4′-(5?-R-pyrimidyl)-2,2′:6′,2″-terpyridyl (R = H, OEt, Ph, Cl, CN) platinum(II) phenylacetylide complexes: Synthesis and photophysics

Article abstract of DOI:10.1021/ic800358w

A series of new square-planar 4′-(5?-R-pyrimidyl)-2,2′: 6′,2″-terpyridyl platinum(II) phenylacetylide complexes (1a-5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer (¹MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer (¹LLCT) character. Photoluminescence arising from the ³MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The ¹MLCT/ ¹LLCT absorption bands and the ³MLCT emission bands for 1a-5a red-shift in comparison to those of the corresponding 4′-toly-2, 2′:6′,2″-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the ¹MLCT/¹LLCT absorption and the ³MLCT emission bands exhibit a linear correlation with the Hammett constant (σ_p) of the 5?-substituent on the pyrimidyl ring. The lifetime of the ³MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a-5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH₃CN than 2a, which has the -OEt substituent, does (λ_T1-Tn^max = 720 nm, τ_T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a-5a vary from 36 600 L·mol ^-1·cm^-1 to 115 090 L·mol ^-1·cm^-1, and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH₃CN solutions, with the singlet oxygen generation quantum yield (Φ_Δ) varying from 0.24 to 0.46.

Full text of DOI:10.1021/ic800358w

Inorg. Chem. 2008, 47, 7599-7607
4′-(5′′′-R-Pyrimidyl)-2,2′:6′,2′′-terpyridyl (R ) H, OEt, Ph, Cl, CN)
Platinum(II) Phenylacetylide Complexes: Synthesis and Photophysics
Zhiqiang Ji, Yunjing Li, and Wenfang Sun*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity, Fargo, North
Dakota 58105
Received February 25, 2008
A series of new square-planar 4′-(5′′′-R-pyrimidyl)-2,2′:6′,2′′-terpyridyl platinum(II) phenylacetylide complexes (1a-5a)
bearing different substituents (R ) H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized.
The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated.
All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively
assigned to the metal-to-ligand charge transfer (1MLCT) transition, possibly mixed with some ligand-to-ligand charge
3
transfer (1LLCT) character. Photoluminescence arising from the MLCT state was observed both in fluid solutions
1
3
at room temperature and in a rigid matrix at 77 K. The MLCT/1LLCT absorption bands and the MLCT emission
bands for 1a-5a red-shift in comparison to those of the corresponding 4′-toly-2,2′:6′,2′′-terpyridyl platinum(II)
phenylacetylide complex. In addition, the energies of the 1MLCT/1LLCT absorption and the 3MLCT emission bands
exhibit a linear correlation with the Hammett constant (σp) of the 5′′′-substituent on the pyrimidyl ring. The lifetime
3
of the MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference
absorption spectra of 1a-5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between
420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band
max
at 785 nm and a shorter lifetime (130 ns) in CH3CN than 2a, which has the -OEt substituent, does (λT1-Tn
)
720 nm, τT ) 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a-5a vary from
36 600 L · mol-1 ·cm-1 to 115 090 L · mol-1 ·cm-1, and the quantum yields of the triplet excited-state formation
range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at
532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH3CN solutions, with
the singlet oxygen generation quantum yield (Φ∆) varying from 0.24 to 0.46.
Introduction
complexes drastically.1c,g Especially when a large aryl
substituent, such as pyrenyl, was attached to the 4′ position
Square-planar platinum terpyridyl (tpy) complexes1 have
attracted great interest in recent decades because of their
unique spectroscopic properties and their potential applica-
tions in photonic devices and photosensitization, such as
organic light-emitting devices,2 photovoltaic cells,3 chemical
sensors,4 photosensitized chemical reactions,5 and so forth.
Recently, our group discovered that some of the platinum(II)
terpyridyl complexes could be promising nonlinear absorbing
materials because of their broad and moderately strong
excited-state absorption in the visible to the near-IR region.1i,6
It has been reported that the substituent on the 4′ position of
the terpyridyl ligand affects the photophysics of the platinum
(1) (a) Aldridge, T. K.; Stacey, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722. (b) Buchner, R.; Field, J. S.; Haines, R. J.; Cunningham,
C. T.; McMillin, D. R. Inorg. Chem. 1997, 36, 3952. (c) Crites, D. K.;
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* Author to whom correspondence should be addressed. Phone: 701-
231-6254. Fax: 701-231-8831. E-mail: Wenfang.Sun@ndsu.edu.
10.1021/ic800358w CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 17, 2008 7599
Published on Web 07/29/2008

Ji et al.
of the terpyridyl ligand, the nature of the lowest excited state
was altered.1g For a relatively small aryl substituent, such
as phenyl, the effect was limited, partially due to the lack of
coplanarity between the phenyl ring and the terpyridyl ligand.
To improve the coplanarity, it is necessary to remove the
repulsive interactions between the four hydrogens adjacent
to the interannular bond. Hanan and co-workers reported that
either replacing the 4′-phenyl substituent by a pyrimidyl ring
or using a triazine ring to substitute the central pyridine ring
could improve the coplanarity.7 Ruthenium complexes using
these approaches were synthesized and investigated.7a–e,g
Coplanarity between the aryl substituent and the terpyridyl
ligand (or the triazine-based terdentate ligand) was realized
in these complexes, and the π conjugation was extended.
Consequently, the metal-to-ligand charge transfer (MLCT)
3
band in their UV-vis absorption spectra and the MLCT-
Figure 1. Chemical structures of 1a-5a and 1b-5b.
based emission band red-shifted. Most importantly, the
emission lifetimes of the ruthenium complexes increased
significantly. To verify whether these strategies are applicable
to the d8 transition-metal complexes and to explore how
significant the substituent on the pyrimidyl ring influences
the photophysics of the d8 transition-metal complexes, a
series of 4′-(5′′′-R-pyrimidyl)-2,2′:6′,2′′-terpyridyl platinu-
m(II) phenylacetylide complexes (1a-5a) bearing different
substituents (R ) H, OEt, Ph, Cl, CN; Figure 1) were
synthesized and fully characterized by 1H NMR, high
resolution mass spectrometry (HRMS), and elemental analy-
sis. Their electronic absorption, photoluminescence, and
triplet excited-state characteristics were systematically in-
vestigated. The efficiency of these complexes to generate
singlet oxygen was also evaluated. For comparison purposes,
4′-(5′′′-R-pyrimidyl)-2,2′:6′,2′′-terpyridyl platinum(II) chlo-
ride complexes (1b-5b) were also synthesized and studied.
Additionally, the nonlinear transmission behavior of these
complexes was investigated.
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Che, C.-M.; Zhu, N. J. Am. Chem. Soc. 2006, 128, 8297. (e) Wong,
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Zhang, L.-P.; Tung, C.-H. Chem.sEur. J. 2007, 13, 1231. (g) Wong,
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Experimental Section
Synthesis. 4′-CN-2,2′:6′,2′′-Terpyridine,8 4′-(5′′′-CN-pyrimidyl)-
2,2′:6′,2′′-terpyridine,7e
4′-(5′′′-Cl-pyrimidyl)-2,2′:6′,2′′-
terpyridine,7e 4′-(5′′′-Ph-pyrimidyl)-2,2′:6′,2′′-terpyridine,7e 4′-
and 2-OEt-trimethinium
pyrimidyl-2,2′:6′,2′′-terpyridine,7e
perchlorate9 were prepared according to the literature procedures.
Silver phenylacetylide (AgCt CPh) was synthesized by modification
of the literature procedure.10 All solvents were purchased from
VWR Scientific Company at analytical grade and used without
further purification unless otherwise stated. All reagents were
purchased from Aldrich and Alfa Aesar and used as-is without
further purification.
All products were characterized by 1H NMR, HRMS, and
elemental analysis. 1H NMR spectra were obtained using a Varian
400 or 500 MHz VNMR spectrometer. All samples were analyzed
by positive ion electrospray ionization (ESI) on a 12T Bruker
APEX-Qe FTICR-MS with an Apollo II ion source at the Old
Dominion University. Elemental analyses were conducted by
NuMega Resonance laboratories, Inc. in San Diego, California.
4′-(5′′′-OEt-Pyrimidyl)-2,2′:6′,2′′-terpyridine. The synthesis is
similar to that reported in the literature for other 4′-(5′′′-R-
(6) (a) Sun, W.; Wu, Z.-X.; Yang, Q.-Z.; Wu, L.-Z.; Tung, C.-H. Appl.
Phys. Lett. 2003, 82, 850. (b) Sun, W.; Guo, F. Chin. Opt. Lett. 2005,
S3, S34. (c) Guo, F.; Sun, W. J. Phys. Chem. B 2006, 110 (30), 15029.
(d) Pritchett, T. M.; Sun, W.; Guo, F.; Zhang, B.; Ferry, M. J.; Rogers-
Haley, J. E.; Shensky, W.; Mott, A. G. Opt. Lett. 2008, 33 (10), 1053.
(7) (a) Passalacqua, R.; Loiseau, F.; Campaigns, S.; Fang, Y.; Hanan, G. S.
Angew. Chem., Int. Ed. 2003, 42, 1608. (b) Polson, M. I. J.; Taylor,
N. J.; Hanan, G. S. Chem. Commun. 2002, 1356. (c) Polson, M. I. J.;
Medlycott, E. A.; Hanan, G. S.; Mikelsons, L.; Taylor, N. J.; Watanabe,
M.; Tanaka, Y.; Loiseau, F.; Passalacqua, R.; Campagna, S.
Chem.sEur. J. 2004, 10, 3640. (d) Medlycott, E. A.; Hanan, G. S.
Chem. Commun. 2007, 4884. (e) Fang, Y.-Q.; Taylor, N. J.; Laverdiere,
F.; Hanan, G. S.; Loiseau, F.; Nastasi, F.; Campagna, S.; Nierengarten,
H.; Leize-Wagner, E.; Dorsselaer, A. V. Inorg. Chem. 2007, 46, 2854.
(f) Medlycott, E. A.; Hanan, G. S. Coord. Chem. ReV. 2006, 250,
1763. (g) Fang, Y.-Q.; Taylor, N. J.; Hanan, G. S.; Loiseau, F.;
Passalacqua, R.; Campagna, S.; Nierengarten, H.; Dorsselaer, A. V.
J. Am. Chem. Soc. 2002, 124, 7912.
(8) Veauthier, J. M.; Carlson, C. N.; Collis, G. E.; Kiplinger, J. K.; John,
K. D. Synthesis 2005, 2683.
(9) Wagner, E. R.; Matthews, D. P.; Barney, C. L. U.S. patent 4788335,
1988.
(10) Agawa, T.; Miller, S. I. J. Am. Chem. Soc. 1961, 83, 449.
7600 Inorganic Chemistry, Vol. 47, No. 17, 2008

Platinum(II) Phenylacetylide Complexes
pyrimidyl)-2,2′:6′,2′′-terpyridine.7e Yield: 54%. 1H NMR (CDCl3,
500 MHz): δ 1.54 (3H, t, J ) 6.5 Hz), 4.26 (2H, q, J ) 7.0 Hz),
7.37 (2H, dt, J ) 1.0, 5.5 Hz), 7.90 (2H, dt, J ) 1.5, 7.5 Hz), 8.58
(2H, s), 8.68 (2H, d, J ) 7.5 Hz), 8.79 (2H, d, J ) 3.5 Hz), 9.41
(2H, s).
product. After removal of the solvent, the solid was recrystalized
in CH3CN/Et2O.
[(5-H-Pm-tpy)Pt(Ct CC6H5)]PF6 (1a). Yield: 74%. H NMR
1
(CD3CN, 500 MHz): δ 7.13 (3H, m), 7.25 (1H, d, J ) 7.5 Hz),
7.53 (2H, dt, J ) 5.5, 1.5 Hz), 7.62 (2H, t, J ) 5.0 Hz), 8.25 (4H,
m), 8.74 (2H, m), 8.99 (2H, d, J ) 12.5 Hz), 9.02 (2H, s). ESI-MS
m/z calcd for [C27H18N5Pt194]+: 606.1189. Found: 606.1183 (73%).
ESI-MS m/z calcd for [C27H18N5Pt195]+: 607.1210. Found: 607.1196
(100%). ESI-MS m/z calcd for [C27H18N5Pt196]+: 608.1211. Found:
608.1209 (85%). Anal. calcd for C27H18F6N5PPt: C, 43.09; H, 2.41;
N, 9.31. Found: C, 42.74; H, 2.54; N, 9.65.
General Procedure for the Synthesis of [(5-R-Pm-tpy)Pt-
Cl]PF6. A total of 1 mmol of 5-R-Pm-tpy ligand and 1 mmol of
Pt(DMSO)2Cl2 was dissolved in 30 mL of CHCl3, and the mixture
was refluxed for 24 h. After that, the reaction mixture was
concentrated and cooled down to room temperature. The solid was
then filtered out and dissolved in a small amount of N,N-
dimethylformamide (DMF). Saturated NH4PF6 aqueous solution was
added, and the mixture was stirred at room temperature for 3 h.
The resultant solid was filtered out; washed with water, methanol,
and ether; and dried. The pure product was obtained by recrystal-
ization in DMF/Et2O.
[(5-H-Pm-tpy)PtCl]PF6 (1b). Yield: 70%. 1H NMR (d6-DMSO,
400 MHz): δ 7.79 (1H, t, J ) 4.8 Hz), 7.99 (2H, t, J ) 6.0 Hz),
8.50 (2H, dt, J ) 1.6, 8.0 Hz), 8.93 (2H, d, J ) 8.0 Hz), 9.00 (2H,
d, J ) 5.2 Hz), 9.15 (2H, d, J ) 4.8 Hz), 9.34 (2H, s). ESI-MS
m/z calcd for [C19H13ClN5Pt194]+: 540.0486. Found: 540.0468
(99%). ESI-MS m/z calcd for [C19H13ClN5Pt195]+: 541.0507. Found:
541.0490 (100%). ESI-MS m/z calcd for [C19H13ClN5Pt196]+:
542.0509. Found: 542.0492 (72%). Anal. calcd for
C19H13ClF6N5PPt.0.5CHCl3: C, 31.37; H, 1.82; N, 9.38. Found: C,
31.64; H, 1.73; N, 9.53.
[(5-OEt-Pm-tpy)Pt(Ct CC6H5)]PF6 (2a). Yield: 35%. 1H NMR
(d6-DMSO, 500 MHz): δ 1.48 (3H, t, J ) 7.0 Hz), 4.39 (2H, q, J
) 7.0 Hz), 7.27 (3H, m, J ) 7.0, 7.5 Hz), 7.40 (2H, dt), 7.85 (2H,
m), 8.41 (2H, m), 8.78 (4H, m), 9.00 (2H, m), 9.20 (2H, m). ESI-
MS m/z calcd for [C29H22N5OPt194]+: 650.1451. Found: 650.1432
(71%). ESI-MS m/z calcd for [C29H22N5OPt195]+: 651.1472. Found:
651.1454 (100%). ESI-MS m/z calcd for [C29H22N5OPt196]+:
652.1474. Found: 652.1452 (85%). Anal. calcd for C29H22F6N5OPPt:
C, 43.73; H, 2.78; N, 8.79. Found: C, 43.55; H, 2.78; N, 8.79.
[(5-Ph-Pm-tpy)Pt(Ct CC6H5)]PF6 (3a). Yield: 32%. 1H NMR
(d6-DMSO, 500 MHz): δ 7.24 (3H, m), 7.48 (2H, m), 7.65 (3H,
m), 7.93 (2H, m), 8.02 (2H, d, J ) 7.5 Hz), 8.49 (2H, m), 8.93
(2H, m), 9.13 (2H, m), 9.45 (2H, m). ESI-MS m/z calcd for
[C33H22N5Pt194]+: 682.1502. Found: 682.1457 (36%). ESI-MS m/z
calcd for [C33H22N5Pt195]+: 683.1523. Found: 683.1489 (100%).
ESI-MS m/z calcd for [C33H22N5Pt196]+: 684.1524. Found: 684.1499
(72%). Anal. calcd for C33H22F6N5PPt·CH2Cl2: C, 44.70; H, 2.65;
N, 7.67. Found: C, 45.02; H, 2.53; N, 8.10.
[(5-Ph-Pm-tpy)PtCl]PF6 (3b). Yield: 75%. 1H NMR (CD3CN,
400 MHz): δ 7.63 (3H, m), 7.88 (4H, m), 8.39 (2H, dt, J ) 1.6,
8.0 Hz), 8.50 (2H, m), 9.05 (2H, m), 9.21 (2H, s), 9.32 (2H, s).
ESI-MS m/z calcd for [C25H17ClN5Pt194]+: 616.0799. Found:
616.0770 (68%). ESI-MS m/z calcd for [C25H17ClN5Pt195]+: 617.0820.
Found: 617.0790 (100%). ESI-MS m/z calcd for [C25H17ClN5Pt196]+:
618.0822. Found: 618.0794 (65%). Anal. calcd for
C25H17ClF6N5PPt: C, 39.36; H, 2.25; N, 9.18. Found: C, 39.03; H,
2.62; N, 9.51.
1
[(5-Cl-Pm-tpy)Pt(Ct CC6H5)]PF6 (4a). Yield: 63%. H NMR
(d6-DMSO, 500 MHz): δ 7.33 (3H, m), 7.35 (2H, m), 7.95 (2H,
m), 8.52 (2H, m), 8.86 (2H, m), 9.17 (2H, m), 9.30 (2H, m), 9.37
(2H, m). ESI-MS m/z calcd for [C27H17ClN5Pt194]+: 640.0799.
Found: 640.0766 (58%). ESI-MS m/z calcd for [C27H17ClN5Pt195]+:
641.0820. Found: 641.0789 (100%). ESI-MS m/z calcd for [C27H17-
ClN5Pt196]+: 642.0822. Found: 642.0787 (81%). Anal. calcd for
C27H17 ClF6N5PPt: C, 41.21; H, 2.18; N, 8.90. Found: C, 41.60;
H, 2.03; N, 9.29.
1
[(5-Cl-Pm-tpy)PtCl]PF6 (4b). Yield: 65%. H NMR (CD3CN,
500 MHz): δ 7.91 (2H, t, J ) 5.0 Hz), 8.41 (2H, t, J ) 4.0 Hz),
8.51 (2H, m), 9.08 (2H, m), 9.12 (2H, s), 9.18 (2H, s). ESI-MS
m/z calcd for [C19H12Cl2N5Pt194]+: 574.0096. Found: 574.0074
(85%). ESI-MS m/z calcd for [C19H12Cl2N5Pt195]+: 575.0117.
Found: 575.0097 (100%). ESI-MS m/z calcd for [C19H12Cl2N5Pt196]+:
576.0119. Found: 576.0096 (69%). Anal. calcd for
C19H12Cl2F6N5PPt: C, 31.64; H, 1.68; N, 9.71. Found: C, 31.40;
H, 1.77; N, 9.73.
[(5-CN-Pm-tpy)PtCl]PF6 (5b). Yield: 59%. 1H NMR (CD3CN,
400 MHz): δ 7.86 (2H, dt, J ) 1.2, 5.6 Hz), 8.41 (2H, dt, J ) 1.2,
8.0 Hz), 8.48 (2H, m), 9.02 (2H, m), 9.19 (2H, s), 9.37 (2H, s).
ESI-MS m/z calcd for [C20H12ClN6Pt194]+: 565.0439. Found:
565.0417 (83%). ESI-MS m/z calcd for [C20H12ClN6Pt195]+: 566.0460.
Found: 566.0440 (100%). ESI-MS m/z calcd for [C20H12ClN6Pt196]+:
567.0461. Found: 567.0440 (66%). Anal. calcd for
C20H12ClF6N6PPt: C, 33.75; H, 1.70; N, 11.81. Found: C, 33.38;
H, 2.02; N, 11.88.
[(5-CN-Pm-tpy)Pt(Ct CC6H5)]PF6 (5a). Yield: 80%. 1H NMR
(d6-DMSO, 500 MHz): δ 6.90 (5H, m), 7.39 (2H, m), 8.10 (2H,
m), 8.25 (2H, m), 8.47 (2H, m), 8.91 (2H, m), 8.30 (2H, m). ESI-
MS m/z calcd for [C28H17N6Pt194]+: 631.1141. Found: 631.1117
(86%). ESI-MS m/z calcd for [C28H17N6Pt195]+: 632.1162. Found:
632.1141 (100%). ESI-MS m/z calcd for [C28H17N6Pt196]+: 633.1164.
Found: 633.1140 (61%). Anal. calcd for C28H17F6N6PPt ·H2O: C,
42.27; H, 2.41; N, 10.56. Found: C, 41.87; H, 2.21; N, 10.74.
Photophysical Studies. The electronic absorption spectra were
recorded on a SHIMADZU 2501 PC UV-vis spectrophotometer.
Complexes 1a-5a, 1b, and 3b-5b were dissolved in CH3CN. The
emission spectra at room temperature, 77 K, and in the solid state
were all recorded on a SPEX Fluorolog-3 fluorometer/phospho-
rometer. The complexes were dissolved in CH3CN for room
temperature measurement. The low-temperature (77 K) emission
was measured in butyronitrile (BuCN) glassy solutions. The
solutions were degassed for 30 min prior to each measurement.
The solid-state emission was measured using a quartz slide with
thin films of complexes 1a-5a formed on it through solvent
evaporation. The slide was placed 45° to the excitation beam. The
emission lifetimes were measured on an Edinburgh LP920 laser
flash photolysis spectrometer. Excitation was provided by the third-
harmonic output (355 nm) of a Quantel Brilliant Q-switched Nd:
YAG laser (fwhm pulsewidth was 4.1 ns and the repetition rate set
at 1 Hz). Sample solutions were degassed for 30 min before each
General Procedure for the Synthesis of [(5-R-Pm-
tpy)Pt(Ct CC6H5)]PF6. A total of 1 mmol of [(5-R-Pm-tpy)Pt-
Cl]PF6 was dissolved in 5 mL of DMF, and 1 mmol of corre-
sponding silver salt was dissolved in 5 mL of pyridine. The two
solutions were mixed together, and the resultant mixture was stirred
under Ar for 24 h. Pyridine was removed, and an excess amount
of ether was added. The solid was separated by centrifuge. The
crude product was then purified by column chromatography using
neutral Al2O3. The column was first flushed by CH2Cl2, and then
CH3CN was used as the eluent. The red layer was collected as the
Inorganic Chemistry, Vol. 47, No. 17, 2008 7601

Ji et al.
measurement. The emission quantum yields of the complexes were
determined by the comparative method,11 in which a degassed
aqueous solution of [Ru(bpy)3]Cl2 (φem ) 0.042, excited at 436
nm)12 was used as the reference.
The triplet transient difference absorption spectra were measured
on an Edinburgh LP920 laser flash photolysis spectrometer.
Excitation was provided by the third-harmonic output (355 nm) of
the Quantel Brilliant Q-switched Nd:YAG laser. The solutions were
degassed with Ar for 30 min before each measurement. The
absorbance of the solution was adjusted to A ) 0.4 at 355 nm in
a 1 cm quartz cuvette. The triplet excited-state absorption coefficient
(εT) at the absorption band maximum was measured by the singlet
depletion method13 (see the Supporting Information for details).
The quantum yields of the triplet excited-state formation (ΦT) were
determined using the comparative method14 (see the Supporting
Information for details). SiNc in benzene (ΦT ) 0.20, ε ) 70 000
15
M-1cm-1
)
was used as the reference.
The singlet oxygen generation quantum yields were evaluated
by the comparative method16 using an Edinburgh TL900 transient
luminescence spectrometer. The third-harmonic output (355 nm)
of the Quantel Nd:YAG laser was used as the excitation source.
An ultrasensitive, ultrafast germanium detector EI-P was used to
monitor the emission of the singlet oxygen at 1270 nm. A silicon
cutoff filter (>1100 nm) was used to eliminate the scattered light
from the laser. The experimental details are provided in the
Supporting Information.
The nonlinear transmission experimental setup and details have
been described previously.17
Results and Discussion
Electronic Absorption. Figure 2 displays the electronic
absorption spectra of 1a-5a, 1b, and 3b-5b. The absorption
band maxima and extinction coefficients are listed in Table
1. In the concentration range of 5 × 10-6 M to 1 × 10-4
M
Figure 2. UV-vis spectra of 1a-5a, 1b, and 3b-5b in CH3CN solutions.
in CH3CN solution, the absorption of 1a-5a obeys
Beer-Lambert’s law, indicating that no ground-state ag-
gregation occurs in this concentration range. All complexes
exhibit intensive absorption between 250 and 350 nm, which
π(Ct CR) f π*(terpyridyl) transitions in the reported 4′-
tolylterpyridyl platinum(II) complexes1f,h,i with respect to
energy and the extinction coefficient. A time-dependent
density functional theory calculation by Feng and co-workers
on 4′-tolylterpyridyl platinum(II) acetylide complexes also
revealed that the highest occupied molecular orbital (HOMO)
has a predominant contribution from the acetylide moiety
and some contribution from the Pt d orbital, while the lowest
unoccupied molecular orbital (LUMO) is dominated by the
terpyridyl ligand.18 Therefore, the lower-energy absorption
band of 1a-5a, 1b, and 3b-5b is tentatively assigned to
the spin-allowed 1MLCT, possibly mixed with some ligand-
to-ligand charge transfer (1LLCT) character. However, due
to the stronger electron-donating ability of the phenylacetyl-
ide ligand in 1a-5a, the dπ(Pt)/π(Ct CR)-based HOMO is
destabilized while the terpyridyl-based LUMO is almost
unaffected. As a result, the energy gap between the HOMO
and the LUMO decreases, resulting in a pronounced broad-
ening and bathochromic shift in their 1MLCT/1LLCT bands
in contrast to their corresponding chloride complexes (1b,
3b-5b).
1
is ascribed to the intraligand π,π* transitions. Complexes
2a, 3a, and 3b with an electron-donating substituent, that
is, -OEt and -Ph, on the pyrimidyl moiety exhibit a slightly
hypsochromic shift in the bands below 300 nm, but an
increased intensity in the band between 300 and 350 nm in
comparison to the complexes with an electron-withdrawing
substituent, that is, -Cl and -CN, and the one without a
substituent on the pyrimidyl moiety. The broad band between
400 and 500 nm for 1a-5a and at ca. 410 nm for 1b and
3b-5b are in line with the dπ(Pt) f π*(terpyridyl)/
(11) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(12) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853.
(13) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(14) (a) Bensasson, R.; Goldschmidt, C. R.; Land, E. J.; Truscott, T. G.
Photochem. Photobiol. 1978, 28, 277. (b) Foley, S.; Jones, G.; Liuzzi,
R.; McGarvey, D. J.; Perry, M. H.; Truscott, T. G. J. Chem. Soc.,
Perkin Trans. 2 1997, 2, 1725. (c) Kumar, C. V.; Qin, L.; Das, P. K.
J. Chem. Soc., Faraday Trans. 1984, 280, 783.
(15) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Keney, M. E.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1988, 110, 7626.
(16) Abdel-Shafi, A. A.; Beer, P. D.; Mortimer, R. J.; Wilkinson, F. Phys.
Chem. Chem. Phys. 2000, 2, 3137.
(17) (a) Sun, W.; Dai, Q.; Worden, J. G.; Huo, Q. J. Phys. Chem. B 2005,
109, 20854. (b) Li, Y.; Dini, D.; Calvete, M. J. F.; Hanack, M.; Sun,
W. J. Phys. Chem. A 2008, 112 (3), 472.
(18) 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.
7602 Inorganic Chemistry, Vol. 47, No. 17, 2008

Platinum(II) Phenylacetylide Complexes
Table 1. Photophysical Data for 1a-5a, 1b, and 3b-5b
a
d
λ
abs/nm (ε/L·mol-1 ·cm-1
)
λ
em/nm (τ /ns; Φem),b R.T. krc/104 s-1, R.T. knr /106 s-1, R.T.
λ
em/nm (τ /µs),e 77K
λem/nm solid, R.T.
1a 275 (44220), 288 (44540)
618 (380; 0.0039)
612 (700; 0.0058)
618 (400; 0.0040)
1.0
0.8
1.0
2.6
1.4
2.5
570 (11.9), 630 (3.4)
665
317 (12420), 334 (10620)
348 (11180), 441 (5980), 469 (5820)
2a 263 (43560), 291 (38240)
315 (27880), 432 (8140)
460 (7720)
3a 263 (33320), 272 (33000)
289 (29840), 318 (23940)
413 (6140), 466 (5300)
572 (10.0), 624 (3.4)
674
691
520 (12.6), 576 (11.1),
619 (6.3)
4a 291 (47400), 349 (10780)
443 (6240), 474 (6270)
5a 291 (58960), 319 (14340)
351 (11320), 453 (7180)
476 (7320)
621 (340; 0.0032)
639 (130; 0.0011)
0.9
0.8
2.9
7.7
528 (12.8), 570 (12.5),
628 (5.1)
599 (8.8), 662 (2.8)
612
668
1b 266 (31710), 287 (42070)
334 (13460), 357 (7680)
387 (4270), 404 (4960)
502 (17.7), 539 (17.1),
577 (7.2)
3b 272 (33970), 285 (42010)
320 (28910), 334 (30230)
390 (9370), 408 (10800)
4b 270 (23740), 288 (34040)
334 (11290), 358 (6130)
389 (3670), 406 (4340)
523 (8.8, 12%; 34.0, 88%),
577 (4.8, 23%; 30.0, 77%),
608 (4.7), 620 (4.5)
505 (19.5), 543 (19.0),
589 (5.0)
5b 290 (51700), 335 (12050)
362 (7190), 409 (5360)
510 (20.4), 551 (19.1),
600 (4.7)
a Electronic absorption band maxima and molar extinction coefficients in CH3CN solutions. b Room temperature emission band maxima and decay lifetimes
measured at a concentration of 5 × 10-5 mol/L, and emission quantum yields measured at solutions with A436 ) 0.1 in CH3CN solutions. c Radiative decay
rate kr ) Φem/τ. d Nonradiative decay rate knr ) (1 - Φem)/τ. e Emission band maxima and decay lifetimes in butyronitrile glassy solutions; the concentration
of the solutions is 1 × 10-4 mol/L.
The substituent at the 5′′′ position of the pyrimidyl ring
1
exhibits a pronounced effect on the energy of the MLCT/
1LLCT band. A linear correlation between the lowest energy
1
of the MLCT/1LLCT band and the Hammett constant (σp)
of the substituent was observed in 1a-5a (see Figure S1 in
the Supporting Information). This is not surprising because
the coplanarity of the pyrimidyl ring with the terpyridyl
ligand would allow for conjugation with the terpyridyl ligand
to occur. The inductive and resonant effects from the 5′′′
substituent would then influence the energy of the terpyridyl-
based LUMO, which in turn causes the change of the lowest
excited-state energy.
The 1MLCT/1LLCT bands are sensitive to solvent polarity.
Figure 3. Room temperature emission spectra of 1a-5a in CH3CN
Lower-polarity solvents, such as CH2Cl2, cause a bathochro-
solutions. The solution concentration was 5 × 10-5 M. The excitation
1
mic shift of the MLCT/1LLCT band in comparison to that
wavelength was 355 nm.
1
in CH3CN. For example, the MLCT/1LLCT band of 2a in
CH2Cl2 appears at 482 nm, but in CH3CN, it is at 437 nm.
energies of these complexes exhibit a linear correlation with
the Hammett σp constant (see Supporting Information Figure
S3), reflecting the electronic delocalization ability of the
pyrimidyl group.7e The emission lifetime and the emission
quantum yield (listed in Table 1) increase with increased
emission energy. For example, 5a, which emits at ca. 640
nm, possesses the shortest lifetime (130 ns) and the lowest
emission quantum yield (0.11%), while 2a, with λem ) 612
nm, exhibits the longest lifetime (700 ns) and the highest
quantum yield (0.58%). These phenomena are consistent with
the energy gap law,19 which describes the exponential
increase of the nonradiative decay rate with decreased
emission energy. As evident in Figure 4, a linear relationship
with a negative slope is obtained on plotting ln knr versus
1
The red-shift of the MLCT/1LLCT band in low-polarity
solvents compared to that in high-polarity solvents indicates
that the ground states of these complexes are more polar
than the excited states, which is consistent with the charge-
transfer nature of the excited state.
Photoluminescence. Complexes 1a-5a emit both at room
temperature in acetonitrile solutions and in a butyronitrile
rigid matrix at 77 K. At room temperature, the emission
appears between 610 and 640 nm (Figure 3 and Table 1),
which is more than a 150 nm Stokes shift with respect to
the excitation spectra (see Supporting Information Figure S2).
This feature along with the relatively long lifetime (hundreds
of nanoseconds) suggests that the emission originates from
a triplet excited state. With reference to the emission of other
platinum terpyridyl complexes,1–9 the emitting state is
tentatively assigned to the 3MLCT state. The emission
max
Eem for 1a-5a.
(19) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7603

Ji et al.
max
Figure 6. Normalized concentration-dependent emission spectra of 4a in
BuCN glassy solutions at 77 K. The excitation wavelength was 412 nm.
Figure 4. Plot of ln knr vs Eem
for 1a-5a.
The emission energy and the emission lifetime remain the
same for 1a-5a at a concentration range of 5 × 10-6 mol/L
to 1 × 10-4 mol/L, and the emission intensity keeps
increasing with increased concentration (see Supporting
Information Figure S4). This implies that no ground-state
aggregation or self-quenching occurs in this concentration
range at room temperature.
Similar to the UV-vis absorption, the emission energy
and quantum yield of 1a-5a exhibit solvent dependence.
Less-polar, noncoordinating solvents, such as CH2Cl2, cause
a bathochromic shift of the emission band and a higher
emission quantum yield. For example, the emission energy
of 3a increases from 625 nm (910 ns) in CH2Cl2 to 622 nm
(290 ns) in acetone and 619 nm (370 ns) in CH3CN with
increased solvent polarity, which is attributed to the charge-
transfer nature of the emitting state. The quantum yield of
4a is 1.25% in CH2Cl2, 0.34% in acetone, and 0.32% in
CH3CN. The reduced emission lifetime and emission quan-
tum yield in CH3CN should arise from the solvent-quenching
effect of CH3CN.
Complexes 1a-5a, 1b, 3b, 4b, and 5b exhibit lumines-
cence in a butyronitrile glassy matrix at 77 K. As shown in
Figure 5 and tabulated in Table 1, the emission bands shift
to higher energies due to the rigidochromic effect20 and
exhibit vibronic structures compared to the emission at room
temperature. The emission energy, decay lifetime, and shape
of the emission band are similar to those of 4-tolylterpyridyl
platinum phenylacetylide and the chloride analogue reported
in the literature.1g–i In addition, the thermally induced Stokes
shift is calculated to be ∆E ≈ 1360 cm-1 for 1a, which is a
characteristic of a charge-transfer state. Therefore, the
emission at 77 K is tentatively assigned to the 3MLCT state
as well. The lowest-energy band above 600 nm for 1a-5a
and above 580 nm for 1b, 3b, 4b, and 5b possesses a much
shorter lifetime and exhibits a concentration dependence. As
exemplified in Figure 6 for 4a, with increased concentration,
the low-energy band at 628 nm increases. However, the
excitation spectra monitored at the 570 nm and the 628 nm
Figure 5. The 77 K emission spectra of 1a-5a, 1b, and 3b-5b in BuCN
glassy solutions. The solution concentration was 1 × 10-4 M, and the
1
excitation wavelength was the MLCT band maximum.
In contrast to 1a-5a, no room temperature emission was
observed in 1b and 3b-5b. The lack of emission of the
platinum terpyridyl chloride complexes is common due to
the presence of a thermally accessible low-lying, nonemissive
metal-centered (MC) excited state,1h which adds an additional
3
decay path for the MLCT state. When the phenylacetylide
ligand replaces the chloride ligand, the stronger π-donating
ability of the phenylacetylide ligand increases the platinum-
based HOMO, while the terpyridyl-based LUMO is es-
sentially unaffected. Consequently, the energy gap between
the HOMO and the LUMO decreases, resulting in a larger
separation of the 3MC state and the 3MLCT state. This gives
rise to the stronger and longer emission of 1a-5a.
(20) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (b) Cummings,
S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949. (c) Polo,
A. S.; Itokazu, M. K.; Frin, K. M.; Patrocinio, A. O. d. T.; Iha,
N. Y. M. Coord. Chem. ReV. 2006, 250, 1669. (d) Lai, S.-W.; Chan,
M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem. 1999, 38, 4262.
7604 Inorganic Chemistry, Vol. 47, No. 17, 2008

Platinum(II) Phenylacetylide Complexes
gated. As shown in Figure 8B, the shape of the transient
absorption spectra for all complexes is quite similar, exhibit-
ing a bleaching band from 420 to 500 nm and a broad,
moderately intense absorption band between 500 and 800
nm. The positions of the bleaching bands are consistent with
1
the MLCT/1LLCT bands in their UV-vis spectra, and the
lifetimes of the transients are in line with the lifetimes of
the emitting 3MLCT state. Therefore, the transient absorption
3
is tentatively assigned to the MLCT state. With reference
to our previous work on 4′-tolylterpyridyl platinum(II)
phenylacetylide complexes1i and other reported work on
platinum terpyridyl complexes3 and ruthenium terpyridyl
complexes,24 the transients that give rise to the observed
spectra are presumably attributed to the terpyridyl anion
radicals. The transient absorption band maximum of 5a with
a stronger electron-withdrawing -CN substituent appears at
a much lower energy (785 nm) in comparison to that of the
complex with the electron-donating substituent (2a, 720 nm)
in CH3CN.
Figure 7. Normalized solid-state emission of 1a-5a. The excitation
wavelength was 460 nm.
emission bands are quite similar (see Supporting Information
Figure S5), indicating that this low-energy band likely arises
from the excimeric emission at higher concentrations. A
similar phenomenon has been observed for the other
complexes and has been reported by Che and co-workers
for platinum(II) 2,6-diphenylpyridine complexes due to the
Using the singlet depletion method,13 the T1-Tn absorption
coefficients of 1a-5a have been determined. As listed in
Table 2, the εT -T values for 1a, 2a, 4a, and 5a are quite
21
3
1
similar, while the nεT -T value for 3a is approximately three
formation of a π,π* excimer at 77 K.
1
times as large as thosenfor 1a, 2a, 4a, and 5a. This feature
should be related to the presence of the phenyl substituent
on the pyrimidyl ring, which could enhance the oscillator
Solid-state emissions of 1a-5a have also been investi-
gated. As displayed in Figure 7 and compiled in Table 1, all
five samples exhibit a broad, structureless emission. Except
for 4a, which exhibits slightly increased emission energy
versus that in CH3CN solution, the emission energies of the
other four complexes all decrease in comparison to those in
CH3CN solutions. In view of the similar energy of the solid
state emission to those observed in the dinuclear and
trinuclear platinum complexes that originate from the triplet
metal-metal-to-ligand charge transfer excited state
(3MMLCT),22 the solid-state emission could be assigned to
the 3MMLCT state due to intermolecular interaction, which
is common for planar platinum(II) terpyridyl complexes in
the solid phase. Hannan and co-workers23 reported that
Br-Br interaction existed in the triazine metal complexes,
which linked neighbor molecules to form one-dimensional
structures. This could also be the case for 4a, which possesses
the chlorine substituent. To fully understand this phenom-
enon, crystal structure of 4a needs to be obtained and
investigated in the near future.
strength of the transient absorption. Nevertheless, the εT -Tn
1
values for these complexes are much higher than that
observed in a platinum 4-tolylterpyridyl pentynyl complex
(εT -T ) 26 650 M-1 · cm-1 at 690 nm),25 reflecting the
n
sig1nificance of coplanarity with the pyrimidyl substituent.
The quantum yield of the triplet excited-state formation for
1a-5a in CH3CN varies from 0.19 to 0.66, which is
significantly higher than that of the 4-tolylterpyridyl pentynyl
complex (ΦT ) 0.16).25
Singlet Oxygen Generation Efficiency. Singlet oxygen
can be generated by an energy transfer from the triplet
excited state of the photosensitizer to the ground state of
oxygen. Recently, several platinum(II) terdentate complexes
have been demonstrated to be efficient photosensitizers for
the oxidation of olefins and oximes Via singlet oxygen.5
Considering the reasonably high triplet excited-state quantum
yield of 1a-5a and similar structure features to the reported
platinum complexes that generate singlet oxygen, it is
expected that 1a-5a could also generate singlet oxygen. To
demonstrate this, the singlet oxygen generation quantum
yields (Φ∆) from 1a-5a have been measured using the
method described in the Experimental Section and in the
Supporting Information, and the results are listed in Table
2. These values are much higher than that measured for the
platinum 4-tolylterpyridyl pentynyl complex (Φ∆ ) 0.019),
Triplet Transient Difference Absorption (TA). To figure
out the characteristics of the triplet excited-state absorption
of 1a-5a and understand the effect of the 4′-pyrimidyl
substitution, the time-resolved triplet transient difference
absorption spectra of 1a-5a in CH3CN have been investi-
(21) Lu, W.; Chan, M. C.; Cheung, K.-K.; Che, C.-M. Organometallics
2001, 20, 2477.
(22) (a) Lu, W.; Chan, M. C. W.; Zhu, N.; Che, C.-M.; Li, C.; Hui, Z.
J. Am. Chem. Soc. 2004, 126, 7639. (b) Kui, S. C. F.; Sham, I. H. T.;
Cheung, C. C. C.; Ma, C. W.; Yan, B. P.; Zhu, N. Y.; Che, C. M.;
Fu, W. F. Chem.sEur. J. 2007, 13, 417. (c) Kui, S. C. F.; Chui, S. S.-
Y.; Che, C.-M.; Zhu, N. J. Am. Chem. Soc. 2006, 128, 8297. (d) Yam,
V. W.-W.; Chan, K. H.-Y.; Wong, K. M.-C.; Chu, B. W.-K. Angew.
Chem., Int. Ed. 2006, 45, 6169. (e) Sun, W.; Zhu, H.; Barron, P. M.
Chem. Mater. 2006, 18, 2602. (f) Shao, P.; Sun, W. Inorg. Chem.
2007, 46, 8603.
3
which is probably related to the reduced MLCT energy as
evident by the red-shifted emission spectra at ca. 610-640
nm for 1a-5a with respect to λem ) 580 nm for the platinum
(24) Co1lin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti, F.; De Cola,
L.; Flamigni, L.; Balzani, V. Inorg. Chem. 1991, 30, 4230.
(23) Medlycott, E. A.; Udachin, K. A.; Hanan, G. S. Dalton. Trans. 2007,
430.
(25) Pritchett, T. M.; Sun, W.; Fuo, F.; Zhang, B.; Ferry, M. J.; Rogers-
Haley, J. E.; Shensky, W.; Mott, A. G. Opt. Lett. 2008, 33 (10), 1053.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7605

Ji et al.
Figure 8. (A) Time-resolved triplet transient difference absorption spectra of 4a in CH3CN solution at room temperature. (B) Triplet transient difference
absorption spectra of 1a-5a in CH3CN solution at room temperature at zero time delay. The excitation wavelength was 355 nm.
Table 2. Triplet Excited-State Characteristics of 1a-5a in CH2Cl2 and
CH3CN Solutions
λ
T1-Tn/nm (τ/ns)a
b
c
d
complex
CH2Cl2 CH3CN
εT -T /M-1 ·cm-1
φT
φ∆
1
n
1a
2a
3a
4a
5a
725(1130) 725(420)
725(1560) 720(660)
755(910) 755(130)
725(920) 730(340)
775(330) 785(130)
42390
40710
115090
39030
36600
0.65 0.40
0.53 0.38
0.19 0.24
0.64 0.46
0.66 0.38
a The triplet transient difference absorption band maxima and triplet
excited-state lifetimes. b The molar extinction coefficients of the triplet
excited-state absorption at the band maximum in CH3CN. c Quantum yields
of the triplet excited-state formation in CH3CN. d Quantum yields of singlet
oxygen generation in CH3CN.
4-tolylterpyridyl pentynyl complex.1h The reduction of the
3MLCT energy could favor the energy transfer from the
3MLCT state to the ground-state oxygen, thus increasing
the singlet oxygen generation efficiency. On the other hand,
the increased singlet oxygen generation efficiency for 1a-5a
could also be attributed to their much longer triplet excited-
state lifetimes (130-660 ns) in comparison to that of the
platinum 4-tolylterpyridyl pentynyl complex (τT ) 62 ns).25
Nonlinear Transmission. As discussed in the transient
absorption section, 1a-5a exhibit a broad triplet excited-
state absorption from 500 to 820 nm and possess relatively
high triplet excited-state quantum yields (except 3a) and long
lifetimes. Therefore, it is expected that these complexes
would exhibit reverse saturable absorption and consequently
nonlinear transmission with increased laser fluence in this
spectral region. As shown in Figure 9, at a linear transmit-
tance of 75% in a 2 mm cell at 532 nm, the transmission of
all five complexes decreases with increased incident fluence,
which is the typical phenomenon of nonlinear transmission,
and it likely arises from the reverse saturable absorption due
to the stronger triplet excited-state absorption versus that of
the ground state at 532 nm. At the same linear transmission
of 75%, 2a exhibits the strongest nonlinear transmission, with
the transmittance dropping to ∼45% at an incident fluence
of 1.5 J/cm2. Complexes 1a and 4a exhibit a similar nonlinear
transmission to that of 2a, while the nonlinear transmission
of 3a and 5a is obviously weaker. Overall, the nonlinear
transmissions of 1a-5a are much weaker than that of the
4-tolylterpyridyl platinum phenylacetylide complex reported
by our group previously.1i
Figure 9. Nonlinear transmission of 1a-5a in CH3CN solution in a 2 mm
cell. The linear transmission was adjusted to 75%.
Table 3. Ground-State and Triplet Excited-State Absorption
Cross-Sections of 1a-5a in CH3CN at 532 nm
a
b
complex
σ0 / cm2
σT /cm2
σT/σ0
σTΦT/σ0
1a
2a
3a
4a
5a
1.30 × 10-18
1.07 × 10-18
1.53 × 10-18
1.69 × 10-18
4.60 × 10-18
6.01 × 10-17
6.09 × 10-17
1.60 × 10-16
5.72 × 10-17
4.54 × 10-17
46.2
56.9
104.5
33.8
9.9
30.0
30.2
19.9
21.6
6.5
a The ground-state absorption cross-section. b The triplet excited-state
absorption cross-section.
To explain these differences, the parameters that are
directly related to the nonlinear transmission, that is, the ratio
of the excited-state absorption cross-section to that of the
ground state (σT/σ0), and the triplet excited-state quantum
yield (ΦT), need to be closely examined. A larger ratio of
σTΦT/σ0 generally leads to a stronger nonlinear transmission.
To figure out this ratio, it is first necessary to estimate the
triplet excited-state absorption coefficient (εT) at 532 nm
using the method described in the Supporting Information.
The εT’s at 532 nm are estimated to be 15 700 M-1cm-1 for
1a, 15 910 M-1cm-1 for 2a, 41 880 M-1cm-1 for 3a, 14 940
M-1cm-1 for 4a, and 11 860 M-1cm-1 for 5a. These εT’s
can then be converted to the triplet excited-state absorption
cross-section (σT) using the conversion equation σ ) 2303ε/
NA, where NA is the Avogadro constant. The results are listed
in Table 3. It is important to note that the key parameter
that determines the nonlinear transmission is the ratio of
σTΦT/σ0, not just σT. σ0 can be calculated from the molar
extinction coefficient of UV-vis absorption at 532 nm,
7606 Inorganic Chemistry, Vol. 47, No. 17, 2008

Platinum(II) Phenylacetylide Complexes
which is also tabulated in Table 3. Comparing the σ0’s of
1a-5a at 532 nm to that of the 4-tolylterpyridyl platinum
phenylacetylide complex (2.43 × 10-19),1i they are 4.4-18.9
times larger. This would cause a decreased ratio of σTΦT/σ0
in the case of a similar σT and ΦT, and therefore a weaker
nonlinear transmission. Among 1a-5a, the σTΦT/σ0 values
for 1a, 2a, and 4a are larger than those for 3a and 5a, which
are consistent with the trend observed from the nonlinear
transmission measurement. Other factors that might contrib-
ute to the difference in nonlinear transmission is the triplet
excited-state lifetime, which is much longer for 2a, compa-
rable for 1a and 4a, and much shorter for 3a and 5a.
anion radical. Except for 3a, the other four complexes exhibit
relatively high quantum yields of the triplet excited-state
formation. This subsequently leads to the formation of singlet
oxygen in a much higher quantum yield in comparison to
the 4-tolyterpyridyl platinum pentynyl complex. In addition,
all complexes exhibit a moderate nonlinear transmission at
532 nm for nanosecond laser pulses, and the difference in
their nonlinear transmission behavior arises from the differing
ratio of σTΦT/σ0.
Acknowledgment. Acknowledgment is made to the
National Science Foundation (CAREER CHE-0449598) and
Army Research Laboratory (W911NF-06-2-0032) for sup-
port. We are also grateful to North Dakota State EPSCoR
(ND EPSCoR Instrumentation Award) for support.
Conclusion
4′-(5′′′-R-Pyrimidyl)-2,2′;6′,2′′-terpyridine platinum(II) phe-
nylacetylide complexes (1a-5a) exhibit moderately intense
1MLCT/1LLCT absorption in their UV-vis spectra, the
energy of which is influenced by the 5′′′ substituent. All
complexes emit at room temperature, 77 K, and in the solid
state, and the emitting state is tentatively assigned to the
3MLCT state. The emission energy at room temperature
exhibits a linear correlation with the Hammett σp constant
of the 5′′′ substituent, indicating the electron delocalization
ability of the pyrimidyl group. All complexes exhibit a broad,
moderately intense triplet excited-state absorption from 500
to 820 nm, which is presumably attributed to the terpyridyl
Supporting Information Available: Plots of the lowest-energy
UV-vis absorption band energy and the emission band energy vs
Hammett σp constant of the 5-pyrimidinyl substituent; concentra-
tion-dependent emission spectra of 1a at room temperature;
concentration-dependent excitation spectra of 2a at room temper-
ature; excitation spectra of 2a monitored at different wavelengths
at 77 K; and time-resolved triplet transient difference absorption
spectra of 1a, 2a, 3a, and 5a in CH3CN. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC800358W
Inorganic Chemistry, Vol. 47, No. 17, 2008 7607