J. Phys. Chem. 1985,89, 3453-3459
(+0.005,-0.010). Moreover, we report that a nonzero value is not ruled out by these data. The implications of these results are probably not more significant than those of the previous evaluation.6 The value reported here for the yield of H2 in the H02 selfreaction of k 2 / k l < 0.0022 is in contrast with the previous measurement. Sahetchian et aL3 have reported a yield of 10.10 for H2in the self-reaction of H02at -500 K. The large difference in the upper limits can be analyzed in several ways by considering the differences in the experimental conditions of the two determinations. The foremost difference is in the nature of the sources of production of the H02radicals. The experiments of Sahetchian and co-workers involved pyrolysis of n-heptyl peroxide to produce n-heptoxy radicals which react with O2to produce H02. It would not be surprising if there were another source of hydrogen in this complicated, radical system. The high temperature and the presence of the long-chain alkoxy radical would seem to make it difficult to rule out definitively other H2 sources. One other experimental aspect should be mentioned; the present experiment was carried out in the presence of -2 torr of water vapor. It is now well-known that water vapor causes an increase in the rate constant of the H 0 2self-rea~tion.'~,'~ It is unknown, however, how water vapor might affect the product distribution. In any case, this new, lower value obtained at ambient temperature would have implications in the understanding of the mechanism of the H 0 2 self-reaction and in its rolqin the chemistry of the strato(14) (a) Hamilton, Jr., E. J.; Lii, R. Int. J. Chem. Kinet. 1977,9,875. (b) Lii, R.; Gorse, Jr., R. A,; Sauer, Jr., M. C.; Gordon, S.J. Phys. Chem. 1980, 84, 813.
3453
sphere. Any appreciable formation of H2would enhance the chain terminating effect of this reaction on the odd-hydrogen cycle as it occurs in the stratosphere. It is also clear that a theory describing the elementary reaction H 0 2 + H 0 2 in the conditions of our experiment should not predict formation of H2in even small amounts, 1-2%. It should also be noted that one other product channel has been investigated. In an FTIR study of the H 0 2 self-reaction, Niki and c o - ~ o r k e r shavk ' ~ determined that the production of 0 )and H 2 0 accounts for less than 0.1% of the reaction. The negative results presented in this paper and by Niki et al.I5 would leave H202and O2as effectively the sole products of H 0 2 + HOz. The remaining question, raised by Patrick et al.," is the degree to which O2 is formed in its electronically excited state, OZ(lAg).
Acknowledgment. We are grateful to Prof. R. Severs and Dr.
R.Barkley for the use of the gas chromatography system. We thank Dr. C. Howard for his comments and Dr. M. Mozurkewich for the copy of his manuscript and helpful discussion. This work was supported in part by the Chemical Manufacturers Association. R.J.G. thanks the Cooperative Institute for Research in Environmental Sciences (CIRES) for a Visiting Fellowship. We must also thank Dr. S. Solomon of NOAA for her helpful discussion on the atmospheric importance of H 0 2 + HO2. Registry No. H202, 7722-84-1; H 2 0 , 7732-18-5; H 0 2 , 3170-83-0; OH, 3352-57-6; Hi, 1333-74-0; 0 2 , 7782-44-7; 0, 17778-80-2.
(15) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1980, 73, 43.
Dimerization Effects on the Optical and Triplet ESR Spectra of Crown Porphyrins T. K. Chandrashekar,+Hans van Willigen,* and M. H. Ebersole Department of Chemistry, University of Massachusetts at Boston, Boston, Massachusetts 021 25 (Received: January 1 1 , 1985)
A study is made of the effect of dimerization on the optical and triplet ESR of a,/3,y,G-tetrakis(benzo-15-crown-5)porphyrin
(TCP) and ZnTCP. Dimerization induced absorption band shifts are compared with those reported for diporphyrins linked covalently. It is found that coupling between the transition moments of the two chromophores fails to account satisfactorily for observed shifts in a number of systems. This is attributed in part to the effect of ~ - 7 r interaction between the porphyrin rings. Another factor that may play a role is the effect of dimerization on the structure of the dimer constituents. Dimerization of the crown porphyrins is found to change the values of the zero field splitting (zfs) parameters of the first excited triplet state. In the case of ZnTCP the change can be interpreted in terms of the triplet exciton theory. The observed dimerization effect indicates that the porphyrin planes are parallel, but that one ring is rotated by about 23' relative to the other. For free base TCP the dimerization effect is small. In this case the triplet excitation energy apparently is localized on one of the porphyrin rings.
Introduction It has been shown that the difference in spectroscopic properties of in vitro chlorophyll and in vivo primary donor chlorophyll in reaction centers of photosynthetic bacteria can be accounted for in terms of a dimer structure (the so-called special pair). Chlorophyll and chlorophyll-pheophytin dimers also may play a role in the photoinduced charge separation reactions taking place in reaction centers of green plants.' This has stimulated studies of dimer systems of well-defined geometry that may mimic the characteristics of the in vivo systems. These investigations are of interest for a number of reasons. A primary goal is to establish, if possible, the relationship between spectroscopic properties and geometric structure of the dimers. This may make it possible to determine the structure of the special 'Present address: Department of Chemistry, Michigan State University, East Lansing, MI.
pair by using spectroscopic data. Secondly, investigations of the photophysics and -chemistry of model dimers may contribute to the understanding of the role the special pair plays in photosynthesis. Finally, some systems may prove suitable for use in man-made solar energy conversion and storage devices. Investigations have been concerned with covalently linked systems as well as dimers formed by spontaneous aggregation of porphyrins and chlorophylls.2-'1 The former have the advantage (1) For a recent review of the field and leading references see: "Photosynthesis: Energy Conversion by Plants and Bacteria", Govindjee, Ed.; Academic Press: New York, 1982; Vol. I. (2) Collman, P. J.; Elliot, C. M.; Halbert, T. R.; Tovrog, B. S. Proc. Nutl. Acad. Sci. U.S.A. 1977, 74, 18. (3) Chang, C. K.;Kuo, M.-S.; Wang, C.-B. J. Heterocycl. Chem. 1977, 14, 943. (4) Chang, C. K. J. Heterocycl. Chem. 1977, 14, 1285. ( 5 ) Kagan, N. E.; Mauzerall, D.; Merrifield, R. B. J . Am. Chem. SOC. 1977, 99, 5484.
0022-3654/8S/2089-3453$01.50/00 1985 American Chemical Society
3454 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
that the dimer geometry is fairly well defined due to the constraints imposed by the linking group. This offers the opportunity to study the relationship between structure and spectroscopic properties. However, it is evident that this relationship can be affected by the covalent links. For instance, the l i n b can modify the geometry of the tetrapyrrole cores. Also, functional groups in the links may interact with the rings. In both cases the electronic structure of the ring systems will be modified. The effect may become more pronounced as the chain length of the bridges is reduced. For this reason it is also of interest to study dimers without covalent links and yet with a well-defined geometry. The Lu,P,y,G-tetrakis(benzo- 15-crown-5)porphyrin (TCP, I) and its metal-substituted
Chandrashekar et al.
1
"
Figure 1. Optical absorption spectra of TCP (Soret, 3 X lod M; Q bands, 6 X M) in C H p C I C H I O H (1:l): (a) before and (b) after addition of KCI (- 10-4M). Note that the vertical scale is based on the
monomer concentration.
R
I.
R = benzo-15-crown-5
TABLE I: .Absorption B a d Positions (in nm) for Porphyrin Monomers and Dimers ~~
derivatives, synthesized recently by Thanabal and Krishnan,I2 are ideally suited for this purpose. As reported by these authors, the porphyrins can be made to dimerize by the addition of cations such as NH4+, K+, Cs', and BaZ+. The structure of the dimers can be deduced from the stoichiometry of the dimerization reaction and data from ESR and ENDOR studies of the Cu(I1)- and VO(1V)-substituted crown porphyrins. In the crown porphyrin dimers the rings are stacked in such a way that they have a "man symmetry axis normal to the planes. Recent ESR studies of copper-substituted, covalently linked diporphyrins show that in these systems the planes may be shifted relative to each other.I3 This difference in structure will affect the interaction between the chromophores. It is of interest to see how this is reflected in the spectroscopic data. This paper reports on the effect of dimerization on the optical (absorption and emission) spectra of TCP and ZnTCP, as well as on the ESR spectra of the photoexcited triplets. The results of the spectroscopic measurements are compared with literature data on covalently linked porphyrins with interplanar spacings similar to those of the crown porphyrin dimers. A comparison with data on the dimerization of tetrakis(4-sulfonatopheny1)porphyrin (TPPS)lo," is presented as well.
Experimental Section Benm15-crown-5 (Strem) was converted to aldehyde following published procedure^.'^ TCP was prepared by condensing pyrrole (Aldrich) with benzo-15-crown-5 aldehyde in propionic acid. The crude product was purified by column chromatography using alumina. Zn2+was inserted by refluxing stoichiometric amounts of zinc acetate and TCP in DMF. The progress of the reaction was followed spectroscopically. The product was purified by column chromatography. Solvents used in this work were of (6) Chang, C. K. Adu. Chem. Ser. 1979, No. 173, 162. Jameson, G. B.; Oakley, R. T.; Rose, (7) Collman, J. P.; Chong, A. 0.; E.; Schmittou, E. R.; Ibers, J. A. J. Am. Chem. SOC.1981, 103, 516. (8) Kaufmann, K. J.; Wasielewski, M.R. Adv. Chem. Phys. 1981,47,579. (9) For a review and leading references see: White, W. I. In "The Porphyrins", Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. Vc, Chapter 7. (10) (a) Chandrashekar,T. K.; Van Willigen, H. J . Am. Chem. Soc. 1983, 105,6323. (b) Chandrashekar, T. K.; Van Willigen, H. Chem. Phys. Letr. 1984, 106, 237.
(1 1) Chandrashekar, T. K.; Van Willigen, H.; Eber.de, M. H. J. Phys. Chem. 1984,88, 4326. (12) (a) Thanabal, V.; Krishnan, V. J . Am. Chorn. Suc. 1982, 104,3643. (b) Thanabal, V. ; Krishnan, V. Znorg. Chem. 1982, 22, 3606. (13) Eaton, S. S.; Eaton, G . R.; Chang, C. K. submitted for publication. (14) Hyde, E. M.;Shaw, B. C.; Shepherd, I. J. Chem. Soc., Dalton Tram. 1978, 1696.
system TCP TCP/K+ ZnTCP ZnTCP/KC TPPS [TPPS]2"
Soret 422 414
Q bands
518 522
556 569
593 595
551
421 413 414
516
415
524
555 552 565
652 665
590 580 598
595 635 657
"Dimerization induced by the addition of Kt and 18-crown-6 to a solution of tetrakis(4-sulfonatophenyl)porphyrin." spectroscopic grade and were used as obtained from commerical sources. Optical absorption spectra of TCP and ZnTCP in chloroformmethanol (1:l) were recorded with a Cary 14 spectrophotometer. A Perkin-Elmer 650-40spectrophotometer was used to record fluorescence spectra. ESR spectra of photoexcited triplets were recorded with a VARIAN E9 X-band spectrometer using field (100 KHz) and light (83 or 13 Hz) modulation with phase-sensitive detection at the two modulation frequencie~.'~This detection method serves to eliminate stationary ESR signals, for instance of doublet free radicals that build up gradually during sample irradiation. It can also lead to strong signal enhancement as a result of spin polarization effects.I5 The field modulation amplitude employed was 20 G, the microwave power was between 0.1 and 0.5 mW. Photoexcited triplets of TCP and ZnTCP in frozen methanol containing about 1% toluene or chloroform were generated with a 1000-W high-pressure Xe lamp powered by a Photochemical Research Associates supply with electronic modulation capability. Alternatively, use was made of a Spectra Physics argon laser (wavelength 514.5 nm, power 0.5 W). Lowtemperature measurements were carried out by cooling with nitrogen gas (down to about 100 K) or with helium (- 10 K) using an Oxford ESR9 gas cryostat. A Nicolet 1180E computer interfaced to the VARIAN and Cary spectrometers was used for data acquisition and analysis. The kinetics of formation and decay of the triplet ESR signals was monitored by exciting the samples with square-wave modulated light. The computer, used in the time-averaging mode, was triggered at the light-on edge and the signal amplitude vs. time profile was stored in a 1K memory block. Results Optical Spectra. Figure l a shows the absorption spectrum of the free base porphyrin TCP (3 X 10-6 M) in chloroform-methanol (1:l). The series of well-defined, intense absorption bands is typical for a porphyrin chromophore. As expected the spectrum has a (15) Levanon, H.; Weissman, S. I. J . Am. chem. SOC.1971, 93, 4309.
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3455
Spectroscopic Study of Crown Porphyrin Dimers
3688
1I
688 658
~
I
28
??e
688
h48
1
---
'i
nn
?68
8E8
--*
.-
/
/f
688
648
hB8
'?B
Nn
:68
988
Figure 2. Fluorescence spectra of TCP (lod M) in C H 3 C l C H 3 0 H (1:l) (A) before addition of KCI, excitation wavelength kx= 426 nm, and (B) after addition of KCl (-loJ M), X, = 412 nm. Slitwidth 5 nm.
close resemblance to that of free base tetraphenylporphyrin (TPP). Addition of Na+ ions to the solution does not have a significant effect on the absorption spectrum even though the cations complex with the crown moieties.16 On the other hand, Figure l b illustrates that the addition of K+ has a strong effect. As reported earlier by Thanabal and Krishnan,'* the addition causes (a) a decrease in the absorbance of the &ret as well as the Q bands, (b) red shifts and broadening of the Q bands, and (c) a blue shift and broadening of the Soret band. Similar effects were found in the absorption spectrum of ZnTCP upon addition of K+. The absorption band positions for TCP and ZnTCP in the absence and presence of cations are listed in Table I. Included are data on the monomer and dimer of TPPS." Both TCP and ZnTCP give fluorescence spectra with welldefined emission bands. The fluorescence spectrum of TCP is shown in Figure 2A. It exhibits emission peaks at 656 and 716 nm. Addition of K+ strongly quenches the fluorescence and the original peaks are replaced by a broad band centered around 704 nm. The effect is illustrated in Figure 2B. Excitation spectra mirror the effect of addition of K+ on the absorption spectrum of TCP. In the case of ZnTCP, addition of K+ leads to complete quenching of the fluorescence. The fluorescence spectra of TCP and ZnTCP are not affected by addition of Na+. ESR Spectra. The ESR spectrum of the photoexcited triplet of TCP randomly oriented in frozen solution, recorded with 83-Hz light modulation at 10 K, is shown in Figure 3a. The effect of addition of Na+ or K+ to the TCP solution is illustrated in Figure 3, b and c, respectively. In general, ESR spectra of randomly oriented triplets will show three pairs of peaks" (labeled x, y, z in Figure 3). One pair for each of the three orientations for which the magnetic field is parallel to a principal axis (x, y, z) of the zero-field-splitting (zfs) tensor. The separations between the pairs of lines are given by 2 0 and D f 3E, where D and E are the zfs (16) Pedersen, C. J. J . Am. Chem. SOC.1967, 89, 7017. (17) Wasserman, E.;Snyder, L.C.; Yager, W. A. J. Chem. Phys. 1964, 41, 1763.
350
325
300
I T
Figure 3. Triplet ESR spectra of TCP (-loJ
M) in CH3CI-CH30H recorded at about 10 K. Microwave power 0.5 mW, field modulation 20 G (100 kHz), excitation with square wave modulated (83 Hz) light of an argon ion laser (514.5 nm, 0.5 W). (a) No addition, (b) with NaCl M), and (c) with KCI (-5 X lo-' M). Absorption and (-5 X emission peaks have been labeled A and E, respectively. TABLE II: Zero-Field-Splitting Parameters (in lo-' cm-') of
Porphyrin Monomers and Dimers svstem TCP TCP/K+ ZnTCP ZnTCP/K+ TPPS [TPPSIt
D
E
X
Y
Z
377 332 298 291 391 334
79 73 99 69 75 80
205 184 199 166 205 191
46 37 0 28 55 31
-251 -221 -199 -194 -261 -223
ref this work this work this work this work 11 11
"TCP and ZnTCP data from triplet ESR spectra recorded at 10 K. Estimated uncertainty f5 X lo4 cm-' for TCP, TCP/K+, and ZnTCP, f7 X lo4 cm-I for ZnTCP/K+. *The dimerization is induced by addition of K+ and 18-crown-6."
parameter^.^' The spectra in Figure 3 show absorption as well as emission peaks as a result of the spin selectivity of the intersystem crossing and the decay from the triplet sub level^.^^^^* The observed pattern of absorption (A) and emission (E) peaks (marked in the figure) corresponds to that found for other TPP t r i p l e t ~ . ~ ~ItJ ~can J ~be attributed to preferential population of the zero-field T, sublevells (following convention the order of the energy levels is given as T, > T, > Tz). It is evident that the introduction of sodium ions does not affect the values of the zfs parameters. It merely causes some enhancement in signal a m -
plitude which may be the result of a slight reduction in line width. On the other hand, upon addition of K+ the original triplet ESR spectrum is replaced by a new spectrum, reflecting a reduction in value of both D and E. The addition also leads to a marked (18) Van der Waals, J. H.; Van Dorp, W. G.; Schaafsma, T. J. In "The Porphyrins", Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. IV, Chapter 5 . (19) Levanon, H.; Wolberg, A. Chem. Phys. Lerr. 1974, 24, 96.
3456 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
,
__
I
LIGHT OFF
LIGHT ON
Chandrashekar et al.
19
I
L I G H T ON
38
LIGHT OFF
1
-
19
Figure 4. Growth and decay of the resonance peaks (Y,Z) in the TCP triplet spectrum, temperature
I
j
38ms
10 K. Solid line, TCP; dotted line, TCP/K*.
reduction in signal amplitude. Values of the zfs parameters derived from the spectra are given in Table 11. Within experimental uncertainty the same values are found at 100 K. The growth and decay of the y and z peaks in the triplet spectra (cf. Figure 3) were studied by recording the time dependence of the signal amplitude at a fixed magnetic field setting while subjecting the sample to square wave modulated light. Figure 4 shows the results obtained for TCP and TCP/K+. No attempt was made to obtain a detailed analysis in terms of the kinetics of triplet sublevel population and decay. It is evident, however, that K+ addition has virtually no effect on the kinetics. An average triplet decay rate of 200 s-l is derived from the kinetic curves. Figure 5 shows the triplet ESR spectra of ZnTCP in the absence and presence of K+ recorded at 100 and 10 K. ZnTCP can give two triplet signals. One, observed at 100 K, is similar to that expected for a triplet species with axial symmetry ( E O).” The other, observed at low temperature, reflects a complete absence of axial symmetry ( D 3E).” Furthermore, this spectrum exhibits spin polarization effects. The triplet ESR spectra of ZnTPP, ZnTPPS,*Oand magnesium tetrabenzoporphyrin*Oshow a similar temperature effect. The effect has been attributed to the fact that the lowest triplet state is orbitally degenerate so that a Jahn-Teller splitting can modify the ESR spectrumst0 If kT is small compared to the splitting of the Jahn-Teller states, the spectrum will reflect the nonaxial spin distribution in the lowest of the two levels. In the high temperature limit the spin distribution will be the average over the two levels giving an E = 0 triplet spectrum. Clearly, addition of K+ affects the spectrum recorded at 100 K. However, due to the broad lines it is impossible to extract meaningful data from the spectrum. At 10 K cation addition results in a distinct change in values of D and E. Table I1 summarizes the ZnTCP triplet data.
-
-
Discussion Thanabal and Krishnan’* have shown that the changes in spectroscopic properties induced by cation (NH4+,K+) addition t o solutions of TCP and its metal-substituted derivatives are due to dimer formation. Addition of cations, which are complexed by the crown ether moieties, but do not induce dimerization of TCP (Le., Na’), does not affect the spectral parameters.12 This (20) Kleibeuker, J. F. Thesis, Landbouwhogeschool Wageningen, 1977.
300
325
350
MT
Figure 5. Triplet ESR spectra of ZnTCP M) in CH,Cl-CH30H recorded at 100 K (top) and 10 K (bottom), before (solid line) and after (dotted line) addition of KCI ( - 5 X IO-) M). Microwave power 0.5 mW, field modulation 20 G (100 kHz), excitation with square wave modulated (1 3 Hz) light from a Xe high pressure arc (1000 W) passed through a CuS04 heat filter.
establishes that cation binding in itself does not affect the electronic structure of the porphyrin core. It follows that the cation induced changes that are discussed here must be attributed to the inter-
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3451
Spectroscopic Study of Crown Porphyrin Dimers TABLE III: Dimerization Induced Absorption Band Shifts (em-’) for Some Porphyrins
Q bands
distance,” A
svstem
Soret
TCP TPPS” tTPPl2b
458 -58 0
-148 -296 -147
-411 -417 -192
-57
-300
-519 +260
-527 -590
916 1121 1684
-200
-105
-155
-156
DP-7‘ DP-6‘ DP-5‘
4.3
6.4 5.4 4.2
“Center-to-center distance derived from ESR spectra of Cu-substituted *Strati-bisprphyrin, two TPP ring systems linked by four 5-atom-long bridges.5 ‘Diporphyrins linked by two 7-, 6-, or 5-atom-long bridges.6 Recent ESR measurements” indicate that the distance between the porphyrin planes in these three dimers is virtually constant (3.8-3.9 A).
action between the porphyrin rings and/or the effect of the crown-cation-crown links on the geometric structure of the porphyrins. The studies by Thanabal and Krishnan have provided a good characterization of the structure of the dimers. The stoichiometry of the dimerization reaction establishes that the two porphyrins are linked by four crown-cation-crown bridges. This forces the porphyrins in a parallel configuration. For steric reasons it is likely that one ring system must be rotated relative to the other, possibly by as much as 450.12 X-band ESR data on g, metal ion hyperfine, and zfs tensor values in CuTCP and VOTCP dimers fit a model of two parallel porphyrins with the metal ions (Cu2+, VOz+) positioned on a common axis perpendicular to the planes.Iz Data from Q-band ESR measurementsZ1are in agreement with the proposed structure. Results of ENDOR measurements2 of proton and nitrogen hyperfine couplings in the monomers and dimers of CuTCP and VOTCP also support this model. An interpretation of the observed zfs values in terms of a point-dipole modelz3 with the unpaired electrons confined to the metal ions yields an apparent interplanar distance of -4.3 A.12 This must be an upper limit since spin delocalization and spin-orbit coupling effects will tend to reduce the value of the zfs parameter D that is used to calculate the distance.I2 In any case, the porphyrin-porphyrin distance is similar to that of a number of covalently linked diporphyrins studied p r e v i ~ u s l y . ~ -The ~ ~ ~similarity - ~ J ~ in structure is reflected, at least qualitatively, in the spectroscopic data. This is illustrated in Table I11 which gives Soret and Q band shifts found for TCP and for covalently linked porphyrins studied by Chang4*6J3 and Kagan et al.5 Included are data for TPPS which can be brought into the dimer form by the addition of cations or cation-crown ether complexes.lO*ll The dimerization-induced blue shifts of the Soret bands found for most of the diporphyrins (cf. Table 111) have been attributed to the effect of dipole-dipole coupling between the transition moments in the two chromophore^.^^ The Coulombic exciton coupling gives rise to a shift (A?) given byz5
A? = D G R 3 Here D represents the transition dipole moment in the monomer. G is a geometric factor accounting for the relative orientation of the dipoles in the dimer constituents and R is the center-to-center distance between the rings. The Soret band shifts listed in Table I11 for the series of diporphyrins studied by Chang agree at least semiquantitatively with those predicted by the exciton coupling m ~ d e l . ~By ? ~contrast, the shifts for the TPP based systems ([TCP],, [TPPSI2, and [TPP],) all are too small to fit the theory. (21) Chasteen, N. D., private communication. (22) Van Willigen, H.; Chandrashekar, T. K.,to be submitted for publication. (23) Chasteen, N. D.; Belford, R. L. Inorg. Chem. 1970, 9, 169. (24) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25, 139. (25) Kasha, M.;Rawls, H. R.; El-Bayoumi, M . A. Pure Appl. Chem. 1965, 11, 371.
The porphyrin-porphyrin separation in [TCP], is virtually identical with that in the 5-atom-bridged dimer (labeled DP-5 in Table 111) studied by Chang.4,6J3 Even so, the Soret band shifts found for these two systems vary by more than a factor of four. The [TPPSI2 and [TPPI2dimers show no shift at all. On the other hand, the three dimers exhibit relatively large Q band shifts (cf. Table 111) for which the exciton coupling theory fails to offer an explanation. The exciton coupling model described above considers only interactions between dipole moments of degenerate transitions. Recently Scherz and Parsonz6published a theoretical analysis of dimerization effects on the optical spectra of bacteriochlorophyll and related molecules. The authors show that in these systems coupling between dipole moments of nondegenerate transitions can be responsible in part for the observed effects. Exciton interactions between nondegenerate transitions in dimer constituents, in this case between transitions giving rise to the Soret and Q bands, will be associated with pronounced changes in relative dipole strengths of the transitions involved. Whereas the dimer spectra considered by Scherz and Parson show clear evidence of such intensity borrowing, the spectra of the porphyrins dimers do not exhibit this effect. Apparently this mechanism does not play a significant role here. The breakdown of the Coulombic exciton coupling approximation may be due to T-T interaction beween the chromophores. In [TCPI2 and [TPPSIz bonding between the dimer constituents is due, at least in part, to attraction between the porphyrin rings. The absence of covalent links allows the rings to assemble in such a way so as to promote the interaction between the T molecular orbitals of the chromophores. The strength of this interaction may be reflected in the magnitude of red shifts of Soret and Q bands. This effect may offset any blue shift of the Soret band due to coupling between the transition moments. In [TPPI2 the four covalent links may fortuitously allow the porphyrins a configuration favorable for T-a interaction. In general, however, the geometric constraints in covalently linked porphyrins give rise to a geometry which is less favorable for electronic interaction between the chromophores. As a consequence, the absorption band shifts may be more in line with predictions based on the Coulombic exciton coupling model. It should be pointed out that other factors may affect the spectroscopic data. For instance, it is known that the porphyrin ring is not a rigid structure and that its geometry can be easily modified by intermolecular interaction^.^' Changes in geometry caused by the bridging groups may be a source of absorption band shifts. Also, interactions beween functional groups in the covalent links and the porphyrin rings can influence band positions. Finally, in the TPP systems that have been studied, dimer formation can lead to a change in orientation of the phenyl rings relative to the porphyrin plane. The resulting change in conjugation between phenyl and porphyrin rings may have some effect on the optical spectra. Evidently, additional studies will be required to assess the relative importance of the factors that can contribute to dimerization induced changes in electronic spectra. Magnetic resonance studies of the photoexcited triplets of chlorophylls and porphyrins have been used to probe the structure of dimers and higher aggregate^.^*-^^ Dimerization can affect the magnitudes of the zfs parameters as well as the kinetics of population and decay of the triplets. The interpretation of these (26) Scherz, A,; Parson, W. W. Biochim. Biophys. Acta 1984, 766, 666. (27) Fleischer, E. B. Accounts Chem. Res. 1970, 3, 105. (28) Dutton, D. L.; Leigh, J. S.; Seibert, M. Biochem. Biophys. Res. Commun. 1972,46,406. Leigh, J. S.; Dutton, D. L. Biochim. Biophys. Acta 1974, 357, 67. (29) Clarke, R. H.; Connors, R. E.; Frank, H. A. Biochem. Biophys. Res. Commun. 1976, 71,671. Clarke, R. H.; Connors, R. E.; Frank, H. A.; Hoch, J. C. Chem. Phys. Lett. 1977, 45, 523. (30) Hagele, W.; Schmid, D.; Wolf, H. C. Z . Naturforsch. A 1978, 33, 83. (31) Hoff, A. J.; Gorter de Vries, H. Biochim. Biophys. Acta 1978, 503, 94. (32) Levanon, H.; Norris, J. R. Chem. Reu. 1978, 78, 185. (33) Giickel, F.; Schweitzer, D.; Collman, J. P.; Bencosme, S.; Evitt, E.; Sessler, J. Chem. Phys. 1984, 86, 161.
3458 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
effects depends on the magntiude of the exchange interaction between the porphyrin^.^^"^ Under weak coupling conditions (exchange interaction J of the order of the zfs interaction) the triplet exciton theory may apply. In that case the orbital of the triplet state can be described by a wave function of the form 3 q D = 2-l/2[3$,A I$,B f 1 A 3 B
J. $ 1
Here IPB are the ground-state wave functions of the monomers (A and B) that make up the dimer, and 3$A.B are the monomer triplet wave functions. If the weak coupling condition is met, the principal components of the zfs tensor in the dimer ( X ,P, Z*) can be related to those in the monomer ( X , Y, 2)by the expre~sion~~
In this expression 1, m, n denote the direction cosines specifying the relative orientations of the principal axes in the monomer and dimer. (The expressions defining the values of D and E are D = -3/2Z and E = I/,( Y - X ) . Rate constants for the intersystem crossing to the triplet sublevels and for decay to the ground state also can be related to the dimer s t r u ~ t u r e . , ~ -The ~ ~ equations make it possible to derive information on dimer geometry from dimerization induced changes in the values of the zfs parameters and the decay rate constants. As the interaction between the dimer constituents becomes stronger (J >> D ) charge-transfer contributions to the wave function describing the triplet electronic state of the dimer must be taken into account. One can predict that charge transfer will reduce the value of the zfs parameter D.1s931-33 It is also expected to increase the triplet decay rate constant^.^^"^ However, it is no longer possible to give an expression relating the dimerization effect on zfs tensor values and triplet decay rates to the dimer geometry. In terms of separation and relative orientation of the porphyrin rings, the dimer structure proposed for [TCP], and [ZnTCPj2 is similar to that of the 4- and 5-atom-amide-chain-linked diporphyrins studied by Guckel et al.33 This group performed magnetic resonance measurements on photoexcited triplets of these molecules. They found that the Z components of the zfs tensor were not affected by dimerization. However, the values for the in-plane components changed drastically, causing a change in the value of E . The experimental data were interpreted in terms of the exciton theory. A rotation of about 60° of one ring relative to the other accounts for the observed values of X and P.33 Rate constants for the decay from the triplet sublevels support this model. The measurements give no information on whether the porphyrins are stacked on top of each other or shifted as suggested by ESR data on Cu-substituted derivative^.'^ If the proposed staggered configurationI2 is correct, dimerization effects on the zfs parameters of TCP and ZnTCP should resemble those found for these covalently linked diporphyrins. The data given in Table I1 show that ZnTCP indeed exhibits the predicted dimerization effects. The value of D stays constant within experimental error. In terms of the exciton model, the reduction in E value can be accounted for by a rotation of -23’ of one porphyrin plane relative to the other. As mentioned earlier, Thanabal and Krishnanlz concluded that the porphyrin rings must be rotated with respect to each other for steric reasons. The ESR data provide experimental support for their conclusion. Unfortunately, available instrumentation did not permit measurement of the triplet formation and decay kinetics which could have confirmed the interpretation of the zfs data. Surprisingly, the TCP triplets do not show the predicted behavior. Dimerization reduces the value of D by about 15% and the E value is virtually unaffected (cf. Figure 3 and Table 11). (34) Schwocrer, M.; Wolf, H. C. Mol. Crysf. 1967, 3, 177. (35) Bowman, M. K.; Norris, .I. R. Chem. Phys. Lett. 1978, 54, 45.
-. . . . Chandrashekar et al.
The kinetic curves show little or no dimierization effect as well (cf. Figure 4). The reason why the dimerization induced changes in triplet parameters differ from those found for ZnTCP and the two covalently linked diporphyrin~~~ is not understood at this time. The data suggest that the triplet state may be localized which would mean that the exchange interaction between the porphyrins is very weak ( J < D).The reduction in D value in that case must be attributed to a dimerization induced change in structure of the dimer constituents. A change in angle between phenyl and porphyrin planes can cause a reduction in D.I8 It is of interest to note that a 6-atom-amide-linked diporphyrin studied by Guckel and c o - ~ o r k e r also s ~ ~showed little or no dimerization effect on the triplet parameters. Assuming that the exciton theory applies, the authors conclude that the dimer must have an eclipsed configuration. The possibility that the triplet excitation energy is localized on one porphyrin ring is not considered. An ESR of photoexcited triplets of chlorophyll phanes with 5- and 6atom-long covalent links establishes that the triplet energy is localized in those dimers. It is evident, therefore, that this alternative deserves serious consideration. A comparison with data on the TPPS systems studied earlier” shows some interesting differences. Dimerization of free base TPPS causes a reduction in D that is more pronounced than that found for TCP. Furthermore, the rate constants of decay from the TPPS triplet sublevels show an increase of more than a factor of two upon dimerization. The strong decrease in triplet lifetime indicates that charge transfer contributes to the triplet electronic state of the TPPS dimer. The ESR data point to a stronger porphyrin-porphyrin interaction in the [TPPS], than in the [TCPI2 triplet. If it is accepted that the Q band shifts stem from the ?M interaction between the chromophores, a similar conclusion can be drawn from the optical absorption spectra. It should be noted, however, that data on excimers show that the interaction between aromatics can be much stronger in the excited singlet state than in the triplet state.37
Conclusion The spectroscopic data presented here do not reveal a simple relationship between dimerization effect and geometric structure. Optical absorption spectra show a wide range of Soret and Q band shifts which are not readily interpreted. The limited data suggest some correlation between Soret and Q band shifts. Namely, systems with an unexpectedly small Soret shift exhibit a relatively large red shift of the Q bands (cf. Table 111). This has been attributed tentatively to the perturbation of the electronic energy levels by the A-A interaction between the porphyrins. The interaction can cause a deviation from the Coulombic exciton coupling model which is invoked frequently to account for dimerization induced Soret band shifts.4~6*24,26~38 Further studies of model systems of known structure will be required to verify the validity of this interpretation. In the case of ZnTCP the effect of dimerization on the triplet ESR spectrum can be used to get information on geometric structure. The porphyrin-porphyrin twist angle deduced from the triplet data agrees reasonably well with that postulated by Thanabal and Krishnan.’, The interpretation of the TCP triplet data is hampered by the fact that it is not known whether or not the triplet state is localized. Absorption band shifts suggest that the exchange interaction between the porphyrins must be strong enough for the exciton model to apply. However, it has not been shown that these shifts are a measure of the strength of the exchange interaction in the triplet state. As noted earlier, there can be a large difference beween the interactions in the singlet and triplet state.” The values of D and E of photoexcited triplets of primary donor bacteriochlorophyll (BChl) in reaction centers of photosynthetic bacteria are much smaller than those of in vitro BCh1.28-29*31v32 (36) Litteken, S. R. Thesis, University of Illinois at Urbana-Champaign, 1983. (37) Birks, J. B. ‘Photophysics of Aromatic Molecules”; Wiley: New York, 1970; Chapter 7. (38) Benthem, L. Thesis, Landbouwhogeschool Wageningen, 1984.
J. Phys. Chem. 1985,89, 3459-3464
This has been attributed to the fact that the primary donor consists of a pair of BChl molecules. The combined effects of triplet exciton "hopping" and a chargetransfer contribution to the triplet state probably account for the reduction. In an earlier it was noted that TPPS represented a model system that appeared to mimic this BChl dimerization effect. It is noteworthy that dimerization of TCP and ZnTCP has relatively little effect on triplet parameters. The data presented here support the conclusion, reached earlier by Litteker~,'~ that the formation of a dimer of closely spaced tetrapyrrole rings does not have to produce a strong effect on zfs parameters and triplet decay kinetics. Evidently, relatively minor changes in dimer geometry can have a profound
3459
effect on the triplet characteristics.
Acknowledgment. We thank Dr. N. D. Chasteen of the University of New Hampshire for performing Q band measurements and D. Kimball of UMB for her assistance in obtaining the fluorescence spectra. Dr. C. K. Chang of Michigan State University is thanked for sending us the results of an ESR study of Cu-substituted diporphyrins prior to publication. Financial support by the Office of Basic Energy Sciences of the Department of Energy under contract DE-FG02-84ER13242 is gratefully acknowledged. Registry No. TCP, 81294-37-3;ZnTCP, 81315-54-0.
A Mass Spectroscopic Study on the Dissociative Photoionization of 1,l-Dichlorodtfiuoroethene: The Adiabatic Ionization Potential of Gaseous Singlet 'A, Dichiorocarbene Klaus Rademann, Hans-Werner Jochims, and Helmut Baumgartel* Institut fiir Physikalische Chemie, Freie Universitht Berlin, D- 1000 Berlin 33, FRG (Received: February 5, 1985) Using monochromatized synchrotron radiation, we have derived the adiabatic ionization potential and the appearance potentials of the most abundant fragment ions of CFZ=CCIZfrom their ion yield curves: IPad(C2F2ClZ) = 9.64 eV, AP(CCl2+)= 12.91 eV, AP(C2FZCl+)= 13.87 eV, AP(CZFCIZ+) = 15.26 eV, APfCCl') = 16.13 eV, and AP(CFt) = 17.0 eV. The ionization potential of gaseous CCIZhas been measured directly: IP,d(cc12) = 9.10 0.10 eV. Reliable heat of formation values = 897.0 12.5 kJ/mol) and CCl2+(AHfoZg8 = 1107.9 f 7.5 kJ/mol). The have been determined for CFZCCl+(AHfoZg8 latter value is about 60 kJ/mol lower than the currently accepted literature value.
*
Introduction The determination of accurate thermochemical characteristics (heat of formation, proton affinities, bond dissociations energies, etc.) of gas-phase ions and radicals is of fundamental and increasing interest.'-9 These data are not only of considerable importance for the elucidation of ion molecule reactions in the broad field of gas-phase ion c h e m i ~ t r ybut ~ * ~they are also of great relevance for the investigation of unimolecular decay kinetics7y8 and fragmentation mechanism^^*^ of isolated gas-phase species. During the past 2 decades, photoionization mass spectrometry (PIMS)lO.llhas become a useful tool to evaluate reliably ionic heat of formation values (AHfo(ion)) from the measured appearance potential (AP). For the general process (carried out at temperature T and with radiation of energy hv) AB+hv+A++B+eenergy conservation requires the relationship APAA+) = AHf"AA+) -IAHf"AB) - AHr"AAB) - AH,,(T)
(1)
(2)
(1) J. L.Holmes and F. P. Lossing, Inr. J. Mass Specrrom. Ion Phys., 58, 113 (1984). (2) J. C. Traeger and R. G. McLoughlin, J . Am. Chem. Soc., 103,3647 (1981). (3) M.L. Fraser-Monteiro, L. Freiser-Monteiro, J. Butler, T. Baer, and J. R. Hass, J. Phys. Chem., 86, 739 (1982). (4) S.G.Lias in "Kinetics of Ion-Molecule Reactions", P. Ausloos, Ed.; NATO Advanced Studies Institute, Series B, Vol 40, Plenum Press, New York, 1979,pp 223-254. ( 5 ) H. Aue and M. T. Bowers in 'Gas Phase ion Chemistry", M. T. Bowers, Ed., Academic Press, New York, 1979. (6) P. Kebarle, Annu. Rev. Phys. Chem., 28, 445 (1977). (7) W.Forst, 'Theory of Unimolecular Reactions", Academic Press, New York, 1973. (8) J. Dannacher, Org. Mass Specrrom., 19,253 (1984). (9) J. C. Lorquet, Organic Mass Spectrom., 16,469 (1981). (10)J. Berkowitz, "Photoabsorption, Photoionisation and Photoelectron Spectroscopy", Academic Press, New York, 1979. (1 1) W.A. Chupka in 'Chemical Spectroscopy and Photochemistry in the Vacuum Ultraviolet", C. Sandorfy, Ed., Reidel, Dordrecht, 1974, p 433.
*
Equation 2, however, holds only if the so-called excess energy12-15 is zero. This energy comprises, in general, two terms: (a) the kinetic shift energy,IZ-l5which depends on the detection sensitivity of the apparatus and occurs in the case of low decomposition rate constants at threshold, and (b) the reverse activation which may be released as kinetic energy of the fragments and is expected to occur when rearrangement processes of the molecular cation precede the fragmentation r e a ~ t i 0 n . l ~ The term AHw( Tj in eq 2 has been derived and discussed in ref 2 and is used to correct for the thermal energies of the species involved in reaction 1, except the electron. By convention, the heat of formation of the electron is taken as zero at all temperatures.2J6 This stationary electron convention is adopted in the present study, too. In this paper we present new experimental results on a PIMS study of CFZ=CC12as part of a comprehensive interrogation of the thermochemistry, spectroscopy, decay dynamics, and fragmentation mechanistics of fluoro-chloro-substituted ethene derivatives. 17-20 Up to the present, vacuum ultraviolet (vacuum UV) studies on CF2=CClZhave provided its absorption spectrum in the range from 6.5 to 11.5 eVZ1and its He I photoelectron spectrum,22but (12) W.A. Chupka, J . Chem. Phys., 30,191 (1959). (13) S.M. Gordon and N. W. Reed, Inr. J. Mass Spectrom. Ion Phys., 18,379 (1975). (14) R. K.Boyd and J. H. Beynon, Int. J . Mass Spectrom. Ion Phys., 23, 163 (1977). (15) R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G. R. Lester, "Metastable Ions", Elsevier, Amsterdam, 1973. (16) H. M. Rasenstock, K.Draxl, B.W. Steiner, and J. T. Herron, J . Phys. Chem. Ref. Data, 6 (1977). (17) D. Reinke, R. Kriissig, and H. Baumgartel, Z . Narurforsch. A , 28, I021 (1973). (18) H. W.Jochims, W. Lohr, and H. Baumgiirtel, Nouu. J. Chim., 3, 109 (1979). (19) G. Frenking, W.Koch, M. Schaale, and H. Baumgartel, Inr. J . Mass Spectrom. Ion Processes, 61,305 (1984). (20) E.Riihl, H.W. Jochims, and H. Baumgartel, Can. J. Chem., in press.
0022-3654/85/2089-3459$01.50/00 1985 American Chemical Society
