Time-resolved dual fluorescence and transient absorption of 9-anthryl oligothiophenes

Abstract

The excited state dynamics of 9-anthryl oligothiophenes (9A-Tn; n ≤ 5) has been studied in solution at temperatures between 100 and 295 K applying ps time-resolved fluorescence and fs transient absorption techniques. The previously discovered dual fluorescence has been verified to occur for 9A-Tn only at n ≤ 3 in liquid environment. For n > 3 always a single oligothiophene-type fluorescence exists. The blue and red components of the dual fluorescence are coupled dynamically on a ps time scale. From analysis in solvents of different polarity and viscosity evidence is provided about the nature of the dual fluorescence. It originates in an intramolecular conformational relaxation between the anthracene and oligothiophene molecular subunits (predominantly controlled by the solvent viscosity) and not in complete charge transfer in the excited state. Therefore, even in polar solvents, the transient absorption spectra show no indication of radical ion formation. Only partial charge transfer takes place during excited state relaxation leading to positive solvatochromism of the red fluorescence component.