Photostable Polynuclear Ruthenium(II) Photosensitizers Competent for Dehalogenation Photoredox Catalysis at 590 nm

Article abstract of DOI:10.1021/jacs.0c01503

Higher nuclearity photosensitizers produced dehalogenation yields greater than 90% in the reported [Ru(bpy)₃]²⁺-mediated dehalogenation of 4-bromobenzyl-2-chloro-2-phenylacetate to 4-bromobenzyl-2-phenylacetate with orange light in 7

Full text of DOI:10.1021/jacs.0c01503

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Communication
Photostable Polynuclear Ruthenium(II) Photosensitizers
Competent for Dehalogenation Photoredox Catalysis at 590 nm
Simon Cerfontaine, Sara A. M. Wehlin, Benjamin Elias, and Ludovic Troian-Gautier
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c01503 • Publication Date (Web): 08 Mar 2020
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Photostable Polynuclear Ruthenium(II) Photosensitizers Competent
for Dehalogenation Photoredox Catalysis at 590 nm
Simon Cerfontaine,† Sara A. M. Wehlin,‡ Benjamin Elias†* and Ludovic Troian-Gautier^*
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† Université catholique de Louvain (UCLouvain), Institut de la Matière Condensée et des Nanosciences (IMCN), Molecular
Chemistry, Materials and Catalysis (MOST), Place Louis Pasteur 1, bte L4.01.02, 1348 Louvain-la-Neuve, Belgium
^ Laboratoire de Chimie Organique, Université libre de Bruxelles (ULB), CP 160/06, 50 avenue F.D. Roosevelt, 1050
Brussels, Belgium.
‡ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599-3290, United
States
ABSTRACT: Higher nuclearity photosensitizers produced dehalogenation yields greater than 90% in the reported [Ru(bpy)3]2+-
mediated dehalogenation of 4-bromobenzyl-2-chloro-2-phenylacetate to 4-bromobenzyl-2-phenylacetate with orange light in 7 hours,
whereas after 72 hours yields of 49% were obtained with [Ru(bpy)3]2+. Dinuclear (D1), trinuclear (T1) and quadrinuclear (Q1) ruthe-
nium 2,2’-bipyridine based photosensitizers were synthesized, characterized and investigated for their photoreactivity. Three main
factors were shown to lead to increased yields: i) the red-shifted absorbance of polynuclear photosensitizers, ii) the more favorable
driving force for electron transfer, characterized by more positive E1/2(Ru2+*/+) and iii) the smaller population of the 3MC state (< 0.5
% for D1, T1 and Q1 vs. 48% for [Ru(bpy)3]2+ at room temperature). Collectively, these results highlight the potential advantages of
using polynuclear photosensitizers in phototriggered redox catalysis reactions.
The increased reactivity of sensitizers upon light excitation
has contributed to the drastic development of the field of pho-
toredox catalysis, where they have been harnessed to perform
challenging organic transformations, difficult to achieve with-
out the use of photoactive catalysts.1-7 While some organic sen-
sitizers have been reported, most are based on mononuclear
transition metal complexes, including iridium, copper and ru-
thenium. Iridium(III) complexes are usually highly photostable
and can be tuned to achieve a wide range of catalytic reactions.5,
8-9 However, their low molar absorption coefficients in the vis-
ible region has limited their widespread application. Copper(I)
bipyridine complexes exhibit long-lived Metal-to-Ligand
Charge Transfer (MLCT) excited states compatible with photo-
redox catalysis.10-13 Nonetheless, copper complexes are often
associated with low molar absorption coefficients (e) in the vis-
ible region and Jahn-Teller distortions accompanying changes
in the oxidation states, which often leads to ligand-loss or
scrambling, particularly problematic for heteroleptic com-
plexes.14-16 In contrast, ruthenium(II) polypyridyl sensitizers ab-
sorb visible light typically with e greater than 10000 M–1cm–1
and exhibit relatively long-lived excited states.17-20 Larger e are
obtained with polynuclear photosensitizers (PS), which have
been known to undergo excited-state electron transfer with cat-
alysts in the presence of sacrificial electron donors or accep-
tors.17, 21-32 Ruthenium complexes exhibit greater stability than
Cu complexes, but are typically more prone to ligand-loss than
iridium complexes. This is often attributed to population of a
metal-centered (3MC) state.33-36 Hence, i) better control of the
deactivation pathways, ii) the development of photosensitizers
that absorb more visible light, allowing to decrease to amount
of photosensitizer, iii) the use of low energy visible light and iv)
the development of stable photosensitizers represent challenges
upon which photoredox catalysis could be further improved.
Here, a series of polynuclear photosensitizers was investi-
gated for the reported visible light-induced dehalogenation of
4-bromobenzyl-2-chloro-2-phenylacetate (1) in the presence of
a sacrificial electron donor and compared to the performance of
the parent compound, [Ru(bpy)3]2+.37 These dehalogenation re-
actions are of synthetic and environmental importance.3, 11, 38-40
Fundamental understanding was garnered through Stern-
Volmer analysis and variable temperature time-resolved photo-
luminescence spectroscopies. In comparison to [Ru(bpy)3]2+,
polynuclear complexes were stronger photo-oxidants, produced
larger yields with low energy visible light and smaller catalyst
loadings. In addition, they exhibited less ligand-loss photo-
chemistry, making them a real alternative to common photore-
dox catalysts.
The syntheses of the polynuclear photosensitizers (Figure
1a) were achieved through coordination of [Ru(bpy)2Cl2] to the
desired bridging ligand, itself synthesized via transition-metal
catalyzed coupling between 4-bromo-2,2’-bipyridine and 4,4’-
dibromo-2,2’-bipyridine derivatives (SI). The photosensitizers
exhibited ground-state absorption spectra (Figure 1b) and
steady-state photoluminescence (Figure 1c-d) typical for ruthe-
nium(II) polypyridyl complexes. The molar absorption coeffi-
cients of the MLCT transition gradually increased from 11,900
M–1cm–1 for [Ru(bpy)3]2+ to 54,900 M–1cm–1 for Q1, concomi-
tant with increased absorption at longer wavelengths (Table 1
and Figure S32). The excited-state lifetime (Table 1) was long-
est for D1 (1920 ns) and decreased according to
T1>Q1>[Ru(bpy)3]2+.
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Figure 1. a) Structures of photosensitizers [Ru(bpy)3]2+, D1, T1 and Q1. b) UV-Vis absorption spectra (solid lines) of the photosensi-
tizers recorded in acetonitrile at room temperature and emission profiles (dashed lines) of the different light sources.41 Steady-state
photoluminescence of the indicated photosensitizers in acetonitrile at room temperature under argon (c) and in butyronitrile at 77K
(d).
Table 1. Photophysical and Electrochemical Characterization of the Photosensitizers (PS).
e
PS
E1/2(L0/–)d
E1/2(RuIII/II d
)
E00
E1/2 (Ru2+*/+ d
)
e
(M–1cm–1)a
tb
Fc
[Ru(bpy)3]2+
11900 (450)
21800 (480)
40100 (485)
54900 (490)
1020
1920
1580
1470
0.094
0.077
0.121
0.098
– 1.31
– 0.99
– 0.93
– 0.83
1.30
1.33
1.38
1.37
579
629
640
644
0.83
0.98
1.01
1.10
D1
T1
Q1
a values in parenthesis indicate the corresponding wavelength in nm. b in ns in argon purged DMF. c measured in CH3CN under argon by
comparative actinometry using [Ru(bpy)3]2+.2PF6– (F = 0.018) in CH3CN under air as a standard.42 d V vs Ag/AgCl. e nm
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Table 2. Photocatalysis Results with White, Blue, Green and Orange LEDs
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3
[PS]
Eq. Ru center
(mol%)
White light
(6500K)a
Blue
Green
Orange
(mol%)
(470 nm)a
(525 nm)a
(590 nm)a
[Ru(bpy)3]2+
2.50
1.25
0.83
0.63
0
2.5
2.5
2.5
2.5
0
77 %
92 %
91 %
93 %
1%c
80 % (5h)
78 % (7h)
31 %b
91 % (6.5h)
94 % (7h)
95 % (7h)
---
4
5
6
7
8
9
D1
T1
91 % (2h) 91 % (2.5h)
93 % (2h) 93 % (2.5 h)
Q1
95% (2h)
---
96% (2.5h)
---
No PS
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aIsolated yields of (2) after column chromatography (time required for full conversion (determined by 1H NMR)). b after 7 hours, 58% of
starting material recovered. The yield reached 49% after 72h of irradiation, with 33% of starting material recovered. c 96% of starting material
recovered.
The photosensitizers were electrochemically characterized in
0.1 M TBAPF6/DMF electrolytes. The first ligand-centered re-
ductions occurred at –0.99, –0.93 and –0.83 V vs Ag/AgCl for
D1, T1 and Q1 respectively, i.e. >300 mV more positive than
[Ru(bpy)3]2+. The metal centered oxidation events were less in-
fluenced by the increased nuclearity, i.e. E1/2(RuIII/II) = 1.33,
1.38 and 1.37 V vs Ag/AgCl for D1, T1 and Q1 respectively.
The excited-state reduction potentials were estimated using
equation 1, with E00, the energy stored in the excited-state, de-
termined through Franck-Condon lineshape analysis of the pho-
toluminescence spectra. These potentials are gathered in Table
1 and highlight that the polynuclear photosensitizers were more
potent photo-oxidants than [Ru(bpy)3]2+.43
The effect of blue, green and orange LED illumination (Fig-
ure 1) on the dehalogenation reaction was then investigated.
Blue light irradiation yielded similar results as white light illu-
mination, which was not surprising considering the spectral
overlap between the LED emission profile and the MLCT ab-
sorption band of the photosensitizers. Noteworthy, the use of
both green, and especially orange LEDs resulted in efficient
dehalogenation, with yields >90% with D1, T1 and Q1. Albeit,
with orange illumination, 7 hours were required compared to
the 2-3 hours required with blue or green LEDs. This was in
stark contrast to [Ru(bpy)3]2+, that only reached 31% yield of 2
after 7 hours orange illumination (Table 2).
Given the promising results obtained with the polynuclear
complexes, the photosensitizer loading was lowered to 0.1
mol% equivalent of ruthenium center (Table 3). This resulted
in a decreased yield and incomplete conversion45 with
[Ru(bpy)3]2+ after 24 hours, while excellent yields of around
90% were still obtained with D1, T1 and Q1.
ꢅ∗/ꢅꢆꢁ
ꢅ/ꢅꢆꢁ
(
)
(
)
+ ꢀꢇꢇ (Eq. 1)
ꢀꢁ/ꢂ ꢃꢄ
= ꢀꢁ/ꢂ ꢃꢄ
The photosensitizers were evaluated as photoredox catalysts
in the dehalogenation reaction reported by C. Stephenson et al.
(Scheme 1).37, 44 This reaction was proposed to occur by photo-
induced electron transfer from diisopropylethylamine (DIPEA)
to [Ru(bpy)3]2+*, followed by dehalogenation of 4-bromoben-
Table 3. Photocatalysis Results at Lower PS Concentration
zyl-2-chloro-2-phenylacetate
to
4-bromobenzyl-2-phe-
nylacetate. Yields around 75% were obtained with [Ru(bpy)3]2+
as photosensitizer. In this model reaction, the effects of the i)
driving force for photoinduced electron transfer ii) impact of
multiple ruthenium centers, iii) irradiation wavelength and iv)
population of the 3MC state on the overall yield and efficiency
were studied.
PS
[PS]
(mol %)
0.1
Eq. Ru center
White light
(6500K)a
(mol %)
0.1
[Ru(bpy)3]2+
66 %b
90 %
87 %
91%
D1
T1
Q1
0.05
0.1
0.03
0.1
0.023
0.1
Scheme 1. Dehalogenation Reaction as Reported by C. Ste-
phenson et al..37
a
b
Isolated yields of (2) after 24h of reaction. Incomplete con-
version, 10% starting material recovered.
O
O
2.5% eq. Ru Center
O
O
The proportions of DIPEA and HE were then modified with
[Ru(bpy)3]2+ as photosensitizer in order to investigate factors
influencing the conversion and the achievable yields (Table S1-
S2). Surprisingly, in the absence of DIPEA, but with increasing
amounts of HE (Table S1, entries 5-7), the reaction reached
quantitative conversion and isolated yields around 80% after
column chromatography. Similar results were obtained with
polynuclear complexes but with yields greater than 90% (Table
S2). Moreover, without HE, but in the presence of DIPEA, a
conversion of 81% was observed, corresponding only to a 33%
isolated yield, which showed that the catalytic cycle proceeded
partially under these conditions, however with predominant
side reactions (Table S1, Entry 9).
Hantzsch Ester (1.1 eq)
DIPEA (2.0 eq)
Cl
Br
Br
DMF, RT, light
1
2
First, the photosensitizers were evaluated in conditions simi-
lar to those reported. The overall concentration of each polynu-
clear photosensitizer was adjusted to 2.5 mol% of ruthenium
center. Therefore, the number of absorbed photons was similar
in all cases. White light illumination during 24h of a DMF so-
lution containing 1 in the presence of 2 equiv. DIPEA, 1.1
equiv. of Hantzsch ester (HE) and 2.5 mol% of [Ru(bpy)3]2+ led
to the formation of 2 in 77% yield, matching the reported
yield.37 Excitingly, with the polynuclear photosensitizers the
isolated yields increased to >90% (Table 2). As expected, in the
absence of any photosensitizer, 96% of starting material was
recovered.
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Table 4. Photophysical Characterization and Excited-State Quenching Rate Constants for the PS
3
4
5
6
7
8
9
a
b
a
b
PS
kq
kq
∆Get
∆Get
% 3MC
(298 K)
(M–1s–1)
2.5 x106
3.3 x107
2.6 x107
1.3 x107
(M–1s–1)
1.3 x106
8.1 x106
4.6 x107
2.6 x108
(eV)
(eV)
[Ru(bpy)3]2+
– 0.07
– 0.22
– 0.24
– 0.33
+ 0.09
– 0.06
– 0.09
– 0.18
48 %
D1
T1
Q1
< 0.5 %
< 0.5 %
< 0.5 %
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a with DIPEA (E1/2 = 0.765 V vs Ag/AgCl)11, b with Hantzsch ester (E1/2 = 0.92 V vs Ag/AgCl in 0.1M TBAPF6 DMF electrolyte)
Figure 2. Excited-state deactivation rate constant for [Ru(bpy)3]2+ (a), D1 (b), T1 (c), Q1 (d); (blue) direct radiative and nonradiative
rate constants (kdr + kdnr), (yellow) thermally activated deactivation via the upper lying 3MLCT, (red) thermally activated deactiva-
tion via the 3MC and (dot) experimental data. Variable temperature measurements were performed in argon purged butyronitrile.
The larger yields obtained with the polynuclear photosensi-
tizers, together with the good yield observed in the absence of
DIPEA, prompted excited-state quenching experiments (Fig-
ures S33-S34).46-47 Quenching rate constants (kq) of 2.53 x106
M–1s–1 and 1.32 x106 M–1s–1 were determined with [Ru(bpy)3]2+
and DIPEA or HE respectively (Table 4). These quenching rate
constants were one order of magnitude larger for D1, T1 and
Q1 than for [Ru(bpy)3]2+. Hence, as absorptivity was similar for
the four photosensitizers, the larger yields could originate from
these larger quenching rate constants. Note that Stern-Volmer
experiments do not provide information about the quenching
mechanism, i.e. electron transfer or energy transfer for example.
Nonetheless, the driving force for electron transfer was esti-
mated using equation 2,48-49 where E1/2(D+/0) is the one-electron
redox potential of the electron donor, and F is Faraday’s con-
stant. Table 4 highlights that the photoinduced electron trans-
fers to the excited-state polynuclear photosensitizers from HE
or DIPEA11 were consistently more favorable than to
[Ru(bpy)3]2+ .
ꢌ/ꢇ
ꢅ∗/ꢅꢆꢁ
(
ꢋ
)
(
)
]ꢍ (Eq. 2)
∆ꢈꢉꢊ = [ꢀꢁ/ꢂ
− ꢀꢁ/ꢂ ꢃꢄ
Additionally, ligand-loss photochemistry was observed for
[Ru(bpy)3]2+ during prolonged white light illumination reac-
tions (Figure S35). This observation was rationalized through
population of the dissociative triplet metal-centered (3MC) state
from the 3MLCT excited state.33-36 [Ru(bpy)2Cl2] and
[Ru(bpy)2(DMF)2]2+ were unable to perform significant cataly-
sis (Table S1, Entries 3-4). Variable temperature time-resolved
photoluminescence measurements (Figures S39-S50) were
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performed to estimate the activation energy required to cross In conclusion, dinuclear, trinuclear and quadrinuclear ruthe-
3
3
3
1
2
3
4
5
6
7
8
from the MLCT to the MC, as well as the percentage MC
population at room temperature, Figure 2 (see SI for additional
explanation).50-51 Polynuclear complexes exhibited < 0.5% of
3MC state population at room temperature, whereas population
reached 48% in [Ru(bpy)3]2+.
nium(II) photosensitizers were synthesized and investigated for
a reported light-induced dehalogenation reaction. Altogether,
the increased photostability, larger quenching rate constants,
shorter reaction time and more favorable driving force for elec-
tron transfer contributed to the increased isolated yields for the
light-induced dehalogenation reaction of 1 into 2. Importantly,
catalyst loadings were decreased down to 0.02 mol% while still
achieving dehalogenation yields greater than 90%. Variable
temperature measurements showed that <0.5% of the polynu-
clear excited-states reached the 3MC, which correlated with in-
creased photostability observed under steady-state illumination.
In addition, this reaction proceeded with similar yields when
orange illumination was used, whereas [Ru(bpy)3]2+ only par-
tially converted 1 into 2 in 72 hours.
The results obtained herein were rationalized according to
three proposed mechanisms (Scheme 2). In all cases, the photo-
reduced ruthenium complexes performed dehalogenation of 1
leading to the corresponding radical. In the absence of HE
(Mechanism 1), this radical performed hydrogen atom transfer
(HAT) from oxidized DIPEA. In the absence of DIPEA (Mech-
anism 3), HE served both as excited-state reductant and as HAT
reagent.52 The intermediate mechanism 2 is similar to the one
reported,53 where DIPEA is a reducing agent prone to photoox-
idation in the presence of excited photosensitizers, whereas HE
acts as a HAT reagent.37
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ASSOCIATED CONTENT
Supporting Information
Scheme 2. Proposed Reaction Mechanisms.
Experimental section, Electrochemistry, Variable temperature
time-resolved photoluminescence measurements, Stern-Volmer
plots and additional references (PDF).50-51, 54-66 The Supporting In-
formation is available free of charge on the ACS Publications web-
site.
AUTHOR INFORMATION
Corresponding Author
Benjamin.Elias@uclouvain.be,
Ludovic.Troian.Gautier@ulb.ac.be
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manu-
script.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
L.T.-G. acknowledges the F.R.S.-FNRS for his “Chargé de Re-
cherches” fellowship. S.C. and B.E. gratefully acknowledge the
UCLouvain, and the Prix Pierre et Colette Bauchau for financial
support. The authors acknowledge Gerald J. Meyer for the scien-
tific generosity and providing access to his equipment. The authors
acknowledge Julien De Winter and Pascal Gerbaux for mass spec-
trometry measurements. Steady-State and time-resolved absorption
and photoluminescence experiments were performed using instru-
mentation in the Alliance for Molecular PhotoElectrode Design for
Solar Fuels (AMPED), an Energy Frontier Research Center
(EFRC) funded by the U.S. Department of Energy (DOE), Office
of Science, Basic Energy Sciences BES, under Award DE-
SC0001011.
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