Synthesis of cyclometallated platinum complexes with substituted thienylpyridines and detailed characterization of their luminescence properties

Article abstract of DOI:10.1021/ic802401j

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl- 2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]py- ridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (6 = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of3jtjt*character, which experience only weak spin- orbit couplings to higher lying singlet states.

Full text of DOI:10.1021/ic802401j

Inorg. Chem. 2009, 48, 4179-4189
Synthesis of Cyclometallated Platinum Complexes with Substituted
Thienylpyridines and Detailed Characterization of Their Luminescence
Properties
Dmitry N. Kozhevnikov,*,†,‡ Valery N. Kozhevnikov,*,§ Maria M. Ustinova,‡,| Amedeo Santoro,§
Duncan W. Bruce,*,§ Burkhard Koenig,| Rafał Czerwieniec,⊥ Tobias Fischer,⊥ Manfred Zabel,⊥
and Hartmut Yersin*,⊥
Institute of Organic Synthesis, Ural Branch of Russian Academy of Sciences, KoValeVskoy 22,
Ekaterinburg, 620219, Russia, Ural State Technical UniVersity, Mira 19, Ekaterinburg, 620002,
Russia, Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K., Institute
of Organic Chemistry and Institute of Physical and Theoretical Chemistry, UniVersity of
Regensburg, UniVersita¨tsstrasse 31, D-93053 Regensburg, Germany
Received December 19, 2008
Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The
final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction.
The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with
aryl ) phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a
series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L ) cyclometallating ligand, acac )
acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] ) [Pt(5-phenyl-
2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] ) [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]py-
ridine)(acac)], and [Pt(L7)(acac)] ) [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined.
Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two
representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization
based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried
out. The studies down to low temperature (T ) 1.2 K) and up to high magnetic fields (B ) 10 T) allowed us to
characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest
triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of 3ππ* character, which experience
only weak spin-orbit couplings to higher lying singlet states.
1. Introduction
photocatalysts,5-7 solar cells,8-11 biological labels,12-14 and,
in particular, as electroluminescent emitters in organic light
During the last two decades, research in the field of the
third-row transition metal complexes has gained additional
momentum since they were found to be important for a wide
range of applications, such as optical chemosensors,1-4
(2) Chen, H.; Zhao, Q.; Wu, Y.; Li, F.; Yang, H.; Yi, T.; Huang, C. Inorg.
Chem. 2007, 46, 11075–11081.
(3) Demas, J. N.; DeGraff, B. A. Coord. Chem. ReV. 2001, 211, 317–
351.
(4) Zhao, Q.; Cao, T.; Li, F.; Li, X.; Jing, H.; Yi, T.; Huang, C.
Organometallics 2007, 26, 2077–2081.
(5) Belmore, K. A.; Vanderpool, R. A.; Tsai, J. C.; Khan, M. A.; Nicholas,
K. M. J. Am. Chem. Soc. 1988, 110, 2004–2005.
(6) Maruyama, T.; Yamamoto, T. J. Phys. Chem. B 1997, 101, 3806–
3810.
(7) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000, 208, 115–
137.
* To whom correspondence should be addressed. E-mail: dnk@ios.uran.ru
(D.N.K.), db519@york.ac.uk (D.B.), hartmut.yersin@chemie.uni-regens-
burg.de (H.Y.).
†
Ural Branch of Russian Academy of Sciences.
Ural State Technical University.
University of York.
Institute of Organic Chemistry, University of Regensburg.
‡
§
|
⊥
Institute of Physical and Theoretical Chemistry, University of Re-
gensburg.
(8) Tse, C. W.; Man, K. Y. K.; Cheng, K. W.; Mak, C. S. K.; Chan,
W. K.; Liu, Z. T.; Yip, C. T.; Djurisˇic, A. B. Chem.sEur. J. 2007,
13, 328–335.
(1) Tang, W. S.; Lu, X.-X.; Wong, K. M.-C.; Yam, V. W.-W. J. Mater.
Chem. 2005, 15, 2714–2720.
10.1021/ic802401j CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 9, 2009 4179
Published on Web 04/01/2009

Kozhevnikov et al.
emitting diodes15-19 (OLEDs). As metal-induced spin-orbit
coupling often allows effective singlet-triplet mixing,
complexes displaying high photoluminescence (phosphores-
cence) quantum yields and relatively short emission decay
times can be engineered. Such triplet emitters represent
attractive candidate materials for conversion of electric
energy into light. Cyclometallated platinum complexes
incorporating 1,3-diketonates (e.g., acetylacetonato, acac) as
spectator ligands were reported to be strongly emissive at
room temperature.19,20 In particular, chemical modification
of the chromophoric cyclometallating ligand should enable
tuning of emitting properties. On the other hand, specific
substitutions may be used for binding of the metal complex
to polymers or biomolecules, which results in new applications.
It is known from textbooks that 1,2,4-triazines can be
converted into pyridines through an aza-Diels-Alder reac-
tion.21 Consequently, any synthetic method that creates 1,2,4-
triazines can be considered as a method for the synthesis of
substituted pyridines. This general idea has been exploited
for decades, and a great variety of interesting examples have
been described.22-24 In 1998, Sauer and co-workers applied
such an approach specifically for the synthesis of polypyri-
dine ligands and demonstrated how useful this methodology
can be in preparing ligands for use in coordination chemis-
try.25 Many other examples followed, and the methodology
evolved considerably, making functionalized polypyridine
ligands readily accessible.26-31 Thus, a combination of the
wide variety of methods for the synthesis of substituted 1,2,4-
triazines, coupled with the aza-Diels-Alder reaction with
various dienophiles, allows for the synthesis of different
functionalized pyridines. In this report, we describe the
application of 1,2,4-triazine chemistry to develop a new,
efficient, flexible, and straightforward methodology for the
synthesis of ligands capable of forming cyclometallated
transition metal complexes. For thienylpyridines, we dem-
onstrate how this methodology can be used for fine-tuning
photophysical properties of [Pt(L)(acac)] complexes, wherein
L is a substituted 2-(2-thienyl)pyridine ligand. Two repre-
sentatives of this series are studied in more detail to obtain
a deeper insight into the nature of the emitting triplet states.
This is carried out on the basis of highly resolved emission
and excitation spectra, emission decay behavior at a low
temperature and under application of high magnetic fields.
For these investigations the Shpol’skii matrix isolation
technique (the complexes are dissolved in n-octane) is
applied. These studies allow us to characterize the orbital
origin of the lowest triplet state, the individual triplet
substates, as well as the importance of spin-orbit coupling
(SOC). In particular, this latter property largely determines
the luminescence behavior of the studied compounds.
2. Experimental Section
2.1. Equipment. Spectroscopic Methods. Nuclear magnetic
resonance spectra were recorded on a Jeol JNM-EX 270 FT NMR
1
system at 270 MHz for H and 68 MHz for 13C. Microanalyses
(9) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisˇic´, A. B.; Yip, C.-T.;
Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nature Mater.
2007, 6, 521–527.
were performed using Carlo Erba 1108 Elemental Analyzer
controlled with CE Eager 200 software. Mass spectra were done
using Varian CH-5 (EI), Finnigan MAT 95 (CI; FAB and FD) or
Finnigan MAT TSQ 7000 (ESI) spectrometers. For measurements
of UV-vis absorption spectra a Varian Cary 300 double beam
spectrometer was used. Luminescence spectra at 300 and 77 K were
recorded with a steady-state fluorescence spectrometer (Jobin Yvon
Fluorolog 3). Emission decay measurements at 300 and 77 K were
performed with a pulsed diode laser (PicoQuant GmbH Model
LDH-P-C-375, λexc ) 372 nm, pulse width 100 ps), attached to
the Fluorolog 3 spectrophotometer. For measurements of the
luminescence quantum yield an integrating sphere purchased from
Labsphere (model 4P-GPS-033-SL with Spectralon inner surface
coating) was used. The solutions for room-temperature measure-
ments were degassed by the freeze-pump-thaw technique. The
[Pt(L)(acac)] doped poly(methyl metacrylate) (PMMA) thin films
were prepared by spin-coating of dichloromethane solutions
containing PMMA and the respective [Pt(L)(acac)] complex onto
a quartz substrate. Experiments at temperatures below 4.2 K were
carried out in a bath cryostat. Temperature was reduced by pumping
off the helium vapor. For magnetic field experiments, an Oxford
Instruments MD10 cryostat equipped with a 12 T magnet was used.
A UV diode laser (Toptica IBeam, 375 nm) was applied as source
for non-selective excitation. For site-selective excitation, a pulsed
dye laser (Lambdaphysik FL2000, Lambdaphysik Scanmate 2C)
was applied using Coumarin 153 and a mixture of Rhodamine B
and Rhodamine 6G, respectively. The spectra were recorded with
a cooled photomultiplier (RCA C7164R), attached to a Spex 1404
(10) Polo, A. S. P.; Itokazu, M. K.; Iha, N. Y. M. Coord. Chem. ReV. 2004,
248, 1343–1361.
(11) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338–2345.
(12) Peyratout, C. S.; Aldridge, T. K.; Crites, D. K.; McMillin, D. R. Inorg.
Chem. 1995, 34, 4484–4489.
(13) Lo, K. K. W.; Chan, J. S. W.; Lui, L. H.; Chung, C. K. Organometallics
2004, 23, 3108–3116.
(14) Lo, K. K. W.; Hui, W. K.; Chung, C. K.; Tsang, K. H. K.; Ng,
D. C. M.; Zhu, N.; Cheung, K. K. Coord. Chem. ReV. 2005, 249,
1434–1450.
(15) Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H.,
Ed; Wiley-VCH: Weinheim, 2008.
(16) Cocchi, M.; Kalinowski, J.; Virgili, D.; Williams, J. A. G. Appl. Phys.
Lett. 2008, 92, 113302–113304.
(17) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403,
750–752.
(18) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I.; Bau,
R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723–8732.
(19) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055–3066.
(20) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205–268.
(21) Joule, J. A.; Mills, K. Heterocyclic Chemistry; Blackwell Science
Limited: Oxford, 2000; Chapter 5.15.
(22) Neunhoeffer, H. In: ComprehensiVe Heterocyclic Chemistry II 6;
Katrizky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon Press:
Oxford, 1996; pp 50-574.
(23) Boger, D. L.; Panek, J. S. J. Am. Chem. Soc. 1985, 107, 5745–5754.
(24) Fernandez-Sainz, Y.; Raw, A.; Taylor, R. J. K. J. Org. Chem. 2005,
70, 10086–10095.
(25) Pabst, G. R.; Sauer, J. Tetrahedron Lett. 1998, 39, 6687–6690.
(26) Sauer, J.; Heldmann, D. K.; Pabst, G. R. Eur. J. Org. Chem. 1999,
313–321.
(27) Rykowski, A.; Branowska, D.; Kielak, J. Tetrahedron Lett. 2000, 41,
3657–3658.
(28) Kozhevnikov, V. N.; Kozhevnikov, D. N.; Nikitina, T. V.; Rusinov,
V. L.; Chupakhin, O. N.; Zabel, M.; Koenig, B. J. Org. Chem. 2003,
68, 2882–2888.
(29) Stanforth, S. P.; Tarbit, B.; Watson, M. D. Tetrahedron Lett. 2003,
44, 693–694.
(30) Kozhevnikov, D. N.; Shabunina, O. V.; Kopchuk, D. S.; Slepukhin,
P. A.; Kozhevnikov, V. N. Tetrahedron Lett. 2006, 47, 7025–7029.
(31) Kozhevnikov, V. N.; Whitwood, A. C.; Bruce, D. W. Chem. Commun.
2007, 3826–3828.
4180 Inorganic Chemistry, Vol. 48, No. 9, 2009

Synthesis of Cyclometallated Platinum Complexes
Scheme 1. Synthesis of Substituted Thienylpyridinesa
combined, and the solvent was evaporated. The residues were
treated with 10 mL of ethanol, upon which solid precipitates formed.
The products were separated by filtration and washed with cold
ethanol (5 mL).
Characterization. 6-Phenyl-3-(2-thienyl)-1,2,4-triazine (3a).
Yield 68%. mp 158-159 °C. 1H NMR (270 MHz, CDCl3): δ 7.21
(dd, 1H, 3JHH 4.8 Hz, 3JHH 3.8 Hz), 7.53-7.60 (m, 4H), 8.08-8.12
(m, 2H), 8.17 (dd, 1H, 3JHH 4.8 Hz, 4JHH 1.2 Hz), 8.95 (s, 1H); 13
C
NMR (68 MHz, CDCl3): δ 126.5, 128.6, 129.3, 130.9, 131.2, 133.1,
139.5, 146.3, 154.4, 159.9.
6-(4-Methoxyphenyl)-3-(2-thienyl)-1,2,4-triazine (3b). Yield
1
68%. mp 209-210 °C. H NMR (270 MHz, CDCl3): δ 3.85 (s,
3
3
3H), 7.15 (m, 2 H), 7.21 (dd, 1H, JHH 5.2 Hz, JHH 3.7 Hz), 7.89
(dd, 1H, 3JHH 5.2 Hz, 4JHH 1.1 Hz), 8.07 (dd, 1H, 3JHH 3.7 Hz, 4JHH
1.1 Hz), 8.19 (m, 2 H), 9.34 (s, 1H). MS (CI+): m/z 270 (MH+).
6-(2-Naphthyl)-3-(2-thienyl)-1,2,4-triazine (3c). Yield 61%. mp
a
Reagents and conditions: (i) EtOH/AcOH, NaOAc, reflux, 12 h; (ii)
2,5-norbornadiene, o-xylene, 200 °C, 12 h; (iii) 1-morpholinocyclopentene,
190 °C, 2 h.
1
3
194-195 °C. H NMR (270 MHz, CDCl3): δ 7.22 (dd, 1H, JHH
5.2 Hz, 3JHH 3.7 Hz), 7.55-7.61 (m, 3H), 7.88-8.03 (m, 3H), 8.19
(dd, 1H, 3JHH 3.7 Hz, 4JHH 1.1 Hz), 8.23 (dd, 1H, 3JHH 8.5 Hz, 4JHH
1.9 Hz), 8.57 (s, 1H), 9.09 (s, 1H). MS (CI+): m/z 290 (MH+).
3,6-Bis(2-thienyl)-1,2,4-triazine (3d). Yield 43%. mp 218-219
double monochromator. Luminescence decay times of the selec-
tively excited probes were measured using a FAST Comtec
(Mu¨nchen) multichannel scaler PCI-card with time resolution of
250 ps. For pulsed excitation, a Nd:YAG laser (355 nm, IB Laser
Inc., DiNY pQ 02) with a pulse width of 7 ns was applied. All low
temperature measurements were carried out with samples of
[Pt(L)(acac)] dissolved in n-octane (Fluka, p.a. quality) at concen-
trations of ≈10-5 mol/L.
1
°C. H NMR (270 MHz, CDCl3): δ 7.24 (m, 2H), 7.56 (m, 2H),
7.74 (dd, 1H, 3JHH 3.7 Hz, 4JHH 1.1 Hz), 8.12 (dd, 1H, 3JHH 3.7 Hz,
4JHH 1.1 Hz), 8.86 (s, 1H). MS (EI): m/z 245 (M+).
5-Phenyl-2-(2-thienyl)pyridine (HL1). Yield 42%. mp 106-108
2.2. Synthesis of Ligands HL1-HL8. General Procedure.
6-Phenyl-3-(2-thienyl)-1,2,4-triazine (3a), 6-(4-methoxyphenyl)-3-
(2-thienyl)-1,2,4-triazine (3b), 6-(2-naphthyl)-3-(2-thienyl)-1,2,4-
triazine (3c), and 3,6-bis(2-thienyl)-1,2,4-triazine (3d) were prepared
from the corresponding bromoacetylarenes 1 and (2-thienyl)car-
boxylic acid hydrazide 2 (see Scheme 1). Mixtures of 1 (10 mmol),
2 (20 mmol), sodium acetate (12 mmol), ethanol (30 mL), and acetic
acid (10 mL) were heated under reflux for 12 h. The reaction
mixtures were allowed to cool to ambient temperature, and the
precipitated solids were filtered off, washed with ethanol, and dried.
The obtained products were used for further reactions as obtained,
without additional purification.
1
3
°C. H NMR (270 MHz, CDCl3): δ 7.12 (dd, 1H, JHH 5.0 Hz,
3JHH 3.8 Hz), 7.35-7.52 (m, 4H), 7.58-7.62 (m, 3H), 7.71 (d, 1H,
3JHH 8.4 Hz), 7.87 (dd, 1H, 3JHH 8.4 Hz, 4JHH 2.3 Hz), 8.80 (d, 1H,
4JHH 2.3 Hz). 13C NMR (68 MHz, CDCl3): δ 118.6, 124.5, 126.7,
127.6, 128.0, 128.1, 129.0, 134.6, 134.9, 137.4, 144.5, 147.8, 151.3.
Found: C, 76.05; H, 4.78; N, 5.71%. C15H11NS requires: C, 75.91;
H, 4.67; N, 5.90%.
5-(4-Methoxyphenyl)-2-(2-thienyl)pyridine (HL2). Yield 34%.
1
mp 154-155 °C. H NMR (270 MHz, CDCl3): δ 3.85 (s, 3H),
3
3
7.00 (m, 2H), 7.11 (dd, 1H, JHH 5.2 Hz, JHH 3.7 Hz), 7.38 (dd,
3
4
3
1H, JHH 5.2 Hz, JHH 1.1 Hz), 7.52 (m, 2H), 7.58 (dd, 1H, JHH
3.7 Hz, 4JHH 1.1 Hz), 7.68 (d, 1H, 3JHH 8.2 Hz), 7.83 (dd, 1H, 3JHH
8.2 Hz, JHH 2.2 Hz), 8.75 (d, 1H, JHH 2.2 Hz). MS (CI+): m/z
268 (MH+). Found: C, 72.01; H, 4.99; N, 5.01%. C16H13NOS
requires: C, 71.88; H, 4.90; N, 5.24%.
Ligands HL1-HL4: 5-phenyl-2-(2-thienyl)pyridine (HL1), 5-(4-
methoxyphenyl)-2-(2-thienyl)pyridine (HL2), 5-(2-naphthyl)-2-(2-
thienyl)pyridine (HL3), and 5,2-bis(2-thienyl)pyridine (HL4) were
prepared from the corresponding 2-(2-thienyl)-1,2,4-triazine precur-
sors 3 according to the following procedure. An autoclave equipped
with a stirring bar was charged with 3 (1 g), o-xylene (10 mL) and
norbornadiene (3 mL). The autoclave was sealed, placed in oil bath
and heated at 200 °C (oil bath) for 12-24 h. The reaction mixture
was filtered through a small pad of silica gel using dichloromethane
as eluent. The solvent was removed under reduced pressure, and
the residue was triturated with methanol, filtered off, and washed
with methanol to give the desired product.
4
4
5-(2-Naphthyl)-2-(2-thienyl)pyridine (HL3). Yield 41%. mp
1
3
150-151 °C. H NMR (270 MHz, CDCl3): δ 7.13 (dd, 1H, JHH
5.2 Hz, JHH 3.7 Hz), 7.41 (bd, 1H,3JHH 5.2 Hz), 7.50-7.52 (m,
2H), 7.63 (bd, 1H, JHH 3.7 Hz), 7.73-7.77 (m, 2H), 7.89-7.96
(m, 3H), 8.00 (dd, 1H, JHH 8.2 Hz, JHH 2.2 Hz), 8.04 (bs, 1H),
8.91 (d, 1H, JHH 2.2 Hz). MS (CI+): m/z 288 (MH+). Found: C,
3
3
3
4
4
79.55; H, 4.69; N, 4.78%. C19H13NS requires: C, 79.41; H, 4.56;
N, 4.87%.
Ligands HL5-ΗL8: 5-phenyl-2-(2-thienyl)-cyclopenteno[c]py-
ridine (HL5), 5-(4-methoxyphenyl)-2-(2-thienyl)cyclopenteno[c]py-
ridine (HL6), 5-(2-naphthyl)-2-(2-thienyl)cyclopenteno[c]pyridine
(HL7), and 5,2-bis(2-thienyl)cyclopenteno[c]pyridine (HL8) were
prepared in reactions of the corresponding 2-(2-thienyl)-1,2,4-
triazine precursors 3 with 1-morpholinocyclopentene. Mixtures of
the respective 6-aryl-3-(2-thienyl)-1,2,4-triazines (4.0 mmol) and
1-morpholinocyclopentene (3 mL, 18.8 mmol) were stirred at 190
°C under argon atmosphere for 1 h, after which a further portion
of 1-morpholinocyclopentene (1 mL, 6.3 mmol) was added and
the reaction mixture was still stirred at 190 °C for 1 h. After cooling
down to room temperature the reaction mixtures were diluted with
dichloromethane and passed through a short silica gel column eluted
with dichloromethane. The fractions containing the product were
5,2-Bis(2-thienyl)pyridine (HL4). Yield 66%. mp 153-154
°C. 1H NMR (270 MHz, CDCl3): δ 7.09-7.12 (m, 2H),
3
7.33-7.36 (m, 2H), 7.39 (bd, 1H, JHH 5.2 Hz), 7.58 (bd, 1H,
3
3
3JHH 3.9 Hz), 7.65 (d, 1H, JHH 8.2 Hz), 7.85 (dd, 1H, JHH 8.2
Hz, 4JHH 2.2 Hz), 8.82 (d, 1H, 4JHH 2.2 Hz). MS (CI+): m/z 244
(M+). Found: C, 64.31; H, 3.89; N, 5.60%. C13H9NS2 requires:
C, 64.16; H, 3.73; N, 5.76%.
5-Phenyl-2-(2-thienyl)cyclopenteno[c]pyridine (HL5). Yield
82%. mp 128-129 °C. 1H NMR (270 MHz, CDCl3): δ 2.14 (m,
2H, 3JHH 7.6 Hz), 3.04 (t, 2H, 3JHH 7.6 Hz), 3.24 (t, 2H, 3JHH 7.6
3
3
Hz), 7.14 (dd, 1H, JHH 5.0 Hz, JHH 4.0 Hz), 7.34-7.45 (m,
3
6H), 7.52 (bd, 1H, JHH 4.0 Hz), 8.44 (s, 1H). 13C NMR (68
MHz, CDCl3): δ 25.0, 32.6, 33.15, 126.0, 127.2, 127.6, 127.9,
Inorganic Chemistry, Vol. 48, No. 9, 2009 4181

Kozhevnikov et al.
128.43, 128.60, 132.2, 135.2, 137.7, 145.3, 146.8, 147.2, 152.6.
Found: C, 78.01; H, 5.58; N, 5.01%. C18H15NS requires: C,
77.94; H, 5.45; N, 5.05%.
3JHH 8.2 Hz, 4JHH 2.2 Hz), 9.59 (d, 1H, 3JHH 2.2 Hz, 3JHPt 37.4 Hz).
MS (EI): m/z 551 (M+); 473 (M+ - OSC2H6); 436 (M+ - OSC2H6
- Cl). Found: C, 32.54; H, 2.70; N, 2.47%. C15H16ClNOPtS3 re-
quires: C, 32.58; H, 2.92; N, 2.53%.
5-(4-Methoxyphenyl)-2-(2-thienyl)cyclopenteno[c]pyridine
(HL6). Yield 44%. mp 131-132 °C. 1H NMR (270 MHz, CDCl3):
δ 2.15 (m, 2H, 3JHH 7.6 Hz), 3.03 (t, 2H, 3JHH 7.6 Hz), 3.24 (t, 2H,
[Pt(L5)Cl(dmso)]. Yield 82%. 1H NMR (270 MHz, CDCl3): δ
3
3
2.22 (m, 2H, JHH 7.5 Hz), 3.10 (t, 2H, JHH 7.5 Hz), 3.15 (t, 2H,
3JHH 7.5 Hz), 3.61 (s, 6H, 3JHPt 24.1 Hz), 7.35-7.50 (m, 5H), 7.71
(d, 1H, 3JHH 5.2 Hz, 3JHPt 18.3 Hz), 9.23 (s, 1H, 3JHH 2.2 Hz, 3JHPt
37.8 Hz). Found: C, 40.90; H, 3.31; N, 2.42%. C20H20ClNOPtS2
requires: C, 41.06; H, 3.45; N, 2.39%.
3
3JHH 7.6 Hz), 3.87 (s, 3H), 6.99 (m, 2H), 7.14 (dd, 1H, JHH 5.2
3
Hz, 3JHH 3.7 Hz), 7.37-7.41 (m, 3H), 7.52 (bd, 1H, JHH 3.7 Hz),
8.42 (s, 1H); MS (CI+): m/z 308 (MH+). Found: C, 74.39; H, 5.63;
N, 4.38%. C19H17NOS requires: C, 74.24; H, 5.57; N, 4.56%.
5-(2-Naphthyl)-2-(2-thienyl)cyclopenteno[c]pyridine (HL7).
Yield 55%. mp 145-146 °C. 1H NMR (270 MHz, CDCl3): δ 2.17
[Pt(L6)Cl(dmso)]. Yield 74%. 1H NMR (270 MHz, CDCl3): δ
3
3
2.21 (m, 2H, JHH 7.5 Hz), 3.11 (t, 2H, JHH 7.5 Hz), 3.17 (t, 2H,
3
3
3
3
(m, 2H, JHH 7.6 Hz), 3.10 (t, 2H, JHH 7.6 Hz), 3.27 (t, 2H, JHH
3JHH 7.5 Hz), 3.60 (s, 6H, JHPt 24.0 Hz), 3.85 (s, 3H), 6.96 (m,
3
3
7.6 Hz), 7.16 (dd, 1H, JHH 5.2 Hz, JHH 3.7 Hz), 7.42-7.60 (m,
5H), 7.84-7.98 (m, 4H), 8.55 (s, 1H); MS (CI+): m/z 328 (MH+).
Found: C, 80.57; H, 5.21; N, 4.05%. C22H17NS requires: C, 80.70;
H, 5.23; N, 4.28%.
2H), 7.35 (m, 2H), 7.45 (d, 1H, 3JHH 5.2 Hz), 7.71 (d, 1H, 3JHH 5.2
Hz), 9.20 (s, 1H, 3JHH 2.2 Hz, 3JHPt 36.5 Hz). Found: C, 40.87; H,
3.81; N, 2.07%. C21H22ClNO2PtS2 requires: C, 41.01; H, 3.61; N,
2.28%.
5,2-Bis(2-thienyl)cyclopenteno[c]pyridine (HL8). Yield 65%.
[Pt(L7)Cl(dmso)]. Yield 53%. 1H NMR (270 MHz, CDCl3): δ
1
3
3
mp 137-138 °C. H NMR (270 MHz, CDCl3): δ 2.21 (m, 2H,
2.24 (m, 2H, JHH 7.6 Hz), 3.18 (t, 2H, JHH 7.6 Hz), 3.21 (t, 2H,
3
3
3JHH 7.6 Hz), 3.18 (t, 2H, JHH 7.6 Hz), 3.24 (t, 2H, JHH 7.6 Hz),
3JHH 7.6 Hz), 3.62 (s, 6H, 3JHPt 23.7 Hz), 7.47-7.53 (m, 4H), 7.73
(d, 1H, JHH 5.2 Hz, JHPt 18.6 Hz), 7.83-7.92 (m, 4H), 9.34 (s,
7.12-7.15 (m, 2H), 7.29 (dd, 1H, JHH 3.7 Hz, 4JHH 1.1 Hz), 7.37
3
3
3
(dd, 1H, 3JHH 5.2 Hz, 4JHH 1.1 Hz), 7.41 (dd, 1H, 3JHH 5.2 Hz, 4JHH
1H, JHPt 36.8 Hz). Found: C, 45.41; H, 3.68; N, 2.13%.
3
3
0.9 Hz), 7.50 (bd, 1H, JHH 3.7 Hz), 8.64 (s, 1H); MS (CI+): m/z
C24H22ClNOPtS2 requires: C, 45.39; H, 3.49; N, 2.21%.
[Pt(L8)(dmso)Cl]. Yield 63%. 1H NMR (270 MHz, CDCl3): δ
284 (M+). Found: C, 67.92; H, 4.68; N, 4.75%. C16H13NS2 requires:
C, 67.81; H, 4.62; N, 4.94%.
3
3
2.28 (m, 2H, JHH 7.6 Hz), 3.17 (t, 2H, JHH 7.6 Hz), 3.27 (t, 2H,
3JHH 7.6 Hz), 3.62 (s, 6H, JHPt 22.6 Hz), 7.11 (dd, 1H, JHH 4.8
3
3
2.3. Synthesis of [Pt(L)Cl(dmso)] Complexes. General Pro-
cedure. To a stirred solution of a cyclometallating agent HL (1
mmol) in acetic acid (30 mL) a solution of potassium tetrachloro-
platinate(II) (415 mg, 1 mmol) in water (1 mL) was added. The
reaction mixture was heated to 70 °C under nitrogen for 24 h.
The chloro-bridged dinuclear intermediate formed in course of the
reaction was filtered off, washed with acetic acid (5 mL), ethanol
(5 mL), and dried under vacuum. Dimethylsulfoxide (dmso, 2 mL)
was then added, and the mixture was heated under reflux for 10
min. Complexes with the cyclometallating ligands which incorporate
the cyclopenteno moiety (L5-L8) precipitated upon cooling to room
temperature. The precipitates were filtered off and washed with
dmso (1 mL) and acetone (3 mL) to give pure compounds. The
product complexes with the ligands L1-L4 remained dissolved in
dmso. The solvent was removed under reduced pressure, and the
residues were dissolved in dichloromethane and purified by column
chromatography (silica gel, dichloromethane).
3
3
4
Hz, JHH 3.7 Hz), 7.32 (dd, 1H, JHH 3.7 Hz, JHH 1.1 Hz), 7.38
(dd, 1H, 3JHH 4.8 Hz, 4JHH 1.1 Hz), 7.47 (d, 1H, 3JHH 5.2 Hz), 7.70
(d, 1H, 3JHH 5.2 Hz, 3JHPt 18.0 Hz), 9.46 (s, 1H, 3JHPt 37.9 Hz). MS
(EI): m/z 591 (M+); 513 (M+ - OSC2H6). Found: C, 36.52; H,
3.07; N, 2.29%. C18H18ClNOPtS3 requires: C, 36.58; H, 3.07; N,
2.37%.
2.4. Synthesis of [Pt(L)(acac)] Complexes. General Proce-
dure. [Pt(thpy)(acac)] was prepared according to the literature
method.19 Other [Pt(L)(acac)] complexes were prepared from the
corresponding [Pt(L)Cl(dmso)] precursors according to the follow-
ing procedure: A mixture of [Pt(L)Cl(dmso)] (0.1 mmol), sodium
acetyl acetonate monohydrate (140 mg, 1.0 mmol) and acetone (10
mL) was stirred at 50 °C for 12 h. Afterward, the reaction mixture
was diluted with water (50 mL), and the precipitated solid was
filtered off, washed with water, and dried under vacuum.
1
Characterization. [Pt(L1)(acac)]. Yield 82%. H NMR (270
Characterization. [Pt(L1)Cl(dmso)]. Yield 63%. 1H NMR (270
MHz, CDCl3): δ 1.91 (s, 3H), 1.94 (s, 3H), 5.43 (s, 1H), 7.15 (d,
MHz, CDCl3): δ 3.61 (s, 6H, 3JHPt 24.0 Hz, CH3SOCH3), 7.21 (m,
1H, JHH 4.8 Hz), 7.21-7.60 (m, 7H), 7.81 (dd, 1H, JHH 8.5, Hz,
3
3
3
4
3
3
5H), 7.60 (m, 3H), 7.96 (dd, 1H, JHH 8.4 Hz, JHH 2.1 Hz), 9.59
4JHH 1.9 Hz), 9.00 (d, 1H, JHH 1.9, Hz, JHPt 41.0 Hz). Found: C,
45.46; H, 3.41; N, 2.56%. C20H17NO2PtS requires: C, 45.28; H,
3.23; N 2.64%.
[Pt(L2)(acac)]. Yield 76%. 1H NMR (270 MHz, CDCl3): δ 1.96
(s, 3H), 1.99 (s, 3H), 3.85 (s, 3H), 5.47 (s, 1H), 7.00 (m, 2H), 7.18
3
3
(d, 1H, JHH 2.1 Hz, JHPt 37.4 Hz). Found: C, 37.48; H, 3.00; N,
2.49%. C17H16ClNOPtS2 requires: C, 37.47; H, 2.96; N, 2.57%.
[Pt(L2)Cl(dmso)]. Yield 71%. 1H NMR (270 MHz, CDCl3): δ
3.61 (s, 6H, 3JHPt 23.5 Hz, CH3SOCH3), 3.83 (s, 3H), 6.99 (m, 2H),
7.36 (d, 1H, 3JHH 4.9 Hz), 7.40 (d, 1H, 3JHH 8.4 Hz), 7.52 (m, 2H),
7.62 (d, 1H, 3JHH 4.9 Hz), 7.90 (dd, 1H, 3JHH 8.4 Hz, 4JHH 2.1 Hz),
9.54 (d, 1H, 3JHH 2.1 Hz, 3JHPt 36.2 Hz). Found: C, 37.40; H, 3.32;
N, 2.34%. C18H18ClNO2PtS2 requires: C, 37.60; H, 3.16; N 2.44%.
[Pt(L3)Cl(dmso)]. Yield 58%. 1H NMR (270 MHz, CDCl3): δ
3
3
3
(d, 1H, JHH 4.9 Hz), 7.31 (d, 1H, JHH 8.5 Hz), 7.47 (d, 1H, JHH
3
4
4.9 Hz), 7.50 (m, 2H), 7.82 (dd, 1 H, JHH 8.5, Hz, JHH 1.9 Hz),
9.00 (d, 1H, 3JHH 1.9, Hz, 3JHPt 43.1 Hz). Found: C, 45.18; H, 3.48;
N, 2.37%. C21H19NO3PtS requires: C, 45.00; H, 3.42; N 2.50%.
[Pt(L3)(acac)]. Yield 90%. 1H NMR (270 MHz, CDCl3): δ 1.97
(s, 3H), 2.00 (s, 3H), 5.49 (s, 1H), 7.21 (d, 1H, 3JHH 4.9 Hz), 7.38
3
3
3.63 (s, 6H, JHPt 24.5 Hz), 7.37 (d, 1H, JHH 4.8 Hz), 7.46-7.53
(m, 3H), 7.64 (bd, 1H, JHH 4.8 Hz), 7.69 (dd, 1H,3JHH 8.5 Hz,
(d, 1H, JHH 8.5 Hz), 7.49-7.54 (m, 3H), 7.69 (dd, 1H JHH 8.5,
3
3
3
4JHH 1.9 Hz), 7.84-7.89 (m, 2H), 7.93 (d, 1H, JHH 8.5 Hz), 8.04
Hz, JHH 1.9 Hz), 7.85-8.03 (m, 5H), 9.18 (d, 1H, JHH 1.9, Hz,
3JHPt 42.1 Hz). MS (EI): m/z 580 (M+); 481 (M+ - acac). Found:
C, 49.67; H, 3.43; N, 2.36%. C24H19NO2PtS requires: C, 49.65; H,
3.30; N 2.41%.
3
4
3
(bs, 1H), 8.08 (dd, 1H, 3JHH 8.9 Hz, 4JHH 2.2 Hz), 9.71 (d, 1H, 3JHH
3
2.2 Hz, JHPt 37.4 Hz). Found: C, 42.49; H, 3.10; N, 2.28%.
C21H18ClNOPtS2 requires: C, 42.39; H, 3.05; N, 2.35%.
[Pt(L4)Cl(dmso)]. Yield 52%. 1H NMR (270 MHz, CDCl3): δ
[Pt(L4)(acac)]. Yield 64%. 1H NMR (270 MHz, CDCl3): δ 1.97
(s, 3H), 2.02 (s, 3H), 5.48 (s, 1H), 7.11 (dd, 1H, 3JHH 5.2 Hz, 3JHH
3
3
3
3.61 (s, 6H, JHPt 24.1 Hz), 7.10 (dd, 1H, JHH 5.2 Hz, JHH 3.7
3
3
3
Hz), 7.34-7.39 (m, 4H), 7.61 (d, 1H, JHH 5.2 Hz), 7.88 (dd, 1H,
3.7 Hz), 7.18 (d, 1H, JHH 4.8 Hz), 7.27 (d, 1H, JHH 8.5 Hz),
4182 Inorganic Chemistry, Vol. 48, No. 9, 2009

Synthesis of Cyclometallated Platinum Complexes
Table 1. Crystal Data, Data Collection, and Structure Refinement Details for Complexes [Pt(L5)(acac)], [Pt(L6(acac)] and [Pt(L7)(acac)]
[Pt(L5)(acac)]
[Pt(L6)(acac)]
[Pt(L7)(acac)]
crystal description
crystal color
empirical formula
formula weight [g mol-1
crystal size [mm]
crystal system
space group
a [Å]
platelike
platelike
orange
C24H23NO3PtS
600.58
prism
orange
orange-red
C23H21NO2PtS
570.56
C27H23NO2PtS
620.61
0.250/0.130/0.070
Monoclinic
P21/c
12.3039(4)
7.7755(4)
23.7483(9)
90
104.809(3)
90
2196.51(16)
4
1.877
13.038
2416
]
0.278/0.226/0.077
triclinic
0.320/0.220/0.120
triclinic
j
j
P1
P1
7.3848(3)
12.2243(5)
12.2985(4)
66.667(4)
73.607(3)
76.079(3)
967.49(7)
2
1.959
7.379
552
173
7.4727(6)
11.6062(10)
12.1314(10)
87.409(10)
83.537(10)
77.083(10)
1018.79(15)
2
1.958
7.016
584
123
b [Å]
c [Å]
R [deg]
ꢀ [deg]
γ [deg]
cell volume [Å3]
Z
density [g cm-3
]
µ [mm-1
F(000)
T [K]
]
123
λ [Å]
0.71073
3.10-32.71
12996
6371
5386
0.71073
3.59-26.87
10536
10536
3849
1.54184
3.40-66.65
19857
11569
5467
Θ range [deg]
reflections collected
unique reflections
observed reflections
[I > 2σ(I)]
absorption correction
semiempirical from
equivalents
1.768/-1.877
0.955
0.0309
0.0579
analytical
semiempirical from
equivalents
1.496/-1.505
0.957
0.0347
0.1050
σfin [e Å-3
]
2.264/-0.908
1.068
0.0230
GOF on F2
final R1 [I g 2σ(I)]
wR2
0.0568
3
3
7.31-7.34 (m, 2H), 7.48 (d, 1H, JHH 4.8 Hz), 7.83 (dd, 1H, JHH
2.5. X-ray Crystallography. Diffraction data for crystals of
[Pt(L6)(acac)] were collected with a STOE-IPDS diffractometer
with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å),
whereas data for crystals of [Pt(L5)(acac)] and [Pt(L7)(acac)] were
collected with an Oxford Diffraction Gemini Ultra CCD diffrac-
tometer with Mo KR (λ ) 0.71073 Å) and Cu KR radiation (λ )
1.5418 Å), respectively. Further crystallographic and refinement
data are collected in Table 1. The structures were solved by direct
methods (SIR-97)32 and refined by full-matrix least-squares on F2
using the SHELXL-97 program.33 The H atoms were calculated
geometrically and a riding model was applied during the refinement
process.
8.5 Hz, JHH 2.1 Hz), 9.09 (d, 1H, JHH 2.1 Hz,3JHPt 42.1 Hz). MS
(EI): m/z 536 (M+); 437 (M+ - acac). Found: C, 40.43; H, 2.97;
N, 2.70%. C18H15NO2PtS2 requires: C, 40.30; H, 2.82; N 2.61%.
[Pt(L5)(acac)]. Yield 54%. 1H NMR (270 MHz, CDCl3): δ 1.95
(s, 3H), 1.97 (s, 3H), 2.21 (m, 2H, JHH 7.6 Hz), 3.09 (t, 2H, JHH
7.6 Hz), 3.22 (t, 2H, JHH 7.6 Hz), 5.46 (s, 1H), 7.26 (d, 1H, JHH
4
3
3
3
3
3
3
4.8 Hz), 7.35-7.50 (m, 5H), 7.58 (d, 1H, JHH 4.8 Hz), 8.68 (s,
1H, JHPt 37.7 Hz). MS (EI) m/z 571 (MH+). Found: C, 48.21; H,
3
3.82; N, 2.31%. C23H21NO2PtS requires: C, 48.42; H, 3.71; N,
2.45%.
[Pt(L6)(acac)]. Yield 78%. 1H NMR (270 MHz, CDCl3): δ 1.93
3
3
(s, 3H), 1.94 (s, 3H), 2.18 (m, 2H, JHH 7.6 Hz), 3.05 (t, 2H, JHH
7.6 Hz), 3.18 (t, 2H, 3JHH 7.6 Hz), 3.84 (s, 3H), 5.44 (s, 1H), 6.97
(m, 2H), 7.22 (d, 1H, 3JHH 4.8 Hz), 7.35 (m, 2H), 7.54 (d, 1H, 3JHH
4.8 Hz), 8.62 (d, 1H, 3JHH 1.9, Hz, 3JHPt 41.4 Hz). Found: C, 47.93;
H, 3.92; N, 2.33%. C24H23NO3PtS requires: C, 48.00; H, 3.86; N
2.33%.
3. Results and Discussion
3.1. Syntheses and Structures. The reaction which forms
the basis for the synthetic method described below was
described first by Saraswathi and Srinivasan in 1971.34
In this investigation, 2-bromoacetophenone was reacted
with 2 equiv of acid hydrazides in the presence of a base
to give 3,6-disubstituted-1,2,4-triazines. In our case
(compare Scheme 1), the key intermediates, 3-thienyl-
1,2,4-triazines 3, were prepared by reaction of readily
available bromoacetylarenes 1 and (2-thienyl)-carboxylic
acid hydrazide 2 with the reaction yields varying from
43% to 68%, depending on the nature of the group Ar of
the 2-bromoacetophenone derivatives. The next step (ii
or iii in Scheme 1) is the transformation of the triazole
[Pt(L7)(acac)]. Yield 84%. 1H NMR (270 MHz, CDCl3): δ 1.92
3
3
(s, 3H), 1.96 (s, 3H), 2.22 (m, 2H, JHH 7.6 Hz), 3.13 (t, 2H, JHH
7.6 Hz), 3.23 (t, 2H, JHH 7.6 Hz), 5.45 (s, 1H), 7.26 (d, 1H,3JHH
3
4.8 Hz), 7.50-7.54 (m, 2H), 7.56 (dd, 1H,3JHH 8.5 Hz, JHH 1.9
4
Hz), 7.58 (d, 1H,3JHH 4.8 Hz), 7.87-7.94 (m, 4H), 8.76 (s, 1H,
3JHPt 39.7). MS (EI): m/z 620 (M+); 521 (M+ - acac). C, 52.06;
H, 3.58; N, 1.98%. C27H23NO2PtS requires: C, 52.25; H, 3.74; N
2.26%.
[Pt(L8)(acac)]. Yield 78%. 1H NMR (270 MHz, CDCl3): δ 1.97
3
(s, 3H), 2.00 (s, 3H), 2.27 (m, 2H, JHH 7.6 Hz), 3.21 (t, 2H,
3JHH 7.6 Hz), 3.24 (t, 2H, 3JHH 7.6 Hz), 5.48 (s, 1H), 7.13 (dd, 1H,
3JHH 5.2 Hz, JHH 3.7 Hz), 7.25 (d, 1H,3JHH 4.8 Hz), 7.27(dd, 1H,
3
(32) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343–350.
(33) Sheldrick G. M., SHELXL-97. Program for crystal structure refine-
ment; University of Go¨ttingen: Germany, 1997.
(34) Saraswathi, T. V.; Srinivasan, V. R. Tetrahedron Lett. 1971, 12, 2315–
2316.
3JHH 3.7 Hz, 4JHH 1.1 Hz), 7.38 (dd, 1H, 3JHH 5.2 Hz, 4JHH 1.1 Hz),
3
3
7.57 (d, 1H, JHH 4.8 Hz), 8.89 (s, 1H, JHPt 40.6 Hz). MS (EI):
m/z 576 (M+); 477 (M+ - acac). Found: C, 43.83; H, 3.40; N,
2.37%. C21H19NO2PtS2 requires: C, 43.74; H, 3.32; N 2.43%.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4183

Kozhevnikov et al.
Scheme 2. Synthesis of the [Pt(L)(acac)] Complexesa
a
Reagents and conditions: (i) K2[PtCl4], acetic acid, reflux, 24 h; (ii)
dmso, reflux, 10 min; (iii) [Na(acac)], acetone, room temperature, 12 h.
Figure 1. Cyclometallating ligands (protonated) prepared in this study.
ligands HL afforded exclusively the very poorly soluble
dinuclear complexes [Pt2(µ-Cl)2L2]. The procedure can be
improved by using an aqueous solution of the platinum
salt, which shortens the reaction time required to give the
dinuclear product. Both mono- and dinuclear complexes
can be easily converted into the respective dmso com-
plexes (dmso ) dimethylsulfoxide) by heating these com-
plexes (a pure dimeric complex or a mixture of the mono-
and dinuclear complexes) in a dimethylsulfoxide, which
yields the mononuclear complexes, [Pt(L)Cl(dmso)]. These
latter complexes can readily be isolated and purified. A
reaction of [Pt(L)Cl(dmso)] with sodium acetyl acetonate,
[Na(acac)], in acetone gives the target [Pt(L)(acac)]
complexes in high yields. More details concerning the
synthetic procedures can be found in our earlier com-
munication (see ref 45).
derivative into the desired ligand in an inverse-electron-
demand Diels-Alder reaction.
Two dienophiles were used in this work: 2,5-norborna-
diene (synthetic analogue of acetylene)35-39 and 1-mor-
pholinocyclopentene (electron-rich enamine)40-42 which
doubles the number of possible structures of the desirable
thienylpyridines. Transformations of the triazines 3 to
monocyclic pyridines 4 can be performed by heating 3 under
reflux with 2,5-norbornadiene in o-xylene for 3 to 5 days.
Application of an autoclave and elevated temperatures (200
°C) allowed the reactions to complete in a relatively short
time (12-24 h) with no adverse effects on the yields (34 to
66%). Reactions of triazines 3 with 1-morpholinocyclopen-
tene proceeded more readily (full conversion was achieved
in 2 h) to give cyclopenteno[c]pyridines 5 in 44 to 82%
yields. The methodology uses no cross-coupling reactions
and therefore expensive reagents and complicated reaction
conditions can be avoided in the preparation of such ligands.
The method is easily scalable and can be performed on a
multigram scale. The chemical structures of all ligands
prepared in this study are shown in Figure 1.
Crystallization of [Pt(L5)(acac)], [Pt(L6)(acac)], and
[Pt(L7)(acac)] via slow diffusion of diethyl ether into
concentrated solutions of these complexes in dichloromethane
afforded orange to orange-red crystals of sufficient quality
for an X-ray structure determination. All of these complexes
j
crystallize in the triclinic P1 space group. Molecules of all
three [Pt(L)(acac)] complexes pack as centrosymmetric head-
to-tail dimers and display a square-planar coordination of
the Pt-ion, typical for d8 transition metal complexes.46,47
Perspective drawings of the [Pt(L5)(acac)], [Pt(L6)(acac)],
and [Pt(L7)(acac)] molecules are shown in Figure 2.
According to literature data, reaction of K2[PtCl4] with
an 2.5 molar excess of a cyclometallating ligand leads to
either a dinuclear halogen-bridged complex [Pt2(µ-Cl)2L2]
(L represents the ortho-metallated η2 ligands) or a mono-
nuclear complex of the formula [Pt(L)(HL)Cl].19,43,44 We
have found that the reactions of K2[PtCl4] with 2.5 mol equiv
of thienylpyridine derivatives HL1-HL8 in acetic acid lead
to mixtures of mononuclear [Pt(L)(HL)Cl] and dinuclear
[Pt2(µ-Cl)2L2] complexes (Scheme 2). In some cases, the
mononuclear complexes were the major products and
could be isolated from the reaction mixtures. On the other
hand, reaction of K2[PtCl4] with equimolar quantities of
In crystals of [Pt(L5)(acac)] and [Pt(L6)(acac)] the mol-
ecules are oriented in layer-like structures (Figure 3a). The
distance between the planes being about 3.5 Å is indicative
of ππ interactions between the aromatic parts of the
[Pt(L5)(acac)] and [Pt(L6)(acac)] molecules. There are no
direct intermolecular Pt-Pt contacts. The quasi-planar parts
of the [Pt(L)(acac)] molecules consisting of the [Pt(acac)]
unit and the coordinating part of the cyclometallating ligand
L (thiophene and pyridine ring) lie on these planes, whereas
the aromatic side-groups attached to the pyridine ring, phenyl
in [Pt(L5)(acac)] and 4-methoxyphenyl in [Pt(L6)(acac)], are
twisted by 44° and 56°, respectively. In crystals of
[Pt(L7)(acac)] the bulkier naphthyl side-groups prevent such
an layer-by-layer orientation of the molecules (Figure 3b),
which may be a reason for the different macroscopic forms
(35) Pabst, G. R.; Schmid, K.; Sauer, J. Tetrahedron Lett. 1998, 39, 6691–
6694.
(36) Pabst, G. R.; Sauer, J. Tetrahedron Lett. 1998, 39, 8817–8820.
(37) Pfu¨ller, O. C.; Sauer, J. Tetrahedron Lett. 1998, 39, 8821–8824.
(38) Pabst, G. R.; Pfu¨ller, O. C.; Sauer, J. Tetrahedron 1999, 55, 8045–
8064.
(39) Kozhevnikov, V. N.; Kozhevnikov, D. N.; Shabunina, O. V.; Rusinov,
V. L.; Chupakhin, O. N. Tetrahedron Lett. 2005, 46, 1521–1523.
(40) Bromley, W. J.; Gibson, M.; Lang, S.; Raw, S. A.; Whitwood, A. C.;
Taylor, R. J. K. Tetrahedron 2007, 63, 6004–6014.
(41) Laphookhieo, S.; Jones, S.; Raw, S. A.; Sainz, Y. F.; Taylor, R. J. K.
Tetrahedron Lett. 2006, 47, 3865–3870.
(42) Kozhevnikov, V. N.; Kozhevnikov, D. N.; Rusinov, V. L.; Chupakhin,
O. N.; Ko¨nig, B. Synthesis 2003, 15, 2400–2405.
(43) Mdleleni, M. M.; Bridgewater, J. S.; Watts, R. J.; Ford, P. C. Inorg.
Chem. 1995, 34, 2334–2342.
(44) Cho, J. Y.; Suponitsky, K. Y.; Li, J.; Timofeeva, T. V.; Barlow, S.;
Marder, S. R. J. Organomet. Chem. 2005, 690, 4090–4093.
(45) Kozhevnikov, V. N.; Ustinova, M. M.; Slepukhin, P. A.; Santoro, A.;
Bruce, D. W.; Kozhevnikov, D. N. Tetrahedron Lett. 2008, 49, 4096–
4098.
(46) Breu, J.; Range, K.-J.; von Zelewsky, A.; Yersin, H. Acta Crystallogr.
1997, C53, 562–565.
(47) Stu¨ckl, A. C.; Klement, U.; Range, K.-J. Z. Kristallogr. 1993, 208,
297–298.
4184 Inorganic Chemistry, Vol. 48, No. 9, 2009

Synthesis of Cyclometallated Platinum Complexes
Figure 2. ORTEP drawings of the [Pt(L5)(acac)] (left), [Pt(L6)(acac)] (middle), and [Pt(L7)(acac)] (right) molecules. Selected bond lengths [Å], angles
[deg], and torsion angles [deg] are listed in Supporting Information, Table S1.
Similarly, the emission behavior of the studied [Pt(L)-
(acac)] complexes strongly resembles that of [Pt(thpy)(acac)].
Degassed dichloromethane solutions of [Pt(L)(acac)] exhibit
red photoluminescence, and the emission bands show some
resolved vibrational satellite structures even at ambient
temperature. The emission decay times lie in the range of
several µs, and the emission quantum yields φPL range from
4% for [Pt(L4)(acac)] up to 28% for [Pt(L5)(acac)]. (Table
2) These properties, in particular in combination with the
relatively long emission decay times of about 20 µs at 77
K, indicate an assignment of the emitting triplet state as being
3
predominantly of ππ* character.
The data collected in Table 2 show clearly that the
photoluminescence properties of the investigated series
of [Pt(L)(acac)] depend on the various substituents of the
pyridine ring of the cyclometallating ligand L. The
emission bands of all [Pt(L)(acac)] complexes studied at
Figure 3. Molecule packing in crystals of [Pt(L5)(acac)] (a) and [Pt(L7)(acac)]
(b).
T ) 77 K are red-shifted with respect to [Pt(thpy)(acac)]
(with λem ) 553 nm). The extent of the shift varies from
of the crystals: prisms ([Pt(L7)(acac)]) and plates ([Pt(L5)-
(acac)] and [Pt(L6)(acac)]; Table 1).
about 400 cm-1 in the case of [Pt(L5)(acac)] (with λem
566 nm) up to 1800 cm-1 for [Pt(L4)(acac)] (with λem
)
)
3.2. Photophysical Trends. Figure 4 shows the absorption
and luminescence spectra of eight [Pt(L)(acac)] complexes
dissolved in dichloromethane and measured at ambient
temperature. Table 2 summarizes the data of resolved
absorption bands and presents information on the emission
behavior, such as positions of luminescence maxima, emis-
sion decay times in different solvents/matrixes, as well as
luminescence quantum yields.
615 nm). Similar trends are also observed at ambient
temperature. Since the lowest electronic transition mainly
involves an intraligand shift of electron density from the
electron-rich thiophene to the electron-deficient pyridine
part of the ligand19 (compare also section 4), the spectral
shifts of the emission and the lowest absorption bands
might be explainable by the electron donating/withdrawing
character of the various substituents. In particular, electron
withdrawing substituents at the pyridine part of the ligands
L should lower and electron donating substituents should
raise the energy of the lowest unnoccupied molecular
orbital (LUMO). However, the relatively strong electron
donating 4-methoxy and 2-thienyl substituents lead to
substantial red shifts of emission and lowest absorption
bands. Thus, the above considerations are not suitable to
describe the observed energy shifts, especially those
induced by these two aromatic side groups. Instead, the
stabilizing effects of the investigated aromatic/heteroaro-
matic substituents on the lowest excited states of the
[Pt(L)(acac)] complexes are probably related to an increase
of a delocalization of the LUMO involving also the aryl/
heteroaryl substituents. In complexes of the series
[Pt(L5)(acac)] to [Pt(L8)(acac)], the stabilization of the
LUMO energy is smaller because of the cyclopenteno
The absorption spectra of the investigated complexes
[Pt(L)(acac)] show similar spectral features as the analogous
complex
with
the
unsubstituted
thpy
ligand
[Pt(thpy)(acac)]19and therefore are assigned equivalently.
The high-energy intense absorption bands largely result
from excitations from the singlet ground state to singlet
excited states of ligand-centered ππ* character. The low-
energy absorption bands with maxima between 420 and
440 nm can be assigned to transitions of mixed metal-
to-ligand charge-transfer (MLCT) and ππ* character.
Compared with [Pt(thpy)(acac)] measured under the same
experimental conditions (dichloromethane solution, ambi-
ent temperature), the lowest absorption bands of the
studied [Pt(L)(acac)] complexes are slightly red-shifted.
The largest shift of about 1200 cm-1 is observed for the
complex with the cyclomatallating ligand substituted with
the 2-thienyl group, [Pt(L4)(acac)], with λabs ) 445 nm
compared to 422 nm for [Pt(thpy)(acac)].
Inorganic Chemistry, Vol. 48, No. 9, 2009 4185

Kozhevnikov et al.
Figure 4. Absorption and luminescence spectra of [Pt(L)(acac)] complexes. All spectra are measured as dichloromethane solutions at ambient temperature.
The emission spectra were recorded upon excitation into the lowest absorption band of each compound (λexc ) 410 to 440 nm). The concentration of the
[Pt(L)(acac)] complexes was about 10-4 mol/L. The absorbances measured at the maximum of the lowest absorption band did not exceed 0.4.
Table 2. Absorption and Luminescence Data of [Pt(L)(acac)] Complexesa
absorption
emission
λem [nm]
τ [µs]d
2-methyl tetrahydrofuran poly(methyl methacrylate)
λabs [nm]b(ε [103 L mol-1 cm-1])
λem [nm] τd [µs] φPL
λem [nm] τ [µs]d φPL
c
e
c
c
e h
,
solvent/matrix
temperature
dichloromethane
dichloromethane
300 K
300 K
77 K
300 K
18
[Pt(thpy)(acac)]f 422 (2.9), 403 (3.2), 360 (4.6), 331
(10.9), 315 (10.6), 285 (10.8)
558
21
0.42
553
20
558
0.45
[Pt(L1)(acac)]
[Pt(L2)(acac)]
[Pt(L3)(acac)]
425 (8.7), 328 (23.8), 292 (51.5)
426 (10.4), 331 (28.7), 294 (22.9)
596
600
608
622
13
9
12
6
0.18
0.10
0.14
0.04
581
587
589
615
17
14
18
8
590
595
601
620
14
11
13
6
0.24
0.16
0.19
0.10
430 (11.9), 334 (31.9), 292 (28.2), 255 (35.8)
[Pt(L4)(acac)]g 445 (11.1), 426 (11.2), 340 (30.5), 298 (21.2)
[Pt(L5)(acac)]g 426 (11.5), 405 (12.3), 361 (10.6), 331
(27.4), 302 (26.8), 284 (26.1), 255 (30.5),
577
18
0.28
566
21
576
17
0.35
[Pt(L6)(acac)]
[Pt(L7)(acac)]
427 (9.3), 406 (9.9), 330 (22.3), 284 (21.4), 252 (24.8)
419 (11.0), 329 (25.3), 283 (26.7),
254 (35.7),
583
587
14
16
0.21
0.23
571
571
19
21
577
584
15
18
0.27
0.30
[Pt(L8)(acac)]
435 (15.9), 416 (16.3), 335 (37.0),
290 (31.0), 260 (34.3)
610
8
0.07
591
15
607
10
0.10
a Structural formulas of the ligands L1-L8 are shown in Figure 1. b λabs ) absorption band maximum, ε ) molar absoptivity (experimental error ( 5%).
c λem ) emission maximum (error ( 1 nm). d τ ) emission decay time measured after pulsed excitation at λ ) 372 nm; experimental error ( 0.5 µs. φPL
e
) photoluminescence quantum yield; experimental error ( 15%. f See also ref 19. g For detailed spectroscopic investigations, see below. h Measured under
nitrogen atmosphere.
group, which sterically forces the ligands L5-L8 to adopt
a less planar conformation. In addition, the presence of
the cyclopenteno goup results in a higher energy of the
molecular frontier orbitals because of the electron donating
character of this aliphatic group. (For DFT optimized
molecular structures of [Pt(L1)(acac)], [Pt(L5)(acac)],
[Pt(thpy)(acac)], and [Pt(cyclopenteno-thpy)(acac)] along
with contour plots and energies of the corresponding
HOMOs and LUMOs see the Supporting Information).
The different substitutions of the cyclometallating ligands
L also lead to changes of other excited-state properties, such
as emission decay times and photoluminescence quantum
yields. (Table 2) The observed changes are related to the
spectral shifts discussed above. In particular, the longer the
emission wavelengths, the shorter is the emission decay time
and the lower the emission quantum yield. Very probably,
these effects can be traced back to the growing importance
of the non-radiative deactivation according to the energy gap
law.48-51
3.3. Detailed Characterization of the Emitting Triplet
State by High Resolution Luminescence Studies. The nature
of the emitting triplet state T1 of the platinum complexes of
the type [Pt(L)(acac)] is not yet sufficiently assigned. In
particular, it is not yet clear, whether the T1 state is of largely
mixed 3MLCT-3LC character or, as indicated by the discus-
3
sions in the preceding section, is rather a LC (3ππ*) state.
(48) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952–957.
(49) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem.
Soc. 1982, 104, 630–632.
(50) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335–4340.
(51) Stufkens, D. J.; Vle`ek, A., Jr. Coord. Chem. ReV. 1998, 177, 127–
179.
4186 Inorganic Chemistry, Vol. 48, No. 9, 2009

Synthesis of Cyclometallated Platinum Complexes
In a series of investigations it has been shown that a reliable
assignment is obtainable when highly resolved spectra and
detailed information on the triplet substates is available (e.g.,
see refs 52 and 53). Therefore, high-resolution investigations
have been carried out for two representative examples,
[Pt(L4)(acac)] and [Pt(L5)(acac)].
At first, we will focus on [Pt(L5)(acac)]. For a frozen
solution of the complex in n-octane at a concentration of c
≈ 10-5 mol/L (Shpol’skii matrix isolation technique) highly
resolved spectra have been obtained. At T ) 4.2 K and with
UV excitation at 375 nm, a manifold of narrow emission
lines with half-widths of about 3 cm-1 and a broad
background can be observed. (See Supporting Information,
Figure S1.) The narrow lines result from [Pt(L5)(acac)]
molecules (dopants) which substitute two or three matrix
molecules of the crystalline host. Since this can occur in
different ways, various sites are built at which the dopants
experience different interactions with their respective envi-
ronments. The background comes from inhomogeneously
distributed dopants. A similar spectroscopic behavior has
been observed for other Pt(II) and also Ir(III) complexes
doped into various Shpol’skii or Shpol’skii-like matrixes and
explained equivalently (e.g., compare refs 54 and 55). The
most intense lines represent electronic 0-0 transitions of the
different sites. The distribution of these sites covers a range
of about 400 cm-1. The intense line of lowest energy, located
at 17199 cm-1, is assigned to site A. This site is well suited
for site selective emission and excitation experiments by use
of a tunable dye laser, because the absorptions of all other
sites lie at higher energies and thus it is possible to excite
molecules of this site selectively.
Figure 5. Emission and excitation spectra of site A of [Pt(L5)(acac)] in
n-octane at a concentration of c ≈ 10-5 mol/L measured at low temperature
and different magnetic fields. The spectrum (a) shows the selectively excited
emission of site A excited at λexc) 17797 cm-1 (17199 cm-1 + 598 cm-1
,
vibrational satellite). The spectra (a) and (b) and (c) to (e) were recorded
at T ) 1.2 K and T ) 1.5 K, respectively. The diagrams at the left-hand
side, (b) to (e), show the region of the electronic origins in an enlarged
scale: (b) enlarged 0-0 transition line of the emission spectrum (a), (c)
excitation spectrum detected at the vibrational satellite at 16573 cm-1
(17199-626 cm-1), (d) emission spectrum in the region of the electronic
origins at B ) 10 T at a selective excitation of the substate III(B) at 17208
cm-1, and (e) excitation spectrum in the region of the electronic 0-0
transitions to I(B), II(B), and III(B) at B ) 10 T detected at λdet ) 16565
cm-1 (17191-626 cm-1, vibrational satellite). Note that with the increase
of the magnetic field, the detection energy was red-shifted by 8 cm-1 to
match the energy of the corresponding vibrational satellite at B ) 10 T.
In Figure 5a, a selectively excited emission spectrum of
site A measured at T ) 1.2 K is shown. The dominating
peak, lying at 17199 cm-1, is found at the same energy as
the dominating peak in the corresponding excitation spec-
trum. (Figures 5b and 5c) This transition represents the
highest energy feature of the low-temperature emission
spectrum and the lowest energy feature of the excitation
spectrum. Therefore, these peaks are assigned to the elec-
tronic 0-0 transition between the lowest triplet state T1 and
the electronic singlet ground state S0. Apart from the
electronic origin line, the emission spectrum shows a number
of weaker lines (Figure 5a), which represent vibrational
satellites. The satellites up to about 100 cm-1 red-shifted
relative to the electronic origin have significant lattice or
chromophore-cage mode character. Overlapping with this
energy range up to ≈ 600 cm-1, one usually finds metal-
ligand (M-L) vibrations. Fundamentals with energies higher
than ≈600 cm-1 can be classified as ligand vibrations. In
this range possibly also M-L progressions and combinations
can occur.52-54
Figure 6. Energy level diagrams and emission decay times for the substates
of the lowest triplet state T1 of [Pt(L5)(acac)] in n-octane (c ≈ 10-5 mol/
L) at B ) 0 T (left) and B ) 10 T (right). The emission decay times refer
to T ) 1.2 K (B ) 0 T) and T ) 1.5 K (B ) 10 T), respectively. Sm is a
higher lying singlet state.
In organo-transition metal compounds, the triplet term T1
is very often split into three substates which are separated
by several wavenumbers even at zero magnetic field. For
[Pt(L5)(acac)], however, at B ) 0 T only one electronic 0-0
transition is observed with the spectral resolution of our
experimental equipment of about 1 cm-1. However, the three
substates manifest themselves by their different emission
decay times. After a pulsed excitation into a higher lying
singlet state (λexc ) 355 nm, ν˜exc ) 28169 cm-1, pulse width
7 ns) the emission decay is strongly non-monoexponential.
The best fit to the experimental decay curve can be obtained
with a tri-exponential decay function. This leads to three
components of τI ) (103 ( 5) µs, τII ) (27 ( 3) µs, and τIII
) (7.6 ( 1) µs (see also Figure 6) with relative contributions
of the integrated emissions of 6, 22, and 72%, respectively.
For T g 50 K, the decay becomes monoexponential with a
(52) Yersin, H.; Donges, D. Top. Curr. Chem. 2001, 214, 81–186.
(53) Yersin, H.; Finkenzeller, W. J. In Highly Efficient OLEDs with
Phosphorescent Materials; Yersin, H., Ed.; Wiley-VCH: Weinheim,
2008; pp 1-97.
(54) Wiedenhofer, H.; Schu¨tzenmeier, S.; von Zelewsky, A.; Yersin, H. J.
Phys. Chem. 1995, 99, 13385–13391.
(55) Finkenzeller, W. J.; Hofbeck, T.; Thompson, M. E.; Yersin, H. Inorg.
Chem. 2007, 46, 5076–5083.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4187

Kozhevnikov et al.
Table 3. Energies of the Emitting T1 State, Emission Decay Times,
Zero-Field and B-field Induced (B ) 10 T) Splittings of T1 State in
[Pt(L4)(acac)] and [Pt(L5)(acac)]a
decay time of τ ) 20 µs (measured at T ) 52 K). This
behavior can be explained as follows: The energy splitting
of the triplet term into three substates is about or less than
1 cm-1. After UV excitation, these substates are populated
individually because of individual and fast intersystem
crossing processes.56 At T ) 1.2 K, these T1 sublevels are
not in a thermal equilibrium because the relaxation rates
between them are very small due to very slow processes of
spin-lattice relaxation (SLR) at small energy separations
between the triplet substates.52,56-58 Therefore, all three
sublevels emit according to their individual decay times. With
temperature increase, for example, to T g 50 K, the processes
of SLR become faster than the individual decay times of
the substates of T1. As consequence, the luminescence decay
becomes monoexponential with an average emission lifetime
of 20 µs. According to detailed investigations of the temperature
dependence of the emission decay behavior carried out with
[Pd(thpy)2],52,59 the relevant SLR process is assigned to a
Raman process. Similar behavior has also been observed for
[Pt(qol)2] (with qol ) 8-hydroxyquinolinate),56 [Ir(ppy)2(CO)Cl]
(with ppy ) 2-phenylpyridine),60 and [Re(pbt)(CO)4] (with pbt
) 2-phenylbenzothiazole).61
The existence of three close-lying triplet sublevels can
further be substantiated by application of high magnetic
fields. With an increase of the magnetic field strength, the
0-0 line at 17199 cm-1 (S0 f T1) splits into three Zeeman
lines (Figure 5e) because of the magnetic field induced
mixings of the corresponding wave functions.59,62 At B )
10 T, the splittings between the B-field disturbed substates
I(B) and II(B), and I(B) and III(B) amount to ∆EI,II(B )
10 T) ) 9 cm-1 and ∆EI,III(B ) 10 T) ) 17 cm-1, res-
pectively. This observation represents also a clear confirma-
tion of the triplet nature of the corresponding state. Detailed
energy level diagrams for [Pt(L5)(acac)] at B ) 0 T and B
) 10 T are depicted in Figure 6.
[Pt(L4)(acac)]
[Pt(L5)(acac)]
T1TS0 0-0 transition
16150 cm-1
≈ 27 µsc
≈ 27 µsc
4 µsc
17199 cm-1
103 µs
27 µs
τIb
b
τII
b
τIII
8 µs
τ (77 K)
∆E(ZFS)
∆EI,III(10 T)
τ (10 T, 1.5 K)
9 µs
20 µs
e1 cm-1
17 cm-1
8.5 µs
e1 cm-1
17 cm-1
21.5 µs
a In n-octane at a concentration of c ≈ 10-5 mol/L. b Determined at T )
1.2 K. c For [Pt(L4)(acac)], a biexponential luminescence decay is observed.
The 0-0 line splitting pattern under external magnetic fields and the result
that the mean decay time of τtherm ) 9.3 µs - calculated according to τtherm
) 3/(1/τI + 1/τII + 1/τIII)59,60 - is nearly equal to the measured decay time
of τ ) 9 µs (at T ) 77 K) lead to the given assignment of τI ≈ τII ) 27 µs
and τIII ) 4 µs. (Details will be published elsewhere).
With respect to emission wavelengths, decay times, and
quantum yields, [Pt(L5)(acac)] shows obvious differences
from [Pt(L4)(acac)]. (Table 2) Therefore, we have chosen
[Pt(L4)(acac)] as a second candidate for the detailed
studies. Table 3 summarizes a series of results. In
particular, the electronic 0-0 transition to the lowest
excited-state is about 103 cm-1 lower for [Pt(L4)(acac)]
than for [Pt(L5)(acac)], as found for the specific sites in
n-octane. This is a result of the combined delocalization
and donor/acceptor effects induced by the different
substituents. (Compare section 3.2.) The significantly
shorter emission decay time of the substate I of [Pt(L4)-
(acac)] compared to that of [Pt(L5)(acac)] is presumably
a consequence of a slightly stronger spin-orbit coupling
to higher lying singlet states. However, the zero-field
splittings of the T1 state for the both complexes are
similarly smaller than about 1 cm-1 and the splittings
induced by external magnetic field have at B ) 10 T the
same magnitude of ∆EI,III(10 T) ≈ 17 cm-1
.
In summary, the emitting states of both compounds, and
most probably of all complexes of the investigated series
exhibit very small splittings of the T1 states. It will be shown
in the next section that in particular the very small splitting
of the T1 state allows us to assign these excited states as
Application of high magnetic fields strongly influences the
emission decay behavior of the T1 excited state. With
increasing B-field, the emission decay time of substate I
decreases from τI ) 103 µs (T ) 1.2 K, B ) 0 T) to τI(B)
)
21.5 µs (T ) 1.5 K, B ) 10 T). This behavior is a con-
sequence of the B-field induced mixings of substate I with
the shorter living substates II and III. At high field, the decay
becomes monoexponential because of the fast thermal
equilibration according to the increasing importance of the
direct processes of SLR57 at larger splitting energies between
the T1 substates. The Zeeman effects as described above are
typical for three substates of one parent T1 term.55-57,61
3
being largely of LC (3ππ*) character.
4. Assignments and Conclusions
A new efficient and straightforward methodology for the
synthesis of cyclometallating ligands, derivatives of 2-(2-
thienyl)pyridine, has been demonstrated. A wide variety of
the ligands substituted on the pyridine fragment can readily
be accessed via triazine intermediates. The different substitu-
tions influence photophysical properties of the [Pt(L)(acac)]
complexes by shifting the energy of the lowest excited-state
and changing emission decay times and quantum yields. A
moderate fine-tuning of the excited-state properties by a
judicious choice of the chemical modifications of the
cyclometallating ligand seems to be possible. Detailed
photophysical investigations carried out for two representa-
tive examples, [Pt(L4)(acac)] and [Pt(L5)(acac)], indicate a
predominantly ligand centered 3ππ* character of the emitting
triplet state.
(56) Donges, D.; Nagle, J. K.; Yersin, H. Inorg. Chem. 1997, 36, 3040–
3048.
(57) Yersin, H.; Strasser, J. Coord. Chem. ReV. 2000, 208, 331–364.
(58) Yersin, H.; Donges, D.; Nagle, J. K.; Sitters, R.; Glasbeek, M. Inorg.
Chem. 2000, 39, 770–777.
(59) Schmidt, J.; Wiedenhofer, H.; von Zelevsky, A.; Yersin, H. J. Phys.
Chem. 1995, 99, 226–229.
(60) Finkenzeller, W. J.; Stoessel, P.; Yersin, H. Chem. Phys. Lett. 2004,
397, 289–295.
(61) Czerwieniec, R.; Finkenzeller, W. J.; Hofbeck, T.; Starukhin, A.;
Wedel, A.; Yersin, H. Chem. Phys. Lett. 2009, 468, 205–210.
(62) Azumi, T.; O’Donnell, C. M.; McGlynn, S. P. J. Chem. Phys. 1966,
45, 2735–2742.
4188 Inorganic Chemistry, Vol. 48, No. 9, 2009

Synthesis of Cyclometallated Platinum Complexes
The magnitude of the ZFS of the lowest triplet state
represents a characteristic value, which can be used to
classify the T1 state with respect to its orbital origin. Such
a splitting generally requires d-orbital involvements in the
excited state, since only the d-orbital contribution provides
mixing paths for spin-orbit coupling (SOC) of the individual
triplet sublevels with higher lying triplet and singlet excited
states.53,63-65 Consequently, ZFS displays the extent of the
MLCT character of the emitting state T1.52,53,57,66 A ZFS-
based empirical ordering scheme developed recently for
organo-transition metal compounds52,57,63,66 provides a useful
scale for the assessment of the MLCT contribution in the
T1 emitting state. In particular, it was found that high ZFS
values, for example, ZFS g 50 cm-1, are indicative of a
predominantly MLCT character, whereas ZFS values in the
range of only a few cm-1 or lower point to a predominantly
ligand-centered character of the emitting triplet state. There-
fore, on the basis of ZFS values found to be e1 cm-1 the
lowest excited triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)]
It is remarked that the triplet state of an emitter for
applications in OLEDs should exhibit a distinct MLCT
character. This is required, since the emitter should have a
short emission decay time and a high luminescence quantum
yield. The requirement of a short emission decay time is
particularly crucial for reduction of saturation and triplet-
triplet annihilation effects to minimize the roll-off of the
device efficiency at high current densities.69,70 Therefore,
complexes of the type [Pt(L)(acac)] do not seem to be the
first choice emitters for OLEDs, at least when high brightness
is required. This, however, does not exclude possible
applications as moderate OLED emitters or as sensors and
bioimaging materials.
Acknowledgment. The authors thank the Bundesminis-
terium fu¨r Bildung and Forschung, BMBF (R.C., T.F., and
H.Y.), the Russian Foundation for Basic Research, RFBR,
the University of York (M.M.U and A.S.), the Royal Society
(D.N.K), the EU and the EPSRC (V.N.K.) for the funding
and Johnson Matthey for generous loans of platinum metal
salts.
3
can be assigned as LC(3ππ*) with only small 1,3MLCT
admixtures.52,53,61,66 This assignment is most probably also
valid for all [Pt(L)(acac)] complexes investigated in this
work. For completeness, it is mentioned that other complexes
involving the Pt(thpy) fragment can exhibit significantly
larger ZFS values. For example, for [Pt(thpy)Cl(CO)]67 and
[Pt(thpy)2]52,54 splittings of 3.8 cm-1 and 16 cm-1, respec-
tively, were observed. These results show that the MLCT
character of the emitting state or the effectiveness of
spin-orbit coupling depends on the chromophoric, as well
as on the anilliary, ligand. (Compare ref 68).
Supporting Information Available: Selected bond lengths,
angles, and dihedral angles of [Pt(L5)(acac)], [Pt(L6)(acac)], and
[Pt(L7)(acac)], photoluminescence spectrum of [Pt(L5)(acac)]
measured in n-octane at 4.2 K under a nonselective excitation at
λexc ) 375 nm, results of DFT computations, and crystallographic
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC802401J
(66) Yersin, H. Top. Curr. Chem. 2004, 241, 1–26.
(67) Yersin, H.; Donges, D.; Humbs, W.; Strasser, J.; Sitters, R.; Glasbeek,
M. Inorg. Chem. 2002, 41, 4915–4922.
(68) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem. A 2009,
in press.
(69) Giebink, N. C.; Forrest, S. R. Phys. ReV. B 2008, 77, 235215.
(70) Reineke, S.; Waltzer, K.; Leo, K. Phys. ReV. B 2007, 75, 125328.
(63) Rausch, A. F.; Homeier, H. H. H.; Djurovich, P. I.; Thompson, M. E.;
Yersin, H. Proc. SPIE 2007, 665566550F (1-16).
(64) Nozaki, K. J. Chin. Chem. Soc. 2006, 53, 101–112.
(65) Ikeda, S.; Yamamoto, S.; Nozaki, K.; Ikeyama, T.; Azumi, T.; Burt,
J. A.; Crosby, G. A. J. Phys. Chem. 1991, 95, 8538–8541.
Inorganic Chemistry, Vol. 48, No. 9, 2009 4189