Ultrafast photoswitched charge transmission through the bridge molecule in a Donor-Bridge-Acceptor system

Article abstract of DOI:10.1021/ja000219d

A Donor-Bridge-Acceptor molecule, D-B-A, was synthesized to probe the effects of changing the electronic-state of the bridge molecule, B, on the rates of electron transfer within D-B-A. Selective photoexcitation of D in a tetrahydrofuran solution of D-B-A

Full text of DOI:10.1021/ja000219d

J. Am. Chem. Soc. 2000, 122, 5563-5567
5563
Ultrafast Photoswitched Charge Transmission through the Bridge
Molecule in a Donor-Bridge-Acceptor System
Ryan T. Hayes,† Michael R. Wasielewski,*,† and David Gosztola‡
Contribution from the Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113,
and the Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439-4831
ReceiVed January 20, 2000
Abstract: A Donor-Bridge-Acceptor molecule, D-B-A, was synthesized to probe the effects of changing
the electronic state of the bridge molecule, B, on the rates of electron transfer within D-B-A. Selective
photoexcitation of D in a tetrahydrofuran solution of D-B-A with 400 nm, 130 fs laser pulses at t ) 0 ps
results in photoinduced electron transfer to yield the ion pair D+-B--A with τ ) 60 ps, which undergoes a
subsequent charge shift with τ ) 140 ps to yield the long-lived ion pair D+-B-A- (τ ) 700 ns). Subsequent
selective photoexcitation of B within D+-B-A- with a 520 nm, 150 fs laser pulse at t ) 500 ps results in
about 20% of the D+-B-A- population undergoing charge recombination with τ ) 100 ps. This charge
recombination rate is about 7000 times faster than the normal recombination rate of the ion pair. The results
demonstrate that formation of the lowest excited singlet state of the bridge molecule B significantly alters the
reaction pathways leading to charge recombination. Thus, D-B-A can be viewed as a molecular switch in
which the D+-B-A- state can be rapidly turned on and off using 400 and 520 nm laser pulses, respectively.
Introduction
A1- -A2. The reduced acceptor A1- possesses an intense optical
absorption that can be irradiated with a second laser pulse to
produce the excited-state D+-1*A1--A2 that is capable of
transferring an electron to the secondary acceptor A2. We have
already demonstrated that sequential application of femtosecond
laser pulses to both linear and branched systems of this type
can propagate the electron up a potential gradient in the linear
case,14 and result in an optically controlled change of electron
transport direction in the branched case.15 The second approach
recognizes that the rates of electron-transfer reactions can be
controlled through the application of electric fields. A consider-
able amount of work has been devoted to the theoretical model-
ing16-18 and experimental realization19-21 of molecular electronic
switches consisting of organic electron donor-acceptor pairs
whose operation is controlled by an external electric field. The
electric field produced by a photogenerated ion pair can have a
large effect on the electronic states of surrounding molecules.
For example, a photogenerated electric field can affect the
optical properties of a nearby molecule, or even alter the rate
of electron transfer between a second donor-acceptor pair. If
each of these processes is photodriven, logic can be developed
based on the number, sequence, and frequencies of optical pulses
used to produce a particular optical observable in these systems.
We have prepared two donor1-acceptor1-acceptor2-donor2
molecular arrays, D1-A1-A2-D2,22,23 in which photoinduced
The rates and mechanisms of electron and hole transfer
through organic molecules are topics of considerable recent
interest.1,2 One aspect of this interest is the question of whether
single molecules can act as molecular wires.3-10 In addition, as
the limits of current silicon-based technologies in electronics
are approached,11 a related question is whether molecular
systems can be used as electronic components such as ultrafast
switches.12,13 Central to both of these issues is the question of
how to control charge transmission through organic molecules.
We have developed three fundamental approaches to exerting
such control over electron-transfer pathways.
One of these strategies uses photoinduced electron transfer
to produce a radical ion pair in the donor-acceptor array: D+-
* Address correspondence to this author. E-mail at wasielew@
chem.nwu.edu.
† Northwestern University.
‡ Argonne National Laboratory.
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(2) Jortner, J.; Bixon, M. Electron Transfer-From Isolated Molecules
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(3) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994,
369, 133-137.
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P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science (Washington,
D.C.) 1996, 271, 1705-07.
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R. J. Am. Chem. Soc. 1996, 118, 8174-8175.
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A. J. Phys. Chem. A 1997, 101, 6158-6164.
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(11) Packan, P. A. Science 1999, 285, 2079-2081.
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(21) Tour, J. M.; Wu, R. Report “Extension of branches for orthogonally
fused molecular switches”, Department of Chemistry, University of South
Carolina, Columbia, SC, 1991.
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10.1021/ja000219d CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/23/2000

5564 J. Am. Chem. Soc., Vol. 122, No. 23, 2000
Hayes et al.
charge separation within one donor-acceptor pair controls the
rate constants for photoinduced charge separation and thermal
Chart 1
charge recombination within a second donor-acceptor pair. In
-
the latter case, photoinduced production of either D1+-A1
-
A2-D2 or D1-A1-A2--D2+ completely inhibits formation of
the second ion pair following absorption of a second laser pulse.
The controlled movement of electrons and/or holes within these
organic molecules can be used to develop logical operations as
well as information storage applications.
In this paper we demonstrate a third strategy in which charge
transmission through the bridging molecule B within a Donor-
Bridge-Acceptor (D-B-A) molecule can be optically con-
trolled by selective photoexcitation of B, thus creating a new
type of molecular switch that depends on altering the electronic
state of B. We have synthesized the D-B-A molecule
illustrated below and have observed that selective photoexci-
tation of D results in the reaction sequence 1*D-B-A f D+-
B--A f D+-B-A-. The intermediate electron carrier B is
also a chromophore that can be selectively photoexcited within
D+-B-A- to produce D+-1*B-A- in which the reaction
pathways leading to charge recombination are significantly
altered. This results in a reaction rate for charge recombination
that is nearly 7000 times more rapid than that observed for
D+-B-A-.
Results and Discussion
The donor-bridge-acceptor molecule, D-B-A, is com-
posed of three electronically distinct subunits, D ) zinc
5-phenyl-10,15,20-tri(n-pentyl)porphyrin, B ) perylene-3,4-
dicarboximide, and A ) 1,8:4,5-naphthalenediimide. To char-
acterize the spectroscopy and electron-transfer reactions of
D-B-A, the model compounds D-B, B-A, and B′-A, where
B′ does not possess the phenyl group between B and A, as well
as the individual species D, B, and A were also studied. The
structures of these molecules are shown below, while the
syntheses of the molecules used in this study are described in
the Supporting Information. The NMR and mass spectra of these
molecules are consistent with their assigned structures. The
ground-state optical absorption spectrum of D-B-A as well
as those of D, B, and A are shown in Figure 1. The data reveal
that the spectrum of D-B-A can be closely approximated by
a sum of the spectra of D, B, and A. The lowest excited singlet
state energies of D, B, and A were each determined from the
average of the lowest energy transition in their absorption spectra
and the highest energy transition in their fluorescence emission
spectra. The absorption maxima of the lowest energy electronic
transitions within D, B, and A are 602, 517, and 380 nm,
respectively, while the corresponding emission maxima for D,
B, and A are 606, 555, and 398 nm, respectively. These data
yield lowest excited singlet state energies of 2.06, 2.32, and
3.20 eV, for D, B, and A, respectively.
The energy levels of the D+-B--A and D+-B-A- ion
pairs in tetrahydrofuran were determined from the sum of the
one-electron redox potentials for formation of the individual
ions in butyronitrile corrected for Coulombic interactions and
ion solvation energy differences between the two solvents using
the Weller equation:24
At 400 nm, the laser wavelength used to generate D+-B-
A- in the time-resolved experiments described below, the Soret
band absorbance of the zinc porphyrin donor is about 3.5 times
larger than that of the perylene-3,4-dicarboximide chromophore,
and is about 10 times larger than that of the 1,8:4,5-naphtha-
lenediimide acceptor. Excitation of the Soret band in D results
in subpicosecond internal conversion leading to the low-lying
e2
rDA‚ꢀs
1
1
1
1
∆GIP ) EOX - ERED
-
+ e2
+
-
(
)(
)
2rD 2rA ꢀS ꢀsp
(1)
The accuracy of this treatment is reasonable for solvents of
moderate polarity, such as THF.25 The one-electron oxidation
potentials of D and B are 0.65 and 1.40 V, respectively, while
the one-electron reduction potentials of B and A are -0.90 and
-0.53V, respectively (all vs SCE in butyronitrile), ꢀs ) 6.9 for
1
1*D state. In addition, since *D is the lowest excited singlet
state within D-B-A, excitation of a small fraction of either B
or A within D-B-A with 400 nm light results in subpicosecond
1
energy transfer to form *D-B-A (Vide infra).
(24) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 6767-6777.
(25) Weller, A. Z. Phys. Chem. 1982, 133, 93-98.
(23) Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem.
Soc. 1998, 120, 5118-5119.

Effects of Electronic State on D-B-A Systems
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5565
Figure 1. Ground-state absorption spectra of the individual chro-
mophores and D-B-A.
Figure 3. One pump pulse transient absorption spectra of 25 µM
D-B-A and D-B in THF at the indicated times following a 130 fs,
400 nm laser pulse.
Figure 4. Spectroelectrochemical absorption difference spectra for
D+-B (‚‚‚) and D+(- - -) obtained in N,N-dimethylformamide contain-
ing 0.1 M tetra-n-butylammonium hexafluorophosphate. The solid line
is the result of subtracting the data for D+ from that of D+-B.
Figure 2. Energy level diagram for D-B-A in tetrahydrofuran.
THF and ꢀsp ) 25 for butyronitrile, rD ) 5 Å, rA ) 4 Å, rDB
)
14.3 Å, and rDA ) 26.7 Å.26 Using these data the ion pair
energies of D+-B--A and D+-B-A- calculated using eq 1
are approximately 1.7 and 1.4 eV, respectively, so that, ∆GCS
for charge separation is about -0.4 eV for 1*D-B-A f D+-
B--A and -0.3 eV for D+-B--A f D+-B-A-, Figure 2.
One and two pump pulse transient absorption spectra and
kinetics of D-B-A and the reference dyads were measured in
THF using an amplified Ti:sapphire laser system, which
provides a 130 fs, 400 nm, 2 µJ pump pulse as well as a second,
520 nm, 150 fs, 1 µJ pump pulse along with a white light
continuum probe pulse as described earlier.15,23 Selective single-
pump excitation of the porphyrin donor, D, at 400 nm in both
540 nm band is due to the formation of D+-B-A-, it is difficult
to assign because A- possesses distinct absorption features at
474 nm (ꢀ ) 26000 M-1 cm-1) and 605 nm (ꢀ ) 7200 M-1
cm-1).24,28
To understand the origin of the long-lived 540 nm band in
the transient absorption spectrum, spectroelectrochemical ex-
periments were carried out on model compounds D, A, D-B,
B-A, and B′-A to obtain the spectra of the relevant radical
ions. The electric field produced by the presence of D+ and A-
positioned at a fixed distance on either side of chromophore B
within D+-B-A- may electrochromically shift the optical
absorption of B resulting in the observed 540 nm band.29 To
test this idea, selective one-electron oxidation of D within D-B
and reduction of A within B-A were carried out. The difference
spectrum (D+)-(D) in Figure 4 displays a band at 445 nm, as
well as bleaches at 560 and 602 nm. This spectrum is typical
of the radical cations of other meso-substituted zinc porphyrins,
such as zinc meso-tetraphenylporphyrin, which has a distinct
absorption at 445 nm (ꢀ ) 32000 M-1 cm-1) as well as a broad,
nearly featureless absorption between 500 and 720 nm (average
ꢀ ) 10000 M-1 cm-1).30 The difference spectrum (D+-B)-
(D-B) in Figure 4 shows that generation of D+-B also
produces a band at 445 nm, as well as bleaches at 517, 560,
1
D-B-A and D-B results in electron transfer from *D to B
with τ ) 60 ps to yield D+-B--A and D+-B-, respectively,
as indicated by the optical spectrum of B-, which exhibits a
peak near 620 nm, Figure 3.27 Following excitation, the Q-bands
of D at 560 and 602 nm bleach with τ < 1 ps, which indicates
that the formation of 1*D either by direct excitation or by energy
1
1
transfer from the small population of *B or *A produced by
the 400 nm laser pulse is complete well before the τ ) 60 ps
initial electron-transfer event takes place. The D+-B- ion pair
recombines with τ ) 220 ps, while in D+-B--A, the 620 nm
B- band decays, and a 540 nm band appears with τ ) 140 ps.
The 540 nm band subsequently decays with τ ) 700 ns. If the
(26) The reported distances are center-to-center distances obtained from
structures calculated using the MM+ model within Hyperchem by Hyper-
cube, Waterloo, Ontario.
(27) Gosztola, D.; Niemczyk, M. P.; Svec, W. A.; Lukas, A. S.;
Wasielewski, M. R. J. Phys. Chem. Submitted for publication.
(28) Penneau, J. F.; Stallman, B. J.; Kasai, P. H.; Miller, L. L. Chem.
Mater. 1991, 3, 791-6.
(29) Liptay, W. Angew. Chem., Int. Ed. Engl. 1969, 8, 177-188.
(30) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J.
Am. Chem. Soc. 1970, 92, 3451-3459.

5566 J. Am. Chem. Soc., Vol. 122, No. 23, 2000
Hayes et al.
Figure 6. One and two pump pulse transient absorption kinetics of
25 µM D-B-A in THF probed at 540 nm. The inset shows an
expansion of the kinetic trace following the second 520 nm pulse at
500 ps.
Figure 5. Spectroelectrochemical absorption difference spectra for
B-A-(- - -), B′-A-(s), and A- (‚‚‚) obtained in N,N-dimethylfor-
mamide containing 0.1 M tetra-n-butylammonium hexafluorophosphate.
τ ) 60 ps. This decay time agrees well with the decay time
found for D+-B--A by selectively monitoring B- at 620 nm.
In addition, the subsequent rise of the signal with τ ) 140 ps
monitors the formation of D+-B-A- and agrees well with the
time constant found for the decay of D+-B--A monitored at
620 nm. Thus, at early times kinetics for all of the key
intermediates in the formation of D+-B-A- are observed at
540 nm, while at longer times 540 nm monitors only the
presence of D+-B-A-.
At t ) 500 ps, bridging molecule B within D+-B-A- is
selectively excited with a 520 nm, 150 fs laser pulse (pump 2).
Following the 520 nm pulse, the transient absorption kinetics
monitored at 540 nm show a sharp decrease in intensity that
occurs with the instrument response τ ) 0.2 ps, Figure 6 and
inset. The magnitude of this decrease is a function of the number
of 520 nm photons absorbed by the sample. Disappearance of
the 540 nm band monitors the loss of D+-B-A-. After
formation of D+-1*B-A-, 45% of the immediate absorption
change recovers with τ ) 50 ps to a new ∆A value that is
20% less than that which D+-B-A- displays when B is
not photoexcited (Figure 6, dashed curve). The new, lower
concentration of D+-B-A- decays with the expected 700 ns
time constant. The ∆A data suggest that D+-1*B-A- decays
by competitive pathways in which about 55% of the population
of D+-1*B-A- leads to D-B-A, while 45% re-forms
D+-B-A-.
There are two principal charge recombination pathways
leading from D+-1*B-A- to D-B-A. Following excitation
of B, if the reaction D+-1*B-A- f D+-B--A (∆G ) -2.0
eV) occurs,14 then the transient absorption spectrum of D+-
B--A should appear following the 520 nm laser pulse.
Subsequently, D+-B-A- should re-appear with the same τ
)140 ps time constant that is observed in the one pulse
excitation experiment. Neither of these characteristics is ob-
served. The observation of the τ ) 50 ps recovery of the 540
nm band suggests that a different reaction occurs following the
formation of D+-1*B-A-. The corresponding hole transfer
reaction D+-1*B-A- f D-B+-A- (∆G ) -1.6 eV) should
be faster than the electron transfer described above because ∆G
for the electron transfer is further into the Marcus inverted region
than is the hole transfer.31 If the hole transfer mechanism of
charge recombination occurs, it should be possible to observe
the loss of D+ and/or the formation of B+ due to the reaction
D+-1*B-A- f D-B+-A-. In addition, the observed τ ) 50
ps kinetic component for the recovery of the 540 nm absorption
and 602 nm. Subtracting the difference spectrum (D+)-(D) from
that of (D+-B)-(D-B), ∆∆A in Figure 4, shows that the 445,
560, and 602 nm bands are due to the presence of D+, while
the effect of D+ on the absorption of B produces the broad
bleach at 500 nm and a positive band at 570 nm. Residual
features due to the Q-bands of D are superimposed on the 570
nm band. The bandshift of the B absorption due to the presence
of D+ is significant even though the ions are generated in polar
media. This most likely occurs because direct attachment of
D+ to B effectively limits solvent access to the space between
D+ and B resulting in diminished screening of the charge by
the solvent.
In a similar manner, selective reduction of A within B-A
and B′-A yields the differential absorption spectra shown in
Figure 5, which are compared to that of A-. The spectra of
B-A- and B′-A- show the presence of a broad band near
530 nm, as well as the 474 and 605 nm bands characteristic of
A-. These spectra are assigned to electrochromic red shifts of
the 517 and 509 nm ground-state absorption bands of B and
B′, respectively. These shifts produce a new positive absorption
near 530 nm and an absorption loss near 475 nm that partially
cancels out the band at 474 nm due to A-. This shift is
proportional to the strength of the electric field,29 so that the
∼4 Å shorter distance between B′ and A- in B′-A relative to
that in B-A- results in a larger red shift in B′-A-. Once again
the magnitudes of these effects are significant and suggest that
the ability of the polar solvent to screen the charge of A- is
diminished by the limited space between B and A. Thus, the
spectroelectrochemical data on both D+-B and B-A- support
the assignment of the 540 nm feature in the transient absorption
to the electrochromically red-shifted ground-state absorption of
the bridge molecule B within D+-B-A-.
Once D+-B-A- is formed, B, which initially serves as an
electron acceptor, then functions as a chromophore as well.
Selective excitation of the perylene-3,4-dicarboximide chro-
mophore, B, can be achieved in the mid-visible region of the
spectrum. Using the data in Figures 4 and 5, the absorbance of
B within D+-B-A- at 520 nm is about 5 times greater than
that of D+ and A- combined at that wavelength, which results
in more than adequate selectivity for the two-pump-pulse
experiments described below. Figure 6 shows the transient
absorption kinetics at 540 nm for both the single-pulse and two-
pulse experiments. In both experiments at t ) 0, a 400 nm, 130
fs laser pulse (pump 1) initiates the formation of D+-B-A-.
The instrument limited absorption increase at t ) 0 is due to
1
formation of *D-B-A, which decays to D+-B--A with
(31) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978.

Effects of Electronic State on D-B-A Systems
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5567
rate-limiting step for charge recombination is the reaction
D-B+-A- f D-B-A, application of the second laser pulse
increases the charge recombination rate by a factor of 7000
over the 1.4 × 106 s-1 natural rate constant for D+-B-A-
recombination.
Conclusions
This work shows that it is possible to significantly change
the electron transfer dynamics within a donor-bridge-acceptor
system by changing the electronic state of the bridge molecule.
Photoexcitation of the bridging molecule results in partially
occupied orbitals that can interact strongly with those of the
adjacent donor and acceptor. We are exploring photonic control
of charge transmission in a variety of related systems to probe
fundamental issues regarding the ability of organic molecules
to exhibit photoswitchable wire-like behavior.
Figure 7. Two pump pulse transient absorption spectra of 25 µM
D-B-A in THF following a 400 nm, 130 fs, laser pulse at t ) 0 ps
and a 520 nm, 150 fs, laser pulse at 500 ps. The times given are
following the second pump pulse.
Experimental Section
Compounds D-B-A, D-B, B-A, and B′-A were synthesized
using methods described in the Supporting Information section. The
syntheses of D and A were described previously.23 The structures of
(inset, Figure 6) should be approximately the sum of the rate
constants for the two reactions:
1
these compounds were confirmed using H NMR and mass spectral
data.
Electrochemistry and Spectroelectrochemistry. Cyclic voltam-
metry (CV) and bulk electrolysis were carried out using a computer
controlled potentiostat (Princeton Applied Research, Model 273, M270
software package) and a standard three-electrode arrangement. CV
measurements used both platinum working and auxiliary electrodes and
a saturated sodium calomel reference electrode (SSCE). All electro-
chemical measurements were carried out in N2 purged DMF or
butyronitrile with 0.1 M Bu4NPF6 as the supporting electrolyte. The
scan rate for CV measurements was typically 50-100 mV/s. Ferrocene
was used as an internal redox standard for all potentiometric measure-
ments. Spectroelectrochemistry of D-B, B-A, B′-A, D, and A each
dissolved in N2 purged N,N-dimethylformamide containing 0.1 M tetra-
n-butylammonium hexafluorophosphate was carried out in an optically
transparent thin layer electrochemical cell using a 2 cm2, 250 lines/in.
gold minigrid electrode.32,33 Controlled potential oxidation of D at 1.0
V vs SCE was used to generate D+, while controlled potential reduction
of A at -0.7 V vs SCE was used to generate A-. These over-potentials
are required to overcome the internal resistance of the cell. Returning
the potentials back to 0 V vs SCE restored the initial optical spectra.
Ground-state optical absorption measurements of the neutral and radical
ion species were recorded using a computer-controlled spectrophotom-
eter (Shimadzu, Model 1601), and the differential absorption spectrum
(∆A) was calculated by subtracting the ground-state spectrum from that
of the radical ion.
Transient Spectroscopy. Femtosecond transient absorption mea-
surements were obtained with pump wavelengths of 400 nm, 520 nm,
or both and a white light continuum probe pulse produced by an
amplified-Ti:sapphire laser system with an optical parametric amplifier
(OPA) as described previously.15 The total instrument function is 180
fs. The concentrations of samples of D-B-A, D-B, and B-A used
for transient absorption measurements were approximately 25 µM in
2 mm path length cells. Both pump beams and the probe beam were
200 mm in diameter with the 400 and 520 nm pump beams having
energies of 2 and 1 µJ at the sample, respectively.
Figure 7 shows the transient absorption spectra that appear
at 5, 20, and 70 ps following the application of the second 520
nm excitation pulse at t ) 500 ps. The spectrum at 5 ps clearly
shows the loss in absorbance at 540 nm that results from
formation of D+-1*B-A-, as well as further loss between 450
and 540 nm, and a modest increase between 590 and 650 nm.
A comparison of the spectral changes between 450 and 540
nm with the (D+)-(D) spectral data in Figure 4 strongly
suggests that these changes are consistent with the loss of D+
that occurs when the reaction of D+-1*B-A- f D-B+-A-
takes place. The optical spectrum of B+ obtained by single 520
nm pulse excitation of B′-A (data not shown) consists of a
broad featureless absorption that extends from 530 to 650 nm
with a maximum at 580 nm. The small residual absorbance
between 590 and 650 nm may be due to the part of the B+
absorption that is not canceled by the strong bleach between
450 and 540 nm. At 20 ps following both laser flashes, the D+
signal recovers somewhat, as does the absorption between 590
and 650 nm. These changes are most likely due to the fraction
of the D-B+-A- population that returns to D+-B-A-.
Finally, a comparison between the transient spectrum of D+-
B-A- generated using a single 400 nm laser flash, Figure 3,
with that at 70 ps following two sequential laser flashes at 400
and 520 nm, Figure 7, shows that the spectra are nearly identical
throughout the measured wavelength range. The only significant
difference between them is the fact that the spectrum in Figure
7 shows an overall loss in D+-B-A- population following
both laser flashes.
Acknowledgment. This work was supported by the National
Science Foundation (CHE-9732840).
Transient absorption spectroscopy following excitation of
B-A with a single 520 nm laser pulse shows that τ ) 100 ps
for the reaction B+-A- f B-A (data not shown). Thus,
referring to eq 2, 1/τ ) kCS + kCR, where kCR ) 1.0 × 1010 s-1
and 1/τ ) 2.0 × 1010 s-1, thus kCS ) 1.0 × 1010 s-1. The kinetic
data suggest that about half of the D-B+-A- population decays
by each pathway. This agrees well with the observed amplitudes
of the absorbance changes depicted in the inset to Figure 6,
where 55% of the initial population of D+-1*B-A- yields
D-B-A, while 45% yields D+-B-A-. Considering that the
Supporting Information Available: Preparation and char-
acterization of D-B-A, D-B, B-A, B′-A, and B (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA000219D
(32) Murray, R. W.; Heineman, W. R.; O’Dom, G. W. Anal. Chem. 1967,
39, 1666-8.
(33) DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-
597.