Photoinduced charge separation and charge recombination to a triplet state in a carotene-porphyrin-fullerene triad

Article abstract of DOI:10.1021/ja9631054

A molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene (C₆₀) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltetrahydrofuran solution, the triad un

Full text of DOI:10.1021/ja9631054

1400
J. Am. Chem. Soc. 1997, 119, 1400-1405
Photoinduced Charge Separation and Charge Recombination to
a Triplet State in a Carotene-Porphyrin-Fullerene Triad
Paul A. Liddell, Darius Kuciauskas, John P. Sumida, Boaz Nash, Dorothy Nguyen,
Ana L. Moore,* Thomas A. Moore,* and Devens Gust*
Contribution from the Center for the Study of Early EVents in Photosynthesis, Department of
Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed September 4, 1996X
Abstract: A molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a
fullerene (C60) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltetrahydrofuran
solution, the triad undergoes photoinduced electron transfer to yield C-P•+-C60•-, which evolves into C•+-P-
•-
C60 with an overall quantum yield of 0.14. This state decays by charge recombination to yield the carotenoid
triplet state with a time constant of 170 ns. Even in a glass at 77 K, C•+-P-C60•- is formed with a quantum yield
of ∼0.10 and again decays mainly by charge recombination to give 3C-P-C60. The fullerene triplet, formed through
normal intersystem crossing, is also observed at 77 K. The generation in the triad of a long-lived charge separated
state by photoinduced electron transfer, the low-temperature electron transfer behavior, and the formation of a triplet
state by charge recombination are phenomena previously observed mostly in photosynthetic reaction centers.
Introduction
Fullerenes absorb light throughout the visible spectrum.
Although their extinction coefficients at visible wavelengths are
small, their low-lying first excited singlet states may be
sensitized by energy transfer from better absorbers. Fullerenes
are excellent electron acceptors and potential electron accumula-
tors. Recently, the interaction of fullerenes with a variety of
covalently attached chromophores has been investigated.1-11
Such dyads undergo singlet energy transfer from the pigment
to the fullerene and/or photoinduced electron transfer to the
fullerene, generating a charge-separated state. Therefore,
fullerenes might be useful electron acceptors in more complex
molecular systems that can demonstrate multiple electron
transfer pathways. In order to investigate this possibility, we
have prepared molecular triad 1, consisting of a diarylporphyrin
(P) covalently linked to a carotenoid polyene (C) and a fullerene
(C60), and model compounds 2-5. As described in detail below,
1 demonstrates curious photochemical behavior. It undergoes
photoinduced electron transfer not only in solution at ambient
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
(1) Liddell, P. A.; Sumida, J. P.; Macpherson, A. N.; Noss, L.; Seely,
G. R.; Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem.
Photobiol. 1994, 60, 537-541.
(2) Imahori, H.; Hagiwara, K.; Akiyama, T.; Taniguchi, S.; Okada, T.;
Sakata, Y. Chem. Lett. 1995, 265-266.
(3) Linssen, T. G.; Durr, K.; Hanack, M.; Hirsch, Q. J. Chem. Soc., Chem.
Commun. 1995, 103-104.
(4) Maggini, M.; Dono, A.; Scorrano, G.; Prato, M. J. Chem. Soc., Chem.
Commun. 1995, 843-845.
(5) Maggini, M.; Karlsson, A.; Scorrano, G.; Sandona, G.; Farina, G.;
Prato, M. J. Chem. Soc., Chem. Commun. 1994, 589-590.
(6) Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. Tetrahedron Lett. 1995,
36, 7971-7974.
(7) Imahori, H.; Cardoso, S.; Tatman, D.; Lin, S.; Macpherson, A. N.;
Noss, L.; Seely, G. R.; Sereno, L.; Chessa de Silber, J.; Moore, T. A.; Moore,
A. L.; Gust, D. Photochem. Photobiol. 1995, 62, 1009-1014.
(8) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc.
1995, 117, 4093-4099.
(9) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.;
Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem.
1996, 100, 15926-15932.
(10) Imahori, H.; Sakata, Y. Chem. Lett. 1996, 199-200.
(11) Williams, R. M.; Koeberg, M.; Lawson, J. M.; An, Y.-Z.; Rubin,
Y.; Paddon-Row, M. N.; Verhoeven, J. W. J. Org. Chem. 1996, 61, 5055-
5062.
S0002-7863(96)03105-8 CCC: $14.00 © 1997 American Chemical Society

Carotene-Porphyrin-Fullerene Triad
J. Am. Chem. Soc., Vol. 119, No. 6, 1997 1401
•-
temperatures but also in a glass at 77 K to yield C-P•+-C60
,
•-
which evolves into C•+-P-C60
. This long-lived charge-
separated state decays by radical pair recombination to yield
the carotenoid triplet state, rather than the molecular ground
state.
Results
Synthesis. The synthesis of triad 1 commenced with the
preparation of porphyrin 6 from 712 via protection of the amino
group as the (benzyloxy)carbonyl derivative, reduction of the
ester with lithium aluminum hydride, and oxidation of the
resulting alcohol with manganese dioxide. Reaction13 of 6 with
C60 and sarcosine gave 3. This reaction is known to produce
only a single isomer; an adduct to a double bond at a 6,6-ring
fusion in C60. Dyad 3 was deprotected with boron tribromide
and allowed to react with the appropriate carotene acid chloride14
to give 1. The triad and intermediates were characterized by
Figure 1. Decay-associated fluorescence emission spectrum obtained
by exciting a ∼1 × 10-5 M solution of triad 1 in toluene with 590 nm,
∼9 ps laser pulses. Global analysis of the data at the indicated
wavelengths gave four exponential decay components with time
constants of 0.012 (3), 1.52 (1), 0.201 (O), and 6.12 (b) ns.
1
mass spectrometry and H NMR spectroscopy, as detailed in
1
the Experimental Section. The H NMR spectra of 1 and 3
displayed broadening of porphyrin and aryl resonances for
protons near the fullerene, suggesting slow conformational
interconversion on the NMR time scale involving single bonds
in the porphyrin-fullerene linkage.
Steady State Absorption and Emission. The visible
absorption spectrum of 1 in toluene solution is essentially a
linear combination of the spectra of model carotenoporphyrin
2 and fullerene 5, with maxima at 412, 455 (sh), 485, 510, 575,
628, and 706 nm. The spectrum in 2-methyltetrahydrofuran
solution is similar, with maxima at 409, 450 (sh), 475, 506,
575, 630, and 705 nm.
of the porphyrin in model carotenoporphyrin 2 decays as a single
exponential with a lifetime of 7.34 ns (λex ) 590 nm, λem
)
700 nm, ø2 ) 1.07). The time-resolved fluorescence results
for 1 are thus compatible with the strong quenching of the
porphyrin fluorescence noted in the steady state measurements,
and the quenching can be attributed at least in part to singlet-
singlet energy transfer to the fullerene.
The only other significant decay component in the decay-
associated spectrum of triad 1, with a lifetime of 1.52 ns, has
the shape of fullerene emission. Model fullerene 5 has a
fluorescence lifetime of 1.40 ns. Thus, in toluene, the fullerene
first excited singlet state is unquenched.
In toluene, porphyrin 4 and carotenoporphyrin 2 have
fluorescence maxima at 631 and 699 nm, whereas fullerene 5
has emission maxima at 715 and 796 nm. Triad 1, excited at
590 nm, yields an emission spectrum with features characteristic
of both the porphyrin and the fullerene moieties. However, the
porphyrin emission is strongly quenched relative to that of the
fullerene. In 2-methyltetrahydrofuran, porphyrin 4 has emission
maxima at 630 and 699 nm, whereas fullerene 5 has maxima
at 714 and ∼800 nm. The emission of 1 in this solvent with
excitation at 590 nm also features spectral shapes characteristic
of both the porphyrin and the fullerene components, but in this
case, the emission from both components is strongly quenched
relative to emission from the model compounds.
Time-Resolved Emission Studies. In order to investigate
further the fluorescence quenching, time-resolved fluorescence
measurements were performed using the single-photon timing
technique. A ∼1 × 10-5 M solution of 1 in toluene was excited
at 590 nm, and the fluorescence decay was monitored at 12
wavelengths in the 625-815 nm region. The results of global
analysis of these data as four exponential processes (ø2 ) 1.12)
are shown in Figure 1. The major decay component, with a
lifetime of 12 ps, has maxima characteristic of porphyrin
emission and represents the decay of C-1P-C60. The negative
amplitude of this decay component in the 800 nm region, where
the fullerene moiety emits, signifies an increase in the amplitude
of fullerene emission with time after the laser pulse. This is
due to singlet-singlet energy transfer from the porphyrin to
the fullerene, yielding C-P-1C60. The first excited singlet state
Similar time-resolved fluorescence measurements were per-
formed on ∼1 × 10-5 M solutions of 1 in 2-methyltetrahydro-
furan. With 600 nm excitation, global analysis (ø2 ) 1.12) of
decays at 772, 787, and 795 nm yielded one significant
exponential decay component of 0.032 ns. At these wave-
lengths, emission is due to the fullerene moiety; the porphyrin
emission is very weak and causes no interference. Thus, the
lifetime of C-P-1C60 is 32 ps. Excitation at 590 nm, where
the porphyrin is the major absorber, gave porphyrin fluorescence
decays at 625, 630, 635, and 640 nm whose global analysis (ø2
) 1.10) yielded one significant lifetime of 10 ps for C-1P-
C60, which is approaching the time resolution of the spectrom-
eter. As the singlet-state lifetime of 2 is 7.22 ns (λex ) 590
nm, λem ) 630 nm, ø2 ) 1.11) and that of 5 is 1.35 ns (λex
)
590 nm, λem ) 712 nm, ø2 ) 1.15) in 2-methyltetrahydrofuran,
the short lifetimes for 1 are consistent with the strong steady
state fluorescence quenching of both the porphyrin and fullerene
moieties.
Nanosecond Transient Absorption in Toluene. The prod-
ucts resulting from decay of the porphyrin and fullerene excited
singlet states of 1 were investigated by nanosecond transient
absorption spectroscopy. Excitation of an argon-purged, ∼1
× 10-4 M solution of 1 in toluene with a 590 nm, 5 ns laser
pulse led to the observation of a transient absorption with a
maximum at ∼550 nm (Figure 2). This transient grows in after
the laser flash with a time constant of 85 ns and decays with a
lifetime of ∼4.2 µs, which decreases in the presence of
increasing amounts of oxygen. This transient species is assigned
to the carotenoid triplet state, 3C-P-C60, by comparison with
(12) Kuciauskas, D.; Liddell, P. A.; Hung, S.-C.; Lin, S.; Stone, S.; Seely,
G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 1997, 101,
429-440.
3
spectra of related compounds.14 The yield of C-P-C60 was
(13) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798-9799.
determined by the comparative method and found to be
essentially unity, using the meso-tetraphenylporphyrin triplet
state (φT ) 0.67, (ꢀT-ꢀG)440 ) 6.8 × 104 L mol-1 cm-1) as the
standard and (ꢀT-ꢀG) ) 1.4 × 105 L mol-1 cm-1 for the
(14) Gust, D.; Moore, T. A.; Bensasson, R. V.; Mathis, P.; Land, E. J.;
Chachaty, C.; Moore, A. L.; Liddell, P. A.; Nemeth, G. A. J. Am. Chem.
Soc. 1985, 107, 3631-3640.

1402 J. Am. Chem. Soc., Vol. 119, No. 6, 1997
Liddell et al.
Figure 2. Transient absorption spectrum of the carotenoid triplet state
determined 200 ns after excitation of a ∼1 × 10-4 M solution of 1 in
toluene with a 590 nm, ∼5 ns laser pulse.
Figure 3. Transient absorption results for a ∼1 × 10-4 M solution of
1 in 2-methyltetrahydrofuran at ambient temperature following excita-
tion with a 5 ns, 590 nm laser pulse. (a) Decay at 950 nm of the
carotenoid. The fullerene triplet state of 5 has an absorption
maximum at ∼700 nm.7,9 Under the experimental conditions
used for observation of 3C-P-C60 in 1, recovery of the
detection system from saturation at 700 nm precluded observa-
tion of the decay of C-P-3C60.
•-
absorption of C•+-P-C60 (τ ) 170 ns) and rise (τ ) 170 ns) and
decay (τ ) 4.9 µs) of the carotenoid triplet absorption at 550 nm. The
smooth curve is a simulation using the rate constants in the text. (b)
By analogy with other porphyrin-fullerene dyads,1,9,10 the
very short lifetime of C-1P-C60 in toluene is consistent with
decay not only by singlet-singlet energy transfer to yield C-P-
1C60 but also by photoinduced electron transfer to give C-P•+-
C60•-. As has been observed in other triads,15 any C-P•+-
C60•- that is formed might evolve by electron transfer from the
carotenoid to give C•+-P-C60•-. In order to investigate this
possibility, transient absorption measurements were also carried
out in the 800-1100 nm region, where the carotenoid radical
cation16 and fullerene radical anion7,17,18 absorb. No significant
transients were observed in this spectral region.
Transient absorption spectrum of 3C-P-C60. (c) Transient absorption
•-
spectrum of C•+-P-C60
.
Nanosecond Transient Absorption in 2-Methyltetrahy-
drofuran and Benzonitrile. In contrast to the results in toluene,
excitation of 1 in argon-purged 2-methyltetrahydrofuran with a
590 nm pulse led to the observation of a strong transient
absorption at ∼940 nm (Figure 3). Similar results were obtained
with excitation at 630 and 650 nm. Both the carotenoid radical
cation and the fullerene radical anion absorb in the 940 nm
region, although the extinction coefficient of the carotenoid
radical cation is ∼10 times larger than that of the fullerene
radical anion.16-18 Therefore, the observed transient is assigned
to C•+-P-C60•-. The charge-separated state has a lifetime of
170 ns and is formed with an overall quantum yield of 0.14, as
determined by the comparative method with the meso-tetra-
phenylporphyrin triplet state as a standard, and using an
extinction coefficient of 1.6 × 105 L mol-1 cm-1 for the
carotenoid radical cation at its absorption maximum. The decay
of C•+-P-C60•- is correlated with the growth (τ ) 170 ns) of
an absorption band at 550 nm characteristic of the carotenoid
Figure 4. Transient absorption results for a ∼1 × 10-4 M solution of
1 in a 2-methyltetrahydrofuran glass at 77 K following excitation with
a 5 ns, 590 nm laser pulse. (a) Decay at 700 nm of the fullerene triplet
state (C-P-1C60). The smooth curve is an exponential decay with a
time constant of 81 µs. (b) Decay of the carotenoid and fullerene radical
ions at 980 nm (smooth curve is the sum of two exponential decays
with time constants of 1.5 µs (73%) and 7 µs (27%)) and rise of the
carotenoid triplet state monitored at 550 nm (smooth curve has a time
constant of 1.4 µs for the rise). (c) Decay of the carotenoid triplet
absorption at 550 nm. The smooth curve is the sum of a 1.4 µs rise
time and 10 µs (87%) and 83 µs (13%) decay lifetimes.
3
triplet state (Figure 3). The quantum yield of C-P-C60 is
3
lifetime was ∼5.7 µs in the argon-purged solution and was
strongly quenched by oxygen.
0.13, and its (oxygen sensitive) lifetime is 4.9 µs. Thus, C-
•-
P-C60 is formed by charge recombination of C•+-P-C60
.
Nanosecond Transient Absorption at 77 K. Excitation of
a ∼1 × 10-4 M solution of 1 in a 2-methyltetrahydrofuran glass
at 77 K with a 590 nm, 5 ns laser pulse gave rise to several
Similar behavior was observed in benzonitrile solution.
Excitation of an argon-purged solution of 1 at 590 nm resulted
in the formation of C•+-P-C60•- with a quantum yield of 0.12.
This charge-separated state decayed with a time constant of 770
ns, with the concurrent formation of 3C-P-C60. The quantum
yield of 3C-P-C60 was 0.12. The carotenoid triplet state
•-
transient species (Figure 4). The C•+-P-C60 charge-
separated state was detected via observation at 980 nm of the
carotenoid radical cation absorption (Figure 4b) and was formed
with a quantum yield of ∼0.10 based on total light absorbed.
The decay of this transient at 77 K is not a true single
exponential, probably due to conformational or environmental
heterogeneity. The decay at 980 nm can be fitted as two
exponentials with a major component (73%) having a 1.5 µs
lifetime and a minor, ∼7 µs component. The 1.5 µs decay is
accompanied by a corresponding rise of the carotenoid triplet
(15) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26,
198-205.
(16) Land, E. J.; Lexa, D.; Bensasson, R. V.; Gust, D.; Moore, T. A.;
Moore, A. L.; Liddell, P. A.; Nemeth, G. A. J. Phys. Chem. 1987, 91, 4831-
4835.
(17) Greaney, M. A.; Gorun, S. M. J. Phys. Chem. 1991, 95, 7241-
7144.
(18) Kato, T. Laser Chem. 1994, 14, 155-160.

Carotene-Porphyrin-Fullerene Triad
J. Am. Chem. Soc., Vol. 119, No. 6, 1997 1403
absorption in the 550 nm region, with a major component of
3
1.4 µs (Figure 4b). The C-P-C60 species is formed with a
quantum yield of 0.07. Its decay can be fit with a lifetime of
10 µs (Figure 4c). The decay also features a small component
with a lifetime of 83 µs. A transient with a maximum
absorbance at 700 nm, corresponding to C-P-3C60, forms with
the excitation pulse in a yield of 0.17 and decays with a time
constant of 81 µs (Figure 4a).
Discussion
Energetics. The photochemical behavior of 1 may be
discussed in terms of Figure 5, which indicates the relevant high-
energy states and kinetic pathways. The energetics of the
excited singlet states in the figure are estimated from the average
of the long-wavelength absorption maxima and short-wavelength
emission maxima of relevant model systems. The energies of
the charge-separated states are estimated from cyclic voltam-
metric measurements in polar solvents of appropriate model
fullerenes,5 porphyrins,12 and carotenoids.16 The triplet state
energies are estimated from those of 58 and model octaalkyl-
porphyrins19 and carotenoids.20 No corrections have been made
for any Coulombic effects.
Figure 5. Transient states of triad 1 and their relevant interconversion
pathways. Spin multiplicities for radical pair states are not indicated.
The quantum yield of 3C-P-C60 in toluene is essentially unity,
•-
and no C•+-P-C60 state is observed. Thus, the quantum
yields of steps 2, 3, 13, and 14 in Figure 5 must be negligible.
Figure 5 indicates a significant thermodynamic driving force
in polar solvents (0.31 eV) for photoinduced electron transfer
from C-P-1C60 to give C-P•+-C60•- (step 2). This does not
occur in toluene, as C-P-1C60 is unquenched relative to the
model compound. The probable reason for this is the low
dielectric constant of toluene. Loss of dielectric stabilization
Photochemistry in Toluene. In toluene, excitation of the
porphyrin is followed by singlet-singlet energy transfer to the
fullerene. Since no charge-separated state was observed in this
solvent, it can be postulated that energy transfer is the only
significant decay pathway for C-1P-C60 (Vide infra). It is
conceivable that some C-P-1C60 is formed from very rapid
charge recombination of C-P•+-C60•-, which in turn might
be produced by photoinduced electron transfer from C-1P-
C60. There is no experimental evidence for such a process in
1. However, charge recombination to excited singlet states has
been observed in other molecules. For example, we recently
reported the observation of dual fluorescence resulting from
charge recombination in porphyrin dyads.21 Picosecond tran-
sient absorption studies may reveal more concerning this
possibility. Thus, k1 is taken as the reciprocal of the observed
•-
of C-P•+-C60 in this solvent reduces the driving force to
the point that the electron transfer is endergonic or only slightly
exergonic, and electron transfer cannot compete with inter-
system crossing. The driving force for electron transfer from
C-1P-C60 to yield C-P•+-C60•- (step 3) is substantially larger
(0.54 eV in polar solvents). This electron transfer process may
well be energetically feasible in toluene, but the very rapid
singlet-singlet energy transfer to yield C-P-1C60 is evidently
the dominant decay process for C-1P-C60 (barring rapid charge
recombination to yield C-P-1C60, as discussed above).
Photochemistry in 2-Methyltetrahydrofuran. In 2-meth-
yltetrahydrofuran at ambient temperatures, C-1P-C60 decays
in 10 ps by some combination of singlet-singlet energy transfer
to the fullerene and electron transfer to give C-P•+-C60•- (k1
+ k3 ) 1.0 × 1011 s-1). The fullerene excited singlet state
C-P-1C60, whether formed by energy transfer or direct
excitation, accepts an electron from the porphyrin to yield
12 ps lifetime: 8.3 × 1010 s-1
.
The C-P-1C60 state lifetime is not quenched relative to the
corresponding state of model fullerene 5. Fullerenes including
58 are known to undergo intersystem crossing to give the triplet
with quantum yields of essentially unity. Thus, C-P-1C60
decays in 1.52 ns, presumably by intersystem crossing to give
C-P-3C60 with k10 ) 6.6 × 108 s-1. However, the carotenoid
triplet, rather than the fullerene triplet, is observed spectroscopi-
cally. Carotenoids such as this one have negligible yields of
•-
C-P•+-C60 (k2 ) 3.1 × 1010 s-1 as determined from the
fullerene fluorescence decays of 1 (32 ps) and 5 (1.35 ns). The
•-
overall quantum yield of C-P•+-C60 by the two paths is
essentially unity. Competing with charge recombination of
C-P•+-C60•- (step 7) is electron transfer (step 4), giving C•+-
3
intersystem crossing, and C-P-C60 is formed with a time
•-
3
P-C60 with an overall yield of 0.14. This final long-lived
constant of 85 ns. Thus, C-P-C60 must form from C-P-
3C60 by triplet-triplet energy transfer. It is postulated that
C-P-3C60 undergoes endergonic triplet energy transfer to the
porphyrin to give C-3P-C60 (k11 ) 1.2 × 107 s-1 based on
the 85 ns rise time of 3C-P-C60). This species in turn transfers
triplet excitation to the carotenoid (k12 > 1 × 108 s-1 based on
the rise time of the carotenoid triplet absorption in 2 following
porphyrin excitation). A similar triplet energy transfer relay
has been observed in a carotenoid-porphyrin-pyropheophor-
bide triad22 and in bacterial photosynthetic reaction centers.23-25
charge-separated state decays in 170 ns, mainly by charge
recombination to produce the carotenoid triplet state (k9 ) 5.9
3
× 106 s-1). The C-P-C60 formed with a quantum yield of
0.13 in this unusual manner returns to the ground state in 4.9
µs (k15 ) 2.0 × 105 s-1).
Photochemistry at 77 K. In a glass at 77 K, C-P-3C60 is
formed by intersystem crossing within 10 ns of excitation with
a quantum yield of 0.17 and decays to the ground state with k13
) 1.2 × 104 s-1. Relay of triplet energy to the carotenoid via
the porphyrin does not occur. This is expected for energetic
reasons, given the estimated triplet energy levels in Figure 5.
With 590 nm excitation, ∼19% of the absorbed light excites
the fullerene directly. As the yield of intersystem crossing in
(19) Gouterman, M.; Khalil, G.-M. J. Mol. Spectrosc. 1974, 53, 88-
100.
(20) Lewis, J. E.; Moore, T. A.; Benin, D.; Gust, D.; Nicodem, D.; Nonell,
S. Photochem. Photobiol. 1994, 59S, 35S.
(21) DeGraziano, J. M.; Macpherson, A. N.; Liddell, P. A.; Noss, L.;
Sumida, J. P.; Seely, G. R.; Lewis, J. E.; Moore, A. L.; Moore, T. A.;
Gust, D. New J. Chem. 1996, 20, 839-851.
(22) Gust, D.; Moore, T. A.; Moore, A. L.; Krasnovsky, A. A., Jr.;
Liddell, P. A.; Nicodem, D.; DeGraziano, J. M.; Kerrigan, P.; Makings, L.
R.; Pessiki, P. J. J. Am. Chem. Soc. 1993, 115, 5684-5691.
(23) Schenck, C. C.; Mathis, P.; Lutz, M.; Gust, D.; Moore, T. A.
Biophys. J. 1983, 41, 123a.
(24) Schenck, C. C.; Mathis, P.; Lutz, M. Photochem. Photobiol. 1984,
39, 407-417.
(25) Takiff, L.; Boxer, S. G. J. Am. Chem. Soc. 1988, 110, 4425-4426.

1404 J. Am. Chem. Soc., Vol. 119, No. 6, 1997
Liddell et al.
5 and other fullerene model compounds is essentially unity, this
suggests that at 77 K the observed fullerene triplet state (φ )
0.17) arises mainly from light absorbed directly by the fullerene
moiety and that C-P-1C60 is not quenched by electron transfer
but decays by intersystem crossing (as occurs in toluene). This
being the case, the majority of the C-1P-C60 states must decay
by the photoinduced electron transfer step 3 to give C-P•+-
C60•-, which goes on to yield C•+-P-C60•- via step 4 with an
overall yield of 0.10 (0.12 based on light absorbed by the
porphyrin). As that which occurs at ambient temperatures, C•+-
Finally, the C•+-P-C60•- charge-separated state recombines
to yield the carotenoid triplet state, rather than the molecular
ground state. In related carotenoid-porphyrin-quinone sys-
tems, only recombination to the ground state is observed.
Charge recombination to triplet states is well-known in photo-
synthetic reaction centers30-33 and has been observed at low
temperatures in liquid crystalline media for a model system
consisting of non-photosynthetic pigments that absorb in the
blue spectral region.34 Recombination of a short-lived charge-
separated state to a local triplet state has been inferred, but not
observed directly, in two fullerene-containing dyads.7,8 The
triplet state in 1 presumably forms by radical pair recombination
•-
P-C60 slowly recombines, mainly to the carotenoid triplet
(k8 + k9 ) 6.7 × 105 s-1, φ ) 0.07), which decays with k15
)
•-
1.0 × 105 s-1. The small 83 µs decay component observed at
wherein most of the initially-formed C•+-P-C60 singlet
550 nm is likely due to absorption of C-P-3C60.
radical pair state evolves into the triplet radical pair, which then
recombines to yield 3C-P-C60. It will be of interest to
investigate 1 using time-resolved EPR spectroscopy in order to
learn more about the birth of this unusual triplet state.
The lack of charge separation from C-P-1C60 at 77 K is
ascribed mainly to loss of Coulombic stabilization of C-P•+-
•-
C60 in the 2-methyltetrahydrofuran glass (dielectric constant
) 2.6026). This makes the free energy change for electron
transfer small or positive. Electron transfer from C-1P-C60
Experimental Section
•-
to yield C-P•+-C60 has a substantially larger driving force,
Synthesis. Model carotenoporphyrin 2,12 model porphyrin 4,12 and
model fullerene 513 were prepared as previously described.
as mentioned above, and this photoinduced electron transfer
reaction still occurs. It is interesting to note that, in the glass
at 77 K, the electron transfer step 3 is the dominant decay
mechanism, rather than energy transfer step 1, whereas in
toluene step 1 predominates. This apparent reversal of behavior
could be rationalized by electron transfer in toluene from
5-(4-(((Benzyloxy)carbonyl)amino)phenyl)-15-(4-(methoxycar-
bonyl)phenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylpor-
phyrin (8). To a stirred solution of 5-(4-aminophenyl)-15-(4-(meth-
oxycarbonyl)phenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylpor-
phyrin (7)12 (300 mg, 0.43 mmol) in 100 mL of dichloromethane under
a nitrogen atmosphere was added 94 µL (0.85 mmol) of N-methyl-
morpholine, followed by 80 µL (0.55 mmol) of carbo(benzyloxy)
chloride. After 4 h of stirring, a second portion of carbo(benzyloxy)
chloride was added, and stirring was continued for a total of 24 h.
Dilute aqueous sodium bicarbonate was added to the reaction vessel,
and the resulting mixture was stirred vigorously for 2 h. The organic
phase was separated, washed with dilute aqueous citric acid, washed
with dilute aqueous sodium bicarbonate, dried over anhydrous sodium
sulfate, and filtered. The filtrate was distilled under reduced pressure
to remove the solvent. The residue was chromatographed on silica
gel (toluene containing 5% ethyl acetate) to give 0.331 g of porphyrin
8 (92% yield): 1H NMR (300 MHz, CDCl3) δ -2.43 (2H, brs, -NH),
1.76 (12H, t, J ) 8 Hz, 2-CH3, 8-CH3, 12-CH3, 18-CH3), 2.46 (6H, s,
13-CH3, 17-CH3), 2.53 (6H, s, 3-CH3, 7-CH3), 4.01 (8H, q, J ) 8 Hz,
2-CH2, 8-CH2, 12-CH2, 18-CH2), 4.13 (3H, s, -OCH3), 5.36 (2H, s,
-CH2Ar), 7.04 (1H, s, -NH), 7.3-7.6 (5H, m, -CH2ArH), 7.79 (2H, d,
J ) 8 Hz, 5Ar-3,5-H), 7.99 (2H, d, J ) 8 Hz, 5Ar-2,6-H), 8.19 (2H,
d, J ) 8 Hz, 15Ar-3,5-H), 8.44 (2H, d, J ) 8 Hz, 15Ar-2,6-H), 10.23
(2H, s, 10-CH, 20-CH); MS m/z 837 (M+); UV/vis (CH2Cl2) 408, 508,
542, 576, 628 nm.
5-(4-(((Benzyloxy)carbonyl)amino)phenyl)-15-(4-(hydroxymeth-
yl)phenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (9).
A stirred solution of porphyrin 8 (320 mg, 0.38 mmol) in 200 mL of
tetrahydrofuran under a nitrogen atmosphere was cooled in an ice bath,
and an excess of lithium aluminum hydride was added. After 20 min,
the mixture was poured into 150 mL of dichloromethane, and the
resulting mixture was washed with water, followed by dilute aqueous
sodium bicarbonate. After drying over anhydrous sodium sulfate, the
solution was filtered, and the solvent was removed by distillation at
reduced pressure. Chromatography of the residue on silica gel (toluene
containing 15% ethyl acetate) gave 9 in 93% yield (290 mg): 1H NMR
(300 MHz, CDCl3) δ -2.42 (2H, brs, -NH), 1.76 (12H, t, J ) 8 Hz,
2-CH3, 8-CH3, 12-CH3, 18-CH3), 2.48 (6H, s, 13-CH3, 17-CH3), 2.53
(6H, s, 3-CH3, 7-CH3), 4.01 (8H, q, J ) 8 Hz, 2-CH2, 8-CH2, 12-CH2,
•-
C-1P-C60 to yield C-P•+-C60 followed by rapid charge
recombination to give C-P-1C60. However, as mentioned
above, the time-resolved fluorescence and nanosecond transient
absorption experiments reported here give no evidence for this
process.
Conclusions
The photochemistry of triad 1 has a number of unusual
features. The results demonstrate that fullerenes can act as
effective primary electron acceptors in multicomponent systems
that generate long-lived charge-separated states with reasonable
quantum yields. Secondly, two-step photoinduced electron
transfer to generate long-lived charge separation in 1 occurs
even at 77 K in a glass. The vast majority of systems based on
other electron acceptors fail to undergo charge separation under
these conditions.15,27,28 It is generally thought that this failure
is due mainly to the fact that upon freezing, the effective solvent
dielectric constant decreases dramatically because the solvent
dipoles cannot reorient to stabilize the newly-formed ions.
(Photosynthetic electron transfer in natural reaction centers does
not suffer from this problem.) Studies of porphyrins linked to
quinones and related acceptors have shown that the loss of
stabilization in going from liquid butyronitrile to a 2-methyl-
tetrahydrofuran glass at low temperatures is ∼0.8 eV and that
only dyads with a greater driving force for electron transfer can
still function at these temperatures.29 The free energy change
•-
for formation of C-P•+-C60 from C-1P-C60 is only 0.54
eV, based on cyclic voltammetry in benzonitrile. Thus, the
fullerene anion in 1 may be less susceptible to destabilization
in nonpolar solvents than these other organic ions. The effect
of solvent reorganization may also be different for fullerenes.
(26) Furutsuka, T.; Imura, T.; Kojima, T.; Kawabe, K. Technol. Rep.
Osaka UniV. 1974, 24, 367.
(27) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(28) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer,
Part D; Fox, M. A., Channon, M., Eds.; Elsevier: Amsterdam, 1988; pp
303-393.
(29) Wasielewski, M. R.; Gaines, G. L., III; O’Neil, M. P.; Svec, W.
A.; Niemczyk, M. P.; Prodi, L.; Gosztola, D. In Dynamics and Mechanisms
of Photoinduced Transfer and Related Phenomena; Mataga, N., Okada, T.,
Masuhara, H., Eds.; Elsevier Science Publishers: New York, 1992; pp 87-
103.
(30) Thurnauer, M. C.; Katz, J. J.; Norris, J. R. Proc. Natl. Acad. Sci.
U.S.A. 1975, 72, 3270-3274.
(31) Regev, A.; Nechushatai, R.; Levanon, H.; Thornber, J. P. J. Phys.
Chem. 1989, 93, 2421-2426.
(32) Rutherford, A. W.; Paterson, D. R.; Mullet, J. E. Biochim. Biophys.
Acta 1981, 635, 205-214.
(33) Dutton, P. L.; Leigh, J. S.; Seibert, M. Biochem. Biophys. Res.
Commun. 1972, 46, 406-413.
(34) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.; Svec,
W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1995, 117, 8055-8056.

Carotene-Porphyrin-Fullerene Triad
J. Am. Chem. Soc., Vol. 119, No. 6, 1997 1405
18-CH2), 5.06 (2H, s, -CH2OH), 5.35 (2H, s, -CH2Ar), 7.03 (1H, s,
-NH), 7.3-7.6 (5H, m, -CH2ArH), 7.72 (2H, d, J ) 8 Hz, 15Ar-3,5-
H), 7.76 (2H, d, J ) 8 Hz, 5Ar-3,5-H), 7.98 (2H, d, J ) 8 Hz, 5Ar-
2,6-H), 8.05 (2H, d, J ) 8 Hz, 15Ar-2,6-H), 10.22 (2H, s, 10-CH,
20-CH); MS m/z 809 (M+); UV/vis (CH2Cl2) 408, 508, 542, 576, 628
nm.
was dissolved in 20 mL of dichloromethane containing 42 µL (0.51
mmol) of pyridine, and the resulting solution added to the solution of
fullerene-aminoporphyrin. After the reaction mixture was stirred for
1 h, it was diluted with toluene, washed with dilute aqueous sodium
bicarbonate, and dried over anhydrous sodium sulfate, and the volume
was reduced to 100 mL by distillation of the solvent at reduced pressure.
The resulting solution was applied to a silica gel column and eluted
with toluene containing 3% ethyl acetate to give 48 mg of triad 1 (48%
yield): 1H NMR (500 MHz, CDCl3) δ -2.40 (2H, brs, -NH), 1.04
(6H, s, Car 16-CH3, Car 17-CH3), 1.44-1.54 (2H, m, Car 2-CH2),
1.58-1.68 (2H, m, Car 3-CH2), 1.73 (3H, s, Car 18-CH3), 1.78 (12H,
t, J ) 8 Hz, 2-CH3, 8-CH3, 12-CH3, 18-CH3), 1.98 (3H, s, Car 19-
CH3), 2.00 (3H, s, Car 20-CH3), 2.02 (3H, s, Car 20′-CH3), 2.03 (2H,
m, Car 4-CH2), 2.10 (3H, s, Car 19′-CH3), 2.55 (12H, brs, 3-CH3,
7-CH3, 13-CH3, 17-CH3), 2.93 (3H, s, -NCH3), 3.94-4.00 (8H, brm,
2-CH2, 8-CH2, 12-CH2, 18-CH2), 3.91 (1H, d, J ) 8 Hz, -CH2N-), 4.73
(1H, s, -CHN-), 4.82 (1H, d, J ) 8 Hz, -CH2N-), 6.0-6.8 (13 H, m,
Car ) CH-), 7.06 (1H, d, J ) 16 Hz, Car 8′-H), 7.61 (2H, d, J ) 8
Hz, Car 1′,5′-H), 7.99 (2H, d, J ) 8 Hz, Car 2′,4′-H), 8.02 (2H, d, J
) 8 Hz, 5Ar-3,5-H or 5Ar-2,6-H), 8.05 (2H, d, J ) 8 Hz, 5Ar-2,6-H
or 5Ar-3,5-H), 8.1 (4H, brs, 15Ar-3,5-H, 15Ar-2,6-H), 8.16 (1H, s,
-NH), 10.21 (2H, s, 10-CH, 20-CH); MS(FAB) m/z 1938.7703 (calcd
for (M + H)+, 1938.7755); UV/vis (CH2Cl2) 410, 482, 510, 574, 628,
702 nm.
5-(4-(((Benzyloxy)carbonyl)amino)phenyl)-15-(4-formylphenyl)-
2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (6). Porphyrin
9 (270 mg, 0.33 mmol) was dissolved in 200 mL of dichloromethane,
and the solution was stirred under nitrogen while excess manganese
dioxide was added. After 1 h of stirring, the solution was filtered
through Celite. A mixture of chloroform and methanol (4:1) was
washed through the filter cake until all of the porphyrin was extracted.
The solvent was removed from the combined filtrates by distillation at
reduced pressure, and the residue was chromatographed on silica gel
(toluene containing 4% ethyl acetate) to give 204 mg of aldehyde 6
(76% yield): 1H NMR (300 MHz, CDCl3 δ -2.43 (2H, brs, -NH),
1.77 (12H, t, J ) 8 Hz, 2-CH3, 8-CH3, 12-CH3, 18-CH3), 2.45 (6H, s,
13-CH3, 17-CH3), 2.53 (6H, s, 3-CH3, 7-CH3), 4.01 (8H, q, J ) 8 Hz,
2-CH2, 8-CH2, 12-CH2, 18-CH2), 5.36 (2H, s, -CH2Ar), 7.04 (1H, s,
-NH), 7.3-7.6 (5H, m, -CH2ArH), 7.78 (2H, d, J ) 8 Hz, 5Ar-3,5-H),
7.98 (2H, d, J ) 8 Hz, 5Ar-2,6-H), 8.26 (2H, d, J ) 8 Hz, 15Ar-3,5-H
or 15Ar-2,6-H), 8.27 (2H, d, J ) 8 Hz, 15Ar-2,6-H or 15Ar-3,5-H),
10.24 (2H, s, 10-CH, 20-CH), 10.39 (1H, s, -CHO); MS m/z 807 (M+);
UV/vis (CH2Cl2) 410, 508, 542, 576, 628 nm.
1
Instrumental Techniques. The H NMR spectra were recorded
Porphyrin-Fullerene 3. Porphyrin aldehyde 6 (100 mg, 0.12
mmol), N-methylglycine (55 mg, 0.62 mmol), and C60 (107 mg, 0.15
mmol) were added to 200 mL of toluene under a nitrogen atmosphere,
and the resulting suspension was heated at reflux for 18 h. The mixture
was cooled to room temperature and applied directly to a silica gel
chromatography column. Elution with toluene containing 4% ethyl
acetate gave an impure product, which was dissolved in carbon
disulfide, diluted with toluene, and chromatographed on silica gel
(toluene containing 3% ethyl acetate) to give pure dyad 3 (119 mg,
62% yield): 1H NMR (500 MHz, 1:1 CDCl3/CS2) δ -2.46 (2H, brs,
-NH), 1.79 (12H, t, J ) 8 Hz, 2-CH3, 8-CH3, 12-CH3, 18-CH3), 2.4
(6H, brs, 13-CH3, 17-CH3), 2.52 (6H, s, 3-CH3, 7-CH3), 3.06 (3H, s,
-NCH3), 4.00 (8H, q, J ) 8 Hz, 2-CH2, 8-CH2, 12-CH2, 18-CH2), 4.27
(1H, d, J ) 8 Hz, -CH2N-), 5.04 (1H, d, J ) 8 Hz, -CH2N-), 5.11 (1H,
s, -CHN-), 5.33 (2H, s, -CH2Ar), 6.97 (1H, s, -NH), 7.3-7.6 (5H, m,
-CH2ArH), 7.78 (2H, d, J ) 8 Hz, 5Ar-3,5-H), 7.96 (2H, d, J ) 8 Hz,
5Ar-2,6-H), 8.13 (4H, brs, 15Ar-3,5-H, 15Ar-2,6-H), 10.14 (2H, s, 10-
CH, 20-CH); MS m/z 1555.6 (M + 1)+; UV/vis (CH2Cl2) 410, 508,
542, 576, 628 nm.
Triad 1. The protecting group of 3 was removed by adding 2 mL
of a 1.0 M solution of boron tribromide in dichloromethane to a stirred
mixture of 3 (80 mg, 0.051 mmol) in 20 mL of toluene and 20 mL of
dichloromethane under a nitrogen atmosphere. After 2 h of stirring,
the solution was diluted with 200 mL of toluene, washed with dilute
aqueous sodium bicarbonate, dried over anhydrous sodium sulfate, and
filtered. The filtrate was applied to a silica gel column and eluted with
toluene containing 3% ethyl acetate. The fraction containing the
fullerene-aminoporphyrin product was concentrated to ∼25 mL and
retained for the coupling reaction.
on Varian Unity spectrometers at 300 or 500 MHz. Unless otherwise
specified, samples were dissolved in deuteriochloroform with tetra-
methylsilane as an internal reference. High-resolution mass spectra
were obtained on a Kratos MS 50 mass spectrometer operating at 8
eV in FAB mode. Other mass spectra were obtained on a Varian MAT
311 spectrometer operating in ET mode or a matrix-assisted laser
desorption/ionization time-of-flight spectrometer (MALDI-TOF). Ul-
traviolet-visible spectra were measured on a Shimadzu UV2100U UV/
vis Spectrometer, and fluorescence spectra were measured on a SPEX
Fluorolog using optically dilute samples and corrected.
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.35 The
instrument response function was 35 ps, as measured at the excitation
wavelength for each decay experiment with Ludox AS-40.
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.36
Acknowledgment. This work was supported by the National
Science Foundation (CHE-9413084). FAB mass spectrometric
studies were performed by the Midwest Center for Mass
Spectrometry, with partial support by the National Science
Foundation (DIR9017262). This is publication 318 from the
ASU Center for the Study of Early Events in Photosynthesis.
A mixture of 27 mg (0.51 mmol) of 7′-apo-7′-(4-carboxyphenyl)-
â-carotene14 (Car) in 20 mL of benzene and 42 µL (0.51 mmol) of
pyridine was stirred under a nitrogen atmosphere, and 19 µL (0.26
mmol) of thionyl chloride was added. After 20 min, the solvent was
removed by distillation at reduced pressure, and an additional 20 mL
of benzene was added and removed in a similar manner. The residue
JA9631054
(35) Gust, D.; Moore, T. A.; Luttrull, D. K.; Seely, G. R.; Bittersmann,
E.; Bensasson, R. V.; Rouge´e, M.; Land, E. J.; de Schryver, F. C.; Van der
Auweraer, M. Photochem. Photobiol. 1990, 51, 419-426.
(36) Davis, F. S.; Nemeth, G. A.; Anjo, D. M.; Makings, L. R.; Gust,
D.; Moore, T. A. ReV. Sci. Instrum. 1987, 58, 1629-1631.