Langmuir 1987, 3, 405-409
405
Photoreversible Intermolecular Reactions of Long-chain Cinnamylideneacetic Acids in Langmuir-Blodgett Films Akira Yabe,* Yasujiro Kawabata, Akihiko Ouchi, and Motoo Tanaka National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, J a p a n Received August 1, 1986. In Final Form: December 17, 1986 Four long-chain derivatives of cinnamylideneacetic acid were synthesized for photochemical studies in Langmuir-Blodgett (LB) films. The long axis of [p-(octyloxy)cinnamylidene]acetic acid (trans, trans) in the LB film was oriented largely perpendicular to the layer plane. The electronic spectra of the LB film showed an exciton splitting as the result of a strong interaction of the chromophores with a side-by-side arrangement of transition dipoles. Upon irradiation of the LB films with light of wavelength >300 nm, photodimerization occurred similarly to the photochemical process for cinnamylideneacetic acid in the solid state. The resulting photodimer was cleaved to form the starting monomer upon irradiation with light at a shorter wavelength (254 nm). Thus, an intermolecular photoreversible reaction in an LB film was found for the first time.
Introduction Photochemical reactions in a monomolecular layer or in multilayers built up by the Langmuir-Blodgett technique have aroused much theoretical and practical In particular, the extension of photoreversible reactions to Langmuir-Blodgett films (LB films) is of interest in connection with photomemory devices that can store binary information a t the molecular level. Although some LB films incorporating photochromic molecules such as spiropyrans4-14or thi~indigos'~ have already been studied, these photoreversible processes are limited to intramolecular reactions. We explore here new photochromic processes in LB films where [2 + 21 photocycloadditions occur reversibly. The [2 + 21 photochemistry of monolayer assemblies containing aromatic olefins or dienes has been investigated by Quina,161e Whitten,'9*20and other^.^'-^^ They have found that such topochemical photodimerizations as observed in the crystals occur in LB films, suppressing completely the cis-trans photoisomerizations which are com(1) Kuhn, H.; Mobius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. I, Part I11 B, Chapter VII, p 577. (2) Kuhn, H. J. Photochem. 1979, 10, 111. (3) Vincett, P. S.;Roberts, G. G. Thin Solid Films 1980, 68, 135. (4) MBbius, D. Acc. Chem. Res. 1981, 14, 63. (5) Roberta, G. G. Sens. Actuators 1983,4, 131. (6) Kuhn, H. Thin Solid Films 1983, 99, 1. (7) Holden, D.A.;Ringsdorf, H.; Haubs, M. J. Am. Chem. SOC.1984, 106,4531. (8) Mobius, D. ACS Symp. Ser. 1985, No. 278, 113. (9) Pommier, H. P.; Baril, J.; Gruda, I.; Leblanc, R. M. Can. J. Chem. 1979,57, 1377. (IO) Polymeropoulos, E. E.; Mobius, D. Ber. Bunsenges. Phys. Chem. 1979,83, 1215. (11) Morin, M.; Leblanc, R. M.; Gruda, I. Can. J.Chem. 1980,58,2038. (12) McArdle, C. B.; Blair, H.; Barraud, A.; Ruaudel-Teixier, A. Thin Solid Films 1983, 99, 181. (13)Holden, D. A.; Ringsdorf, H.; Deblauwe, V.; Smets, G. J. Phys. Chem. 1984,88, 716. (14) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 21. (15) Whitten, D.G. J. Am. Chem. SOC.1974, 96,594. (16) Quina, F.H.; Whitten, D. G. J. Am. Chem. SOC.1975, 97, 1602. (17) Quina, F.H.; Mobius, D.; Carroll, F. A.; Hopf, F. R.; Whitten, D. G. 2.Phys. Chem. Neue Folge 1976,101, 151. (18) Quina, F. H.; Whitten, D. g. J. Am. Chem. SOC. 1977, 99, 877. (19) Whitten, D.G.; Eaker, D. W.; Hosey, B. E.; Schmehl, R. H.; Worsham, P. R. Ber. Bunsenges. Phys. Chem. 1978,82, 858. (20)Whitten, D. G. Angew. Chem. 1979, 91, 472. (21) Enkelmann, V.; Tieke, B.; Kapp, H.; Lieser, G.; Wegner, G. Ber. Bunsenges. Phys. Chem. 1978, 82, 876. (22) Tieke, B.; Enkelmann, V.; Kapp, H.; Lieser, G.; Wegner, G. J. Macromol. Sci., Chem. 1981, A15, 1045. (23) Tanaka, Y.;Nakayama, K.; Iijima, S.; Shimizu, T.; Maitani, Y. Thin Solid Films 1985, 133, 165.
0743-7463/87/2403-0405$01.50/0
mon in solutions. In these systems, the dimerized products formed on the irradiation of aromatic olefins are photostable cyclobutane derivatives. The photochemical cleavage of the dimers has not been reported. We wish to report the photoreversible reaction, that is, the dimerization and the cleavage of dimers, in LB films incorporating cinnamylideneacetic acid. To our knowledge, this is the first report of intermolecular photoreversible reactions in LB films. These photoreversible processes should produce reversible changes in absorption spectra and also in refractive indexes, both of which are important in applications to p h ~ t o m e m o r y .Our ~ ~ results could lead to a new type of photochromism in LB films.
Experimental Section General. A commercially available monolayer trough equipped with a film balance (Messgerate-WerkLauda, West Germany) was used for the measurements of surface pressurearea isotherms and for the preparations of LB films. Electronic absorption and emission spectra were recorded with a Shimadzu UV-300 and a Hitachi MPF-4 spectrophotometer,respectively. IR spectra were recorded by standard transmission spectroscopy with an unpolarized incident beam by use of Shimadzu FTIR-4000 Fourier transform infrared spectrometer. Irradiations were carried out with a 500-W high-pressure mercury lamp irradiated through a Toshiba UV-29 filter for light of wavelengths above 300 nm and a 6-W low-pressure mercury lamp for 254-nm light. Materials. Derivatives of cinnamylideneacetic acid with different long alkyl chains were synthesized as shown in the following equations. The corresponding esters were prepared by the Wittig reaction between three kinds of p-alkoxybenzaldehydes and [3-(ethoxy(ormethoxy)carbonyl)allyl]triphenylphosphonium bromides.25 In the cases of 1,2, and 3, the reaction mixtures (methyl or ethyl esters) contained four geometric isomers (a-d). These isomers (esters) were then separated by use of column chromatography with silica gel and were purified by recrystallization from hexane-ether. The geometrical configuration of each isomer was assigned by means of 'H and 13C NMR spectroscopies. These esters were converted into the corresponding acids by alkaline hydrolysis. The isolations of long-chain cinnamylideneacetic acids la, 2a, 2b, and 3a were performed by column chromatography with silica gel. Their purities were checked by thin-layer chromatography. Melting points (uncorrected) were taken o n a Mettler FP61 apparatus. 'Hand 13C NMR spectra were recorded with a JEOL GX-400 spectrometer at 400 MHz and a JEOL FX-200 spectrometer at 50 MHz, respectively. Mass spectra were recorded (24) Tomlinson, W. J.; Chandross, E. A.; Fork, R. L.; Pryde, C. A.; Lamola, A. A. Appl. Opt. 1972, 11, 533. (25) Howe, R. K. J . Am. Chem. SOC.1971, 93, 3457.
0 1987 American Chemical Society
Yabe et al.
406 Langmuir, Vol. 3, No. 3, 1987 Br C H 2 C H = C H C O O M e
(or
Et 1
.1 Ph3P'CH2CH=CHCOOMe (or Et Bl
+
R - O -~ C H O -
+
R - ~ - C H =CH-~H=CI+-COOH
0
IO
20 3!, 40 50 AREA / A PER MOLECULE
60
Figure 1. Surface pressure-area isotherms of monolayers of (a) la, (b) 2a, (c) 2b, and (d) 3a on the subphase containing 4 X lod mol/L CdClz and 5 X 10" mol/L KHC03 at 17 "C.
1, R = CHB(CH2)S 2, R = CHS(CHJ7
1
.
0
7
1
3, R = CH3(CH&
a, trans-2,trans-4 b, trans-2,cis-4 c, cis-2,trans-4 d, cis-2,cis-4
with a JEOL OISG-2 mass spectrometer at 75 eV. Elemental analwere performed by the Institute of Physical and Chemical Research. [p-(Hexyloxy)cinnamylidene]aceticacid (la; trans,trans):
colorless crystals; mp 161.0 "c; 'H NMR (MezSO-ds)6 12.2 (s, 1 H), 7.50 (d, J = 8.7 Hz, 2 H), 7.33 (ddd, J = 15.0, 8.6, 1.5 Hz, 1H), 6.99 (dd, J = 14.5, 1.5 Hz, 1H), 6.97 (dd, J = 14.5, 8.6 Hz, 1 H), 6.94 (d, J = 8.7 Hz, 2 H), 5.94 (d, J = 15.0 Hz, 1 H), 3.98 (t, J = 6.4 Hz, 2 H), 0.88 (t, J = 7.0 Hz, 3 H);13C NMR (CDC13) 6 167.57, 159.43, 144.71, 139.69, 128.65, 128.48, 124.18, 120.79, 114.75,67.57,30.95,28.56, 25.11, 22.02, 13.84; MS, m / z (relative intensity) 274 (46, M"), 145 (100). Anal. Calcd for C17H2&3: C, 74.42; H, 8.08. Found: C, 74.48; H, 8.09.
0 1 200
,
I
250 300 WAVELENGTH / nm
350
400
Figure. 2. Electronic absorption spectra of 2a in the LB film (Cd salt six layers) (-) and in the 5.64 X lo4 mol/L hexane solution (- - -).
EXCITON BAND SPLITTING
[p-(Octyloxy)cinnamylidene]acetic acid (2a;trans,trans):
colorless crystals; mp 155.9 "C; 'H NMR (Me$O-d6) 6 12.18 (s, 1 H), 7.49 (d, J = 8.8 Hz, 2 H),7.32 (ddd, J = 15.2, 8.8, 1.5 Hz, 1 H), 6.99 (dd, J = 14.8, 1.5 Hz, 1 H), 6.96 (dd, J = 14.8, 8.8 Hz, 1 H), 6.94 (d, J = 8.8 Hz, 2 H), 5.94 (d, J = 15.2 Hz, 1 H), 3.98 (t, J = 6.5 Hz, 2 H), 0.86 (t, J = 6.9 Hz, 1H); '3C N M R ( M 4 o - d ~ ) 6 226.67, 162.54, 159.40, 144.68, 139.63, 128.62, 128.45, 124.13, 120.68, 114.72, 31.16, 28.64, 28.56, 25.40, 21.99, 13.84; MS, m / z (relative intensity) 302 (44, M+), 145 (100). Anal. Calcd for CI9HzeO3:C, 75.46; H, 8.67. Found: C, 75.26; H, 8.66. [p-(Octyloxy)cinnamylidene]acetic acid (2b;trans&): colorless crystals; mp 67.8 "C; 'H NMR (CDC1,) 6 7.91 (ddd, J = 15.2, 11.8, 1.1 Hz, 1 H), 7.28 (d, J = 8.6 Hz, 2 H), 6.91 (d, J = 8.6 Hz, 2 H), 6.80 (dd, J = 11.3, 1.1Hz, 1H), 6.30 (dd, J = 11.3, 11.8 Hz, 1 H), 6.00 (d, J = 15.2 Hz, 1 H), 3.98 (t,J = 6.6 Hz, 2 H), 0.89 (t, J = 6.9 Hz, 3 H); 13C NMR (CDCl,) 6 172.27, 159.49, 143.19, 138.75,130.84, 128.71, 125.35, 121.64, 114.66,68.15, 31.83, 29.37,29.26, 26.07, 22.66, 14.07; MS, m / z (relative intensity) 302 (36, M'), 145 (100). Anal. Calcd for C1gHm03: C, 75.46 H, 8.67. Found: C, 75.17; H, 8.65. [p-(Decyloxy)cinnamylidene]acetic acid (3a; trans,trans): colorless crystals; mp 142.8 "c; 'H NMR (MezSO-d6)6 12.19 (s, 1 H), 7.49 (d, J = 8.7 Hz, 2 H), 7.32 (ddd, J = 15.2, 9.0, 1.5 Hz, 1 H), 6.98 (dd, J = 15.0, 1.5 Hz, 1 H), 6.96 (dd, J = 15.0,g.O Hz, 1 H), 6.93 (d, J = 8.7 Hz, 2 H), 5.94 (d, J = 15.2 Hz, 1 H), 3.97 (t, J = 6.5 Hz,2 H), 0.85 (t,J = 7.0 Hz,3 H);'3C N M R (Me#O-d6) 6 167.57, 159.37, 144.71, 139.66, 128.62, 128.42, 124.13, 120.68, 114.69,67.48, 31.21, 28.94, 28.88, 28.67, 28.62, 28.56, 25.40, 22.02, 13.87; MS, m / z (relative intensity) 330 (48 M+),145 (100). Anal. Calcd for C21H3003: C, 76.33; H, 9.15. Found: C, 75.98; H, 9.18.
Results and Discussion Surface Pressure-Area Isotherms. The properties of the monolayers of la, 2a, 2b, and 3a were studied by measurements of surface pressure-area isotherms. Figure 1 shows the surface pressure-area curves measured at 17 "C. The subphase was doubly distilled water, containing 4X mol/L CdC1, and 5 x mol/L KHC03. The cis-trans isomers 2b occupied larger areas than did the trans-trans isomers. The areas per molecule of la, 2a, and
1 SOLUTION
L B FILM
Figure 3. Energy level schemes of 2a in the LB film and in
hexane solution.
3a calculated from the pressure-area diagram were 20,20, and 23 A2, respectively. This suggests that the long axes of the chromophores of these trans-trans isomers are mainly oriented perpendicular to the surface. The collapse pressures increased in the order of 2b, la, 3a, and 2a. The stability of the monolayers did not increase simply with the increase of chain lengths for the these trans-trans isomers. It may be explained that the alkoxy chains do not orient linearly with respect to the chromophores. Thus,2a, monolayers of which showed the highest stability, was selected as a suitable compound for the preparation of LB films in the following experiments. Electronic Spectra. The LB films of pure 2a and the LB films mixed with arachidic acid were constructed as Y-type films on quartz plates at a surface pressure of 20 mN/m. These LB films were assembled as the cadmium salts because the subphase contained CdClz and KHC03 As shown in Figure 2, the absorption spectrum of the LB film of pure 2a was quite different from that in the hexane solution. The main peak of 2a in the LB film was shifted to shorter wavelengths by 68 nm relative to that in the solution. This remarkable blue shift was also observed in the LB film of 2a deposited on the quartz substrate precoated with five monolayers of cadmium arachidate. In this case, the interaction between the chromophore and the quartz substrate should be excluded. The intense absorption band at A,, 338 nm (the parallel
Intermolecular Reactions of Long-chain Acids
Langmuir, Vol. 3, No. 3, 1987 407
- 0.5
t
4
IAAAOIATION TIME
M
e 0
4
U
u ICL 0
0 200
250
300
350
450
400
WAVELENGTH/nm
Figure 5. Electronic absorption spectra of the LB film of pure 2a (Cd salt) (30 layers on a CaFz substrate) before and after
irradiations with >300-nm light. Numbers refer to the irradiation times in seconds. I ' l ' " ' l ' ' " IRRADIATION TIME
(-1
1
,
WAVELENGTH/ nm
Figure 4. Electronic absorption spectra of the LB film of pure 2a (Cd salt)(l:O)and the LB film (Cd salt) mixed with arachidic acid in ratios of 1:1,1:5,1:10,and 120 and that in hexane solution
-
(- -).
transition to the long axis of chromophore) was split into a strong peak a t A,, 270 nm and a weak shoulder a t 350 nm. These features correspond to the allowed transition and to the forbidden transition from the ground state, respectively. On the other hand, the emission spectrum (A, 407 nm) of 2a in the LB film was red-shifted from that (Am= 390 nm) obtained in hexane solution. The electronic absorption and emission spectra described above can be explained in terms of exciton interaction of the chromophores with the side-by-side arrangement of transition dipoles.26-28 In the case of side-by-side interaction, the absorption from the ground state to the excited state a t the upper level is allowed, while the emission from the excited state a t the lower level is usually observed, as shown in Figure 3. The splitting width between two levels in the excited state is given by 2p2/9,where p is the transition moment of T-T* and r is the distance between the centers of two molecules.% The LB films mixed with arachidic acid in various ratios were prepared to study aggregation of chromophores. Figure 4 shows the relationship between the degree of exciton interaction and the concentration of chromophores. As the chromophore is diluted, the blueshifted peak moves toward the absorption maximum in the hexane solution. However, the absorption maximum is still blue-shifted even in the case of the LB film assembled as a 1:20 mixture, indicating that some extent of aggregation still remains. Photochemical Reactions. Pure LB films of the cadmium salt of 2a were prepared on calcium fluoride plates for photochemical studies. The LB films were stable in the dark a t room temperature and no dark reaction could be detected for a long period (about 1 year). The LB film was irradiated with light of wavelengths above 300 nm under a nitrogen atmosphere. As shown in Figures 5 and 6, photochemical reactions in the LB film were followed by UV-vis and IR spectroscopy on the same sample.
The IR spectrum of an unirradiated LB film (Cd salts of 2a) was assigned on the basis of investigations by Swalen and co-workers.m2 The symmetric stretching vibrations v,(COJ of the carboxylate group is found a t 1478 cm-' and, presumably, the asymmetric stretch va(C02-)may be attributable to the shoulder of this strong band. The C=C stretching vibration and the CH=CH bending are found a t 1628 and 1291 cm-l, respectively. As shown in Figure 5, the main electronic absorption band at 270 nm decreased rapidly up to an irradiation time of 8 s. Drastic changes were also observed in the IR spectrum. With increase of irradiation time, the two peaks
(26) Murrell, J. N. The Theory of the Electronic Spectra of Organic Molecules; Methuen: London, 1963; p 133. (27) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley-Interscience: New York, 1969; p 234. (28) Pant, D.D.;Bhagchandani, C. L.; Pant, K. C.; Verma, S. P. Chem. Phys. Lett. 1971, 9,546.
(29) Allara, D.L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (30) Rabolt, J. F.;Burns, F. C.; Schlotter, N. E.; Swalen, J. D.J. Chem. Phys. 1983, 78, 946. (31) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1986, 82, 2136. (32) Rabe, J. P.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. J . Chem. Phys. 1986, 84, 4096.
1
,
I
0 3000 2000
I
1500
1001
WAVENUMBEA/cm-'
Figure 6. IR spectra of the LB film of pure 2a (Cd salt) before and after irradiations with >300-nm light. The sample for measurements was the same as that used for Figure 5. Numbers refer to the irradiation times in seconds corresponding to those in Figure 5.
408 Langmuir, Vol. 3, No. 3, 1987
Yabe et al.
0.081
WAVELENGTH/ n m
Figure 7. Electronic absorption spectra of the LB film of a mixture of 2a and arachidic acid (Cd salt) in the ratio of 1:lO (160 layers on a quartz substrate): (a) before irradiation; (b) after irradiation for 120 min with >300-nm light; (c) after irradiation of (b) for 3 min with 254-nm light; (d) after irradiation of (c) for 60 min with >300-nm light; (e) after irradiation of (d) for 10 min with 254-nm light.
due to C=C double bonds a t 1628 and 1291 cm-' were decreased and the COz- bands were shifted to higher wavenumbers. The above results suggest that 2a was dimerized or polymerized by intermolecular reactions. The disruption of the conjugation with the aromatic diene group by the photodimerization and/or polymerization would also result in the shift of the Cog- bands to higher wavenumbers. In addition, the shift of the C02- bands may be partly attributable to the changes in molecular orientation resulting from intermolecular reactions. During the initial stages of irradiation, an isosbestic point was observed a t 220 nm (Figure 5). Irradiations longer than 16 s gave rise to apparent deviations from the isosbestic point and the whole spectrum changed. In this irradiation sequence, the IR spectra showed a gradual decrease in the C=C bands and a shift of the C 0 2 - bands (Figure 6). In the photochemical processes of 2a, the cis-trans isomerization was predominant in solution, as with cinnamylideneacetic acid. In the LB film, however, the above spectroscopic data indicate that isomerization to the cis isomer of 2a does not occur, in agreement with the fact that isomerization accompanying an increase in molecular size is generally r e s t r i ~ t e d . ' ~ Many ? ~ ~ reports on photochemistry of LB films have revealed that the most probable photoreaction for aromatic olefins and dienes is a dimeri z a t i ~ n , while ' ~ ~ ~a polymerization is common in the cases of aliphatic olefins and dienes.32y34,35 Tanaka and co-workers have investigated the photochemistry of cinnamylideneacetic acids in polymers and in glassy In their studies the dimerization
/
0.04 I
0
1
40.03
r
I T
120 180 IRRADIATION TIME/ min 60
Figure 8. Photoreversible process of the LB film (2a:arachidic acid = l : l O , Cd salts, 160 layers, the same sample as used for Figure 7): optical densities at 279 (a)and 330 nm ( 0 )as ordinate and irradiationtimes [(-) with >300-nm light; (- - -) with 254-nm light] as abscissa. of cu,@-doublebonds has been confirmed by using 1,4-butanediol bis(cinnamy1ideneacetate). By comparing our UV-vis and IR spectral data in the LB film with those of their model compound, it can be tentatively inferred that the initial photochemical reaction is the dimer formation of 2a. Although the continued photochemical reaction after photodimerization is assumed to be polymerization by remaining double bonds, further details are not certain. Figure 7 shows the photochemical processes for the LB f i s which are prepared in a mixture of 2a and arachidic acid in the ratio of 1:lO. Upon irradiation with light of wavelength above 300 nm, the absorption spectrum having A, 300 nm finally changed to the spectrum having A, 270 nm (a b in Figure 7). Spectrum b was no longer changeable on further irradiation with light of wavelengths above 300 nm. The prolonged irradiation was then performed with 254-nm light and spectrum b changed to spectrum c. Again the irradiation was done with light of wavelength above 300 nm and the spectrum changed to spectrum d, having ,,A 270 nm. This cycle of irradiations was tried twice. These photoreversible processes have similarly been observed in polymer films and in glassy matrices of cinnamylideneacetic acid derivativeass Thus, the phenomenon observed in the LB film can be attributed to such a photoreversible reaction between the photodimerization and the photocleavage as shown in the following scheme.
-
2 R-O~C -H = C H - C H = C H - C O ~ H hv
(33) Nakahara, H.; Fukuda, K. Proceedings of the 37th Symposium on Colloid and Surface Chemistry of Japan; Yamagata, Japan, 1984; p 396. (34) Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1985, 133, 39. (35) Rabe, J. P.; Rabolt, J. F.; Brown, C. A. Thin Solid Films 1985,
j
11
hv'
133. 153. ~~.
(36)Tanaka, H.; Otomegawa, E. J . Polym. Sci., Polym. Chem. Ed. 1974,12, 1125. (37) Tanaka, H.; Tsuda, M.; Nakanishi, H. J. Polym. Sci., Polym. Chem. Ed. 1972,10, 1729. (38) Tanaka, H.; Sato, Y. J . Polym. Sci., Polym. Chem. Ed. 1972,10,
The spectral changes which were monitored a t 279 and 330 nm were plotted as shown in figure 8. The uyo-yo" effect,40that is, the decrease of reversibility for each pro-
29753
(39) Tanaka, H.; Honda, K. J . Polym. Sci., Polym. Chem. Ed. 1977, 15, 2685.
(40) Bertelson, R. C. In Techniques of Chemistry; Brown, G. H., Eds.; Wiley-Interscience: New York, 1971; Vol. 111, p 131.
Langmuir 1987,3,409-413 cess, was observed. This may be due to a side reaction during the irradiation with 254-nm light. The'photodimer may produce photodimerization by another remaining double bond on prolonged irradiation. Thus, the absorption peak a t 270 nm decreased gradually on continued irradiation.
Conclusions Previous studies in the intermolecular reactions of LB films have been limited mainly to photopolymerization and photodimerization. In this study, the intermolecular photoreversible reaction in an LB films has been found for the first time, although the photoreversible reaction of cinnamylideneacetic acid has already been known in polymer film or in low-temperature glassy matrix. Originally, one might expect that these photoreversible systems are more favorable in LB films than in polymers. However, the reversibility was still low for the LB film of
409
2a, similarly to the polymer film of poly(viny1 cinnamyl-
ideneacetate). This is due to the overlapping of the absorption spectra of 2a and the photodimer of 2a and also to the side reaction arising from the chromophore containing two double bonds. Therefore, further investigations are required to find other chromophores for the improvement of reversibility.
Acknowledgment. We thank Drs. A. Kuboyama, S. Matsuzaki, and H. Tanaka for valuable discussions and Dr. N. Wasada and Dr. K. Hayamizu and her co-workers for spectral data.41 (41) MS IR, and 'H and 13C NMR spectra for the compounds (four acids and their methyl or ethyl esters) in this work are saved in the Spectral Data Bank System (SDBS) constructed by our laboratory (NCLI) in the Research Information Processing System (RIPS) of Tsukuba Research Center. The spectral patterns for the compounds are available on request.
Blocking Oriented Monolayers of Alkyl Mercaptans on Gold Electrodes H. 0. Finklea," S. Avery, and M. Lynch Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
T. Furtsch Department of Chemistry, Tennessee Technological University, Cookeville, Tennessee 38505 Received August 11,1986. In Final Form: January 12, 1987 Alkyl mercaptans with long hydrocarbon chains (Clz,C14, Cia, and c18)spontaneously form organized monolayers on gold during adsorption from solution. The oriented monolayers are stable over a wide potential range in aqueous electrolytes. They strongly block electrochemical oxidation of gold and also electron transfer with redox couples in solution. Tafel plots exhibit anomalously low but nonzero slopes for overpotentials up to 0.6 V and are often nonlinear. Electron tunneling across the full width of the oriented monolayer is contraindicated. Faradaic current appears to be composed of electron transfer at defect sites and electron tunneling at "collapsed" sites in the monolayer.
Introduction We are using oriented monolayers to create interesting structures on electrode surfaces. Oriented monolayers offer the possibility of precise control of spacing and orientation on a molecular level, a feature of current interest in electron transfer reacti0ns.l Our initial monolayers were synthesized by using octadecyltrichlorosilane (OTS), a molecule which successfully forms bonded oriented monolayers on alumina and OTS readily self-assembles into a monolayer on a gold or platinum electrode, but despite apparent lateral siloxane bonds between the heat groups, the monolayer is unstable during faradaic processes.8 We hypothesize that a strong interaction between the head group and the electrode is necessary to prevent monolayer desorption during the intense ion flux that accompanies oxidation or reduction. Our hypothesis is supported by the results reported herein: monolayers formed from alkyl mercaptans are * T o whom correspondence should be addressed. Present address: Department of Chemistry, West Virginia University, Morgantown, WV 26506
0743-7463/87/2403-0409$01.50/0
remarkably stable as electrode coatings. The known high affinity of sulfur for goldg accounts for the increased stability. Furthermore, the alkylmercaptans form sufficiently compact monolayers that electrochemical processes, Le., gold oxidation and electron transfer with solution redox molecules, are strongly suppressed. In such circumstances it becomes possible to investigate electron transfer by tunneling across the monolayer. (1)(a) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J.Am. Chem. SOC.
1983,105,670-671. (b) Miller, J. R.; Calcaterra, L. T.; C l m , G. T. J. Am. 1984. 106. 3047-3049. Chem. SOC. (2) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836-1847. (3) Sagiv, J. Isr. J. Chem. 1979,18, 346-353. 1980, 102, 92-98. (4) Sagiv, J. J. Am. Chem. SOC. (5) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235-241. (6) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496. (7) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101, 201-213.
(8) Finklea, H. 0.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239-244. (9) Chambers, J. Q. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York; Vol. XII, pp 329-502.
0 1987 American Chemical Society
