Observation of the room temperature phosphorescence of Bodipy in visible light-harvesting Ru(ii) polyimine complexes and application as triplet photosensitizers for triplet-triplet-annihilation upconversion and photocatalytic oxidation

Article abstract of DOI:10.1039/c3tc30592a

Two Ru(ii) polyimine complexes containing a boron-dipyrromethene (Bodipy) chromophore were prepared. The two complexes are different in the linker which connects the Bodipy part and the Ru(ii) coordination centre. The Bodipy core and the Ru(ii) centre are in π-conjugation in Ru-1, whereas in Ru-2 the Bodipy part is linked in a non-conjugated way to the Ru(ii) centre. Ru(bpy) ₃[PF₆]₂ (Ru-3) was used as a reference complex. Both Ru-1 and Ru-2 show strong absorption in the visible region (ε = 65200 M^-1 cm^-1 at 528 nm for Ru-1 and ε = 76700 M ^-1 cm^-1 at 499 nm for Ru-2). The fluorescence of the Bodipy ligands was almost completely quenched in Ru-1 and Ru-2. Ru-1 shows room temperature phosphorescence of the Bodipy chromophore, as well as the residual fluorescence of the Bodipy ligand. Ru-2 shows only the residual fluorescence of the Bodipy ligand. A long-lived Bodipy-localized triplet excited state was observed for both Ru-1 and Ru-2 upon visible light excitation (τ_T is up to 279.7 μs, the longest T₁ state lifetime observed for the Bodipy moiety in the transition metal complex). Application of the complexes in triplet-triplet-annihilation upconversion and singlet oxygen ( ¹O₂)-mediated photo-oxidation proved that Ru-1 is more efficient (e.g. singlet oxygen quantum yield Φ_Δ = 0.93) as a triplet photosensitizer than Ru-2 (Φ_Δ = 0.64). Therefore, direct connection of the π-core of the Bodipy chromophore to the coordination centre, i.e. by establishing π-conjugation between the visible light-harvesting chromophore and the metal coordination centre is essential to enhance the effective visible light-harvesting of the Ru(ii) complexes. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3tc30592a

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Materials Chemistry C
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Observation of the room temperature phosphorescence
of Bodipy in visible light-harvesting Ru(II) polyimine
complexes and application as triplet photosensitizers
for triplet–triplet-annihilation upconversion and
photocatalytic oxidation†
Cite this: DOI: 10.1039/c3tc30592a
*
Wanhua Wu, Jifu Sun, Xiaoneng Cui and Jianzhang Zhao
Two Ru(II) polyimine complexes containing a boron-dipyrromethene (Bodipy) chromophore were
prepared. The two complexes are different in the linker which connects the Bodipy part and the Ru(II)
coordination centre. The Bodipy core and the Ru(II) centre are in p-conjugation in Ru-1, whereas in Ru-2
the Bodipy part is linked in a non-conjugated way to the Ru(II) centre. Ru(bpy)3[PF6]2 (Ru-3) was used as
a reference complex. Both Ru-1 and Ru-2 show strong absorption in the visible region (3 ¼ 65 200 Mꢀ1
cmꢀ1 at 528 nm for Ru-1 and 3 ¼ 76 700 Mꢀ1 cmꢀ1 at 499 nm for Ru-2). The fluorescence of the Bodipy
ligands was almost completely quenched in Ru-1 and Ru-2. Ru-1 shows room temperature
phosphorescence of the Bodipy chromophore, as well as the residual fluorescence of the Bodipy ligand.
Ru-2 shows only the residual fluorescence of the Bodipy ligand. A long-lived Bodipy-localized triplet
excited state was observed for both Ru-1 and Ru-2 upon visible light excitation (sT is up to 279.7 ms, the
longest T1 state lifetime observed for the Bodipy moiety in the transition metal complex). Application of
the complexes in triplet–triplet-annihilation upconversion and singlet oxygen (1O2)-mediated photo-
oxidation proved that Ru-1 is more efficient (e.g. singlet oxygen quantum yield FD ¼ 0.93) as a triplet
photosensitizer than Ru-2 (FD ¼ 0.64). Therefore, direct connection of the p-core of the Bodipy
chromophore to the coordination centre, i.e. by establishing p-conjugation between the visible light-
harvesting chromophore and the metal coordination centre is essential to enhance the effective visible
light-harvesting of the Ru(II) complexes.
Received 30th March 2013
Accepted 21st May 2013
DOI: 10.1039/c3tc30592a
www.rsc.org/MaterialsC
are detrimental to the applications of Ru(II) polyimine
complexes as triplet photosensitizers in the new applications,
Introduction
Recently Ru(II) polyimine complexes have attracted much
attention owing to the applications in photocatalysis such as
photoredox reactions,1–3 photoinduced H2 production from
water,4 photocatalytic cleavage of DNA,5,6 luminescent molec-
ular probes and bioimaging,7–12 photovoltaics,13–16 and triplet–
triplet-annihilation upconversion.17 However, the conventional
Ru(II) polyimine complexes, such as Ru(bpy)3[PF6]2 (bpy ¼ 2,20-
bispyridine), show weak absorption in the visible region (3 <
20 000 Mꢀ1 cmꢀ1 in the region beyond 400 nm), and the
absorption maxima are usually at 450 nm. Furthermore, the
triplet excited state lifetimes of the typical Ru(II) complexes are
usually shorter than 5 ms.2,8,16,18,19 These photophysical features
such as photocatalysis.
In order to address these challenges, Ru(II) complexes with
long-lived triplet excited states were prepared, based on the
mechanisms of either establishment of triplet state equilib-
rium2,20–27 or population of the ligand-centred IL state19,28–30 or
by optimization of the coordination geometry of the N^N
ligands.31 However, it is still a challenge to enhance the effective
absorption of Ru(II) complexes in the visible region.1–19
Previously the coumarin unit was used as a visible light-
harvesting ligand for the Ru(II) complex, but the absorption
band is in the UV region.32,33 Dipyrrin was used as a ligand for
Ru(II) complexes, but the generation of the triplet excited state
and the application of the resulted visible light-absorbing
complexes were not reported.34 Concerning this aspect, Bodipy
is a versatile chromophore which shows intense absorption in
the visible range.35–52 Bodipy was also attached to the Ru(II)
coordination framework but very oen the phosphorescence
of the Ru(II) coordination center and the Bodipy uorophore
was quenched.25,46 Furthermore, the triplet excited states of
State Key Laboratory of Fine Chemical, School of Chemical Engineering, Dalian
University of Technology, Dalian, 116024, P.R. China. E-mail: zhaojzh@dlut.edu.cn;
Web: http://nechem.dlut.edu.cn/photochem; Fax: +86 411-8498-6236
† Electronic supplementary information (ESI) available: Molecular structure
characterization data, UV/Vis absorption and emission spectra and TTA
upconversion details. See DOI: 10.1039/c3tc30592a
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the complexes were not investigated in detail. Recently we
In order to address these challenges, herein we prepared
attached Bodipy moieties to the Ir(III) coordination center and two Ru(II) polyimine complexes, in which the Bodipy units
long-lived triplet excited states were observed.17c Based on the were either linked in a way of p-conjugation to the coordi-
pioneering studies from other groups, we postulate that the nation center via a C^C bond (Ru-1), or tethered on the N^N
intramolecular energy transfer can be enhanced with selection coordination framework (Ru-2, Scheme 1). Ru(bpy)3[PF6] was
of
a suitable linker between the visible light-harvesting used as a reference complex. The structure of Ru-2 is similar
antenna and the coordination center.46,53,54 Concerted energy to the conventional Ru(II)–Bodipy complexes,25,46 but the
levels for the light-harvesting ligands and the Ru(II) coordi- structural prole of Ru-1 is unprecedented for the Bodipy-
nation center are also important. To date the application of containing Ru(II) complexes.21 The photophysical properties
the long-lived triplet excited state of Ru(II) complexes has of the complexes were studied by steady state and time-
rarely been reported.20,21,28,53 The visible light-absorbing Bod- resolved absorption and photoluminescence spectroscopy.
ipy-containing Ru(II) complexes were not studied for any Both Ru-1 and Ru-2 show strong absorption of visible light
applications.
and long-lived triplet excited states. The complexes were used
Scheme 1 Synthesis of the ligands L1 and L2 and the complexes Ru-1 and Ru-2, and the structure of model compound L3 and complex Ru-3 were also shown. The
complexes are positively charged and the anions PF6ꢀ are omitted for clarity. Reagents and conditions: (i) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8
h; (ii) Bu4NF, THF, r.t. (iii) dry CH2Cl2, CF3COOH, DDQ, BF3–Et2O, NEt3; (iv) Pd(PPh3)2Cl2, PPh3, CuI, THF/NEt3, reflux, 8 h; (v) Pd(PPh3)4, CuI, THF/NEt3, reflux, 14 h; (vi)
ethanol, r.t., 3 h; (vii) 2,20-bipyridine, ethanol–H2O, reflux, 22 h.
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as triplet photosensitizers in triplet–triplet annihilation (TTA) ethynyl-2,20-bipyridine (2) (35.8 mg, 0.2 mmol) were added to
upconversion and singlet oxygen (1O2)-mediated photo- the mixed solvents of THF–triethylamine (5 mL/2 mL). Then
oxidation of organic substrates. The results demonstrated Pd(PPh3)4 (0.02 mmol, 23.1 mg, 10 mol%) and CuI (0.04 mmol,
that the efficiency of Ru-1 as a triplet photosensitizer in these 8.0 mg, 20 mol%) were added under Ar atmosphere. The
applications is much higher than that of Ru-2. These molec- mixture was reuxed and stirred for 14 h under argon. Aer
ular structural design rationales, the photophysical studies completion of the reaction, the mixture was cooled to RT, the
and the applications may be useful for future development of solvents were evaporated under reduced pressure. The crude
visible light-harvesting Ru(II) complexes and for the applica- product was puried by column chromatography (silica gel,
tions of these complexes in photocatalysis and TTA CH2Cl2–CH3OH ¼ 300 : 1, v/v) to give 30.0 mg red solid (30.0%).
upconversion.
1H NMR (400 MHz, CDCl3) d ¼ 8.85 (s, 1H), 8.71 (s, 1H), 8.45 (s,
2H), 7.98–7.96 (d, 1H, J ¼ 4.5 Hz), 7.86 (s, 1H), 7.72 (d, 2H, J ¼
5.2 Hz), 7.34 (m, 3H), 2.56 (s, 6H), 1.44 (s, 6H); 13C NMR (100
MHz, CDCl3) d ¼ 156.0, 155.4, 151.9, 149.4, 143.1, 140.7, 139.7,
137.3, 135.7, 132.5, 131.3, 128.6, 124.3, 123.6, 121.6, 120.7,
120.1, 92.9, 87.9, 14.8. ESI-HRMS: calcd ([C31H25BF2N4 +H]+) m/z
¼ 503.2219; found, m/z ¼ 503.2223.
Experimental section
General analytical measurements
NMR spectra were recorded on a Bruker 400 MHz spectrometer
(CDCl3 or acetone-d6 as solvents, TMS as a standard, d ¼ 0.00
ppm). High resolution mass spectra (HRMS) were recorded on a
LC/Q-TOF MS system (UK). Luminescence spectra were recor-
ded on a RF-5301 PC or a CRT 970 spectrouorometer. Fluo-
rescence lifetimes were measured with an OB920 luminescence
lifetime spectrometer (Edinburgh, UK). Absorption spectra were
recorded on an Agilent 8453A UV/Vis spectrophotometer. The
nanosecond time-resolved transient difference absorption
spectra were measured by Edinburgh analytical instruments
(LP920, Edinburgh Instruments, Livingston, UK) and recorded
on a Tektronix TDS 3012B oscilloscope. The synthesis of
compound 4 was reported elsewhere.55 For detailed synthetic
procedures of compounds 1, 2 and 3, NMR spectra, MS spectra
and spectroscopic experiments, please refer to the ESI.†
General method for the preparation of the Ru(II) complexes
RuCl2(cymene) (0.5 eq.) and ligands (1.0 eq.) were suspended
in ethanol (5.0 mL). The mixture was stirred at r.t. under N2
atmosphere for ca. 2 h until the solution become clear. Then
a solution of 2,20-bipyridine (2.0 eq.) in water (10 mL) was
added and the mixture was reuxed for 22 h. Aer cooling,
the solvent was evaporated under reduced pressure. The
crude product was then subjected to column chromatography
(silica gel eluted with acetonitrile : water : saturated aqueous
NaNO3 ¼ 100 : 9 : 1, v/v) and treated with a saturated aqueous
solution of NH4PF6. Red precipitate was collected with
ltration, the solid was washed with water and dried in a
vacuum.
Ligand L1
Complex Ru-1
Under Ar atmosphere, 2-iodo-1,3,5,7-tetramethyl-8-phenyl-4,4-
diuoro-4-bora-3a-azonia-4a-aza-s-indacene (4) (70.0 mg, 0.16
mmol) and 5-ethynyl-2,20-bipyridine (2) (33.6 mg, 0.19 mmol)
were added to the mixed solvent of THF–triethylamine (5 : 2,
v/v, 7 mL). The ask was vacuumed and back-lled with Ar for
three times, and then Pd(PPh3)2Cl2 (5.6 mg, 0.008 mmol, 5 mol
%), PPh3 (0.016 mmol, 4.2 mg, 10 mol%) and CuI (0.016 mmol,
3.2 mg, 10 mol%) were added under Ar atmosphere. The
mixture was reuxed for 8 h under Ar atmosphere. Aer
completion of the reaction, the mixture was cooled to RT and
the solvents were evaporated under reduced pressure. The
crude product was puried with column chromatography (silica
gel, CH2Cl2–CH3OH ¼ 300 : 1, v/v) to give the red solid, 35.0 mg,
43.8%. 1H NMR (400 MHz, CDCl3) d ¼ 8.72 (s, 1H), 8.44 (s, 1H),
7.87 (d, 2H, J ¼ 5.2 Hz), 7.53–7.52 (m, 4H), 7.31–7.29 (m, 3H),
6.06 (s, 1H), 2.73 (s, 3H), 2.60 (s, 3H), 1.52 (s, 3H), 1.41 (s, 3H);
13C NMR (100 MHz, CDCl3) d ¼ 158.6, 156.4, 155.2, 151.4, 149.0,
145.4, 142.9, 142.4, 139.1, 137.6, 134.7, 130.4, 129.5, 128.0,
124.2, 122.6, 121.7, 120.7, 114.5, 92.9, 87.1, 15.0, 14.8, 13.8, 13.4;
MALDI-HRMS: calcd ([C31H25BF2N4]+), m/z ¼ 502.2140, found,
m/z ¼ 502.2149.
Following the general procedure outlined above, ligand L1 (20.0
mg, 0.04 mmol) was added, and the complex Ru-1 was isolated
in 38.4% yield (18.5 mg). 1H NMR (400 MHz, acetone-d6): 8.83–
8.78 (m, 6H), 8.24–8.20 (m, 7H), 8.11–8.04 (m, 4H), 7.89 (s, 1H),
7.65–7.58 (m, 8H), 7.49–7.43 (m, 2H), 6.32 (s, 1H), 2.57 (s, 3H),
2.46 (s, 3H), 1.45 (s, 5H), 1.38 (s, 3H); 13C NMR (100 MHz,
acetone-d6) d ¼ 160.97, 157.86, 157.70, 156.07, 152.64, 152.38,
147.23, 143.49, 142.10, 139.14, 138.67, 134.63, 134.20, 133.82,
130.08, 129.52, 128.40, 125.24, 124.98, 124.59, 123.98, 91.55,
88.64, 14.57, 14.42, 13.05, 12.75. ESI-HRMS: [(M-2PF6)2+/2]
calcd, m/z ¼ 458.1274; found, m/z ¼ 458.1292.
Complex Ru-2
Following the general procedure outlined above, ligand L2 (15
mg, 0.03 mmol) was added, and the complex Ru-2 was isolated
in 35.4% yield (12.8 mg). 1H NMR (400 MHz, acetone) d ¼ 8.89–
8.82 (m, 5H), 8.37 (d, 1H, J ¼ 8.0 Hz), 8.24–8.22 (m, 7H), 8.11–
8.06 (m, 3H), 8.03 (d, 1H, J ¼ 5.2 Hz), 7.70 (d, 2H, J ¼ 7.8 Hz),
7.60–7.59 (m, 5H), 7.52 (d, 2H, J ¼ 7.9 Hz), 6.13 (s, 2H), 2.50 (s,
6H), 1.41 (s, 6H); 13C NMR (101 MHz, acetone-d6) d ¼ 157.46,
155.88, 152.26, 152.00, 142.99, 140.97, 140.32, 138.33, 132.62,
131.06, 128.23, 127.97, 124.61, 121.67, 95.61, 80.98, 13.97. ESI-
Ligand L2
Under Ar atmosphere, 1,3,5,7-tetramethyl-8-(4-bromophenyl)- HRMS: [(M-2PF6)2+/2] calcd, m/z ¼ 458.1274; found, m/z ¼
4,4-diuoroboradiazaindacene (3) (80.0 mg, 0.2 mmol) and 5- 458.1292.
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irradiation. The irradiation wavelength for the samples and the
reference was the same. Quantum yields for singlet oxygen
generation in CH2Cl2 were determined by monitoring the
photooxidation of 1,3-diphenylisobenzofuran (DPBF) sensitized
by the Ru(II) complexes. The absorbance of DPBF was adjusted
around 1.0 at 414 nm in air saturated CH2Cl2, and absorbance
of the sensitizers was adjusted to 0.2–0.3 at the irradiation
wavelength. The photo-oxidation of DPBF was monitored at the
interval of 10 s. The quantum yields of singlet oxygen genera-
tion (FD) were calculated by a relative method using TPP
(FD ¼ 0.62) and Rose Bengal (FD ¼ 0.80 in MeOH) as the
reference. The following equation was used,
Calculations
The optimization of the ground state geometry and the spin
density surface of the Ru(II) complexes were calculated based on
the optimized triplet state geometry at the B3LYP/6-31
G/LANL2DZ level with Gaussian 09W.56 The UV/Vis absorption,
and the S0/T1 energy gaps of the complexes were calculated with
the time-dependent DFT (TDDFT). MeCN was used as a solvent
in the calculations (PCM model).
TTA upconversion
A diode pumped solid state laser was used in the TTA upcon-
versions. The upconversion quantum yields (FUC) were deter-
mined with the prompt uorescence of 2,60-diiodo-Bodipy (F ¼
9.3% in toluene) as the standard. The upconversion quantum
yields were calculated with eqn (1), where FUC stands for the
upconversion quantum yields, A, I and h represent the absor-
bance, integrated photoluminescence intensity and the refrac-
tive index of the solvent, sam represents the sample, and std
represents the standard (eqn (1)). The equation is multiplied by
factor 2 in order to make the maximum quantum yield to be
unity.17a
msamFstd
mstdFsam
FsDam ¼ FsDtd
(2)
where superscripts ‘sam’ and ‘std’ designate Ru complexes
and the standards, respectively, FD is the quantum yield of
singlet oxygen, m is the slope of a plot of difference in change
in absorbance of DPBF (at 420 nm) with the irradiation time
and F is the absorption correction factor, which is given by
F ¼ 1 ꢀ 10ꢀA
.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
1 ꢀ 10ꢀA
Isam
Istd
hsam
hstd
std
Results and discussion
Molecular design and synthesis of the Ru(II)–Bodipy
FUC ¼ 2Fstd
(1)
1 ꢀ 10ꢀA
sam
complexes
The TTET efficiency was studied by Stern–Volmer quenching
2+
constants. The concentration of the photosensitizers was xed In the typical Ru(II) polyimine complexes, such as Ru(bpy)3
at 1.0 ꢁ 10ꢀ5 M. The lifetimes of the photosensitizers were (bpy ¼ 2,2-bispyridine), p-conjugation between the Ru(II) coor-
measured by LP920 with increasing perylene or DPA concen- dination center and the N^N ligand can be established by the
tration in the solution.
ligand-to-metal bonding and the metal-to-ligand p bonding,
also called “p-backbonding”. For Ru-1, the p-core of the Bodipy
chromophore is directly linked to the bpy ligand via a p-
conjugation linker of the C^C bond. Thus we anticipate red-
shied absorption for Ru-1 vs. the ligand L1. For Ru-2, however,
the linkage is at the meso-phenyl moiety of Bodipy. The p-
conjugation framework of bpy in Ru-2 is not extended signi-
cantly. Therefore we anticipated similar absorption for Ru-2
compared to that for L2. The different linkers between the
Bodipy chromophore and the bpy ligand may impose a signif-
icant effect on the ISC of Ru(II) complexes. Previously Bodipy
was attached to the coordination center of the Ru(II) complexes,
but all those complexes are with the Bodipy chromophore iso-
lated from the coordination center.46 The preparation of the
complexes is based on the iodination of the Bodipy and Pd(0)-
catalyzed Sonogashira coupling reactions. All the compounds
were obtained in satisfactory yields.
Photooxidation
10 mL CH2Cl2–MeOH (9/1, v/v) solution containing DHN (2.0 ꢁ
10ꢀ4 M) and a photosensitizer (2.0 ꢁ 10ꢀ5 M) was placed in a
round-bottom ask and was irradiated by a 35 W xenon lamp
through a 0.72 M NaNO2 solution to cut off the light with
wavelength shorter than 385 nm. At intervals of 2–5 min, 2 mL
of the mixture was sampled for the UV-Vis absorption
measurement and was put back immediately aer recording the
absorption spectra, and UV/Vis absorption spectra were recor-
ded on the Agilent 8453 UV/Vis spectrophotometer. The power
density was tuned to 13 mW cmꢀ2 and was measured with a
solar power meter. The DHN consumption was monitored by a
decrease in the absorption at 301 nm, the concentration of DHN
was calculated by using its molar absorption coefficient
(3 ¼ 7664 Mꢀ1 cmꢀ1) at 301 nm. The juglone production was
monitored by an increase in the absorption at 427 nm, the
concentration of juglone was calculated by using its molar
extinction coefficient (3 ¼ 3811 Mꢀ1 cmꢀ1), and the yield of
juglone was obtained by dividing the concentration of juglone
with the initial concentration of DHN.
UV-Vis absorption of the Ru(II) complexes
The UV-Vis absorption spectra of Bodipy ligands and complexes
were studied (Fig. 1). L1 gives absorption at 527 nm, which is
red-shied compared to the ligand L2 (labs ¼ 499 nm). The
difference in the absorption of L1 and L2 can be attributed to
the larger p-conjugation framework in L1 than L2. We noted
Singlet oxygen (1O2) quantum yields (FD)
FD values of the triplet photosensitizers were calculated that the absorption spectrum of L2 is the sum of the absorption
according to a modied literature method.57 The light source of of Bodipy and phenylethynyl bpy (L3), indicating the lack of
a uorometer was used as the monochromatic light source for p-conjugation between bpy and Bodipy parts in L2.
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solution. The emission band was substantially quenched in
aerated solution, indicates the triplet state origin of this emis-
sive band (phosphorescence). For Ru-2, however, the phos-
phorescence band centered at 650 nm was observed, and the
residual uorescence of the Bodipy unit is more signicant than
that in Ru-1. The reference complex Ru-3 gave the normal
3MLCT phosphorescence at 608 nm and the emission band can
be quenched by O2 (see ESI† for detail). Based on the emission
wavelength of Ru-1, the emission band was attributed to the RT
phosphorescence of Bodipy.25,46,58 For Ru-2, the phosphores-
cence emission band at 648 nm can be attributed to the normal
3MLCT emission of the polyimine Ru(II) complexes.
Fig. 1 UV-Vis absorption spectra of the (a) ligands and (b) complexes Ru-1–Ru-
3. c ¼ 1.0 ꢁ 10ꢀ5 M in MeCN, 20 ꢂC.
To the best of our knowledge, this is the rst time that the RT
phosphorescence of Bodipy was observed with Ru(II)
complexes.25,46,59 The Bodipy–Ru(II) complexes reported previ-
ously share a similar molecular structural prole with Ru-2, as a
result the heavy atom effect is non-efficient and the RT phos-
phorescence of the Bodipy part cannot be observed.25,59 We
propose that the heavy atom effect in Ru-1 is more signicant
than that in Ru-2, therefore the ISC is more efficient in Ru-1. It
should be noted that the emission quantum yields and the
triplet excited state lifetime of the Ru(II) coordination center
would not be altered given the T1 state energy level of the
Compared to the reference complex Ru-3, both Ru-1 and Ru-
2 show much stronger absorption in the red-shied region (at
528 nm and 499 nm, respectively, Fig. 1b). Notably the molar
absorption coefficients (3) are up to 76 700 Mꢀ1 cmꢀ1. To the
best of our knowledge, Ru(II) complexes with such strong visible
light absorption are rarely reported.2,22,25,46 The absorption of
Ru-1 and Ru-2 in the visible range is similar to the ligands L1
and L2, and is comparable to the reported dipyrrin Ru(II)
complexes that show strong absorption in the visible region.16,34
3
organic chromophore is higher than the MLCT state.3
Photoluminescence of the Ru(II) complexes
The luminescence of the ligands and complexes was studied
(Fig. 2). L1 and L2 give strong emission at 563 nm and 512 nm,
respectively. In comparison, the Ru(II) complexes give much
weaker emission, and the uorescence emission of the ligands
was substantially quenched in the complexes. This quenching
effect may be due to the energy transfer, intersystem crossing
(ISC), or electron transfer process.25
Comparison of the luminescence excitation spectra and the
UV-Vis absorption spectra
In order to study the intramolecular energy transfer of the
complexes,33,42,47 the luminescence excitation spectra of the
complexes were recorded and were compared with the UV-Vis
absorption spectra (Fig. 3). For the reference complex Ru-3
(Fig. 3c), the excitation spectrum is superimposable to the UV-
Vis absorption (Fig. 3c), indicating that the 3MLCT emission is
The photoluminescence of the complexes was investigated
(Fig. 2c and d). Ru-1 gives emission at 741 nm in deaerated
1
directly resulted from the S0 / MLCT excitation.
For Ru-1, the emission wavelength was set at 740 nm, i.e. the
3IL state emission localized on the Bodipy part. We found that
the absorption of the Bodipy part at 528 nm is effective to
produce the near IR emission at 740 nm. For Ru-2, with setting
the emission wavelength at 650 nm, only the MLCT absorption
band is effective to produce the emission. The absorption of the
Bodipy part at 500 nm is un-effective to produce the emission at
650 nm. For the emission at 540 nm (emission of the Bodipy
part), the absorption at 500 nm is effective. These results indicate
that the 3MLCT state cannot be accessed with excitation into the
absorption band of Bodipy in Ru-2. That is, the strong absorption
of Ru-2 is not effective to produce the 3MLCT state, which is in
stark contrast to Ru-1 (Fig. 3a). These results indicated that for
Ru-2, the 1Bodipy* / 1MLCT process is non-efficient.
Emission spectra of the complexes at 77 K
The photoluminescence spectra of the complexes at 77 K were
measured (Fig. 4). Compared to the emission spectra at RT, the
uorescence emission of Ru-1 at 550 nm in the RT spectrum
disappeared at 77 K, and only an emission band at 741 nm was
observed, which is slightly blue shied vs. the emission at RT
(Fig. 4a). This small thermally induced Stokes shi (DES) is a
Fig. 2 Emission spectra of (a) Ru-1 and the ligand L1 (lex ¼ 520 nm) in N2, (b)
Ru-2 and L2 (lex ¼ 480 nm) in N2; emission spectra of the complexes (c) Ru-1 (lex
¼ 520 nm) and (d) Ru-2 (lex ¼ 480 nm) under different atmospheres of N2 and air
(MeCN, 1.0 ꢁ 10ꢀ5 M; 20 ꢂC).
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Fig. 3 Comparison of the normalized UV-Vis absorption and the excitation spectra of the complexes (a) Ru-1 (the peak denoted with asterisk is due to the second
order transmission of the monochromator), (b) Ru-2 and (c) Ru-3. The excitation spectra of Ru-1 were recorded with lem ¼ 740 nm, for Ru-2, lem ¼ 650 nm and 540
nm, for Ru-3, lem ¼ 607 nm. c ¼ 1.0 ꢁ 10ꢀ5 M in MeCN, 20 ꢂC.
Fig. 4 Emission spectra of (a) Ru-1 (lex ¼ 520 nm), (b) Ru-2 (lex ¼ 480 nm) and (c) Ru-3 (lex ¼ 450 nm) in ethanol–methanol (4 : 1, v/v) glass at 77 K and in solution at RT.
clear indication of the emissive 3IL state for Ru-1. For Ru-2,
however, no Bodipy phosphorescence emission in the near-IR
region was observed (Fig. 4b), which is in agreement with the
reported Bodipy–Ru(II) complexes that share a similar structural
prole with Ru-2.46 For the reference complex Ru-3 (Fig. 4c), the
emission spectrum become more structured and blue-shied
compared to that at RT. The large thermally induced Stokes
shi of Ru-3 indicated the 3MLCT feature of the emissive triplet
excited state of Ru-3, which is in full agreement with the known
photophysics of Ru-3.21
Nanosecond time-resolved transient difference absorption
spectra
In order to study the triplet excited state of the complexes, the
nanosecond time-resolved transient difference absorption
spectra of the complexes were studied (Fig. 5). Upon 532 nm
photoexcitation, signicant bleaching at 530 nm was observed
for Ru-1 (Fig. 5a), which is due to the depletion of the ground
Fig. 5 Nanosecond time-resolved transient difference absorption spectra of (a)
state of the Bodipy ligand. Therefore the triplet excited state of
Ru-1 and (c) Ru-2 after pulsed excitation (lex ¼ 532 nm); decay traces of (b) Ru-1
at 530 nm and (d) Ru-2 at 500 nm. c ¼ 1.0 ꢁ 10ꢀ5 M in MeCN 20 ꢂC.
Ru-1 is localized on the Bodipy part. The lifetime of the triplet
excited state was determined as 279.7 ms (Fig. 5b). To the best of
our knowledge, this is the longest triplet state lifetime observed
for the Bodipy chromophore.25,46 Previously the lifetime of the excited state of Ru-2 is also localized on the Bodipy part, not the
Bodipy triplet state was observed in the range of 8–30 ms.25,46 Ru(II) coordination center. We propose that this result is due to
3
Interestingly, similar transients were observed for Ru-2 (Fig. 5c), the lower energy level of the Bodipy IL state compared to the
with slightly blue-shied bleaching bands. Thus the triplet 3MLCT state.25,46 The triplet state lifetime of Ru-2 was
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Table 1 Photophysical parameters of the Ru(II) complexes and the ligandsa
the complexes Ru-1 and Ru-2, which contain the Bodipy units,
DFT calculations were carried out.
Emission properties
Firstly the ground state geometry of the complexes was
optimized. For Ru-1, the Bodipy chromophore takes a coplanar
orientation toward the bpy ligand of the Ru(II) coordination
center. As a result, full p-conjugation is expected for the Bodipy
p-core and the Ru(II) coordination center. For Ru-2, however,
the Bodipy p-core takes a perpendicular geometry toward the
bpy coordination ligand. Therefore, there is no full p-conjuga-
tion between the Bodipy chromophore and the bpy coordina-
tion ligand. This different p-conjugation prole will affect the
UV/Vis absorption of the complexes.
The UV/Vis absorption of complexes Ru-1 and Ru-2 was
studied with the time-dependent DFT (TDDFT). The calculated
absorption wavelength of Ru-1 is 499 nm, which is very close to
the experimental results (labs ¼ 528 nm, Fig. 1b and Table 1).
Molecular orbitals of HOMO / LUMO are involved in this S0 /
S1 transition. These molecular orbitals are localized on Bodipy
as well as the bpy moieties (Fig. 6a). Therefore there is efficient
p-conjugation between the Bodipy chromophore and the bpy
coordination ligand. The UV/Vis absorption of Ru-1 was also
calculated without any solvents (in vacuum). The calculated S0
/ S1 transition is with an absorption wavelength of 854 nm (see
ESI† for detail).
labs 3b
lem/nm lemc/nm FLd (%) sFe/ns sT
f
h
h
Ru-1 528 6.52 748
Ru-2 499 7.67 513
Ru-3 451 1.14 608
741
510
578
—
—
279.7 ms/33.9g ms
1.0
9.6
33.6
43.9
0.14 246.6 ms/4.4g ms
h
—
0.8 ms/5.4g ms
h
h
L1
L2
527 5.85 563
499 8.64 512
—
3.5
1.9
—
h
h
—
—
Result of complexes in MeCN solution (1.0 ꢁ 10ꢀ5 mol dmꢀ3), 293 K.
a
Molar absorption coefficient at the absorption maxima, 3 104 Mꢀ1
b
c
d
cmꢀ1
.
Result in ethanol–methanol (4 : 1, v/v) glass at 77 K. With
e
BODIPY (F ¼ 0.72 in THF) as the standard. Fluorescence lifetime.
f
Triplet lifetimes measured by nanosecond time-resolved transient
absorptions. g Luminescence lifetime at 77 K. h Not determined.
determined as 246.6 ms. In comparison, the triplet state lifetime
of the model complex Ru-3 was determined as 0.8 ms (3MLCT
state, see ESI† for details). These results proved that the lowest-
lying triplet states of Ru-1 and Ru-2 are drastically different
1
from that of Ru-3. Since the energy transfer from Bodipy /
1MLCT is inhibited in Ru-2 due to the unmatched energy levels
of the singlet excited states, the heavy atom effect in Ru-2 is
weak, the generation of the 3IL state in Ru-2 may be due to the
charge re-combination process.25 It should be noted that Ru(II)
complexes with a triplet excited state lifetime up to 200 ms were
not reported.2,21,22,29 The photophysical parameters of the Ru(II)
complexes and the ligands are summarized in Table 1.
The energy gap of the S0/T1 states was calculated as 1.57 eV
(792 nm), which is very close to the experimental results of
1.66 eV (748 nm, Fig. 4). Since the major components of the
T1 / S0 state is LUMO / HOMO, the T1 state can be identied
3
as the IL state, which is localized on Bodipy (Fig. 6a, for the
Previously Ru(II) complexes with thermally equilibrated
3MLCT and 3IL states were prepared, but the lifetimes were
much shorter.21 For example, a triplet state lifetime of 42 ms was
observed for a pyrenyl ethynyl polyimine Ru(II) complex.18 The
pyrene unit was attached to the N^N ligand of the Ru(II) complex
by a C–C bond (similar to the prole of Ru-2) and a long-lived
pyrene localized triplet state was observed (148 ꢃ 8 ms).24 Bodipy
was attached to the terpyridine ligand of Ru(II) complexes, but
detail of the electronic transitions, please refer to the ESI†). This
result is in agreement with the transient difference absorption
spectroscopy.
3
the Bodipy-localized IL state is shorter-lived (sT ¼ 8–30 ms).25
Naphthalimide was used for preparation of Ru(II) complexes
that show the 3IL/3MLCT state equilibrium and a lifetime of up
to 115 ms was observed,27 but the molar absorption coefficient of
the complex in the 400–500 nm region is only 20 000 Mꢀ1 cmꢀ1
.
Recently we prepared coumarin-containing Ru(II) complexes
that show the ligand-localized triplet excited state, but the T1
lifetime is much shorter (less than 5 ms).54 Aryl-borane-con-
taining Ru(II) complexes were also reported and the phospho-
rescence lifetime is 12 ms.10,60 Thienyl–dipyrrin Ru(II) complexes
were prepared, the visible light absorption of these complexes
are strong,16,34 but the triplet excited states were not investi-
gated. Recently we prepared cyclometalated Ir(III) complexes
with Bodipy ligands, but the triplet excited state lifetimes
(23.7 ms and 87.2 ms) are much shorter than Ru-1 and Ru-2.17c
DFT calculations
Recently it was shown that density functional theory is useful in
studying the luminescence properties of transition metal
Fig. 6 Frontier molecular orbitals of (a) Ru-1 and (b) Ru-2. Based on the opti-
mized ground state geometry (S0 state). CH3CN was used as a solvent. Calculation
complexes.61 In order to study the photophysical properties of was performed at the B3LYP/6-31G(d)/LanL2DZ level with Gaussian 09W.
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The UV-Vis absorption of Ru-2 was also calculated with the
TDDFT method. Different from Ru-1, the LUMO of Ru-2 is only
distributed on the bpy ligand, the electron density map of the
orbital is completely isolated from the p-conjugation core of
Bodipy (Fig. 6b). The HOMO of Ru-2 is localized on the p-core of
the Bodipy chromophore, which is not spread to bpy (Fig. 6b).
Therefore, S0 / S1 transition is a full charge transfer transition,
which is oen a quantum mechanically forbidden process. This
postulation was supported by the oscillator strength ( f ¼ 0.00)
of the calculated S0 / S1 transition. We identied that S0 / S8
transition (labs ¼ 427 nm, S0 / S8) may be responsible for the
main absorption band of Ru-2 in the visible region (499 nm,
Fig. 1b and Table 1). HOMO / LUMO+1 are involved in the
transition. LUMO+1 is localized on the Bodipy unit (Fig. 6b).
Therefore, the S8 state can be identied as the 1IL state. It
should be pointed out that there is signicant deviation for the
calculated absorption wavelength (427 nm) from the experi-
mental absorption value of 499 nm (Fig. 1b). However, it was
known that very oen the unsubstituted Bodipy chromophore
cannot be well described by the DFT calculations.
The energy gap of the S0/T1 states was calculated as 1.52 eV
(816 nm), which is close to the value of Ru-1 (1.66 eV, 748 nm).
HOMO / LUMO+1 are major components of the T1 state.
Therefore, the T1 state can be described as the 3IL state, which is
localized on the Bodipy unit. This postulation is corroborated
by the nanosecond time-resolved transient difference absorp-
tion spectroscopy of the complexes.
In order to study the localization of the triplet excited states
of the complexes from a point of view of theoretical chemistry,
the spin density surfaces of the complexes were computed with
the DFT method (Fig. 7). This method has not been used for
Bodipy-containing Ru(II) complexes.46 For both Ru-1 and Ru-2,
the spin density surfaces are localized on the Bodipy part. The
bpy ligands and the Ru(II) centre did not contribute to the spin
density surface of the complexes. These results are in full
agreement with the transient absorption spectra and the
TDDFT calculations. Therefore the lowest lying triplet excited
states of Ru-1 and Ru-2 are localized on the Bodipy part. For Ru-
3, however, the spin density surface is localized on the Ru(II)
Scheme 2 Jablonski diagram of the photophysical processes involved in the
photoexcitation and emission of Ru-1.
coordination centre and the bpy ligands, which is in agreement
with the MLCT feature of the triplet excited state of Ru-3.
We studied the difference of the ground state geometry and
the triplet state geometry, and found out that the geometry
changes occurred at the Bodipy p-conjugated core. The Ru(II)
coordination framework and the ethynyl linker did not show
any substantial geometrical changes. This observation is in full
agreement with the transient absorption studies, as well as the
DFT calculation. That is, the triplet states of Ru-1 and Ru-2 are
localized on the Bodipy moiety.
Based on the absorption and emission of the ligands and the
complexes, the photophysical processes involved in Ru-1 can be
summarized in Scheme 2.25,46,58 For Ru-1, the direct 1Bodipy* /
3Bodipy* ISC is efficient, therefore the residual uorescence of
Bodipy is very weak. The 3MLCT* phosphorescence was also
3
quenched due to the 3MLCT* / Bodipy* energy transfer. For
1
3
Ru-2, the Bodipy* / Bodipy* ISC is non-efficient, therefore
the residual uorescence of the Bodipy ligand is more signi-
cant. Based on the excitation spectrum, we propose that the
1MLCT* / 1Bodipy* energy transfer is non-efficient, due to the
fast 1MLCT* / 3MLCT* ISC, therefore excitation into the MLCT
band of Ru-2 cannot produce the uorescence of the Bodipy
3
ligand. Excitation into the Bodipy cannot produce the MLCT
state efficiently, indicating that the CS state has a lower energy
3
level than the MLCT state.25
Application of the complexes in triplet–triplet annihilation
upconversion
Bodipy-containing Ru(II) complexes have never been used for
any photophysical processes.25,46 The new complexes we
prepared show strong absorption of visible light and excep-
tionally long-lived triplet excited states, which are suitable to be
used as triplet photosensitizers. Therefore these complexes
were used for TTA upconversion (Fig. 8). With excitation at
532 nm, weak luminescence was observed for the complexes
(Fig. 8a). With addition of the triplet acceptor perylene, strong
blue emission was observed with Ru-1 as the triplet photosen-
sitizer (Fig. 8b). Excitation of Ru-1 or perylene alone at 532 nm
did not produce this emission band, thus the TTA upconversion
was proved. For Ru-2 and Ru-3, however, no upconversion was
Fig. 7 Isosurfaces of spin density of Ru-1, Ru-2 and Ru-3 at the optimized triplet
state geometries. MeCN was used as a solvent in the calculations (PCM model).
Calculation was performed at the B3LYP/6-31G(d)/LanL2DZ level with Gaussian 09W. observed. Another reference complex Ru(dmb)3[PF6]2 was also
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Fig. 9 Upconversion with Ru-1 and Ru-2 as the photosensitizers (1.0 ꢁ 10ꢀ5 M),
perylene (Py) (4.1 ꢁ 10ꢀ5 M) as the acceptor, excited by the OPO laser at the
isosbestic point of the UV/Vis absorption of Ru-1 and Ru-2 (509 nm) (deaerated
MeCN, 20 ꢂC).
Fig. 8 Upconversion with Ru-1, Ru-2, Ru-3 and Ru(dmb)3[PF6]2 as triplet
sensitizers (1.0 ꢁ 10ꢀ5 M). (a) Emission spectra without perylene and (b) emission
spectra with perylene added. (c) Photographs of the emission of sensitizers alone
and in the presence of perylene (Py) (4.1 ꢁ 10ꢀ5 M). Excited with green laser, lex
¼
532 nm, 11.2 mW (MeCN, 20 ꢂC).
studied but no upconversion was observed under the experi-
mental conditions.
The upconversion is clearly visible to unaided eye (Fig. 8c).
Without the triplet acceptor perylene, the colour of the laser
beam is green, due to the very weak emission of the complex Ru-
1 and the scattered 532 nm laser. In the presence of the triplet
acceptor perylene, strong blue emission was observed. To the
best of our knowledge, this is the rst time that the green light-
excitable Ru(II) complex was used for TTA upconversion.62,63
Fig. 10 Stern–Volmer plots generated from triplet excited state lifetime (sT)
quenching of complexes Ru-1, Ru-2 or phosphorescence intensity quenching of
Ru-3 measured as a function of perylene concentration in MeCN. The lifetimes
were measured with the nanosecond time-resolved transient absorption. The
concentration of the photosensitizers was fixed at 1.0 ꢁ 10ꢀ5 M, 20 ꢂC.
Previously Ru(dmb)3[PF6]2 (dmb ¼ 4,40-dimethyl-2,20-bipyr- the presence of triplet acceptor perylene was studied and the
idine), and Ru(II) complexes with long-lived triplet excited states Stern–Volmer quenching curves were plotted (Fig. 10). We
were used for TTA upconversion, but all these Ru(II) complexes found that the quenching of the triplet excited state of Ru-1 and
show absorption maxima in the blue region.53,54,64–66 One Ru(II) Ru-2 by perylene is signicant. For Ru-3, however, the quench-
containing triplet photosensitizer was reported to be near IR- ing effect is neglectable. These different quenching efficiencies
excitable for TTA upconversion, but the light-harvested chro- are due to the different triplet excited state lifetimes of Ru-1 and
mophore is not the Ru(II) coordination moiety.67
Ru-2. The quenching constants of Ru-1 and Ru-2 are 650–790-
Since the ligand uorescence is almost completely quenched fold of Ru-3. Therefore the long-lived triplet excited state of the
in Ru-2, and the nanosecond time-resolved transient difference triplet photosensitizers is benecial for the TTA upconversion.
absorption spectra conrmed the population of the triplet The TTA upconversion quantum yield of Ru-1 is ca. 2-fold of Ru-
excited states of Ru-2, we propose that TTA upconversion can be 2. The overall upconversion capacity, with consideration of the
observed with appropriate excitation wavelength. Furthermore, absorption of the triplet photosensitizer at the photoexcitation
in order to directly compare the TTA upconversion efficiency wavelength, was also compared (Table 2).
with Ru-1 and Ru-2 as the triplet photosensitizer, the TTA
In order to unambiguously prove that the blue emission is due
upconversion with Ru-1 and Ru-2 as the photosensitizers at the to the TTA upconversion, the lifetime of the blue emission
same excitation wavelength was carried out (Fig. 9). The results observed in the TTA upconversion experiments was studied
show that the TTA upconversion with Ru-1 is ca. 2-fold to that (Fig. 11). ForRu-1andRu-2, thelifetimesoftheemissionat450nm
with Ru-2 as the triplet photosensitizer. This result indicates were determined as 162.0 ms and 180.9 ms, respectively. The life-
that the generation of the triplet excited state in Ru-1 is more time of the prompt uorescence of perylene was determined as
efficient than that in Ru-2. Note that both Ru-1 and Ru-2 show 4.0 ns in a different experiment. Therefore, the exceptionally long-
the same absorbance at the excitation wavelength.
lived luminescence lifetime with Ru-1/perylene and Ru-2/pyrelene
In order to study the efficiency of the TTET process, the is attributed to the TTA upconversion.68–74 The photophysical
quenching of the triplet state lifetime of the Ru(II) complexes in parameters related to the TTA upconversion are listed in Table 2.
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Table 2 Triplet excited state lifetimes (sT), Stern–Volmer quenching constant
(KSV) and bimolecular quenching constants (kq) of Ru-1, Ru-2 and Ru-3 as the
photosensitizers. Perylene was used as the quencher. In deaerated MeCN solu-
tion, 20 ꢂC
hd
a
b
c
sDF/ms
Ksv
kq
FUC
Ru-1
Ru-2
Ru-3
162.0
180.9
—
3718.5
3065.5
4.7
13.3
12.4
5.9
1.2%
0.7%
—
751.2
201.6
—
e
e
e
Quenching constants in 103 Mꢀ1
.
Bimolecular quenching constants
a
b
in 109 Mꢀ1 sꢀ1
.
Excited with a 532 nm laser, with the prompt
c
uorescence of iodo-BDP as the standard (F ¼ 2.7% in MeCN).
d
e
Overall upconversion ability, h ¼ 3 ꢁ FUC, in 103 Mꢀ1 cmꢀ1
.
Too
low to be determined accurately.
Fig. 12 UV-Vis absorption spectral change for DHN using (a) complex Ru-1, (b)
Ru-2, (c) Ru-3, (d) plots of ln(Ct/C0) vs. irradiation time (t) for the photo-oxidation
of DHN using Ru complexes. Irradiated with a 35 W xenon lamp (13 mW cmꢀ2 in
the photoreactor; the UV light with a wavelength shorter than 385 nm was
blocked by 0.72 M NaNO2 solution). In CH2Cl2–CH3OH (9 : 1, v/v); c [DHN] ¼ 2.0 ꢁ
10ꢀ4 M; c [photosensitizer] ¼ 2.0 ꢁ 10ꢀ5 M; 20 ꢂC.
Fig. 11 Time-resolved emission spectra (TRES) of (a) the complex Ru-1 and (b)
the upconverted fluorescence of perylene using Ru-1 as the triplet photosensi-
tizers. Excited with the nanosecond pulsed OPO laser. Ru-1: c ¼ 1.0 ꢁ 10ꢀ5 M. Py:
c ¼ 4.1 ꢁ 10ꢀ5 M. In deaerated MeCN, 20 ꢂC.
localized on Bodipy. The triplet–triplet-energy-transfer (TTET)
between the Ru(II) complexes and the triplet acceptor perylene
will produce the triplet excited state of the acceptor. TTA of the
acceptor will produce the singlet excited state and the radiative
decay of the singlet excited state will give the upconverted
uorescence from perylene (delayed uorescence). The light-
harvesting ability and the lifetime of the Ru(II) photosensitizer
are crucial for TTA upconversion.
The photophysical process involved in the TTA upconversion
with the Ru(II) complexes as triplet photosensitizers can be
summarized in Scheme 3. Excitation into both the MLCT and
the IL bands of Ru-1 will produce the 3IL state, which is
Photosensitization of singlet oxygen (1O2): photoxidation with
the complexes as a triplet photosensitizer
These complexes were also used as a triplet photosensitizer for
other photophysical process that involved TTET, herein we
studied the singlet oxygen (1O2)-mediated photooxidation, with
1
1,5-dihydroxynaphthalene (DHN) as the O2 scavenger to follow
the kinetics of the 1O2 production of the triplet
Table 3 Pseudo-first-order kinetics parameters, 1O2 generation quantum effi-
ciencies and yields of Juglone for the photo-oxidations of DHN using complexes
Ru-1–Ru-3 and MB, TPP as sensitizers
Scheme 3 Jablonski diagram of triplet–triplet-annihilation (TTA) upconversion
with Ru-1 as a triplet photosensitizer and perylene as a triplet acceptor. The effect
of the light-harvesting ability and the triplet lifetimes of the Ru(II) sensitizers on
the efficiency of the TTA-upconversion are also shown. E is the energy. GS is the
ground state (S0). 1MLCT* is the Ru(II) based metal-to-ligand-charge-transfer
singlet excited state, 1IL* is the Bodipy localized intraligand singlet excited state.
3MLCT* is the Ru(II) based metal-to-ligand-charge-transfer triplet excited state.
3IL* is the Bodipy localized intraligand triplet excited state. TTET is the triplet–
triplet energy transfer. 3Py* is the triplet excited state of perylene. TTA is the
triplet–triplet annihilation. 1Py* is the singlet excited state of perylene. The
emission bands observed in the TTA experiment are the singlet emission of per-
ylene (delayed fluorescence).
b
sT/ms
kobs/minꢀ1
via
FD
Yieldd (%)
Ru-1
Ru-2
Ru-3
TPP
MB
279.7
246.6
0.8
82.5
83.3
0.0603
0.0275
0.0108
0.0443
0.0351
1.206
0.55
0.216
0.886
0.702
0.93
0.64
0.66c
0.65
0.57
96.5
93.3
66.2
96.9
83.7
10ꢀ5 M minꢀ1. Initial rate of DHN consumption. b 1O2 generation
a
quantum yield measured using Rose Bengal (FD ¼ 0.80 in MeOH) as a
c
reference, lex ¼ 509 nm. TPP (FD ¼ 0.65 in CH2Cl2) as a reference.
d
lex ¼ 428 nm. Yield of juglone aer 60 min of photoreaction.
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photosensitizers.45,75–78 The photo-oxidation product juglone can lived triplet excited state and for the application of these
be used for preparation of anti-cancer compounds.79,80 The complexes in the area of photocatalysis, photodynamic therapy
conventional triplet photosensitizers meso-tetraphenylporphyrin and triplet–triplet-annihilation upconversion.
(TPP) and methylene blue (MB) were studied for comparison. The
results show that the 1O2 production of Ru-1 is more efficient than
Ru-2, which indicates that the ISC in Ru-1 is more efficient than
Acknowledgements
that in Ru-2. The 1O2 production ability of Ru-1 is more efficient We thank the NSFC (20972024, 21073028 and 21273028), the
than TPP and MB (Fig. 12). To the best of knowledge, very few Royal Society (UK) and NSFC (China-UK Cost-Share Science
triplet photosensitizers were reported to be more efficient than Networks, 21011130154), Science Foundation Ireland (SFI E.T.S.
TPP and MB as 1O2 photosensitizers.
The O2 quantum yields of the triplet photosensitizers were (NCET-08-0077 and SRFDP-20120041130005) for nancial
determined (Table 3). Much higher FD values were found for Ru- support.
Walton Program 11/W.1/E2061) and Ministry of Education
1
1 (0.93) than Ru-2 (0.64). These results indicated that the
molecular structural prole of Ru-1, i.e. the p-conjugation
between the coordination center and Bodipy is favorable for ISC.
Notes and references
1 (a) E. Baranoff, J.-P. Collin, L. Flamigni and J.-P. Sauvage,
Chem. Soc. Rev., 2004, 33, 147–155; (b) J. Xuan and
W.-J. Xiao, Angew. Chem., Int. Ed., 2012, 51, 6828–6838; (c)
Conclusion
In summary, we prepared two Bodipy-containing polyimine
Ru(II) complexes. For Ru-1, the Bodipy and the coordination
center are p-conjugated via a C^C bond, which is a new
molecular structural prole for Bodipy-containing Ru(II)
complexes. For Ru-2, however, the p-core of the Bodipy chro-
mophore and the coordination center are separated by a phenyl
moiety, which is the known structural prole for the Bodipy-
containing Ru(II) complexes. Ru(bpy)3[PF6]2 (Ru-3) was used as a
reference complex. Both Ru-1 and Ru-2 show strong absorption
in the visible region (3 is up to 65 200 Mꢀ1 cmꢀ1 at 528 nm),
which is in stark contrast to Ru-3, which show the typical weak
absorption in the visible region of the normal Ru(II) polyimine
complexes (3 ¼ 11 400 Mꢀ1 cmꢀ1 at 451 nm). The ligand uo-
rescence was substantially quenched in both Ru-1 and Ru-2. RT
near IR emission of the Bodipy part was observed with Ru-1, but
not with Ru-2. This is the rst time that the RT near IR phos-
phorescence of Bodipy was observed in Ru(II) complexes. Bod-
ipy-localized long-lived triplet excited states were populated for
Ru-1 and Ru-2 (sT is up to 279.7 ms), based on the nanosecond
transient absorption spectrum, and was supported by the spin
density analysis. To the best of our knowledge, this is the
longest triplet excited state lifetime observed for the Bodipy
chromophore. The complexes were used as triplet photosensi-
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