Delocalization of Positive and Negative Charge Carriers on Oligo- and Poly-fluorenes Studied by Low-Temperature Matrix Isolation Technique

Oligofluorenes with well-defined chain length (n = 1–5) have been synthesized by step-by-step Suzuki coupling reaction as the basis sets of conjugated segments in polyfluorenes. Cation and anion radicals of oligofluorenes (n > 2) showed intense and clear three infrared absorptions attributed to transitions between SOMO state delocalized over fluorene moieties and valence (HOMOs) or conduction states (LUMOs) in the backbone. The transition energies of the IR band decreased linearly from 1.2 to 0.55 eV in the range of unit number while 1-D exciton transition energies showed apparent saturation at n > 5. This implied that delocalization degree of excess electrons or holes along fluorene chains would saturate at larger number of units than that of neutral excitons.