EP0642678A1

EP0642678A1 - Optically non-linear materials

Info

Publication number: EP0642678A1
Application number: EP93910286A
Authority: EP
Inventors: Geoffrey Joseph Ashwell
Original assignee: BTG International Ltd; British Technology Group Ltd
Current assignee: BTG International Ltd
Priority date: 1992-05-27
Filing date: 1993-05-26
Publication date: 1995-03-15
Also published as: GB2267288A; GB9310843D0; GB9211238D0; JPH07507338A; WO1993024859A1; RU94046100A; GB2267288B

Abstract

The invention relates to novel optically non-linear materials comprising a film forming hemicyanine dye having an amphiphilic anion.

Description

OPTICALLY NON-LINEAR MATERIALS The present invention relates to novel optically non-linear materials, to methods for their synthesis and to optical devices which incorporate such materials.

Film-forming hemicyanines dyes of general formula D-C6H4-CH=CH-C5H4N⁺-R X where D is an electron donating group, R is an al yl group and X~ is a simple inorganic anion such as a halide ion have been extensively studied for second harmonic generation (SHG). Their properties are dependent in part upon the extent of aggregation within the Langmuir-Blodgett film structures and in part upon the position of the anion. Girling e al_. (Thin Solid Films 132, 101 1985) and Schildkraut et al_. (Opt. Lett 13, 134, 1988) have disclosed that the SHG may be enhanced by mixing the dye with a fatty acid. The dilution of the chromophores suppresses the tendency of molecules to aggregate, but as a disadvantage there may be phase separation of the components of the mixed film. We have now discovered that Langmui -Blodgett film structures which generate a signal of greater intensity may be produced by utilising an amphiphilic anion rather than a simple inorganic anion. Film forming hemicyanine dyes comprising an amphiphilic anion are believed to be novel and constitute one aspect of this invention. Preferred anions are a phiphiles of the formula R-Z~ or R-ιr-Z^_ wherein R represents a hydrophobic group comprising from 8 to 30 carbon atoms, -a represents a conjugated or aromatic bridging group and Z represents a negatively charged group such as a carboxylate, sulphate or sulphonate. Preferred examples of bridging group π are -CH=CH-, -CH=N-, -N=N, -Ar- wherein Ar represents an aromatic ring especially a benzene ring or a pyridine ring. Amongst ne compounds which are potentially useful as amphiphilic an jns are anionic surfactants especially the monobasic anionic surfactants such as soaps, alkylbenzene sulphonates, olefin sulphonates, alcohol sulphates and alkoxylated alcohol sulphates, alkoxylated alkylphenol sulphates, sulphated acids, amides and esters and sulphated natural oils and fats. The preferred amphiphiles for use in the present invention are soaps, alkyl benzene sulphonates, alcohol sulphates and ethoxylated alcohol sulphates.

The materials of this invention may comprise any amphiphilic cationic dye of general formula D-ιr-A⁺X~ wherein D represents an electron donating group; ir represents a conjugated bridging group; A⁺ represents an electron accepting group and X^" represents the amphiphilic anion.

One preferred grouping of cationic dyes are compounds having the formula R-A⁺-ιr-D wherein R represents a hydrophobic alkyl or alkenyl group comprising 8 to 30 carbon atoms; A⁺ represents a pyridinium, pyrimidinium, quinoliniu , isoquinoliniu , benzothiazolium or thiazolium cation; the ir-bridge is -CH=CH-, -CH=N-, -N=N-, -Ar- or multiples or combinations thereof, wherein Ar represents an aromatic ring especially a benzene or pyridine ring; D represents an amino, N-alkylamino (R'HN-), N,N-dialkylamino (R'R"N-), alkoxy (R'O-), carboxylate (R'C(O)O) or alkylthio (R'S-) group wherein R' and R" represent an aliphatic, alkyl or alkenyl group comprising from 1 to 8 carbon atoms, an alkyl group substituted by an aromatic group, or an aromatic group. A second preferred group of cationic dyes are those having the formula R-Y-ιr-A⁺ wherein R represents a hydrophobic alkyl or alkenyl group, comprising 8 to 30 carbon atoms; Y represents -0-, -S-, -NH- or -NR'- wherein R' is an alkyl group comprising from 1 to 4 carbon atoms; the ir-bridge is -CH=CH-, -CH=N-, -N=N-, -Ar-, wherein Ar represents an aromatic ring especially a benzene or pyridine ring; A⁺ represents a pyridinium, pyrimidinium, quinolinium, isoquinolinium, benzothiazolium or thiazolium cation. The novel compounds of this invention may conveniently be prepared by a metathesis reaction between a simple inorganic salt of the cation, e.g. the halide and especially the iodide or bromide and a metallic salt of the amphiphilic anion, e.g. the alkali metal and especially the sodium salt. The product may be purified or may be used directly in the production of an optically non-linear Langmuir-Blodgett film. Alternatively the reactants can be mixed during the formation of the film. The inorganic salt produced by the metathesis reaction migrates into the aqueous subphase and the properties of the resulting film are not significantly affected. The novel dyes may be formed into optically non-linear films using conventional techniques. Conveniently they may be spread as a solution in dilute ethanol/chloroform onto the pure water subphase of a Nima Technology Trough and transferred onto a hydrophilically treated glass substrate. Langmuir-Blodgett (LB) films which comprise a monolayer of an amphiphilic cationic dye and an amphiphilic anion are believed to be novel and constitute a further aspect of the invention. Non-centrosymmetric multi-layered films may be produced by alternate layer deposition in which every alternate layer comprises a spacer molecule.

An example of a suitable spacer molecule is 4,4'-dioctadecyl- 3,5,3' ,5'-tetramethyldipyrrylmethene hydrobromide.

The invention is illustrated by the following examples. EXAMPLE 1

The dye, E-N-octadecyl-4-(2-(4-dimethylaminophenyl)ethenyl)- pyridinium octadecylsulphate, obtained by metathesis of E-N-octadeyl-4-(2-(4-dimethylaminophenyl)ethenyl)pyridinium iodide and sodium octadecylsulphate during the spreading process, was spread from a dilute methanol/chloroform solution onto the pure water subphase of a Nima Technology trough, compressed to 30mN/m and transferred to a hydrophilically treated glass slide on the upstroke at a rate of 5mm/min.

SHG measurements were carried out using the apparatus schematically shown in Figure 1 in which 1 represents a Nd:YAG laser, 2 is an attenuator, 3 is an infrared mirror, 4 is a beam splitter, 5 are visible blocking filters, 6 is a Y-cut quartz plate reference, 7 is an infrared blocking filter, 8 is a 532 nm narrow bandpass filter, 9 is a photomultiplier tube, 10 is an infrared mirror, 11 is a Glan Thompson polariser, 12 is a halfwave plate, 13 is a lens, 14 are visible blocking filters, 15 is the Langmuir-Blodgett film, 16 is an infrared blocking filter, 17 is a lens, 18 is a 532 nm narrow bandpass filter and 19 is a photomultiplier tube. Films were irradiated with light from a Q-switched NdrYAG laser (1064nm; pulse width 10 ns; repetition rate 2 Hz) using either s or p polarised light beam incident to the substrate at 45°. SHG from a Y-cut quartz plate reference and LB film were monitored simultaneously using fast rise-time photomultiplier tubes (Philips XP 2020) and a dual channel Hewlett-Packard 541110 digitising oscilloscope. Uniformity of deposition was assessed by using computer-controller translation and vertical stages to map the SH intensity from the LB film. The second order susceptibility, X^^ZZZ» i^s greater than or equal to 500 pm/V. EXAMPLE 2

The following dyes were also prepared:

(1) E-N-octadecyl-4-(2-(4-diethylaminophenyl)ethenyl)pyridinium iodide

(2) E-N-octadecyl-4-(2-(4-methoxyphenyl)ethenyl)pyridinium iodide

(3) E-N-octadecyl-4-(2-(4-phenoxyphenyl)ethenyl)pyridinium iodide

(4) E-N-octadecyl-4-(2-(4-benzyloxyphenyl)ethenyl)pyridinium iodide

(5) E-N-octadecyl-4-(2-(4-acetoxyphenyl)ethenyl)pyridinium iodide The dyes were synthesized by the reaction of N-octadecyl-4- methylpyridinium iodide, the appropriate para-substituted benzaldehyde, e.g. 4-methoxybenzaldehyde for (2), and piperidine in hot methanol.

Each of these dyes together with sodium octadecylsulphate were assessed for SHG measurements as described in Example 1. In each case the SHG from films of the octadecylsulphate salt is higher than from films of the corresponding iodide. EXAMPLE 3

Non-centrosymmetric film structures of the dye, E-N-octadecyl-4-(2-(4-dimethylaminophenyl)ethenyl)pyridinium octadecylsulphate, and the spacer, 4,4'-dioctadecyl-3,5,3' ,5'- tetramethyldipyrrylmethene hydrobromide, were obtained by alternate-layer deposition. The dye was spread from dilute methanol/chloroform solution onto the pure water subphase of compartment A of a Nima Technology Langmuir-Blodgett trough, and the spacer from dilute chloroform solution onto the pure water subphase of compartment B. Both were compressed to 30 mN/m before being transferred to a hydrophilically treated glass slide at 5 mm/min. Non-centrosymmetric structures were obtained by the alternate-layer deposition of the dye (on the upstroke) and the spacer (on the downstroke) on slides initially coated with a bilayer of spacer. The SHG from such alternate-layer films increases quadratically with the number of bilayers.

Claims

1. A film forming hemicyanine dye comprising an amphiphilic anion.

2. A film forming hemicyanine dye which is obtainable by the metathesis of a halide salt of a hemicyanine cation and an alkali metal salt of an amphiphilic anion.

3. A dye according to either of Claims 1 or 2 characterised in that the anion has the formula.

R-Z^" wherein R represents an alkyl or alkenyl group comprising from 8 to 30 carbon atoms and Z represents a carboxylate, sulphate or sulphonate anion.

4. A dye according to Claim 3 characterised in that the anion is an octadecylsulphate anion.

5. A dye according to either of Claims 1 or 2 characterised in that the anion has the general formula

R-*-Z- wherein R represents an alkyl or alkenyl group comprising from 8 to 30 carbon atoms, Z represents a carboxylate, sulphate or sulphonate anion and v represents a conjugated bridging group.

6. A dye according to Claim 5 characterised in that v is selected from the group comprising -CH=CH-, -CH=N-, -N=N- or Ar wherein Ar represents an aromatic group.

7. A dye according to Claim 6 characterised in that Ar represents a benzene or a pyridine ring.

8. A dye according to any of the preceeding claims characterised in that the cation has the formula

D-ιr-A⁺ wherein D is an electron donating group, w is a conjugated bridging group and A⁺ is an electron accepting group.

9. A dye according to Claim 8 characterised in that the cation has the formula

R-A⁺-ιr-D wherein R represents an alkyl or alkenyl group comprising from 8 to 30 carbon atoms; A⁺ represents a pyridinium, pyrimidinium, quinolinium, isoquinolinium, benzothiazolium or thiazolium cation; v represents a group selected from -CH=CH-, -CH=N-, -N=N- or - Ar- or multiples or combinations thereof, wherein Ar represents an aromatic group and D represents an amino, an alkylamino, N,N-dialkylamino, alkoxy, carboxylate or alkylthio group wherein the alkyl groups comprise from 1 to 8 carbon atoms, an aromatic group, or an alkyl group substituted by an aromatic group.

10. A dye according to Claim 9 characterised in that Ar represents a benzene ring or a pyridine ring.

11. A dye according any of Claims 1 to 7 characterised in that the cation has the formula

R-Y-ιr-A⁺ wherein R represents an alkyl or alkenyl group comprising from 8 to 30 carbon atoms, Y represents an oxygen atom, a sulphur atom an amino group or an alkylamino group comprising from 1 to 4 carbon atoms, ir represents a group selected from -CH=CH-, -CH=N-, -N=N- and -Ar- where Ar represents an aromatic ring and A⁺ represents a pyridinium, pyrimidinium, quinolinium, isoquinolinium, benzothiazolium or thiazolium cation.

12. A dye according to Claim 11 characterised in that Ar represents a benzene ring or a pyridine ring.

13. A dye according to either of Claims 11 or 12 characterised in that there is at least one alkyl group having from 1 to 4 carbon atoms attached to the nitrogen atom of the cation A⁺.

14. E-N-octadecyl-4-(2-(4-dimethylaminophenyl)ethenyl)pyridinium octadecylsulphate.

15. An optically non-linear Langmuir-Blodgett film characterised in that it comprises a dye according to any of Claims 1 to 14.

16. An alternate-layer Langmuir-Blodgett film characterised in that it comprises interleaving onolayers of a dye as defined in any of Claims 1 to 14 and a spacer molecule.

17. An alternate-layer Langmuir-Blodgett film according to Claim 16, characterised in that the spacer molecule is

4,4'-dioctadecyl-3,5,3' ,5'-tetramethyldipyrrylmethene hydrobromide.

18. An alternate-layer Langmuir-Blodgett film according to Claim 17, characterised in that the dye is E-N-octadecyl -4-( 2- (4-dimethylaminophenyl)ethenyl)pyridinium octadecylsulphate.