was observed in sive impurities. This is particularly important for cyclomet-
ruthenium-sensitized proton-coupled electron transfer ac- alated iridium(III) complexes because their excitation must
[
33]
ross a guanidinium–carboxylate salt bridges,
and in a occur at much shorter wavelengths than those used for pho-
Ru(bpy)3 -tetrathiafulvalene donor–acceptor molecule.[34]
2
+
toexcitation of Ru(bpy)3
2+
.
If intramolecular long-range charge transfers directly
from a photoexcited state of these sensitizers are to be
probed, the α-diimine ligand should be connected to an oxi-
dative quencher and/or a reductive quencher should be at-
Conclusions
[
9]
Some of the cyclometalated iridium(III) complexes inves- tached to the cyclometalating ligands. This will avoid the
tigated in this work exhibit bimolecular electron-transfer occurrence of slow excited-state electron transfers that are
behavior with 10-methylphenothiazine and methylviologen associated with large electrostatic barriers.
quenchers that is very similar to that observed for Ru-
2
+
(
bpy)3 . The admixture of intraligand excited-state charac-
ter to the photoactive excited state [particularly in the iridi- Experimental Section
um(III) complexes with fluorinated cyclometalating li-
General Information: 1H NMR spectra were measured with a
gands] represents an important difference to Ru(bpy)32
+
Bruker Avance 400 MHz spectrometer. In the NMR spectroscopic
data, all coupling constants are reported in Hz, and the following
abbreviations were used to assign the signals: ph = phenyl, py =
pyridine, bpy = bipyridine, PTZ = phenothiazine. Deuterated sol-
chromophores, but is found to have a negligible impact on
the excited-state reactivity in the cases investigated here. As
2
+
Ru(bpy)3 , the cyclometalated iridium(III) complexes of
this work are much better oxidants and reductants in their vents were bought from Cambridge Isotope Laboratories, Inc.
excited states than in their electronic ground states. For Low-resolution electrospray ionization mass spectrometry was per-
4856
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Eur. J. Inorg. Chem. 2009, 4850–4859

Cyclometalated Iridium(III) Complexes as Photosensitizers
formed with a Finnigan MAT SSQ 7000 instrument, and high-
resolution electrospray mass spectrometry was performed with a
QSTAR XL (AB/MDS Sciex) instrument. Methanol (VWR, HPLC
grade) was used to solubilize the compounds. Elemental analysis
was conducted by Dr. Hansjörg Eder, School of Pharmaceutical
Sciences, University of Geneva, Switzerland. Optical absorption
was measured with a Cary 5000 UV/Vis/NIR spectrophotometer
from Varian, and steady-state luminescence spectroscopy was per-
formed using a Horiba Fluorolog-3 instrument with the use of
quartz cuvettes from Hellma (111-QS). For measurements under
inert atmosphere, home-built quartz cuvettes that can be deoxy-
genated by freeze–pump–thaw cycles were employed. The solvent
used for these measurements was acetonitrile for UV spectroscopy.
The Stern–Volmer experiments were performed in acetonitrile solu-
tions containing 0.1  tetrabutylammonium hexafluorophosphate.
Luminescence lifetimes and transient absorption was measured on
the experimental setup of Professor A. Hauser, Department of
Physical Chemistry, University of Geneva, Switzerland. This setup
is comprised of a Quantel Brilliant Nd:YAG laser with an inte-
grated Magic Prism OPO as an excitation source and a detection
system consisting of a Spex 270M monochromator, a Hamamatsu
photomultiplier, and a Tektronix TDS 540B oscilloscope. The
probe beam used for transient absorption spectroscopy came from
= 8.5, 2.4 Hz, 2 H, bpy C(4)H], 8.56 [d, J = 8.5 Hz, 2 H, bpy C(3)
H], 8.67 [d, J = 2.4 Hz, 2 H, bpy C(6)H] ppm.
Synthetic procedures and characterization data for the monosubsti-
tuted bipyridine ligands used for the IrMe-xy
PTZ dyads have been reported in one of our prior publications.
The first step in the synthesis of the iridium complexes from
1 1
-PTZ and IrF-xy -
[15a]
Scheme 1 is the cyclometalation of IrCl
bridged iridium dimer species of the type [Ir(C N)
3
to afford dichlorido-
∧
2
Cl]
2
. In a typi-
cal procedure, the cyclometalating ligand (2.2 equiv.) was treated
with IrCl ·3H O (1 equiv.) in a 3:1 mixture of 2-ethoxyethanol and
3
2
deionized water at reflux for 24 h. The resulting yellow precipitate
was collected by suction filtration, washed with water and diethyl
∧
ether. Yields varied from 40 to 80%. The two [Ir(C N)
2
Cl]
2
dimers
used in this work have been synthesized and characterized pre-
viously.[
38]
∧
∧
The second step is the reaction of [Ir(C N)
2
Cl]
2
∧
dimers with α-
+
diimine ligands to afford the final [Ir(C N)
2
(N N)] complexes.
This was accomplished by treating the dimer (1 equiv.) with the α-
diimine ligand (2.1 equiv.) in refluxing ethanol overnight. (For the
IrMe-xy -PTZ and IrF-xy -PTZ dyads the solvent was an 8:2 mix-
1 1
ture of ethanol and chloroform). After cooling to room tempera-
ture and subsequent suction filtration, the target complex was pre-
cipitated as its hexafluorophosphate salt through addition of satu-
–
5
a 900-W tungsten lamp. Sample concentrations were ca. 2ϫ10

for luminescence and ca. 10–4  for transient absorption experi- rated aqueous NH
4 6
PF solution. The yellow precipitate was fil-
ments. Cyclic voltammetry was performed with a Versastat3–100
Potentiostat equipped with the K0264 Micro-Cell kit and a plati-
num working electrode from Princeton Applied Research. A silver
wire was used as a quasireference electrode. The supporting electro-
lyte was a 0.1  solution of tetrabutylammonium hexafluorophos-
phate in dry acetonitrile. The solutions were deoxygenated prior to
voltammetry sweeps by bubbling nitrogen gas.
tered, washed with water, cold ethanol, and diethyl ether. Ad-
ditional purification occurred by column chromatography on silica
gel using an eluent mixture composed of acetonitrile/H
2
O/saturated
aqueous KNO (100:10:1). Then the complex was reprecipitated as
3
its hexafluorophosphate salt after acetonitrile evaporation under
reduced pressure and subsequent addition of saturated aqueous
KPF solution. For the IrMe-xy -PTZ and IrF-xy -PTZ dyads, an
6 1 1
additional silica gel column chromatography using an eluent mix-
Synthetic Procedures and Product Characterization Data: Two of
the ligands used in this work are commercially available: 2,2Ј-bipyr-
idine (Fluka, product no. 14453) and 2-(p-tolyl)pyridine (Sigma–
Aldrich, product no. 198870). The other ligands were synthesized
according to the following experimental protocols: The cyclomet-
alating 5-(trifluoromethyl)-2-(2,4-difluorophenyl)pyridine ligand
was obtained in a Suzuki coupling reaction between 2,4-difluo-
rophenylboronic acid and 2-bromo-5-(trifluoromethyl)pyridine.[
The methyl and ethyl 2,2Ј-bipyridine-4,4Јdicarboxylates were ob-
tained from 2,2Ј-bipyridine-4,4Ј-dicarboxylic acid.[
ture composed of dichloromethane and methanol (98:2) was neces-
sary. The yield for this second step varied from 65 to 90%. Charac-
terization data for the eight complexes from Scheme 1 are as fol-
lows:
1
IrMe: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 2.14 (s, 3 H, Me),
6
1
5
1
.09 (m, 2 H), 6.86 (dm, J = 8.4 Hz, 2 H), 6.97 (ddd, J = 8.8, 3.0,
.2 Hz, 2 H), 7.40 (ddd, J = 7.6, 5.2, 1.2 Hz, 2 H), 7.46 (dm, J =
.6 Hz, 2 H), 7.58 (d, J = 8.0 Hz, 2 H), 7.72 (ddd, J = 7.6, 7.6,
.6 Hz, 2 H), 7.85 (dm, J = 7.2 Hz, 2 H), 7.92 (dm, J = 5.2 Hz, 2
35]
36]
H), 8.14 (ddd, J = 8.0, 8.0, 1.6 Hz, 2 H), 8.65 (dm, J = 8.0 Hz, 2
+
H) ppm. HRMS (ESI): calcd. for C34
H
28
N
4
Ir 685.1937; found
The 5,5Ј-diphenothiazine-2,2Ј-bipyridine ligand is accessible in two
consecutive reactions: In the first step, 5,5Ј-dibromo-2,2Ј-bipyr-
idine is synthesized through homocoupling of two 2,5-dibromopyr-
idine molecules in presence of hexabutyldistannane and tetrakis(tri-
6
3
85.1915. C34H F IrN P·0.4CHCl (877.55): calcd. C 47.15, H
.27, N 6.40; found C 47.18, H 3.28, N 6.20.
28 6 4 3
1
IrMe_ester: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 1.44 (t, J =
), 1.99 (s, 6 H, Me), 4.50 (q, J = 7.2 Hz, 4 H
), 6.06 [s, 2 H, ph C(6)H], 6.86 [dd, J = 8.1, 2.0 Hz, 2 H,
ph C(4)H], 7.02 [td, J = 5.6, 1.3 Hz, 2 H, py C(5)H], 7.54 [d, J =
.6 Hz, 2 H, py C(6)H], 7.57 [d, J = 8.1 Hz, 2 H, ph C(3)H], 7.73
td, J = 8.1, 1.3 Hz, 2 H, py C(4)H], 7.84 [d, J = 8.1 Hz, 2 H, py
C(3)H], 7.99 [d, J = 5.6 Hz, 2 H, bpy C(5)H], 8.10 [d, J = 5.6 Hz,
H, bpy C(6)H], 9.24 [s, 2 H, bpy C(3)H] ppm. HRMS (ESI):
phenylphosphane)palladium(0) catalyst.[37] In the second step, the
7.2 Hz, 6 H, CH
CH CH
2 3
CH
2
3
phenothiazine moieties can be coupled to 5,5Ј-dibromo-2,2Ј-bipyr-
idine in a palladium-catalyzed reaction. For this purpose, a solu-
tion of 5,5Ј-dibromo-2,2Ј-bipyridine (0.65 g, 2 mmol), phenothi-
azine (0.83 g, 4 mmol), tris(dibenzylideneacetone)dipalladium(0)
5
[
(30 mg, 0.05 mmol), tri-tert-butylphosphane (20 mg, 0.08 mmol),
2
and potassium tert-butoxide (0.70 g, 5 mmol) in toluene was heated
at reflux for 12 h under a nitrogen atmosphere. Before cooling to
room temperature, the insoluble solids were filtered off the hot
solution, and then the solvent was evaporated under reduced pres-
sure. The crude product was purified by column chromatography
on a silica gel stationary phase (Fluka, product no. 60745) using
+
calcd. for C40
H
36
N
4
O
4
Ir 829.2365; found 829.2351.
1
IrMe_PTZ: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 2.02 (s, 6 H,
Me), 5.86 [s, 2 H, ph C(6)H], 6.77 [d, J = 5.8, 1.5 Hz, 2 H, ph C(4)
H], 7.06 [ddd, J = 7.4, 5.8, 1.5 Hz, 2 H, py C(4)H], 7.19 (m, 16 H,
PTZ), 7.42 [d, J = 8.6 Hz, 2 H, py C(6)H], 7.50 [d, J = 5.8 Hz, 2
H, py C(3)H], 7.57 [dd, J = 8.6, 7.4 Hz, 2 H, py C(5)H], 7.60 [s, 2
pentane/dichloromethane (1:1), thereby yielding a slightly yellow
1
solid (0.80 g, 70%). H NMR (400 MHz, CDCl
(
3
, 25 °C): δ = 6.56 H, bpy C(6)H], 7.75 (dd, J = 8.6, 7.4 Hz, 2 H), 7.81 [d, J = 7.4 Hz,
2 H, ph C(3)H], 8.65 (d, J = 8.6 Hz, 2 H) ppm. HRMS (ESI):
dd, J = 8.0, 1.0 Hz, 4 H, PTZ-H), 6.98 (m, 4 H, PTZ-H), 7.01 (m,
+
4
H, PTZ-H), 7.17 (dd, J = 8.0, 1.0 Hz, 4 H, PTZ-H), 7.80 [dd, J calcd. for C58
42 6 2
H N S Ir 1079.2573; found 1079.2536.
Eur. J. Inorg. Chem. 2009, 4850–4859
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4857

D. Hanss, J. C. Freys, G. Bernardinelli, O. S. Wenger
FULL PAPER
1
IrMe-xy
1
-PTZ: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 1.91 (s, 3
H, Me), 2.07 (s, 3 H, Me), 2.11 (s, 3 H, Me), 2.13 (s, 3 H, Me),
[1] D. M. Roundhill, Photochemistry and Photophysics of Metal
Complexes, Plenum Press, New York, 1994.
6
2
7
1
.11 (s, 1 H), 6.16 (s, 1 H), 6.83 (m, 3 H), 6.94 (m, 2 H), 7.01 (m,
H), 7.41 (m, 2 H), 7.53 (dm, J = 3.2 Hz, 1 H), 7.60 (m, 4 H),
.75 (m, 2 H), 7.88 (dd, J = 8.0, 4.4 Hz, 2 H), 7.97 (dm, J = 7.2 Hz,
[2] a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, M. E.
Thompson, J. Am. Chem. Soc. 2001, 123, 4304–4312; b) S. La-
mansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R.
Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M. E. Thompson,
Inorg. Chem. 2001, 40, 1704–1711; c) J. Li, P. I. Djurovich,
B. D. Alleyne, M. Yousufuddin, N. N. Ho, J. C. Thomas, J. C.
Peters, R. Bau, M. E. Thompson, Inorg. Chem. 2005, 44, 1713–
H), 8.07 (m, 2 H), 8.60 (dm, J = 5.6 Hz, 1 H) ppm. HRMS (ESI):
+
calcd.
for
C
54
H
43
N
5
SIr
986.2863;
found
986.2876.
C
54
H
43
F
6
5 2
IrN PS·H O (1131.21): calcd. C 56.44, H 3.95, N 6.09;
found C 56.21, H 3.77, N 5.96.
IrF: 1H NMR (400 MHz, CDCl
3
, 25 °C): δ = 5.64 (dd, J = 8.0,
1
7
727; d) M. S. Lowry, S. Bernhard, Chem. Eur. J. 2006, 12,
970–7977.
2
7
.0 Hz, 2 H), 6.64 (ddd, J = 8.8, 8.8, 8.0 Hz, 2 H), 7.58 (m, 4 H),
.90 (dd, J = 5.2, 1.2 Hz, 2 H), 8.03 (dd, J = 8.8, 2.0 Hz, 2 H), 8.35
[
3] a) F. Lafolet, S. Welter, Z. Popovic, L. De Cola, J. Mater.
Chem. 2005, 15, 2820–2828; b) V. L. Whittle, J. A. G. Williams,
Inorg. Chem. 2008, 47, 6596–6607; c) C. S. K. Mak, D.
Pentlehner, M. Stich, O. S. Wolfbeis, W. K. Chan, H. Yersin,
Chem. Mater. 2009, 21, 2173–2175.
(
ddd, J = 8.0, 8.0, 1.6 Hz, 2 H), 8.47 (dd, J = 8.8, 3.2 Hz, 2 H), 8.09
+
18 4
(d, J = 8.0 Hz, 2 H) ppm. HRMS (ESI): calcd. for C34H N F10Ir
18 4
865.0995; found 865.1011. C34H F16IrN P (1009.70): calcd. C
40.44, H 1.80, N 5.55; found C 40.77, H 1.98, N 5.41.
1
[4] a) K. K. W. Lo, C. K. Chung, N. Y. Zhu, Chem. Eur. J. 2006,
IrF_ester: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 2.11 (s, 6 H,
12, 1500–1512; b) B. Elias, J. C. Genereux, J. K. Barton, Angew.
Me), 5.57 [d, J = 7.8 Hz, 2 H, ph C(6)H], 6.63 [d, J = 7.8 Hz, 2 H,
ph C(4)H], 7.48 [s, 2 H, bpy C(3)H], 8.04 [d, J = 8.8 Hz, 2 H, py
C(4)H], 8.14 (m, 4 H, bpy), 8.46 [d, J = 8.8 Hz, 2 H, py C(3)H],
Chem. Int. Ed. 2008, 47, 9067–9070.
5] a) M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A.
[
Pascal, G. G. Malliaras, S. Bernhard, Chem. Mater. 2005, 17,
9
C
.12 [s,
2
O
1
H, py C(6)H] ppm. HRMS (ESI): calcd. for
F
5712–5719; b) E. D. Cline, S. E. Adamson, S. Bernhard, Inorg.
10Ir+ 981.1072; found 981.1105.
38
H
22
N
4
4
Chem. 2008, 47, 10378–10388.
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IrF_PTZ: H NMR (400 MHz, CDCl
3
, 25 °C): δ = 5.31 [dd, J =
8
.1, 2.0 Hz, 2 H, ph C(6)H], 6.50 [dd, J = 8.1, 2.0 Hz, 2 H, ph C(4)
H], 7.24 (m, 16 H, PTZ), 7.50 [d, J = 6.0 Hz, 2 H, bpy C(3)H],
.58 [s, 2 H, bpy C(6)H], 7.70 [dd, J = 6.0, 3.0 Hz, 2 H, bpy C(4)
H], 8.06 [d, J = 8.8 Hz, 2 H, py C(4)H], 8.41 [d, J = 8.8 Hz, 2 H,
py C(3)H], 8.72 [d, J = 9.4 Hz, 2 H, py C(6)H] ppm. HRMS (ESI):
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7
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+
calcd. for C58
H
32
N
6
F
10
S
2
Ir 1259.2; found 1259.6.
1
IrF-xy
1
-PTZ: H NMR (400 MHz, CDCl
H, Me), 2.19 (s, 3 H, Me), 5.67 (ddd, J = 8.0, 8.0, 2.4 Hz, 2 H),
.67 (ddd, J = 8.8, 7.6, 2.4 Hz, 2 H), 7.60 (m, 1 H), 7.67 (m, 2 H),
.00 (dd, J = 5.6, 0.8 Hz, 2 H), 8.01 (d, J = 2.4 Hz, 1 H), 8.09 (m,
H), 8.11 (m, 1 H), 8.33 (ddd, J = 7.6, 7.6, 1.6 Hz, 1 H), 8.38 (dd,
J = 8.4, 2.0 Hz, 2 H), 8.52 (ddd, J = 8.8, 8.8, 3.2 Hz, 2 H), 8.84
dm, J = 8.4 Hz, 1 H), 8.96 (d, J = 8.4 Hz, 1 H) ppm. HRMS
ESI): calcd. for C53
16IrN PS·3CH
N 4.57; found C 50.88, H 3.45, N 4.39.
–·CH
Crystal Data for IrMe_ester: [C40H N O Ir] PF Cl , M =
36 4 4 6 2 2
058.90, monoclinic, space group Pc, a = 8.6551(7) Å, b =
5.0012(8) Å, c = 15.8243(11) Å, β = 93.416(9)°, U = 2050.9(2) Å ,
α
Z = 2, Mo-K radiation (λ = 0.7103 Å), µ = 3.50 mm , T = 150 K,
Stoe IPDS-II diffractometer, 25450 reflections measured, 9784
unique (Rint = 0.039) of which 7190 with |F |Ͼ4σ(F ). Final values
R = 0.030, ωR = 0.031, and S = 1.85(2). CCDC-737590 contains
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
, 25 °C): δ = 2.06 (s, 3
6
8
1
(
(
+
H
33
N
5
F
10SIr 1166.1920; found 1166.1925.
C
53
H
33
F
5
3
COCH (1299.10): calcd. C 50.98, H 3.75,
3
2007, 4808–4814; g) M. Tavasli, S. Bettington, M. R. Bryce,
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Chem. 2005, 15, 4963–4970; h) H. Yersin, Top. Curr. Chem.
2004, 241, 1–26.
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Supporting Information (see footnote on the first page of this arti-
cle): Cyclic voltammograms of the IrF-xy
dyads; Stern–Volmer plots used to determine the quenching con-
stants k ; luminescence decays used to determine the excited-state
lifetimes.
1 1
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Q
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Published Online: October 12, 2009
Eur. J. Inorg. Chem. 2009, 4850–4859
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4859