Rhenium(i) tricarbonyl polypyridine complexes showing strong absorption of visible light and long-lived triplet excited states as a triplet photosensitizer for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c2dt30804e

The preparation of rhenium(i) tricarbonyl polypyridine complexes that show a strong absorption of visible light and long-lived triplet excited state and the application of these complexes as triplet photosensitizers for triplet-triplet annihilation (TTA) based upconversion are reported. Imidazole-fused phenanthroline was used as the N^N coordination ligand, on which different aryl groups were attached (Phenyl, Re-0; Coumarin, Re-1 and naphthyl, Re-2). Re-1 shows strong absorption of visible light (ε = 60800 M ^-1 cm^-1 at 473 nm). Both Re-1 and Re-2 show long-lived T₁ states (lifetime, τ_T, is up to 86.0 μs and 64.0 μs, respectively). These properties are in contrast to the weak absorption of visible light and short-lived triplet excited states of the normal rhenium(i) tricarbonyl polypyridine complexes, such as Re-0 (ε = 5100 M^-1 cm^-1 at 439 nm, τ_T = 2.2 μs). The photophysical properties of the complexes were fully studied with steady state and time-resolved absorption and emission spectroscopes, as well as DFT calculations. The intra-ligand triplet excited state is proposed to be responsible for the exceptionally long-lived T₁ states of Re-1 and Re-2. The Re(i) complexes were used as triplet photosensitizers for TTA based upconversion and an upconversion quantum yield up to 17.0% was observed. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30804e

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PAPER
Rhenium(I) tricarbonyl polypyridine complexes showing strong absorption of
visible light and long-lived triplet excited states as a triplet photosensitizer for
triplet–triplet annihilation upconversion†
Xiuyu Yi, Jianzhang Zhao,* Wanhua Wu, Dandan Huang, Shaomin Ji and Jifu Sun
Received 13th April 2012, Accepted 17th May 2012
DOI: 10.1039/c2dt30804e
The preparation of rhenium(I) tricarbonyl polypyridine complexes that show a strong absorption of visible
light and long-lived triplet excited state and the application of these complexes as triplet photosensitizers
for triplet–triplet annihilation (TTA) based upconversion are reported. Imidazole-fused phenanthroline
was used as the N^N coordination ligand, on which different aryl groups were attached (Phenyl, Re-0;
Coumarin, Re-1 and naphthyl, Re-2). Re-1 shows strong absorption of visible light (ε = 60 800 M−1 cm−1
at 473 nm). Both Re-1 and Re-2 show long-lived T1 states (lifetime, τT, is up to 86.0 μs and 64.0 μs,
respectively). These properties are in contrast to the weak absorption of visible light and short-lived
triplet excited states of the normal rhenium(I) tricarbonyl polypyridine complexes, such as Re-0
(ε = 5100 M−1 cm−1 at 439 nm, τT = 2.2 μs). The photophysical properties of the complexes were fully
studied with steady state and time-resolved absorption and emission spectroscopes, as well as DFT
calculations. The intra-ligand triplet excited state is proposed to be responsible for the exceptionally long-
lived T1 states of Re-1 and Re-2. The Re(I) complexes were used as triplet photosensitizers for TTA based
upconversion and an upconversion quantum yield up to 17.0% was observed.
microseconds (τ, μs). It is clear that much room is left for the
development of new rhenium(I) tricarbonyl complexes that show
Introduction
Rhenium(I) tricarbonyl polypyridine complexes have attracted
much attention due to their tunable photophysical properties and
applications in luminescent biological molecular probes, photo-
voltaics and photocatalysis, etc.1–6 For example, rhenium(I)
tricarbonyl polypyridine complexes have been used for
luminescent bio-labeling, DNA probes and luminescent
materials.3,5,7–9 However, similar to the conventional Pt(II) and
Ir(III) complexes,11,12 the rhenium(I) tricarbonyl polypyridine
complexes show weak absorption in the visible range, which is a
disadvantage for the application of these rhenium(I) complexes
as luminescent molecular probes.1,3,5,8,9 Visible light-harvesting
rhenium(I) tricarbonyl complexes with dipyrrinato or porphyrin
ligands have been prepared,13 but none of these complexes have
been used to sensitize a photophysical process.13 Previously
Re(I) complexes were used for the photosensitizing of singlet
oxygen (1O2), but the complex showed weak absorption in the
visible range.10 Furthermore, the phosphorescent Re(I) com-
plexes usually show short luminescence times of a few
a visible light-harvesting effect and long-lived triplet excited
states.
On the other hand, a recent application of transition metal
complexes as triplet photosensitizers for triplet–triplet annihil-
ation (TTA) upconversion have attracted much attention.14–19
However, no rhenium(I) tricarbonyl polypyridine complexes
have been used as triplet photosensitizers in TTA
upconversion.15–17 Concerning this aspect, the rhenium(I) tricar-
bonyl polypyridine complexes are of particular interest because
these complexes give intense phosphorescence emission, indicat-
ing that the non-radiative decay is inhibited and efficient inter-
system crossing (ISC) occurs, which are beneficial for the triplet-
triplet-energy-transfer (TTET), a crucial step for TTA upconver-
sions and other photophysical processes.15–17
In order to prepare rhenium(I) tricarbonyl complexes that
show strong absorption of visible light, and to utilize these com-
plexes in a practical photophysical process, herein we used a
coumarin-containing imidazole N^N ligand for the preparation
of new rhenium(I) tricarbonyl complexes (Re-1, Re-0 and Re-2
as control compounds, Scheme 1). Previously a similar ligand
was used for the preparation of visible light-harvesting Ru(II)
and Ir(III) complexes,20,21 but these ligands have not been used
for the preparation of Re(I) complexes. Re-2 shows strong
absorption in visible range. All the complexes are phosphores-
cent at room temperature (RT). Re-1 and Re-2 show long-lived
triplet excited state (τT are 86.0 μs and 64.0 μs, vs. the normal
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, E 208 Western Campus,
2 Ling-Gong Road, Dalian 116012, P. R. China.
E-mail: zhaojzh@dlut.edu.cn
†Electronic supplementary information (ESI) available: The synthesis
and the structure characterization data of the Re(I) complexes, the details
of TTA upconversion and DFT calculation information. See DOI:
10.1039/c2dt30804e
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Scheme 1 Synthesis of Re-0, Re-1 and Re-2. (a) NaIO4, RuCl3·3H2O; (b) POCl3, DMF, 0–5 °C; (c) CH3COOH, Ar, NH4OAc, reflux, 12 h; (d)
toluene, reflux, 1 h.
lifetime of the triplet states of Re(I) complex of a few μs).1 The
Synthesis of L-1
complexes were used as triplet photosensitizers for TTA
upconversions.
7-Diethylamino-2-oxo-2H-chromene-3-carboxyldehyde (128 mg,
0.68 mmol), 1,10-phenanthroline-5,6-dione (104 mg, 0.5 mmol),
and ammonium acetate (805 mg, 11.3 mmol) were dissolved in
glacial acetic acid (13 mL). The mixture was heated and refluxed
for 6 h under Ar atmosphere. The color of the mixture turned to
orange. After completion of the reaction, the mixture was cooled
to room temperature, and concentrated NH3·H2O was added
until the pH of the mixture was brought to about 7.0 to give a
yellow precipitate. The precipitate was filtered, washed with
water, and then dried under vacuum overnight. The crude
product was further purified by column chromatography (silica
Experimental
Materials and reagents
All the chemicals were analytically pure and were used as
received. Solvents were dried and distilled for synthesis.
Re(CO)5Cl was purchased from Sigma-Aldrich.
1
gel, CH2Cl2–methanol = 10 : 1, v/v). Yield: 167 mg, 77%. H
Analytical measurements
NMR (400 MHz, CDCl3): δ = 9.16 (d, J = 4.2 Hz, 2H), 8.99 (s,
1H), 8.78 (d, J = 5.8 Hz, 2H), 7.72–7.69 (m, 2H), 7.48 (d, J =
8.0 Hz, 1H), 6.66–6.63 (m, 1H), 6.50 (d, J = 2.3 Hz, 1H),
3.48–3.43 (m, 4H), 1.26 (t, J = 7.0 Hz, 6H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 162.1, 156.4, 151.7, 148.3, 146.9,
144.4, 141.5, 130.1, 129.4, 122.9, 110.0, 108.7, 107.7, 96.8,
45.1, 12.5 ppm. ESI-HRMS: calcd ([C26H22N5O2 + H]+): m/z =
436.1774, found, m/z = 436.1767.
NMR spectra were recorded on a 400 MHz Varian Unity Inova
NMR spectrophotometer. Mass spectra were recorded with
Q-TOF Micro MS spectrometer. UV-Vis absorption spectra
were measured with a HP8453 UV-vis spectrophotometer.
Fluorescence spectra were recorded on Shimadzu 5301 PC
spectrofluorometer. Upconversion was carried out on a customized
Sanco 970 CRT spectrofluorometer. Phosphorescence quantum
yields were measured with [Ru(dmb)3(PF6)2] as the standard (Φ
= 7.3% in acetonitrile, dmb = 4,4′-dimethyl-2,2′-bipyridine).
Luminescence lifetimes were measured on a OB920 lumines-
cence lifetime spectrometer (Edinburgh Instruments, UK) and
FLS920 spectrofluorometer (Edinburgh Instruments, UK).
Synthesis of L-0
The synthetic procedure is similar to that of L-1, except that ben-
zaldehyde (72 mg, 0.68 mmol) was used. The crude product was
8932 | Dalton Trans., 2012, 41, 8931–8940
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purified by column chromatography (silica gel, CH2Cl2–metha-
nol = 8 : 1, v/v). Yield: 105 mg, 71%. 1H NMR (400 MHz,
methanol-d4): δ = 8.74 (d, 1H, J = 3.9 Hz), 8.59–8.32 (m, 1H),
7.98 (d, 1H, J = 7.8 Hz), 7.48–7.41 (m, 3H), 4.87 (m, 5H) ppm.
13C NMR (100 MHz, methanol-d4): δ = 151.42, 147.42, 143.14,
129.77, 129.63, 128.81, 126.47, 123.10 ppm. ESI-HRMS: calcd
([C19H12N4 + H]+): m/z = 297.1140, found, m/z = 297.1135.
red solid after column chromatography (silica gel, CH2Cl2–
CH3OH = 25 : 1, v/v). Yield: 41 mg, 89%. M.p. > 250 °C. H
1
NMR (400 MHz, DMSO-d6): δ = 9.41 (d, 2H, J = 5.0 Hz); 9.31
(d, 2H, J = 8.2 Hz); 9.10 (d, 1H, J = 8.4 Hz); 8.14–8.19 (m,
4H); 8.08 (d, 1H, J = 8.0 Hz); 7.62–7.77 (m, 3H) ppm. 13C
NMR (100 MHz, DMSO-d6): δ = 197.8, 190.1, 152.5, 151.4,
143.8, 133.6, 132.9, 130.7, 128.5, 128.3, 127.4, 126.6, 126.0,
125.3 ppm. ESI-HRMS: calcd ([C26H14N4O3ClRe + Na]+): m/z
= 675.0210, found, m/z = 675.0216. IR (KBr, cm−1) ν: 2022 (s),
1909 (s), 1622 (w), 1541 (w), 1455 (m), 1406 (m), 1261 (m),
1093 (m), 807 (m), 726 (m).
Synthesis of L-2
The synthetic procedure is similar to that of L-1 except that
1-naphthaldehyde (63.7 mg, 0.6 mmol) was used. The crude
product was purified with column chromatography (silica gel,
dichloromethane–methanol = 8 : 1, v/v), and a pink solid was
Nanosecond time-resolved transient difference absorption
spectroscopy
1
obtained. Yield: 128.5 mg, 74.3%. H NMR (400 MHz, metha-
nol-d4): δ = 9.02 (d, 2H, J = 4.0 Hz), 8.87 (m, 1H), 8.79 (m,
1H), 8.59 (d, 1H, J = 8.1 Hz), 8.08 (d, 1H, J = 8.2 Hz),
8.00–7.95 (m, 2H), 7.78–7.75 (m, 2H), 7.67–7.56 (m, 3H) ppm.
ESI-HRMS: calcd ([C23H15N4 + H]+): m/z = 347.1297, found,
m/z = 347.2496.
The nanosecond time-resolved transient absorption spectra were
measured on LP 920 laser flash photolysis spectrometer
(Edinburgh Instruments, UK) and recorded on a Tektronix TDS
3012B oscilloscope. The lifetime values (by monitoring the
decay trace of the transients) were obtained with the LP900 soft-
ware. All samples in flash photolysis experiments were de-
aerated with argon for ca. 15 min before measurement and the
argon gas flow was kept during the measurement.
Synthesis of Re-0
Re(CO)5Cl (25 mg, 0.07 mmol) and L-0 (21 mg, 0.07 mmol)
were dissolved in dry toluene and heated to 110 °C, and stirred
under Ar for 1 h. The solvent was then removed under reduced
pressure, and the product was obtained as a yellow solid by
column chromatography (silica gel, CH2Cl2). Yield: 39 mg,
Triplet–triplet annihilation upconversions
1
A diode pumped solid state (DPSS) continuous laser (473 nm)
was used as the excitation source for the upconversions. The
diameter of the laser spot was ca. 3 mm. The power of the laser
beam was measured with a VLP-2000 pyroelectric laser power
meter. For the upconversion experiments, the mixed solution of
the compound (triplet sensitizer) and 9,10-diphenylanthracene
(DPA) was degassed with N2 or Ar for at least 15 min (note the
upconversion can be significantly quenched by O2). Then the
solution was excited with laser. The upconverted fluorescence of
DPA was observed with a spectrofluorometer. In order to repress
the scattered laser, a black box was put behind the fluorescent
cuvette to trap the laser beam.
93%. M.p. > 250 °C. H NMR (400 MHz, DMSO-d6): δ = 9.39
(d, 2H, J = 4.6 Hz); 9.29 (d, 2H, J = 8.4 Hz); 8.32 (m, 6H);
8.14–8.22 (m, 2H); 7.64–7.68 (m, 2H); 7.57–7.60 (m, 1H) ppm.
13C NMR (100 MHz, DMSO-d6): δ = 203.5, 197.8, 152.6,
151.4, 143.7, 136.3, 132.9, 130.4, 129.7, 129.3, 126.8, 126.5,
125.6 ppm. ESI-HRMS: calcd ([C22H12N4O3ClRe + Na]+): m/z
= 625.0053, found, m/z = 625.0047. IR (KBr, cm−1) ν: 2025 (s),
1894 (s), 1622 (m), 1545 (w), 1514 (w), 1458 (m), 1406 (w),
1366 (w), 807 (m), 726 (m), 697 (m).
Synthesis of Re-1
The synthetic procedure was the same as that for Re-0 except
that L-1 (31 mg, 0.07 mmol) was used. The product was
obtained as a red solid by column chromatography (silica gel,
CH2Cl2–CH3OH = 25 : 1, v/v). Yield: 46 mg, 88%. M.p.
> 250 °C. 1H NMR (400 MHz, CDCl3): δ = 9.39 (d, 2H, J = 4.6
Hz); 9.29 (d, 2H, J = 8.4 Hz); 8.32 (m, 6H); 8.14–8.22 (m, 2H);
7.64–7.68 (m, 2H); 7.57–7.60 (m, 1H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 197.5, 189.8, 162.3, 157.2, 152.6,
151.3, 150.7, 149.5, 145.2, 143.2, 137.2, 132.3, 131.4, 131.0,
125.9, 121.1, 110.7, 108.9, 106.9, 106.8, 97.2, 45.4, 12.7 ppm.
ESI-HRMS: calcd ([C29H21N5O5ClRe + Na]+): m/z = 764.0687,
found, m/z = 764.0673. IR (KBr, cm−1) ν: 2978 (w), 2927 (w),
2871 (w), 2020 (s), 1917 (s), 1718 (m), 1619 (s), 1590 (s), 1521
(s), 1424 (m), 1358 (m), 1263 (s), 1188 (m), 1133 (m), 1078
(w), 811 (m), 775 (w).
The upconversion quantum yields (ΦUC) were determined
with the prompt fluorescence of coumarin-6 (ΦF = 78% in
C2H5OH). The upconversion quantum yields were calculated
with the following equation, where ΦUC, Asam, Isam and ηsam rep-
resents the quantum yield, absorbance, integrated photolumines-
cence intensity and the refractive index of the solvents (eqn (1),
where the subscript “std” is for the standard used in the measure-
ment of the quantum yield and “sam” for the samples to be
measured). The equation is multiplied by a factor of 2 in order to
make the maximum quantum yield be unity.15 All these data
were independently measured three times (with different solu-
tions samples).
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
Astd
Isam
ηsam
ηstd
ΦUC ¼ 2Φstd
ð1Þ
Asam
Istd
The CIE coordinates (x, y) of the emission of the sensitizers
alone and the emission of the upconversion were derived from
the emission spectra with the software of CIE color Matching
Linear Algebra.
Synthesis of Re-2
The synthetic procedure is similar to that of Re-0 except that L-2
(24 mg, 0.07 mmol) was used. The product was obtained as a
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Delayed fluorescence
The delayed fluorescence feature of the upconversion was
measured with a nanosecond pulsed laser (Opolett™ 355II+ UV
nanosecond pulsed laser, typical pulse length: 7 ns, pulse repe-
tition: 20 Hz, peak OPO energy: 4 mJ, wavelength is tunable in
the range of 210–355 nm and 410–2200 nm, OPOTEK, USA),
which is synchronized to a FLS920 spectrofluorometer (Edin-
burgh, U.K.). The pulsed laser is sufficient to sensitize the TTA
upconversion. The decay kinetics of the upconverted fluor-
escence (delayed fluorescence) was monitored with a FLS920
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 is synchronized to the FLS 920
spectrofluorometer.
Fig. 1 UV-Vis absorption of Re(I) complexes and the ligand L-1.
c = 1.0 × 10−5 M in toluene, 20 °C.
DFT calculations
The density functional theory (DFT) calculations were used for
optimization of both singlet states and triplet states. The energy
level of the T1 state (energy gap between S0 state and T1 state)
were calculated with the time-dependent DFT (TDDFT), based
on the optimized singlet ground state geometries (S0 state). All
the calculations were performed with Gaussian 09W.22
Fig. 2 The emission spectra of Re(I) complexes under different atmos-
phere. (a) Re-0 (λex = 439 nm) (b) Re-1 (λex = 472 nm). c = 1.0 ×
10−5 M in toluene, 20 °C.
Results and discussion
Design and synthesis of complexes
bodipy.25–27 The strong absorption of Re-1 is similar to the Re(I)
complexes with a dipyrrinato ligand.13 Previously a Re(I) tricar-
bonyl diimine complex with napthalimide appendent was pre-
pared which showed much weaker absorption in the visible
range (ε = 11 980 M−1 cm−1 at 408 nm).28
Ligand L-1 gives weak absorption in the visible range (ε =
23 344 M−1 cm−1). Thus metalation of the N^N ligand gives
enhanced absorption. This red-shifted and enhanced absorption
of the complex compared to the free ligand indicate that the elec-
tronic interaction of the coumarin unit with the Re(I) coordi-
nation center is strong at the ground state.
The emission of the complexes were studied (Fig. 2). All the
complexes give RT phosphorescence (see ESI† for the results of
Re-2). However, different emission properties were found for the
complexes. For Re-0 and Re-2, broad and structureless emission
bands were observed, thus the emission is basically an MLCT
feature.7,8,29 For Re-1, however, the emission band shows sig-
nificant vibrational progression.9a Furthermore, the sensitivity of
the phosphorescence intensity to O2 is different for the com-
plexes. The phosphorescence emission intensity of Re-1 and
Re-2 are more significantly quenched in an aerated solution than
that of Re-0. Thus we propose that the lifetime of the triplet
excited states of Re-1 and Re-2 are longer than that of Re-0. All
these results indicated that the emissive triplet excited states of
Re-0, Re-1 and Re-2 are different from each other.
Imidazole fused phenanthroline was used as the N^N ligand for
the preparation of the Re(I) complexes (Scheme 1). Different aryl
units can be introduced on to the imidazole moieties to tune the
photophysical properties of the complexes. Previously a similar
strategy was used for the preparation of Ir(III) complexes and
Ru(II) complexes.20,21 However, this method has not been used
for the preparation of visible light-harvesting Re(I) complexes.1
A coumarin unit was attached to the imidazole unit (Re-1). The
intramolecular hydrogen bond will enhance the absorption. Re-2
was prepared as a control complex, in which the naphthalene
unit was attached to the imidazole moiety. All the compounds
were obtained in moderate to satisfying yields. The molecular
structures of the compounds were fully characterized with
1H NMR, 13C NMR and HR MS spectroscopy. Notably all the
Re(I) complexes show the characteristic CuO stretchings at
2020 cm−1 and 1900 cm−1 in the IR spectra.
Absorption and emission spectra
The UV-vis absorption of the complexes and the ligand were
studied (Fig. 1). The model complex Re-0 gives weak absorption
in the visible range (ε = 5100 M−1 cm−1 at 439 nm). The
absorption maximum is at 288 nm (ε = 46 646 M−1 cm−1). The
control compound Re-2 also gives weak absorption in the visible
range (ε = 5300 M−1 cm−1 at 447 nm). This absorption property
is typical for the normal Re(I) tricarbonyl complexes.1,2,5,7–9,23,24
Interestingly, Re-1 gives strong absorption in the visible range
(ε = 60 800 M−1 cm−1 at 473 nm). This strong absorption is
comparable to some typical organic fluorescent dyes, such as
The luminescence lifetimes and the luminescence quantum
yields of the complexes were studied and the data are listed in
Table 1. The model complex Re-0 gives intense phosphor-
escence emission (ΦP = 23.4%) and a short luminescence
8934 | Dalton Trans., 2012, 41, 8931–8940
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Table 1 Photophysical parameters of Re-0, Re-1 and Re-2
τPe (μs)
τTf (μs)
a
c
d
λabs
εb
λem
ΦL
77 K
298 K
Air
N2
2.2
86.0
64.0
Re-0
Re-1
Re-2
439
473
447
0.51
6.08
0.53
554
605
552
23.4%
0.03%
3.3%
5.14 μs
63.1 ms
1.4 ms
0.1 μs
70.4 μs
80.1 μs
0.14
0.43
0.34
a In toluene, c = 1.0 × 10−5 M. b Molar extinction coefficient at the absorption maxima. ε: 104/M−1 cm−1 c In toluene. d Luminescence quantum yield.
.
In toluene, with [Ru(dmb)3(PF6)2] (Φ = 0.073 in acetonitrile) as the standards. e Luminescence lifetime, at RT (20 °C) and 77 K (in C2H5OH–CH3OH
= 4 : 1). f Triplet state lifetime, measured by time-resolved transient absorption. c = 1.0 × 10−5 M in aerated and de-aerated toluene.
lifetime (τP = 0.1 μs). For the Re(I) complexes with the cou-
marin- or naphthalene-containing ligands, the phosphorescence
quantum yields are much lower, but the phosphorescence life-
times are much longer than that of Re-0. For example, phosphor-
escence lifetimes up to 70.4 μs and 80.1 μs were observed for
Re-1 and Re-2, respectively. The phosphorescence lifetimes of
Re-1 and Re-2 are much longer than the normal phosphorescent
Re(I) complexes.1,4,8,9,29 Re-1 shows very long-lived phosphor-
escence at 77 K (63.1 ms, Table 1), which is an indication of a
3IL state. Previously a Re(I) tricarbonyl complex with an excep-
tionally long-lived triplet excited state was reported (τ =
651 μs).28 The long lifetime of that complex is due to the intra-
molecular “ping-pong” energy transfer, or 3MLCT/3IL excited
state equilibrium. For Re-1, however, it is due to the low-lying
3IL excited state.
Re-1 and Re-2 show much lower phosphorescence quantum
yields than Re-0 (Table 1). However, the low phosphorescence
quantum yields do not necessarily mean the triplet excited state
was inefficiently populated, or the non-radiative decay of the
triplet excited state is significant.13 Rather, it is possible that the
Fig. 3 Photoluminescence spectra of the complexes at RT and 77 K.
triplet excited state of the complexes was efficiently populated.
We will demonstrate the efficient production of the triplet excited
states by the TTA upconversion experiments (see later
section).16c
(a) Re-0 (λex = 440 nm), (b) Re-1 (λex = 470 nm), (c) Re-2 (λex
430 nm). c = 1.0 × 10−5 M in C2H5OH–CH3OH (4 : 1, v/v).
=
Re(I) complex.3a Both the fluorescence of the anthracene and the
3MLCT emission of the Re(I) coordination center were
quenched. Intramolecular Föster energy transfer from the anthra-
cene to Re(I) coordination center is responsible for the quenching
of the anthracene fluorescence, whereas the Dexter triplet excited
state energy transfer from the Re(I) coordination center to anthra-
cene is responsible for the quenching of the 3MLCT emission.3a
77 K emission spectra
In order to study the emissive triplet excited states of the com-
plexes, the emission spectra at 77 K were studied (Fig. 3). The
emission of Re-0 is blue-shifted compared to that at RT by
1756 cm−1 (thermally induced Stokes shifts, ΔES). The emission
band of Re-0 are broad at both RT and 77 K. Thus the emissive
triplet excited state of Re-0 is mainly metal-to-ligand-charge-
transfer (3MLCT) in character.7 For Re-2, a similar large ΔES
was observed (2816 cm−1). The emission at 77 K is blue-shifted
compared to that at RT. Furthermore, the emission band at 77 K
is more structured compared to that at RT.
A different emission profile was observed for Re-1 at 77 K
(Fig. 3b). The emission at 77 K shows no substantial blue-shift
compared to RT. Both the emission at RT and 77 K are highly
structured, which is different from the normal phosphorescent
Re(I) complexes that show the MLCT emission.7 Based on these
results, the emissive triplet excited state of Re-1 can be assigned
mainly as intraligand (3IL state).30 Previously a ligand-localized
triplet excited state was observed for an anthracene-containing
Nanosecond time-resolved transient difference absorption
spectroscopy
In order to study the triplet excited states, the nanosecond time-
resolved transient difference absorption spectra of the complexes
were studied (Fig. 4). Upon pulsed laser excitation at 355 nm,
significant bleaching at 470 nm was observed for Re-1 (Fig. 4a),
which is assigned to the depletion of the ground state of the
ligand. At the same time, positive transient absorption bands in
the range 300 nm–400 nm and 500 nm–750 nm were observed.
The lifetime of the transient was determined as 86.0 μs. This
long-lived transient species is assigned to a triplet excited state
of Re-1 because the lifetime is significantly reduced in an
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Fig. 5 Spin density of the complexes Re-0, Re-1 and Re-2. Calculated
based on the optimized triplet state by DFT at the B3LYP/6-31G/
LanL2DZ level using Gaussian 09W.
Fig. 4 Nanosecond time-resolved transient absorption difference
spectra of (a) Re-1 and (c) Re-2 after pulsed excitation (λex = 355 nm).
Decay traces of (b) Re-1 at 470 nm and (d) Re-2 at 580 nm. In de-
aerated toluene, c = 1.0 × 10−5 M. 20 °C.
The electronic transitions of the Re-1 was calculated based on
the optimized ground state geometry, i.e. the UV-vis absorption
spectra. The absorption of Re-1 in the visible range is due to the
transitions of S0 → S2, and S0 → S4, the related transitions are
characterized as intraligand H → L, and H → L + 1. These calcu-
lation results are in good agreement with the experimental results
(ESI†).
The triplet excited states of Re-1 were studied with the
TDDFT method (approximated with the optimized ground state
geometry). The energy level of the T1 excited state was calcu-
lated as 597 nm (ESI†), which is close to the experimental
results of the RT phosphorescence at 605 nm (Fig. 2). The major
electronic transition of the T1 state is H → L + 1, which is loca-
lized on the coumarin ligand. Thus the T1 state can be identified
as the coumarin-imidazole localized 3IL excited state,20,30,32
which is in full agreement with the result observed with the
time-resolved transient absorption spectra (Fig. 4a).
aerated solution to 0.43 μs. The time-resolved transient absorp-
tion of Re-2 was also studied (Fig. 4c). A significant positive
absorption band in the range 500 nm–700 nm was observed,
which is different from Re-1. The lifetime of the transient of Re-
2 was determined as 64.0 μs. These long-lived transients of Re-1
and Re-2 are drastically different from that of Re-0 (τT = 2.2 μs,
ESI†). Thus the aryl groups on the imidazole moiety impart sub-
stantial influence on the triplet excited states of Re-1 and Re-2.
We propose the long-lived T1 excited state of Re-1 is due to the
3
ligand-localized IL excited state. Previously an anthracene-con-
taining Re(I) complex showed a long-lived triplet excited state
(42 μs), which is localized on the anthracene, due to the energy
transfer from the Re(I) coordination center to the anthracene
ligand.6c However, that complex shows the typical weak absorp-
tion in the visible range.6c
Similar DFT/TDDFT calculations were carried out for Re-0
and Re-2 (ESI†). Generally the calculations support the exper-
imental observations.
DFT calculations on the complexes: spin density surfaces and
electronic transitions
Triplet-triplet annihilation upconversion
In order to study the triplet state of the complexes from a theor-
etical perspective, the spin density surfaces of the complexes
were calculated (Fig. 5). For the model complex Re-0, the spin
density is distributed on the Re(I) atom, the CO ligand and the
phen unit of the imidazole moiety, which is in agreement with
the MLCT feature of the triplet excited state of Re-0. For Re-1,
however, the spin density is distributed on the coumarin ligand,
and the Re(I) made no contribution to the spin density, thus the
T1 excited state of Re-1 can be assigned as 3IL state, which is in
full agreement with the transient absorption spectroscopy studies
(Fig. 4).31 Re-2 gives a similar spin density to that of Re-0.
The electronic transitions of the complexes were calculated on
DFT and TDDFT calculations (ESI†). The Re(I) coordination
center of Re-1 takes a octahydro geometry. The C–Re–C angle
is 92.1°, which is close to the single crystal structures of the
Re(I) carbonyl complexes.
TTA upconversion has attracted much attention, due to its poten-
tial applications in photovoltaics and photocatalysis.15–18,33–36
Transition metal complexes, such as Pt(II)/Pd(II) porphyrin com-
plexes, or the complexes with visible light-harvesting cyclometa-
lated Ir(III), Pt(II) and Ru(II) complexes have been used as the
triplet photosensitizers.16 However, much room is left to explore
other transition metal complexes as triplet photosensitizers for
TTA upconversion. For example, to the best of our knowledge,
no Re(I) tricarbonyl complexes have been used as triplet photo-
sensitizers for TTA upconversion.16 Herein the Re(I) complexes
were used as triplet photosensitizers for TTA upconversion and
significant TTA upconversion was observed.
Firstly the emission of complexes upon 473 nm laser exci-
tation were studied (Fig. 6a). Re-0 and Re-2 gave similar emis-
sion, whereas Re-1 shows red-shifted emission. A bench mark
2+
Ru(II) complex, Ru(dmb)3
(dmb
=
4,4′-dimethyl-2,2′-
8936 | Dalton Trans., 2012, 41, 8931–8940
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Table 2 Upconversion related photophysical properties of the Re(I)
complexes
τ/μsa
ΦP
Ksv/(105 M−1 c
)
ΦUC
b
d
[Ru(dmb)3]2+
Re-0
Re-1
0.84
0.12
70.4
80.1
7.3%
23.4%
0.03%
3.3%
0.017
0.048
2.67
—
—
17.0%
16.9%
Re-2
2.76
a Phosphorescence lifetime. b Phosphorescence quantum yield.
c Quenching constant. d Upconversion quantum yields, and with
coumarin-6 as the standard (ΦF = 78% in EtOH).
We found that non-coherent excitation light with lower power
density, such as the deem light of the excitation beam of a spec-
trofluorometer, is sufficient to impart the TTA upconversion with
Re-1 as the triplet photosensitizer (Fig. 7). Upon excitation with
the spectrofluorometer, significant TTA upconversion was
observed. Lower upconversion quantum yield (10.3%) than that
with the laser excitation was observed. This finding is useful for
the application of the upconversion in photocatalysis or photo-
voltaics, for which the terrestrial solar radiance can be used as
the excitation source (100 mW cm−2, AM 1.5 G).
In order to unambiguously prove that the blue emission
observed in Fig. 6 and 7 are due to the TTA upconversion, the
luminescence lifetimes of the upconverted fluorescence emission
of DPAwas measured (Fig. 8).18 For the upconversion with Re-1
as the triplet photosensitizer, a luminescence lifetime of 225.0 μs
was observed. The prompt fluorescence lifetime of DPA was
determined as 5.5 ns. Therefore, the long-lived blue emission in
Fig. 6b is the typical delayed fluorescence of TTA upconversion.
Thus the TTA upconversion with Re-1 as the triplet photosensiti-
zer was unambiguously confirmed. Similar long-lived delayed
fluorescence was observed with Re-2 as the triplet photosensiti-
zer (τDF = 210.4 μs, ESI†).
The upconversion is significant to unaided eyes (Fig. 9). For
example, in the presence of DPA, intense blue emission was
observed with Re-1 as the triplet photosensitizer. For Re-2,
much weaker blue emission was observed. For other triplet
photosensitizers, such as Re-0 and Ru-1([Ru(dmb)3]2+), no
upconversion was observed under the same experimental con-
ditions. The different TTA upconversion efficiency of the triplet
photosensitizers can be rationalized by the efficiency of the
TTET process. TTET efficiency was studied by the quenching of
the luminescence or the triplet state lifetime of the photosensiti-
zers by the triplet acceptor DPA (Fig. 10).
Fig. 6 Emission and upconversion with the Re(I) complexes as triplet
photosensitizers. Excited with 473 nm (5 mW) laser. (a) Emission of the
photosensitizers alone. (b) The upconverted DPA fluorescence and the
residual phosphorescence photosensitizers [Ru(dmb)3]2+, Re-1, Re-2 or
Re-0 respectively. c[DPA] = 1.5 × 10−5 M; c[sensitizers] = 5.0 ×
10−6 M. In de-aerated toluene. The asterisks in (a) and (b) indicate the
scattered 473 nm excitation laser. 20 °C.
bipyridine) was also studied for comparison (Fig. 6a).37 We
noted the weak phosphorescence of Re-1 (ΦP = 0.03%. Table 2).
However, as we proved previously,16c the weak phosphorescence
do not necessarily mean an inefficient producing of the triplet
state of the complexes upon photoexcitation.
Next, the emission in the presence of 9,10-diphenylanthracene
(DPA, triplet acceptor) were studied (Fig. 6b). In the presence of
DPA, intense blue emission in the range 400 nm–550 nm were
observed with Re-1 as the triplet photosensitizer. Irradiation of
DPA or the sensitizer alone did not produce this blue emission
band, thus the TTA upconversion was confirmed. For Re-2,
much weaker upconverted emission was observed, due to its
weak absorption in the visible range. For Re-0 and Ru-1-
([Ru(dmb)3]2+), no upconversions were observed under the same
experimental conditions. The upconversion quantum yields
(ΦUC) for Re-0, Re-1, Re-2 as triplet photosensitizers were
determined as 0.0%, 17.0% and 16.9%, respectively.
It should be pointed out that the upconverted fluorescence
emission peak area with Re-1 is much larger than the quenched
phosphorescence band of Re-1. This result is in contrast to the
Pt(II)/Pd(II) porphyrin complex sensitizers, which give intense
phosphorescence.14–17 Our finding with Re-1 as the triplet
photosensitizer confirms our previous postulation that the weakly
phosphorescent transition metal complex, i.e. the dark triplet
excited states, can be used for TTA upconversion.16c
The TTET efficiency of Re-1 and Re-2 are similar, due to the
similar T1 state lifetime of the complexes. Re-0 and Ru-1
([Ru(dmb)3]2+) gave much weaker quenching, due to their short-
lived T1 excited states. The quenching study also confirms that
the light harvesting effect of the triplet photosensitizers is crucial
for the TTA upconversion. For example, Re-2 shows weak
absorption in the visible range, thus its TTA upconversion is
much lower than that of Re-1, despite the T1 state lifetime, or
the TTET efficiency, is close to that of Re-1.
Upconversion with Re-1 as the triplet photosensitizer can be
summarized in Scheme 2. Upon excitation, the singlet excited
3
state of Re-1 was produced. In turn the IL triplet excited state
was populated due to the heavy atom effect of Re. The TTET
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Fig. 7 Upconversion with excitation by the light of a Shimadzu
RF5301 spectrofluorometer. Light power density: 0.5 mW cm−2. The
asterisk indicates the scattered 473 nm excitation light of the spectro-
fluorometer. c[Re-1] = 5.0 × 10−6 M; c[DPA] = 1.5 × 10−5 M. In
de-aerated toluene. 20 °C.
Fig. 9 (a) Photographs of the emission of sensitizers alone and (b) the
upconversion. (c) CIE diagram of the emission of sensitizers alone and
(d) in the presence of DPA (upconversion). λex = 473 nm (laser power:
5 mW). In de-aerated toluene, c[sensitizer] = 5.0 × 10−6 M, c[DPA] =
1.5 × 10−5 M, 20 °C.
Fig. 8 (a) Delayed fluorescence observed in the TTA upconversion
with Re-1 as the triplet photosensitizer and DPA as the triplet acceptor.
Excited at 473 nm (nanosecond pulsed OPO laser synchronized with
spectrofluorometer) and the decay of the emission was monitored at
410 nm. Under this circumstance the Re-1, not DPA, is selectively
excited and the emission is due to the upconverted emission of DPA. (b)
The prompt fluorescence decay of DPA determined in a different exper-
iment (excited with a picosecond 405 nm laser, the decay of the emis-
sion was monitored at 410 nm). In de-aerated toluene. c[Sensitizers] =
5.0 × 10−6 M; c[DPA] = 1.5 × 10−5 M; 20 °C.
Fig. 10 Stern–Volmer plots for phosphorescence or lifetime quench-
ing. Quenching of the lifetime of the triplet photosensitizers with
increasing the concentration of DPA. c[photosensitizers] = 1.0 ×
10−5 M, in de-aerated toluene. 20 °C.
between the triplet photosensitizer and triplet acceptor will
produce the triplet excited state of the acceptor (DPA). TTA of
DPA at the triplet excited state will give the singlet excited state
of DPA. Radiative decay from the singlet excited state of DPA
will give the delayed fluorescence, i.e. the upconverted fluor-
escence emission. It is clear that both the light-harvesting ability
and the lifetime of the triplet excited state of a photosensitizer
are crucial for the improvement of the biomolecular TTET
process, a key step of TTA upconversion. Therefore, the Re(I)
complexes that show strong absorption of visible light and long-
lived triplet state are ideal triplet photosensitizers for TTA
upconversions.
prepared. These properties are in stark contrast to the weak
visible light absorption and short-lived triplet excited states of
the conventional rhenium(I) tricarbonyl polypyridine complexes.
The light-harvesting antenna in the Re(I) complex is the cou-
marin-integrated imidazole ligands. Intra-ligand triplet excited
state (3IL) is proposed to be responsible for the exceptionally
long-lived T1 state, proved by nanosecond time-resolved transi-
ent absorption spectroscopy, spin density analysis and emission
spectra at 77 K. It is for the first time that Re(I) complexes were
used as triplet photosensitizers for triplet–triplet annihilation
based upconversion, and an upconversion quantum yield up to
17.0% was observed. Our results may pave the way for prep-
aration of the Re(I) tricarbonyl polypyridine complexes that
show strong absorption of visible light and long-lived triplet
excited states and for explorations of the application of these
Conclusions
In conclusion, rhenium(I) tricarbonyl polypyridine complexes
with strong absorption of visible light (ε = 60 800 M−1 cm−1 at
473 nm) and long-lived triplet excited state (86.0 μs) were
8938 | Dalton Trans., 2012, 41, 8931–8940
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Scheme 2 Jablonski diagram of TTA upconversion with Re-1 as the
triplet photosensitizer. GS stands for ground state. E is energy. GS is
1
3
ground state (S0). IL* is intraligand singlet excited state. IL* is intra-
ligand triplet excited state. TTET is triplet-triplet energy transfer. 3DPA*
is the triplet excited state of DPA. TTA is triplet–triplet annihilation.
1DPA* is the singlet excited state of DPA. The emission bands observed
for the sensitizers is the 3MLCT/3IL emission. The emission bands
observed in the TTA upconversion experiment is the delayed emission
of DPA.
14 A. Monguzzi, J. Mezyk, F. Scotognella, R. Tubino and F. Meinar, Phys.
Rev. B: Condens. Matter Mater. Phys., 2008, 78, 195112.
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Acknowledgements
17 P. Ceroni, Chem.–Eur. J., 2011, 17, 9560–9564.
We thank the NSFC (20972024 and 21073028), Ministry of
Education (NCET-08-0077), the Royal Society (UK) and NSFC
(China-UK Cost-Share Science Networks, 21011130154) for
financial support.
18 Y. Y. Cheng, T. Khoury, R. G. C. R. Clady, M. J. Y. Tayebjee,
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