Long-lived room-temperature near-IR phosphorescence of BODIPY in a visible-light-harvesting N^C^N PtII-acetylide complex with a directly metalated BODIPY chromophore

Article abstract of DOI:10.1002/chem.201102634

Room-temperature long-lived near-IR phosphorescence of boron-dipyrromethene (BODIPY) was observed (λ_em=770 nm, φ_P=3.5%, τ_P=128.4 μs). Our molecular-design strategy is to attach Pt^II coordination centers directly

Full text of DOI:10.1002/chem.201102634

FULL PAPER
DOI: 10.1002/chem.201102634
Long-Lived Room-Temperature Near-IR Phosphorescence of BODIPY in a
Visible-Light-Harvesting N^C^N PtII–Acetylide Complex with a Directly
Metalated BODIPY Chromophore**
Wanhua Wu, Jianzhang Zhao,* Huimin Guo, Jifu Sun, Shaomin Ji, and Zilong Wang[a]
Abstract: Room-temperature long-
lived near-IR phosphorescence of
boron-dipyrromethene (BODIPY) was
observed (lem =770 nm, FP =3.5%,
tP =128.4 ms). Our molecular-design
strategy is to attach PtII coordination
centers directly onto the BODIPY p-
core using acetylide bonds, rather than
on the periphery of the BODIPY core,
thus maximizing the heavy-atom effect
of PtII. In this case, the intersystem
crossing (ISC) is facilitated and the ra-
diative decay of the T1 excited state of
BODIPY is observed, that is, the phos-
phorescence of BODIPY. The complex
shows strong absorption in the visible
range (e=53800mꢀ1 cmꢀ1 at 574 nm),
which is rare for PtII–acetylide com-
plexes. The complex is dual emissive
with 3MLCT emission at 660 nm and
let annihilation based upconversion
and an upconversion quantum yield of
5.2% was observed. The overall upcon-
version capability (h=eꢀFUC) of this
complex is remarkable considering its
strong absorption. The model complex,
without the BODIPY moiety, gives no
upconversion under the same experi-
mental conditions. Our work paves the
way for access to transition-metal com-
plexes that show strong absorption of
3
the IL emission at 770 nm. The T1 ex-
cited state of the complex is mainly lo-
calized on the BODIPY moiety (i.e.
3IL state, as determined by steady-state
and time-resolved spectroscopy, 77 K
emission spectra, and spin-density anal-
ysis). The strong visible-light-harvesting
ability and long-lived T1 excite state of
the complex were used for triplet-trip-
3
visible light and long-lived IL excited
states, which are important for applica-
tions in photovoltaics, photocatalysis,
and upconversions, etc.
Keywords: BODIPY
·
chromo-
phores · phosphorescence · photo-
chemistry · platinum
Introduction
transition-metal complexes that show intense absorption in
the visible region and, at the same time, show long-lived T1
excited states, to enhance the photophysical processes that
these complexes are involved in. Concerning these demands,
attaching a light-harvesting organic chromophore to the
transition metal coordination center to prepare a dyad is
one straight-forward strategy.[14,16–20] However, the photo-
physics of these dyads are very often elusive; for example,
the phosphorescence was quenched and short T1 lifetimes
were observed when perylenediimide (PDI) and boron-di-
pyrromethene (BODIPY) were used for the preparation of
PtII and IrIII complexes.[21,22]
On the other hand, BODIPY is a particularly interesting
chromophore,[23,24] and has been incorporated into PtII, RuII,
and IrIII complexes to afford visible-light harvesting and
long-lived T1 excited states.[18,22,25] However, the photophysi-
cal property of the components often collapse in the dyad.
For example, for a BODIPY-containing RuII complex, the
intrinsic metal-to-ligand charge-transfer (3MLCT) emission
of the RuII coordination center was quenched and no long-
lived triplet excited state was observed at room temperature
(RT).[18] Recently, a BODIPY-containing IrIII complex was
reported, in which again both the emission of the IrIII coor-
dination center and the fluorescence of the BODIPY
Transition-metal complexes with populated triplet excited
states upon excitation have attracted much attention, owing
to their applications in electroluminescence.[1–3] The repre-
sentative compounds include PtII and IrIII complexes, etc.[1,4]
Recently, these complexes have been found to have new ap-
plications in photocatalysis,[5] optical limiting,[6] molecular
probes,[7,8] and upconversions.[9–14]
Concerning these new applications, the ability to have
strong visible-light harvesting and to access the long-lived
triplet-excited states are often very crucial. However, con-
ventional N^N PtII–bis-acetylide complexes usually show
weak absorption in the visible range and short T1 excited-
state lifetimes (t<5 ms).[1,4,15] Thus, it is important to devel-
op new molecular design rationales for the preparation of
[a] W. Wu, Prof. J. Zhao, Dr. H. Guo, J. Sun, S. Ji, Z. Wang
State Key Laboratory of Fine Chemicals
School of Chemical Engineering
Dalian University of Technology
Dalian 116024 (P. R. China)
Fax : (+86)411-8498-6236
E-mail: zhaojzh@dlut.edu.cn
[**] BODIPY=boron-dipyrromethene.
moiety were quenched, the BODIPY-localized intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand
Supporting information, including the synthesis of complex Pt-2 and
the spectroscopic data, for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102634.
(3IL) state was populated (lifetime=25 ms, measured with
time-resolved transient absorption difference spectrosco-
Chem. Eur. J. 2012, 18, 1961 – 1968
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1961

py).[22] Thus, in all of these reported BODIPY dyads, the RT
phosphorescence of BODIPY was never observed and the
T1 excited-state lifetime was usually not very long.
Pt-1 were ideal for sensitizing photophysical or photochemi-
cal processes; herein this sensitization is demonstrated by
upconversion based on triplet–triplet annihilation (TTA).
After careful examination of the molecular structures of
those pioneering BODIPY-containing dyads, we realized
that the metal centers were attached onto the peripheral
moieties of BODIPY, rather than to the p-conjugated
BODIPY core. As a result, we envisaged that the heavy-
atom effect of the transition metals in these dyads was not
maximized, and thus the RT phosphorescence of BODIPY
was never observed. Observation of RT phosphorescence
usually requires a strong heavy-atom effect to facilitate the
radiative 3IL!S0 intersystem crossing (ISC), that is, the
phosphorescence.[19]
Results and Discussion
Design and synthesis: To avoid non-radiative decay, the re-
cently reported cyclometalated platinum(II) scaffold em-
ployed in the C^N^N complex of 2,6-bis(N-alkylbenzimida-
zol-2’-yl)benzene was used instead of the PtII–terpyridine-
coordinated scaffold.[1,26] The synthesis of complex Pt-1 uses
the BODIPY chromophore as the starting material. After
di-iodation at the 2- and 6-positions, two acetylide groups
were attached to the BODIPY core (3) using Sonogashira
coupling. Finally, a CuI-catalyzed reaction between the
building block Pt-2 and BODIPY acetylide afforded the de-
sired Pt-1 in good yield (64.9%; for more details, see the
Experimental Section). It should be pointed out that the PtII
centers are connected to the p-core of the BODIPY fluoro-
phore. This unique structural profile has never been em-
ployed in the preparation of phosphorescent transition-
metal complexes with BODIPY.
To this end, herein we report the synthesis of complex Pt-
1 (Scheme 1), in which two N^C^N PtII-coordinated units
are connected to the p-core of BODIPY to maximize the
heavy-atom effect of the Pt atom. Thus, we expected that
the RT phosphorescence of BODIPY could be observed.
We used the PtII-coordinated unit of complex Pt-2 for the
construction of complex Pt-1.[26] Complex Pt-2 was also used
as a model complex in the study of the photophysical prop-
erties of these compounds. Complex Pt-1 shows strong ab-
sorption in the visible-light range and a long-lived T1 excited
state, as well as RT BODIPY phosphorescence. The popula-
Steady-state electronic spectroscopy (absorption and emis-
sion): The UV/Vis absorption spectra of compounds Pt-1,
Pt-2, and the acetylide ligand BDP were studied (Figure 1).
Compound Pt-2 showed a weak absorption in the visible
range (absorption maxima centered at 414 nm, molar extinc-
tion coefficient e=16200mꢀ1 cmꢀ1). However, for complex
Pt-1, a strong absorption at 574 nm was observed (e=
3
tion of the BODIPY-localized IL excited state for complex
Pt-1 was confirmed by nanosecond time-resolved transient
absorption difference spectroscopy, emission spectroscopy at
77 K, and DFT calculations (analysis of the spin-density sur-
faces and the molecular orbitals). The strong absorption of
visible light and the long-lived T1 excited state of complex
Scheme 1. Synthesis of complexes Pt-1 and Pt-2, the structure of 2,6’-diiodo-bis-BODIPY (4; used as the standard for measuring the upconversion quan-
tum yields), and the triplet-acceptor perylene (used in TTA upconversion). Reagents and conditions: i) H3PO4, 2308C, 4 h; ii) NaH, 1-bromobutane,
DMF, 1008C, overnight; iii) K2PtCl4, HOAc, microwave; iv) dry CH2Cl2, BF3·OEt2, NEt3; v) NIS (4.0 equiv), RT; vi) trimethylsilylacetylene, [Pd-
(PPh3)2Cl2], PPh3, CuI, NEt3, reflux, 8 h; vii) Bu4NF, THF, ꢀ788C; viii) CuI, CH2Cl2, (iPr)2NH.
1962
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N^C^N PtII–Acetylide Complex with a BODIPY Chromophore
FULL PAPER
Table 1. Photophysical parameters of the PtII complexes and the
BODIPY ligand.
[a]
labs
e[b]
Emission Properties
lem [nm][c]
FL [%][f]
t[g]
Pt-1
406/574
3.39/5.38
660/770[d]
3.5[e]
128.4 ms (RT)
1.5 ms (77 K)
6.5 ms
Pt-2
BDP
395/414
543
1.34/1.62
8.58
513/554
560
18.3
54.4
6.0 ns
[a] In toluene (1.0ꢀ10ꢀ5 m). [b] Molar extinction coefficient.
104 mꢀ1 cmꢀ1. [c] In MeCN. [d] In MeOH. [e] In deaerated MeOH, with
[Ru(phen)(bpy)2](PF6)2 (F=0.06 in MeCN) as a standard. [f] In MeCN,
e
:
A
R
ACHTUNGTRENNUNG
with BODIPY (F=0.72 in THF) as a standard. [g] Luminescence life-
time at RT.
Figure 1. UV/Vis absorption spectra of complexes Pt-1, Pt-2, and acety-
lide ligand BDP (toluene, 1.0ꢀ10ꢀ5 m; 208C).
730 nm),[22] which supports our assignment of the RT emis-
3
sion of complex Pt-1 at 770 nm as the IL emission of the
BODIPY ligand.
53800mꢀ1 cmꢀ1). This remarkably strong absorption in the
visible range is very rare for PtII–acetylide complexes.[1] The
acetylide ligand BDP showed an absorption at 543 nm. Thus
the significantly red-shifted absorption of complex Pt-1 com-
pared to the BDP ligand indicates a strong electronic inter-
action between the PtII center and the p-conjugated core of
the BODIPY chromophore.[21] Notably, the absorption of
complex Pt-1 in the visible region is more intense than most
N^N PtII–bis-acetylide complexes.[1,27,28]
Interestingly, besides the emission band at 770 nm, an
emission band was also observed at 660 nm for complex Pt-
1 (Figure 2). This emission was sensitive to O2 and was sig-
nificantly quenched in air, thereby indicating that it was a
phosphorescence emission (Figure 2). Thus, we propose that
the dual emissions of complex Pt-1 at 660 nm and 770 nm
3
3
are due to MLCT and the IL emissive state, respectively,
and that the two triplet excited state are in equilibrium at
RT.[29] The energy difference between the two states is at
least 2164 cmꢀ1 (0.27 eV, approximated as the distance be-
tween the two peaks).
Next, we studied the RT photoluminescence (Figure 2).
Unlike complex Pt-2, which showed structured emission at
500–650 nm, highly red-shifted emission peaks at 660 nm
To confirm that the dual emission of complex Pt-1 at
660 nm/770 nm is due to the simultaneous emission from the
equilibrated 3MLCT and 3IL states, rather than from one
dual-emissive T1 excited state, we measured the emission
spectra of complex Pt-1 at 77 K (Figure 3). Only the emis-
sion band at 775 nm was observed at 77 K. The thermally in-
duced Stokes shift (DES) for this emission band was as small
3
as 84 cmꢀ1, which is a clear indication of IL emission for
this emission band at 770 nm.[16b] We propose that, at 77 K,
3
3
the MLCT state and the low-lying IL excited state are no
longer in equilibrium because the upward transformation
3
3
from the IL to the MLCT state is more difficult at 77 K
Figure 2. Normalized emission spectra of complexes Pt-1 and Pt-2. Pt-1:
MeOH, lex =590 nm; Pt-2: MeCN, lex =420 nm. 1.0ꢀ10ꢀ5 m. 208C.
3
than at RT; thus, only the emission from the IL state was
observed.[29]
and 770 nm were observed for complex Pt-1. Because the
N^N PtII bis-acetylide complexes usually show emission
peaks in the range 500–600 nm,[1,27,28] we tentatively assigned
3
the emission of complex Pt-1 at 770 nm to the IL emission
of the BOIDPY core (1.61 eV, see below). This assignment
was supported by the T1 energy level of the BODIPY acety-
lide ligand (BDP), which was calculated to be 821 nm
(1.51 eV; see the Supporting Information). The RT phos-
phorescence quantum yield (FP) of complex Pt-1 was 3.5%
(Table 1). To the best of our knowledge, this is the first time
that the RT phosphorescence of BODIPY has been ob-
served. None of the previously reported BODIPY-contain-
ing PtII and IrIII complexes showed RT phosphores-
cence.[18,22] One of those complexes showed phosphores-
Figure 3. Emission spectra of complex Pt-1 in EtOH/MeOH (4:1 v/v)
glass was at 77 K and solution at RT. lex =590 nm. The asterisk indicates
a spike owing to the spectrometer.
cence of the BODIPY chromophore at 77 K (lem
=
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1963

J. Zhao et al.
3
Nanosecond time-resolved transient difference absorption
spectroscopy: Next, nanosecond time-resolved transient dif-
ference absorption spectra were studied (Figure 4). Upon
complex Pt-1. Typically, the IL state has a longer lifetime
3
than the MLCT state.[31–33]
To assign the T1 state of complex Pt-1 from the perspec-
tive of molecular orbitals, the UV/Vis absorption and the
phosphorescence of the complexes were studied by time-de-
pendent DFT (TDDFT) analysis. The calculated absorption
bands of complex Pt-1 were at 570 nm and 429 nm
(Table 2); these values were in good agreement with the ex-
Table 2. Electronic excitation energies (eV), oscillator strengths (f), main
configurations, and CI coefficients of the low-lying electronically excited
states of complex Pt-1.[a]
Electronic
transition
TDDFT//B3LYP/6-31G(d)
Composition CI[d]
Energy
f[c]
Character
[b]
N
]
S0!S1
S0!S3
S0!S10
0.0003
0.6361
0.4982
Hꢀ1!L+1 0.1605
H!L+1 0.1343
H!L+2 0.6681
LLCT
LLCT
LLCT
LLCT
Figure 4. a) Time-resolved transient difference absorption spectra of com-
plex Pt-1 in deaerated MeOH (lex =532 nm), and b) a decay trace of
complex Pt-1 at 550 nm, 258C.
2.17/570
2.89/429
Hꢀ4!L
Hꢀ2!L
H!L
0.1230
0.1191 ILCT,MLCT
0.6769 ILCT,MLCT
Hꢀ4!L+1 0.3293
Hꢀ3!L+2 0.4689
Hꢀ2!L+1 0.3857
LLCT
LLCT
LLCT
ILCT
pulsed-laser excitation, intense bleaching was found at
550 nm for complex Pt-1, which was due to depletion of the
ground state of the BDP ligand. DFT calculations on the
transient absorption spectra supported the assignment of the
3IL state for complex Pt-1 (see the Supporting Information).
The lifetime (t) of the T1 state of complex Pt-1 was 125.8 ms
(RT), which was consistent with the lifetime determined by
luminescence (128.4 ms). This lifetime is the longest RT trip-
let excited-state lifetime ever reported for BODIPY. The
previous values for BODIPY-localized 3IL states were 8–
30 ms.[18,22,25,30]
[e]
S0!T1
1.39/892
2.01/617
0.0000[f] Hꢀ6!L
H!L
0.1910
0.6814 ILCT,MLCT
0.1229 LLCT
0.6771 ILCT,MLCT
[e]
S0!T2
0.0000[f] Hꢀ3!L
Hꢀ1!L
[a] calculated at the TDDFT//B3LYP/6-31G(d)/LanL2DZ level. [b] Only
selected low-lying excited states are presented. [c] Oscillator strength.
[d] The CI coefficients are absolute values. [e] Based on optimized excit-
ed-state geometries. [f] No spin-orbit coupling was considered, thus f=0.
perimental values (Table 1 and Figure 1). The main compo-
nents of the 570 nm absorption band were due to the
HOMO!LUMO transition, which was localized on the
BODIPY units (Figure 6 and Table 2).
DFT calculations for assignment of the T1 excited state: To
confirm the IL nature of the T1 state of complex Pt-1, the
3
spin-density surfaces of complexes Pt-1 and Pt-2 were ana-
lyzed (Figure 5).[19] For complex Pt-2, the spin density was
distributed on the PtII center and on the N^C^N ligand;
thus, the T1 state of complex Pt-2 could be safely assigned
as a 3MLCT. However, the spin density for complex Pt-1
was located on the BODIPY chromophore, whilst the
N^C^N ligand and the PtII center made very small contribu-
tions. Thus, the lowest lying triplet state of complex Pt-1 is
3
the IL state.[13,14,19] This theoretical prediction is consistent
with the experimental results for the near-IR BODPY phos-
phorescence and with the long-lived triplet excited state of
Figure 5. Spin-density surfaces of complexes Pt-1 and Pt-2 at the opti-
mized triplet-state geometry. Calculated at the DFT/B3LYP/6-31G(d)/
LANL2DZ level with Gaussian 09W.
Figure 6. Frontier molecular orbitals for complex Pt-1, at the optimized
S0 state geometry. Calculated by DFT at the B3LYP/6-31G(d)/LanL2DZ
level using Gaussian 09W.
1964
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N^C^N PtII–Acetylide Complex with a BODIPY Chromophore
FULL PAPER
The triplet excited states were also studied by TDDFT
calculations (Table 2). The calculated T1 and T2 states gave
energy levels that were close to the experimental results
(Figure 2 and Table 1). The molecular orbitals involved in
the T1 and T2 states were mainly localized on the BODIPY
ligand. These results were in agreement with the time-re-
solved transient absorption difference spectra and the spin-
density analysis of the complex (Figure 4 and Figure 5).
The electronic spectra of the model complex, Pt-2, were
also studied with the TDDFT methods (Table 3 and
Figure 7). The calculated absorption maximum was located
at 423 nm, which is close to the experimentally observed
remarkably different to that of the T1 state energy level for
the calculated complex Pt-1 (1.39 eV); these results con-
firmed that the TDDFT method is reliable for the current
complexes. Furthermore, the TDDFT calculation indicated
that the frontier molecular orbitals of complex Pt-2 are sub-
stantially different from those of complex Pt-1. For example,
the S0!S1 transition of complex Pt-2 can be recognized as a
MLCT/IL transition (Figure 7), which is drastically different
from that of complex Pt-1, for which an IL feature is promi-
nent (Figure 6). Moreover, the T1 excited state of complex
Pt-2 is mainly a 2,6-bis(N-alkylbenzimidazol-2’-yl)benzene-
3
localized IL state, which is entirely different from that of
complex Pt-1, for which the T1 state is localized on the
BODIPY ligand (Figure 6 and Table 2).
Table 3. Electronic excitation energies (eV), oscillator strengths (f), main
configurations, and CI coefficients of the low-lying electronically excited
states of complex Pt-2.[a]
Application in upconversion based on triplet–triplet annihi-
lation: The intense visible-light absorption and the long-
lived triplet excited state of complex Pt-1 are ideal for appli-
cations in sensitizing photophysical processes that need trip-
let excited states for initiation, such as upconversion based
on triplet–triplet annihilation (TTA; see Scheme 2 for Ja-
Electronic
transition
TDDFT//B3LYP/6-31G(d)
Composition CI[d]
Energy
f[c]
Character
[b]
N
]
S0!S1
S0!S9
2.93/423
3.74/332
2.43/510
0.1109
0.2185
H!L
0.7005
0.1230 LLCT/ILCT
0.1476
0.1658
0.4624 MLCT/ILCT
MLCT
Hꢀ4!L
[e]
S0!T1
0.0000[f] Hꢀ5!L
Hꢀ4!L+1
Hꢀ2!L
ILCT
LLCT
Hꢀ1!L
0.4317
0.1233
LLCT
MLCT
H!L+1
[a] Calculated at TDDFT//B3LYP/6-31G(d)/LanL2DZ level [b] Only se-
lected low-lying excited states are presented. [c] Oscillator strength.
[d] The CI coefficients are absolute values. [e] Based on the optimized
excited state geometries. [f] No spin-orbit coupling was considered, thus
f=0.
Scheme 2. Jablonski diagram of triplet–triplet annihilation (TTA) upcon-
version, with complex Pt-1 as the triplet sensitizer. TTET=triplet–triplet
energy-transfer.
blonski diagram).[9–12,34] Unlike other upconversion methods,
such as two-photon absorption dyes or rare-earth materi-
als,[35–37] TTA upconversion is particularly interesting, owing
to the low excitation power (a few mWcmꢀ2 is sufficient,
the same scale as terrestrial solar-light intensity), the tunable
excitation/emission wavelengths, and the high upconversion
quantum yields. Triplet–triplet energy transfer (TTET) be-
tween the triplet sensitizer and the triplet acceptor/annihila-
tor is the crucial process in TTA upconversion.
Figure 7. Frontier molecular orbitals for complex Pt-2, at the optimized
S0 state geometry. Calculated by DFT at the B3LYP/6-31G(d)/LanL2DZ
level using Gaussian 09W.
The current development of TTA upconversion faces
some challenges; for example, triplet sensitizers are limited
to PtII–porphyrin complexes to a large extent, for which the
energy level of the T1 state is difficult to change.[9–11,34] Thus,
new triplet sensitizers with intense visible-light absorptions,
readily tunable T1 state energy levels, and long-lived T1
states were highly desired. PtII–acetylide complexes are
ideal candidates because the aforementioned photophysical
properties of these complexes can be readily tuned by
changing the acetylide ligand, which is synthetically feasi-
ble.[27,28,38]
value (414 nm; Figure 1 and Table 1). Furthermore, the os-
cillator strength for the S0!S1 transition was much smaller
than that of the corresponding transition in complex Pt-1
(Table 2); this trend is in agreement with the stronger ab-
sorption of complex Pt-1, compared to the UV/Vis absorp-
tion of complex Pt-2 (Figure 1).
The T1 energy level of complex Pt-2 was calculated to be
510 nm, which was close to the experimentally observed
phosphorescence of complex Pt-2 (513 nm; Figure 2 and
Table 1). The T1 energy level of complex Pt-2 (2.43 eV) was
Chem. Eur. J. 2012, 18, 1961 – 1968
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1965

J. Zhao et al.
Recently, a N^N^N PtII–phenylacetylide complex was
used as a triplet sensitizer for TTA upconversion.[39] Howev-
er, that complex showed weak absorption in the visible-light
region (e=7600mꢀ1 cmꢀ1 at 476 nm) and a short lifetime of
the T1 state (t=4.6 ms). As a result, the low upconversion
quantum yield was observed (FUC <1.1%).[39]
The Jablonski diagram of the TTA upconversion
(Scheme 2) indicates that the visible-light-harvesting ability
and the lifetime of the triplet excited state are two crucial
properties for improving the efficiency of TTA upconver-
sion. In principle, TTET is a bimolecular quenching process
that follows the Stern–Volmer equation; thus the lifetime of
the T1 excited state of the triplet sensitizer is important:
long-lived T1 excited states will enhance the TTET process.
Furthermore, strong absorption of the excitation light by the
triplet sensitizer would produce more-concentrated sensitiz-
ers at the triplet excited state, thus the TTET process can
also be enhanced. Therefore, the ideal triplet sensitizers for
TTA upconversion would be those complexes that show
strong absorption and long-lived T1 excited states.
tion, the overall upconversion capability (h=eꢀFUC, where
e is the molar extinction coefficient of the triplet sensitizer
and FUC is the upconversion quantum yield) of complex Pt-
1 is significant. Previously, only the upconversion quantum
yield (FUC) was used to evaluate the upconversion capabili-
ty of triplet sensitizers,[9] but we propose that the h value is
a more-reasonable parameter, especially where practical ap-
plications of the TTA upconversion are concerned, such as
for photovoltaics.
Herein, we noticed two interesting results: 1) The dual
emission bands of complex Pt-1 were both quenched with
addition of perylene (Figure 8a); this observation indicates
that both emission bands are due to triplet emissive excited
states, which is in line with our previous conclusions. 2) The
quenched phosphorescence peak area of complex Pt-1 is
smaller than the peak area of the upconverted fluorescence
band. This result goes against the traditional observations of
TTA upconversion,[9–11,32,34] where the generation of upcon-
verted fluorescence is accompanied by significant quenching
of the phosphorescence of the triplet sensitizer. Our obser-
vation implies that triplet sensitizers at the T1 excited states
that are otherwise non-emissive were involved in the TTET
process. Thus, we suggest that weakly phosphorescent, or
even non-phosphorescent, transition-metal complexes can
be used as triplet sensitizers for TTA upconversion.[12b,20] At
present, most of the utilized triplet sensitizers are phosphor-
escent.[9,11] In fact, the phosphorescence of these triplet sen-
sitizers are detrimental to the TTA upconversion because
the radiative decay of the T1 excited state of sensitizer (i.e.
the phosphorescence) is in competition with TTET
(Scheme 2).
Because complex Pt-1 shows precisely these desired pho-
tophysical properties, that is, intense absorption in visible
region (e=53800mꢀ1 cmꢀ1 at 574 nm) and a long-lived T1 ex-
cited state (tP =128.4 ms, RT), it was used as a triplet sensi-
tizer for TTA upconversion. Perylene was used as a triplet
acceptor (ET1 =1.53 eV, 810 nm). With excitation at 635 nm,
complex Pt-1 showed dual emission at 660 nm and 770 nm
(Figure 8). In the presence of perylene, intense blue emis-
Quenching of the dual phosphorescence of complex Pt-1
with perylene was studied quantitatively (Figure 9). The
emission at 770 nm was quenched more-significantly than
that at 660 nm. In this case, Fçster quenching of the emis-
sion of complex Pt-1 by perylene is impossible, owing to the
unmatched singlet excited state energy levels and the dis-
tance between complex Pt-1 and perylene molecules in solu-
tion. Thus, the Dexter mechanism is responsible for the
phosphorescence quenching of complex Pt-1, that is, by
TTET from complex Pt-1 to perylene. The Stern–Volmer
Figure 8. Upconversion with complex Pt-1 as a triplet sensitizer (1.0ꢀ
10ꢀ5 m). a) Emission spectra and b) photographs of the emission of sensi-
tizers Pt-1 and Pt-2, alone and in presence of perylene (Py; 4.0ꢀ10ꢀ5 m).
Excited with a red laser (lex =635 nm, 20 mW), in deaerated MeOH,
208C.
sion in the range 400–550 nm was observed and both phos-
phorescence bands of complex Pt-1 were completely
quenched. Irradiation of perylene alone did not produce the
blue emission, thereby verify the upconverted fluorescence.
No upconversion was observed with complex Pt-2 under the
same conditions (see the Supporting Information). The up-
conversion with complex Pt-1 as the triplet sensitizer was
visible to the naked eye (Figure 8b).
Figure 9. a) Quenching of the phosphorescence of complex Pt-1 by pery-
lene; b) Stern–Volmer plots of complexes Pt-1 and Pt-2 with perylene
(for Pt-1) or 9,10-diphenylanthracene (DPA, for Pt-2) as triplet acceptors
(quencher).
430 nm for Pt-2. In deaerated MeOH, 208C.
[Pt-2]=1.0ꢀ10ꢀ5 m. lex =590 nm for Pt-1 and
cACTHNGUTRENN[UG Pt-1]or cACHTGNUTRENNUNG
The upconversion quantum yield with complex Pt-1 as the
triplet sensitizer was 5.2%. Considering its intense absorp-
1966
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1961 – 1968

N^C^N PtII–Acetylide Complex with a BODIPY Chromophore
FULL PAPER
(TCSPC) instrument. The nanosecond time-resolved transient absorption
difference spectra were measured by Edinburgh analytical instruments
(LP 920, Edinburgh Instruments, Livingston, U.K.) and recorded on a
Tektronix TDS 3012B oscilloscope. All samples in flash photolysis experi-
ments were deaerated with argon for approximately 15 min before mea-
surement and the gas flow was kept constant during the measurement.
fitting showed a downward curvature (Figure 9), which
clearly indicated that two excited species with different life-
times were responsible for the quenching (Table 4). For
complex Pt-2, only one quenching constant was observed
Table 4. Upconversion-related parameters of the PtII complexes.[a]
Luminescence quantum yield of complex Pt-1 was measured with [Ru-
AHCTNUGTREUN(GNN bpy)2ACHTUNTREGN(GNUN Phen)]CAHTUNGTREN(NUNG PF6)2 (F=6.0% in deaerated CH3CN) as the reference
[b]
Ksv [ꢀ10ꢀ3 mꢀ1
]
FUC [%][c]
and the other values were measured with BODIPY as the reference (F=
0.72 in THF). The luminescent photographs were obtained using a Sam-
sung NV 5 digital camera. The exposure time was the default value of
the camera. A diode-pumped solid-state (DPSS) laser was used for the
upconversions. The samples were purged with N2 or Ar for 15 min before
measurement. The upconversion quantum yields were determined with
2,6’-diiodo-BODIPY (F=9.3% in toluene; see Scheme 1 for the molecu-
lar structure) as the standard.
Pt-1
Pt-2
Ksv1 =3.6 (13.3)[d]
Ksv2 =1232.2 (86.7)[d]
40.7
5.2
0.0
[a] In deaerated CH3OH (1.0ꢀ10ꢀ5 m; 208C). [b] Stern–Volmer quench-
ing constant. [c] Upconversion quantum yields. [d] The two quenching
constants (the numbers in the parenthesis are the percentage ratios of
the components).
The density functional theory (DFT) calculations were used for optimiza-
tion of the ground-state and excited-states geometries. The energy level
of the singlet and triplet states of the acetylide ligand (BDP) and the
complexes were calculated with by time-dependent DFT (TDDFT) anal-
ysis. The spin-density surfaces of complexes Pt-1 and Pt-2 were calculat-
ed with the optimized triplet-state geometries.
(Figure 9b). It should be pointed out that 9,10-diphenylan-
thracene (DPA) was used as triplet acceptor for the quench-
ing studies of complex Pt-2 rather than perylene. This
change was because the excitation wavelength of complex
Pt-2 is shorter than that of complex Pt-1 (Figure 1), and so
perylene can be excited at the excitation wavelength of com-
plex Pt-2.[9b]
1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (3): Under a
nitrogen atmosphere, benzoyl chloride (2.8 g, 0.021 mol) and 2,4-dime-
thylpyrrole (4 mL, 3.7 g, 0.04 mol) were added to dry CH2Cl2 (150 mL)
using a syringe. The mixture was stirred at RT overnight, then Et3N
(20 mL) and BF3·Et2O (20 mL) were added under ice-cold conditions,
and the reaction mixture was stirred for 1 h. Next, the mixture was
poured into water (200 mL), and the organic layer was collected, dried
over anhydrous Na2SO4, and evaporated under reduced pressure. The
crude product was purified by column chromatography on silica gel
(CH2Cl2/n-hexane, 1:1 v/v) to give compound 3 as a red powder (2.3 g,
yield: 33.3%). 1H NMR (400 MHz, CDCl3): d=7.49–7.47 (m, 3H), 7.29–
7.26 (m, 2H), 5.98 (s, 2H), 2.56 (s, 6H), 1.37 ppm (s, 6H).
Conclusion
Room-temperature long-lived near-IR phosphorescence of
BODIPY was observed (lem =770 nm, FP =3.5%, tP =
128.4 ms). Our unique molecular-design strategy is to attach
PtII coordination centers directly onto the BODIPY p-core
using acetylide bonds, thereby maximizing the heavy-atom
effect of PtII. Intersystem crossing (ISC) is facilitated and
the radiative decay of the T1 excited state of BODIPY is ob-
served, that is, the phosphorescence of BODIPY. The com-
plex shows intense absorption in visible range (e=
53800mꢀ1 cmꢀ1 at 574 nm). The complex is dual emissive at
660 nm and 770 nm. The T1 excited state of the complex is
2,6-Diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene
(4):[14b] To a solution of compound 3 (200 mg, 0.62 mmol) in dry CH2Cl2
(25 mL) was added excess N-iodosuccinimide (NIS; 558 mg, 2.48 mmol).
The mixture was stirred at RT for about 30 min (monitored by TLC until
the starting material had been completely consumed). The reaction mix-
ture was then concentrated under vacuum, and the crude product was pu-
rified by column chromatography on silica gel (n-hexane/CH2Cl2, 2:1 v/
v). The red band was collected and the solvent was removed under re-
duced pressure to obtain a red solid (300.0 mg, yield: 84.0%). 1H NMR
(400 MHz, CDCl3): d=7.54–7.51 (m, 3H), 7.26–7.24 (m, 2H), 2.65 (s,
6H), 1.38 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=156.91, 145.52,
141.52, 134.44, 129.69, 129.62, 127.91, 85.84, 17.12, 16.22 ppm; HRMS
(MALDI): m/z calcd for [C19H17BF2I2N2]ꢀ 575.9542; found: m/z:
575.9528.
3
mainly localized on the BODIPY moiety (i.e. IL state, as
determined by steady-state and time-resolved spectroscopy,
77 K emission spectra, and spin-density analysis). The in-
tense visible-light-harvesting ability and long-lived T1 excite
state of the complex were used for triplet–triplet annihila-
tion upconversion and an upconversion quantum yield of
5.2% was observed. Our work paves the way for access to
transition-metal complexes that show strong absorption of
2,6-Diethynyl-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene
(BDP): Under an argon atmosphere, compound 4 (280.0 mg, 0.49 mmol),
[PdACHTUNGTRNEU(GN PPh3)2Cl2] (34.0 mg, 0.049 mmol), PPh3 (26.8 mg, 0.10 mmol), and
CuI (19.4 mg, 0.10 mmol) were mixed in (iPr)2NH (5 mL) and THF
(10 mL). After stirring, trimethylsilylacetylene (190.6 mg, 1.94 mmol) was
added using a syringe. The solution was stirred at 608C for 4 h. The sol-
vent was removed under reduced pressure and the crude product was pu-
rified by column chromatography on silica gel (CH2Cl2) to obtain a dark-
red solid. Bu4NF·3H2O (1m in THF, 1.2 mL) was added dropwise to a so-
lution of the above trimethylsilyl-protected intermediate (150 mg,
0.29 mmol) in 5 mL THF at ꢀ788C and the solution was kept at ꢀ788C
for about 1 h. Next, CH2Cl2 (100 mL) and water (50 mL) were added.
The organic layer was separated, and the aqueous layer was extracted
with CH2Cl2 (3ꢀ15 mL). The combined organic layers were washed with
brine (200 mL), dried over anhydrous MgSO4, and the solvent was re-
moved under reduced pressure. The residue was purified by passing
through a silica plug (CH2Cl2) to give a red solid (80 mg, 0.21 mmol,
yield: 73.6%). 1H NMR (400 MHz, CDCl3): d=7.53–7.52 (m, 3H), 7.26–
7.24 (m, 2H), 3.31 (s, 2H), 2.65 (s, 6H), 1.46 ppm (s, 6H). 13C NMR
3
visible light and long-lived IL excited states, which are im-
portant for applications in photovoltaics, photocatalysis, and
upconversions.
Experimental Section
All of the chemicals used were analytical pure and used as received. Sol-
vents were dried and distilled before use. UV/Vis absorption spectra
were measured with an Agilent 8453 UV/Vis spectrophotometer. Fluo-
rescence spectra were recorded on a Shimadzu 5301 PC spectrofluorome-
ter or on a Sanco 970 CRT spectrofluorometer. Fluorescence/phosphores-
cence lifetimes were measured on a Horiba Jobin Yvon Fluoro Max-4
Chem. Eur. J. 2012, 18, 1961 – 1968
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1967

J. Zhao et al.
(100 MHz, CDCl3): d=159.00, 145.70, 143.30, 134.40, 131.10, 129.64,
129.56, 127.86, 115.32, 84.30, 76.07, 13.68, 13.40 ppm. HRMS (EI): m/z
calcd for [C23H19N2F2B]+ 372.1609; found: 372.1612.
H. Guo, Dalton Trans. 2011, 40, 11550–11561; d) L. Huang, L.
Zeng, H. Guo, W. Wu, W. Wu, S. Ji, J. Zhao, Eur. J. Inorg. Chem.
2011, 4527–4533; e) W. Wu, H. Guo, W. Wu, S. Ji, J. Zhao Inorg.
Chem. 2011, 50, 11446–11460; f) Y. Liu, Q. Li, J. Zhao, H. Guo.
RSC Adv. 2011, DOI:10.1039C1A00434D.
Complex Pt-1: Under an argon atmosphere, complex Pt-2 (48.0 mg,
0.074 mmol) and BDP (13.2 mg, 0.037 mmol) were dissolved in a mixed
solvent system of diisopropylamine (1.0 mL) and CH2Cl2 (5 mL). The
mixture was deaerated with argon, then CuI (5 mg, 0.09 mmol) was
added and the mixture was stirred at RT overnight. The mixture was
evaporated to dryness and the residue was purified by column chroma-
tography on silica gel (CH2Cl2/CH3OH, 100:1, v/v). The purple band was
collected, and 38.0 mg of purple powder was obtained, yield: 64.9%.
1H NMR (400 MHz, CDCl3/[D6]DMSO): d=8.39 (d, 2H, J=8.0 Hz),
7.94–7.92 (m, 3H), 7.68–7.62 (m, 9H), 7.45–7.37 (m, 7H), 7.19–7.17 (m,
1H), 7.12–7.08 (m, 3H), 6.94 (t, 1H, J=7.8 Hz), 6.60 (t, 1H, J=8.4 Hz),
4.67–4.29 (m, 8H), 2.91 (s, 6H) 2.02–1.79 (m, 8H), 1.76 (s, 6H), 1.53–1.40
(m, 8H), 1.02–0.85 ppm (m, 12H); 13C NMR (100 MHz, CDCl3 +
[D6]DMSO): d=160.96, 156.13, 143.20, 132.40, 132.22, 131.87, 123.94,
122.83, 122.47, 122.30, 116.41, 116.25, 112.88, 109.86, 108.83, 89.60, 76.15,
45.94, 43.77, 31.18, 28.66, 19.05, 12.96 ppm. HRMS (MALDI): m/z calcd
for [C79H75N10Pt2BF2]+ 1602.5533; found: m/z 1602.5460.
[15] J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R.
Bau, M. E. Thompson, Inorg. Chem. 2002, 41, 3055–3066.
[16] a) S. M. Borisov, I. Klimant, Anal. Chem. 2007, 79, 7501–7509;
b) I. E. Pomestchenko, C. R. Luman, M. Hissler, R. Ziessel, F. N.
Castellano, Inorg. Chem. 2003, 42, 1394–1396.
[17] P.-H. Lanoe, J.-L. Fillaut, L. Toupet, J. A. G. Williams, H. Le Bozeca,
V. Guerchais, Chem. Commun. 2008, 4333–4335.
[18] M. Galletta, S. Campagna, M. Quesada, G. Ulrich, R. Ziessel,
Chem. Commun. 2005, 4222–4224.
[19] K. Hanson, A. Tamayo, V. V. Diev, M. T. Whited, P. I. Djurovich,
M. E. Thompson, Inorg. Chem. 2010, 49, 6077–6084.
[20] J. Sun, W. Wu, H. Guo, J. Zhao, Eur. J. Inorg. Chem. 2011, 3165–
3173.
[21] A. A. Rachford, S. Goeb, F. N. Castellano, J. Am. Chem. Soc. 2008,
130, 2766–2767.
[22] A. A. Rachford, R. Ziessel, T. Bura, P. Retailleau, F. N. Castellano,
Inorg. Chem. 2010, 49, 3730–3736.
[23] a) G. M. Fischer, A. P. Ehlers, A. Zumbusch, E. Daltrozzo, Angew.
Chem. 2007, 119, 3824–3827; Angew. Chem. Int. Ed. 2007, 46, 3750–
3753; b) X. Qi, E. J. Jun, L. Xu, S.-J. Kim, J. S. J. Hong, Y. J. Yoon, J.
Yoon, J. Org. Chem. 2006, 71, 2881–2884.
[24] G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem. 2008, 120, 1202–
1219; Angew. Chem. Int. Ed. 2008, 47, 1184–1201.
[25] F. Nastasi, F. Puntoriero, S. Campagna, S. Diring, R. Ziessel, Phys.
Chem. Chem. Phys. 2008, 10, 3982–3986.
[26] A. Y.-Y. Tam, D. P.-K. Tsang, M.-Y. Chan, N. Zhu, V. W.-W. Yam,
Chem. Commun. 2011, 47, 3383–3385.
Acknowledgements
We thank the NSFC (20972024 and 21073028), the Royal Society (UK)
and the NSFC (China-UK Cost-Share Science Networks, 21011130154),
the Fundamental Research Funds for the Central Universities
(DUT10ZD212 and DUT11LK19), and the Ministry of Education
(SRFDP-200801410004 and NCET-08–0077) for their financial support.
[27] M. Hissler, W. B. Connick, D. K. Geiger, J. E. McGarrah, D. Lipa,
R. J. Lachicotte, R. Eisenberg, Inorg. Chem. 2000, 39, 447–457.
[28] C. E. Whittle, J. A. Weinstein, M. W. George, K. S. Schanze, Inorg.
Chem. 2001, 40, 4053–4062.
[29] A. Harriman, A. Khatyr, R. Ziessel, Dalton Trans. 2003, 2061–2068.
[30] M. Galletta, F. Puntoriero, S. Campagna, C. Chiorboli, M. Quesada,
S. Goeb, R. Ziessel, J. Phys. Chem. A 2006, 110, 4348–4358.
[31] F. N. Castellano, I. E. Pomestchenko, E. Shikhova, F. Hua, M. L.
Muro, N. Rajapakse, Coord. Chem. Rev. 2006, 250, 1819–1828.
[32] H. Guo, M. L. Muro-Small, S. Ji, J. Zhao, F. N. Castellano, Inorg.
Chem. 2010, 49, 6802–6804.
[1] J. A. G. Williams, Top. Curr. Chem. 2007, 281, 205–268.
[2] Y. Chi, P.-T. Chou, Chem. Soc. Rev. 2010, 39, 638–655.
[3] G. S. M. Tong, P. K. Chow, C.-M. Che, Angew. Chem. 2010, 122,
9392–9395; Angew. Chem. Int. Ed. 2010, 49, 9206–9209.
[4] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti,
Top. Curr. Chem. 2007, 281, 143–203.
[5] J. Zhang, P. Du, J. Schneider, P. Jarosz, R. Eisenberg, J. Am. Chem.
Soc. 2007, 129, 7726–7727.
[6] W.-Y. Wong, C.-L. Ho, Acc. Chem. Res. 2010, 43, 1246–1256.
[7] Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 2010, 39, 3007–3030.
[8] Y. Chen, K. Li, W. Lu, S. S. Chui, C.-W. Ma, C.-M. Che, Angew.
Chem. 2009, 121, 10093–10097; Angew. Chem. Int. Ed. 2009, 48,
9909–9913.
[9] a) T. N. Singh-Rachford, F. N. Castellano, Coord. Chem. Rev. 2010,
254, 2560; b) J. Zhao, S. Ji, H. Guo, RSC Adv. 2011, 1, 937–950.
[10] Y. Y. Cheng, B. Fꢂckel, T. Khoury, R. G. C. R. Clady, M. J. Y. Tayeb-
jee, N. J. Ekins-Daukes, M. J. Crossley, T. W. Schmidt, J. Phys.
Chem. Lett. 2010, 1, 1795–1799.
[11] A. Monguzzi, J. Mezyk, F. Scotognella, R. Tubino, F. Meinardi,
Phys. Rev. B 2008, 78, 195112–1–195112–5.
[12] a) S. Ji, W. Wu, W. Wu, H. Guo, J. Zhao, Angew. Chem. 2011, 123,
1664–1667; Angew. Chem. Int. Ed. 2011, 50, 1626–1629; b) S. Ji, H.
Guo, W. Wu, W. Wu, J. Zhao, Angew. Chem. 2011, 123, 8433–8436;
Angew. Chem. Int. Ed. 2011, 50, 8283–8286.
[33] N. Armaroli, ChemPhysChem 2008, 9, 371–373.
[34] S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S. Chernov, S.
Aleshchenkov, G. Nelles, A. Cheprakov, A. Yasuda, K. Mꢂllen, G.
Wegner, Angew. Chem. 2007, 119, 7837–7840; Angew. Chem. Int.
Ed. 2007, 46, 7693–7696.
[35] H. M. Kim, B. R. Cho, Acc. Chem. Res. 2009, 42, 863–872.
[36] F. Gao, Q. Liao, Z.-Z. Xu, Y.-H. Yue, Q. Wang, H.-L. Zhang, H.-B.
Fu, Angew. Chem. 2010, 122, 744–747; Angew. Chem. Int. Ed. 2010,
49, 732–735.
[37] M. Haase, H. Schꢃfer, Angew. Chem. 2011, 123, 5928–5950; Angew.
Chem. Int. Ed. 2011, 50, 5808–5829.
[38] X. Han, L. Z. Wu, G. Si, J. Pan, Q.-Z. Yang, L.-P. Zhang, C.-H.
Tung, Chem. Eur. J. 2007, 13, 1231–1239.
[39] P. Du, R. Eisenberg, Chem. Sci. 2010, 1, 502–506.
[13] W. Wu, W. Wu, S. Ji, H. Guo, J. Zhao, Dalton Trans. 2011, 40, 5953–
5963.
[14] a) Y. Liu, W. Wu, X. Zhang, H. Guo, J. Zhao, Dalton Trans. 2011,
40, 9085–9089; b) W. Wu, H. Guo, W. Wu, S. Ji, J. Zhao, J. Org.
Chem. 2011, 76, 7056–7064; c) W. Wu, J. Sun, S. Ji, W. Wu, J. Zhao,
Received: August 24, 2011
Published online: January 16, 2012
1968
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1961 – 1968