6144
J. Phys. Chem. 1986,90, 6144-6148
Two-Dimensional Arrangement of Chromophores In J Aggregates of Long-Chaln Merocyanlnes and Its Effect on Energy Transfer In Monolayer Systems Hiroo Nakahara,* Kiyoshige Fukuda, Faculty of Science, Saitama University, Urawa 338, Japan
Dietmar Mobius, and Hans Kuhn Max-Planck Institut fur biophysikalische Chemie, 0-3400 Gottingen-Nikolausberg, Bundesrepublik, West Germany (Received: November 4, 1985; In Final Form: May 14, 1986)
Aggregate formation in mixed monolayers with methyl arachidate and n-hexadecane of several long-chain merocyanine dyes with different unsymmetrical structures and modied hydrophilic g o u p has been studied by measuring the surface pressurearea isotherms and the absorption spectra of the built-up films. The dye with a benzothiazol ring as an electron-donating group and one methylene carbon between the rhodanine ring and the carboxyl group gives a more condensed monolayer and is more favorable to produce J aggregates in mixed films. The geometry and aggregation number of the chromophore have been considered by applying an extended dipole model to the two-dimensional arrangement of the transition moments. Furthermore, the mechanism of energy transfer from the aggregate sensitizer in the monolayer systems has also indicated a characteristic of the J aggregate.
Introduction Dye aggregation characterized by close packing of chromophores and strong coupling is of practical and theoretical interest because of a particular photophysical behavior and distinct spectral Typical examples are the large J and H aggregates of cyanine dyes.3 However, relatively few studies on the aggregation of merocyanine dyes with an unsymmetrical structure have been reported.4 It is important to obtain a correlation between the dye structure and its ability to form J aggregates. Monolayer techniques are particularly suited for studying the structure and functional behavior of dye aggregates. Kuhn and wworkers have revealed the geometry of aggregates for sensitizing dyes with long chains in the monomolecular layers5v6and proposed a coherent exciton model for the excited state.7 Nakahara and Fukuda have constructed molecular assemblies with a well-defined orientation of chromophores by introducing long hydrocarbon chains together with appropriate hydrophilic groups to different positions of the chromophores such as anthraquinone and azobenzene.* Recently, MBbius et al. have developed monolayer organizates with cooperating functional units in inert matrix m o l e c ~ l e s . ~ JIn~ the previous paper," it has been shown that orientation and aggregation of the long-chain merocyanine dyes (1) Gilman, P. B., et al: In Photographic Sensitiuity, Cox, R. J., Ed.; Academc Press: London, 1973; p 187. (2) Sturmer, D. M. In Special Topics in Heterocyclic Chemistry; Weissberger, A., Taylor, E. C., Eds.; Wiley: New York, 1977; p 540. (3) Herz, A. H. Phofogr. Sci. Eng. 1974, 18, 323. (4) Steiger, R.; Kitzing, R.; Junod, P. In Photographic Sensitiuity, Cox, R. J., Ed.; Academic: London, 1973; p 221. (5) Kuhn, H.; Mabius, D. Angew. Chem. 1971,83,672. Angew. Chem., Inf. Ed. Engl. 1972, 10, 620. (6) Kuhn, H.; Mabius, D.; Bather, H. In Physical Methods of Chemistry, Weissberger, A,, Ed.; Interscience: New York, 1972; Part 111-B,Chapter VII, n r
-511. .
(7) Kuhn, H. Isr. J . Chem. 1979, 18, 375. (8) Nakahara, H.; Fukuda,K.J . Colloid Interface Sci. 1979,69, 24. 1983, 93, 530. (9) Mabius, D. Ber. Bunsenges. Phys. Chem. 1978, 82, 848. (IO) Miibius, D. Acc. Chem. Res. 1981, 14, 63. (11) Nakahara, H.; MBbius, D. J. Colloid Interface Sci., in press
in mixed monolayers transferred to glass plates depend strongly on the matrix used. In particular, mixed films of the merocyanine dye with arachidic acid or methyl arachidate and n-hexadecane show the typical absorption and fluorescence spectra of the J aggregates. In this paper, we report on aggregate formation in mixed monolayers with methyl arachidate and n-hexadecane of several long-chain merocyanine dyes with different unsymmetrical structures and modified hydrophilic groups, as studied by measuring the surface pressurwrea isotherms and the absorption spectra of the built-up films. Furthermore, the geometry and aggregation number have been considered by application of an extended dipole model to the two-dimensional arrangement of the transition moments. In addition, energy transfer from the aggregate sensitizer of merocyanine to a long-chain crystal violet derivative acceptor in the monolayer assemblies has been investigated for characterization of the J-type aggregate.
Experimental Section Several unsymmetrical merocyanine dyes with an alkyl chain, Mc[X-m,n] (X = 0, S, Se; m = 2 or 18; n = 1, 2, 3), used in this work are indicated in Figure 1, in which different electrondonating (basic) groups are linked to the rhodanine ring as an acidic group by methine carbons and the number, n, of methylene carbons between the rhodanine and the carboxyl group is varied. These dyes were synthesized by Dr. S. Yasui, Japanese Research Institute for Photosensitizing Dyes, Co. (Okayama, Japan). The monolayers were spread from a chloroform solution onto the aqueous subphase containing 3 X IO4 M CdC12and 5 X lo-' M NaHCO, (pH 6.3). A modified Wilhelmy type film balance, which consists of a thin hanging glass plate connected with a digital precision balance, was used for measurement of surface pressure as a function of molecular area. According to the LangmuirBlodgett method, the monolayers were transferred onto a quartz plate that was previously covered with cadmium arachidate films (3-5 layers) to make the surface hydrophobic, with a deposition ratio of about unity. Absorption and fluorescence spectra of the built-up films were measured with a Hitachi spectrophotometer
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The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6145
J Aggregates of Long-chain Merocyanines
Mc [X-m.n]
( X = 0 , S, Se) Figure 1. Structure of the long-chain merocyanine dyes used in this work.
I \
(a 1
W
a
I
44
i
- I
u W
LL 4
5'0 v)
Figure 2. Pressure-area isotherms for monolayers of long-chain merocyanine dyes: (a) Mc[X-18,1] (X = 0, S,Se) and (b) Mc[S-l8,n] (n = 1, 2, 3).
(Model 340) and a fluorometer (Model MPF-3), respectively.
Results and Discussion For the long-chain merocyanine dyes Mc[X-18,1] (X = 0, S, Se) and Mc[S-l8,n] (n = 1, 2, 3), the surface pressure-area isotherms of the monolayers on the water surface are shown in Figure 2, a and b, respectively. In all cases, the limiting areas Ad, obtained from extrapolation of the linear part to zero pressure, fall into the range of values of 60-70 A2/molecule, which suggests a chromophore orientation with'the long axis lying nearly flat to the surface and the short axis standing almost vertically.11J2 When the benzothiazol ring as the electron-donating (basic) group is linked to the rhodanine derivative by methine carbons, the monolayer is found tb be more condensed than the other cases. Furthermore, as the number of methylene carbons between the rhodanine ring and the carboxyl group increases, the A- and particularly the occupied area in the high pressure region become larger and the monolayer exhibits less compressibility with a higher collapsed pressure. These facts are considered to reflect the most closely packed arrangement of chromophores in the Mc[S-l8,1] monolayer. These monolayers can be transferred onto the solid surface by the Langmuir-Blodgett method, forming an alternating (Y-type) (12) Smith, D.L.Phorogr. Sci. Eng. 1974, 18, 309.
WAVELENGTH ( nm 1
Figure 3. Electronic spectra for built-up films (single layer) of Mc[X18,1] (X = 0, S,Se) deposited at 25 dyn/cm. The arrows (AX)indicate the band shifts from each chloroform solution to the film spectrum.
built-up film by themselves under a surface pressure of 25 dyn/cm. Figure 3 shows the electronic spectra of the multilayers for Mc[X-18,1] (X = 0,S, Se), where the arrows represent the band shifts, AX, from each chloroform solution to the film spectrum. As to features of the film spectrum of each dye alone, broadened bands with some shoulders are observed over the range of 450-650 nm. This is considered to be attributable to inhomogeneities in the films containing some kinds of aggregates of the chromophores. Nevertheless, the spectral shift Ahx is found to be most significant for the Mc[S-18,1] film with a benzothiazol ring in comparison with others. According to the fact that the spectral shifts can be related to the orderly array of the oriented chromophores induced by dipole-dipole intera~tion,'~ the benzothiazol group on the merocyanine dye may be favorable for a linear aggregate of electronic transition moments closely packed in the monolayer. This fact corresponds well to the result speculated from surface pressure-area isotherms. As pointed in the previous paper," by incorporating n-hexadecane with mixed monolayers of merocyanine dyes the more homogeneous films with well-controlled orientation and aggregation of the chromophore can be obtained. The inert components such as hexadecane are considered to be lubricants to enhance the stable molecular arrangement through compressing the spread monolayer. Figure 4 shows the electronic spectra of the built-up films obtained from the mixed monolayers of Mc[X-18,1] (X = 0, S, Se)/[methyl arachidate (MA)]/[hexadecane (HD)] (1:l:l molar ratio) 15-30 min after compression at 15 dyn/cm. In comparison with the spectra of the individual dye films without the mixed components of [MA] and [HD] (Figure 3), the absorption bands are sharpened and red shifts (AX) are somewhat extended. Also, in these spectra of the dye mixed films, the Mc[S-18,1]/ [MA]/[HD] monolayer is found to give the most significant red-shifted and sharpened band. Furthermore, in the series of dyes Mc[S-l8,n] (n = 1, 2, 3), mixed monolayers with the equimolecular amounts of [MA] and [HD], the sharpest band and the largest shift to longer wavelength are observed for the film spectrum of the Mc[S-18,1] containing only one methylene (1 3) McRae, E. G.; Kasha, M. In Physical Processes in Radiation Biology, Augenstein, L. G., Mason, R., Rosenberg, B., Eds.; Academic: New York, 1964; p 23.
Nakahara et al.
6146 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
a
4
0
6
0
Dielectric " t a n t
Figure 4. Electronic spectra for mixed monolayers of Mc[X-18,1] (X = 0, S,Se)/[methyl arachidate (CzoMe)]/[hexadecane](1:l:l molar ratio), deposited at 15 dyn/cm. The arrows (AX) indicate the band shifts from the chloroform solution to the film spectrum of each dye. Figure 6. Solvent effects on the absorption spectra of the merocyanine dye Mc[S-2,1].
Figure 5. Electronic spectra for the mixed monolayers of Mc[S-l8,n] (n = 1,2, 3)/[methyl arachidate (CzoMe)]/[hexadecane](1:l:l molar ratio), deposited at 15 dyn/cm. The arrows (AX) indicate the band shifts from the chloroform solution to the film spectrum of each dye.
carbon between the rhodanine and the carboxyl groups, as shown in Figure 5 . The increase of the number of methylene carbons which causes some diminuation in the amount of the red shifts seems to be unsuitable for dye aggregation with the closely packed chromophores. This is well consistent with the result of the pressure-area isotherms. By annealing the built-up film of Mc[S-l8,1]/[MA]/ [HD] (1:l:l) on a hot plate at 50 O C , the optical density of the redshifted band decreased by one-half and the shoulder band a t 500-545 nm was slightly enhanced. Recently, with the built-up film of the 6-methylmerocyanine derivative mixed with arachidic acid, Inoue has examined the absorption coefficients at temperatures between 79 and 270 K, which is expressed by the Urbach rule with weak exciton-phonon coupling, and also observed F-type
emission from the free exciton state.I4 Similar results should be expected with the film Mc[S-l8,l]/[MA]/[HD], though a low temperature spectrum of this film has not yet been measured. On the other hand, the preliminary behavior of the solvent effects on the absorption spectra has been examined for a short alkyl chain derivative of the merocyanine dye Mc[S-2,1]. By the use of dioxane-H20, ethanol-H20, and dioxaneethanol mixtures as solvents, small red shifts (longer wavelength) of the peak position (Amx) are observed and simultaneously the extinction coefficients (emax) are enhanced with an increase of the solvent polarity, as shown in Figure 6, a and b. According to Brooker's characterization with these patterns of solvent effects, this type of merocyanines appears to be classified as a weakly polar dye.15 The red-shifted band for the Mc[S-l8,l]/[MA]/[HD] mixed film, in any event, can be considered to result from a so-called J aggregation of the dyes. For a Mc[0-18,1] or Mc[Se-18,1] mixed film, the chromophores may form other linear aggregates together with somewhat smaller J aggregates. Thereupon, it is attributable to strong interactions of the electronic transition moments which are parallel to the long-axis of the chromophore in two-dimensional arrangements. Some possible models for J aggregates have been proposed by Kuhn and co-workers,'6 which utilized geometrical arrangements for dye molecules in close-packed monolayers. In accordance with their treatments, the molecules are replaced by extended dipoles of length 1 and charges +e and -e, and therefore, the interaction integral J12among these charges is obtained by
where D is the dielectric constant of medium ( D = 2.5 for hydrocarbons) and ri,the distance between two charges (Figure 7). The charge t and the dipole length 1 are assumed to be related (14) Inoue, T. Thin Solid Films 1985, 132, 21. (15) McRae, E. G. Spectrochim. Acta 1958, 12, 192. (16) Czikklely, V.; Fbrsterling, H. D.; Kuhn, H . Chem. Phys. Lett. 1970, 6, 207.
J Aggregates of Long-chain Merocyanines
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6147 Donor Mc[S-18, I) :C&le :C.H,r= I :I: I
/
aoioC AcceDfor
F m e 7. Calculation of J 1 2for an arrangement of two extended dipoles of charge c and length 1.
'-
0
ea, WAVELENGTH ( n m )
400
I 600
Figure 9. Absorption spectrum for the multilayer of a long-chain crystal violet derivative (acceptor) and fluorescence spectrum for a J aggregate of the merocyanine dye Mc[S-18,1] (donor).
I t/ t
D"-+A
(aggregate)
fl
-
.................../.
Id
0
Figure 8. Two-dimensionalarrangement of dye molecules with the long axis horizontally oriented and parallel to each other ( I = 9 A, d = s = 5 A); the calculated values EJI2for various arrangements are indicated. (p and q are the column and row numbers, respectively, CY is the angle between the transition moments and these centers.)
with the transition moment M by e l = M. Thus, the excitation energy AE'of a dye aggregate is represented with the excitation energy of the dye monomer, AE, and the interaction integral JI2 approximated by
AE'= AE
f 2CJ12
(2)
For the dye in a two-dimensional arrangement with the long axes horizontally oriented and parallel to each other, as shown in the insert of Figure 8, the values of J12can be calculated by assuming 1 = 9 A and also by estimating the distance between the adjacent transition moments at about d = s = 5 A, which corresponds to the diameter of the cross section of a hydrocarbon chain, from the result of the pressure-area isotherms and the transition moment M = 8.256 D from the solution spectrum for Mc[S-18,1]. Varying the numbers p and q of the column and the row for the arrangement of the transition moments, respectively, and the angle a between the transition moments and these centers gives the values of JI2that are depicted by the curves in Figure 8. These exhibit minima around a = 30°, corresponding to a brick stone work arrangement at which the red shifts of the bands are considered to be the most significant. The results of erg, for the fitting to the observed red shift, 2 X (-2.55 X dye Mc[S-18,1] film', have been obtained for q = 10 with p = 1, q = 5 with p = 2, and q = 3 with p = 3. Consequently, values of the farthest distance between the positive and the negative charges are 98,61, and 55 8,for (p,q) = (1, lo), (2, 5), and (3, 3), respectively. In any case, the aggregation number of the dye Mc[S-18,1] in the film falls between 9 and 10, which is somewhat larger than that obtained in an ethanol -water mixture by Mizutani et al." This fact suggests that the two-dimensional arrangement of the dye is of greater advantage than the bulk solution (17) Mitzutani, F.; Iijima, S.;Tsuda, K.Bull. Chem. SOC.Jpn. 1982, 55, 1295.
lo
Figure 10. Energy transfer from the J-aggregate sensitizer (D) to the acceptor (A) in the monolayer assemblies.
for producing a large size of the chromophore aggregates such a J type. As previously reported,' the built-up film of J-aggregating Mc[S-18,1]/ [MA]/ [HD] mixed monolayer exhibits the narrow fluorescence band a t 610 nm together with the characteristic absorption. A long-chain crystal violet derivative was used as an acceptor so that the absorption band has a sufficient overlap with the above fluorescence in the films, as shown in Figure 9, and the energy transfer from the J-type aggregate sensitizer (S) to the acceptor (A) monolayers has been examined. Figure 10 shows the fluorescence quenching as a function of the distance d between S and A, which was controlled by the insertion of cadmium arachidate multilayers. These data are approximately followed with the solid curve given by the following equation with do = 115 A: (3) do = a( ~ ) 2 L - 1 ( A , q ) 1 / n
where Ioand q are the intensity and quantum yield respectively of the S fluorescence in the absence of A, n is the refractive index of the medium, A, is the absorption of A, L is the length of the aggregate, and a is the orientation factor which is 61/2/(4.rr)2= 0.016 in the case of a statistical distribution of transition moments of the acceptor A in the layer plane.6 Equation 3 has been given for the energy transfer from a row of many dipoles, oscillating in phase, to a weakly absorbing acceptor layer a t a distance d.'* Using the values A , = 0.01, X = 610 nm, n = 1.5, and 0.05 I I0.3 for the merocyanine aggregate, the value 51 IL I130 has been obtained. This result seems to correspond nearly to the aggregation number described above. Furthermore, the Jaggregate sensitizer has been found to provide approximately the Coulomb field with the uncompensated charge of a chain of the
1
J . Phys. Chem. 1986, 90, 6148-6154
6148
dipole moments, as proposed by Kuhn.Is In conclusion, among the unsymmetrical merocyanine with one alkyl chain the dye Mc[S-18,1] containing the benzothiazol ring as an electron-donating group and one methylene carbon between the rhodanine ring and the carboxyl group forms a more condensed monolayer with smaller occupied area, which is more favorable to produce the J aggregates in the mixed monolayers with methyl arachidate and n-hexadecane than the other dyes. According to (18) Kuhn, H. J . Chem. Phys. 1970,53, 101.
the extended dipole model for the spectral properties of the resultant aggregate, the aggregation number has been estimated to be 9-10 in the twedimensional arrangement with the transition moments closely packed parallel to each other. In addition, the mechanism of the energy transfer from the aggregate sensitizer in the monolayer systems has appeared distinct from behaviors of the monomeric sensitizer. Registry No. Mc[0-18,1], 77404-31-0; Mc[S-18,1], 75983-37-8; Mc[Se-18,1], 77392-73-5; Mc[S-18,2], 87454-74-8; Mc[S-18,3], 104069-98-9; Mc[S-2,1], 25962-03-2; C2,,Me, 1120-28-1;Me(CH2)14Me, 544-76-3.
Reactlons of Gallium Atomst S. A. Mitchell,* P. A. Hackett,* D. M. Rayner, and M. Cantint Laser Chemistry Group, Division of Chemistry, National Research Council Canada, Ottawa, Ontario, Canada KIA OR6 (Received: April 14, 1986)
Ground-state Ga(4p' 2PI,2)atoms are produced by visible multiphoton dissociation of trimethylgallium and monitored by resonance fluorescence excitation in a pulsed laser photolysis-laser fluorescence arrangement. Reactions with CF3X (X = F, C1, Br, I), SF6, C2F4, N20,CzH2, C2H4, l-C4Hs, and Ga(CH& are studied under pseudo-first-order conditions in a gas cell at room temperature. Abstraction and association reactions are observed and characterized with respect to Ar buffer gas pressure dependence. For several of the association reactions an equilibration is observed between free Ga atoms and Ga atoms bound in complexes with the reactant molecules. From measured equilibrium constants and estimated partition functions, approximate gallium atom binding energies (kcal.mol-') are obtained for C2H4(9 f 2), l-C4Hs(9 2), and Ga(CH,), (14 iz 2). Bimolecular and termolecular rate constants are reported and discussed in relation to reaction products and mechanisms.
Introduction At present there is little information available on gas-phase chemical reactions of the heavier group 13 atoms Ga, In, and T1, although the lighter members B and AI have been more extensively investigated.' Atomic Al, Ga, and In are intermediates in a number of chemical vapor deposition processes with applications in microelectronics fabrications2 Condensed-phase reactions of A1 atoms and to a lesser extent Ga atoms have been investigated by low-temperature condensation techniques involving spectroscopic characterization of reaction These techniques are however not well suited for application to chemical kinetics. There appears to be no data available on rate coefficients for gallium atom reactions. An interesting feature of A1 atom chemistry is the occurrence of both a- and a-type complexes of A1 with unsaturated organic molecules. ESR studies of cryogenic matrix deposits have shown that a-complexes are formed with ethylene6 and benzene,' and AI-C u-bonded complexes with acetylene6 and buta- 1,3-dienee3 Similar studies have shown that both AIS and Ga4 atoms react with carbon monoxide to form a-complexes of stoichiometry M(C0)2. There is an indication from the ESR work that the singly occupied p-orbital of A1 or Ga (with configuration ns2npi) may become involved in a-bonding with unsaturated organic molecules in a manner analogous to that of the d-orbitals of certain transition metals, for example, in complexes with 01efins.~ There is however no indication from presently available results as to the stability of the A1 or Ga a-complexes. In this paper we present results on gas-phase reaction kinetics of ground-state Ga(42Pi/2) atoms with a variety of reactants including several olefins, acetylene, benzene, and carbon monoxide. For a number of these systems equilibration is observed between free Ga atoms and Ga atoms bound in complexes with the reactant, which allows estimates of the associated binding energies. For 'Issued as NRCC No. 25485. *Sherbrooke University co-op student, Sherbrwke, Quebec, Canada.
0022-3654/86/2090-6148$01.50/0
ethylene and 1-butene we find binding energies of -9 kcal-mol-'. Benzene and carbon monoxide were found to be unreactive with respect to complex formation with Ga under the conditions of our experiments, indicating considerably lower binding energies for these complexes. A number of rate constants are also reported, including termolecular rate constants for the association reactions and bimolecular rate constants for several abstraction reactions involving halogen and oxygen atom transfers. In our experiments ground-state Ga(42Pi 2) atoms are produced by visible multiphoton dissociation (MPDJ of Ga(CH3),. Measurements of the nascent-state distribution of gallium atoms1° and of cross sections for collisional relaxation of the metastable state" (1) (a) Tabacco, M. B.;Stanton, C. T.; Sardella, D. J.; Davidovits, P. J . Chem. Phys. 1985,II,5595. (b) Ishikawa, T.; Parson, J. M. J. Chem. Phys. 1983,79,4261. (c) Gole, J. L.; Pace, S.A. J . Phys. Chem. 1981,85,2651. (d) Rettner, C. T.; WBste, L.; Zare, R. N. Chem. Phys. 1981,58,371. (e) Schwenz, R. W.; Geiger, L. C.; Parson, J. M. J. Chem. Phys. 1981,74,1736. (0 Levy, M. R. Prog. React. Kinet. 1979,10, 1. (g) Fontijn, A.; Felder, W. In Reactive Intermediates in the Gas Phase; Setser, D. W., Ed.; Academic: New York, 1979; Chapter 2. (h) Klabunde, K. J. Chemistry ojFree Atoms and Particles; Academic: New York, 1980. (2) Ehrlich, D. J.; Tsao, J. Y. J. VUC.Sci. Techno/. B 1983, 1 , 969. (3) Chenier, J. H. B.;Howard, J. A.; Tse, J. S.; Mile, B. J . Am. Chem. SOC.1985,107,7290. (4) (a) Kasai, P. H.; Jones, P. M. J. Phys. Chem. 1985,89, 2019. (b) Howard, J. A.; Sutcliffe, R.; Hampson, C. A.; Mile, B. J . Phys. Chem. 1986, 90,4268. ( 5 ) (a) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1984,106,8018. (b) Chenier, J. H. B.; Hampson, C. A.; Howard, J. A.; Mile, B.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1524. (6) Kasai, P. H. J . Am. Chem. SOC.1982,104, 1165. (7) Kasai, P. H.; McLeod, D. Jr. J . Am. Chem. Soc. 1979,101,5860. (8) (a) Sonchik, S.M.; Andrews, L.; Carlson, K. D. J. Phys. Chem. 1983, 87,2004. (b) Tanaka, Y.;Davis, S. C.; Klabunde, K. J. J. Am. Chem. SOC. 1982,104, 1013. (c) Hauge, R. H.; Kauffman, J. W.; Margrave, J. L. J. Am. Chem. SOC.1980,101, 6005. (d) Zehe, M. J.; Lynch, D. A. Jr.; Kelsall, B. J.; Carlson, K. D. J. Phys. Chem. 1979,83,656.(e) Hinchcliffe, A. J.; Ogden, J. S.; Oswald, D. D. J. Chem. SOC.,Chem. Comm. 1972,338. (9) Hartley, F. R. Angew. Chem., Int. Ed. Engl. 1972, J J , 596. (10) Mitchell, S. A.; Hackett, P. A.; Rayner, D. M.; Humphries, M. R. J . Chem. Phys. 1985,83,5028.
Published 1986 bv the American Chemical Society
