GB2219098A - Optically anisotropic materials and applications - Google Patents

Abstract

An optical device is described having a polarisation state which is externally controllable and comprises a layer of a photochromic material which possesses a polar state in its bleached or coloured form and which has been subjected to molecular orientation. In one typical form of the device, the photochromic layer is disposed between transparent substrates and subjected to an electrical field to cause molecular alignment of the photochromic material.

Description

OPTICALLY ANISOTROPIC PHOTOCHROMIC MATERIALS & APPLICATIONS This invention relates to optical devices having a polarisation state which is externally controllable and which are useful, for example, as switching devices in optical computing and signal processing.

In our co-pending British Patent Application No.8625513 (Publication No. 2197495), birefringent optical devices are described in which photochromic materials are incorporated into a plastics matrix. The devices can be switched between stable states having different refractive indices by increasing or decreasing the incident light intensity. Such devices can be used to perform logic functions, e.g. in optical signal processing and computing.

The present invention is based on the realisation that if certain photochromic compounds are subjected to molecular orientation by alignment in an electric field while in a polar state, a layer of the photochromic material will be optically anisotopic. Furthermore, the molecularly ordered layer can be selectively switched between states in which it is optically isotropic, transmitting light in all planes, and an anisotropic one in which it transmits light polarised in only one orthogonal plane.

According to one aspect of the present invention there is provided an optical device having a polarisation state which is externally controllable and comprises a layer of a photochromic material which possesses a polar state in its bleached or coloured form and which has been subjected to an electric or magnetic field or to mechanical stretching while in such state to cause molecular orientation of said photochromic material.

A photochromic material is characterised by its ability to undergo a reversible change between two different chemical species, each with its characteristic optical absorption spectrum such a change is induced, in at least one direction, by the action of light. In many photochromic systems, absorption of light in the U.V. or blue region of the spectrum results in increased absorption in the visible region (colouring); this coloured state decays (bleaching) gradually in the dark or more rapidly on heating or exposure to visible radiation. Ouantitatively, the change in optical absorption under either colouring or bleaching is proportional to the absorbed optical energy, or proportional to exposure time under conditions of constant illumination. In the latter case, colouring or bleaching proceeds continuously to saturation or complete bleaching respectively.

Certain organic photochromic materials possess planar heterocyclic molecular structures in both coloured and bleached states. The optical absorption is dominated by the presence of carbonyl (-C-O) groups together with a chain of conjugated double bonds, the position of the absorption maximum depending on the length of this chain.

Essentially, the photochromic transition between the two optical states proceeds via photo-induced bondcleavage/ring-closure, with corresponding changes in the length of the conjugated region.

The molecular asymmetry, particularly that associated with the planar structure, suggests that optical properties on the molecular scale will be anisotropic; in particular, if it is assumed that the polarisability of a particular bond is anisotropic, more specifically, exhibiting rotational symmetry parallel to the bond axis and varying with the angle between the bond axi.s and the appli.ed electric field, then the overall molecular polarisability will show a related angular dependence. More generally, the degree of anisotropy of a molecule depends on the properties of the individual bonds and their spatial arrangement, and in a relatively planar structure, may be expected to exhibit a symmetry axis normal to the molecular plane.

This postulated structural behaviour leads to two important characteristics on the molecular scale, theoretically observable in single crystal form: (a) Birefringence The refractive index of the photochromic material exhibits anisotropy, typically with axial symmetry.

(b) Dichroism The optical absorption properties of the material, in both coloured and bleached states, exhibits anisotropy, again generally with axial symmetry.

Previously considered photochromic materials, and optical devices derived from them, have been discussed exclusively in terms of planar layers formed by solvent casting, spin-coating or solvent-assisted in-diffusion (imbibing) techniques, as described in our British Patent Application No.86 25513 (Publication No.2197495). In each case, the resultant photochromic layer is formed of material at relatively low dilution in a suitable polymer matrix. Under such conditions, the photochromic molecules can be considered to be disordered, with molecular axes randomly and uniformly distributed in space. In order to exploit intrinsic optical anisotropies exhibited by these materials, a technique is required for preparation of molecularly ordered layers; since the preparation of single crystals in this context is generally impracticable.

In accordance with this invention, the preparation of highly aligned thin films of photochromic materials may be carried out by various methods described below, depending on the class of compound in question.

Examples of photochromic compounds which exhibit the required optical anisotropic properties are fulgides which possess a dipole in their bleached form. These include fulgides disclosed in British Patents Nos. 1442628; 1464603 and 1602755. Preferred fulgides may be represented by the general formula:

where R3 represents an aromatic group such as phenyl and R1, R2 and R4 represent hydrogen, alkyl, aryl or aralkyl groups or a heterocyclic ring, e.g. a furan, benzofuran, thiophene or benzothi.opene ring. Specific examples are 2, 5-di methyl-3 -furyl-ethyl idene (isopropylidene succinic anhydride and 2,5-dimethyl-3furyl-ethylidene (adamantylide) succinic anhydride.

In practice, many photochromic materials exhibit extinction coefficients in excess of 10,000, with correspondingly low transmission through thin si.ngle crystal plates, where these are avai.lable. In physicochemical terms, the dipole moment associated with the C=O bonds in the fulgides li.es parallel to the c-axi.s, while those relating to the aromatic bonds lie in the ab plane.

Also useful for the production of devices in accordance with the invention are photochromic benzo- or naphthopyrans which undergo thermally induced colouring to a polar form. Such pyran derivatives may be represented, for example, by the general formula (2):

where R and R1 may be independently selected from hydrogen, alkyl, alkoxy, chloro, alkyl or dialkyl amino or hydroxy, and R2 and R3 are preferably alkyl or acryl but may together represent a spiro heterocyclic ring.

Photochromic pyrans and methods for their preparation are disclosed in our copending British patent applications Nos.

86 14680 and 86 11837 (Publication Nos: 2193005 and 2190379). Examples of specific compounds of this kind are shown in formulae (A), (B), (C), & (D) below in which one of R1 and r2 is hydrogen or methyl and the other is phenyl substituted with amino and R3 and R4 are methoxy.

In the case of photochromic compounds which undergo thermal colouring to a polar state, such as the naphthopyrans mentioned above, the compound may be crystallised from a melt while being subjected to an appropriate electric field. A suitable structure for carrying out the molecular alignment during cooling from the melt is shown diagrammatically in the accompanying Figures l(a) and (b). As can be seen from Figure l(a), a continuous metal film may be formed on a pai.r of opposed substrates (e.g. a pair of glass slides) and a perpendicular alignment of the molecules of photochromic compound achieved within the molten photochromic layer by establishing a potential difference between the metal films forming the electrodes.Figure l(b) shows a modified arrangement in which electrode films are deposited at the ends of corresponding glass plates and a potential difference established to encourage alignment parallel to the plates.

Photochromic compounds which exhibit a dipole or thermal colour at 1000C or less, including certain of those mentioned above, may be aligned in a plastics matrix. A doped matrix may be prepared by dissolving the photochromic compound in a suitable polymer solution and solvent casting a thin film between substrates which are furnished with electrodes according to the alignment required. The film is heated to approximately 1000C and allowed to cool under the influence of the electric field.

A particularly suitable range of polymers for photochromic compounds which show high solubility, such as the naph-thopyrans, are polymers of difluoroethylene, trifluoroethylene and vinylidene fluoride.

Apart from polarisation selectivity, the general properties of aligned photochromic materials, e.g.

sensitivity, spectral response and spatial resolution, are typical of the unaligned materials.

BIREFRINGENT PHOTOCHROMIC DEVICE APPLICATIONS Nonlinear device applications associated with the use of materials exhibiting optical birefringence have been described in our British Patent Specification No. 2192071.

In the configurations outlined therein, a family of nonlinear devices exhibiting polarisation selective properties was implemented by incorporating discrete nonlinear and birefringent photochromic elements into a Fabry-Perot etalon. The availability of a birefringent photochromic material permits implementation of polarisation selective nonlinear and related functions (with a single photochromic layer instead of the double element structure hitherto considered). Other devices and functions which could be adapted using the polarisation techniques of this invention are described in our British Patent Specifications Nos: 2180360 & 2197495.

DICHROIC PHOTOCHROMIC DEVICE APPLICATIONS Conventional plastic polarising sheet comprise arrays of dichroic molecules, selectively attached to aligned polymer molecules. In this arrangement light polarised in one plane is absorbed strongly whi.le the orthogonal polarisation suffers relatively little attenuation; the resultant beam is linearly polarised. The present invention provides comparable materials, with the important difference that the selective absorption of one polarisation can be directly controlled by external optical means.

Figure 2 shows an example of an optically controlled polariser based on an aligned photochromic layer in accordance with this invention. In Figure 2(a), the bleached layer is optically isotropic, transmitting all polarisations equally. On colouring by means of external U.V. illumination, the aligned molecules absorb one polarisation selectively, and the resultant transmitted light is linearly polarised in the orthogonal direction.

Bleaching of the layer either via visible illumination or thermally restores the initial, unpolarising state.

Figure 3 shows a modification of the configuration shown in Figure 2 in which the input radiation is polarised in the ultimate photochromic extinction direction. This arrangement functions as an optically controlled switch, since conversion of the photochromic layer into its coloured form causes absorption of the single incident polarisation state.

The two-beam photochromic bistability described in our British Patent Application No.85 19711 (Publication No.2180360), operates via the absorptive properties of the photochromic layer. The provision of an intrinsically polarisation-dependent photochromic layer according to this invention, permits the fabrication of new classes of polarisation-selective devices based on this principle.

The molecules of the photochromic material may be aligned by mechanical stretching of a film containing the photochromic material. Particularly sui.table film materials are poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene (VDF:TrFE copolymers). PVDF is a semi-crystalline polymer having CF2CH2 repeating units. Some of the properties of PVDF and techniques for polarization of PVDF films are described in 'Piezoelectricity in Polyvinylidene Fluoride', G.M. Sessler, J.Acoust.Soc.Am. 70, 1596-1608 (1981). PVDF can exist in a number of crystalline forms which may be interconverted. In particular, drawing the trans-gauchetrans-gauche' material (Form II) yields the all trans planar zigzag Form I which transformation may be utilised to orientate contained molecules of photochromic material.

Form II may also be converted to Form I by drawing.

The conversion is reversible on high temperature annealing.

Form II can be converted to Form IIp and hence to Form I by the influence of electromagnetic fields. These transformations may also be used to effect alignment.

Procedures for producing PVDF homopolymer and copolymer films and effecting molecular orientation of dipoles in the polymers are also described in British Patent No. 1589746.

A VDF:TrFE copolymer (70:30 VDF:TrFE molar ratio) was dissolved in methyl ethyl ketone (M.E.K.) and the photochromic compound having the formula (C) above was added to the polymer solution to form a solution containing approximately 1% by weight of the photochromic compound.

The solution was filtered and cast onto a glass plate onto which an aluminium electrode had previously been deposited by vacuum deposition. After drying at room temperature for 15 minutes and then in an oven for 1 hour at 600C, an aluminium electrode was evaporated onto the exposed surface of the film. The film waspoled by applyi.ng an electrical field of about 20 MVm'l while maintained at a temperature of 1000C to cause simultaneous alignment and colouring of the photochromic compound. In the coloured, ali.gned state the film is isotropic, but is reversible converted to an ani.sotropi.c state by illumination with visible light.

Although the particular means for effecting alignment of the molecules of the photochromic material mentioned in the foregoing description has been restricted to electric or magnetic fields, or mechanical stretching of a film containing the photochromic material, other methods are available for effecting such alignment.

For example, the photochromic material may be incorporated into a host polymer (e.g.by imbibition, as described in British Patent Specification No. 2197495, using as host polymers the fluorine containing polymers mentioned above or polymers described in British Patent Specification No. 2197495), in which the molecules are prealigned (e.g. by stretching), and the molecules of the photochromic material becoming orientated by proximity to aligned dipoles in the host polymer. Further alignment may be achieved by stretching of the doped host polymer or by use of an electric field.

Another possible method of achieving alignment of the photochromic material is by crystallisation of the material from a suitable solvent.

Claims (9)

1. An optical device having a polarisation state which is externally controllable and comprises a layer of a photochromic material which possesses a polar state in its bleached or coloured form and which has been subjected to molecular orientation.

2. An optical device according to claim 1 wherein the photochromic material has been subjected to an electric field or to mechanical stretching sufficient to cause alignment of molecules of the material.

3. A device according to claim 1 or 2, in which the polarisation state is controllable by illuminating the photochromic layer with a light beam.

4. A device according to any one of claims 1 to 3, i.n which the photochromic compound is a fulgide which possesses a dipole in its bleached form.

5. A device according to any one of claims 1 to 3, in which the photochromic compound is a photochromic benzoor naphthopyran which undergoes thermal colouring to a polar state.

6. A device according to any one of the preceding claims, which is in the form of an optically controlled switch and includes means for projecting a light beam onto the photochromic layer (said layer transmitting said light beam while the photochromic material is in its bleached condition), and means for selectively illuminating the layer with U.V. light so as to convert the layer reversibly to its coloured state in which it absorbs light polarised in one plane.

7. A device according to claim 6 i.n which the projected light beam is polarised in a first plane and the photochromic layer absorbs light in said first plane when converted to its coloured state.

8. A device according to any one of the preceding claims in which the photochromic material is dispersed in a light-transmitting polymer layer.

9. A device according to claim 8 in which the polymer is polyvinylidene fluoride or a copolymer of vinylidene fluoride with di.- or trichloroethylene.