Maniga Sumida Stone Moore Moore Gust J Porphyr Phthalocyan 3 1999 32

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 3, 32–44 (1999)

Increasing the Yield of Photoinduced Charge Separa-
tion through Parallel Electron Transfer Pathways

NYANGENYA I. MANIGA, JOHN P. SUMIDA, 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, AZ 85287-1604, USA

Received 29 January 1998
Accepted 6 March 1998

ABSTRACT: A strategy for increasing the yield of long-lived photoinduced charge separation in artificial
photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has
been investigated. Excitation of the porphyrin moiety of a carotenoid (C)–porphyrin (P)–naphthoquinone (Q)
. .
molecular triad leads to the formation of a charge-separated state C ‡–P–Q À with an overall quantum yield of
0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives
. .
C–P ‡–Q À with a quantum yield of $1.0. However, electron transfer from the carotenoid to the porphyrin
. .
radical cation to form the final state does not compete well with charge recombination of C–P ‡–Q À, reducing
the yield. The related pentad C3–P–Q features carotenoid, porphyrin and quinone moieties closely related to
. .
those in the triad. Excitation of this molecule gives a C ‡–P(C2)–Q À state with a quantum yield of 0.073. The
enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with
charge recombination. The yield does not increase by the statistically predicted factor of three owing to small
differences in thermodynamic driving force between the two compounds. # 1999 John Wiley & Sons, Ltd.

KEYWORDS: porphyrin; photoinduced electron transfer; spectroscopy; synthesis; quinone; carotenoid

INTRODUCTION [7, 8]. Excitation of the porphyrin moiety generates
C–1P–Q, which decays mainly by photoinduced
. .
A large number of model photosynthetic reaction electron transfer to the quinone, giving C–P ‡–Q À
centers based on porphyrins or related chromophores with a quantum yield of 0.95 in benzonitrile (step 2 in
covalently linked to electron donor and/or acceptor Fig. 1). This state recombines to the ground state
moieties have been described in the last two decades within a few picoseconds (step 3), but competing with
[1–6]. Although dyads consisting of porphyrins linked charge recombination is electron donation from the
to quinones or other electron acceptors can undergo carotene to the porphyrin radical cation to produce the
. .
photoinduced electron transfer to produce charge- C ‡–P–Q À charge-separated state (step 4). Owing to
separated states in high yield, generating such states the large spatial separation of the charges in this
wherein charge recombination is slow, and thus species and the weak electronic coupling between the
facilitating the harvesting of the stored energy by anion and cation radicals, charge recombination (step
diffusional or other processes, has in general required 5) requires 67 ns. The two-step electron transfer
molecular triads or more complex supermolecular process has tremendously increased the lifetime of
constructs which employ multistep electron transfer the charge separation. Photosynthetic reaction centers
pathways conceptually related to those found in also use a multistep electron transfer cascade to
natural reaction centers. Carotene (C)–porphyrin separate charge across the thickness of a lipid bilayer
(P)–quinone (Q) triad 1 exemplifies this strategy membrane, generating long-lived charge separation.
. .
The yield of C ‡–P–Q À for triad 1 in benzonitrile is
——————— only 0.044 [8]. The low yield results from inefficient
Correspondence to: D. Gust, Center for the Study of Early Events in
Photosynthesis, Department of Chemistry and Biochemistry, competition of step 4 with charge recombination by
Arizona State University, Tempe, AZ 85287-1604, USA. step 3. In our laboratories we have used a variety of
CCC 1088–4246/99/010032–13 $17.50
strategies to increase the yield of the final, long-lived
# 1999 John Wiley & Sons, Ltd. charge-separated state in artificial reaction centers [2].

INCREASING THE YIELD OF PHOTOINDUCED CHARGE SEPARATION 33

Fig. 1. Transient states and relevant interconversion path-
ways for C–P–Q triad 1.

nation reactions in the Marcus inverted region [9, 10].
The electronic coupling between donor and acceptor
moieties has been tuned in order to alter electron
transfer rate constants in favorable directions [8, 11–
13]. The effects of medium (solvent) and temperature
have also been investigated [14, 15]. Although the
yields of long-lived charge separation can be increased
substantially using these approaches, one of the most
successful has been the incorporation of additional
secondary electron donors and acceptors [12, 13, 16–
19]. For example, we have reported a carotene–
diporphyrin–diquinone molecular pentad in which the
. .
final C ‡–PZn–P–Q–Q À charge-separated state is
generated with an overall yield of 0.83 and a lifetime
of several hundred microseconds [12, 19].
It was proposed that one reason for the high yield of
charge separation in the pentads was the use of a
parallel multistep electron transfer strategy. Two
electron transfer steps compete with charge recombi-
. .
nation of the initially formed C–PZn–P ‡–Q À–Q and
lead eventually to the final state. Thus competition
with charge recombination is more efficient than
would have been the case if either pathway were
operating alone (as in the triad discussed above).
Subsequently, this strategy was investigated in a
porphyrin–diquinone triad, where two photoinduced
electron transfer steps compete with decay of an
excited singlet state [20]. In the present work the
Structures parallel multistep electron transfer approach has been
investigated quantitatively through the preparation
and study of C3–P–Q pentad 2 and related model
The energies of the various species have been adjusted compounds 3–5. This pentad is structurally very
in order to increase the rates of favorable electron similar to triad 1. However, it was expected that
. .
transfer steps occurring in the normal region of the charge recombination of the C3–P ‡–Q À state, formed
Marcus relationship and slow down charge recombi- by photoinduced electron transfer from the porphyrin

# 1999 John Wiley & Sons, Ltd. J. Porphyrins Phthalocyanines 3, 32–44 (1999)

34 N. I. MANIGA ET AL.

first excited singlet state, would have to compete with
electron donation from all three carotenoid moieties to
the porphyrin radical cation. Statistically, this should
. .
increase the yield of a final C ‡–P(C2)–Q À state by a
factor of three if the parallel strategy were successful.

RESULTS

Synthesis
Fig. 2. Absorption spectra of C3–P–Q pentad 2 (—),
The preparation of triad 1 has been reported previously tricarotenoporphyrin 5 (…) and carotenoporphyrin 4 (- - -)
[21]. Although several general approaches to the in benzonitrile solution.
preparation of pentad 2 were investigated, the most
successful involved attachment of the three carotenoid
moieties to the tetraarylporphyrin, followed by linkage
of the quinone. Basic partial hydrolysis of 5,10,15,20- features are apparent in the spectra of triad 1 and of
tetrakis(4-trifluoroacetamidophenyl)porphyrin yielded model carotenoporphyrin 4 and model tricarotenopor-
the porphyrin bearing a single trifluoroacetyl protect- phyrin 5 (Fig. 2). The spectra of 1 and 4 are closely
ing group. This porphyrin was linked to three approximated in the 400–800 nm region by a linear
carotenoid moieties via amide bonds. The remaining combination of the spectra of model porphyrin 3 and a
trifluoroacetyl group was then removed and the model carotenoid (7'-apo-7'-(4-methoxycarbonylphe-
resulting aminoporphyrin was coupled to the naphtho- nyl)-b-carotene). The spectra of pentad 2 and
quinone derivative via the quinone acid anhydride. tricarotenoporphyrin 5 are approximated by a similar
The synthetic details and characterization data appear linear combination employing three times as much
in the Experimental section. carotenoid absorption. Thus the absorption spectra of
the dyad, triad and pentad are not indicative of strong
electronic interactions among the chromophores. The
Electrochemistry
chromophores in the multicomponent molecules will
Cyclic voltammetric studies of model porphyrins, therefore be treated as essentially separate entities
quinones and carotenoids were performed in order to with weak electronic interactions, rather than as single
obtain estimates of the energies of the various charge- species with extended conjugation. In this connection,
separated states in the triad and pentad. The first the electronic interaction among the chromophores is
oxidation potential of a model for the carotenoid limited in part by the fact that the aryl rings on the
moiety common to 1 and 2 is ‡0.65 V vs SCE [22]. porphyrin moiety reside at steep (45 °–90 °) angles to
The first oxidation potential in benzonitrile solution of the plane of the macrocycle, owing to steric repulsions
a model for the porphyrin moiety of 1, 5,15-bis [8].
(4-acetamidophenyl)-10,20-bis(4-methylphenyl)por-
phyrin, is ‡0.93 V,[8] whereas that for a model of the
Emission Spectra
porphyrin in 2, 5,10,15,20-tetrakis(4-acetamidophe-
nyl)porphyrin (3), is ‡0.91 V. The first reduction The emission spectrum of porphyrin 3 in benzonitrile
potential of a model for the quinone moiety, 6- is typical of those of tetraarylporphyrins and features
phenylcarbamyl-1,4-naphthoquinone, is À0.58 V, maxima at 665 and 730 nm in a ratio of intensities of
measured under the same conditions [8]. 2.3:1. The emission spectra of pentad 2 and tricar-
otenoporphyrin 5, with excitation at 590 nm where
most of the absorption is due to the porphyrin moiety,
Absorption Spectra
are virtually identical to that of 3 in shape, but the
Figure 2 shows the absorption spectrum of pentad 2 in quantum yield is substantially reduced. No emission
benzonitrile solution. The maxima at 430, 590 and due to the carotenoid moiety was observed, as
653 nm are characteristic of porphyrin absorption, expected since the fluorescence quantum yields of
whereas the shoulder at 460 nm and the maxima at 488 carotenes are infinitesimal. The fluorescence excita-
and 517 nm signify carotenoid absorption. Similar tion spectrum for porphyrin emission in 5 indicated



Fig. 3. Decay-associated spectra obtained after excitation of Fig. 4. Transient states and relevant interconversion path-
a $1 Â 10À5 M solution of pentad 2 in benzonitrile with a ways for C3–P–Q pentad 2.
590 nm laser pulse. The lifetimes of the components are
0.092 (*), 0.83 (~) and 2.88 ns (&).

resulting from electron transfer, singlet energy transfer
that the efficiency of singlet–singlet energy transfer or perturbation of the porphyrin p-electron system by
from the carotenoid moieties to the porphyrin was the carotenoid. Much stronger quenching is observed
negligible (` 10%). in the quinone-bearing pentad 2. This increase is
More information concerning the fluorescence attributed to photoinduced electron transfer by step 2
quenching in 2 and 5 was obtained using time- in Fig. 4. The C3–1P–Q excited state donates an
. .
resolved fluorescence spectroscopy. The samples in electron to the quinone, yielding a C3–P ‡–Q À
benzonitrile solution were excited at 590 nm with $ charge-separated state. Time-resolved fluorescence
10 ps laser pulses and the fluorescence decays were and absorption experiments on triad 1 and related
measured using the single-photon-timing technique porphyrin–quinone species have documented this
(see Experimental section). In the case of 2, decays electron transfer behavior. For example, the C–1P–Q
were measured at nine wavelengths in the 640–755 nm state of 1 in benzonitrile solution has a lifetime of 110
region and the decay profiles were analyzed globally ps and decays by photoinduced electron transfer to
. .
as the sum of three exponential processes (w2 = 1.13) yield C–P ‡–Q À, whereas a comparable carotenopor-
to yield the decay-associated spectra in Fig. 3. The phyrin model system has a fluorescence lifetime of
major component consists of a decay with the 2.2 ns [7, 8, 21].
emission band shape of the porphyrin and a lifetime
of 0.092 ns. The only other significant component has
Transient Absorption Spectroscopy
a lifetime of 0.83 ns and likely represents a small
. .
amount of material in which the quinone has been By analogy with 1, the C3–P ‡–Q À species may either
reduced (see below). The fluorescence lifetime of undergo charge recombination to the ground state by
model porphyrin 3, measured under similar condi- step 3 in Fig. 4 or experience electron donation from
.
tions, is 8.6 ns (w2 = 1.16). The fluorescence decay of one of the carotenoid polyenes to generate a final C ‡–
.À
model tricarotenoporphyrin 5, measured at seven P(C2)–Q state (steps 4). Transient absorption
wavelengths in the 650–750 nm region, was analyzed spectroscopy on the nanosecond time scale was used
globally to yield a major (! 92%) component with a to investigate the fate of the initial charge-separated
lifetime of 0.99 ns and a minor decay with a time state. A sample of 2 in benzonitrile solution
constant of 2.74 ns (w2 = 1.14). The major component ($4 Â 10À5 M) was excited at 650 nm (where only
is assigned to the decay of C3–1P and the minor one to the porphyrin absorbs) with a 5 ns laser pulse and the
an impurity or conceivably a minor conformation. absorbance of any transients produced was monitored
Thus the fluorescence lifetimes of C3–1P and by a steady state probe beam. A strong transient
C3–1P–Q are strongly quenched compared with that absorption was observed in the long-wavelength
of model porphyrin 3, as expected from the steady region, with a maximum at 945 nm (Fig. 5). This
state emission results. The quenching of the porphyrin was assigned to the carotenoid radical cation of the
. .
first excited singlet state by attached carotenoids has C ‡–P(C2)–Q À charge-separated state [22].
been previously reported and discussed [23–28]. It is Excitation of triad 1 under similar conditions
due to some form of enhanced internal conversion yielded a similar carotenoid radical cation transient



. .
Fig. 5. Transient absorption spectrum of C ‡–P(C2)–Q À Fig. 6. Decay of transient in Fig. 5 measured at 940 nm (*).
taken 20 ns after laser excitation of a $4 Â 10À5 M solution Data were taken every 5 ns. The full line is a single-
of pentad 2 with a 5 ns laser pulse at 650 nm. The transient exponential fit to the decay with a time constant of 67 ns.
absorption is due to the carotenoid radical cation.
. . . .
energies of the final C ‡–P(C2)–Q À and C ‡–P–Q À
.‡ .À
absorption due to formation of C –P–Q by steps 2 states are both 1.23 eV. These estimates do not attempt
and 4 of Fig. 1. Quantitative comparison of the two to correct for the influence of coulombic stabilization
.
compounds revealed that the quantum yield of C ‡– of each ion by its counterpart in the radical pairs or for
.À .
P(C2)–Q in 2 was 1.7 times larger than that of C ‡– any effects due to differences in solvent dielectric
.À
P–Q in 1. This determination was made by taking properties. Any such effects would be expected to be
the ratio of the slopes of the linear dependences of the small and essentially identical for 1 and 2 in a polar
transient absorbance of the carotenoid radical cation solvent such as benzonitrile. The energies of the
immediately after the laser pulse on laser power. relevant transient states of the triad and pentad are
Given the reported quantum yield for 1 of 0.044, the shown schematically in Figs 1 and 4.
. .
quantum yield of C ‡–P(C2)–Q À in benzonitrile is
0.075.
. Photoinduced Electron Transfer
The decay of the carotenoid radical cation of C ‡–
.À
P(C2)–Q was also observed (Fig. 6). The decay was The yield and lifetime data reported above can be used
exponential with a time constant of 67 ns. An identical to evaluate rate constants for the photochemical steps
. .
lifetime was measured for the C ‡–P–Q À state of triad shown in Figs 1 and 4. The rate constant for
1. photoinduced electron transfer step 2 may be esti-
mated from
1
DISCUSSION k2 ˆ À k1 …1†
(s
As the main point of this work is the comparison of the where ts is the lifetime of the porphyrin first excited
photochemistry of pentad 2 and triad 1, the properties singlet state as derived from the time-resolved
of these two molecules will be discussed below in fluorescence studies. For triad 1, ts is 110 ps and k1
tandem. may be estimated as 4.5 Â 108 sÀ1, as determined from
the 2.2 ns porphyrin singlet state lifetime of a
carotenoporphyrin model compound. Thus k2 is
Energetics . .
8.6 Â 109 sÀ1. The quantum yield of C–P ‡–Q À is
The wavenumber average of the long-wavelength given by
absorption maximum and short-wavelength fluores- k2
cence maximum of pentad 2 yields an energy of È2 ˆ …2†
1.88 eV for C3–1P–Q. The corresponding spectra of k1 ‡ k2
triad 1 yield an energy of 1.89 eV for C–1P–Q. The and equals 0.95. In the case of pentad 2 the same
electrochemical results for the model compounds equations yield a value for k2 of 9.9 Â 109 sÀ1 (based
discussed above allow estimation of the energies of on a k1 value of 1.0 Â 109 sÀ1 from the 0.99 ns lifetime
. . . .
the initial C3–P ‡–Q À and C–P ‡–Q À charge-sepa- of the porphyrin first excited singlet state in 5) and a
rated states as 1.49 and 1.51 eV respectively. The Φ2 value of 0.91.



Secondary Electron Transfer k4a ‡ k4b ‡ k4c
Èfin ˆ È2 …4†
In 1 and 2 the initial charge-separated states have two k3 ‡ k4a ‡ k4b ‡ k4c
possible fates. Charge recombination to the ground Assuming that k4a = k4b = k4c = 2.0 Â 1011 sÀ1,
state (step 3 in Figs 1 and 4) competes with forward k3 = 4.2 Â 1012 and Φ2 = 0.91, the quantum yield of
. .
electron transfer by step 4, which leads to the final, C ‡–P(C2)–Q À should be 0.11, or 2.5 times larger
long-lived charge-separated species. For triad 1 the than that for triad 1. In fact, the experimental yield in 2
rate constant for charge recombination by step 3 has is only 1.7 times that in 1. As discussed in the next
been estimated as 4.2 Â 1012 sÀ1, based on subpico- subsection, this apparent discrepancy can be rationa-
second transient absorption studies of a closely related lized.
porphyrin–quinone dyad [8]. The quantum yield of the
. .
final C ‡–P–Q À species in triad 1 was previously
Effect of Driving Force on Quantum Yield
found to be 0.044 in benzonitrile, as determined by the
comparative method using the triplet state of From the above discussion it is clear that the parallel
5,10,15,20-tetraphenylporphyrin as a standard [8]. multistep electron transfer strategy employed in
The rate constant for step 4 may be calculated from pentad 2 does increase the quantum yield of the final
charge-separated state relative to triad 1, but that the
k4
Èfin ˆ È2 …3† increase is less than the statistically expected factor of
k3 ‡ k4 three. A small part of the effect is due to the increased
non-productive quenching of the porphyrin first
where Φfin is the overall quantum yield of the final
excited singlet state in 2 by the additional carotenoids,
charge-separated state. Substituting the values for Φfin,
relative to that in 1. The lifetime of the porphyrin first
Φ2 and k3 given above yields a k4 of 2.0 Â 1011 sÀ1 for
excited singlet state in a carotenoporphyrin model for
triad 1.
triad 1 is 2.2 ns in benzonitrile, whereas the corre-
In the case of pentad 2 there are three carotenoids
sponding lifetime in tricarotenoporphyrin 5 is 0.99 ns.
capable of electron donation to the porphyrin radical
. . Even though the rate constants k2 for photoinduced
cation of the C3–P ‡–Q À state, and three correspond-
electron transfer in 1 and 2 are very similar, the
ing electron transfer steps, 4a, 4b and 4c, as per Fig. 4.
increase in the rate constant of the competing non-
In the kinetic analysis of the results for 2 we will
productive processes decreases the quantum yield of
assume that the rate constants for all three of these
the initial charge-separated state from 0.95 in 1 to 0.91
steps are essentially identical. Of course, the carote-
in 2.
noid at the 15-position of the porphyrin has a different
The majority of the difference in the measured and
relationship to the quinone from that of the carotenoids
expected quantum yields in pentad 2 must lie in the
at the 10- and 20-positions. However, electron transfer
yield of electron transfer from the carotenoid to the
in step 4 involves only the carotenoid and porphyrin
porphyrin radical cation to produce the final charge-
moieties and would be expected to be relatively
separated state (steps 4 in Figs 1 and 4). This yield
insensitive to this structural difference. In fact, in a
depends upon the ratio of the rate constants for steps 3
previous study of two carotenoporphyrin–quinone
and 4. The rate constants for electron transfer reactions
triads differing only in the relationship of the
(ket) are conveniently discussed in terms of
carotenoid and quinone (5, 10 vs 5,15 on the porphyrin
r 2 3
skeleton), it was found that the two triads had identical % 2 À…ÁG0 ‡ !†2
. .
quantum yields of the C ‡–P–Q À state at both 295 and ket ˆ jV j exp …5†
"2 !kB T
h 4!kB T
220 K [11, 29]. This suggests that the rate constants
for the relevant electron transfer steps are insensitive which has been developed for non-adiabatic transfers
to the structural difference. [30, 31]. The pre-exponential factor includes the
As a first approximation, we make the further electronic matrix element V that describes the
assumption that the rate constants for steps 3 and 4 coupling of the reactant state with that of the product.
_
determined for triad 1 are also appropriate for pentad It also includes Planck’s constant h, Boltzmann’s
2. This might seem reasonable, as the two molecules constant kB, the absolute temperature T and the
differ very little both structurally and energetically, reorganization energy for the reaction, !. The
with the exception of the number of carotenoid reorganization energy is associated with the nuclear
. .
moieties. The quantum yield of the C ‡–P(C2)–Q À motions necessary to carry the molecule from the
species in pentad 2 is given by initial to the final state. It is convenient to express ! as



the sum of a solvent-independent term !i, which dynamic driving force values for 2 discussed above
originates from internal molecular structural differ- may be used to calculate values for k3 and
ences between the reactant and product, and !s, the k4a = k4b = k4c, which may be associated with the
solvent reorganization energy, which is due to corresponding steps in Fig. 4 for pentad 2. The
differences in the orientation and polarization of calculations yield k3 = 4.5 Â 1012 sÀ1 and a rate
solvent molecules around the initial and final states. constant of 1.5 Â 1011 sÀ1 for each of steps 4. With
The exponential term includes the standard free these estimates, equation (4) gives a theoretical
. .
energy change for the reaction, DG0, as well as !. quantum yield for C ‡–P(C2)–Q À of 0.083. This is
In triad 1 and pentad 2 the similarity in molecular close to the measured yield of 0.075 and gives a ratio
structure suggests that the electronic coupling terms in of quantum yield for pentad 2 to that for triad 1 of 1.9,
the various electron transfer steps would be essentially as compared with the experimental ratio of 1.7. Thus
identical, since the steps in question occur between the apparent discrepancy between the statistically
directly joined chromophores and the linkages are expected and observed quantum yields is explained by
identical in the two cases. Similarly, the internal and the small differences in thermodynamic driving force
solvent reorganization energies are expected to be resulting from differences in the oxidation potential of
nearly the same. Given these assumptions, any the porphyrin moieties involved.
differences in electron transfer rate constants for the
two molecules must arise from differences in thermo-
dynamic driving force. At first glance, these differ- CONCLUSIONS
. .
ences would be expected to be small. The C–P ‡–Q À
state of triad 1 lies 1.51 eV above the ground state, Comparison of the results for triad 1 and pentad 2
. .
whereas the corresponding C3–P ‡–Q À state of 2 is at shows that the three forward electron transfer path-
. .
1.49 eV. The energies of the final C ‡–P–Q À and ways 4 (Fig. 4) operating in parallel and competing
.‡ .À
C –P(C2)–Q states should be essentially equiva- with charge recombination by step 3 enhance the yield
lent. Can this small difference in DG0 lead to the of the final, long-lived charge-separated state by the
observed difference in quantum yields? statistically expected factor of three, after adjustment
Equation (5) may be used to investigate this for changes in thermodynamic driving force and in the
question. The approach is to use the known values of rate constants for steps 1 (Figs 1 and 4). The relatively
k3 and k4 for triad 1 to calculate values for the small change in thermodynamic driving force
electronic coupling V in equation (5), then to use these (0.02 eV) has a rather large effect on quantum yield
same coupling values to calculate corresponding rate because it affects two sets of rate constants. Forward
constants for pentad 2 and thus the theoretical ratio of electron transfer by steps 4 occurs in the normal region
the quantum yields of the final charge-separated states. of the Marcus relationship, equation (5), and thus the
The calculation requires an estimate of the total reduction in driving force for this step in pentad 2
reorganization energy !. A study of photoinduced relative to triad 1 leads to a reduction in the rate
electron transfer in porphyrin–quinone systems carried constant for steps 4. This in turn reduces the yield of
. .
out in butyronitrile solution yielded a value for ! of the final C ‡–P(C2)–Q À state relative to that expected
1.25 eV [32]. Using this value and values for k4 and statistically. On the other hand, charge recombination
. .
DG0 of 2.0 Â 1011 sÀ1 and À0.28 eV respectively for of C3–P ‡–Q À by step 3 occurs in the inverted region
triad 1 at 298 K, equation (5) yields V = 1150 cmÀ1 for of the Marcus relationship. A reduction in driving
step 4 in triad 1. A similar calculation employing force thus results in an increase in the rate constant for
k3 = 4.2 Â 1012 sÀ1 and DG0 = À1.51 eV yields charge recombination, and this also reduces the
. .
V = 175 cmÀ1 for charge recombination step 3 in the quantum yield of C ‡–P(C2)–Q À relative to that of
triad. (It should be noted that the numerical values of V the corresponding state in 1. The two effects operating
calculated in this way depend strongly on the choice of together account for the relatively large effect on
! and should not be taken as true estimates of quantum yield.
electronic coupling. However, the choice of ! has Most photosynthetic reaction centers of known
relatively little effect on the calculated rate constants structure approach C2 symmetry. Electron transfer
for 2 and quantum yield ratio for 2 relative to 1 generally proceeds down only one of the two possible
discussed below.) electron transfer pathways, although there is the
Assuming that these same values of ! and V apply suggestion that in some reaction centers transfer may
equally to pentad 2, equation (5) and the thermo- occur via both paths [33–37]. Parallel electron transfer



down both branches could make use of the principle (0.19 mmol) of 6, 1.5 mL of pyridine and 100 mL of
discussed above to compete with other pathways for dichloromethane. The resultant mixture was stirred at
deactivation of chlorophyll excited states and thus in room temperature for 6 h. The mixture was then mixed
principle enhance the quantum yield of charge with 200 mL of 10% aqueous sodium bicarbonate and
separation. In most modern reaction centers the extracted with four 100 mL portions of
electron transfer process is so efficient that any such dichloromethane. The organic extracts were further
increase would be of little significance. On the other washed with two 100 mL portions of water. The
hand, this strategy could have been employed by more organic phase was dried over sodium sulfate and
primitive organisms. In fact, if modern reaction filtered. The filtrate was distilled under reduced
centers evolved from homologous dimeric proteins, pressure to remove the solvent. The resultant solid
then the original dimeric proteins, with strict C2 residue was purified by chromatography on a silica gel
symmetry, would have demonstrated parallel electron column eluted with a dichloromethane/methanol (9:1)
transfer pathways by necessity. As mentioned earlier, mixture to yield 0.15 g (74%) of pure 7; 1H NMR
the parallel multistep electron transfer strategy does (300 MHz, CDCl3/trace of CD3OD) 7.77–7.80 (8H,
account for enhanced quantum yields in some d, J = 8 Hz, Ar3,5-H), 8.07–8.10 (8H, d, J = 8 Hz,
complex artificial reaction centers and can be applied Ar2,6-H), 8.80 (8H, s, pyrrole-H); MS m/z 1058 (M‡).
to other multicomponent molecular systems that
demonstrate electron, proton and/or energy transfer. 5-(4-Trifluoroacetamidophenyl)-10,15,20-tris(4-amino-
phenyl)porphyrin (8). Porphyrin 7 (0.20 g, 0.19 mmol)
was dissolved in 200 mL of a 3:1 mixture of
tetrahydrofuran and methanol in a 500 mL round-
EXPERIMENTAL bottom flask. Then 3 mL of 10% aqueous KOH were
added and the mixture was stirred under a nitrogen
Synthesis
atmosphere at room temperature for 72 h. The
The preparation of triad 1 has been reported in the hydrolysis reaction was quenched by dilution with
literature [21]. 100 mL of water and extracted with 100 mL portions
of chloroform/methanol (4:1) until the aqueous layer
5,10,15,20-Tetrakis(4-aminophenyl)porphyrin (6). A was clear. The combined organic extracts were
1.96 g portion (2.33 mmol) of 5,10,15,20-tetrakis(4- washed with 20 mL of 5% sodium bicarbonate, dried
acetamidophenyl)porphyrin 3 (prepared by the method over sodium sulfate and filtered. The solvent was
of Adler et al. [38]) was dissolved in 50 mL of distilled from the filtrate under reduced pressure and
trifluoroacetic acid and the mixture was placed in a the residue was purified by chromatography on a silica
500 mL flask. A 300 mL portion of 12N hydrochloric gel column eluted with dichloromethane/hexane/ethyl
acid was added and a reflux condenser was attached. acetate (55:25:20) to yield pure 8 (0.05 g, 34%); 1H
The mixture was heated at reflux for 24 h, cooled and NMR (300 MHz, CDCl3) À2.74 (2H, s, pyrrole-NH),
neutralized with 10% aqueous sodium hydroxide. The 4.03 (6H, brs, ArNH2), 7.07 (6H, d, J = 8 Hz,
purple crystals that formed on neutralization were 10,15,20Ar3,5-H), 7.98 (8H, m, 10,15,20Ar2,6-H,
removed by filtration. The crystals were dissolved in 5Ar3,5-H), 8.24–8.27 (3H, m, 5Ar2,6-H, ArNHCO),
300 mL of chloroform and the mixture was heated to 8.77 (2H, d, J = 5 Hz, 3,7-H), 8.92–8.94 (6H, m,
reflux for 1 h. Undissolved material was removed by pyrrole-H); MS m/z 770 (M‡).
filtration and the solvent was removed from the filtrate
by distillation under reduced pressure. The residue Tricarotenoporphyrin 9. In a 100 mL round-bottom
was recrystallized from methanol to yield 1.64 g flask was placed 0.13 g (0.24 mmol) of 7'-apo-7'-(4-
(98%) of 6 as needle-like purple crystals; 1H NMR carboxyphenyl)-b-carotene [39] dissolved in 20 mL of
(300 MHz, CDCl3) À2.72 (2H, brs, pyrrole-NH), toluene. Pyridine (0.24 mL, 3 mmol) was added and
4.01 (8H, brs, ArNH2), 7.07 (8H, d, J = 8 Hz, Ar3,5- the mixture was stirred under an argon atmosphere.
H), 7.99 (8H, d, J = 8 Hz, Ar2,6-H), 8.90 (8H, s, Then 8.8 mL (1.22 mmol) of thionyl chloride were
pyrrole-H); MS m/z 674 (M‡). added dropwise. The mixture was stirred for 20 min
and the solvent and excess thionyl chloride were
5,10,15,20-Tetrakis(4-trifluoroacetamidophenyl)- removed by distillation under reduced pressure. The
porphyrin (7). A 1.0 mL portion of trifluoroacetic resulting acid chloride was dissolved in 15 mL of
anhydride was added dropwise to a solution of 0.13 g dichloromethane, and 0.12 mL (0.15 mmol) of

# 1999 John Wiley Sons, Ltd. J. Porphyrins Phthalocyanines 3, 32–44 (1999)


pyridine was added. The flask was connected to a 1.98 (9H, s, 19C-CH3), 2.00 (9H, s, 20C-CH3), 2.01
dropping funnel that contained 0.040 g (0.052 mmol) (9H, s, 20'C-CH3), 2.03 (6H, m, 4C-CH2), 2.10 (9H, s,
of porphyrin 8 in 10 mL of dichloromethane and 19'C-CH3), 6.12–6.20 (9H, m, 7,8,10C-H), 6.27 (3H,
0.12 mL (0.15 mmol) of pyridine. The porphyrin d, J = 11 Hz, 14C-H), 6.34 (3H, d, J = 12 Hz, 14'C-H),
solution was added slowly over 5 min. The mixture 6.37 (3H, d, J = 16 Hz, 12C-H), 6.45 (3H, d, J = 12 Hz,
was stirred under an argon atmosphere for 12 h, after 10'C-H), 6.49 (3H, d, J = 16 Hz, 12'C-H), 6.60–6.72
which time the reaction was quenched with 50 mL of (15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H, 15'C-H),
10% aqueous sodium bicarbonate. The solution was 7.07 (3H, d, J = 16 Hz, 8'C-H), 7.08 (2H, d, J = 7.0 Hz,
extracted with three 100 mL portions of a 9:1 mixture 5Ar3,5-H), 7.63 (6H, d, J = 8 Hz, 1',5'C-H), 8.00 (8H,
of chloroform and methanol. The organic extract was d, J = 8 Hz, 5Ar2,6-H, 2'C-H, 4'C-H), 8.05 (6H, d,
washed twice with 100 mL of water, dried over J = 8 Hz, 10,15,20Ar3,5-H), 8.15 (3H, ArNHCO),
sodium sulfate and filtered and the solvent was 8.23 (6H, d, J = 8 Hz, 10 15,20Ar2,6-H), 8.86–8.98
distilled under reduced pressure. The residue was (8H, m, pyrrole-H); MS m/z 2225 (M‡).
purified by chromatography on a silica gel column
eluted with dichloromethane/hexane/ethyl acetate
6-Carboxy-1,4-dimethoxynaphthalene. A portion of 6-
(65:25:10) to yield 0.025 g (21%) of tricaro-
carbomethoxy-1,4-dimethoxynaphthalene [18]
tenoporphyrin 9; 1H NMR (500 MHz, CDCl3)
(0.1739 g, 0.71 mmol) was dissolved in 25 mL of a
À2.76 (2H, brs, pyrrole-NH), 1.04 (18H, s, 16C-
tetrahydrofuran/methanol mixture (1:4), and 4 mL of
CH3, 17C-CH3), 1.45–1.50 (6H, m, 2C-CH2), 1.58–
10% aqueous potassium hydroxide were added. The
1.66 (6H, m, 3C-CH2), 1.73 (9H, s, 18C-CH3), 1.98
mixture was stirred at room temperature for 48 h and
(9H, s, 19C-CH3), 2.00 (9H, s, 20C-CH3), 2.01 (9H, s,
then diluted with 150 mL of chloroform and
20'C-CH3), 2.03 (6H, m, 4C-CH2), 2.10 (9H, s, 19'C-
transferred into a separatory funnel. A 100 mL
CH3), 6.1–6.2 (9H, m, 7,8,10C-H), 6.27 (3H, d,
portion of 6N HCl was added, the whole was
J = 12 Hz, 14C-H), 6.34 (3H, d, J = 12 Hz, 14'C-H),
thoroughly mixed and let stand until separated into
6.37 (3H, d, J = 16 Hz, 12C-H), 6.45 (3H, d, J = 12 Hz,
two layers, and the organic phase was removed. The
10'C-H), 6.48 (3H, d, J = 16 Hz, 12'C-H), 6.60–6.72
organic extract was washed with 100 mL of water,
(15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H, 15'C-H),
dried over sodium sulfate and filtered. The solvent was
7.07 (3H, d, J = 16 Hz, 8'C-H), 7.62 (6H, d, J = 8 Hz,
removed from the filtrate by distillation under reduced
1'C-H, 5'C-H), 7.99 (6H, d, J = 8 Hz, 2'C-H, 4'C-H),
pressure to yield the desired acid (0.149 g, 91%); 1H
7.99 (2H, m, 5Ar3,5-H), 8.05 (6H, m, 10,15,20Ar3,5-
NMR (300 MHz, CDCl3) 3.98 (3H, s, 1-OCH3), 3.99
H), 8.16 (4H, s, ArNHCO), 8.21 (8H, m,
(3H, s, 4-OCH3), 6.76 (1H, d, J = 8.4 Hz, 3-H), 6.85
5,10,15,20Ar2,6-H), 8.75–8.94 (8H, m, pyrrole-H);
(1H, d, J = 8.4 Hz, 2-H), 8.1 (1H, dd, J = 8 Hz, 8-H),
MS m/z 2321 (M‡).
8.23 (1H, d, J = 8 Hz, 7-H), 9.0 (1H, s, 5-H); MS m/z
231 (M‡-1).
Tricarotenoporphyrin 10. Tricarotenoporphyrin 9
(0.05 g, 0.022 mmol) was dissolved in 30 mL of
tetrahydrofuran/methanol (3:1), and 4 mL of 10% 6-Carboxy-1,4-naphthoquinone. A portion of 6-
aqueous potassium hydroxide were added. The carboxy-1,4-dimethoxynaphthalene (0.042 g, 0.181
mixture was stirred under nitrogen at room mmol) was dissolved in 15 mL of acetonitrile. Ceric
temperature for 36 h. The mixture was then diluted ammonium nitrate (0.288 g, 0.525 mmol) in 2 mL of
with 100 mL of water and extracted with three 100 mL water was slowly added dropwise. The resultant
portions of a 9:1 chloroform/methanol mixture. The mixture was stirred for 8 min. The reaction was
combined organic extracts were washed with three quenched by addition of 30 mL of 6N hydrochloric
100 mL portions of water. The organic phase was acid. The resultant mixture was extracted with
removed, dried over sodium sulfate and filtered and chloroform/methanol (93:7). The organic extract was
the solvent was distilled under reduced pressure. The dried over sodium sulfate and filtered and the solvent
residue was purified by chromatography on a silica gel was removed by distillation under reduced pressure.
column eluted with dichloromethane/methanol (97:3), The yield of quinone was 0.035 g (96%); 1H NMR
yielding 0.04 g (83.5%) of 10; 1H NMR (500 MHz, (300 MHz, CDCl3/trace of CD3OD) 7.06 (2H, s, 2-H,
CDCl3) À2.77 (2H, brs, pyrrole-NH), 1.03 (18H, s, 3-H), 8.17 (1H, d, J = 8 Hz, 8-H), 8.42 (1H, dd,
16C-CH3, 17C-CH3), 1.45–1.50 (6H, m, 2C-CH2), J = 8 Hz, 1.5 Hz, 7-H), 8.75 (1H, d, J = 1.5 Hz, 5-H);
1.60–1.65 (6H, m, 3C-CH2), 1.72 (9H, s, 18C-CH3), MS m/z 202 (M‡).



Pentad 2. A 0.036 g (0.176 mmol) portion of 6- eluted with a dichloromethane/hexane/ethyl acetate
carboxy-1,4-naphthoquinone was dissolved in 10 mL (65:25:10) mixture. The leading fraction was
of dichloromethane, 25 mL (0.176 mmol) of unreacted 7 and the second fraction was the desired
triethylamine were added and the mixture was compound 11 (0.12 g, 44%); 1H NMR (500 MHz,
stirred. To the solution was added 0.03 g CDCl3) À2.79 (2H, brs, pyrrole-NH), 4.02 (2H, brs,
(0.112 mmol) of phenyl N-phenylphosphoamido- 20ArNH2), 7.07 (2H, d, J = 8.5 Hz, 20Ar3,5-H), 7.99
chloridate. The mixture was stirred at room (10H, m, J = 8.5 Hz, 5,10,15Ar3,5-H, 20Ar2,6-H), 8.2
temperature for 20 min. A solution of (3H, brs, ArCONH), 8.25 (6H, d, J = 8 Hz,
tricarotenoporphyrin 10 (0.044 g, 0.02 mmol) in 5,10,15Ar2,6-H), 8.8 (6H, m, pyrrole-H), 8.96 (2H,
4 mL of dichloromethane and 25 mL (0.176 mmol) of d, pyrrole 3-H and 7-H); MS m/z 962 (M‡).
triethylamine was added and the mixture was stirred
under argon for 20 h, whereupon TLC analysis showed 5-(4-Benzamidophenyl)-10,15,20-tris(4-trifluoroacet-
that the starting material had been completely amidophenyl)porphyrin (12). In a 50 mL round-bottom
consumed. The mixture was diluted with 100 mL of flask was placed 0.044 g (0.045 mmol) of porphyrin 11
chloroform and washed twice with 100 mL portions of dissolved in 20 mL of toluene. A 2.5 mL portion of
water. The organic phase was dried over sodium pyridine was added and the mixture was stirred under
sulfate and filtered and the solvent was distilled under nitrogen. Benzoyl chloride (0.030 g) was added
reduced pressure. The crude product was purified by dropwise. The mixture was stirred and the progress
chromatography on a silica gel column eluted with a of the reaction was monitored by TLC. The reaction
dichloromethane/methanol (97:3) mixture. The yield was complete in 6 h and the mixture was diluted with
of 2 was 0.045 g (94.5%); 1H NMR (500 MHz, 100 mL of water and transferred into a separatory
CDCl3) À2.76 (2H, brs, pyrrole-NH), 1.04 (18H, s, funnel. The organic phase was removed and the
16C-CH3, 17C-CH3), 1.45–1.50 (6H, m, 2C-CH2), aqueous phase was washed with two 100 mL portions
1.58–1.65 (6H, m, 3C-CH2), 1.72 (9H, s, 18C-CH3), of chloroform. The combined organic extracts were
1.98 (9H, s, 19C-CH3), 2.00 (9H, s, 20C-CH3), 2.01 washed with 100 mL of saturated sodium bicarbonate
(9H, s, 20'C-CH3), 2.03 (6H, m, 4C-CH2), 2.09 (9H, s, and two 100 mL portions of water and dried over
19'C-CH3), 6.12–6.20 (9H, m, 7C-H, 8C-H, 10C-H), sodium sulfate. After filtration and distillation of the
6.27 (3H, d, J = 11 Hz, 14C-H), 6.34 (3H, d, J = 12 Hz, solvent at reduced pressure the residue was purified by
14'C-H), 6.37 (3H, d, J = 15 Hz, 12C-H), 6.45 (3H, d, chromatography on silica gel eluted with a mixture of
J = 11 Hz, 10'C-H), 6.48 (3H, d, J = 16 Hz, 12'C-H), dichloromethane, hexane and ethyl acetate (65:25:10)
6.58–6.72 (15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H, to yield 0.047 g (97%) of 12; 1H NMR (300 MHz,
15'C-H), 7.02–7.18 (5H, m, Q2,3-H, 8'C-H), 7.62 (6H, CDCl3) À2.82 (2H, s, pyrrole-NH), 7.6 (3H, m,
brs, 1'C-H, 5'C-H), 8.00 (6H, d, J = 8 Hz, 2'C-H, 4'C- 3,4,5Bn-H), 7.97 (6H, d, J = 8 Hz, 5,10,15Ar3,5-H),
H), 8.07 (8H, brs, 10,15,20Ar3,5-H, 5Ar3,5-H), 8.17 8.05 (5H, m, ArCONH, Bn2,6-H, 20Ar3,5-H), 8.15–
(4H, s, ArNHCO), 7.95–8.35 (3H, m, Q5,7,8-H), 8.20– 8.2 (5H, m, CF3CONH, 20Ar2,6-H), 8.24 (8H, d,
8.38 (8H, m, 10,15,20Ar2,6-H), 5Ar2,6-H, 8.92 (8H, J = 8 Hz, 5,10,15Ar2,6-H), 8.8–8.9 (8H, m, pyrrole-
brs, pyrrole-H); MS m/z 2409 (M‡). H); MS m/z 1066 (M‡).

5-(4-Aminophenyl)-10,15,20-tris(4-trifluoroacetamido- 5-(4-Benzamidophenyl)-10,15,20-tris(4-aminophenyl)-
phenyl)porphyrin (11). Porphyrin 7 (0.30 g, porphyrin (13). Porphyrin 12 (0.140 g, 0.13 mmol) was
0.28 mmol) was dissolved in 150 mL of a mixture of dissolved in 50 mL of a mixture of tetrahydrofuran and
tetrahydrofuran and methanol (3:2), and 2.0 mL of methanol (1:1), and 5 mL of 10% aqueous KOH were
20% aqueous potassium hydroxide were added. The added. The mixture was stirred at room temperature
mixture was stirred at room temperature under a for 36 h, after which time there was no trace of
nitrogen atmosphere. The progress of the formation of porphyrin 12. The solution was diluted with 100 mL of
11 was monitored by TLC and was found to have water and extracted with three 100 mL portions of
peaked after 17 h. The reaction mixture was diluted chloroform. The organic extract was dried over
with 100 mL of water and extracted with three 100 mL sodium sulfate and filtered and the solvent was
portions of chloroform. The organic layer was dried distilled under reduced pressure. The residue was
over sodium sulfate and filtered and the solvent was purified by chromatography on a silica gel column
distilled under reduced pressure. The residue was eluted with dichloromethane/hexane/ethyl acetate
purified by chromatography on a silica gel column (65:25:20) to give 0.098 g (97%) of 13; 1H NMR



(300 MHz, CDCl3) À2.8 (2H, s, pyrrole-NH), 4.01 carboxyphenyl)-b-carotene dissolved in 20 mL of
(6H, brs, Ar NH2), 7.05 (6H, d, J = 8 Hz, toluene. Pyridine (0.11 mL, 1.35 mmol) was added
10,15,20Ar3,5-H), 7.56–7.62 (1H, m, Bn4-H), 7.59 and the mixture was stirred under an argon
(2H, d, J = 8 Hz, 5ArH-3,5), 7.98 (6H, d, J = 8 Hz, atmosphere. Thionyl chloride (34 mL, 0.45 mmol)
10,15,20Ar2,6-H), 8.01–8.03 (4H, m, Bn2,3,5,6-H), was added dropwise. The mixture was stirred for
8.14 (1H, brs, ArNHCO), 8.22 (2H, d, J = 8 Hz, 15 min and the excess thionyl chloride was removed
5Ar2,6-H), 8.83–8.93 (8H, m, pyrrole-H); MS m/z 778 by distillation under reduced pressure. The crude acid
(M‡). chloride was dissolved in 15 mL of dichloromethane,
and 0.11 mL (1.35 mmol) of pyridine was added. The
Tricarotenoporphyrin 5. A portion (0.30 g, mixture was stirred under an argon atmosphere as
0.654 mmol) of 7'-apo-7'-(4-carboxyphenyl)-b- 0.030 g (0.030 mmol) of porphyrin 11 in 10 mL of
carotene [39] was dissolved in 70 mL of toluene, and dichloromethane was added over a 5 min period. The
0.65 mL (7.8 mmol) of pyridine was added. The mixture was stirred under an argon atmosphere for 6 h
mixture was stirred and 0.2 mL (3.27 mmol) of and the reaction was quenched with 50 mL of 5%
thionyl chloride was added dropwise. The mixture aqueous sodium bicarbonate. The solution was
was stirred under a nitrogen atmosphere for 20 min. extracted with three 100 mL portions of a 9:1
The solvent and excess thionyl chloride were removed mixture of chloroform and methanol. The organic
by distillation under reduced pressure. To the residue extract was washed with three 50 mL portions of
were added 20 mL of toluene and 0.1 mL (1.2 mmol) water, dried over sodium sulfate and filtered and the
of pyridine. The solvent was again distilled at reduced solvent was removed by distillation under reduced
pressure to ensure complete removal of excess thionyl pressure. Chromatography on a silica gel column
chloride. The residue was dissolved in 40 mL of eluted with dichloromethane/hexane/ethyl acetate
dichloromethane, and 0.45 mL (5.4 mmol) of pyridine (65:25:10) gave 0.032 g (70%) of 14; NMR
was added. To this mixture was added dropwise (300 MHz, CDCl3) À2.81 (2H, brs, pyrrole-NH),
0.099 g (0.116 mmol) of porphyrin 13 dissolved in 1.04 (6H, s, CH3-16,17C-CH3), 1.45–1.50 (2H, m, 2C-
10 mL of dichloromethane. The mixture was stirred at CH2), 1.60–1.66 (2H, m, 3C-CH2), 1.73 (3H, s, 18C-
room temperature for 16 h and then quenched by CH3), 1.99 (3H, s, 19C-CH3), 2.00 (3H, s, 20C-CH3),
addition of 100 mL of 10% sodium bicarbonate. The 2.01 (3H, s, 20'C-CH3), 2.04 (2H, m, 4C-CH2), 2.10
mixture was extracted with four 100 mL portions of (3H, s, 19'C-CH3), 6.10–6.72 (13H, m, vinylic-H),
chloroform. The organic extract was washed with 7.07 (1H, d, J = 16 Hz, 8'C-H), 7.62 (2H, d, J = 8 Hz,
100 mL of water, dried over sodium sulfate and 1',5'C-H), 7.99 (2H, m, 2',4'C-H), 8.00 (6H, d,
filtered and the solvent was distilled under reduced J = 8 Hz, 10,15,20Ar3,5-H), 8.05 (2H, d, J = 8 Hz,
pressure. The residue was purified by chromatography 5Ar3,5-H), 8.19 (4H, brs, ArNHCO), 8.25 (6H, d,
on silica gel. Dichloromethane (100%) was used to J = 8 Hz, 10,15,20Ar2,6-H), 8.30 (2H, m, 5Ar2,6-H),
elute a lead fraction that was not fluorescent. Then a 8.83–8.93 (8H, m, pyrrole-H); MS m/z 1479 (M‡).
dichloromethane/hexane/ethyl acetate (67:25:8)
mixture was used to elute the desired compound 5 Carotenoporphyrin 15. To a 50 mL round-bottom flask
(0.06 g, 20%); 1H NMR (500 MHz, CDCl3) À2.74 was added 0.032 g (0.022 mmol) of carotenoporphyrin
(2H, brs, pyrrole-NH), 1.03 (18H, s, 16C-CH3, 17C- 14 dissolved in 20 mL of a 3:1 mixture of
CH3), 1.45–1.50 (6H, m, 2C-CH2), 1.60–1.65 (6H, m, tetrahydrofuran and methanol. A solution of 10%
3C-CH2), 1.97 (9H, s, 19C-CH3), 1.99 (9H, s, 20C- aqueous potassium hydroxide (2 mL) was added and
CH3), 2.00 (9H, s, 20'C-CH3), 2.02 (6H, m, 4C-CH2), the mixture was stirred under an argon atmosphere for
2.09 (9H, s, 19'C-CH3), 6.0–6.5 (42H, m, vinylic-H), 17 h. The mixture was diluted with 100 mL of
7.05 (3H, d, J = 16 Hz, 8'C-H), 7.6 (5H, m, 5Ar3,5-H), chloroform and extracted with three 100 mL portions
7.6 (1H, m, Bn3-H), 7.62 (6H, d, J = 8 Hz, 1'C-H, 5'C- of water. The organic layer was dried over sodium
H), 7.98 (6H, d, J = 8 Hz, 2'C-H, 4'C-H), 7.94–8.00 sulfate and filtered and the solvent was removed by
(10H, m, 10,15,20Ar3,5-H, Bn2,3,5,6-H), 8.14 (4H, distillation under reduced pressure. The yield of 15
brs, ArNHCO), 8.20 (8H, m, 5,10,15,20Ar2,6-H,), obtained was 0.023 g (89%); NMR (300 MHz, CDCl3)
8.80–8.96 (8H, m, pyrrole-H); MS m/z 2329 (M‡). À2.72 (2H, brs, pyrrole-NH), 1.04 (6H, s, 16C-CH3,
17C-CH3), 1.45–1.50 (2H, m, 2C-CH2), 1.58–1.66
Carotenoporphyrin 14. To a 50 mL round-bottom flask (2H, m, 3C-CH2), 1.73 (3H, s, 18C-CH3), 1.98 (3H, s,
was added 0.048 g (0.09 mmol) of 7'-apo-7'-(4- 19C-CH3), 2.00 (3H, s, 20C-CH3), 2.02 (3H, s, 20'C-



CH3), 2.04 (2H, m, 4C-CH2), 2.10 (3H, s, 19'C-CH3), Shimadzu UV2100U UV-vis spectrometer, and fluor-
4.10 (6H, brs, ArNH2), 6.10–6.72 (13H, m, vinylic-H), escence spectra were measured on a SPEX Fluorolog
7.07 (H, m, 8'C-H), 7.07 (6H, d, J = 8 Hz, using optically dilute samples and corrected.
10,15,20Ar3,5-H), 7.62 (2H, d, J = 9 Hz, 1',5'C-H), Cyclic voltammetric measurements were carried
7.99 (2H, m, 2',4'C-H), 8.00 (6H, d, J = 8 Hz, out with a Pine Instrument Company Model AFRDE4
10,15,20Ar2,6-H), 8.03 (2H, d, J = 9 Hz, 5Ar3,5-H), potentiostat. The electrochemical measurements were
8.16 (1H, brs, ArNHCO), 8.22 (2H, d, J = 9 Hz, performed in benzonitrile at ambient temperatures
5Ar2,6-H), 8.85–8.94 (8H, m, pyrrole-H); MS m/z with a glassy carbon working electrode, an Ag/Ag‡
1191 (M‡). reference electrode and a platinum wire counter
electrode. The electrolyte was 0.1 M tetra-n-butylam-
Carotenoporphyrin 4. In a 50 mL round-bottom flask monium hexafluorophosphate, and ferrocene was
was placed 0.023 g (0.019 mmol) of 15 dissolved in employed as an internal reference redox system.
2 mL of pyridine. To this mixture were added 2 mL of Fluorescence decay measurements were performed
acetic anhydride and the mixture was stirred under an on $1 Â 10À5 M solutions by the time-correlated
argon atmosphere for 10 h. The reaction mixture was single-photon-counting method. The excitation source
diluted with 100 mL of chloroform and extracted with was a cavity-dumped Coherent 700 dye laser pumped
100 mL of saturated sodium bicarbonate and three by a frequency-doubled Coherent Antares 76s
100 mL portions of water. The organic extract was Nd:YAG laser [40]. The instrument response function
dried over sodium sulfate and filtered and the solvent was 35 ps, as measured at the excitation wavelength
was removed by distillation under reduced pressure. for each decay experiment with Ludox AS-40.
The residue was purified by chromatography on a Nanosecond transient absorption measurements
silica gel column eluted with a 9:1 mixture of were made with excitation from an Opotek optical
chloroform and methanol to yield 0.025 g (97%) of parametric oscillator pumped by the third harmonic of
carotenoporphyrin 4; NMR (500 MHz, CDCl3/5% a Continuum Surelite Nd:YAG laser. The pulse width
CD3OD) À2.80 (2H, brs, pyrrole-NH), 1.04 (6H, s, was $5 ns and the repetition rate was 10 Hz. The
16C-CH3, 17C-CH3), 1.45–1.50 (2H, m, 2C-CH2), detection portion of the spectrometer has been
1.60–1.65 (2H, m, 3C-CH2), 1.73 (3H, s, 18C-CH3), described elsewhere [41].
1.99 (3H, s, 19C-CH3), 2.00 (3H, s, 20C-CH3), 2.02
(3H, s, 20'C-CH3), 2.04 (2H, m, 4C-CH2), 2.10 (3H, s,
Acknowledgements
19'C-CH3), 2.34 (9H, s, acetamido-CH3), 6.12–6.21
(3H, m, 7,8,10C-H), 6.28 (H, d, J = 10 Hz, 14C-H), This work was supported by a grant from the US
6.34 (H, d, J = 13 Hz, 14'C-H), 6.38 (H, d, J = 16 Hz, Department of Energy (DE-FG03-93ER14404). This
12C-H), 6.45 (H, d, J = 12 Hz, 10'C-H), 6.48 (H, d, is publication 350 from the ASU Center for the Study
J = 15 Hz, 12'C-H), 6.60–6.72 (5H, m, of Early Events in Photosynthesis.
11,11',15,15',7'C-H), 7.07 (H, d, J = 16 Hz, 8'C-H),
7.62 (2H, d, J = 8 Hz, 1',5'C-H), 7.93 (6H, d, J = 9 Hz,
10,15,20Ar3,5-H), 8.02 (2H, d, J = 8 Hz, 2',4'C-H), REFERENCES AND NOTES
8.07 (2H, d, J = 8 Hz, 5Ar3,5-H), 8.15 (6H, d,
J = 8 Hz, 10,15,20Ar2,6-H), 8.21 (2H, d, J = 8 Hz, 1. D. Gust and T. A. Moore, Top. Curr. Chem. 159, 103–
5Ar3,5-H), 8.81–8.90 (8H, m, pyrrole-H); MS m/z 151 (1991).
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Chem. Lett. 145–148 (1993). Nemeth, J. Am. Chem. Soc. 107, 3631–3640 (1985).
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