Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex

Article abstract of DOI:10.1021/jacs.8b00944

The complex [Ru(deeb)(bpz)₂]²⁺ (RuBPZ²⁺, deeb = 4,4′-diethylester-2,2′-bipyridine, bpz = 2,2′-bipyrazine) forms a single ion pair with bromide, [RuBPZ²⁺, Br^-]⁺, with K_eq = 8400 ± 200 M^-1 in acetone. The RuBPZ²⁺ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ²⁺ acetone solution led to significant PL quenching and Stern-Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [RuBPZ²⁺, Br^-]⁺ and free RuBPZ²⁺, respectively. The single ion-pair [RuBPZ²⁺, Br^-]⁺? had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k_et = (2.2 ± 0.3) × 10⁷ s^-1. In contrast, RuBPZ²⁺? was dynamically quenched by bromide with a quenching rate constant, k_q = (8.1 ± 0.1) × 10¹⁰ M^-1 s^-1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ⁺ and Br₂^?- products that underwent recombination to regenerate the ground state with a second-order rate constant, k_cr = (2.3 ± 0.5) × 10¹⁰ M^-1 s^-1. Kinetic analysis revealed that RuBPZ⁺ was a primary photoproduct, while Br₂^?- was secondary product formed by the reaction of a Br^? with Br^-, k = (1.1 ± 0.2) × 10¹⁰ M^-1 s^-1. Marcus theory afforded an estimate of the formal reduction potential for E⁰(Br^?/-) in acetone, 1.42 V vs NHE. A ¹H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru^II center. Prolonged steady state photolysis of RuBPZ²⁺ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br₂. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ¹-bpz)(Br)]⁺, was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br₂ photoproducts.

Full text of DOI:10.1021/jacs.8b00944

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Visible Light Driven Bromide Oxidation and Ligand
Substitution Photochemistry of a Ru Diimine Complex
Guocan Li, Matthew David Brady, and Gerald J. Meyer
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Mar 2018
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Visible Light Driven Bromide Oxidation and
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Ligand Substitution Photochemistry of a Ru Diimine Complex
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Guocan Li, Matthew D. Brady, Gerald J. Meyer*
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Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599−3290, United States
Abstract: The complex [Ru(deeb)(bpz)2]2+ (RuBPZ2+, deeb = 4,4’-diethylester-2,2’-bipyridine, bpz = 2,2’-
bipyrazine) forms a single ion pair with bromide, [RuBPZ2+, Br−]+, with Keq = 8400 ± 200 M−1 in acetone.
The RuBPZ2+ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 µs. The
addition of bromide to a RuBPZ2+ acetone solution led to significant PL quenching and Stern-Volmer plots
showed upward curvature. Time-resolved PL measurements identified two excited state quenching
pathways, static and dynamic, which were operative towards [RuBPZ2+, Br−]+ and free RuBPZ2+,
respectively. The single ion-pair [RuBPZ2+, Br−]+* had a lifetime of 45 ± 5 ns, consistent with an electron
transfer rate constant, ket = (2.2 ± 0.3) × 107 s−1. In contrast, RuBPZ2+* was dynamically quenched by
bromide with a quenching rate constant, kq = (8.1 ± 0.1) ×1010 M−1s−1. Nano-second transient absorption
•−
revealed that both the static and dynamic pathways yielded RuBPZ+ and Br2 products that underwent
recombination to regenerate the ground state with a second-order rate constant, k = (2.3 ± 0.5) ×1010 M−1s−1.
•−
Kinetic analysis revealed that RuBPZ+ was a primary photoproduct, while Br2 was secondary product
formed by the reaction of a Br• with Br−, k = (1.1 ± 0.2) ×1010 M−1s−1. Marcus theory afforded an estimate
of the formal reduction potential for E0(Br•/−) in acetone, 1.42 V vs. NHE. A 1H NMR analysis indicated
that the ion-paired bromide was preferentially situated close to the RuII center. Prolonged steady state
photolysis of RuBPZ2+ and bromide yielded two ligand-substituted photoproducts, cis- and trans-
Ru(deeb)(bpz)Br2. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ1-bpz)(Br)]+, was found
to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br2 photoproducts.
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Introduction
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Excited state electron transfer reactions are central to molecular level approaches to solar energy
conversion.1-5 Recently hydrohalic acid splitting has re-emerged as a promising target.6-11 Hydrobromic
acid is of particular interest in this regard,12-16 Equation 1, as the hydrogen and bromine products are solar
fuels amenable to storage in flow batteries17-18 and fuel cells.19-22 Here we report bromide photo-oxidation
sensitized to visible light with a Ru diimine complex.
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2 HBr + hv  H2 + Br2
(1)
Dye-sensitized HBr splitting provides significant challenges: 1) Bromide oxidation requires a strong
photo-oxidant;23 2) Dye excited states generally provide one-electron transfer chemistry that in this case
yields reactive Br atom;24-25 3) Bromide is a coordinating ligand that may deactivate transition metal
complexes that serve as dyes;26-27 and 4) Bromine is reactive and may oxidatively add to transition metals
or undergo other unwanted reactions.
O
O
EtO
OEt
N
N
N
N
N
N
bpz
deeb
This report provides new insights into the first three of these challenges. A potent photo-oxidant
2+
Ru(deeb)(bpz)2 (RuBPZ2+), where bpz is 2,2’-bipyrazine and deeb is 4,4’-diethylester-2,2’-bipyridine
shown above, was synthesized and reacted with bromide in acetone. Lever has previously shown that RuII
bipyrazine complexes are strong photo-oxidants,28-29 and bromide oxidation has been found to be more
facile in organic solvents than in water.24-25 Clear evidence for excited state bromide oxidation was provided
by photoluminescence and absorption spectroscopies. Both static and dynamic excited state electron
transfer pathways were identified with compelling evidence of a bromine atom intermediate. Finally,
prolonged steady state photolysis led to the formation of both cis- and trans-Ru(deeb)(bpz)Br2, which
stands in contrast to a vast literature where only one isomer was formed. A photochemical intermediate,
tentatively assigned as [Ru(deeb)(bpz)(κ1-bpz)(Br)]+, was found to absorb a second photon and yielded the
isomers. The observation of two isomer provided new insights into ligand loss photochemistry
mechanism(s) that may be exploited for synthetic chemistry.
RESULTS
To our knowledge the photochemistry of the tri-bromide ion in non-aqueous solution has not been
previously reported and was investigated here to provide insights into the dye-sensitized bromide oxidation
reactions. The absorption spectra of tetrabutylammonium (TBA) salts of bromide and tri-bromide in
acetonitrile are shown in Figure 1a. Tri-bromide absorbs only weakly in the visible region with a strong
absorption in the ultraviolet region centered at 270 nm. Acetonitrile (CH3CN) was utilized here as it
provided a wider ultraviolet spectral window than did acetone, which was the solvent utilized for all the
dye-sensitized studies.
Nanosecond transient absorption spectroscopy was used to study the intermediates after pulsed 355 nm
laser excitation of tri-bromide in acetonitrile. The kinetics and spectra were dependent on the bromide
concentration. In the absence of bromide irreversible photochemistry was observed that precluded signal
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averaging, which was absent when bromide was intentionally added to the solution. Pulsed laser excitation
of 50 µM tri-bromide and 50 µM bromide in acetonitrile produced an intermediate consisting of two
positive absorptions centered at 370 nm and 770 nm, along with a tri-bromide ground state bleach below
•−
340 nm (Figure 1b). The positive absorption bands were consistent with previous reports of Br2 in
aqueous solutions.30 Representative kinetic data monitored at 770 nm (Figure 1b inset) revealed a biphasic
feature, a growth that could not be time resolved (> 108 s−1), and a slower growth that reached maximum in
several microseconds. The amplitudes of both features were equal.
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Figure 1. (a) Molar extinction coefficient of bromide (red), tri-bromide (green) and dibromide (blue) in acetonitrile.
Note the dibromide extinction coefficients were estimated from transient absorption experiments. (b) Transient
absorption spectra at indicated time delay after 355-nm pulsed laser excitation of an argon purged acetonitrile solution
containing 50 µM TBABr3 and 50 µM TBABr. Inset: absorption change at 770 nm after pulsed light excitation. (c)
Absorption change at 370 nm after 355-nm pulsed laser excitation of 50 µM TBABr3 and X µM TBABr in acetonitrile.
X = 50 (black), 100 (orange), 150 (blue), 200 (magenta), 1000 (green). Overlaid in red are the mono-exponential fits.
Inset: kobs as a function of bromide concentration.
The slow component appeared more quickly when the bromide concentration was increased. The data
were fit to an exponential function that provided kobs. Plots of kobs versus the bromide concentration afford
a second-order rate constant, kCH3CN = (1.7 ± 0.1) ×1010 M−1s−1 (Figure 1 c inset). The corresponding data
measured in acetone is given in Figure S1, with kacetone= (9.2 ± 0.7) × 109 M−1s−1. On a longer timescale
than is shown, this feature returned cleanly to baseline with a second-order equal concentration kinetics, k
= (5.0 ± 0.1) ×108 M−1s−1 and (2.3 ± 0.2) ×108 M−1s−1, in CH3CN and acetone, respectively (Figure S2).
Based on the initial and final growth amplitudes and the tri-bromide bleach, the molar extinction coefficient
of this species assigned as Br2•− was estimated (Figure 1a).
The absorption of RuBPZ2+ in acetone displayed two overlapping metal-to-ligand charge transfer
(MLCT) bands centered at 425 and 450 nm with an absorption onset around 600 nm (Figure 2). Bromide
addition led to significant changes in the RuBPZ2+ absorption spectra indicative of a ground state ion pair,
Figure 2. A substantial growth appeared around 360 nm, accompanied by slight suppression and red-shift
of the MLCT bands. Isosbestic points were preserved at 420 and 460 nm throughout the titration, consistent
with the formation of a single ion pair, denoted as [RuBPZ2+, Br−]+. A Benesi-Hildebrand analysis of the
absorption change at 360 nm afforded an equilibrium constant, Keq = 8400 ± 200 M−1, for the formation of
the ion pair.
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Figure 2. UV-Vis absorption of 55 µM RuBPZ2+ in acetone with increasing bromide concentration (0 − 1 mM).
To gain more insights into the ground state ion pair, 1H NMR spectroscopy was utilized in deuterated
acetone. Nine aromatic hydrogen atoms in RuBPZ2+ are labeled in Figure 3 due to the intrinsic C2
symmetry. A set of seven peaks were observed beyond 8.0 ppm that were assigned by analysis of peak
position, peak multiplicity and 2-dimensional COSY NMR spectra (Figure S3). In the presence of bromide,
a, a’, b, and f, shifted downfield, while d, c, and c’ shifted in the opposite direction. Negligible shifts were
observed for b’ and e. The hydrogen atoms that associate with bromide were expected to shift downfield
due to deshielding that occurs as the carbon-hydrogen bond is lengthened. At the same time, other protons
may shift upfield as they gain electron density from the bromide. In this sense, bromide preferentially
associated with a, a’, b, and f. Note that a and a’ are the most electron deficient and hence acidic protons
in the complex. The presence of bromide near b and f may be due to enhanced Coulombic attraction to the
formally dicationic RuII center. In the presence of four equivalents of bromide, a notably larger shift was
observed for proton b and f, 0.09 and 0.12 ppm respectively, relative to 0.03 ppm for a and a’, providing
compelling evidence that bromide was preferentially located near the RuII center rather than on the exterior
of the diimine ligands.
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4.0 eq
3.5 eq
3.0 eq
2.5 eq
2.0 eq
1.5 eq
1.0 eq
0 5 eq
10.2 10.0 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0
Figure 3. Upper: Molecular structure of RuBPZ2+ withft1h(e ar)omatic hydrogen atoms labeled. Bottom: NMR spectra
of RuBPZ2+ in the presence of indicated equiv of bromide in deuterated acetone, [RuBPZ2+] = 200 µM.
Visible light excitation of RuBPZ2+ resulted in orange photoluminescence (PL) centered at 622 nm
with a quantum yield of 0.09. The PL intensity was greatly attenuated when bromide was titrated into the
solution (Figure 4). Care was taken to excite at an isosbestic point so that the number of absorbed photons
remained unchanged throughout the titration. At high enough bromide concentrations, a red shift in the PL
spectrum was observed with a maximum at 635 nm that was largely unaffected by further addition of
bromide. The red shift was correlated with the low energy absorption change. At low bromide
concentrations, the Stern-Volmer plot was linear with KSV = (1.8 ± 0.1) × 105 M−1. At higher bromide
concentrations, upward curvature was apparent. A Franck-Condon line-shape analysis of the RuBPZ2+ and
[RuBPZ2+, Br−]+ PL spectra afforded estimates of the free energies stored in the excited states, ∆GES = 2.17
eV for both species (Figure S4). The photophysical properties of RuBPZ2+ and [RuBPZ2+, Br−]+ in acetone
are summarized in Table 1.
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Figure 4. Steady state photoluminescence spectra of RuBPZ2+ as bromide was titrated into a 10 µM acetone solution.
The inset shows a Stern-Volmer plot of this same data.
Table 1. Photophysical Properties of RuBPZ2+ and [RuBPZ2+, Br−]+ in Acetone.
kr
knr
PLmax
(nm)
622
τ
φPL
(%)
9.0
∆GES
(eV)
2.17
2.17
Compound
(104 s−1) (105 s−1)
(ns)
1750
45
RuBPZ2+
5.1
6.7
5.2
[RuBPZ2+, Br−]+
635
0.3
220
To better understand the upward curvature in the Stern-Volmer plots of the steady-state PL spectra,
excited state relaxation was quantified on a nanosecond timescale after pulsed laser excitation. In neat
acetone first-order relaxation was observed with a lifetime of 1.75 µs. Bromide addition led to the
appearance of bi-exponential kinetics with a short lifetime of τs = 45 ± 5 ns that was independent of the
bromide concentration and a longer lived component, τd, that was quenched by bromide (Figure 5a). The
kinetic data was fit to Equation 2 where As and Ad represent the pre-exponential coefficients.
PLI(t) = Asexp(-t/45 ns) + Adexp(-t/τd)
(2)
At low bromide concentration, the PL decay was dominated by τd. As more bromide was added, τs became
dominant and eventually at very high bromide concentration (> 100-fold), the excited state decay returned
to mono-exponential with a lifetime that was consistent with τs = 45 ns. Significantly, the ratio of As/Ad
agreed well with the relative concentration of [RuBPZ2+, Br−]+/RuBPZ2+ calculated based on Keq = 8400
M−1 (Figure 5b). These results provided compelling evidence that τs and τd represent the lifetimes of
[RuBPZ2+, Br−]+ and RuBPZ2+, respectively and that the ground and excited state equilibrium constants
for ion-pairing are very similar. A linear Stern-Volmer plot was produced based on τd (Figure 5a inset),
which afforded a KSV = (1.41 ± 0.01) × 105 M−1, corresponding to a dynamic quenching rate constant, kq =
(8.1 ± 0.1) × 1010 M−1s−1 based on kq = KSV/τ0.
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b)
Figure 5. (a) Time-resolved PL decays measured after pulsed light excitation of RuBPZ2+ in acetone with added
bromide. Overlaid in red are the bi-exponential fits to the data based on eq 2. The inset shows a Stern-Volmer plot
based on dynamic quenching, with an overlaid best fit line. (b) The pre-exponential coefficient ratio As/Ad with respect
to the calculated concentration ratio [RuBPZ2+, Br−]+/RuBPZ2+ based on Keq = 8400 M−1.
Nanosecond transient absorption was employed to elucidate the reaction mechanism. Pulsed 532 nm
laser excitation of RuBPZ2+ in acetone resulted in the prompt appearance of a transient absorption spectrum
consistent with formation of MLCT excited state. At low bromide concentrations, the MLCT absorption
spectrum was observed at early observation times with the appearance of new absorption features centered
at 370 and 500 nm that lived for hundreds of microseconds (Figure 6a). When the bromide concentration
was four times greater than the RuBPZ2+ concentration, this long-lived absorption was well modeled by
•−
equal concentration of Br2 and RuBPZ+ (Figure 6b). At lower bromide concentration, the long-lived
absorption spectra could not be satisfactorily modeled due to a smaller amplitude absorption around 370
•−
nm (Figure 6b). This discrepancy was consistent with less than 1 eq Br2 per RuBPZ+ at low [Br−],
suggested Br2•− was not a direct photoproduct of excited state electron transfer reactions.
•−
The formation of RuBPZ+ was monitored at 500 nm, where it absorbs strongly relative to Br2 , which
also represents a wavelength where a ground state bleach is observed. The absorption change was bi-
exponential and modelled with a fixed lifetime of τs = 45 ns and a lifetime, τd, that was bromide
concentration dependent (Figure 6c). At high bromide concentrations (≥ 1 mM), the absorption signal at
500 nm became mono-exponential with τs = 45 ns (Figure 6a inset), which represents the excited state
lifetime of [RuBPZ2+, Br−]+*. Plots of 1/τd with the free bromide concentration were linear affording a
second order rate constant of (8.6 ± 0.7) × 1010 M−1s−1 (Figure 6c inset), that agreed with kq values extracted
from time-resolved PL experiment, indicating τd was the lifetime of RuBPZ2+*. Hence RuBPZ+ was a
direct product of excited state electron transfer in both static and dynamic reactions. Cage escape yields
measured based on the concentration of RuBPZ+ remained at 0.055 ± 0.005 over a wide range of bromide
concentration, 10 µM to 5 mM, indicating the same cage escape yield for both static and dynamic reactions.
•−
The formation of Br2 was monitored at 400 nm, which represented an isosbestic point between the
RuBPZ2+ ground and excited state. As shown in Figure 6d, the appearance of Br2•− was delayed relative
to RuBPZ+, providing strong evidence that Br2•− was not a primary photochemical product of the excited
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state reaction. Indeed, the formation of Br2 was bromide concentration dependent. Varying bromide
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concentration allowed to abstract the second order rate constant for the formation of Br2 , (1.1 ± 0.2) ×
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1010 M–1s–1 (Figure 6d inset), which was 8 times smaller than that of RuBPZ+. At long time delay, Br2
and RuBPZ+ were found to recombine to yield ground state products with a second-order rate constant, k
= (2.3 ± 0.5) × 1010 M−1s−1.
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Figure 6. (a) Transient absorption spectra measured at the indicated time delays after pulsed 532 nm light excitation
of a 50 µM RuBPZ2+ and 1 mM bromide acetone solution. The inset shows the absorption change monitored at 500
nm with an overlaid mono-exponential fit, τ = 45 ns. (b) Absorption spectra measured 5 µs after pulsed light excitation
of 50 µM RuBPZ2+ and the indicated bromide concentrations. Overlaid as solid lines are simulated spectra based on
equal concentration of RuBPZ+ and Br2•−. (c) The absorption change monitored at 500 nm after pulsed light excitation
of a 50 µM RuBPZ2+ bromide acetone solution. Overlaid in red are fits to a bi-exponential kinetic model with one
time constant fixed at 45 ns. The inset shows kobs as a function of the free bromide concentration with an overlaid best
fit line, k = (8.6 ± 0.7) × 1010 M−1s−1. (d) The absorption change monitored at 400 nm (black) and at 500 nm (red)
after pulsed 532 nm laser excitation of 50 µM RuBPZ2+ and 300 µM bromide solution. The inset shows kobs as a
function of the free bromide concentration with an overlaid best fit line, k = (1.1 ± 0.2) × 1010 M−1s−1.
Prolonged 460 nm steady-state photolysis of RuBPZ2+ bromide acetone solutions led to significant
changes in the ground state absorption spectra (Figure 7a). In the first 10 minutes, a set of clean isosbestic
points at 357, 407, and 474 nm were preserved, consistent with the formation of a single photoproduct that
had two absorption bands centered at 380 and 510 nm. After 10 minutes, these isosbestic points were lost
and a new set of absorption bands appeared at 585, 670, and 870 nm. The initially formed photoproduct at
380 nm and 510 nm reached the maximum after 2 h, and further irradiation led to a decrease. After 20 h of
irradiation, the absorption spectra became time independent and three photoproducts were isolated, free bpz
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ligand and two isomeric Ru complexes, cis-Ru(deeb)(bpz)Br2 (purple), and trans-Ru(deeb)(bpz)Br2 (dark
green). A quantum yield of 0.4% was measured for the ligand loss reaction. The identity of these two
isomers was confirmed by 1H NMR (Figure 7b). In acetone, cis-Ru(deeb)(bpz)Br2 shows two absorption
peaks at 410 and 572 nm, while trans-Ru(deeb)(bpz)Br2 displays multiple absorption peaks at 400, 440,
490, 588, 670, and 870 nm (Figure 7c). It is worth noting the absorption onset of trans-Ru(deeb)(bpz)Br2
is 950 nm, one of the longest for reported RuII diimine complexes.4 The photo and thermal stability of these
two isomers were investigated, while cis-Ru(deeb)(bpz)Br2 showed remarkable photo and thermal stability,
trans-Ru(deeb)(bpz)Br2 slowly converted to cis-Ru(deeb)(bpz)Br2 under prolonged light irradiation or
thermal heating (Figure S5).
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When the photolysis was terminated prior to complete conversion, a dark orange intermediate was
identified and isolated by column chromatography. The UV-vis maxima of this orange intermediate at 382
and 504 nm (Figure 7c) were consistent with the absorption maxima of the intermediate observed in steady
state photolysis of RuBPZ2+ bromide acetone solutions with 380 and 510 nm absorption maxima. They
were also consistent with a mono-bromo-bound Ru complex.29, 31 The electrospray ionization mass
spectrum ((ESI-MS) of this intermediate in methanol solution revealed a peak at m/z = 798.0 (Figure S6),
consistent with chemical composition of [Ru(deeb)(bpz)(bpz)Br]+. The theoretical mass spectrum of
[Ru(deeb)(bpz)(bpz)Br]+ with natural isotope distributions agreed well with the experimental data (Figure
1
S6 inset). The H NMR spectrum of the intermediate showed aromatic resonances associated with the
diimine ligands with the ethyl ester groups below 5 ppm (Figure 7b). Integration in the aromatic region
gave a total of 18 aromatic hydrogen atoms, indicating that all three diimine ligands were still bound to Ru
center (Figure S7). The ethyl resonances showed two multiplets, which would be expected for a cis-
configuration rather than trans, Figure 7b. As continued light excitation resulted in loss of a bpz ligand,
we suspect that this intermediate is cis-[Ru(deeb)(bpz)(κ1-bpz)(Br)]+. However, the stereochemistry is
uncertain and an ion-paired species may have been observed in the ESI-MS. Therefore, the intermediate is
referred to as [Ru(deeb)(bpz)(bpz)(Br)]+.
b)
a)
c)
10.5
10.0
9.5
9.0
8.5
8.0
7.5
4.5
9.5
9.0
8.5
8.0
4.5
10.5
10.0
9.5
9.0
8.5
8.0
7.5 4.5
Figure 7. (a) UV-Vis absorption of 55 µM RuBPZ2+ and 110 µM TBABr in acetone under irradiation of a blue light
laser (460 nm, power = 3.36 mW) at selected time delay. Inset: absorption change spectra during photolysis. (b) NMR
of cis-Ru(deeb)(bpz)Br2 in CD3CN (purple), trans-Ru(deeb)(bpz)Br2 in CD2Cl2 (green), and the intermediate
[Ru(deeb)(bpz)(bpz)(Br)]+ (orange) in CDCl3. (c) Molar extinction coefficients of cis-Ru(deeb)(bpz)Br2 (purple),
trans-Ru(deeb)(bpz)Br2 (green), and [Ru(deeb)(bpz)(bpz)(Br)]+ in acetone.
To verify the orange product was indeed a photochemical intermediate, it was irradiated in a bromide
acetone solution. As shown in Figure 8a, steady state photolysis led to significant suppression of
[Ru(deeb)(bpz)(bpz)(Br)]+ absorptions, with concomitant formation of new absorption bands at long
wavelengths that were consistent with the formation of cis- and trans-Ru(deeb)(bpz)Br2. Standard addition
of the authentic absorption spectra of [Ru(deeb)(bpz)(bpz)(Br)]+, cis- and trans-Ru(deeb)(bpz)Br2 modelled
the absorption data accurately and showed that as the [Ru(deeb)(bpz)(bpz)(Br)]+ disappeared, the yield of
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cis- and trans-Ru(deeb)(bpz)Br2 increased cleanly with no evidence of other photoproducts (Figure 8b).
There was no evidence of a thermal reaction for 3 hours at room temperature (Figure 8c) or for 30 minutes
at 50 °C in the dark (Figure S8).
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Figure 8. a) UV-vis of an acetone solution of [Ru(deeb)(bpz)(κ1-bpz)Br]+ and bromide after irradiated with white
light using a Xenon lamp (150 W), the UV part of which was filtered off using a GG-420 filter. Overlaid in red
represent the modeled spectra based on standard addition of the authentic spectra of [Ru(deeb)(bpz)(bpz)Br]+, cis- and
trans-Ru(deeb)(bpz)Br2. b) The concentration of each species during photolysis based on spectra modeling. c) UV-
vis of an acetone solution of [Ru(deeb)(bpz)(bpz)Br]+ and bromide before (black) and after (red) storing in the dark
for 3 h. Inset: UV-vis of the same solution after irradiated with a Xenon lamp.
To test generality, photolysis of bromide acetone solutions of [Ru(deeb)2(bpz)]2+, [Ru(bpy)3]2+ or
[Ru(deeb)3]2+ were conducted. For [Ru(bpy)3]2+ only the cis-Ru(bpy)2Br2 product was observed while the
other two complexes yielded both cis- and trans-isomers. Relative to bpy, both deeb and bpz are weaker
σ-donors, which may facilitate cleavage of the second Ru−N bond in the trans-position to form the trans-
isomer.
Discussion
A mechanistic study of dye-sensitized bromide oxidation is reported in acetone solutions and revealed
two reaction pathways that enabled the bromine formal reduction potential in acetone to be approximated.
Ligand loss photochemistry produced both cis and trans isomers, behavior that we are unaware of in the
vast literature of RuII diimine compounds. Before discussion of this excited state reactivity, the
photochemistry of tri-bromide is first presented.
Tri-bromide Photochemistry
−
The direct excitation of tri-bromide, Br3 , was investigated in CH3CN solution as it provided a better
−
spectroscopic window than did acetone in the ultraviolet region. Pulsed 355 nm light excitation of Br3
led to the prompt formation of a transient spectra consistent with the formation of the bromine radical anion,
•−
often called dibromide Br2 . Mass balance implies that a bromine atom, Br• (eq 3), whose presence was
•−
indicated by bromide titration that yielded a second equivalent of Br2 with a rate constant, (1.7 ± 0.1)
×1010 M−1s−1, that was within a factor of two of that measured in acetone, (9.2 ± 0.7) × 109 M−1s−1 (eq 4),
•−
but was significantly larger than that reported in dichloromethane, (5.4 ± 1.0) × 108 M−1s−1.25 The Br2
underwent disproportionation to reset the ground state (eq 5).
h
→
−
−
Br3
[Br3 ]* → Br2•− + Br•
k3 > 108 s−1
(3)
(4)
(5)
•−
Br• + Br− → Br2
k4 = (1.7 ± 0.1) ×1010 M−1s−1
k5 = (5.0 ± 0.1) ×108 M−1s−1
Br2•− + Br2•− → Br3 + Br−
−
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Excited State Electron Transfer
The spectroscopic data provided compelling evidence for the ground state ion-pair equilibrium shown
in Scheme 1. Metal-to-ligand charge transfer excitation of bromide solutions of RuBPZ2+ or [RuBPZ2+,
Br−]+ formed identical products suggesting that the excited state solvent cage [RuBPZ2+*, Br−] formed by
diffusional interactions of RuBPZ2+* and Br− are similar to that formed when the ion-paired complex was
directly excited. Excited state quenching formally involved electron transfer from bromide to the RuIII
excited state to yield a bromine atom and the reduced ruthenium complex. This reaction was quantitative,
yet the yield of products measured was only about 6%, φce = 0.055. In the presence of excess Br− the cage
escaped bromine atoms reacted quantitatively to yield dibromide whereas in the absence of excess Br− some
irreversible photochemistry occurred. Dibromide and the reduced Ru complex returned to ground state
products with second-order equal concentration kinetics.
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Scheme 1. Summary for Excited State Electron Transfer Reactions of RuBPZ2+ and [RuBPZ2+, Br−]+.
= 0.055
ϕce
−
0.005
1
107
s
×
−
−
±
=
kdiff
2.2
ket
+ Br
•
•
]
RuBPZ2+
*, Br
RuBPZ+
, Br
RuBPZ+
RuBPZ2+
* + Br
[
]
[
+ Br−
±
− 0−.2)
k = (1.1
Μ
×1010
1
1s
+ Br
RuBPZ+
•−
= 1.75
= 45 ns
2
−
s
τ
ν
ν
h
τ
0
s
µ
1
−
Μ
h
0
1
10
×10
0.5)
±
(2.3
=
kcr
−
= 8400 M
1
−
Keq
−
+ Br
+
RuBPZ2+
RuBPZ2+
, Br
[
]
The quenching rate constant extracted from Stern-Volmer analysis, kq, is related to the true electron
transfer rate constant ket through eq 6,32 in which kdiff is the diffusion rate constant and KA is the association
constant. The value of kdiff has been calculated through eq 7, in which DRu and DBr represent the diffusion
coefficient of RuBPZ2+* and Br− in acetone, respectively. Based on a 7.0 and 1.96 Å radii for RuBPZ2+*
and Br−,33 DRu and DBr were found to be 9.75 × 10−10 and 3.48 × 10−9 m2s−1, respectively, through the
Einstein-Stokes equation. The parameter β is the effective reaction radius described by
Rcexp(Rc*κ)/[exp(Rc/R)−1], in which R is the sum of the radii of RuBPZ2+ and of Br−. The Onsager radius
Rc is ZRuZBre2/4πεrε0kbT, and κ−1 is the Debye length, (εrε0kbT/2000e2NAI)1/2.34 Based on an ionic strength
I = 2.5 × 10−4 M−1 and KA = 8400 M−1, an estimated electron transfer rate from Br− to RuBPZ2+* was
calculated to be ket = 4.1 × 107 s−1, which is within a factor of two of that measured for the ion-paired
complex.
1
1
1
=
+
ꢇ ꢀ
ꢈ ꢉꢊ
(6)
(7)
(8)
ꢀ
ꢀ
ꢁꢂꢃ
ꢄꢅꢆꢆ
(
)
ꢗ
ꢋꢌꢍꢎꢎ = 4ꢏꢐ ꢒꢓꢔ + ꢒꢕꢖ
ꢑ
2
ꢁ 2
2π |ꢚ
|
ꢈꢛ
ꢋꢘꢙ
=
exp(− (λ+∆ꢝ )
)
ℏ
4λꢀ ꢜ
4πλꢀ ꢜ
�
ꢂ
ꢂ
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9
When typical values for a pre-exponential factor, 1011 s−1, and a reorganization energy λ = 1 eV are
assumed, Marcus theory and eq. 8 indicate that ∆G° = −0.11 eV. The free energy change associated with
electron transfer reaction is given by eq 9. In this equation E(Br•/−) is the bromine formal reduction potential
and E(Ru2+*/+) is the excited state reduction potential of RuBPZ2+, calculated to be 1.68 V vs NHE with a
previously reported ground state reduction potential of −0.49 V vs. NHE,24 and the free energy stored in
the excited state ∆GES is 2.17 eV. The work term, ∆Gw, accounts for the Coulombic energy associated with
electron transfer, and is defined in eq 10, where ke is Coulomb’s constant, ε is the relative permittivity of
acetone (20.7), ZBr is the charge of bromide, Zi is the partial charge of atom i in RuBPZ2+, and ri is the
distance between bromide and atom i in RuBPZ2+.35 Since the bromine atom is uncharged, the Coulombic
potential energy after electron transfer is zero. For the reactants, ∆Gw was approximated by assuming point
charges in the center of mass for RuBPZ2+ and Br− with a sum of the ionic radii as 8.96 Å, from which ∆Gw
was calculated to be 0.15 eV. This afforded an estimate of E(Br•/−) = 1.42 V vs. NHE, which is substantially
more negative that the accepted value in water of 1.92 V vs. NHE.23
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∆Grxn = [E(Br•/−) − E(Ru2+*/+)]F + ∆Gw
(9)
k
Z
Z
Br
i
e
∑
∆Gw =
∆
(10)
ꢍ=1
ε
r
i
Ligand Exchange Mechanism
An Achilles heel of Ru diimine complexes is their susceptibility to ligand loss photochemistry,
especially in the presence of halides. To our knowledge, in all such previously reported reactions one
isomeric photoproduct of the form Ru(LL)2X2, where X stands for halides, were formed.36 The vast
majority of studies reported the cis-product,26-27, 29, 31, 37-53 while [Ru(bpy’)(btz)2]2+ (btz=1,1’-dibenzyl-4,4’-
bi-1,2,3-triazolyl) yielded the trans-[Ru(bpy’)(btz)(CH3CN)2]2+ isomer as the only photoproduct.54-56
Recently unprecedent ligand loss photochemistry was reported for an OsII compound, [Os(btz)3]2+, that
yielded both cis- and trans-[Os(btz)2(CH3CN)2]2+ photoproducts.57 It was therefore of interest to understand
the reaction mechanism in the present ligand loss photochemistry, a reaction that is unwanted for HBr
splitting but, may have synthetic utility for preparing trans-Ru complexes.
To identify the photochemical reaction mechanism(s), acetone solutions of bromide and RuBPZ2+ were
photolyzed and the reaction products were separated chromatographically and were characterized. This
provided authentic samples of [Ru(deeb)(bpz)(bpz)(Br)]+, cis- and trans-Ru(deeb)(bpz)Br2 that were used
as reference spectra for global analysis of the photolysis mixtures. It is notable that the trans isomer has
significant absorption beyond 900 nm.
Figure 9 shows the visible absorption spectra of a 460-nm light illuminated bromide and RuBPZ2+
solution measured from 10 minutes to 20 hours. Note that the initial spectrum was used as a reference such
that these represent difference spectra. It was not possible to simulate these spectra with standard addition
of cis- and trans- Ru(deeb)(bpz)Br2 and the spectra of [Ru(deeb)(bpz)(bpz)(Br)]+ was therefore included.
Remarkably, standard addition of these three spectra afforded very good agreement with the experimental
spectra throughout the entire photolysis experiment, suggesting these three species were the only visible
light absorbing photoproducts involved.
As shown in Figure 9b, the concentration of
[Ru(deeb)(bpz)(bpz)(Br)]+ first increased and then decreased, consistent with its intermediate nature.
Although ligand photosubstitution chemistry of Ru tris-diimine complexes has been known for several
decades, the reaction mechanism is still unclear. Therefore, the present work represents one of the very
few examples that enable isolation and direct observation of the photosubstitution intermediate. To the best
of our knowledge, the only other reports managing to isolate the intermediate were the CH3CN-substituted
photochemistry of a series of [Ru(bpy’)(btz)2]2+ complexes.54-55
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b)
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20 h
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10 min
Figure 9. a) Absorption changes measured from 10 min to 20 h during the 460-nm photolysis of a 55 µM RuBPZ2+
and 110 µM TBABr acetone solution. Overlaid on the data in red are modeled spectra. b) The concentration of each
species as a function of time based on spectra modeling.
It was a surprise to find that trans-Ru(deeb)(bpz)Br2 was the dominant photoproduct, particularly
because thermal experiments indicated that cis-Ru(deeb)(bpz)Br2 was the thermodynamically more stable
isomer. Furthermore, only cis-photoproducts have been reported for the prototypical Ru(bpy)3 , consistent
with these studies where cis-Ru(bpy)2Br2 was the only photoproduct observed. However, photolysis of
acetone bromide solutions of [Ru(deeb)2(bpz)]2+ or [Ru(deeb)3]2+ revealed that both cis- and trans-dibromo
isomers were formed. Therefore, the presence of the electron withdrawing ethyl ester or nitrogen groups
appears to influence this photochemistry and prompted a closer investigation.
To rationalize the photochemistry, a mechanism wherein light absorption forms the MLCT excited state
that undergoes activated crossing to a dissociative ligand field excited state39 that dissociates a N from a
bpz ligand followed by Br− coordination to yield [Ru(deeb)(κ1-bpz)(bpz)Br]+, Scheme 2. This intermediate
was isolated yet characterization failed to provide a conclusive stereochemistry. The inequivalence of the
resonances associated with the ethyl esters and the large number of aromatic resonances associated with
the diimmine ligands, suggests a cis-geometry, but this could also be a result of the lower symmetry of the
κ1-bpz ligand and/or a mixture of both isomers which precludes a definitive assignment. The coordination
of Br− to Ru will weaken the Ru-N bond trans- to Br− and as a weaker σ-donor the bpz ligand would be
preferentially released. Full release of the bpz ligand required a second photon. If the intermediate was the
cis isomer, a cis- product would be expected followed by isomerization to yield the trans-product. The
proposed kinetic competition between Br− coordination and isomerization implies that a higher yield Br−
concentration would yield more cis-isomer which was consistent with experimental findings (Figure S9).
2+
Scheme 2. Proposed Mechanism for Ligand Substitution Photochemistry Between RuBPZ2+ And Br−.
O
O
+
Br
Br
+ Br−
EtO
EtO
EtO
EtO
Br
N
N
N
N
RuII
N
RuII
N
N
N
N
2+
ν
N
N
h
O
O
O
O
N
N
−
N
bpz
N
N
EtO
EtO
N
N
N
N
ν
h
[Ru(deeb)(bpz)(bpz)Br]+
RuII
N
+ Br−
cis-isomer
ν
h
−
O
O
O
bpz
+
Br
Br
N
N
N
+ Br−
N
EtO
EtO
EtO
EtO
N
N
N
N
RuII
RuII
N
N
N
N
N
Br
O
trans-isomer
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Conclusion
In summary, visible light driven bromide oxidation by RuBPZ2+* in acetone was investigated
comprehensively by UV-vis, NMR, steady-state/time-resolved photoluminescence, and nanosecond
transient absorption techniques. A ground state equilibrium between RuBPZ2+ and a single bromide ion
pair [RuBPZ2+, Br−]+ was evident. Static and dynamic excited state electron transfer pathways were
identified for [RuBPZ2+, Br−]+ and RuBPZ2+, respectively. Transient absorption spectroscopy revealed
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•−
two photo-products: the reduced ruthenium complex, RuBPZ+, and Br2 . The RuBPZ+ was a primary
•−
photochemical product while an intermediate Br• was involved in the formation of Br2 . Prolonged laser
irradiation afforded cis- and trans-Ru(deeb)(bpz)Br2. A tentatively assigned [Ru(deeb)(bpz)(κ1-bpz)Br]+
intermediate was proposed in the photochemical ligand loss mechanism. This study provides new insights
into visible light driven halide oxidation and halide substitution reactions of coordination compounds in
fluid solution.
Experimental
Materials. Tetrabutylammonium bromide (TBABr, Sigma-Aldrich, ≥99.0%), tetrabutylammonium tri-
bromide (TBABr3, Sigma-Aldrich, 98%), acetone (Burdick & Jackson, HPLC grade, >99.9%), acetonitrile
(Burdick & Jackson, HPLC grade, >99.9%) and deuterated acetone (Cambridge Isotope Laboratories) were
used as received. Argon gas (Airgas, 99.998%) was passed through a Drierite drying tube prior to use.
[Ru(deeb)(bpz)2](PF6)2 was prepared following literature procedure.58
Preparation of cis- and trans-Ru(deeb)(bpz)Br2. A mixture of RuBPZ2+ (50 mg, 0.05 mmol) and TBABr
(32 mg, 0.1 mmol) were dissolved in 100 mL of acetone in a round bottom flask sealed with a rubber septum.
The solution was purged with argon for 20 min and illuminated with a white light lamp for 4 hours. The
solvent was removed on rotary evaporator and the crude was loaded onto a silica gel column using
DCM:MeOH = 20:1 (v: v) as eluent. After collecting a fast-moving green and purple bands, the eluent was
changed to DCM:MeOH = 10: 1 (v: v), and the following orange band was collected. After removing the
solvent on rotary evaporator, the three fractions were recrystallized in DCM/Hexane to afford the trans-
Ru(deeb)(bpz)Br2 (green), cis-Ru(deeb)(bpz)Br2 (purple), and the intermediate [Ru(deeb)(bpz)(bpz)Br]+,
respectively.
Characterization. NMR spectra were measured on a Bruker Avance III 600 MHz spectrometer at room
temperature. NMR spectra were processed using MNOVA software. UV−Vis absorption spectra were
recorded on a Varian Cary 60 spectrophotometer in a quartz cuvette with a 1.0 cm pathlength. Mass
spectrometry was performed on a Shimadzu LCMS and an LTQ VELOS Thermo LCMS (positive mode).
Photoluminescence and Transient Absorption. Solutions were sparged with argon for at least 20 min
prior to steady-state, time-resolved PL, and transient absorption experiments.
Steady-state PL spectra were obtained on a HORIBA Fluorolog 3 spectrofluorometer equipped with a
450 W Xe arc lamp as the excitation source. Samples were excited at 460 nm, which is the isosbestic point
of RuBPZ2+ and [RuBPZ2+, Br−]+. The intensity was integrated for 0.1 s at 1 nm resolution and averaged
over 3 scans. Quantum yields were measured using [Ru(bpy)3](PF6)2 in dichloromethane as the standard
(ϕPL = 0.029) with the optically dilute method.59
Time-resolved PL data were acquired with pulsed laser excitation at 500 nm (time resolution: 10 ns)
by pumping a dye laser with a PTI GL-3300 nitrogen laser. The PL was detected by a Hamamatsu R928
PMT optically coupled to a ScienceTech Model 9010 monochromator terminated into a LeCroy
Waverunner LT322 oscilloscope. PL decays were monitored at 630 nm and averaged over 180 scans.
For nanosecond transient absorption, a Q-switched, pulsed Nd:YAG laser (BigSky Brilliant B 5−6 ns
fwhm, 1 Hz, ∼ 1 cm in diameter) was doubled to 532 nm and served as the excitation light source. A 150
W xenon arc lamp (Applied Physics) pulsed at 1 Hz with 70 V was used as the probe at a right angle. The
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light from the lamp was collected by a monochromator (SPEX 1702/ 04) optically coupled to an R928
photomultiplier tube (Hamamatsu). The signal from the PMT was sent to a digital oscilloscope (LeCroy
9450, Dual 330 MHz) and averaged 30 times to acquire the kinetic data.
Bromide Titration Experiments. Acetone solution of 4 mL 50 µM RuBPZ2+ in a quartz cuvette was
sparged with argon for 20 min. Argon was passed through acetone prior to the sample to minimize solvent
evaporation. 5 mM argon-purged TBABr acetone solution was added to the RuBPZ2+ solution using a
micro-syringe at an interval of 10 µL until UV-Vis absorption/time-resolved PL became unchanged. The
solution volume change was less than 5% throughout the titration. Note that in the UV-Vis titration
experiments, argon purge was not performed.
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Spectral Modeling. Franck-Condon lineshape analysis was performed following a published procedure.58
The absorption of the photolysis solution was globally modelled using the molar extinction coefficient of
the intermediate, cis-isomer, and trans-isomer as key spectra in a self-written Mathematica program, which
afforded the concentration of each species.
Determination of the Extinction Coefficient of RuBPZ+. Acetone solution of 4 mL 50 µM RuBPZ2+ was
sparged with argon for 20 min in a quartz cuvette. Argon was passed through acetone prior to the sample
to minimize solvent evaporation. 10 µL argon-purged triethanolamine was added to the RuBPZ2+ solution
using a micro-syringe and the mixture was irradiated with a lamp. The UV-Vis absorption of the solution
was intermittently recorded until no further change, at which point the solution turned from orange to purple.
The newly formed solution was the one-electron reduced product, RuBPZ+.
Determination of the Extinction Coefficient of Dibromide. Nanosecond transient absorption (TA)
spectra of 50 µM TBABr and 50 µM TBABr3 in CH3CN was collected by pulsed excitation at 355 nm. The
50 ns and 10 µs TA spectra, ∆Abs (50 ns) and ∆Abs (10 µs), corresponds to reaction in eq 3 and eq 11,
•−
respectively, which gives rise to eq 12 and 13, in which C(Br2 ) represents the concentration of Br2•− at 50
ns after laser excitation and ε represents molar extinction coefficient.
−
•−
Br3 + Br− → 2Br2
(11)
(12)
(13)
•−
•−
•−
−
∆Abs (50 ns) = C(Br2 )*ε(Br2 ) − C(Br2 )*ε(Br3 )
•−
•−
•−
−
∆Abs (10 µs) = 2*C(Br2 )*ε(Br2 ) − C(Br2 )*ε(Br3 )
•−
Based on eq 12 and 13, C(Br2 ) was calculated in eq 14.
•−
−
C(Br2 ) = [∆Abs (10 µs) − 2*∆Abs (50 ns)]/ε(Br3 )
(14)
(15)
•−
ε(Br2 ) was therefore calculated using eq 15.
•−
•−
ε(Br2 ) = [∆Abs (10 µs) − ∆Abs (50 ns)]/ C(Br2 )
Photolysis Experiment. Acetone solutions containing 55 µM RuBPZ2+ and 110 µM or 1 mM TBABr in
a quartz cuvette were sparged with argon gas for 20 mins in the dark prior to photolysis and sealed under
positive pressure. Argon was passed through acetone prior to the sample to minimize solvent evaporation.
The solutions were illuminated using a Coherent Genesis MX 460 nm solid state laser (power = 3.36 mW)
and were stirred constantly during photolysis. The UV-Vis absorption of the solution was intermittently
recorded on a Varian Cary 60 spectrophotometer with 1 nm resolution.
Acknowledgement. This material is based upon work solely supported as part of the UNC EFRC: Center
for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number DE-SC0001011.
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Supporting Information. Nanosecond transient absorption spectra of TBABr3 in acetone; disappearance
kinetics of dibromide in acetone and CH3CN; COSY NMR of RuBPZ2+ in acetone; steady-state PL of
RuBPZ2+ and [RuBPZ2+, Br−]+ and overlaid Franck-Condon line-shape analysis; photo- and thermo-
stability of cis- and trans- Ru(deeb)(bpz)Br2 in acetone; mass spectrometry of [Ru(deeb)(bpz)(bpz)Br]+;
NMR of [Ru(deeb)(bpz)(bpz)Br]+ in CDCl3 with integration; thermal stability test of
[Ru(deeb)(bpz)(bpz)Br]+ at 50 °C for 30 min in the dark; concentration of each species during 460-nm
photolysis of 50 µM RuBPZ2+ and 1 mM TBABr acetone solution.
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Corresponding Author
* gjmeyer@email.unc.edu
Notes
The authors declare no competing financial interest.
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Visible Light Driven Bromide Oxidation and
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Ligand Substitution Photochemistry of a Ru Diimine Complex
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Guocan Li, Matthew D. Brady, Gerald J. Meyer*
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Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599−3290, United States
N
+
Br
O
N
N
RuII
N
EtO
EtO
N
N
N
N
O
2.2
N
Br
O
N
×
+
static
Br
EtO
10 7
Br
−
1
N
N
O
O
s
N
RuII
N
pathway
N
N
N
RuII
N
= 45 ns
EtO
EtO
τ
N
N
N
N
N
EtO
N
ligand-substitution
O
O
N
+
Br
+
•
Br
Photochemistry
Bromide Oxidation
N
−
1
8400 M
Br
Br
−
1s 1
N
−
Br
Br
EtO
Br
N
N
N
N
ligand-substitution
Br
−
1s−
N
M
109
M
RuII
Br
10
×
1
2+
9.2
10
×
EtO
O
pathway
N
N
N
N
8.1
EtO
EtO
O
N
N
N
N
RuII
dynamic
N
O
N
= 1.75
τ
s
µ
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