Dynamics of photoinduced electron transfer in a carotenoid - porphyrin - dinitronaphthalenedicarboximide molecular triad

Article abstract of DOI:10.1021/jp970682l

An electron acceptor moiety based on the 4,5-dinitro-1,8-naphthalenedicarboximide system has been prepared and used as the basis for synthesis of a porphyrin - imide dyad (P-NIm) and a carotenoid - porphyrin - imide triad molecule (C-P-NIm). Excitation of

Full text of DOI:10.1021/jp970682l

5214
J. Phys. Chem. B 1997, 101, 5214-5223
Dynamics of Photoinduced Electron Transfer in a
Carotenoid-Porphyrin-Dinitronaphthalenedicarboximide Molecular Triad
Quan Tan, Darius Kuciauskas, Su Lin, Simon Stone, Ana L. Moore,* Thomas A. Moore,* and
Devens Gust*
Center for the Study of Early EVents in Photosynthesis, Department of Chemistry and Biochemistry,
Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed: February 25, 1997; In Final Form: April 22, 1997X
An electron acceptor moiety based on the 4,5-dinitro-1,8-naphthalenedicarboximide system has been prepared
and used as the basis for synthesis of a porphyrin-imide dyad (P-NIm) and a carotenoid-porphyrin-imide
triad molecule (C-P-NIm). Excitation of the porphyrin moiety in either compound with visible light leads
to rapid photoinduced electron transfer to generate in high yield a charge-separated state consisting of the
porphyrin radical cation and imide radical anion. In the triad, this C-P•+-NIm•- state decays in part by a
second electron transfer from the carotenoid to yield a final C•+-P-NIm•- charge-separated state. In
benzonitrile, this state is formed with a quantum yield of 0.33 and has a lifetime of 430 ns. The 4,5-dinitro-
1,8-naphthalenedicarboximide moiety is conveniently synthesized and undergoes facile and reversible one-
electron reduction. The NIm•- ion has a readily observable spectroscopic signature in the visible. In contrast
to a series of closely related triads reported by other investigators, the triad studied here shows no evidence
for photoinduced electron transfer from the carotenoid first excited singlet state.
Introduction
CHART 1: Structures 1, 4-6
Dyads, triads, and other multicomponent supermolecules
consisting of covalently linked chromophores and electron
donors and acceptors have proven useful as mimics of photo-
synthetic reaction centers, which convert light energy into
chemical potential via energy transfer and photoinduced electron-
transfer processes.1-6 In principle, such molecules can also be
made to function as molecular-scale optoelectronic switches and
logic elements.7-9 A typical system of this type is a donor-
pigment-acceptor triad such as carotenoid-porphyrin-quinone
(C-P-Q) triad 1 (see Chart 1) first reported by our group 14
years ago.10,11 The porphyrin pigment absorbs light, and its
first excited singlet state, C-1P-Q, donates an electron to the
quinone acceptor to form C-P•+-Q•-. This state evolves by
electron transfer from the carotenoid secondary electron donor
to yield a final C•+-P-Q•- charge-separated state. The final
state lives for microseconds, is formed with a reasonable
quantum yield, and preserves a substantial fraction of the photon
energy as chemical potential. The key to the function of this
triad is the two-step electron-transfer sequence, which allows
formation of the final charge-separated state in high yield but
electronically and energetically separates the oxidizing and
reducing elements to retard charge recombination to the ground
state.
In photosynthesis, quinones perform useful roles in electron
and proton transport, but they are not the primary electron
acceptors in the photoinduced electron-transfer reaction. In
artificial systems, other electron acceptors including porphyrins,
fullerenes,12-18 and aromatic imides19-34 have proven useful in
dyad, triad, and more complex devices. Imide acceptors can
be more chemically robust than quinones and can be linked to
other molecules in ways that limit conformational mobility. In
some cases, their radical anions have characteristic absorption
spectra that allow relatively easy spectroscopic detection.
Synthetic and photophysical studies of an interesting series
of triad molecules consisting of porphyrins covalently linked
to carotenoids and pyromellitimide (Im) moieties have recently
been reported by Osuka, Mataga, and co-workers.27 Excitation
of the carotenoid moiety of C-P-Im triads 2 and 3 (Chart 2)
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
S1089-5647(97)00682-2 CCC: $14.00 © 1997 American Chemical Society

Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5215
CHART 2: Structures 2 and 3
Figure 1. Absorption spectra of triad 4 (-), porphyrin 7 (- - -), imide
8 (- ‚ -), and carotenoid 9 (‚‚‚) in dichloromethane solution. The
porphyrin Soret band is truncated.
which was coupled with the appropriate porphyrin to produce
a P-NIm dyad. Reaction with the acid chloride of the requisite
carotenoid yielded 4. The model compounds were prepared
using related methods, as described in the Experimental Section.
Electrochemical Studies. Cyclic voltammetric studies of
model imide 8 in benzonitrile solution gave two reversible
reductions at -0.88 and -1.20 V vs ferrocene/ferrocenium as
an internal reference. The first one-electron reduction potential
of the imide moiety of 2 is -1.24 V.32 The first oxidation
potential of a model for the porphyrin moiety of 4 is +0.47 V
in benzonitrile, whereas the carotenoid is oxidized at +0.18 V
(determined in dichloromethane).36
CHART 3: Structures 8 and 9
Steady-State Absorption Spectra. The absorption spectra
of triad 4, meso-tetraphenylporphyrin (7), model imide 8, and
model carotenoid 9 in dichloromethane are shown in Figure 1.
Porphyrin 7 exhibits a Soret absorption at 417 nm and Q-bands
at 516, 552, 591, and 647 nm. The carotenoid moiety has major
absorption maxima at 450 (sh), 481, and 513 nm. The imide
absorptions are found in the 250-380 nm region. The spectrum
of the triad is similar to a linear combination of the spectra of
the component chromophores. Thus, the absorption spectrum
shows no indication of strong electronic interactions between
the various pigments. In polar solvents such as benzonitrile,
all absorption bands are shifted to longer wavelengths by 1-8
nm.
Fluorescence Spectra. The fluorescence emission spectra
of triad 4 and dyad 5 in benzonitrile solution, with excitation
at 590 nm, are similar in shape to that of model porphyrin 7
with maxima at 656 and 721 nm, but the fluorescence quantum
yields are much lower than that of 7 (see next paragraph). The
fluorescence excitation spectrum of 4 in the 430-700 nm region,
with detection at 720 nm, is nearly identical with that of model
porphyrin 7, with little or no contribution from light absorbed
by the carotenoid polyene. From these results, it can be
calculated that singlet-singlet energy transfer from the caro-
tenoid to the porphyrin is <10% efficient.
was found to initiate photoinduced electron transfer from the
carotenoid first excited singlet state, 1C-P-Im, to yield C•+-
P-Im•- in a direct, one-step electron-transfer reaction. Excita-
tion of the porphyrin moiety of 2 was followed by rapid singlet-
1
singlet energy transfer to the carotenoid to give C-P-Im,
which then evolved directly to the C•+-P-Im•- charge-
separated state. The authors thus suggested the single-step, long-
range charge separation mechanism as an alternative to the two-
step mechanism proposed for the quinone-containing triads.
To investigate the generality of the intriguing single-step long-
range electron transfer observed in the C-P-Im triads, we
initially studied the photochemistry of several C-P-Q triads
and found no evidence for significant photoinduced electron
transfer from the carotenoid first excited singlet state.35 Now
we turn our attention to triads with imide-based electron acceptor
moieties. We report the preparation of triad 4 and model
compounds 5, 6, 8 and 9 (Charts 1 and 3), electrochemical
investigations of these compounds, and the study of their
photochemical properties using time-resolved absorption and
emission techniques. Note that 4 includes a carotenoid moiety
with an amide link to the porphyrin meso-aryl ring similar to
that in 1 and 3. The acceptor is a 4,5-dinitro-1,8-naphthalene-
dicarboximide (NIm). This electron acceptor has the structural
advantages of pyromellitimides such as that in 2 and 3, but is
more easily reduced (see below), and can therefore serve as an
effective acceptor under a wider variety of conditions.
The strong porphyrin fluorescence quenching observed for 4
and 5 indicates that attachment of the carotenoid and imide
moieties to the porphyrin provides new pathways for deactiva-
tion of the porphyrin first excited singlet state. Quantitative
information concerning the quenching was obtained from time-
resolved fluorescence studies using the single-photon timing
technique, with excitation at 590 nm. The lifetime of the first
excited singlet state of porphyrin 7 in benzonitrile is 9.7 ns.
Results
Synthesis. The porphyrin and carotenoid moieties of 4 were
prepared as previously described. Nitration of commercially
available 4-nitronaphthalic anhydride yielded the 4,5-analogue,

5216 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Tan et al.
TABLE 1: Time Constants for Photochemical Processes in Triad 4 and Model Dyads
dyad 5
τR (ps)
124c
dyad 6
τSb (ns)
3.70
triad 4
τRg (ps)
NO
solvent
ꢀa
τSb (ps)
τD (ps)
807e
τSb (ps)
τDh (ns)
toluene
2.4
6.0
7.6
8.9
25.2
116
432
264
60
110
NO
190
173
340
430
ethyl acetate
2-methyltetrahydrofuran
dichloromethane
benzonitrile
12.7d
240f
3.48
3.00
2.21
243
59
93
103
12.9d
107f
99
a Dielectric constant from ref 44. b Lifetime of the porphyrin first excited singlet state measured by time-resolved fluorescence techniques. c Rise
time of transient absorbance corresponding to formation of P•+-NIm•-
.
d Rise time of transient absorbance corresponding to decay of P•+-NIm•-
f
(see text). e Decay time of P•+-NIm•- measured by subpicosecond transient absorption techniques. Decay time of transient absorbance corresponding
to decay of the porphyrin first excited singlet state and concurrent formation of P•+-NIm•- g Rise time of the carotenoid radical cation measured
.
by subpicosecond transient absorption techniques. h Decay time of the carotenoid radical cation measured by nanosecond transient absorption
techniques.
position was observed during these experiments. There is a
net transient absorption increase over the whole 450-750 nm
region due to absorption by the porphyrin first excited singlet
state. Spectral minima at 516, 552, ∼590, and 650 nm
correspond to porphyrin ground-state absorption (Figure 1) and
represent depletion of the ground state. Stimulated emission
from the porphyrin is observed as minima at 650 and 726 nm,
corresponding to porphyrin fluorescence bands. Thus, at 5 ps,
the transient absorption spectrum is essentially that of the first
excited singlet state of the porphyrin moiety.
The inset in Figure 2 shows the transient absorption kinetics
at 500 and 650 nm measured on different time scales. At 500
nm, where absorption by the porphyrin first excited singlet state
is strong, the transient absorbance decays monoexponentially
with a time constant of 107 ps. This is equal to the 103 ps
lifetime of 1P-NIm determined from the single-photon timing
fluorescence measurements, within experimental error. Figure
2 shows that at 650 nm, the net absorption change resulting
Figure 2. Transient absorption spectrum of dyad 5 in benzonitrile taken
5 ps after excitation with a 200 fs, 590 nm laser pulse. The spectrum
from excitation of the porphyrin to its first excited singlet state
is nearly zero. Thus, the kinetic behavior of states with smaller
contributions to the transient absorption spectrum can be
observed at this wavelength. On a short time scale, the rise of
the transient absorption at 650 nm is biexponential with lifetimes
of ∼400 fs and 12.9 ps. The time constant for decay of the
absorbance is again ∼100 ps. The 400 fs rise is ascribed to
is an average of several thousand experiments. The inset shows the
rise of the transient absorption at 650 nm and the corresponding two-
exponential fit to time constants of 400 fs and 12.9 ps (top and right
axes). Also shown in the inset is the decay of the transient at 500 nm
and the corresponding fit with a time constant of 107 ps (bottom and
left axes).
The fluorescence decay of carotenoporphyrin 6 was measured
in benzonitrile at seven wavelengths in the 640-750 nm region
and the results analyzed globally (ø2 ) 1.04). The only
significant decay component has a lifetime of 2.2 ns. Thus,
the carotenoid quenches the porphyrin first excited singlet state.
This state is quenched even more strongly by the imide moiety.
The emission decays for porphyrin-imide dyad 5 were mea-
sured in benzonitrile at five wavelengths in the 650-740 nm
region. Global analysis (ø2 ) 1.10) yielded a single significant
decay component with a lifetime of 103 ps for 1P-NIm. Similar
experiments with triad 4 gave a lifetime for C-1P-NIm of 93
ps (global analysis at seven wavelengths in the 640-750 nm
region, ø2 ) 1.07). Fluorescence lifetimes were also determined
in toluene, ethyl acetate, 2-methyltetrahydrofuran, and dichlo-
romethane. In all cases, satisfactory goodness-of-fit parameters
were obtained, and only one significant decay component was
observed. The lifetimes are listed in Table 1.
Picosecond Transient Absorption Experiments. The strong
quenching of the porphyrin first excited singlet state of P-NIm
dyad 5 noted above suggests photoinduced electron transfer to
yield P•+-NIm•-, as has been observed for a variety of other
porphyrin-imide dyads. This possibility was investigated using
transient absorption techniques. Figure 2 shows the transient
absorption spectrum of 5 in benzonitrile solution obtained 5 ps
after excitation at 590 nm (where only the porphyrin moiety
absorbs) with a ∼200 fs laser pulse, averaged over several
hundred thousand laser flashes. No significant sample decom-
1
formation of P-NIm with the laser pulse, and the decay of
the transient at 650 nm (not shown) is due to the decay of 1P-
NIm, as confirmed by the time-resolved fluorescence measure-
1
ments. The decay of P-NIm is attributed to photoinduced
electron transfer to yield the P•+-NIm•- charge-separated state,
which is therefore formed with a time constant of 100 ps.
The 12.9 ps rise time of the transient absorption at 650 nm
is actually associated with the decay of the P•+-NIm•- charge-
separated state. The time constant for decay of the charge-
separated species appears as a rise time in the transient
absorption experiment because the rate constant for charge
recombination of P•+-NIm•- to the ground state is larger than
that for its formation from 1P-NIm. Related kinetic behavior
has been observed in dyads consisting of porphyrins linked to
quinone acceptors.35 Similar transient absorption experiments
were carried out in 2-methyltetrahydrofuran and in toluene. The
results are tabulated in Table 1. It will be noted that in toluene,
the lifetime of the P•+-NIm•- state is ∼7 times longer than
1
that of P-NIm, and normal kinetic behavior is observed.
Figure 3 shows picosecond transient absorption spectra
obtained from benzonitrile solutions of triad 4 after excitation
at 590 nm with a ∼200 fs laser pulse. The spectrum obtained
5 ps after excitation is characteristic of the porphyrin first excited
singlet state, with minima at 650 and ∼720 nm due to ground-
state depletion and stimulated emission (see Figure 2). At 300
ps after excitation, the signature of the excited singlet state is

Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5217
Figure 4. Decay of the transient absorbance of the carotenoid radical
cation of the C•+-P-NIm•- charge-separated state following excitation
with a ∼5 ns, 650 nm laser pulse. The decays were measured at the
maxima of the radical cation absorptions. The decays, in order of
decreasing maximum amplitude, were obtained in benzonitrile, ethyl
acetate, 2-methyltetrahydrofuran, and dichloromethane and represent
samples with equal absorbance at the excitation wavelength.
Figure 3. Transient absorption spectra of triad 4 in benzonitrile taken
5 and 300 ps after excitation with a 200 fs, 590 nm laser pulse. The
inset shows the rise of the carotenoid radical cation absorption at 930
nm with a time constant of 99 ps and the decay of the transient
absorbance at 630 nm, which includes three exponential components
(see text).
absent, and the spectrum is dominated by a strong absorption
in the 850-1000 nm region that is characteristic of the
carotenoid radical cation.36 This transient represents the C•+-
P-NIm•- charge-separated state.
maximum of the carotenoid radical cation absorption is solvent
dependent and occurs at 920 nm in ethyl acetate, 930 nm in
2-methyltetrahydrofuran, 960 nm in dichloromethane, and 950
nm in benzonitrile. The decay of the carotenoid radical cation
due to charge recombination is a single-exponential process;
the lifetimes are 190 ns in ethyl acetate, 173 ns in 2-methyltet-
rahydrofuran, 340 ns in dichloromethane, and 430 ns in
benzonitrile. Quantum yields for the C•+-P-NIm•- state in
the various solvents were determined from linear regression
analyses of the dependence of the amplitude of the carotenoid
The inset in Figure 3 shows the transient absorption kinetics
measured at 630 and 930 nm. Global analysis of the data in
the 850-1032 nm region demonstrated that the carotenoid
radical cation absorption rises from zero as a single exponential
with a time constant of 99 ps (Table 1). The absorption does
not decay on the subnanosecond time scale. Global analysis
of the data in the 604-750 nm region, taken on different time
scales, showed that the decay is best fit by three exponential
components: 7.7 ps (80%), 99 ps (14%), and >1 ns (6%). The
spectrum for the 7.7 ps component has a maximum at 624 nm,
and this spectrum and the lifetime indicate that this component
is due to the carotenoid first excited singlet state. The carotenoid
transient absorption arises because the carotenoid is responsible
for about 10% of the total excitation light absorption at 590
nm. The 99 ps component has the spectrum of the porphyrin
first excited singlet state, and its lifetime is the same as that
determined for C-1P-Q by time-resolved fluorescence spec-
troscopy. The small, long-lived component (τ >1 ns) is due to
the NIm radical anion, as discussed below.
Picosecond transient absorption experiments were also per-
formed with excitation at 556 nm, where the carotenoid is
responsible for 43% of the total light absorption. Global
analysis of the data in the 850-992 nm region showed that the
rise of the carotenoid radical cation was again a single
exponential with a time constant of 99 ps. No formation of
carotene radical cation with a time constant in the 7.7 ps range
was observed.
Picosecond transient absorption experiments with excitation
at 590 nm were also carried out in 2-methyltetrahydrofuran
solution, and the relevant time constants appear in Table 1.
Experiments were performed in toluene solution, but no
carotenoid radical cation was observed under these conditions.
Nanosecond Transient Absorption Experiments. Excited-
state dynamics on a longer time scale were investigated using
nanosecond transient absorption spectroscopy. Figure 4 shows
the decay of the carotenoid radical cation absorption of the C•+-
P-NIm•- state following excitation of triad 4 in various
deoxygenated solvents at 650 nm with a ∼5 ns laser pulse. The
radical cation transient absorption (ꢀ ) 1.6 × 105 L mol-1 cm-1
)
on excitation intensity, using the triplet state of porphyrin 7 as
a standard (φT ) 0.67, (ꢀT - ꢀG)440 ) 6.8 104 L mol-1 cm-1).
The quantum yields were 0.23, 0.20, 0.16, and 0.33 for ethyl
acetate, 2-methyltetrahydrofuran, dichloromethane, and ben-
zonitrile, respectively.
With 650 nm excitation, only the porphyrin first excited
singlet state is produced. Triad 4 in 2-methyltetrahydrofuran
was also excited at 550 nm, where the carotenoid moiety absorbs
60% of the light. Under these conditions, the quantum yield
of C•+-P-NIm•- was only ∼9%. This yield can be attributed
to light absorbed by the porphyrin, in accord with the results
from 650 nm excitation discussed above; excitation of the
carotenoid moiety does not lead to significant (>1%) formation
of the C•+-P-NIm•- charge-separated state.
Nanosecond transient absorption measurements of solutions
of 4 in deoxygenated benzonitrile with 650 nm excitation were
also performed in the 550-580 nm spectral region (Figure 5).
The kinetics at 550 nm feature the prompt appearance of an
absorbance decrease, which recovers with a 430 ns time constant
superimposed on a much weaker decay component of 4.4 µs.
The 430 ns component of the decay is not sensitive to oxygen,
has the lifetime of the carotenoid radical cation, and is assigned
to recovery from the carotenoid ground-state bleaching as C•+-
P-NIm•- decays back to the ground state. The 4.4 µs
component is due to the carotenoid triplet state, as shown by
its spectrum, which is shown 2 µs after excitation as an inset in
Figure 5. The component is quenched by oxygen, as expected
3
for this triplet state. The quantum yield of C-P-NIm is
∼0.03. Formation of 3C-P-NIm is ascribed to triplet-triplet
energy transfer from C-3P-NIm, which is generated by normal
intersystem crossing. Consistent with this interpretation is the

5218 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Tan et al.
first excited singlet state of model porphyrin 7; kcs equals 9.4
× 109 s-1
.
1
kcs )
- km
(1)
( )
τs
The quantum yield of P•+-NIm•- (τskcs) equals 0.99. The P•+-
NIm•- state recombines to the ground state in 12.9 ps, with kcr
) 7.8 × 1010 s-1. The fact that charge recombination is more
rapid than charge separation gives rise to the “inverse kinetic”
spectroscopic behavior discussed above. Rate constants for
charge separation and recombination in benzonitrile and other
solvents appear in Table 2.
Turning to triad 4, the kinetic behavior in benzonitrile may
be discussed with reference to Figure 7, which shows the
relevant high-energy states and their interconversion pathways.
The energies of the excited states have been estimated from
spectroscopic data, and the energies of the charge-separated
states are based on the cyclic voltammetric results presented
above. No corrections for Coulombic effects have been made.
Figure 5. Transient absorption kinetics of triad 4 in benzonitrile
measured after excitation with a ∼5 ns, 650 nm laser pulse. The solid
lines show multiexponential fits to the data at 580 nm (430 ns and 4.4
µs decay times) and 550 nm (430 ns rise, 4.4 µs decay). The insets
show the transient absorption spectra measured 100 ns and 2 µs after
excitation.
Excitation of the porphyrin moiety, as was achieved by 650
nm laser pulses, produces C-1P-NIm, which decays in 96 ps
(an average of the 93 ps lifetime from fluorescence studies and
the 99 ps lifetime from transient absorption measurements.) In
the absence of the imide acceptor, the lifetime of the porphyrin
first excited singlet state is 2.21 ns, as determined from studies
of model carotenoporphyrin 6. Thus, k1, the rate constant for
step 1 in Figure 7, may be estimated as 4.5 × 108 s-1. The
drastic shortening of the lifetime of the porphyrin first excited
singlet state upon introduction of the imide moiety is due to
photoinduced electron-transfer step 2, which yields C-P•+-
NIm•-. Calculation analogous to that indicated in eq 1 yields
k2 ) 1.0 × 1010 s-1, which is close to the rate constant obtained
for photoinduced electron transfer in dyad 5. The quantum yield
of C-P•+-NIm•-, Φ2, is 0.96.
3
fact that in 2-methyltetrahydrofuran the yield of C-P-NIm
increases to ∼0.09. This increase is due to the increased lifetime
of C-1P-NIm in this solvent (Table 1) and thus a higher yield
of C-3P-NIm (Table 2).
The absorption decay at 580 nm following excitation in
benzonitrile can be fitted with two exponential components of
430 ns (79%) and 4.4 µs (21%). The 4.4 µs component is
quenched by oxygen and is assigned to 3C-P-NIm as discussed
above. The spectrum of the 430 ns decay component taken
100 ns after excitation (Figure 5) and its lifetime suggest that
it arises from the NIm•- moiety of the C•+-P-NIm•- charge-
separated state superimposed on carotenoid ground-state bleach-
ing. The spectrum of the radical anion of model imide 8,
generated electrochemically, features absorption in the 500-
650 nm region with a broad maximum at 520 nm (Figure 6).
Comparison of the transient absorbance at 580 nm with that of
the carotenoid radical cation at 950 nm (ꢀ950 ) 1.6 × 105 L
mol-1 cm-1) allows a rough estimation of the extinction
coefficient of NIm•- as ∼2.2 × 104 L mol-1 cm-1 at 580 nm.
This estimate in turn may be used to calculate an extinction
coefficient of ∼3.4 × 104 L mol-1 cm-1 for NIm•- at its 520
nm absorption maximum, which is obscured by carotenoid
ground-state bleach in the spectrum of triad 4. In aerated
samples, the lifetime of the 580 nm decay is reduced by a factor
of up to 2, depending upon the oxygen concentration. This
likely signals some reaction of NIm•- with molecular oxygen.
The intermediate C-P•+-NIm•- state decays by two
routes: charge recombination to the ground state (step 3) and
electron transfer from the carotenoid to give the final C•+-P-
NIm•- species (step 4). The overall quantum yield of C•+-
•-
+
P-NIm , ΦC• -P-NIm•-, was measured to be 0.33 and is given
by eq 2:
k4
ΦC
•- ) Φ
(2)
•+-P-NIm
2 k3 + k4
If we estimate k3 as 7.8 × 1010 s-1, as was found for model
dyad 5, then k4 ) 4.1 × 1010 s-1. The final C•+-P-NIm•-
state decays to the ground state with k5 ) 2.3 × 106 s-1. Rate
constants for some steps in Figure 7 may be calculated from
the time-resolved data in other solvents using these same
considerations. The results are given in Table 2.
Discussion
Photoinduced Electron Transfer from the Porphyrin First
Excited Singlet State. We will first discuss the spectroscopic
results obtained from excitation of the porphyrin moiety in
benzonitrile solution. The first excited singlet state of model
porphyrin 7 has a lifetime of 9.7 ns in this solvent and decays
by intersystem crossing to the triplet, fluorescence, and internal
conversion. Attaching the imide moiety, as in dyad 5, reduces
the lifetime of the porphyrin first excited singlet state, τs, to
105 ps (Table 1) by introducing a new decay pathway:
photoinduced electron transfer to the attached NIm moiety to
yield P•+-NIm•-. (The lifetime noted here is an average of
the 103 ps lifetime measured by fluorescence techniques and
the 107 ps lifetime determined by transient absorption.) The
rate constant for photoinduced electron transfer, kcs, is given
by eq 1, where km is the reciprocal of the 9.7 ns lifetime of the
Photoinduced Electron Transfer from a Carotenoid Ex-
cited Singlet State. Several of the experiments described above
bear on the question of possible photoinduced electron transfer
from 1C-P-NIm to yield the final C•+-P-NIm•- state. When
triad 4 is excited at 590 nm, where the carotenoid absorbs 10%
of the light, the picosecond transient absorption experiments
show that the carotenoid first excited singlet state absorption is
readily observable and has a lifetime of 7.7 ps (Figure 3). The
lifetime of the unperturbed first excited singlet state of a
carotenoid such as that in 4 is ∼10 ps.27,38 Thus, quenching of
the carotenoid first excited singlet state by the porphyrin or imide
moieties of the triad must be minimal. In addition, any such
quenching that was due to photoinduced electron transfer must

Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5219
TABLE 2: Rate Constants and Quantum Yields for Photochemical Processes in Dyad 5 and Triad 4
dyad 5
triad 4
b
c
d
e
f
g
h
kcs
kcr
k1
k2
k3
k4
k5
×10-9 ×10-9 ×10-9 ×10-9 ×10-9 ×10-9 ×10-9
ꢀa
s-1
s-1
1.2
s-1
s-1
8.8
s-1
1.2
s-1
s-1
ΦC-P
ΦC
Φ3
C-P-NIm
•+
•-
•+
•-
-P-NIm
solvent
-NIm
toluene
ethyl acetate
2-methyltetrahydrofuran
dichloromethane
benzonitrile
2.4
6.0
7.6
8.9
25.2
8.5
2.2
3.7
16.6
9.4
0.27
NO
NO
5.3
5.8
2.9
2.3
0.97
NO
0.02
0.23
0.20
0.16
0.33
79
78
0.29
0.33
0.45
3.8
16.6
10.0
79
78
22
41
0.92
0.98
0.96
0.09
0.02
0.03
a Dielectric constant from ref 44. b Determined from the lifetime of the porphyrin first excited singlet state using eq 1. c Determined from
subpicosecond transient absorption results. d Estimated from the lifetime of the porphyrin first excited singlet state of carotenoporphyrin 6 as determined
from time-resolved fluorescence studies. e Determined from the lifetime of the porphyrin first excited singlet state using an equation analogous to
eq 1. f Estimated from corresponding rate in dyad 5. g Calculated according to eq 2. h Calculated from the lifetime of the carotenoid radical cation
as determined by nanosecond transient absorption spectroscopy.
This conclusion is bolstered by the results of excitation at
556 nm, where the carotenoid moiety absorbs 43% of the
excitation. The carotenoid radical cation absorption was again
found to rise with the time constant of the decay of C-1P-
NIm (99 ps), and no rise time corresponding to the 7.7 ns
1
lifetime of C-P-NIm was observed.
Finally, the transient absorption experiments on the nano-
second time scale demonstrate that excitation of the sample at
550 nm, where the carotenoid moiety absorbs approximately
one-half of the light, produced a much lower quantum yield of
the final C•+-P-NIm•- state than did excitation at 650 nm,
where only the porphyrin absorbs. Thus, the porphyrin first
excited singlet state is responsible for the vast majority of the
long-lived charge-separated species.
It has been reported that excitation of the porphyrin moiety
of triad 2 is followed by singlet-singlet energy transfer to the
carotenoid, which then rapidly donates an electron to the imide
to form the C•+-P-Im•- charge-separated state in a single
Figure 6. Absorption spectrum of the radical anion of imide 8 in
electron-transfer step.27 This is not the case with triad 4. It is
true that the porphyrin first excited singlet state of 6, and
benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate
generated electrochemically as described in the text.
therefore that of 4 as well, is quenched by the carotenoid. This
quenching is presumably due to photoinduced electron transfer
or singlet-singlet energy transfer. However, in 4, the quantum
yield of C-P•+-NIm•- from C-1P-NIm is 0.94, the yield of
C•+-P-NIm•- is 0.33, and that of carotenoid quenching process
is only 0.06. This alone requires that the majority of the C•+-
P-NIm•- state be formed via the two-step route involving the
porphyrin first excited singlet state. Charge separation from
the carotenoid first excited singlet state produced by any energy
transfer process may also be ruled out on the basis of the
transient absorption experiments described above, which show
1
that C-P-NIm formed by direct excitation does not lead to
detectable (>1%) formation of the carotenoid radical cation.
Solvent Dependence of Photoinduced Electron Transfer.
Perusal of Tables 1 and 2 shows that the rate of photoinduced
electron transfer from the porphyrin first excited singlet state
to NIm does not correlate in any simple way with the solvent
Figure 7. Relevant high-energy states of triad 4 and their intercon-
version pathways. The energies of the charge-separated states are
estimated from cyclic voltammetric data for model compounds. Rate
constants for the various processes are given in Table 2.
dielectric constant. For example, photoinduced electron transfer
is substantially faster in dichloromethane (dielectric constant
) 8.9) than in benzonitrile (dielectric constant ) 25.2). In this
respect, the NIm acceptor parallels the behavior of quinone
electron acceptors in porphyrin-quinone dyads, which also
show unexpectedly large rates of photoinduced electron transfer
in chlorinated solvents.39-43 In contrast, the rates of photoin-
duced electron transfer between porphyrin moieties in porphyrin
dyads correlate relatively smoothly with the solvent dielectric
constant using the dielectric continuum model for solvent effects
on thermodynamic driving force.44
lead to concurrent formation of the carotenoid radical cation.
Figure 3 shows that at 5 ps, no carotenoid radical cation
absorption in the 930 nm region is detectable. In fact, as shown
in the inset, the carotenoid radical cation absorption rises
exponentially from zero with a time constant equal to the
lifetime of the porphyrin first excited singlet state. Therefore,
the porphyrin first excited singlet state gives rise to the charge-
separated state by the two-step mechanism discussed above, and
the carotenoid singlet state is not involved.
Charge Recombination. The lifetimes of the C•+-P-NIm•-
state in various solvents (Table 1) are roughly 4 orders of

5220 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Tan et al.
magnitude longer than the lifetimes of the precursor C-P•+-
NIm•- states. This tremendous lifetime extension is typical of
triads, tetrads, and pentads that demonstrate multistep electron-
transfer behavior.2,45-47 Indeed, it is at the heart of the
mechanism of action of natural photosynthetic reaction centers.
In principle, recombination of C•+-P-NIm•- may be either
a single-step, direct electron transfer from the imide moiety to
the carotenoid radical cation, or a two-step process. The two-
step mechanism would involve, for example, slow, thermally
activated recombination of C•+-P-NIm•- to C-P•+-NIm•-
followed by rapid recombination of this species. The two-step
process has been observed in a series of carotenoid-porphyrin-
quinone triads,48 whereas both kinds of behavior have been
observed in previously studied carotenoid-porphyrin-imide
triads such as 2 and 3.32
The solvent dependence observed for charge recombination
in 4 is qualitatively consistent with the two-step process. When
solvent polarity is decreased, the energies of both the C•+-P-
NIm•- and C-P•+-NIm•- states should increase because of
loss of solvent dielectric stabilization of the charges. However,
the increase should be smaller for C-P•+-NIm•- because of
compensation due to increased mutual Coulombic stabilization
of the adjacent ions in less polar solvents. This mutual effect
is not as effective in C•+-P-NIm•-, where the charges are
much farther apart. The net result of this is that the energy
difference between C•+-P-NIm•- and C-P•+-NIm•- should
decrease in nonpolar solvents, and this should speed up two-
step charge recombination in less polar solvents, as is observed.
On the other hand, the energy of the final C•+-P-NIm•-
state is expected to increase in nonpolar solvents and solvent
reorganization energy to decrease; direct, single-step charge
recombination in these solvents would likely occur in the
inverted region of the Marcus equation.49,50 Thus, the increase
in driving force with decreasing solvent polarity should result
in a decrease in the rate of charge recombination, in contrast to
the observations. Further information on the charge recombina-
tion process must await studies of temperature dependence.
However, it is interesting to note that charge recombination in
triad 3 has been found to occur by the two-step mechanism at
ambient temperatures.32
that the quantum yield of any such process be exceedingly low
relative to that of the two-step sequence. Since quantum yields
were not reported in the earlier study,27 direct comparison is
not facile. An additional complicating factor is that in triads 2
and 3, photoinduced electron transfer from C-1P-Im to yield
C-P•+-NIm•- is very slow because the thermodynamic driving
force is much lower than in triad 4, and electronic coupling is
likely not as great as in 4. This makes it potentially easier to
detect formation of the final charge-separated states by alterna-
tive routes. In addition, the porphyrin moiety used in the
previous study differs from that in 4, and thus, the energies of
porphyrin excited states, porphyrin redox potentials, and the
degree of electronic coupling of peripheral groups both to and
through the porphyrin moiety will differ. Photoinduced electron
transfer originating from carotenoid excited singlet states is
certainly possible when it can compete with the very rapid
relaxation of these states, and has been observed in other cases.52
Finally, the 4,5-dinitro-1,8-naphthalenedicarboximide moiety
used as an electron acceptor in the molecules discussed here
has been demonstrated to be an easily prepared, electrochemi-
cally reversible, one-electron acceptor with a convenient reduc-
tion potential. It is slightly more easily reduced than benzo-
quinone, and its radical anion has a characteristic absorption
spectrum in the visible region. This imide should be useful in
a variety of similar electron-acceptor applications.
Experimental Section
Synthesis. The preparation of carotenoid 9 has been previ-
ously reported.51
N-(4-Chlorophenyl)-4,5-dinitro-1,8-naphthalenedicarbox-
imide (8). A mixture of 100 mg (0.35 mmol) of 4,5-dinitro-
1,8-naphthalenedicarboxylic anhydride53 and 60 mg (0.47 mmol)
of 4-chloroaniline in 5 mL of glacial acetic acid was refluxed
overnight and then poured into 50 mL of water. The off-white
precipitate was collected by filtration to yield 110 mg of crude
product. This was recrystallized from chloroform and acetone
to give 50 mg of white, silky crystalline imide (8) (36% yield),
mp 296-298 °C. 1H NMR (300 MHz, CDCl3): δ 7.26 (2H,
d, J ) 9 Hz, chlorophenyl), 7.56 (2H, d, J ) 9 Hz, chlorophen-
yl), 8.51 (2H, d, J ) 8 Hz, naphthyl), 8.87 (2H, d, J ) 8 Hz,
naphthyl). MS: m/z 397 (M+).
Conclusions
5-(4-Acetamidophenyl)-10-(4-aminophenyl)-15,20-bis(4-
methylphenyl)porphyrin (10). A 57 mg portion (0.085 mmol)
of 5,10-bis(4-aminophenyl)-15,20-bis(4-methylphenyl)porphy-
rin51 and 38 mg of triethylamine were dissolved in 10 mL of
chloroform. A solution of acetyl chloride in chloroform was
slowly added dropwise with stirring. The reaction was moni-
tored by thin-layer chromatography, and addition of acetyl
chloride was stopped when a maximum amount of 10 was
detected. The chloroform solution was washed with saturated
aqueous sodium bicarbonate and water and dried with potassium
carbonate. The solvent was removed by distillation at reduced
pressure and the resulting material was chromatographed on
silica gel (dichloromethane containing 5% methanol) to give
16 mg porphyrin 10 (26% yield). 1H NMR (300 MHz, CDCl3):
δ -2.76 (2H, s, pyrrole NH), 2.34 (3H, s, acetamido CH3), 2.70
(6H, s, Ar-CH3), 7.04 (2H, d, J ) 8 Hz, 10Ar3,5-H), 7.49
(1H, s, amide NH), 7.54 (4H, d, J ) 8 Hz, 15,20Ar3,5-H), 7.85
(2H, d, J ) 8 Hz, 5-Ar3,5-H), 7.98 (2H, d, J ) 8 Hz, 10Ar2,6-
H), 8.08 (2H, d, J ) 8 Hz, 15(20)Ar2,6-H), 8.09 (2H, d, J ) 8
Hz, 20(15)Ar2,6-H), 8.15 (2H, d, J ) 8 Hz, 5Ar2,6-H), 8.82-
8.93 (8H, m, pyrrole H). MS: m/z 715 (M)+.
In triad 4 photoinduced electron transfer originating from the
porphyrin first excited singlet state has been found to initiate a
two-step electron-transfer sequence that generates the C•+-P-
NIm•- state in high yield. In benzonitrile, the C-P•+-NIm•-
initial charge-separated state is produced with a quantum yield
of 0.96, and C•+-P-NIm•- is formed with an overall yield of
0.33. The high yield, long lifetime, and high energy of the final
C•+-P-NIm•- state in 4 suggests that triads of this general
type may be useful in the design of more complex systems for
harvesting photon energy or for optoelectronic applications.
In contrast to the results reported for closely related triads 2
and 3,27 photoinduced electron transfer from the carotenoid first
excited singlet state to yield C•+-P-NIm•- was not detected
in 4. The reasons for this difference in behavior are not clear.
It is unlikely that the fact that 4 features a 5,10 relationship of
the carotenoid and acceptor on the porphyrin macrocycle
whereas 2 and 3 have the 5,15 relationship is significant. Earlier
studies have shown that carotenoid-porphyrin-quinone triads
differing only in this respect have virtually identical photo-
chemical properties.48,51 Several factors may contribute to the
difference in behavior. In the first place, the results for 4 do
not completely rule out all photoinduced electron transfer from
the carotenoid first excited singlet state; they simply require
Dyad 5. A mixture of 20 mg (0.028 mmol) of porphyrin 10
and 20 mg (0.069 mmol) of 4,5-dinitro-1,8-naphthalenedicar-
boxylic anhydride in 5 mL of glacial acetic acid was refluxed

Photoinduced Electron Transfer
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5221
gently for 1 h. The reaction mixture was then poured into 20
mL of water and extracted with three 10 mL portions of
dichloromethane. The organic phase was washed with 5%
aqueous sodium carbonate and with water and then dried over
potassium carbonate. The solvent was removed by distillation
at reduced pressure and the residue was chromatographed (silica
gel, dichloromethane containing 5% methanol) to give 19.8 of
mg dyad 5 (72% yield). 1H NMR (500 MHz, CDCl3): δ -2.69
(2H, s, pyrrole NH), 2.37 (3H, s, acetamido CH3), 2.71 (3H, s,
15(20)Ar-CH3), 2.72 (3H, s, 20(15)Ar-CH3), 7.54 (2H, d, J
) 8 Hz, 15(20)Ar3,5-H), 7.57 (1H, s, amide NH), 7.57 (2H, d,
J ) 8 Hz, 20(15)Ar3,5-H), 7.67 (2H, d, J ) 8 Hz, 10Ar3,5-H),
7.91 (2H, d, J ) 8 Hz, 5Ar3,5-H), 8.10 (4H, d, J ) 8 Hz,
15,20Ar2,6-H), 8.17 (2H, d, J ) 8 Hz, 5Ar2,6-H), 8.41 (2H, d,
J ) 8 Hz, 10Ar2,6-H), 8.48 (2H, d, J ) 8 Hz, naphthalimido
H), 8.90 (2H, d, J ) 8 Hz, naphthalimido H), 8.86-8.95 (8H,
m, pyrrole H). MS: m/z 985 (M)+.
8 Hz, naphthalimido H), 8.89-8.96 (8H, m, pyrrole H). MS:
m/z 1459 (M)+.
Dyad 6. A portion of 7′-apo-7′-(4-carboxyphenyl)-â-
carotene54 (10 mg, 0.019 mmol) was dissolved in a mixture of
2 mL of dry toluene and 1 mL of dry pyridine. An excess of
thionyl chloride was added and the mixture was stirred under a
nitrogen atmosphere at room temperature for 10 min. All
solvents and excess thionyl chloride were removed by vacuum
distillation. The carotene acid chloride was redissolved in a
mixture of 2 mL of dichloromethane and 0.5 mL of pyridine.
The resulting solution was added to a dichloromethane solution
containing 6.6 mg (0.0092 mmol) of aminoporphyrin 10. The
reaction mixture was stirred under a nitrogen atmosphere
overnight at room temperature and worked up by washing with
saturated sodium bicarbonate and water and drying with sodium
sulfate. The solvent was removed by distillation at reduced
pressure and the residue was chromatographed (silica gel,
dichloromethane containing 3% methanol) to yield 10.2 mg of
dyad 6 (90% yield). 1H NMR (500 MHz, CDCl3): δ -2.77
(2H, s, pyrrole NH), 1.04 (6H, s, Car 16-CH3, Car 17-CH3),
1.46-1.50 (2H, m, Car 2-CH2), 1.60-1.65 (2H, m, Car 3-CH2),
1.72 (3H, s, Car 18-CH3), 1.98 (3H, s, Car 19-CH3), 1.99 (3H,
s, Car 20-CH3), 2.01 (3H, s, Car 20′-CH3), 2.03 (2H, m, Car
4-CH2), 2.10 (3H, s, Car 19′-CH3), 2.36 (3H, s, acetamido CH3),
2.71 (6H, s, 15,20Ar-CH3), 6.1-7.1 (14H, m, Car dCH-), 7.48
(1H, s, amide NH), 7.55 (4H, d, J ) 8 Hz, 15,20Ar3,5-H), 7.63
(2H, d, J ) 8 Hz, Car 1′,5′-H), 7.89 (2H, d, J ) 8 Hz, 10Ar3,5-
H), 7.99 (2H, d, J ) 8 Hz, Car 2′,4′-H), 8.04 (2H, d, J ) 8 Hz,
5Ar3,5-H), 8.09 (4H, d, J ) 8 Hz, 15,20Ar2,6-H), 8.15 (1H, s,
amide NH), 8.16 (2H, d, J ) 8 Hz, 10Ar2,6-H), 8.22 (2H, d, J
) 8 Hz, 5Ar2,6-H), 8.85-8.88 (8H, m, pyrrole H). MS: m/z
1232 (M)+.
Absorption Spectrum of NIm•-. A 50 mL glass cuvette
was fitted for controlled-potential electrolysis; the three-electrode
setup consisted of an optically transparent indium tin oxide
(ITO) working electrode, a platinum foil counter electrode, and
a silver/silver nitrate electrode to which all of the following
potentials are referenced. To the cuvette was added 20 mL of
a 0.1 M solution of tetrabutylammonium hexafluorophosphate
in benzonitrile. This electrolyte solution was purged with high-
purity nitrogen and exhibited no electroactivity within the usable
range of the ITO electrode in this system (-1.2 to +0.6 V).
Approximately 4 mg of N-(4-chlorophenyl)-4,5-dinitro-1,8-
naphthalenedicarboximide (8) was dissolved in this solution.
Cyclic voltammetry indicated a half-wave potential for reduction
of 8 near -0.9 V. The cathodic and anodic sweeps exhibited
significant dissimilarities in their wave shapes, indicating unique
kinetics for each charge-transfer process. A small quantity of
ferrocene was added to the cell in a subsequent cyclic volta-
mmetry run after the spectrometric analysis, and this couple
also displayed some quasi-reversibility in its waveforms. Thus,
the anomalous kinetics may be attributable to the use of the
semiconducting working electrode.
Dyad 11. A suspension of 11.8 mg (0.012 mmol) of dyad 5
in 3 mL of 6 N hydrochloric acid was heated on a steam bath
for 1 h. After the reaction mixture was cooled to room
temperature, the green solid was removed by filtration and
dissolved in 20 mL of dichloromethane. The solution was
washed with 5% sodium carbonate and water and dried with
potassium carbonate, and the solvent was removed by distillation
under reduced pressure. The purple residue was chromato-
graphed (silica gel, dichloromethane containing 3% methanol)
to give 8.7 mg of aminoporpyrin-imide dyad 11 (77% yield).
1H NMR (400 MHz, CDCl3 + CD3OD): δ 2.72 (3H, s, 15-
(20)Ar-CH3), 2.73 (3H, s, 20(15)Ar-CH3), 7.11 (2H, d, J )
8 Hz, 5Ar3,5-H), 7.57 (2H, d, J ) 8 Hz, 15(20)Ar3,5-H), 7.58
(2H, d, J ) 8 Hz, 20(15)Ar3,5-H), 7.71 (2H, d, J ) 8 Hz,
10Ar3,5-H), 8.02 (2H, d, J ) 8 Hz, 5Ar2,6-H), 8.11 (4H, d, J
) 8 Hz, 15,20Ar2,6-H), 8.44 (2H, d, J ) 8 Hz, 10Ar2,6-H),
8.54 (2H, d, J ) 8 Hz, naphthalimido H), 8.96 (2H, d, J ) 8
Hz, naphthalimido H), 8.88 (8H, brs, pyrrole H). MS: m/z 943
(M)+.
Triad 4. A portion of 7′-apo-7′-(4-carboxyphenyl)-â-
carotene54 (12.7 mg, 0.024 mmol) was dissolved in a mixture
of 2 mL of dry toluene and 1 mL of dry pyridine. An excess
of thionyl chloride was added and the mixture was stirred under
a nitrogen atmosphere at room temperature for 10 min. All
solvents and excess thionyl chloride were removed by vacuum
distillation. The carotene acid chloride was redissolved in a
mixture of 2 mL of dichloromethane and 0.5 mL of pyridine.
The resulting solution was added to a dichloromethane solution
containing 8.7 mg (0.0093 mmol) of aminoporphyrin-imide
dyad 11. The reaction mixture was stirred under a nitrogen
atmosphere overnight at room temperature and worked up by
washing with saturated sodium bicarbonate and water and drying
with sodium sulfate. The solvent was removed by distillation
at reduced pressure and the residue was chromatographed (silica
gel, dichloromethane containing 3% methanol) to yield 6.1 mg
of triad 4 (44% yield). 1H NMR (500 MHz, CDCl3): δ -2.77
(2H, s, pyrrole NH), 1.04 (6H, s, Car 16-CH3, Car 17-CH3),
1.42-1.51 (2H, m, Car 2-CH2), 1.60-1.64 (2H, m, Car 3-CH2),
1.72 (3H, s, Car 18-CH3), 1.98 (3H, s, Car 19-CH3), 1.99 (3H,
s, Car 20-CH3), 2.01 (3H, s, Car 20′-CH3), 2.03 (2H, m, Car
4-CH2), 2.10 (3H, s, Car 19′-CH3), 2.71 (3H, s, 15(20)Ar-CH3),
2.72 (3H, s, 20(15)Ar-CH3), 6.1-7.1 (14H, m, Car dCH-),
7.54-7.60(4H, m, 15,20Ar3,5-H), 7.61 (2H, d, J ) 8 Hz, Car
1′,5′-H), 7.65 (2H, d, J ) 8 Hz, 10Ar3,5-H), 7.99 (2H, d, J )
8 Hz, Car 2′,4′-H), 8.06 (2H, d, J ) 8 Hz, 5Ar3,5-H), 8.10
(4H, d, J ) 8 Hz, 15,20Ar2,6-H), 8.17 (1H, s, amide NH), 8.23
(2H, d, J ) 8 Hz, 5Ar2,6-H), 8.40 (2H, d, J ) 8 Hz, 10Ar2,6-
H), 8.47 (2H, d, J ) 8 Hz, naphthalimido H), 8.89 (2H, d, J )
The cell was placed in the sample compartment of a Shimadzu
UV2100U UV-vis spectrometer so that the probe beam passed
through the solution normal to the ITO electrode. Changes in
the absorbance of the cell were observed only when a potential
below -0.8 V was applied to the working electrode. Figure 6
shows spectra recorded at 0.0 and -1.0 V; the latter is an
average of four scans acquired once the cathodic current reached
its diffusion-limited value (after about 20 s). Returning the
applied potential to a value below E1/2 allowed eventual
reoxidation of NIm•-, whereupon the spectrum matched that
of the original solution at 0.0 V.
Instrumental Techniques. The 1H NMR spectra were

5222 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Tan et al.
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Fluorescence decay measurements were performed on ∼1 ×
10-5 M solutions by the time-correlated single-photon counting
method. The excitation source was a cavity-dumped Coherent
700 dye laser pumped by a frequency-doubled Coherent Antares
76s Nd:YAG laser.55 The instrument response function was
35 ps, as measured at the excitation wavelength for each decay
experiment with Ludox AS-40.
Transient absorption measurements on the picosecond time
scale were made using the pump-probe technique. The sample
was dissolved in purified solvent and the resulting solution was
circulated by magnetic stirring in a cuvette having a 2 mm path
length in the beam area. Excitation was at 590 nm with 150-
200 fs, 30 µJ pulses at a repetition rate of 540 Hz. The signals
from the continuum-generated white-light probe beam were
collected by an optical spectrometric multichannel analyzer with
a dual diode array detector head.56
Nanosecond transient absorption measurements were made
with excitation from an Opotek optical parametric oscillator
pumped by the third harmonic of a Continuum Surelight Nd:
YAG laser. The pulse width was ∼5 ns, and the repetition rate
was 10 Hz. The detection portion of the spectrometer has been
described elsewhere.57
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Acknowledgment. This work was supported by the National
Science Foundation (CHE-9413084). This is Publication 323
from the ASU Center for the Study of Early Events in
Photosynthesis.
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