J. Phys. Chem. 1988,92, 409-414
409
Photophysics of a Cerium( 1V)-Porphyrin Sandwich Complex: Picosecond Deactivation via Neutral Exciton or Charge-Transfer Excited States Xinwei Yan and Dewey Holten* Department of Chemistry, Washington University, St. Louis, Missouri 631 30 (Received: May 22, 1987: In Final Form: July 24, 1987)
We have investigated, using time-resolved and steady-state optical techniques, some of the photophysical properties of a lanthanide-porphyrin dimer. The complex contains a Ce(1V) ion sandwiched between two octaethylporphyrin macrocycles. The dimer is nonfluorescent (q+ < 2 X lo"). The absorption changes produced by pulse excitation decay largely (>90%) by return of the molecules to the ground state in < l o ps. There is evidence for low energy quenching states in the ground state absorption spectrum. In order to explain these observations, we have considered. the importance of neutral (?r,?r*) exciton states of the two rings, ring-to-ring charge-transfer states and ring-to-metal charge-transfer states. We believe that excited states having substantial charge-transfer character may provide the most effective routes for rapid radiationless decay.
Introduction This article is concerned with the photophysical behavior of a complex consisting of a cerium(1V) ion sandwiched between two porphyrin macrocycles which are very close together (-3.4 8, average separation'). Rapid radiationless deactivation of the initially prepared excited state may proceed via (?r,?r*) exciton states of the two rings, ring-to-ring charge-transfer (RRCT) excited states, and/or ring-to-metal charge-transfer (RMCT) excited states. Our results and discussion are relevant to the spectroscopy and excited-state dynamics of biologically important porphyrin complexes and to metalloporphyrin monomers and dimers in general. The primary electron donor in bacterial photosynthetic reaction centers is a dimer of bacteriochlorophyll (BChl) molecules, also known as the special pair.* The interplane separation of the two macrocycles in this dimer is -3.8 A.2d It is thought that the (?r,?r*) excited states of this complex are derived from electronic interaction of the two BChl's of the special pair with one another and possibly also with the neighboring pigments3 In addition, it has been proposed recently, on the basis of photon echo: hole and Stark effect6 measurements, that the lowest excited singlet state of the special pair may contain C T character. Mixing of the neutral (?r,?r*) exciton and RRCT states has been considered in recent attempts to calculate the ground state absorption spectrum of the reaction However, the details of how pigment-pigment interactions influence the electronic properties of the prosthetic groups and the overall charge separation process, are not completely understood. The effects of exciton coupling on photophysical behavior of porphyrin dimers have received some Excitonic interactions are thought, for example, to be responsible for the or Soret (B), band in the ground state blue-shifted S2(?r,?r*), absorption spectrum of cofacial porphyrin dimers. A puzzling issue, one that is not understood, is the variable yield of fluorescence (as compared to the corresponding monomers) from the Sl(?r,x*),or Q, state in cofacial dimers. Fluorescence quenching has been observed in some dimers? and not in others? even though the dimers are expected to have comparable exciton s littings on the basis of similar equilibrium ring separations (-4.5
1). The central metal ion also strongly influences the electronic
properties of metalloporphyrins. In numerous nonluminescent transition metal porphyrins, metal-centered excited states (i.e., MRCT, RMCT, and (d,d) states) appear to provide effective routes for deactivation of the normally emissive ring (?r,?r*) states.'*'2 Essentially all of the photodissociation/association behavior of metalloporphyrins is thought to proceed via these
* Author to whom correspondence should be addressed. 0022-36S4/88/2092-0409$01.50/0
metal-centered excited ~ t a t e s . ' ~ 3 ~However, ~-'~ the pathways, rates, and mechanisms of the formation and decay of porphyrin met(1)Buchler, J. W.; De Cian, A.; Fischer, J.; Kihn-Botulinski, M.; Paulus, H.; Weiss, R. J. Am. Chem. SOC.1986,108,3652-3659. (2)(a) Norris, J. R.; Uphaus, R. A.; Crespi, H. L.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1971,68,625-628.(b) Norris, J. R.; Scheer, H.; Druyan, M. E.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1974,71,4897-4900. (c) Feher, G.; Hoff, A. J.; Isaacson, R. A.; Ackerson, L. C. Ann. N.Y. Acad. Sci. 1975, 244, 239-259. (d) Diesenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984,180,385-398. (3)(a) Parson, W. W.; Scherz, A.; Warshel, A. In Antennas and Reaction Centers of Photosynthetic Bacteria; Michel-Beyerle, M. E.; Ed.; SpringerVerlag, Berlin, 1985;pp 122-130. (b) Zinth, W.; Knapp, E. W.; Fischer, S. F.; Kaiser, W.; Deisenhofer, J.; Michel, H. Chem. Phys. Lett. 1985,119,1-4. (c) Parson, W. W.; Scherz, A. J . Am. Chem. SOC.1987,109,6152-6153. (4)(a) Meech, S. R.; Hoff, A. J.; Wiersma, D. A. Chem. Phys. Lett. 1985, 121,287-292. (b) Meech, S. R.; Hoff, A. J.; Wiersma, D. A. Proc. Natl. Acad. Sci U.S.A. 1986,83,9464-9468. (5)(a) Boxer, S. G.; Lockhart, D. J.; Middendorf, T. R. Chem. Phys. Letr. 1986,123,476-482. (b) Boxer, S. G.; Middendorf, T. R.; Lockhart, D. J. FEBS Lett. 1986,200, 237-241. (c) Hayes, J. M.; Small, G. J. J . Phys. Chem. 1986,90,4928-493 1. (6)(a) De Leeuv, D.; Malley, M.; Buttermann, G.; Okamura, M. Y.; Feher, G. Biophys. J . 1982,37, llla. (b) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987,26,664-668. (7)(a) Chang, C. K. J . Heterocycl. Chem. 1977,14, 1285-1288. (b) Giickel, F.; Schweitzer, D.; Collman, J. P.; Bencosme, S.; Evitt, E.; Sessler, J. Chem. Phys. 1984,86,161-172,and references cited therein. (c) Kagan, N. E.; Mauzerall, D.; Merrifield, R. B. J . Am. Chem. SOC. 1977, 99, 5484-5486. (d) Pellin, M. J.; Wasielewski, M. R.; Kaufmann, K. J. J. Am. Chem. SOC.1980,102, 1868-1873. (e) Boxer, S. G.; Closs, G. L. J . Am. Chem. Soc. 1976,98,5406-5408.(f) Petke, J. D.; Maggiora, G. M. J. Chem. Phys. 1986,84, 1640-1652. (g) Fujita, I.; Fajer, J.; Chang, C. K.; Wang, C.-B.; Bergkamp, M. A.; Netzel, T. L. J . Phys. Chem. 1982,86,3754-3759, and references cited therein. (8)(a) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977,25, 139-153. (b) Selensky, R.; Holten, D.; Windsor, M. W.; Paine, J. B.; Dolphin, D.; Gouterman, M.; Thomas, J. Chem. Phys. 1981,60,33-46. (9) (a) Katz, J. J.; Norris, J. R.; Shipman, L. L.; Thurnauer, M. C.; Wasielewski, M. R. Annu. Rev. Biophys. Bioeng. 1978,7,393-434. (b) Boxer, S. G. Biochim. Biophys. Acra 1983,726,265-292. (10)Holten, D.; Gouterman, M. In Optical Properties and Structure of Tetrapyrroles; Blauer, G., Ed.; de Gruyter: Berlin, 1985;pp 63-90. (11)(a) Kobayashi, T.; Straub, K. D.; Rentzepis, P. M. Phorochem. Photobiol. 1979,29,925-931. (b) Kobayashi, T.; Huppert, D.; Straub, K. D.; Rentzepis, P. M. J . Chem. Phys. 1979,70, 1720-1726. (c) Dzhagarov, B. M.; Timinskii, Y. V.; Chirvonyi, V. S.; Gurinovich, G. P. Dokl. Biophys. 1979,247,138-140. (d) Chirvonyi, V. S.; Dzhagarov, B. M.; Timinskii, Y. V.; Gurinovich, G. P. Chem. Phys. Lett. 1980,70,79-83. (e) Kim, D.-H.; Holten, D.; Gouterman, M.; Buchler, J. W. J. Am. Chem. SOC.1984,106, 4015-4017. ( f ) Tait, C. D.; Holten, D.; Gouterman, M. Chem. Phys. Lett. 1983,100,268-272. (g) Serpone, M.; Netzel, T. L.; Gouterman, M. J . Am. Chem. SOC.1982,104,246-252. (12)(a) Kim, D.-H.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305-322. (b) Kim, D.-H.; Holten, D. Chem. Phys. Lett. 1983,98,584-589. (c) Tait, C.D.; Holten, D.; Gouterman, M. J . Am. Chem. SOC.1984,106, 6653-6659. (d) Kim, D.-H.; Holten, D.; Gouterman, M. J . Am. Chem. SOC. 1984,106,2793-2798. (e) Dixon, D. W.; Kirmaier, C.; Holten, D. J . Am. Chem. SOC.1985,107,808-813.
0 1988 American Chemical Society
410
Yan and Holten
The Journal of Physical Chemistry, Vol, 92, No. 2, 1988
TABLE I: Ground State Absorption Spectra Data' compd
8
solvent
Ln(OEP)(OH)'
MeOH
Ce(OEP),
cyclohexand
B(O,O)
B( 1
33
400 (33) 378 (12) 377 379 377 377 376 314
382 (5.2)
2.0 1.9 2.4 7.0 9.1 21 38
3-methylpentane toluene 2-methyl-THF CHZC12 acetone acetonitrile
344 346 345 346 345 344
compd
solvent
Cb
B(1,O)'
B(O,O)
HzOEP
toluene 2-methyl-THF CH2C12 acetone acetonitrile
2.4 7.0 9.1 21 38
313 371 312 370 369
399 396 396 393 391
?d
Q(l,O)
Q(O,O)
?d
466 466 466 466 466 466
534 (1.3) 530 (0.6) 530 530 529 530 529 521
57 1 (2.1) 513 (1.7) 512 573 57 1 57 1 570 569
-661 (0.14) -642 -647 -637 -642 -642 -658
Qy(l,O) 497 496 495 494 494
QA 1 8 )
Qy(O,O) 530 527 530 526 527
Q,(O,O) 622 622 617 618 616
568 565 564 564 563
'Peak wavelength in nanometers (ztl nm). bDielectric constant. 'Shoulder. dBand of unknown origin. See text. 'Average spectral data for 11 lanthanide O E P monomers; extinction coefficients in parentheses ( X lo4 M-' cm-' ).Is /Extinction coefficients in parentheses (X104 M-' cm-').15a,c
al-centered excited states are, in general, not well understood. Recently, Buchler and co-workers have synthesized a series of lanthanide-porphyrin complexes, both dimers (double-deckers) and trimers (triple-decker~).l,'~The ground state absorption spectra contain evidence for excitonic interaction between the rings, i.e., blue-shifted Soret bands. For complexes containing a trivalent metal ion (e.g., E u ~ " ( O E P ) ~ the ) , ground state is, in essence, a RMCT state: one electron has been transferred from the octaethylporphyrin (OEP) ring(s) to the metal to form a neutral c o m p l e ~ . ' ~(Each ~ , ~ porphyrin ring is a dianion.) These particular dimers show a near-infrared band (1 100-1400 nm) that is similar to the one observed in the spectrum of the oxidized reaction-center special pair;I6 such a band may be a signature of a delocalized dimer cation. In fact, the one-electron-oxidation product of Ce1V(OEP)2is thought to be a cation radical of the ring(s) and has an absorption band at 1270 nm.lScvd The various types of metal-centered states are likely to be important for many of the lanthanide-porphyrins. However, the photophysical properties of these dimers have not been explored in much detail. Thus, we have begun to examine the lanthanide-porphyrin dimers as a system in which we can address some of the unresolved issues regarding porphyrin electronic states outlined above. For our initial study we chose Ce1V(OEP)2for several reasons: (1) the X-ray crystal structure of the complex has been solved;' (2) the ground state absorption spectrum contains evidence of possible excitonic and/or CT interactions;' (3) Ce(1V) has no low-lying (d,d), (f,f), or MRCT excited states-the only possible metalcentered excited states of consequence are RMCT states. Here we present the results of ground state absorption, excited state (time-resolved) absorption, and steady state emission measurements on this complex. We also discuss some results on the related dimer, CerV(TTP)2. (13) (a) Chirvonyi, V. S.;Dzhagarov, B. M.; Shul'ga, A. M.; Gurinovich, G. P. Dokl. Biophys. 1982, 259, 144-148. (b) Chemoff, D. A,; Hochstrasser, R.M.; Steele, A. W. Proc. Natl. Acad. Sci. U.S.A.1980, 77, 5606-5610. (c) Waleh, A,; Loew, G. H.J . Am. Chem. SOC.1982, 104, 2346-2351. (d) Eisenstein, L.; Frauenfelder, H. In Biological Events Probed by Ulrrafast Laser Spectroscopy; Alfano, R. R., Ed.; Academic: New York, 1982, pp 321-337. (14) (a) Kim, D.-H.; Spiro, T. G . J . Am. Chem. Sot. 1986, 108, 2099-2100. (b) Findsen, E. W.; Alston, K.; Shelnutt, J. A.; Ondrias, M. R. J. Am. Chem. SOC.1986, 108,4009-4017. (15) (a) Buchler, J. W.; Knoff, M.; In Oprical Properties and Structure of Tetrapyrroles; Blauer, G.; Ed.; de Gruyter: Berlin, 1985; pp. 91-105. (b) Buchler, J. W.; Kapellman, H.-G.; Knoff, M.; Lay, K.-L.; Pfeifer, S . Z . Naturforsch., B Anorg. Chem., Org. Chem. 1983, 388, 1339-1345. (c) Buchler, J. W.; E l h e r , K.; Kihn-Botulinski, M.; Scharbert, B.; Tamil, S . In Excited States and Dynamics of Porphyrins; Gouterman, M.; Rentzepis, P. M.; Straub, K. D., Eds.; ACS Symposium Series 321; American Chemical Society: Washington, DC, 1986; pp 94-104. (d) Buchler, J. W.; Elsasser, K.; Kihn-Botuliuski, M.; Scharbert, B. Angew. Chem., Inr. Ed. Engl. 1986, 25, 286-287. (16) Kirmaier, C.; Holten, D. Photosynth. Res. 1987, 13, 225-260.
1
I
0.8
I
Ce(OEP),
c
.0 c
e
9$
0.6
0.4 1600
1200
800
I
0.2
0
300
400
500
800
700
800
900
1000
Wavelength (nm)
Figure 1. Ground-state absorption spectrum of Ce(OEP), in CH2CI,. The inset shows the near-infrared region on an expanded scale. The magnitude of the absorption in the three regions should be multiplied by the scale factors to obtain the correct spectrum.
Experimental Section
Bis(octaethylporphinato)cerium(IV), Ce(OEP),, was prepared, purified, and characterized by 'H N M R and optical absorption spectroscopy as described by Buchler and co-w0rkers.l A sample of Ce(TTP)2 was kindly provided by Dr. J. W. Buchler. (TTP is meso-tetratolylporphyrin). Spectral grade solvents were used for all measurements, which were carried out at room temperature. Picosecond transient absorption and kinetic studies were performed basically as described previously.12a Samples flowing through a 2-mm path length cell were excited with a 30-ps flash at either 355 nm (-200 pJ) or 532 nm ( - 2 mJ) and probed at various delay times with a weak 30-ps broad-band (450-950 nm) pulse. Steady state emission measurements were carried out on a home-built or a Spex Fluorolog spectrofluorometer. Ground state absorption spectra were measured on a Perkin-Elmer 330 or a Cary 14 spectrometer. N M R spectra were recorded on a Varian XL-300 spectrometer in toluene-d8. Results Ground-State Absorption. Figure 1 shows the ground electronic state absorption spectrum of Ce(OEP)2 in CH2C12." Small shifts observed in peak positions as a function of solvent are generally no greater than those found in the spectrum of the metal-free monomer, H 2 0 E P (Table I). The absorption spectrum for Ce(OEP)2 contains the characteristic metalloporphyrin (T,T*)ab(17) There appears to be a factor of 2 error in the scale factor relating the visible and near-UV regions of the spectrum of Ce(OEP), in ref 1 . The spectrum does not agree with the extinction coefficients reported in ref 1 Sa or our spectrum of Figure 1, which themselves are in good agreement.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 411
Photophysics of Ce(1V)-Porphyrin Sandwich Complex 0.08
0.02 -4
a
7-
-0.02
-0.06 -0.10 n nn
"'"V 0.06
i
I
500
660
!
h
v
following excitation of Ce(OEP), with a 30-ps 355-nm flash. The spectrum at 27 ps consists of a broad excited-state absorption broken by bleachings of the Q(1,O) and Q(0,O) ground state absorption bands at 530 and 573 nm. The shape of the spectrum remains essentially the same as a function of time, and the spectrum contains isosbestic points (AA = 0) near 560 and 580 nm. By 143 ps, the transient absorption and the bleachings have decayed to 10% of their amplitudes at 27 ps. These residual absorption changes do not decay appreciably over the 8-11s time scale of the measurements. Both the excited-state absorption and the ground-state bleachings develop and decay, to the level shown in the 143-ps spectrum, essentially with the excitation flash profile. This places an instrument-limited time constant of 30 ps on the relaxation of the initially observed excited state. The small amplitudes of the initial absorption changes, together with the relatively high excitation intensities needed to obtain them, are also consistent with a transient state having a lifetime much shorter than the 30-ps duration of the excitation pulse. We estimate that the lifetime of the initial transient is (10 ps. Because of the potential for recycling during the excitation flash, the quantum yield of the longer lived transient is likely to be much less than 10%. In order to investigate the absorption changes at wavelengths longer than 600 nm, spectra were acquired on a more concentrated sample than could be used for probing through the ground-state Q bands, and the sample was excited with stronger flashes at 532 nm (Figure 2B). The spectrum at 27 ps in this longer wavelength region contains evidence for bleaching in the broad ground state absorption band centered near 660 nm (see Figure 1). Similar to what was found at shorter wavelengths, the weak featureless transient absorption beyond 650 nm, and extending to near 950 nm, also decays essentially with the excitation flash. Similar measurements were carried out on Ce(TTP),. The broad transient absorption and Q-band bleachings also grow and decay with the flash profile; there is no evidence for a longer lived transient.
-
366 nm Pump
I
i
600
650
Ce(OEP), B
/
532 nm Pump
0.02 0.00
2.4 n8
Discussion Possible Deactivation Pathways. Three observations on Ce(OEP), are very telling with regard to the photophysical behavior of this complex: (1) The lack of measurable fluorescence (& < 2X implies an extremely short lifetime for the normally fluorescent ring excited singlet state 'Q(r,r*). (For monomeric diamagnetic metalloporphyrins, which have a natural fluorescence 60 ns,19 this quantum yield would correspond lifetime of T? to a lQ(r,r*) lifetime rQ = q"& < 1.2 ps). (2) The picosecond measurements indicate that >90% of the decay of the initial transient absorption is accompanied by return of the molecules to the ground state in < l o ps (Figure 2). (3) The ground electronic state absorption spectrum reveals clear evidence for possible low energy quenching states (Figure 1). These three observations, taken together, suggest that the most likely fate of photoexcited Ce(OEP)2 is as follows: the initially prepared excited singlet state decays very rapidly, and nonradiatively, to the ground state via one or more excited singlet states at lower energy than the normally emissive ring 'Q(r,r*) state. In this scheme, the 27-ps transient absorption spectrum (Figure 2) probably contains a contribution from lQ(r,r*) and one or more of the excited singlet states through which it decays. The 143-ps spectrum could represent the ring triplet state 3 T ( r , r * ) formed in very low (
