Strongly emissive long-lived 3IL excited state of coumarins in cyclometalated Ir(iii) complexes used as triplet photosensitizers and application in triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c3dt52306c

Three different coumarin chromophores were used to prepare the Ir(iii) complexes, i.e. coumarin 6 (Ir-1), 7-diethylamino coumarin (Ir-2) and x-phenyl-y-diethylamino coumarin (Ir-3). Ir(ppy)₂(bpy)[PF ₆] was used as the reference compl

Full text of DOI:10.1039/c3dt52306c

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Strongly emissive long-lived 3IL excited state of
coumarins in cyclometalated Ir(III) complexes used
as triplet photosensitizers and application in
triplet–triplet annihilation upconversion†
Cite this: Dalton Trans., 2014, 43,
1672
Xiuyu Yi, Caishun Zhang, Song Guo, Jie Ma and Jianzhang Zhao*
Three different coumarin chromophores were used to prepare the Ir(III) complexes, i.e. coumarin 6 (Ir-1),
7-diethylamino coumarin (Ir-2) and x-phenyl-y-diethylamino coumarin (Ir-3). Ir(ppy)2(bpy)[PF6] was used
as the reference complex (Ir-0). The coumarin units were connected to the coordination center of the
complexes by using a CuC π-conjugation bond. The photophysical properties of the complexes were
studied with steady state and time-resolved absorption and luminescence spectroscopy, low-temperature
luminescence (77 K), as well as DFT calculations. All the three new complexes show strong absorption of
visible light (molar absorption coefficient ε is up to 42 000 M−1 cm−1 at 487 nm) and a long-lived triplet
excited state (τT = 65.9 μs), compared to the reference complex Ir(ppy)2(bpy)[PF6], which shows the
typical weak visible light-absorption (ε < 5000 M−1 cm−1 in the region beyond 400 nm) and a short triplet
excited state (τT = 0.3 μs). Interestingly the long-lived triplet excited states are strongly phosphorescent
(quantum yield is up to 18.2%, with emission maxima at 607 nm), which is rare for phosphorescent
transition metal complexes. With nanosecond time-resolved transient difference absorption spectroscopy
we proved that a coumarin-localized triplet excited state (3IL) was produced upon photoexcitation. The
complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion and upcon-
version quantum yields up to 22.8% were observed. Our results are useful for the preparation of visible
light-harvesting transition metal complexes, the study of the triplet excited state of organic chromo-
phores, as well as the application of these visible light-harvesting transition metal complexes as efficient
triplet photosensitizers.
Received 22nd August 2013,
Accepted 30th October 2013
DOI: 10.1039/c3dt52306c
www.rsc.org/dalton
they are detrimental to the new applications concerning visible
Introduction
light-harvesting, such
as photocatalysis1 and
TTA
Transition metal complexes such as cyclometalated Ir(III) com- upconversion.9–13 Therefore, it is highly desired to design
plexes have attracted much attention due to the applications cyclometalated Ir(III) complexes that show strong absorption of
in photocatalysis,1–3 photodynamic therapy,3–5 phosphor- visible light and long-lived triplet excited states.17–20
escence and molecular probes,6–8 and more recently in the
To attach a visible light-harvesting ligand to the coordi-
triplet–triplet annihilation (TTA) upconversion.9–13 However, nation center, either by a π-conjugation linker or a non-conju-
most of the known Ir(III) complexes show weak absorption of gate linker, is the effective method to prepare the
visible light (molar absorption coefficients are smaller than aforementioned transition metal complexes.3,17,21–23 A π-conju-
10 000 M−1 cm−1 in the region >400 nm), and the lifetime of gation linker between the organic chromophore and the Ir(III)
the triplet excited state is short (usually less than 5 μs).14–17 coordination center is preferred, so that the intersystem cross-
These properties do not hamper the complexes from being ing (ISC) will be improved.24–26 Concerning this aspect,
used in conventional areas such as electroluminescence,8 but Bodipy,24,27 naphthaimide,28 naphthalenediimide,29 and peryl-
enebisimide have been attached to the Ir(III) coordination
center.26 The common feature of these organic chromophore-
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
containing Ir(III) complexes is the strong absorption of visible
light and long-lived triplet excited states.3 We have used these
University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024,
P. R. China. E-mail: zhaojzh@dlut.edu.cn; http://finechem.dlut.edu.cn/photochem
complexes in photocatalysis and TTA upconversion.10 Gener-
†Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and
HRMS data of the compounds, the photophysical data and DFT calculations of
ally much improved efficiencies were observed for these com-
plexes compared to those conventional Ir(III) complexes such
the Ir(III) complexes. See DOI: 10.1039/c3dt52306c
1672 | Dalton Trans., 2014, 43, 1672–1683
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as Ir(ppy)3, which shows weak absorption of visible light and molecular probes or light-harvesting arrays.31–33 However, its
short-lived triplet excited states.30 However, much room is left application in the triplet excited state manifold is rare.25,34–36
to explore the preparation of visible light-absorbing Ir(III) com- Previously coumarin 6 was used for the preparation of Ir(III)
plexes. For example, coumarin is a versatile organic chromo- complexes, by the direct cyclometalation method (coumarin 6
phore, which has been extensively used in fluorescent as the C donor of the C–Ir bond, Ir–Cou–a, Scheme 1),14a,37
Ru(II) complexes.34,35 However, coumarin derivatives were
never connected to the Ir(III) coordination center via a π-conju-
gation linker, such as a CuC bond. Furthermore, the 3IL emis-
sion of coumarin was rarely reported.37 Previously we prepared
coumarin-containing Ir(III) complexes with the imidazole
linker, and normally the complexes are weakly phosphorescent
(Ir–Cou–b–d, Scheme 1).25,38 In these complexes, the π-conju-
gation framework of the coumarin is not directly connected to
the Ir(III) center.
In order to overcome the aforementioned challenges,
herein we prepared C^N cyclometalated Ir(III) complexes in
which three different coumarin chromophores were connected
to the Ir(III) coordination center with the π-conjugation linker
(Schemes 2 and 3). Strong absorption of visible light and
3
long-lived triplet excited IL state were observed for the com-
plexes. Interestingly, strong red phosphorescence originating
from the 3IL excited state was observed. The photophysical pro-
perties were investigated by steady state and time-resolved
spectroscopy, as well as DFT calculations. The phosphor-
escence lifetimes of Ir-1–Ir-3 are much longer than that of the
reference complex Ir-0. The complexes were used as triplet
photosensitizers for TTA upconversion. Our results are useful
for the preparation of visible light-harvesting cyclometalated
Ir(III) complexes and for the application of these complexes in
photocatalysis and TTA upconversion.
Scheme 1 Some known coumarin-containing cyclometalated Ir(III)
complexes.
Scheme 2 Synthesis of Ir-0, Ir-1, Ir-2 and Ir-3. (a) IrCl3·3H2O, 2-ethoxyethanol–water = 3 : 1 (v/v) as solvent, 110 °C, 24 h; (b) CH2Cl2–MeOH = 2 : 1
(v/v), reflux, 5–6 h.
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Scheme 3 Synthesis of Ir-0, Ir-1, Ir-2 and Ir-3. (a) CH3COOCOCH3, HCl/CH3COOH, pyridine, 1 h, room temperature; (b) Br2, CH3COOH, room temperature;
(c) K2CO3, CH3OH, 10 min, room temperature; (d) 130 °C, 6 h; (e) CH3OH, 30 °C, 4 h; (f) pyridine, reflux; (g) tributyltin chloride, n-BuLi, DEE; (h) 2,5-dibromo-
pyridine, Pd(PPh3)4, xylene; (i) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, THF/NEt3; (j) TBAF, THF, Ar, room temperature; (k) Pd(PPh3)4, CuI, THF/NEt3.
Compounds 1–8 were synthesized according to the litera-
Experimental
General information
ture methods.39,40 9–12 were prepared by the reported
method.41
NMR spectra were taken on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with a Q-TOF
Compound L1
Micro MS spectrometer. UV-Vis spectra were taken on a
HP8453 UV-Visible spectrophotometer. Fluorescence spectra
were recorded on a Shimadzu RF-5301PC spectrofluorometer.
Luminescence quantum yields were measured with [Ru(dmb)3]
[PF6]2 (Ru-1) as standard (ΦP = 0.073 in deaerated acetonitrile).
Luminescence lifetimes were measured on an OB 920 lumine-
scence lifetime spectrometer (Edinburgh, UK).
5 (120 mg, 0.28 mmol) and 5-ethynyl-2,2-bipyridine (12)
(50.0 mg, 0.28 mmol) were added to deaerated triethylamine
(20 mL). Then Pd(PPh3)4 (8.0 mg, 5 mol%) and CuI (3 mg,
5 mol%) were added under an Ar atmosphere. The mixture was
stirred and refluxed for 8 h under Ar. After completion of the
reaction, the mixture was cooled to RT and the light red pre-
cipitate was collected by filtration. The crude product was
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purified with column chromatography (silica gel, ethyl Complex Ir-0
acetate–petroleum ether = 1 : 2, v/v) to give an orange solid.
A solution of [Ir(ppy)2Cl]2 (43 mg, 0.04 mmol) and 2,2′-bipyri-
dine (16 mg, 0.1 mmol) in CH2Cl2–MeOH (12 mL, 2 : 1 v/v) was
refluxed under an Ar atmosphere. After 5–6 h, the orange solu-
tion was cooled to rt, and then a 10-fold excess of ammonium
hexafluorophosphate was added. The suspension was stirred
for 15 min and then filtered to remove insoluble inorganic
salts. The solution was evaporated to dryness under reduced
pressure to obtain a crude orange solid. The solid was dis-
solved in CH2Cl2 and filtered to remove the residual traces of
inorganic salts, then purified with column chromatography
(silica gel, CH2Cl2–MeOH = 25 : 1) to give a light yellow solid.
Yield: 48 mg (76.2%). M.p. >250 °C. 1H NMR (400 MHz, CDCl3,
CH3OD), δ = 8.49 (d, 4H, J = 7.8 Hz), 8.18 (t, 1H, J = 9.0 Hz),
7.94–7.87 (m, 6H), 7.78 (t, 1H, J = 7.8 Hz), 7.70 (d, 1H, J =
7.3 Hz), 7.52 (d, 1H, J = 7.8 Hz), 7.42–7.36 (m, 6H), 7.07–7.00
(m, 2H), 6.94 (t, 1H, J = 7.1 Hz), 6.31 (d, 1H, J = 7.6 Hz).
ESI-HRMS: [(M − PF6)]+, calc (C32H24N4Ir): m/z = 657.1630,
found, m/z = 657.1625.
Yield: 101 mg (68.2%). M.p.: 201.3–201.9 °C. 1H NMR
(400 MHz, CDCl3), δ = 8.91 (s, 1H); 8.81 (s, 1H); 8.72 (d, 1H, J =
2.0 Hz); 8.46 (t, 2H, J = 8.7 Hz); 8.06 (d, 1H, J = 7.6 Hz);
7.96–7.85 (m, 3H); 7.79 (s, 1H); 7.51 (t, 1H, J = 7.6 Hz); 7.39 (m,
2H); 6.74 (s, 1H); 3.71 (m, 4H); 1.34 (t, 6H, J = 5.8 Hz). 13C
NMR (100 MHz, CDCl3): δ = 161.0, 160.5, 155.9, 155.3, 154.8,
152.6, 151.4, 149.3, 141.0, 140.5, 139.1, 137.4, 136.6, 131.1,
129.0, 126.4, 124.2, 121.8, 121.6, 120.7, 120.6, 118.8, 115.2,
110.6, 108.4, 102.0, 92.5, 90.5, 46.1, 13.4 ppm. EI-HRMS: calcd
(C32H24N4O2S): m/z = 528.1620, found, m/z = 528.1628.
Compound L2
The synthetic procedure is similar to that of L1 except that
compound 7 (63 mg, 0.28 mmol), instead of 5, was used. The
crude product was purified with column chromatography
(silica gel, ethyl acetate–petroleum ether = 1 : 2, v/v) to give an
orange solid. Yield: 81 mg (73.6%). M.p.: 183.7–184.1 °C. 1H
NMR (400 MHz, CDCl3), δ = 8.83 (s, 1H); 8.70 (s, 1H); 8.42 (t,
2H, J = 4.9 Hz); 7.97 (d, 1H, J = 9.0 Hz); 7.85 (m, 3H); 7.79 (m,
Complex Ir-1
2H); 7.33 (d, 1H, J = 4.1 Hz); 7.27 (d, 1H, J = 7.9 Hz); 6.61 (d, The synthetic procedure is the same as that for Ir-0, except that
1H, J = 8.7 Hz); 6.50 (s, 1H); 3.46–3.41 (m, 4H); 1.23 (t, 6H, J = L1 (53 mg, 0.1 mmol) was used. The solution was evaporated
7.2 Hz). 13C NMR (100 MHz, CDCl3): δ = 160.9, 156.6, 154.8, to dryness under reduced pressure to obtain a crude orange
151.6, 149.4, 146.3, 139.6, 137.2, 132.3, 129.3, 128.7, 128.6, solid. The solid was dissolved in CH2Cl2 and filtered to remove
124.1, 121.6, 120.5, 109.5, 108.5, 103.9, 97.4, 90.3, 89.1, 45.1, the residual inorganic salts, then purified by column chromato-
12.6 ppm. EI-HRMS: calcd (C25H21N3O2): m/z = 395.1634, graphy (silica gel, CH2Cl2–MeOH, 25 : 1, v/v) to give a red solid.
found, m/z = 395.1644.
Yield: 70 mg, (75.1%). M.p. >250 °C. 1H NMR (400 MHz,
DMSO-d6), δ = 9.00 (s, 1H); 8.89 (d, 2H, J = 8.4 Hz); 8.32 (t, 4H,
J = 16.4 Hz); 8.09 (d, 1H, J = 7.9 Hz); 8.03–7.93 (m, 6H); 7.89 (s,
2H); 7.83 (d, 1H, J = 5.5 Hz); 7.74 (t, 1H, J = 12.8 Hz); 7.68 (d,
1H, J = 5.5 Hz); 7.55 (t, 1H, J = 15.3 Hz); 7.42 (t, 1H, J = 15.0
Hz); 7.23–7.17 (m, 2H), 7.07–7.02 (m, 2H); 6.96–6.91 (m, 2H);
6.81(s, 1H); 6.24 (t, 2H, J = 17.1 Hz); 3.50 (m, 4H); 1.04 (t, 6H,
J = 13.6 Hz). 13C NMR (100 MHz, DMSO-d6): δ 166.9, 166.8,
160.4, 159.6, 155.8, 154.7, 154.0, 153.8, 152.0, 150.7, 150.2,
149.9, 149.2, 143.9, 143.8, 141.2, 140.6, 139.8, 139.1, 135.6,
131.2, 130.4, 126.6, 125.5, 124.7, 124.2, 123.8, 122.6, 122.1,
120.2, 113.9, 110.0, 105.0, 100.8, 96.1, 87.7, 45.5, 13.1 ppm.
Compound L3
The synthetic procedure is similar to that of L1 except that
compound 8 (104 mg, 0.28 mmol), instead of 5, was used. The
crude product was purified with column chromatography
(silica gel, ethyl acetate–petroleum ether = 1 : 2, v/v) to give an
orange solid. Yield: 66 mg (50.1%). M.p.: 181.2–181.7 °C. 1H
NMR (400 MHz, CDCl3), δ = 8.82 (s, 1H); 8.69 (d, 1H, J =
3.6 Hz); 8.43–8.40 (m, 2H); 7.95 (d, 1H, J = 8.3 Hz); 7.83–7.73
(m, 4H); 7.60 (d, 2H, J = 8.1 Hz); 7.29 (t, 2H, J = 9.7 Hz); 6.59
(d, 1H, J = 10.2 Hz); 6.51 (s, 1H); 3.44–3.38 (m, 4H); 1.23 (t, 6H,
J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 161.4, 156.4, 155.6,
154.9, 151.7, 150.9, 149.4, 140.9, 139.4, 137.1, 136.4, 131.7,
129.3, 128.2, 124.0, 121.7, 121.5, 120.4, 119.8, 109.2, 109.1,
97.1, 93.8, 87.2, 45.0, 12.6 ppm. EI-HRMS: calcd (C31H25N3O2):
m/z = 471.1947, found, m/z = 471.1953.
ESI-HRMS: [(M
1029.2563, found, m/z = 1029.2568.
− =
PF6)]+, calc (C54H40N6IrO2S): m/z
Complex Ir-2
The synthetic procedure is the same as that for Ir-0, except that
L2 (40 mg, 0.1 mmol) was used. The solution was evaporated
to dryness under reduced pressure to obtain a crude red solid.
The solid was dissolved in CH2Cl2 and filtered to remove the
General method for synthesis of the Ir(III) complexes
All procedures were carried out under an Ar atmosphere. Cyclo- residual inorganic salts, then purified by column chromato-
metalated Ir(III) chloro-bridged dimers were synthesized graphy (silica gel, CH2Cl2–MeOH, 25 : 1, v/v) to give a red solid.
according to a literature method.38 IrCl3·3H2O and 2.5 equiv. Yield: 65.8 mg, (79.0%). M.p. >250 °C. 1H NMR (400 MHz,
of ligand were heated in a 3 : 1 mixture of 2-ethoxyethanol and CDCl3/CD3OD), δ = 8.59 (d, 2H, J = 4.4 Hz); 8.27 (d, 1H, J =
water (v/v). The slurry was heated at 110 °C for 24 h. After 7.6 Hz); 8.17 (t, 1H, J = 15.8 Hz); 8.07 (d, 2H, J = 6.8 Hz); 7.88
cooling to rt, the crude product was filtered. The solid residue (s, 2H); 7.64–7.59 (m, 4H); 7.54 (s, 3H); 7.31 (d, 2H, J = 7.9 Hz);
was washed with water, followed by two portions of n-hexane 7.10–7.06 (m, 4H); 6.96 (d, 2H, J = 7.1 Hz); 6.71 (d, 1H, J =
and ether. Then the solid was dried under vacuum.
9.3 Hz); 6.51(s, 1H); 6.33–6.28 (m, 2H); 3.52–3.47 (m, 4H); 1.27
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(t, 6H, J = 13.8 Hz). 13C NMR (100 MHz, CDCl3/CD3OD): δ 78.0% in CH3CH2OH) and the quantum yields were calculated
167.7, 166.6, 156.7, 153.8, 152.3, 150.7, 148.3, 143.3, 139.3, with eqn (2), where Φunk, Aunk, Iunk and ηunk represents the
138.3, 132.4, 131.4, 130.6, 129.8, 129.2, 128.7, 124.7, 123.2, quantum yield, absorbance, integrated photoluminescence
122.8, 120.5, 120.0, 113.7, 110.0, 108.2, 96.6, 88.6, 87.3, 44.9, intensity of the samples and the refractive index of the
12.0 ppm. ESI-HRMS: [(M − PF6)]+, calc (C47H37N5IrO2): m/z = solvents.9
896.2576, found, m/z = 896.2562.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Astd
Asam
Isam
Istd
ηsam
ηstd
Φsam ¼ 2Φstd
ð2Þ
Complex Ir-3
The synthetic procedure is the same as that for Ir-0, except that
L3 (47 mg, 0.1 mmol) was used. The solution was evaporated
to dryness under reduced pressure to obtain a crude red solid.
The solid was dissolved in CH2Cl2 and filtered to remove the
residual inorganic salts, then purified by column chromato-
graphy (silica gel, CH2Cl2–MeOH, 25 : 1, v/v) to give a red solid.
Yield: 67.2 mg (75.2%). M.p. >250 °C. 1H NMR (400 MHz,
CDCl3), δ = 8.65 (d, 2H, J = 8.3 Hz); 8.17 (m, 2H); 7.98–7.92 (m,
4H); 7.80–7.70 (m, 7H); 7.57 (d, 1H, J = 5.3 Hz); 7.52–7.46 (m,
3H); 7.42–7.36 (m, 2H); 7.07–7.01 (m, 4H); 6.96–6.90 (m, 2H);
6.70 (s, 1H); 6.61 (s, 1H); 6.32–6.27 (m, 2H); 3.47–3.42 (m, 4H);
1.25 (t, 6H, J = 14.2 Hz). 13C NMR (100 MHz, DMSO-d6): δ
166.7, 166.6, 160.2, 156.0, 154.7, 154.5, 150.8, 150.1, 149.8,
149.4, 149.0, 139.7, 138.9, 138.8, 137.2, 131.4, 131.1, 130.9,
130.4, 130.2, 130.0, 128.1, 125.5, 125.0, 124.9, 124.0, 123.6,
122.4, 120.2, 120.0, 119.0, 117.0, 109.3, 108.3, 96.4, 85.1, 44.1,
12.3. ESI-HRMS: [(M − PF6)]+, calc (C53H41N5IrO2): m/z =
972.2890, found, m/z = 972.2897.
The delayed fluorescence of the upconversion was
measured with nanosecond pulsed laser (Opolett™ 355 II
nanosecond pulsed laser, pulse length: 7 ns; pulse repetition:
20 Hz; peak OPO energy: 4 mJ; OPOTEK, USA), which is syn-
chronized to a FLS 920 spectrofluorometer (Edinburgh, U.K.).
The pulsed laser is sufficient to sensitize the TTA upconver-
sion. The decay kinetics of the upconverted fluorescence
(delayed fluorescence) was monitored with a FLS 920 spectro-
fluorometer (synchronized to the OPO nanosecond pulsed
laser). The prompt fluorescence lifetimes were measured with
EPL picosecond pulsed laser (405 nm) which is synchronized
to the FLS 920 spectrofluorometer. The photographs of the
upconversion were taken with Sumsung NV 5 digital camera.
DFT calculations
The density functional theory (DFT) calculations were used for
the optimization of the singlet states and triplet states. The
spin density surfaces of the complexes were calculated based
on the optimized triplet state geometries. All the calculations
were performed with Gaussian 09W.42
Singlet oxygen (1O2) quantum yields (ΦΔ)
The 1O2 quantum yields (ΦΔ) of the photosensitizers were
determined with 2,6-diiodo-bodipy as standard (ΦΔ = 0.83 in
dichloromethane, DCM). Air saturated DCM was obtained by
Results and discussion
Design and synthesis of the complexes
1
bubbling air for 15 min. The absorbance of the O2 scavenger
1,3-diphenylisobenzofuran (DPBF) was adjusted around 1.0 in
air saturated DCM. Then, the photosensitizer was added to the
cuvette and the photosensitizer’s absorbance was adjusted to
around 0.2–0.3. The cuvette was irradiated with monochro-
matic light at the peak absorption wavelength for 10 seconds.
Absorbance was measured several times after each irradiation.
The slopes of absorbance maxima of DPBF at 414 nm versus
time graph for each photosensitizer were calculated. Singlet
oxygen quantum yield (ΦΔ) were calculated according to a
modified equation:
Three different coumarin chromophores were selected, i.e. the
coumarin 6, coumarin and the phenylcoumarin. All
the chromophores were connected to the bpy ligand via the
π-conjugation linker, so that the ISC can be enhanced. Cou-
marin 6 and diethylamino coumarin are known to be highly
fluorescent. The subtle difference between the two chromo-
phores may cause different effects on the photophysical pro-
perties of the complexes. In Ir-3, the diethylamino coumarin
moiety was isolated from the Ir(III) coordination center by a
phenyl unit. In order to prepare coumarin 6, 5-bromo-4-diethyl-
salicylaldehyde was prepared. Then the bromo coumarin 6 was
prepared (Scheme 3). Sonogashira coupling between compound
5 and 5-ethynyl-2,2′-bipyridine gives ligand L-1. Ligands L-2 and
L-3 were prepared by similar methods. All the compounds were
obtained in satisfactory yields (Scheme 2). Complex Ir-0 was
used as reference complex.
ksam Fref
Φsam ¼ Φref
ꢀ
ꢀ
ð1Þ
kref
Fsam
where ‘sam’ and ‘ref’ designate the photosensitizers and ‘MB’
respectively. k is the slope of the plots of the absorbance of
DPBF (414 nm) against the irradiation time, F is the absorp-
tion correction factor, which is given by F = 1–10−O.D. (O.D. at
the irradiation wavelength).
UV-Vis absorption and emission spectra
Triplet–triplet annihilation upconversion
The UV-Vis absorption spectra of the complexes were studied
Diode pumped solid state laser was used for the upconver- (Fig. 1). The reference complex Ir-0 gives weak absorption in
sions. The samples were purged with N2 for 15 min before the visible region, with ε < 5000 M−1 cm−1 in the region
measurement. The upconversion quantum yields were deter- beyond 400 nm. In contrast, all the coumarin-containing com-
mined with coumarin-6 as the quantum yield standard (Φ = plexes Ir-1, Ir-2 and Ir-3 give strong absorption in the visible
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Fig. 3 Photoluminescence spectra of the complexes at RT and 77 K. (a)
Ir-1 (λex = 470 nm), (b) Ir-2 (λex = 470 nm). In C2H5OH–CH3OH (4 : 1, v/v),
1.0 × 10−5 M.
Fig. 1 UV-Vis absorption of Ir(III) complexes. c = 1.0 × 10−5 M in CH2Cl2,
20 °C.
measured and were compared with those at room temperature
(Fig. 3).24,25,27,43 For Ir-1, the emission spectra became more
structured at 77 K compared to that at RT. The thermally
induced Stokes shift is very small. Therefore, the emission of
Ir-1 can be attributed to the intraligand triplet excited state
(3IL state) localized on the coumarin unit.25 For Ir-2, the struc-
tured emission spectrum at 77 K was observed, with a small
thermally induced Stokes shift of 481 cm−1. The emissive
region. For example, Ir-1 shows a broad absorption band
in the region of 350 nm–450 nm, with maximum at 439 nm
(22 440 M−1 cm−1). Ir-2 gives stronger absorption at 487 nm
(42 000 M−1 cm−1). In comparison Ir-3 gives slightly blue-
shifted absorption at 437 nm (45 800 M−1 cm−1).
The luminescence spectra of the complexes were studied
(Fig. 2). Ir-1 gives emission at 607 nm. Similar emission bands
were observed for Ir-2 and Ir-3. Furthermore, we found that the
emission of the complexes Ir-1–Ir-3 can be dramatically
quenched by O2. Therefore, the luminescence can be attribu-
ted to phosphorescence. Moreover, the emissions of Ir-1 and
Ir-2 are almost completely quenched by O2, indicating the
longer phosphorescence lifetime of Ir-1 and Ir-2 than that of
Ir-3.
3
triplet excited state of Ir-2 can also be assigned as an IL state.
Similar results were observed for Ir-3 (see ESI† for detail). For
the reference complex Ir-0, however, the emission band at 77 K
is structureless and the thermally induced Stokes shift is
larger. DFT calculations on the T1 state energy level support
the assignment of the triplet excited states as the intraligand
triplet excited state.44,45
Nanosecond time-resolved transient absorption spectra
77 K emission spectra
In order to study the triplet excited state of the complexes, the
nanosecond time-resolved transient difference absorption
spectra of the complexes were investigated (Fig. 4). For Ir-1,
In order to study the emissive states of the complexes, the
phosphorescence spectra of the complexes at 77 K were
Fig. 4 Nanosecond time-resolved transient difference absorption
spectra of (a) Ir-1, (c) Ir-2 after pulsed excitation (λex = 532 nm). Decay
trace of (b) Ir-1 at 470 nm and (d) Ir-2 at 490 nm. 1.0 × 10−5 M in deaer-
ated CH2Cl2. 20 °C.
Fig. 2 The emission spectra of the complexes under different atmos-
phere. (a) Ir-1 (λex = 470 nm), (b) Ir-2 (λex = 475 nm), (c) Ir-3 (λex
438 nm), (d) Ir-0 (λex = 403 nm). c = 1.0 × 10−5 M in CH2Cl2, 20 °C.
=
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Table 1 Photophysical parameters of Ir-0, Ir-1, Ir-2 and Ir-3
τL e/μs
ε b
λem
ΦL d/% 77 K
298 K
τT f (μs) ΦΔ
a
c
g
λabs
Ir-0 405
Ir-1 473
Ir-2 487
Ir-3 437
0.50 578
2.01 607
4.20 645
4.58 640
4.3
18.2
6.5
4.8
0.3
0.3
65.9
34.8
2.7
—
9962.8 102.2
137.6
1241.0
0.803
0.721
0.585
36.7
7.37
1.1
a In CH2Cl2 (1.0 × 10−5 M). b Molar extinction coefficient at the
absorption maxima. ε: 104/cm−1 mol−1 dm3. c In CH2Cl2. d In CH2Cl2,
with [Ru(dmb)3][PF6]2 (Ru-1) (Φ = 0.073 in deaerated acetonitrile) as
the standards. e Luminescence lifetime, at 298 K (in CH2Cl2) and 77 K
(in C2H5OH–CH3OH = 4 : 1). f Triplet state lifetime, measured by time-
resolved transient absorption. 1.0 × 10−5 M in deaerated CH2Cl2.
g Singlet oxygen quantum yield. With 2,6-diiodo-Bodipy as a standard
(ΦΔ = 0.83 in DCM). Photoexcitation wavelengths of 494 nm, 509 nm,
490 nm were used.
Fig. 5 Spin density of the complexes Ir-0, Ir-1, Ir-2 and Ir-3. Calculated
by DFT at the CAM-B3LYP/6-31G/LanL2DZ level using Gaussian 09W.
upon visible light excitation, a bleaching band at 444 nm was
observed, which is due to the depletion of the ground state.
Positive absorption in the region of 550–750 nm was observed.
The lifetime of the transient was determined as 65.9 μs, which
was substantially quenched in an aerated solution. Therefore,
the observed long-lived transient species is due to the triplet
excited state of the complex, which is localized on coumarin
6. These results indicated a subtle variation of the molecular
structure of the moiety.24,25 A similar transient was observed
for Ir-2, but the lifetime was reduced to 34.6 μs. For Ir-3,
however, the triplet excited state lifetime is much shorter,
2.7 μs (Table 1). Since the main application of the triplet
photosensitizers is the triplet–triplet energy transfer (TTET),
the efficiency of the TTET processes will be affected by the life-
time of the triplet excited state of the complexes.10,12,13,46,47
The photophysical properties of the compounds are listed
in Table 1. Notably, the complexes have long-lived triplet
excited states, yet the phosphorescence quantum yields are
high. Such long-lived strong emissive T1 excited states are
rarely reported for cyclometalated Ir(III) complexes. Normally
the long-lived triplet excited states and the strong emissive T1
state are exclusive for Ir(III) complexes.14,15,17 We noted the
difference between the phosphorescence lifetimes and the
triplet excited state lifetimes measured by the nanosecond
time-resolved transient absorption spectroscopy. The discre-
pancy can be attributed to the different TTA effect for the
phosphorescence lifetime and the triplet excited state lifetime
measurement, for which the laser power is different, as a
result, the concentrations of the excited molecules are
different. The TTA effect is concentration dependent.
Firstly the spin density surfaces of the complexes were
studied (Fig. 5).25,27,53–56 This approach is helpful to reveal the
localization of the triplet excited state of the transition metal
complexes, especially for those containing different chromo-
phores, such as the metal coordination center and a remote
organic chromophore.25,27,29,36,41,43,54,57 The spin density sur-
faces of Ir-1–Ir-3 are all localized on the coumarin ligand of
the complexes. Therefore, the triplet excited states of the com-
plexes can be assigned as intraligand triplet excited states,
which are supported by the nanosecond transient absorption
spectroscopy (Fig. 4) in which the bleaching band at the steady
state absorption of the coumarin ligand was observed.
The ground state geometry of the complexes was optimized
(Fig. 6). The coumarin chromophore is almost coplanar to the
bpy coordination ligand, thus the π-conjugation between the
coumarin moiety and the coordination center was expected.
Interestingly, the frontier molecular orbital of Ir-1 indicated
that the π-conjugation between the coumarin moiety and the
Ir(III) coordination framework is limited. For example, the
HOMO of Ir-1 is localized on the coumarin 6 moiety. Conver-
sely, the LUMO of Ir-1 is localized on the coordination bpy
ligand.
DFT calculations
The photophysical properties of the transition metal com-
plexes can be studied by DFT calculations. For example, the
UV-Vis absorption and luminescence emission properties of
transition metal complexes have been studied with DFT and
TDDFT calculations.47–52 The photophysical properties of the
complexes Ir-1–Ir-3 were studied with DFT calculations.
Fig. 6 Frontier molecular orbitals of Ir-1. Calculated by DFT at the
CAM-B3LYP/6-31G/LanL2DZ level using Gaussian 09W.
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Table 2 Electronic excitation energies (eV) and the corresponding oscillator strengths ( f ), main configurations and CI coefficients of the low-lying
electronically excited states of complex Ir-1, calculated by TDDFT//B3LYP/6-31G/LanL2DZ, based on the DFT//CAM-B3LYP/6-31G/LanL2DZ opti-
mized ground state geometries. CH2Cl2 was used as a solvent in the calculations
TDDFT//B3LYP/6-31G
Electronic transition
Energya (eV)
fb
Compositionc
CId
Character
Singlet
Triplet
S0→S1
3.11 eV 398 nm
1.5630
H → L
0.4704
0.4564
0.6563
0.4899
0.6478
0.3202
0.6019
0.6568
0.3716
LLCT
ILCT
ILCT
ILCT
ILCT
ILCT
ILCT
LLCT
ILCT
H → L+1
H−1 → L
H−1 → L
H−1 → L+2
H−8 → L
H → L+1
H → L
S0→S2
S0→S3
S0→S4
S0→S8
T1→S0
T2→S0
T3→S0
3.31 eV 374 nm
3.38 eV 367 nm
3.74 eV 332 nm
4.12 eV 301 nm
2.03 eV 609 nm
2.28 eV 545 nm
2.78 eV 446 nm
0.0177
0.6317
0.1644
0.4319
0.0000e
0.0000
0.0000
H−1 → L+2
a Only the selected low-lying excited states are presented. b Oscillator strength. c H stands for HOMO and L stands for LUMO. 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 are
zero.
The UV-Vis absorption of Ir-1 was calculated based on the opti- Ir(III), Ru(II) and Re(I) complexes as triplet photosensitizers for
mized ground state geometry (Table 2). The calculated absorption TTA upconversion.17 However, more triplet photosensitizers
band is located at 398 nm, which is close to the experimental for TTA upconversion are desired so that the tunability of the
value of 439 nm (Fig. 1). The molecular orbitals involved in the TTA upconversion can be increased.
S0→S1 transition are HOMO, LUMO and LUMO+1. Inspection of
The ideal triplet photosensitizers for TTA upconversion are
these orbitals (Fig. 6) indicated that the S0→S1 transition can be those compounds that show strong absorption of visible light
identified as IL and LLCT transition. Interestingly, the Ir(III) and long-lived triplet excited states, so that the triplet–triplet-
center does not contribute to this transition.
energy-transfer (TTET) can be enhanced.3,17 Complexes Ir-1,
The calculated T1 state energy level is 2.03 eV (609 nm), Ir-2 and Ir-3 were used as triplet photosensitizers for TTA
which is close to the experimental value (1.99 eV, 622 nm, upconversion (Fig. 7). Firstly, the emissions of the complexes
approximated by the 77 K emission wavelength of Ir-1). were recorded upon 473 nm laser excitation. With the addition of
HOMO/LUMO+1 are involved in the T1 state electronic con- the triplet acceptor 9,10-diphenylanthracene, DPA, strong blue
figuration. Therefore, the T1 state can be assigned as an IL emission bands in the region 400 nm–500 nm were observed
state, which is in agreement with the nanosecond time- (Fig. 7b). Under the same experimental conditions, a convention-
resolved transient absorption spectra and the long-lived triplet al complex Ru(bmb)3[PF6]2 gave very weak upconversion emission
excited state of Ir-1. Similar calculations were carried out for intensity.63 Ir-1 gives the strongest upconverted emission inten-
Ir-2 and Ir-3 (see ESI† for details). T1 state energy levels of sity, which is followed by Ir-2. Ir-3 gives the weakest upconverted
1.85 eV and 1.96 eV were found for Ir-2 and Ir-3, respectively. emission intensity among the three complexes.
3IL excited states were identified for these complexes.
Notably, Ir-1 and Ir-3 give almost similar absorbances at the
Interestingly, we found that using a solvent is necessary in excitation wavelength, but there is almost two-fold difference
the calculation of the triplet excited state energy levels for the
applied functional and the base set. For example, without
solvent CH2Cl2, the T1 state energy level of the complexes Ir-1,
Ir-2 and Ir-3 were calculated as 1.93 eV, 1.76 eV and 1.85 eV,
which are different from the experimental values.
Triplet–triplet annihilation upconversion
Triplet–triplet annihilation upconversion has attracted much
attention due to its advantage of tunable photophysical para-
meters (excitation and emission), strong absorption of exci-
tation light and high upconversion quantum yields.3,9–13 TTA
upconversion has been used for solar cells58–61 and photo- Fig. 7 Emission and upconversion of the complexes with 473 nm
catalytic reactions.62 One of the major challenges in the devel-
(5 mW) continuous laser excitation. (a) Emission of the Ir(III) complexes.
(b) The up-converted DPA fluorescence and the residual phosphor-
opment of the TTA upconversion is the availability of the
escence of the mixture of DPA and the photosensitizers [Ru(dmb)3]-
triplet photosensitizers.3,11,17 Conventionally, triplet photo-
(PF6)2, Ir-2, Ir-1, or Ir-0 respectively. c[DPA] = 6.0 × 10−5 M; c [sensi-
tizers] = 1.0 × 10−5 M. In deaerated CH2Cl2. The asterisks in (a) and (b)
sensitizers are limited to the Pt(II)/Pd(II) porphyrin complexes.
Recently, we prepared a series of visible light-harvesting Pt(II), indicate the scattered 473 nm excitation laser, 20 °C.
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Table 3 Photophysical parameters of Ir-0, Ir-1, Ir-2 and Ir-3
KSV d/M−1
Kq e/M−1S−1
εa
τT b (μs)
ΦUC
(×105)
(×109)
ηf
c
g
g
Ir-0
Ir-1
Ir-2
Ir-3
0.50
2.01
4.20
4.58
0.3
65.9
34.8
2.7
—
0.05
31.6
3.21
1.86
16.6
48.0
9.2
—
22.8%
9.8%
9.3%
4.6
3.9
4.3
68.8
a Molar extinction coefficient at the absorption maxima. ε: 104/cm−1
mol−1 dm3. b Triplet state lifetime, measured by time-resolved
transient absorption. 1.0 × 10−5 M in deaerated CH2Cl2. c Excited with
473 nm laser. d Quenching constants. e Bimolecular quenching
constants. f Overall upconversion ability (brightness). η = ε × ΦUC. In
103 M−1 g Too low to be determined accurately.
.
between the upconverted emission intensity. This result is
attributed to the long-lived triplet excited state of Ir-1 (65.9 μs)
vs. the short-lived triplet excited state of Ir-3 (34.8 μs). The
upconversion quantum yields with Ir-1–Ir-3 were calculated as
22.8, 9.8 and 9.3%, respectively (Table 3).
Fig. 9 Delayed fluorescence observed in the TTA upconversion with (a)
Ir-1, (b) Ir-2, and (c) Ir-3 as triplet photosensitizers and DPA as the triplet
In order to study the triplet–triplet-energy-transfer (TTET) acceptor. Excited at 473 nm (nanosecond pulsed OPO laser synchro-
nized with spectro-fluorometer) and monitored at 410 nm. Under this
circumstance the Ir-1, Ir-2, and Ir-3 are selectively excited and the emis-
sion is due to the upconverted emission of DPA. (d) Prompt fluorescence
decay of DPA determined in a different experiment (excited with picose-
efficiency, the quenching of the phosphorescence of the com-
plexes in the presence of DPA was studied (Fig. 8). In the same
DPA concentration range, complex Ir-1 gave the most signifi-
cant quenching effect. Ir-2 gave a slightly decreased quenching
cond 405 nm laser, the decay of the emission was monitored at
effect. For Ir-3 and Ru(dmb)3[PF6]2 (Ru-1), however, quenching 410 nm). In deaerated CH2Cl2. c [Sensitizers] = 1.0 × 10−5 M; c [DPA] =
6.0 × 10−5 M; 20 °C.
was hardly observed. The quenching constants of Ir-1–Ir-3
were determined as 3.16 × 106 M−1, 3.21 × 105 M−1 and 1.86 ×
105 M−1, respectively.
determined as 282.6 μs. This exceptionally long-lived lumines-
cence lifetime proved the TTA upconversion feature of the blue
emission band. Similar results were observed for Ir-2 and Ir-3.
For the DPA alone, the fluorescence lifetime was determined
as 5.5 ns (Fig. 9d).
In order to unambiguously prove that the blue emission in
Fig. 7b is due to the TTA upconversion, the luminescence life-
times of the blue emission bands were measured (Fig. 9). The
fluorescence lifetime for the Ir-1/DPA upconversion was
In order to unambiguously verify the delayed fluorescence
feature of the upconverted emission, the time-resolved emission
spectra of the complexes and the upconversion were studied. Ir-1
alone shows long-lived phosphorescence emission (τ = 79.9 μs).
In the presence of the triplet acceptor DPA, an emission
band at 400–500 nm was observed. The lifetime was deter-
mined as 282.6 μs (Fig. 9a). Similar long-lived upconverted
fluorescence emissions were also observed with Ir-2 and Ir-3.
The delayed fluorescence is characteristic of the TTA upconver-
sion. Therefore, the TTA upconversion with the Ir(III) com-
plexes Ir-1, Ir-2 and Ir-3 was proved. The time-resolved
emission spectra of the photosensitizers alone and the TTA
upconversion are presented in Fig. 10.
The TTA upconversion with the triplet photosensitizers is
visible to un-aided eyes (Fig. 11). For Ir(III) complexes alone, red
emission was observed upon 473 nm laser excitation (Fig. 11a).
In the presence of triplet acceptor DPA, a strong blue emission
was observed (Fig. 11b), which is due to the TTA upconversion.
For the reference complexes Ir-0 and Ru-1, however, no upcon-
verted emission was observed. Previously Ir-0 and Ru-1 were
used as triplet photosensitizers for TTA upconversion.63 These
results indicated that the Ir(III) complexes (Ir-1–Ir-3) are more
efficient triplet photosensitizers for TTA upconversion.
Fig. 8 Quenching of the phosphorescence of the triplet photosensiti-
zers with increasing the concentration of DPA, (a) Ir-1 (λex = 470 nm), (b)
Ir-2 (λex = 470 nm), (c) Ir-0 (λex = 445 nm), (d) Stern–Volmer plots for
phosphorescence quenching. c[photosensitizers] = 1.0 × 10−5 M. In
deaerated CH2Cl2, 20 °C.
1680 | Dalton Trans., 2014, 43, 1672–1683
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strong absorption of visible light and a long-lived triplet
excited state. Three different coumarin chromophores were
used to prepare the Ir(III) complexes, i.e. coumarin 6 (Ir-1),
7-diethylamino coumarin (Ir-2) and 3-phenyl-7-diethylamino
coumarin (Ir-3). Ir(ppy)2(bpy)[PF6] was used as the reference
complex (Ir-0). All the three new complexes show strong
absorption of visible light and the long-lived triplet excited
state. We found that the triplet excited state lifetime (65.9 μs)
is longer than the complexes prepared with the direct metala-
tion method (visible light-harvesting chromophore as the C
donor of the C–Ir bond, <15 μs). Interestingly, the long-lived
triplet excited states have a strong phosphorescence emission
(quantum yield is up to 18.2%), which is rare for phosphores-
cent transition metal complexes. The photophysical properties
of the complexes were studied in detail with steady state and
time-resolved spectroscopy, as well as DFT calculations. The
population of the coumarin-localized triplet excited state (3IL)
was produced upon photoexcitation, but the triplet excited
state lifetimes are drastically different (65.9 μs, 34.8 μs and
2.7 μs). The complexes were used as triplet photosensitizers
for triplet–triplet annihilation upconversion, and upconversion
quantum yields up to 22.8% were observed. Our results are
useful for the preparation of visible light-harvesting transition
metal complexes, the study of the triplet excited state of
organic chromophores, as well as the application of the visible
light-harvesting transition metal complexes as efficient triplet
photosensitizers.
Fig. 10 Time-resolved emission spectra (TRES) of Ir(III) complexes alone
and the TTA upconversion. With DPA as the triplet acceptor. (a) Ir-1
alone (τP = 79.9 μs). (b) TRES of Ir-1 in the presence of DPA. Upcon-
verted emission in the range of 380 nm–750 nm was observed (τDF
=
282.6 μs). (c) Ir-2 alone (τP = 30.7 μs). (d) TRES of Ir-2 in the presence of
DPA. Upconverted emission in the range of 380 nm–750 nm was
observed (τDF = 186.5 μs). Note for Ir-1–Ir-2 the phosphorescence wave-
lengths are much longer than the upconverted emission wavelength.
c [DPA] = 6.0 × 10−5 M; c [sensitizers] = 1.0 × 10−5 M. In deaerated
CH2Cl2. Excited with nanosecond pulsed OPO laser synchronized with
spectro-fluorometer (λex = 473 nm). 20 °C.
Acknowledgements
We thank the NSFC (21073028 and 21273028), the Royal
Society (UK) and NSFC (21011130154), Science Foundation
Ireland (SFI E.T.S. Walton Program 11/W.1/E2061), Ministry of
Education (SRFDP-20120041130005) and Dalian University of
Technology for financial support.
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