Controlling electron transfer in donor-bridge-acceptor molecules using cross-conjugated bridges

Article abstract of DOI:10.1021/ja107420a

Photoinitiated charge separation (CS) and recombination (CR) in a series of donor-bridge-acceptor (D-B-A) molecules with cross-conjugated, linearly conjugated, and saturated bridges have been compared and contrasted using time-resolved spectroscopy. The photoexcited charge transfer state of 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) is the donor, and naphthalene-1,8:4,5-bis(dicarboximide) (NI) is the acceptor in all cases, along with 1,1-diphenylethene, trans-stilbene, diphenylmethane, and xanthone bridges. Photoinitiated CS through the cross-conjugated 1,1-diphenylethene bridge is about 30 times slower than through its linearly conjugated trans-stilbene counterpart and is comparable to that observed through the diphenylmethane bridge. This result implies that cross-conjugation strongly decreases the π orbital contribution to the donor-acceptor electronic coupling so that electron transfer most likely uses the bridge σ system as its primary CS pathway. In contrast, the CS rate through the cross-conjugated xanthone bridge is comparable to that observed through the linearly conjugated trans-stilbene bridge. Molecular conductance calculations on these bridges show that cross-conjugation results in quantum interference effects that greatly alter the through-bridge donor-acceptor electronic coupling as a function of charge injection energy. These calculations display trends that agree well with the observed trends in the electron transfer rates.

Full text of DOI:10.1021/ja107420a

Published on Web 10/13/2010
Controlling Electron Transfer in Donor-Bridge-Acceptor
Molecules Using Cross-Conjugated Bridges
Annie Butler Ricks, Gemma C. Solomon, Michael T. Colvin, Amy M. Scott,
Kun Chen, Mark A. Ratner,* and Michael R. Wasielewski*
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern UniVersity, EVanston, Illinois 60208-3113, United States
Received August 17, 2010; E-mail: m-wasielewski@northwestern.edu
Abstract: Photoinitiated charge separation (CS) and recombination (CR) in a series of donor-bridge-acceptor
(D-B-A) molecules with cross-conjugated, linearly conjugated, and saturated bridges have been compared
and contrasted using time-resolved spectroscopy. The photoexcited charge transfer state of 3,5-dimethyl-
4-(9-anthracenyl)julolidine (DMJ-An) is the donor, and naphthalene-1,8:4,5-bis(dicarboximide) (NI) is the
acceptor in all cases, along with 1,1-diphenylethene, trans-stilbene, diphenylmethane, and xanthone bridges.
Photoinitiated CS through the cross-conjugated 1,1-diphenylethene bridge is about 30 times slower than
through its linearly conjugated trans-stilbene counterpart and is comparable to that observed through the
diphenylmethane bridge. This result implies that cross-conjugation strongly decreases the π orbital
contribution to the donor-acceptor electronic coupling so that electron transfer most likely uses the bridge
σ system as its primary CS pathway. In contrast, the CS rate through the cross-conjugated xanthone bridge
is comparable to that observed through the linearly conjugated trans-stilbene bridge. Molecular conductance
calculations on these bridges show that cross-conjugation results in quantum interference effects that greatly
alter the through-bridge donor-acceptor electronic coupling as a function of charge injection energy. These
calculations display trends that agree well with the observed trends in the electron transfer rates.
contacts.9,10 In a formal sense, the two approaches are related
Introduction
in that the D and A molecules in the D-B-A systems serve a
similar role as that of the metallic contacts in the conductance
measurements.
D-B-A systems provide a means of systematically studying
electron transfer through bridge molecules such as DNA base
pairs,11-13 peptides in proteins,14 porphyrins,15,16 saturated
alkane σ-systems,17,18 and unsaturated π-conjugated alkene,
alkyne, and aromatic spacers.19-27 In order to design D-B-A
systems with predictable electron transfer properties, it is
The ability to promote and control charge transport over long
distances is essential to the development of solar energy
conversion systems and molecule-based electronics.1-6 Photo-
initiated multistep electron transfer in photosynthetic reaction
center proteins, where charge is transferred across about 4 nm
with near unity efficiency, is an important model for designing
molecular donor-bridge-acceptor (D-B-A) systems to achieve
efficient, long-lived, long-distance charge separation.7,8 The
electron transfer characteristics of D-B-A systems are usually
determined by time-resolved spectroscopic measurements of
photoinitiated charge separation (CS) leading to D+•-B-A-•
radical ion pairs (RPs) that subsequently undergo dark charge
recombination (CR). In comparison, studies of charge transport
in wire-like molecules for molecular electronics often focus on
the dark conductance of bridge molecules that link two metallic
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J. AM. CHEM. SOC. 2010, 132, 15427–15434 15427

A R T I C L E S
Ricks et al.
essential to understand how the electronic structure and com-
position of the bridge plays a role in governing the rates of
electron transfer. Electron transfer in D-B-A systems occurs
most often by a superexchange mechanism involving virtual
bridge states.19,28-31 Qualitatively, the superexchange mecha-
nism results in three observed trends in electron transfer rates:
(1) increasing bridge length decreases the rate; (2) charge
transport through a fully conjugated bridge is faster than through
a saturated bridge; (3) a larger energy gap between the starting
state of the charge transfer process and the relevant virtual bridge
states leads to decreased electron transfer rates.32
ways.33 The ꢀ value is affected by bridge length34,35 and
conformational rigidity,36 as well as the electronic properties
of the donor and acceptor.22,36
Recently, theory has predicted that certain molecules will have
quantum interference effects that will strongly influence electron
transfer rates. Cross-conjugated molecules are one such class
of compounds where it is thought that these interference effects
will be large.32 Cross-conjugation is defined as “a compound
possessing three unsaturated groups, two of which although
conjugated to a third unsaturated center are not conjugated to
each other. The word conjugated is defined here in the classical
sense of denoting a system of alternating single and double
bonds.”37 Although the term cross-conjugation is used infre-
quently, molecules that exhibit this particular type of conjugation
are common in chemistry, e.g., quinones, radialenes, fulvalenes,
and various fused aromatics.38 The effects of cross-conjugation
on charge transfer states in molecular systems have been
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magnetic materials,41,42 as well as donor-acceptor interac-
tions.23,43,44 In addition, many oligomers and polymers with
cross-conjugated frameworks have recently been synthesized
for advanced materials.38,45,46
In more quantitative terms, through-bond electron transfer
rate constants most often decay exponentially with donor-
acceptor distance as described by eq 1
k ) k0e-ꢀ(r-r )
(1)
0
where k0 is the rate constant at the van der Waals contact
distance r0 (3.5 Å), and ꢀ is an exponential damping factor.
This behavior is characteristic of the superexchange mechanism
in which the ꢀ values depend on both the electronic coupling
matrix element for the charge transfer process, VDA, as well as
Theoretical work has examined the relationship between
charge transfer and charge transport47 in D-B-A molecules
in which D and A are attached to metallic contacts. This work
has shown that it is possible to separate the contributions from
the metallic contacts in molecular conductance measurements
from the intrinsic charge transfer rates from D to A, revealing
the underlying commonality: the electronic coupling through
the bridge. In this way, it can be shown that molecular
conductance is approximately proportional to the electron
transfer rate, with the contributions from the metallic contacts
and the donor and acceptor scaling the result and preventing a
direct equality.
the energy gap for charge injection from the donor to the virtual
28,29
bridge state, ∆EDB
,
N-1
VDBVBA VBB
VDA
)
(2)
(
)
∆EDB ∆EDB
where ꢀ is described by
∆EDB
2
r
ꢀ ) ln
(3)
(
)
VBB
8e2
π2Γ(DL)Γ(DR)
and where VDB and VBA are the matrix elements that couple the
donor to the bridge and the bridge to the acceptor, respectively,
VBB is the electronic coupling between bridge sites, N is the
number of identical bridge sites, and r is the length of one bridge
segment. Equation 2 is approximate and does not take into
account non-nearest neighbor interactions and multiple path-
g ≈
kDfA
(4)
F
Here g is the conductance, e is the charge on an electron, Γ is
the influence of the leads in perturbing the bridging molecule,
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15428 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010

Electron Transfer in Donor-Bridge-Acceptor Molecules
A R T I C L E S
Figure 1. Chemical structures of the molecules used in this study.
F is the thermally averaged Franck-Condon weighted density
of nuclear states of the donor and acceptor, and kDfA is the
rate. This result leads to the supposition that there may be
circumstances where a ratio of charge transfer rates shows the
same trends as ratios of conductance or charge transport. The
extent to which such a relationship will quantitatively hold is
limited by a number of factors. Equation 4 is derived under a
number of assumptions that should be noted at this point. First,
this relationship was derived for the case in which the full
D-B-A system is bound between metallic electrodes, rather
than binding a bridge directly to electrodes as is more common
in molecular electronics experiments. Second, the relationship
is derived with the assumption that coherent tunneling (super-
exchange) dominates both electron transfer and transport.
Finally, although not an assumption of the derivation, for a
comparison between the different bridges to hold, the influence
of the electrodes and the donor and acceptor must be equal in
both cases to ensure that the prefactors cancel. Comparing
electron transport theory with electron transfer experiments
seems unlikely to yield a quantitative comparison. However,
the relationship in eq 4 provides a basis for a qualitative
comparison, and we expect to see the trends observed in the
electron transfer rates through the series of bridges reflected in
the trends for electron transport.
Although a metallic electrode is chemically distinct from
molecular donor and acceptor components, the results show that
there is significant common ground between the two contexts.
Experimental Section
Synthesis and Steady-State Spectroscopy. The synthesis and
characterization of 1-4 are described in the Supporting Information.
All reagents were purchased from Sigma-Aldrich and used as
received. All final products were purified by normal-phase prepara-
tive thin layer chromatography prior to characterization. All solvents
were spectrophotometric grade or distilled prior to use. Intermediates
1
and the resulting products were characterized by H NMR, HR-
MS, and UV-vis spectroscopy.
Steady-state absorption spectroscopy was performed using a
Shimadzu (UV-1601) spectrophotometer. All solvents were spec-
troscopic grade and used as is, except for tetrahydrofuran (THF),
which was further purified by passing it twice through alumina
(GlassContour) immediately prior to use.
Energy Levels and Molecular Structures. Given that photo-
excitation of DMJ-An results in quantitative subpicosecond charge
separation to produce DMJ+•-An-• with a spectroscopically
determined energy of 2.89 eV in toluene,49 the energy levels for
the charge separated states, DMJ+•-An-B-NI-•, DMJ-An-
B+•-NI-•, and DMJ+•-An-B-•-NI were determined in toluene
using eq 5
In this study, we report the results of our investigation into
photoinduced electron transfer in a D-B-A system where
different bridges are used to probe the effects of cross-
conjugation on CS and CR rates. These rates are compared with
the predictions of molecular conductance calculations for the
same bridges. The 3,5-dimethyl-4-(9-anthracenyl)julolidine
(DMJ-An) donor and the naphthalene-1,8:4,5-bis(dicarboxim-
ide) (NI) acceptor are linked using one of four different bridges
(Figure 1). In molecule 1, the 1,1-diphenylethene bridge is cross-
conjugated, since both phenyls are linearly conjugated to the
double bond, but not to each other. In molecule 2, the trans-
stilbene bridge is linearly conjugated,48 while in 3, the diphe-
nylmethane bridge possesses a saturated CH2 group that breaks
the direct conjugated pathway between the phenyls. In molecule
4, the xanthone bridge is also cross-conjugated, since both
phenyls are linearly conjugated to the ketone, but not to each
other. The xanthone bridge tests how substituent effects, as
represented by the oxygen atom of the carbonyl group, alter
charge transport through a cross-conjugated bridge.
e2
εs r1
1
1
rF
∆GF ) ∆GI + sign(EI - EF) +
-
(5)
(
)
where ∆GI and ∆GF are the energies above ground state for the
initial and final ion pairs, respectively, EI and EF are the redox
potentials for the initial and final ions, respectively, between which
the electron is transferred, rI and rF are the initial and final ion pair
distances, respectively, e is the electronic charge, and εs is the static
dielectric constant of the solvent (εs ) 2.38 for toluene), and the
sign ) (-) if EF > EI and the sign ) (+) if EI > EF. The distances
rI and rF between the donor, bridge, and acceptor components were
determined from the energy-minimized structures of DMJ-An-
B-NI determined with density functional theory (DFT) using
Becke’s50 three-parameter hybrid functional with Lee, Yang, and
Parr51 correction functional (B3LYP) and the STO-3G basis set.
The redox potentials of DMJ, An, the bridges, and NI are given in
the Supporting Information, and the ion pair distances and energies
are listed in Table S1 and illustrated in Figure S1 of the Supporting
Information.
The trends predicted by the molecular conductance calcula-
tions are clearly reflected in the observed CS and CR rates.
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9
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A R T I C L E S
Ricks et al.
Transient Absorption Spectroscopy. Femtosecond transient
absorption measurements were made using the 416-nm frequency-
doubled output from a regeneratively amplified titanium:sapphire
laser system operating at 2 kHz.21 A white light continuum probe
pulse was generated by focusing the IR fundamental into a 1-mm
sapphire disk.52 Detection with a CCD spectrograph has previously
been described.52 The optical density of all samples was maintained
between 0.3 and 0.5 at 416 nm, and the samples were placed in a
2 mm path length quartz cuvette equipped with a vacuum adapter
and subjected to five freeze-pump-thaw degassing cycles prior
to transient absorption measurements. The samples were irradiated
with 1.0 µJ per pulse focused to a 200 µm spot. Typically, 5 to 7 s
of averaging was used to obtain the transient spectrum at a given
delay time. The total instrument response function (IRF) for the
pump-probe experiments was 180 fs. Transient absorption kinetics
were determined at a given wavelength by using a nonlinear least-
squares fit to a general sum of exponentials, using the Levenberg-
Marquardt algorithm, while accounting for the presence of the finite
instrument response.
Samples for nanosecond transient absorption spectroscopy were
placed in a 10 mm path length quartz cuvette and freeze-pump-
thawed five times. The samples were excited with 7 ns, 2.5 mJ,
416 nm using the frequency-tripled output of a Continuum Precision
II 8000 Nd:YAG laser to pump a Continuum Panther OPO. The
excitation pulse was directed to a 5 mm diameter spot and matched
to the diameter of the probe pulse generated using a xenon flashlamp
(EG&G Electro-Optics FX-200). Kinetic traces were detected from
430-800 nm every 5 nm using a monochromator and photomul-
tiplier tube with high voltage applied to only four dynodes
(Hamamatsu R928). The total instrument response time is 7 ns and
is determined primarily by the laser pulse duration. Analysis of
the kinetic data was performed at multiple wavelengths using a
Levenberg-Marquardt nonlinear least-squares fit to a general sum-
of-exponentials function with an added Gaussian function to account
for the finite instrument response.
Hz, QuantaRay Lab-150). The polarization of the laser was set to
54.7° relative to the direction of the static magnetic field to avoid
magnetophotoselection effects on the spectra. Following photoex-
citation, kinetic traces of the transient magnetization were ac-
cumulated under CW microwave irradiation (6-20 mW). The field
modulation was disabled to achieve a Q/πν ≈ 30 ns instrument
response function (IRF), where Q is the quality factor of the
resonator and ν is the resonant frequency, while microwave signals
in emission (e) and/or enhanced absorption (a) were detected in
both the real and the imaginary channels (quadrature detection).
Sweeping the magnetic field gave 2D spectra versus both time and
magnetic field. For each kinetic trace, the signal acquired prior to
the laser pulse was subtracted from the data. Kinetic traces recorded
at magnetic field values off-resonance were considered background
signals, whose average was subtracted from all kinetic traces. The
spectra were subsequently phased into a Lorentzian part and a
dispersive part, and the former, also known as the imaginary
magnetic susceptibility ꢁ”, is presented.
Transport Calculations. In the coherent tunneling limit the
conductance through a molecule is frequently calculated with what
is known as the Landauer approach. At this level the current through
a molecule is defined as:
∞
2e
h
I(V) )
-∞ dE Tr[T(E, V)](fL(E) - fR(E))
(6)
∫
where e is the charge on an electron, h is Planck’s constant, T(E,V)
is the transmission through the molecule, and fL and fR are the Fermi
functions for the two electrodes. The Fermi functions define the
occupation of the electrodes and set the energy window over which
transport occurs at a given voltage. The differential conductance is
given in eq 7.
dI(V)
dV
g(V) )
(7)
For the magnetic field effect experiments, the sample cuvette
was placed between the poles of a Walker Scientific HV-4W
electromagnet powered by a Walker Magnion HS-735 power
supply, and the field strength was measured by a Lakeshore
gaussmeter with a Hall effect probe. The electromagnet and
gaussmeter were interfaced with Labview, allowing measurements
and control of the magnetic field to (1 × 10-5 T during the data
acquisition. To maintain sample integrity during the experiment, a
probe light shutter was used to block the sample from irradiation
when transient absorption kinetics were not being collected. The
triplet yield was monitored at 480 and 430 nm and kinetic traces
were collected in increments of 0.3, 1.5, or 5.0 mT with zero field
∆A(B ) 0) collection after four or five steps. To compensate for
possible sample degradation, zero field kinetics were collected
during the experiment in four- or five-step increments and plotted
and fit with polynomial or linear trend lines. These functions were
used to calculate the relative RP yield or triplet yield as a function
of applied field strength (B) and plotted as ∆A(B)/∆A(B ) 0). The
results presented are an average of three or more experiments
conducted on separate days with freshly prepared samples in
spectrophotometric or freshly distilled ACS-grade toluene.
Time-Resolved EPR Measurements. Toluene solutions of 1,
3, and 4 (∼10-4 M) were loaded into quartz tubes (4 mm o.d. × 2
mm i.d.), subjected to five freeze-pump-thaw degassing cycles
on a vacuum line (10-4 mBar), and sealed using a hydrogen torch.
Time-resolved EPR (TREPR) measurements were made at X-band
using continuous wave (CW) microwaves and direct detection on
a Bruker E-580 spectrometer. Sample temperatures were controlled
by an Oxford Instruments CF935 cryostat using liquid N2. Samples
were photoexcited at 416 nm using the output from a frequency-
tripled, H2-Raman shifted Nd:YAG laser (1-2 mJ/pulse, 7 ns, 10
As the applied bias approaches zero, the differential conductance
can be approximated as the transmission at the (now common)
Fermi energy of the two electrodes:
g(0) ≈ T(EF)
(8)
In order to understand the relationship in eq 4 we assume that the
transmission at some unknown energies can be related to the
observed rates for charge separation and recombination. Conse-
quently, the electronic transmission will be plotted over a substantial
energy range, and the trends across the bridges will be examined
for energy windows in which the trends match the observed rates.
All transport calculations were performed for the bridges with
thiol groups substituted in the place of the donor and acceptor
(designated 1S, 2S, 3S, and 4S), to allow binding to gold electrodes.
Geometries for thiol-substituted bridges were optimized with
Qchem3.053 using density functional theory with B3LYP and
6-311G**. The thiol hydrogen atoms were removed, and these
systems were bound to planar gold electrodes with gold-sulfur
distances taken from the literature.54 The transport calculations were
performed using gDFTB.55-58 In the cases where the dependence
(53) Shao, Y.; et al. Phys. Chem. Chem. Phys. 2006, 8, 3172–3191.
(54) Bilic, A.; Reimers, J. R.; Hush, N. S. J. Chem. Phys. 2005, 122,
094708-094715.
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Frauenheim, T.; Suhai, S.; Seifert, G. Phys. ReV. B 1998, 58, 7260–
7268.
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Porezag, D.; Suhai, S.; Scholz, R. Phys. Status Solidi B 2000, 217,
41–62.
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Amkreutz, M.; Sternberg, M.; Hajnal, Z.; Di Carlo, A.; Suhai, S. J.
Phys.: Condens. Matter 2002, 14, 3015–3047.
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2002, 124, 8530–8531.
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9
15430 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010

Electron Transfer in Donor-Bridge-Acceptor Molecules
A R T I C L E S
Figure 3. Normalized ground-state absorption spectra of 1-4.
Figure 2. General synthetic scheme for 1-4. (a) N-(2,5-di-tert-butylphe-
nyl)-naphthalene-1,8-dicarboxyanhydride-4,5-dicarboximide, pyridine reflux;
(b) bis(pinacolato)diboron, DMF, KOAc, PdCl2(dppf), 75 °C; (c) 3,5-
dimethyl-4-(10-bromoanthracenyl)julolidine, Pd2(dba)3, P(o-tol)3, Ag2O,
THF, 70 °C.
of the transmission on the molecular conformation was investigated,
the optimized structures were modified by varying the dihedral as
noted, but not reoptimized in each step.
Figure 4. Transient absorption spectra of 1 in toluene at 293 K at the
indicated times following excitation. Inset: transient absorption kinetics at
480 nm.
Results
which indicates that in each case the bridge is weakly electroni-
cally coupled to the donor and acceptor.
Synthesis and Steady-State Spectroscopy. The syntheses of
compounds 1-4 are summarized in Figure 2, and the details
are given in the Supporting Information. Briefly, the bridges
are synthesized with one bromo and one amino substituent at
the appropriate positions. Each bridge is then reacted with
N-(2,5-di-tert-butylphenyl)naphthalene-1,8-dicarboxyanhydride-
4,5-dicarboximide59 in pyridine to form Br-bridge-NI. The
bromo group is then converted into a boronic ester using
bis(pinacolato)diboron and a Pd catalyst in toluene, and the
resulting boronic ester is linked to DMJ-An-Br using Pd-
catalyzed Suzuki coupling to give compounds 1-4.
The ground-state absorption spectrum of DMJ-An in toluene
exhibits a broad CT absorption maximum at 367 nm with a
broad emission maximum at 519 nm, resulting in an excited
singlet CT state energy of 2.89 eV.49,60 The ground-state
electronic absorption spectra of compounds 1-4 (Figure 3) have
maxima at 360 nm, 380 nm, and 398 nm, with the prominent
vibronic structure resulting from overlapping contributions of
the DMJ-An charge transfer (CT) absorption49 and the NI
acceptor Franck-Condon progressions.4 Additionally, for 2,
there is a broad feature between 300 and 350 nm due to the
trans-stilbene bridge. The remainder of the bridges absorb
primarily to the blue of 300 nm.48,61,62 As the bridge is changed,
no significant changes are observed in the absorption spectrum,
Transient Absorption Spectroscopy. Femtosecond transient
absorption measurements were performed in toluene on mol-
ecules 1-4 using 130-fs, 416-nm pulses to selectively excite
the DMJ-An CT state. The transient absorption spectrum for
1 is shown in Figure 4, while the spectra for 2-4 are shown in
Figure S2, Supporting Information. At early times, the broad
63
absorption features centered at 490 nm (ε ) 4500 M-1cm-1
)
and 680 nm (ε ) 10 000 M-1cm-1) are attributed to DMJ+•
and An-•, respectively. As the final step of the charge separation
occurs for 1-4, DMJ+•-An-•-B-NI f DMJ+•-An-B-NI-•,
the formation of prominent absorption bands due to NI-• are
distinctly seen at 480 and 610 nm (ε480 ) 25 500 M-1cm-1).64
No large spectral differences were observed between the
molecules in this series, but the charge separation times (τCS)
differ significantly. The rise of NI-• was monitored at 480 nm,
giving the τCS values for 1-4 shown in Table 1.
Nanosecond transient absorption measurements were per-
formed in toluene using 7-ns, 416-nm pulses to measure the
charge recombination times (τCR) through the various bridges
for molecules 1-4. Formation of the NI-• absorption bands at
480 and 610 nm was again observed, and the values of τCR were
determined at these wavelengths (Figure S3, Supporting Infor-
mation). The absorption features at later times in the 400 to
500 nm range are attributed to 3*An, which is generated upon
charge recombination. The charge recombination times are listed
in Table 1.
(59) Carmieli, R.; Mi, Q.; Butler Ricks, A.; Giacobbe, E. M.; Mickley,
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Phys. Chem. Lett. 2010, 1, 215–218.
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9
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A R T I C L E S
Ricks et al.
Table 1. Charge Separation (τCS) and Recombination Times (τCR
)
The dihedral angle was varied from 0° to 90° for the cases of
1S, 2S, and 3S and from 0° to 10° in the case of 4S. Changing
the dihedral angle in 4S stretches the C-O-C bonds that tether
the rings, and consequently the structure is much more restrict-
ed. The dihedral angles that are energetically allowed tend to
vary only 5-10° from kT at room temperature, suggesting that
the dihedral angle is not changing significantly.
Measured by Transient Absorption, and the Spin-Spin Exchange
Interaction, 2J, Measured by MFE and TREPR
compound
τCS (ps)
τCR (ns)
2J (mT)
1
2
3
4
395 ( 10
14 ( 1
460 ( 6
6 ( 1
1044 ( 36
107 ( 5
287 ( 18
155 ( 3
0.4 ( 0.2a
29.0 ( 1b
2 ( 0.5a
3.5 ( 0.5a b
,
The transmission through the thiol-substituted bridges as a
function of dihedral angle was calculated in order to determine
how the change in dihedral angle affects the transmission (Figure
8). The linearly conjugated system 2S shows the largest
dependence on dihedral angle, while 3S and 4S show the least
dependence on dihedral angle. However, the dihedral angle of
4S cannot be greatly varied due to the tethering of the rings, as
mentioned previously. The cross-conjugated system, 1S, never
approaches the high transmission of 2S even when the dihedral
approaches 0°.
a Measured by TREPR. b Measured by MFE.
Discussion
The photoinduced charge transfer dynamics of 1-4 are highly
dependent on the bridge conjugation. Charge transfer through
the cross-conjugated 1,1-diphenylethene bridge in 1 is nearly
30 times slower than through its linearly conjugated trans-
stilbene counterpart in 2. According to the transmission spectra,
interference in cross-conjugated molecules suppresses π system
transport, thereby revealing the much slower σ system transport.
Therefore, it is expected that 1 should have a rate similar to a
D-B-A system that uses a σ-system bridge, such as compound
3. This holds true, as τCS is slightly faster in 1 versus 3. This
slight increase in rate for 1 could be due to multiple factors
such as either inelastic transport or the partial π conjugation
that exists in cross-conjugation. This is counterbalanced some-
what by direct π-π overlap between the phenyls in diphenyl-
methane. However, what is most important is the similarity in
τCS for 1 and 3, which strongly suggests that the σ system is
the primary transfer pathway through the cross-conjugated
bridge in 1.
Figure 5. TREPR spectra of 1, 3, and 4 in toluene at 295 K, following a
7 ns, 416 nm laser pulse. Simulations are shown in red.
Figure 6. The transmission through the thiol-substituted bridges bound
between metallic electrodes. Over a large range the transmission through
1S and 3S is significantly lower than that through 2S and 4S.
Bridge 4 provides another example of a cross-conjugated
bridge; however, the cross-conjugated component is a ketone
rather than an ethylene group. The transmission curve of 4
strongly implies that the CS and CR times should be faster than
1, and more like that of a linearly conjugated molecule. This is
borne out by the data which show that both τCS and τCR are
similar for 2 and 4. This result may seem counterintuitive, but
previous transmission curve calculations have shown that having
an electron-withdrawing substituent directly bonded to the
double bond shifts the energy of the interference feature and
hence changes the conductance significantly.66,67
The charge recombination rates do not follow the same trend
observed for charge separation. In order to better understand
this difference, the magnitude of the magnetic spin-spin
exchange interaction, 2J, was obtained using either TREPR
spectroscopy on the RPs or MFE measurements on the yield of
3*An resulting from CR, both of which have been described
previously.65,68 As mentioned above, the electronic coupling,
VDA, gives the effective interaction energy between the donor
Time-Resolved EPR Measurements. We employed TREPR
to obtain the spin-spin exchange interaction (2J) in the
photogenerated RPs within 1, 3, and 4. Following charge
separation at 295 K, we observed an intense spin-polarized RP
signal (Figure 5). The spectra were fit with the model developed
by Till and Hore,65 using the hyperfine coupling constants
calculated by DFT for (DMJ+•-An) and measured by EPR for
NI-• (see Supporting Information). The 2J values obtained from
the fits are presented in Table 1. The 2J value for 2 was
determined directly from magnetic field effects (MFEs) on the
3*An yield formed upon charge recombination because 2J for 2
is too large to be measured accurately using the spin-polarized
RP signal obtained by TREPR.
Transport Calculations. The electronic transmission through
the thiol-substituted bridges shows a considerable difference in
the magnitude of the transmission between 2S and 4S and the
much lower 1S and 3S (Figure 6). The relationship between 1S
and 3S is more involved, with 1S lower over a limited energy
range near the Fermi energy and 3S lower as the system
approaches the resonances.
(66) Solomon, G. C.; Andrews, D. Q.; Van Duyne, R. R.; Ratner, M. A.
ChemPhysChem 2009, 10, 257–264.
The energy of the isolated molecules as a function of the
dihedral angle was calculated in order to determine how much
conformational flexibility is energetically feasible (Figure 7).
(67) Andrews, D. Q.; Solomon, G. C.; Van Duyne, R. P.; Ratner, M. A.
J. Am. Chem. Soc. 2008, 130, 17309–17319.
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E. M.; Colvin, M. T.; Wasielewski, M. R. J. Am. Chem. Soc. 2009,
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15432 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010

Electron Transfer in Donor-Bridge-Acceptor Molecules
A R T I C L E S
Figure 7. The energy of the thiol-substituted bridges, above the geometrically optimized structures, as a function of the dihedral between the phenyl rings
and the bridging unit. In each case the dashed horizontal line indicates kT at room temperature and a vertical line indicates the dihedral in the optimized
structures. In the case of 4S only a very limited range is shown, as this dihedral change is stretching the C-O-C bonds tethering the rings together.
Figure 8. The transmission through the thiol-substituted bridges at a range of dihedral angles, showing the extent to which the electronic coupling is
perturbed by any of this kind of conformational flexibility in the systems. The dashed vertical line indicates the Fermi energy in the calculations.
and acceptor, which will influence the charge separation and
the CR lifetimes to increase as 2J decreases, which is the trend
observed in this series. Molecule 1 has the smallest 2J value
and has the longest CR time, and consequently we expect VDA
to be the smallest for 1. In contrast, molecule 2 has the largest
2J value and the shortest CR time, and we expect VDA to be
large, as the bridge is fully linearly conjugated. While the CR
lifetime trend does not follow that of the CS lifetimes, it does
follow what is expected based on 2J values.
2
recombination rates. Since 2J ∝ VDA and depends on orbital
overlap,69,70 variations in 2J yield information on the depen-
dence of VDA on bridge structure. Since the bridges in 1-4 have
some torsional flexibility about their single bonds, their equi-
librium geometries pre- and postcharge separation may be
different. Since orbital overlap depends on this geometry, it is
likely that VDA for charge separation and recombination are
different. As 2J is only measured for the CR reaction, it only
yields information related to CR, and not CS. It is common for
The results from the electron transport calculations show
energy regions where the trends observed in the CS and CR
times are reflected in the magnitude of the transmission; but
any sort of quantitative agreement is poor. Above the Fermi
energy, but below the energy at which the resonant transport
peaks occur, in the range of -3.5 to -4.0 eV, the trends
(69) Kobori, Y.; Sekiguchi, S.; Akiyama, K.; Tero-Kubota, S. J. Phys.
Chem. A 1999, 103, 5416–5424.
(70) Paddon-Row, M. N.; Shephard, M. J. J. Phys. Chem. A 2002, 106,
2935–2944.
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A R T I C L E S
Ricks et al.
observed in the CS rates are somewhat evident. In this region,
1S and 3S have much lower transmission than 2S and 4S,
although the transmission through 4S never exceeds 2S in these
calculations. Closer to the Fermi energy, in the range between
-5.5 and -4.5 eV, the transmission through 1S drops signifi-
cantly and the trends in the rates of CR are reflected in the
transmission.
Bridges 1, 2, and 3 have more rotational freedom than bridge
4. This is of concern, as the calculated electronic transmission
curves (Figure 6) neglect the role that conformational fluctua-
tions can have in changing the coupling through a molecule by
only taking into account the minimum energy structures for the
bridges. Further, in addition to the topological differences in
the bonding between 1 and 2, there are also significant
differences in the planarity of the molecules, which is a factor
that has direct bearing on the electronic coupling in conjugated
systems.71,72 The effects of twisting in the structure, as well as
a sense of the conformational flexibility at room temperature,
was assessed by examining the bridges as the dihedral angle
between the phenyl rings and the central component of the
bridge is varied. As expected, there is very little conformational
flexibility in 4S compared with the other structures, while 3S
showed the greatest range of accessible motion.
Conformational flexibility influences electron transfer and
transport rates by modulating the electronic coupling through
the system. The transmission through the thiol-substituted
bridges as a function of dihedral angle was also determined
(Figure 8). The combination of these two results gives insight
into one aspect that is expected to lead to a lack of agreement
between the calculations and measured rates. The linearly
conjugated system 2S shows the largest dependence on dihedral
angle, which is expected for a system where the π-system
dominates the transport. Any deviation from the optimal planar
structure results in a decrease in the transmission and therefore
the conductance. This increased flexibility may contribute to
the fact that the observed CS rate is faster in 4 than in 2, whereas
there is higher calculated transmission through 2S compared
with 4S across a large energy range.
two cases arise as the decreasing dihedral angle shifts the
proximate resonance closer to the Fermi energy in the case of
1S, whereas it decreases the magnitude of the resonant transmis-
sion in 3S. In both cases, however, the conformationally
averaged transmission would not be expected to shift so far from
the minimum energy value as it would in 2S. It is also interesting
to note that while 3S has the largest conformational flexibility,
this does not result in a significant change in the transport, as
the σ system mediates transport through the saturated central
part of the bridge and is significantly less sensitive to confor-
mational changes.
Conclusions
We have prepared a series of molecules to test the effect of
cross-conjugated bridges on charge transfer in D-B-A systems.
We found that charge transfer through a cross-conjugated 1,1-
diphenylethene bridge occurs about 30 times slower through
its linearly conjugated counterpart trans-stilbene. Comparisons
of the charge transfer rates obtained for the 1,1-diphenylethene
bridge with that of a diphenylmethane bridge shows that the
former most likely uses its σ system as its primary charge
transfer pathway. The data obtained for the cross-conjugated
xanthone bridge illustrates how substituent effects can shift the
energy of the cross-conjugation antiresonance and thereby
control the electron transfer rate through the bridge. Computa-
tions allow us to qualitatively compare electron transport theory
with electron transfer experiments. From these results we were
able to observe the trends in conductance through bridges
1S-4S reflected in the charge transfer rates through bridges
1-4. Long RP lifetimes are often the goal of research on
D-B-A molecules, but the synthesis of long bridges generally
involves many steps to make molecules having long RP
lifetimes. Our results show that changing the electronic coupling
of the bridge can change the electron transfer rates dramatically
so that much simpler molecules can be used to prolong RP
lifetimes.
Acknowledgment. This work was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, DOE under grant no. DE-FG02-99ER14999
(M.R.W.). M.A.R. thanks the NSF Chemistry Division. M.T.C.
thanks the Link Foundation for a fellowship. We thank Mr. Thomas
Michl for assistance in synthesizing compound 1.
While 1S and 3S both have a reasonable degree of confor-
mational flexibility, the variation in the transmission with
dihedral angle change is not unidirectional. Above the Fermi
energy, the transmission through 1S increases with decreasing
dihedral angle and decreases with increasing dihedral angle,
whereas in 3S the behavior is reversed. The differences in these
Supporting Information Available: Details regarding the
synthesis and characterization of 1-4, additional transient
absorption data, and complete ref 53. This material is available
free of charge via the Internet at http://pubs.acs.org.
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