Observation of the β-phase in two short-chain oligofluorenes

The polymer poly(9,9-dioctylfluorene) (PF8) can be driven into a morphological form termed the beta-phase that has enhanced planarity and increased structural rigidity. We show that the beta-phase can be generated in two different fluorene oligomers; a fluorene pentamer, and a short (statistical) oligomer composed of chains having a maximum length of 19 monomer units. By comparing the energy of the zero-zero phonon line in fluorescence from the beta-phase oligomers with that in the PF8 polymer, we show that the electronic conjugation length of the beta-phase is (30 +/- 12) monomer-units, a value consistent with the persistence length of the molecular chain.