SPECTROSCOPIC STUDIES ON THE RADICAL-CATION DIMER OF A MODEL-COMPOUNDOF POLY(P-PHENYLENEVINYLENE) - SIMILARITIES BETWEEN THE DIMER AND THESTATE OF POSITIVE POLARONS IN THE SULFURIC-ACID-TREATED POLYMER

Reversible dimerization of the radical cations of a model compound [CH 3(C6H4CH=CH)(2)C6H4CH3] of poly-(p-phenylenevinylene) (PPV) in dichlor omethane solution has been studied by electronic absorption, electron spin resonance, and resonance Raman spectroscopy. The electronic absor ption spectrum of the radical-cation monomer drastically changes upon dimerization; the absorption maxima of the monomer are located at 636 and 1200 nm (15 700 and 8330 cm(-1)) and those of the dimer at 527 and 981 nm (19 000 and 10 200 cm(-1)) The monomer-dimer equilibrium const ants (K's) at various temperatures have been determined from the obser ved temperature dependence of the electronic absorption spectra. From the van't Hoff plot of ln K against 1/T, the heat of dimerization Delt a H has been evaluated to be -12.6 kcal mol(-1). Electron spin resonan ce measurements have shown that the unpaired spins of the radical-cati on monomers make a pair in the dimer. Such a radical-cation dimer is a good model of interchain interaction between polarons that are create d on neighboring chains of PPV. The resonance Raman spectra of sulfuri c-acid-treated PPV have been analyzed on the basis of the spectra of t he radical-cation monomer and dimer. Sulfuric-acid-treated PPV is cons idered to contain positive polarons that interact with other polarons, in addition to positive polarons without such interaction.