Spectroscopy and Electrochemistry of Langmuir-Blodgett Films

Three. (1) Roberta, G., Ed. Langmuir-Blodgett Films; Plenum Press: New. York, 1990. Ulman, A. Ultrathin Organic Films; Academic Press: San. Diego, CA,...
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Langmuir 1992,8, 876-886

876

Spectroscopy and Electrochemistry of Langmuir-Blodgett Films Formed from Rigid Rod Oligoimides Vince Cammarata, Ljiljana Atanasoska, Larry L. Miller,* Chad J. Kolaskie, and Barbara J. Stallman Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received July 11, 1991. In Final Form: October 21, 1991

Nine oligoimideswith structures based on 1,4,5,8-naphthalenetetracarboxylicdianhydride (A) and 3,3'dimethoxybenzidine (B) were synthesized and characterized. The end groups X on XPh-A-B-A-PhX were varied and the number of repeat units (therefore the length) was also changed. The longest oligoimide was B-A-B-A-B-A-B-A-B, a rigid rod 78 8, long. Applied to the water subphase of the Langmuir trough, pressure/area curves were recorded. Langmuir-Blodgett films were transferred by horizontal lifting to a variety of solid surfaces. The transfer ratios were 0.95 f 0.05. The transferred films were analyzed by UV, X-ray photoelectronspectroscopy,grazingangle reflectanceIR, attenuated total reflectance IR, and transmission IR and were studied by cyclic voltammetry. In several cases anisotropic films were produced in which the average long molecular axis of the oligoimide was tilted up from the surface of the substrate by 10-25'. In other cases the films were nearly isotropic. The structure and electrochemistry are compared to thin films of oligoimides formed by other methods.

Introduction Although the Langmuir-Blodgett method continues to be of interest for the formation of oriented thin films, the types of molecules used in this method are largely limited to amphiphiles.' It is typical to use a molecule which has an active group, e.g. a chromophore or electrophore, and a long aliphatic chain, which provides organization within each layer.' In order to expand the scope of LB materials, it is of interest to study compounds which lack the aliphatic tail. LB films from such compounds would, in principle, afford a much higher density of active groups in each layer. The question is whether or not such compounds will lead to organized and transferable films and, if so, how they are organized. Our work in this direction has involved rigid rod molecule^.^^^ For other reasons we prepared some rigid molecules of unusual length and several examples of such compounds, e.g. 1 and 2, produced surface pressuretarea 0

&

0

A r O

o

ococn, COO€)

0

M

0

1

A r O

COOL1 0

2

diagrams indicating that close-packed, stable layers were formed by compression on a Langmuir trough. Because molecules with nonpolar end groups behaved similarly to those with polar end groups, the surface activity seemed to stem from the rod structure, not the molecules' amphiphilicity.2 Subsequent studies on 3 and 9 demonstrated that rigid rod oligoimides behaved similarly and that the films could be transferred by the horizontal lifting method.3 Here we report on LB films from nine oligoimides. We have varied the end groups on these oligomers and we have studied molecules that are up to 78 8, long. Three (1) Roberta, G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. Ulman, A. Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (2) Kenny, P. W.; Miller, L. L.; Rak, S. F.; Jozefiak, T. H., Christopfel, W. C.; Kim, J. H.; Uphaus, R. A. J . Am. Chem. SOC.1988, 110,4445. (3) Cammarata, V.; Kolaskie, C. J.; Miller, L. L.; Stallman, B. J. J. Chem. Soc., Chem. Commun. 1990, 1290.

IRmethods, X-ray photoelectron spectroscopy (XPS), and UV spectroscopy have been used to elucidate the structure of transferred multilayer films. These materials have some special interest because of the importance of polyimide thin films in microelectroni~a.~ In addition we are interested in comparing the structure and properties of these LB films with 'self-assembled" monolayers formed by attaching oligoimides covalently to surfaces.6 Properties of some interest accrue to films containing oligoimide anion radicals.6 It is known that electrically conducting solids can be formed from these anion radicals and there is interest in preparing conducting LB films.' Here we report preliminary electrochemical studies which delineate conditions under which an oligoimide LB film on an electrode can be cathodically reduced, forming anion radicals that do not desorb from the surface. The literature contains few examples of LB films composed of molecules lacking long aliphatic tails. The early study of quinquethiophene6 has been followed by several investigations of thiophene ~ligomers.~ When mixed layers of fatty acids and the oligothiophene were studied by linear dichroism UV measurements, it was concluded that the oligomer was standing more or less perpendicular to the substrate surface. Several reports of LB films formed from phthalocyanines have appeared. When the compound has an attached alkyl chain, or(4) Mittal, K. L., Ed. Polyimides; Plenum Press: New York, 1982. (5) Kwan, W. S. V.; Penneau, J. F.; Miller, L. L. J.Electroanal. Chem. 1990,291, 295. (6) Dietz, T. M.; Stallman, B. J.; Kwan, W. S. V.; Penneau, J. F.; Miller, L. L. J. Chem. SOC.,Chem. Commun. 1990,367. (7) Conducting LB films: Richard, J.; Vandevyver, M.; Lesieur, P.; Ruaudel-Teixier, A.; Barraud, A. J. Chem. Phys. 1987,86, 2428. Ikeg-

ami, K.; Kuroda, %I.; Saito, M.; Saito, K.; Sugi, M. Thin Solid Films 1988, 160, 139. Vandevyer, M.; Richard, J.; Barraud, A.; Ruaudel-Teixier, A. J . Chem. Phys. 1987, 87, 6754. Conducting imide anion radical J.;Muller, salts: Heywang,C.;Born,L.;Fitzky,H.G.;Hassel,T.;Hocker, H. K.; Pittel, B.; Roth, S. Angew. Chem., Int. Ed. Engl. 1989,28,483. (8) Schoeler, U.;Tews, K. H.; Kuhn, H. J. Chem. Phys. 1974,61,5009. (9) Yamamoto, N.; Ohnishi,T.; Hatakeyama, M.; Tsubomura, H. Thin Solid Films 1980,68,191. Tusaka, S.; Tatz, H. E.; Hutton, R. S.; Orenstein, J.; Fredrickson, G. H.; Wang, T. T. Synth. Met. 1986,16,17. Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. Thin Solid Films 1988, 160, 87.

0743-7463/92/2408-0816$03.00/00 1992 American Chemical Society

Langmuir, Vol. 8, No. 3, 1992 877

LB Films Formed from Rigid Rod Oligoimides

ganization is apparent.1° Without this addend, it has been proposed that organized layers are formed in one case." The effect of alkylgroups on the LB f i i forming properties of smaller aromatic hydrocarbons has been studied.12 It was shown that organization required some alkyl group, for example, butyl and propanoic acid groups on an anthracene ring. Rigid rod polymers have been investigated quite successfully by Wegner and co-workers,l3who prepared poly(siloxyphthalocyanines),which were substituted with hexyl groups to provide solubility. These compounds formed transferred LB films which were shown to be anisotropic. The rods were found to lie down on the surface of the substrate and t o become oriented with their long axis more or less parallel to the dip direction. Also pertinent to the present work are recent publications in which poly(amic acids) were formed into transferred LB films then imidized to form polyimide films.14 Experimental Section Chemicals. Millipore systems, filtered, distilled water (18 Ma resistivity) or Mallinkrodt HPLC grade water gave identical results. BaC12, KC1, HzSO,, and NaHC03 were reagent grade, used as received from Mallinkrodt. CHzClz and CHC13 were reagent grade from Fischer Chemical and passed down an alumina column (Neutral Brockman activity 1, Fischer Chemical) to remove acidic impurities. Trifluoroacetic acid (TFA) was obtained from Aldrich and used as received. Au-coated slides were prepared as in ref 15. Quartz slides, 3 mm thick, were obtained from GM Associates, Inc., and were cut to 2.54 cm X 1 cm. Ge ATR crystals (Wilmad Glass Co.) had 45' entrance and exit faces with overall dimensions of 50 mm X 10mm X 2 mm. Preliminary UV-vis data were obtained on borosilicate glass slides (Curtin Matheson Scientific, Inc.) which were rendered hydrophobic by reacting them with 10% (v/v)Me3SiCl in CH3CN overnight,then rinsing with CHCl3. The compounds 3, 4, and 11 were synthesized as previously described.6m6 The following chemicals were purchased from Aldrich and used without further purification: 1,4,5,8-naphthalenetetracarboxylic dianhydride (A); 3,3'-dimethoxybenzidine (B); sulfanilic acid, sodium salt hydrate; 4-aminobenzoic acid; 4-aminobenzonitrile; tetrabutylammonium bromide (TBABr); NJV-dimethylacetamide (DMA) (Sure/Sealor anhydrous stored over activated 4-A molecular sieves (MCB)). Zinc acetate dihydrate was purchased from Mallinkrodt and 4-aminothiopheno1 from Lancaster Synthesis, Ltd. Infrared spectra were obtained with a Perkin-Elmer 1600 FTIR instrument. lH NMR spectra were obtained with either an IBM-AC 200 or an IBM-AC 300 instrument or a Varian Unity 300 or Unity 500 instrument. Fast atom bombardment (FAB) masa spectra were recorded on a VG 7070E-HFinstrument. HPLC analyses were done using a system composed of two Waters Associates Model 6000A pumps with a Model 660 solvent programmer and a UV detector manufactured by Applied Bio~~

~

(10)Barger, W. R.; Snow,A. W.; Wohltjen, H.; Jarvis, N. L. ThinSolid Films 1985,133,197.Kalina, D. W.; Crane, S. W. Thin Solid Films 1985, 134,109. Ogawa, K.; Kinoshita, S.-I.;Yonehara, H.; Nakahara, H.; Fukuda, K. J. Chem. Soc., Chem. Commun. 1989,477. (11)Hann, R.A.; Barlow, W. A.; Eyres, B. L.; Twigg, M. V.; Roberta, G. G. Proceedings of the 2nd International Workshop on Molecular Electronic Deuices, Washington,1983; Marcel Dekker: New York, 1987. (12)Vincett, P.S.;Barlow, W. A.; Boyle, F. T.; Finney, J. A.; Roberts, G. G. Thin Solid Films 1979,60,265.Vincett, P. S.;Barlow, W. A. Thin Solid Films 1980, 71,305. Steven, J. H.; Hann, R. A.; Barlow, W. A.; Laird, T. Thin Solid Films 1983,99, 71. (13)Sauer,T.; Wegner,G.Mol.Cryst.Liq. Cryst. l988,162B,97.Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A,; Wegner, G. Thin Solid Films 1990,187,357.Kalachev, A. A,; Sauer, T.; Vogel, V.; Plate, N. A.; Wegner, G. Thin Solid Films 1990,188,341. (14)Suzuki, M.-A.; Kakimoto, M.-A.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986,395. Kakimoto, M.-A.; Suzuki, M.-A.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986,823. (15) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991,7, 1419.

systems, Model 757. The solvent used was a 1:lmixture of CH8CN/H20 with 0.08% TFA (v/v). The detection wavelength was 254 nm. A-B-A. 1,4,5,8-Naphthalenetetracarboxylicdianhydride, A (0.6273 mmol, 0.1681 g), and 10 mL of DMA were added to a two-necked, 25-mL round-bottom flask. A reflux condenser, fitted with a drying tube containing anhydrous calcium sulfate, was inserted into one neck of the flask. A dropping funnel containing 3,3'-dimethoxybenzidine, B (0.2091 mmol, 0.0510 g), dissolved in 2 mL of DMA was inserted into the other neck. The mixture of A was stirred under a nitrogen atmosphere and heated at 90 OC. Upon dissolution, diamine B was added dropwise over a 10-minperiod. The mixture was then stirred and heated at 135 "C for 16 h. After cooling to room temperature, 40 mL of diethyl ether was added to precipitate out the product. This was filtered through a fine porosity glass frit and rinsed with diethyl ether (three 10-mL portions). The crude product was purified by dissolving it in methylene chloride and filtering off the excess A. The solvent was evaporated off under reduced pressure, yielding 0.1110 g (71%)of golden colored powder. IR (KBr) 2980 cm-l, 1786, and 1743 anhydride C = O , 1718 and 1677 imide C - 0 , 1636,1604,1583,1498,1446,1370,1343,1287,1244,1190,1152, 1119,1028,874,852,808,762,735,705,569. 'H NMR (CDCl3/ TFA-d) 3.87 ppm (e, 6 methoxy H), 7.31-7.41 (m, 6 phenyl H), 8.93 (d, J = 7 Hz, 4 naphthyl H), 8.99 (d, J = 7,4 naphthyl H). FAB-MS (MNBAmatrix) calculated for C42H20N2012: 744.1015; observed (M H)+745.1125. HPLC retention time 18.0 min; flow rate 0.5 mL/min. Na03SPh-A-B-A-PhS03Na (4). A-B-A (0.1082 mmol, 0.0805 g), sulfanilicacid, sodium salt hydrate (1.082 mmol, 0.2110 g), and 6 mL of DMA were added to a 10-mL round-bottom flask equipped with a magnetic stir bar, reflux condenser, and drying tube containing anhydrous CaSOd. The mixture was stirred at 135 OC under a nitrogen atmosphere for 16 h. After cooling to room temperature, 30 mL of diethyl ether was added to precipitate out the product. The crude product was filtered through a fine porosity glass frit, rinsed with diethyl ether (three 10-mLportions) and then water (one, 10-mLportion), and dried. This compound was very insoluble in water and organic solvents. IR (KBr) 3072 cm-l, 1716, and 1673 imide C=O, 1603,1580,1498,1447,1420, 1396,1349,1249,1196,1129,1040,1014,980,881,843,823,767, 752, 716, 669, 644, 626, 570, 519. Bu,NOsSPh-A-B-A-PhSO&h,N (3). Methylene chloride (20 mL) and 4 (0.0903 mmol, 0.0992 g) were placed in a separatory funnel. BaNBr (0.1805 mmol, 0.0581 g) was dissolved in 20 mL of water and the solution was transferred to the separatory funnel. The mixture was shaken over the course of several hours. The organic layer was drawn off, extracted with water (three 10-mLportions), and dried with anhydrous MgSO,. The solvent was removed under reduced pressure. The resulting brown crystals were further dried under high vacuum for 12 h giving 0.0858 g (62%) of golden tan crystals. IR (KBr) 2961 cm-1, 2874,1717, and 1676 imide C=O, 1601,1579,1500,1448, 1349,1250,1198,1120,1030,1014,983,879, 858,786,768,754, 711,668,643,623,569,519. lH NMR (CDCl3) 1.01 ppm (t,J = 7 Hz, -CHZCH~CH~CH~), 1.42 (sextet,J = 7, -CH2CHzCH&Hs), 3.26 (m, J = 7, -CHzCHz1.66 (m, J = 7, -CH~CHZCH~CH~), CHzCHs), 3.88 (s,6 methoxy H), 7.26-7.4 (m, 10 phenyl H), 8.1 (d, 4 phenyl H), 8.85 (s,8naphthyl H). FAB-MS (MNBAmatrix) calculatedfor C&I~N&&, 1536.6637;observed M-, 1536.6659. HPLC retention time 12.98 min; flow rate 0.5 mL/min. A-B-A-PhS03Na (5). The same procedure was used as for 4 except that 0.020 g of A-B-A (0.027 mmol), 0.0053 g of sulfanilic acid, sodium salt hydrate (0.027 mmol),and 5 mL of DMA were heated at 135 OC for 24 h. The crude product was rinsed with water (10mL) and then diethyl ether (three 10-mLportions). After drying, 0.0153 g (60%) of yellowish powder was isolated. IR (KBr) 1786 cm-l and 1746 anhydride C=O, 1717 and 1676 imide C=O, 1498, 1346, 1247. 1H NMR (CDClaITFA-d) 3.88 ppm (bs, 6 methoxy H), 7.3-7.5 (m, 8 phenyl H), 7.6 (m, 2 phenyl H), 8.96 (m, 8 naphthyl H). FAB-MS (MNBA matrix) calculated 899.1129. for C&%N3014SNa,921.087;observed (M- Na+ + H)+, HOOCPh-A-B-A-PhCOOH (6). The same procedure was used as for 4 except that a solution of 0.0707 mmol(O.5260 g) of A-B-A and 0.424 mmol(O.5810 g) of 4-aminobenzoicacid in 10 mL of DMA was heated at 135 "C for 24 h. The crude product

+

Cammarata et al.

878 Langmuir, Vol. 8, No. 3, 1992 was purified by rinsing it with water (two 10-mL portions) and ethanol (10 mL); 0.0501 g (73%) of a dark yellow powder was isolated. IR (KBr) 3448 cm-l -OH, 1715 and 1679 imide C=O and carboxyl C=O, 1605, 1579, 1500, 1448, 1425, 1346, 1248, 1197, 1119, 1026,981, 860, 767,745,715,544. 'H NMR (CD2ClJTFA-d) 3.92 ppm (s,6 methoxy H), 7.3-7.5 (m, 6 phenyl H from benzidine moiety), 7.6 (d, J = 8 , 4 phenyl H), 8.4 (d, J = 8,4 phenyl H), 9.02 (s,8 naphthyl H). FAB-MS (H3P04 matrix) calculated for C56HmN4014,982.1750; observed (M + H)+, 983.1860. HPLC retention time 5.82 and 6.12 min. (perhaps due to diacid and its monoionized salt); flow rate 0.9 mL/min. NCPh-A-B-A-PhCN (7). The general procedure was modified in that a Lewis acid catalyst, Zn(OAc)z, was added.16 AB-A (0.0574 mmol, 0.0427 g), 4-aminobenzonitrile (0.350 mmol, 0.0413 g), and Zn(OAc)2*2HzO(0.116 mmol, 0.0255 g), in 10 mL of DMA were reacted a t 135 "C for 25 h. Beige powder (0.0382 g, 71%) was isolated. The product was sparingly soluble in a solution of methylene chloride and trifluoroacetic acid. IR (KBr) 2229 cm-1 -CN, 1718 and 1676 imide C=O, 1579, 1500, 1447, 1381,1346,1247,1195,1029,982,860,768,754,557. 'H NMR (CD2C1dTFA-d)3.90 ppm (s,6methoxy H), 7.38-7.43 (m, 6 benzidine H), 7.66 (d, J = 7, 4 phenyl H), 8.03 (d, J = 7, 4 phenyl H), 8.97 (dd, 8 naphthyl H). FAB-MS (H3POdmatrix)calculated for C,&I2&Olo, 944.1860; observed (M H)+,945.1976. HPLC retention time 21.84 min; flow rate 0.9 mL/min. HSPh-A-B-A-PhSH (8). The same procedure was used as for 4 except that 4-aminothiophenol (0.0605 mmol, 0.0076 g), and A-B-A (0.0202 mmol, 0.0150 g) were heated in 5 mL of DMA at 135 "C for 24 h. In order to precipitate out the product, a mixture of hexane and diethyl ether was used. The pale yellow precipitate was washed with ether yielding 0.0131 g (67%) of product. IR (KBr) 1715 cm-l and 1675 imide C=O, 1490,1346, 1247. 1H NMR (CDCldTFA-d) 3.88 ppm (s,6 methoxy H), 7.27.4 (m, 6 phenyl H from benzidine moiety), 7.6-7.8 (dd, 8 phenyl H), 8.95 (bs, 8 naphthyl H). FAB-MS (H3P04matrix) calculated for C@HmN,O&, 958.1396; observed (M - HI+, 957.3. The compound did not elute from the HPLC column. B-A-B-A-B (9). 3,3'-Dimethoxybenzidine (0.4098 mmol, 0.1000 g) and 8 mL of DMA were added to a 25-mL, two-necked round-bottom flask. A reflux condenser, fitted with a drying tube containing anhydrous CaS04, was inserted into one of the necks and a stopper in the other. The mixture was stirred and heated at 80 OC under a nitrogen atmosphere. Upon dissolution, 0.04032 mmol of A-B-A (0.0300 g) was added in small portions over a 20-min period. The temperature was then increased to 135 "C and the mixture was heated and stirred for 15 h. After cooling to room temperature, 30 mL of diethyl ether was added. The gray precipitate was filtered and rinsed with acetone (10 mL)and then diethyl ether (three 10-mLportions), yielding 0.0275 g (57%) of gray powder. IR (KBr) 1714 and 1676 cm-l imide C=O, 1579, 1500,1448,1397,1348, 1249,1199,1027,859,768. 1H NMR (CDC13/TFA-d)3.88 ppm (m, 12 internal methoxy H), 4.03 (a, 6 external methoxy H), 7.3-7.6 (m, 18phenyl H), 9.00 (8, 8 naphthyl H). FAB-MS (MNBA matrix) calculated for C70&&01~, 1196.3216; observed (M H)+, 1197.3268. HPLC retention time 6.54 min; flow rate 0.5 mL/min. B-A-B-A-B-A-B-A-B (10). The same procedure was used as for 9 except that B-A-B (0.278 mmol, 0.200 g) and 5 mL of DMA were heated to 135 OC. Upon dissolution, A-B-A (0.1008 mmol, 0.0750 g) solid was added slowly. The mixture was stirred at 135 OC for 15 h. Black material (0.160 g, 74%) was isolated. IR (KBr) 3426 cm-l -NH, 1718and 1676imide C=O, 1602,1578, 1499,1447,1395,1346,1246,1195,1113,1024,981,859. 'H NMR (CDCldTFA-d) 3.89 ppm (d, 24 internal methoxy H), 4.03 (s,6 external methoxy H), 7.35-7.40 (m, 30 phenyl H), 9.00 (b, 16 naphthyl H). HPLC retention time 10.63min; flow rate 0.9 mL/ min. Langmuir-Blodgett Experiments. LB experiments were performed on a KSV5000 LB trough from Oriel Corp. with computer control and temperature regulation. Differential surface pressures were measured with a Wilhelmy balance constructed from a 3 cm x 1 cm Pt/Ir (90%/10%) plate. Isotherms were measured a t 20 "C, with a compression speed of

+

+

(16) Grazer, F.; Kilpper, G. Ger. Patent 2,146,027;Chem. Abstr. 1973, 79,80340.

5 mm/min. Transfer was accomplishedusing the horizontallifting technique (see below) a t 23 "C after compressing the film a t 10 mm/min and holding the required pressure for 1h. The substrates for deposition were cleaned as follows: Ge crystals were argon ion plasma etched for 30 min a t 100W, quartz and glass substrates were sonicated in CHCl3 for 1h and then rinsed 10 min prior to film transfer, and Au-coated slides were rinsed with CHCls 10 min prior to film transfer. IR and UV-Vis Measurements. GIR experiments were performed as in ref 15on a Mattson Instruments Sirius 100FTIR spectrometer with a Harrick Scientific Corp., external reflectance attachment set at a constant (-8") grazing angle. In order to free the carbonyl region from absorption due to water, it was necessary to purge the spectrometer with dry N2 for >4 h and carry out all manipulations in the sample compartment through a glovebag. Background spectra were taken on clean Au-coated slides, and generally 3000 scans were coadded to increase the signal to noise ratio. Attenuated total reflectance (ATR) and transmission experiments were performed on Ge ATR crystals using the same inert conditions as above, also with 3000 scans, coadded to increase signal to noise. Polarized IR experiments utilized a KRS-5 substrate/0.2 pm aluminum wire grid polarizer from Cambridge Physical Sciences. The polarizer was >96% efficient, but calculations assumed 100% efficiency. UV-vis experiments were performed on a Shimadzu UV-160 spectrophotometer using quartz substrates held 90"to the incident beam. The sample was referenced in a double beam experiment to a clean quartz slide. XPS. XPS was performed using a Perkin-Elmer Physical Electronics 555 spectrometer (Mg K a X-ray source, 250 W)with a PDP-11 computer for dataacquisition and analysis. The surface compositions of oligoimide LB films were calculated from the areas of photoelectron peaks corrected by the following sensitivity factors: C Is, 0.205; 0 Is, 0.67; N, Is, 0.43; and S 2p, 0.53.16 Analysis of the high-resolution core level spectra included background subtraction and iterative lineshape decomposition, as described previously in detail.16 The length of the acquisition time had no measurable effect on core level lineshapes and the atomic ratios. Angular-resolved XPS studies were carried on a small spot Perkin-Elmer Physical Electronics 5400 spectrometer. Electrochemistry. Measurements 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 counterelectrode was a Pt mesh of 10 cm2 area. The reference electrode was a saturated KCl calomel electrode (SCE). The electrolyte solution was degassed with welding grade argon for a minimum of 15 min.

-

Results and Discussion Compounds. Because so little is known about LB films from rigid compounds which lack an aliphatic tail, it was of interest to examine a range of oligoimideswith different end groups and with different lengths. With this in mind oligomers were synthesized by first condensing together naphthalene dianhydride (A) and dimethoxybenzidine (B) to form the diimide A-B-A (Scheme I). For the preparation of derivatives with different end groups (3-8) this compound was then capped with variously substituted anilines yielding the compounds XPh-A-B-A-PhX, and XPh-A-B-A. In the case of the compounds where X = SOs-, both sodium and tetrabutylammonium (BmN+)salts were prepared. Longer rigid rod compounds (9, 10) were prepared starting from A-B-A (24.2 8, long). Thus, reaction of AB-A with excess B produced B-A-B-A-B (9, 44.3 &. Reaction of excess B-A-B with A-B-A gave B-A-B-A(10, 78.5 A). This compound and the B-A-B-A-B (11,75.0 previously reported B-A-B-A-B-A-B-A-PhSH A) are of extraordinary lengths5 It was key to this work that we were able to solubilize the larger compounds in a mixture of chloroform and trifluoroacetic acid for purification and handling. The sol-

Langmuir, Vol. 8, No. 3, 1992 879

LB Films Formed from Rigid Rod Oligoimides Scheme I

A

I

1

I

B

"3

3 28

'40

u.

3 '68

'88

'128

'188

Mean M o l e c u l a r A r e a

Figure 1. LB isotherms of 3,4,and 5,recorded at 293 K. The barrier speed was 5 mm/min with 3 spread from a CHCla solution

A-B-A

and 4 and 5 spread from 20% TFA/CHCl3 solutions.

f

\ J A l

D

X-Ph-A-B-A-Ph-X

'148

(2)

-

X- Ph- A- B A

ubilizing effect of TFA relies on the large number of basic sites on these oligoimides. It has been found, for example, that the solubility of A-B-A was decreased substantially by replacing dimethoxybenzidine (B) with benzidine. The methoxy groups on the benzidine units also increase twisting of the rings along the rod and this may improve solubility by interfering with packing of the molecules in the solid. Identification of oligomers 4-11 relied on IR, to show that the compounds were fully imidized, and NMR, to compare the number of A, B, and end groups. All the new compounds reported here, except 5 and 8 which did not elute, where shown to be more than 95% pure by HPLC. With the exception of the very involatile compounds 10 and 11, the oligomers gave satisfactory high-resolution mass spectra.

Cammarata et al.

880 Langmuir, Vol. 8, No. 3, 1992 Table I. Langmuir Trough Data no. 3

4

compound B&N+ -S03Ph-A-B-A-PhSOs-Bu,N+ Na+ -S03Ph-A-B-A-PhS03-Na+

4d

5 6

Na+ -SOsPh-A-B-A HOOCPh-A-B-A-PhCOOH

6d

7 8 9 10 11

NCPh-A-B-A-PhCN HSPh-A-B-A-PhSH B-A-B-A-B B-A-B-A-B-A-B-A-B B-A-B-A-B-A-B-A-PhSH

a Length not including counterion. * From extrapolation of rising curveto zero pressure. Recorded at 20 OC. Area of substrate/change of area of film at 23 "C. d The parent compound w a spread ~ across an aqueous 1 mM BaC12/10 mM NaHC03 subphase.

-

of the slope of the curve at high pressure, but several compounds showed inflection points along the curves. Considering only data obtained on the first compression, the entire curve was quite reproducible as evidenced by variation in the intercept of only about 5%. The oligomer with sulfonate end groups was studied as the BudN+salt (3) and the Na+ salt (4). Surprisingly, the bulk BwN+ gave a smaller extrapolated molecular area than the Na+ salt (110 A2). When an aqueous (75 BaClz solution (pH 8) was used as the subphase, both 3 and 4 gave an extrapolated molecular area of 120 A2, suggesting that the film contained the Ba2+salt in both cases. Clearly, the counterion is an important part of these films. Compound 5, which has one sulfonate and one anhydride end group, gave a curve quite similar to that of 3. Sensitivity of the pressure-area curve to the end group was similarly illustrated by the oligomer 6, which has carboxylic acid end groups. On the basis of previous studies of fatty acids, it was expected that the presence of acid or Ba2+ in the aqueous subphase would have a substantial effect. In fact, on a H 3 subphase the curve rises from an intercept of 108 2 to a plateau a t 22 mN m-l. On a Ba2+ pH 8 subphase there is no plateau and the extrapolated area is 85 A2. Such behavior, smaller molecular area for the Ba2+ subphase, is opposite that observed for aliphatic fatty acids.' Compounds 7 and 8 have the same oligomer structure as the previous compounds but have nonionic and nonionizable end groups. With extrapolated areas of 80 and 90 A2, they behave very similarly to their amphiphilic counterparts 3-6. This suggests that an ionic or ionizable end group is not necessary to get surface activity from rigid rod molecules of this type. Indeed, our earlier study2 showed that the terminal groups of rigid molecules could be totally nonpolar and yet yield sharply rising pressure/ area curves. It is of interest that although some of the oligomers 3-8 exhibit intercepts at about 115A2molecule-l and a plateau, while others show simple curves with intercepts at 70-90 A2 molecule-', all these oligomers are compressed to an area of about 55 A2 molecule-l when the applied surface pressure reaches 25-30 mN m-l. This suggests a similar film structure for all these similarly sized molecules. Compounds 9 (44.3 A), 10 (78.5 A), and 11 (75.0 A) are longer than 3-8 (37-39 A, without counterion). The extra length is not important for compound 9, which shows a pressure/area curve and extrapolated molecular area (70 A2) that is quite like that of the slightly shorter oligoimides. Compounds 10 and 11, on the other hand show extrapolated molecular areas of about 300 A2, which are much larger (Figure 3).

i2)

K

I

11

length molecular transfer CPK," A area,' A* ratioC 39.1 75 1.00 39.1 110 1.10 120 0.95 31.5 78 1.00 37.7 108 0.94 85 0.98 37.8 80 1.08 36.9 90 1.11 44.3 70 78.5 300 1.05 75.0 300 0.98 Mean M o l e c u l a r A r e a

02

(A )

Figure 3. LB isotherms of 10 and 11 recorded a t 293 K. The barrier speed was 5 mm/min. All compounds were spread from 30% TFA/CHC&. The subphase was HzO.

Interpretation of the pressure-area curves must be tempered by consideration of the following experiments which show that the curves are determined kinetically, not thermodynamically. As reported earlier: there is hysteresis upon compression and expansion of these films. When the barrier is moved back after compression the pressure/area curve drops more steeply than expected for a reversible system. If the film is then recompressed, the curve nearly retraces the expansion curve. These results demonstrate kinetic control and can be interpreted as indicating that the molecules strongly aggregate and only slowly spread, once they have been compressed. When films were compressed to 7 mN m-l and held at that pressure, the molecular area decreased about 10% in 30 min. Thereafter, the area was stable. This indicates that some reordering occurred during the first 30 min. Preliminary experiments on oligoimides 3 and 6 and on compounds 1 and 2 demonstrated that the films would not transfer to solid substrates using the usual vertical dipping method. Substrates with different surface polarity, as well as different dip speeds and surface pressures were employed. The horizontal lifting method was, however, successful for all the compounds, 1-11. 3 was successfully transferred to gold, borosilicate glass, silylated glass, quartz, aluminum, silylated silicon,germanium, and glassy carbon with a transfer ratio (change in area of trough/area of substrate) of 1.0. In the horizontal lifting method" the plane of the substrate is lowered nearly parallel to the surface. When the substrate just touches the surface, noted by the positive meniscus, the travel of the substrate is reversed until the film-water adhesion is broken. Typically the angle between the substrate and film was 5-10', but it was not measured accurately and, indeed, did not appear to affect the results. The literature describes some rather complicated methods for horizontal lifting.17 These are designed to limit the amount of compound transferred to only those molecules in contact with the substrate and not those moleculesjust beyond the edge of the substrate. We chose to avoid these cumbersome approaches and to carefully analyze the material transferred. However, there is some concern (see below) about material contained in the droplet which adheres to the corner of the substrate when it is lifted. Figure 4 shows the surface pressure and the barrier position (proportional to film area) as a function of time for a film of 3. Lifting (manually operated) occurred (17) Kawaguchi, T.;Nakahara, H.; Fukuda, K. Thin S o l i d F i l m 1985, 133,29.Langmuir, I. V.; Schaefer,V. J. J.Am. Chem. SOC. 1938,60,1351. Tamm, L. K.; McConnell, H. M. Biophys. J. 1985, 47, 105.

LB Films Formed from Rigid Rod Oligoimides

Langmuir, Vol. 8, No. 3, 1992 881 Chart I z Laboratow

'5

'0

'15

io

'20

25

30

I

35

d=

Time (min)

Figure 4. Plot of film pressure and movable barrier position during the film transfer process at a fixed pressure of 7 dyn/cm for 3. Recorded at 296 K.

projection Of p on

xy plane

Solution +0.7

I

I

I

t

t

I

0 U

m f

f0

view of the short axis in p'z plane

v)

n

Q

z Relationship between laboratory coordinates and Euier angles for perpendicularlyoriented transition dipole moments

+R .0

L a r g m u i r - B l o d g e t t Film

+ o .3

>

W Disorder in short axes around long axis

a, V

c n

L

(0

D

Q

C0.D

I 30Q

I

I

Wavelength

1

500

Figure 5. UV-vis spectra of 3 at 296 K: (a) 8.2 X 104 M in CHC1, with 1cm path length quartz cell; (b) LB film of 18layers of 3 transferred aa in Figure 4.

approximately every 2 min and is signified on the curve by the rapid change in the surface pressure and film area. The key point is that at 7 mN m-l the ratio of material transferred to substrate area was 1.0 f 0.05 and reproducible even after many previous depositions. Transfer ratios to gold or glass for oligomers 4-11 (Table I) were also close to 1.0. Curiously, the Langmuir films of 3 and 4 did not transfer well to substrates within the first 30 min of compression and only after an hour of constant compression were transfer ratios of 1.0 observed. Therefore, this 1-hwaiting period was employed for all the other oligoimides as well. UV-Vis Spectroscopy. Transferred films of oligoimides 3,6,and 10 on quartz substrates showed bands at 383,363, and 345 nm (Figure 5). These same bands, but with slightly different positions and intensities, are present in the spectra of these compounds in chloroform solution. It is unclear whether the changes are due to orientational effects or intermolecular forces. Comparison with the spectra of the monomers A and B shows that these bands are due to the A unit, not the B unit. A Lambert-Beers law plot of absorbance as a function of the number of LB transfers is linear to 20 transfers for

3. Compounds 6 (transferred from water or from Ba2+) and 10 were also studied and showed linearity to 12 transfers. Thicker multilayers were not investigated. The molar extinction coefficient per A unit was constant for the three oligomers studied. These data coupled with the consistent transfer ratios from the first to the last dip implies that a consistent amount is deposited on each dip throughout the process. The horizontal transfer process leaves a small droplet on the corner of the slide, which was removed by vacuum aspiration. When the absorbance was measured on this corner, it was smaller by 10-50% than for any other part of the film. This suggests that some material was lost by the suction procedure. For future applications involving small substrates or in situations where a uniform film is needed across the entire surface, this could be a significant problem with the method used here. IR Spectroscopy. Transferred LB films have been studied by three types of IR spectroscopy: transmission, reflection at a grazing angle (GIR), and attenuated total reflectance (ATR). Our attention has focused on the carbonyl stretching region. The two bands of the imide group have been assigned as the symmetric (1715 cm-l) and the asymmetric (1675 cm-') carbonyl stretches.ls The s y m metric band is polarized along the long molecular axis. The asymmetric band is polarized along the short axis, which is perpendicular to the long axis and in the plane of the naphthalene ring. Two other strong bands appear in the spectra at 1350 and 1250 cm-l. These bands have been previously assigned as the symmetric and asymmetric (18) D e b e , M. J. Vac. Sci. Technol. 1982, 21 (l), 74.

(19)H a r r i c k , N.J.I n t e r n a l R e f l e c t i o n Spectroscopy; H a r r i c k Scientific Corp.: N e w York, 1967. M i r a b e l l a , F.M., Jr. Appl. Spectrosc. Rev. 1985, 21. 45.

Cammarata et al.

882 Langmuir, Vol. 8, No. 3,1992

1

n

---

Table 11. IR Data from Thin Films on Ge Crystals

I

T I

P

tilte twist0 compound transmission' ATRb 81716, ATRb w w, no. film D1716m-1 Dl716 deg Dl67S deg 3 LB 1.19 i 0.03 1.23 f 0.05 79 0.61 f 0.05 54 3 syringed 0.98 i 0.02 0.71 f 0.05 0.73 f 0.05 6 LB 1.04 i 0.03 0.88f 0.03 63 6 syringed 1.00 f 0.04 0.77 f 0.05 6 LB, Ba2+ 1 . 1 O i 0.05 0.75 fO.02 54

D = A,I(A, + Az). Calculated using the measured a D = A,/A,. n = 1.22 for this organic film. Isotropic films are produced by syringing -100 p L of solution of the compound onto -1 cm2 area of a Ge crystal.

,

,

,

,

,

,

,

,

,

,

,

,

,

,

l

,

,

l

,

I

,

,I

I

,

I

l

l

I

l

l

1

I

I

I

I

The same LB-coated germanium crystals which were used for transmission spectra of 3 were used for polarized ATR spectra. In ATR the internally reflected light is attenuated at those frequencies at which the surface LB film absorbs. The two polarization directions are y (p polarized) and [x sin (45') + z cos (45O)I (s polarized) and the dichroic ratio D = A,/[A, + A,] can be computed. Since the LB films of 3 were not isotropic in the x, y plane (see above), the usual uniaxial model for analysis of ATR data was not applicable.20 The equations used and their derivation are available as supplementary material. In essence the analysis uses the transmission data and treats absorbances in the x and y directions separately. It leads to eqs 2 and 3 for the long axis and short axis molecular vibrational dipoles, respectively.

D,, = (sin' 0 sin2 cp)[(l+ n3l2)sin2 u - n3121X [(sin2u - n312)(sin2e cos2cp) + (sin2u n324) cos2el-' (2)

+

= (cos20 cos2w sin2 cp + sin' w cos2cp 2 cos e cos w sin cp cos cp sin w ) [(I + n3,2)sin2 u - n312~ x [(sin2u - n312)(~os2 e cos2w cos' cp + sin2w sin2cp 2 cos e cos w sin cpcos cpsin w ) + (sin2u n324)sin2 e cos2WI-' (3)

D,,

+

In these equations n31 is the ratio n3/n1, where n3 is the index of refraction of Ge, n2 the LB film, and nl the N2 atmosphere. To use eqs 2 and 3 the refractive index of Ge was taken to be 4.03.21 The refractive index of the film was taken to be 1.22. This value was obtained from the 01715 = 01675 = 0.72 (Table 11) of the syringed-on sample of 3 using eqs 2 and 3 assuming the sample was isotropic so that 8 = 54.7O,w = 45O, and cp = 45O. The equal dichroic ratios for the two bands indicated that the syringed-on sample was, indeed, isotropic. Analysis of the ATR spectra for LB films of 3, gave an average long axis tilt angle, 8 = 79O. This indicates that the rods tend to lie down on the surface. The 01676 gave a short axis twist of 54O. LB films of 3 transferred a t 20 mN m-l showed similar ATR spectra with e = 8 1 O and w = 52O.

A concern with this calculation is the assumed refractive index of the LB film. An alternative value, n = 1.45 can be estimated by extrapolating the refractive index of polyimide films from the UV to the IR.22 Using n = 1.45 in eq 1 changed the average tilt angle 0 by 2O and w by 5O. This difference will not change any qualitative conclusions. It is noted that the data are nearly within experimental (20) Zbinden, R. Infrared Spectroscopy of High Polymers; Academic Press: New York, 1964. (21) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: New York, 1985. (22) Russel, T. P.; Gugger, H.; Swalen, J. D. J . Polym. Sci., Polym. Phys. Ed. 1983, 21, 1745.

Langmuir, Vol. 8, No. 3, 1992 883

LB Films Formed from Rigid Rod Oligoimides

n

The spectra of LB films of 3, transferred at either 7 or 20 mN m-l, have very small 1715-cm-l bands relative to their 1675-cm-' bands. Indeed, the intensity of the 1715-cm-l band can only be estimated because of interference from residual water. Calculation, using the Ge surface, ATR-determined w = 54O, and R > 10.2, gives the average tilt angle 8 > 76'. This is in good agreement with the ATR-determined 8 = 79O. An alternative calculation can be made assuming that the twist angle w = 45O, i.e., that there exists a random distribution of twist angles. This leads to 8 > 73O and illustrates that in this range this GIR method for determination of 8 is insensitive to small variations in w. For all other GIR analyses, where w is usually not determined independently, a twist angle of 45O will be assumed. GIR spectra were recorded for the oligoimide 4, which has the BQN+ of 3 exchanged with Na+ for films transferred from water to gold and from a pH 8 subphase containing 1mM Ba2+. In the former case the transferred films had GIR spectra with a ratio of carbonyl intensities, R = 3.0, very similar to the R = 2.0 expected for an isotropic sample. The LB film transferred as the Ba2+ salt (see XPS)was anisotropic, R > 7, where the intensity at 1715 cm-' from residual water significantly interfered with the determination. The average long axis tilt angle is estimated to be > 69O. As concluded from the pressurelarea curves, it is clear that the counterion has strong effects on the structure. For compound 6, which has carboxylic acid end groups, LB films were transferred from both an acidic and an 1800 1600 1400 1200 WAVENUMEERfCm" aqueous Be2+subphase. For both films the transmission, ATR, and GIR spectral intensities increased linearly with Figure 7. Grazing angle FTIR spectra of 3: (top) isotropic the number of layers from 2 to 10. Although distinct peaks sample, evaporated from a CHCls solution onto a Au-coated glass slide; (bottom) LB film of four layers compressed at 7 dyn/cm. could be distinguished, the resolution was not sufficient Angle of incidence = 85O, 3000 scans. to fully separate the imide 1675 cm-' and the carboxylic acid carbonyl peaks. Therefore, the GIR data were not error of an average twist angle of 45O, expected for a random useful for calculation of molecular orientation and only distribution of short axis vectors. the long axis tilt was calculable from the transmission and GIR spectroscopy selects for molecular vibrational ATR data. As indicated in Table I1 the x,y anisotropy dipoles that are perpendicular to the surface, that is the from transmission spectra was small, on the order of the z direction.18~~3 The absorbance measured in GIR is related experimental error. to the absorbance measured with transmission spectrosATR data were analyzed as above. The film transferred copy taking into account factors due to the enhancement from the acid subphase to Ge gave a tilt angle of 63O. The of the electric field at the surface and orientational effects. results for LB films transferred from the basic, aqueous Although the absolute intensities of the bands have been Ba2+subphase to Ge and analyzed by ATR have essentially used to estimate molecular 0rientations,2~ there was the same dichroic ratio as syringed-on films. considerable concern about the validity and utility of this LB films from the unsymmetrical 5, which has sulfonate approach for oligoimide films. Therefore, the relative (Na+) and anhydride end groups, and the symmetrical 7, intensities of the two carbonyl bands were used to analyze which has nonionizable, nitrile end groups, gave very the data and compute tilt angles, 8. Details of the similar carbonyl band ratios in GIR. The data lead to a treatment are given in the supplementary material, which calculated average long molecular axis tilt of about 65'. show that the ratio of the two bands in the LB film, R = Compound 10 is of particular interest because of its A1675/A1715 can be used with eq 4 to determine the tilt length and because it has so many rings, each with an angle, 8, where Aisois the isotropic absorbance determined independent twist about the long molecular axis, that the from a syringed film or a KBr pellet spectrum and w is the assumption of a 45' short axis twist angle is fortified. rotational angle defined above. Studied by GIR (Table 111)R = 13.7, and the average tilt angle in the LB film is calculated to be greater than 75O R = ( A ~ ~ ~ ~ tan2 ~ , 8~ cos2 / Aw ~ ~ ~ ~(4), ~ ~(the ) water peak contributes to absorption at 1715 cm-'1. An isotropic film (syringed on) gave the same R = 2.0 observed for other oligoimides. Figure 7 shows the GIR spectrum of an isotropic sample (a syringed-on film) of 3 on a Au-coated slide. The relative Compound 11, also quite long, is special because it is intensities of the carbonyl bands, R = 2.0, are the same thiol terminated and we suspected that the first monoas in the transmission spectrum of 3 as a KBr pellet (not layer might covalently bind to a gold substrate during the shown). transfer process. since we have previously used GIR to study the tilt angle of a chemisorbed monolayer of 11 on (23) Porter, M. A. Anal. Chem. 1988, 60, 1143A. gold, a comparison between LB and "self-assembled"films (24) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 1 1 , was of particular i n t e r e ~ t .The ~ GIR spectra show that 1215. Allara, D. L.; Swalen, J. D. J.Phys. Chem. 1982,86,2700. Allara, the average orientation of six to eight layered LB films D. L.; Nuzzo, R.G . Langmuir 1985, 1 , 52. T

I

884 Langmuir, Vol. 8, No. 3, 1992

Cammarata et al.

Table 111. G I R Data from Au-Coated Glass Slides compound no. 3 3

filma LB syringe LB LB, Ba2+ LB LB LB LB syringe

4 4 5 7 10 11 11

R = Aiei5IAimC >10.2 f 0.5 2.0 f 0.2 3.0 f 0.2 >7 5.1 f 0 4.6 f 0.3 >13.7 f 0.5 2.6 f 0.2 2.0 f 0.2

tilt angled 0, deg >73

A

1 LE layer

A

3 LB layers

0

4 LE layers

0 5 LE layers

60

>69

6 LE layers

66 65 >75 58

-

-

a LB films transferred at 7 dyn cm-l. Isotropic films are produced by syringing 100 pL of a CHC13 solution of the compound onto 1 cm*area of Au-coated slide. Subphase contained 1 mM BaC12 and 10 mM NaHC03. A minimum of three samples were measured. dcalculated using eq 2 assuming w = 45O, reproducible to f3'.

Table IV. Elemental Ratios from XPS c:s C:N c:o

NS

cmpd expt theory expt theory expt theory expt theory 4.2 5.4 2.9 3 3 44.3 43 15.4 14.3 3.8 4 6 14.8 14 3.3 5.4 1.9 2 8 24.5 27 12.7 13.5

have a tilt angle, 8, of 58". This can be compared to a tilt angle of 60" found by Kwan et al. for 11 self-assembled as a monolayer on Au from chloroform solution. This is also similar to the angle expected for an isotropic sample. In summary, compounds 3,4,5,7and 10 form LB films in which the long molecular axis is tilted up from the surface by 10-30". The tilt angles of LB films from 6 and 11 indicate that the films are almost isotropic. XPS. The spectra were obtained and analyzed as described in the Experimental Section. The atomic ratios for compounds 3,6,and 8 deposited as LB films on gold are compared in Table IV with the values calculated from the molecular formulas. As previously found for polyimides25v26and for self-assembled monolayers of thiophenol-terminated oligoimides on gold16 the agreement of experimental and expected atomic ratios is satisfactory (usually within 10%). The chemical shift values for the several kinds of carbon, nitrogen, sulfur, and oxygenswere consistent with previous assignments.16 No evidence for trifluoroacetic acid, chloroform, or large amounts of water was found, although they could have been outgassed in the XPS chamber. In the case of 3, the contribution of the tetrabutylammonium nitrogen cannot be resolved from the imide nitrogen emission. Even so, the experimental atomic ratios clearly demonstrate the presence of the countercations. If the presence of two Bu4N+ ions was not taken into account, the values for C:S, C:N, C:O, and N:S atomic ratios calculated from the molecular formula would be 27,13.5, 3.4, and 2, which are far from the experimental ratios. The average thickness per LB layer of 3 was evaluated from the angular-resolved photoemission of the gold substrate. The attenuation behavior of the Au 4f7p peak was monitored as a function of the number of LB layers and take-off angles. The method was based on a uniform overlayer model.27The signal from an ideally flat substrate attenuates exponentially with the increasing thickness of a uniform, homogeneous overlayer according to the equation (25) Atanasoska, Lj.; Anderson, S.G.; Meyer, H. M., 111; Zhanda, Lin; Weaver, J. H. J. Vac. Sci. Technol., A 1987, A5, 3325. (26) Atanasoska, Lj.; Meyer, H. M., 111; Anderson, Steven G.; Weaver, J. H.J. Vac. Sci. Technol., A 1988, A6, 2175. (27) Fadley, C. S. h o g . Solid State Chem. 1976, 2, 265.

0

4

8

Nlsln

12

16

8

Figure 8. Reduced Au 4f,/2 photoemission intensities, In ZA,,(~)/ZA,,(O), as a function of number of LB layers corrected for l/sin 6 (grazing emission data, below 40° were excluded). Id

= Io exp(-d/X(sin 8')

(5)

where I d and IO are the intensities of the Au signal with and without an overlayer of thickness d, 8' is the photoemission take-off angle (measured between the sample surface and axis of the detection cone), and Xis the length of the inelastic mean free path of the substrate photoelectrons in the overlayer. Equation 5 can be rearranged to a form which correlates experimentally derivable parameters by a linear relationship In I d / & = -don/X(sin 8')

(6)

The total overlayer thickness, d, can be expressed as ndo, where n is the number of LB layers (dips) and do is the thickness per layer. Thus a plot of nisin 8' vs In Id/IO should give a straight line with a slope equal to -&/A. Such a plot for LB films of 3 prepared by one to six dips is shown in Figure 8. The predicted linear variation of the Au 4f7/2 reduced core level photoemission intensities with the number of LB layers was obeyed for photoemission between 90" and 35" take-off angles.27 This allowed a close fit of the data (which involve both the wide range of take-off angles and LB films of various thicknesses) to a straight line and a determination of the slope, do/X. The thickness per dip was obtained from the slope assuming the attenuation length X of the Au 4f7/2 photoelectrons to be 66 A. This attenuation length was calculated by adopting X of 60 A for 1-keVphotoelectrons and an energy dependence of A(&) 0: EO.65 as determined by Cartier et aL28 for saturated hydrocarbon films on platinum and by Schrech et al.29for LB films of cadmium arachidate on gold. Thus, the estimated thickness per LB layer is 22 f 3 A. If it is assui led that the BmN+ ions are situated at the ends of the long axis of the oligoimide, the molecular length of 3 would be 55 A. Since the tilt angle is -79O, the thickness is -25 A. The thickness calculated from XPS is directly dependent upon the value assumed for A. The value adopted (28) Cartier, E.; Pfluger, P.; Pireaux, J.-J.; Rei Vilar, M. AppE. Phys. A 1987, A44, 43.

(29) Schreck, M.; Schmeisser, D.; Gopel, W.; Schier, H.; Habermeier, H. U.; Roth, S.; Dulog, L. Thin Solid Films 1989, 175, 95.

Langmuir, Vol. 8, No. 3, 1992 885

LB Films Formed from Rigid Rod Oligoimides

0.0

-0.4

0.0

-0.8

Potential (V vs SCE)

Figure 9. Cyclic voltammetry of a four-layer LB film of 3 in 0.1

M aqueous CaClz solution. The film was compressed at 7 dyn/ cm and 296 K. The solution was degassed with argon. Scan rates are as shown.

here has but it is much larger than some other values in the literature. An alternative value of X = 22 A, which has been suggested for organic gave a thickness per layer of 7.5 A. Considering the lengths and tilt angles of the molecules in these LB films, this is an unreasonable conclusion. Electrochemistry. The electrochemistry of oligoimides in solution and in self-assembled monolayers has been previously r e p ~ r t e d . ~The . ~ cyclic voltammogram of 3 in DMF, BmNBF4 solution shows two reversible couples with cathodic-anodic peak separations, AE, = 60mV and peak currents, I, 0: (scan rateV2. The cyclic voltammogram of self-assembled monolayers of 11on gold in DMF, B U & I ~ is F characteristic ~ of surface-confined species.The two couples are reversible with AEp 5 20 mV and I, 0: (scan rate). In water, the film shows only one couple before the onset of water reduction, and the peaks are broadened, with a peak separation of 110 mV and a peak width at half-height of 260 mV. For this initial investigation, LB films of 3 on gold electrodes were used. The LB films are slightly soluble in DMF, so the electrochemistry was performed in aqueous solution using an SCE reference. In 0.1 M aqueous KC1 electrolyte, the first cyclic voltammetric scan revealed a symmetric cathodic peak at -0.67V. No reoxidation peak was observed upon reversal. Using the known area/ molecule on the trough during transfer, the transfer ratio and the electrode area, it can be calculated that the integrated charge under the cathodic peak corresponds to 2 e-/molecule. The second voltammetric scan reveals no faradaic current and it seems likely that the anionic species formed cathodically are soluble enough to desorb. When 0.1 M CaC12 was substituted for KC1 as the electrolyte, cyclic voltammograms become repetitively reproducible. Figure 9 shows the cyclic voltammogram of (30)Cadman, P.;Gossedge, G.; Scott, J. D. J. Electron Spectrosc. Relat. Phenom. 1988,47, 197. (31)Evans, S.;Pritchard, R. G.;Thomas, J. M. J.Phys. C: Solid State Phys. 1977, C10,2483. (32)Ohnishi, T.; Ishitani, A.; Ishida, H.; Yamamoto, N.; Tsubomura, H. J . Phys. Chem. 1978.82, 1989. (33)Henke, B. L. Adu. X-ray Anal. 1971, 13, 1. (34)Cohen, S. L.;Brusic, V. A.; Kaufman, F. B.; Frankel, G.S.; Motakef, S.; Rush, B. J. Vac. Sci. Techno[. A 1990, A8, 2417. (35)Clark, D.T. Aduances in Polymer Science; Springer Verlag: New York, 1977; pp 126-187.

a four-layer LB film of 3 on a Au-coated glass slide. A reduction peak at -0.54V, with distorted reoxidation peaks at -0.4and-0.22 V, shows an electrochemicallyirreversible process. The ratio of integrated cathodic to anodic charge is 0.96 and this information coupled with the repetitive reproducibility indicates chemical reversibility. The linear dependence of I, on scan rate further indicates that the electroactive species is surface confined. The integrated charge corresponds to two electrons per molecule, i.e. one electron per A unit. This suggests that all the LB-coated material is electroactive. The chemical reversibility of the redox process is important, because it demonstrates that the LB film remains on the electrode even though electrons and counterions are being inserted and removed from the film. Thus the film morphology is changing reversibly during the redox process. Although self-assembled monolayers of 11 show reversible electrochemistry,these LB filmshave more complex and more interesting processes. The pronounced shift of the anodic peak to -0.22 V indicates that the anion radical is stabilized in the film and is slow to reoxidize. This probably results from the formation of radical anion dimers or stacks. It has recently been shown that similar imide anion radicals do form stacks in solution or in electroprecipitated films and that this shifts the redox potential to values near -0.2 V.36 The electroprecipitated films are conductive, suggesting that the reduced LB films may prove to be anisotropic conductors. These aspects are being pursued.

Conclusion This study provides the first information available on LB fiis formed from well-defined, nonaliphatic, rigidrod oligoimides. It was found that kinetically controlled but reproducible surface pressure-area curves resulted and that horizontal lifting was a suitable transfer method. It gave reproducible 0.95f 0.05 transfer ratios and all the spectroscopic evidence confirms that at least each of the first ten transfers gives consistent results. The simplicity of the horizontal lifting approach is appealing and suggests that it can be useful. Consider all the data on 3. The UV, IR, and XPS spectra make it quite clear that the salt has been transferred intact and the questions of interest then concern the molecular orientation in transferred films. It is concluded that the short axis is essentially disordered, the long axis tends to lie along the plane of the substrate, and there is a slight in-plane x,y anisotropy. This is entirely different from syringed-on samples, which are isotropic. The data suggest that the area/molecule in films of 3 transferred from water at 7 mN m-l is 60-65A2/molecule. If one molecule of 3 lies with tilt angle 90’ and twisting its ’footprint” is about 300 A2. This orientation would require a layer about five molecules high (Chart I1upper). The smaller tilt angle measured experimentally might then result from the surface roughness and expected disorder in the film. An alternative limiting structure is one in which every molecule touches the water surface forming a true monolayer, with an average molecular tilt angle of about 80° (Chart 11, lower). In either model the average thickness per transferred layer is about 27 A, in agreement with the XPS determination. Imagine traveling up from the surface on the z axis. In either limiting model an average of four to five molecules will be encountered in each transferred layer. An important aspect that we (36)Zhong, C.-J.; Zinger, B.; Cammarata, V.; Kasai, P.; Miller, L. L. Chem. Mater. 1991, 3, 787.

886 Langmuir, Vol. 8, No. 3,1992

Cammarata et al.

Chart I1 B

TBA'

U

In contrast to the anisotropy which is clear for 3 and 10, the films from compounds 6 and 11 appear to be almost isotropic. There is nothing in the pressure/area curves to suggest this difference and it seems clear that the different aggregation kinetics which lead to different film structures are rather subtle. The effects of adding Ba2+to the aqueous subphase are evident in both the IR and pressure/area data, and XPS demonstrates the incorporation of the Ba2+into a transferred film of 4. Again, however, the relationship between molecular structure and film organization is obscure. From the XPS work, spectroscopic data and pressure/ area curves, a film density can be calculated. For 3 the density, p = 1.48 g/cm3is consistent with a structure based on polyimide ( p = 1.4).37Again with 6, the density is 1.4 g/cm3, a rather dense film. However, for 10 and 11, the calculated densities are -0.45 and -0.28 g/cm3, respectively. This low density is surprising in light of the high density of the shorter rods. Since the stoichiometry from XPS is good for all the films, it is unclear why the density is low or, alternatively, the free volume is high. A t this stage of our work little attention has been paid to the nonspectroscopic properties of the transferred films. Because of the importance of polyimide films, such properties are worthy of investigation. The electrochemical results on 3 are intriguing. Conditions were found under which the film could be cycled between oxidized and reduced forms and the voltamograms suggest that, unlike self-assembled films of 11on gold, the reduced LB films contain dimers or stacks of anion radicals. Therefore, the LB films could have interesting electrical properties. It is known that some polyimide films can be reduced and they may have a similar structure.38

-

cannot evaluate is the roughness of each layer, that is, dispersion in the height. It is emphasized that the structure is determined by kinetics, not thermodynamics. It is suggested that the kinetic phenomenon of interest is aggregation and that once molecules aggregate they only slowly reorganize at room temperature. Thus, we propose that islands are formed on the trough when the chloroform solvent evaporates. The short height of the islands in this model depends on the relatively rapid growth rate along the x,y surface compared to that in the z direction. When compressed, these islands form "continents" and then a continuous, but not defect-free, film, on the surface. When the barrier was moved back, there was pressure-area hy~teresis,~ indicating that the more compressed structure is retained to some extent. In geologic language the continents remain largely intact. Similar behavior to that detailed for LB films of 3, is exhibited by the monosulfonate salt, 5, and by the dinitrile, 7. This demonstrates that a similar anisotropic structure can be obtained for a variety of end groups. Of some importance is the result obtained for the longest rod, compound 10. It also adopts a large long axis tilt angle and it is proposed that a similar aggregation model explains its behavior. The molecular area of 10 is about 700 A2 and the molecules that transferred at 250 A2 were tilted > 7 5 O .

Acknowledgment. This work was supported by the National Science Foundation and the Office of Naval Research. Compounds 8 and 11were synthesized by Wing Sum V. Kwan and compound 10 was synthesized by Timothy Dietz. The Langmuir trough was made available by the Center for Interfacial Engineeringat the University of Minnesota. Helpful conversations with Drs. J. Valentine, J. Swalen, and H. Brockman are acknowledged. SupplementaryMaterial Available: Equations used to determine polarization directions and computethe tilt angles (5 pages). Orderinginformationis givenon any current masthead page. Registry No. 3,129250-10-8;4,138313-07-2;5,138313-08-3; 6,138313-09-4;7,138313-10-7;8,132054-16-1;9,129271-85-8;10, 138313-11-8;A-B-A, 129250-07-3;A-B (copolymer), 134334-886; A-B (SRU),138313-12-9. ~~

~

(37) Takahashi, N.; Yoon, D. Y.; Parrish, W. Macromolecules 1984, 17, 2583.

(38)Viebeck, A.; Goldberg, M. S.; Kovac, C. A. J . Electrochem. SOC. 1990,137,1460.