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

• Published in: The journal of physical chemistry. B Vol. 101; no. 10; pp. 1726 - 1732
• Main Authors: Sakamoto, Akira, Furukawa, Yukio, Tasumi, Mitsuo
• Format: Journal Article
• Language: English
• Published: American Chemical Society 06-03-1997
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp9619649

Reversible dimerization of the radical cations of a model compound [CH3(C6H4CHCH)2C6H4CH3] 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 chains 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.