2. PHOTORESPONSNE AZO POLYMERS. CONCEPTS,

CHARACTERISTICS AND APPLICATIONS

The first report of photochromic low molecular weight organic

compound was originated by ~ a r c k w a l d ~ in 1899. Following this a

good number of photoresponsive polymers having stilbene system 15-18

and spirobenzopyran groups have been developed. In polymers

with stilbene system reversible -- cis-trans isomerisation is responsible

for the photochromic behaviour, whereas in spirobenzopyrans a reversible

ring opening or ring closure. This thermally or photochemically reversible

isomerisation or the ring o'pening phenon~enon is always acco~npanied

by some physical and chemical changes in the chromophores. Among

such changes the following properties, ( i ) a change in dipole moment

( i i ) a difference in geometrical structure and ( i i i ) the generation of

a charged species, seem to be useful as a driving forcc to induce

reversible conformational changes in the polymer chain. The change

in conformation is cxpccted to produce a concomitant changc in physical

and chemical properties of the polymers. In view of thc possible

cis-trans isomerisation of the azo group, though azobenzene isomerises -- by inversion, and the expected importance of azo photochromes as

materials, a detailed study of photoresponsive polymers having azo

groups was initiated following the report of Lovrien and Waddington 24

in 1964.

Azobenzenc and many of its derivatives ure charticterized by

reversible transformations from the generally more stable trans form (1) - t o the less stable - cis form (2) upon irradiation w i th UV-visible l ight

hv A 7

hv, A

to yield a photostationary cornposition which is wavelength and tempera-

ture dependent 25-28. The lack o f coplanarity due t o steric hindrance

makes the - cis form less stable. while the - trans configuration is essentially

2 9 planar and ful ly conjugated . In the act ivated state the double bond

character between the two nitrogen atoms is decreased and very low

energy is required t o twis t the molecule around the N-N bond so

that the - cis and trans isomer w i l l rapidly interconvcrt.

Azobenzeric containing systems have one advantage in that there

is no known evidence o f emission from the cxc i tcd stntcs o f uzobcnzcne

c i thcr in the - c i s ' o r trnns form, and therefore no proccsscs o f spcctro-

scopic ineff iciency competing w i th photochemical isomerisation is known.

The -- cis-trans isomerisation o f azobenzene moiety represents vir tual ly

a model photochemical process i n which one stereoisonicr is favoured

thermally and the other photochemically. The isomerisation involves

a decrease in the distance between the para carbon atoms from 9 A0

in the trans-azobenzene t o 5.5 Pi0 in the - cis form. 'I'he photo-~somerisation

causes a decrease in the intense absorption around 320 nm due to

;g - IT* transition and an increase in absorption around 440 nm due

to n-T* t r a n ~ i t i o n ~ ~ , in addition to a change in dipole moment. Moreover,

the quantum yields are generally high for the isomerisation. As a

result, azobenzene moiety has been conveniently used by many groups

of researchers to enforce changes in conformation of polymers under

appropriately controlled conditions. Nowadays, the trend is to employ

the azobenzene group as a trigger to induce changes in morphology

which can be light driven.

2.1. Spectral Response

UV-visible spectroscopy has been extensively exploited to study

the photo-isomerisations involved in polymers 30-32. The absorption

spectrum of azo group changes as the configuration alters from - trans

to - cis upon irradiation. Irie, Menju and ~ a ~ a s h i ~ ~ in 1979 observed

similar types of spectral changes in photoresponsive polyamides having

azobenzene groups in the backbone of the polymer chain. The intense

absorptions at 390 nm decreased when an N,N-dimethylacetarnide solution

of the polyamide (3) was irradiated with ultraviolet light, while the

band at 510 nm increased in intensity. The intense band at 390 nm

was attributed to the trans form of the polyamide having ~zobenzene - in the backbone of the polymer chain and band at 510 nrn to the

cis form. The change in inlensity of the two bands is a clear cvidencc -

of the photo-isomerisation of the azobenzene residues in thc chain

backbone. It was also observed that the intensity of absorption a t

390 n m increases gradually, after cutting off the light and keeping

the polymer solution a t room temperature.

I<amogawa and c o - ~ o r k e r s ~ ~ studied the spectral changes in 4-vinyl-

4'-dimethylaminoazobenzene polymer (4) with light irradiations from

a 500 :V tungsten lamp spot light. It was observed that upon irradiation,

absorption peaks for the polymer shift towards shorter wavelengths

and the absorbance a t the original wavelength decreases simultaneously.

The recovery of the original peak sites was also observed when the

irradiated polymer was exposed to darkness. Following this, R detailed

study on the spectral characteristics of a series of aminoazobcnzene

polymers were carried out by the same research groups.

Absorption spectra of azoaromatic polyureas (5) i n dimethyl sulphoxide

showing thermal recovery after irradiation with ultraviolet light

5

(410 > h > 350 n m ) have also been investigated by Sudhesh Kumar

e t 8 1 . ~ ~ According to them, the intensity of peak a t 400 n m decreased

after irradiation. The thermal isomerisation from the photogenerated

cis form to the trans form was the explanation offered for the gradual - - increase in intensity a t 400 nm of the polymer solutions in the dark.

The photoresponsive spectral behaviour of the azo modified poly

3 5 (I,-glutamic acid) (6) has been reported . In hexafluoro-2-propanol

the - trans form showed an intense band at 338 n m , which is due to

- f l * transition and a weak band a t 430 n m which corresponds to

n-a* transition. Formation of - cis isomer is revealed by the appearance

of a decrease of the 338 nm band together with its shifting towards

shorter wavelength region. At .the same timc the intensity of the

430 nm band was found to increase.

trans 6 cis

Analogous spectral change was observed by l r i c e t a136 in polystyrene

huving uzobenzcnc pendant groups. On irradiation o f a cyclohcxanc

solution o f the polymcr w i th ul traviolet l ight (410 > X > 350 nm), the

intcnsc absorption a t 353 nm decreased while the absorptions at 440

and 255 nm increased. The absorption band a t 350 nm is ascribable

t o thc 7 - T * transit ion o f the trans form o f the pendunt uzobcnzenc - rcsiducs u ~ ~ d the band a t 440 nm t o the n- 77 * transition o f thc cis - form. The intensity change o f the 350 nm band and the 440 nm band is

R clear evidence o f the isomerisation o f the pendant azobcnzene rcsiducs.

Copolymers o f polystyrenc and 4-(methacryloy1amino)azobenzene (7) also

showed n chnngc in nbsorption spectra that can be ascribed to - trnns +cis -

isomerisntion o f the azo groupsJ'. On irrndiation by n flnsh o f 347 nm

light, appreciably di f ferent absorption spectra were observed for the

isomers. The trans form has an absorption maximum a t 353 nm due - to a i'T-T* transition, whereas the - cis form absorbs relat ively weakly

i n this wavelength range.

Absorption spectra o f several polypeptides w i th azobenzene moieties,

subjected t o irradiation, have been studied i n deta i l 38-44. The absorption

spectra o f copolymers resemble that o f azobenzcne showing the absorp-

tions associated wi th the T- 7* and n - Z * transitions o f thesidc chain

nzobenzene moieties around 320 and 450 nm, respectively.

The e f fec t o f photochromism on the absorption spectrum o f azo-

44 benzene modified crown ethers have also been reported .

2.2 Viscosity

The solution viscosity o f a polymer system is a direct ref lect ion

o f the polymer conformation. The change i n conformation o f linear

macromolecules i n solution has at t racted the attent ion o f many researchers

in the past45146. The constitution o f the azo units i n the polymer

backbone hns great significance i n deciding the conformation o f the

polymers. I f each repeating uni t o f the polymer contains an azobenzene

group and a major portion o f these groups is in the - trans configuration,

the polymer chains are extended. Conversely, the chains form rather

conlpact coils when a major portion of the azobenzene groups is converted

to the - cis form. The extended conformation of thc polymer chain

contracts rapidly to a compact form when the configurtltion of thc

attached azobenzene groups isomerises to the form. Thus, a change

in viscosity of the polymer solution results when the polymers having

azobenzcne groups are irradiated.

The reversible photoviscosity effect of a solution of a photochromic

7 polymer was first observed by Lovrien . Irie e t al . 22'26 synthesised

different polyamides having azobenzene residues i n the backbone of

polymers. They observed a decrease in viscosity on U V irradiation

(410 ) h ) 350 nm) and the recovery of the initial viscosity in 30 hrs

in the dark a t 20°C. The slow recovery of the viscosity in the dark

was accelerated by visible light irradiation ( h > 4 7 0 nm). With alternating

irradiation using U V and v~sible light, the reversible viscosity change

was as much as 60%. The reversible photo-induced change i n viscosity

correlates well with the isomcrisatiot~ of the azobenzcnc rcsiducs i n

the backbone. Bcforc photo-irradiation, thc polyamidc has u rod like

conformation. The isomerisation from thc trans to the cis form on - -

irradiation kinks the polymer chain, resulting in a compact conformation

and a decrease i n viscosity. The compact conformation returns to

the initial extended form either thermally or by visible light irradiation,

thereby causing an increase in viscosity.

The photochemical viscosity decrease for a new class of azoaromatic

polyureas (8) was thoroughly investigated by Sudhesh Kumar and co-

3 4 workers . According to them the constitution of the stiff segments

of nzobenzcnc polymcr behaves like semiflexible rods in solution. The

extended rod like shape of the semiflexible chain transforms

readily to a compact conformation, when thc configuration of the

constituent azobenzcne residue changes from - trans to cis Corm. It -

h r ~ s r ~ l s o i)ccri obscrvcd tho1 thc viscosily cornplctcly t~ccovc~~s to tlic

initial valuc after 8 hrs in the dark a t 20 OC. Therefore, it can be

concluded that the photodecrease in viscosity is a rcsult of the change

i n conformation of the polymer chain and not from any interchain

interactions. The relationship of the thermal recovery of the original

viscosity and the recovery of absorption intensity indicates that the

contraction of the macromolecular volume is indeed induced by the

isomerisation of the azobenzene residues.

Photochemical control of macromolecular conformation as a result

of photostimulated changes of electrostatic forces was first observed

2 4 by Lovrien and Waddington . They designed polyelectrolyte which

bears covalently bound side chains capable of undergoing -- trans-cis

isomerisation. The - trans form of the side chains generates nonpolar

interaction forces which tend to contract the polymer chain. Irradiation

produces - cis form, changing these hydrophobic interactions somewhat

and causing the polymer to reach new equilibrium by expansion of

the coil.

A pronounced reversible photodecrease i n viscosity was observed

in the case of a copolymer of maleic anhydride with styrene containing

28 aromatic azo groups (91 in 1,4-dioxane solution . it was explained

that, radiation induces a very strong dipole moment in the azo bond 47

and these dipoles may become mutually oriented and attracted each

other so that more coiled conformations are preferred. A smaller

photoviscosity effect was observed in tetrahydrofuran solution when

compared to 1,4-dioxane which can be explained by the higher dielectric

constant of the former.

2.3. pH and Conductivity

Photostimulated changes in conformation of polymers give rise

changes not only to viscosity but to other solution propertics like

pH and conductivity. Light induced changes in pH is of particular

4 8 interest in connection with the function of bacteriorhodopsin .

Photocontrol of the conformation of poly(methacrylic acid) in

solution a t degrees of ionisation of 0.2 and at pH 4.5-5.5 was possible

with the use of cationic ligands based on (p-phenyla~ophenyl)trimethyl-

49 ammonium iodide . The -- trans-cis photo-isomerisation leads to a change

in the degree of coiling of the polyelectrolyte molecules and thus

the display of an induced photochemical jump in pK value has been

explained.

lrie and co-workers26 achieved photocontrol of conductivity and

pH values on photoresponsive polyamides having terephthalic acid groups.

Analogous to the photoviscosity effect, the response of the conductivity

correlates well with the isomerisation of the azobenzene residues in

the polymer backbone. Here, the cot~formalionnl chnngc of the polymer

chain influences the dissociation equilibrium of amide substituted

terephthalic acid residues in the polymer backbone. Dissociation o f

the acid is stimulated i n the compact conformation, while the extended

conformation depresses the dissociation. A plausible explanation for

the increasc i n dissociation i n the compact conformation is that the

cis form o f the azobenzene residues i n the compact conformation -

has a polar structure compared w i th the - trans form and hcnce the

polar structure assists i n dissociation o f the acid. 'i'hc changc i n the

cquilibriunl consttint o f dissociation o f the acid in organic solvcnt

by photo-irradiation suggests the possibility o f pH control in aqueous

system. The photocontrol o f pH using a synthetic polymer has special

interest in view o f i t s importance i n constructing photoreccptor

5 0 analogues .

2.4 Solubility

When photo-isomerisable chromophores are incorporated into the

backbone o f a polymer chain or as pendant groups, photo-isomerisation

of the chromophores w i l l a f fec t the physical properties o f thc polymers

and polymer solutions, especially i f the isomerisation involves a change

in polurity. Changc in dipole moment o f pcndunt groups upon photo-

irradiation alters the balance o f intcrchain internction, resulting in

expansion or contraction o f polymer chain. 'I'he studics rc lutcd to

the rcversiblc solubility chunge o f polystyrcnc in cyc lohcx~~nc clre ctlso

o f thc contributions of Id. ~ r i e ~ ~ , the pioneer in this field. Polystyrene

with n small amount o f azobenzenc pcndant groups became insoluble

in cyclohexane upon irradiation with UV-light (410 > X > 350 nm) , while

low molecular weight azobenzene itself did not show any solubility

change on photo-irradiation. On visible irradiation ( > > 470 n m ) the

polymer again became soluble. In cyclohexane, intermolecular interaction

between polystyrene and the solvent is in balance with the intra-inter

polymer interactions. As a result of i~omerisat ion '~ the dipole moment

increases from 0.5 D to 3.1 D. The dipolc moment incrcusc of the

pendant groups by U V irradiation is considered to alter the balance

of polymer-solvent and polymer-polymer interactions. The introduction

of nonpolar trans form of azobenzene into polystyrene as pendant

groups scarcely affects the polymer-solvent interaction in cyclohexane

while the polar - cis form tends to decrease the polymer-solvent interaction.

Therefore, onUV-irradiation the polymer-solvent interaction decreases

considerably until the polymer precipitates. It was also reported that

isomerisation of a few mole percent of azobenzene units in the polymer

chain is large enough to cause a solubility change of the polymer,

though the precipitation behaviour depends on the nature of the solvent.

The occurrence of photostimulated aggregation-disaggregation

processes in aged solutions of azo modified poly(L-glutamic acid) containing

about 20 mol % of azobenzene units in the side chains and reversible

variations of solubility induced by light in analogous azo polypeptide

having R much higher content of azobenzene groups were also reported 5 1 3 2

These photoresponsive effects have been investigated in samples of

azo modified poly(L-glutamic acid) having various contents of azobenzene

units i n the side chains a t different solvent conditions and ut different

wavelengths of incident light. In suitable conditions, depending on

the azo content, the trans-cis photo-isomerisation of the azobenzene -- units was accompanied by aggregation-disaggregation processes among

macromolecules or precipitation-dissolution of the polymers. The phenomena

were fully reversible and can be photomodulated by irradiating a t

the appropriate wavelength. Findings are consistent with a molecular

mechanism i n which aggregation and precipitation processes occur

through hydrophobic interactions and ordered stacking between the

azotenzene moieties. Such interactions can be favoured or inhibited

depending on the -- trans-cis photo-isomerisation of the azo units, thus

causing the photorcsponse effects.

Several studies on naturally occurring photoreceptors have provided

evidence of changes in aggregation of protein matrices upon photo-cxcita-

tion of the attached photochromic moiety 53'54. A photo-induced nggrega-

tion-di.saggregation process has been reported in human immunoglobulin

5 5 labcllcd with an azobenzcne reagent .

2.5. Surface Free Energy

'I'he surface free energy of a solid is an important parameter

for printing, dyeing and adhesion. If surface frcc energy which is

an inherent value of thc material,. can be controlled by external physical

signnls such as light, w~de variety of new materials of commercial

applications could be derived.

Ishihara and co-workers 56-58 have synthesised a series of photo-

responsive polymers containing azobenzene groups in their side chains.

In one such example, the change in contact angle formed by water

on the surface of the f i lm prepared from p(phenylazo)acrylanilide-2-

hydroxyethyl~methacrylate copolymer, by the photo-isomerisation of the

azobenzene moiety has been described. In addition to this, photo-induced

changes in the surface free energy of the azoaromatic polymer prepared

by the introduction of azobenzene groups into the side chains of hydrophilic

poly(hydroxyethy1 inelhacrylate) was investigated. When a fi lm of

the above azoaromatic polymers was irradiated, the absorbance at

325 nm which corresponds to a peak for the - trans form decreased

with irradiation time and the photo-equilibrium was reached within

15 min. The absorbance change was accompanied by an increase in

the wettability of the surface of the polymer f i lm and the absorbance

returned to its original value and the wettability decreased when the

irradiation was continued with visible light. The results indicate that

the wettability of the polymer surface can be regulated by the photo-

isomerisation of the azobenzene moiety. lrie and lga5' also reported

thut the corltact angle of water on butyl methacrylnte-2-(hydroxypheny1)-

4-(viny1phenyl)benzyI alcohol copolymer surfaccs increased significantly

upon UV-irradiation and reversed in the dark.

'l'he regulation of the hydrophobic chromatography for proteins

6 0 by light was reported by Ishihara and co-workers , Polymeric hydrophobic

adsorbents containing an azobenzene moiety as a ligand were prepared

and the photo-induced adsorptionldesorption behaviour of proteins was

investigated. The separation of protein mixtures was also investigated

using a gradient column which consisted of two polymeric hydrophilic

adsorbents having different hydrophobicities.

2.6. Photomechanical Effects

In light sensitive polymers containing photochromic components,

photo-isomerisation of the photochrome under suitable conditions leads

to a change in the conformation of the macromolecule. In solution,

for example, this effect is reflected in a change in viscosity after

irradiation 22'61 , while in solid samples there is a change in macroscopic

dimensions and mechanical characteristic^^^-^^. The changes in configura-

tion of the chromophore and connected chain segments of a photochromic

polymer certainly induce a change in dimensions of bulk polymers,

which eventually leads to reversible photomechanical effects, is an

area of technological importance.

From an Organic chemist's point of view, a reversible dilation1

contraction phenomenon should be observed in pliotochromic rletworks

above T the glass transition temperature. Photo-isomerisable groups g

incorporated into a polymcr framework have been shown to cause

reversible contraction or expansion of polymcr samples on irrndiation.

'rhc mechanism of contraction is explnined again on thc btlsis of isomcrisu-

tion from - trans to the - cis form which involves a decrease in the distance

between the para carbon atoms in the azobenzene derivative from

nbout 9 A " to 5.5 A" 12'13. During isomerisation thc chromophore

gets preferential ly oriented parallel t o the stretching axis. Upon irradia-

t ion o f such an oriented sample in the maximum - trans absorption

range ().= 365 nm), the change i n the conformation o f the azo chromo-

phore causes a change i n the conformation o f an adjacent chain segment

vrhicil is considered t o be the mnin e f fec t responsible for contraction.

Reversible contractionldilation experiments under corlstant load

were recently performed by ~ isenbach" on stretched poly(ethy1 ncrylarc)

networks crosslir~ked wi th 4,4'-bis(dimethacryloylamino)azobenzene.

The photochemical nnd thermal cis-trans isomerisation o f azo cllromophores -- of either dissolved, a pendant group or part o f n crosslink in rubbery

poly(ethy1 acrylate) networks have been investigated. For stretched

polymer f i lms w i th ttzoarornatic crosslinks a photomechanical e f fec t -

a reversible contraction or expansion - was observed, which is mainly

attr ibuted t o the conformational change o f the azo chromophore.

Blair, Pogue and Riordan 25 described photoresponsive effects

in polyamides in which every monomer unit contains an azo group.

'I'he polymers investigated were that o f 3,3'-azodibenzoyl-trans-3,5- -

dimethylpiperazinc (10) and i t s 4,4'-isomer (11). They have also carried

out Langmuir f i lm balance measurements on monolayers of the photochromic

polyomides mentioned above. On changing from dark to l ight, reduction

in area per mononler uni t was observed. Again, the interpretation

given is based on the trans to cis isomerisation of the azo group. - - The more extended conformation of the trans isorner forms a larger -

apparent molecular area as indicated by the film balance measurements.

Besides the photomechanical effect observed on rubbery networks and

swollen gels, photocontractility of the photochromic systems in the

solid state without the chemical crosslinking agents has been described.

'I'he possible partial crystallinity and eventually hydrogen bonding ensure

physical crosslinlting which leads to the formation of a network

structure.

The effect of radiation on photochromic crosslinked polymers

containing azo side chains has been investigated by Matejka e t al. 66

After irradiation a t constant sample length, the swollen gels of light

sensitive polymers exhibit a reversible increase in the elastic retractive

force. 'l'hc photomechanical effect increased with an increase in the

content of photochromic groups.

2.7. Membrane Permeability

~Vluch effort has been concentrated on the characterization of

the photoresponsive properties of membranes entrapping photochromic

moieties such as azobenzene and spiropyran derivatives. More recently,

it has been shown that the membrane potentials and permeabilities

of polymer membranes containing photochromic compounds in the polymer

chains could be regulated by light irradiation. Photo-induced -- trans-cis

isomerisation of azobenzene derivatives has been widely used to control

67 metal ion permeabilities through poly(viny1 chloride) membranes , 70 liquid crystalline liposomes69 and bilayer membranes .

Anzai e t a1 71'72 applied the -- cis-trans isomerisation of azobenzene

modified crown ethers to regulate the membrane potential across a

poly(viny1 chloride) membrane. Deal and c o - ~ o r k e r s ~ ~ used azobenzene

derivative photochromism to obtain photoregulation i n membranes by

means of light energy. Tanaka e t a 1 7 ~ i n 1981 prepared artificial

photoresponsive lipid membrane by embedding amphiphatic alkyl ammonium

salts containing azobenzene chromophore in dipalmitoyl phosphatidyl

choline liposomes. The configurational change due to -- trans-cis photo-

isonierisation of the azobenzene chromophore provided the perturbation

of the membrane structure to obtain an increase in the water and

bromothymol blue permeability of the liposomal membranes.

2.8. Photoresponsive Metal Ion Chelation

I-lost molecules have drawn considerable attention nowadays, since

they act as simplified enzyme model systems. 'The specificity of

enzymes is based on the spatial "fitness" between host and guest

n~olecules. It is, therefore, expected that i f the conformation of

a host molecule is somewhat distorted by a change i n the photo-induced

"trigger", it would lead to a change in the binding ability.

A photoresponsive crown ether that combines with a molecule

changes its conformation in response to the photo-induced configurational

change i n the chromophore 75-78 which leads to photocontrol of chemical

and physical functions of a crown ether just like an "on-off light switch".

This event~lally results in a change in the complexation ability of

crown ethers. This finding has been applied to the novel idea of

photocontrol of solvent extraction and ion transport across membranes 79,80

If the binding ability of crown ether easily changes in response to

reaction conditions it would function as an efficient ion-transport

carrier.

81-84 developed With the above objects in view, Shinkai e t al.

crown ethers with azobenzene groups as a 'photoantenna' and have

attempted the photocontrol of the crown ether functions. The synthesis

of a polystyrene derivative bearing pendant 4-aza(benzo-15-crown-5) (12)

and its monomeric model and estimation of the photoresponsive affinity

7 5 toviards alkali metal ions have been reported . The study revealed

that the ion binding ability of the crown ether immobilized in the

polymer alters in response to the changes in the conformation and

the side chain configuration. They have also studied the effect of

photo-irradiation on the extraction equilibrium of alkali metal salts

75 between water and benzene with an azobenzene-bridged azacrown ether . It follows that the binding ability can be controlled by photo-induced

cis-trans isomerisation of the azobenzene moiety. Two new classes of -- photoresponsive crown ethers, cylindrical and phone ionophores, have

84 been developed by Shinkai and co-workers . This novel photoresponsive

behaviour has been applied to determine the spatial distance between two

metal ions i n addition to the photocontrol of solvent extraction and

membrane transport.

The synthesis of crown-type polyether azo dyes and the ion-pair

extraction of sodium and potassium i o n s have been carried out by

76 Shiga et a1 . The extraction behaviour resembles those of benzo-type

crown ether but changes on light illumination due to trans-cis isomerisa-

tion of the dyes. Another macrocyclic compound with a similar function

in which the diaza crown ether bridged with azobenzene-3,3'-dicarbonyl

7 5 group through amido linkages has been reported . Here the azo group

which is not involved in the co-ordination, induces a conformational

change in the crown ether ring on photo-isomerisation.

2.9. Helix Reversal of Photochromic Polypeptides

Photoresponsive polypeptides are especially interesting since light

induces transitions between distinctly different conformations such

as left- and right-handed helices, )?-structure and random coil. It

has been found that photo-isomerisation of the side chain changes

the chiroptical properties of the polymer without variation i n the backbone

8 5 conformcltion .

The idea of studying conformation changes induced by light on

modified polypeptides containing azobenzene residues i n their side

chains was originally due to Goodman e t a1 86-89. Howcver, detailed

studies on photo-induced conformational changes of polypeptides were

carried out by Ucno et al. with azobenzene containing polyaspartates

in solution. They found that photoresponsive polyaspartates can undergo

photo-induced conversion of secondary structure in solution. Successful

cases of whole reversal of the helix sense from left to right were

reported. It was suggested that the trans to cis photo-isomerisation

of a small amount of azobenzene groups incorporated into the polypeptides

as side chains causes helix reversal 41'94. Moreover, photochromic

amino acids tend to form a left-handed &-helix in a polypeptide chain

when the side chain of azobenzene group is trans, whereas they tend

to form a right handed K-helix when it is cis. Ueno et al.developed -

a series of copolymers i n which n-octadecyl-L-aspartate has been used

as a light insensitive cornonomer to form membrane-mimic environment

around the photochromic azobenzene groups. Since in the rhodopsin

system, retinal-bound opsin is embedded i n a lipid bilayer membrane,

this system bears some resemblance to the native system. Together

with other reported photoresponsive polypeptide^^^-'^^, these polymers

may find application to the construction of photoswitches in which

5 1 chemical and physical phenomena can be controlled .

Ueno et al?' in 1981 investigated the effect of light on polypeptide

conformation using circular dichroism measurements on a series of

copolymers composed of p-benzyl-L-aspartate and 8-(m-pheny1azo)benzyl-

L-aspartate dissolved in the mixed solvents of 1,2-dichloroethane and

trimethyl phosphate. Photo-isomerisation of the side chain azobenzene

moieties causes helix reversal in these copolymers a t adequate solvent

compositions.

Most of the investigations are concerned with the photo-induced

conformational change of the polypeptides i n solution, and only scant

reports on the polymer membrane have been appeared. A photoresponsive

porous mcmbranc with adsorbed poly(L-glutamic acid) conteining azobcnzcne

101 sulphonate moieties on the porcwall was reported . Photoirradiation

induced the irreversible helix to coil transition of the adsorbed poly-

(L-glutamic acid) with the photo-isomerisation of azobenzene sulfonate

moieties.

2.10. Photoehromism in Polymer Matrix

Photochemical reactions which occur i n small molecules can also

be induced to occur in macromolecules. Though in macromolecular

environments there are limitations associated with it , one can very

well apply the fundamental principles to macromolecules. The investiga-

tions on the chemical and physical behaviour of the photochromic

moieties i n polymeric matrices and the influence of polymers on photo-

chromism is relatively recent. Literature study 102'103 reveals that

the nature and morphology of a polymer chain significantly influence

the photo- and thcrmochromism of a chromophore in a given polymer

matrix. The size of a chromophore, its shape, conformation and point

of attachment to a polymer chain all have a significant role in determining

the effective free volume available. The regional arrangement of

the polymeric environment i.e. macroscopic and microscopic values

of polarity, viscosity, T and tacticity, affect photochemical and photo- g

physical events. However, no meaningful theories exist to know the

extent to which the photochromic properties are related to matrix

characteristics.

The photo-isomerisation of - trans azo compounds present i n polymeric

matrix was initially studied by Morawetz and co-workers 104-106

Copolyamides were synthesised in which azodibenzoic acid or azodianiline

was incorporated into Nylon6,6. In dilute solution, the photo-isomerisation

rates of the polymers were very similar to that observed in low molecular

weight analogs, suggesting that incorporation into the backbone of

a high molecular weight chain polymer does not hinder the isomerisation

process. There was no evidence of the isomerisation being slowed

down at polymer concentrations corresponding to extensive chain entangle-

107 ments. This study supports the earlier conclusion by Morawetz e t al.

that the conformational transitions involving rotation around a bond

in the backbone of a long chain molecule take place easily than the

corresponding rotation in small molecules.

Following this a number of research groups carried out

n detailed investigation on the photochromism of azo groups present

in polymer matrix, the observed results go hand in hand with the predic-

tions.

2.11. Applications

Photoresponsive polymers represent a new class of speciality polymers.

In biological systems, light acts not only as an energy source for photo-

chemical transformations but also. as an information source or trigger

for the reversible control of the physical and chemical properties.

The manipulations

systems are not

apply them t o the molecular design o f synthetic photoresponsive polymers,

so that the physical and chemical properties can be changed reversibly

by photo-irradiation. Biological photoresponsive systems contain photo-

chromic moieties embedded i n biopolymer matrices and use mainly

the photo-isomerisation for controll ing the conformation and assembly

of the biopolymers.

As discussed elsewhere, the introduction o f photochromic moieties

into the polymer chains can produce sudden changes in conformational

properties as a result o f photo-irradiation. For instance, several biological

systems and photochromic azo dyes 24'1 14-' 1 6 , either covtllcntly

or noncovalently connected t o macromolecules, have been used t o

control polymer conformations and t o obtain photoregulation i n membranes.

The photo-induced reversible chiral i ty inversion in polypeptides as

a result o f the photo-isomerisation o f the azobenzene moieties, might

permit photocontrol i n ch i ra l recognition or chiral catalyt ic reactions,

when binding or catalyt ic functional groups are incorporated in to the

polypeptide sequences.

The photomechanical and photoviscosity ef fects and pH control

systems are important by themselves and have possible commercial

applications. Although they have not yet achieved pract ical i ty, it

is possible for them t o f ind applications i n several fields such as printing,

photocopying, photolithography and photosensors. An example of such

an application which has attracted attention is that of materials which

can store memories in the molecular level. Recently, much interest

has been focussed on photoresponsive polymers which can be applied

in photomechanical engines or photoswitches. The photomechanical

effect allovrs the direct conversion of light energy into mechanical

energy.

Aromatic polyamides containing azo groups having liquid crystalline

behaviour were found to offer high strength high modulus fibres 117-119

In addition to this, azobenzene moieties act as suitable mediators

between light energy and chemical functions via the structural change.

Hence, photocontrolled complexation was performed between azobenzene-

capped cyclodextrin and various kinds of guest molecules 120-125 and

also between azo modified crown ethers and alkaline cations 126,127

The combination of a chelating agent and a readily isomerisable group

in a macromolecule ensures the possibility of a structurally modifiable

photosensitive ion switch. Investigations on conformational changes

induced by the absorption of light of specific wavelength being used

to influence the ability of a macromolecule Lo complex with metal

ions or to form complexes of specific shape and stereochcmistry are

thc follow-up studies in this field 79-82,121

SYNTHESIS AND THERMAL STUDIES OF

PHOTORESPONSNE AZO POLYMERS