3188
J . Phys. Chem. 1990, 94, 3188-3192
mation on D values, but there is controversy on the interpretation of data regarding L values. Using accepted D values one arrives at the rather unlikely result of L < D. A wav to eive meaning to such result is to Drowse semiflexibleIg long ro& f i r N, phase; (and large semiflex'ible disks for Nd phases). X-ray diffraction result^^^^ show that the diffraction associated with distances in the L direction is much weaker and broader than diffraction associated with distances in the D direction. So an alternative to the more generally accepted view (19) Khokhlov. A. R.; Semenov. A. N. Physica 1982, IIZA, 605.
of small micelles with small anisotropies would be a picture of semiflexible micelles made up either of large defective micelles or of interconnected small micelles. Such a picture is in agreement with the alternative interoretation of X-rav data.20*21 Acknowledgment. Thanks are due to Dr. T. R. Taylor for discussions. (20) Amaral, L. Q.; Helene, M. E. M.; Bittencourt, D. R.; Itri, R. J . Phys. Chem. 1987, 91, 5949. (21) Amaral, L. Q.; Marcondes Helene, M. E. J . Phys. Chem. 1988, 92, 6094.
Radlolytic Generation of Radical Cations in Xenon Matrices. Tetramethylcyciopropane Radical Cation and Its Transformations X.-Z. Qin and A. D. Trifunac* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: August 8, 1989; In Final Form: October 30, 1989)
Radiolytic generation of radical cations in xenon matrices containing electron scavengers is illustrated by studying the 1,1,2,2-tetramethylcyclopropaneradical cation. Dilute and concentrated solutions of tetramethylcyclopropane in xenon without electron scavengers and neat tetramethylcyclopropaneyielded neutral radicals upon y-irradiation. Speculationon the mechanisms of radical formation is presented. The radical species observed in the y-irradiation of neat tetramethylcyclopropane appears to be identical with the paramagnetic species observed in CF2CICFCI2above 120 K, suggesting that a neutral radical rather than the ring-opened distonic radical cation is observed in the CF2CICFC12matrix.
Introduction
Formed by one-electron loss from the corresponding neutral compounds, radical cations are important chemical intermediates in the chemistry induced by ionizing or photoionizing radiation and in various electron- and charge-transfer processes. Radical cations are usually highly reactive and short-lived. The development of matrix-isolation methods in the past decade has allowed EPR spectroscopy to be used to study these intermediates.' The principle of these methods is to trap radical cations in different matrix cages separated by a large number of matrix atoms or molecules at low temperatures, hence slowing down and/or preventing their reactions. By use of the method pioneered by Shida and Kat0 in 1979,2 many organic radical cations have been studied by EPR. In this method, radical cations are generated by y-irradiation of a halocarbon solvent containing a small amount of substrate.la< The temperature range for halocarbon solvents is from 2 to 160 K. However, these matrices often show strong interaction with the substrate radical cations.lb&The chemistry as well as the structure of radical cations can be dependent on such matrix interaction. In 1982, a more inert neon matrix was employed for the EPR investigation of small radical cations by Knight.Id The technique involves photoionization of substrates in the gas phase during the process of deposition on a neon matrix at 4 K. The temperature range for neon is, however, limited (2-10 K). In halocarbon matrices the substrate radical cations are formed by hole transfer from the solvent cation to the substrate, while in the neon matrix the substrate molecules are probably directly photoionized at the site of deposition. Due to the large ionization potential difference between the rare gases (IPnmn= 21.6 eV) and most organic compounds (IPS 11 eV), the exothermicity of the hole-transfer process in a rare gas matrix may cause rearrangement or frag( I ) (a) Shida, T.; Haselbach, E.; Bally, T. Arc. Chem. Res. 1984, 17, 180. (b) Symons, M. C. R. Chem. SOC.Rev. 1984, 393. (c) Shiotani, M. Magn. Reson. Rev. 1987, 12, 333. (d) Knight, L. B. Acc. Chem. Res. 1986, 19, 313. (2) Shida, T.; Kato, K. Chem. Phys. Lerr. 1979, 68, 106.
0022-3654/90/2094-3 188$02.50/0
mentation of substrate radical cation^.^ At this time, only a few transition-metal carbonyl cations have been generated by y-irradiation and studied by EPR in the krypton m a t r i ~ . ~ The heaviest rare gas, xenon, has the smallest ionization potential of 12.1 eV. It is thus expected that the fragmentation of radical cations during the hole-transfer process in this matrix should be least likely. Surprisingly, no radical cation study in xenon has appeared in the literature so far. Here, we report the first EPR study of a radical cation in xenon. The 1,1,2,2-tetramethylcyclopropane (TMCP) radical cation was generated by y-irradiation of a xenon matrix containing the parent compound and a small amount of electron scavengers. A mechanism of its formation in xenon is proposed. The radical cation of TMCP has been studied in several freon mat rice^.^.^ Both the ring-closed and ring-opened radical cation of this compound have been reported in a CF2CICFC12m a t r i ~ . ~ The ring-closed cation (1) is characterized by an elongated one-electron bond between the two gem-dimethyl-substituted carbon atoms, while the ring-opened cation (2) is suggested as an orthogonal distonic species (CH3)2CCH2C+(CH3)2 with the spin confined on one dimethyl-substituted carbon.
1
2
(3) (a) Bally, T.; Roth, K.; Straub, R. Helv. Chim. Acra 1989, 72, 73. (b) Bally, T.;Haselbach, E.; Nitsche, S.; Roth, K. Tetrahedron 1986, 42, 6325. (4) (a) Morton, J. R.; Preston, K. F.; Strach, S . J.; Adrian, F. J.; Jette, A. N. J . Chem. Phys. 1979, 70, 2889. (b) Morton, J. R.; Preston, K. F. Organometallics 1984, 3, 1386. (5) Qin, X.-2.: Snow, L. D.; Williams, F. J . Am. Chem. SOC.1984, 106, 7640.
0 1990 American Chemical Societv
Generation of Radical Cations in Xe Matrices
In the subsequent study of the parent cyclopropane in CF2CICFCI,, the paramagnetic species observed was assigned to the distonic trimethylene radical The formation of these distonic ions is attributed to a special solvent effect of CF2CICFC12. However, this was questioned by Symons,8 who suggested that the ring opening of the cyclopropane radical cation is due to its complexation with a freon molecule. The problem has also attracted theoretical investigations, and several ab initio calculations at different levels9-" suggest that the preferred ring opening of the cyclopropane radical cation occurs by either a special solvent effect of CF2CICFCI2or the complexation of the cation with a C FzCICFCl, molecule. The present study sheds more light on this problem. In this work, only the ring-closed radical cation of TMCP (1) is observed in the y-irradiated xenon matrix containing electron scavengers within the accessible temperature range (40-105 K). The ring opening of TMCP was observed by y-irradiation of xenon containing higher concentrations of TMCP in the absence of.electron scavengers, where the localized radical species ((CH,),CR) was observed. This species has a different conformation at the radical center from that of 2 and is assigned to a neutral radical rather than a distonic radical cation. Furthermore, the y-irradiation of pure TMCP gave a localized neutral radical, which shows the same hyperfine coupling constants as the presumed radical cation 2. On the basis of these results, it is suggested that a neutral radical is formed by ion-molecule reactions in the CF2CICFCI2matrix. Experimental Section Xenon (99.999%) is purchased from MSD Isotope at natural isotopic abundance. It was purified by passing through a column of 5A molecular sieve and then by a bulb-to-bulb distillation on the vacuum line. Tetramethylcyclopropane (99%) from Wiley and CH2Clz and CF2CICFCI2 from Aldrich were all used as purchased. The substrate (2 vol W ) , electron scavenger (2 vol %), and xenon were mixed in the gas phase and then condensed at 7 7 K into a 2.5-mm-0.d. Suprasil sample tube. The sample was immersed in an isobutyl alcohol slush bath at 165 K, allowing the solution to stay in liquid form for I / * h to assure thorough mixing. The sample was then irradiated by y-rays from a @Co source a t 77 K for a dose of 0.5 Mrad. For EPR studies a Varian E-IO9 spectrometer interfaced with an Apple IIe computer for recording the spectra was employed. EPR studies were carried out from 4 to 110 K using an AirProducts Heli-Tran cryostat. Results Xenon Solution with Electron Scavengers. The TMCP radical cation was generated radiolytically in xenon in the presence of electron scavengers, CF2C1CFCI2 and CH2CI2. The y-irradiation of a xenon solution containing TMCP (2 vol %) and CF2ClCFClz (2 vol %) at 77 K gave rise to an EPR spectrum ( g = 2.0030 f 0.0002) shown in Figure la. Eleven equally spaced lines can be recognized, although the central lines are distorted by quartz signals from the sample tube and unidentified signals from the solvent. As shown in Figure IC, this multiline spectrum can be satisfactorily simulated by the known = 14.9 G and aZH= 18.7 G) hyperfine coupling constants ( a 1 2 H for the ring-closed TMCP radical cation (TMCP'+), l.5 In both simulated and experimental spectra, the expected wing lines are too weak to be observed. The spectrum in Figure l a can, therefore, be confidently assigned to 1. The y-irradiation of a xenon solution containing TMCP (2 vol %) and CH2C12(2 vol %) at 7 7 K gave essentially the same EPR (6) Qin, X.-2.; Williams, F. Tetrahedron 1986, 42, 6301. (7) (a) Qin, X.-Z.; Williams, F. Chem. Phys. Lett. 1984, 112, 79. (b) Qin, X.-Z.; Snow, L. D.; Williams, F. Chem. Phys. Lett. 1985, 117, 383. (8) Symons, M. C. R. Chem. Phys. Lett. 1985, 117, 381. (9) Wayner, D. D. M.; Boyd, R. J.; Arnold, D. R. Can. J . Chem. 1985, 63, 3283. (IO) Hrovat, D. A.; Du, P.; Borden, W. T. Chem. Phys. Left. 1986, 123, 337. ( 1 1 ) Lunell, S.; Yin, L.; Huang, M.-B. Chem. Phys. 1989, 139, 293
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3189
, 50G
A
b
A
I
111 \I
H-
Figure 1. EPR spectra obtained from y-irradiated xenon solutions of tetramethylcyclopropane containing (a) CF2CICFCI2and (b) CH2C12at 77 K, and (c) computer-simulated spectrum using 0 2 H = 18.7 G and 0 , 2 H = 14.9 G and a Lorentzian line width of 4.0 G. i
,
50G
I
H---*
Figure 2. (a) EPR spectrum recorded at 40 K after y-irradiation of a xenon solution containing tetramethylcyclopropaneand CF2CICFCI2at 77 K. (b) Computer-simulatedspectrum using 0 2 H = 18.7 G and ugH= 22.4 G and a Lorentzian line width of 5.0 G. The arrows in (a) point to the quartz signals. spectrum ( g = 2.0030 f 0.0002), shown in Figure Ib. When CHzC12 was used as the electron scavenger, the signal from TMCP'+ was more intense; therefore, the distortion in the central region of the spectrum is less in Figure l b than in Figure la. Hence, Figure l b is also assigned to 1. Radical cation 1 persisted in xenon to about 100 K. But the EPR spectra at elevated temperatures are not much changed from that at 77 K. In other words, well-resolved spectra as obtained in freon matrices at higher temperatures ( > I 10 K) were not observed in xenon. A careful temperature study reveals information about the conformational change of the methyl groups of 1 below 7 7 K. In the experiment using CF,ClCFCI, as electron scavenger, when the sample was cooled down, Figure l a reversibly and gradually changed to Figure 2a recorded at 40 K. In Figure 2a, the central line is distorted by the strong quartz signals, while the right side showing five peaks is somewhat more intense than the left side showing four peaks. The outermost peak at the left side is buried within the noise. Therefore, Figure 2a is analyzed to consist of
3190
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Qin and Trifunac
30 G
H
H -
Figure 3. EPR spectrum (upper) obtained from a y-irradiated xenon solution of tetramethylcyclopropane at 77 K and the simulated stick (absorption) spectrum (lower) using = 23.3 G. The arrows indicate signals from the solvent.
I 1 lines. The reversible spectral change indicates a rotational change of the methyl groups of 1. The 18.7-G coupling constant due to the interaction of the two methylene protons should remain unchanged. A simulation based on aZH= 18.7 G and a8H = 22.3 G shown in Figure 2b fits the spectrum best, demonstrating that the rotational motion of the methyl groups occurring at 77 K is frozen out at 40 K. Only eight protons out of 12 methyl protons are strongly coupled. The obtained hyperfine data indicate that the conformation of methyl protons relative to the p orbitals of the SOMO is that, for each of the four methyl groups, one proton lies in the nodal plane of the p orbital, giving a zero coupling constant, and the other two protons possess a dihedral angle 0 of 30' between the p axes and the C-H bonds, giving the same coupling constant of 22.3 G. The relation (2 X 22.3 + 0)/3 G = 14.9 G yields the coupling constant for the free-rotating methyl protons, as required by the above analysis. Below 40 K, the EPR signals due to 1 broadened while the signals from the unidentified species in the central region from the solvent became more and more dominant. Below 10 K, the EPR signals due to I were too broad to be observed. TMCP-Xenon Solution without Electron Scavengers. In contrast to the above results, without an electron scavenger, yirradiation of T M C P (5 vol %) in xenon at 77 K gave rise to an EPR spectrum shown in Figure 3a, which consists of a binomial eight-line (a7H = 23.3 G) pattern as indicated by the stick spectrum. Since these signals were not observed by y-irradiation of the pure xenon, they are assigned to the paramagnetic species from TMCP. The eight-line pattern showed no significant change between 20 and 100 K. The 23.3-G seven-proton coupling constant indicates the interaction of six protons of two methyl groups plus one proton of the methylene group, which points to a spin-localized radical (CH,),CR, 3.12 This radical has to be formed by the opening of the cyclopropane ring. However, it is a neutral radical rather than a radical cation since it has been well-established that yirradiation of a rare gas matrix containing higher concentration of organic substrates results in the formation of organic neutral
radical^.'^-'^ It is also well-known that y-irradiation of pure alkane solutions produces neutral radicals.1618 Figure 4a shows the EPR spectrum (12) Heller, C.; McConnell, H. M. J . Chem. Phys. 1960, 32, 1535. (13) Kinugawa, K.; Miyazaki, T.; Hase, H. J . Phys. Chem. 1978,82, 1697. (14) Bhattacharya, D.; Willard, J . E. J . Phys. Chem. 1981, 85, 154. (15) Gotoh, K.; Miyazaki, T.:Fueki, K.; Kwang-Pill, L. Radiat. Phys. Chem. 1987, 30, 89. (16) Gillbro, T.;Lund, A. Radiat. Phys. Chem. 1976, 8, 625. (17) Iwasaki, M.; Toriyama, K.; Muto, H.:Nunome, K . J . Chem. Phys. 1976. 65. 596.
(18) Miyazaki, T.; Kasugai, J.; Wada, M.; Kinugawa, K. Bull. Chem. Soc Jpn. 1978, 51, 1676.
H-
Figure 4. (a) EPR spectrum recorded at 130 K after y-irradiation of tetramethylcyclopropane at 77 K. (b) Computer-simulated spectrum using a2H = 11.7 G and u~~ = 23.3 G and a Lorentzian line width of 3.5
G. TABLE I: EPR Parameters for the Radical Cations and Radicals from TMCP radical cation or radical matrix T, K hyperfine couplings, G ref 1
xenon/
77
18.7 (2 H), 14.9 (12 H) this work
CF2CICFCI, xenon/ CF,CICFCI,
40
18.7 (2 H), 22.3 (8 H )
this work
18.7 (2 H), 15.0 (12 H) 18.7 (2 H), 14.9 (12 H) 23.3 (6 H), 11.7 (2 H) 23.3 (7 H) 23.3 (6 H), 11.7 ( 2 H)
5 5 5 this work this work
CFCli 2 3
xenon
145 109 117 77
4
TMCP
130
CF2CICFC12
CF2CICFC12
recorded at 130 K after y-irradiation of pure TMCP at 77 K. The well-resolved EPR spectrum can be analyzed as a septet (a6H= 23.3 G) of triplets (a2H = 11.7 G). This is confirmed by the simulation in Figure 4b. The spectrum is assigned to the neutral radical (CIi,),CR' (4), which is also a ring-opened species with the spin residing at only one (CH3)2C group. The imperfect agreement between the relative line intensities in the experimental and simulated spectra can be attributed to a small contribution from the neutral radical 3 (see Discussion). It is noteworthy that the radical conformation as well as the hyperfine data of 4 is virtually identical with that reported for the species assigned to the ring-opened radical cation of TMCP (2) in CF2CICFC1,.5 The EPR data of 1-4 are collected in Table I.
Discussion Radical Cation Formation in Xenon. To the best of our knowledge, this is the first EPR observation of an organic radical cation (RH) in a xenon matrix. This example describes a new way to study radical cations in a nonpolar matrix, xenon, which is known to have many excellent properties as a s o l ~ e n t . ' ~ ~ ~ ~ An electron scavenger (e.g., CH2C12) is necessary for observation of a radical cation in xenon as demonstrated in the Results section. The TMCP radical cation was only formed in the presence of an electron scavenger. In the absence of an electron scavenger, only neutral radicals from TMCP were produced. These results are consistent with the fact that xenon is not a good electron (19) Rentzepis, P. M.; Douglass, D. C. Nature (London) 1981, 293, 165. (20) Cook, M. D.; Roberts, B. P. J . Chem. SOC.,Chem. Commun. 1983, 264.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3191
Generation of Radical Cations in Xe Matrices acceptor. The mechanism of the radical cation formation in xenon is suggested to be similar to that occurring in a freon matrix: Xe
'7
--
+ Xe Xes+ + R H e- + CH2C12 Xe*+
CH2C12'-
+ eXe + Xe*+ Xe + RH'+
Xe*+
-
-
(1)
(2) (3)
CH2CI2'-
(4)
+ CI-
(5)
'CH2CI
First, xenon cations and electrons (e-) are produced by radiolytical ionization. Electrons are then captured by electron scavengers, and xenon cations that escape geminate recombination with the electrons can exhibit charge migration. Thus, the positive charge is transferred to substrates to generate substrate radical cations provided that xenon has a higher ionization potential than the solute used. Other organic radical cations have been produced radiolytically in xenon,2i supporting this mechanism, e.g., tetramethylethylene radical cation. It is evident that EPR spectra of 1 in xenon are fairly well resolved and without substantial background interference. Very recently, hydrocarbon radical cations have been generated and characterized in other nonpolar solvents such as alkane matrices.22 However, the EPR signals due to the matrix radicals in these matrices are so strong that the detection of the substrate radical cations has to be assisted by subtraction of matrix radicals by a microcomputer. In this respect xenon matrix as a more inert solvent is superior to alkane matrices. The relatively broad line width of the EPR spectra of 1 in xenon is attributable to its limited temperature range (40-105 K), since the EPR spectra of 1 in freon matrics were also not well-resolved below 105 K. However, it may also be due, in part, to the superhyperfine interaction with xenon nuclei. Neutral Radical Formation in Xenon. In the present study the paramagnetic species observed in the y-irradiation of xenon containing TMCP is assigned to the ring-opened radical species (CH3)2CR(3), and in the y-irradiation of neat TMCP, another very similar radical was observed, (CH,),CR' (4). We speculate here on the possible pathways that would give rise to such ring-opened radical species. Alkyl radical formation - ' ~ a mechanism in xenon has been studied by several g r ~ u p s , ' ~and was proposedI5 where the excited species in an inert gas matrix would fragment to yield a radical and an H' atom following the neutralization reaction (eq 6). The radical produced would
molecule reaction of TMCP" at 77 K. Electron scavenging was also found to be necessary to stabilize radical cations in argon matrices at 20 K.3a This problem needs further study. In the absence of the stabilizing influence of the electron scavenger, TMCP'+ could undergo several types of ion-molecule reactions, giving rise to the ring-opened neutral radicals. Two types of ion-molecule reactions are conceivable: ( 1 ) ion-molecule reaction of the radical cation T M C P " with the neutral to give rise to radical cation dimers or oligomers and (2)proton transfer within the dimer or oligomer radical cation or with the neutral species to give the radical. The competition between these two reactions could produce differing radicals 3 and 4 where the R group would reflect the degree of dimer/oligomer formation before the proton transfer occurs. Thus, in a neat TMCP experiment one observes one type of radical and in xenon solution another since the concentrations of TMCP in the two solutions are different, yielding different amounts of the two radicals. That is, in the dilute solution in xenon, the "clusters" of TMCP are smaller, allowing different opportunities for dimerization/oligomerization versus proton transfer, while in the neat TMCP, the many available TMCP molecules allow for more efficient proton transfer with less dimer/oligomer formation. To test these ideas, our experiment in xenon was carried out using higher (30vol %) concentrations of TMCP. In this case, upon y-irradiation both species (3 and 4) were observed. Presumably, in freon matrices such ion-molecule processes are occurring as well. Other ion-molecule reactions of TMCP'+ radical cation could also be considered, namely, hydride ion transfer between the radical cation and the neutral.25 The various proposed mechanisms are quite speculative since the nature of the R group in radicals 3 and 4 is not known. However, the evidence supports the idea that in xenon without the electron scavengers, in neat TMCP, and in the previous work in the CF2ClCFClzmatrix, the paramagnetic species observed was the neutral ring-opened radical, not the ring-opened distonic radical cation. The ring-opened radical species observed in the y-irradiation of neat TMCP and the CFzClCFC12matrix containing TMCP have identical coupling parameters (see Table I), indicating that the same neutral radical is being observed. Conclusion
We have shown that the xenon matrix can be utilized in the low-temperature study of radiolytically produced radical cations. The advantage of a xenon matrix over the widely used freon matrices is that there appears to be less chemistry involving matrix TMCP'+ e(TMCP)* ((CH3)2-CR) H' (6) molecules. A disadvantage is that the range of temperatures depend on which bond.in TMCP* ruptured, yielding the ringavailable for study is more limited with xenon than in freons. opened species (CH3)2CCH2C(CH3)=CH2(5) assuming bond However, the xenon matrix has the widest useful temperature rupture at the methyl group followed by ring opening. Loss of range of all rare gas matrices. The role of the electron scavenger H' atom at the methylene group followed by the ring opening in xenon is crucial, since it appears that solute (TMCP) radical would give (CH,)2C=CH-C(CH3)2 ( 6 ) . cation reactions are prevented by the presence of the electron The radical observed does not fit the known 5 coupling constants scavenger. This could be due to some sort of complex formation a6" = 23.0 G and a2H= 17.5 G S z 3 Radical 6 is not known between the radical cation and the chloride anion produced by experimentally but was studied theoretically by Davis and K o ~ h i ~ ~the dissociative electron capture with the scavengers used. using the INDO method. Furthermore, Kochi et aLz3showed that The transformation reactions of the tetramethylcyclopropane the precursor of radical 6, 2,2,3,3-tetramethylcyclopropylradical, radical cation observed in freons, xenon, and neat TMCP matrices does not undergo ring opening to 6 below 200 K. can be explained by considering the varying extent of ion-molecule Since the two radicals (5 and 6 ) are thus ruled out by previous reactions yielding different cations which then give rise to the work on such systems, we must consider alternate pathways ring-opened radical centers. In neat TMCP proton transfer is whereby a ring-opened radical is produced by ion-molecule revery facile to give an opened-ring radical while in xenon matrices, actions of the TMCP'+ radical cation. depending on the TMCP concentration, cation dimer and oligomer Since the radical is only seen in xenon when the electron scaformation occurs in competition with the proton transfer. Thus, venger is absent, this requires that the species produced by electron the radical center ultimately produced will reflect the extent of scavenging perform the very essential role of preventing ionthis condensation reaction before the proton transfer yields the final radical center. Finally, the radical species observed in the y-irradiation of neat (21) Qin, X.-Z.; Trifunac, A . D. Unpublished results. TMCP is identical with the paramagnetic species in CF2CICFC12 (22) Ichikawa, T.; Shiotani, M.; Ohta, N.; Katsumata, S.J . Phys. Chem.
+
-
-
+
1989, 93, 3826. (23) Chen, K . S.; Edge, D. J.; Kochi, J. K. J . Am. Chem. SOC.1973, 95, 7036. (24) Davis, W. H . , Jr.; Kochi, J . K . Tetrahedron Lett. 1976, 2 / , 1761.
(25) Toriyama, K.; Nunome, K.; Iwasaki, M.; Shida. T.; Ushida, K. Chem. Phys. Leu. 1985, 122, 118.
J . Phys. Chem. 1990, 94, 3192-3196
3192
above 120 K, suggesting that a neutral radical rather than the ring-oDened distonic radical cation is observed in CF,CICFCI,. - . Acknowledgment. We are indebted to Drs. Martin Bakker and David Werst for technical assistance and for numerous useful discussions. Useful discussions with other members of the Radiation and Photochemistry Group and with Prof. Ffrancon
Williams of the University of Tennessee are acknowledged as well. This work was cerformed under the ausDices of the Office of Basic Energy Sciences, Division of Chemicai Science, US-DOE, under Contract No, w-3 -109-ENG-38. Registry No. I , 56324-44-8; T M C P , 41 27-47-3; Xe, 7440-63-3; CF,CICFCI,, 76-1 3-1; CH2CI,, 75-09-2.
Aqueous Reactivity of Polyacetylene: pH Dependence Anthony Guiseppi-Elie* AAI-ABTECH, P.O. Box 376, Yardley, Pennsylvania 19067 and Gary E. Wnek Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 (Received: July 11, 1989; In Final Form: November I , 1989)
The stability of iodine-doped polyacetylene in aqueous environments and the particular effect of variations in pH upon the stability of the intrinsically conducting polymer has been investigated in a variety of aqueous environments. Stability performance was established by simultaneously monitoring the four-probe electrical conductivity and the steady-state electrode rest potential during exposure of the metallically doped, [CHI,,l,4,20],, polymer over an approximate IO-day period. Initial polymer conductivities were in all cases between 100 and 300 S cm-’. The initially measured electrode potential was in all cases ca. 0.45 V vs SCE irrespective of the doping levels studied; y = 0.007, 0.168,0.210 or the pH of the test solution in the range 1-9. The rate and extent of degradation in these material properties was found to increase at the extremes of pH. Alkaline conditions were found to be generally more aggressive to the p-doped polymer compared to acidic conditions. The normally slow but ever present degradation of iodine-doped polyacetylene which occurs even under inert atmosphereconditions is accelerated in all aqueous environments leading to an often precipitous degradation of electrically based material properties.
Introduction The reactivity of doped and undoped plyacetylene in aqueous environments is relevant to its application as a fuel cell electrode,’ as a transducer-active material in chemical and biological sens o r ~ , *and , ~ in aqueous-based rechargeable storage b a t t e r i e ~ . ~ Equally important, the reactivity of doped and undoped p l y acetylene with water is relevant to a fundamental understanding of its environmental stability because moisture is an ubiquitous component of ambient. Considerable work has been reported on the stability and degradation of polyacetylene, but few have addressed its stability in aqueous environments. Polyacetylene is characterized both as an electrode and as an electroactive polymer. Implicit in this definition is the ability of polyacetylene to be an intrinsic source of charge carriers,j to act as a conduit for the movement of charge,6 and be redox active,’%* that is, be electrochemically oxidized or reduced. The redox properties of undoped trans-plyacetylene have been investigated by MacDiarmid et al.9 and the following summary has been provided: ( I ) Shirakawa, H.: Ikeda, S.; Aizawa, M.; Yoshitake, J.; Suzuki, S. Synth. Met. 1981, 4 , 43. (2) Guiseppi-Elie, A . Biosensor Applications of Polyacetylene. Paper presented before the NOBECCHE, Philadelphia, PA, April 1988. ( 3 ) Malmros, M. K.; Gulbrinski, J.; Gibbs Jr., W. B. Biosensors 1987/ 1988, 3, 7 1 . (4) Nigrey, P. J.; MacDiarmid, A. G.; Heeger, A . J. J. Chem. Soc., Chem. Commun. 1979. 594. (5) Maclnnes Jr.. D.; Druy, M. A.; Nigrey, P. J.; Nairns, D. P.; MacDiarmid, A. G.; Heeger, A . J . J . Chem. Sot., Chem. Commun. 1981, 317. ( 6 ) Shirakawa, H.; Louis, E. J.; MacDiarmid, A . G.; Chiang, C. K.; Heeger. A . J . J . Chem. Soc., Chem. Commun. 1977, 578. (7) Nigrey, P. J.; MacDiarmid, A. G.; Heeger, A. J. J . Chem. Soc.. Chem. Commun. 1979. 594. (8) Diaz, A . F.; Clarke, T. C. J . Electroanal. Chem. 1980, I I I , 1 1 5. ( 9 ) MacDiarmid, A. G.;Mammone, R. J.; Krawczyk, J . R.; Porter, S. J . Mol. Cryst. Liq. Cryst. 1984, 105, 89.
0022-3654/90/2094-3 192$02.50/0
(CH+q+‘)x + (ax)e--
(CH+q),
Eo vs Li+ ( 1 M)/Li = 3.1; Eo vs H+ ( 1 M)/f/,H2 = 0.0
-
(CH-99, + (ax)e(CH-9), E” vs Li+ ( 1 M ) / L i = 1.8; Eo vs H+ (1 M)/f/,H2 = -1.3 These reduction potentials define the valence and conduction band edges, respectively, of this narrow band gap (1.3-1.4 eV), polymeric, semiconductor. This implies, as illustrated in Figure I , that all redox-active species that have reduction potentials more positive than the oxidation potential of (CH),, Le., greater than 0.0 V vs NHE, will spontaneously react with polyacetylene and result in charge-transfer oxidation or p-doping. Similarly, all redox-active species that have reduction potentials less than the reduction potential of plyacetylene, Le., more negative than -1.3 V vs NHE, will spontaneously react with polyacetylene resulting in charge-transfer reduction or n-doping. The term “doping” is used with the understanding that this process, while it produces a result similar to the doping of inorganic semiconductors, that is, a change in electrical properties, is indeed a redox process involving charge transfer. Since reduced or ndoped polyacetylene is extremely reactive with water,I0 this will not be discussed here. Oxidized or p-type polyacetylene is considerably less reactive, can be handled in air, and has been demonstrated to be stabilized by immersion in aqueous chloride solutions.”.l2 We have been engaged in a study of the aqueous environmental stability or reactivity of polyacetylene for some time now11-13 and in developing ways to chemically functionalize and (IO) Chiang, C . K.; Gau, S. C.; Fincher Jr., C. R.; Park, Y. W.; MacDiarmid, A . G.; Heeger, A . J. Appl. Phys. Left. 1978, 33(1), 18. ( I I ) Guiseppi-Elie, A.; Wnek, G. E. Stabilization of Conductive Polymers in Aqueous Environments. U.S.Patent 4,499,007, 1985. (12) Guiseppi-Elie. A.; Wnek, G. E. J . Chem. Soc.,Chem. Commun. 1983. 63. ( 1 3 ) Guiseppi-Elie, A.; Wnek,
G.E. J . Phys., Colloq. 1983, C3, C3-193.
0 1990 American Chemical Society
