Z-type films for second harmonic generation

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428 1, 428704, 428910, 428913, B32B 904





The present invention relates to optically non-linear materials suitable for forming Z-type films for second harmonic generation, to methods for their synthesis and to optical devices which incorporate such materials.
WO 89/05843 discloses amphiphilic hemicyanines such as iodide, useful as non-linear optical media, which may form Langmuir-Blodgett films of non-centrosymmetric chromophores.
EP-A-203780 discloses a multi-layer Langmuir-Blodgett film having non-linear optical properties, consisting of alternating layers of differently substituted asymmetric hemicyanine dyes.
EP-A423619 discloses various compounds which can be deposited by Langmuir-Blodgett techniques to form films for non-linear-optics purposes.
Film-forming hemicyanine dyes of general formula D--C.sub.6 H.sub.4 --CH.dbd.CH--C.sub.5 H.sub.4 N.sup.+ --R X.sup.- where D is an electron donating group, R is a hydrophobic alkyl group and X.sup.- is a simple inorganic anion such as a halide ion have been extensively studied for second harmonic generation (SHG). Their properties are dependent upon the donor group (Bubeck et al, Advanced Materials 1991, 3, 54), the counterion (Liu et al, Thin Solid Films 1992, 219 221; Ashwell et al Nature 1992, 357 393) and the extent of aggregation (Girling et al, Thin Solid Films 1985, 132, 101; Schildkraut et al., Opt. Lett 1988, 13 134). In our International PCT Publication WO 93/24859 we have disclosed that Langmuir-Blodgett (LB) film structures which generate a signal of greater intensity may be produced by utilising an amphiphilic anion rather than a simple inorganic anion. For these known LB film-forming materials, it has been assumed that there is a requirement for the cation to be amphiphilic with a long hydrophobic tail group attached to one end of the hydrophilic chromophore, this being necessary so that the film-forming molecules may be aligned at the air/water interface, with the head groups downwards. Such films typically display centrosymmetric interlayer arrangements with the molecules packing head-to-head and tail-to-tail (Y-type). The molecules tend to "flip" over to retain the hydrophobic parts together but a non-centrosymmetric structure has been obtained by the incorporation of spacer layers between the active LB layers (International Patent Application WO 92/09374).
We have now discovered that, in film forming hemicyanine dyes, a more favourable non-centrosymmetric, head-to-tail, Z-type, packing arrangement may be produced by ensuring that the hydrophilic chromophore is substituted at opposite ends by hydrophobic alkyl, alkenyl or alkylaryl groups comprising from 4 (less than 4 would be insufficiently hydrophobic) to 30 carbon atoms, in which the groups at each end differ in size by four or more carbon atoms, e.g. dihexyl (12C) and octadecyl (18C), difference=6C. A difference of at least four carbon atoms is necessary for Z-type packing, if the inconvenience of intermediate layers of a different compound is to be avoided.
Preferred cationic dyes of use in the present invention are those of general formula R'D--.pi.--A--R (neutral) or R'D--.pi.--A.sup.+ R (cationic) wherein R'D represents a hydrophobic donor group; .pi. represents a conjugated .pi.-electron bridging group; A and A.sup.+ represent electron acceptor groups having hydrophobic substituents R.
Preferably R'D represents an N-alkylamino, N,N-diallylamino, alkoxy, carboxylate R'C(O)O--, alkylthio or alkylseleno group and A-R represents a keto (RC(O)--) or ester (ROC(O)--) group. A.sup.+ represents a heterocyclic group such as a pyridinium, pyrimidinium, quinolinium, isoquinolinium, benzothiazolium or thiazolium cation and R is preferably attached to the heteroatom. A.sup.+ may be optionally substituted.
R and R' represent hydrophobic alky, alkenyl or alkylaryl groups comprising 4 to 30 carbon atoms, R and R' being the same or different, optionally differing by four or more carbon atoms. The .pi.-bridge is --CH.dbd.CH--, --CH.dbd.N--, --N.dbd.N--, --Ar-- or multiples or combinations thereof, wherein Ar represents an aromatic ring especi

patent: 5510200 (1996-04-01), Ashwell


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