Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing Pt(II)-Schiff Base Complex: Synthesis, Photophysical Studies, and Anti-Stokes Shift Increase in Triplet-Triplet Annihilation Upconversion

Article abstract of DOI:10.1021/acs.inorgchem.7b02989

A boron dipyrromethane (BDP)-containing Pt(II)-Schiff base complex (Pt-BDP), showing ping-pong singlet-triplet energy transfer, was synthesized, and the detailed photophysical properties were investigated using various steady-state and time-resolved transient spectroscopies. Femtosecond/nanosecond transient absorption spectroscopies demonstrated that, upon selective excitation of the BDP unit in Pt-BDP at 490 nm, F?rster resonance energy transfer from the BDP unit to the Pt(II) coordination center occurred (6.7 ps), accompanied by an ultrafast intersystem crossing at the Pt(II) coordination center (<1 ps) and triplet-triplet energy transfer back to the BDP moiety (148 ps). These processes generated a triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet-triplet annihilation (TTA) upconversion through selective excitation of the BDP unit or the Pt(II) coordination center at lower excitation energy. An upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift of 0.65 eV was observed upon excitation of the lower-energy band of the Pt(II) coordination center. We propose that using triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes shift of TTA upconversion.

Full text of DOI:10.1021/acs.inorgchem.7b02989

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing
Pt(II)−Schiff Base Complex: Synthesis, Photophysical Studies, and
Anti-Stokes Shift Increase in Triplet−Triplet Annihilation
Upconversion
Syed S. Razi,†,§ Yun Hee Koo,‡,§ Woojae Kim,‡ Wenbo Yang,† Zhijia Wang,† Habtom Gobeze,⊥
Francis D’Souza,⊥ Jianzhang Zhao,*,† and Dongho Kim*,‡
†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling Gong Road, Dalian 116024, P. R. China
‡Spectroscopy Laboratory for Functional π−Electronic Systems, Department of Chemistry, Yonsei University, Seoul 120-749, Korea
⊥Department of Chemistry, University of North Texas, 1155 Union Circle, P.O. 305070, Denton, Texas 76203-5017, United States
S
* Supporting Information
ABSTRACT: A boron dipyrromethane (BDP)-containing
Pt(II)−Schiff base complex (Pt-BDP), showing ping-pong
singlet−triplet energy transfer, was synthesized, and the
detailed photophysical properties were investigated using
various steady-state and time-resolved transient spectroscopies.
Femtosecond/nanosecond transient absorption spectroscopies
demonstrated that, upon selective excitation of the BDP unit in
Pt-BDP at 490 nm, Forster resonance energy transfer from the
̈
BDP unit to the Pt(II) coordination center occurred (6.7 ps),
accompanied by an ultrafast intersystem crossing at the Pt(II)
coordination center (<1 ps) and triplet−triplet energy transfer
back to the BDP moiety (148 ps). These processes generated a
triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a
complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet−triplet annihilation (TTA)
upconversion through selective excitation of the BDP unit or the Pt(II) coordination center at lower excitation energy. An
upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift
of 0.65 eV was observed upon excitation of the lower-energy band of the Pt(II) coordination center. We propose that using
triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes
shift of TTA upconversion.
excitation and the upconverted emission wavelength (note that
the application of the term anti-Stokes shifts is different from
the original meaning of Stokes shift, which means that the same
electronic states are involved for the excitation and
emission).33−36 Castellano and co-workers used a Ru(II)/
Zn(II) trinuclear complex for TTA upconversion, and the anti-
Stokes shift is up to 0.71 eV.37 Normally, TTA upconversion
with transition-metal complexes as triplet photosensitizers is
with anti-Stokes shifts smaller than 0.5 eV.38−42 Recently, one
promising method was reported to increase the anti-Stokes shift
by using photosensitizers with a relatively strong S0 → T1
absorption because such direct excitation to the T1 state is
possible, the energy loss of the S1 → T1 ISC process in normal
aromatic triplet photosensitizers is avoided, and the anti-Stokes
shift is up to 0.86 eV.43 However, such complexes are limited,
INTRODUCTION
■
Transition-metal complexes have attracted much attention in
recent years because of their potential applications in
photocatalysis,1 photovoltaics,2,3 electroluminescence,4,5 lumi-
nescence bioimaging and molecular sensing,6−13 photodynam-
ics,14−16 fundamental photochemistry studies,17−22 and, more
recently, triplet−triplet annihilation (TTA) upconversion.23−28
Although many transition-metal complexes have been proposed
as triplet photosensitizers for TTA upconversion, developing
efficient triplet photosensitizers is still challenging. One
example is Pt(II) complexes,29−32 which are promising
candidates because of their efficient intersystem crossing
(ISC) and strong absorption of visible light.26,27 Furthermore,
their feasible preparation and derivatization, chemical stability,
and photostability make Pt(II) complexes suitable for TTA
upconversion.
One of the remaining problems in TTA upconversion is the
Received: November 28, 2017
small anti-Stokes shift, i.e., the energy difference between
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02989
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a
Scheme 1. Synthesis of the Complexes
a
The molecular structures of the ligands and reference complex Pt-Ph are also shown: (i) EtOH, acetic acid, reflux; (ii) K2CO3, K2PtCl4, DMSO, 75
°C, 12 h; (iii) pinacol, toluene, 120 °C, 12 h; (iv) (a) 2,4-dimethylpyrrole, trifluoroacetic acid, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, CH2Cl2,
room temperature, 12 h; (b) Et3N, BF3·OEt2, room temperature, 2 h; (v) Pd(PPh3)4, toluene/EtOH/water, Na2CO3; (vi) phenylboronic acid,
Pd(PPh3)4, toluene/EtOH/water, K2CO3.
and in most compounds, the S0 → T1 transition is strongly
forbidden, which leads to a small oscillator strength and thus
weak absorption. Furthermore, because of the strong heavy-
atom effect, which is crucial for the S0 → T1 transition, the
triplet state is short-lived (T1 → S0 ISC is enhanced). Although
S0 → T1 absorptivity is important to improve the anti-Stokes
shifts of TTA upconversion, the S0 → T1 absorptivity is weak,
which is detrimental to the efficiency of TTA upconversion.
Moreover, the triplet-state lifetime of the Os(II) coordination
center is short; thus, much room is left for using Os(II)
complexes as triplet photosensitizers to attain efficient TTA
upconversion with large anti-Stokes shifts and efficient TTA
upconversion. Another strategy to increase the anti-Stokes shift
of TTA upconversion is to use photosensitizers with a small S1/
T1 energy gap and thus a high-T1 triplet-state energy level.
However, for most π-conjugated organic chromophores, such
as boron dipyrromethane (BDP) dyes,44 the S1/T1 energy gap
is large because of the strong electron-exchange effects [J
values, which are proportional to the spatial overlap of the
highest occupied molecular orbital (HOMO)/lowest unoccu-
pied molecular orbital (LUMO); the S1/T1 state energy gap is
2J]. Thus, the energy loss, via the S1 → T1 ISC of the
photosensitizer, is unavoidable for TTA upconversion.
state energy level but a higher T1 state energy level than the
BDP appendant. Finally, the energy is transferred to the T1
state of the BDP unit through intramolecular triplet−triplet
energy transfer (TTET). Ping-pong energy transfer (singlet−
triplet energy transfer) is a known concept, and several
examples have been reported, but it was rarely used for TTA
upconversion.45−55
To achieve the aim of being a spin converter, the
coordination center moiety should satisfy the following
conditions: it should be with a lower S1 state energy level but
a T1 state energy level higher than BDP and, most importantly,
an efficient ISC. Pt(II)−Schiff base complexes can be good
candidates for the spin-converter moiety because they satisfy all
conditions listed above. Although the Pt(II)−Schiff base
complexes are potent spin converters, studies on the photo-
physical properties of this type of phosphorescent Pt(II)−Schiff
base complex are rare.24 Previously, Che et al. reported a series
of Pt(II)−Schiff base complexes and the photoluminescence,
triplet excited state, and electroluminescence properties were
studied.56 Borisov et al. prepared red-light-absorbing Pt(II)−
Schiff base complexes, the application for TTA upconversion
was studied, and the anti-Stokes shift was 0.36 eV.57 These
complexes contained a single chromophore; thus, there was
only one major absorption band in the visible range. Previously,
we prepared Pt(II)−Schiff base complexes attached with
pyrenyl moieties, in which intramolecular triplet energy transfer
was observed and the T1 state was the pyrenyl moiety-localized
3IL state (IL = intraligand), not the conventional 3MLCT state
(MLCT = metal-to-ligand charge transfer).58
Herein, we prepared a Pt(II)−Schiff base complex containing
a visible-light-harvesting BDP appendent (complex Pt-BDP;
Scheme 1). The complex shows broad-band absorption in the
visible spectral region, and it is used for TTA upconversion to
attain a large anti-Stokes shift. Our method it to attach a spin-
converter moiety to the BDP moiety, thus “inserting” singlet
and triplet energy levels between the S1 and T1 state energy
levels of BDP. The Pt(II) coordination framework attached to
BDP serves as a singlet energy acceptor and spin converter to
In this study, we prepared an exemplar Pt(II)−Schiff base
complex containing a BDP chromophore on the coordination
framework (Pt-BDP; Scheme 1). With femtosecond/nano-
second transient absorption (fs/ns-TA) spectroscopy, we
elucidated the kinetics of the forward singlet energy transfer
and backward triplet energy transfer to the BDP chromophore
and the ISC process. We examined the complex as a triplet
collect the S state energy of BDP via intramolecular Forster
̈
1
resonance energy transfer (FRET) and convert the singlet spin
state to the triplet state with the heavy-atom effect of Pt(II).
Note that the Pt(II) coordination framework is with a lower S1
B
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photosensitizer for TTA upconversion. Indeed, through the
ping-pong energy-transfer pathway, we obtained upconversion
quantum yields of up to 10% with the excitation of BDP.
Furthermore, a large anti-Stokes shift of 5290 cm−1 (0.65 eV)
was observed, compared to the direct excitation of the BDP
moiety (anti-Stokes shift of 2660 cm−1, 0.33 eV) upon
excitation of the lowest absorption band of the Pt(II)−Schiff
base complex.
complexes.56 The bond angles among N1−Pt1−N2, N1−Pt1−
O1, N2−Pt1−O2, N2−Pt1−O1, N1−Pt1−O2, and O1−Pt1−
O2 were 83.9(3)°, 94.7(3)°, 95.6(3)°, 176.8(3)°, 176.7(3)°,
and 86.0(2)°, respectively. The bond lengths of Pt1−O1, Pt1−
O2, Pt1−N1, and Pt1−N2 were 1.964, 1.973, 1.940, and 1.921
Å, respectively, which were close to the previously reported
values (1.921−2.015 Å).4 The C−N, C−O, and C−C bond
lengths were in the range of 1.193−1.470 Å, which are close to
the literature values (1.192−1.233 Å).57 The Pt-Ph packing
structure shows an arrangement of molecules at the corners,
which is specific to orthorhombic systems in the dorsal and
front views, and the presence of bulky tert-butyl groups
effectively inhibits intermolecular π−π-stacking interactions of
the aryl rings.
RESULTS AND DISCUSSION
■
Molecular Design and Synthesis. Previously, Pt(II)−
Schiff base complexes containing no linked chromophores were
reported and examined as triplet sensitizers for TTA
upconversion.57 Recently, we prepared a pyrenyl-containing
Pt(II)−Schiff base complex and observed the triplet-state
Steady-State UV−Vis Absorption and Emission Spec-
tra. The UV−vis absorption of Pt-BDP and the reference
compounds Pt-Ph and BDP was studied (Figure 2). The
58
3
equilibrium and population of the IL state. To improve the
light-harvesting efficiency of chromophores and attain efficient
triplet-state localization on the linked chromophore rather than
3
the Pt−Schiff base complex-localized MLCT state thus to
extend the triplet-state lifetime, we synthesized a new system
combining a Pt(II)−Schiff base coordination center and a BDP
appendant (Pt-BDP; Scheme 1). The BDP chromophore was
selected as an appendant because of its feasible derivatization,
high photostability, strong absorption of visible light, and, most
importantly, lower triplet-state energy and longer triplet-state
lifetime than those of the Pt(II)−Schiff base complex.59−65
Pt(II),66−69 Ir(III),70−72 Ru(II),73−75 and Re(I) complexes76
containing BDP ligands have been reported, but this is the first
time that a Pt(II)−Schiff base complex linked with BDP has
been reported.
Figure 2. (a) UV−vis absorption spectra of Pt-BDP, Pt-Ph, and BDP.
(b) Photoluminescence spectra of Pt-BDP under air and N2
atmospheres. λex = 475 nm. c = 1.0 × 10−5 M in toluene, 20 °C.
The synthesis of Pt-BDP is outlined in Scheme 1. The ligand
1 was synthesized from 3,5-di-tert-butylsalicylaldehyde and 4-
bromo-1,2-diaminobenzene under reflux conditions. Then the
metalation of the Schiff base with K2PtCl4 gives the
intermediate Pt(II) complex 2. The Suzuki coupling reaction
of complex 2 with phenylboronic acid gave the reference
complex Pt-Ph. The Suzuki coupling reaction of BDP 4 with 2
gave the target Pt(II) complex (Pt-BDP) in 38% yield. All
compounds were well characterized using H NMR and high-
resolution mass spectrometry (HRMS). Single-crystal X-ray
analysis was carried out for Pt-Ph.
Single-Crystal Molecular Structure. The molecular
structure of Pt-Ph was determined by single-crystal X-ray
diffraction (Figure 1 and Table S1). For Pt-Ph, the Pt(II)
coordination centers formed an orthorhombic crystal system
with square-planar geometries. Bond lengths and angles were
consistent with those reported for typical Pt(II)−Schiff base
complex Pt-Ph shows a low-energy band at 575 nm (ε = 6.0 ×
103 M−1 cm−1), which can be assigned to the S0 → 1MLCT/1IL
transitions of the Pt(II) coordination center, while the higher-
energy bands at 486 nm (ε = 5.0 × 103 M−1 cm−1) and 393 nm
(ε = 2.4 × 104 M−1 cm−1) can be assigned to the π−π*
transition of the coordinated Schiff base ligand.56 Pt-BDP
shows one major absorption band at 578 nm (ε = 1.1 × 104
1
1
M−1 cm−1), due to the S0 → MLCT/1IL transition, and a
second strong absorption band at 504 nm (ε = 7.8 × 104 M−1
cm−1), which originates from the BDP chromophore. The same
strong band is observed in the BDP absorption spectrum.
The absorption spectrum of Pt-BDP resembles the sum of
the Pt-Ph and BDP spectra, and the peaks from neither the Pt−
Schiff base complex nor the linked BDP unit are shifted. These
features suggest that there are no significant electronic
interactions between the Pt(II) coordination center and the
BDP chromophore at the ground state.45−47
Under aerated conditions, Pt-BDP shows a single emission
band at 515 nm, which can be assigned to the residual
fluorescence of the BDP unit (Figure 2b). Under a N2
atmosphere, an additional red-shifted emission band at 652
nm was observed, which can be assigned to phosphorescence
from the Pt(II) coordination center. This assignment is
supported by the phosphorescence of Pt-Ph observed at 650
nm (Table 1). Similar results were observed in other solvents
such as dichloromethane (CH2Cl2), acetonitrile (CH3CN), and
methanol (MeOH) (Figure S13). We found that Pt-BDP is
unstable upon photoirradiation; the resulting slight decom-
position may induce a stronger emission at 515 nm.
To investigate the characteristics of the emissive states in Pt-
BDP and Pt-Ph, the emission spectra at 77 K are also studied
Figure 1. Perspective view of the single-crystal molecular structure of
Pt-Ph with 50% thermal ellipsoid (H atoms are omitted for clarity).
C
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Table 1. Photophysical Properties of the Pt(II) Complexes and Reference Compounds
a
b
a
e
g
j
λabs /nm
ε
λem /nm
ΦL/%
ΦΔ /%
ΦT / %
τL/ns
τT /μs
c
d
e
f
h
Pt-BDP
Pt-Ph
BDP
396
504
578
393
486
575
473
503
4.5
515/650
0.20 /0.28
77 /51
89
τF = 0.085 ns (99%), 4.8 ns (1%)
103.2
i
7.8
τP = 18.3 ns (98%) 3.9 μs (2%)
1.1
c
d
e
f
h
2.4
650
4.4 /11
60 /58
40
0.77 ns
3.5
i
0.05
0.06
2.12
3.7 μs
c
526
83
3.8 ns
10.5
a
b
c
In toluene (1.0 × 10−5 M), 20 °C. Molar extinction coefficient at the absorption maxima. ε = 104 M−1 cm−1. Fluorescence quantum yields.
d
DiiodoBDP (ΦF = 2.7% in CH3CN) was used as the standard. Phosphorescence quantum yields. DiiodoBDP (ΦF = 2.7% in CH3CN) was used as
e
f
the standard. The quantum yield of 1O2 with diiodoBDP (ΦΔ = 70% in CH3OH; λex = 500 nm) is used as the standard. The quantum yield of 1O2
g
with styrylBDP (59% in toluene; λex = 580 nm) is used as the standard. Triplet-state quantum yield, with diiodoBDP (ΦT = 87% in CH2Cl2; λex
=
h
i
j
532 nm) as the standard. Fluorescence lifetimes. Phosphorescence lifetimes. Triplet-state lifetime determined with ns-TA spectroscopy. λex = 532
nm. c = 1.0 × 10−5 M in degassed toluene. The estimated error is ca. 5%.
(Figure 3). For both Pt-BDP and Pt-Ph, the emission band at
Pt(II) coordination center. By calculating the Gibbs free-energy
changes of the putative PET process, we find that PET is
prohibited in toluene (see the electrochemical study section);
thus, we expect that fluorescence is quenched by the FRET
process. Similarly, phosphorescence quenching of the Pt(II)
coordination center in Pt-BDP can be attributed to TTET to
the BDP moiety because PET is also prohibited in this case.
The excitation spectra of Pt-BDP and Pt-Ph are compared
with the UV−vis absorption spectra (Figure 5).78−80 The
645 nm is blue-shifted by about 470 cm−1 at 77 K compared to
Figure 3. Normalized emission spectra of (a) Pt-BDP and (b) Pt-Ph
in degassed MeOH/EtOH (1:4, v/v) at room temperature and 77 K
(frozen solution). λex = 445 nm. c = 1.0 × 10−5 M.
the room temperature emission band (Figure 3). This large
thermally induced increase in the emission energy observed in a
rigid matrix indicates the MLCT feature of the emissive triplet
state of the Pt(II)−Schiff base complex. The emissive triplet
state of the pendant BDP chromophore in Pt-BDP is expected
to be located in the lower-energy region (>730 nm).77
Compared to BDP and Pt-Ph, Pt-BDP shows significantly
quenched fluorescence and phosphorescence emission (Figures
4 and S14). In Pt-BDP, because BDP and the Pt(II)
coordination center do not show electronic interactions, the
possible deactivation pathways of the emissive singlet and
triplet states can be attributed to two major processes: FRET
and photoinduced electron transfer (PET) from BDP to the
Figure 5. Comparison of the normalized UV−vis absorption and
luminescence excitation spectra of (a) Pt-BDP (λem = 650 nm) and
(b) Pt-Ph (λem = 650 nm). c = 1.0 × 10−5 M in degassed toluene, 20
°C.
phosphorescence emission of the Pt(II) coordination center at
650 nm in Pt-BDP and Pt-Ph was monitored in the excitation
spectra. Both complexes show decreased excitation bands above
400 nm compared to the UV−vis absorption spectrum (with
normalization at the absorption maximum, ca. 570 nm). Similar
phenomena have been observed for naphthalenediimide and
perylenebisimide chromophores.81−83
Besides the change at higher-energy bands, Pt-BDP shows a
comparably large excitation peak at 503 nm, which corresponds
to the absorption band of the BDP moiety (Figure 5a). Because
the Pt(II) coordination center and BDP moiety have almost no
electronic interaction, phosphorescence emission of the Pt(II)
coordination center upon excitation of BDP clearly indicates
FRET from BDP to the Pt(II) coordination center. The FRET
efficiency is estimated to be approximately 47%.78
The singlet-oxygen (1O2) photosensitizing ability of the
complexes was studied (Table 1; for details, see Figure S15).
Figure 4. Comparison of the emission spectra of (a) Pt-BDP and
BDP (λex = 475 nm) and (b) Pt-BDP and Pt-Ph (λex = 550 nm).
Optically matched solutions were used. c = ca. 1.0 × 10−5 M in
degassed toluene, 20 °C.
1
1,3-Diphenylisobenzofuran was used as an O2 scavenger for
monitoring 1O2 production. Given that no intramolecular
energy transfer occurred, photoexcitation of Pt-BDP at 500 nm
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will not show significant 1O2 photosensitizing because the
absorption band at 500 nm corresponds to BDP excitation.
1
However, the results show that Pt-BDP gives O2 quantum
yields (ΦΔ) of 77% upon photoexcitation at 500 nm, similar to
Pt-Ph (Table 1). Thus, we can deduce that singlet energy
transfer would occur upon excitation of the BDP moiety in Pt-
BDP, followed by a TTET that would finally localize the triplet
state on the BDP unit.
Time-Resolved Photoluminescence Spectroscopy. For
BDP, a luminescence decay with a monoexponential profile was
observed (Figure 6a), with the lifetime determined as 3.8 ns;
Figure 7. Cyclic voltammograms of (a) Pt-BDP and BDP and (b) Pt-
BDP and Pt-Ph. Ferrocene (Fc0/+) is used as an internal reference. In
degassed CH2Cl2 solutions containing 1.0 mM photosensitizers and Fc
(0.5 mM), 0.10 M Bu4NPF6 is the supporting electrolyte and Ag/
AgNO3 is the reference electrode. Scan rate: 50 mV s−1. T = 20 °C.
reduction waves were observed, and the E1/2 values are −1.53
and −1.86 V, respectively. Reversible oxidation waves were
observed, with potentials of +0.63 and +0.80 V, respectively
(Figure 7b). This result indicates that there is no electronic
interaction between the two units in Pt-BDP; otherwise, the
redox potentials of Pt-BDP would be different from those of
the reference compounds (Table 2).
Figure 6. (a) Fluorescence decay curves of Pt-BDP (λem = 515 nm)
and BDP (500 nm). Excited with a 445 nm picosecond pulsed laser.
(b) Phosphorescence decay curves of Pt-BDP (λem = 650 nm) and Pt-
Ph (λem = 650 nm). Excited with a 445 nm picosecond pulsed laser; c
= 1.0 × 10−5 M in deaerated toluene, 20 °C.
ΔGCS = e[EOX − ERED] − E00 + ΔGS
(1)
e2
e2
8πε0
1
1
RA
1
1
εS
⎛
⎞⎛
⎞
⎟
⎠
ΔGS = −
−
+
−
⎜
⎝
⎟⎜
⎠⎝
4πεSε0RCC
R
ε
D
REF
(2)
this is assigned as the S1 state lifetime. For Pt-Ph, the
phosphorescence lifetime was determined to be 3.7 μs, also
with a monoexponential decay feature (Figure 6b).
Table 2. Electrochemical Redox Potentials of Pt-BDP, Pt-Ph,
a
and BDP
On the other hand, for Pt-BDP, the fluorescence decay of the
BDP moiety is with biexponential character of 0.085 ns (ca.
99%) and 4.8 ns (ca. 1%). Because the BDP moiety is far away
from the Pt(II) center (9.4 Å), the π core of the BDP
fluorophore is not π-conjugated with the coordination center
and PET is thermodynamically inhibited (see the later section).
Thus, we propose that the main fluorescence-quenching
pathway is FRET from the BDP moiety to the Pt(II)
coordination center. On the basis of the fs-TA spectral changes,
i.e., growing in of the spectral features associated with the Pt(II)
center and decay of the BDP-based absorbance, we calculate
the FRET rate constant to be kFRET = 1.5 × 1011 s−1 (see the fs-
TA study section).
In the case of phosphorescence (Figure 6b), we attribute the
quenched phosphorescence of the Pt(II) coordination center in
Pt-BDP to intramolecular TTET from the Pt(II) coordination
center to the BDP moiety. The shorter phosphorescence
lifetime at the Pt(II) coordination center of Pt-BDP (18.3 ns/
98% and 3.9 μs/2%), compared to Pt-Ph (3.7 μs), indicates the
existence of quenching channels for the emissive triplet state of
the Pt(II) coordination center, presumably TTET.
compound
EOX/V
ERED/V
Pt-BDP
Pt-Ph
BDP
+0.80, +0.63
+0.61
−1.53, −1.86
−1.83
−1.55
+0.76
a
Cyclic voltammetry in Ar-saturated CH2Cl2 containing a 1.0 mM
photosensitizer and a 0.10 M Bu4N[PF6] supporting electrolyte. A Pt
counter electrode, a glassy carbon working electrode, and an Ag/
AgNO3 reference electrode were used. Fc was used as an internal
reference. Scan rate: 50 mV s−1. T = 20 °C.
To study the possibility of intramolecular PET, the ΔGCS
values of Pt-BDP in several solvents were calculated using the
Weller equation (eqs 1 and 2).84−87 ΔGS is the static
Coulombic energy (eq 2), where e = electronic charge, EOX is
the half-wave potential for one-electron oxidation of the
electron-donor unit (+0.58 V), ERED is the half-wave potential
for one-electron reduction of the electron-acceptor unit (−1.57
V), E00 is the energy level for the singlet excited state
approximated by the fluorescence emission (510 nm and 2.43
eV) or the triplet excited state approximated with the
phosphorescence emission (562 nm and 2.21 eV), εS is the
static dielectric constant of the solvent, RCC is the center-to-
center separation distance determined by density functional
theory (DFT) optimization of the geometry (10.63 Å), RD is
the radius of the electron donor (4.36 Å), RA is the radius of the
electron acceptor (6.86 Å), εS is the static dielectric constant of
the solvent used for electrochemical studies, and ε0 is the
permittivity of free space. The solvents used are toluene (εS =
2.4), CH2Cl2 (εS = 8.9), and CH3CN (εS = 37.5). The energies
of the CSS (ECS) and the free-energy change of the charge
The photophysical properties of Pt-BDP, Pt-Ph, and BDP
1
are compiled in Table 1. The O2 quantum yield (ΦΔ) of Pt-
BDP is as high as 77% (λex = 500 nm), and the quantum yield
of triplet formation in Pt-BDP is ΦT = 89% (λex = 532 nm).
Electrochemical Study: Gibbs Free-Energy Changes
(ΔGCS) of PET and the Charge-Separated State (CSS)
Energy Levels. The redox properties of the complexes are
studied using cyclic voltammetry (Figure 7). For Pt-Ph, a
reversible oxidation wave was observed, and the half-wave
potential (E1/2) is +0.61 V. A reversible reduction is observed
with E1/2 of −1.83 V (Figure 7b). For Pt-BDP, reversible
E
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a
Table 3. Free-Energy Changes of Pt-BDP: ΔGCS, ECSS, and ΔGS
electron transfer
solvent
ΔGCS/eV
ECSS/eV
ΔGS/eV
b
c
d
e
Pt(II) center → BDP
toluene
CH2Cl2
CH3CN
+0.77 /+0.55
+2.9/+2.9
+2.0/+1.9
+1.8/+1.9
+1.9/+1.9
+0.83/+0.83
−0.14/−0.14
−0.26/−0.27
d
e
e
e
−0.19 /−0.42
d
−0.32 /−0.54
d
MeOH
−0.31 /−0.53
−0.26/−0.26
a
b
c
d
e
The arrow indicates the direction of charge transfer. Electron donor. Electron acceptor. Energy level of the triplet state of BDP as E00. Energy
level of the singlet state of BDP as E00.
recombination process (ΔGCR) are calculated using eqs 3 and
section). This process is a TTET from the T1 state of the Pt
coordination center to the T1 state of the BDP moiety.
However, ISC in the Pt(II) coordination center of Pt-BDP was
not observed, presumably because it is extremely fast, similar to
other Pt−Schiff base complexes that exhibit subpicosecond ISC
time constants.88 This ultrafast ISC process at the Pt−Schiff
base is further supported by the fs-TA spectra of Pt-Ph upon
photoexcitation at 560 nm (Figure S17), which shows the
ultrafast evolution of the T1 state spectral features of Pt-Ph
within 1 ps.
Using a global target analysis assuming four states [the S1
state of BDP, S1 and T1 states of the Pt(II)−Schiff base
complex, and T1 state of BDP] that are involved in the excited-
state dynamics, consecutively, and assuming that ISC at the Pt−
Schiff base coordination framework and TTET are too fast for
the S1 and T1 states of the Pt−Schiff base to be resolved, we can
extract the species-associated difference spectroscopy (SADS)
spectra of each state and the rate constants of the FRET and
TTET processes (Figure 8c and Table 4). Species A shows a
4.
ECS = e[EOX − ERED] + ΔGS
(3)
ΔGCR = −(ΔGCS + E00)
(4)
The Gibbs free-energy changes for putative electron transfer
are positive (Table 3), indicating that the intramolecular PET
in Pt-BDP and Pt-Ph is thermodynamically unfavorable. This
conclusion agrees well with the luminescence emission spectra
and triplet-state study of the complexes.
fs-TA Spectroscopy: Intramolecular Singlet−Triplet
Energy Transfer (Ping-Pong Energy Transfer). To
investigate the excited-state dynamics of the Pt-BDP complex,
fs-TA measurements were carried out (Figure 8). First, the
Table 4. FRET, TTET, and ISC Rate Constants of the
Complexes Using Ultrafast TA Spectroscopy (in Toluene)
a
b
compound
kFRET /s−1
kTTET /s−1
Pt-BDP
Pt-Ph
1.5 × 1011
6.8 × 109
a
b
FRET rate constants. TTET rate constants.
strong GSB at 510 nm and a relatively weak ESA, which is
assigned to the S1 state of BDP, while species B is assigned to
the T1 state of the Pt−Schiff base coordination center because
it shows spectral features similar to those of the T1 state of Pt-
Ph observed in the ns-TA spectra (see the ns-TA section).
Thus, the spectral change A → B represents a FRET process
from the S1 state of the BDP moiety to that of the Pt−Schiff
base coordination framework with a time constant of 6.7 ps
(kFRET = 1.5 × 1011 s−1), followed by ultrafast ISC in the Pt−
Schiff base coordination framework. Species C corresponds to
the T1 state of the BDP moiety, which can be identified by the
resolved spectrum being similar to the T1 state of the BDP
moiety (see the ns-TA section). Therefore, process B → C
represents triplet−triplet back-energy transfer from the T1 state
of the Pt−Schiff base framework to the T1 state of the BDP
moiety with a time constant of 148 ps (kTTET = 6.8 × 109 s−1).
Next, the excitation wavelength is tuned to 560 nm,
corresponding to the S0 → S1 transition of the Pt(II)−Schiff
base coordination framework (Figure 9a). From 1 ps to 5 ns,
we observed a rise in the GSB band of the BDP moiety at 510
nm and a simultaneous decay of the ESA band of the complex
at 600−700 nm. This spectral change can be ascribed to TTET
from the T1 state of the Pt−Schiff base complex to that of the
BDP moiety. Because of the fast time constant, the ISC of the
Figure 8. fs-TA spectra of Pt-BDP (a) from 1 to 20 ps and (b) from
30 ps to 5 ns. (c) SADS spectra obtained by global target analysis. In
toluene (490 nm excitation).
complex Pt-BDP was excited at 490 nm, which corresponds to
the S0 → S1 transition of the BDP ligand. From 1 to 20 ps, we
observed a decay of the ground-state bleach (GSB) band of the
BDP moiety at 510 nm and a simultaneous rise in the excited-
state absorption (ESA) band in the 600−700 nm region. These
spectral changes are ascribed to FRET from the BDP moiety to
the Pt−Schiff base coordination framework. After the initial
spectral evolution, from 30 ps to 5 ns, we observed a rise in the
BDP GSB band at 510 nm and the ESA band at ca. 640 nm,
which corresponds to the spectral features of the BDP-moiety-
localized T1 state, as observed in ns-TA studies (see the ns-TA
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Figure 9. (a) fs-TA spectra measured from 1 ps to 5 ns for Pt-BDP in
toluene (560 nm excitation). (b) SADS spectra of Pt-BDP in toluene
obtained by global target analysis.
Figure 10. Time-resolved ns-TA spectra: (a) Pt-Ph, (b) decay trace of
Pt-Ph at 391 nm, (c) Pt-BDP, and (d) decay trace of Pt-BDP at 504
nm. λex = 532 nm laser. c = 1.0 × 10−5 M in degassed toluene, 20 °C.
Pt(II) coordination center in Pt-BDP was not observable at
560 nm.
Schiff base complex exerted on the BDP unit is not very
significant.
The photophysical process of Pt-BDP is summarized in
Scheme 2. Upon selective excitation of the BDP moiety, FRET
Furthermore, by using a global target analysis assuming that
three states (the S1 and T1 states of the Pt(II) coordination
center and T1 state of the BDP moiety) are involved in the
excited-state dynamics, the generation of these states is in a
consecutive order, and because the ISC process in the Pt−
Schiff base is ultrafast, we can extract the SADS spectrum of
each state and the ISC rate constants (Figure 9b and Table 4).
Species B and C can be assigned as the S1 state of the Pt−Schiff
base complex and the T1 state of the BDP moiety, respectively,
because they show SADS spectra almost identical with those of
species B and C obtained at 490 nm. Therefore, B → C
represents TTET in the Pt−Schiff base complex with a time
constant of 143 ps (kTTET = 7.0 × 109 s−1), following the
ultrafast ISC of the Pt(II)−Schiff base coordination center.
ns-TA Spectroscopy. To study the triplet-state properties,
i.e., the lifetime and spatial localization of the triplet state of the
complexes, the ns-TA spectra of Pt-Ph and Pt-BDP were
measured (Figure 10). For Pt-Ph, upon pulsed-laser excitation
at 532 nm, GSB bands at 391 nm as well as broad ESA bands in
the range of 400−700 nm were observed (Figure 10a). The
lifetime of this transient signal is measured to be 3.5 μs (Figure
10b) in degassed solutions and 0.5 μs in aerated solutions
(Figure S18). From the significantly reduced lifetime in aerated
solutions, we can assign this transient signal as the triplet state
of Pt-Ph.
For Pt-BDP, an intense GSB band at 504 nm was observed
upon nanosecond pulsed-laser excitation at 532 nm (Figure
10c). This excitation wavelength corresponds to the S0 → S1
transition of the Pt(II)−Schiff base coordination center;
however, the resulting transient signal corresponds to the T1
state of the BDP moiety, as identified by a GSB band at 504
nm, which corresponds to the ground-state absorption of BDP
(Figure 10c). Furthermore, the ESA bands also agree with
absorption of the BDP triplet state reported previously.89−91
This result further supports fast TTET from the Pt(II)
coordination center to the BDP moiety, as observed in fs-TA
studies. The triplet-state lifetime was determined to be 103.2
μs, which is much longer than the BDP triplet state accessed
with the heavy-atom effect, such as in 2,6-diiodoBDP (57.1
μs).44 This implies that the heavy-atom effect from the Pt−
Scheme 2. Photophysical Processes of Pt-BDP upon
a
Excitation at 490 nm and 570 nm
a
Note the ping-pong energy-transfer processes.
from the BDP moiety to the Pt−Schiff base complex occurs.
This is followed by fast ISC at the Pt−Schiff base complex.
Backward TTET from the Pt−Schiff base complex to the BDP
moiety is observed with a time constant of ca. 140 ps. Thus, we
can conclude that the T1 state of complex Pt-BDP is exclusively
localized on the BDP moiety.
DFT Calculations. The photophysical properties of the
complex are rationalized with DFT and time-dependent DFT
(TDDFT) calculations (Figure 11). We find that the molecular
orbitals are highly localized, on either the Pt(II) center or the
BDP moiety. This result is in accordance with the spectroscopic
measurements. There are almost no electronic interactions
between the two moieties in the ground state. The lowest
singlet state (S1 state, HOMO → LUMO+1) of Pt-BDP is
localized on the Pt(II) coordination framework, and the
calculated transition energy of 2.29 eV (542 nm) is close to the
experimental values (2.17 eV and 530 nm). The lowest-energy
transition between the singlet states localized on the BDP
moiety is the S0 → S6 transition (HOMO−1 → LUMO), with
G
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internal conversion T3 → T1 may occur, which corresponds to
the intramolecular TTET process from the Pt(II) coordination
center to BDP. The heavy-atom effect for this BDP-localized
triplet state is confirmed to be weak because there are only
small electronic interactions between BDP and the Pt(II)
coordination center. This has been proven both spectroscopi-
cally and computationally. Detailed information on the excited-
state energy levels and electronic configurations is presented in
Table S2.
To determine where the lowest-energy triplet states are
localized in Pt-BDP and Pt-Ph, the spin-density surfaces of
these complexes are calculated (Figure 12). The spin-density
surface of Pt-Ph is mainly distributed on the Schiff base ligand,
the Pt(II) metal center, and the pendent phenyl ring. This
calculation agrees with the MLCT/IL state feature. For Pt-
BDP, on the other hand, the spin-density surface is exclusively
localized on the BDP moiety, in accordance with the ns-TA
results, which proposes a long-lived triplet state localized at
BDP in Pt-BDP.
TTA Upconversion: Benefit of Ping-Pong Energy
Transfer for Attaining a Large Anti-Stokes Shift.
Complexes Pt-BDP and Pt-Ph are examined as triplet
photosensitizers for TTA upconversion (Figure 13). On the
Figure 11. Electron density maps of the frontier molecular orbitals of
Pt-BDP, calculated using DFT at the B3LYP/GENECP/LANL2DZ
level with Gaussian 09W.
an excitation energy of 2.86 eV (433 nm), which is higher than
the experimental value of 530 nm (Figure 11). The calculated
energy is higher than the experimental value, which can be
explained by well-known reports that the DFT method usually
overestimates the excitation energy for BDP chromo-
phores.92−94
For the T3 state (2.01 eV) localized on the Pt(II)
coordination center, it is close in energy to the coordination-
center-localized S1 state (2.29 eV). The small energy difference
between the S1 and T3 states shows the possibility for fast ISC
between the two states. Interestingly, the lowest excited triplet
state (T1 state) is a BDP-localized state; thus, nonradiative fast
Figure 13. TTA upconversion with (a) Pt-BDP and (b) Pt-Ph as
triplet photosensitizers (c = 1.0 × 10−5 M) and perylene as the triplet
acceptor (c = 4.0 × 10−5 M) in degassed toluene, 25 °C. P stands for
photoexcitation of both Pt-BDP and Pt-Ph at 510 and 589 nm
without perylene. P1 stands for photoexcitation at 589 nm with the
addition of perylene. P2 stands for photoexcitation at 510 nm with the
addition of perylene. P3 stands for simultaneous photoexcitation with
510 and 589 nm using a 4 mW continuous-wave (cw) laser (the two
laser beams are in quasi-collinear geometry) with the addition of
perylene. Asterisks indicate laser scattering.
Figure 12. Spin-density surfaces of (a) Pt-Ph and (b) Pt-BDP at the optimized triplet-state geometry calculated at the B3LYP/GENCEP/
LANL2DZ level with Gaussian 09W.
H
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basis of the excited-state energy levels of the complexes, we
select perylene as the triplet acceptor/emitter for the
upconversion studies.44,91 The excitation wavelength was
selected as 510 and 589 nm, which correspond to the S0 →
S1 transition of the BDP moiety and the Pt(II)−Schiff base
coordination framework, respectively.
The upconversion quantum yields of Pt-Ph and Pt-BDP
were calculated from the upconverted fluorescence with 510
and 589 nm excitation (Table 5 and Figure 13). For Pt-Ph,
It should be pointed out that a large anti-Stokes shift in the
TTA upconversion can also be achieved with other molecular
systems, for instance, those based on the combination of a
perylene acceptor and benzoporphyrin sensitizers having rather
small S1/T1 state energy gaps.95 However, the large anti-Stokes
shifts of the TTA upconversion of the current molecular
systems are based on excited-state tuning of the triplet
photosensitizer; i.e., the T1 state of the BDP ligand was kept,
but the excitation wavelength was red-shifted by attachment of
the Pt(II) center and the FRET in the complex.
Excitation with the quasi-collinear geometry of 510/589 nm
laser beams, i.e., the simultaneous excitation with a two-color
laser, was also studied for Pt-Ph and Pt-BDP (Figure 13).
Interestingly, the upconversion intensity is measured to be
roughly the sum of the upconversions upon excitation by a 510
or 589 nm laser individually.
To study the origin of the efficient upconversion with Pt-
BDP as a triplet photosensitizer versus that with Pt-Ph, the
intermolecular TTET between the Pt(II) complexes and
perylene was studied with triplet-state lifetime changes in the
complexes in the presence of perylene (Table 5 and Figure 15).
Table 5. Triplet Excited-State Lifetimes (τT), Stern−Volmer
Quenching Constant (KSV), and Bimolecular Quenching
Constant (Kq) of Pt-BDP and Pt-Ph in Degassed Toluene at
20 °C
b
c
KSV/×104 kq /×108 k0 /×1010
d
g
τT/μs
M−1
8.3
M−1 s−1
7.8
M−1 s−1
1.46
ΦUC /% f /%
a
e
Pt-
BDP
107.7
10.0 /
5.34
f
1.0
a
e
Pt-Ph
3.87
0.3
7.8
1.25
2.0 /
0.02
6.24
f
a
b
Excited with a 532 nm cw laser. Bimolecular quenching constant.
c
KSV = kqτ0. Diffusion-controlled bimolecular quenching rating
constant. Upconversion quantum yields. Excited with a 510 nm
d
e
cw laser (4 mW), with 2,6-diiodoBDP as the standard (ΦF = 3.6% in
f
CH2Cl2). Excited with a 589 nm cw laser (4 mW), with styrylBDP as
g
the standard (ΦF = 59% in CH2Cl2). Quenching efficiency.
barely any upconversion was observed and the upconversion
quantum yields were determined as 2% and 0.02% at the two
excitation wavelengths, respectively. On the other hand, under
the same conditions (laser power, photosensitizer concen-
tration, and triplet acceptor), the upconversion quantum yields
with Pt-BDP as the triplet photosensitizer were measured to be
10% and 1%, respectively, which are almost 5 times higher than
those of Pt-Ph. The upconversion of Pt-BDP is clearly visible
to the unaided eye (Figure 14).
Figure 15. Stern−Volmer plots generated from the triplet excited-state
lifetime (τT) quenching curves of Pt-BDP (c = 1.0 × 10−5 M) and Pt-
Ph (c = 1.0 × 10−5 M) measured with increasing concentrations of
perylene (c = 1.0 × 10−5 M). The triplet-state lifetime of the
complexes is measured with a 532 nm nanosecond-pulsed-laser
excitation in degassed toluene at 20 °C.
Note that Pt-BDP shows a 28 times larger Stern−Volmer
quenching constant than that of Pt-Ph (Ksv = 8.3 × 104 M−1 for
Pt-BDP and Ksv = 0.3× 104 M−1 for Pt-Ph). This can be
attributed to the longer triplet-state lifetime of Pt-BDP due to
localization of the triplet state on the BDP unit, which is crucial
for an efficient upconversion.
Interestingly, the anti-Stokes shift for the upconversion with
Pt-BDP at 589 nm excitation increases to 0.65 eV, which is
comparable to a previously reported 0.86 eV for an OsII(bpy)3
complex showing a large anti-Stokes shift.43 However, the
reported OsII(bpy)3 complex shows very low upconversion
yields in the solution phase (ΦUC = 0.0014%), at a very high
excitation power density (212 W cm−2) because of its small
extinction coefficient and short triplet-state lifetime (ε = 3200
M−1 cm−1 at 888 nm for the S0 → T1 transition and τT = 12
ns).43 In contrast, Pt-BDP shows a higher upconversion yield,
reaching 1%. The upconversion quantum yield of Pt-BDP is
even higher than the previously reported sensitizers excited
with near-IR light (<1%), which also have comparable anti-
Stokes shifts of 0.70 and 0.86 eV.37
Figure 14. Photographs of the emissions of triplet photosensitizers
alone and TTA upconversion: (a) TTA upconversion with Pt-BDP
and Pt-Ph excited with a 510 nm laser. (b) CIF diagram of the
upconversion with 510 nm laser excitation. (c) TTA upconversion
with Pt-BDP and Pt-Ph at 589 nm laser excitation. (d) CIF diagram of
the upconversion with 589 nm laser excitation with Pt-BDP and Pt-Ph
as the triplet photosensitizers (c = 1.0 × 10−5 M) and perylene (Py) as
the acceptor (4.0 × 10−5 M). Excited with a 4 mW cw laser (λex = 510
and 589 nm), 20 °C in toluene.
Herein with Pt-BDP, we propose a new method to obtain
TTA upconversion from a broad-band light source, high
upconversion quantum yields, and large anti-Stokes shift
utilizing ping-pong energy transfer. By linking a Pt−Schiff
I
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(d, 2H, J = 8.0 Hz), 7.40 (s, 1H), 7.36 (s, 1H), 6.02 (s, 2H), 2.58 (s,
6H), 1.60 (s, 18H), 1.50 (s, 6H), 1.37 (s, 18H). 13C NMR (125 MHz,
CDCl3): δ 164.5, 155.8, 148.5, 148.4, 145.8, 143.0, 141.6, 138.6, 137.8,
134.8, 131.4, 129.0, 127.7, 125.4, 121.4, 120.8, 115.1, 113.1, 36.2, 34.0,
31.3, 29.7. TOF MALDI HRMS. Calcd ([C55H63BF2N4O2Pt]+): m/z
1055.4660. Found: m/z 1055.4642.
base coordination framework to the BDP moiety, we can
efficiently generate a triplet state localized in the BDP moiety
over a wide range of visible-light sources through efficient ISC
and TTET. A large anti-Stokes shift can be obtained by
excitation at 589 nm, where the Pt(II) coordination center only
absorbs moderately. In this case, an anti-Stokes shift up to 5290
cm−1 (0.65 eV) is obtained, which is larger than the anti-Stokes
shift obtained by excitation of the BDP moiety (2660 cm−1,
0.33 eV), with perylene as the triplet acceptor/emitter.
Synthesis of Pt-Ph. Under an Ar atmosphere, 2 (80.0 mg, 0.10
mmol), phenylboronic acid (18.0 mg, 0.15 mmol), K2CO3 (42.0 mg,
0.30 mmol), and EtOH/toluene/water (20 mL, 2:4:1, v/v/v) were
mixed together. Then, Pd(PPh3)4 (9.0 mg, 0.007 mmol, 5 mol %) was
added. The mixture was refluxed and stirred under an Ar atmosphere
for 10 h. Then the mixture was cooled to room temperature. The
reaction mixture was extracted with CH2Cl2, washed with water (2 ×
100 mL), and dried over anhydrous Na2SO4. The solution was
evaporated to dryness under reduced pressure to give a crude solid,
which was further purified with column chromatography (silica gel,
CH2Cl2) to yield Pt-Ph as a dark red solid (38 mg, yield 40%). Mp:
>200 °C. 1H NMR (CDCl3, 500 MHz): δ 8.92 (s, 1H), 8.87 (s, 1H),
8.10 (s, 1H), 8.03 (d, 1H, J = 8 Hz), 7.69 (d, 4H, J = 8.0 Hz), 7.52 (t,
3H, J1 = 4 Hz, J2 = 8 Hz,), 7.44 (t, 1H, J1 = 8 Hz, J2 = 4 Hz), 7.34 (d,
2H, J = 8.0 Hz), 1.59 (s, 18H), 1.36 (s, 18H). 13C NMR (125 MHz,
CDCl3): δ 164.4, 164.3, 148.5, 145.6, 144.5, 141.5, 140.1, 139.6, 137.6,
131.0, 129.0, 127.1, 125.6, 120.8, 115.0, 113.2, 36.3, 34.1, 31.3, 29.5.
TOF MALDI HRMS. Calcd ([C42H50N2O2Pt]+): m/z 810.3598.
Found: m/z 810.3570.
X-ray Crystallography. Pt-Ph (20 mg) was dissolved in CH2Cl2
(4 mL) in small test tube, and hexane (6 mL) was added slowly. After
a few days of slow evaporation of the solvent at room temperature, red
needlelike crystals were obtained, and the crystals were used for single-
crystal X-ray diffraction analysis. The crystal data of Pt-Ph were
collected on a Bruker SMART APEX CCD diffractometer equipped
with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation
source; the data were acquired using the SMART and SAINT
programs. The structure was solved by direct methods and refined on
F2 using full-matrix least-squares methods by the SHELXTL, version
5.1, software.96 The non-H atoms were refined anisotropically. Crystal
data and details of the data collection and structural refinements for
Pt-Ph are summarized in Table S1.
Tables of atomic coordinates, isotropic thermal parameters, and
complete bond distances and angles have been deposited with the
Cambridge Crystallographic Data Center. Copies of this information
can be obtained free of charge by quoting the deposition number
CCDC 1565649 (Pt-Ph).
Cyclic Voltammetry. Cyclic voltammetry was measured using a
CHI610D electrochemical workstation (CH instruments, Inc.,
Shanghai, China). Tetrabutylammonium hexafluorophosphate (Bu4N-
[PF6], 0.1 M) was used as the supporting electrolyte, and glassy
carbon and platinum electrodes were used as the working and counter
electrodes, respectively. CH2Cl2 was used as the solvent. Fc was used
as the internal reference. The solution was purged with N2 before the
measurements, and the N2 gas flow was maintained during the
measurements. The measurements were performed under a 50 mV s−1
scan rate at room temperature.
CONCLUSIONS
■
In summary, we prepared a Pt(II)−Schiff base complex with a
BDP moiety attached to the Schiff base ligand. The
photophysical properties of the complex and reference
compounds were studied with steady-state UV−vis absorption,
luminescence spectroscopy, time-resolved fs/ns-TA spectros-
copies, and cyclic voltammetry and DFT/TDDFT calculations.
The ultrafast intramolecular singlet−triplet energy transfer in
Pt-BDP was observed using fs-TA spectroscopy. A forward
singlet energy transfer from the BDP moiety to the Pt(II)
coordination center (FRET, kFRET = 1.5 × 1011 s−1) is
accompanied by an ultrafast ISC (<1 ps, confirmed with Pt-Ph)
at the Pt(II) coordination center and a backward TTET from
the Pt(II) coordination center to the BDP moiety (TTET,
kTTET = 6.8 × 109 s−1). Localization of the triplet state at BDP
upon excitation of Pt-BDP is confirmed by ns-TA spectrosco-
py. Pt-BDP has a longer triplet-state lifetime (τT) of 103.2 μs
compared to the reference complex Pt-Ph, which shows a
triplet state localized mainly on the Pt(II)−Schiff base
coordination center with τT = 3.5 μs. Finally, Pt-BDP is
examined as a triplet photosensitizer for TTA upconversion.
We find that the complex shows high upconversion quantum
yields upon excitation of BDP, up to 10%, and a large anti-
Stokes shift upon excitation of the lowest absorption band
(increased to 5290 cm−1, 0.65 eV). We propose the use of
triplet photosensitizers showing ping-pong energy transfer as a
method to increase the anti-Stokes shift of TTA upconversion.
EXPERIMENTAL SECTION
■
Materials. Solvents were dried or distilled before use. K2PtCl4 was
purchased from Aladdin Chemical Co., Ltd. (China). Compounds 1,
2, Pt-BDP, and Pt-Ph were prepared according to literature
procedures.58
Analytical Measurements. NMR spectra were recorded on a
Bruker 400 MHz spectrometer with CDCl3 or dimethyl sulfoxide
(DMSO)-d6 as solvents. HRMS was determined using a TOF MALDI
HRMS system (U.K.). Fluorescence spectra were measured on a
RF5301 PC spectrofluorometer (Shimadzu, Japan) and a FS5
spectrofluorometer (Edinburgh Instruments, U.K.). UV−vis absorp-
tion spectra were recorded on a UV2550 UV−vis spectrophotometer
(Shimadzu, Japan).
Synthesis of Pt-BDP. Under an Ar atmosphere, 2 (80.0 mg, 0.10
mmol), 4 (112.0 mg, 0.15 mmol), Na2CO3 (65.0 mg, 0.45 mmol), and
solvents of ethanol (EtOH)/toluene/water (20 mL, 2:4:1, v/v/v) were
mixed. Then, Pd(PPh3)4 (9.0 mg, 0.007 mmol, 5 mol %) was added.
The reaction mixture was refluxed and stirred under an Ar atmosphere
for 8 h. After the reaction was complete, the mixture was cooled to
room temperature, then extracted with CH2Cl2, and washed with
water (2 × 100 mL), and the combined organic layers were dried over
anhydrous Na2SO4. The solution was evaporated to dryness under
reduced pressure to give a crude solid, which was further purified with
column chromatography (silica gel, CH2Cl2) to yield Pt-BDP as a red
solid (32 mg, yield 38%). Mp: >250 °C. 1H NMR (CDCl3, 400 MHz):
δ 9.01 (s, 1H), 8.93 (s, 1H), 8.23 (s, 1H), 8.11 (d, 1H, J = 8.0 Hz),
7.89 (d, 2H, J = 8.0 Hz), 7.69 (s, 2H), 7.64 (d, 1H, J = 12.0 Hz), 7.47
ns-TA Spectroscopy. The ns-TA spectra were recorded on an
LP920 laser flash photolysis spectrometer (Edinburgh Instruments,
U.K.). The signal was digitized with a Tektronix TDS 3012B
oscilloscope. All samples were purged with N2 for 15 min before the
measurements and excited with an Opolette 355II+UV nanosecond
pulse laser. The data were processed with the LP900 software.
fs-TA Spectroscopy. The ultrafast TA (pump−probe) spectros-
copy measurements were performed using a Ti:sapphire laser
amplifier-optical parametric amplifier system (Spectra Physics, Spitfire
Pro XP, TOPAS) and a commercial ultrafast TA spectrometer setup
(Spectra Physics, Helios). The pulse duration was 100 fs. Wavelengths
of the pump beam were chosen according to the steady-state
absorption spectra of the studied compounds. A white-light continuum
was generated with the sapphire crystal and used as the probe beam.
DFT Calculations. Geometry optimizations were carried out using
the B3LYP functional with 6-31G(d) basis sets for the C, H, O,97 and
N elements and the LANL2DZ basis set for the Pt atom,98 while the
J
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Article
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02989.
1H and 13C NMR data, HRMS and photophysical
spectra, and DFT/TDDFT calculations of the complexes
(PDF)
Accession Codes
CCDC 1565649 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhaojzh@dlut.edu.cn (J.Z.).
*E-mail: dongho@yonsei.ac.kr (D.K.).
■
ORCID
Zhijia Wang: 0000-0002-8309-0050
Francis D’Souza: 0000-0003-3815-8949
Jianzhang Zhao: 0000-0002-5405-6398
Dongho Kim: 0000-0001-8668-2644
Author Contributions
§These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (Grants 21473020, 21673031,
21761142005, 21603021, 21421005, and 21273028), the
State Key Laboratory of Fine Chemicals (ZYTS201801), and
the Fundamental Research Funds for the Central Universities
(Grants DUT16TD25, DUT15ZD224, and DUT2016TB12)
for financial support.
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