Strong main-chain length-dependence for the β-phase formation of oligofluorenes†
Poly-/oligofluorenes are promising luminescent materials applicable in organic light emitting diodes, lasers, etc. In particular, the β-phase structure, in which the fluorene main chain adopts a planar conformation with extended conjugation, shows characteristic optical properties. In this study, we have synthesized oligo(9,9′-dioctylfluorenes) whose chain lengths (n) are n = 9, 12, 15, 18, 21, 24 and 27 mers. Interestingly, the β-phase formation of the oligofluorenes is observed remarkably for oligofluorenes with n > 15, which is confirmed by the UV/vis and fluorescence spectroscopy. This study elucidates the structure–property relationship required to design high performance fluorene-based materials. Such a chain-length-dependent optical property provides useful information for utilizing the materials in many applications.