Tuning photophysical properties and improving nonlinear absorption of Pt(II) diimine complexes with extended π-conjugation in the acetylide ligands

Article abstract of DOI:10.1021/jp309841x

Two new Pt(II) 4,4′-di(5,9-diethyltridecan-7-yl)-2,2′- bipyridine complexes (1 and 2) bearing 9,9-diethyl-2-ethynyl-7-(2-(4- nitrophenyl)ethynyl)-9H-fluorene ligand and N-(4-(2-(9,9-diethyl-7-ethynyl-9H- fluoren-2-yl)ethynyl)phenyl)-N-phenylbenzeneamine ligand, respectively, were synthesized and characterized. Their photophysical properties were investigated systematically by UV-vis absorption, emission, and transient absorption (TA) spectroscopy, and the nonlinear absorption was studied by nonlinear transmission technique. Theoretical TD-DFT calculations using the CAM-B3LYP functional were carried out to determine the nature of the singlet excited electronic states and to assist in the assignment of significant transitions observed in experiments. Complex 1 exhibits an intense, structureless absorption band at ca. 397 nm in CH₂Cl₂ solution, which is attributed to mixed metal-to-ligand charge transfer (¹MLCT)/ligand-to-ligand charge transfer (¹LLCT)/intraligand charge transfer (¹ILCT)/ ¹π,π* transitions, and two ¹MLCT/ ¹LLCT transitions in the 300-350 nm spectral region. Complex 2 possesses an intense acetylide ligand localized ¹π,π* absorption band at ca. 373 nm and a moderately intense ¹MLCT/ ¹LLCT tail above 425 nm in CH₂Cl₂. Both complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the predominant ³π,π* state for 1, whereas the emitting state of 2 exhibits a switch from ³π,π* state in high-polarity solvents to ³MLCT/³LLCT state in low-polarity solvents. Both 1 and 2 exhibit strong singlet excited-state TA in the visible to NIR region, where reverse saturable absorption (RSA) is feasible. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these Pt complexes can be tuned drastically by extending the π-conjugation of the acetylide ligands. In addition, strong RSA was observed at 532 nm for nanosecond (ns) laser pulses from 1 and 2, demonstrating that the RSA of the Pt(II) diimine complexes can be improved by extending the π-conjugation of the acetylide ligands.

Full text of DOI:10.1021/jp309841x

Article
pubs.acs.org/JPCA
Tuning Photophysical Properties and Improving Nonlinear
Absorption of Pt(II) Diimine Complexes with Extended π‑Conjugation
in the Acetylide Ligands
Rui Liu,† Alexander Azenkeng,‡ Dapeng Zhou,§ Yuhao Li,† Ksenija D. Glusac,§ and Wenfang Sun*,†
†Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
‡Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9018, United States
§Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403-0001, United States
S
* Supporting Information
ABSTRACT: Two new Pt(II) 4,4′-di(5,9-diethyltridecan-7-yl)-2,2′-bipyri-
dine complexes (1 and 2) bearing 9,9-diethyl-2-ethynyl-7-(2-(4-nitrophenyl)-
ethynyl)-9H-fluorene ligand and N-(4-(2-(9,9-diethyl-7-ethynyl-9H-fluoren-2-
yl)ethynyl)phenyl)-N-phenylbenzeneamine ligand, respectively, were synthe-
sized and characterized. Their photophysical properties were investigated
systematically by UV−vis absorption, emission, and transient absorption (TA)
spectroscopy, and the nonlinear absorption was studied by nonlinear
transmission technique. Theoretical TD-DFT calculations using the CAM-
B3LYP functional were carried out to determine the nature of the singlet
excited electronic states and to assist in the assignment of significant
transitions observed in experiments. Complex 1 exhibits an intense,
structureless absorption band at ca. 397 nm in CH2Cl2 solution, which is
attributed to mixed metal-to-ligand charge transfer (1MLCT)/ligand-to-ligand
charge transfer (1LLCT)/intraligand charge transfer (1ILCT)/1π,π* tran-
sitions, and two 1MLCT/1LLCT transitions in the 300−350 nm spectral region. Complex 2 possesses an intense acetylide ligand
1
1
localized π,π* absorption band at ca. 373 nm and a moderately intense MLCT/1LLCT tail above 425 nm in CH2Cl2. Both
complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the predominant
3π,π* state for 1, whereas the emitting state of 2 exhibits a switch from π,π* state in high-polarity solvents to MLCT/3LLCT
state in low-polarity solvents. Both 1 and 2 exhibit strong singlet excited-state TA in the visible to NIR region, where reverse
saturable absorption (RSA) is feasible. The spectroscopic studies and theoretical calculations indicate that the photophysical
properties of these Pt complexes can be tuned drastically by extending the π-conjugation of the acetylide ligands. In addition,
strong RSA was observed at 532 nm for nanosecond (ns) laser pulses from 1 and 2, demonstrating that the RSA of the Pt(II)
diimine complexes can be improved by extending the π-conjugation of the acetylide ligands.
3
3
observed on the emission energy, lifetime, and quantum yield
upon variation of the substituents on the acetylide ligands. The
latter phenomenon is attributed to the admixture or switching
order of multiple excited states in proximity. Our group
recently reported the influence of the auxiliary substituent on
the photophysics of Pt(II) bipyridyl bisfluorenylacetylide
complexes21 and Pt(II) bipyridyl bisstilbenylacetylide com-
plexes,22 respectively. When a strong electron-withdrawing
substituent is introduced to the acetylide ligand, the lowest
singlet and triplet excited states of the Pt(II) bipyridyl complex
are dominated by the acetylide ligand localized π,π* transitions,
resulting in long-lived triplet excited state and broadband triplet
excited-state absorption.21,22 Castellano and co-workers23,24
discovered that when the π-conjugation of the acetylide ligand
INTRODUCTION
■
Pt(II) diimine bisacetylide complexes1 are interesting photo-
functional materials. They have shown potential applications in
a variety of photonic processes and devices, such as solar energy
conversion via photoinduced charge separation,2−4 low-power
upconversion,5−9 luminescent materials,10−12 chemosen-
sors,13,14 and nonlinear optics.15−17 These applications are
primarily based on the square-planar configuration and the rich
photophysical properties of the Pt(II) diimine complexes,
which can be readily tuned by structural modifications to meet
the specific requirements for a predetermined application.
It has been reported that the lowest-energy absorption
band(s) in many of the Pt(II) diimine complexes is the metal-
to-ligand charge transfer (1MLCT)/ligand-to-ligand charge
transfer (1LLCT) absorption band(s), which can be tuned by
the substituent on the bipyridine ligand or on the acetylide
ligands.18−20 Although the effect of the substituents on the
bipyridine ligand is well understood, no monotonic trend was
Received: October 4, 2012
Revised: February 8, 2013
Published: February 11, 2013
© 2013 American Chemical Society
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Scheme 1. Structures of Pt(II) Complexes 1−4 and Ligands 1L and 2L
Scheme 2. Synthetic Route for Complexes 1 and 2
is extended, the contribution of acetylide ligand localized 3π,π*
state to the lowest triplet excited state becomes more
significant, and the configurational mixing and triplet excited-
state inversion can be induced by solvent with different polarity.
Zhao and co-workers also demonstrated that extending the π-
conjugation of the acetylide ligand in Pt(II) diimine complexes
electron-donating diphenylamino substituent was introduced to
the acetylide ligand, both the singlet and triplet excited-states of
the resultant Pt(II) complexes absorb strongly in the visible
spectral region with λmax ≈ 520 nm, resulting in remarkably
strong reverse saturable absorption at 532 nm.
Considering these exciting results and the effect of extended
π-conjugation of the acetylide ligand, we envision that
extending the π-conjugation of the acetylide ligands in Pt(II)
diimine bisfluorenylacetylide complexes with nitro or dipheny-
lamino substituents would prolong the triplet excited state
lifetime and further improve their reverse saturable absorption.
With these considerations in mind, two Pt(II) bipyridyl
complexes (complex 1 and 2 in Scheme 1) with terminal NO2
or NPh2 substituent on the acetylide ligand were synthesized.
In order to reduce the intermolecular aggregation and
consequently improve the solubility of the Pt(II) complexes,
branched alkyl chains were introduced on bipyridine ligand.
Complexes 3 and 4 (Scheme 1) have been reported previously
by our group21 and are included in this paper as references for
better understanding of the effect of extended π-conjugation.
3
led to long-lived acetylide ligand localized π,π* state and one
of the complexes is suitable for triplet−triplet-annihilation
based upconversion application.9 Very recently, our group
revealed that when a strong electron-withdrawing aromatic
substituent naphthalimide was introduced to the fluorenylace-
tylide ligand, the lowest singlet and triplet excited states were
dominated by the intraligand charge transfer (ILCT) characters
in CH2Cl2.25 Moreover, the triplet excited-state lifetimes of
these complexes become very long and their reverse saturable
absorption is significantly enhanced due to the extended π-
conjugation in the acetylide ligand.
On the other hand, our previous studies on the Pt(II)
bipyridyl bisfluorenylacetylide complexes21 and Pt(II) bipyridyl
bisstilbenylacetylide complexes22 manifested that when a strong
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The singlet and triplet excited-state characteristics of 1 and 2
are systematically investigated via UV−vis absorption, steady-
state emission, and TA techniques and TD-DFT calculations.
The influence on RSA has been demonstrated by nonlinear
transmission at 532 nm using ns laser pulses.
developed by Handy and co-workers.29 The B3LYP method
was used for geometry optimizations, and the CAM-B3LYP
method was used for calculating excited electronic states
transition energies. The CAM-B3LYP method is one of the
recently developed DFT functionals that takes into account the
long-range correction effects often encountered in the
calculation of charge transfer molecules such as those involved
in this work. The basis sets used in all calculations includes
functions from the 6-31G* set30−34 used to describe all light
element atoms and the LANL2DZ set35−37 for the Pt atom; the
particular combination is abbreviated in this work as LANG631.
LANL2DZ is an effective core potential (ECP) basis set, which
was used to provide some corrections for the scalar relativistic
effects of the Pt atom.
Full equilibrium geometry optimizations were performed for
the ground electronic states of all complexes using a tightened
self-consistent field (SCF) convergence threshold of 10−9 a.u.
To simplify the calculatuions on these large complexes, methyl
groups were used to replace the branched alkyl chains on the
bipyridine ligand and on the fluorenyl components in
complexes 1 and 2.
Excited state calculations were performed at the fully
optimized ground state molecular geometry of the complexes.
In addition, bulk solvent effects have been included in all
calculations using the polarizable continuum model (PCM).38
For consistency with experiments, dichloromethane (CH2Cl2)
that was used as the solvent for experimental measurements
was selected as the solvent in theoretical calculations as well. All
calculations were performed using the Gaussian 09 (Revision
A.1) software suite39 running on a 96-node distributed-memory
cluster with 192 Dual 3.06 GHz Xeon-HT processors at North
Dakota State University.
Photophysical Measurements. Ground-state UV−vis
absorption spectra of Pt complexes and ligands were measured
in a 1 cm quartz cuvette on a Shimadzu UV-2501
spectrophotometer in different solvents (HPLC grade). The
steady-state emission spectra were carried out using a HORIBA
FluoroMax 4 fluorometer/phosphorometer. Relative actino-
metry method40 was applied to measure the emission quantum
yields in degassed solutions. A degassed aqueous solution of
[Ru(bpy)3]Cl2 (Φem = 0.042, λex = 436 nm)41 was used as the
reference for Pt complexes and an aqueous solution of quinine
bissulfate (Φem = 0.546, λex = 365 nm)42 was used as the
reference for ligands.
EXPERIMENTAL SECTION
■
Synthesis. The synthetic route for complexes 1 and 2 is
provided in Scheme 2. The synthesis and characterization of 3
and 4 were reported by our group previously.21 All reagents and
solvents for synthesis were purchased from Aldrich or Alfa
Aesar and used as received. The precursor complex 4,4′-di(5,9-
diethyltridecan-7-yl)-2,2′-bipyridyl Pt(II) dichloride (Pt-
(ddtbpy)Cl2),22 ligand 1L26 and 2L26 were prepared according
to literature procedures. All HPLC grade solvents used for
spectroscopic studies were purchased from VWR International
and used without further purification unless otherwise stated.
Silica gels used for chromatography were purchased from
Sorbent Technology (230−400 mesh). Complexes 1 and 2
1
1
were characterized by H NMR and elemental analyses. H
NMR spectra were obtained on a Varian Oxford-400 VNMR
spectrometer or a Varian Oxford-500 VNMR spectrometer.
Elemental analyses were carried out by NuMega Resonance
Laboratories, Inc. in San Diego, California. HRMS data for 1
and 2 were not obtained due to the difficulty in ionizing the
complexes.
General Procedure for Synthesis of Complexes 1 and
2. The ligand 1L or 2L (0.70 mmol) and Pt(ddtbpy)Cl2 (0.33
mmol) were dissolved in degassed mixture of dry CH2Cl2 (30
mL) and diisopropyl amine (20 mL). The catalyst CuI (∼5
mg) was then added. The reaction mixture was refluxed under
argon for 12 h. After cooling to room temperature, the reaction
solution was washed with brine and dried with MgSO4 and the
solvent was removed. The residual solid was purified by column
chromatography on silica gel using CH2Cl2 as the eluent. The
product was further purified by recrystallization from CH2Cl2
and hexane to yield desired product.
Complex 1. A total of 332 mg of yellow solid was obtained
1
(yield: 63%). H NMR (400 MHz, CDCl3) δ 9.84 (d, J = 5.5
Hz, 2H), 7.73 (s, 2H), 7.67 (d, J = 8.0 Hz, 2H), 7.62−7.56 (m,
6H), 7.45 (d, J = 4.5 Hz, 2H), 7.33−7.30 (m, 4H), 7.28−7.25
(m, 2H), 2.95 (br, 2H), 2.09−1.98 (m, 8H), 1.62−1.56 (m,
12H), 1.33−1.09 (m, 36H), 0.93−0.80 (m, 22H), 0.36−0.33
(m, 10H). Anal. Calcd. (%) for C82H110N2Pt·1/3CH2Cl2: C,
73.40; H, 8.30; N, 2.08. Found: C, 73.21; H, 8.37; N, 2.35.
Complex 2. A total of 261 mg of yellow solid was obtained
The femtosecond (fs) TA spectra and the singlet excited-
state lifetimes were measured using a fs pump−probe UV−vis
spectrometer. The laser system for the ultrafast TA measure-
ment was described previously.43 The laser system output
consists of a Ti:sapphire oscillator/regenerative amplifier
(Hurricane, Spectra Physics) as a source of 800 nm light with
full width at half-maximum (fwhm) of 110 fs operating at a
repetition rate of 1 kHz. The 800 nm light was split into pump
(85%) and probe (10%) beams. The probe beam (800 nm) was
delayed by a delay stage (MM4000, Newport). A white light
continuum probe beam was produced using a CaF2 crystal. The
sample solution in a 2-mm flow cell was excited using 400 nm
light (∼ 2 μJ/pulse) produced by a second harmonic generator
(Super Tripler, CSK).
1
(yield: 43%). H NMR (400 MHz, CDCl3) δ 9.77 (d, J = 6.0
Hz, 2H), 7.68 (s, 2H), 7.59−7.51 (m, 8H), 7.45−7.36 (m,
10H), 7.27−7.34 (m, 8H), 7.09 (d, J = 7.6 Hz, 8H), 7.05−6.98
(m, 8H), 2.91 (sh, 2H), 2.03−1.98 (m, 8H), 1.57 (sh, 8H),
1.34−1.05 (m, 36H), 0.82−0.75 (m, 24H), 0.31 (t, J = 7.4 Hz,
12H). Anal. Calcd (%) for C122H136N6Pt: C, 79.06; H, 7.40; N,
3.02. Found: C, 79.07; H, 7.57; N, 2.97.
DFT Calculations. The ground electronic states of
complexes 1 and 2 were simulated by density functional theory
(DFT) and their excited singlet electronic states were
calculated using the time-dependent DFT (TDDFT) method
to simulate their theoretical UV−vis absorption spectra. The
specific DFT methods used include Becke’s exchange (B3)
functional, in conjunction with Lee−Yang−Parr (LYP)
correlation functional, commonly known as the B3LYP
method27,28 and the Coulomb attenuating model (CAM)
modified version of the B3LYP method (CAM-B3LYP)
The ns TA spectra, triplet excited-state lifetimes, and triplet
excited-state quantum yields were measured on an Edinburgh
LP920 laser flash photolysis spectrometer. The excitation
source was the third harmonic output (355 nm) of a Nd:YAG
laser (Quantel Brilliant, pulsewidth ∼4.1 ns, 1 Hz). Before each
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measurement, all of the sample solutions were degassed with Ar
for 30 min.
molar extinction coefficient and similar feature. Therefore, this
band should contain significant acetylide ligand based π,π*
1
The triplet excited-state molar extinction coefficients (εT) at
the TA band maximum was determined by singlet depletion
method,44 in which εT was calculated by the following
equation:44
character. However, the band in 1 is substantially red-shifted
and broadened. On one hand, the red-shift could partially arise
from the delocalization of ligand-centered molecular orbital due
to the interaction with the Pt(II) dπ orbital. On the other hand,
the broadening and red-shift imply admixture of other charge
εS[ΔODT]
εT =
1
transfer transitions with the π,π* transition, which is partially
ΔODS
reflected by the structureless feature of this band. With
reference to the UV−vis absorption spectra of complex 321
and Pt(II) diimine complex with nitrostilbenylacetylide
ligands22 (which all exhibit the similar intense structureless
absorption band), this band could be tentatively attributed to
where εS is the ground-state molar extinction coefficient at the
wavelength of the bleaching band minimum in TA spectrum;
ΔODS and ΔODT are the optical density changes at the
minimum of the bleaching band and the maximum of the
positive band, respectively. When the εT value was obtained,
the triplet excited-state quantum yield can be calculated by the
relative actinometry,45 in which SiNc in benzene was used as
the reference (ε590 = 70 000 M−1 cm−1, ΦT = 0.20).46
Nonlinear Transmission Measurement. Using 4.1 ns
laser pulses at 532 nm, the nonlinear transmission experiments
for Pt complexes were carried out in CH2Cl2 solution in a 2-
mm cuvette. The linear transmission of the solution was
adjusted to 80%. A Quantel Brilliant ns laser with a repetition
rate of 10 Hz was used as the light source. The experimental
setup and details were described previously.47 An f = 40 cm
plano-convex lens was used to focus the beam to the sample
cuvette. The radius of the beam waist at the focal point was
approximately 96 μm measured by knife edge.
1
acetylide ligand localized π,π* transition mixed with some
1MLCT/1LLCT transitions. In addition, considering the
extended π-conjugation of the acetylide ligands and the strong
1
electron-withdrawing ability of the NO2 substituent, ILCT
from the ethynylfluorene component to the nitrophenyl
component also likely to contribute to this band. The
configurationally mixed transition nature of this absorption
band is partially reflected by the minor solvent effect (Figure
2a) and is supported by the TDDFT calculation results that will
be discussed later.
The UV−vis absorption spectrum of 2 mainly consists of an
intense absorption band at ca. 373 nm and a broad tail from
425 to 525 nm. The similar shape and energy of the 373 nm
band to the major absorption band in ligand 2L suggests that
this band should predominantly emanate from the acetylide
ligand based 1π,π* transitions. The minor solvent effect
observed for this band (Figure 2b) supports this assignment.
In contrast, the broad tail from 425 to 525 nm displays a
negative solvatochromic effect (i.e., the band bathochromically
shifts when the polarity of solvent decreases, such as the case in
toluene and hexane) (Figures 2b). The negative solvatochromic
effect is indicative of a charge-transfer transition in which the
dipole moment of the ground state is larger than that of the
excited state. Based on the similar results reported for other
Pt(II) diimine acetylide complexes2,4,11,12 and for complex 4,21
RESULTS AND DISCUSSION
■
Electronic Absorption. The UV−vis absorption spectra of
the complexes 1 and 2 and ligands 1L and 2L in CH2Cl2 are
presented in Figure 1. The absorption band maxima and molar
1
this band can be attributed to the MLCT/1LLCT transitions.
This assignment is also supported by the TDDFT calculation
results discussed later.
The major absorption bands in 1 and 2 are influenced
significantly by the extended π-conjugation in the acetylide
ligands. A comparison of 1 and 3 and 2 and 4, respectively,
indicates that the molar extinction coefficients in 1 and 2 are
substantially increased; especially in 2 the enhancement is
dramatic. In addition, for complex 2, the 1MLCT/1LLCT band
is slightly blue-shifted compared to that in 4. The blue-shift of
the 1MLCT/1LLCT absorption band with increased π-
conjugation of the phenylacetylide ligand has also been
reported by Che’s group48 and by our group49 on cyclo-
metalated Pt(II) C∧N∧N complexes.
Figure 1. UV−vis absorption spectra of Pt complexes 1−4 in CH2Cl2.
Inset shows the UV−vis absorption spectra of ligands 1L and 2L in
CH2Cl2..
Calculation of Singlet Excited Electronic States. The
ground state equilibrium geometry and the electronic structure
of the complexes 1 and 2 were calculated using the B3LYP
DFT method in conjunction with LANG631 basis set. All
calculations were performed in the condensed phase using the
polarizable continuum model (PCM)38 and CH2Cl2 as solvent
to maintain close similarity to the experimental conditions.
Results of the ground state electronic structure calculations
were used to generate the frontier molecular orbital (FMO)
electron density plots of four highest occupied molecular
orbitals (HOMOs) and four lowest unoccupied molecular
extinction coefficients are listed in Table 1. The UV−vis
absorption of the complexes and ligands obey Beer’s law in the
concentration range of 1 × 10−6 to 1 × 10−4 mol L−1,
suggesting that no dimerization or oligomerization occurs in
this concentration range.
The absorption spectra of 1 and 2 are quite distinct. For
complex 1, the spectrum is dominated by an intense, broad,
structureless absorption band at λmax = 397 nm (ε ≈ 7.1 × 104
L mol−1 cm−1) in CH2Cl2. Comparing this absorption band to
the corresponding band in ligand 1L, both bands exhibit similar
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Table 1. Photophysical Data for Ligands 1L and 2L and Pt(II) Complexes 1 and 2
λem/nm (τ /ns; Φem
)
λem/nm (τem/μs)
λT1‑Tn/nm (τT/ns;
a
emb
d
e
f
λabs/nm (ε/104 L/mol cm)
R.T.
77 K
λS1−Sn/nm (τS/ps)
εT1‑Tn /104 L/mol cm; ΦT)
g
1L 303 (5.39), 369 (7.64)
549 (c, 0.049)
540 (c)
476 (108 2), 552
(sh.,101 2)
478 (1870, 6.51, 0.17)
g
2L 305 (4.05), 369 (6.07)
448 (c, 0.81)
410 (c)
555 (sh., 8 1), 651 (17 1)
475 (31200, 3.51, 0.11)
h
1
2
314 (6.03), 338 (5.64),
397 (7.13)
576 (25; 0.001)
570 (109), 615 (75) 489 (40 2)
500 (3220, 7.20, 0.15)
h
300 (6.44), 373 (15.0),
450 (1.37)
568 (970; 0.070)
561 (93), 605 (93) 646 (13 1)
550 (610, i, i)
a
b
Electronic absorption band maxima and molar extinction coefficients in CH2Cl2 at room temperature. Room temperature emission band maxima
and decay lifetimes measured in CH2Cl2 at a concentration of 1 × 10−5 mol/L. A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem = 0.042, excited
c
d
at 436 nm) was used as the reference. Not measured. The emission band maxima and decay lifetimes at 77 K measured in MTHF glassy solution at
e
f
a concentration of 1 × 10−5 mol/L. fs TA band maxima and singlet excited-state lifetimes in 10:1 (v/v) CH3CN/CH2Cl2 mixture. ns TA band
maxima, triplet extinction coefficients, triplet excited-state lifetimes and quantum yields. SiNc in C6H6 was used as the reference. (ε590 = 70 000 L
g
h
i
mol−1 cm−1, ΦT = 0.20). In CH3CN. In toluene. Too weak to be measured.
Figure 2. Normalized UV−vis absorption spectra of (a) complex 1 and (b) complex 2 in different solvents. The concentration of each solution was
adjusted in order to obtain A = 0.08 at 436 nm in a 1-cm cuvette.
Figure 3. Top: Contour plots of the highest occupied molecular orbital (HOMO), HOMO-1, HOMO-2, HOMO-3, lowest unoccupied molecular
orbital (LUMO), LUMO+1, LUMO+2, and LUMO+3 for complex 1. Bottom: Contour plots of the highest occupied molecular orbital (HOMO),
HOMO-1, HOMO-2, and HOMO-3, and lowest unoccupied molecular orbital (LUMO), LUMO+1, LUMO+2, and LUMO+3, for complex 2.
orbitals (LUMOs) displayed in Figure 3, to assist in the
interpretation and assignment of significant electron transitions
in the experimentally observed UV−vis absorption spectra.
For complex 1, the electron density of the HOMO includes
contributions from the Pt metal and the ethynylfluorene unit of
only one of the acetylide ligands and that of the LUMO is
located almost exclusively on the nitrophenylacetylene
component of the same acetylide ligand. The HOMO-1
electron density comes from the Pt metal and the other
acetylide ligand that does not contribute to the HOMO.
Similarly, the electron density of the LUMO+1 is located
almost exclusively on the nitrophenylacetylene component of
the other acetylide ligand that shows dominant contribution to
the HOMO-1 density. For complex 2, the electron density
distributions on the HOMO and HOMO-1 orbitals have
similar features; with almost equal contributions from both
acetylide ligands and considerable contributions from the Pt
metal. The LUMO and LUMO+1 electron densities are quite
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Table 2. Excitation Energies (eV), Wavelengths (nm), Oscillator Strengths, Main Contributing Transitions, and the Associated
Configuration Coefficients of Five Singlet Electronic States of Complexes 1 and 2 Obtained at the CAM-B3LYP/LANG631//
a
b
B3LYP/LANG631 Level of Theory
excitation energy
complex
Sn
eV
nm
f
main contributing transitions
configuration coefficients
1
1
3.21
386
2.6303
HOMO → LUMO
0.48
0.26
0.53
0.55
0.59
0.56
0.42
0.38
0.37
0.34
0.40
0.37
0.35
0.35
0.32
0.48
0.36
0.30
HOMO → LUMO+3
HOMO-1 → LUMO+1
HOMO → LUMO+2
HOMO-1 → LUMO+2
HOMO-2 → LUMO+2
HOMO → LUMO
2
3
4
5
1
3.30
3.49
3.61
3.85
3.22
375
355
343
322
384
1.8968
0.1099
0.0001
0.1609
1.9284
2
HOMO-2 → LUMO
HOMO-3 → LUMO
HOMO-1 → LUMO
HOMO → LUMO+1
HOMO-1 → LUMO+2
HOMO → LUMO+2
HOMO-1 → LUMO+1
HOMO-3 → LUMO
HOMO-5 → LUMO
HOMO-4 → LUMO
HOMO-6 → LUMO
2
3
4
3.34
3.48
3.57
370
356
346
1.3008
2.5675
0.2158
5
3.88
319
0.0091
a
b
LANG631 basis set refers to 6-31G* for C, H, N, and S and LANL2DZ for Pt. All calculation results are corrected for solvent effects using the
PCM model.
different; the dominant contribution to the LUMO is almost
exclusively from the π*(bipyridine) component, while the
LUMO+1 orbital shows dominant contributions from the
phenylethynylfluorenylacetylene components of the two
acetylide ligands.
for predicting excited state energies of molecules involving
charge transfer transitions as has been observed in other
organic dye systems.50
Based on the electron density distribution on the FMOs and
the predicted vertical excitation energies, the lowest-energy
absorption bands for these complexes should arise from
1LLCT/1ILCT/1MLCT/1π,π* transitions for 1, and the
1LLCT/1MLCT transitions for 2. This is consistent with the
experimental data of the ground-state absorption, and further
supports the absorption band assignments. It is worth noting
that for complex 1, the 1ILCT and acetylide ligand-based 1π,π*
transitions (HOMO → LUMO+3) partially overlap with the
1LLCT/1MLCT transition bands and the charge transfer band
observed from the UV−vis measurement is an intense,
structureless absorption band. Additionally, because the
FMOs are localized only on one of the acetylide ligands in 1,
the molar extinction coefficient for the major absorption band
in 1 is much smaller than that for the major absorption band in
2, in which the HOMOs are delocalized on both acetylide
ligands.
Emission. The steady-state emission spectra of complexes 1
and 2 were measured in different solvents at R.T. and in
butyronitrile matrix at 77 K. The normalized emission spectra
of the Pt complexes and their respective acetylide ligands in
CH2Cl2 are illustrated in Figure 4. The fluorescence spectra of
L1 and L2 exhibit structureless feature and positive
solvatochromic effect (Supporting Information, Figure S3 and
S4), indicating an intramolecular charge-transfer (ICT)
emission. In contrast, excitation of 1 and 2 in CH2Cl2 solution
at their respective charge-transfer band produces orange
luminescence, which can be quenched by oxygen. Their
emission spectra exhibit significant Stokes shifts, relatively
sharp and narrow shape, with emission lifetimes of 25 ns for 1
and approximately 1 μs for 2 in deaerated CH2Cl2. Taking into
account these pieces of information, we believe that their
emission emanates from the first triplet excited state (T1).
Theoretical UV−vis absorption spectra were calculated at the
CAM-B3LYP/LANG631//B3LYP/LANG631 model chemis-
try; with the inclusion of solvent correction effects using the
PCM model and CH2Cl2 as solvent. The excitation energies
(eV), wavelengths (nm), oscillator strengths, dominant
contributing transitions, and the associated configuration
coefficients of several singlet excited electronic states of
complexes 1 and 2 are presented in Table 2. Results of the
TDDFT calculations for complex 1 indicate that the lowest
energy absorption band involves two main electronic
transitions: HOMO → LUMO and HOMO → LUMO+3,
which correspond to the first excited state with excitation
wavelength of 386 nm. The second lowest energy absorption
band maximum (2nd excited state) occurs at 375 nm and
involves the HOMO-1 → LUMO+1 electron transition. For
complex 2, three significant low-lying electronic transitions
were obtained from the calculations: HOMO → LUMO and
HOMO-2 → LUMO (1st excited state, occurred at 384 nm),
HOMO-3 → LUMO and HOMO-1 → LUMO (2nd excited
state, occurred at 370 nm), and HOMO → LUMO+1 and
HOMO-1 → LUMO+2 (3rd excited state, occurred at 356
nm). The calculated excitation wavelengths for these complexes
are in semiquantitative agreement with the experimentally
measured λmax shown in Table 1; which can be expected
considering that the complexes are large and, thus, limit the use
of larger basis sets that can improve the predicted excitation
energies. The results obtained for these complexes for the first
three excited states show an absolute deviation from experi-
ment of 11−41 nm for complex 1 and 3−66 nm for complex 2.
These are actually nice results for such large molecules and
indicate that the CAM-B3LYP functional is a good performer
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Castellano’s group11 and our group.15 It is noted that complex
4 only shows a very weak emission band in CH2Cl2, which was
ascribed to the enhanced ICT (intramolecular charge transfer)
3
state, particularly the LLCT state caused by the electron-
donating NPh2 group.21 With the extension of π-conjugation in
the acetylide ligands, the nature of emitting state could be
3
3
altered by admixing π,π* character with the MLCT/3LLCT
excited states in 2. This configurational mixing is probably
accounted for the blue-shift of the emission band in 2
compared to that in 4. The blue-shift of the emission band
with extended π-conjugation in the acetylide ligand has also
been observed in Pt(II) C∧N∧N complexes by Che and co-
workers previously.48
The emission of 1 and 2 in BuCN glassy matrix at 77 K was
investigated to better understand the nature of their emissive
triplet excited states. As shown in Figure 5, their emission
spectra at 77 K are blue-shifted and become narrower and more
structured. The lifetimes are also much longer compared to
those at room temperature. The thermally induced Stokes shift
(ΔEs) is relatively small for both complexes (∼ 554 cm−1 for 1
and ∼933 cm−1 for 2). The small ΔEs values indicate that the
triplet emitting states of 1 and 2 indeed possess substantial
3π,π* characters.
Transient Absorption. The fs (Figure 6) and ns (Figure 7)
TA spectra of complexes 1 and 2 were investigated to further
understand the properties of both singlet and triplet excited
states. The time-resolved fs TA spectra of 1 and 2 in CH3CN/
CH2Cl2 are presented in Figure 6a,b, and the singlet excited-
state lifetimes deduced from the fs TA decay profiles are listed
in Table 1. The fs TA spectroscopic study not only provides the
singlet excited-state difference absorption spectrum, but also
deduces the singlet excited state lifetime that cannot be
obtained from the decay of fluorescence due to the
undetectable fluorescence in many Pt(II) complexes.
Figure 4. Normalized emission spectra of Pt complexes 1 (λex = 463
nm), 2 (λex = 450 nm), 3 (λex = 397 nm), and 4 (λex = 475 nm) in
CH2Cl2 (c = 1 × 10−5 mol L−1). Inset shows the normalized emission
spectra of ligand 1L (λex = 379 nm) and 2L (λex = 370 nm) in CH2Cl2..
However, the nature of the emitting state of 1 and 2 varies in
different solvents. The emission energies, lifetimes and
quantum yields in different solvents are summarized in Table
3. As shown in Figure 5a for complex 1, a minor solvatochromic
effect was observed. The emission was significantly quenched in
polar solvents, such as CH3CN and CH2Cl2. In contrast, in low-
polarity solvents like toluene and hexane, the lifetime becomes
much longer compared to those in polar solvents, which is
indicative of a charge-transfer emitting state in polar solvents.
However, compared to 3,21 the emission spectrum of 1 is more
structured and relatively narrower; and the emission lifetime is
also significantly longer. Considering that the emission of
complex 3 was attributed to a triplet charge transfer (CT) state
caused by the strong electron-withdrawing NO2 substituent,21
and the extended π-conjugation could significantly admix the
3π,π* character into the lowest triplet excited state, we attribute
As illustrated in Figure 6, complexes 1 and 2 both exhibit
positive absorption bands from 450 to 800 nm, which resemble
the spectral region of their respective ligands (Supporting
Information, Figures S5 and S6) but with somewhat different
shapes. This indicates that their singlet excited state absorptions
3
the emitting state of 1 to mixed CT/3π,π* states (where the
1
should partially originate from the ILCT/1π,π* excited states
charge transfer states could be 3MLCT/3LLCT/3ILCT). In the
case of complex 2 (shown in Figure 5b and Table 3), a
structured emission band in CH2Cl2 (vibronic spacing of
∼1159 cm−1) with a longer emission lifetime (∼1 μs in
CH2Cl2) was observed. This indicates that the emitting state in
polar solvent could be dominated by the 3π,π* excited state that
is mainly localized on the acetylide ligands, possibly mixed with
some 3MLCT/3LLCT characters. On the contrary, the negative
solvatochromic effect, structureless feature, and shorter lifetime
in low-polarity solvents imply that the emitting state is
predominantly 3MLCT/3LLCT in low-polarity solvents. Similar
of the acetylide ligands. Immediately following the 400 nm
excitation, ultrafast intersystem crossing occurs for complexes 1
and 2. The shapes of the TA spectra of complex 1 remain the
same over the whole time range of the ∼1.4 ns delay line after
the 400-nm excitation, which are quite similar to those
measured by ns laser flash photolysis (Figure 6d). The minor
difference between the fs TA and ns TA spectra could be due to
the different solvents used in these two measurements. For
complex 2, the intensity of the fs TA spectrum decreases within
1.7 ps after excitation; then the intensity of the spectrum
increases to the maximum at 23 ps delay and then decay
relatively slowly. This feature implies a fast intersystem crossing
in less than 23 ps from the singlet to the triplet excited state
3
solvent-induced switch between the π,π* dominated emission
and 3MLCT/3LLCT dominated emission in other Pt(II)
diimine complexes has also been reported previously by
Table 3. Emission Energies, Lifetimes and Quantum Yields of Complexes 1 and 2 in Different Solvents at R.T.
a
λem/nm (τem/ns; Φem
)
CH3CN
CH2Cl2
MTHF
toluene
hexane
1
2
b
b
576 (25; 0.001)
568 (970; 0.070)
595 (190; 0.005)
592 (225; 0.046)
581 (1330; 0.061)
594 (365; 0.12)
578 (2790; 0.071)
597 (440; 0.14)
a
The spectra and lifetimes were measured in dilute solutions with an absorbance of approximately 0.08 at 436 nm in a 1-cm cuvette. A degassed
b
aqueous solution of [Ru(bpy)3]Cl2 (Φem = 0.042, excited at 436 nm) was used as the reference. Too weak to be measured.
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Figure 5. Normalized emission spectra of (a) complex 1 and (b) complex 2 in different solvents at room temperature and in BuCN glassy matrix at
77 K, λex = 436 nm.
Figure 6. (a) Time-resolved fs TA spectrum of complex 1 in 10:1 (v/v) CH3CN/CH2Cl2; (b) Time-resolved fs TA spectrum of complex 2 in 12:1
(v/v) CH3CN/CH2Cl2; (c) Comparison of the fs TA spectra of complexes 1−4 in CH3CN/CH2Cl2 mixture (1, 10:1; 2, 12:1; 3, 12:1; 4, 1:1) at
zero delay after excitation; (d) Comparison of the fs and ns TA spectra of 1 at zero delay after excitation. λex = 400 nm for fs TA and λex = 355 nm
for ns TA measurements. The fs TA spectrum was measured in mixed 10:1 (v/v) CH3CN/CH2Cl2 solution; while the ns TA spectrum was
measured in toluene solution with A355 = 0.4 in a 1-cm cuvette. The inset shows the time-resolved ns TA spectrum of 1 in toluene.
and the triplet excited state exhibits a very similar absorption
spectrum to that of the singlet excited state. Kinetic study of the
triplet excited-state absorption at 550 nm reveals that the decay
of the absorbing triplet excited state is approximately 610 ns.
Unfortunately, the full ns TA spectrum of 2 was unable to be
measured due to the very weak signal under our ns TA
experimental condition. However, the fs TA spectra at longer
decay times (>23 ps) indicate that the triplet excited state of 2
absorbs broadly from 430 to 800 nm. For complex 1, both fs
TA and ns TA spectral features resemble the ns TA spectrum of
1L (Supporting Information, Figure S7) and the fs TA
spectrum of 1L at longer delay (Supporting Information,
Figure S5). Considering this character and the much longer
lifetime (τT = 3.2 μs), the excited states that contribute to the
observed fs and ns TA for complex 1 should be dominated by
the 1π,π*/1ILCT and 3π,π*/3ILCT states, respectively, possibly
mixed with some MLCT/LLCT characters. For complex 2,
considering the nature of the lowest singlet excited state being
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Article
Figure 8. Nonlinear transmission curves for Pt complexes 1−4 in
CH2Cl2 for 4.1 ns laser pulses at 532 nm. The linear transmission for
all samples was adjusted to 80% in a 2-mm cuvette.
Figure 7. Comparison of the triplet transient difference absorption
spectra of 1−4. Complexes 1 and 3 were measured in toluene, and 4
was measured in 9:1 (v/v) toluene/CH2Cl2 with A355 = 0.4 in a 1-cm
cuvette, λex = 355 nm, the delay time was zero. The spectrum of
complex 2 was measured by fs TA in 12:1 (v/v) CH3CN/CH2Cl2 at a
time delay of 1361 ps after excitation, λex = 400 nm. The spectrum of 2
was scaled to 1/5 of the original intensity for comparison purpose.
for complex 2, its RSA threshold decreases to 5.1 μJ and the
transmission decreases to 0.15 when the incident energy
reaches ∼170 μJ. The respective 5- and 2-fold decreases in the
RSA threshold and transmission suggests that extension of π-
conjugation of the acetylide ligand dramatically increases the
degree of RSA in the studied complexes; and, hence,
demonstrating that RSA in Pt(II) diimine complexes can be
improved by extending the π-conjugation of the acetylide
ligands. This provides a useful approach for developing efficient
nonlinear absorbing materials.
1MLCT/1LLCT as discussed earlier and the similar spectral
region to its corresponding ligand, the observed fs TA is
1
presumably attributed to MLCT/1LLCT probably mixed with
1
some π,π*/1ILCT characters.
It is worth noting that the fs TA spectra of 1 and 3 (Figure
6c) are similar in that both of them possess a strong absorption
band in the visible and another band in the NIR spectral region.
However, the NIR band in 3 is much broader and stronger than
that in 1 although the NIR band in 1 is red-shifted. In contrast,
the fs TA spectrum of 2 at zero delay is pronouncedly
broadened and red-shifted compared to that of 4. The same
trend is observed for the triplet transient absorption spectra of
1 and 3, and 2 and 4 (Figure 7). The different features in the
respective fs and ns TA spectra of 1 and 3, and 2 and 4 reflect
the influence of the extended π-conjugation of the acetylide
ligand on the singlet and triplet excited-state absorption.
Meanwhile, the observed trend implies that the transient
absorbing species in 2 should be closely related to the acetylide
ligand, which could possibly be the LLCT, π,π*, or ILCT states.
Reverse Saturable Absorption. Our TA study indicates
that complexes 1 and 2 exhibit stronger excited-state absorption
than ground-state absorption in the visible to near-IR spectral
region. Thus, reverse saturable absorption (RSA, i.e., trans-
mission of sample solution decreases with increased incident
energy) is anticipated to occur in this spectral region. To
demonstrate this, the transmission vs incident energy experi-
ment at 532 nm was carried out in a 2-mm cuvette in CH2Cl2
solutions using 4.1 ns laser pulses, and the results are shown in
Figure 8. For easy comparison, the concentrations of 1 and 2
were adjusted in order to obtain a linear transmission of 80% at
532 nm. To manifest the effect of extended π-conjugation of
the acetylide ligand on the RSA, the nonlinear transmission
curves of 3 and 4 at the identical experimental conditions are
provided in Figure 8 as well. Obviously, with increased incident
energy, the transmission of 1−4 all decrease drastically, which
clearly indicates the occurrence of RSA at 532 nm. Compared
to 3 and 4, the strength of the RSA at 532 nm for complexes 1
and 2 is significantly increased. For example, complex 4 shows a
RSA threshold (defined as the incident energy at 70% of the
linear transmittance) of 26.1 μJ and the transmission decreases
to 0.31 when the incident energy reaches ∼170 μJ. In contrast,
CONCLUSION
■
Two new Pt(II) diimine bis(phenylethynylfluorenylacetylide)
complexes with terminal NO2 and NPh2 substituents (1 and 2)
were synthesized and their photophysical properties were
systematically investigated. Complex 1 exhibits intense
structureless absorption at ∼397 nm, which is attributed to
the 1π,π*/1ILCT/1LLCT/1MLCT transitions. Complex 2
possesses a very intense acetylide ligands localized 1π,π*
absorption band at ∼373 nm and broad 1MLCT/1LLCT
transition tail from 425 to 525 nm. The ground state electronic
structure and vertical excitation energies of several excited
singlet electronic states were computed using DFT (B3LYP)
and TDDFT (CAM-B3LYP) methods, respectively, and the
results of the calculations assisted in the interpretation and
assignment of the UV−vis absorption bands. The emitting state
of 1 was found to be mixed 3CT/3π,π* states; while the
3
emitting state of 2 exhibits a switch from predominantly π,π*
state in high-polarity solvents to 3MLCT/3LLCT states in low-
polarity solvents. The fs TA study indicates that 1 and 2 exhibit
ultrafast intersystem crossing and broadband excited-state
absoprtion in 450−800 nm. Their triplet excited states appear
to be populated via rapid intersystem crossing in only a few
picoseconds after laser excitation. In addition, strong RSA was
observed at 532 nm for ns laser pulses from 1 and 2, which are
drastically stronger than each of their corresponding complex
with shorter π-conjugation in their acetylide ligands, i.e., 3 and
4, respectively. The photophysical and nonlinear transmission
studies revealed that extending the π-conjugation of the
acetylide ligand can alter the singlet and triplet excited state
properties substantially and improve the RSA at 532 nm
drastically. The broad excited-state absorption and strong
nonlinear transmittance performance at 532 nm for 1 and 2
suggest that these complexes could be promising candidates as
broadband nonlinear absorbing materials.
1915
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ASSOCIATED CONTENT
■
S
* Supporting Information
Solvent-dependency UV−vis and emission spectra of 1L and
2L, time-resolved fs and ns transient difference absorption
spectra of 1L and 2L, TDDFT calculation results for 1 and 2,
and the full author list for ref 39. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Wenfang.Sun@ndsu.edu. Phone: 701-231-6254. Fax:
701-231-8831.
Notes
The authors declare no competing financial interest.
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E.; Mackie, D. M.; Shensky, W., III; Mott, A. G. Excited-state
absorption of a bipyridyl platinum(II) complex with alkynyl-
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Ugrinov, A.; Haley, J. E.; Li, Z.; Hoffmann, M. R.; Sun, W. Synthesis,
Structural Characterization, Photophysics, and Broadband Nonlinear
Absorption of a Platinum(II) Complex with the 6-(7-Benzothiazol-2′-
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ACKNOWLEDGMENTS
■
This work is partially supported by the National Science
Foundation (CAREER CHE-0449598) and partially supported
by the Army Research Laboratory (W911NF-10-2-0055).
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