P-carborane-bridged bipyridine ligands for energy transfer between two iridium centers

Article abstract of DOI:10.1021/ic302222q

Two iridium(III) complexes displaying for one a high HOMO-LUMO gap and for the other a weaker gap were linked in a controlled and logical manner to closo-p-carborane spacers. The bridging ligand is composed of 5-ethynyl-2,2′-bipyridine units, and the peripherical Ir-ligands are orthometalated 2′,4′-difluoro-2-phenylpyridine (dfppy) (λ_abs at 400 nm for the Ir(dfppy)₂(bpy′ ) ) for the energy donor fragment and dibenzo[a,c]phenazine (dbpz) (λ_abs at 525 nm for Ir(dbpz)₂(bpy′) ) for the energy acceptor fragment.Redox, spectroscopic, and photophysical properties for models and the donor-carborane-acceptor complex were determined. Efficient energy transfer from the Ir(dfppy)₂(bpy′) moiety to the Ir(dbpz)₂(bpy′) fragment is occurring with a rate constant of 3.3 × 10⁸ s^-1 despite weak electronic coupling through the inert p-carborane spacer. From flash photolysis experiments it is shown that, by excitation of the donor, a low lying triplet state localized on the acceptor bridging ligand side is formed which decays by conversion to the ³MLCT of the acceptor fragment which phosphoresces at 644 nm.

Full text of DOI:10.1021/ic302222q

Article
pubs.acs.org/IC
p‑Carborane-Bridged Bipyridine Ligands for Energy Transfer
between Two Iridium Centers
M. Teresa Indelli,*,† Thomas Bura,‡ and Raymond Ziessel*,‡
†Dipartimento di Chimica, Universita
‡Laboratoire de Chimie Organique et Spectroscopies Avancee
Chimie, Polymeres et Materiaux, Universite de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
* Supporting Information
̀
di Ferrara, INSTM Sezione di Ferrara, 44100 Ferrara, Italy
́
s (ICEEPS-LCOSA), UMR 7515 au CNRS, Ecole Europeenne de
́
̀
́
́
S
ABSTRACT: Two iridium(III) complexes displaying for one a high HOMO−
LUMO gap and for the other a weaker gap were linked in a controlled and
logical manner to closo-p-carborane spacers. The bridging ligand is com-
posed of 5-ethynyl-2,2′-bipyridine units, and the peripherical Ir-ligands are
orthometalated 2′,4′-difluoro-2-phenylpyridine (dfppy) (λabs at 400 nm for
the “Ir(dfppy)2(bpy′)”) for the energy donor fragment and dibenzo[a,c]phenazine
(dbpz) (λabs at 525 nm for “Ir(dbpz)2(bpy′)”) for the energy acceptor fragment.
Redox, spectroscopic, and photophysical properties for models and the donor−
carborane−acceptor complex were determined. Efficient energy transfer from
the “Ir(dfppy)2(bpy′)” moiety to the “Ir(dbpz)2(bpy′)” fragment is occurring
with a rate constant of 3.3 × 108 s−1 despite weak electronic coupling through the inert p-carborane spacer. From flash photolysis
experiments it is shown that, by excitation of the donor, a low lying triplet state localized on the acceptor bridging ligand side is
3
formed which decays by conversion to the MLCT of the acceptor fragment which phosphoresces at 644 nm.
chromophores is usually not innocent because it will not only
control the spatial arrangement of the chromophores, and
consequently the intercomponent distances and angles, but also
influence the electronic communication between the partners.31,32
In some cases the bridging ligands can participate to the energy
transfer processes and can promote cascade energy transfer events
over large distances.33
INTRODUCTION
■
Designing and powering synthetic methodologies for the con-
struction of rigid multisite ligands in which weak electronic inter-
actions are effective represent major challenges and opportunities.
Considerable efforts have been devoted to the preparation of
ligands and transition metal complexes in order to quantify the
level of interaction using spectroscopic tools.1−3 Multichromo-
phoric systems have been designed and scrutinized with the view-
point of collecting, transferring, and converting solar energy into
useful energy source.4,5 In the design of such systems the bridg-
ing ligands are crucial not only from the structural viewpoint
but also for the electronic coupling between the separated
subunits.6 Along these lines, polypyridine transition metal com-
plexes have been largely employed due to their rich photophysics
and long-lived triplet excited states that can act as energy donors or
energy acceptors for fast and efficient energy transfer processes.7
Particularly attractive are molecular systems where photoin-
duced energy or electron transfer processes can be realized over
large distances and in a preferred direction.8,9 Earlier discoveries
have shown that the nature of the spacing units separating photo-
active terminals plays a crucial role in the efficiency and mecha-
nism of information transfer.10,11 Unsaturated systems have been
explored and appear to be the most attractive. Among these systems,
p-phenylenevinylene oligomers,12 polyenes,13,14 polyalkynes,15−19
polyphenylenes,20−24 polyphenyl/alkyne,25,26 and polythio-
phene27−29 units have been extensively studied due to their
chemical stability and synthetic accessibility.3,30 Generally, the
chemistry required to construct and purify such molecular objects
is highly demanding, and up-to-now very few applications have
been foreseen. The role of the bridging ligands that anchors the
Rigid rodlike spacers are attractive building blocks for the
construction of supramolecular arrays. For instance, carboranes
have been extensively used as templates for the preparation of
soft matter,34 polymers,35 nonlinear materials,36 rigid rods,37 self-
assembled molecular structures,38 and supramolecular species
containing carborane derivatives assembled through appropriate
metal centers.31,39 Their derivatives such as metallacarboranes are
analogues of metallocenes40 and, like them, find applications,
for example, in medicinal chemistry.41 Carboranes of the closo
form, such as 1,12-dicarbadecaborane (C2B10H12), are geometrically
rigid species with an electronic structure making them chemically
and thermodynamically very robust. Furthermore, they are trans-
parent to visible and UV light down to about 200 nm and do not
exhibit redox activity in standard windows.42 These properties
prompted us to use closo-1,12-dicarbadecaborane as a platform
to link organic chromophores in a rodlike manner and promote
through space Forster resonance energy transfer (FRET).43
̈
In a single case, para-carborane has been used as a bridging
unit between two phosphorescent ruthenium-trisbipyridine units.44
Mixed valence homometallic complexes have been investigated, and
Received: October 11, 2012
© XXXX American Chemical Society
A
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Chart 1
powder by crystallization in a mixture dichloromethane/diethylether
(160 mg, 87%).
little delocalization through the bridge has been evidenced.
However photoinduced electron transfer in mixed valence com-
plexes highlights the effectiveness of such processes. Up to now,
no energy transfer between two dissimilar complexes bridged
by carborane scaffoldings has been investigated. Within this con-
text we devise a synthetic protocol capable to control by a con-
vergent procedure the construction of donor-bridge-acceptor
system bridged by a bipy-Carbo-bipy framework (bipy accounts
for 5-ethynyl-2,2′-bipyridine and Carbo for closo-p-carborane)
and capable to promote electronic energy transfer between two
remote cationic Ir(III) centers having dissimilar energy levels of
the lowest excited states (Chart 1).
1H NMR ((CD3)2CO, 300 MHz): 5.76 (dd, 1H J = 8.6 Hz, J =
2.85 Hz), 5.84 (dd, 1H 3J = 8.4 Hz, 4J = 2.30 Hz), 6.70−6.79 (m, 2H),
7.21−7.28 (m, 2H), 7.74−7.78 (m, 1H), 7.91−7.95 (m, 1H), 8.04−
8.10 (m, 3H), 8.19−8.22 (m, 2H), 8.31−8.40 (m, 3H), 8.50−8.53 (m,
1H), 8.80−8.88 (m, 2H). EI-MS m/z 809.0 ([M − PF6]+ 100); 807.0
([M − PF6]+ 100). Anal. Calcd for C32H19BrF4IrN4(PF6) (Mr =
952.6): C, 40.35; H, 2.01; N, 5.88. Found: C, 40.11; H, 1.87; N, 5.67.
Compound 6. In a Schlenk tube compound 2 (40 mg, 0.073 mmol)
and complex 5 (69 mg, 0.073 mmol) were dissolved in a mixture of
benzene (5 mL), CH3CN (5 mL), and triethylamine (2 mL). Argon
was bubbled through the mixture for 30 min, then [Pd(PPh3)4] (9 mg)
was added, and the mixture was stirred at 65 °C for 36 h. The solution
was cooled down to RT and the solvent evaporated to dryness. The
residue was dissolved in DMF (1 mL) and was added dropwise
through a pad of Celite into an aqueous solution of KPF6 (500 mg in
20 mL of water). The precipitate was collected on paper and washed
with water (3 × 50 mL). The complex was dried in air and was
purified by column chromatography on alumina. The desired complex
was eluted with a gradient of methanol (0−1%) in dichloromethane as
mobile phase. The analytically pure complex 6 was obtained as yellow
powder by hot crystallization in acetone (70 mg, 68%).
3
4
EXPERIMENTAL SECTION
■
Synthesis and Characterization. Compound 1. To a degazed
solution of A (47.1 mg, 0.077 mmol) and 5-ethynyl-2,2′-bipyridine
(13.9 mg, 0.077 mmol) in benzene (5 mL) and triethylamine (1.5 mL)
was added Pd(PPh3)4 (8.9 mg, 0.008 mmol). The resulting mixture
was stirred over a weekend at 60 °C under argon. It was then cooled
down to RT (room temperature), dissolved in CH2Cl2, washed with
water, and extracted with dichloromethane. Organic layers were dried
over Na2SO4 and evaporated under vacuum. The product was purified
by silica gel chromatography (from 5/95 to 10/90 ethyle acetate/
1H NMR ((CD3)2CO, 300 MHz): 1.63−3.15 (broad B−H
3
4
absorption overlapping with H2O), 5.76 (dd, 1H J = 8.6 Hz, J =
2.31 Hz), 5.82 (dd, 1H 3J = 8.6 Hz, 4J = 2.31 Hz), 6.73−6.81 (m, 2H),
7.23−7.27 (m, 2H), 7.41−7.49 (m, 7H), 7.59−7.61 (m, 2H), 7.75−
7.79 (m, 1H), 7.91−7.98 (m, 2H), 8.05−8.10 (m, 4H), 8.22−8.28 (m,
2H), 8.34−8.54 (m, 6H), 8.69−8.70 (m, 1H), 8.82 (s broad, 1H),
8.89−8.94 (m, 2H). EI-MS m/z 1273.4 ([M − (PF6)]+ 100). Anal.
Calcd for C64H44B10F4IrN6(PF6) (Mr = 1418.36): C, 54.20; H, 3.13;
N, 5.93. Found: C, 54.08; H, 3.25; N, 5.77.
1
petroleum ether) to obtain 46.2 mg (90%) of compound 1. H NMR
(200 MHz, CDCl3) δ (ppm): 0.66 (q, 6 H, 3J = 7.8 Hz), 1.03 (t, 9 H,
3J = 7.9 Hz), 7.18−7.39 (m overlapping with slovent, 7 H), 7.42 (d,
2H, 3J = 8 Hz), 7.75−7.97 (m, 2 H), 8.41 (d, 2 H, 3J = 8.2 Hz), 8.61−
8.87 (m, 2 H).
Compound 2. In the round-bottom flask with THF (5 mL) and
MeOH (1 mL) was added compound 1 (45 mg, 0.068 mmol) and
NaOH (14 mg, 0.34 mmol). The reaction was agitated during 1 h at
RT. Then, the solvent was evaporated to dryness. The crude material
was purified by column chromatography on alumina eluted with a
mixture of CH2Cl2/petroleum ether (40/60) and afforded compound
2 as a white powder (35 mg, 95%).
Compound 8. The dimeric complex 7 (84 mg, 0.054 mmol) and
5-ethynyl-2,2′-bipyridine (20 mg, 0.088 mmol) were dissolved in di-
chloromethane (6 mL) and methanol (6 mL). The mixture was heated
at 60 °C during the night. The solution was cooled down to RT and
the solvent evaporated to dryness. The residue was dissolved in DMF
(1 mL) and was added dropwise through a pad of Celite into an
aqueous solution of KPF6 (500 mg in 20 mL of water). The precipitate
was collected on paper and washed with water (3 × 100 mL). The
complex was dried in air and was purified by column chromatography on
alumina. The desired complex was eluted with a gradient of methanol (0−
1%) in dichloromethane as mobile phase. The analytically pure complex 8
was obtained as orange powder by crystallization in a mixture dichloro-
methane/diethylether (20 mg, 35%).
1H NMR (CDCl3, 300 MHz): 1.63−3.15 (broad B−H absorption
3
10H), 3.17 (s, 1H), 7.25−7.40 (m, 7H), 7.47 (d, 2H J = 8.2 Hz),
7.81−7.94 (m, 2H), 8.42 (d, 2H, 3J = 7.9 Hz), 8.69−8.80 (m, 2H). EI-
MS 546.2 ([M] 100). Anal. Calcd for C32H26B10N2 (Mr = 546.67): C,
70.31; H, 4.79 ; N, 5.12. Found: C, 70.20; H, 4.52; N, 4.88.
Compound 5. The dimeric complex 3 (100 mg, 0.084 mmol) was
dissolved in dichloromethane (6 mL) and methanol (6 mL), and
5-bromo-2,2′-bipyridine (42 mg, 0.176 mmol) was added as a solid. The
mixture was heated at 60 °C during the night. The solution was cooled
down to RT and the solvent evaporated to dryness. The residue was
dissolved in DMF (1 mL) and was added dropwise through a pad of
Celite into an aqueous solution of KPF6 (500 mg in 20 mL of water).
The precipitate was collected on paper and washed with water (3 ×
100 mL). The complex was dried in air and purified by column
chromatography on alumina. The desired complex (yellow band) was
eluted with a gradient of methanol (0−1%) in dichloromethane as
mobile phase. The analytically pure complex 5 was obtained as yellow
1H NMR (CD3)2CO, 300 MHz): 3.95 (s, 1H), 6.62 (d, 1H, 3J = 7.6
3
Hz), 6.73 (d, 1H, J = 7.6 Hz), 7.12−7.21 (m, 2H), 7.44−7.57 (m,
3
5H), 7.78−8.01 (m, 6H), 8.11−8.24 (m, 4H), 8.32 (t, 2H, J = 7.9
3
3
Hz), 8.43 (d, 2H, J = 8.6 Hz), 8.68 (d, 2H, J = 8.6 Hz), 8.73−8.77
(m, 2H), 9.39−9.43 (m, 2H). EI-MS m/z 931.4 ([M − PF6]+ 100).
Anal. Calcd for C52H30IrN6(PF6) (Mr = 1076.02): C, 58.04; H, 2.81;
N, 7.81. Found: C, 58.12; H, 3.04; N, 7.62.
Compound [Ir(dfppy)2-Carbo-Ir(dfppy)2](PF6)2. The dimeric com-
plex 3 (12.2 mg, 0.0103 mmol) and compound 6 (29 mg, 0.0206 mmol)
B
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Scheme 1
were dissolved in dichloromethane (4 mL) and methanol (4 mL). The
mixture was heated at 60 °C during the night. The solution was cooled
down to RT and the solvent evaporated to dryness. The residue was
dissolved in DMF (1 mL) and was added dropwise through a pad of
Celite into an aqueous solution of KPF6 (500 mg in 20 mL water).
The precipitate was collected on paper and washed with water (3 ×
100 mL). The complex was dried in air and was purified by column
chromatography on alumina. The desired complex was eluted with a
gradient of methanol (0−3%) in dichloromethane as mobile phase.
The analytically pure compound [Ir(dfppy)2-Carbo-Ir(dfppy)2](PF6)2
was obtained as yellow powder by hot crystallization in acetone (35 mg,
80%).
8.66−8.68 (m, 2H), 8.75−8.93 (m, 4H), 9.38−9.43 (m, 2H). EI-MS
m/z 2169.2 ([M − (PF6)]+ 100). Anal. Calcd for C104H66B10-
F4Ir2N10(PF6)2 (Mr = 2314.17): C, 53.98; H, 2.87; N, 6.05. Found: C,
54.19; H, 3.07; N, 5.84.
RESULTS AND DISCUSSION
■
Synthesis. The preparations of the target [Ir(dbpz)2-Carbo-
Ir(dfppy)2](PF6)2 and reference complexes are sketched in
Schemes 1 and 2. The synthesis of the pivotal starting material
A bearing one protected alkyne function and a reactive
iodophenyl fragment has been previously described.45 Cross-
coupling A with 5-ethynyl-2,2′-bipyridine B46 produces ligand 1
in excellent yield. This molecule is easily deprotected under
basic conditions to provide the terminal alkyne 2 in good yields.
Cross-coupling complex 5 bearing a reactive bromo-aryl func-
tion with ligand 2 provides the cationic iridium complex 6. The
key precursor 5 was prepared as a single isomer from the
neutral dimer 3 and 5-bromo-2,2′-bipyridine 4 (Scheme 1).47
The use of 2′,4′-difluoro-2-phenylpyridine (dfppy) allows us to
prepare pale yellow complexes displaying optical transitions at
about 400 nm.48
1H NMR (CDCl3, 400 MHz): 2.01−3.30 (broad B−H absorption
3
4
3
10H) 5.65 (dd, 2H J = 8.2 Hz, J = 2.04 Hz), 5.70 (dd, 2H J = 8.6
Hz, 4J = 2.4 Hz), 6.54−6.61 (m, 4H), 7.11−7.14 (m, 4H), 7.29 (d, 4H,
3J = 8.2 Hz), 7.39 (d, 4H, 3J = 8.2 Hz), 7.49−7.56 (m, 6H), 7.81−7.85
(m, 4H), 7.91−7.94 (m, 4H), 8.18−8.24 (m, 4H), 8.30−8.35 (m, 4H),
8.67−8.72 (m, 4H). EI-MS m/z 1990.9 ([M − (PF6)]+ 100). Anal.
Calcd for C86H56B10F8Ir2N8(PF6)2 (Mr = 2135.88): C, 48.36; H, 2.64;
N, 5.25. Found: C, 48.59; H, 2.77; N, 5.02.
Compound [Ir(dbpz)2-Carbo-Ir(dfppy)2](PF6)2. The dimeric com-
plex 7 (30 mg, 0.0170 mmol) and compound 6 (24 mg, 0.170 mmol)
were dissolved in dichloromethane (4 mL) and methanol (4 mL). The
mixture was heated at 60 °C during the night. The solution was cooled
down to RT and the solvent evaporated to dryness. The residue was
dissolved in DMF (1 mL) and dropwise added through a pad of Celite
into an aqueous solution of KPF6 (500 mg in 20 mL water). The
precipitate was collected on paper and washed with water (3 × 100 mL).
The complex was dried in air and was purified by column chromatography
on alumina. The desired complex was eluted with a gradient of methanol
(0−2%) in dichloromethane as mobile phase. The analytically pure
compound [Ir(dbpz)2-Carbo-Ir(dfppy)2](PF6)2 was obtained as
orange powder by crystallization in a mixture CH3CN/Et2O (25 mg,
63%). 1H NMR ((CD3)2CO, 300 MHz): 1.63−3.15 (broad B−H
absorption overlapping with H2O), 5.76 (dd, 1H 3J = 8.6 Hz, 4J = 2.00
With complex 6 in hand it was possible to chelate the free
bipyridine residue with a second iridium center by reacting 6
with dimer 7 under standard conditions providing the dicationic
[Ir(dbpz)2-Carbo-Ir(dfppy)2]2+ in good yields (Scheme 2). The
use of dibenzo[a,c]phenazine (dbpz) allows us to prepare deep
red complexes displaying optical transitions above 530 nm.49
On the other hand the preparations of the donor and acceptor
complexes that are useful for the spectroscopic measurements
were completed as depicted in Scheme 2. The acceptor complex 8
was easily prepared by reacting the dimer 7 with ligand B and the
acceptor complex [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+ (D) by reac-
ting the mononuclear complex 6 with dimer 3. For all charged
complexes the counteranion is hexafluorophosphate which facilitates
the purification by column chromatography. The molecular structures
3
4
Hz), 5.82 (dd, 1H J = 8.6 Hz, J = 2.31 Hz), 6.68−6.80 (m, 4H),
7.10−7.33 (m, 8H), 7.40−7.57 (m, 6H), 7.73−7.78 (m, 2H), 7.81−
7.93 (m, 5H), 7.97−8.09 (m, 6H), 8.13−8.27 (m, 7H), 8.32−8.45 (m, 8H),
C
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Scheme 2
of the intermediates and final compounds were unambiguously
assigned by NMR, ESI-MS, and optical measurements.
Absorption Spectra. The compound studied is made of
two different chromophoric units, Ir(dbpz)2- and Ir(dfppy)2-
bridged by a p-carborane-containing ligand (Carbo). In order
to discuss the photophysical and electrochemical results ob-
tained for the bichromophoric complex, it is useful to study
simple compounds that model the intrinsic behavior of the
individual chromophoric units. For Ir(dfppy)2-unit, an appropriate
model is represented by the binuclear [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+
complex, whereas for Ir(dbpz)2-unit, the mononuclear [Ir(dbpz)2-
(bpy′)]+ complex 8 is available as model.
The spectroscopic, photophysical, and redox properties of
the bichromophoric complex were investigated in CH2Cl2 solu-
tion and compared to those of the model compounds. The absorp-
tion spectra of the model compounds are shown in Figure 1.
In the spectrum of [Ir(dbpz)2(bpy′)]+, beside the ligand centered
(LC) bands in the UV region, there are two bands in the visible
region at 410 and 525 nm, which can be assigned as metal to
ligand charge transfer MLCT transitions, localized on the bpy-
based ligand (higher energy band) and on the cyclometalating
dbpz ligands (lower energy band), respectively. These assign-
ments are supported by redox results that show that the ex-
tended aromatic dbpz ligand is easier to reduce than the bpy-
based ligand (vide infra). On the other hand, the binuclear
[Ir(dfppy)2-Carbo-Ir(dfppy)2]2+ complex does not absorbs in
the visible (Figure 1), showing intense bands in the UV region
due to the ligand centered (LC) transitions (possibly overlapping
less intense metal to ligand charge transfer, MLCT bands). The
spectrum of bichromophoric complex, [Ir(dfppy)2-Carbo-
Ir(dbpz)2]2+, shown in Figure 1, clearly exhibits the typical absorp-
tions of both the Ir(dbpz)2- and Ir(dfppy)2-units. The substantial
Figure 1. Absorption spectra of [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+, donor
model (×1/2, blue line); [Ir(dppz)2(bpy′)]+, acceptor model (black line);
and [Ir(dfppy)2-Carbo-Ir(dbpz)2]2+ bichromophoric complex (red line)
in CH2Cl2 solution at room temperature.
additivity of the spectroscopic properties of the molecular com-
ponents in the complex points toward an absence of extensive
electronic delocalization across the bridging ligand38−44 and
warrants a localized description of the system studied.6 Figure 1
shows that fully selective excitation of the Ir(dbpz)2-unit can be
achieved at wavelength longer than 500 nm, whereas the
Ir(dfppy)2-unit can be excited only partially (e.g., 60% at 355 nm).
For the roles that they play in the photophysical experiments
described below, the Ir(dfppy)2- and Ir(dbpz)2-units are thereafter
denoted, respectively, as the donor and acceptor units.
Redox Properties. The electrochemical properties of the
bichromophoric complex were examined by cyclic voltammetry
in dichloromethane solution (electrochemical window from +1.6
to −2.2 V, TBAPF6 0.1 M supporting electrolyte, Pt microdisk
D
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working electrode, SCE reference electrode, silver wire count-
erelectrode). For purposes of comparison, the electrochemical
behavior of the model compounds was studied under the same
experimental conditions (Figure 2). The results are reported
in Table 1.
of the waves observed for the model components confirming a
very weak degree of electronic communication between the two
moieties through the p-carborane bridge and justifying the con-
sideration of the bichromophoric complex as true supramolecular
species.7a
Emission. Stationary emission measurements were carried
out in room temperature fluid solution and in a rigid matrix
(EtOH/MeOH 4/1) at 77 K. The relevant emission properties
of the systems studied are collected in Table 2. At room
temperature, the model complexes as well as the bichromophoric
complex give rise to relatively intense emissions (Φ = (1−6) ×
10−2, see Table 2).
The emission spectra in room temperature CH2Cl2 solution
of the bichromophoric complex together with those of the two
models are shown in Figure 3. The acceptor model ([Ir(dbpz)2-
(bpy′)]+) exhibits a broad, structureless band with a maximum
at 640 nm. The lifetime of this emission, strongly quenched by
oxygen, is 2.0 μs in deaerated solution with a strictly mono-
exponential decay, regardless of monitoring wavelength. The
spectral shape, together with the properties reported in Table 2,
clearly indicate that the nature of this emission is a phos-
phorescence from a metal to ligand charge transfer (MLCT)
triplet state. In this complex two types of MLCT states are
present, one localized on the bpy-like ligand and the other
involving the dbpz cyclometalating ligand. On the basis of the
spectroscopic (Figure 1) and redox results (Table 1) we can
easily assign the emitting state as MLCT triplet localized on the
dbpz ligand.
Figure 2. Cyclic voltammogram of 0.5 mM: (blue trace) complex 8;
(red trace) complex 5; (black trace) binuclear complex [Ir(dfppy)2
-Carbo-Ir(dbpz)2]2+ in dichloromethane with 0.1 M TBAPF6. Working
electrode, Pt disk; counterelectrode, Pt coil; reference electrode, SCE.
Scan rate was 0.2 V/s.
Table 1. Redox Potentials of the Bichromophoric Complex
a
and of the Model Compounds
By contrast, the emission of the donor model, [Ir(dfppy)2-
Carbo-Ir(dfppy)2]2+, shows different properties with respect the
acceptor emission (Figure 3): it is structured with a maximum
at 550 nm, a longer lifetime (3.3 μs), and a significantly smaller
radiative rate constant (see Table 2). These features clearly sug-
gest that in this complex the emission is a phosphorescence
from the lowest ligand centered (LC) triplet localized on the
bridging ligand.52,53
In a rigid matrix (EtOH/MeOH 4/1) at 77 K, for both
model complexes, the emissions become more intense (Figure 4)
with shapes and the lifetimes (see Table 2) that confirm the room
temperature assignments.
a
c
complex
E°red, V (Ep − Ep , mV)
b
b
[Ir(dfppy)2-Carbo-
Ir(dfppy)2]2+
[Ir(dbpz)2(bpy′)]+
−1.12 (65); −1.75 (irr)
- 0.96 (60); −1.17 (60); −1.50 (60)
b
[Ir(dfppy)2-Carbo-
−0.95 (60); −1.14 (70); −1.41 (70); −1.55 (irr);
Ir(dbpz)2]2+
−1.76 (irr); 1.95 (irr)
a
CH2Cl2 solution with 0.1 M TBAPF6. Potential values vs SCE, values
calculated as average of the cathodic and anodic peaks, one-electron
process unless otherwise noted. Two-electron process.
b
In the anodic region (0 to +1.6 V vs SCE) the complexes are
not active. As usual for Ir(III) complexes with imine ligands,50
the cathodic region is characterized by ligand centered reduc-
tion processes. For the [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+ binuclear
donor model, a reversible bielectronic wave is observed at −1.12 V
versus SCE. This process can be attributed to reduction of the
bpy-like moiety of the bridging ligand by comparison with the
redox behavior of related Ir(III) complexes containing ppy cyclo-
metalating and bpy ligands.51 The bielectronic nature of the reduc-
tion process indicates that the two bpy-like moieties of the bridge
are reduced simultaneously, again consistent with a weak through-
bridge interaction between the two metallic units. A subsequent
irreversible wave is observed likely due to reduction of one of
the dfppy terminal ligands. The [Ir(dbpz)2(bpy′)]+ acceptor
model exhibits three well resolved reversible reduction waves.
The first falls at a potential definitely less negative (−0.96 V vs
SCE) than that for the first reduction process in the donor
model, and, as a consequence, can be safely assigned to reduc-
tion of the dbpz ligand. In the more negative region, the reduc-
tion of bpy′ ligand occurs at −1.17 V versus SCE, a potential
value very close to reduction of bpy-like moiety of the bridge.
The third wave can be tentatively assigned to the reduction of the
second dbpz ligand. The voltammetric behavior of [Ir(dfppy)2-
Carbo-Ir(dbpz)2]2+ bichromophoric complex is a nice combination
As far as the behavior of the [Ir(dfppy)2-Carbo-Ir(dbpz)2]2+
bichromophoric complex is concerned, a single emission practi-
cally identical (spectral region, shape and quantum yield) to
that exhibited by the acceptor model appears (Figure 3), regard-
less of excitation wavelength. In particular, upon substantial excita-
tion of the donor unit (λ = 355 nm), (i) no emission with char-
acteristic features of the donor model is detected, clearly indicating
that the emitting state of the donor unit is completely quenched,
and (ii) the acceptor emission is efficiently sensitized (as shown
by the close correspondence of the excitation spectrum to the
absorption spectrum, Figure S8 of Supporting Information).
These two results provide strong evidence for the occurrence of
an efficient energy transfer process (eq 1):
*
Donor−Carbo−Acceptor → Donor−Carbo−Acceptor*
(1)
On the basis of the emitting features of the two chro-
mophoric units, this energy transfer process is quite exergonic.
A value of −0.35 eV can be obtained from the energy of the
lowest emitting excited states of the two components (see E0−0
reported in Table 2). From the excitation spectrum (Figure S8
of Supporting Information) the efficiency of the energy transfer
process is estimated to be practically unitary. The fact that,
E
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Table 2. Photophysical Properties of the Bichromophoric Complex and of the Model Species
a
b
298 K
77 K
d
e
f
c
kr (s−1
)
λem (nm)
τ (μs)
E0−0 (eV)
max
max
complex
λem (nm)
τ
(μs)
3.3
Φem
[Ir(dfppy)2-Carbo-Ir(dfppy)2]2+
561
640
644
0.015
0.059
1.7 × 104
7.9 × 104
8.6 × 104
542
630
30
17
18
2.29
1.59
1.59
[Ir(dbpz)2(bpy′)]+
2.0
1.7
f
[Ir(dfppy)2-Carbo-Ir(dbpz)2]2+
0.060
630
a
b
c
d
e
In CH2Cl2 solution. In EtOH/MeOH, 4/1, rigid matrix. Obtained from the maximum of the 77 K emission. Deaerated solution. Measured in
f
2+
aerated solution, using Ru(bpy)3 as reference. Radiative rate constant, kr = Φem/τaer; independent of the excitation wavelength.
decay was strictly monoexponential with a lifetime of 1.7 μs in
deaerated acetonitrile, a value very similar to the lifetime of the
model acceptor (Table 2). These results are consistent with the
occurrence of an energy transfer process from donor to acceptor
unit and, in addition, clearly demonstrate that this process takes
place within the laser pulse (8 ns).
In order to evaluate the rate of the energy transfer process,
single photon counting experiments were performed by excita-
tion at 380 nm where the light is substantially (50%) absorbed
by the donor unit. The emission decay, measured at λ = 550 nm
(maximum of the donor emission), is not monoexponential
(Figure S9 of Supporting Information). It consists of two com-
ponents: a major (85%) short-lived component with a lifetime
of 3.2 ns and a minor (15%) component with a lifetime >30 ns.
The short-lived component maximizes at 550 nm, decreasing in
amplitude at shorter and longer wavelengths. This component
is assigned to the decay of the donor emission, while the latter
Figure 3. Emission spectra of [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+, donor
model (blue line); [Ir(dbpz)2(bpy′)]+, acceptor model (black line);
and [Ir(dfppy)2-Carbo-Ir(dbpz)2]2+ bichromophoric complex (red
line) in CH2Cl2 solution at room temperature.
one can be easily assigned to long-lived emission from the acceptor
unit (not completely negligible at this emission wavelength).54
From the lifetime of the short-lived component, a value of 3.3 ×
108 s−1 can be estimated for the rate of the energy transfer process.
As for the mechanism of this process is concerned, two types
of mechanisms, Forster-type or Dexter-type, must be considered.7a
̈
The rate constant for Forster energy transfer can be calculated
̈
according to Forster equation55 which contains the spectro-
̈
scopic properties of the two molecular components (absorption
spectrum of the acceptor, see Figure 1, and emission spectrum
of the donor, see Figure 3) with the following appropriate
parameters: ΦD and τD (luminescence quantum yield and life-
time of the donor, see Table 2), n (solvent refractive index) =
2
1.424, k2 (statistical orientation factor) = /3. As far as the
Figure 4. Emission spectra of [Ir(dfppy)2-Carbo-Ir(dfppy)2]2+, donor
model (blue line); [Ir(dbpz)2(bpy′)]+, acceptor model (black line);
and [Ir(dfppy)2-Carbo-Ir(dbpz)2]2+ bichromophoric complex (red
line) in rigid matrix (EtOH/MeOH 4/1) at 77 K.
donor−acceptor distance is concerned, since the two chromo-
phores are delocalized partly over the bridge, it is difficult to assign
an exact value, but a range of distances between a maximum
(32.36 Å, Ir−Ir distance) and a minimum value (25.69 Å, Ir−
bridge distance) must be taken into consideration in the
calculation. The calculations, performed using Photochem
CAD,56 lead to values in the range 1.3 × 105 to 3.4 × 104 s−1,
much lower than the experimental value (3.3 × 108 s−1). The
at 77 K, a single emission with the same features as that of the
acceptor model is observed (Figure 4 and Table 2) shows that
the energy transfer process is efficient even at low temperature.
Time Resolved Transient Emission and Absorption
Spectroscopy. In order to investigate the kinetic and the me-
chanism of the energy transfer process, time-resolved experi-
ments in nanosecond time scale were performed by using laser
flash photolysis and single photon counting techniques.
Emission. Time resolved emission experiments on bichro-
mophoric complex solutions were carried out by using nano-
second laser excitation at 355 nm. The emission spectra were
measured at different delay times after the laser pulse. All the
spectra are identical to the emission spectrum observed in spec-
trofluorimetric experiments showing the distinctive features of
the emission of the acceptor unit. Despite the fact that at the
excitation wavelength efficient light absorption by the donor
takes place, no donor emission was detected, even with the
shortest possible delay (5 ns) after laser excitation. The emission
Forster mechanism can, therefore, be easily ruled out, and it is
̈
reasonable to assume that the Dexter-type mechanism57 is active
in the energy-transfer process.
Absorption. The transient absorption spectra obtained in the
laser photolysis (λexc = 355 nm) of the model complexes are
shown in Figure 5a,b.
The transient observed for the acceptor model shows the
typical features of the absorption spectrum of a MLCT state:58
(i) positive absorptions (a strong, sharp absorption band at 475
nm and a weaker broad absorption in the 600−850 nm region)
characteristic of the formation of a ligand radical anion, and (ii)
bleaching at 550 nm that corresponds very closely to the
ground-state absorption and an apparent bleaching around 650 nm
due to emission. The transient decay was strictly monoexponential
F
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Figure 6. Transient absorption spectra in CH2Cl2 of [Ir(dfppy)2-Carbo-
Ir(dbpz)2]2+ bichromophoric complex, measured at different delay times
(5 ns, black; 10 ns, red line; 50 ns, blue line; 300 ns, green line) after the
laser pulse (λexc = 355 nm; half-width = 8 ns). Inset shows spectrum
obtained by subtraction of the trace measured at 300 ns delay time from
the initial trace.
basis of the lifetime and spectral features, the long-lived transient
can be assigned to lowest emitting state of the acceptor unit that is
formed by direct excitation as well as by an energy transfer process
from excited donor unit accordingly to emission results. On the
other hand, the attribution of the short-lived component of the
transient absorption is not straightforward.59 The spectrum of the
short component (see inset of Figure 6), obtained by subtraction
of the trace measured at 300 ns delay time (with the appropriate
correction for the decrease of the signal due to the decay) from
the initial trace, exhibits a broad absorption with a maximum
around at 550 nm. In order to get more insight into the nature
of this transient, laser flash photolysis experiments were also
performed at 532 nm, where light selectively excites the acceptor
unit. Very similar time dependent spectral changes were obtained
(Figure S10 of Supporting Information) clearly indicating that
the donor unit is not responsible for the short-lived transient.60
We tentatively attribute the short-lived transient to a low-lying
LC triplet state localized on the acceptor bridging ligand moiety,
which decays by conversion to the MLCT triplet of the acceptor.
The low energy position of such a state is consistent with the high
degree of conjugation typical of phenyl-ethynylene-substituted
bipyridine ligands.61 Its nonemissive behavior (no transient
with 100 ns lifetime is detected in time-resolved emission ex-
periments) is consistent with the LC nature and short lifetime.
The fact that this transient is not observed in the laser photolysis
Figure 5. Transient absorption spectrum in CH2Cl2 of [Ir-
(dbpz)2(bpy′)]+, acceptor model (a) and of [Ir(dfppy)2-Carbo-
Ir(dfppy)2]2+, donor model (b) taken immediately after laser pulse
(λexc = 355 nm; half-width = 8 ns) in CH2Cl2 solution at room
temperature.
and matches well the emission decay, indicating that the tran-
sient spectrum is associated with the emissive MLCT charge
transfer triplet state localized on the dbpz ligand.
The transient spectrum of the binuclear donor model (Figure 5b)
fully differs from that observed for the acceptor model. It is
dominated by an intense broad absorption band at 600 nm. In
addition, a bleaching corresponding to the ground state absorp-
tion appears in the near UV region, and an apparent bleaching
due to emission is observed at 560 nm. The decay is mono-
exponential with the same lifetime as emission confirming that
the transient is associated to the LC emitting state localized on
the bridging ligand.
Laser flash photolysis of the bichromophoric complex was
carried out using 355 nm as excitation wavelength, which
corresponds to the absorption by the bridging ligand. In Figure 6
the transient spectra, measured at different delay times after the
laser pulse, are reported.
The important result is that, for this system, unlike the case
of the model complexes, the shape of the transient observed
was found to be time dependent. The spectrum measured im-
mediately after the laser pulse (5 ns delay time) shows an
positive absorption in the whole region investigated (400−850 nm)
dominated by two intense bands peaking at 472 and 575 nm
and a weak broad band around 600 nm. On a longer time scale
(300 ns delay time), the absorption at 575 nm practically dis-
appears and the transient evolves to a spectrum identical to the
excited state spectrum of the model acceptor (Figure 5a). The
decay of the transient absorption is clearly not exponential and
consists of two components, a short component with a lifetime
of ca. 100 ns and a long component with a lifetime of 1.6 μs, a
value very similar to the emission lifetime. The relative ampli-
tudes of the two components are strongly wavelength-dependent:
at 550 nm the short component is predominant (ca. 80%)
whereas at 475 nm it is practically negligible (ca. 10%). On the
Figure 7. Schematic energy-level diagram and photophysical processes
taking place in [Ir(dfppy)2-Carbo-Ir(dbpz)2]2+ bichromophoric complex.
G
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of [Ir(dbpz)2(bpy′)]+ simply shows that this molecule, which
lacks a substantial fragment (ethynyl-phenyl) of the bridging
ligand moiety, is not a fully appropriate model. Upon excitation
of the bichromophoric complex at 532 nm, the bridging-ligand
localized LC triplet is assumed to be directly populated, in
competition with the MLCT triplet, by intersystem crossing
from the MLCT singlet manifold. Upon excitation at 355 nm,
where both the donor and acceptor units absorb, it could be in
principle populated either by direct absorption of the acceptor
or by energy transfer from the donor in a time scale (ns)
shorter than that of the experiment (8 ns). A schematic repre-
sentation of the processes that occur upon laser excitation of the
bichromophoric complex is reported in Figure 7.
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CONCLUSION
■
We succeed in the synthesis of a multichromophoric Ir(III)
complexes in which two disparate Ir fragments of different
energy are linked by an inert and transparent para-closo carborane
spacer. The logical construction of the mixed complex is con-
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ASSOCIATED CONTENT
* Supporting Information
General methods, traces for NMR spectra, excitation spectrum of
[Ir(dfppy)2-Carbo-Ir(dbpz)2]2+, emission decay of the [Ir(dfppy)2-
Carbo-Ir(dbpz)2]2+, and transient absorption spectra of [Ir(dfppy)2-
Carbo-Ir(dbpz)2]2+. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: idm@unife.it (M.T.I.); ziessel@unistra.fr (R.Z.).
Notes
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ACKNOWLEDGMENTS
■
We thank the Centre National de la Recherche Scientifique
(CNRS) for financial support of this work. M.T.I. thanks
R. Argazzi for the 77 K emission measurements and F. Scandola
for fruitful discussions.
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descriptions seem to be more appropriate.7
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dx.doi.org/10.1021/ic302222q | Inorg. Chem. XXXX, XXX, XXX−XXX