Spiro-Linked Ter-, Penta-, and Heptafluorenes as Novel Amorphous Materials for Blue Light Emission

A series of spiro-linked oligofluorenes were synthesized that exhibit blue emission with photoluminescence quantum yields in some cases exceeding 0.50 in neat spin-coated films. Differential scanning calorimetry revealed highly variable glass transition temperatures. A longer conjugation length is desired for better stability against thermally activated crystallization, and a shorter pendant is desired for a higher glass transition temperature at a given conjugation length. In contrast to poly(2,7-(9,9-di-n-octylfluorene)), prolonged heating of spiro-oligo(fluorene) films resulted in no changes in morphology, emissive color, and photoluminescence quantum yield. The propensity of spiro-oligo(fluorene)s to thermally activated crystallization was rationalized with molecular shapes predicted by molecular mechanics simulations.