Luminescence switching of a cyclometalated iridium(III) complex through a redox-active tetrathiafulvalene-based ligand

Article abstract of DOI:10.1002/chem.200901483

A tetrathiafulvalene (TTF) moiety through an acetyl acetonate bridge to a phosphorescent Iridium (Ir) core with a luminescent active MLCT excited state to observe redox controlled electronic coupling between the inorganic metal luminophore and TTF spacer through distinct luminescent behaviours has been reported. The TTF structure was confirmed by mass spectrometry and NMR spectrocopy. The phosphorescence property of complexes can be modulated by ET-acac ligand, which exhibits different photo-induced electron transfer efficiencies of TTF in distinct oxidation states. The target compound may serve as a novel model compound to investigate the mechanism of charge transfer between Ir based luminescent layer and conductive layer, such as derivative, in OLED materials. The singlet-triplet interactions at excited states contribute to the dynamic π-d coupling in TTF-based metal compounds.

Full text of DOI:10.1002/chem.200901483

COMMUNICATION
DOI: 10.1002/chem.200901483
Luminescence Switching of a Cyclometalated IridiumACTHNUTRGNE(NUG III) Complex through a
Redox-active Tetrathiafulvalene-based Ligand
Chun-Hu Xu, Wei Sun, Chao Zhang, Can Zhou, Chen-Jie Fang, and Chun-Hua Yan*[a]
In the last 30 years, tetrathiafulvalene (TTF) and its deriv-
atives have attracted considerable attention because of their
potential application in organic conductors or superconduc-
tors resulting from their redox-controlled charge-transfer
behaviours.[1,2] By reversible modulation of frontier-orbital
occupancy through chemical or electrochemical redox pro-
cesses, the TTF moiety tends to exhibit distinct electron- or
energy-transfer efficiencies within the donor-acceptor en-
sembles.[3–5] For this purpose, TTF and its derivatives have
been recently highlighted as spacers in organic photo-voltaic
devices, such as those bearing C60 photo-sensitive units.[4]
Studies on the spectral properties of these compounds have
also unveiled many principles for the charge-transfer behav-
iours of TTF. Particularly, the electronic coupling between
TTF spacer at excited state and organic fluorophore gener-
ally produces an ON–OFF switching emission behaviour,
which provides a highly sensitive probe to investigate the
study the d–p electron interactions between the metal core
and TTF moiety and afford new transport properties, owing
to the restriction principle of orbital symmetry and spin
multiplicity. Although some results have been reported so
far concerning the interaction between d electrons in transi-
tion metals and p-electrons in TTF moiety, there are few re-
ports on the luminescent features of inorganic luminophore
switched by the charge-transfer process with TTF at distinct
oxidation states.[6] Cyclometalated IrIII complexes that bear
high stability, high photoluminescence efficiency and long-
lived excited states are widely explored in organic light-
emitting diode (OLED) materials.[7] If an electro-active TTF
moiety is attached to a luminescent IrIII core, investigation
of the photophysical properties of such complexes is helpful
to the study the charge-transfer mechanisms in OLED devi-
ces. In this communication, we conjugate a TTF moiety
through an acetyl acetonate bridge to a phosphorescent IrIII
core with a luminescent-active MLCT excited state, expect-
ing to observe redox-controlled electronic couplings be-
tween the inorganic metal luminophore and TTF spacer
through distinct luminescent behaviours.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
minescence properties with high luminescence efficiencies
and long excited-state lifetimes, because their d orbitals con-
tribute a lot to the frontier orbitals and their luminescence
properties mainly originate from the triplet metal-to-ligand
charge transfer (MLCT) excited states. If a TTF moiety is
covalently attached to these metal complexes instead of the
organic fluorophores, they may provide a model system to
Under this strategy, a new TTF-derived ligand 3-{4-[4’,5’-
(ethylenethio)-5-(methylthio)-tetrathiafulvalenyl]-thio}-
acetyl acetonate (ET-acac) comprising an acetyl acetonate
(acac) moiety (Scheme 1) was prepared first. Conjugated
with an electron-rich TTF moiety, the acac subunit may ex-
hibit improved coordination activity to many metal ions.
And the negative charge on acac unit would ensure the co-
ordination stability with the positively charged luminescent
core. Following the synthetic strategy reported in the litera-
ture,[7] an electro-active[8] IrIII complex [Ir
and its derivative [Ir(dbq)2A(ET-acac)] (2) (dbq=dibenzo-
[f,h]quinoxaline[7g]) bearing a TTF moiety was obtained
ACHTUNGRTEN(NUNG dbq)2acac] (1)
[a] C.-H. Xu, Dr. W. Sun, C. Zhang, C. Zhou, Dr. C.-J. Fang,
Prof. Dr. C.-H. Yan
A
CHTUNGTRENNUNG
Beijing National Laboratory for Molecular Sciences
State Key Laboratory of Rare Earth Materials
Chemistry and Applications
PKU-HKU Joint Laboratory in Rare Earth Materials
and Bioinorganic Chemistry
Peking University, Beijing 100871 (China)
Fax : (+86)10-6275-4179
E-mail: yan@pku.edu.cn
AHCTUNGTRENNUNG
(Figure 1), and its structure is confirmed by mass spectrome-
try and NMR spectroscopy (see the Experimental Section).
The cyclic voltammogram (CV) and differential pulse vol-
tammogram (DPV) of ET-acac in dichloromethane show
two reversible oxidation peaks, with an E1/2 separation of
358 mV, while no reduction process is observed within the
solvent window (Figure 2a and Figure S1a in Supporting In-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901483.
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ure 2b and Figure S1b in Sup-
porting Information). The acac
unit coordinates to the IrIII
core, and the electro-withdraw-
ing inductive effect of the IrIII
core is enhanced by the
p
backbonding connected to the
cyclometalated ligand, so both
the two oxidation potentials
within ET-acac are anodically
shifted. Similarly, the oxidation
potential of IrIII/IrIV is also pre-
sumed to be positively shifted
compared to 1,[6] owing to the
electrostatic interactions be-
tween the dicationic TTF
centre and the IrIII core, hence
the corresponding oxidation
potential may exceed the sol-
vent window and thus is not
Scheme 1. The synthetic route of the ancillary ligand ET-acac starting from bis(tetraethylammonium)bis(2-
thioxo-1,3-dithiole-4,5-dithilato)zincate (DMIT). Reagents and conditions: a) CH3CN, BrCH2CH2CN, reflux
for 1 h; b) CHCl3, CH3COOH, Hg
d) P(OEt)3, reflux for 2.5 h; e) CH3CN, CsOH H2O/CH3OH, stirred at RT for 1 h; CH3I, stirred at RT for 2 h;
f) CH3CN, CsOH H2O/CH3OH, stirred at RT for 1 h; CH3COCHClCOCH3, stirred at RT overnight.
ACHTNUGTNER(UNNG OAc)2, stirred at RT overnight; c) CH3CN, BrCH2CH2Br, reflux for 1 h;
C
ACHTUNGTRENNUNG
C
detected. This redox process can be repeated for several
times without significant change, suggesting that the com-
plex remain stable in solution even at the dicationic state.
Neutral ET-acac shows a strong absorption band at
315 nm and two clear shoulder peaks around 338 and
386 nm respectively, as depicted in Figure 3a, which is the
characteristic p–p* transition of the TTF skeleton.[1,10] After
oxidation by 1.0 equiv of NOBF4, two distinct absorption
bands at about l=440 and 580 nm emerge, and the initial
Figure 1. Molecular structures of [IrACTHUNGTREN(NNGU dbq)2acac] (1) and [IrACHTUNRTEG(NGNUN dbq)2ACHTUNGTREN(NUNG ET-
acac)] (2).
formation). The two reversible oxidation steps are attributed
to the sequential oxidation of the TTF unit, from neutral
ET-acac to cationic radical ET+ -acac, and further to dicat-
C
ionic ET2+-acac. E11=2 value is 565 mV, showing a 143 mV
anodic shift compared to that of the pristine TTF
(422 mV),[9] owing to the electron-withdrawing feature of
the acac group connected to the ET moiety. E12=2 value is
923 mV, also largely positively shifted compared to that of
pristine TTF (810 mV).
The electro-activity of ET-acac moiety is retained in 2.
Two reversible oxidation processes of ET-acac are observed,
while the oxidation of the IrIII core is not observed (Fig-
Figure 2. Cyclic voltammogram of a) the ancillary ligand ET-acac and
b) the IrIII complex 2 in dichloromethane (c=1ꢁ10À4 molLÀ1).
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Cyclometalated IridiumACHTNUTRGNE(NUG III) Complex
COMMUNICATION
the ET unit is oxidized to the cationic radical state, the
MLCT absorption band is not obvious because it significant-
ly overlaps the absorption of ET+ -acac between l=460 and
C
575 nm. If the ET unit is oxidized to the dicationic state, the
characteristic absorption peak of ET+ -acac at l=885 nm
C
disappears, but the case is different for the bands at l=460
and 575 nm, indicating the maintenance of the MLCT bands
even after oxidation, similar to that of 1 (Figure S4 in the
Supporting Information).
Without the presence of NOBF4, 2 displays a very weak
phosphorescence emission band (Figure 4a) with a relative
luminescence efficiency of 0.02 referenced to 1 (Figure S4 in
the Supporting Information). However, 1 mixed with neutral
TTF remains the strong luminescence (Figure S5 in the Sup-
porting Information). And the absorption peak of the mix-
ture is similar to that of 2, revealing that only a weak
ground-state electronic coupling exists between TTF and lu-
minescent core, although the two moieties are closely
linked. Thus the luminescence quenching is assigned to the
photo-induced intramolecular electron transfer from ET
moiety to excited-state IrIII center, similar to those in organ-
ic TTF-fluorophore dyads.[5] With oxidant added, the emis-
sion intensity is remarkably enhanced and reaches the maxi-
Figure 3. UV-Vis-NIR absorption spectral profiles of neutral, cationic
radical, and dicationic a) ET-acac and b) 2 in dichloromethane (c=5ꢁ
10À5 molLÀ1) with consecutive addition of NOBF4 in acetonitrile. Neutral
state, no NOBF4 introduced. Cationic radical state, one equivalent of
NOBF4 introduced. Dicationic state, two equivalents of NOBF4 intro-
duced.
absorbance at l=315 nm decreases gradually (Figure 3a
and Figure S2 in the Supporting Information), indicating
[10]
that ET-acac has been converted into ET+ -acac. As for
C
the shoulder peak around l=490 nm, it originates from the
electronic transition from the inner molecular orbitals to the
single occupied molecular orbital (SOMO).[5k] A very strong
and broad absorption band between l=650 and 1100 nm
centred at l=930 nm appears in the near-infrared (NIR)
region, and is attributed to the singlet intra-ligand charge
transfer (1ILCT), similar to those in other TTF-acceptor dy-
ads.[3b,e,5k,11] The relative intensities of the above several ab-
sorption peaks change very little with respect to the concen-
tration changes (Figure S3 in the Supporting Information),
which excludes the possibility that the NIR region absorp-
tion arises from the formation of intermolecular dimers.[5k,12]
The absorbance in the visible and NIR regions correspond-
ing to the cationic radical ET-acac is decreased when the
TTF moiety is further oxidized to the dicationic state, which
1
leads to the emergence of a new ILCT band centred at l=
Figure 4. Phosphorescence emission changes of 2 in dichloromethane (c=
5ꢁ10À5 molLÀ1) with stoichiometric addition of a) NOBF4 and b) NaBH4
in acetonitrile, subsequently under pumped-in argon to remove the dis-
solved oxygen, lex =365 nm. Starting from the solution of neutral 2, in
the oxidation process, two equivalents of NOBF4 were added sequentially
to produce cationic radical and dicationic states in turn; in the reduction
process, likewise, two equivalents of NaBH4 were introduced consecutive-
ly to recover cationic radical and neutral states.
710 nm, indicating the formation of the dicationic ET
moiety after addition of two equivalents of NOBF4.
As for 2, a distinct 1MLCT transition band around l=
3
450 nm and a MLCT band around l=500 nm are observed
3
in the absorption spectra, but the dbq-localized LC (ligand-
centred charge transfer) is still not distinct (Figure 3b). If
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C.-H. Yan et al.
mum after the ancillary ligand ET-acac is oxidized to the di-
cationic state by the addition of stoichiometric oxidant (Fig-
ure 4a), probably caused by some prohibition of the reduc-
tive electron transfer from IrIII core to TTF moiety. Refer-
enced to that of neutral 1, the luminescence efficiencies of
cationic radical and dicationic 2 are about 0.15 and 0.35 re-
spectively. As the oxidized TTF moiety is an electron-with-
drawing group herein, the electron density localized on IrIII
core is reduced and the energy level of ground state is de-
creased compared with that of 1, thus the energy gap be-
tween the frontier MLCT state and the ground state is en-
larged and the emission peak of dicationic 2 is significantly
blue-shifted by about 40 nm to 550 nm in comparison with
that of 1. In the air-equilibrated solution, the luminescence
efficiencies were reduced greatly (Figure S6 in Supporting
Information). The emission lifetime of 2 monitored at the
dicationic state in oxygen-removed solution at room temper-
ature is 544 ns, whereas it is 354 ns in air-equilibrated solu-
tion. The oxygen-sensitivity of both intensity and lifetime in-
dicate the phosphorescence nature of dicationic 2.
As described above, after oxidation by NOBF4, 2 displays
a much enhanced phosphorescence emission with a intensity
ratio of 40 at 550 nm. This process can be regarded as a
“turn-on” switching operation in the luminescence mode.
Next, “turn-off” of the luminescence was accomplished by
consecutively adding NaBH4, as the reductant to reduce the
dicationic 2 to the neutral state (Figure 4b). With addition
of NaBH4, the phosphorescence emission is drastically
quenched, suggesting that the ancillary ligand ET-acac is re-
stored to the neutral state from the dicationic state. During
the reduction process, the luminescence efficiencies of cat-
ionic radical and neutral 2 are about 0.15 and 0.06, respec-
tively, referenced to that of neutral 1. Thus, a redox-con-
trolled phosphorescence switch is realized. Presumably,
other oxidants and reductants can also modulate the phos-
phorescence emission behaviours, so it is expected that this
kind of compounds may serve as a probe for detecting the
presence of some interesting oxidative species, such as
oxygen and nitric monoxide in vitro and even in vivo,[13] or
execute chemical-driven digital functionalities.[5j,k,10]
between charge transfer efficiency and MLCT energy level.
Studying the singlet-triplet interactions at excited states may
also contribute to the dynamic p–d coupling in TTF-based
metal compounds.
Experimental Section
Synthesis of the cyclometalated ligand dbq: Dbq was obtained through a
classical Schiff-base synthetic reaction from phenanthrene-9,10-dione and
g]
1,2-ethylenediamine.[7
Synthesis of the target ancillary ligand ET-acac: The target ancillary
ligand 3-{4-[4’,5’-(ethylenethio)-5-(methylthio)-tetrathiafulvalenyl]-thio}-
acetyl acetonate (ET-acac) was synthesized from bis(tetraethylammo-
nium)bis(2-thioxo-1,3-dithiole-4,5-dithilato)zincate (DMIT) following the
route in Scheme 1 in the text. All the precursors of ET-acac were synthe-
sized following the reported procedures.[14] The synthetic procedures of
the last step was exploited from the literature with minor alteration[15] as
follows. A solution of CsOH·H2O (181 mg, 1.1 mmol) in MeOH (3 mL)
was slowly added to a solution of 2-(2-cyanoethylthio)-6,7-(ethylenetho)-
3- methyl thiotetrathiafulvalene (425 mg, 1.0 mmol) in acetonitrile
(30 mL) under Ar atomsphere. One hour later, 3-chloro-2, 4-pentadione
(0.14 mL, 1.2 mmol) was added. After stirred overnight, the mixture was
concentrated under reduced pressure, extracted with dichloromethane
and purified by silica-gel column chromatography with dichloromethane/
petroleum ether (v/v=3:1) as the eluent. The obtained oil was recrystal-
lized from dichloromethane and petroleum ether to afford the target
product (1.0 g), as
a
red powder. Yield: 85%. 1H NMR (CDCl3,
400 MHz): d=17.19 (s, 1H), 3.29 (s, 4H), 2.44 (s, 6H), 2.40 ppm (s, 3H);
13C NMR (CDCl3, 100 MHz): d=197.6, 132.0, 119.7, 114.0, 113.9, 109.5,
102.8, 67.0, 30.2, 24.9, 19.3 ppm; FT-IR (KBr): u=2920 (s), 2854 (w),
1719 (m), 1560 (vs), 1407 (vs), 1257 (s), 1125 (w), 1018 (s), 1246 (s), 911
(m), 889 (s), 773 cmÀ1 (s); HRMS: m/z calcd. for C14H14O2S8: 469.8760;
found: 469.8744; EI MS: m/z=470, 442, 382, 364, 340, 332, 312, 296, 280,
252, 233, 224; elemental analysis (%) calcd for C14H14O2S8: C 35.72, H
3.00; found: C 35.61, H 3.03.
Synthesis of the IrIII complexes: [{(dbq)2Ir(m-Cl)}2][16] and [Ir
ACHTUNGTRENUNNG AHCTUNRTEGN(NGUN dbq)2ACHTUNGTRENNUNG(acac)]
(1)[17] were synthesized according to the method reported. The new li-
gands were reacted with IrCl3·3H2O to yield chloro-bridged dimeric com-
plexes [{(dbq)2IrACTHNUTRGNEG(NU m-Cl)}2], and these species were subsequently cleaved
by reaction with acetyl acetone in 1,2-dichloroethane in presence of
Na2CO3 to give clean conversion to the target compound 2: bright red
1
powders. Yield: 23%. H NMR (CDCl3, 400 MHz): d=9.26 (d, 1H), 9.21
(d, 1H), 9.05 (d, 1H), 8.91 (d, 1H), 8.87 (d, 1H), 8.77 (d, 1H), 8.54
2H), 8.05 (d, 1H), 7.96 (d, 1H), 7.81 (m, 2H), 7.19 (t, 1H), 7.05 (t, 1H),
6.37(m, 2H), 5.34 (s, 1H), 2.52 (s, 4H), 2.41 (s, 6H), 2.01 ppm (s, 3H);
ACHTUNGTRENNUNG(m,
AHCTUNGTRENNUNG
FT-IR (KBr): u=3060 (w), 2920 (w), 1580 (vs), 1566 (m), 1524 (m), 1464
(vs), 1444 (vs), 1424 (vs), 1403 (s), 1383 (vs), 1289 (s), 1209 (s), 1136 (m),
1053 (m), 1017 (m), 951 (m), 846 (w), 763 cmÀ1 (vs); MALDI-TOF MS:
m/z=649, 1120. elemental analysis (%) calcd for C46H31IrN4O2S8: C
49.31, H 2.79, N 5.00; found: C 48.45, H 2.99, N 5.02.
In conclusion, an electro-active IrIII complex 2 was synthe-
sized, and its electrochemical and spectral properties as well
as the ancillary ligand ET-acac were measured and analyzed.
Based on the reversible conversion of 2 in different oxida-
tion states, a reversible redox-controlled “OFF-ON-OFF”
phosphorescence switch is realized through alternating addi-
tion of NOBF4 and NaBH4. The phosphorescence property
of IrIII complexes can be modulated by ET-acac ligand,
which exhibits different photo-induced electron transfer effi-
ciencies of TTF in distinct oxidation states. In addition, the
target compound may serve as a novel model compound to
investigate the mechanism of charge transfer between IrIII-
based luminescent layer and conductive layer, such as TTF
derivative, in OLED materials. Further investigation on the
photophysical mechanisms is in progress, including theoreti-
cal calculation and spectral characterization of other IrIII
compounds with different MLCT levels for the correlation
Acknowledgements
The authors acknowledge the support for this research through the
NSFC under contract numbers 20771009, 20821091, and 20731160001.
Keywords: iridium
· luminescence · redox switching ·
tetrathiafulvalene
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Cyclometalated IridiumACHTNUTRGNE(NUG III) Complex
COMMUNICATION
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Received: June 2, 2009
Published online: July 23, 2009
Chem. Eur. J. 2009, 15, 8717 – 8721
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8721