Trans-Bis(alkylphosphine) platinum(ii)-alkynyl complexes showing broadband visible light absorption and long-lived triplet excited states

Article abstract of DOI:10.1039/c4tc01675k

Heteroleptic trans-bis(alkylphosphine) platinum(ii) bisacetylide complexes Pt-1 and Pt-2 were prepared to achieve broadband absorption of visible light. Two different ethynylBodipy ligands, 2-ethynylBodipy and 2,6-diethynylBodipy or 8-(4′-ethynylphenyl)Bodipy, were used in each complex. Each Bodipy ligand gives strong absorption in the visible spectral region, but at different wavelengths, thus broadband absorption in the visible spectral region was achieved for the Pt(ii) complexes (ε is up to 1.85 × 10⁵ M^-1 cm^-1 in the region of 450-700 nm). Singlet energy transfer from the peripheral coordinated Bodipy to the central coordinated Bodipy (with 2,6-diethynyl substitution) was confirmed by steady state absorption/luminescence spectroscopy, fluorescence excitation spectroscopy and nanosecond/femtosecond ultrafast time-resolved transient absorption spectroscopy. Long-lived triplet excited states were observed for both complexes (τ_T = 63.13 μs for Pt-1 and τ_T = 94.18 μs for Pt-2). Nanosecond time-resolved transient absorption spectroscopy indicated that the triplet excited state of Pt-1 is distributed on both Bodipy units. For Pt-2, however, the T₁ state is confined to the central coordinated Bodipy ligand. These complexes show high singlet oxygen (¹O₂) quantum yields (Φ_Δ = 76.0%). With nanosecond pulsed laser excitation, delayed fluorescence was observed for the complexes (τ_DF = 43.8 μs for Pt-1 and τ_DF = 111.0 μs for Pt-2), which is rarely reported for transition metal complexes. The complexes were used as efficient multi-wavelength excitable triplet photosensitizers for triplet-triplet annihilation upconversion. This journal is

Full text of DOI:10.1039/c4tc01675k

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Materials Chemistry C
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PAPER
trans-Bis(alkylphosphine) platinum(II)-alkynyl
complexes showing broadband visible light
Cite this: J. Mater. Chem. C, 2014, 2,
absorption and long-lived triplet excited states†
9720
Huiru Jia,a Betul Kuçukoz,b Yongheng Xing,c Poulomi Majumdar,a Caishun Zhang,a
¨
¨ ¨ ¨
b
b
b
a
*
Ahmet Karatay, Gul Yaglioglu, Ayhan Elmali, Jianzhang Zhao
d
*
and Mustafa Hayvali
Heteroleptic trans-bis(alkylphosphine) platinum(II) bisacetylide complexes Pt-1 and Pt-2 were prepared to
achieve broadband absorption of visible light. Two different ethynylBodipy ligands, 2-ethynylBodipy and
2,6-diethynylBodipy or 8-(40-ethynylphenyl)Bodipy, were used in each complex. Each Bodipy ligand
gives strong absorption in the visible spectral region, but at different wavelengths, thus broadband
absorption in the visible spectral region was achieved for the Pt(II) complexes (3 is up to 1.85 ꢀ 105 Mꢁ1
cmꢁ1 in the region of 450–700 nm). Singlet energy transfer from the peripheral coordinated Bodipy to
the central coordinated Bodipy (with 2,6-diethynyl substitution) was confirmed by steady state
absorption/luminescence spectroscopy, fluorescence excitation spectroscopy and nanosecond/
femtosecond ultrafast time-resolved transient absorption spectroscopy. Long-lived triplet excited states
were observed for both complexes (sT ¼ 63.13 ms for Pt-1 and sT ¼ 94.18 ms for Pt-2). Nanosecond
time-resolved transient absorption spectroscopy indicated that the triplet excited state of Pt-1 is
distributed on both Bodipy units. For Pt-2, however, the T1 state is confined to the central coordinated
Bodipy ligand. These complexes show high singlet oxygen (1O2) quantum yields (FD ¼ 76.0%). With
nanosecond pulsed laser excitation, delayed fluorescence was observed for the complexes (sDF ¼ 43.8
ms for Pt-1 and sDF ¼ 111.0 ms for Pt-2), which is rarely reported for transition metal complexes. The
complexes were used as efficient multi-wavelength excitable triplet photosensitizers for triplet–triplet
annihilation upconversion.
Received 29th July 2014
Accepted 10th September 2014
DOI: 10.1039/c4tc01675k
www.rsc.org/MaterialsC
production of triplet excited states and the readily derivatizable
molecular structures.17,18 Since most of the applications of these
Introduction
complexes are related to the triplet excited states, with which
the consequential electron transfer or triplet energy transfer is
initiated, the visible light harvesting ability and the lifetime of
the T1 state of these complexes are crucial, because the above-
mentioned intermolecular processes are capable of being
enhanced with the abundant molecules at the excited states and
the long-living triplet excited states. The weak absorption of
conventional transition metal complexes may be attributed to
Transition metal complexes have been used in photocatalysis,
hydrogen production (H2), photoredox catalytic organic reac-
tions, molecular devices, photodynamic therapy (PDT), non-
linear optics, and electroluminescence,1–13 as well as in photo-
chemistry studies, such as investigations of singlet or triplet
exciton migration.14–16 Concerning these aspects, the Pt(II)
complexes are of particular interest because of the efficient
the weakly allowed S0
character).13 The visible light-absorbing ability is capable
/
1MLCT transition (charge transfer
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E-208 West Campus, 2 Ling Gong Rd, Dalian 116024,
P. R. China. E-mail: zhaojzh@dlut.edu.cn; Web: http://nechem.dlut.edu.cn/
photochem
of being enhanced by switching the S0 / S1 transition to the
1
S0 / IL (intraligand) transition, with the latter as a strongly
allowed p–p* transition. In recent years Pt(II) complexes with
visible light-harvesting ligands were reported.5,11 Our group
reported Pt(II) complexes which contain visible light-harvesting
ligands such as Bodipy, coumarin, naphthalenediimide, per-
ylene bisimide, etc.13 These complexes show strong visible light
absorption and long-lived triplet excited states, and have been
used in triplet–triplet annihilation (TTA) upconversion,
bDepartment of Engineering Physics, Faculty of Engineering, Ankara University, 06100
Be¸sevler, Ankara, Turkey
cCollege of Chemistry and Chemical Engineering, Liaoning Normal University, No. 850
Huanghe Road, Dalian 116029, P.R. China
dDepartment of Chemistry, Faculty of Science, Ankara University, 06100 Be¸sevler,
Ankara, Turkey. E-mail: hayvali@science.ankara.edu.tr
† Electronic supplementary information (ESI) available. CCDC 973944. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4tc01675k
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photoredox catalytic organic reactions and photosensitization method is exemplied with two heteroleptic trans-bis(alkyl-
of singlet oxygen (1O2).13
phosphine) platinum(II)-alkynyl complexes (Pt-1 and Pt-2,
We noted that a common disadvantage of these complexes is Scheme 1).5,19–24 For transition metal complexes containing
le to overcome, that is, there is usually only a single kind of multi-chromophores to achieve broadband absorption of visible
visible light-harvesting ligand in these complexes, as a result, light, it is important to control the direction of the singlet
there is only one major absorption band in the visible spectral energy transfer so that the spin converter acts as the singlet
region.5,13,17 This is clearly a drawback if a panchromatic light energy acceptor to ensure efficient transformation of the
source was used, such as solar light. Therefore, it is important photoexcitation energy into triplet state energy.13b,28,29
to prepare Pt(II) complexes that show broadband absorption of
Herein we prepared two heteroleptic trans-bis(alkylphos-
visible light. This goal cannot be achieved by using the phine) platinum(II)-alkynyl complexes (Pt-1 and Pt-2, Scheme 1).
conventional molecular structural protocol based on using a In these complexes, different Bodipy units are connected to the
single kind of visible light-harvesting ligand in the complex Pt(II) atom either at the 2-position or the meso(40-phenyl) posi-
molecule.13
tion of the Bodipy unit, so that different absorption wavelengths
On the other hand, the trans-bis(alkylphosphine) platinum(II)- and T1 state energy levels resulted, and intramolecular RET is
alkynyl complexes have attracted much attention.5,19–24 The ensured with the central 2,6-coordinated Bodipy moiety as the
molecular structure of these compounds can be easily modied by singlet energy acceptor, which is an efficient spin converter to
using different acetylide ligands, providing tunability of both the produce triplet excited states.13b
photophysical and redox properties.1,19 However, trans-bis(alkyl-
With steady state and nanosecond or femtosecond ultrafast
phosphine) platinum(II)-alkynyl complexes showing strong visible time-resolved transient absorption spectroscopy techniques, we
light absorption were rarely reported.20,25,26 We have shown the conrmed the singlet and triplet excited state energy transfer in
rich triplet excited state-related photochemistry of organic uo- these complexes. By nanosecond time-resolved transient
rophores, such as Bodipy, coumarin, rhodamine, NDI and PBI absorption spectroscopy, we found that the triplet excited state
with transition metal complexes,13 thus it is interesting to study of Pt-1 is delocalized on the three Bodipy ligands, due to
the intramolecular excited state energy transfer with the trans- degenerated T1 and T2 states, whereas in Pt-2 the triplet excited
bis(alkylphosphine) platinum(II)-alkynyl complexes that contain state is exclusively conned to one Bodipy ligand. Long-lived
organic chromophores.
triplet excited states (sT is up to 94.18 ms) were observed for the
In organic molecular dyads, uorescence resonance energy complexes. The complexes were used as multi-wavelength
transfer (FRET) was very oen used to achieve a broadband excitable triplet photosensitizers for TTA upconversion.
visible light absorption.27 However, to the best of our knowl-
edge, this strategy was rarely used to achieve broadband visible
Experimental section
Analytical measurements
light absorption in transition metal complexes.13,17,18 Recently
we prepared
a Pt(II) complex with Bodipy and naph-
All the chemicals used in synthesis were analytically pure and
were used as received. Solvents were dried and distilled before
synthesis. cis-Pt[P(n-Bu)3]2Cl2 was prepared according to a
literature method,5 and the synthesis of compounds 1, 2, and 3
was reported previously.30,31
thalenediimide ligands, but the absorption wavelength and
intensity are not satisfactory.19
In this paper we propose a strategy to achieve broadband
visible light-absorption in transitional metal complexes. Our
Scheme 1 Pt(II) complexes used in the study. The known complex Pt-3 is also presented.
This journal is © The Royal Society of Chemistry 2014
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128.9, 128.2, 122.5, 121.5, 120.4, 115.7, 114.4, 100.0, 99.7, 29.9,
26.5, 24.5, 24.42, 24.36, 23.9, 23.8, 23.6, 14.4, 14.0, 13.5, 13.4.
MALDI-HRMS: calcd ([C113H161B3F6N6P4Pt2]+), m/z ¼ 2263.1212,
found, m/z ¼ 2263.1389. Anal. calcd for [C113H161B3F6N6P4Pt2 +
C6H14]: C, 60.82; H, 7.51; N, 3.58. Found: C, 60.79; H, 7.24;
N, 3.38.
Complex Pt-a. Under an Ar atmosphere, compound 1 (34.8
mg, 0.1 mmol) and cis-Pt[P(n-Bu)3]2Cl2 (80 mg, 0.12 mmol) were
dissolved in a mixed solvent of THF/Et2NH (1 : 1, v/v, 6 mL), and
the ask was placed in a Dewar ask (ethyl acetate/liquid
nitrogen low temperature bath). The ask was evacuated and
back-lled with Ar. Then CuI (3.8 mg, 0.02 mmol) was added.
The temperature of the mixture was allowed to rise from ꢁ78 ꢂC
to RT within 8 hours. Then deionized water was used to quench
the reaction. The mixture was extracted with dichloromethane
(DCM, 3 ꢀ 15 mL). The organic layer was dried over anhydrous
Na2SO4, ltered and the solvent was removed under reduced
pressure. The crude product was puried using column chro-
matography (silica gel, CH2Cl2 : petroleum ether ¼ 1 : 2, v/v) to
give compound Pt-a as a purple solid. Yield: 44.5 mg (45.3%).
M.p.: 89.9–91.5 ꢂC. 1H NMR (400 MHz, CDCl3): 7.48–7.28
(m, 5H), 5.93 (s, 1H), 2.64 (s, 3H), 2.53 (s, 1H), 1.93 (s, 12H),
1.43–1.35 (m, 30H), 0.89 (t, 18H, J ¼ 8.0 Hz). 13C NMR (100 MHz,
CDCl3): d 158.4, 154.1, 142.0, 140.9, 140.7, 135.5, 131.5, 131.1,
129.2, 129.0, 128.2, 122.2, 120.7, 92.0, 90.0, 29.9, 26.3, 24.4, 22.2,
22.1, 21.9, 14.4, 14.0, 13.3. MALDI-HRMS: calcd ([C45H72BClF2-
N2P2Pt]+), m/z ¼ 981.4568, found, m/z ¼ 981.4590. Anal. calcd
for [C45H72BClF2N2P2Pt] + 1.6C6H14]: C, 58.50; H, 8.48; N, 2.50.
Found: C, 58.79; H, 8.09; N, 2.31.
Complex Pt-2. Complex Pt-2 was prepared by a procedure
similar to that for complex Pt-1, with Pt-b (29.0 mg, 0.03 mmol)
as the starting material, instead of Pt-a. Complex Pt-2 was
obtained as a dark green solid. Yield: 12.0 mg (33.3%). M.p.:
226.4–228.6 ꢂC. 1H NMR (400 MHz, CDCl3): d 7.49–7.48 (m, 3H),
7.36 (d, J ¼ 10.0 Hz, 4H), 7.29–7.27 (m, 2H), 7.08 (d, J ¼ 10.0 Hz,
4H), 5.97 (s, 4H), 2.64 (s, 6H), 2.55 (s, 12H), 2.13–2.08 (m, 24H),
1.61–1.55 (m, 24H), 1.45 (s, 12H), 1.43–1.37 (m, 30H), 0.89 (t, J ¼
10 Hz, 36H). 13C NMR (100 MHz): 159.1, 157.5, 145.8, 144.9,
142.2, 142.0, 137.8, 133.7, 133.3, 133.1, 132.3, 131.3, 131.1,
130.6, 123.3, 117.3, 117.1, 117.0, 113.3, 113.2, 113.1, 111.0,
102.0, 32.1, 28.7, 26.7, 26.6, 26.4, 26.3, 26.1, 16.7, 15.9, 15.4.
MALDI-HRMS: calcd ([C113H161B3F6N6P4Pt2]+), m/z ¼ 2263.1212,
found, m/z ¼ 2263.1409. Anal. calcd for [C113H161B3F6N6P4Pt2]:
C, 59.95; H, 7.17; N, 3.71. Found: C, 60.05; H, 6.87; N, 3.87.
Crystal structure. The crystal was mounted on glass bers for
X-ray measurement. Reection data were collected at room
temperature on a Bruker AXS SMART APEX II CCD diffractom-
eter with graphite-monochromatized Mo-Ka radiation (l ¼
Complex Pt-b. Under an Ar atmosphere, compound 2 (34.8
mg, 0.1 mmol) and cis-Pt[P(n-Bu)3]2Cl2 (79.6 mg, 0.12 mmol)
were dissolved in Et2NH (8 mL), and the ask was evacuated and
back-lled with Ar for several times. The mixture was heated at
45 ꢂC for 9 h. The solvent was removed under reduced pressure.
The crude product was further puried using column chroma-
tography (silica gel, CH2Cl2 : petroleum ether ¼ 1 : 1, v/v) to give
Pt-b as a yellow solid. Yield: 36.9 mg (37.6%). M.p.: 118.6–120.4 ꢂC.
1H NMR (400 MHz, CDCl3): 7.35–7.08 (m, 4H), 5.97 (s, 2H), 2.55
(s, 6H), 2.04 (s, 12H), 1.59 (m, 12H), 1.49–1.40 (m, 18H), 0.92 (t,
18H, J ¼ 8.0 Hz). 13C NMR (100 MHz, CDCl3): d 155.4, 143.3,
142.3, 131.6, 129.8, 127.7, 121.2, 106.2, 86.2, 86.0, 85.9, 29.9,
26.3, 24.6, 24.5, 24.4, 22.4, 22.3, 22.1, 14.7, 13.9. MALDI-HRMS:
calcd ([C45H72BClF2N2P2Pt]+), m/z ¼ 981.4568, found, m/z ¼
˚
0.71073 A) and a u scan mode. All the measured independent
reections (I > 2s(I)) were used in the structural analyses, and
semi-empirical absorption corrections were applied using the
SADABS program. The structures were solved by the direct
method using SHELXL-97. The hydrogen atoms of the organic
frameworks were xed at calculated positions geometrically
and rened by using a riding model. In particular, the atoms
(C22–C45) of the ligand are found to be disordered and
modelled over two split positions with occupancies in a ratio of
0.3 : 0.7. All corresponding non-hydrogen atoms were rened by
using the “Sadi” and “Simu” restraints to make the parameters
of the disordered atoms more reasonable. A single crystal of
Pt-b suitable for X-ray diffraction was grown by layering n-
hexane onto the dichloromethane solution of the complex. The
ORTEP drawing of Pt-b is depicted in Fig. 1. Selected atomic
distances and bonding angles are summarized in Table 1.†
981.4578. Anal. calcd for [C45H72BClF2N2P2Pt] + 0.6C6H14
+
0.3CH2Cl2]: C, 55.43; H, 7.71; N, 2.64. Found: C, 55.45; H, 7.44;
N, 2.49.
Complex Pt-1. Under an Ar atmosphere, compounds Pt-a
(24.5 mg, 0.025 mmol) and 3 (4.6 mg, 0.0125 mmol) were dis-
solved in a mixed solvent of THF/Et2NH (3 mL/3 mL), and the
ask was evacuated and back-lled with Ar several times. CuI
(3.0 mg, 0.015 mmol) was added, and the mixture was stirred at
0–5 ꢂC for 1 h. Deionized water was used to quench the reaction.
The mixture was extracted with DCM (3 ꢀ 15 mL). The
combined organic layer was dried over Na2SO4, ltered and the
solvent was removed under reduced pressure. The crude
product was puried using column chromatography (silica gel,
CH2Cl2 : petroleum ether ¼ 1 : 1, v/v) to give complex Pt-1 as a
dark blue solid. Yield: 17.7 mg (62.5%). M.p.: >250 ꢂC. 1H NMR
(400 MHz, CDCl3): 7.47–7.27 (m, 12H), 5.91 (s, 2H), 2.64–2.61 (d,
12H, J ¼ 12.0 Hz), 1.39–1.33 (m, 66H), 2.52 (s, 6H), 0.84 (t, 36H, J
¼ 8.0 Hz). 13C NMR (100 MHz, CDCl3): d 159.1, 157.1, 153.5,
141.5, 141.0, 140.5, 139.9, 139.5, 135.8, 135.5, 131.3, 130.9,
Fig. 1 The ORTEP view of the single crystal structure of Pt-b with 35%
thermal ellipsoids.
9722 | J. Mater. Chem. C, 2014, 2, 9720–9736
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Table 1 Selected bond lengths (A˚) and the angles (ꢂ) for complex Pt-b
absorption wavelengths, in one Pt(II) complex molecule. More-
over, the energy acceptor acts as the spin converter,13b thus to
convert the harvested excitation energy to the triplet state.
Bodipy was selected as the visible light harvesting chromophore
because of its strong absorption of visible light, high uores-
cence quantum yields and easiness of derivatization.32–39 High
uorescence quantum yield indicates that the internal conver-
sion is weak and it is more likely the ISC will be efficient if the
Bodipy is attached to the Pt(II) centre. The Bodipy ligands in the
complex molecule are with different coordination proles,
either at the 2-position of the p-core of Bodipy, or the 40-position
of the meso-phenyl group (Scheme 1). Thus the two coordina-
tion moieties give different absorption wavelengths.12,13
The central coordinated Bodipy units with 2,6-diethynyl
presumably will show a longer absorption wavelength than the
peripheral Bodipy ligands, which are metalated at the 2-posi-
tion (Pt-1), or at the 40-position of the meso-phenyl group of the
Bodipy unit (Pt-2). Thus with spectral overlap, singlet energy
transfer from the peripheral coordinated Bodipy ligands to the
central coordinated Bodipy ligand is envisaged. With the central
coordinated Bodipy moiety as the efficient spin converter to
produce triplet excited states, effective broadband absorption
was achieved for Pt-1 and Pt-2. The central coordinated Bodipy
unit and the peripheral coordinated Bodipy at the 2-position
(Pt-1) are expected to undergo efficient ISC because the metal-
ation is directly on the p-core of the Bodipy chromophore.13
In Pt-2, the metalation of the peripheral Bodipy unit is on the
meso-phenyl moiety, not on the p-core of the Bodipy. As a result,
the UV-vis absorption of this ligand is not perturbed, i.e. it is at
ca. 500 nm and the emission band overlaps with the absorption
spectra of the central coordinated Bodipy unit. Thus singlet
energy transfer from the peripheral Bodipy unit to the central
Bodipy unit is expected. Complexes Pt-a and Pt-b were prepared
as reference complexes. The previously reported Pt-3 was used
as a reference for photophysical studies.
˚
Bond lengths (A)
Bond angles (ꢂ)
Pt(1)–C(21)
Pt(1)–Cl(1)
Pt(1)–P(1)
Pt(1)–P(2)
C(20)–C(21)
1.965(7)
2.358(3)
2.318(2)
2.294(2)
1.18(1)
Cl(1)–Pt(1)–C(21)
Cl(1)–Pt(1)–P(1)
Cl(1)–Pt(1)–P(2)
P(1)–Pt(1)–P(2)
P(1)–Pt(1)–C(21)
P(2)–Pt(1)–C(21)
178.2(2)
93.6(1)
88.54(9)
175.69(9)
86.2(2)
91.6(2)
Nanosecond time-resolved transient difference absorption
spectroscopy. Nanosecond time-resolved transient difference
absorption spectra were recorded on a LP920 laser ash
photolysis spectrometer (Edinburgh Instruments, Livingston,
UK). The sample solution was purged with N2 or argon for
30 min before measurement. The samples were excited with a
nanosecond pulsed laser (Vibrant 355II, wavelength tunable in
the range of 410–2400 nm), and the transient signals were
recorded on a Tektronix TDS 3012B oscilloscope.
Femtosecond ultrafast transient difference absorption
spectroscopy. The ultrafast wavelength dependent pump probe
spectroscopy measurements were performed using a Ti:sap-
phire laser amplier-optical parametric amplier system
(Spectra Physics, Spitre Pro XP, TOPAS) and a commercial
setup (Spectra Physics, Helios). Pulse duration was measured to
be 100 fs. Wavelengths of the pump beam were chosen
according to the absorption spectra of complex Pt-2 as 500 nm
(selective excitation into the peripheral Bodipy moiety) and 640
nm (selective excitation into the central coordinated Bodipy
moiety). White light continuum was used as a probe beam.
TTA upconversion. A diode-pumped solid state continuous
laser was used for upconversion (632 nm and 589 nm). The laser
power was measured with a phototube. The mixed solution of
the Pt(II) complex (triplet photosensitizers) and PBI (triplet
acceptor) was degassed for at least 15 min with N2 or argon
before measurement. The spectra were recorded on an adapted
RF5301PC spectrouorometer (Shimadzu, Japan).
The delayed uorescence of the upconversion (sDF). The
delayed uorescence lifetimes (sDF) and spectra were recorded
with an Opolette™ 355II + UV nanosecond pulsed laser (typical
pulse length: 7 ns, pulse repetition: 20 Hz, peak OPO energy:
4 mJ, and wavelength is tunable in the range of 210–355 nm and
410–2200 nm. OPOTEK, USA), which is synchronized to a FLS
920 spectrouorometer (Edinburgh, U.K.). The decay kinetics of
the upconverted uorescence (delayed uorescence) was
monitored with a FLS 920 spectrouorometer (Edinburgh
Instruments, Livingston, UK). The prompt uorescence lifetime
of the triplet acceptor perylene was measured with an EPL
picosecond pulsed laser (405 nm) which is synchronized to the
FLS 920 spectrouorometer.
The preparation of the ligands is with Bodipy as the starting
material (Scheme 2). Iodination and the Sonogashira coupling
reactions readily introduce either mono- or bis-acetyl units at
the 2- and 6-positions of the Bodipy chromophore. The Bodipy
ligand with the ethynyl group at the meso-position was prepared
with 4-bromobenzenaldehyde.
Complexes Pt-a and Pt-b are the synthons for complexes Pt-1
and Pt-2 (Scheme 2). CuI catalyzed acetylization readily gives the
desired complexes Pt-1 and Pt-2 with moderate to satisfactory
yields. The reference complexes Pt-a and Pt-b were prepared by
similar methods. All the complexes were fully characterized by 1H
NMR, 13C NMR and HR MS.
Single crystal molecular structures
The single crystal structure of complex Pt-b was determined
(Fig. 1). The coordination geometry of the Pt(II) center is square
planar with two PBu3 ligands, an ethynyl ligand and a Cl atom in a
trans-arrangement.40 The meso-phenyl moiety takes a perpendic-
ular geometry toward the p-core of the Bodipy chromophore.
Results and discussion
Molecule design and synthesis
The principal molecular designing strategy of heteroleptic Pt-1 Thus there is no efficient p-conjugation across this phenyl group.
and Pt-2 is to use different Bodipy ligands showing different This postulation was conrmed by the unchanged UV-vis
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Scheme 2 Preparation of the ligands and the Pt(II) complexes. The model complex Pt-3, triplet acceptor PBI used in TTA upconversion and the
standards (B-6 and DCM) for determination of the fluorescence quantum yields are also presented. Reagents and conditions: (i) dry CH2Cl2,
BF3$OEt2, NEt3; (ii) NIS, RT; (iii) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8 h; (iv) Bu4NF, THF, ꢁ78 ꢂC; (v) K2CO3, MeOH, RT, 3
h; (vi) dry CH2Cl2, CF3COOH, BF3$OEt2, NEt3; (vii) distilled THF, NHEt2, CuI, ꢁ78 ꢂC to RT, 8 h; (viii) NHEt2, 45 ꢂC, 8 h; (ix) THF/HNEt2, CuI, 0–5 ꢂC.
absorption of the Bodipy unit upon coordination with the Pt(II) in Pt-a, for which the p-conjugation core of the Bodipy moiety
˚
center in complex Pt-b. The C(21)–Pt(1) bond length is 1.965 A. is connected to the coordination centre.
˚
The P(1)–Pt(1) bond length is 2.318 A. The C^C bond length,
The redox potentials of complex Pt-1 were studied (Fig. 2c).
˚
C(21)–C(20), is 1.18 A. All these bond lengths are similar to the A reversible reduction wave was observed at E1/2 ¼ ꢁ1.56 V. By
reported linear Pt(II) complexes.41
comparison with the redox potential of Pt-3, the reduction
wave at ꢁ1.56 V can be attributed to the sum of complexes Pt-a
and Pt-1. A reversible oxidation wave at E1/2 ¼ +0.43 V was
observed. By comparison with Pt-3, this wave can be attrib-
uted to the central coordination Bodipy moiety. The oxidation
wave of the peripherally coordinated Bodipy moiety in Pt-1 is
at +0.70 V, which is anodically shied as compared with that
of complex Pt-a. This change can be attributed to the weaker
p-backbonding in Pt-1 than in complex Pt-a. Similar results
were observed for complex Pt-2 (Fig. 2d). Generally the redox
potentials of the complexes Pt-1 and Pt-2 are the sum of the
central coordinated Bodipy moiety and the respective
peripheral coordinated Bodipy units (Table 2). Therefore,
there is no strong electronic interaction, such as p-conjuga-
tion, between the chromophores in Pt-1 and Pt-2 at ground
Redox properties
The redox properties of the complexes were studied (Fig. 2).
Complex Pt-a shows reversible oxidation wave at E1/2 ¼ +0.62
V, and a reversible reduction wave at E1/2 ¼ ꢁ1.52 V (Fig. 2a).
The reversible reduction potential of Pt-b is at ꢁ1.51 V
(Fig. 2b), which is similar to Pt-a. Irreversible oxidation wave
at +0.91 V was observed for Pt-b. The energy gap of Pt-a is
smaller than that of complex Pt-b. These differences can be
attributed to the different linkage between the Pt(II) coordi-
nation centre and the Bodipy chromophore in complexes Pt-a
and Pt-b.13 The cathodically shied oxidation wave of complex
Pt-a as compared with Pt-b may be due to the p-backbonding
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Fig. 3 UV-vis absorption spectra of the complexes. (a) Pt-a, Pt-1 and
Pt-3. (b) Pt-b, Pt-2 and Pt-3. c ¼ 1.0 ꢀ 10ꢁ5 M in toluene, 20 ꢂC.
as Pt-a and Pt-3. The absorption band at 643 nm of Pt-1 is
attributed to the central coordinated ligand, as demonstrated
by Pt-3. The minor absorption bands at 606 nm and 565 nm
are attributed to the peripheral Bodipy ligands, as demon-
strated by complex Pt-a.
Fig. 2 Cyclic voltammogram of the complexes. (a) Pt-a, (b) Pt-b, (c)
Pt-1, and (d) Pt-2. The data of Pt-3 are presented in (c) and (d) for
No long range p-conjugation exists between the two
difference Bodipy units across the Pt(II) centers in Pt-1. This
postulation is supported by the same absorption band at 643
nm of Pt-3 as compared with the absorption band at 644 nm
for Pt-1. This nding is different from the previously reported
Pt(II) complexes,42 for which the absorption of one coordina-
tion ligand can be affected by the other coordination ligands
in the molecule, demonstrated with the platinum–acetylide
copolymers.43 Pt-2 shows two well-separated absorption
bands at 504 nm and 640 nm (Fig. 3b). By comparison of the
shape and energy of the absorption bands with that of the
references Pt-b and Pt-3, the two major absorption bands of
Pt-2 can be assigned to the peripheral 40-phenyl coordinated
Bodipy unit and the central coordinated Bodipy unit,
respectively.43 The absorption of Pt-2 is the sum of the refer-
ence compounds Pt-3 and Pt-b (Fig. 3b).
comparison. Ferrocene (Fc) was used as an internal reference (E1/2
¼
+0.64 V (Fc+/Fc) vs. standard hydrogen electrode). In deaerated
CH2Cl2 containing 0.5 mM photosensitizers with ferrocene, 0.10 M
Bu4NPF6 as a supporting electrolyte, and Ag/AgNO3 a reference
electrode. Scan rates: 100 mV sꢁ1 20 ꢂC.
Table 2 Electrochemical data of Pt(II) complexesa
Compound
Oxidation (V)
Reduction (V)
Pt-a
Pt-b
Pt-1
Pt-2
Pt-3
+0.62
+0.91
+0.43, +0.70
+0.45, +0.93
+0.44
ꢁ1.52
ꢁ1.51
ꢁ1.56
ꢁ1.54
ꢁ1.56
a
Cyclic voltammetry in Ar saturated CH2Cl2 containing 0.10 M Bu4NPF6
as a supporting electrolyte; counter electrode as the Pt electrode;
working electrode as the glassy carbon electrode; Ag/AgNO3 couple as
the reference electrode. c [Ag+] ¼ 0.1 M. 0.5 mM photosensitizers in
CH2Cl2, 25 ꢂC.
Therefore we propose that there is no signicant electronic
interaction between the two different coordination units in Pt-2.
No S0
/ T1 absorption bands were observed for the
complexes.44 We found that the absorption bands of Pt-1 and
Pt-2 are not broader than those of the complexes Pt-a and Pt-b,
thus the numbers of the conformers of Pt-1 and Pt-2 in liquid
solution are limited otherwise the broadening of the absorption
bands should be observed.45
states. This conclusion is supported by the UV-vis absorption
and the luminescence studies.
The UV-vis absorption maxima on the higher energy side
of the absorption spectra of complexes Pt-1 and Pt-2 are
identical to those of complexes Pt-a and Pt-b, respectively.
Conversely, the absorption maxima on the lower energy side
of the spectra are identical to that of complex Pt-3. There-
fore, the Franck–Condon singlet excited states of the
complexes are spatially conned to respective acetylide
ligands, and there is no p-conjugation across the Pt(II) atom,
otherwise red-shied absorption bands will be observed for
Pt-1 or Pt-2. This nding is different from the previously
reported delocalized Franck–Condon singlet excited states
in Pt(II) acetylide copolymers containing phenyl and thienyl
acetylide ligands.43,45–47 Previously Pt(II) complexes with a
Steady-state electronic spectroscopy (UV-vis absorption and
luminescence emission spectra)
The UV-vis absorption spectra of the complexes were studied
(Fig. 3). Reference Pt-3 gives an absorption band centered at
643 nm, whereas complex Pt-a gives absorption at 584 nm
(Fig. 3a). The effect of metalation of the p-core of Bodipy on
the absorption is clear, that is, with metalation, the absorp-
tion of the Bodipy unit is red-shied as compared with that of
the free ligand.13 For Pt-1, the absorption band covers the
range 450–700 nm, which is clearly much broader than the
complexes with a single visible light-absorbing ligand, such
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Fig. 5 Emission of the complexes to demonstrate intramolecular
singlet energy transfer. (a) Pt-a (c ¼ 2.0 ꢀ 10ꢁ5 M), Pt-1 (c ¼ 0.5 ꢀ 10ꢁ5
M) and Pt-3 (c ¼ 1.0 ꢀ 10ꢁ5 M), lex ¼ 424 nm. (b) Pt-b (c ¼ 1.0 ꢀ 10ꢁ5
M), Pt-2 (c ¼ 0.3 ꢀ 10ꢁ5 M) and Pt-3 (c ¼ 1.0 ꢀ 10ꢁ5 M). lex ¼ 490 nm.
In (a) or (b), optically matched solutions at the excitation wavelength
were used therefore the concentrations are different; in deaerated
toluene, 20 ꢂC.
Fig. 4 Emission of the complexes under N2 and air atmospheres. (a)
Pt-a (lex ¼ 550 nm), (b) Pt-b (lex ¼ 470 nm), (c) Pt-1 (lex ¼ 570 nm), and
(d) Pt-2 (lex ¼ 590 nm). c ¼ 1.0 ꢀ 10ꢁ5 M in aerated or deaerated
toluene at 20 ꢂC.
plausible reason for the quenching effect of the emission of Pt-a
and Pt-b in the triads.
To evaluate the intramolecular energy transfer efficiency,
the luminescence excitation spectra of the complexes were
compared with the UV-vis absorption spectrum (Fig. 6).27b
Good superimposition between the excitation spectrum and
the UV-vis absorption spectrum was observed, indicating an
efficient RET effect in Pt-1. For Pt-2, two well-separated bands
in the excitation spectrum were found. With normalization of
the band at 640 nm, the excitation band at 502 nm is weaker
than the corresponding UV-vis absorption band. The singlet
energy transfer in Pt-2 from the peripheral Bodipy coordi-
nated ligand to the central coordinated Bodipy part was
calculated to be 44.8%.27b The inefficient intramolecular
energy transfer can be attributed to the intramolecular elec-
tron transfer or inappropriate dipole moment orientation, the
poor spectral overlap, etc.49,50 We noted the discrepancy
between the results obtained by quenching studies and the
excitation/absorption comparison, which is a lasting question
in RET.27b The photophysical properties of the complexes are
listed in Table 3. Pt-1, Pt-2 and Pt-a show weak uorescence
(FF < 5%), but complex Pt-b shows much stronger uores-
cence (FF ¼ 27.7%). The singlet oxygen (1O2) quantum yields
Bodipy ligand were reported, but these complexes
show ‘narrow’ absorption bands in the visible spectral
region.48
The luminescence spectra of the complexes were studied
(Fig. 4). Pt-a gives emission at 627 nm. The emission intensity is
the same in both aerated and deaerated solutions. The lumi-
nescence lifetime of this emission band was determined to be
0.24 ns. These features and the small Stokes shi indicate that
the luminescence is uorescence. A minor emission band at 800
nm was observed and the emission intensity is reduced in
aerated solution. Thus this emission band is most likely origi-
nated from a triplet emissive state, i.e. the emission band at
800 nm is phosphorescence. A similar emission feature was
observed for Pt-1 (Fig. 4c). For complexes Pt-b and Pt-2 (Fig. 4b
and d), only uorescence emission bands were observed. The
major emission band of both Pt-1 and Pt-2 is at 670 nm. In
contrast, the emission of complexes Pt-a and Pt-b shows blue-
shiing of 42 nm and 46 nm, respectively, as compared with
that of Pt-1 and Pt-2, probably due to the different coordination
prole of the Pt(II) centers.
Pt-1 shows a similar minor emission band at 825 nm, but
Pt-2 does not show such emission. We propose that the emis-
sion band of Pt-1 at 825 nm is due to the peripheral coordina-
tion Bodipy ligand. Note that the energy gap between the S1 and
T1 states of Pt-1 is 0.26 eV, much smaller than those of the linear
Pt(ii) ethynylene complexes with simple organic acetylene
ligands (ca. 0.7 eV).42 A small S1–T1 state energy gap is benecial
for fast ISC.42
In order to study the intramolecular singlet energy transfer,
the luminescence of the complexes was compared (Fig. 5: the
solutions are with the same optical density at the excitation
wavelength). The emission of Pt-1 is, at 669 nm, different from
that of Pt-a (Fig. 5a). Similarly, the emission of Pt-b at 516 nm
was signicantly quenched in Pt-2. Singlet energy transfer is the
Fig. 6 Comparison of the normalized UV-vis absorption and the
luminescence excitation spectra of the complexes. (a) Pt-1 (lem ¼ 695
nm) and (b) Pt-2 (lem ¼ 690 nm). c ¼ 1.0 ꢀ 10ꢁ5 M in deaerated
toluene at 20 ꢂC.
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Table 3 Photophysical properties of the ligands and Pt(II) complexes
labsa/nm
3b
lem
FFc/%
sFd/ns
sTe/ms
FD/%
a
f
f
2
3
Pt-a
Pt-b
Pt-1
Pt-2
505
543
432/584
504
428/607/644
504/640
8.70
11.0
1.75/3.80
8.427
5.28/10.3/13.5
18.55/8.18
519
560
627
516
669
518/662
20.9
—
0.89
27.7
2.81
3.3
2.71
5.52
0.24
2.68
—
—
f
f
f
—
—
62.48
607.84
83.87
69.8%g
79.3%h
0.48
76.0%g
—f/0.66
100.31
49.7%g/78%h
In toluene (1.0 ꢀ 10ꢁ5 M). b Molar absorption coefficient, 3: 104 Mꢁ1 cmꢁ1
.
Fluorescence quantum yields: B-6 (FF ¼ 10.5% in toluene) was used as
d
a
c
a standard for Pt-a, Pt-1, and Pt-2. DCM (FF ¼ 10% in CH2Cl2) was used as a standard for Pt-b. Luminescence lifetimes, c ¼ 1.0 ꢀ 10ꢁ5 M in
e
f
g
toluene. Triplet state lifetimes, measured by transient absorptions, c ¼ 5.0 ꢀ 10ꢁ6 M in toluene. Not applicable. Singlet oxygen quantum
yields. Methylene blue (MB, FD ¼ 57% in DCM) was used as a standard. The excitation wavelength for Pt-a (lex ¼ 590 nm), for Pt-1 (lex
¼
h
597 nm) and for Pt-2 (lex ¼ 573 nm). Singlet oxygen quantum yields, 2,6-diiodo-Bodipy (FD ¼ 83% in DCM) was used as a standard. Different
excitation wavelengths for Pt-b (lex ¼ 486 nm) and for Pt-2 (lex ¼ 505 nm) were used.
(FD) of the complexes were also measured (Table 3). The high
values indicated efficient ISC in these complexes. Further-
more, the FD of Pt-2 is dependent on excitation wavelength,
which is an indication of non-unity intramolecular electron
transfer.
The uorescence lifetimes of Pt-1 and Pt-2 were deter-
mined to be 0.48 ns and 0.66 ns, respectively. These values are
very close to the uorescence lifetime of the reference
complex Pt-3 which contains only the central coordinated
Bodipy ligand (0.49 ns).51 Therefore, we propose that there is
no signicant photoinduced intramolecular electron transfer,
otherwise the uorescence lifetimes will be reduced. This
postulation is also supported by the similar triplet state life-
times of Pt-1 and Pt-2 as compared to the reference complex
Pt-3 (see the later section).52
Nanosecond time-resolved transient difference absorption
spectroscopy: localization of the triplet excited state
In an effort to study the triplet excited state of the complexes,
nanosecond time-resolved transient difference absorption
spectra of the complexes were studied (Fig. 7). For Pt-a (Fig. 7a),
a bleaching band at 566 nm was observed upon nanosecond
pulsed laser excitation. This band is due to the depletion of the
ground state of the Bodipy moiety upon photoexcitation. A
minor bleaching band at 428 nm was also observed, which can
be assigned to the ground state depletion of the same species.
Transient absorption bands at 366 nm and 470 nm and a
broadband in the region of 600–800 nm were observed. These
features indicated that triplet excited state localized on the
Bodipy part was populated. The decay curve gives the lifetime of
50.0 ms (at c ¼ 1.0 ꢀ 10ꢁ5 M, Fig. 7b).
Fig. 7 Nanosecond time-resolved transient difference absorption spectra and the decay curves of the Pt(II) complexes. (a) Complex Pt-a, (b)
decay of Pt-a at 584 nm; (c) Pt-b, (d) decay of Pt-b at 504 nm (lex ¼ 503 nm pulsed laser); (e) Pt-1, (f) decay curve of Pt-1 at 644 nm; (g) Pt-2, (h)
decay curve of Pt-2 at 640 nm. With nanosecond pulsed laser excitation (lex ¼ 532 nm). c ¼ 1.0 ꢀ 10ꢁ5 M in deaerated toluene at 20 ꢂC.
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For Pt-b, a bleaching band at 501 nm was observed (Fig. 7c), linked to the Bodipy chromophores,59 compared to platinum–
where the Bodipy moiety gives strong steady state absorption phenyl acetylide oligomers (normally <25 ms).45 The observation
(Fig. 3b). Thus the triplet excited state of Pt-b is localized on the of the long-lived triplet state with the complexes is unusual
Bodipy ligand, not on the Pt(II) coordination center. The triplet considering the low energy level of the coordinated Bodipy
state lifetime is up to 597.75 ms (c ¼ 1.0 ꢀ 10ꢁ5 M). To the best of ligand (ca. 1.6 eV). Usually the triplet state lifetime will be short
our knowledge, this is the longest RT triplet excited state lifetime because the non-radiative decay will be signicant with smaller
observed for the Bodipy chromophore in transition metal T1–S0 energy gap.60 These long-living T1 states of the complexes
complexes. Previously this value is normally shorter than are benecial for the application of these complexes as the
100 ms.30,53–58
The transient absorption spectra of Pt-1 show a bleaching processes.59
triplet state energy donor to initiate the cascade photophysical
band in the region of 550–700 nm (Fig. 7e), which is in agree-
Linear Pt(II) acetylide complexes showing a long-lived T1
ment with the steady state absorption of Pt-1. The bleaching state were rarely reported.17 Previously Pt(II) acetylide complexes
peak at 620 nm is stronger than that at 658 nm. Based on the with uorenyl ligands were reported to show the T1 state life-
steady state UV-vis absorption spectrum of Pt-1, we propose that time up to 178–220 ms, but those complexes show weak
the triplet excited state of Pt-1 is distributed on the three Bodipy absorption of visible light.47 Notably the long-living triplet state
ligands, and it is more localized on the peripheral coordinated of Pt-1 and Pt-2 is not compromised by any low triplet state yield
Bodipy unit, than on the central coordinated Bodipy part. This indicated by the high singlet oxygen (1O2) quantum yields (FD,
apparent delocalization is most probably due to the degenerated Table 3).44 A similar triplet state lifetime of the complexes Pt-1,
T1 and T2 states of the complexes, i.e. triplet state equilibrium. Pt-2, and Pt-a and the reference complex Pt-3 (64.3 ms)51 indi-
This postulation is in agreement with the observation that the cated that there is no signicant photoinduced intramolecular
810 nm phosphorescence band of complex Pt-a was not electron transfer for Pt-1 and Pt-2, otherwise the triplet state
quenched in complex Pt-1. The triplet state lifetime of 63.13 ms lifetimes will be substantially reduced.52 Furthermore, all the
was observed for Pt-1 (1.0 ꢀ 10ꢁ5 M). The uniform decay of the transient absorption features (T1 / Tn transitions) are similar
two triplet excited states indicated that the two isoenergetic to that observed for the Pt(II) Bodipy acetylide complex, thus the
triplet excited states are in fast equilibrium, i.e. there is efficient compelling evidence shows that the transient species are due to
triplet state energy transfer between the different Bodipy the triplet excited state of the coordinated Bodipy ligands, not
ligands in the molecular chain.41 This conclusion is in agree- charge separated states.52
ment with the previous report of triplet excited state energy
transfer across the central coordination Pt(II) center.43 To the
best of our knowledge, this is the rst time that the excited state
equilibrium was observed for the linear Pt(II) arylacetylide
Calculation of the free energy changes of the photoinduced
intramolecular electron transfer
complexes.43 Previously, the T1 triplet excited state of the Pt(II) The study of the triplet excited state implies that no signicant
acetylide polymer or oligomers was reported to be spatially electron transfer exists for complexes Pt-1 and Pt-2, because the
conned to one ligand.15 Although triplet state equilibrium was triplet state lifetimes of Pt-1 and Pt-2 are close to those of
proposed previously, no direct spectral evidence was observed complexes Pt-a and Pt-3. This conclusion was supported by
due to other competitive processes such as electron transfer calculation of the free energy changes (DGꢂCS) of the photo-
(charge separation).52
The transient absorption of Pt-2 shows a major absorption equation.
induced intramolecular electron transfer with the Weller
band at 635 nm (Fig. 7g). Interestingly, no bleaching band at
First, we assume that the triplet excited state of Pt-1 and Pt-2
501 nm was observed. Therefore, the triplet state of Pt-2 is is responsible to initiate the photo-induced intramolecular
exclusively conned to the central coordinated Bodipy ligand, electron transfer. Based on the electrochemical properties of the
not on the peripheral coordinated Bodipy ligand.43 Since we complexes (Table 2), the central coordinated Bodipy moiety was
OX
have shown with Pt-b that the triplet state of the peripheral considered as the electron donor (E
¼ +0.43 V), and the
1/2
RED
1/2
coordinated Bodipy part can be produced upon pulsed laser peripheral coordinated Bodipy as the electron acceptor (E
¼
excitation at 503 nm (Fig. 7c), the lack of the bleaching band at ꢁ1.56 V). The E0,0 value was approximated by the phosphores-
500 nm of Pt-2 with pulsed laser excitation at 503 nm (see ESI, cence of Pt-1 (ca. 1.55 eV). Therefore the free energy change
Fig. S18†) indicated intramolecular triplet–triplet-energy-trans- of electron transfer of Pt-1 was approximated as DGCꢂ S
¼
fer (TTET) from the peripheral Bodipy moiety to central Bodipy e[EOX ꢁ ERED] ꢁ E0,0 ¼ 0.43 ꢁ (ꢁ1.56) ꢁ 1.55 ¼ +0.44 eV (the
in Pt-2. No triplet state equilibrium was established in Pt-2 static Columbic energy is usually small and was not considered
because of the large energy gap between the triplet states here). Thus, the triplet excited state of Pt-1 is unlikely to drive
localized on the central and the peripheral Bodipy moieties. The intramolecular electron transfer. Similarly a positive value of
triplet state energy level of the unsubstituted Bodipy was +0.45 eV was determined for Pt-2. If the singlet excited state of
determined to be 1.70 eV by phosphorescence,53 whereas the Pt-1 and Pt-2 was responsible to drive intramolecular electron
central coordinated Bodipy moiety is with the triplet state transfer (E0,0 is approximated with the uorescence emission
energy level of 1.61 eV.30 The triplet state lifetime of Pt-2 was wavelength as ca. 1.85 eV), DGCꢂ S ¼ +0.14 eV and DGCꢂ S ¼ +0.15 eV
determined to be 94.18 ms (c ¼ 1.0 ꢀ 10ꢁ5 Mꢁ1). The T1 state were obtained for Pt-1 and Pt-2, respectively. Thus, no photo-
lifetime is very long, even with the Pt(II) center p-conjugatedly induced electron transfer is expected for Pt-1 and Pt-2.
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Femtosecond time-resolved transient difference absorption
spectroscopy: singlet energy transfer
The two kinds of Bodipy ligand in Pt-2 show substantially
different absorption wavelengths (Fig. 3b), thus Pt-2 is ideal for
the study of the ultrafast singlet energy transfer. Femtosecond
ultrafast pump–probe experiments were performed for Pt-2 at
504 nm and 640 nm pump wavelength to excite the singlet state
of the peripheral BODIPY part and central coordinated BODIPY
part, respectively (S0 / S1) (Fig. 8). Upon excitation of Pt-2 at
504 nm, bleaching bands were observed around 505 nm and
650 nm where the peripheral BODIPY part and central coordi-
nated BODIPY part give the steady state absorption, respec-
tively. While the bleaching signal around 505 nm decreases, the
bleaching signal around 650 nm increases concomitantly
(Fig. 8). The decay time of the bleaching signal of the peripheral
Bodipy part (around 505 nm) is about 25 ps, which is approxi-
mately equal to the rise time of the bleaching signal of the
central coordinated Bodipy part (around 650 nm, Fig. 8b). This
process indicates singlet energy transfer from the peripheral
Bodipy part to central coordinated Bodipy part (kEnT ¼ 4.0 ꢀ
1010 sꢁ1). On the other hand, a positive signal (excited state
absorption) appears above 680 nm probe wavelength aer 800
ps delay time (independent of the pump wavelength). This
signal can be attributed to the T1–Tn transition.
Fig. 9 Isosurfaces of spin density of the complexes Pt-a, Pt-b, Pt-1
and Pt-2. Calculated at the optimized triplet state geometries. Toluene
was used as a solvent in the calculations (PCM model). Calculation was
performed at the B3LYP/GENECP/LANL2DZ level with Gaussian 09W.
In addition, an extra pump probe experiment was performed
with 640 nm pump wavelength (see ESI, Fig. S19†). There is a
single bleaching signal at 650 nm, which corresponds to the
singlet state of the central coordinated BODIPY part and no
singlet energy transfer is expected. Furthermore, a similar T1–Tn
transition was observed as a positive signal at the same wave-
length (680 nm).
the nanosecond time-resolved transient difference absorption,
which indicates that the triplet state is more localized on the
peripheral Bodipy moiety (Fig. 7), as well as the fact that the
phosphorescence of the peripheral coordinated Bodipy (Pt-a) is
retained in Pt-1 (Fig. 4).
For Pt-2, the spin density is exclusively localized on the
central coordinated Bodipy ligand, and the peripheral coordi-
nated Bodipy ligand does not contribute to the spin density
surface at all. This nding is in full agreement with the tran-
sient absorption spectrum (Fig. 7g). Based on the spin density
surface, the T1 state of Pt-2 is highly conned to the central
Bodipy ligand (Fig. 9).46
For Pt-1 and Pt-2, the common feature of the spin density
surface is the minor contribution of the Pt(II) atom. With the
large p-conjugation ligand, the S1 and T1 states are more
localized on the ligand, therefore the 3IL / S0 transition is
assumed to be a strongly forbidden transition, which may be
responsible for the long lifetime of the T1 state of the
complexes.44
DFT calculations on the photophysical properties of the
complexes
The spin density surfaces of the complexes were calculated
(Fig. 9). For Pt-a and Pt-b, the spin density is localized on the
Bodipy unit, not on the Pt(II) coordination centre. This result is
in agreement with the transient absorption spectra (Fig. 7). For
Pt-1, the spin density surface is localized on the peripheral
coordinated Bodipy part. Since the spin density surface shows
the location of only the T1 state, this result is in agreement with
The electronic structures of the singlet excited states and the
triplet excited states of the complexes Pt-1 and Pt-2 were studied
in detail by DFT and TDDFT methods. HOMO / LUMO+1 is
involved in the S0 / S1 transition of Pt-1 (Fig. 10), thus the S1
state is featured with a charge transfer character. The S3 state is
localized on the central coordinated Bodipy unit. The S18 state is
mainly localized on the peripheral Bodipy ligands.
Fig. 8 Picosecond ultrafast transient absorption spectra of Pt-2 upon
femtosecond pulsed laser excitation at 504 nm. (a) Transient absorp-
tion spectra after pulsed laser excitation at 504 nm. (b) Decay traces at
500 nm and 650 nm. c ¼ 1.0 ꢀ 10ꢁ5 M in toluene at 20 ꢂC.
The triplet state of Pt-1 was studied by the TDDFT method
based on the ground state geometry (Table 4). HOMO /
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Fig. 10 Electron density maps of the frontier molecular orbitals of complex Pt-1 based on the optimized ground state geometry. The solvent
toluene was considered in the calculations (PCM model). Calculated at the B3LYP/GENCP/LANL2DZ level with Gaussian 09W.
LUMO+2 is involved in the T1 state, thus the T1 state is localized the DFT theory usually overestimated the excitation energy of
on the central coordinated Bodipy unit to a large extent. Bodipy compounds.61
HOMOꢁ1 and LUMO+1 are involved in the T2 state. Thus the T2
The triplet states of Pt-2 were studied by similar methods
based on the ground state geometry (Fig. 11 and Table 5).
state is localized on the peripheral Bodipy unit.
The electronic structure of the singlet excited state and the HOMO and LUMO are involved in the T1 state (Table 5). Thus it
triplet excited state of B-2 was also studied by the DFT and the is clear that the T1 state is localized on the central coordinated
TDDFT calculations (Fig. 11 and Table 5). The S0 / S1 transi- Bodipy unit. This conclusion is in agreement with the nano-
tion is composed of HOMO / LUMO, both orbitals are local- second time-resolved transient difference absorption spectra of
ized on the central coordination Bodipy unit. We found Pt-2 (Fig. 7). Degenerated T2 and T3 states were observed for Pt-2
degenerated S2 and S3 states (Table 5), for which HOMOꢁ3 / (energy level is 1.39 eV). Inspection of the MOs involved in T2
LUMO+1 and HOMOꢁ2 / LUMO+1 are involved, respectively. S2 and T3 states indicates that these triplet states are localized on
and S3 states are localized on the peripheral coordinated Bodipy the peripheral coordinated Bodipy units.
units (Fig. 11). We noted the discrepancy between the calculated
The photophysical process of Pt-1 and Pt-2 are summarized
excitation energy (UV-vis absorption) and the experimental in Scheme 3. For Pt-1, the UV-vis absorption spectrum indicated
values. This result is within expectation because it is known that that the peripheral coordinated Bodipy part shows higher
Table 4 Excitation energies (eV) and corresponding oscillator strengths (f), main configurations and CI coefficients of the low-lying elec-
tronically excited states of Pt-1, calculated by TDDFT//B3LYP/LANL2DZ, based on the DFT//B3LYP/LANL2DZ optimized ground state geometries
TDDFT/B3LYP/LANL2DZ
Electronic transition
Energya [eV / nm]
f b
Compositionc
CId
Character
Singlet
S0 / S1
S0 / S2
S0 / S3
S0 / S4
S0 / S18
S0 / S23
S0 / T1
S0 / T2
S0 / T3
1.98/626
2.00/617
2.12/585
2.18/568
2.87/432
3.10/400
1.35/919
1.40/884
1.43/868
0.4889
0.1335
0.7147
0.0823
0.7453
0.5790
0.0000e
0.0000e
0.0000e
H / L+1
H / L
H / L+2
Hꢁ1 / L+1
Hꢁ6 / L+1
Hꢁ8 / L+2
H / L+2
0.6277
0.6462
0.6651
0.6115
0.5981
0.5041
0.6168
0.5452
0.5379
MLCT/ILCT
MLCT/ILCT
MLCT
MLCT
MLCT/ILCT
ILCT
MLCT
MLCT
MLCT/ILCT
Triplet
Hꢁ1 / L+1
Hꢁ2 / L
a
Only the selected low-lying excited states are presented. b Oscillator strengths. c Only the main congurations are presented. d The CI coefficients
are in absolute values. e No spin–orbital coupling effect was considered, thus the f values are zero.
9730 | J. Mater. Chem. C, 2014, 2, 9720–9736
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Fig. 11 Electron density maps of the frontier molecular orbitals of Pt-2 based on the optimized ground state geometry. The solvent toluene was
considered in the calculations (PCM model). Calculated at the CAM-B3LYP/SDD/LANL2DZ level with Gaussian 09W.
singlet state energy (1[BDP*-p] in Scheme 3) than the central triplet state equilibrium was observed for Pt-2. Thus we propose
coordinated Bodipy part (1[BDP*-c] in Scheme 3). The lumi- that the 3[BDP*-p] state was quenched by the efficient triplet
nescence emission studies show that the uorescence of the state energy transfer to 3[BDP*-c]. This photophysical process is
1[BDP*-p] part was quenched in Pt-1, i.e. the singlet excited state different from that of Pt-1.
of the peripheral coordinated Bodipy is with a slightly higher
energy level than the central coordinated Bodipy moiety does,
thus the singlet energy transfer from the 1[BDP*-p] to 1[BDP*-c]
Triplet–triplet annihilation upconversion with the complexes
as triplet photosensitizers
takes place. As a result, only the uorescence emission from the
1[BDP*-c] part can be observed. Both coordination parts can
undergo ISC to give the triplet excited state. The two triplet
excited states localized on the central and the peripheral Bodipy
parts are in fast equilibrium, since the two triplet excited states
decay uniformly (Fig. 7e). Triplet excited state equilibrium in
transition metal complexes was rarely reported.62
For Pt-2, the difference between the S1 state energy levels of
the peripheral and the central Bodipy parts is more distinct
than that of Pt-1. Based on the transient absorption spectra, no
The complexes show strong absorption of visible light and long-
lived triplet excited states, thus these complexes can be used as
triplet photosensitizers for TTA upconversion.13,63 Among the
upconversion methods, TTA upconversion is of particular
interest, due to its strong absorption of visible light, high
upconversion quantum yields, and tunable excitation/emission
wavelength. Non-coherent, low power excitation light is suffi-
cient for this kind of upconversion, such as solar light.64–68
Conventional triplet photosensitizers for TTA upconversion are
Table 5 Excitation energies (eV) and corresponding oscillator strengths (f), main configurations and CI coefficients of the low-lying electron-
ically excited states of complex Pt-2. Calculated by TDDFT//B3LYP/LANL2DZ, based on the DFT//B3LYP/LANL2DZ optimized ground state
geometries
TDDFT/CAM-B3LYP/SDD
Electronic transition
Energya [eV / nm]
f b
Compositionc
CId
Character
MLCT
Singlet
S0 / S1
S0 / S2
S0 / S3
S0 / S4
S0 / S5
S0 / T1
S0 / T2
S0 / T3
2.49/497
2.90/426
2.91/426
3.34/370
3.48/356
1.30/951
1.39/891
1.39/891
1.1904
0.0665
1.3281
0.1169
0.4370
0.0000e
0.0000e
0.0000e
H / L
0.5334
0.4587
0.4759
0.4774
0.5125
0.4904
0.4554
0.3462
Hꢁ3 / L+1
Hꢁ2 / L+1
Hꢁ5 / L
Hꢁ6 / L
H / L
ILCT
MLCT
MLCT
MLCT
ILCT
Triplet
Hꢁ2 / L+1
Hꢁ3 / L+1
Only the selected low-lying excited states are presented. b Oscillator strengths. c Only the main congurations are presented. d The CI coefficients
are in absolute values. e No spin–orbital coupling effect was considered, thus the f values are zero.
a
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Scheme 3 Qualitative energy level diagram of complexes (a) Pt-1 and (b) Pt-2. BDP stands for Bodipy. BDP-c stands for the central coordination
Bodipy part. BDP-p stands for the peripheral coordination Bodipy part.
limited to Pt(II) or Pd(II) porphyrin complexes.64,65,69,70 Recently with the complexes Pt-1 and Pt-2, which may be suitable for TTA
we developed a series of Ru(II), Pt(II), Ir(III) and Re(I) complexes upconversion with a broadband excitation source.
for TTA upconversion; all these complexes show strong
First a 635 nm laser was used for excitation. All the
absorption of visible light and long-lived triplet excited complexes Pt-1–Pt-3 give strong uorescence at 670 nm
states.13,65 However, to the best of our knowledge, no complexes (Fig. 12a). In the presence of a triplet energy acceptor PBI,
showing broadband visible light absorption have been used as a emission bands in the region of 510–600 nm were observed
triplet photosensitizer for TTA upconversion. It will become (Fig. 12b). Excitation of PBI alone with a 635 nm laser did not
possible to carry out multi-wavelength excitable upconversion produce any emission bands in the same region. Thus, these
new emission bands can be attributed to the upconverted
emission of PBI. Upconversion quantum yields of 1.53%, 1.19%
and 2.42% were determined for Pt-1–Pt-3, respectively.
Since these complexes show broadband absorption in the
region of 500–700 nm, we also used another excitation wave-
length (589 nm) for the TTA upconversion (Fig. 12c and d).
Interestingly, we found that the TTA upconversion is stronger as
compared with that excited at 635 nm. For example, the
upconverted emission intensity in the region of 500–600 nm is
stronger than the residual uorescence of the complexes, which
can be treated as an internal standard. The upconversion
quantum yields were determined to be 9.72%, 4.89% and 9.84%
for Pt-1–Pt-3, respectively with excitation at 589 nm.
The upconversion can be performed in dim light, for
example, the light from
a spectrouorometer (see ESI,
Fig. S20†). TTA upconversion was observed with Pt-1 and Pt-2 by
excitation at 600–620 nm with a spectrouorometer. Other
upconversion methods failed to be carried out with weak light
as an excitation source.71
The TTA upconversion with the complexes as triplet photo-
sensitizers is visible to the naked eye (Fig. 13). For the
complexes alone, deep red emission was observed upon 635 nm
laser excitation. In the presence of a triplet acceptor/emitter
PBI, strong green emission was observed. Such strong and un-
aided eye-visible upconversion will be promising for applica-
tions in luminescence bioimaging72–75 or solar cells.76–78
The delayed uorescence of the TTA upconversion was studied
(Fig. 14). Firstly, we studied the uorescence emission of
Pt-1 alone. Interestingly, we observed long-lived luminescence
Fig. 12 Upconversion with Pt-1, Pt-2 and Pt-3 as the triplet photo-
sensitizers and PBI as a triplet acceptor. (a) Photoluminescence
spectra of the photosensitizers alone at 635 nm excitation (10 mW). (b)
Photoluminescence spectra in the presence of PBI. Excited with a 635
nm laser (10 mW). (c) Photoluminescence spectra of the photosensi-
tizers alone at 589 nm excitation (5 mW). (d) Photoluminescence
spectra of the complexes in the presence of PBI. Excited with a 589 nm
laser (5 mW). For Pt-1, c [PBI] ¼ 2.6 ꢀ 10ꢁ5 M; for Pt-2, c [PBI] ¼ 4.0 ꢀ
10ꢁ5 M; for Pt-3, c [PBI] ¼ 4.1 ꢀ 10ꢁ5 M. The asterisks indicate the
scattered laser (589 nm and 635 nm). c [photosensitizers] ¼ 1.0 ꢀ 10ꢁ5
M in deaerated toluene at 20 ꢂC.
9732 | J. Mater. Chem. C, 2014, 2, 9720–9736
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Journal of Materials Chemistry C
luminescence spectrum of Pt-1. The emission of Pt-1 was also
studied in aerated solution, and no signal was observed.
Considering that the power of a nanosecond pulsed OPO laser is
high, and the long-lived triplet state of Pt-1, we propose that the
long-lived luminescence is due to the delayed uorescence, via the
TTA mechanism. This postulation was supported by the fact that
no such long-lived uorescence can be observed with the pico-
second pulsed laser, for which the laser power is much lower.
Thus, another mechanism for the delayed uorescence, i.e. the
thermally activated reverse ISC, is not responsible for the delayed
luminescence of Pt-1.79,80
Similar delayed luminescence was observed for Pt-2 (sDF
¼
111.0 ms, in aerated solution, no signal was detected). Delayed
luminescence was observed for Pt-a (13.7 ms) and Pt-b (189.2 ms).
The delayed uorescence of the complexes is due to the long-
lived triplet excited states. To the best of our knowledge, the
delayed uorescence of Pt(II) complexes is rarely reported.17,81–84
Previously it was observed for Pt-3.51 Delayed uorescence may
nd applications in time-gated luminescence bioimaging.85
In the presence of triplet energy acceptor PBI, uorescence
emission in the region of 500–600 nm was observed (Fig. 14b
and d), which is the featured uorescence of PBI. The lumi-
nescence lifetime was determined to be 130.5 ms and 43.5 ms
with Pt-1 and Pt-2 as the triplet photosensitizers, respectively.
Thus the TTA upconversion was conrmed by the delayed
uorescence of PBI.69
Fig. 13 Photographs of the TTA upconversion. Emission of triplet
photosensitizers alone and the TTA upconversion. Excited with a
continuous 635 nm laser (10 mW). c [complexes] ¼ 1.0 ꢀ 10ꢁ5 M. For
Pt-1, c [PBI] ¼ 2.6 ꢀ 10ꢁ5 M; for Pt-2, c [PBI] ¼ 4.0 ꢀ 10ꢁ5 M in dea-
erated toluene at 20 ꢂC.
Conclusions
In summary, two heteroleptic trans-bis(alkylphosphine) plat-
inum(II)-alkynyl complexes with three Bodipy ligands were
prepared to achieve the goal of broadband absorption of
visible light (Pt-1 and Pt-2). Sequential intramolecular reso-
nance energy transfer (RET) and intersystem crossing (ISC)
were concerted to transform the harvested broadband
photoexcitation energy to triplet excited state energy. Both the
singlet energy donor and the acceptor give strong visible light
absorption, but at different wavelengths. The singlet energy
acceptor is also a spin converter to produce triplet excited
states. Two reference complexes with only one visible light-
absorbing Bodipy ligand in the molecule were prepared for
comparison. Singlet energy transfer from the peripheral
coordination Bodipy moiety to the central coordinated Bodipy
(with 2,6-diacetyl prole) moiety was conrmed by steady
state luminescence spectroscopy, uorescence excitation
spectroscopy and femtosecond ultrafast transient difference
absorption spectroscopy. Nanosecond time-resolved transient
difference absorption spectroscopy indicated that the triplet
excited state of Pt-1 is delocalized on both the peripheral and
the central coordinated Bodipy units. For Pt-2, however, the T1
state is conned to the central coordinated Bodipy segment.
Long-lived triplet excited states were observed for both
complexes (sT ¼ 63.13 ms for Pt-1 and sT ¼ 94.18 ms for Pt-2 at
1.0 ꢀ 10ꢁ5 M). With pulsed laser excitation, delayed uores-
cence was observed for the complexes (sDF ¼ 43.8 ms for Pt-1,
sDF ¼ 111.0 ms for Pt-2), which is rarely reported for transition
metal complexes. The complexes were used for multi-
Fig. 14 Time-resolved emission spectra (TRES) of Pt-1, Pt-2 alone and
the TTA upconversion with PBI as a triplet acceptor. (a) Pt-1 alone, the
fluorescence and phosphorescence regions were measured (550–
850 nm, sDF ¼ 43.8 ms, sP ¼ 47.6 ms). (b) TRES of Pt-1 in the presence
of PBI. Upconverted emission in the range of 500–600 nm was
observed (sDF ¼ 130.5 ms). (c) TRES of Pt-2 alone. The fluorescence
region was measured (550–850 nm, sDF ¼ 111.0 ms). (d) TRES of Pt-2 in
the presence of PBI. Upconverted emission in the range of 500–600
nm was observed (s ¼ 43.5 ms). For Pt-1, c [PBI] ¼ 4.0 ꢀ 10ꢁ5 M, for Pt-
2, c [PBI] ¼ 4.0 ꢀ 10ꢁ5 M. c [sensitizers] ¼ 1.0 ꢀ 10ꢁ5 M in deaerated
toluene. Excited with a nanosecond pulsed OPO laser synchronized
with a spectrofluorometer (lex ¼ 560 nm), 25 ꢂC.
(sDF ¼ 43.8 ms). The emission features, such as the wavelength and
the band shape, are superimposable to the steady state
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wavelength excitable triplet–triplet annihilation upconver- 14 E. O. Danilov, I. E. Pomestchenko, S. Kinayyigit, P. L. Gentili,
sion. Furthermore, the efficient singlet and triplet energy
transfer for the chromophores across the center coordinating
M. Hissler, R. Ziessel and F. N. Castellano, J. Phys. Chem. A,
2005, 109, 2465–2471.
¨
Pt(II) center will be fundamentally important for investigation 15 K. Glusac, M. E. Kose, H. Jiang and K. S. Schanze, J. Phys.
of the photochemistry of Pt(II) complexes and for construction Chem. B, 2007, 111, 929–940.
of functional molecular arrays with these photoresponsive 16 T. Kindahl, P. G. Ellingsen, C. Lopes, C. Brannlund,
units. Our results are useful for preparation of transition
metal complexes that show broadband absorption of visible
M. Lindgren and B. Eliasson, J. Phys. Chem. A, 2012, 116,
11519–11530.
light and long-lived triplet excited states, for the study of the 17 J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205–268.
fundamental photochemical properties of transition metal 18 (a) S. C. F. Kui, S. S.-Y. Chui, C.-M. Che and N. Zhu, J. Am.
complexes and the applications in photocatalysis, lumines-
cence bioimaging photodynamic therapy and upconversion.
Chem. Soc., 2006, 128, 8297–8309; (b) W. Y. Wong and
C. L. Ho, Coord. Chem. Rev., 2006, 250, 2627–2690.
19 L. Liu, S. Guo, J. Ma, K. Xu, J. Zhao and T. Zhang, Chem.–Eur.
J., 2014, DOI: 10.1002/chem.201403780, accepted.
20 K. Y. Kim, A. H. Shelton, M. Drobizhev, N. Makarov,
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Acknowledgements
We thank the NSFC (21073028, 21273028, 21421005 and
21473020), the Royal Society (UK) (China–UK Cost-Share Science 21 J. Chen, G. Zhao, T. R. Cook, X. Sun, S. Yang, M. Zhang,
Networks), the Ministry of Education (SRFDP-20120041130005),
the Program for Changjiang Scholars and Innovative Research
K. Han and P. J. Stang, J. Phys. Chem. A, 2012, 116, 9911–
9918.
Team in University [IRT_13R06], the State Key Laboratory of Fine 22 C. K. M. Chan, C. Tao, K. Li, K. M. C. Wong, N. Zhu,
Chemicals (KF1203), the Fundamental Research Funds for the
Central Universities (DUT14ZD226) and Dalian University of
Technology (DUT2013TB07) for nancial support.
K. W. Cheah and V. W. W. Yam, Dalton Trans., 2011, 40,
10670–10685.
23 J. E. Rogers, J. E. Slagle, D. M. Krein, A. R. Burke, B. C. Hall,
A. Fratini, D. G. McLean, P. A. Fleitz, T. M. Cooper,
M. Drobizhev, N. S. Makarov, A. Rebane, K. Y. Kim,
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