Improved Visible Light Absorption of Potent Iridium(III) Photo-oxidants for Excited-State Electron Transfer Chemistry

Article abstract of DOI:10.1021/jacs.9b12108

Three iridium photosensitizers, [Ir(dCF₃ppy)₂(N-N)]⁺, where N-N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino[2,3-a]phenazine (pzph), or benzo[a]pyrazino[2,3-h]phenazine (bpph) and dCF₃ppy is 2-(3,5-bis(trifluoromethyl-phenyl)pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to Ir-bpph, with a molar absorption coefficient, ? = 9800 M^-1 cm^-1, at 450 nm and an excited-state reduction potential, E(Ir^+*/0) = 1.76 V vs NHE. These complexes were successful in performing light-driven charge separation and energy storage, where all complexes photo-oxidized seven different electron donors with rate constants (0.089-3.06) × 10¹⁰ M^-1 s^-1. A Marcus analysis provided a total reorganization energy of 0.7 ± 0.1 eV for excited-state electron transfer.

Full text of DOI:10.1021/jacs.9b12108

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Communication
Improved Visible Light Absorption of Potent Iridium(III)
Photooxidants for Excited-State Electron Transfer Chemistry
Robin Bevernaegie, Sara A. M. Wehlin, Eric J. Piechota, Michael Abraham,
Christian Philouze, Gerald J. Meyer, Benjamin Elias, and Ludovic Troian-Gautier
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12108 • Publication Date (Web): 15 Jan 2020
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Improved Visible Light Absorption of Potent Iridium(III) Photooxi-
dants for Excited-State Electron Transfer Chemistry
Robin Bevernaegie,† Sara A. M. Wehlin,‡ Eric J. Piechota,‡ Michael Abraham,† Christian Philouze,G
Gerald J. Meyer,‡ Benjamin Elias†* and Ludovic Troian-Gautier‡^*
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†UCLouvain, Institut de la Matière Condensée et des Nanosciences (IMCN), Molecular Chemistry, Materials and Catalysis
(MOST), Place Louis Pasteur 1 box L4.01.02, B-1348 Louvain-la-Neuve, Belgium.
‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599-3290, United
States
GDépartement de Chimie Moléculaire, Université Grenoble-Alpes (UGA), UMR CNRS 5250, CS 40700, 38058 Grenoble
^Laboratoire de Chimie Organique, Université Libre de Bruxelles (ULB), CP 160/06, 50 avenue F.D. Roosevelt, B-1050
Brussels, Belgium.
Supporting Information Placeholder
potentials, E1/2(Ir(n/n-1)). Potent (photo)oxidants with pos-
ABSTRACT:
Three
iridium
photosensitizers,
itive E1/2(Ir(IV/III)) or E1/2(Ir+*/0) are desired for applica-
tions such as water oxidation, hydrohalic acid oxidation,
or photoredox catalysis.19-26 Of particular interest is the
energetically demanding aqueous halide oxidation which
occurs at E1/2(X•/–) of 1.33, 1.93 and > 2V vs. NHE for
iodide, bromide and chloride, respectively.25 Unfortu-
nately, due to limited visible light absorption and unsatis-
factory thermodynamic properties, existing iridium(III)
photosensitizers are of limited use. Efforts to circumvent
their poor visible light harvesting include covalently
linked additional chromophores27-29, engineering the
HOMO-LUMO gap through ligand modifications30-33 and
changing the nature of the emissive excited states.34-35
Here, we report access to iridium complexes (Chart 1 and
Figure 1) that are strong photo-oxidants with molar ab-
sorption coefficient suitable for visible-light-driven appli-
cations. Furthermore, excited-state electron transfer stud-
ies reported herein reveal a small reorganization energy
demonstrating that these new Ir complexes are promising
for visible-light-driven applications from both a kinetic
and thermodynamic point of view.
[Ir(dCF3ppy)2(N-N)]+, where N-N is 1,4,5,8-tetraazaphe-
nanthrene (TAP), pyrazino[2,3-a]phenazine (pzph) or
benzo[a]pyrazino[2,3-h]phenazine (bpph) and dCF3ppy
is 2-(3,5-bis(trifluoromethyl-phenyl)pyridine), were
found to be remarkably strong photo-oxidants with en-
hanced light absorption in the visible region. In particular,
judicious ligand design provided access to Ir-bpph, with
a molar absorption coefficient, e = 9800 M–1cm–1, at 450
nm and an excited-state reduction potential, E(Ir+*/0) =
1.76 V vs. NHE. These complexes were successful in per-
forming light-driven charge separation and energy stor-
age where each excited-state oxidized seven different
electron donors with rate constants between 0.089 to 3.06
x1010 M–1s–1. A Marcus analysis provided a total reorgan-
ization energy of 0.7±0.1 eV for excited-state electron
transfer.
Since the early 2000s, cyclometalated iridium(III) com-
plexes have emerged as powerful light-emitting materi-
als, with exceptional photostability and highly tunable
optoelectronic properties. They have found broad appli-
cations in lighting devices,1-3 life sciences4-10 and photo-
redox chemistry,11-16 however their poor light harvesting
in the visible region has largely precluded applications in
solar energy conversion. Heteroleptic complexes with the
architecture [Ir(C-N)2(N-N)]+ (C-N = cyclometalated lig-
and, N-N = diimine ligand) have been the focus of intense
research because they possess spatially-separated frontier
orbitals.17-18 This is a key advantage that enables the in-
dependent tuning of the C-N centered HOMO and N-N
centered LUMO levels and hence the related reduction
The synthetic approach to yield photo-oxidizing irid-
ium complexes with increased visible light absorption re-
lied on several factors. Two trifluoromethyl-substituted
phenylpyridine ligands (dCF3ppy) were chosen to de-
crease the Ir–C bond electron density and thus increase
the photo-oxidizing character. The N-N ligand was sys-
tematically extended from the electron-deficient 1,4,5,8-
tetraazaphenanthrene (TAP) backbone.36-37 Conjugation
of π-electron density was increased through the addition
of one (pyrazino[2,3-a]phenazine, pzph) or two
(benzo[a]pyrazino[2,3-h]phenazine, bpph) phenyl rings
to TAP. Such annular modifications stabilized the LUMO
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level and hence increased visible light absorption inde-
bronic structure evident in steady-state photolumines-
cence spectra (Figures 2 and S26-27), as well as their
long excited-state lifetimes, τ, and their corresponding
small radiative (kr) and non-radiative (knr) decay rate con-
stants (Table 1).
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pendently of the excited-state reduction potential,
E1/2(Ir+*/0). Moreover, the additional phenyl rings induced
intra-ligand charge transfer (ILCT) bands between the
phenyl donor groups and the electron-accepting pyrazine
cores.38
Chart 1. Structures of Ir-TAP, Ir-pzph and Ir-bpph.
+
+
+
F3C
F3C
F3C
9
N
N
N
C
C
C
N
N
N
N
N
N
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N
N
N
N
N
N
CF3
CF3
CF3
CF3
CF3
CF3
IrIII
IrIII
IrIII
C
C
C
N
N
N
F3C
F3C
F3C
Ir-TAP
Ir-pzph
Ir-bpph
Figure 2. Absorption (solid) and steady-state photolumines-
cence (dash) spectra of the iridium complexes measured in
acetonitrile at room temperature. An excitation wavelength
between 400 and 470 nm was used, where the absorbance
was below 0.1. * Raman peak of CH3CN
Reversible electrochemical reduction associated with
the N-N ligands were determined by cyclic voltammetry
in 0.1 M TBAPF6/CH3CN electrolyte and reported vs.
NHE (Figure S25), E1/2(Ir(+/0)) = –0.41, –0.13 and –0.25
V for Ir-TAP, Ir-pzph and Ir-bpph respectively (Table
1). In all cases, E1/2(Ir(IV/III)) was > 2.2 V. An excited-state
reduction potential (E1/2(Ir+*/0)) of ca. 1.77 V vs. NHE was
determined for all three complexes using the Ir(+/0) reduc-
tion potentials and a E00 value determined through
Franck-Condon lineshape analysis of the photolumines-
cence spectra (Figures S26-27).42-45
Figure 1. ORTEP diagram for Ir-bpph with the ellipsoids
drawn at the 50% probability level. A PF6– counter ion as
well as a diethylether molecule were omitted for clarity.
The synthesis of the three planar ligands was achieved
by condensation of 5,6-diamino-quinoxaline with the
suitable dione (Figure S1). For pzph, this synthetic ap-
proach was an improvement compared to the previously
published procedure.39 For the newly-reported bpph, this
condensation yielded a mixture of regioisomers in 4:6
proportions, which were separated by chromatography on
silica. Subsequently, each complex was formed by suc-
cessive chelation of the cyclometalated ligands, as de-
scribed by Nonoyama,40 followed by addition of the N-N
ligand. The three complexes were characterized by strong
ligand-centered (LC) absorption bands from 200 to 350
nm while transitions at wavelengths > 350 nm are most
often attributed to mixed metal-to-ligand (MLCT) and
ligand-to-ligand (LLCT) charge transfer bands from the
dCF3ppy-Ir moiety to the N-N ligand (Figure 2).8, 41 For
Ir-pzph and Ir-bpph, the greater electronic conjugation
within the diimine ligand stabilized the Ir(III) LUMO and
resulted in both a redshift of the lowest energy absorption
feature as well as a significant increase in molar absorp-
tion coefficients which reached values of ε450 =500 M–
1cm–1, 1400 M–1cm–1 and 9800 M–1cm–1 for Ir-TAP, Ir-
pzph and Ir-bpph respectively.
Excited-state reactivity of Ir-TAP, Ir-pzph and Ir-
bpph was investigated by time-resolved photolumines-
cence quenching experiments with seven quenchers. In
each case, excited-state quenching was solely dynamic in
nature, as indicated by the initial amplitude of the time-
resolved photoluminescence.25-26, 46 A representative ex-
ample of Ir-bpph excited-state quenching with halides is
shown in Figure 3. Quenching by all other com-
plex/quencher combinations are presented in Figures
S28-S34. The corresponding quenching rate constants
(kq) obtained through Stern-Volmer analysis ranged from
0.089 to 3.06 x1010 M–1s–1 (Table 2).
Transient absorption spectroscopy was performed with
Ir-TAP, Ir-pzph and Ir-bpph and tri-tolylamine (Me-
TPA) in argon purged CH3CN solutions (Figures S36).
Pulsed light excitation of an Ir complex resulted in elec-
tron transfer from Me-TPA as evident through the obser-
vation of spectral features unambiguously attributed to
the formation of Me-TPA+, that also provided access to
the spectra of the monoreduced iridium species. (Figures
S35-S36).
Time-dependent density functional theory indicated in-
creased contribution from pzph or bpph intra-ligand
charge transfer to these visible absorption bands (Figures
S19-24). These assignments were strengthened by vi-
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Table 1. Photophysical and electrochemical properties of the Ir(III) complexes in acetonitrile.
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kr (s–1)
knr (s–1)
Ir(+/0) c
-0.41
-0.13
-0.25
Ir(+*/0) c
1.78
E00 (eV) d
2.19
e (M–1cm–1)a
500
t (µs)b
1.4
Complex
Ir-TAP
λem (nm)
572
Φem
0.154e
0.028f
0.057f
1.1 x105 6.0 x105
2.9 x103 1.0 x105
1.2 x103 2.0 x104
Ir-pzph
Ir-bpph
1400
651
9.7
1.77
1.90
9800
618
47.0
1.76
2.01
a at 450 nm. measured at a concentration of 10 µM. V vs. NHE. d Determined by Franck-Condon lineshape analysis. e
b
c
Measured vs. Quinine (Φ = 0.546).[47] f Measured vs. [Ru(bpy)3]2+.[48]
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Table 2. Excited-state quenching rate constants (kq) for the
Ir(III) complexes by various quenchers in CH3CN.
saturate at intermediate driving forces. Much more inter-
esting was that beyond –ΔGο > 0.8 eV, kq was found to
decrease, suggestive of Marcus inverted behavior which
is rarely observed in bimolecular reactions.53 For Cl– and
Br–, the expectation was that they should be more difficult
to oxidize than I–, which has a known one-electron reduc-
tion potential of 1.23 eV vs. NHE.25-26, 51 This expectation
was reflected in the Stern-Volmer analysis as smaller kq
for Br– and Cl– were observed.
E(Dn/n-1
)
kq (x 1010 M–1s–1)
V vs NHE
Ir-
Ir-pzph
Ir-bpph
TAP
H2Q
MeO-TPA
TPA
0.70a
0.72b
0.93b
1.23c
0.84
1.15
1.17
2.85
0.67
1.25
1.34
3.06
2.29
1.04
0.16
0.65
1.14
1.21
2.97
2.16
0.67
0.089
I–
Br–
Cl–
1.35 ± 0.04d 1.89
1.46 ± 0.05d 0.82
tBu-Phenol
1.50e
0.26
afrom Ref [49]. bfrom Ref [50]. cfrom Ref [51]. d this work.
efrom Ref [52].
Additional transient absorption spectroscopy experiments
were performed using Ir-bpph with iodide, bromide and
chloride (Figure 3). With iodide and bromide, transient
absorption changes were consistent with the formation of
•–
•–
monoreduced iridium complex and either I2 or Br2 .
UV-Visible spectra before and after transient absorption
experiments were superimposable, indicative of the high
stability of these iridium complexes. In the case of chlo-
ride, excited-state quenching by transient absorption
spectroscopy was evident only by a drastic decrease in
excited-state lifetime. Unfortunately, photoproducts re-
sulting from electron transfer were not observed even
when 100 mJ/pulse of 532 nm light was used.
Quenching rate constants, kq, were then compiled as a
function of the free energy change for the reactions be-
tween the quenchers and iridium complexes, ΔGo =
{E1/2(Dn/n-1) – E1/2(Ir+*/0)}F as shown in Figure 4A, when
possible as one-electron redox potentials for Cl•/– and Br•/–
have not been previously reported in acetonitrile. Though
the quenching species were not a homologous series
where reaction exergonicities were systematically varied,
the driving force for quenching spanned nearly 1 eV and
provided impetus to investigate electron transfer reactiv-
ity more closely.
Figure 3. (top) Time-resolved photoluminescence of Ir-
bpph with increasing amount of TBACl in argon purged ac-
etonitrile at room temperature. The inset represents the
Stern-Volmer plots determined for the excited-state quench-
ing of Ir-bpph by iodide (purple), bromide (orange) and
Quenching rate constants were found to qualitatively
increase with reaction exergonicity before appearing to
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chloride (green). (bottom) Transient absorption spectros-
Page 4 of 7
theory provided a small total reorganization energy of 0.7
eV implying small kinetic barriers for excited-state elec-
tron transfer. Hence the Ir-bpph complex combines en-
hanced visible light absorption, strong photo-oxidizing
power and a small reorganization energy for rapid elec-
tron transfer.
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copy of Ir-bpph in the presence of TBAI (purple) and
TBABr (orange). Overlaid are results of spectral modelling
•–
based on spectra of I2 , Br2•– and monoreduced Ir-bpph.
To develop a quantitative investigation into the kinetics
of electron transfer, kq was corrected for diffusional con-
tributions as it is related to the second-order activated rate
constant, kact, and the diffusion rate constant, kdiff, through
Equation 1.
9
ꢀ
ꢀ
ꢀ
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=
+
Eq. 1.
ꢁ
ꢁ
ꢁ
ꢂ
ꢃꢄꢅ
ꢆꢇꢈꢈ
Diffusion limits for neutral and charged quenchers were
calculated using the Stokes-Einstein equation, and in-
cluded a Coulombic work term correction.25, 54-55 In
agreement with literature values for diffusion of neutral
species in acetonitrile, this was calculated as kdiff
=
1.9×1010 M–1s–1,55 while the value for oppositely charged
species, i.e. a halide and Ir+ complex, was estimated to be
5.0×1010 M–1s–1. Each series appeared to be near a diffu-
sion limit, Figure 4A, and application of Eq. 1 allowed to
estimate kact for each complex/quencher combination (Ta-
ble S7).
Activated rate constants were determined using kdiff for
both halide and neutral quenchers and were similarly an-
alyzed as a function of –ΔGo, Figure 4B. Non-adiabatic
Marcus theory, Equation 2, was used to model kact as a
product of one pre-factor, A’, total reorganization energy,
λ, and free energy change for the reaction, ΔGo.
ꢗ
ꢉꢊꢋꢌ = ꢍꢎꢉꢏꢐ = ꢑꢒ (ꢓ)exp ꢔ−(
ꢚꢝEq. 2.
)
ꢕꢖ ꢘꢙ
ꢛꢙꢁ ꢐ
ꢜ
Two methods were used to examine the experimental
data. First, a least-squares analysis of kact with Equation
2 provided λ = 0.62 eV and A’ = 8×1010 M–1s–1, while ex-
cluding the quenching data for Cl– and Br–. A second
method utilized values of λ = 0.7 eV and a transition state
collision frequency of A’ = 1×1011 M–1s–1 commonly uti-
lized for bimolecular electron transfer reactions.56 From
there, kact was calculated over a 1.5 eV driving force range
centered on –ΔGo = 0.7 eV for both neutral and charged
species, shown as the black solid line in Figure 4B, under
an assumption that A’ and λ were independent of the
quencher identity. It was evident that a maximum in kact
alone revealed that oft-quoted magnitudes of A’ = 1×1011
M–1s–1 for bimolecular reactions was entirely appropri-
ate.56 Finally, an average kact for Cl– and Br– and Eq. 1
were used to estimate the driving force for the reaction
using Cl– and Br– which subsequently provided one-elec-
tron redox potentials shown in Table 1.
Figure 4. (Top) Experimentally determined kq values for
neutral (empty) and charged (dot-filled shape) species with
three Ir+ complexes as a function of driving force for electron
transfer, –ΔGo. Horizontal dotted lines represent calculated
diffusion limits for neutral (orange) and charged (purple)
quenchers in CH3CN solution. The dashed lines represent
solutions to Eq. 1 using the determined values of kdiff and
simulated values of kact. (Bottom) Activated rate constants
calculated by removing the influence of diffusion on exper-
imentally determined kq. The black solid line is a result from
Marcus theory simulations using λ = 0.7 eV and A’ = 1×1011
M–1s–1. kact used to determine ∆G° and E1/2(Br•/–) and
E1/2(Cl•/–) through Eq. 1 and 2 are indicated in red.
ASSOCIATED CONTENT
In conclusion, the results highlight that through rational
ligand design, iridium complexes with E1/2(Ir(IV/III)) > 2.2
V, E(Ir+*/0) = 1.76 V vs NHE and strong visible light har-
vesting (e = 9800 M–1cm–1 at 450 nm) can be realized and
utilized for visible-light-driven chemistry. Nanosecond
transient absorption spectroscopy revealed that the pho-
tosensitizer excited-states were able to oxidize a variety
of organic substrates and halides. Application of Marcus
Supporting Information. Experimental section, Electro-
chemistry, X-Ray crystallography, DFT calculations, Time-
resolved photoluminescence quenching, Stern-Volmer plots,
Transient absorption spectroscopy. The Supporting Infor-
mation is available free of charge on the ACS Publications
website.
AUTHOR INFORMATION
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Journal of the American Chemical Society
12.
Twilton, J.; Le, C.; Zhang, P.; Shaw, M. H.; Evans, R. W.;
Corresponding Author
MacMillan, D. W. C., The merger of transition metal and
photocatalysis. Nat. Rev. Chem 2017, 1, 0052.
1
Ludovic.Troian.Gautier@ulb.ac.be, Benjamin.Elias@uclou-
2
3
4
5
6
7
8
13.
Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T.
vain.be
H.; Bernhard, S., Discovery and High-Throughput Screening of
Heteroleptic Iridium Complexes for Photoinduced Hydrogen
Production. J. Am. Chem. Soc. 2005, 127, 7502-7510.
Funding Sources
14.
Choi, G. J.; Zhu, Q.; Miller, D. C.; Gu, C. J.; Knowles, R.
No competing financial interests have been declared.
R., Catalytic alkylation of remote C–H bonds enabled by proton-
coupled electron transfer. Nature 2016, 539, 268.
15.
ACKNOWLEDGMENT
Zheng, J.; Swords, W. B.; Jung, H.; Skubi, K. L.; Kidd, J.
9
B.; Meyer, G. J.; Baik, M.-H.; Yoon, T. P., Enantioselective
Intermolecular Excited-State Photoreactions Using a Chiral Ir Triplet
Sensitizer: Separating Association from Energy Transfer in
Asymmetric Photocatalysis. J. Am. Chem. Soc. 2019, 141, 13625-
13634.
R. B. and B.E acknowledge the Fonds National pour la
Recherche Scientifique (F.R.S.-FNRS), the Fonds pour la
Formation à la Recherche dans l’Industrie et dans
l’Agriculture (F.R.I.A). L.T.-G. is a Postdoctoral researcher
of the Fonds de la Recherche Scientifique – FNRS. Steady-
State and time-resolved absorption and photoluminescence
experiments were performed using instrumentation 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. The authors wish to acknowledge the sup-
port from the ICMG FR2607, Chemistry Nanobio Platform,
Grenoble and in particular Prof. Eric Defrancq.
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