Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics

Article abstract of DOI:10.1039/c7sc03839a

Two sensitizers, [Ru(bpy)₂(dcb)]²⁺ (RuC) and [Ru(bpy)₂(dpb)]²⁺ (RuP), where bpy is 2,2′-bipyridine, dcb is 4,4′-dicarboxylic acid-2,2′-bipyridine and dpb is 4,4′-diphosphonic acid-2,2′-bipyridine, were anchored to mesoporous TiO₂ thin films and utilized to sensitize the reaction of TiO₂ electrons with oxidized triphenylamines, TiO₂(e^-) + TPA⁺ → TiO₂ + TPA, to visible light in CH₃CN electrolytes. A family of four symmetrically substituted triphenylamines (TPAs) with formal E^o(TPA^+/0) reduction potentials that spanned a 0.5 eV range was investigated. Surprisingly, the reaction followed first-order kinetics for two TPAs that provided the largest thermodynamic driving force. Such first-order reactivity indicates a strong Coulombic interaction between TPA⁺ and TiO₂ that enables the injected electron to tunnel back in one concerted step. The kinetics for the other TPA derivatives were non-exponential and were modelled with the Kohlrausch-William-Watts (KWW) function. A Perrin-like reaction sphere model is proposed to rationalize the kinetic data. The activation energies were the same for all of the TPAs, within experimental error. The average rate constants were found to increase with the thermodynamic driving force, consistent with electron transfer in the Marcus normal region.

Full text of DOI:10.1039/c7sc03839a

View Article Online
View Journal
Chemical
Science
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: B. N. DiMarco, L.
Troian-Gautier, R. Neiva Sampaio and G. J. Meyer, Chem. Sci., 2017, DOI: 10.1039/C7SC03839A.
This is an Accepted Manuscript, which has been through the
Volume
7 Number 1 January 2016 Pages 1–812
Royal Society of Chemistry peer review process and has been
accepted for publication.
Chemical
Science
Accepted Manuscripts are published online shortly after
www.rsc.org/chemicalscience
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2041-6539
EDGE ARTICLE
Francesco Ricci et al.
Electronic control of DNA-based nanoswitches and nanodevices
rsc.li/chemical-science

Page 1 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
Introduction
Motivation for the study of electron transfer at semiconductor interfaces originates from
both applications as well as to enhance fundamental knowledge.1–3 In dyeꢀsensitized solar
cells (DSSCs), mesoporous thin films of anatase TiO2 nanocrystallites are functionalized with
molecular chromophores, or “sensitizers”, that extend the spectral response of the materials
into the visible region. In the accepted mechanism for power generation with DSSCs, light
absorption induces electron “injection” from the sensitizer excited state to the metal oxide
acceptor states.4 A donor present in the electrolyte solution then reduces the oxidized
sensitizer, a process often termed ‘dye regeneration’.5 Competition between the collection of
the injected electron and back electron transfer, or “charge recombination”, to the oxidized
sensitizer or the oxidized donor often lowers the efficiency of DSSCs. This paper seeks to
better understand the unwanted charge recombination reaction of the injected electrons with
the oxidized donor through use of four symmetrically substituted triphenylamines whose
formal reduction potentials span a range of 0.5 eV.
Scheme 1. Mechanism for photo-initiating the desired reaction. Visible light absorption by the ruthenium
sensitizer induced rapid excitedꢀstate electron injection to the acceptor state of TiO2, kinj >108 sꢀ1. The oxidized
sensitizer is then regenerated by triphenylamine (TPA) with a rate constant kreg. This sequence provides the
reactants for the desired charge recombination reaction of the injected electron with the oxidized triphenylamine
redox mediator (krec) that was quantified over a 0.5 eV change in driving force.
(-)
kinj
e-
RuIII/II
*
krec
MeO-TPA
Me-TPA
Cl-TPA
{
TPA+/0
kreg
Br-TPA
RuIII/II
TiO2
(+)
When compared to the wealth of literature reports focused on electron transfer from TiO2
to the oxidized sensitizer, there are a remarkably small number of systematic studies that seek
2

Chemical Science
Page 2 of 23
View Article Online
DOI: 10.1039/C7SC03839A
to understand the factors that influence charge recombination to oxidized donors present in
ꢀ
the electrolyte.6–13 This is likely due to the dominance of Iꢀ/I3 as the prototypical redox
mediator employed in DSSCs that is not easily amenable to systematic studies.14,15 However,
alternative redox mediators, such as Co(III/II) polypyridyl complexes,16–23 triphenylamine or
phenothiazine24,25, have provided new opportunities to gain fundamental information on how
sensitive charge recombination is to the electrolyte composition,6,26 molecular structure,12,27
or the thermodynamic driving force.7–9
The influence of reaction driving force on recombination has been previously studied by
several groups7,8,10,28 and data consistent with the Marcus normal kinetic region has been
reported.29,30 Recall that in the Marcus normal region the total reorganization energy λ for
electron transfer is greater than the absolute value of the Gibbs free energy change, λ > |ꢀGo|.
Under normal conditions, an increase in the driving force results in an increase in the rate of
electron transfer. In a study of recombination to a series of substituted ferrociniums, Hupp et
al. noted that normal region behavior was peculiar given the small reorganization energies and
the large reaction driving forces8 that should have placed the reaction in the Marcus inverted
region, λ > |ꢀGo|.
Hamann et al. have suggested that electron transfer from different
electronic states in the TiO2 may mask inverted behavior similar to the case for metallic
donors.31 Further complicating the analysis is a considerable literature that indicates that the
observed rate constants abstracted from transient spectroscopic and/or electrochemical data
reports mainly on the diffusion of the injected TiO2 electron and/or acceptor.32–38 It is possible
that diffusion does rate limit interfacial electron transfer in some cases, however this does not
explain the reported driving force dependencies or the recent observation of specific
interfacial electron transfer pathways at sensitized TiO2 interfaces.39,40
Nevertheless, an experimental challenge is to abstract the actual interfacial electron
transfer rate constant from the transient data that are usually highly nonꢀexponential.32,33,41
A
3

Page 3 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
fitting function that has been phenomenally successful is the soꢀcalled KohlrauschꢀWilliamsꢀ
Watts (KWW) function that was proposed empirically by Kohlrausch, and later popularized
by Williams and Watts.42 This function was first derived by Scher and Montroll based on a
random walk model and has since become a paradigm for charge transport in disordered
media.43 In particular the work of Nelson and coworkers have extended the KWW model to
the trapping/detrapping electron transport in the dye sensitized mesoporous TiO2 thin films
that are commonly used in DSSCs.32,37,44–46 It should be emphasized however, a quality fit to
the KWW function does not necessarily indicate that an underlying transport mechanism is
operative. For example, Anderson has derived the KWW function based on serially linked
rate constants that are not necessarily associated with charge transport in disordered solids.47
In this study, a series of four symmetrically substituted triphenylamines (TPAs) were
utilized to establish a correlation between the reaction kinetics and driving force, Figure 1.
The TPA molecules were substituted at the paraꢀposition of each phenyl ring with electron
donating/withdrawing functional groups that allowed the TPA+/0 reduction potential to be
varied by ~ 0.5 V. A key finding was that the interfacial electron transfer became firstꢀorder
when the reaction driving force was large. Such firstꢀorder behavior has not been previously
reported and was maintained when the two different ruthenium sensitizers were used to
initiate the reaction as well as for the two different acetonitrile electrolytes. A Perrinꢀlike
model is proposed to rationalize this kinetic behavior. Additionally, an increase in driving
force resulted in an increase in the rate of interfacial electron transfer, consistent with the
reaction occurring in the Marcus normal region.
4

Chemical Science
Page 4 of 23
View Article Online
DOI: 10.1039/C7SC03839A
R
R
N
X
X
N
N
N
R
N
N
RuII
N
RuP: X = PO3H2
RuC: X = COOH
R = MeO, Me, Cl, Br
Figure 1. Ruthenium sensitizers (RuP and RuC) and triphenylamines (TPAs) used in this study.
Results
Mesoporous nanocrystalline TiO2 thin films were sensitized to visible light with
[Ru(bpy)2(dcb)]2+ (RuC) or ([Ru(bpy)2(dpb)]2+ (RuP) (bpy = 2,2’ꢀbipyridine; dcb = 4,4’ꢀ
dicarboxylicꢀ2,2’ꢀbipyridine and dpb = 4,4’ꢀdiphosphonicꢀ2,2’ꢀbipyridine) by immersion into
concentrated sensitizer CH3CN solutions. These films were soaked for a minimum of 24
hours to help ensure that saturation surface coverages (~10ꢀ8 mol cmꢀ2) were obtained, as
determined by a previously reported spectral method.48 The sensitized films, abbreviated
TiO2|RuC and TiO2|RuP, immersed in neat acetonitrile displayed a broad absorption band in
the visible region characteristic of a metalꢀtoꢀligand charge transfer (MLCT) feature. A
bathochromic shift of the absorption spectrum was observed upon the addition of 0.1 M
NaClO4 or LiClO4 to the acetonitrile that surrounded the thin film. Such spectral shifts have
previously been assigned to a change in the local electric field upon cation adsorption to the
TiO2 surface.6,49,50 The TPAs did not appreciably absorb visible light in their neutral forms,
however the one electron oxidized forms exhibited strong absorption between 600ꢀ800 nm,
Figure 2b. The peak position and the molar absorption coefficient for these absorbances are
reported in Table 1.
The TPA+/0 reduction potentials were determined by cyclic voltammetry in 0.1 M NaClO4
acetonitrile, Figure 2a. The halfꢀwave potentials were taken as an estimate for the formal
reduction potentials that varied between 0.72 V (MeOꢀTPA) and 1.25 V vs NHE (BrꢀTPA),
Table 1.
5

Page 5 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
Table 1: Electrochemical and spectroscopic properties of the Ru sensitizers and the
triphenylamines.
Oxidation Potential
(V vs NHE)a
TPA+ Peak
Absorbance (nm)
TPA+ Extinction Coefficient
(Mꢀ1 cmꢀ1)
MeOꢀTPA
MeꢀTPA
ClꢀTPA
BrꢀTPA
RuP
0.72 ± 0.01
0.93 ± 0.01
1.24 ± 0.01
1.25 ± 0.01
1.54b
717 ± 1
668 ± 1
685 ± 1
702 ± 1
2.8 × 104 ± 300
2.4 × 104 ± 300
2.7 × 104 ± 300
2.6 × 104 ± 300
1.48c
RuC
a Measured in 0.1 M NaClO4 CH3CN electrolyte unless otherwise noted. b Ref 51, measured
in 0.1 M LiClO4 CH3CN electrolyte on nanoITO. c Ref 48, measured in 0.1 M LiClO4 CH3CN
electrolyte on nanocrystaline TiO2.
Figure 2: Cyclic voltammograms (a) and TPA+ absorption spectra (b) of the indicated TPA measured in 0.1 M
NaClO4 acetonitrile electrolyte.
Nanosecond transient absorption spectroscopy was used to quantify the recombination
reaction. Experiments were performed in 0.1 M NaClO4 acetonitrile electrolyte for TiO2|RuC
and in 0.1 M LiClO4 acetonitrile electrolyte for TiO2|RuP. The Lewis acidic Na+ or Li+
cations were present in the external electrolyte to improve the excited state injection yield.
49,52,53
It is worth noting that no specific interaction between the cations and the TPA is
expected based on previous literature6 but these cations have been shown to influence the rate
for recombination from TiO2(eꢀ) to MeꢀTPA+, with rate constants following the trend
ꢀ 26
Na+≥Li+>Mg2+> Ca2+.6 Such a trend was also observed for the recombination to I3 . In a
typical experiment, pulsed 532 nm light excitation of a sensitized TiO2 thin films submerged
in argon purged 0.1 M NaClO4 or 0.1 M LiClO4 acetonitrile electrolytes with 8 mM of a
6

Chemical Science
Page 6 of 23
View Article Online
DOI: 10.1039/C7SC03839A
selected TPA derivative formed the initial charge separated state, TiO2(eꢀ)|RuC+ or TiO2(eꢀ
)|RuP+. The subsequent absorption changes were monitored on 10 ns and longer time scale.
Absorption changes associated with the oxidized sensitizer were observed within the
instrument response time consistent with rapid excited state injection (kinj > 108 sꢀ1).2,4 Under
all conditions studied, the transient spectra were simulated with a linear combination of
standard spectra comprised of: (1) TiO2|RuC and the oxidized form of the sensitizer,
abbreviated TiO2|RuC+; (2) TPA+; and/or (3) the Stark effect.49,54,55 The Stark effect results
from local electric fields generated by cation adsorption and/or the injected electron and are
themselves well simulated by a first derivative of the ground state sensitizer absorption. In
principle, the injected electron also contributes to the transient spectra, however its weak
absorption was obscured by the more intense absorption associated with TPA+.
Figure 3 shows representative absorption difference spectra measured after pulsed light
excitation of TiO2|RuC immersed in 0.1 M NaClO4 acetonitrile solutions with 8 mM MeOꢀ
TPA (Figure 3a) or 8 mM ClꢀTPA (Figure 3b). Contributions to the spectra from the
oxidized sensitizer were absent after 1
ꢁs with MeOꢀTPA (or MeꢀTPA) and the relative
amplitudes were consistent with quantitative regeneration of the oxidized sensitizer. This was
true for either sensitizer in both electrolytes and enabled the desired reaction, TiO2(eꢀ) + TPA+
→
TiO2 + TPA, to be quantified on microsecond and longer time scales without contributions
from slow regeneration and the oxidized sensitizer. In contrast, regeneration by ClꢀTPA (or
BrꢀTPA) were not quantitative on any time scale and contributions from the oxidized
sensitizer were evident on all delay times.
7

Page 7 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
Figure 3. Transient absorption spectra measured at indicated delay times after pulsed 532 nm excitation of
TiO2|RuC submerged in 0.1M NaClO4 acetonitrile electrolytes with 8 mM MeOꢀTPA (a) or 8 mM ClꢀTPA (b).
Overlaid as solid lines are simulated spectra.
The time dependent concentration of TPA+ was biphasic, consisting of an initial rise
followed by a decay, Figure 4. For transient absorption data acquired with MeOꢀ and Meꢀ
TPA, a sum of two KohlrauschꢀWilliamꢀWatts (KWW) functions adequately modelled the
kinetic data, n = 2 Equation 1. The regeneration of the oxidized sensitizer was quantified
independently from single wavelength absorption data monitored at 402 nm, which
represented an isosbestic point for the Stark effect, that was fit to a single KWW function; the
obtained value for β and k were used to constrain fits to the biꢀKWW function used to model
the TPA+ kinetics as β1 and k1. An “average” rate constant, kkww, was then calculated as the
first moment of this distribution using Equation 2, where Γ is the Gamma function.
ꢋ
ꢈ
ꢃꢌꢍ ꢎꢏ ꢐ
ꢑꢌꢍ
ꢀꢁꢃꢄꢅꢆꢇ ꢉꢊ
(Eq 1)
ꢈ
∑
ꢀꢁ ꢂ
8

Chemical Science
Page 8 of 23
View Article Online
DOI: 10.1039/C7SC03839A
ꢇ ꢕ
ꢔ
ꢒꢇꢓꢓ ꢂ ꢖꢗ
(Eq 2)
ꢍ
ꢙ
ꢘ
ꢕ
Figure 4. Single wavelength absorption changes measured after pulsed 532 nm laser excitation of TiO2|RuP in
0.1M LiClO4 acetonitrile electrolyte (a) and TiO2|RuC in 0.1M NaClO4 acetonitrile electrolyte (b) with 8 mM of
the indicated TPA mediator. Kinetics were monitored at the TPA+ absorption peak. Fits to the KWW model are
overlaid on the data as solid yellow lines.
Fits of the recombination data for electron transfer from TiO2(eꢀ) to Clꢀ or BrꢀTPA+ using
the KWW function revealed a ꢚ value of 1, while the corresponding data obtained with MeOꢀ
TPA and MeꢀTPA required ꢚ values ranging between 0.62 and 0.85. A ꢚ value of unity
corresponds to the single exponential behavior expected for a firstꢀorder kinetic reaction. The
kinetics were independent of the excitation irradiances and hence the initial concentrations
resulting in normalizable kinetics that allowed a single rate constant to model the desired
reaction. A summary of the fitting parameters is given in Table 2. Note that the rate constants
for recombination from TiO2(eꢀ) to all TPA+ derivatives were larger in Na+ than in Li+ as
previously reported.6
Table 2: Charge recombination rate constants abstracted from the KWW model.a
kkww (105sꢀ1)
kkww (105sꢀ1)
k (105 sꢀ1), β
k (105 sꢀ1), β
TiO2|RuCb
TiO2|RuPc
TiO2|RuCb
TiO2|RuPc
MeOꢀTPA 0.42 ± 0.05, 0.67
0.47 ± 0.04
0.27± 0.03, 0.62
0.19 ± 0.02
9

Page 9 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
MeꢀTPA
ClꢀTPA
BrꢀTPA
1.5 ± 0.1, 0.74
12.0 ± 0.3, 1
9.7 ± 0.1, 1
1.7 ± 0.1
12 ± 0.1
9.7 ± 0.1
0.74± 0.07, 0.85
3.1± 0.1, 1
0.68 ± 0.02
3.1 ± 0.1
3.3 ± 0.1
3.3± 0.1, 1
a Rate constants (krec) for electron transfer from TiO2(eꢀ) to TPA+, where k represents the rate constant
extracted from fits to equation 1 and kkww represents the average rate constant calculated with equation 2. Note
that for ClꢀTPA and BrꢀTPA, the recombination was firstꢀorder such that β =1 such that kcr and kkwww are
equivalent. b Measured in 0.1M NaClO4 CH3CN electrolyte. c Measured in 0.1M LiClO4 CH3CN electrolyte
Kinetic data acquired as a function of temperature were modeled using the procedure
described for the room temperature data, Figure 5a. Arrhenius plot of the firstꢀorder rate
constant (ClꢀTPA or Br-TPA) or the average rate constants (kkww) are shown in Figure 5b,
and revealed a common activation energy Ea = 0.13 ± 0.01 eV for all TPA+ acceptors. The Ea
values obtained for the recombination to Br-TPA+ in 0.1M MgClO4 and 0.1M TBAClO4
CH3CN electrolytes were within experimental error the same (Figure 5c). The recombination
to Br-TPA+ in 0.1M MgClO4 was slower that in LiClO4 or NaClO4. Interestingly, even
though the identity of the cation changed the rate of charge recombination, the activation
energy remained the same.
Figure 5. Single wavelength absorption changes measured after pulsed 532 nm laser excitation of TiO2|RuP
0.1M LiClO4 with 8 mM of BrꢀTPA at indicated temperatures (a). Kinetics were monitored at the TPA+
absorption peak. Arrhenius plots for the recombination from TiO2|RuP to indicated TPAs in 0.1 M LiClO4 (b)
and from TiO2|RuP to BrꢀTPA in indicated 0.1 M CH3CN electrolytes (c).
Discussion
The intent of this study was to quantify the driving force dependence of the interfacial
charge recombination reaction of dyeꢀsensitized TiO2 electrons, TiO2(eꢀ)s, with oxidized
triphenylamines to yield ground state products, Equation 3.
10

Chemical Science
Page 10 of 23
View Article Online
DOI: 10.1039/C7SC03839A
TiO2(eꢀ) + TPA+ → TiO2 + TPA
(Eq 3)
If one assumes that the reducing power of the TiO2(eꢀ) is insensitive to the TPA derivative
used, the Gibbs free energy change was varied by over 500 meV. The reactants were
produced by pulsed laser excitation of the sensitizers (RuC or RuP) that initiated rapid
excited state injection, kinj > 108 sꢀ1, into TiO2 followed by diffusional ‘regeneration’ through
TPA oxidation in an acetonitrile electrolyte. The time required for regeneration of the
oxidized sensitizer by TPA was about 1 microsecond and occurred quantitatively for the more
easily oxidized TPA derivatives.
The data provide new insights into the origin(s) of the nonꢀexponential kinetics that
are often reported for interfacial charge recombination reactions in dyeꢀsensitized TiO2
materials. Specifically, the observation of a firstꢀorder reaction indicates a strong Coulombic
attraction between the TiO2(eꢀ) and the TPA+ that provides a pathway for charge
recombination. Consistent with previous reports, the kinetic data indicates that Equation 3
occurs in the Marcus normal kinetic region, despite the large expected driving forces. Below
we discuss in more detail a kinetic reaction sphere model for charge recombination followed
by a description of the driving force dependence.
Kinetic Model. The spectroscopic approach described provided equal numbers of injected
electrons and oxidized TPA compounds. A rate law that is firstꢀorder in TPA+ and firstꢀorder
in TiO2(eꢀ), and hence secondꢀorder overall might be expected, Equation 4.
Rate = k [TPA+][TiO2(eꢀ)] ~ k [TPA+]nꢕ
(Eq 4)
The order in TiO2(eꢀ)s has been quantified by many groups, through transient photovoltage
spectroscopy, and is mysteriously often reported to be nonꢀintegral56,57 although under some
condition it is indeed unity.58 Since the molar concentrations asserted by the brackets in
11

Page 11 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
Equation 4 are illꢀdefined in the mesoporous TiO2 thin films, the total number of injected
as an approximation to the [TiO2(eꢀ)].2
electrons, n, is often raised to the exponent
ꢚ
A striking result from this study was that when charge recombination was highly
exothermic, the kinetic data were most accurately modelled as a firstꢀorder kinetic model and
displayed single exponential kinetics even when the initial concentrations was varied by over
a factor of five. Such data is inconsistent with the rate law implied by Equation 4. Instead,
firstꢀorder recombination is expected for a unimolecular electron transfer like that observed in
covalently linked DonorꢀAcceptor (DꢀA) compounds. Therefore, electron transfer is to a
surface adsorbed TPA+ proximate to the injected electron, Equation 5.
TiO2(eꢀ)|TPA+ ꢀ TiO2|TPA
(Eq 5)
Hence, diffusional regeneration of the oxidized dye yields TPA+ that adsorbs to the TiO2
surface prior to charge recombination. It is likely that the Coulombic attraction between the
injected electron and TPA+ stabilizes the proposed adduct. Indeed, the electric field produced
by excited state injection has been estimated to be ~ 2 MV/cm under oneꢀsun illumination.50,51
The field is sufficiently large to electrostatically bind the cationic TPA+ and the injected
electron is suitably close to afford the electronic coupling necessary for electron transfer.
In order to rationalize why the reaction becomes nonꢀexponential at smaller driving
forces, a “reaction sphere” model is proposed that shares some similarities with those of
Perrin and Onsager (Scheme 2).59,60 In this model, it is assumed that the injected electrons
reside in localized trap states as Ti(III) species that can either hop to a Ti(IV) site or reduce a
surface adsorbed TPA+. Such Ti(IV/III) hopping is conceptually equivalent to previously
reported trapping/detrapping mechanisms46,61 and is expected to be independent of the TPA+
derivative. A key aspect of this model is that the TiO2(eꢀ) reacts with any TPA+ within a
sphere whose radius r increases with driving force. Recombination within the reaction sphere
gives rise to firstꢀorder kinetics; recombination to TPA+ outside the sphere requires Ti(IV/III)
12

Chemical Science
Page 12 of 23
View Article Online
DOI: 10.1039/C7SC03839A
hopping, manifest as dispersive kinetics and β < 1 in the KWW function, until the rate
constant for electron transfer to TPA+ is sufficiently large to compete with transfer to another
Ti(IV) site.
Scheme 2. A Reaction Sphere Model for interfacial charge recombination. Schematic representation of a
proposed Perrinꢀlike model. When the driving force for recombination is large, as seen for Brꢀ and ClꢀTPA,
electron transfer occurs over relatively large distances (Blue sphere). With smaller driving force, the electron
must hop closer to the TPA+ acceptor before electron transfer can occur (Red sphere), leading to a decreased β
value in fits to the KWW function.
rrecBr, Cl
rrecMe
X
ß = 1
rrecMeO
N
# of hops
X
X
ß < 1
TiO2
X = MeO, Me,
As first derived by Scher and Montroll,40 electron transport in disordered media
naturally gives rise to dispersive kinetics that follow the KohlrauschꢀWilliamsꢀWatts model,
Equation 6.
ꢆ ꢊ
ꢊꢕ
ꢁ ꢛ ꢂ ꢁꢜ exp −ꢒt
ꢆ
(Eq 6)
where β is inversely related to the width of the underlying Lévy distribution of rate constants,
0 < β < 1, A0 is the initial absorbance, and k is the characteristic rate constant. When β =1, a
firstꢀorder reaction is recovered. The inverse Laplace transform of Equation 6 is known
analytically for certain β values and has been determined by saddleꢀpoint approximation for
others. Figure 6 shows the distributions for charge recombination to the TPA+ derivatives
13

Page 13 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
under study as well as their average rate constants (Equation 2) and the firstꢀorder rate
constants for Clꢀ and BrꢀTPA+. While inverse Laplace transforms are illꢀconditioned and
should be viewed with caution, the highly dispersive kinetics that span at least three orders of
magnitude in time likely reflect the heterogeneous Ti(IV/III) hopping transport in the
mesoporous thin films. When the injected electron is within the reaction sphere, electron
transfer to TPA+ kinetically outcompetes hopping to another Ti(IV) site. When the injected
electron is outside the reaction sphere, a wide range of rate constants is possible due to the
weak coupling and hence small electron transfer rate constants to TPA+ We emphasize that
.
this model, like those of Perrin’s and Onsager’s, are approximations. Electron transfer to
TPA+ likely occurs at a distribution of distances that are not marked by a sharp turn on/off at
radius r.
Figure 6: Calculated Lévy distribution of the charge recombination rate constants abstracted from transient data
for electron transfer from TiO2(eꢀ)|RuC (top) and TiO2(eꢀ)|RuP (bottom) to MeOꢀTPA+ (red) or to MeꢀTPA+
(black). An average rate constant based on Equation 2 is shown as a vertical line. Also shown as vertical lines
are the firstꢀorder rate constants for recombination to BrꢀTPA+ (blue) and ClꢀTPA+ (green).
It is interesting to note that the Ea value of 0.13 ± 0.01 eV were independent of the
identity of the TPA+ acceptor. A similar activation energy of 0.13 eV was reported for the
45
electron transport within mesoporous TiO2 as well as for Li+ hops in Li10SnP2S12ꢀbased
14

Chemical Science
Page 14 of 23
View Article Online
DOI: 10.1039/C7SC03839A
composited62. Cation hopping was excluded since the same activation energy was measured
in 0.1M MgClO4 and 0.1M TBAClO4 electrolytes for recombination to Br-TPA+ (Figure 5c).
Since the observed rate constant is the true rate constant for recombination to Clꢀ and Brꢀ
TPA+, the activation energy must reflect the barrier for interfacial electron transfer reaction.
For the other TPA+ derivatives, the same Ea value should be taken as the average of a
distribution of activation energies as it was extracted from rate constants based on Lévy
distributions. The fact that these average activation energies are within experimental error the
same indicates that the barrier for Ti(IV/III) hopping must be very similar to that for electron
transfer to the oxidized TPA+, a conclusion that is in agreement with a recent publication.45
Marcus Normal Electron Transfer. Regardless of how the time resolved data is analyzed,
they clearly show that the recombination rate becomes larger as the Eo(TPA+/0) potential
increases, consistent with Marcus normal kinetic behavior. There was no evidence for
activationless or Marcus inverted kinetic behaviors even when the mediator potentials were
tuned to the most positive values. Indeed, the electron transfer was clearly activated, Ea =
0.13 ± 0.01 eV. Normal kinetic behavior has also been reported for Co(III) complexes and
for ferrocinium acceptors.7,8,28 While a precise value of the formal EoTi(IV/III) reduction
potential is unknown, the onset for spectroscopic changes attributed to TiO2(eꢀ) is around 0V
vs NHE in 0.1M LiClO4.6,49 Hence, the spectroelectrochemical studies suggest that the
driving force for recombination to XꢀTPA+ acceptors, where X = Cl or Br, is greater than 1.2
eV. We emphasize however, that the true free energy change for charge recombination is
unknown and may be complicated by the time dependent electric field26,63 or the exponential
density of donor states that are often invoked to model such data.49,64 Nevertheless, the data is
fully consistent with Marcus normal electron transfer and suggests that either the
reorganization energy must exceed this value or, as has been previously described, that
15

Page 15 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
recombination occur through lower energy “trap” states with a significantly smaller driving
force.10,31
The total reorganization energy for this interfacial recombination reaction includes
contributions from both the TiO2, the triphenylamine acceptors, and the electrolyte solution.
Theoretical calculations indicate that λtot = 0.15ꢀ0.25 eV for TPA+/0 which is consistent with
the rapid selfꢀexchange in this class of compounds.65 The reorganization energy change
associated with Ti(IV/III) hopping is unknown. In molecular compounds, the d0/d1 redox
chemistry is subject to a large JahnꢀTeller distortion and reported reorganization energies are
quite large.66–68 While molecular Ti compounds and TiO2 are quite different, the
spectroscopic properties of Ti(III) in the solid state often show clear evidence of JahnꢀTeller
distortions.69,70 Hence, a significant inner sphere contribution to the reorganization energy is
expected.66–68 Dielectric continuum theory provides outerꢀsphere reorganization of ~ 1 eV in
acetonitrile.71,72 Recently, reorganization energies near 1.2 eV for electron hopping have been
calculated using DFT+U method.73 Hence it is not unreasonable to suggest that the total
reorganization energy is greater than 1.2 eV. Alternatively, and as has previously been
discussed28,31, a lower driving force for the reaction would be expected if the energetic
position of the TiO2 donor states that participate in charge recombination are at much more
positive potentials than the spectroelectrochemical data indicates. This would allow for the
observation of normal behavior when the total reorganization energy is less than 1.2 eV.
In the present study, the firstꢀorder recombination observed for the halogenated TPAs
rules out diffusion or transport of the injected electrons as being a rate limiting process.
Furthermore, recent studies have shown that charge recombination to oxidized dyes are
sensitive to the bridge that separates them from the surface or the orientation of the surface
linker.39,40 Indeed, specific bridge mediated pathways to oxidized dyes have been identified.37
Taken together, these findings indicate that the observed rate constants do, at least partially,
16

Chemical Science
Page 16 of 23
View Article Online
DOI: 10.1039/C7SC03839A
report upon the interfacial electron transfer rate constant(s). Even when |∆Go| was as large as
that reported here there was no evidence for firstꢀorder kinetics or rate constants that were
highly sensitive to the sensitizer Eo(S+/0) reduction potentials. This very different behavior for
electron transfer to oxidized sensitizers versus redox mediators is not fully understood. A
working hypothesis is that the anionic phosphonate (or carboxylate) binding groups preclude
closeꢀencounters of the injected electrons with the oxidized sensitizers resulting in more
dispersive kinetics with longer distance interfacial electron transfer.
Conclusion
Electron transfer from TiO2 to a series of symmetrically substituted oxidized
triphenylamines were quantified over a driving force range of 0.5 eV. The rates increased
with the thermodynamic driving force, consistent with the reaction occurring in the Marcus
normal. This conclusion was robust and occurred for two different CH3CN electrolytes
(LiClO4 and NaClO4) and when sensitized to visible light by two different sensitizers (RuC
and RuP). An unprecedented firstꢀorder electron transfer from TiO2 to Clꢀ or BrꢀTPA+
indicated a Coulombic interaction that provided sufficient coupling for unimolecularꢀlike
recombination. A reaction sphere model was proposed to account for this behavior and the
dispersive kinetics observed for the other TPA+ acceptors wherein the recombination radius is
related to the Gibbs free energy change.
Experimental
Materials: The following reagents and solvents were purchased from the listed commercial
sources and used without further purification: acetonitrile (Burdick and Jackson,
spectrophotometric grade), methanol (Fischer, ACS Reagent grade), sodium perchlorate
(NaClO4, SigmaꢀAldrich, 99%), argon gas (Airgas, >99.998%), [Ru(bpy)2(dcb)]2+.2PF6
17

Page 17 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
(Solaronix), triꢀpꢀtolylꢀ amine (MeꢀTPA, SigmaꢀAldrich, 97%), tris(4ꢀbromophenyl)amine
(BrꢀTPA, SigmaꢀAldrich, 98%). [Ru(bpy)2(dpb)]2+.2PF6, Triꢀpꢀanisylamine (MeOꢀTPA)74
and tris (4ꢀchlorophenyl)amine (ClꢀTPA)75 were prepared according to previously reported
methods.
Materials Preparation: Anatase TiO2 nanocrystallites were prepared through a previously
described solꢀgel method.76 The sols were casts as thin films by doctor blading onto methanol
cleaned glass substrates. Scotch tape (~ 10 ꢁm) was used as a spacer and aided in achieving a
uniform film thickness. The films were allowed to stand covered for 30 min before being
transferred to a tube furnace. The furnace was first purged with pure O2, then heated at 450°C
for 30 min. After annealing, the films were either kept in a 70°C oven for later use, or
immediately submerged in concentrated acetonitrile solution of RuC or RuP.
Electrochemistry: All electrochemical experiments were performed in acetonitrile solutions
containing 0.1 M NaClO4 as the supporting electrolyte. Potentials were applied against a selfꢀ
contained Ag wire pseudo reference electrode containing the same electrolyte solution. The
reference electrode was calibrated externally against the Fc+/0 reduction potential (0.31 V vs
SCE in 0.2 M LiClO4). Cyclic voltammetry employed Pt disk electrodes (BASi, 1.6 mm
diameter) as both working and auxiliary electrodes. Spectroelectrochemical studies were
performed using a gold Honeycomb Spectroelectrochemical Cell (Pine Research
Instrumentation), and allowed absorption changes to be quantified after application of an
electrochemical bias.
Absorption Spectra: Ground state absorption spectra were obtained from a Varian Cary 50
spectrophotometer. Serial dilution of stock solution was used to determine the extinction
coefficient of each compound.
Transient Absorption: Nanosecond transient absorption measurements were performed on a
previously described apparatus. A pulsed (1 Hz) 150 W xenon arc lamp (Applied
18

Chemical Science
Page 18 of 23
View Article Online
DOI: 10.1039/C7SC03839A
Photophysics) was used as the probe beam. Appropriate filters were placed before the sample
to minimize sample excitation. After passing through the sample, the probe beam was focused
into a Spex monochromator coupled to a R928 Hamamatsu photomultiplier tube. A pulsed (1
Hz) Nd:YAG laser (Quantel U.S.A. Brilliant B; 5ꢀ6 ns full width at halfꢀmaximum, spot size
~0.8 cm2) provided sample excitation. Excitation power was measured at the sample using a
thermopile power meter (Molectron), with typical excitation powers varying between 500 ꢁJ
and 5 mJ/pulse. Typically, 30 pumpꢀprobe measurements were averaged over the range of
400 – 800 nm. Data generated for kinetic modeling were typically averaged between 150 –
210 measurements in order to improve signal to noise. Full spectra were generated at a set
time after laser excitation by averaging 3 – 21 data points around the timepoint of interest.
Data Modeling: Kinetic modeling was performed with OriginPro 9, which utilizes a
LevenbergꢀMarquardt iteration method. A custom Mathematic 9 script was used to modeling
the spectral signatures observed in transient absorption. Fitting was achieved through standard
addition of steady state absorption spectra.
Acknowledgments: The authors acknowledge support by a grant from the Division of
Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S.
Department of Energy (Grant DEꢀSC0013461). L.T.ꢀG. would like to acknowledge the
Belgian American Educational Foundation (BAEF) as well as the Bourse d’Excellence
WallonieꢀBruxelles (WBI.World) for generous support.
References
1
2
3
4
N. S. Lewis, J. Phys. Chem. B, 1998, 102, 4843–4855.
S. Ardo and G. J. Meyer, Chem. Soc. Rev., 2009, 38, 115–164.
T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 43, 7520–7535.
D. F. Zigler, Z. A. Morseth, L. Wang, D. L. Ashford, M. K. Brennaman, E. M. Grumstrup,
E. C. Brigham, M. K. Gish, R. J. Dillon, L. Alibabaei, G. J. Meyer, T. J. Meyer and J. M.
Papanikolas, J. Am. Chem. Soc., 2016, 138, 4426–4438.
5
A. Listorti, B. O’Regan and J. R. Durrant, Chem. Mater., 2011, 23, 3381–3399.
19

Page 19 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
6
B. N. DiMarco, R. M. O’Donnell and G. J. Meyer, J. Phys. Chem. C, 2015, 119, 21599–
21604.
7
8
9
T. W. Hamann, O. K. Farha and J. T. Hupp, J. Phys. Chem. C, 2008, 112, 19756–19764.
M. J. DeVries, M. J. Pellin and J. T. Hupp, Langmuir, 2010, 26, 9082–9087.
S. M. Feldt, G. Wang, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2011, 115, 21500–
21507.
10
S. M. Feldt, P. W. Lohse, F. Kessler, M. K. Nazeeruddin, M. Grätzel, G. Boschloo and A.
Hagfeldt, Phys. Chem. Chem. Phys., 2013, 15, 7087.
11
12
13
14
B. M. Klahr and T. W. Hamann, J. Phys. Chem. C, 2009, 113, 14040–14045.
Y. Xie, J. Baillargeon and T. W. Hamann, J. Phys. Chem. C, 2015, 119, 28155–28166.
Y. Xie and T. W. Hamann, J. Phys. Chem. Lett., 2013, 4, 328–332.
J. G. Rowley, S. Ardo, Y. Sun, F. N. Castellano and G. J. Meyer, J. Phys. Chem. C, 2011,
115, 20316–20325.
15
16
J. G. Rowley, B. H. Farnum, S. Ardo and G. J. Meyer, J. Phys. Chem. Lett., 2010, 1,
3132–3140.
J.-H. Yum, E. Baranoff, F. Kessler, T. Moehl, S. Ahmad, T. Bessho, A. Marchioro, E.
Ghadiri, J.-E. Moser, C. Yi, M. K. Nazeeruddin and M. Grätzel, Nat. Commun., 2012,
631.
3
,
17
18
T. W. Hamann, Dalt. Trans., 2012, 41, 3111.
E. Mosconi, J.-H. Yum, F. Kessler, C. J. Gómez García, C. Zuccaccia, A. Cinti, M. K.
Nazeeruddin, M. Grätzel and F. De Angelis, J. Am. Chem. Soc., 2012, 134, 19438–53.
S. M. Feldt, E. a. Gibson, E. Gabrielsson, L. Sun, G. Boschloo and A. Hagfeldt, J. Am.
Chem. Soc., 2010, 132, 16714–16724.
19
20
21
22
T. Daeneke, A. J. Mozer, T.-H. Kwon, N. W. Duffy, A. B. Holmes, U. Bach and L. Spiccia,
Energy Environ. Sci., 2012, , 7090.
T. Daeneke, T.-H. Kwon, A. B. Holmes, N. W. Duffy, U. Bach and L. Spiccia, Nat. Chem.
2011, , 211–215.
5
,
3
Y. Saygili, M. Söderberg, N. Pellet, F. Giordano, Y. Cao, A. B. Muñoz-García, S. M.
Zakeeruddin, N. Vlachopoulos, M. Pavone, G. Boschloo, L. Kavan, J.-E. Moser, M.
Grätzel, A. Hagfeldt and M. Freitag, J. Am. Chem. Soc., 2016, 138, 15087–15096.
M. Freitag, F. Giordano, W. Yang, M. Pazoki, Y. Hao, B. Zietz, M. Grätzel, A. Hagfeldt
and G. Boschloo, J. Phys. Chem. C, 2016, 120, 9595–9603.
Y. Hao, W. Yang, L. Zhang, R. Jiang, E. Mijangos, Y. Saygili, L. Hammarström, A.
Hagfeldt and G. Boschloo, Nat. Commun., 2016, 7, 13934.
S. Cazzanti, S. Caramori, R. Argazzi, C. M. Elliott and C. A. Bignozzi, J. Am. Chem. Soc.
2006, 128, 9996–9997.
R. N. Sampaio, R. M. O’Donnell, T. J. Barr and G. J. Meyer, J. Phys. Chem. Lett., 2014,
3265–3268.
23
24
25
26
27
28
,
5
,
S. A. Sapp, C. M. Elliott, C. Contado, S. Caramori and C. A. Bignozzi, J. Am. Chem. Soc.
2002, 124, 11215–11222.
S. M. Feldt, P. W. Lohse, F. Kessler, M. K. Nazeeruddin, M. Gratzel, G. Boschloo and A.
Hagfeldt, Phys. Chem. Chem. Phys., 2013, 15, 7087–7097.
,
29
30
31
32
33
N. Sutin, Acc. Chem. Res., 1982, 15, 275–282.
P. F. Barbara, T. J. Meyer and M. a. Ratner, J. Phys. Chem., 1996, 100, 13148–13168.
J. W. Ondersma and T. W. Hamann, J. Am. Chem. Soc., 2011, 133, 8264–8271.
M. Ansari-Rad, J. A. Anta and J. Bisquert, J. Phys. Chem. C, 2013, 117, 16275–16289.
M. Ansari-Rad, Y. Abdi and E. Arzi, J. Phys. Chem. C, 2012, 116, 3212–3218.
20

Chemical Science
Page 20 of 23
View Article Online
DOI: 10.1039/C7SC03839A
34
35
J. Bisquert, A. Zaban and P. Salvador, J. Phys. Chem. B, 2002, 106, 8774–8782.
S. A. Haque, Y. Tachibana, D. R. Klug and J. R. Durrant, J. Phys. Chem. B, 1998, 102
1745–1749.
,
36
37
A. V. Barzykin and M. Tachiya, J. Phys. Chem. B, 2002, 106, 4356–4363.
R. Katoh, A. Furube, A. V. Barzykin, H. Arakawa and M. Tachiya, Coord. Chem. Rev.
2004, 248, 1195–1213.
,
38
39
R. Katoh and A. Furube, J. Phys. Chem. Lett., 2011,
K. Hu, A. D. Blair, E. J. Piechota, P. A. Schauer, R. N. Sampaio, F. G. L. Parlane, G. J.
Meyer and C. P. Berlinguette, Nat. Chem., 2016, , 853–859.
K. E. Spettel and N. H. Damrauer, J. Phys. Chem. C, 2016, 120, 10815–10829.
A. N. M. Green, E. Palomares, S. A. Haque, J. M. Kroon and J. R. Durrant, J. Phys. Chem.
, 2005, 109, 12525–12533.
2, 1888–1891.
8
40
41
B
42
43
44
G. Williams and D. C. Watts, Trans. Faraday Soc., 1970, 66, 80.
H. Scher and E. W. Montroll, Phys. Rev. B, 1975, 12, 2455–2477.
J. Bisquert, F. Fabregat-Santiago, I. Mora-Seró, G. Garcia-Belmonte and S. Giménez, J.
Phys. Chem. C, 2009, 113, 17278–17290.
45
46
47
G. Boschloo and A. Hagfeldt, J. Phys. Chem. B, 2005, 109, 12093–12098.
J. Nelson, S. A. Haque, D. R. Klug and J. R. Durrant, Phys. Rev. B, 2001, 63, 205321.
R. G. Palmer, D. L. Stein, E. Abrahams and P. W. Anderson, Phys. Rev. Lett., 1984, 53
,
958–961.
48
49
50
51
52
B. N. DiMarco, T. C. Motley, R. S. Balok, G. Li, M. A. Siegler, R. M. O’Donnell, K. Hu and
G. J. Meyer, J. Phys. Chem. C, 2016, 120, 14226–14235.
R. M. O’Donnell, R. N. Sampaio, T. J. Barr and G. J. Meyer, J. Phys. Chem. C, 2014, 118
16976–16986.
,
C. L. Ward, R. M. O’Donnell, B. N. DiMarco and G. J. Meyer, J. Phys. Chem. C, 2015,
119, 25273–25281.
B. H. Farnum, Z. A. Morseth, M. K. Brennaman, J. M. Papanikolas and T. J. Meyer, J.
Am. Chem. Soc., 2014, 136, 15869–15872.
C. A. Kelly, F. Farzad, D. W. Thompson, J. M. Stipkala and G. J. Meyer, Langmuir, 1999,
15, 7047–7054.
53
54
D. F. Watson and G. J. Meyer, Coord. Chem. Rev., 2004, 248, 1391–1406.
S. Ardo, Y. Sun, A. Staniszewski, F. N. Castellano and G. J. Meyer, J. Am. Chem. Soc.
2010, 132, 6696–6709.
,
55
U. B. Cappel, S. M. Feldt, J. Schöneboom, A. Hagfeldt and G. Boschloo, J. Am. Chem.
Soc., 2010, 132, 9096–9101.
56
57
E. C. Brigham and G. J. Meyer, J. Phys. Chem. C, 2014, 118, 7886–7893.
P. R. F. Barnes, K. Miettunen, X. Li, A. Y. Anderson, T. Bessho, M. Gratzel and B. C.
O’Regan, Adv. Mater., 2013, 25, 1881–1922.
58
59
60
61
62
T. J. Barr and G. J. Meyer, ACS Energy Lett., 2017,
F. Perrin, Ann. Phys. (Paris)., 1929, 10, 169–275.
L. Onsager, J. Am. Chem. Soc., 1936, 58, 1486–1493.
J. Nelson, Phys. Rev. B, 1999, 59, 15374–15380.
M. Kaus, H. Stöffler, M. Yavuz, T. Zinkevich, M. Knapp, H. Ehrenberg and S. Indris, J.
Phys. Chem. C, 2017, acs.jpcc.7b08350.
2, 2335–2340.
63
64
65
R. M. O’Donnell, S. Ardo and G. J. Meyer, J. Phys. Chem. Lett., 2013, 4, 2817–2821.
B. C. O’Regan and J. R. Durrant, Acc. Chem. Res., 2009, 42, 1799–1808.
P. Cias, C. Slugovc and G. Gescheidt, J. Phys. Chem. A, 2011, 115, 14519–14525.
21

Page 21 of 23
Chemical Science
View Article Online
DOI: 10.1039/C7SC03839A
66
67
68
69
E. A. Robinson and J. E. Earley, Inorg. Chem., 1999, 38, 4128–4131.
O. Olubuyide, K. Lu, A. O. Oyetunji and J. E. Earley, Inorg. Chem., 1986, 25, 4798–4799.
C. Creutz and M. H. Chou, Inorg. Chem., 2008, 47, 3509–3514.
W. R. Entley, C. R. Treadway, S. R. Wilson and G. S. Girolami, J. Am. Chem. Soc., 1997,
119, 6251–6258.
70
71
R. Ameis, D. Reinen and S. Kremer, Inorg. Chem., 1985, 24, 2751–2754.
E. H. Yonemoto, G. B. Saupe, R. H. Schmehl, S. M. Hubig, R. L. Riley, B. L. Iverson and T.
E. Mallouk, J. Am. Chem. Soc., 1994, 116, 4786–4795.
72
73
74
Y.-P. Liu and M. D. Newton, J. Phys. Chem., 1994, 98, 7162–7169.
N. A. Deskins and M. Dupuis, Phys. Rev. B, 2007, 75, 195212.
C. Quinton, V. Alain-Rizzo, C. Dumas-Verdes, F. Miomandre, G. Clavier and P.
Audebert, RSC Adv., 2014, 4, 34332.
75
76
A. P. Davis and A. J. Fry, J. Electrochem. Soc., 2013, 160, G3091–G3096.
T. A. Heimer, S. T. D’Arcangelis, F. Farzad, J. M. Stipkala and G. J. Meyer, Inorg. Chem.
1996, 35, 5319–5324.
,
22

Chemical Science Page 22 of 23
rrecBr, Cl
rrecMe
X
ß = 1
rrecMeO
N
# of hops
X
X
ß < 1
TiO2
X = MeO, Me, Cl, Br