Photoinduced electron transfer in platinum(II) terpyridyl acetylide chromophores: Reductive and oxidative quenching and hydrogen production

Article abstract of DOI:10.1021/jp072187n

A series of luminescent platinum(II) terpyridyl acetylide complexes, ([Pt(tpy)(C≡CPh)]ClO₄ (1) and [Pt-(ttpy)(C≡C-p-C ₆H₄R)]ClO₄, where tpy = terpyridine, ttpy = 4′-p-tolylterpyridine, R = H, Cl, Me) (2-4) were studied with regard to excited-state quenching by dialkylated bipyridinium cations as electron acceptors and triethanolamine (TEOA) as an electron donor and the photogeneration of hydrogen from systems containing the chromophore, the dialkylated bipyridinium cations, TEOA, and colloidal Pt as a catalyst. The dialkylated bipyridinium cations include methyl viologen (MV²⁺) and a series of diquats prepared from 2,2′-bipyridine or 4,4′-dimethyl-2, 2′-bipyridine. The quenching rates for the diquats for one of the chromophores (2) are close to the diffusion-controlled limit. The most effective electron acceptor and relay for hydrogen evolution has been found to be 4,4′-dimethyl-1,1′-trimethylene-2,2′-bipyridinium (DQ4) which on photoreduction by the chromohore provides the strongest reducing agent of the diquats studied. The rate of hydrogen evolution depends in a complex way on the concentration of the bipyridinium electron relay, increasing with concentration at low concentrations and then decreasing at high concentrations. The rate of H₂ photogeneration also increases with TEOA concentration at low values and eventually reaches a plateau. The most effective system examined to date consists of the chromophore 2 (2.2 × 10^-5 M), DQ4 (3.1 × 10^-4 M), TEOA (2.7 × 10^-2 M), and Pt colloid (6.0 × 10^-5 M), and has produced 800 turnovers of H ₂ (67% yield based on TEOA as sacrificial electron donor) after 20 h of photolysis with λ > 410 nm.

Full text of DOI:10.1021/jp072187n

J. Phys. Chem. B 2007, 111, 6887-6894
6887
Photoinduced Electron Transfer in Platinum(II) Terpyridyl Acetylide Chromophores:
Reductive and Oxidative Quenching and Hydrogen Production†
Pingwu Du, Jacob Schneider, Paul Jarosz, Jie Zhang, William W. Brennessel, and
Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
ReceiVed: March 19, 2007; In Final Form: April 5, 2007
A series of luminescent platinum(II) terpyridyl acetylide complexes, ([Pt(tpy)(CtCPh)]ClO4 (1) and [Pt-
(ttpy)(CtC-p-C6H4R)]ClO4, where tpy ) terpyridine, ttpy ) 4′-p-tolylterpyridine, R ) H, Cl, Me) (2-4)
were studied with regard to excited-state quenching by dialkylated bipyridinium cations as electron acceptors
and triethanolamine (TEOA) as an electron donor and the photogeneration of hydrogen from systems containing
the chromophore, the dialkylated bipyridinium cations, TEOA, and colloidal Pt as a catalyst. The dialkylated
bipyridinium cations include methyl viologen (MV2+) and a series of diquats prepared from 2,2′-bipyridine
or 4,4′-dimethyl-2,2′-bipyridine. The quenching rates for the diquats for one of the chromophores (2) are
close to the diffusion-controlled limit. The most effective electron acceptor and relay for hydrogen evolution
has been found to be 4,4′-dimethyl-1,1′-trimethylene-2,2′-bipyridinium (DQ4) which on photoreduction by
the chromohore provides the strongest reducing agent of the diquats studied. The rate of hydrogen evolution
depends in a complex way on the concentration of the bipyridinium electron relay, increasing with concentration
at low concentrations and then decreasing at high concentrations. The rate of H2 photogeneration also increases
with TEOA concentration at low values and eventually reaches a plateau. The most effective system examined
to date consists of the chromophore 2 (2.2 × 10-5 M), DQ4 (3.1 × 10-4 M), TEOA (2.7 × 10-2 M), and Pt
colloid (6.0 × 10-5 M), and has produced 800 turnovers of H2 (67% yield based on TEOA as sacrificial
electron donor) after 20 h of photolysis with λ > 410 nm.
Introduction
SCHEME 1
Artificial photosynthesis and the conversion of sunlight into
stored chemical energy in the form of fuels represent one of
science’s great challenges in this century.1 The particular
reaction of paramount importance in this regard is water splitting
into its constituent elements with hydrogen as the fuel. The
subsequent, efficient recombination of H2 and O2 through a fuel
cell then represents an ideal way in which to generate usable
energy in a completely carbon-free way. The development of
an artificial photosynthetic system for water splitting has been
tackled over the last three decades in numerous ways, beginning
with Fujishima and Honda’s seminal work2 on water splitting
over irradiated large band gap semiconductors and the reports
by Lehn,3 Kagan,4 and Gra¨tzel5,6 on a molecular approach to
the problem, focusing on H2 generation from aqueous protons
and a sacrificial electron source. The latter are thus detailed
molecular approaches to the reductive side of the water splitting
reaction.
In the systems pioneered by Lehn, Kagan, and Gra¨tzel, an
approach represented in Scheme 1 was followed in which Ru-
(bpy)32+ was employed as a visible light photosensitizer, methyl
viologen (MV2+) served as an oxidative quencher and electron
relay, triethanolamine (TEOA) or ethylenediaminetetraacetic
acid (EDTA) functioned as the electron source, and a colloidal
Pt catalyst was used to generate the hydrogen. During the
ensuing three decades, different steps and components of the
various systems have been investigated in detail. For example,
sensitizers have been extensively varied ranging from d6 Ru-
(II) trisdiimine complexes to Zn(II) porphyrins,7-12 cyclometa-
lated [Ir(CˆN)2(NˆN)]Cl complexes,13,14 methyl acridine or-
ange,15 and conjugated polymers.16 Electron relays have been
modified and extended to include other viologens,17,18 various
alkylated pyridines and diquats,19-21 Rh diimine complexes,3
cobalt(II) complexes,22,23 and semiconductor particles.24,25 Other
system components have been studied as well.26-28
Solution-luminescent platinum(II) terpyridyl acetylide com-
plexes were first reported in 2001 by Yam and co-workers.29
Like the analogous Pt diimine bis(acetylide) complexes,30-33
3
these cationic systems possess a MLCT emissive state and
undergo electron-transfer quenching with both donor and
acceptor molecules. Since the initial report, papers from different
laboratories have appeared dealing with these complexes and
related derivatives with respect to (a) vapochromic behavior for
selected volatile organic compounds,34 (b) sensing capabilities
for simple inorganic cations29,35,36 and (c) photocatalysis of
hydrogen generation from Hantsch dihydropyridines.37 In 2006,
we reported that complex 2 can be employed as part of a
multiple component system containing a chromophore, a
sacrificial electron donor, an electron relay, and a colloidal
catalyst for the light-driven generation of H2 from aqueous
protons. The system has both advantages and disadvantages from
previously reported multiple component mixtures for light driven
hydrogen generation. In this paper, we describe details and
† Part of the special issue “Norman Sutin Festschrift”.
10.1021/jp072187n CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/14/2007

6888 J. Phys. Chem. B, Vol. 111, No. 24, 2007
Du et al.
TABLE 2: Photophysical Data of the Chromophores in
Acetonitrile/Water (2:3, v/v) at Room Temperature
λ
abs (MLCT)
τ
λem
(nm)
Eoo
(eV)
cmpd
(ꢀ/dm3mol-1cm-1
)
(µs)a
φem
1
2
3
4
430 (4980)
420 (7700)
424 (6430)
1.9
4.6
4.7
0.8
597
609
591
625
0.010
0.025
0.024
0.004
2.46
2.48
2.48
2.46
433, 461 (5165)
a Taken from ref 38.
TABLE 3: Excited State Energy, Rate of Quenching, and
Electrochemical Dataa of the Pt Chromophores
b
kq (by TEOA)b kq (by MV2+
)
cmpd × 109 M-1 s-1 × 109 M-1 s-1
E2+/+
E+/0
1
2
3
4
2.4
1.4
1.4
1.5
5.1
3.3
3.4
0.4
+1.49c -0.63, -1.16d
+1.34c -0.59, -1.07d
+1.41c -0.61, -1.09e
+1.50c -0.59, -1.05e
a Electrochemical data were obtained in degassed DMF solution with
0.10 M nBu4NPF6 (TBAH) as supporting electrolyte at room temper-
ature; scan rate 100 mV s-1. Potentials (vs NHE) converted from
internal Fc+/0 using ref 55. b Measured in acetonitrile/water (2:3 v/v).
c Irreversible oxidation peak estimated vs NHE. d Reversible reductions
measured with Fc+/0 as an internal standard and adjusted to NHE.
e Taken from ref 38.
Figure 1. Perspective drawing of complex 1 with atomic numbering
scheme.
TABLE 1: Selected Bond Lengths (Å) and Angles (deg) for
Complex 1
Pt(1)-N(2)
1.9652(15) Pt(1)-N(3)
2.0231(15) Pt(1)-C(1)
2.0211(15)
1.9734(19)
1.379(2)
1.345(2)
1.381(2)
80.65(6)
80.70(6)
161.35(6)
127.66(12)
Pt(1)-N(1)
well to values of 1.977(6)39 and 1.98(1)40 for [Pt(tpy)(CtC-
C6H4-p-N(CH3)2)]BF4 and [Pt(tpy)(CtCPh)]PF6, respectively.
The crystal structure of 1 shows the cations pack in pairs with
head-to-tail stacking and a nearest intermolecular Pt‚‚‚Pt distance
of 3.4143(2) Å.
N(1)-C(9)
1.345(2)
1.345(2)
1.345(2)
178.74(7)
99.78(7)
98.87(7)
N(1)-C(13)
N(2)-C(18)
N(2)-C(14)
N(3)-C(23)
N(3)-C(19)
N(2)-Pt(1)-C(1)
C(1)-Pt(1)-N(3)
C(1)-Pt(1)-N(1)
N(2)-Pt(1)-N(3)
N(2)-Pt(1)-N(1)
N(3)-Pt(1)-N(1)
Steady-State Absorption and Emission Spectra. The
electronic spectra of 1-4 have been reported previously in
dichloromethane solutions.38 However, since all of the reactions
for light-driven hydrogen production presented here were carried
out in a 2:3 acetonitrile/water mixture, the steady-state absorp-
tion and emission spectra of the chromophores were remeasured
in this solvent system (see Figures 2 and 3). As in dichlo-
romethane solutions, all of the complexes exhibit vibronic-
structured absorption bands at high energy (λ < 350 nm) with
extinction coefficients (ꢀ) on the order of 104 dm3 mol-1 cm-1
that are attributable to intraligand transitions.35,36,38 A less
intense, solvatochromic band corresponding to a dπ(Pt)-π*-
(terpy) metal-to-ligand charge transfer (MLCT) transition is seen
for all of the complexes in the range of 375-520 nm with ꢀ on
the order of 103 dm3 mol-1 cm-1. For 2 as an example, Figure
2 reveals that the MLCT absorption maximum in acetonitrile/
water occurs at 23 800 cm-1, which is ca. 700 cm-1 higher in
energy than that in methylene chloride (23 100 cm-1). Similar
solvatochromism of the MLCT band has been reported previ-
ously for the Pt(diimine)(CtCAr)2 complexes30 and related Pt-
(diimine)(dithiolate) complexes,43 consistent with a highly polar
ground state, an MLCT state of reduced (and possibly inverted)
polarity, and increasing solvent stabilization of the former with
increasing solvent dielectric constant.
C(9)-N(1)-C(13) 119.05(15) C(9)-N(1)-Pt(1)
C(13)-N(1)-Pt(1) 113.28(12) C(18)-N(2)-C(14) 123.69(16)
C(18)-N(2)-Pt(1) 118.24(12) C(14)-N(2)-Pt(1) 117.95(12)
aspects of the earlier report and expand the scope of the study
through variation of the chromophore, the electron relay, the
concentration of sacrificial donor (TEOA) and the concentration
of electron relay (diquats). The results lead to conclusions about
different steps in the overall energy storing reaction and allow
comparison between different systems used to carry it out.
Results and Discussion
The Platinum Terpyridyl Acetylide Chromophore: Struc-
tural Characterization. The synthesis and solution character-
ization of all of the Pt terpyridyl acetylide chromophores used
in the present study (1-4) have been described in previous work,
as have their absorption and emission spectra.38 The molecular
structure of 1 obtained from a crystal grown by acetonitrile
(MeCN) evaporation is shown in Figure 1 with important bond
lengths and angles summarized in Table 1. A complete
tabulation of distances and angles is found in the Supporting
Information in CIF format. As anticipated, the Pt(II) coordination
geometry is a square planar with modest distortion imposed by
the constraint of the terpyridyl ligand (the cis N-Pt-N angles
average 80.67 (6)° and the one trans N-Pt-N angle is 161.35
(6)°). For the same reason, the bond distance from Pt to the
central terpy N atom (Pt-N2 ) 1.9652(15) Å) is slightly shorter
than that to the other two nitrogen atoms (Pt-N1 ) 2.0231-
(15) Å, Pt-N3 ) 2.0211(15) Å), which is consistent with the
corresponding bond distances in [Pt(tpy)(CtC-C6H4-p-N(CH3)2)]-
BF4.39 The Pt-N2 distance of 1.9652(15) Å agrees well with
the values of 1.956(4),39 1.969(5),35 1.97(10),40 1.961(13),41 and
1.935(6) Å42 in other related platinium(II) alkynyl complexes,
while the Pt-C (acetylide) distance of 1.9734(19) Å corresponds
The [Pt(terpyridyl)(acetylide)]+ complexes 1-4 exhibit strong
photoluminescence with λmax ranging from 595 to 625 nm and
relative emission quantum yields, φem, given in Table 2.
Complexes 2 and 3 give higher quantum yields, 0.025 and 0.024,
respectively, than complexes 1 and 4. The values of φem were
obtained using [Ru(bpy)3](PF6)2 in degassed MeCN as the
reference solution (φ ) 0.062).44 The excited-state energy of
the 3MLCT emissive state, Eoo, can be estimated most directly
from the onset of the emission at 77 K.45 In this way, complexes
2 and 3 were found to have the same Eoo of 2.48 eV while 1
and 4 were determined to have Eoo of 2.46 eV. These values

Photoinduced Electron Transfer in Chormophores
J. Phys. Chem. B, Vol. 111, No. 24, 2007 6889
Figure 2. Absorption spectra of the Pt chromophores in acetonitrile/water (v/v ) 2:3) at 1.0 × 10-5 M.
Figure 3. Normalized emission spectra of the Pt chromophores in acetonitrile/water (v/v ) 2:3) at 1.0 × 10-5 M.
compare well with Eoo estimates obtained in the same way for
Pt(diimine)(arylacetylide)2 complexes having similar substitu-
ents. The excited-state energies Eoo of the latter systems were
also estimated independently by Whittle et al., using a single
mode analysis,33 yielding values that, while 5-7% lower, were
essentially in agreement given the estimated errors on these
measurements.
The reductive quenching of 3 in acetonitrile has recently been
studied by Schmehl and co-workers47 who determined a good
estimate of its excited-state reduction potential as +1.4 V vs
SCE (+1.6 V vs NHE) using a series of reductive quenchers
including triphenylamine (TPA), diphenylamine (DPA), triethy-
lamine (TEA), and diisopropylamine (DIPA), diethyl-1,4-
dihydro-2,6-dimethyl-3,5-pyridinecarboxylate (DHP), N,N,N′,N′-
tetramethylphenylenediamine (TMPD), N-methylphenothiazine
(MPTH) and N,N,N′,N′-tetramethylbenzidine. The determined
value for the excited-state reduction potential indicates that 3
and the other chromophores are strong excited-state oxidants.
Because 1-4 undergo irreversible oxidations, only an estimate
of the excited-state oxidation potential can be made using the
value of the observed oxidation wave and the excited-state
energy Eoo. When written as a formal reduction, the estimate
Reductive Quenching. Complexes 1-4 were all found to
undergo reductive quenching by triethanolamine (TEOA) which
had been used as a sacrificial electron donor in earlier reported
Ru(bpy)32+-based systems for light-driven hydrogen generation
(Table 3). For all of the chromophores, quenching studies were
conducted by measurements of steady-state emission intensities.
The quenching of eq 1 is dynamic in nature and was well
modeled by the Stern-Volmer equation46 (eq 2), where Io is
the integrated MLCT emission intensity in the absence of
quencher, τo is the excited-state lifetime in the absence of
quencher, I and τ are the corresponding values in the presence
of quencher, kq is the bimolecular quenching rate constant, and
[Q] is the quencher concentration. Linear plots of Io/I vs [Q]
together with excited-state lifetimes for the Pt chromophores
yielded values of the quenching rate constants kq that for 2, 3,
and 4 were approximately 1.4 × 109 M-1 s-1 and for 1 was
for the excited-state potential E (complex2+ +*
mophores 1-4 are -0.97, -1.14, -1.07, and -0.96 V,
/ ) for chro-
respectively, versus NHE where the potential E (complex2+
/
+*) ) E (complex2+/+) - (the excited-state energy Eoo in eV).
Please note that no correction is given for the work term in this
simple estimation.
The related Pt(diimine)(CtCAr)2 complexes are also known
to undergo reductive electron-transfer quenching, and they have
also been studied quantitatively in this regard.32,48 In the case
of Pt(dbbpy)(CtCC6H5)2, for example, 10-methylphenothiazine
and N,N,N′,N′-tetramethylbenzidine were found to give es-
sentially diffusion-controlled kq values of 9.6 × 109 M-1 s-1
and 1.3 × 1010 M-1 s-1, respectively. Phenothiazine and N,N′-
dimethylaniline gave similar results. It is noteworthy that the
slightly greater at 2.4 × 109 M-1 s-1
.
*[Pt(terpyridyl)(acetylide)]+ + TEOA f
*[Pt(terpyridyl)(acetylide)] + TEOA+ (1)
Io/I ) τo/τ ) 1 + kqτ[Q] (2)

6890 J. Phys. Chem. B, Vol. 111, No. 24, 2007
Du et al.
Figure 5. Comparison of hydrogen production using different chro-
mophores at 2.2 × 10-5 M (together with 3.1 × 10-4 M MV2+, 5.6 ×
10-3 M TEOA, phosphate pH 7 buffer, and 6.0 × 10-5 M Pt colloid).
Figure 4. Formation of MV+• upon irradiation by visible light in the
presence of 2.2 × 10-5 M 2, 3.1 × 10-4 M MV2+, and 6.0 × 10-5
M
Pt colloid.
SCHEME 2
quenching rate constants for these Pt chromophores is around
3 orders of magnitude faster with these quenchers than are found
2+
with Ru(bpy)3 (6.5 × 106 M-1 s-1).5
Oxidative Quenching. We previously reported that 2 un-
dergoes oxidative quenching by MV2+ with a rate constant kq
of 3.3 × 109 M-1 s-1 in acetonitrile/water (3:2). This result
was in accord with earlier reported studies of related Pt diimine
bis(acetylide) complexes that exhibit oxidative as well as
reductive quenching of their respective excited states. In those
earlier studies,30 the rates of quenching using 4-nitrobenzalde-
hyde and dinitrobenzene were essentially diffusion controlled
with kq values of 1.0 × 1010 M-1 s-1, while the rate using
nitrobenzene was slightly slower with a kq value of 5.2 × 108
M-1 s-1
.
Hydrogen Production Using Platinum(II) Terpyridyl
Acetylide Chromophores. In a preliminary communication, we
reported that photolysis of [Pt(ttpy)(CtCPh)]ClO4 (2) in MeCN/
H2O in the presence of MV2+, TEOA, and colloidal Pt leads to
the photogeneration of H2.49 With a cutoff filter that allows only
λ > 410 nm, initial results yielded up to 84 turnovers and a
34% yield based on the initial amount of TEOA used as the
electron donor. The reported yield is a lower limit estimate since
unreacted TEOA remains in solution, as determined by liquid
chromatography-mass spectrometry (LCMS). This multiple
component system closely paralleled earlier ones based on Ru-
(II) tris diimine sensitizers,5,6 but with some mechanistic
differences. The success of 2 as a chromophore for the
photogeneration of H2 is thought to relate to the long-lived (4.6
In the present study, the potential electron relays DQ1-DQ4
in addition to MV2+ were examined. Specifically, these dica-
tionic diquat systems were found to give essentially diffusion
controlled quenching rates of ∼1.0 × 1010 M-1 s-1 with
complex 2. While the oxidative quenching of complexes 1, 3-4
was not examined quantitatively for DQ1-DQ4 in the present
study, similar quenching rate constants may be expected on the
basis of the similar reaction chemistry seen in subsequent
studies, particularly with regard to hydrogen generation (vide
infra).
Photoreduction of MV2+. While MV2+ is an effective
quencher of chromophores 1-4, the characteristic blue color
of MV+• is not observed during simple quenching experiments,
presumably because of rapid back-electron transfer. However,
when the sacrificial electron donor TEOA is also present in the
photolysis solutions, the blue color of MV+• develops within 2
min and continues to increase in intensity up to a limiting value
after ca. 50 min. Time dependence of the MV+• concentration
in a system containing 5.6 × 10-3 M TEOA, 2.2 × 10-5 M of
2 and 3.1 × 10-4 M MV2+ under visible light irradiation (λ >
410 nm) is shown in Figure 4. When irradiation is stopped, the
blue color persists under an inert atmosphere but dissipates upon
exposure of the solution to air within 1-2 min. The persistence
of MV+• in solution as judged by the solution’s blue color when
TEOA is present is due to the fact that upon oxidation, TEOA
undergoes decomposition leading to di(ethanol)amine and
glycolaldehyde, along with transfer of a second, highly reducing
3
µs) MLCT excited-state of 2, its energy (2.48 eV, Table 2),
and its ability to undergo both oxidative and reductive quench-
ing. The reduction potentials of excited-state redox processes
for 2 can be estimated as -1.14 and +1.89 V (vs NHE) for
oxidation and reduction, respectively. From quenching studies
using a graded series of electron-donor quenchers, Schmehl has
shown that 3 is a strong excited-state oxidant, albeit at a slightly
lower excited-state reduction potential (ca. +1.4 V vs SCE or
1.65 V vs NHE).47
In the present study, the scope of the earlier work has been
enlarged. A series of experiments using chromophores 1-4 have
been carried out employing similar irradiation conditions (λ >
410 nm) and the following concentrations: chromophore, 2.2
× 10-5 M; TEOA, 5.6 × 10-3 M; MV2+, 3.1 × 10-4 M; and
colloidal Pt catalyst, 6.0 × 10-5 M in acetonitrile/water (2:3
v/v). Quantitative determination of generated H2 was conducted
by GC analysis with added methane as an internal standard.
Figure 5 shows the time dependence of hydrogen production
in this reaction system for the different Pt chromophores. The
plot shows that hydrogen generation follows the order 2 > 1 >
3 > 4, which is dependent on both the absorptivities of the
chromophores and their quenching rate constants. The molar
electron to another MV2+
.
When MV2+ as the oxidative quencher and electron relay is
replaced by the diquats DQ1-DQ4 (Scheme 2), similar
quenching is observed with a decrease in the photoluminescence
of 2, and in the presence of TEOA, the formation of persistent
solution colors characteristic of the corresponding diquat radical
cations (green, tan, yellow-green, and pink for DQ1-DQ4,
respectively).

Photoinduced Electron Transfer in Chormophores
J. Phys. Chem. B, Vol. 111, No. 24, 2007 6891
Figure 6. Comparison of hydrogen production using different diquat electron relays at 3.1 × 10-4 M (together with 2.2 × 10-5 M 2, 5.6 × 10-3
M TEOA, phosphate pH 7 buffer, and 6.0 × 10-5 M Pt colloid).
Figure 7. Comparison of hydrogen production under different concentration of DQ4 (together with 2.2 × 10-5 M 2, 5.6 × 10-3 M TEOA,
phosphate pH 7 buffer, and 6.0 × 10-5 M Pt colloid).
TABLE 4: Quenching of 2 by Different Diquat Electron
Relays Acetonitrile/Water (2:3, v/v)
No such correlation, however, exists between the quenching
rate constants and the rates of H2 generation. The excited-state
oxidation potential of 2 estimated above as -1.14 V vs NHE
from E00 and the irreversible oxidation found for the complex
by cyclic voltammetry indicates that the excited chromophore
2* is capable of reducing all of the diquats. It is evident,
however, that factors other than simple driving force dependence
go into determining the quenching rates in light of the absence
of such a correlation. If in fact the driving force dependence of
the quenching reaction were the dominant factor in determining
the rate of H2 production, the order shown in Figure 6 would
be reversed. Instead, it appears that both the effectiveness of
DQ4 and the observed order for H2 production for the diquats
relate to the reducing ability of the corresponding radical cation
(DQ4+• is most reducing and DQ1+• is least). This suggests
that electron transfer from the radical cation of the electron relay
to the Pt colloid may in fact be a limiting step in the system.
E1/2
(V vs NHE)
kq (by 2)
∆E°
cmpd
(× 109 M-1 s-1
)
Va
MV2+
DQ1
DQ2
DQ3
DQ4
-0.44
-0.37
-0.55
-0.65
-0.70
3.3
10.0
7.1
6.1
7.0
0.67
0.74
0.56
0.46
0.41
a Electrochemical driving force for the quenching reaction estimated
using an excited-state oxidaiton potential of -1.14 V.
extinction coefficients (ꢀ) of the MLCT maxima of these
complexes exist in the order of 2 > 1 > 3 ≈ 4. The observation
that 4 is least effective for hydrogen generation may relate to
the smallest quenching rate constant it has for oxidative
quenching by MV2+ of the four chromophores. The slight
induction periods in Figure 5 relate to charging of the Pt colloid
through transfer of multiple electrons before the onset of H2
generation as well as to instrument sensitivity at low H2 levels.
In a second series of experiments, the nature of the electron
relay was probed by using different diquats (DQ1-DQ4) in
place of MV2+. These experiments were all conducted using 2
as the chromophore with similar concentrations and conditions
to those given above in the previous experiments. The results
in terms of H2 production are shown in Figure 6. The rate of
hydrogen generation is greatest for DQ4 with the overall order
being DQ4 > DQ2 ≈ DQ3 > MV2+> DQ1. In Table 4 both
the reduction potential and the quenching rate constant with 2
are shown for each electron relay. There exists an obvious
correlation between the reduction potentials of DQ1-DQ4 and
MV2+ and the respective rates of H2 photogeneration. DQ4,
which is the most difficult to reduce, exhibits the fastest rate of
H2 generation, while DQ1, which is most easily reduced, shows
the slowest rate.
In a study of the effect of viologen and diquat electron relays
on the photogeneration of H2 using Zn(II) meso-tris(sul-
fonatophenyl)porphyrin (Zn(TPPS3) as the sensitizer, Okura et
al., found that the largest rates of H2 production were obtained
with diquats that were most difficult to reduce, as we find in
the present study.9 The reaction conditions in Okura’s study
were adjusted so that an excess of the hydrogen evolving catalyst
(hydrogenase) was present to make the photoreduction of the
electron relay rate-determining, but further examination showed
that the competitive back reaction and forward electron transfer
to the hydrogenase catalyst were clearly involved in determining
the effectiveness of the system for the photogeneration of H2.
The results we describe in this paper, while in accord with those
earlier conclusions, show even more clearly that the rate of H2
photogeneration correlates most closely with the reducing power
of the diquat radical anion generated by quenching or a
subsequent reduction.

6892 J. Phys. Chem. B, Vol. 111, No. 24, 2007
Du et al.
but when TEOA is also present, the color of the respective
radical cation appears within minutes. When a colloidal platinum
catalyst is also added to the system, hydrogen is evolved. The
most effective electron acceptor and relay for hydrogen evolu-
tion has been found to be DQ4 (4,4′-dimethyl-1,1′-trimethylene-
2,2′-bipyridinium) which is also the hardest to reduce and which
on photoreduction provides the strongest reducing agent. The
rate of hydrogen evolution depends in a complex way on the
concentration of the bipyridinium electron relay, increasing with
concentration at low concentrations and then decreasing at high
concentrations. The rate of H2 photogeneration also increases
with TEOA concentration at low values and eventually reaches
a plateau. The most effective system examined to date consists
of the chromophore 2 (2.2 × 10-5 M), DQ4 (3.1 × 10-4 M),
TEOA (2.7 × 10-2 M), and Pt colloid (6.0 × 10-5 M) in pH 7
acetonitrile/water (2:3 v/v) adjusted with concentrated HCl, and
has produced 800 turnovers of H2 (67% yield based on TEOA
as sacrificial electron donor) after 20 h of photolysis with λ >
410 nm.
Figure 8. Comparison of hydrogen production under different
concentrations of TEOA (together with 2.2 × 10-5 M 2, 3.1 × 10-4
M
MV2+, phosphate pH 7 buffer, and 6.0 × 10-5 M Pt colloid).
In the preliminary communication describing part of the
present study, it was noted that the rate of H2 evolution in
controlled photolysis varied in nonmonotonic ways for both
MV2+ concentration and solution pH. The results were explained
based on a reaction scheme in which MV2+ and proton-transfer
entered into two or more elementary steps. Examination of the
concentration dependence of the most effective diquat, DQ4,
was also carried out and is shown in Figure 7. While the rate
of H2 evolution increases with increasing DQ4 concentration
below 1 × 10-4 M, it appears to reach a limiting value in the
range 3 × 10-4-1.5 × 10-3 M, after which the rate declines
as DQ4 concentration increases further. This finding is similar
to what we reported for MV2+ as the electron relay. Possible
reasons for the decline in H2 production rate at higher
Experimental Section
Chemicals. All of the chromophores were synthesized
following details reported previously49 by the reaction of [Pt-
(R-terpy)Cl]Cl with different arylacetylenes in the presence of
CuI as the catalyst in triethylamine/DMF and were characterized
by 1H NMR and ESI-MS spectroscopies. The perchlorate salts
were obtained by exchange of chloride with potassium perchlo-
rate. Warning: Perchlorate salts of metal complexes are
known to be explosive and should therefore be handled with
extreme care and proper protection. Methyl viologen (MV2+
)
dichloride and triethanolamine (TEOA) were used as purchased
(Aldrich). The diquat salts (DQ1-DQ4) were prepared as
published.54 HPLC grade acetonitrile was purchased from Fisher
and used without further purification.
concentrations of MV2+ included possible association complexes
3+•
such as (MV)2
of diminished reducing ability50,51 and
destructive hydrogenation of the viologen52,53 when colloidal
Pt is employed as the H2 generating catalyst. Both factors may
be at work in the system having DQ4 as the electron relay.
The optimal value of the diquat concentration found for the
system composed of 2/TEOA/DQ4 was found to be 3.1 × 10-4
M at pH 7. With this system and an initial concentration of
TEOA of 5.6 × 10-3 M, 64 turnovers of H2 relative to 2 were
generated after 180 min of irradiation with λ > 410 nm
compared with the corresponding MV2+ system that yielded
32 turnovers.
Physical Measurements. Absorption spectra were recorded
using a Hitachi U2000 scanning spectrophotometer (200-1100
nm). Emission spectra were obtained using a Spex Fluoromax-P
fluorimeter corrected for instrument response. Monochromators
were positioned with a 2-nm band-pass, and solution samples
were degassed by at least three freeze-pump-thaw cycles. The
77 K frozen glass samples (1:4 MeOH/EtOH) were prepared
in NMR tubes using a circular quartz-tipped immersion Dewar
filled with liquid nitrogen for measuring the emission spectra
and estimating the excited-state energy of chromophores. Cyclic
voltammetry experiments were conducted on an EG&G PAR
263A potentiostat/galvanostat using a three-electrode single-
compartment cell. A glassy carbon working electrode, a Pt wire
auxiliary electrode, and a Ag wire reference electrode were used.
For all measurements, samples were dissolved in DMF and
degassed with argon. Tetrabutylammonium hexafluorophosphate
(Fluka) was used as the supporting electrolyte (0.1 M), and
ferrocene (ROC/RIC) was employed as an internal redox
reference. All redox potentials were reported relative to the
ferrocenium/ferrocene (Fc+/Fc) couple (0.45 V vs SCE55 and
SCE vs NHE ) 0.24 V). All scans were done at 100 millivolts
per s.
Examination of the influence of TEOA concentration on the
rate of H2 production was also examined. As shown in Figure
8, the rate increases up to a value of ∼2.7 × 10-2 M, beyond
which the rate levels off, which is comparable to the system
photocatalyzed by Ru(bpy)3 .
2+ 52 When the TEOA concentration
was 0.11 M, photolysis led to more than 300 turnovers, and in
the absence of a phosphate buffer with a TEOA concentration
of 2.7 × 10-2 M, 800 turnovers were achieved with a yield of
67% based on TEOA. A reason for the increased turnover
number in the absence of buffer is unclear and requires testing
but may relate to buffer anion binding to the colloidal Pt surface
thus decreasing the catalyst’s effectiveness.
Crystal Structure Determination of [Pt(tpy)(CtCPh)]-
ClO4. Crystals were grown by slow evaporation of a concen-
trated acetonitrile solution. A crystal (0.32 × 0.32 × 0.28 mm3)
was placed onto the tip of a glass fiber and mounted on a Bruker
SMART APEX II CCD Platform diffractometer for data
collection at 100.0(1) K. A preliminary set of cell constants
and an orientation matrix were calculated from 876 reflections
harvested from three sets of 20 frames. These initial sets of
Conclusions
The platinum(II) terpyridyl acetylide chromophores 1-4
exhibit fast reductive quenching with TEOA and near diffusion-
controlled oxidative quenching with MV2+ and a series of
diquats DQ1-DQ4. In the simple oxidative quenching experi-
ments, no evidence of the photochemically reduced radical
cation of the dialkylated bipyridinium electron acceptor is seen,

Photoinduced Electron Transfer in Chormophores
J. Phys. Chem. B, Vol. 111, No. 24, 2007 6893
frames were oriented such that orthogonal wedges of reciprocal
space were surveyed. The data collection was carried out using
Mo KR radiation (graphite monochromator) with a frame time
of 5 s and a detector distance of 5.03 cm. A randomly oriented
region of reciprocal space was surveyed: four major sections
of frames were collected with 0.50° steps in ω at four different
φ settings and a detector position of -33° in 2θ. The intensity
data were corrected for absorption. Final cell constants were
calculated from the xyz centroids of 3975 strong reflections from
the actual data collection after integration. Additional crystal
and refinement information can be obtained from the CIF file
deposited as Supporting Information.
absence of quencher, τo is the excited-state lifetime in the
absence of quencher, I and τ are the corresponding values in
the presence of quencher, kq is the bimolecular quenching rate
constant, and [Q] is the molar concentration of the quencher.
Acknowledgment. This work is dedicated to Norman Sutin
for his leadership in electron transfer and recognition of the
potential of metal complex excited states for solar energy
conversion. We wish to thank the Division of Basic Sciences,
U.S. Department of Energy for financial support of this research
(Grant DE-FG02-90ER14125).
Supporting Information Available: X-ray crystallographic
data of complex 1 in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
Quenching Experiments. For all of the quenching experi-
ments, stock solutions of the chomophores were made in
acetonitrile/water (2:3 v/v) containing 5 mg of the particular
complex. Samples with known quencher concentrations were
prepared by adding an appropriate amount of solid quencher to
10 mL volumetric flasks containing the diluted Pt chromophore
stock solutions (1.0 × 10-5 M). The solutions were deaerated
by three freeze-pump-thaw cycles. Steady-state luminescence
spectra were then collected for each sample. UV-vis spectra
were taken for each experiment to inspect potential photochemi-
cal products. In the simple bimolecular quenching reactions,
there was no evidence of any change in absorption spectra under
all experimental conditions. However, in the presence of both
TEOA and MV2+ reagents, visible excitation of the Pt chro-
mophore generated the blue methyl viologen radical cation,
which was detected by UV-vis spectra.
References and Notes
(1) Lewis, N. S.; Nocera, A. G. Proc. Natl. Acad. Sci. 2006, 103,
15729-15735.
(2) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38.
(3) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. HelV. Chim. Acta 1979, 62,
1345-1384.
(4) Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. NouV. J. Chim.
1978, 2, 547-549.
(5) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. HelV. Chim. Acta 1978,
61, 2720-2730.
(6) Gra¨tzel, M. Acc. Chem. Res. 1981, 14, 376-384.
(7) Amao, Y.; Hiraishi, T.; Okura, I. J. Mol. Catal. A: Chem. 1997,
126, 21-26.
(8) Okura, I. Coord. Chem. ReV. 1985, 68, 53.
(9) Okura, I.; Kaji, N.; Aono, S.; Kita, T. Inorg. Chem. 1985, 24, 451-
453.
Details of Hydrogen Generating Experiments. For photo-
induced hydrogen evolution, each sample was made in a 50
mL round-bottom flask with a volume of 25 mL in acetonitrile/
(10) Amao, Y.; Kamachi, T.; Okura, I. J. Photochem. Photobiol., A:
Chem. 1996, 98, 59.
(11) Persaud, L.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.;
Webber, S. E.; White, J. M. J. Am. Chem. Soc. 1987, 109, 7309-7314.
(12) Adar, E.; Degani, Y.; Goren, Z.; Willner, I. J. Am. Chem. Soc.
1986, 108, 4696-4700.
water (2:3 v/v). Typically, the sample contained 2.2 × 10-5
M
M
Pt chromophore, 6.0 × 10-5 M Pt colloids, and 3.1 × 10-4
electron carrier. Phosphate buffer (0.20 M) was used to maintain
the pH, and the Pt colloids were stabilized using sodium
polyacrylate polymer.56 The flask was sealed with a rubber
septum and degassed by bubbling nitrogen for 15 min under
atmospheric pressure at room temperature, after which 5 mL
of nitrogen was removed from the flask and replaced with 5
mL of methane (760 Torr) to serve as the internal standard.
The samples were irradiated under a 200 W Mercury Xexon
lamp. A cutoff filter was used to remove light with λ < 410
nm. The amount of H2 generated was determined by GC analysis
using a chromatograph (Shimadzu) with a molecular sieve 5 Å
column (30 m × 0.53 mm) and thermal conductivity detector.
(13) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, J.,
R. A.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712-5719.
(14) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.;
Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502-7510.
(15) Islam, S. D. M.; Yoshikawa, Y.; Fujitsuka, M.; Ito, O.; Komatsu,
S.; Usui, Y. J. Photochem. Photobiol., A: Chem. 2000, 134, 155-161.
(16) Jiang, D. L.; Choi, C. K.; Honda, K.; Li, W. S.; Yuzawa, T.; Aida,
T. J. Am. Chem. Soc. 2004, 126, 12084-12089.
(17) Mandler, D.; Degani, Y.; Willer, I. J. Phys. Chem. 1984, 88, 4366-
4370.
(18) Brugger, P.-A.; Cuendet, P.; Gra¨tzel, M. J. Am. Chem. Soc. 1981,
103, 2923-2927.
(19) Schmalzl, K. J.; Summers, L. A. Aust. J. Chem. 1977, 30, 657-
652.
(20) Amouyal, E.; Zidler, B. Chem. Phys. Lett. 1980, 74, 314-317.
(21) Miller, D.; McLendon, G. Inorg. Chem. 1981, 20, 950-953.
(22) Krishnan, C. V.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Am.
Chem. Soc. 1985, 107, 2005-2015.
(23) Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Matsubara, T.; Sutin,
N. J. Am. Chem. Soc. 1979, 101, 1298-1300.
(24) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141-145.
(25) Gra¨tzel, M. Acc. Chem. Res. 1981, 14, 376-384.
(26) Hiroaki, K.; Toshiya, O.; Kei, O.; Shunichi, F. Phys. Chem. Chem.
Phys. 2007, 9, 1487-1492.
(27) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639-1641.
(28) Esswein, A. J.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc.
2005, 127, 16641-16651.
(29) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K. K.
Organometallics 2001, 20, 4476-4482.
(30) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355-4365.
(31) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000, 208, 115-137.
(32) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.
(33) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S.
Inorg. Chem. 2001, 40, 4053-4062.
(34) Wadas, T. J.; Wang, Q.-M.; Kim, Y.-J.; Flaschenreim, C. N.;
Blanton, B. T.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841-16849.
(35) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.;
Cheung, K.-K. Inorg. Chem. 2001, 40, 571-574.
To investigate concentration effects of the different Pt
chromophores, electron relays, the sacrificial donor, and the
solution pH, one of the reaction components was varied while
the others were held constant. General reaction conditions and
concentrations included 2.2 × 10-5 M Pt chromophore, 5.6 ×
10-3 M TEOA, 6.0 × 10-5 M Pt colloid, 3.1 × 10-4 M MV2+
or diquat, and pH 7. When examining the reductive quenching
effects on H2 production for chromophore 2, concentrations of
TEOA were varied in the range 5.6 × 10-3- 0.11 M. Likewise,
for oxidative quenching effects on H2 production for Pt
chromophore 2, concentrations of the electron relay DQ4 were
varied in the range 1 × 10-4- 6.2 × 10-3 M.
Determination of Rate Constants from Quenching Experi-
ments. In all samples, quenching rate constants (kq) were
evaluated from steady-state luminescence experiments. Addition
of quenchers to the Pt(II) compounds resulted in concentration-
dependent luminescence intensity quenching. Bimolecular quench-
ing was well-modeled by the Stern-Volmer equation46 (eq 2),
where Io is the integrated MLCT emission intensity in the

6894 J. Phys. Chem. B, Vol. 111, No. 24, 2007
Du et al.
(36) 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.
(37) Zhang, D.; Wu, L. Z.; Zhou, L.; Han, X.; Yang, Q.-Z.; Zhang,
L.-P.; Tung, C.-H. J. Am. Chem. Soc. 2004, 126, 3440-3441.
(38) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653-5655.
(46) Vincze, L.; Sandor, F.; Pem, J.; Bosnyak, G. J. Photochem.
Photobiol. 1999, 120, 11-14.
(47) Narayana-Prabhu, R.; Schmehl, R. H. Inorg. Chem. 2006, 45,
4319-4321.
(48) Wadas, T. J.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2003,
42, 3772-3778.
(49) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726-7727.
(50) Kosower, E. M.; Teuerstein, A. J. Am. Chem. Soc. 1973, 95, 6127-
6128.
(51) Itoh, M.; Kosower, E. M. J. Am. Chem. Soc. 1967, 89, 3655-
3656.
(52) Keller, P.; Moradpour, A. J. Am. Chem. Soc. 1980, 102, 7193-
7196.
(53) Ebbesen, T. W. J. Phys. Chem. 1984, 88, 4131-4135.
(54) Homer, R. F.; Tomlinson, T. E. J. Chem. Soc. 1960, 2498-2503.
(55) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(56) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed,
M. A. Science 1996, 272, 1924-1925.
(39) Yang, Q.-Z.; Tong, Q.-X.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.;
Tung, C.-H. Eur. J. Inorg. Chem. 2004, 1948-1954.
(40) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476-4482.
(41) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002,
124, 6506-6507.
(42) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg.
Chem. 1999, 38, 4262-4267.
(43) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118,
1949-1960.
(44) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T.
D.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627.
(45) Turro, N. J. Modern Molecular Photochemistry; University Press:
Menlo Park, CA, 1978.