Films, dithiolene complexes

With a view to preparing polymerizable complexes, thiophene-substituted nickel-dithiolene complexes [Ni(L)(L )] have been synthesized and used to prepare films by electrochemical polymerization. The features of the complexes and of the polymers depend on the number of thiophene substituents. In particular, the complex with four thiophene substituents (L = L = thpdt, 12) shows a narrower HOMO-LUMO gap as compared to complexes with two thiophene and two phenyl groups or four phenyl groups [Xmax, nm (e, M- cm- ) 976 (38800) L = L = 12 931 (37700) L = 12, L = 9, R = Ph 866 (30900) L = L = 9, R = Ph] and gave a polymer whose electrochemical features are similar to those of poly[l,2-di(2,5-thienylene)ethane], suggesting that similar extended chains are formed"". 

This chapter first gives background on compounds that can be qualified as benchmarks in the field of molecular organic and inorganic conductors and superconductors. Focus is then further given to systems based on metal bis-dithiolene complexes. The last section considers the processing methods that can be applied to grow films of molecular conductors and superconductors. 

A mixed-valence complex of metal(dmit)2 (H2dmit = 4,5-dimercapto-l,3-dithiolo-2-thione) system is one of the promising candidates for the preparation of conductive LB films with higher dimensionality [27]. This sulfur-rich 1,2-dithiolene complex has provided three molecular superconductors, TTF[Ni(dmit)2]2 [28], TTF[Pd(dmit)2l2 [29], and Me4N[Ni(dmit)2]2 [30]. By the introduction of the alkylammonium group as a counter cation, the complexes become amphiphilic, and suitable for the construction of LB films. 

The Ni and Pt complexes can also be incorporated into polymer films of quaternized poly(vinylpyridine) (PVP) and deposited onto the transparent electrode (84). Photocurrents are enhanced to microamps (pA), an increase that may be attributed to either the effect of immobilization of the complexes near the electrode surface or an increase of the excited-state lifetimes in the polymer matrix. However, the effective concentrations of the complexes in this study were much greater than for the acetonitrile solutions in their earlier work. The polymer films are not stable to continuous photolysis, and voltammograms of the films are quite sensitive to anions used in the supporting electrolyte. The system can be stabilized by using a polymer blend of PVP and a copolymer containing quaternary ammonium ion and including [Fe(CN)6]4- in the electrolyte solution (85). Upon irradiation of the visible MLCT bands of [M(mnt)2]2 (M = Ni, Pt), photocurrents are produced. The mechanism (Scheme 4) is believed to involve photooxidation of the metal bis(dithiolene) triplet state by the Sn02 electrode, followed by [Fe(CN)6]4 reduction of the monoanion, with completion of the ET cycle as ferricyanide, Fe(CN)6 3, diffuses to the other electrode and is reduced. 

Kaye and Underhill [81] studied polypyrrole films containing redox-active anions. The anionic nickel and palladium complexes of l,2-dicyanoethylene-l,2-dithiolene served as the anions and produced films that were more stable than those containing tetrafluoroborate anions. 

---