Electron Transfer around a Molecular Corner

Article abstract of DOI:10.1002/anie.201800396

The distance dependence of electron transfer (ET) is commonly investigated in linear rigid rod-like compounds, but studies of molecular wires with integrated corners imposing 90° angles are very rare. By using spirobifluorene as a key bridging element and by substituting it at different positions, two isomeric series of donor-bridge-acceptor compounds with either nearly linear or angled geometries were obtained. Photoinduced ET in both series is dominated by rapid through-bond hole hopping across oligofluorene bridges over distances of up to 70 ?. Despite considerable conformational flexibility, direct through-space and through-solvent ET is negligible even in the angled series. The independence of the ET rate constant on the total number of fluorene units in the angled series is attributed to a rate-limiting tunneling step through the spirobifluorene corner. This finding is relevant for multidimensional ET systems and grids in which individual molecular wires are interlinked at 90° angles.

Full text of DOI:10.1002/anie.201800396

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Accepted Article
Title: Electron Transfer around a Molecular Corner
Authors: Hauke C. Schmidt, Christopher B. Larsen, and Oliver S.
Wenger
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800396
Angew. Chem. 10.1002/ange.201800396
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Electron Transfer around a Molecular Corner
Hauke C. Schmidt,[a] Christopher B. Larsen,[a] and Oliver S. Wenger*[a]
Abstract: The distance dependence of electron transfer (ET) is
commonly investigated in linear rigid rod-like compounds, but
studies of molecular wires with integrated corners imposing 90°
angles are very rare. By using spirobifluorene as a key bridging
element and by substituting it at different positions, two isomeric
series of donor-bridge-acceptor compounds with either nearly linear
or angled geometries were obtained. Photoinduced ET in both series
is dominated by rapid through-bond hole hopping across oligo-
fluorenes over distances of up to 70 Å. Despite considerable
conformational flexibility, direct through-space and through-solvent
ET is negligible even in the angled series. The independence of the
ET rate constant on the total number of fluorene units in the angled
series is attributed to a rate-limiting tunneling step through the
spirobifluorene corner. This finding is relevant for multi-dimensional
ET systems and grids in which individual molecular wires are
interlinked at 90° angles.
compounds,[7] but we are unaware of prior studies that explored
long-range electron transfer in spirobifluorene-based systems
with both linear and angled geometries. This comparison should
provide direct insight into the relative efficiencies of different
charge transfer pathways, and this is relevant in the greater
context of a future molecular electronics technology.[8] Both
linear and -stacked oligo-fluorenes were previously identified as
molecular wires that mediate long-range electron and energy
transfer very efficiently.[9] Spirobifluorene plays a key role in hole
transport materials for solar cells,[10] and it has also been
explored in the context of luminescent materials.[11]
Photoinduced electron transfer has been widely explored in
artificial and natural systems. Whilst numerous studies focused
on the influence of driving-force and reorganization energy on
electron transfer rates,[1] many other investigations concentrated
on their distance dependence.[2] A key goal was often the
generation of long-lived charge-separated states and the
mimicry of elementary processes in natural photosynthesis.[3]
Linear rigid rod-like donor-acceptor compounds and straight
molecular wires were frequently preferred,[4] because this
permits the investigation of through-bond electron transfer over
well-defined donor-acceptor distances and a separation of
charges over maximal distance. By contrast, rigid molecular
bridges or wires containing 90° angles are very uncommon.[5]
Prior investigators employed octahedral metal complexes as
photosensitizers with donor and acceptor units attached to
different ligands that were coordinated at ca. 90° angles relative
to one another,[6] but this is conceptually much different from the
situation in which a corner is integrated into the molecular wire.
We hypothesized that spirobifluorene as a key bridging unit
between a donor and an acceptor could provide access to linear
and angled isomers of rigid rod-like donor-acceptor dyads
(Scheme 1). When complemented with additional fluorene
bridging units on both sides of the central spirobifluorene
element, donor and acceptor branches of variable lengths
should be accessible (Scheme 1). Both branches would then
have a rigid rod-like structure, but depending on the substitution
pattern at the spirobifluorene unit either linear or angled isomer
series should be accessible. Electron delocalization across
spiro-centers has been investigated in mixed-valence
Scheme 1. Molecular dyads investigated in this work. C1 has the same angled
structure as A1, A2 and A3, but with different relative bridge lengths between
donor and acceptor branches. The linking position of the donor and the
acceptor is the same in all 6 compounds.
A triarylamine (TAA) unit and a [Ru(bpy)3]2+ (bpy = 2,2’-
bipyridine) complex (Ru) were chosen as a donor and an
acceptor, respectively, because this combination has favorable
optical spectroscopic properties and there is enough driving-
force for long-range electron transfer after photoexcitation of the
metal complex, at least in so-called flash-quench experiments
(see below). For the linear series of dyads, we managed to
synthesize compounds with either 3 or 5 fluorene units between
TAA and Ru (L1, L2). For the angled series, dyads with 3, 5, and
7 fluorene units between TAA and Ru could be synthesized and
explored (A1 – A3). Synthetic details and characterization data
are in the Supporting Information (SI pages S2-S30).
Not too surprisingly, in vacuo Merck Molecular Force Field
(MMFF) energy minimization calculations quickly dispersed the
notion of the angled series having a strictly rigid rod-like right-
angled structure. Instead, two low energy conformers can be
readily identified: an open ‘winged’ conformer, and a closed
conformer in which the TAA and [Ru(bpy)3]2+ units approach one
another, with NTAA-Ru distances of 7.6, 8.9 and 9.0 Å for A1, A2
and A3, respectively (SI page S40-S41). This second conformer
is predicted to be lower in energy by 8 kJ mol-1 for A1, 26 kJ mol-
1 for A2, and 27 kJ mol-1 for A3, compared to the open conformer
(SI page S41). MMFF is a low, computationally inexpensive level
[a]
Dr. H. C. Schmidt, Dr. C. B. Larsen, Prof. Dr. O. S. Wenger
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: oliver.wenger@unibas.ch
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201800396
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of theory and these stabilization values are therefore likely
exaggerated by the calculations.[12] Furthermore, as the
calculations are performed in vacuo, solvent stabilization effects
are not taken into consideration. The key message from these
calculations is that in the angled isomer series conformers with
relatively short through-space but long through-bond distances
can form. Electron transfer in such systems can either proceed
through the long bonding system, or across the short space
between donor and acceptor.[13] In the linear series, by contrast,
only the through-bond pathway is available. To further separate
the role of through-bond and through-space electron transfer, an
additional angled control compound, C1, was prepared, in which
a close contact between the donor and acceptor cannot form
(Scheme 1).
CH3CN at room temperature. The duration of the laser pulses
was ca. 10 ns, and transient absorption spectra were recorded
by time-integration over a period of 200 ns immediately after
excitation. The key observation is the transient absorption band
at 730 nm, which is diagnostic for TAA+ (SI page S38),[15] in
addition to a band at 605 nm caused by MV+.[16] Evidently,
selective excitation of the Ru sensitizer at 532 nm leads to the
formation of both TAA+ and MV+, and this can be explained by
rapid oxidative 3MLCT excited-state quenching by MV2+,
followed by intramolecular electron transfer from TAA to the
[Ru(bpy)3]3+ unit, as noted above. When performing the same
experiment with the commercial [Ru(bpy)3]2+ reference complex,
one still observes the MV+ band at 605 nm (black trace in Figure
1), but instead of the TAA+ absorption at 730 nm there is now an
MLCT bleach at 450 nm signaling the disappearance of
[Ru(bpy)3]2+ and the formation of [Ru(bpy)3]3+, in line with
expectation.
The TAA and [Ru(bpy)3]2+ redox potentials are very similar in the
6 dyads (SI pages S32-S34), leading to essentially identical
driving-forces for intramolecular electron transfer in all cases.
3
For photoinduced electron transfer from TAA to MLCT-excited
[Ru(bpy)3]2+ the reaction free energy is relatively small (GET0  -
0.2 eV), making the observation of long-range charge transfer
challenging because the rates can get slow compared to the
3MLCT lifetime. However, in presence of excess methylviologen
3
(MV2+), the MLCT-excited complex is oxidatively quenched to
yield [Ru(bpy)3]3+ within less than 10 ns (SI page S35),[14] and
the Ru(III) species is then reduced by TAA with substantially
0
greater driving-force (GET  -0.7 eV). Bimolecular thermal
reverse electron transfer from MV+ to TAA+ subsequently takes
places on a longer timescale (~ 40 μs, SI page S35).[14] This so-
called flash-quench technique was most useful to explore long-
range electron transfer in our dyads.
Figure 2. Temporal evolution of the transient absorption signal at 730 nm for
compounds A1 – A3 following excitation at 532 nm with laser pulses of ca. 10
ns duration. The dyad and methylviologen concentrations were 15 M and 80
mM, respectively, in all cases. The solvent was de-aerated CH3CN at 22 °C.
The dashed red lines are single-exponential fits.
Figure 1. Transient absorption spectra obtained after excitation at 532 nm with
laser pulses of ca. 10 ns duration: 15 M dyad A1 with 80 mM MV2+ (red trace),
and 15 M [Ru(bpy)3]2+ with 80 mM MV2+ (black trace) in de-aerated CH3CN at
22 °C.
Given these spectral characteristics, the rate constants for
intramolecular electron transfer (kET) from TAA to [Ru(bpy)3]3+ in
the 6 dyads can be determined by monitoring the temporal
evolution of the transient absorption signal at 730 nm. In the
linear dyads L1 and L2, and in the control angled dyad C1, the
risetime of the signal at 730 nm is instrumentally limited, and we
can only estimate a lower limit of 108 s-1 for kET (SI, page S39).
For the angled dyads A1, A2, and A3, risetimes of 17±3, 25±3,
and 21±3 ns, respectively, are measured (Figure 2, Table 1).
Thus, electron transfer is faster in the linear than in the angled
isomers, but this is not particularly surprising because the former
All dyads exhibit intense fluorene-localized -* absorption
bands between 370 and 385 nm, but at 532 nm photoexcitation
occurs selectively into MLCT absorptions of the photosensitizer
units (SI page S36). The transient absorption spectrum of A1
recorded in presence of MV2+ (red trace Figure 1) is typical for
all 6 dyads (SI page S37). In all cases, the dyad concentration
was 15 M, whilst MV2+ was present at 80 mM in de-aerated
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have a more extensively -conjugated bridging system than the
latter. The most significant observations are clearly the distance
independence of kET along the series of angled isomers (A1, A2,
A3) and the faster electron transfer in the control compound C1
compared to A1, A2 and A3 (Table 1).
through-space tunneling plays only a minor role in A1, A2 and
A3 if any at all. This is not overly surprising, as the flash-quench
measurements were undertaken in acetonitrile, a highly polar
solvent that can efficiently stabilize charges and therefore
minimize the energy difference between open and closed
conformers compared to what is obtained with the (crude) in
vacuo MMFF calculations (see above and SI page S40-S41).
For example, we recently reported electron transfer across a
close-contact ion-pair which was significantly disrupted by even
modestly polar solvents (CH2Cl2, THF).[19] Given that the
coulombic attraction in an ion-pair is significantly greater than for
an ion-dipole system such as presented herein, it is unlikely that
enough molecules exist in a close-contact conformation in
acetonitrile. Electron tunneling through solvent is very inefficient
compared to through-bond hopping. For example, the distance
Table 1. Through-bond (rtb) and through-space (rts) distances between the
triarylamine N-atom and the Ru-atom in the 6 dyads from Scheme 1, along
with time (ET) and rate constants (kET) for electron transfer between TAA and
photogenerated Ru(III). The through-space distances represent the lowest
energy close-contact conformation of donor and acceptor calculated for
geometry optimized structures using the MMFF level of theory (SI page S40).
dyad
L1
rtb / Å
35.9
52.8
38.6
55.4
72.3
38.6
rts / Å

ET / ns
kET / s-1
< 10
< 10
17±3
25±3
21±3
< 10
> 108
decay constant () for tunneling through 2-methyl-THF is 1.6 Å-
> 108
1 [20]
L2
,
whilst for hopping processes the (phenomenological) -
values are usually below 0.1 Å-1.[18] Thus, through-space and
through-solvent tunneling seem to play a negligible role in our
dyads, and through-bond pathways seem to be dominant.
A1
A2
A3
C1
7.6
8.9
9.0
19.5
(5.9±1.2)·107
(4.0±0.5)·107
(4.8±0.8)·107
> 108
The greater rate of electron transfer in C1 compared to A1, A2
and A3 further gives implications for the mechanism of charge
transfer. The linear series of complexes (L1, L2) clearly
demonstrates that the hopping mechanism of hole transfer
across fluorene bridges is extremely rapid. In the angled series
however, the hole must tunnel through the spirobifluorene corner
unit. This is likely to be the rate-limiting step of the hole transfer
despite some spiroconjugation,[21] and is corroborated by the
more rapid rate of charge transfer in C1, which has the TAA
electron donor located much closer to the spirobifluorene unit
than any of the angled dyads A1, A2 and A3, leading to stronger
electronic coupling between TAA and spirofluorene.
Consequently, the tunneling rate through the spirobifluorene
corner is expected to increase in C1, as observed
experimentally. However, hole tunneling between Ru(III) and its
neighboring fluorene unit (rather than electron tunneling
between TAA and the spirobifluorene corner) is likely to remain
the initial key step of long-range charge transfer between Ru(III)
and TAA (see SI pages S42-S43 for further details).
In summary, charge transfer in both the linear and angled dyads
seems to be dominated by through-bond (hole) hopping
mechanisms over distances of up to 70 Å. A lack of dependence
of the electron transfer rate on the number of fluorene bridging
units in the angled isomers is attributed to a rate-limiting hole
tunneling step through the spirobifluorene corner. Despite the
possible formation of conformers with relatively short donor-
acceptor contacts as a consequence of the relatively flexible
long fluorene bridges, no evidence for direct through-space or
through-solvent tunneling was observed, presumably due to the
ability of acetonitrile to disrupt the ion-dipole attraction that
would lead to such phenomena and the comparatively large -
values for tunneling through solvent.[2g, 20] Thus, the kinetics of
charge transfer occurring over 40-70 Å can be governed by a
single molecular corner unit. This key insight is relevant for the
Prior studies on linear oligo-fluorene bridged donor-acceptor
systems already reported very shallow distance dependences of
through-bond electron transfer rates.[9a, 9b] However, the distance
dependence of kET
, commonly captured by the so-called
distance decay constant (), is not a bridge-specific property but
rather it is governed by the entire combination of donor, bridge,
and acceptor.[17] It is therefore difficult to make a general
statement about the distance dependence of kET for a given
bridging structure. However, the hopping mechanism identified
previously in some linear fluorene-bridged systems is likely to be
9b]
operative in our dyads as well,[9a,
because the oxidation
potentials of the Ru photosensitizer and the oligo-fluorenes are
very close to one another, ca. 1.3 V vs. SCE in both cases (SI
page S33). Consequently, after photo-generation of [Ru(bpy)3]3+,
hole transfer to the molecular bridge is expected to be possible,
and such a hopping mechanism is in line with the distance
insensitivity of kET observable in the angled series.[9a, 18] However,
for the angled dyads A1 – A3, through-space or through-solvent
charge transfer pathways between TAA and [Ru(bpy)3]3+ should
also be considered, especially in view of the MMFF results
discussed above.
The control compound C1 was specifically designed to probe the
possibility of through-space electron transfer, because it cannot
adopt conformations with short TAA-[Ru(bpy)3]2+ contacts
(Figure S9 on page S41) due to the different lengths of the
donor and acceptor branches (n = 0, m = 2, Scheme 1).
According to MMFF calculations the shortest possible through-
space NTAA-Ru distance in C1 is 19.5 Å. If direct through-space
tunneling from the donor to the acceptor were to contribute
significantly to kET in any of the angled isomers (A1, A2, A3), one
would expect the charge transfer rate to decrease for compound
C1 (see SI page S42 for further details). The opposite
phenomenon is instead observed (kET > 108 s-1 for C1, Table 1
and SI page S39), and thus it seems plausible to conclude that
construction of multi-dimensional electron transfer systems or
[22]
molecular grids
in which individual molecular wires are
connected by spirobifluorene or other corner-forming units.[23]
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Acknowledgements
Financial support by the Swiss National Science Foundation
through grant number 200021_146231/1 is acknowledged. C. B.
L. acknowledges a Swiss Government Excellence Postdoctoral
Scholarship for Foreign Scholars.
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Keywords: electron transfer • time-resolved spectroscopy •
donor-acceptor systems • molecular electronics • charge transfer
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Entry for the Table of Contents
COMMUNICATION
The kinetics of hole hopping over 40-
70 Å are governed by a rate-
Hauke C. Schmidt, Christopher B.
Larsen, Oliver S. Wenger*
determining tunneling step across a
spirobifluorene corner unit. This
finding is relevant for the construction
of multi-dimensional electron transfer
systems or grids in which individual
molecular wires are interlinked at 90°
angles.
Page No. – Page No.
Electron Transfer around a Molecular
Corner
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