GB2204053A - Azo and stilbene compounds - Google Patents

Abstract

A compound of the following structural formula: Where, Y= -N= or -C= (trans> X= -R, -OR, -NO2, -CnN, F, Cl, Br or I R= CmH2m+1- m= 0 to 30 Ar= and A & B are selected from the following two groups, one from each group, Group 1 R-, RO-, R2N- (same or different R), RS- or -CONR2 (same or different R) Group 2 -NO2, -CN, -SO3H, -CO2R, -OCOR, -COR or -R (except when R- selected from Group 1) The compounds have high nonlinear optical coefficients, and can be used for the construction of devices such as planar or channel waveguides when supported as a Langmuir-Blodgett film on a substrate body.

Description

AZO COMPOUND This invention relates to an azo compound which has been found to be capable of use as an optical material. This can lead to the construction of an optical device, such as a planar or channel waveguide structure.

Organic materials are attractive for nonlinear optical devices due to their potentially large, fast-acting second and third order optical nonlinearities. For second-order nonlinear effects, such as frequency doubling and parametric amplification, ordered material structures are required. Additional benefits may be derived when the material can be formed as a thin film that can be used as an optical waveguide, in which high optical intensities can be achieved and maintained over long interaction lengths. Recent techniques for fabricating organic thin film structures have included single crystal growth and indiffusion. Langmuir-Blodgett (LB) multilayer deposition is an attractive alternative technique, enabling the fabrication of well controlled, highly ordered thin films, suitable for integrated waveguide and thin film optical devices.However, certain molecular design criteria have to be met to exploit this approach successfully. The molecular structure of most of the commonly reported nonlinear organic materials are unsuitable for deposition by the LB technique and molecular modifications are therefore necessary before they can be used. The present invention was devised to provide a new class of organic molecule, some examples of which can allow the fabrication of Idngmuir-Blodgett films.

Not all the examples of the azo compound of the invention are capable of being deposited by the LB technique and in this case a thin film structure could be formed by an alternative process such as single crystal growth or indiffusion.

According to the present invention, there is provided an azo compound of the following general structural formula:

Wh ere, Y= -N= or < = (trans) X= -R, -OR, -NO2, -cN, F, CL Br or I R= CmH2m+1 m= O to 30

and A & B are selected from the following two groups, one from each group, Group 1 R-, RO-, R2N- (same or different R), RS or - CON'R2 (same or different R) Group 2 -N02, CN, -S03H, -C02R, OCOR, -COR or -R (except when R- selected from Group 1) Some particular embodiments falling within the scope of this formula are as follows::

where,

x x W= X 0 (1, 2, 4 trisubstituted naphthalene) A = C14H29 -, X= OH, B= -H (Compound 1) A = C14H29-, X= -H, B -OH (Compound 2) A = C14H29-, X= -H, B= -NH2 (Compound 3) A = -NT02 X= -H, B= C14H29NH- (Compound 4) A = -CO2H, X= -H, B= C14H29NH- (Compound 5) A = -CN, X= -H, B= C14H29NH- (Compound 6) A = -CN, X= -H, B= C7H15NH- (Compound 7) or where,

(1, 2, 4 - trisubstituted benzene)

(Compound 8) A = C14 H29 NH- X= -H, B= -CN (Compound 9) It has been found that the compound of this general formula has a high nonlinear optical (NLO) coefficient which makes it particularly suitable for the construction of an optical element, such as an optical waveguide structure.

According to a feature of the invention, there is provided an optical element having non-linear optical properties comprising a substrate body supporting a layer of an organic material having the composition of any one of Compounds 1 to 9.

Preferably, the organic material is deposited on the substrate body by a technique which gives a high degree of control of the resulting film order and thickness. A deposition in the form of a multilayer coating may be used. One technique which has been found to be suitable is a dipping bath which enables a Langmuir Blodgett type of deposition to be effected. This allows the successive deposition of organised organic monolayers, the molecules of which have high nonlinear optical coefficients. Other possible techniques include crystal growth and a chemical vapour deposition process.

The invention further comprises a method for the preparation of an optical element having nonlinear optical properties, which comprises the step of passing a' surface of a suitable substrate body into and out of a Langmuir trough containing a liquid carrying a superficial monomolecular layer of a material having the composition of any one of Compounds 1 to 9.

The invention further comprises an optical device, in which the optical element comprises multiple layers of a Langmuir-Blodgett deposited film having the composition of any one of Compounds 1 to 9, and which forms a region of high refractive index, which is supported on a substrate body formed of a lower refractive index material. The resulting device which provides definition of a channel of high refractive index material will result in a two-dimensional confinement of the light which makes possible the construction of a range of planar waveguide structures. By using a photolithographic definition process, a channel waveguide structure can be produced which can have a width of a few microns.

Planar and channel waveguide structures provide a high degree of optical confinement which can be maintained over long lengths. This makes them very suitable for producing electro-optic and all-optical switching and signal processing devices with a low electrical or optical power consumption.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 shows the chemical route used for synthesising Compounds 1, 2 and 3; Figure 2 shows the route used for synthesising Compounds 4 to 9; Figure 3 is a sketch of a planar waveguide structure including a Langmuir-Blodgett deposited film of the invention; and, Figure 4 shows a channel waveguide structure including a similar film.

The evaluation of the azo compounds and optical elements of the invention begins with the synthesis of the Compounds 1 to 9 which are derived from the aforementioned general structural formula.

The preparations of the compounds will frrst be described with reference to the chemical routes depicted in Figure 1 (for Compounds 1, 2 and 3) and Figure 2 (Compounds 4 to 9) of the drawing.

COMPOUND 1 A mixture of 4-tetradecylaniline (2.84g, 0.01 mol) in water (20 ml) was stirred at a temperature of 50 C until the amine had melted.

The hydrochloride was precipitated by addition of concentrated hydrochloric acid (3 ml, 0.03 mol). The mixture was cooled to 10 C and treated dropwise over a period of ten minutes with a solution of sodium nitrite (0.7g, 0.01 mol) in water (5 ml). Upon raising the temperature to 20-25 C a clear solution of the diazonium salt was obtained.

Naphth-2-ol (0.01 mol) was dissolved in a solution of sodium hydroxide (2g) in water (40 ml), and treated at 0-5 C with the previously prepared aqueous diazonium salt. After being stirred overnight the mixture was neutralized and this caused precipitation of the product which was filtered off and dried. The crude dye was crystallised three times from ethanol, giving Compound 1 as orange needles (yielding 2.62g, 59%), melting point 87-90C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3440 cm-1 (br;OH).

In the accompanying descriptions, melting point values are uncorrected. All dyes were found to exhibit satisfactory purity as determined by HPLC.

COMPOUND 2 The diazonium salt was prepared as for Compound 1 above.

Naphth-l-ol (0.01 mol) was dissolved in ethanol and added over a period of five minutes to the aqueous diazonium salt at 20 25OC and the mixture was stirred for four hours. The isolated crude dye was treated with chloroform and filtered off. Crystallisation of the filtrand three times from toluene gave pure Compound 2, (yielding 2.10g, 47%), melting point 158-600C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3400 cm-1 (br;OH).

COMPOUND3 The diazonium salt was prepared as for Compound 1 above.

The coupling reaction was carried out as for Compound 1 above, but using l-naphthylamine (0.01 mol) and at a temperature of 10-15 C. The crude dye was subjected to flash chromatography (silica, applying the sample in warm toluene and eluting with toluene). The resulting red oil was crystallised twice from ethanol giving Compound 3 as an orange powder (yielding 2.5g, 56tic), melting point 77-9OC; maximum frequency of light absorption (when supported between plates of potassium chloride) 3460 and 3370cm-1 (NH2 bond).

COMPOUND 4 A quantity of 4-nitroaniline was diazotised by standard procedure as described in H.E. Fiery-David and L. Blangen, 'Fundamental Processes of Dye Chemistry, Interscience, London, 5th Edn., 1949.

A mixture of l-naphthylamine (14.3g, 0.1 mol), tetradecyl bromide (27.7g, 0.1 mol), sodium carbonate (11 g, 0.1 mol) and hexamethylphosphoramide (HMPA; 200ml) was stirred at 100oC for twentyfour hours and then poured into water (400 ml), with stirring. The resulting 1 -(N-tetradecyl)naphthy lamine was filtered off, washed with water, dried, and crystallised twice from ethanol yielding a white crystalline solid (17.4g, 51%), melting point 52-40C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3400 cm-l(NH bond).

The l-(N-tetradecyl)naphthylamine (0.01 mol) was added to a stirred solution of acetic acid (10 ml) and sodium acetate (5g) in water (10 ml), and treated at 0-5 C with the aqueous diazonium salt.

After being stirred overnight, the mixture was neutralized and the product filtered off and dried. The crude dye was washed with (50:50) chloroform/light petroleum (b.p. 60-80 C, in quantities of 3 x 50 ml) giving an almost pure residue, which was crystallised twice from toluene to give Compound 4 as shiny green leaflets (yielding 2.8g. 58%), melting point 146-8 C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3430 cm-1(NH bond).

COMPOUND 5 A quantity of 4-aminobenzoic acid was diazotised by standard procedure as used for Compound 3 and converted to the diazonium tetrafluoroborate.

The coupling reaction (using the 4-aminobenzoic acid diazonium salt and 1-(N-tetradecyl)naphthylamine) was carried out at pH 8-9 (using potassium hydroxide), then adjusted to pH 6-7 and the product filtered off. The crude dye was washed with light petroleum (b.p. 60-80 C; in quantities of 2 x 50 ml) to remove unreacted coupler, then taken up in boiling chloroform (800 ml), filtering off any insoluble material (this process was repeated on the filtrand). The combined filtrates were evaporated to dryness and cr-stallised three times from toluene, to give Compound 5 as dark red feathery needles (yielding 0.97g, 20%), melting point 174-60C: maximum frequency of light absorption (when supported between plates of potassium chloride) 3440(NH bond) and 1685 cm-1 (carboxylic C = 0 bond).

COMPOUND 6 A quantity of 4-aminobenzonitrile was diazotised by standard procedure as used for Compound 3.

The crude dye (coupled as for Compound 4 above using the 4aminobenzonitrile diazonium salt and l-(N-tetradecyl) naphthylamine was washed with light petroleum (boiling point 60 80oC; in quantities of 2 x 30 ml) and crystallised twice from toluene, furnishing Compound 5 as a dark red crystalline powder (yielding 2.9g, 62%), melting point 141-3 C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3410(NH bond) and 2230 cm-l (C N bond).

COMPOUND 7 An efficiently stirred mixture of l-naphthylamine (64.3a, 0.45 mol) and heptyl bromide (26.9g, 0.15 mol) was heated on a boiling water bath for three hours. The resulting solid mass was added to 10% aq. sodium hydroxide (150 ml) and the l-(N-heptyl) naphthylamine extracted into chloroform, dried (MgSO4) and obtained as an oil, boiling point 161-50C/0.25mm Hg (yielding 25.5g, 71%); maximum frequency of light absorption (when supported between plates of potassium chloride) 3440cm-1 (NH bond).

A quantity of 4-aminobenzonitrile was diazotised as for Compound 6.

The crude dye (coupled as for Compound 4 using the 4aminobenzonitrile diazonium salt and l-(N-heptyl)naphthylamine) was crystallised from ethanol, then subjected to flash chromatography (silica, eluting with a mixture of (50:50) chloroformtoluene). Crystallisation of the pure fractions from toluene yielded Compound 7 as shiny green blocks (yielding 2.1 it, 57%), melting point 1446or; maximum frequency of light absorption (when supported between plates of potassium chloride) 3410 (NH bond) and 2230cm-1 (CN bond).

COMPOUND 8 A quantity of 4-aminobenzoic acid was converted to the diazonium tetrafluoroborate as for Compound 5 above.

N-Decyl-N-methyl-m-toluidine was prepared from N-methylm-toluidine (by the method proposed in R.M. Roberts and P.J. Vogt, Org. Synth., Coll. vol. IV. 420, 1963) and decyl bromide using standard procedure (as disclosed in A.I. Vogel, 'Textbook of Practical Organic Chemistry, Longman, London, vol. 4, p 657, 1978) in 572 yield.

N-Decyl-N-methyl-m-toluidine (4.20g, 0.01 mol), followed by sodium acetate (1.35g) in water (5 ml), was added to a solution of 4carboxybenzene diazonium tetrafluoroborate (2.36g, 0.01 mol) in water (20 ml) and ethanol (5 ml) at 3-50C. The mixture was stirred overnight, poured into water and extracted into chloroform (in quantities of 2 x 200 ml). The isolated crude dye was subjected to flash chromatography (silica, eluting with a mixture of (4:1) chloroform-methanol) giving Compound 8 as orange prisms from ethanol (yielding 3.18g, 78%), melting point 1920C (dec.).

COMPOUND 9 A quantity of 4-aminobenzonitrile was diazotised as for Compound 6.

An efficiently stirred mixture of aniline (41.8g, 0.45 mol) and tetradecyl bromide (41 .5g, 0.15 mol) was heated on a boiling water bath for four hours, then added to 10% aq. sodium hydroxide (150 ml). Chloroform (200 ml) was added and the organic layer was separated and dried (MgSO4). Evaporation of the solvent gave an oil which was distilled, yielding three fractions: (i) starting materials, boiling point 90 C/0.2mm Hg, (ii) N-tetradecylaniline, boiling point 180-840C/0.2mm Hg, (29.lg, 67%) and (iii) N, N-ditetradecylaniline (by 'H-nmr and ms), boiling point about 2500C/0.2mm Hg, (l.9g, 4%).

The N-tetradecylaniline had melting point 41-30C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3380cm-1 (NH bond).

Coupling of the 4-aminobenzonitrile diazonium salt and Ntetradecylaniline in buffered acid failed in this case. The coupling component was dissolved in ethanol at 400C and treated with the aqueous diazonium salt, raising the temperature gradually up to 780C to keep the coupler in solution. After ten minutes the mixture was cooled and the resulting crystalline produce filtered off. Three crystallisations from ethanol yielded Component 9 as red leaflets (yielding 2.49g, 60%), melting point 115-70C; maximum frequency of light absorption (when supported between plates of potassium chloride) 3380 (NH bond) and 2220cm-1 (CN bond).

Following synthesis and purification of each compound, a solution of known concentration of the material under test is made up using a suitable solvent, for example chloroform.

By use of the Langmuir Trough unit it is possible to form a monomolecular layer on a substrate base and this operation is capable of being carried out under accurately reproducible conditions. The organic material to form the monomolecular layer in a suitable solution is first deposited on the surface of a suitable liquid subphase (usually highly purified water), the temperature and pH of which have been correctly optimised. The liquid subphase is contained in the trough of the Langmuir unit and the accurately measured amount of organic material solution is deposited within part of the subphase surface that is capable of being confined within a movable surface barrier which defines a working area on the liquid surface. Upon evaporation of the solvent, individual molecules of the organic material are left floating on the subphase surface.By suitably moving the surface barrier, the working surface area may be compressed and the molecules can be made to form a quasi-solid layer one molecule in thickness.

The stability of the test monolayer is capable of being monitored by measuring the change in surface area with time at a preset surface pressure under given subphase conditions.

To form a multilayer of the material of the test monolayer requires the use of a dipping mechanism which is part of the Langmuir unit and which is normally housed on a gantry located above the trough. The mechanism includes a screw thread drive arrangement to which an appropriate body of a selected substrate material can be attached. Examples of common substrates are silicon wafers (n- type or p- type), glass or evaporated metals, the surfaces of which have been chemically treated to ensure cleanliness and the presence of a hydrophilic or hydrophobic surface condition. The substrate is lowered and raised through the subphase-monolayer interface such that a transfer of the monolayer material from the subphase to the substrate surface takes place. By repeating this cycle, a multilayer deposit of the organic material can be built up upon the substrate body.

In this way, each of the Compounds 1 to 9 was examined for its film forming and its optical properties.

The results were as follows: Compound 1 Spontaneous collapse of the monolayer occurred under various subphase liquid conditions, even at relatively low surface pressures.

A second harmonic generation (SHG) effect was observed from the powder at an intensity of approximately ten percent relative to lithium niobate.

Compound 2 The monolayer material was relatively stable. Second harmonic generation was observed from the monolayer deposited onto a glass substrate at an intensity approximately three times greater than that observed from a silver coated substrate. It has not yet been established that the film is truly monomolecular and there may be some doubt as to the nature of the monolayer material, which appears as a pink coloured film. The second harmonic generation effect was observed from the powder at an intensity of slightly less than ten percent relative to that of lithium niobate.

Compound 3 The monolayer material was found to be unstable under standard conditions, but at a low pH value and with sulphate ions incorporated into the subphase liquid composition, the monolayer was highly stable. Under these conditions, protonation of the molecule was found to take place. No second harmonic generation effect was observed from a monolayer deposited onto either a hydrophobic or a hydrophilic glass substrate, or from a powder sample.

Compound 4 A wide range of subphase liquid conditions were used to try to produce a stable monolayer, but in each case spontaneous film collapse resulted. No second harmonic generation effect was observed from the powder.

Compound 5 The monolayer material was most stable at neutral pH values with either sulphate or cadmium ions incorporated into the subphase liquid composition. At the extremes of pH the monolayer material was found to be destabilised, possibly by increased dissolution into the subphase liquid as a result of film ionisation. Monolayers were deposited onto hydrophobic and hydrophilic glass substrates, but no second harmonic generation effect could be detected. No second harmonic generation was observed from a powder sample.

Compound 6 A powder sample of this material exhibited the highest second harmonic generation intensity value for this family of materials, namely 3.3 times greater relative to that of lithium niobate. Under normal or high subphase liquid pH conditions rapid collapse of the monolayer occurs, but at low subphase liquid pH values (when protonation of the molecule will occur) a relatively stable monolayer was produced. The corresponding isotherm had some unusual features but it appeared to be reproducible. A monolayer deposited onto either a hydrophobic or a hydrophilic glass substrate exhibited a second harmonic generation effect with an intensity approximately five times greater than that observed from a silver coated substrate body.

Compound 7 The monolayer material at neutral and low pH (pH = 2.0) values exhibits low stability. It is not possible to achieve a sufficient surface pressure to enable detailed studies and film deposition to be carried out. The heptyl chain destabilises the system relative to the tetradecyl homologue (Compound 6).

Compound 8 Assesment of this compound was covered over a subphase pH value range of 1.8 to 10.4 with added ions (S042- or Cd2+) as appropriate. At the extremes of pH the monolayer is highly unstable, and even at an optimum intermediate pH value the monolayer is not fully stable. Consequently, it was not found possible to form multilayers of this material, and even monolayer formation results in a large error in calculation of the amount of material removed from the subphase surface.

However, monolayer deposition was attempted using either a glass hydrophobic or hydrophilic substrate. In each case, levels of the second harmonic generation effect were observed from the deposited monolayer comparable to that produced by a hemicyanine dye for which a second order molecular polarisability, beta = (95 plus or minus 37) x 10-50 C3 M3 J-2. See I.R. Girling, N.A. Code, P.V.

Kolinskys J.D. Earls, G.H. Cross and I.R. Peterson, Thin Solid Films. 132.

101, (1985).

In addition, the level of SHO was found to be dependent upon the square of the input intensity as would be theoretically expLd.

Compound 9 No monolayer is formed either under neutral or acidic subphase conditions. Compared with Compound 6, the effects of replacing the naphthalene ring system by the benzenoid system has had a serious destabilising effect on the monolayer.

Thus, not- all of the Compounds 1 to 9 just described are suitable for being deposited by the Langmuir-Blodgett technique.

Compounds 1, 4, 7 and 9 exhibit poor or non-existant LB properties.

The substrate body carrying a mono or multilayer deposit of the organic compound according to the invention formed an optical element having non-linear optical properties. The element can be used to construct various optical devices as depicted in Figures 3 and 4.

Figure 3 shows a planar waveguide formed from a substrate body 1 which supports a Langmuir-Blodgett film 2 multilayer of one of the aforementioned Compounds 1 to 9. The substrate 1 had a lower refractive index than that of the film 2, and the provision of a high refractive index region bounded by a lower refractive index region thus gave the necessary confinement to provide the required waveguide effect.

A typical thickness for the material of the film 2 for use in guiding visible and near infrared light was of the order of one micron.

From the planar waveguide construction, the definition of a narrow channel in the high refractive index material will result in a two-dimensional confinement of the light and make possible the construction of a range of channel waveguide structures. One such construction is shown in Figure 4 where the substrate body 1 supports a Langmuir-Blodgett deposited film 2 multilayer which by photolithographic definition has been cut away to leave a width of only a few microns.

The constructions of the planar and channel waveguide structures which have just been described can lead to other electrooptic and all-optical switching and signal processing devices which make use of the organic compounds of the invention.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims.

For instance, it is not essential that the azo compound of the invention should be deposited by the dipping bath technique that has been specifically described. In a different embodiment the technique of film assembly might use processes such as crystal growth, chemical vapour deposition etc.

Before the deposit of azo compound is placed on the substrate body, the body might be given an initial coating such as one of silicon nitride or to form a reflecting surface on the body. The layers of azo compound on the substrate body also might be provided with intervening layers of different substances.

Claims (7)

1. An azo compound of the following general structural formula:

Where, Y= -N= or -C= (trans) X= -R, OR, -NO2, -CN, F, Cl, Br or I R= CmH2m+ m= O to 30

and A & B are selected from the following two groups, one from each group, Group 1 R-, RO-, R2N- (same or different R), RS or - CONR2 (same or different R) Group

2 -N02, CN, -SO3H, -C02R, -OCOR, -COR or -R (except when R- selected from Group 1) 2.An azo compound as claimed in Claim 1 having the following general structural formula:

where,

x x S= X 0 (1, 2, 4 tri substituted naphthalene) A = C14H29 -, X= -OH, B= -H (Compound 1) A = C14H29-, X= -H, B -OH (Compound 2) A = C14H29-, X= -H, B= .-NH2 (Compound 3) A = -NO2 X= -H, B= Cl4H2gX=H- (Compound 4) A = -CO2H, X= -H, B= C14H29NH- (Compound 5) A = -CN, X= -H, B= C14H,N'H- (Compound 6) A = -CN, X= -H, B= C7H1SSE- (Compound 7) or where,

(1, 2, 4 - trisubstituted benzene)

(Compound 8) A = C14 H29 NH- X= -H, B= -CN (Compound 9)

3. An optical element having non-linear optical properties, the element comprising a substrate body supporting a layer of an azo compound having the composition of any one of the compounds claimed in Claim 1 or Claim 2.

4. An optical element as claimed in Claim 3, in which the substrate body supports multiple layers of said azo compound, the layers constituting a film of high refractive index material.

5. An optical element as claimed in Claim 3 or 4, in which the azo compound layer is formed by deposition from a liquid subphase surface.

6. An optical device, such as a planar or channel waveguide structure, including an optical element as claimed in any one of Claims 3 to 5.

7. An optical device, substantially as hereinbefore described with reference to the accompanying drawing.