Synthesis, structural characterization, photophysics, and broadband nonlinear absorption of a platinum(II) complex with the 6-(7-benzothiazol- 2′-yl-9,9-diethyl-9-H-fluoren-2-yl)-2,2′-bipyridinyl ligand

Article abstract of DOI:10.1002/chem.201103095

A platinum complex with the 6-(7-benzothiazol-2′-yl-9,9-diethyl-9H- fluoren-2-yl)-2,2′-bipyridinyl ligand (1) was synthesized and the crystal structure was determined. UV/Vis absorption, emission, and transient difference absorption of 1 were systematically investigated. DFT calculations were carried out on 1 to characterize the electronic ground state and aid in the understanding of the nature of low-lying excited electronic states. Complex 1 exhibits intense structured ¹π-π* absorption at λ_abs<440 nm, and a broad, moderate ¹MLCT/ ¹LLCT transition at 440-520 nm in CH₂Cl₂ solution. A structured ³π-π*/³MLCT emission at about 590 nm was observed at room temperature and at 77 K. Complex 1 exhibits both singlet and triplet excited-state absorption from 450 nm to 750 nm, which are tentatively attributed to the ¹π-π* and ³π-π* excited states of the 6-(7-benzothiazol-2′- yl-9,9-diethyl-9H-fluoren-2-yl)-2,2′-bipyridine ligand, respectively. Z-scan experiments were conducted by using ns and ps pulses at 532 nm, and ps pulses at a variety of visible and near-IR wavelengths. The experimental data were fitted by a five-level model by using the excited-state parameters obtained from the photophysical study to deduce the effective singlet and triplet excited-state absorption cross sections in the visible spectral region and the effective two-photon absorption cross sections in the near-IR region. Our results demonstrate that 1 possesses large ratios of excited-state absorption cross sections relative to that of the ground-state in the visible spectral region; this results in a remarkable degree of reverse saturable absorption from 1 in CH₂Cl₂ solution illuminated by ns laser pulses at 532 nm. The two-photon absorption cross sections in the near-IR region for 1 are among the largest values reported for platinum complexes. Therefore, 1 is an excellent, broadband, nonlinear absorbing material that exhibits strong reverse saturable absorption in the visible spectral region and large two-photon-assisted excited-state absorption in the near-IR region.

Full text of DOI:10.1002/chem.201103095

FULL PAPER
DOI: 10.1002/chem.201103095
Synthesis, Structural Characterization, Photophysics, and Broadband
Nonlinear Absorption of a Platinum(II) Complex with the 6-(7-Benzothiazol-
2
’-yl-9,9-diethyl-9H-fluoren-2-yl)-2,2’-bipyridinyl Ligand
[
a]
[a]
[a]
[b]
Bingguang Zhang, Yunjing Li, Rui Liu, Timothy M. Pritchett,
[
c]
[a]
[d]
[a]
Alexander Azenkeng, Angel Ugrinov, Joy E. Haley, Zhongjing Li,
[
e]
[a]
Mark R. Hoffmann, and Wenfang Sun*
Abstract: A platinum complex with the
-(7-benzothiazol-2’-yl-9,9-diethyl-9H-
absorption from 450 nm to 750 nm,
which are tentatively attributed to the
p–p* and p–p* excited states of the
spectral region and the effective two-
photon absorption cross sections in the
near-IR region. Our results demon-
strate that 1 possesses large ratios of
excited-state absorption cross sections
relative to that of the ground-state in
the visible spectral region; this results
in a remarkable degree of reverse satu-
rable absorption from 1 in CH Cl solu-
tion illuminated by ns laser pulses at
532 nm. The two-photon absorption
cross sections in the near-IR region for
1 are among the largest values reported
for platinum complexes. Therefore, 1 is
an excellent, broadband, nonlinear ab-
sorbing material that exhibits strong re-
verse saturable absorption in the visi-
ble spectral region and large two-
photon-assisted excited-state absorp-
tion in the near-IR region.
6
1
3
fluoren-2-yl)-2,2’-bipyridinyl ligand (1)
was synthesized and the crystal struc-
ture was determined. UV/Vis absorp-
tion, emission, and transient difference
absorption of 1 were systematically in-
vestigated. DFT calculations were car-
ried out on 1 to characterize the elec-
tronic ground state and aid in the un-
derstanding of the nature of low-lying
excited electronic states. Complex 1 ex-
6-(7-benzothiazol-2’-yl-9,9-diethyl-9H-
fluoren-2-yl)-2,2’-bipyridine ligand, re-
spectively. Z-scan experiments were
conducted by using ns and ps pulses at
532 nm, and ps pulses at a variety of
visible and near-IR wavelengths. The
experimental data were fitted by
a five-level model by using the excited-
state parameters obtained from the
photophysical study to deduce the ef-
fective singlet and triplet excited-state
absorption cross sections in the visible
2
2
1
hibits intense structured p–p* absorp-
tion at l <440 nm, and a broad, mod-
abs
1
1
erate MLCT/ LLCT transition at 440–
20 nm in CH Cl solution. A struc-
5
2
2
3
3
tured p–p*/ MLCT emission at about
90 nm was observed at room tempera-
Keywords: luminescence · nonlin-
ear optics · photophysics · platinum ·
two-photon absorption
5
ture and at 77 K. Complex 1 exhibits
both singlet and triplet excited-state
Introduction
Square-planar platinum(II) complexes have drawn great at-
tention in the last two decades because of the intriguing
photophysical properties and the resulting, versatile, poten-
tial applications in photonic devices, such as in organic light-
[
a] Dr. B. Zhang, Dr. Y. Li, Dr. R. Liu, Dr. A. Ugrinov, Z. Li,
Dr. W. Sun
Department of Chemistry and Biochemistry
North Dakota State University, Fargo, ND 58108-6050 (USA)
Fax : (+1)701-231-8831
[1]
[2]
emitting diodes (OLEDs), dye-sensitized solar cells, pho-
[3]
[4]
E-mail: Wenfang.Sun@ndsu.edu
tochemical cells, and optical sensors. These applications
are essentially based on the complexesꢀ long-lived phosphor-
escence at room temperature, moderate to strong metal-to-
ligand charge transfer absorption in the visible spectral
region, and multiple intramolecular charge-transfer transi-
tions. More importantly, these properties can be modulated
by chemical or physical stimuli, such as pH, temperature,
[
b] Dr. T. M. Pritchett
US Army Research Laboratory, AMSRD-SE-EM
2
800 Powder Mill Road, Adelphi, MD 20783-1197 (USA)
[
c] Dr. A. Azenkeng
Energy and Environmental Research Center
University of North Dakota, Grand Forks, ND 58202-9018 ACHTUTNGRNENUNG( USA)
[
d] Dr. J. E. Haley
[4]
Materials and Manufacturing Directorate
Air Force Research Laboratory
Wright Patterson Air Force Base, Dayton, OH 45433 (USA)
cation, anion, and so forth. In recent years, our group and
several other groups have reported the potential of utilizing
the broad and moderately strong excited-state absorption of
some platinum complexes in the visible to the near-IR
region for nonlinear optical applications, including optical
[
e] Dr. M. R. Hoffmann
Department of Chemistry
University of North Dakota, Grand Forks, ND 58202-9024 AHCUTNGRNENUG( USA)
[5]
switching and laser-pulse compression.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103095.
Chem. Eur. J. 2012, 18, 4593 – 4606
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4593

II
[5a,d,l,p,r]
Another attractive feature of square-planar Pt complexes
tends from the visible to the near-IR region.
However,
is the ease of structural modification. A variety of monoden-
tate, bidentate, terdentate, and tetradentate ligands have
the lack of ground-state absorption above 570 nm prevents
population of the excited state through one-photon absorp-
tion; this limits the potential application of these complexes
as broadband nonlinear absorbing materials. If the excited
state could be populated through TPA, it would not only
allow the utilization of the broadband excited-state absorp-
tion, but the TPA itself could also reduce the transmission
of light.
Other important structural modifications include intro-
duction of the benzothiazolyl (BTZ) substituent on the fluo-
rene unit. Because the BTZ group is an electron-withdraw-
ing aromatic component, it would not only extend the p
conjugation in the target molecule, that is, complex 1 in
Scheme 1, but also make the complex possess the acceptor
(BTZ)–p (fluorenyl component)–acceptor (bipyridinyl Pt
[6]
been reported to coordinate with the platinum ion. Photo-
physical properties of the resultant complexes are drastically
modulated by the different types of ligands. Among the vari-
ous ligands, terpyridyl and 6-phenyl-2,2’-bipyridyl (C^N^N)
[6a]
ligands have been widely investigated. It has been shown
that due to the p-donating ability of the phenyl ring and re-
duced distortion from the square-planar configuration in the
6
-phenyl-2,2’-bipyridyl platinum complexes, the [Pt-
(C^N^N)] complexes exhibit stronger emission than the cor-
responding platinum terpyridyl complexes. Studies in our
group further revealed that the [Pt(C^N^N)] complexes ex-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
hibit broader triplet excited-state absorption in the visible to
the near-IR region and stronger reverse saturable absorp-
tion at 532 nm for ns laser
pulses than the terpyridyl coun-
[5a,d,l,p,r]
terparts.
Therefore, [Pt-
ACHTUNGTRENNUNG( C^N^N)] complexes are prom-
ising broadband nonlinear-
transmission materials. By in-
troducing electron-donating al-
koxyl substituents on the
phenyl ring of the C^N^N
ligand, the lowest excited state
incorporates more intra ACHUNTGRENNUGl i ACHTUNGRENNUGg and-
charge-transfer character, lead-
ing to a longer triplet excited-
state lifetime and enhanced
[5l]
emission.
Recently, we reported that
incorporation of a p-donating
fluorenyl substituent on the 4-
position of the central pyridine
ring of the C^N^N ligand in-
creased the triplet-excited-state
lifetimes, the emission quantum
yields, and the ratios of the ex-
cited-state absorption cross-sec-
tion relative to that of the
ground-state for these complex-
Scheme 1. Synthetic route for 1.
[
5r]
es.
Consequently, stronger reverse saturable absorption
component) (A–p–A) feature. The A–p–A structural fea-
was observed from the platinum(II) chloride complex con-
taining the 4-fluorenyl substituent on the central pyridine
ring than in the corresponding complexes without the fluo-
renyl substituent. Intrigued by these results, we are inter-
ested in how the photophysics and nonlinear absorption
would be influenced by introducing the substituted fluorenyl
component at the 6-position of the pyridine ring to form
a new type of C^N^N ligand. In addition, incorporating the
fluorenyl component into the platinum complex would
greatly improve the probability of two-photon absorption
ture has been found to be an effective approach to enhance
[7]
TPA in organic molecules. Previous studies by our group
II
and by others have demonstrated that Pt complexes con-
[5r]
taining the BTZ-substituted fluorenyl acetylide ligands ex-
[5q,t]
hibit broadband TPA in the near-IR region.
By taking all
these factors into consideration, complex 7 in Scheme 1 was
initially designed. However, due to the limited solubility of
7 in organic solvent, the chloride ligand in 7 was replaced by
the 4-tolylacetylide ligand to improve the solubility. Thus,
complex 1 in Scheme 1 became the target complex in this
study, and its synthesis, crystal structure, photophysics, and
TPA are reported herein.
(
TPA) of the complex in the near-IR region; thus allowing
for the possibility to populate the excited state through
II
TPA. In earlier studies, we found that [Pt
A
H
U
G
E
N
N
(C^N^N)] com-
Because the ultimate goal of our research is to optimize
the reverse saturable absorption of platinum complexes, it is
plexes exhibit broadband excited-state absorption that ex-
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Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
FULL PAPER
critical to understand the photophysical parameters that in-
fluence the reverse saturable absorption. Key characteristics
of an ideal reverse saturable absorber include: 1) considera-
ble ground-state absorption to enable population of the ex-
cited state; 2) high excited-state quantum yield; 3) long ex-
cited-state lifetime that exceeds the excitation pulse width;
and 4) most importantly, the excited-state absorption should
be much stronger than the ground-state absorption, that is,
the ratio of the excited-state absorption cross section rela-
tive to that of the ground state should be significantly larger
than 1. The ground-state absorption cross section can be de-
duced from the electronic absorption measurement, whereas
the excited-state absorption characteristics need to be ob-
tained from the transient difference absorption measure-
ments. Emission measurements would provide information
on the energy level and the lifetime of the excited state.
Quantitative characterizations of the excited-state absorp-
tion and TPA cross sections in this study were achieved by
performing the Z-scan experiments and the data were fitted
by using a five-level model. Nonlinear transmission meas-
urements at 532 nm were conducted to demonstrate the re-
verse saturable absorption of the complex in response to ns
laser pulses.
Results and Discussion
Synthesis: The precursor compounds 2, 3, 4, and 6 were syn-
thesized according to the procedures reported in the litera-
ture. Lithiation of 4 in dry THF at low temperature, fol-
lowed by reaction with isopropyl pinacolyl borate gave the
key precursor 5 in 70% yield. Suzuki coupling reaction be-
tween 5 and 6 led to the formation of ligand 1-L in 70%
yield. The cyclometalated platinum(II) chloride complex 7
was prepared by following the reported procedures for [Pt
Figure 1. ORTEP view of 1-L (top) and 1 (bottom; with 50% probability
for thermal ellipsoids). Hydrogen atoms are omitted for clarity.
[8]
Crystal structures of 1-L and 1: Figure 1 shows an ORTEP
view of the crystal structures of ligand 1-L and complex 1;
with selected bond lengths and angles listed in Table 1 and
Table 2. The full list of bond lengths and bond angles for 1-
(
C^N^N)Cl] complexes from the ligand 1-L and [Pt-
Table 1. Selected bond lengths (ꢂ) for 1 and 1-L.
ACHTUNGTRENNUNG( DMSO) Cl ] in DMF, in which a few drops of water was
2
2
[a]
Atom1
Atom2
1 XRD
Calcd
1-L XRD
[9]
added as catalyst to activate the CꢀH bond. Although
complex 7 was obtained in a very high yield, the solubility
of 7 was so poor that it could not be further purified and
characterized. Therefore, it was used directly, without fur-
ther purification, in the following step to prepare 1. The
target complex 1 was synthesized by replacing the chloride
ligand by 1-ethynyl-4-methylbenzene in DMF with KOH as
base for deprotonation of 1-ethynyl-4-methylbenzene and
a catalytic amount of CuI to facilitate the substitution reac-
Pt
Pt
Pt
Pt
S
S
N1
N2
2.112(8)
1.980(7)
1.997(8)
2.00(1)
1.757(9)
1.711(9)
1.37(1)
1.36(1)
1.35(1)
1.36(1)
1.32(1)
1.39(1)
1.49(1)
1.49(1)
1.44(1)
1.40(1)
1.46(1)
1.48(1)
1.53(1)
1.55(1)
1.18(1)
1.46(1)
2.189
2.025
1.996
1.963
1.794
1.750
1.339
1.363
1.349
1.359
1.299
1.380
1.484
1.466
1.437
1.397
1.461
1.465
1.534
1.534
1.228
C12
C35
C28
C29
C1
1.748(3)
1.724(3)
1.337(4)
1.342(3)
1.345(3)
1.344(3)
1.318(4)
1.395(3)
1.490(3)
1.488(3)
1.397(4)
1.377(4)
1.466(3)
1.466(3)
1.526(4)
1.528(2)
N1
N1
N2
N2
N3
N3
C5
C10
C11
C12
C14
C18
C20
C21
C35
C36
C5
C6
C10
C28
C34
C6
C11
C12
C13
C15
C28
C21
C22
C36
C37
[5d,10]
tion.
The resultant complex 1 shows much better solu-
bility than chloride complex 7. It is soluble in CH Cl ,
2
2
CHCl , acetonitrile, acetone, DMF, and DMSO; and slightly
3
soluble in methanol and ethanol.
The new compound/complex 1-L and 1 were fully charac-
1
terized by H NMR spectroscopy, HRMS, and elemental
analyses. High-quality single crystals of 1-L and 1 suitable
for X-ray crystallography were also obtained by slow diffu-
sion of heptane vapor to dilute CH Cl solutions of the
2
2
sample. The crystal structures for 1-L and 1 will be discussed
in the following section.
[a] From DFT/LANG631 calculations.
Chem. Eur. J. 2012, 18, 4593 – 4606
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4595

W. Sun et al.
Table 2. Selected bond angles (8) for 1 and 1-L.
contrast, the Pt atom lies almost in the same plane as the bi-
[
a]
II
Atom1
Atom2
Atom3
1 XRD
Calculated
1-L XRD
pyridinyl–Pt plane, with a deviation of 0.020 ꢂ from the
II
plane. The bond lengths around Pt are 2.112(8) ꢂ for Ptꢀ
Pt
Pt
Pt
Pt
Pt
Pt
Pt
S
N1
N1
N2
N2
C12
C12
C35
C28
C28
C29
Pt
Pt
C5
Pt
Pt
Pt
C6
C10
C28
C34
C11
Pt
C18
C18
S
C1
C5
C6
125.9(6)
113.9(6)
120.7(6)
118.2(6)
111.9(6)
130.0(6)
176.8(9)
115.6(7)
122.2(7)
110.0(7)
160.3(3)
102.0(3)
115.1(8)
82.3(3)
127.3
112.7
119.9
116.3
112.4
129.4
177.5
114.6
121.2
109.2
159.2
102.0
116.0
81.9
N1, 2.00(1) ꢂ for PtꢀC35, 1.997(8) ꢂ for PtꢀC12, and
1
.980(7) ꢂ for PtꢀN2. The longer PtꢀN1 distance could pos-
C10
C11
C13
C36
N3
C18
C34
C12
C35
C6
sibly arise from a strong trans influence from the fluorene C
II
donor at the trans position. The bond angles around Pt ,
which are 82.3(3)8 for N2-Pt-C12, 78.0(3)8 for N1-Pt-N2,
102.0(3)8 for N1-Pt-C35, and 97.7(3)8 for C12-Pt-C35, devi-
ate somewhat from the ideal square-planar configuration
angle (908). In general, the bond lengths and bond angles
115.5(2)
121.6(2)
109.7(2)
S
S
N1
N1
N1
N2
N2
N2
N2
N2
N3
N3
C10
C12
C17
C19
C28
C28
C35
II
around the Pt atom are consistent with those reported for
II
[11]
116.8(2)
other [Pt CHTUTGNERNNUN(G C^N^N)] and Pt terpyridyl complexes.
IIA
C12
C35
N1
Figure 2 shows the 3D packing of 1, with the two neigh-
boring molecules stacked parallel to each other in a head-
to-head but back-to-back fashion. The shortest distance be-
tween two neighboring Pt atoms is 3.867 ꢂ, which is slightly
longer than the metal–metal interaction distance (3.09–
178.9(3)
78.0(3)
179.2
77.2
C5
112.0(8)
111.5(7)
122.1(8)
115.0(9)
116.0(7)
97.7(3)
121.1(8)
118.8(8)
89.6(4)
114.2
113.1
124.1
115.4
116.3
98.9
118.7
118.7
88.7
115.5(2)
115.9(2)
122.9(2)
115.2(2)
121.8(2)
C11
C18
C29
C12
C35
C28
C28
C29
C34
C37
3
.50 ꢂ) usually observed in extended, linear-chain-arranged,
II
[12]
monomeric, Pt structures.
Therefore, no intermolecular
Pt–Pt interactions are expected from complex 1 even in the
solid state. However, the distances between two adjacent
layers are approximately 3.4 ꢂ, which would allow for
strong intermolecular Pt–p and p–p interactions to occur.
The presence of intermolecular Pt–p and p–p interactions
119.2(2)
121.7(2)
89.5(1)
N3
C36
109.8(8)
173(1)
112.1
179.2
110.0(2)
[
a] From DFT/LANG631 calculations.
1
was also supported by H NMR analysis of 1 at various con-
centrations, as discussed in detail in the next section. Note
that such interactions would alter the electronic structure of
the complex in the solid state and make it quite different
from those in the gas phase or in a solution. However, be-
cause all of the photophysical properties and nonlinear ab-
sorption of 1 discussed in this paper are investigated in solu-
tions, consideration of the solid-state electronic properties is
not only less important, but also beyond the scope of this
work.
L and 1 are provided in the Supporting Information, Ta-
bles S1 and S2. The X-ray structure of 1-L indicates that the
two pyridine rings that adopt a trans conformation are not
coplanar, with a dihedral angle of 13.168 between them.
Both pyridine rings are twisted out of the plane of fluorene,
with dihedral angles of 43.868 and 18.628 relative to the fluo-
rene plane for the end pyridine ring and the middle pyridine
ring, respectively. In addition, the benzothiazole group is
not coplanar with the fluorene plane either. A dihedral
angle of 17.188 is displayed between these two components.
The C28ꢀS bond length is 1.748(3) ꢂ and that of C29ꢀS is
1
.724(3) ꢂ. The CꢀN bond lengths in the pyridine rings are
similar, generally around 1.34 ꢂ. However, those in the ben-
zothiazole component are significantly different (i.e.,
1.318(4) ꢂ and 1.395 ꢂ). The C28-S-C29 bond angle is close
to 908.
II
After coordination with Pt , the bipyridine and fluorene
components become almost coplanar. The dihedral angle
between the plane of the bipyridinyl–Pt component (de-
II
Figure 2. 3D packing of 1. The distances between neighboring planes, de-
fined by Pt, N1, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, and N2 atoms,
are as follows: 3.396 ꢂ if the Pt–Pt distance is 3.867 ꢂ, and 3.430 ꢂ if the
Pt–Pt distance is 5.507 ꢂ. These distances represent p–p interaction be-
tween bipyridine fragments.
fined by Pt, N1, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10,
and N2 atoms) and the fluorene plane (defined by C11, C12,
C13, C14, C15, C16, C17, C18, C19, C20, C21, C22, and C23
atoms) is 3.398; whereas the dihedral angle between the
II
plane of the bipyridinyl–Pt component and the benzothia-
zole plane (defined by the nine atoms of benzothiazole (N3,
C28, S, C29, C30, C31, C32, C33, C34)) is 15.568. Between
the fluorene component and the benzothiazole component,
1
H NMR spectrum of 1: Figure 3 shows the aromatic region
1
of the H NMR spectra of 1 in CDCl at different concentra-
3
the dihedral angle is 18.658. The toluene plane is twisted by
tions. The assignment of the peaks in the spectrum is based
on the chemical shift and the splitting pattern. Hydrogen
II
17.658 from the plane of the bipyridinyl–Pt component. In
4596
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Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
FULL PAPER
Figure S1, and selected bond lengths and bond angles are
given in Table 1 and Table 2 together with those obtained
from X-ray crystallography analysis for comparison.
Figure 4 shows the ground-state electron-density distribution
of the five HOMOs (HOMO, HOMOꢀ1, HOMOꢀ2,
HOMOꢀ3, and HOMOꢀ4) and the five LUMOs (LUMO,
LUMO+1, LUMO+2, LUMO+3, and LUMO+4) for
complex 1. The relative contributions of the ligand/metal
fragments to HOMOꢀ1, HOMO, LUMO and LUMO+
1
are listed in Table 3. Table 4 lists the excitation energies
(
eV), wavelengths (nm), oscillator strengths, and coefficients
of dominant contributing configurations of ten, low-lying,
singlet, electronic states of complex 1 based on time-depen-
dent DFT (TDDFT) calculations at the MPW3LYP/
LANG631 level of theory.
1
Figure 3. H NMR spectra of complex 1 at different concentrations in
ꢀ
2
ꢀ3
ꢀ3
ꢀ3
ꢀ1
3
CDCl : a) 1.0ꢃ10 , b) 5.0ꢃ10 , c) 2.5ꢃ10 , d) 1.0ꢃ10 molL .
atoms c and r (for labels see Scheme 1), which are at the
ortho positions of the coordinating atoms on the terminal
pyridine ring, and the fluorene ring are most deshielded due
to the reduced electron density on the respective carbon
atoms after coordination of the pyridine and phenyl rings
Table 3. Contribution of fragments to the frontier molecular orbitals
(
FMOs) in complex 1.
Fragment
Contribution (%)
HOMOꢀ1
10.48
15.52
2.56
HOMO
LUMO
LUMO+1
[a]
Bipyridine
Pt
Tolylacetylide
BTZ-F2
1.69
21.14
76.67
0
92.26
5.53
1.27
0.94
49.37
II
with Pt . With increased concentration of the solution, the
1.35
0
49.31
peaks corresponding to protons r, c, d, e, f, g, h, i, and j shift
to high field. The up-field shift appears to be indicative of
significant intermolecular p–p interactions at high concen-
trations. The head-to-head stacking pattern observed in the
crystal structure analysis indicates that the C^N^N compo-
nents are on top of each other between the neighboring
molecules, thus placing the C^N^N component of 1 in the
shielding zone of the aromatic rings on the other molecule,
which results in the up-field shift of the protons on the
C^N^N ligand.
71.44
[a] This value is falsely similar to that for the BTZ-F2 fragment because
of an artifact in the calculation that stem from the apparently large
atomic-orbital coefficients of a few atoms in the bipyridine fragment. The
correct prediction is shown qualitatively in the MO plots in Figure 4, in
which the electron density of LUMO+1 is predominantly located on the
BTZ-F2 fragment.
Results of ground-state properties show that the calculat-
ed bond lengths and angles are generally in good agreement
with those obtained from X-ray crystal analysis (see Table 1
and Table 2). The largest difference between the calculated
and observed bond lengths is 0.077 ꢂ for the distance be-
tween the Pt atom and N1 atom, whereas the smallest devia-
DFT calculations: DFT calculations were performed to
characterize the ground electronic state of complex 1 and
low-lying singlet excited states. The optimized structure of
1
in the gas phase is shown in the Supporting Information,
Figure 4. Contour plots of the five HOMOs (HOMOꢀ1, HOMOꢀ2, HOMOꢀ3, HOMOꢀ4) and the five LUMOs (LUMO, LUMO+1, LUMO+2,
LUMO+3, LUMO+4) for complex 1.
Chem. Eur. J. 2012, 18, 4593 – 4606
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4597

W. Sun et al.
Table 4. Excitation energies, wavelengths, oscillator strengths, dominant
contributing configuration, and the associated configuration coefficient of
LUMO+1 transition is the dominant contributing configu-
ration to the fourth excited state, which occurs at 526 nm.
For the transition at 420 nm, which has the strongest oscilla-
tor strength (i.e., the ninth excited state), the dominant con-
tributing configuration involves the HOMOꢀ1!LUMO+
ten low-lying electronic states (S
MPW3LYP/LANG631 level of theory.
n
)
of complex
1
obtained at the
[
a]
S
n
Excitation energy
f
Dominant
configuration
Configuration
coefficient
1
orbital pair. Excitations to the second, third, and fifth ex-
[eV]
[nm]
cited states emanate from deeper-lying MOs, that is,
HOMOꢀ2, HOMOꢀ1 and HOMOꢀ4, respectively, and
have small oscillator strengths. According to the electron-
density distribution analysis, the electron density of the
LUMO is almost exclusively localized on the bipyridine
component (92%), with about 6% on Pt (d orbitals) and
only about 1% each on the 4-tolylacetylide and BTZ-F2
components. The HOMO shows a very different electron-
density distribution pattern, with about 77% contribution
from the 4-tolylacetylide fragment, 21% from the Pt (d or-
bitals) and about 2% from bipyridine; whereas the contribu-
tion from the BTZ-F2 component is zero. In contrast, the
HOMOꢀ1 has dominant contribution from the BTZ-F2
fragment and considerable contributions from the metal and
bipyridine components; with a small contribution of about
2.6% from 4-tolylacetylide. The distribution on the
LUMO+1 is almost evenly split between the bipyridine and
BTZ-F2 components; the metal adds only approximately
1% of the electron density and the contribution from 4-toly-
lacetylide is zero. Overall, the contribution from Pt (d orbi-
tals) is largest in the HOMO and HOMOꢀ1 (21.1% and
15.5%, respectively), small but important in the LUMO,
whereas it is marginal in the LUMO+1 (see Table 3).
Therefore, the dominant transitions that contribute to the
three major transitions at 673, 526, and 420 nm are Pt(dp)+
1
2
3
4
5
6
7
8
9
1.84
2.25
2.29
2.36
2.67
2.78
2.79
2.83
2.95
3.21
673
550
541
526
464
447
444
438
420
386
0.0591 HOMO!LUMO
0.67
0.64
0.63
0.64
0.70
0.53
0.46
0.0098 HOMOꢀ2!LUMO
0.0016 HOMOꢀ1!LUMO
0.0591 HOMO!LUMO+1
0.0015 HOMOꢀ4!LUMO
0.0089 HOMO!LUMO+2
0.0081 HOMOꢀ3!LUMO
0.0885 HOMOꢀ2!LUMO+1 0.60
0.5445 HOMOꢀ1!LUMO+1 0.47
1
0
0.0715 HOMO!LUMO+4
0.67
and
[
a] LANG631 basis set refers to 6-31G* for C, H,
N
S and
LANL2DZ for Pt.
tion is 0.001 ꢂ for the PtꢀC12 bond. Although the X-ray
structure depicts the PtꢀN1 connection with a solid bond,
calculations show a rather nonbonded interaction as shown
in Figure S1 in the Supporting Information, with bond order
significantly less than a single bond. The Wiberg bond index
for this bond, based on natural bond orbital (NBO) analy-
[
13]
sis calculations, is determined to be only 0.23. The large
difference between the calculated and measured PtꢀN1 dis-
tance could be just an artifact in the DFT calculations. Al-
ternatively, it could be due to the lack of consideration of in-
termolecular interactions in the DFT calculations. Devia-
tions in bond angles range from 0 to 6.28; with the largest
difference observed for the C35-C36-C37 angle in the 4-toly-
lacetylide ligand, for which the experimental value is 1738
and the calculated value is 179.28. The benzene ring on the
acetylide ligand is found to be twisted away from the molec-
ular plane by about 118. This additional strain in the acety-
lide ligand can be expected to contribute to the slightly
larger deviation seen in the C35-C36-C37 angle. The mean
absolute deviation (MAD) from the experimental bond
lengths is 0.020 ꢂ, whereas that for bond angles is 1.28. The
molecule is so large that theoretical calculations that
employ a larger basis set than 6-31G* for atoms other than
Pt were computationally unfeasible. Slight differences from
the X-ray crystallography data can be expected because the-
oretically predicted geometrical parameters are generally
sensitive to the basis set size. Even though the inclusion of
relativistic effects through the effective core potential of Pt
is helpful, it is far from complete. In addition, theoretical
calculations were performed for molecules in the gas phase
in vacuo and do not take into account the interactions that
occur in solid-state compounds used in the X-ray crystallog-
raphy experiments.
tolylacetylide(p)!bpy(p*),
Pt(dp)+tolylacetylide(p)!
BTZ-F2(p*)+bpy(p*), and BTZ-F2(p)!BTZ-F2(p*)+
bpy(p*) transitions, respectively.
Based on theoretical calculations, the lowest-energy elec-
tronic transition (which is the HOMO!LUMO as expect-
ed) should correspond to the lowest-energy absorption band
maximum in the measured UV/Vis absorption spectrum.
However, the experimental absorption band maximum was
observed at 459 nm, whereas the value predicted from calcu-
lations was 673 nm; this represents an overestimation of
about 200 nm. The experimental value was obtained in
a polar solvent (CH Cl ), which can usually cause the
2
2
lowest-energy absorption band maximum to be hypsochrom-
ically shifted. Generally, energetic parameters that are deter-
mined based on differences in the total electronic energy
are sensitive to both theoretical method and basis set size.
Therefore, part of the deviation can be explained by the fact
that a relatively small basis set of 6-31G* for C, N, S, and H
atoms in conjunction with LANL2DZ for the Pt atom was
used for computing the properties of complex 1; as noted
earlier, the molecule is large and calculations with a larger
basis set were not computational feasible. In addition, the
theoretical prediction is based on gas-phase calculations in
vacuo with no consideration of solvent effects, whereas ex-
perimental measurements were conducted in the condensed
phase in CH Cl . Although polarized continuum model
To understand the nature of the low-lying, excited, singlet,
electronic states of complex 1, TDDFT calculations were
performed. The results shown in Table 4 indicate that elec-
tronic transitions to the first excited state involve the
HOMO!LUMO orbital pair as the dominant contributing
configuration and occurs at 673 nm, whereas the HOMO!
2
2
4598
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Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
FULL PAPER
(
PCM) calculations are often used in theoretical calculations
compared with those in 1-L, indicating the delocalization of
the ligand-centered molecular orbitals through interactions
with the platinum dp orbitals; this is consistent with obser-
vations from the DFT study. Considering the similarity in
energy of these bands for 1 and 1-L and the large molar ex-
tinction coefficients of these bands in 1, we can assign these
to account for solvation effects, the molecule considered in
this study was large and such calculations were not feasible.
Even if the above-mentioned effects were allowed, the devi-
ation of the TDDFT result (i.e., 0.86 eV) is larger than typi-
cal for TDDFT descriptions of excitations that are local in
nature (i.e., typically 0.2–0.3 eV) and provide an important
clue that the excitation has Rydberg or charge-transfer char-
acter (i.e., errors in the 1–2 eV range are not uncommon).
Consequently, although the predicted value of the absorp-
tion band maximum represents a semi-quantitative estimate,
TDDFT calculations still provide valuable information on
the electronic structure of the lowest excited states; this is
useful in interpreting and understanding the photophysics
observed for complex 1 described in the following sections.
1
bands to p–p* transitions within the 6-(7-benzothiazol-2’-yl-
9,9-diethyl-9H-fluoren-2-yl)-2,2’-bipyridine ligand (predomi-
nantly within the benzothiazolylfluorene component as pre-
dicted by the DFT calculations) as well. In addition,
a broad, structureless tail between 430 and 530 nm is ob-
served in complex 1, but not in ligand 1-L. With reference
II
to other Pt C^N^N and terpyridyl acetylide complexes,
1
1
this tail could be attributed to the MLCT/ LLCT (metal-to-
ligand/ligand-to-ligand charge transfer) transitions. The as-
signment of this low-energy absorption band is bolstered by
DFT calculations, in which the HOMO is dominated by the
tolylacetylide ligand and the Pt components, and the
LUMO has major contribution from the bipyridine compo-
nent (see Table 3). Another piece of evidence that supports
the charge transfer nature of the low-energy absorption
band is the negative solvatochromic effect. As demonstrated
in the Supporting Information, Figure S4, this band shifts to
a longer wavelength in less polar solvents, such as toluene
and hexane, in comparison with the bands in more polar sol-
vents (CH CN, CH Cl and DMSO). This is indicative of the
Electronic absorption: The electronic absorption of 1-L and
1
used in our study (5ꢃ10 –5ꢃ10 molL , see the Support-
ing Information, Figure S2). The UV/Vis absorption spectra
of 1-L and 1 in CH Cl are shown in Figure 5. The absorp-
obeys the Beer–Lambert law in the concentration range
ꢀ
6
ꢀ3
ꢀ1
2
2
tion band maxima and molar extinction coefficients are pre-
sented in Table 5. The absorption of 1-L is dominated by
structured bands in the UV region, which emanate from the
1
p–p* transitions. The polarity of solvent exhibits minor
effect on the UV/Vis spectrum of 1-L (see the Supporting
Information, Figure S3a), which is consistent with the p–p*
3
2
2
1
charge-transfer character of the ground state.
assignment. For complex 1, the dominant absorption also
appears in the UV region, however, the bands are redshifted
Emission: 1-L and 1 are both emissive in solutions at room
temperature and in glassy matrix at 77 K. As shown in
Figure 6, upon excitation of 1-L at 356 nm in CH Cl solu-
2
2
tion, it exhibits structured emission at 380 and 410 nm,
which decays with a lifetime of 752 ps. The quantum yield of
the emission is 73%. Because of the mirror relationship be-
tween the emission spectrum and the UV/Vis absorption
spectrum and the lifetime, the observed emission from 1-L
1
is attributed to fluorescence from the p–p* state. The as-
1
signment of the p–p* state as the emitting state of 1-L is
supported by the minor solvent effect. As shown in the Sup-
porting Information, Figure S5, in different solvents with
a broad range of polarity, the emission energies and quan-
tum yields (except in hexane) are quite similar. The only dif-
ference is the relative intensity of the vibronic peaks.
For complex 1, upon excitation at 388 nm, a broad, some-
what structured emission appears at 591 nm with a shoulder
Figure 5. UV/Vis absorption spectra of 1-L and 1 in CH
Table 5. Photophysical parameters of 1-L and 1.
2
Cl
2
.
[
a]
ꢀ1
ꢀ1
[b]
[c]
[d]
l
abs [nm] (loge [M cm ])
l
em [nm] (Fem; t
0
)
l
em [nm] (t/ms)
l
S1ꢀSn [nm] (t
S
/ps)
lT ꢀT [nm] (eT ꢀTn
1
1
n
1
ꢀ
ꢀ1
[e]
T
AHCTUNGTNERNU[G M cm ]; tTA/ms; F )
[
f]
1
1
-L
282 (4.39), 358.5 (4.86), 373 (4.73)
265 (4.36), 286 (4.34), 352 (4.51),
380, 410 (0.73; 752 ps)
379, 402 (0.011 (16%),
.090 (84%)), 427, 453
575 (19.4), 625 (21.1)
646 (796ꢁ96)
585 (83290; 32.8; 0.36)
633 (44650; 14.0; 0.28)
0
[
g]
591 (0.067; 1.43 ms),
634 (–; 1.38 ms)
633 (24.6ꢁ14.8)
3
87.5 (4.57), 458 (3.63)
[
a] UV/Vis absorption band maxima and molar extinction coefficients in CH
2
Cl
2
. [b] Emission-band maximum, quantum yield, and lifetime in CH
2
Cl
2
at
.
ꢀ
6
ꢀ1
ꢀ6
ꢀ1
a concentration of 5ꢃ10 molL . [c] Emission-band maxima and lifetime in a butyronitrile matrix at 77 K at a concentration of 5ꢃ10 molL
[
d] Femtosecond transient difference absorption (fs TA) band maximum and singlet excited-state lifetime in CH
2 2
Cl . [e] ns TA band maximum, triplet ex-
tinction coefficient, triplet excited-state lifetime and quantum yield. [f] Measured in butyronitrile. [g] Measured in CH
2
Cl
2
. SiNc in C
6
H
6
was used as the
ꢀ
1
ꢀ1
reference. (e590 =70000 Lmol cm , F
T
=0.20).
Chem. Eur. J. 2012, 18, 4593 – 4606
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4599

W. Sun et al.
The emission spectra of 1-L and 1 in butyronitrile matrix
at 77 K are given in Figure 6. For 1-L, the emission spectrum
at 77 K exhibits clear vibronic structures, but remains at the
same energy as that at room temperature, and the lifetime is
shorter than 100 ns. Therefore, it is still fluorescence from
1
the p–p* state. Our attempt to measure the phosphores-
cence at 77 K was unsuccessful even if we added an excess
amount of CH I to the solution. The emission spectrum of
3
1
at 77 K becomes narrower and blueshifted compared with
that at room temperature; this is due to the rigidochromic
[14]
ꢀ1
effect.
The vibronic spacing is approximately 1390 cm ,
which is also consistent with the stretching vibration of the
aromatic ligand. Considering the small thermally induced
ꢀ
1
Stokes shift (DE ffi470 cm ), the similar shape and vibronic
s
Figure 6. Normalized emission spectra of 1-L and 1 in CH
at room temperature and in butyronitrile matrix at 77 K for 1.
2 2
Cl solutions
spacing of the spectra at 77 K and at room temperature for
, the emission of 1 at 77 K is tentatively assigned as the p–
p* state, possibly mixed with some MLCT character.
3
1
3
at approximately 634 nm. The vibronic spacing between the
ꢀ
1
peak and the shoulder is approximately 1150 cm , corre-
sponding to the ring-breathing mode of the aromatic rings
in the ligands. The lifetime of the emission is approximately
Transient absorption spectroscopy: Transient difference ab-
sorption spectroscopy measures the difference between the
excited-state absorption and the ground-state absorption.
Thus, it can provide information on the spectral region in
which the excited-state absorption is stronger than that of
the ground-state and predict the wavelength region in which
reverse saturable absorption could occur. From the decay of
the transient absorption, the lifetime of the excited state
that gives rise to the excited-state absorption is obtained.
This is especially important for measuring the singlet excit-
1
.43 ms. In view of the large Stokes shift of the emission and
the long lifetime, the emission from 1 at room temperature
should originate from a triplet excited state. The vibronic
structure in the emission spectrum suggests that the p–p*
state should be involved in the emission, possibly mixed
with some MLCT character; this is partially supported by
the negative solvatochromic effect of the emission (see the
Supporting Information, Figure S5). In less polar solvents,
such as in toluene and hexane, the emission spectra are red-
shifted and become less structured in contrast to those in
more polar solvents, such as ethanol, acetonitrile, CH Cl ,
and acetone. This probably is indicative of different degrees
of mixing the MLCT character into the p–p* state in sol-
vents with different polarities. In polar solvents, the less
polar MLCT state (in comparison to the more polar MLCT
3
3
II
ed-state lifetime of the Pt complexes that cannot be ob-
tained from the decay of fluorescence because of the lack of
fluorescence at room temperature in many cases. By esti-
mating the triplet excited-state molar extinction coefficient
at the triplet excited-state absorption band maximum with
2
2
3
3
[15]
the singlet depletion method, and by using the relative ac-
[16]
tinometry, with SiNc in benzene as the reference, the trip-
let excited-state quantum yield can be obtained. Both the
singlet and triplet transient difference absorption spectra of
1-L and 1 were measured with fs and ns pump-probe UV/
Vis spectrometers, respectively.
3
ground state) is less stabilized than the ground state; this
3
would cause the blueshift of the MLCT emission. In con-
3
trast, the influence of the solvent polarity on the p–p* ex-
3
cited state is not as significant as that on the MLCT state.
This would allow for the MLCT state and the p–p* state
Figure 7 shows the time-resolved singlet transient differ-
ence absorption spectra of 1-L and 1 in CH Cl . For 1-L, im-
3
3
2
2
to be energetically more close to each other in polar sol-
mediately after the excitation at 400 nm with ultrafast fem-
tosecond laser pulses (150 fs), a broad, slightly structured
absorption band appears at about 650 nm, which decays rap-
idly and redshifts to about 675 nm. At longer decay time,
a new broad band occurs at approximately 580 nm, accom-
panied by an isosbestic point at 480 nm. This reflects the in-
tersystem crossing from the singlet excited state to the trip-
let excited state. The spectrum at longer delay times is con-
sistent with that measured by ns laser flash photolysis
(shown in Figure 8). The singlet lifetime measured from the
decay of fs TA is quite similar to that obtained from fluores-
cence decay (see Table 5). Therefore, the observed singlet
3
vents, resulting in more configurational mixing of p–p* and
3
MLCT characters in the emission in polar solvents. On the
3
other hand, in less polar solvents, the MLCT state would be
more stabilized and the energy level would be lowered,
3
whereas the energy of the p–p* state is less affected. Con-
sequently, the energy gap between these two states becomes
3
larger and the contribution from the p–p* state is reduced.
This is reflected by the less structured emission spectra and
shorter lifetime in less polar solvents (see the Supporting In-
formation, Table S3). When the concentration of the CH Cl
2
2
ꢀ
6
ꢀ4
ꢀ1
solution increases from 1ꢃ10 to 1ꢃ10 molL , the inten-
sity of the emission keeps increasing (see the Supporting In-
formation, Figure S6) and the lifetime remains the same,
suggesting that no self-quenching occurs in the concentra-
tion range used herein.
1
TA is attributed to the p–p* state, whereas the TA at long
3
decay time should arise from the p–p* state. For complex
1, the fs TA spectra changes very little in the whole spec-
trometer decay window (6 ns) although it exhibits a very
4600
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Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
FULL PAPER
Figure 7. Time-resolved singlet transient difference absorption spectra of
-L (top) and 1 (bottom) in CH Cl . lex =400 nm.
Figure 8. Time-resolved triplet transient difference absorption spectra of
1
2
2
1
3
-L (top) in butyronitrile and 1 (bottom) in CH CN. lex =355 nm.
small change right after the excitation, which correlates to
a lifetime of approximately 25 ps. We assume that this is due
to a decay of the singlet excited state, including intersystem
crossing to the triplet excited state. At longer decay time,
the spectrum is essentially the same as that measured by ns
flash photolysis and is attributed to the triplet excited-state
absorption. Because there is not much difference in the
spectral properties of the singlet and triplet excited states,
we can assume that the geometry of the molecule does not
change much upon intersystem crossing. In addition, the TA
spectrum of 1 is quite similar to that of 1-L, therefore, the
singlet excited state that contributes to the observed TA
sorption band maximum, that is, 620 nm for 1 and 585 nm
for 1-L. The redshifted TA spectrum of 1 compared with
that of 1-L suggests delocalization of the ligand-centered
molecular orbitals through interactions with the platinum
dp orbitals; this is similar to that observed in the UV/Vis
absorption spectrum for the singlet excited state. A similar
feature has been reported for platinum diimine complex
[17]
with acetylide ligands.
The extinction coefficients at the
band maximum, the lifetimes deduced from the decay of the
transient absorption, and the triplet excited-state quantum
yield determined from relative actinometry for 1-L in butyr-
onitrile and 1 in CH Cl are listed in Table 5 (the data for
2
2
1
could be assigned as the p–p* and the triplet state as the
1 in CH Cl , rather than those in CH CN, are listed because
2
2
3
3
p–p* as well. However, the singlet lifetime of the Pt com-
we need to use the triplet excited-state parameters in
plex 1 is much shorter compared with that of the ligand 1-L.
This is attributed to the increased intersystem crossing
through spin-orbital coupling through Pt; this makes the
decay of the singlet excited state more rapid.
CH Cl to fit the Z-scan data measured in CH Cl solution).
2
2
2
2
The triplet extinction coefficients for both 1-L and 1 are
quite large, accompanied by long triplet lifetimes. The long
lifetime of 32.8 ms for 1-L implies that the transient absorp-
3
The time-resolved triplet transient difference absorption
tion likely arises from the p–p* state. For complex 1, al-
spectra of 1-L in butyronitrile and 1 in CH CN are present-
though the lifetime (14.0 ms in CH Cl ) of the excited state
3
2
2
ed in Figure 8. The ns TA spectra of 1 in CH Cl and tolu-
that gives rise to the transient absorption is shorter than
that of the ligand, probably due to the increased decay from
T to S by spin-orbital coupling through Pt, or possibly be-
2
2
ene are provided in the Supporting Information, Figure S7.
The spectral features for 1-L and 1 are quite similar, with
a positive absorption band appearing in the visible spectral
region and a bleaching band below 400 nm for 1-L and
below 415 nm for 1. However, the spectrum of 1 is redshift-
ed compared with that of 1-L, which is reflected by the ab-
1
0
3
cause the reduced energy level of the p–p* excited state
(evident by the redshifted TA band) in 1 leads to reduced
[18]
lifetimes according to the energy-gap law, it is still much
longer than that deduced from the decay of emission. This
Chem. Eur. J. 2012, 18, 4593 – 4606
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4601

W. Sun et al.
indicates that the transient species would likely arise from
3
the p–p* state and is supported by the similar features of
the TA spectra of 1 and 1-L. The assignment of the absorb-
3
ing excited state in CH Cl solution to p–p*, rather than
2
2
3
the MLCT state or a mixed state, is also partially based on
the fact that the lifetimes deduced from the decay of TA in
toluene (13.5 ms) is similar to that in CH Cl . This phenom-
2
2
enon is distinct from the solvent-dependency lifetime de-
duced from the decay of emission, in which the emitting
3
3
state has different composition of p–p*/ MLCT character
with varied solvent polarity. Therefore, we conclude that the
excited state that gives rise to the transient absorption in
3
CH Cl and toluene solutions is the ligand-centered p–p*
2
2
state that is not affected pronouncedly by the polarity of sol-
vents. However, in more polar, coordinating solvent, such as
CH CN, the lifetime (2.0 ms) deduced from the decay of the
3
transient absorption is much shorter than those in CH Cl
and toluene solutions, suggesting that the MLCT state plays
2
2
3
a role in transient absorption in CH CN solution.
3
Z-scan study and nonlinear absorption cross sections: Z
[19]
scan is a simple, nonlinear-optical, characterization tech-
nique that is used to separately measure the contributions of
nonlinear absorption and nonlinear refraction to the ob-
served optical nonlinearity of a material, with nonlinear ab-
sorption evaluated by open-aperture Z-scan measurement.
In general, nonlinear absorption is dominated by the singlet
excited-state absorption if the intersystem crossing time of
the molecule is longer than the laser pulse width. In con-
trast, triplet excited-state absorption plays the major role
when the intersystem crossing time is shorter than the laser
pulse width. Therefore, the observed nonlinear absorption
often is a combination of singlet and triplet excited-state ab-
sorption. In the case of a molecule that exhibits TPA at the
excitation wavelength, two-photon initiated excited-state ab-
sorption would contribute to the observed open-aperture Z-
scan data. To separate the contribution of singlet excited-
state absorption from triplet excited-state absorption, and
the contribution of TPA from excited-state absorption (in
the near-IR region), open-aperture Z scans were performed
in a CH Cl solution at 532 nm by using both ns and ps laser
Figure 9. Plots of Z-scan experimental data (symbols) and fitting curves
(
solid lines) for 1 in CH
a 2 mm cell. The spot size at the focal plane is 33 mm for ps pulses and
7 mm for ns pulses at 532 nm, and 38 mm for ps pulses at 680 nm. The
2 2
Cl solution at 532 (top) and 680 nm (bottom) in
3
energy used is 5.2 mJ for ns measurement and 3.1 mJ for ps measurement
at 532 nm, and 10.7 mJ for ps measurement at 680 nm. The solution con-
ꢀ4
ꢀ4
ꢀ1
centration is 6.72ꢃ10 and 3.82ꢃ10 molL for the ns and ps measure-
ꢀ
3
ꢀ1
ments at 532 nm, respectively; and 4.71ꢃ10 molL for the 680 nm
measurement.
[5,21]
saturable absorbers.
The combination of strong excited-
state absorption and weak ground-state absorption in the
visible and near-IR regions leads to large ratios of the excit-
ed-state to ground-state absorption cross sections. The ratio
becomes extremely large at longer wavelengths; this places
it among the largest ratios reported to date for reverse satu-
rable absorbers. Comparisons of the ratios of 1 to those of
other reported compounds are provided in the Supporting
Information, Tables S4 and S5. It is obvious that the ratios
of 1 are larger than most of the reported reverse saturable
absorbers. Although the ratios of 1 are smaller than those
for platinum 2,2’-bipyridine complexes with 2-(benzothiazol-
2’-yl)-9,9-diethyl-7-ethynylfluorene ligands at multiple wave-
2
2
pulses and at a variety of visible and near-IR wavelengths
by using ps pulses. The experimental data were then fitted
[5g,n,q,s,20]
by using the five-level model described previously
with the input parameters (s , t , t , F ) obtained from the
0
s
T
T
photophysical studies described above to abstract both the
singlet and triplet excited-state absorption cross sections (ss
and s ) and the TPA cross sections (s ). Representative Z-
T
2
scan data and fitting curves are provided in Figure 9. Table 6
lists the resulting values of the excited-state absorption cross
sections at wavelengths in the visible region, together with
the ratio of the excited-state absorption cross section rela-
tive to that of the ground-state; also shown in Table 6 are
the values of the TPA cross sections in the near-IR region.
The data in Table 6 show that the excited-state absorption
cross sections at all of the wavelengths studied are in the
[5q]
lengths,
the reverse saturable absorption spectral region
for 1 (430–680 nm) is broader than that for the platinum
2,2’-bipyridine complex (450–600 nm). This feature along
with the large absorption cross-section ratios, as well as the
long triplet excited-state lifetime makes complex 1 a very
promising broadband reverse saturable absorber.
ꢀ
17
ꢀ16
2
range of 10 to 10 cm , which is comparable to, or even
larger than, those reported in the literature for other reverse
In the near-IR region, in which the ground-state absorp-
tion is extremely weak, the excited state is populated by
4602
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
Table 6. Excited-state absorption cross sections and TPA cross sections
FULL PAPER
creases drastically from 80% at low incident fluence to 24%
at 1.8 Jcm . From the fractional populations of the affected
of 1 at selected wavelengths in CH
2
Cl
2
solution.
ꢀ2
c
S
l [nm]
s
0
s
s
T
s
S
/s
0
s
T
/s
0
s
2
[GM]
excited states (bottom part of Figure 10) it is clear that the
triplet excited state is the dominant contributor to the ob-
served decrease in transmission. Because the triplet excited-
state absorption cross section greatly exceeds that of the
ground-state (s /s =69.6, see Table 6), strong reverse satu-
ꢀ
18
2
1
0
cm
[
c]
430
475
500
532
550
575
600
630
680
740
800
850
875
910
29.6
14.3
6.75
1.48
1.25
0.344
0.0956
0.0318
0.01
40
30
48
92
1.4
2.1
7.1
3.1
7.1
15.9
69.6
92
448
2.04ꢃ10
7.96ꢃ10
1.48ꢃ10
–
–
–
–
–
–
–
–
–
–
–
–
–
–
600
650
1200
220
200
[
c]
101
107
[
c]
40ꢁ5 103ꢁ10 27
T
0
[
[
[
[
[
c]
c]
c]
c]
c]
40
40
45
200
180
40
27
–
–
–
115
154
195
253
148
32
116
471
6.29ꢃ10
1.80ꢃ10
–
–
–
–
–
rable absorption occurs.
3
3
4
3
4
Conclusion
[
[
[
[
[
a]
a]
a]
a]
a]
[b]
[c]
[c]
0
0
0
0
0
86
89
–
–
–
[b]
A platinum(II) complex (1) with a new type of C^N^N
ligand, that is, 6-(7-benzothiazol-2’-yl-9,9-diethyl-9H-fluo-
ren-2-yl)-2,2’-bipyridine, was synthesized and the structure
was characterized with spectroscopic techniques and single
crystal X-ray crystallography. The C^N^N ligand forms
[d]
[e]
[e]
[
ACHTUNGTRENNUNG
a] Approximate value. [b] s
S
(l) n˜ = are determined from the value s
(532 nm)=4.0ꢃ10 cm and the fs TA spectrum at zero time delay. Be-
cause the fs TA will include contributions from both S and S , these
values should be considered effective cross sections for the singlet excited
S
-
ꢀ
17
2
II
a distorted square-planar configuration with the Pt ion.
1
2
1
The resulting complex exhibits ligand-centered p–p* transi-
ꢀ
16
2
states. [c] s
fitting of nanosecond and picosecond Z-scan data. For other wavelengths,
(l) is determined from the value of s (532 nm) and the fs TA spectrum
at 5.8 ns time delay. [d] Effective TPA cross section for the Z scan of
T
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(532 nm)=1.03ꢃ10 cm was determined from combined
tion in the UV and blue region, and a broad, structureless
MLCT/ LLCT absorption band in the visible region; this is
1
1
s
T
T
ACHTUNGTRENNUNG
supported by DFT calculations. The emission of 1 at room
3
ꢀ
2
temperature and at 77 K is dominated by p–p* character,
lowest energy (0.5 Jcm ). Z scans at a progression of higher energies
(
ꢀ
2
3
0.7 and 1.1 Jcm ) yield effective s
2
values of 1900 GM and 2000 GM,
possibly mixed with some MLCT character. The degree of
respectively, clear evidence for two-photon-initiated excited-state absorp-
ꢀ
2
tion. [e] Effective TPA cross section for the Z scan of 0.3 Jcm fluence
on axis.
TPA. The observed nonlinear absorption is thus two-
photon-initiated excited-state absorption. The singlet and
triplet excited-state absorption cross sections at 740 and
800 nm were estimated from the values at 532 nm and the fs
TA spectra at zero and 5.8 ns time delays, respectively.
These estimated values were used as parameters in the
model and allowed the TPA cross sections at these wave-
lengths to be obtained by fitting the Z-scan data. At wave-
lengths of 850 nm and above, fs TA data were not available,
so this method could not be used to provide an estimate of
the excited-state absorption cross sections. As the relative
contributions of two-photon and excited-state absorption
cannot be unambiguously deconvolved for wavelengths of
850, 875, and 910 nm, the TPA cross sections given in
Table 6 for these wavelengths represent effective values.
Comparison of the deconvolved or the effective TPA cross
sections of 1 in the spectral region of 630–910 nm with those
of the other reported platinum complexes is provided in the
Supporting Information, Table S6. It is clear that the TPA
cross sections of 1 are among the largest values reported for
[5t,22]
platinum complexes.
Reverse saturable absorption: Both the fs and ns transient
absorption measurements suggest that complex 1 exhibits
stronger excited-state absorption than ground-state absorp-
tion in the visible-to-near-IR region. Therefore, reverse satu-
rable absorption in this spectral region should occur. To
demonstrate this, a nonlinear transmission experiment was
carried out at 532 nm with 4.1 ns laser pulses. The result is
shown in Figure 10. The transmission of the solution de-
2 2
Figure 10. Top) Nonlinear transmission of 1 in CH Cl solution for 4.1 ns
laser pulses at 532 nm. The linear transmission is 80%, and the path
length of the cuvette is 2 mm. Bottom) The fractional population of the
affected excited states on the input face of the sample within a laser
ꢀ
2
pulse. The fluence used for the calculation is 0.24 Jcm
.
Chem. Eur. J. 2012, 18, 4593 – 4606
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4603

W. Sun et al.
3
3
p–p* and MLCT mixing varies in solvents with different
8.21 (dd, J=8.0 and 1.6 Hz, 1H), 8.20 (s, 1H), 8.14 (d, J=8.0 Hz, 1H),
8
1
2
.05 (dd, J=8.0 and 1.6 Hz, 1H), 7.79–7.90 (m, 6H), 7.47 (dt, J=8.0 and
.2 Hz, 1H), 7.36 (dt, J=8.0 and 1.2 Hz, 1H), 7.26–7.32 (m, 1H), 2.20–
polarities. In contrast, the excited state that gives rise to the
transient absorption (both fs and ns TA) is predominantly
the p–p* state. Z-scan experiments with ns and ps laser
pulses at 532 nm and ps laser pulses at a variety of visible
and near-IR wavelengths reveal that complex 1 exhibits
large ratios of excited-state absorption to ground-state ab-
sorption from 430 to 680 nm and strong TPA from 740 to
.28 (m, 4H), 0.46 ppm (t, J=7.6 Hz, 6H); ESI-HRMS: m/z: calcd for
+
[
C
34
H
27
N
3
S+H] : 510.1998; found: 510.1980; elemental analysis calcd
(%) for C H N S: C 80.12, H 5.34, N 8.24; found: C 79.85, H 5.66, N
3
4
27
3
8.40.
Compound 7: Ligand 1-L (130 mg, 0.26 mmol) and [Pt
126 mg, 0.30 mmol) were dissolved in DMF (5 mL) with a few drops of
2 2
AHCTUNGTRNNEUG( DMSO) Cl ]
(
water. The solution was stirred at 808C for 30 h under argon. The formed
yellow solid was collected by filtration, washed with water, methanol and
ether, and dried in vacuum. The crude product of 7 (190 mg) was ob-
tained in a quantitative yield. Due to the poor solubility of 7 (insoluble
in ethanol, ether, hexane and toluene, slightly soluble in DMSO, DMF
and CH Cl ), it could not be further purified by column chromatography
2 2
or recrystallization. Therefore, it was used directly in the following step
for preparation of 1 without further purification and/or characterization.
9
10 nm, which are among the largest values reported for
platinum complexes. As one might expect from the large
value of the ratio at 532 nm, strong reverse saturable ab-
sorption of ns laser pulses was observed at this wavelength.
Therefore, complex 1 could potentially be used as a broad-
band nonlinear absorbing material.
Compound 1: To a degassed suspension of 7 (128 mg, 0.16 mmol) and 1-
ethynyl-4-methylbenzene (23 mg, 0.20 mmol, 25 mL) in DMF (70 mL),
powder KOH (14 mg, 0.25 mmol), and a catalytic amount of CuI were
added. The reaction mixture was heated and stirred at 808C for 48 h
under argon. After removing the solvent, the residue was washed with
water and ether, dried in vacuum, and purified by column chromatogra-
phy on silica gel (Sorbent Technologies, 60 ꢂ, 230–450 mesh) column. Di-
chloromethane with 3% methanol (v/v) was used as the eluent. The pure
Experimental Section
Synthesis: All solvents and reagents were purchased from Aldrich or
Alfa Aesar and used as received, unless otherwise stated. 6-Bromo-2,2’-
[
23]
[24]
bipyridine (6),
9
2,7-dibromo-9,9-diethyl-9H-fluorene (2),
7-bromo-
[
24]
,9-diethyl-9H-fluorene-2-carbaldehyde (3),
and 2-(7-bromo-9,9-dieth-
[
24]
yl-9H-fluoren-2-yl)-benzothiazole (4) were prepared according to the
procedures published in the literature. The procedures for the synthesis
of 5, 7, 1-L, and 1 are described below. Compounds 5, 1-L, and 1 were
product of complex 1 was obtained as an orange solid (44 mg, yield:
1
31%). H NMR (500 MHz, CDCl
3
): d=9.24 (s, 1H), 8.44 (t, J=42 Hz,
1H), 8.15 (d, J=1.5 Hz, 1H), 8.12 (d, J=8.0 Hz, 1H), 8.03 (dd , J=8.0
and 1.5 Hz, 1H), 8.01 (d, J=10 Hz, 1H), 7.88–7.96 (m, 3H), 7.82 (t, J=
8.0 Hz, 1H), 7.63 (brs, 1H), 7.57 (d, J=8.0 Hz, 1H), 7.52 (dt, J=8.0 and
1.0 Hz, 3H), 7.48 (dt, J=8.0 and 1.0 Hz, 1H), 7.36 (dt, J=8.0 and 1.0 Hz,
1H), 7.33 (s, 1H), 7.18 (d, J=8.0 Hz, 2H), 2.37 (s, 3H), 2.06–2.26 (m,
4H), 0.43 ppm (t, J=7.5 Hz, 6H); ESI-HRMS: m/z: calcd for
1
characterized by H NMR spectroscopy and elemental analyses. Addi-
tional characterization by ESI-HRMS was carried out on 1-L and 1.
H NMR spectra were measured on a Varian Oxford-400 VNMR spec-
trometer or a Varian Oxford-500 VNMR spectrometer; and ESI-HRMS
analyses were conducted on a Bruker Daltonics BioTOF III mass spec-
trometer. Elemental analyses were performed by NuMega resonance labs
in San Diego, CA.
1
+
[C H N PtS+H] : 819.2119; found, 819.2105; elemental analysis calcd
4
3
33
3
(%) for C H N PtS·CH Cl : C 58.47, H 3.90, N 4.65; found: C 58.33, H
4
3
33
3
2
2
Compound 5: Compound 4 (3.05 g, 7.00 mmol) was dissolved in degassed
dry THF (40 mL) and the solution was cooled down to ꢀ788C in a dry
ice–heptane bath. n-Butyl lithium (5.1 mL) in hexane (1.60m, 8.20 mmol)
was then added dropwise under argon. After stirring for 30 min, isopro-
pyl pinacolyl borate (1.70 mL, 1.52 g, 8.20 mmol,) was added with a sy-
ringe. The reaction mixture was stirred overnight, first at ꢀ788C and
then slowly warmed up to room temperature. After reaction, the mixture
was again cooled down to 58C, and treated with a hydrochloric acid solu-
tion (15 mL, 6.00m). Then THF was removed by distillation, and the
aqueous phase was extracted three times with diethyl ether (3ꢃ50 mL).
3.93, N 4.91.
Crystal structure determination: Single crystals of 1-L and 1 were ob-
tained by slow diffusion of heptane into dilute dichloromethane solutions
of the sample. The single crystal X-ray data were collected on a SIE-
MENS diffractometer with a 1K CCD area detector (graphite-monochro-
mated MoKa radiation) at ambient temperature. The structures were
2
solved by direct methods and refined on F by using the SHELXTL
V6.14 package (after absorption corrections with SADABS). All of the
non-hydrogen atoms were refined with anisotropic displacement coeffi-
cients. Hydrogen atoms were idealized using the SHELXTL software.
Details of the data collections and refinements are given in Table 7.
The organic layer was washed with brine, dried with Na
2 4
SO , and the sol-
vent was removed. The residual solid was recrystallized from toluene to
1
Computational details for DFT calculations: DFT calculations were per-
formed for complex 1 to characterize the FMOs, and to understand the
nature of the ground and low-lying, excited, electronic states. The low-
lying, excited, electronic states were characterized by performing
give 2.66 g yellow crystals (yield: 70%). H NMR (CDCl
1
1
3
): d=8.13 (s,
H), 8.11 (d, J=8.0 Hz, 1H), 8.04 (d, J=8.0 Hz, 1H), 7.92 (d, J=8.0 Hz,
H), 7.80–7.86 (m, 2H), 7.74–7.80 (m, 2H), 7.51 (t, J=9.0 Hz, 1H), 7.40
(
t, J=9.0 Hz, 1H), 2.10–2.22 (m, 4H), 1.40 (s, 12H), 0.28–0.33 ppm (m,
[
25]
TDDFT calculations.
A hybrid generalized gradient approximation
6
2
H); elemental analysis calcd (%) for C30
.9; found: C 74.8, H 7.1, N 3.2.
2
H32BNO S: C 74.8, H 6.7, N
(
hybrid GGA) exchange-correlation functional was used for all DFT cal-
culations. The particular hybrid GGA used is known by the acronym
MPW3LYP, which consists of the three-parameter modified Perdew-
Wang (MPW) exchange functional and the Lee–Yang–Parr (LYP) corre-
Compound 1-L: Compounds 5 (0.48 g, 1.00 mmol), 6 (0.35 g, 1.50 mmol),
and K CO (5.50 g, 0.04 mol) were dissolved in a mixed solvent of diox-
ane (40 mL), toluene (40 mL), and water (20 mL), and the solution was
degassed with argon for 30 min. [Pd(PPh ] (33 mg, 0.03 mmol) and PPh
16 mg, 0.06 mmol) were then added. After refluxing for 72 h under
argon, the aqueous phase was extracted with diethyl ether (3ꢃ50 mL).
The organic layer was washed with brine, dried with Na SO , and the sol-
2
3
[
26]
lation functional.
The basis sets used include functions from the
A
H
U
G
R
N
U
G
3
)
4
3
[
27]
[28]
LANL2DZ set and from the 6–31G* set; the particular combination
is abbreviated in this work as LANG631. LANL2DZ is an effective core
potential (ECP) basis set and was used to describe the platinum atom;
this provides some correction for scalar relativistic effects of the platinum
atom. The 6–31G* basis set was used for the descriptions of all other
atoms.
(
2
4
vent was removed. The residual solid was purified by chromatography on
silica gel (Sorbent Technologies, 60 ꢂ, 230–450 mesh) column. The by-
product was removed first by toluene, then the desired product was ob-
tained by using dichloromethane or ether as the eluent. The crude prod-
uct was purified by recrystallization from dichloromethane and heptane
Full geometry optimizations were performed for the ground state of com-
plex 1 and excited singlet electronic states were calculated at the ground-
state-optimized geometry by using the TDDFT method, with the
MPW3LYP exchange-correlation functional. The Gaussian 03 (revi-
to give colorless crystal (0.35 g, yield: 70%) suitable for X-ray diffraction
1
analysis. H NMR (CDCl
3
): d=8.72 (d, J=4.0 Hz, 1H), 8.70 (d, J=
[
29]
8
.0 Hz, 1H), 8.44 (dd, J=8.0 and 1.2 Hz, 1H), 8.24 (d, J=1.6 Hz, 1H),
sion C.02) software suite,
running on a 96-node distributed-memory
4604
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4593 – 4606

Broadband Nonlinear Absorption of a Platinum(II) Complex
FULL PAPER
Table 7. Single-crystal X-ray parameters and refinement data for 1-L and
taining the e
T
value, the triplet excited-state quantum yield was measured
[
16]
1
.
by relative actinometry, in which SiNc in benzene was used as the ref-
ꢀ1
ꢀ1
[33]
erence (e590 =70000m cm , F
Nonlinear optical characterizations: The nonlinear absorption of complex
was characterized by open-aperture Z-scan experiments by using ns
T
=0.20).
1-L
1
T [K]
273(2)
273(2)
1
l [ꢂ]
0.71073
0.71073
laser at 532 nm and ps laser from 450 to 900 nm, and by nonlinear-trans-
mission experiments at 532 nm by using ns laser. For the open-aperture
Z-scan measurements, the experimental setup and experimental details
formula
C
34
H
27
N
3
S
43 33 3
C H N PtS
M
r
509.65
818.87
crystal size [mm]
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
0.66ꢃ0.3ꢃ0.26
monoclinic
C2/c
34.358(4)
7.4355(9)
24.366(3)
90
120.523(2)
90
5362.00(11)
8
0.22ꢃ0.16ꢃ0.06
triclinic
P1
[5s,20]
were similar to those reported in our previous publications.
The ex-
perimental data were fitted by using a five-level model to extract the ex-
cited-state absorption cross sections and the TPA cross sections. The de-
tails of the model and fitting procedure were the same as those reported
¯
7.527(7)
13.206(12)
17.357(15)
84.284(16)
84.008(16)
86.178(15)
1705(3)
2
[5s,20]
previously.
The nonlinear-transmission experiment for complex 1 was conducted in
CH Cl in a 2 mm cuvette by using 4.1 ns laser pulses at 532 nm. The
2 2
a [8]
b [8]
light source was a Quantel Brilliant ns laser with a repetition rate of
10 Hz. The experimental setup and details are the same as previously de-
g [8]
V [ꢂ ]
3
[5c]
scribed. A 20 cm plano-convex lens was used to focus the beam to the
2 mm thick sample cuvette. The linear transmission of the solution was
adjusted to 80% at 532 nm.
Z
ꢀ
3
1
calc [gcm
]
1.263
0.149
1.595
4.213
ꢀ
1
m [mm
]
F
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(000)
2144
812
reflns collected
14846
15113
unique reflections
no. of reflns (Iꢂ2s)
goodness-of-fit on F
5243
3469
1.006
52
0.0468/0.1422
0.0797/0.1721
2
7320
5879
1.076
52
0.0464/0.1362
0.0668/0.1757
Acknowledgements
2
This work is supported by National Science Foundation (CAREER
CHE-0449598 and CHEM-0946990) and the Army Research Laboratory
(W911NF-06–2–0032).
2
R
R
q max [8]
[
a]
1
/wR
/wR
2
(Iꢂ2s(I))
[
a]
1
2
(all data)
2
2
2
2
1/2
2
[
2
0
a] R
1
=Sj jF
o
jꢀjF
c
j j/SjF
o
j, wR
2
=[S(w
A
H
N
R
N
U
G
o
ꢀF
c
) )/Sw
A
H
U
G
R
N
U
G
o
) ] for F
o
>
2
2
2
2
2
2
s
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(F
o
), w=1/[s
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(F
o
)+(AP) +BP], for which P=(F
o
+2F
c
)/3; A(B)=
[
1] a) W. Lu, B. X. Mi, M. C. W. Chan, Z. Hui, C. M. Che, N. Zhu, S. T
Lee, J. Am. Chem. Soc. 2004, 126, 4958–4971; b) H. Yersin, D.
Donges, W. Humbs, J. Strasser, R. Sitters, M. Glasbeek, Inorg.
Chem. 2002, 41, 4915–4922; c) J. Brooks, Y. Babayan, S. Lamansky,
P. I. Djurovich, I. Tsyba, R. Bau, M. E. Thompson, Inorg. Chem.
.19323 (0.000) for 1-L and A(B)=0.1176 (0.000) for 1.
cluster with 192 Dual 3.06 GHz Xeon-HT processors at North Dakota
State University, was used to perform all calculations.
2
002, 41, 3055–3066; d) J. C. Shi, H. Y. Chao, W. F. Fu, S. M. Peng,
Photophysical measurements: The UV/Vis spectra of 1-L and 1 in differ-
ent solvents (spectrophotometric grade) were acquired by using a UV-
C. M. Che, J. Chem. Soc. Dalton Trans. 2000, 3128–3132; e) L. Chas-
sot, A. von Zelewsky, D. Sandrini, M. Maestri, V. Balzani, J. Am.
Chem. Soc. 1986, 108, 6084–6085; f) M. Maestri, D. Sandrini, V. Bal-
zani, L. Chassot, P. Jolliet, A. von Zelewsky, Chem. Phys. Lett. 1985,
2
501 spectrophotometer. The steady-state emission spectra in different
solvents were obtained on a SPEX fluorolog-3 fluorometer/phosphorom-
eter. The emission quantum yields were measured by the relative actino-
1
22, 375–379; g) W. Y. Wong, Z. He, S. K. So, K. L. Tong, Z. Lin,
[
30]
metry method in degassed solutions. A degassed aqueous solution of
Organometallics 2005, 24, 4079–4082; h) S. C. F. Kui, I. H. T. Sham,
C. C. C. Cheung, C. W. Ma, B. Yan, N. Zhu, C. M. Che, W. F. Fu,
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[
31]
[
Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy)
3
]Cl
2
(Fem =0.042, lex =436 nm) was used as the reference for
=0.546, lex
47.5 nm) was used as the reference for ligand 1-L. The excited-state
complex 1 and an aqueous solution of quinine sulfate (F
3
f
=
[
32]
lifetimes, the triplet TA spectra, and the triplet excited-state quantum
yields were measured on an Edinburgh LP920 laser flash photolysis spec-
trometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quan-
tel Brilliant, pulsewidth ca. 4.1 ns, repetition rate is set to 1 Hz) was used
as the excitation source. Sample solutions were purged with Ar for
4
180–4190.
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0 min prior to each measurement. The femtosecond transient difference
absorption spectra and the singlet excited-state lifetime were measured
with a femtosecond pump-probe UV/Vis spectrometer (HELIOS) manu-
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Physics Hurricane, 1 kHz repetition rate, 1 mJ per pulse at 800 nm) and
the absorption was probed from 425 to 800 nm with sapphire generated
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1
2244–12254.
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The triplet excited-state molar extinction coefficients (e
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) at the TA band
[
15]
maximum were determined by the singlet depletion method, in which
[
15]
T
the following equation was used to calculate the e :
e
S
½DOD
DOD
T
Š
e
T
¼
S
in which DOD
mum of the bleaching band and at the maximum of the positive band in
the TA spectrum, respectively, and e is the ground-state molar extinction
coefficient at the wavelength of the bleaching band minimum. After ob-
S T
and DOD are the optical-density changes at the mini-
S
Chem. Eur. J. 2012, 18, 4593 – 4606
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Published online: March 8, 2012
4606
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4593 – 4606