1. ICI Chemicals and Polymers, P.O. Box 8, The Heath, Runcorn, Cheshire WA7 4QD, U.K., Department of Chemistry, School of Engineering, and Centre for Molecular Electronics, University of Durham, South Road, Durham DH1 3LE, U.K., ICI Wilton Research Centre, P.O. Box 90, Wilton, Middlesbrough, TS90 8JE, U.K., Zeneca Specialities, P.O. Box 42, Hexagon House, Blackley, Manchester M9 3DA, U.K., and Materials Research Institute, Sheffield Hallam University, City Campus, Pond Street, Sheffield S1 1WB, U.K.