Spectroscopic studies on the radical-cation dimer of a model compound of poly(p-phenylenevinylene). Similarities between the dimer and the state of positive polarons in the sulfuric-acid-treated polymer

Reversible dimerization of the radical cations of a model compound [CH₃(C₆H₄CH=CH)₂C₆H ₄CH₃] of poly-(p-phenylenevinylene) (PPV) in dichloromethane solution has been studied by electronic absorption, electron spin resonance, and resonance Raman spectroscopy. The electronic absorption 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 constants (K's) at various temperatures have been determined from the observed temperature dependence of the electronic absorption spectra. From the van't Hoff plot of ln K against 1/T, the heat of dimerization ΔH has been evaluated to be -12.6 kcal mol^-1. Electron spin resonance measurements have shown that the unpaired spins of the radical-cation monomers make a pair in the dimer. Such a radical-cation dimer is a good model of interchain interaction between polarons that are created on neighboring chain of PPV. The resonance Raman spectra of sulfuric-acid-treated PPV have been analyzed on the basis of the spectra of the radical-cation monomer and dimer. Sulfuric-acid-treated PPV is considered to contain positive polarons that interact with other polarons, in addition to positive polarons without such interaction.