Excited-state absorption properties of platinum(II) terpyridyl acetylides

Article abstract of DOI:10.1021/ic0618652

A comprehensive photophysical study is presented which compares the ground- and excited-state properties of four platinum(II) terpyridyl acetylide compounds of the general formula [Pt(^tBu₃tpy)(C≡CR)] ⁺, where ^tBu₃tpy is 4,4′,4″-tri- tert-butyl-2,2′:6′,2″-terpyridine and R is an alkyl or aryl group. [Ru(^tBu₃tpy)₃]²⁺ and the pivotal synthetic precursor [Pt(^tBu₃tpy)Cl]⁺ were also investigated in the current work. The latter two complexes possess short excited-state lifetimes and were investigated using ultrafast spectrometry while the other four compounds were evaluated using conventional nanosecond transient-absorption spectroscopy. The original intention of this study was to comprehend the nature of the impressive excited-state absorptions that emanate from this class of transition-metal chromophores. Transient-absorbance- difference spectra across the series contain the same salient features, which are modulated only slightly in wavelength and markedly in intensity as a function of the appended acetylide ligand. More intense absorption transients are observed in the arylacetylide structures relative to those bearing an alkylacetylide, consistent with transitions coupled to the π system of the ancillary ligand. Reductive spectroelectrochemical measurements successfully generated the electronic spectrum of the ^tBu₃tpy radical anion in all six complexes at room temperature. These measurements confirm that electronic absorptions associated with the ^tBu₃tpy radical anion simply do not account for the intense optical transitions observed in the excited state of the Pt(II) chromophores. Transient-trapping experiments using the spectroscopically silent reductive quencher DABCO clearly demonstrate the loss of most transient-absorption features in the acetylide complexes throughout the UV, visible, and near-IR regions following bimolecular excited-state electron transfer, suggesting that these features are strongly tied to the photogenerated hole which is delocalized across the Pt center and the ancillary acetylide ligand.

Full text of DOI:10.1021/ic0618652

Inorg. Chem. 2007, 46, 3038−3048
Excited-State Absorption Properties of Platinum(II) Terpyridyl Acetylides
Elena Shikhova,† Evgeny O. Danilov,‡ Solen Kinayyigit,† Irina E. Pomestchenko,†
Alexander D. Tregubov,† Frank Camerel,§ Pascal Retailleau,| Raymond Ziessel,*,§ and
Felix N. Castellano*,†
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403, Ohio Laboratory for Kinetic Spectrometry, Bowling Green State
UniVersity, Bowling Green, Ohio 43403, Laboratoire de Chimie Mole´culaire associe´ au Centre
National de la Recherche Scientifique (CNRS), Ecole de Chimie, polyme´res, Mate´riaux (ECPM),
25 rue Becquerel, 67087 Strasbourg Cedex, France, and Laboratoire de Cristallochimie,
ICSN-CNRS, Baˆt 27, 1 aVenue de la Terrasse, F-91198 Gif-sur-YVette, France
Received September 28, 2006
A comprehensive photophysical study is presented which compares the ground- and excited-state properties of
four platinum(II) terpyridyl acetylide compounds of the general formula [Pt(tBu3tpy)(C CR)]+, where tBu3tpy is 4,4
,4′′
tri-tert-butyl-2,2 :6
,2′′-terpyridine and R is an alkyl or aryl group. [Ru(tBu3tpy)3]2+ and the pivotal synthetic precursor
t
′
-
′
′
[Pt(tBu3tpy)Cl]+ were also investigated in the current work. The latter two complexes possess short excited-state
lifetimes and were investigated using ultrafast spectrometry while the other four compounds were evaluated using
conventional nanosecond transient-absorption spectroscopy. The original intention of this study was to comprehend
the nature of the impressive excited-state absorptions that emanate from this class of transition-metal chromophores.
Transient-absorbance-difference spectra across the series contain the same salient features, which are modulated
only slightly in wavelength and markedly in intensity as a function of the appended acetylide ligand. More intense
absorption transients are observed in the arylacetylide structures relative to those bearing an alkylacetylide, consistent
with transitions coupled to the
π system of the ancillary ligand. Reductive spectroelectrochemical measurements
successfully generated the electronic spectrum of the tBu3tpy radical anion in all six complexes at room temperature.
t
These measurements confirm that electronic absorptions associated with the Bu3tpy radical anion simply do not
account for the intense optical transitions observed in the excited state of the Pt(II) chromophores. Transient-
trapping experiments using the spectroscopically silent reductive quencher DABCO clearly demonstrate the loss of
most transient-absorption features in the acetylide complexes throughout the UV, visible, and near-IR regions
following bimolecular excited-state electron transfer, suggesting that these features are strongly tied to the
photogenerated hole which is delocalized across the Pt center and the ancillary acetylide ligand.
Introduction
band in the visible region with corresponding high quantum
yield photoluminescence spanning the visible1-4 and near-
IR (NIR)1,5 regions facilitates a variety of applications such
as dye-sensitized solar cells,6,7 biosensors,8 nonlinear trans-
mission,9 electroluminescent materials,4 molecular switches,10
The fundamentally interesting excited-state properties of
Pt(II) charge-transfer (CT) complexes based on bipyridyl and
terpyridyl acetylides continue to attract the attention of
researchers. Indeed, the location of the low-energy absorption
(1) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro,
M. L.; Rajapakse, N. Coord. Chem. ReV. 2006, 250, 1819-1828.
(2) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.
(3) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053-4062.
* To whom correspondence should be addressed. E-mail: castell@bgsu.edu
(F.N.C.), ziessel@chimie.u-strasbg.fr (R.Z.).
† Department of Chemistry and Center for Photochemical Sciences,
Bowling Green State University.
‡ Ohio Laboratory for Kinetic Spectrometry, Bowling Green State
University.
(4) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.;
Zhu, N. Chem.sEur. J. 2001, 7, 4180-4190.
§ Laboratoire de Chimie Mole´culaire associe´ au Centre National de la
Recherche Scientifique.
(5) Adams, C. J.; Fey, N.; Weinstein, J. A. Inorg. Chem. 2006, 45, 6105-
6107.
| Laboratoire de Cristallochimie, ICSN-CNRS.
3038 Inorganic Chemistry, Vol. 46, No. 8, 2007
10.1021/ic0618652 CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/23/2007

Platinum(II) Terpyridyl Acetylides
and photocatalysis.11-13 The optical properties of these Pt-
(II) chromophores are generally tied to the low-lying CT
excited states inherent in these molecules. The variability
of the CT states to modulation in the electronic properties
of the ligands and solvent polarity allows substantial flex-
ibility in tuning the absorption/emission spectra, excited-state
lifetimes, emission quantum yields, and transient-absorption
(TA) features. The net result is an array of structurally similar
complexes with programmable optical properties useful for
a variety of applications harnessing their excited-state
properties.
resent a MLCT complex with “pure” MLCT excited states.
[Pt(tBu3tpy)Cl]ClO4 represents another pivotal model chro-
mophore since it is the closest structural analogue of the
compounds under investigation and it is known to possess
mainly low-energy MLCT excited states.19-22
Experimental Section
1. General Procedures. All synthetic manipulations were
performed under an inert and dry argon atmosphere using standard
techniques. Anhydrous CH2Cl2 (EMD Chemicals) and diisopropyl-
amine (Aldrich) were obtained by distillation over CaH2. The
reagents p-tolylacetylene, triethylsilylacetylene, and K2PtCl4 were
purchased from Fluka and Strem Chemicals, and sodium perchlorate
was purchased from Acros. Copper(I) iodide, 4,4′,4′′-tri-tert-butyl-
2,2′:6′,2′′-terpyridine, and phenylacetylene were purchased from
Aldrich Chemical Co. and used as received. All other reagents and
materials from commercial sources were used without further
purification. The silica gel used in the chromatographic separations
of 1 and 2 was obtained from EM Science (Silica Gel 60, 230-
400 mesh). In compounds 4 and 5, chromatographic purification
was conducted using aluminum oxide 90 standardized, deactivated
with 6% water by weight. Thin-layer chromatography was per-
formed on aluminum oxide plates coated with a fluorescent
indicator. All mixtures of solvents are given in a v/v ratio. The
300 MHz 1H, 400 MHz 1H, and 100 MHz 13C NMR spectra were
recorded on Bruker Advance spectrometers at room temperature
with perdeuterated solvents, with residual protiated solvent signals
providing internal references. All splitting patterns are designated
as s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q
(quartet), m (multiplet), and br (broad). MALDI mass spectra were
measured using a Bruker-Daltonics Omniflex spectrometer. Elec-
trospray mass spectra were acquired using an Agilent MSD
analytical apparatus in positive mode; an automatic calibration was
used with CH2Cl2 as the solvent. FT-IR spectra were recorded on
neat liquids or as thin films, prepared with a drop of CH2Cl2, and
evaporated to dryness on KBr pellets. Elemental analyses were
performed by Atlantic Microlab, Norcross, GA. The experimental
procedures for each reaction were tested several times and are
reported optimized for the best conditions.
Recent theoretical studies using DFT analysis have
revealed that the low-lying CT absorptions involve combina-
tions of dπ(Pt) f π* metal-to-ligand CT (MLCT) mixed
with π(CtCR) f π*(tpy) ligand-to-ligand CT (LLCT)
transitions.14-16 Thus, it is this interplay between CT states
of different origin that dramatically alters the excited-state
properties of the Pt(II) complexes as a function of the
electronic properties of the various appended ligands. The
excited-state absorption bands exhibit a complexity that likely
represents an admixture of CT transitions. The current work
seeks to identify the nature of the excited-state absorptions
in platinum(II) terpyridyl aryl- and alkylacetylides by cor-
relating a combination of ground-state spectroelectrochemical
measurements and reductive-transient-trapping experiments
with excited-state-difference spectra. In this work, four
platinum(II) terpyridyl acetylide compounds with the general
t
structure [Pt(tBu3tpy)(CtCR)]+, where Bu3tpy is 4,4′,4′′-
tri-tert-butyl-2,2′:6′,2′′-terpyridine ( R ) Ph (phenyl), SiEt3
t
(triethylsilyl), Tol (tolyl), and Bu), are synthesized and
structurally characterized. Comprehensive photophysical
measurements including the investigation of ground-state
absorption, steady-state and time-resolved photolumines-
cence, electrochemistry, reductive spectroelectrochemistry,
and time-resolved excited-state absorption measurements
were performed. To support this study, we prepared two
model complexes that possess low-energy MLCT transitions
2. Syntheses and Structures. Preparations. Caution! Perchlo-
rate salts are potential explosion hazards so appropriate precau-
tions should be taken when handling them. The synthon [Pt(tBu3-
tpy)Cl]ClO4 (1) was prepared according to the well-established
literature procedure,21 whereas [Pt(tBu3tpy)Cl]Cl was prepared via
adaptation of the same reference. 1H (400.129 MHz) NMR: δ 8.94
(s, 2H), 8.91 (d, 3J ) 6.2 Hz, 2H), 8.89 (d, 4J ) 1.8 Hz, 2H), 7.63
and measured their photophysics in parallel with the other
17,18
molecules. Ru(tBu3tpy)2(PF6)2
was selected to best rep-
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M.; Takahashi, Y.; Murata, S.; Arakawa, H. Inorg. Chem. 2001, 40,
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2005, 44, 242-250.
3
4
(dd, J ) 6.2 Hz, J ) 1.8 Hz, 2H), 1.63 (s, 9H), 1.52 (s, 18H).
MALDI-TOF: m/z 1049.67 [M+ - ClO4].
(8) Wong, K. M.-C.; Tang, W.-S.; Chu, B. W.-K.; Zhu, N.; Yam, V. W.-
W. Organometallics 2004, 23, 3459-3465.
[Pt(tBu3tpy)(CtCPh)]ClO4 (2). [Pt(tBu3tpy)Cl]ClO4 (0.150 g,
0.205 mmol) was reacted with phenylacetylene (0.06 g, 0.615
mmol) in distilled CH2Cl2 (100 mL) in the presence of freshly
distilled diisopropylamine (30 mL) and with CuI (0.004 g, 0.022
(9) Guo, F.; Sun, W.; Liu, Y.; Schanze, K. Inorg. Chem. 2005, 44, 4055-
4065.
(10) Yutaka, T.;Mori, I.; Kurihara, M.; Mizutani, J.; Tamai, N.; Kawai,
T.; Irie, M.; Nishihara, H. Inorg. Chem. 2002, 41, 7143-7150.
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Tung, C.-H. J. Am. Chem. Soc. 2004, 126, 3440-3441.
(12) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726-7727.
(18) Abrahamsson, M.; Ja¨ger, M.; Osterman, T.; Eriksson, L.; Persson, P.;
Becker, H. C.; Johansson, O.; Hammarstrom, L. J. Am. Chem. Soc.
2006, 128, 12616-12617.
(13) Narayana-Prabhu, R.; Schmehl, R. H. Inorg. Chem. 2006, 45, 4319-
4321.
(19) Michalec, J. F.; Bejune, S. A.; McMillin. D. R. Inorg. Chem. 2000,
39, 2708-2709.
(14) Zhou, X.; Zhang, H.-X.; Pan, Q.-J.; Xia, B.-H.; Tang, A.-C. J. Phys.
Chem. A 2005, 109, 8809-8818.
(20) Ratilla, E. M. A.; Brothers, H. M., II.; Kostic, N. M. J. Am. Chem.
Soc. 1987, 109, 4592-4599.
(15) Liu, X.-J.; Feng, J.-K.; Meng, J.; Pan, Q.-J.; Ren, A.-M.; Zho, X.;
Zhang, H.-X. Eur. J. Inorg. Chem. 2005, 1856-1866.
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2006, 45, 4304-4306.
(21) Yip, H. K.; Cheng, L. K.; Cheung, K. K.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1993, 2933-2938.
(22) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin D. R. Inorg.
Chem. 2001, 40, 2193-2200.
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(2), 338-347.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3039

Shikhova et al.
mol) as catalyst. The reaction mixture was stirred under an inert
argon atmosphere at room temperature for 17 h while excluding
light. Crude product 2 was purified by column chromatography (5
vol % methanol in CH2Cl2, SiO2), yielding a dark-orange crystalline
solid in 67% yield.
1H NMR (300 MHz, CDCl3): δ 9.15 (d, 2H, J ) 6.0 Hz), 8.44
(s, 2H), 8.37 (d, 2H, J ) 2.0 Hz), 7.6 (dd, 2H, J ) 6.0 and J ) 2.0
Hz), 7.51 (dd, 2H, J ) 8.0 and J ) 2.0 Hz), 7.31 (m, 3H), 1.59 (s,
9H), 1.51 (s, 18H). MALDI-MS: calculated for C35H40N3Pt, 697.29;
found, 697.36. Anal. Calcd for C35H40ClN3O4Pt: C, 52.69; H, 5.02;
N, 5.27. Found: C, 51.50; H, 5.06; N, 5.03.
1505, 1463, 1366, 1255, 1046, 1019, 915. ES-MS: m/z 711.2
(100%, [M - BF4]+) in CH2Cl2. Anal. Calcd for C36H42N3PtBF4
(Mr ) 798.62): C, 54.14; H, 5.30; N, 5.26. Found: C, 54.00; H,
5.12; N, 4.93.
[Ru(tBu3tpy)2](PF6)2 (6).23 Ruthenium(III) chloride monohydrate
(170 mg, 0.754 mmol) and 680 mg (1.514 mmol) of 4,4′,4′′-tri-
tert-butyl-2,2′:6′,2′′-terpyridine were refluxed in 20 mL of anhy-
drous ethanol for 5 h. After the solution was cooled to room
temperature, a concentrated aqueous solution of ammonium hexafluo-
rophosphate was added and the mixture was stirred for 1 h. Upon
the addition of 25 mL of water, a dark-red solid precipitated. The
solid was collected by vacuum filtration and washed with diethyl
ether. The title compound was purified by column chromatography
over neutral Al2O3 using a gradient elution of toluene/acetonitrile
from 1:1 to 1:4. The resulting red powder was obtained by rotary
[Pt(tBu3tpy)(CtCtBu)]ClO4 (3). [Pt(tBu3tpy)Cl]ClO4 (0.160 g,
0.218 mmol) was reacted with 3,3-dimethyl-1-butyne (0.054 g,
0.654 mmol) in distilled CH2Cl2 (150 mL) in the presence of freshly
distilled diisopropylamine (50 mL) and with CuI (0.004 g, 0.022
mmol) as catalyst. The reaction mixture was stirred under an inert
argon atmosphere at room temperature for 18 h. The volatiles were
then removed under reduced pressure. The crude product 3 was
recrystallized from chloroform. Yield: 87%. MALDI-MS: calcu-
lated for C33H44N3Pt, 677.80; found, 677.38. 1H NMR (300 MHz,
CDCl3): δ 9.13 (d, 2H, J ) 6.0 Hz), 8.40 (s, 2H), 8.33 (d, 2H, J
) 1.8 Hz), 7.64 (dd, 2H, J ) 6.0 and J ) 2.1 Hz), 1.63 (s, 9H),
1.53 (s, 18H), 1.43 (s, 9H). Anal. Calcd for C33H44ClN3O4Pt: C,
50.96; H, 5.66; N, 5.41. Found: C, 49.64; H, 5.61; N, 5.23.
[Pt(tBu3tpy)(CtCSiEt3)]BF4 (4) and [Pt(tBu3tpy)(CtCTol)]-
BF4 (5). The preparation begins with the dissolution of [(tBu3tpy)-
PtCl]Cl (0.200 g, 0.300 mmol) in a mixture of DMF (10 mL) and
triethylamine (3 mL), followed by the addition of p-tolylacetylene
(0.042 g, 0.359 mmol) or triethylsilylacetylene (0.050 g, 0.359
mmol). The solution was degassed by three freeze-pump-thaw
cycles to exclude residual oxygen. The addition of CuI (0.0017 g,
0.009 mmol) to the deep-yellow solution resulted in an instanta-
neous color change to red. After it was stirred at room temperature
for 2 days, the deep-red solution was concentrated to roughly 1
mL, filtered over Celite, and then dropped into an aqueous solution
(10 mL) containing NaBF4 (1.00 g). The complex was recovered
by filtration using standard filter paper and washed with water (3
× 100 mL), and the red solid was dried under high vacuum.
Purification was ensured by column chromatography using alumina
as a solid support and a gradient of methanol (0-1%) in CH2Cl2
as the mobile phase. Ultimate recrystallization by slow evaporation
of CH2Cl2 from a CH2Cl2/cyclohexane solution afforded pure 4
and 5.
1
evaporation of the desired fraction. H NMR (MeCN-d3): δ 8.77
(s, 4H), 8.36 (s, 4H), 7.17 (dd, 8H), 1.75 (s, 18H), 1.33 (s, 36H).
MALDI-TOF: m/z 1194.46 [M+], 1049.67 [M+ - PF6], 904.72
[M+ - 2PF6].
Single-Crystal X-ray Crystallography. X-ray diffraction data
for 4 were recorded from a single, brown (dark-yellow) crystal of
dimensions 0.4 mm × 0.2 mm × 0.2 mm, at ambient temperature
on an Enraf-Nonius Kappa-CCD diffractometer, using graphite-
monochromated Mo KR (λ ) 0.71073 Å) radiation, and performed
with the COLLECT software.24 A æ scan with an increment of 1.8°
over 183° was performed first and completed by four subsequent
ω scans to fill the asymmetric unit. The images were interpreted,
and the intensities were integrated using the program DENZO.25
Unique reflections numbering 6421 were obtained from a total of
16 005 measured reflections (Rint ) 0.029), in the range of h, -11
to 11; k, -14 to 13; l, -19 to 18, with 2θmax ) 56.7°. The structure
was solved by Patterson methods, expanded to all non-H atoms by
the Fourier method (PATTY),26 and refined by full-matrix least-
squares on F2 values using SHELX-L9727 as implemented into
Crystalbuilder-GUI.28 In the least-squares refinement, all non-H
atoms were refined anisotropically despite the strong dynamic
disorder of certain groups (the tert-butyl group (C20 > C23), the
-
three ethyl groups attached to Si, the BF4 counterion, and the
cyclohexane solvent molecule), necessitating geometrical and
anisotropic displacement parameter (ADP) restraints. Additionally,
the apparent free rotation of the aforementioned tert-butyl group
was treated by considering two sites per atoms with the occupancy
2
rate refined to /3:1/3. Similarly, the C atoms of the cyclohexane
were allowed to take two positions to account for large ADP. H
atoms from the pyrrole groups were located on difference Fourier
syntheses and then treated like those placed in calculated positions,
as riding atoms, with Uiso set to 1.2 times that of the attached C
atom (1.5 times when it was of a methyl group). Convergence for
541 variable parameters by least-squares refinement on F2 with w
1
Complex 4: 0.222 g, 90% isolated yield. H (400.129 MHz)
3
3
NMR: δ 9.09 (d, J ) 6.2 Hz, 1H), 8.97 (d, J ) 6.2 Hz, 1H),
4
3
4
8.49 (s, 2H), 8.44 (d, J ) 1.5 Hz, 2H), 7.55 (dd, J ) 6.2 Hz, J
3
) 1.5 Hz, 2H), 1.52 (s, 9H), 1.46 (s, 18H), 1.08 (t, J ) 8.0 Hz,
9H), 0.66 ppm (q, 3J ) 8.0 Hz, 6H). 13C{1H} (100.612 MHz
NMR): δ 168.8, 167.5, 158.8, 153.8, 153.5, 125.1, 123.4, 121.7,
119.6 (CtC), 105.8 (CtC), 37.4, 36.4, 30.3, 30.1, 7.9, 5.4. FT-IR
(cm-1): 2947, 2920, 2870, 2050 (νC≡C), 1613, 1555, 1478, 1464,
1368, 1258, 1064, 1018, 916. ES-MS: m/z 735.2 (100%, [M -
2
) 1/[σ2(Fo ) + (0.0591P)2 + 1.5933P], where P ) (Fo2 + 2Fc2)/3
(23) Schubert, U. S.; Eschbaumer, C.; Andres, P.; Hofmeier, H.; Weidl,
C. H.; Herdtweck, E.; Dulkeith, E.; Morteani, A.; Hecker, N. E.;
Feldmann, J. Synth. Met. 2001, 121, 1249-1252.
BF4]+) in CH2Cl2. Anal. Calcd for C35H50N3PtSiBF4 (Mr
)
(24) Enraf-Nonius; Delft, The Netherlands, 1997.
822.70): C, 51.09; H, 6.13; N, 5.11. Found: C, 50.78; H, 5.94; N,
4.96.
(25) Otwinovski, Z.; Minor, W. Meth. Enzymology A 1997, 307-326.
(26) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.;
Gould, R. O.; Israel, R. and Smits, J. M. M. The DIRDIF-99 program
system; Crystallography Laboratory, University of Nijmegen: Nijmegen,
The Netherlands, 1999.
(27) Sheldrick, G. M. SHELX97; Program for the Refinement of Crystal
Structures from Diffraction Data; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(28) Welter, R.; Romang, J. F. Crystalbuilder-GUI for SHELXL97 on
MACOSX; Louis Pasteur University-Strasbourg: Strasbourg, France,
2002. http://www-chimie.u-strasbg.fr/∼decmet/crystalbuilder/crystal-
b.html.
1
Complex 5: 0.229 g, 96% isolated yield. H (400.129 MHz)
3
3
NMR: δ 9.15 (d, J ) 6.0 Hz, 1H), 8.93 (d, J ) 6.1 Hz, 1H),
3
4
8.44 (s, 2H), 8.42 (m, 2H), 7.61 (dd, J ) 6.1 Hz, J ) 1.3 Hz,
2H), 7.39 (d of the ABq, J ) 8.1 Hz, 2H), 7.13 (d of the ABq, J
) 8.1 Hz, 2H), 2.37 (s, 3H), 1.54 (s, 9H), 1.48 (s, 18H). 13C{1H}
(100.612 MHz NMR): δ 168.5, 167.8, 159.2, 154.6, 151.2, 136.9,
132.2, 129.2, 125.3, 123.6, 122.5, 104.9, 96.8, 37.8, 36.8, 30.6,
30.5, 21.8. FT-IR (cm-1): 2921, 2853, 2060 (νC≡C), 1606, 1554,
3040 Inorganic Chemistry, Vol. 46, No. 8, 2007

Platinum(II) Terpyridyl Acetylides
for 5480 reflections with I > 2σ(I), was reached at R ) 0.0396
and Rw ) 0.0958 with a goodness-of-fit value of 1.043. (∆/σ)max
< 0.001. The final difference Fourier map was featureless, with
maximum positive and negative peaks of +0.692 and -0.843 e
Å-3, respectively.
UV-vis Spectroelectrochemistry. Spectroelectrochemical mea-
surements were performed in a home-built cell with an optically
transparent thin-layer electrode (OTTLE) cell.33 This home-built
capillary apparatus consists of a gold-grid working electrode, a
platinum counter electrode, and a Ag/AgCl (3 M NaCl) reference
electrode. The optical cell was placed in a plexiglass box with quartz
windows and mounted in the HP 8453 such that the light beam
passes through the gold-grid working electrode. Solutions were
prepared by dissolving 1 mM metal complexes in 0.1 M TBAP/
DMF or 0.5 M TBAH/DMF, depending on the counterion of the
metal complex. Samples were degassed with argon gas prior to
the experiments, and the inert-gas purge was maintained through
the mounted box during the measurements to exclude oxygen and
water vapor. The ground-state absorption spectrum of the sample
contained within the cell at 0 V was used as the spectrophotometer
blank. In this configuration, any changes associated with bulk
reduction are observed as differences in absorption (∆A). In all
cases, the potentials were maintained at a value 100-200 mV more
negative than the first-reduction wave, using the BAS Epsilon
system. The electrolysis times ranged between 5 and 9 min.
Reductive spectroelectrochemistry yielded clearly observable and
stable one-electron-reduction products, whereas the oxidative
process performed on the first irreversible wave generated ill-
defined, unstable oxidation products at ambient temperature. The
original UV-vis spectrum from the [Pt(tBu3tpy)Cl]+ complex could
not be completely restored after reductive spectroelectrochemistry
in CH2Cl2 or DMF.
Spectroscopic Measurements. UV-vis absorption spectra were
measured with a Hewlett-Packard 8453 diode array spectropho-
tometer, accurate to (2 nm. Uncorrected steady-state photolumi-
nescence spectra were obtained with a single-photon-counting
spectrofluorimeter from Edinburgh Analytical Instruments (FL/FS
900). All photophysical experiments used optically dilute (OD)
solutions (absorbance range ) 0.09-0.11) prepared in spectroscopic-
grade solvents unless otherwise stated. All luminescence samples
in 1 cm2 anaerobic quartz cells (Starna Cells) were deoxygenated
with solvent-saturated argon for at least 20 min prior to measure-
ment. Photoluminescence quantum yields were determined using
[Ru(bpy)3]2+ in deaerated CH3CN (Φem ) 0.062)29 as the quantum
counter. Emission lifetimes were measured by means of a nitrogen-
pumped broadband dye laser (2-3 nm fwhm) from a PTI instrument
(model GL-3300 N2 laser and model GL-301 dye laser) using the
experimental apparatus previously described.30
The experimental apparatus for nanosecond TA spectroscopy,
operating under the control of LabView, was described earlier.31
TA spectra and decay kinetics were obtained using the unfocused
second or third harmonic provided by a Continuum Surelite I Nd:
YAG laser (532 or 355 nm and 5-7 ns fwhm) or a Spectra Physics
GCR 230 laser system. The data consisting of a 16- or 32-shot
average were analyzed by Origin 6.1 software. All flash photolysis
measurements were conducted at the ambient temperature of 22 (
2 °C. All samples were thoroughly degassed prior to measurements
with high-purity argon and kept under an argon atmosphere
throughout the experiment. Samples with absorbances of 0.3-0.4
at the excitation wavelength were typically employed in these
experiments. To verify the stability of the complexes, UV-vis
absorbance spectra were taken before and after all transient
experiments. Ultrafast TA spectra and kinetics were obtained using
the same apparatus as that described in an earlier publication.32
Electrochemistry. Electrochemical measurements were per-
formed in anhydrous DMF solutions with 0.1 M nBu4NClO4
(TBAP) or 0.5 M nBu4NPF6 (TBAH) as the supporting electrolyte
at room temperature, depending on the counterion of the complex.
A platinum disk working electrode, a platinum wire auxiliary
electrode, and a Ag/AgCl (3M NaCl) reference electrode were used
for all the measurements at Bowling Green State University
(BGSU). The work at ULP utilized a platinum pseudo-reference
electrode. Anhydrous DMF was purchased from Acros or distilled
over KOH under reduced pressure, and TBAP and TBAH were
recrystallized from ethanol. The solutions were degassed with argon
prior to each measurement. The ferrocenium/ferrocene couple
(FeCp2+/0) was used as an internal reference for all measurements
from both laboratories, and potentials are reported relative to
FeCp2+/0, which has E1/2 ) +0.55 V vs aqueous Ag/AgCl with Epa
- Epc ) 81 mV. No iR compensation was used. The cyclic- and
differential-pulse voltammograms measured at BGSU were recorded
with a Bioanalytical Systems Epsilon controller interfaced with a
Pentium PC.
Results and Discussion
Syntheses and Structures. All complexes presented in
this study are air-stable solids and are reasonably soluble in
both CH3CN and CH2Cl2 (Chart 1). All newly synthesized
Pt(II) compounds were structurally characterized by NMR,
mass spectrometry, elemental analysis, and FT-IR spectrom-
etry. Dark-yellow prismatic crystals suitable for X-ray
analysis were obtained in pure single phase by slow
evaporation from a CH2Cl2/cyclohexane mixture at room
temperature. The asymmetric unit contains one Pt complex,
-
one BF4 counteranion, and one disordered cyclohexane
molecule in general position. Figure 1 shows the atomic
numbering and labeling scheme of the Pt(II) complex. The
tert-butyl fragment on the central pyridine is disordered, and
only the position with a 2/3 occupancy is represented on the
ORTEP drawing. The geometry of the Pt center is distorted
square planar with Pt-C1′ ) 1.967(7) Å, Pt-N1 ) 2.020-
(5) Å, Pt-N2 ) 1.962(6) Å, and Pt-N3 ) 2.016(6) Å
distances and C1′-Pt-N1 ) 98.14(26)°, N1-Pt-N2 )
79.95(21)°, N2-Pt-N3 ) 81.08(21)°, and N3-Pt-C1′ )
100.82(25)° angles. The whole molecule is planar, and the
N2, Pt, C1′, C2′, and Si atoms are all contained in the mean
plane of the molecule. Only a slight tilt of 9.6° between the
N2-Pt-C1′ and the C1′-C2′-Si axes can be observed,
showing that the triethylsilylalkyne fragment did not bind
along the pseudo-C2 geometrical axis of the terpyridine
fragment. Additional crystallographic parameters are pro-
vided as Supporting Information (Table S1 and Figure S1).
Steady-State Absorption and Photoluminescence Prop-
erties. The ground-state absorption spectra of the platinum-
(29) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583-5590.
(30) Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano,
F. N. Inorg. Chem. 2003, 42, 1394-1396.
(31) Pomestchenko, I. E.; Castellano, F. N. J. Phys. Chem. A 2004, 108,
3485-3495.
(32) Danilov, E. O.; Pomestchenko, I. E.; Kinayyigit, S.; Gentili, P. L.;
Hissler, M.; Ziessel, R.; Castellano, F. N. J. Phys. Chem. A 2005,
109, 2465-2471.
(33) DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-
597.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3041

Shikhova et al.
Chart 1
(II) terpyridyl acetylide complexes 1-5 were measured in
CH2Cl2 at room temperature and are presented in Figure 2.
In the spectral region between 300 and 600 nm, all
investigated complexes display a strong absorption band
centered at ∼320 nm and a broad absorption between 360
and 520 nm. For 3 and 4, this low-energy band is centered
at 420-430 nm, whereas in 2 and 5, the same band
apparently consists of two components centered at 410 and
470 nm and 400 and 490 nm, respectively. The intense
higher-energy band with extinction coefficients of 15000-
30000 M-1 cm-1 is assigned to ligand-localized π-π*
transitions.21 On the basis of the recent results of DFT
calculations on similar compounds,14-16 the broad absorption
bands at lower energy with extinction coefficients of
∼3200-5000 M-1 cm-1 are most likely composed of
overlapping MLCT and LLCT transitions. Previous experi-
mental and theoretical work has revealed that the nature of
the substituents on the terpyridyl and acetylide ligands
dramatically affects the lowest-energy excited states of the
Pt(II) complexes by modulating the HOMO-LUMO energy
gap. The two major controlling influences are the σ-donating
ability of the substituents and the extent of conjugation in
the acetylide group. Increasing the σ-donating ability of the
acetylide causes destabilization of the HOMO and a con-
comitant decrease of ∆E, shifting the low-energy absorption
band to the red region. This is the primary reason why the
low-energy band of 3 is red-shifted when directly compared
to the absorption spectrum of 4. Extending the conjugation
in the acetylide group provides additional low-energy
electronic states which then participate in LLCT transitions.
As a result, the low-energy CT bands in complexes 2 and 5
are substantially broadened relative to the corresponding
absorptions in compounds 3 and 4. Since a more substantial
contribution of acetylide electron density to the HOMO
increases the role of the LLCT transitions, it may be possible
to alter the character of the lowest-energy transition from
mixed to pure MLCT or pure LLCT by regulating these
factors.
Figure 1. ORTEP drawing of 4 with thermal ellipsoids drawn at the 50%
probability level.
Figure 2. Normalized room-temperature absorption spectra of the
complexes 1 (black line), 2 (green line), 3 (blue line), 4 (magenta line),
and 5 (red line) in CH2Cl2.
The photoluminescence spectra of 2-5 collected at room
3042 Inorganic Chemistry, Vol. 46, No. 8, 2007

Platinum(II) Terpyridyl Acetylides
Table 1. Photophysical Properties of the Platinum(II) Terpyridyl
Acetylide Complexes 2-5 Measured at Room Temperature in
Argon-Saturated CH2Cl2
λ
abs, nm
compound (ꢀ, M-1 cm-1
)
λ
em, nm τ0 em,a µs Φem
kr, s-1 c
knr, s-1 d
b
2
3
315, 340,
592
2.9
0.10 3.4 × 104 3.1 × 105
411 (4750),
458 (4380)
315 (9547),
548
2.2
0.17 7.7 × 104 3.8 × 105
341 (8804),
406 (3224),
426 (3873)
4
5
315, 339,
400 (4539),
424 (4781)
528
608
1.9
0.08 4.2 × 104 4.8 × 105
0.035 4.1 × 104 1.1 × 106
312, 332,
0.85
380 (3913),
398 (4136),
463 (2302)
a Emission intensity decay lifetime, (5%. Quantum yield of photolu-
b
minescence measured relative to [Ru(bpy)3](PF6)2 (Φem ) 0.062)29 in
Figure 3. Normalized room-temperature emission spectra of the complexes
2 (green line), 3 (blue line), 4 (magenta line), and 5 (red line) in deaerated
CH2Cl2.
CH3CN. c Radiative decay rate, calculated by kr ) Φem/τem d Nonradiative
.
decay rate, calculated by knr ) (1 - Φem)/τem; it is assumed that the emitting
excited state is produced with unit efficiency.
temperature in degassed CH2Cl2 solutions measured under
OD conditions are shown in Figure 3. Upon photoexcitation
into the CT transitions, 2-5 exhibit broad structureless
photoluminescence over the range of 475-800 nm. [Pt(tBu3-
tpy)Cl]ClO4, complex 1, is completely nonluminescent in
fluid solution at 298 K,22 resulting from radiationless decay
of the excited state via a low-lying metal-centered ligand
field state.34 On the basis of the large Stokes shifts, relatively
long lifetimes (1-3 µs), and dynamic quenching by air (O2),
the emission can be assigned as triplet charge transfer in
nature in all the Pt(II) molecules. All emission spectra were
completely invariant to the excitation wavelength in each
instance. The emission maximum in 5 is 20 nm red-shifted
relative to that of 2, illustrating the stronger σ-donating ability
of a p-tolylacetylide versus the phenylacetylide substituent,
confirming that the acetylide electronic structure plays a
significant role in determining the position of the HOMO in
these molecules. The same reasoning can be used to explain
the red shift of the emission maximum in 3 compared with
that of 4; however, the stabilizing nature (electron withdraw-
ing) of the SiEt3 moiety is predominately responsible for the
green emission displayed by 4.35 As mentioned above,
expansion of the π conjugation in the acetylide ligands also
aids in decreasing the energy gap, and therefore, the emission
spectra of complexes 2 and 5 are red-shifted compared with
those of complexes 3 and 4. The room-temperature absorp-
tion and photoluminescence properties of the investigated
complexes are collected in Table 1, along with their
calculated values of kr and knr. An interesting observation is
that the value of knr increases by a factor of 3.5 between 2
and 5. If one correlates knr differences between Pt(dbbpy)-
(CtCPh)2 and Pt(dbbpy)(CtCTol)2 using literature data in
the same solvent,3,31 an increase of 2.2-fold is revealed,
similar to the present case but slightly attenuated. These
somewhat surprising differences in knr are not obvious based
on the structural differences between phenyl- and tolylacetyl-
Table 2. Cyclic Voltammetry Data at Room Temperature in DMFa
+/0 g
complex
E1/2/V vs FeCp2
1b
+0.45e
-1.34,
-1.91,
-2.66 (DPV)h
-1.38,
-1.92
-1.43,
-1.99
-1.34,
-1.91
-1.35,
-1.93
-1.78,
-2.02
2b
3b
4d
5d
6c,f
>+1.15
>+1.15
>+1.17
>+1.14
+0.65
a All measurements made at room temperature using a scan rate of 200
mV/s unless otherwise indicated. b Measured in 0.1 M TBAP. c Measured
e
in 0.5 M TBAH. d Measured in 0.1 M TBAH. Epa refers to the anodic
f
peak potential for the irreversible oxidation waves. E1/2 oxidation potential;
g
cyclic voltammogram was completely reversible. E1/2 ) (Epa + Epc)/2;
E
pa and Epc are anodic and cathodic peak potentials, respectively. h Reduction
measured using differential-pulse voltammetry.
ide nor are they anticipated based on energy-gap consider-
ations. Therefore, they must have their origin in subtle
electronic structure differences, particularly in how these
acetylide ligands interact and mix with the metal-based
orbitals. Since these electronic differences are not revealed
in electrochemistry experiments (see below), we believe that
this problem deserves a detailed theoretical investigation.
Electrochemistry and Spectroelectrochemistry. The
electrochemical data of the compounds in this study are
summarized in Table 2. All the complexes show two
reversible couples at ca. -1.35 and -1.92 V. For [Pt(tBu3-
tpy)Cl]ClO4, a third reduction at -2.66 V was observed by
differential-pulse voltammetry (DPV). The relative insensi-
tivity of the reduction potentials toward the nature of the
acetylide ligand suggests that these reductions arise mainly
from the terpyridine reductions with some mixing of Pt(II)
metal character.36,37 No oxidation process could be observed
(34) McMillin, D. R.; Moore, J. J. Coord. Chem. ReV. 2002, 229, 113-
(36) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585-4590.
(37) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta
1998, 273, 346-353.
121.
(35) Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg. Chem.
2005, 44, 471-473.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3043

Shikhova et al.
Figure 4. UV-vis absorption spectral changes accompanying the reductive-controlled-potential electrolysis as a function of time at room temperature: (a)
1 mM of 2 at -0.93 V vs Ag/AgCl in 0.1 M TBAP/DMF; (b) 1 mM of 6 at -1.42 V vs Ag/AgCl in 0.5 M TBAH/DMF.
beyond +1.15 V due to the limited potential range of the
TBAP/DMF electrolyte solution. When the potential range
of the supporting electrolyte solution permits, an irreversible
anodic wave is observed, which is assigned to a HOMO-
centered oxidation, with most substantial contributions
coming from the oxidation of Pt(II) to Pt(III).38,39
This reduction process is somewhat reversible on the
spectroelectrochemical time scale (minutes), and the initial
absorption spectrum is partially (∼70%) regenerated after
removal of the negative bias on the OTTLE cell. Similar
data were obtained using the CH2Cl2 solvent except that the
lower-energy visible band shifts to the red (421 nm),
becoming better defined relative to the similar feature
observed as a shoulder on the 362 nm band in DMF. Since
the electrolysis is not completely reversible in either solvent,
comparison to the TA-difference spectrum will not be
definitive. Controlled-potential electrolysis of 2 and 3 shows
behavior similar to that of 1. In both absorption-difference
spectra, there is a decrease in the transitions around 327 and
Spectroelectrochemical measurements were performed on
compounds 1-3 and 6. Since the data for 1-3 are quite
similar, only the data generated on compounds 2 and 6 are
presented in Figure 4 for illustrative purposes. The remaining
reductive spectroelectrochemical data are provided as Sup-
porting Information (Figures S2-S5). Compounds 1-3 are
unstable toward oxidative-controlled-potential electrolysis,
but all compounds are stable with regards to reductive
spectroelectrochemistry. In the case of 6, which possesses
t
341 nm that originate from the Bu3tpy ligand. New bands
are observed for both reduced complexes at 362, 562, and
t
no acetylide ligands but only the Bu3tpy ligand, the π-π*
763 nm in 2 and at 368, 568, and 777 nm for 3, which result
transitions of the tBu3tpy ligand attenuate upon one-electron
reduction of the complex while two new absorption bands
develop at 361 and 537 nm, corresponding to intraligand
t
from one-electron reduction of the Bu3tpy ligand, which is
essentially the same trend as that seen in the spectroelec-
trochemical data of model compound 1. The major difference
between the spectra generated upon the reduction of the
model compound 1 and those of the platinum(II) terpyridyl
acetylide complexes is that the new bands produced in the
region of 425-600 nm are more structured in the acetylide
complexes. In the acetylide complexes, more complete
regeneration of the original UV-vis spectrum was observed
in each case following electrolysis.
t
transitions of the Bu3tpy radical anion (tBu3tpy• -), similar
to the absorptions observed in the electrochemically gener-
ated one-electron-reduced ruthenium(II) terpyridyl and plati-
num(II) bipyridyl complexes previously studied.40-44 UV-
vis spectroelectrochemical reduction of 1 mM 1 was
performed in both CH2Cl2 (Figure S2a) and DMF (Figure
S2b) to better correlate these data with ultrafast TA measure-
ments (see below). In DMF, as the potential is stepped just
past the first reduction, the absorption bands at 256, 329,
and 344 nm and the CT band between 360 and 400 nm
disappear while new absorptions appear at 362, 560, and 770
nm, indicative of the formation of tBu3tpy• -. There is also
a shoulder to the red side of the 362 nm band at 418 nm.
The second reduction of 2 has been studied spectroelec-
trochemically and compared to the first-reduction-product
absorption-difference spectrum. With the increase of voltage
beyond the second-reduction wave, a decrease and slight red
shift in the absorption of a newly formed band at 365 nm
during the first reduction is observed. Another band originally
resolved between 670 and 1000 nm during the first-electron
addition to the complex also starts decreasing during the
second-reduction process. On the other hand, the broad band
in the region of 450-600 nm loses its structure but grows
stronger with a shift to the red upon the second reduction.
Given the fact that the tBu3 groups likely suppress dimeriza-
tion36 in solution at room temperature, we believe that this
second-reduction process does indeed represent double reduc-
tion of the tBu3tpy ligand,45,46 which is not accessible optically.
(38) Blanton, C. B.; Murtaza, Z.; Shaver, R. J.; Rillema, D. P. Inorg. Chem.
1992, 31, 3230-3235.
(39) von Zelewsky, A.; Gremaud, G. HelV. Chim. Acta 1988, 71, 1108-
1115.
(40) Berger, R. M.; McMillin, D. R. Inorg. Chem. 1988, 27, 4245-4249.
(41) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176-1186.
(42) Collison, D.; Mabbs, F. E.; McImnes, E. J. L.; Taylor, K. J.; Welch,
A. J.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1996, 329-334.
(43) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. Chem. Soc., Chem.
Commun. 1981, 287-289.
(44) Weinstein, J. A.; Zheligovskaya, N. N.; Mel’nikov, M. Y.; Hartl, F.
J. Chem. Soc., Dalton. Trans. 1998, 2459-2466.
3044 Inorganic Chemistry, Vol. 46, No. 8, 2007

Platinum(II) Terpyridyl Acetylides
Figure 5. TA-difference spectra of (a) 2, (b) 3, (c) 4, and (d) 5 measured in CH2Cl2 following a 3 mJ, 355 nm laser pulse. The delay times are specified
on each graph. The strong bleaching signals between 500 and 600 nm in b and c are photoluminescence artifacts.
Nanosecond UV-vis TA Spectroscopy. Nanosecond TA
measurements were performed on 2-5 at room temperature
in CH2Cl2 purged with an argon stream. The TA-difference
spectra determined at different delay times after a 355 nm
laser pulse are shown in Figure 5. No transient absorptions
from [Pt(tBu3tpy)Cl]ClO4 were observed in the nanosecond
experiments due to its short excited-state lifetime (τ e 1 ns,
vide infra). The TA spectra for all compounds look similar,
with two distinct positive absorption peaks near 320 nm and
between 500 and 550 nm, a negative signal centered around
the emission maxima of the compounds (592 (2), 548 (3),
528 (4), and 608 nm (5)) corresponding to the emission from
the triplet CT state, and a very broad positive absorption
band which extends into the NIR region (690-800 nm),
continuing beyond our current detection range. In the valley
between the first two bands on the blue-green side of the
transient spectra, the signal minimizes, going negative in
compounds 2 and 5. This spectral region is associated with
the maxima of the respective low-energy ground-state
absorption. Accordingly, the shapes of the transient spectra
in this region are determined by a combination of positive
excited-state absorptions and negative ground-state absorp-
tion bleaching bands. Interpretation is complicated by the
fact that in all cases the data result from a combination of
the transient signals of opposite signs. For all compounds,
both positive and negative components of the transient signal
decay monoexponentially, producing very clear isosbestic
points around 420, 470, 580, and 610 nm (2), 525 and 655
nm (3), 500 and 650 nm (4), and 430, 475, 580, and 665 nm
(5). No difference in lifetimes was observed as a function
of the detection wavelength, regardless of whether they
correspond to TA or bleaching.
Upon laser excitation, the platinum(II) terpyridyl acetylide
complexes undergo electron density redistribution toward the
terpyridyl moiety. It has therefore been common to assign
the positive absorptions throughout the visible and NIR
regions to terpyridyl radical anion formation resulting from
MLCT.11,47 However, the excited-state absorption spectra
comprise all elements contributing to the CT state, the
terpyridyl ligand with increased electron density, and also a
“hole” with decreased electron density delocalized over the
Pt center and acetylide framework. To isolate the contribution
of the terpyridyl radical anion to the TA signals, we
performed a reductive quenching experiment with the idea
to restore the electron density on the platinum-acetylide
moiety lost in the act of photoexcitation. A transient spectrum
taken after reductive quenching would therefore reflect a
more distilled contribution from the terpyridyl radical anion.
It is well-known that aliphatic amines serve as good
(45) Mamo, A.; Stefio, I.; Parisi, M. F.; Credi, A.; Venturi, M.; Di, Pietro,
C.; Campagna, S. Inorg. Chem. 1997, 36, 5947-5950.
(46) Bassani, D. M.; Lehn, J. M.; Serroni, S.; Puntoriero, F.; Campagna,
S. Chem.sEur. J. 2003, 9, 5936-5946.
(47) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim, C.; Schmehl,
R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284-6293.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3045

Shikhova et al.
Scheme 1. Charge-Separation and Charge-Recombination Electron-Transfer Reactions in the Pt/DABCO Systema
a Represented with the hole localized on the metal center to simplify the presentation.
reveal characteristic absorption features of the tBu3tpy radical
anion while suppressing all other contributions from the CT
excited state. Time-resolved TA-difference spectra for com-
plexes 2-5 were measured in CH2Cl2 in the presence of
DABCO (3 mM solution). Immediately after the laser pulse,
the transient spectra in the presence of DABCO resemble
those in the absence of the quencher (Figure 5). In a few
tens of nanoseconds, the spectra substantially change as a
result of reductive quenching in the presence of large quench-
er concentration. The triplet-state lifetimes become much
shorter, dropping below the instrument response time in the
cases of 4 and 5 (Figure S7). At delay times 3-fold longer
than the quenched lifetime, one can ignore the contributions
to the transient spectra from the triplet CT excited state. The
spectra taken after this time delay (Figure 6) demonstrate
that only one distinct peak remains at high energy between
350 and 360 nm. Since the only common structural feature
Table 3. Rate Constants for the Reductive Quenching of the
Platinum(II) Terpyridyl Acetylide Complexes with DABCO, Measured
in CH2Cl2
Pt(II) complex
kq (109 M-1 s-1
)
2
3
4
5
9.6 ( 0.6
8.9 ( 0.2
7.3 ( 0.5
6.2 ( 0.2
intermolecular reductive quenchers of transition-metal
complexes,48-51 so we focused our attention on this class of
quenchers. In our experiments we selected 1,4-diazabicyclo-
[2.2.2]octane (DABCO) as the quencher, since the one-
electron-oxidation product does not have appreciable ab-
sorption in the visible and NIR regions.50 Photoluminescence-
lifetime quenching experiments with DABCO showed that
the amine successfully suppresses emission of the investi-
gated complexes through a dynamic quenching mechanism.
Degassed solutions of 2-5 containing varying concentrations
of DABCO were excited at wavelengths corresponding to
the CT band. The triplet decay following excitation in the
CT region was then monitored at wavelengths where the com-
plexes produced maximum emissions. The luminescence life-
times of the platinum(II) terpyridyl acetylide complexes were
well modeled using single exponentials over the entire range
of DABCO concentrations (0.03-0.5 M), demonstrating that
the bimolecular quenching cleanly obeys pseudo-first-order
kinetics. The values of the rate constants for the lifetime
quenching of the triplet state of the platinum(II) terpyridyl
acetylide complexes were calculated using the Stern-Volmer
equation and are presented in Table 3. These constants ap-
proach the value expected for a diffusion-controlled reaction
(1.6 × 1010 M-1 s-1 in CH2Cl2),52 and no significant devia-
tions from linear Stern-Volmer behavior were observed over
the entire quencher concentration range (Supporting Informa-
tion, Figure S6). The steps of the photoinduced electron-
transfer reaction from DABCO as an electron donor to the
triplet state of the Pt(II) complexes are shown in Scheme 1.
Upon 355 nm laser excitation, all compounds undergo a rapid
CT leading to the quenching and reduction of the Pt(II)
complex to form DABCO• + and [Pt(tBu3tpy• -)(CtCR)]+.
DABCO• + can be considered a spectroscopically silent rea-
gent since its absorption in the visible region (at 450 nm)
has a low extinction coefficient (<1500 M-1 cm-1).53 Conse-
quently, the resulting excited-state-difference spectra should
t
in the studied compounds is the Bu3tpy ligand, we assign
t
the band at 350-360 nm to the Bu3tpy radical anion. The
long-lived component of the kinetic traces in the presence
of the quencher accounts for the charge-recombination
reaction between [PtII(tBu3tpy• -)(CtCR)]+ and DABCO• +
,
which presumably follows diffusion-controlled second-order
equal-concentration kinetics (∼109-1010 M-1 s-1). We did
not investigate the charge-recombination kinetics since this
aspect is far beyond the scope of this work. The most
important facet of the reductive quenching data is that in all
cases the excited-state-difference spectra of the platinum-
(II) terpyridyl acetylide complexes is not solely attributable
t
to the Bu3tpy• - species. Therefore, the predominately
absorbing components in the excited state result from
transitions within the photogenerated hole or those associated
with the intramolecular charge separated species, i.e., MLCT
or LLCT transients.
Ultrafast TA Spectroscopy. A spectral signature of
untainted tBu3tpy• - is also expected to appear in TA spectra
of model compounds whose structure exclusively produces
only low-energy MLCT transitions. A good example of this
motif is the Ru(II) complex 617,18 or, more appropriately,
the Pt(II) complex 1. Since no transients can be observed at
room temperature on the nanosecond time scale for these
compounds, ultrafast TA spectra were measured in both
instances. Figure 7 displays the TA spectral evolution
recorded at different delay times for 6 in CH2Cl2 after a 100
fs, 490 nm excitation by laser pulse. The transient spectrum
is characterized by an intense positive absorption near 360
nm, ground-state absorption bleaching centered around 485
nm, and a weak broad TA band between 550 and 750 nm.
The bleaching corresponds to the lowest-energy ground-state
absorption band assigned to the MLCT transition, and the
sharp feature at 490 nm is simply due to the scattering from
the optical pump pulse. The transient spectrum decays with
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3046 Inorganic Chemistry, Vol. 46, No. 8, 2007

Platinum(II) Terpyridyl Acetylides
Figure 6. TA spectra and time-resolved-absorption kinetics (monitored
at 370 nm) of 2 in the presence of DABCO in CH2Cl2; λexc ) 355 nm, 3
mJ/pulse.
Figure 8. Excited-state absorption-difference spectra of 1 in CH2Cl2,
excited at 340 nm (∼100 fs fwhm), with delay times specified on the graph.
TA corresponding to tBu3tpy• - formed upon photoexcitation
find their match in reductive spectroelectrochemistry. As we
have seen above, the electrochemical-difference spectra
correlate remarkably well with the ultrafast TA spectra for
6. This, however, is not the case for Pt(II) compounds.
Whereas the ultrafast TA spectrum of 1 presented in Figure
8 has a shape similar to that of the reductive spectroelec-
trochemical data for the complex (Supporting Information,
Figure S2), the two major band positions in the visible region
are significantly shifted from the corresponding features
observed in the ground-state one-electron-reduced species.
At the moment, we do not know the origin of this discrep-
ancy but speculate that some of the difference may simply
result from attempting to correlate the excited-state-difference
spectrum to that of an ill-defined ground-state reduced
species. In addition, this experiment does not account for
any transients associated with excited-state transitions be-
tween species generated after the laser pulse. These issues,
combined with the fact that we cannot perform the comple-
mentary oxidative electrolysis experiment, only permit us
to conclude that the excited-state spectrum of 1 cannot be
Figure 7. Excited-state absorption-difference spectra of 6 in CH2Cl2,
excited at 490 nm (∼100 fs fwhm), with delay times specified on the graph.
The sharp feature at 490 nm superimposed on the low-energy side of the
ground-state bleaching present at all delay times is an artifact from pump
beam scattering.
first-order kinetics, with a lifetime of 115 ps at all examined
probe wavelengths either corresponding to positive TA or
bleaching features. The ultrafast TA-difference spectrum
resembles the absorption-difference spectrum of the elec-
trochemically obtained terpyridyl radical anion (Figure 4b).
Even the asymmetric, shark-fin shape of the ground-state
absorption bleaching is retained. On the basis of the
spectroelectrochemical data and previous literature reports,
we assign the positive TA to the electronic transitions of
t
definitively traced to solely Bu3tpy radical anion compo-
nents. Although the complete origins of the transient-
difference spectrum in 1 remain under investigation at this
point, those measured in 2-5 clearly indicate that the large
excited-state absorptions associated with these structures are
not simply composed of reduced-ligand- and oxidized-metal-
center constituents.
Conclusions
t
the one-electron-reduced Bu3tpy ligand.36,39,40
The TA spectrum for 1 in CH2Cl2 following a 100 fs
excitation laser pulse at 340 nm is shown in Figure 8. The
transient spectrum shows positive absorptions around 360,
470, and 600 nm. The transient spectrum of 1 decays
monoexponentially with τ ) 650 ( 50 ps at all examined
wavelengths, and the features likely correspond to the triplet
MLCT excited state. One interesting observation is that the
dip in the spectrum centered at 520 nm may be due to
stimulated emission from 1. We are currently investigating
how the Pt(II) metal center may facilitate such a process by
permitting strong mixing of the singlet and triplet CT
manifolds. As in the case of 6, one would anticipate that the
Several [Pt(tBu3tpy)(CtCR)]+ complexes (2-5) with
substituted acetylide ligands have been synthesized along
with the relevant model compounds 1 and 6. Bright photo-
luminescence and long-lived excited states (up to 3 µs) of
the whole series of compounds make them attractive for
numerous applications. The electronic nature of the acetylide
ligands is largely responsible for the absorption/emission
characteristics of these complexes and can be used to readily
modulate photophysical properties. Not surprisingly, the
reductive spectroelectrochemical data for all the Pt(II)
t
complexes are almost identical, since the Bu3tpy radical
anion is the primary absorbing species and does not seem to
Inorganic Chemistry, Vol. 46, No. 8, 2007 3047

Shikhova et al.
be influenced by the ancillary acetylide ligand. The TA
experiments in the presence of excess DABCO gave similar
results, since the positive charge localized on the Pt-
acetylide framework is reductively quenched by the amine,
and the origin of the resulting transient is essentially the
tBu3tpy• - moiety. Thorough analysis of data collected from
TA studies on the microsecond scale, reductive quenching
of the excited state, and spectroelectrochemical experiments
allow us to assert that the tBu3tpy radical anion formation in
the CT excited state does not account for the intense TA
features throughout the visible and NIR regions. The excited-
state absorptions seem to be strongly tied to the photoge-
nerated hole that is delocalized across the Pt center and the
ancillary acetylide ligand. However, the instability of these
complexes toward one-electron oxidation prohibits an exact
assignment of whether the excited-state absorptions result
from transitions localized in the photogenerated hole or
whether they result from transitions between the intramo-
lecular charge-separated species. Experiments that probe the
optical transitions in the photogenerated hole will be neces-
sary to unravel this dilemma. Work on related Pt(II)
chromophores possessing stable one-electron oxidations is
currently underway to resolve this issue.
Acknowledgment. The BGSU portion of the work was
supported by the AFOSR (FA9550-05-1-0276), the NSF
(CAREER Award CHE-0134782), the ACS-PRF (44138-
AC3), and the BGSU Technology Innovation Enhancement
Program. All TA measurements were performed in the Ohio
Laboratory for Kinetic Spectrometry on the BGSU campus.
The CNRS/ULP/ECPM and the ANR (FCP-OLEDs ANR-
05-BLAN-0004-01) are acknowledged for partial financial
support.
Supporting Information Available: Spectroelectrochemical
measurements, additional transient-absorption spectra, electron-
transfer quenching studies, and single-crystal X-ray analysis of 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0618652
3048 Inorganic Chemistry, Vol. 46, No. 8, 2007