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

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.

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 switch-

ing 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 cross-

sectional area and molecular conformation that are associated

with azobenzene isomerization necessitate availability of

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 deriva-

tives 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 advan-

tage 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

298

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-(4-

hexylphenyl)azo]phenoxyundecanoic acid(6A10) and 11-(4-

phenyl azo)phenoxyundecanoic acid(0A10), and diamine

random grafted poly(methylmetha acrylate) (PMMA) polymers

(GPn, 6–16% graft) by LB method and study of photoisome-

rization 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 photoisome-

rization 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-bromoundeca-

nol followed by its oxidation using PDC ( 6A10, mp=106 -C,reported mp=106–108 -C, 0A10, mp=116–119 -C, reportedmp=116–118 -C) [31,32]. 4-substituted anilines, 11-bromoun-

Fig. 1. Structures of azo acids

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-diamino-

propane (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 diaminopro-

pane 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 PMMAwas 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 dimen-

sions 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

and the grafted polymer.

299

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

X-rays of wavelength 1.5406 A. The samples were scanned at

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.

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–Aisotherms 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)=Aava+Apvp where A is the area

of composite film and Aa, Ap areas and va, vp being the

respective mole fractions of 6A10 and the graft polymers GPn,

respectively (thus AGP1 Lmma is ¨18.5 A2 compared to

19.5 A2). This is suggestive of attractive interaction between

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),

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.

300

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

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).

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) mono-

layer 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 non-

uniform LB films.

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

Table 1

Transfer ratio (TR) 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 6A10–GP1 0.9T0.1 0.5T0.1

2 6A10–GP2 0.8T0.1 0.4T0.13 6A10–GP3 0.8T0.1 0.5T0.1

4 6A10–GP4 0.7T0.1 0.3T0.1

5 6A10–GP5 1.0T0.1 0.6T0.1

301

FTIR spectrum typical of AGPn films is shown in Fig. 6.

Absorption bands at 2924, 2855 cm�1 (CH2 mas and msstretching) and at 1586 cm�1 (C(O)O–mas stretching) establishthe 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, respec-

tively. A typical as deposited LB film of trans 6A10 exhibits

Fig. 6. FT-IR spectrum of AGP1 LB film on CaF2 (22

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, respec-

tively [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, respec-

tively. 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 exam-

ined by irradiation at 350 nm and subsequently at 254 nm and

the absorption spectrum after these irradiations are given by

layers) showing presence of both 6A10 and GP1.

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

λλ(nm) λ(nm)

λ(nm) λ(nm)

λ(nm) % Graft

0.070II(AGP2)I(AGP1)

III(AGP3) IV(AGP4)

V(AGP5) VI

0.035

0.000

0.10

0.05

0.00

0.20

0.10

0.00

0.40

0.20

0.00

0.12

0.06

0.00

0.20

0.30

0.00

200 400 600 190 400 600

190 400 600 200 400 600

200 400 600 6 11 16

350

350300

300abc a

bc

abc

abc

ab

c

300

249

300

350

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.

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 o

irradiation. Inset shows the cis– trans photoisomerization cycles in a AGP2, 22

monolayer film.

302

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 kmax¨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 (kmax¨355 nm)

in equilibrium with aggregated species (kmax¨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-

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

)

f

0 2.5 5.0 nm

0

2.5

5.0 nm

0 nm

0.5 nm

1.0 nm

a

0 2.5 5.0 nm

0

2.5

5.0 nm

0 nm

0.9 nm

1.8 nm

b

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

deposited on quartz.

303

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 isomeriza-

tion. 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 isomeri-

zation 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

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.

(a), the Fourier transform of which indicates a distorted

hexagonal structure with a =5.1T0.2 A and b =3.6T0.2 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 photoisome-

rization. In some cases the photoisomerization process caused

enhanced irreversible disaggregation of the azo acid molecules.

Acknowledgements

Authors wish to thank Dr. M. Subramaniam for providing

1,2-diamino propane and valuable suggestions about grafting

304

of PMMA, Dr. P. Bhimalapuram for valuable discussions on

isotherm analysis, Prof. R.Srinivas and Professor S. Vitta for

discussions regarding AFM. SST thanks Sukhvinder Singh for

assistance in processing of figures.

References

[1] D.G. Whitten, Angew. Chem., Int. Ed. Engl. 18 (1979) 440.

[2] E. Ando, J. Miyazaki, K. Morimoto, K. Fukuda, Thin Solid Films 133

(1985) 21.

[3] T. Kawai, Thin Solid Films 301 (1997) 225.

[4] A. Yabe, Y. Kawabata, H. Niino, M. Matsumoto, A. Ouchi, H. Takahashi,

S. Tamura, W. Tagaki, H. Nakahara, K. Fukuda, Thin Solid Films 160

(1988) 33.

[5] H. Tachibana, T. Nakamura, M. Matsumoto, H. Komizu, E. Manda, H.

Niino, A. Yabe, Y. Kawabata, J. Am. Chem. Soc. 111 (1989) 3080.

[6] T. Seki, T. Tamaki, Y. Suzuki, Y. Kawanishi, K. Ichimura, Macromole-

cules 22 (1989) 3505.

[7] Z.F. Liu, K. Hashimoto, A. Fujishima, Nature 347 (1990) 658.

[8] J. Anzai, N. Sugaya, T. Osa, J. Chem. Soc., Perkin Trans. 2 (1994) 1897.

[9] D.G. Whitten, J. Am.Chem. Soc. 96 (1974) 594.

[10] L. Collins-Gold, D. Mobius, D.G. Whitten, Langmuir 2 (1986) 191.

[11] H. Nakahara, K. Fukuda, M. Shimomura, T. Kunitake, Nippon Kagaku

Kaishi (1998) 1001.

[12] W.F. Mooney, P.E. Brown, J.C. Russell, S.B. Costa, L.G. Pederson, D.G.

Whitten, J. Am. Chem. Soc. 106 (1984) 5659.

[13] O. Befort, D. Mobius, Thin Solid Films 243 (1994) 553.

[14] R.C. Ahuja, J. Maack, J. Phys. Chem. 99 (1995) 9221.

[15] K. Shirota, K. Kajikawa, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 29

(1990) 750.

[16] X. Xu, M. Era, T. Tsutsui, S. Saito, Thin Solid Films 178 (1989)

541.

[17] X. Xu, M. Era, T. Tsutsui, S. Saito, Thin Solid Films 178 (1989)

L138.

[18] M. Tanaka, Y. Ishizuka, M. Matsumoto, T. Nakamura, A. Yabe, H.

Nakanishi, Y. Kawabata, H. Takahashi, S. Tamura, W. Tagaki, H. Nakara,

K. Fukuda, Chem. Lett. (1987) 1307.

[19] Z.F. Liu, B.H. Loo, R. Baba, A. Fujishima, Chem. Lett. (1990) 1023.

[20] M. Matsumoto, D. Miyazaki, M. Tanaka, R. Azumi, E. Manda, Y. Kondo,

N. Yoshino, H. Tachibana, J. Am. Chem. Soc. 120 (1998) 1479.

[21] T. Kunitake, A. Tsuge, N. Nakashima, Chem. Lett. (1984) 1783.

[22] K. Nishiyama, M. Fujihira, Chem. Lett. (1988) 1257.

[23] O.N. Oliveira, Polymer 43 (2002) 3753.

[24] E.D. Goddard, Colloids Surf. 19 (1986) 301.

[25] R.A. Hall, M. Hara, W. Knoll, Langmuir 12 (1996) 2551.

[26] K. NIshiyama, M. Kurihara, M. Fujihira, Thin Solid Films 179 (1989)

477.

[27] M. Shimomura, T. Kunitake, Thin Solid Films 132 (1985) 243.

[28] H. Tachibana, R. Azumi, M. Tanaka, M. Matsumoto, S. Sako, H. Sakai,

M. Abe, Y. Kondo, N. Yoshino, Thin Solid Films 284–285 (1996) 73.

[29] N. Kimizuka, T. Kunitake, Chem. Lett. (1988) 827.

[30] R.A. Hall, M. Hara, W. Knoll, Langmuir 12 (1996) 2551.

[31] T. Seki, M. Sakuragi, Y. Kawanishi, Y. Suzuki, T. Tamaki, R. Fukuda,

K. Ichimura, Langmuir 9 (1993) 211.

[32] M. Era, T. Tsutsui, S. Saito, Langmuir 5 (1989) 1410.

[33] S.T. Kowel, G.G. Zhou, M.P. Srinivasan, P. Stoeve, B.P. Huggins, Thin

Solid Films 134 (1986) 209.

[34] M. Shimomura, R. Ando, T. Kunitake, Ber. Bunsenges. Phys. Chem. 87

(1983) 1134.

[35] Monolayer behaviour and LB film deposition of GP2 with 11-[4-

(-dodecyl phenyl)azo]phenoxyundecanoic acid (12A10), 4-[4-dodecy-

loxyphenyl)azo]benzoic acid (12A0) composites was also studied.

Vishakha Shembekar, Ph.D Thesis, IIT Bombay, 2000.

[36] W.F. Mooney, D.G. Whitten, J. Am. Chem. Soc. 108 (1986) 5712.

[37] J. Heesemann, J. Am. Chem. Soc. 102 (1980) 2167.

[38] D.L. Beveridge, H.H. Jaffe, J. Am. Chem. Soc. 88 (1966) 1948.

[39] X. Xu, M. Era, T. Tsutsui, S. Saito, Chem. Lett. (1988) 773.

[40] S. Dante, R. Advincula, C.W. Frank, P. Stroeve, Langmuir 15 (1999) 193.