Oxidation of Bromide to Bromine by Ruthenium(II) Bipyridine-Type Complexes Using the Flash-Quench Technique

Article abstract of DOI:10.1021/acs.inorgchem.7b01238

Six ruthenium complexes, [Ru(bpy)₃]²⁺ (1), [Ru(bpy)₂(deeb)]²⁺ (2), [Ru(deeb)₂(dmbpy)]²⁺ (3), [Ru(deeb)₂(bpy)]²⁺ (4), [Ru(deeb)₃]²⁺ (5), and [Ru(deeb)₂(bpz)] ²⁺ (6) (bpy: 2,2′-bipyridine; deeb: 4,4′-diethylester-2,2′-bipyridine; dmbpy: 4,4′-dimethyl-2,2′-bipyridine, bpz: 2,2′-bipyrazine), have been employed to sensitize photochemical oxidation of bromide to bromine. The oxidation potential for complexes 1-6 are 1.26, 1.36, 1.42, 1.46, 1.56, and 1.66 V vs SCE, respectively. The bimolecular rate constants for the quenching of complexes 1-6 by ArN₂⁺ (bromobenzenediazonium) are determined as 1.1 × 10⁹, 1.6 × 10⁸, 1.4 × 10⁸, 1.2 × 10⁸, 6.4 × 10⁷, and 8.9 × 10⁶ M^-1 s^-1, respectively. Transient kinetics indicated that Br^- reacted with photogenerated Ru(III) species at different rates. Bimolecular rate constants for the oxidation of Br^- by the Ru(III) species derived from complexes 1-5 are observed as 1.2 × 10⁸, 1.3 × 10⁹, 4.0 × 10⁹, 4.8 × 10⁹, and 1.1 × 10¹⁰, M^-1 s^-1, respectively. The last reaction kinetics observed in the three-component system consisting of a Ru sensitizer, quencher, and bromide is shown to be independent of the Ru sensitizer. The final product was identified as bromine by its reaction with hexene. The last reaction kinetics is assigned to the disproportionation reaction of Br₂^-? ions, for which the rate constant is determined as 5 × 10⁹ M^-1 s^-1. Though complex 6 has the highest oxidation potential in the Ru(II)/Ru(III) couple, its excited state fails to react with ArN₂⁺ sufficiently for subsequent reactions. The Ru(III) species derived from complex 1 reacts with Br^- at the slowest rate. Complexes 2-5 are excellent photosensitizers to drive photooxidation of bromide to bromine.

Full text of DOI:10.1021/acs.inorgchem.7b01238

Article
pubs.acs.org/IC
Oxidation of Bromide to Bromine by Ruthenium(II) Bipyridine-Type
Complexes Using the Flash-Quench Technique
Kelvin Yun-Da Tsai and I-Jy Chang*
Department of Chemistry, National Taiwan Normal University, 88 Tingchow Road, Section 4, Taipei 11677 Taiwan
*
S Supporting Information
2
+
2+
ABSTRACT: Six ruthenium complexes, [Ru(bpy) ] (1), [Ru(bpy) (deeb)] (2),
3
2
2
+
2+
2+
[
Ru(deeb) (dmbpy)] (3), [Ru(deeb) (bpy)] (4), [Ru(deeb)3] (5), and [Ru-
2
2
2+
(
deeb) (bpz)]
(6) (bpy: 2,2′-bipyridine; deeb: 4,4′-diethylester-2,2′-bipyridine;
2
dmbpy: 4,4′-dimethyl-2,2′-bipyridine, bpz: 2,2′-bipyrazine), have been employed to
sensitize photochemical oxidation of bromide to bromine. The oxidation potential for
complexes 1−6 are 1.26, 1.36, 1.42, 1.46, 1.56, and 1.66 V vs SCE, respectively. The
+
bimolecular rate constants for the quenching of complexes 1−6 by ArN
2
9
8
8
(
bromobenzenediazonium) are determined as 1.1 × 10 , 1.6 × 10 , 1.4 × 10 , 1.2 ×
8 7 6
−1 −1
−
1
0 , 6.4 × 10 , and 8.9 × 10 M s , respectively. Transient kinetics indicated that Br
−
reacted with photogenerated Ru(III) species at different rates. Bimolecular rate constants for the oxidation of Br by the Ru(III)
8
9
9
9
10
−1 −1
species derived from complexes 1−5 are observed as 1.2 × 10 , 1.3 × 10 , 4.0 × 10 , 4.8 × 10 , and 1.1 × 10 , M s ,
respectively. The last reaction kinetics observed in the three-component system consisting of a Ru sensitizer, quencher, and
bromide is shown to be independent of the Ru sensitizer. The final product was identified as bromine by its reaction with hexene.
−
•
The last reaction kinetics is assigned to the disproportionation reaction of Br ions, for which the rate constant is determined as
2
9
−1 −1
5
× 10 M s . Though complex 6 has the highest oxidation potential in the Ru(II)/Ru(III) couple, its excited state fails to
+
−
react with ArN2 sufficiently for subsequent reactions. The Ru(III) species derived from complex 1 reacts with Br at the slowest
rate. Complexes 2−5 are excellent photosensitizers to drive photooxidation of bromide to bromine.
INTRODUCTION
TON photoproduction of chlorine and bromine with most of
1
■
5−19
them conducted under UV irradiation.
Photochemical reactions take the advantage of photon energy
to drive otherwise energetically unfeasible reactions. However,
other than energy utilized to overcome reaction barriers,
surplus energy generates reactive species that cause undesired
reactions. For example, a photoinduced electron transfer (ET)
reaction generates potent oxidants and/or reductants that may
2+
Ruthenuim trisbipyridine ([Ru(bpy) ] ; complex 1) com-
3
plex has been widely incorporated in photoinduced electron
transfer (ET) reactions. With a suitable redox partner,
2+
[
Ru(bpy)3] (1) easily undergoes ET reactions from its
excited state and produces strong oxidants or reductants.
Substituents on the bpy ligand shift the redox potentials of the
complexes and hence modify the driving forces of the ET
1
,2
either cause back ET or side reactions. Sacrificial reagents
e.g., EDTA, triethanolamine) have been incorporated to react
(
20
reactions. Recently we have reported a visible-light driven,
with one of the photogenerated ET products to avoid the back
reaction; however, use of such reagents usually results in low
high TON, photocatalytic oxidation of bromide to bromine
2+
21
sensitized with [Ru(deeb) (dmbpy)] complex (complex 3).
3
−5
2
yields.
Additional, degradation of the photosensitizer may
To fully understand the reaction mechanism, we have
synthesized four other ruthenium complexes with strong
electron withdrawing groups on the bipyridine, [Ru-
result in low turn-over number (TON).
A flash-quench reaction scheme has been developed to study
the long-range electron transfer in biological systems.
central concept in the flash-quench technique is to design
reactions of the photogenerated species instead of the
photoexcited compounds. Thus, the back ET reaction may be
suppressed by fast subsequent reactions.
6
−10
The
2+
22
2+
22
(
(
bpy) (deeb)] (2), [Ru(deeb) (bpy)] (4), [Ru-
2 2
22 23
2+
2+
deeb)3] (5), and [Ru(deeb) (bpz)]
(6) together
2
2+
with [Ru(bpy)3] (1) (Scheme 1), to further investigate the
application of the flash-quench technique in the photo-
production of bromine.
Photochemical splitting of HX to hydrogen and halogen has
been demonstrated as a sustainable method for solar energy
RESULTS AND DISCUSSION
11,12
■
conversion and storage.
However, the efficiency of this
Photophysical and Redox Properties of Ruthenium
Complexes. The ruthenium complexes prepared in this work
exhibit similar absorption and emission spectra, while
reaction is often limited by the oxidation of halide to halogen.
Although the oxidation of iodide to iodine has been efficiently
13,14
coupled into photoreaction systems,
chlorine and bromine
productions remain an overwhelming challenge due to high
potentials required to oxidize chloride and bromide. Only a few
examples have been reported on either stoichiometric or low
Received: May 14, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01238
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Inorganic Chemistry
Article
Scheme 1
complexes with substituted bpy show red shifts in both+
absorption and emission bands compared to Ru(bpy)32
withdrawing ligands tends to afford a smaller HOMO−LUMO
24
gap in these type of complexes. The homoleptic complexes 1
(Figure 1). It is not surprising that the use of electron-
and 5 have sharper metal to ligand charge transfer (MLCT)
absorption bands, while heteroleptic complexes have wider
peak-to-shoulder separation. The absorption and emission
maxima are listed in Table 1. It has been shown that ruthenium
25
complexes with the deeb ligand have much longer lifetimes.
The same behavior has been observed in complexes 2−6.
Despite the low emission maximum of 668 nm, [Ru-
2+
(
bpy) (deeb)] has a lifetime of 910 ns. The long-lived
2
excited-state facilitates photoinduced ET reactions.
Redox potentials, crucial in understanding in ET reactions,
were measured for complexes 1−6 in acetonitrile solution and
listed in Table 1. Cyclic voltammograms of all complexes
(
Figure 2) exhibit one oxidation and three reduction peaks. The
oxidation process has been assigned to the oxidation of Ru(II)
to Ru(III). The higher oxidation potentials for complexes 2−6
2+
in comparison to [Ru(bpy) ] indicate electron-poor character
3
at the metal centers of 2−6 due to the coordination of the
strong electron withdrawing deeb ligand. Complex 6, [Ru-
2+
(
deeb) (bpz)] , judged to bear the highest electron-with-
2
drawing effect of the ligands exhibits the highest oxidation
potential (1.66 V vs SCE) among these sensitizers. The
reduction potentials were assigned to the ligand reduction
processes. Each diimine ligand is capable of one electron
reduction; therefore, three reduction peaks are observed. For
the homoleptic complexes 1 and 5, the reduction potential
increases about 0.2 V for each further reduction, though at very
different potentials. The first reduction potential of complex 5,
Figure 1. Absorption (a) and emission (b) spectra for complexes 1−6
in acetonitrile solution. Complex 1 (−), 2 (red line), 3 (dark blue
line), 4 (green line), 5 (purple line), and 6 (light blue line). Emission
spectra are not corrected but the intensities are normalized.
Table 1. Absorption and Emission Maxima, Lifetime, and Redox Potentials for Complexes 1−6 in Acetonitrile Solution
2+
λabs, nm (ε, 10 M cm−1
4
−1
E1/2(RuIII/II) , V
a
E1/2(Ru2+/+) , V
a
E1/2(Ru+/0) , V
a
E1/2(Ru0/−) , V
a
[
Ru(LL)] , LL =
)
λem, nm
τ, ns
1
2
3
4
5
6
(bpy)3
451 (1.46)
476 (1.40)
486 (1.22)
478 (1.26)
466 (1.65)
456 (1.08)
608
668
656
645
626
637
740
910
1.26
1.36
1.42
1.46
1.56
1.66
−1.36
−1.02
−0.96
−0.96
−0.91
−0.84
−1.55
−1.47
−1.15
−1.15
−1.06
−1.06
−1.78
−1.68
−1.63
−1.59
−1.27
−1.27
(bpy) (deeb)
2
b
(deeb) (dmbpy)
1130
1400
2020
1050
2
(deeb) (bpy)
2
(deeb)3
(deeb) (bpz)
2
a
b
Potentials are vs SCE. Electrolyte: 0.1 M Bu NBF . Ref 21.
4
4
B
DOI: 10.1021/acs.inorgchem.7b01238
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Inorganic Chemistry
Article
Figure 2. Cyclic voltammogram of complexes 1−6 (from top to
bottom) in acetonitrile solution. Standard three component cell that
2
consists of a glassy-carbon (3 mm ) working electrode, Pt and Ag
wires as counter and reference electrodes, respectively, are utilized.
Tetrabutylammonium tetrafluoroborate (0.1 M) is utilized as the
supporting electrolyte. Scan rate is 100 mV/s. Potentials are obtained
from separate scans with ferrocene as the internal standard.
2
+
[
Ru(deeb) ] , shows a substantial anodic shift relative to
Figure 3. (a) Emission decay observed at 668 nm for complex 2 with
3
2
+
+
+
complex 1, [Ru(bpy) ] . These well-separated reduction
various concentrations of ArN . [ArN ] = 0 (−), 0.5 (red line), 1.0
3
2
2
processes indicate that the electron is localized on individual
ligands, and each ligand reduction makes the further ligand
reduction harder. For heteroleptic complexes, three reduction
potentials show one large and one small gap. It is helpful to
assign the site of reduction based on these peak-separation
tendencies. For example, it is conclusive that the first reduction
for complex 2 and the first two reduction for complex 4 are the
reduction of deeb, while the subsequent peaks for complex 2
and the last reduction for complex 4 are the reduction of the
bpy ligand. The first reduction potential of −0.84 V for
complex 6 confirms that bpz is the ligand possessing the highest
electron-withdrawing effect employed in this study. The
electron-withdrawing effect decreases in the order of bpz,
(dark blue line), 2.0 (green line), 3.3 (purple line), 5.0 (light blue
line), and 10.0 (brown line) mM. (b) Stern−Volmer plots for I /I
o
+
(
black) and τ /τ (red) vs [ArN ]. Solid lines are fits obtained from eq
o
2
2
.
where V is the effective reaction volume. By using τ of 910 ns,
o
8
−1 −1
−1
k and V are determined as 1.6 × 10 M
s
and 81.1 M ,
q
respectively. Other complexes show similar behavior, and the
−1
calculated effective reaction volume is around ∼80 M , which
gives the effective reaction radius as around 3.2 nm. Quenching
rate constants determined are listed in Table 2.
Transient kinetic measurements of the quenching reaction at
MLCT band exhibit ground state bleach recovery. However,
deeb, bpy, and dmbpy.
+
Generation of Long-Lived Ru3+ by Using 4-Bromo-
the recovery does not complete. For reactions with ArN
2
, the
bleaching region shows a fast partial recovery from the
nonreacted excited complexes, and the remaining bleach is
held at the same magnitude up to 1 ms. This result indicates
benzenediazonium Tetrafluoroborate (ArN BF ). To
produce long-lived Ru species, an irreversible oxidative
quencher of 4-bromobenzenediazonium tetrafluoroborate
ArN BF ) was utilized.
ground-state ruthenium complexes with various concentrations
of ArN2 exhibit no change at all. Upon irradiation, the
emission intensity and the lifetime decrease as the concen-
tration of quencher are increased, as shown in Figure 3a. The
Stern−Volmer equation (eq 1) is generally utilized to describe
the linear relationship of emission intensity and lifetime versus
quencher concentration.
2
4
3
+
2
6,27
that a long-lived intermediate is formed. Considering the nature
(
The absorption spectra of
2
4
+
of the ArN , the intermediate is attributed to the oxidized form
2
+
of each ruthenium sensitizer, Ru(III) species. The long-lived
character of the Ru(III) species further supports the irreversible
nature of the quenching of the excited state of the Ru(II)
+
2
sensitizer by ArN . The reactions of ruthenium complexes with
ArN2+ are summarized in eqs 3−5.
II
II*
Ru + hν → Ru
(3)
(4)
I /I = τ /τ = 1 + k τ [Q]
Ru + ArN2+ → Ru + Ar + N2
II*
III
•
o
o
q o
(1)
2+
A Stern−Volmer plot of complex 2 [Ru(bpy) (deeb)] with
ArN2 is shown in Figure 3b. However, instead of a linear
2
•
Ar + solvent → Ar − H
(5)
+
+
relationship of the quencher concentration and lifetime, an
exponential behavior is observed. A sphere-of-action model can
be adopted to explain this phenomenon. The ET reaction
may take place before reactants reach their van der Waals
contact distance and may give extra contribution to the
reaction. At higher concentrations of the reactants, the sphere-
of-action is more pronounced, resulting in increase in the I /I
factor in an exponential fashion. A modified Stern−Volmer
equation (eq 2) is therefore employed to fit the observed data:
Upon reduction, ArN
2
quickly releases a nitrogen molecule
and an aryl radical. Dinitrogen evolves out of the solution for its
low solubility in acetonitrile, and the aryl radical abstracts a
hydrogen atom from solvent to produce Ar−H. This
28
3+
photoinduced electron-transfer produces Ru species in the
2
9
solution for at least 1 ms.
3
+
−
Reaction of Ru with Br . The formation of long-lived
o
Ru3 species gives the possibility to directly observe its reaction
+
+
with bromide. Mixing 50 μM of complex 2, 10 mM of ArN
2
and various bromide concentrations (50 to 1000 μM) showed
no change in the MLCT region of the absorption spectra and
I /I = τ /τ = (1 + k τ [Q]) exp(V[Q])
o
o
q o
(2)
C
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Article
Table 2. Bimolecular Quenching Rate Constants, Excited-State Potential Energies and Reduction Potentials, Work Term and
Reaction Gibbs Energies of Ruthenium Complexes with ArN2+
2
+
kq, M−1 s−1
V, M−1
a
b
E1/2(RuIII/II*), V
c
[
Ru(LL)] , LL =
r, nm
E , eV
ΔG , eV
−ΔGET(q), eV
00
w
9
1
2
3
4
5
6
(bpy)3
1.1 × 10
83.0
81.1
83.5
74.4
84.1
3.20
3.18
3.21
3.09
3.22
f
2.18
1.94
1.98
2.02
2.09
2.05
−0.92
−0.58
−0.56
−0.56
−0.53
−0.39
0.12
0.11
0.10
0.11
0.10
0.12
0.70
0.37
0.36
0.35
0.33
0.17
8
(bpy) (deeb)
1.6 × 10
2
d
8
(deeb) (dmbpy)
1.4 × 10
2
8
(deeb) (bpy)
1.2 × 10
2
7
(deeb)3
6.4 × 10
6
e
(deeb) (bpz)
8.9 × 10
f
2
a
b
c
= −(E (RuIII/II*) −
Effective reaction radius, determined from volume V. Calculated from the time-dependent DFT method. −ΔG
ET(q)
1/2
+
/0
+/0
d
e
E(ArN2 ) + ΔG ), see text, where E(ArN ) = −0.10 V vs SCE. Ref 21. The emission intensity and lifetime changes with various quencher
w
2
concentrations are within the instrument limit. The quenching rate constant is estimated from the yields of Ru(III) measured at MLCT bleach
f
spectra. Fits linear Stern−Volmer equation.
III
maintained the same emission intensities and lifetimes.
Although bromide is capable of quenching the excited-state
of ruthenium complexes, the quenching-rate constants are in
Ru formed from the excited-state quenching is about 1 μM
30
in this reaction condition. With a bromide concentration of at
least 50 μM, the slow bleach-recovery signal is treated as a
pseudo-first order reaction and is fitted to a single-exponential
decay function. By plotting the observed rates vs the bromide
concentration, the bimolecular reaction rate constant can be
obtained (Figure 4b). Complexes 1, 3−5 show similar kinetics,
and the rates are listed in Table 3. Complex 6 has very low
quenching to generate a sufficient amount of the corresponding
Ru(III) species, and therefore, is not subject to the bromide
reaction.
6
−1 −1 21
the proximity of ∼10 M s . At the highest bromide
concentration of 1 mM, it cannot compete with the quenching
+
reaction of 10 mM of ArN . Transient kinetics of the three-
2
components solution of complex 2 exhibits a dramatic change
at the MLCT bleach region (Figure 4). The long-lived bleach
3
+
−
signals recovery back to baseline due to Ru reacting with Br .
Direct Observation of Products by Transient Absorp-
tion. Transient kinetics monitored at 390 nm provide
important information about the intermediates and reaction
products. Figure 5 exhibits the excited-state absorption of 50
+
2
−
μM of complexes, 10 mM of ArN , and 1 mM Br at 390 nm.
Complexes 1 (Figure 5a) and 5 (Figure 5e) show clear biphasic
processes. In this reaction condition, the excited-state of
complex 1 is completely quenched by ArN2+ within 0.5 μs. The
observed biphasic kinetics must be due to subsequent reactions.
As shown in Figure 5a, the growth rate matches its Ru(III)
decay rate monitored at 451 nm; therefore, this growth process
−
is assigned to the bimolecular reaction of Ru(III) with Br . The
positive signal indicates a new species is formed and absorbs at
3
90 nm with a larger extinction coefficient than Ru(III) species.
After ∼5 μs, the positive signal begins to decay. The decay
process is attributed to the reaction of the new species and has
been followed to its completion (Figure 5f, black line).
Complex 5, on the other hand, shows two decay kinetics
(
Figure 5e). Since the calculated reaction rate for the excited-
−
5
+
5
−1
Figure 4. (a) Transient kinetics monitored at 476 nm of 5.0 × 10
M
state quenching of complex 5 with ArN2 (6.4 × 10 s ) is
much slower than the calculated reaction rate of Ru(III) with
Br (1.1 × 10 s ), the fast decay signal is the excited-state
decay, but the rate is dominated by the fast reaction between
+
−
of complex 2, 10 mM of ArN2 and various Br concentrations in
acetonitrile. [Br ] = 0 (−), 50 (red line), 200 (dark blue line), 500
green line), 750 (purple line), and 1000 (light blue line) μM. (b) Plot
−
−
7
−1
(
−
of observed rate constants vs [Br ].
−
Ru(III) and Br . For complexes 2−4, the two reactions, the
+
2
oxidative quenching of the excited state by ArN and the
oxidation of Br− by Ru(III), have comparable rates within
Table 3. Reaction Rate Constants, Work Term, Gibbs Energies, and Turnover Numbers for Bimolecular Reaction of
III
Photogenerated Ru Complexes with Bromide
2
+
kET(Br), M−1 s−1
E1/2(RuIII/II), V
a
[
Ru(LL)] , LL =
ΔG , eV
-ΔGET(Br)
,
eV
TON
w
8
1
2
3
4
5
(bpy)3
1.2 × 10
1.26
1.36
1.42
1.46
1.56
−0.15
−0.14
−0.13
−0.14
−0.13
0.19
0.28
0.33
0.38
0.47
116
142
160
152
140
9
(bpy) (deeb)
1.3 × 10
2
b
9
(deeb) (dmbpy)
4.0 × 10
2
9
(deeb) (bpy)
4.8 × 10
2
1
0
(deeb)3
1.1 × 10
a
ΔGET(Br) = −(E(Br /Br ) − E (RuIII/II) + ΔG ), where E(Br /Br ) = +1.22 V vs SCE. Ref 21.
•
−
•
−
b
−
1/2
w
D
DOI: 10.1021/acs.inorgchem.7b01238
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Inorganic Chemistry
Article
34
excited at 476 ± 16 nm from a 150 W Xe lamp. When the
reaction was completed, 10 mM of 1-hexene was added into the
solution. NMR sample was prepared by vacuum distillation of
1
the reaction solution. As shown in Figure 6, H NMR spectrum
Figure 6. Proton NMR spectra for (a) 1-hexene, (b) 10 mM of 1-
hexene reacted with a steady-state photochemical product of 5.0 ×
−5
+
2
−
1
0
M complex 2, 20 mM of ArN , and 20 mM Br in acetonitrile-d
solution, and (c) 10 mM of 1-hexene reacted with Br (l).
2
Figure 5. Transient absorption monitored at 390 nm. (a−e) 5.0 ×
of the solution from photoreaction exhibits both 1-hexene and
1,2-dibromohexane peaks. From the peak integration of Figure
6b, the TON is obtained as 144. Similar steady-state
−
5
+
−
1
0
M of complexes 1−5, 10 mM of ArN and 1 mM Br in
2
35
acetonitrile solution. Solid lines are calculated spectra using rate
−•
constants listed in Table 3 and Br disproportionation rate constant
2
photochemistry has been performed for other complexes (1,
9
−1 −1
of 5 × 10 M s . (f) Same kinetics of (a−e) (top to bottom)
3
−5). The TONs obtained are listed in Table 3.
Bimolecular Electron Transfer Reactions. In this study,
recorded to 1 ms.
bimolecular electron transfer reactions are keys to utilize solar
energy. Marcus ET theory describes the relationship between
ET rates and reaction Gibbs energy. The reaction Gibbs
individual complexes. The kinetics traces in Figure 5b−d
exhibit a slight hint of multiphasic behavior.
36
Notably, the initial rapid process possesses a rate that is
sensitive to the choice of complexes among 1−5, while the rates
for the slower process in the sub-millisecond time domain is
rather insensitive to the choice of complex (Figure 5f). The
similarity in the latter slow decay rates supports that the
ruthenium species does not participate in the latter slow
process. Molar extinction coefficient at 390 nm of bromine
energies for the electronically excited ruthenium complexes and
ArN2+ are calculated using ΔG
= E1/2(Ru *) −
III/II
ET(q)
+/0
E(ArN2 ) + ΔG , where ΔG is the work term that brings
w
w
37
two reactants together and is calculated from eq 9.
k Z Z
ε r12
e
1 2
ΔG =
w
(9)
−•
−
related species has been reported for Br , Br , and Br3 of
2
2
−1
−1
31,32
where k is Coulomb’s constant, ε is the relative permittivity of
e
3
800, 225, and 770 M cm , respectively,
coefficients of Br and Br are too small to be the major
The extinction
−
acetonitrile (37.5), Z and Z are the charges of the reactants,
1
2
2
3
and r12 is the van der Waals contact distance for the reactants.
The calculated results are listed in Table 2. The reaction Gibbs
energies for photogenerated Ru(III) species and bromide are
component in the microsecond time range. One-electron
−
•
•
oxidation of Br should give Br . It has been reported that Br
−
−• 31
combines with Br in diffusion limit to form Br
.
Therefore,
2
•
−
III/II
calculated using ΔG
= E(Br /Br ) − E (Ru ) + ΔG
−
ET(Br)
1/2
w
we assign the species formed in the reaction with Ru(III) that
•
−•
and are listed in Table 3, where E(Br /Br ) is estimated from
absorbs at 390 nm to Br2 . The slow decay at 390 nm has been
31
ET reactions of +1.22 V vs SCE.
fitted to an equal concentration bimolecular reaction with a rate
9
−1 −1
Reaction rate constants for both bimolecular reactions exhibit
a positive linear relationship with reaction Gibbs energies. By
constant of 5 × 10 M s . This reaction is attributed to the
−•
−
−
disproportionation reaction of Br2 to Br3 and Br . There is a
small residue signal at 1 ms for all reactions. Transient
absorption indicates the reactions followed by Ru(III) reacts
36
fitting into classical Marcus electron transfer theory (eq 10),
reorganization energies for the reactions are obtained.
−
◦ 2
with Br are summarized in eqs 6−8.
−(λ+ΔG ) /4λk T
B
kET = νne
(10)
RuIII + Br → Ru + Br
−
II
·
(6)
where ν is nuclear motion frequency (10 s−1) and λ is the
13
n
fast
reorganization energy for the reaction.
As shown in Figure 7, the reorganization energy for the
•
−
−•
Br + Br ⎯ ⎯→ Br
(7)
(8)
2
Br2 + Br2 → Br − + Br ⇌ Br + 2Br
−
•
−•
−
−
excited-state and quencher reaction (eq 4) is 1.82 eV. The self-
3
2
3+/2+
exchange reorganization (λ ) of Ru(bpy)
has been
11
3
33
38
Hexene has been routinely utilized to detect bromine.
reported to be 0.57 eV. By using the Marcus cross-relation
Since Br3− and Br are in equilibrium with Br + 2Br , the
−
−
21
(λ = 1/2 λ + 1/2 λ ), the self-exchange reorganization (λ )
2
12
11
22
22
−
+
hexene reaction is utilized to confirm the production of Br3
for ArN is estimated to be 3.07 eV. Since the reaction involves
2
+/0
and/or Br . Steady-state photochemistry has been performed to
a bond breaking in ArN , large reorganization energy is
2
39
2
acquire a suitable amount of product. Deuterated acetonitrile
expected. This large reorganization energy also explains the
smaller rate constants for the excited-state reaction compared
solution of complex 2 with 20 mM of ArN2+ and Br has been
−
E
DOI: 10.1021/acs.inorgchem.7b01238
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
concentration, as the only reactive species in the solution, it
quantitatively disproportionates to Br3− and Br .
−
CONCLUSION
■
Conversion of solar energy into chemical forms is the central
topic of photochemistry. Bimolecular reactions combined with
the flash-quench technique ransom part of the photon energy
to generate a low concentration reactive species that reacts
further with additional high concentration reactants. In this
design, part of the photon energy is stored in Br−Br bond
Figure 7. Plot of ET reaction rate constants vs negative Gibbs energy.
−/
formation. The last step for the Br3 Br formation is the slow
2
Black squares: Data from the excited-state quenching reactions. Red
equal-concentration bimolecular reaction. Since the production
−
circles: Data from the Ru(III) and Br reactions. Solid lines are the fits
−•
of its precursor, Br2 , consumes the reactive species in
to classical Marcus theory (eq 10). Dash line is the diffusion limit of 2
10
−1 −1
solution, the slow reaction can proceed to afford the final
product without interference. The photosensitizer, Ru com-
plexes, cycled back to the original state in the ground state
within 5 μs.
×
10
M
s .
−
to the Ru(III) and Br reaction with similar reaction Gibbs
energy. Performing the same calculation for Ru(III) and Br−
reaction of λ12 = 1.45 eV, we estimate the self-exchange
To achieve high TON, it is important to balance the driving
III
•
−
force of *Ru and Ru reactions. Complex 1 has the highest
reorganization (λ ) for Br / is 2.33 eV. This value is smaller
22
36,38
driving force for excited-state quenching, but it has the lowest
driving force for the oxidation of bromide and gives a minimal
yield. Complex 6 is prepared to have the highest driving force
than that calculated from eq 11 of 2.85 eV.
This result
indicates that bromide is partially solvated in acetonitrile.
⎛
⎞
⎠
−
⎛
⎞
for the reaction with Br . Nevertheless, it shows low efficiency
1
2ared
+ 2 a1 − 1 ⎟⎜
1
1
Ds
2
λout = Δe ⎜
−
⎟
⎟
⎜
in quenching to generate its strongly oxidizing Ru(III)
oxidation state. Complexes 2−5 have the best balance that
gives satisfied reaction rate constants for both reactions. TON
around 150 for complexes 2−5 support the argument. Bromine
production ultimately results in a drastic increase in optical
density leading to loss in the excitation efficiency by the Ru
sensitizers. The methodology for the efficient removal of the
⎝
r ⎠ D
ox
⎝
op
(11)
where Δe is the charge transferred from one reactant to the
other, a and a are the diameter of reduced and oxidized
red
ox
species, in this case Br (185 pm) and Br• (115 pm),
−
respectively; r is the center-to-center separation distance, Dop
and D are the optical (square of refractive index) and static
s
photogenerated Br during the photolysis must be developed in
dielectric constants of the solvent, respectively.
The overall reactions are summarized in Scheme 2.
Incorporating bipyridine ligands with electron withdrawing
2
order to gain a long-term productivity of the system.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Scheme 2
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01238.
General instruments, synthesis, time-dependent DFT
1
method, H NMR spectrum, ESI mass spectrum of
complexes 1−6. Quenching experiments, bromide
reaction of complexes 1, 4, and 5, hexene reaction with
photoproduct of complexes 1, 3−5 (PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: changijy@ntnu.edu.tw.
■
*
groups alternates both the MLCT excited-state and oxidation
potentials for complexes 1−6. The oxidation potentials have
bigger changes than the excited-state potential energy changes.
While keeping the irradiation in the visible region, the flash-
quenched products have 0.4 eV variants. High oxidation
potentials of the complexes are essential to oxidize bromide.
ORCID
I-Jy Chang: 0000-0001-5565-8174
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Although the quenching rate constants for ruthenium
■
+
complexes 1−5 with ArN differ by about 20-fold, 10 mM
The authors thank Prof. Ming-Kang (Brad) Tsai for his DFT
calculation of the excited-state potentials. Financial support
from National Taiwan Normal University is acknowledged.
2
+
3+
of ArN2 quenched the excited ruthenium complexes to Ru in
−
3+
less than 5 μs. With 1 mM Br present, Ru completely
reacted in about 10 μs. These fast reactions ensure no side
reactions that may occur from either ruthenium excited-states
or oxidized species. Bimolecular reactions allowed us to adjust
the concentrations of each reactant to give reactions in
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