Photochemistry and Photobiology, Vol. 60, No. 6, pp. 537-541, 1994 Printed in the United States. All rights reserved

003 1-8655/93 $05.00+0.00 0 1994 American Society for Photobiology

RAPID COMMUNICATION

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C~O DYADS

PAUL A. LIDDELL, JOHN P. SUMIDA, ALISDAIR N. MACPHERSON, LORI N o s , GILBERT 5. SEELY, KRISTINE N. CLARK, ANAL. MOORE,* THOMAS A. MOORE* AND DEVENS GUST

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 Ca 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 -2x 10 s 11 -?

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 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 suggest that fullerenes might serve as useful electron acceptor moieties in multicomponent photosynthesis mimics. In order to investigate this possibility, we have prepared two porphyrin-C, dyads and studied their photochemical properties.

MATERIALS AND METHODS

Synthesis. Preparation of the porphyrin-C, dyads (Fig. 1) began with reduction of a mixture of ester 1 and its diastereomer (having the opposite relative configurations at the carbon atoms bearing the carbethoxy groups)' with lithium aluminum hydride in tetrahydrofuran at ambient temperature. The resulting mixture of stereoisomeric diols (produced in 90% yield) was treated with methanesulfonyl chloride in pyridine at 0°C to yield a mixture of

* To whom correspondence should be addressed.

C,. toluene 1

3: M=Zn \ 4: M = H ,

Figure 1. Synthesis and conformation of the dyads.

diastereomeric dimesylates (9 1% yield). Treatment of a dichloromethane solution of the dimesylates with excess zinc acetate in methanol produced the metallated porphy- rim in quantitative yield. Dehydromesylation with potas- sium t-butoxide in dimethylformamide at 25 "C gave diene 2 in 42% yield. A toluene solution of the diene containing an excess of C a (Mer Corp., Tucson, AZ) was purged with nitrogen, sealed in a glass tube and heated at 120 "C for 3 h. Porphyrin-C, dyad 3 was isolated in 34% yield from the reaction mixture. Removal of the zinc with trifluoroacetic acid in dichloromethane gave free-base dyad 4. Treatment of a dichloromethane solution of 1 with excess methanolic

537

538 PAUL A. 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 SPEX Fluorolog-2. Excitation was produced by a 450 W xenon lamp and single grating monochromator. Fluorescence was detected at a 90” angle to the excitation beam via a single grating monochromator and an R928 photomultiplier tube having S-20 spectral response and operating in the photon counting mode. Fluorescence decay measurements were made using the time-correlated single photon counting method. The excitation source was a frequency-doubled, mode-locked Nd-YAG laser coupled to a synchronously pumped, cavity dumped dye laser with excitation at 590 nm. Detection was via a microchannel plate photomultiplier (Hamamatsu R2809U- 1 9, and the instrument response time was ca. 35 ps. Cyclic voltammetric measurements were performed in benzonitrile using a three-electrode system and a Pine Instruments Model AFRDE4 potentiostat. The cell featured a glassy carbon working electrode and salt bridges to an SCE refer- ence electrode and a platinum wire counter electrode. The tetra-n-butylammonium hexafluorophosphate electrolyte was recrystallized and dried before use, and the cell was kept under an atmosphere of nitrogen. The nanosecond transient absorption apparatus has been previously described.“

RESULTS

Structure and conformation The 500-MHz ‘H NMR spectra of 2 - 4 in deuterio-

chloroform solution were assigned with the aid of COSY, NOESY and HMBC results. The presence of the Ca moiety in 3 was verified through the detection of the two C a carbon nuclei bearing the bridge to the porphyrin (6 66 ppm) and the adjacent four C a carbon nuclei (153, 155, 156, 156 ppm) via HMBC and HMQC experiments. It is known that Cm reacts as a dienophile with 1,3-dienes to yield derivatives bridged across the 6-6 ring junction.“”*

Molecular mechanics calculations using the Discover program in the Insight11 molecular-modeling package from Biosym Technologies yielded the structure shown in Fig. 1 as the lowest-energy conformation of 4. This folded conformation is consistent with the ‘H NMR spectrum, in which the hydrogens at the 5 and 15 meso positions of the porphyrin ring are shifted upfield by -0.50 and 0.30 ppm, respectively, relative to model o h in 1 due to shielding by components of the C a ring. fFyr

Cyclic voltammety Cyclic voltammetric studies on model porphyrins 1

and its zinc analog 6 yielded reversible waves, with first oxidation potentials of +0.359 and +O. 177 V, respectively, relative to a ferrocene internal reference redox system. The first and second reduction potentials of model fullerene 5 were -1.047 V and -1.468 V.

Absorption spectra The absorption spectra of 4, model fullerene 5 and

model porphyrin 1 in toluene solution are shown in Fig. 2a. The spectrum of 4 features a broad band at 325 nm corresponding to fullerene absorption. A similar band is seen with 5. The absorption spectrum of 5 tails off slowly to the red, with a weak absorption at 715 nm. The spectrum of 4 has additional bands at 423,5 19,557,589 and 643 nm which are characteristic of free-base porphyrins. Also shown in Fig. 2a is the linear combination of the spectra of models 1 and 5 which best approximates that of dyad 4 in the 320 - 450 nm region. It will be noted that the Soret and Q-bands of the dyad are shifted to longer wavelengths by 1 1 - 15 nm relative to the porphyrin model.

Fig. 2b shows the absorption spectra for zinc dyad 3, model fullerene 5, and porphyrin 6. In the spectrum of 3, the fullerene absorption at -325 is apparent, as are bands at 425, 550 and 590 nm which are characteristic of metallated porphyrins. The summed spectra indicate that the Soret and Q-bands of the porphyrin are shifted to longer wavelengths by 9 - 12 nm relative to model porphyrin 6.

Steady-state fluorescence spectra Fig. 3 presents the fluorescence emission spectra of

free base dyad 4, model hllerene 5, and model porphyrin 1 in toluene solution. The fluorescence quantum yield of 1, measured using tetraphenylporphyrin ($I~ = 0.11) as a standard, is 0.081, whereas that of fillerene 5 is 0.0014. Dyad 4 features only very weak emission from the porphyrin moiety, indicating strong quenching of the porphyrin excited singlet state by the attached fullerene. Fullerene emission is observed in the dyad with a quantum yield (0.00072) which is only about a factor of two less than that of the model compound. Thus, the fullerene excited singlet state is not significantly quenched.

The corrected fluorescence excitation spectrum of dyad 4 in toluene, measured at 800 nm where the porphyrin moiety does not emit significantly, is identical to the absorption spectrum within experimental error. Thus, singlet-singlet energy transfer fiom the porphyrin to the fidlerene occurs with a quantum yield close to unity and is responsible for the observed quenching of the porphyrin first excited singlet state.

The fluorescence spectrum of zinc dyad 3 (excitation at 550 nm) shows that emission from both moieties is strongly quenched. The fluorescence quantum yield for the porphyrin moiety is 2.0 x lo9 and that for the fullerene is about 1.8 x 10”.

The corrected fluorescence excitation spectrum of zinc dyad 3 in toluene, measured at 800 nm, is identical with the absorption spectrum within experimental error, signaling singlet-singlet energy transfer with a quantum yield close to unity.

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0.4 r-- L

0.3

8 5 0.2 e 51 2 0.1

0.0

- 4 5 I 5 + 1

- - ........

t

400 500 600 700 0.4 I

0.3

8 5 0.2

a 0.0

e 0 v) n

0.0

400 500 600 700

Wavelength (nm) Figure 2. Absorption spectra in toluene of fiee base dyad 4 (a), zinc dyad 3 (b), and model compounds.

Time-resolved fluorescence 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 g l~bal ly '~ to give the decay-associated spectra shown in Fig. 4. There are two significant components to the decay. The 7-ps decay is of high amplitude in the 700-nm region, where most of the emission is due to the porphyrin, and negative at 800 nm, where the fullerene emits. The first excited singlet state of a model porphyrin diester similar to 1 has a lifetime of 1 1.5 ns in benzene solution. Thus, the 7- ps component represents the rapid decay of the porphyrin first excited singlet state by singlet-singlet energy transfer and concurrent rise of the fullerene singlet state. The 1.4-ns decay has the shape of the fuIlerene emission, and denotes the lifetime of the fullerene first excited singlet state. Fullerene model 5 has a fluorescence lifetime of 1.2 ns in toluene. Thus, the fullerene excited singlet state in dyad 4 is unquenched by the attached porphyrin, as also revealed by the steady-state fluorescence studies.

In benzonitrile solution, the fluorescence lifetimes of

600 650 700 750 800 850

Wavelength (nm) Figure 3. Fluorescence emission spectra of porphyrin model 1 ( ), fullerene 5 (- -, x30), and fiee base dyad 4 (-, x30) in toluene solutions having equal absorbance at the 550-nm excitation wavelength.

120

80 a, U 3 .cI .- -

40 a

0

-40

0.007 ns

* 0.11 ns 1.4 ns

4.8 ns

680 720 760 800 840

Emission Wavelength (nm)

Figure 4. Decay-associated spectra for dyad 4 in toluene with excitation at 590 nm. The x2 value of the fit was 1.25.

the model porphyrin and fullerene 5 were 12.6 and 1.1 ns, respectively. In this solvent, the fluorescence decays of dyad 4 at 12 wavelengths in the 630 - 800 nm region were analyzed globally to give only one significant component with a lifetime of -2 ps (x2 = I . 12). In the 740 - 800 nm region, where most of the emission is due to the fullerene, the data yielded a lifetime of -6 ps for the only significant decay component. Thus, emission fiom both the porphyrin and fullerene moieties is strongly quenched in this solvent.

The time-resolved fluorescence of zinc dyad 3 was also studied in toluene. In the 680 - 840 nm region, a single

540 PAUL A. LIDDELL et al.

2.0 -

9 Q)

)r

Q) C W

v

P i . 0 -

0.0 -

significant emission with a lifetime of -5 ps was observed. Thc zinc porphyrin model 6, on the other hand, has a fluorescence lifetime of 1.9 ns in benzene. Thus, fluorescence from both the porphyrin and fullerene moieties of the dyad is very strongly quenched, as also revealed by the steady-state emission studies.

Trunsient absorption studies Excitation of an argon-purged toluene solution of 4

with 590-nm, -5 ns laser pulses resulted in the formation of a transient species whose absorption spectrum is shown in Fig. 5. This transient decayed with a lifetime of -10 ps, and the lifetime was strongly quenched by the admission of atmospheric oxygen. The spectral shape is identical to that of the transient absorption obtained after excitation of fullerene 5 under the same conditions (Fig. 5). The transient is ascribed to the fullerene triplet state. The C a 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 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 one assumes that the quenching of the C a triplet state by oxygen to yield singlet oxygen is quantitative the quantum yield of singlet oxygen and of the triplet state for 5 must also be 0.88 [1.0]. Similarly, the quantum yield of fkllerene triplet state for 4 is estimated to be 0.20 E0.231. For C a in benzene, ET = 20,200 mol-' cm-' at the 750-nm ma~imum. '~ 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

0.004

0.002 a

0.000

400 600 800 1000 Wavelength (nm)

Figure 5. Transient absorption spectra of -5 x 10" M dyad 4 (0 ) and fullerene 5 (0 ) in argon-purged toluene following excitation at 590-nm with a -5-11s laser pulse.

Discussion The spectroscopic results for the porphyrin-Ca dyads

can be discussed in terms of Fig. 6. The energies of the first excited singlet states have been estimated from the absorption and emission spectra. The energies of the charge-separated states are based on the cyclic voltammetric measurements in benzonitrile reported above.

The results for 3 will be discussed first. Excitation of the zinc porphyrin moiety gives 'Pa-Ca, which decays by singlet-singlet energy transfer to the fullerene to yield Pzn- 'Ca, with kl -2 x 10" s-', as estimated from the time- resolved fluorescence data in toluene. This rate constant is only approximate, as lifetimes of -5 ps are subject to significant uncertainty due to instrumental limitations and interference from solvent Raman scattering. Although electron transfer via step 2 is energetically feasible, it evidently does not compete favorably with energy transfer. The decay of the fullerene excited singlet state, which can also be produced by direct excitation, is ascribed to

Figure 6 . Transient states and interconversion paths for dyad 3 in a polar solvent. Dashed lines indicate levels in free base dyad 4.

photoinduced electron transfer from the PO hyrin ground state to yield Phof-Cao- with k3 -2 x 10 s ' and 4 3 = 1.0. This assignment of the quenching mechanism is consistent with the electrochemical data and the results for 4 discussed below, but confirmation must await transient absorption experiments. The rapid energy and electron transfer is consonant with the folded conformation shown in Fig. 1, where the n-electron systems of the donor and acceptor moieties approach van der Waals contact, and with the perturbations of the absorption spectra, which indicate interaction between the chromophores.

Extremely rapid and efficient singlet-singlet energ transfer (kl -5 x 10" s-l) and electron transfer (k3 -2 x 10 s-') are also observed for free base dyad 4 in benzonitrile. In this case, the 'P-Ca and Po+-Cao- states lie at 1.90 and 1.41 eV, respectively. In toluene, rapid (-1.4 x 10" s-')

' T : -

x

Rapid Communication 54 1

singlet-singlet energy transfer with unity quantum yield is also observed for 4. However, the P - k a state is unquenched relative to the fullerene model, and electron transfer to yield Pof-Cao- does not occur. Instead, the fillerene triplet state is produced. In a polar solvent such as benzonitrile AGO for electron transfer step 3 in Fig. 6 is -0.31 eV, and electron transfer is facile. The driving force is reduced in non-polar toluene to the point where transfer becomes thermodynamical1 unfavorable. For example, the dielectric continuum model yields a AGO value in toluene that is positive by several tenths of an electron volt. In the case of zinc dyad 3, AGO is -0.50 eV in benzonitrile, and electron transfer is still feasible in toluene.

The conformation of 4 and the observation of very rapid singlet-singlet energy transfer and electron transfer (in benzonitrile) suggest that triplet-triplet energy transfer between the porphyrin and fullerene should be facile. Thus, the observation of the fullerene triplet state (Fig. 5) suggests that the energy of the fullerene triplet in 4 is comparable to or below that of the porphyrin triplet state.

Y8

CONCLUSIONS

The porphyrin-C, dyads, which are readily synthe- sized from diene 2 and C,, exhibit rapid and efficient singlet-singlet energy transfer from the porphyrin moiety to the fullerene. Zinc dyad 3 undergoes very rapid photoin- duced electron transfer to yield P%'+-C,'- in both ben- zonitrile and toluene, whereas with free base dyad 4, elec- tron transfer occurs only in the polar benzonitrile due to thermodynamic constraints. Excitation of dyad 4 in toluene solution produces the fullerene triplet state by normal intersystem crossing. The rapid and efficient electron transfer observed in these molecules suggests that they may be useful in various areas of photoinduced electron transfer, including modeling of photosynthetic solar energy conversion and molecular-scale opto-electronics.

Acknowledgements -This work was supported by the National Science Foundation (CHE-94 13084). This is publication 209 from the ASU Center for the Study of Early Events in Photosynthesis.

REFERENCES

1. Gust, D., T. A. Moore and A. L. Moore (1993) Molecular mimicry of photosynthetic energy and electron transfer. Acc. Chem. Res 26, 198-205.

2. Gust, D. and T. A. Moore (1991) Mimicking photosynthetic electron and energy transfer. Advances in Photochemistry 16, 1-65.

3. Wasielewski, M. R. (1992) Photoinduced electron transfer in supramolecular systems for artificial photosynthesis. Chem. Rev. 92,435-461.

4. Zhou, F., C. Jehoulet and A. J. Bard (1992) Reduction and electrochemistry of C, in liquid ammonia. J. Am. Chem. Soc. 114, 11004-1 1006.

5. Dubois, D., K. M. Kadish, S. Flanagan, R. E. Haufler, L. P. F. Chibante and L. J. Wilson (1991) Spectroelectrical study of the C a and Cm fullerenes and

their mono-, di-, tri-, and tetraanions. J. Am. Chem. SOC.

6. Guldi, D. M., P. Neta and K.-D. Asmus (1994) Electron transfer reactions between Ca and radical ions of metalloporphyrins and arenes. J. Phys. Chem. 98,4617- 462 1 .

7. Hwang, K. C. and D. Mauzerall (1993) Photoinduced electron transport across a lipid bilayer mediated by (2%. Nature 361, 138-140.

8. Liddell, P. A., L. J. Demanche, S. Li, A. N. Macpherson, R. A. Nieman, A. L. Moore, T. A. Moore and D. Gust (1 994) A new porphyrin derivative for use as a diene in the Diels-Alder reaction. Tetrahedron Lett.

9. Gust, D., T. A. Moore, D. K. Luttrull, G. R. Seely, E. Bittersmann, R. V. Bensasson, M. Rougte, E. J. Land, F. C. de Schryver and M. Van der Auweraer (1990) Photophysical properties of 2-nitro-5,10,15,20-tetra-p- tolylporphyrins. Photochem. Photobiol. 51,4 19-426.

10. Davis, F. S., G. A. Nemeth, D. M. Anjo, L. R. Makings, D. Gust and T. A. Moore (1987) Digital back off for computer controlled flash spectrometers. Rev. Sci. Instrum. 58, 1629- 163 1 .

1 I. Diederich, F., U. Jonas, V. Gramlich, A. Herrmann, H. Ringsdorf and C. Thilgen (1993) Synthesis of a fullerene derivative of benzo[ 1 81crown-6 by Diels- Alder reaction: Complexation ability, amphiphilic properties, and x-ray crystal structure of a dimethoxy- 1 ,9-(methano[ 1,2] benzomethano)fUllerene[60] benzene clathrate. Helv. Chim. Actu 76,2445-2453.

12. Kahn, S. I., A. M. Oliver, M. N. Paddon-Row and Y . Rubin (1993) Synthesis of a rigid "ball-and-chain" donor-acceptor system through Diels-Alder functionalization of buckminsterfullerene (C,). .I Am. Chem. SOC. 115,4919-4920.

13. Prato, M., T. Suzuki and F. Wudl (1993) Experimental evidence for segregated ring currents in C,. J. Am. Chem. SOC. 115,7876-7877.

14. Wendler, J. and A. Holzwarth (1987) State transitions in the green alga Scenedesmus obliquus probed by time-resolved chlorophyll fluorescence spectroscopy and global data analysis. Biophys. J. 52,7 17-728.

15. Bensasson, R. V., T. Hill, C. Lambert, E. J. Land, S. Leach and T. G. Truscott (1 993) Pulse radiolysis study of buckminsterfullerene in benzene solution. Assignment of the Ca triplet-triplet absorption spectrum. Chem. Phys. Lett. 201,326-335.

16. Gust, D., T. A. Moore, A. L. Moore, A. A. Krasnovsky, P. A. Liddell, D. Nicodem, J. M. DeGraziano, P. Kerrigan, L. R. Makings and P. J. Pessiki (1993) Mimicking the photosynthetic triplet energy transfer relay. J. Am. Chem. SOC. 115,5684-569 1.

17. Hung, R. R. and J. J. Grabowski (1991) A precise determination of the triplet energy of C a by photoacoustic calorimetry. J. Phys. Chem. 95, 6073- 6075.

18. Weller, A. (1982) Photoinduced electron transfer in solution: Exciplex and radical ion pair formation ftee enthalpies and their solvent dependence. 2. Physik. Chem. NF 133,93-98.

113,4364-4366.

35,995-998.