Platinum(ll) terpyridyl acetylide complexes on platinized TiO2: Toward the photogeneration of H2 in aqueous media

Article abstract of DOI:10.1021/ic9001913

New platinum(ll) terpyrldyl acetylide complexes having the ability to bind to TiO₂ have been synthesized and assayed In their ability to sensitize platinized titanium dioxide for the photogeneration of H₂ using visible light (λ > 410

Full text of DOI:10.1021/ic9001913

Inorg. Chem. 2009, 48, 9653–9663 9653
DOI: 10.1021/ic9001913
Platinum(II) Terpyridyl Acetylide Complexes on Platinized TiO2: Toward the
Photogeneration of H2 in Aqueous Media
Paul Jarosz, Pingwu Du, Jacob Schneider, Soo-Hyun Lee, David McCamant, and Richard Eisenberg*
Department of Chemistry, University of Rochester, Rochester, New York 14627
Received January 29, 2009
New platinum(II) terpyridyl acetylide complexes having the ability to bind to TiO2 have been synthesized and assayed
in their ability to sensitize platinized titanium dioxide for the photogeneration of H2 using visible light (λ > 410 nm).
Specifically, the complexes [Pt(tpy-phen-COOH)(CtC-C6H5)]Cl (1), where tpy-phen-COOH = 40-(4-carboxyphe-
nyl)-[2,20;60,200]terpyridine and CtC-C6H5 = phenylacetylide, and [Pt(tpy-COOH)(CtC-C6H5)]Cl (2), where tpy-
COOH = 40-carboxy-2,20;60,200-terpyridine, were prepared to investigate the effectiveness of attachment and proximity
to the TiO2 surface on hydrogen yield. Both complexes 1 and 2 sensitize the photogeneration of hydrogen, but produce
fewer turnovers than the unbound chromophore, [Pt(ttpy)(CtC-C6H5)]PF6 (5). On the basis of these observations
and electrochemical data, a major limitation to the effectiveness of these chromophores is their instability upon
oxidation. To attempt to remedy this problem, two donor-chromophore (D-C) dyads, [Pt(tpy-phen-COOH)-
(CtC-C6H4CH2-PTZ)]PF6 (3), where CtC-C6H4CH2-PTZ = N-(4-ethynylbenzyl)-phenothiazine and [Pt(tpy-
COOH)(CtC-C6H4CH2-PTZ)]Cl (4) were prepared to function as TiO2-attached sensitizers. Transient absorption
measurements have shown that the PTZ moiety reductively quenches the Pt center in several picoseconds. While the
resultant PTZþ radical cation is capable of oxidizing rapidly the triethanolamine sacrificial electron donor, dyads 3 and 4
attached to platinized TiO2 do not function to generate hydrogen upon irradiation, in contrast with results seen for 1 and 2.
Introduction
dye-sensitized semiconductors materials.1-18 Pursued in
parallel, but with less frequency, has been the conversion of
The drive to convert solar energy into more usable forms of
energy has been a powerful stimulus in transition metal
photochemistry and photophysics. Many of these efforts
have focused on the conversion of solar energy to electrical
power through the use of semiconductor arrays and
(15) Gholamkhass, B.; Koike, K.; Negishi, N.; Hori, H.; Sano, T.;
Takeuchi, K. Inorg. Chem. 2003, 42, 2919–2932.
(16) Wolpher, H.; Sinha, S.; Pan, J.; Johansson, A.; Lundqvist, M. J.;
Persson, P.; Lomoth, R.; Bergquist, J.; Sun, L.; Sundstrom, V.; Akermark,
B.; Polivka, T. Inorg. Chem. 2007, 46, 638–651.
(17) Lundqvist, M. J.; Nilsing, M.; Lunell, S.; Akermark, B.; Persson, P.
J. Phys. Chem. B 2006, 110, 20513–20525.
(18) Abuabara, S. G.; Cady, C. W.; Baxter, J. B.; Schmuttenmaer, C. A.;
Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. J. Phys. Chem. C 2007, 111,
11982–11990.
(19) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38.
(20) Khaselev, O.; Turner, J. A. Science 1998, 280, 425–427.
(21) Kim, H. G.; Borse, P. H.; Choi, W.; Lee, J. S. Angew. Chem., Int. Ed.
2005, 44, 4585–4589.
*To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu.
::
(1) Gratzel, M. Philos. Trans. R. Soc. A 2007, 365, 993–1005.
(2) Overstraeten, R. V.; Mertens, R.; Nijs, J. Rep. Prog. Phys. 1982, 1041.
(3) Moser, J. E.; Bonnote, P.; Gratzel, M. Coord. Chem. Rev. 1998, 171,
245–250.
::
(4) Gratzel, M. Coord. Chem. Rev. 1991, 111, 167–174.
::
(5) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737.
(6) David, F. W.; Gerald, J. M. Annu. Rev. Phys. Chem. 2005, 56, 119–156.
(7) Yan, S. G.; Prieskorn, J. S.; Kim, Y.; Hupp, J. T. J. Phys. Chem. B
2000, 104, 10871–10877.
(8) Piotrowiak, P.; Galoppini, E.; Wei, Q.; Meyer, G. J.; Wiewior, P. J.
Am. Chem. Soc. 2003, 125, 5278–5279.
(9) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G.
J. Langmuir 1999, 15, 7047–7054.
(10) Prezhdo, O. V.; Duncan, W. R.; Prezhdo, V. V. Acc. Chem. Res. 2008,
41, 339–348.
(22) Hyun Gyu, K.; Euh Duck, J.; Pramod, H. B.; Seongho, J.; Kijung,
Y.; Jae Sung, L.; Wei, L.; Se, H. O. Appl. Phys. Lett. 2006, 89, 064103.
(23) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue,
Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13107–13112.
(24) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.;
Domen, K. J. Phys. Chem. B 2006, 110, 13753–13758.
(25) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828–6840.
::
(26) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. J. Am.
(11) Moser, J. E.; Noukakis, D.; Bach, U.; Tachibana, Y.; Klug, D. R.;
Durrant, J. R.; Humphry-Baker, R.; Gratzel, M. J. Phys. Chem. B 1998, 102,
3649–3650.
Chem. Soc. 1981, 103, 6324–6329.
(27) Du, P.; Schneider, J.; Li, F.; Zhao, W.; Patel, U.; Castellano, F. N.;
Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 5056–5058.
(28) Elvington, M.; Brown, J.; Arachchige, S. M.; Brewer, K. J. J. Am.
(12) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Moser, J. E.; Klug, D. R.;
Durrant, J. R. J. Phys. Chem. B 2001, 105, 7424–7431.
Chem. Soc. 2007, 129, 10644–10645.
::
::
(13) Tachibana, Y.; Moser, J. E.; Gratzel, M.; Klug, D. R.; Durrant, J. R.
(29) Rau, S.; Schafer, B.; Gleich, D.; Anders, E.; Rudolph, M.; Friedrich,
::
J. Phys. Chem. 1996, 100, 20056–20062.
M.; Gorls, H.; Henry, W.; Vos, J. G. Angew. Chem., Int. Ed. 2006, 45, 6215–
(14) Kroeze, J. E.; Hirata, N.; Koops, S.; Nazeeruddin, M. K.; Schmidt-
Mende, L.; Gratzel, M.; Durrant, J. R. J. Am. Chem. Soc. 2006, 128, 16376–
16383.
6218.
(30) Ozawa, H.; Haga, M.; Sakai, K. J. Am. Chem. Soc. 2006, 128, 4926–
4927.
r
2009 American Chemical Society
Published on Web 09/17/2009
pubs.acs.org/IC

9654 Inorganic Chemistry, Vol. 48, No. 20, 2009
Jarosz et al.
solar energy directly into chemical fuel by means of photo-
electrochemical processes.3,19-39 Within these reports, the
light-driven reduction of water to H2 has been a foremost
objective because of hydrogen’s high specific enthalpy of
combustion as a fuel and the low environmental impact of its
combustion product, water.
The strategy of using charge transfer chromophores to
sensitize or inject electrons into the conduction band (CB) of
TiO2 has been pursued for more than three decades.11-13,18
Moreover, if the TiO2 is platinized (TiO2-Pt), proton reduc-
tion to H2 becomes possible when a sacrificial electron donor
such as triethanolamine or another tertiary amine is also
present.26,35-40 While most of these studies have involveda d6
Ru(II) tris diimine sensitizer, a few reports have examined d8
Pt(II) diimine chromophores in this capacity. Recently, we
described one of these systems with a Pt(II) bipyridyl dithio-
late chromophore and TiO2-Pt for electron separation and
H2 catalysis, and found that with λ > 455 nm light, the
system was photostable as well.40 This and other reports
demonstrate that platinum chromophores are competent as
dye sensitizers for TiO2, and with TiO2-Pt, are capable of the
photogeneration of H2.41-44
Figure 1. Chromophores 1, 2, and 5 used for the sensitization of
platinized titanium dioxide for the photogeneration of hydrogen, as well
as the methyl ester precursors 1a and 2a.
With the intention of making these systems more effective
in producing H2, efforts have been directed to the construc-
tion of integrated systems for hydrogen generation. Such a
system would remove the constraint of diffusional quenching
kinetics, and thereby eliminate the need for higher concen-
trations of quenching species to achieve efficient charge
separation and productive photochemistry. However, the
actual effects of chromophore attachment to TiO2 in these
constructions is often overlooked. In this paper, we describe
the use of Pt(II) terpyridyl acetylide chromophores for the
sensitization of TiO2 in systems for the photogeneration of
hydrogen (Figure 1). Complexes 1 and 2 possess carboxylate
groups for attachment to platinizedTiO2 (Figure 2), and their
effectiveness is compared to that of the parent chromophore
5, which has no such anchoring group for TiO2 binding.
Previous reports have shown that upon excitation of
suitably anchored Ru(bpy) or Ru(tpy) derivatives, electron
injection into the TiO2 particle proceeds from the excited
chromophore to the TiO2 conduction band at a rate ranging
Figure 2. Schematic representation of a system for the photogeneration
of hydrogen, using platinum terpyridyl acetylide chromophores 1 (n = 1)
and 2 (n = 0) to sensitize platinized TiO2. The processes of charge
injection, back electron transfer, charge migration through the particle,
and reduction by the sacrificial donor have rate constants denoted as kinj
kbet, kmig, and kred, respectively.
,
from ∼102 fs to picoseconds, depending on the distance
between the chromophore and the particle.11-13,18 In these
systems, the nature of the excited state from which charge
injection occurs is a metal-to-ligand charge transfer (MLCT),
which is similar to that of the platinum terpyridyl acetylides
exemplified by 5.45-53 For both sets of chromophores, the
lowest unoccupied orbital is primarily the π* orbital of a bpy
or tpy ligand, and therefore, similar rates of charge injection
from the excited state of the Pt(II) chromophore excited state
into TiO2 are expected.
(31) Lei, P.; Hedlund, M.; Lomoth, R.; Rensmo, H.; Johansson, O.;
Hammarstrom, L. J. Am. Chem. Soc. 2008, 130, 26–27.
::
(32) Kiwi, J.; Gratzel, M. J. Am. Chem. Soc. 1979, 101, 7214–7217.
::
(33) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim. Acta 1978,
61, 2727–2730.
::
(34) Kiwi, J.; Gratzel, M. Angew. Chem., Int. Ed. Engl. 1979, 18, 624–626.
::
(35) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269–277.
(36) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141–145.
(37) Abe, R.; Hara, K.; Sayama, K.; Domen, K.; Arakawa, H. J.
Photochem. Photobiol., A 2000, 137, 63–69.
(38) Bae, E.; Choi, W. J. Phys. Chem. B 2006, 110, 14792–14799.
(39) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J. Phys.
Chem. B 2004, 108, 14093–14101.
(40) Zhang, J.; Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am.
Chem. Soc. 2007, 129, 7726–7727.
(41) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida,
M.; Takahashi, Y.; Murata, S.; Arakawa, H. Inorg. Chem. 2001, 40, 5371–
5380.
(42) Geary, E. A. M.; Yellowlees, L. J.; Jack, L. A.; Oswald, I. D. H.;
Parsons, S.; Hirata, N.; Durrant, J. R.; Robertson, N. Inorg. Chem. 2005, 44,
242–250.
(43) Geary, E. A. M.; Hirata, N.; Clifford, J.; Durrant, J. R.; Parsons, S.;
Dawson, A.; Yellowlees, L. J.; Robertson, N. Dalton Trans. 2003, 3757–
3762.
(44) Geary, E. A. M.; McCall, K. L.; Turner, A.; Murray, P. R.; McInnes,
E. J. L.; Jack, L. A.; Yellowlees, L. J.; Robertson, N. Dalton Trans. 2008,
3701–3708.
(45) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Lu, X.-X.;
Cheung, K.-K.; Zhu, N. Chem.-Eur. J. 2002, 8, 4066–4076.
(46) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002,
124, 6506–6507.
(47) Yam, V. W.-W.; Lee, V. W.-M. J. Chem. Soc., Dalton Trans. 1997,
3005–3010.
(48) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.;
Cheung, K.-K. Inorg. Chem. 2001, 40, 571–574.
(49) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Angew. Chem., Int. Ed.
2003, 42, 1400–1403.
(50) Hui, C.-K.; Chu, B. W.-K.; Zhu, N.; Yam, V. W.-W. Inorg. Chem.
2002, 41, 6178–6180.
(51) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653–5655.
(52) Chakraborty, S.; Wadas, T. J.; Hester, H.; Schmehl, R.; Eisenberg,
R. Inorg. Chem. 2005, 44, 6865–6878.
(53) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim, C.;
Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284–6293.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9655
Figure 3. D-C dyads 3 and 4 used for the sensitization of platinized titanium dioxide for the photogeneration of hydrogen as well as the methyl ester
precursors 3a and 4a and the previously reported D-C dyad 6.
Once in the conduction band, these electrons are able to
migrate to the catalytic sites of platinum metal and effect
proton reduction. Rates of charge transport through single
particles of TiO2 are becoming increasingly well understood,
and several physical models have been proposed to describe
the mobility of electrons in these particles.18,54-60 These rates
have been found to depend on the energy of the injected
electron as well as the density of trap sites in the particle.
With charge injection taking place on a picosecond time
scale, it is unlikely that reductive quenching by a sacrificial
donor will occur prior to electron injection. It is more
Figure 4. Schematic representationofD-C dyads3 (n = 1) and 4 (n = 0)
as sensitizers for the photogeneration of hydrogen. kD is the rate constant
for reductive quenchingof the chromophore bythe phenothiazine moiety.
(Figure 3) by transient absorption spectroscopy.52 In simi-
larly motivated efforts, donor-chromophore dyads have been
explored for photocurrent production in Gratzel-type cells,
probable that, after charge injection, the oxidized chromo-
phore will be reduced by the sacrificial donor, or undergo
back electron transfer with electrons in the conduction band,
at a rate that is again affected by the distance and pathway
between the chromophore and the particle. For oxidatively
unstable chromophores, this scenario leads to “long-term”
decomposition as a consequence of light driven electron
injection into TiO2. Recent work of Zhiyong et al., on the
::
though such systems are relatively uncommon compared to
pure chromophore sensitizers.16,63,64
Experimental Section
Materials. The chemicals N,N-dimethylformamide (DMF),
acetonitrile (MeCN), tetrahydrofuran (THF), dimethylsulfox-
ide, copper(I) iodide, ammonium acetate, ammonium hexa-
fluorophosphate (NH4PF6), 2-acetylpyridine, triethylamine,
potassium hydroxide, 2-furaldehyde (Aldrich), electrochemical
grade tetrabutylammonium hexafluorophosphate and pota-
ssium tert-butoxide (Fluka), potassium hexachloroplatinate,
(Strem Chemical), and titanium dioxide P25 (Degussa) were
used without further purification.
The synthesis of Pt(DMSO)2Cl2 was carried out according to
a literature procedure.52 [Pt(ttpy)(CtC-C6H5)]PF6 (5) was
used as previously prepared.52 Platinized TiO2 catalysts were
prepared by the photodeposition method, using potassium
hexachloroplatinate as the platinum source by a method pre-
viously described.40 Syntheses were performed under nitrogen
with solvents purified by passing the degassed solvent through
columns containing activated molecular sieves and activated
alumina.65 All other reagents were of spectroscopic grade and
used without further purification.
degradation of Methyl Orange dye on TiO2 addresses this
::
issue directly,61 and a 2006 report by Gratzel describes this as
one of the chief concerns in designing systems to achieve high
turnovers in photovoltaic systems.62
To counter the issue of Pt chromophore oxidative instabi-
lity and to expand the study of charge transfer complexes on
TiO2, donor-chromophore dyads 3 and 4 bearing the pheno-
thiazine moiety (Figure 3) have also been synthesized and
examined as sensitizers for the photogeneration of hydrogen
(Figure 4). The results are also described herein and are
compared to those obtained for 1 and 2. Phenothiazine
donors have previously been found to be effective reductive
quenchers of MLCT excited states of Ru and Pt polypyridyl
complexes, such as in the case of the donor-chromophore
(D-C) dyad, [Pt(ttpy)(CtC-C6H4CH2-PTZ)]PF6 [6]
Characterization. 1H NMR spectra were recorded on a
Bruker Avance-500 spectrometer (500 MHz). Mass determina-
tions were accomplished by electrospray ionization mass spectro-
metry (ESIMS) using a Shimadzu mass spectrometer equipped
with a quadrupole mass filter. Mass spectrometric data for
(54) Weng, Y. X.; Wang, Y. Q.; Asbury, J. B.; Ghosh, H. N.; Lian, T. J.
Phys. Chem. B 2000, 104, 93–104.
(55) Du, L. C.; Weng, Y. X. J. Phys. Chem. C 2007, 111, 4567–4577.
(56) Turner, G. M.; Beard, M. C.; Schmuttenmaer, C. A. J. Phys. Chem. B
2002, 106, 11716–11719.
(57) Nelson, J.; Chandler, R. E. Coord. Chem. Rev. 2004, 248, 1181–1194.
(58) Barzykin, A. V.; Tachiya, M. J. Phys. Chem. B 2002, 106, 4356–4363.
::
(63) Meyer, G. J. Inorg. Chem. 2005, 44, 6852–6864.
(59) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49–68.
(64) Bonhote, P.; Moser, J. E.; Humphry-Baker, R.; Vlachopoulos, N.;
(60) Katoh, R.; Furube, A.; Barzykin, A. V.; Arakawa, H.; Tachiya, M.
Coord. Chem. Rev. 2004, 248, 1195–1213.
::
Zakeeruddin, S. M.; Walder, L.; Gratzel, M. J. Am. Chem. Soc. 1999, 121,
(61) Zhiyong, Y.; Laub, D.; Bensimon, M.; Kiwi, J. Inorg. Chim. Acta
1324–1336.
(65) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
2008, 361, 589–594.
::
(62) Gratzel, M. C. R. Chim. 2006, 9, 578–583.

9656 Inorganic Chemistry, Vol. 48, No. 20, 2009
Jarosz et al.
1-4 are provided in the Supporting Information. Cyclic
voltammetry experiments were conducted on an EG&G PAR
263A potentiostat/galvanostat using a three electrode single
compartment cell, with DMF as the solvent. A glassy carbon
working electrode, Pt wire auxiliary electrode, and Ag wire
reference electrode were used. For all measurements, samples
were degassed with nitrogen. Tetrabutylammonium hexafluoro-
phosphate (0.10 M) was used as the supporting electrolyte
while ferrocene was employed as an internal redox reference.
All redox potentials are reported relative to NHE using the
ferrocenium/ferrocene (Fcþ/Fc) couple (0.45 V vs SCE for
DMF)66 as an internal standard, and all scans were done at
100 mV/s. Absorption spectra were recorded using a Hitachi
U2000 scanning spectrophotometer (300-800 nm). Emission
spectra were obtained using a Spex Fluoromax-P fluorimeter.
Monochromators were positioned with a 2 nm band-pass, and
solution samples were degassed by sparging with N2. Frozen
glass samples were prepared in butyronitrile using NMR tubes
in a circular quartz-tipped immersion Dewar filled with liquid
nitrogen. Emission spectra were uncorrected for spectral
response. Infrared spectra of titanium dioxide samples were
obtained using a Shimadzu 8400 S FT-IR spectrometer in air
with the background signal subtracted.
Femtosecond transient absorption measurements were per-
formed with a regeneratively amplified titanium/sapphire laser
producing 1.7 mJ 100 fs duration pulses at 800 nm with a 1 kHz
repetition rate. The 400 nm pump pulse was generated by
frequency doubling the Ti:sapph. fundamental, resulting in a
∼120 fs pulse with 0.5-1 μJ per pulse. The probe continuum
extended from 320 to 600 nm and was produced by focusing a
small portion of the 400 nm pulse into a 2 mm thick CaF2 crystal.
The relative polarization of the pump and probe beams was 55°,
to eliminate artifacts from molecular rotation. The sample was
degassed with nitrogen and sealed in a 10 mm cuvette. After
being focused through the sample with the pump pulse, the
probe was collected, dispersed by a spectrograph (Acton 320
mm fl, 150 g/mm), and measured with a 1024 pixel diode array
detector. Each transient absorption spectrum is the average of
1000 laser pulses, in which the alternating pump-off and pump-
on spectra are compared in 100 ms exposures to produce the
transient absorption spectrum for a single pump-probe time
delay. The time delay was controlled by optically delaying the
pump pulse and the time resolution was 250-300 fs, depending
on wavelength.
but has not been previously reported by this procedure. Alter-
native procedures have been reported, with comparable
yields.67,68 A 500 mL flask was charged with potassium tert-
butoxide (6.8 g, 59 mmol) and THF (60 mL). This was stirred
vigorously until most of the base had dissolved. To this was
added dropwise a solution of 2-acetyl pyridine (5.5 mL,
49 mmol) in THF (60 mL) over 1 h, yielding an off white
suspension. A solution of 4-methylcarboxy-benzaldehyde (4 g,
24 mmol) in THF (60 mL) was then added dropwise over 1 h,
and the resulting orange solution was left to stir for 3 h, during
which it became dark red. A solution of ammonium acetate
(10 g, excess) in methanol (50 mL) and glacial acetic acid (25 mL)
was added, and the reaction stirred at reflux for 16 h. After
removing volatiles by rotary evaporation, water was added to
the green slurry, and the pH adjusted to 7 with sodium hydro-
xide. The resulting yellow precipitate was collected by filtration,
washed with acetonitrile, diethyl ether, and dried in vacuo.
Yield: 2.485 g (28%). Characterization data are consistent with
those previously reported.
tpy-phen-COOH. A 25 mL flask was charged with tpy-phen-
COOMe (0.200 g, 0.54 mmol), sodium hydroxide (200 mg,
excess), acetonitrile (8 mL), and water (8 mL). The suspension
was stirred at reflux for 16 h. The solution was neutralized with
dilute HCl, and the white solid collected by precipitation,
washed with THF/diethyl ether (1:1/v:v), and dried in vacuo.
Yield: 0.170 g (89%). 1H NMR data were consistent with those
previously reported.67
[Pt(tpy-phen-COOMe)Cl]Cl. A 25 mL flask was charged
with Pt(DMSO)2Cl2 (0.080 g, 0.20 mmol), tpy-phen-COOMe
(0.080 g, 0.22 mmol), DMF (8 mL), and methanol (8 mL). The
resulting suspension was stirred at reflux under N2 for 16 h. A
yellow suspension formed, and complete precipitation was
effected by the addition of water. The solid was collected by
filtration and washed with acetone and diethyl ether. Yield:
0.094 g (74%). 1H NMR (DMSO-d6): δ 9.12 (2H, s), 9.02 (2H, d,
J = 5 Hz), 8.93 (2H, d, J = 8), 8.60 (2H, t, J = 8), 8.37 (2H, d,
J = 8), 8.24 (2H, d, J = 8), 8.03 (2H, t, J = 7), 3.96 (3H, s).
[Pt(tpy-phen-COOMe)(CtC-C6H5)]Cl (1a). A 10 mL flask
was charged with [Pt(tpy-phen-COOMe)Cl]Cl (0.080 g, 0.13
mmol), ammonium hexafluorophosphate (0.100 g, excess), CuI
(0.010 g, 0.06 mmol), and DMF (4 mL). Ammonium hexafluoro-
phosphate was required to enhance the solubility of the complex
and reduce reaction time (see Results and Discussion). The
reaction flask was sealed with a septum, and the solution
sparged with N2. Phenylacetylene (0.10 mL, 0.9 mmol) and
triethylamine (0.10 mL, 0.8 mmol) were added via syringe, and
the reaction mixture was stirred in the dark for 48 h. A red
precipitate that formed upon the addition of diethyl ether was
collected by centrifugation and dissolved in acetonitrile. The
addition of tetrabutylammonium chloride resulted in complete
precipitation as the chloride salt, which was washed with
acetone and diethyl ether, then dried in vacuo. Yield: 0.079 g
For photoinduced hydrogen evolution experiments, each
sample was prepared in a 50 mL round-bottom flask with a
solution volume of 25 mL of acetonitrile/water (3:2 v/v). Typi-
cally, the sample contained 2.0 ꢀ 10-5 M photosensitizer, 25 mg
platinized TiO2 (TiO2/Pt, Pt% = 0.05%), and triethanolamine
(50-500 mg). The pH of the solution was adjusted to 7 with 10%
HCl. The flask was sealed with a septum and degassed by
bubbling nitrogen for 20 min under atmospheric pressure at
room temperature. Following degassing, a gas syringe was used
to remove 5 mL nitrogen from the flask headspace and 5 mL
methane (760 Torr) was likewise injected into the flask, to serve
as an internal standard. The samples were irradiated with a 200
W Mercury Xenon lamp. A long-pass filter (λ > 410 nm) was
used to exclude higher energy light, capable of exciting the
bandgap of TiO2. The hydrogen generated by the system was
measured by periodically withdrawing 0.1 mL of the headspace
atmosphere of the reaction vessel and analyzing it with a GC-
1
(84%). H NMR (DMSO-d6): δ 9.21 (2H, d, J = 5 Hz), 9.14
(2H, s), 8.90 (2H, d, J = 8), 8.58 (2H, t, J = 8), 8.33 (2H, d, J =
8), 8.24 (2H, d, J = 8), 7.98 (2H, t, J = 8), 7.53 (2H, d, J = 7),
7.37 (2H, t, J = 7), 7.30 (1H, t, J = 7), 3.96 (3H, s). MS (positive-
API-ES): m/z 663.10 [M - Cl]þ.
[Pt(tpy-phen-COOH)(CtC-C6H5)]Cl (1). A test tube was
charged with 1a (0.030 g, 0.32 mmol), potassium hydroxide
(0.060 g, 1 mmol), THF (1.0 mL), and water (1.0 mL). The tube
was sealed with a septum, and the suspension was stirred for 2
days in the dark. The suspension was adjusted to pH 7 with
dilute HCl, and additional water was added. The precipitate was
collected by centrifugation and washed with water, acetone, and
˚
17A (Shimadzu) gas chromatograph equipped with a 5 A
molecular sieves column (30 m ꢀ 0.53 mm) and a thermal
conductivity detector. Pre-purified nitrogen was employed as
the carrier gas.
40-(4-Methylcarboxyphenyl)-[2,20;60,200]terpyridine (tpy-phen-
COOMe). Thisligand was prepared using standard methodology,
(67) Storrier, G. D.; Colbran, S. B. Inorg. Chim. Acta 1999, 284, 76–84.
(68) Figgemeier, E.; Aranyos, V.; Constable, E. C.; Handel, R. W.;
Housecroft, C. E.; Risinger, C.; Hagfeldt, A.; Mukhtar, E. Inorg. Chem.
Commun. 2004, 7, 117–121.
(66) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9657
dried in vacuo. Yield: 0.020 g (90%). 1H NMR (DMSO-d6): δ
9.15 (2H, d, J = 5 Hz), 9.10 (2H, s), 8.88 (2H, d, J = 8), 8.55 (2H,
t, J = 8), 8.28 (2H, d, J = 8), 8.20 (2H, d, J = 8), 7.95 (2H, t, J =
6), 7.50 (2H, d, J = 7), 7.36 (2H, t, J = 7), 7.29 (1H, t, J = 7).
HRMS - ESI (exp; calc): m/z (648.11725, 649.12134; 648.1177,
649.1198) for [194PtC30H20N3O2, 195PtC30H20N3O2]þ. Anal.
Calcd for PtC30H20N3O2Cl: C, 52.63; H, 2.92; N, 6.14. Found:
C, 49.35; H, 3.73; N, 5.66.
solution of 2-furaldehyde (3.0 mL, 36.2 mmol) in THF (80 mL)
was then added dropwise to the white suspension over the course
of 1 h becoming dark red after an additional 2 h of stirring. A
solution of ammonium acetate (10 g, excess) in ethanol (50 mL)
and glacial acetic acid (25 mL) was added. The resultant solution
was heated at reflux for 48 h. Volatiles were removed by rotary
evaporation and distilled water was added to yield a brown
solid. This was collected by filtration and washed with water,
50% aqueous ethanol, diethyl ether, then dried in vacuo. Yield:
5.420 g (50%). 1H NMR data were consistent with those
previously reported.
[Pt(tpy-phen-COOMe)(CtC-C6H4CH2-PTZ)]Cl (3a). A
10 mL flask was charged with [Pt(tpy-phen-COOMe)Cl]Cl
(0.040 g, 0.063 mmol), N-(4-ethynylbenzyl)-phenothiazine
(0.049 g, 0.16 mmol), ammonium hexafluorophosphate (0.050
g, excess), CuI (0.010 g, 0.06 mmol), and DMF (4 mL).
Ammonium hexafluorophosphate was required to enhance the
solubility of the complex and reduce reaction time. The flask was
sealed with a septum, and the solution sparged with N2.
Triethylamine (0.10 mL, 0.8 mmol) was added via syringe, and
the reaction mixture was stirred in the dark for 48 h. A dark red
precipitate formed upon the addition of diethyl ether, which was
collected by centrifugation and dissolved in acetonitrile. The
addition of tetrabutylammonium chloride resulted in complete
precipitation as the chloride salt, which was washed with
acetone and diethyl ether, then dried in vacuo. Yield: 0.043 g
40-Carboxy-2,20;60,200-terpyridine (tpy-COOH). This ligand
was synthesized from tpy-furyl by a reported method without
significant modification.16 During workup, it was observed that
precipitation of the product from this filtrate after adjusting to
pH 4 with HCl was slow, taking several hours to complete. 1H
NMR data were consistent with those previously reported.
40-Methylcarboxy-2,20;60,200-terpyridine (tpy-COOMe).
A
flame-dried 25 mL flask was charged with tpy-COOH (0.800
g, 2.9 mmol), K2CO3 (0.800 g, 6.0 mmol), and dry, anhydrous
THF. The flask was sealed with a septum, and stirred vigorously
for 2 h. At this time, iodomethane (0.70 mL, 11 mmol) was
delivered to the suspension via syringe. The mixture was allowed
to stir overnight. Water was added, and the resulting white
precipitate was collected by filtration. This was washed with
water and dried in vacuo. Yield: 0.372 g (44%). 1H NMR
(DMSO-d6): δ 8.91 (2H, s), 8.80 (2H, d, J = 4 Hz), 8.70 (2H,
d, J = 7), 8.08 (2H, t, J = 8), 7.58 (2H, t, J = 5), 4.02 (3H, s).
[Pt(tpy-COOMe)Cl]Cl. A 25 mL round-bottom flask was
charged with tpy-COOMe (0.080 g, 0.27 mmol), Pt(DMSO)2Cl2
(0.100 g; 0.24 mmol), 8 mL DMF, and 4 mL MeOH. The
mixture was stirred at reflux for 16 h. Acetone was added to
the resulting yellow suspension, and the yellow precipitate was
collected by filtration and washed with acetone and diethyl ether
before drying in vacuo. In some instances, solvatochromism
(red, purple) was observed upon precipitation with acetone.
Repeated washing with acetone gave a yellow solid. Yield:
0.088 g (58%). 1H NMR (DMSO-d6): δ 9.08 (2H, s), 9.05 (2H,
d, J = 5 Hz), 8.99 (2H, d, J = 8), 8.57 (2H, t, J = 8), 8.05 (2H, t,
J = 5), 4.08 (3H, s).
1
(74%). H NMR (DMSO-d6): δ 9.15 (2H, d, J = 5 Hz), 9.11
(2H, s), 8.87 (2H, d, J = 8), 8.54 (2H, t, J = 7), 8.32 (2H, d, J =
8), 8.23 (2H, d, J = 8), 7.92 (2H, t, J = 7), 7.45 (2H, d, J = 8 Hz),
7.33 (2H, d, J = 8 Hz), 7.20 (2H, d, J = 8 Hz), 7.13 (2H, t, J = 8
Hz), 6.96 (2H, t, J = 7 Hz), 6.86 (2H, d, J = 8 Hz), 5.18 (2H, s),
3.96 (3H, s). MS (positive-API-ES): m/z 874.20 [M - Cl]þ.
[Pt(tpy-phen-COOH)(CtC-C6H4CH2-PTZ)]Cl (3). A test
tube was charged with 3a (0.020 g, 0.022 mmol), potassium
hydroxide (0.060 g, 1 mmol), THF (1.0 mL), and water (1.0 mL).
The tube was sealed with a septum, and the suspension was
stirred for 2 days in the dark. The suspension was adjusted to pH
7 with dilute HCl, and additional water was added. The pre-
cipitate was collected by centrifugation and washed with water,
1
acetone, and dried in vacuo. Yield: 0.010 g (49%). H NMR
(DMSO-d6): δ 9.16 (2H, d, J = 5 Hz), 9.10 (2H, s), 8.90 (2H, d,
J = 7), 8.55 (2H, t, J = 8), 8.24 (2H, d, J = 8), 8.16 (2H, d, J = 7),
7.93 (2H, t, J = 6), 7.46 (2H, d, J = 8), 7.33 (2H, d, J = 8), 7.19
(2H, d, J = 8), 7.13 (2H, t, J = 7), 6.96 (2H, t, J = 7), 6.86 (2H, d,
J = 8), 5.18 (2H, s). MS (positive-API-ES): m/z 860.25 [M - Cl]þ.
HRMS data were not obtained but the isotope pattern for the MS
data is consistent with the formula of the cation (see Supporting
Information). Anal. Calcd for PtC43H29N4O2SCl: C, 57.65; H,
3.24; N, 6.26. Found: C, 53.34; H, 3.39; N, 5.82.
[Pt(tpy-COOMe)(CtC-C6H5)]Cl (2a). A 10 mL flask was
charged with [Pt(tpy-COOMe)Cl]Cl (0.080 g, 0.14 mmol), CuI
(0.010 g, 0.06 mmol), and DMF (4 mL). Ammonium hexafluoro-
phosphate was required to enhance the solubility of the complex
and reduce reaction time (see Results and Discussion). The
reaction flask was sealed with a septum, and the solution
sparged with N2. Phenylacetylene (0.10 mL, 0.9 mmol) and
triethylamine (0.10 mL, 0.8 mmol) were added via syringe, and
the reaction mixture was stirred in the dark for 48 h. A red
precipitate formed upon the addition of diethyl ether, was
collected by centrifugation, washed with a mixture of aceto-
nitrile/diethyl ether (1:1/v:v) followed by pure diethyl ether, and
dried in vacuo. Yield: 0.079 g (93%). 1H NMR (DMSO-d6): δ
9.25 (2H, d, J = 6 Hz), 9.18 (2H, s), 8.98 (2H, d, J = 8 Hz), 8.56
(2H, t, J = 8 Hz), 8.00 (2H, d, J = 8 Hz), 7.55 (2H, d, J = 8),
7.39 (2H, t, J = 8), 7.31 (1H, t, J = 8), 4.08 (3H, s).
[Pt(tpy-phen-COOH)Cl]PF6. A 25 mL flask was charged with
Pt(DMSO)2Cl2 (0.053 g, 0.13 mmol), tpy-phen-COOH (0.050 g,
0.14 mmol), DMF (8 mL). The resulting suspension was stirred
at reflux under N2 for 16 h. A yellow suspension formed, and
complete precipitation was effected by the addition of excess of
NH4PF6 in water. The solid was collected by filtration and
washed with tetrahydrofuran/diethyl ether (1:1/v:v), diethyl
1
ether, and dried in vacuo. Yield: 0.047 mg (49%). H NMR
(DMSO-d6, with added drop HCl): δ 9.12 (2H, s), 9.02 (2H, d,
J = 5 Hz), 8.93 (2H, d, J = 8), 8.60 (2H, t, J = 8), 8.37 (2H, d,
J = 8), 8.24 (2H, d, J = 8), 8.03 (2H, t, J = 7).
[Pt(tpy-COOH)(CtC-C6H5)]Cl (2). A test tube was charged
with 2a (0.020 g, 0.032 mmol), potassium hydroxide (0.040 g, 1
mmol), THF (1.0 mL), and water (1.0 mL). The tube was sealed
with a septum, and the suspension was stirred for 2 days in the
dark. The suspension was adjusted to pH 7 with dilute HCl,
and acetone was added. The precipitate was collected by centri-
fugation, washed with water followed by acetone, and dried in
vacuo. Yield: 0.011 g (56%). 1H NMR (DMSO-d6): δ 9.18
(2H, d, J = 6 Hz), 9.02 (2H, s), 8.89 (2H, d, J = 8 Hz),
8.51 (2H, t, J = 8 Hz), 7.96 (2H, t, J = 6 Hz), 7.52 (2H, d,
J = 7), 7.38 (2H, t, J = 8), 7.30 (1H, t, J = 7). HRMS - ESI
(exp; calc): m/z (572.08478, 573.08911; 572.0864, 573.0885)
for [194PtC24H16N3O2, 195PtC24H16N3O2]þ. Anal. Calcd for
40-(2-furyl)-2,20;60,200-terpyridine (tpy-furyl). This ligand was
synthesized by the same method as described for tpy-phen-
COOMe, and differs from reported literature methods.69,70
A
500 mL flask was charged with potassium tert-butoxide (10.00 g,
89.3 mmol) and THF (80 mL). A solution of 2-acetylpyridine
(8.0 mL, 71.4 mmol) in THF (80 mL) was added dropwise. A
(69) Zhao, L.-X.; Kim, T. S.; Ahn, S.-H.; Kim, T.-H.; Kim, E.-K.; Cho,
W.-J.; Choi, H.; Lee, C.-S.; Kim, J.-A.; Jeong, T. C.; Chang, C.-j.; Lee, E.-S.
Bioorg. Med. Chem. Lett. 2001, 11, 2659–2662.
(70) Husson, J.; Beley, M.; Kirsch, G. Tetrahedron Lett. 2003, 44, 1767–
1770.

9658 Inorganic Chemistry, Vol. 48, No. 20, 2009
Jarosz et al.
PtC24H16N3O2Cl: C, 47.37; H, 2.63; N, 6.91. Found: C, 45.09;
H, 2.73; N, 6.65.
[Pt(tpy-COOMe)(CtC-C6H4CH2-PTZ)]Cl (4a). A 10 mL
flask was charged with [Pt(tpy-COOMe)Cl]Cl (0.040 g, 0.072
mmol), N-(4-ethynylbenzyl)-phenothiazine (0.045 g, 0.14
mmol), CuI (0.010 g, 0.06 mmol), and DMF (4 mL). This was
sealed with a septum, and sparged with N2. Triethylamine (0.10
mL, 0.8 mmol) was added via syringe, and the reaction mixture
was stirred in the dark for 48 h. A dark red precipitate which
formed upon the addition of diethyl ether was collected by
centrifugation, washed with acetonitrile/diethyl ether (1:1/v:v)
followed by diethyl ether, and dried in vacuo. Yield: 0.043 g
1
(71%). H NMR (DMSO-d6): δ 9.18 (2H, d, J = 5 Hz), 9.09
Figure 5. Roomtemperature absorption spectra of2 in DMF/H2O (1:1)
at 1 ꢀ 10-5 M, adjusted to various pH values.
(2H, s), 8.93 (2H, d, J = 8), 8.52 (2H, t, J = 8), 7.96 (2H, t, J =
8), 7.47 (2H, d, J = 7), 7.35 (2H, d, J = 7), 7.19 (2H, d, J = 8),
7.13 (2H, t, J = 7), 6.96 (2H, t, J = 7), 6.87 (2H, d, J = 8), 5.19
(2H, s), 4.05 (3H, s).
tetrabutylammonium chloride to solutions of the hexa-
fluorophosphate salt.
[Pt(tpy-COOH)(CtC-C6H4CH2-PTZ)]Cl (4). A test tube
was charged with 4a (0.030 g, 0.032 mmol), potassium hydroxide
(0.060 g, 1 mmol), THF (1.0 mL), and water (1.0 mL). The tube
was sealed with a septum, and the suspension was stirred for 2
days in the dark. The suspension was adjusted to pH 7 with
dilute HCl, and additional water was added. The precipitate was
collected by centrifugation, washed with water and acetone, and
dried in vacuo. Yield: 0.011 g (36%). 1H NMR (DMSO-d6): δ
9.22 (2H, d, J = 6 Hz), 9.07 (2H, s), 8.94 (2H, d, J = 8), 8.53 (2H,
t, J = 8), 7.97 (2H, t, J = 8), 7.48 (2H, d, J = 8), 7.34 (2H, d, J =
8), 7.18 (2H, d, J = 7), 7.12 (2H, t, J = 7), 6.94 (2H, t, J = 7),
6.85 (2H, d, J = 8), 5.18 (2H, s). MS (positive-API-ES): m/z
785.15 [M - Cl]þ. HRMS data were not obtained but the
isotope pattern for the MS data is consistent with the formula
of the cation (see Supporting Information). Anal. Calcd for
PtC37H25N4O2SCl: C, 54.21; H, 3.05; N, 6.84. Found: C, 51.44;
H, 3.26; N, 6.18.
1H NMR data for all of the compounds are consistent
with the assigned structures and previously reported
compounds of this type.52,53,71-76 All of the ligands and
complexes have distinct, well-resolved patterns in their
aromatic proton resonances, which are attributable to the
protons of the pyridine rings and phenyl substituents. The
pyridine resonances (δ 7.8-9.4 ppm) are particularly
informative, as they shift substantially upon coordination
of the Pt(II) metal ion and upon exchange of chloride for
acetylide, particularly the protons nearest the acetylide
moiety. Hydrolysis of the methyl esters was easily fol-
lowed by monitoring the proton resonance of the methyl
ester group.
Mass spectrometry is particularly informative in ana-
lyzing platinum containing compounds, as platinum gives
a characteristic isotopic pattern. Analysis by mass spectro-
metry gave data consistent with the proposed formulas
and included the expected isotope pattern for Pt contain-
ing complexes. However, elemental analyses of the plati-
num terpyridyl acetylide complexes reported here were
always unsatisfactory in carbon for which experimental
results were always low. Poor elemental analyses for
compounds of this type have been noted previously by
us and others working on these systems despite high
purity of samples as determined by other characterization
Results and Discussion
Syntheses and Characterization. All of the compounds
were synthesized using standard methodology.52,53,71-76
It was observed that having a carboxylic acid on the
terpyridine ligand impeded the copper-catalyzed ligand
exchange of acetylide for chloride in the final step. Thus,
these reactions took much longer than usual, and re-
quired purification to remove any remaining chloride
complex. Alternatively, if the terpyridyl ligand contained
the methyl ester, acetylide coordination proceeded
smoothly, and the esters could be subsequently hydro-
lyzed in strong aqueous base without degradation of the
chromophores. Therefore, the latter method was em-
ployed in the syntheses of compounds 1-4.
In addition, attempts to substitute chloride by acetylide
ligands in the complex [Pt(tpy-phen-COOMe)Cl]Cl were
slow and frequently stopped short of completion because
of poor solubility of the chloride salt of the cationic Pt(II)
complex. Therefore, NH4PF6 was added to the reaction
solution to improve solubility. After reaction, conversion
back to the chloride salt was effected by the addition of
1
methods, the most notable of which is H NMR spec-
troscopy.
Absorption and Emission Spectra. Dimethylforma-
mide/water (1:1 v/v) was used as the “mixed” solvent
for characterization of complexes 1 and 2 because of the
complexes’ poor solubility in other solvents. All of the
complexes exhibit features in the range of 300-800 nm
which are consistent with previously studied platinum
terpyridyl acetylide complexes.45-53 The higher energy
absorptions (300-370 nm) are assigned as intraligand
transitions, while the broad, less intense absorptions (ε ∼
7000-9000 M-1 cm1) at lower energies (410-500 nm) are
assigned to MLCT (dπ(Pt)-π*(tpy)) transitions.
The electronic absorption spectrum of compound 2 was
found to be dependent on the pH of the solution, red
shifting at lower pH as the carboxylate group is proto-
nated (Figure 5). The observed red shift at lower pH is
consistent with the MLCT assignment, as -COOH is a
stronger electron withdrawing group, lowering the energy
of the lowest unoccupied molecular orbital (LUMO)
which is π*(tpy). Similar behavior has been observed
previously in platinum bipyridyl dithiolate complexes
(71) Chan, C.-W.; Che, C.-M.; Cheng, M.-C.; Wang, Y. Inorg. Chem.
1992, 31, 4874–4878.
(72) Chan, C.-W.; Cheng, L.-K.; Che, C.-M. Coord. Chem. Rev. 1994,
132, 87–97.
(73) Howe-Grant, M.; Lippard, S. J. In Inorganic Syntheses; Busch, D. H.,
Ed.; John Wiley & Sons: New York, 1980; Vol. 20.
(74) Romeo, R.; Scolaro, L. M. Inorg. Synth. 1998, 32, 153–158.
(75) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L. M.; Ricevuto,
V.; Romeo, R. Inorg. Chem. 1998, 37, 2763–2769.
(76) Wadas, T. J.; Wang, Q.-M.; Kim, Y.-j.; Flaschenreim, C.; N., B. T.;
Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841–16849.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9659
Figure 6. Room temperature emission spectra of complexes 1 (a) and 2
(b) in DMF/H2O (1:1) at 1 ꢀ 10-5 M, adjusted to various pH values.
Excitation wavelength 420 nm.
with carboxylic acid moieties on the bipyridyl ligand.40
Additionally, the spectrum of the methyl ester derivative
2a is similar to that observed for 2 at low pH. In contrast,
the electronic spectrum of compound 1 is not very sensi-
tive to pH, which may be expected given the increased
distance of the carboxylic acid moiety from the terpyridyl
ligand (Supporting Information, Figure S1). This in-
creased distance and the fact that the central tpy ring
and the attached p-benzoate ring will not be coplanar lead
to poorer electronic communication between the tpy
ligand and the carboxylic acid or carboxylate substituent.
The absorption spectra of complexes 3 and 4 are similar
to those of 1 and 2, with slight broadening and red-
shifting of the MLCT transitions, similar to what has
been observed for dyad 6 (Figure 7).52 Dyad 4 also
exhibits a pH sensitivity regarding its absorption spec-
trum that resembles that seen with complex 2.
Figure 7. 77 K emission spectra of (a) 3a and (b) 4a at 1 ꢀ 10-5 M in
butyronitrile. Excitation wavelength 480 nm.
conducted with compounds 3 and 4 because of their poor
solubility in butyronitrile. Therefore, compounds 3a and
4a were examined. Both were found to be brightly emis-
sive in frozen butyronitrile (Figure 7). The low tempera-
ture emission spectrum of 3a is similar to those observed
for related complexes, with both exhibiting evidence of
vibronic progression, similar to that previously reported
for closely related dyad 6.51-53,77 The spectrum of 4a is
red-shifted relative to 3a because of lowering of the
terpyridyl π* orbital by the electron-withdrawing ester
in the former. That complexes 3a and 4a are emissive in a
frozen glass but are non-emissive in a room temperature
solution is consistent with a thermally activated non-
radiative decay process such as solvent reorganization
that is inhibited in a rigid matrix.
Quenching Studies. The emission intensities of com-
pounds 2 and 2a were too weak to allow reliable quanti-
fication of reductive quenching by triethanolamine. On
the other hand, the emission intensities of 1 and 1a were
sufficient to allow Stern-Volmer analyses of quenching
by triethanolamine. Linear plots of (Io/I) versus [TEOA]
yielded KSV values of 1 ꢀ 105 M-1 and 3 ꢀ 105 M-1 for 1
and 1a, respectively, with no evidence of static quenching
from absorption spectra in the presence of increasing
amounts of TEOA. If one assumes that the excited state
lifetime for these complexes is the same as that for the
parent chromophore 5 (∼1 μs), then the resultant quench-
ing rate constants kq are calculated to be greater than
diffusion controlled rates (∼1 ꢀ 1011 M-1 s-1 and ∼3 ꢀ
1011 M-1 s-1 respectively), which means that the actual
emission lifetimes of 1 and 1a must be shorter than that of
chromophore 5.
Emission studies were conducted on compounds 1 and
2 and both were found to be weakly emissive, with
luminescence profiles typical of platinum terpyridyl acet-
ylide complexes, that is, broad, structureless emissions at
λmax ∼ 610 nm.51-53,77 The quantum yields for emission
for 1 and 2 were found to be 0.011 and 0.004, respectively,
2þ
using Ru(bpy)3 as a standard with an emission quan-
tum yield φem of 0.062.78 There appears to be some
dependence of the emission intensity of these compounds
upon pH, although this effect may be due to changes in
the solvent (Figure 6). The emission properties of the
methyl esters 1a and 2a were also examined. These were
found to be similar to the acids with low quantum yields
of 0.0048 and 0.0017, respectively.
Dyads 3 and 4 were found to be non-emissive in fluid
solution, consistent with previous observations made
with the closely related dyad 6.52 The methyl ester analo-
gues, 3a and 4a, were also found to be non-emissive.
Low-temperature emission studies at 77 K could not be
Electrochemistry. The methyl esters 1a-4a were found
to be more amenable to electrochemical characterization
than the free acids 1-4 because of their superior solubi-
lities and the absence of acidic protons to complicate
analysis. However, even the methyl esters exhibited weak,
(77) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476–4482.
(78) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620–6627.

9660 Inorganic Chemistry, Vol. 48, No. 20, 2009
Jarosz et al.
Table 1. Electrochemical Data (E1/2) for Pt Complexes 1a-4a
Reduction of the PTZ Radical Cation. It has been
previously shown that trialkylamines can reduce the
phenothiazine cation radical.79 Triethanolamine was also
demonstrated to function in this capacity. A solution of
the phenothiazine ligand was made in MeCN, to which
was added a small amount (0.1 equiv) of ceric ammonium
nitrate in MeCN in air.80 This produced a rapid color
change from nearly colorless to bright red, indicative of
formation of the PTZ radical cation. When a small
amount of neat triethanolamine was added, this color
dissipated rapidly, leaving the solution nearly colorless,
confirming that triethanolamine can transfer an electron
rapidly to the phenothiazine radical cation.
compound
oxidation E1/2/V
reduction E1/2/V
1a
3a
2a
4a
1.30a
-0.61, -1.05
-0.54, -1.03
-0.66
1.05b, 1.33a
1.03a, 1.26a
0.92a, 1.05b, 1.39a
-0.62
a Irreversible under experimental conditions. b Quasi-reversible with
Au working electrode and scan limited to 1.1 V. Potential is given in volts
vs NHE as determined using the ferrocenium/ferrocene (Fcþ/Fc) couple
(0.45 V vs SCE for DMF)66 as an internal standard.
broad redox waves in most cases. All of the complexes
exhibit a reversible reduction at ∼-0.62 V based on NHE
as determined using the ferrocenium/ferrocene (Fcþ/Fc)
couple (0.45 V vs SCE for DMF)66 as an internal stan-
dard, with 1a and 3a showing a second reduction
at ∼-1.05 V (Table 1). These reductions have been
assigned in earlier studies to be tpy-based.52,53 Most
importantly, the complexes all exhibit irreversible oxida-
tion waves that in previous studies were assigned as Pt-
based. Square planar Pt(II) compounds are well-known
to undergo irreversible oxidation processes, but the nat-
ure of possible Pt(III) species and ultimate degradation
products is unknown. The Pt(II) oxidations in com-
pounds 1-4 appear near 1.3 V, similar to values found
for previously studied platinum terpyridyl acetylides.52,53
Additionally, the dyads 3a and 4a show quasi-reversible
oxidations attributable to the phenothiazine moiety at
∼1 V, while both 2a and 4a exhibit one additional
oxidation wave each.
Transient Absorption Spectroscopy. The phenothia-
zine-attached ligand employed in this work has been
reported previously and established as a good reductive
quencher of the excited state of the platinum terpyridyl
acetylide chromophore.52 To obtain values for the rate of
this reductive quenching, the previously reported dyad 6
was studied with transient absorption spectroscopy as
shown in Figure 8. This dyad was more convenient to
study than dyads 3 and 4 because of the higher solubility
of 6. It is anticipated that the rates of reductive quenching
are similar for all three dyads.
The transient absorption spectrum (Figure 8a) clearly
shows the initial formation of the chromophore excited
state, apparent as the broad absorption at 600 nm,
and the subsequent rapid formation of the phenothia-
zine radical cation, with a distinctive absorption band
at 510 nm. The electron transfer thus occurs by the
reductive quenching of the chromophore excited state,
and is consistent with previous observations described
for this dyad.52 The signal assigned to the PTZ radical
cation appears in the transient spectrum less than 50 ps
after excitation. Kinetic analysis of the risetime at
510 nm and the decay time at 600 nm (Figure 8b) reveals
that the cation forms in less than 6 ps. The latter
result sets a lower limit for the rate constant for
intramolecular reductive quenching of the chromophore
by PTZ of k > 1011 s-1. The transient signals then
decay with time-constants of 1.1 and 1.5 ns for the
600 nm and 510 nm signals, respectively, indicating
charge recombination on this time scale. The negative
signal observed at 400 nm is due to scattering from the
excitation pulse.
Binding to TiO2 and Platinized TiO2 (TiO2-Pt). To
quantify the degree of binding of the complexes to the
TiO2 nanoparticles, solutions were analyzed by UV/vis
spectroscopy before and after stirring with TiO2. After
stirring, the supernatant solution was separated from the
TiO2 suspensions by centrifugation and filtered 2-4 times
through glass fiber filter paper until clear, before analysis.
It was observed that binding of the complexes to TiO2-Pt
depends on the treatment applied to the sample after the
photodeposition of platinum metal, as well as the solvent
used for the binding study. All binding studies were
conducted with quantities of the chromophore falling
well below the saturation limit of the TiO2 (∼50 μmol/g),
which was predicted on the basis of previous studies
verified by independent analysis of the saturation limit
of tpy-phen-COOH on TiO2.81
Specifically, when TiO2-Pt was prepared using the
photoreduction method, heat treatment following the
photoreduction was found to be important to remove
byproducts capable of inhibiting chromophore binding.
A thorough investigation of this process and the effect of
heat treatment on the activity of platinized TiO2 has
recently been reported by Millon et al.82 Briefly, in the
photoreduction method, an alcohol (methanol in the
present case) serves as the reductant. This leads to the
formation of formic acid and formaldehyde, which bind
to the free surface of the TiO2. This can be seen clearly in
the IR spectrum of a sample of platinized TiO2 freshly
prepared by this method, and subjected only to washing
with ethanol and drying overnight in vacuo. This is
compared with the spectrum of the same sample, heated
to 500 °C in vacuo for 3 h (Figure 9).
When attempts were made to assess the efficacy of
chromophore binding to these samples of TiO2-Pt,
DMSO was initially chosen as the solvent, to avoid
complications due to the low solubility of complexes 1
and 2. It was clear from the visible spectrum of the
supernatant DMSO solutions that binding to TiO2-Pt
not subjected to heat treatment was incomplete, even
after stirring for several hours. After filtration, absor-
bance spectra of the supernatant solutions from these
experiments would exhibit features characteristic of the
chromphore. In contrast, binding to TiO2-Pt that had
(79) Bandlish, B. K.; Padilla, A. G.; Shine, H. J. J. Org. Chem. 1975, 40,
2590–2595.
(80) Masti, S. P.; Seetharamappa, J.; Melwanki, M. B.; Motohashi, N.
Anal. Sci. 2002, 18, 167–169.
(81) Cherian, S.; Wamser, C. C. J. Phys. Chem. B 2000, 104, 3624–3629.
ꢀ
(82) Millon, C.; Riassetto, D.; Berthome, G.; Roussel, F.; Langlet, M. J.
Photochem. Photobiol., A 2007, 189, 334–348.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9661
Figure 8. (a) Transient absorption spectra of dyad 6 in acetonitrile, collected at various intervals after excitation. (b) Kinetics of the photoinduced
absorption changes at 510 and 600 nm showing the rapid loss of absorption at 600 nm and the long-time decay of both signals in 1-1.5 ns. The excitation
wavelength was 400 nm.
Figure 9. Transmission FTIR spectra of pellets of platinized TiO2 (5% Pt) prepared by the photodeposition method before heat treatment (left) and after
.
heating at 500 °C in vacuo for 3 h (right). Spectral window 2000-1000 cm-1
been heated was complete after 1 h, and no absorbance
due to the chromophore was detected in solution.
However, after several cycles, a gray color would persist
indefinitely. Upon washing and drying of the TiO2 from
these experiments, the gray color was similar to platinized
TiO2 prepared by the photodeposition method. These last
observations are consistent with the notion of slow
photodecomposition of Pt terpyridyl complexes to form
colloidal Pt when irradiated with λ > 410 nm for several
hours.27 As discussed previously, the mechanism for this
decomposition subsequent to oxidation has yet to be
determined.
Hydrogen Production. Previously we reported that the
Pt tpy acetylide complex 5 promotes the photogeneration
of H2 in a system containing TiO2-Pt and TEOA as the
sacrificial electron donor when irradiated with λ > 410
nm.86,87 Experiments were done to compare the effective-
ness of TiO2-bound chromophores 1 and 2 with that of 5
for the photogeneration of H2 under the same conditions.
The platinum metal loading on the TiO2 employed was
0.05% Pt wt/wt, and the resultant material was nearly
white, allowing for visual confirmation of chromophore
binding. The activity of the TiO2 platinized at 0.05 wt %/
wt loading of metal yielded activity very similar to that of
Upon a change to an acetonitrile/water solvent
(3:2 v/v), it was found that the complexes bind strongly
to TiO2-Pt, including samples that had not undergone
heat treatment. This solvent effect is possibly due to an
increased exchange rate with surface bound species in
aqueous media versus DMSO, and is consistent with the
predicted increase of the pKa of the carboxylic acid group
(and decreased acidity) in DMSO versus MeCN/H2O.
Evidence of Electron Injection into TiO2. To demon-
strate that both the bound and free chromophores are
able to inject electrons into TiO2, solutions of 1 and 5 (5
mL at 1 ꢀ 10-4 M) were added in different experiments to
0.100 g TiO2 in a flask to which 0.500 g TEOA, 10 mL
H2O, and 10 mL of MeCN were added. The flasks were
sealed with septa and the suspensions sparged with N2.
Upon irradiation with a Hg lamp (λ > 410 nm), the
suspensions were observed to change in color from yellow
or orange to blue-gray. When briefly exposed to air, this
blue-gray color would rapidly dissipate, and the original
yellow or orange color would return, suggesting that the
color observed was due to reduced titanium dioxide.83-85
::
(86) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726–7727.
(83) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1985, 89, 4495–4499.
::
(84) Highfield, J. G.; Gratzel, M. J. Phys. Chem. 1988, 92, 464–467.
(85) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97,
11064–11070.
(87) Du, P.; Schneider, J.; Jarosz, P.; Zhang, J.; Brennessel, W. W.;
Eisenberg, R. J. Phys. Chem. B 2007, 111, 6887–6894.

9662 Inorganic Chemistry, Vol. 48, No. 20, 2009
Jarosz et al.
reductive quenching by TEOA prior to possible charge
injection into the TiO2. Electrochemical analysis shows
that the first reduction of 5 and related platinum(II)
terpyridyl acetylides is reversible, indicating that the
complexes are at least moderately stable when reduced.
Thus, a system involving unbound chromophore in which
reductive quenching by TEOA is faster than oxidative
quenching by electron transfer into TiO2 may be expected
to achieve a greater number of turnovers than if only the
latter were operative. In contrast, chromophores 1 and 2
achieve H2 generation through a mechanism that involves
oxidative quenching by photoinjection of an electron
from the chromophore into TiO2. That these bound
chromophores, 1 and 2, exhibit lower rates of H2 genera-
tion and achieve fewer H2 turnovers than 5 supports the
notions that (a) an oxidative quenching path is less
efficient for H2 photogeneration, possibly because of
rapid back reaction, and (b) these TiO2-anchored chromo-
phores undergo oxidative decomposition as suggested by
the electrochemical data and other observations.
In an attempt to increase the rates of reductive quench-
ing and reduction of the oxidized chromophore, the
concentration of TEOA was increased. This change in
the system yielded a substantially greater number of
turnovers in the system containing complex 1 as the
sensitizer. In a trial where the concentration of TEOA
was set to 1.3 ꢀ 10-1 M, 231 turnovers were achieved after
25 h irradiation, compared to 16 turnovers achieved by a
system otherwise identical with the concentration of
TEOA being 1.3 ꢀ 10-2 M.
Figure 10. Hydrogen turnovers as a function of irradiation time in
systems containing chromophores 1, 2, or 5. Error bars are approximate.
TiO2-Pt with higher loadings, consistent with the obser-
vations of others.88
Both 1 and 2 promote the photogeneration of hydrogen
when bound to platinized TiO2 upon λ>410 nm irradia-
tion in the presence of triethanolamine. For both of these
systems, as well as for the one using unbound chromo-
phore 5, hydrogen production decreases and eventually
stops, at which point the suspensions were changed in
color from yellow or orange to gray, similar to that
observed when a higher loading of Pt metal (0.5-1%) is
present on TiO2. Additionally, when the gray material
thus produced was used in place of TiO2-Pt prepared by
the photodeposition method, hydrogen was produced
without an induction period upon irradiation of a system
also containing chromophore 5 and TEOA, supporting
the conclusion that the product of decomposition was
colloidal Pt, similar to what we have noted previously.27
When the three chromophores were compared directly
in their ability to produce hydrogen upon irradiation, the
result was unexpected (Figure 10). The highest rate and
overall yield were achieved with the unbound chromo-
phore 5. While the system with 5 yielded 115 turnovers
after 12 h of irradiation, the system with bound chromo-
phore 2 generated only 16 turnovers after 13 h of photo-
In contrast with the results observed with 1 and 2, dyads
3 and 4 do not produce H2 when used as the photosensi-
tizer, despite the fact that phenothiazine has been
shown to reductively quench the chromophore.52 The
results of transient absorption spectroscopy demonstrate
that this reductive quenching is very rapid. Despite the
evidence that electron transfer reaction between the
phenothiazine radical cation and the triethanolamine
occurs readily to reduce the PTZ radical cation, back
electron transfer between the reduced chromophore and
PTZ radical cation is also very rapid. The decay rate for
PTZ radical cation from the TA experiment study of dyad
6 is >1 ꢀ 109 s-1. This decay is due to charge recombina-
tion, and the rate of this recombination is likely very
similar in dyads 3 and 4. This result suggests that while
attachment of the PTZ moiety to the sensitizer may
stabilize the latter to photodecomposition, the strategy
is also counter-productive to the photogeneration of H2
by anchored dyads because back electron transfer will be
competitive with the processes that lead to hydrogen
production.
Geometry on the Surface and Exchange Reactions with
[Pt(tpy-phen-COOH)Cl]PF6. In the course of investi-
gating dyads 3 and 4 as sensitizers for the photogeneration
of hydrogen, it was proposed that the complexes may
adopt an arrangement in which the dyads lie flat on the
surface of the TiO2, as shown in Figure 4, making them
unreactive for net electron transfer. The orientation of
chromophores and complexes on the surface can influence
the rates of charge transfer in ways that will affect the
desired photocatalysis. Evidence for such geometry depen-
dent behavior has been observed in other dye-sensitized
systems.15
lysis. Chromophore
2
produced hydrogen more
effectively than 1, which may be due to its closer proxi-
mity to and superior electronic communication with
TiO2. The apparent but slight decline in turnover num-
bers at longer irradiation times for 5 and 2 may be due to
leakage through the septa of the reaction vessels, but the
relative ability of the two chromophores for promoting
the photogeneration of hydrogen is clear. While the
observed turnover numbers for the TiO2-anchored sys-
tems are modest, particularly when compared to the
unattached multiple component systems, they are similar
to other reported integrated systems for hydrogen pro-
duction. A recent example of such a system based on a
porphyrin sensitizer and a hydrogenase model compound
is reported by Li et al., in which 16 turnovers were
achieved.89
For the excited unbound chromophore 5, the rate of
bimolecular electron injection into TiO2 is much slower
than those for the anchored chromophores 1 and 2, and
hence photoexcited 5 is much more likely to undergo
(88) Kondarides, D. I.; Daskalaki, V. M.; Patsoura, A.; Verykios, X. E.
Catal. Lett. 2008, 122, 26–32.
(89) Li, X.; Wang, M.; Zhang, S.; Pan, J.; Na, Y.; Liu, J.; Akermark, B.;
Sun, L. J. Phys. Chem. B 2008, 112, 8198–8202.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9663
cation. Other sacrificial donors were tried in the system,
which have been shown to undergo electron transfer
reactions with the PTZ radical cation. These include
diisopropylethylamine and ascorbate.79,92 However, sys-
tems employing these sacrificial donors were not ob-
served to generate hydrogen upon irradiation.
Conclusions
Two new platinum(II) complexes, [Pt(tpy-phen-COOH)-
(CtC-C6H5)]Cl (1) and [Pt(tpy-COOH)(CtC-C6H5)]Cl
(2) have been prepared and characterized. These were used as
sensitizers in a system with platinized titanium dioxide
nanoparticles for the photogeneration of hydrogen. The
ability of these complexes to act as sensitizers in this capacity
is limited in the long term by decomposition upon oxidation.
When compared to the related chromophore [Pt(ttpy)-
(CtC-C6H5)]PF6 (5) which does not possess an anchoring
group for attachment to TiO2, the unbound chromophore
achieveda greater numberof turnovers than 1 or 2, consistent
with the notion that 5 undergoes reductive quenching by
TEOA to a greater extent than either 1 or 2. Instead, 1 and 2
by virtue of their attachment to TiO2 undergo oxidative
quenching with electron injection into TiO2 preferentially.
To probe this hypothesis, the concentration of the sacrificial
donor was increased, and this modification led to a greater
number of turnovers of H2. In further efforts to address
the problem of oxidative instability, D-C dyads [Pt(tpy-
phen-COOH)(CtC-C6H4CH2-PTZ)]PF6 (3) and [Pt(tpy-
COOH)(CtC-C6H4CH2-PTZ)]Cl (4) were prepared
for use as sensitizers in the photogeneration of hydrogen.
Despite evidence of rapid reductive quenching of the
excited state of the chromophore and of an electron transfer
reaction occurring between phenothiazine radical cation
and triethanolamine, these dyads were unable to function
for hydrogen production. There is some evidence that this
may be due to an unfavorable orientation on the surface of
TiO2, although it may also be due to slow kinetics of the
reaction between triethanolamine and the phenothiazine
radical cation.
Figure 11. Depiction of the possible orientations of an adsorbed plati-
num species in the absence (left) and presence (right) of coadsorbate, and
the effect on displacement of the chloride ligand by an incoming nucleo-
phile, X.
Support for this hypothesis is provided in observations
that involve ligand exchange reactions with [Pt(tpy-phen-
COOH)Cl]PF6 bound to the surface of TiO2. In fluid
solution, exchange of the chloride ligand for a thiophe-
nolate is very rapid at room temperature, and is accom-
panied by a rapid color change from yellow to dark
purple. Similar observations have been made of other
platinum(II) terpyridyl chloride complexes.90,91
However, when anchored to TiO2 at ∼20% monolayer
coverage (20% = 10 μmol/g TiO2),81 there is no evidence
of color change upon the addition of a solution of sodium
thiophenolate to the material, even after prolonged stir-
ring. However, if benzoic acid or 4-tert-butyl-benzoic
acid is added as a coadsorbate on the TiO2 surface (50
μmol acid/g TiO2), a color change (yellow to purple) is
observed upon the addition of thiophenolate. It seems
likely that the addition of the coadsorbate forces the
bound complex PtCl(tpy-phen-COO-)þ to adopt a geo-
metry more perpendicular to the TiO2 surface, thereby
enabling the ligand exchange, which may occur through
an associative mechanism as illustrated below
(Figure 11).
With this result, attempts were made to utilize coad-
sorbates to manipulate the geometry of the D-C dyads on
the surface, to enable the photocatalysis. However, even
with coadsorbates added, neither of the dyads were found
to be capable of producing hydrogen in the system.
Another hypothesis for the inability of 3 and 4 to
promote the photogeneration of hydrogen may reside in
the slow kinetics of charge transfer between the triethanol-
amine and the PTZ radical cation. No definitive rate
constants have yet been determined for the electron
transfer between trialkylamines and the PTZ radical
Acknowledgment. We wish to thank the Division of
Chemical Sciences, Geosciences, and Biosciences, Office
of Basic Sciences of the U.S. Department of Energy
for financial support of this research (DE-FG02-
90ER14125).
Supporting Information Available: Plots of the absorption
spectra of the Pt chromophores; cyclic voltammograms of 1a
and 2a; mass spectrometric data for 1-4. This material is
available free of charge via the Internet at http://pubs.acs.org.
(90) Howe-Grant, M.; Wu, K. C.; Bauer, W. R.; Lippard, S. J. Bio-
chemistry 1976, 15, 4339–4346.
(91) Chen, W. H.; Reinheimer, E. W.; Dunbar, K. R.; Omary, M. A.
Inorg. Chem. 2006, 45, 2770–2772.
(92) Pelizzetti, E.; Meisel, D.; Mulac, W. A.; Neta, P. J. Am. Chem. Soc.
1979, 101, 6954–6959.