Influence of energetics and electronic coupling on through-bond and through-space electron transfer within U-shaped donor-bridge-acceptor arrays

Article abstract of DOI:10.1021/jp037756f

We present here a study of multistep electron transfer mechanisms within donor-bridge-acceptor arrays consisting of functionalized aromatic imide and diimide donors and acceptors arranged in rodlike linear structures and in U-shaped folded structures on xanthene scaffolds. Femtosecond and nanosecond transient absorption spectroscopy is used to explore the relative efficiency of through-bond and through-space electron transfer in these molecules. The magnitude of the electronic coupling between the oxidized donor and the reduced acceptor is probed specifically by direct measurements of the singlet-triplet splitting, 2J, within the radical ion pairs using magnetic field effects on the yield of triplet states resulting from radical ion pair recombination. These data are used to quantitatively assess the effects of both energetics and electronic coupling on the electron transfer mechanism. Through-space electron transfer is found to be a viable mechanism in the U-shaped structures when reduction of the acceptor that is folded back toward the donor is energetically more favorable than reduction of the acceptor directly bonded to the donor.

Full text of DOI:10.1021/jp037756f

J. Phys. Chem. B 2004, 108, 10309-10316
10309
Influence of Energetics and Electronic Coupling on Through-Bond and Through-Space
Electron Transfer within U-Shaped Donor-Bridge-Acceptor Arrays†
Emily A. Weiss, Louise E. Sinks, Aaron S. Lukas, Erin T. Chernick, Mark A. Ratner,* and
Michael R. Wasielewski*
Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly,
Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: December 8, 2003; In Final Form: February 23, 2004
We present here a study of multistep electron transfer mechanisms within donor-bridge-acceptor arrays
consisting of functionalized aromatic imide and diimide donors and acceptors arranged in rodlike linear
structures and in U-shaped folded structures on xanthene scaffolds. Femtosecond and nanosecond transient
absorption spectroscopy is used to explore the relative efficiency of through-bond and through-space electron
transfer in these molecules. The magnitude of the electronic coupling between the oxidized donor and the
reduced acceptor is probed specifically by direct measurements of the singlet-triplet splitting, 2J, within the
radical ion pairs using magnetic field effects on the yield of triplet states resulting from radical ion pair
recombination. These data are used to quantitatively assess the effects of both energetics and electronic coupling
on the electron transfer mechanism. Through-space electron transfer is found to be a viable mechanism in the
U-shaped structures when reduction of the acceptor that is folded back toward the donor is energetically
more favorable than reduction of the acceptor directly bonded to the donor.
Introduction
The ability of photosynthetic reaction center proteins to
convert photon energy into chemical potential with near unity
quantum yield and the availability of crystal structures for these
proteins have resulted in an extensive effort to decipher and
mimic elements of the complex series of electron-transfer
reactions that occur within them.1-3 An important feature of
these proteins that differs from most model systems is the fact
that none of their energy or electron transport cofactors are
covalently linked to one another. The protein structure provides
specific distances and orientations between the cofactors as well
as an electronic environment that is critical to proper functioning
of the system. Thus, it is important to understand electron
transfer between donors and acceptors that are not covalently
linked to one another as well as the more typical situation in
which they are covalently linked by a bridging molecule. In
the noncovalent case, electron transfer occurs by the interaction
of the molecular orbitals of the electron donor with those of
the acceptor, whereas in the covalent case, the interaction
involves participation of the orbitals of bridging molecules in a
superexchange interaction.4-6 A corollary to the noncovalent
case is electron transfer using the orbitals of solvent molecules
or nonbonded protein functional groups positioned between the
donor and the acceptor.7-19 The possibility that the highly
efficient electron-transfer reactions in the reaction center may
involve all of these mechanisms has motivated the synthesis
and study of a host of model compounds designed to probe the
relative efficiency of such mechanisms under specific structural
and environmental conditions.4-6,20-28
Figure 1. Structures of the donor-acceptor arrays.
electronic coupling on the pathway utilized for electron transfer.
These arrays consist of functionalized aromatic imide and
diimide donors and acceptors arranged in rodlike structures and
in U-shaped structures on xanthene scaffolds.29 The xanthene
scaffold provides a means of folding the structure so that specific
electron acceptors are spatially proximate to the electron donors,
yet have many covalent bonds between them. Since electron
transfer via the long through-bond pathway is expected to be
very slow,4 these structures provide a means to study the
through-space interaction between the donor and the nearby
We present here a series of donor-bridge-acceptor arrays,
Figure 1, designed to explore the effects of both energetics and
† Part of the special issue “Gerald Small Festschrift”.
* To whom correspondence should be addressed. E-mail: wasielew@
chem.northwestern.edu.
10.1021/jp037756f CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/22/2004

10310 J. Phys. Chem. B, Vol. 108, No. 29, 2004
Weiss et al.
acceptor in the folded leg of the structure. Specifically, radical
ion pairs (RPs) are created in these systems through a series of
rapid, nonadiabatic charge separation reactions starting from the
lowest excited singlet state of their common chromophore, 4-(N-
piperidinyl)naphthalene-1,8-dicarboximide (6ANI).28 The through-
space and through-bond charge separation and recombination
mechanisms between the donor and the acceptor vary throughout
the series.
maintained between 0.3 and 1.0 at 416 nm, (ꢀ6ANI, 416 nm ) 7000
cm-1 M-1).28 Femtosecond and nanosecond transient absorption
data, as well as MFE experiments were obtained with apparatus
described elsewhere.33,46,47,51 For the femtosecond experiments,
cuvettes with a 2 mm path length were used and the samples
were irradiated with 80 fs, 400 nm 0.5-1.0 µJ laser pulses
focused to a 200 µm diameter spot. The optical density at 400
nm was typically 0.4-0.8. The total instrument response
function (IRF) for the pump-probe experiments was 130 fs.
For the nanosecond experiments, samples were placed in a 1
cm 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 excited
with 5 ns, 1 mJ, 416 nm laser pulses focused to a 5 mm diameter
spot and the IRF was 7 ns. Between 50 and 80 shots were
averaged for each kinetic trace with a LeCroy 9384 digital
oscilloscope and sent to a microcomputer, which calculated the
∆A. Kinetic analyses were performed at several wavelengths
using a nonlinear least-squares fit to a general sum-of-expo-
nentials using the Levenberg-Marquardt algorithm accounting
for the presence of the finite instrument response.
The 6ANI molecule serves as the photoexcited chromophore
within all of the donor-acceptor arrays presented here. In some
arrays 1*6ANI is the primary electron donor, whereas in others,
it is the primary electron acceptor. p-Methoxyaniline (MeOAn),
acts as the primary or secondary electron donor, whereas
naphthalene-1,8:4,5-bis(dicarboximide) (NI) and pyromellitimide
(PI) are primary or secondary electron acceptors. The NI and
PI acceptors are covalently linked to 6ANI either directly using
a N-N single bond or 2,5-dimethylphenyl (dmp) or 4,5-
disubstituted xanthene (xan) spacers.30 Energy minimized
ground-state structures calculated using the semiempirical AM1
method show that the π systems of the NI and PI acceptors
adopt a nearly cofacial arrangement when they are linked
through their imide bonds to the 4 and 5 positions of the
xanthene spacer in 5-8, 12, and 13.31 Direct linkage of 6ANI
to NI or PI in 1, 3, 5, 7, and 10-13 using N-N single bonds
at their imide groups results in a near perpendicular orientation
of their respective π systems, minimizing electronic com-
munication, whereas linkage of 6ANI to NI or PI through the
dmp spacer in 2, 4, 6, and 8 results in a nearly coplanar
arrangement of their π systems. The nature of the amino group
functionality within 4-diakylaminonaphthalene-1,8-dicarbox-
imides is known to play a critical role in the photophysics of
this chromophore.32 The conformational effects of 4-cycloalkyl-
amino rings on the photophysics of 6ANI derivatives in Debye
solvents has been explored in detail.33 All molecules in this
series have either piperidine or piperazine derivatives attached
to the 4-position of the naphthalene-1,8-dicarboximide to
produce the 6ANI chromophore, so that conformational effects
on the electron-transfer rates will be similar across the set.
The electron transfer dynamics in the molecules presented
here are investigated in both nonpolar toluene and polar
2-methyltetrahydrofuran (MTHF). Charge separation and re-
combination rates are measured using femtosecond and nano-
second transient absorption spectroscopy. In addition, the
magnitude of the electronic coupling between the oxidized donor
and the reduced acceptor is probed specifically by direct
measurements of the RP singlet-triplet splitting, 2J, using
magnetic field effects (MFEs) on the yield of triplet states
resulting from radical ion pair recombination. The MFEs are
due to the radical pair intersystem crossing (RP-ISC) mech-
anism, which is well-known to account for triplet production
within photosynthetic reaction centers34-39 and has been de-
scribed in detail elsewhere.40-48
For the MFE experiment, the sample cuvette was placed
between the poles of a Walker Scientific HV-4W electromagnet
powered by a Walker Magnion HS-735 power supply. The field
strength was measured by a Lakeshore 450 gaussmeter with a
Hall effect probe. Both the electromagnet and the gaussmeter
were interfaced with the data collection computer, allowing
measurement and control of the magnetic field to (1 × 10-5
T
during data acquisition. Kinetic traces at the characteristic
wavelength of the localized triplet state were taken at each
magnetic field, which was changed by a constant increment
depending on desired resolution. Due to the length of the sample
runs (>5 h), a small amount of sample degradation was
observed, resulting in a decrease in the triplet yield at zero field,
∆A(B ) 0), over the course of the experiments. To compensate
for this, the magnetic field was reset to B ) 0 mT every five
kinetic traces for increments of 5 mT and every three kinetic
traces for increments of 0.5 mT and 1 mT and ∆A(B ) 0) was
plotted and fit with a polynomial or series of polynomials. These
functions were used to calculate the relative triplet yield as a
function of applied field strength. The relative triplet yield is
thus
∆A(B)
∆A(B ) 0)
T
T0
)
(1)
The results presented are an average of two or more experiments
conducted on separate days with freshly prepared samples.
Results
The photophysics of rodlike donor-acceptor arrays contain-
ing the 6ANI chromophore have been characterized previously
in detail.28 The oxidation potential of 6ANI (1.2 V vs SCE) is
similar to that of piperidine, whereas its reduction potential
(-1.4 V vs SCE) is similar to that of naphthalene-1,8-
dicarboximide. The oxidation potential of the MeOAn electron
donor (0.79 V) is substantially less positive than that of 6ANI
due to resonance stabilization of the aniline radical cation by
the p-methoxy group. Greenfield et al.28 found that attachment
of the MeOAn electron donor to 6ANI via a piperazine bridge
results in strong quenching of the 6ANI emission, consistent
with the rapid electron-transfer reaction: MeOAn-1*6ANI f
MeOAn+•-6ANI-•. The reduction potentials of the NI and PI
chromophores are -0.5 and -0.79 V vs SCE, respectively.52
Experimental Section
The synthesis and characterization of compounds 1,46 2,28
3,33 4,46 9,28 10,49 11,50 and 1329 have been reported previously,
and those of 5-8 and 12 can be found in the Supporting
Information. Characterization was performed with a Gemini 300
MHz, Varian 400 MHz, or INOVA 500 MHz NMR and a PE
BioSystems MALDI-TOF mass spectrometer. All solvents were
spectrophotometric grade or distilled prior to use. Immediately
before use, the 2-methyltetrahydrofuran was additionally purified
over a basic alumina column.
Absorption measurements were made on a Shimadzu (UV-
1601) spectrophotometer. The optical density of all samples was

ET in Donor-Bridge-Acceptor Arrays
J. Phys. Chem. B, Vol. 108, No. 29, 2004 10311
TABLE 1: Reorganization Energies (λ) and ∆G’s for Each Electron Transfer Step in Toluene and MTHFa
toluene
MTHF
λCR
a
b
a
b
λS*
λCS1
λCS2
λCR
∆GCS1
∆GCS2
∆GCRS
∆GCRT
λS*
λCS1
λCS2
∆GCS1
∆GCS2
∆GCRS
1
2
3
4
5
6
7
8
0.08
0.09
0.08
0.09
0.08
0.09
0.08
0.09
0.06
0.07
0.07
0.07
0.74
0.19
0.19
0.19
0.74
0.19
0.19
0.19
0.19
0.74
0.81
0.74
1.11c
0.34
0.40
0.41
1.11c
0.34
0.40
0.34
0.54
0.55
0.61
0.79
0.54
0.55
0.61
0.55
0.29
1.05
1.12
1.05
-0.53
-0.32
-0.32
-0.32
-0.53
-0.32
-0.32
-0.32
-0.32
-0.56
-0.27
-0.62
-0.36
-0.47
-0.28
-0.18
-0.41
-0.50
-0.35
-0.53
-1.91
-2.01
-2.20
-2.30
-1.86
-1.98
-2.13
-1.95
-2.48
-2.24
-2.53
-2.18
0.12
0.02
-0.15
-0.25
0.17
0.05
-0.08
0.08
1.10
1.19
1.10
1.19
1.10
1.20
1.10
1.18
0.81
1.01
1.01
1.01
1.64
0.93
0.89
0.93
1.64
0.93
0.89
0.93
0.93
1.64
1.71
1.64
1.19c
0.69
0.67
0.76
1.19c
0.70
0.67
0.68
1.56
1.65
1.63
1.72
1.56
1.66
1.63
1.64
1.24
1.99
2.06
1.99
-0.78
-0.41
-0.41
-0.41
-0.78
-0.41
-0.41
-0.41
-0.41
-0.83
-0.54
-0.88
-0.43
-0.76
-0.51
-0.47
-0.48
-0.81
-0.58
-0.82
-1.51
-1.60
-1.85
-1.89
-1.59
-1.55
-1.78
-1.54
-2.36
-1.94
-2.23
-1.89
9
10
11
12
13
(PI)
(NI)
0.07
0.08
0.81
0.75
1.12
1.06
-0.33
-0.55
-2.47
-2.25
1.03
1.07
1.73
1.70
2.08
2.05
-0.61
-0.86
-2.16
-1.91†
a All values are in eV. Charge recombination in MTHF takes place too fast for RP intersystem crossing and so all CR is from the singlet RP.
The methods and schemes used to calculate the reorganization energies, denoted by superscripts a, b, and c in the table entries, are provided in the
Supporting Information.
Figure 2. Femtosecond transient spectra for compound 8 in toluene
illustrating through-space electron transfer; inset: kinetic trace of
compound 8 at 480 nm.
Redox potentials for all relevant chromophores within com-
pounds 1-13 can be found in Table S1. The electronic spectra
of all NI-containing compounds in toluene, Figure S1, exhibit
a broad charge transfer (CT) absorption centered near 400 nm
in both toluene and MTHF due to the 6ANI chromophore, and
a second band displaying vibronic structure at 343, 363, and
382 nm arising from a π-π* transition of the NI acceptor. The
ground-state absorption maximum of PI occurs at 307 nm and
overlaps higher energy absorption bands of 6ANI. The charge
separation and charge recombination free energies for 1-13,
calculated using the spectroscopic method outlined by Greenfield
et al.,28 which is based on the Weller’s dielectric continuum
Figure 3. Normalized nanosecond kinetic traces at radical anion peaks
treatment for the energy of an RP in an arbitrary solvent,53 as
(480 nm for NI-• and 720 nm for PI-•) for compounds in toluene, (A)
well as the solvent reorganization energies calculated for the
various electron-transfer reactions using the Marcus formula-
tion,54 are listed in Table 1. Internal reorganization energies,
λI, calculated from DFT-energy minimized structures of both
the neutral and charged donors and acceptors are given in the
Supporting Information, Table S1.
Figure 2 illustrates the time evolution of the transient spectra
observed following photoexcitation of 8 in toluene and is
representative of similar data obtained for 1-13. At early times
the transient bleach at 500 nm is due to stimulated emission
from 1*6ANI and is followed by rapid formation of an absorption
band at 520 nm due to the formation of MeOAn+•-6ANI-•. The
520 nm feature is due principally to MeOAn+•.55,56 At longer
times, the 520 nm feature persists and is accompanied by
formation of an intense band with maxima at 480 and 610 nm
1, 3, 5, and 7; (B) 2, 4, 6, and 8.
characteristic of NI-•.57 The inset to Figure 2 shows the transient
kinetics at 480 nm. Figure 3 shows the corresponding nano-
second transient kinetics for 1-8 in toluene monitoring either
NI-• at 480 nm or PI-• at 720 nm. The transient absorption
kinetics show a single-exponential decay component followed
by a residual low amplitude absorption which decays on a much
longer time scale. This long-lived absorption is due to formation
of 3*6ANI in molecules having only a PI acceptor and 3*NI in
those that have a NI acceptor.58 The transient absorption kinetics
for 1-13 are summarized in Table 2.
The data in Figure 4 show the relative triplet yields resulting
from radical ion pair recombination for 1-8 in toluene as a
function of applied magnetic field. These MFE plots exhibit

10312 J. Phys. Chem. B, Vol. 108, No. 29, 2004
Weiss et al.
Figure 4. MFE plots for compounds in toluene: (a) 5, inset: 1; (b) 6, inset: 2; (c) 7, inset: 3; (d) 8, inset: 4. The value of 2J is given by the field
at which the maximum triplet yield (T/T0) is achieved.
TABLE 2: Charge Recombination Time Constants and in
Toluene and MTHF
donor-acceptor electronic coupling to the rate of charge
recombination within these systems. The compounds with
MeOAn donors attached to the 6ANI chromophore (1-8) have
2J values within the range measurable by our magnet
(0-1.2 T), but for those where 6ANI serves as the electron
donor (10-13), the stronger coupling between the radical cation
and radical anion spins results in a short-lived RP and
presumably a S-T splitting that is much larger than 1.2 T.
τ
CR (ns) toluene
τ
CR (ps) MTHF
1
2
3
4
28
210
18
150
57a
280
73
53a
5
6
7
8
27
350
14
230
5.0
170
57a
280
Discussion
53a
9
52a
In the following discussion, the influence of reaction free
energy, reorganization energy, and electronic coupling on the
electron-transfer dynamics within the multiple donor-acceptor
arrays 1-8 are examined by focusing on the competition
between (1) reduction of 1*6ANI by the adjacent MeOAn donor
and oxidation of 1*6ANI by PI or NI acceptors in the primary
charge separation step, (2) secondary charge separation and
primary RP charge recombination, and (3) through-bond and
through-space charge separation/charge recombination pathways.
Compounds 9-13 serve as reference molecules that allow us
to dissect the contributions of competing electron-transfer
processes to the overall charge separation and recombination
within 1-8. The free energies for the electron-transfer reactions
are changed by using acceptors with different reduction
potentials, NI and PI, as well as solvents having different
polarities, toluene and MTHF. The donor-acceptor electronic
coupling is altered by changing both the distance and orientation
between the donor and acceptor. Scheme 1 outlines the primary
charge separation steps and, depending on the compound, either
the subsequent secondary charge separation step or the charge
recombination of the primary RP for 1-13, as discussed below.
Table 1 gives charge recombination time constants for the RPs
formed in these systems.
10
11
12
13
12
19
21
12 (NI)
10 (PI)
100
230
88
250 (NI)
250 (PI)
a τCR is for recombination from the MeOAn+-6ANI- ion pair.
TABLE 3: Singlet-Triplet Splitting (2J) and Full Width at
Half Maximium of the 2J Distribution for Compounds with
MeOAn Donors ((0.2 mT)
2J
∆2Jfwhm
2J
∆2Jfwhm
1
2
3
4
48 mT
1 mT
66 mT
2, 19 mT
12 mT
<1 mT
48 mT
5
6
7
8
44 mT
<1 mT
63 mT
<1 mT
22 mT
<1 mT
48 mT
<1 mT
1.5, 7 mT
resonances whose maxima directly give the singlet-triplet
splitting of the RP, 2J. The magnitude of 2J changes consider-
ably as the electronic coupling between the radical ions within
the RPs varies as a function of molecular structure. The values
of 2J measured for 1-8 in toluene are listed in Table 3 along
with the full width at half-maximum of the resonances. As has
been discussed previously,46,47 the strength of the magnetic
superexchange interaction between the spins of the RP, as
indicated by the position of the 2J resonance in the MFE plot,
Figure 4, is a good gauge of the overall contribution of the
Primary Charge Separation. Following photogeneration of
1*6ANI in 1-8, a competition occurs between oxidation of

ET in Donor-Bridge-Acceptor Arrays
J. Phys. Chem. B, Vol. 108, No. 29, 2004 10313
SCHEME 1: Primary and Secondary Charge Transfer
Steps for Compounds 1-13 in Toluene (Black) and
MTHF (Red)a
must therefore conclude that the errors in our calculations of
∆G and λ are too great to predict the observed mechanism in
this case, where one mechanism is not significantly energetically
favored over the other. When the same experiment is carried
out in MTHF, the data for compounds 1 and 5 show that the
formation of MeOAn+• and NI-• both occur with the same time
constants (<1ps), so that both reactions occur simultaneously.
In MTHF, -∆G ≈ λ/2 for both mechanisms, so that both
reactions are in the normal region and should have similar rates.
All other compounds, 2-4 and 6-8, show exclusive forma-
tion of MeOAn+•-6ANI-• as the initial RP with time constants
given in Scheme 1. This includes 3 and 7 in which 6ANI is
covalently bound directly to PI using an N-N bond. The
formation of MeOAn+•-6ANI-• in both 3 and 7 in toluene occurs
with -∆G ) 0.32 eV, whereas λ is only 0.19 eV, Table 1, so
that the reaction lies near the maximum of the Marcus rate vs
free energy profile. On the other hand, the data for reference
molecule 11 shows that -∆G ) 0.27 for electron transfer from
1*6ANI to PI is comparable to that for the oxidation of MeOAn,
yet λ for the reduction of PI is 0.81 eV, resulting in a relatively
slow rate of PI reduction compared to the rate of MeOAn
oxidation. The energy gap law is therefore most likely respon-
sible for determining whether 1*6ANI is either oxidized or
reduced by the adjacent acceptor or donor, respectively, in these
compounds.
Secondary Charge Separation vs Primary RP Recombi-
nation. As discussed above, 1 and 5 within the series 1-8
generates a primary RP in which 6ANI+• forms. This implies
that the secondary electron-transfer step for these two molecules
MeOAn-6ANI+•-NI-• f MeOAn+•-6ANI-NI-• must compete
with charge recombination within MeOAn-6ANI+•-NI-•. For
both 1 and 5, the charge shift reaction is more than 10 times
faster than the charge recombination of 6ANI+•-NI-•, as
indicated by the data for reference molecule 10, Scheme 1,
leading to a high yield of the distal RP MeOAn+•-6ANI-NI-•.
In addition, following photogeneration of the primary RP,
MeOAn+•-6ANI-• in 2-4 and 6-8, secondary charge separation
from 6ANI-• to the adjacent NI or PI acceptor competes well
with charge recombination within the primary RP. The data for
reference molecule 9 in toluene show that the charge recom-
bination reaction, MeOAn+•-ANI-• f MeOAn-6ANI, occurs
with τCR ) 5 ns. As a consequence of the relatively long lifetime
of this RP in toluene, secondary charge separation from 6ANI-•
to NI or PI, for 2-4 and 6-8 in toluene, even for the slowest
case (τCS ) 410 ps for 7), occurs with >90% yield. In contrast,
the secondary electron transfer does not occur at all in MTHF
for the compounds having the dmp spacer between 6ANI and
the acceptor (2, 4, 6, and 8), because it cannot compete
kinetically with charge recombination of the primary RP, which
occurs in 53-57 ps, closely matching τCR ) 52 ps for model
compound 9. However, the secondary charge shift reaction in
MTHF is competitive with charge recombination of the primary
RP within the compounds in which 6ANI is directly linked to
NI or PI via a N-N bond (1, 3, 5, and 7). Once again, the
slowest time constant for the secondary electron transfer is 4.7
ps (7), which implies that the yield of the secondary RP is >90%
in 1, 3, 5, and 7. The solvent reorganization energies for the
electron-transfer reactions in these molecules, 0.8 eV < λS <
1.2 eV, in MTHF (ꢀ ) 6.9761) are much larger than those in
toluene, 0.06 eV < λS < 0.09 eV. In addition, the RP is
stabilized in the more polar solvent, which makes the charge
recombination reaction of MeOAn+•-6ANI-• lie much further
in the Marcus inverted region in toluene than in MTHF resulting
in a much slower recombination rate in toluene.
a D ) MeOAn and C ) 6ANI and A, A1, and A2 refer to acceptors
in the positions indicated in the diagram.
1*6ANI by NI or PI and reduction of 1*6ANI by MeOAn.
Electron transfer from 1*6ANI to NI to form MeOAn-6ANI+•-
NI-• occurs in e1 ps in toluene within MeOAn-6ANI-NI (1)
and MeOAn-6ANI-NI-xan-PI (5), as noted by observing the
formation of NI-•, which matches the time constant for charge
separation in the model compound 6ANI-NI (10). Formation
of the initial RP is followed by electron transfer from MeOAn
to 6ANI+• to yield the final RPs, MeOAn+•-6ANI-NI-• (1) and
MeOAn+•-6ANI-NI-•-xan-PI (5), in τCS ) 1.3 ps. Measurements
on reference molecule 9 show that the time constant for the
alternative process MeOAn-1*6ANI-NI f MeOAn+•-6ANI-•-
NI should be about 11 ps,59 which is therefore not kinetically
competitive with the initial reduction of NI by 1*6ANI. The
Marcus energy gap law60 states that the electron-transfer rate is
fastest when -∆G for the reaction is equal to λ, the total nuclear
reorganization energy. Calculations of ∆G and λ for these
reactions (see Table 1 and the Supporting Information) show
that the reaction MeOAn-1*6ANI-NI f MeOAn+•-6ANI-•-NI
in toluene is in the Marcus inverted region (-∆G ) 0.32 eV >
λ ) 0.19 eV), whereas the opposing reaction, MeOAn-1*6ANI-
NI f MeOAn-6ANI+•-NI-•, is in the normal region (-∆G )
0.53 eV < λ ) 0.74 eV). Neither mechanism is conclusively
favored based on these calculations. However, electronic
coupling is most likely not responsible for the preference to
reduce NI because, in general, the PI acceptor, having greater
torsional freedom around the imide-imide linkage, is more
strongly coupled to 6ANI than NI (see Table 3 and ref 46). We

10314 J. Phys. Chem. B, Vol. 108, No. 29, 2004
Weiss et al.
Through-Space vs Through-Bond Electron Transfer. The
folded donor-acceptor molecules 5-8, 12, and 13 offer the
possibility of competitive through-space electron transfer to a
second adjacent NI or PI that is part of the folded leg of the
xanthene structure. The term “through-space” is meant to
encompass both a direct transfer from 6ANI-• to the distal NI
or PI as well as a superexchange interaction involving toluene
or MTHF positioned between them. We have presented evidence
previously implicating aromatic solvents in such an interaction
in related molecules.29 In both toluene and MTHF, the electron-
transfer dynamics of MeOAn-6ANI-NI-xan-PI (5) and MeOAn-
6ANI-PI-xan-NI (7) are very similar to those of the linear model
compounds, MeOAn-6ANI-NI (1) and MeOAn-6ANI-PI (3),
respectively. There is no evidence of through-space electron
transfer to the “opposite-side” acceptors in 5 (PI) and 7 (NI).
In addition, there is no evidence of a charge shift equilibrium
between NI and PI attached to the xanthene scaffold.
pathways, plus the equilibrium constant. From model compound
6ANI-PI (11), we know that formation of the radical pair
6ANI+•-PI-• occurs in 11 ps in MTHF. This is significantly
slower than the rate of radical anion formation in 13, indicating
that the formation of the radical anion is dominated by a fast
through-space rate. The situation is similar in toluene, though
the charge separation in 13 is biphasic, with a 1.5 ps component
(30%) and a 7.6 ps component (70%). This may represent a
separation of the observed rates into intrinsic rates, but assign-
ment of the components to particular processes is difficult.
Again, through comparison with the reference compound 11, it
is appears that the dominate electron-transfer pathway is
through-space.
Charge recombination can also occur via these two pathways,
and therefore, the observed charge recombination rates are
composite rates. In MTHF, both radical anion signals decay in
250 ps, which is very similar to the 230 ps time constant
observed in 11 for the decay of PI-•. This indicates that the
covalent pathway is the major recombination route. In toluene,
however, the recombination time constant in 13 is approximately
11 ns for both anions, which is significantly shorter than the
19 ns recombination time constant seen in 11. This may indicate
that a faster through-space recombination pathway is accessible
in toluene and not in MTHF. However, the MFE indicates that
covalent pathway must still be important. In compound 8, the
value of 2J decreases sharply as the through-space pathway is
utilized. In compound 13, no magnetic field effect was observed,
indicating that the strongly coupled through-bond pathway is a
major component of the overall recombination process.
Comparing the results for MeOAn-6ANI-dmp-NI-xan-PI (6)
to those for the linear model compound MeOAn-6ANI-dmp-
NI (2), the formation of the primary RPs occur with similar
time constants, whereas the secondary charge separation from
6ANI-• to NI across the dmp spacer is somewhat faster for 6
than it is for 2. There is no evidence for electron transfer to PI
in any of the electron-transfer dynamics of 6. In MTHF, the
primary RP recombines in only 57 and 54 ps in 6 and 2,
respectively, so that secondary electron transfer to NI does not
occur.
The situation is more interesting in the case of MeOAn-6ANI-
dmp-PI-xan-NI (8) in toluene, where the secondary charge
separation results in formation of NI-• with τCS ) 360 ps. In
fact, there is no evidence of formation of PI-• at any time during
the electron-transfer dynamics leading to MeOAn+•-6ANI-dmp-
PI-xan-NI-•. The corresponding data for model compound 4
shows that electron transfer from 6ANI-• to PI across the dmp
spacer does in fact occur, albeit slowly with τCS ) 5 ns. Thus,
the reduction of NI in 8 is more than 10 times faster than the
reduction of PI in 4. It is unlikely that the energetically
accessible MeOAn+•-6ANI-dmp-PI-•-xan-NI intermediate is a
precursor to NI reduction because the formation of this
intermediate would be rate limiting, which is not consistent with
the observed rapid formation of NI-•. Thus, the formation of
NI-• in 8 most likely occurs by through-space electron transfer
either directly from 6ANI-• to NI or through a superexchange
interaction involving the orbitals of solvent molecules between
6ANI-• and NI. The free energy for electron transfer from
6ANI-• to NI within 8 is more negative by 0.35 eV than that
for transfer to PI, Table 1. Even though the energetics favor
through-space electron transfer from 6ANI-• to NI in 8, the
electronic coupling matrix element for this process should be
smaller than that for through-bond transfer to PI. Nevertheless,
the dominance of electron transfer to NI implies that the
electronic coupling between 6ANI-• and NI cannot be signifi-
cantly smaller, otherwise the free energy advantage for the
reduction of NI would be canceled out.
Electronic Coupling. The charge recombination reactions
within 1-8 take place in a single kinetically resolvable step in
both toluene and MTHF. We have shown previously that the
overall charge recombination process is dominated by the
electronic coupling matrix element for recombination to the
lowest excited triplet state of the system in other compounds
of this type,46 so that that the singlet-triplet splitting within
the RP, 2J, accurately reflects the relative electronic coupling
for charge recombination. The magnitude of 2J depends
exponentially on distance, so that as mentioned earlier, the short
distances between the RPs generated within 9-13 result in larger
values of 2J than we can measure. However, values of 2J are
readily obtained for 1-8 in which the distances between the
radical ions in the final RPs are a minimum of 13.5 Å, Table
S2. The insertion of the dmp spacer between the 6ANI
chromophore and the NI or PI acceptor greatly increases rDA to
an average of about 18 Å resulting in a sharp decrease in 2J in
all cases, Table 3. The value of 2J goes from 48 mT for
MeOAn+•-6ANI-NI-• (1) to <1 mT for MeOAn+•-6ANI-dmp-
NI-• (2) and MeOAn+•-6ANI-dmp-NI-•-xan-PI (6), within
which NI serves as the only acceptor. The value of 2J drops
from 66 mT for MeOAn+•-6ANI-PI-• (3) to either 2 or 19 mT
for MeOAn+•-6ANI-dmp-PI-• (4) depending on the conforma-
tion of the 6ANI piperazine ring.46 The values of 2J are
exquisitely sensitive to changes in RP structure which, in turn,
modulate the electronic interaction between the two radical ions.
For example, comparing the data for 1 and 3, the larger value
of 2J for 3 relative to that of 1 most likely results from the
greater torsional freedom of PI around the imide-imide linkage
with 6ANI as compared to NI.46
In the reference compound 6ANI-PI-xan-NI (13), spectro-
scopic signatures of both PI-• and NI-• are observed. We will
discuss the MTHF case first, where these features develop with
the same time constants, τ ) 3.2 ps. Formation of the radical
anion can occur via two pathways: 1) direct electron transfer
through the covalent bonds to form PI-• and 2) through-space
electron transfer to form NI-•. The electron can then move back
and forth between the NI and PI, resulting in detectable
populations of both radical anions. The rate observed is most
likely a composite rate consisting of all electron-transfer
The values of 2J observed for the MeOAn+•-NI-• and
MeOAn+•-PI-• RPs within 5-7 are very similar to those
observed in the corresponding linear model compounds 1-3.
This is reasonable given that the transient kinetic data shows
that the electron transfer rates, RP intermediates, and final RP

ET in Donor-Bridge-Acceptor Arrays
J. Phys. Chem. B, Vol. 108, No. 29, 2004 10315
products are essentially the same for a given donor-acceptor
array in both the linear series, 1-3, and the folded series, 5-7.
Once again, MeOAn-6ANI-dmp-PI-xan-NI (8) proves to be the
exception. The value of 2J observed for the MeOAn+•-6ANI-
dmp-PI-xan-NI-• RP within 8 is < 1 mT, significantly less than
the 2 or 19 mT interaction observed for MeOAn+•-6ANI-dmp-
PI-• (4) (please see ref 46 for a detailed discussion of this double
resonance and an additional MFE plot for this compound),
consistent with having the electron strongly localized on NI as
indicated by the transient absorption data.
measurements of the RP singlet-triplet splitting, 2J, using
magnetic field effects (MFEs) on the yield of triplet states
resulting from radical ion pair recombination. These data were
used to quantitatively assess the effects of both energetics and
electronic coupling on the electron transfer mechanism. Through-
space electron transfer was found to be a viable mechanism in
the U-shaped structures when reduction of the acceptor that is
folded back toward the donor is energetically more favorable
than reduction of the acceptor directly bonded to the donor.
Future work will focus on developing a better understanding
how residual structural motions, such as rotations about single
bonds joining the donors and acceptors influence the electron-
transfer reactions, as well as elucidating the role of potential
“antennae” in electron-transfer processes, e.g., the alkyl chain
attached to the opposite-side acceptor in the xanthene molecules.
Preliminary rate and coupling measurements suggest that the
alkyl chains may play a role in mediating through-space electron
transfer in these molecules, a result that could clarify long-
standing speculation regarding the contribution of the phytyl
chain of the bacteriochlorophyll in the initial charge separation
events of photosynthesis.
Structural Effects of the Xanthene Spacer on Electron
Transfer. Finally, it is important to comment on the potential
effects of single bond rotations within these structures on the
observed electron-transfer reactions. Attaching the bulky xan-
thene-acceptor moiety to the linear donor-acceptor segment
containing 6ANI most likely changes the torsional motions of
the PI or NI acceptor on the linear segment. This may change
the electronic coupling matrix elements for the various electron-
transfer reactions, altering the reaction rates. The kinetic data
in Table 2 show that the presence or absence of the xanthene-
acceptor moiety attached to the linear donor-acceptor segment
containing 6ANI has a significant effect the primary or
secondary charge separation time constants in toluene and
MTHF. For example, comparing the data for MeOAn-6ANI-
dmp-NI-xan-PI (6) and MeOAn-6ANI-dmp-NI (2) in toluene,
the secondary charge separation reaction producing NI-• occurs
faster in 6 (270 ps) as opposed to 2 (410 ps). A similar
comparison between 8 and 4 is not valid because different
acceptors are reduced to form the final RP in each molecule.
The charge recombination data also shows that attachment
of the xanthene spacer has an impact on the time constants.
Comparing the data for 6 and 2, the time constant for
recombination reaction in 6 (350 ns) is significantly slower than
that for 2 (210 ns). Once again, no valid comparison can be
made between 8 and 4 as mentioned above. Comparisons
between compounds 5 vs 1 and 7 vs 3 in toluene as well as
MTHF show that the presence of the xanthene-acceptor moiety
has less of an impact on the rates of charge recombination. Large
amplitude torsional motions about the single bonds joining the
redox components in 1, 3, 5, and 7 should have higher barriers
due to significant steric interactions between the carbonyl groups
of 6ANI with those of NI or PI at their point of attachment.
The torsional barriers should be somewhat lower for 2, 4, 6,
and 8 in which 6ANI and NI or PI are attached to dmp. Thus,
attachment of the bulky xanthene-acceptor moiety with its own
steric requirements is more likely to influence the torsional
motion about single bonds which have lower torsional barriers.
This model is consistent with our observation that the time
constants for both the secondary charge separation and charge
recombination reactions in 6 vs 2 differ significantly, whereas
those for charge recombination in 5 vs 1 and 7 vs 3 do not. In
summary, the variations that we see depend both on torsional
barriers and on the time scales at which large amplitude torsions
occur relative to the observed electron-transfer rates.
Acknowledgment. M.R.W. acknowledges support by the
Division of Chemical Sciences, Office of Basic Energy Sciences,
U.S. Department of Energy under Grant No. DE-FG02-99ER-
14999. M.A.R. acknowledges support from the National Science
Foundation, and E.A.W. acknowledges a Fellowship from the
Link Energy Foundation.
Supporting Information Available: Details regarding the
synthesis and characterization of the molecules used in this
study. This material is available free of charge via the Internet
at http://pubs.acs.org.
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