Platinum diimine bis(acetylide) complexes: Synthesis, characterization, and luminescence properties

Article abstract of DOI:10.1021/ic991250n

A new set of luminescent platinum(II) diimine complexes has been synthesized and characterized. The anionic ligands in these complexes are arylacetylides. The complexes are brightly emissive in fluid solution with relative emission quantum yields φ(em) ranging from 3 x 10^-3 to 10^-1. Two series of complexes have been investigated. The first has the formula Pt(Rphen)(C≡CC₆H₅)₂ where Rphen is 1,10-phenanthroline substituted in the 5-position with R = H, Me, Cl, Br, NO₂, or C≡CC₆H₅, while the second has the formula Pt(dbbpy)(C≡CC₆H₄X)₂ where dbbpy = 4,4'-di(tert-butyl)bipyridine and X = H, Me, F, or NO₂. From NMR, IR, and electronic spectroscopies, all of the complexes are assigned a square planar coordination geometry with cis-alkynyl ligands. The crystal structure of Pt(phen)(C≡CC₆H₄CH₃)₂ confirms this assignment. All of the complexes exhibit an absorption band at ca. 400 nm that corresponds to a Pt d → π*(diimine) charge-transfer transition. The variation of λ(max) for this band with substituent variation supports this assignment. From similar changes in the energy of the solution luminescence as a function of substituents R and X, the emissive excited state is also of MLCT origin, but with spin-forbidden character on the basis of excited-state lifetime measurements (0.01-5.6 μs). The complexes undergo electron-transfer quenching, showing good Stern-Volmer behavior using 10-methylphenothiazine and N,N,N',N'-tetramethylbenzidine as reductive quenchers. Excited-state reduction potentials are estimated on the basis of a simple thermochemical analysis. Crystal data for Pt(phen)(C≡CC₆H₄CH₃)₂: monoclinic, space group C2/c, a = 19.0961(1) A, b = 10.4498(1) A, c = 11.8124(2) A, β = 108.413(1)°, V = 2236.49 A³, number of reflections 1614, number of variables 150, R1 = 0.0163, wR2 (I > 2σ) = 0.0410.

Full text of DOI:10.1021/ic991250n

Inorg. Chem. 2000, 39, 447-457
447
Platinum Diimine Bis(acetylide) Complexes: Synthesis, Characterization, and Luminescence
Properties
Muriel Hissler,† William B. Connick,†,‡ David K. Geiger,*,§ James E. McGarrah,†
Donald Lipa,§ Rene J. Lachicotte,† and Richard Eisenberg*,†
Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, and
Department of Chemistry, State University of New York-College at Geneseo,
Geneseo, New York 14454
ReceiVed October 25, 1999
A new set of luminescent platinum(II) diimine complexes has been synthesized and characterized. The anionic
ligands in these complexes are arylacetylides. The complexes are brightly emissive in fluid solution with relative
emission quantum yields φem ranging from 3 × 10-3 to 10-1. Two series of complexes have been investigated.
The first has the formula Pt(Rphen)(CtCC6H5)2 where Rphen is 1,10-phenanthroline substituted in the 5-position
with R ) H, Me, Cl, Br, NO2, or CtCC6H5, while the second has the formula Pt(dbbpy)(CtCC6H4X)2 where
dbbpy ) 4,4′-di(tert-butyl)bipyridine and X ) H, Me, F, or NO2. From NMR, IR, and electronic spectroscopies,
all of the complexes are assigned a square planar coordination geometry with cis-alkynyl ligands. The crystal
structure of Pt(phen)(CtCC6H4CH3)2 confirms this assignment. All of the complexes exhibit an absorption band
at ca. 400 nm that corresponds to a Pt d f π*diimine charge-transfer transition. The variation of λmax for this band
with substituent variation supports this assignment. From similar changes in the energy of the solution luminescence
as a function of substituents R and X, the emissive excited state is also of MLCT origin, but with spin-forbidden
character on the basis of excited-state lifetime measurements (0.01-5.6 µs). The complexes undergo electron-
transfer quenching, showing good Stern-Volmer behavior using 10-methylphenothiazine and N,N,N′,N′-
tetramethylbenzidine as reductive quenchers. Excited-state reduction potentials are estimated on the basis of a
simple thermochemical analysis. Crystal data for Pt(phen)(CtCC6H4CH3)2: monoclinic, space group C2/c, a )
19.0961(1) Å, b ) 10.4498(1) Å, c ) 11.8124(2) Å, â ) 108.413(1)°, V ) 2236.49 Å3, number of reflections
1614, number of variables 150, R1 ) 0.0163, wR2 (I > 2σ) ) 0.0410.
Introduction
for collecting and funneling photon energy, electron-hole pair
creation, charge separation by vectorial electron transfer, charge
accumulation, and catalysis of the desired energy-storing
reaction. The directional nature of charge-transfer excited states
in square planar diimine complexes is ideal for electron-hole
creation and separation, and consequently, these systems merit
consideration for use in molecular photochemical devices and
related supramolecular systems. The coordinative unsaturation
of square planar complexes also renders them of interest for
sensor applications.16
Complexes exhibiting excited-state properties consistent with
a high degree of charge separation are of inherent interest for a
number of reasons. One is their potential use in emerging
technologies such as the design of molecular-scale electronic
devices.1-5 Another is their applicability in the design of
supramolecular systems for the conversion of light energy to
chemical energy.3,6-15 Such systems will require components
† University of Rochester.
‡ Present address: Department of Chemistry, University of Cincinnati,
Cincinnati, OH 45221-0172.
For more than a decade, platinum diimine complexes of the
general formula Pt(diimine)X2 have been known to be lumi-
nescent in fluid solution, with the energy and nature of the
emitting state dependent on the anionic ligand X.17-22 For the
§ State University of New York-College at Geneseo.
(1) Ziessel, R. F. J. Chem. Educ. 1997, 74, 673-679.
(2) Balzani, V.; Moggi, L.; Scandola, F. In Towards a Supramolecular
Photochemistry: Assembly of Molecular Components to Obtain
Photochemical Molecular DeVices; Balzani, V., Eds.; D. Reidel
Publishing Co.: Dordrecht, Holland, 1987; pp 1-28.
(3) Balzani, V.; DeCola, L. In Supramolecular Chemistry; Balzani, V.,
DeCola, L., Eds.; Kluwer Academic Publishers: Dordrecht, Holland,
1992.
(11) Baxter, S. M.; Jones, J., W. E.; Danielson, E.; Worl, L.; Strouse, G.;
Younathan, J.; Meyer, T. J. Coord. Chem. ReV. 1991, 111, 47-71.
(12) Yonemoto, E. H.; Riley, R. L.; Kim, Y. I.; Atherton, S. J.; Schmehl,
R. H.; Mallouk, T. E. J. Am. Chem. Soc. 1992, 114, 8081-8087.
(13) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S. M.; Riley,
R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 6,
4786-4795.
(4) Balzani, V. In Supramolecular Photochemistry; Balzani, V., Ed.; D.
Reidel Publishing Co.: Dordrecht, Holland, 1987.
(5) Balzani, V.; Gomez-Lopez, M.; Stoddart, J. F. Acc. Chem. Res. 1998,
31, 405-414.
(6) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170.
(7) Meyer, T. J. Photochem. Energy ConVers., Proc. Int. Conf. Photochem.
ConVers. Storage Solar Energy, 7th 1989, 75-95.
(8) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100,
13148-13168.
(9) Meyer, T. J. Abstr. Pap. Am. Chem. Soc. 1996, 212, 222-INOR.
(10) Treadway, J. A.; Chen, P. Y.; Rutherford, T. J.; Keene, F. R.; Meyer,
T. J. J. Phys. Chem. A 1997, 101, 6824-6826.
(14) Kaschak, D. M.; Mallouk, T. E. Abstr. Pap. Am. Chem. Soc. 1997,
214, 358-PHYS.
(15) Hasenknopf, B.; Hall, J.; Lehn, J. M.; Balzani, V.; Credi, A.;
Campagna, S. New J. Chem. 1996, 20, 725-730.
(16) Kunugi, Y.; Mann, K. R.; Miller, L. L.; Exstrom, C. L. J. Am. Chem.
Soc. 1998, 120, 589-590.
(17) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529-1533.
(18) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.; Wang, Y. Inorg.
Chem. 1993, 32, 2518-2524.
(19) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625-5627.
10.1021/ic991250n CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/20/2000

448 Inorganic Chemistry, Vol. 39, No. 3, 2000
Hissler et al.
ethynylbenzene, 1-methoxy-4-ethynylbenzene, and 4-ethynyltoluene
(Aldrich) were used as received. 1-Nitro-4-ethynylbenzene,32 5-nitro-
systems in which a dithiolate is the anion, the excited state has
been assigned as a charge transfer from an orbital of mixed
metal/dithiolate character to a π* orbital localized on the diimine
ligand.23-25 For complexes in which X ) CN, the emission is
often structured and usually occurs at substantially higher
phenanthroline,33 Pt(phen)Cl2,34 and Pt(dbbpy)Cl2 were prepared
34
according to literature methods. Pt(Xphen)Cl2 complexes, where X is
NO2, Cl, CH3, or Br in the 5-position of phenanthroline, were prepared
in a fashion analogous to the synthesis of Pt(phen)Cl2.34 The syntheses
of Pt(Rphen)(alkynyl)2 complexes were performed under nitrogen with
all solvents degassed prior to use.
3
energy, corresponding to a (ππ*) excited state, although in
more concentrated solutions, the di(cyanide) complexes exhibit
a broad red-shifted excimeric emission.20,26,27
5-Bromophenanthroline (Brphen). The compound was synthesized
using a modification of the preparation described by Mlochowski.35
The yield of product is quite sensitive to the amount of bromine
employed, the temperature, and the reaction time. Higher temperatures
result in the formation of varying amounts of 5,6-dibromophenanthroline
and 1,10-phenanthroline-5,6-dione. In our hands, the following prepara-
tion resulted in the maximum yield of product. A 3.6 g (20 mmol)
sample of phenanthroline was placed in a heavy-walled glass reaction
tube with a Teflon screw top fitted with a Viton O-ring. The reaction
vessel was placed in an ice bath, and 12 mL of oleum (15%) and 0.60
mL (11.6 mmol) of bromine were added. The reaction tube was placed
in a silicon oil bath, and the temperature was slowly raised to 135 °C.
After 23 h, the reaction mixture was cooled to room temperature, poured
over ice, and neutralized with NH4OH. The mixture was extracted with
CHCl3. The extracts were stirred with charcoal and then dried over
Na2SO4. The crude reaction mixture contained about 5% unreacted
phenanthroline as judged by 1H NMR spectroscopy. The solid was
recrystallized from hot diethyl ether with a minimum amount of
CH2Cl2. Yield: 4.67 g (90%). 1H NMR (CDCl3): δ 9.17 (m, 2H), 8.64
(dd, 1H, J ) 8.0 Hz, J ) 1.6 Hz), 8.15 (pseudo-dd, 1H, J ) 8.4 Hz,
J ) 2.0 Hz, J ) 8.0 Hz, J ) 1.6 Hz), 8.12 (s, 1H), 7.72 (dd, 1H, J )
8.4 Hz, J ) 4.0 Hz), 7.61 (dd, 1H, J ) 8.0 Hz, J ) 4.4 Hz).
Pt(phen)(CtCC6H5)2 (1). A 100 mg (0.22 mmol) sample of Pt-
(phen)Cl2, 10 mg of CuI, 0.2 mL (2.0 mmol) of phenylacetylene, 4
mL of DMF, and 3 mL of diethylamine were sonicated for 4 h. The
flask was chilled, and the bright yellow precipitate was collected by
filtration and washed with ether. The material was recrystallized from
methylene chloride and methanol. Yield: 100 mg (77%). 1H NMR
(DMSO-d6): δ 9.71 (2H, d, J ) 5.2 Hz), 9.04 (2H, d, J ) 8.0 Hz),
8.30 (2H, s), 8.23 (2H, dd, J ) 8.4 Hz, J ) 5.2 Hz), 7.51 (4H, d, J )
7.2 Hz), 7.32 (4H, pseudo-t, J ) 7.2 Hz, J ) 7.6 Hz), 7.22 (2H, pseudo-
t, J ) 7.6 Hz, J ) 7.2 Hz). FT-IR (mull): ν/cm-1 2106, 2116 (νCtC).
FAB+ (m-NBA): m/z 578.0 [M + H]+. UV-vis (CH3CN): λmax/nm
(ꢀ/M-1 cm-1) 392 (8000), 292 (sh) (32 400), 270 (51 000). Anal. Calcd
for C28H18N2Pt (Mr ) 577.54): C, 58.23; H, 3.14; N, 4.85. Found: C,
57.95; H, 2.90; N, 4.80.
In 1994, Che published a brief account of a different
luminescent platinum diimine complex in which X ) phenyl-
acetylide.28 This complex, Pt(phen)(CtCC6H5)2, was said to
be brightly emissive with an excited-state energy between those
3
of the di(cyanide) and the dithiolate complexes, and a MLCT
emissive state was proposed. The bright luminescence and the
change in excited state from those of the dithiolate and
di(cyanide) complexes made this system and its derivatives
worthy of further study. The notion of how different X ligands
influence the relative positions of ligand-centered, ligand field,
and charge-transfer excited states in platinum diimine complexes
has been discussed by Miskowski et al.18
Luminescence in other platinum acetylide complexes has been
noted as well.29 The specific systems investigated were trans-
Pt(CtCR)2(PEt3)2, where R ) H or C6H5, and the luminescence
of those systems was studied mainly in the solid state and frozen
glass solutions at 77 K. While site heterogeneity was noted for
the frozen glass samples, the emission was assigned as arising
2
from a charge-transfer excited state involving Pt dz and
acetylenic π* orbitals. In fluid solution, emission from the two
acetylide complexes (R ) H, C6H5) was described as “weak”
and “barely above background”.29
In this paper, we describe the synthesis, characterization, and
luminescence properties of the highly emissive platinum diimine
bis(arylacetylide) complexes. Through ligand variation, the
nature of the excited state has been probed. The complexes are
found to undergo electron-transfer quenching, and through the
use of a simplified thermochemical cycle, excited-state reduction
potentials are estimated. In the course of studying the photo-
physical properties of the Pt(diimine)(CtCAr)2 complexes, it
was found that solution excited-state lifetimes decreased with
increasing metal complex concentration indicative of self-
quenching, which in turn has led to a separate investigation of
self-quenching by platinum diimine complexes in general.30
While the work described in this paper was in progress, a
separate report by James et al. on the synthesis of the
Pt(diimine)(CtCAr)2 complexes appeared.31
Pt(phen)(CtCC6H4CH3)2 (2). The procedure followed for 1 was
used except that 0.22 mL of ethynyltoluene was used. Yield: 120 mg
1
(88%). H NMR (DMSO-d6): δ 9.83 (2H, dd, J ) 5.1 Hz, J ) 1.2
Hz), 9.04 (2H, dd, J ) 8.2 Hz, J ) 1.3 Hz), 8.32 (2H, s), 8.25 (2H,
dd, J ) 8.3 Hz, J ) 5.2 Hz), 7.22 (AB quartet, 8H, JAB ) 8.0 Hz, ∆ν
) 56.9 Hz), 2.31 (6H, s). FT-IR (mull): ν/cm-1 2110, 2120 (sh) (νCt
Experimental Section
C). FAB+ (m-NBA): m/z 606.0 [M + H]+. UV-vis (CH3CN): λmax
/
nm (ꢀ/M-1 cm-1): 392 (9800), 292 (sh) (46 400), 270 (70 500). Anal.
Calcd for C30H22N2Pt‚0.5 CH3OH (Mr ) 605.55 + 16.02): C, 58.93;
H, 3.89; N, 4.51. Found: C, 59.03; H, 3.66; N, 4.66.
Reagents. Phenanthroline (Aldrich) and K2[PtCl4] (Johnson Matthey)
were used without further purification. Ethynylbenzene, 1-fluoro-4-
(20) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton Trans.
1991, 1077-1080.
Pt(phen)(CtCC6H4F)2 (3). The procedure followed for 1 was
(21) Zuleta, J. A.; Chesta, C. A.; Eisenberg, R. J. Am. Chem. Soc. 1989,
111, 8916-8917.
used except that 0.18 mL of 1-ethynyl-4-fluorobenzene was used.
1
Yield: 100 mg (91%). H NMR (DMSO-d6): δ 9.79 (2H, d, J ) 5.4
(22) Zuleta, J. A.; Bevilacqua, J. M.; Eisenberg, R. Coord. Chem. ReV.
1992, 111, 237-248.
Hz), 9.05 (2H, d, J ) 8.4 Hz), 8.31 (2H, s), 8.24 (2 H, dd, J ) 8.4 Hz,
J ) 5.4 Hz), 7.46 (4H, dd, J ) 8.4 Hz, J ) 5.7 Hz), 7.15 (4H, t, J )
9.0 Hz). FT-IR (mull): ν/cm-1 2112, 2123 (sh) (νCtC). FAB+
(m-NBA): m/z 614 [M + H]+. UV-vis (CH3CN): λmax/nm (ꢀ/M-1
cm-1) 391 (7100), 286 (sh) (29 600), 265 (46 800). Anal. Calcd for
(23) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-
1960.
(24) Zuleta, J. A.; Bevilacqua, J. M.; Proserpio, D. M.; Harvey, P. D.;
Eisenberg, R. Inorg. Chem. 1992, 31, 2396-2404.
(25) Cummings, S. D.; Eisenberg, R. Inorg. Chem. 1995, 34, 2007-2014.
(26) Che, C.-M.; Wan, K.-T.; He, L.-Y.; Poon, C.-K.; Yam, V. W.-W. J.
Chem. Soc., Chem. Commun. 1989, 943-945.
(31) James, S. L.; Younus, M.; Raithby, P. R.; Lewis, J. J. Organomet.
Chem. 1997, 543, 233-235.
(32) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627-630.
(33) Smith, G. F.; Cagle, J., F. W. J. Org. Chem. 1947, 12, 781-784.
(34) Hodges, K. D.; Rund, J. V. Inorg. Chem. 1975, 14, 525-528.
(35) Mlochowski, J. Roczniki Chem., Ann. Soc. Chim. Polonorum 1974,
48, 2145.
(27) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625-5627.
(28) Chan, C.-W.; Cheng, L.-K.; Che, C.-M. Coord. Chem. ReV. 1994, 132,
87-97.
(29) Sacksteder, L.; Baralt, E.; DeGraff, B. A.; Lukehart, C. M.; Demas,
J. N. Inorg. Chem. 1991, 30, 2468-2476.
(30) Connick, W. B.; Geiger, D. P.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264-3265.

Platinum Diimine Bis(acetylide) Complexes
Inorganic Chemistry, Vol. 39, No. 3, 2000 449
C28H16N2F2Pt (Mr ) 613.52): C, 54.82; H, 2.63; N, 4.57. Found: C,
54.64; H, 2.56; N, 4.59.
and 3 mL of Et3N was deaerated and added to the flask. A 44 µL (0.40
mmol) portion of ethynylbenzene was added to the mixture, which was
stirred at room temperature for 2 h. After filteration, the remaining
Et3N base was removed via rotary evaporation and the DMF solvent
was removed under vacuum. The golden residue was recrystallized from
CHCl3 and EtOH. A 120 mg sample of material was collected. A bright
yellow fraction was isolated by preparative TLC (silica gel; 1% MeOH/
Pt(NO2phen)(CtCC6H5)2 (4). Method 1: To 100 mg (0.20 mmol)
of Pt(phenNO2)Cl2 and 10 mg of CuI were added 4 mL of DMF, 4 mL
of triethylamine, and 0.2 mL of ethynylbenzene. The mixture was
sonicated for 31/2 h, during which time most of the starting material
dissolved. The solution was filtered, removing ca. 10 mg of yellow
solid identified as starting material. The product was isolated from the
filtrate with the addition of diethyl ether and dried under vacuum.
Yield: 96 mg (76%). Method 2: A 50 mg (0.102 mmol) sample of
Pt(5-NO2phen)Cl2 and 1 mg of CuI were placed in a Schlenk flask
under nitrogen. A mixture of dichloromethane (20 mL), acetonitrile
(10 mL), and diisopropylamine (3 mL) was degassed and added to the
flask. Then 0.03 mL of ethynylbenzene was added to the mixture, which
was stirred for 24 h at room temperature. The solvent was removed
under vacuum. The crude precipitate was chromatographed on neutral
alumina using dichloromethane/methanol as eluent with a gradient of
methanol (0-10, v/v). The product was further purified by recrystal-
lization from acetone/hexane. Yield: 39 mg (61%). 1H NMR (DMSO-
d6): δ 9.82 (d, 2H, J ) 4.5 Hz), 9.39 (s, 1H), 9.36 (d, 1H, J ) 9.3
Hz), 9.24 (d, 1H J ) 7.8 Hz), 8.34 (m, 2H), 7.42 (d, 4H, J ) 7.5 Hz),
7.32 (pseudo-t, 4H, J ) 7.2 Hz, J ) 7.7 Hz), 7.21 (pseudo-t, 2H, J )
7.2 Hz). FT-IR (mull): ν/cm-1 2116, 2126 (sh) (νCtC); 842, 1334, 1484
1
CH2Cl2). Yield: 45 mg (25%). H NMR (DMSO-d6): δ 9.89 (d, 1H,
J ) 5.2 Hz), 9.80 (d, 1H, J ) 5.2 Hz), 9.30 (dd, 1H, J ) 8.4 Hz, J )
1.2 Hz), 9.01 (d, 1H, J ) 8.4 Hz), 8.65 (s, 1H), 8.35 (dd, 1H, J ) 8.4
Hz, J ) 5.2 Hz), 8.25 (dd, 1H, J ) 8.4 Hz, J ) 5.2 Hz), 7.84 (m, 2H),
7.54 (m, 3H), 7.43 (m, 4H), 7.31 (t, 4H, J ) 7.6 Hz), 7.20 (pseudo-t,
2H, J ) 8.0 Hz, J ) 7.2 Hz). FT-IR (mull): ν/cm-1 2106, 2115, 2124
(νCtC). FAB+ (m-NBA): m/z 677 [M]+. UV-vis (CH3CN): λmax/nm
(ꢀ/M-1 cm-1) 396 (9600), 336 (23 600), 273 (66 500). Anal. Calcd for
C36H22N2Pt‚CH3OH (Mr ) 677.66 + 32.04): C, 62.62; H, 3.69; N,
3.95. Found: C, 62.71; H, 3.47; N, 4.02.
Pt(dbbpy)(CtCC6H5)2 (9). A 150 mg (0.28 mmol) sample of Pt-
(dbbpy)Cl2 and 5 mg of CuI were placed in a Schlenk flask under
nitrogen. A solution of 15 mL of dichloromethane and 5 mL of
diisopropylamine was degassed and added to the flask. Ethynylbenzene
(0.08 mol) was then added to the mixture, which was stirred for 24 h
at room temperature. The solvent was removed under vacuum. The
crude precipitate was chromatographed on neutral alumina using
hexane/dichloromethane as eluent with a gradient of dichloromethane
(20-100, v/v). The yellow product was further purified by recrystal-
lization from dichloromethane/methanol and hexane. Yield: 110 mg
(58%). Rf ) 0.50 (20 vol % hexane in dichloromethane, silica gel).1H
NMR (d6-acetone): δ 9.66 (d, 2H, J ) 6 Hz), 8.61 (d, 2H, J ) 1 Hz),
7.86 (dd, 2H, J ) 6 Hz, J ) 2 Hz), 7.38 (dd, 4H, J ) 8 Hz), 7.23 (t,
4H, J ) 8 Hz), 7.11 (t, 2H, J ) 8 Hz), 1.46 (s, 18H). FT-IR (KBr):
ν/cm-1 2119, 2106 (νCtC). FAB+ (m-NBA): m/z 666 [M + H]+. UV-
vis (CH3CN): λmax/nm (ꢀ/M-1 cm-1) 386 (5800), 281 (sh) (42 200),
262 (44 300). Anal. Calcd for C34H34N2Pt‚CH3OH (Mr ) 665.73 +
32.04): C, 60.25; H, 5.49; N, 4.01. Found: C, 60.43; H, 5.44; N, 3.83.
Pt(dbbpy)(CtCC6H4CH3)2 (10). The procedure followed for 9 was
used except that 0.09 mL of ethynyltoluene was used. Yield: 120 mg
(61%). Rf ) 0.51 (20 vol % hexane in dichloromethane, silica gel). 1H
NMR (CDCl3): δ 9.53 (d, 2H, J ) 5 Hz), 7.89 (d, 2H, J ) 2 Hz), 7.40
(dd, 2H, J ) 6 Hz, J ) 2 Hz), 7.15 (AB quartet, 8H, JAB ) 8.0 Hz, ∆ν
) 139.8 Hz), 2.41 (s, 6H), 1.34 (s, 18H). FT-IR (KBr): ν/cm-1 2112,
2124 (sh) (νCtC). FAB+ (m-NBA): m/z 693 [M + H]+. UV-vis
(CH3CN): λmax/nm (ꢀ/M-1 cm-1) 389 (8200), 273 (73 000). Anal. Calcd
for C36H38N2Pt (Mr ) 693.79): C, 62.32; H, 5.52; N, 4.04. Found: C,
62.28; H, 5.64; N, 3.80.
(νNO ). FAB+ (m-NBA) m/z: 623 [M + H]+. UV-vis (CH3CN): λmax
/
2
nm (ꢀ/M-1 cm-1) 397 (5000), 323 (sh) (6200), 272 (31 000). Anal.
Calcd for C28H17N3O2Pt‚C3H6O (Mr ) 622.54 + 58.08): C, 54.71; H,
3.41; N, 6.17. Found: C, 54.63; H, 3.23; N, 6.31.
Pt(Brphen)(CtCC6H5)2 (5). A 100 mg (0.15 mmol) sample of Pt-
(Brphen)Cl2 and 10 mg of CuI were placed in a round-bottom flask
under nitrogen. A mixture of 4 mL of DMF and 3 mL of Et3N was
degassed and added to the flask. A 2 mL portion of ethynylbenzene
was added to the mixture, which was sonicated for 3 h. After filtration,
the remaining Et3N was removed under vacuum. The DMF solution
was added to CH2Cl2 and extracted with water. The CH2Cl2 fraction
was washed with water and then evaporated to dryness. Yield: 69 mg
(86%). The NMR spectrum of the material showed a small amount of
starting material; the sample was purified by preparative TLC (silica
1
gel; 1% MeOH/CH2Cl2). H NMR (DMSO-d6): δ 9.77 (d, 1H, J )
5.2 Hz), 9.70 (d, 1H, J ) 4.8 Hz), 8.99 (d, 1H, J ) 8.4 Hz), 8.92 (d,
1H, J ) 8.4 Hz), 8.74 (s, 1H), 8.28 (dd, 1H, J ) 8.0 Hz, J ) 5.2 Hz),
8.19 (dd, 1H, J ) 8.4 Hz, J ) 5.4 Hz), 7.41 (m, 4H), 7.31 (pseudo-t,
4H, J ) 7.6 Hz), 7.20 (pseudo-t, 2H, J ) 7.2 Hz). FT-IR (mull): ν/cm-1
2116, 2125 (sh) (νCtC). FAB+ (m-NBA): m/z 656 [M + H]+. UV-
vis (CH3CN): λmax/nm (ꢀ/M-1 cm-1) 395 (8000), 262 (86 000). Anal.
Calcd for C28H17N2PtBr (Mr ) 656.44): C, 51.23; H, 2.61; N, 4.27.
Found: C, 50.94; H, 2.71; N, 4.09.
Pt(dbbpy)(CtCC6H4OCH3)2 (11). The procedure followed for 9
was used except that 0.09 mL of 1-methoxy-4-ethynylbenzene was used.
Yield: 130 mg (64%). Rf ) 0.48 (dichloromethane, silica gel). 1H NMR
(d6-acetone): δ 9.68 (d, 2H, J ) 6 Hz), 8.59 (d, 2H, J ) 1.6 Hz), 7.84
(dd, 2H, J ) 5.6 Hz, J ) 1.6 Hz), 7.06 (AB quartet, 8H, JAB ) 8.8 Hz,
∆ν ) 196.2 Hz), 3.77 (s, 6 H), 1.46 (s, 18H). FT-IR (KBr): ν/cm-1
2111, 2121 (sh) (νCtC). FAB+ (m-NBA): m/z 725 [M + H]+. UV-
vis (CH3CN): λmax/nm (ꢀ/M-1 cm-1) 393 (6600), 279 (38 000). Anal.
Calcd for C36H38N2PtO2‚CH3OH (Mr ) 725.79 + 32.04): C, 58.64;
H, 5.59; N, 3.70. Found: C, 58.46; H, 5.57; N, 3.31.
Pt(Clphen)(CtCC6H5)2 (6). The procedure for Pt(Brphen)-
(CtCC6H5)2 was followed except that 132 mg (0.275 mmol) of Pt-
(Clphen)Cl2 and 10 mL of degassed MeCN were added to the reaction
mixture. Yield: 116 mg (69%). 1H NMR (DMSO-d6): δ 9.85 (d, 1H,
J ) 4.8 Hz), 9.75 (d, 1H, J ) 4.8 Hz), 9.12 (d, 1H, J ) 8.4 Hz), 8.95
(d, 1H, J ) 8.0 Hz), 8.61 (s, 1H), 8.33 (dd, 1H, J ) 8.4 Hz, J ) 5.2
Hz), 8.23 (dd, 1H, J ) 8.0 Hz, J ) 5.2 Hz), 7.42 (m, 4H), 7.31 (pseudo-
t, 4H, J ) 7.6 Hz), 7.19 (pseudo-t, 2H, J ) 7.6 Hz). FT-IR (mull):
ν/cm-1 2121, 2130 (sh) (νCtC). FAB+ (m-NBA): m/z 612 [M + H]+.
UV-vis (CH3CN): λmax/nm (ꢀ/M-1 cm-1) 397 (7200), 272 (49 500).
Anal. Calcd for C28H17N2ClPt‚CH3OH (Mr ) 611.98 + 32.04): C,
54.08; H, 3.29; N, 4.35. Found: C, 54.46; H, 2.95; N, 4.32.
Pt(CH3phen)(CtCC6H5)2 (7). The procedure for 6 was followed
except 70.0 mg (0.152 mmol) of Pt(CH3phen)Cl2 was used. Yield: 57.6
mg (64%). 1H NMR (DMSO-d6): δ 9.81 (d, 1H, J ) 5.1 Hz), 9.71 (d,
1H, J ) 5.1 Hz), 9.06 (d, 1H, J ) 8.5 Hz), 8.88 (d, 1H, J ) 8.3 Hz),
8.24 (dd, 1H, J ) 8.4 Hz, J ) 5.2 Hz) 8.17 (dd, 1H, J ) 8.2 Hz, J )
5.2 Hz), 8.10 (s, 1H), 7.41 (m, 4H), 7.31 (m, 4H), 7.19 (m, 2H), 2.87
(s, 3H). FT-IR (mull): ν/cm-1 2117, 2126 (sh). FAB+ (m-NBA): m/z
592 [M + H]+. UV-vis (CH3CN) λmax/nm (ꢀ/M-1 cm-1) 396 (7600),
272 (49 400). Anal. Calcd for C29H20N2Pt‚H2O (Mr ) 591.57 +
18.02): C, 57.14; H, 3.64; 4.60. Found: C, 56.88; H, 3.44; N, 4.65.
Pt(C6H5CtCphen)(CtCC6H5)2 (8). A 140 mg (0.27 mmol) sample
of Pt(Brphen)Cl2, 16 mg of Pd(PPh3)2Cl2, and 2 mg of CuI were placed
in a round-bottom flask under nitrogen. A mixture of 5 mL of DMF
Pt(dbbpy)(CtCC6H4F)2 (12). The procedure followed for 9 was
used except that 0.08 mL of 1-fluoro-4-ethynylbenzene was used.
Yield: 146 mg (74%). Rf ) 0.61 (20 vol % hexane in dichloromethane,
1
silica gel). H NMR (d6-acetone): δ 9.61 (d, 2H, J ) 6 Hz), 7.88 (s,
2H), 7.51 (d, 2H, J ) 6 Hz), 7.41 (dd, 4 H, J ) 8.0 Hz, J ) 5.6 Hz),
6.86 (pseudo-t, 4 H, J ) 8.8 Hz) 1.38 (s, 18H). FT-IR (KBr): ν/cm-1
2114, 2126 (sh) (νCtC). FAB+ (m-NBA): m/z 701 [M + H]+. UV-
vis (CH3CN) λmax/nm (ꢀ/M-1 cm-1) 385 (7500), 271 (55 800). Anal.
Calcd for C34H32N2PtF2‚0.5 CH2Cl2 (Mr ) 701.71 + 42.47): C, 55.68;
H, 4.47; N, 3.76. Found: C, 55.63; H, 4.80; N, 3.43.
Pt(dbbpy)(CtCC6H4NO2)2 (13). The procedure followed for 9 was
used except that 0.100 mg (0.57 mmol) of 1-nitro-4-ethynylbenzene
was used. Yield: 139 mg (61%). Rf ) 0.81 (10 vol % hexane in
dichloromethane, silica gel). 1H NMR(CDCl3): δ 9.50 (d, 2H, J ) 6.2
Hz), 7.94 (d, 2H, J ) 2.0 Hz), 7.58 (dd, 2H, J ) 6.2 Hz, J ) 2.0 Hz),
7.80 (AB quartet, 8 H, JAB ) 9.1 Hz, ∆ν ) 214.2 Hz), 1.41 (s, 18H).

450 Inorganic Chemistry, Vol. 39, No. 3, 2000
Hissler et al.
Table 1. Crystallographic Data for Pt(phen)(CtCC6H4CH3)2 (2)
empirical formula
fw
T, K
C30H22N2Pt
605.59
193(2)
abs correction
transm range
F(000)
empirical (SADABS)
0.633-0.928
1176
λ, Å
0.710 73
monoclinic
C2/c (no. 15)
4
19.0961(1)
10.4498(1)
11.8124(2)
108.413(1)
2236.49
1.799
2θ range, deg
5.5 to 46.8
-21 e h e 19
-11 e k e 10
-12 e l e 13
4857
1614/1574
1614/0/150
1.088
cryst syst
space group
Z
limiting indices
a, Åa
no. of reflns collected
b, Åa
no. of independent/obsd (I > 2σ) reflns
no. of data/restraints/params
goodness of fitb
c, Åa
â, dega
V, Å3
R1, wR2 (I > 2σ)c
0.0163, 0.0410
0.0170, 0.0413
0.584 and -0.603
F
calcd, g cm-3
R1, wR2 (all data)c
µ, mm-1
6.295
largest difference peak and hole, e/Å3
a It has been noted that the integration program SAINT produces cell constant errors that are unreasonably small, since systematic error is not
included. More reasonable errors might be estimated at 10× the listed value. b GOF ) [Σ[w(Fo - Fc2)2]/(n - p)]1/2, where n and p denote the
2
number of data and parameters, respectively. c R1 ) (Σ||Fo| - |Fc||)/Σ|Fo|; wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo ]]1/2, where w ) 1/[σ2(Fo ) + (aP)2
2
2
2
2
+ bP] and P ) f(max) or Fo + (1 - f)Fc2.
FT-IR (KBr): ν/cm-1 2111, 2121 (sh) (νCtC) 852, 1337, 1504 (νNO ).
10-5 M, while quencher concentrations varied in the range 5 × 10-6
to 3 × 10-4 M. N,N,N′,N′-Tetramethylbenzidine (TMB) was sublimed
just prior to use.
2
FAB+ (m-NBA): m/z 756 [M + H]+. UV-vis (CH3CN): λmax/nm (ꢀ/
M-1 cm-1) 371 (38 100), 320 (26 800), 278 (19 600). Anal. Calcd for
C34H32N4PtO4‚2CH3OH (Mr ) 755.73 + 64.08): C, 52.74; H, 4.92;
N, 6.83. Found: C, 52.58; H, 4.53; N, 6.52.
Crystal Structure Determination of Pt(phen)(CtCC6H4CH3)2 (2).
Crystals were grown by slow vapor diffusion of hexane into a
concentrated solution of 2 in CH2Cl2. A single crystal (0.18 × 0.14 ×
0.14 mm) coated with Paratone-8277 was mounted on a glass fiber
and immediately placed in a cold nitrogen steam at -80 °C. The X-ray
intensity data were collected on a standard Siemens SMART CCD area
detector system equipped with a normal focus molybdenum-target X-ray
tube operated at 2.0 kW (50 kV, 40 mA). A total of 1321 frames of
data (1.3 hemispheres) were collected using a narrow frame method
with scan widths of 0.3° in ω and exposure times of 10 s/frame using
a detector-to-crystal distance of 5.088 cm (maximum 2θ angle of
56.54°). The total data collection time was approximately 13 h. Frames
were integrated with the Siemens SAINT program to 0.90 Å to yield
a total of 4857 reflections, 1614 of which were independent. Crystal,
data collection,40 and refinement parameters are summarized in Table
1. The structure was solved using direct methods and refined by full-
matrix least-squares on F2. All non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were included in
idealized positions, giving a data:parameter ratio of about 11:1 in final
least-squares refinements. Final refined atomic coordinates and aniso-
tropic thermal parameters are given in the Supporting Information.
Molecular Orbital Calculations. Iterative extended Hu¨ckel (IEH)
calculations were performed using the program FORTICON8.41 The
Hii values obtained from valence-orbital ionization potentials (VOIP)
for H, C, and N were included in the program. For Pt, the double-ú
expansion was used and the Hii values were charge-iterated for square
planar complexes.42 The geometries of the complexes were optimized
using PCSpartan plus,40 giving C2V symmetry for Pt(phen)(CtCH)2,
Pt(phen)(CtCC6H5)2, and Pt(phen)(CtCF)2 and Cs symmetry for Pt-
(phenNO2)(CtCH)2. The structures were optimized with Pt-N and
Pt-C bond distances constrained at 1.948 and 2.062 Å, respectively,
as observed for the structure of 2.
Physical Measurements. Infrared spectra were obtained from
tetrachloroethylene mulls using a Perkin-Elmer 1600 FTIR spectrometer
or from KBr pellets using a Mattson Galaxy 6020 FTIR spectrometer.
1H NMR spectra were recorded on a Bruker AMX-400 or DPX 300
spectrometer (400 and 300 MHz, respectively). Cyclic voltammetry
experiments were carried out on either a BAS CV-37 electrochemical
analyzer or an EG&G PAR 263A potentiostat/galvanostat using a three-
electrode single-cell compartment. All samples were degassed with
nitrogen or argon. A Pt working electrode, Pt auxiliary electrode, and
Ag/AgNO3/acetonitrile reference electrode were used. For all of the
measurements, the ferrocene/ferrocenium couple at 0.40 V vs NHE
was used to calibrate the cell potential.
Absorption spectra were recorded using either a Hewlett-Packard
HP8452A or a Hitachi U2000 spectrometer. Luminescence spectra were
obtained using a Spex Fluorolog-2 spectrophotometer and corrected
for instrumental response. Fluid solution emission samples were freeze-
pump-thaw cycled at least four times with a vacuum of 10-4 Torr or
lower. Frozen glass emission samples at 77 K were prepared by inserting
a 3 mm (internal diameter) quartz tube containing a ca. 10-5 M solution
(butyronitrile or ethanol/methanol (3:1)) of the complex into a quartz-
tipped finger dewar of liquid nitrogen. Luminescence quantum yields
were referenced to tris(2,2′-bipyridyl)ruthenium(II) in acetonitrile (Φ
) 0.062),36 as previously described.37 Long emission lifetimes were
measured using an excimer pumped dye laser system (1-3 mJ/pulse)
and fitted to single-exponential decays.38 Shorter luminescent lifetimes
were determined using time-correlated single-photon counting.39
For all the compounds studied, unless otherwise noted, self-
quenching was observed.30 Over a range of concentrations (5 × 10-6
to approximately 2 × 10-4 M), the emission decays (kobs ) 1/τ) are
well modeled by a modified Stern-Volmer expression (eq 1),
kobs ) kq[Pt] + ko
(1)
Results and Discussion
Synthesis and Characterization. The synthetic strategy for
making the complexes Pt(diimine)(CtCAr)2 shown below
where kq is the self-quenching rate constant, [Pt] is the concentration
of Pt complex, and ko (1/τo) is the rate of excited-state decay at infinite
dilution. For the self-quenching studies, kobs values at each concentration
were taken at the emission maximum or at higher energy and averaged.
Bimolecular reductive quenching studies were performed by lumines-
cence lifetime measurements and fit by Stern-Volmer kinetics (eq 2),
(36) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar,
J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755-2763.
(37) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108,
1067-1071.
(38) Chen, L.; Farahat, M. S.; Gaillard, E. R.; Farid, S.; Whitten, D. G. J.
Photochem. Photobiol., A 1996, 95, 21-25.
(39) Jukabiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B.
R. J. Phys. Chem. A 1999, 103, 2394-2398.
(40) Wavefunction, Inc., 18401 Von Karman, Suite 370, Irvine, CA 92612.
(41) QCPE Program No. 344, Indiana University Chemistry Department.
(42) Hoffmann, R.; Summerville, R. H. J. Am. Chem. Soc. 1976, 98, 7240-
7254.
τo/τ ) 1 + kqτo[quencher]
(2)
where kq is the quenching rate constant, [quencher] is the concentration
of quencher, and τï and τ are the lifetimes of the chromophore in the
absence and presence of quencher, respectively. The concentration of
Pt chromophore used in these studies was held constant at ca. 3 ×

Platinum Diimine Bis(acetylide) Complexes
Inorganic Chemistry, Vol. 39, No. 3, 2000 451
involves a CuI-catalyzed chloride-to-alkyne metathesis that has
been used for synthesizing other Pt alkynyl derivatives.43 The
complexes Pt(dbbpy)(CtCC6H4X)2, with X ) H, CH3, OCH3,
NO2, or F, were obtained in good yield by this strategy from
stoichiometric reaction of Pt(dbbpy)Cl2 with the corresponding
terminal alkyne HCtCC6H4X. For the dbbpy systems, the CuI
catalyst (10 mol %) was used with dichloromethane as solvent
and diisopropylamine as base. The preparation of the 5-substi-
tuted phenanthroline derivatives Pt(Rphen)(CtCC6H4X)2 re-
quired sonication because of low solubility of the dichloride
starting compounds Pt(Rphen)Cl2 in the dimethylformamide/
diethylamine solvent/base combination employed.
1
All of the complexes were characterized by H NMR, FT-
IR, electronic absorption, and emission spectroscopies, mass
spectrometry, and elemental analyses. The spectroscopic data
support the anticipated structure assignment of a square planar
coordinated platinum(II) ion with two cis-alkynyl ligands and
a diimine chelate. The cis disposition of the alkynyl ligands
leads to two bands at ca. 2100 cm-1 corresponding to the sym-
metric and asymmetric νCtC stretches of the alkynyl ligands.43
The structural assignment for the Pt(diimine)(CtCAr)2 com-
plexes was confirmed by a structure determination of the bis-
(tolylacetylide) derivative Pt(phen)(CtCC6H4CH3)2 (2). While
the structure of Pt(phen)(CtCC6H5)2 (1) has been mentioned
in the literature,28 details of that determination have not yet
appeared.
Figure 1. Molecular structure and atom numbering scheme for Pt-
(phen)(CtCC6H4CH3)2 (2) (50% probability ellipsoids).
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2a
Pt(1)-C(1)
Pt(1)-N(1)
N(1)-C(10)
N(1)-C(14)
C(1)-C(2)
C(2)-C(3)
C(10)-C(11)
1.948(3) C(11)-C(12)
2.063(3) C(12)-C(13)
1.347(4) C(13)-C(14)
1.361(4) C(13)-C(15)
1.201(5) C(14)-C(14)′
1.452(5) C(15)-C(15)′
1.381(5)
1.365(5)
1.406(4)
1.397(4)
1.436(5)
1.425(6)
1.334(8)
C(1)′-Pt(1)-C(1)
C(1)-Pt(1)-N(1)′
C(1)-Pt(1)-N(1)
N(1)′-Pt(1)-N(1)
92.5(2)
C(10)-N(1)-Pt(1) 128.1(2)
94.32(12) C(14)-N(1)-Pt(1
170.83(12) C(2)-C(1)-Pt(1)
79.64(14) C(1)-C(2)-C(3)
113.6(2)
170.4(3)
177.8(4)
N(1)-C(14)-C(14)′ 116.5(2)
C(10)-N(1)-C(14) 118.0(3)
a Symmetry transformations used to generate equivalent (primed)
3
atoms: -x + 1, y, -z + /2.
1.948(3) Å, which compares well with values of 2.02(4) and
1.958(7) Å for the two independent molecules of [Pt-
(CtCtBu)4]2Ag4,45 2.01(3) Å in [cis-{Pt(C6F6)2(CtCSiMe3)2}-
Pd(η3-C3H5)]-,46 2.026(9) Å in [(PPh3)2Pt(µ-η1-η2-CtCtBu)2Pd-
(η3-C3H5)]+,47 1.998(4) Å in trans-Pt(2,2′:6′,2′′-terpyridin-
4′-ylethynyl)(PBu3)2,48 and 2.013(1) Å in [{cis-Pt(C6F5)2-
(CtCPh)2}2Ag2]2- 49
Only the Pt-C distance of 2.17(2) Å
.
found in [Pt2(µ-CdCHPh)(CtCPh)(PEt3)4]+ 50 is significantly
longer compared to the present value. The phenanthroline ligand
is planar to within 0.061 Å, while the phenyl rings are tilted at
an angle of 56° from the phenanthroline plane.
Deviations from square planar coordination geometry are
relatively minor. The C-Pt-C angle is 92.5(2)°, while a 9.6°
twist angle exists between the PtNN and PtCC planes defined
by the donor atoms. An examination of the crystal packing
shown in Figure 2 reveals an intricate network containing
cofacial π stacking between phenyl rings on adjacent molecules
and perpendicular aromatic interactions between phenyl rings
and neighboring phenanthroline ligands. The closest intermo-
Molecular Structure of Pt(phen)(CtCC6H4CH3)2 (2). The
molecular structure of 2, which is crystallographically required
to have C2 symmetry, is shown in Figure 1 along with the atom
numbering scheme used. Important bond lengths and angles are
given in Table 2, while a complete tabulation of distances and
angles may be found in the Supporting Information. The Pt-N
distance is 2.063(3) Å and compares well to those of other Pt-
(phen) complexes, such as [Pt(phen)2]Cl2, which exhibits a
Pt-N bond length of 2.033(6) Å.44 The Pt-C bond distance is
(45) Espinet, P.; Fornies, J.; Martinez, F.; Tomas, M.; Lalinde, E.; Moreno,
M. T.; Ruiz, A.; Welch, A. J. J. Chem. Soc., Dalton Trans. 1990,
791-798.
(46) Berenguer, J. R.; Fornies, J.; Lalinde, E.; Martinez, F. J. Organomet.
Chem. 1994, 470, C15-C18.
(47) Berenguer, J. R.; Fornies, J.; Lalinde, E.; Martinez, F. Organometallics
1996, 15, 4537-4546.
(48) Harriman, A.; Hissler, M.; Ziessel, R.; DeCian, A.; Fisher, J. J. Chem.
Soc., Dalton Trans. 1995, 4067-4080.
(43) Sonogashira, K.; Fujikura, Y.; Yatake, T.; Toyoshima, N.; Takahashi,
S.; Hagihara, N. J. Organomet. Chem. 1978, 145, 101-108.
(44) Hazell, A.; Mukhopadhyay, A. Acta Crystallogr. 1980, B36, 1647-
1649.
(49) Espinet, P.; Fornies, J.; Martinez, F.; Sotes, M.; Lalinde, E.; Moreno,
M. T.; Ruiz, A.; Welch, A. J. J. Organomet. Chem. 1991, 403, 253-
267.

452 Inorganic Chemistry, Vol. 39, No. 3, 2000
Hissler et al.
varied. Nevertheless, it is evident that the aggregate transitions
comprising the 400 nm band shift to lower energy as the
electron-accepting properties of the phenanthroline increase. In
particular, it is striking that the low-energy tail of this absorption
manifold systematically shifts to lower energy as the electron-
withdrawing properties of the diimine substituents increase (Me
< H < Cl, Br < NO2).51 This behavior is entirely consistent
with charge transfer to diimine transitions.
The influence of varying the para substituent of the aryl-
acetylide ligands on the ca. 400 nm feature is shown in Figure
4B. While the difference in absorption maxima is nearly
indiscernible for X ) H and F, the band does exhibit a
significant shift to lower energy for X ) OCH3 and CH3.
Moreover, it is noteworthy that the low-energy tail of the
absorption manifold systematically shifts to lower energy as the
electron-donating properties of the acetylide substituents increase
(F < H < Me < OCH3). These results are consistent with the
notion that the HOMO is predominantly metal-based. As
arylacetylide donicity increases, it is expected that the energies
of nonbonding and weakly π bonding metal orbitals will
increase,18 leading to the observed red shifts for the Pt(dbbpy)-
(CtCC6H4OCH3)2 (11) and Pt(dbbpy)(CtCC6H4CH3)2 (10)
derivatives.
Figure 2. Packing diagram for Pt(phen)(CtCC6H4CH3)2 (2). The
nearest Pt‚‚‚Pt distance is 5.92 Å.
The p-nitrophenylacetylide complex Pt(dbbpy)(CtCC6H4-
NO2)2 (13) exhibits an absorption spectrum that is significantly
different from those of the rest of the series and is omitted from
Figure 4B. Specifically, the lowest energy absorption band,
which is significantly blue shifted, possesses a molar extinction
coefficient that is substantially higher (ꢀ ∼ 37 000 M-1 cm-1
)
than those of the rest of the series. While the shift of the MLCT
absorption to higher energy (ca. 380 nm) is expected for the
nitro derivative 13 on the basis of reduced donor ability for the
acetylide ligands, the change in molar absorptivity suggests that
the observed band actually corresponds to a different transition,
obscuring the MLCT band. The intense 380 nm absorption is
attributed to a transition based on the p-nitrophenylacetylide
ligand. Free p-NO2C6H4CtCH exhibits a similar band at 286
nm that shifts to lower energy upon coordination. Support for
this notion is seen in the electrochemical results discussed below.
Figure 3. Normalized absorption and RT emission spectra of Pt(phen)-
(CtCC6H5)2 in CH2Cl2 (s), acetonitrile (‚‚‚), and 2:1 Et2O/CH2Cl2
(- - -).
lecular contact of the former type is between C7 and C7′ at
3.20 Å, and that of the latter type is between C5 and C12′ at
3.48 Å. The closest Pt‚‚‚Pt contact is 5.92 Å, indicating the
absence of any interaction between metal centers.
Emission Spectra. Figure 5 presents the room temperature
and 77 K emission spectra of Pt(dbbpy)(CtCC6H5)2 (9) and
Pt(CH3phen)(CtCC6H5)2 (7) as well as their absorption spectra.
The room-temperature emission is broad and asymmetric, while
the 77 K spectrum in butyronitrile glass shows a rigidochromic
shift and resolvable vibronic components of the emission. The
room-temperature luminescence mirrors the absorption spectrum
with good 0-0 overlap. For the 77 K spectrum, the vibronic
interval is ∼1230 cm-1, which is diagnostic of diimine
involvement in the emissive excited state, and the Huang-Rhys
constant S, I(1,0)/I(0,0), is about 0.65 and 0.90 for 7 and 9,
respectively. The latter compares to values of 1.4 observed for
the 3IL emission of the Pt(bpy)(en)2+ cation17 and slightly greater
than 1 observed for the solid-state emission of Pt(3,3′-(CH3-
OCO)2bpy)Cl2.18
Absorption Spectra. The absorption spectra of the complexes
exhibit a slightly solvatochromic band near 400 nm, as shown
in Figure 3 for 1, in addition to higher energy bands (λ < 310
nm) that correspond to diimine- and acetylide-based intraligand
transitions. The slightly solvatochromic transition with a molar
extinction coefficient ꢀ of ca. 8000 M-1 cm-1 has been assigned
by Che as a charge-transfer excitation from a filled metal d
orbital to a vacant π*diimine orbital (MLCT).28 The dependence
of this absorption maximum on the diimine and acetylide
substituents has been studied for both series of complexes, and
these results are tabulated in Table 3 and illustrated in Figure
4. Interpretation of these spectra is complicated by the fact that
this asymmetric band clearly is comprised of more than one
transition. In fact, a higher energy shoulder (350-370 nm) is
resolved in absorption spectra of 1, 5, 6, and 7 (Figure 4A).
This shoulder shifts strongly to lower energy as the electron-
withdrawing properties of the substituent on the phenanthroline
ligand increase (CH3, H, ∼350 nm; Cl, ∼370 nm); for the NO2-
phen complex 4, this feature presumably is buried in the 400
nm band envelope. In contrast, the absorption maximum for the
Pt(Rphen)(CtCPh)2 series shows only a modest shift from 392
nm for 1 to 397 nm for 4 as the phenanthroline substituent is
The influence of substituent variation on the emission energies
for the Pt(Rphen)(CtCC6H5)2 and Pt(dbbpy)(CtCC6H4X)2
series of complexes is shown in Figure 6 and tabulated in Table
3. The extent of the observed shifts is greater than those seen
(50) Afzal, D.; Lenhert, P. G.; Lukehart, C. M. J. Am. Chem. Soc. 1984,
106, 3050-3052.
(51) In Figure 4, panel A, the Brphen complex 5 is omitted for clarity
since it and the Clphen derivative 6 have very similar absorption
spectra as would be expected on the basis of the similar electron-
withdrawing ability of the chloro and bromo substituents.

Platinum Diimine Bis(acetylide) Complexes
Inorganic Chemistry, Vol. 39, No. 3, 2000 453
Table 3. Absorption Maxima, Emission Maxima, Luminescence Quantum Yields, Excited-State Lifetimes, and Self-Quenching Rate Constants
for Pt(diimine)(CtCAr)2 Complexes in Acetonitrile
a
compound
λ
abs (nm)
λ
em (nm)
φem
τo (ns)b
kq (×109 s-1
)
Pt(phen)(CtCC6H3)2 (1)
Pt(phen)(CtCC6H4CH3)2 (2)
Pt(phen)(CtCC6H4F)2 (3)
392 (8000)
392 (9800)
391 (7100)
397 (5000)
395 (8000)
397 (7200)
396 (7600)
396 (9600)
386 (7900)
389 (8200)
393 (6600)
385 (7500)
371 (38100)
575
578
584
582
605
605
575
590e
570
592
640
570
570
0.090
907c
549c
814c
∼400
366
390
972
5600
691
6.3c
6.2c
6.7c
Pt(NO2phen)(CtCC6H5)2 (4)
Pt(Brphen)(CtCC6H5)2 (5)
Pt(Clphen)(CtCC6H5)2 (6)
Pt(CH3phen)(CtCC6H5)2 (7)
Pt(C6H5CtCphen)(CtCC6H5)2 (8)
Pt(dbbpy)(CtCC6H5)2 (9)
Pt(dbbpy)(CtCC6H4CH3)2 (10)
Pt(dbbpy)(CtCC6H4OCH3)2 (11)
Pt(dbbpy)(CtCC6H4-F)2 (12)
Pt(dbbpy)(CtCC6H4-NO2)2 (13)
0.003
0.036
0.040
0.098
4.6 + 0.2
5.5 ( 0.7
5.5 ( 0.2
4.2 ( 0.3
1.4 ( 0.2
1.0 ( 0.2
0.11
0.07
0.002
0.14
0.04
440
14d
663
∼4500
1.6 ( 0.1
a Luminescence quantum yields are extrapolated to infinite dilution. Estimated error is (15%. b τo is the lifetime at infinite dilution ( 5%.
c Measured in acetonitrile (ref 30). d Measured by single-photon counting at one concentration (2.9 × 10-5 M). e There is a shoulder to higher
energy.
Figure 5. Absorption and emission spectra at room temperature in
CH3CN (s) and 77 K emission spectra in butyronitrile (- - -) for (A)
Figure 4. Absorption spectra for the Pt(Rphen)(CtCC6H5)2 series (A)
Pt(dbbpy)(CtCC6H5)2 (9) and (B) Pt(CH3phen)(CtCC6H5)2 (7).
and Pt(dbbpy)(CtCC6H4X)2 series (B) in CH3CN at room temperature.
and diimine-centered LUMO. More dramatic perturbations in
in the absorption spectra of Figure 4 with trends that are exactly
the emission are realized when the acetylide ligands are re-
parallel. Shifts to lower energy in emission bands occur as the
placed with other carbanion ligands, namely, phenyl or methyl
electron-withdrawing ability of the diimine substituent increases
groups. Figure 7 shows the 77 K ethanol/methanol (3:1) glass
and as the donicity of the arylacetylide ligand increases. Both
emission spectra for a series of Pt(phen)R2 complexes where R
changes are consistent with a mainly metal-based HOMO and
a π*diimine LUMO. The influence of substituent variation on the
emission energies for the Pt(Rphen)(CtCC6H5)2 and Pt(dbbpy)-
(CtCC6H4X)2 series of complexes is shown in Figure 6 and
is CtCC6H5, C6H5, or CH3. The emissions from the methyl52
and aryl53 derivatives previously have been identified as
originating from lowest 3MLCT excited states. The onset of the
3MLCT emission from the Pt(phen)(CtCC6H5)2 occurs ca. 400
tabulated in Table 3. The extent of the observed shifts is greater
and 1000 cm-1 to higher energy of the onset of the emission
than those seen for the ca. 400 nm absorption maximum with
from the phenyl and methyl complexes, respectively. It is
trends that are exactly parallel. The emission maximum shifts
striking that the energy of the onset of emission from these
to lower energy as the electron-withdrawing ability of the
diimine substituent increases and as the donicity of the aryl-
acetylide ligands increases. This behavior is consistent with a
mainly metal-based HOMO and a π*diimine LUMO. From the
limited data set tested, the HOMO-LUMO gap appears to be
much more sensitive to substitution on the diimine than on the
aryl group of the alkynyl ligands; the relative insensitivity to
para substituents of the arylacetylide ligands is in keeping with
their distance from the predominantly metal-centered HOMO
complexes decreases (CtCC6H5 > C6H5 > CH3) with increas-
ing basicity of the carbanion ligands (sp < sp2 < sp3). These
observations are consistent with more electron-donating ligands
stabilizing MLCT states as discussed by Miskowski and co-
workers.18
(52) Hill, R. H.; Puddephatt, R. J. J. Am. Chem. Soc. 1985, 107, 1219-
1225.
(53) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176-1186.

454 Inorganic Chemistry, Vol. 39, No. 3, 2000
Hissler et al.
concentrations is shown in Figure 8. The red-shifted emission
at higher concentration is assigned as an excimer emission
consistent with what has been described by Connick et al. for
Pt(phen)(CtCC6H5)2 (1) and by Vogler and Che for the di-
(cyanide) derivatives.19,20,30
Electrochemistry. Cyclic voltammograms recorded for the
Pt(diimine)(CtCAr)2 complexes all exhibit one reversible
reduction wave between -1.20 and -1.50 V, corresponding to
the addition of an electron to an orbital on the respective diimine
ligand (Table 5).54 Substitution at the 5-position of phenanthro-
line while keeping the alkynyl ligand fixed (1, 5-7) has a more
substantial effect on the E1/2 reduction potentials than does a
corresponding variation of the arylacetylide substituent while
keeping the diimine fixed (9-12). For the former series, methyl
substitution at the 5-position makes the reduction more difficult
while chloro, bromo, and phenylethynyl substituents shift E1/2
very modestly to less negative values. Substitution at the
5-position of phenanthroline clearly influences the energy of
the π*diimine LUMO, which is in turn probed by one-electron
reduction. The phenylethynyl substituent in 8 may achieve this
effect through greater delocalization.
The compounds containing nitro groups (4 and 13) exhibit
additional reduction waves that are not observed in the cyclic
voltammograms of the other complexes. On the basis of the
electrochemical behavior of the free ligands,55a these waves are
attributed to nitro group reductions. For both NO2phen and
p-NO2C6H4CtC ligands, the observed ArNO2-based reduction
waves occur at more positive values when coordinated to Pt
than when not coordinated, as expected on the basis of donation
of electron density upon coordination. Further support for
assigning the additional, less cathodic reduction waves to the
nitro groups is obtained from the electrochemical study of the
related Pt(4,4′-(NO2)2bpy)Cl2 complex, which shows facile
reductions that are highly localized on the nitro groups.55b The
shift in reduction potential between 13 and free p-nitrophenyl-
acetylene is completely consistent with the observed shift in
their absorption spectra for the intense low-energy band. The
380 nm absorption band in 13 is based on p-nitrophenylacetyl-
ide, and it is red-shifted relative to that of free ligand with a
higher energy LUMO. This is the orbital that serves as the
acceptor function upon one electrochemical reduction.
Oxidation of the Pt(diimine)(CtCAr)2 complexes is not
observed in most cases up to 1.6 V vs NHE. When it is seen,
the oxidation wave is irreversible. The results can be compared
with previous reports on platinum diimine dithiolate systems
and other Pt complexes containing heteroaromatic or related
chelating ligands that show only irreversible oxidations.23,56-58
Excited-State Lifetimes and Emission Quantum Yields.
In Table 3 are presented the lifetimes at infinite dilution (τo)
and the self-quenching rate constants (kq) for all of the
compounds studied except the nitro-containing derivatives 4 and
13. These two compounds were too weakly emissive to be
analyzed by the same procedure as done for the rest of the
compounds (i.e., similar concentration ranges and excitation
power per pulse). The methoxyphenylacetylide compound 11
is also weakly emissive (φem ) 0.002). While its short excited-
state lifetime was measured by single-photon counting (∼14
ns in a 2.9 × 10-5 M acetonitrile solution), a quenching study
Figure 6. Normalized emission spectra for the Pt(Rphen)(CtCC6H5)2
series (A) and Pt(dbbpy)(CtCC6H4X)2 series (B) in CH3CN at room
temperature.
Figure 7. Emission spectra (77 K) of Pt(phen)R2 in ethanol/methanol
(3:1) glass where R is CtCC6H5 (s), C6H5 (‚‚‚), and CH3 (-‚‚‚-).
IEH molecular orbital calculations also support the assignment
of the emission to a lowest 3MLCT excited state. As shown in
Table 4, the HOMO is primarily metal dσ in character and the
LUMO is localized on the phenanthroline ligand. The HOMO-
LUMO gaps obtained for Pt(phen)(CtCC6H5)2, Pt(phen)(Ct
CH)2, Pt(phen)(CtCF)2, and Pt(phenNO2)(CtCH)2 are 1.59,
1.65, 1.70, and 0.76 eV, respectively. This trend is consistent
with that observed for the low-energy MLCT absorptions at
ca. 400 nm and for the room-temperature emission at ca. 580
nm in MeCN solution for the Pt(dbbpy)(CtCC6H4X)2 series
of complexes. The fact that the calculated HOMO-LUMO gap
is at least 0.7 eV less energetic than the observed emission
energies indicates that these calculations are of value mainly in
the comparison of electronically similar systems and not in any
absolute sense.
Whereas all of the emission spectra presented in Figure 6
show one asymmetric but structureless band, the emission spec-
trum of 8 gives evidence of a vibrational progression with a
Huang-Rhys parameter of greater than 1 and, at higher con-
centrations, of a substantially red-shifted emission at 750 nm.
The emission spectrum for 8 in MeCN solutions at two different
(54) Ohsawa, K. W.; Hanck, K. W.; DeArmond, M. K. J. Electroanal.
Chem. Interfacial Electrochem. 1984, 175, 229-240.
(55) (a) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten,
D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101,
4815-4824. (b) McInnes, E. J. L.; Welch, A. J.; Yellowlees, L. J.
Chem. Commun. 1996, 2393-2394.

Platinum Diimine Bis(acetylide) Complexes
Inorganic Chemistry, Vol. 39, No. 3, 2000 455
Pt(phenNO2)(CtCH)2, Cs symmetry
Table 4. Relative Percent Atomic Contribution to the MOs
Pt(phen)(CtCH)2, C2V symmetry
energy of
energy of
MO, eV
MO, eV
Pt
CtC
phen π
Pt
CtC
phen π
NO2
-7.45
-7.57
0.1
16.5
1.5
0.3
2.2
61.7
17.7
66.9
61.7
59.6
21.1
0.2
72.8
3.5
0.1
1.2
8.4
4.2
25.5
15.0
15.5
5.2
99.7
10.7
95.0
99.6
96.7
0.0
78.1
0.0
23.3
24.9
0.0
-7.57
-8.07
16.2
1.8
0.3
2.1
0.1
61.3
66.7
31.8
63.0
20.0
45.2
71.7
4.0
0.1
1.1
0.0
8.4
25.4
7.6
15.3
5.2
12.1
12.1
93.0
83.0
96.7
32.7
28.2
7.1
54.2
21.3
60.6
38.4
0.0
1.2
16.6
0.0
67.3
0.3
0.1
6.4
0.4
8.4
4.3
-8.11
-9.31
-9.60
-9.73
-9.73
-10.62
-11.38
-12.22
-12.25
-12.34
-12.49
-12.51
-11.38
-12.16
-12.22
-12.33
-12.47
-12.52
Pt(phen)(CtCPh)2, C2V symmetry
Pt(phen)(CtCF)2, C2V symmetry
energy of
MO, eV
energy of
MO, eV
Pt
CtC
phen π
Pt
CtC
phen π
2F
-8.27
-8.71
0.0
0.2
3.3
0.8
2.5
65.3
50.6
56.0
46.0
0.5
0.0
25.4
26.1
0.1
0.0
0.5
0.1
99.1
96.5
20.7
3.2
16.4
32.1
1.7
-7.4
-7.4
46.3
0.1
1.1
0.3
2.2
61.8
65.5
23.5
62.1
50.0
21.3
24.5
0.1
1.5
0.1
0.9
6.1
27.3
6.5
19.5
16.4
3.0
27.8
99.8
97.4
99.6
96.8
30.3
4.6
69.5
16.8
32.3
68.5
1.0
0.0
0.1
0.0
0.1
0.3
2.3
0.5
1.6
1.3
0.2
-8.77
-8.1
-9.52
-9.6
-9.7
-9.82
0.9
-11.41a
-11.93
-12.16
-12.16
-12.27
-12.42
10.9
21.6
21.5
17.0
37.8
8.4
-11.4a
-12.1
-12.1
-12.3
-12.4
-12.5
16.8
71.9
a Highest occupied molecular orbital.
Table 5. Electrochemical Data (E1/2 Values for Reductions and
Irreversible Oxidation Waves) for Pt(diimine)(CtCAr)2 Complexes
compound
Eox (V)
Ered (V)
Pt(phen)(CtCC6H5)2 (1)a
-1.31, -2.00
-1.30
Pt(phen)(CtCC6H4CH3)2 (2)a
Pt(phen)(CtCC6H4F)2 (3)b
Pt(NO2phen)(CtCC6H5)2 (4)a
Pt(Brphen)(CtCC6H5)2 (5)a
Pt(Clphen)(CtCC6H5)2 (6)b
Pt(CH3phen)(CtCC6H5)2 (7)b
Pt(dbbpy)(CtCC6H5)2 (9)b
Pt(dbbpy)(CtCC6H4CH3)2 (10)b
-1.29
-0.62, -0.89, -1.36
-1.17
-1.20
1.15 (irr) -1.41
-1.40
1.18 (irr) -1.42
Pt(dbbpy)(CtCC6H4OCH3)2 (11)b 1.02 (irr) -1.42
Figure 8. Normalized emission spectra of Pt(C6H5CtCphen)-
(CtCC6H5)2 (8) at 135 and 13.5 µM. The difference between these
spectra is also shown (‚‚‚).
Pt(dbbpy)(CtCC6H4NO2)2 (13)c
Pt(dbbpy)(CtCC6H4F)2 (12)b
Pt(C6H5CtCphen)(CtCC6H5)2 (8)b
p-NO2-C6H4-CtCH
-0.95, -1.20, -1.53
-1.39
-1.21
was not performed. Because the measurement was made at low
concentration, there is an insignificant deviation from the
measured lifetime compared to a value that might be extrapo-
lated to infinite dilution.59
Table 3 shows that some of the systems have luminescence
quantum yields nearly twice that of Ru(bpy)32+ in acetonitrile.
Luminescent quantum yields in Table 3 were corrected for
infinite dilution using a modified Stern-Volmer equation60 (eq
3), where φ and φo are the measured quantum yield and the
quantum yield at infinite dilution, respectively.
-1.07
5-nitrophenantrolinea
-0.96, -1.67
a In dimethylformamide solution containing 0.1 M (TBA)PF6 at 293
K. b In acetonitrile solution containing 0.1 M (TBA)PF6 at 293 K. c In
dichloromethane solution containing 0.1 M (TBA)PF6 at 293 K.
Potentials in volts are versus NHE as calibrated using the ferrocene/
ferrocenium couple at 0.40 V vs NHE (see the Experimental Section).
-1
kr ) φoτo
(4)
(5)
knr ) kr(φo-1 - 1)
φo/φ ) 1 + kqτo[Pt]
(3)
In Table 6 are shown Eemmax, kr, and knr for the Pt(Rphen)-
(CtCC6H5)2 and Pt(dbbpy)(CtCC6H4X)2 series. The radiative
From the lifetime and emission quantum yields, the radiative
and nonradiative rate constants can be calculated using eqs 4
and 5.
(59) If one assumes that the self-quenching constant for compound 11 is
similar to those of the rest of the compounds in the Pt(dbbpy)-
(CtCC6H4X)2 series obtained from eq 2, it can be seen that the dif-
ference in τ vs τo is less than 0.1%. Compared to the estimated error
of 5% in the value of τo, this error is negligible. Likewise, the correc-
tion for the quantum yield at infinite dilution will be negligible too.
(60) Vincze, L.; Sandor, F.; Pem, J.; Bosnyak, G. J. Photochem. Photobiol.
A 1999, 120, 11-14.
(56) Zuleta, J. A.; Burberry, M. S.; Eisenberg, R. Coord. Chem. ReV. 1990,
97, 47-64.
(57) Cornioley-Deuschel, C.; von Zelewsky, A. Inorg. Chem. 1987, 26,
3354-3358.
(58) Blanton, C. B.; Murtaza, Z.; Shaver, R. J.; Rillema, D. P. Inorg. Chem.
1992, 31, 3230-3235.

456 Inorganic Chemistry, Vol. 39, No. 3, 2000
Hissler et al.
Table 6. Radiative and Nonradiative Rates for the Pt(Rphen)(CtCPh)2 and Pt(dbbpy)(CtCC6H4X)2 Seriesa
max
em
compound
E
(cm-1
)
kr (×106 s-1
)
k (×106 s-1
)
knr (×106 s-1
)
Pt(phen)(CtCC6H5)2 (1)
17 300
0.099
0.008
0.129
0.103
0.101
0.159
0.159
0.143
0.211
0.007
1.103
2.500
3.497
2.564
1.029
1.447
2.273
71.429
1.508
0.177
1.00
2.49
3.37
2.46
0.93
1.29
2.11
71.3
1.23
0.17
Pt(NO2phen)(CtCC6H5)2 (4)
Pt(Brphen)(CtCC6H5)2 (5)
Pt(Clphen)(CtCC6H5)2 (6)
Pt(CH3phen)(CtCC6H5)2 (7)
Pt(dbbpy)(CtCC6H5)2 (9)
Pt(dbbpy)(CtCC6H4CH3)2 (10)
Pt(dbbpy)(CtCC6H4OCH3)2 (11)
Pt(dbbpy)(CtCC6H4F)2 (12)
Pt(dbbpy)(CtCC6H4NO2)2 (13)
17 180
16 530
16 530
17 390
17 540
16 890
15 630
17 540
17 540
a All rates are estimated to have an error of (15%.
Table 7. Ground- and Excited-State Redox Properties of
Pt(diimine)(CtCAr)2 Complexesa
Eoo
E1/2 (red,
compound
(eV) V vs NHE) E(Pt*/Pt-)
Pt(phen)(CtCC6H5)2 (1)
2.50
2.44
2.44
2.50
2.55
2.52
-1.31
-1.17
-1.20
-1.41
-1.38
-1.37
-1.39
-1.36
1.19
1.27
1.24
1.09
1.17
1.15
0.99
1.19
Pt(Brphen)(CtCC6H5)2 (5)
Pt(Clphen)(CtCC6H5)2 (6)
Pt(CH3phen)(CtCC6H5)2 (7)
Pt(dbbpy)(CtCC6H5)2 (9)
Pt(dbbpy)(CtCC6H5CH3)2 (10)
Pt(dbbpy)(CtCC6H4OCH3)2 (11) 2.38
Pt(dbbpy)(CtCC6H4F)2 (12)
2.55
Figure 9. Stern-Volmer plot for the quenching of Pt(dbbpy)-
(CtCC6H5)2 (9) in CH3CN with 10-methylphenothiazine.
a All potentials in volts vs NHE.
rate constants (103-105 s-1) indicate that the observed emission
state reduction potentials for the Pt(diimine)(CtCAr)2 com-
plexes have been estimated. Table 7 presents these results. As
expected for a lowest MLCT excited state, the smallest
E(Pt*/Pt-) value for the Pt(Rphen)(CtCC6H5)2 series occurs
for the complex which has an electron-donating CH3 substit-
uent on the phenanthroline ligand. Likewise, the smallest
E(Pt*/Pt-) value for the Pt(dbbpy)(CtCC6H4X)2 series occurs
for complex 11, which has an electron-donating methoxy
substituent on each arylacetylide ligand. The observed ordering
is consistent with the notion that the excited-state reduction
potential reflects the driving force for filling the vacancy in
the ground-state HOMO. A better donating ligand will in-
crease electron density on the metal center more, thereby raising
in energy the predominantly metal-centered HOMO, while
shifting the excited-state reduction potential to less positive
values.
has a a significant degree of spin-forbiddedness consistent with
3
a MLCT assignment. Except for the nitro compounds 4 and
13, all of the compounds obey the energy gap law from plots
of log knr versus Eemmax for each series. This includes the short
lifetime and low emission quantum yield for the electron-
donating p-MeOC6H4 acetylide derivative 11. These results lend
support to the assertion that the lowest energy excited state in
all of these complexes originates from a metal-to-diimine ligand
charge-transfer transition.
The phototophysical, electrochemical, and spectroscopic data
for Pt(dbbpy)(CtCC6H4NO2)2 (13) suggest that the emission
from this complex originates from a different lowest excited
state (involving a LUMO centered on the acetylide ligand) than
observed for the other complexes in the Pt(dbbpy)(CtC(C6H4-
X)2 series. Further spectroscopic studies on this system are in
progress.
Estimation of Excited-State Reduction Potentials. The Pt-
(diimine)(CtCAr)2 complexes undergo electron-transfer quench-
ing of both reductive and oxidative types. As with the
corresponding diimine dithiolate complexes,23 the systems are
relatively photostable in the presence of an electron donor,
whereas photodegradation occurs under oxidative quenching.
For example, photolysis of Pt(dbbpy)(CtCC6C4CH3)2 (10) in
acetonitrile containing p-nitrobenzaldehyde with Hg(Xe) ir-
radiation (λex > 345 nm) leads to substantial photodegradation
after 30 min.
For the complex Pt(dbbpy)(CtCC6H5)2 (9), reductive quench-
ing was examined quantitatively by measuring the emission
lifetime in the presence of the electron donors 10-methylphe-
nothiazine and N,N,N′,N′-tetramethylbenzidine. Stern-Volmer
behavior was observed using both quenchers with essentially
diffusion-controlled kq values of 9.6 × 109 and 1.3 × 1010 M-1
s-1, respectively, and τo of 715 ( 20 ns. A plot of τo/τ vs the
concentration of 10-methylphenothiazine is shown in Figure 9.
Through the use of a simple thermochemical cycle involving
the excited-state energies Eoo (estimated from the 77 K emission
spectra) and the reversible reduction potentials E1/2, the excited-
Conclusions
The Pt(diimine)(CtCAr)2 complexes comprise a new and
potentially useful set of solution luminescent square planar
complexes, with some members exhibiting greater emission
intensity than comparable solutions of Ru(bpy)32+. The nature
of the excited state in these systems has been investigated
through systematic ligand variation and its influence on excited-
state properties. In this context, two series of complexes were
synthesized and studied, one having the diimine held constant
as di-tert-butylbipyridine while the para substituent on the
arylacetylide ligand was changed and the other having the
alkynyl fixed as phenylacetylide with the diimine varying.
Through this investigation the emitting state is confirmed to be
a Pt-to-diimine charge transfer in accord with that originally
proposed by Che. This assignment is consistent with the notion
that variation of the diimine affects the energy of the lowest
unoccupied molecular orbital, and that variation of the aryl-
acetylide leads to only minor changes in the Pt-based HOMO.
Luminescence decay measurements reveal excited-state lifetimes
between 14 and 5600 ns and a degree of spin-forbiddenness
corresponding to a 3MLCT assignment.28 Unlike the octahedral

Platinum Diimine Bis(acetylide) Complexes
Inorganic Chemistry, Vol. 39, No. 3, 2000 457
d6 diimine complexes such as Ru(diimine)32+ and Re(diimine)-
(CO)3X (where X ) halide or pyridyl group),61 these square
planar d8 complexes exhibit self-quenching of their excited state.
The Pt(diimine)(CtCAr)2 complexes undergo electron-
transfer quenching with good Stern-Volmer behavior seen for
Pt(phen)(CtCC6H5)2 when quenched by the electron donors
10-methylphenothiazine and N,N,N′,N′-tetramethylbenzidine. A
simple thermochemical analysis leads to estimates for the
excited-state reduction potentials in the range of ca. 1.0-1.3
V. The long-lived excited states of the Pt(diimine)(CtCAr)2
complexes, their redox properties, and the synthetic flexibility
which these systems offer make these complexes very attractive
for application in molecular photochemical devices and the
construction of multicomponent molecular systems.
(R.E.), and the Research Corporation (D.K.G.). We thank Dr.
Witek Paw and Dr. Adnan Mansour for valuable discussions,
Dr. Gary Dombrowski for expert technical assistance, and Dr.
Chris Collison for assistance with single-photon counting
measurements.
Supporting Information Available: Tables giving crystal data and
structure refinement, atomic and hydrogen coordinates, isotropic and
anisotropic displacement parameters, and bond lengths and angles for
2. This material is available free of charge via the Internet at http://
pubs.acs.org.
IC991250N
(61) Roundhill, D. M. In Photochemistry and Photophysics of Metal
Complexes; Fackler, J. P. J., Ed.; Plenum Press: New York, 1994; p
356.
Acknowledgment. This research was supported by the
Department of Energy, Division of Basic Chemical Sciences