J. Phys. Chem. 1992, 96, 8374-8377
8374
Transient Absorption, Time-Resolved Fluorescence, and Resonance Raman Spectroscopic Studies of Zr(TPP), and Zr(TPP)2(SbC16) Minyung Lee, Ok-Keun Song, Jung-Cbul Seo, Dongho Kim,* Spectroscopy Laboratory, Korea Research Institute of Standards and Science, Taedok Science Town, Taejon, 305-606, Korea
Hee-Joon Kim, and Kimoon Kim* Department of Chemistry and Center for Biofunctional Molecules, Pohang Institute of Science and Technology, P.O. Box 125, Pohang, 790-330, Korea (Received: February 5, 1992; In Final Form: June 8, 1992)
Transient absorption, time-resolved fluorescence, and resonance Raman measurements are reported for Zr(TPP)2and for the complex Zr(TPP)*(SbC&)in which an electron is removed from the porphyrin *-system. The singlet excited-state lifetime of Zr(TPP)2 in dichloromethane was measured to be 1.9 f 0.1 ns through the transient absorption technique and 1.95 f 0.05 ns using the time-correlated single photon counting method, respectively. By contrast, the excited-state lifetime of Zr(TPP)2(SbCl6) was found to be 40 f 10 ps, which is much shorter than that of the neutral species. The fast nonradiative deactivation of the cationic complex is likely caused by the rapid quenching of the initially excited doublet (*,**)state via one or more of the states seen in the near-IR ground-stateabsorption spectrum. These low energy states cormpond to transitions between the molecular orbitals derived from the ab and al, monomer orbitals through the T,* interaction of the two porphyrin subunits. In addition, re-sonance Raman spectroscopic data confm that in Zr(TPP)2(SbCls) one electron is removed from a dimer orbital derived from the monomer a2" orbital with a hole delocalized over the two porphyrin rings.
Introduction Metal-porphyrin sandwich complexes,'J in which two porphyrin ligands encompass a metal ion, have drawn great attention in recent years because of the similarities in their spectroscopic properties with those3 of the bacteriochlorophyll dimer ("special pair")4 in the photosynthetic reaction center. For example, similar to the special pair, the sandwich complexes exhibit a broad long-wavelength absorption'^^*^ in the neutral form and a nearinfrared absorption band'S2v6in the oxidized form. Until recently, the sandwich complexes have been prepared mainly with the lanthanide' or actinide2metal ions whose ionic radii are generally larger than 1 A. Recently, Buchler et ale7and one of us* independently reported the first transition-metal sandwich complex, Zr(TrP)2 (TPP = mesetetraphenylporphyrin). In this compound, the two porphyrin rings are held in unusually close proximity (2.56 A between the two porphyrin N4 planes) due to the small ionic radius or Zr4+(ca. 0.84 A); therefore, the electronic interaction between the two porphyrin systems is extremely strong which is indicated by a much higher energy shifted near-IR band of the oxidized species compared to those of the previously known metal-porphyrin sandwich complexes.'*2*6 Very recently, Holten et al.9 reported the time-resolved and steady-state optical measurements on Zr(TPP)2 and Hf(TPP)2. These porphyrin dimers showed new features in both ground- and excited-state absorption spectra as well as a broad fluorescence red shifted to lo00 nm.9 They concluded that the electronic states responsible for these optical features arise from strong interactions between the two porphyrin rings. In order to provide further insights into the effect of the strong T,T interaction on the optical properties of the sandwich dimers, we carried out time-resolved transient absorption, fluorescence, and steady-state Raman spectroscopic studies of Zr(TPP)2 and Zr(TPP)2(SbC16). This study provides further evidence for the strong T,T interaction of the porphyrin rings, the excited-state decay processes of the neutral and the oxidized species, and the electron delocalization of the oxidized species.
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Experimental Section Zirconium(1V) bis(tetraphenylporphyrinate), Zr(TPP),, and its oneelectron oxidized species, Zr(TPP),(SbCb), were prepared as previously described.* Dichloromethane was dried and distilled over P205under nitrogen before being used for all measurements. To whom correspondence should be addressed.
0022-365419212096-8374$03.00/0
Zr(TPP)2(SbC&)is rather stable in the dried solvent, but it turned into the neutral precursor within 1 h in the unpurified solvent. Ground-state absorption spectra were recorded on a HewlettPackard 8452A spectrophotometer,and the near-IR absorption was measured on KBr pellets by a Bomem DA 3.01 FT-IR spectrometer. Transient absorption data on the picmxond to 5-ns time scale with a 2-mm path length flow cell were recorded by excitation with 20-Hz, 0.8-ps, 0.3-mJ pulses at 585 nm and probing with a continuum pulse generated by focusing the same laser pulses into a 1 cm X 1 cm cell containing water.1° These laser pulses were obtained from a combination system consisting of a continuous wave (CW) mode-locked Nd:YAG laser, hybridly mode-locked synchronously pumped dye laser, Nd:YAG regenerative amplifier, and tunable picosecond dye amplifier. The concentrations of Zr(TF'P)2 and its cationic analog were adjusted to show about 0.5 absorbance in the Q band region for transient absorption measurements. The time-correlated single photon counting method was employed to determine the lifetime of the fluorescent singlet excited state of Zr(TPP)*. The laser source consisted of the CW mode-locked Nd:YAG laser and a cavity-dumped dual-jet dye laser. The dye laser pulses had the repetition rate of 3.8 MHz, 2-ps pulse width, and a 10-18 nJ/pulse energy at 570-605 nm. The electronic components for this system were from Tennelec and EG & G. With a photomultiplier tube (Hamamatsu Model R928), the instrument response function of 580 ps was obtained." For Raman measurements,'2 a Spectra-Physics 165 AI ion laser was used for sample excitation. A l-m Jobin-Yvon Raman Ulo00 double monochromator, a Hamamatsu R943-02 photomultiplier with a PMT cooler, and a Hamamatsu C1230 photon counter/discriminator were used for the signal detection. The backscattering geometry was employed for the KBr pellet sample excitation.
Results and Discussion Absorption Spectra of Zr(TPP)* and Zr(TPP)2(SbQ). The absorption spectra of Zr(TPP)2 and Zr(TPP)2(SbC16)are presented in Figure 1 (UV-visible region) and Figure 2 (near-infrared region). As Holten and co-workers reported recently: the UVvisible spectrum of Zr(TPP)2 exhibits a broad absorption band at -700 nm (Q band)I3 which is a characteristic of closely spaced porphyrin dimers: such a band has not been observed in the spectra of mon~porphyrins'~ and porphyrin dimers with larger
8 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8375
Spectra of Zr(TPP)2 and Zr(TPP)2(SbC16)
I
0.3 I
."6
0.0
Zr(Tl'P),
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500
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800
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Wavelength ( nm )
F i p e 1. Absorption spectra of Zr(TPP)2 and Zr(TPP)2(SbC16)in dichloromethane at room temperature.
960
930 900
1800
1400
loo0
Wavelength ( nm )
Figure 2. Near-IRabsorption spectra of Zr(TPP), and Zr(TPP)2(SbC4) in KBr pellets. The spectral resolution is 5 cm-I.
spacing1*such as the pox0 dimer or cofacial dimers linked with covalent bridges. The Soret band of the one-electron-oxidation product Zr(TPP),(SbCl,) is blue shifted relative to that of the neutral precursor (376 vs 396 nm). This is a typical behavior observed in oxidized metalloporphyrins when oxidation occurs at the porphyrin ring.16 Another notable feature of the cationic species spectrum is a broad absorption around 1110 nm (fwhm = -200 nm) (Figure 2) which also has been observed in other single-hole sandwich complexes.1*2s6A molecular orbital model for the sandwich complexes, initially p r o m by Man and a+worke& and modified for Zr(TPP)2 by Holten and c0-workers? provides an explanation for the near-IR band. The W,T interaction between the porphyrin rings results in the splitting of the monomer HOMO a2, orbital into bonding (b2) and antibonding (a,) orbitals of the dimer (see Figure 5 of ref 9). Similarly, the a,, monomer orbital splits into bl (bonding) and a2(antibonding) orbitals of the dimer. The hole of Zr(TPP)2+appears to reside on the a, dimer orbital derived from the a2, monomer orbital (see the resonance Raman data discussed below). The promotion of an electron from the filled b2 orbital to the half-filled a, orbitals gives rise to the 11IO-nm absorption band in the simple model. Further inspection of the 1 1IO-nm band of Zr(TPP)2+reveals the presence of a reproducible weak fine structure. A complex fine structure on the near-IR bands of similar singlehole sandwich complexes at 77 K has been reported recently and attributed to multiple vibrational progressions involving intermolecular modes of the dimer.6b*cThe fine structure seen in Figure 2 may have a similar origin, although a more extensive intestigation will be required before detailed conclusions can be drawn. In addition to the 1100-nm band, the spectrum of Zr(TPP)2+ also contains a weaker absorption band near 1800 nm (Figure
840
810
780
Wavelength (nm)
Figure 3. Absorption difference spectra of Zr(TPP)2in dichloromethane at room temperature acquired 13 ps, 0.8 ns, and 4.5 ns after a 0.8-ps flash at 585 nm.
2). Although the nature of this band is unclear at present, several dipole-forbidden transitions that may be lower in energy than the 1100-nm band warrant consideration. Perhaps the lowest energy transition is that corresponding to electron promotion from the filled al,-derived antibonding HOMO a2 to the half-filled a2,derived antibonding HOMO a]. It appears that the splitting between the a,,- and a2,derived dimer HOMOs is comparable9J9 to the splitting between the alu and a2, HOMOs of porphyrin monomers, which is typically on the order of 3000 ~ m - l . l *If~this assessment is correct, the a2 al transition would occur in the vicinity of 3 pm, much lower in energy than the 1800-nm band. Perhaps a more likely candidate is the transition corresponding to electron promotion from the bl to a, orbitals. A definitive assignment must await more detailed information on the ordering and spacing of the dimer HOMOs and the magnitude of twoelectron terms that contribute to the transition energies. However, the observation of the 1800-nm band and consideration of the possible low energy excited states of Zr(TPP)2+are very useful for the discussion of the deactivation pathways of this molecule discussed below. Transient Absorption and Time-Resolved Fluorescence of Zr(TPP)*. Visible/near-infrared excited-state absorption spectra at various time delays for Zr(TPP)2 in dichloromethane are presented in Figure 3. The spectrum at 13 ps is assigned to the fluorescent IQ (*,a*)excited state, and the 4.5-11s spectrum is state.g The 0.8-ns spectrum can be assigned assigned to 3T (a,**) to the mixture of the singlet and the triplet excited-state absorption spectrum. These spectra are similar to the previously reported the ST(a,**)spectrum shows a distinct broad absorption near 930 nm and the rising absorption on the shorter wavelength edge around 800 nm of the IQ (a,a*). The presence of the transient absorption band near 930 nm in the 3T (a,**)spectrum (4.5-11s time delay spectrum in Figure 3) was in accord with the MO description given by Holten et ala9The l Q ' (x,T*) lifetime was measured through the rise at 930 nm and the decay at 800 nm; both of them give the lifetime of 1.9 f 0.1 ns in dichloromethane. In order to measure the lifetime of the fluorescent IQ' (r,r*) state more accurately, a time-resolved fluorescence study was carried out on Zr(TPP)2 using the time-correlatedsingle photon counting method (Figure 4). Although the fluorescence maximum of Zr(TPP), was reported to be at 970 nm? the emission was collected at 900 nm because the sensitivity of the photomultiplier tube used in the experiment dropped quickly past 900 nm. The lifetime obtained here was 1.95 f 0.05 ns in dichloromethane, which is in good accord with the one measured by the transient absorption measurement mentioned above. The lifetimes in both toluene and n-hexane were measured to be 1.7 f 0.1 ns. The lifetimes in toluene and n-hexane appear to be slightly shorter than in dichloromethane,but our value is somewhat longer than that reported previouslyg from transient absorption measurements in toluene (1.1 f 0.1 ns).
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Lee et al.
0376 The Journal of Physical Chemistry, Voi. 96, No. 21. 1992
l i
\
25
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Zr(TPP), in CH,CI, IRF 560ps EXT 580nm EMS 900nm Lifetime : 1.95 N X' : 1.14
5
75
I 10
TIME (N)
Figure 4. Fluorescence decay curve of Zr(TPP)2in dichloromethane excited by 2-p, 10-nJ, and 580-nm flash. The instrument response function of 580 ps was obtained by a laser pulse, and the lifetime was calculated by the deconvolution of instrument response function and the fluorescencedecay profile. The x 2 value of 1.14 was obtained after the curve fitting. ni
bi
0108 0.06
i
1
Raman Shift (an") Figure 6. Resonance Raman spectra of Zr(TPP)2and Zr(TPP),(SbC&) in K#04 pellet. # mark denotes a sulfonate peak due to K$O,. Other conditions: laser power, 30 mW; slit width, 5-cm-l resolution;scan speed, 1 cm-'/s.
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Wavelength (nm)
F i p e 5. Absorption difference spectra of Zr(TPP)2(SbC16)in dichloromethane at room temperature acquired 6.7 and 33.4 ps after a 0.8-ps flash at 585 nm.
Transient Absorption Spectra of Zr(TPP)2(SbQ). Visible/ near-infrared transient absorption spectra of Zr(TPP)2+ [Le. Zr(TPP)2(SbCl6)] in dichloromethane are presented in Figure 5. These spectra show a featureless absorption up to 930 nm and start to build a bleaching past 930 nm caused by ground-state near-IR absorption (Figure 2). As discussed previously, the Zr(TPP),(SbC16) has a broad ground-state absorption band centered at 11 10 nm. The lack of measureable fluorescence (@f < 10-9 and the small amplitude of the transient absorption signal under the same experimental conditions used for Zr(TPP), (the same optical density at the excitation wavelength, 585 nm) imply an extremely short lifetime for the excited state. Indeed, the picosecond transient absorption measurements indicate that the molecules in the initial transient state return to the ground state almost completely in 40 f 10 ps. Clearly, the oxidized species Zr(TPP)z(SbC16) has a deactivation channel not accessible to the neutral complex. The ground-state absorption spectrum of the oxidized complex in the near-IR region (Figure 2) reveals the presence of at least two low energy quenching states. As noted above, these low energy states correspond to electron promotion from filled dimer HOMOS to the top, half-filled HOMO a,. Thus, after Zr(TPP)2+is excited states in the visible region, into the higher energy doublet (r,~*) then it relaxes rapidly via these lower energy doublet states. The first step in this process is likely a decay to the state responsible for the llOa-nm near-IR ground-state absorption, and this may be followed by deactivation to the state responsible for the 1800-nm
band. If, as noted above, the state involving electron promotion from the a2 to a, orbitals lies at even lower energy, then the relaxation process could proceed further through this state. This last step is analogous to that proposed to be responsible for the fast decay of excited Ru porphyrin cations." Additionally, some of the 4 0 - p decay of the transient absorption may not simply reflect electronic relaxations but equilibration of vibrationally 'hot" low energy doublet excited states or of the ground electronic state itself. A great deal of excess electronic energy must be dissipated during the rapid decay of the excited state produced by excitation in the visible region. Such vibrational "cooling" on the 10-20-p time scale has been reported in a number of porphyrin systems.'* Our data do not allow us to assign rate constants to the individual step in the deactivation of Zr(TPP)2(SbCl& but clearly show that the overall relaxation of the photoexcited complex requires about 40 p. Again, that this relaxation is so much faster than in the Zr(TPP)2 is a quite interesting observation and one that can be understood in terms of the presence of low energy excited states involving transitions to the half-filled top HOMO of the oxidized species. Such low energy states are not possible for the neutral complex Zr(TPP),, which has significantly longer lived excited states. Reaolna~eRnnnrn S ~ ~ C & Of E B(TPP)*urd Zr(TPP)@KQ. Resonance Raman (RR) spectra of Zr(TPP)2 and Zr(TPP),(SbC16) obtained with 457.9- and 514.5-nm excitations are presented in Figure 6. The RR spectrum of Zr(TPP),(SbC&) is comprised of a single set of peaks rather than two sets due to one neutral and one oxidized porphyrin chromophore. This suggats that the hole is delocalized over the two porphyrin rings of the cationic species as is the case of all the previously studied single-hole sandwich complexesi6 (with the exception of the asymmetrical Ce(OEP)(TPP)+ complex6). Comparison of the RR spectra of Zr(TPP)z and Zr(TPP)2(SbC&)reveals large down shifts of the v2 (20-m-I) and vI1(36tm-') bands of the o x i d i d complex relative to those of the neutral species (Table I). The
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8377
Spectra of Zr(TPP)2 and Zr(TPP)2(SbC16) TABLX I: Rescwsee Ram;m Frequencies (em-') of the Zr(TPP)2 md Zr(TPP)*+Complexes in Comparison with Tbose of the Cerium
Anrlons V, VI1
V,
Zr(TPP)2 1562 1510 1362
Zr(TPP)2+ 1542 1474 1362
A -20 -36 0
CC(TPP)~O Ce(TPP)Z+" 1543 1535 1344
1331
A -8 -13
"Taken from ref 6c.
v4 band shows a negligible shift. These shifts are consistent with the fact that the hole of Zr(TPP)2+resides on a dimer a l orbital derived from a& monomer orbitalm(see Figure 5 of ref 9). Similar down shifts of these bands have been observed for Ce(TPP), (see Table I) and Ce(TFW) (W= msetetrapentylporphyrin) upon one-electron oxidation, and the resulting hole in these oxidized complexes has been assigned to reside on a dimer orbital derived from monomer aZuorbital." A smaller shift of the bands of Ce(TPP)*+ compared to those of Zr(TPP)2+appears to be due to the fact that the HOMO of Ce(TPP), contains the metal f orbital character as well as the porphyrin ab character (see Figure 7 of ref 6a).
Conclusions The lifetime of the fluorescent singlet excited state of Zr(TPP)2 in dichloromethanewas measured through the transient absorption technique (1.9 f 0.1 ns) and the time-correlated single photon counting method (1 -95 f 0.05 ns). The excited-state lifetime of Zr(TPP)2(SbC16) in dichloromethane was found to be 40 f 10 ps. This fast nonradiative deactivation of the excited state of the cationic species is probably due to the near-IR absorption state lying lower than the normal x,x* transition. Resonance Raman frequency shifts confirmed that in Zr(TPP)@Ch) one electron is removed from a dimer orbital derived from the monomer aZu orbital with a hole delocalized over the two porphyrin rings. Acknowledgment. This work was supported by the Ministry of Science and Technology, Korea Science and Engineering Foundation through the Center for Molecular Science (D.K.) and through the Center for Biofunctional Molecules (K.K.). We thank Prof. Dewey Holten for helpful and stimulating discussion and for sending us a manuscript related to our work prior to publication. References and Notes (1) (a) Buchler, J. W.; Kapellmann, H.-G.; Knoff, M.; Lay, K.-L.; Pfeifer, S . Z . Naturforsch. 1983,386, 1339. (b) Buchler, J. W.; De Cian, A.; Fischer, J.; Kihn-Botulinski, M.; Paulus, H.; Wiess, R. J. Am. Chem. Soc. 1986,108, 3652. (c) Buchler, J. W.; De Cian, A.; Fischer, J.; Kihn-Botulinski, M.; Wiess, R. Inorg. Chem. 1988,27,339. (d) Buchler, J. W.; Huttermann, J.; Loffler,
J. Bull. Chem. Soc. Jpn. 1988,61,11. (2) (a) Girolami, G. S.;Milam, S,M.; Suslick, K. S . Inorg. Chem. 1987, 26,343. (bl Girolami. G. S.: Milam. S.N.; Suslick. K. S . J. Am. Chem. Soc. 1988,110,2011. (3) (a) Kirmaier, C.; Holten, D. fhorosynth. Res. 1987, 13, 225. (b) Budill, D.; Gast, P.; Chang, C. H.; Schifer, M.; Norris, J. R. Annu. Rev.fhys. Chem. 1987, 38, 561. (c) Hanson, L. K. fhotochem. Phorobiol. 1988, 47, 903. (d) Friesner, R. A.; Won, Y. Biochim. Biophys. Acta 1989, 977, 99. (4) For a review, see: Deinhofer, J.; Michel, H. Science 1989,245, 1463. (5) (a) Yan, X.;Holten, D. J. Phys. Chem. 1988, 92, 409. (b) Bilsel, 0.; Rodriguez, J.; Holten, D. J . Phys. Chem. 1990, 94, 3508. (c) Bilsel, 0.; Rodriguez, J.; Holten, D.; Girolami, G. S.; Milam, S. N.; Suslick, K. S.J . Am. Chem. Soc. 1990, 112,4075. (6) (a) Donohot, R. J.; Duchowski, J. K.; Bocian, D. F. J . Am. Chem. Soc. 1988,110,6119. (b) Duchowski, J. K.; Bocian, D. F. J . Phys. Chem. SOC. 1990,112,3312. (c) Perng, J.-H.; Duchowski. J. K.; Bocian, D. F. J . fhys. Chem. 1990, 94, 6684. (d) Duchowski, J. K.; Bocian, D. F. Inorg. Chem. 1990, 29, 4158. (e) Perng, J.-H.; Duchowski, J. K.; Bocian, D. F. J. Phys. Chem. 1991,95, 1319. (7) Buchler, J. W.; De Cian, A,; Fischer, J.; Hammerschmitt, P.; Weiss, R. Chem. Ber. 1991, 124, 1051. (8) Kim, K.; Le,W. S.;Kim, H.-J.; Cho, S.-H.; Girolami, G.S.; Gorlin, P. A.; Suslick, K. S . Inorg. Chem. 1991, 30, 2652. (9) Bilsel, 0.;Buchler, J. W.; Hammerschmitt, P.; Rodriguez, J.; Holten, D. Chem. fhys. Lett. 1991,182,415. (10) (a) Seo. J.-C.: Lee. M.: Kim. D.: JeonP. H. S.:Park. S.H.: Kim. U. J . Kor: bpt. Soc. 1991,20s. (b) Seo, J.-C.; Cldung, Y . B.; Kim, D.; J&g, H. S.; Kim, U.New Phys. 1991, 31 (9,530. (11) Chung, Y. B.; Jang, D.-J.; Kim, D.; Lee, M.; Kim, H. S.;Boo, B. H. Chem. Phys. Lett. 1991, 176.453. (12) Ha, J.-S.; Yoon, M.; Lee, M.; Jang, D.-J.; Kim, D. J . RamanSpectrosc. 1991, 22, 597. (13) The notations of Q' and Q bands at 700 and 504 nm, respectively, of Zr(TPP)z follow those presented in ref 9. (14) (a) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 111, Chapter 1. (b) Goutcrman, M.; Hanson, L. K.; Khalil, G.-E.; Buchler, J. W.; Rohbock, K.; Dolphin, D. J . Am. Chem. Soc. 1975, 97, 3142. (c) Buchler, J. W.; Folz, M.; Habets, H.; van Kaam, J.; Rohbwk, K. Chem. Eer. 1976, 109, 1477. (15) (a) Gouterman, M.; Holten, D.; Lieberman, E. Chem. fhys. 1977, 25, 139. (b) Chang, C. K. J. Heterocyclic Chem. 1977, 14, 1285. (c) Collman, J. P.; Barnes, C. E.; Collins, T. J.; Brothers, P. J. J. Am. Chem. Soc. 1981,103.7030. (d) Osuka, A.; Maruyama, K. J. Am. Chem. Soe. 19%8,110, 4454. (16) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic h, New York, 1978; Vol. V, pp 53-125. (17) Barley, M.; Dolphin, D.; James, B. R.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1984,106, 3937. (18) (a) Rodriguez, J.; Holten, D. 1.Chem. Phys. 1989,91 (6), 3525. (b) Rodriguez, J.; Holten, D. J. Chem. Phys. 1990,92 (lo), 5944. (c) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Chem. Phys. 1991, 94 (9), 6020. (19) Bilsel, 0.;Rodriguez, J.; Milam, S. N.; Gorlin, P. A.; Girolami, G. S.; Suslik, K. S.;Holten, D. J. Am. Chem. Soc. 1992, 114, 6528. (20) (a) Yamaguchl, H.; Nakano, M.; Itoh, K. Chem. ,511. 1982, 1397. (b) Kim, D.; Miller, L. A.; Rakhit, G.; Spiro, T. G. J . Phys. Chem. 1986,90, 3320. (c) Salehi, A.; Ocrtling, W. A.; Babcock, G.T.; Chang, C. K. J . Am. Chem. SOC.1986,108,5630. (d) Oertling, W. A,; Salehi, A,; Chung, Y. C.; Leroi, G. E.; Chang, C. K.; Babcock, G. T. J . fhys. Chem. 1987,91,5887. (e) Czernuszewicz, R. S.; Macor, K. A.; Li, X.-Y.; Kincaid, J. R.; Spiro, T. G. J . Am. Chem. Soc. 1989,111, 3860.
