Comments On The Possible Solvent E ffect.

It has been suggested that there is a possible effect due to trapped solvent11, producing a step in the magnetic and conductivity data for complexes such as NMP- TCHQ. Work carried out by the author at Cranfield Institute of Technology showed that when acetonitrile was the solvent in the preparations there was a transition at approximately its freezing point (229K). When using other solvents the results were inconclusive.

It was possible that transitions observed at

Cranfield Institute of Technology could have been

attributed to errors in the magnetic balance used. However, similar results have been observed in some of the magnetic (Fig.5.29) and electrical conductivity (Fig.30) data obtained at Sheffield City Polytechnic.

The only commom piece of equipment at Sheffield City Polytechnic and Cranfield Istitute of Technology was the C.I.Robal microbalance. It is, therefore, the view of the author that instrumental error in this matter has been eliminated, and this possible effect (the focus of debate) warrants further investigation.

8.3 O£ CM LlI X_\ 5.3 ■a. a 155.0 TCKD Fig.5.29.

Magnetic Susceptibility Of MG-TCNQ Showing An Anomaly At Approximately 229K Possibly Due To Trapped Solvent.

Fig.5.30.

Electrical Conductivity Data For TB-TCNQ Showing An Anomaly At Approximately 229K Possibly Due To Trapped Solvent.

RESULTS.

CHAPTER 6

6.Results. Discussion For CT Polymer Synthesis

The aim of this part of the project was to investigate the possibility of preparing a CT polymer composite with a structure somewhat similar to a side chain liquid crystal polymer blended with a complementary, small molecular species (see sections 6.1 and 6.2).

It was not intended to take one synthetic pathway and persuade it to work, as this could have taken the whole three years and not produced a result, but rather to look at a number of possible synthetic routes to assess their viability.

Over the course of this project a number of pathways and options were investigated. These are outlined below:

6.1. Types Of CT Polymer Considered.

Three types of constitution were envisaged:

6.1.1. Polymer Bound Donors And Accentors.

This was not considered viable as the entropy of the system in the necessary configuration, (i.e. segregated and long range stacking), would be very unfavourable in the context of normal polymer conformational behaviour.

6.1.2. Polymer Bound Donors Blended With Monomeric Acceptors.

Again this was not investigated because the important criteria for conduction in many CT systems is a continuous stack of CT electron acceptor molecules. Blending free acceptors with a CT donor polymer would not guarantee the correct stacking of the acceptors.

The only advantages in this strategy were, firstly, that it may have been simpler to attach the spacer groups to the type of chemical species found on donor molecules and secondly, that in some systems donors are associated with conduction (eg via holes). This approach was rejected on the grounds of both cost (many donors and their precursors are very expensive) and of the need to adopt a target and concentrate upon it.

6.1.3. Polymer Bound Acceptors Blended With Monomeric Donors.

This was the strategy eventually adopted in the hope of producing a TC

UQ

derivative chemically bound to a suitable polymer. It was envisaged that the polymer backbone would influence the long range ordering of the CT units, whilst short range CT stacking may be expected to be influenced by both spacer design, and factors that would effect unbound crystal formation, such as lattice energy. At this stage we do not know what effect the combination of these factors will have on the overall ordering of the system. Conductive (probably semi-conductive) polymer

blends night be obtained if the polymer could be made to produce appropriate ordered phases and the TCNQ moeities could be doped by blending with appropriate donor species.

The sought for ordered phase would be encouraged by slow cooling of the composite material from the molten or

solution state. Using this approach side chain

crystallisation, (eg by stacking of acceptors), should be optimised, as in the case of analogous LC systems possessing long spacer linkages.

The aim was to look at poly(alkylmethylsiloxane) or

monosubstituted polyethylene as selected polymer

candidates with siloxane the first choice since it is a particularly flexible polymer, and should allow the TCNQ moeities greater freedom to form ordered stacks than would be the case for monosubstituted polyethylene. In both cases the TCNQ derivative is attached to the polymer via an alkyl spacer group as shown below in Fig.6.1.

CH,

Si-0_I

R

Poly(alkyl nethylsiloxane)

Where R = - t CH£-i^TCN(!

R' = -f-CH H r - T C N G

CH — CH

_{£ |}

R'

Nonosubstituted

Polyethylene

Fig.6.1.

The synthesis of such a system required TCNQ units to be attached to a siloxane polymer backbone via a flexible alkyl spacer. The initial step in this process was to synthesise a TCNQ derivative in which an oligoalkyl spacer containing a terminal alkene suitable for reaction onto the siloxane polymer, was attached to TCNQ.

6.2. Synthetic Pathways Attempted.

Outlined below are three reaction schemes that were investigated in order to find a viable synthesis for the TCNQ derivative.

1. Claisen rearrangement reaction on a precursor of TCNQ, followed by conversion to the TCNQ derivative (scheme 1A).

2. Friedel-Crafts reactions on precursors of TCNQ, followed by conversion to the TCNQ derivatives (scheme IB).

3. Similar to 1 except a Stork enamine synthesis was to be used (scheme 2).

4. Replacement of a cyano group in TCNQ with an amino-alkyl spacer (scheme 3).

The details of the reaction schemes are outlined below:

In document The investigation of novel charge transfer systems (Page 182-189)