Spectral stability study and molecular modeling of fluorence based conjugated polymers 1

The following sections in this chapter give a description of stability and processing, as well as optical and electronic properties of conjugated polymers, followed by a discussion on th

CHAPTER 1 GENERAL INTRODUCTION

1-1 Scope of thesis

The work of this thesis could be roughly categorized into two parts The first part examines spectral and thermal spectral stability in films and aggregation in solutions for fluorene-based conjugated polymers The second part reports influence of donor and acceptor substituents on the electronic characteristics of poly(fluorene-phenylene)

The following sections in this chapter give a description of stability and processing, as well as optical and electronic properties of conjugated polymers, followed

by a discussion on the development of polymer light-emitting diodes Polyfluorene derivatives and light-emitting diodes are also discussed

Chapter 2 discusses experimental and calculation methods used in this work Ultraviolet-visible absorption spectroscopy, photoluminescence spectroscopy and differential scanning calorimetry are described It is then followed by a discussion on semiempirical molecular orbital calculation

Chapter 3 discusses in details spectral and thermal spectral stability of seven fluorene-based conjugated polymers in film states Ultraviolet-visible absorption and fluorescence spectra of these polymers were presented Their differential scanning calorimetry and crystallization analysis results were discussed

Chapter 4 investigates aggregation behavior of polyfluorene derivatives in solutions Five representative polyfluorene derivatives were examined with respect to their absorption and emission spectra in chloroform/methanol mixtures

Chapter 5 investigates theoretically influence of presence of acceptor or donor group(s) along poly(9,9-dihexylfluorene-1,4-phenylene) backbone The present quantum chemistry calculations report on changes in geometric and electronic properties of poly(9,9-dihexylfluorene-1,4-phenylene) unit cell induced by substitution with cyano, methoxy or amino group(s)

1-2 Introduction

1-2-1 Conducting polymers

There is hardly an aspect of our lives that is not touched by synthetic polymers The role of polymers in the electronics industry has been traditionally associated with insulating properties, whether these are for isolating metallic conductors or for use in photoresist technology From that starting point it was the pioneering work of MacDiarmid, Heeger, and Shirakawa et al that inspired chemists and physicists to consider the opportunity of using polymers as conductors The report in 1977 from the University of Pennsylvania of high conductivity in charge-transfer complexes formed with a polymer, polyacetylene, which exhibited extended π conjugation along the

polymer chain, provoked considerable excitement.1 Conjugated polymers derive their conducting properties by having delocalized π-electron bonding along the polymer chain The π (bonding) and π* (antibonding) orbitals form delocalized valence and conduction wavefunctions, which support mobile charge carriers As the length of the conjugated sequence is increased, the energy gap between the filled π and empty π*

states falls, though in the long chain limit the gap remains finite, and takes a value of

about 1.5 eV Polyacetylene has served as the prototypical conjugated polymer; the simplicity of its structure has allowed theoretical modeling

This was the first demonstration of metallic behavior within the intramolecular π

electron system along the polymer chain, and the significance of these results was quickly picked up by many other groups worldwide

Conducting polymers have tremendous potential for innovation After 20 years

of progress, these unusual polymeric materials can now be used as transparent antistatic coatings, electromagnetic shielding, superconductors, modified electrodes, electrochromic windows, supercapacitors, transistors, light-emitting diodes, lasers, conducting photoresists, photovoltaic cells, biosensors, and so forth.2,3 The significance

of this class of polymers was recently highlighted by the awarding of the 2000 Nobel Prize in Chemistry to H Shirakawa, A G MacDiarmid, and A J Heeger, the three scientists who pioneered this novel materials field

Whereas conventional polymers are readily processed in solution or in the melt and can be cheaply manipulated into desirable forms, this is not in general possible for conjugated polymers The delocalized π electron system makes the molecular chains

rigid, with resultant high melting points and low solubilities The dilemma of a potentially attractive material that cannot be processed is not new in materials science in general or polymer science in particular Two well-established lines of attack on such problems involve either modifying the molecular structure so as to retain the property of interest while rendering the material processible, or carrying out the processing stages with a more tractable precursor, which can be converted subsequently to the desired

material Both of these approaches, have been successfully applied to the processing of most classes of conjugated polymers, and the methods adopted are summarized in Section 1-3 for the major structural classes

Conjugated polymers behave as “molecular materials,” and there is a considerable reorganization of the local π electron bonding in the vicinity of extra charges added to the chains This results in self-localization of the added charge, to

form, in general, polarons, though for the particular symmetry of the trans isomer of

polyacetylene, these take the form of bond-alternation defects, or solitons The theoretical models developed to describe these processes are discussed in Section 1-4, along with a description of fluorescence from conducting polymers

1-2-2 Polymer light-emitting diodes

The availability of film-forming conducting polymers in the late 1970s resulted

in attempts to fabricate a range of semiconductor devices, principally two-terminal diodes formed as sandwich structures with metallic electrodes to either side of a film of

polymer The first report on polyacetylene formed by the Shirakawa route, of in situ

polymerization of acetylene gas onto the bottom electrode, revealed that Schottky barriers could be formed against metals with appropriate work functions (here the

polyacetylene was functioning as a p-type semiconductor),4 However, these early experiments were constrained by the poor processibility of the polymers then available

Thin-film electroluminescence (EL), that is the emission of light when excited

by flow of an electric current, in conjugated polymers has provided the other major area

of device-related activity for display The discovery that conjugated polymers could act

as both transport and emissive layers, reported in 1990,5 has generated a very high level

of interest Device fabrication can be very straightforward, with a layer (or layers) of polymer sandwiched between two electrodes, one of which is transparent This is illustrated in Figure 1.1 for the case of the first EL diodes.5 The polymer was prepared as

a thin film, of thickness of order 100 nm, by spin-coating a “precursor” polymer from solution, using a standard photoresist spin-coater, and subsequently converting the

“precursor” polymer to the semiconducting PPV by heating Spin-coating from solution has been demonstrated to be capable of producing highly uniform layer thickness, with a thickness variation of no more than a few ångströms spread over several cm2 This polymer layer was formed on a glass substrate coated with indium-tin oxide (ITO), which has a relatively high workfunction and is therefore suitable for use as a hole-injecting electrode, and the other electrode then formed by thermal evaporation of the selected low workfunction metal such as Al, Mg or Ca, which are suitable for injection of electrons, as shown in Figure 1.1 Organic electroluminescent displays represent an alternative to the well-established display technologies based on cathode-ray tubes and liquid-crystal displays (LCDs), particularly with respect to large-area displays for which the existing methods are not well suited Rapid progress has since been made, and EL diodes with a wide range of emission colors and with quantum efficiencies (photons/electron) of several percent are now reported The development of polymer light-emitting diodes is discussed in Section 1-5

Figure 1.1 Structure of an electroluminescent diode based on a conjugated polymer

The polymer film is formed on a glass substrate coated with indium-tin oxide, and the top electrode is then formed by thermal evaporation

1-2-3 Polyfluorenes

Polyfluorenes are an important class of electroactive and photoactive materials

In the last few years this research field has literally exploded because of polyfluorenes’ exceptional electrooptical properties for applications in light-emitting diodes This is the only family of conjugated polymers that emit colors spanning the entire visible range with high efficiency and low operating voltage The development of this area is discussed in Section 1-6

1-3 Chemical structures, stability and processing of conducting polymers

The chemical structures of some common conjugated polymers are shown in Table 1.1

Table 1.1 Some common conjugated polymers

S

R S

R

n

n S

N

NH n

a The band gap is taken as the energy at the maximum slope, ∂α/∂E, of the

spectrum of the optical absorption (α)

In order to successfully exploit the properties of conducting polymers in

commercial applications, it is imperative that the candidate materials exhibit good

environmental stability and be amenable to a wide variety of processing techniques

A polymer with poor environmental stability is essentially unstable in its doped

state under normal atmospheric conditions Compared to polyacetylene, polyheterocycles (such as polythiophene and polypyrrole) have demonstrated much better environmental stability6 For example, polypyrrole displays only minor changes

in its conductivity state even after exposing in air to temperatures as high as 200 °C for extended periods of time In general, the stability of a conducting polymer depends on a number of factors including its susceptibility and accessibility to external chemical species, the nature and type of counterion present in the material, the reactivity of its doped sites to surrounding chains, and the flexibility and conformational states of its backbone A great deal of progress has been made towards the development of stable conducting polymers; however, this issue continues to be of paramount importance to the successful utilization of these materials in commercial applications

Many of the initially prepared conducting polymers were formed as intractable, insoluble films or powders that, once synthesized, could not be further manipulated into forms with more ordered, controllable structures In fact, the structural attributes that give rise to the interesting electrical and optical properties of the conducting polymers, namely their rigid, planar conjugated backbones, severely limit the ways in which the polymer can be processed To overcome these limitations, a number of structurally modified polymers and novel processing schemes have been developed that allow substantially more control over the state of the final product These processing schemes can be conveniently divided into four categories

The first category is the manipulation of soluble precursor polymers This scheme is based on the synthesis and manipulation of a processible, nonconducting precursor polymer that, once fabricated into a suitable form using conventional polymer

processing techniques (usually by thermal treatment), can be converted into an insoluble electrically conducting polymer This route has been successfully utilized to prepare highly oriented thin films and fibers of polyacetylene7, poly(phenylene vinylene)8, poly(thienylene vinylene)9 and some other similar polymers10

The second processing scheme is the manipulation of soluble conducting polymer derivatives and copolymers This is to modify the structure of the polymer in such a way to improve the processibility without compromising its electrical or optical properties For example, it is possible to dramatically modify the processibility of the polythiophenes without severely compromising the electrical properties By simply substituting the hydrogen atom attached to the three position of the thiophene ring with

an alkyl group containing at least four carbons, conjugated polythiophenes that are both solution and melt processible can be achieved11 Meanwhile, the conductivities of the doped derivatives are also comparable to the parent polymer and generally range from 1-200 S/cm

The third processing scheme is the in-situ polymerization of conducting polymers in insulting matrix polymers This processing scheme focuses on the growth of insoluble and intractable conjugated polymers within a performed polymer matrix In this case, a processible, insulating polymer impregnated with a catalyst system is fabricated into a desired form such as a thin film or fiber This activated polymer matrix

is then exposed to the monomer, usually in the form of a gas or vapor, resulting in a blend typically comprised of an isolated or semi-continuous conjugated polymer phase dispersed throughout a continuous phase of the host polymer For example, stretched aligned blends of polyacetylene/polybutadiene exhibit conductivities at least one order

of magnitude larger than that of the unstretched material12 This enhancement in conductivity reflects a higher state of order resulting from the deformation process

The last processing scheme is the manipulation of conducting polymers via the Langmuir-Blodgett (LB) technique This technique relies on the ability of the LB trough

to manipulate surface active molecules into highly ordered thin films with structures and film thickness that are controllable at the molecular level The true promise of the LB processing technique is its unique ability to allow control over the molecular architecture of conducting polymer thin films

1-4 Optical and electronic properties of conjugated polymers

1-4-1 Introduction

The electronic structure of conjugated polymers can be conveniently described

in terms of σ bonding formed by overlap of sp 2 hybrid orbitals, and π bonding formed by

overlap of P z orbitals on adjacent carbons This description then allows a useful

parameterization of the electronic properties in which the contribution of the σ electrons

provide the force constant for the carbon–carbon bonds, and in which the π electrons are

described using Hückel (tight-binding) methods This approach has proved to be particularly important for describing the coupling of the lattice to the electronic excitations in the π electrons caused by photoexcitation from π to π* or by charge

injection However, the input parameters in such models need to be chosen empirically, and the use of more sophisticated quantum chemical calculations has been important in

gaining an accurate description of the electronic structure We briefly review both approaches here

It is appreciated that the effect of the electron-lattice and electron-electron interactions is to cause localization of excited electronic states on the polymer chain These are variously described as solitons, polarons, bipolarons, or excitons, depending

on the symmetry of the polymer chain and charge on the excitation, as we discuss in more detail in Section 1-4-3 The application of models for infinite isolated chains to measurements made on materials in which the polymer chains comprise relatively short straight conjugated sections, separated by conformational or chemical defects, requires some caution, and we discuss later how interchain interactions and disorder may modify these isolated-chain descriptions

A model for the electronic properties of an infinite one-dimensional chain of the

polymer trans-polyacetylene was developed by Su, Schrieffer, and Heeger.13,14 This model and its refinements aimed at modeling polymers are described in Sections 1-4-2 and 1-4-3

One unique property of conducting polymers is the efficient photoluminescence

in thin film states This property makes conducting polymers attractive and most suitable for the applications as active elements in PLEDs Section 1-4-4 gives the general conception of the fluorescence from the conducting polymers The formation of excimers and quenching center is also discussed in order to emphasize the importance of achieving high fluorescence efficiency of the conducting polymers The basic

conception of band gap is also described

1-4-2 Electronic structure – Ground state

1-4-2a Tight-binding models

The band structure of trans-polyacetylene has been modeled by several

groups15,16; the most widely used model was developed by Su, Schrieffer and Heeger (SSH),13,14 and involves a tight-binding calculation for a polymer chain with cyclic boundary conditions, neglecting electron-electron interactions

1-4-2b Quantum chemical calculations

The usefulness of the empirical Hückel models discussed in Section 1-4-2a derives from their correspondence to the results obtained from more sophisticated calculations A wide variety of computational techniques has been used, ranging from

ab initio calculations to highly parameterized semiempirical methods.17 The details of these techniques are described elsewhere.18

These calculations provide many of the basic parameters that define the electronic structure relevant to the operation of polymer light-emitting diodes, including the energy gap between the π and π* states, the widths of the π and π* bands, and the

energies of these bands with respect to the vacuum level

1-4-3 Electronic structure – Excited states

1-4-3a SSH model

The fundamental excitations of the Peierls-distorted chain with a half-filled band are known to be phase kinks, or solitons, in the pattern of the bond alternation This was shown for polyacetylene13-15 to take the form of the bond alternation defects An important insight into the nature of these excitations from the work of Su et al.13,14 is that

the bond alternation defect is not localized at a single carbon site, but is spread over some 10 to 15 carbon sites This delocalization is crucial to the energetics of the stabilization of the soliton and is clearly demonstrated experimentally

1-4-3b Polymers with a nondegenerate ground state

The charged excitations of a nondegenerate ground state polymer are termed

polarons or bipolarons and represent localized charges on the polymer chain, with an

accompanying local rearrangement of bond alternation These states may be considered

as being equivalent to a confined soliton pair, and in this model, the two nonbonding mid-gap soliton states form bonding and antibonding combinations, thus producing two gap states symmetrically displaced about mid-gap (± ω 0) These levels can be occupied

by 0, 1, 2, 3, or 4 electrons, giving a positive bipolaron (bp2+), positive polaron (p+), polaron exciton, negative polaron (p-), or negative bipolaron (bp2-), respectively

If the intermolecular contacts are optimized via a geometrical change following excitation, such excitons are described as excimers (where the exciton extends over identical molecular units) or exciplexes (where the exciton extends over two or more different molecular units)

1-4-4 Fluorescence from conducting polymers

1-4-4a General conception

Figure 1.2 illustrates the scheme of photoluminescence (PL) and electroluminescence (EL) of the conjugated polymers Irradiation of a fluorescent polymer excites an electron from HOMO to LUMO In a typical conjugated polymer, two new energy states are generated upon relaxation within the original HOMO – LUMO energy gap and are each filled with one electron of opposite sign (singlet excited state) The excited polymer may then relax to the ground state with emission of light at a longer wavelength than that absorbed This process is photoluminescence

In a polymer LED, electrons are injected into the LUMO (to form radical anions) and holes into the HOMO (to form radical cations) of the electroluminescent polymer The resulting charges migrate from polymer chain to polymer chain under the influence

of the applied electric field When a radical anion and a radical cation combine on a single conjugated segment, singlet and triplet exited states are formed, of which the singlets can emit light This process is electroluminescence

Figure 1.2 Scheme for a) photoluminescence (PL) and b) electroluminescence (EL)

of the conjugated polymers [Kraft, A.; Grimsdale, A C.; Holmes, A B.,

Angew Chem Int Ed., 1998, 37, 402.]

1-4-4b Excimers and quenching center

As described above, photoluminescence is believed to be the radiative decay of singlet excited states Therefore, photoluminescence efficiency of conjugated polymer is one of the most important factors for the photonic devices However, this efficiency may

be limited by quenching of extrinsic or conformational defects and intermolecular interaction that includes excimer formation19 Excimer can be formed when the

backbones of neighboring chains are packed very closely In this case, the wavefunctions spread out across more than one chain Usually, the emission from the excimers is red shifted, spectrally broad, and inefficient20 Elimination of the excimer formation can be achieved by modifying the polymers’ structures in order to get higher photoluminescence efficiency of the photonic devices

The nature of quenching sites in polymers is mainly due to the non-radiative recombination through the structure defects A small concentration of the defects will greatly reduce the photoluminescence efficiency of a polymer because excitons can migrate and find the location of the defects, which have an energy level within the band gap of the polymer Taken as a whole, stable and higher photoluminescence efficiencies

of light emitting diodes can be achieved by encapsulating the conjugated polymers whose side chains will prevent neighboring backbones from forming excimers in an inert atmosphere and a hermetic packaging

1-4-4c Band gap

Band gap of the conjugated polymers can be tuned by modifying the polymers’ structures to obtain emission colors of the polymers in the visible and near infrared regions of spectra Band gap can be determined by the bond alternation and torsion angles between rings in the polymers’ backbone Also, the extent of electron delocalization is the main factor that may determine the band gap of the conjugated polymers By tuning these factors, the band gap of conjugated polymers can be fluctuated in fine increments from 1.1 eV to 3.3 eV Polyparaphenylene (PPP) has one of the largest band gaps (3.0 eV) of all conjugated polymers because its excited state