1. Photochemistry and Photobiology, Vol. 60, No. 6, pp. 537-541, 1994 0031-8655/93 $05.00+0.00
Printed in the United States. All rights reserved 0 1994 American Society for Photobiology
RAPID COMMUNICATION
PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C~O
DYADS
PAUL LIDDELL,
A. JOHN SUMIDA,
P. ALISDAIR MACPHERSON, N o s , GILBERT SEELY,
N. LORI 5.
N.
KRISTINE CLARK, ANAL.MOORE,* THOMAS MOORE* AND DEVENS GUST
A.
Department of Chemistry and Biochemistry,
Center for the Study of Early Events in Photosynthesis,
Arizona State University, Tempe, AZ 85287-1604.
(Received 15 September 1994; accepted 6 October 1994)
Abstract - Porphyrin-Ca dyads in which the two chromophores are linked by a bicyclic bridge have
been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz,-C,)
and free base (P-C,) dyads, determined by time-resolved fluorescence measurements, are 1 7 ps in
toluene. This substantial quenching is due to singlet-singlet energy transfer to C,. The lifetime of Pzn-
'Cm is -5 ps in toluene, whereas the singlet lifetime of an appropriate C a model compound is 1.2 ns. This
quenching is attributed to electron transfer to yield Pa'+-C,'-. In toluene, P - k a is unquenched; the
lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an
absorption maximum at 700 nm and a lifetime of -10 ps was detected using transient absorption methods.
This state was quenched by oxygen, and is assigned to the C a triplet. In the more polar benzonitrile, P-
'C, under oes photoinduced electron transfer to give P'+-Cm'-. The electron transfer rate constant is
- 2 x 1011 s-?
INTRODUCTION
One approach to mimicry of photosynthetic energy
conversion is the construction of synthetic molecular
systems containing chromophores, electron donors and
electron acceptors linked by covalent bonds. These bonds
control the distances, angles and electronic coupling
between the moieties, and thus the rates of electron and
energy transfer. Typically, such molecules are based upon
1 C,
. toluene
porphyrins or other chlorophyll derivatives linked to
various organic donors and acceptors. 1-3 C, readily
undergoes one-electron reduction to the radical anion>5
and accepts electrons from metalloporphyrin radical
anions.6 In addition, fullerenes have been found to serve as
electron carriers in lipid bilayer membranes.' These facts 3: M=Zn
suggest that fullerenes might serve as useful electron 4: M = H ,
acceptor moieties in multicomponent photosynthesis
mimics. In order to investigate this possibility, we have Figure 1. Synthesis and conformation of the dyads.
prepared two porphyrin-C, dyads and studied their
photochemical properties. diastereomeric dimesylates (9 1% yield). Treatment of a
dichloromethane solution of the dimesylates with excess
MATERIALS AND METHODS zinc acetate in methanol produced the metallated porphy-
rim in quantitative yield. Dehydromesylation with potas-
Synthesis. Preparation of the porphyrin-C, dyads (Fig. sium t-butoxide in dimethylformamide at 25 "C gave diene
1) began with reduction of a mixture of ester 1 and its 2 in 42% yield. A toluene solution of the diene containing
diastereomer (having the opposite relative configurations at an excess of C a (Mer Corp., Tucson, AZ) was purged with
the carbon atoms bearing the carbethoxy groups)' with nitrogen, sealed in a glass tube and heated at 120 "C for 3
lithium aluminum hydride in tetrahydrofuran at ambient h. Porphyrin-C, dyad 3 was isolated in 34% yield from the
temperature. The resulting mixture of stereoisomeric diols reaction mixture. Removal of the zinc with trifluoroacetic
(produced in 90% yield) was treated with methanesulfonyl acid in dichloromethane gave free-base dyad 4. Treatment
chloride in pyridine at 0°C to yield a mixture of of a dichloromethane solution of 1 with excess methanolic
* To whom correspondence should be addressed.
537

2. 538 PAULA. LIDDEU et al
zinc acetate yielded the zinc analog, 6 , quantitatively.
Model fullerene 5 was prepared by refluxing a toluene
solution of Cm and the corresponding diene under nitrogen
for 30 h. The structures of 2 , 3 and 5 were verified by mass
spectrometric, UV-VIS and NMR data (see Results).
Spectroscopic studies. Steady state fluorescence and
fluorescence excitation spectra were measured using a Absorption spectra
SPEX Fluorolog-2. Excitation was produced by a 450 W The absorption spectra of 4, model fullerene 5 and
xenon lamp and single grating monochromator. model porphyrin 1 in toluene solution are shown in Fig. 2a.
Fluorescence was detected at a 90” angle to the excitation The spectrum of 4 features a broad band at 325 nm
beam via a single grating monochromator and an R928 corresponding to fullerene absorption. A similar band is
photomultiplier tube having S-20 spectral response and seen with 5. The absorption spectrum of 5 tails off slowly
operating in the photon counting mode. Fluorescence decay to the red, with a weak absorption at 715 nm. The spectrum
measurements were made using the time-correlated single of 4 has additional bands at 423,5 19,557,589 and 643 nm
photon counting method. The excitation source was a which are characteristic of free-base porphyrins. Also
frequency-doubled, mode-locked Nd-YAG laser coupled to shown in Fig. 2a is the linear combination of the spectra of
a synchronously pumped, cavity dumped dye laser with models 1 and 5 which best approximates that of dyad 4 in
excitation at 590 nm. Detection was via a microchannel the 320 - 450 nm region. It will be noted that the Soret and
9,
plate photomultiplier (Hamamatsu R2809U- 1 and the Q-bands of the dyad are shifted to longer wavelengths by
instrument response time was ca. 35 ps. Cyclic 11 - 15 nm relative to the porphyrin model.
voltammetric measurements were performed in benzonitrile Fig. 2b shows the absorption spectra for zinc dyad 3,
using a three-electrode system and a Pine Instruments model fullerene 5, and porphyrin 6. In the spectrum of 3,
Model AFRDE4 potentiostat. The cell featured a glassy the fullerene absorption at -325 is apparent, as are bands at
carbon working electrode and salt bridges to an SCE refer- 425, 550 and 590 nm which are characteristic of metallated
ence electrode and a platinum wire counter electrode. The porphyrins. The summed spectra indicate that the Soret and
tetra-n-butylammonium hexafluorophosphate electrolyte Q-bands of the porphyrin are shifted to longer wavelengths
was recrystallized and dried before use, and the cell was by 9 - 12 nm relative to model porphyrin 6.
kept under an atmosphere of nitrogen. The nanosecond
transient absorption apparatus has been previously Steady-state fluorescence spectra
described.“ Fig. 3 presents the fluorescence emission spectra of
free base dyad 4, model hllerene 5, and model porphyrin 1
RESULTS in toluene solution. The fluorescence quantum yield of 1,
Structure and conformation measured using tetraphenylporphyrin ($I~ 0.11) as a
=
The 500-MHz ‘H NMR spectra of 2 - 4 in deuterio- standard, is 0.081, whereas that of fillerene 5 is 0.0014.
chloroform solution were assigned with the aid of COSY, Dyad 4 features only very weak emission from the
NOESY and HMBC results. The presence of the Ca porphyrin moiety, indicating strong quenching of the
moiety in 3 was verified through the detection of the two porphyrin excited singlet state by the attached fullerene.
C a carbon nuclei bearing the bridge to the porphyrin (6 66 Fullerene emission is observed in the dyad with a quantum
ppm) and the adjacent four C a carbon nuclei (153, 155, yield (0.00072) which is only about a factor of two less
156, 156 ppm) via HMBC and HMQC experiments. It is than that of the model compound. Thus, the fullerene
known that Cm reacts as a dienophile with 1,3-dienes to excited singlet state is not significantly quenched.
yield derivatives bridged across the 6-6 ring junction.“”* The corrected fluorescence excitation spectrum of
Molecular mechanics calculations using the Discover dyad 4 in toluene, measured at 800 nm where the porphyrin
program in the Insight11 molecular-modeling package from moiety does not emit significantly, is identical to the
Biosym Technologies yielded the structure shown in Fig. 1 absorption spectrum within experimental error. Thus,
as the lowest-energy conformation of 4. This folded singlet-singlet energy transfer fiom the porphyrin to the
conformation is consistent with the ‘H NMR spectrum, in fidlerene occurs with a quantum yield close to unity and is
which the hydrogens at the 5 and 15 meso positions of the responsible for the observed quenching of the porphyrin
porphyrin ring are shifted upfield by -0.50 and 0.30 ppm, first excited singlet state.
respectively, relative to model o h in 1 due to shielding The fluorescence spectrum of zinc dyad 3 (excitation
by components of the C a ring. fFyr at 550 nm) shows that emission from both moieties is
strongly quenched. The fluorescence quantum yield for the
Cyclic voltammety porphyrin moiety is 2.0 x lo9 and that for the fullerene is
Cyclic voltammetric studies on model porphyrins 1 about 1.8 x 10”.
and its zinc analog 6 yielded reversible waves, with first The corrected fluorescence excitation spectrum of zinc
oxidation potentials of +0.359 and +O. 177 V, respectively, dyad 3 in toluene, measured at 800 nm, is identical with the
relative to a ferrocene internal reference redox system. The absorption spectrum within experimental error, signaling
first and second reduction potentials of model fullerene 5 singlet-singlet energy transfer with a quantum yield close to
were -1.047 V and -1.468 V. unity.

3. Rapid Communication 539
0.4r--
L
-4
0.3 -- 5
........ I
8 5+1
5 0.2
e
51
2 0.1
0.0
t
400 500 600 700
0.4 I 600 650 700 750 800 850
Wavelength (nm)
0.3 Figure 3. Fluorescence emission spectra of porphyrin
model 1 ( ), fullerene 5 (- - x30), and fiee base
,
8 dyad 4 (
-
, x30) in toluene solutions having equal
5
e 0.2
absorbance at the 550-nm excitation wavelength.
0
v)
n
a 0.0 0.007 ns
120
1.4 ns
* 0.11 ns
0.0 4.8 ns
80
a,
U
400 500 600 700 3
-
.-
. I
c
Wavelength (nm) 40
Figure 2. Absorption spectra in toluene of fiee base dyad 4 a
(a), zinc dyad 3 (b), and model compounds.
0
Time-resolvedfluorescence studies
The fluorescence decay of dyad 4 in toluene solution
with excitation at 590 nm was measured at 6 wavelengths
in the 700 - 840 nm region, and the results were analyzed -40
g l ~ b a l l y 'to give the decay-associated spectra shown in
~ 680 720 760 800 840
Fig. 4. There are two significant components to the decay. Emission Wavelength (nm)
The 7-ps decay is of high amplitude in the 700-nm region,
where most of the emission is due to the porphyrin, and Figure 4. Decay-associated spectra for dyad 4 in toluene
negative at 800 nm, where the fullerene emits. The first with excitation at 590 nm. The x2 value of the fit was 1.25.
excited singlet state of a model porphyrin diester similar to
1 has a lifetime of 11.5 ns in benzene solution. Thus, the 7- the model porphyrin and fullerene 5 were 12.6 and 1.1 ns,
ps component represents the rapid decay of the porphyrin respectively. In this solvent, the fluorescence decays of
first excited singlet state by singlet-singlet energy transfer dyad 4 at 12 wavelengths in the 630 - 800 nm region were
and concurrent rise of the fullerene singlet state. The 1.4-ns analyzed globally to give only one significant component
decay has the shape of the fuIlerene emission, and denotes with a lifetime of -2 ps (x2 = I . 12). In the 740 - 800 nm
the lifetime of the fullerene first excited singlet state. region, where most of the emission is due to the fullerene,
Fullerene model 5 has a fluorescence lifetime of 1.2 ns in the data yielded a lifetime of -6 ps for the only significant
toluene. Thus, the fullerene excited singlet state in dyad 4 decay component. Thus, emission fiom both the porphyrin
is unquenched by the attached porphyrin, as also revealed and fullerene moieties is strongly quenched in this solvent.
by the steady-state fluorescence studies. The time-resolved fluorescence of zinc dyad 3 was
In benzonitrile solution, the fluorescence lifetimes of also studied in toluene. In the 680 - 840 nm region, a single

4. 540 A.
PAUL LIDDELL al.
et
significant emission with a lifetime of -5 ps was observed. can be discussed in terms of Fig. 6. The energies of the first
Thc zinc porphyrin model 6, on the other hand, has a excited singlet states have been estimated from the
fluorescence lifetime of 1.9 ns in benzene. Thus, absorption and emission spectra. The energies of the
fluorescence from both the porphyrin and fullerene charge-separated states are based on the cyclic
moieties of the dyad is very strongly quenched, as also voltammetric measurements in benzonitrile reported above.
revealed by the steady-state emission studies. The results for 3 will be discussed first. Excitation of
the zinc porphyrin moiety gives 'Pa-Ca, which decays by
Trunsient absorption studies singlet-singlet energy transfer to the fullerene to yield Pzn-
Excitation of an argon-purged toluene solution of 4 'Ca, with kl -2 x 10" s-', as estimated from the time-
with 590-nm, -5 ns laser pulses resulted in the formation of resolved fluorescence data in toluene. This rate constant is
a transient species whose absorption spectrum is shown in only approximate, as lifetimes of -5 ps are subject to
Fig. 5. This transient decayed with a lifetime of -10 ps, and significant uncertainty due to instrumental limitations and
the lifetime was strongly quenched by the admission of interference from solvent Raman scattering. Although
atmospheric oxygen. The spectral shape is identical to that electron transfer via step 2 is energetically feasible, it
of the transient absorption obtained after excitation of evidently does not compete favorably with energy transfer.
fullerene 5 under the same conditions (Fig. 5). The The decay of the fullerene excited singlet state, which can
transient is ascribed to the fullerene triplet state. The C a also be produced by direct excitation, is ascribed to
triplet state in benzene has an absorption maximum at 750
No long-lived transient absorption was observed
following excitation of dyad 3 under similar conditions.
Excitation of 4 and 5 in aerated toluene solution with
2.0 -
590-nm, -5 ns laser pulses led to the production of singlet
oxygen, which was detected via its characteristic emission
at 1270 nm.16 The quantum yield of singlet oxygen was
identical for fullerene 5 and for Cm. The triplet quantum
yield for C a in benzene is reported to be 0.8815 [l.O]." If
9Q)
v
one assumes that the quenching of the C a triplet state by
oxygen to yield singlet oxygen is quantitative the quantum )r
yield of singlet oxygen and of the triplet state for 5 must
also be 0.88 [1.0]. Similarly, the quantum yield of fkllerene
Pi.0
Q)
-
triplet state for 4 is estimated to be 0.20 E0.231. For C a in C
benzene, ET = 20,200 mol-' cm-' at the 750-nmma~imum.'~ W
Using this value, the triplet quantum yields mentioned
above, and the transient absorption spectra, ETEG is
estimated to be 9,300 mol-' cm-' for hllerene 5.
0.006
0.0 -
0.004 Figure 6 . Transient states and interconversion paths for
dyad 3 in a polar solvent. Dashed lines indicate levels in
free base dyad 4.
a 0.002 photoinduced electron transfer from the PO hyrin ground
state to yield Phof-Cao- with k3 -2 x 10 :- and 4 3 =
'T s'
1.0. This assignment of the quenching mechanism is
consistent with the electrochemical data and the results for
0.000 4 discussed below, but confirmation must await transient
absorption experiments. The rapid energy and electron
transfer is consonant with the folded conformation shown
400 600 800 1000 in Fig. 1, where the n-electron systems of the donor and
Wavelength (nm) acceptor moieties approach van der Waals contact, and
with the perturbations of the absorption spectra, which
Figure 5. Transient absorption spectra of -5 x 10" M dyad indicate interaction between the chromophores.
4 ( 0 ) and fullerene 5 ( 0 ) in argon-purged toluene
following excitation at 590-nm with a -5-11s laser pulse.
Extremely rapid and efficient singlet-singlet energ
transfer (kl -5 x 10" s-l) and electron transfer (k3 -2 x 10 x
s-') are also observed for free base dyad 4 in benzonitrile.
Discussion In this case, the 'P-Ca and Po+-Cao- states lie at 1.90 and
The spectroscopic results for the porphyrin-Ca dyads 1.41 eV, respectively. In toluene, rapid (-1.4 x 10" s-')

5. Rapid Communication 54 1
singlet-singlet energy transfer with unity quantum yield is their mono-, di-, tri-, and tetraanions. J. Am. Chem. SOC.
also observed for 4. However, the P - k a state is 113,4364-4366.
unquenched relative to the fullerene model, and electron 6. Guldi, D. M., P. Neta and K.-D. Asmus (1994) Electron
transfer to yield Pof-Cao- does not occur. Instead, the transfer reactions between Ca and radical ions of
fillerene triplet state is produced. In a polar solvent such as metalloporphyrins and arenes. J. Phys. Chem. 98,4617-
benzonitrile AGO for electron transfer step 3 in Fig. 6 is 462 1 .
-0.31 eV, and electron transfer is facile. The driving force 7. Hwang, K. C. and D. Mauzerall (1993) Photoinduced
is reduced in non-polar toluene to the point where transfer electron transport across a lipid bilayer mediated by
becomes thermodynamical1 unfavorable. For example, the (2%. Nature 361, 138-140.
dielectric continuum modelY8 yields a A O value in toluene
G 8. Liddell, P. A., L. J. Demanche, S. Li, A. N.
that is positive by several tenths of an electron volt. In the Macpherson, R. A. Nieman, A. L. Moore, T. A. Moore
case of zinc dyad 3, A O is -0.50 eV in benzonitrile, and
G and D. Gust (1 994) A new porphyrin derivative for use
electron transfer is still feasible in toluene. as a diene in the Diels-Alder reaction. Tetrahedron Lett.
The conformation of 4 and the observation of very 35,995-998.
rapid singlet-singlet energy transfer and electron transfer 9. Gust, D., T. A. Moore, D. K. Luttrull, G. R. Seely, E.
(in benzonitrile) suggest that triplet-triplet energy transfer Bittersmann, R. V. Bensasson, M. Rougte, E. J. Land,
between the porphyrin and fullerene should be facile. Thus, F. C. de Schryver and M. Van der Auweraer (1990)
the observation of the fullerene triplet state (Fig. 5) Photophysical properties of 2-nitro-5,10,15,20-tetra-p-
suggests that the energy of the fullerene triplet in 4 is tolylporphyrins. Photochem. Photobiol. 51,4 19-426.
comparable to or below that of the porphyrin triplet state. 10. Davis, F. S., G. A. Nemeth, D. M. Anjo, L. R. Makings,
D. Gust and T. A. Moore (1987) Digital back off for
CONCLUSIONS computer controlled flash spectrometers. Rev. Sci.
Instrum. 58, 1629-163 1.
The porphyrin-C, dyads, which are readily synthe- 1 I. Diederich, F., U. Jonas, V. Gramlich, A. Herrmann, H.
sized from diene 2 and C,, exhibit rapid and efficient Ringsdorf and C. Thilgen (1993) Synthesis of a
singlet-singlet energy transfer from the porphyrin moiety to fullerene derivative of benzo[ 181crown-6 by Diels-
the fullerene. Zinc dyad 3 undergoes very rapid photoin- Alder reaction: Complexation ability, amphiphilic
duced electron transfer to yield P%'+-C,'- in both ben- properties, and x-ray crystal structure of a dimethoxy-
zonitrile and toluene, whereas with free base dyad 4, elec- 1 ,9-(methano[ 1,2]benzomethano)fUllerene[60] benzene
tron transfer occurs only in the polar benzonitrile due to clathrate. Helv. Chim. Actu 76,2445-2453.
thermodynamic constraints. Excitation of dyad 4 in toluene 12. Kahn, S. I., A. M. Oliver, M. N. Paddon-Row and Y .
solution produces the fullerene triplet state by normal Rubin (1993) Synthesis of a rigid "ball-and-chain"
intersystem crossing. The rapid and efficient electron donor-acceptor system through Diels-Alder
transfer observed in these molecules suggests that they may functionalization of buckminsterfullerene (C,). .Am. I
be useful in various areas of photoinduced electron Chem. SOC.115,4919-4920.
transfer, including modeling of photosynthetic solar energy 13. Prato, M., T. Suzuki and F. Wudl (1993) Experimental
conversion and molecular-scale opto-electronics. evidence for segregated ring currents in C,. J. Am.
Chem. SOC. 115,7876-7877.
Acknowledgements -This work was supported by the 14. Wendler, J. and A. Holzwarth (1987) State transitions
National Science Foundation (CHE-9413084). This is in the green alga Scenedesmus obliquus probed by
publication 209 from the ASU Center for the Study of time-resolved chlorophyll fluorescence spectroscopy
Early Events in Photosynthesis. and global data analysis. Biophys. J. 52,7 17-728.
15.Bensasson, R. V., T. Hill, C. Lambert, E. J. Land, S.
REFERENCES Leach and T. G. Truscott (1993) Pulse radiolysis study
of buckminsterfullerene in benzene solution.
1. Gust, D., T. A. Moore and A. L. Moore (1993) Assignment of the Ca triplet-triplet absorption
Molecular mimicry of photosynthetic energy and spectrum. Chem. Phys. Lett. 201,326-335.
electron transfer. Acc. Chem. Res 26, 198-205. 16. Gust, D., T. A. Moore, A. L. Moore, A. A. Krasnovsky,
2. Gust, D. and T. A. Moore (1991) Mimicking P. A. Liddell, D. Nicodem, J. M. DeGraziano, P.
photosynthetic electron and energy transfer. Advances Kerrigan, L. R. Makings and P. J. Pessiki (1993)
in Photochemistry 16, 1-65. Mimicking the photosynthetic triplet energy transfer
3. Wasielewski, M. R. (1992) Photoinduced electron relay. J. Am. Chem. SOC.115,5684-569 1.
transfer in supramolecular systems for artificial 17. Hung, R. R. and J. J. Grabowski (1991) A precise
photosynthesis. Chem. Rev. 92,435-461. determination of the triplet energy of C a by
4. Zhou, F., C. Jehoulet and A. J. Bard (1992) Reduction photoacoustic calorimetry. J. Phys. Chem. 95, 6073-
and electrochemistry of C, in liquid ammonia. J. Am. 6075.
Chem. Soc. 114, 11004-1 1006. 18. Weller, A. (1982) Photoinduced electron transfer i n
5. Dubois, D., K. M. Kadish, S. Flanagan, R. E. Haufler, solution: Exciplex and radical ion pair formation ftee
L. P. F. Chibante and L. J. Wilson (1991) enthalpies and their solvent dependence. 2. Physik.
Spectroelectrical study of the C a and Cm fullerenes and Chem. NF 133,93-98.