Remote control of electronic coupling-modification of excited-state electron-transfer rates in Ru(tpy)2-based donor-acceptor systems by remote ligand design

Article abstract of DOI:10.1039/c8cc10075f

A comprehensive understanding of how the molecular structure influences the electronic coupling is crucial in optimizing (supra) molecular assemblies for photoinduced electron transfer. Here, we report that the electronic coupling underlying electron transfer from a phenothiazine donor to a photoexcited Ru(tpy)₂ acceptor is modulated by substitution of the second (remote) tpy-ligand.

Full text of DOI:10.1039/c8cc10075f

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2019, DOI: 10.1039/C8CC10075F.
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Remote control of electronic coupling – modification of excited-
state electron-transfer rates in Ru(tpy)2-based donor-acceptor
systems by remote ligand design
Received 00th January 20xx,
Accepted 00th January 20xx
Yusen Luo,ab Jens H. Tran,a Maria Wächtler,ab Kevin Barthelmes,cd Andreas Winter,cd Sven Rau,e
DOI: 10.1039/x0xx00000x
Ulrich S. Schubert,cd and Benjamin Dietzek*abd
www.rsc.org/
A comprehensive understanding of how the molecular structure
influences the electronic coupling is crucial in optimizing (supra)
molecular assemblies for photoinduced electron transfer. Here,
we report that the electronic coupling underlying electron transfer
from a phenothiazine donor to a photoexcited Ru(tpy)2 acceptor is
modulated by substitution of the second (remote) tpy-ligand.
Photoinduced electron transfer is a fundamental process in
natural and artificial photosynthesis.1-4 Compared to natural
photosynthesis where electron transfer proceeds with unity
quantum efficiencies and long-lived charge-separated states
(CSS) are generated,4 the artificial (supra)molecular assemblies
need to be optimized to achieve efficient electron transfer and
to produce long-lived CSS.4-7 Optimization of such man-made
systems requires an in depth understanding of the interplay
between molecular structure and key parameters for electron
transfer, i.e. electronic coupling (HDA), reorganization energy
(λ) and driving force (–ΔG°) according to the semi-classical
Marcus theory.1,3-5 While the impact of molecular structure on
λ and –ΔG° is quite well understood4,5,8,9 and / or can be
estimated quite well from e.g. electrochemical measurements
(see ESI for a more detailed description), the factors governing
HDA are not fully comprehended yet: The nature of electron
donor (D) and electron acceptor (A), the structure of molecular
spacers (e.g. length1,3,10,11 and substituents12-17) separating D
and A as well as the molecular conformation18-21 have
significant effects on HDA: HDA generally decreases with
increasing D–A distance.1,3,10,11 Carbonera and coworkers
Fig 1. Molecular dyads D1–D4 and triad T1 studied in this work. Upon excitation of
RuII(tpy)2 center, electron transfer (ET) takes place from phenothiazine (PTZ) donor to
photo-oxidized Ru(II) in the Ru(tpy)2 center.23,24 This process is of particular interest
and we will focus on it in this work. For detailed decay processes after
photoexcitation see Fig. S1. The UV-Vis absorption spectra shown here were
recorded at room temperature in dichloromethane. For D1 and T1 the spectra were
taken from ref 22.
pointed out that substitution of a molecular spacer in
structurally related carotenoid–porphyrin–fullerene triads
increased HDA and, thus, significantly increased the charge
recombination rate.13 Albinsson and coworkers reported
conformer-dependent electron-transfer rates in a (Zinc
porphyrin)2–fullerene dyad.20 The significantly different
electron-transfer rates for different molecular conformers
were ascribed to the significantly different HDA in the two
conformers.20 Despite these and other careful studies on the
structural impact on the electron-transfer rates1,3,4,10-18 it is
difficult to isolate the impact of various structural factors on
HDA in photoactive transition metal complexes based D–A
systems: Altering e.g. the chemical nature of D or A will impact
not only electronic coupling but also the driving force for
electron transfer.
a. Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller
University Jena, Helmholtzweg 4, 07743 Jena, Germany
b. Department Functional Interfaces, Leibniz Institute of Photonic Technology (IPHT),
Albert-Einstein-Straβe 9, 07745 Jena, Germany.
E-mail: benjamin.dietzek@leibniz-ipht.de
c. Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller
University Jena, Humboldtstraβe 10, 07743 Jena, Germany
d. Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich
Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany
e. Institute for Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081
Ulm, Germany
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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is not prominent in D1 (Fig. 2a) which is due toVitehweArtsicpleeOcntlrinael
overlap with the residual GSB. These obDseOrI:v1a0t.i1o0n39s/Cag8CreCe10w07it5hF
our previous reports on D1 at room temperature23 and
indicate an electron transfer from the PTZ donor to the excited
·-
acceptor Ru(tpy)2*, i.e. the formation of PTZ·+–Ru(tpy)2 . For
T1 at 1700 ps, two distinct ESA bands are observed at 475, 585
and a rather broad feature at around 700 nm (Fig. 2c). The two
bands are assigned to the absorption of PTZ·+ and the far-red
feature is attributed to the absorption of the fullerene triplet
state (3C60*). Different to D1 (Fig. 2a), the absorption band of
PTZ·+ at 365 nm is not pronounced in fs TA spectra of T1 (Fig.
2c) due to the spectral overlap with the negative absorption of
reduced fullerene (C60·-) below 400 nm.23 Hence, in T1 the fully
charge-separated state, PTZ·+–Ru(tpy)2–C60·-, is formed.
Temperature-dependent electron-transfer time constants
were obtained by global fits of the fs TA data (see Fig. 2b, d,
Fig. S3, S5, S7, S9 and S11). Previous experiments combining
transient absorption and resonance Raman spectroscopy
showed that the processes associated with τ1 and τ2 (Fig. 2b, d)
Fig 2. (a, c) fs TA spectra at selected delay times and (b, d) decay-associated spectra
resulting from the global fit of fs TA data collected in a 1 cm cuvette in
dichloromethane at 270 K for D1 (a, b) and T1 (c, d). The grey dashed line represents
the shape of the corresponding inverted ground state absorption spectrum which is
scaled to fit the respective figure.
3
occur from two distinct MLCT states (Fig. S1).23 The first
kinetic component (τ1) is characterized by an increased
absorption at 365 nm and at 550 to 590 nm corresponding to
the absorption of PTZ·+.23 Thus, the respective kinetic
component is attributed to the PTZRu(tpy)2* electron
transfer. It should be noted that the characteristic time
constant of the electron transfer is in the same range as
previously described rotational motion around the terpyridine-
phenyl (tpy-ph) bond (typically also observed on a some-ps
timescale for Ru(tpy)2-derived systems25-27). This indicates that
the PTZRu(tpy)2* electron transfer is likely accompanied by
planarization of the tpy-ph ligand in D2–D4. The second
component (τ2) represents the relaxation of 3MLCT state
In this contribution, we discuss how electronic coupling
underlying the photoinduced electron transfer in
a
phenothiazine donor–Ru(tpy)2 acceptor complex (i.e. D1, Fig.
1) can be modified without changing the D–A distance or the
chemical nature of the linkage. Tuning HDA is simply achieved
by modifying the substituents at the 4’-position of the remote
terpyridine ligand (i.e. –R, Fig. 1). This ligand does not link the
photoactive Ru(II) core to the phenothiazine donor, and hence,
it presents a convenient handle to tune the electronic coupling
within the paradigm D–A dyad. We vary the substituent R from
H atom to C60, phenyl and more extended phenylmethyl,
phenylmethoxy and show that HDA values in the systems
change significantly albeit similar driving forces, fixed chemical
nature of D and identical D–A linkage and consequently fixed
distance and mutual arrangement of D and A.
3
localized on the terminal tpy ligand, i.e. a MLCTtpy-R state: For
D1–D4 it deactivates directly to ground state. For T1 3MLCTtpy-R
undergoes an energy transfer yielding a 3C60* state.23 The third
component in T1 is attributed to the formation of PTZ·+–
Ru(tpy)2–C60 .
·- 23,24 To investigate how intramolecular electronic
To quantify the HDA values temperature-dependent
electron-transfer rates were determined between 300 and
240 K by transient absorption spectroscopy (see Fig. S2–S11
for the full sets of data): The Ru(tpy)2-core was excited at 520
nm, i.e. the red edge of the MLCT band. For each of the
compounds, the TA spectra do not change significantly upon
decreasing the temperatures. Hence, the previously developed
model to account for the photoinduced processes in D1 and T1
at room temperature (Fig. S1)23,24 is applied to analyze the
temperature-dependent data. Fig. 2 exemplarily show the fs
TA spectra of D1 and T1 at 270 K: At short delay times, the
spectra are dominated by strong ground state bleach (GSB) at
around 500 nm and two excited-state absorption (ESA) bands
below 450 nm and above 550 nm. At long delay times, e.g. at
1700 ps for D1, the ESA band shifts from 600 to 580 nm
meanwhile a new ESA band appears at 365 nm (Fig. 2a). Both
features are indicative of the formation of one-electron
coupling is influenced by substitution of the remote ligand, we
focus on the PTZRu(tpy)2* electron transfer for which all
compounds have the same D and D–A distance.
To obtain HDA values for the photoinduced electron transfer
PTZRu(tpy)2*, the Marcus equation was used:28-32
2
1
°
(
)
휋
λ + ∆G
2
ln kET ∙ T2 = ln
∙ HDA
―
(1)
(
)
ћ2 ∙ λ ∙ kB
(
)
4 ∙ λ ∙ kB ∙ T
Plotting ln(kET·T1/2) vs. 1/T yields a straight line (see Fig. 3b)
indicating that both λ and (λ+ΔG°)2/4λ are temperature-
independent.33,34 This is in line with the estimated λ (according
to Marcus’ dielectric continuum model) and –ΔG° in the
temperature region of 300 to 240 K that λ and the term
(λ+ΔG°)2/4λ (i.e. the activation energy barrier ΔGǂ) are nearly
temperature independent (see Table S3–4). Recently, Wenger
showed experimentally that –ΔG° is indeed temperature
independent in a related donor-photosensitizer-acceptor
system.35 Under such conditions λ and HDA can be extracted
from the slope and the intercept of the linear regression,
respectively.28,29 Experimentally, τ1 increases with decreasing
oxidized
phenothiazine
(PTZ·+)
according
to
spectroelectrochemistry revealing three distinct absorption
bands at 365, 473 and 580 nm for PTZ·+.23 The band at 473 nm
2 | J. Name., 2012, 00, 1-3
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temperature (Fig. 3a and Table S1) indicating reduced
electron-transfer rates (kET) at lower temperatures. The full set
of parameters describing electron transfer within Marcus’
theory, i.e. λ, –ΔG°, HDA and ΔGǂ extracted from Fig. 3b, is
summarized in Table 1.
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DOI: 10.1039/C8CC10075F
Table 1. Summary of driving force (–ΔG°), reorganization energy (λ), electronic coupling
(HDA) and activation energy barrier (ΔGǂ) obtained from the experimental results shown
→
in Fig. 3b for the PTZ Ru(tpy)2* electron transfer process in D1–D4 and T1.
D1
D2
D3
D4
T1
–ΔG° / eV
λ / eV
HDA / cm-1
0.29a
0.72
94
0.26b
0.67
90
0.25b
0.65
74
0.24b
0.95
371
0.23a
0.71
157
ΔGǂ / eV
0.064
0.063
0.062
0.13
0.072
a
b
→
Values were taken from ref 24.
Values were obtained from the
Fig 3. (a) Temperature dependence of the time constant associated with PTZ
electrochemical data (Fig. S12) according to Rehm-Weller equation (eq S3-4).
Ru(tpy)2* electron transfer. The dashed lines are only guides to the reader. (b) Plots
of ln (kET·T1/2) vs. 1/T for D1–D4 and T1 with the corresponding linear fit. The
adjusted R2 value of the linear fit is 0.90 (D1), 0.99 (D2), 0.98 (D3), 0.92 (D4) and
0.90 (T1).
°
→
–ΔG for PTZ Ru(tpy)2* electron transfer varies by only
20% within the investigated series from 0.29 (D1) to 0.23 eV
(T1). Likewise, due to identical D, D–A distance and mutual D–
A orientation the reorganization energy (λ) is rather similar for
D1–D3 and T1 varying from 0.65 (D3) to 0.72 eV (D1).
However, D4 exhibits a roughly 40% higher  which indicates
that the inner-sphere reorganization energy λi must be
significantly changed, as the outer-sphere reorganization
energy λo is not supposed to vary a lot within the assumptions
of the dielectric continuum model (eq S1–2). In contrast to 
and –ΔG°, HDA values are quite different within the series of
complexes investigated. They vary by more than 400% from D3
to D4 despite of minimal structural differences (see Table 1):
D4, with the strongly electron-donating substituent –OCH3,
shows the strongest coupling (371 cm-1); T1 with the directly
connected strongly electron withdrawing –C60 reveals
moderate coupling (157 cm-1); For D1–D3 the HDA values are
significantly lower (i.e. between 90 and 70 cm-1). Thus, for
example, replacing the –H atom at the 4’-position of the
terminal tpy ligand in D1 by –C60 (i.e. T1) causes a roughly 70%
increase of HDA between PTZ and Ru(tpy)2* despite the fact
that D1 and T1 have the same D, D–A linkage and D–A
distance. The electron rich –OCH3 group in para position of the
phenyl ring increases the coupling by a factor of 3 comparing
D2 and D4.
Similar effects have been reported for mixed-valance
RuII/RuIII complexes.36,37 Here, the properties of intervalence
charge-transfer (IVCT) transitions were altered by design of
remote ligands: While this strategy ensured almost identical
driving forces in a broad range of substitutions the electronic
coupling underlying the IVCT could be altered by 20%.
However, this work36 does not consider photoinduced excited-
state electron-transfer reactions in the mixed-valence systems.
To rationalize the control of the donor-acceptor coupling
by remote ligand design, we will consider the influence of the
substituents –R on the charge densities. Such consideration is
based on the fact that HDA will be largely determined by the
electronic structure of the molecular fragments involved.38
Previously published calculations have studied the ground39
1
and MLCT excited-states40 of two model Ru(tpy)2 complexes
with strongly electron donating (–NH2) and electron with-
drawing (–NO2) groups attached to the 4’-position of the tpy
ligand via a phenyl spacer (ph, see Fig. S13).39,40 Calculations
showed an increased π-character and a consequently
shortened tpy-ph bond upon introducing the –NH2 substituent.
Furthermore, the electron donating group reduces the
dihedral angle between ph and tpy in the ground state of the
complex.39 The –NO2 facilitates long-range charge de-
localization both in the ground- and excited-state albeit a
larger tpy-ph dihedral angle in the ground state.39,40 Similar
effects on the electronic levels of a Ru(II) complex were
calculated by Kupfer, who considered the effect of coligand
exchange on the electronics of charge-accumulation within a
photoactive Ru(II)-complex.41
The above discussed literature details the impact of substi-
tuents on the electronic situation in related Ru(II) complexes,
leaving the question unanswered if such substitutions can be
utilized to affect electronic coupling of the photoactive Ru(II)
core with an electron donor linked via a second non-modified
ligand. Comparing the PTZ-Ru(tpy)2* electronic coupling within
the series of complexes investigated here, will address this
issue (to the best of our knowledge for the first time in a
systematic experimental approach). Within the series of the
dyads investigated the electron rich –OCH3 substituent
drastically increases the PTZ-Ru(tpy)2* coupling as reflected in
the HDA value. This is accompanied by a significant increase of
 associated with the PTZRu(tpy)2* electron transfer. This
increase in  is likely associated with a decreased tpy-ph bond
length and smaller tpy-ph dihedral angle in D4 as λi can be
related to the free energy change associated with bond length
changes42 which upon electron transfer would be different.
Thus, the electron rich –OCH3 substituent in D4 leads to
structural changes within the modified (remote) tpy ligand
affecting λ but also to electronic changes altering the
electronic coupling underlying PTZ-Ru(tpy)2* electron transfer.
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12 C. Lambert, G. Nöll and J. Schelter, Nat. Mater., 2002, 1, 69–
The slight (~18%) decrease in HDA comparing D3 to D2 might
stem from the weak electron-donating ability of the CH3 group.
This property causes the tpy-ph unit to be more planar than in
D2.39 Consequently, upon photoexcitation of the Ru(tpy)2-
photocenter in its MLCT transition, the access charge on the
formally reduced ligand(s) becomes somewhat more
delocalized, hence, reducing the coupling for the PTZ
Ru(tpy)2* electron transfer. Alterations of the remote ligand
upon introduction of C60, i.e. comparing D1 and T1, increases
HDA by 70%. The effect of the C60-containing tpy ligand also
shifts the RuIII/RuII oxidation anodically by 120 mV compared
to D1 (Table S2). The shifted HOMO apparently impacts the
electronic levels on the PTZ-tpy ligand and thus increases the
coupling underlying the PTZRu(tpy)2* electron transfer.
An experimental investigation on the electronic coupling
underlying the photoinduced electron transfer in D–A dyads of
the form PTZ–(tpy)Ru(tpy–R) is presented. The data reveal the
possibility to modulate HDA between the PTZ donor and the
Ru(II) acceptor/photosensitizer by a factor of four by changing
the remote substituent –R. Altering the electronics in the
photo-excited Ru(tpy)2*-photosensitizer, either by deloca-
lization of the relaxed excited-state within the ligand sphere or
by modifying the HOMO level of the Ru(II) ion, impacts the
electronic coupling for photoinduced electron transfer in the
dyad. Thus, the data point towards an additional design
parameter for molecular systems, in which realizing efficient
and specific electron transfer paths is key to improved function.
View Article Online
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