WO1993014177A1

WO1993014177A1 - Conjugated polymer

Info

Publication number: WO1993014177A1
Application number: PCT/GB1993/000131
Authority: WO
Inventors: Donal Donat Conor Bradley; Paul Leslie Burn; Richard Henry Friend; David Alan Halliday; Andrew Bruce Holmes; Arno Kraft
Original assignee: Cambridge Display Technology Limited
Priority date: 1992-01-21
Filing date: 1993-01-21
Publication date: 1993-07-22
Also published as: AU3361793A; GB9201240D0

Abstract

A conjugated polymer which is preparable from a solution-processible precursor polymer and which exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition. A conjugated poly(arylenevinylene) polymer is also provided, such as poly(p-phenylenevinylene). The conjugated polymer may be used in a variety of applications where an improved electronic structure is useful, such as in electroluminescent devices.

Description

TITLE OF THE INVENTION

CONJUGATED POLYMER

The present invention relates to conjugated polymers, more particularly to conjugated poly(arylenevinylene) polymers having improved electronic properties. The present invention also relates to methods of making the polymers.

BACKGROUND TO THE INVENTION

Conjugated polymers are of considerable interest for applications as electronically or optically active materials because they can combine the ease and cheapness of processing of a processible polymer together with the semiconducting or conducting properties usually associated with inorganic materials. Processing of these polymers, usually to form a thin film, is conveniently performed from solution by such methods as spin or dip coating, or from a melt. Although the conjugated polymer itself may commonly be insoluble in convenient solvents or infusible below its decomposition temperature, methods are available to overcome this problem. The method of Lenz et al describes how poly(p-phenylenevinylene) , PPV, an intractable polymer, can be prepared via a solution-processible "precursor" polymer formed as a sulphonium polyelectrolyte (R.W. Lenz, C-C Han, J. Stenger-Smith, and F.E. Karasz in Journal of Polymer Science: Part A: Polymer Chemistry 1988, 26_, 3241). PPV produced by this method is shown in Figure 1 as PPV(a),or via a methoxy leaving group precursor polymer (S. Tokito, T. Momii, H. Murata, T. Tsutsui and S. Saito, Polymer, 1990, 31,1137). The sulphonium polyelectrolyte (shown as [1] in Figure 1) is soluble in both water and methanol and it is possible to obtain films of high quality from a solution in methanol. These films have been used as the emissive layer in large-area light-emitting diodes as discussed in International Patent Application No. W090/13148 of the present applicant. Another strategy to achieve a soluble polymer is to attach flexible side-groups to the main chain and this has been achieved for PPV with attachment of alkoxy groups to the 2 and 5 positions on the phenylene ring. An example of such a polymer is poly(2,5-dihexyloxy- phenylene vinylene) as disclosed in S.H. Askari, S.D. Rughooputh and F. Wudl, Synthetic Metals 1989, .29., E129.

Both of these methods for the preparation of the conjugated polymer suffer from the disadvantage that the soluble phase is likely to be one in which the polymer is disordered. Where the polymer is in the form of a random coil in solution, it is difficult to remove this disorder in the conjugated form of the polymer once it has been converted. Electronic and optical properties are very sensitive to the presence of defects, including conformational defects on the polymer chain.

In P.L. Burn et al (P.L. Burn, D.D.C. Bradley, A.R. Brown, R.H. Friend, D.A. Halliday, A.B. Holmes, A. Kraft, J.H.F. Martins, Proceedings of Kirchberg Winterschool on Conducting Polymers, March 1991 Springer Ser. Solid-state Sci., in press), various precursor routes to the synthesis of PPV were discussed. It was found that by increasing the bulk of the leaving group and/or the rigidity of precursor polymers to PPV, the intra-chain order increases whereas the inter-chain order decreases. It has now been surprisingly found that conjugated polymers can be produced in a controlled manner with greatly improved electronic and optical properties. The improved properties are expected to be of value in several areas of application.

SUMMARY OF THE INVENTION

The present invention provides a conjugated polymer which is preparable from a solution-processible precursor polymer and which exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition. A conjugated poly(arylenevinylene) polymer is also provided which exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition. Preferably, the conjugated poly(arylenevinylene) polymer is preparable from a solution-processible precursor polymer.

Throughout this specification, the term arylene is intended to include in its scope all types of arylenes including heteroarylenes as well as arylenes incorporating more than one ring structure, including fused ring structures.

Typically, the poly(arylenevinylene) polymer is a poly(phenylenevinylene) polymer which may be substituted or unsubstituted. A preferred example of the poly(phenylene- vinylene) polymer is poly(p-phenylenevinylene) , PPV. - A -

An important characteristic of the electronic structure of the conjugated polymer of the present invention is provided by the spectrally-resolved optical absorption and photoluminescence. Referring in particular to PPV, in the absorption spectrum, a sharp onset is observed at around 2.4 eV and a peak in absorption at about 2.45 eV. At higher photon energies, the absorption spectrum shows a series of subsidiary peaks or shoulders. These are assigned to optical transitions that couple to the vibrational quanta for the polymer chain. The luminescent spectra are complementary, showing a peak in emission just below the absorption edge and subsidiary maxima at lower energies, again spaced by the vibrational quanta for the polymer chain. The peak in emission is at about 2.35 eV.

These peaks are identified as the transitions between the vibrational ground state of the electronic ground and excited states and are termed (0,0) transitions. Thus, the conjugated polymer of the present invention has a greater absorption amplitude in the (0,0) transition than in any of the other vibronic side-bands. The conjugated polymers of the present invention appear to have a high level of chain order in comparison with polymers found in the prior art. This is discussed in further detail below.

The present invention also provides a process for preparing a conjugated polymer, which process comprises providing a leaving group substituted precursor polymer comprising saturated and unsaturated units, the saturated units of which include a leaving group, reacting the leaving group substituted precursor polymer in a solvent comprising a modifier group at a temperature whereby the modifier group substitutes some or all of the leaving groups leaving groups, and converting the solution-processible precursor polymer to the conjugated polymer under conditions to eliminate the modifier group, wherein the solvent and temperature are selected such that the conjugated polymer produced exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition.

It is particularly important that the precursor polymer has introduced in its structure a sufficient amount of unsaturation to minimise the amount of disorder within the polymer chain. Standard spectroscopic techniques may be used to characterise the precursor polymers. It has been found that a typical proportion of unsaturated units in the precursor polymer should be up to 40%. If too much unsaturation is introduced the precursor polymer may cease to be solution processible and precipitate from solution.

The modifier group present in the precursor polymer must be capable of elimination from the precursor polymer so as to yield an unsaturated unit typically conjugated with further unsaturated units in the polymer. The conditions of elimination must be such that the polymer is not decomposed. Preferably an uncharged modifier group is used such as a methoxy group, although charged modifier groups such as sulphonium moieties may be used.

In the process, the solution-processible precursor polymer is selected so that it may be converted into one of the conjugated polymers as described above. Where the solution-processible precursor polymer comprises a poly(arylenevinylene) polymer, a proportion of the vinylic groups of the polymer are typically substituted with the modifier group. Standard conditions of elimination may be employed, such as heating in the presence of acid substantially in the absence of oxygen. Typically, an inert gas atmosphere is used.

The solution-processible polymer is provided by reacting a leaving group substituted precursor polymer, advantageously in a solvent comprising the modifier group so that the modifier group replaces the leaving group, so as to form the solution-processible precursor polymer. Advantageously, no co-solvent is present with the modifier group. A preferred solvent is methanol whereby a methoxy modifier group is provided directly as the solvent. The leaving group substituted precursor polymer is preferably provided in a different solvent from that comprising the modifier group and may be formed by any suitable reaction of monomer units. Preferably, conversion of the leaving group substituted precursor polymer into the solution-processible precursor polymer takes place with an increase in the proportion of unsaturated units present in the polymer. Control of the degree of unsaturation in the solution-processible precursor polymer may be achieved by appropriate variation of the time and temperature for reaction. Where a methoxy modifier group is used, the preferred reaction temperature is over 50 C, more preferably around 55 C.

In a preferred embodiment, an initial precursor polymer is formed. This initial precursor polymer is then reacted in solution in the presence of base so as to form the leaving group substituted precursor polymer. Advantageously, the reaction conditions can be tailored so as further to control the degree of unsaturation in the leaving group substituted precursor polymer. By varying the concentration of base and, where necessary, the time and temperature of the reaction, leaving group substituted precursor polymers may be formed having an appropriate degree of unsaturation for subsequent formation into the solution-processible precursor polymer. A preferred base for this reaction is an organic base, typically one which is soluble in organic solvents. Preferably, a tetra alkylammonium hydroxide base is used such as tetra-n-butylammonium hydroxide.

Where tetra-n-butylammonium hydroxide is used, an amount of less than one molar equivalent is preferred so as to prevent conversion into an insoluble product.

Typically, the leaving group present in the solution-processible and leaving group substituted precursor polymers is a sulphonium leaving group.

It will be apparent that the polymers of the present invention may be exploited in many applications including electroluminescent devices. In such devices, where PPV according to the present invention is used, the energy of the peak emission is blue-shifted. This enables some control of colour of emission in comparison with conventional PPV. Furthermore, the ranges of wavelengths for the electroluminescence emissions is narrowed so that a greater fraction of the emitted light falls within a given colour range.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described further by way of example only with reference to the drawings in which:

Figure 1 shows the various synthetic routes for forming PPV both as described in the prior art and in the present invention; Figure 2 shows optical absorption spectra measured at room temperature for the PPVs (a) to (i) of Figure 1;

Figure 3 shows the photoluminescence spectra at 10K for selected PPVs;

Figures 4 to 12 shows the infra-red spectra of each of the PPVs (a) to (i);

Figures 13 to 16 show respectively the infra-red spectra of precursor polymers ([4], [9], [10], and [11] of Figure- 1) ; and

Figure 17 shows the UV/visible spectra of the precursor polymers ([4], [9], [10], and [11] of Figure 1).

Examples

All reactions were carried out under argon.

Comparative Example 1 (PPV Prepared in Accordance with International Application Publication No. WO90/13148)

De-oxygenated aqueous sodium hydroxide (0.4 M, 29.5 ml) was added dropwise to a stirred, deoxygenated ice cold solution of l,l'-[l,4-phenylenebis(methylene)]bis[tetrahydrothiophenium] dichloride [2](4.00g, 11.8 mmol) in methanol (30ml). The reaction mixture was stirred for 1 hour and then neutralised with dilute hydrochloric acid. The product was then dialysed against distilled water (3 x 1000 ml) at room temperature, under argon for 3 days, using a cellulose membrane dialysis tubing with molecular weight cut-off of 12400 (supplied by Sigma Chemical Company Limited, Dorset, U.K.), the dialysis medium being changed every 24 hours. The solvent was completely removed from the material in the dialysis tubing and the polymer residue [1] was redissolved in dry methanol. Films of [1] were obtained by spin-coating or free casting and then thermally converted (220°C, 12 hours, under vacuum) to give films of PPV(a) . For all conversions from the various precursors to the PPVs detailed below, films were spin-coated onto spectrosil substrates from either methanol or chloroform solutions, and were converted by heat treatment at 220 C for 12 hours, under flowing Ar with HCl.

Comparative Example 2

To a stirred, ice cold, deoxygenated solution of 1,1'-[1,4-phenylenebis(methylene)]bis[tetrahydrothiophenium] dichloride [2] (4.00 g, 11.8 mmol) in dry methanol (30 ml), was added dropwise a deoxygenated methanolic solution of tetrabutylammonium hydroxide (0.4 M, 29.5 ml). The reaction mixture was stirred for 1 hour and then neutralised with a methanolic solution of hydrochloric acid. The product was then dialysed against methanol (3 x 1000 ml) at room temperature under argon for 3 days, using a cellulose membrane dialysis tubing with molecular weight cut-off of 12400 (supplied by Sigma Chemical Company Limited, Dorset, U.K.), the dialysis medium being changed every 24 hours.

A solution of polymer [3] in methanol (11 mg/ml, 120 ml) was prepared and the solution was divided into 6 aliquots.

Two aliquots of [3] were purified by precipitation by pouring the polymer solution into hexane (150 ml) and were then redissolved in the minimum quantity of dry methanol. One of these aliquots was used directly to form PPV(b) . Example 1

The second aliquot [3], from Comparative Example 2, was added to dry methanol (40 ml) and then the reaction mixture was stirred at 55°C for 12 hours. The reaction mixture was allowed to cool to room temperature. The precipitate [4] was collected, washed with dry methanol (100 ml), air dried and dissolved in the minimum quantity of dxy chloroform. PPV(f) was obtained from [4] by thermal treatment under acidic conditions.

Examples 2 - 7 (Via Base Treated Precursor Polymers)

The remaining 4 aliquots of [3] were de-oxygenerated and tetrabutylammonium hydroxide in methanol (1.0 M, 0.25, 0.50, 0.75 and 1.0 molar equivalents respectively) was added and the reaction mixtures were stirred for 10 minutes at room temperature to form polymers [5], [6], [7], and [8] . Polymer [8] was insoluble and hence was not further investigated. The other reactions were terminated with a methanolic solution of hydrochloric acid. Polymers [5], [6] , and [7] were purified by precipitating the polymer solution into hexane (150 ml) and were then redissolved in the minimum quantity of dry methanol.

Half of each of the polymer solutions of [5], [6], and [7] were converted to PPV by thermal treatment under acidic conditions to form, PPV(c), PPV(d) , and PPV(e) respectively.

The second half of the polymer solutions of [5], [6], and [7] were in each case added to dry methanol (40 ml) , deoxygenated, and the solutions stirred at 55°C for 12 hours under argon. They were then allowed to cool to room temperature, and the precipitates, polymers [9], [10], and [11] respectively, were collected, washed with dry methanol (100 ml) air dried and each dissolved in the minimum quantity of dry chloroform. Polymers [9], [10], and [11] were converted to PPV by thermal conversion under acidic conditions to form PPV(g), PPV(h) , and PPV(i) respectively.

Characterisation of PPVs (a) ^'to (i)

As previously discussed, an important characterisation of the electronic structure of the polymer is provided by the spectrally-resolved optical absorption and photoluminescence. These are shown respectively in Figures 2 and 3 for the various forms of PPV described above, including the prior art PPVs (a) and (b) . None of the films had been subject to uniaxial stretching.

The absorption spectra for the improved PPVs; PPV(f), PPV(g) , PPV(h) and PPV(i) (uncorrected for reflectivity) show a sharp onset at around 2.4 eV and a peak in absorption a little below 2.5 eV. At higher photon energies the absorption spectra shows a series of subsidiary peaks, or shoulders, and these are assigned to optical transitions that couple to the vibrational quanta for the chain. The luminescence spectra are complementary, showing the peak in emission just below the absorption edge, and subsidiary maxima at lower energies, again spaced by the vibrational quanta for the chain. The peaks in absorption and emission at 2.45 and 2.35 eV are identified respectively, as the transitions between the vibrational ground states of the electronic ground and excited states, termed the (0,0) transitions.

These spectra are very different from those that are obtained for PPV prepared by other methods such as that indicated for the tetrahydrothiophene-eliminating precursor polymer, PPV(a) . These are reported in for example "Photoexcited States in Poly(p-phenylenevinylene) : Comparison with trans, rans-distyrylbenzene, a Model Oligomer", N.F. Colaneri, D.D.C. Bradley, R.H. Friend, P.L. Burn, A.B. Holmes and C.W. Spangler, Phys. Rev. B42., 11671-11681 (1990) and P.L. Burn, A.B. Holmes, D.D.C. Bradley, A.R. Brown and R.H. Friend, Synthetic Metals, 1991, 41-43, 261. The effect of disorder in the polymer is to broaden the absorption and emission features, and also to shift oscillator strength away from the (0,0) transitions, that is, to higher energy for absorption and to lower energy for emission.

The spectra obtained for the improved PPV are readily distinguished from those seen for previous forms of PPV. Polymers with the high level of chain order seen here can be quantitatively differentiated from those with more normal levels of order through the relative strengths of the vibronic side-bands seen in the emission and absorption spectra. Specifically the material made here is identified as having a greater absorption amplitude at the photon energy equal to that of the (0,0) transition than at the energies of any of the other vibronic side-bands.

Evidence for a high level of order may be seen in electron diffraction measurements of free-standing films of the polymer. The films are prepared by spin-coating the precursor polymer onto KBr discs from solution which are then dissolved to leave the free standing film. The powder patterns, of the films converted to PPV, obtained for the random orientation of microcrystallites in the film show sharp diffraction rings, an indication of high order. It is important to note two points in relation to the synthetic process apart from the improved optical spectra. First, the fully conjugated polymer formed in each case is described by the same chemical structure as PPV. A comparison of the infra-red spectra of each of the PPVs shows that the same chemical structural units are present, (Figures 4 - 12) but the relative intensities of the nodes indicate that there are subtle differences in the electronic structure. Second, the relative intensity of the absorption amplitude in the (0,0) transition can be controlled. The precursor polymers were characterised by infra-red, UV/visible, and H n. .r. spectroscopy. The infra-red spectra of the precursor polymers ([4], [9], [10], and [11]), which gave the greatest absorption amplitude for the (0,0) transition in the formed PPV are illustrated in Figures 13 - 16 respectively and the UV/visible spectra of the same are shown in Figure 17. The H n.m.r. gave the percent ratio of the non-conjugated to conjugated sequences and these are respectively 68:32, 64:36, 61:39 and 61:39 for polymers [4], [9], [10], and [113.

The improvements in the electronic structure of PPV can be exploited in a range of applications:

(a) Improved mobility for charge carriers present on the chains of the polymers. These can be introduced by chemical doping with for example oxidising agents such as sulphuric acid and iodine, and reducing agents such as sodium or potassium. Another method for introducing carriers is to form a field-effect device such as a Metal-Insulator-Semiconductor Field Effect Transistor, in which carriers are introduced through formation of space or surface charge layers. Another method is through optical absorption across the energy gap to produce separated electron-hole pairs.

(b) Improved optical transmission below the optical absorption edge, i.e. below 2.4 eV. This is important for many applications which exploit both the linear and non-linear optical properties, and which require low propagation of light through the sample with low loss of intensity.

(c) Improved non-linear optical coefficients. The transfer of oscillator strength in the absorption spectrum to lower photon energies in the improved PPV will increase both linear and non-linear optical polarisabilities of the polymer.

(d) Control of colour of luminescence emission is achieved, and a greater fraction of the output appears in the part of the emission spectrum due to the (0,0) emission. This is useful for applications in electroluminescent devices. These have been described in PCT Patent Application PCT International Patent Application No. W090/13148.

(e) Alignment of films of the PPV through stretch orientation during conversion from the precursor polymer is known to give highly ordered material. See for example "Infra-red Characterisation of Oriented Poly(phenylenevinylene)", D.D.C. Bradley, R.H. Friend, H. Lindenberger and S. Roth, Polymer 27, 1709 (1986). Improved alignment achieved with the improved PPV will give better performance _ in applications which exploit the anisotropy. These include high strength fibres and films, and polarisation-sensitive optical devices.

Claims

CLAIMS :

1. A conjugated polymer which is preparable from a solution-processible precursor polymer and which exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition.

2. A conjugated polymer according to claim 1, which comprises a poly(arylenevinylene) polymer.

3. A conjugated polymer according to claim 2, wherein the poly (arylenevinylene) polymer is a substituted or unsubstituted poly(phenylenevinylene) polymer.

4. Poly(p_-phenylenevinylene) which exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor band-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition.

5. A process for preparing a conjugated polymer according to claim 1, which process comprises providing a leaving group substituted precursor polymer comprising saturated and unsaturated units, the saturated units of which include a leaving group, reacting the leaving group substituted precursor polymer in a solvent comprising a modifier group at a temperature whereby the modifier group substitutes the leaving group, and converting the solution-processible precursor polymer to the conjugated polymer under conditions to eliminate the modifier group, wherein the solvent and temperature are selected such that the conjugated polymer produced exhibits in optical absorption spectroscopy its greatest amplitude of absorption at the photon energy equal to that of the (0,0) electronic transition across the semiconductor energy-gap in comparison to that at the energies of any of the vibronic side-bands of the (0,0) electronic transition.

6. A process according to claim 5, wherein the modifier group is a methoxy group.

7. A process according to claim 6, wherein the temperature is greater than 50 C.

8. A process according to any one of claims 5 to 7 wherein the leaving group substituted precursor polymer comprises a poly(arylenevinylene) polymer in which a proportion of the vinylic groups of the polymer are saturated with the leaving group.

9. A process according to claim 8, wherein the poly(arylenevinylene) polymer is a substituted or unsubstituted poly(phenylenevinylene) polymer.

10. A process according to claim 9, wherein the ρoly(ρhenylenevinylene) polymer is poly(p_-phenylene- vinylene) .

11. A process according to claim 10, which further comprises providing the leaving group substituted precursor polymer by reacting in solution in the presence of base an initial precursor polymer substituted with the leaving group whereby the proportion of unsaturated units in the initial precursor polymer is increased so as to form the leaving group substituted precursor polymer.

12. A process according to claim 11, wherein the base comprises a tetraalkylammonium hydroxide base.

13. A process according to any one of claims 10 to 12, wherein the leaving group comprises a sulphonium salt.

14. A process substantially as hereinbefore described with reference to any one of Examples 1 to 7.

15. A conjugated polymer obtainable by the process of any one of claims 5 to 14.

16. An electroluminescent device incorporating a conjugated polymer according to any one of claims 1 to 4 and 15.