Cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2, 2′-bipyridine complexes: Synthesis, photophysics, and nonlinear absorption

Article abstract of DOI:10.1021/ic902281a

A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investigated. All complexes exhibit strong ¹π, π*absorption bands in the UV region, and a broad, structureless charge transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ( ³MLCT/³ILCT). For emission at 77 K, the emitting state is tentatively assigned as ³MLCT for F-2-F-4, and ³MLCT/ ³π,π*for F-1 and F-5 taking into account the emission energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 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. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, 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. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and increased ratios of the excited-state absorption cross-section to that of the ground-state.

Full text of DOI:10.1021/ic902281a

Inorg. Chem. 2010, 49, 4507–4517 4507
DOI: 10.1021/ic902281a
Cyclometalated Platinum(II) 6-Phenyl-4-(9,9-dihexylfluoren-2-yl)-2,20-bipyridine
Complexes: Synthesis, Photophysics, and Nonlinear Absorption
Pin Shao,† Yunjing Li,† Jing Yi,† Timothy M. Pritchett,‡ and Wenfang Sun*,†
†Department of Chemistry and Molecular Biology, North Dakota State University, Fargo,
North Dakota 58108-6050, and ‡U.S. Army Research Laboratory, AMSRD-ARL-SE-EM,
2800 Powder Mill Road, Adelphi, Maryland 20783-1197
Received November 17, 2009
A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,20-
bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investi-
1
gated. All complexes exhibit strong π,π* absorption bands in the UV region, and a broad, structureless charge
transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to
those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand
charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those
for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes
are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer
absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet
metal-to-ligand charge transfer/intraligand charge transfer emission (3MLCT/3ILCT). For emission at 77 K, the emitting
state is tentatively assigned as 3MLCT for F-2-F-4, and 3MLCT/3π,π* for F-1 and F-5 taking into account the emission
energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 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. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns
laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the
experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross
sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, 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. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt
complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their
low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and
increased ratios of the excited-state absorption cross-section to that of the ground-state.
Introduction
by structural modification of the ligands to meet the different
requirements for diverse applications.1,2 To date, there have
been a large number of reports on the potential use of
platinum terdentate complexes in light emitting diodes,1,3
photovoltaic cells,4 chemosensors,5 and light modulation via
nonlinear absorption.6
Square-planar platinum terdentate complexes have drawn
great attention in the past two decades because of their
versatile spectroscopic properties. Many of these complexes
exhibit moderate charge-transfer absorption in the visible
region and long-lived emission at room temperature in
solution. The charge-transfer absorption band and the emiss-
ion characteristics of these complexes can be readily tuned
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*To whom correspondence should be addressed. E-mail: wenfang.sun@
ndsu.edu. Phone: 701-231-6254.
(1) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Che, C. M.; Zhu, N.; Lee,
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2006, 45, 4304. (b) Paw, W.; Cummings, S. D.; Mansour, M. A.; Cennick, W. B.;
Geiger, D. K.; Eisenberg, R. Coord. Chem. Rev. 1998, 171, 125. (c) Thomas,
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r
2010 American Chemical Society
Published on Web 04/20/2010
pubs.acs.org/IC

4508 Inorganic Chemistry, Vol. 49, No. 10, 2010
Shao et al.
Among the square-planar ∧pla∧tinum complexes reported,
(C∧N∧N)PtX complexes (C N N denotes 6-phenyl-2,20-
bipyridyl, X = Cl, PPh3, acetylide, etc.)1,3g,6c,d,7,8 are parti-
cularly interesting because of their relatively intense emission.
The stronger emission from these complexes with respect to
their corresponding platinum terpyridyl complexes is par-
tially attributed to the less-distorted square-planar geometry
that reduces the radiationless decay. On the other hand,
because of the extended π system and the stronger σ-donating
ability of the deprotonated carbon donor on the 6-phenyl
ring, the energy gap between the metal-to-ligand charge
transfer (MLCT) excited state and the nonemissive metal-
centered d,d state is increased, which would enhance the
radiative decay from the MLCT state. The better π-donating
ability of the 6-phenyl ring could also introduce some
intraligand charge transfer character into the emitting state,8a
which would enhance the emission as well. Moreover, the
easier structural modification of the C∧N∧N ligands favors
the tuning of the photophysical properties of the complexes.
We have demonstrated that introducing an electron-donat-
ing alkoxyl substituent on the 6-phenyl ring of the C∧N∧N
ligand significantly increases the emission lifetime and emis-
sion efficiency through the admixture of intraligand charge
transfer (ILCT) and π,π* characters into the emitting excited
state.9 In addition to the improved emission of the
(C∧N∧N)PtX complexes, (C∧N∧N)PtCꢀCPh and (C∧N∧N)-
PtC5H7 have been demonstrated to exhibit broader triplet
excited-state absorption, higher triplet excited-state quantum
yield, and thus enhanced reverse saturable absorption for
ns laser pulses compared to their platinum terpyridyl analo-
gues.6a,d Therefore, 6-phenyl-2,20-bipyridyl ligand is consid-
ered as a better terdentate ligand for nonlinear absorption
applications. One more advantage for using the 6-phenyl-
2,20-bipyridyl ligand over the terpyridyl ligand for platinum
complexation is that it will form neutral platinum chloride or
acetylide complexes to increase the solubility of the com-
plexes in common organic solvents, such as CH2Cl2 and
CH3CN, and therefore facilitate the purification of the
platinum products.
Fluorene and its derivatives have been widely studied as
building blocks for photoluminescent and electrolumines-
cent materials10 and two-photon absorbing materials11
because of their rigid conjugated structures and excellent
thermal and photochemical stabilities.11 In addition, the
facile substitution at the 9-position of the fluorene unit
allows for easy control of the materials’ properties such
as solubility, processability, and morphology. Because of
the aforementioned advantageous properties and the
π-donating ability of fluorene, we intend to incorporate
the 9,9-dihexylfluorene unit to the 4-position of the central
pyridine ring of the C∧N∧N ligand to improve the solubility
of the platinum complexes and increase the excited-state
lifetime. More importantly, by introducing the fluorene
unit to the platinum complexes, we expect to enhance the
two-photon absorption (TPA) cross sections of the plati-
num complexes in the near-IR region to populate the
excited state via TPA and then utilize the broad and
relatively strong excited-state absorption in this region for
photonic devices application.
In this work, two new C∧N∧N ligands containing the
fluorene unit at the 4-position of the central pyridine ring
and their platinum complexes with different monodentate
co-ligands were synthesized and characterized. Structures of
these complexes and their synthetic routes are displayed in
Scheme 1. The photophysics of these complexes and their
reverse saturable absorption for ns laser pulses at 532 nm
were systematically investigated and analyzed.
Experimental Section
Synthesis. The compounds pyridacylpyridinium iodide,12
1-2,13 3,14 69 and 9-1215 were prepared according to the
literature procedures. All the solvents and reagents were pur-
chased from Alfa Aesar and used as received unless otherwise
stated. Column chromatography was carried out on silica gel
˚
(Sorbent Technologies, 60 A, 230 ꢁ 450 mesh) or neutral
˚
aluminum oxide (Sigma-Aldrich, 58 A, ∼150 mesh).
1H NMR spectra were measured on a Varian 300 or 400 MHz
VNMR spectrometer. High-resolution mass spectrometry was
carried out on a Bruker BioTOF III mass spectrometer. Ele-
mental analyses were carried out by NuMega Resonance La-
boratories, Inc. in San Diego, California.
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Hester, H.; Flaschenreim, C.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005,
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L.-P.; Tung, Z.-H. Inorg. Chem. 2004, 43, 5195. (b) Wu, L. Z.; Cheung, T. C.;
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4. To a mixture of 3 (1.43 g, 4 mmol) and 2-acetylpyridine
(0.48 g, 4 mmol) in 20 mL of ethanol, 4 mL of 1.5 M aqueous
NaOH solution was added. After stirring at room temperature
for 4 h, the solid formed was collected by filtration. The crude
product was purified by recrystallization from methanol to
afford 0.21 g yellow solid as the pure product (yield: 11%).
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Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4509
Scheme 1. Synthetic Routes for Complexes F-1-F-5
1H NMR (CDCl3): δ 8.79 (d, J=4.5 Hz, 1H), 8.32 (d, J=16.2
Hz, 1H), 8.23 (d, J=8.1 Hz, 1H), 8.06 (d, J=16.2 Hz, 1H),
7.88-7.93 (m, 1H), 7.72 (d, J=5.1 Hz, 4H), 7.50-7.54 (m, 1H),
7.35 (d, J=3.3 Hz, 3H), 1.98-2.04 (m, 4H), 1.04-1.14 (m, 12H),
0.75 (t, J=6.6 Hz, 6H), 0.59-0.63 (m, 4H) ppm. ESI-HRMS: m/
z calcd for [C33H39NOþH]þ: 466.3104; found, 466.3124. Anal.
Calcd (%) for C33H39NO: C, 85.11; H, 8.44; N 3.01. Found: C,
84.75; H, 8.90; N, 3.05.

4510 Inorganic Chemistry, Vol. 49, No. 10, 2010
Shao et al.
5. A mixture of 4 (1.42 g, 3 mmol), N-(benzoylmethyl)-
pyridinium bromide (0.85 g, 3 mmol) and NH4OAc (3.00 g,
39 mmol) in 70 mL of methanol was refluxed for 6 h. Methanol
was removed and 40 mL of water was added to the residue. The
resultant mixture was extracted with CH2Cl2. The organic layer
was washed with brine (40 mLꢁ2) and dried over Na2SO4. The
crude product was purified using a silica gel column eluted with
hexane/ethyl acetate (v/v=20:1) to yield 0.75 g of a colorless
lected by filtration. The crude product was purified by a neutral
alumina gel column eluted by CH2Cl2, and then recrystallized
from CH2Cl2/hexane. A 0.25 g portion of orange solid was
obtained as the pure product (yield: 76%). 1H NMR (CDCl3):
δ 8.72 (d, J=5.1 Hz, 1H), 7.77-7.82 (m, 5H), 7.71 (d, J=8.1 Hz,
1H), 7.46 (s, 1H), 7.36-7.39 (m, 3H), 7.18-7.23 (m, 2H), 7.15 (d,
J=8.4 Hz, 1H), 6.98 (d, J=2.4 Hz, 1H), 6.49 (d, J=8.4 Hz, 1H),
3.72 (d, J=5.7 Hz, 2H), 2.06-2.12 (m, 4H), 1.67-1.72 (m, 1H),
1.26-1.51 (m, 8H), 1.05-1.15 (m, 12H), 0.85-0.90 (m, 6H),
0.72-0.79 (m, 10H) ppm. ESI-MS: m/z calcd for [C49H59-
N2O195PtþCH3CN]þ, 927.4539; found, 927.4551. Anal. Calcd
(%) for C49H59ClN2OPt: C, 63.79; H, 6.45; N, 3.04. Found: C,
63.82; H, 6.22; N, 3.35.
1
viscous oil (yield: 44%). H NMR (CDCl3): δ 8.77-8.80 (m,
1H), 8.73-8.76 (m, 2H), 8.29 (dd, J=1.5, 8.4 Hz, 2H), 8.09 (d,
J=1.5 Hz, 1H), 7.91 (dd, J=2.1, 7.5 Hz, 1H), 7.86-7.89 (m, 2H),
7.83 (d, J=1.2 Hz, 1H), 7.78-7.81 (m, 1H), 7.59 (td, J=1.5,
6.6 Hz, 2H), 7.47-7.53 (m, 1H), 7.35-7.44 (m, 4H), 2.02-2.13
(m, 4H), 1.11-1.19 (m, 12H), 0.73-0.82 (m, 10H) ppm. ESI-
HRMS: m/z calcd for [C41H44N2þH]þ: 565.3577; found,
565.3572. Anal. Calcd (%) for C41H44N2: C, 87.19; H, 7.85;
N, 4.96. Found: C, 86.91; H, 8.05; N, 4.97.
F-3. A mixture of F-1 (0.13 g, 0.16 mmol), 1-pentyne (40 μL,
0.41 mmol), CuI (3.0 mg, 0.01 mmol), and KOH (60 mg,
1 mmol) in degassed CH2Cl2/CH3OH (40 mL/20 mL) was
stirred at room temperature under argon for 18 h. After the
reaction, the solvent was removed, and the crude product was
purified by a neutral alumina gel column eluted by CH2Cl2/
hexane (v/v =3:1), and then recrystallized from CH3OH. Red
solid was obtained as the pure product (89 mg, yield: 67%). 1H
NMR (CDCl3): δ 9.05 (d, J=5.4 Hz, 1H), 7.92-7.98 (m, 2H),
7.69-7.86 (m, 5H), 7.60 (s, 1H), 7.53 (s, 1H), 7.34-7.38 (m, 5H),
6.96-7.05 (m, 2H), 2.68 (t, J=7.2 Hz, 2H), 2.02-2.08 (m, 4H),
1.67-1.77 (m, 2H), 1.07-1.17 (m, 15H), 0.67-0.77 (m, 10H)
ppm. ESI-MS: m/z calcd for [C46H50N2195PtþH]þ, 826.3698;
found, 826.3695. Anal. Calcd (%) for C46H50N2Pt: C, 66.89; H,
6.10; N, 3.39. Found: C, 66.51; H, 6.37; N, 3.54.
7. A mixture of 3 (1.17 g, 3.23 mmol), 6 (0.80 g, 3.23 mmol),
and KOH (0.84 g, 15 mmol) in dry MeOH (80 mL) was refluxed
for 24 h. The solvent was removed, and 40 mL of water was
added to the residue. The resultant mixture was extracted with
ether. The organic layer was washed with brine and dried over
Na2SO4. The crude product was purified by a silica gel column
eluted with CH2Cl2/hexane (v/v=1:1) to afford 0.71 g of yellow
oil (yield: 37%). 1H NMR (CDCl3): δ 8.09 (d, J=8.4 Hz, 2H),
7.93 (d, J=15.6 Hz, 1H), 7.71-7.73 (m, 2H), 7.59-7.66 (m, 3H),
7.34 (d, J=2.4 Hz, 3H), 7.00 (d, J=8.4 Hz, 2H), 3.93 (d, J=
5.4 Hz, 2H), 1.98-2.04 (m, 4H), 1.75-1.77 (m, 1H), 1.33-1.54
(m, 8H), 1.05-1.14 (m, 12H), 0.86-0.97 (m, 6H), 0.73-0.78
(m, 6H), 0.63 (s, 4H) ppm. ESI-HRMS: m/z calcd for [C42H56-
O2þH]þ: 593.4353; found, 593.4363. Anal. Calcd (%) for
F-4. A mixture of F-1 (0.13 g, 0.16 mmol), phenylacetylene
(24 μL, 0.24 mmol), CuI (3.0 mg, 0.01 mmol), and KOH (60 mg,
1.00 mmol) in degassed CH2Cl2/CH3OH (50 mL/25 mL) was
stirred at room temperature under argon for 24 h. The solvent
was removed, and the crude product was purified by a neutral
alumina gel column with CH2Cl2/hexane (v/v =3:1) used as the
eluent. Recrystallization from CH2Cl2/ether yields 82 mg of a
red solid as the pure product (yield: 59%). 1H NMR (CDCl3): δ
9.10 (d, J=4.8 Hz, 1H), 8.01 (d, J=4.4 Hz, 2H), 7.75-7.81 (m,
4H), 7.71 (d, J=8.0 Hz, 1H), 7.64 (s, 1H), 7.58 (d, J=8.0 Hz,
3H), 7.37-7.43 (m, 6H), 7.29 (t, J=8.0 Hz, 2H), 7.18-7.20 (m,
1H), 7.03 (dd, J=3.2, 5.6 Hz, 1H), 2.01-2.05 (m, 4H), 1.07-1.14
(m, 12H), 0.76 (t, J=7.2 Hz, 6H), 0.66 (br. s, 4H) ppm. ESI-MS:
m/z calcd for [C49H48N2195PtþNa]þ, 882.3362; found, 882.3343.
C42H56O2 C6H14: C, 84.89; H, 10.40. Found: C, 85.35; H, 10.70.
3
8. A mixture of 7 (1.46 g, 2.5 mmol), pyridacylpyridinium
iodide (0.80 g, 2.5 mmol) and NH4OAc (2.00 g, 26 mmol) was
refluxed in 50 mL of MeOH for overnight. The solvent was
removed, and 40 mL of ether was added to the residue. The
resultant mixture was washed with brine and dried over Na2SO4.
The crude product was purified by a silica gel column eluted with
CH2Cl2/hexane (v/v=1:1) to give 0.22 g of a colorless viscous oil
(yield: 13%). 1H NMR (CDCl3): δ 8.73 (d, J=4.8 Hz, 1H), 8.68
(d, J=7.8 Hz, 1H), 8.61 (d, J=1.2 Hz, 1H), 8.17 (d, J=8.7 Hz,
2H), 7.97 (d, J=1.2 Hz, 1H), 7.84-7.90 (m, 1H), 7.80 (t, J=
1.2 Hz, 2H), 7.73-7.76 (m, 2H), 7.32-7.37 (m, 4H), 7.05 (d, J=
9.0 Hz, 2H), 3.93 (d, J = 5.7 Hz, 2H), 2.00-2.05 (m, 4H),
1.73-1.80 (m, 1H), 1.33-1.55 (m, 8H), 1.05-1.14 (m, 12H),
0.91-0.97 (m, 6H), 0.75 (t, J=6.3 Hz, 6H), 0.64-0.66 (m, 4H)
ppm. ESI-HRMS: m/z calcd for [C49H60N2OþH]þ: 693.4778;
Anal. Calcd (%) for C49H48N2Pt 0.1CH2Cl2: C, 67.90; H, 5.59;
3
N, 3.23. Found: C, 67.70; H, 5.26; N, 3.38.
F-5. AmixtureofF-2 (0.48 g, 0.52 mmol), 12 (0.10 g, 0.26 mmol),
CuI (6.0 mg, 0.02 mmol), and KOH (56 mg, 1.0 mmol) in degassed
CH2Cl2/CH3OH (50 mL/40 mL) was stirred at room temperature
under argon for 18 h. The solvent was removed, and the crude
product was purified by a neutral alumina gel column (CH2Cl2/
hexane (v/v =3:1) was used as the eluent), and then recrystallized
from CH2Cl2/ether. Red solid was obtained as the pure product
(0.34 g, yield: 30%). 1H NMR (CDCl3): δ 9.38 (d, J=5.7 Hz, 2H),
8.05 (s, 4H), 7.57-7.82 (m, 22H), 7.45 (d, J=8.4 Hz, 2H), 7.39 (br.
s, 6H), 6.65 (d, J=9.0 Hz, 2H), 4.03 (br. s, 4H), 2.02 (br. s, 12H),
1.78 (br. s, 2H), 1.35-1.52 (m, 16H), 1.08-1.23 (m, 36H),
0.91-0.98 (m, 12H), 0.75-0.78 (m, 30H) ppm. ESI-MS: m/z calcd
for [C127H150N4O2Pt2þ2Na]2þ, 1100.0432; found, 1100.0466.
Anal. Calcd (%) for C127H150N4O2Pt2: C, 70.79; H, 7.02; N,
2.60. Found: C, 70.73; H, 6.69; N, 2.64.
found, 693.4809. Anal. Calcd (%) for C49H60ON2 1/3CH2Cl2:
3
C, 82.14; H, 8.48; N, 3.88. Found: C, 82.23; H, 8.27; N, 3.56.
F-1. A mixture of ligand 5 (0.20 g, 0.35 mmol) and K2PtCl4
(0.15 g, 0.35 mmol) was refluxed in 60 mL of AcOH for 24 h.
After being cooled to room temperature, the yellow precipitant
was collected by filtration. The crude product was purified by a
neutral alumina gel column eluted with CH2Cl2, and then
further purified by recrystallization from CH2Cl2/ether. Orange
solid was obtained as the pure product (0.23 g, yield: 83%). 1H
NMR (CDCl3): δ 8.73 (d, J=5.4 Hz, 1H), 7.92 (d, J=3.6 Hz,
2H), 7.85 (d, J=8.4 Hz, 1H), 7.78-7.81 (m, 1H), 7.72-7.74 (m,
2H), 7.60 (d, J=1.8 Hz, 1H), 7.44 (dd, J=7.2, 1.2 Hz, 1H),
7.37-7.41 (m, 3H), 7.31 (t, J=4.2 Hz, 2H), 7.22-7.24 (m, 1H),
6.96 (td, J=1.2, 7.8 Hz, 1H), 6.88 (td, J=1.2, 7.5 Hz, 1H),
2.07-2.16 (m, 4H), 1.09-1.14 (m, 12H), 0.76 (t, J=6.6 Hz, 6H),
0.69 (br. s, 4H) ppm. ESI-MS: m/z calcd for [C41H43N2-
195PtþCH3CN]þ, 799.3338; found, 799.3396. Anal. Calcd (%)
for C41H43ClN2Pt: C, 61.99; H, 5.46; N, 3.53. Found: C, 62.30;
H, 5.10; N, 3.64.
Photophysical Measurements. The UV-vis absorption spec-
tra were measured using an Agilent 8453 spectrophotometer in a
1 cm or 1 mm quartz cuvette. The steady-state emission spectra
were obtained on a SPEX fluorolog-3 fluorometer/phosphoro-
meter. The emission quantum yields were determined by the
optical dilute method16 in degassed solutions, and a degassed
aqueous solution of [Ru(bpy)3]Cl2 (Φem=0.042, λex=436 nm)17
F-2. A mixture of 8 (0.25 g, 0.36 mmol) and K2PtCl4 (0.15 g,
0.36 mmol) was refluxed in 60 mL of AcOH for 24 h. After being
cooled down to room temperature, the solid formed was col-
(16) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(17) Van Houten, J.; Watts, R. J. Am. Chem. Soc. 1976, 98, 4853.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4511
was used as the reference. The excited-state lifetimes, the triplet
excited-state quantum yields, and the triplet transient difference
absorption spectra were measured in degassed solutions on an
Edinburgh LP920 laser flash photolysis spectrometer. The third
harmonic output (355 nm) of a Nd:YAG laser (Quantel Bril-
liant, pulsewidth ∼4.1 ns, repetition rate was set at 1 Hz) was
used as the excitation source. Each sample was purged with Ar
for 30 min before measurement.
The self-quenching rate constants (kQ) in CH2Cl2 were de-
duced followed the Stern-Volmer equation:
kobs ¼ kQ½Cꢂ þ k0
ð1Þ
where kobs is the decay rate constant of the emission (kobs
=
1/τem), kQ is the self-quenching rate constant, [C] is the concen-
tration of the complex in mol/L, and k0 (k0=1/τ0) is the decay
rate constant of the excited state at infinite dilute solution. A
plot of the observed decay rate constant versus concentration
should give rise to a straight line. The slope of the straight line
corresponds to kQ and the intercept corresponds to k0.
Figure 1. UV-vis absorption spectra of F-1-F-5 in CH2Cl2. c = 5 ꢁ
10-5 mol/L. The inset shows the expansion of the charge transfer band.
model and the fitting procedure were reported previously by our
group.19
The triplet excited-state molar extinction coefficient and
triplet quantum yield were determined by the partial saturation
method.18 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 ΦT.18
Results and Discussion
Electronic Absorption. The UV-vis absorption spectra
of F-1-F-5 were investigated in CH2Cl2 at different
concentrations (5 ꢁ 10
-6
-4
to 5 ꢁ 10
mol/L), and the
results are presented in Figure 1 for a concentration of 5ꢁ
10 -5 mol/L. The absorption of all five complexes obeys
Lambert-Beer’s law in the concentration range studied,
indicating that no ground-state aggregation occurs in this
concentration range. The intense absorption below 400 nm
for these complexes could be assigned to the intraligand
π,π* transitions within the 4-fluorenyl-C∧N∧N ligand
because these bands are essentially independent of the
monodentate co-ligand. The broad, moderately intense
absorption band in the visible region, that is, 400-500 nm
for F-1 and F-2, 410-550 nm for F-3 and F-4, and
420-600 nm for F-5, could be tentatively attributed to
the admixture of 1MLCT (metal-to-ligand charge trans-
fer)/1ILCT (intraligand charge transfer) transitions for
F-1 and F-2, and 1MLCT/1ILCT/1LLCT (ligand-to-
ligand charge transfer) for F-3-F-5 considering the simi-
lar shape and energy of this band to those reported in the
literature for other platinum C∧N∧N and terpyridyl
acetylide complexes.1,3g,6d,7-9,20 The involvement of the
intraligand charge transfer (1ILCT) character into the
charge-transfer band of these complexes should be taken
into account because of the π-donating ability of the
fluorenyl unit on the C∧N∧N ligand, which is similar to
that previously reported by our group for platinum
C∧N∧N complexes with an electron-donating alkoxyl
substituent on the 6-phenyl ring of the C∧N∧N ligand9
and for platinum terpyridyl complexes with a dimethyla-
mino substituent on the 40-position of the terpyridine
ligand.21 Another piece of evidence that supports this
hypothesis arises from the enhanced intensity of the
charge transfer band of these complexes compared to
their respective platinum C∧N∧N complexes without
the 4-fluorenyl substituent9 (exemplified by F-1-F-3 in
Supporting Information Figure S1). The charge transfer
band becomes broadened and red-shifted for F-3- F-5
ΔOD ¼ að1 - expð - bIpÞÞ
ð2Þ
where ΔOD is the optical density at the interested wavelength,
Ip is the pump intensity in Einstein cm-2, a=(εT - ε0)dl, and
3
b=2303 εe0xΦT/A. εT and ε0 are the absorption coefficients of the
excited state and the ground state at the interested wavelength,
εe0x is the ground state absorption coefficient at the excita-
tion wavelength of 355 nm, d is the concentration of the sample
(mol L-1), l is the thickness of the sample, and A is the area of the
sample irradiated by the excitation beam.
Nonlinear Transmission Measurements. The experimental
setup was similar to what had been described previously,6c with
a 20 cm focal-length lens used to focus the beam to the 2 mm
thick sample cuvette.
Z-Scan Measurements. The open-aperture experimental set-
up is similar to the one reported previously.19 For ns Z-scan
measurements, the second harmonic output (532 nm) from a
Quantel Brilliant Nd:YAG laser with a pulsewidth of 4.1 ns and
a repetition rate of 10 Hz was used as the light source. The laser
beam was focused by a 30 cm focal-length plano-convex lens to a
beam waist of 30 μm at the focal point, which corresponds to a
2
Rayleigh length (z0=πω0 /λ, where ω0 is the radius at the beam
waist) of 5.31 mm. For ps Z scans, the light source was the
second harmonic output of an EKSPLA PL2143A passively
mode-locked, Q-switched Nd:YAG laser (pulsewidth=21 ps,
repetition rate=10 Hz). A 15 cm plano-convex lens was used to
focus the beam to a beam waist of 34 μm at the focal point, which
gave rise to a Rayleigh length of 6.82 mm. Therefore, the sample
solution placed in a 1 mm thick quartz cuvette for ns measure-
ments and in a 2 mm cuvette for ps measurements could be
considered as thin samples. A 50 cm plano-convex lens was
placed at approximately 30 cm after the linear focal plane to
collect all of the transmitted light into the Molectron J4-09
joulemeter probe.
To fit the Z-scan experimental data, we used a five-band
model that consists of a ground state, two singlet excited states
and two triplet excited states. The detailed descriptions of the
(20) Fan, Y.; Zhang, L.-Y.; Dai, F.-R.; Shi, L.-X.; Chen, Z.-N. Inorg.
Chem. 2008, 47, 2811.
(21) Ji, Z.; Azenkeng, A.; Hoffmann, M.; Sun, W. Dalton Trans.
2009, 7725.
(18) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(19) Li, Y.; Pritchett, T. M.; Huang, J.; Ke, M.; Shao, P.; Sun, W. J. Phys.
Chem. A 2008, 112, 7200.

4512 Inorganic Chemistry, Vol. 49, No. 10, 2010
Shao et al.
comparing to those of F-1 and F-2. Such a feature
could be rationalized by two factors: First, the stronger
π-donating ability of the acetylide in F-3-F-5 could raise
the Pt d orbitals, and thus decreases the energy gap
between the bipyridine-based lowest unoccupied molec-
ular orbital (LUMO)9 and the d orbital. This would
1
consequently decrease the energy of the MLCT excited
state. Second, the acetylide ligand would admix more
LLCT character into the lowest excited state with the
increased π-donating ability of the acetylide ligand, which
would also cause the broadening and red-shift of the
charge-transfer band.
A point worthy of addressing is the influence of the
4-fluorenyl substituent on the UV-vis absorption spectra
of F-1-F-5. As shown in Supporting Information, Figure S1
for F-1-F-3, the transition energies in these complexes
are quite similar to those of their corresponding platinum
C∧N∧N complexes without the 4-fluorenyl substituent,9
especially for the lowest-energy charge transfer bands,
indicating that the fluorenyl 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.9 However,
the presence of the fluorenyl substituent significantly
increases the molar extinction coefficients for the bands
above 300 nm. This phenomenon likely arises from the
extended π-system in the fluorenyl-substituted com-
plexes, which could increase the oscillator strength for
these transitions by increasing the transition dipoles.6b
It is also interesting to note that the extinction coeffi-
cients for the dinuclear complex F-5 are much larger than
those of the mononuclear complexes F-1-F-4. The ex-
tinction coefficients of the bands at about 360 and 490 nm
are more than double of the respective bands in the
mononuclear complexes, and the ε value at the low-
energy absorption band apex is approximately 2.4 ꢁ
104 M-1 cm-1, which is similar to the value of a binuclear
platinum terpyridyl 1,10-phenanthrolineethynyl complex
(2.2ꢁ104 M-1 cm-1).20 This indicates that interactions
between the two Pt(C∧N∧N) components could occur
through the rigid conjugated bridging ligand.
The charge transfer nature of the low-energy absorption
bands in F-1-F-5 is supported by the solvent-dependency
studies. As exemplified in Figure 2 for F-4, the low-energy
absorption band energy bathochromically shifts to longer
wavelengths in solvents with lower polarity (such as
hexane and CH2Cl2) compared to those in more polar
solvents (i.e., CH3CN and CH3OH). This negative solva-
tochromic effect is a characteristic of a charge-transfer
transition, in which the ground state is more polar than
the excited state, and is in line with many of the plati-
num C∧N∧N or terpyridyl complexes reported in the
literature.1,3g,6d,7,8
Figure 2. UV-vis absorption spectra of F-4 in different solvents. c =
5 ꢁ 10-5 mol/L.
Figure 3. Normalized emission spectra of F-1-F-5 in CH2Cl2 at a
concentration of 5 ꢁ 10-5 mol/L. The excitation wavelength is 440 nm.
in Figure 4) upon excitation at λ
e 370 nm. The
ex
lifetimes of these two emission bands are quite distinct.
The lifetime of the high-energy band is too short to be
measured on our instrument, which is expected to be
shorter than 5 ns; while the lifetime of the orange emission
band is hundreds of nanoseconds. In addition, the excita-
tion spectra corresponding to these two emission bands
are different. The excitation spectrum monitored at the
high-energy band is consistent with the π,π* trans-
ition bands in the UV region (Supporting Information,
Figure S2). In contrast, the excitation spectrum measured
for the orange emission corresponds to the charge trans-
fer band in the UV-vis absorption spectrum (Supporting
Information, Figure S2). Considering the different life-
times, the distinct excitation spectra, and the different
Stokes shifts, we tentatively attribute the high-energy
1
Emission. F-1-F-5 are emissive in solutions at room
temperature and in glassy solutions at 77 K. Figure 3
show the emission spectra of F-1-F-5 in CH2Cl 2 solution
at a concentration of 5ꢁ10 -5 mol/L at room temperature
when excited at their respective charge transfer band.
When excited at their ∼350 nm π,π* transition bands,
F-1-F-4 give rise to the same red/orange emission band
as that observed from excitation at their charge transfer
bands. In contrast, F-5 exhibits a structured emission in
the 320-500 nm regions in addition to the broad struc-
tureless emission band in the 500-750 nm regions (shown
emission from F-5 to fluorescence from the π,π* state
of the 2,7-diethynyl-9,9-dihexylfluorenyl bridging ligand;
whereas the red/orange emission from F-1-F-5 is as-
signed as a triplet excited-state with charge transfer
3
character, likely to be MLCT/3ILCT with reference to
the other platinum C∧N∧N complexes reported in the
literature.1,3g,6d,7-9,20 The charge transfer nature of the
red/orange emission band could be supported by the fact
that this band exhibits a negative solvatochromic effect.
As shown in Figure 5 for complex F-4, the emission
energy decreases in less polar solvents such as hexane

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4513
Figure 4. Emission spectra of F-5 in CH2Cl2 at room temperature with different excitation wavelengths.
and toluene in comparison to those in polar solvents
CH2Cl2 and CH3CN. The same solvent effect was ob-
served for the other four complexes as well.
It is important to note that the high-energy emission
band observed in the solution of F-5 cannot be attributed
to a trace amount of uncoordinated 4-fluorenyl-C∧N∧N
ligand or fluorenyl impurity because the free 4-fluorenyl-
C∧N∧N ligand exhibits a structureless fluorescence at
378 nm in CH2Cl2 upon excitation at 325 nm and the
free fluorene shows a slightly structured fluorescence at
312 nm in CH2Cl2 upon excitation at 265 nm (Supporting
Information, Figure S3). Neither the emission energy nor
the spectral feature from the free 4-fluorenyl-C∧N∧N
ligand and free fluorene is consistent with the high-energy
band observed in F-5 solution. However, this high-energy
fluorescence band is essentially the same as the fluorescence
spectrum of 2,7-diethynyl-9,9-dihexylfluorene (Supporting
Information, Figure S3(c)). The fact that this band is only
observed from F-5 solution but not from the solutions of
the other four complexes also suggests that this band is
associated with the unique structural feature of F-5, that
is, the presence of the 2,7-diethynyl-9,9-dihexylfluorenyl
bridging ligand. We believe that this high-energy emission
band in F-5 solution indeed emanates from the coordi-
nated bridging ligand, rather than the free bridging ligand
impurity because the excitation spectrum (Supporting
Information, Figure S2(a)) giving rise to this band is
quite different from that measured in the free ligand
(Supporting Information, Figure S3(d)).
The possibility of the low-energy red/orange emission
band arising from fluorescence resonance energy transfer
(FRET) from the fluorenyl component to the Pt(C∧N∧N)
component could be excluded based on the fact that no
low-energy red/orange emission band appears in the
emission spectrum of the free 4-fluorenyl-C∧N∧N ligand
(Supporting Information, Figure S3(a)).
The concentration-dependent emission was investi-
gated for all five complexes. When the concentration is
increased from 5 ꢁ 10-6 mol/L to 5 ꢁ 10-4 mol/L, the
emission energies and the shapes of the spectra of F-1-F-
4 remain essentially the same (exemplified in Supporting
Information, Figure S4 for F-4); however, the emission
intensity decreases at high concentrations and the emis-
sion lifetimes decrease with increased solution concentra-
tion. Because these complexes have almost no ground-
state absorption at the emission band apex and the shapes
Figure 5. Emission spectra of F-4 in different solvents at room tempera-
ture. c = 5 ꢁ 10-5 mol/L. λex = 440 nm.
of the spectra are the same, self-absorption of emission
could be excluded. As discussed in the previous section,
the UV-vis absorption obeys the Lambert-Beer’s law in
the concentration range of 5ꢁ10-6 mol/L to 5ꢁ10-4 mol/
L, suggesting that no ground-state aggregation occurs in
this concentration range. Therefore, the intensity de-
crease at high concentrations cannot be attributed to
the ground-state aggregation. In view of the decreased
lifetime at high concentrations, it is clear that self-quench-
ing occurs at high concentrations. The self-quenching rate
constants (kQ) were measured according to the
Stern-Volmer relation, and the results are listed in
Table 1. The kQ for F-1-F-4 varies between 1.12 ꢁ 109
and 1.63ꢁ109 L mol-1 s-1, which falls in the same order of
values reported for the terpyridine platinum phenylace-
tylide complexes (1.45ꢁ109-1.74ꢁ109 L mol-1 s-1).22
A
feature worthy of mention is that the self-quenching
constant for F-1 is much higher than that for F-2. This
could be explained by the presence of the branched
alkoxyl chain in F-2, which could reduce the π,π stacking
and thus the formation of excimer.
Complex F-5 exhibits a slightly different concentration-
dependent behavior from the other four complexes. As
shown in Figure 6, the emission band maximum gradually
red-shifts with increased concentration accompanied by
reduced intensity and shortened lifetime. In view of the
(22) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476.

4514 Inorganic Chemistry, Vol. 49, No. 10, 2010
Shao et al.
Table 1. Photophysical Parameters of F-1-F-5
CH3CNa
C3H7CNb
a
CH2Cl2
λ
T1-Tn/ nm
(τTA/ ns; εT1-Tn
L mol-1 cm-1; ΦT)
λabsc/ nm
λemc/nm
(τ0/ns; Φem
kQd/109
λemc/ nm
(τem/ ns, Φem)
/
λemc/nm
(τem/μs)
(ε/103 L mol-1 cm-1
)
)
L mol-1 s-1
3
3
3
3
3
3
F-1
F-2
F-3
F-4
F-5
282 (38.2), 312 (25.9),
568 (960; 0.075)
591 (950; 0.047)
593 (680; 0.073)
593(980; 0.076)
1.63
1.12
1.41
1.37
1.94
566 (190; 0.067)
588 (570; 0.042)
590 (510; 0.065)
592 (758; 0.067)
387 (140; 9070),
656 (210; 4980; 0.08)
548 (23.2),
588 (23.9), 635
336 (29.6), 354 (32.3),
421 (8.6), 439 (8.8)
291 (37.5), 323 (30.8),
354 (38.1), 419 (7.7),
439 (6.8)
288 (37.7), 339 (35.7),
355 (30.8), 443 (9.2),
463 (8.8), 529 (1.0)
284 (48.2), 341 (31.9),
355 (30.9), 441 (10.3),
465 (9.2), 530 (1.2)
293 (80.4), 324 (79.6),
361 (126.0), 441 (24.4),
486 (24.6), 510 (22.3)
475 (620; 5500; 0.16),
665 (680; 4820), 800 (480)
542(16.0),
586 (15.5),
400 (670; 11480),
635 (660; 3790; 0.11)
550 (14.0),
590 (14.2)
400 (880; 11000),
645 (800; 1670; 0.24)
554 (14.0),
584 (14.5)
602e (890 (97%),
30 (3%)); 0.015)
607f (825 (94%),
20 (6%)); 0.008g)
h
588 (16.0),
629 (14.7)
a Measured at room temperature. b In butyronitrile glassy solutions at 77 K. c At a concentration of 5 ꢁ 10-5 mol/L. d Self-quenching rate constant.
e At a concentration of 5 ꢁ 10-6 mol/L. f At a concentration of 5 ꢁ 10-6 mol/L in acetone. g In acetone solution. h Too weak to be measured.
considerable ground-state absorption from 550 to 600 nm
in F-5, it is reasonable to believe that the red-shift is
induced by self-absorption of the emission. The reduced
intensity and lifetime at high concentrations could be attri-
buted to a combination of self-quenching, self-absorption
of emission and inner filter effect. The self-quenching
constant is higher for F-5 than those for F-1-F-4, possi-
bly because of the extended π-system that facilitates the
formation of excimer. This notion could be partially
supported by the biexponential decay of the emission
for F-5 (shown in Table 1), in which the minor component
may arise from an excimer emission. For the 1π,π*
emission at the UV region for F-5, it also exhibits sig-
nificant intensity decrease when the concentration in-
creases. This emission is completely quenched when the
concentration reaches 5ꢁ10-5 mol/L or higher. Because
Figure 6. Normalized emission spectra of F-5 at different concentra-
tions in CH2Cl2 at room temperature. λex = 441 nm.
of the intense absorption of F-5 in this spectral region, the
reduced emission intensity should be caused predomi-
nately by self-absorption of the emission, although the
inner filter effect due to the intense absorption could also
contribute.
study, and indicates that the fluorenyl substituent likely
twists from the plane of the Pt(C∧N∧N) component.
Consequently, electronic interaction between the fluor-
enyl substituent and the C∧N∧N ligand is reduced. How-
ever, introducing the fluorenyl 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
fluorenyl substituent,6d,9 which is testified by F-1, F-2,
and F-3.
With respect to the emission spectra shown in Figure 3
for F-1-F-5, it is obvious that replacing the chloride co-
ligand by the acetylide co-ligand reduces the emitting
state energy, which has been seen in many reports for
platinum C∧N∧N or terpyridyl complexes,1,3g,6d,7-9,20
and could be attributed to the π-donating ability of the
3
acetylide ligand that would decrease the MLCT state
energy. However, it is noted that the emission energy for
F-2, F-3, and F-4 is quite similar, which probably reflects
the admixture of 3ILCT character that is independent of
the nature of the co-ligand into the emitting state. The
reduced emission energy of F-2 compared to that of F-1 is
similar to that observed for the platinum C∧N∧N com-
plexes without the fluorenyl substituent and could also be
explained by the involvement of the 3ILCT character into
the emitting state.9 Another point should be noticed is
that the emission energies for all five complexes are quite
similar to their respective platinum C∧N∧N congeners
without the 40-fluorenyl substituent.6d,7,9 This feature is
consistent with that observed from the UV-vis absorption
F-1-F-4 are emissive in butyronitrile at 77 K. Com-
pared to the emission at room temperature in CH2Cl2, the
emission spectra shift to higher-energy and exhibit clear
vibronic structures, as exemplified in Figure 7 for F-1 and
Supporting Information, Figure S5 for F-3 and listed in
Table 1 for the other three complexes. The vibronic
progression is in the range of 930 cm-1 to 1390 cm-1
for these complexes, which falls into the skeletal stretch-
ing modes of the C∧N∧N ligand.7 These spectral fea-
tures are similar to those of C∧N∧N platinum acetylide
complexes reported by Che.1 The hypsochromic shift
of the emission spectra at 77 K could be due to the
rigidochromic effect, which is commonly seen in many

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4515
transition-metal complexes including platinum com-
plexes.23 The lifetimes of these complexes at 77 K are also
comparable to those reported in the literature for other
platinum C∧N∧N or terpyridyl complexes.1,3g,6d,7-9 How-
the lack of observable TA from F-5 could be hypotheti-
cally related to its lower triplet excited-state quantum
yield, which is partially reflected by its significantly lower
emission quantum yield compared to those for F-1-F-4.
F-1 and F-2 exhibit broad positive absorption from 380
to 830 nm. The profiles of the TA spectra of F-3 and F-4
are similar to that of F-1, with the exception that some
bleaching occurred in the region of 420-480 nm and a
flatter and stronger absorption in the NIR region of
770-850 nm. The facts that the bleaching band position
is consistent with the charge transfer band in the UV-vis
absorption spectra and the different features in the NIR
region indicate that the TA could possibly originate from
the 3MLCT/3ILCT/3LLCT state for F-3 and F-4. For F-1
and F-2, the excited-state that gives rise to the transient
ever, the thermally induced Stokes shift for F-1 (580 cm-1
)
and F-5 (395 cm-1) is much smaller than those for F-2
(1440 cm-1), F-3 (1230 cm-1), and F-4 (1160 cm-1). Consi-
dering the emission energy, the shape of the spectrum, the
lifetime, and the thermally induced Stokes shift, we tenta-
tively assign the emitting state at 77 K as 3MLCT for F-2-
F-4, and 3MLCT/3π,π* for F-1 and F-5.
Transient Difference Absorption. The lifetimes mea-
sured from the decay of the emission of F-1- F-5 suggest
that the triplet excited state is much long-lived than the
laser pulse width. Therefore, triplet excited-state absorp-
tion is expected to be observed for these complexes.
Figure 8 displays the triplet transient difference absorp-
tion (TA) spectra of F-1-F-4 in degassed CH3CN solu-
tion at zero time-delay after the excitation. The time-
resolved TA spectrum is exemplified in Figure 8 for F-2 as
well. The transient absorption from F-5 is too weak to be
detected. This is partially attributed to the very poor
solubility of this complex in CH3CN, which limits the
highest concentration we could use for the TA experiment
and prevents efficient population of the excited state
because of the very dilute solution. On the other hand,
3
absorption could be MLCT/3ILCT. All the TA decays
monoexponentially throughout the whole spectral range
monitored, indicating that the transient absorption arises
from the same excited state or excited states in close
proximity that are in equilibrium. In addition, the lifetime
measured from the decay of the transient absorption and
from the decay of the emission are essentially consistent,
suggesting that the TA might arise from the same excited
state that emits, or a state that is in equilibrium with the
emitting state. These pieces of evidence further sup-
port the aforementioned assignment for the TA of these
complexes.
Figure 9. Transmittance vs incident fluence curves of F-1-F-5 in
CH2Cl2 for 4.1 ns laser pulses at 532 nm in a 2 mm cell. The linear
transmission was adjusted to 80%.
Figure 7. Emission spectra of F-1 at room temperature and at 77 K.
λ
ex = 423 nm. c = 5 ꢁ 10-5 mol/L.
Figure 8. (a) Triplet transient difference absorption spectra of F-1-F-4 in argon-degassed CH3CN solution at room temperature at zero time delay
following 355 nm excitation. (b) Time-resolved TA spectra for F-2 in degassed CH3CN solution.

4516 Inorganic Chemistry, Vol. 49, No. 10, 2010
Shao et al.
Figure 10. Nanosecond and picosecond open-aperture Z-scan experimental data and fitting curves for F-1 at 532 nm. The energy used was 3.9 μJ for ns
Z scan and 1.5 μJ for ps Z scan, and the beam radius at the focal point was 30 μm for ns Z scan and 34 μm for ps Z scan.
Table 2. Excited-State Absorption Cross Sections of F-1-F-4 in CH2Cl2 at 532 nm
σ0a (10-18 cm2)
σSb (10-18 cm2)
σTc (10-18 cm2)
σS/σ0
σT/σ0
ΦσT/σ0
F-1
F-2
F-3
F-4
0.765
0.536
3.29
4.21
1.60
62 ( 2
80 ( 3
130 ( 5
100 ( 5
19 ( 1
245 ( 5
103 ( 3
145 ( 5
75 ( 5
81
149
40
24
12
320
192
44
18
29
25.6
30.8
4.8
4.3
14.7
(C∧N∧N)PtC5H7
46 ( 2
a Ground-state absorption cross-section. b Singlet excited-state absorption cross-section. c Triplet excited-state absorption cross-section.
Reverse Saturable Absorption and Z-Scan Measurements.
As discussed in the TA section, complexes F-1- F-4
exhibit broad positive TA in most of the visible to the
near-IR region, which implies that their excited-state
absorption cross-sections are larger than the ground-state
absorption cross-sections. In addition, the TA decay times
are hundreds of nanoseconds. Therefore, reverse satur-
able absorption (RSA) could occur for these platinum
complexes under the irradiation of ns laser pulses. To
demonstrate this, the nonlinear transmission experiments
were carried out for F-1-F-5 in CH2Cl2, and the results
are illustrated in Figure 9.
5.4ꢁ10-19 cm2 for F-2, 3.3ꢁ10-18 cm2 for F-3, and 4.2ꢁ
10-18 cm2 for F-4 by using the ε values obtained from
their UV-vis absorption spectra and the conversion
equation σ=2303ε/NA, where NA is the Avogadro con-
stant. To obtain the excited-state absorption cross-sections,
open-aperture Z-scan24 experiments that measure the
transmission changes due to nonlinear absorption were
carried out at 532 nm using both ns and ps laser pulses for
F-1-F-4. Fitting of these ns and ps open-aperture Z-scan
experimental data using the 5-band model will allow us to
extract both the singlet and triplet excited-state absorp-
tion cross-section.19
When the incident fluence is increased, the transmit-
tance of F-1-F-4 decreases significantly. The RSA
threshold, defined as the incident fluence at which the
transmittance decreases to 70% of the linear transmit-
tance, is 0.29 J/cm2 for F-1 and F-2, 0.48 J/cm2 for F-3,
and 0.65 J/cm2 for F-4. When the incident fluence is
increased to 1.8 J/cm2, the transmittance drops to 0.28
for F-1 and F-2, 0.37 for F-3, and 0.42 for F-4. For F-5,
the transmittance almost keeps constant even at high
fluences, indicating that almost no RSA occurs, which
is consistent with the TA results discussed in the previous
section. The strength of the RSA for these five complexes
obviously follows this trend F-1=F-2 > F-3 > F-4 > F-5.
According to our previous studies, one of the key
parameters that determines the degree of RSA is the ratio
of the excited-state absorption cross-section to that of the
ground-state. The ground-state absorption cross-sections
at 532 nm for these complexes are 7.7ꢁ10-19 cm2 for F-1,
Figure 10 shows the open-aperture Z-scan experimen-
tal data and the fitting curves for F-1. F-2-F-4 exhibit the
similar results, while F-5 shows negligible nonlinear
absorption. The transmission for F-1-F-4 decreases
when the samples are moved close to the focal plane, that
is, the incident fluence increases, indicating the occur-
rence of reverse saturable absorption. By applying the
ground-state absorption cross-sections determined from
the UV-vis absorption, the triplet and singlet excited-
state lifetimes obtained from the decay of the respective ns
and fs transient absorption (the τT’s are listed in Table 1,
the τS is 12.1 ps for F-1, 6.9 ps for F-2, and 5.3 ps for F-3
and F-4), and the triplet excited-state quantum yields
(listed in Table 1) to the five-band model, and using the
procedure described previously,19 we obtained a single
pair of excited-state absorption cross-section values (σs,
σT) that simultaneously fit both the nanosecond and
picosecond Z scans. These results are compiled in Table 2.
For comparison purpose, Z scans of the corresponding
(C∧N∧N)PtC5H7 complex6d has also been carried out,
and the fitting results are listed in Table 2.
(23) (a) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625.
(b) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton Trans. 1991,
1077. (c) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa, D.;
Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447. (d) Lai, S.-W.; Chan,
M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M. Inorg. Chem. 1999, 38, 4046.
(24) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagen, D. J.; Stryland,
E. W. V. IEEE J. Quantum Electron. 1990, 26, 760.

Article
It is obvious that the excited-state absorption cross-
Inorganic Chemistry, Vol. 49, No. 10, 2010 4517
visible to the near-IR region; therefore, they give rise to
a strong reverse saturable absorption for ns laser pulses at
532 nm. The degree of reverse saturable absorption follows
this trend: F-1=F-2 > F-3 > F-4 > F-5, 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 the fluorenyl sub-
stituent on the C∧N∧N ligand causes a pronounced effect on
the photophysics of these complexes, which is reflected by
their much larger molar extinction coefficients for their low-
energy charge transfer absorption band, longer triplet ex-
cited-state lifetimes, higher 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 the fluorenyl
substituent. Additionally, F-2 shows much improved solubi-
lity in CH2Cl2 (∼150 mg/mL), making it possible to prepare
high-concentration solutions for two-photon absorption
study in the near-IR region. Therefore, two-photon induced
excited-state absorption was observed in the near-IR region
for this complex. These results will be reported later.
section values (both σs and σT) at 532 nm for F-1-F-4 are
much larger than those for the (C∧N∧N)PtC5H7 complex,
which is in line with the trend observed in the UV-vis
absorption, reflecting the influence of fluorenyl unit on
the C∧N∧N complexes. The ratios of σS/σ0 and σT/σ0 for
F-1 and F-2 are among the largest values reported in the
literature.6g,25 When fitting the Z-scan results, the popu-
lation fraction on related excited states are calculated
versus time. The results are exemplified in Supporting
Information, Figure S6 for F-1. It is apparent that for ns
excitation, the triplet excited states (T1 and T2) are much
more populated than the singlet excited states (S1 and S2).
Therefore, the excited-state absorption for ns laser pulses
is dominated by the triplet excited-state absorption. In
such a case, the RSA is not only predominantly deter-
mined by the ratio of σT/σ0, but also affected by the triplet
excited-state quantum yield. Taking these factors into
account, we found that the ratio of ΦσT/σ0 (listed in
Table 2) correlates very well with the observed RSA trend
shown in Figure 9. Therefore, to improve the RSA for ns
laser pulses, improving the ratio of ΦσT/σ0 is crucial. In
the case of complexes having similar excited-state absorp-
tion cross-section, the complex with the minimum
ground-state absorption cross-section and higher triplet
quantum yield would give rise to a larger ratio of ΦσT/σ0
and thus stronger RSA.
Acknowledgment. Financial support from the National
Science Foundation (CAREER CHE-0449598) and the
Army Research Laboratory (W911NF-06-2-0032) is
greatly appreciated. We are grateful to Dr. Joy E. Haley
at the UES Inc. and the Air Force Research Laboratory
for measuring the singlet excited-state lifetime and to
Zhongjing Li at Sun’s group at NDSU for measuring
the fluorescence spectra of the related ligands and fluorene.
Conclusion
Mononuclear and dinuclear platinum(II) 6-phenyl-4-(9,9-
dihexylfluoren-2-yl)-2,20-bipyridine complexes F-1-F-5 were
synthesized, and their photophysics were systematically in-
vestigated. Because of the electron-donating ability of the
fluorenyl substituent, the lowest-energy singlet and triplet
excited states possess intraligand charge transfer character,
Supporting Information Available: Comparisonof the UV-vis
absorption spectra of F-1-F-3 to their respective platinum C∧N∧N
complexes without the fluorenyl substituent, the excitation spectra
of F-5 in CH2Cl2, the fluorescence spectra of ligand 5, 2,7-diethynyl-
9,9-dihexylfluorene, and fluorenes in CH2Cl2, the normalized emis-
sion spectra of F-4 at different concentrations at room temperature,
the emission spectra of F-3 in CH2Cl2 at room temperature and in
butyronitrile at 77 K, and the population fraction versus time on
different excited states for F-1 upon ns laser excitation. This material
is available free of charge via the Internet at http://pubs.acs.org.
3
possibly mixed with some MLCT character. F-1-F-4 ex-
hibit broad triplet transient difference absorption in the
(25) Pritchett, T. M.; Sun, W.; Zhang, B.; Ferry, M. J.; Li, Y.; Haley, J. E.;
Machie, D. M.; Shensky, W., III; Mott, A. G. Opt. Lett. 2010 (in press).