Dipyridylketone as a versatile ligand precursor for new cationic heteroleptic cyclometalated iridium complexes

Article abstract of DOI:10.1039/c1dt11279a

Three new bis-cyclometalated iridium(iii) complexes, of general formula [Ir(2-phenylpyridine)₂(L)]⁺, are reported. The compounds contain a dipyridine-type ligand (L) derived from di-2-pyridylketone (dipyridin-2-ylmethanol, 2,2′-(hydrazonomethylene)dipyridine and 3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile) and were synthesized through two different reaction pathways. The alternative synthetic pathway herein proposed, namely the direct reactions on the complex [Ir(2-phenylpyridine) ₂(2,2′-dipyridylketone)]⁺, overcame the inconveniences encountered with the standard reaction between the dimeric precursor [Ir(2-phenylpyridine)₂(μ-Cl)]₂ and the ancillary ligands (L). The photophysical characterization of the iridium complexes reveals that modifications on the ancillary ligand introduce large changes in the photophysical behaviour of the complexes. High emission quantum yield is associated with the presence of a saturated carbon between the two pyridyl moieties: [Ir(2-phenylpyridine)₂(2,2′-dipyridylketone)] ⁺ and [Ir(2-phenylpyridine)₂(2,2′- (hydrazonomethylene)dipyridine)]⁺ are extremely low emissive, while [Ir(2-phenylpyridine)₂(dipyridin-2-ylmethanol)]⁺ and [Ir(2-phenylpyridine)₂(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile) ]⁺ are good photoemitters. DFT and TD-DFT calculations confirmed the mixed LC/MLCT character of the excited states involved in the absorption and emission processes and highlighted the role of the π-conjugation between the two subunits of the ancillary ligand in determining the nature of the LUMO.

Full text of DOI:10.1039/c1dt11279a

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PAPER
Dipyridylketone as a versatile ligand precursor for new cationic heteroleptic
cyclometalated iridium complexes†
Giorgio Volpi,‡ Claudio Garino,‡ Emanuele Breuza, Roberto Gobetto* and Carlo Nervi*
Received 5th July 2011, Accepted 3rd October 2011
DOI: 10.1039/c1dt11279a
Three new bis-cyclometalated iridium(III) complexes, of general formula [Ir(2-phenylpyridine)2(L)]+, are
reported. The compounds contain a dipyridine-type ligand (L) derived from di-2-pyridylketone
(dipyridin-2-ylmethanol, 2,2¢-(hydrazonomethylene)dipyridine and 3-hydroxy-3,3-di
(pyridine-2-yl)propanenitrile) and were synthesized through two different reaction pathways. The
alternative synthetic pathway herein proposed, namely the direct reactions on the complex
[Ir(2-phenylpyridine)2(2,2¢-dipyridylketone)]+, overcame the inconveniences encountered with the
standard reaction between the dimeric precursor [Ir(2-phenylpyridine)2(m-Cl)]2 and the ancillary ligands
(L). The photophysical characterization of the iridium complexes reveals that modifications on the
ancillary ligand introduce large changes in the photophysical behaviour of the complexes. High
emission quantum yield is associated with the presence of a saturated carbon between the two pyridyl
moieties: [Ir(2-phenylpyridine)2(2,2¢-dipyridylketone)]+ and [Ir(2-phenylpyridine)2(2,2¢-(hydrazonome-
thylene)dipyridine)]+ are extremely low emissive, while [Ir(2-phenylpyridine)2(dipyridin-2-ylmethanol)]+
and [Ir(2-phenylpyridine)2(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile)]+ are good photoemitters.
DFT and TD-DFT calculations confirmed the mixed LC/MLCT character of the excited states
involved in the absorption and emission processes and highlighted the role of the p-conjugation
between the two subunits of the ancillary ligand in determining the nature of the LUMO.
The general structure of a cyclometalated iridium(III) complex
Introduction
is characterized by an octahedral coordination arrangement with
Second- and third-row transition-metal complexes have attracted
strong Ir–C bonds between the metal ion and the cyclometalating
significant attention due to their outstanding luminescence
ligands. This compact framework guarantees a good thermal-
properties.1,2 Among them, cyclometalated iridium(III) complexes
and photo-stability, together with a strong electronic interaction
exhibit highly desirable features in terms of thermal- and photo-
between the d orbitals of the metal and the p orbitals of the ligands,
stability, emission quantum yields and lifetime, and emission
increasing the d–d energy gap and preventing the radiationless
wavelength that can be tuned to cover a full range of visible
quenching. Furthermore, the large spin-orbit coupling effect of the
colours.3,4 All these features make iridium(III) complexes highly
promising for various optical applications, in particular as phos-
phors in multicolor organic light-emitting devices (OLEDs) and in
light-emitting electrochemical cells (LECs), but also as biological
labelling agents, as photocatalysts for CO2 reduction and water-
splitting, and as singlet oxygen sensitizers.5–10
Ir atom assures efficient room temperature phosphorescence.11,12
Cationic bis-cyclometalated iridium(III) complexes combine
promising photophysical properties with good solubility in polar
solvents.6,13 In these systems, the HOMO (highest occupied
molecular orbital) is generally centred over the Ir atom and the
cyclometalating ligands, while the LUMO (lowest unoccupied
molecular orbital) is preferentially localized on ligands. For this
reason the photophysical properties of Ir(III) complexes can be
readily perturbed by modifying the cyclometalating ligand with
proper substituents, as well as accurately choosing the ancillary
ligand.11,14 However, if the metal and ligand orbitals do not mix
Department of Chemistry IFM, University of Turin, Via P. Giuria 7, 10125,
Turin, Italy. E-mail: carlo.nervi@unito.it; Fax: +39 011 670 7855; Tel: +39
011 670 7507
† Electronic supplementary information (ESI) available: Sketch of the
synthetic pathways to IrL4; Frontier orbitals for IrL1–IrL4; Experimental
absorption and emission spectra of IrL2 and IrL3, together with calculated
singlet and triplet excited state transitions, and estimated emission energy
using DSCF and TD-DFT approaches; Contour plots of the spin density
of the lowest-lying triplet-state geometry of IrL1–IrL4; Lowest energy
singlet–triplet electronic transition of IrL1, IrL2, and IrL4 in their lowest-
lying triplet state geometries. See DOI: 10.1039/c1dt11279a
3
3
efficiently, ligand-based p–p* or LLCT states can be obtained,
and the emission efficiency decreases resulting in lower quantum
yields and longer lifetimes.15
The use of certain types of dipyridine ligands can lead to the for-
mation of six-membered chelated metal complexes and sometimes
has the advantage to break the crosstalk between the pyridine
rings, allowing in principle to shift the HOMO and LUMO
‡ These authors contributed equally to this work.
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energies independently.16,17 However, if dipyridine type ligands are
characterized by the presence of multiple coordinating sites, they
provide a variety of structural motifs and coordination modes.
For example, 2,2¢-dipyridylimines (2,2¢-dipyridyl-N-methylimine
or 2,2¢-dipyridylmethylideneaniline) or di-2-pyridylmethanamine
can act as tridentate ligands18,19 or behave as bidentate ligands.20
They may form a six-membered chelate ring by using the two pyri-
dinic nitrogen or may coordinate to the metal by the iminic/aminic
nitrogen and just one pyridinic nitrogen, forming a five-membered
chelate ring. This variety of coordination modes may lead to
the formation of several compounds, preventing the synthesis, or
lowering the reaction yield, of the desired complex. Furthermore,
the standard synthesis of cationic heteroleptic cyclometalated
Ir(III) complexes is generally accomplished by reacting a chloride-
bridged Ir(III) dimer (containing four cyclometalating ligands)
with an appropriate organic ligand. This approach can not
always be pursued because in some cases the ancillary ligand
contains reactive units or embeds s-coordinating groups that
interact with the metal precursor, preventing the synthesis of
the desired complex. This is also the major drawback of ligands
containing sensitive functionality that cannot necessarily tolerate
harsh synthetic conditions.
and
[Ir(ppy)2(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile)]+
(IrL4) were obtained by nucleophilic addition of hydrazine (in
methanol) and acetonitrile (activated by potassium hydroxide),
respectively. All the complexes were obtained in reasonable or
almost quantitative yields. It is worth noting that modification
of the ketone moiety introduces relatively large changes in
the photophysical behaviour of the iridium complexes. A
similar phenomenon has been previously reported by Lehn and
coworkers in a ruthenium bipyridyl complex containing a quinone
group.23 In its oxidized form the quinone selectively quenches the
luminescence whereas the luminescence is not quenched in the
hydroquinone form.
Ligands and related iridium complexes were fully characterized
by NMR, UV-vis absorption and emission spectroscopy, mass
spectrometry, and electrochemical measurements. DFT (density
functional theory) and TD-DFT (time-dependent density func-
tional theory) studies were undertaken to rationalize the key role
of the ketone group, and its modifications to form non-conjugated
ancillary ligands, on the photophysical properties of these closely
related iridium(III) complexes.
Results and discussion
In our previous work, we studied the complex [Ir(ppy)2(di-
2-pyridylketone)]+ (IrL1) where ppy = 2-phenylpyridine.21 This
compound is extremely low emissive but we predicted that the
replacement of C O by a non electroreducible conjugated group
should lead to complexes having high quantum yields, analogously
to that observed for the dipyridylamine Ir complex.22
Common ancillary ligands incorporate 2,2¢-bipyridine subunits
and coordinate the metal forming a stable five-membered chelate
rings. Much less attention has been dedicated to the study of
ligands containing two pyridines linked by a spacer. The simplest
of these ligands is the di-2-pyridylmethane while the most studied
is probably the di-2-pyridylketone. [Ir(ppy)2(di-2-pyridylketone)]+
(IrL1) can be easily obtained in good yield using the same
synthesis employed for the complex [Ir(ppy)2(bipyridine)]+.24 The
bipyridine derivative shows good photophysical properties but
lacks functional groups which are easy to modify; conversely,
[Ir(ppy)2(di-2-pyridylketone)]+ bears a carbonyl group that is
widely versatile from the synthetic point of view.
Herein we extend those studies and explore the possibility
of carrying out new synthetic strategies in order to obtain
mononuclear complexes via clean reactions, at low temperature,
and with good reaction yield. The syntheses and the photophysical
characterization of new phosphorescent bis-cyclometalated
iridium(III) phenylpyridinato complexes [Ir(ppy)2(L)]+, where
L (L2–L4) is a ligand derived from the di-2-pyridylketone
(L1) carrying two pyridines separated by a single atom spacer
(Scheme 1), are reported. The usual synthetic procedure of
reacting the chloride-bridged Ir(III) dimer [Ir(ppy)2(m-Cl)]2
with an appropriate ancillary ligand (L) was substituted by
exploiting the wide and versatile reactivity of the carbonyl group
of the [Ir(ppy)2(2,2¢-dipyridylketone)]+ (IrL1) complex (Scheme
2 and 3). The reduction of the carbonyl group with NaBH4
on IrL1 afforded [Ir(ppy)2(dipyridin-2-ylmethanol)]+ (IrL2),
whereas [Ir(ppy)2(2,2¢-(hydrazonomethylene)dipyridine)]+ (IrL3)
Synthesis
The classical synthesis of bis-cyclometalated iridium(III) phenyl-
pyridinato complexes [Ir(ppy)2(L)]+ has been followed to get the
complex IrL1, i.e. by refluxing a mixture of [Ir(ppy)2(m-Cl)]2
and 2,2¢-dipyridylketone (L1) in CH3OH/CH2Cl2 solution for
2 h. This procedure affords IrL1 as a pure product in good
yield (91%), as published before.21 However, the same synthetic
approach was not effective when we tried to synthesize IrL2, IrL3,
and IrL4 by reacting [Ir(ppy)2(m-Cl)]2 with dipyridin-2-ylmethanol
(L2), 2,2¢-(hydrazonomethylene)dipyridine (L3), and 3-hydroxy-
3,3-di(pyridine-2-yl)propanenitrile (L4), respectively.
Employing the ligand L2, obtained from the reduction of L1
with NaBH4 in methanol (Scheme 2a), only a very low amount
of IrL2 was obtained (reaction yield 10%) with the concomitant
formation of several side products. To avoid this drawback, the
reaction strategy has been changed by performing the carbonyl
group reduction directly on IrL1 (Scheme 2b). The reaction of
IrL1 with NaBH4 in methanol afforded IrL2 as a pure product in
high yield (97%).
By using the same reaction strategy employed to synthesize
IrL2, we set up a nucleophilic addition between hydrazine and IrL1
in methanol (Scheme 3a). This approach afforded a good amount
Scheme 1
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Scheme 2 Sketch of the synthetic pathways to IrL2.
Scheme 3 Sketch of the synthetic pathways to IrL3.
of IrL3 that was easily purified by thin layer chromatography (yield
43%).
any other compounds. The problem was solved by introducing the
cyanomethyl group directly on the di-2-pyridylketone unit of IrL1.
The reaction of IrL1 in acetonitrile with potassium hydroxide
at reflux afforded the complex IrL4 in good yield (40%), after
chromatographic purification.
We also obtained IrL3 as an unexpected side product from
the reaction of [Ir(ppy)2Cl]2 with di-2-pyridylketoneazine (1,2-
bis(dipyridin-2-ylmethylene)hydrazine) performed in order to ob-
tain the corresponding dinuclear complex (Scheme 3b).25–28 In
our case the reaction of di-2-pyridylketoneazine with [Ir(ppy)2Cl]2
in refluxing MeOH/CH2Cl2 did not produce the expected cor-
responding dinuclear complex, but afforded a plethora of com-
pounds, where IrL3 was isolated only as a minor product (yield
<5%).
The free ligand L4 (3-hydroxy-3,3-di(pyridine-2-yl)propane-
nitrile) can be obtained by introducing a cyanomethyl group on
L1; the synthesis is straightforward and was performed by reacting
L1 with potassium hydroxide in acetonitrile (see Scheme ESI 1 in
the ESI†).
The same reaction, performed on benzophenone affords the
dehydration product (3,3-diphenylacrylonitrile).29 By using di-
2-pyridylketone (L1) the elimination reaction was not observed
and we obtained the stable b-hydroxy-nitrile intermediate, as
confirmed by ESI mass spectrometry (molecular peak at m/z 226.3
[M + H]+) and by NMR spectroscopy (hydroxyl signal at 6.70 ppm
and methylene signal at 3.60 ppm in [D6]acetone).
Several attempts to react L4 with the iridium dimeric precursor
[Ir(ppy)2(m-Cl)]2 at reflux in different solvents (CH2Cl2/CH3OH,
CH3CH2OH, and CH3CH2OH/CH3COOH) failed. In all the
conditions the two reagents did not give the expected product nor
Computed geometries and electronic structures
Geometry optimization of the singlet ground state (S0) of the
complexes IrL1–IrL4 was performed in gas phase using the B3LYP
method, as described in the Computational details section. The
main optimized geometrical parameters are reported in Table
1. All the complexes have a pseudo-octahedral coordination
structure; the Ir–N(ppy), Ir–C(ppy), and Ir–N(L) bond lengths
˚
are in the range 2.085–2.099, 2.021–2.024, and 2.249–2.352 A, re-
spectively. In the case of IrL1, for which X-ray data are available,21
the bond lengths and angles obtained by DFT calculations are in
excellent agreement with experimental data. For complexes IrL2–
IrL4, we were not able to get suitable crystals for X-ray diffraction
analysis. In all optimized geometries, N–Ir–C and N–Ir–N angles
are found to be very similar. The bite angle of the ppy ligands (N1–
Ir–C1 and N2–Ir–C2) is relatively small, close to 80◦, whereas
the bite angle of the L ligands (N3–Ir–N4) spans from 84.48◦
to 86.67◦. The main geometrical parameters of IrL1–IrL4 in the
lowest-lying triplet state (T1) are also calculated and results are
given in Table 1. Compared to the data of the ground state, there
are some variations. In particular, the Ir–N3/N4 bond lengths
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◦
˚
Table 1 Selected bond lengths [A] and angles ( ) of IrL1–IrL4 on calculated ground state (S0) and lowest-lying triplet state (T1) geometries, and X-ray
crystallography
Complex
Ir–N1
Ir–C1
Ir–N2
Ir–C2
Ir–N3
Ir–N4
N1–Ir–C1
N2–Ir–C2
N3–Ir–N4
IrL1
X-ray
S0
T1
S0
T1
S0
T1
S0
T1
2.039
2.090
2.093
2.087
2.075
2.085
2.086
2.098
2.068
2.007
2.024
2.005
2.022
1.990
2.022
2.024
2.021
1.980
2.041
2.085
2.082
2.092
2.090
2.086
2.081
2.099
2.113
2.015
2.023
1.997
2.023
1.985
2.022
2.024
2.023
2.018
2.179
2.264
2.243
2.287
2.315
2.276
2.252
2.352
2.400
2.153
2.249
2.209
2.266
2.295
2.259
2.238
2.307
2.325
80.45
79.99
80.65
79.90
80.89
79.97
79.94
79.81
81.49
80.96
80.12
80.67
80.14
81.15
80.17
80.15
80.16
80.17
86.96
85.67
83.32
86.67
82.90
85.25
85.47
84.48
83.37
IrL2
IrL3
IrL4
˚
are shortened in IrL1 (0.021 and 0.040 A) and IrL3 (0.024 and
preferentially localized on the ligands (Fig. 1 and Fig. ESI 1†). The
LUMO of IrL1 and IrL3 is a p* orbital localized on the whole
L ligand, with antibonding features along the double C O and
˚
˚
0.021 A) and are lengthened in IrL2 (0.028 and 0.029 A) and IrL4
(0.048 and 0.018 A). With the exception of IrL3, the Ir–C1/C2
˚
bond lengths are shortened for all the complexes, while the Ir–
N1/N2 bond lengths remain unaltered. It is worth noting that,
in the triplet state of IrL1 and IrL3, the double C O (L1) and
C
N bonds, respectively. As in the case of most Ir(III) complexes,
the LUMO of IrL2 resides on the cyclometalating ligands, being a
p* orbital delocalized on one ppy. In a different way, the LUMO
of IrL4 is a p* orbital delocalized on one ppy and on the aromatic
rings of L4, with no contribution from the aliphatic portion. The
sp2 hybridized carbon atom in L1 and L3 is likely to promote
the direct electronic crosstalk, i.e. p-conjugation, between the two
pyridyl moieties. Such conjugation seems to stabilize the L-centred
p* frontier orbital; conversely the saturated carbon in L2 and L4
effectively interrupts the p-conjugation, increasing the energy of
the L-centred p* orbital over the p* orbital of the cyclometalating
ligands. For all complexes, the LUMO is followed in energy by a
series of ppy and L p* orbitals.
C
N (L3) bonds are significantly lengthened. With respect to
˚
˚
the ground state, the two bonds are 0.045 A and 0.106 A longer,
in agreement with the population of orbitals with antibonding
features along the C O and the C N bonds.
To provide insight into the electronic structure of IrL1–IrL4, we
performed single-point calculations on the optimized geometries,
taking into account the solvent effect (acetonitrile). The complexes
have common orbital features. For all complexes, the HOMO is an
antibonding combination of Ir(d) and ppy(p) orbitals, centred on
the Ir atom and on the two phenyl groups of the ppy ligands (Fig.
1 and Fig. ESI 1†). The HOMO is followed, at lower energy (0.5–
0.7 eV below), by five or six combinations of Ir (d) and ppy and/or
L (p) orbitals. These orbitals are very close in energy (within
0.464–0.470 eV) and are followed by the p-bonding framework
of the L ligand (0.8–0.9 eV below). The LUMO of IrL1–IrL4 is
Photophysical properties
Electronic absorption spectra of IrL1–IrL4 were recorded at room
temperature in acetonitrile solution. Results are summarized in
Table 2.
Table 2 Absorption and emission data for complexes IrL1–IrL4 in
deareated acetonitrile solutions
Complex
l
abs /nm
l
em/nm
U
tav /ms
IrL1
262
288a
333a
376
480
260
290a
330a
380
480
260
300a
361
405a
465
263
290a
338
390
458
(678)
<0.005
—
IrL2
IrL3
IrL4
477
507
547
0.10
1.335
2.411
1.600
480
510
<0.005
0.49
535
a Shoulders.
Fig. 1 Frontier orbitals for complexes IrL3 and IrL4.
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Typical Ir(III) complexes have a multitude of concurrent elec-
tronic transitions. In principle, singlet and triplet metal-to-ligand
charge-transfer (1MLCT and 3MLCT) transitions and singlet
L2 transitions, and in the case of the first shoulder a Ir-ppy →
L2 transition is also contributing. The less intense feature at 380
1
nm can be ascribed to a mixed LC/1MLCT (Ir-ppy → ppy-L2)
3
and triplet ligand-centered (1LC and LC) transitions should be
transition. The weak band, experimentally found at 480 nm, can
be related to the presence of two singlet–triplet transitions having a
mixed 3LC/3MLCT character (Ir-ppy → ppy transition at 449 nm,
Ir-ppy → ppy-L2 transition at 445 nm).
considered.
The complexes IrL1–IrL3 show intense absorption bands in the
range 250–300 nm, less intense features in the range 300–400 nm
with tails which extend into the visible region, and no absorption
above 500 nm. IrL4 has a similar absorption spectrum, plus a
pronounced shoulder at l = 458 nm (Fig. 2).
In the case of IrL3, calculations attribute the UV band at l =
1
260 nm to mixed LC/1MLCT transitions involving all ligands.
However, L3 is the unit receiving electron density. The shoulder
falling at l = 300 nm can be ascribed to Ir-ppy → ppy transitions.
The less intense feature at 361 nm is due to mixed 1LC/1MLCT (Ir-
ppy → L3) transitions, while the shoulder at l = 405 nm is mainly
due to an Ir-ppy → ppy transition, with a minor contribution from
a Ir-ppy → L3 transition. The weak band, experimentally found
at 465 nm, can be related to the presence of three singlet–triplet
transitions having mixed 3LC/3MLCT character (Ir-ppy → L3 at
483 nm, Ir-ppy → ppy at 449 nm, and Ir-ppy → ppy at 447 nm).
Finally, IrL4 displays an intense absorption band at l = 263 nm
(Ir-ppy → ppy transition), and a shoulder at l = 290 nm (Ir-ppy →
L4 and Ir-ppy → ppy-L4 transitions). Less intense absorptions
are present at l = 338 nm and l = 390 nm (Ir-ppy → L4
transition), plus a pronounced shoulder at l = 458 nm, neglected
by singlet excited state calculation. This shoulder can be ascribed
to singlet–triplet transitions (452 and 444 nm) having mixed
3LC/3MLCT character (Ir-ppy → ppy-L4 and Ir-ppy → ppy,
respectively).
When excited in acetonitrile solution at room temperature, the
luminescence of all complexes is strongly quenched by oxygen.
IrL1 and IrL3 are very weak emitters, both showing emission
quantum yields lower than 0.005. The former displays a wide and
unresolved emission centred at 678 nm21 while the latter presents
a structured emission profile with rather well defined maxima at
480 and 510 nm (t = 2.411 ms). On the contrary, IrL2 and IrL4
exhibit intense emission spectra. IrL2 shows a structured emission
with prominent vibrational progression, with maximum at 477,
507, and 547 nm (U = 0.10, t = 1.335 ms), while IrL4 presents
a structureless emission centred at 535 nm with an excellent
quantum yield (U = 0.49, t = 1.600 ms).
The lowest triplet state, which is responsible for phosphores-
cence emission, can be either 3MLCT transition state or 3LC
transition state and it is generally accepted to describe the
phosphorescent state as a mix of MLCT and LC transition states.11
The emission lifetimes and the effective quenching by oxygen
molecules of the luminescence quantum yields are indicative of
emission properties due to triplet excited states. In terms of spectral
shape, we can attribute the structureless emission of IrL1 and IrL4
to an emitting excited state with predominant MLCT character.
On the contrary, in the case of IrL2 and IrL3 an increase of
the content of p–p* character in the emitting excited state gives
rise to more structured emission profile that we label as LC
phosphorescence.
To gain insights into the nature of the excited states involved
in the emission process, we used two different computational
approaches. The first consisted of TD-DFT calculations on triplet
excited states from the ground state, using the lowest-lying triplet
state geometries; the second employed the DFT/UKS calcula-
tion (unrestricted Kohn–Sham) of lowest-lying triplet geometries
(DSCF method, see the Computational details).
Fig. 2 Experimental absorption (blue line) and emission (green line)
spectra, calculated singlet (purple bars) and triplet (blue bars) excited state
transitions, and estimated emission energy using DSCF (dashed green bar)
and TD-DFT (solid green bar) approaches, of complex IrL4 in acetonitrile.
The vertical bar height of singlet transitions is equal to the oscillator
strength, while the other bars have arbitrary intensity to indicate their
position.
The nature of the electronic transitions responsible for the
absorption bands was assigned using TD-DFT calculations. Sixty-
four singlet and eight triplet excited states were computed starting
from the gas-phase optimized geometry. The solvent effect was
taken into account by using the CPCM method.
The high-energy absorptions are well described by TD-DFT,
but for these Ir systems the efficient spin-orbit coupling make it
possible to experimentally observe the normally forbidden singlet–
triplet absorptions.30 Triplet excited states were calculated in order
to consider the contribution of the singlet–triplet transitions
that dominate the low-energy portion of the spectra. Since the
TD-DFT calculation used here is known to neglect the spin-
orbit coupling, the singlet–triplet transitions have zero oscillator
strength and are reported in Fig. 2 with arbitrary intensity.
The absorption spectrum of IrL1 shows an intense UV band at
1
l = 262 nm. This feature can be ascribed to mixed LC/1MLCT
transitions involving all ligands; where L1 is the unit receiving
electron density. A second band (shoulder) falls at l = 376 nm
and Ir-ppy → ppy singlet transitions are responsible of such
absorption. At l = 480 nm IrL1 has a large and weak absorption
band assigned to a singlet Ir-ppy → L1 transition, according to
calculations.
The complex IrL2 has an absorption band centred at l = 260 nm
that can be ascribed to mixed 1LC/1MLCT transitions, where L2
is the unit receiving electron density. The two shoulders at l =
290 nm and l = 330 nm are essentially due to Ir-ppy → ppy-
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TD-DFT correctly estimates the emission energy of IrL2 (ex-
perimental 477 nm, calculated 485 nm) and slightly overestimates
the emission of IrL4 (experimental 535 nm, calculated 571 nm).
DSCF also gives satisfactory results, predicting the emission of
IrL2 at 448 nm and the emission of IrL4 at 533 nm.
the case of IrL3; both methods predict negative emission energy.
However, the spin density obtained for the lowest-lying triplet-
state shows a triplet state centred over L3, involving the C
N
bond (Fig. 4). This is in agreement with the spectral shape and with
the oxygen quenching of the emission observed experimentally.
According to the triplet excited state calculations obtained by
TD-DFT, for both IrL2 and IrL4 the luminescence is due to
a mixed 3LC/3MLCT (Ir-ppy → ppy) emissive state (Fig. 3),
confirmed by the spin density of the lowest-lying triplet-state
obtained with the unrestricted Kohn–Sham formalism (Fig. 4 and
Fig. ESI 4†). Due to the qualitative nature of the computational
methods used to evaluate the emission, calculations are not able
to discriminate between the LC or MLCT nature of the state
responsible for the emission and describe it as a mix of MLCT
and LC states. Although, from the experimental data it is clear
that the emissive state of IrL2 has a greater LC character while
the emissive state of IrL4 can be mainly considered as a MLCT
transition state.
Electrochemistry
Cationic [Ir(ppy)2(N–N)]+ derivatives (where N–N is a bipyridine
type ligand) are known to undergo one-electron ligand-based
reduction and one-electron metal-based oxidation.13,31 These are
not strict assignments, because metal and ligand orbitals are
strongly mixed. Furthermore, one should consider that in some
cases the presence of electro-reducible moieties, like the nitro
group,21 can significantly alter the electrochemical behaviour in
reduction, and sometimes, the reversible Ir(III)/Ir(IV) oxidation
step can become quasi-reversible due to an increased contribution
of the formal anionic ppy ligands to the HOMO.31,32 The elec-
trochemical behaviour in acetonitrile solutions of the complexes
herein studied is similar to that of this class of cationic complexes,
namely [Ir(C–N)2(N–N)]+ (where C–N is a phenyl–pyridine type
ligand). In cyclic voltammetry IrL1 undergoes an additional
reversible 1e reduction centred on the carbonyl moiety at -1.26 V
vs. ferrocene/ferrocinium redox couple.21 Also IrL3 carries an
electro-reducible group (C N) that undergoes a reversible 1e
reduction process; however its reduction potential, besides the
Ir(ppy)2 fragment, is located at -1.72 V, a more negative value
because nitrogen is less electronegative than oxygen.33 IrL3 shows
a chemical irreversible oxidation process at Ep = 1.01 V vs. Fc/Fc+,
that probably involves the organic hydrazone fragment. At more
negative potentials appear chemically irreversible multielectron
reductions, which are very likely centred on the organic ligands.
Fig. 3 Electron density difference maps (EDDMs) of the lowest energy
singlet–triplet electronic transition of IrL2 and IrL4 in their lowest-lying
triplet state geometries. Blue indicates a decrease in electron density, while
orange indicates an increase.
Conclusions
In conclusion, we have synthesized and fully characterized three
new cationic bis-cyclometalated Ir(III) complexes (IrL2–IrL4).
The standard synthetic strategy, i.e. the reaction of the ancillary
ligands (L2–L4) with the chloride-bridged Ir(III) dimer [Ir(ppy)2(m-
Cl)]2 did not yield the related complexes. Therefore, we changed
the synthetic strategy, exploiting the reactivity of the carbonyl
group directly on the complex IrL1. The reduction with NaBH4,
the nucleophilic addition of hydrazine in methanol, and the nucle-
ophilic addition of acetonitrile activated by potassium hydroxide
gave IrL2, IrL3, and IrL4, respectively. The modification of the
carbonyl group directly on the complex IrL1 allowed us to obtain
the related complexes in good yields. These examples show how
to take advantage of the synthon versatility of di-2-pyridylketone
moiety already coordinated to the metal.
Photophysical data reveal that slight modifications of the ketone
moiety introduce relatively large changes in the photophysical
behaviour of the iridium complexes. The complex IrL3 is extremely
low emissive, as well as the precursor IrL1. On the contrary, the
complexes IrL2 and IrL4 are good photoemitters; in particular
IrL4 affords an emission quantum yield value of 49%.
Fig. 4 Contour plots of the spin density of the lowest-lying triplet-state
geometry of complexes IrL3 and IrL4 (iso value = 0.004).
As already reported,21 the structureless weak emission observed
in IrL1 can be ascribed to a 3LC/3MLCT transition (Ir-ppy → L1)
involving the electron withdrawing carbonyl group on L1 (Fig. ESI
4†); this is consistent with the quenching of the emission properties
in such compound.
The very weak emission observed for IrL3 could be related
to an analogous electronic situation; both the lowest singlet and
3
triplet absorptions have mixed LC/3MLCT character involving
The results of DFT and TD-DFT calculations allowed us
to characterize the nature of the excited states involved in the
absorption and emission properties. Frontier orbital analysis
highlighted the role of the p-conjugation, between the two subunits
the population of the LUMO (p* orbital localized on the L3
ligand). Unfortunately, the DSCF and the TD-DFT approaches
are unable to describe the emission process that takes place in
1070 | Dalton Trans., 2012, 41, 1065–1073
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of the ancillary ligand, in determining the nature of the LUMO and
data were collected using a spectral bandwidth of 2–10 nm. The
data were collected into 2048 channels to 10 000 counts in the peak
channel. The sample was maintained at 20 ◦C in an automated
sample chamber (F-3004 Peltier Sample Cooler from Horiba Jobin
Yvon IBH) for ambient temperature measurements. Emission
decay data were analysed using the software DAS6 (TCSPC Decay
Analysis Software).
Mass spectra were recorded using an XCT PLUS electrospray
ionisation-ion trap (ESI-IT) mass spectrometer (Agilent Italy,
Milan). In the spectra description the abbreviation [M] was used
for the molecular ion. The reported values are in atomic mass units.
Samples were dissolved in methanol. Scan range was 50–2000 m/z.
thus influencing the radiationless decay through C O and C
N
3
units. Calculations confirmed the mixed LC/3MLCT character
of the states responsible for the structured emission of IrL2 and
for the structureless emission of IrL4.
Experimental
Materials and methods
Di-2-pyridylketone (L1), 2-phenylpyridine, iridium trichloride,
and all other reagents and solvents were of reagent grade and
used as received without any further purification. Acetonitrile was
distilled over calcium hydride just before use. All the reactions
involving the metal complexes or precursor were routinely per-
formed under nitrogen atmosphere by using standard Schlenk
techniques.
Electrochemistry was performed by a PC-controlled Autolab
PGSTAT302N electrochemical analyser in the usual conditions,34
using a standard three electrode cell configuration (glassy carbon
working electrode, Pt counter electrode, aqueous 3 M KCl Calomel
reference electrode). All measurements were carried out under
Ar atmosphere, in acetonitrile solution with tetrabutylammonium
hexafluorophosphate (TBAPF6) 0.1 M as supporting electrolyte,
obtained as previously reported.35 Positive feedback iR compen-
sation was applied routinely and ferrocene (Fc) was used as an
internal standard (half-wave potentials are reported against the
Fc(0/+1) redox couple).
Computational details
All calculations were performed with the Gaussian 09 program
package.39 Geometry optimization was performed in the gas
phase, employing the DFT method for the ground state and the
unrestricted Kohn–Sham formalism (UKS) for the lowest-lying
triplet state. The nature of all stationary points was confirmed by
normal-mode analysis. The conductor-like polarizable continuum
model (CPCM)40–42 with acetonitrile as the solvent was used
to calculate the electronic structure and the excited states in
solution. A total of 64 singlet and 8 triplet excited states were
determined with a TD-DFT43,44 calculation, employing ground
state geometries optimized in the gas phase. The emission energy
was evaluated by the DSCF45 and TD-DFT46,47 approaches, taking
into account the solvent effect with the CPCM method. The DSCF
approach calculates the vertical energy gap between the ground
state and the lowest-lying triplet state, both evaluated at the ge-
ometry optimized for lowest-lying triplet state and both computed
using unrestricted wave functions (UKS). The TD-DFT approach
calculates 8 triplet excited states as electronic transitions from the
ground state, evaluated with lowest-lying triplet state geometry.
This TD-DFT calculation uses a restricted wave function. The
program GaussSum 2.2.448 was used to simulate the electronic
spectra and to visualize the excited state transitions as electron
density difference maps (EDDMs).49,50 All calculations employed
the Becke three-parameter hybrid functional,51 and Lee Yang
Parr’s gradient-corrected correlation functional (B3LYP).52 The
LanL2DZ basis set53 and effective core potential were used for the
Ir atom, and the 6-31G** basis set54 was used for all other atoms.
NMR spectra were recorded on a JEOL EX 400 spectrometer
1
(B0 = 9.4 T, H operating frequency 399.78 MHz) with chemical
shifts referenced to residual protons in the solvent. The following
abbreviations are used: s, singlet; d, doublet; t, triplet; m, multiplet.
UV-Vis absorption spectra were measured with a double-beam
Perkin-Elmer Lambda 20 UV-Vis spectrophotometer equipped
with a 1 cm quartz cell. Room temperature emission spectra as well
as luminescence lifetimes were obtained using a HORIBA Jobin
Yvon IBH Fluorolog-TCSPC spectrofluorimeter. Fluorescence
quantum yields (U) were determined by the comparative method,36
using quinine bisulfate (0.1 N H2SO4) as standard (U = 0.546).37
Refractive index corrections were made to adjust for different
solvents used. Five solutions with increasing concentrations
(Absorbance at the excitation wavelength ranging from 0/solvent
blank to 0.10) were prepared for the standard and for each sample.
A graph of integrated fluorescence intensity vs. absorbance was
plotted for each compound and the gradients of the graphs
obtained, which are proportional to the quantum yield of the
different samples,38 were used to provide the quantum yields
according to the following equation:
Synthesis
Dipyridin-2-ylmethanol (L2),55 di-2-pyridylketoneazine (1,2-
bis(dipyridin-2-ylmethylene)hydrazine),25 [Ir(ppy)2(m-Cl)]2,56 and
IrL121 were synthesized as previously reported.
3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile
(L4). Di-2-
2
⎛
⎜
⎜
⎞⎛
⎞
⎟
⎟
⎟
⎟
⎠
GradX hX
⎟
⎜
pyridylketone (1.000 g, 5.43 mmol) and KOH (360 mg,
6.42 mmol) were refluxed in acetonitrile (5 ml) for 20 min,
under N2. After cooling at room temperature (r.t.), 6 ml of H2O
were added and the solution was stirred for 10 min, extracted
3 times with 3 ml of Et2O and the organic phase evaporated
to obtain a purple solid. Chromatographic purification on
silica (CH2Cl2/CH3OH 98 : 2 v/v as eluent) gave 277 mg of L4
(1.23 mmol, 22.6% yield).
⎟
FX = FST
⎜
⎟
2
⎜
⎜
⎝
⎜
⎜
ST
⎟
Grad
h
ST ⎠⎝
where the subscripts ST and X denote standard and test respec-
tively, U is the fluorescence quantum yield, Grad the gradient from
the plot of integrated fluorescence intensity vs. absorbance, and h
is the refractive index of the solvent. Luminescence lifetimes were
determined by time-correlated single-photon counting. Excitation
with nanosecond pulses of 297 nm light (repetition rate of 100 kHz)
generated by a NanoLED pulsed diode was used. The emission
1H NMR ([D6]acetone, 400 MHz) d: 8.58 (d, J = 4.69 Hz, 2H),
7.82 (t, J = 7.68, 2H), 7.76 (d, J = 8.05 Hz, 2H), 7.34 (t, J = 6.22 Hz,
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2H), 6.70 (s, 1H), 3.60 (s, 2H). 13C NMR ([D6]acetone, 100 MHz)
d: 161.93, 148.88, 138.28, 123.94, 121.69, 118.30, 77.29, 31.32. MS
(ESI+): m/z 226.1 [M + H]+.
v/v as eluent) and the yellow solid recovered (yield: 43%, 44 mg,
0.060 mmol).
1H NMR ([D6]acetone, 400 MHz) d: 8.86 (d, J = 4.39 Hz, 1H),
8.59 (d, J = 5.86, 1H), 8.27 (t, J = 9.52 Hz, 2H), 8.15 (t, J = 7.91 Hz,
1H), 8.08–8.00 (m, 3H), 7.96 (d, J = 4.98, 1H), 7.90–7.85 (m, 4H),
7.65 (d, J = 9.08 Hz, 2H), 7.52 (t, J = 6.00 Hz, 1H), 7.44 (t, J =
6.88, 1H), 7.33 (t, J = 6.44 Hz, 1H), 7.04–6.97 (m, 2H), 6.92 (t,
J = 7.18 Hz, 1H), 6.86 (t, J = 7.47, 1H), 6.38 (d, J = 7.61 Hz,
1H), 6.31 (d, J = 7.03 Hz, 1H). 13C NMR ([D6]acetone, 100 MHz)
d: 168.76, 168.35, 157.43, 151.77, 151.14, 150.69, 150.35, 149.67,
148.49, 144.85, 139.85, 139.71, 139.31, 132.83, 132.30, 131.24,
131.14, 127.44, 126.97, 126.78, 126.42, 125.80, 125.76, 124.97,
124.87, 123.88, 123.61, 120.89, 120.83. MS (ESI+): m/z 699.2 [M]+.
[Ir(ppy)2(dipyridin-2-ylmethanol)]Cl ([IrL2]Cl). Two different
reaction approaches have been carried out. In the first method,
a suspension of [Ir(ppy)2(m-Cl)]2 (150 mg, 0.140 mmol) and L2
(65 mg, 0.350 mmol) in CH2Cl2/MeOH (20 ml, 1 : 1 v/v) was
heated to reflux while stirring under N2. After 2 h, the resulting
yellow solution was cooled to r.t. The solution was filtered and the
solvent removed by rotary evaporation. The resulting yellow solid
was purified by thin layer chromatography (TLC) (CH2Cl2/MeOH
96 : 4 v/v). IrL2 was the slowest yellow band out of nine. The
complex was extractedfrom silicawithMeOH, the solutionfiltered
and the solvent removed by rotary evaporation. The yellow solid
was collected (yield: 10%, 20 mg, 0.028 mmol).
[Ir(ppy)2(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile)]Cl ([Ir-
lL4]Cl). The precursor IrL1 (100 mg, 0.139 mmol) and KOH
(25 mg, 0.44 mmol) were refluxed in acetonitrile (10 ml) for
15 min, under nitrogen atmosphere. The resulting red solution was
evaporated to dryness under vacuum, the solid dissolved in CH2Cl2
(5 ml) and washed with H2O (2 ml for 3 times). Evaporation of the
organic layer gave a red solid, which resulted to be a mixture
of IrL4 and IrL1. The yellow solid IrL4 (yield: 40%, 41 mg,
0.056 mmol) was obtained after TLC separation (CH2Cl2/MeOH
96 : 4 v/v).
In the second reaction approach, IrL2 was prepared by reacting
IrL1 (100 mg, 0.139 mmol), dissolved in 15 ml of MeOH at
◦
0 C, with a large excess of NaBH4 (25 mg, 5 eq.). The solution
was stirred at 0 ◦C for 1 h, evaporated under vacuum, and the
obtained pale yellow solid dissolved in CH2Cl2 (5 ml) and H2O
(5 ml). The resulting solution was neutralized with HCl 0.1 M
and stirred for 10 min. The organic layer was washed four times
with 1 ml of H2O, filtered, dried over Na2SO4 and the solvent was
removed under vacuum to give a yellow solid (yield: 97%, 97 mg,
0.135 mmol).
1H NMR ([D6]acetone, 400 MHz) d: 10.00 (d, J = 5.86 Hz,
1H), 8.23 (d, J = 4.69, 1H), 8.16 (d, J = 8.20 Hz, 1H), 8.06 (t, J =
7.91 Hz, 1H), 8.00 (d, J = 7.91 Hz, 1H), 7.92 (t, J = 7.91, 1H),
7.85 (t, J = 7.91 Hz, 1H), 7.76 (d, J = 7.61 Hz, 1H), 7.72 (d, J =
7.61 Hz, 1H), 7.64 (d, J = 4.98, 1H), 7.50 (d, J = 7.91 Hz, 1H),
7.35 (d, J = 8.20 Hz, 1H), 7.28 (t, J = 6.44 Hz, 1H), 7.21 (t, J =
6.74, 1H), 7.03 (t, J = 7.61 Hz, 1H), 6.92 (d, J = 5.56 Hz, 1H),
6.85 (t, J = 6.30 Hz, 1H), 6.81 (t, J = 7.47 Hz, 3H), 6.68 (t, J =
7.61, 1H), 6.62 (t, J = 7.47 Hz, 1H), 6.31 (t, J = 6.59 Hz, 1H)
7.26 (d, J = 7.61 Hz, 1H), 6.12 (d, J = 7.61 Hz, 1H). 13C NMR
([D6]acetone, 400 MHz) d: 172.09, 150.01, 149.03, 148.79, 147.69,
136.77, 136.36, 136.26, 133.16, 132.62, 130.18, 129.60, 128.03,
125.19, 125.12, 124.59, 122.01, 121.69, 121.51, 121.06, 121.01,
119.49, 118.95, 90.13, 80.30, 54.89, 38.11. MS (ESI+): m/z 726.2
[M]+.
1H NMR (CDCl3, 400 MHz) d: 8.91 (d, J = 5.86 Hz, 1H), 8.39 (d,
J = 9.09 Hz, 1H), 8.33 (d, J = 5.27 Hz, 1H), 8.02 (d, J = 7.91 Hz,
1H), 7.79–7.70 (m, 6H), 7.54 (t, J = 6.15 Hz, 1H), 7.49 (d, J =
8.20 Hz, 1H), 7.41 (d, J = 5.56 Hz, 1H), 7.07 (t, J = 6.44 Hz, 1H),
6.96 (t, J = 7.61 Hz, 1H), 6.90 (d, J = 6.15 Hz, 2H), 6.86–6.78 (m,
4H), 6.52 (s, 1H), 8.58 (d, J = 4.69 Hz, 2H), 6.19 (d, J = 7.50 Hz,
1H), 6.11 (d, J = 7.50 Hz, 1H). 13C NMR (CDCl3, 400 MHz)
d: 168.35, 167.65, 163.20, 162.15, 153.60, 151.60, 151.55, 149.90,
148.80, 148.30, 148.05, 144.90, 143.35, 139.15, 138.90, 138.55,
138.00, 132.15, 131.55, 130.75, 130.40, 124.85, 124.60, 124.45,
124.15, 123.40, 123.05, 122.65, 122.40, 120.20, 119.40, 72.50. MS
(ESI+): m/z 687.2 [M]+, 501.1 [M–L2]+.
Ir(ppy)2(2,2¢-(hydrazonomethylene)dipyridine)]Cl
This complex has been synthesized using two different
reaction approaches. In the first reaction method,
([IrL3]Cl).
Acknowledgements
a
suspension of [Ir(ppy)2(m-Cl)]2 (150 mg, 0.140 mmol) and
di-2-pyridylketoneazine (51 mg, 0.140 mmol) in CH2Cl2 and
MeOH (20 ml, 1 : 1 v/v) was heated to reflux while stirring under
N2. After 2 h, the resulting yellow solution was cooled to r.t.
When a precipitate was obtained, the solution was filtered and
the solid eliminated. The solvent was removed under vacuum,
the resulting yellow solid washed 3 times with 1 ml of Et2O
and purified by TLC (CH2Cl2/MeOH 96 : 4 v/v as eluent). The
complex IrL3, the third red band out of five, was collected (yield:
5%, 10 mg, 0.014 mmol). In the second method, the precursor
IrL1 (100 mg, 0.139 mmol) was dissolved in methanol (15 ml) and
glacial acetic acid (0.5 ml) with hydrazine monohydrate (2 ml).
The mixture was heated to reflux, stirring under N2. After 1 h the
resulting red solution was cooled to r.t. and the solvent removed
under vacuum. The resulting crude product was dissolved in 3 ml
of CH2Cl2 and washed 3 times with 1 ml of H2O. The orange
organic solution was filtered on silica (CH2Cl2/CH3OH 94 : 6
C.G. thanks Regione Piemonte for financial support.
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