Spectroscopic Characterization and Mechanistic Studies on Visible Light Photoredox Carbon-Carbon Bond Formation by Bis(arylimino)acenaphthene Copper Photosensitizers

Article abstract of DOI:10.1021/acscatal.8b02502

Currently, the most popular molecular photosensitizers used for synthetic organic chemistry and energy applications are still the noble-metal-based ruthenium and iridium complexes that usually require expensive metal and ligand precursors. In contrast, bi

Full text of DOI:10.1021/acscatal.8b02502

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Article
Spectroscopic Characterization and Mechanistic Studies on
Visible Light Photoredox Carbon-Carbon Bond Formation
by Bis(Arylimino)Acenaphthene Copper Photosensitizers
Yik Yie Ng, Lisa Jiaying Tan, Shue Mei Ng, Yoke Tin Chai, Rakesh
Ganguly, Yonghua Du, Edwin K. L. Yeow, and Han Sen Soo
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02502 • Publication Date (Web): 24 Oct 2018
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ACS Catalysis
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Spectroscopic Characterization and Mechanistic Studies on
Visible Light Photoredox Carbon-Carbon Bond Formation by
Bis(Arylimino)Acenaphthene Copper Photosensitizers
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†
Yik Yie Ng,‡ Lisa Jiaying Tan, ‡, Shue Mei Ng,‡ Yoke Tin Chai,‡ Rakesh Ganguly,‡ Yonghua Du,⊥
,§
Edwin Kok Lee Yeow,‡ and Han Sen Soo* ‡
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, 21 Nanyang Link, Nanyang
Technological University, Singapore 637371.
⊥Institute of Chemical and Engineering Sciences A*STAR, 1 Pesek Road, Singapore 627833.
§Solar Fuels Laboratory, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
ABSTRACT: Currently, the most popular molecular photosensitizers used for synthetic organic chemistry and energy applications
are still the noble metal-based ruthenium and iridium complexes that usually require expensive metal and ligand precursors. In con-
trast, bis(arylimino)acenaphthene (Ar-BIAN) are established redox non-innocent π-accepting ligands that are easily assembled in one
condensation step from affordable and commercially available precursors. Herein, we have developed a series of Ar-BIAN CuI com-
plexes as visible light harvesting photosensitizers. Notably, one of these panchromatic, homoleptic Ar-BIAN CuI complexes exhibits
a radiative recombination lifetime that is longer than diffusion control, as observed by time-correlated single photon counting spec-
troscopy. The Ar-BIAN CuI facilitates visible-light promoted atom transfer radical addition reactions via carbon-carbon bond for-
mation with CBr3 radicals in good yields of up to 75%. Steady-state and transient absorption spectroscopic measurements, together
with spectroelectrochemical experiments and intermediate isolation studies, were performed to obtain insights into this photoredox
catalysis and provide guidelines for the general deployment of Ar-BIAN CuI photosensitizers in synthetic organic chemistry and
renewable energy applications.
KEYWORDS: copper photosensitizer, bis(arylimino)acenaphthene, transient absorption spectroscopy, time-correlated single
photon counting, visible-light photoredox catalysis, carbon-carbon bond formation
1. INTRODUCTION
nines, and corroles5 have remained the mainstay of photosensi-
tizers most commonly employed. However, the prohibitive high
costs and toxicity of heavy metal-based chromophores have
added impetus to the development of more sustainable and af-
fordable alternatives, including organic or copper (Cu) dyes.6
The storage of solar energy in the form of chemical bonds, also
known as artificial photosynthesis or the production of solar
fuels,1 is an increasingly attractive and complementary ap-
proach to photovoltaics for sustainable energy production. Fur-
thermore, (visible) light is progressively being adopted as an al-
ternative, eco-friendly energy source to drive photoredox catal-
ysis in a renaissance for the application of organic radical chem-
istry in synthetic methodology under mild reaction conditions.2
One of the critical components in sophisticated artificial photo-
synthetic units, photoredox reactions, and even photovoltaics
such as dye-sensitized solar cells3 is the light harvester, which
is necessary for the conversion of photon energy into chemical
potential. Effective solar energy harvesters should ideally be
panchromatic and absorb even some of the near infrared (NIR)
wavelengths without compromising too much on the anodic and
cathodic chemical potentials that can be derived. Moreover, to
serve as a photosensitizer that can power the multi-electron cat-
alytic processes in artificial photosynthesis, the excited state
lifetimes must be sufficiently long to ensure that charge injec-
tion or transfer occurs before unproductive recombination pro-
cesses. In this respect, molecules comprising precious metals
like ruthenium (Ru), iridium (Ir), and platinum (Pt),4 or syn-
thetically laborious ligands such as porphyrins, phthalocya-
Our team has recently been involved in assembling various
components of an artificial photosynthetic construct, such as the
multi-electron (photo)catalysts and the light harvesters, with ex-
clusive use of only earth-abundant elements.7 CuI chromo-
phores with suitable π-acceptor ligands have been likened to
RuII polypyridyl complexes, since both classes of compounds
can absorb visible light by metal-to-ligand (MLCT) charge
transfer transitions, while minimizing non-radiative losses
through energetically accessible dark states in other spin mani-
folds.8 Ligand systems that have been explored for supporting
CuI chromophores include oligopyridines,8b halides,9 phos-
phanes,8c N-heterocyclic carbenes (NHCs),10 and bis(ar-
ylimino)acenaphthene (Ar-BIAN).7b
However, unlike the RuII complexes, there are several current
challenges to developing effective CuI photosensitizers that
have amply long lifetimes for photochemistry. Since CuI has a
d10 electronic configuration, the complexes typically have up to
only four coordination sites, with the most common geometry
being distorted tetrahedral, although some trigonal planar and
linear complexes exist.11 When the CuI complexes absorb light
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via MLCT, the electron is usually promoted from an almost
Page 2 of 21
BIAN in the same solvent under N2 gave rise to the respective
Ar-BIAN CuI complexes: [(ArNHOH-BIANBr)2CuI]PF6 (4),
[(ArBr,OMe-BIAN)2CuI]PF6 (5), and [(ArOMe-BIANBr)2CuI]PF6
(6) (Scheme 1).
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doubly or triply degenerate antibonding orbital to the ligand,
which results in a pseudo Jahn-Teller distortion.8b Conse-
quently, the excited state lifetimes of CuI chromophores with
small ligands such as halides or simple unhindered pyridines
will be very short, since the distortion will provide many non-
radiative relaxation pathways for the excited state to return to
the ground state.8b Some of the most successful examples pos-
sess bulky pyridine cores with branched hydrocarbon chains on
the periphery,8b, 12 which can avert the structural reorganizations
that are expected of the transient CuI to CuII excitations.13 How-
ever, these ligands will then require expensive precursors and
consist of demanding synthetic routes with palladium-catalyzed
coupling reactions, which negate the benefits from utilizing Cu
instead of Ru or Ir. Another approach was to use bulky, strongly
-donating ligands including phosphanes and NHCs, but the
larger ligand fields resulted in CuI complexes that do not absorb
beyond the blue to green region of the visible spectrum.8c, 10
Scheme 1. Synthesis of the Ar-BIAN ligands and CuI Com-
plexes. Inset: ORTEP from single crystal X-ray diffraction
experiment of 4. The PF6¯ counter-anion and the hydrogen
atoms have been omitted for clarity.
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On the contrary, Ar-BIAN ligands are recognized as versatile,
redox non-innocent π-acceptors14 that are easily assembled by
condensation reactions between affordable, commercial ani-
lines and acenaphthenequinone.15 A number of metal com-
plexes supported by Ar-BIAN ligands have been reported re-
cently,16 including several from our team.7b, 7f, 17 Previous at-
tempts to develop Ar-BIAN CuI photosensitizers revealed that
these ligands possessed low-lying, non-emissive triplet excited
states, which would facilitate undesired thermal relaxation to
the ground state.18 To understand the photophysical behavior of
the Ar-BIAN CuI dyes, we have re-designed and prepared a se-
ries of new Ar-BIAN CuI complexes, and conducted electro-
chemical and transient absorption spectroscopic measurements
on them. The complexes contain only earth-abundant elements,
and noble metals have been excluded even during the synthetic
process. Gratifyingly, a new homoleptic Ar-BIAN CuI chromo-
phore that exhibits radiative recombination lifetimes longer
than diffusion control is observed for the first time, and has been
employed in photocatalytic carbon-carbon bond formation re-
actions as a proof of concept.
Herein, we explain our experimental findings on the CuI photo-
sensitizers, provide examples of their application in photoredox
atom transfer radical addition (ATRA),19 and present detailed
spectroscopic and mechanistic studies to probe elementary
steps of the catalysis. Critically, we can use the insights from
this study to improve the photocatalyst design for future appli-
cations in other photoredox organic transformations such as
atom transfer radical polymerization (ATRP), which is an es-
tablished method for the production of polymeric materials of
well-defined compositions.20 In addition, ATRA reactions have
also been instrumental for the highly enantioselective addition
of CX4 reagents to olefins, which can be another potentially im-
pactful application for the CuI photosensitizers presented
here.19a
Crystals of 4 suitable for single crystal X-ray diffraction were
obtained by recrystallization from DMF layered below a solu-
tion of diethyl ether (Et2O) and 1,2-dimethoxyethane (DME).
However, the crystal contained disordered solvent molecules in
the unit cell, resulting in C-C bond distances that may not be
very accurate. Nevertheless, the Oak Ridge thermal-ellipsoid
plot (ORTEP) of 4 in Scheme 1 exhibits the expected distorted
tetrahedral geometry around CuI, which is consistent with other
tetrahedral Ar-BIAN CuI complexes.16b, 16c, 22
To determine the feasibility of complexes 4-6 in redox catalysis,
we performed cyclic voltammetry (CV) experiments on them.
Electrochemical measurements were performed for all the com-
pounds 1-6 (Figures S12-S14, Table S11). Notably, a quasi-re-
versible redox wave was observed for 6 at E1/2 = +0.29 V (Fig-
ure 1a) and is assigned to the CuII/I couple. During a cathodic
scan, a similarly quasi-reversible reduction wave was detected
at E1/2 = –0.99 V, as depicted in Figure 1b. Application of more
negative potentials to 6 led to irreversible reductions. In addi-
tion, very similar redox waves were observed for 5 (Figure S14)
since both complexes have similar structures except for the po-
sition of the bromide atoms. Due to the poor solubilities of 1
and 4 in DCM, the CV experiments were conducted in DMF
instead. Hence, cathodically shifted redox potentials were ob-
served for 4. Nevertheless, the reversibility of the redox waves
in 5 and 6 suggest that the regeneration of the complexes after
photocatalytic activity should be plausible, which prompted us
to explore if they could be applied as photoredox catalysts.
2. RESULTS AND DISCUSSION
The synthetic steps for the substituted Ar-BIAN ligands and de-
tailed characterization data are described in the supporting in-
formation (SI). Briefly, the protocol reported by Gasperini and
co-workers15 was adapted for the synthesis of ArNHOH-BIANBr
(1), while ArBr,OMe-BIAN (2) and ArOMe-BIANBr (3) were syn-
thesized via a TiCl4-promoted condensation7b, 21 to give moder-
ate yields. Subsequently, treatment of suspensions of
[(CH3CN)4CuI]PF6 in N,N-dimethylformamide (DMF) or di-
chloromethane (DCM), with solutions of the corresponding Ar-
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615 nm is probably due to non-emissive low-lying states local-
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ized at the electron-withdrawing naphthyl part of the Ar-BIAN
ligand.16c The longer-lived emission signal for 6 could arise
from the presence of the electron-withdrawing and heavy atom,
Br, on the acenaphthene ring. The Br localizes the photoexcited
negative charges on the acenaphthene motif, thus improving
spatial separation of the LUMO from the HOMO on Cu, while
the heavy atom facilitates intersystem crossing, both of which
lead to longer-lived radiative excited states.
Despite these favorable modifications, ligand 3 and complex 6
have emission quantum yields, φr, of only 1.2 x 10-3 and 2.6 x
10-4 respectively, with [Ru(bpy)3](PF6)2 (bpy = tris(bipyridine))
in DCM (φr = 0.029) used as a reference.24 The excitation
wavelength used for all the samples was 460 nm, while the
integrated area was from 480 – 800 nm. Therefore, the
Figure 1. Cyclic voltammograms for 6 in DCM solutions at differ-
9
ent scan rates: under (a) an anodic and (b) a cathodic scan. The pink
arrows indicate the scan directions, with vertical dashed lines rep-
resenting the initial potentials for each scan.
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Next, UV-visible-NIR spectroscopy was employed to study the
absorption characteristics of the Ar-BIAN ligands 1-3 (Figure
S18) and the complexes 4-6 (Figure 2a), in order to explore their
utilization as photocatalysts. Complexes 4-6 displayed panchro-
matic properties since their optical absorptions extend into the
NIR region up to about 1000 nm. To evaluate if the structure of
the Cu complex changed when dissolved in a coordinating sol-
vent such as DMF, the Cu K-edge X-ray absorption near-edge
structure (XANES) spectra were obtained under ambient tem-
perature for 6 at the XAFCA beamline of the Singapore Syn-
chrotron Light Source.23 The XANES spectra displayed almost
identical profiles in both the solid state and after dissolution in
DMF (Figure S20), implying that the oxidation state and coor-
dination environment of Cu remained unchanged even when 6
was dissolved.
emission quantum yield measured for
sion feature consisting of both the emission at 510 nm and
the weak shoulder at 615 nm. The low quantum yield of
6 is the total emis-
6
is consistent with the fact that only a small fraction of the
MLCT transitions are emissive, while there are lower en-
ergy non-emissive states in the NIR region. Although 4 also
has a Br atom at the same position on the ligand as 6, quenching
due to exciplex formation with DMF might have rendered it
non-photoluminescent.25 A strongly binding, polar solvent such
as DMF was required for 4 due to its poor solubility in non-
coordinating solvents such as DCM.
Considering the favorable photoluminescent property of 6, we
decided to ascertain the excited state lifetime of the CuI com-
plexes using nanosecond time-resolved optical spectroscopy to
evaluate their suitability for photoredox catalysis. For the tran-
sient absorption and emission spectroscopy (TAS and TES re-
spectively) experiments, the samples were irradiated with UV
or visible light pulses of 5 – 8 ns durations. They were then
probed by an electronically synchronized broadband xenon
lamp beam before and after the pulses, and the intensity of the
transmitted light was detected. In all the TAS data in this man-
uscript, the logarithm of the ratio (ΔA) of the transmitted light
intensity from the probe beam after laser excitation to the trans-
mitted intensity before laser excitation are presented. The
change in optical density (ΔOD) of the transient photoexcited
state relative to the ground state thus indicates increased absorp-
tion (positive ΔA) or reduced absorption/emission (negative
ΔA). The time evolution of the transient absorption signals of 6
are presented in Figure 3, while those of 4 and 5 are in Figures
S21a and S22. Compound 6 was excited with 460 nm laser
pulses to obtain the excited state absorption and emission dif-
ference spectra after increasing time intervals as labelled by the
different colors in Figure 3.
Figure 2. (a) Electronic absorption spectra of 4 in DMF, and 5 and
6 in DCM; (b) steady state emission spectra of 6 in DCM, with the
excitation wavelengths labelled in different colors.
Like most previously documented homoleptic CuI Ar-BIAN
complexes,16b, 16c, 18a no steady-state emission signal was de-
tected for 4 and 5. On the contrary, the steady-state lumines-
cence spectra of 6 contained emission signals as illustrated in
Figure 2b. Upon excitation between 430 to 460 nm, 6 displayed
an emission band at 510 nm, and a weaker broad shoulder
around 615 nm. The emission at 510 nm is likely attributed to a
ligand-centered emission since 3 shows a similar signal (Figure
S21b) with comparable lifetimes (vide infra). This is further
corroborated by previous reports of Ar-BIAN metal complexes
with similar weak luminescence attributed to intraligand fluo-
rescence.17b, 18b On the other hand, the broad band at 615 nm
arises from luminescence via the MLCT excited state, which
has been corroborated by the transient absorption and emission
spectra obtained after 580 nm photoexcitation (Figure S24).
Since the ligand 3 does not absorb at 580 nm and CuI is a d10
system, the absorption maximum at 580 nm in 6 is likely due to
MLCT. The absorption by 6 beyond the emission shoulder at
Figure 3. Nanosecond transient (a) absorption and (b) emission
spectra of 6 in DCM, at increasing delay times of 20 (red), 40
(blue), 60 (green), and 80 (orange) ns after the 460 nm pulse.
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Visible-light assisted ATRA reaction. With a photoexcited
An absorbance (ΔA) bleach centered at ~550 nm was observed
(Figure 3a), indicating a ground state bleach of 6 superimposed
on the excited state emission, as confirmed by the TES obtained
between 20 to 80 ns (Figure 3b). The latter concurs with the
steady-state emission spectra. Since the lifetimes of 6 were in
the same order of magnitude as the time-resolution of our TAS
experiments, time-correlated single photon counting (TCSPC)
measurements with higher resolution down to 100 ps were con-
ducted on 6 in DCM at 510 nm with 466 nm irradiation (Figure
4a). The photoluminescence decay of 6 can be fitted with a bi-
exponential decay function with a faster component (3.6 ± 0.1
ns) that can be attributed to relaxation of the MLCT excited
state of CuI. A slower component (11 ± 0.3 ns) is similar to the
luminescence lifetime of the ligand, 3 (9.9 ± 0.4 ns, Figure S25).
To determine only the lifetime of the MLCT state, a longer ex-
citation wavelength of 562 nm where 3 did not absorb was used
to photoexcite 6. From the time-resolved photoluminescence at
620 nm, we obtained a single exponential decay fit with a life-
time of 2.0 ± 0.1 ns (Figure 4b), which is similar to the faster
decay component observed in Figure 4a. Compared to the ul-
trafast optical spectroscopic studies previously reported, when
some heteroleptic Ar-BIAN complexes were excited at higher
energies, the transient optical spectrum was mainly dominated
by a triplet ligand-centered relaxation.18 Therefore, the photolu-
minescence decay of 6 consists of two contributions. The ob-
servation of more than one emission constituent is not uncom-
mon. As described by Casadonte et al., in a rigid matrix, two
emission signals were observed from a triplet intraligand (3IL)
and a triplet charge transfer (3CT) excited state.26
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6
7
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lifetime in the nanosecond regime, the unimolecular recombi-
nation rate of 6 is now longer-lived than diffusion-controlled
reactions, giving us the opportunity to apply it in electron trans-
fer processes such as photoredox catalysis. As a proof-of-con-
cept, the ability of 6 to photocatalyze carbon-carbon bond for-
mation via ATRA30 between styrenes and tetrabromomethane
(9) was investigated. Table 1 summarizes the ATRA reaction
carried out using visible light.
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Table 1. ATRA Reaction Photocatalyzed by 6a
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Alkene
Product
%
Yield
(%)c
conv.b
1
100
30
74
<3
<1
7
2d
3e
4f
5g
6h
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100
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100
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8g
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10i
11g
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100
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100
100
44j
32j, k
Figure 4. Photoluminescence decay of 6 in DCM. (a) The sam-
ple was excited at 466 nm and the photoluminescence was rec-
orded at 510 nm. The red line represents the bi-exponential fit
for the decay signal. (b) The sample was excited at 562 nm and
the photoluminescence was recorded at 620 nm. The red line
represents the single exponential fit for the decay signal.
a Conditions: Alkene (1 eq.), CBr4 (1 eq.), 6 (0.60 mol%) in DCM
(1.0 mL), irradiation with two white LEDs (48 W each) for 24 h. b
The % conversion of styrene was monitored by 1H NMR spectros-
copy. c The 1H NMR yields were calculated by comparing against
1,1,2,2-tetrachloroethane as the internal standard. d Dark reaction. e
f
No catalyst. [(CH3CN)4Cu]PF6, no Ar-BIAN ligand. g Ar-BIAN
ligand (0.6 mol%) only as the catalyst. h Irradiation by one LED
upon installation of a longpass filter to cut off wavelengths below
490 nm. i Reaction time: 48 h. j The desired ATRA product was not
isolated due to hydrolysis upon purification. k The NMR spectra of
both the crude reaction mixture and the hydrolyzed product have
been presented as Figures S35-37.
Although the excited state lifetime of 6 (~ 4 ns) is shorter than
those of the prototypical ruthenium-based photosensitizers such
2+
as Ru(bpy)3 (488 ns in DCM),24 it is still one of the longest-
lived panchromatic chromophores among first-row transition
metal complexes. For example, the excited state lifetime of the
longest-lived iron-based photosensitizer is 100 ps,27 while the
rest are on the order of tens of picoseconds or shorter.28 The
excited state lifetime of 6 is short compared to the state-of-the-
art CuI-based photosensitizers,29 an example being
[Cu(dsbtmp)2]+ (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetrame-
thyl-1,10-phenanthroline), which has an excited state lifetime
ranging from 1.2 to 2.8 µs.29a However, [Cu(dsbtmp)2]+ is not
panchromatic and absorbs only up to 520 nm, and also has a
more costly, multi-step ligand synthetic route.29a
Upon irradiation with two white light LEDs and a low catalyst
loading (0.60 mol%), the reaction proceeded with moderate
NMR yields (32-75%), calculated by comparison with an inter-
nal standard in the NMR. Notably, we could conduct the photo-
catalytic reaction even when light of shorter wavelengths (≤ 490
nm) was cut off (entry 6), highlighting an advantage over
longer-lived Cu photosensitizers such as [Cu(dsbtmp)2]+. The
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control experiments (entries 2-4) showed minimal product for-
mation. Although irradiation with the Ar-BIAN ligand alone led
1
to some products, the yields and selectivities were poorer (en-
tries 5, 8, and 11), especially for the slower substrate. Electron-
donating methyl (entry 12) and electron-withdrawing chloro
and cyano (entries 7 and 10) were all tolerated. In addition,
fused rings such as naphthalene (entry 13) were also compatible
for this reaction.
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8
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Evaluating the Reaction Pathway of the Photocatalyzed
ATRA Reaction. The products obtained agree with those from
previously reported CuI-photocatalyzed ATRA reactions and
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other photoredox catalytic reactions where a radical (ion) is of-
19d, 31
ten proposed as the key intermediate.19c,
As shown in
Scheme 2, we propose that there are two possible reaction path-
ways. In the closed photoredox catalytic cycle, the photoexcited
CuI complex transfers an electron to 9 to generate a CBr3 radical
(10), which adds to the styrene. The resulting intermediate (11)
is then oxidized by the CuII species (8) to regenerate 6, while
intermediate 12 combines with Br– to form the desired product
(13). On the other hand, as shown by the pathway indicated by
the blue arrows in Scheme 2, another plausible mechanism in-
volves radical chain propagation, whereby 11 reacts with CBr4
to generate 13, while regenerating 11.
Figure 5. A “light-on light-off” experiment on the ATRA reaction.
overall quantum yield for the formation of 13 was found to be
φ = 3, indicating that for every photon absorbed by 6, three
equivalents of 13 were produced. Yoon and coworkers demon-
strated that a reaction with φ greater than one would be con-
sistent with a radical chain propagation.32 Therefore, we pro-
pose that 6 may be undergoing a short chain propagation cycle,
more consistent with the pathway illustrated by the blue arrows
in Scheme 2.
In order to distinguish between the closed photoredox catalytic
cycle (red arrows) and the chain propagation cycle (blue ar-
rows) in Scheme 2, we conducted a series of experiments to
evaluate if 6 was behaving as a radical initiator. Firstly, the pro-
gress of the reaction was monitored in alternating periods of ir-
radiation and darkness. We observed that 13 was formed only
when the reaction mixture was under irradiation as shown in
Figure 5. To confirm whether this trend was due to a rapidly
terminating chain process or the absence of chain propagation,
quantum yield measurements were performed. The reaction
quantum yield (φ) was calibrated against the photodecomposi-
tion of potassium ferrioxalate as the chemical actinometer. The
To detect radical intermediates during the catalytic cycle, the
stable radical (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO)
was employed as a radical trap. We observed inhibition of the
ATRA reaction since none of desired product was detected in
the 1H NMR spectrum (Figure S41). A side reaction leading to
the formation of benzaldehyde (green circles) was observed in-
stead of the trapped intermediate. To obtain further insights into
the proposed mechanism, UV-visible spectra of the different
components (6, styrene, and CBr4) were collected at 1 h inter-
vals (Figure 6). The UV-visible spectra of the reaction mixture
containing only 6, and both 6 and styrene showed minimal de-
creases in the absorbance at 580 nm, as illustrated in Figure S42.
However, in the absence of styrene, the λmax at 580 nm de-
creased rapidly (Figure 6a), suggesting that 6 quickly depleted,
due to the lack of 11 to reduce 8 back to 6. Therefore, after ir-
radiating for 3 h, the MLCT band of 6 at 580 nm diminished,
which was then followed by the formation of a new species at
Scheme 2. Proposed Mechanism of the CuI-Photocatalyzed
ATRA of CBr4 and Styrenes
λ
max of 470 nm, preventing the completion of the catalytic cycle.
In contrast, when styrene was present, the absorption band at
580 nm persisted (with modest reduction due to photobleach-
ing) even after 5 h of irradiation (Figure 6b), which indicated
the regeneration of 6 during the catalytic cycle via the reduction
of the CuII in 8.
We then attempted to identify 8 formed during the photoredox
reaction by carrying out spectroelectrochemical measurements
on 6.33 The UV-visible spectra obtained in Figure 6c shows the
changes in the absorption bands of 6 upon its oxidation when a
potential of more than 0.50 V (versus Fc+/Fc) was applied. This
potential was chosen since the CV experiments indicated that 6
had a one-electron oxidation wave at 0.29 V (Table S11). The
absorption band at 580 nm was diminished whereas the band at
434 nm grew slightly after 6 was oxidized electrochemically
(black to orange, Figure 5c). Remarkably, the final spectrum af-
ter one-electron oxidation (orange line, Figure 6c) appears to be
very similar to the intermediate from the reaction between 6 and
CBr4 after 3 h (grey dashed line, Figure 6c, reproduced from
Figure 6a). When a reductive potential at -0.20 V was applied,
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Page 6 of 21
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Figure 7. (a) ORTEP from the single crystal X-ray diffraction ex-
periment of 14. The structure of 14 has not been fully refined due
to severe disorder in the position of the Br atoms. A model with
equal occupancy of the Br as Br1 and Br1a provided adequate re-
finement, but the disordered solvent molecules could not be refined
any further. The two counter-anions and the hydrogen atoms have
been omitted for clarity. (b) UV-visible spectra comparison be-
tween 8, 14 after stirring with two equivalents of LiBr, and 14.
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Figure 6. UV-visible spectra of the reaction mixture containing (a)
only 6 and CBr4, and (b) 6, styrene, and CBr4 at 1 h intervals. UV-
visible spectra of 6 obtained by spectroelectrochemical measure-
ments (c) when an oxidative potential is applied at more than 0.50
V versus Fc+/Fc with the time points at 0 (black), 5 (red), 10
(green), 15 (blue), and 20 (orange) min; and (d) when a reductive
potential of -0.20 V versus Fc+/Fc is applied on the orange sample
in (c) with the time points at 0 (orange), 3 (green), 6 (red), and 9
(black) min. In both (c) and (d), the intermediate from the reaction
between 6 and CBr4 after 3 h (grey dashed line) has been repro-
duced and normalized from (a).
their d-d absorption bands and a decrease in coordination num-
ber from six.34 To further corroborate this proposed identity of
8, we added 2.0 equivalents of LiBr to 14. The steady-state ab-
sorption spectrum is displayed in black in Figure 7b. Gratify-
ingly, this product now shows red-shifted absorption bands in
the NIR region, intermediate between 8 and 14, although it is
still not coincident with 8. Thus, we propose that 8 may be a
CuII complex with CBr3 or Br– coordinated, with a lower square
pyramidal or trigonal bipyramidal symmetry. Additional in situ
X-ray absorption spectroscopic experiments with photolysis
will be necessary, which is beyond the scope of this report.
the characteristic absorption at 580 nm was recovered, while the
band at 434 nm decreased slightly to regenerate the original
UV-visible spectrum of 6 (Figure 6d). Therefore, we postulate
that 8 in the photoredox catalytic cycle could be the one-elec-
tron oxidized intermediate obtained during the spectroelectro-
chemical experiments.
3. CONCLUSION
In conclusion, we have developed and characterized several
panchromatic Ar-BIAN CuI complexes, one of which displays
radiative recombination rates that are longer-lived than diffu-
sion control. Notably, we demonstrated that this luminescent
Ar-BIAN CuI complex is capable of photocatalytically facilitat-
ing ATRA carbon-carbon bond formation reactions under am-
bient conditions. Our insights into the reaction pathway of the
photocatalytic cycle indicate that exciplex formation by coordi-
nation of photogenerated radicals may be occurring. Conse-
quently, in addition to the introduction of electron-withdrawing
heavy atoms on the acenaphthene motif to increase the photo-
excited lifetimes, more steric protection of the CuI center will
be necessary in future generations of Ar-BIAN CuI complexes
for them to become generally exploitable in solar energy har-
vesting and synthetic organic chemistry applications.
Attempts were made to isolate and identify 8 by irradiating a
mixture of 6 and a stoichiometric equivalent of CBr4. Brown
precipitate was collected, but single crystals could not be ob-
tained after multiple recrystallization attempts. Therefore, the
UV-visible spectrum of the brown precipitate was obtained and
compared to the chemically oxidized 6, which was formed by
mixing tris(2,4-dibromophenyl)aminium hexafluoroantimonate
(‘Magic Green’) with 6 to obtain a CuII product (14). However,
as illustrated in Figure 7, the UV-visible spectra of 8 and 14 are
patently distinct. Complex 14 consists of an octahedral CuII cen-
ter with acetonitrile (ACN) molecules bound in the axial posi-
tions. A fully refined structure of 14 was not obtained due to the
presence of disorder in the location of the Br atoms and also
disordered molecules. ACN was necessary for the recrystalliza-
tion of 14 due to its poor solubility in less polar solvents.
As anticipated for octahedral and distorted octahedral CuII com-
plexes with ACN bound,34 the higher symmetry and orbitally
forbidden nature of the d-d transitions of 14 result in weak vis-
ible absorption bands. On the other hand, the optical absorption
bands of 8 are dramatically red-shifted with intensity even in
the NIR region up to around 1000 nm. Therefore, we anticipate
that 8 is unlikely to have a distorted octahedral geometry similar
to 14, but may have a lower symmetry or could have at least one
Br– bound. This has been supported by Olshin and co-workers,
who reported that CuII complexes in ACN, with increasing
equivalents of added Br–, demonstrated a marked red shift in
4. EXPERIMENTAL SECTION
Instrumental Details
The 1H and 13C spectra were recorded at room temperature on a
Bruker AVANCE 400 MHz spectrometer. The 1H and 13C
chemical shifts (δ reported in ppm) are referenced to the resid-
ual solvent signal(s). Elemental analyses were performed with
an Elementar vario MICRO cube analyzer. High-resolution
mass spectra (HR-MS) were obtained with a Q-TOF Premier
LC HR mass spectrometer.
UV-visible spectroscopic measurements were performed using
a Shimadzu UV-3600 UV-Vis-NIR Spectrophotometer. Photo-
luminescence decay profiles were recorded on a time-correlated
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ACS Catalysis
(φr = 0.029) was used as the reference; I is the integrated fluo-
single-photon counting (TCSPC) spectrofluorimeter (Fluo-
1
2
3
4
5
6
7
8
rocube, Horiba Jobin Yvon) with a time resolution of 100 ps.
The samples were excited at 466 nm using a pulsed diode laser
(NanoLED-466 L, Horiba Jobin Yvon) and the photolumines-
cence decay signals were recorded at 510 nm. The transient ab-
sorption and emission measurements were performed using an
Edinburgh Instruments model LP920 transient absorption spec-
trometer equipped with a pulsed Xe probe lamp in conjunction
with a Nd:YAG laser (Continuum model Surelite II-10) as the
excitation source. The laser pulse width is 5-8 ns and the repe-
tition rate is 10 Hz. During transient absorption measurements,
the pulses are synchronized with the LP920 system at a fre-
quency of 1 Hz. The pulse energy used was between 6-23
mJ/pulse.
rescence intensity and A is the absorbance at the excitation
wavelength of the sample (no subscripts) and the reference
compound (subscript ref).
Time-correlated single photon counting (TCSPC)
The errors corresponding to the fits of the transient signal life-
times were determined via the principle of error propagation by
calculating the root-mean-square deviation from the sum of
squares of the uncertainties in each measured value. For each
time-resolved measurement, the associated uncertainties in-
cluded the spectrofluorimeter time-resolution, mass of samples,
and volumes of samples. Hence, the error of the transient signal
lifetime, δτ, was calculated according to the following equation:
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2
2
2
Electrochemical measurements
δ휏푎
휏푎
δ푚푘
푚푘
δ푉
푛
√
(
δ휏 = 휏
) + (
) + (
)
Cyclic voltammetry (CV) experiments were conducted using a
Biologic SP-300 potentiostat with 1.0 mM solutions of each
sample and 0.10 M of tetrabutylammonium hexafluorophos-
phate (TBAH) as the electrolyte. The measurements of 1 and 4
were conducted in DMF while the measurements of 2, 3, 5, and
6 were conducted in DCM, each at a scan rate of 100 mV s–1 in
a glovebox. A standard three-electrode electrochemical cell was
used with a glassy carbon working electrode (3 mm in diameter
from BAS), a Pt wire as the counter electrode, and another Pt
wire as the pseudoreference electrode. The potentials were cal-
ibrated by addition of ferrocene as an internal reference (0 V)
after the CV measurements on the compound have been con-
ducted to avoid obscuring signals from our samples. The data
are reported relative to the ferrocenium/ferrocene redox couple
(Fc+/Fc). Before each experiment, the working electrode (glassy
carbon) was polished using a polishing pad infused with a 0.05
μm alumina suspension, followed by sonication in DI water for
10 min, washed with methanol and acetone, before drying in
air.
푉
푛
where τ = transient signal lifetime; δτa = uncertainty of the spec-
trofluorimeter time-resolution; δm6 = uncertainty of the mass of
the dissolved 6; δVn = uncertainty of the nth volume measure-
ment, where n represents the n different volume measurements
including stock solution and sample preparations.
General procedure for ATRA reaction catalyzed by 6
A solution of alkene (0.13 mmol), CBr4 (0.13 mmol), and 6
(0.60 mol%) in DCM (1.0 mL) was degassed using two freeze-
pump-thaw cycles, refilled with N2, and irradiated with two
white LEDs for 24 h. The reaction mixture was concentrated in
vacuo. The 1H NMR yields were calculated using the following
formula: yield (%) = 100% • [ν(product) / ν(standard)] • n,
where ν(product)) / ν(standard) is the integral ratio of the corre-
1
sponding H NMR peaks and n is the ratio of the standard to
starting material (in mol). Some of these organic compounds
have been previously reported and we have verified the identity
of our products by comparing our spectroscopic data with those
from the references.
Spectroelectrochemical experiments
Reaction quantum yield measurements32
The spectroelectrochemical cell was assembled by fitting a cus-
tomized three-electrode setup (Latech Scientific Supply Pte.
Ltd. and Tianjin AIDA Hengsheng Science-Techonology De-
velopment Co., Ltd.) into a 1 mm-path length customized cu-
vette (Starna Scientific Ltd.). The spectroelectrochemical cell
was connected to a Bio-Logic SP-300 potentiostat. The experi-
ments were carried out at RT. The concentration of 6 used was
0.50 mM with 0.10 M of TBAH as the electrolyte in DCM. The
three electrodes used consisted of a Pt mesh working electrode,
a Pt wire as the counter electrode immersed in a Chloropore
jacket containing a solution of p-benzoquinone (0.010 M) and
TBAH (0.10 M), and another Pt wire as a pseudoreference elec-
trode in a Chloropore jacket containing a solution of TBAH
(0.10 M). The data are reported relative to the ferrocenium/fer-
rocene redox couple (Fc+/Fc).
The photon flux of the light source used for the reaction was
determined by standard ferrioxalate actinometry. A 0.15 M so-
lution of ferrioxalate was prepared and 2.0 mL of the solution
was added to a cuvette and irradiated for 5 min. After irradia-
tion, 0.35 mL of a 1,10-phenanthroline solution was added. Af-
ter the solution was allowed to rest for 1 h, the absorbance of
the solution at 510 nm was recorded. The absorbance at 510 nm
of another sample that was kept in the dark was also recorded.
The following expression was used for calculation:
푉 × ∆퐴
mol Fe2+
=
푙 × 휀
where V is the total volume (0.00235 L) of the solution, ΔA is
the difference in absorbance of the irradiated and non-irradiated
solutions at 510 nm, l is the pathlength (1 cm), and ε is the molar
extinction coefficient at 510 nm (1.11 x 104 M-1 cm-1).32, 35 The
photon flux can be obtained from the following equation:
Emission quantum yield measurements
Radiative quantum yields, φr, were measured in DCM solutions.
Each sample was prepared in a glovebox under a N2 atmos-
phere. The φr values were obtained by comparing the photolu-
minescence intensities to Ru(bpy)3](PF6)2 and calculated using
the following expression:
mol Fe2+
photon flux =
휑 × 푡 × 푓
where φ is the quantum yield of the ferrioxalate actinometer
(1.01), t is the time of irradiation (300 s), and f is the fraction of
light absorbed at λ = 436 nm (0.998). The photon flux was cal-
culated (average of three experiments) to be 3.78 x 10-10 einstein
s-1.
퐴
퐼
푟푒푓
φr = φref
퐴 퐼
푟푒푓
where φref is the known quantum yield of the reference com-
pound. The photosensitizer [Ru(bpy)3](PF6)2 dissolved in DCM
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Page 8 of 21
Efficient Dye-Sensitized Solar Cells – A Goal Toward a “Bright”
Radical trapping experiment
Future. Coord. Chem. Rev. 2012, 256, 3008-3035; (b) Henwood,
A. F.; Zysman-Colman, E., Lessons Learned in Tuning the
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M.; Slattery, S. A.; Miller, A. J. M., Photochemical Formic Acid
Dehydrogenation by Iridium Complexes: Understanding
Mechanism and Overcoming Deactivation. ACS Catal. 2015, 5,
6320-6327; (d) Yam, V. W.; Au, V. K.; Leung, S. Y., Light-
Emitting Self-Assembled Materials Based on d8 and d10 Transition
Metal Complexes. Chem. Rev. 2015, 115, 7589-7728; (e) Yu, S.;
Zeng, Y.; Chen, J.; Yu, T.; Zhang, X.; Yang, G.; Li, Y.,
Intramolecular Triplet–Triplet Energy Transfer Enhanced Triplet–
Triplet Annihilation Upconversion with a Short-Lived Triplet
State Platinum(II) Terpyridyl Acetylide Photosensitizer. RSC Adv.
2015, 5, 70640-70648.
1
2
3
4
5
6
7
8
To detect any radical intermediates during the catalytic reac-
tion, the stable radical (2,2,6,6-tetramethylpiperidin-1-yl)oxy
(TEMPO) was employed as a radical trap. A solution of styrene
(0.13 mmol), CBr4 (0.13 mmol), 6 (0.60 mol%), and TEMPO
(0.65 mmol) in DCM (1.0 mL) was degassed using two freeze-
pump-thaw cycles, refilled with N2, and irradiated with two
white LEDs for 24 h. The reaction mixture was concentrated in
1
vacuo and analyzed by H NMR spectroscopy to obtain the
yields.
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ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge via the In-
(5) (a) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod,
B. F.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.;
Nazeeruddin, M. K.; Gratzel, M., Dye-Sensitized Solar Cells with
13% Efficiency Achieved Through the Molecular Engineering of
Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247; (b) Salvatori,
P.; Amat, A.; Pastore, M.; Vitillaro, G.; Sudhakar, K.; Giribabu,
L.; Soujanya, Y.; De Angelis, F., Corrole Dyes for Dye-Sensitized
Solar Cells: The Crucial Role of the Dye/Semiconductor Energy
Level Alignment. Comput. Theor. Chem. 2014, 1030, 59-66; (c)
Chen, Z.; Lohr, A.; Saha-Moller, C. R.; Wurthner, F., Self-
Assembled -stacks of Functional Dyes in Solution: Structural
and Thermodynamic Features. Chem. Soc. Rev. 2009, 38, 564-
584.
(6) (a) Romero, N. A.; Nicewicz, D. A., Organic Photoredox Catalysis.
Chem. Rev. 2016, 116, 10075-10166; (b) Miller, A. J.; Dempsey,
J. L.; Peters, J. C., Long-Lived and Efficient Emission from
Mononuclear Amidophosphine Complexes of Copper. Inorg.
Chem. 2007, 46, 7244-7246; (c) Zhang, Y.; Petersen, J. L.;
Milsmann, C., A Luminescent Zirconium(IV) Complex as a
Molecular Photosensitizer for Visible Light Photoredox Catalysis.
J. Am. Chem. Soc. 2016, 138, 13115-13118; (d) Tokumasu, K.;
Yazaki, R.; Ohshima, T., Direct Catalytic Chemoselective alpha-
Amination of Acylpyrazoles: A Concise Route to Unnatural
alpha-Amino Acid Derivatives. J. Am. Chem. Soc. 2016, 138,
2664-2669.
(7) (a) Gazi, S.; Hung Ng, W. K.; Ganguly, R.; Putra Moeljadi, A. M.;
Hirao, H.; Soo, H. S., Selective Photocatalytic C–C Bond
Cleavage under Ambient Conditions with Earth Abundant
Vanadium Complexes. Chem. Sci. 2015, 6, 7130-7142; (b) Kee, J.
W.; Ng, Y. Y.; Kulkarni, S. A.; Muduli, S. K.; Xu, K.; Ganguly,
R.; Lu, Y.; Hirao, H.; Soo, H. S., Development of
Bis(arylimino)acenaphthene (BIAN) Copper Complexes as
Visible Light Harvesters for Potential Photovoltaic Applications.
Inorg. Chem. Front. 2016, 3, 651-662; (c) Shao, H.; Muduli, S.
K.; Tran, P. D.; Soo, H. S., Enhancing Electrocatalytic Hydrogen
Evolution by Nickel Salicylaldimine Complexes with Alkali
Metal Cations in Aqueous Media. Chem. Commun. 2016, 52,
2948-2951; (d) Gazi, S.; Đokić, M.; Moeljadi, A. M. P.; Ganguly,
R.; Hirao, H.; Soo, H. S., Kinetics and DFT Studies of Photoredox
Carbon–Carbon Bond Cleavage Reactions by Molecular
Vanadium Catalysts under Ambient Conditions. ACS Catal. 2017,
7, 4682-4691; (e) Kee, J. W.; Shao, H.; Kee, C. W.; Lu, Y.; Soo,
H. S.; Tan, C. H., Mechanistic Insights for the Photoredox
Organocatalytic Fluorination of Aliphatic Carbons by
Anthraquinone Using Time-Resolved and DFT Studies. Catal.
Sci. Technol. 2017, 7, 848-857; (f) Wang, J.; Ganguly, R.;
Yongxin, L.; Diaz, J.; Soo, H. S.; Garcia, F., A Multi-Step
Solvent-Free Mechanochemical Route to Indium(III) Complexes.
Dalton Trans. 2016, 45, 7941-7946.
ternet at http://pubs.acs.org.
Detailed synthetic procedures of the ligands, CuI complexes, and
the ATRA products, their thorough characterization data, and es-
sential crystallographic data. (PDF)
X-ray crystallographic data for 4 and 14 are deposited under CCDC
1848314 and 1866188 respectively. (CIF)
AUTHOR INFORMATION
Corresponding Author
* Email: hansen@ntu.edu.sg; Tel: +65 65923182.
Present Addresses
† School of Mechanical and Aerospace Engineering, Nanyang
Technological University, 50 Nanyang Avenue, Singapore 639798.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
HSS is supported by MOE Tier 1 grants M4011611 and M4011791.
HSS also acknowledges the Agency for Science, Technology and
Research (A*STAR), AME IRG grants A1783c0002,
A1783c0003, and A1783c0007 for funding this research. The au-
thors are grateful for support from the Solar Fuels Lab at NTU). EY
is supported by MOE Tier 1 grants (RG115/16). YYN thanks Mr.
Xian Liang Ho for his help in the spectroelectrochemical experi-
ments and Mr. Andrew Yun Ru Ng for his assistance with sample
preparation for the XAS experiments.
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