Photophysics and nonlinear absorption of cyclometalated 4,6-diphenyl-2,2′-bipyridyl platinum(II) complexes with different acetylide ligands

Article abstract of DOI:10.1021/jp107348h

The photophysical properties of a series of 4,6-diphenyl-2,2′- bipyridyl platinum(II) complexes bearing different σ-alkynyl ancillary ligands (1a-1k) were systematically investigated. All complexes exhibit strong ¹π,π* absorption bands in the U

Full text of DOI:10.1021/jp107348h

J. Phys. Chem. A 2010, 114, 12639–12645
12639
Photophysics and Nonlinear Absorption of Cyclometalated 4,6-Diphenyl-2,2′-bipyridyl
Platinum(II) Complexes with Different Acetylide Ligands
Rui Liu,†,‡ Yunjing Li,† Yuhao Li,‡ Hongjun Zhu,‡ and Wenfang Sun*,†
Department of Chemistry and Biochemistry, North Dakota State UniVersity, Fargo,
North Dakota 58108-6050, United States, and Department of Applied Chemistry, College of Science, Nanjing
UniVersity of Technology, Nanjing 210009, People’s Republic of China
ReceiVed: August 4, 2010; ReVised Manuscript ReceiVed: October 26, 2010
The photophysical properties of a series of 4,6-diphenyl-2,2′-bipyridyl platinum(II) complexes bearing different
1
σ-alkynyl ancillary ligands (1a-1k) were systematically investigated. All complexes exhibit strong π,π*
absorption bands in the UV region; and broad, structureless charge-transfer band(s) in the visible region,
which systematically red-shift(s) when the electron-donating ability of the para substituent on the
phenylacetylide ligand increases. All complexes are emissive in solution at room temperature. When excited
at the charge-transfer absorption band, the complexes exhibit long-lived orange emission (λmax: 555 - 601
nm), which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission (3MLCT/
3ILCT). Most of these complexes exhibit broad triplet transient difference absorption in the visible to the
near-IR region, with a lifetime comparable to those measured from the decay of the 3MLCT/3ILCT emission.
The reverse saturable absorption (RSA) of these complexes were demonstrated at 532 nm using nanosecond
laser pulses. The degree of RSA follows this trend: 1k ≈ 1a > 1c > 1f ≈ 1i > 1h ≈ 1b > 1e > 1d > 1g,
which is mainly determined by the ratio of the triplet excited-state absorption cross section to that of the
ground-state and the triplet excited-state quantum yield.
Introduction
extinction coefficients for their low energy charge-transfer
absorption bands, longer triplet excited-state lifetimes, higher
Cyclometalated square-planar platinum(II) complexes have
attracted a great deal of attention owing to their versatile
photophysical properties and potential applications in light-
emittingdevices,1-5photocatalysis,6-8nonlinearopticaldevices,9-12
and chemosensors.13,14 Many of these complexes exhibit moder-
ate charge-transfer absorption in the visible region and long-
lived emission at room temperature in solution. The square-
planar coordination geometry around Pt(II) reduces D2d distortion
that is likely to lead to radiationless decay.3,5 In addition, their
photophysical properties can be tuned by structural modification
of the ligands to meet the different requirements for diverse
applications.
emission quantum yields, and significantly increased ratios of
the excited-state absorption cross section to that of the ground
state compared to those of their corresponding Pt complexes
without these substituents.9,11 In these works we also demon-
strated the influence of the monodentate ligand on the photo-
physics and nonlinear absorption of the C∧N∧N platinum
complexes.9,11,18 A prior work reported by Che and co-workers
studied the effect of substituent on the acetylide ligand on the
UV-vis and emission properties of the Pt(II) C∧N∧N complexes
as well.1 The metal-to-ligand charge transfer (1MLCT) absorp-
tion band energy and the emission energy of the complexes were
systematically affected by the electron-donating or electron-
withdrawing ability of the para substituent on the phenylacetyl-
ide ligand. In addition, our group have demonstrated that
(C∧N∧N)PtCt CPh and (C∧N∧N)PtCt CC3H7 exhibit broader
triplet excited-state absorption, higher triplet excited-state
quantum yield, and thus enhanced reverse saturable absorption
for nanosecond laser pulses compared to their platinum terpy-
ridyl analogues.17,19
The reported work are quite intriguing, however, the study
on the effect of monodentate ligand on the photophysics,
especially on the nonlinear absorption of platinum C∧N∧N
complexes is still limited. To remedy this deficiency, our groups
have designed and synthesized a series of mononuclear cyclo-
metalated platinum(II) 4,6-diphenyl-2,2′-bipyridyl acetylide
complexes (Chart 1). Their photophysical properties have been
systematically investigated with the aim to provide a basis for
elucidating the structure-property correlations and developing
new nonlinear absorbing materials.
Among the cyclometalated square-planar platinum(II) com-
plexes, 6-phenyl-2,2′-bipyridyl (C∧N∧N) platinum acetylide
complexes are particularly intriguing by virtue of their relatively
intense emission.1-3,15-17 The stronger emission from these
complexes compared to their corresponding platinum terpyridyl
complexes is partially attributed to the less-distorted square-
planar geometry that reduces the radiationless decay. Addition-
ally, the ease of structural modification on the 4′-position of
the central pyridine ring and on the 6-phenyl ring facili-
tates the tuning of the photophysical properties of the complexes.
In the past, our group introduced alkoxyl substituent on the
6-phenyl ring11 and fluorenyl substituent on the 4′-position of
the central pyridine ring of the C∧N∧N ligand.9,18 These
modifications not only improve the solubility of the complexes
but also cause a pronounced effect on the photophysics of these
complexes, which is reflected by their much larger molar
* To whom correspondence should be addressed. Phone: 701-231-6254.
Fax: 701-231-8831. E-mail: Wenfang.Sun@ndsu.edu.
† North Dakota State University.
The synthesis and structural characterization of the cyclom-
etalated platinum(II) complexes with different acetylide ligands
have been reported previously.20 In this work, we focus on the
‡ Nanjing University of Technology.
10.1021/jp107348h  2010 American Chemical Society
Published on Web 11/15/2010

12640 J. Phys. Chem. A, Vol. 114, No. 48, 2010
CHART 1: Structures for Pt(II) Complexes 1a-1k
Liu et al.
photophysical and nonlinear absorption studies of these com-
plexes for nanosecond laser pulses.
was the second harmonic output (λ ) 532 nm) of a 4.1 ns
(fwhm), 10 Hz, Q-switched Quantel Brilliant Nd:YAG laser.
A f ) 30 cm plano-convex lens was used to focus the laser
beam to the center of a 2-mm-thick sample cuvette. The radius
of the beam waist was ∼75 µm. Two Molectron J4-09
pyroelectric probes and an EPM2000 energy/power meter were
used to monitor the incident and output energies.
Experimental Section
Photophysical Measurements. An Agilent 8453 spectro-
photometer was used to measure the UV-vis absorption spectra,
and a SPEX fluorolog-3 fluorometer/phosphorometer was
utilized to obtain the steady-state emission spectra. The emission
quantum yields were determined by the comparative method,21
Results and Discussion
in which a degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem
)
Synthesis. The synthesis of the 4,6-diphenyl-2,2′-bipyridyl
platinum(II) chloride complex ((C∧N∧N)PtCl) followed the
procedures reported in the literature.25 The (C∧N∧N)PtCl
complex was then reacted with para-substituted ethynylbenzene
with a base (diisopropylethylamine (DIEA)) and catalytical
amount of CuI in degassed CH2Cl2 solution at room temperature
for 12 h. The crude product was purified via recrystallization
in CH2Cl2/Et2O to yield the product for photophysical study.
The complexes were characterized by 1H and 13C NMR, HRMS,
and elemental analysis. The detailed synthetic procedure and
characterization data were reported earlier.20
0.042, excited at 436 nm)22 was used as the reference. An
Edinburgh LP920 laser flash photolysis spectrometer was used
to measure the excited-state lifetimes, the triplet transient
difference absorption spectra and the triplet excited-state
quantum yields and the molar extinction coefficients in degassed
solutions. The excitation source was the third harmonic output
(355 nm) of a Nd:YAG laser (Quantel Brilliant, pulsewidth ∼4.1
ns, repetition rate was set at 1 Hz). Each sample was purged
with Ar for 30 min before measurement.
The triplet excited-state molar extinction coefficient and triplet
quantum yield were determined by the partial saturation
method,23 in which the optical density at the interested
wavelength was monitored when the excitation energy at 355
nm was gradually increased. Saturation was observed when the
excitation energy was higher than 10 mJ. The following equation
was then used to fit the experimental data to obtain the εT and
UV-Vis Absorption. The UV-vis absorption spectra of
1a-1k in CH3CN solution are presented in Figure 1, and the
band maxima and molar extinction coefficients for each complex
are compiled in Table 1. The absorption obeys Lambert-Beer’s
law in the concentration range studied (1 × 10-6 through 1 ×
10-4 mol/L), suggesting no dimerization or oligomerization of
the complexes within this concentration range. In line with
the previous work on cyclometalated platinum(II) complexes,
the intense absorption bands below 370 nm are assigned to the
intraligand (IL) π,π* transitions, while the broad, moderately
intense absorption bands at 400-550 nm are assigned to the
mixed 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-
to-ligand charge transfer)/1ILCT (intraligand charge transfer)
transitions considering the similar energy and intensity of this
band to those reported in the literature for other platinum C∧N∧N
23
ΦT
∆OD ) a(1 - exp(-bIp))
(1)
where ∆OD is the optical density at the interested wavelength,
Ip is the pump intensity in Einstein cm-2, a ) (εT - ε0)dl, and
b ) 2303 ε0exΦT/A. εT and ε0 are the absorption coefficients of
the excited state and the ground state at the interested wave-
length, ε0 the ground-state absorption coefficient at the
acetylide complexes.5,9,11,15,17,18 The involvement of the ILCT
ex
1
1
excitation wavelength of 355 nm, d the concentration of the
sample (mol L-1), l the thickness of the sample excited by
the excitation beam (namely the diameter of the excitation
beam), and A the area of the sample irradiated by the excitation
beam.
and LLCT characters into the low-energy absorption band of
these complexes should be taken into account because of the
π-donating ability of the 6-phenyl ring on the C∧N∧N ligand
and the acetylide ligand, which has been suggested by the DFT
calculation previously reported by our group for the 4,6-
diphenyl-2,2′-bipyridine platinum chloride complex and plati-
num 4,6-diphenyl-2,2′-bipyridine complexes with an alkoxyl
Nonlinear Transmission Measurement. The experimental
setup was similar to that described previously.24 The light source

4,6-Diphenyl-2,2′-bipyridyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12641
Figure 1. UV-vis absorption spectra of Pt(II) complexes 1a-1k in CH3CN solution.
TABLE 1: Photophysical Parameters of 1a-1k in CH3CNa
λabs b/nm
λem b/nm
λT1-Tn/nm
(ε/103 L·mol-1 ·cm-1
)
(τem/ns, Φem
)
(τTA/ns; εT1-Tn/L·mol-1 ·cm-1; ΦT)
1a
1b
1c
1d
1e
1f
1g
1h
1i
285 (43.2), 332 (13.8), 365 (9.4), 432 (6.36), 450 (6.42)
289 (45.2), 332 (16.6), 365 (10.8), 429 (7.42), 448 (7.09)
286 (50.0), 332 (25.2), 372 (31.0), 432 (22.1)
281 (42.7), 333 (12.0), 367 (8.0), 440 (5.18), 466 (5.40)
290 (36.0), 366 (9.0), 436 (6.14), 455 (6.08)
289 (21.6), 327 (21.7), 422 (4.50), 440 (4.57)
287 (25.7), 322 (21.5), 437 (4.68), 472 (4.55)
291 (10.4), 341 (3.2), 364 (1.8), 435 (1.33), 455 (1.28)
287 (24.6), 332 (27.6), 430 (5.19), 453 (5.19)
285 (10.4), 339 (9.6), 427 (1.73)
584 (500, 0.048)
570 (360,0.073)
555 (380, 0.024)
584 (90, 0.001)
592 (250, 0.018)
582 (410, 0.062)
601 (190, 0.003)
593 (70, 0.004)
568 (280, 0.015)
572 (550, 0.047)
569 (210, 0.021)
615 (510, 3004, 0.41)
615 (490, 10455, 0.18)
610 (400, 9305, 0.16)
c
540 (450, 2422, 0.13)
605 (840, 15873, 0.08)
c
c
c
1j
1k
580 (15480, 2758, 0.18)
580 (260, 5432, 0.11)
282 (40.3), 319 (31.5), 340 (29.1), 430 (sh. 2.56)
a Measured at room temperature. b At a concentration of 5 × 10-5 mol/L. c Too weak to be measured.
substituent on the 6-phenyl ring.11 The transition energies in
these complexes are quite similar to those of their corresponding
platinum C∧N∧N complexes without the 4-phenyl substituent,
especially for the low-energy charge-transfer bands. This
indicates that the phenyl substituent has a negligible effect on
the energy level of the bipyridine based LUMO (π*(N∧N)) for
the Pt(C∧N∧N)X complexes.11 However, introducing the 4-phe-
nyl ring on the C∧N∧N ligand pronouncedly increases the molar
extinction coefficients of the low-energy charge-transfer absorp-
tion band compared to those of their corresponding Pt complexes
without the 4-phenyl substituent,1 which is in line with the
influence of the 4-tolyl substituent on the terpyridyl ligand on
the UV-vis absorption of the corresponding platinum terpyridyl
complexes.26
resonant effects from the para substituent, the energy of the
acetylide-based HOMO is affected, which in turn causes the
change of the lowest excited-state energy. An electron-donating
substituent raises the acetylide-based HOMO, while an electron-
withdrawing substituent stabilizes the acetylide-base HOMO.
Consequently, the energy difference between the acetylide-based
HOMO and the bipyridine (N∧N)-based LUMO will be varied
and the bathochromic shift or hypsochromic shift occurs.
Although the influence of the para substituent at the pheny-
lacetylide ligand on the MLCT transition is not as pronounced
as it does on the LLCT transition, a similar trend was observed
for the MLCT transition. A linear correlation also exists between
the energy of the 1MLCT band and the Hammett constant (σp)
of the para substituent (see Figure S1 of Supporting Informa-
tion). This is because an acetylide ligand with electron-donating
substituent is more electron rich and raises the Pt d orbital
energy, which leads to a decrease of the energy gap between
the bipyridine-based LUMO and the Pt-based HOMO and results
in the red-shift. In contrast, an electron-withdrawing substituent
would have the opposite effect. A similar effect of the para
substituent at the phenylacetylide ligand on the charge-transfer
band energy has been reported by Che group for Pt(C∧N∧N)
complexes without the 4-phenyl substituent1 and by our group
and Yam group for platinum terpyridyl complexes.26,27
As shown in Figure 1 and Table 1, the low-energy absorption
band for most of the complexes appears to show two distinct
transitions. The transitions occurring in the region of 424-440
nm are not quite sensitive to the ancillary substituents on the
phenyl ring of the acetylide ligand, which likely emanate from
the dπ (Pt) f π*(N∧N) MLCT excited state. In contrast, the
transitions appearing at lower energies are influenced signifi-
cantly by the nature of the ancillary substituents. Electron-
donating substituents, such as OBu, NPh2, and 9H-carbazolyl,
cause a pronounced red-shift of this transition, while electron-
withdrawing substituents such as SAc and NO2 induce a blue-
shift compared to complex 1a. A linear correlation is observed
between the energy of this band and the Hammett constant (σp)
of the substituent on the phenyl ring (see Figure S1 of
Supporting Information). Considering the sensitivity of the
lower-energy transition to the para-substituent at the pheny-
lacetylide ligand, we tentatively assign this transition to a spin
In addition, it is noted that complex 1c has an additional
intense absorption band at λmax ) 378 nm (ε ≈ 3.5 × 104 dm3
mol-1 cm-1). With reference to the work reported by Eisenberg28
and Che1 groups, this band is attributed to the intraligand charge-
transfer transition (1ILCT) within the 4-nitrophenylacetylide
ligand. It is also worthy of mention that extension of the
conjugation of the acetylide ligand affects the charge transfer
band pronouncedly. In comparison of the spectra for 1a, 1f and
1j, and 1i and 1k, we notice that with the increased conjugation
1
allowed LLCT transition (1LLCT). However, the ILCT may
also contribute to this region. Because of the inductive and

12642 J. Phys. Chem. A, Vol. 114, No. 48, 2010
Liu et al.
Figure 4. Normalized emission spectra of 1f in different solvents. The
excitation wavelength was 468 nm for CH2Cl2, toluene, and hexane
solutions, 450 nm for CH3CN solution, and 420 nm for CH3OH solution.
Figure 2. UV-vis absorption spectra of 1f in different solvents
of the oligoyne ligand the charge-transfer band blue-shifts and
the molar extinction coefficient decreases. Especially the lower-
energy transition corresponding to the 1LLCT transition is
significantly weakened. This phenomenon could probably be
ascribed to the lowering of the oligoyne 1π,π* transition energy
with the increased conjugation length, which could change the
lifetimes and the emission quantum yields are summarized in
Table 1. As shown in Figure 3 and Table 1, excitation of these
complexes in CH3CN solution at their respective charge-transfer
band at room temperature results in orange to red luminescence,
corresponding to a Stokes shift of 4220-5940 cm-1. The
emission lifetimes in degassed CH3CN solutions are all several
hundred nanoseconds except for 1d and 1h. The drastic Stokes
shift and relatively long lifetime suggest that the observed
emission from these complexes is from a triplet excited state.
With reference to the other platinum C∧N∧N complexes reported
in the literature, the emission could be tentatively assigned to
the 3MLCT/3ILCT emission.1,2,9-11,15 The charge transfer nature
of the emitting state could be supported by the structureless
feature of the emission spectra and the solvent-dependency
study. Except for 1g and 1h, all of the other complexes exhibit
a negative solvatochromic effect (exemplified in Figure 4 for
complex 1f and in Figures S12-S21 of Supporting Information
for the other complexes), i.e., when the dielectric constant (ε)
or ET(30) value of the solvent increases, the emission energy
of the complexes decreases, which is a characteristic of the
3MLCT excited state. However, no linear correlation is observed
between the ε or ET(30) value and the emission energy. For
complex 1g, a positive solvatochromic effect was observed (see
Figure S17 of Supporting Information), indicating that the
excited state is more polar than the ground state. This suggests
that the emitting state for 1g could possibly be dominated by
the intraligand charge-transfer character. For complex 1h, minor
solvatochromic effect was observed, which probably reflects the
1
parentage of the lowest excited state by mixing some π,π*
character into the lowest excited state for 1j and 1k. The
participation of the acetylenic π,π* character in the emissive
state of C∧N∧N complexes bearing oligoyne ligand has been
suggested by Che previously.1 We believe it is also likely to be
the case for the lowest singlet excited state of 1j and 1k.
The charge transfer nature of the low-energy absorption bands
in 1a-1k is supported by the solvent-dependency studies. As
exemplified in Figure 2 for 1f, the low-energy absorption band
bathochromically shifts to longer wavelengths in solvents with
lower polarity (i.e., hexane and toluene) compared to those in
more polar solvents (i.e., CH3CN and CH3OH). This 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. This phenomenon is also in accordance
with that observed in many of the platinum C∧N∧N or terpyridyl
complexes reported in the literature.9-14,17,18,24,26-28 The same
solvent effect is observed for all of the other complexes studied
in this work, and the results are provided in the Figures S2-S11
of the Supporting Information.
Emission. The emission characteristics of complexes 1a-1k
in a variety of solvents and at different concentrations in CH2Cl2
at room temperature were investigated, and the normalized
emission spectra of these complexes in CH3CN at a concentra-
tion of 1 × 10-5 mol/L are illustrated in Figure 3. The emission
3
3
interplay of the MLCT and ILCT configuration. In addition
to the solvatochromic effect discussed above, the different
Figure 3. Normalized emission spectra of complexes 1a-1k in CH3CN solution (1 × 10-5 mol/L). The excitation wavelength was 450 nm for 1a,
1b, 1e, 1f, 1h, and 1j; 421 nm for 1c, 420 nm for 1d, 466 nm for 1g and 1i, and 436 nm for 1k.

4,6-Diphenyl-2,2′-bipyridyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12643
solvents also influence the emission lifetime and quantum yield
pronouncedly. As shown in Table 2, all of the complexes exhibit
a longer emission lifetime and a higher quantum yield in
noncoordinating solvents, such as CH2Cl2 in comparison to those
in coordinating solvents like CH3CN and CH3OH. This solvent
induced quenching by coordinating solvents is commonly seen
in Pt(II) complexes with a 3MLCT emitting state, which provides
the (C∧N∧N)Pt component. In X-ray diffraction crystallography
study of complex 1b,20 the dihedral angle between the phenyl
ring at the 4-position of the C∧N∧N ligand and the plane of the
(C∧N∧N)Pt moiety is 142.8°. Obviously, electronic interaction
between the phenyl substituent and the C∧N∧N ligand is
minimized. However, introducing the phenyl substituent on the
C∧N∧N ligand pronouncedly enhances the emission efficiency
and increases the emission lifetime for these complexes
compared to their respective Pt complexes without the phenyl
substituent in CH2Cl2.1,2
3
another piece of evidence for the involvement of the MLCT
character in the emitting state of these complexes.
The emission energies (λmax/nm) of 1a-1k are influenced by
the electron-donating ability of the para substituent on the
phenylacetylide ligand, which increase in the order of NPh2 >
carbazolyl > BuO > SAc > NO2. This trend is consistent with
Transient Difference Absorption. Transient difference
absorption (TA) spectroscopy measures the absorption difference
between the ground state and the excited state. A positive TA
band indicates stronger excited-state absorption than that of the
ground-state, while a negative band (bleaching band) suggests
stronger ground-state absorption than that of the excited state.
Additionally, when monitoring the decay of the TA at a given
wavelength, the lifetime of the excited state contributing to the
TA can be obtained. Because many of the platinum C∧N∧N
complexes exhibit broad and moderately strong excited-state
absorption in the visible to the near-IR region,9,11,17,18 and the
lifetimes measured from the decay of the emission of 1a-1k
suggest that the triplet excited state is much more long-lived
than the excitation pulsewidth (4.1 ns). Therefore, triplet excited-
state absorption is expected to be observed for these complexes.
Figure 5 displays the triplet TA spectra of 1a, 1e, 1j, and 1k at
the zero-time delay and the time-resolved TA spectra of 1c in
degassed CH3CN solution. The TA spectra of 1b and 1f are
similar to that of 1k (see Figure S23 of Supporting Information).
The TA absorption band maxima, the excited-state lifetimes
deduced from the decay of the TA, the excited-state absorption
coefficients, and the triplet quantum yields for these complexes
are listed in Table 1. The TA spectra of 1d, 1g, 1h, and 1i were
unable to be measured partially due to the shorter excited-state
lifetime as indicated by the emission lifetime. Another possible
reason could be related to their lower triplet excited-state
quantum yields, which is suggested by their significantly lower
emission quantum yields compared to the other complexes in
this series.
1
that observed for the MLCT band in the UV-vis absorption
spectra. It provides another piece of evident that the emission
3
has MLCT character. Similar to that observed for the low-
energy charge-transfer band(s) in the UV-vis absorption
spectra, the emission energy for 1a, 1b, 1c, 1d, 1e, and 1g
exhibits a linear correlation to the Hammett constant (σp) of
the para substituent on the phenylacetylide ligand (see Figure
S22 of Supporting Information). The effects of the π-conjugation
length of the oligo phenylacetylide ligands on the emission of
(C∧N∧N)Pt(C≡CC6H4)n-R in CH3CN solution is salient. When
the conjugation length of the phenylacetylide ligand increases
from ethynylbenzene monomer in 1a to phenylene ethynylene
trimer in 1j, the corresponding emission energy blue-shifts from
584 nm (1a) to 572 nm (1j). The same trend was observed for
complexes 1d and 1k. For complexes with stronger electron-
donating substituent on the phenylacetylide ligand, i.e., 1d, 1g,
and 1h, the lifetimes are much shorter and the emission quantum
yields are much lower. This could be attributed to the presence
of low-lying nonemissive 3LLCT state, which adds an additional
decay pathway.
Concentration-dependency study in CH2Cl2 reveals that from
2.5 × 10-6 mol/L to 5 × 10-5 mol/L the intensity of the
emission keeps increasing with the increased concentration.
However, the lifetime keeps decreasing at higher concentrations,
indicating a self-quenching effect. The self-quenching rate
constant deduced from the Stern-Volmer plot is 1.63 × 109
for 1a, 5.19 × 109 for 1c, 5.40 × 109 for 1f, and 4.04 × 109 for
1k, which are of the same order as those reported for many
other platinum terpyridyl or C∧N∧N complexes by our group
and several other groups.1,9,11,26,29
The spectral features for these complexes are quite different.
1a and 1c exhibit a bleaching band below 475 nm and broad,
structureless absorption bands in the visible to the NIR region.
The bleaching bands occur at the similar position as the lowest-
energy absorption bands in their respective UV-vis absorption
spectrum. The lifetimes obtained from the decay of the TA in
CH3CN are in line with those measured from the decay of the
emission. These facts suggest that the TA for 1a and 1c possibly
emanate from the same excited state that emits or a state in
It is also interesting to note that the emission energies for all
the complexes are similar to their respective platinum C∧N∧N
congeners without the 4-phenyl substituent.1,2 This feature is
consistent with that observed from the UV-vis absorption study,
indicating the twist of the phenyl substituent from the plane of
TABLE 2: Emission Energy, Lifetime, and Quantum Yield of 1a-1k in Different Solvents at Room Temperature
λ
em/nm (τem/ns; Φem)
CH3CN
CH3OH
CH2Cl2
hexane
toluene
1a
1b
1c
1d
1e
1f
1g
1h
1i
584 (500; 0.048)
570 (360; 0.073)
555 (380; 0.024)
584 (90; 0.0007)
592 (250; 0.018)
582 (410; 0.062)
601 (190; 0.003)
593 (70; 0.004)
568 (280; 0.015)
572 (550; 0.047)
569 (210; 0.021)
582 (150; 0.014)
588 (750; 0.073)
576 (1060; 0.127)
558 (1060; 0.180)
576 (430; 0.003)
595 (390; 0.034)
588 (820; 0.072)
576 (530; 0.007)
595 (170; 0.016)
595 (460; 0.036)
586 (890; 0.078)
583 (890; 0.101)
604 (a; 0.002)
592 (670; 0.084)
577 (430; 0.103)
565 (470; 0.185)
618 (210; 0.004)
600 (510; 0.035)
591 (740; 0.110)
578a
595 (390; 0.051)
599 (660; 0.076)
589 (530; 0.082)
588 (1040; 0.105)
575 (100; 0.011)
587 (a; 0.0005)
a
571 (; 0.0004)
593 (a; 0.0008)
593 (180; 0.014)
584 (210; 0.021)
595 (a; 0.0007)
576 (60; 0.007)
589 (200; 0.016)
583 (270; 0.02)
578 (140; 0.01)
a
592 (370; 0.001)
602 (540; 0.001)
a
a
594 (480; 0.003)
592 (a; 0.001)
609 (800; 0.003)
1j
1k
a Signal too weak to be detected.

12644 J. Phys. Chem. A, Vol. 114, No. 48, 2010
Liu et al.
Figure 5. (a) Triplet transient difference absorption spectra of 1a, 1e, 1j, and 1k at zero delay after the laser excitation in CH3CN solution. (b)
Time-resolved TA spectra of 1c in CH3CN solution. λex ) 355 nm.
equilibrium with the emitting state. Thus, this state possibly
3
consists of MLCT/3ILCT characters. The TA spectra of 1b,
1f, and 1k are quite similar, with two absorption bands at 400
and 600 nm respectively. Almost no bleaching was observed
for these three complexes. The TA lifetime and the emission
lifetime for these three complexes are of the same order, but
the lifetime deduced from the decay of TA is somewhat longer
than that obtained from the decay of emission. This implies
that the emitting state and the TA state possibly have different
compositions of the 3MLCT and 3ILCT characters. Alternatively,
there could be some extent of configurationally mixing of the
acetylenic 3π,π character. The TA spectrum of 1e composes of
two obvious absorption bands, one appearing at ∼395 nm and
Figure 6. Transmittance vs incident fluence curves for 1a-1k in
another broad, somewhat structured one from 500 to 800 nm.
CH2Cl2 for 4.1-ns laser pulses at 532 nm. The linear transmission was
The TA lifetime of 1e is also longer than the emission lifetime,
which could be attributed to the similar reason as we just
adjusted to 80% for each sample in a 2-mm cell.
discussed for 1b, 1f and 1k.
TABLE 3: Ground-State (σ0) and Excited-State (σex)
Absorption Cross Sections of the Platinum Complexes in
CH3CN at 532 nm
The TA spectrum of 1j is drastically different from all the
aforementioned complexes, with a bleaching band in the UV
region and a relatively narrow positive band at 580 nm. The
bleaching band occurs in the region of the acetylenic ligand
1π,π* absorption band, and the lifetime of 1j obtained from the
decay of the TA in CH3CN is ∼15.5 µs, which is much longer
than that for a charge-transfer band and is quite distinct from
that obtained from the decay of the emission for this complex.
Taking these facts into account, we believe that the TA for 1j
1a 1b
1c 1d 1e
1f 1g 1i 1j 1k
σ0/10-18 cm2 1.53 1.76 1.27 4.13 1.83 1.37 7.65 3.83 1.53 0.63
σex/10-18 cm2 6.91 39.2 20.7
10.3 50.4
5.63 36.8
0.73 2.94
7.15 19.3
4.67 30.6
0.84 3.37
σex/σ0
4.52 22.3 16.3
1.85 4.01 2.61
ΦTσex/σ0
transmission decrease with the increased incident fluence,
indicating the occurrence of RSA. Among these complexes 1a
and 1k exhibit the strongest RSA, with a RSA threshold (defined
as the incident fluence at which point the transmittance drops
to 70% of the linear transmittance) of 0.14 J/cm2 and the
transmission decreases to 0.28 when the incident fluence reaches
1.73 J/cm2. The strength of the RSA for these ten complexes
follows this trend: 1k ≈ 1a > 1c > 1f ≈ 1i > 1h ≈ 1b > 1e >
1d > 1g.
Our previous studies on RSAs reveal that two of the key
factors that determine the strength of RSA are the ratio of the
excited-state absorption cross section to that of the ground-state
and the triplet excited-state quantum yield. The ground-state
absorption cross sections at 532 nm for these complexes are
listed in Table 3, which are deduced from the ε values obtained
from their UV-vis absorption spectra and the conversion
equation σ ) 2303ε/NA, where NA is the Avogadro constant.
The triplet excited-state absorption cross sections at 532 nm
are estimated from the ∆OD at zero time delay and the εT -T
3
arises from the π,π* state of the oligo phenylene ethynylene
ligand. Although it is unusual that the emitting state (3MLCT)
and the excited state giving rise to the TA (3π,π*) is different
for 1j, this phenomenon is not unprecedented. The presence of
an emissive 3MLCT state and a nonemissive lower-lying 3π,π*
3
state than MLCT state has been reported by Turro and co-
workers for a [Ru(bpy)2(dppn)]2+ complex.30 Unfortunately, the
reason for this phenomenon is still not well understood.
Reverse Saturable Absorption. The TA spectra discussed
above show that most of the complexes studied in this work
exhibit broad positive absorption band in most of the visible to
the near-IR region. This implies that their excited-state absorp-
tion is stronger than the ground-state absorption. Therefore,
reverse saturable absorption (RSA) are expected to occur from
these complexes under the irradiation of nanosecond laser pulses.
To demonstrate this, the nonlinear transmission experiment was
conducted for these complexes in CH2Cl2 solution. The experi-
ment results are illustrated in Figure 6. Except for 1j that has
too low solubility in CH2Cl2 to obtain solution with 80% linear
transmission; thus the data was not included for comparison,
all of the other 10 complexes exhibit moderate to strong
1
n
at the TA band maximum according to the method described
previously by our group.31 The results are also compiled in Table
3. The ΦTσex/σ0 value follows this trend: 1b > 1k > 1f > 1c >

4,6-Diphenyl-2,2′-bipyridyl Platinum(II) Complexes
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12645
1a > 1j > 1e. This trend essentially parallels the RSA trend
except for 1a and 1b, implying that extending the conjugation
length of the acetylide ligand and introducing electron-
withdrawing substituent on the acetylide could reduce the
ground-state absorption cross section while improving the triplet
excited-state absorption cross section, which consequently
increases the ratio of the σex/σ0 and improves the RSA. The
disparity between the ΦTσex/σ0 value and the RSA result for
1a and 1b could be due to the fact that the σex/σ0 values listed
in Table 3 are measured or estimated in CH3CN solution, while
the RSA measurement was conducted in CH2Cl2. Both the
ground-state absorption and the excited-state absorption param-
eters could vary in different solvents.
at room temperature for 1a-1e and 1g-1k in different solvents,
and the time-resolved triplet transient difference absorption
spectra of 1a, 1b, 1e, 1f, 1j, and 1k in CH3CN. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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Conclusions
The photophysics of mononuclear cyclometalated platinum(II)
4,6-diphenyl-2,2′-bipyridyl acetylide complexes 1a-1k were
systematically investigated. The broad, moderately intense
1MLCT/1ILCT/1LLCT absorption bands at 400-550 nm are
systematically red-shifted in accordance with the electron-
donating ability of the para substituent on the phenylacetylide
ligand. Excitation of these complexes in solution at their
respective charge-transfer band at room temperature results in
orange luminescence, which can be assigned as 3MLCT/3ILCT
emission. Except for 1d, 1g, 1 h, 1i, and 1j, all of the other
complexes exhibit broad and relatively strong triplet excited-
state absorption in the visible extending to the near-IR region,
which is tentatively attributed to the MLCT, ILCT, and/or
3π,π* states. Most of the complexes exhibit reverse saturable
absorption for nanosecond laser pulses at 532 nm, with the RSA
strength decreases following this trend: 1k ≈ 1a > 1c > 1f ≈ 1i
> 1h ≈ 1b > 1e > 1d > 1g, which is mainly determined by the
ratio of the triplet excited-state absorption cross section to that
of the ground state and the triplet excited-state quantum yield.
Introducing electron-withdrawing substituent or extending the
conjugation length of the acetylide ligand could reduce the
ground-state absorption cross section while increasing the triplet
excited-state absorption cross section. Consequently, the RSA
for these complexes is improved. Moreover, introducing the
phenyl substituent on the C∧N∧N ligand pronouncedly increases
the molar extinction coefficients for the low energy charge-
transfer absorption band, enhances the emission efficiency and
increases the emission lifetime.
3
3
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Acknowledgment. W. Sun acknowledges the financial sup-
port from the National Science Foundation (CAREER CHE-
0449598). R. Liu, Y. Li, and H. Zhu acknowledge the
Postgraduate Innovation Fund of Jiangsu Province (2009,
CX09B_141Z) and the Doctor Thesis Innovation Fund of
Nanjing University of Technology (2008, BSCX200812) for
financial support.
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Supporting Information Available: The linear correlation
between the various excited-state energies and the Hammett
constant (σp), UV-vis absorption spectra and emission spectra
(31) Shao, P.; Li, Y.; Sun, W. Organometallics 2008, 27, 2743.
JP107348H