INFRARED AND RAMAN-STUDY OF POLYANILINE .2. INFLUENCE OF ORTHO SUBSTITUENTS ON HYDROGEN-BONDING AND UV/VIS NEAR-IR ELECTRON CHARGE-TRANSFER

IR and Raman spectra of chemically (electrochemically) prepared protonated forms of polymers of 2-methyl (PANIME), 2-methoxy (PANIMEO), 2-chloro (PANICL) and 2-fluoroanilines (PANIF) and their corresponding bases have been investigated in the 4000-100(200)cm(-1) region. Exciting lines at 457.94, 514.53, 632.81 and 1064nm have been used for the investigation of Raman resonance spectra in order to characterize the benzenoid and (semi)quinoid (SQ) parts of the repeat unit. Highly conducting poly-2-methyl and poly-2-methoxyaniline on the one hand show spectral features which can be interpreted in terms of a strong N-H ... N hydrogen bond (d(N-N)approximate to 0.25nm) with a NH stretching broad absorption centred near 1300 cm(-1) cut by numerous Evans transmission bands. On the other hand, poly-2-chloro and poly-2-fluoroaniline which have much lower conductivity have much weaker N-H ... X hydrogen bonds characterized by NH stretching bands near 3000cm(-1) (d(N-N)approximate to 0.28/0.285nm). Conductivity mechanisms are discussed. UV/Vis-IR spectra show that conductivity is essentially associated with interchain coupling. IR spectra suggest that in PANIME and PANIMEO as in PANI the interchain charge carriers constitute bipolaronic species, i.e. -NH+=SQ=NH+-, cross-linked by NK ... N hydrogen bonds. In weakly conducting PANIF and PANICL, however, disproportionation of the bipolaronic species, no longer stabilized by H-bonding, leads to localized polaronic moieties. Possible mechanisms for proton conduction are discussed.