Photochemistry and Photobiology, Vol. 60, No. 6, pp. 537-541, 1994 0031-8655/93 $05.00+0.00Printed 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-? INTRODUCTIONOne approach to mimicry of photosynthetic energyconversion is the construction of synthetic molecularsystems containing chromophores, electron donors andelectron acceptors linked by covalent bonds. These bondscontrol the distances, angles and electronic couplingbetween the moieties, and thus the rates of electron andenergy transfer. Typically, such molecules are based upon 1 C, . tolueneporphyrins or other chlorophyll derivatives linked tovarious organic donors and acceptors. 1-3 C, readilyundergoes one-electron reduction to the radical anion>5and accepts electrons from metalloporphyrin radicalanions.6 In addition, fullerenes have been found to serve aselectron 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 photosynthesismimics. In order to investigate this possibility, we have Figure 1. Synthesis and conformation of the dyads.prepared two porphyrin-C, dyads and studied theirphotochemical 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 diene1) began with reduction of a mixture of ester 1 and its 2 in 42% yield. A toluene solution of the diene containingdiastereomer (having the opposite relative configurations at an excess of C a (Mer Corp., Tucson, AZ) was purged withthe carbon atoms bearing the carbethoxy groups) with nitrogen, sealed in a glass tube and heated at 120 "C for 3lithium aluminum hydride in tetrahydrofuran at ambient h. Porphyrin-C, dyad 3 was isolated in 34% yield from thetemperature. 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. Treatmentchloride 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
538 PAULA. LIDDEU et alzinc acetate yielded the zinc analog, 6 , quantitatively.Model fullerene 5 was prepared by refluxing a toluenesolution of Cm and the corresponding diene under nitrogenfor 30 h. The structures of 2 , 3 and 5 were verified by massspectrometric, UV-VIS and NMR data (see Results). Spectroscopic studies. Steady state fluorescence andfluorescence excitation spectra were measured using a Absorption spectraSPEX Fluorolog-2. Excitation was produced by a 450 W The absorption spectra of 4, model fullerene 5 andxenon 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 nmbeam via a single grating monochromator and an R928 corresponding to fullerene absorption. A similar band isphotomultiplier tube having S-20 spectral response and seen with 5. The absorption spectrum of 5 tails off slowlyoperating in the photon counting mode. Fluorescence decay to the red, with a weak absorption at 715 nm. The spectrummeasurements were made using the time-correlated single of 4 has additional bands at 423,5 19,557,589 and 643 nmphoton counting method. The excitation source was a which are characteristic of free-base porphyrins. Alsofrequency-doubled, mode-locked Nd-YAG laser coupled to shown in Fig. 2a is the linear combination of the spectra ofa synchronously pumped, cavity dumped dye laser with models 1 and 5 which best approximates that of dyad 4 inexcitation 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 byinstrument 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 atcarbon working electrode and salt bridges to an SCE refer- 425, 550 and 590 nm which are characteristic of metallatedence electrode and a platinum wire counter electrode. The porphyrins. The summed spectra indicate that the Soret andtetra-n-butylammonium hexafluorophosphate electrolyte Q-bands of the porphyrin are shifted to longer wavelengthswas 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 nanosecondtransient absorption apparatus has been previously Steady-state fluorescence spectradescribed.“ 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 theNOESY and HMBC results. The presence of the Ca porphyrin moiety, indicating strong quenching of themoiety 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 quantumppm) and the adjacent four C a carbon nuclei (153, 155, yield (0.00072) which is only about a factor of two less156, 156 ppm) via HMBC and HMQC experiments. It is than that of the model compound. Thus, the fullereneknown 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 porphyrinprogram in the Insight11 molecular-modeling package from moiety does not emit significantly, is identical to theBiosym 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 theconformation is consistent with the ‘H NMR spectrum, in fidlerene occurs with a quantum yield close to unity and iswhich the hydrogens at the 5 and 15 meso positions of the responsible for the observed quenching of the porphyrinporphyrin 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 (excitationby components of the C a ring. fFyr at 550 nm) shows that emission from both moieties is strongly quenched. The fluorescence quantum yield for theCyclic 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 zincoxidation potentials of +0.359 and +O. 177 V, respectively, dyad 3 in toluene, measured at 800 nm, is identical with therelative to a ferrocene internal reference redox system. The absorption spectrum within experimental error, signalingfirst and second reduction potentials of model fullerene 5 singlet-singlet energy transfer with a quantum yield close towere -1.047 V and -1.468 V. unity.
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) 40Figure 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 solutionwith excitation at 590 nm was measured at 6 wavelengthsin the 700 - 840 nm region, and the results were analyzed -40g l ~ b a l l y to give the decay-associated spectra shown in ~ 680 720 760 800 840Fig. 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 toluenenegative 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 to1 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 offirst excited singlet state by singlet-singlet energy transfer dyad 4 at 12 wavelengths in the 630 - 800 nm region wereand concurrent rise of the fullerene singlet state. The 1.4-ns analyzed globally to give only one significant componentdecay has the shape of the fuIlerene emission, and denotes with a lifetime of -2 ps (x2 = I . 12). In the 740 - 800 nmthe 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 significanttoluene. Thus, the fullerene excited singlet state in dyad 4 decay component. Thus, emission fiom both the porphyrinis 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
540 A. PAUL LIDDELL al. etsignificant emission with a lifetime of -5 ps was observed. can be discussed in terms of Fig. 6. The energies of the firstThc zinc porphyrin model 6, on the other hand, has a excited singlet states have been estimated from thefluorescence lifetime of 1.9 ns in benzene. Thus, absorption and emission spectra. The energies of thefluorescence from both the porphyrin and fullerene charge-separated states are based on the cyclicmoieties 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 byTrunsient 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 isa transient species whose absorption spectrum is shown in only approximate, as lifetimes of -5 ps are subject toFig. 5. This transient decayed with a lifetime of -10 ps, and significant uncertainty due to instrumental limitations andthe lifetime was strongly quenched by the admission of interference from solvent Raman scattering. Althoughatmospheric oxygen. The spectral shape is identical to that electron transfer via step 2 is energetically feasible, itof 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 cantransient is ascribed to the fullerene triplet state. The C a also be produced by direct excitation, is ascribed totriplet state in benzene has an absorption maximum at 750 No long-lived transient absorption was observedfollowing 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 singletoxygen, which was detected via its characteristic emissionat 1270 nm.16 The quantum yield of singlet oxygen wasidentical for fullerene 5 and for Cm. The triplet quantumyield for C a in benzene is reported to be 0.8815 [l.O]." If 9Q) vone assumes that the quenching of the C a triplet state byoxygen to yield singlet oxygen is quantitative the quantum )ryield of singlet oxygen and of the triplet state for 5 mustalso 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 Cbenzene, ET = 20,200 mol- cm- at the 750-nmma~imum.~ WUsing this value, the triplet quantum yields mentionedabove, and the transient absorption spectra, ETEG isestimated 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, whichFigure 5. Transient absorption spectra of -5 x 10" M dyad indicate interaction between the chromophores.4 ( 0 ) and fullerene 5 ( 0 ) in argon-purged toluenefollowing 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-)
Rapid Communication 54 1singlet-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) Electrontransfer to yield Pof-Cao- does not occur. Instead, the transfer reactions between Ca and radical ions offillerene 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) Photoinducedis reduced in non-polar toluene to the point where transfer electron transport across a lipid bilayer mediated bybecomes 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. Moorecase of zinc dyad 3, A O is -0.50 eV in benzonitrile, and G and D. Gust (1 994) A new porphyrin derivative for useelectron 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 asinglet-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, amphiphilicduced 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 benzenetron 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-Aldertransfer observed in these molecules suggests that they may functionalization of buckminsterfullerene (C,). .Am. Ibe 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) Experimentalconversion 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 transitionsNational Science Foundation (CHE-9413084). 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