Langmuir 1992,8, 3003-3007
3003
Electrochemistry of Langmuir-Blodgett and Self -Assembled Films Built from Oligoimides Vincent Wing Sum Kwan, Vince Cammarah,+Larry L. Miller,* Michael G. Hill, and Kent R. M a n * Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received December 16,1991. In Final Form: April 29,1992
Linear oligoimides of defined structure were assembled into multilayer structures by the LangmuirBlodgett (LB) method. Self-assembled monolayer films of thiol-terminated oligoimides were prepared by chemisorptionon gold. The imide groups can be reduced electrochemically, and cyclic voltammetry was used to examine the complex electrochemistryof the LB films, and the reduction kinetics of the self-assembledfilms. Spectroelectrochemistrywas used to elucidatethe reductionprocess of the LB films. Comparison of the results for isotropic, self-assembled, and LB films is made in terms of the differing film structures and the mobility of the molecules in the films. There is considerable interest in the synthesis and properties of organized mono- and multilayer molecular thin films. These films are usually formed by the Langmuir-Blodgett (LB) method of film transfer from water to the substrate,’ by “self-assembly”chemisorption (SA)? or by vapor dep~sition.~ This paper concerns the electrochemistry of oligoimide films formed by the LB and SA methods. A synthesis of pure imide oligomers with lengths up to 8 nm has been developed: and we have reported that these oligomers can be assembled into anisotropic fiis using either the LB50rSA6method. These films were characterized using IR, UV, and XPS methods. The oligoimide rods in the SA monolayers were tilted up from the surface an average of about 35’. LB films were transferred from water to solid substrates by horizontal lifting. Depending on the molecule, the average tilt angle of the rods up from the surface varied from < 1 5 O to 35O. Initial electrochemicalstudies of the SA films indicated that the thiol-terminated oligomers were strongly bound to gold and that each and every imide group along the oligomer could be reversibly reduced with one or two electrons in DMF solvent.6 Only a very limited electrochemical study of one LB film was reported.5b Conditions were found which prevented desorption of the reduced film, and the cyclic voltammetry (CV)curves revealed a chemically reversible but more complex response than that from the SA films. Using oligomers 1-7, this paper follows up on those results to reveal some more detailed aspects of electron transfer to the SA monolayers, expands the scope of electrochemical studies of LB films, and reports spectrochemistryresults on the LB film reduction process. A general goal is to compare the behavior of LB, SA, and isotropic films (formed by syringing on the oligoimides). Oligoimide films are unique in that the compounds are rigid rods. Previous studies of LB and SA films have + Present address: Department of Chemistry, Auburn University, Auburn, AL 36849. (1) Roberta, G., Ed. Langmuir-Blodgett F i l m ; Plenum Press: New York, 1990. Ulman, A. Ultrathin Organic F i l m ; Academic Press: San Diego, 1991. (2) Ulman, A. Adu. Mater. 1991,3, 298 and references therein. (3) Debe, M. J. Vac. Sci. Technol. 1982,21 (l),74. S.V.;Penneau, J.F.;Miller, (4) Dietz,T.M.;Stallman,B.J.;Kwan,W. L. L. J. Chem. SOC.,Chem. Commun. 1990, 367. (5) (a) Cammarata, V.; Kolaskie, C. J.; Miller, L. L.; Stallman, B. J. J. Chem. SOC.,Chem. Commun. 1990, 1290. (b) Cammarata V.; Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992,8,876. (6) (a) Kwan, W. S.V.; Penneau, J. F.; Miller, L. L. J. Electroanal. Chem. 1990,291,295. (b) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1990, 7, 1419.
HSPh-A-PhtBu
HSPh-A-8.A-PhSH
1
2
HSPh-A-CH2CHzCH,-APhSH
HSPh-A-8-A-8-A-8-A-8
J
‘03SPh-A-B-A-PhS0,’ 5
B-A-B-A-B-A-B-A-B I
4
HOzCPh-A-8-A-PhCOzH
6
‘03SPh-A-PhSOJ’, 2 Na+ 8
usually used flexible compounds with long alkyl chains, and oligoimides expand the repertoire of thin f i i materials. We imagine that the rigidity of oligoimides can make them useful as rigid bridges for studying electron transfer between an electrode and an electrophore because the conformational flexibility of alkyl chains would be superceded. A necessary preliminary to that study is complete characterization of the electrochemistry of the oligomer films. It is further noted that charge transport through polyimide films is of practical concern for microelectronic~.~ We believe that the ease of reduction of the imide group may play a role in the unwanted degradation of devices formed with polyimide overlayers. Previous work on the electrochemistry of SA and LB films has primarily focused on their qualities as insulating layers which can block electrochemical events. Recently, severalother electroactive,surface-bound thioWor related compoundsg have been reported, and in one case the interfacial electron transfer rate was studied.gc Electroactive LB films have been investigated by a number of groups.10 (7) Mittal, K. L., Ed. Polyimides; Plenum Press: New York, 1982. (8) (a) Hickman, J. J.; Oe, D.; Zou, C.-F.; Wrighton, M. S.;Laibinis, P. E.; Whitesides, G. M. J.Am. Chem. SOC.1991,113,1128. (b) Fox,M. A.; Collard,D. M.Langmuir 1991,7,1192. (c) Chidsey, C. E. D.; Bertozzi, C. R.; Putzoinski,T. M.;Mujsce, A. M. J.Am. Chem.Soc. 1990,112,4301. Harvey, P. D.; Wrighton, M. S.; (d) Hickman, J. J.; Oe, D.; Zou, C.-F.; Laibinis, P. E.; Bain, C. D.; Whitesides, G.M. J.Am. Chem. SOC.1989, 111,7271. (9) (a) Lee, K. A. B. Langmuir 1990,6,709. (b) Delong, H.C.; Butry, D. A. Langmuir 1990, 6, 1319. (c) Edward, T. R. G.; Cunnane, V. J.; Parsons, R.; Gani, D. J. Chem. SOC.,Chem. Commun. 1989, 1041.
0743-7463/92/2408-3003$03.00/00 1992 American Chemical Society
Kwan et al.
3004 Langmuir, Vol. 8, No. 12, 1992
Results and Discussion The oligomers of interest are designated '-A-B-A-", where the A group comes from naphthalene dianhydride and the B group from dimethoxybenzidine. Compound 3 has a flexible propyl group linking the two A units. The synthesis and characterization of compounds 1 and 4-8 have been Important for understanding the resulta are electrochemical studies of similar molecules in solution. In DMF it has been shown that the A group can be reduced to form stable anion radicals and dianions. Using 1, the apparent Eos for these processes are -0.56 and-1.04V (SCE).ll In aqueous solution it has been shown by ESR and vis-near-IR spectroscopy that the watersoluble radical anions 8- form spin-paired 7r-dimers. In aqueous NaCl solution or in the presence of polycations, 8- aggregates into 7r-sta~ks.l~ Important here is the observation of so-called Davidov shifts of the visible -*band of 8-. The monomer absorbs at 473 nm, the dimer at 453 nm, and the stacks as low as 439 nm for the largest aggregatesobserved. Electrochemistryin aqueous solution is complicated by these aggregation phenomena and by precipitation of the 7r-stacks into conducting films.12 Self-Assembled Films of 1-4. Monolayer films were formed by soaking gold disk electrodes or vapor-deposited gold films on glass in CHCl3 solutions of the oligomers. A small amount of trifluoroacetic acid was added to enhance the solubility of the oligomers. After soaking for a few hours, the samples were removed, rinsed, and used for electrochemical studies in DMF and tetramethylammonium fluoroborate ((TMA)BF3. CV at sweep rates of 200 mVs-' or less showed two reversible reductions. The peak potentials and widths were independent of sweep rate, and equal anodic and cathodic currents were measured for each couple. As described previously the surface concentration calculated from the CV data for all four oligomers was calculated to be 0.95 X 10-lomol cm-2.The anodic-cathodic peak separation AI3 for the first couple increased from 20 mV when the sweep rate was increased above about 0.5 V s-' (Figure 1). This allowed the kinetic investigation that follows. Followingthe method of Laviron,13anodic-cathodic CV peak separationswere used to measure the average electron transfer rate constant from the electrode to the surfaceconfined A units. The results are interpreted following the accepted model for charging redox polymer films on In essence it is hypothesized that the process e1e~trodes.l~ is initiated by electron transfer from gold to A units located closest to the gold surface. The charging of the layer then can propogate to other A units by hopping either along one chain or between chains. Because the chains are tilted, A groups are brought into close proximity. Thus, interchain hopping should be facilitated. Importantly, all of (10) (a) Park,S. G.;Aoki, K.;Tokuda, D.; Matsuda, H. J.Electroanal.
Chem. 1986,195,167. (b) Fujihira, M.; Araki, T. Bull. Chem. SOC.Jpn.
1986,59,2376. (c) Lee,C.-W.; Bard, A. J. J. Electroanal. Chem. 1988, 239,441. (d) Facci, J. S.; Faleigno, P. A,; Gold, J. M. Langmuir 1986,2, 732. (e) Widrig, C. A.; Majda, M. Langmuir 1989,5,689. (0Ueyama, S.; M a , S.; Maeda, M. J . Electroanul. Chem. 1989, 264149. (g) Shimomwa, M.; Kaeuga, K.; Tsukada, T. J . Chem. Soc., Chem. Commun. 1991, 845. (h) Liu, J. F.; Loo, B. H.; Hashimoto, K.; Fujiehima, A. J. Electroanal. Chem. 1991,297,133. (i) Ueyama, S.; Isoda, S.;Maeda, M. J. Electroanal. Chem. 1990,293, 111. (11) Penneau, J.-F.; Stallman, B. J.; K d , P. H.; Miller, L. L. Chem. Mater. 1991, 3, 791. (12) Zhong, C. J.; Zinger, B.; Cammarata, V.; Kasai, P. H.; Miller, L. L. Chem. Mater. 1991,3, 797. (13) Laviron, E. J . Electroanal. Chem. 1979,101,19. (14)For an early report and recent review, see: Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q. J . Am. Chem. SOC.1980,101,483. Rubenstein, I. In Applied Polymer Analysis and Characterization; Mitchell, J., Ed.;Hanser: New York 1991; Vol. 11, Chapter 1.
\
0.2~4A
\
, ,
0
0.9
0.5
E ( V, SCE ) Figure 1. CV of a SA f i b from 2 at 6 0 , 1 0 0 , 2 0 0 , and 600 mV s-l in DMF and 0.1 M (TMA)BFh. Electrode area 0.016 cm*. Table I. Rates of Interfacial Electron Transfer for 1-4 on Gold thiol kea (8-l) thiol k$ (8-9 1 2 a
181 120
3 4
197 92
The reproducibility is &20%.
these electron transfers should be accompanied by movement of countercations into the film. Maintenance of electrical neutrality is an important energetic consideration, and it is hypothesized that cations must penetrate to ion pair even with the inner A groups during the process and certainly by the time the film is completely reduced. Using this model, it is clear that the rate could be limited by the heterogeneouselectron transfer or the rate of charge propagation through the film, and that each of these can involve counterion movement. Since they coupled, it has generally proven impossible to differentiate electron or ion transport as the rate-limiting process. Table I lists the various rate constantsfor the compounds studied. The single A group of compound 1reduces with k = 181 s-l. This is a reasonable value for an organic electrophore16 initially held 6 A (on average) away from the surface.6 The rate constant depends only slightly on the length of the rod, with the longest oligomer 4 showing k = 92 s-l. The smaller rate for the longer rod may be a consequence of the greater difficulty in movement of counterions through a thicker film during the redox process. However,this effect is small,and the data suggest that, for the longest oligomer 4, TMA cations can penetrate through 38 A of film to ion pair with an anion radical (16) Sharp,M.; Peterson, M. J. Electroanal. Chem. 1981, 409.
Langmuir, Vol. 8, No. 12, 1992 3006
LB and Self-Assembled Films Built from Oligoimides
-I--
I
I
I
I
n ZOO mv/s
100 50
20
0.0
0
E ( V, SCE ) Figure 2. CV of a SA film from 4 at 100,200,and 500 mV s-l in DMF and 0.1 M Ph& BFI. Table 11. CV Peak Separations for Thiols 2 and 4 electrolvte
Y
(mV 8-1)
2
4
30 40 70 60 60 70
40 67 120 40 67 100
~~
PhrAs
THA
100 200 500 100
200 500
I 0.0
I
I -0.4
I
I -0.8
E ( V, SCE ) Figure 3. Lek CV of an LB film from 5 (four layere). Transferredat 7 mN m-l at 296 K. Measured in 0.1 M aqueous CaC12. Right: CV of a syringed-on sample of 5 measured in 0.1 M CaC12.
0.8
0.4
-0.4
formed on the inner imide. Compound 3, which has two A groupsjoined by a propyl, reduces as fast as 1and faster than 2. We suggest that the propyl link in 3 is shorter and more flexible than the benzidine (B) unit in 2 and that this facilitates the reduction of the second A group in 3. T w o larger electrolyte cations, tetrahexylammonium and tetraphenylarsonium (9.4-A diameter),were also studied. For these cations, the CV peaks were quite broad at higher sweep rates and it was not possible to measure the rate constant. The peak-to-peak separation (ap)for several sweep rates for monolayers of 2 and 4 are listed in Table 11. With a smaller cation, e.g., TMA, AP exceeds 30 mV only with sweep rates greater than 1 V s-l. The onset of greater separation occurred around 0.1 V s-l for both thiols 2 and 4 when the two bigger ions were used (Figure 2). This observation signifies a smaller electron transfer rate. Compound 4 seemsslightly slower as expected. We suggest that the cations must enter the space between the molecules. TMA cations can do this readily, especially if the rods straighten up, giving a larger tilt angle and larger intermolecular spaces, but the larger cations cannot. LB Films. The films were produced by the horizontal transfer method as previously described.6 A surface pressure of 7 mN m-l was employed for the transfer from water to gold-coated glass slides. Consider first the results using four LB layers of compound 5. CV, using 0.1 M aqueous KC1 electrolyte, showed on the first scan a symmetric cathodic peak at -0.67 V. The area under the
Table 111. Cyclic Voltammetry of LB and Evaporated Films. E, pwhhb pwhh' compd method (V) (mV) E, (mV) 70 -0.22,-0.4 120,200 6 LB -0.54 evap -0.55 200 -0.31 390 6 LB -0.58 70 -0.23,-0.36 90,190 LB,Ba2+d -0.67 60 -0.22.4.38 100,150 evap -0.63 120 -0.24,-0).39 150, -190 7 LB -0.70 120 -0).48,-0.58 broad 4 LB -0.73 100 -0.43,-0.55 130,150 evap -0.69 205 -0.45,-0.51 150,180 a Sweep rate (mV s-l) in 0.1 M aqueous BaC12. Cathodic peak width. Anodic peak width. Baa+(pH 8.9) aqueous aubphase.s
peak corresponded to four layers of electroactive LB material (two A units per molecule, each accepting one electron) with a surface concentration of 0.22 nmol cm-2. (The surface concentration can be independently calculated from the area per molecule at 7 mN m-l, and the transfer ratio of 0.95, to be 0.22 nmol cm-2.) Upon reversal of the scan direction no reoxidation peak is observed. The second and subsequent scans reveal no further faradaic current. These observations can be most readily explained if the reduced f i is soluble and dissolves in the solution. When CaC12 or BaClz is substituted for KC1 as the electrolyte, both anodic and cathodic peaks are observed on the first and succeeding scans (figure 3, Table 111). The second and following scans are repetitively reproducible. For the first scan the cathodic peak is located at -0.7 V with a peak width at half-height (pwhh) of 40 mV. T w o anodic peaks occur at -0.4 and -0.22 V. The second reductive sweep has a cathodic peak at -0.64 V and the same two anodic peaks. The voltammogram is reproducible thereafter, with little loss of current from scan to scan. The second and later scans have a cathodic pwhh of 70 mV with the anodic pwhh's of 120 and 200 mV. The ratio of integrated cathodic to anodic current is 0.96, indicating chemical reversibility. The peak currents are proportional to the scan rate in the 10-500 mV/s range. This indicates the electroactive species is surface confined. The integrated current corresponds to 1 e-/A group, for the entire LB film. Four, six, and eight layers gave proportionately
3006 Langmuir, Vol. 8, No. 12, 1992
Kwan et al.
larger currents (hence charge), indicating the whole film is electroactive. The peak positions are independent of sweep rate, indicating that the process is not kinetically controlled. Scanning the electrode potential more negative than -0.70V reveals a second peak at -0.75 V with a reoxidation wave at -0.68 V, after which further scanning reveals far less current than the previous scan. Since the film appeared to be unstable (decreased current upon cycling) after the second reduction, we did not explore it further. In contrast to the LB film, an evaporated, isotropic film Of 5 under the same conditions exhibits one cathodic peak at -0.55 V with a pwhh of 200 mV and a single anodic peak at -0.31 V with a pwhh of 390 mV. This broad peak may include both of the anodic peaks found for the LB films, but an important difference is that the cathodic peak width for the isotropic film is roughly 3 times that of the LB film. A variety of spectroscopic techniques including grazing angle IR (GIR) have shown5 that the average long-axis oreintation of 5 deposited by the LB method is tilted 15O up from the electrode surface before electrochemical cycling. GIR spectra on films that were reduced and reoxidized show that the average tilt angle of the film increased to ~ 3 5 This ~ . angle is similar to those for solution-evaporated samples and is consistent with an isotropic orientation. It is clear that redox cycling disrupts the original organization. UV-vis spectroelectrochemistry16was employed to identify the reduced species in a six-layer LB film of 5 in aqueous 0.1 M BaC12. In the experiment, the potential was stepped from 0.1 V (vs Pt wire pseudoreference) in -50-mV increments and spectra were recorded at each potential out to -0.15 V. At 0.05 V, the spectrum of 5 diminished and a reduced species (5-1 was formed with ,A, = 450 nm. At -0.15 V a second reduced species (assigned as dianion 52-) was formed with absorbances at 415,545, and 590 nm. When the potential was stepped positively to -0.05 V, the dianion disappeared and the anion radical reappeared. Only at the rather positive potential of 0.3 V did the spectrum of the neutral reappear. This spectral behavior correlates well with the expectation from CV. We note that the absorbance at 450 nm is not appropriate for a monomeric anion radical. Aqueous solution studies of &and other solublediimides have shown that dimer anion radicals absorb at 453 nm ( E = 25 OOO M-l cm-l) and 1140 nm (3900) (vs 473 nm for monomeric anion radicals).'l In light of the excellent agreement between the expected and observed band at 450 nm for a radical dimer, we tentatively conclude that the first electrochemical process of the film involves the formation of dimer anion radicals, not monomers or stacks. We were unable to observe the band at 1140 nm due to its small predicted absorbance (-O.OOO6 absorbance units). In similar fashion, we assign the second reduction product with absorptions at 415 (0.021, 545 (0.005),and 590 nm (0.009) to the dianion P.The dianion 82- in DMF has peaks at 409 nm (293001, 538 nm (8500),and 582 nm (13 2OOI.l' Both peak positions and ratios of peak heights observed in the spectroelectrochemistry are within expectations for dianion formation. The electrochemistry of 7 in aqueous 0.1 M BaClz is similar to that of 5. Upon repetitive cycling, the first reduction in the LB film occurs at -0.7 V vs SCE with a peak width of 120 mV. The reoxidation gives peaks at -0.48 and -0.58. GIR of the LB film before electrochemical
11'
-
~~~
~
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(16) Bullock, J. P.;Mann, K.R.Znorg. Chem. 1989,28,4006.
(17) Unpublished work from this laboratory.
0.0
0.4
0.8
E ( V, SCE ) Figure 4. CV of 4 (six LB layers). Transferred at 7 mN m-l at 296 OC. Measured in 0.1 M aqueous BaC12.
cycling shows a tilt angle of loo from the surface. After cycling, it is 35') i.e., essentially isotropic. LB films of 6 were lifted from both a pH 3 subphase and a pH 8 subphase which contained Ba2+. The voltammograms of these films (4-8 layers) were quite similar and similar to those for the other oligoimide LB films (see Table 111). Isotropic films from 6 have a larger cathodic pwhh (-2X), but the peak positions are again at the same potential as the LB films. A second reduction at 4 . 7 5 V with a reoxidation at -0.67 V is observed. As in the case of 3 there is a significant loss of signal upon cycling to these more negative potentials. Thiol-terminated 4 is interesting since SA, LB, and evaporated films have been studied electrochemically. It is found (Table 111)that the evaporated and LB films (six layers) behave the same as the other oligoimides studied. The cathodic and anodic waves occur at approximately the same potentials, and the cathodic peak width is substantially larger in the evaporated films. The peak current varies proportionatelywith sweep rate expected for a surface-confiied redox couple, and the totalnumber of faradays passed is equivalent to the number of moles of A groups deposited on the electrode. The CV of a SA monolayer of 4 in H2O and (TMA)Cl is quite different from that for the LB film. The SA film shows broad peake at -0.63 V (cathodic) and -0.43 V (anodic). Addition of BaClz does not lead to the unsymmetrical shape of Figure 4.
Summary Self-assembled oligoimide films are quite stable to electrochemicalreduction in both DMF and water solvents.
Langmuir, Vol. 8, No. 12, 1992 3007
LB and Self-Assembled Film Built from Oligoimides
A 0.02
I3 4
Wavelength (nm)
Figure 5. Spectroelectrochemical reduction of 5 (six LB layers transferred at 7 mN m-l) using 0.1 M aqueous BaC12. Difference spectra plotted as a function of applied potential (see text). Negative absorbance which appears at 350 nm due to depleted starting material. Peaks which successively grow in at 450 nm and at 415, 6-46, and 590 nm due to 5'- and P,respectively.
Study of the rate of electron transfer to the A units in the self-assembledfilms showed that in DMF the longer rigid oligomides (2,4) reduced slightly slower than the shortest (I). Very large cations like tetraphenylarsonium slowed thereaction. These results are understandable if the films admit cations, but (as expected) the rate of ion incorporation into the film is important to the overall rate. LB films insolubilized with alkaline-earth ions give chemically reversible voltammograms. The voltammogram shapes are quite different from those of the SA films, but similar to those for syringed-on films. Spectroelectrochemistry shows that anion radical dimersand dianions are formed. IR analysis of the reoxidized films shows that the redox cycle changes the film morphology, making it more isotropic. The difference between SA and LB film electrochemistry may arise because the LB or syringedon films can adopt a more stable morphology as they reduce. SA films are constrained in their morphological changes because of the Au-thiol tether. This makes the SA films more open in structure and more permeable to cations, but d m not allow such strong interactions between oligoimide molecules.
Experimental Section The preparation of the various thiols (except for 2 and 3) and the gold thin films on glass and the details of the preparation of LB and SA fiis on gold have been published.SsB Synthesis of HS-Ph-A-B-A-Ph-SH (2). To a two-necked flask charged with 19.7 mg of A-B-A (0.027 mmol) and 10 mg of 4-aminothiophenol (0.078 mmol) was added 5 mL of dimethylformamide, and the resulting solution was heated under nitrogen at 135 OC for 24 h. Upon completion, ether was added and the product was filtered, washed with a copious amount of diethyl ether, and dried to yield 15.2 mg (60%) of yellowish brown powder. IR (KBr pellet): 1716,1676,1447,1344,1246 cm-l. lH NMR (CDCh, trifluoroacetic acid): 8.94 (8 H, br s), 7.6-7.8 (4 H, m), 7.2-7.4 (6 H,m), 3.87 (6 H, br 8). MS-FAB (Hapod): calcd for C54H3&0&2 958.1, found 958.1 (M+).
Synthesis of A-CHKH&HrA. To a 50-mL two-necked round-bottom flask were added 0.257 g of naphthalene dianhydride (0.96m o l ) , 15mL of dimethylacetamide, and a magnetic stir bar. A reflux condenser, a Cas04 drying tube, and a NZ source were connected to one neck of the flask. An addition funnel containing 0.01 mL of 1,3-diaminopropane (0.12 "01) and 5 mL of DMA was connected to the other neck of the flask. The solution was stirred and heated at 135 OC under a nitrogen atmosphere until dissolution occurred. Subsequently, the 1,3diaminopropane solution was added dropwise over a 3-h period. After the addition was complete, the reaction was stirred and heated at 135 OC under a nitrogen atmosphere for an additional 1h. After cooling to room temperature, 150 mL of diethyl ether was added and the precipitate was filtered through a fine fritted scintered glass funnel and rised well with diethyl ether (5X, 15 mL). The product was obtained by dissolving the precipitate in CHzClz and filtering off the insoluble excw dianhydride. The product (0.064g, 79 % ) was isolated by evaporatingthe methylene chloride. IR (KBr pellet): 1785,1741,1706,1663,1498, 1340, 1240 cm-l. lH NMR (DMSO-&): 8.69 (dd, 8 H), 4.19 (t, 4 H), 2.20 (m, 2 HI. Synthesis of HS-Ph-A-Propyl-A-Ph-SH (3). The procedure for synthesizing2 was used except that A-CHzCH2CHrA was used instead of A-B-A. The reaction gave a 70% yield as a yellowish brown precipitate. IR (KBr pellet): 1706,1664,1340, 1248 cm-'. lH NMR (CDCls, TFA): 8.86 (br s, 8 H), 7.7 (m, 4 H), 7.2-7.3 (m, 4 H), 2.3 (m, 2 H). MS-DC1 (CH& calcd for ClsHuNiO& 788.1, found 790.3 (M+ 2H). Electrochemistry. Measurementa were made in a onecompartment cell using a PAR Model 175 potentiostat and a Model 173 programmer in a three-electrode configuration with a Houston X-Y recorder. The counter electrode was a Pt mesh of 10 cm2area. The reference electrode was a saturated KC1 calomel electrode (SCE). The electrolyte Solution was degassed with welding grade argon for a minimum of 15 min. UV-Vis Spectroelectrochemical Experiments. UV-vis spectroeledzochemiatrywas carried out in a previously described optically transparent flow through thin-layer cell (100-pm path length).ls Data were collected on a Tracor Northern TN-6600 rapid-scan diode-array apparatus, employing a Xe arc lamp as the light source. Electrolyses were controlled by a BAS-100 electrochemical analyzer. Electron Transfer Kinetics. The anodic peak positions at various sweep rates were noted and plotted out as a function of In (sweeprate). The various rate constants were then calculated according to the Laviron methodlawith 8, the transfer coefficient for the oxidation, set at 0.5.
+
-
Acknowledgment. This work was supported by the National Science Foundation and the Office of Naval Research. Registry No. 1,134334-87-5;2,132054-16-1;3,144018-00-8; 4,132029-01-7; 5,129250-09-5; 6,138313-09-4;7,129250-12-0; 8, 64005-86-3; A-B-A, 129250-07-3; Au, 7440-57-5; F&C02H, 7605-1; CHCla, 67-66-3; DMF, 68-12-2; MerNBF,, 661-36-9; (CsHlz)rN+, 20256-54-6; P W + , 15912-80-8; KCl, 7447-40-7; CaC12, 10043-52-4; BaCl2, 10361-37-2; 4-aminophenol, 1193-028; naphthalene dianhydride, 81-30-1; dimethylacetamide, 12719-5; 1,3-diaminopropane, 109-76-2.
