Near - IR Broadband-Absorbing trans -Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Article abstract of DOI:10.1021/acs.inorgchem.5b01107

Broadband near-IR absorbing trans-bis(trialkylphosphine) Pt(II) bisacetylide binuclear complex (Pt-1) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. Pt-1 shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand Pt-0 or Bodipy ligand Pt-2). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for Pt-1. Singlet EnT in Pt-1 was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant k_EnT = 2.2 × 10¹⁰ s^-1). With nanosecond transient absorption spectra, intramolecular triplet-state energy transfer in Pt-1 was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in Pt-1 is thermodynamically prohibited. Intermolecular triplet transfer between Pt-2 and L-1 was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10⁴ s^-1 and 94.5%, respectively. The singlet oxygen (¹O₂) photosensitizing of Pt-1 was improved as compared to the complexes containing only a single visible-light-absorbing chromophore.

Full text of DOI:10.1021/acs.inorgchem.5b01107

Article
pubs.acs.org/IC
Near-IR Broadband-Absorbing trans-Bisphosphine Pt(II) Bisacetylide
Complexes: Preparation and Study of the Photophysics
†
‡
,†
§
†
,§
Wenbo Yang, Ahmet Karatay, Jianzhang Zhao,* Jian Song, Liang Zhao, Yongheng Xing,*
†
†
‡
,∥
‡
Caishun Zhang, Cheng He, Halime Gul Yaglioglu, Mustafa Hayvali,* Ayhan Elmali,
‡
and Betu
̈
̈ ̈ ̈
l Kuc u̧ koz
†
State 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
‡
∥
Department of Engineering Physics, Faculty of Engineering and Department of Chemistry, Faculty of Science, Ankara University,
6100 Besevler, Ankara, Turkey
College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, P. R. China
0
§
̧
*
S Supporting Information
ABSTRACT: Broadband near-IR absorbing trans-bis-
trialkylphosphine) Pt(II) bisacetylide binuclear complex (Pt−
) was prepared with boron-dipyrromethene (Bodipy) and
(
1
styrylBodipy acetylide ligands. Pt−1 shows strong absorption
bands at 731 and 503 nm. Singlet energy transfer (EnT) and
efficient intersystem crossing of the central coordinated Bodipy
ligand were proposed to be responsible for the efficient
funneling of the excitation energy to the triplet-state manifold.
Reference complexes containing only a single Bodipy ligand
were prepared for comparison (with styrylBodipy ligand Pt−0 or
Bodipy ligand Pt−2). The molecular structures were confirmed
by single-crystal X-ray diffraction. The photophysical properties
were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and
density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for Pt−1.
Singlet EnT in Pt−1 was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient
10
−1
absorption spectra (energy transfer rate constant k
= 2.2 × 10 s ). With nanosecond transient absorption spectra,
EnT
intramolecular triplet-state energy transfer in Pt−1 was proved. Gibbs free energy changes of charge separation indicate that the
photoinduced intramolecular electron transfer in Pt−1 is thermodynamically prohibited. Intermolecular triplet transfer between
Pt−2 and L−1 was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were
4
−1
1
determined as 3.6 × 10 s and 94.5%, respectively. The singlet oxygen ( O ) photosensitizing of Pt−1 was improved as
2
compared to the complexes containing only a single visible-light-absorbing chromophore.
INTRODUCTION
In this context, the conventional Pt(II) complexes are
suffering from some limitations. First, the complexes usually
■
1
Transition metal complexes, such as Pt(II) complexes, have
attracted much attention due to the applications in photo-
show very weak absorption in visible spectral region, and the
20−26
1
−3
4,5
absorption is usually in UV or blue range of the spectrum.
Very few complexes show absorption in visible spectra
catalysis,
hydrogen (H ) production, photoredox catalytic
2
6
,7
organic reactions,
bioimaging,
dynamic therapy,
electroluminescence and luminescence
molecular sensors and devices,
2
7−30
8
−11
12,13
range.
Second, the lifetime of the triplet excited state is
photo-
1
1
4,15
usually short, less than 10 μs. Third, the molecular structure
profile of the complexes is based on a single visible-light-
harvesting chromophore; thus, there is only one major
absorption band in visible spectral range for these complexes.
As a result, harvesting of the excitation energy of a broadband
light source, such as solar light, is inefficient.
To address the above challenges, recently we prepared a
series of Pt(II),
and more recently in triplet−triplet
1
6−19
annihilation (TTA) upconversion.
these applications, the complexes play the pivotal role via
Concerning most of
photoinduced intermolecular energy transfer (EnT) or electron
1
9
transfer (ET). Thus, the visible-light-harvesting ability and
the lifetime of the triplet excited state of the complexes are the
two crucial parameters for the transition metal complexes to be
used as triplet photosensitizers, because the intermolecular EnT
and ET can be enhanced by the strong light absorption (a
prerequisite for formation of abundant photosensitizers at
1
,19
2
8,29,31−33
34,35
36−39
Ru(II),
Ir(III),
and Re(I)
Received: May 16, 2015
1
9
excited states) and the long-lived triplet excited states.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01107
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Inorganic Chemistry
Article
a
Scheme 1. Preparation of the Complexes Pt−0 and Pt−1
a
The molecular structures of the ligands and reference complexes are also shown. (a) Deionized water, Ar, RT, 3 h, yield 57.7%. (b) Dried NHEt2,
Ar, 45 °C, 8 h, yield: 79.8%. (c) Benzaldehyde, acetic acid, piperidine, Ar, microwave irradiation 150 °C, 6 min, yield: 85.1%. (d)
i
Trimethylsilylacetylene, Pd(PPh ) Cl , PPh , CuI, distilled THF, ( Pr) NH, Ar, 68 °C, 6 h, yield: 77.0%. (e) Bu NF·3H O, distilled THF, Ar, −78
3
2
2
3
2
4
2
°
C, 0.5 h, yield: 58.4%. (f) CuI, distilled THF, dried NHEt , Ar, RT, 0.5 h, yield: 61.9%. (g) CuI, distilled THF, dried NHET , Ar, RT, 3 h, yield
2
2
4
3.0%. (h) Dried NHEt , Ar, 45 °C, 9 h, yield: 33.4%.
2
4
0,41
complexes,
using organic chromophores of boron dipyrro-
Second, the limited examples of the broadband absorbing
methene (Bodipy), naphthaleneimide, naphthalenediimide,
complexes show only the energy acceptor’s emission. Dual
emission is rare for Pt(II) complexes.
1
9,42
9,44,45
perylenediimide, rhodamine, and fluorescein.
Strong
absorption of visible light and long-lived intraligand triplet
state ( IL) was observed for these complexes. These complexes
To address these limitations, herein we prepared near-IR
broadband absorbing binuclear trans-bis(tributylphosphine)
Pt(II) bisacetylide complex, which contains two types of
different Bodipy ligands (Bodipy and styrylBodipy. Pt−1.
Scheme 1). Bodipy was selected as the light-harvesting antenna
due to its strong absorption of visible light, high fluorescence
quantum yields (inhibited non-radiative decay), good photo-
3
have been used for photocatalysis and TTA upconver-
1
7,19,42
sion.
We also prepared Pt(II) complexes that contain
two different visible-light-absorbing chromophores; thus,
30,43
broadband visible-light-absorption was observed.
However,
there is still much room left to fully explore the molecular
structural diversity of this kind of complex; for example, the
absorption wavelength of these complexes are usually shorter
than 600 nm. This short absorption wavelength is detrimental
for luminescence bioimaging in vivo or photodynamic therapy
46−52
stability, and feasible derivatization.
The central coordi-
nated styrylBodipy in Pt−1 is with larger π-conjugation
framework than the peripheral coordinated Bodipy ligands
(Scheme 1). The central coordinated styrylBodipy section
shows absorption/emission in the near-IR region (660−800
3
0,43
(PDT), for which near-IR absorption/emission is desired.
B
DOI: 10.1021/acs.inorgchem.5b01107
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Article
Figure 1. ORTEP view of the single-crystal structure of (a) Pt−0 and (b) Pt−1 with 30% thermal ellipsoids.
nm), and the peripheral coordinated Bodipy ligands give strong
absorption at 500 nm. The photophysical properties of the
complexes were studied with steady-state and time-resolved
transient spectroscopy. Ultrafast singlet-energy transfer via the
through-bond energy-transfer (TBET) mechanism was pro-
posed for the broadband visible-light-absorbing complex,
because the spectral overlap integral is too small to ensure a
fast fluorescence-resonance energy-transfer (FRET) EnT
The attachment of ethynyl moieties to Bodipy is via
3
8,58
Sonogashira coupling reactions (Scheme 1).
StyrylBodipy
ligand L−1 was obtained by the condensation of Bodipy with
50
benzaldehyde. Pt−1 was prepared with Pt−2 and L−1 as the
starting materials. All the compounds were obtained in
moderate-to-satisfactory yields. The molecular structures were
1
13
fully verified with H NMR, C NMR, and high-resolution
mass spectrometry (HRMS).
process (Fo
̈
rster mechanism). Dual emission was observed,
Single-Crystal Molecular Structures. The molecular
structures of Pt−0 and Pt−1 were determined by single-crystal
which is unprecedented for trans-bisphosphine Pt(II) bisacety-
59
X-ray diffraction (Figure 1 and Table 1). For Pt−0, the two
Pt(II) coordination centers take planar square geometries. For
example, the bond angle of C(96)−Pt(1)−P(3) is 91.22(13)°,
the bond angle of C(95)−Pt(1)−C(96) is 178.79(18)°, and
the P(3)−Pt(1)−P(4) bond angle is 176.84(4)°. A similar
result was observed for the Pt(2) coordination center. These
values are close to a previously reported trans-bis-
lide complexes. The complex shows efficient singlet oxygen
1
(
O ) photosensitizing ability (Φ = 89.0%). Intramolecular
2
Δ
and intermolecular triplet-energy transfers were studied with
nanosecond time-resolved transient absorption spectroscopy.
RESULTS AND DISCUSSION
■
(
53
Design and Synthesis of the Complexes. trans-Bis-
trialkylphosphine) Pt(II) bisacetylide complexes are of
(
trialkylphosphine) Pt(II) bisacetylide complex. The Pt(1)−
C(95) bond length is 1.992(4) Å, which is close to the
53
particular interest because it is feasible to introduce heteroleptic
previously reported values (1.981−2.015 Å). The CC bond
(
different) visible-light-absorbing ligands into the molecules,
lengths are in the range of 1.193−1.209 Å, which is close to the
53−55
59
simply by using different acetylide ligands.
Recently we
literature values (1.192−1.233 Å).
prepared N^N Pt(II) bisacetylide complex with heteroleptic
For the coordinated styrylBodipy, the two styryl moiety
planes twisted toward the π-core of the Bodipy chromophore,
presumably due to the steric hindrance exerted by the two
Pt(II) coordination centers and the phosphine ligands. For
example, the dihedral angles of C(49)−C(50)−C(51)−C(52)
and C(57)−C(58)−C(59)−C(60) were observed as 12.611°
and 22.140°, respectively. Previously in an uncoordinated
styrylBodipy molecule, the dihedral angles of the two styryl
moieties with the π-core of the Bodipy chromophore are much
5
6
acetylide ligands. But two different acetylide ligands are
unable to be introduced in a stepwise way. Previously
unmodified Bodipy ligands were used for preparation of the
Pt(II) complexes, but the absorption wavelength is shorter than
2
8,30,43
600 nm.
Herein we used styrylBodipy as an acetylide
ligand, which shows absorption at 648 nm. Upon coordination
with Pt(II), the absorption is shifted to 731 nm, that is, in the
near-IR region. To the best of our knowledge, no trans-
bis(trialkylphosphine) Pt(II) bisacetylide complexes have been
60
smaller. Pt−0 shows a rodlike long molecule axial of 30.6527
53−55,57
reported showing absorption in near-IR spectral region.
Å (the distance of C(76) to C(82)), with the C−Pt−C bond
C
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Inorganic Chemistry
Article
Table 1. Crystallographic Data for Pt−0 and Pt−1
complexes
sum formula
M (g mol−1)
temperature/K
crystal system
space group
a (Å)
Pt−0
C101H143BF N P Pt
Pt−1
C127H169B F N P Pt
2
2
4
2
3
6
6
4
2
1948.04
296(2)
2440.17
296(2)
triclinic
monoclinic
P2 /n
1
P1
̅
21.7912(18)
14.8974(12)
33.153(3)
90.00
15.292(3)
17.398(3)
25.540(4)
b (Å)
c (Å)
α (deg)
102.092(3)
94.011(3)
103.690(3)
6403.9(19)
2
Figure 2. UV−vis absorption spectra of the compounds. c = 1.0 × 10−
5
β (deg)
108.1610(10)
90.00
M in toluene, 20 °C.
γ (deg)
volume/Å3
10 226.4(14)
4
Z
Dcalc/g·cm−3
position of the ethynyl on Bodipy moiety and thus the different
perturbation on the π-conjugation framework of the
chromophore.
1.265
1.265
crystal size (mm)
0.34 × 0.28 × 0.26
4008
0.35 × 0.27 × 0.15
2516
F (000)
−
1
For Pt−1, which contains two different types of Bodipy
ligands as visible-light-harvesting antenna, two major absorp-
tion bands were observed at 503 and 731 nm (Figure 2). The
absorption maxima of Pt−1 is the sum of Pt−0 and Pt−2; thus,
there is no significant electronic interaction between the two
kinds of Bodipy ligands in Pt−1 at the ground state. Moreover,
there is no π-conjugation across the Pt(II) atoms, otherwise the
absorption wavelength of Pt−1 will be drastically different from
that of Pt−0 and Pt−2. This finding is different from a previous
result that delocalized Franck−Condon singlet-excited state
μ (Mo Kα)/mm
θ (deg)
2.841
2.288
1.88−28.27
62 775
1.52−28.41
47 943
reflections collected
independent reflections
parameters
24 775
31 660
1239
1526
largest diff. peak and hole
0.822, −0.543
1.358, −0.721
−
3
(
e Å )
goodness of fit
1.012
1.014
a
b
b
R
0.0477
0.0496
a
b
b
ωR2
0.0823
0.1113
a
2
o
2
2
2
o
2
25
R = ∑||F | − |F ||/∑|F |, wR = [∑(w(F − F ) )/∑(w(F ) )]1/2;
was proposed. To the best of our knowledge, trans-
o
c
o
2
c
b
[F > 4σ(F )]. On the basis of all data.
bisphosphine Pt(II) bisacetylide complexes showing near-IR
o
o
5
5,62,63
absorption, like Pt−1, were not reported.
Recently we
angles of C(95)−Pt(1)−C(96) and C(99)−Pt(2)−C(100) at
prepared N^N Pt(II) bisacetylide complex with heteroleptic
acetylide ligands. But the complex does not show any
5
9
56
1
78.79(18)° and 177.3(2)°, respectively.
On the basis of the single-crystal structure of Pt−1, a rodlike
absorption in near-IR range.
geometry was found, with the B(2)−B(3) distance of 39.4989
Å. The two Pt(II) coordination centers are close to planar
square geometry. For example, the P(1)−Pt(1)−P(2) bond
angle is 173.44(5)°, and the C(109)−Pt(1)−C(110) bond
angle is 176.80(18)°. The bond lengths of the acetylide CC
bonds are in the range of 1.201−1.205 Å, which is close to the
Notably the complexes Pt−0, Pt−1, and Pt−2 show very
strong absorption in visible region, which was rarely reported
5
3,55,62−64
for trans-bisphosphine Pt(II) bisacetylide complexes.
For example, the molar absorption coefficients (ε) of Pt−1 at
−1
5
−1
−1
4
503 and 731 nm are 1.4 × 10 M cm and 9.7 × 10 M
−1
cm , respectively.
5
9
literature values. Another distinct feature is the non-coplanar
geometry of the styryl moiety with the Bodipy chromophore.
For example, the dihedral angle of C(18)−C(19)−C(20)−
C(21) is 18.998°, and the dihedral angle of C(26)−C(27)−-
C(28)−C(33) is 0.127°. Furthermore, we found that the
coordinated moieties on the long axial of the Pt(II) coordinated
rod are not in a straight line. For example, C(110)−C(111)−
C(112) is 176.0(5)°, C(111)−C(110)−Pt(1) is 173.2(4)°,
C(123)−C(122)−Pt(2) is 173.3(5)°, and C(122)−C(123)−
C(124) is 173.3(6)°. Similar deviation was also found for other
The UV−vis absorption spectra and the luminescence
spectra of the compounds were studied (Figure 3). For ligand
L−1, emission band at 663 nm was observed (Figure 3a),
which is a mirror to the absorption band on lower-energy side.
The lifetime of the emission was determined as 5.55 ns. Note
the small Stokes shift of the emission is typical for Bodipy
5
0,65−67
fluorophores.
For Pt−0, the emission band is at 758 nm
(Figure 3b). The small Stokes shift and the short luminescence
lifetime of Pt−0 (1.84 ns) indicated that the emission of Pt−0
is fluorescence of the coordinated styrylBodipy ligand, not
53
46
trans-bis(trialkylphosphine) Pt(II) bisacetylides complexes. In
phosphorescence. This result reveals the interesting photo-
less congested Pt(II) complexes, a nearly straight coordination
axial was found. Similar geometries were observed for Pt−0
physics of Pt(II) complexes; that is, no phosphorescence was
observed even with two Pt(II) atoms coordinated to the π-
conjugation framework of the same styrylBodipy ligand, which
is in stark contrast to a previous observation that strong room-
temperature (RT) phosphorescence of Bodipy ligand was
6
1
(
Tables S1 and S2, see Supporting Information).
UV−vis Absorption Spectra and Fluorescence Spec-
tra. The UV−vis absorptions of the complexes were studied
(Figure 2). The references Pt−0 and Pt−2, which contain only
observed with a N^C^N auxiliary coordination ligand (Φ =
P
29
the styrylBodipy or the Bodipy chromophore, give only one
major absorption band at 734 or 504 nm, respectively. Notably,
the absorption of the ligand L−1 is red-shifted by 86 nm upon
coordination with Pt(II). For Pt−2, however, the absorption
wavelength is similar to the free ligand L−2. The different
absorption of the two complexes is due to the derivatization
3.5%).
For Pt−1, two emission bands were observed (Figure 3c).
The small Stokes shifts and the short luminescence lifetimes of
the two emission bands (2.22 and 1.95 ns) suggest that the
emission bands are fluorescence. Previously we prepared a
Pt(II) complex that contains two different coordinated Bodipy
D
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Inorganic Chemistry
Article
variation of the solvent polarity. For the coordinated L−1, that
is, complex Pt−0, however, the emission intensity is dependent
on the solvent polarity; the emission intensity was reduced in
polar solvents as compared with that in nonpolar solvents such
as toluene.
It should be pointed out that the quenched luminescence
intensity of Pt−1 in polar solvents is unlikely due to
intramolecular charge transfer (ICT); otherwise, the emission
wavelength of the emission bands will change substantially with
68
variation of the solvent polarity. The photophysical properties
of the compounds were summarized in Table 2.
Electrochemical Studies: Cyclic Voltammetry. The
electrochemical properties of the complexes were studied by
cyclic voltammetry versus Fc(+/0) (Figure 4). For the
Figure 3. Comparison of the normalized UV−vis absorption spectra
and emission spectra of (a) L−1 (λ = 362 nm), (b) Pt−0 (λ = 670
ex
ex
nm), (c) Pt−1 (λ = 480 nm), (d) Pt−2 (λ = 470 nm). c = 1.0 ×
ex
ex
−
5
1
0
M in toluene, 20 °C.
ligands; only the fluorescence of the energy acceptor was
4
3
Figure 4. Cyclic voltammogram of Pt−1, Pt−0, and Pt−2. Ferrocene
observed.
We propose a through-bond singlet-energy-transfer for Pt−
+
(
Fc) was used as internal reference (E1 = +0.17 V, Fc /Fc). In
/2
48
deaerated DCM solution containing 0.5 mM photosensitizers with the
ferrocene, 0.10 M Bu N[PF ] as supporting electrolyte, Ag/AgNO
3
1
, instead of the resonance energy transfer, since the emission
4
6
of the peripheral Bodipy ligand overlap poorly with the
absorption band of the central coordinated styrylBodipy ligand.
The poor spectral overlap between the singlet-energy donor
and the energy acceptor may result in inefficient FRET. Similar
emission band with small Stokes shift was observed for Pt−2
reference electrode, scan rates: 100 mV/s. 20 °C.
reference complex Pt−0, a reversible oxidation wave was
observed for which the half-wave potential E1 is +0.18 V,
/2
(
Figure 3d). The emission band can be attributed to the
whereas in the cathodic scan, a reversible reduction was
observed in which the half-wave potential E1/2 is −1.51 V
(Figure 4a). For Pt−2, a reversible reduction wave was
fluorescence emission of the Bodipy ligand.
The solvent polarity-dependency of the UV−vis absorptions
of the compounds were studied (Figure S21, see Supporting
Information). No substantial changes were observed for the
UV−vis absorption of the compounds with variation of the
solvent polarity. Thus, we conclude that there is no substantial
interaction or charge transfer for the compounds at ground
states. The Franck−Condon states of the compounds were not
affected by the solvent polarity. The solvent dependency of the
luminescence of the compounds was studied (Figure S22, see
Supporting Information). For ligand L−1, the emission
intensity and wavelength do not change significantly with
observed for which the half-wave potential E1 is −1.70 V,
/2
whereas in the oxidation region, an irreversible oxidation wave
was observed in which the potential is +0.91 V (Figure 4b).
For Pt−1 in the cathodic scanning, two reversible waves at
−1.50 and −1.70 V were observed, which is the sum of the
reduction waves of the reference complexes Pt−0 and Pt−2. In
the anode region, a reversible oxidation wave at +0.20 V was
observed for Pt−1. This result indicates that there is no
electronic interaction between the chromophores in Pt−1;
otherwise, the reduction potentials of Pt−1 should be
a
Table 2. Photophysical Properties of the Ligands and the Pt(II) Complexes
a
b
a
c
d
λabs
,
nm
ε
λem,
nm
Φ , %
F
Φ , %
Δ
τ , ns
τ , μs
F
T
e
g
i
L−1
362/648
734
6.9/12.7
12.4
663
758
23.6
2.26
f
5.55
f
h
Pt−0
Pt−1
Pt−2
37.0 ± 0.6
89.0 ± 1.3 /27.8 ± 0.4
12.1 ± 0.2
1.84
5.82
6.09
708.55
i
g
j
h
503/731
504
14.0/9.7
7.7
518/754
516
1.46 /1.77 /2.65
2.22/1.95
2.67
i
j
36.7
a
−5
b
4
−1
−1
c
−5
d
In toluene (c = 1.0 × 10 M). Molar absorption coefficient. ε: 1 × 10 M cm . Luminescence lifetimes. c = 1.0 × 10 M. Triplet state
−5
e
lifetimes. c = 1.0 × 10 M, in deaerated toluene. Fluorescence quantum yields. B−7 (Φ = 9.3%) was used as standard for L−1 (λ = 584 nm).
F
ex
f
g
Not applicable. Fluorescence quantum yields. B−3 (Φ = 9.5%) was used as standard for Pt−0 and Pt−1 (λ = 640 nm). For Pt−1 the
F
ex
h
fluorescence quantum yield belongs to the emission band at 754 nm. Singlet oxygen quantum yields. Methylene blue (MB Φ = 57% in DCM) was
Δ
i
used as standard for Pt−0 and Pt−1 (λ = 670 nm). Fluorescence quantum yields. B−0 (Φ = 72% in THF) was used as standard for Pt−1 (λ =
ex
F
ex
4
83 nm) and Pt−2 (λ = 470 nm). For Pt−1, the first quantum yield is for the emission at 518 nm, and the second quantum yield is for the
ex
j
emission at 754 nm. Singlet oxygen quantum yields, in toluene. Rose Bengal (RB Φ = 80% in MeOH) was used as standard for Pt−1 and Pt−2
Δ
(λ = 516 nm).
ex
E
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substantially different from that of the reference compounds.
The redox potentials of the complexes were summarized in
Table 3.
Pt−0; note that Pt−0 is an analogue of the energy acceptor in
Pt−1.
However, the experiments demonstrated that the fluores-
cence emission intensity of Pt−1 is similar to that of Pt−0
(Figure 5a; optically matched solutions were used). Similar
results in solvents with different polarity were observed (Figure
S23, see Supporting Information). Same result was observed
even in polar solvent such as acetonitrile (Figure 5b). It is
known that electron transfer is more favored in polar solvents.
Therefore, we conclude that there is no significant photo-
a
Table 3. Electrochemical Potentials Versus Fc(+/0)
compound
oxidation, V
reduction, V
Pt−0
Pt−1
Pt−2
+0.18
+0.20
+0.91
−1.51
−1.50, −1.70
−1.70
7
0,74,75
a
induced electron-transfer process in Pt−1,
which is in
Cyclic voltammetry in deaerated DCM containing a 0.10 M Bu NPF
4
6
supporting electrolyte; counter electrode is Pt electrode; working
electrode is glassy carbon electrode; Ag/AgNO3 couple as the
agreement with the electrochemical studies and the Gibbs free
energy changes (ΔG ) of the electron transfer.
CS
+
reference electrode. [Ag ] = 0.1 M, 0.5 mM photosensitizers in
To study the energy-transfer process in Pt−1, the
fluorescence intensities of Pt−2 and Pt−1 were compared
deaerated DCM. 20 °C.
(
Figure 6). Pt−2 is the analogue of the energy donor unit in
Intramolecular photoinduced electron transfer may exert
significant effect on the photophysical properties of multi-
chromophore transition metal complexes, such as the
6
9−72
luminescence property or the triplet-state property.
The
Gibbs free energy changes (ΔG ) of the photoinduced
CS
electron transfer were calculated with the Rehm−Weller
7
2,73
equation.
The results show that the electron transfer in
Pt−1 is thermodynamically prohibited (Table 4). This
conclusion is in agreement with the spectroscopy studies.
Table 4. Free Energy Changes of Charge Recombination
(
ΔG ), Charge Separation (ΔG ), and Charge Separated
CR
CS
Figure 6. Fluorescence emission spectra of Pt−1 and Pt−2 (λ = 473
States Energy Level (E ) of Pt−1 with the Coordinated
StyrylBodipy as the Electron Donor and the Peripheral
Bodipy as the Electron Acceptor
ex
CST
nm; optically matched solutions were used) in (a) toluene and (b)
a
MeCN. 20 °C.
electron transfer
1StyrylBodipy* → Bodipy
ΔGCR, eV
ΔGCS, eV
ECST, eV
Pt−1. It was found that the fluorescence emission intensity of
Pt−2 was significantly quenched in Pt−1. This quenching
effect of the fluorescence of the peripheral Bodipy ligand is
attributed to energy transfer, since we proved with the Gibbs
b
b
c
c
c
−2.32
+0.67
+2.32
d
d
d
−
−
1.81
1.67
+0.16
+1.81
e
c
e
e
+0.02
+1.67
3StyrylBodipy* → Bodipy
c
c
−2.32
+1.37
+2.32
free energy changes (ΔG ) that intramolecular electron
d
e
d
d
CS
−
−
1.81
1.67
+0.86
+1.81
74
transfer is prohibited in Pt−1.
e
e
+0.72
+1.67
Same quenching effect was observed in toluene, as well as in
dichloromethane (DCM), MeCN, and MeOH; thus, this
quenching effect is unlikely due to polarity-enhanced ICT effect
a
b
The arrow means the direction of charge transfer. Electron transfer
c
d
e
process in Pt−1. In toluene. In dichloromethane. In MeCN.
(
Figure S24, see Supporting Information). The EnT rate
9
−1
74
constants were estimated at kEnT = 9.0 × 10 s using eq 5.
This predicted value agrees with the femtosecond transient
absorption spectroscopy (kEnT = 2.2 × 10 s , see later
section). This value is smaller as compared with the previously
Intramolecular Energy and Electron Transfer. To study
the photoinduced electron transfer in Pt−1, the emission
intensity of Pt−0 and Pt−1 was compared (Figure 5). Given an
electron transfer occurred after photoexcitation, the fluores-
cence emission intensity of Pt−1 should be weaker than that of
10
−1
11
−1
reported 7.8 × 10
s
for a Bodipy−Pt(II) porphyrin
76
complex. For a Bodipy−AzaBodipy triad, for which the
spectral overlap between the emission of the energy donor and
the absorption of the energy acceptor is not significant, the rate
11
−1 70
constant of the EnT is 1.2 × 10 s . These large FRET rate
constants, despite the poor spectral overlap, may be attributed
to through-bond-energy-transfer (TBET).
Recently Akkaya et al. proposed to use the fluorescence
77−80
75
excitation spectra to evaluate the singlet EnT efficiency. Thus,
the fluorescence excitation spectra of the compounds were
studied. The excitation spectra of the compounds were
recorded (Figure 7). For Pt−1, the excitation at 503 nm is
half of the UV−vis absorption; therefore, we propose the FRET
efficiency is ca. 50%. We noted the discrepancy for the EnT
efficiency evaluated by the luminescence quenching and the
Figure 5. Fluorescence emission spectra of Pt−0 and Pt−1 (λ = 670
ex
81
nm, optically matched solutions were used) in different solvents of (a)
toluene and (b) MeCN. 20 °C.
excitation spectra. However, the comparison of the excitation
spectra with the UV−vis absorption spectrum in Figure 7
F
DOI: 10.1021/acs.inorgchem.5b01107
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Inorganic Chemistry
Article
triplet-state lifetime may be due to the different steric hindrance
of the styryl moiety; with bulky substitution on the 2,6-position,
the coplanarity of the π-conjugation framework is compromised
(
which is confirmed by the molecular structures determined
with the single-crystal X-ray diffraction). As a result, the lifetime
of the triplet state of styrylBodipy was reduced.
For Pt−2, significant bleaching band at 504 nm was observed
upon nanosecond pulsed laser excitation at 503 nm (Figure
8
e). The lifetime was determined as 709 μs. Thus, we conclude
that the triplet-excited state is localized on the Bodipy moiety.
For Pt−1, as previously confirmed, singlet-energy transfer
from the peripheral Bodipy ligands to the central coordinated
styrylBodipy ligand occurs. Thus, Pt−1 was selectively excited
Figure 7. Comparison of the normalized UV−vis absorption and the
luminescence excitation spectra of Pt−1 (λ = 770 nm). c = 1.0 ×
em
−
5
1
0
M in toluene at 20 °C.
into the peripheral coordinated Bodipy parts (λ = 500 nm).
ex
Interestingly, no bleaching band at 504 nm was observed, which
is different from Pt−2. Instead, intensive bleaching band at 741
nm was observed, which is similar to Pt−0. Thus, we conclude
that triplet state is confined on the central coordinated Bodipy
ligand upon selective excitation into the peripheral Bodipy
unambiguously confirmed that there is singlet energy transfer
from the peripheral Bodipy ligands to the central coordinated
7
0,74,75
styrylBodipy ligand.
Nanosecond Transient-Absorption Spectroscopy. To
study the triplet-excited state of the complexes, the nanosecond
transient-absorption spectroscopy of the complexes was studied
86
part. Since we confirmed the triplet-state formation in Pt−2,
and the fact that no triplet state is localized on the peripheral
Bodipy in Pt−1, we propose that intramolecular triplet−triplet-
energy transfer (TTET) exists for Pt−1. Since no increasing
stage was observed for the transient signal at 741 nm, we
5
8,82,83
(
Figure 8).
For Pt−0, significant bleaching band at 741
nm was observed upon nanosecond pulsed laser excitation.
Minor bleaching band at 369 nm was also observed; thus, the
T triplet state of Pt−0 is localized on the Bodipy unit. The
1
8
−1
propose the triplet state EnT in Pt−1 is fast (k ≫ 1 × 10 s .
The time-resolution of the nanosecond transient absorption
spectrometer is 10 ns). Previously it was reported that the
intramolecular triplet-state energy-transfer rate constant is 1.0
transient was significantly quenched in aerated solution (Figure
S25a, see Supporting Information). Thus, the signal can be
attributed to the triplet state of the complex.
The triplet-state lifetime was determined as 5.8 μs, which is
much shorter than a similar trans-bis(trialkylphosphine)
bisacetylide complex with Bodipy ligand (not styrylBodipy
10
−1
76
× 10
s
for a Bodipy−Pt(porphyrin) complex.
The above result confirms that the excitation energy
harvested by the energy donor, that is, the peripheral
coordinated Bodipy part, can be funneled efficiently into the
energy acceptor, either through intramolecular singlet energy
transfer or TTET. Pt−1 was also excited with pulsed laser at
2
9
ligand. τ = 128 μs). However, this lifetime is close to that
T
84
observed with heavy atom effect (τ = 1.8 μs). Previously we
T
prepared a C −styrylBodipy dyad, for which a much longer
6
0
85
triplet-state lifetime was observed (71−123 μs). The different
Figure 8. Nanosecond transient absorption spectra. (a) Pt−0 and (b) decay curve at 420 nm (λ = 510 nm); (c) Pt−1 and (d) decay curve at 490
ex
−5
nm (λ = 500 nm); (e) Pt−2 and (f) decay curve at 490 nm (λex = 503 nm). c = 1.0 × 10 M in deaerated toluene at 20 °C.
ex
G
DOI: 10.1021/acs.inorgchem.5b01107
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
25 nm, that is, selectively into the energy-acceptor part.
Exactly the same transient spectra was observed, and the triplet-
state lifetime was determined as 6.5 μs (Figure S26, see
Supporting Information).
Article
87
7
Intermolecular triplet-state energy transfer is rarely studied.
Intermolecular triplet-state energy transfer is normally
inefficient due to the short lifetime of the excited state of the
energy donor. For Pt−2, however, the triplet-state lifetime is
very long (709 μs); thus, long-range intermolecular triplet-
energy transfer with Pt−2 is possible. To study the triplet-
energy transfer in Pt−1, complex Pt−2 was also used as triplet-
state energy donor, and the ligand L−1 was used as triplet-state
energy acceptor. This mixture can be used as a model for
studying of the triplet-state energy transfer in Pt−1.
The triplet-state property of Pt−1 in polar solvent such as
acetonitrile was studied (Figure 9). Triplet-excited state was
With increasing concentration of L−1, the lifetime of the
bleaching band at 504 nm was reduced, and a new bleaching
band at 642 nm was observed (Figure 11, and Supporting
Figure 9. (a) Nanosecond transient absorption spectra of Pt−1; (b)
−5
decay curve of Pt−1 at 490 nm (λ = 500 nm). c = 1.0 × 10 M in
ex
deaerated MeCN at 20 °C.
observed with lifetime of 5.9 μs, which is close to the triplet-
state lifetime in nonpolar solvent such as toluene (6.1 μs,
Figure 8d). Thus, the photoinduced electron-transfer effect in
Pt−1 is not significant, which is in agreement with the study of
the Gibbs free energy changes (Table 4) and the fluorescence
spectra (Figure 5). A recently reported broadband visible-light-
absorbing N^N Pt(II) bisacetylide complex with heteroleptic
acetylide ligands shows longer triplet-state lifetime of 68.21
5
6
μs.
The triplet state of the ligand L−1 was produced with the
sensitization method; that is, 2,6-diiodoBodipy (B−2, Scheme
1
) was used as the triplet-state energy donor (photosensitizer)
87
and L−1 as the triplet-state energy acceptor (Figure 10).
The triplet-state lifetime of B−2 was reduced to 22.8 μs in
the presence of L−1. Note the intrinsic triplet-state lifetime of
B−2 is 179.8 μs (Figure S27, see Supporting Information). The
bleaching band at 647 nm was determined with a lifetime of
Figure 11. Nanosecond transient absorption spectra and the decay
curves of the mixture of complex Pt−2 and ligand L−1. (a) Molar
ratio is 1:0.3 (Pt−2/L−1); decay curves at (b) 490 nm and (c) 640
nm (λ = 503 nm), molar ratio is 1:0.3 (Pt−2/L−1); (d) Stern−
ex
1
52 μs, which is much longer than that of Pt−1. This value is
Volmer plots for lifetime quenching of Pt−2 with increasing the
concentration of L−1. λ = 503 nm. [Pt−2] = 1.0 × 10− M. In
5
close to a previously reported triplet-state lifetime of
ex
deaerated toluene at 20 °C.
styrylBodipy compounds observed with C −StyrylBodipy
6
0
dyads, that is, intramolecular sensitization method (71.5−
9
1
123.2 μs).
Information Figure S28). The change of the transient
absorption spectra upon increasing the triplet-energy acceptor
L−1 was attributed to the intermolecular triplet-energy transfer
and the produce of the triplet state of L−1.
8
7,88
Figure 10. (a) Nanosecond time-resolved transient absorption spectra of the mixture of B−2 (sensitizer) and L−1 (triplet-energy acceptor). (b)
−5
Decay curve at 647 nm (λ = 528 nm) and (c) decay curve at 532 nm (λ = 528 nm). c = 1.0 × 10 M in deaerated toluene at 20 °C.
ex
ex
H
DOI: 10.1021/acs.inorgchem.5b01107
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
To study the kinetics of the intermolecular energy transfer,
the decay trace at 640 nm was monitored. Interestingly, the
decay is featured with biphase character; that is, there is first a
sharp increase of the bleaching at 640 nm, and then a slow
decay process was observed (Figure 11c). Similar results were
observed in Figure 10b. The fast increase of the transient is
attributed to the production of the triplet state of L−1 via the
intermolecular triplet-energy transfer, and thereafter the
relatively slow decay of the transient is attributed to the
decay of the triplet state of L−1. The exceptionally long
lifetime of the triplet state is probably due to the low
concentration of L−1 molecule at triplet state. The triplet-state
intermolecular energy-transfer rate constant was calculated as
Figure 12. Femtosecond transient absorption spectra of Pt−1 (λ
=
ex
−5
5
05 nm). c = 1.0 × 10 M in deaerated toluene at 20 °C.
4
−1
on 1 × 10 s scale (which is concentration-dependent)
according to eq 6 (Table 5). The triplet-state energy-transfer
Table 5. Rate Constant (k ) and Efficiency (Φ ) for the
ET
ET
Triplet−Triplet Energy Transfer in the Mixture of Pt−1 and
a
Pt−2
Pt−2/L−1
kET (s−1)
ΦET (%)
4
1.0:0.3
1.0:0.5
1.0:0.7
1.0:1.0
1.6 × 10
2.0 × 10
2.4 × 10
3.6 × 10
92.2
92.3
91.4
94.5
4
4
4
Figure 13. Decay curves of Pt−1 at 505 and 730 nm (λ = 505 nm). c
ex
a
−5
In deaerated toluene. 20 °C.
= 1.0 × 10 M in deaerated toluene at 20 °C.
efficiency was calculated as ca. 92% according to eq 7. These
values are similar to that observed with intra-assembly
spectra of Pt−1 show that the bleaching signal around 505
nm decreases, while the bleaching signal around 730 nm
increases, simultaneously.
(hydrogen-bonded molecular assembly) triplet-state energy
transfer in a C −ferrocene hydrogen-bonding molecular
The decay time of the bleaching signal for Bodipy (505 nm)
6
0
5
−1
10 −1
assembly (9.2 × 10 s ), which indicates that the
is ∼45 ps (k = 2.2 × 10 s ), which is approximately equal
EnT
88
intermolecular triplet-energy transfer is efficient. Previously
a sub-phthalocyanine/zinc tetra-tert-butylphthalocyanine trip-
let-state energy transfer was observed to show similar triplet-
to the rise time of the bleaching signal for styrylBodipy (730
nm) in Pt−1 (Figure 13). These results give strong evidence of
singlet EnT from Bodipy to styrylBodipy. Previously a Bodipy−
AzaBodipy triad was studied, for which the spectral overlap
between the emission of the energy donor and the absorption
of the energy acceptor is not significant; the rate constant of the
5
−1 87
state energy transfer k = 4.2 × 10 s .
The Stern−Volmer quenching constant of Pt−2/L−1 was
6
−1
calculated as K = 1.78 × 10 M (Figure 11d). The
SV
9
11 −1 70
bimolecular quenching constant was calculated as 2.5 × 10
EnT is 1.2 × 10 s . Recently we prepared N^N Pt(II)
−
1
−1
56
M
s
according to eq 8. Quenching efficiency was given by
bisacetylide complex with heteroleptic acetylide ligands. The
1
1
eq 9, where k is the diffusion-controlled bimolecular quenching
singlet-energy transfer rate constant is 10-fold faster, 2.6 × 10
0
−
1
rating constants, which can be calculated with the Smoluchow-
s .
8
9
ski eq 10.
The positive signal (excited-state absorption, ESA) repre-
The molecular radius of the energy donor (Pt−2) is 8.7 Å,
sents S → S transition of Bodipy and styrylBodipy and T →
1
n
1
and that of quencher (L−1) is 8.6 Å. According to eq 11 the
T transition of styrylBodipy. The ESA signals around 430 and
n
−6
diffusion coefficient of the energy donor (Pt−2) is 4.21 × 10
2 −1
620 nm can be attributed to S → S transition of Bodipy due
1
n
2
−1
−6
cm s , and that of the quencher (L−1) is 4.26 × 10 cm s
to appearance of these signals at zero time delay upon
excitation of Bodipy. The other ESA signals around 445 and
635 nm increased after singlet EnT process; therefore, these
signals can be attributed to S → S transition of styrylBodipy.
(
×
2
in deaerated toluene at 20 °C). Thus, k was calculated as 1.11
10
0
10
−1 −1
M
s . The quenching efficiency was calculated as
2.7% according to eq 9, which indicates that there is an
1
n
efficient triplet-state energy transfer between Pt−2 and L−1 in
In addition to that, evidence of T → T transition is obtained
1 n
the mixture.
from positive signals that were placed around 420 and 560 nm
probe wavelengths. These signals appear after ∼1 ns.
We also checked whether any energy transfer from
styrylBodipy to Bodipy with an extra pump−probe experiment
upon excitation styrylBodipy (730 nm) for Pt−1. There are
bleach signals of ground state (730 nm) and photoinduced
charge-transfer state (810 nm) of styrylBodipy, and no energy
transfer is seen as expected (Figures S29a and S31, see
Supporting Information).
Femtosecond Transient Absorption Spectroscopy.
Ultrafast pump−probe experiments were performed for study
of the singlet EnT (Figure 12). Ultrafast pump−probe
experiments were performed for Pt−0, Pt−1, and Pt−2 at
appropriate pump wavelengths (Figures 12 and 13 and
Supporting Information, Figures S29−S33). It is expected
that singlet EnT may occur only for Pt−1, which contains two
different kinds of Bodipy unit. Upon excitation of Pt−1 at 505
nm, bleaching bands were observed around 505 and 730 nm,
where the Bodipy and styrylBodipy give steady-state
absorption, respectively (Figure 12). Transient absorption
Density Functional Theory Computations. The ground-
state geometry and the excited states of the near-IR absorbing
complex Pt−0 were studied with density functional theory
I
DOI: 10.1021/acs.inorgchem.5b01107
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Inorganic Chemistry
Article
(
(
DFT) and time-dependent density functional theory
TDDFT) computations (Figure 14). The optimized P−Pt−
Figure 15. Isosurfaces of spin density of Pt−0. Calculated at the
optimized triplet-state geometries at B3LYP/GENECP/LANL2DZ
level with Gaussian 09W.
1
more efficient than Pt−0 to produce O (Figure 16a). Thus,
2
singlet EnT in Pt−1 is confirmed.
Similar results were obtained with white light excitation
(
Figure 16b). Pt−1 is more efficient than Pt−0 and Pt−2 for
1
production of O . These results confirm the singlet EnT in
2
Pt−1 and that the broadband absorption of Pt−1 is
advantageous for Pt−1 to be used for photocatalysis.
Figure 14. Electron density maps of the frontier molecular orbitals of
Pt−0 based on the optimized ground-state geometry. Calculated at the
B3LYP/GENECP/LANL2DZ level with Gaussian 09W.
1
The O photosensitizing ability of Pt−1 and Pt−2 was also
2
compared, upon excitation at the same wavelength (Figure
1
O
6c). The result shows that Pt−1 is more efficient than Pt−2 as
2
1
photosensitizer upon excitation at 503 nm. Thus,
P bond angles are 179.779° and 179.782°, which is similar to
the structure determined with the single-crystal molecular
structure. The CC bond lengths are in the range of 1.2277−
intramolecular singlet EnT was confirmed for Pt−1, and the
intersystem crossing is weak in Pt−2.
CONCLUSIONS
trans-Bis(tributylphosphine) Pt(II) bisacetylide complex (Pt−
1) was prepared, which shows strong broadband visible-light/
1.2290 Å, also in agreement with the experimental values.
■
The singlet and triplet excited states of Pt−0 were studied
with TDDFT computations. The highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
5
−1
−1
near-IR absorption at both 503 nm (ε = 1.4 × 10 M cm )
4
−1
−1
(
LUMO) are involved in the low-lying excited states (S state
and 731 nm (ε = 9.7 × 10 M cm ). The complex contains
two peripheral coordinated Bodipy ligands and a centrally
coordinated styrylBodipy ligand, which show absorption at 503
and 731 nm, respectively. Reference complexes (Pt−0 and Pt−
2), which contain a single visible-light-harvesting Bodipy ligand,
were prepared for comparison. The photophysical properties of
the complexes were studied with steady-state and femtosecond/
nanosecond time-resolved transient-absorption spectroscopies.
Dual fluorescence at 518 and 754 nm was observed for Pt−1,
which is rare for Bodipy-containing Pt(II) complexes. No
phosphorescence was observed for Pt−1. With fluorescence
emission spectra and excitation spectra, singlet EnT in Pt−1
1
and T state. Table 6). Thus, S state, as well as the T state, is
1
1
1
localized on the coordinated styrylBodipy moiety, which is in
agreement with the steady-state and the time-resolved transient
absorption spectra of the complex. The spin density surface of
Pt−0 was also studied; the results show the spin density is
localized on the coordinated Bodipy (Figure 15).
Singlet Oxygen Photosensitizing. The singlet oxygen
O ) photosensitizing ability of the complexes was compared
2
(1
(
Figure 16). 1,3-Diphenylisobenzofuran (DPBF) was used as
1
1
1
O scavenger for monitoring the O production. First the O
2
2
2
photosensitizing ability of Pt−0 and Pt−1 was compared upon
selective photoexcitation into the energy donor in Pt−1. At 503
was confirmed (Φ = 50%). TBET mechanism was proposed
EnT
nm, Pt−1 shows stronger absorption than Pt−0. However, no
to be responsible for the ultrafast singlet EnT in Pt−1, due to
the poor spectral overlap, yet fast energy transfer. Femtosecond
transient absorption indicated the intramolecular singlet EnT is
1
enhanced O photosensitizing should be observed given no
2
energy transfer exists in Pt−1. The results show that Pt−1 is
Table 6. Excitation Energies (eV) and Corresponding Oscillator Strengths (f), Main Configurations, and CI Coefficients of the
Low-Lying Electronically Excited States of Pt−0, Calculated by TDDFT//B3LYP/LANL2DZ, Based on the DFT//B3LYP/
LANL2DZ Optimized Ground-State Geometries
TDDFT/B3LYP/LANL2DZ
a
b
c
d
electronic transition
energy [eV/nm]
f
composition
CI
character
MLCT
singlet
S0→S1
S0→S3
S0→S9
S0→S19
S0→T1
S0→T2
S0→T3
1.91/648
2.46/503
3.28/378
3.73/333
0.95/1311
1.92/645
1.945/637
0.8572
0.4345
0.8707
0.6393
0.0000
0.0000
0.0000
H→L
0.6852
0.6297
0.4953
0.5331
0.6989
0.5501
0.4799
H−2→L
H→L+1
H−1→L+1
H→L
MLCT/ILCT
MLCT
MLCT/ILCT
MLCT
e
triplet
H−1→L
H−2→L
MLCT/ILCT
MLCT/ILCT
a
b
c
d
Only the selected low-lying excited states are presented. Oscillator strengths. Only the main configurations are presented. The CI coefficients are
e
in absolute values. No spin−orbital coupling effect was considered; thus, the f values are zero.
J
DOI: 10.1021/acs.inorgchem.5b01107
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Inorganic Chemistry
Article
1
−5
Figure 16. Comparison of the singlet oxygen ( O ) photosensitizing of (a) Pt−0 and Pt−1 (λ = 503 nm). c = 1.0 × 10 M, respectively. (b) Pt−0,
2
ex
−5
−5
Pt−1, and Pt−2 (xenon lamp was used as light source). [Pt−0] and [Pt−1] = 1.0 × 10 M; [Pt−2] = 2.0 × 10 M. (c) Pt−1 and Pt−2 (λ = 503
nm, optically matched solutions were used), [Pt−1] = 1.0 × 10 M. In toluene. 20 °C.
ex
−5
1
0
−1
84
fast: kEnT = 2.2 × 10 s . Gibbs free energy changes (ΔG )
Synthesis of 3. B−2 (28 mg, 0.05 mmol) was dissolved in 3 mL
CS
of dried dimethylformamide. Benzaldehyde (21 mg, 0.2 mmol) was
added, followed by three drops of acetic acid and three drops of
piperidine before the mixture was saturated by Ar. Then the mixture
was subjected to microwave irradiation for 6 min at 150 °C under Ar.
After the reaction was finished, 25 mL of water was added. The solid
was filtrated out and purified by column chromatography (silica gel,
CH Cl /PE = 1:4, v/v). 3 was collected as a dark green solid (32 mg,
and steady-state luminescence studies show that the photo-
induced intramolecular electron transfer in Pt−1 is prohibited.
With nanosecond time-resolved transient-absorption spectra,
intramolecular triplet-state energy transfer was proved, and the
T state of Pt−1 is only confined on the central coordinated
Bodipy ligand. Intermolecular triplet-state energy transfer was
studied with nanosecond time-resolved transient absorption
1
2
2
1
yield: 85.1%). H NMR (400 MHz, CDCl ): 8.16 (d, 2H, J = 16.7
3
4
−1
spectra (k = 3.6 × 10 s and Φ = 94.5%). With singlet
oxygen ( O ) photosensitizing experiments, we proved that the
Hz), 7.74−7.66 (m, 6H), 7.56−7.54 (m, 3H), 7.45−7.41 (m, 4H),
EnT
EnT
1
7
.38−7.34 (m, 2H), 7.31−7.29 (m, 2H), 1.46 (s, 6H). MALDI-TOF-
2
+
HRMS ([C H BF I N ] ): calcd m/z = 752.0168, found m/z =
broadband visible-light-absorbing of Pt−1 is beneficial for the
complex to be used as triplet photosensitizers. These studies
will be useful for designing near-IR absorbing Pt(II) complexes
and for the application of these complexes in luminescent
bioimaging and photodynamic therapy studies.
33 25
2 2
2
7
52.0184.
Synthesis of 4. 3 (70 mg, 0.09 mmol) was dissolved in 5 mL of
29
i
distilled tetrahydrofuran (THF) before 2.5 mL of ( Pr) NH was added
2
under Ar. Pd(PPh ) Cl (17 mg, 0.02 mmol) and PPh (13 mg, 0.05
3
2
2
3
mmol) were added, followed by CuI (9.4 mg, 0.05 mmol).
Trimethylsilylacetylene (35 mg, 0.36 mmol) was injected before the
mixture was stirred under Ar for 6 h at 68 °C. After the reaction was
finished, the solvent was removed under reduced pressure. The residue
was purified by column chromatography (silica gel, CH Cl /PE = 1:4,
EXPERIMENTAL SECTION
■
2
Materials. Solvents were dried or distilled for synthesis. K PtCl4
was purchased from Aladdin Chemical Co., Ltd. (P. R. China). B−2
was synthesized according to literature methods.
Analytical Measurements. All chemicals are analytically pure and
used as received. NMR spectra were recorded by Bruker 500 MHz
spectrometer and OXFPRD NMR 400 MHz spectrometer with
2
2
1
v/v). 4 was collected as a dark green solid (48 mg, yield: 77.0%). H
NMR (400 MHz, CDCl ): 8.44 (d, 2H, J = 16.3 Hz), 7.79 (d, 2H, J =
58
3
1
4
1
6.5 Hz), 7.65 (d, 4H, J = 7.5 Hz), 7.53−7.52 (m, 3H), 7.45−7.41 (m,
H), 7.37−7.34 (m, 2H), 7.30−7.28 (m, 2H), 1.51 (s, 6H), 0.27 (s,
13
8H). C NMR (100 MHz, CDCl ): δ 152.7, 146.9, 140.4, 139.4,
3
CDCl as solvent and tetramethylsilane (TMS) as standard at 0.00
3
136.8, 134.6, 132.5, 129.5, 129.4, 129.3, 128.8, 128.1, 127.8, 118.6,
12.9, 86.4, 29.7, 1.0. MALDI-TOF-HRMS ([C H BN F Si ] ):
calcd m/z = 692.3026, found m/z = 692.3005.
Synthesis of L−1. 4 (34.6 mg, 0.05 mmol) was dissolved in 3 mL
of distilled THF. Bu NF·3H O (0.2 M in THF, 1 mL) was added
dropwise at −78 °C under Ar, and the solution was kept at −78 °C for
0.5 h (monitored by thin-layer chromatography until the starting
material had been completely consumed). After the reaction was
finished, 20 mL of water and 30 mL of CH Cl were added. The
aqueous layer was extracted with CH Cl (3 × 10 mL). The combined
organic layers were dried over anhydrous Na SO . The solvent was
removed under reduced pressure. The residue was purified by column
chromatography (silica gel, CH Cl /PE = 1:4, v/v). L−1 was collected
as a dark green solid (16 mg, yield: 58.4%). H NMR (400 MHz,
CDCl ): 8.37 (d, 2H, J = 16.6 Hz), 7.78 (d, 2H, J = 16.2 Hz), 7.67 (d,
ppm. HRMS was accomplished with GCT time-of-flight (TOF) mass
spectrometer (Waters, U.K.) and matrix-assisted laser desorption/
ionization (MALDI) micro MALDI TOF mass spectrometer (Waters,
U.K.). Fluorescence spectra were measured on an RF5301 PC
spectrofluorometer (Shimadzu, Japan). Absorption spectra were
recorded on UV2550 UV−vis spectrophotometer (Shimadzu, Japan).
+
1
43 43 2 2 2
29
4
2
∼
90
Synthesis of 1. K PtCl (1.04 g, 2.5 mmol) was dissolved in 20
2
4
n
mL of deionized water under argon atmosphere (Ar). P( Bu) (1.07 g,
.3 mmol) was injected. Then the mixture was stirred under Ar at
room temperature (RT) for 3 h. After the reaction was finished, the
solid was filtrated out and purified by column chromatography (silica
gel, CH Cl ). 1 was collected as a white solid (0.97 g, yield: 57.7%).
3
2
2
5
2
2
2
4
2
2
2
2
1
1
H NMR (400 MHz, CDCl ): 2.02−1.96 (m, 12H), 1.55−1.50 (m,
3
1
(
2H), 1.49−1.40 (m, 12H), 0.95 (t, 18H, J = 7.2 Hz). TOF-HRMS
3
+
[C H Cl P Pt] ): calcd m/z = 669.2726, found m/z = 669.2731.
4H, J = 7.6 Hz), 7.55−7.54 (m, 3H), 7.45−7.41 (m, 4H), 7.37−7.36
24
54
2 2
3
0
13
Synthesis of 2. 1 (435 mg, 0.65 mmol) was dissolved in 30 mL of
dried NHEt under Ar. Phenylacetylene (61 mg, 0.60 mmol) was
(m, 2H), 7.31−7.30 (m, 2H), 3.57 (s, 2H), 1.52 (s, 6H). C NMR
2
(100 MHz, CDCl ): δ 152.5, 146.2, 140.0, 139.2, 137.0, 134.7, 132.6,
3
injected. Then the mixture was stirred under Ar at 45 °C for 8 h. After
the reaction was finished, the solvent was removed under reduced
pressure. The residue was purified by column chromatography (silica
gel, CH Cl /petroleum ether (PE) = 1:5, v/v). 2 was collected as a
129.4, 129.3, 129.2, 128.8, 127.7, 118.8, 114.0, 104.1, 98.9, 77.2, 29.7.
+
MALDI-TOF-HRMS ([C H BF N ] ): calcd m/z = 548.2235,
3
7
27
2
2
found m/z = 548.2269.
28
2
2
Synthesis of Pt−0. 2 (30 mg, 0.04 mmol) and L−1 (10 mg, 0.02
1
light yellow solid (382 mg, yield: 79.8%). H NMR (400 MHz,
mmol) were dissolved in the mixture of distilled THF (2 mL) and
CDCl ): 7.25−7.18 (m, 4H), 7.16−7.11 (m, 1H), 2.04−1.98 (m,
dried NHEt (2 mL) under Ar. CuI (2 mg, 0.01 mmol) was added
before the mixture was stirred for 0.5 h at RT. After the reaction was
3
2
1
2H), 1.62−1.52 (m, 12H), 1.49−1.40 (m, 12H), 0.92 (t, 18H, J = 7.3
+
Hz). MALDI-TOF-HRMS ([C H ClP Pt − Cl] ): calcd m/z =
finished, 20 mL of water and 30 mL of CH Cl were added. The
32
59
2
2
2
7
00.3740, found m/z = 700.3727.
aqueous layer was extracted with CH Cl (3 × 15 mL). The combined
2 2
K
DOI: 10.1021/acs.inorgchem.5b01107
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
organic layers were dried over anhydrous Na SO . The solvent was
[PF ], 0.1 M) as the supporting electrolyte, glassy carbon electrode as
2
4
6
removed under reduced pressure. The residue was purified by column
chromatography (silica gel, CH Cl /PE = 1:3, v/v). Pt−0 was
the working electrode, and platinum electrode as the counter
electrode. Dichloromethane was used as the solvent, and ferrocene
(Fc) was added as the internal reference. The solution was purged
with N before measurement, and the N gas flow was kept constant
2
2
collected as a dark green solid (22 mg, yield: 61.9%). Crystals were
obtained by slow liquid diffusion of n-hexane into a concentrated
2
2
1
CH Cl solution. H NMR (400 MHz, CDCl ): 8.93 (d, 2H, J = 16.5
during the measurement.
2
2
3
Hz), 7.82 (d, 2H, J = 16.5 Hz), 7.62 (d, 4H, J = 7.8 Hz), 7.48 (t, 3H, J
5.5 Hz), 7.37−7.27 (m, 12H), 7.20 (t, 4H, J = 15.3 Hz), 7.11 (t, 2H,
J = 15.0 Hz), 2.09−2.03 (m, 24H), 1.55−1.50 (m, 24H), 1.43 (s, 6H),
The Gibbs Free Energy Changes (ΔG ) of the Photoinduced
CS
=
Electron Transfer. ΔGcs was calculated with the Rehm−Weller
7
2,73
equation.
13
1
.37−1.27 (m, 24H), 0.82 (t, 36H, J = 14.5 Hz). C NMR (100 MHz,
ΔGCS = e[EOX − ERED] − E00 + ΔGS
(eq 1)
CDCl ): δ 150.8, 140.8, 138.0, 137.6, 137.1, 136.3, 133.2, 131.0, 129.2,
3
1
1
2
29.0, 128.9, 128.7, 128.5, 128.0, 127.7, 125.0, 120.1, 120.0, 119.8,
19.6, 109.7, 108.2, 108.0, 107.9, 102.5, 29.9, 26.5, 24.6, 24.5, 24.4,
e2
e2
⎛
⎜
⎞⎛
⎟⎜
⎞
⎟
1
1
1
1
ΔG = −
−
+
−
+
S
4.3, 24.1, 24.0, 13.9. MALDI-TOF-HRMS ([C101H143BF N P Pt ] ):
4πε ε R
8πε ⎝ R
R ⎠⎝ ε
ε ⎠
2
2
4
2
S
0
CC
0
D
A
REF
S
calcd m/z = 1946.9559, found m/z = 1946.9431.
(
eq 2)
2
8
Synthesis of Pt−1. Pt−2 (30 mg, 0.03 mmol) and L−1 (8 mg,
where ΔG is the static Coulombic energy, e is electronic charge, E
0
.015 mmol) were dissolved in the mixture of distilled THF (3 mL)
S
OX
is half-wave potential for one-electron oxidation of the electron-donor
unit, ERED is half-wave potential for one-electron reduction of the
electron-acceptor unit, and E00 is energy level for the singlet-excited
state approximated with the fluorescence emission wavelength or for
the T state energy of styrylBodipy. ε is static dielectric constant of
and dried NHEt (3 mL) under Ar. CuI (3 mg, 0.015 mmol) was
added before the mixture was stirred for 3 h at RT. After the reaction
was finished, 20 mL of water and 30 mL of CH Cl were added. The
aqueous layer was extracted with CH Cl (3 × 15 mL). The combined
organic layers were dried over anhydrous Na SO . The solvent was
removed under reduced pressure. The residue was purified by column
chromatography (silica gel, CH Cl /PE = 1:1, v/v). Pt−1 was
collected as a dark green solid (16 mg, yield: 43.0%). Crystals were
2
2
2
2
2
1
S
2
4
the solvent, RCC is center-to-center separation distance determined by
DFT optimization of the geometry, RD is the radius of the electron
donor determined by DFT optimization of the geometry, R is the
radius of the electron acceptor determined by DFT optimization of the
geometry, εREF is the static dielectric constant of the solvent used for
the electrochemical studies, and ε is the permittivity of free space. The
solvents used in the calculation were toluene (ε = 2.4), DCM (ε =
.1), and MeCN (ε = 37.5).
2
2
A
obtained by slow liquid diffusion of n-hexane into a concentrated
CH Cl solution. H NMR (500 MHz, CDCl ): 8.91 (d, 2H, J = 16.3
Hz), 7.83 (d, 2H, J = 16.2 Hz), 7.62 (d, 4H, J = 7.6 Hz), 7.49 (t, 3H, J
1
2
2
3
0
S
S
=
6.2 Hz), 7.38−7.34 (m, 8H), 7.32−7.31 (m, 2H), 7.28 (d, 2H, J =
9
S
7.4 Hz), 7.09 (d, 4H, J = 8.0 Hz), 5.97 (s, 4H), 2.55 (s, 12H), 2.11−
2.06 (m, 24H), 1.60−1.53 (m, 24H), 1.45 (d, 18H, J = 3.8 Hz), 1.36−
1
.29 (m, 24H), 0.82 (t, 36H, J = 14.6 Hz). 13C NMR (100 MHz,
ECS = e[EOX − ERED] + ΔGS
(eq 3)
(eq 4)
CDCl ): δ 155.2, 150.7, 143.2, 142.3, 140.7, 137.8, 137.5, 137.0, 136.0,
3
ΔGCR = −(ΔGCS + E00)
1
1
1
33.0, 131.5, 131.2, 130.9, 129.7, 128.9, 128.8, 128.7, 128.5, 128.4,
27.5, 121.1, 119.8, 118.9, 118.8, 118.7, 110.7, 110.6, 110.5, 109.4,
The energy transfer rate constant was calculated with eq 5.
02.6, 29.7, 26.4, 24.4, 24.3, 24.3, 24.2, 24.1, 23.9, 14.6, 13.7, 12.7.
+
⎡ Φ (Bodipy)
⎤
⎦
MALDI-TOF-HRMS ([C127H169B F N P Pt ] ): calcd m/z =
PL
3
6
6
4
2
kEnT = ⎢
− 1⎥/τ(Bodipy)
2
439.1838, found m/z = 2439.1865.
⎣
Φ
PL
(eq 5)
43
Synthesis of Pt−2. L−2 (35 mg, 0.10 mmol) and 1 (80 mg, 0.12
mmol) were dissolved in 8 mL of dried NHEt under Ar. The mixture
where [ΦPL(Bodipy)/ΦPL] is the fluorescence quantum yield ratio of
2
was stirred for 9 h at 45 °C. After the reaction was finished, the solvent
was removed under reduced pressure. The residue was purified by
column chromatography (silica gel, CH Cl /PE = 1:1, v/v). Pt−2 was
the Bodipy unit in Pt−2 and Pt−1, and τ(Bodipy) is the S
lifetime of Pt−2 (Table 2).
state
1
The triplet-state intermolecular energy-transfer rate constant was
calculated with eq 6. The triplet-state energy-transfer efficiency was
calculated with eq 7.
2
2
1
collected as a yellow solid (33 mg, 33.4%). H NMR (500 MHz,
CDCl ): 7.33 (d, 2H, J = 8.2 Hz), 7.09 (d, 2H, J = 8.2 Hz), 5.97 (s,
3
2
H), 2.55 (s, 6H), 2.06−2.01 (m, 12H), 1.63−1.55 (m, 12H), 1.48−
kET = (1/τ ) − (1/τ )
(eq 6)
(eq 7)
2
1
1
.41 (m, 18H), 0.92 (t, 18H, J = 7.3 Hz). MALDI-TOF-HRMS
+
(
[C H BF N ClP Pt] ): calcd m/z = 981.4568, found m/z =
45
72
2
2
2
ΦET = 1 − τ /τ
2
1
9
81.4573.
Nanosecond Transient Absorption Spectroscopy. Nano-
where τ and τ are triplet-state lifetime of the rising stage and decline
1
2
second transient absorption spectroscopy was studied with a LP920
laser flash photolysis spectrometer (Edinburgh Instruments, Living-
ston, U.K.). The samples were purged with N for 15 min before
measurements, and the N gas flow was kept constant during the
measurement. The signal was digitized with a Tektronix TDS 3012B
oscilloscope.
stage of decay curves, respectively (Figure 11 and Supporting
Information Figure S28).
The bimolecular quenching constant was calculated according to eq
2
2
8.
kq = KSV/τ
0
(eq 8)
Femtosecond Transient Absorption Spectroscopy. The
ultrafast pump−probe transient absorption spectroscopy measure-
ments were performed using a Ti:sapphire laser amplifier−optical
parametric amplifier system (Spectra Physics, Spitfire Pro XP,
TOPAS) and a commercial setup of ultrafast transient absorption
spectrometer (Spectra Physics, Helios). Pulse duration was measured
as 100 fs. Wavelengths of the pump beam were chosen according to
the steady-state absorption spectra of the studied compounds. White
light continuum generated with a sapphire crystal was used as a probe
beam.
where τ is the triplet-state lifetime of the triplet-energy donor.
0
Quenching efficiency was given by eq 9, where k is the diffusion-
0
controlled bimolecular quenching rating constants, which can be
89
calculated with the Smoluchowski eq 10.
fQ = k /k
q
0
(eq 9)
4
πN
k0 = 4πRND/1000 =
(R + R )(D + D )
f
q
f
q
1000
(eq 10)
Cyclic Voltammetry. Cyclic voltammetry was performed under a
where D is the sum of the diffusion coefficients of the energy donor
(D ) and quencher (D ), and N is Avogadro’s number. R is the
collision radius, the sum of the molecular radii of the energy donor
1
(
00 mV/s scan rate, in CHI610D Electrochemical workstation
Shanghai, China). The measurements were performed at room
temperature with tetrabutylammonium hexafluorophosphate (Bu N-
f
q
(R ) and the quencher (R ).
4
f
q
L
DOI: 10.1021/acs.inorgchem.5b01107
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Diffusion coefficients can be obtained from Stokes−Einstein
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́
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D = kT/6πηR
(eq 11)
(
where k is Boltzmann’s constant, η is the solvent viscosity, and R is the
molecule radius.
137, 4885−4901.
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Density Functional Theory Calculations. Geometry optimiza-
tion was calculated by using the B3LYP functional, the vertical
excitation energy was calculated with the TDDFT method based on
the singlet ground-state geometry. All the calculations were performed
91
with Gaussian 09W software (Gaussian, Inc.).
Single-Crystal X-ray Diffractions. The crystal was mounted on
glass fibers for X-ray measurement. Reflection data were collected at
RT on a Bruker AXS SMART APEX II CCD diffractometer with
graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) and an
ω scan mode. All the measured independent reflections (I > 2σ(I))
were used in the structural analyses, and semiempirical absorption
corrections were applied using SADABS program. The structures were
solved by the direct method using SHELXL-97. The hydrogen atoms
of the organic frameworks were fixed at calculated positions
geometrically and refined by using a riding model. In particular, the
atoms on the phosphine ligand are found to be disordered and
modeled over two split positions. All corresponding non-hydrogen
atoms were refined by using the “Sadi” and “Simu” restraints to make
the parameters of the disordered atoms more reasonable. CCDC No.
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9
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ASSOCIATED CONTENT
■
2013, 49, 5423−5425.
*
S
Supporting Information
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1
13
H NMR and C NMR data, selected bond lengths and angles,
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HRMS and photophysical spectra, and DFT/TDDFT calcu-
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available free of charge on the ACS Publications website at
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Lyons, A. M. J. Phys. Chem. A 2014, 118, 10364−10371.
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AUTHOR INFORMATION
Corresponding Authors
E-mail: zhaojzh@dlut.edu.cn. (J.-Z.Z.)
E-mail: xingyongheng2000@163.com. (Y.-H.X.)
E-mail: hayvali@science.ankara.edu.tr. (M.H.)
■
*
*
*
(19) Zhao, J.; Wu, W.; Sun, J.; Guo, S. Chem. Soc. Rev. 2013, 42,
5323−5351.
(20) Liu, R.; Azenkeng, A.; Li, Y.; Sun, W. Dalton Trans. 2012, 41,
12353−12357.
(21) Soliman, A. M.; Abdelhameed, M.; Zysman-Colman, E.; Harvey,
P. D. Chem. Commun. 2013, 49, 5544−5546.
22) Kim, K.-Y.; Liu, S.; Kose, M. E.; Schanze, K. S. Inorg. Chem.
006, 45, 2509−2519.
23) Chan, C. K. M.; Tao, C.-H.; Tam, H.-L.; Zhu, N.; Yam, V. W.-
W.; Cheah, K.-W. Inorg. Chem. 2009, 48, 2855−2864.
24) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus,
M.; Khan, M. S.; Raithby, P. R.; Kohler, A.; Friend, R. H. J. Am. Chem.
Soc. 2001, 123, 9412−9417.
25) Liu, Y.; Jiang, S.; Glusac, K.; Powell, D. H.; Anderson, D. F.;
Schanze, K. S. J. Am. Chem. Soc. 2002, 124, 12412−12413.
26) Glusac, K.; Kose, M. E.; Jiang, H.; Schanze, K. S. J. Phys. Chem. B
007, 111, 929−940.
27) Li, Y.; Kose, M. E.; Schanze, K. S. J. Phys. Chem. B 2013, 117,
025−9033.
28) Wu, W.; Zhao, J.; Sun, J.; Huang, L.; Yi, X. J. Mater. Chem. C
Notes
(
2
(
̈
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the NSFC (20972024, 21273028, 21421005, and
1473020), Ministry of Education (SRFDP−-
0120041130005), Program for Changjiang Scholars and
■
2
2
(
̈
(
Innovative Research Team in University [IRT_13R06], the
Fundamental Research Funds for the Central Universities
(
2
(
9
̈
(
2
DUT14ZD226), and Dalian University of Technology (DUT-
013TB07) for financial support.
̈
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