Platinum(II) terpyridyl-acetylide dyads and triads with nitrophenyl acceptors via a convenient synthesis of a boronated phenylterpyridine

Article abstract of DOI:10.1021/ic801769v

Four new Pt(II) terpyridyl acetylide complexes which possess a covalently linked nitrophenyl moiety were prepared and studied. Specifically, the chromophore-acceptor (C-A) dyads reported here include [Pt(ptpy-ph-p-NO ₂)(C≡C-C₆H₅

Full text of DOI:10.1021/ic801769v

Inorg. Chem. 2009, 48, 2420-2428
Platinum(II) Terpyridyl-Acetylide Dyads and Triads with Nitrophenyl
Acceptors via a Convenient Synthesis of a Boronated Phenylterpyridine
Paul Jarosz,† Kenneth Lotito,† Jacob Schneider,† Duraisamy Kumaresan,‡ Russell Schmehl,*,‡
and Richard Eisenberg*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Department
of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118
Received September 15, 2008
Four new Pt(II) terpyridyl acetylide complexes which possess a covalently linked nitrophenyl moiety were prepared
and studied. Specifically, the chromophore-acceptor (C-A) dyads reported here include [Pt(ptpy-ph-p-
NO2)(Ct C-C6H5)](PF6)3 (1), where ptpy-ph-p-NO2 ) 4′-{4-(4-nitrophenyl)-phenyl}-[2,2′;6′,2′′]terpyridine, and
Ct C-C6H5 ) phenylacetylide and [Pt(ptpy-ph-m-NO2)(Ct C-C6H5)](PF6)2 (2), where ptpy-ph-m-NO2 ) 4′-(4-
m-nitrophenyl-phenyl)-2,2′;6′,2′′-terpyridine, as well as the related donor-chromophore-acceptor (D-C-A) triads
[Pt(ptpy-ph-p-NO2)(Ct C-C6H4CH2-PTZ)]PF6 (3), where Ct C-C6H4CH2-PTZ ) 4-ethynylbenzyl-N-phenothiazine,
and [Pt(ptpy-ph-m-NO2)(Ct C-C6H4CH2-PTZ)]PF6 (4). Transient absorption spectroscopy and electrochemical
analyses were used to characterize these compounds. In contrast to previous observations for closely related
multicomponent systems, it appears that, in the current systems, the nitrophenyl group is not an effective quencher
of the excited state. The luminescence and transient absorption properties of the C-A dyads are virtually identical
to those of the parent chromophore, [Pt(ttpy)(Ct C-C6H5)]PF6 (5), where ttpy ) 4′-p-tolyl-[2,2′;6′,2′′]terpyridine.
Introduction
This chromophore has the benefit of directionality in the
3MLCT excitation, with the energetic electron residing
primarily on the terpyridyl unit. This directionality can be
exploited in the design of systems for photoinduced charge
separation (PICS) and placement of electron-donating and
-accepting components of such systems.42-46 Once a viable
In the systems that achieve light-to-chemical energy
conversion, photoinduced charge separation is an essential
step. In efforts to better understand this process, much work
has been done to construct two- and three-component systems
comprised of a chromophore, an electron donor, and an
electron acceptor.1-41 Our recent work in this field focuses
on square planar terpyridyl acetylide complexes of Pt(II).
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* To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu (R.E.).
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†
University of Rochester.
Tulane University.
‡
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2420 Inorganic Chemistry, Vol. 48, No. 6, 2009
10.1021/ic801769v CCC: $40.75  2009 American Chemical Society
Published on Web 02/11/2009

Platinum(II) Terpyridyl-Acetylide Dyads and Triads
PICS system has been designed, it can then be coupled to
moieties for electron or hole collection and catalysts to drive
energy-storing processes.1,3,8,24,30,32,33,35-39,47-49
The first triad based on a platinum chromophore, T-1, was
reported by McGarrah et al.42,50 This complex was based
on a diimine bis(acetylide) platinum coordination sphere and
employed a nitrophenyl acceptor and a phenothiazine donor.
This triad was observed to form a fully charge-separated state
upon excitation with a lifetime of 70 ns. Inspired by this
result, the syntheses and characterization of several dyads
and triads based on the platinum terpyridyl acetylide chro-
mophore were undertaken and have been described previ-
ously.45,46 One such triad was [Pt(NO2stil-tpy)(Ct C-C6-
H4CH2-PTZ)](PF6) (T-2) where NO2stil-tpy ) 4′-{4-
[2-(4-nitrophenyl)vinyl]-phenyl}-[2,2′;6′,2′′]terpyridine.
Triad T-2 was found to exhibit complete luminescence
quenching, thought to occur by a charge-transfer mechanism,
similar to that observed in the study of triad T-1. The charge-
separated state formed upon excitation of T-2 was found to
have a lifetime of 230 ns. However, the efficiency of
formation of the charge-separated state was modest with only
25% of the excited molecules leading to the charge-separated
state on the basis of a biexponential analysis of transient
decay.46 The related C-A dyad, [Pt(NO2stil-tpy)(Ct C-
C6H5)](PF6) (D-2), also exhibited significant quenching of
luminescence, which was attributed to a charge-transfer
process in which the nitrostilbene moiety was reduced by
electron transfer from the excited state of the chromophore.46
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Though this result was very encouraging, the stability
of the vinylene bridge remained a concern with the
synthetic methodology. Steps were therefore taken to
append as an electron acceptor the nitrophenyl unit through
a more stable and robust aryl-aryl linkage. In this paper,
we describe the syntheses of these new dyads (1 and 2)
and triads (3 and 4), obtained as their PF6- salts and based
on chromophore 5, as well as their spectroscopic and
electrochemical properties.
These efforts have also led to the development of a facile
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Inorganic Chemistry, Vol. 48, No. 6, 2009 2421

Jarosz et al.
equipped with a quadrupole mass filter. Cyclic voltammetry
experiments were conducted on an EG&G PAR 263A poten-
tiostat/galvanostat using a three-electrode single compartment
cell. A glassy carbon working electrode, Pt wire auxiliary
electrode, and Ag wire reference electrode were used. For all
measurements, samples were degassed with nitrogen. Also, 0.10
M tetrabutylammonium hexafluorophosphate was used as the
supporting electrolyte, while ferrocene was employed as an
internal redox reference. All redox potentials are reported relative
to the ferrocenium/ferrocene (Fc+/Fc) couple (0.4 V vs SCE for
MeCN, 0.45 V vs SCE for DMF),55 and all scans were done at
100 mV/s. Absorption spectra were recorded using a Hitachi
U2000 scanning spectrophotometer (200-1100 nm).
Emission and excitation spectra were obtained using a Spex
Fluoromax-P fluorimeter corrected for instrument response. Mono-
chromators were positioned with a 2 nm band-pass, and solution
samples were degassed by at least four freeze-pump-thaw cycles.
Frozen glass samples were prepared in butyronitrile using NMR
tubes in a circular quartz-tipped immersion Dewar filled with liquid
nitrogen.
Transient absorption spectra were carried out using an Applied
Photophysics LKS60 flash photolysis system with a Quantel
Brilliant B Nd:YAG laser pumped OPOTEK OPO at various delays
following excitation at various wavelengths between 420 and 500
nm. The spectra were assembled from decays measured at every
10 nm. Individual decays were analyzed as single or double
exponentials using a Marquardt-Levenberg algorithm included with
the LKS60 software. Kinetic analysis, where appropriate, was made
by global analysis using the PRO-K software package (Applied
Photophysics).
Experimental Section
Materials. The chemicals N,N-dimethylformamide, acetoni-
trile, tetrahydrofuran, dimethylsulfoxide, copper(I) iodide, am-
monium acetate, ammonium hexafluorophosphate (NH4PF6),
2-acetylpyridine, pinnacol, triethylamine, triphenylphosphine,
cesium carbonate, 4-bromonitrobenzene, and 3-iodonitrobenzene
(Aldrich); electrochemical-grade tetrabutylammonium hexafluo-
rophosphate and potassium tert-butoxide (Fluka); 4-formyl-
phenylboronic acid (Boron Molecular), potassium tetrachloro-
platinate; and palladium(II) acetate (Strem Chemical) were used
without further purification.
The syntheses of N-(4-ethynylbenzyl)-phenothiazine and
Pt(DMSO)2Cl2 were carried out according to literature proce-
dures.42,46 The chromophore [Pt(ttpy)(Ct C-C6H5)]PF6 (5) and
the donor-chromophore dyad [Pt(ttpy)(Ct C-C6H4CH2-PTZ)]-
PF6 (D-2′) were used as previously prepared.46 Syntheses were
performed under nitrogen with solvents purified by passing the
degassed solvent through columns containing activated molecular
sieves and activated alumina.54 All other reagents were of
spectroscopic grade and used without further purification.
Characterization. 1H NMR spectra were recorded on a Bruker
Avance-500 spectrometer (500 MHz). Mass determinations were
accomplished by electrospray ionization mass spectrometry and
atmospheric pressure chemical ionization (APCI) using a
Hewlett-Packard Series 1100 mass spectrometer (model A)
4′-(4-Boronato-phenyl)-2,2′;6′,2′′-terpyridine, ptpy-B(OH)2
(L1). A solution of 4-formylphenylboronic acid (4.00 g, 26.7
mmol) and pinnacol (4.70 g, 39.8 mmol) was prepared in THF
(60 mL) and stirred for 1 h. A 500 mL, two-necked flask was
charged with potassium tert-butoxide (8.00 g, 71.4 mmol) and
THF (60 mL). The flask was then equipped with a reflux
condenser and a 100 mL pressure-equalizing addition funnel.
This addition funnel was charged with THF (60 mL) and
2-acetylpyridine (6.2 mL, 55.3 mmol). The reaction flask was
then placed under a N2 atmosphere, and the THF solution of
2-acetylpyridine was added dropwise to the solution of potassium
tert-butoxide over 1 h, with vigorous stirring. This produced a
pale pink suspension. The addition funnel was then charged with
the solution of 4-formylphenylboronic acid and pinnacol. This
solution was added dropwise to the white suspension over the
course of 1 h, with vigorous stirring, producing a yellow-orange
suspension, which gradually became an opaque, cherry red
solution with an additional 1.5 h of stirring. A solution of
ammonium acetate (10 g, excess) was prepared in ethanol (60
mL) and glacial acetic acid (50 mL). This solution was added
directly to the red solution in the reaction flask to give a
translucent, yellow solution. The addition funnel was removed
and replaced with a glass stopper. This solution was then heated
at reflux for 16 h. After cooling, the yellow solution was decanted
into a flask, and the persisting crystals of ammonium acetate
were washed with THF, which was likewise decanted into the
same flask. The volatiles were removed by rotary evaporation
to give a brown sludge. This was treated with water and a 10%
HCl solution and heated until the sludge had completely
dissolved. After cooling, the solution was neutralized with
aqueous ammonium hydroxide. To drive hydrolysis of the
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2422 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum(II) Terpyridyl-Acetylide Dyads and Triads
pinnacol ester to give the free boronic acid, the suspension was
twice more acidified with HCl and heated, then neutralized with
ammonium hydroxide. After the final neutralization, the suspen-
sion was filtered to give a gummy solid. This material was
sonicated in a mixture of THF/Et2O (50:50 v/v) to give an off-
white, powdery solid which was collected by filtration and
washed with an addition of Et2O, then dried in vacuo. Yield:
g, 0.10 mmol), CuI (0.006 g, 0.03 mmol), and DMF (4 mL). The
flask was sealed with a septum and placed under N2. Phenylacety-
lene (0.02 mL, 0.2 mmol) was added via syringe, followed by
triethylamine (0.03 mL, 0.3 mmol). The reaction mixture was stirred
in the dark for 48 h. The brown solution was treated with Et2O (50
mL) to effect precipitation of an ochre solid, which was collected
by filtration, washed with Et2O, and dried in vacuo. Yield: 0.052 g
(60%). 1H NMR (DMSO-d6): δ 9.25 (2H, d, J ) 6 Hz), 9.18 (2H,
s), 8.94 (2H, d, J ) 8 Hz), 8.60 (2H, t, J ) 8 Hz), 8.40 (4H, m),
8.17 (4H, t, J ) 9 Hz), 8.00 (2H, t, J ) 7 Hz), 7.54 (2H, d, J ) 8
Hz), 7.38 (2H, t, J ) 7 Hz), 7.30 (1H, t, J ) 7 Hz). HRMS calcd
for PtC35H23N4O2: 726.1463. Found: 726.14911. Anal. calcd for
PtC35H23N4O2PF6: C, 48.16; H, 2.64; N, 6.42. Found: C, 45.50; H,
2.52; N, 6.11.
1
3.000 g (32%). H NMR (DMSO-d6): δ 8.79 (2H, d, J ) 5 Hz),
8.76 (2H, s), 8.71 (2H, d, J ) 8 Hz), 8.24 (2H, s), 8.07 (2H, td,
J ) 8, 2 Hz), 8.03 (2H, d, J ) 8 Hz), 7.93 (2H, d, J ) 8 Hz),
7.56 (2H, dd, J ) 5, 1 Hz). MS (positive APCI, methanol
solvent): m/z 382 [ptpy-B(OMe)2]+
4′-(4-p-Nitrophenyl-phenyl)-2,2′;6′,2′′-terpyridine, ptpy-ph-p-
NO2 (L2). A 100 mL, two-necked flask was charged with Cs2CO3
(0.550 g, 1.70 mmol), Pd(OAc)2 (0.017 g, 0.076 mmol), and PPh3
(0.080 g, 0.30 mmol). This was fitted with a reflux condenser and
a septum, then placed under N2. THF (50 mL) was added via
syringe, and the resulting suspension was stirred for 30 min. To
this was added a solution of L1 (0.600 g, 1.70 mmol) in THF (10
mL) via syringe. The reaction mixture rapidly became emerald green
and was allowed to stir for 10 min before the addition of a solution
of 4-bromonitrobenzene (0.310 g, 1.54 mmol) in THF (10 mL) via
syringe. The resulting mixture was stirred at 80 °C for 48 h. A
total of 150 mL of water was added to the resulting tan suspension,
and the solids were separated by filtration. The solid was washed
by vigorous sonication in MeOH/H20 (10:1, v:v) and filtered again,
whereupon it was washed with cold methanol followed by diethyl
ether/hexane (1:1, v:v) and dried in vacuo. Yield: 0.391 g (59%).
1H NMR (CDCl3): δ 8.84 (2H, s), 8.79 (2H, d, 5 J ) Hz), 8.74
(2H, dt, J ) 8, 1 Hz), 8.38 (2H, dt, J ) 9, 2 Hz), 8.09 (2H, dt, J
) 8, 2 Hz), 7.94 (2H, td, J ) 8, 2 Hz), 7.86 (2H, dt, J ) 9, 2 Hz),
7.82 (2H, dt, J ) 7, 2 Hz), 7.42 (2H, ddd, J ) 8, 5, 1 Hz). MS
(positive APCI): m/z 431 [M]+.
[Pt(ptpy-ph-m-NO2)(Ct C-C6H5)]PF6 (2). This compound was
prepared in a way directly analogous to that of 1, excepting the
use of [Pt(ptpy-ph-m-NO2)Cl]PF6 in place of [Pt(ptpy-ph-p-
1
NO2)Cl]PF6. Yield: 0.054 g (62%). H NMR (DMSO-d6): δ 9.23
(2H, d, J ) 6 Hz), 9.17 (2H, s), 8.94 (2H, d, J ) 8 Hz), 8.64 (1H,
s), 8.59 (2H, t, J ) 8 Hz), 8.40 (2H, d, J ) 8 Hz), 8.34 (2H, m),
8.16 (2H, d, J ) 8 Hz), 7.99 (2H, t, J ) 7 Hz), 7.87 (1H, t, J ) 8
Hz), 7.54 (2H, d, J ) 8 Hz), 7.38 (2H, t, J ) 7 Hz), 7.30 (1H, t,
J ) 7 Hz). HRMS calcd for PtC35H23N4O2: 726.1463. Found:
726.14563. Anal. calcd for PtC35H23N4O2PF6: C, 48.16; H, 2.64;
N, 6.42. Found: C, 49.86; H, 2.68; N, 6.33.
[Pt(ptpy-ph-p-NO2)(Ct C-C6H4CH2-PTZ)]PF6 (3). A 10 mL
round-bottom flask was charged with [Pt(ptpy-ph-p-NO2)Cl]PF6
(0.080 g, 0.10 mmol), CuI (0.006 g, 0.03 mmol), N-(4-ethynyl-
benzyl)-phenothiazine (0.047 g, 0.15 mmol), and DMF (4 mL).
The flask was sealed with a septum and placed under N2.
Triethylamine (0.04 mL, 0.4 mmol) was added via syringe, and
the reaction mixture was stirred in the dark for 48 h. The resultant
red solution was treated with Et2O (100 mL) to effect precipitation
of a red solid, which was collected by filtration, washed with Et2O,
and dried in vacuo. Yield: 0.082 g (76%). 1H NMR (DMSO-d6): δ
9.00 (2H, s), 8.95 (2H, s), 8.77 (2H, d, J ) 7 Hz), 8.42 (2H, t, J
) 8 Hz), 8.35 (2H, d, J ) 9 Hz), 8.28 (2H, d, J ) 8 Hz), 8.09 (2H,
d, J ) 8 Hz), 8.02 (2H, d, J ) 8 Hz), 7.80 (2H, t, J ) 6 Hz), 7.34
(2H, d, J ) 8 Hz), 7.26 (2H, d, J ) 8 Hz), 7.18 (2H, d, J ) 8 Hz),
7.10 (2H, t, J ) 7 Hz), 6.95 (2H, t, J ) 8 Hz), 6.81 (2H, d, J ) 8
Hz), 5.11 (2H, s). HRMS calcd for PtC48H32N5O2S: 937.1919.
Found: 937.18848. Anal. calcd for PtC48H32N5O2SPF6: C, 53.23;
H, 2.96; N, 6.47. Found: C, 52.46; H, 2.71; N, 6.40.
4′-(4-m-Nitrophenyl-phenyl)-2,2′;6′,2′′-terpyridine, ptpy-ph-m-
NO2 (L3). This was prepared in the same way as L2, substituting
3-bromonitrobenzene for 4-bromonitrobenzene (0.384 g, 1.54
1
mmol). Yield: 0.410 g (62%). H NMR (CDCl3): δ 8.84 (2H, s),
8.79 (2H, d, J ) 5 Hz), 8.74 (2H, d, J ) 8 Hz), 8.58 (1H, t, J )
2 Hz), 8.28 (1H, dd, J ) 8, 2 Hz), 8.10 (1H, dt, J ) 8, 2 Hz), 8.04
(1H, d, J ) 8 Hz), 7.94 (2H, td, J ) 8, 2 Hz), 7.83 (2H, dt, J ) 9,
1 Hz), 7.70 (1H, t, J ) 8 Hz), 7.58 (1H, m), 7.42 (2H, ddd, J ) 8,
5, 1 Hz). MS (APCI): m/z 431 [M]+.
[Pt(ptpy-ph-p-NO2)Cl]PF6. A 25 mL round-bottom flask was
charged L2 (0.281 g, 0.652 mmol), Pt(DMSO)2Cl2 (0.250 g, 0.607
mmol), and MeCN (10 mL). The flask was equipped with a reflux
condenser, and the suspension was stirred at 80 °C for 48 h. At
this time, excess NH4PF6 was added, followed by water (20 mL),
and the mixture was stirred for 15 min. The ochre solid was
collected by filtration and washed with chloroform and ether to
[Pt(ptpy-ph-m-NO2)(Ct C-C6H4CH2-PTZ)]PF6 (4). This com-
pound was prepared in a way directly analogous to that of 3, starting
with [Pt(ptpy-ph-m-NO2)Cl]PF6 in place of [Pt(ptpy-ph-p-
1
NO2)Cl]PF6. Yield: 0.087 g (80%). H NMR (DMSO-d6): δ 8.98
(2H, s), 8.94 (2H, s), 8.76 (2H, d, J ) 8 Hz), 8.55 (1H, s), 8.41
(2H, t, J ) 7 Hz), 8.28 (4H, m), 8.07 (2H, d, J ) 8 Hz), 7.80 (3H,
m), 7.34 (2H, d, J ) 8 Hz), 7.26 (2H, d, J ) 8 Hz), 7.18 (2H, d,
J ) 8 Hz), 7.10 (2H, t, J ) 7 Hz), 6.95 (2H, t, J ) 8 Hz), 6.81
(2H, d, J ) 8 Hz), 5.11 (2H, s). HRMS calcd for PtC48H32N5O2S:
937.1919. Found: 937.19073. Anal. calcd for PtC48H32N5O2SPF6:
C, 53.23; H, 2.96; N, 6.47. Found: C, 53.44; H, 2.73; N, 6.20.
1
remove any free ligand. Yield: 0.355 g (72%). H NMR (DMSO-
d6): δ 9.10 (2H, s), 8.97 (2H, d, J ) 6 Hz), 8.91 (2H, d, J ) 8 Hz),
8.60 (1H, t, J ) 2 Hz), 8.58 (2H, t, J ) 8 Hz), 8.40 (2H, d, J ) 6
Hz), 8.38 (2H, d, J ) 7 Hz), 8.17 (2H, d, J ) 9 Hz), 8.14 (2H, d,
J ) 9 Hz), 8.00 (2H, t, J ) 6 Hz), 7.86 (1H, t, J ) 8 Hz).
[Pt(ptpy-ph-m-NO2)Cl]PF6. This was prepared in the same way
as [Pt(ptpy-ph-p-NO2)Cl]PF6, excepting the substitution of L2 with
Results and Discussion
1
L3. Yield: 0.427 g (87%). H NMR (DMSO-d6): δ 9.06 (2H, s),
Syntheses and Characterization. The phenyl terpyridine
bearing a boronic acid substituent, ptpy-B(OH)2, was pre-
pared via a synthetic methodology developed by Constable
et al. for the preparation of phenyl terpyridines.56,57 By
8.93 (2H, d, J ) 6 Hz), 8.89 (2H, d, J ) 7 Hz), 8.61 (1H, t, J )
2 Hz), 8.58 (2H, td, J ) 8, 2 Hz), 8.37 (2H, d, J ) 8 Hz), 8.33
(2H, m), 8.13 (2H, d, J ) 9 Hz), 7.97 (2H, t, J ) 7 Hz), 7.85 (1H,
t, J ) 8 Hz).
[Pt(ptpy-ph-p-NO2)(Ct C-C6H5)]PF6 (1). A 10 mL round-
bottom flask was charged with [Pt(ptpy-ph-p-NO2)Cl]PF6 (0.080
(56) Constable, E. C.; Kariuki, B.; Mahmood, A. Polyhedron 2003, 22,
687–698.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2423

Jarosz et al.
protecting the boronic acid moiety as a pinnacolate ester,
this method proved both facile and amenable to multigram
scale synthesis of the desired ligand. Deprotection in aqueous
acid furnished the free acid form in reasonable yield (eq 1).
Suzuki couplings between ptpy-B(OH)2 and various aryl
halides proved successful under standard conditions and gave
acceptable yields of pure products without the need for
chromatography (eq 2).52,53
The new complexes were characterized by 1H NMR
spectroscopy and HRMS, with each giving data consistent
with the assigned structure. All of the ligands and complexes
have distinct, well-resolved patterns in the their aromatic
proton resonances, which are attributable to the protons on
the pyridine and substituted phenyl rings. The pyridine
resonances (δ 7.8-9.4) are particularly informative, as they
shift substantially upon the coordination of Pt(II) and upon
the exchange of chloride for acetylide, particularly the
protons nearest the acetylene moiety. Mass spectroscopy is
also particularly convenient in analyzing these platinum-
containing compounds, as platinum gives a characteristic
isotopic pattern. Elemental analyses of platinum terpyridyl
acetylide complexes are frequently found to be low in carbon,
as is the case here, because of difficulties with incomplete
combustion; despite this problem, a high purity of the
compounds is indicated by other characterization methods,
particularly NMR spectroscopy.
Coordination of Pt(II) proceeded smoothly under standard
conditions to give the platinum(II) terpyridyl chlorides (eq
3).45,46,58-63 Subsequent CuI-catalyzed exchange of chloride
for acetylide ligands was somewhat slow compared to
previously studied compounds, requiring slightly higher CuI
loadings for timely reaction (eq 4).45,63-68
(57) Storrier, G. D.; Colbran, S. B.; Hibbert, D. B. Inorg. Chim. Acta 1995,
239, 1–4.
(58) Chan, C.-W.; Che, C.-M.; Cheng, M.-C.; Wang, Y. Inorg. Chem. 1992,
31, 4874–4878.
(59) Chan, C.-W.; Cheng, L.-K.; Che, C.-M. Coord. Chem. ReV. 1994, 132,
Absorption and Emission Spectra. The room-tempera-
ture absorption spectra of the complexes were measured in
MeCN and are shown in Figure 1. All of the complexes
exhibit features in the range of 300-800 nm which are
87–97.
(60) Howe-Grant, M.; Lippard, S. J. In Inorganic Syntheses; Busch, D. H.,
Ed.; John Wiley and Sons: New York, 1980; Vol. XX.
(61) Romeo, R.; Scolaro, L. M. Inorg. Syn. 1998, 32, 153–158.
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V.; Romeo, R. Inorg. Chem. 1998, 37, 2763–2769.
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Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841–16849.
(64) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653–5655.
(66) Sonogashira, K.; Yatake, T.; Tohda, Y.; Takahashi, S.; Hagihara, N.
Chem. Commun. 1977, 291.
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10, 879.
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S.; Hagihara, N. J. Organomet. Chem. 1978, 145, 101–108.
(68) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627–630.
2424 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum(II) Terpyridyl-Acetylide Dyads and Triads
Figure 1. Room-temperature absorption spectra of the complexes 1-4 in
1 × 10-5 M MeCN.
Figure 3. Emission spectra of complexes 3 and 4 in butyronitrile at 77 K.
Figure 4. Cyclic voltamogram of complex 1 in DMF with 0.1 M NBu4PF6
and ferrocene added as a reference, recorded at 100 mV/s. Values have
been adjusted to NHE, 1.8 - -1.6 V.
Figure 2. Room-temperature emission spectra of complexes 1-5 in 1 ×
10-5 M MeCN. Overlap exists in the spectra of 3 and 4.
emission features similar to that previously reported for the
related D-C dyad D-2′ (Figure 3).46 A faint vibronic
progression is also observable in the frozen glass emission
spectrum of 3, consistent with previous observations for
platinum terpyridyl acetylides.45,46,64,76 That the triads are
emissive in a frozen glass but are nonemissive in a room-
temperature solution is consistent with a charge-transfer
quenching process having a substantial solvent reorganiza-
tional barrier.
consistent with previously studied platinum terpyridyl acetyl-
ide complexes.45,46 The higher-energy absorptions (300-370
nm) are assigned as intraligand transitions, while the broad,
weaker absorptions (ε ∼ 9000-11000 M-1cm-1) at lower
energies (410-500) nm are assigned as 1MLCT, (dπ(Pt) f
π*(tpy)), absorptions.45,46,64,69-74
Room-temperature emission studies of the dyads and triads
were conducted in degassed MeCN (Figure 2). The C-A
dyads 1 and 2 were found to be emissive in solution, with
luminescence profiles virtually identical to that of the parent
chromophore 5, and at slightly lower intensities. The
quantum yields for emission for dyads 1 and 2 were found
2+
to be 0.011 and 0.012, respectively, using Ru(bpy)3 as a
standard with φrelem ) 0.062. These values can be compared
to the quantum yield of 0.025 measured for 5.75 The triads
3 and 4 were nonemissive at room temperature in an
acetonitrile solution. However, when studied in a frozen
butyronitrile glass at 77 K, both triads were found to have
Electrochemistry. Cyclic voltammetry studies were per-
formed on all of the complexes in DMF with 0.1 M
tetrabutylammonium hexafluorophosphate as the supporting
electrolyte (Figure 4 gives a cyclic voltamogram for 1). All
of the redox potentials are reported relatiVe to NHE with
the Fc/Fc+ couple used as an internal redox standard and a
Value of 0.4 V Versus SCE taken for the Fc/Fc+ couple in
MeCN and 0.45 V Versus SCE for the Fc/Fc+ couple in DMF.
The electrochemical behavior of the parent chromophore
5 and D-C dyad D-2′ have been reported previously.46 The
(69) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Lu, X.-X.; Cheung,
K.-K.; Zhu, N. Chem.sEur. J. 2002, 8, 4066–4076.
(70) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002,
124, 6506–6507.
(71) Yam, V. W.-W.; Lee, V. W.-M. J. Chem. Soc., Dalton Trans.: Inorg.
Chem. 1997, 3005–3010.
(72) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.; Cheung,
K.-K. Inorg. Chem. 2001, 40, 571–574.
(73) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Angew. Chem., Int. Ed.
2003, 42, 1400–1403.
(74) Hui, C.-K.; Chu, B. W.-K.; Zhu, N.; Yam, V. W.-W. Inorg. Chem.
2002, 41, 6178–6180.
(75) Du, P.; Schneider, J.; Jarosz, P.; Zhang, J.; Brennessel, W. W.;
Eisenberg, R. J. Phys. Chem. B 2007, 111, 6887–6894.
(76) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476–4482.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2425

Jarosz et al.
Table 1. Electrochemical Data (E1/2) for Pt Complexes and Free
Ligands
compound
oxidation E1/2/V
reduction E1/2/V
1
2
3
4
5
D-2′
1.56a
1.50a
-0.54, -0.82, -1.11
-0.54, -0.83, -1.08
-0.55, -0.82, -1.12
-0.53, -0.85, -1.10
-0.58, -1.10
1.5,a b1.02c
,
1.51,a1.03c
1.54a
1.49,a1.09c
-0.60, -1.10
a Irreversible under experimental conditions. b Weak couple, difficult to
determine potential. Potential in volts vs NHE (see the Experimental
Section). c Reversible when anodic scan limited to 1.4 V.
Figure 7. Transient absorption spectrum of D-C-A triad 3 (0.08 mM in
MeCN) at room temperature in MeCN (λexc ) 440 nm).
excitation at 470 nm (pulse < 10 ns). The spectra were
assembled from decays measured at every 10 nm.
The transient absorption spectra of the C-A dyads 1 and
2 (Figures 5 and 6) are virtually indistinguishable from that
previously observed for the parent chromophore 5.45,46 All
three complexes display positive absorptions at 360 nm and
broad unresolved absorptions between 520 and 800 nm with
maxima at ca. 740 nm and a bleach at 420-460 nm, which
is still observed when the excitation wavelength is varied.
All of these features decay with a rate constant of 1.6 ×
106 s-1.
Figure 5. Transient absorption spectrum of C-A dyad 1 (0.08 mM in
MeCN) at room temperature in MeCN (λexc ) 440 nm).
The transient absorption features of the D-C-A triads 3
and 4 are also virtually identical to those previously reported
for the related D-C dyad D-2′, which correspond to a
(PTZ+-Pt-tpy-) charge-separated state.46 Both 3 and 4
exhibit absorptions with maxima at 510 nm and broad
envelopes between 550 and 770 nm, which decay in less
than 30 ns (Figure 7). These features are consistent with the
phenothiazine radical cation. Most importantly, in none of
the transient absorption spectra of triads 3 and 4 is there
ever observed an absorption which can be attributed to the
nitrophenyl radical anion. If formed by charge transfer, this
radical would be expected to give absorptions with a λmax
near 350 and 460 nm.77
The absence of transient absorptions resulting from the
nitrophenyl radical anion for complexes 1-4 is somewhat
surprising, as the nitrophenyl moiety has previously
functioned as an electron acceptor in the closely related
triad systems T-1 and T-2.43,46 That the nitrophenyl unit
does not accept an electron from the excited state of the
chromophore is consistent with the absence of lumines-
cence quenching in dyads 1 and 2 and the similarity of
the transient absorption spectra of 1 and 2 with that of
the central chromophore 5.
From the transient absorption data of triads 3 and 4, it
appears that reductive quenching is rapid (<10 ns) and
produces a charge-separated species in which the phenothi-
azine moiety is oxidized and the chromophore reduced, as
was observed previously in the study of dyad D-2′. Subse-
quent charge transfer from the reduced chromophore to the
nitrophenyl unit would not be predicted to be a thermody-
namically favorable process on the basis of electrochemical
data (Table 1). The chromophore moiety undergoes reduction
at -0.55 V and would not be expected to transfer an electron
Figure 6. Transient absorption spectrum of C-A dyad 2 (0.08 mM in
MeCN) at room temperature in MeCN (λexc ) 440 nm).
dyads in the present study show the expected number of
oxidation and reduction waves, and the results are listed in
Table 1. The first oxidation for the phenothiazine moiety
proved to be reversible when the anodic limit of the scan
was kept lower than 1.4 V, but it exhibited a loss in the
current intensity of the cathodic wave when the scan was
extended to higher potentials. The Pt-based oxidations were
irreversible and had greater intensity when using a Au disk
working electrode.
Transient Absorption Studies. To probe the dynamics
of charge transfer in these systems, nanosecond transient
absorption studies were performed on 1-4 at 298 K in
MeCN. The results for 1-3 are shown in Figures 5-7; the
results for 4 are virtually identical with the results for 3 in
Figure 7. The spectra were collected immediately after
2426 Inorganic Chemistry, Vol. 48, No. 6, 2009

Platinum(II) Terpyridyl-Acetylide Dyads and Triads
to the nitrophenyl unit (Ered ) -0.82 V). Therefore, a
pathway in which the chromophore first undergoes reductive
quenching by the donor cannot lead to full charge separation.
Two explanations can be offered to explain the differ-
ence in behavior of the nitrophenyl moiety in triads 3 and
T-1. One relates to the difference in chromophore oxida-
tion potential between a neutral Pt(phenanthroline)(Ct C-
C6H4R)2 entity as in T-1 and a cationic Pt(terpyridyl)(Ct C-
C6H4R)+ complex as in 3. On the basis of the charge, the
relative donating capacity of the ligand set for each, and
the excited-state oxidation potentials for the chromophores,
T-1 will possess a greater tendency for oxidative quench-
ing and electron transfer to nitrobenzene upon excitation
than 3. A second factor to consider is the charge-transfer
pathway in the two systems. More specifically, there is
likely to be greater electronic communication between the
phenanthroline moiety and the nitrophenyl acceptor in triad
T-1 than between the terpyridine moiety and the nitro-
phenyl acceptor in triad 3. Aside from the increased
distance between these species in the latter, the p-
phenylene unit in 3 is expected to be orthogonal or twisted
relative to a coplanar orientation of the chromophore and
the nitrophenyl acceptor, which would retard electron
transfer. This orientation would not be expected in triad
T-1, where steric factors will not be significant, owing to
the intervening acetylene bridge.
Intermolecular Quenching Studies. The difference in
acceptor behavior between triads 3 and T-2 has a different
rationalization. In T-2, the acceptor moiety can be viewed
as nitrostilbene rather than nitrobenzene. The former will
undergo reduction more easily than the latter and will also
have better conjugation for connecting chromophore with
acceptor and transferring charge than is likely for the latter
in 3 (and 4). To further probe the energetics of charge
transfer, nitrostilbene and nitrobenzene were compared in
their ability to quench the luminescence of the parent
chromophore, 5, in fluid solution. Nitrostilbene was found
to quench with a rate approaching the diffusion limit, k )
4.6 × 109 s-1. Nitrobenzene, however, was not observed to
reduce the emission intensity of the chromophore at con-
centrations up to 1000 times that of the chromophore. This
result was unexpected, given the minor difference in reduc-
tion potentials of nitrobenzene and nitrostilbene, with the
latter undergoing reduction at less than 0.1 V more positive
than nitrobenzene. As possible oxidative quenchers of the
central chromophore, nitrobenzene and nitrostilbene may lie
near the threshold for thermodynamically favorable charge
transfer, as discussed further below.
previously studied dyad D-2, transient absorption data are
consistent with the triplet state of nitrostilbene.46,79 In contrast,
energy transfer to the nitrophenyl moiety would not be expected,
as the lowest triplet energy for nitrobenzene has been reported
at >2.6 eV.80
Additional insight is gained by a thermodynamic analysis
of the charge-transfer process using the Rehm-Weller
equation (eq 5). With this equation, it is found that oxidative
quenching of the excited state of the chromophore with the
nitrophenyl acceptor would be unfavorable. For this calcula-
tion, the oxidation potential of the chromophore, ED°, is
estimated as 1.5 V versus NHE from the onset of the anodic
wave. However, this value introduces a significant source
of uncertainty in the calculations to follow, as the process is
irreversible. The excited triplet energy, ∆E*, is 2.2 eV, as
described above, and the work term is set to 0 for an
intramolecular process.
∆G ) |ED° - EA°|-∆E* + w
(5)
From the electrochemical behavior of complexes 1 and 2,
as well as ligands L1 and L2, the nitrophenyl reduction can
be taken as -0.82 V versus NHE. Thus, oxidative quenching
of the central chromophore by this species would be predicted
to be unfavorable by ∼0.1 eV for the process. While the
result may be viewed as unexpected in light of earlier
observations, it is consistent with all of the current data for
complexes 1-4.46
Conclusions
The D-C-A triads [Pt(ptpy-ph-p-NO2)(Ct C-C6H4CH2-
PTZ)]PF6 (3) and [Pt(ptpy-ph-m-NO2)(Ct C-C6H4CH2-
PTZ)]PF6 (4) have been prepared along with the correspond-
ing C-A dyads 1 and 2. The characterization data of each
of these compounds support the assigned structures and
are similar to previously reported analogous multicom-
ponent systems. The syntheses of these complexes were
greatly facilitated by the improved preparation of the
previously reported terpyridyl derivative, 4′-(4-boronato-
phenyl)-2,2′;6′,2′′-terpyridine, ptpy-B(OH)2.51,52 These
compounds were prepared for comparison with earlier
studies of the closely related triad T-2 and dyad D-2
bearing a nitrostilbene acceptor.46 On the basis of the
previous findings, it was expected that the nitrophenyl
acceptor would function to oxidatively quench the excited
state of the central chromophore. However, the current
data indicate that, in contrast to the behavior of triad T-2
and dyad D-2, the complexes described here do not exhibit
any charge-transfer quenching to the nitrophenyl moiety
or electron transfer to nitrophenyl from the reduced
chromophore. This may be due to the slight difference in
electrochemical potential for the reduction of the accep-
An energy-transfer quenching mechanism involving nitros-
tilbene is also possible with the platinum terpyridyl acetylide
chromophore. The triplet energy of nitrostilbene has been
reported as ∼2.0 eV.78 The triplet energy of the chromophore
may be taken as the λmax of the 77 K frozen glass emission, or
550 nm (2.2 eV). Thus, a triplet-triplet energy transfer from
the chromophore to the nitrostilbene would be expected to be
energetically favorable and may account for the quenching
behavior observed here and in the previously reported
chromophore-acceptor dyad D-2 and triad T-2.46 Also, for the
(77) Armstrong, N. R.; Vanderborgh, N. E.; Quinn, R. K. J. Phys. Chem.
1976, 80, 2740–2745.
(78) Goerner, H. J. Phys. Chem. 1989, 93, 1826–1832.
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450.
(80) (a) Takezaki, M.; Hirota, N.; Terazima, M. J. Phys. Chem. A 1997,
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Chem. Phys. 2000, 2, 947–954.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2427

Jarosz et al.
tors, or differences in the pathway for charge transfer.
Additional analysis also suggests that the nitrostilbene
moiety may quench the chromophore through an energy-
transfer process rather than the charge-transfer process
initially proposed. With the synthetic methodology de-
scribed here, it will be possible to systematically vary the
acceptor, with the ultimate aim of generating multicom-
ponent systems that can achieve long-lived charge-
separated states.
Acknowledgment. We wish to thank the Division of Basic
Sciences, U.S. Department of Energy, for financial support
of this research (Grant DE-FG02-90ER14125). We also want
to thank Dr. Paul Merkle for his help with the lifetime
measurements.
IC801769V
2428 Inorganic Chemistry, Vol. 48, No. 6, 2009