Two-photon spectroscopy of cyclometalated iridium complexes

Article abstract of DOI:10.1039/c1dt11164g

The near-infrared two-photon absorption (TPA) spectra of a series of cyclometalated iridium complexes have been measured. These complexes exhibit moderately large TPA cross-sections of approximately 20 GM at the biological relevant wavelength of 800 nm. A new complex has been designed and synthesised, and found to have an increased cross-section of 44 GM at 800 nm. Full photophysical characterisation of this complex is presented.

Full text of DOI:10.1039/c1dt11164g

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PAPER
Two-photon spectroscopy of cyclometalated iridium complexes†
Robert M. Edkins, Sylvia L. Bettington, Andre´s E. Goeta‡ and Andrew Beeby*
Received 20th June 2011, Accepted 1st September 2011
DOI: 10.1039/c1dt11164g
The near-infrared two-photon absorption (TPA) spectra of a series of cyclometalated iridium
complexes have been measured. These complexes exhibit moderately large TPA cross-sections of
approximately 20 GM at the biological relevant wavelength of 800 nm. A new complex has been
designed and synthesised, and found to have an increased cross-section of 44 GM at 800 nm. Full
photophysical characterisation of this complex is presented.
ns, 1064 nm pulses, although no value of s2 was reported. More
Introduction
recently there has been a report of TPA observed in iridium(III)
terpyridine systems.14 In these systems TPA cross-sections of up to
67 GM were reported for a styryl extended system which undergoes
an intraligand charge transfer (ILCT) transition in the case of a
methoxy substituted example or ligand-centred (LC) transition
for the parent compound. There have also been two reports15
of two-photon induced luminescence of cyclometalated Ir(III)
species, but importantly these did not investigate the potential
for excitation in the NIR region, instead exciting the sample at
438 nm. In these earlier studies the TPA induced luminescence can
be attributed to ligand based emission rather than lower energy
MLCT transitions, and was short-lived, with a lifetime of 3–4 ns as
part of a complicated multi-exponential decay when doped at low
concentration in a poly(methylmethacrylate) (PMMA) film. This
is a crucial distinction since MLCT transitions may be expected
to enhance s2 as charge transfer is usually regarded as a requisite
for strongly two-photon absorbing chromophores.16
Over the past decade cyclometalated iridium complexes have
been extensively studied for their tunable phosphorescence, high
photoluminescence quantum yields (PLQY)17 for use in organic
light emitting devices (OLEDs).18 To date, there have been no
reports of NIR-TPA of neutral iridium complexes. These materials
would possess several favourable properties for time-resolved two-
photon imaging: namely their high quantum yields and relatively
long-lived excited states (typically 1–5 ms)17 compared to the
autofluorescence encountered in biological systems. Additionally,
they have a large quantum yield for the generation of cytotoxic
singlet oxygen,19 which if generated by TPA, could be used for high
resolution photodynamic therapy with good tissue penetration.
In this paper we report the TPA spectra measured for a
series of neutral cyclometalated Ir(III) compounds. The se-
lected compounds are summarised in Fig. 1. These were
chosen to include examples of various structural types and
include the tris-cyclometalated homoleptic complex Ir(ppy)3
(ppyH = 2-phenylpyridine) and the related Ir(ppm)3 (ppmH =
2-phenylpyrimidine), allowing the influence of the heterocycle
to be probed. A heteroleptic complex containing two different
The design of materials for the application of two-photon absorp-
tion (TPA) to biological imaging,1 photodynamic therapy2 and
optical power limiting at telecommunication wavelengths3 contin-
ues to be a topic of active research. These applications demand
materials with large two-photon absorption cross-sections (s2),
especially in the near-infrared (NIR) region where biological tissue
is most transparent and least susceptable to photodamage.4
A recent review by He et al.5 summarised the TPA properties of
a large number of chromophores, but was predominantly focused
on organic compounds. This is not surprising because many
organic species have been highly engineered to induce large two-
photon cross-sections. However, lanthanide and transition metal
complexes often exhibit other favourable properties, in addition
to the potential for large cross-sections, that make them attractive
TPA materials, for example, long phosphorescence lifetimes (tP),
large Stokes’ shifts and high quantum yields.
Previous studies of metal complexes exhibiting enhanced two-
photon absorption include those of Pt(II)6 and Ru(II)7 acetylides,
Ru(II) with modified 2,2¢-bipyridyl (bipy) ligands,8 lanthanide
antennae systems in particular those of Eu(III)9,10 and Tb(III),11
as well as Zn(II) porphyrins.12 In another example, a bipyridine-
based ligand with a large TPA cross section was complexed to
iridium to promote intersystem crossing (ISC) to the triplet excited
state and resulted in a system which exhibits a mixed metal to
ligand charge-transfer (MLCT) and ligand centred (LC) p–p*
transition.13 In this system a dual mechanism reverse saturable
absorption and TPA process was observed when irradiated with 5
Address, Department of Chemistry, Durham University, South Road,
Durham, UK, DH1 3LE. E-mail: Andrew.beeby@durham.ac.uk; Tel: +44
(0) 191 33 42023
† Electronic supplementary information (ESI) available: Full experimental
and CIF file of Ir(4-pe-2-ppy)2(acac). CCDC reference numbers 831044.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1dt11164g
‡ It is with great sadness that we report the death of Dr Andre´s Goeta on
29th July 2011.
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this, the potential ligand 2-phenyl-4-(phenylethynyl) pyridine, 4-
pe-2-ppyH, was identified as a synthetic target. The complex
Ir(4-pe-2-ppy)2(acac) would be expected to have trans substituted
pyridines giving the simplest A-p-D-p-A linear structure for these
compounds.
The novel ligand 4-pe-2-ppyH was synthesised in a two step
procedure as outlined in Scheme 1. Commercially available 4-
bromopyridine hydrochloride was cross-coupled with pheny-
lacetylene using standard Sonogashira methodology. The resul-
tant pyridine was reacted with phenyllithium under anaerobic
conditions to afford the ligand in moderate yield (26%). Com-
plexation of the ligand was achieved using standard protocols;
first formation of a di-m-chloro bridged dimer by reaction with
IrCl3·3H2O in aqueous 2-ethoxyethanol26 and second, reacting the
dimer material with acetylacetone in basic acetone/ethanol mixed
solvent followed by purification by column chromatography.27
Fig. 1 Iridium cyclometalated compounds studied by two-photon ab-
sorption spectroscopy, including the newly designed Ir(4-pe-2-ppy)2(acac).
cyclometalated ligands Ir(ppy)2(fppy) (where fppyH is 4-(2-
pyridyl)-benzaldehye) was selected along with Ir(ppy)2(acac),
which includes the widely used ancillary ligand acacH (acety-
lacetone). The s2 values were measured using the two-photon
luminescence method using the focussed output of a mode-locked
Ti:sapphire laser (t = 150 fs fwhm). A compound with an increased
cross-section was then designed based on the principles governing
the magnitude of s2 in organic systems, that is an extended p-
system with the capacity to act as a multipolar system. The
synthesis, structure, photophysics and a computational study
of this new complex, Ir(4-pe-2-ppy)2(acac) (4-pe-2-ppyH is 4-
phenylethynyl-2-phenylpyridine), are first presented, followed by
the results of the TPA experiments.
Scheme 1 Synthetic route to Ir(4-pe-2-ppy)2(acac).
When the styryl analogues were synthesised, the authors re-
ported that the double bonds were reduced under the experimental
conditions if the reaction was carried out at high temperatures,
catalysed by the intermediate dimer.28 Under the conditions
employed here for the synthesis of Ir(4-pe-2-ppy)2(acac) we found
no evidence for reduction of the acetylene bond. This was
confirmed by the presence of characteristic bands associated with
the acetylene in the Raman spectra at 2217 and 2234 cm-1 and the
absence of styryl or alkyl resonances in the 1H NMR spectrum.
Results and Discussion
Synthesis
Organic materials have been shown to have large TPA cross-
sections when they have dipolar,20 quadrupolar21 or octapolar22
ground state charge distributions and undergo a large charge
redistribution in the excited state. Quadrupolar A-p-D-p-A (A, p
and D are acceptor, pi-conjugated bridge and donor, respectively)
for example have been especially well studied. In the MLCT
transition of iridium ppy systems, the iridium can be considered
a donor to the accepting pyridyl unit, whose p* orbitals have
been shown by density functional theory (DFT) calculations to
be the major contributor to the LUMO.23 It therefore seemed
reasonable to suggest that if the conjugation of the pyridyl frag-
ment was increased, greater charge separation of the MLCT state
could occur, resulting in larger TPA cross-sections. p-Conjugated
substituents at the 4 position of the pyridine trans to the
iridium in the complex, were considered for this purpose. Iridium
complexes of simple derivatives of 2-phenyl-4-styrylpyridine have
been reported,24 and, although it has been shown that olefinic
p-bridges give larger TPA cross sections than their acetylinic
equivalents,25 it was the acetylenes that were chosen for this study.
This is because incorporation of olefins introduces the possibility
of cis-trans photo-induced isomerism, which could complicate
the interpretation of the photophysical measurements. Based on
X-ray crystallography
Crystallisation of Ir(4-pe-2-ppy)2(acac) by slow diffusion between
CDCl3 and cyclohexane resulted in the formation of orange
needles, allowing the molecular structure to be fully elucidated
by single-crystal X-ray diffraction (Fig. 2).
Fig.
2 The molecular structure of Ir(4-pe-2-ppy)2(acac). Element
(colour): iridium (green), carbon (grey), nitrogen (blue) and oxygen (red).
Hydrogen atoms are omitted for clarity. Thermal ellipsoids drawn at 50%
probability. Crystal data in footnote.29 See ESI for full CIF.†
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The two symmetry related chelating 4-pe-2-ppy ligands have
trans Ir–N bonds as a consequence of the geometry of the
intermediate dichloride-bridged dimer that does not rearrange
under the conditions of the reaction with acetylacetone. This
results in an extended structure centred on the iridium atom with
an end to end length of 2.50 nm. The complex deviates from
linearity with a C17–Ir–C17A angle of 169.22(5)◦ due to the
constraint of the C1–Ir–N1 and O1–Ir–O1 angles to 80.90(10)◦
and 87.39(11)◦ respectively, resulting in a distorted octahedral
environment about the iridium centre.
Computational study
The ground state optimised structure of Ir(4-pe-2-ppy)2(acac)
was calculated using DFT, employing the exchange–correlation
functional B3LYP and a split basis set 6-31+G/LANL2DZ (see
experimental for full details). The calculated Ir–O, Ir–C and Ir–N
bond lengths agree with those determined experimentally from the
X-ray structure to within 2%. A time-dependent DFT (TD-DFT)
calculation was also carried out with the CAM-B3LYP functional,
chosen because of its better description of charge transfer states
compared to B3LYP,31 and the same 6-31+G/LANL2DZ basis
set. This calculation showed that the T1 ← S0 excitation is
best described by a multi-component set of orbital transitions.
The greatest contribution is LUMO+1 ← HOMO-2, which is a
transition onto the phenylethynylpyridine moiety from an orbital
based predominantly on the Ir atom with a small proportion
on the 4-pe-2-ppy ligand (Table 1 for orbital plots). The second
most important contribution comes from a transition from the
HOMO-1 based on the Ir atom and acac ancillary ligand, to the
LUMO distributed across the 4-(phenylethynyl)pyridine moiety.
One-photon photophysics
The UV-visible absorption spectrum (Fig. 3) of Ir(4-pe-2-
ppy)2(acac) is typical of this type of material in both shape and
position of features. In analogy with previous assignments,30 these
include a p–p* 1LC band below 320 nm, 1MLCT bands between
3
370 and 420 nm and spin-forbidden MLCT transitions above
450 nm. These features are also reproduced in the excitation
spectrum.
Table 1 DFT 6-31+G/LANL2DZ calculated orbitals of Ir(4-pe-2-
ppy)2(acac). Hydrogen atoms have been removed for clarity
Fig. 3 Absorption, excitation and emission spectra of Ir(4-pe-2-ppy)2-
(acac). lex = 400 nm, lem = 610 nm. All spectra are normalised and the
absorption spectrum offset in intensity for clarity. All measurements were
made in degassed solvent.
The presence of a MLCT contribution to the emissive state of
Ir(4-pe-2-ppy)2(acac) is supported by a positive solvatochromic
emission shift on changing from toluene (lem = 570 nm) to
acetonitrile (lem = 603 nm), a shift totalling some 960 cm-1. Further,
the broad and essentially structureless emission profile is indicative
of an MLCT component. Compared to the parent complex
Ir(ppy)2(acac), the emission maximum is bathochromically shifted
by approximately 1800 cm-1.
The luminescence lifetime of Ir(4-pe-2-ppy)2(acac) in degassed
and aerated toluene solution was measured as being 0.93 and
0.068 ms, respectively. The relatively short phosphorescence life-
time is also typical of a MLCT state and of a red emitter in
which non-radiative decay is enhanced due to the energy gap
law.
The value of UP was measured in degassed toluene and found
to be 0.28. For comparison, the parent compound Ir(ppy)2(acac)
in the same solvent has a longer lifetime and higher quantum yield
of 1.6 ms and 0.46, respectively.
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The lowest energy excitation is therefore best described as an
admixture of MLCT, ligand to ligand charge transfer (LLCT) and
ligand-centred (LC) transitions, with MLCT being the principle
constituent. This is in agreement with the observed photophysics
which indicate some degree of charge transfer. Importantly, it
shows a large spatial extent of the charge transfer as desired for a
potential TPA material. The energy of this transition is calculated
to be 2.41 eV (515 nm), which is within the band assigned 3MLCT
in the UV-visible spectrum.
Two-photon absorption spectroscopy
The TPA spectrum of the cyclometalated iridium compounds
outlined in Fig. 1 were measured to understand the effect of
structural change on the magnitude of s2. The TPA spectrum
of the compound Ir(4-pe-2-ppy)2(acac), which was designed to
imitate the A-p-D-p-A organic chromophores, vide supra, was
recorded to confirm whether similar design principles for the
increase of s2 for organic two-photon absorbers were applicable
to these organometallic systems. Fig. 4 depicts the TPA spectra of
the 4 known complexes of various structural types (IrL3, IrL2L¢
and IrL2(acac)) and the novel compound Ir(4-pe-2-ppy)2(acac),
overlain with the OPA spectra. A comparison of the one-photon
Fig. 5 Emission spectrum of Ir(4-pe-2-ppy)2(acac) under one-photon
(400 nm) and two-photon (800 nm) excitation.
and two-photon emission profiles, Fig. 5, indicates that emission
is from the same state independent of the excitation method.
The use of a 670 nm short-pass filter in the detection optics to
eliminate stray laser light reaching the detector results in the
spectrum being cut off at this point. A correction was applied
to the integrated emission intensity to account for this. A two-
photon process could be confirmed by a quadratic dependence of
the integrated emission, F(l), on the incident laser intensity, P. By
plotting ln(F(l)) against ln(P) a linear fit was found, the gradient
of which gave the argument of the power dependence, see Table 2
and representative fit in Fig. 6. In all cases a gradient of 2, within
experimental error, confirmed that TPA was being observed. These
data were recorded at 800 nm, the optimum operating wavelength
of the system, which is also the wavelength chosen for comparison
of cross-sections, s2, and TPA action cross-sections, the product
of UP and s2.
Fig. 6 A representative example of integrated luminescence intensity as
a function of laser power for Ir(4-pe-2-ppy)2(acac) at 800 nm. Gradient is
2.1 0.2 (R = 0.96).
The fit of the OPA to the TPA excitation spectra is crude,
which in part will be due to the inherent uncertainty in measuring
TPA spectra. The region of spectrum measured covers several
transitions of LC and MLCT character and these have will
different cross sections for OPA and TPA, due to the different
selection rules of the two processes, and hence different intensities
in the two spectra. A similar explanation has been employed
previously in the case of Eu complexes which show differences
Fig. 4 TPA spectra (black, bottom abscissa, left ordinate) and OPA
(UV-vis) spectra (red, top abscissa, right ordinate) for a series of iridium
cyclometalated complexes (ppm = 2-phenylpyrimidine). Estimated error
20%.
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Table 2 Two-photon absorption spectroscopic data and phosphorescence quantum yields (UP) in degassed toluene
Compound
Gradienta
Ir(ppy)3
Ir(ppm)3
Ir(ppy)2(acac)
Ir(ppy)2(fppy)
Ir(4-pe-2-ppy)2(acac)
1.9 0.1 (R = 0.99)
2.2 0.1 (R = 0.96)
0.47
9.5
2.0 0.1 (R = 0.96)
2.0 0.1 (R = 0.95)
2.1 0.2 (R = 0.96)
0.28
44
b
UP
0.64c
20
0.46
17
8
0.29
18
6
s2/GMd
U2.s2/GMe
13
4.5
12
a Gradient of plot of log(P/mW) against log(F(l)) at 800 nm measured over 100 data points, where P is laser power in mW and F(l) is the integrated
emission spectrum. b Estimated error 0.1. c Literature values range from 0.40 (ref. 32) to 0.90 (ref. 33). d TPA cross section at 800 nm. 1 GM = 10-50 cm4
s photon-1. Estimated error 20%. e TPA action cross section at 800 nm.
between their OPA and TPA spectra.9 Comparing the magnitude
of the TPA cross-sections at 800 nm, it can be seen that all the
complexes show relatively low values compared to some of the
best organic chromophores,2,3 however they are still appreciable
when compared to other coordination and organometallic com-
pounds measured with femtosecond excitation pulses. For instance
phenylpyridine and ancillary ligands have been measured. In all
cases TPA was observed and a value of s2 of 20 GM was measured
for the parent complex Ir(ppy)3 at 800 nm which compares
favourably with materials of a similar structural nature. In general
it was found that there was little difference between complexes
of the form IrL2(acac) and IrL3 for the same ligand, L and thus
the synthetically simpler IrL2(acac) sub-class should be developed
further. In light of the structural features leading to large TPA cross
sections in organic molecules and by considering the electronic
structure of iridium complexes, the extended compound Ir(4-pe-
2-ppy)2(acac) was synthesised and found to have a cross-section
more than twice that of Ir(ppy)2(acac) at 800 nm. This implies
the extent of charge separation upon excitation must be increased
in order to enhance the ability of these molecules to undergo
TPA. It has therefore been shown that the design principles
commonly employed for organic chromophores are applicable
to these materials and thus large cross-section materials should
be possible with tunable emission properties for a wide range of
applications.
Ru(bpy)3 has been reported to have a s2 value of only 4 GM,34
2+
while platinum NŸCŸN complexes that could be successfully used
in time-resovled two-photon cell imaging have values that range
between 4 and 20 GM.35,36 The three complexes Ir(ppy)2(fppy),
Ir(ppy)2(acac) and Ir(ppy)3 show little variation in their s2 values.
It appears the exchange of the ligand 2-phenylpyridine for 2-
phenylpyrimidine has a strong negative effect on the TPA, with
Ir(ppm)3 having half the value of s2 of Ir(ppy)3, although the
reason for this is not known. The most noteworthy measurement
is for Ir(4-pe-2-ppy)2(acac) which has a s2 value more than
double that of than Ir(ppy)2(acac) simply with the introduction
of phenylethynyl appendages. This ability to improve the TPA
properties by rational design is important if these species are
to fulfil their potential applications. This also suggests the same
design principles seen for large TPA susceptibilities in organic
chromophores are relevant here and it can be expected that greater
s2 values would be obtained with further increase in the conju-
gation and/or donor/acceptor strength. The magnitude of s2 is
maximal at 820 nm, with a value of 62 GM (estimated error 20%).
It is however the action cross-section that is arguably the impor-
tant parameter when evaluating species for imaging applications.
Any potential TPA imaging agent would ideally have a large cross-
section to excite a high proportion of the molecules that will be
in a low concentration environment in any particular part of
a sample, but would also have a large quantum yield to aide
detection. The action cross-sections of Ir(ppy)3 and Ir(4-pe-2-
ppy)2(acac) are of comparable magnitude (Table 2) and therefore
exemplify why it must be borne in mind that while improving the
two-photon cross-section is highly desirable, it cannot be at the
expense of a diminished quantum yield. It is worth comparing
these compounds therefore to the recently reported terpyridine
iridium complexes which have comparable two-photon cross-
sections but emission quantum yields two orders of magnitude
smaller (U = 0.0026–0.0056). Thus, the cyclometalated complexes
presented here are potentially more useful for certain applications
and are arguably synthetically less challenging.
Acknowledgements
R.M.E. thanks Durham University for funding through a Durham
Doctoral Fellowship. Dr Lars-Olof Pa˚lsson is acknowledged for
help in operating the Ti:sapphire laser.
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˚
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