J. Phys. Chem. 1982, 86, 2691-2695
2691
Site-Selected Fluorescence Study of Anthracene and Its Halogenated Derivatives in Shpol'skii Matrices Thomas P. Carter' and 0. D. Qililspie** DqhWtm6vIt of Chembtp', State University of New York at Albany, Albany, New York 12222 (Received: November 18, 1981;
I n Finel Farm: Februety 23, 1982)
Well-resolved fluorescence spectra have been measured in n-alkane Shpol'skii matrices for nine simply substituted anthracenes. Seven of the nine anthracene ap fundamentals (exclusive of C-H stretches) carry appreciable vibronic activity in the fluorescence. The remaining two, predicted to lie at 1007 and 1476 cm-' by normal coordinate analysis, are greatly intensified in 9,lO-dichloroanthracene (9,lO-DCA)and 9,lO-dibromoanthracene (9,lO-DBA). Thus, all of the anthracene % fundamental frequencies can now be assigned with confidence. In the two 9,lO-dihaloanthracenespectra there are also strong low-frequency bands which correspond to carbon-halogen stretching vibrations. From a simple Franck-Condon factor analysis we estimate 0.003 nm for the carbon-halogen bond length change upon electronic excitation. Evidence for S1-SBvibronic coupling and Fermi resonance in the So state are discussed. We find that symmetry breaking by chemical substitution has a fairly small effect on the fluorescence vibronic patterns.
Introduction Fluorescence spectroscopy offers certain advantages over the more usual techniques of infrared and Raman spectroscopy for the study of molecular vibrations. If the Sl-So transition is allowed, only the totally symmetric vibrations are strongly active in the fluorescence. Moreover, it is the normal coordinate displacements of the totally symmetric modes which relate So and S1 equilibrium geometries. Thus, it may be possible to deduce excited-state geometries from known ground-state geometries and Franck-Condon factors extracted from the fluorescence spectra. Systematic studies of the influence of chemical substitution on vibronic spectra are well-known for, inter alia, benzenes, naphthalenes, and azabenzenes. However, the extensive vibronic coupling characteristic of these molecules is a complication. We report here the fluorescence spectra of simply substituted anthracenes, in which vibronic coupling is relatively unimportant. The anthracene class is also of interest because of the opportunities to probe the effect of symmetry breaking on the fluorescence spectra. Surprisingly, the only anthracenes for which vibrationally well-resolved fluorescence spectra have been previously reported are anthracene, itself, and anthracene-&.'-" We have restudied these two parent compounds and, in addition, all three monochlorinated anthracenes, three symmetrically substituted meso derivatives (9,10-dichloro-, 9,10-dibromo-, and 9,lO-dimethylanthracenes), and 1,5dichloroanthracene. Here we present an overview of the work performed to date. Experimental Section Details of the experimental approach have been published previously.'2 Most spectra were acquired with a Spex Model 1802 l-m monochromator under conditions for which the resolution was not instrument limited. Some of the earlier work was done with a McKee-Pedersen Model 1018B 0.45-m monochromator with limiting resolution of ca. 0.08 nm. The narrowest features observed with the Spex are 0.04 nm wide (fwhm). For accurate analysis of the anthracene spectrum the output of a Fe-Ne hollow cathode lamp was directed 'Department of Chemistry, Iowa State University, Amee, IA. *Also Department of Physics, State University of New York at Albany.
collinear with the anthracene fluorescence into the Spex emhion monochromator. The wavelengths of the stronger vibronic features were determined to within fO.O1 nm by interpolation of their positions between the well-characterized Fe lines of the hollow cathode lamp. This wavelength uncertainty corresponds to an uncertainty of 0.6 cm-' in the wavenumbers (uncorrected to vacuum) of the anthracene bands. For the other spectra the emission mon&hromators were wavelength calibrated with the lines of a low-pressure Hg lamp. In these cases we estimate that vibrational intervals are uncertain by less than 5 cm-', a figure confirmed by combination band assignments.
Results and Discussion In Table I we summarize for each compound the n-alkane Shpol'skii solvent, the wavelength of the 0-0 band of the most prominent site used in the vibronic analysis, the molecular point group, and the number of vibrational modes (exclusive of C-H stretching vibrations) which transform as the totally symmetric irreducible representation for that point group. The investigation of Shpol'skii solvents was not exhaustive; once we had found a solvent which gave bandwidths of less than 0.1 nm for a given molecule, we generally did not test other solvents. However, we did observe the following: n-hexane and n-heptane gave essentially equally narrow bands for anthracene, with slightly poorer resolution in n-octane. Octane is superior to either hexane or heptane for 2chloroanthracene, whereas n-heptane is inferior to n-hexane for the meso-substituted anthracenes. The 1 2 K site-selected fluorescence spectrum of anthracene in n-heptane is shown in Figure 1. The band(1)J. Sidman, J. Chem. Phys., 26, 115 (1956). (2)A. V. Bree and S. Katagiri, J. Mol. Spectrosc., 17,24 (1965). (3)T.N. Bolotnikova, L. A. Klimova, G. N. Neresova, and L. F. Utkina, Opt. Spectrosc. (Engl. Trawl.),21, 420 (1966). (4) A. V.Bree, S. Katagiri, and S.R. Suart, J. Chem. Phys., 44,1788 (19fiRl. \--_-,.
(5)R.H.Clarke, J. Chem. Phys., 62, 2328 (1970). (6)R. M. Macnab and K. Sauer, J. Chem. Phys., 63, 2805 (1970). (7)L. E.Lyons and L. J. Warren, Aust. J. Chem., 26, 1411 (1972). (8)J. J. Dekkers, G. Ph. Hoornweg, C. Maclean, and N. H. Velthorst, Chem. Phys., 6,393 (1974). (9) J. Ferguson and A. W.-H. Mau, Mol. Phys., 28, 469 (1974). (10)N. Kesri and R. Ostertag, Chen. Phys. Lett., 39, 431 (1976). (11)G. J. Small,J. Chem. Phys., 62,656(1970). (12)T.P.Carter,G. D. Gillispie, and M. A. Connolly,J. Phys. Chem., 86, 192 (1982).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
Carter and Giilispie
TABLE I: General Information for Anthracene Fluorescence Studies no. of
totally molecular symmetric point group fundamentalsC
Shpol'skii wavelength of solvent major site, nm anthracene (A) n-heptane 381.3 Dzh 9 n-heptane 380.4 D,h 9 anthracene-d,, ( A d , , ) 1-chloroanthracene(1-CA) n-heptane 383.6 C8 36 2-chloroanthracene (2-CA) n-octane 386.2 c, 36 9-chloroanthracene (9-CA) n-hexane 390.6 CZ" 18 1,5-dichloroanthracene(1,B-DCA) n-hexane 390.2 Czh 19 403.7 D2h 10 9,lO-dichloroanthracene(9,IO-DCA) n-hexane 9,lO-dibromoanthracene(9,lO-DBA) n-hexane 405.6 Dzhb 10 399.3 Dzh 1O b 9,lO-dimethylanthracene(9,lO-DMeA) n-hexane Abbreviations given in parentheses. The influence of the methyl protons has been ignored. Excluding C-H stretching vibrations. moleculea
.-.i W 0 0 h
VI:!
405 Figure 1. Site-selected fluorescence spectrum of anthracene in nheptane at 12 K. The origin band is labeled by its wavenumber in air and the other bands are labeled by their wavenumber displacements from the o w n . This spectrum was measured with the Spex 1802 emission monochromator.
widths of the individual vibronic features are approximately 3 cm-' (fwhm). Within the limitations of the Born-Oppenheimer and Condon approximations, only totally symmetric vibrational modes will be active in the low-temperature fluorescence spectra. If D% symmetry holds for both the So and S1states, there are 12 ag modes. Three of these correspond to unobserved C-H stretching vibrations, leaving nine fundamentals expected in the region less than 1700 cm-' from the origin. The bands at 394.4,627, 759.4, 1163.3, 1257.3, ca. 1405, and 1566.1cm-' are readily assigned as % fundamentals, based on infrared13J4 and Raman'*-17 studies and normal coordinate analyses.1sm These valuea are also in good agreement with previous luminescence s t ~ d i e s ' ~for~ a~ variety ~ ~ ~ ~ofJ ~ different hosts. The predictions for the remaining ag fundamental frequencies by the accurate normal coordinate analysis of Ohno20are 1007 and 1476 cm-'. Several g r ~ u p s ~have ~ ~ ~previously ~J' observed a weak luminescence band within the range 1007-1027 cm-', as we do also at (13)S.Califano, J. Chem. Phys., 36,903 (1962). and J.-M. Lebas, Spectrochim. Acta, Part A , 27, 1315,1325 (1971). (15)N. Abasbegovic,N.Vukotic, and L. Colombo, J. Chem. Phys., 41, 2575 (1964). (16)M. Suzuki, T.Yokoyama, and M. Ito, Spectrochim. Acta, Part A, 24, 1091 (1968). (17) J. Rasanen, F. Stenman, and E. Penttinen, Spectrochim. Acta, Part A , 29, 395 (1973). (18)E. P. Krainov, Opt. Spectrosc. (Engl. Transl.),16, 532 (1964). (19) N. Neb, M. Scrocco,and S. Califano, Spectrochim. Acta, 22, 1981 (1966). (20)K.Ohno, J . Mol. Spectrosc., 72, 238 (1978). (14) M. Brigodiot
41 5
h (NM)
425
435
Figure 2. Site-selected fluorescence spectrum of 9,1Odibromoanthracene in n-hexane at 12 K. Bands are labeled in the same fashion as for Figure 1. The spectrum was also taken with the Spex monochromator.
1015 cm-l. No luminescence spectrum has revealed the 1476-cm-' fundamental. Ohno's predictions for the two missing 88 fundamentals are nicely confirmed by the fluorescence spectrum of 9,lO-dibromoanthracene (9,lO-DBA) in n-hexane shown in Figure 2. In 9,lO-DBA there are 10 ag vibrations (exclusive of C-H stretches) since a C-H stretching mode of anthracene is replaced by a low-frequency C-Br stretching mode by the meso substitution. Bands in the experimental spectrum at 404,651,1178,1262,1392, and 1548 cm-l are assigned to the % symmetry class by analogy with anthracene. The features at 1030 and 1475 cm-' correspond very closely in frequency to the "missing" ag modes of anthracene and are therefore also assigned to this symmetry class. The C-Br stretching vibration is associated with the strong band at 207 cm-' and the tenth, and final, ag mode is not observed. To check these assignments we have performed a normal coordinate analysis for 9,lO-DBA under the assumption of transferability of the anthracene force field constructed by Net0 et al.19 The root mean square error for the nine % fundamental assignments made in the paragraph above is 16 cm-l, comparable to the accuracy found for anthracene. The unobserved mode has a calculated frequency of 846 cm-'. The fluorescence spectrum of 9,lO-dichloroanthracene (9,lO-DCA) is almost identical with that of 9,lO-DBA except that the 207-cm-' band is replaced by one at 321 cm-'. For the other eight observed % modes, the frequencies in 9,lO-DBA and 9,lO-DCA differ by no more than 10 cm-'. The frequency ratio of the 321-cm-' band in 9,lO-DCA to the 207-cm-' band in 9,lO-DBA is nearly equal to ( m ~ ~ /
SRaSelected Fluorescence Study of Anthracene
The Journal of Physical Chemisfry, Vol. 86, No. 14, 1982 2693
TABLE 11: Vibrational Analysis of Anthracene-h lo Fluorescence in n-Heptane A
381 2.89 3871.11 3906.2 3926.59 3931.11 3966.4 3967.6 3989.86 3993.39 4000.94 4004.89 4006.55 4019.5 4026.2 4028.25 4029.11 4030.2 4032.0 4048.0 4053.70 4055.04 4064.6 4067.03 4069.0 4070.7 4093.39 4094.08 4095.4 4119.47 4 1 20.94 4133.54
-
v , cm-I
AT, cm''
26 226.8 25 832.4 25 6 0 0 25 467.4 25 4 3 8 . 1 25 212 25 204 25 063.5 25 041.4 24 994.1 25 969.5 25 959.2 24 8 7 9 24 837 24 824.7 24 819.4 24 8 1 3 24 8 0 1 24 704 24 6 6 8 . 8 24 660.7 24 6 0 3 24 558.0 24 576 24 566 24 429.6 24 425.5 24 4 1 8 24 274.9 24 266.3 24 192.4
394.4 627 759.4 788.7 1015 1023 1163.3 1185.3 1232.7 1257.3 1267.6 1348 1389 1402.1 1407.4 1414 1426 1523 1558.0 1566.1 1624 1638.8 1651 1661 1797.2 1801.3 1809 1951.9 1960.5 2034.4
'
assignment
4elative
300-400 100 6 7 14 2 2 35 6 4 19 20 3 4
7
255
5 15 52 3 24 3 3 15 14 6 3 12 6
0-0 band 394.4 (a,) 627 (a ) 759.4 fag) 2 x 394.4 ( - 0 . 1 ) 1 0 1 5 (a,) 394.4 + 6 2 7 ( + 2 ) 1163.3 (a,) 1185.3 (big) ? 1257.3 (a,) 1267.6 (b,,?) ?
a, fundamental split by Fermi resonance? ?
394.4 + 1163.3 ( + 0 . 3 ) 1566.1 (a,) 394.4 + 1232.7 (-3)? 1638.8 (b,,) 394.4 + 1257.3 (-1) 394.4 + 1267.6 (-1) 394.4 + 1402.1 ( + 0 . 7 ) 394.4 + 1407.4 ( - 0 . 5 ) 394.4 + 1 4 1 4 (+l) 2 X 394.4 + 1163.3 (-0.2) 394.4 + 1566.1 (0.0) 394.4 + 1638.8 ( + 1 . 2 )
Bands for which the wavelength is given t o only 0.1 A were too weak to measure accurately in the Fe-Ne hollow cathode calibration. Wavelengths are n o t corrected t o vacuum values.
Q ~ ) ' / ~which , shows the predominately carbon-halogen stretch character of these vibrations. Whereas ag fundamentals (and overtone and combination bands built from them) provide an entirely satisfactory analysis of the 9,lO-DBA and 9,lO-DCA spectra, the anthracene fluorescence does not admit such a simple interpretation. In particular, the doublet at 1257/ 1267 cm-', the triplet of bands at 1402/1407/1414 cm-', and a fairly strong fundamental at 1639 cm-' must still be explained. However, we choose to defer this discussion until after the presentation of spectra for the other substituted anthracenes. Owing to the large number of compounds studied, the data have been condensed for easy visualization. In the schematic spectra plotted in Figure 3 obvious combination bands and overtones have been eliminated for clarity. The "intensities" are actually relative peak heights with no correction for any variations in the individual vibronic bandwidths except for a simple deconvolution of obviously overlapping bands. The 0-0 bands are not shown in Figure 3; even at concentrations as low as 1p M there can still be significant reabsorption of the origin band. From a few experiments in which the concentration was as low as 0.1 pM,we estimate that the (to band is typically 3 times as strong as the most intense vibronic bands at ca. 400 and 1400 cm-'. The overall picture which emerges from Figure 3 is that simple chemical substitution of anthracene does not change the fluorescence vibronic pattern in any drastic way. In the region 1100-1700 cm-' from the origin are to be found the ring-stretching vibrations and the higher-frequency in-plane C-H bends. Characteristic bands at ca. 1170, 1260,1400,1550,1630 cm-l can easily be traced through the entire series. To be sure, there are various band splittings and substantial redistribution of intensity among these fundamentals. In particular, the bands at ca. 1260,
1400, and 1550 cm-l are of nearly equal intensity in the 9,lO-disubstituted compounds, whereas 1400 cm-' is much the strongest for the other derivatives. The extreme case is 1-chloroanthracene, where all other bands in the ringstretching region are very much weaker than the one at 1404 cm-l. There is more of a variation in the region less than 500 cm-l from the origin. For those molecules of Czuor higher symmetry there is a strong fundamental band at 385 f 20 cm-l. In the less symmetric 1,5-dichloroanthracene (C, symmetry) and in 1-chloro- and 2-chloroanthracenes (both C8),this single band is apparently split into two or more. We note that the summed intensity of the split bands, as well as the "center of gravity" of their intensity distribution, is similar to the intensity and position of the single band in the more symmetric compounds. Finally, in the region between 500 and 1100 cm-' from the origin there are also variations in the vibronic patterns. However, this is a region of low vibronic intensity in general. Let us now consider the extra features in the anthracene spectrum. The vibrational analysis given in Table I1 eliminates combination bands as a possible explanation. The 1638.8-cm-' band has been observed in several previous high-resolution spectra and is usually assigned as a blg fundamental.21 The most likely source of non-a vibronic activity is Herzberg-Teller coupling of the S1 lLJ state with the S3 ('BsU,lBb) state responsible for the intense ultraviolet absorbance near 250 nm.= By referring
(bk,
(21) The bl, vibronic activity can also be seen in low-resolution photoselection polarization experiments,e.g.,see D. M. Friedrich, R. Mathies, and A. C. Albrecht, J. Mol. Spectrosc., 61, 166 (1974). (22) Note that we are using the axis labeling convention traditionalto polyacene electronic spectroscopy (X is long in-plane axis, Y is short in-plane axis). If the X and 2 axes are reversed, as is often done by IR and Raman workers, then the bl (B,)and b3 (B3) labels should also be interchanged.
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The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
Carter and Gillispie
1
I
I
I I
I
Ill
I
t
t 2
4
6
8
-
9, IO-DBA
10 12 Al02,
li
16
Flgwe 4. Schematic fluorescence spectra for anthracene-h (middle panel) with site origin at 26 227 cm-l, anthracenad,, (upper panel) with site origin at 26 291 cm-I, and anthracenah (lower panel) with site origin at 26619 cm-l. The three spectra have been normalized to the same intensity for the strong band near 400 cm-'.
the 1162-cm-' dloband corresponds to one or both members of the hlo doublet at 1257/1267 cm-'. These isotope frequency shifts are reproduced very well by the normal mode analysis of Ohno.20 All of the other fluorescence active modes are shifted to a much lesser degree upon perdeuteration of the molecule. The probable sources of the extra bands are vibronic Figure 3. Schematic fluorescence vibronic patterns for the anthracenes of this study. Obvious combination and overtone bands have coupling and Fermi resonance. Presumably, the former been omltted for purposes of clarity. Molecule abbreviations and the mechanism would involve S,-S3 coupling so that bl, fun0-0 wavelengths are gben in Table I. The abscissa represents damentals would be generated, as is the case for the wavenumber displacement from the origin band. 1639-cm-l band. In contrast, the Fermi resonance mechanism results when combination or overtone levels of ag to Figure 3, one sees that the vibronic coupling is also vibrational symmetry borrow intensity from nearby zeroperative in the substituted anthracenes, as signified by oth-order % fundamentals. In the absence of polarization a fundamental within the range 1610-1645 cm-l. measurements to establish the symmetries, analysis and The doublet at 1257/1267 cm-l and the multiplet around interpretation beyond this point are speculative. Unfor1400 cm-' are consistently found in anthracene-hlo tunately, the mixed-crystal studies1,2J1reveal a much lower fluorescence spectra. We have observed them in n-heptane degree of polarization for the fluorescencethan is predicted solvent (two sites), in n-hexane, and in n-octane (two sites). by the oriented gas model and are inconclusive. They are also present in the phosphorescence spectra From the substitution studies summarized in Figure 3, measured in n-heptaneB and in a phenazine mixed cry~tal.~ it is clear that 1400 cm-l is always the strongest (or nearly Similar features can be seen in Small's spectra taken in so) vibronic fluorescence band. In anthracene this intena p-Grphenyl host.'l In consideration of the wide variety sity is distributed over three or more bands, which results of environments which admit these extra bands, we conin an apparent reduction in intensity. Such an intensity clude that they are intrinsic to anthracene and probably redistribution is consistent with a Fermi resonance not induced by mechanisms such as a reduction in site mechanism. Other evidence supporting Fermi resonance symmetry of the guest molecule. is the fact that the 1400-cm-l multiplet structure is nearly On the other hand, there are some variations in the identical in the phosphorescence in n-heptane even though multiplet structure intensity distributions from one enthe overall vibronic pattern changes considerably. The vironment to another. A few examples are shown in Figure most likely candidate for a blg mode induced by vibronic 4. The middle panel is constructed from the spectrum coupling would be the vibration of frequency 1376 cm-' of Figure 1,the upper panel is the schematic spectrum of found in the Raman spectrum of crystalline anthracene." the corresponding site in anthracene-dlo, and the lower We believe, however, that the weak fluorescence feature panel is the spectrum of anthracene-hlo, but now for the at 1389 cm-' most probably represents this b,, mode. site whose origin is 26 619 cm-'. The individual vibronic Both Fermi resonance and vibronic coupling mechaband intensities have been scaled for each spectrum to give nisms have been proposed for the 1257/1267-cmT1doublet. the same relative intensity for the lowest-frequency ag The Raman experimental datal7 and normal coordinate fundamental. In most cases the relative intensities of calculations are in agreement that there is a b,, fundacorresponding vibronic bands in the three spectra are quite mental about 15 cm-l higher in frequency than an a, similar. With regard to the anthracene-& spectrum, we fundamental near 1260 cm-l. However, there is no obvious note that the 839-cm-I feature is the analogue of the counterpart in the anthracene-dlofluorescence spectrum 1163-cm-' C-H in-plane bend of anthracene-h,,. Similarly, to a 1267-cm-' blg mode of anthracene-h,,. The Fermi resonance assignment of Clarke5is that the overtone of the (23) G. D. Gillispie, unpublished work from Wayne State University, Detroit, MI, 1977. 627-cm-l% fundamental gains intensity by interaction with 9,lO-DMeA
J. Phys. Chem. 1982, 86, 2695-2698
2895
a 1269-cm-' % fundamental. We would point out here that, unless the 627-cm-' mode has a large positive anharmonicity (which seems unlikely), at least one of the Fermi split bands would be lower in frequency than 2 X 627 = 1254 cm-'. However, there are many other 88 overtones and combination leveh of approximately the correct energy to enter into Fermi mixing with the fundamental whose zero-order energy is near 1260 cm Of the remaining unassigned weak bands, the one at 1185 cm-' is assigned as bl, as indicated by the Raman data.17 Three other bands a t 1233,1348, and 1523 cm-' are consistently observed in all three Shpol'skii solvents, but the frequencies do not fit well with either an a, combination band or bl, vibronically induced fundamental interpretation. Given the uncertainty in the preceding analysis, there is little point in further speculating about these minor features at this time.
have only a small effect on the number of fluorescence active vibrations. The evidence for S1-S3 vibronic coupling and So Fermi resonance in the parent compound has been discussed, although further work will be required for a more definitive analysis. We are currently performing Franck-Condon factor calculations using the experimental fluorescence intensities to determine SIequilibrium geometries. If the intensity of the carbon-halogen stretches in 9,lO-DCA and 9,lO-DBA is assumed to depend only on the carbon-halogen bond length change upon electronic excitation, a bond length change of 0.003 nm is inferred. Preliminary studies of the fluorescence spectra of amino- and nitroanthracenes are in progress. These substituents, and others such as CN, OH,, and OCH,, are of interest because they can be expected to exert more substantial perturbations on the electronic and vibronic structure.
Summary and Conclusions The influence of chemical substitution on fluorescence vibronic patterns has been systematically studied in nine simply substituted anthracenes. All nine totally symmetric fundamental vibrational frequencies of anthracene (exclusive of C-H stretches) have been assigned, and the results are in agreement with Raman data.17 Symmetry breaking by the chemical substitution has been shown to
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the Research Foundation of SUNY for additional support. The closed cycle refrigerator used in this work was purchased with funds provided by the Research Corp. We thank Elizabeth Dennison, who performed the normal coordinate calculations.
3.
Reaction of Tetrathiafulvaiene with Haloalkanes Behnam Vesoal and John 0. Miller' Department of Chembtty and Laboratory for Research on the Structure of Matter, Universky of Pennsylvanla, Phlladelphie, Pennsylvania 19 104 (Received November 25, 198 1)
A kinetic study of the photochemical reaction of TTF with haloalkanes has been made. The results are largely in agreement with a mechanism suggested by Scott and cO-workers, but some additional findings of fundamental importance were obtained. An improved photosynthesis of TTFClo,68is reported.
Introduction A strong interest has been shown in the cation radical complexes formed by tetrathiafulvalene (TTF) with halogen acceptors,' especially since these salts have high electron conductivity in the crystalline state2 Scott and co-workers3discovered that TTF and several of its derivatives undergo photooxidation in solutions containing halocarbons, producing mixed-valence salts, TI'FT,where y lies in a narrow range, e.g., 0.68 f 0.03 for X = C1. Later: it was found that this photochemical procedure was the best for preparing mFCb.68. The IBM group suggested3that the mechanism of that photooxidation involved excitation of the charge-transfer (1) Wudl, F.; Smith, G. M.; Hufnagel, E. J. Chem. C O M ~ U1970, ~. 1453-4. (2) Wudl, F.; Wobschall, D.; Hufnagel, E. J. J. Am. Chem. SOC.1972, 94,670-2. (3) Scott, B. A.; Kaufman, F. B.; Engler, E. M. J . Am. Chem. SOC. 1976,98,4342-4. (4) Scott, B. A.; LaPlaca, S. J.; Torrance, J. B.; Silverman, B. D.; Welber, B. J . Am. Chem. SOC.1977,99,6631-9. 0022-365418212088-2695$01.25/0
(CT) complex expected to form between TTF and a haloalkane such as CCl,, followed by decomposition of the complex into the radical cation which would then combine with excess TTF to form the mixed-valence salt. Their evidence for that mechanism was limited to the behavior of the CT absorption band at different concentrations of CCll as acceptor. As they pointed out, detailed kinetic study would be required to establish the process. They proposed the mechanism in analogy to the mechanism proposed earlier for photochemical CT reactions in halocarbon solutions for ferrocene6i6and, in this laboratory, for alkylamines.' We have made a kinetic study of the photooxidation of TTF in solutions of CCl,, CHCl,, and C2C4. The results are shown to be largely in accord with the prediction of Scott and co-workers3but with some additional findings of fundamental importance. Our work has also led to an (5) Brand, J. C. D.; Sneddon, W. Trans. Faraday SOC.1957, 53, 894-900. (6) Traverso, 0.;Scandola, F. Znorg. Chim. Acta 1970, 4, 493-8. (7) Biaselle, C. J.; Miller, J. G. J . Am. Chem. SOC.1974, 96,3813-6.
0 1982 American Chemical Society
