A small cationic donor-acceptor iridium complex with a long-lived charge-separated state

Article abstract of DOI:10.1039/b820744e

A cationic cyclometalated iridium complex with a triarylamine donor attached to the primary ligand sphere showed long-lived (0.04 and 1.7 μs) charge-separated states after photoexcitation which are due to a combined Marcus inverted region effect and spin-

Full text of DOI:10.1039/b820744e

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A small cationic donor–acceptor iridium complex with a long-lived
charge-separated statew
Barbara Geiß and Christoph Lambert*
Received (in Cambridge, UK) 20th November 2008, Accepted 23rd January 2009
First published as an Advance Article on the web 11th February 2009
DOI: 10.1039/b820744e
A cationic cyclometalated iridium complex with a triarylamine
donor attached to the primary ligand sphere showed long-lived
(0.04 and 1.7 ls) charge-separated states after photoexcitation
which are due to a combined Marcus inverted region effect and
spin-selection rules.
flexibility. The Ir complex or, more precisely, the 3,4,7,8-
tetramethyl-1,10-phenanthroline (tmp) ligand serves as the
acceptor (A).
Redox cascades that display long-lived charge-separated (CS)
states upon photoexcitation of a chromophore are model
compounds for the initial steps in photosynthesis and may
further be used in artificial photovoltaic devices.1–4 Major
traps that shorten the lifetime of the otherwise long-lived CS
states are low-lying triplet states, obstacles that are inherent to
all closed-shell organic chromophores as the triplet states are
always lower in energy than the associated singlet states.5 This
circumstance usually limits the energy stored in the
charge-separated state which therefore has to be lower than
the lowest triplet state of the light absorbing chromophore.
Consequently, this leads to energy loss because the lowest
photon energy that can be absorbed by an organic
chromophore is that of the lowest lying singlet state.
Furthermore, in terms of energy conservation, it is generally
advantageous if the energy of the lowest excitable state is as
close as possible to the charge-separated state. In this
communication we seek to circumvent the above mentioned
problem by using a simple cyclometalated iridium complex as
the chromophore which, owing to strong spin–orbit coupling,
shows moderately strong singlet–triplet absorption and all
other states may involve strong triplet components.6 Thus,
the lowest energy state directly accessible by photoexcitation is
a triplet state whose energy can directly be used to induce
charge separation. A further goal of this study is to keep the
redox cascades as small as possible while still maintaining a
long-lived CS state, an option that requires special design and
is generally hard to achieve.7,8
The TAA-substituted cyclometalating ligand ppzTAA (C^N)
for complex 1 was synthesised from 3,7-dibromo-10,11-
dihydro-5H-dibenzo[a,d]cycloheptene: pyrazole was connected
to this dibromo-compound via copper-catalysed coupling and
the dianisylamine was attached by a palladium-catalysed
Buchwald–Hartwig amination.
The target complexes were synthesised via the dichloro-
bridged Ir(III) dimers (C^N)2Ir(mꢀCl)2Ir(C^N)2 which were
synthesised following the classical method of Nonoyama by
reacting the cyclometalating ligands with IrCl3 in ethoxyethanol–
water 3 : 1 at 110 1C for 12 h.14 The mixed-ligand complexes
were then prepared in dry dichlormethane by reaction of the
corresponding dichloro-bridged iridium(III) dimer with the
tmp (N^N) ligand, followed by metathesis with NH4PF6.12,15
The electrochemical properties of the bis-cyclometalated
complex, its reference and the free ligand were examined by
cyclic voltammetry, see Table 1.
The complexes show one reduction at about ꢀ2000 mV that
is assigned to the reduction of the tmp ligand, an interpretation
supported by electrochemical studies of related Ir(III)
compounds. Furthermore, the complexes exhibit one
oxidation wave at about 800–900 mV that is associated with
While
cationic
mixed-ligand
Ir
complexes
[(C^N)2Ir(N^N)]+ similar to Ref were recently used in organic
light-emitting diodes (OLEDs),9–12 we used this type of Ir
complex chromophore directly linked to two equivalent donor
components in order to construct a relatively small and simple
(donor)2–acceptor (D2–A) molecule which may display a
long-lived CS state. Triarylamine (TAA) was chosen as the
donor (D) because of its high-lying triplet state (3.05 eV).13 It
was connected to the phenylpyrazole via a methylene- and
an ethylene-bridge in order to minimise conformational
Table 1 Redox properties of the ligand, Ref and 1 vs. Fc/Fc+ in MeCN
(0.2 M tetrabutylammonium hexafluorophosphate at n = 100 mV sꢀ1
)
Ox
Ox
Red
E /
1/2
mV
E
1/2
mV
(Ir)/
E
1/2
mV
(TAA)/
DE1/2 =
EOx ꢀ ERed/mV
—
ppzTAA
Ref
1
—
890rev
800
180rev
—
180rev
—
ꢀ2010rev
ꢀ2050rev
a
2900
2230
Institut fur Organische Chemie, Am Hubland, Wurzburg, 97074,
¨
¨
Germany. E-mail: lambert@chemie.uni-wuerzburg.de
w Electronic supplementary information (ESI) available: Synthesis
and emission spectra. See DOI: 10.1039/b820744e
a
rev = reversible process.
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Fig. 2 Transient absorption spectra of Ref (excitation at 28 200 cmꢀ1
)
Fig. 1 Absorption spectra of 1, Ref and ppzTAA in MeCN solution at
and 1 (excitation at 24 000 cmꢀ1) in 2-MeTHF at RT. Early spectra are
shown in blue–green and late spectra are shown in orange–red colours
(0–190 ns). Inset: time scan at 13 700 cmꢀ1 for 1.
298 K. The spectra in 2-MeTHF are virtually identical.
the bis-cyclometalated phenyl–Ir moiety.12,16–18 Compound 1
shows one additional oxidation wave at 180 mV that we assign
to the oxidation of the triarylamine by comparison with the
free ligand.
triarylamine to the tmp ligand yielding the CS state
D+(D)–Aꢀ. This process has a quantum yield of about unity
as was estimated by comparison of the luminescence quantum
yield with that of Ref.23 To prove this assumption transient
absorption spectra were measured at RT in degassed
2-MeTHF (Fig. 2). Ref shows three absorption bands each
with a lifetime of 3.5 ms, respectively. These bands can be
assigned to the 3[Ir(C^N)2(tmp)]+ states and are in good
agreement with the luminescence lifetime at RT.
The absorption spectra of the compounds in acetonitrile are
given in Fig. 1. The complexes show the characteristic intense
bands for the spin-allowed 1p–p* ligand-centred (LC)
transition in the C^N and N^N ligands between 35 000 and
40 000 cmꢀ1. The absorption bands between 17 000 and
26 000 cmꢀ1 can be assigned to the spin-forbidden 3MLCT
absorption transitions caused by spin–orbit coupling
associated with the iridium centre.12,18 The moderately intense
For 1 the redox potential difference DE = EOx ꢀ ERed
=
bands between 26 000 cmꢀ1 and 35 000 cmꢀ1 are
a
2.23 V (see Table 1), which is an estimate for the energy of the
CS state, is smaller than the lowest excited state energy of
3[Ir(C^N)2(tmp)]+ as determined from the 00-transition of the
luminescence spectrum at 77 K which yields 2.60 eV. Thus,
charge separation is exothermic by ca. 0.4 eV after
photoexcitation of the lowest excited triplet state. The
transient absorption spectra of the CS state are expected to
display characteristic features of the spectra of the isolated
radical anion and the radical cation of 1. Thus, for
comparison, the spectra of the isolated radical ion species
were generated by oxidation and reduction of 1 in acetonitrile
by spectroelectrochemistry (Fig. 3).
superposition of the characteristic localised transitions of the
triarylamine subunit5,19 and the typical spin-allowed metal-
to-ligand charge-transfer (1MLCT) dp(Ir) - p* absorption
shoulders.12,18
While Ref shows a strong and broad luminescence centred
at 19 200 cmꢀ1 in 2-MeTHF at RT the triarylamine-
substituted complex 1 shows no emission at all. Obviously
there is a very fast quenching process which inhibits lumines-
cence from the Ir core. However, both complexes show intense
luminescence with vibrational fine structure at 77 K with a
lifetime in the ms regime which indicates the phosphorescent
nature of the luminescence (see Table 2 and Fig. S3 in ESIw).
Interestingly, unlike at RT, at 77 K complex 1 shows the same
luminescence spectrum as Ref which proves that the quenching
process at RT is a thermal reaction which is slowed down at
77 K. The structured blue-shifted emission spectra of 1 and
3
Ref suggests a p–p* LC transition.9,17,18,20–22
We assume that the missing luminescence from the Ir core
in 1 at RT is due to charge-transfer quenching from the
Table 2 Emission data
Compound
T/K
n~em/cmꢀ1
t/ms
Fem
Ref
2-MeTHF (298)a
2-MeTHF (77)a
2-MeTHF (298)a
2-MeTHF (77)a
CH2Cl2 (298)b
19 200
19 600/21 100
—
19 600/21 000
25 600
3.6
18.3
—
10.0
—
0.31
—
—
—
1
Fig. 3 Absorption spectra of radical cation 1+ and radical anion 1ꢀ.
The cyan line shows the superimposed spectra of cation and anion.
The pink line shows the transient absorption spectrum of 1 in
2-MeTHF with excitation at 24 000 cmꢀ1 (416 nm).
ppzTAA
0.06
a
b
Excitation at 28 200 cmꢀ1 [355 nm]. Excitation at 40 000 cmꢀ1
[300 nm].
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In fact, pump probe spectroscopy at 24 000 cmꢀ1 excitation
yields transient signals as expected for the CS state, that is an
intense band at 13 500 cmꢀ1 which is typical of triarylamine
cations,19 and two less intense bands at 16 400 cmꢀ1 and
22 600 cmꢀ1 that can be assigned to the tmp radical anion.24
Both signals decay biexponentially with the same time
constants of 41 ns (95%) and 1.7 ms (5%).z Both components
display the same spectral features and relative intensities.
Thus, the two species from which these two processes originate
must be very similar. Measurements in air equilibrated
solutions reduce the time constants to 21 ns (95%) and
160 ns (5%). Obviously, triplet species play a major role in
the deactivation pathway. However, we can exclude localised
triplet species by comparison with the transient signals of
Ref (Fig. 2) and with that of triphenylamine which shows a
emitter based OLED devices if triarylamines are used as hole
transporting components despite their high local triplet
energies.
We thank the Deutsche Forschungsgemeinschaft for
financial support.
Notes and references
z A somewhat better fit may be obtained by three time constants 26 ns
(73%), 87 ns (22%) and 1.7 ms (5%).
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1
recombination from either 3CS or CS state to S0, both the
spin-allowed 1CS - S0 and the spin-forbidden 3CS - S0
processes are likely to be in the Marcus inverted region.29,30
This is in agreement with the observation that back electron
transfer is much faster in the polar MeCN (t1 = 7 ns (94%);
t2 = 218 ns (6%)) in which both the reorganisation energy is
larger and the free energy of back electron transfer is smaller.
Both terms should decrease the inverted region effect as
observed. In addition, back electron transfer from 3CS to
the singlet ground state is spin-forbidden, which is the reason
for the large time constant. Both lifetimes of these CS states
even in MeCN are unusually long for such a small dyad and
much longer than that of the recently published D–A dyad
(70 ps) in which the iridium is coordinated by terpy ligands
and where the donor also is a triarylamine. In this case, the
energy of the CS state is only 1.50 eV which might be the
reason for the shorter lifetime.31
In conclusion, our study demonstrates that relatively small
D–A Ir complexes can show long-lived CS states due to a
combination of inverted region effects and spin-selection rules.
Quenching due to low-lying states is avoided because optical
excitation already populates the lowest possible triplet states.
Equally important, our study also demonstrates a severe
quenching effect that might reduce the efficiency in triplet
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This journal is The Royal Society of Chemistry 2009
1672 | Chem. Commun., 2009, 1670–1672