J. Phys. Chem. 1983, 87, 1490-1493
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formation of the Ar('S) H + H products. The existence of the 3A1potential well is important because it enables the 3A1/3B2crossing to occur at energetically accessible geometries. Whether it is also important because some trajectoriers are trapped in the region of the well enhancing either the probability of the 3A1/3B2surface hopping or a3Zg+excitation cannot be answered at present. Although the attractive nature of the 3A1 surface and the accessible 3Al/3B2surface crossing are probably responsible for the much higher total quenching cross section for the 3P2level of Ar compared to that of Xe or Kr,2,3,5the magnitude (3.6-5.7 A2) of the Art3P) quenching cross section suggests that either relatively few trajectories make it into the region of the deep well or that of those that do only a small fraction go on to react. Both these possibilities are consistent with the finding that the 3A1 surface has a barrier for RHH= Re: The barrier could turn back those trajectories for which the H2 bond length remains close to Re, and if the H2 bond stretches appreciably near R h = 3.3 A (resulting in an attractive rather than a repulsive interaction) and remains stretched as the collision proceeds to small Rh, then the 3B2and 3A1surfaces would no longer cross in an energetically accessible region. Experimentally it has been found that the total cross section for quenching of Ar(3P0)by H2is 1.2-1.6 times that of Ar(3P2).2,14The larger cross section for the 3P0state
may be due in part to its greater energy. However, part of the difference in the 3P2and 3P0quenching rates may arise from differences in the potential energy surfaces for the individual sublevels. In particular when spin-orbit interactions are included, it is found in C2, symmetry that there are five surfaces correlating with Ad3P2)+ H2,three to Ar(3Pl) H2, and one to Ad3Po)+ H2. The attractive 13A1surface correlates with the 3P2limit, while the surface correlating to the 3P0limit (assuming RHH = 0.735 A) is derived from the repulsive 23B2surface. Hence, the larger quenching cross section for the 3P0case may indicate involvement of additional potential energy surfaces, in particular the 3B1surfaces arising from Ar(3P) H2 and Ar + H2(c3II,,)or one or more of the surfaces arising from the Ar('P,) + H2 limit. In order to establish the detailed mechanisms for the quenching of the individual sublevels we plan to extend the present study to include these additional surfaces and to include the effects of spin-orbit interactions.
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Acknowledgment. This work has been supported by the National Science Foundation. It is a pleasure to acknowledge stimulating discussions with Professors M. Golde and P. Siska. (14)J. Balamuta and M. F. Golde, unpublished results.
ARTICLES Transfer of Excitation Energy from Zinc to Copper Porphyrln in Methylcyclohexane Rigid Solution S. Konishl,' M. Hoshlno, and M. Imamura The Institute of Physical and Chemical Research, Wako-shi, Saitama 35 1, Japan (Received: July 29, 1982; I n Final Form: November 1, 1982)
The emission spectrum of methylcyclohexane (MCH) rigid solutions of a mixture of zinc(I1) and copper(I1) mesoporphyrin dimethyl ester (ZnMPDE and CuMPDE) consists of two components at 77 K: the fluorescence of ZnMPDE and the phosphorescence of the CuMPDE aggregate. The excitation spectrum of the zinc fluorescence is in good agreement with the absorption spectrum of solutions of only ZnMPDE, whereas that of the copper phosphorescence corresponds to the sum of the absorption spectra of solutions of the separate components. On the other hand, no aggregation of the solute(s), nor contribution from the zinc porphyrin to the excitation spectrum of the copper phosphorescence, was observed for 2-methyltetrahydrofuran (MTHF) rigid solutions of the mixed components. These results are interpreted in terms of transfer of excitation energy from ZnMPDE to CuMPDE in MCH rigid solutions,which provide favorable conditions for the energy transfer.
(1) Birks, J. B. In 'Photophysics of Aromatic Molecules"; Wiley-Interscience: New York, 1970; Chapter 11.
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(2) Govindjee, Ed. "Bioenergetics of Photosynthesis"; Academic Press: New York, 1975.
0 1983 American Chemical Society
The Journal of Physical Chemistty, Vol. 87, No. 9, 1983 1491
Transfer of Excitation Energy from ZnMPDE to CuMPDE
400
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Figure 1. Optical spectra of MCH solutions containing ZnMFDE (A) and CuMPDE (B). Absorption spectra at room temperature (..e) and at 77 K (-). Emission spectra at 77 K (---).
systems. Since efficient energy transfer is not expected between dispersed and randomly oriented molecules, covalently linked porphyrin dimers were used in a few studies to investigate the efficiency of the energy transfer between porphyrin moieties within a m o l e c ~ l e . ~Despite *~ the short distance between the chromophores, high efficiency of energy transfer was not always observed for such dimers. This fact suggests that the two chromophores are not necessarily in a condition for efficient energy transfer in hybrid porphyrin dimers. This paper reports clear evidence for the energy transfer from ZnMPDE to CuMPDE in MCH rigid solutions of a mixture of the two monomeric components. Energy transfer as efficient as the present case was not observed for the covalently linked hybrid dimers of zinc and copper porphyrin^.^ A probable mechanism for the energy transfer occurring between the mixed monomers is discussed.
Experimental Section Free base of mesoporphyrin IX dimethyl ester was obtained from Sigma Chemical Co. Its Zn(I1) and Cu(I1) complexes were prepared according to the literature5 and p d i e d by column chromatography using neutral alumina. MCH and MTHF were dried over sodium-potassium alloy after fractional distillation. Sample solutions of the order of M solute(s) were degassed by repeated freezepump-thaw cycles on a vacuum line. Absorption and emission spectra were measured with a Cary 14 spectrophotometer and a Hitachi MPF-4 spectrofluorimeter, respectively. ESR spectra were obtained with a JEOL JES-FE3AX spectrometer operating in the X band with 100-kHz modulation. Measurements at 77 K were achieved by placing the sample cells in a Dewar vessel filled with liquid nitrogen. No corrections were made to the observed emission spectra. Results and Discussion The absorption and emission spectra of ZnMPDE and CuMPDE in MCH solutions are shown in Figure 1. The absorption spectrum of ZnMPDE at 77 K shows a small red shift, sharpening, and an increase in intensity in both the Soret (near-UV) and the Q (visible) bands compared to that at room temperature. Such small spectral changes are usually observed due to low temperatures and volume contraction of the solvents. The emission of ZnMPDE observed at 77 K is fluorescence; phosphorescence, al(3) Schwarz, F. p.; Gouterman, M.; Muljiani, Z.; Dolphin, D. H. Bioinorg. Chem. 1972, 2, 1-32. (4) Anton,J. A.; Loach, P. A.; Govindjee Photochem. Photobiol. 1978, 28, 235-42. (5) Alder, A. D.; Longo, F. R.; Kampus, F.; Kim, J. J. Znorg. N u l . Chem. 1970,32, 2443-5.
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Figure 2. ESR spectrum of an MCH solution containing CuMPDE at 77 K.
A
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(nm)
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Figure 3.
Absorption and excitation spectra of an MCH solution containing ZnMPDE and CuMPDE. (A) Absorption spectra at room temperature (. .) and at 77 K (-). (B) Excitation spectrum at 77 K monitored at 698 nm. The meaning of the various arrows is given in the text.
though previously reported,’ is not detected by the present detection system. The absorption spectra of CuMPDE show temperature dependence markedly different from those of ZnMPDE. The sharp Soret band at room temperature changes to the blue-shifted, very broad band at 77 K. The changes in the Soret band are accompanied by the red-shifted Q band with an increase in width. Since the ESR spectrum at 77 K, as represented in Figure 2, shows only a single resonance line due to the fast spin exchange, the species at 77 K is ascribed to the CuMPDE aggregate. Accordingly, the emission observed at 77 K is ascribed to the phosphorescence of the aggregate. Details of the spectroscopic studies on the aggregation of CuMPDE are reported elsewhere.8 Figure 3A shows the absorption spectra of a mixture of ZnMPDE and CuMPDE in a MCH solution in which two solutes have approximately equal absorbances in the Soret band at room temperature. The Q band may be regarded as only the s u m of the spectra of the separate solutes both at room temperature and at 77 K. In the Soret band, however, a small but distinct peak indicated by an arrow appears at 77 K. Since such a peak is not observed at all (6) Gouterman, M. In “The Porphyrins”; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111, Chapter I. (7) Allison, J. B.; Becker, R. S. J . Chem. Phys. 1960, 1410-7. (8) Konishi, S.; Hoshino, M.; Imamura, M. Chem. Phys. Lett., submitted.
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for solutions of the separate solutes as evidenced in Figure 1, this peak must be due to a new species having strong interaction between the two solutes. In other words, the peak is attributable to a hetero aggregate which includes a hetero dimer formed between ZnMPDE and CuMPDE. The lack of a substantial decrease in the absorption intensity of ZnMPDE in the Soret band, however, indicates that only a small portion of the ZnMPDE molecules take part in the formation of the hetero aggregate with the CuMPDE molecules. The important point to be noted is that an affinity between ZnMPDE and CuMPDE becomes noticeable in MCH solutions at low temperatures. An affinity between different kinds of metalloporphyrins is not very unusual. The hetero-dimer formation between CuMPDE and AgMPDE in tetrachloroethane rigid solution is reported el~ewhere.~ The MCH solution of a mixture of the two solutes shows both the fluorescence of ZnMPDE and the phosphorescence of the CuMPDE aggregate, which are the same as those shown in Figure 1. The excitation spectrum of the ZnMPDE fluorescence is identical with that observed for solutions of only ZnMPDE. On the other hand, the excitation spectrum of the CuMPDE phosphorescence shown in Figure 3B is the sum of the excitation spectra of the emissions of the separate solutes. The peaks indicated by the broken arrows are due to CuMPDE and those indicated by the solid arrows due to ZnMPDE. These peaks correspond well to the peaks of the absorption spectra of the mixed solutes shown in Figure 3A. These results manifest that transfer of excitation energy occurs from ZnMPDE to the CuMPDE aggregate in MCH rigid solutions. The absence of the hetero-aggregate emission may be due to the weak intensity of the absorption and/or of the emission of the hetero aggregate. When MTHF is used as a solvent instead of MCH, the absorption spectra of the separate solutes show no sign of aggregation. In fact, the MTHF solution of CuMPDE at 77 K gives an ESR spectrum solely due to the m0n0mer.l~ The absorption spectra of the mixed solutes are nothing but the sum of the spectra of the separate solutes both at room temperature and at 77 K. Accordingly, the excitation spectra of the ZnMPDE fluorescence and the CuMPDE phosphorescence of solutions of the mixed solutes at 77 K are the same as those observed for solutions of the separate solutes. These results indicate that the energy transfer between ZnMPDE and CuMPDE observed for MCH rigid solutions does not take place in MTHF rigid solutions. Therefore, it is evident that solvents play an important role in the transfer of excitation energy between the two metalloporphyrins. The different nature between MTHF and MCH may be understood as follows. MTHF molecules have an oxygen atom which can coordinate to the central metal of metalloporphyrins. This coordination prevents metalloporphyrins from aggregating and they are likely to be uniformly dispersed and randomly oriented as well. No efficient energy transfer is expected between molecules under such conditions. On the other hand, MCH molecules are incapable of coordinating to metal(9)Konishi, S.;Hoshino, M.; Imamura, M. J.Phys. Chem., in press. (IO) Ogoshi, H.; Sugimoto, H.; Yoshida, Z. Tetrahedron Lett. 1977, 164-72. (11)Kagan, N.E.;Mauzerall, D.; Merrifield, R. B. J. Am. Chem. SOC. 1977,99,5484-6. (12)Chang, C. K. J. Chem. SOC.,Chem. Commun. 1977,800-1. (13)Collman, J. P.;Elliott, C. M.; Halbert, T. R.; Tovrog, B. S. Proc. Natl. Acad. Sci. U.S.A. 1977,74, 18-22. (14)Collman, J. P.;Chong, A. 0.;Jameson, G. B.; Oakley, R. T.; Rose, E.; Schmittou, E. R.; Ibers, J. A. J. Am. Chem. SOC.1981,103,516-33. (15) Konishi. S.:Hoshino. M.; Imamura, M. Chem. Phys. Lett. 1982, 86,'228-30.
ZnPor
CuPor
i
I
i 'Aig
Bw
FIgm 4. Energy level dlagrams for zinc and copper porphyrins. Three possible pathways of energy transfer from the zinc singlet to the copper tripdoublet state are shown by the dotted, broken, and curly arrows.
loporphyrins and provide them with a favorable condition for the formation of dimers or higher aggregates, the formation constants of which become larger with decreasing temperature. The experimental results in the present study show that CuMPDE has a tendency to aggregate much larger than ZnMPDE does in MCH solutions. Furthermore, there exists an affinity between ZnMPDE and CuMPDE in MCH solutions at low temperatures as evidenced in Figure 3A. The formation of the hetero aggregate indicates that the two kinds of metalloporphyrins are so close and in such good orientation with each other that they give a distinct absorption peak different from those of either the monomers or the aggregate of CuMPDE. Although most of the ZnMPDE molecules do not form the hetero aggregate with the CuMPDE molecules, they are also likely to be in close proximity and in good orientation to the CuMPDE molecules so that transfer of excitation energy can easily take place. The occurrence of such conditions cannot be detected by absorption spectra, because only strong interactions between porphyrins, for example, in the case of sandwich dimers with interfacial distances of less than several angstroms,l*15 are observable by absorption spectra. The energy transfer between zinc and copper porphyrins which are covalently linked has been investigated in one s t ~ d y Despite .~ the short distance between the two porphyrins, the excitation spectrum of the emission of each porphyrin moiety was found to be identical with that of each monomeric porphyrin. This result clearly indicates that the two porphyrins are not in favorable geometry for efficient energy transfer in the covalently linked porphyrins. Both short distance and some favorable mutual orientation seem essential to the efficient energy transfer between metalloporphyrins. We now proceed to a brief discussion about possible pathways of the energy transfer. Figure 4 shows the energy level diagram of the monomeric zinc and copper porphyr i m 6 Although CuMPDE exists in the form of an aggregate in MCH solutions, at 77 K this diagram seems still useful for a qualitative discussion. The upward solid arrow indicates excitation of the zinc porphyrin and the downward solid arrow that of the copper porphyrin phosphorescence, most of which arises from the tripdoublet (TI) state at 77 K.l69l7 Other arrows denote three possible pathways of the energy transfer from the zinc singlet to the copper tripdoublet. The dotted arrow indicates a pathway via the copper doublet, while the broken arrow shows a direct one. The wavy arrow depicts another (16)Gouterman, M.;Mathies, R. A.; Smith, B. E.; Coughey, W.S.J. Chem. Phys. 1970,52,3795-802. (17)Noort, M.; Jansen, G.; Canters, G. W.; van der Waals, J. H. Spectrochim. Acta, Part A 1976,32,1371-5.
J. PhyS. Chem. 1983, 87, 1493-1498
pathway via the zinc triplet. The ZnMPDE fluorescence intensity of MCH solutions of the mixed monomers at 77 K, when excited at 543 mm, shows ca. 40 f 15% decrease compared to that of only ZnME'DE after correction for the CuMPDE absorption. The ESR intensity of the Am8 = 2 transition due to the ZnMPDE triplet state of solutions of the mixed solutes under white-light irradiation also shows ca. 30 f 10% decrease compared to that of only ZnMPDE. These results are interpreted by assuming either energy transfer from the singlet state of ZnMPDE to CuMPDE or enhancement of the intersystem crossing rate in the excited singlet state of ZnMPDE due to the adjacent CuMPDE followed by efficient triplet energy
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transfer from the triplet ZnMPDE to CuMPDE. For the case of the covalently linked zinc and copper porphyrins, no decrease in the zinc fluorescence quantum yield was observed and only a slight shortening of the zinc triplet lifetime was detected in comparison with the zinc monomer.3 Therefore, in order to elucidate the details of the energy transfer in the present case, quantitative studies are needed on the quantum yields and lifetimes of the excited states of ZnMPDE and CuMPDE for solutions of both the separate and mixed solutes. Registry No. ZnMPDE, 15376-02-0; CuMPDE, 14710-65-7; methylcyclohexane, 108-87-2; 2-methyltetrahydrofuran, 96-47-9.
Photoionization of Bis(dimethy1amino)tetrahydropyrene. Importance of Solvent-Solute Exciplex Interactions Yoshlnorl Hlrala, Noboru Mataga, Department of Ctmmisby, Faculty of Englnmring Sclence, Osaka Unlverslty, Toyonaka, Osaka 560, Japan
Yoshlteru Sakaia, and Solchl Mlsuml The InstiMe of Scientific and Industrlel Research, Osaka Unlverslty, Sulta, Osaka 565, Japan (Received: September 2, 1982; I n Final Form: November 16, 1982)
The photoionization of 2,7-bis(dimethylamino)-4,5,9,1O-tetrahydropyrene(BDATP) has been studied by means of transient absorption and transient photoconductivity measurements with the picosecond laser photolysis method. The precursor of the photoionization, which consisted of the BDATP cation and the acetonitrile dimer anion, has been directly observed in acetonitrile solution. It has been demonstrated clearly that solvent-solute exciplex interactions are quite important for the photoionization processes of BDATP in benzonitrile, pyridine, as well as acetonitrile solutions. In contrast to these solutions, no free radical ion has been detected in a,a,a-trifluorotoluene (TFT) solution, where, however, the solute-solvent ion-pair state has been confirmed to be in equilibrium with the first excited singlet (SI)state of BDATP.
Introduction There has been a great deal of work, both experimental and theoretical, concerning the electron photoejection of aromatic molecules in organic liquids and solids. However, most of the work has been performed in nonpolar solvents as well as alcoholic glasses, and studies pertaining to electron photoejection in polar liquids have been rather scarce in spite of their importance in photochemical and photobiological primary processes. The process of electron photoejection of an aromatic compound in a condensed medium was first demonstrated by Lewis and collaborators.' They irradiated N,N,N',N'-tetramethyl-p-phenylenediamine(TME'D) in EPA glass at low temperature and obtained its cation radical, which is known as Wurster's blue. Since then, the electron photoejection of TMPD has been studied extensively in order to elucidate the mechanistic details of electron photodetachment, electron-cation recombination, and and other related phenomena.2 Another type of photoinduced charge separation of aromatic compounds is ionic photodissociation, which has
been studied for the typical exciplex and excited donoracceptor complex systems such as pyrene-N,N-dipyrene-p-dicyanobenzene,6 and tetrameth~laniline,~-~ cyanobenzene-toluene' in polar solvents. Ionic photodissociation is an important decay channel of exciplexes, which quenches their fluorescence in polar solvents leading to the formation of anion and cation radicals in the case of the above typical systems. The electron photoejection of some aromatic amines like TMPD occurs monophotonically with near-UV irradiation even in nonpolar solvents. We can expect that the lower energy photon causes the ionization of such compounds because the energy of the cation-electron pair state can be lowered by the interaction with the surrounding polar solvent. Several aromatic amines such as 2,7-bis(dimethylamino)-4,5,9,1O-tetrahydropyrene (BDATP) benzidine derivative^,^ and N,N,N',N'-tetramethylpyrenedi-
(1) Lewis, G. N.; Lipkin, D. J. Am. Chem. SOC.1942,64,2801. Lewis, G. N.; Bigeleisen, J. Zbid. 1943, 65, 520.
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(2) Lesclaux, R.; Joussot-Dubien, J. 'Organic Molecular Photophysics";Birks, J. B., Ed.; Wiley-Interscience: London, 1973; Vol. 1, pp 455-587.
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(3) Taniguchi, Y.; Nishina, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972, 45, 764.
(4) Taniguchi, Y.; Mataga, N. Chem. Phys. Lett. 1972,13, 596. (5) Masuhara, H.; Hino, T.; Mataga, N. J . Phys. Chem. 1975, 79,994. (6) Hino, T.; Masuhara, H.; Mataga, N. Bull. Chem. SOC.Jpn. 1976,
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(7) Masuhara, H.; Shimada, M.; Tsujino, N., Mutugu, N. Bull. Chem. SOC.Jpn. 1971, 44, 3310. ( 8 ) Hirata, Y.; Mataga, N.; Skata, Y.; Misumi, S. J . Phys. Chem. 1982, 86. 1508.
0 1983 American Chemical Society
