The synthesis and screening of substituted sulphur heterocycles for Langmuir-Blodgett films and non-linear optics

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Routledge, P., 1995. The synthesis and screening of substituted sulphur heterocycles for Langmuir-Blodgett films and non-linear optics. PhD, Nottingham Trent University.

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Abstract

Over recent years there has been a great increase in research into Langmuir- Blodgett (LB) thin films, as the LB method is an organic fabrication technique appropriate for the formation of monolayer and multilayer structures that have a high degree of anisotropy and structural order as well as a precise and uniform thickness. In particular, considerable interest in the second-order non-linear optical properties of molecules that can be deposited as LB films has arisen, since the technique enables them to be readily assembled into non-centrosymmetric structures that ensures that the individual second-order hyperpolarisabilities do not cancel one another.

One group of materials that have been investigated extensively, and which have produced exciting results are hemicyanine dyes, which have a D-π-A system (donor and acceptor groups linked by a conjugated π system) containing a stilbene unit bonded to a heterocyclic group containing nitrogen. In this study a thiophene moiety was incorporated into the molecule instead of the nitrogen heterocycle. The inclusion of a π-excessive centre into a stilbene type molecule of this type may enhance the second-order non-linear optical properties of the material. Background experiments have been performed in which the thiophene units were incorporated into various simple long chain molecules, in order to establish how this moiety was likely to effect the film forming capabilities, film stability at the air/subphase interface and the transfer ratio and mode of transfer onto the solid substrate.

The design, synthesis and subsequent systematic characterisation of several members of three homologous series of novel thiophene based compounds, which are amphiphilic in nature, is described. The characterisation of these films involved the production of isotherms under subphase conditions that were varied systematically; subsequently the area per molecule, decay rate of the film on the subphase, transfer ratio of the film to a substrate and the mode of transfer of the films have been determined. A systematic investigation into the effects on monolayer stability and transferability of introducing a thiophene moiety at various positions in the alkyl chain of stearic acid is also reported.

Subsequent work focused on the design, synthesis and characterisation of a few members of another homologous series of thiophene based amphiphilic compounds. The members of this series were somewhat more complicated in structure as compared to the first few series. These molecules incorporated an extended region of π-conjugated bonds, which was bounded at one end by an electron donating group and at the other end by an electron accepting group. This gives a Donor-π-Acceptor (D-π-A) system. There was also a long alkyl chain to confer the required water insolubility to the system.

Once again these molecules have been subjected to a systematic investigation of LB characteristics; this screening was extended to include a study of the characteristics of heterogeneous films incorporating various percentages of arachidic acid. Since these films adopted the centrosymmetric Y-type mode of deposition onto a solid substrate, alternate layer structures would be required if any second-order nonlinear properties of the molecules are to be exploited. Preliminary second harmonic generation experiments on a series of such structures, performed in collaboration with a group at Durham University, indicate that the material is not significantly more active than other materials reported in the literature.

Item Type:

Thesis

Creators:

Routledge, P.

Date:

1995

ISBN:

9781369312881

Identifiers: