Photocatalytic hydrogen evolution with a self-assembling reductant-sensitizer-catalyst system

Article abstract of DOI:10.1002/chem.201300133

A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by nanosecond laser flash photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequencies (TOF=10.8 or 3.6 min^-1, relative to the sensitizer or the catalyst, respectively) and the quantum yield (Φ=4.6 %, that is, 9.2 % of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst. Copyright

Full text of DOI:10.1002/chem.201300133

FULL PAPER
DOI: 10.1002/chem.201300133
Photocatalytic Hydrogen Evolution with a Self-Assembling
Reductant–Sensitizer–Catalyst System
Mirco Natali,*[a] Roberto Argazzi,[b] Claudio Chiorboli,[b] Elisabetta Iengo,*[c] and
Franco Scandola*[a]
Abstract: A noble-metal-free system
for photochemical hydrogen produc-
tion is described, based on ascorbic
acid as sacrificial donor, aluminium
pyridyl porphyrin as photosensitizer,
and cobaloxime as catalyst. Although
the aluminium porphyrin platform has
docking sites for both the sacrificial
donor and the catalyst, the resulting as-
sociated species are essentially inactive
because of fast unimolecular reversible
electron-transfer quenching. Rather,
the photochemically active species is
the fraction of sensitizer present, in the
aqueous/organic solvent used for hy-
drogen evolution, as free species. As
shown by nanosecond laser flash pho-
tolysis experiments, its long-lived trip-
let state reacts bimolecularly with the
ascorbate donor, and the reduced sen-
sitizer thus formed, subsequently reacts
with the cobaloxime catalyst, thereby
triggering the hydrogen evolution proc-
ess. The performance is good, particu-
larly in terms of turnover frequencies
(TOF=10.8 or 3.6 minꢀ1, relative to
the sensitizer or the catalyst, respec-
tively) and the quantum yield (F=
4.6%, that is, 9.2% of maximum possi-
ble value). At high sacrificial donor
concentration, the maximum turnover
number (TON=352 or 117, relative to
the sensitizer or the catalyst, respec-
tively) is eventually limited by hydro-
genation of both sensitizer (chlorin for-
mation) and catalyst.
Keywords: cobaloximes · hydrogen ·
porphyrinoids · self-assembling · so-
lar fuels
Introduction
such strategies, “soft” methods potentially leading to self-as-
sembling are particularly interesting from the viewpoint of
the synthetic ease and the combinatorial flexibility. One of
such soft strategies is the metal-mediated approach,[18] by
which chromophores and other functional units, suitably
functionalized with peripheral ligand groups, are assembled
by means of coordinative bonds to appropriate metal cen-
ters.
Conversion of solar energy into fuels by photochemical
water splitting is considered as an attractive potential solu-
tion to the global energy problem.[1] Thus, large efforts are
being devoted worldwide to the development of efficient
molecular devices for photocatalytic hydrogen[2–6] or oxy-
gen[1c,7–17] evolution. Various types of synthetic strategies are
applied in this field, to suitably assemble molecular compo-
nents (light-absorbing units, electron donors or acceptors,
catalysts) into functional supramolecular systems. Among
AluminiumACTHNUTRGNEUG(N III) pyridylporphyrins are interesting molecu-
lar components for the construction of functional supramo-
lecular arrays.[19,20] What makes them particularly attractive
is their bifunctional nature, that is, the simultaneous pres-
ence of a Lewis acid (the Al center) and a Lewis basic (the
meso-pyridyl group) function (Figure 1). The Al center can
[a] M. Natali, Prof. Dr. F. Scandola
Department of Chemical and Pharmaceutical Sciences
University of Ferrara, Via Fossato di Mortara 17-19
44121 Ferrara (Italy)
and
Centro Interuniversitario per la Conversione Chimica dellꢀEnergia
Solare (SOLARCHEM), sezione di Ferrara, Via L. Borsari 46
44121 Ferrara (Italy)
E-mail: mirco.natali@unife.it
snf@unife.it
Figure 1. Schematic representation of the aluminium monopyridyl por-
phyrin pointing out the two docking sites: the Lewis acidic aluminium
center (X) and the Lewis basic pyridyl group (Y).
[b] Dr. R. Argazzi, Dr. C. Chiorboli
ISOF-CNR, sezione di Ferrara
Via L. Borsari 46, 44121 Ferrara (Italy)
[c] Dr. E. Iengo
Department of Chemical and Pharmaceutical Sciences
University of Trieste, Via L. Giorgeri 1, 34127 Trieste (Italy)
E-mail: eiengo@units.it
axially bind a variety of ligands, with a particular affinity for
oxygen-based functionalities, whereas the pyridyl function
can coordinate to a large variety of transition metals. By
using the acidic and basic functionalities and exploiting se-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300133.
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lective coordination, aluminium pyridylporphyrins can be
used to tackle the non-trivial problem of obtaining intrinsi-
cally asymmetric three-component supramolecular systems
by self-assembly.
We have recently described the self-assembly of a triad
for photoinduced charge separation based on such a plat-
form.[20] In that case, the Lewis acidic and basic functions
were used to bind, respectively, an electron acceptor (a car-
boxyl derivative of naphthalene bisimide) and an electron
donor unit (a ruthenium porphyrin). Selective coordination
(of carboxyl to Al and of pyridyl to Ru) ensured exclusive
formation of the required triad by simple mixing of the mo-
lecular components. Stepwise photoinduced charge separa-
tion was verified by ultrafast spectroscopic techniques. We
report now on the implementation of the same assembling
strategy to obtain a noble-metal-free system for photocata-
lytic hydrogen evolution. On the same aluminium porphyrin
platform, a hydrogen evolution
The absorption spectrum of AlP(OH) in tetrahydrofuran
is typical of metallo porphyrins, with a Soret band at l=
425 nm and Q-bands at l=521, 559, and 600 nm (Figure S1
in the Supporting Information).[21] Both [Co
ACTHNUTRGNNEG(U dmgH)ACHTUNGTRENNUNG(H2O)]
catalyst can be bound through
the meso-pyridyl group (see for
example, in Figure 2a a coba-
loxime unit).[2] On the other
hand, axial coordination at the
Al center can be used to rever-
sibly tether a variety of sacrifi-
Table 1. Electrochemical data for the molecular units and dyads.[a]
Oxidation [V]
0.96[b]
Reduction [V]
AlP(OH)
Co
(dmgH)[c]
1.22[b]
ꢀ1.22[b]
ꢀ1.59[b]
ꢀ1.66[b]
A
ꢀ0.68[d]
ꢀ1.22[d]
ꢀ1.13[d]
AlP(OH)–Co
AlP–Asc
E
1.21[b]
1.38[b]
1.02[b]
1.12[b]
ꢀ0.63[d]
ꢀ1.23[b]
0.61[e,f]
ꢀ1.20[b]
[a] Obtained by differential pulse voltammetry in THF at 298 K, 0.1m tetrabutylammonium hexafluorophos-
phate (TBA(PF6)) as supporting electrolyte, a standard calomel electrode SCE as reference electrode, ferro-
cene (0.56 V vs. SCE)[22] as internal standard, 50 mV pulse width. [b] Process attributed to the porphyrin unit.
[c] For solubility reasons, [Co(dmgH)2Cl(4-ethylpyridine)] was used as a model instead of the aquo complex.
cial
oxygen-based
electron
AHCTUNGTRENNUNG
donors (see for example, in
Figure 2 ascorbate).
AHCTUNGTRENNUNG
[d] Process attributed to the cobaloxime unit. [e] Process attributed to ascorbate. [f] Irreversible process.
and AscH have negligible absorption at l>400 nm. The flu-
orescence of AlP(OH) (l=615, 663 nm, Figure S2 in the
Supporting Information) has a lifetime t=6.4 ns in THF.
Relevant electrochemical data on these molecular compo-
nents, as well as on their assemblies (see below), are given
in Table 1.
Self-assembling of the molecular components on the alumi-
nium porphyrin platform: The assembling of the triad
system can be appropriately characterized in THF, a solvent
that provides the best balance between the solubility of the
three molecular units, by separately studying the formation
of the two component dyads (see the Supporting Informa-
tion for experimental details). The association between the
aluminium porphyrin and the cobaloxime [Eq. (1)], taking
place by coordination of the meso-pyridyl group of the por-
phyrin to the cobalt center, with displacement of the labile
water ligand,[23] can be followed both spectrophotometrically
(5 nm red shift of the porphyrin Soret and Q-bands, Fig-
ure S1 in the Supporting Information) and spectrofluorimet-
rically (quenching of the porphyrin fluorescence by the coor-
dinated cobaloxime, Figure S2 in the Supporting Informa-
tion).
Figure 2. Schematic representation of the target self-assembling triad for
photoinduced hydrogen production.
Results and Discussion
The molecular components used for the assembling of the
reductant–photosensitizer–catalyst (R–P–C) target triad
system are: 1) an aluminium monopyridyl porphyrin bearing
an axial hydroxo group, [5-(4’-pyridyl)-10,15,20-(3,5-di-tert-
butyl)-triphenylporphyrinato-hydroxo]aluminium
(OH)); 2) a cobaloxime with a chloride and a labile water
ligand in the axial positions, chloro(aquo)bis-
(dimethylglioximate)cobalt(III), ([Co(dmgH)(H2O)]); and
3) ascorbic acid (AscH).
ACHTUNGTRENUN(NG III), (AlP-
ACHTUNGTRENNUNG
AlPðOHÞþ½CoðdmgHÞðH2OÞꢁ Ð AlPðOHÞ ꢀ CoðdmgHÞþH2O
ð1Þ
A
E
N
ACHTUNGTRENNUNG
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Photocatalytic Hydrogen Evolution
FULL PAPER
With both techniques, the association equilibrium in
by ultrafast charge recombination to reform the ground
state. Oxidation of ascorbate is known from electrochemical
studies[26] to be irreversible in aqueous media, and an irre-
versible electrochemical behavior is also observed in THF
(Table 1). The time scales of the electrochemical and time-
resolved photochemical experiments are so different, how-
ever, that no extrapolation between the two domains can be
made. The proof that photoinduced electron transfer is
largely, but not fully reversible, derives from the observation
of a permanent chemical change upon continuous visible ir-
radiation. The spectral variations, with development of a
characteristic band at l=630 nm (Figure S7 in the Support-
ing Information), indicate the formation of chlorin,[27–30]
likely originating, upon further reduction or disproportiona-
tion and protonation, from the metal porphyrin radical
anion.[31] The quantum yield of chlorin formation in THF is
estimated to be approximately 10ꢀ2, an order of magnitude
which is fully compatible with the failure to observe irrever-
sibility by time-resolved spectroscopy.
10ꢀ4 m THF solution is seen to require approximately a two-
fold molar excess of [CoACHTNUTRGNENUG(dmgH)ACHUTGNTREN(NUNG H2O)] in order to obtain
complete formation of the dyad. The fluorescence quench-
ing in the associated dyad takes place on a time scale short-
er than the time resolution of the time-correlated single
photon counting technique, that is, <250 ps. Time-resolved
femtosecond spectroscopy shows a clean decay of the ab-
sorption of the aluminium porphyrin excited singlet state to
the ground state, with a time constant of 65 ps (Figure S3 in
the Supporting Information). The most likely quenching
mechanism is an oxidative electron transfer [Eq. (2)], which,
as estimated[24] from the AlP(OH) singlet excited-state
energy (2.04 eV) and the electrochemical data given in
Table 1, is thermodynamically allowed by approximately
0.4 eV.
1
þ
ꢀ
*
ð2Þ
AlPðOHÞ ꢀ CoðdmgHÞ ! AlPðOHÞ ꢀ CoðdmgHÞ
The failure to observe transient accumulation of the prod-
The results obtained on the two dyads suggest complete
ucts of Equation (2) by time-resolved spectroscopy most
likely indicates that the back reaction (charge recombina-
tion) to the ground state is faster than the forward one
[Eq. (2), charge separation].[25]
formation of the Asc–AlP–CoACTHNUGRTNEUNG(dmgH) triad in a THF solu-
tion containing 10ꢀ4 m AlP(OH), ꢂ10ꢀ3 m AscH, and ꢂ2ꢂ
10ꢀ4 m [Co
ACTHNUTRGNEN(UG dmgH)ACHTUNGTRENN(UGN H2O)]. In the triad, as well as in the com-
ponent dyads, the aluminium porphyrin fluorescence is com-
pletely quenched.
The association between the aluminium pyridyl porphyrin
and ascorbic acid [Eq. (3)] can also be followed spectropho-
tometrically (red shifts of the porphyrin B- and Q-bands,
Figure S4 in the Supporting Information) and spectrofluori-
metrically (quenching of the porphyrin fluorescence by the
pendent ascorbate, Figure S5 in the Supporting Informa-
tion).
Energy levels and allowed processes: From the behavior of
the two dyad systems, reasonable predictions can be made
about plausible photochemical routes in a fully assembled
triad system. Figure 3a shows the energy level diagram for
an Asc–AlP–CoACHTUNTRGNEUNG(dmgH) triad in THF. The energies of the
various states are calculated by using the AlP(OH) singlet
excited-state energy (2.04 eV) and the electrochemical data
in Table 1 for the two dyads AlP–Asc (Eox values) and AlP–
AlPðOHÞþAscH Ð AlP ꢀ AscþH2O
ð3Þ
CoACTHNUTRGNEUNG
(dmgH) (Ered values).[24] The diagram shows that two
The association in 10ꢀ4 m THF solution is efficient, with
approximately 70% dyad formed with stoichiometric
amounts and complete formation with a tenfold excess of
ascorbic acid. The quenching in the associated dyad very
likely takes place by reductive photoinduced electron trans-
fer [Eq. (4)], which, as estimated[24] from the chromophore
excited-state energy (2.04 eV) and the electrochemical data
given in Table 1, is thermodynamically feasible by approxi-
mately 0.2 eV.
photoinduced charge-separation paths, of a similar driving
force, are available to the triad following excitation of the
aluminium porphyrin unit: 1) reductive quenching by ascor-
bate followed by electron shift to the cobaloxime, 2) oxida-
tive quenching by the cobaloxime followed by hole transfer
to ascorbate. Both pathways are consistent with the quench-
ing processes observed in the two dyads, although the ultra-
fast measurements do not permit any decision about their
relative importance in the triad. For hydrogen generation,
two-electron reduction of the cobaloxime unit is required,
and thus a second photoinduced electron-transfer cycle
(after replacement of the oxidized ascorbate unit) should be
envisioned. Figure 3b show the energy level diagram for ex-
1
ꢀ
þ
*
ð4Þ
AlP ꢀ Asc ! AlP -Asc
The process in the dyad takes place on a time scale short-
er than the time resolution of the time-correlated single
photon counting technique, that is, <250 ps. Time-resolved
femtosecond spectroscopy (Figure S6 in the Supporting In-
formation) shows a clean decay of the absorption of the alu-
minium porphyrin excited singlet state to the ground state
(bi-exponential, with time constants of 5 ps, 60%, and 26 ps,
40%), without apparent accumulation of significant
amounts of transient intermediate products. This implies
that the reductive electron transfer is largely reversible, in
the sense that photoinduced charge separation is followed
citation of an Asc–AlP–CoACHTNUGRTNEUNG
(dmgH)ꢀ triad,[24] containing a
one-electron-reduced cobaloxime unit. It can be seen that,
at least by a type 1) path, the second reduction step of the
cobaloxime unit is thermodynamically allowed.
Hydrogen evolution experiments: The hydrogen evolution
experiments were performed by irradiating deaerated solu-
tions (5 mL) with a 175 W Xe arc-lamp and a cut-off filter
at 400 nm, and by monitoring the hydrogen evolution by gas
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M. Natali, E. Iengo, F. Scandola et al.
Figure 4. a) Effect of the nature of the organic solvent on the hydrogen
evolution in 30:70 water/organic solvent mixtures (1ꢂ10ꢀ4 m AlP(OH),
3ꢂ10ꢀ4 m [Co
N
(H2O)], 1ꢂ10ꢀ2 m AscH). b) Effect of the amount
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Figure 3. Energy levels and elementary steps potentially involved in pho-
tocatalytic cycles for hydrogen generation based on an Asc–AlP–Co-
ACHTUNGTRENNUNG(dmgH) triad. Parts a) and b) show subsequent one-electron photoin-
duced charge-separation steps (charge-recombination steps omitted for
clarity). For space reasons, the following short-hand notation is used: A=
ascorbate, P=aluminium porphyrin, Co=cobaloxime.
acetone solvent mixtures, the hydrogen-evolving perform-
ance first improves with increasing the water content up to
30%, but then decreases upon further addition of water
(Figure 4b). Therefore, the remaining experiments were car-
ried out in 30:70 water/acetone mixtures. The effect of the
concentration of the sacrificial reductant and the pH value
are intertwined as, for example, increasing the concentration
of ascorbic acid from 1ꢂ10ꢀ2 to 5ꢂ10ꢀ2 m brings a change in
the measured pH value from 4.5 to 3.5. The effects can be
disentangled by controlling the pH upon addition of appro-
priate amounts of NaOH or HClO4. At pH 6, an increase in
chromatography (for details, see the Experimental Section).
In neat THF, some hydrogen was detected upon visible irra-
diation of a solution containing 1ꢂ10ꢀ4 m AlP(OH), 3ꢂ
10ꢀ4 m [Co (H2O)], and 1ꢂ10ꢀ2 m AscH. It was pro-
ACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG
duced, however in small, non-catalytic amounts (0.75 mmol,
corresponding to turnover number (TON) values of 0.5 rela-
the concentration of ascorbic acid from 1ꢂ10ꢀ2 to 5ꢂ10ꢀ2
m
tive to [Co
(dmgH)
E
brings about a steady increase in both the quantum yield
and the maximum chemical yield of the hydrogen evolution
(Figure 5a).
The effect of the pH is a complex one, however, with
quantum yields (Figure 5b) and limiting chemical yields
(Figure S9 in the Supporting Information) peaking at differ-
ent pH values depending on the ascorbic acid concentration.
The best hydrogen evolving conditions were obtained by
working at pH 6 with 5ꢂ10ꢀ2 m ascorbic acid. Under these
conditions, the effect of the concentration of the catalyst
quantum yield, ca. 4ꢂ10ꢀ4). It was soon realized that a large
gain in hydrogen production could be obtained by adding
water to the organic solvent (Figure S8 in the Supporting In-
formation). Therefore, optimization of the system was per-
formed in water/organic solvent mixtures containing 1ꢂ
10ꢀ4 m AlP(OH), by checking the effects on hydrogen evolu-
tion of systematic changes in 1) the nature of the organic
solvent, 2) the fractional amount of water, 3) the pH value
of the irradiated solution, 4) the concentration of ascorbic
acid, 5) the concentration of the cobaloxime catalyst, and
6) the concentration of added dimethylglyoxime ligand.
Among various water/organic solvent mixtures tested, those
containing acetone were found to give the best results in
terms of both quantum yield (slope) and maximum chemical
yield (long time asymptotic behavior) (Figure 4a). In water/
[CoACHTNUTGRENNUG(dmgH)ACHTUNGTRNEN(UGN H2O)] was then checked (Figure 6a). It can be
seen that a considerable gain in performance can be ob-
tained by increasing the catalyst concentration from 1ꢂ
10ꢀ4 m (stoichiometric with the AlP(OH) sensitizer) up to a
threefold excess (no further gain observed upon further in-
crease). Finally, as already observed by Eisenberg and cow-
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Photocatalytic Hydrogen Evolution
FULL PAPER
orkers in related work with cobaloxime catalysts,[32] the addi-
tion of free dimethylglyoxime ligand (dmgH2) brings about
a further increase in the maximum chemical yield of the hy-
drogen evolution (Figure 6b). Following these optimization
experiments, the best hydrogen evolving conditions can be
summarized as follows: 1ꢂ10ꢀ4 m AlP(OH), 3ꢂ10ꢀ4 m [Co-
A
E
m
AscH, pH 6, and 5ꢂ10ꢀ3
m
dmgH2. Under these conditions, the quantum yield of the
hydrogen evolution (as measured from the slope in the ini-
tial 20 min of irradiation, see the Supporting Information) is
0.046, which, given the two-photon–two-electron nature of
the photocatalytic process, amounts to approximately 9.2%
of the maximum possible value. The TON values, referred
to the possible limiting components (see below), are 352 rel-
ative to AlP(OH) and 117 relative to [CoACHTUNRTGENNUG(dmgH)ACHTUNGTRENNUNG(H2O)].
The corresponding turnover frequency (TOF) values are
10.8 and 3.6 minꢀ1, respectively.
Photocatalytic mechanism: The process of the photocatalyt-
ic hydrogen evolution is certainly a complex one, involving
a variety of possible photochemical and thermal steps. As to
the nature of the primary photochemical process, two path-
ways are available to the aluminium porphyrin excited state
(Figure 3): 1) oxidative quenching by the cobaloxime or
2) reductive quenching by ascorbate. The following discus-
sion aims at an experimental discrimination between such
possibilities. The first point to be emphasized is that in THF,
where the association equilibria between the three molecu-
lar components are studied and the electrochemical data de-
fining the thermodynamics of the system are obtained, prac-
tically no photocatalytic activity is observed (Figure S8 in
the Supporting Information). This is no doubt the conse-
quence of the fast charge recombination and largely reversi-
ble behavior observed by ultrafast spectroscopy in this sol-
vent.
Figure 5. a) Effect of the ascorbic acid concentration on the hydrogen
evolution at pH 6 (30:70 water/acetone, 1ꢂ10ꢀ4 m AlP(OH), 3ꢂ10ꢀ4
m
[Co
bic acid concentration on the hydrogen evolution (30:70 water/acetone,
1ꢂ10ꢀ4 m AlP(OH), 3ꢂ10ꢀ4 m [Co
(dmgH)(H2O)]).
ACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG(H2O)]). b) Combined effects of the pH value and the ascor-
A
ACHTUNGTRENNUNG
When the solvent is changed to water/organic solvent mix-
tures, efficient photocatalytic hydrogen evolution is ob-
served (Figure 4). In such solvents, however, the association
equilibria are clearly very different from the ones in THF.
For instance, in a 30:70 water/acetone pH 6 solution contain-
ing 1ꢂ10ꢀ4 m AlP(OH) and 5ꢂ10ꢀ2 m AscH the porphyrin
fluorescence is only quenched to an extent of 38% (Fig-
ure S10 in the Supporting Information), indicating that the
axial bond is hydrolyzed under these conditions, with the
porphyrin being just partially coordinated by ascorbate. A
57% quenching is observed, on the other hand, when 3ꢂ
10ꢀ4 m [Co
ACTHNUTRGNEN(UG dmgH)ACHTUNGTREN(NUGN H2O)] is added to such a solution (Fig-
ure S10 in the Supporting Information), implying that under
these conditions coordination of the cobaloxime to the pyr-
idyl porphyrin takes place only partially. This is likely
caused by the excess of the ascorbate ligand competing effi-
ciently with the pyridyl porphyrin for cobalt coordination.
In summary, under conditions of photocatalytic hydrogen
evolution, the catalyst and the sacrificial donor are only par-
tially coordinated to the aluminium porphyrin chromophore,
which is largely (ca. 43%) present in solution as a free spe-
cies.
Figure 6. a) Effect of the concentration of cobaloxime on the hydrogen
evolution (30:70 water/acetone, pH 6, 1ꢂ10ꢀ4 AlP(OH), 5ꢂ10ꢀ2
m
m
AscH). b) Effect of added dimethylglioxime ligand on the hydrogen evo-
lution (30:70 water/acetone, pH 6, 1ꢂ10ꢀ4 m AlP(OH), 3ꢂ10ꢀ4 m [Co-
ACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG
(H2O)], 5ꢂ10ꢀ2 m AscH).
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M. Natali, E. Iengo, F. Scandola et al.
Figure 7. a) Different absorption spectra at different time delays obtained
by ultrafast spectroscopy (UFS) (excitation at l=550 nm) on a 30:70
water/acetone solution containing 1ꢂ10ꢀ4
m m
AlP(OH) and 5ꢂ10ꢀ2
AscH at pH 6, and kinetic analysis at l=480 nm (inset). b) Different ab-
sorption spectra at different time delays obtained by UFS (excitation at
l=550 nm) on
AlP(OH), 3ꢂ10ꢀ4 m [Co
kinetic analysis at l=480 nm (inset).
a m
30:70 water/acetone solution containing 1ꢂ10ꢀ4
(H2O)], and 5ꢂ10ꢀ2 m AscH at pH 6, and
ACHTUNGTRENNUNG(dmgH)ACHTUGNTRENNUGN
Figure 8. Different absorption spectra at a) 0.1–5 and b) 5–100 ms time
delays obtained by laser flash photolysis (excitation at l=532 nm) on a
Under these conditions, the behavior of the aluminium
porphyrin chromophore upon excitation can be investigated
by ultrafast spectroscopy. It can clearly be seen that, both in
the AlP(OH)/AscH (Figure 7a) and AlP(OH)/AscH/[Co-
30:70 water/acetone solution containing 1ꢂ10ꢀ4 m AlP(OH) and 5ꢂ10ꢀ2
AscH at pH 6, and c) kinetic analysis at l=700 nm.
m
A
E
mation). The evolution of the spectral changes is clearly bi-
phasic: in the earlier period (0.1–5 ms), a new spectrum with
a sharp maximum at l=720 nm develops, characteristic for
the radical anion of metal tetraphenylporphyrins.[33] Then
this transient decays to the baseline on a much longer time
scale with complex kinetics (from a bi-exponential fit to the
black trace of Figure 9a, lifetimes t=60 and 400 ms are ob-
tained). This experiment provides clear evidence that free
aluminium porphyrin reacts bimolecularly upon excitation
with the ascorbate donor at the triplet level, giving rise to
the reduced form of the sensitizer. The reaction of the alu-
minium porphyrin radical anion with the catalyst can then
be monitored by laser flash photolysis in the AlP(OH)/
ly formed excited singlet state (roughly corresponding to the
fluorescence-quenched fraction under the same conditions)
decays completely on a time scale of tens of picosecond,
whereas the remaining part is appreciably constant over the
whole time window of the experiment (2 ns).
By comparison with what observed in THF (Figures S3
and S6 in the Supporting Information), the fast decay of the
singlet excited state can be easily assigned to rapidly reversi-
ble electron-transfer quenching taking place in the associat-
ed fraction of the aluminium porphyrin chromophore. The
fate of the long-lived fraction of excited states, correspond-
ing to the free chromophore, can be investigated by nano-
second laser flash photolysis. In the AlP(OH)/AscH system
(Figure 8), the initial spectrum (100 ns time delay) can be at-
tributed to the aluminium porphyrin triplet state, populated
in a few nanoseconds by intersystem crossing from the sin-
glet (the analogous spectrum obtained with AlP(OH) is
shown for comparison in Figure S11 in the Supporting Infor-
AscH/[CoACTHNUTRGENN(UG dmgH)ACHTUGNTERN(NGUN H2O)] system. The transient spectral
changes (Figure S12 in the Supporting Information) are
qualitatively similar to those obtained for the AlP(OH)/
AscH system, except for a faster decay of the long-lived alu-
minium porphyrin radical anion spectrum. As expected, the
decay rate shows an appreciably first-order dependence on
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Photocatalytic Hydrogen Evolution
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Figure 10. Schematic representation of the mechanism leading to coba-
loxime reduction and hydrogen generation. For clarity reasons the fol-
lowing abbreviations are used: AlP=aluminium porphyrin, Asc=ascor-
bate, and Co=cobaloxime.
Figure 9. a) Single-wavelength kinetics (l=700 nm) obtained by laser
flash photolysis (excitation at l=532 nm) on a 30:70 water/acetone solu-
tion containing 1ꢂ10ꢀ4 m AlP(OH) 0–3ꢂ10ꢀ2 m [Co
ACTHNUTRGNENU(G dmgH)HCATUNGTREN(NGUN H2O)], and
sponsible for the observed quenching, as shown by a com-
parison between the behavior of the two AlP(OH)/[Co-
ACHUTNRGEN(NUG dmgH)ACHTUGNTREN(NUNG H2O)] and AlP(OH)/AscH subsystems (Figure S13
5ꢂ10ꢀ2 m AscH at pH 6. b) Plot of the rate constant (obtained by mono-
exponential fitting of the kinetic traces) versus the cobaloxime concentra-
tion for the calculation of the bimolecular rate constant, slope=1.4ꢂ
in the Supporting Information). Under these conditions,
with strong quenching by coordinated cobaloxime, a low
triplet yield and thus a low photocatalytic activity should be
expected (Figure 10). In fact, at natural pH a good hydrogen
production efficiency is nevertheless observed (Figure 4), al-
though with a prominent induction period. A key to this
issue is the behavior of the aluminium porphyrin fluores-
cence in the early time lag of the irradiation experiments at
pH 4.5 (Figure 11). The fluorescence, initially completely
quenched, rises rapidly upon irradiation, reaching in a few
minutes the level of emission characteristic of a two-compo-
nent mixture of AlP(OH) and AscH. The likely explanation
is that, in the initial stage of the irradiation, a photochemical
detachment of the cobaloxime quenching unit from the alu-
minium pyridyl porphyrin takes place.[35,36] The blue shift
(ca. 5 nm) of the Soret and Q-bands of the porphyrin that
accompanies this rise in the emission (Figure S14 in the Sup-
porting Information), symmetric to the red shift observed
upon complexation in THF, confirms this hypothesis. As
CoII centers are generally known to be substitutionally
labile,[37,38] some detachment of the reduced cobaloxime is
likely to take place, in competition with charge recombina-
tion, following oxidative quenching of the sensitizer singlet
state. In the hydrogen evolution experiments carried out
under these conditions (Figure 4 and, in more detail, Fig-
ure S15 in the Supporting Information), an induction period
is clearly present. Part of this induction period (the first 2–
3 min, according to the quenching data of Figure 11) can be
accounted for by the photochemical detachment of the co-
108 mꢀ1 sꢀ1
.
the [CoACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG(H2O)] concentration (Figure 9), with a cal-
culated bimolecular rate constant of 1.4ꢂ108 mꢀ1 sꢀ1.
The main mechanistic conclusions from these experiments
can be summarized as schematized in Figure 10. 1) Contrary
to what was initially envisioned, association of the catalyst
and the sacrificial donor with the sensitizer is not useful to-
wards photocatalytic hydrogen evolution. In fact, it turns
out to be detrimental, to the extent to which the associated
species are rapidly quenched by largely reversible electron
transfer. 2) The photocatalytic hydrogen evolution comes
from a bimolecular reaction of the triplet state of the free
sensitizer with the sacrificial donor. This primary photoreac-
tion produces the reduced photosensitizer, which further
reacts bimolecularly with the catalyst, thereby triggering the
photocatalytic hydrogen production.[34] The reaction se-
quence experimentally demonstrated here is similar to that
proposed by Eisenberg[2c,32] for a number of hydrogen-evolv-
ing systems based on organic sensitizers and cobaloximes.
An interesting comparison is that between the hydrogen
evolution experiments carried out at pH 6 and at natural pH
(i.e., pH 3.5 with 5ꢂ10ꢀ2 m AscH and pH 4.5 with 1ꢂ10ꢀ2
m
AscH). It should be remarked that at low pH a much more
efficient quenching of the aluminium porphyrin fluorescence
takes place (e.g., ꢂ95% for a solution containing 1ꢂ10ꢀ4
m
m
AlP(OH), 3ꢂ10ꢀ4 m [Co (H2O)], and 1–5ꢂ10ꢀ2
ACTHNUTRGENNU(G dmgH)ACHTUTGNRENNUGN
AscH). Efficient coordination of cobaloxime is the main re-
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M. Natali, E. Iengo, F. Scandola et al.
Information).[40] In terms of kinetics, the dependence of the
initial rate of the hydrogen production on the cobaloxime
concentration is appreciably a first-order one (Figure S18 in
the Supporting Information).[41] As in similar cases,[30c,32,42]
this could be taken as an indication that, among the hydro-
gen evolution mechanisms available to the two-electron-re-
duced form of cobaloxime,[43] a heterolytic pathway, involv-
ing protonation of the CoIII hydrido complex, is prevailing
here.
The effect of increasing the ascorbic acid concentration
on the hydrogen production rates (Figure 5a) can be mainly
attributed to two effects: the above-mentioned decrease in
singlet quenching by cobaloxime, with a consequent in-
creased triplet formation yield, and an increasing efficiency
of the bimolecular reaction of ascorbate with the aluminium
porphyrin triplet. The bell-shaped effects of the pH value
(Figure 5b) are likely the result of at least two factors play-
ing in opposite directions as the pH is increased: the above-
mentioned positive effects of increasing the ascorbate con-
centration and the decreasing thermodynamic driving force
for water reduction.
An interesting observation is that in several experiments
(e.g., with 1ꢂ10ꢀ2 m ascorbic acid, Figures 4b and S16 in the
Supporting Information) the total amount of hydrogen
evolved equals the total amount of ascorbic acid present in
solution. This means that, although reductive quenching of
the aluminium porphyrin by ascorbate is a one-electron
transfer process, each sacrificial donor molecule eventually
undergoes a two-electron oxidation to dehydroascorbic acid.
This most likely occurs by disproportionation[44,45] of the as-
corbate radical formed as primary product.
As far as the TON limiting reactions are concerned, as
discussed above, formation of permanent reduction products
of the sensitizer (chlorins) is certainly one of the main fac-
tors. Another one is clearly the consumption of the catalyst,
presumably by hydrogenation of the dimethylglyoximate li-
gands.[32,46,47] The improvement in performance obtained by
addition of free dimethylglyoxime (Figure 6b) is consistent
with this notion.
Figure 11. a) Time-dependent fluorescence spectra obtained upon contin-
uous irradiation (175 W Xe arc-lamp, cut-off filter at l=400 nm) of a
30:70 water/acetone solution containing 1ꢂ10ꢀ4 m AlP(OH), 3ꢂ10ꢀ4
m
[CoACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG
(H2O)], and 1ꢂ10ꢀ2 m AscH, at natural pH (i.e., pH 4.5),
compared with the fluorescence (dashed line) of a corresponding solution
lacking the cobaloxime catalyst. b) Normalized emission intensity as a
function of the irradiation time.
baloxime quenching unit, whereas the remaining part (up to
ca. 10 min) is as expected because an appreciable fraction of
reduced cobaloxime must accumulate at stationary state
before a constant hydrogen formation rate is established. It
can be noticed that the induction period for the hydrogen
formation is apparently absent, or much less pronounced, at
higher pH (Figures 5, 6, and S9 in the Supporting Informa-
tion). Two reasons can account for this observation: 1) as
discussed before, the higher concentration of ascorbate
chemically removes the quenching cobaloxime from the alu-
minium porphyrin, thus largely suppressing that part of the
induction period related to the need for photochemical re-
lease of this unit and 2) the overall much higher rate is ex-
pected to compress any induction period to a time scale
shorter than that of the actual measurements.[39]
In terms of the mechanism shown in Figure 10, the effect
of the cobaloxime concentration on the hydrogen produc-
tion rates and yields (Figures 6a and S16 in the Supporting
Information) can be easily related to the efficiency of its bi-
molecular reaction with the porphyrin radical anion. This re-
action competes with the evolution of the latter to perma-
nent reduction products, such as chlorins. Therefore, fast
electron scavenging is expected to be crucial to minimize
the photosensitizer degradation. As a matter of fact, in all
the experiments an inverse relationship has been observed
between the extent of chlorin product formation and the ef-
ficiency of hydrogen evolution (Figure S17 in the Supporting
Conclusion
The sacrificial donor/sensitizer/catalyst system described
here was originally conceived with the idea that self-assem-
bly of the three components would favor the photocatalytic
performance of the system. In practice, this expectation is
not confirmed. In fact, association of the sensitizer with
either the catalyst or the sacrificial donor leads to fast and
efficient quenching of the singlet excited state of the sensi-
tizer, with little if any net chemical change. In aqueous/or-
ganic solvent mixtures, however, a substantial amount of
sensitizer is present as free, unquenched species. Under
these conditions, the relevant photochemically active species
is the triplet state of the sensitizer. Photocatalytic hydrogen
evolution arises from a bimolecular reaction of the sensitizer
triplet state with the sacrificial donor. This primary photore-
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Photocatalytic Hydrogen Evolution
FULL PAPER
equipped with sub-nanosecond LED sources (280, 380, 460, and 600 nm,
500–700 ps pulse width) powered by a PicoQuant PDL 800-B variable
(2.5–40 MHz) pulsed power supply. The decays were analyzed by means
of PicoQuant FluoFit Global Fluorescence Decay Analysis Software.
action yields the reduced photosensitizer, which further
reacts bimolecularly with the catalyst, thereby triggering the
photocatalytic hydrogen production. Similar conclusions
have been recently reached by Eisenberg and coworkers
when comparing the photocatalytic efficiency of bound
versus free fluorescein/cobaloxime systems.[36]
A system similar to the one studied in this work has been
recently described by Sun and coworkers.[2d] It features the
same cobaloxime catalyst as used here, a zinc (instead of
aluminium) pyridyl porphyrin as sensitizer, and triethyla-
mine (instead of ascorbic acid) as sacrificial donor. In that
case, the mechanism was assumed to be unimolecular and
thought to consist of two consecutive steps: oxidative
quenching by the catalyst followed by hole transfer to the
sacrificial donor. From the viewpoint of hydrogen evolution,
that system (reported values in 80:20 THF/water, TON=22
in 5 h of irradiation)[2d] is considerably less efficient than the
present one.
In conclusion, a new noble-metal-free sacrificial donor/
photosensitizer/catalyst system for photochemical hydrogen
production, based on ascorbic acid, aluminium pyridyl por-
phyrin, and cobaloxime is presented. The hydrogen evolving
performance is good, particularly in terms of turnover fre-
quencies[48] (TOF=10.8 or 3.6 minꢀ1, with respect to the sen-
sitizer or the catalyst, respectively) and quantum yield (F=
4.6%, that is, 9.2% of maximum possible value). The turn-
over numbers (maximum TON=352 or 117, with respect to
the sensitizer or the catalyst, respectively) is limited by both
the permanent reduction of the sensitizer (formation of
chlorins) and the hydrogenation of the ligand of the cata-
lyst.
Ultrafast spectroscopy (UFS): Femtosecond time-resolved experiments
were performed by using a pump-probe setup based on the Spectra-Phys-
ics Hurricane Ti:sapphire laser source and the Ultrafast Systems Helios
spectrometer.[49] The 550 nm pump pulses were generated with a Spectra
Physics 800 OPA. Probe pulses were obtained by continuum generation
on a sapphire plate (useful spectral range: 450–800 nm). Effective time
resolution approximately 300 fs, temporal chirp over the white-light 450–
750 nm range approximately 200 fs, temporal window of the optical delay
stage 0–2000 ps. The time-resolved spectral data were analyzed with the
Ultrafast Systems Surface Explorer Pro software.
Nanosecond laser flash photolysis: Nanosecond transient measurements
were performed with a custom laser spectrometer comprised of a Contin-
uum Surelite II Nd:YAG laser (FWHM 6–8 ns) with frequency doubled,
(532 nm, 330 mJ) or tripled, (355 nm, 160 mJ) option, an Applied Photo-
physics xenon light source including a mod. 720 150 W lamp housing, a
mod. 620 power controlled lamp supply and a mod. 03-102 arc lamp
pulser. Laser excitation was provided at 908 with respect to the white
light probe beam. Light transmitted by the sample was focused onto the
entrance slit of a 300 mm focal length Acton SpectraPro 2300i triple gra-
ting, flat field, double exit monochromator equipped with a photomulti-
plier detector (Hamamatsu R3896) and a Princeton Instruments PIMAX
II gated intensified CCD camera, using a RB Gen II intensifier, a ST133
controller and a PTG pulser. An Edmund optics notch filter centered at
532 nm was used in order to avoid eventual scattered laser by the sample
to hit the detector. Signals from the photomultiplier (kinetic traces) were
processed by means of a LeCroy 9360 (600 MHz, 5 Gssꢀ1) digital oscillo-
scope.
Photolysis apparatus: The hydrogen evolution experiments were carried
out upon continuous visible-light irradiation with
a 175 W xenon
CERMAX arc-lamp (cut-off filter at 400 nm) of a reactor (a 10 mm path
length pyrex glass cuvette with head space obtained from a round-
bottom flask) containing the solution. The measuring cell was sealed
during the photoreaction: the head to which the cell is attached has
indeed four ports, closed with Swagelok connections, two of them are
part of a closed loop involving a GC gas inlet and a sample vent in order
to analyze the head space content without an appreciable gas consump-
tion, and the other two are for the degassing procedure (input and
output). Regarding the calculation of the photoreaction quantum yields
the irradiation was performed with an array of four high power Roithner
Lasertechnik orange (590 nm) 350 mA LEDs instead of white light (for
more details see the Supporting Information).
Experimental Section
Materials: Solvents for spectroscopic, photophysical, and photolysis
measurements were of spectroscopic grade, all the other chemicals were
of reagent grade quality, and were used as received.
Gas chromatography: The gas phase of the reaction vessel was analyzed
on an Agilent Technologies 490 microGC equipped with a 5 ꢃ molecular
sieve column (10 m), a thermal conductivity detector, and by using Ar as
carrier gas. An aliquot of 5 mL from the headspace of the reactor was
sampled by the internal GC pump and 200 nl were injected in the
column maintained at 608C for the separation and detection of gases.
The unused gas sample was then re-introduced in the reactor in order to
minimize its consumption along the whole photolysis. The amount of hy-
drogen was quantified through the external calibration method. This pro-
cedure was performed, prior to analysis, through a galvanostatic (typical-
ly 1 mA) electrolysis of a 0.1m H2SO4 solution in an analogous cell (same
volume) equipped with two Pt wires sealed in the glass at the bottom of
the cell. A 100% faradaic efficiency was assumed leading to a linear cor-
relation between the amount of H2 evolved at the cathode and the elec-
trolysis time.
NMR spectroscopy: 1H spectra were recorded at 400 MHz on a Bruker
Avance 400 QNP and at 500 MHz on a Bruker Avance 500. All spectra
were run at room temperature (298 K) in [D5]pyridine and [D3]acetoni-
trile. Proton peak positions were referenced to the peak of residual non-
deuterated solvent peaks.
Electrochemical measurements: Differential pulse voltammetry (DPV)
measurements were carried out with a PC-interfaced Eco Chemie Auto-
lab/Pgstat 30 potentiostat. Argon-purged 10ꢀ4 m sample solutions in THF,
containing 0.1m TBAPF6 (Fluka, electrochemical grade, 99%, dried in an
oven), were used. A conventional a three-electrode cell assembly was
adopted: a saturated calomel electrode (SCE Amel) and a platinum elec-
trode, both separated from the test solution by a glass frit, were used as
reference and counter electrodes, respectively; a glassy carbon electrode
was used as the working electrode. The thermodynamic reduction poten-
tials (half-wave potentials) were calculated from the relation E1/2
peak+DE/2, where DE is the pulse potential.
=
Hydrogen evolution experiments: In a typical experiment, samples of
5 mL were prepared in 20 mL scintillation vials starting from a solution
of AlP(OH) (1ꢂ10ꢀ4 m in water/organic solvent), and further adding
E
Steady-state absorption/emission measurements: UV/Vis absorption
spectra were recorded on a Jasco V-570 UV/Vis/NIR spectrophotometer.
Emission spectra were taken on a Horiba-Jobin Yvon Fluoromax-2 spec-
trofluorimeter, equipped with a Hamamatsu R3896 tube.
(H2O)] (small aliquots from a 3ꢂ10ꢀ2
AscH (as solid) and [CoACTHNUTRGENNU(G dmgH)ACHUTNGTRENNUNG m
mother solution in acetonitrile). The change in volume upon addition of
the latter can always be considered negligible (dilution ꢃ1%). The pH
of the final solution was adjusted (when required) upon further addition
of different aliquots of 1m NaOH or 1m HClO4 solution (total volume
Time-correlated single photon counting (TCSPC): Fluorescence lifetimes
were measured by using a TCSPC apparatus (PicoQuant Picoharp 300)
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M. Natali, E. Iengo, F. Scandola et al.
changes are negligible). The solution was then put in the reactor, de-
gassed by bubbling Ar for 30 min, and thermostated at 158C. The cell
was then irradiated and the solution continually stirred during the photol-
ysis. The gas phase of the reaction was analyzed through GC and the
amount of hydrogen was quantified.
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AlP(OH):[19,20] 5-(4’Pyridyl)-10,15,20-(3,5-di-tert-butyl)-triphenylporphyr-
in (P) was synthesized and purified according to literature methods.[50]
Compound P (300 mg, 2.82 mmol) was then dissolved in dry toluene
(100 mL) and trimethylaluminium (0.2 mL, 2.0m in toluene, 5.6 mmol)
was added under a N2 atmosphere. The solution was stirred at room tem-
perature, under a N2 atmosphere, for 4 h, after which time H2O (5 mL)
was added and stirring was continued overnight. The toluene was re-
moved and the violet solid thus formed was re-dissolved in CH2Cl2. The
solution was filtered, dried over Na2SO4, and passed over a column of
alumina to give the product (265 mg, 85%). 1H NMR (400 MHz,
[D5]pyridine): d=9.28 (m, 6H,; b4, b3, b2), 9.12 (m, 4H; b1, pyHa), 8.2
(brd, 6H; oH, oH’), 8.15 (d, 2H; pyHb), 7.84 (brd, 2H; pH, pH’), 1.54 (s,
36H; tBu), 1.52 ppm (s, 18H; tBu).
[Co
cording to literature procedures.[51]
[Co(dmgH)(H2O)]: CoCl2·6H2O (1.10 g, 5 mmol) and dmgH2 (dimethyl-
ACHTUNGTRENNUNG(dmgH)ACHTUNGTRENNUNG(EtPy)]: [CoACHTUNGTRENNUNG(dmgH)2Cl(4-ethylpyridine)] was synthesized ac-
A
ACHTUNGTRENNUNG
glioxime, 1.18 g, 11 mmol) were dissolved in 95% ethanol (50 mL), and
the solution was heated to 708C. A 1 m NaOH solution (5 mL, 5 mmol)
was then added to the whole mixture and the solution was stirred for 1 h
at 708C. After cooling to room temperature, a stream of air was blown
through the solution for 30 min. The solution was filtered to remove side-
reaction products and concentrated to allow precipitation. The so ob-
tained brown crystals were collected by filtration on a Bꢄchner funnel
and washed with water (5 mL) and then with diethyl ether (10 mL). The
solid was then dried at room temperature to yield the product (590 mg,
35%). 1H NMR (400 MHz, [D3]acetonitrile): d=18.40 (brs, 2H; OHO),
2.45 ppm (s, 12H; CH3).
Acknowledgements
The authors thank Dr. Michele Orlandi for help with preliminary experi-
ments. Financial support from the Italian MIUR (PRIN 20085ZXFEE
and FIRB RBAP11C58Y ꢅꢅNanoSolar’’) is gratefully acknowledged. E.I.
gratefully acknowledges financial support from Reintegration Grant
PERG02-GA-2007–224823, and the Fondazione Beneficentia Stiftung.
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Photocatalytic Hydrogen Evolution
FULL PAPER
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[34] The bimolecular nature of the reaction with the catalyst is con-
firmed by the observation that comparable hydrogen yields are ob-
tained if, instead of [Co
N
ACHTUNGTNER(NNUG H2O)], a coordinatively saturated
analogue such as [Co(dmgH)ACHTGNUTRENNUNG
A
used as cobaloxime component of the reaction mixture. An alterna-
tive check could be performed, in principle, by substituting the
AlP(OH) sensitizer with a tetraphenylporphyrin analogue lacking
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conditions, arises from self-association through formation of alumi-
de-quenching behavior observed for the AscH/AlP/Co
three-component system is not observed for the AlP/Co
dyad clearly speaks for the electron transfer case.
ACHTUNGTRENNUNG
[20]
ꢀ
nium pyridyl bonds.
ACHTUNGTRENNUNG
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in the present case, such a possibility.
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electrostatic work terms tend to be strongly overestimated.
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[25] A possible alternative quenching mechanism could be energy trans-
fer. This mechanism is unlikely for the following reasons. The ab-
tion on the [Co
meaningful.
ACHUTGTNERN(NUG dmgH)ACHTUNGTREN(NUNG H2O)] concentration is not mechanistically
sorption spectrum of [CoACHTNUTRGENNUG(dmgH)CAHTUNGTREN(NGUN H2O)] displays no features of ap-
preciable intensity at lꢂ500 nm. Thus, spectral overlap between the
donor emission and the acceptor absorption (a requisite for fast
energy transfer) is negligible. Also, the photochemical stability of
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Received: January 14, 2013
Revised: April 12, 2013
Published online: June 3, 2013
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Chem. Eur. J. 2013, 19, 9261 – 9271
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www.chemeurj.org
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