Photoisomerization of amphiphilic azobenzene derivatives in Langmuir Blodgett films prepared as polyion complexes, using ionic polymers

Thin Solid Films 510 (2006) 297 – 304 www.elsevier.com/locate/tsf

Photoisomerization of amphiphilic azobenzene derivatives in Langmuir Blodgett films prepared as polyion complexes, using ionic polymers Vishakha R. Shembekar a,1, A.Q. Contractor a, S.S. Major b, S.S. Talwar b,* a

Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai-400 076, India Department of Physics, Indian Institute of Technology, Bombay, Mumbai-400 076, India

b

Received 5 November 2004; received in revised form 22 August 2005; accepted 14 December 2005 Available online 19 January 2006

Abstract Polyion complexation in mixed Langmuir and Langmuir Blodgett (LB) films of photochromic amphiphilic azobenzene carboxylic acids, 11-[4(4-hexylphenyl)azo] phenoxyundecanoic acid, 11-(4-phenylazo)phenoxyundecanoic acid, and diamine grafted poly(methylmethaacrylate) polymers has been studied. Monolayer behaviour of the pure components and mixed films was studied through pressure – area isotherms and LB films were characterized by spectroscopic, X-ray diffraction and Atomic force microscopy techniques. Aggregation (H-type), often observed in LB films of pure amphiphilic azo acids, was partly avoided in the mixed LB films as indicated by absorption spectral studies. Photoisomerization of the polyion complexed LB films was also studied. The results altogether demonstrate that amine grafted polymer enter into a polyion complexation with azo acid carboxylate group. LB films could be obtained by transfer of the composite monolayers and these LB films exhibited different levels of aggregation of the azo acids. Reversible photoisomerization was observed in LB films with unaggregated azo acid. D 2005 Elsevier B.V. All rights reserved. Keywords: Polyion complexes; Azobenzenes; Langmuir-Blodgett films; Photoisomerization

1. Introduction Photochrome containing Langmuir Blodgett (LB) films have been explored as media for photochromic reactions aimed at constructing molecule based information storage or switching devices [1– 3]. In this context LB films of azobenzene derivatives have attracted increasing attention in recent years due to their diverse photofunctional applications [4 –8]. These derivatives undergo facile cis –trans photoisomerization in solution, however isomerization is inhibited [9 –12] in pure LB films due to insufficient Ffree volume_ in the higher order structure of LB films. Photoisomerization not only involves electronic changes but also involves changes in the molecular conformation and cross-sectional area due to difference in the shape of the cis and the trans isomers. Changes in the crosssectional area and molecular conformation that are associated with azobenzene isomerization necessitate availability of * Corresponding author. Tel.: +91 22 25767556; fax: +91 22 25767552. E-mail address: [email protected] (S.S. Talwar). 1 Current address: 217 Biotechnology Bldg., Cornell University Ithaca, NY 14853-2703, United States. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.210

adequate Ffree volume_ within the two dimensional LB film structure since the cross-sectional area of the cis isomer is larger than that of the trans isomer. Another factor limiting many potential applications of LB films is the poor chemical and thermal stability. Amongst various strategies [13 – 21] used to provide additional Ffree volume_, complexation of azobenzene derivatives with suitable polyions has received much attention partly due its versatility and convenience. This approach obviates the need for synthesis of specially designed complex amphiphilic molecules [22,23]. Polyion complex approach has been investigated extensively in last two decades for enhancing the stability of monolayers and influencing the orientation of the amphiphiles [24]. The polyion complex method is useful in stabilizing monolayers at the air – water interface by the electrostatic interaction between the monolayers and the water-soluble polyions [25 – 27]. This strategy has the advantage of providing azobenzene a favorable environment for the reversible cis – trans photoisomerization [25,26]. It has been reported by Nishiyama et al. [26] that polymers with ionic pendent groups are capable of forming complexes with carboxylic acid groups of acid at the air water interface. This

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phenomenon could be used for hooking the azo acids on polymer backbone at a certain intervals thereby breaking the aggregates of azo acid being formed in the monolayer [28]. LB films of azobenzene containing polyion complexes have been shown to exhibit reversible photoisomerization. The photo stationary state cis isomer composition and the free volume availability in such films depend on the nature of polyions and the experimental conditions under which the film are prepared [29,30]. In this paper we report investigations on monolayers and transferred multilayers prepared as polyion complexes made from amphiphilic azobenzene carboxylic acids, 11-[4-(4hexylphenyl)azo]phenoxyundecanoic acid(6A10) and 11-(4phenyl azo)phenoxyundecanoic acid(0A10), and diamine random grafted poly(methylmetha acrylate) (PMMA) polymers (GPn, 6– 16% graft) by LB method and study of photoisomerization in LB films. The chemical structures of the azo acids and the amine grafted polymers are shown in Fig. 1. The protonated basic amino centers in the grafts are expected to function as polyions for complexation with the ionized acid. Monolayer behaviour has been examined by detailed pressure – area isotherms (p – A) isotherm studies. p – A isotherm studies of the monolayers essentially indicated absence of aggregation in the monolayers. LB films were obtained by transfer of the monolayers by vertical transfer methodology and characterized by Fourier Transform Infrared spectroscpoy (FTIR), electronic absorption spectroscopy (UV-Vis), X-ray diffraction (XRD) and Atomic Force microscopy (AFM). The results demonstrate that suitable pendent group grafts on polymers enter into complexation with the carboxylate group on azo acid and can serve as a basis for thin film media for reversible photoisomerization of the azo group. 2. Experimental details 2.1. Materials 11-[4-(4-hexylphenyl)azo]phenoxyundecanoic acid [31], and 11-(4-phenylazo)phenoxyundecanoic acid [32], were synthesized by diazotization of respective 4-substituted aniline and coupling of the diazotized product with 11-bromoundecanol followed by its oxidation using PDC ( 6A10, mp = 106 -C, reported mp = 106 –108 -C, 0A10, mp = 116 –119 -C, reported mp = 116 – 118 -C) [31,32]. 4-substituted anilines, 11-bromoun-

decanol and dodecylbromide were bought from Aldrich. The azo acids were repeatedly recrystallized to achieve a purity of 99.9%. Purity of the azo acids was monitored using high performance liquid chromatography (HPLC) (Shimadzu LC-8A). Grafted PMMA was prepared by refluxing 1, 2-diaminopropane (10 –50% of the PMMA) and PMMA in toluene for 8 h in a Dean – Stark apparatus to remove methanol. Subsequently grafted polymers were precipitated in hexane. The precipitated polymers were purified and freed from unreacted diaminopropane by repeated dissolution in toluene and precipitation in hexane. The levels of grafting achieved were much lower than the attempted graft level. Extent of grafting was determined by amine value method. Grafting achieved and attempted in % for different samples were GP1, 6.7(10); GP2, 8.7(20); GP3, 11(30); GP4, 13.1(40); GP5, 15.3(50), respectively. The grafted PMMA was further characterized by FTIR and Nuclear Magnetic Resonance spectroscopy. Measurements of surface pressure –area isotherms and the automated deposition of the monolayers were carried out with Langmuir trough (KSV 3000) equipped with an electronic microbalance and a platinum Wilhelmy plate, kept in a clean room, class 10,000. Deionized water with resistivity 18.2 MV cm from Millipore system was employed for subphase preparation. Pure azo acid monolayer behaviour and deposition of LB films was studied with water subphase and subphase containing 10 4 M CdCl2. Composite monolayers films of azo acids and GPn polymers were made by spreading solutions of the azo acid and polymer made by mixing parent solutions of azo acid (0.56 mg ml 1) and GPn (0.62 mg ml 1) in HPLC grade chloroform in 1 : 1 M proportion of the presumed graft. 100 Al of the mixed solution of azo acid and GPn was spread on the LB trough with water subphase at ambient temperature and chloroform was allowed to evaporate. An equilibration time of 40 min was allowed. The monolayers at the air – water interface were compressed with the help of barriers (barrier speed 3 mm/min). Formation of the condensed monolayer was followed by p – A isotherms. Stability of the monolayers was studied by noting the change in mean molecular area while maintaining the monolayer at a specific pressure. Monolayers were transferred onto quartz plates of dimensions 1 1 in. (and on CaF2 plates of dimensions 1  0.5 in. for FTIR) at a surface pressure of 15 mN/m with a dipping speed of 3 mm/min. For all films either 11 or 22 monolayers were

Fig. 1. Structures of azo acids and the grafted polymer.

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transferred. Electronic absorption spectra of the films were recorded using Shimadzu 260 UV-Vis spectrophotometer. A clean identical quartz plate was used as reference. XRD studies were carried out using a Philips X-ray Diffractometer (model PW 1729) with Ka line from a copper target which produces ˚ . The samples were scanned at X-rays of wavelength 1.5406 A the rate of 2.4-/min in the 2h range of 4 –20-. AFM studies were done using Nanoscope III scanning probe microscope with a microfabricated Si3N4 cantilever (force constant 0.5 N m 1) stylus assembly. 5 nm to 2 Am images were obtained under constant force conditions with a net tip force in the region of 10 – 100 nN. The AFM measurements were performed in the Fnon-contact_ mode to obtain molecular resolution images. 3. Results and discussion 3.1. Monolayer studies This section reports behaviour of composite monolayers of azo acids and grafted PMMA. Monolayer behaviour of the pure azo acid and pure grafted polymers was also examined for comparison. Grafted PMMA monolayers were prepared by spreading a chloroform solution of the polymer over water subphase and composite monolayers of azo acids and polymers GPn were prepared by spreading a chloroform solution of the mixture of appropriate composition on the water subphase. Surface pressure – area isotherms of diamine grafted PMMA are shown in Fig. 2. Area per molecule axis is based on the number of graft weighted PMMA monomer molecules. Increased diamine grafting causes a shift of the p –A curve to larger mean molecular area (Mma) values while retaining the shape of the PMMA p – A curve. The curves exhibit two phase transitions similar to PMMA. The first phase transition probably involves folding of polymer chains on the water surface and the second transition, which occurs at pressures above ¨30 mN/m, probably involves an irreversible collapse. The first phase transition in the grafts occurs around 15 – 17 mN/m. The limiting mean molecular area (Lmma) obtained from p – A isotherms of GPn by extrapolation of the steep part of the curves before and after the first transition to zero surface

Fig. 2. p – A isotherms of the grafted polymers, GP1 – GP5.

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pressure, are plotted as function of graft composition in Fig. 3 (a) and (c), respectively. Collapse pressure for all the GPn monolayers was ¨45 mN/m. In PMMA the ester groups function as hydrophilic groups and it has been shown that in the compressed monolayer the polymer backbone is stretched out horizontally along the barrier at the interface [33]. The gradual increase in the Lmma with the increase in the percentage of amine graft as shown in Fig. 3(a) (except for GP2 where the increase in Lmma is much larger) suggests that grafting anchors grafted PMMA on water surface and polymer chains in grafted polymers are probably more extended than in PMMA. This may also result from increase in the area of the grafted repeat unit and enhanced polarity due to presence of free amino groups in the polymer. It is noted that the sharpness of the first transition is less marked in the p – A curves as the grafting level increases, however, the transition occurs at almost the same surface pressure. 6A10 –GPn (AGPn) Composite monolayers and LB films were studied in greater detail as the amphiphilic azo acid 6A10 is well studied by the LB methodology. p –A isotherm of composite 6A10 –GP1 (AGP1) monolayer is shown in Fig. 4(a). The area per molecule axis is based on the total number of GP1 and 6A10 molecules. Fig. 4(a) also shows the p – A isotherms of pure GP1 and 6A10 under similar conditions. Compression –decompression cycles up to pressure of 15 mN/ m did not show any hysteresis for the composite monolayer. Also the monolayer was found to be quite stable at a pressure of 15 mN/m. p – A isotherm of the composite monolayer shows an expanded nature. The composite isotherm AGP1 is more characteristic of the grafted polymer than that of the 6A10 isotherm. It showed two phase transitions similar to those observed in pure grafted polymer monolayers. However, the p – A isotherm curve shifts to larger mean molecular area values compared to those for pure GP1 monolayer whereas the collapse pressure was almost comparable to 6A10. Similar monolayer behaviour was observed for composite films containing polymers grafted to different extents Fig. 4(b) – (e). Pre and post transition Lmma values obtained from the composite curves AGPn are plotted as Fig. 3 (b) and (d), respectively. Experimentally determined Lmma values from the pre-transition region of the AGPn curves are smaller than that expected for a simple mixture of components calculated using the expression A (calculated) = A av a + A pv p where A is the area of composite film and A a, A p areas and v a, v p being the respective mole fractions of 6A10 and the graft polymers GPn, ˚ 2 compared to respectively (thus AGP1 Lmma is ¨18.5 A 2 ˚ ). This is suggestive of attractive interaction between 19.5 A the azo acid and the polyion indicating polyion complex formation and avoidance of aggregation of 6A10 molecules in the closely packed composite monolayer at the air – water interface [34]. Polyion complex formation is also supported by the FTIR of the transferred films (vide infra). Comparison of other isotherms of GPn and composite 6A10 – GPn (1 : 1), respectively, also indicate similar behaviour. Lmma obtained from the AGPn isotherms for the post transition region are smaller for some of the cases than that obtained from the graft polymer GPn isotherms as indicated in Fig. 3 (d) and (c),

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Fig. 3. Variation of least mean molecular area for GPn and AGPn with graft percent. Before transition (a) GPn and (b) AGPn; after transition (c) GPn and (d) AGPn.

respectively. It may be noted that the phase obtained after the first transition is less compressible in contrast with corresponding GPn phase. Compressibility is least for AGP5

which contains only 15% grafted polymer and thus may have large amount of uncomplexed azo acid in the monolayer. The similarity of the AGPn curves with the GPn curves may be a reflection of the fact that the polymer is horizontally spread out and makes large contribution to the mean molecular area in contrast with azo acid which is oriented in the vertical direction. Monolayer behaviour of some other amphiphilic azo acids [35] and polymer GP2 composite systems were studied. GP2 was selected as a representative graft polymer for mixing with the amphiphilic acids due to its potential for providing relatively higher azo acid loading capacity, (GP3 – GP5 have higher loading capacities, but have shown greater extent of aggregation of the azo acid in LB films ). Behaviour of the composite monolayers was similar to that observed for AGP2 monolayer despite differences in the structure of the azo acids. The p – A isotherm of composite 0A10 –GP2 (AzGP2) monolayer is shown in Fig. 5 along with those of pure components. Both, 0A10 and the composite p – A isotherms showed considerably expanded nature and the p – A isotherm of composite monolayer shows characteristics similar to that of the graft polymer GP2 rather than that of the azo acid. The composite monolayer was more stable in contrast with the monolayer of the 0A10 with collapse pressure of 30 mNm 1. No hysteresis was seen when the monolayer was compressed and decompressed repeatedly to a surface pressure lower than the collapse pressure. AzGP2 p –A isotherm curve shifted to larger mean molecular area compared to pure GP2 monolayer. Lmma observed for the composite film and FTIR of the of the LB film of AZGP2 support the formation of polyion complex. 3.2. LB film studies The composite monolayers of AGPn were transferred on to the substrate at ¨15 mN m 1. This was done due to larger mean molecular area prior to the first phase transition. All the AGPn composite monolayers showed Y type transfer tending towards Z type. The transfer ratios observed up to 22 layers are given in Table 1. On deposition of still larger number of layers, the transfer ratio decreased considerably with increasing number of layers. The deposition ratios observed for AGPn composites suggest the formation of poorly ordered and nonuniform LB films.

Fig. 4. (a) p – A isotherms of GP1, AGP1 and 6A10 on subphase water; (b) – (e) p – A isotherms of AGPn and GPn (n = 2 – 5).

Fig. 5. p – A isotherms of GP2, AzGP2 and 0A10 on subphase water.

V.R. Shembekar et al. / Thin Solid Films 510 (2006) 297 – 304 Table 1 Transfer ratio (T R) obtained for lifting and dipping runs during transfer of AGPn diamine grafted monolayer at air – water interface Sr. no.

6A10 – GPn

TR (lifting)

TR (dipping)

1 2 3 4 5

6A10 – GP1 6A10 – GP2 6A10 – GP3 6A10 – GP4 6A10 – GP5

0.9 T 0.1 0.8 T 0.1 0.8 T 0.1 0.7 T 0.1 1.0 T 0.1

0.5 T 0.1 0.4 T 0.1 0.5 T 0.1 0.3 T 0.1 0.6 T 0.1

FTIR spectrum typical of AGPn films is shown in Fig. 6. Absorption bands at 2924, 2855 cm 1 (CH2 m as and m s stretching) and at 1586 cm 1 (C(O)O – m as stretching) establish the presence of the azo acid component in the film and bands at 1728 cm 1 (ester carbonyl peak), ¨1660 cm 1 (amide carbonyl) point to the presence of the grafted polymer in the film. Polyion complex formation of 6A10 and the polymer in the film is supported by the absence of 1710 cm 1 band for carbonyl of the carboxylic acid and appearance of band at 1586 cm 1 for carboxylate anion and weak bands for protonated amine at ¨3000 – 2800 cm 1 region. The carbonyl stretching band at ¨1710 cm 1 present in the 6A10 azo acid LB films is absent in the AGP1 – AGP5 films despite the expected presence of some azo acid in the uncomplexed form in the composite films. The masking of this band by the large ester carbonyl absorption is likely in films with excess acid. UV-Vis spectra of the as deposited trans 6A10 – GP1-5 (AGP1 –AGP5) films (22 layers) are shown in Fig. 7 (I – V). Curves (a) in each frame represent the spectrum of the as deposited film while the curves (b) and (c) are the spectra obtained after photoirradiation of the films at 350 and 254 nm sequentially for each of the films. In solution spectrum of 6A10 absorption bands are seen with maxima at ¨450 nm (assigned as n – k* transition), ¨350, ¨238 nm (assigned to k – k* transitions) for azobenzene group and benzene ring, respectively. A typical as deposited LB film of trans 6A10 exhibits

301

absorption bands with maxima at ¨300 and ¨250 nm and a very weak absorption in the ¨400 nm region. These changes in the LB film spectrum have been attributed to H-aggregation with parallel stacking of the azobenzene chromophores of the azo acid molecules in the LB film [36,37]. It is noteworthy that as deposited AGP1, AGP2 LB films exhibit significant absorbance in the 350 nm region besides absorption at 300 nm region suggesting presence of unaggregated azo acid. Increased loading of the grafted polymer with the azo acid AGP3 –AGP5, Fig. 7 (III – V) curves (a), respectively, suggest that only a small fraction of unaggregated acid is present in AGP3 –AGP5. The relative absorption intensities at 300 and 350 nm may be seen as an indication of the fraction of molecules in the aggregated and unaggregated state, respectively [29,38 – 40]. Fig. 7(VI) is a plot of absorbance of azo acid in polyion composite films at 300 and 350 nm, respectively, with the extent of graft. Examination of the relative intensity at 300/350 nm indicates a gradual increase in the fraction of aggregated 6A10 molecules in the films. Also, there is finite and small increase in the fraction of complexed azo acid molecules with increase in graft level although it is not proportional to increase in the graft level. The increase in the fraction of aggregated azo molecules from AGP1 to AGP5 is to be expected in view of the excess of azo acid present above the graft levels in the composite films AGP1 to AGP5, respectively. Further, the number of monolayers deposited was also found to affect the extent of the azo acid aggregation observed in the film. An eleven layer LB film was found to show greater fraction of the unaggregated acid in comparison with a 22 layer film. This was noticed for LB films of AGP1, AGP2. 3.3. Photoisomerization in LB films Photoisomerization of the composite LB films was examined by irradiation at 350 nm and subsequently at 254 nm and the absorption spectrum after these irradiations are given by

Fig. 6. FT-IR spectrum of AGP1 LB film on CaF2 (22 layers) showing presence of both 6A10 and GP1.

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0.000 200

λ(nm)

0.20

600

0.20

0.10 c a b

400

λ(nm)

0.40

600

249

0.20 300 a b c

400

λ(nm)

600

a b c

400

λ(nm)

600

IV(AGP4)

0.30 a b c

0.00 200

400

λ(nm)

0.12

V(AGP5)

0.00 200

300

0.00 190

III(AGP3)

0.00 190

350

0.05

Absorbance

Absorbance

a b c

400

II(AGP2)

Absorbance

350 300

0.035

Absorbance

0.10

I(AGP1)

Absorbance

Absorbance

0.070

600 VI

0.06

300 350

0.00

6

11

nantly cis isomer containing film when irradiated with 254 nm, showed a spectrum characteristic of trans isomer with complete absence of aggregated azo acid. Further alternate irradiation cycles showed good reversible isomerization in an AGP2 composite film as shown in inset Fig. 8. Electronic absorption spectra of composite films of 6A10 – GP3, 6A10 – GP4 and 6A10 –GP5 (Fig. 7), showed relatively greater degree of aggregation of 6A10. These films showed very low levels of photoisomerization (Fig. 7). The azo acid aggregation remained almost unaffected on irradiation, in contrast with composite films AGP1 and AGP2. LB films of 0A10-GP2 (AzGP2) were also obtained by transfer of the composite monolayer on the substrate at 15 mN/ m. The composite monolayer showed poor transfer with transfer ratios of 0.5 for lifting and 0.15 for dipping. On deposition of larger number of layers, the transfer ratio decreased considerably. Poor transfer ratios were also observed in the transfer of two other polyion-azo acid complexes (not discussed here) [35]. FTIR spectrum of AzGP2 film was very similar to that in Fig. 6, and showed all the signatures for the formation of polyion complex. The UV-Vis spectrum of the as deposited trans 0A10-GP2 film showed a broad absorption with absorption maxima at 350 and 300 nm (Fig. 9) indicating presence of substantial amount of unaggregated azo acid along with the aggregated azo acid in the film. Curves (b) and (c) in Fig. 9 represent UV-Vis spectra obtained after photoirradiation of the film with 350 nm followed by 254 nm, respectively. On irradiation with 350 nm, the trans isomer was converted predominantly to the cis isomer with characteristic absorption at 300 nm (trace b). This was then back converted to trans by irradiation with 254 nm (trace c) showing absorption only at

16

% Graft

Fig. 7. UV-Visible absorption spectra of AGP1 – AGP5 films (I – V). Curves Fa_ are for as deposited films, curves Fb_ after irradiation of the as deposited films at ¨350 nm and curves Fc_ obtained after subsequent irradiation with 254 nm in each case; (VI ) Plot of absorbance at 300 nm (>) and 350 nm(r) against level of grafting for the as deposited films.

curves (b) and (c) in frames I– V in Fig. 7, respectively. Composite films of AGP1 and AGP2 were found to exhibit reversible photoisomerization. Films AGP3 – AGP5, with greater grafting and also greater amount of azo acid loading, showed low and decreasing levels of photoisomerization. As deposited trans 6A10 – GP1 films (11 and 22 layers) and 11 monolayer 6A10 – GP2 film showed broad band absorption with k max¨355 nm characteristic of unaggregated 6A10 (as usually seen in solutions of 6A10) and typified in Fig. 8. Thus UV spectrum of the as deposited trans 6A10 –GP1 film (22 layers) showed presence of monomer species (k max¨355 nm) in equilibrium with aggregated species (k max¨304 nm). On irradiation of 6A10 – GP1 film (22 layers) with ¨350 nm, absorption spectra of the film indicated conversion to predominantly cis isomer. It is of interest that this predomi-

Fig. 8. Absorption spectrum of AGP1 (22 monolayers), (a) as deposited, (b) after irradiation at 350 nm, (c) after irradiation at 254 nm after several cycles of irradiation. Inset shows the cis – trans photoisomerization cycles in a AGP2, 22 monolayer film.

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350 nm suggesting absence of aggregates of the azo acid. Further irradiation of the film showed good reversible isomerization. Storage of the film did not lead to reaggregation of molecules. In summary we note that low graft levels and low azo acid loaded composite LB films exhibit presence of unaggregated and aggregated azo molecules in proportions depending on the graft level, the number of deposited layers and azo acid loading. Composite films with low loading of azo acid exhibit higher fraction of unaggregated azo molecules due to polyion complexation and such films show facile reversible isomerization. Further partial aggregation present in the as deposited composite films was irreversibly destroyed in the isomerization process. Films with higher loading of azo acid do not exhibit facile reversible photoisomerization and do not indicate significant effect on the aggregate proportion in the isomerization process. It may be speculated that even with higher graft levels of polymers, higher loading of azo acid may not lead to larger proportion of unaggregated azo acid although it requires further experimental confirmation. To understand the structural changes associated with polyion complexation in the LB films, XRD and AFM of the composite films were studied. XRD of the composite AGPn and AzGP2 as deposited films did not show any diffraction pattern pointing to absence of any layered order. In-plane molecular packing of 6A10 and AGP1 composite film was analyzed using atomic force microscopy. Scanning a 2  2 Am2 area showed the presence of 100 nm diameter domains with internal structure (not shown). AFM results of the fine structure within single domains for 6A10 and AGP1 composite film are shown in Fig. 10 (a) and (b). The AFM of 6A10 film shows an ordered molecular arrangement, Fig. 10

303

b

5.0 nm

1.8 nm 2.5

0.9 nm 0 nm

0 0

2.5

5.0 nm 5.0 nm

a

1.0 nm 2.5

0.5 nm 0 nm

0 0

2.5

5.0 nm

Fig. 10. AFM of 17 layered LB film of (a) 6A10 and (b) AGP1 composite deposited on quartz.

(a), the Fourier transform of which indicates a distorted ˚ and b = 3.6 T 0.2 A ˚. hexagonal structure with a = 5.1 T 0.2 A The sharp and symmetric reflections seen in the Fourier transform is an indicator of long range molecular order in the film. The AFM image from the composite film, Fig. 10 (b) shows a heavily deformed structure with lack of long range molecular order. The presence of short range order in the composite film clearly indicates that inclusion of randomly grafted polymer into the LB film does not disrupt the molecular order completely. 4. Conclusion The above results demonstrate that amine containing pendent group random PMMA grafts enter into complexation with the carboxylate group of azo acids, to form stable and transferable monolayers. LB films of the polyion complexes obtained show partial deaggregation of amphiphilic azo acids; the extent of aggregation depends on graft levels and the film preparation conditions. Greater aggregation was noticed in LB films with larger number of deposited monolayers. Although no layered order was observed, AFM measurements indicated that some degree of short range in plane order is observed in the composite films. Unlike LB films of pure azo acids, LB films of polyion complexes exhibited reversible photoisomerization. In some cases the photoisomerization process caused enhanced irreversible disaggregation of the azo acid molecules. Acknowledgements

Fig. 9. UV-Visible spectrum of 0A10-GP2 (a) as deposited film with broad absorption at 350 nm indicating presence of unaggregated species, (b) after irradiation at 254 nm.

Authors wish to thank Dr. M. Subramaniam for providing 1,2-diamino propane and valuable suggestions about grafting

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