Magnetic properties of quinoidal oligothiophenes: more than good candidates for ambipolar organic semiconductors

A series of quinoidal oligothiophenes have been investigated by means of solid-state Fourier-transform (FT)-Raman and electron spin resonance (ESR) spectroscopies complemented with density functional theory calculations. FT-Raman spectra recorded as a function of temperature show that, upon laser irradiation, the molecules undergo a reversible structural evolution from a quinoid-type pattern at low temperature to an aromatic-type pattern at high temperature. Moreover, ESR spectra show that a portion of these compounds exists in a biradical state at room temperature. These seemingly disconnected findings and others, such as conformational isomerism, are consistently explained by the consideration of biradical species associated with the presence of low-lying triplet electronic states. In addition to the well-established versatility of quinoidal oligothiophenes regarding ambipolar electrical actuation in field-effect transistors, the exhibition of dual electrical and magnetic behavior leads to the prospect of new materials that have tunable electrical, optical, and magnetic properties.