Accelerated hole transfer across a molecular double barrier

Article abstract of DOI:10.1039/c0cc01591a

We report on a dyad in which photoinduced hole transfer through a non-uniform molecular double barrier is more than one order of magnitude more rapid than hole transfer across a comparable uniform (rectangular) tunneling barrier.

Full text of DOI:10.1039/c0cc01591a

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Accelerated hole transfer across a molecular double barrierw
David Hanss,a Mathieu E. Walthera and Oliver S. Wenger*b
Received 26th May 2010, Accepted 19th July 2010
DOI: 10.1039/c0cc01591a
We report on a dyad in which photoinduced hole transfer
through a non-uniform molecular double barrier is more than
one order of magnitude more rapid than hole transfer across a
comparable uniform (rectangular) tunneling barrier.
Long-range charge transfer may occur via hopping or tunneling
mechanisms.1 Hopping plays an important role in artificial
molecular wires such as oligo-p-phenylene vinylenes and
DNA.2–8 Tunneling processes occur in less p-conjugated
Scheme 2 Formulae of the molecules investigated herein.
molecular bridges and in many different proteins.1,9,10 Recent
investigations indicate that biological electron transfer may
regularly involve a sequence of tunneling and hopping
processes,1,11,12 in which amino acids with aromatic side
chains serve as hopping stations. Such hopping stations can
also be introduced deliberately into artificial oligo-peptide
bridges in order to accelerate the long-range charge transfer
rates.13 Despite recent work on molecular diodes and molecular
bridges with redox gradients,14 chemists have been mostly
concerned with the two extreme mechanistic cases of one-step
tunneling across uniform (rectangular) barriers and multi-step
hopping.1 In semiconductor physics, tunneling through
so-called double barriers plays a very important role.15,16 In
this work, we created a molecular bridge that imposes such a
double barrier to hole transfer and investigated the charge
transfer kinetics across this molecular bridge.
decrease of the donor–bridge energy gap and a lowering of
the tunneling barrier for holes.
Here, we report on hole transfer between photoexcited
rhenium(I) complexes and PTZ redox partners that are
separated by a combination of p-xylene and methoxybenzene
bridging units (Scheme 2). Dyad 1 contains five identical
p-xylene spacers and serves as
a reference molecule,
whereas dyads 2 and 3 with central p-dimethoxybenzene and
tetramethoxybenzene bridging units are the actual compounds
of interest. Dyad 4 is a reference molecule with the PTZ redox
partner at the same position where the methoxybenzene units
are located in dyads 2 and 3.
Optical absorption spectroscopy shows that the lowest
excited state is the rhenium/phenanthroline MLCT state in
all cases, and the differences between the spectra of the
individual compounds are relatively minor (see ESIw).
Fig. 1a shows the steady-state MLCT luminescence spectra
measured on dichloromethane solutions of the five molecules
from Scheme 2 after excitation at 410 nm, where all samples
had an identical optical density of 0.1. Reference complex 5
shows the strongest emission, followed by dyads 1 and 3. Only
the emissions of dyads 2 and 4 are strongly quenched. The
temporal evolution of the luminescence signals from Fig. 1a
after excitation at 410 nm with 10 ns pulses is shown in Fig. 1b.
In a recent study, we found that phototriggered hole
3+
transfer between photogenerated Ru(bpy)3
(bpy = 2,20-
bipyridine) and phenothiazine (PTZ) occurs three orders of
magnitude more rapidly across a tetra-p-dimethoxybenzene
bridge than across a structurally very similar tetra-p-xylene
spacer (Scheme 1).17 This is due to the fact that p-dimethoxy-
benzene is oxidized at significantly lower potential (1.35 V vs.
SCE) than p-xylene (2.06 V vs. SCE),18 which leads to a
Scheme 1 Hole tunneling across tetra-p-xylene and tetra-p-dimethoxy-
benzene bridges.17
a Department of Inorganic, Analytical and Applied Chemistry,
University of Geneva, 30 quai Ernest-Ansermet, CH-1211, Geneva,
Switzerland
b Institut fur Anorganische Chemie, Georg-August-Universitat
¨
Gottingen, Tammannstr. 4, D-37077, Gottingen, Germany.
¨
¨
¨
E-mail: oliver.wenger@chemie.uni-goettingen.de;
Fax: +49 551 39 3373; Tel: +49 551 39 19424
w Electronic supplementary information (ESI) available: Experimental
procedures, synthetic protocols and product characterization data,
additional spectroscopic and electrochemical data. See DOI: 10.1039/
c0cc01591a
Fig. 1 (a) Steady-state emission of molecules 1–5 in deoxygenated
CH2Cl2 after excitation at 410 nm. (b) Temporal evolution of these
emissions under the same experimental conditions.
c
7034 Chem. Commun., 2010, 46, 7034–7036
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(dyad 1), revealing an exponential drop-off with a distance
decay constant (b) of 0.52 Aꢁ1 20
However, the most important
.
observation is the large kCT-value of dyad 2 with respect to
dyads 1 and 3 despite identical donor–acceptor distances in all
three cases.
A hole transfer rather than electron transfer picture is useful
to understanding charge transfer in our dyads.21,26 While in its
3
long-lived MLCT excited state, rhenium complex 5 (Re) is a
potent oxidant with Ered Z 1.3 vs. SCE,27 whereas the
relevant PTZ reduction potential is B0.8 V vs. SCE (see
ESIw).16 The p-xylene bridge units (xy) are oxidized at
2.06 V vs. SCE.18 Hence, in dyad 1 a large rectangular
tunneling barrier of roughly 0.76 eV height is imposed to hole
transfer across the five p-xylene spacers (Scheme 3).
Fig. 2 (a) Transient absorption spectra of molecules 1–5 measured
under the conditions described in the text. (b) Plot of the charge
transfer rate constants in dyads 1–4. The four open circles stand for a
homologous series of Re–(p-xylene)n–PTZ molecules, and the straight
line is a linear regression fit to these data yielding a distance decay
In dyad 2, the central p-dimethoxybenzene (dmb) unit is
structurally similar to p-xylene, but it is significantly easier to
oxidize. Free p-dimethoxybenzene is oxidized at a potential of
1.35 V vs. SCE,18 which results in a local lowering of the
barrier height to only B0.05 eV. Hence, the overall shape of
the barrier associated with hole transfer from Re to PTZ in
dyad 2 is that of a double barrier (Scheme 3). With an
estimated local barrier height of only B0.05 eV at the central
bridging unit, a two-step hopping mechanism may become
potentially relevant for Re to PTZ hole transfer in dyad 2, but
transient absorption spectroscopy has failed to provide direct
evidence for the dmbꢀ+ radical cation. Moreover, we note that
for two-step hopping, charge transfer in dyad 2 is relatively
slow (kCT = 4.1 ꢂ 106 sꢁ1). Both the hole transfer from Re to
dmb and the hole transfer from dmb to PTZ would involve
tunneling across a bi-p-xylene spacer, and in dyad 4 hole
tunneling across such a bi-p-xylene bridge occurs with
kCT = 5 ꢂ 107 sꢁ1. Therefore, despite the fact that the two
consecutive steps involved in the (hypothetic) hopping
mechanism in dyad 2 would involve different driving-forces,
it appears plausible that charge transfer in this molecule occurs
via (accelerated) one-step tunneling across all five bridging
units. Indeed, to explain the physical origin of the rate
acceleration of the charge transfer process in dyad 2 with
respect to dyad 1, it is not necessary to invoke a hopping
mechanism. According to superexchange theory, the electronic
donor–acceptor coupling (HDA) is a function of donor–
constant (b) of 0.52 Aꢁ1 20,21
.
The time-resolved data show exactly the same trend with
respect to emission quenching as the steady-state spectra: weak
MLCT excited-state quenching for dyads 1 and 3, intermediate
quenching for dyad 2, and strong quenching for dyad 4.
Luminescence quenching of rhenium(I) tricarbonyl diimines
by PTZ has long been known to occur by electron transfer,
whereas energy transfer can be excluded on thermodynamic
grounds.19 Direct evidence for an electron transfer product is
provided by Fig. 2a, which shows transient absorption spectra
measured in a 5 ms time window starting 4 ms after pulsed
excitation at 410 nm of the molecules from Scheme 2.
Dyads 1–4 all exhibit a nearly identical transient absorption
+
spectrum, which can be attributed to the PTZꢀ radical
cation.22 Of key interest in this study are the rates with which
the charge transfer processes occur. The preferred method for
determining these rates is usually to monitor the build-up of
the transient absorption signals associated with the charge
transfer products.17 Unfortunately, this method is technically
not feasible in our case: in the accessible visible spectral range
only the phenothiazine radical cation has a clear spectroscopic
signature, but it coincides spectrally with the emission
originating from the rhenium complex. Thus, we are limited
to determining the sought-after rate constants indirectly from
luminescence decay data, a method that has been applied
successfully in many different previous instances.23–25 In
this method, the rate constant for charge transfer (kCT) is
approximated by:23–25
bridge (hDb), bridge–bridge (hbb), and bridge–acceptor (hbA
)
ꢁ1
ꢁ1
kCT = tdyad ꢁ tdyad
(1)
In deoxygenated dichloromethane solution, the luminescence
of complex 5 decays with a lifetime of treference = 2.90 ms.
Under the same conditions (data in Fig. 1b), tdyad = 2.50 ms
for dyad 1, 0.24 ms for dyad 2, 2.48 ms for dyad 3 and 0.02 ms
for dyad 4. The calculated kCT-values for the individual
compounds from Scheme 2 are displayed graphically in
Fig. 2b along with two data points that originate from a prior
investigation.20 The four open circles mark kCT-values for
rhenium–(p-xylene)n–phenothiazine molecules with bridge
lengths (n) varying from two (dyad 4) to five p-xylene units
Scheme 3 Energy level diagrams for hole transfer from the photo-
excited rhenium complex to PTZ through the molecular bridges
of dyads 1–3 (xy = p-xylene; dmb = p-dimethoxybenzene; tmb =
1,2,4,5-tetramethoxybenzene).
c
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couplings, as well as the so-called tunneling energy gap De that
Notes and references
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ꢀ
ꢁ
nꢁ1
hDb hbb
HDA
¼
hbA
ð2Þ
De De
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contacts since the expected equilibrium torsion angles between
these para-disubstituted units are similar. However, at the
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In dyad 3, the overall Re to PTZ hole transfer is
equally slow as in dyad 1 despite the presence of a central
tetramethoxybenzene (tmb) unit. Free tmb is oxidized at an
electrochemical potential of 0.81 V vs. SCE (Scheme 3 and
ESIw),18 and therefore our initial expectation was that this
would permit Re to tmb hole transfer and enable an efficient
two-step hopping process for the overall Re to PTZ charge
transfer. However, the slow MLCT quenching rate observed
for this dyad indicates that our expectation is not fulfilled.
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Funding from the Swiss National Science foundation
through grant number PP002-110611 is acknowledged.
c
7036 Chem. Commun., 2010, 46, 7034–7036
This journal is The Royal Society of Chemistry 2010