XPS AND SIMS STUDIES OF CHARGE TRANSFER INTERACTIONS IN NITROGEN CONTAINING NON-CONJUGATED ELECTROACTIVE POLYMERS : POLY(N-VINYLCARBAZOLE) AND POLYVINYLPYRIDINE

Abstract

The 'charge-transfer (CT) interactions between non-conjugated polymers such as poly(N vinylcarbazole) (PVK), poly(2-vinylpyridine) (P2VP) and poly(4-vinylpyridine) (P4VP) and various electron acceptors, such as tetrachloro-o-benzoquinone ( o-chloranil), tetrachloro-p-benzoquinone (pchloranil), tetrabromo-o-benzoquinone (o-bromanil), tetraf!uoro-pbenzoquinone (p-fluoranil), tetracyanoethylene (TCNE), 2,4,7,trinitrofluoronone (TNF), and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) were studied using X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS). XPS provides a convenient means for elucidating the extent of CT and the structure of a polymeric complex. The CT interactions involving the polymeric electron donors (PVK, P2VP and P4VP) and the various electron acceptors are reflected in the XPS core-level spectra of the functional species. Static SIMS on the other hand provides confirmation to the plausible molecular structures resulting from the CT interactions. In the PVK-halobenzoquinone (o-chloranil, p-chloranil and obromanil) er complexes, the halogen species were found to exist in two distinct chemical environments. The covalently bonded halogen species are associated with the dopant molecules while the anionic species are the result of the CT interactions between the dopants and the PVK molecule. A partial delocalization of charges around the halobenzoquinone species was observed and a higher degree of CT could be seen in the complex with less steric hindrance. A partially charged state was also observed in PVK TCNE and PVK-DDQ complexes. However, CT in these complexes are relatively weak. Negligible CT interaction was observed in the PVK-TNF complex. In the PVK-halogen CT complexes, a predominant presence of covalently bonded halogen species was observed. This is indicative that CT interactions have progressed beyond the mere formation of molecular complexes. In the P2VP and P4VP CT complexes studied, the positions of the pyridinium nitrogens in the two polymers and the various cyano- and halobenzoquinones with functional groups at different positions in the ring provide a good basis for the investigation of the effect of steric hindrance on the CT interactions. The XPS data show that the pyridinium nitrogen in P2VP is in a more hindered configuration than in P4VP. Also, the para- halobenzoquinones in general encounter more steric hindrance than their ortho- counterpart. Amongst the various halobenzoquinone species, fluoranil has the highest reactivity followed by bromanil and chloranil. A similar two-environment component halogen species was observed in the P2VP and P4VP-halobenzoquinone CI' complexes. The presence of positively charged nitrogens and the halogen and cyano anions suggest the cleavage of the carbon-halogen or carbon-cyano bonds of the acceptor and the formation of linkages between the pyridinium nitrogens and the acceptors at the site of cleavage. The XPS data of the P2VP and P4VP CT complexes indicate that the reactivity of the functional groups towards complex formation with polyvinylpyridine decreases in the order C=N>C)>C1. The static SIMS data provide further evidence that the CT involving p-chloranil is sterically more hindered than that involvh1g o-chloranil acceptors. P4VP-o-chloranil complex with least steric hindrance further confirms a molecular structure involving the interaction between the pyridinium nitrogen and the halogen species of the acceptor. Another molecular structure involving the interaction between the pyridinium nitrogen and the oxygen species of the acceptor is also present but with less dependence on the steric hindrance. In the case of P4VP-o-chloranil, each dopant molecule can interact with more than one pyridinium nih·ogen. The pyridinium nitrogens may originate from a single molecular chain resulting in chain distortion or from neighbouring polymer chains giving rise to chain cross-linking.