INTRODUCTION AND OBJECTIVES

K.R. Girijan “Development and property changes of photoresponsive polymers with azo groups”, Department of chemistry, University of Calicut, 2006

CHAPTER I

INTRODUCTION AND OBJECTIVES

Organic photochemistry has made tremendous advances as a science

during the last few decades. The proliferation of research articles including

monographs and patents in this field, adds testimony to this. Apart from

academic interest, many photochemical and photophysical processes have

extraordinary significance to life on earth and industrial applications as

energy converting materials. The harvesting of Sun's energy during

photosynthesis by antenna pigments of chlorophyll to create carbohydrate

from atmospheric carbon dioxide as well as the liberation of oxygen to the

atmosphere is the nature's choice of photochemical energy conversion to

chemical energy. In addition, the delicate mechanism involved in one of the

senses represented by vision (photo), the primary stimulations are eventually

converted into electrical signals to excite nerves. Nature, thus relies

essentially on light to affect some of her most important energy converting

materials to sustain life on earth. Photochemical principles are not confined to

illustrating delicate mechanisms of natural processes, but it open new vistas of

industrial significance. The dye sensitized electrochemical solar cells for the

conversion of visible light to electricity, the optoelectronic properties of

oligosilanes and polysilanes, photoinduced metallmetal oxide decomposition

on semi-conductors and redox reactions of transition metal complexes are all

concerned with inorganic photochemistry.

The industrial aspects of organic photochemistry are also numerous.

The photoinduced cross-linking of epoxies, both the prevention and

promotion of polymer degradation, photopolymerisation, the effect of

simultaneous photoconductivity and the electro-optic response

(photorefractive) polymers are all based on absorption of either visible or

UV irradiation. Application of emission properties of excited molecules are

utilized in optical brighteners and chemiluminescence. The excellent acceptor

ability of recently developed fullerenes is a key feature of photoconductivity

of fullerene - dopped polymer films. Understanding the photochemistry of

drugs is essential for rationalizing the significant phototoxicity of many drugs

and also in the field of photoimmunology, photodynamic therapy and

photomedicine.

The ramifications of photochemical principles find considerable

technological significance, where the combination of various phenomena of

principles that are meaningful and influential to our actual society are

envisaged. The most important among them is in the development of

functional materials such as erasable photomemories and photomechanical

devices. Photochromic polymers represent an appropriate choice in which the

polymer matrix serve as a carrier of the photosensitive species. Photochrornic

processes are widely distributed in nature and represent the basic molecular

'triggers' for every important photoregulated biological processes such as

vision in animals, phototropism in plants and photomovements (phototaxis) in

microorganisms. Even light insensitive natural substances are made

photoresponsive by chemical modifications with photochromic moieties. The

information of light captured by this photochromic biological molecules is

then transferred and amplified to physiological processes by surrounding

proteins and membranes in a complex manner. Although, artificially it is not

possible to reach such a high level of complexity, one can mimic in part the

molecular amplification systems. The most essential feature of photochromic

system is the ability to provide dynamic processes utilizing the reversible

structural changes.

There have already been great advancement in polymer systems

incorporating photochromic units or inole~ules '~~. Photochromic molecules

supply us with ample information on the mobility of polymer matrix, free

volume involved and thermal/photophysical properties7. Photochromic liquid

crystalline systems have attracted a great deal of attention in which light

switching of physical states and molecular orientations are altered through

photochemical processes8g. Several light sensitive chromophores like azo

groups, stilbene systems, spiro-benzopyran groups are known for their

important photoresponsive beha~iour '~ ' '~ . When these chromophoric groups

become part of the polymer in the backbone or as pendant groups, the

photoresponsive character is induced to the macromolecules too. In the case

of low molecular weight compounds, this photochemical phenomena is

related to structural modifications such as valence isomerisation, tautomerism,

geometrical isomerisation, ring opening and ring closure. The same

phenomena is expected when these groups become part of the polymer

matrix. Triggered by this molecular processes, physical properties and

functions of the solutions such as viscosity, pH, solubility, metal ion capture,

membrane potential, permeability, surface wettability, material shape, sol-gel

transitions, miscibility of blends, glass transition temperature etc. in polymer

films and gels are alteredl4.I5.

From the fundamental scientific point of view, it is obvious that

photochemically reversible cis-trans isomerisation is responsible for many

changes in physical properties of the polymers having stilbene systems or azo

groups16*'7. During photoirradiation, a change in the chemical structure of the

chromophore of the photoresponsive polymer occurs, which is then

transferred to the polymer chain, causing reversible confomational change

with a concomitant change in physical and chemical properties of the polymer

without any change in the molecular weight."

Although, many chromophores were reported to undergo physical

property changes upon photoirradiation conditions, only a limited number of

photoactive molecules are used as photoresponsive polymers. One of the

important criteria of the photoactive molecules is to exhibit a property change

during isomerisation which is large enough to cause conformational change of

19,20 polymer chain . Polymers containing azobenzene derivatives rank the best

known photoresponsive systems among them. Azobenzene containing

2 1 polymers were first reported by Blair , Umeda etc" in which azobenzene

units introduced into the main chain undergo isomerisation from trans to cis

form under UV irradiation. The cis form can return thermally and

photochemically to the trans fonn which is thermodynamically more stable.

The photoinduced isomerisation of azobenzene proceeds with large structural

changes as reflected in the dipole moment and changes in geometry and when

the photochrome is a part of the polymer, a change in the configuration affects

23,24 the chain conformation .

Nowadays, ortho substituted azo systems have attracted considerable

attention due to their outstanding photoconducting properties. Afier the

energy crisis in 1970's, an explosion of research activities aimed at the

development of cheaper and better organic photoconducting materials for

xerographic imaging in copier applications have dominated the field of

organic chemistry. Since then, a number of organic photoconducting materials

such as phthalocyanines, squaraines, perylene pigments, perinones, PVK-TNF

charge transfer complexes, thiapyrilium salts and azopigments were

synthesized 25-35 and studied the photoconductivity overshadowing the

developments made in the field of inorganic photoconducting materials such

as silicon. It was explained that azo pigments from 2-naphthol couplers

exhibited consistently moderatelhigh photoconductivity owing to the well

documented ortho-hyroxyazo to ketohydrazone tautomerism both in solution

and in solid state36937. Theoretical calculations revealed that ketohydrazone

form is thermodynamically favoured. Electron withdrawing hnctionality and

the hydrogen bonding are the key driving forces that shift the equilibrium into

38,39 ketohydrazone form . The advances made in the field of organic

photoconducting materials and the photochromic phenomenon associated

with azo systems prompted to design polymers having ortho-hydroxyazo

group in polymer backbone. There is no doubt that, the development of such

smart and highly controllable amplification systems will be a perpetual

challenge in the future. Thus, the present thesis deals with the development of

photoresponsive polymers and the property changes occurring in some

hitherto unreported polyesters with ortho-hydroxyazo chromophores in the

main chain.

The main objectives of the investigations are:

to synthesise photoresponsive polyesters having ortho-hydroxyazo

groups.

o to characterize the synthesized polymers by different analytical and

spectral methods.

o to determine the dilute solution photoinduced viscosity changes that

are occurring in the polymers.

o to determine the dilute solution conductivity changes of the polymers.

o to correlate the photoinduced viscosity and conductivity changes of the

polymers with the conformation of the polymeric chain in the dilute

solutions.

o to assess the photoconductivity (if any) in the polymers in the solid

state.

o to propose a mechanism of photoconduction in the solid state of ortho-

hydroxyazo benzene containing polymers

The thesis is divided into 6 Chapters. Instead of giving an extensive

review of the synthesis of the azo polymers and pigments, an overview of the

important properties of the photoresponsive azo pigments and polymers

touching upon their applications are described in Chapter 2. Chapter 3

describes the synthesis and characterization of different ortho-hydroxy

azobenzene containing polymers. The photoinduced property changes

(viscosity and conductivity) of the synthesized azo polymers in dilute

solutions are described in Chapter 4. The solid state properties of ortho-

hydroxyazo polymers prepared as films and pellets are discussed in chapter 5.

The experimental methods used are described in Chapter 6. Summay of the

results and the conclusion that are arrived at, touching upon their future

implications are also included. The references are given at the end of the

thesis.