Long-lived room-temperature deep-red-emissive intraligand triplet excited state of naphthalimide in cyclometalated IrIII complexes and its application in triplet-triplet annihilation-based upconversion

Article abstract of DOI:10.1002/chem.201200224

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2-phenylpyridine, bpy=2,2'-bipyridine) in which naphthal (Ir-2) and naphthalimide (NI) were attached onto the ppy (Ir-3) and bpy ligands (Ir-4) through acetyli

Full text of DOI:10.1002/chem.201200224

DOI: 10.1002/chem.201200224
Long-Lived Room-Temperature Deep-Red-Emissive IntraACTHNUTRGNENGlU iACHUTNGTRENNUGgand Triplet
Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its
Application in Triplet-Triplet Annihilation-Based Upconversion
Jifu Sun, Wanhua Wu, and Jianzhang Zhao*[a]
Abstract: Cyclometalated IrIII com-
plexes with acetylide ppy and bpy li-
gands were prepared (ppy=2-phenyl-
lem =627 nm, Fp =0.3%; cf. Ir-1: e=
16600mꢀ1 cmꢀ1 at 382 nm, tem =1.16 ms,
Fp =72.6%). Ir-3 was strongly phos-
phorescent in non-polar solvent (i.e.,
toluene), but the emission was com-
pletely quenched in polar solvents
(MeCN). Ir-4 gave an opposite re-
sponse to the solvent polarity, that is,
stronger phosphorescence in polar sol-
vents than in non-polar solvents. Emis-
sion of Ir-1 and Ir-2 was not solvent-po-
larity-dependent. The T1 excited states
of Ir-2, Ir-3, and Ir-4 were identified as
(3IL) by their small thermally induced
Stokes shifts (DEs), nanosecond time-
resolved transient difference absorp-
tion spectroscopy, and spin-density
analysis. The complexes were used as
triplet photosensitizers for triplet-trip-
let annihilation (TTA) upconversion
and quantum yields of 7.1% and
14.4% were observed for Ir-2 and Ir-3,
respectively, whereas the upconversion
was negligible for Ir-1 and Ir-4. These
results will be useful for designing visi-
pyridine,
bpy=2,2’-bipyridine)
in
which naphthal (Ir-2) and naphthali-
mide (NI) were attached onto the ppy
(Ir-3) and bpy ligands (Ir-4) through
acetylide bonds. [Ir
also prepared as a model complex.
Room-temperature phosphorescence
ACHTUNGRTEN(NGNU ppy)3] (Ir-1) was
was observed for the complexes; both
neutral and cationic complexes Ir-3 and
Ir-4 showed strong absorption in the
visible range (e=39600mꢀ1 cmꢀ1 at
402 nm and e=25100mꢀ1 cmꢀ1 at
404 nm, respectively), long-lived triplet
ble-light-harvesting
transition-metal
mainly intra
G
N
complexes and for their applications as
triplet photosensitizers for photocataly-
sis, photovoltaics, TTA upconversion,
etc.
Keywords: density functional calcu-
lations · iridium · naphthalimide ·
photochemistry · upconversion
excited
states
(tT =9.30 ms
and
16.45 ms) and room-temperature red
emission (lem =640 nm, Fp =1.4% and
Introduction
sorption in the visible-light range,[8–10,15,26,30–32] and only very
few cyclometalated IrIII complexes have shown intense visi-
ble-light absorption.[7,18a,33] Moreover, the lifetimes of the T1
excited states of the normal IrIII complexes are short (usual-
ly less than 5.0 ms).[1,8–12,15,18b,19,34,35] We proposed that these
photophysical processes could be enhanced by the long-
lived T1 excited states, as has been demonstrated in lumines-
cent oxygen sensing[36–45] and in triplet-triplet annihilation
(TTA) upconversion.[43–53] Thus, the preparation of cyclome-
talated IrIII complexes with intense visible-light absorption
and long-lived T1 excited states is of great interest.
Cyclometalated IrIII complexes have attracted much atten-
tion owing to their applications in electroluminescence,[1–15]
molecular sensing,[1,16–23] and photocatalysis.[24–29] The photo-
physics of these IrIII complexes, such as UV/Vis absorption,
phosphorescence, and the lifetimes of the triplet excited
states (T1), are pivotal for their applications. For example,
cyclometalated IrIII complexes have been used as photocata-
lysts in hydrogen (H2) production[24–28] and oxygen (O2) sens-
ing,[18–21] and as luminescent molecular sensors.[30] One of the
main objectives in these applications is to prepare IrIII com-
plexes that show intense absorption of visible light, or to
access the long-lived T1 excited state. However, convention-
al cyclometalated IrIII complexes typically show weak ab-
However, compared to PtII complexes and RuII com-
plexes, the photophysics of cyclometalated IrIII complexes
that show intense visible-light absorption and long-lived T1
excited state have been less investigated. The absorption
and emission wavelengths, as well as the lifetimes of the T1
excited states of RuII complexes, are readily tuned by ligand
modification.[38,54–56] However, for cyclometalated IrIII com-
plexes, the relationship between the molecular structure and
photophysical properties is far from clear. For example,
ligand modification is known to have a significant effect on
the emission property of the IrIII complexes, but sometimes
the photophysics of the IrIII coordination center and the
chromophore have collapsed in their dyads. For example,
boron dipyrromethene (BODIPY) was attached onto cyclo-
[a] J. Sun, W. Wu, Prof. J. Zhao
State Key Laboratory of Fine Chemicals
E-208, West Compus
Dalian University of Technology
Dalian 116024 (China)
Fax : (+86)411-8498-6236
E-mail: zhaojzh@dlut.edu.cn
Homepage: http://finechem.dlut.edu.cn/photochem
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200224.
8100
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

FULL PAPER
metalated IrIII complexes but the phosphorescence emission
was completely quenched.[33a]
Recently, by using a direct-cyclometalation strategy or the
connection of a fluorophore to the transition-metal center
ligands on the IrIII complex through acetylide bonds. This
ꢁ
C C linker assured efficient electronic communication be-
tween the NI moiety and the coordination center; thus, the
T1 excited state of the IrIII coordination center could be
3
ꢁ
through an ethynyl bond (C C), we prepared cyclometalat-
drastically perturbed and it was more likely to attain the IL
ed PtII complexes with a naphthalimide (NI) ligand,[36,52] and
we found that the photophysical properties of the PtII com-
plexes were tuned significantly by the NI moiety. For exam-
ple, compared to the model complexes, intensified visible-
light absorption, red-shifted emission, and prolonged T1 ex-
cited-state lifetimes were obtained for the NI-containing PtII
complexes. We also obtained an exceptionally long T1 excit-
ed-state lifetime with an N^N PtII bis-acetylide complex that
contained NI acetylide ligands.[37,57] Coumarin and BODIPY
were incorporated into the IrIII complexes and long-lived
triplet excited states were observed.[7a,18a,33a,49] However, to
the best of our knowledge, the NI ligand has never been
used for synthesis of the cyclometalated IrIII complexes.
Herein, for the first time, the NI moiety was used for the
preparation of both neutral (Ir-3) and cationic (Ir-4) cyclo-
metalated IrIII complexes. Cyclometalated neutral IrIII com-
plex with a naphthal ligand (Ir-2) and the model complex
(Ir-1) were also prepared for comparison. By using steady-
state absorption and -emission, 77 K emission, nanosecond
time-resolved transient difference absorption spectroscopy,
and density functional theory (DFT) and time-dependent
DFT (TDDFT) calculations, we confirmed that the emissive
excited states of complexes Ir-2, Ir-3, and Ir-4 were mainly
excited states because the 3IL excited state resided in a
3
lower energy level than the MLCT excited state of the IrIII
coordination center.
4-Bromo-1,8-naphthalic anhydride (2) was used as the
starting material and functionalized with alkylamines to pre-
pare the N-alkyl NI moiety to improve the solubility of the
final complexes (Scheme 1). Attachment of the NI moiety
onto the ppy or bpy ligands was achieved with the Pd0-cata-
lyzed Sonogashira coupling reactions. Complex Ir-2, which
contained a naphthalene moiety, and parent complex Ir-1
were also prepared for comparison of the photophysical
properties. All of the complexes were obtained in moderate
to satisfactory yields.
UV/Vis absorption of cyclometalated iridium (III) com-
plexes: UV/Vis absorption spectra of the cyclometalated IrIII
complexes are shown in Figure 1. Ir-1 (e=16600mꢀ1 cmꢀ1 at
382 nm) and Ir-2 (e=49100mꢀ1 cmꢀ1 at 348 nm) showed
intraACHTUNGTRENNUNGliACHTUNGTRENNUNG
gand triplet excited states (3IL), whereas for Ir-1, the
T1 excited state was mainly a metal-to-ligand-charge-transfer
triplet excited state (3MLCT). These new complexes showed
intense visible-light absorption and the lifetimes of the T1
excited states were much longer than that of the model com-
plex (Ir-1). The cyclometalated IrIII complexes were used as
triplet photosensitizers for TTA-based upconversion and an
upconversion quantum yield of up to 14.4% was observed
for Ir-3. Our complementary experimental and theoretical
data will be useful for the design of visible-light-harvesting
cyclometalated IrIII complexes that show long-lived T1 excit-
ed states, as well as for the application of these complexes
in photovoltaics, photocatalysis, photodynamic therapies
(PTD), and upconversion, etc.
Figure 1. UV/Vis absorption spectra of complexes Ir-1, Ir-2, Ir-3, and Ir-4
(toluene, 1.0 ꢁ10ꢀ5 m, 258C).
maxima in the UV range and only very weak absorption in
the visible-light range. Strong absorption bands at 285 nm
are typical for cyclometalated IrIII complexes. For Ir-3 and
Ir-4, the absorption in the visible-light range was more-in-
tense than that of Ir-1 and Ir-2. For example, the molar ex-
tinction coefficient of Ir-3 at 402 nm was 39600mꢀ1 cmꢀ1,
which was much larger than that of Ir-1 and Ir-2. Ir-4 also
showed strong absorption at 404 nm (e=25100mꢀ1 cmꢀ1).
The absorption of Ir-3 in the visible-light range was much
stronger than the typical absorption of the IrIII com-
plexes.[10,26,31,32,35]
Results and Discussion
Synthesis of the cyclometalated iridium (III) complexes: To
enhance the UV/Vis absorption of these complexes and to
3
access their long-lived IL excited states, our principle strat-
egy was to extend the p-conjugation of the ppy or bpy li-
gands by adding arylacetylide ligands (Scheme 1).[36,38] The
aryl acetylides were judiciously selected to perturb the
3MLCT excited state of the parent coordination structure
Photoluminescence of the cyclometalated iridium (III) com-
plexes: Room-temperature (RT) phosphorescence of the NI
ligand: All of the IrIII complexes were phosphorescent at
room temperature, but the phosphorescence quantum yields
varied dramatically (Figure 2 and Table 1). The phosphores-
cence quantum yields for Ir-1, Ir-2, Ir-3 and Ir-4 were FP =
0.726, 0.186, 0.014, and 0.003, respectively. The parent (un-
3
(e.g., Ir-1), thereby allowing access to the long-lived IL ex-
cited states.[38,57–63] Arylacetylides are rarely used for the
preparation of cyclometalated IrIII complexes.[33a] The NI
and the naphthal moieties were connected to the ppy or bpy
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8101

J. Zhao et al.
Scheme 1. Preparation of IrIII complexes Ir-1, Ir-2, Ir-3, and Ir-4; the molecular structures of the triplet acceptor 9,10-diphenylanthracene (DPA) and
ruthenium(II) complex [Ru
2-ethoxyethanol, Ar, 1008C, 18 h; b,e) [{Ir
ethynylphenyl)pyridine, [Pd(PPh3)2Cl2], PPh3, CuI, NEt3, EtOH, Ar, reflux, 8 h; f) 5-ethynyl-2,2’-bipyridine, [Pd
reflux, 8 h; g) [{Ir(ppy)2Cl}2], CH2Cl2/MeOH (2:1 v/v), Ar, reflux, 6 h.
ACHUTGTNRNENUG(dmb)3]ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
the NI ligand. We noticed that the emission band of Ir-1 was
much-less structured than the other complexes. Typically, a
3
structured emission band indicates a IL emission, whereas a
structure-less emission band indicates that the emission is
3
from a MLCT excited state.
Notably, the fluorescence of ligands L-2, L-3, and L-4 was
completely quenched in complexes Ir-2, Ir-3, and Ir-4, re-
spectively (for the emission of the ligands, see the Support-
ing Information), which indicated efficient intersystem
crossing (ISC) from the singlet excited states to the triplet
excited states. However, this ISC is not always the case for
Figure 2. Emission spectra of complexes Ir-1, Ir-2, Ir-3, and Ir-4: a) Com-
parison of the emission intensities of Ir-1, Ir-2, Ir-3, and Ir-4 under the
same conditions; b) normalized emission spectra of Ir-3 and Ir-4. Ir-
1: lex =383 nm; Ir-2: lex =348 nm; Ir-3: lex =402 nm; Ir-4: lex =405 nm (in
deaerated toluene, 1.0 ꢁ10ꢀ5 m, 258C).
cyclometalated IrIII complexes. For example, with
a
BODIPY fluorophore was attached to an IrIII coordination
center by using a similar approach, the fluorescence emis-
sion of the BODIPY could not be completely quenched;
rather, the phosphorescence of the IrIII coordination center
was quenched instead.[33a] Recently, cyclometalated IrIII com-
plexes with thiophene ligands were found to be fluores-
cence/phosphorescence dual emissive.[64] Thus, the efficient
ISC in Ir-2–Ir-4 may have been due to the direct connection
substituted) complex Ir-1 gave an intense emission at
510 nm. The emission band of Ir-2 was red-shifted by 61 nm
(571 nm). For Ir-3, a weak emission band was observed at
640 nm. However, the emission of cationic complex Ir-4 was
the weakest and was blue-shifted by 13 nm compared to the
neutral complex (Ir-3, Figure 2b). We proposed that both Ir-
3 and Ir-4 showed room-temperature phosphorescence of
8102
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

Triplet Excited State of Naphthalimide in IrIII Complexes
FULL PAPER
Table 1. Photophysical parameters of the ligands and cyclometalated IrIII complexes (in toluene, 1.0ꢁ10ꢀ5 m,
258C).
center.[65] These cyclometalated
IrIII complexes adopted octahe-
dral geometries; thus the
quenching of the emission of Ir-
3 was unlikely to have been
caused by the coordination of
solvent molecules to the IrIII
center.[65] We proposed that the
sensitivity of the emission of Ir-
3 and Ir-4 to solvent polarity
was due to the charge-transfer
components of the T1!S0 tran-
sition (see below).
[f]
labs [nm][a]
e[b]
lem [nm][c]
F
tem
tT [ms][g]
tT [ns][h]
[i]
[i]
L2
L3
L4
Ir-1
Ir-2
Ir-3
Ir-4
235/334
382/402
379/400
286/382
286/348/363
291/402
53400/47700
48200/43300
37700/33700
427
428
418
510
571
640
627
0.826[d]
0.897[d]
0.731[d]
0.726[e]
0.186[e]
0.014[e]
0.003[e]
0.90 ns
–
–
–
–
–
–
[i]
[i]
1.74 ns
1.47 ns
1.16 ms
6.98 ms
10.15 ms
16.36 ms
[i]
[i]
54600/16600
1.34
6.94
9.30
29.4
92.3
313.7
387.3
48300/49100/46300
48300/39600
21000/28300/25100
291/383/404
16.45
[a] Absorption maxima (in toluene, 1.0ꢁ10ꢀ5 m, 258C); [b] molar extinction coefficient at the absorption
maxima (e: mꢀ1 cmꢀ1); [c] emission maxima; [d] fluorescence quantum yield with quinine sulfate as the stand-
ard (FF =0.547 in 0.05m sulfuric acid); [e] phosphorescence quantum yield with complex [RuACHTNUTRGNEN(UG dmb)3]CAHUTNGTRENN[UNG PF6]2 as
a standard (Fp =0.073 in MeCN); [f] luminescence lifetimes; [g] triplet excited state lifetimes, measured by
nanosecond time-resolved transient difference absorption in N2; [h] triplet excited state lifetimes, measured by
nanosecond time-resolved transient difference absorption in air; [i] not applicable.
The photophysical properties
of these complexes are summar-
ized in Table 1. The T1 excited
of the p-core of the fluorophore to the coordination centers,
which was not the case for some of the previous IrIII com-
plexes that contained a bulky organic chromophore.[33a,64]
The emission of Ir-3 was highly sensitive to the polarity of
the solvent (Figure 3c). For example, the emission in tol-
uene was relatively strong but it was completely quenched
state lifetime of Ir-4 was the longest (16.45 ms). The phos-
phorescence lifetimes of Ir-2, Ir-3, and Ir-4 were longer than
those of Ir-1 and other typical cyclometalated IrIII com-
plexes.[1,10,12,19,22,23,25,31,32,35,66–68] Thus, we proposed that the T1
3
excited states of Ir-2, Ir-3, and Ir-4 were mainly due to IL
excited states. Usually, emission from 3IL states gives low
phosphorescent quantum yields.[69,70] Furthermore, the T1 ex-
cited-state lifetimes of all of the complexes in air-saturated
solutions were much shorter than those in N2-saturated solu-
tions; thus, these results confirmed that the long-lived excit-
ed states were T1 states (see the Supporting Information).
The photophysical properties of the complexes were com-
pared with known IrIII complexes that contained ligands
with extended p-conjugation frameworks.[33] The BODIPY-
containing cyclometalated IrIII complex showed no RT phos-
phorescence,[33a] whereas an IrIII complex with styryl-tpy li-
gands showed much-shorter phosphorescence lifetimes
AHCTUNGTRENNUNG
(<2 ms) and lower phosphorescence quantum yields.[33b]
Emission spectra of the complexes at 77 K: To study the
emissive excited states of these IrIII complexes, their emis-
sions at 77 K and RT were compared (Figure 4).[34,71] Basi-
cally, the vibration progression of the emission spectra
became more significant at 77 K. The emission of Ir-1 at
77 K was blue-shifted by 19 nm compared to that at RT;
thus, the thermally induced Stokes shift (DES) of Ir-1 was
755 cmꢀ1. However, for Ir-2, Ir-3, and Ir-4, the DES values
were 31, 148, and 156 cmꢀ1, respectively, which were much
smaller than that of Ir-1. A large DES value is an indication
of a 3MLCT emissive state, whereas small DES values usually
indicate a 3IL excited state.[34,57,71] Based on the emission
spectra at 77 K and RT, we proposed that the emission of Ir-
Figure 3. Solvent-polarity-dependence of the emission of the complexes
(1.0ꢁ10ꢀ5 m): a) Ir-1, lex =383 nm; b) Ir-2, lex =348 nm; c) Ir-3, lex
=
402 nm; d) Ir-4, lex =405 nm. The solutions were bubbled with N2 for
about 30 min before the measurements were taken, 258C.
in MeCN. However, the UV/Vis absorption of Ir-3 was
almost independent of solvent polarity (see the Supporting
Information). The emission of cationic complex Ir-4 also
varied dramatically in different solvents but there was no
clear trend with variation of the solvent polarity. For exam-
ple, the emission of Ir-4 was strongest in CH2Cl2 but it was
weakest in toluene (Figure 3d). Unlike Ir-3 and Ir-4, the
emission of Ir-1 and Ir-2 was independent of solvent polarity
(Figure 3a,b). It has been reported that the phosphores-
cence of a C^N PtII acac complex can be significantly
quenched by a weak Lewis base, such as CH2Cl2 and
MeCN; in this case, the quenching was due to the coordina-
tion of the solvent molecules to the electron-deficient PtII
3
2, Ir-3, and Ir-4 were due to IL emissive states, whereas the
emissive state of Ir-1 was characterized by a more-significant
3MLCT state.
Nanosecond time-resolved transient difference absorption of
the IrIII complexes: Assignment of the emissive excited
state: To study the T1 excited states of the IrIII complexes,
nanosecond time-resolved transient difference absorption
spectroscopy was performed (Figure 5). For Ir-1 (Figure 5a),
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8103

J. Zhao et al.
Figure 6. Electron-density maps of the frontier molecular orbitals of Ir-1;
based on the optimized ground-state geometry calculated by DFT calcu-
lations at the B3LYP/6-31 g(d)/LanL2DZ level with Gaussian 09W.
Figure 4. Normalized emission intensity of the IrIII complexes at RT
(258C) and 77 K in 2-methyltetrahydrofuran (1.0ꢁ10ꢀ5 m): a) Ir-1, lex
=
400 nm; b) Ir-2, lex =410 nm; c) Ir-3, lex =402 nm; d) Ir-4, lex =405 nm.
The solutions were purged with N2 for about 15 min before the measure-
ments were taken.
transient absorption spectra, we concluded that the emissive
excited states of Ir-3 and Ir-4 were 3IL emissive excited
states, whilst that of Ir-1 was characterized by a more-signifi-
3
cant MLCT state.
TDDFT calculations: Assignment of the UV/Vis absorption
and emissive excited states: TDDFT calculations have been
used to rationalize the photophysics of transition-metal com-
plexes.[8,12,30,66,72–74] Previously, we have used TDDFT calcu-
lations to explain the UV/Vis absorption and emission prop-
erties of PtII–bis-acetylide complexes, polyimine RuII com-
plexes, and phosphorescent molecular probes.[36–53,75,76]
Herein, we carried out TDDFT calculations to rationalize
the UV/Vis absorption and phosphorescence emission prop-
erties of the IrIII complexes.
The vertical excitations (i.e., UV/Vis absorption) of Ir-1
are listed in Table 2. The calculations predicted weak transi-
tions at 428, 398, 387, and 361 nm. The calculated data were
in agreement with the experimental results (Figure 1). The
IrIII atom contributed to the transitions, which could be as-
signed as MLCT or LLCT transitions (Figure 6).
The T1 excited state of Ir-1 was calculated and the major
S0!T1 transition was a HOMO!LUMO transition, which
could be assigned as LLCT or MLCT; this result was in
agreement with the photophysics of the typical cyclometalat-
ed IrIII complexes. The calculated S0!T1 energy gap was
2.64 eV (470 nm), which was in good agreement with the ob-
served RT phosphorescence emission at 510 nm (Figure 2a
and Table 1).
The excitations of Ir-2 were also studied by TDDFT cal-
culations (Table 3 and Figure 7), and the UV/Vis absorption
bands could be assigned as MLCT, IL, or LLCT. The calcu-
lated excitations were in good agreement with the experi-
mental data (Figure 1). The S0!T1 energy gap was calculat-
ed to be 590 nm, which was close to the experimentally ob-
served RT phosphorescence emission of Ir-2 (571 nm, Fig-
ure 2a). Based on the components of the T1 excited states,
the transitions were assigned as LLCT/MLCT/IL. Noticea-
bly, the 3IL that was localized on the naphthalene-ppy
Figure 5. Nanosecond time-resolved transient difference absorption spec-
tra of a) Ir-1; b) Ir-2; c) Ir-3; and d) Ir-4 (in deoxygenated toluene, 1.0ꢁ
10ꢀ5 m, 258C). Arrows indicate the elapsed time after a 355 nm laser
flash.
a transient absorption was observed at 320 nm. The signifi-
cant bleaching at 525 nm was due to the strong phosphores-
cence emission of the complex (FP =0.726). However, for
Ir-3, a notably different transient profile was observed (Fig-
ure 5c): Significant bleaching was observed at 405 nm and
380 nm, which were due to bleaching of the ground state;
this bleaching was consistent with the strong UV/Vis absorp-
tion of the complex in this range (Figure 1). Furthermore,
strong transient absorption of Ir-3 was found at 620 nm and
658 nm, which was characteristic of an NI-localized T1 excit-
ed state.[57] Thus, we concluded that the T1 excited state in
3
Ir-3 was an NI-localized IL excited state. Ir-2 and Ir-4 also
showed similar transient absorption (Figure 5b,d). From the
8104
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

Triplet Excited State of Naphthalimide in IrIII Complexes
FULL PAPER
Table 2. Electronic excitation energies [eV], oscillator strengths [f], main configura-
tions, and CI coefficients of the low-lying electronic excited states of Ir-1; calculated
by TDDFT//B3LYP/6-31G(d)/LanL2DZ based on the optimized ground-state geome-
tries.
The calculated UV/Vis absorption of Ir-3 was in
good agreement with the experimental results
(Table 4 and Figure 1). The weak absorption band
at 537 nm was attributed to the HOMO!LUMO
and HOMO!LUMO+1 transitions. Based on the
calculated frontier molecular orbitals (Figure 8),
these transitions were mixed LLCT/MLCT (ppy!
NI-ppy and Ir!ppy-NI) states. The intense absorp-
tion at 433 nm was assigned to the IL transition
(HOMOꢀ3!LUMO). This result showed that the
intensified absorption at 433 nm was due to the NI-
ppy ligand (for the UV/Vis absorption of NI-ppy).
TDDFT//B3LYP/6-31G(d)
Electronic
transition
Energy[a]
[eV/nm]
f[b]
Composition[c]
CI[d]
Character
ACHTUNGTRENNUNG
S0!S1
S0!S5
2.90/428
3.11/398
0.0006
0.0225
H!L
0.6963
0.6482
0.1521
0.1607
0.4371
0.2384
0.1517
0.4369
0.3343
0.3316
0.1519
0.1803
0.1598
0.5557
LLCT/MLCT
MLCT/LLCT
LLCT
LLCT/MLCT
LLCT/MLCT
LLCT
Hꢀ2!L
Hꢀ2!L+2
Hꢀ1!L+1
Hꢀ2!L+1
Hꢀ1!L
S0!S8
3.20/387
0.0540
Hꢀ1!L+1
Hꢀ1!L+2
Hꢀ2!L+2
Hꢀ1!L+1
Hꢀ2!L+1
Hꢀ1!L+1
Hꢀ1!L+2
H!L
LLCT/MLCT
LLCT/MLCT The absorption at 390 nm was assigned to
LLCT
S0!S10
S0!T1
3.43/361
2.64/470
0.0478
HOMOꢀ2!LUMO+1,
HOMOꢀ2!LUMO+2,
LLCT/MLCT
LLCT/MLCT
LLCT/MLCT
LLCT/MLCT
LLCT/MLCT
HOMOꢀ2!LUMO+3 transitions, etc., with mixed
0.0000[e]
IL/MLCT/LLCT states.
The calculated S0!T1 energy gap was 671 nm
(1.85 eV), which was close to the experimentally
observed RT phosphorescence emission of Ir-3
(640 nm, Figure 2a). Based on the components of
the T1 excited state (HOMOꢀ3!LUMO,
[a] Only selected low-lying excited states are presented; [b] oscillator strengths;
[c] only the main configurations are presented; [d] the CI coefficients are in absolute
values; [e] no spin-orbital coupling effect was considered, thus the f value was zero.
HOMOꢀ2!LUMO,
HOMO!LUMO,
etc.;
Table 4), the major components of the transitions
were assigned as NI-localized. Thus, we proposed
Table 3. Electronic excitation energies [eV], oscillator strengths [f], main configura-
tions, and CI coefficients of the low-lying electronic excited states of Ir-2; calculated
by TDDFT//B3LYP/6-31G(d)/LanL2DZ based on the optimized ground-state geome-
tries.
3
that the T1 excited state was in a IL state, which
was consistent with the long-lived T1 excited state
of Ir-3 (tT =9.3 ms, Table 1).
TDDFT//B3LYP/6-31G(d)
Electronic
transition
Energy[a]
[eV/nm]
f[b]
Composition[c]
CI[d]
Character
The calculated UV/Vis absorption of Ir-4 (see the
Supporting Information) was similar to that of Ir-3,
which was in good agreement with the experiment
results (Table S1 in the Supporting Information and
Figure 1). The calculated S0!T1 energy gap was
663 nm (1.85 eV), which was close to the experi-
mentally observed RT phosphorescence emission of
Ir-4 (627 nm, Figure 2). The T1 excited state of Ir-4
ACHTUNGTRENNUNG
S0!S1
2.70/459
0.0399
Hꢀ3!L
Hꢀ2!L
H!L
0.1123
0.1606
0.6536
0.5182
0.2008
0.1283
0.1840
0.1539
0.1364
0.2204
0.1236
0.1173
0.1208
0.3833
0.1149
0.2726
0.1816
0.3629
0.4007
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
LLCT/MLCT
LLCT/MLCT
MLCT/ILCT
MLCT/ILCT
LLCT/MLCT
LLCT/MLCT
LLCT MLCT
MLCT/ILCT
LLCT/MLCT
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
ILCT
S0!S8
3.19/389
0.7678
0.1319
Hꢀ3!L
Hꢀ2!L
Hꢀ2!L+2
Hꢀ1!L+2
H!L
3
was also in the IL state, which was consistent with
S0!S10
3.27/379
2.10/590
Hꢀ3!L
Hꢀ3!L+2
Hꢀ2!L+1
Hꢀ2!L+2
Hꢀ1!L+1
Hꢀ1!L+2
H!L
the long-lived T1 excited state of Ir-4 (t=16.45 ms,
Table 1).
The difference between complexes Ir-2, Ir-3, and
Ir-4 was that the IrIII center in Ir-2 was more in-
volved in the transitions than in Ir-3 and Ir-4; that
is, the IL feature of the excitation transitions of Ir-2
was less than those of Ir-3 and Ir-4.
S0!T1
0.0000[e]
Hꢀ3!L
Hꢀ2!L
Hꢀ1!L
H!L
MLCT/ILCT
Spin-density surfaces of the triplet excited states:
To support the assignment of the T1 excited states
of the IrIII complexes from a theoretical perspective,
we studied the spin-density surfaces of the triplet
states of the complexes (Figure 9). For Ir-1, the
[a] Only selected low-lying excited states are presented; [b] oscillator strength;
[c] only the main configurations are presented; [d] the CI coefficients are in absolute
values; [e] no spin-orbital coupling effect was considered, thus the f value was zero.
ligand was clearly recognizable (Figure 7). Thus, we pro-
posed that the T1 excited state was a 3IL state to some
extent, which was consistent with the long-lived T1 excited
state of Ir-2 (t=6.94 ms, Table 1) and it was in agreement
with the 77 K emission spectrum (Figure 4b) and the transi-
ent difference absorption spectrum (Figure 5b). The lifetime
of Ir-2 was longer than those of typical IrIII complex-
es.[1,8–12,15,18b,19,34,35]
spin-density was distributed on the IrIII atom and on the ppy
ligand. This result was in agreement with the LLCT/MLCT
assignment of Ir-1. For Ir-2, Ir-3, and Ir-4, only the acetylide-
linked ppy-naphthalene, bpy-naphthalene, or ppy-NI moiet-
3
ies contributed to the triplet excited states. Thus, IL T1 ex-
cited states were assigned for Ir-2, Ir-3 and Ir-4.[74]
The 77 K emission spectra, nanosecond time-resolved
transient difference spectra, and DFT calculations suggested
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8105

J. Zhao et al.
required T1 excited states for sensitization. We chose to
study TTA upconversion. Among the various upconversion
schemes,[77,78] TTA upconversion was of particular interest
owing to its low excitation power (0.1 Wcmꢀ2, the same
scale as terrestrial solar radiation).[46,79–82] Furthermore, the
excitation and the emission wavelength of the TTA upcon-
version could be readily tuned by independent selection of
the triplet sensitizers and the acceptors/emitters, two compo-
nents of the TTA upconversion.
The basic TTA-upconversion mechanism was the photo-
excitation of the triplet photosensitizer (e.g., S0!S1), fol-
lowed by population of the T1 excited state by ISC. Triplet-
triplet energy-transfer (TTET) from the sensitizer to the ac-
ceptor would produce the T1 excited state of the acceptor.
Collision of the triplet acceptors would produce the singlet
excited state of the acceptor, with an overall proba-
Figure 7. Electron-density maps of the frontier molecular orbitals of Ir-2;
based on the optimized ground-state geometry calculated by TDDFT cal-
culations at the B3LYP/6-31 g(d)/LanL2DZ level with Gaussian 09W.
Table 4. Electronic excitation energies [eV], oscillator strengths [f], main configura-
tions, and CI coefficients of the low-lying electronic excited states of Ir-3; calculated
by TDDFT//B3LYP/6-31G(d)/LanL2DZ based on the optimized ground-state geome-
tries.
bility of at least 11.1%, as determined by the spin
statistical law.[79,83]
Currently, the typical triplet photosensitizers for
TTA upconversion are PtII/PdII porphyrin com-
plexes.[79,81,84] Previously, the cyclometalated IrIII
TDDFT//B3LYP/6-31G(d)
Electronic
transition
Energy[a]
[eV/nm]
f[b]
Composition[c]
CI[d]
Character
ACHTUNGTRENNUNG
complex [IrACTHUNRGTNEUNG(ppy)3] was used as a triplet sensitizer
S0!S1
2.31/537
0.0251
H!L
H!L+1
Hꢀ3!L
Hꢀ2!L+1
Hꢀ2!L+2
Hꢀ1!L+2
Hꢀ3!L
Hꢀ2!L
Hꢀ1!L
H!L
0.6845
0.1552
0.6823
0.5229
0.1282
0.2925
0.1721
0.5440
0.2919
0.1124
LLCT/MLCT
MLCT/LLCT
LLCT/MLCT
MLCT/IL
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
ILCT
for TTA upconversion but the lifetime of the com-
plex was short (t=1.34 ms) and the excitation had
to be performed in the UV range; the upconversion
quantum yield was not provided.[85] Because Ir-2, Ir-
3, and Ir-4 showed intense absorption in the visible-
light range, these complexes with long-lived T1 ex-
cited states could be used as triplet photosensitizers
for the TTA upconversion (for the Jablonski dia-
gram of the TTA upconversion, see Scheme 2).
9,10-Diphenylanthracene (DPA) was used as the
triplet acceptor owing to its appropriate T1 excited-
S0!S4
2.86/433
3.19/390
0.9062
0.0542
S0!S10
S0!T1
1.85/671
0.0000[e]
MLCT/ILCT
LLCT/MLCT
[a] Only selected low-lying excited states are presented; [b] oscillator strength;
[c] only the main configurations are presented; [d] the CI coefficients are in absolute
values; [e] no spin-orbital coupling effect was considered, thus the f values were zero.
Figure 8. Electron-density maps of the frontier molecular orbitals of Ir-3;
based on the optimized ground-state geometry calculated by TDDFT cal-
culations at the B3LYP/6-31 g (d)/LanL2DZ level with Gaussian 09W.
Figure 9. Isosurfaces of the spin density of IrIII complexes Ir-1, Ir-2, Ir-3,
and Ir-4 (in toluene) at the optimized triplet-state geometry; calculated
at the B3LYP/6-31G/LANL2DZ level with Gaussian 09W.
that the T1 excited states of Ir-2, Ir-3, and Ir-4 were intra
ACHTUNGERTNlNUNG i-
ACHTUNGTRENNUNG
3
3
of Ir-1 was mainly a MLCT state. The long-lived IL states
are significant for the application of these IrIII complexes.
state energy level (1.77 eV, 700 nm).[86] Upon laser excitation
at 445 nm, the complexes showed different phosphorescence
emission intensities (Figure 10a): Ir-1 gave the strongest
emission owing to its high phosphorescence quantum yield.
Ir-2 gave a similar e value at 445 nm, but its emission was
much weaker than Ir-1. Ir-3 gave very weak emission, whilst
Application of the visible-light-harvesting and long-lived T1
excited states for TTA upconversion: Next, we used the en-
hanced visible-light absorption and the long-lived T1 excited
states of Ir-2, Ir-3, and Ir-4 in photophysical processes that
8106
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

Triplet Excited State of Naphthalimide in IrIII Complexes
FULL PAPER
sion. However, for Ir-1, no upconverted emission was ob-
served in the range 380–550 nm, which indicated that Ir-1
was not efficient at sensitizing the TTA upconversion under
these experimental conditions because of its poor absorption
at 445 nm (e=4559mꢀ1 cmꢀ1, Figure 1) and short T1 excited-
state lifetime (tT =1.34 ms, Table 1). Because excitation of Ir-
4 at 445 nm was not suitable, Ir-4 could not be excited effi-
ciently, and no upconversion was observed (Figure 10b), al-
though the lifetime of the T1 excited state was long (tT =
16.45 ms, Table 1).
Scheme 2. Qualitative Jablonski diagram that shows the sensitized TTA
process between the IrIII complexes (exemplified by Ir-3) and the triplet
acceptor DPA; the effect of visible-light-harvesting ability and the lumi-
nescence lifetime of the IrIII sensitizer on the efficiency of TTA-based up-
conversion is also shown. E is energy; GS is the ground state (S0);
1MLCT* is the IrIII-based metal-to-ligand charge-transfer singlet excited
The variation in the emission of the sensitizers with in-
creasing DPA concentration was investigated (Figure 11).
The emission of Ir-1 was partially quenched in the presence
of DPA, but no upconverted emission of DPA was observed
(Figure 11a). We proposed that the lack of upconversion
was due to the poor absorption of Ir-1 at 445 nm and to the
short-lived T1 excited state (tT =1.34 ms). However, for Ir-2,
the upconverted fluorescence of DPA was observed with
quenching of the phosphorescence of Ir-2 (Figure 11b). Sim-
ilar to Ir-1, Ir-2 also showed weak absorbance at 445 nm;
however, the T1 excited-state lifetime of Ir-2 (tT =6.94 ms)
was much longer than that of Ir-1. The upconversion quan-
tum yield with Ir-2 as the triplet sensitizer was 7.1%
(Table 5). Because excitation at 445 nm was not suitable for
state; ISC is intersystem crossing; 3IL* is the intra
ACTHUNRTGENNlGU iACHUTGTNERNNUGgand triplet excited
3
state (NI localized); TTET is triplet-triplet energy transfer; DPA* is the
1
triplet excited state of DPA; TTA is triplet-triplet annihilation; DPA* is
the singlet excited state of DPA. The emission band for the sensitizer
alone is the 3IL emissive excited state. The emission bands in the TTA
experiment are the simultaneous 3IL* emission (phosphorescence) and
1
the DPA* emission (fluorescence).
Figure 10. Upconversion with IrIII complexes as triplet sensitizers and
DPA as triplet acceptors (in deaerated toluene): a) The emission of the
complexes alone (laser excitation: 445 nm); b) upconversion emission
spectra of mixtures of the sensitizers and DPA upon selective excitation
of the sensitizers at 445 nm. Note the residual phosphorescence of the
sensitizers; [sensitizers]=1.0ꢁ10ꢀ5 m, [DPA]=6.0ꢁ10ꢀ5 m, 258C.
Ir-4 gave almost no emission because the absorption band of
Ir-4 in the visible-light range was very narrow and the ab-
sorption at 445 nm was weak (e=1128mꢀ1 cmꢀ1, Figure 1).
However, we considered that this property did not necessa-
rily mean that the efficiency of the S1!T1 ISC was low. In
other words, the low phosphorescence quantum yields of Ir-
3 and Ir-4 should not necessarily negate their application in
photophysical processes that are sensitized with triplet excit-
ed states. To confirm this concept, and to explore the appli-
cation of visible-light harvesting, as well as the long-lived T1
excited states of Ir-3 and Ir-4, the complexes were used as
triplet sensitizers for TTA-based upconversion (Figure 10).
In the presence of DPA, blue emission in the range 380–
550 nm was observed for Ir-2 and Ir-3, which was due to the
upconverted fluorescence of DPA (Figure 10b). Laser irra-
diation of DPA alone at 445 nm failed to produce the emis-
sion at 380–550 nm; thus, the blue emission of the mixed Ir-
2/DPA and Ir-3/DPA solutions was due to TTA upconver-
Figure 11. Upconverted fluorescence of DPA and the residual phosphor-
escence of the IrIII complexes with increasing DPA concentration (selec-
tive excitation at 445 nm): a) Ir-1; b) Ir-2; c) Ir-3; d) Ir-4 (in deaerated tol-
uene, 1.0ꢁ10ꢀ5 m, 258C).
Table 5. Upconversion-related parameters of the complexes.[a]
[b]
[c]
[d]
Fuc [%]
Ksv (ꢁ103) [mꢀ1
]
Kq (ꢁ109) [mꢀ1 Sꢀ1
]
Ir-1
Ir-2
Ir-3
Ir-4
Ru-1
0.0
7.1
14.4
0.0
5.2
24.1
25.8
30.9
3.9
4.50
3.46
2.54
1.89
5.30
1.6
[a] Measured in toluene, 1.0ꢁ10ꢀ5 m, 258C; [b] upconversion quantum
yields, measured with [Ru(dmb)3][PF6]2 as a standard (Fp =0.073 in
A
ACHTUNGTRENNUNG
CH3CN); [c] Stern–Volmer quenching constants (Ksv) of the quenching of
phosphorescence of IrIII complexes by triplet acceptor DPA; [d] bimolec-
ular quenching constants (kq), Ksv =Kq.
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8107

J. Zhao et al.
Ir-4, neither the phosphorescence of Ir-4 nor the upconvert-
ed fluorescence of DPA could be observed (Figure 11d), de-
spite the longest T1 excited lifetime (tT =16.45 ms, Table 1).
For Ir-3, a more-significant upconversion was observed (Fig-
ure 11c), which was attributed to the intense absorbance at
445 nm compared to that of Ir-1, Ir-2, and Ir-4 (Figure 1).
Furthermore, the T1 excited-state lifetime of Ir-3 (tT =
9.30 ms) was longer than that of Ir-1 and Ir-2. The upconver-
sion quantum yield with Ir-3 as the triplet sensitizer was up
to 14.4%.
Interestingly, the peak area of the upconverted fluores-
cence was much larger than the diminished quenched phos-
phorescence peak area; this result was apparently inconsis-
tent the photophysics of the TTA upconversion with phos-
phorescent triplet photosensitizers (Scheme 2).[79] We pro-
posed that the Ir-3 molecules at the T1 excited state that
were otherwise non-phosphorescent were involved the
TTET process, which was a key step in the cascade photo-
physical process of TTA upconversion (Scheme 2). Thus, we
expect that weakly phosphorescent or even non-phosphores-
cent complexes can be used as triplet sensitizers for TTA up-
conversion, as long as the T1 excited state of the complexes
can be populated upon photoexcitation.[48]
Figure 13. Stern–Volmer plots that were generated from the quenching of
the photoluminescence intensity of the IrIII complexes as a function of
DPA concentration in toluene: Ir-1 (lex =445 nm); Ir-2 (lex =460 nm); Ir-
3 (lex =445 nm); Ir-4 (lex =425 nm). [complex]=1.0ꢁ10ꢀ5 m in deaerated
toluene, 258C.
Next, the upconversion quantum yields were determined
(Table 5). For Ir-3, the upconversion quantum yield was
14.4%. For Ir-2, the upconversion quantum yield was 7.1%.
The upconversion quantum yields with Ir-1 and Ir-4 as the
sensitizers were almost zero. We attributed the high upcon-
version quantum yield of Ir-3 to its intense absorption at the
excitation wavelength and to the long-lived T1 excited state.
Previously, the theoretical limit of the TTA upconversion
quantum yield was predicted to be 11.1%. However, exam-
ples that exceeded this limit have been reported.[79]
The upconversion was visible to the naked eye
(Figure 14). The emission color changes were characterized
by CIE coordinates (Figure 14c,d). For Ir-1, no upconver-
sion was observed; thus, only the green emission of Ir-1 was
observed (laser excitation at 445 nm) and there was almost
no change in the CIE coordinates. For Ir-2, a yellow emis-
The efficiency of the TTET process of TTA upconversion
was quantitatively studied by investigating the quenching of
the phosphorescence emission of the IrIII complexes by DPA
(Figure 12). Stern–Volmer curves are shown in Figure 13. Ir-
2, Ir-3, and Ir-4 showed similar quenching effects in the pres-
ence of DPA with similar quenching constants (Table 5).
However, for Ir-1, the quenching curve had a much shallow-
er slope and the quenching constant was much smaller
(KSV =5.22ꢁ103 mꢀ1) than those of Ir-2 (KSV =2.41ꢁ104 mꢀ1),
Ir-3 (KSV =2.58ꢁ104 mꢀ1), and Ir-4 (KSV =3.09ꢁ104 mꢀ1
;
Table 5).
Figure 14. a) Photographs of the emission of the sensitizers alone and
b) photographs of the upconversion (mixed solution with triplet acceptor
DPA); c) CIE diagram of the emissions of the sensitizers alone and
d) CIE diagram of the emissions in the presence of DPA (upconversion).
The data of Ir-4 are not shown owing to the weak emission; lex =445 nm
(5 mW), in deaerated toluene, 1.0ꢁ10ꢀ5 m, 258C.
Figure 12. Phosphorescence emission spectra of the IrIII complexes with
increasing DPA concentration in deaerated toluene: a) Ir-1, lex
445 nm); b) Ir-2, lex =460 nm; c) Ir-3, lex =445 nm; d) Ir-4, lex =425 nm
(in toluene, 1.0 ꢁ10ꢀ5 m, 258C).
=
8108
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

Triplet Excited State of Naphthalimide in IrIII Complexes
FULL PAPER
sion was observed for the sensitizer alone. White emission
(CIE coordinates of 0.40, 0.36) was observed for Ir-2 in the
presence of DPA, owing to the upconverted fluorescence
emission of DPA in the blue region and to the residual
phosphorescence emission of Ir-2 at 571 nm (Figure 14b,d).
For Ir-3, a red emission was observed without DPA (Fig-
ure 14a). In the presence of DPA, a blue emission was ob-
served, owing to the upconversion (Figure 14b). There was
a notable change in the CIE coordinates of Ir-3 (Fig-
ure 14c,d). For Ir-4, there was no emission and no upcon-
verted fluorescence was observed. The weak blue light in Ir-
4 was after laser excitation at 445 nm (Figure 14a,b).
plex (Ir-1). RT phosphorescence was observed for all of the
IrIII complexes. Ir-3 showed intense absorption in the visible-
light range (e=39600mꢀ1 cmꢀ1 at 402 nm), RT deep-red
emission (640 nm, Fp =1.4%), and a long-lived T1 excited
state (tT =9.30 ms). The phosphorescence of Ir-3 was strongly
dependent on the polarity of the solvent: it was strongly
phosphorescent in non-polar solvents, such as toluene, but
the emission was completely quenched in polar solvents,
such as MeCN. Complexes Ir-1 and Ir-2 showed solvent-po-
larity-independent emission. The T1 excited states of Ir-2, Ir-
3
3, and Ir-4 were assigned as a IL state from the emission
spectra at 77 K (small thermally induced Stokes shifts),
nanosecond time-resolved transient difference absorption
spectra, and by TDDFT calculations (spin density of the
triplet states). These complexes were used for TTA-based
upconversion and upconversion quantum yields of 7.1%
and 14.4% were observed for Ir-2 and Ir-3, respectively,
whereas the TTA upconversion was negligible for Ir-1 and
Ir-4 under the same conditions. The high upconversion
quantum yield of Ir-3 was attributed to the strong absorp-
tion of the complex at the excitation wavelength and to the
long-lived T1 excited state. Our results will be useful for the
design of visible-light-harvesting transition-metal complexes
that show long-lived triplet excited states and for their appli-
cations as triplet sensitizers for various photophysical proc-
esses, such as TTA upconversion, photovoltaics, photocataly-
sis, etc.
We noticed a slow upconversion process in the upconver-
sion with Ir-3 as the triplet sensitizer, that is, the upconver-
sion was weak at the beginning of the laser irradiation and
it became more significant with prolonged irradiation (see
the Supporting Information). We tentatively attributed the
slow kinetics to the consumption of trace amounts of O2 in
the solution. Previously, we observed a similar phenomenon
for the TTA upconversion with RuII–polyimide complexes
3
that showed long-lived IL excited states.[47,48]
The photophysics of TTA with the IrIII complexes as trip-
let sensitizers and DPA as the triplet acceptor are summar-
ized in Scheme 2. The principle processes that were involved
in the cascade photophysical processes of TTA upconversion
were: photoexcitation of the sensitizer and population of
the T1 excited state by ISC and TTET, which led to popula-
tion of the acceptor DPA at the T1 excited state. Collision
and annihilation of two DPA molecules at the T1 excited
state produced the S1 excited states and the unconverted flu-
orescence emanate from the S1 excited state of DPA. Note
that a few processes were crucial for the upconversion effi-
ciency; that is, the light-harvesting ability of the triplet sensi-
tizer, the efficiency of ISC, and the lifetimes of the T1 excit-
ed state of the sensitizer, as well as the energy level of the
T1 excited state of the sensitizer. These processes could be
enhanced by the light-harvesting ability, the heavy-atom
effect. Ir-3 fulfilled these requirements; therefore, the up-
conversion quantum yield with Ir-3 as the triplet sensitizer
was as high as 14.4%. It should be pointed out that this
Experimental Section
All of the chemicals were analytical pure and used as received. The sol-
vents were dried and distilled. 2-(4-Ethynylphenyl)pyridine,[36] 1-ethynyl-
naphthalene,[36] 2-ethylhexyl-4-bromo-1,8-naphthalimide,[36] ligand L-2,[36]
ACTHNUTRGNEUNG
the cyclometalated IrIII chloro-bridged dimers [{Ir(ppy)2Cl}2],[87] and 5-
ethynyl-2,2’-bipyridine[88] were synthesized according to literature proce-
dures (for characterization data, see the Supporting Information).
1H and 13C NMR spectra were recorded on a Varian Unity Inova NMR
spectrophotometer at 400 and 100 MHz, respectively, with total proton
decoupling. Mass spectra were recorded on Q-TOF Micro and MALDI
micro MX spectrometers. UV/Vis absorption spectra were measured on a
HP8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded
on a RF-5301 PC or on a Sanco 970 CRT spectrofluorometer (modified
for the upconversion experiments). Fluorescence quantum yields were
measured with quinine sulfate as a standard (FF =0.547 in 0.05m sulfuric
quantum yield was far beyond that of the model IrACTHNUTRGENUG(N III) com-
plexes (Ir-1, whose upconversion quantum yields was almost
zero). We are now actively working along these lines to pre-
pare new cyclometalated IrIII complexes that showed strong
absorption in the visible-light range, and long-lived triplet
excited states to enhance the application of these complexes
for photocatalysis, photovoltaics, photodynamic therapy
(PDT), and TTA upconversions.
acid). Phosphorescence quantum yields were measured with [Ru
ACHTUNGTRENNUNG(dmb)3]-
AHCTUNGTRENNUNG
times were measured on a Horiba Jobin Yvon Fluoro Max-4 (TCSPC) in-
strument. The emission spectra at 77 K were measured on an Oxford Op-
tistat DN cryostat (filled with liquid N2) and a FLS920 fluorospectrome-
ter (Edinburgh Instruments Ltd. U.K.).
Synthesis: Ligands L3 and L4 were synthesized according to a modified
literature procedure.[36]
L3: 2-ethylhexyl-4-bromo-1,8-naphthalimide (1.08 g, 2.78 mmol), 2-(4-
ethynylphenyl)pyridine (497.7 mg, 2.78 mmol), dry EtOH (40 mL), and
Conclusion
triethylamine (12 mL) were mixed together. Then, [PdACHTUNGTRENNUNG(PPh3)2Cl2]
We have prepared IrIII complexes with ethynylated ppy and
bpy ligand (ppy=2-phenylpyridine, bpy=2,2’-bipyridine).
Naphthal acetylide (Ir-2) and NI acetylide (Ir-3 and Ir-4)
were attached to the ppy or bpy ligands of the IrIII com-
(0.139 mmol, 97.6 mg, 5 mol%), PPh3 (0.139 mmol, 97.6 mg, 5 mol%),
and CuI (26.5 mg, 0.139 mmol, 5 mol%) were added. The reaction mix-
ture was heated to reflux and stirred under an argon atmosphere for 8 h.
After completion of the reaction, the mixture was cooled to RT, the
yellow precipitate was collected by filtration, and the crude product was
purified by column chromatography on silica gel (CH2Cl2) to give the
plexes. Unsubstituted [IrACTHNUTRGNE(UNG ppy)3] was studied as a model com-
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8109

J. Zhao et al.
product as
a
yellow solid (984.6 mg, 72.8%). 1H NMR (400 MHz,
132.35, 131.96, 131.73, 131.02, 130.20, 128.61, 128.37, 128.16, 128.00,
127.72, 127.18, 125.13, 125.01, 123.76, 123.53, 123.13, 122.89, 120.05,
119.89, 93.44, 92.89, 44.34, 37.96, 30.77, 28.73, 24.08, 23.14, 14.19,
10.70 ppm; HRMS (ESI): m/z calcd for [C54H45IrN5O2]+: 988.3203; found:
988.3183; elemental analysis calcd (%) for [C54H45F6IrN5O2P+0.02C6H14]:
C 57.28, H 4.02, N 6.17; found: C 57.15, H 4.11, N 6.34.
CDCl3): d=8.73–8.77 (m, 2H), 8.64 (d, 1H, J=7.2 Hz), 8.56 (d, 1H, J=
7.7 Hz), 8.10 (d, 2H, J=8.3 Hz), 7.99 (d, 1H, J=7.6 Hz), 7.87 (t, 1H, J=
7.8 Hz), 7.79 (d, 4H, J=7.8 Hz), 7.27–7.30 (m, 1H), 4.07–4.18 (m, 2H),
1.92–2.00 (m, 1H), 1.30–1.44 (m, 8H), 0.86–0.95 ppm (m, 6H); HRMS
(ESI): m/z calcd for [C33H31N2O2]+: 487. 2386; found: 487.0323.
L4: Yellow solid (350.4 mg, 71.9%). 1H NMR (400 MHz, CDCl3): d=
8.96 (s, 1H), 8.75–8.47 (m, 6H), 8.10–8.01 (m, 2H), 7.90–7.85 (m, 2H),
7.37 (t, 1H, J=5.5 Hz), 4.19–4.08 (m, 2H), 1.97–1.93 (m, 1H), 1.43–1.26
(m, 8H), 0.96–0.87 ppm (m, 6H); HRMS (ESI): m/z calcd for
[C32H30N3O2]+: 488.2338; found: 488. 2326.
TTA upconversion: A diode pumped laser was used for the upconver-
sions. The laser power was measured with a phototube. A mixed solution
of the complex (triplet photosensitizer) and 9,10-diphenylanthracene
(DPA, triplet acceptor) was degassed for at least 15 min with N2 or Ar
before the measurements were taken. The absorption of DPA at 445 nm
was very weak, thus the triplet acceptor could not be excited with 445 nm
laser irradiation (only the sensitizers, that is, the IrIII complexes, were se-
lectively excited). The upconversion quantum yields were determined
Complexes Ir-1, Ir-2, and Ir-3 were synthesized according to a modified
literature procedure.[19]
Ir-1: 2-phenylpyridine (46.5 mg, 0.3 mmol), [{IrACTHNUGRTNEUNG(ppy)2Cl}2] (64.3 mg,
with [Ru
methyl-8-phenyl-4,4-difluoro-4-bora-3a-azonia-4a-aza-s-indacene
3.6% in aerated MeCN) as the standards.[53] It was important to keep the
concentration of [Ru(dmb)3][PF6]2 low, otherwise the upconversion quan-
tum yield would be overestimated (self-quenching was observed for [Ru-
(dmb)3]
[PF6]2 in highly concentrated solutions, such as at 2.0ꢁ10ꢀ5 m).
The quantum yields were calculated by using Equation (1), where Funk
ACHUTGTNRNENUG(dmb)3]ACHTUNGTRENNUNG
0.06 mmol), and silver triflate (30.7 mg, 0.12 mmol) were dissolved in 2-
ethoxyethanol (10 mL) and heated at 1008C in an oil bath under an
argon atmosphere overnight. The deep-yellow solution was cooled and
gravity-filtered to remove the gray AgCl precipitate. The solvents were
removed under reduced pressure and the residue was purified by column
chromatography on silica gel (CH2Cl2/n-hexane=2:1, v/v) to afford the
product as a yellow solid (66.9 mg, 85.1%). 1H NMR (400 MHz, CDCl3):
d=7.96 (d, 3H, J=8.0 Hz), 7.59–7.71 (m, 9H), 6.76–6.97 ppm (m, 12H);
HRMS (MALDI): m/z calcd for [C33H24IrN3]+: 655.1600; found:
655.1634; elemental analysis calcd (%) for [C33H24IrN3+0.05C6H14]:
C 60.68, H 3.78, N 6.38; found: C 60.75, H 3.76, N 6.36.
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
,
Aunk, Iunk, and hunk represent the quantum yield, absorbance, integrated
photoluminescence intensity, and the refractive index of the samples, re-
spectively.[79a] The photography of the upconversion were taken with a
Samsung NV 5 digital camera.
Ir-2: Column chromatography on silica gel (CH2Cl2/n-hexane, 1:1 v/v) af-
forded the product as a yellow solid (95.3 mg, yield: 59.2%). 1H NMR
(400 MHz, CDCl3): d=8.30 (s, 1H), 6.88–7.87 ppm (m, 29H); 13C NMR
(100 MHz, CDCl3): d=166.82, 166.00, 160.83, 160.56, 160.33, 147.16,
147.06, 146.93, 144.40, 143.63, 140.15, 137.38, 137.18, 135.95, 133.46,
133.21, 129.92, 128.08, 126.56, 126.17, 125.15, 124.12, 123.91, 123.46,
122.16, 121.82, 120.05, 119.93, 119.17, 118.90, 118.75, 96.70 ppm; HRMS
(MALDI): m/z calcd for [C45H30IrN3]+: 805.2069; found: 805.2096; ele-
mental analysis calcd (%) for [C45H30IrN3+0.15C6H14]: C 67.40, H 3.96,
N 5.14; found: C 67.61, H 3.81, N 5.06.
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
Astd
Aunk
Iunk
Istd
hunk
hstd
ð1Þ
ꢀunk ¼ 2ꢀstd
Nanosecond time-resolved transient difference absorption spectroscopy:
Nanosecond time-resolved transient difference absorption spectra were
recorded on a LP 920 laser flash photolysis spectrometer (Edinburgh In-
struments, Livingston, UK). The samples were purged with N2 or Ar for
30 min before any measurements were taken. The samples were excited
with a 355 nm laser and the transient signals were recorded on a Tektro-
nix TDS 3012B oscilloscope.
Ir-3: Column chromatography on silica gel (CH2Cl2/n-hexane, 4:1 v/v) af-
forded the product as a red solid (56.9 mg, 57.7%). 1H NMR (CDCl3,
400 MHz): d=8.48–8.61 (m, 3H), 7.90 (t, 3H, J=8.1 Hz), 7.83 (d, 1H,
J=7.6 Hz), 7.76 (t, 1H, J=7.8 Hz), 7.52–7.71 (m, 9H), 7.20 (t, 2H, J=
8.1 Hz), 6.87–6.99 (m, 9H), 4.06–4.16 (m, 2H), 1.89–1.97 (m, 1H), 1.30–
1.43 (m, 8H), 0.86–0.95 ppm (m, 6H); 13C NMR (100 MHz, CDCl3): d=
166.89, 165.83, 164.71, 164.42, 160.94, 160.59, 160.33, 147.49, 147.19,
145.71, 143.81, 140.64, 137.32, 136.26, 132.93, 131.88, 131.62, 130.60,
130.24, 130.09, 128.52, 128.26, 127.33, 124.26, 124.11, 123.69, 122.97,
122.78, 122.08, 121.65, 120.24, 119.58, 119.15, 119.02, 102,09, 86.85, 44.34,
38.10, 30.93, 28.87, 24.23, 23.22, 14.22, 10.80 ppm; HRMS (MALDI): m/z
calcd for [C55H45IrN4O2]+: 986.3172; found: 986.3104; elemental analysis
calcd (%) for [C55H45IrN4O2+0.09C6H14]: C 67.11, H 4.69, N 5.64; found:
C 67.05, H 4.43, N 5.42.
TDDFT computational methods: Geometry optimizations were calculat-
ed by using the B3LYP functional with the 6–31G(d)/LanL2DZ basis set.
The vertical excitation energy was calculated with the TDDFT method
based on the singlet ground-state geometry. The spin-density of the trip-
let excited state was calculated with their energy-minimized triplet geo-
metries.[89] The solvents were used in the calculations (CPCM model). All
calculations were performed with the Gaussian 09W software (Gaussian,
Inc.).
Acknowledgement
Ir-4: Ir-4 was synthesized according to our previously reported proce-
dure.[49] [{Ir
ACHTUNGTRENNUNG(ppy)2Cl}2] (53.6 mg, 0.05 mmol) and L4 (60.0 mg, 0.12 mmol)
We thank the NSFC (20972024 and 21073028), the Fundamental Re-
search Funds for the Central Universities (DUT10ZD212), the Royal So-
ciety (UK), the NSFC (China) (China-UK Cost-Share Program,
21011130154), the Ministry of Education (SRFDP-200801410004 and
NCET-08–0077), the Education Department of Liaoning Province
(2009T015), the State Key Laboratory of Fine Chemicals (KF0802), and
the Dalian University of Technology for financial support.
were dissolved in CH2Cl2/MeOH (12 mL, 2:1, v/v). The mixture was
heated at reflux for 6 h under an Ar atmosphere. The reaction mixture
turned an orange color. After completion of the reaction, the mixture
was cooled to RT and a 10-fold excess of ammonium hexafluorophos-
phate was added. The suspension was stirred for 15 min and then filtered
to remove the insoluble inorganic salts. The solution was evaporated to
dryness under reduced pressure to obtain a crude orange solid. The
crude product was purified by column chromatography on silica gel
(CH2Cl2/MeOH=15:1, v/v) to afford the product as an orange solid
(72.4 mg, 73.3%). 1H NMR (CDCl3, 400 MHz): d=9.89 (d, 1H, J=
7.6 Hz), 9.65 (d, 1H, J=7.9 Hz), 8.65 (d, 1H, J=7.1 Hz), 8.53 (d, 2H, J=
7.6 Hz), 8.42 (d, 1H, J=8.3 Hz), 8.28 (t, 1H, J=7.7 Hz), 8.07 (s, 1H),
7.99–7.70 (m, 9H), 7.58 (d, 1H, J=5.3 Hz), 7.51 (d, 1H, J=5.2 Hz), 7.44
(t, 1H, J=6.4 Hz), 7.15–6.93 (m, 6H), 6.39–6.30 (m, 2H), 4.17–4.06 (m,
2H), 2.03 (d, 1H), 1.39–1.26 (m, 8H), 0.95–0.86 ppm (m, 6H); 13C NMR
(100 MHz, CDCl3): d=167.89, 164.18, 163.91, 155.73, 155.59, 155.44,
152.01, 150.15, 149.77, 148.63, 143.61, 143.43, 142.62, 140.39, 138.44,
[1] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti,
Top. Curr. Chem. 2007, 281, 143–203.
[2] Y. Chi, P. T. Chou, Chem. Soc. Rev. 2010, 39, 638–655.
[3] Z. Liu, M. Guan, Z. Bian, D. Nie, Z. Gong, Z. Li, C. Huang, Adv.
Funct. Mater. 2006, 16, 1441–1448.
[4] C. H. Yang, C. C. Tai, I. W. Sun, J. Mater. Chem. 2004, 14, 947–950.
[5] C. L. Ho, Q. Wang, C. S. Lam, W. Y. Wong, D. Ma, L. Wang, Z. Q.
Gao, C. H. Chen, K. W. Cheah, Z. Lin, Chem. Asian J. 2009, 4, 89–
103.
8110
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112

Triplet Excited State of Naphthalimide in IrIII Complexes
FULL PAPER
[6] S. Huo, J. C. Deaton, M. Rajeswaran, W. C. Lenhart, Inorg. Chem.
2006, 45, 3155–3157.
[7] a) K. Hanson, A. Tamayo, V. V. Diev, M. T. Whited, P. I. Djurovich,
M. E. Thompson, Inorg. Chem. 2010, 49, 6077–6084; b) Y. Liu, K.
Ye, Y. Fan, W. Song, Y. Wang, Z. Hou, Chem. Commun. 2009,
3699–3701.
[8] B. S. Du, C. H. Lin, Y. Chi, J. Y. Hung, M. W. Chung, T. Y. Lin,
G. H. Lee, K. T. Wong, P. T. Chou, W. Y. Hung, H. C. Chiu, Inorg.
Chem. 2010, 49, 8713–8723.
Chem. Eur. J. 2009, 15, 136–148; c) G. Volpi, C. Garino, L. Salassa,
J. Fiedler, K. I. Hardcastle, R. Gobetto, C. Nervi, Chem. Eur. J.
2009, 15, 6415–6427.
[32] K. Y. Zhang, S. P. Y. Li, N. Zhu, I. W. S. Or, M. S. H. Cheung, Y. W.
Lam, K. K. W. Lo, Inorg. Chem. 2010, 49, 2530–2540.
[33] a) A. A. Rachford, R. Ziessel, T. Bura, P. Retailleau, F. N. Castella-
no, Inorg. Chem. 2010, 49, 3730–3736; b) L. S. Natrajan, A. Toul-
min, A. Chew and S. W. Magennis, Dalton Trans. 2010, 39, 10837–
10846.
[9] J. C. Deaton, R. H. Young, J. R. Lenhard, M. Rajeswaran, S. Huo,
Inorg. Chem. 2010, 49, 9151–9161.
[34] K. K. W. Lo, C. K. Chung, T. K. M. Lee, L. H. Lui, k. H. K. Tsang,
N. Zhu, Inorg. Chem. 2003, 42, 6886–6897.
[10] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest, M. E. Thompson, J. Am.
Chem. Soc. 2001, 123, 4304–4312.
[11] S. C. Lo, R. E. Harding, C. P. Shipley, S. G. Stevenson, P. L. Burn,
I. D. Samuel, J. Am. Chem. Soc. 2009, 131, 16681–16688.
[12] S. Ladouceur, D. Fortin, E. Zysman-Colman, Inorg. Chem. 2010, 49,
5625–5641.
[13] Q. Zhao, S. Liu, M. Shi, C. Wang, M. Yu, L. Li, F. Li, T. Yi, C.
Huang, Inorg. Chem. 2006, 45, 6152–6160.
[14] S. Aoki, Y. Matsuo, S. Ogura, H. Ohwada, Y. Hisamatsu, S. Moromi-
zato, M. Shiro, M. Kitamura, Inorg. Chem. 2011, 50, 806–818.
[15] M. S. Lowry, W. R. Hudson, R. A. Pascal, S. Bernhard, J. Am.
Chem. Soc. 2004, 126, 14129–14135.
[16] a) Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 2010, 39, 3007–3030;
b) V. Fernꢂndez-Moreira, F. L. Thorp-Greenwood, M. P. Coogan,
Chem. Commun. 2010, 46, 186–202; c) H.-F. Shi, S.-J. Liu, H.-B.
Sun, W.-J. Xu, Z.-F. An, J. Chen, S. Sun, X.-M. Lu, Q. Zhao, W.
Huang, Chem. Eur. J. 2010, 16, 12158–12167.
[17] L. H. Fischer, M. I. J. Stich, O. S. Wolfbeis, N. Tian, E. Holder, M.
Schꢃferling, Chem. Eur. J. 2009, 15, 10857–10863.
[18] a) S. M. Borisov, I. Klimant, Anal. Chem. 2007, 79, 7501–7509;
b) C. S. K. Mak, D. Pentlehner, M. Stich, O. S. Wolfbeis, W. K. Chan,
H. Yersin, Chem. Mater. 2009, 21, 2173–2175.
[35] J. S. Y. Lau, P. K. Lee, K. H. K. Tsang, C. H. C. Ng, Y. W. Lam, S. H.
Cheng, K. K. W. Lo, Inorg. Chem. 2009, 48, 708–718.
[36] W. Wu, W. Wu, S. Ji, H. Guo, P. Song, K. Han, L. Chi, J. Shao, J.
Zhao, J. Mater. Chem. 2010, 20, 9775–9786.
[37] H. Guo, S. Ji, W. Wu, W. Wu, J. Shao, J. Zhao, Analyst 2010, 135,
2832–2840.
[38] S. Ji, W. Wu, W. Wu, P. Song, K. Han, Z. Wang, S. Liu, H. Guo, J.
Zhao, J. Mater. Chem. 2010, 20, 1953–1963.
[39] W. Wu, W. Wu, S. Ji, H. Guo, J. Zhao, J. Organomet. Chem. 2011,
696, 2388–2398.
[40] W. Wu, W. Wu, S. Ji, H. Guo, X. Wang, J. Zhao, Dyes Pigm. 2011,
89, 199–211.
[41] W. Wu, C. Cheng, W. Wu, H. Guo, S. Ji, P. Song, K. Han, J. Zhao, X.
Zhang, Y. Wu, G. Du, Eur. J. Inorg. Chem. 2010, 4683–4696.
[42] W. Wu, S. Ji, W. Wu, H. Guo, X. Wang, J. Zhao, Z. Wang, Sensors
and Actuators B. 2010, 149, 395–406.
[43] W. Wu, J. Sun, S. Ji, W. Wu, J. Zhao, H. Guo, Dalton Trans. 2011, 40,
11550–11561.
[44] W. Wu, W. Wu, S. Ji, H. Guo, J. Zhao, Dalton Trans. 2011, 40, 5953–
5963.
[45] H. Sun, H. Guo, W. Wu, X. Liu, J. Zhao, Dalton Trans. 2011, 40,
7834–7841.
[46] J. Zhao, S. Ji, H. Guo, RSC Adv. 2011, 1, 937–950.
[47] S. Ji, W. Wu, W. Wu, H. Guo, J. Zhao, Angew. Chem. 2011, 123,
1664–1667; Angew. Chem. Int. Ed. 2011, 50, 1626–1629.
[48] S. Ji, H. Guo, W. Wu, W. Wu, J. Zhao, Angew. Chem. 2011, 123,
8433–8436; Angew. Chem. Int. Ed. 2011, 50, 8283–8286.
[49] J. Sun, W. Wu, H. Guo, J. Zhao, Eur. J. Inorg. Chem. 2011, 3165–
3173.
[19] M. C. DeRosa, D. J. Hodgson, G. D. Enright, B. Dawson, C. E. B.
Evans, R. J. Crutchley, J. Am. Chem. Soc. 2004, 126, 7619–7626.
[20] N. Tian, D. Lenkeit, S. Pelz, L. H. Fischer, D. Escudero, R. Schie-
wek, D. Klink, O. J. Schmitz, L. Gonzꢂlez, M. Schꢃferling, E.
Holder, Eur. J. Inorg. Chem. 2010, 4875–4885.
[21] Z. Xie, L. Ma, K. E. DeKrafft, A. Jin, W. Lin, J. Am. Chem. Soc.
2010, 132, 922–923.
[50] L. Huang, L. Zeng, H. Guo, W. Wu, W. Wu, S. Ji, J. Zhao, Eur. J.
Inorg. Chem. 2011, 4527–4533.
[22] L. Murphy, A. Congreve, L. O. Pꢄlsson, J. A. G. Williams, Chem.
Commun. 2010, 46, 8743–8745.
[51] Y. Liu, W. Wu, J. Zhao, X. Zhang, H. Guo, Dalton Trans. 2011, 40,
9085–9089.
[23] S. P. Y. Li, H. W. Liu, K. Y. Zhang, K. K. W. Lo, Chem. Eur. J. 2010,
16, 8329–8339.
[52] W. Wu, H. Guo, W. Wu, S. Ji, J. Zhao, Inorg. Chem. 2011, 50,
11446–11460.
[53] W. Wu, H. Guo, W. Wu, S. Ji, J. Zhao, J. Org. Chem. 2011, 76, 7056–
7064.
[24] a) X. Wang, S. Goeb, Z. Ji, N. A. Pogulaichenko, F. N. Castellano,
Inorg. Chem. 2011, 50, 705–707; b) R. S. Khnayzer, J. Blumhoff,
J. A. Harrington, A. Haefele, F. Deng, F. N. Castellano, Chem.
Commun. 2012, 48, 209–211.
[25] S. Metz, S. Bernhard, Chem. Commun. 2010, 46, 7551–7553.
[26] J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson, S.
Bernhard, J. Am. Chem. Soc. 2005, 127, 7502–7510.
[54] G. J. Wilson, A. Launikonis, W. H. F. Sasse, A. W. H. Mau, J. Phys.
Chem. A 1997, 101, 4860–4866.
[55] D. S. Tyson, J. Bialecki, F. N. Castellano, Chem. Commun. 2000,
2355–2356.
[56] A. F. Morales, G. Accorsi, N. Armaroli, F. Barigelletti, S. J. A. Pope,
M. D. Ward, Inorg. Chem. 2002, 41, 6711–6719.
[57] H. Guo, M. L. Muro-Small, S. Ji, J. Zhao, F. N. Castellano, Inorg.
Chem. 2010, 49, 6802–6804.
[27] a) B. F. DiSalle, S. Bernhard, J. Am. Chem. Soc. 2011, 133, 11819–
11821; b) F. Gꢃrtner, D. Cozzula, S. Losse, A. Boddien, G. Anilku-
mar, H. Junge, T. Schulz, N. Marquet, A. Spannenberg, S. Gladiali,
M. Beller, Chem. Eur. J. 2011, 17, 6998–7006; c) K. Mori, K. Wata-
nabe, K. Fuku, and H. Yamashita, Chem. Eur. J. 2012, 18, 415- 418.
[28] F. Wang, W. G. Wang, X. J. Wang, H. Y. Wang, C. H. Tung, L. Z.
Wu, Angew. Chem. 2011, 123, 3251–3255; Angew. Chem. Int. Ed.
2011, 50, 3193–3197.
[58] A. Harriman, A. Khatyr, R. Ziessel, Dalton Trans. 2003, 2061–2068.
[59] I. E. Pomestchenko, C. R. Luman, M. Hissler, R. Ziessel, F. N. Cas-
tellano, Inorg. Chem, 2003, 42, 1394–1396.
[29] a) S. Takizawa, R. Aboshi, S. Murata, Photochem. Photobiol. Sci.
2011, 10, 895–903; b) J. Lalevꢅe, M. Peter, F. Dumur, D. Gigmes, N.
Blanchard, M.-A. Tehfe, F. Morlet-Savary, J. P. Fouassier, Chem.
Eur. J. 2011, 17, 15027–15031.
[30] Q. Zhao, F. Li, S. Liu, M. Yu, Z. Liu, T. Yi, C. Huang, Inorg. Chem.
2008, 47, 9256–9264.
[31] a) K. K. W. Lo, K. Y. Zhang, C. K. Chung, K. Y. Kwok, Chem. Eur.
J. 2007, 13, 7110–7120; b) R. Ragni, E. Orselli, G. S. Kottas, O. H.
Omar, F. Babudri, A. Pedone, F. Naso, G. M. Farinola, L. De Cola,
[60] A. A. Rachford, S. Goeb, F. N. Castellano, J. Am. Chem. Soc. 2008,
130, 2766–2767.
[61] P. H. Lanoꢆ, J. L. Fillaut, L. Toupet, J. A. G. Williams, H. L. Bozeca,
V. Guerchais, Chem. Commun. 2008, 4333–4335.
[62] F. Nastasi, F. Puntoriero, S. Campagna, S. Diring, R. Ziessel, Phys.
Chem. Chem. Phys. 2008, 10, 3982–3986.
[63] C. Bronner, S. A. Baudron, M. W. Hosseini, C. A. Strassert, A.
Guenet, L. D. Cola, Dalton Trans. 2010, 39, 180–184.
Chem. Eur. J. 2012, 18, 8100 – 8112
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8111

J. Zhao et al.
[64] D. N. Kozhevnikov, V. N. Kozhevnikov, M. Z. Shafikov, A. M. Pro-
khorov, D. W. Bruce, J. A. G. Williams, Inorg. Chem. 2011, 50, 3804–
3815.
[65] M. D. Perez, P. I. Djurovich, A. Hassan, G. Y. Cheng, T. J. Stewart,
K. Aznavour, R. Bauz, M. E. Thompson, Chem. Commun. 2009, 45,
4215–4217.
[66] C. Dragonetti, L. Falciola, P. Mussini, S. Righetto, D. Roberto, R.
Ugo, A. Valore, Inorg. Chem. 2007, 46, 8533–8547.
[67] E. Baranoff, E. Orselli, L. Allouche, D. D. Censo, R. Scopelliti, M.
Grꢃtzel, M. K. Nazeeruddin, Chem. Commun. 2011, 47, 2799–2801.
[68] D. C. Goldstein, Y. Y. Cheng, T. W. Schmidt, M. Bhadbhadec, P.
Thordarson, Dalton Trans. 2011, 40, 2053–2061.
[69] C. Goze, D. V. Kozlov, D. S. Tyson, R. Ziessel, F. N. Castellano, New
J. Chem. 2003, 27, 1679–1683.
[70] D. V. Kozlov, D. S. Tyson, C. Goze, R. Ziessel, F. N. Castellano,
Inorg. Chem. 2004, 43, 6083–6092.
[71] T. Hofbeck, H. Yersin, Inorg. Chem. 2010, 49, 9290–9299.
[72] L. Shi, Y. Geng, H. Gao, Z. Su, Z. Wu, Dalton Trans. 2010, 39,
7733–7740.
[73] Y. Li, H. Li, X. Zhao, M. Chen, J. Phys. Chem. A 2010, 114, 6972–
6977.
Wegner, Angew. Chem. 2007, 119, 7837–7840; Angew. Chem. Int.
Ed. 2007, 46, 7693–7696.
[82] H. C. Chen, C. Y. Hung, K. H. Wang, H. L. Chen, W. S. Fann, F. C.
Chien, P. Chen, T. J. Chow, C. P. Hsu, S. S. Sun, Chem. Commun.
2009, 4064–4066.
[83] Y. Y. Cheng, B. Fꢇckel, T. Khoury, R. G. C. R. Clady, M. J. Y. Tayeb-
jee, N. J. Ekins-Daukes, M. J. Crossley, T. W. Schmidt, J. Phys.
Chem. Lett. 2010, 1, 1795–1799.
[84] A. Monguzzi, R. Tubino, F. Meinardi, Phys. Rev. B. 2008, 77,
155122/1–155122/4.
[85] W. Zhao, F. N. Castellano, J. Phys. Chem. A 2006, 110, 11440–11445.
[86] T. N. Singh-Rachford, F. N. Castellano, J. Phys. Chem. Lett. 2010, 1,
195–200.
[87] S. Sprouse, K. A. King, P. J. Spellane, R. J. Watts, J. Am. Chem. Soc.
1984, 106, 6647–6653.
[88] E. K. Pefkianakis, N. P. Tzanetos, J. K. Kallitsis, Chem. Mater. 2008,
20, 6254–6262.
[89] Gaussian 09, Revision A.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢈ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT, 2009.
[74] E. Baranoff, S. Fantacci, F. D. Angelis, X. Zhang, R. Scopelliti, M.
Grꢃtzel, M. K. Nazeeruddin, Inorg. Chem. 2011, 50, 451–462.
[75] S. Ji, H. Guo, X. Yuan, X. Li, H. Ding, P. Gao, C. Zhao, W. Wu, W.
Wu, J. Zhao, Org. Lett. 2010, 12, 2876–2879.
[76] S. Ji, J. Yang, Q. Yang, S. Liu, M. Chen, J. Zhao, J. Org. Chem. 2009,
74, 4855–4865.
[77] M. Haase, H. Schꢃfer, Angew. Chem. 2011, 123, 5928–5950; Angew.
Chem. Int. Ed. 2011, 50, 5808–5829.
[78] H. M. Kim, B. R. Cho, Acc. Chem. Res. 2009, 42, 863–872.
[79] a) T. N. S. Rachford, F. N. Castellano, Coord. Chem. Rev. 2010, 254,
2560–2573; b) P. Ceroni, Chem. Eur. J. 2011, 17, 9560–9564; c) G.
Bergamini, P. Ceroni, P. Fabbrizi, S. Cicchi, Chem. Commun. 2011,
47, 12780–12782.
[80] P. Du, R. Eisenberg, Chem. Sci. 2010, 1, 502–506.
[81] S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S. Chernov, S.
Aleshchenkov, G. Nelles, A. Cheprakov, A. Yasuda, K. Mꢇllen, G.
Received: January 20, 2012
Published online: May 21, 2012
8112
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8100 – 8112