Green light-excitable naphthalenediimide acetylide-containing cyclometalated Ir(iii) complex with long-lived triplet excited states as triplet photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c3dt32815e

Naphthalenediimide (NDI) was connected to the ligand of a cyclometalated Ir(iii) complex (Ir-1) via a CC triple bond to enhance the absorption in the visible region and to access long-lived triplet excited states. Ir(ppy) ₂(bpy)[PF₆] (Ir-2, ppy = 2-phenylpyridine and bpy = 2,2′-bipyridine) was used as a model complex. The photophysical properties of the complexes were studied with steady state and time-resolved spectroscopy. Ir-1 shows strong absorption in the visible region (ε = 11 000 M ^-1 cm^-1 at 542 nm) and in comparison Ir-2 shows typically weak absorption in the visible region (ε < 3000 M^-1 cm ^-1 above 400 nm). Room temperature near IR emission at 732 nm (Φ_P = 0.1%) was observed for Ir-1, which is attributed to the NDI localized emissive triplet excited state, by transient absorption spectra and DFT calculations on the spin density surface. The lifetime of the NDI-localized triplet excited state is up to 130.0 μs, which is rarely reported for Ir(iii) complexes. In comparison, Ir-2 shows phosphorescence at 578 nm and the triplet state lifetime is a typical value of 0.3 μs. The complexes were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion and an upconversion quantum yield of 6.7% was observed with Ir-1. No upconversion was observed with Ir-2 as the triplet photosensitizer at the same experimental conditions.

Full text of DOI:10.1039/c3dt32815e

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Green light-excitable naphthalenediimide
acetylide-containing cyclometalated Ir(III) complex
with long-lived triplet excited states as triplet
photosensitizers for triplet–triplet annihilation
upconversion†
Cite this: Dalton Trans., 2013, 42, 6478
Lihua Ma, Song Guo, Jifu Sun, Caishun Zhang, Jianzhang Zhao* and Huimin Guo*
Naphthalenediimide (NDI) was connected to the ligand of a cyclometalated Ir(III) complex (Ir-1) via a
CuC triple bond to enhance the absorption in the visible region and to access long-lived triplet excited
2 6
states. Ir(ppy) (bpy)[PF ] (Ir-2, ppy = 2-phenylpyridine and bpy = 2,2’-bipyridine) was used as a model
complex. The photophysical properties of the complexes were studied with steady state and time-
resolved spectroscopy. Ir-1 shows strong absorption in the visible region (ε = 11 000 M− cm at 542 nm)
1
−1
and in comparison Ir-2 shows typically weak absorption in the visible region (ε < 3000 M− cm−1 above
1
4
P
00 nm). Room temperature near IR emission at 732 nm (Φ = 0.1%) was observed for Ir-1, which is
attributed to the NDI localized emissive triplet excited state, by transient absorption spectra and DFT cal-
culations on the spin density surface. The lifetime of the NDI-localized triplet excited state is up to
1
30.0 μs, which is rarely reported for Ir(III) complexes. In comparison, Ir-2 shows phosphorescence at
78 nm and the triplet state lifetime is a typical value of 0.3 μs. The complexes were used as triplet photo-
Received 25th November 2012,
Accepted 14th February 2013
5
sensitizers for triplet–triplet annihilation (TTA) upconversion and an upconversion quantum yield of 6.7%
was observed with Ir-1. No upconversion was observed with Ir-2 as the triplet photosensitizer at the
same experimental conditions.
DOI: 10.1039/c3dt32815e
www.rsc.org/dalton
unique photophysical properties, such as the strong absorp-
tion of visible light and long-lived triplet excited states, com-
Transition metal complexes, such as cyclometalated Ir(III) pared to analogues which are without rylene moieties.
Introduction
2
2,24,25
1
–9
complexes, have attracted much attention due to their appli- For example, Pt(II) complexes with rylene acetylide ligands
2
4,26–28
cations in organic light-emitting diodes (OLED) and lumines- were reported to show long-lived triplet excited states.
1
0
11–13
cence, photocatalysis (e.g. H production from water),
Castellano et al. reported perylenebisimide (PBI) acetylide
2
1
4,15
16,17
luminescent bioimaging,
recently triplet–triplet annihilation (TTA) upconversion.
However, conventional Ir(III) complexes show weak absorption (0.246 μs). A Re(I) complex was reported to show exceptionally
molecular probes
and more Pt(II) complexes which show strong absorption of visible light
1
8–23
but interestingly, the lifetime of the triplet state is very short
2
9
24b
of visible light and short triplet excited state lifetimes (a few long-lived triplet excited states (651 μs). Recently, we reported
2
,3
μs). These features are disadvantages for the application of naphthalenediimide (NDI)- and coumarin-containing Pt(II) com-
2
8,30
27,31
Ir(III) complexes.
plexes,
naphthalimide (NI) Pt(II) and Ir(III) complexes,
32
Concerning these aspects, transition metal complexes with which show long-lived triplet excited states. However, the
rylene ligands have attracted much attention due to their absorption of the Ir(III) complexes we prepared is limited to the
blue region (<500 nm). Recently, diketopyrrolopyrrole (DPP)-
containing Ir(III) complexes were reported to show absorption in
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116012,
P.R. China. E-mail: zhaojzh@dlut.edu.cn, guohm@dlut.edu.cn;
http://finechem.dlut.edu.cn/photochem
the 500–600 nm range. The triplet excited state lifetime is
33
3 μs. Therefore, much room is left for the preparation of Ir(III)
complexes that show strong absorption of visible light and
long-lived triplet excited states because these photophysical
features are crucial for the application of Ir(III) complexes, such
†
Electronic supplementary information (ESI) available: The synthesis and struc-
ture characterization data of the Ir(III) complexes; the details of the up-
conversion and calculation information. See DOI: 10.1039/c3dt32815e
1
1–13,32
as in photocatalysis and photo upconversion.
6
478 | Dalton Trans., 2013, 42, 6478–6488
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Naphthalenediimide (NDI) is a versatile chromophore for
Herein, for the first time, the NDI moiety was used for the
which the photophysical properties, such as the absorption preparation of cyclometalated Ir(III) complexes. Following the
wavelength, can be readily tuned by molecular structural concept of maximizing the heavy atom effect by direct metala-
3
4–39
28,30–32,43–46
modifications.
NDI has been widely used in fluorescence tion of a chromophore,
the π-conjugated core of
3
5–39
studies
of NDI in phosphorescence or triplet excited states is rare.
The T state energy level of NDI is high (ca. 2.0 eV) and thus it triplet excited states may be quenched by the possible cis/trans
is suitable for use as a triplet photosensitizer (triplet energy isomerization of the CvC double bonds. Strong absorption
but to the best of our knowledge, the application NDI is linked to the Ir(III) centre via CuC triple bonds (Ir-1,
4
0–42
Scheme 1). We did not use CvC double bonds because the
1
4
7
donor) to initiate photophysical processes, such as TTA up- of visible light (ε = 11 000 M− cm at 542 nm) and a long-
1
−1
4
0
conversion. Previously, we studied the photophysical pro- lived triplet excited state (τ = 130.0 μs) were observed for Ir-1.
perties of bromo-substitued NDIs as well as the application of These values are in stark contrast to conventional cyclometa-
1
,2,48
the compounds as triplet photosensitizers for TTA upconver- lated Ir(III) complexes,
such as Ir-2 (Scheme 1), which
sion. We used NDI for the preparation of a Pt(II) complex, show weak absorption of visible light and short-lived triplet
which shows moderate triplet excited state lifetime excited states (τ = 0.3 μs). The complexes were used as triplet
22.3 μs). NDI was also used for the preparation of a Ru(II) photosensitizers for TTA upconversion. We demonstrated that
4
0
a
2
8
(
4
2b
complex.
However, to the best of our knowledge, NDI has the triplet excited state of Ir-1 is efficiently populated and that
never been used for the preparation of cyclometalated Ir(III) the performance of Ir-1 is much better than Ir-2. Our results
complexes. are useful for the design of new Ir(III) complexes that show
Scheme 1 Preparation of the Ir(III) complex Ir-1. The molecular structure of the model complex Ir(ppy)
2
(bpy) (Ir-2) is also presented. Both complexes are cationic
Cl , PPh , CuI, Et N, refluxed
3 4
, 2 h; (f) diglycolamine (DGA), 2-methoxyethanol, 120 °C, 8 h; (g) Pd(PPh ) ,
−
and the counter ion is [PF
6
] . Reagents and conditions: (a) n-BuLi, DEE, −80 °C, 6 h; (b) Pd(PPh
3
)
4
, xylene, refluxed for 24 h; (c) Pd(PPh
3
)
2
2
3
3
for 8 h; (d) DBI, oleum (20% SO
CuI, Et N, reflux, 8 h; (h) CH CH
3
), 40 °C, 5 h; (e) 2-ethylhexylamine, 120 °C, under N
OH–CH Cl (v/v = 1 : 2), refluxed for 6 h.
2
3
3
2
2
2
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strong absorption of visible light and long-lived triplet excited (200 mL), dried over anhydrous MgSO
4
, filtered and then the
states. These complexes can be used for photocatalysis and solvent was removed. The residue was purified by passing it
TTA upconversion.
through a silica plug using CH
to give a white solid (180.0 mg, yield: 78.3%). H NMR
400 MHz, CDCl ): 8.71 (d, 2H), 8.45 (t, 3H), 7.90 (t, 3H), 7.39
2 2
Cl –hexane = 5 : 1 as the eluent
1
(
3
+
Experimental
Materials and reagents
(s, 1H), 3.31 (s, 1H). MS-TOF EI : calcd (C12
80.0687, found m/z = 180.0689.
8 2
H N ) m/z =
1
All the chemicals were analytically pure and used as received.
2
-Bromopyridine was purchased from Aladdin Co. The cyclo- Synthesis of 9
III
49
40
metalated Ir chloro-bridged dimers[{Ir(ppy) Cl} ], 6,
7
2
2
Under an argon atmosphere, compounds
.074 mmol) and 4 (26.7 mg, 0.149 mmol) were dissolved in
triethylamine (15 mL). Then Pd(PPh (8.6 mg, 0.0075 mmol)
and CuI (5 mg, 0.026 mmol) were added. The mixture was
refluxed for 8 h. The reaction mixture was cooled to room
temperature (RT) and the solvent was removed under reduced
pressure. Water was added and the mixture was extracted with
8 (50.0 mg,
and 8 were synthesized according to literature procedures (for
characterization data, see the ESI†). Compounds 2 and 3 were
0
3 4
)
5
0
synthesized according to
a
literature method.
1,4,5,8-
Naphthalenetetracarboxylic dianhydride (compound 5) is a
product of Aladdin Co. (China).
Analytical measurements
2 2
CH Cl . The combined organic layer was dried over anhydrous
NMR spectra were recorded on a Bruker 400 MHz spectrometer Na SO . After the removal of the solvent, the crude product
2
4
3
2 2
(CDCl as the solvent, TMS as the standard, δ = 0.00 ppm). was purified with column chromatography (silica gel, CH Cl –
High resolution mass spectra (HRMS) were determined on a CH OH = 25 : 1 as eluent, v/v) to give a red solid 9 (30.0 mg,
3
1
MALDI micro MX. Luminescence spectra and the TTA upcon- 52.6%). H NMR (400 MHz, CDCl ): 10.45 (s, 1H), 8.98 (s, 1H),
3
version were measured on a RF-5301PC spectrofluorometer 8.71 (d, 2H), 8.39 (d, 3H), 8.12 (s, 1H), 7.86 (s, 1H), 7.35 (s,
(
Shimadzu, Japan) or an adapted CRT970 spectrofluorometer. 1H), 4.13 (d, 4H), 3.70 (t, 8H), 1.92–1.98 (m, 2H), 1.25–1.37
+
The nanosecond time-resolved transient difference absorption (m, 16H), 0.89–0.94 (m, 12H), MS-TOF LD : calcd
spectra were measured with a laser flash photolysis spec- ([C H B F N ] ), m/z = 772.4074, found, m/z = 772.4061.
+
4
0
36 2 4 4
trometer (LP 920, Edinburgh Instruments, UK) and recorded
on a Tektronix TDS 3012B oscilloscope. The lifetime values (by
monitoring the decay trace of the transients) were obtained
Synthesis of Ir-1
with the LP920 software. Luminescence quantum yields of the A solution of compound 9 (30.0 mg, 0.039 mmol) and 10
complexes were measured with B-1 as the standard (Φ = 2.7% (58.7 mg, 0.055 mmol) in CH OH–CH Cl (15 mL, 1 : 2, v/v)
F
3
2
2
in CH CN). The emission spectra at 77 K were measured with was heated under reflux. After 6 h, the red solution was cooled
3
TM
an Oxford Optistat DN cryostat (with liquid nitrogen filling) to RT and then a 10-fold excess of ammonium hexafluorophos-
and a FLS920 spectrofluorometer (Edinburgh Instruments Ltd, phate was added. The suspension was stirred for 1 h and then
UK).
filtered to remove the insoluble inorganic salts. The solution
was evaporated to dryness under reduced pressure to obtain a
crude red solid. The crude product was purified by column
Synthesis of 4
Under an argon atmosphere, compound
3
(300.0 mg, chromatography (silica gel, CH Cl –CH OH = 30 : 1, v/v) and a
2
2
3
1
1
.283 mmol), Pd(PPh
3
)
2
Cl
2
(9.1 mg, 0.013 mmol), PPh
3
red solid was obtained (35.0 mg, 45.5%). H NMR (400 MHz,
): 10.51 (s, 1H), 8.78 (s, 2H), 8.64 (s, 1H), 8.37 (s, 2H),
(6.8 mg, 0.026 mmol) and CuI (5.2 mg, 0.026 mmol) were dis- CDCl
3
solved in triethylamine (15 mL). After stirring, trimethylsilyl- 8.10 (d, 2H); 7.77–7.79 (m, 4H), 7.94 (s, 4H), 7.45–7.80 (m, 8H),
acetylene (125.5 mg, 1.280 mmol) was added via a syringe. The 6.93 (d, 6H), 4.06–4.09 (s, 4H), 3.69 (t, 8H), 3.55 (s, 1H), 1.90 (s,
+
mixture was then heated at 88 °C for 8 h. The solvent was 1H), 1.26–1.34 (m, 16H), 0.88 (s 12H), MS-TOF LD : calcd
+
removed under reduced pressure, water was added and the ([C H N O S Ir-PF ] ) m/z
=
1272.4939, found m/z
=
6
9
56
8
6
2
6
mixture was extracted with dichloromethane (4 × 20 mL). The 1272.5001.
combined organic layer was dried over anhydrous Na SO
After the removal of the solvent, the crude product was puri-
fied with column chromatography (silica gel, CH Cl –hexane =
: 1, v/v) and a colourless liquid was obtained. Tetrabutyl- The geometry optimizations were calculated using the B3LYP
2
4
.
Computational methods
2
2
5
−1
ammonium fluoride (3.2 mL, 1 mol L ) was added to a solution functional with the GENECP basis set with density functional
of the above trimethylsilane protected intermediate (200 mg, theory (DFT). The excitation energy was calculated using the
0
.800 mmol) in THF (25 mL) and the solution was stirred at time-dependent DFT (TDDFT) method based on the optimized
room temperature under argon for 3 h. CH Cl (100 mL) and singlet ground state geometry. The spin density surfaces of the
2
2
water (50 mL) were added. The organic layer was separated complexes were calculated with the energy minimized triplet
and the aqueous layer was extracted with CH Cl (3 × 15 mL). state geometries. All the calculations were performed using
The combined organic layers were washed with brine Gaussian 09W (Gaussian, Inc.).
2
2
5
1
6
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TTA upconversions
A diode pumped solid state laser was used as the excitation
source for the TTA upconversions. The samples were purged
with N or Ar for 15 min before the measurement. The upcon-
2
version quantum yields were determined with the fluorescence
of B-1 as the quantum yield standard (Φ = 0.027 in CH
3
CN)
and the quantum yields were calculated using eqn (1),
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
ꢀ
O:D:
2
1
1
ꢀ 10
std
Isam
Istd
ηsam
ηstd
ΦUC ¼ 2Φstd
ð1Þ
ꢀO:D:
ꢀ 10
sam
Fig. 1 The UV-Vis absorption and emission spectra of Ir-1, ligand 9 and Ir-2.
where ΦUC, 1–10−O.D., Isam and ηsam represent the upconver-
sion quantum yield, the absorbance correction factor, the inte-
grated photoluminescence intensity and the refractive index of
(a) UV-Vis absorption. (b) Normalized emission spectra (with excitation at λex =
−5
5
42 nm, 542 nm, 400 nm respectively), 1.0 × 10 M, 20 °C.
the solvents, respectively (sam means samples). The photo- used to connect the NDI to the bpy ligand. All the compounds
graphs of the upconversion were taken with a Samsung NV 5 were obtained in moderate to satisfactory yields.
digital camera. The exposure times are the default values of
the camera.
UV-vis absorption and photoluminescence spectra of the
complexes
Delayed fluorescence of the TTA upconverted emission
The UV-Vis absorption of the complexes was studied
The delayed fluorescence of the TTA upconversion was
2 2
with CH Cl as the solvent (Fig. 1a). Ir-2 shows the typical
measured with
a nanosecond pulsed laser (Opolette™
absorption of the normal Ir(III) complexes, i.e. a very weak
5
3
3
55II+UV nanosecond pulsed laser, typical pulse length: 7 ns;
absorption in the visible region. The absorption maxima is
at 269 nm (35 000 M− cm ). However, with the ethynyl NDI
1
−1
pulse repetition: 20 Hz; peak OPO energy: 4 mJ; the wavelength
is tunable in the range of 210–355 nm and 410–2200 nm.
OPOTEK, USA), which was synchronized to a FLS 920 spectro-
fluorometer (Edinburgh Instruments, UK). The decay kinetics
of the upconverted fluorescence (delayed fluorescence) was
monitored with a FLS 920 spectrofluorometer (synchronized to
the OPO nanosecond pulse laser). The prompt fluorescence
lifetime of the triplet acceptor DPA was measured with an EPL
picosecond pulsed laser (405 nm) which was synchronized to
the FLS 920 spectrofluorometer.
−
1
ligand, Ir-1 shows a strong absorption at 542 nm (11 000 M
−
1
cm ). This absorption band can be attributed to the NDI
ligand, which shows a similar absorption at 544 nm (Fig. 1a).
Furthermore, we noticed a difference between the UV-Vis
absorption of Ir-1 and the NDI-conjugated bpy ligand (com-
pound 9, the free ligand) and therefore, we propose an elec-
tronic interaction between the NDI moiety and the Ir(III)
coordination center.
To the best of our knowledge, this is the first time that the
NDI moiety has been used to prepare cyclometalated Ir(III)
complexes. Previously, we used the NI moiety to prepare Ir(III)
Time-resolved emission spectra
The nanosecond and the microsecond time-resolved emission complexes, but those complexes gave absorption in a much
3
1,54
spectra of the triplet photosensitizers alone and the TTA shorter wavelength range.
Furthermore, cyclometalated
upconversion were recorded with a OPO nanosecond pulsed Ir(III) complexes showing strong absorption of visible light are
2
,32
laser. The pulsed laser was synchronized to the FLS 920 spec- rarely reported.
Previously, a bodipy-tethered Ir(III) complex
trofluorometer (Edinburgh Instruments, UK).
was reported but a different, non-π-conjugated linker between
the light-harvesting ligand and the coordination ligand was
5
5
used. We prepared Ir(III) complexes with a coumarin-contain-
ing ligand but these complexes show absorption in the rela-
tively blue-shifted spectral region.
The photoluminescence of the complexes was recorded
Results and discussion
Design and synthesis of the complexes
3
2
The design of the complexes is based on a few rationalizations. (Fig. 1b). The model complex Ir-2 shows the typical structure-
First, the π-conjugated framework of NDI is linked to the less emission band of the Ir(III) complexes at 578 nm (ΦP
=
coordination centre of the Ir(III) complexes via CuC triple 4.3%). For Ir-1, two emission bands at 572 nm and 732 nm
bonds to ensure an efficient heavy atom effect and thus ISC, were observed. The luminescence lifetimes of Ir-1 at 572 nm
otherwise the strong absorption of visible light will not result and 732 nm were determined as 5.0 ns and 220.3 μs, respect-
in efficient funneling of the energy to the triplet excited ively (Table 1). The phosphorescence quantum yield of Ir-1
5
2
38,40
states. Second, NDI alone gives a very weak absorption.
(0.1%, Table 1) is much lower than Ir-2. Room temperature
On introducing an amine substituent to the NDI core, the near IR emission has rarely been reported for Ir(III) complexes.
absorption in the visible range will be enhanced. Third, an Bodipy is a versatile chromophore and has been used for the
ether chain was introduced to improve the solubility of the preparation of Ir(III) complexes but no RT near IR emission
5
5
complex Ir-1 (Scheme 1). A Sonogashira coupling reaction was was observed. Recently, a DPP conjugated Ir(III) complex was
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Table 1 Photophysical parameters of the Ir(III) complexes
In order to rule out any impurities responsible for the
observation of the fluorescence band of Ir-1, the photolumi-
nescence of Ir-1 and ligand 9 at 77 K were compared to that at
RT (Fig. S16 in the ESI†). For ligand 9, the emission at 77 K
(623 nm) is red-shifted compared to that at RT (583 nm). For
Ir-1, however, the same emission maxima were observed for
the solutions at 77 K (599 nm) and RT (597 nm) (in EtOH–
MeOH, 4 : 1, v/v, Fig. S16 in the ESI†). Therefore, we propose
that the fluorescence band of Ir-1 is not due to any impurities,
a
b
a
c
e
λ
abs (nm)
ε
λ
em (nm)
τ
L
Φ (%)
λem (77 K)
f
Ir-1 542
Ir-2 269
1.11 572
32
3.47 578
1.71 578
5.0 ns
220.3 μs
0.3 μs
574
724
585
621
g
d
7
0.1
d
h
4.3i
9
544
9.8 ns
76.2
a
10−5 mol
−1
b
In CH Cl
2
2
solution (1.0 × L ). Molar absorption
4 −1 −1 c
coefficient at the absorption maxima. ε: 10 M cm
.
Luminescence
at RT. Quantum yield of
, B-1 as the standard (Φ = 0.027 in CH
3
CN). at least not due to the free ligand. This conclusion is sup-
lifetime. c = 1.0 × 10− M in CH
phosphorescence in CH Cl
Luminescence maxima at 77 K, c = 1.0 × 10 M, EtOH–MeOH, (4 : 1,
5
d
2 2
Cl
2
2
e
−5
ported by the different luminescence lifetimes of Ir-1 and 9 at
540 nm (Table 1). The excitation spectra with the two emission
lifetime with the decay monitored at 578 nm. Quantum yield of the maxima at 572 nm and 732 nm were measured (Fig. S18 in the
f
v/v).
Luminescence lifetime of 572 nm in nanoseconds.
g
h
Luminescence lifetime of 732 nm in microseconds. Fluorescence
i
fluorescence with bodipy as the standard (Φ = 0.72 in THF).
ESI†). The two spectra are similar to each other, indicating
that both emission bands at 572 nm and 732 nm originate
from one species, that is, the emission band at 572 nm is not
due to any impurities.
Nanosecond time-resolved transient difference absorption
spectroscopy
In order to study the triplet excited state of the complexes, the
nanosecond time-resolved transient difference absorption of
3
3
the complexes was investigated (Fig. 3). For Ir-2, no signifi-
cant bleaching bands were observed in the visible region upon
pulsed laser excitation (Fig. 3c), which is in agreement with
the weak absorption of Ir-2 in the visible region. The bleaching
band at 620 nm is due to the phosphorescence of Ir-2. Positive
transient absorption bands at 380 nm and 760 nm were
observed. The lifetime of the transient was determined as
Fig. 2 Emission spectra of (a) Ir-1 (λex = 542 nm) and (b) Ir-2 (λex = 400 nm);
−5
2 2
c = 1.0 × 10 M in CH Cl , 20 °C.
reported, which shows absorption in the 500–600 nm region
0.3 μs, which was further quenched to 67.7 ns in an aerated
but near IR emission was only observed at low temperature
solution. Therefore, the triple feature of the transient species
is verified.
3
3
(
77 K). Previously, we used NDI for a N^N Pt(II) bis(acetylides)
2
8
complex, which shows emission at 784 nm.
Similarly, the transient absorption of Ir-1 was studied
In order to investigate the singlet/triplet feature of the emis-
sion bands of the complexes, the emission sensitivity to
(
Fig. 3a). Upon pulsed laser excitation, bleaching bands at
oxygen (O ) was studied (Fig. 2). For Ir-2, the emission can be
2
quenched to a large extent in an aerated solution compared to
that in a deaerated solution (Fig. 2b). Thus, the emission of
Ir-2 can be attributed to the triplet emission, i.e. phosphor-
escence. For Ir-1, the emission band at 732 nm is more sensi-
tive to the presence of O than that of Ir-2. For example, the
2
emission was completely quenched in an aerated solution. The
emission band at 732 nm shows structural features. Interest-
ingly, the emission at 572 nm is not sensitive to O . Therefore,
2
we concluded that the emission at 572 nm is fluorescence and
the emission band at 732 nm can be assigned to phosphor-
escence. With the increased bulkiness of the ligand, transition
metal complexes can demonstrate the residual fluorescence of
3
3,56,57
the ligands.
Since the emission wavelength is signifi-
cantly longer than the normal cyclometalated Ir(III) complexes,
we therefore propose that the emission at 732 nm is due to the
3
32,33,43
NDI-localized IL state.
It should be noted that RT
3
ligand-centered IL emission has rarely been reported for
cyclometalated Ir(III) complexes. For example, with bodipy,
coumarin and DPP-containing ligands, no ligand-localized
Fig. 3 Nanosecond time-resolved transient difference absorption spectra of (a)
Ir-1. (b) Decay trace at 542 nm. (c) Ir-2. (d) Decay trace at 310 nm in deaerated
3
33,55
−5
IL emission has been observed at RT.
CH
2
Cl
2
after pulsed laser excitation (λex = 355 nm), 2.0 × 10 M, 20 °C.
6
482 | Dalton Trans., 2013, 42, 6478–6488
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70 nm and 510 nm were observed. The minor bleaching band Triplet–triplet annihilation upconversion
Paper
3
at 510 nm compared to that at 370 nm is in agreement with
the fact that the steady-state molar absorption coefficient at
TTA upconversion has attracted much attention due to the
advantages of the requirement of low excitation power, strong
absorption of photoexcitation energy, high quantum yields
5
10 nm is much smaller than that at 370 nm. Positive transi-
ent absorptions in the 300–500 nm region and 600–700 nm
region were found. Note that the bleaching and the positive
absorption bands may be distorted by each other due to the
overlap of the bleaching and the positive transient absorption
bands. This situation may be responsible for the weak bleach-
ing band at 542 nm and the lack of a bleaching band at
and
wavelengths.
photocatalytic H production, solar cells,
readily
changeable
excitation
and
emission
18–23,62–69
TTA upconversion has been used for
70
71–73
luminescent
2
sensing. The triplet photo-
2
74
75
bioimaging and luminescent O
22
sensitizers are crucial for the TTA upconversion. However, to
date the triplet photosensitizers are limited to on-the-shelf
compounds (which have been known for a long time) and few
7
50 nm (due to the phosphorescence of Ir-1).
The triplet excited state lifetime of Ir-1 was determined as
30.0 μs. Such a long-lived triplet excited state has rarely been
reported for cyclometalated Ir(III) complexes.
tailor-designed triplet photosensitizers have been used for TTA
1
22,23
upconversion.
We developed a series of visible light-
2
,33,54,55,58a,59,60
absorbing complexes that show a long-lived triplet excited
We prepared coumarin-containing Ir(III) complexes which
22
state
complexes,
plexes,
for
TTA
upconversion,
such
as
Pt(II)
Ru(II) com-
and Re(I) complexes. However, the Ir(III) com-
plexes used for TTA upconversion are limited to those which
5
9
show a triplet state lifetime of 75.5 μs. With a bodipy-con-
28,30,44–46,76–78
31,54,59,69
Ir(III) complexes,
5
5
taining ligand, the triplet excited state lifetime is 25 μs. With
DPP-π-conjugated N^N Ir(III) complexes, the lifetime of the
52,61,79,80
80
3
3
triplet excited state was determined as 3 μs. With the ethynyl
31,54,59,60
show absorption in the blue range (<500 nm).
There-
NI ligand, the lifetime of the triplet excited state of the cyclo-
fore, much room is left for the preparation of Ir(III) complexes
that show absorption in longer wavelength regions for TTA
upconversion.
3
1
metalated Ir(III) complex was determined as 16.45 μs. We also
used NDI for the preparation of a N^N Pt(II) bis(acetylides)
complex, which shows a much shorter T state lifetime of
1
Ir-1 shows strong absorption of visible light and a long-
lived triplet excited state, which is suitable for use as a triplet
photosensitizer for TTA upconversion (Fig. 5). Upon 532 nm
laser excitation, Ir-1 gives a weak emission (Fig. 5a). In the
presence of the triplet acceptor perylene, intensive blue emis-
sion in the 450–500 nm region was observed (Fig. 5b). The
emission band is superimposable with the steady state fluor-
escence emission spectrum of perylene (Fig. S17 in the ESI†).
Furthermore, excitation of Ir-1 or perylene alone did not
produce this emission band and therefore, the upconverted
2
8
2
2.3 μs. The photophysical properties of the complexes are
listed in Table 1.
Spin density surfaces: assignment of the excited states
The triplet state of the complexes was studied from a point of
view of theoretical calculations on the spin density surface of
3
2,43,58a,59,61
the complexes (Fig. 4).
For Ir-2, the spin density
surface was distributed on the bpy and ppy ligand as well as
the Ir(III) metal center. This spin density distribution is in
agreement with the known assignment of the triplet excited
22
emission was proved. For Ir-2, however, no TTA upconversion
was observed due to the failure to excite Ir-2 at 532 nm (the
absorption of Ir-2 in the visible region is very weak). To the
best of our knowledge, this is the first time that a green light-
excitable cyclometalated Ir(III) complex was used as a triplet
photosensitizer for TTA upconversion. Previously, Ir(III) com-
plexes used for TTA upconversion have only be excited at
shorter wavelengths. For example, Ir(ppy)3 was used as a
5
9
state of cylcometalated Ir(III) complexes as a MLCT/IL state.
For Ir-1, the spin density is mainly distributed on the NDI
ligand, the Ir(III) center and the bpy ligands made no contri-
bution to the spin density surface, which is in agreement with
the long-lived IL triplet excited state of Ir-1. Therefore, the spin
density surface analysis of the complexes confirmed that the
3
T triplet state of Ir-1 is a IL state, whereas the T state of Ir-2
is a MLCT/ IL state.
1
1
3
3
Fig. 5 Upconversions with Ir-1 and Ir-2 as the triplet photosensitizers. (a) Emis-
sion of the triplet photosensitizers or perylene (Py) alone, (b) emission of the
sensitizers in the presence of perylene. Excited with a 532 nm laser (5.6 mW).
Fig. 4 Isosurfaces of the spin density of Ir-1 and Ir-2 at the optimized triplet
state geometries (isovalue = 0.0004). CH Cl was used as the solvent in the cal-
2
2
c [Ir-1 or Ir-2] = 1.0 × 10− M; c [perylene] = 1.3 × 10 M in deaerated
5
−5
culations. The calculations were performed at the B3LYP/6-31G(d)/LanL2DZ
level with Gaussian 09W.
CH Cl , 20 °C.
2
2
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T
Table 2 Triplet excited state lifetimes (τ ), Stern–Volmer constants (KSV) and
quenching constants (k
0 °C
q
) of the sensitizers. In a deaerated CH
2
Cl
2
solution,
2
a
b
c
d
e
f
τ
T
/μs
K
sv
k
q
Φ
UC /%
η
τ
DF /μs
Ir-1
Ir-2
130.0
0.3
1000
0.001
7692.3
3.3
6.7
0.0
7.4
0
238.6
—
a
Triplet state lifetime. Determined with nanosecond time-resolved
b
3
−1
−1
−1
transient absorption spectroscopy. Stern–Volmer constants. In 10 M
.
.
.
c
d
f
6
−1
Bimolecular quenching constants. k
q
= Ksv/τ
Upconversion quantum yield. η = ε × ΦUC. In 10
Lifetime of the delayed fluorescence (upconverted fluorescence).
T
. In 10
M
s
e
2
−1
Fig. 6 (a) Delayed fluorescence observed in the TTA upconversion with Ir-1 as
the triplet photosensitizer and perylene as the triplet acceptor. DF stands for
delayed fluorescence. Excited at 532 nm (nanosecond pulsed OPO laser synchro-
nized with spectrofluorometer) and the decay trace was monitored at 470 nm.
Under these circumstances, Ir-1 was selectively excited and the emission is due
to the upconverted emission of perylene. (b) Prompt fluorescence of perylene
M
cm
excited with an EPL picosecond laser (445 nm). PF stands for prompt fluor-
. c [sensitizers] = 1.0 × 10− M; c [perylene] = 1.3 ×
5
escence. In deaerated CH
2
Cl
2
−5
1
0
M; 20 °C.
T
Fig. 7 Stern–Volmer plots generated from the triplet excited state lifetime (τ )
quenching of compounds Ir-1 (λex = 532 nm) and Ir-2 (λex = 355 nm) measured
as a function of perylene concentration. Measured with the nanosecond time-
resolved transient absorption. c [sensitizers] = 2.0 × 10− M in deaerated CH
5
Cl
2
2
.
2
0 °C.
Fig. 8 (a) Photographs of the emission of the sensitizers alone and (b) the
upconversion. (c) CIE diagram of the emission of the sensitizers alone and (d) in
−
5
the presence of perylene (upconversion). c [Ir-1 or Ir-2] = 1.0 × 10 M; c [pery-
lene] = 1.3 × 10− M in deaerated CH
5
Cl
2
, λex = 532 nm (5.6 mW); 20 °C.
2
photosensitizer for upconversion but the excitation was at
8
1
4
45 nm. Previously, we used coumarin-containing Ir(III) com-
plexes for TTA upconversion with excitation in the blue range
3
1,54,59,60
(445 nm or 473 nm).
the variation of the triplet state lifetime of the Ir(III) complexes
In order to prove the TTA upconversion, the luminescence Ir-1 and Ir-2. Much larger Stern–Volmer constants (1.0 ×
6
−1
lifetime of the upconversion was measured (Fig. 6a). It is 10 M ) were observed for Ir-1 than for Ir-2. Therefore, the TTET
known that the luminescence lifetime of the TTA upconverted process of the Ir-1/perylene is much more efficient than that of
1
9,44,45,80,82
emission is exceptionally long.
A lifetime of Ir-2/perylene. This result indicates that the lifetime of the
2
38.6 μs was observed for the blue emission band of the Ir-1/ triplet excited state of triplet photosensitizers is important for
perylene mixture. For the perylene alone, however, an emission their application. The photophysical parameters related to the
lifetime of 4.8 ns was observed (Fig. 6b). Therefore, the TTA TTA upconversion process are listed in Table 2. From Table 2,
upconversion feature of the blue emission of the Ir-1/perylene it is clear that the Stern–Volmer constants and the quenching
mixture was unambiguously proved. The long-lived upcon- constants of Ir-1 are much larger than that of Ir-2. The
verted luminescence may be useful for time-resolved lumines- enhanced TTET of Ir-1 is due to its long-lived triplet excited
7
4,75
cent bioimaging or oxygen sensing.
In order to reveal the effect of the triplet state lifetime on triplet state lifetime (0. 3 μs).
the TTA upconversion, the triplet–triplet-energy-transfer
The TTA upconversion is clearly visible to un-aided eyes
state (130.0 μs). In comparison, Ir-2 gives a much shorter
(
(
TTET) efficiency was studied by a quenching experiment (Fig. 8). For Ir-1 alone, the emission is yellow. In the pres-
Fig. 7), i.e. the quenching of the triplet excited state of the ence of the triplet acceptor perylene, intensive blue emission
complexes by the triplet acceptor perylene was monitored by is observed (Fig. 8b). For Ir-2, however, no such emission
6
484 | Dalton Trans., 2013, 42, 6478–6488
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Paper
Fig. 9 Time-resolved emission spectra (TRES) of Ir-1 alone and the TTA upcon-
Scheme 2 Jablonski diagram of the triplet–triplet-annihilation (TTA) upconver-
version with perylene as the triplet acceptor. (a) The phosphorescence region
sion with Ir-1 as the triplet photosensitizer. GS stands for ground state (S
0
); E is
was measured (500 nm–780 nm, τ
pulsed laser (532 nm). (b) Upconverted emission in the range of 400 nm–
00 nm (τDF = 252.1 μs) with nanosecond pulsed laser (532 nm) excitation.
P
= 207.1 μs), excited with a nanosecond
1
energy; IL* is the intraligand singlet excited state (NDI localized); ISC is intersys-
3
tem crossing; IL* is the intraligand triplet excited state (NDI localized); TTET is
8
3
triplet–triplet energy transfer; Perylene* is the triplet excited state of perylene;
−5
−5
c [photosensitizers] = 1.0 × 10 M, c [perylene] = 1.3 × 10 M. In deaerated
CH Cl ; 20 °C.
1
TTA is triplet–triplet annihilation; Perylene* is the singlet excited state of pery-
2
2
lene. The emission bands observed in the TTA upconversion experiment is the
delayed upconverted emission.
color is observed, which is in agreement with the spectral
results of the TTA upconversion (Fig. 6). The change in the
emission color of the triplet photosensitizers with and
Conclusions
without the triplet acceptor perylene were quantified with the Naphthalenediimide (NDI) was connected to the coordination
CIE coordinates. For Ir-1, the CIE coordinates changed from center of cyclometalated Ir(III) complexes via a π-conjugated
(
0.44, 0.50) to (0.14, 0.048). For Ir-2, no significant change linker (CuC triple bonds. Ir-1). Strong absorption of visible
light (542 nm, ε = 11 000 M− cm ) was observed for Ir-1 com-
(bpy) (Ir-2), which shows
sion spectra (TRES, Fig. 9). For complex Ir-1 alone, a long-lived typical weak absorption of visible light. Room temperature near
phosphorescence emission band was observed at 730 nm (τ
IR emission was observed for Ir-1 (732 nm) compared to the
07.1 μs). The lifetime is close to that obtained in the lifetime normal emission at 578 nm for Ir-2. Furthermore, with nano-
1
−1
was observed.
The upconversion was also studied with time-resolved emis- pared to the model complex Ir(ppy)
2
P
=
2
measurement (220.3 μs. Table 1). It should be pointed out that second time-resolved transient absorption spectroscopy and DFT
the short-lived fluorescence emission band of Ir-1 cannot be calculations on the spin density surfaces of the complex, we con-
recorded in the microsecond resolved TRES (the lifetime of the firmed the long-lived triplet excited state (130.0 μs) of Ir-1 is due
3
fluorescence band of Ir-1 is 5.0 ns, Table 1). In the presence of to the NDI-localized intraligand triplet excited state ( IL state).
the triplet acceptor perylene (Fig. 9b), the phosphorescence at In comparison, the model complex shows a typical short
7
32 nm was diminished. However, strong emission in the blue lifetime of 0.3 μs. The complexes were used for triplet–triplet
region (460 nm) was observed. The lifetime of this long-lived annihilation upconversion and an upconversion quantum yield
emission band is 252.1 μs (Fig. 9b), which is very close to that of 6.7% was observed for Ir-1 but no upconversion was observed
obtained with different measuring modes (Fig. 6a, 238.6 μs). with Ir-2 under the same experimental conditions. Our method
These results unambiguously confirmed the TTA upconversion of preparing Ir(III) complexes shows that visible light-absorption
of Ir-1 in the presence of the triplet acceptor DPA as well as the and a long-lived triplet excited state will be useful for the devel-
kinetic feature of the emissions.
opment of Ir(III) complexes and for their applications in photo-
The photophysical processes involved in the TTA upcon- catalysis and upconversion.
1
8
version are presented in Scheme 2. Upon visible light
1
photoexcitation, first the IL state of Ir-1 was populated.
3
Then direct ISC produces the NDI-localized IL excited state.
It should be pointed out that IL→ MLCT energy transfer is
Acknowledgements
1
1
impossible due to the unmatched energy levels. The long- We thank the NSFC (20972024, 21073028, 21103015 and
3
lived IL excited state ensures efficient TTET process from 21273028), the Royal Society (UK) and NSFC (China-UK Cost-
Ir-1 to perylene, the triplet energy acceptor in the TTA upcon- Share Science Networks, 21011130154), the Science Foundation
version. Annihilation of the perylene triplet excited states Ireland (SFI E.T.S. Walton Program 11/W.1/E2061) and the Minis-
produces the singlet excited state and the delayed fluor- try of Education SRFDP-(20120041130005) for financial support.
escence was observed. A notable feature of the present Ir(III)
complex for TTA upconversion is the strong absorption of
visible light (542 nm) and the exceptionally long-lived triplet
excited states, these features are rarely reported for cyclo-
Notes and references
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