[page 5↓]

1.  Introduction

The first report about the generation of electrochemiluminescence (ECL) in conducting polymers (CP) came in the year 1994 [1] from the group of A.J. Bard. After that there have been only three publications about the electrogenerated chemiluminescence [2] in conducting polymers until to date [3-5]. But there have been several works done on the ECL in the solution phase containing organic molecules [6,7]. The generation of electroluminescence (EL) in conducting polymers is another field of research that is widely reported in the literature [8]. However ECL in conducting polymers is different from that in the solution phase and the EL process. In order to understand the significance of the ECL in conducting polymers, first the principles of the ECL process in the solution phase will be discussed, as it is relatively simple. Then the unique properties of conducting polymers such as the nature and transport of charges in them will be described. The characteristics of the electroluminescence (EL) process in CP will be described in brief to understand the uniqueness of the ECL process.

1.1. Electrogenerated chemiluminescence

Electrogenerated chemiluminescence is the production of light by the reaction between the charged species generated by electrochemical means. The first report of this kind appeared in 1964 [9]. The electrochemical generation of reactants may be formulated (for organic compound Og) as:

(1.1a)

(1.1b)

The redox process between the reactants produces neutral molecules in an excited electronic state that relaxes by emission of photons.

.

(1.1c)

This chemiluminescence reaction is electron transfer luminescence since the sequence oxidation-reduction can be as effective as reduction-oxidation. The heterogeneous electron transfer reactions at the electrode/solution interface are fast. The luminescence observed is fluorescence. Therefore the schemes to be considered must be the ones, which provide sufficient energy to yield an [page 6↓]aromatic hydrocarbon in its first excited singlet state, 1Og*.

The mechanisms considered to date for the formation of 1Og* in these systems can be classified into four types:

(1) The first mechanism postulates the direct formation of 1Og* via the annihilation of electrogenerated Og+. by electrogenerated Og¯.. The process may yield 1Og* or an excimer 1Og2* [10].

,

(1.1d)

.

(1.1e)

(2) The second mechanism envisions the formation of 3Og* via the same annihilation reaction as above. This is then followed by the known process of triplet-triplet annihilation (TTA) to yield 1Og* provided that sufficient energy is available from two triplets. The presumably efficient quenching of 3Og* by Og+. and/or Og¯. must be recognized in discussing mechanisms involving triplets.

(1.1f)

(1.1g)

(3) A third mechanism was postulated describing the direct generation of excited states, 3Og* or 1Og*, by a heterogeneous electron transfer reaction at the electrode [11]. Oxidation of Og¯. or reduction of Og+., under certain polarization conditions was said to generate 3Og*.

(4) The fourth type is the chemiluminescence reaction of Og¯. with products resulting from the decomposition of Og+. and/or the solvent. Chemiluminescence reaction of stable Og+., with the decomposition products of Og¯. or with the solvent also belongs to this class. This mechanism was operative in a number of other cases, particularly those in which Og+. is able of oxidizing or otherwise reacting with the solvent, e.g., 9,10-diphenylanthracene (DPA) or 9,10-dimethylanthracene in dimethylformamide (DMF). In the rubrene system, as in the DPA in acetonitrile, luminescence was observed only when both Og+. and Og¯. are electrogenerated. The pre-annihilation ECL reported [page 7↓]earlier in some cases [12] is absent when the solute-solvent electrode system yields stable Og+. and Og¯..

1.2. Energetics of the ECL

In the usual thermal electron transfer reaction, the products are formed in their electronic ground states, for only these are usually conveniently accessible energetically. Nevertheless, the potential energy surface of the reactants can "cross" the surface of the products, in which one (or more) product(s) is (are) electronically excited in some other region of configuration space. If this latter intersection region is easily accessible (energetically and entropically, Marcus [13]), a reaction to form an excited product can occur. The excited product may either emit light or react. An example of such a reaction is a possible triplet-triplet annihilation to form an excited singlet, which later fluoresces. [14].

The electron transfer theory yields the rate constant to form a particular electronic state of the products as

(1.2a)

where Z is about 1011 l mole-1 s-1, κ is a factor close to unity unless the splitting is extremely small, ρ≈ 1, and the ΔF* reflects the accessibility of the intersection region.

(1.2b)

(1.2c)

In these equations wr is the work required to bring the reactants together to the most probable separation distance R in the intersection region, i.e., in the "activated complex"; wp is the corresponding work to bring the products together, each 'w' refers to the given state of excitation. ΔF°' is the "standard" free energy of reaction in the prevailing medium, λ is a reorganization term. It is expressible in terms of differences in equilibrium bond lengths of each reacting species in its initial and final electronic states in terms of dielectric properties [page 8↓](related to differences in equilibrium orientation polarization) in these electronic states; λ is typically on the order of 0.4-0.6 eV for solvents such as acetonitrile or dimethylformamide. The activation energy has a minimum value at ΔF° = -λ. If ΔF° is larger than -λ, we are dealing with a normal activation process. If, on the other hand, ΔF° is smaller than -λ, the intersection of the adiabatic potential curves occurs in the abnormal region.

In a sufficiently exothermic electron transfer reaction the region where the surface of ground-state products "intersects" that of ground-state reactants is not readily accessible and becomes less accessible with increasing exothermicity. Under such conditions the region where the potential energy surface involving an excited product intersects that for the unexcited reactants may be readily accessible. The formation of an excited product can then occur easily. This formation of an electronically excited Og is allowed because it represents a favourable way of accommodating the energy released by the electron transfer reaction. The actual transfer presumably does not involve any large changes in molecular geometry or co-ordination and could occur rapidly on the time scale of molecular vibrations. Such a time scale makes it difficult to convert this energy into thermal energy. Thus, the creation of an excited state and emission from it are possible in an electron transfer reaction under special conditions. This process is analogous to the combination of an electron with a hole in the solid state. An even closer analogy is the phenomenon of recombination luminescence of organic compounds held in rigid matrices at low temperatures [15].


[page 9↓]

Fig 1.2a: Scheme of potential energy surfaces for an electron transfer reaction.

To understand the mechanism of electron transfer leading to the singlet or triplet excited state, we consider the simple molecular orbital picture representing the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO).

Fig. 1.2b: Molecular orbital representation of an electron transfer reaction.


[page 10↓]

An initial one-electron transfer with the formation of a radical ion generally characterizes the redox activity of an aromatic molecule in an aprotic solvent. Reductions are pictured as an addition of an electron to the lowest vacant molecular orbital (MO), and oxidations as a removal of an electron from the highest filled MO. Good correlations are found between electrochemical half-wave potentials and eigen values of these MO's which has been shown in the following table [reproduced from 16]:

Process energetics has weighed heavily in mechanistic arguments, because it is easy to characterize the redox reactions thermodynamically via the standard potentials of the couples involved.

(1.2d)

From the cyclic voltammogram of the organic compound, e.g., DPA (9,10- diphenylanthracene) the difference in the standard potentials for oxidation and reduction can be calculated using Eq. (1.2d). This corresponds to the free energy of the radical ion pair compared to the ground state DPA pair, which is ≈ 3.2 eV in acetonitrile. The free energy of 1Og* + Og was calculated from the 0-0 [page 11↓]transition energy of the fluorescence spectrum to be 3.0 eV. Thus, the energy supplied in the electrochemical generation of the radical ions is sufficient for the production of DPA excited state. Such systems are called energy sufficient systems, and the corresponding electron transfer reaction can result in the ECL.

1.3. Analysis of the ECL reaction mechanism

1.3.1. Distinguishing singlet route from triplet route

In section 1.1 four possible ways of ECL generation were mentioned. The identification of the actual mechanism is indeed a difficult task, especially differentiating the mechanisms 1 from 2. The ECL emission spectrum yields the energy of the emitted light, which can be used as a tool to identify the nature of the excited state, whether it is a singlet or triplet excited state. But the problem is the identification of the triplet-triplet annihilation (TTA, Eq. (1.1g)) giving rise to the singlet-excited state. If the lowest triplet has less than half the energy of the first excited singlet, then TTA will not occur, as in the case of naphthacene [17]. However it cannot be generalised so easily. Especially in systems like DPA, where 3Og* + Og has larger energy, 1.8 eV than the half of the singlet energy.

Feldberg [18,19] did a kinetic analysis of the processes to identify the mechanism. The reactions (1.1a) and (1.1b) corresponding to the generation of radical cation and radical anion respectively can be achieved by the application of two successive potentiostatic steps of appropriate magnitudes. In which case reaction(s) (1.1d) (singlet) or (1.1f) and (1.1g) (triplet) take place in the second potential step. Let tf and tr be the duration of the first and second potential steps. Feldberg has calculated that when k1 tf C is larger than 103, the intensity of the fluorescence radiation will be given by:

(1.3.1a)

for times where [(t - tf) / tf]1/2 > 0.4 and where P is the photometer output and C is the bulk concentration. This equation is valid for direct formation of excited [page 12↓]singlets (reaction (1.1d)) or for triplet formation followed by TTA to give an excited singlet (reactions (1.1 f) and (1.1g)). For the latter case the constant is smaller by a factor of 2 than for the case of direct singlet formation. Thus, this method though helped to identify the production of the singlet state; a conclusion about whether it is a direct singlet formation or singlet formation through TTA is not possible. The details of the theoretical analysis will be discussed in detail in chapter 4.

Since the triplet-excited state has a paramagnetic spin, method of electron spin resonance (ESR) spectroscopy coupled with the ECL experiments was performed [20, 21]. TTA was tried to be identified by this technique. An initial report dealt with two energy-deficient systems, TMPD(+) / DPA(-) and TMPD(+)/anthracene(-), and one energy-sufficient systems, DPA(+) / DPA(-). Monotonically increasing intensities with field strength were observed for the former cases, but the latter did not show any effect. These results demonstrated the presence of a field influenced step in the emission scheme for the two TMPD(+) oxidations and the absence of such a process in the DPA(+) / DPA(-) case. The influenced reactions were presumed to involve paramagnetic species, and triplet-triplet annihilation was suggested as an appealing candidate. Thus, these findings were cited as possible evidence that the two energy-deficient systems proceed via the T-route, whereas the S-route yields the emission from DPA(+) / DPA(-) [22]. This method was not successful in all the cases. In DPA the ECL was ESR silent, but the ECL efficiency (ratio of the number of emitted photons in unit time to the faradaic current) was just about 1.5 % when it is expected to be 100% based on a pure singlet route [23]. Thus, the differentiation between direct singlet formation and that due to TTA is still uncertain. However the other possibilities can be identified. The complications due to excimer formation were often eliminated by phenyl substituents at reactive sites.

Also on the basis of the magnetic field effects it is not really possible to confirm or deny this mechanistic position, because one cannot predict the magnitude of an ECL field effect in any qualitative fashion. In principle, it could be calculated if a host of rate constants, the quencher concentrations, and molecular parameters in the spin Hamiltonians were all available [24]. In ECL the quencher concentrations within the reaction zone have unknown (and possibly [page 13↓]unknowable) magnitudes and time dependencies. Even though a quantitative description is not feasible, this technique is still a very powerful qualitative aid for distinguishing ECL processes involving triplets from those that do not.

A very sensitive means for probing T-route systems involved intercepting a triplet intermediate 3D* by triplet energy transfer to an acceptor species A [25,26]:

(1.3.1b)

The acceptor triplet can itself undergo triplet annihilation; hence, the acceptor's addition may transform the ECL spectrum from donor emission to that of the acceptor. Alternatively, it might undergo rapid radiation less decay resulting in quenching of the ECL emission. Either effect provides support for the operation of the T-route in any case for which it is suspected. Moreover, the acceptor triplet can undergo a "photochemical annihilation" created by a known number of redox events and can give triplet yields in an attractively straightforward manner. Although it is a common tool in photochemistry, it is very difficult with ECL systems, because the conditions for unambiguous transfer are quite stringent. The basic problem is that the acceptor triplet will ordinarily get reduced or oxidized more easily than one of the ion precursors. It therefore interferes severely with ECL generation, and any modification in behaviour upon its addition is practically uninterpretable. A less severe, but important problem is the need for a chemical inertness of the acceptor toward the ion radicals. Very few systems have met these demands so far, and all of them have involved the fluorantharene triplet as a donor. Since this molecule has a small S1-T1 splitting, it exhibits high triplet energy (2.3 eV) for a species that reduces so easily (- 1.70 V vs. SCE). There are several suitable acceptors for its triplet, and interception studies of oxidations of its anion have proved quite fruitful. However, quantitative calculation of the triplet yield φt is difficult, even if the productions of the radical ions are controlled because of the interference by any quenchers present in the system [27].


[page 14↓]

1.3.2. Identification of other possible mechanisms for the production of an excited state

Regarding the formation of an excited state, by the reaction of the radical ions with the electrode, which is the possibility 3 mentioned in section 1.1: It depends on the free energy for the formation of the radical ion. If it is larger (≈ 3 eV), then there will be no chance to form an excited species in the solution. If the standard energy for the formation of the radical anion is higher, then when the potential of the electrode is switched to a positive value, the electron from the radical anion can go easily to one of the unoccupied orbitals of the metal. Thus, the radical anion becomes oxidized to the neutral species instead of becoming excited. Similar remarks apply to a very exothermic electron transfer from an electrode to a molecule or a radical cation in solution. In this case the electron can relieve the exothermicity by coming from one of the filled half of the conduction band of the metal. Thus the molecule or the radical cation becomes, reduced when the polarity of the electrode becomes more negative [13].

The possibility 4, i.e., the reaction between one of the radical ions with the decomposed products of the electrochemical system, depends on the stability of the system. In such cases ECL is observed before reaching the appropriate potentials for the electrogeneration of one of the radical ions. This phenomenon is termed as pre-annihilation ECL For example, Rubrene in dimethyformamide (DMF) has the pre-annihilation ECL due to the reaction of the radical cation with the solvent. Whereas DPA or Rubrene in acetonitrile yields luminescence, only when both Og+. and Og-. are electrogenerated [28].

1.4. Factors affecting the ECL and the precautionary measures

The electrogenerated chemiluminescence is affected by the stability of the exciton as well as that of the precursor radical ions. The definition of quenching is a process, which reduces the lifetime of the excited state. A reduction in the lifetime usually implies a decrease in the quantum yield. Some of the processes that reduce the quantum yield are: collisional quenching, static quenching and energy transfer. Light scattering can appear as quenching (loss of fluorescence signal). Both, collisional and static quenching require a contact between the [page 15↓]fluorophore and the quenchers. Thus, these methods are useful to measure rates of diffusion and exposure of fluorescent species to the quencher. A large number of quenchers are known, and a partial list is: molecular oxygen, amides, BrO4 -, xenon, peroxides, nitroxides and acrylamide.

The mechanism of quenching was difficult to determine. In some cases, such as acrylamide, the quenching mechanism seems to involve a transfer of charge from the excited fluorophore to the quenching agent.

In the case of the conducting polymers, the chemical nature of the polymer plays a crucial role in determining the stability of the exciton. The movement of the exciton along the polymer, which is enhanced by the presence of delocalized bonds in CP, as well as the dissociation of the exciton, which is caused by the lack of delocalization, lead to the loss of the excitons.

Considering the stability of the radical ions, again, oxygen is the foremost threat to it. It is invariably present in the chemicals and in the environment. This even in a minute amount is a threat to the excited state because of its paramagnetic nature [29]. And hence, traces of water in the solvent or the supporting electrolyte is also detrimental. It can produce the superoxide during the electrochemical potential sweep/step necessary for the production of ECL. Hence, the success of ECL experiments depends strongly on the removal of oxygen and water from the experimental system. The radical ions, especially those with extensive delocalization and blocked reactive sites, may be stable or they may undergo a number of different types of reaction [30]. Electrogeneration of dianions and dications at potentials beyond the initial reduction and oxidation waves is also possible. But these species are usually quite unstable in the solvents employed for ECL.

The solvent supporting electrolyte systems are chosen for their lack of reactivity with the electrogenerated species, their wide potential limits and their good conductivities. Purity of the medium is important in determining both, the quality of the electrolytic background and the stabilities of the ECL reactants, hence, special attention must be given to it. The common solvents used for the ECL experiments are acetonitrile (AN), benzonitrile (BN), N,N-dimethylformamide (DMF), methylene chloride, propylene carbonate (PC) and tetrahydrofuran [page 16↓](THF). The supporting electrolytes are tetraalkylammonium salts with the anions as tetrafluoroborate (BF4 -), hexafluorophosphate (PF6 -) and perchlorate (ClO4 -).

Excellent purification procedures are described in the literature for dimethylformamide (DMF) and acetonitrile [31]. Since these solvents are hygroscopic, they must be isolated from the atmosphere during storage. These solvents are kept under argon and when needed can be dispensed through the spigot by slightly pressurizing the flask with the inert gas on a vacuum line. Greaseless fittings are used when dealing with solvents. The supporting electrolytes can often be used as received. Recrystallisation may be required [32]. Hygroscopic electrolytes must be dried for 24-28 hours at 90-100 °C and should be kept in a desiccator. Drying them overnight prior to the experiment also helps to reduce the moisture content.

Vacuum systems are generally used which offer convenience in continuous removal of water from the system. The vacuum level is approximately 10-5 Torr. The system should be filled with argon during the experiment. Bubbling the electrolytic solution with argon also helps to remove the traces of oxygen along with the above said procedure.

Platinum is the most frequently used electrode material for ECL studies. Although the oxide films that occur in aqueous solutions do not affect it, some pre-treatment is generally done before use. Polishing the platinum to a high finish, with very fine emery paper or diamond paste, are some of the regular procedures.

1.5. Conducting polymers

Conducting polymers (CP) contain electronic states that can be reversibly occupied and emptied with electrochemical techniques. In fact, the electrochemistry of conducting polymers has enormous potential for wide ranging practical applications, e.g., in batteries, displays, etc. [33-35]. As polymers, these materials have a highly anisotropic quasi-one-dimensional electronic structure that is fundamentally different from the structures of conventional inorganic semiconductors. This has two consequences: First, their chain like structure leads to strong coupling of the electronic states to [page 17↓]conformational excitations peculiar to the one-dimensional (1D) system [36], and second, the relatively weak interchain binding allows diffusion of dopant molecules into the structure (between chains), whereas the strong intrachain carbon-carbon bond maintains the integrity of the polymer. In their neutral form, these polymers are semiconductors with an energy gap of ≈ 2 eV. However, when they are in the doped state (i.e., oxidized or reduced), their conductivity increases by many orders of magnitude; conductivities in the range of ≈ 103 - 104 S/cm are not unusual [37].

The conductivity in doped polymers, optical and magnetic properties that are far from those of traditional metals provided the direct evidence that the description of electronic excitations in CPs is very different from that of three-dimensionally bonded materials. The anisotropic bonding in the polymers allows a local rearrangement of the chain geometry to better accommodate an electronic excitation, without requiring a large lattice strain in the directions perpendicular to the chain. And it allows, in general, formation of "polaronic" excited states, for both charged and neutral excitations (excitons).

The character of these polaronic states was first established for the case of polyacetylene, for which, in the trans isomer, the degeneracy with respect to the sense of bond alternation causes these states to take the form of topological, soliton-like chain excitations, which have associated with them a nonbonding π level that is situated at the middle of the π-π* semiconductor gap [38]. In polymers with a nondegenerate ground state, such as PPV, the two alternative senses of bond alternation do not have equivalent energies; 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. The neutral excited state shows similar structural reorganization, as shown in the following Fig. 1.5a.


[page 18↓]

Fig. 1.5a: Configuration of the exciton in PPV.

These states may be considered equivalent to a confined soliton pair, and in this model the two nonbonding midgap "soliton" states form bonding and antibonding combinations, thus producing two gap states symmetrically displaced about the midgap as shown in Fig.1.5b. 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.

Fig. 1.5b: Scheme of the valence and conduction bands of polarons and bipolarons.

The case of near degeneracy allows the polarons to be relatively extended with [page 19↓]electronic levels near the center of the gap, while strong breaking of the degeneracy keeps them much more compact with levels near the band edges. The singly charged polaron is expected to have gap states that are more closely tied to the band edges than is the case for a bipolaron or exciton. The greater degree of relaxation of the bipolaron in comparison to the polaron stabilizes the coalescence of two like-charged polarons to form a bipolaron. There are many theoretical works concerned with the modelling of these polaronic levels [39, 40]. The success of these models lies in their ability to account for the appearance of new optical absorption bands within the semiconductor gap on chemical doping (e.g., at 0.6 eV and 1.6 eV for the case of PPV [41]) and at the same time to account for low paramagnetic response (bipolarons being low spin). The quantum chemical techniques that have been used to describe the ground state electronic structure have also been used with considerable success to model the electronic structure and chain geometry of charged excited states [42, 43]. But these predict that the energy of the photons emitted by luminescence should be equal to the spacing between the two-bipolaron levels. Whereas experimentally the luminescence is found to occur at significantly higher energy than the bipolaron spacing as determined from photo induced absorption. Such discrepancies are overcome by taking the electron-electron interactions into account in the model calculations [44]. Further improvements are made by considering interchain interactions [45]. Describing the excited states, especially the neutral excited states that are excitonic, is still complex. Excitons are electron hole pairs, which may either be localized on one molecular unit or spread over many molecular units. A complete description of the excitons was done by considering the electron-electron (coloumbic) and the electron-lattice interactions. One consequence of the electron-electron is that singlet and triplet excitons are no longer of the same energy or of the same size. The triplet exciton is considerably more localized than the singlet exciton, which is confirmed for PPV and its oligomers [46]. Based on these theoretical studies, the arrangement of ground and excited state energies for PPV is depicted as shown in Fig. 1.5c.


[page 20↓]

Fig. 1.5c: Ground and excited states of PPV polymer.

Thus, the energy levels in the case of CPs are different from the organic monomer molecules and are strongly influenced by the electronic and lattice interactions. The understanding of this is necessary to assign the spectroscopic features of these polymers. Polymers with larger π-π* energy gaps have poor intrachain delocalization and can show changes in optical properties as they are brought together in the solid. Interchain interactions are therefore important for these materials and lead to the formation of more extended excited states, which would be described as charge transfer excitons within the framework of molecular semiconductors. 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).

The other important factor need to be known is the movement of the charges and excitons in CP. Let us consider excitons alone at first. Excitons are mobile within the solid, and their motion, either coherent or diffusive, plays a very important part in the photophysics of conjugated polymers. When there are appreciable nearest-neighbor interactions between the molecular units of CP, the exciton becomes delocalized, the exciton then drifts along the delocalized band. When there is a significant electron-phonon interaction, the exciton loses [page 21↓]its coherence, and it becomes a localized state that moves by hopping between sites. In practice, disorder in site energies plays a very important part in causing this loss of coherence.

The transport of charges (positive polaron, negative polaron and the corresponding bipolarons) resembles that of the exciton. The presence of ions from the supporting electrolyte with CP in an electrochemical environment yields a characteristic charge transport mechanism in them. This is discussed in detail in chapter 4.

Since the electron-electron and interchain interactions determine the polaronic and excitonic energy levels in the CP, they can be tuned to achieve the desired emission. This is done by changing the substituents of the conducting polymer to achieve characteristics like the emission in the desired energy range and of desired solubility [47].

1.6. Electroluminescence

Due to the tunability of the energy of emission and the ease of handling them, conducting polymers are used in solid-state light emitting devices (LED), emission of which is called electroluminescence (EL). Electroluminescence (EL) from conjugated polymers was first reported in 1990 [48] using poly(p-phenylenevinylene) (PPV) as the single semiconductor layer between metallic electrodes, as illustrated in Fig. 1.6a.

Fig. 1.6a: Construction of a light emitting diode (LED).

PPV has an energy gap between π and π* states of about 2.5 eV and produces [page 22↓]luminescence in a band below this energy, as shown in Fig. 1.6b. Operation of LED is achieved when the diode is sufficiently biased to achieve injection of positive and negative charge carriers from opposite electrodes. Capture of oppositely charged carriers within the region of the polymer layer can then result in the formation of the singlet exciton, which is generated by photo excitation across the π-π* gap, and this can then decay radiatively to produce the same emission spectrum as that produced by photo excitation. The absorption and emission spectra for PPV are shown in Fig 1.6b. Note that the absorption rises rapidly above the onset of the π-π* threshold and that the emission spectrum appears on the low energy side of the absorption. Both, absorption and emission spectra show a broadening due to vibronic coupling, as is characteristic for optical transitions in molecular semiconductors where the excited state is a singlet exciton. The similarity of the emission spectra produced by photo excitation and by charge injection establishes that the excited state responsible for light generation in the LED is the same as that produced by photoexcitation.

Fig. 1.6b: Absorption, photoluminescence (PL) and electroluminescence (EL) spectra of PPV.

The levels of efficiency of the first, simple LEDs based on PPV, which were fabricated with aluminium negative electrodes, were relatively low, on the order of 1 photon generated within the device per 104 charges injected (an internal quantum efficiency of 0.01 %) [48]. External quantum efficiencies are strongly [page 23↓]affected by the refractive index of the emissive layer, and the relationship between the two has been discussed by Greenham et al. [49]. These values have risen rapidly over the past few years as improved understanding of the operation of these devices, aided in considerable measure by parallel developments made with sublimed molecular film devices, has allowed considerable optimization of the devices characteristics. The use of negative electrodes with lower work functions was shown to improve efficiency to as high as 1 % in devices made with ITO/MEH-PPV/Ca [50].

1.7. Objectives of the Present Study

The conducting polymers behave electrically different in the solid state and in contact with the solution. Since CPs yield both, electroluminescence (EL) in the solid state and electrogenerated chemiluminescence (ECL) in contact with electrochemical solution. Therefore, it is of interest to analyze the mechanism of the ECL process in the conducting polymers, to find out whether is it similar to the EL process or to the ECL process in solution phase. If it is different, in what way does it differ from the other two processes? In order to achieve that, theoretical modeling of the ECL reaction and charge transport mechanism and comparison with the experimental findings are required. This study can also throw some light on the charge transport mechanism of CP in electrochemical conditions. These are the objectives of the present study.


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