Langmuir 1994,10, 3311-3314
3311
(Methylacy1amino)azobenzeneDerivatives for Photoresponsive Monomolecular Layers J. Freimanis,* E. Markava, and G. Matisova Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia
L. Gerca, I. Muzikante, M. Rutkis, and E. Silinsh Institute of Physical Energetics, Latvian Academy of Sciences, Riga LV-1006, Latvia Received June 14, 1993. In Final Form: February 14, 1994@ Faur azobenzene amphiphiles containing tertiary amide groups were synthesized and characterized. Reversible cisltrans photoisomerization of these compounds has been studied in solutions by irradiation with W light at 335 nm and visible light at 452 nm. Stability of the monomolecular layer, the collapse pressure, and the molecular area have been investigated as a function of chemical structure of azobenzene derivative and of equilibrium concentration of the cis and trans form in spreading solution.
Introduction In the last decade Langmuir-Blodgett (LB) films have played a n important role because of their possibility of being applied to functional molecular devices such as electronic, nonlinear optical, pyroelectric elements, and biological sensors. Azobenzene derivatives with long alkyl chains have attracted much attention in this field. Azobenzene derivatives, possesing extended z-electron systems with electron acceptor (NOz, COOH)and electron donor (alkylamino, alkoxy) groups a t opposite ends of the molecule, deposited as LB films exhibit large nonlinear optical hyperp~larizability.l-~ In the present study four photoresponsive amphiphiles bearing a n electron acceptor (carboxyl or carbomethoxy) group and a N-methylamino group, in which the electron donor character was weakened by acylation, have been synthesized (Scheme 1). Type of deposition and molecular orientation in LB films of azobenzene-containing amphiphiles have been inv e ~ t i g a t e d . ~Barnik ~ ~ ~ ~et - ~al. ~ have obtained LB films exhibiting photoinduced optical anisotropy effect from azobenzene-containing NH-alkyl group.ll Unique application of azobenzene LB monolayer film in “photoelectrochemical memories” has been suggested by Liu et a1.12 Photochromism in the azobenzene system has received considerable attention because of its potential application in areas such as density optical memories and molecular switching devices.13-19 Photochromism in LB films is very Abstract published in Advance ACS Abstracts, April 15,1994. (1)Ledoux, I.; Josse, D.; Vidakovic, P.; Zyss, J.;Hann, R. A.; Gordon, P. F.; Bothwell, B. D.; Gupta, S. K.; Allen, S.; Robin, P.; Chastaing, E.; Dubois, J. C. Europhys. Lett. 1987, 3 , 803. (2)Ledoux, I.; Josse, D.; Fremaux, P.; Piel, J. P.; Post, G.; Zyss, J.; McLean, T.; Hann, R. A.; Gordon, P. F.;Allen, S. Thin Solid Films 1988, 160, 217. (3) Kalina, D. W.; Grubb, S. G. Thin Solid Films 1988, 160, 363. (4)Loubergue, J. C.; Dumont, M.; Levy, Y.; Robin, P.; Pochelle, J. P.; Papuchon, M. Thin Solid Films 1988,160, 399. (5) Kajzar, F.; Ledoux, I. Thin Solid Films 1989, 179, 359. (6) Mobius, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 848. (7) Nakahara, H.; Fukuda, K. J. Colloid Znterfme Sci. 1983,93,530. (8)Xu, X.; Kawamura, S.; Era, M.; Tsutsui, T.; Saito, S. Nippon Kagaku Kaishi 1987,2083. (9)Xu, X.; Era, M.; Tsutsui, T.; Saito, S. Thin Solid Films 1989,178, 541. (10)Kawai, T.; Umemura, J.;Takenaka, T. Langmuir 1990,6,672. (11)Barnik, M. I.; Palto, S. P.; Khavrichev, V. A.; Shtykov, N. M.; Yudin, S. G. Thin Solid Films 1989, 179, 493. (12)Liu,Z.F.;Loo,B. H.;Hashimoto,K;Fujishima,A.J.Electroanal. Chem. 1991,297, 133. (13) Quina, F. H.; Whitten, D. G . J.Am. Chem. SOC.1977,99, 877. @
different from t h a t in solutions. The cisltrans isomerization occurs predominantly, whereas the reverse reaction accompanied with increase in the molecular area is significantly retarded due to the close packing of chromophores.20 By using the polyion complex technique, Nishiyama et al. have succeeded in producing reversible cisltrans photoisomerization of long chain azobenzenes.15J6 Liu et al. have achieved excellent reversible photochromic behavior of 4-octyl-4’-(5-carboxylpentamethylene0xy)azobenzene after its LB films were “pretreated” with UV irradiation.18 In this work the properties of the monomolecular layers of long chain (N-methyl-N-acy1amino)azobenzene derivatives as a potential material for photochromic LB films have been studied.
Experimental Section Materials. All solvents were reagent grade and distilled before use. Dimethylformamide(DMFA)was dried and distilled from BaO under reduced pressure. F’yridine was dried and distilled from KOH. N-Methylaniline(Fluka)was distilled before use. Lauroyl chloride, stearoyl chloride, and palmitoyl chloride were used as received from Fluka. p-Aminobenzoic acid was reagent grade, recrystallized from water. (14) Sandhu, S. S. Biochim. Biophys. Acta 1988, 860, 253. (15) Nishyjama, IC;Fujihira, M. Chem. Lett. 1988, 1257. (16) Nishyjama, K.; Kurihara, M. A.; Fujihira, M. Thin Solid Films 1989,179, 417. (17) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J.Am. Chem. SOC.1989, 11I, 3080. (18) Liu, Z. F.; Loo, B. H.; Baba, R.; Fujishima, A. Chem. Lett. 1990, 6, 1023. (19) Yamamoto, H.; Nishida, A.; Takimoto, T.; Nagai, A. J. Polym. Sci. 1990, A28, 67. (20)Nakahara, H.; Fukuda, K.; Shimomura,M.; Kunitake, T.Nippon Kagaku Kaishi 1988, 1001.
0743-746319412410-3311$04.5010 0 1994 American Chemical Society
3312 Langmuir, Vol. 10,No. 9,1994
Freimanis et al.
of 6c (trans form) was 56%, mp 149-151 "C, Rf (C) 0.37, Rf (F) Instruments. Melting points reported were detected on 0.46, purity established by HPLC 99.5%. 'H NMR (DMSO&), Boetius apparatus, NMR spectra were taken on a Brucker WH ppm: 0.83 (3 H, CHs), 1.15 (30 H, CH21, 2.1 (2 H, CHzCO), 3.2 9O/DS spectrometer. (The chemical shifts were given in 6 scale (3 H, CH3N), 7.51 (2 H, Ph), 7.78, (2 H, Ph), 7.95 (2 H, Ph), 8.10 using tetramethylsilane as internal standard.) IR spectra were (2 H, Ph). IR(cm-l): 1689,1658. UV(CHC13)1,,340(~ 26 4001, recorded on a Perkin-Elmer 580 B spectrometer, a Nujol mull 453 ( E 810). Cis form: Rf (C) 0.18, Rf (F) 0.3. technique was employed. Electronic absorption spectra were 4'-Carbomethoxy-4-(methylpalmitoylamino)azorecorded on a Specord W-VIS spectrometer. HPLC analysis benzene (7). The same acylation procedure was used for 5 as was performed on a Du Pont (8800)apparatus with Wdetector. for 3 in the case of 6a. The crude reaction product was dissolved (The conditionsfor the analyses were as follows: columnZorbaxin EtOAc, washed successively with 10% Na2C03 and water, Sil, 4.6 x 250 mm, eluent A a 10:1:89 mixture of i-PrOWCH3dried over Na2S04, evaporated, and purified on Silicagel L loo/ COOWhexane(v/v). The detection wavelength was 254 nm. TLC 160 column with eluent hexane-EtOAc (3:l). Yield of 7 (trans was carried out on Silufol with the following solvent systems: form) was 69%, mp 94-95 "C. Rf (C) 0.9, Rf (F)0.75, purity benzene-EtOAc 1:l (B); hexane-EtOAc 2 : l (C); chloroformestablished by HPLC 99.1%. IR (cm-l): 1729,1657. W(CHC13) EtOAc 3 : l (D);benzene-EtOAc 9 : l (E);benzene-EtOAc 7:3 (F). I,, 340 ( E 27 loo), 454 ( E 830). Cis form: Rf(C)0.7, Rf(F)0.6. After being sprayed with phosphomolybdic acid, plates were Elemental analyses (C,H, N) were obtained for all compounds heated at 120 "C for 5 min. 3-7. 4'-Carboxy4(methylamiao)azobenzene(3). 4-AminobenPhotoisomerization in Solutions. Solutionsof azobenzenes zoic acid (1) (2.74 g, 20 mmol) was suspended in 5.5 mL (56 6a-c and 7 in chloroform and eluent (hexane/2-propanol/acetic mmol)of hydrochloricacid along with 40 mL of water and stirred Mand (5-6) x 10-4Mwere irradiated acid (89:lO:l))(2-3) x for 15 min at f 2 "C, then cold solution of NaNOz (1.38 g, 20 from the mercury lamp using the filter (the transmittance range mmol) in 20 mL of water was added dropwise and stirred at +2 320-380 nm) or interference filter (452 & 9 nm) at room "C for 15 min. After dropwise addition of N-methylaniline (2) temperature. The light intensity was of the order Z = 4 x 1013 (2.18mL, 20 mmol),the mixture was stirred for 2 h. Red-orange photons/(cm2s). precipitate was filtered off, washed with water, dried, and Monomolecular Layer Experiments. Surface pressure vs recrystallized from ethanol. Yield 2.72 gof triazene (4),mp 190area per molecule isotherms were obtained on the computer192 "C,RAB) 0.75,RAC) 0.56, RAD) 0.65. W (CH30H)Imax 241 controlled Langmuir trough by the 'step by step" method nm ( E 18 200), 357 nm ( E 28 600). described in a previous paper.21 Filtrate was diluted with water. m e r 24 h a red-violet As spreading solvents benzene, hexane, and chloroform, and precipitate was filtered off and stirred with 100 mL of 10%Nazin some experiments eluent (see above)was used. The spreading COSsolution for 1 h. Sodium salt was filtered off, dissolved in solutions were freshly prepared and stored in a dark box. water, and acidified with 1 M HC1 to give orange crystals of 3. t o 2.40 x M. Concentration was in range from 1.90 x Yield 1.43 g, mp 225-227 "C, RAB) 0.55, RAC) 0.24, RAD) 0.36. All investigations were made at ambient temperature (18-22 Purity established by HPLC 98.6%. 'H NMR (DMSO-d,j),ppm: "C). A Langmuir trough was mounted in dark box with red light 2.78 (3 H, CH3N), 6.65 (2 H, Ph), 7.68 (2 H, Ph), 7.78 (2 H, Ph), illumination. As subphase, double distilled water (pH = 5.8) 8.00 (2 H, Ph). IR cm-l: 3290,1685,1545. U V (CH3OH)Amax: was used. When the trans& isomerization effect on isotherms 274 nm (E 10 5001, 428 nm (E 30 800). was investigated, a freshly prepared solution was irradiated Triazene 4 (2.72g) was mixed with 2 (15 mL) and HCl(3mL), under the same conditions as in previous experiments. Spreading the mixture was stirred at 50-60 "C for 24 h, and acetic acid (20 solutions were irradiated in time interval steps (20 min) to mL) and water (20 mL) were added. Stirring was continued for produce different trandcis isomer ratios in solution and then 15 min and the precipitate was filtered off and purified by the spread on aidwater interface. In some experiments monolayer NazCO3-HCl procedure above to give 2.12 g of 3. Total yield of was illuminated with, a high-pressure mercury lamp (filter 345 3 was 3.55 g (69.6%). nm) to produce trans/cis isomerization in the monolayer at the 4-Carbomethoxy-4-(methylamino)azobenzene (5). 3 (0.5 aidwater interface. Light in these experiments covered all g) was dissolved in methanol (30 mL), concentrated HzS04 (3 monolayerarea. In order to observepossible photoisomerization mL) was added, and the mixture was heated at reflux for 2 h. effects reflected on changes of monolayer covered area in a After cooling, 50 mL of 10%Na2C03 was added dropwise. Red Langmuir trough, the experiment was performed at constant precipitate was filtered off, washed with water, and recrystallized pressure mode (0.5 or 10 mN/m). from 2-propanol to give 0.39 g(74%)of5, mp 173-175 "C, Rf(E) 0.45,purity established by HPLC 99.1%. IR(cm-l): 3420,1712, Results and Discussion 1531. W (CHC13)I,, 274 nm (E 12 660), 421 nm ( E 29 300). 4'-Carboxy-4-(methyllauroylamino)azobenzene (6a). 3 Photoisomerization of azobenzenes 6a-c and 7 in (0.2 g, 0.78 mmol) was dissolved in DMFA (4 mL) and pyridine chloroform and eluent solution was studied. When the (0.13 mL, 1.6 m o l ) and lauroyl chloride (0.24 mL, 1 mmol) in solution was irradiated with an UV light A.1 centered a t 2 mL of DMFA were added dropwise with stirring. The reaction 335 nm, the absorbance a t 460 nm increased and at 340 mixture was stirred for 24 h and then water (30 mL) was added. nm decreased (Figures 1 and 2). Trans and cis isomers The precipitate was separated, dissolved in 15 mL of acetic acid were identified by HPLC (in %) and TLC (Rfgiven in the and diluted with water, filtered, washed with hexane, and purified Experimental Section). on a Silicagel L 40/100 column with eluent D. Yield was 0.28 g After irradiation with UV and visible light alternately (82%)of6a (trans form),mp 154-155 "C,Rf (C) 0.28,Rf(D)0.48, Rf(F)0.39,purity established by HPLC 99.4%. lHNMR(DMS0the solutions were found to give reversibility between de), ppm: 0.8 (3 H, CH3), 1.13 (18 H, CHz), 2.06 (2 H, CHzCO), steady states of the cis and trans isomers. The switching 3.22 (3 H, CHsN), 7.50 (2 H, Ph), 7.86 (2 H, Ph), 7.94 (2 H, Ph), reversibility between both steady states could be repeated 8.10 (2 H, Ph). IR (cm-l): 1688,1660, W(CHsOH)I,, 231 nm for at least three cycles. From Figure 3 it is clear that the (E 14 400), 335 nm (E 27 7001,448 nm (E 980). W (CHC13)Amax changes in absorbance a t 460 nm of the compound 6a in 340 nm (E 27 4001,455 nm ( E 810). In solutions after irradiation the chloroform with the concentration 6.1x M were with UV light at 335 nm cis form was observed: Rf (C) 0.13, Rf reversible and the steady-state position was reached (D) 0.31, Rf (F)0.25. approximately after half a n hour in all cases. A similar 4-Carboxy-4-(methylpalmitoylamino)azobenzene(6b). reversible behavior was also observed in the films of the The reaction procedure analogous to that for 6a was followed. similar systems.18 Yield of 6b (trans form) was 71% mp 151-152 "C, Rf (D) 0.60, Rf(F)0.51,purityestablished by HPLC 99.5%. lHNMR(DMS0In order to calculate the trandcis ratio ofthe azobenzene dd, ppm: 0.82 (3 H, CH3) 1.12 (26 H, CHz), 2.07 (2 H, CHzCO), isomers in solution, the absorption spectra were compared 3.20 (3 H, CH3N), 7.48 (2 H, Ph), 7.86 (2 H, Ph), 7.94 (2 H, Ph), with the solution HPLC data. For this purpose the 8.10 (2 H, Ph). IR (cm-l): 1688,1656. W (CHCl3)I,, 340 nm (E 31 6001, 455 nm (E 960). Cis form: Rf (D) 0.4, Rf (F) 0.35. (21) Markava, E.; Freimanis, J.; Gailite,V.; h u l a , I.; Matisova, G.; 4 ' - C a r ~ x y 4 ( m e ~ y ~ ~ ~ y ~ o (&I. ) a zThe o b e Rutkis, M.; Aleksandrov, S.B.; Ilchenko, A. Ya.; Kolotilo, N. V. Latu. J. Chem. 1992, 604. reaction procedure analogous to that for 6a was followed. Yield
Photoresponsive Monomolecular Layers
Langmuir, Vol. 10, No. 9, 1994 3313 1500 1
I
I 3
d 1000
250
0
350
300
500
400
500
WAVELENGTH /nm
Figure 1. UV-vis absorption spectra of azobenzene6b solution ineluentA(2-MH-CH3COOH-hexane (10:1:89)):9.2 x M (a),2.3 x M (b); curve 1,before irradiation 100%trans form; curve 2, after 40min of irradiation by UV light at 335 nm 27% trans and 73% cis form.
25
50
I
75
100
Trans Isomer Concentration (%) Figure 4. Absorbance at 460 nm as a function of trans isomer concentration for azobenzene 6a solution in eluent A (2-PrOHCH3COOH-hexane (10:1:89)), 50.0 4
"
300
250
350 403 WAVELENCiTH/nm
500
Figure 2. UV-vis absorption spectra of azobenzene 7 solution in eluentA(2-PrOH-CH3COOH-hexane10:1:89): 1.86 x M (a),2.32 x M (b);curve 1,before irradiation 100%trans form; curve 2, after 40min of irradiation by UV light at 335 nm, 29% trans and 71% cis form.
-2 1
0.9 I
I 335 nm
I 4 5 2 nm
~
v
0.54
0
'
'
!
30
'
'
1
60
I 335 nm
1
1
I
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Time (min)
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Figure 3. Changes in absorbance at 460nm of azobenzene 6a solution in chloroform on alternate irradiation. solution of compound 6a was made directly in the eluent and irradiated, and then simultaneously both absorption spectra and HPLC pattern were recorded. At first the solution consisted of 90% of the trans isomer. After U V irradiation the portion of the trans isomer decreased to 34%, whereas the absorption at 460 nm reached its final equilibrium value. On the other hand, when the solution was irradiated with visible light (12 = 452 nm), cis-totrans isomerization was observed. Figure 4 shows that the concentration of the trans isomers influenced the absorbance a t the maximum 460nm and this relationship
10.0
20.0
30.0
40.0
50.0
60.0
Area per Molecule ( E ) Figure 5. Surface pressure-area per molecule isotherms of azobenzenes 6a (l),6b (2), 6c (31, and 7 (4)on water surface. was linear. It means that trandcis isomerization can be exactly controlled by the absorption changes of the 460nm band. Monomolecular Layers. The monolayers spread at the airlwater interface have been investigated by means of surface pressure-area (JC-A) isotherms and monolayer stability examination at the surface pressure 20 mN/m. The monolayer quality depends on the spreading solvent and the best results were obtained with chloroform and mixture hexane/2-propanol/aceticacid (89:1O:l).The bad spreading of azobenzene surfactants from low polarity solvent solutions may be connected with aggregation of azobenzene dyes in dimeres or larger aggregates. The spreading is also affected by the concentration of the surfactant on the surface. Smaller spreading volumes give higher collapse pressures, but not drastically influenced limiting area per molecule. As seen from isotherms (Figure 5) compound 6a with a shorter hydrophobic chain had a lower collapse pressure than 6b, 6c, and 7. The tail length of the surfactant also affects the stability of the monolayer (see Table 11,as well as esterification of carboxyl group which decreases the solubility of surfactant in subphase. As known from l i t e r a t ~ r e ~ the J ~ 8limiting ~~ area of trans and cis isomers of azobenzene-containing amphiphiles is rather different. In Table 1 we report the smallest area per molecule which is obtained in real experiments. We try to estimate the areas for pure cis and trans isomers. Light irradiation was used to change the cis/trans ratio
3314 Langmuir, Vol. 10,No. 9,1994
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Table 1. Monolayer Data
area per molecule, compound
k
collapse pressure, mN/m
monolayer stability (area change), % per h
6a
30.5 30.4 30.7 30.8
27 35 42 33
38 8 10 4
6b 6c
7
5
Freimanis et al. 70.0 0
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30.0.
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0
20.0.
Trans Isomer Concentration (Z) Figure 7. Area per molecule dependence on cidtrans isomer spread from eluent A (2-PrOHratio 'for azobenzenes 6a (0) CHaCOOH-hexane (10:1:89))and 6b (W) spread from CHCls.
15.0.
u
*3
10.0-
0.0 80.0
30.0
40.0
50.0
50.3 70.3 80.0
Area per Molecule (A') Figure 6. Surface pressure-area per molecule isotherms of azobenzene 6b on water surface: freshly prepared solution (1); after irradiation for 60 min (2); for 40 min (3); for 20 min (4). in spreading solution. The cidtrans ratio was estimated from the absorbances at 460 nm. In Figure 6 are shown isotherms of one ofthe investigations. ks seen from Figure 6,a n increase of cis isomer concentration increases area per molecule and decreases collapse pressure. There is correlation between limiting area per molecule and cis/ trans isomers ratio in spreading solution (Figure 7). From this linear correlation there are estimated limiting areas per molecule for cis (60f5 A) and trans (25 f5 A) isomers. These values correspond to nearly vertical orientation of azobenzene moiety in trans isomer molecules in monolayers and almost horizontal in cis isomer. We also made some attempts a t photoisomerization of azobenzene 6a in a monolayer at water/air interface. A large difference in surface area per molecule for cis and trans isomers means that photoisomerization should produce considerable effect on total area covered by the monolayer a t constant surface pressure. However irradiation with W light from a highpower mercury lamp a t constant pressures (0.5 and 10 mN/m) for more than 1 h produced no change of the monolayer covered area. That means that the trans to cis
isomerization did not take place under this experimental condition. Similar experimentsz2 in monolayers with azobenzene moiety located in aliphatic tail of surfactant show significant reversible changes of surface pressure in the mixed monolayers where azobenzene parts have space for isomerization. In our experiments, a t least a t the pressure 10mN/m, the trans to cis photoisomerization probably is blocked by close packing of azobenzene groups.
Conclusions The azobenzene derivatives 6a-c and 7 synthesized for this study have been shown to undergo a reversible trandcis isomerization in solutions with light irradiation. I t was found t h a t there is a linear correlation between limiting area per molecule and cis/trans isomers ratio in spreading solution. Area per molecule estimated from this correlation is 25 F 5 for the trans isomer and 60 F 5 Az for the cis isomer. These amide-type amphiphiles posessing photochromic azobenzene moiety and giving stable monolayers could be recommended for obtaining photoresponsive LB multilayers. However, it is reasonable in this case to apply some kind of molecular spacers between the azobenzene molecules which should help to avoid steric hindrance for trans to cis photoisomerization.
Az
(22)Maack, I.; Ahuja, R. C.; Matsumoto, M.; Mobius, D. 6th International Conferenceon Organized Molecular Films, July 4-9,1993, Quebec, Canada, Abstracts, p 106.
