An N^N Platinum(II) Bis(acetylide) Complex with Naphthalimide and Pyrene Ligands: Synthesis, Photophysical Properties, and Application in Triplet–Triplet Annihilation Upconversion

Article abstract of DOI:10.1002/ejic.201700656

Two different chromophores, pyrene (Py) and naphthalimide (NI), were introduced as acetylide ligands in a heteroleptic diimine N^N Pt^II bis(acetylide) complex (Pt-NI-Py) to broaden the absorption bands and enable the intramolecular energy-transfer process. Steady-state and transient spectroscopy were used to study the photophysical properties of the complexes. The lowest triplet state of Pt-NI-Py (τ_T = 34 μs) is localized on the pyrene part, although the singlet-state energy level of the naphthalimide unit is lower. Intramolecular energy-transfer processes were observed. Moreover, Pt-NI-Py shows enhanced visible-light harvesting compared with the bis(pyrenylacetylide) complex (Pt-Py) and a lower triplet-state energy level than that of the bis(ethynylnaphthalimide) complex (Pt-NI). The results demonstrate that the relative energy levels of the singlet excited states of the different chromophores do not agree with the relative energy levels of the triplet states of the same chromophores. Efficient triplet-state generation (Φ_Δ = 0.8) gives Pt-NI-Py a high triplet–triplet annihilation (TTA) upconversion efficiency (34 %).

Full text of DOI:10.1002/ejic.201700656

A Journal of
Accepted Article
Title: N^N Pt(II) Bisacetylide Complex with Naphthalimide and Pyrene
Ligands: Synthesis, Photophysical Properties and Application in
Triplet-Triplet Annihilation Upconversion
Authors: Jianzhang Zhao and Fangfang Zhong
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700656
Link to VoR: http://dx.doi.org/10.1002/ejic.201700656

10.1002/ejic.201700656
European Journal of Inorganic Chemistry
FULL PAPER
N^N Pt(II) Bisacetylide Complex with Naphthalimide and Pyrene
Ligands: Synthesis, Photophysical Properties and Application in
Triplet-Triplet Annihilation Upconversion
Fangfang Zhong[a] and Jianzhang Zhao*[a]
Abstract: Two different chromophores of pyrene and naphthalimide
were introduced as acetylide ligands into diimine N^N heteroleptic
Pt(II) bisacetylide complex (Pt-NI-Py) to broaden the absorption
bands and study the intramolecular energy transfer process. Steady
state and transient spectroscopies were used for study of the
photophysical properties of the complexes. The results show that the
lowest triplet state of Pt-NI-Py (τT = 34 μs) is localized on the pyrene
part, although the singlet state energy level of naphthalimide unit is
lower. Intramolecular energy transfer processes were observed.
Moreover, Pt-NI-Py shows enhanced visible light harvesting property
compared with bis(pyrenylacetylide) complex (Pt-Py) and relatively
band of the photosensitizer is extremely important.[22] Introducing
an extra chromophore is an easy method to enhance the
absorption and form new molecules that display a wealth of
fundamentally interesting and potentially useful properties.[23]
Previously, we reported a broad-band N∧N Pt(II) bisacetylides
complex which contains two different Bodipy ligands.[24] The two
ligands have different singlet and triplet energy levels, thus the
photoexcitation energy collected by the two ligands can be
funneled onto one Bodipy ligand. Although heteroleptic Pt(II)
complex was obtained, the energy donor and acceptor were
derivatives from one kind of chromophore, i.e. Bodipy.
lower
triplet
state
energy
level
compared
The
to
Room temperature phosphorescence from Pt(II) diamine
bis(pyrenylacetylide) complex (Pt-Py) was observed.[13]
Subsequently, the strong room temperature phosphorescence of
Pt(II) diamine complex contains 1,8-naphthalimide ligands (Pt-
NI) was also reported.[14] The efficiencies of their applications in
oxygen sensing or TTA upconversion show that both Pt-NI and
Pt-Py have good triplet state generation properties.[7,14,25] The
absorption and phosphorescence emission spectra of Pt-NI and
Pt-Py demonstrate the small energy gap between the lowest
singlet and triplet states of this two complexes.
Herein, two different chromophores (1,8-naphthalimide and
pyrene) are introduced into the Pt(dbbpy) (dbbpy = 4,4′-di-tert-
butyl-2,2′-bipyridine) coordination center by triple bonds to form
the heteroleptic complex Pt-NI-Py (Scheme 1). We proposed
that the pyrene ligand in its singlet excited state can transfer its
photoexcitation energy to the naphthalimide ligand, while the
naphthalimide ligand in its triplet excited state could transfer
energy backward to the triplet state of pyrene. By this ‘ping-
pong’ energy transfer process, complex Pt-NI-Py can make
better use of the photoexcitation energy of broad-band photo
source.
bis(ethynylnaphthalimide)
complex
(Pt-NI).
results
demonstrated that the relative singlet excited state energy levels of
the different chromophores do not agree with the relative triplet state
energy levels of the same chromophores. The efficient triplet state
generation property (ΦΔ
upconversion efficiency (34%).
=
0.8) gives Pt-NI-Py high TTA
Introduction
The transition metal complexes, such as those contain Pt(II)
atoms, usually show efficient intersystem crossing (ISC)
properties,[1] and these complexes attract increasing attention in
applications of photocatalysis,[2] photovoltaics,[3] photodynamic
therapy,[4] oxygen sensing,[5] and triplet-triplet annihilation (TTA)
upconversion.[6–10] However, the conventional platinum (II)
complexes show weak absorption of visible light.[11,12] In order to
address this problem, visible light harvesting Pt(II) complexes
have been widely investigated by coupling with organic
chromophores, such as pyrene,[13] naphthalimide,[14,15]
perylene,[16,17] and boron-dipyrromethene (Bodipy) ligands.[18]
N∧N Pt(II) bisacetylide complexes have rich excited states,
good photostability and feasible derivatization,[12,19] which make
them good triplet photosensitizers.[20,21] However, those
complexes contain only one kind of ligand, as a result there is
only one absorption band in the visible region. To make better
use of the broad-band light source, broadening the absorption
Results and Discussion
2.1. Design and Synthesis of the Compounds. Our
hypothesis is that the coordinate pyrene part (referred as Py in
the following text) with higher singlet and lower triplet energy
levels than coordinate naphthalimide part (referred as NI in the
following text) can be selected as the singlet energy donor and
triplet energy acceptor, while NI could use as the singlet energy
acceptor and triplet energy donor in heteroleptic complex Pt-NI-
Py. By these two opposite energy transfer processes, Pt-NI-Py
have two absorption antennae under the excitation of broad-
band photo source.
[a]
State Key Laboratory of Fine Chemicals,
School of Chemical Engineering,
Dalian University of Technology,
E-208 Western Campus, 2 Ling-Gong Road, Dalian 116024, China
E-mail: zhaojzh@dlut.edu.cn
http://finechem2.dlut.edu.cn/photochem
The molecular structures and synthesis procedures of the
ligands (NI-H and Py-H) and complexes (Pt-NI-Py, Pt-NI and Pt-
Py) were displayed in Scheme 1. Heteroleptic complex Pt-NI-Py
Supporting information for this article is given via a link at the end of
the document
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10.1002/ejic.201700656
European Journal of Inorganic Chemistry
FULL PAPER
Table 1. Photophysical Parameters of the Acetylide Ligands and Pt(II) Complexes
λabs
ε[b]
λem
τ[c]
Φ[d]
τT
ΦΔ
[a]
[a]
[e]
[f]
[g]
[g]
[g]
NI-H
350
2.70
400
0.7 ns
14.3 ns
−
−
−
−
[g]
[g]
[g]
Py-H
360
4.33
384
−
−
42 μs, 179 μs[h]
95 μs, 365 μs[h]
39 μs, 176 μs[h]
34 μs
78 μs
32 μs
Pt-NI-Py
385, 420
4.63, 2.58
661, 731
4.4%[i] 4.0%[j]
0.76[i] 0.84[j]
Pt-NI
Pt-Py
414
386
5.06
5.95
624, 679
662, 731
14%
0.74[j]
2.1%
0.75[i]
1
[a] In toluene (1.0 × 10−5 M), in nm. [b] Molar extinction coefficient at the absorption maxima. ε : 104 M− cm−1. [c] Luminescence lifetime at room temperature.
[d] Luminescence quantum yield, [Ru(dmb)3]2+ (ΦP = 0.073 in deaerated acetonitrile) as the reference. [e] Triplet state lifetime. [f] Singlet oxygen generation
quantum yield, [Ru(bpy)3]2+ (ΦΔ = 0.57 in dichloromethane) as standard. [g] Not measured. [h] Luminescence lifetime at 77 K. [i] Excited at 350 nm. [j] Excited
at 470 nm.
was synthesized with
a
mixture of the two ligands and
were compared (Figure 1a). For Pt-NI-Py, broad absorption
band from 320 nm to 500 nm was observed, which is due to the
two different coordinated ligands. There is no shift of the
absorption bands of Pt-NI-Py as compared to Pt-NI and Pt-
Py,[13,14] indicating that there is no electronic interaction between
the two ligands at ground state.
Pt(dbdbpy)Cl2 under the catalysis of CuI under N2
atmosphere.[26,27] The ligands and reference complexes Pt-NI
and Pt-Py were synthesized according to the reported
methods.[13,14,28]
The absorption spectra of the two ligands and three Pt(II)
complexes in different solvents were also studied. With the
polarity of the solvents increases, the absorption bands do not
change obviously, which proved no significant electron
interaction at the ground states (Figure S1). The absorption
peaks and corresponding molar extinction coefficients at the
absorption maxima in toluene were summarized in Table 1.
No obvious fluorescence was detected while room
temperature phosphorescence was observed for Pt-NI-Py. The
strongly quenched singlet state emission of Pt-NI-Py is result
from the efficient ISC by the heavy atom effect of platinum, since
no fluorescence emission was observed in both Pt-NI and Pt-Py.
To investigate the triplet state emission properties of Pt-NI-Py,
the normalized phosphorescence emission spectra of the three
complexes were compared as well (Figure 1b).
O
O
N
O
O
O
O
N
O
Trimethylsilylacetylene
K2CO3
n-butylamine
CH2Cl2/MeOH
r. t., 1 h
Pd(PPh3)2Cl2, PPh3, CuI
NEt3, reflux, 8 h
ethanol, reflux, 6 h
Br
Br
NI-H
Trimethylsilylacetylene
K2CO3
Pd(PPh3)2Cl2, PPh3, CuI CH2Cl2/MeOH
r. t., 1 h
NEt3, reflux, 12 h
Br
Py-H
N
N
HCl / H2O
reflux, 2 h
N
N
Cl
Cl
K2PtCl4
Pt
Pt(
dbb
py)C
l2
O
N
O
N
N
(i-Pr)2NH, CH2Cl2
1.2
0.8
0.4
0.0
0.8
0.6
0.4
0.2
0.0
NI-H
Py-H
Pt
Pt(dbbpy)Cl2
a
b
CuI, r. t. 10 h
Pt-NI-Py
Pt-NI
Pt-Py
Pt-NI-Py
Pt-NI
Pt-Py
Pt-NI-Py
O
N
O
N
N
N
Pt
Pt
N
300
400
λ / nm
500
600
600 650 700 750 800 850
O
λ / nm
N
Pt-NI
Pt-Py
O
Figure 1. The UV−vis absorption (a) and normalized phosphorescence
5
emission spectra (b) of Pt-NI-Py, Pt-NI and Pt-Py, c = 1.0 × 10− M in
deaerated toluene at 20 °C.
Scheme 1. Synthesis of NI-H, Py-H and Pt-NI-Py, the molecular structures of
Pt-NI and Pt-Py are also shown.
As reported by the literatures, Pt-NI and Pt-Py have strong
room temperature phosphorescence emission at 624 nm and
662 nm, respectively.[13,14] Interestingly, the phosphorescence
emission bands of Pt-NI-Py (661 nm and 731 nm in toluene) are
2.2. UV−vis Absorption and Emission Spectroscopies.
The UV−vis absorption spectra of Pt-NI-Py, Pt-NI and Pt-Py
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10.1002/ejic.201700656
European Journal of Inorganic Chemistry
FULL PAPER
almost the same as that of Pt-Py (Figure 1b). The
phosphorescence emission (624 nm in toluene) quenching of
the NI part suggests the possible intramolecular energy transfer
process from the NI part to the Py part.
reflects the high intramolecular energy transfer efficiency. The
emission lifetime of Pt-NI-Py is 34 μs, almost the same as that
of Pt-Py (32 μs), while Pt-NI has
phosphorescence lifetime of 78 μs.
a
much longer
The emission properties of the ligands and complexes in
different solvents were studied (Figure S2 and Figure S3). For
NI-H and Py-H, the emission intensities do not change
significantly with the solvent. On the contrary, phosphorescence
of the complexes is quenched in polar solvents at different
extent. This indicates the photoinduced electron transfer process
in polar solvents, such as acetonitrile and methanol. The
following electrochemistry study also supported this conclusion.
The heteroleptic complex Pt-NI-Py show similar emission
wavelength and lifetime with Pt-Py, indicating that the lowest
triplet state of Pt-NI-Py is pyrene-localized 3IL. Thus, we
proposed intramolecular triplet energy transfer happened when
excited at the NI part. To prove this photophysical process, we
determined the excitation spectrum of Pt-NI-Py in deaerated
toluene (Figure 3). Under excitation into the NI part, the
phosphorescence of the Py part (with higher singlet energy level,
thus singlet energy transfer is excluded here) was detected. The
overlapping of the absorption and excitation spectra indicates
the high efficient intramolecular triplet energy transfer. This
conclusion agrees with the nanosecond transient spectroscopies.
1.2
0.9
0.6
0.3
0.0
1.2
0.9
0.6
0.3
0.0
a
b
RT
77 K
77 K
RT
2.0
-A
1-10
1.5
1.0
0.5
600
650
λ / nm
700
750
600
650
λ / nm
700
750
Ex
Figure 2. The emission spectra of (a) Pt-NI-Py (λex = 400 nm), (b) Pt-Py (λex
=
0.0
380 nm) in deaerated toluene at room temperature (RT) and 77 K, c = 1.0 ×
300
400
500
600
10−5 M.
Wavelength / nm
Figure 3. The normalized excitation and UV−vis absorption spectra of Pt-NI-
6
Py determined with emission wavelength at 662 nm, c = 1.0 × 10− M in
To get a better understanding of the emissive triplet state, the
low temperature (77 K) emission spectra were measured (Figure
deaerated toluene. 20 °C.
2
and Figure S4). Compared to the emission at room
temperature, the phosphorescence bands of the three
complexes were all slightly blue-shifted at 77 K, which is
attribute to the thermally induced Stokes shift (ΔEs). The small
values of ΔEs (0.033 eV, 0.020 eV and 0.036 eV for Pt-NI-Py,
Pt-NI, Pt-Py, respectively) exclude the charge transfer at
emissive triplet state and reveal the 3IL excited-state
assignments.[29,30] The phosphorescence lifetime at 77 K was 3-
fold longer than which measured at room temperature (Table 1).
The phosphorescence quantum yield of Pt-NI-Py was
measured under different excitation wavelength (Table 1). The
2.3. Electrochemical Characterization: Redox Properties of
the Complexes. To illuminate the photo-induced electron
transfer, the redox potentials of the complexes were studied by
cyclic voltammograms (Figure 4) or differential pulse
voltammetry (DPV, Figure S5).
For the reference complexes Pt-NI and Pt-Py, irreversible
metal-based oxidation waves at +0.83 V and +0.35 V were
observed, while a very weak oxidation wave was observed in Pt-
NI-Py (Figure 4). To obtain the accurate redox potential value,
results indicate that either excite the NI part or excite the Py part, DPV curve was determined, which shows the oxidation wave of
the emission quantum yields are the same (ca. 0.04), which
Table 2. Electrochemical Redox Potentials (V vs. Fc/Fc+) and Driving Forces of Charge Recombination (ΔGCR) and Charge Separation (ΔGCS) and Static
Coulombic Energy (ΔGS) for Pt-NI-Py in Toluene, CH2Cl2 and CH3CN.
oxidation / V
reduction / V
ECS / eV[b]
2.75
εS
ΔGCS / eV[b]
−0.39
ΔGCS / eV[c]
+0.88
ΔGS / eV
+0.58
ΔGCR / eV
−2.75
Toluene
CH2Cl2
CH3CN
2.4
8.9
37.5
−
−
+0.38[a]
−1.79[a]
−1.10
−0.13
−2.04
+0.17
2.04
1.84
−
−
−1.30
−0.04
−0.33
−1.84
[a] Data from DPV measurement in CH2Cl2, 1Py* unit as the electron donor while Pt(dbbpy) * unit as the electron acceptor. [b] E00 = energy level for the singlet
excited state, approximated with the absorption maxima peak of pyrene ligand at 395 nm. [c] E00 = energy level for the triplet excited state, approximated with
the phosphorescence emission wavelength at 662 nm.
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10.1002/ejic.201700656
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This reasonable in consideration of the charge transfer energy
level. The charge-transfer state (CS) is the lowest singlet state,
which has higher energy level than the lowest triplet state. But
the situation changed when high polar solvent is used. For
instance, the CS state can quench the phosphorescence in
acetonitrile or methanol (Figure S3).
Pt-NI-Py at +0.38 V (DPV, Figure S5). Comparably, the
reduction waves of the three complexes are all near −1.79 V,
which is attributed to reduction of the Pt(dbbpy) unit.[19,24]
The redox potential data of Pt-NI-Py, together with the
spectral measurements and geometry optimization data were
used for evaluation of the Gibbs free energy changes (ΔGCS) of
the photoinduced intramolecular electron transfer process, as
well as calculation of the energy level of the charge-transfer
state (ECS, Table 2). The Weller equation (Eq. 1 and Eq. 2) for
the ΔGCS and static Coulombic energy (ΔGS), as given in the
following:
2.4. DFT Calculations. The ground state geometry and the
electronic excited states of Pt-NI-Py were calculated by the
Density Functional Theory (DFT) and Time-dependent Density
Functional Theory (TDDFT) methods with the Gaussian 09W
software.[33] Due to the charge transfer process and the
presence of platinum heavy atom, the CAM-B3LYP/GENECP
basis is selected for the calculation.[34–36]
ΔGCS = e EOX − ERED − E00 + ΔGS
[
]
(Eq. 1)
(Eq. 2)
e2
e2
1
1
1
1
⎛
⎜
⎞⎛
⎟⎜
⎞
⎟
⎠
ΔGS = −
−
+
−
4πεSε0RCC 8πε0 RD RA εREF εS
⎝
⎠⎝
where e is the electronic charge, EOX is half-wave potential for
one-electron oxidation of the electron-donor unit (+0.38 V), ERED
is half-wave potential for one-electron reduction of the electron-
acceptor unit (−1.79 V); E00 is the energy of the 0–0 transition
energy gap between the lowest excited state and the ground
state, εS is static dielectric constant of the solvents, RD is the
radius of the electron donor (pyrene, 3.51 Å), RA is the radius of
the electron acceptor (Pt(dbbpy), 4.90 Å), RCC is the center-to-
center separation distance between the electron donor and
electron acceptor (from the DFT optimized geometry, 12.3 Å),
εREF is the static dielectric constant of the solvent used for the
electrochemical studies, ε0 is permittivity of free space.[31,32]
(Eq. 3)
(Eq. 4)
ECS = e[EOX − ERED ]+ ΔGS
ΔGCR = −(ΔGCS + E00 )
Figure 5. Selected frontier molecular orbitals involved in the excitation and
triplet excited states of Pt-NI-Py, calculated by DFT at the CAM-
B3LYP/GENECP level with Gaussian 09W, the butyl group was simplified as
Energies of the charge-separated states (ECS) and charge
recombination energy state (ΔGCR) of Pt-NI-Py were calculated
by the reported methods (Eq. 3 and Eq. 4).[32]
methylamine in the calculation. Key: CT
intersystem crossing.
= charge-transfer state, ISC =
+
Fc / Fc
At the ground state, the naphthalimide ligand has a coplanar
geometry toward the Pt(dbbpy) coordination center, while the
pyrene ligand is noncoplanar with the platinum core. Moreover,
there is no π conjugation between the two ethynyl ligands. The
selected frontier molecular orbitals of Pt-NI-Py are presented in
Figure 5. The HOMO is located on the pyrene ligand while the
LUMO is spread over the Pt(dbbpy) part, which is in agree with
the electrochemical results.
Pt-NI-Py
Pt-NI
Pt-Py
1
0
-1
-2
The S3 and S4 states are featured as the mixed MLCT and IL
transition states (Table 3). The energy level of the first singlet
excited state (S1) is 2.87 eV, in accordance with the result of the
electrochemistry calculation (2.75 eV). The second triplet excited
state (T2) is a typical 3IL state, attributed by the transition
localized on the NI part. After activated by the intersystem
crossing (ISC) from S1 state, the T2 state transfer the triplet
energy to the lowest triplet excited state (T1) by internal
conversion, which finally gives the phosphorescence emission.[5]
Potential / V
Figure 4. Cyclic voltammograms of (a) Pt-Py, Pt-NI and Pt-NI-Py. Ferrocene
(Fc) was used as internal reference (E1/2 = +0.17 V, Fc+/Fc). In deaerated
CH2Cl2 solutions containing 0.5 mM photosensitizers and ferrocene (0.2 mM),
0.10 M Bu4NPF6 as supporting electrolyte, Ag/AgNO3 reference electrode.
Scan rates: 100 mV/s.
In toluene, charge transfer state is exo-energetic or
thermodynamically allowed from the singlet state (ΔGCS = −0.39
eV), but completely inhibited from triplet state (ΔGCS = +0.88 eV).
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2.5. Nanosecond Transient Absorption Spectroscopy. In
order to investigate the triplet excited state of the complexes, the
nanosecond transient absorption spectra and decay traces were
studied (Figure 6).
For Pt-NI, upon 355 nm pulsed laser photoexcitation, a
bleaching band at 420 nm was observed, which is in agreement
with the steady UV absorption band (Figure 6c).[6] The broad
absorption band in the range of 470 nm − 720 nm is the excited
state absorption (ESA) of T1→Tn transitions, which is the similar
as the reported results (broad transient absorption band at 650
nm).[14] The triplet lifetime of Pt-NI is detected as 78 μs (Figure
6d). For Pt-Py, much more structured ESA bands were
determined, while the ground state bleaching signal was
observed at 380 nm (Figure 6e) and the corresponding decay
time is 32 μs (Figure 6f). Note that the triplet lifetime is
concentration dependent.
a
0.01
0.00
0.00
117 μs
...
13 μs
0 μs
-0.01
-0.02
τ = 34 μs at 370 nm
-0.01
b
-0.02
300 400 500 600 700
0
100
200
300
400
1[Py*]
τ / μs
λ / nm
c
1[NI*]
CSS
0.005
3.22 eV
0.000
TN
ISC
2.95 eV
2.75 eV
0.000
-0.005
-0.010
3[NI*]
DCM
-0.005
TTET
3[Py*]
288 μs
...
32 μs
0 μs
τ = 78 μs at 420 nm
2.04 eV
1.99 eV
624 nm
ACN
-0.010
0
420 nm
385 nm
1.88 eV
661 nm
d
1.84 eV
200
400
600
800
400 500 600 700
τ / μs
λ / nm
e
S0
S0
0.01
0.00
0.000
-0.005
-0.010
Scheme 2. Simplified Jablonski diagram illustrating the photophysical
processes involved in Pt-NI-Py. The energy levels of the excited state are
derived from the spectroscopic data. [NI] stands for the ethynyl-1,8-
naphthalimide moiety while [Py] stands for the ethynylpyrene moiety in Pt-NI-
Py. TN, DCM and ACN stand for toluene, dichloromethane and acetonitrile,
respectively.
90 μs
...
10 μs
0 μs
τ = 32 μs at 380 nm
-0.01
-0.02
f
-0.015
0
400 500 600 700
200
400
600
800
τ / μs
λ / nm
Figure 6. The nanosecond transient absorption spectra and decay trace of Pt-
5
NI-Py (a and b), Pt-NI (c and d), Pt-Py (e and f), λex = 355 nm, c = 1.0 × 10−
M, in deaerated toluene, 20 °C.
Table 3. Electronic Excitation Energies (eV) and Oscillator Strengths (f), Main Configurations and CI Coefficients of the Low-lying Electronic Excited States of
Complex Pt-NI-Py, Calculated by TDDFT // CAM-B3LYP / GENECP, Based on the DFT // CAM-B3LYP / GENECP Optimized Ground State Geometries.
Electronic transition
Energy[a]
f[b]
Composition[c]
CI[d]
Character
Singlet
S0 → S1
S0 → S3
2.87 eV / 432 nm
3.48 eV / 356 nm
0.085
0.217
H → L
0.632
0.617
0.196
0.442
0.342
0.483
0.602
0.528
0.509
MLCT
MLCT
ILCT
H − 3 → L
H − 1 → L + 1
H − 1 → L + 1
H → L + 4
H → L + 4
H → L + 4
H − 1 → L + 1
H → L
3.54 eV / 351 nm
0.587
ILCT
S0 → S4
ILCT, MLCT
ILCT
3.70 eV / 335 nm
1.78 eV / 695 nm
1.95 eV / 635 nm
2.75 eV / 451 nm
0.452
S0 → S6
S0 → T1
S0 → T2
S0 → T3
Triplet
0.000[e]
0.000[e]
0.000[e]
ILCT
ILCT
MLCT
[a] Only the selected low-lying excited states are presented, the butyl group was simplified as methylamine in the calculation. [b] Oscillator strengths. [c] Only the
main configurations are presented. [d] The CI coefficients are in absolute values. [e] No spin-orbital coupling effect was considered, thus the f values are zero.
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Interestingly, the transient absorption spectrum of Pt-NI-Py is
approximately the same as Pt-Py, with single bleaching band at
370 nm (Figure 6a) with a decay time of 34 μs (Figure 6b). The
absence of NI bleaching signal in Pt-NI-Py and similar triplet
lifetime of Pt-NI-Py and Pt-Py may owe to the efficient triplet-
triplet energy transfer (TTET) from the NI part to the Py part,
which is in consistent of the excitation spectroscopy (Figure 3).
Furthermore, we studied the intermolecular triplet energy
transfer process. With the adding of Pt-Py into the solution of Pt-
NI, the bleaching signal at 420 nm decreased while the
bleaching signal at 370 nm generated simultaneously
(Supporting Information, Figure S6). The increasing phase of the
decay curve at 370 nm indicated the formation and accumulation
of the triplet state of Pt-Py via intermolecular TTET, whereas the
decreasing phase at 420 nm demonstrated the enhanced decay
of the triplet excited state of Pt-NI. Notably, the decay time
decreased fast with the raise of the Pt-Py / Pt-NI molar ratio
(Supporting Information, Figure S7). The intramolecular TTET
rate constants are ca. 8.0 × 103 s−1 − 8.7 × 104 s−1, depending on
the molar ratio of the triplet acceptor. The intermolecular energy
transfer efficiency is up to ca. 0.87.[37,38]
The photophysical processes in Pt-NI-Py were illustrated in a
simplified Jablonski diagram (Scheme 2). The two acetylide
ligands were excited at different wavelength. Then ISC occurred,
the NI part is with triplet state energy level of 1.99 eV, higher
than the Py unit. Thus, the T1 excited state of Pt-NI-Py will be
localized on the Py part. This conclusion is supported by the
phosphorescence emission spectrum (Figure 1b) and
nanosecond transient absorption spectroscopy (Figure 6a). In
acetonitrile, the charge separated state has a lower energy level
than Pt-NI, although the triplet lifetime of Pt-NI-Py (34 μs) is
much shorter than Pt-NI (78 μs). We propose this is due to the
higher triplet state generation ability of Pt-NI-Py (approximated
as the singlet oxygen quantum yields. Table 1) as well as the
better matched energy levels of photosensitizer and acceptor
(0.11 eV for Pt-NI-Py/DPA and 0.22 eV for Pt-NI/DPA).
According to our previous study, larger triplet sates energy gap
do increase the bimolecular quenching process between the
sensitizer and acceptor, but this is not sufficient for a high
upconversion quantum yield, since the triplet state generation
ability of the photosensitizer is also important for
upconversion.[40]
1200
0.9
a
b
*
Pt-NI-Py
Pt-NI
900
600
300
0
0.6
0.3
0.0
Pt-Py
DPA
Pt-NI-Py
(0.65, 0.31)
*
(0.15, 0.04)
Pt-NI-Py+DPA
400
500
λ / nm
600
700
0.0 0.2 0.4 0.6 0.8
x
Figure 7. (a) Upconverted DPA fluorescence and the residual
5
phosphorescence emission of the Pt(II) sensitizers (c = 1.0 × 10− M in
deaerated toluene), excited by blue laser (λex = 443 ± 3 nm, 83 mW / cm2) at
room temperature, the asterisks indicate the scattered excitation laser. For Pt-
5
NI-Py, c [DPA] = 3.8 × 10− M; for Pt-NI, c [DPA] = 3.7 × 10−5 M; for Pt-Py, c
[DPA] = 5.7 × 10−5 M. (b) CIE diagram of the upconversion emission.
than the Py localized triplet state. As
a
result, the
phosphorescence is quenched by the charge separation in
acetonitrile, which was revealed by the emission spectrum
(Supporting Information, Figure S3a).
103
102
Slope = 1.06
2.6. TTA Upconversion. Owing to the absorption of the
complexes in visible range and the long triplet state lifetime, the
three complexes Pt-NI-Py, Pt-NI and Pt-Py were used as triplet
photosensitizers in application of triplet-triplet annihilation (TTA)
101
Slope = 1.92
100
upconversion. 9,10-diphenylanthracene (DPA) with
a lower
triplet state energy of 1.77 eV was chosen as the triplet energy
acceptor.[8,39] As discussed in the steady emission part, the
phosphorescence of the three complexes were quenched at
different levels in acetonitrile or methanol (Figure S3). This
means the triplet states of the complexes can be quenched in
polar solvents, thus toluene was selected as the solvent to avoid
the CS state (1.84 eV in acetonitrile) quenching process.
Strong upconverted DPA fluorescence emission was observed
under the sensitizing of Pt-NI and Pt-Py, as well as Pt-NI-Py
(Figure 7). The TTA quenching parameters and upconversion
quantum yields (ΦUC) were calculated and presented in Table 4.
The quantum yields of Pt-NI and Pt-Py are 28% and 15%,
respectively. The much higher quantum yields than the reported
values (18% for Pt-NI and 14% for Pt-Py in acetonitrile)[7] is
reasonable, since the quenching of triplet states is prohibitive in
toluene.
10
100
Power Density / mW cm-2
Figure 8. Upconverted DPA emission intensity at 410 nm as a function of the
443 ± 3 nm incident laser power in a mixture of Pt-NI-Py (1.0 μM) and DPA
(3.8 μM) in deaerated toluene. The solid blue and red lines are fitting results
with slopes of 2.0 and 1.0 in the low- and high-density regions, respectively.
To assess the triplet state generation abilities of Pt-NI-Py, Pt-
NI and Pt-Py, the singlet oxygen generation quantum yields (ΦΔ)
of the three photosensitizers were measured with DPBF as the
1O2 scavenger (Table 1). Upon excitation of the complexes at
350 nm, similar singlet oxygen quantum yields (ΦΔ) were
observed for Pt-NI-Py (0.76), Pt-NI (0.74) and Pt-Py (0.75).
With higher triplet state generation ability, the more efficient
The higher ΦUC value of Pt-NI than Pt-Py can be attributed to
longer triplet lifetime. Interestingly, the ΦUC of Pt-NI-Py is larger
upconversion of Pt-NI-Py than Pt-NI is rationalized. It should be
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10.1002/ejic.201700656
European Journal of Inorganic Chemistry
FULL PAPER
platinum complexes Pt-NI-Py, Pt-NI and Pt-Py (Figure 9). With
the increasing molar ratio of DPA acceptor, the
phosphorescence intensities of photosensitizes are all
quenching at different rate. This trend is well linear fitted as the
Stern–Volmer quenching curves (Figure 9d).[44,45] The Stern-
Volmer quenching constant of Pt-NI is much larger than Pt-NI-
Py and Pt-Py (Table 4). This is main caused by the big lifetime
differences of Pt-NI-Py (34 μs), Pt-NI (78 μs) and Pt-Py (32 μs).
On the other hand, Pt-NI-Py, Pt-NI and Pt-Py have similar
bimolecular quenching constants and quenching efficiencies,
which reflect the similar triplet generation abilities of the three
complexes.
noted that the major absorption band of Pt-NI-Py is under 500
nm, and small anti-Stokes shift was obtained in TTA
upconversion. Thus, much room is left for designing of new
photosensitizers giving larger anti-Stokes shift.
Table 4. Upconversion-Related Parameters of the Photosensitizers[a]
[d]
1[b]
1
1[c]
[e]
KSV / 104 M−
kq / 109 M− ⋅s−
ΦUC
fQ
Pt-NI-Py
Pt-NI
Pt-Py
8.5
23
7.0
2.5
2.9
2.2
0.22
0.26
0.19
0.34
0.28
0.15
[a] In deaerated toluene, DPA as the triplet acceptor, excited at 460 nm,
20 °C. [b] Stern-Volmer quenching constants. [c] Bimolecular quenching
constants. KSV = kq×τ0, τ0 is the triplet state lifetime of the sensitizer without
acceptors. [d] The quenching efficiency. [e] The TTA upconversion
quantum yields, Ru(dmb)3 (Φ = 7.3%) was used as the standard, excited
with 445 nm laser.
Conclusions
In summary, we have prepared new N∧N platinum bisacetylide
complex (Pt-NI-Py) with two different acetylide ligands, which
has broad absorption band and high triplet state generation
ability (ΦΔ = 0.8). The intramolecular charge-transfer and triplet
energy transfer processes were studied in detail with steady
state and time-resolved spectroscopies, as well as theoretical
computation. The intramolecular energy transfer process results
Figure 8 shows the double-logarithm plots generated by the
intensity of the upconverted emission from DPA at 410 nm as a
function of incident power density at 445 nm. A slope change
from 1.92 to 1.06 was observed, indicative of quadratic
dependence for low power density and linear dependence for
high power density, respectively.[41,42] The laser power chosen in
the TTA upconversion quantum yield measurement is within the
liner part to guarantee the max value is obtained.[43]
3
in the pyrene localized IL as the emissive triplet state in Pt-NI-
Py. Thus, the introducing of naphthalimide ligand broaden the
absorption band to the visible range without changing the lowest
triplet state. High TTA upconversion quantum yield (ΦUC = 0.34)
shows the potential application of Pt-NI-Py as a good triplet
photosensitizer. Moreover, we found that chromophore (e.g.
ethynyl naphthalimide) with lower singlet state energy level may
have higher triplet state energy levels. Thus, it is not reliable to
predict the relative triplet state energy level based on the relative
singlet state energy levels of different chromophores, and vice
versa. Our results are useful for the design of platinum
photosensitizers with broad-band absorption properties and high
triplet state quantum yields.
400
800
a
b
0 eq. DPA
1 eq. DPA
...
300
200
100
0
600
400
200
0
0 eq. DPA
1 eq. DPA
...
7 eq. DPA
7 eq. DPA
550 600 650 700 750 800
600 650 700 750 800
λ / nm
λ / nm
25
Experimental Section
d
200
c
20
15
10
5
Pt-NI-Py
Pt-NI
Pt-Py
0 eq. DPA
1 eq. DPA
...
Materials. The solvents used for synthesis were analytically pure and
were dried before use, while chromatographically pure solvents were
150
100
50
used for measurements. K2PtCl4 was purchased from Aladdin Chemical
7 eq. DPA
[14]
Co., Ltd (P. R. China). Ligands NI-H
and Py-H [46], as well as
[14]
[13]
complexes Pt-NI
reported procedures.
and Pt-Py
were prepared according to the
0
0
Analytical Measurements. All the 1H NMR spectra were recorded in
CDCl3 solutions with TMS as standard at 0.00 ppm by a Bruker 400 MHz
spectrometer. The high-resolution mass spectrum (HRMS) for the
platinum complex was detected in a TOF MALDI-HR MS system (UK).
Fluorescence and phosphorescence emission spectra were determined
600 650 700 750 800
0
2
4
6
8
10
λ / nm
c [DPA] / μM
Figure 9. The quenching phosphorescence intensities of the complexes (a)
Pt-NI-Py, (b) Pt-NI and (c) Pt-Py as a function of the increasing DPA
concentration. (d) The Stern–Volmer quenching curves in deaerated toluene.
The concentration of triplet photosensitize is 1.0 × 10−5 M, λex = 460 nm.
on
a
RF5301 PC spectrofluorometer (Shimadzu, Japan) while the
absorption spectra were measured on
a
UV2550 UV−vis
spectrophotometer (Shimadzu, Japan). The excitation spectrum was
collected by FS5 fluorescence spectrophotometer (Edinburgh
Instruments Ltd). The nanosecond transient absorption spectra and
The intermolecular TTET processes between the triplet
photosensitizes and DPA energy acceptor were quantitatively
studied by the quenching phosphorescence intensities of the
decay curves were recorded on
a LP920 laser flash photolysis
spectrometer (Edinburgh Instruments, Livingston, UK).
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10.1002/ejic.201700656
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of Pt(dbbpy)Cl2. The complex was synthesized by a modified
literature procedure.[47] To a mixture of 4,4′-di-tert-butyl-2,2′-bipyridine
(135 mg, 0.5 mmol) and K2PtCl4 (208 mg, 0.5 mmol) in 15 mL distilled
water, HCl aqueous solution (1.0 mL, 4 M) was added under stirring.
Then the reaction mixture was refluxed for 2 h. After the mixture was cool
to room temperature, light yellow product precipitated. Pure product was
obtained after filtration, washing with hot water, methanol and ether (240
mg, yield: 90%). 1H NMR (400 MHz, CDCl3) δ 9.46 (d, J = 4.0 Hz, 2H),
7.89 (s, 2H), 7.49 (d, J = 4.0 Hz, 2H), 1.47 (s, 18H).
Keywords: Energy transfer • Naphthalimide • Pyrene • Triplet
State • Upconversion
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2
⎛
⎜
⎝
⎞⎛
⎞⎛
ηsam
⎞
⎟
⎠
A
I
std
⎟⎜ sam ⎟⎜
(Eq. 5)
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A
Istd ηstd
sam ⎠⎝
⎠⎝
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Entry for the Table of Contents
FULL PAPER
Heteroleptic Pt(II) complex with
naphthalimide and pyrene acetylide
ligands (Pt-NI-Py) was prepared.
Broad absorption band and pyrene
localized room temperature
Luminescent Materials
Fangfang Zhong, Jianzhang Zhao*
Page No. – Page No.
N^N Pt(II) Bisacetylide Complex with
Naphthalimide and Pyrene Ligands:
Synthesis, Photophysical Properties
and Application in Triplet-Triplet
Annihilation Upconversion
phosphorescence was observed (ΦP =
4%). Efficient singlet oxygen
generation (ΦΔ = 80%) and high TTA
upconversion quantum yield (ΦUC
=
34%) verified Pt-NI-Py a good triplet
photosensitizer.
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