Visible light-absorbing rhenium(i) tricarbonyl complexes as triplet photosensitizers in photooxidation and triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c2dt32420b

We prepared N^N Re(i) tricarbonyl chloride complexes (Re-1 and Re-2) that give very strong absorption of visible light. To this end, it is for the first time that boron dipyrimethane (Bodipy) was used to prepare Re(i) tricarbonyl chloride complexes. The π-conjugation linker between the π-conjugation framework of the antenna Bodipy and the Re(i) coordination centre ensures efficient intersystem crossing (ISC). Re-0 without visible light-harvesting ligand was prepared as a model complex in the photophysical studies. Re-1 (with Bodipy) and Re-2 (with carbazole-ethynyl Bodipy) show unprecedented strong absorption of visible light at 536 nm (ε = 91700 M^-1 cm ^-1) and 574 nm (ε = 64600 M^-1 cm^-1), respectively. Interestingly, different from Re-0, Re-1 and Re-2 show fluorescence of the ligand, not the phosphorescence of the Re(i) coordination centre. However, long-lived triplet excited states were observed upon visible light excitation (τ_T = 104.0 μs for Re-1; τ_T = 127.2 μs for Re-2) vs. the short lifetime of Re-0 (τ_T = 26 ns). With nanosecond time-resolved transient absorption spectroscopy and DFT calculations, we proved that the triplet excited states of Re-1 and Re-2 are localized on the Bodipy ligands. The complexes were used as triplet photosensitizers for two triplet-triplet-energy-transfer (TTET) processes, i.e.¹O₂ mediated photooxidation and triplet-triplet annihilation (TTA) upconversion. With the strong visible light-harvesting ability, Re-1 proved to be a better ¹O₂ photosensitizer than the conventional triplet photosensitizer tetraphenylporphyrin (TPP). Significant upconversion was observed with Re-1 as the triplet photosensitizer. Our result is useful for preparation of Re(i) tricarbonyl chloride complexes that show strong absorption of visible light and long-lived triplet excited states and for the application of these complexes as triplet photosensitizers in photocatalysis, photodynamic therapy and TTA upconversion. The Royal Society of Chemistry 2013.

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Visible light-absorbing rhenium(I) tricarbonyl
complexes as triplet photosensitizers in photooxidation
Cite this: Dalton Trans., 2013, 42, 2062
and triplet–triplet annihilation upconversion†
Xiuyu Yi, Jianzhang Zhao,* Jifu Sun, Song Guo and Hongli Zhang
We prepared N^N Re(I) tricarbonyl chloride complexes (Re-1 and Re-2) that give very strong absorption
of visible light. To this end, it is for the first time that boron dipyrimethane (Bodipy) was used to prepare
Re(I) tricarbonyl chloride complexes. The π-conjugation linker between the π-conjugation framework of
the antenna Bodipy and the Re(I) coordination centre ensures efficient intersystem crossing (ISC). Re-0
without visible light-harvesting ligand was prepared as a model complex in the photophysical studies.
Re-1 (with Bodipy) and Re-2 (with carbazole-ethynyl Bodipy) show unprecedented strong absorption of
visible light at 536 nm (ε = 91 700 M−1 cm−1) and 574 nm (ε = 64 600 M−1 cm−1), respectively. Interest-
ingly, different from Re-0, Re-1 and Re-2 show fluorescence of the ligand, not the phosphorescence of
the Re(I) coordination centre. However, long-lived triplet excited states were observed upon visible light
excitation (τT = 104.0 μs for Re-1; τT = 127.2 μs for Re-2) vs. the short lifetime of Re-0 (τT = 26 ns). With
nanosecond time-resolved transient absorption spectroscopy and DFT calculations, we proved that the
triplet excited states of Re-1 and Re-2 are localized on the Bodipy ligands. The complexes were used as
triplet photosensitizers for two triplet–triplet-energy-transfer (TTET) processes, i.e. 1O2 mediated photo-
oxidation and triplet–triplet annihilation (TTA) upconversion. With the strong visible light-harvesting
ability, Re-1 proved to be a better 1O2 photosensitizer than the conventional triplet photosensitizer tetra-
phenylporphyrin (TPP). Significant upconversion was observed with Re-1 as the triplet photosensitizer.
Our result is useful for preparation of Re(I) tricarbonyl chloride complexes that show strong absorption of
visible light and long-lived triplet excited states and for the application of these complexes as triplet
photosensitizers in photocatalysis, photodynamic therapy and TTA upconversion.
Received 11th October 2012,
Accepted 5th November 2012
DOI: 10.1039/c2dt32420b
www.rsc.org/dalton
shows strong visible-light absorption, but the absorption is
still limited to the blue range (<500 nm).3b
Introduction
Re(I) complexes have attracted much attention due to the appli-
The straightforward method to prepare transition metal
cation as luminescent bioprobes, photovoltaics and triplet complexes that show strong absorption of visible light is to
photosensitizers for photocatalysis.1 However, most of these attach a fluorophore to the coordination center.4 The fluoro-
complexes show weak absorption in the visible region, which phore should show strong absorption in the visible range,
is a clear disadvantage for the application of these complexes such as boron dipyrromethane (Bodipy).5 However, the
in the circumstances which require visible light excitation, linker between the chromophore and the coordination
such as photocatalysis.2 Thus Re(I) complexes with strong centre is also crucial. In some cases the strong absorption
absorption of visible light are highly desired, but similar to of the complexes is ineffective, that is, the excitation energy
other conventional transition metal complexes, these com- harvested by the chromophore cannot be efficiently fun-
plexes were rarely reported.3 Previously we reported a Re(I) nelled to the triplet state manifold of the complexes,6 and
complex with coumarin-imidazole phenanthroline ligand that the respective photophysics of the fluorophore ligand and
the coordination centre collapse. For example, the emissive
triplet excited state of the coordination center can be
quenched.7
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024,
Basically there are two methods for attachment of a fluoro-
P. R. China. E-mail: zhaojzh@dlut.edu.cn; http://finechem.dlut.edu.cn/photochem
phore to a coordination center. One is to tether the fluorophore
†Electronic supplementary information (ESI) available: General experimental
methods, 1H and 13C NMR data of the compounds, and the photophysical data
of the Re(I) complexes. See DOI: 10.1039/c2dt32420b
to the coordination center, without π-conjugation linker
between the two units.7,8 In this case the respective energy
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levels of the excited state of the ligands and the coordination not very long. To the best of our knowledge, no Bodipy fluoro-
centre have to be concerted in order to ensure an efficient phore was used to prepare Re(I) complexes.1h,i
energy transfer from the ligand to the coordination centre.6,9
In order to address the above challenges, herein we
There will be a limitation on the maximal effective absorption designed two Re(I) complexes with Bodipy ligands (Re-1, Re-2,
wavelength for the ligand, that is, the excitation light with Scheme 1). The attachment of Bodipy to the coordination
longer wavelength than this limit can be harvested by the centre is based on the aforementioned π-conjugation method,
ligand, but the energy will be trapped on the ligand and it which ensures effective absorption in the visible region. We
cannot be transferred to the triplet excited state manifold of used two Bodipy-conjugated N^N ligands, L-1 shows absorp-
the complexes.6
tion in 460 nm–560 nm and L-2 shows absorption in 485 nm–
Another method to enhance the absorption of transition 650 nm range. We investigated the photophysical properties of
metal complexes in the visible range is to connect a fluoro- the complexes with steady state and time-resolved spec-
phore to the coordination centre via a π-conjugation linker, troscopy, as well as DFT calculations. Both complexes show
such as a –CuC– triple bond or the direct metalation metho- strong absorption of visible light, compared to the model
d.3a,4b,d,e,10 In this case the effective absorption wavelength of complex Re-0, which is a typical Re(I) tricarbonyl chloride com-
the complex can be pushed further into the red end of the plex.1i Furthermore, the application of the complexes in two
spectrum. One should be cautious because limitations also triplet–triplet-energy-transfer (TTET) processes was studied.
exist in this case, that is, the intersystem crossing (ISC) tends We proved the triplet photosensitizing performance of the Re
to be diminished with increasing the bulkiness of the (I) complexes are better than the known triplet photosensitizer
ligand.10d,11,12
Only a few Re(I) complexes were reported to show strong Scheme 1). Furthermore, the upconversion quantum yield of
visible light-absorption.1i,3a–c but the absorption wavelength is the Re(I) complexes are higher than
known bisiodo-
tetraphenylporphyrin (TPP) and Methylene Blue (MB,
a
Scheme 1 Molecular structures of the Re(I) complexes Re-1 and Re-2 and the ligands L-1 and L-2. The known complex Re-0 was used as a model complex in the
study. Note the π-core of Bodipy is conjugated to the coordination centre in Re-1 and Re-2. The model triplet photosensitizers tetraphenylporphyrin (TPP), MB and
B-2 are also shown (see ESI† for the synthetic details).
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Fig. 1 UV-Vis absorption of Re(I) complexes and the ligands. c = 1.0 × 10−5 mol
dm−3 in toluene, 25 °C.
substituted Bodipy.13 These results confirmed that the strong
visible absorption of the Re(I) complexes is effective. Our
results are useful for the design of transition metal complexes
that show a strong absorption of visible light and long-lived
triplet excited states, as well as the application of these com-
plexes as triplet photosensitizers in photocatalysis, TTA Fig. 2 The normalized emission spectra of the Re(I) complexes and the ligands.
(a) L-1 (λex = 525 nm), L-2 (λex = 578 nm), (b) Re-0 (λex = 400 nm), Re-1 (λex
=
upconversion, luminescent bio/molecular probing and photo-
dynamic therapy, etc.
510 nm), Re-2 (λex = 570 nm) c = 1.0 × 10−5 mol dm−3 in deaerated toluene,
25 °C.
in the visible range is broader than in Re-1, the absorption pro-
files are similar to the respective ligands L1 and L2. Attaching
an electron-donating unit to the Bodipy moiety can broaden
the absorption band.15 Previously we prepared coumarin-
containing Re(I) complex, which shows absorption maxima at
473 nm (ε = 60 800 M−1 cm−1).3b To the best of our knowledge,
Re-1 and Re-2 are the Re(I) complexes that show to date the
strongest absorption at the longest wavelength.1i,3b
Results and discussions
Design and synthesis of the complexes
The Bodipy chromophore was connected to the coordination
centre Re(I) via a π-conjugation linker in both complexes Re-1
and Re-2.10a,c,d,14 We envision that Re-2 will show red-shifted
absorption compared to Re-1, due to the larger π-conjugation
framework of the ligand of Re-2 (see ESI† for the synthetic
details).
The emission of the ligands and the Re(I) complexes were
studied (Fig. 2). Ligands L1 and L2 give intense emission at
566 nm (ΦF = 34.0%) and 637 nm (ΦF = 36.4%), respectively
(Fig. 2a). Interestingly, similar emission profiles were
observed for the Re(I) complexes Re-1 and Re-2. However, the
luminescence quantum yields of Re-1 and Re-2 decreased
compared to that of free ligands (Table 1). Furthermore, the
emission of Re-1 and Re-2 are not sensitive to O2, which is
different from the emission of Re-0, for which the emission
intensity can be substantially decreased in the presence of O2
(see ESI† for detail). We also measured the luminescence life-
times of the ligands as well as the Re(I) complexes (Table 1).
Luminescence lifetimes in the range of ns were found for
Re-1 and Re-2. For Re-0, which is known to be phosphores-
cent, the luminescence lifetime is 23.6 ns. Based on these
results, we concluded that the major luminescence bands of
Re-1 and Re-2 are fluorescence, not phosphorescence. The
characteristic phosphorescence of the Re(I) coordination
centre, demonstrated by Re-0, was completely quenched in
Re-1 and Re-2. Fluorescent Re(I) tricarbonyl chloride com-
plexes have been rarely reported.1p
Ligands L1 and L2 were prepared by Stille and Sonogashira
coupling reactions. The Pd(0) catalyzed Sonogashira coupling
leads to ligand L1. In order to push the absorption wavelength
to the red end of the spectra, the carbazole moiety was con-
nected to the Bodipy core via a CuC triplet bond linker.15
Complex Re-0 was used as a control compound in the photo-
physical studies. All the compounds were obtained in mode-
rate to good yields (see ESI† for the synthetic details).
UV-vis absorption and emission spectra
The UV-vis absorption spectra of the ligands and the Re(I)
complexes were compared (Fig. 1). The model complex Re-0
shows very weak absorption in the visible range, which is
typical for Re(I) tricarbonyl chloride complexes.1g,i,16 The
Bodipy ligands L1 and L2 give absorption bands at 537 nm (ε =
62 400 M−1 cm−1) and 579 nm (ε = 54 900 M−1 cm−1), respect-
ively. Upon complexation with Re(I), slightly enhanced absorp-
tion was observed for both Re-1 and Re-2, which show
absorption at 536 nm (ε = 91 700 M−1 cm−1) and 574 nm (ε =
64 600 M−1 cm−1). We noted that the absorption band of Re-2
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Table 1 Photophysical parameters of Re-0, Re-1 and Re-2
e
τF
εb
λem
ΦF
77 K
298 K
τT
a
c
d
f
λabs
Re-0
Re-1
Re-2
L-1
401
536
574
537
579
0.36
9.17
6.46
6.24
5.49
604
556, 742
647
0.58%
0.13%
17.1%
34.0%
36.4%
4.1 μs
23.6 ns
0.2 ns
0.1 ns
4.34 ns
1.56 ns
26 nsg
104.0 μs
127.2 μs
—
41.2, 114.8 μs
1.7, 282.0 μs
—
—
566
637
L-2
—
a In toluene (1.0 × 10−5 mol dm−3). b Molar absorption coefficient at the absorption maxima. ε: 104/cm−1 mol−1 dm3. c In toluene. d In toluene,
with quinine sulfate (Φ = 0.546 in H2SO4 (0.05 M)) for Re-0, bis-iodo-BODIPY (Φ = 0.027 in CH3CN) for Re-1 and Rhodamine B (Φ = 0.65 in
CH3OH) for Re-2, L-1, L-2 as the standards. e Luminescence lifetime, at 298 K and 77 K (in C2H5OH–CH3OH = 4 : 1). f Triplet state lifetime,
measured by time-resolved transient absorption. 1.0 × 10−5 mol dm−3 in deaerated toluene. g Literature values.18
We noted the fluorescence quantum yield of Re-2 is much
higher than that of Re-1, indicating the ISC in Re-2 is less
efficient than Re-1. This result indicated that with the increase
of the bulkiness of the ligand, the ISC becomes less efficient
and the residual fluorescence of the ligand will be more
significant.10d,11a–d However, since the fluorescence emission
of the ligands was substantially reduced, the heavy atom effect
of the Re(I) does exist, thus we postulate that the triplet excited
state will be populated upon photoexcitation. The excitation
spectra of the complexes are roughly superimposable to the
UV-Vis absorption spectra (see ESI† for detail). Furthermore, it
should be pointed out that the 1MLCT → 1Bodipy energy trans-
fer cannot be excluded. The coumarin–Re(I) complex we pre-
pared previously show only phosphorescence, no fluorescence
was observed.3b
Nanosecond time-resolved transient difference absorption
spectra
In order to elucidate the triplet excited states of the Re(I) com-
plexes, the time-resolved transient absorption of the complexes
was studied (Fig. 3). Upon pulsed laser excitation at 532 nm, a
significant bleaching band at 536 nm was observed for Re-1,
which is due to the bleaching of the ground state of the
Bodipy ligand in Re-1. TA bands in the range of 350 nm–
450 nm and 600–800 nm were observed. The lifetime of the
Fig. 3 Nanosecond time-resolved transient difference absorption spectra of (a)
transient was determined as 104.0 μs by following the decay
trace at 530 nm. The transient can be substantially quenched
Re-1, inset is the decay trace at 530 nm, (b) Re-2, inset is the decay trace at
570 nm. In deaerated toluene at room temperature after pulsed excitation (λex
=
in air saturated solution (lifetime of the transient was reduced
to 0.42 μs). These results indicated that the transient observed
is due to the triplet excited state. This conclusion was sup-
ported by the DFT calculated transient absorption of Re-1,
which indicated absorption at 400 nm and 660 nm (see ESI†
for detail), which is in agreement with the experimental obser-
vations. Similarly, a long-lived triplet excited state was also
observed for Re-2 upon pulsed photoexcitation at 532 nm (τT =
127.2 μs). The bleaching band at 570 nm is in agreement with
the steady-state UV-vis absorption of the ligand L2. DFT calcu-
lations indicated broad transient absorption bands in the
range of 350–500 nm and in the range of 650–850 nm (see ESI†
for detail), which are in agreement with the experimental
results.
532 nm).
These triplet excited state lifetimes are among the longest
values for Re(I) complexes.1h,i Previously triplet excited state
lifetimes up to 651 μs were observed for a Re(I) complex with
an excited state equilibrium.1k But the complex shows rela-
max
tively weak absorption in the visible range (λabs
= 408 nm,
ε = 11 980 M−1 cm−1).1k Re(I) complexes with dipyrrinato
ligand were reported,3a show strong absorption in the visible
range (λex = 490 nm with ε up to 42 100 M−1 cm−1), but the
triplet excited states of the complexes were not studied.
Recently we prepared Re(I) complexes with coumarin-phenan-
throline ligands, the complexes show long-lived triplet excited
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Fig. 5 Frontier molecular orbitals of Re-1. Calculated by DFT at the B3LYP/6-
31G/LanL2DZ level using Gaussian 09W.
The photophysics of the Re(I) complexes can be summar-
ized in Scheme 2. Note the energy level of the Bodipy ligand
localized S1 state (1IL) is lower than the Re(I) coordination
centre-localized state (MLCT state, based on the DFT calcu-
lations, Table 2). Therefore the energy transfer from the
1Bodipy* state to the 1MLCT* state is prohibited. The only
available channel for the transformation from the 1Bodipy*
state to the triplet state is ISC. In this case the heavy atom
effect is crucial.
Non-efficient ISC will lead to the observation of more pro-
found residual fluorescence of the Bodipy ligands (Fig. 2).
Based on the quenching extent of the ligand fluorescence in
Re-1 and Re-2, we tentatively conclude that the ISC is more
efficient for Re-1 than that in Re-2. Also noticeable is the lower
energy level of the 3Bodipy* state than the Re(I)-centred
3MLCT* state, which ensures an efficient energy transfer from
3MLCT to the 3Bodipy* state, as a result the normal 3MLCT
emission in complexes Re-1 and Re-2 were completely
quenched. This postulation is in full agreement with the
experimental results (Fig. 2).
The Re(I) complexes Re-1 and Re-2 show strong absorption
of visible light and long-lived triplet excited states, thus these
complexes were used as triplet photosensitizers. Herein we
studied two processes, the photooxidation via photosensitizing
of the singlet oxygen (1O2) and the TTA upconversion. The
application of the Re(I) complexes in these processes will
reveal the ISC efficiencies in Re-1 and Re-2. Furthermore, the
performance of the Re(I) complexes will also be compared with
some known efficient triplet photosensitizers, such as methyl-
ene blue (MB), tetraphenylporphyrin (TPP),20 and B-2.13
Fig. 4 Spin density of the complexes Re-0, Re-1 and Re-2.
states (τ = 86.0 μs),3b but the complexes show blue-shifted
absorption (in the range of 447–473 nm) compared to Re-1
and Re-2. Re(I) complexes with strong absorption of visible
light and long-lived triplet excited states will be beneficial for
the application of these complexes in photocatalysis, photo-
luminescent biolabeling or probing and TTA upconversion. It
should be noted that some of these applications do not need
the complexes to be phosphorescent.17
DFT calculations on the photophysical properties of the Re(I)
complexes
The triplet excited states of the Re(I) complexes were studied
by DFT calculations on the spin density surfaces.4d,e,17a,19 For
Re-0, the spin density is localized on the Re(I) centre and the
bpy ligand (Fig. 4). For Re-1 and Re-2, however, the spin
density surfaces are exclusively localized on the Bodipy
ligands, the Re(I) centre and the bpy ligand do not contribute
to the spin density surfaces. Therefore, the spin density analy-
sis indicates that the triplet state is localized on the Bodipy
3
ligands (intraligand state, IL), which is in agreement with the
transient absorption spectra (Fig. 3).
The geometry of the ground state of Re-1 was optimized
and the front molecular orbitals are presented (Fig. 5). The
UV-vis absorption of the complexes were calculated with the
TDDFT method based in the optimized S0 state geometry
(Frank–Condon principle). The absorption maximum is
located at 506 nm (Table 2), which is close to the experimental
result at 536 nm (Fig. 1). The components of the S0 → S1 tran-
sition are mainly the HOMO and LUMO, both are localized on
the Bodipy ligand. HOMO − 1 is also involved in the transi-
Photooxidation with the Re(I) complexes as the triplet
photosensitizer
tion. The energy gap between the S0 state and the T1 state was Firstly we studied the application of Re(I) complexes as triplet
also estimated with the DFT calculation (Table 2). The S0 − T1 photosensitizers for 1O2 sensitizing. 1,5-Dihydronaphthalene
energy gap is estimated as 1.60 eV (776 nm), which is close to (DHN) was used as the 1O2 scavenger (Scheme 3).2b,20 DHN can
the phosphorescence band of Re-1 at 742 nm (Fig. 2). Similar be oxidized with 1O2 to produce Juglone, a synthetically impor-
studies were carried out for Re-2 (see ESI† for detail).
tant compound. The reaction kinetics can be easily followed
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Table 2 Electronic excitation energies (eV) and corresponding oscillator strengths (f), main configurations and CI coefficients of the low-lying electronically excited
states of complex Re-1, calculated by TDDFT//B3LYP/6-31G/LanL2DZ, based on the DFT//B3LYP/6-31G/LanL2DZ optimized ground state geometries
TDDFT//B3LYP/6-31G
Electronic transition
S0 → S1
Energy (eV)a
f b
Compositionc
CId
Character
Singlet
Triplet
2.45 eV 506 nm
2.49 eV 498 nm
0.5885
0.6173
H → L
0.5136
0.3495
0.4238
0.4237
0.5529
0.5726
0.3226
0.3732
0.3408
0.4506
0.4170
LLCT
MLCT
MLCT
LLCT
MLCT
LLCT
LLCT
LLCT
LLCT
MLCT
MLCT
H − 1 → L
H − 1 → L
H → L
H − 2 → L
H → L
H → L + 1
H → L + 1
H → L
H − 1 → L
H − 1 → L + 1
S0 → S2
S0 → S3
T1 → S0
2.62 eV 473 nm
1.60 eV 776 nm
0.1505
0.0000e
T2 → S0
T3 → S0
2.15 eV 577 nm
2.43 eV 511 nm
0.0000
0.0000
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.
Scheme 2 Qualitative Jablonski diagram for the photophysical process of the
Re(I) complexes. Exemplified with Re-1. Solid lines represent excitation or radia-
tive decay, dotted lines represent non-radiative processes. Note the 3MLCT phos-
phorescence is quenched by the low-lying 3Bodipy* state. The energy levels are
approximated by absorption or luminescence of the chromophores.
Fig. 6 Photosensitizing of 1O2 with the complexes as photosensitizers.
Irradiation time-dependent decrease of absorbance at 301 nm of DHN (2.0 ×
10−4 M) with different 1O2 sensitizers. (a) Re-1, (b) Re-2, (c) Re-0, (d) plots of ln
(At/A0) vs. irradiation time. c [sensitizers] = 2.0 × 10−5 M. c [DHN] = 2.0 × 10−4
M. In CH2Cl2–MeOH (9 : 1, v/v). Broadband irradiation with 35 W xenon lamp
(15 mW cm−2) at 25 °C.
light-absorbing chromophores is much better than the normal
1
Ir(III) complex O2 photosensitizers.20a Recently we used cyclo-
Scheme 3 Mechanism for the photooxidation of DHN with a singlet oxygen
(1O2) photosensitizer.20a
metalated Ir(III) complexes showing a strong absorption of
visible light for the photooxidation of DHN, but the absorption
wavelength is in the blue range.2b To the best of our knowl-
by monitoring the decay of the absorption of DHN at 301 nm, edge, no visible light-absorbing Re(I) complexes have been
1
thus the O2 photosensitizing ability of the complexes can be used as triplet photosensitizers for photoxidation. Herein we
compared.20 Previously cyclometalated Ir(III) complexes were will show that Re-1 is more efficient than the well-known
used for the photooxidation of DHN.20a But those conventional triplet photosensitizer TPP and RB (Scheme 1).
Ir(III) complexes show weak absorption in the visible range.20
The three Re(I) complexes were used as 1O2 photosensitizers
Organic triplet photosensitizers such as MB and TPP were also (Fig. 6). With Re-1, the change of UV-vis absorption is much
used for photooxidation and the performance of these visible more significant than that with Re-0 as the photosensitizer
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Fig. 8 Emission and upconversion of the complexes with perylene as the
triplet acceptor with 532 nm (5 mW) laser excitation. (a) Emission of the com-
plexes alone. (b) The upconverted perylene fluorescence and the residual lumi-
nescence of the photosensitizers Re-1, Re-0 or B-2, respectively. c [perylene] =
3.0 × 10−5 M; c [photosensitizers] = 1.0 × 10−5 M in deaerated toluene. The
asterisks in (a) and (b) indicate the scattered 532 nm excitation laser at 25 °C.
Fig. 7 Plots of chemical yield as a function of irradiation time for the photooxi-
dation of DHN using Re-1, Re-0, TPP, MB, [Ir(ppy)2(dpy)]+ as triplet photosensiti-
zers. c [sensitizers] = 2.0 × 10−5 M. Broadband irradiation with 35 W xenon
lamp (light power 15 mW cm−2). In CH2Cl2–MeOH (9 : 1 v/v) at 25 °C.
B-2 was used as a triplet photosensitizer as previously.13,22 ΦΔ
= 0.79 was determined for B-2. Based on these results, we con-
cluded that the ISC is highly efficient in Re-1, at least 88%.
Table 3 Photophysical parameters of the Re(I) and the organic triplet
photosensitizersa
kobs/min−1a
vi/b
Yieldc (%)
ΦΔ
d
Triplet–triplet annihilation upconversion
The Re(I) complexes were also used as triplet photosensitizers
for TTA upconversion.23 Compared to other upconversion
methods, such as those with two-photon absorption dyes,24 or
with rare earth materials,25 TTA upconversion shows the
advantages of the requirement of low and non-coherent exci-
tation light, strong absorption of the excitation light, high
upconversion quantum yield and tunable excitation/emission
wavelength.23g,i,j
Re-1
Re-2
TPP
65
6
37
21
2
6.5
0.6
3.7
2.1
0.2
99.3
43.4
98.5
81.1
27.5
0.88
0.06
0.62e
0.57
—
MB
Ir(ppy)2bpy+
a Photoreaction rate constants: 10−3 min−1 b Initial rate: 10−5 M. In
.
min−1 c Yield of Juglone after photoirradiation for 45 min. d Quantum
.
yield of singlet oxygen (1O2), with Rose Bengal as standard (ΦΔ = 0.8 in
CH3OH). e Literature values.21
One of the major areas of TTA upconversion research is to
develop new triplet photosensitizers.23i Conventionally the
(Fig. 7c). For Re-2, however, the change of UV-vis absorption is triplet photosensitizers are limited to Pt(II)/Pd(II) porphyrin
not significant upon photoirradiation (Fig. 6b). Thus we complexes.23a,f,26 Recently we developed a series of Ru(II),6,17a
1
propose that the O2 sensitizing ability of Re-1 is more signifi- Pt(II),10c,d,14b,27c,d Ir(III),27b and Re(I) complexes3b as triplet
cant than Re-0 and Re-2. The weak 1O2 sensitizing ability of photosensitizers for TTA upconversion.23i,27 These complexes
Re-0 can be attributed to its weak absorption of visible light. share the common features of strong absorption of visible
For Re-2, we propose the non-efficient ISC may be responsible light and long-lived triplet excited states. The molecular diver-
1
for the weak O2 photosensitizing ability, which is supported sity we developed for the triplet photosensitizers may offer an
by the strong fluorescence of Re-2 rather than that of Re-1.
opportunity for further facile adaptation of the TTA upconver-
We also studied photooxidation with the known triplet sion for applications in other areas, such as photocatalysis,28
photosensitizers TPP and MB (Fig. 6 and 7). The photoxidation photovoltaics,29 luminescent bioimaging,23l,30 and lumines-
rate with Re-1 is more faster than that with TPP and MB as the cent O2 sensing.31
triplet photosensitizers. It should be noted that TPP and MB
Firstly the emission of the triplet photosensitizers alone
1
proved to be much more efficient as O2 photosensitizers than upon 532 nm laser excitation were studied (Fig. 8a). Re-2 gives
the normal Ir(III) complex photosensitizers.20 To the best of intense emission, whereas Re-0 and Re-1 give very weak emis-
our knowledge, there is no transition metal complex sion. The weak emission of Re-0 and Re-1 is due to the weak
reported to be more efficient than TPP and MB as 1O2 absorption of the complex at the excitation wavelength or the
photosensitizers.20
low fluorescence quantum yields. With addition of the triplet
The kinetic data of the photooxidation were compiled in acceptor perylene, intense blue emission in the range of
Table 3. kobs value of Re-1 is 10-fold that of Re-2, and it is also 430–550 nm was observed for Re-1. For Re-2, however, the
at least 2-fold that of TPP. Furthermore, the 1O2 quantum emission band at the same position is much weaker. No
yields (ΦΔ) of the Re(I) complexes were also determined. We upconverted emission was found with Re-0 as the triplet
found that Re-1 gives the highest value (ΦΔ = 0.88). Re-2 gives photosensitizer. The new emission band is similar to the
a ΦΔ value of 0.06. The strong absorption of visible light and prompt fluorescence of perylene (see ESI†), therefore the new
the high ΦΔ value of Re-1 may be responsible for the signifi- emission band was assigned as the TTA upconversion, i.e. the
cant photooxidation capability of Re-1. The bis-iodo-BODIPY delayed fluorescence of perylene.
2068 | Dalton Trans., 2013, 42, 2062–2074
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Table 4 Photophysical parameters of Re-0, Re-1 and Re-2
Ksv/M−1 kq/M−1 s−1
εa
τT/μsb ΦF
(×105)d
(×109)e
ΦUC
ηg
c
f
Re-1 8.83 104.0
Re-2 2.66 127.2
0.13% 6.75
6.49
5.05
4.87
8.5% 7.5
2.0% 0.5
5.3% 4.7
17.1%
2.7%
6.42
3.28
B-2
8.90
67.3
a Molar extinction coefficient at the excited wavelength (532 nm). ε:
104/cm−1 mol−1 dm3. b Triplet state lifetimes. c Fluorescence quantum
yield. d Quenching constant. e Bimolecular quenching constants.
f Upconversion quantum yields, and with bis-iodo-BODIPY (B-2) as the
standard (Φ = 2.7% in CH3CN). g η = ε × ΦUC. In 103 M−1 cm−1
.
The upconversion quantum yields (ΦUC) were determined
(Table 4). Re-1 shows an upconversion quantum yield (ΦUC) of
8.5%, which is more than 4-fold that of Re-2. We proposed the
overall upconversion capability of a triplet photosensitizer
should be η = ε × ΦUC (where ε is the molar extinction coeffi-
cient of the triplet photosensitizer at the excitation wave-
length).23i The η value of Re-1 is 15-fold that of Re-2 with the
532 nm laser excitation.
Fig. 9 (a) Photographs of the emission of the photosensitizers alone and (b)
the upconversion. (c) CIE diagram of the emission of sensitizers alone and (d) in
the presence of perylene (upconversion). λex = 532 nm (laser power: 5 mW). In
deaerated toluene, c [sensitizer] = 1.0 × 10−5 M, c [perylene] = 3.0 × 10−5 M,
25 °C.
The upconversions were compared with iodo-BODIPY (B-2,
Scheme 1), which shows a similar molar extinction coefficient
at 532 nm compared to Re-1. With bis-iodo substitution in B-2,
the ISC of B-2 is efficient (ΦΔ = 79%).13,22 B-2 was used as a
triplet photosensitizer.13
A slightly smaller upconversion
quantum yield of 5.3% was determined for B-2. This lower
upconversion quantum yield can be attributed to its shorter
triplet excited state lifetime (τT = 57.1 μs) compared with that
of Re-1 (τT = 104.0 μs).
The TTA upconversion with the Re(I) complexes as the
triplet photosensitizers are clearly visible by the naked eye
(Fig. 9). For example, the emission color of Re-1 or Re-2 alone
is green and cherry-red, which are due to the fluorescence
emission of the complexes. In the presence of the triplet accep-
tor of perylene, the emission color of Re-1 turns to bright-blue,
which is due to the upconverted fluorescence of perylene. For
Re-2, however, the emission color barely changed, which is in
full agreement with the spectral observations (Fig. 9b). For Re-
0, no upconversion was observed (the green color of Re-0 or
Re-0/perylene mixed solution is due to the scattered excitation
laser). The changes of the emission color were quantified with
the CIE coordinates (Fig. 9c and d).
Fig. 10 Quenching of the lifetime of the triplet photosensitizers with increas-
ing concentration of perylene. Stern–Volmer plots for the lifetime quenching. c
[photosensitizers] = 1.0 × 10−5 mol L−1, in deaerated toluene. The triplet excited
state lifetimes were measured with a transient absorption spectrometer upon
355 nm nanosecond laser excitation, 25 °C.
In order to quantify the efficiency of the TTET process, the
quenching of the triplet excited state lifetime of the triplet
photosensitizers by the triple acceptor perylene was studied
(Fig. 10). The quenching constants of Re-1 (KSV = 6.75 × 105 luminescence lifetime of the upconversion observed in Fig. 8
M−1) and Re-2 (KSV = 6.42 × 105 M−1) are close to each other were measured (Fig. 11). For Re-1 as the triplet photosensitizer,
(Table 4), which is due to the similar triplet excited state life- the luminescence lifetime of the blue emission band observed
time of the two complexes. The quenching constant of B-2 is in the presence of perylene was determined as 111.8 μs. The
only half that of Re-1 and Re-2, which is due to the shorter T1 prompt fluorescence lifetime of perylene determined in a
state lifetime of B-2. Despite the similar quenching constants, different experiment was measured as 4.5 ns. Similar long-
Re-2 shows much weaker TTA upconversion than Re-1, which lived lifetimes were also observed with Re-2 (τ = 52.85 μs) and
can be assigned to the weak absorption of Re-2 at the exci- B-2 (τT = 154.1 μs) as the triplet photosensitizer in the TTA
tation wavelength and the less efficient ISC of Re-2.
upconversion experiments (see ESI† for more details). The
In order to unambiguously confirm the TTA upconversion long-lived luminescence is the characteristic feature of the TTA
observed with Re-1 as the triplet photosensitizer, the upconversion.3b,14c,d,23d,27c,32
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luminescence lifetime (for Re-1, the lifetime of the delayed
fluorescence is 111.8 μs, vs. the 4.5 ns for the prompt fluore-
scence of perylene).
Conclusions
In conclusion, N^N Re(I) tricarbonyl chloride complexes with
to date the strongest absorption in the visible range (ε is up to
91 700 M−1 cm−1 at 536 nm) and long-lived triplet excited state
(104 μs) were prepared. It is also for the first time that the
well-known fluorophore of Bodipy was used to enhance the
visible-light-absorption of the Re(I) tricarbonyl chloride Re(I)
complexes. Different from the model complex (Re-0), in which
no visible-light-harvesting chromophore is presented and
showing phosphorescence, the new Re(I) complexes show the
fluorescence of Bodipy ligands. Furthermore, Bodipy ligand-
localized triplet excited states were identified with nanosecond
time-resolved transient difference absorption spectroscopy and
spin density analysis. The complexes were used as triplet
photosensitizers in two different triplet–triplet-energy-transfer
(TTET) processes, i.e. the singlet oxygen (1O2) mediated photo-
oxidation and the triplet–triplet annihilation (TTA) upconver-
Fig. 11 (a) Delayed fluorescence observed in the TTA upconversion with Re-1
as triplet photosensitizer and perylene as the triplet acceptor. Excited at 532 nm
(nanosecond pulsed OPO laser synchronized with spectrofluorometer) and mon-
itored at 445 nm. Under these circumstances Re-1 is selectively excited and the
emission is due to the upconverted emission of perylene. (b) The prompt fluor-
escence decay of perylene determined in a different experiment (excited with
EPL picosecond 405 nm laser, the decay of the emission was monitored at
445 nm). In deaerated toluene. c [sensitizers] = 1.0 × 10−5 M; c [perylene] = 3.0
× 10−5 M; 25 °C.
1
sion. We found that the O2 photosensitizing efficiency of the
new Re(I) complexes are up to 3-fold that of the known
efficient triplet photosensitizers such as tetraphenylporphyrin
(TPP) and methylene blue (MB). The TTA upconversion
quantum yields (ΦUC) of the new Re(I) complexes are 1.6-fold
that of the efficient bis-iodo-BODIPY triplet photosensitizer.
Therefore we conclude that the strong visible-light absorption
of the new Re(I) complex is effective, that is, the excitation
energy harvested by the Bodipy ligand can be efficiently fun-
nelled to the triplet excited states of the complexes. Our results
are useful for preparation of Re(I) tricarbonyl complexes that
show strong absorption of visible light and long-lived triplet
excited states, as well as for the application of these complexes
as triplet photosensitizers in photocatalysis, photodynamic
therapy (PDT) and triplet–triplet annihilation upconversion.
Scheme 4 Jablonski diagram of TTA upconversion with Re-1 as triplet photo-
sensitizer. E is energy. GS is ground state (S0). 1Bodipy* is intraligand singlet
excited state localized on Bodipy ligand. 3Bodipy* is intraligand triplet excited
state localized on the Bodipy ligand. TTET is triplet–triplet energy transfer. 3pery-
lene* is the triplet excited state of perylene. TTA is triplet–triplet annihilation.
1perylene* is the singlet excited state of perylene. The emission bands observed
for the sensitizers along is the 1IL/3IL emission. The emission bands observed in
the TTA upconversion experiment is the delayed emission of perylene.
The photophysical processes involved in the photoexcita-
tion and the TTA upconversion with Re-1 can be summarized
in Scheme 4. After photoexcitation of Re-1, the singlet excited
state was obtained, which is localized by the Bodipy ligand
Experimental
Analytical measurements
(1Bodipy* state). Then through the process of intersystem NMR spectra were recorded on a Bruker 400 MHz spectrometer
crossing (ISC), the triplet excited state was produced (CDCl3 as solvent, TMS as standard, 0.00 ppm). High resolu-
(3Bodipy*), facilitated by the heavy atom effect of Re(I). With tion mass spectra (HR MS) were determined on a LC/QTOF MS
the triplet–triplet energy transfer (TTET) from the triplet system (UK). Fluorescence spectra were measured on
a
excited state of Re-1 to perylene, the triplet excited state of per- RF-5301 PC fluorospectrometer (Shimadzu, Japan). Fluore-
ylene was populated. Note the energy level of the triplet accep- scence lifetimes were measured with a OB920 lifetime spectro-
tor must be lower than the energy level of the triplet fluorometer (Edinburgh instruments, UK). Absorption spectra
photosensitizer. The biomolecular quenching constants are on were recorded on
a UV2550 UV/vis spectrophotometer
the scale of 109 M−1 s−1, indicating diffusion-controlled (Shimadzu) and an Agilent 8453 UV/vis near-IR spectropho-
quenching, i.e. the TTET is efficient. In turn the TTA process tometer. The nanosecond time-resolved transient absorption
gives the singlet excited state of perylene. The delayed fluore- spectra were measured on a LP920 laser flash photolysis
scence was produced from the radiative decay of the S1 state of spectrometer (Edinburgh Instruments, UK) and recorded on a
perylene, which was proved by the measurement of the Tektronix TDS 3012B oscilloscope. The lifetime values (by
2070 | Dalton Trans., 2013, 42, 2062–2074
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monitoring the decay trace of the transients) were obtained CDCl3), δ = 9.09 (d, 2H, J = 5.4 Hz); 8.22 (d, 2H, J = 8.1 Hz);
with LP900 software.
8.10 (t, 2H, J = 15.8 Hz); 7.58 (t, 2H, J = 12.9 Hz). HRMS
(MALDI): calcd [C13H8ClN2O3Re–Cl]+: m/z = 427.0093; found:
m/z = 427.0082. IR (KBr, cm−1): ν = 2023 (s), 1890 (s), 1601 (m),
Materials
All the chemicals were analytically pure and used as received. 1471 (m), 1444 (m), 777 (m), 765 (m).
Solvents were dried and distilled for synthesis. Re(CO)5Cl was
SYNTHESIS OF RE-1. The synthetic procedure was the same as
purchased from Sigma-Aldrich Corp. Compounds 1 to 6 were that for Re-0 except that L1 (50 mg, 0.08 mmol) was used. The
synthesized according to literature methods (see ESI† for product was obtained as a red solid by column chromato-
detail).33 7 to 10 were prepared by the reported method.34
graphy (silica gel, CH2Cl2–CH3OH = 100 : 1). Yield: 56 mg
SYNTHESIS OF L1. 4 (80 mg, 0.14 mmol) and 5-ethynyl-2,2′- (75.7%). M.p. > 250 °C. 1H NMR (400 MHz, CDCl3), δ =
bipyridine (10) (25 mg, 0.14 mmol) were added to deaerated 9.02–9.05 (m, 2H); 8.10–8.15 (m, 2H); 7.95–8.04 (m, 2H); 7.50
triethylamine (15 mL). Then Pd(PPh3)2Cl2 (5 mg, 5 mol%), (t, 1H, J = 12.6 Hz); 7.17 (t, 2H, J = 13.2 Hz); 7.07 (d, 2H, J =
PPh3 (4 mg, 10 mol%) and CuI (3 mg, 10 mol%) were added 8.4 Hz); 6.09 (s, 1H); 4.22 (t, 2H, J = 7.9 Hz); 3.93 (t, 2H, J =
under an Ar atmosphere. The mixture was stirred and refluxed 7.9 Hz), 3.76 (t, 2H, J = 8.4 Hz); 3.61 (t, 2H, J = 8.4 Hz); 3.42 (s,
for 12 h under argon. After completion of the reaction, the 3H); 2.71 (s, 3H); 2.60 (s, 3H); 1.58 (s, 3H); 1.48 (s, 3H) ppm.
mixture was cooled to RT and the light red precipitate was col- 13C NMR (100 MHz, CDCl3): δ = 196.74, 189.46, 159.83, 154.59,
lected by filtration. The crude product was purified with 153.33, 153.09, 146.37, 142.84, 140.05, 138.99, 133.82, 129.12,
column chromatography (silica gel, CH2Cl2–ethyl acetate = 127.00, 126.52, 125.21, 123.29, 122.69, 115.60, 112.65, 92.32,
1
20 : 1, v/v) to give red solid, 59 mg (67.8%). M.p. > 250 °C. H 90.36, 83.57, 72.08, 71.00, 69.87, 67.66, 59.23, 29.83, 15.08,
NMR (400 MHz, CDCl3): δ = 8.78 (d, 2H, J = 13.7 Hz); 8.50–8.43 13.60 ppm. HRMS (MALDI): calcd ([C39H35N4O6ClBF2Re]−):
(m, 2H); 7.92–7.87 (m, 2H); 7.41–7.38 (m, 1H); 7.23 (d, 2H, J = m/z = 926.1864, found: m/z = 926.1855. IR (KBr, cm−1): ν = 2924
8.3 Hz); 7.12 (d, 2H, J = 8.1 Hz); 6.10 (s, 1H); 4.27 (t, 2H, J = (m), 2200 (m), 2022 (s), 1914 (s), 1887 (s), 1608 (m), 1540 (s),
3.2 Hz); 3.98 (t, 2H, J = 3.2 Hz), 3.82 (t, 2H, J = 3.2 Hz); 3.67 (t, 1516 (s), 1478 (m), 1317 (m), 1277 (m), 1191 (s), 1108 (m), 1060
2H, J = 4.0 Hz); 3.46 (s, 3H); 2.77 (s, 3H); 2.63 (s, 3H); 1.62 (s, (m), 990 (m), 791 (m). Elemental analysis calcd (%) for
3H); 1.51 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3): δ = 159.63, [C39H35N4O6ClBF2Re + 1.5 H2O]: C 49.11, H 3.99, N 5.88;
158.23, 155.37, 154.11, 151.26, 149.11, 145.30, 142.42, 139.00, found: C 49.05, H 4.06, N 5.78.
137.31, 133.18, 130.60, 129.14, 126.76, 120.44, 115.40, 114.23,
SYNTHESIS OF RE-2. The synthetic procedure was the same as
92.75, 86.82, 71.99, 70.91, 69.79, 67.50, 59.21, 29.73, 14.78, that for Re-0 except that L2 (60 mg, 0.08 mmol), which
13.46 ppm. HRMS (MALDI): calcd [C36H35BF2N4O3 + H]+: m/z = replaced L0 was used. The product was obtained as a dark
621.2849; found: m/z = 621.2795.
purple solid by column chromatography (silica gel, CH2Cl2–
1
SYNTHESIS OF L2. The synthetic procedure was the same as CH3OH = 100 : 1). Yield: 61 mg, (72.6%). M.p. > 250 °C. No H
that for L1 except that compound 11 (97 mg, 0.14 mmol),13 NMR and 13C NMR spectrum were not measured due to the
instead of 4, was used. The crude product was purified by poor solubility of the compound in common organic solvents.
column chromatography (silica gel, CH2Cl2–CH3OH = 100 : 1) HRMS (MALDI): calcd ([C52H40N5O3ClBF2Re]−): m/z
=
to give a dark purple solid. Yield: 67 mg (63.8%). M.p. > 1053.2438, found: m/z = 1053.2410. IR (KBr, cm−1): ν = 3056
1
250 °C. H NMR (400 MHz, CDCl3), δ = 8.74 (s, 1H); 8.71 (d, (w), 2924 (m), 2853 (m), 2197 (m), 2024 (s), 1899 (s), 1597 (w),
1H, J = 4.1 Hz); 8.46 (d, 2H, J = 8.4 Hz); 8.20 (s, 1H); 8.08 (d, 1530 (s), 1474 (m), 1322 (m), 1278 (m), 1185 (s), 1013 (m),
1H, J = 7.9 Hz); 7.88 (d, 2H, J = 8.5 Hz); 7.56–7.55 (m, 4H); 7.50 745 (w), 725 (m). Elemental analysis calcd (%) for [C52H40-
(t, 1H, J = 6.1 Hz); 7.41 (d, 1H, J = 7.9 Hz); 7.36–7.32 (m, 4H); N5O3ClBF2Re + 2.5 H2O]: C 56.83, H 4.10, N 6.38; found: C
7.18–7.08 (m, 1H), 4.32 (t, 2H, J = 14.2 Hz); 2.79 (d, 6H, J = 56.75, H 4.21, N 6.34.
16.5 Hz); 1.89–1.81 (m, 2H); 1.59 (d, 6H, J = 14.5 Hz); 1.44–1.34
Triplet–triplet annihilation upconversions
(m, 2H); 0.96–0.92 (m, 3H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 157.57, 155.77, 154.04, 152.15, 148.51, 147.49, 145.87, Diode pumped solid state (DPSS) continuous laser (532 nm)
145.50, 144.54, 143.69, 142.64, 141.71, 141.00, 140.26, 139.19, was used as an excitation source for the upconversions. The
138.00, 134.57, 131.98, 129.23, 126.30, 123.08, 121.93, 119.52, power of the laser beam was measured with VLP-2000 pyroelec-
116.49, 113.35, 109.58, 109.14, 108.92, 93.14, 90.29, 87.05, tric laser power meter. For the upconversion experiments, the
86.12, 61.91, 59.18, 43.17, 29.89, 20.71, 14.08, 13.79 ppm. mixed solution of the compound (triplet photosensitizer) and
HRMS (MALDI): calcd [C49H40BF2N5]+: m/z = 747.3345; found: perylene was degassed with N2 or Ar for at least 15 min (note
m/z = 747.3300.
the upconversion can be significantly quenched by O2). Then
SYNTHESIS OF RE-0. Re(CO)5Cl (30 mg, 0.08 mmol) and 2,2′- the solution was excited with laser. The upconverted fluor-
bipyridine (L0, 13 mg, 0.08 mmol) were dissolved in dry escence of perylene was observed with a spectrofluorometer. In
toluene and heated to 110 °C, the mixture was stirred under Ar order to repress the scattered laser, a black box was put behind
for 1 h. The solvent was then removed under reduced pressure, the fluorescent cuvette to trap the laser beam.
and the product was obtained as a light yellow solid by
column chromatography (silica gel, CH2Cl2–CH3OH = 100 : 1). with the prompt fluorescence of bis-iodo-BODIPY (B-2, ΦF
Yield: 34 mg (91.9%). M.p. > 250 °C. 1H NMR (400 MHz, 2.7% in CH3CN). The upconversion quantum yields were
The upconversion quantum yields (ΦUC) were determined
=
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calculated with the following equation, where ΦUC, Aunk, Iunk spectra were recorded at intervals of 2–5 min. The consump-
and ηunk represents the quantum yield, absorbance, integrated tion of DHN was monitored by the decrease of the UV absorp-
photoluminescence intensity and the refractive index of the tion at 301 nm, and the concentration of DHN was calculated
solvents (eqn (1), where the subscript “std” is for the standard based on its molar extinction coefficient (ε = 7664 M−1 cm−1).
used in the measurement of the quantum yield and “sam” for The Juglone production was determined by monitoring the
the samples to be measured). Factor 2 is used in the equation increase of the absorption at 427 nm. The concentration of
in order to make the maximum quantum yield to be unity.23j Juglone was calculated by using ε = 3811 M−1 cm−1 at 427 nm,
Furthermore, the absorption correction factors 1–10−A are and the yield of Juglone was obtained by dividing the concen-
used in the equation, instead of the absorption (A) in the pre- tration of Juglone with the initial concentration of DHN.
viously reported equation,23g due to the strong absorbance of
the samples compared to that of the dilute solutions, which
are normally used for determination of the luminescence
quantum yields.
Acknowledgements
We thank the NSFC (20972024, 21073028 and 21273028), the
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
1 ꢀ 10ꢀA
Iunk
Istd
ηunk
ηstd
std
Royal Society (UK) and NSFC (China-UK Cost-Share Science
Networks, 21011130154), Science Foundation Ireland (SFI E.T.
S. Walton Program 11/W.1/E2061) and Ministry of Education
(NCET-08-0077) for financial support.
ΦUC ¼ 2Φstd
ð1Þ
1 ꢀ 10ꢀA
unk
The CIE coordinates (x, y) of the emission of the triplet
photosensitizers alone and the emission of the TTA upconver-
sion in the presence of a triplet acceptor were derived from the
emission spectra with the software of CIE color Matching
Linear Algebra.
Notes and references
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The delayed fluorescence of the TTA upconversion was
measured with nanosecond pulsed laser (OpoletteTM 355II+UV
nanosecond pulsed laser, typical pulse length: 7 ns. Pulse
repetition: 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 FLS920 spectrofluoro-
meter (Edinburgh, UK). The decay kinetics of the upconverted
fluorescence (delayed fluorescence) was monitored with
FLS920 spectrofluorometer (synchronized to the OPO nano-
second pulse laser). The prompt fluorescence lifetime of the
triplet acceptor DPA was measured with EPL picosecond
pulsed laser (405 nm) which is synchronized to the FLS 920
spectrofluorometer.
DFT calculations
The density functional theory (DFT) calculations were used for
optimization of both singlet and triplet states. The UV-vis
absorption and 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). The spin density surface of
the complexes were calculated based on the optimized triplet
state. All the calculations were performed with Gaussian
09W.35
Photooxidation
A CH2Cl2–MeOH (9 : 1, v/v) mixed solvent containing DHN
(2.0 × 10−4 M) and triplet photosensitizer (10 mol% vs. DHN)
was put into a two neck round bottom flask (25 mL). The solu-
tion was then irradiated using a 35 W xenon lamp through a
cut off filter (0.72 M NaNO2 aqueous solution, which is trans-
parent for light with wavelength >385 nm). UV-vis absorption
2072 | Dalton Trans., 2013, 42, 2062–2074
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