Synthesis, photophysics, and optical limiting of platinum(II) 4′-tolylterpyridyl arylacetylide complexes

Article abstract of DOI:10.1021/ic049266n

A series of 4′-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide, 4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4- dimethylaminophenylacetylide, 1-naphthylacetylide, and 3-quinolinylacetylide, were synthesized. Their photophysical properties, such as electronic absorption spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (¹MLCT) transition at ca. 420-430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C ₆H₄Br-4, ttpy-C₆H₄OCH _3--4, ttpy-C₆H₄N(CH₃)₂-4, and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460-540 nm. This band is sensitive to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-terpyridyl charge-transfer transition (i.e., a ¹MLCT or/and ¹LLCT transition). All of the complexes exhibit room-temperature phosphorescence. The emission can be attributed to a ³MLCT state except for ttpy-C₆H₄NO ₂-4, for which the emission likely originates from an intraligand ³π,π* state involving the nitrophenylacetylide ligand. For ttpy-C₆H₄OCH₃-4, ttpy-C₆H ₄N(CH₃)₂-4, and ttpy-Np, there probably is more than one low-energy state in close energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference spectra of ttpy-Ph, ttpy-C₆H₄Br-4 ttpy-C₆H ₄NO₂-4, and ttpy-Quin exhibit moderately intense, broad absorption bands in the visible region and extending into the near-IR region, which likely originate from the same excited state that emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetvlide, cause a decrease of the emission efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical limiting for nanosecond laser pulses at 532 nm.

Full text of DOI:10.1021/ic049266n

Inorg. Chem. 2005, 44, 4055−4065
Synthesis, Photophysics, and Optical Limiting of Platinum(II)
-Tolylterpyridyl Arylacetylide Complexes
4′
Fengqi Guo and Wenfang Sun*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity,
Fargo, North Dakota 58105-5516
Yao Liu and Kirk Schanze
Department of Chemistry, UniVersity of Florida, P.O. Box 117200,
GainesVille, Florida 32611-7200
Received June 5, 2004
A series of 4′-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide,
4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4-dimethylaminophenylacetylide, 1-naph-
thylacetylide, and 3-quinolinylacetylide, were synthesized. Their photophysical properties, such as electronic absorption
spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have
been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (1MLCT) transition at ca. 420
430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4,
and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460 540 nm. This band is sensitive
−
−
to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-
terpyridyl charge-transfer transition (i.e., a 1MLCT or/and 1LLCT transition). All of the complexes exhibit room-
temperature phosphorescence. The emission can be attributed to a 3MLCT state except for ttpy-C6H4NO2-4, for
which the emission likely originates from an intraligand 3π
,π* state involving the nitrophenylacetylide ligand. For
ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, there probably is more than one low-energy state in close
energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference
spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-4, and ttpy-Quin exhibit moderately intense, broad absorption bands
in the visible region and extending into the near-IR region, which likely originate from the same excited state that
emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide
ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetylide, cause a decrease of the emission
efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption
bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical
limiting for nanosecond laser pulses at 532 nm.
Introduction
as DNA intercalators9,10 and protein probes.11 Recently,
platinum(II) terpyridyl phenylacetylide complexes were
Square-planar platinum(II) terpyridyl complexes have
attracted great interest in recent years because of their unique
spectroscopic properties1-8 and their potential applications
(4) Bu¨chner, R.; Cunningham, C. T.; Field, J. S.; Haines, R. J.; McMillin,
D. R.; Summerton, G. C. J. Chem. Soc., Dalton Trans. 1999, 711.
(5) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem.
1999, 38, 4262.
(6) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476.
(7) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193.
* To whom correspondence should be addressed. Tel.: 701-231-6254.
Fax: 701-231-8831. E-mail: Wenfang.Sun@ndsu.nodak.edu.
(1) Aldridge, T. K.; Stacey, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722.
(2) Bu¨chner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.; McMillin,
D. R. Inorg. Chem. 1997, 36, 3952.
(3) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta
1998, 273, 346.
(8) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653.
10.1021/ic049266n CCC: $30.25
Published on Web 05/04/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 11, 2005 4055

Guo et al.
Chart 1. Chemical Structures of 4′-Tolylterpyridyl Arylacetylide
Complexes
bromo-4-ethynylbenzene were purchased from Aldrich Chemical
Co. Potassium tetrachloroplatinate and trans-dichlorobis(tri-
phenylphosphine)palladium(II) were obtained from Alfa Aesar.
Phenylacetylene, 1-bromo-4-nitrobenzene, 4-bromoanisole, 4-bromo-
N,N-dimethylaniline, 1-iodonaphthalene, 3-bromoquinoline, and
trimethylsilylacetylene were obtained from Avocado Research
Chemical Ltd. All solvents purchased from VWR Scientific
Products were analytical grade and were used directly without
further purifications. p-Nitroethynylbenzene, p-ethynylanisole, p-
(dimethylamino)phenylacetylene, 1-ethynylnaphthalene, and 3-
ethynylquinoline were synthesized according to literature proce-
dures.14,15
The synthesized compounds were characterized by UV-vis, IR,
NMR, ESI-HRMS, and elemental analyses. UV-vis spectra were
obtained using a CARY 500 dual-beam scanning UV-vis-NIR
spectrophotometer. IR spectra were measured on a Thermo Nicolet
1
Fourier transform infrared spectrophotometer. H NMR and 13C
NMR spectra were measured on an Oxford 300 MHz VNMR
spectrometer. Electrospray ionization (ESI) high-resolution mass
spectrometry analyses were conducted on a 3-Tesla Finnigan
FTMS-2000 Fourier transform mass spectrometer. Elemental
analyses were performed on a Perkin-Elmer 2400 Series II CHNS/O
analyzer.
p-Ethynylanisole. p-Ethynylanisole was synthesized according
to the literature procedure.14 Yield: 61%. 1H NMR (CDCl3) δ: 3.01
(s, 1H), 3.80 (s, 3H), 6.90 (d, 2H, J ) 8.5 Hz), 7.50 (d, 2H, J )
8.5 Hz) ppm. 13C NMR (CDCl3) δ: 55.5, 76.1, 84.0, 115.0, 129.0,
134.0, 134.7, 160.5 ppm.
demonstrated by our group12 to exhibit strong reverse
saturable absorption (RSA) for nanosecond laser pulses,
suggesting that this class of complexes represents promising
broadband optical limiting materials. These results are
intriguing; however, our previous study focused only on the
effect of conjugation length of the phenylacetylide ligand
and the possible influence of the 4′-substituent on the
4-phenyl ring of the terpyridyl ligand. The effect of different
aryl substituents on the acetylide ligand on the RSA and the
photophysics of the complexes has not been evaluated. It
has been demonstrated that the nature of the acetylide ligand
has a significant effect on the emission properties of platinum
complexes.3,6-8 Therefore, it is expected that different
arylacetylide ligands will have a significant effect on their
excited-state absorption and, in turn, on their optical limiting
properties. To demonstrate this, a series of platinum 4′-
tolylterpyridyl arylacetylide complexes (Chart 1) has been
synthesized and characterized. The effects of the arylacetylide
ligands on the ground-state and excited-state energy levels
and lifetimes and triplet excited-state transient difference
absorption spectra have been systematically investigated. The
optical limiting performances of several complexes have also
been explored using 4.1-ns laser pulses at 532 nm. From
these studies, we intend to better understand the correlation
between the electronic characteristics of the arylacetylide
ligand and the photophysical properties in order to develop
new platinum(II) terpyridyl arylacetylide complexes with
long-lived excited states and strong excited-state absorption
for application in optical power limiting.
1-Ethynylnaphthalene. 1-Ethynylnaphthalene was synthesized
by modification of the literature procedure.14 1-Iodonaphthalene
(1.28 g, 5 mmol) and trimethylsilylacetylene (0.99 g, 10 mmol)
were dissolved in 20 mL of triethylamine. Bis(triphenylphosphine)
palladium dichloride (70 mg, 0.1 mmol) and CuI (5 mg, 0.026
mmol) were added to this mixture. The reaction mixture was stirred
at room temperature for 16 h, and then the mixture was heated to
reflux for 2 h. After being allowed to cool, the solvent was removed
under reduced pressure. A 15 mL portion of methanol was added
to dissolve the residue, and 1 N aqueous potassium hydroxide
solution (5 mL) was added. The mixture was stirred at room
temperature for 2 h. Methanol was removed, and the aqueous
solution was extracted with ether (3 × 50 mL). The ether layer
was dried over MgSO4, and then the solvent was removed. The
crude product was purified using column chromatography (silica
gel 60 mesh), with a 1:1 benzene/hexane mixture used as the eluent.
1
Yield: 27%. H NMR (CDCl3) δ: 3.55 (s, 1H), 7.48 (t, 1H, J )
8.3 Hz), 7.62 (m, 2H), 7.82 (d, 1H, J ) 6.7 Hz), 7.90 (d, 2H, J )
8.3 Hz), 8.47 (d, 1H, J ) 8.3 Hz) ppm. 13C NMR (CDCl3) δ: 82.2,
82.5, 120.0, 125.5, 126.5, 127.0, 127.5, 128.8, 129.6, 131.9, 133.5,
133.8 ppm.
3-Ethynylquinoline. The procedure was similar to that for
1-ethynylnaphthalene, except that 3-bromoquinoline was used to
replace 1-iodonaphthalene. Yield: 27%. 1H NMR (CDCl3) δ: 3.20
(s, 1H), 7.59 (m, 1H), 7.68 (m, 2H), 8.10 (d, 1H, J ) 7.2 Hz), 8.25
(s, 1H), 8.95 (s, 1H) ppm. 13C NMR (CDCl3) δ: 80.8, 81.2, 101.0,
116.5, 127.5, 127.7, 129.8, 130.8, 139.7, 147.5, 152.1 ppm.
[Pt(ttpy)Cl]Cl. [Pt(ttpy)Cl]Cl was synthesized by a slight
modification of the literature procedure.5 After being refluxed for
24 h in acetonitrile/water, the yellow solid was separated by
filtration, dried under reduced pressure, and recrystallized in ethanol.
Experimental Section
Synthesis. The platinum 4′-tolylterpyridyl arylacetylide com-
plexes were synthesized by modification of literature proce-
dures.5,6,13 4′-(4-Methylphenyl)-2,2′:6′,2′′-terpyridine (ttpy) and 1-
(9) Lippard, S. J. Acc. Chem. Res. 1978, 11, 211.
(10) Arena, G.; Monsu´ Scolaro, L.; Pasternack, R. F.; Romeo, R. Inorg.
Chem. 1995, 34, 2994.
(11) Ratilla, E. M. A.; Brothers, H. M., II.; Kostiæ, N. M. J. Am. Chem.
Soc. 1987, 109, 4592.
(12) Sun, W.; Wu, Z.-X.; Yang, Q.-Z.; Wu, L.-Z.; Tung, C.-H. Appl. Phys.
Lett. 2003, 82(6), 850.
(13) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
(14) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627.
(15) Leonard, K. A.; Nelen, M. I.; Anderson, L. T.; Gibson, S. L.; Hilf,
R.; Detty, M. R. J. Med. Chem. 1999, 42, 3942.
4056 Inorganic Chemistry, Vol. 44, No. 11, 2005

Platinum(II) 4′-Tolylterpyridyl Arylacetylide Complexes
General Procedure for the Synthesis of [Pt(ttpy)(CtCC6H4R)]-
Cl [R ) Br, N(CH3)2, OCH3, NO2]. The procedure was modified
from literature procedures.6,8 Potassium hydroxide (20 mg, 0.36
mmol) was added to 50 mL of a methanol solution of arylacetylene
(0.24 mmol), and the mixture was stirred at room temperature for
30 min. [Pt(ttpy)Cl]Cl (120 mg, 0.2 mmol) and CuI (5 mg, 0.026
mmol) were added to the reaction mixture, and the resultant solution
was stirred at room temperature for 12 h. During the reaction, solid
gradually precipitated. The precipitate was isolated by filtration and
recrystallized from DMF and ether.
General Procedure for the Synthesis of [Pt(ttpy)(CtCC6H4R)]-
PF6. The synthesized [Pt(ttpy)(CtCC6H4R)]Cl complexes (0.2
mmol) were dissolved in 20 mL of DMF, and 65 mg of NH4PF6
(0.4 mmol) and 40 mL of CH3OH were added. After the mixture
had been stirred for 2 h at room temperature, the solids were
separated by filtration and recrystallized from acetronitrile and ether.
[Pt(ttpy)(CtCC6H5)]PF6 (ttpy-Ph). Yield: 76%. 1H NMR
(CD3CN) δ: 2.49 (s, 3H), 7.12-7.28 (m, 5H), 7.36-7.48 (m, 4H),
7.63 (d, 2H, J ) 8.1 Hz), 8.00-8.20 (m, 6H), 8.55 (m, 2H) ppm.
Anal. Calcd for C30H22N3PtPF6‚H2O: C, 46.03; H, 3.06; N, 5.37.
Found: C, 46.34; H, 2.93; N, 5.47.
[Pt(ttpy)(CtCC6H4Br)]PF6 (ttpy-C6H4Br-4). Yield: 60%. 1H
NMR (CD3CN) δ: 2.51 (s, 3H), 7.09 (d, 2H, J ) 7.5 Hz), 7.30 (d,
2H, J ) 7.5 Hz), 7.40 (m, 4H), 7.60 (d, 2H, J ) 7.2 Hz), 8.10-
8.24 (m, 6H), 8.58 (2H) ppm. Anal. Calcd for C30H21N3BrPtPF6‚
H2O: C, 41.60; H, 2.67; N, 4.87. Found: C, 41.27; H, 2.50; N,
4.87.
[Pt(ttpy)(CtCC6H4Br)]Cl. Yield: 68%. 1H NMR (DMSO-d6)
δ: 2.48 (s, 3H), 7.15 (d, 2H, J ) 4.5 Hz), 7.33 (m, 4H), 7.61 (m,
2H), 7.87 (d, 2H, J ) 5 Hz), 8.30 (t, 2H), 8.59 (m, 4H), 8.75 (m,
2H) ppm. IR (KBr) ν: 2110 [w, ν(CtC)] cm-1. ESI-MS: m/z calcd
for [C30H21BrN3Pt194]+ 697.0902, found 697.0523 (58%); calcd for
[C30H21BrN3Pt195 +
] 698.0923, found 698.0467 (100%); calcd for
[C30H21BrN3Pt196]+ 699.0925, found 699.0469 (72%). Anal. Calcd
for C30H21BrClN3Pt‚3H2O: C, 45.60; H, 3.43; N, 5.46. Found: C,
45.97; H, 3.53; N, 5.65.
[Pt(ttpy)(CtCC6H4NO2)]PF6 (ttpy-C6H4NO2-4). Yield: 55%.
1H NMR (CD3CN) δ: 2.40, 7.39, 7.50, 7.71, 8.00, 8.45, 8.82, 8.90
ppm. Anal. Calcd for C30H21N4O2PtPF6‚H2O: C, 43.60; H, 2.66;
N, 6.77. Found: C, 43.26; H, 2.52; N, 6.78.
[Pt(ttpy)(CtCC6H4N(CH3)2)]Cl. Yield: 67%. 1H NMR (DMSO-
d6) δ: 2.45 (s, 3H), 2.90 (s, 6H), 6.60 (d, 2H, J ) 8.7 Hz), 7.15 (d,
2H, J ) 8.7 Hz), 7.45 (d, 2H, J ) 8.7 Hz), 7.75 (t, 2H, J ) 9.0
Hz), 8.00 (d, 2H, J ) 8.7 Hz), 8.40 (t, 2H, J ) 9.0 Hz), 8.72 (d,
2H, J ) 8.7 Hz), 8.80-9.00 (m, 4H) ppm. IR (KBr) ν: 2110 [w,
ν(CtC)] cm-1. ESI-MS: m/z calcd for [C32H27N4Pt194]+ 661.2623,
found 661.1880 (72%); calcd for [C32H27N4Pt195]+ 662.2644, found
[Pt(ttpy)(CtCC6H4OCH3)]PF6 (ttpy-C6H4OCH3-4). Yield:
1
72%. H NMR (CD3CN) δ: 2.49 (s, 3H), 3.75 (s, 3H), 6.60 (d,
2H, J ) 6.3 Hz), 6.90 (d, 2H, J ) 6.3 Hz), 7.30-7.40 (m, 4H),
7.50 (d, 2H, J ) 5.7 Hz), 7.90-8.20 (m, 6H), 8.34 (m, 2H) ppm.
Anal. Calcd for C31H24N3OPtPF6‚H2O: C, 45.87; H, 3.22; N, 5.17.
Found: C, 45.69; H, 3.09; N, 5.28.
662.1885 (100%); calcd for [C32H27N4Pt196 +
663.2646, found
]
663.1871 (92%). Anal. Calcd for C32H27N4PtCl‚3H2O: C, 51.06;
H, 4.35; N, 5.58. Found: C, 50.66; H, 3.62; N, 5.60.
[Pt(ttpy)(CtCC6H4N(CH3)2)]PF6 (ttpy-C6H4N(CH3)2-4). Yield:
45%. H NMR (CD3CN) δ: 2.49 (s, 3H), 2.95 (s, 6H), 6.54 (d,
2H, J ) 8.4 Hz), 7.06 (d, 2H, J ) 7.2 Hz), 7.40 (d, 2H, J ) 6.6
Hz), 7.49 (m, 2H), 7.70 (d, 2H, J ) 6.9 Hz), 8.10-8.24 (m, 6H),
8.69 (m, 2H) ppm. Anal. Calcd for C32H27N4PtPF6: C, 47.54; H,
3.34; N, 6.93. Found: C, 47.19; H, 3.39; N, 6.91.
1
[Pt(ttpy)(CtCC6H4OCH3)]Cl. Yield: 64%. 1H NMR (DMSO-
d6) δ: 2.41 (s, 3H), 3.75 (s, 3H), 6.85 (d, 2H, J ) 9.0 Hz), 7.33 (d,
2H, J ) 9.0 Hz), 7.43 (d, 2H, J ) 9.0 Hz), 7.79 (t, 2H), 8.05 (d,
2H), 8.40 (m, 2H), 8.75 (m, 2H), 8.90 (m, 4H) ppm. IR (KBr) ν:
2110 [w, ν(CtC)] cm-1. ESI-MS: m/z calcd for [C31H24N3OPt194 +
]
[Pt(ttpy)(CtCC10H7)]PF6 (ttpy-Np). The synthesis was similar
to the general procedure for the synthesis of [Pt(ttpy)(CtCC6H4R)]-
Cl, except that 1-ethynylnaphthalene was used. After being stirred
for 12 h at room temperature, the mixture was filtered, and a
saturated solution of ammonium hexafluorophosphate in methanol
was added to the filtrate. The product was isolated and recrystallized
from acetonitrile and ether. Deep red product was obtained. Yield:
648.2203, found 648.1500 (74%); calcd for [C31H24N3OPt195 +
649.2224, found 649.1571 (100%); calcd for [C31H24N3OPt196 +
]
]
650.2226, found 650.1569 (74%). Anal. Calcd for C31H24N3OClPt‚
3H2O: C, 50.38; H, 4.06; N, 5.60. Found: C, 50.70; H, 3.60; N,
5.46.
1
[Pt(ttpy)(CtCC6H4NO2)]Cl. Yield: 31%. H NMR (DMSO-
1
70%. H NMR (CD3CN) δ: 2.47 (s, 3H), 7.30-7.50 (m, 4H),
d6) δ: 2.46 (s, 3H), 7.33 (d, 2H, J ) 8.2 Hz), 7.47 (d, 2H, J ) 8.2
Hz), 7.66 (t, 2H, J ) 6.8 Hz), 7.95 (t, 4H, J ) 8.2 Hz), 8.40 (t,
2H, J ) 8.2 Hz), 8.60-8.90 (m, 6H) ppm. IR (KBr) ν: 2110 [w,
ν(CtC)] cm-1. ESI-MS: m/z calcd for [C30H21N4O2Pt194]+ 663.1917,
7.52-7.70 (m, 7H), 7.76-7.96 (m, 2H), 8.20-8.40 (m, 6H), 9.04
(m, 2H) ppm. IR (KBr) ν: 2110 [w, ν(CtC)] cm-1. ESI-MS: m/z
calcd for [C34H24N3Pt194]+ 668.2539, found 668.1463 (76%); calcd
for [C34H24N3Pt
[C34H24N3Pt196 +
]
195 + 669.2560, found 669.1563 (100%); calcd for
670.2562, found 670.1628 (99.9%) Anal. Calcd
found 663.1180 (71%); calcd for [C30H21N4O2Pt195 +
]
664.1938,
found 664.1273 (100%); calcd for [C30H21N4O2Pt196 +
] 665.1940,
]
for C34H24N3PtPF6: C, 50.0; H, 2.97; N, 5.15. Found: C, 49.70;
H, 3.03; N, 4.92.
found 665.1392 (79%). Anal. Calcd for C30H21N4O2ClPt‚5H2O: C,
45.50; H, 3.90; N, 7.01. Found: C, 45.14; H, 3.33; N, 7.06.
[Pt(ttpy)(CtCC6H5)]Cl. The synthesis of [Pt(ttpy)(CCC6H5)]-
Cl followed the general procedure described above, except for the
following: After 12 h of reaction at room temperature, the mixture
was filtered, and 100 mL of ether was added to the filtrate, yielding
the crude product. The crude product was isolated and recrystallized
from methanol and ether. Dark purple powder was collected
[Pt(ttpy)(CtCC9H6N)]PF6 (ttpy-Quin). The procedure was
similar to that for complex [Pt(ttpy)(CtCC10H7)]PF6, except that
3-ethynylquinoline was used to replace 1-ethynylnaphthalene.
1
Yield: 32%. H NMR (CD3CN) δ: 2.45 (s, 3H), 7.15 (d, 2H),
7.40-8.10 (m, 10H), 8.30 (m, 2H), 8.50-8.90 (m, 6H) ppm. IR
(KBr) ν: 2110 [w, ν(CtC)] cm-1. ESI-MS: m/z calcd for
[C33H23N4Pt194 +
[C33H23N4Pt195 +
]
]
669.2417, found 669.1483 (79%); calcd for
670.2438, found 670.1533 (100%); calcd for
1
(yield: 57%). H NMR (CD3OD) δ: 2.42 (s, 3H), 6.91 (d, 2H, J
) 6.6 Hz), 7.10 (m, 3H), 7.30 (m, 4H), 7.55 (d, 2H, J ) 9.3 Hz),
8.00 (t, 2H), 8.09 (d, 2H, J ) 9.3 Hz), 8.20 (2H), 8.25 (d, 2H, J )
6.3 Hz) ppm. IR (KBr) ν: 2150 [w, ν(CtC)] cm-1. ESI-MS: m/z
calcd for [C30H22N3Pt194]+ 618.1941, found 618.1515 (68%); calcd
[C33H23N4Pt196]+ 671.2440, found 671.1492 (98%). Anal. Calcd for
C33H23N4PtPF6: C, 48.58; H, 2.82; N, 6.87. Found: C, 48.18; H,
3.15; N, 6.91.
for [C30H22N3Pt
]
195 + 619.1962, found 619.1453 (100%); calcd for
Photophysical Measurements. The electronic absorption spectra
were measured on a CARY 500 dual-beam scanning UV-vis-
NIR spectrophotometer. The [Pt(ttpy)(CtCAr)]PF6 complexes were
dissolved in acetonitrile. The concentration of the solutions used
[C30H22N3Pt196]+ 620.1964, found 620.1426 (84%). Anal. Calcd for
C30H22ClN3Pt‚2H2O: C, 52.10; H, 3.70; N, 6.06. Found: C, 51.90;
H, 3.84; N, 6.07.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4057

Guo et al.
for the measurements was 1 × 10-5 mol/L. The room-temperature
emission and excitation spectra were measured in acetonitrile
solutions on a SPEX Fluorolog-3 fluorimeter/phorsphorimeter. The
low-temperature emission (77 K) spectra were measured in buty-
ronitrile solutions (glass). The excitation wavelength was 436 nm
for all samples. The solutions were purged with argon for 20 min
before each measurement. The emission quantum yields of the
samples were determined by the comparative method,16 in which a
degassed aqueous solution of [Ru(bpy)3]Cl2 (φem ) 0.042, excited
at 436 nm)17 was used as the reference. The room-temperature
emission lifetimes were determined by a time-correlated single
photon counting technique on an instrument that has been previously
described,18 and the low-temperature (77 K) emission lifetimes were
measured on an Edinburgh LP920 laser flash photolysis spectrom-
eter. The samples were excited by the third-harmonic output (355
nm) of a Quantel Brilliant Nd:YAG laser; the laser pulse width
(fwhm) was 4.1 ns, and the repetition rate used was 1 Hz.
Transient absorption spectroscopy was carried out on an instru-
ment described previously.19 Samples were degassed with argon
for 30 min. Excitation was provided by a Nd:YAG laser (Spectra
Physics GCR-14) at 355 nm. The typical pulse energy was 5 mJ/
pulse, corresponding to a fluence of 20 mJ/cm2 in the pump-probe
region. Transient absorption decay lifetimes were determined from
the multiwavelength difference-absorption data by using the
SPECFIT/32 factor analysis software.20
Optical Limiting Measurements. The optical limiting measure-
ments were carried out using a Q-switched Quantel Brilliant Nd:
YAG laser operated at the second-harmonic wavelength (532 nm)
as the light source. The repetition rate of the laser was 10 Hz.
Energies of the incident laser beam were attenuated by a combina-
tion of a half-wave plate and a polarizable cubic beam splitter. The
beam was then split by a wedged beam splitter. One of the reflected
beams was used to monitor the incident energy. The diameter of
the transmitted beam was reduced to one-half of the original size
by a telescope and was focused by a 25-cm plano-convex lens (f/
65.6) to the center of a 2-mm sample cell. The radius of the beam
waist was approximately 22.2 µm. The incident energy and the
output energy were monitored by two Molectron J4-09 pyroelectric
joule meters.
Figure 1. Electronic absorption spectra of ttpy-Ar in acetonitrile solution
at a concentration of 1.0 × 10-5 mol/L in a 1-cm cell.
assigned to the dπ(Pt) f π*(ttpy) metal-to-ligand charge-
transfer (1MLCT) transition based on the previous studies
on platinum(II) terpyridyl acetylide complexes.6,8,22 For most
of the complexes, there appears to be two distinct bands in
this region, indicating that there might be two orbitally
distinct MLCT transitions, e.g., dxz(Pt) f π*(ttpy) and dyz(Pt)
f π*(ttpy) (assuming that the acetylide ligand is along the
x axis with z perpendicular to the coordination plane), which
has been suggested by Schanze and co-workers for diimine
platinum(II) bis-acetylide complexes.23
The band at ca. 420-430 nm appears to be insensitive to
the para substituents on the phenylacetylide ligand, suggest-
ing that it probably arises from the dyz(Pt) f π*(ttpy)
transition, in which the dyz orbital is perpendicular to the
coordination plane and is unaffected by the para substituents
on the phenylacetylide ligand. In contrast, the lowest-energy
band above 460 nm is affected dramatically by the electron-
donating ability of the ancillary substituents. Strong electron-
donating groups, such as -OCH3 and -N(CH3)2, cause a
significant bathochromic shift of this band. This indicates
that the lowest-energy band could originate from the dxz(Pt)
f π*(ttpy) transition, which is sensitive to the para substit-
uents in view of the possible resonance forms when strong
electron-donating substituents are attached at the para posi-
tion (Chart 2). In this view, dxz is raised in energy by the
Results and Discussion
Electronic Absorption Spectra. Figure 1 shows the
electronic absorption spectra of platinum(II) 4′-tolylterpyridyl
arylacetylide complexes in acetonitrile solutions at the
concentration of 1.0 × 10-5 mol/L. The absorption obeys
Beer’s law when the concentration is lower than 1 × 10-4
mol/L. The absorption band maxima and the extinction
coefficients are presented in Table 1. All of the spectra
exhibit intense absorption bands at 260-360 nm and broad
bands above 400 nm. According to previously reported
spectroscopic studies,1-8 the UV absorption bands are due
to the intraligand transitions of the terpyridyl and acetylide
ligands and the charge-transfer transition involved in the Pt-
CtCR moieties.8,21 The bands above 400 nm are tentatively
(16) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(17) van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 4853.
(18) Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003, 19, 6523.
(19) Wang, Y. S.; Schanze, K. S. Chem. Phys. 1993, 176, 305.
(20) Binstead, R. A. SPECFIT/32, version 1.0; Spectrum Software As-
sociates: Chapel Hill, NC, 2000.
(22) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722.
(21) Masai, H.; Sonogashiram, K.; Hagihara, N. Bull. Chem. Soc. Jpn. 1971,
44, 2226.
(23) Whittle, C. Ed; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053.
4058 Inorganic Chemistry, Vol. 44, No. 11, 2005

Platinum(II) 4′-Tolylterpyridyl Arylacetylide Complexes
Table 1. Photophysical Parameters of the Platinum(II) 4′-Tolyterpyridyl Arylacetylide Complexes
λ
abs/nma
λ
em/nm
τ
em/ns
τ
TA/ns
λem/nm
d
complex
ttpy-Ph
(ꢀ/dm3‚mol-1‚cm-1
)
(RT)b
605
∆Es/cm-1 c
φem
(RT)e
(RT)f
(τem/µs) (77 K)g
266 (69600)
289 (59700)
312 (26800)
430 (8000)
462 (9940)
1820
0.0081
947
907
545 (14.52)
588 (14.18)
680 (2.00)
ttpy-C6H4Br-4
273 (50300)
309 (27800)
427 (9810)
460 (7080)
597
552
597
1632
752
0.0092
0.0020
0.0014
873
805
544 (15.64)
579 (13.81)
632 (2.00)
ttpy-C6H4NO2-4
ttpy-C6H4OCH3-4
270 (27600)
288 (34400)
338 (36700)
423 (13400)
4 (10%)
60 (61%)
895 (29%)
76 (40%)
851 (60%)
530 (21.49)
618 (1.95)
700 (1.46)
270 (48700)
311 (25200)
418 (4880)
478 (5660)
1431
3 (36%)
43 (10%)
817 (54%)
550 (13.81)
582 (12.91)
-
ttpy-C6H4N(CH3)2-4
ttpy-Np
289 (57800)
414 (3580)
540 (4410)
595
617
2241
1746
0.0005
0.0030
7 (37%)
112 (24%)
936 (39%)
525 (25.49)
560 (20.83)
-
-
286 (39400)
311 (37700)
328 (37600)
412 (8250)
475 (7090)
23 (42%)
231 (39%)
805 (19%)
557 (20.10)
594 (11.32)
ttpy-Quin
262 (38700)
288 (33120)
330 (20900)
343 (20000)
429 (8600)
597
1235
0.021
893
855
556 (11.90)
587
688
a Linear absorption band maximum in CH3CN solution. b Room-temperature emission band maximum in CH3CN solution. c Thermally induced Stokes
shift [∆Es) E00(77 K) - E00(298 K)]. d Room-temperature emission quantum yield in CH3CN solution. e Emission decay lifetime(s) at room temperature
in argon-degassed CH3CN solution. f Transient absorption decay lifetime(s) in CH3CN solution. g Emission band maxima and decay lifetimes at 77 K in
argon-degassed butyronitrile glass.
Chart 2. Resonance Structures for ttpy-C6H4N(CH3)2-4
π-donating ability of the acetylide ligand, whereas the energy
of the ttpy-ligand-based LUMO remains the same. Such a
change reduces the energy gap between the HOMO and the
LUMO, resulting in the red shift of this band.
has a mixed MLCT/LLCT character, as reported by Eisen-
berg and co-workers for Pt(diimine)(dithiolate) com-
plexes.26,27 The possibility of the lowest-energy absorption
2
2
band being due to a π(CtCR) f dx -y (Pt) LMCT transition
can be excluded because of the fact that no absorption band
is observed above 350 nm for a platinum bis(tributylphos-
phine) bis(phenylacetylide) complex,28 indicating that LMCT
occurs at a wavelength shorter than 350 nm. The assignment
of this band to an intraligand charge-transfer transition within
the -CtC-C6H4N(CH3)2 ligand can be ruled out on the
basis of the fact that the intraligand transition for the
compound H-CtC-C6H4N(CH3)2 appears at a much high
energy (ca. 291 nm in acetonitrile).29
On the other hand, the sensitivity of the lowest-energy
band to the para substituents on the phenylacetylide ligand
is also consistent with an acetylide-to-terpyridyl ligand-to-
ligand charge-transfer (1LLCT) transition, in which electron-
donating groups increase the energy of the acetylide-ligand-
based HOMO, whereas the ttpy-ligand-based LUMO remains
the same. For ttpy-C6H4N(CH3)2-4, the LLCT transition can
alternatively be viewed as occurring from the amino group
to the terpyridyl ligand through the metal center,24,25 con-
sidering the fact that the LLCT band disappears upon
protonation of the amino group (shown in Figure 2a).
Therefore, it is equally probable that the lowest absorption
Figure 2 displays the electronic absorption spectra of ttpy-
C6H4N(CH3)2-4 and ttpy-C6H4OCH3-4 in CH2Cl2 and CH3-
1
1
(26) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949.
(27) Paw, W.; Lachiocotte, R. J.; Eisenberg, R. Inorg. Chem. 1998, 37,
4139.
band results either from MLCT or from LLCT, or even
(24) Wong, K. M.-C.; Tang, W.-S.; Lu, X.-X.; Zhu, N.; Yam, V. W.-W.
Inorg. Chem. 2005, 44, 1492.
(25) Yang, Q.-Z.; Wu, L.-Z.; Zhang, H.; Chen, B.; Wu, Z.-X.; Zhang, L.-
P.; Tung, Z.-H. Inorg. Chem. 2004, 43, 5195.
(28) Liu, Y.; Jiang, S.; Glusac, K.; Powell, D. H.; Anderson, D. F.; Schanze,
K. S. J. Am. Chem. Soc. 2002, 124, 12412.
(29) Zachariasse, K. A.; Grobys, M.; Tauer, E. Chem. Phys. Lett. 1997,
274, 372.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4059

Guo et al.
Figure 3. Excitation and emission spectra of ttpy-Ph in acetonitrile solution
in a 1-cm cell. Excitation wavelength ) 436 nm, concentration ) 5.0 ×
10-6 mol/L.
Figure 2. Normalized linear absorption spectra of (a) ttpy-C6H4N(CH3)2-4
and (b) ttpy-C6H4OCH3-4 in different solvents. The spectra for ttpy-C6H4-
OCH3-4 in acetonitrile and acidified acetonitrile overlap each other.
Figure 4. Normalized emission spectra of ttpy-Ar in CH3CN solution at
room temperature. Excitation wavelength ) 436 nm, concentration ) 8.55
× 10-6 mol/L.
CN. The lowest-energy band exhibits a substantial batho-
chromic shift in CH2Cl2 in contrast to that in CH3CN,
indicating that the ground state of the complex is more polar
than the excited state. Such a negative solvatochromic effect
is consistent with either an MLCT or an LLCT assignment.
It is also reminiscent of the PtII(diimine)(dithiolate) com-
plexes26,27 and suggests a similar polar ground state in these
complexes with a mixed MLCT/LLCT character. In addition,
upon protonation of the -N(CH3)2 group by addition of dilute
HCl to the acetonitrile solution, the band at 540 nm
completely disappears. This suggests the involvement of the
lone pair of electrons on nitrogen in the MLCT/LLCT
transition. A similar solvent effect was observed for ttpy-
Np, with an approximate 30-nm red shift of the lowest-energy
band in CH2Cl2 solution.
Comparison of the charge-transfer bands of the 4′-
tolylterpyridyl platinum complexes with those of the corre-
sponding terpyridyl platinum complexes reported in the
literature6,24 shows that they appear at almost identical
positions. This indicates that the 4′-tolyl substituent on the
terpyridyl ligand has a small effect on the energy level of
the LUMOs [π*(ttpy) orbital] of the complexes, which is
consistent with the reported work in Pt(4′-X-ttpy)Cl+ sys-
tems.3 Interestingly, the molar extinction coefficients for these
transitions are significantly enhanced for the 4′-tolyl-
substituted complexes. This is likely due to the fact that the
extended π-conjugation in the 4′-tolyl-substituted complexes
increases the oscillator strength for these transitions by
increasing the transition dipoles. A similar effect was
observed by McMillin and co-workers in a series of phenyl-
substituted Cu(I) phenanthroline complexes.30
Photoluminescence. Figure 3 shows the room-temperature
excitation and emission spectra for ttpy-Ph in an acetonitrile
solution. The Stokes shift is approximately 4500 cm-1, and
the lifetime is ∼950 ns. With reference to the previous
spectroscopic studies on similar platinum(II) terpyridyl
complexes, the emitting state is tentatively assigned as the
3MLCT state.5,6,8 The six other complexes all exhibit room-
temperature emission in acetonitrile solution, as shown in
Figure 4 and Table 1. Except for ttpy-C6H4NO2-4, the
emitting states of the other five complexes are also tentatively
3
assigned as the MLCT state because of the similarities in
the emission spectra to that of ttpy-Ph. However, there
appears to be a lack of a systematic trend in the emission
energy and lifetime. Unlike the electronic absorption spectra,
the emission bands of ttpy-C6H4OCH3-4 and ttpy-C6H4N-
(CH3)2-4 are slightly blue-shifted compared to that of ttpy-
Ph. This might result from a raising of the terpyridyl π*
orbital by π back-bonding with dyz because the emitting state
3
might involve the dyz-π* state that lies in close energy
3
proximity to dxz-π*.
(30) Phifer, C. C.; McMillin, D. R. Inorg. Chem. 1986, 25, 1329.
4060 Inorganic Chemistry, Vol. 44, No. 11, 2005

Platinum(II) 4′-Tolylterpyridyl Arylacetylide Complexes
Figure 6. Concentration-dependent emission spectra of ttpy-C6H4NO2-4
in acetonitrile solution at room temperature (λex ) 436 nm). In order of
increasing intensity, the concentrations are: 5.2 × 10-6, 1.1 × 10-5, 3.0 ×
10-5, 5.3 × 10-5, and 1.6 × 10-4 mol/L.
Figure 5. Time-resolved emission spectra of ttpy-Np complex in CH3CN
solution. Excitation wavelength ) 355 nm.
The existence of other low-energy states in close energy
proximity is further suggested by the multiexponential
emission decays from the -N(CH3)2-, -OCH3-, and Np-
containing complexes, as shown in Table 1. From these
multiexponential decays, it is clear that multiple excited states
For ttpy-C6H4NO2-4, as will be discussed later, the
3
emission probably emanates from the π,π* state localized
at the nitrophenylacetylide ligand. The multiexponential
decays for this complex are likely due to excimer formation
at relatively low concentration (∼5 × 10-5 mol/L), which
is demonstrated by the concentration dependence study of
the emission spectra. As shown in Figure 6, the emission
band exhibits a substantial red shift when the concentration
is increased, changing from ca. 553 nm at a concentration
of 5.2 × 10-6 mol/L to ca. 626 nm when the concentration
of the solution is increased to 5.3 × 10-5 mol/L.
3
3
exist concomitantly, which could be dxz-π*, dyz-π*,
3
3
intraligand π-π*, and/or LLCT. These states might mix
with each other, as suggested by McMillin and co-workers
for platinum(II) 4′-arylterpyridyl chloride complexes,7 or they
might not mix. In either case, multiexponential decays are
possible. As suggested by Schmehl and Thummel for
ruthenium(II) diimine donor/pyrene acceptor complexes, the
multiexponential decays probably result from either several
of these closely spaced states or luminescence from a single
state with the fast component reflecting the preequilibrium
relaxation of the emitting state and the longer decay processes
involving relaxation of the equilibrated states.31
In contrast to ttpy-C6H4NO2-4, no excimer emission was
observed from the rest of the complexes, even at the very
high concentration of ca. 10-3 mol/L. For example, for ttpy-
Quin, the emission band maximum in CH3CN solution
remains the same as the concentration is increased from 10-7
to 10-3 mol/L. However, the intensity of the emission
increases with concentration for c e 1.03 × 10-4 mol/L,
and then at higher concentration, it decreases with increasing
concentration, presumably as a result of self-quenching. A
similar result was obtained for solutions of ttpy-Ph. The
notion of the decreased emission intensity at higher concen-
trations being due to self-quenching is confirmed by emission
lifetime measurements. For example, the lifetime for ttpy-
Ph is 780 ns at c ) 1 × 10-5 mol/L, 740 ns at c ) 5 × 10-5
mol/L, and 476 ns at c ) 5 × 10-4 mol/L, and the lifetime
decreases to 397 ns when the concentration is increased to
1 × 10-3 mol/L. The self-quenching constant obtained from
the slope of the kobs versus complex concentration plot is
1.3 × 109 M-1 s-1. For ttpy-C6H4OCH3-4/CH3CN and ttpy-
C6H4N(CH3)2-4/CH3CN solutions, the self-quenching occurs
at concentrations greater than 1.3 × 10-5 mol/L. Self-
quenching has also been observed in the mono platinum(II)
complexes of 6-phenyl-2,2′-bipyridine by Che and co-
workers34 and in (diimine)Pt(-CtC-Ar)2 complexes by
Eisenberg and co-workers.13 The bimolecular self-quenching
rate constants were reported to be on the order of 109 M-1
s-1.13,34
The presence of closely spaced excited states could also
be accountable for the low emission quantum yields for ttpy-
C6H4OCH3-4 and ttpy-C6H4N(CH3)2-4. As was proposed
previously for the donor-acceptor-substituted complex
[(bpy)ReI(CO)3DMABA]+ (DMABA ) dimethylaminoben-
zonitrile), MLCT emission can be strongly quenched via the
efficient LLCT radiationless decay pathway in polar solvents
because of the significantly increased MLCT f LLCT
interconversion rate.32,33
Figure 5 displays the time-resolved emission spectra for
ttpy-Np. It is clear that the decay rates vary at different
wavelengths. Although such wavelength dependence is also
consistent with impurities, this assignment for the multiex-
ponential behavior is not favored because similar emission
spectra are obtained when the complex is excited at different
excitation wavelengths and the excitation spectra monitored
at different emission wavelengths remain the same. This
suggests that no emissive impurities are present in the
solution.
(31) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012.
(32) Perkins, T. A.; Pourreau, D. B.; Netzel, T. L.; Schanze, K. J. Phys.
Chem. 1989, 93, 4511.
(33) Perkins, T. A.; Humer, W.; Netzel, T. L.; Schanze, K. J. Phys. Chem.
1990, 94, 2229.
(34) Lai, S.-W.; Chan, M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M.
Inorg. Chem. 1999, 38, 4046.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4061

Guo et al.
Figure 7. Emission spectra of ttpy-Ar in butyronitrile solution (glass) at
77 K. Excitation wavelength ) 436 nm, concentration ≈ 10-4 mol/L.
Figure 8. (a) Concentration-dependent emission spectra of ttpy-Ph‚Cl in
methanol/ethanol glass at 77 K. Excitation wavelength ) 436 nm. (b)
Excitation spectra of 1.26 × 10-4 mol/L methanol/ethanol glass of ttpy-
Ph‚Cl monitored at different emission wavelengths.
It is noteworthy that, in contrast to the very small influence
of the 4′-tolyl substituent on the electronic absorption spectra,
there appears to be a pronounced effect in emission upon
introduction of the 4′-tolyl substituent on the terpyridyl
ligand. For example, the ttpy-C6H4OCH3-4 and ttpy-C6H4N-
(CH3)2-4 complexes exhibit detectable emission in solution,
whereas the corresponding terpyridyl complexes without the
4′-tolyl substituent do not.6,24
ttpy-C6H4NO2-4 complex but not in the diimine platinum
bis-4′-nitrophenylacetylide complex are due to the formation
of ground-state aggregates at the concentration used for the
measurement, as discussed further below.
The low-energy peaks at ca. 680 nm for ttpy-Ph, 618 and
700 nm for ttpy-C6H4NO2-4, and 688 nm for ttpy-Quin
exhibit a concentration dependence, as exemplified by ttpy-
Ph in Figure 8a. This feature suggests that an excimer or
ground-state complex is responsible for these low-energy
bands. Che and co-workers ascribed the structureless low-
energy band at high concentration at 77 K for platinum(II)
The luminescence spectra of these complexes in butyroni-
trile at 77 K are presented in Figure 7. At 77 K, the emission
bands shift to higher energy and exhibit a clear vibronic
structure. For ttpy-Ph, the major band appears at ca. 545 nm
with a shoulder near 588 nm. This is consistent with the
previous spectroscopic studies on similar platinum(II) terpy-
ridyl complexes.5,6,8 The spacing of the vibronic progression
(∼1340 cm-1) corresponds to the aromatic vibrational mode
of the terpyridyl ligand.6 Because the rigidochromic shift (i.e.,
large thermally induced Stokes shift ∆Es ≈ 1820 cm-1) is a
characteristic of the charge-transfer emission, we attribute
the emitting state at 77 K as the 3MLCT state. The 3MLCT
assignment for low-temperature emission has been suggested
by Eisenberg and co-workers for diimine bis(acetylide)
complexes.13 This assignment also applies to the other
complexes except for ttpy-C6H4NO2-4. For ttpy-C6H4NO2-
4, the lack of a significant thermally induced Stokes shift
(∆Es ≈ 752 cm-1) indicates that the 77 K emission does not
originate from an MLCT manifold. Rather, it probably arises
3
2,6-diphenylpyridine complexes to the formation of π,π*
excimer.35 However, this assignment does not apply to the
aforementioned complexes. A study of the excitation spectra
of ttpy-Ph, ttpy-C6H4NO2-4, and ttpy-Quin at different
emission wavelengths shows that the excitation spectrum
monitored at the low-energy band is different from those
monitored at the high-energy structured bands, as exemplified
in Figure 8b for the ttpy-Ph complex. With reference to the
work reported by Yam and co-workers for the platinum(II)
2,6-biphenylpyridyl complexes,36 the broad low-energy band-
(s) at 77 K originate(s) from an emissive state resulting from
ground-state aggregation of the complexes. The appearance
3
from an intraligand π,π* state that involves the nitrophe-
(35) Lu, W.; Chan, M. C.; Cheung, K.-K.; Che, C.-M. Organometallics
2001, 20, 2477.
(36) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Lu, X.-X.; Cheung,
K.-K.; Zhu, N. Chem. Eur. J. 2002, 8, 4066.
nylacetylide ligand, as proposed by Schanze and co-workers
for the diimine platinum bis-4′-nitrophenylacetylide com-
plex.23 The two low-energy emission bands observed in the
4062 Inorganic Chemistry, Vol. 44, No. 11, 2005

Platinum(II) 4′-Tolylterpyridyl Arylacetylide Complexes
Figure 9. Transient absorption difference spectra of ttpy-Ar in argon-degassed acetonitrile solution at room temperature following 355-nm excitation. The
time increments are 160 ns for ttpy-Ph, ttpy-C6H4Br-4, and ttpy-Quin and 40 ns for ttpy-C6H4NO2-4 beginning immediately after the laser pulse. Arrows
indicate the direction of change of ∆A with increasing time.
of the low-energy emission band(s) at 77 K for glasses but
not at room temperature for solutions at similar concentra-
tions indicates that the association constant for the aggrega-
tion process could be too small to be observed at room
temperature. This hypothesis was further supported by the
electronic absorption spectra measurement at different con-
centrations. No bathochromic shift or peak broadening was
observed for ttpy-Ph from the concentration of 10-6 to 10-4
mol/L. Unfortunately, the measurement of the concentration-
dependent electronic absorption spectra at 77 K could not
be carried out because of our current instrument limitations.
Transient Absorption (TA) Spectroscopy. The time-
resolved transient difference absorption spectra are illustrated
in Figure 9 for complexes ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4-
NO2-4, and ttpy-Quin. Transient absorption from complexes
ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np was too
weak to be observed. The TA spectra of ttpy-Ph and ttpy-
C6H4Br-4 are similar, with each spectrum featuring a
moderately intense, narrow absorption band in the near-UV
region (λmax ≈ 390 nm); a bleaching band from 430 to 500
nm; and a broad, strong absorption throughout the visible
region and extending into the near-IR region. The bleaching
band occurs in the region corresponding to the MLCT
transition, and this feature is consistent with the excited state
having MLCT character. In addition, τTA coincides with τem,
indicating that the transient absorption arises from the same
excited state that is responsible for the emission, i.e., from
is present in the MLCT state, which is in line with the TA
absorptions of diimine platinum(II) bis-acetylide complexes.23
The MLCT assignment is further supported by the fact that
the shapes of these TA spectra are very similar to those of
the ruthenium dinuclear complexes (ttp)Ru(tpy-tpy)Ru(ttp)4+
and (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, which are known to have
lowest excited states based on the metal-to-bridged ligand
charge-transfer transition.37 For ttpy-Quin, the bleaching band
is not as obvious as those for ttpy-Ph and ttpy-C6H4Br-4.
Nevertheless, the appearance of a moderately intense, broad
absorption band in the visible to near-IR region and the
coincidence of τTA with τem suggest that the transients might
also represent the 3MLCT excited-state complex. On the other
hand, the similarity of the shape of the spectrum in the visible
to near-IR region to that of the diimine platinum bis-4′-
nitrophenylacetylide complex (complex 2b)23 and the ap-
pearance of the bleaching band in the UV region also indicate
that the excited state involves the acetylide ligands, i.e., the
lowest excited state for TA absorption involves a 3π,π* state
that is localized on the quinolinyl acetylide ligand. This is
particularly true for the ttpy-C6H4NO2-4 complex, in which
3
even the emitting state is dominated by the π,π* state
localized on the nitrophenylacetylide ligand as described
earlier.
Optical Limiting of Nanosecond Laser Pulses. It is well-
known that reverse saturable absorption can lead to optical
limiting, which reduces the transmittance when the laser
intensity is higher than the limiting threshold. One criterion
3
a state that exhibits MLCT character. Alternatively, the
absorbing transient might be in equilibrium with the emitting
excited state. The transient absorption in the visible to near-
IR region likely results from the terpyridyl anion radical that
(37) Hammarstro¨m, L.; Barigelletti, F.; Flamigni, L.; Indelli, M. T.;
Armaroli, N.; Calogero, G.; Guardigli, M.; Sour, A.; Collin, J.-P.;
Sauvage, J.-P. J. Phys. Chem. A 1997, 101, 9061.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4063

Guo et al.
for reverse saturable absorption to occur is that the excited-
state absorption cross section must exceed the ground-state
absorption cross section. The higher the ratio of the cross
section of the excited-state absorption to that of the ground-
state absorption, the stronger the reverse saturable absorption.
To increase this ratio, one can either decrease the ground-
state absorption or increase the excited-state absorption. In
addition, optical limiting of nanosecond laser pulses is
generally dominated by the triplet excited-state absorption
when the intersystem crossing time is shorter than the laser
pulse width. It has been reported that the intersystem crossing
time for a platinum ethynyl complex is ∼330 ps.38 It is
reasonable to assume that the intersystem crossing rate is
also rapid for platinum(II) terpyridyl complexes. This
hypothesis is supported by the fact that the emission of
platinum(II) terpyridyl complexes originates from the triplet
excited state, as discussed in the Photoluminescence section.
Thus, the optical limiting of nanosecond laser pulses by
platinum(II) terpyridyl complexes should be dominated by
the triplet excited-state absorption. In this case, a long
lifetime, high quantum yield, and large absorption cross
section for the triplet excited state would be expected to
enhance the optical limiting of nanosecond laser pulses.
As shown in Figure 1, the absorption spectra of the
complexes studied exhibit weak or no ground-state absorption
above 500 nm, whereas the transient difference absorption
spectra of ttpy-Ph, ttpy-C6H4Br-4, and ttpy-Quin, shown in
Figure 9, exhibit a broad and moderately intense absorption
band in this spectral region. This indicates that reverse
saturable absorption could occur throughout the visible to
near-IR region. In addition, as discussed earlier in the
Photoluminescence section and shown in Table 1, the triplet
excited-state lifetimes of ttpy-Ph, ttpy-C6H4Br-4, and ttpy-
Quin complexes are quite long. The quantum yields of the
triplet excited states were measured by laser-induced optoa-
coustic spectroscopy (LIOAS) as described previously39 and
were found to be 88 ( 7% for ttpy-Ph, 77 ( 8% for ttpy-
C6H4Br-4, and 60 ( 5% for ttpy-Quin. Such high quantum
yields are the result of the heavy-metal-enhanced spin-
orbital coupling effect. These characteristics combined with
the broadband reverse saturable absorption suggest that these
complexes are promising candidates for broadband optical
limiting applications.
Figure 10. Optical limiting of platinum(II) 4′-tolylterpyridyl arylacetylide
complexes in acetonitrile in a 2-mm cell. The linear transmission for each
of the solutions is 70%.
Table 2. Optical Limiting Parameters of the Platinum(II)
4′-Tolylterpyridyl Arylacetylide Complexes in Acetonitrile Solution
a
σ0
Fthb (J/cm2)
(at T/T0 ) 0.9)
complex
ttpy-Ph
ttpy-C6H4Br-4
ttpy-Quin
(cm2)
Tlim
σeff/σg
c
2.43 × 10-19
7.18 × 10-19
1.12 × 10-18
0.048
0.144
1.09
0.28
0.34
0.44
>3.57
>3.02
>2.30
a Ground-state absorption cross section at 532 nm. b Optical limiting
threshold when the transmittance drops to 90% of the linear transmittance.
c Nonlinear transmittance at incident fluence of 2.5 J/cm2.
at the incident fluence of 2.5 J/cm2. The strength of the
optical limiting appears to be in the order ttpy-Ph > ttpy-
C6H4Br-4 > ttpy-Quin. The stronger optical limiting of ttpy-
Ph can be attributed to its higher triplet excited-state quantum
yield.
To quantitatively compare the strength of optical limiting
abilities of these complexes, the figure of merit σeff/σg is used,
where σeff is the effective excited-state absorption cross
section and σg is the ground-state absorption cross section.
At the incident fluence of 2.5 J/cm2, which is much higher
than the saturable fluence defined as Fsat ) hν/σgΦt,40 the
ground state is greatly bleached, and the excited-state
population is mainly distributed at the triplet state. In this
case, σeff/σg can be defined as σeff/σg ) (ln Tsat)/(ln Tlin),40
where Tsat is the transmittance at the saturable fluence and
Tlin is the linear transmittance. However, the damage
threshold of the quartz cell (5 J/cm2) limited the maximum
incident fluence that could be used in our experiments;
therefore, we were unable to reach the saturable transmit-
tance. Nevertheless, the lowest transmittance at the highest
incident fluence can be used to calculate a lower bound to
σeff/σg for these complexes. The results are listed in Table
2.
To demonstrate this possibility, a study of the the optical
limiting of ttpy-Ph, ttpy-C6H4Br-4, and ttpy-Quin for 4.1-ns
laser pulses was conducted, and the results are shown in
Figure 10. It is obvious that the curves for these samples
deviate significantly from the linear absorption line, sug-
gesting the occurrence of optical limiting. The limiting
threshold at T/T0 ) 0.9 and the nonlinear transmittance at
incident fluence of 2.5 J/cm2 are listed in Table 2. As can
be seen from Figure 10 and Table 2, the optical limiting
performances of these complexes are influenced dramatically
by the nature of the chemical structures. ttpy-Ph yields the
strongest optical limiting, with transmittance dropping to 28%
In addition to the excellent optical limiting performance,
these three complexes show exceptional thermal and pho-
tochemical stability in the solid and in CH3CN solution. After
2 years of storage in air and exposure to UV and visible
laser light radiations, the complexes remain unchanged.
(38) McKay, T. J.; Bolger, J. A.; Staromlynska, J.; Davy, J. R. J. Chem.
Phys. 1998, 108, 5537.
(39) Walters, K. A.; Schanze, K. S. Spectrum 1998, 11 (2), 1.
(40) Perry, J. W.; Mansour, K.; Marder, S. R.; Perry, K. J.; Alvarez, D.
Jr.; Choong, I. Opt. Lett. 1994, 19, 625.
4064 Inorganic Chemistry, Vol. 44, No. 11, 2005

Platinum(II) 4′-Tolylterpyridyl Arylacetylide Complexes
Conclusions
exhibit no transient absorption. Moreover, because of the
high quantum yield and the relatively long lifetime of the
triplet excited state, as well as the broad, moderately intense
triplet excited-state absorption in the visible spectral range,
ttpy-Ph, ttpy-C6H4Br-4, and ttpy-Quin have been demon-
strated to exhibit significant optical limiting for nanosecond
laser pulses at 532 nm. They are promising candidates for
broadband optical limiting of nanosecond laser pulses.
Platinum(II) 4′-tolylterpyridyl arylacetylide complexes
exhibit a low-energy solvatochromic charge-transfer transi-
tion (1MLCT or/and LLCT) in their electronic absorption
spectra that is sensitive to the para substituents on the
phenylacetylide ligand. Except for ttpy-C6H4NO2-4, which
likely emits from an IL π,π* state involving the nitrophe-
nylacetylide ligand, the emission of the other complexes
emanates from the MLCT state. Complexes containing
electron-rich arylacetylide ligands, i.e., ttpy-C6H4OCH3-4,
ttpy-C6H4N(CH3)2-4, and ttpy-Np, exhibit multiexponential
emission decays due to the presence of multiple low-energy
states in close energy proximity. The transient absorption
difference spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-
4, and ttpy-Quin exhibit moderately intense, broad absorption
bands in the visible region and extending into the near-IR
region that might originate from the same excited state that
emits or from a state that is in equilibrium with the emitting
state. The complexes with electron-rich arylacetylide ligands
1
3
3
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for support of
this research. W.S. also acknowledges the North Dakota
EPSCoR for support (EPSCoR Seed Award, Instrumental
Award, and Faculty Start-up Award). K.S. and Y.L. ac-
knowledge the National Science Foundation for support of
their work (CHE- 0211252). We also thank the editor and
the anonymous reviewers for valuable comments and
suggestions.
IC049266N
Inorganic Chemistry, Vol. 44, No. 11, 2005 4065