Proton-Coupled Electron-Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen-Bond Stabilization on Photoinduced Electron Transfer

Article abstract of DOI:10.1002/chem.201605594

The proton-coupled electron-transfer (PCET) reaction is investigated for a newly synthesized imidazole-anthraquinone biomimetic model with a photoactive Ru^II-polypyridyl moiety that is covalently coupled to the imidazole fragment. Intramolecular H-bonding interactions between imidazole and anthraquinone moieties favor the PCET process; this can be correlated to an appreciable positive shift in the one-electron reduction potential of the coordinated anthraquinone moiety functionalized with the imidazole fragment. This can also be attributed to the low luminescence quantum yield of the Ru^II-polypyridyl complex used. The dynamics of the intramolecular electron-transfer (ET) and PCET processes are studied by using femtosecond transient absorption spectroscopy. The steady-state spectroscopic studies and the results of the time-resolved absorption studies confirm that H-bonded water molecules play a major role in both ET and PCET dynamics as a proton relay in the excited state. The electron-transfer process is followed by a change in the H-bonding equilibrium between AQ and imidazole in acetonitrile solvent, and protonation of AQ^.? by water leads to PCET in the presence of water. A slower forward and backward electron-transfer rate is observed in the presence of D₂O compared with that in H₂O. These results provide further experimental support for a detailed understanding of the PCET process.

Full text of DOI:10.1002/chem.201605594

A Journal of
Accepted Article
Title: Proton Coupled Electron Transfer Processes in Ultrafast
Time Domain: Evidence for Effects of H-bond Stabilization on
Photoinduced Electron Transfer
Authors: Hirendra Nath Ghosh, Ananta Dey, Jayanta Dana, Sunil Auta,
Partha Maity, and Amitava Das
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201605594
Link to VoR: http://dx.doi.org/10.1002/chem.201605594
Supported by

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
Proton Coupled Electron Transfer Processes in Ultrafast Time
Domain: Evidence for Effects of H-bond Stabilization on
Photoinduced Electron Transfer
[a]
[b]
[a]
[b]
[a],[c],
[b],
Ananta Dey, Jayanta Dana, Sunil Aute, Partha Maity, Amitava Das,
* Hirendra N. Ghosh *
Dedication ((optional))
Abstract: Proton coupled electron transfer
PCET) reaction has been investigated for a
moiety
functionalized
with
imidazole
water molecule played a major role in both
ET and PCET dynamics as a proton relay in
excited state. The electron transfer process
was followed by changed in H-bonding
equilibrium between AQ and imidazole in
(
fragment. This could also be attributed to
the low luminescence quantum yield of
Ru(II)-polypyridyl complex that was used for
the present study. Dynamics of the
intramolecular electron transfer (ET) and
PCET processes were studied using
newly synthesized imidazole-anthraquinone
bio mimetic model having a photoactive
Ru(II)-polypyridyl moiety that is covalently
coupled to the imidazole fragment.
Intramolecular hydrogen bonding interaction
between imidazole and anthraquinone
moieties favoured the PCET process and
this could be correlated to an appreciable
positive shift of the one-electron reduction
potential of the coordinated anthraquinone

acetonitrile solvent and protonation of AQ ¯
from water makes the PCET in presence of
water. We observed a slower forward and
backward electron transfer rate in presence
femtosecond
transient
absorption
spectroscopy. Consequences of the steady
state spectroscopic studies as well as the
results of the time resolved absorption
studies confirmed that hydrogen bonded
of
2 2
D O compaired to H O. Our results
provide further experimental support for
understanding of the PCET process in detail.
In bacterial photosynthetic reaction also the fast electron transfer
[
12]
process occurs through the reduction of quinone,
which is
Introduction
facilitated due to the formation of a hydrogen bonded adduct
with amino acids in immediate vicinity having appropriate spatial
orientation. Semiquinone radical, a partially reduced form of
Proton coupled electron transfer (PCET), is an important
process and is generally operational in many natural processes
like water driven reduction of CO
quinone, is generally highly reactive and decays quickly when
2
to methane, nitrogen fixation,
[13],[14]
produced through a chemical transformation.
In biological
and four-electron reduction of dioxygen by cytochrome c oxidase
processes such semiquinone radicals participate in relay
process to extend their lifetimes and this also prevent such
radicals participating in undesired side reactions. However, such
processes are not well understood. The conversion of water to
dioxygen in the photo-system(II) is associated with the four
electron four proton coupled process and a detailed insight is
still lacking. In particular, understanding of PCET process in
aqueous medium is even more challenging, as water can act as
proton acceptor or donor depending on the media condition as
in respiration, hydrogenase reductase, water oxidation at the
[1],[2],[3]
oxygen evolving centre in photosynthesis etc.
The simplest
concept of proton coupled electron transfer can be described as
a process wherein an electron and a proton transferred from one
[
4],[5]
redox centre to another redox centre.
As two species, such
as proton and electron are being associated with the PCET
process, fundamentally it is more complicated than simple
electron transfer or the proton transfer mechanism. The PCET
process could be defined by either one of the following
processes, namely, proton transfer-electron transfer (PT-ET),
stepwise electron transfer-proton transfer (ETPT) and concerted
[15]
well as the other participating reactant. There are attempts in
understanding such
a process with appropriate synthetic
analytes that can mimic the process associated with
photosynthesis.
[
6]
proton-electron transfer (CPET). PCET is associated with
[
7],[8]
several crucial biological processes.
quinones play a key role as a primary electron acceptor
the reduction process of quinone is facilitated by hydrogen-bond
For photosynthesis,
[
9],[10]
and
Haga and his co-workers had reported some imidazole
functionalised Ru(II)-polypyridyl complexes immobilized on gold
surface, which showed the PCET phenomena and the process
[
11]
donors or by protonation.
[
16],[17]
was
studied by cyclic voltammetric
technique.
[
a]
Ananta Dey, Sunil Aute, Dr. Amitava Das
Organic Chemistry Division
Hammarström and his co-workers had reported an
intramolecular PCET process that happened in micro and
nanosecond time domain for ruthenium-tyrosine and tryptophan
complexes involving electron transfer from tyrosine and
tryptophan to the oxidised Ru-centre and water molecule
CSIR-National Chemical laboratory
Pune, Maharashtra, 411008.
E-mail: a.das@ncl.res.in
[
[
b]
Jayanta Dana, Partha Maity, Dr. Hirendra N. Ghosh
Radiation & Photochemistry Division, Bhabha Atomic Research Centre,
Mumbai – 400085, India. E-Mail: hnghosh@barc.gov.in
CSIR-Central salt & Marine Chemicals Research Institute, Bhavnagar:
[
18],[19]
participated in the process as proton acceptor.
Costentin et.
c
]
al. discussed about mechanistic aspects of the PCET having
364002, Gujarat, India. Fax: +91 278 2577562; E-Mail:
[20],[21],[22]
phenol in the proton relay networks.
Porphyrin-
a.das@csmcri.org
benzoquinone dyads that mimic the function of P680 in biological
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
[
9]
systems were also reported in literature. Wenger and co-
workers have described an Ir-polypyridyl for achieving long lived
charge separated species and other systems to describe the
also favour the ET process from photo-excited Ru(II)-polypyridyl
centre to the anthraquinone moiety (AQ) by lowering the energy
level (LUMO) of anthraquinone. We have used different
analytical and spectroscopic techniques such as cyclic
voltammetry, steady state as well as time-resolved
[
23],[24]
PCET phenomena.
Many theoretical models have been
proposed correlating free energy for reaction, donor-acceptor
distance, spatial orientation electronic coupling and biomimetic
systems that mimics the tyrosine–histidine pair in PSII, in order
luminescence
and
femto-second
transient
absorption
spectroscopy to study the ET and PCET process in organic as
well as in presence of water.
[
25]
to develop better insight in PCET phenomena.
The range of
theoretical and computational treatments of hydrogen tunneling
in enzymatic and bio mimetic systems are described in the
[
26]
previously reported literature.
Despite several literature Results and Discussion
reports, majority of the studies on the mechanistic understanding
of PCET process were focussed on the electronic ground
and there are only limited reports that discussed the
PCET process associated with electronically excited state.
[
27],[28]
state
[
29],[30]
[
30]
[31]
Mayer and co-workers
and others
had demonstrated the
electron transfer and proton coupled electron transfer reaction in
Ru(II)-polypyridyl based complexes and many other
[
32],[33]
+
molecules
having H -donor fragments through transient
absorption studies with nanosecond and micro-second time
resolution and cyclic voltammetric studies. Though it has been
argued that studies on excited state dynamics and femtosecond
transient absorption spectroscopy are crucial for developing a
better insight in PCET process, systematic studies in the
ultrafast time domain is still lacking. Most of the literature reports
[34],[35]
Scheme 1. Methodology that was adopted for synthesis of RuQ.
unable to observe the transient absorption photoproducts.
In general, the electronic absorption spectra of transition
metal complexes depends on the oxidation state of the
corresponding metal ion and such complexes with well defined
redox and excited state properties are ideal for developing a
better insight on PCET process. Ru(II)-polypyridyl based
complexes comply with both such requirements. However, to the
best of our knowledge, no such reports are available in the
contemporary literature that describes the excited state
dynamics of the PCET process involving Ruthenium(II)-
polypyridyl complex in ultrafast time scale. Keeping these in
mind, we have designed and synthesized a Ru(II)-polypyridyl
complex (RuQ) functionalized with an imidazole moiety,
covalently linked to a quinone moiety for studying the PCET
process in the ultrafast time domain.
Cyclic voltammetric studies of RuQ complex
To understand the thermodynamic feasibility of the electron
transfer processes in the RuQ complex, the cyclic voltammetric
(II)/(III)
measurements were carried out. Redox potentials for Ru

and bpy/bpy ¯ couples were evaluated and compared with
2
+
Ru(bpy)
degassed and pre-dried acetonitrile solution of respective
complexes and in the presence of tetrabutyl
) as the supporting
3
.
Cyclic voltammograms were recorded using
ammoniumhexafluoro phosphate (TBAPF
electrolyte.
6
Previous reports on the PCET processes involving
intermolecular electron and proton transfer process suggest that
dynamics of such processes are largely dependent on the rate
of diffusion process. To avoid such a problem, we designed a
triad system having photoactive Ru(II)-polypyridyl core
conjugated with anthraquinone through an imidazole linker
(
Scheme 1). RuQ was synthesized in 3 steps. Oxidation of 4,4′-
dimethyl-2,2′-bipyridine in the presence of SeO resulted in the
aldehyde (L ). Condensation of with 1,2-diamino
anthraquinone yielded the ligand L . Complexation of L with
Ru(bpy) Cl resulted the RuQ complex. All the intermediates and
2
1
L
1
2
2
2
2
the final compound were well characterized by standard
analytical and spectroscopic techniques. We envisioned that in
the RuQ complex, Ru acts as a photo-active centre. Imidazole
proton is expected to form H-bond with one of the two carbonyl
functionalities of the quinone moiety. H-bonding interaction is
expected to facilitate the reduction of anthraquinone. H-bonding
2
+
Figure 1. Cyclic voltammogram of (a) RuQ (red trace), (b) Ru(bpy)
3
(blue
trace) and (c) (olive trace) differential pulsed voltammetry of RuQ in de-
oxygenated and pre-dried acetonitrile in the presence of 0.1 (M) tetra butyl
ammonium hexafluorophosphate (TBAHF ) as supporting electrolyte using
Ag/AgCl (saturated KCl) electrode as the reference electrode. Ferrocene (Fc)
was used as an internal standard.
6
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
(
II)/(III)
It is evident from Figure 1 that Ru
couple appeared at a
Addition of water to acetonitrile solutions of RuQ complex
studied results in positive shifts of both the reduction steps (-
+
potential of 0.89 V vs Fc/Fc , which is reversible under these
experimental conditions. The two subsequent reversible


2-
0.81V, -0.99V for AQ/AQ ¯ and AQ ¯/ AQ respectively)
compared to the pure acetonitrile solvent. Representative cyclic
voltammograms are shown in Figure S28, S29 (In the supporting


2
AQ/AQ ¯ and AQ ¯/ AQ ¯ couple appeared at -0.95 V and -1.24
+
II/III
V (vs Fc/Fc ), respectively. The Ru
couple provide us some
2
+
estimate about the HOMO energy level. For RuQ complex, two
more bpy-based reductions were observed at -1.77 V and -1.97
V, (vs Fc/Fc ) respectively. These two processes are attributed
information) for the RuQ and Ru(bpy)
3
complexes respectively.
In presence of water the positive shift is much larger for the
second wave associated with the anthraquinone reduction. The
shifts in the potentials of quinine couples caused by the addition
of water to aprotic solvent are mainly due to the fast protonation
of the dianion. The significant positive shift of the AQ couples in
presence of water suggests that the involvement of water
molecules plays an important role in the electron transfer
phenomena by lowering the LUMO energy level of
anthraquinone moiety. This was studied in details by time
resolved absorption spectroscopy and is discussed in
subsequent sections.
+
[
36]
to the two coordinated bpy-ligands in RuQ.
Please note that
second bpy reduction was quasi reversible and for this
differential pulsed voltammetry (DPV) plot for these two redox
processes are shown in Figure 1. Table 1 shows that reduction
potential of Ru(II)/Ru(III) of RuQ dye is similar to that of
(0.89 V) vs Fc/Fc in acetonitrile), which is rather
anticipated. Literature reports suggest that free anthraquinone
reduction occurs at -1.32 V (vs Fc/Fc ). In the present study,
AQ/AQ ¯ couple appeared at -0.95 V (vs. Fc/Fc ) and this facile
reduction is attributed to the coordination of L to the cationic
2
+
+
Ru(bpy)
3
+
[35]

+
2
Ru(II)-centre. Further, intra-molecular H-bonding interaction
between one of the two C=OImidazole group and NHimidazole
facilitates protonation of the resulting radical (AQ ¯) intermediate
Optical studies and luminescence spectroscopy of RuQ
complex

and we observed the positive shift of the anthraquinone
Prior to the monitoring of ET and PCET processes in RuQ, it
was essential to understand and compare the spectral
[
37]
couple. Scheme 2 represents the redox relative energy levels
of different excited states of Ru-complex (RuQ) and
anthraquinone moiety and it is evident that photo-excited
2
+
(absorption and emission) properties of RuQ and Ru(bpy)
3
.
Figure 2 shows optical absorption and emission spectra of
2
+
electron
transfer
from
Ru(II)
to
Anthraquinone
is
Ru(bpy)
3
(Figure 2a) and RuQ (Figure 2b) in acetonitrile
thermodynamically feasible.
medium. For both the complexes, the absorption band at ~ 290
[
38]
nm was attributed primarily for -* based transition.
For
2
+
Ru(bpy)
3
, band at ~ 450 nm was assigned to a predominant
Table 1. Half-wave potential of the Ru(bpy)32+ and RuQ complex for the
oxidation and reduction processes.
3
[39]
Ru dto bpy* based MLCT transition. For RuQ, this transition
appeared at ~ 480 nm. In addition to that a prominent absorption
band was observed at ~ 350 nm for RuQ and this could be
Species
E
1/2
E
1/2
E
1/2
E
1/2
E
1/2
E
1/2
RuII/III
(AQ/AQ ¯) (AQ ¯/ AQ ¯) (bpy/bpy-) (bpy/bpy-) (bpy/bpy-)


2
attributed to a predominant intra-ligand [bpy] -[L ]
transition.
2 -*
based
(
)
-*
[36]
2+
Ru(bpy)
RuQ
3
0.89V
0.89V
-
-
-
-1.71V
-1.77V
-
-1.91V
--1.97V
-
-2.16V
-0.95V
-1.32V
-1.24V
-
-
-
AQ
E vs Fc/Fc+
E vs Fc/Fc+
-
2.0
-
2.0
1.0
1MLCT
-1.18eV
(
LUMO
AQ
(-1.32 eV)
-1.0
-
3MLCT
LUMO
-1.07 eV)
(
AQ
(
-0.90 eV)
0.0
0.0
+1.0
+1.0
+
S/S
1
.16eV
+
2.0
+2.0
2
+
Figure 2. Optical absorption spectra of (a) Ru(bpy)
Steady state luminescence spectra of (a′) Ru(bpy)
time-resolved luminescence spectra of (a′′) Ru(bpy)
3
complex and (b) RuQ.
and (b′) RuQ. Inset:
and (b′′) RuQ after 445
2
+
3
2
+
3
Scheme 2. Energy levels of RuQ complex and free anthraquinone from cyclic
voltammetry measurements and feasibility of the electron transfer from Ru(II)
centre to anthraquinone moiety.
nm laser excitation. All the emission decay traces are monitored at 610 nm. L
stands for profile of 445 nm laser excitation source. The concentration of both
the complex solutions is 1x10 (M) in acetonitrile solvent.
-
5
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
Presence of the extended conjugation and the electron
withdrawing groups like imidazole/anthraquinone is expected to
reaction energetic are favourable (dexter energy transfer).
[
42],[43]
However, the lowest triplet excited state of anthraquinone
3
lower the L
2
-based LUMO energy in RuQ and this further
is at 2.72 eV which is much higher than the MLCT state of the
2
+
2+
support the observed longer wavelength absorption band
Ru(bpy)
3
complex (2.12 eV). So, Ru(bpy)
3
to anthraquinone
2
+
maximum for RuQ as compared to Ru(bpy)
3
. Steady state
(Figure. (2a′)) and RuQ
Figure (2b′)) in acetonitrile (at 298 K) are shown in Figure 2.
triplet-triplet energy transfer is an endergonic process. On the
other hand with increasing base concentration the fluorescence
intensity of RuQ complex (Figure S13 in the Supporting
2
+
luminescence spectra for Ru(bpy)
3
(
2
+
Luminescence spectra for Ru(bpy)
3
shows an anticipated
Information) increases and
addition of 2equvalent base
broad luminescence band with maximum at 610 nm (emi = 0.1)
(TBAOH) the fluorescence quantum yield increases to emi
=
following excitation at 450 nm. This emission is attributed to
0.004. The increased quantum yield in presence of base also
suggests that triplet-triplet energy transfer from Ru-centre to
3
[40]
MLCT-based emission.
appears in the same spectral region with much lower emission
quantum yield (emi 0.003). This appreciably lower
luminescence quantum yield for RuQ as compared to pure
For RuQ, luminescence band
anthraquinone is
a unlikely quenching process. Here the
=
decrement of emission lifetime can be mainly attributed to photo-
excited electron transfer from Ru-complex to anthraquinone
moiety which is thermodynamically viable process. While
addressing photoluminescence quenching in RuQ, it is clear that
charge separation can take place through electron transfer from
2
+
Ru(bpy)
3
could be attributed to energy or electron transfer
3
process involving MLCT phtoexcited state of Ru(II)-centre and
the anthraquinone moiety as the acceptor fragment. Steady
state redox potential data confirms the thermodynamic feasibility
2
+
photo excited Ru(bpy)
3
to anthraquinone forming cation radical
[
35]

for the electron transfer pathway.
For RuQ, intramolecular
of Ru-complex and anion radical of anthraquinone (AQ ¯). The
hydrogen bonding between one of the two carbonyl moieties of
anthraquinone fragment and NHImidazole further lowers the energy
level of anthraquinone LUMO, which eventually favours the
possibility of the electron transfer process. These suggest that
the decrease in photoluminescence quantum yield for the
ligand L
with emission maxima at 500 nm. The emission decay trace of
can be best fitted with two time constants τ = 0.071 ns (95%)
and 1.382 ns (5%) (Figure S11 in the Supporting
Information). To understand the intramolecular electron transfer
dynamics in shorter time scale we have carried out femtosecond
transient absorption studies in the above systems, which are
discussed below.
2
shows a broad emission spectrum (450 nm – 650 nm)
L
2
1
τ
2
=
3
MLCT-based excited state in Ru(II)-imidazole-anthraquinone
triad is due to intramolecular electron transfer process. In order
to check such a possibility and to monitor the intramolecular
electron transfer dynamics (if any), time resolved emission
2
+
studies were carried out for both Ru(bpy)
3
and RuQ following
Transient absorption studies of RuQ complex
excitation at 445 nm and monitoring the emission decay for
respective complex at 610 nm in dry and thoroughly degassed
Femtosecond transient absorption studies have been carried out
2
+
(
with argon) acetonitrile solution. These decay profiles are
for RuQ, Ru(bpy)
3
2
complexes and L ligand separately after
shown as an inset in Figure 2. Figure 2a'' shows the emission
exciting at 400 nm. For both the complexes, transient absorption
due to triplet state in 500-700 nm with very long lifetime (>> 1
2+
decay trace of pure Ru(bpy)
3
complex which can be fitted
[
44]
mono-exponentially with time constant of τ = 156.37 ns. On the
other hand Figure 2b'' shows the emission decay trace of RuQ,
ns) and bleach due to ground state absorption below 500 nm
were observed (FigureS18 in the Supporting Information). The
broad band (580 nm - 700 nm) can be assigned as the excited
which can be fitted bi-exponentially with time constant τ
1
= 1.772
3
ns (81.721%); τ = 111.031 ns (18.279%). It is interesting to see
2
triplet state absorption of the dπRu-π*bpy based MLCT state and
[
45]
that the Ru-complex emission decay occurs in much faster rate
once it is coupled with H-bonded anthraquinone moiety. This
decrement of emission lifetime in RuQ complex suggests a
decays till 1 ns (FigureS20 in the Supporting Information).
Figure 3 shows transient absorption spectra of RuQ complex at
different time delay and these set of time resolved spectra looks
2
+
2+
strong electronic interaction between Ru(bpy)
anthraquinone moiety. The fast 1.772 ns component may
indicate photoinduced electron transfer process (PET). However, absorption below 515 nm and broad absorption band beyond
3
and
3
completely different as compared to those for Ru(bpy) .
Transient absorption spectra of RuQ complex show negative
TCSPC cannot resolve the PET timescale as it occurs in
ultrafast time domain. To confirm that we have performed femto
second transient absorption study which are described later in
detail. From the previous report, one can also predict that the
515 nm with two peaks at ~ 530 nm and ~ 650 nm respectively.
Time-resolved photoluminescence and cyclic voltammetric
studies indicated that the intra-molecular electron transfer takes
place from Ru-complex to anthraquinone moiety (AQ). In the
present study, the transient peak at 530 nm can be attributed to
3
2+
MLCT excited state of Ru(bpy)
3
can also be quenched due to

the presence of molecular components through energy transfer.
However it is known that anthraquinone moiety absorbs light
radical anion of anthraquinone (AQ ¯) which has been confirmed
[46],[47],[48]
from earlier reports.
On the other hand broad transient
[
41]
below 450 nm
and there will be no overlap between
and photo-luminescence spectra of Ru-
absorption band 580-700 nm region peaking at 600 nm can be
3
+
absorption spectra of L
2
3
attributed to cation radical of Ru-complex Ru(bpy) . Cation
complex, (Figure S10 in the Supporting Information) as a result
we can discard Förster energy transfer mechanism. The
emissive MLCT excited states of Ruthenium complexes can be
radical band was confirmed on the basis of the data obtained in
a complementary pulsed radiolysis experiment (Figure S24 in
3
3+
the Supporting Information) where Ru(bpy)
3
was generated
2
radical in N O
2
+
quenched efficiently by triplet – triplet energy transfer when the
selectively by the reaction of Ru(bpy)
3
with N
3
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER

saturated aqueous solution (5% acetonitrile + 95% water). The
negative absorption band in 450-500 nm region peaking at 480
nm can be attributed to the bleach due to ground state
absorption.
After accepting the electron, pK
a
of AQ ¯ form increases and the
H-bonding equilibrium changes with the imidazole proton and
forms strong H-bond with AQ . However the transient decay
kinetics at 600 nm can be fitted with bi-exponential growth with
.
-
time constants of τ
1
= < 100 (±15) fs (95%), τ
5%), and bi-exponential decay with time-constants of τ
±3) ps (75%), τ = > 1 (±0.05) ns (25%) (Table 2). Although we
2
= 10 (±0.5) ps
(
(
1
= 60
2
have attributed that transient at 600 nm is due to metal centered
oxidation of Ru- complex however it is wondering that 2 (±0.2)
ps time constant is absent which we have attributed to forward
electron transfer time. This might be due to both photo-excited
3
triplet state ( MLCT) and oxidised state (cation radical) absorb in
[
49]
the same spectral region.
Table 2. Parameters for multi-exponential fits for transient kinetics of RuQ dye
at different wavelengths after 400 nm laser excitation.
Figure 3. Femtosecond transient absorption spectrum of RuQ in acetonitrile
solvent after exciting the samples at 400 nm laser light.
Wavelength
nm)
τ1 growth
τ2 growth
τ3 growth
τ1
τ2
τ3
(
4
90
<100 (±15) fs
-
-
1.5 (0.15) ps
(40%)
60 (±3) ps
>1 (±0.05)
Now, to monitor the electron transfer dynamics transient kinetics
have been monitored at key wavelengths and shown in Figure 4.
The transient decay kinetics at 530 nm can be fitted with tri-
(100%)
(17%)
ns
(43%)
530
<100 (±15) fs
45%)
2 (±0.2) ps 10 (±0.5) 250 (±12.5) ps >1(±0.05) ns
-
(
(50%)
ps
5%)
-
(55%)
(45%)
exponential growth with time constants of τ
45%), τ = 2 (±0.2) ps (50%), τ = 10 (±0.5) ps (5%), and bi-
exponential decay with time-constants of τ = 250 (±12.5) ps
55%), τ = >1 (0.05) ns (45%) (Table 2). Here the first growth
1
= <100 (±15) fs
(
(
2
3
6
00
<100 (±15) fs 10 (±0.5) ps
95%) (5%)
60 (±3) ps
>1(±0.05) ns
-
1
(
(75%)
(25%)
(
2
component < 100 (±15) fs can be attributed to the formation of
triplet state of Ru-complex as the MLCT state has absorption in
3
So, on transfer of electron from triplet state, metal centered
oxidation of Ru-complex occurs which absorbs in the same
spectral region with almost similar molar extinction co-efficient.
As a result we do not see much change in transient absorption
in early time scale. The bi-exponential decay components at
both 530 nm and 600 nm can be attributed to back electron
this wavelength region. The 2 (±0.2) ps time constant can be
attributed to electron transfer from photo-excited Ru-complex to
anthraquinone moiety with the formation of anthraquinone

radical (AQ ¯) (Scheme 3). The important observation in the
transient absorption spectrum is that, at 530 nm there is no
obvious band at 0.5 ps, but there is a clear peak after 1 ps, and
this is essentially fully developed after 5 ps. This observation

transfer time from anthraquinone radical (AQ ¯) to Ru(III). The
bi-exponential decay of 530 nm and 600 nm kinetic traces are
due to the multiple conformations of the bpy-attached Im-AQ
fragment which can rotate around the single bond. The bleach
kinetics at 490 nm can be fitted with pulse-width limited growth <
.-
strongly suggests that the 2 ps is the major component of AQ
formation.
1
00 (±15) fs and multi-exponential recovery with time-constants
of τ = 1.5 ps (40%), τ = 60 (±3) ps (17%) and τ = > 1 (±0.05) ns
43%). Bleach recovery generally gives the indication of back
1
2
3
(
electron transfer however in the present investigation as we are
monitoring the bleach at 490 nm which has overlap with the

transient spectra of anthraquinone radical AQ ¯ so the faster
component 1.5 (±0.15) ps can be attributed to electron transfer
time from photo-excited Ru-complex to anthraquinone. However,
the longer components 60 (±3) ps and > 1 (±0.05) ns are
attributed to back electron transfer from anthraquinone radical

AQ ¯ to Ru(III).
Figure 4. Transient kinetic traces of (a) cation radical (at 600 nm) of Ru-

complex; (b) anion radical of anthraquinone (AQ ¯) (at 530 nm); and (c) bleach
recovery kinetics 490 nm of RuQ complex.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
Figure 5. Transient absorption spectrum of RuQ complex in presence of
tetrabutyl ammonium hydroxide in acetonitrile at different time delay after
exciting the sample at 400 nm laser light.
In Scheme 3 we have discussed that the intramolecular H-
bonding of imidazole proton and the anthraquinone moiety
2
+
facilitates the intramolecular electron transfer from Ru(bpy)
3
to
Scheme 3. Mechanistic scheme of electron transfer and proton coupled
electron transfer (PCET) pathways and involvement of H-bonding in the
transient states.
the anthraquinone moiety. Now to re-confirm the above process
we have carried out transient absorption studies of RuQ in
presence of tetra butyl ammonium hydroxide (TBAOH in
methanolic solution) and shown in Figure 5. The TA spectra of
RuQ in presence of base show broad bleach at 490 nm and a
broad positive absorption at (560-700 nm). The transient spectra
showed that the bleach slightly shifted to the longer wavelength
The above forward and backward electron transfer are
demonstrated in Scheme 3. Before discussing the intramolecular
electron transfer and back electron transfer process, it is worth
noting that FTIR spectra showed the strong evidence for
intramolecular H-bonding between anthraquinone carbonyl
functional group and adjacent imidazole N-H functional group

and there is no signature of AQ ¯ radical. It is interesting to see
that the transient spectra of RuQ in presence of TBAOH exactly
2
+
matches with transient spectra of Ru(bpy)
3
(Figure S18 in the
(
Figure S25 in the Supporting Information). In the excited state
Supporting Information). In addition to that the transient
absorption and bleach kinetics also matches exactly as that of
this H-bonding equilibrium changes followed by easier
anthraquinone reduction process. Then subsequently the ground
state bleach recovers as Ru(II) is regenerated in the course of
intramolecular ET recombination with AQ ¯ unit and Ru(III)
(
produces AQ and Ru(III). The forward electron transfer reaction
innitialy leads to a charge separated state after photo excitation
of Ru(bpy)
2
+
Ru(bpy)
3
(Figure S23 in the Supporting Information). This
observation clearly suggests that no intramolecular electron
transfer takes place from Ru-centre to AQ-centre in RuQ in
presence of TBAOH. Herein TBAOH deprotonate the imidazole
proton making the electron transfer process energetically
unfavourable, as a result no electron transfer takes place from
photo-excited Ru-complex to anthraquinone moiety.

Scheme 3). Ultrafast intramolecular electron transfer transiently
.
-
2
+
3
centre, finally back electron transfer from carbonyl
to Ru(III) i.e. the radical recombination occurs. The positive
absorption band at 530 nm indicates the formation of additional
Scheme 3 supported by the above experimental results
suggested that intra-molecular H-bonding facilitates the excited
state electron transfer of RuQ and followed by the proton motion
and produces a charge separated state. Electron transfer from
.
-
intermediate (AQ ) shown in Scheme 3. The significant
observation is that the 530 nm band decays with the same
3
+
a
kinetics as that of the Ru absorption and ground state recovery. photo excited Ru-centre results in the remarkable pK increase
.
-
Therefore there is no proton transfer from imidazole to AQ .
To confirm that there is no interference of the L ligand excited
at quinone centre, that generates a new state leading to a H-
bond rearrangement reaction between imidazole to the
2
[
50]
state dynamics in the electron transfer and H-bonding
equilibrium dynamics we had taken the transient absorption
spectrum of L ligand in acetonitrile separately (Figure S21 in
2
the Supporting Information).The spectrum shows broad positive
absorption peaking at 550 nm and the kinetic trace monitored at
anthraquinone moiety.
So the overall reaction can be
considered as a proton coupled electron transfer (PCET)
wherein a considerable amount of proton density transferred
from imidazole to anthraquinone rather than full proton transfer,
finally it goes to ground state due to radical recombination. It has
been observed that H-bonding play a major role in PCET
reaction of photo-excited RuQ complex.
550 nm is totally different from the kinetic trace of the RuQ
complex monitored at 530 nm.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
Therefore, for the transient absorption measurements RuQ was
dissolved in acetonitrile-water (2:3, v/v) solvent mixture instead
of pure water. Figure 6 shows the transient absorption spectra of
RuQ in acetonitrile-water (2:3, v/v) mixture at different time delay
after exciting the sample at 400 nm laser. The transient spectra
consist of negative absorption below 510 nm peaking at 490 nm
and a broad positive absorption band from 520-700 nm and
beyond. The negative absorption band is attributed to bleach
due to ground state absorption which matches with steady state
optical absorption spectra (Figure S7 in the Supporting
Information). It is interesting to see that with time, the broad
positive absorption band was converted into two separate
transient absorption bands peaking at 542 nm and 630 nm,
Photoinduced electron transfer in the RuQ complex in
presence of water
Water is an interesting molecule for H-bonding, it can act as
[
51]
both H-bond donor and H-bond acceptor and stabilises many

excited state species. After accepting electron, AQ ¯ is known to
[
46]
be highly reactive with water molecule
and the exact
mechanism and the photoproduct is not well understood.
Mechanistic demonstration of the PCET is always a challenging
task. Most of the natural processes which involve PCET
reactions avoid intermediate charge build-up states, and follows
the accumulation of various types of oxidizing equivalents which
can stabilise the high energy intermediates. So in the present
study, it is important to monitor both ET and PCET in the
presence of water. Literature reports also suggest that both the
benzoquinone and anthraquinone are excited state oxidants,

which can be attributed to anthraquinone radical (AQ ¯) and
cation radical of Ru-complex respectively. More importantly, the

transient absorption band due to AQ ¯ in acetonitrile-water (2:3,
[
46]
v/v) was shifted to the red region of spectra (542 nm) as
compared to that in pure acetonitrile (530 nm) (Figure 3). This
shift could be attributed to strong H-bonding interaction between
and can oxidise aliphatic alcohols, and even water.
In the present investigation we have carried out steady state
absorption and emission spectroscopic study of RuQ complex in
presence of water. Figure S7 (In the supporting information)
shows the optical absorption spectra of RuQ complex in water-
acetonitrile (99:1, v/v) solvent mixture. The ~ 10 nm shift of
optical absorption spectra in water-acetonitrile (99:1, v/v) solvent
mixture as compared to pure acetonitrile can be attributed to the
hydrogen bonding interaction between N-H functional group of
the imidazole and water molecules. For better understanding it is
.
-
AQ and water as water is a strong H-bond donor. It is
noteworthy to see that both bleach recovery and transient
kinetics decay was much faster in acetonitrile-water (2:3, v/v)
mixture as compared to pure acetonitrile. This observation
clearly suggests that water plays a major role in both ET and
PCET reaction in the present investigation. To monitor both ET
and PCET dynamics, transient kinetics was monitored at key
wavelengths as shown in Figure 7. The transient decay kinetics
at 540 nm can be fitted with single exponential growth with time
a
necessary to calculate the pK of the RuQ complex. We did the
pH titration experiment using photoluminescence technique,
exciting the RuQ complex at 450 nm, with increasing pH of the
constants of τ
time-constants of
1
= 500 (±50) fs and single exponential decay with
20 (±2) ps (Table 3) due to the
~
3 2
solution from pH 2 to 12 (1:1, v/v) in CH CN–H O mixture. We
recombination reaction. We have already mentioned that growth
calculated pK of RuQ complex, from the inflection point of the
a

at 540 nm due to anthraquinone radical (AQ ¯) can be attributed
titration curve monitoring the emission maximum at 612 nm as a
function of pH is shown in (Figure S8 in the Supporting
Information). The inflection point of the titration curve obtained is
to electron transfer time from photo-excited Ru-complex to
anthraquinone moiety.
It is interesting to see that in the presence of water, electron
at pH
.45, obtained from pH
excited-state lifetimes of the protonated (τHB) and deprotonated
forms (τ ) of the RuQ complex. Since we used PF salt of RuQ
i
= 6.75. The true excited-state ionization constant (pK
a
*) is
3 2
transfer become faster 500 (±50) fs. CH CN−H O leads to an
7
i
while taking into account the different
even more important increase in electron transfer rates. The
stabilization of the anthraquinone radical anion through the
formation of H-bonding with water and the considerable positive
shifting of the AQ reduction potential compared to pure
acetonitrile indicates that LUMO energy level of anthraquinone
become further lowers that eventually facilitate both forward and
backward electron transfer. The free anthraquinone moiety in
B
6
complex, the solubility of RuQ is extremely poor in pure water.
RuQ complex cannot be protonated by water (pK
H O (pK
3 a
a
= 15.7) or
= - 1.7) as the conjugate acid of anthraquinone
possess the pK = - 8.2 in H O. On the other hand, in N, N′-
dimethylformamide solvent medium the conjugate acid form of
+
a
2
+
anthraquinone mono anion has pK
protonate the anthraquinone mono anion.
a
= 5.3, hence H
3
O
can
[
35],[52]
In addition to this,
as we change the solvent medium from acetonitrile to
acetonitrile-water (2:3, v/v) mixture, the dielectric constant of the
[
53]
medium increases from 35.94 to 55.87,
which in turn
increases the driving force for intramolecular electron transfer
[
35]
from Ru-centre to AQ centre in RuQ complex.
Figure 6. Femtosecond transient absorption spectra of RuQ in acetonitrile-
water (2:3, v/v) solvent mixture at different time delay after 400 nm laser
excitation.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
spectra and transient kinetics are similar with acetonitrile-water
(
2:3, v/v) mixture. This observation clearly suggests that, if the
protonation of the anthraquinone takes place after forward
electron transfer from Ru-centre to AQ unit, and there is no
influence of pH in the forward electron transfer rate.
Scheme 4 demonstrates the involvement of hydrogen-bonding
.
-
of water molecules to AQ and AQ . Water acts as proton relay in
the forward electron transfer from Ru(II) to anthraquinone moiety.
Table 3. Parameters for multi-exponential fits for the transient of RuQ dye in
acetonitrile-Water (2:3, v/v) mixture at different wavelengths after 400 nm laser
excitation.
Wavelength
nm)
τ1 growth
τ1
τ2
τ3
(
490
540
600
<100 (±15) fs
200 (±26) fs
(30%)
2 (±0.2) ps
(30%)
-
20 (±2) ps
(40%)
-
(
100%)
500 (±50) fs
100%)
<100 (±15) fs
100%)
20 (±2) ps
( 100%)
(
Figure 7. Transient kinetic traces of (c) cation radical of RuQ-complex (at 600
nm); (b) anion radical of anthraquinone (AQ ¯) (at 540 nm); and (a) bleach
200 (±26) fs
(24%)
2 (±0.2) ps
(43.7%)
20 (±2) ps
(32.3%)

(
recovery kinetics at 490 nm of RuQ complex in acetonitrile-water (2:3, v/v)
solvent mixture.
To examine the proton coupled electron transfer process it is
common to distinguish the effect of protonation and
The single exponential decay time constant 20 (±2) ps could be
.
-
attributed to back electron transfer from AQ radical anion to Ru
III) as shown in Scheme 4. The bleach recovery kinetics at 490
nm can be fitted with pulse-width limited single exponential
growth and multi-exponential recovery with time constants of τ
00 (±26) fs (30%), τ = 2 (±0.2) ps (30%) and τ = 20 (±2) ps
40%). Due to the overlap between transient bleach (at 490 nm)
deprotonation in presence of D
slidely slower forward electron transfer rate (700 fs) in presence
of D O. Transient absorption spectra of RuQ in acetonitrile-D
2:3, v/v) are shown in (Figure S16 in the Supporting
Information). To monitor the difference between H O and D O in
2
O. Interestingly we observed a
(
2
2
O
1
=
(
2
2
3
2
2
(
the back electron transfer reaction transient kinetics at both 490
nm and at 600 nm are compared and shown in Figure 8. As
expected, both the transient decay and bleach recovery were

and anthraquinone radical (AQ ¯) (at 540 nm), both the faster
components at 490 nm can be attributed to forward electron
transfer. However, the longer component can be attributed to
2 2
slower in presence of D O as compared to H O. The transient
.-
back electron transfer from AQ to Ru(III) (Scheme 4). Kinetics
at 600 nm can be fitted with pulse-width limited < 100 (±15) fs
data are fitted multi-exponentially and are shown in Table 4. At
both the wavelengths (490 nm and 600 nm) two longer
components 2 (±0.2) ps and 20 (±2) ps have been observed in
water, which we attributed to back electron transfer reaction
from anthraquinone to Ru(III) centre. Additionally in the transient
kinetics it is clear from the data shown in Figure 8 that the
isotope effect is only present in long time components of 20 ps.
While for the shorter timescales the (2 ps and 3 ps) two time-
growth and multi-exponential decay with time constants of τ
00 (±26) fs (24%), τ = 2 (±0.2) ps (43.7%) and τ = 20 (±2) ps
32.3%). Here the faster components can be attributed to
1
=
2
2
3
(
electron transfer from the triplet state of Ru-complex to
anthraquinone however the longer component can be attributed
to back electron transfer time due to ground state recombination.
Probable reaction mechanism was predicted (Scheme 4) and
the effect of water molecule has been demonstrated in both
forward ET and back ET reaction. Upon photo-excitation,
electron will be transferred to anthraquinone moiety. After this
electron transfer reaction, electron will be localized on oxygen
atom of anthraquinone, and there occurs an inter molecular
proton relay with anthraquinone and water molecules.
constants are pretty much the same. Interestingly, in D
the wavelengths the longer components can be fitted with 3
±0.3) ps and 30 (±3) ps respectively. T. J. Meyer and his co-
workers had reported EPT and PCET phenomena in the sub-
2
O in both
(
[54],[55]
picosecond time scale.
In our Transient Absorption study,
we have clearly identified the transient absorption photoproducts
even in presence of water. It has been observed that
To confirm that, if there is any effect of pH in the forward
electron transfer rate we also performed the Transient
absorption experiment in presence of acidic pH solution (pH 4),
.-
intramolecular H-bonding and H-bonding between AQ and
water play a major role in PCET reaction of photo-excited RuQ
complex. The experimental observations suggest the unique
example of proton coupled electron transfer reaction in presence
of water in the ultrafast time scale for the Ru-polypyridyl complex
as sensitizer.

using aqueous HCl. Literature reports suggest that the AQ ¯ is
stable only in the pH range of 10-4. With decrease in the pH

below 2, the transient absorption spectrum of AQ ¯ decreases
[
47]
and finally vanishes. The transient absorption measurements
in acetonitrile-water (2:3, v/v) mixture at pH 4 are shown in
(
Figure S14 in the Supporting Information). It is interesting to
see that at pH 4 there is no change in transient absorption
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
Ruthenium polypyridyl complex covalently linked with imidazole-
anthraquinone (RuQ) has been designed, synthesized and
characterized by FTIR, Mass and NMR spectroscopy. Imidazole
H-atom forms H-bond with carbonyl group which suggests that
RuQ molecule is an ideal system for demonstrating proton
coupled electron transfer reaction in the photo-excited state.
Cyclic voltammetric studies of RuQ complex suggest that photo-
excited electron can be transferred from Ru-polypyridyl moiety to
anthraquinone moiety (AQ). Water plays an important role in
both forward ET and PCET reactions, where both the reaction
rate gets facilitated in presence of water. The significant
lowering of LUMO energy level of anthraquinone in presence of
water facilitates both forward and backward electron transfer
process which has been confirmed by femtosecond transient
absorption studies. We have successfully identified the transient
.
-
3+
absorption photo products AQ and Ru(bpy)
3
in acetonitrile and
even in presence of water. Electron transfer time from photo-
excited Ru(II) to anthraquinone moiety has been monitored after
following growth time of anthraquinone radical (AQ ¯) at 530 nm
resulting time constants of 2 (±0.2) ps and 10 (±0.5) ps. Forward
ET reaction time has been determined to be 500 (±50) fs in
presence of water. Our study reveals that excitation of the RuQ
complex is followed by electron transfer to the anthraquinone.
Our results reveal the effects of H-bond stabilization on
photoinduced electron transfer.To the best of our knowledge,
this type of example is rare in contemporary literature.
Scheme 4. Mechanistic scheme for electron transfer and proton coupled
electron transfer (PCET) pathways and involvement of water molecule through
H-bonding in the transient states.

Acknowledgements
A. D. acknowledges SERB (India) Grant (SB/S1/IC-23/2013)
and CSIR-CSMCRI network project (CSC 0134) for funding.
This work was also supported by DST (Project No. 303726) and
A.D. acknowledges DST and J.D. S.A. acknowledge CSIR and
P.M. acknowledges DAE for research fellowship. We also thank
Organic Chemistry Division and Centralized Instrument Facilities,
NCL, for assistance with different analytical and spectroscopic
measurements.
2
Figure 8. Transient decay kinetics at 600 nm for RuQ in (a) acetonitrile- H O
(2:3, v/v) and (a′) in acetonitrile- D
2
O (2:3, v/v); bleach recovery kinetics at 490
nm for RuQ in (b) acetonitrile- H
v/v).
2 2
O (2:3, v/v) and (b′) in acetonitrile- D O (2:3,
Keywords: Proton Coupled Electron Transfer, Intramolecular
Table 4. Parameters for multi-exponential fit for the transients of RuQ dye at
different wavelengths in acetonitrile-H O (2:3, v/v) and in acetonitrile-D O (2:3,
v/v) solvent mixtures after exciting the sample at 400 nm laser light.
Electron Transfer, H-bond rearrangement.
2
2
[
[
[
1]
2]
3]
C. W. Hoganson, G. T. Babcock, Science 2008, 277, 1953–1956.
G. T. Babcock, Pnas 1999, 96, 12971–12973.
Medium
Acetonitrile- H2O
l(nm)
490
τgrowth
<100 (±15) fs
(100%)
τ1
τ2
τ3
M. W. Kanan, Y. Surendranath, D. G. Nocera, Chem.Soc. Rev. 2009,
38, 109–114.
200 (±26) fs
2 (±0.2) ps
20 (±2) ps
(2:3,v/v)
(30%)
(30%)
(40%)
Acetonitrile- D2O
490
<100 (±15) fs
200 (±26) fs
3(±0.3) ps
30 (±3) ps
[4]
C. Turrd, C. K. Chang, G. E. Leroi, R. I. Cukier, D. G. Nocera, J. Am.
Chem. Soc. 1992, 114, 4013–4015.
(
2:3, v/v)
Acetonitrile- H2O
2:3 v/v)
Acetonitrile- D2O
(100%)
<100 (±15) fs
(100%)
(20%)
200 (±26) fs
(24%)
(33%)
2 (±0.2) ps
(43.7%)
(47%)
20 (±2) ps
(32.3%)
[
[
5]
6]
C. J. Chang, M. C. Y. Chang, N. H. Damrauer, D. G. Nocera, Biochim.
Biophys. Acta. 2004, 1655, 13–28.
600
600
(
M. T. Zhang, T. Irebo, O. Johansson, L. Hammarström, J. Am. Chem.
Soc. 2011, 133, 13224–13227.
<100 (±15) fs
200 (±26) fs
3 (±0.3) ps
30 (±3) ps
(
2:3, v/v)
(100%)
(16.66%)
(42.5%)
(40.8%)
[7]
8]
B. A. Barry, Nat. Chem. 2014, 6, 376–377.
[
J. D. Megiatto, D. D. Mendez-Hernandez, M. E. Tejeda-Ferrari, A.
L.Teillout, M. J. Llansola-Portoles, G. Kodis, G. O. Poluektov, T. Rajh,
V. Mujica, L. T. Groy, D. Gust, T. A. Moore, A. L. Moore, Nat Chem.
Conclusion
2014, 6, 423–428.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
[
[
9]
M. R. Wasielewski, cham. Rev. 1992, 92, 435–461.
[46] S. Y. Reece, M. R. Seyedsayamdost, J. Stubbe, D. G. Nocera, J. Am.
Chem. Soc. 2007, 129, 8500–8509.
10] M. Borgström, O. Johansson, R. Lomoth, H. B. Baudin, S. Wallin, L.
Sun, B. Åkermark, L. Hammarström, Inorg. Chem. 2003, 42, 5173–
[47] B. E. Hume, E. J. Land, G. O. Phillips, Chem. Commun. 1969, 370,
518–519.
5
184.
11] T. Yago, M. Gohdo, M. Wakasa, J. Phys. Chem. B. 2010, 114, 2476–
483.
12] K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, Science 2004, 43,
831–1839.
[
[
[48] R. L. Willson, Chem.Comms. 1971, 1349, 1249–1250.
[49] G. Benkö, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev, V.
Sundström, J. Am. Chem. Soc. 2002, 124, 489–493.
2
1
[50] S. Hung, A. N. Macpherson, S. Lin, P. A. Liddell, G. R. Seely, A. L.
Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 1995, 117, 1657–1658.
[51] S. Sinnecker, E. Reijerse, F. Neese, W. Lubitz, J. Am. Chem. Soc.
2004, 126, 3280–3290.
[
[
[
13] H. I. S. K.P Clark, J. Chem. Soc. Faraday Trans. 1972, 68, 1676–1686.
14] P. R. Study, D. Salts, Faraday Trans. 2002, 756, 295–300.
15] C. Costentin, M. Robert, J. M. Savéant, J. Am. Chem. Soc. 2007, 129,
5
870–5879.
[52] A. Babaei, P. A. Connor, A. J. McQuillan, J. Chem. Educ. 1997, 74,
1200–1204.
[
16] M. Haga, H. Hong, Y. Shiozawa, Y. Kawata, H. Monjushiro, T. Fukuo, R.
Arakawa, Inorg. Chem. 2000, 39, 4566–4573.
[53] G. G. Gurzadyan, S. Steenken, Chem. Eur. J. 2001, 7, 1808–1815.
[54] C. J. Gagliardi, B. C. Westlake, C. A. Kent, J. J. Paul, J. M. Papanikolas,
T. J. Meyer, Coord. Chem. Rev. 2010, 254, 2459–2471.
[
[
17] A. M. Bond, M. Haga, Inorg. Chem. 1986, 25, 4507–4514.
18] M. Sjödin, H. Wolpher, Y. Xu, L. Sun, L. Hammarström, J. Am. Chem.
Soc. 2005, 127, 3855–3863.
[55] B. C. Westlake, M. K. Brennaman, J. J. Concepcion, J. J. Paul, S. E.
Bettis, S. D. Hampton, S. A. Miller, N. V. Lebedeva, M. D. E. Forbes, A.
M. Moran, T. J. Meyer, J. J. Papanikolas, PNAS 2011, 108, 8554–8558.
[
[
[
19] M. T. Zhang, L. Hammarström, J. Am. Chem. Soc. 2011, 133, 8806–
8
809.
20] C. Costentin, C. Louault, M. Robert, J. M. Savéant, J. Am. Chem. Soc.
008, 130, 15817–15819.
21] C. Costentin, M. Robert, J. Savéant, J. Am. Chem. Soc. 2006, 128,
552–4553.
2
4
[
[
22] C. Costentin, Chem. Rev. 2008, 108, 2145–2179.
23] J. Freys, G. C. Bernardinelli, O. S. Wenger, Chem. Commun. 2008,
4
267–4269.
24] A. Pannwitz, O. S. Wenger, Phys. Chem. Chem. Phys. 2016, 18,
1374–11382.
[
1
[
[
[
25] J. Nomrowski, O. S. Wenger, Inorg. Chem. 2015, 54, 3680–3687.
26] J. P. Lay, S. Hammes-schi, Chem. Rev. 2014, 114, 3466–3494.
27] L. Muñoz-Rugeles, J. R. Alvarez-Idaboy, Phys. Chem. Chem. Phys.
2
015, 17, 28525–28528.
28] W. Zhang, I. J. Burgess, Phys. Chem. Chem. Phys. 2011, 13, 2151–
159.
[
[
2
29] C. J. Gagliardi, B. C. Westlake, C. A. Kent, J. J. Paul, J. M. Papanikolas,
T. J. Meyer, Coord. Chem. Rev. 2010, 254, 2459–2471.
[
[
30] V. W. Manner, J. M. Mayer, J. Am. Chem. Soc. 2009, 131, 9874–9875.
31] M. Kuss-Petermann, O. S. Wenger, J. Phys. Chem. Lett. 2013, 4,
2535–2539.
[
32] J. M. Hodgkiss, N. H. Damrauer, S. Presse, J. Rosenthal, D. G. Nocera,
J. Phys. Chem. B. 2006, 110, 18853–18858.
[
[
33] A. A. Pizano, J. L. Yang, D. G. Nocera, Chem. Sci. 2012, 3, 2457–2461.
34] R. Hönes, M. Kuss-Petermann, O. S. Wenger, Photochem. Photobiol.
Sci. 2013, 12, 254-261.
[
[
35] J. Hankache, D. Hanss, O. S. Wenger, J. Phys. Chem. A. 2012, 116,
3347–3358.
36] T. Banerjee, A. K. Biswas, T. S. Sahu, A. Das, B. Ganguly, H. N. Ghosh,
J. Phys. Chem. C. 2014, 118, 3864–3877.
[
[
37] I. J. Rhile, J. M. Mayer, Angew. Chemie - Int. Ed. 2005, 44, 1598–1599.
38] J. Chen, M. Kuss-Petermann, O. S. Wenger, J. Phys. Chem. B. 2015,
119, 2263–2273.
[
39] P. Kar, S. Verma, A. Sen, A. Das, B. Ganguly, H. N. Ghosh, Inorg.
Chem. 2010, 49, 4167–4174.
[
[
40] R. J. Houten, J. V. Watts, J. Am. Chem. Soc. 1976, 98, 4853–4858.
41] S. M. Gasper, G. B. Schuster, J. Am. Chem. Soc. 1997, 119, 12762–
12771.
[
[
[
[
42] S. L. Mecklenburg, D. G. McCafferty, J. R. Schoonover, B. M. Peek, B.
W. Erickson, T. J. Meyer, Inorg. Chem. 1994, 33, 2974–2983.
43] J. Hankache, O. S. Wenger, Phys. Chem. Chem. Phys. 2012, 14,
2
685–2692.
44] T. Banerjee, S. Kaniyankandy, A. Das, H. N. Ghosh, Inorg. Chem. 2013,
2, 5366–5377.
45] S. Verma, P. Kar, A. Das, D. K. Palit, H. N. Ghosh, Chem. Eur. J. 2010,
6, 611–619.
5
1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605594
FULL PAPER
FULL PAPER
Ultrafast electron transfer (ET) and
proton coupled electron transfer
Ananta Dey, Jayanta Dana, Sunil
Aute, Partha Maity, Amitava Das*,
Hirendra N. Ghosh*.
(
PCET)
reaction
has
been
e
-
demonstrated in Ru(II)-polypyridyl
complex, with the help of
Femtosecond transient absorption
spectroscopy. Water found to play
an active role in both ET and PCET
reaction. The electron transfer
process was followed by proton
reequilibration.
Page No. – Page No.
Title: Proton Coupled Electron
Transfer Processes in Ultrafast
Time Domain: Evidence for
Effects of H-bond Stabilization on
Photoinduced Electron Transfer
This article is protected by copyright. All rights reserved.