Influence of alkoxyl substituent on 4,6-diphenyl-2,2'-bipyridine ligand on photophysics of cyclometalated platinum(II) complexes: Admixing intraligand charge transfer character in low-lying excited states

Article abstract of DOI:10.1021/ic801589q

A series of platinum 4,6-diphenyl-2,2'-bipyridine complexes (6-10) with alkoxyl substituent on the 6-phenyl ring have been synthesized and characterized. The influence of the alkoxyl substituent on the nature of the low-lying excited states, and thus the photophysical properties, have been systematically investigated spectroscopically and theoretically. Complexes 6-10 exhibit a broad low-energy charge-transfer absorption band from 400 to 500 nm, which shows weak negative solvatochromic effect. They all emit at about 590 nm in fluid solutions at room temperature, with the emission energy essentially independent of the nature of the monodentate ligand and the polarity of solvent. The excited-state lifetimes of 6 and 7 are much longer (460-570 ns) than those of their corresponding "alkoxyl free" analogues 12 and 13 (40-100 ns) in CH3CN. Additionally, the emission quantum yields of 7-9 in CH₂Cl₂ are quite high (0.15-0.21). Spectroscopic studies and Time-Dependent Density Functional Theory (TDDFT) calculations indicate that these unique photophysical properties are induced by the electrondonating ability of the alkoxyl substituent, which causes a mixture of the intraligand charge transfer (ILCT) with the metal-to-ligand charge transfer (MLCT)/ligand-to-ligand charge transfer (LLCT) in their low-lying excited states. Complexes 6-10 exhibit broad triplet transient difference absorption in the near-UV to the near-IR region, where reverse saturable absorption (RSA) could occur. Nonlinear absorption experiments at 532 nm for nanosecond laser pulses demonstrate that 6-9 are strong reverse saturable absorbers, while 10 exhibits weak RSA because of its larger ground-state absorption cross-section and its low triplet excited-state quantum yield.

Full text of DOI:10.1021/ic801589q

Inorg. Chem. 2009, 48, 2407-2419
Influence of Alkoxyl Substituent on 4,6-Diphenyl-2,2′-bipyridine Ligand
on Photophysics of Cyclometalated Platinum(II) Complexes: Admixing
Intraligand Charge Transfer Character in Low-Lying Excited States
Pin Shao,† Yunjing Li,† Alexander Azenkeng,‡ Mark R. Hoffmann,§ and Wenfang Sun*,†
Department of Chemistry and Molecular Biology, North Dakota State UniVersity,
Fargo, North Dakota 58105-5516, Energy and EnVironmental Research Center, UniVersity of
North Dakota, Grand Forks, North Dakota 58202-9018, and Department of Chemistry, UniVersity
of North Dakota, Grand Forks, North Dakota 58202-9024
Received August 20, 2008
A series of platinum 4,6-diphenyl-2,2′-bipyridine complexes (6-10) with alkoxyl substituent on the 6-phenyl ring
have been synthesized and characterized. The influence of the alkoxyl substituent on the nature of the low-lying
excited states, and thus the photophysical properties, have been systematically investigated spectroscopically and
theoretically. Complexes 6-10 exhibit a broad low-energy charge-transfer absorption band from 400 to 500 nm,
which shows weak negative solvatochromic effect. They all emit at about 590 nm in fluid solutions at room
temperature, with the emission energy essentially independent of the nature of the monodentate ligand and the
polarity of solvent. The excited-state lifetimes of 6 and 7 are much longer (∼460-570 ns) than those of their
corresponding “alkoxyl free” analogues 12 and 13 (∼40-100 ns) in CH3CN. Additionally, the emission quantum
yields of 7-9 in CH2Cl2 are quite high (0.15-0.21). Spectroscopic studies and Time-Dependent Density Functional
Theory (TDDFT) calculations indicate that these unique photophysical properties are induced by the electron-
donating ability of the alkoxyl substituent, which causes a mixture of the intraligand charge transfer (ILCT) with the
metal-to-ligand charge transfer (MLCT)/ligand-to-ligand charge transfer (LLCT) in their low-lying excited states.
Complexes 6-10 exhibit broad triplet transient difference absorption in the near-UV to the near-IR region, where
reverse saturable absorption (RSA) could occur. Nonlinear absorption experiments at 532 nm for nanosecond
laser pulses demonstrate that 6-9 are strong reverse saturable absorbers, while 10 exhibits weak RSA because
of its larger ground-state absorption cross-section and its low triplet excited-state quantum yield.
Introduction
is usually reflected in the low-energy absorption band of their
UV-vis absorption spectra and in their emission spectra.
The versatile spectroscopic properties of these complexes
In recent years, a variety of platinum(II) complexes with
polypyridine ligands have been synthesized, and their
spectroscopic properties have been extensively studied.1-9
The square-planar configuration of these complexes could
facilitate electron delocalization and thus lead to various
intramolecular charge transfers, such as metal-to-ligand
charge transfer (MLCT), ligand-to-ligand charge transfer
(LLCT), or intraligand charge transfer (ILCT). This character
(2) (a) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Che, C. M.; Zhu, N.;
Lee, S. T. J. Am. Chem. Soc. 2004, 126, 4958. (b) Yesin, H.; Donges,
D.; Humbs, W.; Strasser, J.; Sitters, R.; Glasbeek, M. Inorg. Chem.
2002, 41, 4915. (c) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich,
P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41,
3055. (d) Shi, J. C.; Chao, H. Y.; Fu, W. F.; Peng, S. M.; Che, C. M.
J. Chem. Soc., Dalton Trans. 2000, 18, 3128. (e) Maestri, M.; Sandrini,
D.; Balzani, V.; Chassot, L.; Jolliet, P.; von Zelewsky, A. Chem. Phys.
Lett. 1985, 122, 375. (f) Wong, W.-Y.; He, Z.; So, S.-K.; Tong, K.-
L.; Lin, Z. Organometallics 2005, 24, 4079.
(3) (a) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida,
M.; Takahashi, Y.; Murata, S.; Arakawa, H. Inorg. Chem. 2001, 40,
5371. (b) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim,
C.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284. (c) Geary,
E. A. M.; Yellowlees, L. J.; Jack, L. A.; Oswald, I. D. H.; Parsons,
S.; Hirata, N.; Durrant, J. R.; Robertson, N. Inorg. Chem. 2005, 44,
242.
*
To whom correspondence should be addressed. E-mail:
wenfang.sun@ndsu.edu.
†
North Dakota State University.
Energy and Environmental Research Center, University of North
‡
Dakota.
§
Department of Chemistry, University of North Dakota.
(1) Eryazici, I.; Moorefield, C. N.; Newkome, G. R. Chem. ReV. 2008,
108, 1834.
10.1021/ic801589q CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 6, 2009 2407
Published on Web 02/18/2009

Shao et al.
enable their potential applications in organic light-emitting
devices,2 dye-sensitized solar cells,3 photocatalysis,4 bio-
sensors,5 and nonlinear optical materials.6
complexes directly affect their nonlinear absorption proper-
ties.6 The broadest and strongest nonlinear absorption
observed so far arises from the platinum(II) 4,6-diphenyl-
2,2′-bipyridine complex.6f However, because of the very
small ground-state absorption cross-sections above 550 nm
for this complex, it is very difficult to sufficiently populate
the excited state unless very high concentration solutions are
used. Unfortunately, the solubility of this complex in
common organic solvents is limited, which prevents its
potential application in photonic devices that require ap-
preciable ground-state absorption. To solve this problem, it
is extremely important to increase the solubility of this
complex while maintaining its broad and strong excited-state
absorption. To realize this goal, a branched alkoxyl sub-
stituent is proposed to be incorporated on the 6-phenyl ring.
It is expected that the alkoxyl substituent would reduce
the intermolecular π,π stacking and therefore increase the
solubility. Moreover, the electron-donating ability of the
alkoxyl substituent could increase the electron density on
the 6-phenyl ring where it is attached, which would in turn
influence the lowest-energy excited-state characteristics and
thus alter the nonlinear absorption of the resultant complexes.
Scheme 1 displays the synthetic procedures and the
structures of the target complexes. Five monodentate ligands
with different π-electron systems (except for chloride ligand)
were used in these complexes to investigate the electronic
effect of the ancillary ligand. All these new platinum
complexes were fully characterized, and their photophysical
properties and nonlinear absorption were systematically
investigated. To understand the effect of the alkoxyl sub-
stituent on photophysical properties of 5-10, photophysics
of their corresponding “alkoxyl free” analogues 11, 12 and
13 (Chart 1) were also investigated. In addition, Density
Functional Theory (DFT) calculations were carried out on
6-8 and 12 with the aim to characterize the highest few
occupied and lowest few unoccupied molecular orbitals, and
Time-Dependent (TD) DFT calculations were performed to
understand the natures of the low-lying excited states, and
to verify and elucidate the observed photophysical charac-
teristics.
In general, the lowest-energy excited states play an
important role in the photophysical properties of the com-
plexes, such as the absorption/emission spectra, emission
quantum yields, lifetimes, and transient difference absorption
(TA). As reported in the literature for most of the platinum(II)
terpyridyl complexes, the low-lying absorption band in the
UV-vis absorption spectrum mainly arises from the metal-
to-ligand charge transfer (MLCT) transition, possibly mixed
with some LLCT character in case of the presence of an
acetylide ancillary ligand.1,7a,10 In these transitions, the lowest
unoccupied molecular orbital (LUMO) has predominant
contribution from the terpyridyl ligand, while the highest
occupied molecular orbital (HOMO) is dominated by the
platinum d-orbital in the case of MLCT transition and/or
centered on the alkynyl ligand for the LLCT transition.
Therefore, structural modification is pivotal for tuning the
excited-state properties. Our previous studies on the nonlinear
absorption of the platinum(II) terdentate complexes have
demonstrated that the excited-state characteristics of the
(4) (a) Zhang, D.; Wu, L. Z.; Zhou, L.; Han, X.; Yang, Q. Z.; Zhang,
L. P.; Tung, C. H. J. Am. Chem. Soc. 2004, 126, 3440. (b) Du, P.;
Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128,
7726. (c) Narayana-Prabhu, R.; Schmehl, R. H. Inorg. Chem. 2006,
45, 4319.
(5) Wong, K. M. C.; Tang, W. S.; Chu, B. W. K.; Zhu, N.; Yam, V. W. W.
Organometallics 2004, 23, 3459.
(6) (a) Guo, F.; Sun, W.; Liu, Y.; Schanze, K. Inorg. Chem. 2005, 44,
4055. (b) Sun, W.; Wu, Z.-X.; Yang, Q.-Z.; Wu, L.-Z.; Tung, C.-H.
Appl. Phys. Lett. 2003, 82, 850. (c) Sun, W.; Guo, F. Chin. Opt. Lett.
2005, S3, S34. (d) Guo, F.; Sun, W. J. Phys. Chem. B 2006, 110 (30),
15029. (e) Pritchett, T. M.; Sun, W.; Guo, F.; Zhang, B.; Ferry, M. J.;
Rogers-Haley, J. E.; Shensky, W., III; Mott, A. G. Opt. Lett. 2008,
33 (10), 1053. (f) Shao, P.; Li, Y.; Sun, W. J. Phys. Chem. A. 2008,
112, 1172. (g) Shao, P.; Li, Y.; Sun, W. Organometallics 2008, 27,
2743. (h) Ji, Z.; Li, Y.; Sun, W. Inorg. Chem. 2008, 47, 7599.
(7) (a) Shikhova, E.; Danilov, E. O.; Kinayyigit, S.; Pomestchenko, I. E.;
Tregubov, A. D.; Camerel, F.; Retailleau, P.; Ziessel, R.; Castellano,
F. N. Inorg. Chem. 2007, 46, 3038. (b) Wong, K. M.-C.; Tang, W.-
S.; Lu, X.-X.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2005, 44, 1492.
(8) (a) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708. (b) Ratilla, E. M. A.; Brothers, H. M., II; Kostic, N. M.
J. Am. Chem. Soc. 1987, 109, 4592. (c) Yip, H. K.; Cheng, L. K.;
Cheung, K. K.; Che, C. M. J. Chem. Soc., Dalton Trans. 1993, 2933.
(d) 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.
(9) (a) Cheung, T. C.; Cheung, K. K.; Peng, S. M.; Che, C. M. J. Chem.
Soc., Dalton Trans. 1996, 1645. (b) Neve, F.; Ghedini, M.; Crispini,
A. J. Chem. Soc., Chem. Commun. 1996, 2463. (c) Lai, S. W.; Chan,
M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C. M. Inorg. Chem. 1999,
38, 4046. (d) Lai, S. W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M.
Organometallics 1999, 18, 3327. (e) Yip, J. H. K.; Suwarno; Vittal,
J. J. Inorg. Chem. 2000, 39, 3537.
(10) (a) Aldridge, T. K.; Stacey, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722. (b) Bu¨chner, R; Field, J. S.; Haines, R. J.; Cunningham, C. T.;
McMillin, D. R. Inorg. Chem. 1997, 36, 3952. (c) Crites, D. K.;
Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta 1998, 273,
346. (d) 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. (e) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg.
Chem. 1999, 38, 4262. (f) Yam, V. W.-W.; Tang, R. P.-L.; Wong,
K. M.-C.; Cheung, K.-K. Organometallics 2001, 20, 4476. (g) Yang,
Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg. Chem.
2002, 41, 5653. (h) Liu, X.-J.; Feng, J.-K.; Meng, J.; Pan, Q.-J.; Ren,
A.-M.; Zhou, X.; Zhang, H.-X. Eur. J. Inorg. Chem. 2005, 1856. (i)
Zhou, X.; Zhang, H.-X.; Pan, Q.-J.; Xia, B.-H.; Tang, A.-C. J. Phys.
Chem. A 2005, 109, 8809. (j) Hua, F.; Kinayyigit, S.; Cable, J. R.;
Castellano, F. N. Inorg. Chem. 2006, 45, 4304.
Experimental Section
Synthesis. Precursors 1, 2 and Pt(DMSO)2Cl2 were synthesized
following the literature procedures.11 The synthesis of 11-13 were
reported previously.6f,12 All the other chemicals and solvents were
purchased from Alfa Aesar and used as is unless otherwise stated.
Column chromatography was carried out using silica gel (Sorbent
Technologies, 60 Å, 230 × 450 mesh) or neutral aluminum oxide
(Sigma-Aldrich, 58 Å, ∼150 mesh).
1H NMR spectra were measured on a Varian 300 or 400 MHz
VNMR spectrometer. Elemental analyses were conducted by
(11) (a) Chang, J. Y.; Nam, S. W.; Hong, C. G.; Im, J.-H.; Kim, J.-H.;
Han, M. J. AdV. Mater. 2001, 13, 1298. (b) Neve, F.; Crispini, A.;
Campagna, S.; Serroni, S. Inorg. Chem. 1999, 38, 2250. (c) Price,
J. H.; Williamson, A. N.; Schramm, R. F.; Wayland, B. B. Inorg.
Chem. 1972, 11, 1280.
(12) Shao, P.; Sun, W. Inorg. Chem. 2007, 46, 8603.
2408 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
Scheme 1. Synthetic Routes and Structures for Complexes 6-10
Chart 1
1
NuMega Resonance Laboratories, Inc. in San Diego, CA. ESI-
HRMS analyses were conducted on a Bruker Daltonics BioTOF
III mass spectrometer.
3a. A mixture of 1a (1.24 g, 3.70 mmol), 2 (1.20 g, 3.70 mmol),
and NH4OAc (2.60 g, 37.70 mmol) in 20 mL of methanol was
refluxed for 6 h. The solvent was removed under reduced pressure,
and the residue was extracted with CH2Cl2. The CH2Cl2 solution
was washed with water, and the solvent was then removed. The
crude product was purified by passing a silica column with hexane/
ethyl acetate (v/v: 20/1) used as the eluent. A 0.24 g colorless liquid
was obtained (yield: 15%). H NMR (CDCl3) δ: 8.68-8.72 (m,
2H), 8.60 (d, J ) 0.80 Hz, 1H), 8.16 (d, J ) 8.40 Hz, 2H), 8.17
(d, J ) 1.20 Hz, 1H), 7.81-7.86 (m, 3H), 7.43-7.52 (m, 3H),
7.30-7.32 (m, 1H), 7.05 (d, J ) 8.80 Hz, 2H), 3.93 (d, J ) 5.60
Hz, 2H), 1.75-1.81 (m, 1H), 1.28-1.63 (m, 8H), 0.88-0.99 (m,
6H).
3b. The synthetic procedure was similar to that for 3a except
1b instead of 1a was used. 3b was obtained as white solid with a
yield of 69%. 1H NMR (CDCl3) δ: 8.74 (m, 1H), 8.68 (d, J ) 7.80
Hz, 1H), 8.41 (d, J ) 1.50 Hz, 1H), 8.13 (d, J ) 9.00 Hz, 2H),
Inorganic Chemistry, Vol. 48, No. 6, 2009 2409

Shao et al.
7.91 (td, J ) 7.80 and 1.50 Hz, 1H), 7.86 (d, J ) 1.50 Hz, 1H),
7.59 (d, J ) 8.40 Hz, 2H), 7.39 (m, 1H), 7.04 (d, J ) 9.00 Hz,
2H), 6.95 (d, J ) 9.00 Hz, 2H), 6.84 (s, 1H), 3.93 (d, J ) 6.00 Hz,
2H), 1.77 (m, 1H), 1.47 (m, 4H), 1.36 (m, 4H), 0.95 (t, J ) 7.80
Hz, 3H).
) 8.40 Hz, 1H), 6.58 (dd, J ) 2.70 and 8.40 Hz, 1H), 3.94 (d, J
) 6.30 Hz, 2H), 2.67 (t, J ) 6.90 Hz, 2H), 1.69-1.76 (m, 3H),
1.35-1.53 (m, 8H), 1.15 (t, J ) 7.20 Hz, 3H), 0.95 (t, J ) 0.75
Hz, 6H). ESI-MS: m/z calcd for [C35H38N2O195Pt-H]+, 696.2551;
found, 696.2633. Anal. Calcd for C35H38N2OPt: C, 60.25; H, 5.49;
N, 4.01. Found: C, 59.95; H, 5.48; N, 4.01.
4. 3b (2.26 g, 5.00 mmol) was dissolved in 10 mL of dry pyridine
at 0 °C, and (F3CSO2)2O (2.10 g, 7.50 mmol) was added slowly.
The mixture was stirred at 0 °C for 30 min and then at room
temperature for 2 days. The resultant mixture was poured into 100
mL of ice water, and the solid was collected by filtration. The crude
product was dissolved in hexane, and the undissolved solid was
removed by filtration. Removal of hexane from the filtrate yielded
8. A mixture of phenylacetylene (15.0 µL, 0.14 mmol) and KOH
(9.0 mg, 0.15 mmol) in 10 mL of MeOH and 25 mL of CH2Cl2
was purged with argon for 20 min, and then 6 (46.0 mg, 0.07 mmol)
and CuI (3.0 mg, 0.016 mmol) were added. The mixture was stirred
at room temperature for 24 h under argon protection. The solvents
were removed, and the crude product was purified by aluminum
oxide column with CH2Cl2 as the eluent. A 32.0 mg quantity of
yellow solid was obtained (yield: 71%). 1H NMR (CDCl3) δ: 9.21
(d, J ) 4.80 Hz, 1H), 8.01 (t, J ) 8.00 Hz, 1H), 7.93 (d, J ) 8.00
Hz, 1H), 7.67-7.69 (m, 2H), 7.62 (s, 1H), 7.49-7.57 (m, 7H),
7.33 (d, J ) 8.00 Hz, 1H), 7.23-7.25 (m, 3H), 7.16 (t, J ) 7.60
Hz, 1H), 6.58 (dd, J ) 2.80 and 8.40 Hz, 1H), 3.91 (d, J ) 6.00
Hz, 2H), 1.69-1.74 (m, 1H), 1.29-1.49 (m, 8H), 0.86-0.94 (m,
6H). ESI-MS: m/z calcd for [C38H36N2O195Pt + H]+, 732.2552;
found, 732.2557. Anal. Calcd for C38H36N2OPt: C, 62.37; H, 4.96;
N, 3.83. Found: C, 61.99; H, 4.84; N, 3.84.
1
2.54 g of white solid (yield: 87%). H NMR (CDCl3) δ: 8.68 (m,
2H), 8.52 (s, 1H), 8.14 (d, J ) 9.00 Hz, 2H), 7.84 (m, 4H), 7.40
(d, J ) 9.00 Hz, 2H), 7.34 (m, 1H), 7.05 (d, J ) 8.40 Hz, 2H),
3.93 (d, J ) 5.70 Hz, 2H), 1.78 (m, 1H), 1.48 (m, 4H), 1.37 (m,
4H), 0.97 (t, J ) 7.50 Hz, 3H).
5. A mixture of 4 (1.17 g, 2.00 mmol), Pd(PPh3)2Cl2 (82.0 mg,
0.12 mmol), CuI (14.0 mg, 0.07 mmol), and (trimethylsily)acetylene
(0.54 mL, 3.90 mmol) in 1.4 mL of freshly distilled NEt3 and 4
mL of N,N-dimethylformamide (DMF) was stirred at 55 °C for
24 h under argon protection. After the reaction, the mixture was
poured into 100 mL of water and extracted with CH2Cl2 three times.
The CH2Cl2 layer was washed with brine (30 mL × 3) and dried
over anhydrous MgSO4. Trimethylsily protected intermediate was
obtained after passing a silica column eluted with hexane/ethyl
acetate (v/v: 10/1). The white intermediate (266 mg, 0.50 mmol)
was dissolved in 50 mL of tetrahydrofuran (THF), and then 1.5
mL of 10% NaOH aqueous solution and 50 mL of MeOH were
added. The mixture was stirred for 10 min and then neutralized
with 1.0 M HCl. The organic solvents were removed, and the
residue was extracted with CH2Cl2. The CH2Cl2 solution was
washed with water and dried over Na2SO4. After removal of the
9. A mixture of KOH (28.0 mg, 0.50 mmol), 6 (230.0 mg, 0.35
mmol), 5 (160.0 mg, 0.35 mmol), and CuI (3.0 mg, 0.016 mmol)
in MeOH/CH2Cl2 was stirred at room temperature for 24 h under
the protection of argon. The solvent was removed under reduced
pressure, and the crude product was purified by passing a short
aluminum oxide column with CH2Cl2 as the eluent and recrystalized
by diffusing ether into dilute CH2Cl2 solution. A 246 mg quantity
1
of red solid was obtained (yield: 64%). H NMR (CDCl3) δ: 9.10
(d, J ) 5.00 Hz, 1H), 8.76 (dd, J ) 4.50 and 1.00 Hz, 1H), 8.72
(d, J ) 8.00 Hz, 1H), 8.66 (d, J ) 1.00 Hz, 1H), 8.20 (d, J ) 9.00
Hz, 2H), 7.99 (d, J ) 1.00 Hz, 1H), 7.97 (d, J ) 7.50 Hz, 1H),
7.88-7.92 (m, 2H), 7.81 (d, J ) 8.00 Hz, 2H), 7.73-7.75 (m,
2H), 7.69 (d, J ) 7.50 Hz, 2H), 7.61 (s, 1H), 7.51-7.52 (m, 4H),
7.45 (s, 1H), 7.42-7.44 (m, 1H), 7.35-7.38 (m, 1H), 7.30 (s, 1H),
7.08 (d, J ) 8.50 Hz, 2H), 6.60 (dd, J ) 2.50 and 8.50 Hz, 1H),
3.97 (d, J ) 6.00 Hz, 2H), 3.92 (d, J ) 5.50 Hz, 2H), 1.77-1.82
(m, 2H), 1.37-1.60 (m, 16H), 0.92-1.01 (m, 12H). ESI-MS: m/z
calcd for [C62H62N4O2195Pt + H]+, 1090.4598; found, 1090.4568.
Anal. Calcd for C62H62N4O2Pt: C, 68.30; H, 5.73; N, 5.14. Found:
C, 68.31; H, 6.12; N, 5.32.
1
solvent, 0.67 g white solid was obtained (yield: 73%). H NMR
(CDCl3) δ: 8.69 (d, J ) 4.40 Hz, 1H), 8.65 (d, J ) 8.00 Hz, 1H),
8.55 (d, J ) 1.20 Hz, 1H), 8.13 (d, J ) 8.40 Hz, 2H), 7.86 (d, J )
1.20 Hz, 1H), 7.82-7.85 (m, 1H), 7.76 (d, J ) 8.00 Hz, 2H), 7.61
(d, J ) 8.00 Hz, 2H), 7.29-7.32 (m, 1H), 7.03 (d, J ) 8.40 Hz,
2H), 3.92 (d, J ) 5.60 Hz, 2H), 3.17 (s, 1H), 1.75-1.78 (m, 1H),
1.33-1.54 (m, 8H), 0.90-0.97 (m, 6H).
6. K2PtCl4 (0.22 g, 0.54 mmol) and ligand 3a (0.24 g, 0.54 mmol)
was refluxed in 50 mL of acetic acid for 24 h. The resultant orange
solid was collected by filtration, and washed with water. The crude
product was recrystalized from CH2Cl2/ether to obtain 0.30 g of
orange solid (yield: 83%). 1H NMR (CDCl3) δ: 8.96 (s, 1H),
7.91-7.99 (m, 2H), 7.70 (br. s, 2H), 7.53 (d, J ) 6.00 Hz, 6H),
7.38 (s, 1H), 7.16 (s, 1H), 6.58 (d, J ) 11.2 Hz, 1H), 3.84 (br. s,
2H), 1.69 (br., 1H), 1.34-1.51 (m, 8H), 0.94 (t, J ) 9.20 Hz, 6H).
ESI-MS: m/z calcd for [C30H31N2O195Pt + CH3CN]+, 671.2347;
found, 671.2364. Anal. Calcd for C30H31ClN2OPt: C, 54.09; H, 4.65;
N, 4.21. Found: C, 54.36; H, 4.25; N, 4.17.
10. A mixture of 9 (170.0 mg, 0.16 mmol) and Pt(DMSO)2Cl2
(68.0 mg, 0.16 mmol) in 15 mL of CHCl3 was refluxed for 24 h.
The solvent was removed by rotator evaporation, and the crude
product was purified by passing a short aluminum oxide column
eluted with CH2Cl2, and then further purified by diffusing ether
into dilute CH2Cl2 solution to yield 100 mg of a red solid (yield:
1
47%). H NMR (CDCl3) δ: 8.85 (br. s, 2H), 7.87-7.92 (m, 4H),
7.67 (s, 4H), 7.49 (s, 8H), 7.40 (br., 2H), 7.31 (s, 2H), 7.18 (d, J
) 8.80 Hz, 2H), 7.10 (s, 1H), 6.53 (d, J ) 8.40 Hz, 2H), 3.80 (br.,
4H), 1.66 (m, 2H), 1.32-1.55 (m, 16H), 0.91-0.93 (m, 12H). Anal.
Calcd for C62H61ClN4O2Pt2: C, 56.42; H, 4.66; N, 4.25. Found: C,
56.04; H, 4.26; N, 4.30.
Photophysical Measurements. The UV-vis absorption spectra
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/phosphorometer. The
emission quantum yields were determined by the comparative
method13 in degassed CH2Cl2 and CH3CN solutions, respectively.
A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem ) 0.042, λex
7. A mixture of KOH (15.0 mg, 0.27 mmol), 6 (70.0 mg, 0.11
mmol), pentyne (15.0 µL, 0.15 mmol), and CuI (3.0 mg, 0.016
mmol) in MeOH/CH2Cl2 was stirred at room temperature for 24 h
under argon protection. The solvent was removed under reduced
pressure, and the crude product was purified by passing a short
aluminum oxide column (CH2Cl2 was used as the eluent), and then
recrystalized by diffusing ether into dilute CH2Cl2 solution. A 43.0
1
mg quantity of orange solid was obtained (yield: 56%). H NMR
(CDCl3) δ: 9.17 (d, J ) 5.10 Hz, 1H), 7.98 (td, J ) 1.80 and 7.20
Hz, 1H), 7.90 (d, J ) 7.80 Hz, 1H), 7.70 (d, J ) 4.50 Hz, 1H),
7.67 (d, J ) 1.50 Hz, 1H), 7.59 (d, J ) 1.20 Hz, 1H), 7.56 (d, J )
2.40 Hz, 1H), 7.50-7.54 (m, 3H), 7.44-7.49 (m, 2H), 7.30 (d, J
(13) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
2410 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
lens used to focus the beam to the sample cuvette. The thickness
of the cuvette was 2 mm.
Theoretical Calculations. DFT calculations were performed for
complexes 6-8 and 12 to characterize the highest few occupied
and lowest few unoccupied molecular orbitals, and to understand
the nature of the low-lying excited states. Additional TDDFT
calculations were performed to better characterize the excited states.
A hybrid generalized gradient approximation (hybrid GGA) exchange-
correlation functional was used for all DFT calculations. The
particular hybrid GGA used is known by the acronym MPW3LYP,
which consists of the three-parameter modified Perdew-Wang
(MPW) exchange functional and the Lee-Yang-Parr (LYP)
correlation functional.17 The basis sets used include functions from
the LANL2DZ set18 and from the 6-31+G** set;19 the particular
combination is abbreviated in this work as LANG6. LANL2DZ is
an effective core potential (ECP) basis set and was used to describe
the platinum atom; this provides some correction for relativistic
effects of the platinum atom. The 6-31+G** basis set was used
for descriptions of all other atoms.
Since complex 8 is much larger than complexes 6, 7, and 12,
the smaller 6-31G* basis set19a,b was used for other atoms except
Pt in this complex, while the same LANL2DZ ECP basis set was
used for the Pt atom. This combination of basis sets has been
abbreviated as LANG631. Full geometry optimizations were
performed for complexes 6-8 and 12. Excited singlet electronic
states were calculated using the Time-Dependent Density Functional
Theory (TDDFT) method20 with the MPW3LYP exchange-cor-
relation functional. The Gaussian 03 (Rev. C.02) software suite,21
running on two different computers [SunFire V1280 (OS Solaris
10) and an SGI (OS IRIX64 6.5)], was used to perform all
calculations.
Figure 1. Electronic absorption spectra of 6-10 in CH2Cl2. The inset shows
the normalized low-energy absorption band for 7-9.
) 436 nm) was used as the reference.14 The excited-state lifetimes,
triplet excited-state quantum yields, and the triplet transient
difference absorption spectra were measured in CH3CN solutions
on an Edinburgh LP920 laser flash photolysis spectrometer. The
third harmonic output (355 nm) of a Nd:YAG laser (Quantel
Brilliant, pulsewidth (fwhm) ) 4.1 ns, repetition rate ) 10 Hz)
was used as the excitation source. Each sample was purged with
Ar for 30 min before measurement.
The intrinsic excited-state lifetime (τ0) of the complex and the
emission self-quenching rate constants (kq) in CH2Cl2 were obtained
from the Stern-Volmer equation:
kobs ) kq[C] + k0
(1)
where kobs is the observed emission decay rate constants (kobs
)
1/τem) at a given concentration, kq is the self-quenching rate constant,
[C] is the concentration of the solution, and k0 (k0 ) 1/τ0) is the
decay rate constant of the excited state at infinite dilute solution.
The lifetimes of the complex in a series of solutions with different
concentrations were measured. The obtained kobs’s were plotted
against the solution concentrations, and a straight line was obtained.
The slope of the straight line corresponds to the kq, and the intercept
is the k0.
The molar extinction coefficient of the triplet excited state
(εT1-Tn) and triplet quantum yield (ΦT) were determined by the
partial saturation method.15 The optical density at the absorption
band maximum was monitored when the excitation energy at 355
nm was gradually increased. The following equation was then used
to fit the experimental data to obtain the εT and ΦT.15
Results
Electronic Absorption. The electronic absorption of 6-10
in CH2Cl2 and CH3CN obey Beer-Lambert’s law in the
concentration range of 10-6-10-4 mol/L. As depicted in
Figure 1, all spectra consist of two groups of electronic
transitions. With reference to the reported polypyridine
platinum complexes,1,2a,7-10 the intensive absorption bands
in the UV region (<400 nm) are assigned to the intraligand
π,π* transition, while the broad low-energy (400-550 nm)
band is tentatively assigned to a charge-transfer transition.
The shape and the extinction coefficient of complexes 6-8
are quite similar except that 7 and 8 exhibit stronger and
broader charge-transfer absorption. As will be discussed later
in the discussion section, the broadening of this charge-
transfer band could emanate from the mixture of the LLCT
∆OD ) a(1 - exp(-bIp))
(2)
where ∆OD is the optical density at the corresponding band
maximum, Ip is the pump intensity in Einstein.cm-2, a ) (εT -
ε0)dl, and b ) 2303εe0xΦT/A. εT and ε0 are the absorption coefficients
of the excited state and the ground state at the band maximum, ε0ex
is the ground-state absorption coefficient at the excitation wave-
length 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.
(17) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2004, 108, 6908.
(18) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
(19) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (c)
Clark, T.; Chandrasekhar, J.; Schleyer, P. V. R. J. Comput. Chem.
1983, 4, 294. (d) Krishnam, R.; Binkley, J. S.; Seeger, R.; Pople, J. A.
J. Chem. Phys. 1980, 72, 650. (e) Gill, P. M. W.; Johnson, B. G.;
Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1992, 197, 499.
(20) (a) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett.
1996, 256, 454. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub,
D. R. J. Chem. Phys. 1998, 108, 4439.
Nonlinear Absorption Measurements. The experimental setup
was similar to what had been described previously,16 with a 20 cm
(14) Van Houten, J.; Watts, R. J. Am. Chem. Soc. 1976, 98, 4853.
(15) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(16) Sun, W.; Zhu, H.; Barron, P. M. Chem. Mater. 2006, 18, 2602.
(21) Frisch, M. J. et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2411

Shao et al.
Figure 2. Electronic absorption spectra of 6 and 9 in different solvents.
Figure 3. Comparison of the electronic absorption spectra of 3a and 11, and 7 and 13 in CH2Cl2.
transition into the charge-transfer band. Complex 9, which
possesses a second 4,6-diphenyl-2,2′-bipyridine unit on the
ethynyl ligand, has extinction coefficients that almost double
those for complex 8 in most wavelengths. From 7 to 9, the
charge-transfer band becomes slightly broader (see inset in
Figure 1), and the extinction coefficients gradually increase,
which reflects the increased π-donating ability of the ancillary
ligand. However, for complex 10 that has the second
terdentate ligand coordinated with platinum(II) ion, its
electronic absorption spectrum, especially the charge-transfer
band, is quite different from that of 9. Rather than that it
almost parallels the spectrum of 6, except that its extinction
Figure 4. Normalized emission spectra of complexes 6-10 in degassed
CH2Cl2 solutions at room temperature. Excitation was at the charge-transfer
band maximum for each complex.
coefficients double those of 6 in the UV region, and triple
for the charge-transfer transition.
The charge-transfer bands of 6-10 exhibit some solvent
dependence. As exemplified in Figure 2 for complexes 6 and
9, the charge-transfer band displays some extent of batho-
chromic shift in less polar solvents such as CH2Cl2 in
comparison to that in more polar solvents, such as CH3CN.
This negative solvatochromic effect is also observed in
complexes 7, 8, and 10 (Supporting Information, Figure S1)
and is typical for platinum(II) polypyridine complexes,
including the cyclometallated C∧N∧N platinum complexes
reported by Che’s group.2a,7-10
Another finding from the electronic absorption measure-
ment is the effect of alkoxyl substitution. When comparing
the spectrum of the ligand 3a to that of the “alkoxyl free”
ligand 11 (Figure 3), the former exhibits an obvious
bathochromic shift with somewhat enhanced intensity. A
similar trend was observed in the UV spectral region (<400
nm) of the corresponding platinum complexes, as demon-
strated by complexes 7 and 13 in Figure 3. However, the
energy of the low-energy charge-transfer band is essentially
unaffected by the alkoxyl substitution. The same phenomena
have been observed for 6 and 12 (see Supporting Information,
Figure S2).
Emission. Complexes 6-10 are emissive in fluid solutions
at room temperature. In contrast to the influence of the
ancillary ligand on the electronic absorption spectra, the
effect of these ligands on the emission energy is nominal.
As shown in Figure 4 and listed in Table 1, the emission
energy of 6 and 8-10 appears at about 590 nm. Complex 7
exhibits slightly reduced emission energy at 596 nm in
CH2Cl2. Except for 6 and 10, the quantum yields of 7-9
are on the order of 15-21% (see Table 1), which is much
higher than most of the platinum terdentate complexes
reported in the literature,7-10 and comparable to some of
2412 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
the highly emissive diimine platinum(II) bis(acetylide)
complexes (Φem ) 0.13-0.27) reported by Eisenberg et al.22
and Schanze et al.23 in CH2Cl2. In particular, these values
are 2-3 times higher than that of the corresponding “alkoxyl
free” C∧N∧N platinum phenylacetylide complex (Φem
)
0.07).2a Although the emission efficiency of 6 and 10 is quite
distinct from those of 7-9, the lifetimes deduced from the
decay of the emission are quite similar for 6-10. They are
all on the order of several hundred nanoseconds. Considering
such a long lifetime and the drastic Stokes shift of the
emission, the emission should originate from a triplet excited
state. The lower emission efficiency of 6 and 10 could
emanate from the different degrees of mixture of the 3MLCT
configuration in the emitting state in comparison to those in
7-9, which is evident from the TDDFT calculations and
will be discussed in more details in the discussion section.
Different solvents display a minor effect on the emission
energy. Figure 5 depicted the emission spectra of complexes
6 and 9 in different solvents. A similar phenomenon was
observed for 7, 8, and 10. Although the solvatochromic effect
on the emission energy is insignificant, it is still evident that
less polar solvents, such as CH2Cl2, cause a slight batho-
chromic shift of the emission energy compared to polar
solvents, such as CH3CN. This is in line with the charge-
transfer transition in the electronic absorption spectra,
although the effect is minor. The insensitivity of the emission
energy to the polarity of the solvent is in accord with the
behavior of the corresponding “alkoxyl free” C∧N∧N plati-
num chloride and phenylacetylide complexes reported by
Che’s group,2a,9c but is quite different from the platinum
terpyridyl complexes without electron-donating substituent
on the terpyridyl ligand reported in literature.1,7,8a-c,10 Such
a disparity suggests that the emitting states for 6-10 could
3
be different from the pure MLCT state that is commonly
assigned for most of the platinum terpyridyl complexes.1,7,8,10
The effect of the alkoxyl substitution on emission is
noteworthy for complexes 6, 7 and the ligand 3a in
comparison to their corresponding “alkoxyl free” analogues
12, 13, and 11. As listed in Table 1, the emission energy of
6 in both CH2Cl2 and CH3CN is approximately 20 nm red-
shifted with respect to that of 12. The lifetime of 6 in CH2Cl2
doubles that of 12, while in CH3CN it is more than 1 order
of magnitude longer. The lifetimes of 7 in CH2Cl2 and
CH3CN are also much longer than those of 13 in corre-
sponding solvent. In addition, the solvent quenching effect
on the lifetime is more drastic for 12 and 13 than for 6 and
7. Coordinating solvents like CH3CN reduce the lifetime of
12 and 13 remarkably, that is, approximately 10 times for
12 and 4 times for 13, while the lifetime reduction for 6 and
7 in CH3CN is less than one half. For the analogous ligands
3a and 11, alkoxyl substitution induces an approximately
30 nm bathochromic shift of the emission energy for 3a.
More significantly, 3a exhibits a pronounced solvatochromic
effect, that is, the emission energy gradually decreases with
increased solvent polarity (Figure 6), while 11 shows almost
(22) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355.
(23) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2413

Shao et al.
Figure 5. Normalized emission spectra of complexes 6 and 9 in different solvents. The excitation wavelength was 432 nm for 6 and 431 nm for 9.
attributed to a self-quenching effect (the quenching of the
excited state because of interactions between an excited-state
molecule and a ground-state molecule of the same type;24
in the case of the platinum terdentate complexes, such
interactions could result in nonemissive excimers.), which
is commonly seen in many organometallic complexes
including platinum complexes9c,10d,25 and is confirmed from
the lifetime measurement in different-concentration solutions.
The lifetimes of 6-10 keep decreasing with increasing
concentration. Plotting of the decay rate constant versus
complex concentration results in a straight line for each
complex (see Supporting Information, Figure S3 for complex
8). The slope of the straight line corresponds to the self-
quenching rate constant (kq), which is listed in Table 1 and
is of the order of 109 L mol-1 s-1 for each complex. These
values are in line with those reported for “alkoxyl free”
platinum C∧N∧N complexes (1.6 × 109-5.7 × 109 L mol-1
s-1)2a,9c and platinum terpyridy phenylacetylide complexes
(1.45 × 109-1.74 × 109 L mol-1 s-1).10f
Figure 6. Normalized emission spectra of 3a and 11 in different solvents.
Table 2. Lifetime of 3a in Different Solvents at a Concentration of ∼1
× 10-5 mol/L
solvent
hexane
toluene
CH2Cl2
CH3CN
τ/ns
2.51 ( 0.004 2.76 ( 0.015 2.95 ( 0.015 3.62 ( 0.015
no solvent dependence. In addition, as listed in Table 2, the
lifetime of 3a tended to increase as the polarity of the solvent
increased.
DFT Calculations. TDDFT calculations were carried out
on 6-8 and 12 to provide insight into the effect of the
alkoxyl substitution and explain the ancillary-ligand-
independent and solvent-insensitive emission characteristics
of 6-10. At the least, the TDDFT calculations are expected
to help us to understand the nature of the low-lying excited
electronic states.
The emission energy of 6-10 remains constant in the
concentration range of 10-6 to 10-4 mol/L; however, the
emission intensity decreases at high concentrations. As
exemplified in Figure 7 for complex 9 in CH2Cl2, its
normalized emission spectrum overlaps very well at various
concentrations between 1.1 × 10-6 and 2.8 × 10-4 mol/L.
The emission intensity increases when the concentration
increases from 1.1 × 10-6 to 5.5 × 10-5 mol/L. However,
the intensity decreases dramatically when the concentration
reaches 2.8 × 10-4 mol/L. This phenomenon should be
Figure 8 presents the electron density distribution of the
HOMO, HOMO-1, LUMO and LUMO+1 for complex 6,
while similar information is shown in the Supporting
Information for complexes 7, 8, and 12 (Supporting Informa-
tion, Figure S4). The relative contributions of each moiety
to the corresponding MOs are listed in Table 3. The
calculations show that the LUMO is almost exclusively
π*(bipyridine) for all complexes. In contrast, the composition
of the HOMO is quite different for these complexes. The
HOMO is essentially on the 6-phenyl and the central pyridine
rings (referred to as C∧N component) for complex 6, with
4.5% contribution from the d(Pt). For complex 7, the
dominant contribution is also from the C∧N component, and
(24) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991; p 357.
(25) (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.
Figure 7. Emission spectra of complex 9 in degassed CH2Cl2 solutions at
different concentrations at room temperature.
2414 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
Table 4. Excitation Energies (eV), Wavelengths (nm), Oscillator
Strengths, Dominant Contributing Configuration, and the Associated
Configuration Coefficient of Five Low-Lying Electronic States of
Complex 6 Obtained at the TDDFT/LANG6 Level of Theory
excitation energy
active orbital pair of
configuration
Sn
[eV]
[nm]
f
dominant configuration
coefficient
1
2
3
4
5
2.23
2.52
2.56
2.66
2.93
555
492
484
465
423
0.0016
0.0325
0.0003
0.0007
0.0079
HOMO f LUMO
HOMO-1 f LUMO
HOMO-2 f LUMO
HOMO-4 f LUMO
HOMO f LUMO+1
0.66
0.65
0.67
0.67
0.48
LUMO+1 orbital pair in their dominant configurations
appear at 423 nm (5th excited state) for 6, 454 nm (5th
excited state) for 7, and 463 nm (4th excited state) for 8.
Although the calculated electronic excitation energies (in nm)
to the lowest-lying electronic states for 6-8 show deviations
in the range of 114-173 nm compared to the observed
lowest-energy absorption band maxima, the trend is in line
with the experimental results. The observed differences
between theoretical and experimental results reported in this
work are not entirely surprising because the theoretical
calculations were performed for compounds in the gas-phase,
while the experiments were conducted in solutions. However,
information obtained from the electronic structure of the
molecules combined with the agreement between theory and
experiment in the overall trends of the excitation wavelengths
indicates that the TDDFT calculations can be expected to
provide additional insight into the nature of the lowest excited
states. Such information is useful and aids in the interpreta-
tion of the unusual photophysical properties observed in the
platinum(II) complexes reported in this paper.
Triplet Transient Difference Absorption. The time-
resolved triplet transient difference absorption (TA) spectra
of 6-9 in CH3CN are displayed in Figure 9. All the
complexes exhibit broad positive absorption from 370 to 820
nm. The spectrum of 10 (Supporting Information, Figure S5)
resembles that of 6, and the spectra of 8 and 9 resemble
each other. The TA spectrum of 7 possesses similar features
to those of 6, but with a red-shifted band maximum for the
band between 440 and 600 nm and an enhanced absorption
in the visible spectral region. Specifically, the peak appears
at 468 nm (i.e., 21367 cm-1) for complex 6, but 502 nm
(i.e., 19920 cm-1) for 7. On the other hand, the bands from
440 to 700 nm become broader and flatter for complexes 8
and 9 in comparison to those for 6, 7, and 10.
In keeping with the charge-transfer absorption energy and
the emission energy, the TA absorption energy in the visible
spectral region (440-700 nm) is also influenced notably by
the alkoxyl substitution. The band maximum of the com-
plexes with an alkoxyl substituent is blue-shifted with respect
to those of their corresponding “alkoxyl free” complexes.
In particular, the corresponding visible TA band energy
decreases from 468 nm (i.e., 21367 cm-1) for 6 to 515 nm
(i.e., 19417 cm-1) for 12 in CH3CN (Supporting Information,
Figure S6). For complexes 7 and 13, the shift appears to be
even larger: it is 502 nm (i.e., 19920 cm-1) for 7 but 572
nm (i.e., 17483 cm-1) for 13 in CH3CN. The similar trend
was observed in CH2Cl2 solution as exemplified by 7 and
13 in Figure 10. The TA decay lifetimes for 6-10 are listed
Figure 8. Contour plots of the calculated highest occupied molecular orbital
(HOMO), HOMO-1, lowest unoccupied molecular orbital (LUMO) and
LUMO+1 for complex 6.
Table 3. Relative Percent Contribution of Fragments to the MOs in
Complexes 6-8 and 12
contribution of
complex fragments (%) HOMO-1 HOMO LUMO LUMO+1
6
Pt
37.7
32.0
30.3
4.5
0.0
95.5
0.3
0.0
0.0
0.0
Cl
C∧N
Bipyridine
Pt
99.7
0.4
0.5
100.0
0.0
1.2
7
4.4
15.5
80.0
15.7
19.9
64.4
Pr-Acetylide
C∧N
Bipyridine
Pt
98.8
4.5
0.0
98.8
0.0
0.0
8
29.3
5.6
65.1
28.8
71.2
0.0
Ph-Acetylide
C∧N
Bipyridine
Pt
94.2
0.3
0.0
100.0
0.2
0.0
12
24.8
13.4
61.9
42.4
19.4
38.2
Cl
C∧N
Bipyridine
99.7
99.8
the d(Pt) and π(C≡C) components contribute 16% and 20%,
respectively. However, the HOMO of complex 8 has no
contribution from the C∧N component; the dominant con-
tribution emanates from the π(C≡CPh) (71%) component
and d(Pt) contributes to the remaining 29%. An apparent
trend is noted that with the increased π-donating ability of
the ancillary ligand, the contribution from the C∧N compo-
nent to the HOMO gradually diminishes, while the contribu-
tion from the π(C≡CR) component and the d(Pt) drastically
increase. For “alkoxyl free” complex 12, the π(C∧N) and
the d(Pt) components contribute almost equally (∼40%) to
the HOMO, and the remaining 20% contribution comes from
the Cl ligand. The electron-donating alkoxyl substituent
apparently increases the contribution of the C∧N component
to the HOMO in complex 6.
The excitation energies (eV), wavelengths (nm), oscillator
strengths, and dominant contributing configurations obtained
at the TDDFT/LANG6 level of theory for complexes 6, 7,
8, and 12 are provided in Table 4 and in the Supporting
Information, Table S1, respectively. The calculation results
show that electronic transitions with dominant contributing
configurations involving the HOMOfLUMO orbital pair
occur at 555 nm (1st excited state) for 6, 578 nm (2nd excited
state) for 7, 637 nm (1st excited state) for 8, and 534 nm
(1st excited state) for 12. Transitions involving the HOMOf
Inorganic Chemistry, Vol. 48, No. 6, 2009 2415

Shao et al.
Figure 9. Time-resolved triplet transient difference absorption spectra of complexes 6-9 in CH3CN at room temperature in a 1 cm cuvette. For all complexes,
the linear absorptions at 355 nm were adjusted to 0.4. (λex ) 355 nm).
Figure 11. Transmission vs incident fluence curves for 6-10 and 12 in
Figure 10. Comparison of the TA spectra of 7 and 13 in CH2Cl2 solutions
CH2Cl2 solutions for 4.1 ns laser pulses at 532 nm in a 2 mm cell. The
at zero time delay. The linear absorption at 355 nm was adjusted to 0.4 for
linear transmission was adjusted to 80%.
both samples, and the excitation wavelength was 355 nm.
quartz cuvette, all complexes exhibit significant transmission
decreases with increased incident fluence, which is clearly
in Table 1. Except for 10 that exhibits a much longer lifetime,
the TA lifetimes of 6-9 are all similar to those measured
from the decay of emission.
a characteristic of reverse saturable absorption (RSA).
Although the RSA of 6-9 and 12 are quite similar, a small
difference is still discernible. The degree of RSA follows
this trend: 7 ≈ 6 > 8 ≈ 12 ≈ 9 . 10. The RSA of 6 is
clearly stronger than that of its “alkoxyl free” analogue 12.
Using the partial saturation method, the triplet excited-
state absorption coefficients (εT1-Tn) and the quantum yields
of triplet excited-state formation (ΦT) were measured, and
the results are given in Table 1. Compounds 6-9 show
moderate to strong triplet excited-state absorption coefficients
at about 400 nm, with εT1-Tn varying from 8.2 × 103 to 2.0
× 104 L mol-1 cm-1, while the εT1-Tn for 10 is much smaller.
In line with this trend, ΦT for 10 is quite small, being only
0.06, while the ΦT’s for 6-9 are in the range of 0.11-0.42.
Reverse Saturable Absorption. The nonlinear absorption
measurements were conducted at 532 nm using nanosecond
pulses. Figure 11 shows the results of 6-10 and 12 in CH2Cl2
solutions. With a linear transmission of 80% in a 2 mm thick
Discussion
Nature of the Excited States and the Substituent and
Solvent Effects. For the reported platinum terpyridyl com-
plexes,1,7,10 the ancillary substituent has salient effect on the
electronic structure of the complex. The electron densities
of the LUMOs of these complexes are essentially localized
on the terpyridyl ligand, and those of the HOMOs are
dominated by the d(Pt) and the π(C≡CR) components.10h-j
2416 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
Therefore, the lowest singlet excited state is generally
regarded as a 1MLCT/1LLCT state, and the emitting state is
considered as a 3MLCT state.1,7,10 As a result, the low-lying
1MLCT/1LLCT transition in the electronic absorption spec-
different compositions. For complex 6, the HOMO is almost
exclusively distributed on the C∧N component, with 4.5%
contribution from the d(Pt). For complex 7, the majority
contribution is still from the C∧N component, with the d(Pt)
and the π(C≡C) components contributing 16% and 20%,
respectively. Conversely, the dominant contribution for
complex 8 emanates from the π(C≡CPh) (71%) component
and d(Pt) contributes to the remaining 29%, while the C∧N
component has no contribution. On the basis of the results
of these calculations, the nature of the lowest singlet excited
state can be assigned tentatively to intraligand-charge-transfer
(ILCT, π(C∧N) f π*(bipyridine)) and intraligand π,π*
transitions mixed with a little MLCT (d(Pt) f π*(bipyri-
dine)) character for complex 6. For complex 7, it is a mixture
of ILCT, π,π*, MLCT and LLCT (π(C≡C) f π*(bipyri-
dine)). For complex 8, the lowest excited state likely admixes
LLCT (π(C≡CPh) f π*(bipyridine)) and the MLCT
characters.
Experimental results coincide with these assignments. First,
the involvement of LLCT transition in acetylide containing
complexes is manifested by the broadening and the enhance-
ment of the molar extinction coefficients of the low-energy
absorption band in complexes 7-9 with respect to that of 6,
in which the shoulder of the low-energy absorption band
could be mainly attributed to the LLCT. With the increased
π-donating ability of the acetylide ligand from 7 to 9, the
relative contribution from the LLCT transition increases.
Consequently the relative intensity of the shoulder increases
(see the inset plots in Figure 1). In complex 10, because of
the coordination of the second C∧N∧N ligand with platinum,
the electronic density of the C∧N∧N ligand decreases, which
diminishes the contribution from the π[C≡C(C∧N∧N)] f
π*(bipyridine) LLCT transition. As a result, the profile of
the low-energy absorption band in 10 is quite similar to that
of 6. Second, the experimental result that strongly indicates
the mixture of MLCT configuration in the lowest singlet
excited state is that the low-energy absorption band exhibits
negative solvatochromism, which is typical for the charge
transfer band in platinum terpyridyl complexes with dominant
MLCT character.1,7,8,10 Third, the involvement of the ILCT
and π,π* transitions in the lowest excited states has been
implied by the aforementioned insensitivity of the emissions
of 6-10 to the nature of the ancillary ligand, and can also
be seen from the red-shift and broadening of the low-energy
charge-transfer absorption bands and the bathochromic shift
of the emissions of 7 and 13 in comparison to those of the
corresponding 4′-tolylterpyridyl platinum pentynyl
complex.6f,10g Other experimental evidence supporting the
admixture of the ILCT and π,π* characters into the lowest
excited state includes the minimal solvatochromism of 6-10
and the increased excited-state lifetime and reduced solvent
quenching effect of 6, which will be discussed later. Fourthly,
the emission quantum yields of 6 and 10 are quite different
from those of 7-9. This indicates the different compositions
of the emitting excited state, which in turn leads to the
different radiative decay rate constants for 6 and 10.
Resembling most of the platinum polypyridine complexes
reported in the literature,1,7-10 complexes 6-10 all emit at
3
trum usually becomes broadened and the MLCT emission
band is red-shifted when the electron donation ability of the
ancillary ligand increases. For example, Yam and co-workers
reported that for platinum terpyridyl acetylide complexes
with different auxiliary substituents on the phenylacetylide
ligand, electron-donating substituents cause bathochromic
1
3
shifts in their MLCT/1LLCT absorption band and MLCT
emission band, while an electron-withdrawing substituent
induces hyposochromic shifts.10f Wu and Tung et al. also
discovered that in the 4′-tolylterpyridyl platinum(II) acetylide
complexes, the complex with the pentynyl ligand possesses
a low-energy absorption band at about 441 nm, and the
emission appears at 580 nm in CH2Cl2.10g In contrast, the
1MLCT/1LLCT transition is much broader for the complex
with the phenylacetylide ligand, where the band maximum
occurs at 433 nm and a shoulder at 482 nm, and the emission
is observed at 619 nm.10g The similar phenomenon has been
observed by Che and co-workers in “alkoxyl free” platinum
C∧N∧N complexes2a and by us from complexes 12 and 13,
in which the emission of 12 appears at 566 nm in CH3CN,
but complex 13 emits at about 589 nm.6f However, for
complexes 6-10 studied in this work, the ancillary ligand
does not show prominent effect on the low-lying absorption
and emission energies. The emission energy of 6-10 is
essentially the same, and the energy of the low-lying
absorption band remains almost the same for 7-9. Moreover,
the emission of these complexes did not manifest as
significant solvatochromism as the platinum terpyridyl com-
plexes.1,7,8,10 All these facts imply that the nature of the
1
lowest excited state could be different from the MLCT/
3
1LLCT and MLCT that are commonly regarded for the
platinum terpyridyl complexes. Features that are independent
of the acetylide ligand, such as the intraligand π,π* or ILCT
transitions within the C∧N∧N ligand, must be involved in
the lowest excited state.
Che and co-workers have revealed that the emissions of
the heteroatom-containing platinum S∧N∧N and O∧N∧N
complexes are insensitive to the substituent(s) on the
arylacetylide ligand, and they tentatively attributed these
1
emissions to the ILCT transitions within the S∧N∧N and
O∧N∧N ligands.2a For the platinum 4′-pyrenylterpyridyl
chloride complex reported by McMillin et al., the emitting
state was considered as an admixture of 3MLCT/3ILCT/3π,π*
transitions.8d A common feature found in these three systems
is that they all contain an electron donating component, that
is, thienyl, furyl, and pyrenyl rings, in the terdentate ligand.
Considering the electron donating ability of the alkoxyl
substituent, we speculate that the transition involved in the
lowest excited state is the ILCT transition.
To support this hypothesis, TDDFT calculations were
carried out on 6-8. The calculations show that the electron
density distribution on the LUMOs of these complexes all
have dominant contribution from the π*(bipyridine), but
distributions on the HOMOs of these three complexes have
Inorganic Chemistry, Vol. 48, No. 6, 2009 2417

Shao et al.
room temperature in fluid solutions, with an emission lifetime
of several hundreds of nanoseconds and a Stokes shift of
more than 4600 cm-1. Therefore, the emitting state could
be attributed to a triplet excited state. However, unlike most
of the platinum terpyridyl complexes, the emitting state of
of 7 and 8 would increase the radiative decay rate constants
(kr) for these complexes, which in turn increases the emission
quantum yields for 7 and 8. The emitting-state character of
10 is assumed to be similar to that of 6 in view of the similar
feature of the electronic absorption spectra of 6 and 10 as
discussed earlier. Consequently, the emission quantum yield
of 10 is similar to that of 6.
3
3
6-10 cannot be a pure MLCT state because the MLCT
emission usually exhibits a salient negative solvatochromic
effect, and the energy is affected by the electron donating
ability of the acetylide ligand, like the cases mentioned earlier
for the platinum(II) terpyridyl acetylide complexes.10g,f With
reference to the character of the singlet excited state, we
speculate that the emitting states of 6-10 likely admix the
3MLCT, 3ILCT, and 3π,π* characters. Experimental evidence
Effect of the Alkoxyl Substituent. The alkoxyl substituent
on the 6-phenyl ring of the C∧N∧N ligand plays an important
role in the electron density distribution and thus on the
photophysical properties. Because of the electron-donating
ability of the alkoxyl substituent, the electron density of the
6-phenyl ring increases. On the other hand, the bipyridyl
component is electron deficient, which would allow for the
occurrence of electron transfer from the 6-phenyl component
to the bipyridyl component. Our speculation is supported by
the TDDFT calculations and several experimental results.
The calculation results indicate that for 6-8 the LUMOs’
electron densities are almost exclusively on the bipyridyl
component. For complexes without strong π-donating acetyl-
ide ligand, such as 6 and 7, the HOMO density has
predominant contribution from the C∧N component. In
particular, the HOMO density is essentially all from the C∧N
component in complex 6. Comparing the relative contribution
from each component to the HOMOs of 6 and 12 (Table 3),
it is obvious that the alkoxyl substituent remarkably increases
the contribution of the C∧N component from ∼38% in 12
to ∼96% in 6. This in turn increases the ILCT character in
the lowest excited states. Consequently, the lifetime of the
excited state, particularly the triplet excited state, increases,
which is evident by the lifetimes of 6 and 7 with respect to
those of 12 and 13 listed in Table 1. Except for the prolonged
lifetime of the platinum complexes, the increased charge-
transfer character within the C∧N∧N ligand also leads to an
enhanced solvatochromic effect in the ligand, as demon-
strated by 3a in Figure 6. Moreover, the alkoxyl substitution
affects the π,π* transitions of the C∧N∧N ligand. As
exemplified in Figure 3 for ligand 3a and complex 7 with
respect to their “alkoxyl free” analogues 11 and 13,
respectively, alkoxyl substitution causes a bathochromic shift
of the absorption bands in the UV region and an increase of
the molar extinction coefficients. This reveals that alkoxyl
substitution reduces the energy gap but enhances the oscil-
lator strengths of the π,π* transitions because of the electron
donating ability of the alkoxyl substituent and the increased
asymmetry of the C∧N∧N ligand, respectively.
3
that partially supports the notion of admixing ILCT con-
figuration into the emitting state arises from the fact that the
emission of the ligand 3a manifests a prominent positive
solvatochromic effect (Figure 6). The pronounced solvato-
chromism of 3a suggests that the emission is from a charge-
transferred state, which can only be intraligand charge
transfer in the ligand. Although the emission observed from
the ligand is fluorescence in view of its very short lifetime,
this result still provides strong implication of the involvement
of the ILCT to the emitting state of the complexes. The
nominal solvatochromic effect for 6-10 could be simply
attributed to the delocalization of the electron density in the
excited state that is supported by the TDDFT calculation
result for the HOMO; or alternatively, to the admixture of
3
configurationally distinct transitions (3MLCT, ILCT, and
3π,π*) into the emitting state. Further support of the mixture
of the ILCT configuration into the emitting state comes from
the lifetime measurement of these complexes in CH2Cl2 and
CH3CN. It is known that CH3CN is a Lewis base, which
can coordinate with the platinum ion. As a result of this
coordination, the 3MLCT emission of the platinum complexes
usually is quenched, which has been reported in the literature
for most of the platinum polypyridyl complexes.1-10 How-
ever, this is not the case for complexes 6-10. Although the
lifetimes of 6-10 become somewhat shorter in CH3CN, the
reduction is not as drastic as that in the platinum terpyridyl
complexes. Considering the independence of the ILCT and
π,π* transitions on the alternation of the metal center, we
believe that the emitting state possesses some ILCT and π,π*
characters. The mixture of configurationally distinct transi-
3
3
3
tions, such as ILCT, π,π*, and MLCT into the emitting
state has been proposed by McMillin and co-workers for a
4′-pyrenylterpyridyl platinum chloride complex that contains
a strong π-donating pyrenyl substituent; a similar nominal
solvent-induced exciplex quenching was observed for this
complex in butyronitrile because of the admixture of the
Transient Absorption and RSA. Like many platinum
polypyridine acetylide complexes,2a,4b,6,7a complexes 6-10
exhibit broad and moderately strong triplet transient differ-
ence absorption from the near-UV to the near-IR region.
Except for complex 10, the TA lifetimes of complexes 6-9
are comparable to those measured from the decay of the
emission, indicating that the transient absorption originates
from the emitting state or a state that is in equilibrium with
the emitting state. Therefore, the transient absorption is
tentatively attributed to the 3MLCT/3ILCT/3LLCT state.
Although the alkoxyl substituent influences the position of
8d
3
3ILCT and π,π* configurations in the emitting state.
The notion of the admixture of configurationally distinct
excitations into the emitting state can be used to explain the
low emission quantum yields of 6 and 10. As discussed
earlier for the lowest singlet excited states of 6-8, the
contribution of MLCT character gradually increases from 6
to 8. It is reasonable to assume that the triplet emitting states
of these complexes have the similar compositions. The
3
increased degree of MLCT character in the emitting state
2418 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum 4,6-Diphenyl-2,2′-bipyridyl Complexes
the maximum of the transient absorption spectra, the overall
features of the TA spectra of 6, 7, and 8 resemble those of
their corresponding “alkoxyl free” analogues (Figure 10 and
Supporting Information, Figure S6).6b,f However, the aux-
iliary substituent at the acetylide ligand seems to affect the
profile of the transient absorption. In particular, complexes
8 and 9, which possess π-donating aryl substituents at the
acetylide ligand, exhibit a flatter visible absorption band, and
the absorption in the visible region for 7 is stronger than
that of 6. These features are likely related to the acetylide
ligand. The contribution from the ancillary acetylide ligand
cation to the transient absorption of platinum terpyridyl
acetylide complexes has been verified by Castellano and co-
workers through transient-trapping experiments.7a This pro-
substituent enriches the electron density of the 6-phenyl ring,
leading to the occurrence of the ILCT. TDDFT calculations
corroborate the experimental results. Admixture of the ILCT
character into the lowest excited states results in enhanced
emission, prolonged excited-state lifetime, and reduced
solvatochromism and solvent quenching effect. It also
contributes to the independence of the emission energies on
the ancillary ligand in 6-10. Notably, complexes 7-9 show
remarkably high emission quantum yields in CH2Cl2 (Φem
) 0.15-0.21), which are among the highest values reported
in the literature for platinum polypyridine complexes. Not
only this, 6-10 show broad positive transient absorption
from the near-UV to the near-IR region, and exhibit strong
RSA at 532 nm for nanosecond laser pulses except for 10.
The studies on the emission energy, lifetime, and RSA clearly
demonstrate that, unlike the influence on the platinum
terpyridyl complexes, the ancillary acetylide ligand does not
influence the triplet excited-state characteristics pronounced-
ly. Therefore, for future photonic device applications, it
would be unnecessary to convert the chloride ligand into
acetylide ligand as what is usually required for the platinum
terpyridyl chloride complexes. This clearly demonstrates the
beneficial consequence of the alkoxyl substitution.
3
vides evidence for the notion of the LLCT participation in
the transient absorption of 7-9. Nevertheless, the broad
positive absorption band in the visible to the near-IR region
suggests that RSA could occur in this spectral region, which
has been verified by the nonlinear absorption measurement
at 532 nm (Figure 11).
All mononuclear complexes (6-9) exhibit significant RSA
when the incident fluence increases. The weak RSA of 10
could be ascribed to its large ground-state absorption cross-
section (σ0), which reduces the ratio of the excited-state
absorption to ground-state absorption cross-sections (σex/σ0),
and the low quantum yield of the triplet excited-state
formation. Both of these parameters are found to be critical
for RSA of nanosecond laser pulses from our previous
studies.6,16 Disregarding the difference in the triplet excited-
state formation quantum yield, the observed trend of the RSA
strength for 6-10 essentially coincides with their relative
ground-state absorption cross-sections at 532 nm, which is
4.2 × 10-19 cm2 for 6, 1.6 × 10-18 cm2 for 7, 2.4 × 10-18
cm2 for 8, 4.0 × 10-18 cm2 for 9, and 7.2 × 10-18 cm2 for
10. It is worthy of mention that 6 exhibits noticeably stronger
RSA than 12 does, which is likely related to the quite
different triplet excited-state lifetimes of these two com-
plexes. The excellent RSA of 6 demonstrates another
beneficial consequence of the alkoxyl substitution.
Acknowledgment. Acknowledgment is made to the
National Science Foundation (CAREER CHE-0449598 and
CHE-0313907) and Army Research Laboratory (W911NF-
06-2-0032) for support. We are also grateful to North Dakota
State EPSCoR (ND EPSCoR Instrumentation Award) for
support.
Supporting Information Available: Normalized electronic
absorption spectra of complexes 7 and 8 in different solvents,
normalized electronic absorption and emission spectra of 6 and 12
in CH2Cl2, plot of kobs versus concentration for complex 8 in CH2Cl2,
contour plots of the calculated MOs for complexes 7, 8, and 12,
time-resolved triplet transient difference absorption spectra of 10
in CH3CN, comparison of the TA spectra of 6 and 12 in CH3CN
solutions at zero time delay, the excitation energies (eV), wave-
lengths (nm), oscillator strengths, dominant contributing configu-
ration, and the associated configuration coefficient obtained at the
TDDFT/LANG6 level of theory for complexes 7, 8, and 12, and
the full author list for ref 21. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusion
The 4,6-diphenyl-2,2′-bipyridine platinum complexes
6-10 with alkoxyl substituent on the 6-phenyl ring exhibit
enhanced solubility in CH3CN and CH2Cl2. The alkoxyl
IC801589Q
Inorganic Chemistry, Vol. 48, No. 6, 2009 2419